Selective-area growth of GaN nanocolumns on Si(111) substrates for application to nanocolumn emitters with systematic analysis of dislocation filtering effect of nanocolumns

NASA Astrophysics Data System (ADS)

Kishino, Katsumi; Ishizawa, Shunsuke

2015-06-01

The growth of highly uniform arrays of GaN nanocolumns with diameters from 122 to 430 nm on Si (111) substrates was demonstrated. The employment of GaN film templates with flat surfaces (root mean square surface roughness of 0.84 nm), which were obtained using an AlN/GaN superlattice (SL) buffer on Si, contributed to the high-quality selective-area growth of nanocolumns using a thin Ti mask of 5 nm thickness by rf-plasma-assisted molecular beam epitaxy. Although the GaN template included a large number of dislocations (dislocation density ˜1011 cm-2), the dislocation filtering effect of nanocolumns was enhanced with decreasing nanocolumn diameters (D). Systematic transmission electron microscopy (TEM) observation enabled us to explain the dependence of the dislocation propagation behavior in nanocolumns on the nanocolumn diameter for the first time. Plan-view TEM analysis was performed for nanocolumns with D = 120-324 nm by slicing the nanocolumns horizontally at a height of ˜300 nm above their bottoms and dislocation propagation through the nanocolumns was analyzed by the cross-sectional TEM observation of nanocolumns with D ˜ 200 nm. It was clarified that dislocations were effectively filtered in the bottom 300 nm region of the nanocolumns, the dislocation density of the nanocolumns decreased with decreasing D, and for narrow nanocolumns with D < 200 nm, dislocation-free crystals were obtained in the upper part of the nanocolumns. The dramatic improvement in the emission properties of GaN nanocolumns observed with decreasing diameter is discussed in relation to the decreased dislocation density. The laser action of InGaN/GaN-based nanocolumn arrays with a nanocolumn diameter of 170 nm and a period of 200 nm on Si under optical excitation was obtained with an emission wavelength of 407 nm. We also fabricated red-emitting InGaN-based nanocolumn light-emitting diodes on Si that operated at a wavelength of 652 nm, demonstrating vertical conduction through the AlN/GaN SL buffer to the Si substrate.

Integrating AlGaN/GaN high electron mobility transistor with Si: A comparative study of integration schemes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mohan, Nagaboopathy; Raghavan, Srinivasan; Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore 560012

2015-10-07

AlGaN/GaN high electron mobility transistor stacks deposited on a single growth platform are used to compare the most common transition, AlN to GaN, schemes used for integrating GaN with Si. The efficiency of these transitions based on linearly graded, step graded, interlayer, and superlattice schemes on dislocation density reduction, stress management, surface roughness, and eventually mobility of the 2D-gas are evaluated. In a 500 nm GaN probe layer deposited, all of these transitions result in total transmission electron microscopy measured dislocations densities of 1 to 3 × 10{sup 9}/cm{sup 2} and <1 nm surface roughness. The 2-D electron gas channels formed atmore » an AlGaN-1 nm AlN/GaN interface deposited on this GaN probe layer all have mobilities of 1600–1900 cm{sup 2}/V s at a carrier concentration of 0.7–0.9 × 10{sup 13}/cm{sup 2}. Compressive stress and changes in composition in GaN rich regions of the AlN-GaN transition are the most effective at reducing dislocation density. Amongst all the transitions studied the step graded transition is the one that helps to implement this feature of GaN integration in the simplest and most consistent manner.« less

Structural investigations of GaN grown by low-pressure chemical vapor deposition on 6H{endash}SiC and Al{sub 2}O{sub 3} from GaCl{sub 3} and NH{sub 3}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Koynov, S.; Topf, M.; Fischer, S.

1997-08-01

GaN films grown on (0001) 6H{endash}SiC and (0001) Al{sub 2}O{sub 3} substrates using low-pressure chemical vapor deposition with GaCl{sub 3} and NH{sub 3} as precursors are comparatively explored by optical, scanning tunneling, and transmission electron microscopy. Independent of the substrate material used, the surface of the GaN layers is covered by hexagonally shaped islands. For GaN on 6H{endash}SiC, the islands are larger in diameter ({approx}50 {mu}m) and rather uniformly distributed. An atomically flat interface is observed for GaN on Al{sub 2}O{sub 3} in contrast to GaN grown on 6H{endash}SiC, where the interface is characterized by large steps. For both substrates,more » faceted holes (named as pinholes) are observed in near-surface regions of the GaN layers occurring with a density of about 7{times}10{sup 8} cm{sup {minus}2}. No unequivocal correlation between the density of pinholes and the density of threading dislocations ({approx}1.6{times}10{sup 10} cm{sup {minus}2} for GaN/Al{sub 2}O{sub 3} and {approx}4{times}10{sup 9} cm{sup {minus}2} for GaN/6H{endash}SiC) can be found. Rather, different types of defects are identified to be correlated with the pinholes, implying a dislocation-independent mechanism for the pinhole formation. Despite the small lattice mismatch between GaN and 6H{endash}SiC, the pronounced original surface roughness of this substrate material is believed to account for both the marked interfacial roughness and the still existing high density of threading dislocations. {copyright} {ital 1997 American Institute of Physics.}« less

High-Brightness Blue Light-Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer.

PubMed

Chen, Zhaolong; Zhang, Xiang; Dou, Zhipeng; Wei, Tongbo; Liu, Zhiqiang; Qi, Yue; Ci, Haina; Wang, Yunyu; Li, Yang; Chang, Hongliang; Yan, Jianchang; Yang, Shenyuan; Zhang, Yanfeng; Wang, Junxi; Gao, Peng; Li, Jinmin; Liu, Zhongfan

2018-06-08

Single-crystalline GaN-based light-emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid-state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high-quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high-brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal-organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and In x Ga 1- x N/GaN multiple quantum well structures. The as-fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one-step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high-brightness LEDs. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Relationship between dislocation and the visible luminescence band observed in ZnO epitaxial layers grown on c-plane p-GaN templates by chemical vapor deposition technique

NASA Astrophysics Data System (ADS)

Saroj, Rajendra K.; Dhar, S.

2016-08-01

ZnO epitaxial layers are grown on c-plane GaN (p-type)/sapphire substrates using a chemical vapor deposition technique. Structural and luminescence properties of these layers have been studied systematically as a function of various growth parameters. It has been found that high quality ZnO epitaxial layers can indeed be grown on GaN films at certain optimum conditions. It has also been observed that the growth temperature and growth time have distinctly different influences on the screw and edge dislocation densities. While the growth temperature affects the density of edge dislocations more strongly than that of screw dislocations, an increase of growth duration leads to a rapid drop in the density of screw dislocation, whereas the density of edge dislocation hardly changes. Densities of both edge and screw dislocations are found to be minimum at a growth temperature of 500 °C. Interestingly, the defect related visible luminescence intensity also shows a minimum at the same temperature. Our study indeed suggests that the luminescence feature is related to threading edge dislocation. A continuum percolation model, where the defects responsible for visible luminescence are considered to be formed under the influence of the strain field surrounding the threading edge dislocations, is proposed. The theory explains the observed variation of the visible luminescence intensity as a function of the concentration of the dislocations.

Self-organization of dislocation-free, high-density, vertically aligned GaN nanocolumns involving InGaN quantum wells on graphene/SiO2 covered with a thin AlN buffer layer.

PubMed

Hayashi, Hiroaki; Konno, Yuta; Kishino, Katsumi

2016-02-05

We demonstrated the self-organization of high-density GaN nanocolumns on multilayer graphene (MLG)/SiO2 covered with a thin AlN buffer layer by RF-plasma-assisted molecular beam epitaxy. MLG/SiO2 substrates were prepared by the transfer of CVD graphene onto thermally oxidized SiO2/Si [100] substrates. Employing the MLG with an AlN buffer layer enabled the self-organization of high-density and vertically aligned nanocolumns. Transmission electron microscopy observation revealed that no threading dislocations, stacking faults, or twinning defects were included in the self-organized nanocolumns. The photoluminescence (PL) peak intensities of the self-organized GaN nanocolumns were 2.0-2.6 times higher than those of a GaN substrate grown by hydride vapor phase epitaxy. Moreover, no yellow luminescence or ZB-phase GaN emission was observed from the nanocolumns. An InGaN/GaN MQW and p-type GaN were integrated into GaN nanocolumns grown on MLG, displaying a single-peak PL emission at a wavelength of 533 nm. Thus, high-density nitride p-i-n nanocolumns were fabricated on SiO2/Si using the transferred MLG interlayer, indicating the possibility of developing visible nanocolumn LEDs on graphene/SiO2.

Movement of basal plane dislocations in GaN during electron beam irradiation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yakimov, E. B.; National University of Science and Technology MISiS, Leninskiy pr. 4, Moscow 119049; Vergeles, P. S.

The movement of basal plane segments of dislocations in low-dislocation-density GaN films grown by epitaxial lateral overgrowth as a result of irradiation with the probing beam of a scanning electron microscope was detected by means of electron beam induced current. Only a small fraction of the basal plane dislocations was susceptible to such changes and the movement was limited to relatively short distances. The effect is explained by the radiation enhanced dislocation glide for dislocations pinned by two different types of pinning sites: a low-activation-energy site and a high-activation-energy site. Only dislocation segments pinned by the former sites can bemore » moved by irradiation and only until they meet the latter pinning sites.« less

Dislocation filtering in GaN nanostructures.

PubMed

Colby, Robert; Liang, Zhiwen; Wildeson, Isaac H; Ewoldt, David A; Sands, Timothy D; García, R Edwin; Stach, Eric A

2010-05-12

Dislocation filtering in GaN by selective area growth through a nanoporous template is examined both by transmission electron microscopy and numerical modeling. These nanorods grow epitaxially from the (0001)-oriented GaN underlayer through the approximately 100 nm thick template and naturally terminate with hexagonal pyramid-shaped caps. It is demonstrated that for a certain window of geometric parameters a threading dislocation growing within a GaN nanorod is likely to be excluded by the strong image forces of the nearby free surfaces. Approximately 3000 nanorods were examined in cross-section, including growth through 50 and 80 nm diameter pores. The very few threading dislocations not filtered by the template turn toward a free surface within the nanorod, exiting less than 50 nm past the base of the template. The potential active region for light-emitting diode devices based on these nanorods would have been entirely free of threading dislocations for all samples examined. A greater than 2 orders of magnitude reduction in threading dislocation density can be surmised from a data set of this size. A finite element-based implementation of the eigenstrain model was employed to corroborate the experimentally observed data and examine a larger range of potential nanorod geometries, providing a simple map of the different regimes of dislocation filtering for this class of GaN nanorods. These results indicate that nanostructured semiconductor materials are effective at eliminating deleterious extended defects, as necessary to enhance the optoelectronic performance and device lifetimes compared to conventional planar heterostructures.

Transmission electron microscopy study of microstructural properties and dislocation characterization in the GaN film grown on the cone-shaped patterned Al2O3 substrate.

PubMed

Park, Jung Sik; Yang, Jun-Mo; Park, Kyung Jin; Park, Yun Chang; Yoo, Jung Ho; Jeong, Chil Seong; Park, Jucheol; He, Yinsheng; Shin, Keesam

2014-02-01

Growing a GaN film on a patterned Al2O3 substrate is one of the methods of reducing threading dislocations (TDs), which can significantly deteriorate the performance of GaN-based LEDs. In this study, the microstructural details of the GaN film grown on a cone-shaped patterned Al2O3 substrate were investigated using high-resolution transmission electron microscopy and weak-beam dark-field techniques. Various defects such as misfit dislocations (MDs), recrystallized GaN (R-GaN) islands and nano-voids were observed on the patterned Al2O3 surfaces, i.e. the flat surface (FS), the inclined surface (IS) and the top surface (TS), respectively. Especially, the crystallographic orientation of R-GaN between the GaN film and the inclined Al2O3 substrate was identified as $[\\overline 1 2\\overline 1 0]_{{\\rm GaN}} \\hbox{//}[\\overline 1 101]_{{\\rm R - GaN} \\,{\\rm on}\\,{\\rm IS}} \\hbox{//}[\\overline 1 100]_{ {{\\rm Al}} _{\\rm 2} {\\rm O}_{\\rm 3}} $, $(\\overline 1 012)_{{\\rm GaN}} \\hbox{//}(1\\overline 1 02)_{{\\rm R - Ga}\\,{\\rm Non}\\,{\\rm IS}} \\hbox{//}(\\overline {11} 26)_{ {{\\rm Al}} _{\\rm 2} {\\rm O}_{\\rm 3}} $. In addition, a rotation by 9° between $(10\\overline 1 1)_{{\\rm R - GaN}} $ and $(0002)_{{\\rm GaN}} $ and between $(10\\overline 1 1)_{{\\rm R - GaN}} $ and $(0006)_{ {{\\rm Al}} _{\\rm 2} {\\rm O}_{\\rm 3}} $ was found to reduce the lattice mismatch between the GaN film and the Al2O3 substrate. Many TDs in the GaN film were observed on the FS and TS of Al2O3. However, few TDs were observed on the IS. Most of the TDs generated from the FS of Al2O3 were bent to the inclined facet rather than propagating to the GaN surface, resulting in a reduction in the dislocation density. Most of the TDs generated from the TS of Al2O3 were characterized as edge dislocations.

Structural anisotropic properties of a-plane GaN epilayers grown on r-plane sapphire by molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lotsari, A.; Kehagias, Th.; Katsikini, M.

2014-06-07

Heteroepitaxial non-polar III-Nitride layers may exhibit extensive anisotropy in the surface morphology and the epilayer microstructure along distinct in-plane directions. The structural anisotropy, evidenced by the “M”-shape dependence of the (112{sup ¯}0) x-ray rocking curve widths on the beam azimuth angle, was studied by combining transmission electron microscopy observations, Raman spectroscopy, high resolution x-ray diffraction, and atomic force microscopy in a-plane GaN epilayers grown on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy (PAMBE). The structural anisotropic behavior was attributed quantitatively to the high dislocation densities, particularly the Frank-Shockley partial dislocations that delimit the I{sub 1} intrinsic basal stacking faults,more » and to the concomitant plastic strain relaxation. On the other hand, isotropic samples exhibited lower dislocation densities and a biaxial residual stress state. For PAMBE growth, the anisotropy was correlated to N-rich (or Ga-poor) conditions on the surface during growth, that result in formation of asymmetric a-plane GaN grains elongated along the c-axis. Such conditions enhance the anisotropy of gallium diffusion on the surface and reduce the GaN nucleation rate.« less

Free-carrier mobility in GaN in the presence of dislocation walls

NASA Astrophysics Data System (ADS)

Farvacque, J.-L.; Bougrioua, Z.; Moerman, I.

2001-03-01

The free-carrier mobility versus carrier density in n-type GaN grown by low-pressure metal-organic vapor- phase epitaxy on a sapphire substrate experiences a particular behavior that consists of the appearance of a sharp transition separating a low- from a high-mobility regime. This separation appears as soon as the carrier density exceeds a critical value that depends on the growth process. Using low-field electrical transport simulations, we show that this particular mobility behavior cannot be simply interpreted in terms of dislocation scattering or trapping mechanisms, but that it is also controlled by the collective effect of dislocation walls (the columnar structure). As the free-carrier density increases, the more efficient screening properties result in the transition from a barrier-controlled mobility regime to a pure-diffusion-process-controlled mobility regime. The model permits us to reproduce the experimental mobility collapse quantitatively.

Resistivity control of unintentionally doped GaN films

NASA Astrophysics Data System (ADS)

Grzegorczyk, A. P.; Macht, L.; Hageman, P. R.; Rudzinski, M.; Larsen, P. K.

2005-05-01

GaN epilayers were grown on sapphire substrates via low temperature GaN and AlN nucleation layers (NL) by metalorganic chemical vapor phase epitaxy (MOCVD). The morphology of the individual NLs strongly depends on the carrier gas used during the growth and recrystallization and this is the key factor for control of the resistivity of the GaN layer grown on it. The GaN nucleation layer grown in presence of N2 has a higher density of islands with a statistically smaller diameter than the samples grown in H2 atmosphere. The NL grown in N2 enables the growth GaN with a sheet resistivity higher than 3×104 cm as opposed to a 0.5 cm value obtained for the NL grown in H2. Introduction of an additional intermediate (IL) low temperature (GaN or AlN) nucleation layer changes the GaN epilayer resistivity to about 50 cm, regardless of the carrier gas used during the growth of the IL. Defect selective etching demonstrated that control of the type and density of the dislocations in GaN enables the growth of highly resistive layers without any intentional acceptor doping (Mg, Zn). It will be demonstrated that by changing the ratio of edge type to screw dislocations the resistivity of the layer can be changed by a few orders of magnitude.

Nano-indentation used to study pyramidal slip in GaN single crystals

NASA Astrophysics Data System (ADS)

Krimsky, E.; Jones, K. A.; Tompkins, R. P.; Rotella, P.; Ligda, J.; Schuster, B. E.

2018-02-01

The nucleation and structure of dislocations created by the nano-indentation of GaN samples with dislocation densities ≈103, 106 or 109 ⊥/cm2 were studied in the interest of learning how dislocations can be created to relieve the mismatch strain in ternary nitride films grown on (0001) oriented binary nitride substrates. Using transmission electron microscopy and stress analyses to assist in interpreting the nano-indentation data, we determined that the pop-ins in the indenter load vs. penetration depth curves are created by an avalanche process at stresses well above the typical yield stress. The process begins by the homogeneous formation of a basal plane screw dislocation that triggers the formation of pyramidal and other basal plane dislocations that relieve the excess stored elastic energy. It appears that pyramidal slip can occur on either the {1122} or {0111} planes, as there is little resistance to the cross slip of screw dislocations.

High-electron-mobility GaN grown on free-standing GaN templates by ammonia-based molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kyle, Erin C. H., E-mail: erinkyle@umail.ucsb.edu; Kaun, Stephen W.; Burke, Peter G.

2014-05-21

The dependence of electron mobility on growth conditions and threading dislocation density (TDD) was studied for n{sup −}-GaN layers grown by ammonia-based molecular beam epitaxy. Electron mobility was found to strongly depend on TDD, growth temperature, and Si-doping concentration. Temperature-dependent Hall data were fit to established transport and charge-balance equations. Dislocation scattering was analyzed over a wide range of TDDs (∼2 × 10{sup 6} cm{sup −2} to ∼2 × 10{sup 10} cm{sup −2}) on GaN films grown under similar conditions. A correlation between TDD and fitted acceptor states was observed, corresponding to an acceptor state for almost every c lattice translation along each threading dislocation. Optimizedmore » GaN growth on free-standing GaN templates with a low TDD (∼2 × 10{sup 6} cm{sup −2}) resulted in electron mobilities of 1265 cm{sup 2}/Vs at 296 K and 3327 cm{sup 2}/Vs at 113 K.« less

Selective heteroepitaxy on deeply grooved substrate: A route to low cost semipolar GaN platforms of bulk quality

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tendille, Florian, E-mail: florian.tendille@crhea.cnrs.fr; Vennéguès, Philippe; De Mierry, Philippe

2016-08-22

Semipolar GaN crystal stripes larger than 100 μm with dislocation densities below 5 × 10{sup 6} cm{sup −2} are achieved using a low cost fabrication process. An original sapphire patterning procedure is proposed, enabling selective growth of semipolar oriented GaN stripes while confining the defects to specific areas. Radiative and non-radiative crystalline defects are investigated by cathodoluminescence and can be correlated to the development of crystal microstructure during the growth process. A dislocation reduction mechanism, supported by transmission electron microscopy, is proposed. This method represents a step forward toward low-cost quasi-bulk semipolar GaN epitaxial platforms with an excellent structural quality which will allowmore » for even more efficient III-nitride based devices.« less

Epitaxial GaN layers formed on langasite substrates by the plasma-assisted MBE method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lobanov, D. N., E-mail: dima@ipmras.ru; Novikov, A. V.; Yunin, P. A.

2016-11-15

In this publication, the results of development of the technology of the epitaxial growth of GaN on single-crystal langasite substrates La{sub 3}Ga{sub 5}SiO{sub 14} (0001) by the plasma-assisted molecular-beam epitaxy (PA MBE) method are reported. An investigation of the effect of the growth temperature at the initial stage of deposition on the crystal quality and morphology of the obtained GaN layer is performed. It is demonstrated that the optimal temperature for deposition of the initial GaN layer onto the langasite substrate is about ~520°C. A decrease in the growth temperature to this value allows the suppression of oxygen diffusion frommore » langasite into the growing layer and a decrease in the dislocation density in the main GaN layer upon its subsequent high-temperature deposition (~700°C). Further lowering of the growth temperature of the nucleation layer leads to sharp degradation of the GaN/LGS layer crystal quality. As a result of the performed research, an epitaxial GaN/LGS layer with a dislocation density of ~10{sup 11} cm{sup –2} and low surface roughness (<2 nm) is obtained.« less

Macrodefect-free, large, and thick GaN bulk crystals for high-quality 2–6 in. GaN substrates by hydride vapor phase epitaxy with hardness control

NASA Astrophysics Data System (ADS)

Fujikura, Hajime; Konno, Taichiro; Suzuki, Takayuki; Kitamura, Toshio; Fujimoto, Tetsuji; Yoshida, Takehiro

2018-06-01

On the basis of a novel crystal hardness control, we successfully realized macrodefect-free, large (2–6 in.) and thick +c-oriented GaN bulk crystals by hydride vapor phase epitaxy. Without the hardness control, the introduction of macrodefects including inversion domains and/or basal-plane dislocations seemed to be indispensable to avoid crystal fracture in GaN growth with millimeter thickness. However, the presence of these macrodefects tended to limit the applicability of the GaN substrate to practical devices. The present technology markedly increased the GaN crystal hardness from below 20 to 22 GPa, thus increasing the available growth thickness from below 1 mm to over 6 mm even without macrodefect introduction. The 2 and 4 in. GaN wafers fabricated from these crystals had extremely low dislocation densities in the low- to mid-105 cm‑2 range and low off-angle variations (2 in.: <0.1° 4 in.: ∼0.2°). The realization of such high-quality 6 in. wafers is also expected.

Self-assembled Multilayers of Silica Nanospheres for Defect Reduction in Non- and Semipolar Gallium Nitride Epitaxial Layers

PubMed Central

2015-01-01

Non- and semipolar GaN have great potential to improve the efficiency of light emitting devices due to much reduced internal electric fields. However, heteroepitaxial GaN growth in these crystal orientations suffers from very high dislocation and stacking faults densities. Here, we report a facile method to obtain low defect density non- and semipolar heteroepitaxial GaN via selective area epitaxy using self-assembled multilayers of silica nanospheres (MSN). Nonpolar (11–20) and semipolar (11–22) GaN layers with high crystal quality have been achieved by epitaxial integration of the MSN and a simple one-step overgrowth process, by which both dislocation and basal plane stacking fault densities can be significantly reduced. The underlying defect reduction mechanisms include epitaxial growth through the MSN covered template, island nucleation via nanogaps in the MSN, and lateral overgrowth and coalescence above the MSN. InGaN/GaN multiple quantum wells structures grown on a nonpolar GaN/MSN template show more than 30-fold increase in the luminescence intensity compared to a control sample without the MSN. This self-assembled MSN technique provides a new platform for epitaxial growth of nitride semiconductors and offers unique opportunities for improving the material quality of GaN grown on other orientations and foreign substrates or heteroepitaxial growth of other lattice-mismatched materials. PMID:27065755

Role of dislocations and carrier concentration in limiting the electron mobility of InN films grown by plasma assisted molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Tangi, Malleswararao; De, Arpan; Shivaprasad, S. M.

2018-01-01

We report the molecular beam epitaxy growth of device quality InN films on GaN epilayer and nano-wall network (NWN) templates deposited on c-sapphire by varying the film thickness up to 1 μm. The careful experiments are directed towards obtaining high mobility InN layers having a low band gap with improved crystal quality. The dislocation density is quantified by using high resolution X-ray diffraction rocking curve broadening values of symmetric and asymmetric reflections, respectively. We observe that the dislocation density of the InN films grown on GaN NWN is less than that of the films grown on the GaN epilayer. This is attributed to the nanoepitaxial lateral overlayer growth (ELOG) process, where the presence of voids at the interface of InN/GaN NWN prevents the propagation of dislocation lines into the InN epilayers, thereby causing less defects in the overgrown InN films. Thus, this new adaptation of the nano-ELOG growth process enables us to prepare InN layers with high electron mobility. The obtained electron mobility of 2121 cm2/Vs for 1 μm thick InN/GaN NWN is comparable with the literature values of similar thickness InN films. Furthermore, in order to understand the reasons that limit electron mobility, the charge neutrality condition is employed to study the variation of electron mobility as a function of dislocation density and carrier concentration. Overall, this study provides a route to attaining improved crystal quality and electronic properties of InN films.

Use of hydrogen etching to remove existing dislocations in GaN epitaxial layers

NASA Astrophysics Data System (ADS)

Yeh, Yen-Hsien; Chu, Chung-Ming; Wu, Yin-Hao; Hsu, Ying-Chia; Yu, Tzu-Yi; Lee, Wei-I.

2015-08-01

In this paper, based on the anisotropic nature of hydrogen (H2) etching on GaN, we describe a new approach to the removal of threading dislocations in GaN layers. The top surfaces of c-plane (Ga-face) and a-plane GaNs are considered stable in H2; therefore, H2 etches only crystal imperfections such as dislocation and basal plane stacking fault (BSF) sites. We used H2 to etch undoped c-plane GaN, n-type c-plane GaN, a-plane GaN, and an InGaN/GaN multiple quantum well structure. Several examinations were performed, indicating deep cavities on the c-plane GaN samples after H2 etching; furthermore, gorge-like grooves were observed on the a-plane GaN samples. The deep cavities on the c-plane GaN were considered the etched dislocation sites, and the gorge-like grooves on the a-plane GaN were considered the etched BSF sites. Photoluminescence measurements were performed and the results indicated that the H2-etched samples demonstrate superior optoelectronic properties, probably because of the elimination of dislocations.

The Effect of Growth Environment on the Morphological and Extended Defect Evolution in GaN Grown by Metalorganic Chemical Vapor Deposition

NASA Astrophysics Data System (ADS)

Fini, P.; Wu, X.; Tarsa, E.; Golan, Y.; Srikant, V.; Keller, S.; Denbaars, S.; Speck, J.

1998-08-01

The evolution of morphology and associated extended defects in GaN thin films grown on sapphire by metalorganic chemical vapor deposition (MOCVD) are shown to depend strongly on the growth environment. For the commonly used two-step growth process, a change in growth parameter such as reactor pressure influences the initial high temperature (HT) GaN growth mechanism. By means of transmission electron microscopy (TEM), atomic force microscopy (AFM), and high resolution X-ray diffraction (HRXRD) measurements, it is shown that the initial density of HT islands on the nucleation layer (NL) and subsequently the threading dislocation density in the HT GaN film may be directly controlled by tailoring the initial HT GaN growth conditions.

Influence of different aspect ratios on the structural and electrical properties of GaN thin films grown on nanoscale-patterned sapphire substrates

NASA Astrophysics Data System (ADS)

Lee, Fang-Wei; Ke, Wen-Cheng; Cheng, Chun-Hong; Liao, Bo-Wei; Chen, Wei-Kuo

2016-07-01

This study presents GaN thin films grown on nanoscale-patterned sapphire substrates (NPSSs) with different aspect ratios (ARs) using a homemade metal-organic chemical vapor deposition system. The anodic aluminum oxide (AAO) technique is used to prepare the dry etching mask. The cross-sectional view of the scanning electron microscope image shows that voids exist between the interface of the GaN thin film and the high-AR (i.e. ∼2) NPSS. In contrast, patterns on the low-AR (∼0.7) NPSS are filled full of GaN. The formation of voids on the high-AR NPSS is believed to be due to the enhancement of the lateral growth in the initial growth stage, and the quick-merging GaN thin film blocks the precursors from continuing to supply the bottom of the pattern. The atomic force microscopy images of GaN on bare sapphire show a layer-by-layer surface morphology, which becomes a step-flow surface morphology for GaN on a high-AR NPSS. The edge-type threading dislocation density can be reduced from 7.1 × 108 cm-2 for GaN on bare sapphire to 4.9 × 108 cm-2 for GaN on a high-AR NPSS. In addition, the carrier mobility increases from 85 cm2/Vs for GaN on bare sapphire to 199 cm2/Vs for GaN on a high-AR NPSS. However, the increased screw-type threading dislocation density for GaN on a low-AR NPSS is due to the competition of lateral growth on the flat-top patterns and vertical growth on the bottom of the patterns that causes the material quality of the GaN thin film to degenerate. Thus, the experimental results indicate that the AR of the particular patterning of a NPSS plays a crucial role in achieving GaN thin film with a high crystalline quality.

MOCVD growth of gallium nitride with indium surfactant

NASA Astrophysics Data System (ADS)

Won, Dong Jin

In this thesis research, the effect of indium surfactant on Ga-polar and N-polar GaN films grown at 950 °C by MOCVD on various substrates such as Si-face SiC, bulk GaN, Si(111), and C-face SiC was studied to investigate the stress relaxation mechanism, structural, and optical properties of GaN films which were modified by the indium surfactant. The effect of indium surfactant on GaN films grown on SiC was studied first. In the 1.8 microm thick Ga-polar GaN films grown on lattice-mismatched Si-face SiC substrates utilizing indium surfactant at 950 °C, inverted hexagonal pyramid surface defects, so-called V-defects which consist of six (1011) planes, formed at threading dislocations on the GaN surface, which gave rise to the relaxation of compressive misfit stress in an elastic way. Simultaneously, enhanced surface mobility of Ga and N adatoms with indium surfactant lead to improved 2D growth, which may be contradictory to the formation of surface defects like V-defects. In order to find the driving force for V-defect formation in the presence of indium, a nucleation and growth model was developed, taking into consideration the strain, surface, and dislocation energies modified by indium surfactant. This model found that the V-defect formation can be energetically preferred since indium reduces the surface energy of the (1011) plane, which gives rise to the V-defect formation and growth that can overcome the energy barrier at the critical radius of the V-defect. These Ga-polar GaN films were found to be unintentionally doped with Si. Thus, an investigation into the effect of intentional Si doping at a constant TMIn flow rate on GaN films was also performed. Si turned out to be another important factor in the generation of V-defects because Si may be captured at the threading dislocation cores by forming Si -- N bonds, acting as a mask to locally prevent GaN growth. This behavior appeared to assist the initiation of the V-defect which enables V-defects to easily grow beyond the critical radius. Thus, introduction of indium surfactant and Si doping was found to be the most favorable conditions for V-defect formation in Ga-polar GaN films grown on Si-face SiC substrates. The nucleation and growth model predicted that V-defects may not form in homoepitaxy because the energy barrier for V-defect formation approaches infinity due to zero misfit stress. When indium surfactant and Si dopant were introduced simultaneously during the homoepitaxial growth, V-defects did not form in 1.8 microm thick Ga-polar GaN films grown at 950 °C on bulk GaN that had very low threading dislocation density, as predicted by the nucleation and growth model. Ga-polar GaN films grown on Si(111) substrates using indium surfactant showed that additional tensile stress was induced by indium with respect to the reference GaN. Since cracking is known to be a stress relaxation mechanism for tension, the In-induced additional tensile stress is thus detrimental to the GaN films which experience the tensile thermal stress associated with the difference in coefficient of thermal expansion between GaN and the substrate during cooling after growth. The generation of tensile stress by indium seemed correlated with a reduction of V-defects since a high density of V-defects formed under the initial compressive stress at the GaN nucleation stage and then V-defect density decreased as the film grew. Even though the initial misfit stress of the GaN film grown on Si(111) was lower than that of GaN grown on SiC, a high density of V-defects were created under the initial compressive stress. Therefore, the high density of threading dislocations was believed to strongly drive the V-defect formation under In-rich conditions. Consequently, without using high quality bulk GaN substrates, V-defects could not be avoided in Ga-polar GaN films grown on foreign substrates such as Si-face SiC and Si(111) in the presence of indium surfactant and Si dopants during growth. Thus, N-polar GaN films were investigated using vicinal C-face SiC substrates because a theoretical study utilizing first-principles calculations predicted that V-defects are not energetically favored on the N-face GaN. When indium surfactant and Si doping were used during N-polar GaN growth, V-defects did not form, as predicted by theory. This observation suggests that V-defect free N-polar InGaN alloys also can be achieved, which may enable stable green laser diodes with long lifetime to be fabricated using the high indium composition N-polar InGaN films. (Abstract shortened by UMI.)

Anisotropic structural and optical properties of semi-polar (11-22) GaN grown on m-plane sapphire using double AlN buffer layers.

PubMed

Zhao, Guijuan; Wang, Lianshan; Yang, Shaoyan; Li, Huijie; Wei, Hongyuan; Han, Dongyue; Wang, Zhanguo

2016-02-10

We report the anisotropic structural and optical properties of semi-polar (11-22) GaN grown on m-plane sapphire using a three-step growth method which consisted of a low temperature AlN buffer layer, followed by a high temperature AlN buffer layer and GaN growth. By introducing double AlN buffer layers, we substantially improve the crystal and optical qualities of semi-polar (11-22) GaN, and significantly reduce the density of stacking faults and dislocations. The high resolution x-ray diffraction measurement revealed that the in-plane anisotropic structural characteristics of GaN layer are azimuthal dependent. Transmission electron microscopy analysis showed that the majority of dislocations in the GaN epitaxial layer grown on m-sapphire are the mixed-type and the orientation of GaN layer was rotated 58.4° against the substrate. The room temperature photoluminescence (PL) spectra showed the PL intensity and wavelength have polarization dependence along parallel and perpendicular to the [1-100] axis (polarization degrees ~ 0.63). The realization of a high polarization semi-polar GaN would be useful to achieve III-nitride based lighting emission device for displays and backlighting.

Density of bunched threading dislocations in epitaxial GaN layers as determined using X-ray diffraction

NASA Astrophysics Data System (ADS)

Barchuk, M.; Holý, V.; Rafaja, D.

2018-04-01

X-ray diffraction is one of the most popular experimental methods employed for determination of dislocation densities, as it can recognize both the strain fields and the local lattice rotations produced by dislocations. The main challenge of the quantitative analysis of the dislocation density is the formulation of a suitable microstructure model, which describes the dislocation arrangement and the effect of the interactions between the strain fields from neighboring dislocations reliably in order to be able to determine the dislocation densities precisely. The aim of this study is to prove the capability of X-ray diffraction and two computational methods, which are frequently used for quantification of the threading dislocation densities from X-ray diffraction measurements, in the special case of partially bunched threading dislocations. The first method is based on the analysis of the dislocation-controlled crystal mosaicity, and the other one on the analysis of diffuse X-ray scattering from threading dislocations. The complementarity of both methods is discussed. Furthermore, it is shown how the complementarity of these methods can be used to improve the results of the quantitative analysis of bunched and thus inhomogeneously distributed threading dislocations and to get a better insight into the dislocation arrangement.

Dislocation-induced nanoparticle decoration on a GaN nanowire.

PubMed

Yang, Bing; Yuan, Fang; Liu, Qingyun; Huang, Nan; Qiu, Jianhang; Staedler, Thorsten; Liu, Baodan; Jiang, Xin

2015-02-04

GaN nanowires with homoepitaxial decorated GaN nanoparticles on their surface along the radial direction have been synthesized by means of a chemical vapor deposition method. The growth of GaN nanowires is catalyzed by Au particles via the vapor-liquid-solid (VLS) mechanism. Screw dislocations are generated along the radial direction of the nanowires under slight Zn doping. In contrast to the metal-catalyst-assisted VLS growth, GaN nanoparticles are found to prefer to nucleate and grow at these dislocation sites. High-resolution transmission electron microscopy (HRTEM) analysis demonstrates that the GaN nanoparticles possess two types of epitaxial orientation with respect to the corresponding GaN nanowire: (I) [1̅21̅0]np//[1̅21̅0]nw, (0001)np//(0001)nw; (II) [1̅21̅3]np//[12̅10]nw, (101̅0)np//(101̅0)nw. An increased Ga signal in the energy-dispersive spectroscopy (EDS) profile lines of the nanowires suggests GaN nanoparticle growth at the edge surface of the wires. All the crystallographic results confirm the importance of the dislocations with respect to the homoepitaxial growth of the GaN nanoparticles. Here, screw dislocations situated on the (0001) plane provide the self-step source to enable nucleation of the GaN nanoparticles.

Detection of edge component of threading dislocations in GaN by Raman spectroscopy

NASA Astrophysics Data System (ADS)

Kokubo, Nobuhiko; Tsunooka, Yosuke; Fujie, Fumihiro; Ohara, Junji; Hara, Kazukuni; Onda, Shoichi; Yamada, Hisashi; Shimizu, Mitsuaki; Harada, Shunta; Tagawa, Miho; Ujihara, Toru

2018-06-01

We succeeded in measuring the density and direction of the edge component of threading dislocations (TDs) in c-plane (0001) GaN by micro-Raman spectroscopy mapping. In the micro-Raman spectroscopy mapping of the E2 H peak shift between 567.85 and 567.75 cm‑1, six different contrast images are observed toward directions of < 1\\bar{1}00> . By comparing X-ray topography and etch pit images, the E2 H peak shift is observed where the edge component of TDs exists. In contrast, the E2 H peak is not observed where the screw component of TDs exists.

Formation of definite GaN p-n junction by Mg-ion implantation to n--GaN epitaxial layers grown on a high-quality free-standing GaN substrate

NASA Astrophysics Data System (ADS)

Oikawa, Takuya; Saijo, Yusuke; Kato, Shigeki; Mishima, Tomoyoshi; Nakamura, Tohru

2015-12-01

P-type conversion of n--GaN by Mg-ion implantation was successfully performed using high quality GaN epitaxial layers grown on free-standing low-dislocation-density GaN substrates. These samples showed low-temperature PL spectra quite similar to those observed from Mg-doped MOVPE-grown p-type GaN, consisting of Mg related donor-acceptor pair (DAP) and acceptor bound exciton (ABE) emission. P-n diodes fabricated by the Mg-ion implantation showed clear rectifying I-V characteristics and UV and blue light emissions were observed at forward biased conditions for the first time.

Defect reduction in Si-doped Al{sub 0.45}Ga{sub 0.55}N films by SiN{sub x} interlayer method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Yang; Chen, Shengchang; Kong, Man

2014-01-28

The dislocation density in AlGaN epitaxial layers with Al content as high as 45% grown on sapphire substrates has been effectively reduced by introducing an in-situ deposited SiN{sub x} nanomask layer in this study. By closely monitoring the evolution of numerous material properties, such as surface morphology, dislocation density, photoluminescence, strain states, and electron mobility of the Si-Al{sub 0.45}Ga{sub 0.55}N layers as the functions of SiN{sub x} interlayer growth time, the surface coverage fraction of SiN{sub x} is found to be a crucial factor determining the strain states and dislocation density. The dependence of the strain states and the dislocationmore » density on the surface coverage fraction of SiN{sub x} nanomask supports the very different growth models of Al-rich AlGaN on SiN{sub x} interlayer due to the reduced nucleation selectivity compared with the GaN counterpart. Compared with GaN, which can only nucleate at open pores of SiN{sub x} nanomask, Al-rich AlGaN can simultaneously nucleate at both open pores and SiN{sub x} covered areas. Dislocations will annihilate at the openings due to the 3D growth initiated on the opening area, while 2D growth mode is preserved on SiN{sub x} and the threading dislocations are also preserved. During the following growth process, lateral overgrowth will proceed from the Al{sub 0.45}Ga{sub 0.55}N islands on the openings towards the regions covered by SiN{sub x}, relaxing the compressive strain and bending the dislocations at the same time.« less

Influence of dislocation density on internal quantum efficiency of GaN-based semiconductors

NASA Astrophysics Data System (ADS)

Yu, Jiadong; Hao, Zhibiao; Li, Linsen; Wang, Lai; Luo, Yi; Wang, Jian; Sun, Changzheng; Han, Yanjun; Xiong, Bing; Li, Hongtao

2017-03-01

By considering the effects of stress fields coming from lattice distortion as well as charge fields coming from line charges at edge dislocation cores on radiative recombination of exciton, a model of carriers' radiative and non-radiative recombination has been established in GaN-based semiconductors with certain dislocation density. Using vector average of the stress fields and the charge fields, the relationship between dislocation density and the internal quantum efficiency (IQE) is deduced. Combined with related experimental results, this relationship is fitted well to the trend of IQEs of bulk GaN changing with screw and edge dislocation density, meanwhile its simplified form is fitted well to the IQEs of AlGaN multiple quantum well LEDs with varied threading dislocation densities but the same light emission wavelength. It is believed that this model, suitable for different epitaxy platforms such as MOCVD and MBE, can be used to predict to what extent the luminous efficiency of GaN-based semiconductors can still maintain when the dislocation density increases, so as to provide a reasonable rule of thumb for optimizing the epitaxial growth of GaN-based devices.

Dislocation blocking by AlGaN hot electron injecting layer in the epitaxial growth of GaN terahertz Gunn diode

NASA Astrophysics Data System (ADS)

Li, Liang; Yang, Lin'an; Zhang, Jincheng; Hao, Yue

2013-09-01

This paper reports an efficient method to improve the crystal quality of GaN Gunn diode with AlGaN hot electron injecting layer (HEI). An evident reduction of screw dislocation and edge dislocation densities is achieved by the strain management and the enhanced lateral growth in high temperature grown AlGaN HEI layer. Compared with the top hot electron injecting layer (THEI) structure, the bottom hot electron injecting layer (BHEI) structure enhances the crystal quality of transit region due to the growth sequence modulation of HEI layer. A high Hall mobility of 2934 cm2/Vs at 77 K, a nearly flat downtrend of Hall mobility at the temperature ranging from 300 to 573 K, a low intensity of ratio of yellow luminescence band to band edge emission, a narrow band edge emission line-width, and a smooth surface morphology are observed for the BHEI structural epitaxy of Gunn diode, which indicates that AlGaN BHEI structure is a promising candidate for fabrication of GaN Gunn diodes in terahertz regime.

High-Quality GaN Epilayers Achieved by Facet-Controlled Epitaxial Lateral Overgrowth on Sputtered AlN/PSS Templates.

PubMed

He, Chenguang; Zhao, Wei; Zhang, Kang; He, Longfei; Wu, Hualong; Liu, Ningyang; Zhang, Shan; Liu, Xiaoyan; Chen, Zhitao

2017-12-13

It is widely believed that the lack of high-quality GaN wafers severely hinders the progress in GaN-based devices, especially for defect-sensitive devices. Here, low-cost AlN buffer layers were sputtered on cone-shaped patterned sapphire substrates (PSSs) to obtain high-quality GaN epilayers. Without any mask or regrowth, facet-controlled epitaxial lateral overgrowth was realized by metal-organic chemical vapor deposition. The uniform coating of the sputtered AlN buffer layer and the optimized multiple modulation guaranteed high growth selectivity and uniformity of the GaN epilayer. As a result, an extremely smooth surface was achieved with an average roughness of 0.17 nm over 3 × 3 μm 2 . It was found that the sputtered AlN buffer layer could significantly suppress dislocations on the cones. Moreover, the optimized three-dimensional growth process could effectively promote dislocation bending. Therefore, the threading dislocation density (TDD) of the GaN epilayer was reduced to 4.6 × 10 7 cm -2 , which is about an order of magnitude lower than the case of two-step GaN on the PSS. In addition, contamination and crack in the light-emitting diode fabricated on the obtained GaN were also effectively suppressed by using the sputtered AlN buffer layer. All of these advantages led to a high output power of 116 mW at 500 mA with an emission wavelength of 375 nm. This simple, yet effective growth technique is believed to have great application prospects in high-performance TDD-sensitive optoelectronic and electronic devices.

Zn-dopant dependent defect evolution in GaN nanowires

NASA Astrophysics Data System (ADS)

Yang, Bing; Liu, Baodan; Wang, Yujia; Zhuang, Hao; Liu, Qingyun; Yuan, Fang; Jiang, Xin

2015-10-01

Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chemical vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calculations. Undoped GaN nanowires show a hexagonal wurtzite (WZ) structure with good crystallinity. Several kinds of twin boundaries, including (101&cmb.macr;3), (101&cmb.macr;1) and (202&cmb.macr;1), as well as Type I stacking faults (...ABABC&cmb.b.line;BCB...), are observed in the nanowires. The increasing Zn doping level (<1 at%) induces the formation of screw dislocations featuring a predominant screw component along the radial direction of the GaN nanowires. At high Zn doping level (3-5 at%), meta-stable cubic zinc blende (ZB) domains are generated in the WZ GaN nanowires. The WZ/ZB phase boundary (...ABABAC&cmb.b.line;BA...) can be identified as Type II stacking faults. The density of stacking faults (both Type I and Type II) increases with increasing the Zn doping levels, which in turn leads to a rough-surface morphology in the GaN nanowires. First-principles calculations reveal that Zn doping will reduce the formation energy of both Type I and Type II stacking faults, favoring their nucleation in GaN nanowires. An understanding of the effect of Zn doping on the defect evolution provides an important method to control the microstructure and the electrical properties of p-type GaN nanowires.Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chemical vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calculations. Undoped GaN nanowires show a hexagonal wurtzite (WZ) structure with good crystallinity. Several kinds of twin boundaries, including (101&cmb.macr;3), (101&cmb.macr;1) and (202&cmb.macr;1), as well as Type I stacking faults (...ABABC&cmb.b.line;BCB...), are observed in the nanowires. The increasing Zn doping level (<1 at%) induces the formation of screw dislocations featuring a predominant screw component along the radial direction of the GaN nanowires. At high Zn doping level (3-5 at%), meta-stable cubic zinc blende (ZB) domains are generated in the WZ GaN nanowires. The WZ/ZB phase boundary (...ABABAC&cmb.b.line;BA...) can be identified as Type II stacking faults. The density of stacking faults (both Type I and Type II) increases with increasing the Zn doping levels, which in turn leads to a rough-surface morphology in the GaN nanowires. First-principles calculations reveal that Zn doping will reduce the formation energy of both Type I and Type II stacking faults, favoring their nucleation in GaN nanowires. An understanding of the effect of Zn doping on the defect evolution provides an important method to control the microstructure and the electrical properties of p-type GaN nanowires. Electronic supplementary information (ESI) available: HRTEM image of undoped GaN nanowires and first-principles calculations of Zn doped WZ-GaN. See DOI: 10.1039/c5nr04771d

Dislocations limited electronic transport in hydride vapour phase epitaxy grown GaN templates: A word of caution for the epitaxial growers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chatterjee, Abhishek, E-mail: cabhishek@rrcat.gov.in; Khamari, Shailesh K.; Kumar, R.

2015-01-12

GaN templates grown by hydride vapour phase epitaxy (HVPE) and metal organic vapour phase epitaxy (MOVPE) techniques are compared through electronic transport measurements. Carrier concentration measured by Hall technique is about two orders larger than the values estimated by capacitance voltage method for HVPE templates. It is learnt that there exists a critical thickness of HVPE templates below which the transport properties of epitaxial layers grown on top of them are going to be severely limited by the density of charged dislocations lying at layer-substrate interface. On the contrary MOVPE grown templates are found to be free from such limitations.

Compositionally graded relaxed AlGaN buffers on semipolar GaN for mid-ultraviolet emission

DOE Office of Scientific and Technical Information (OSTI.GOV)

Young, Erin C.; Wu Feng; Haeger, Daniel A.

In this Letter, we report on the growth and properties of relaxed, compositionally graded Al{sub x}Ga{sub 1-x}N buffer layers on freestanding semipolar (2021) GaN substrates. Continuous and step compositional grades with Al concentrations up to x = 0.61 have been achieved, with emission wavelengths in the mid-ultraviolet region as low as 265 nm. Coherency stresses were relaxed progressively throughout the grades by misfit dislocation generation via primary (basal) slip and secondary (non-basal) slip systems. Threading dislocation densities in the final layers of the grades were less than 10{sup 6}/cm{sup 2} as confirmed by plan-view transmission electron microscopy and cathodoluminescence studies.

Compositionally graded relaxed AlGaN buffers on semipolar GaN for mid-ultraviolet emission

NASA Astrophysics Data System (ADS)

Young, Erin C.; Wu, Feng; Romanov, Alexey E.; Haeger, Daniel A.; Nakamura, Shuji; Denbaars, Steven P.; Cohen, Daniel A.; Speck, James S.

2012-10-01

In this Letter, we report on the growth and properties of relaxed, compositionally graded AlxGa1 - xN buffer layers on freestanding semipolar (202¯1) GaN substrates. Continuous and step compositional grades with Al concentrations up to x = 0.61 have been achieved, with emission wavelengths in the mid-ultraviolet region as low as 265 nm. Coherency stresses were relaxed progressively throughout the grades by misfit dislocation generation via primary (basal) slip and secondary (non-basal) slip systems. Threading dislocation densities in the final layers of the grades were less than 106/cm2 as confirmed by plan-view transmission electron microscopy and cathodoluminescence studies.

Role of low-temperature AlGaN interlayers in thick GaN on silicon by metalorganic vapor phase epitaxy

NASA Astrophysics Data System (ADS)

Fritze, S.; Drechsel, P.; Stauss, P.; Rode, P.; Markurt, T.; Schulz, T.; Albrecht, M.; Bläsing, J.; Dadgar, A.; Krost, A.

2012-06-01

Thin AlGaN interlayers have been grown into a thick GaN stack on Si substrates to compensate tensile thermal stress and significantly improve the structural perfection of the GaN. In particular, thicker interlayers reduce the density in a-type dislocations as concluded from x-ray diffraction (XRD) measurements. Beyond an interlayer thickness of 28 nm plastic substrate deformation occurs. For a thick GaN stack, the first two interlayers serve as strain engineering layers to obtain a crack-free GaN structure, while a third strongly reduces the XRD ω-(0002)-FWHM. The vertical strain and quality profile determined by several XRD methods demonstrates the individual impact of each interlayer.

Recombination properties of dislocations in GaN

NASA Astrophysics Data System (ADS)

Yakimov, Eugene B.; Polyakov, Alexander Y.; Lee, In-Hwan; Pearton, Stephen J.

2018-04-01

The recombination activity of threading dislocations in n-GaN with different dislocation densities and different doping levels was studied using electron beam induced current (EBIC). The recombination velocity on a dislocation, also known as the dislocation recombination strength, was calculated. The results suggest that dislocations in n-GaN giving contrast in EBIC are charged and surrounded by a space charge region, as evidenced by the observed dependence of dislocation recombination strength on dopant concentration. For moderate (below ˜108 cm-2) dislocation densities, these defects do not primarily determine the average diffusion length of nonequilibrium charge carriers, although locally, dislocations are efficient recombination sites. In general, it is observed that the effect of the growth method [standard metalorganic chemical vapor deposition (MOCVD), epitaxial lateral overgrowth versions of MOCVD, and hydride vapor phase epitaxy] on the recombination activity of dislocations is not very pronounced, although the average diffusion lengths can widely differ for various samples. The glide of basal plane dislocations at room temperature promoted by low energy electron irradiation does not significantly change the recombination properties of dislocations.

III-V compound semiconductor material characterization of microstructures and nanostructures on various optoelectronic devices with analytical transmission electron microscopy and high resolution electron microscopy

NASA Astrophysics Data System (ADS)

Zhou, Wei

Analytical Transmission Electron Microscopy (TEM) and High Resolution Electron Microscopy have been carried out to characterize microstructures and nanostructures in various III-V compound semiconductor devices by metalorganic chemical vapor deposition (MOCVD). The low-defect GaN nonplanar templates by lateral epitaxial overgrowth (LEO) has a trapezoidal cross-section with smooth (0001) and {112¯2} facets. Penetration of threading dislocations (TDs) beyond mask windows is observed in ordinary LEO substrates. In two-step LEO substrates, where TDs are engineered to bend 90° in the TD bending layer after the first LEO step, only perfect a-type dislocations with Burgers vector b = 1/3 <112¯0> are generated in the upper Post-bending layer with a density of ˜8 x 107cm-2. The demonstrated 3-dimensional dislocation spatial distribution in the LEO nonplanar substrate substantiates the dislocation reaction mechanism. Al0.07GaN/GaN superlattice can further decrease dislocations. InGaN QW thickness enhancement on top of GaN nonplanar templates has been verified to influence the optoelectronic properties significantly. Dense arrays of hexagonally ordered MOCVD-grown (In)(Ga)As nano-QDs by block copolymer nanolithography & selective area growth (SAG), approximately 20nm in diameter and 40nm apart with a density of 1011/cm 2, are perfect crystals by TEM. V-shaped defects and worse InAs growth uniformity have been observed in multiple layers of vertically coupled self-assembled InAs nanostructure arrays on strain-modulated GaAs substrates. TEM shows a smooth coalesced GaN surface with a thickness as thin as ˜200nm after Nano-LEO and a defect reduction of 70%-75%. The (In)GaAs 20 nm twist bonded compliant substrates have almost no compliant effect and higher dislocation density, but the 10nm compliant substrates are on the contrary. A 60nm oxygen-infiltrated crystallized transition layer is observed between the amorphous oxidized layer and the crystallized unoxidized aperture in Al xGa1-xAs wet lateral oxidation, potentially influencing the current confinement characteristic of the sub-micron oxide aperture. Almost no dislocation is aroused by the wet lateral oxidation of In0.52Al 0.48As in the InP microresonator waveguides. XTEM was performed to compare InP SAG regions with 10˜50mum masks, which shows the performance deterioration of laser threshold current densities in the case of 50mum mask results from high density of dislocations induced from the highly strained QW structures caused by the high enhancements.

Dislocation-induced stress in polycrystalline materials: mesoscopic simulations in the dislocation density formalism

NASA Astrophysics Data System (ADS)

Berkov, D. V.; Gorn, N. L.

2018-06-01

In this paper we present a simple and effective numerical method which allows a fast Fourier transformation-based evaluation of stress generated by dislocations with arbitrary directions and Burgers vectors if the (site-dependent) dislocation density is known. Our method allows the evaluation of the dislocation stress using a rectangular grid with shape-anisotropic discretization cells without employing higher multipole moments of the dislocation interaction coefficients. Using the proposed method, we first simulate the stress created by relatively simple non-homogeneous distributions of vertical edge and so-called ‘mixed’ dislocations in a disk-shaped sample, which is necessary to understand the dislocation behavior in more complicated systems. The main part of our research is devoted to the stress distribution in polycrystalline layers with the dislocation density rapidly varying with the distance to the layer bottom. Considering GaN as a typical example of such systems, we investigate dislocation-induced stress for edge and mixed dislocations, having random orientations of Burgers vectors among crystal grains. We show that the rapid decay of the dislocation density leads to many highly non-trivial features of the stress distributions in such layers and study in detail the dependence of these features on the average grain size. Finally we develop an analytical approach which allows us to predict the evolution of the stress variance with the grain size and compare analytical predictions with numerical results.

Zn-dopant dependent defect evolution in GaN nanowires.

PubMed

Yang, Bing; Liu, Baodan; Wang, Yujia; Zhuang, Hao; Liu, Qingyun; Yuan, Fang; Jiang, Xin

2015-10-21

Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chemical vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calculations. Undoped GaN nanowires show a hexagonal wurtzite (WZ) structure with good crystallinity. Several kinds of twin boundaries, including (101¯3), (101¯1) and (202¯1), as well as Type I stacking faults (…ABABCBCB…), are observed in the nanowires. The increasing Zn doping level (<1 at%) induces the formation of screw dislocations featuring a predominant screw component along the radial direction of the GaN nanowires. At high Zn doping level (3-5 at%), meta-stable cubic zinc blende (ZB) domains are generated in the WZ GaN nanowires. The WZ/ZB phase boundary (…ABABACBA…) can be identified as Type II stacking faults. The density of stacking faults (both Type I and Type II) increases with increasing the Zn doping levels, which in turn leads to a rough-surface morphology in the GaN nanowires. First-principles calculations reveal that Zn doping will reduce the formation energy of both Type I and Type II stacking faults, favoring their nucleation in GaN nanowires. An understanding of the effect of Zn doping on the defect evolution provides an important method to control the microstructure and the electrical properties of p-type GaN nanowires.

Structural defects in bulk GaN

NASA Astrophysics Data System (ADS)

Liliental-Weber, Z.; dos Reis, R.; Mancuso, M.; Song, C. Y.; Grzegory, I.; Porowski, S.; Bockowski, M.

2014-10-01

Transmission Electron Microscopy (TEM) studies of undoped and Mg doped GaN layers grown on the HVPE substrates by High Nitrogen Pressure Solution (HNPS) with the multi-feed-seed (MFS) configuration are shown. The propagation of dislocations from the HVPE substrate to the layer is observed. Due to the interaction between these dislocations in the thick layers much lower density of these defects is observed in the upper part of the HNPS layers. Amorphous Ga precipitates with attached voids pointing toward the growth direction are observed in the undoped layer. This is similar to the presence of Ga precipitates in high-pressure platelets, however the shape of these precipitates is different. The Mg doped layers do not show Ga precipitates, but MgO rectangular precipitates are formed, decorating the dislocations. Results of TEM studies of HVPE layers grown on Ammonothermal substrates are also presented. These layers have superior crystal quality in comparison to the HNPS layers, as far as density of dislocation is concern. Occasionally some small inclusions can be found, but their chemical composition was not yet determined. It is expected that growth of the HNPS layers on these substrate will lead to large layer thickness obtained in a short time and with high crystal perfection needed in devices.

Study of recombination characteristics in MOCVD grown GaN epi-layers on Si

NASA Astrophysics Data System (ADS)

Gaubas, E.; Ceponis, T.; Dobrovolskas, D.; Malinauskas, T.; Meskauskaite, D.; Miasojedovas, S.; Mickevicius, J.; Pavlov, J.; Rumbauskas, V.; Simoen, E.; Zhao, M.

2017-12-01

The radiative and non-radiative recombination carrier decay lifetimes in GaN epi-layers grown by metal-organic chemical vapour deposition technology on Si substrates were measured by contactless techniques of time-resolved photoluminescence and microwave-probed transients of photoconductivity. The lifetime variations were obtained to be dependent on growth regimes. These variations have been related to varied densities of edge dislocations associated with growth temperature. It has been also revealed that the lateral carrier lifetime and photoluminescence intensity distribution is determined by the formation of dislocation clusters dependent on the growth conditions. For low excitation level, the asymptotic component within the excess carrier decay transients is attributed to carrier trapping and anomalous diffusion through random-walk processes within dislocation cluster regions and barriers at dislocation cores. The two-componential decay process at high excitation conditions, where excess carriers may suppress barriers, proceeds through a nonlinear recombination, where band-to-band transitions determine the nonlinearity of the process, while the asymptotic component is ascribed to the impact of D-A pair PL within the long-wavelength wing of the UV-PL band.

Effect of growth pressure on the morphology evolution and doping characteristics in nonpolar a-plane GaN

NASA Astrophysics Data System (ADS)

Song, Keun Man; Kim, Jong Min; Kang, Bong Kyun; Shin, Chan Soo; Ko, Chul Gi; Kong, Bo Hyun; Cho, Hyung Koun; Yoon, Dae Ho; Kim, Hogyoung; Hwang, Sung Min

2012-02-01

Nonpolar a-plane GaN layers grown on r-plane sapphire substrates were examined by using a two-step growth process. The higher initial growth pressure for the nucleation layer resulted in the improved crystalline quality with lower density of both threading dislocations and basal stacking faults. This was attributed to the higher degree of initial roughening and recovery time via a growth mode transition from three-dimensional (3D) to quasi two-dimensional (2D) lateral growth. Using Hall-effect measurements, the overgrown Si doped GaN layers grown with higher initial growth pressure were found to have higher mobility. The scattering mechanism due to the dislocations was dominant especially at low temperature (<200 K) for the lower initial growth pressure, which was insignificant for the higher initial growth pressure. The temperature-dependent Hall-effect measurements for the Mg doped GaN with a higher initial growth pressure yielded the activation energy and the acceptor concentration to be 128 meV and 1.2 × 1019 cm-3, respectively, corresponding to about 3.6% of activation at room temperature. Two-step growth scheme with a higher initial growth pressure is suggested as a potential method to improve the performance of nonpolar a-plane GaN based devices.

Morphological evolution and characterization of GaN pyramid arrays fabricated by photo-assisted chemical etching

NASA Astrophysics Data System (ADS)

Zhang, Shiying; Xiu, Xiangqian; Xu, Qingjun; Li, Yuewen; Hua, Xuemei; Chen, Peng; Xie, Zili; Liu, Bin; Zhou, Yugang; Han, Ping; Zhang, Rong; Zheng, Youdou

2016-12-01

GaN pyramid arrays have been successfully synthesized by selective photo-assisted chemical etching in a K2S2O8/KOH solution. A detailed analysis of time evolution of surface morphology has been conducted, which describes an etching process of GaN pyramids. Room temperature cathodoluminescence images indicate that these pyramids are composed of crystalline GaN surrounding dislocations, which is caused by the greater recombination rate of electrons and holes at dislocation than that of crystalline GaN. The Raman results show a stress relaxation in GaN pyramids compared with unetched GaN. The optical property of both unetched GaN and GaN pyramids has been studied by photoluminescence. The formation mechanism and feature of GaN pyramids are also rationally explained.

Defect analysis of the LED structure deposited on the sapphire substrate

NASA Astrophysics Data System (ADS)

Nie, Qichu; Jiang, Zhimin; Gan, Zhiyin; Liu, Sheng; Yan, Han; Fang, Haisheng

2018-04-01

Transmission electron microscope (TEM) and double-crystal X-ray diffraction (DCXRD) measurements have been performed to investigate dislocations of the whole structure of the LED layers deposited on both the conventional (unpatterned sapphire substrate, UPSS) and patterned sapphire substrates (PSS). TEM results show that there exists a dislocation-accumulated region near the substrate/GaN interface, where the dislocation density is much higher with the UPPS than that with the PSS. It indicates that the pattern on the substrate surface is able to block the formation and propagation of dislocations. Further analysis discloses that slope of the pattern is found to suppress the deposition of GaN, and thus to provide more spaces for the epitaxially lateral overgrowth (ELO) of high temperature GaN, which significantly reduces the number of the initial islands, and minimizes dislocation formation due to the island coalescence. V-defect incorporating the threading dislocation is detected in the InGaN/GaN multi-quantum wells (MQWs), and its propagation mechanism is determined as the decrease of the surface energy due to the incorporation of indium. In addition, temperature dependence of dislocation formation is further investigated. The results show that dislocation with the screw component decreases monotonously as temperature goes up. However, edge dislocation firstly drops, and then increases by temperature due to the enhanced thermal mismatch stress. It implies that an optimized range of the growth temperature can be obtained to improve quality of the LED layers.

Electronic and optical properties of GaN/AlN quantum dots with adjacent threading dislocations

NASA Astrophysics Data System (ADS)

Ye, Han; Lu, Peng-Fei; Yu, Zhong-Yuan; Yao, Wen-Jie; Chen, Zhi-Hui; Jia, Bo-Yong; Liu, Yu-Min

2010-04-01

We present a theory to simulate a coherent GaN QD with an adjacent pure edge threading dislocation by using a finite element method. The piezoelectric effects and the strain modified band edges are investigated in the framework of multi-band k · p theory to calculate the electron and the heavy hole energy levels. The linear optical absorption coefficients corresponding to the interband ground state transition are obtained via the density matrix approach and perturbation expansion method. The results indicate that the strain distribution of the threading dislocation affects the electronic structure. Moreover, the ground state transition behaviour is also influenced by the position of the adjacent threading dislocation.

Epitaxy of GaN in high aspect ratio nanoscale holes over silicon substrate

NASA Astrophysics Data System (ADS)

Wang, Kejia; Wang, Anqi; Ji, Qingbin; Hu, Xiaodong; Xie, Yahong; Sun, Ying; Cheng, Zhiyuan

2017-12-01

Dislocation filtering in gallium nitride (GaN) by epitaxial growth through patterned nanoscale holes is studied. GaN grown from extremely high aspect ratio holes by metalorganic chemical vapor deposition is examined by transmission electron microscopy and high-resolution transmission electron microscopy. This selective area epitaxial growth method with a reduced epitaxy area and an increased depth to width ratio of holes leads to effective filtering of dislocations within the hole and improves the quality of GaN significantly.

Curvature and bow of bulk GaN substrates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Foronda, Humberto M.; Young, Erin C.; Robertson, Christian A.

2016-07-21

We investigate the bow of free standing (0001) oriented hydride vapor phase epitaxy grown GaN substrates and demonstrate that their curvature is consistent with a compressive to tensile stress gradient (bottom to top) present in the substrates. The origin of the stress gradient and the curvature is attributed to the correlated inclination of edge threading dislocation (TD) lines away from the [0001] direction. A model is proposed and a relation is derived for bulk GaN substrate curvature dependence on the inclination angle and the density of TDs. The model is used to analyze the curvature for commercially available GaN substratesmore » as determined by high resolution x-ray diffraction. The results show a close correlation between the experimentally determined parameters and those predicted from theoretical model.« less

Luminescence from defects in GaN

NASA Astrophysics Data System (ADS)

Reshchikov, M. A.; Morkoç, H.

2006-04-01

We briefly review the luminescence properties of defects in GaN and focus on the most interesting defects. In particular, the blue luminescence band peaking at about 3 eV is assigned to different defects and even different types of transitions in undoped, Zn-, C-, and Mg-doped GaN. Another omnipresent luminescence band, the yellow luminescence band may have different origin in nearly dislocation-free freestanding GaN templates, undoped thin layers, and carbon-doped GaN. The Y4 and Y7 lines are caused by recombination at unidentified point defects captured by threading edge dislocations.

Ion Beam Assisted Deposition of Thin Epitaxial GaN Films.

PubMed

Rauschenbach, Bernd; Lotnyk, Andriy; Neumann, Lena; Poppitz, David; Gerlach, Jürgen W

2017-06-23

The assistance of thin film deposition with low-energy ion bombardment influences their final properties significantly. Especially, the application of so-called hyperthermal ions (energy <100 eV) is capable to modify the characteristics of the growing film without generating a large number of irradiation induced defects. The nitrogen ion beam assisted molecular beam epitaxy (ion energy <25 eV) is used to deposit GaN thin films on (0001)-oriented 6H-SiC substrates at 700 °C. The films are studied in situ by reflection high energy electron diffraction, ex situ by X-ray diffraction, scanning tunnelling microscopy, and high-resolution transmission electron microscopy. It is demonstrated that the film growth mode can be controlled by varying the ion to atom ratio, where 2D films are characterized by a smooth topography, a high crystalline quality, low biaxial stress, and low defect density. Typical structural defects in the GaN thin films were identified as basal plane stacking faults, low-angle grain boundaries forming between w-GaN and z-GaN and twin boundaries. The misfit strain between the GaN thin films and substrates is relieved by the generation of edge dislocations in the first and second monolayers of GaN thin films and of misfit interfacial dislocations. It can be demonstrated that the low-energy nitrogen ion assisted molecular beam epitaxy is a technique to produce thin GaN films of high crystalline quality.

Oxygen induced strain field homogenization in AlN nucleation layers and its impact on GaN grown by metal organic vapor phase epitaxy on sapphire: An x-ray diffraction study

NASA Astrophysics Data System (ADS)

Bläsing, J.; Krost, A.; Hertkorn, J.; Scholz, F.; Kirste, L.; Chuvilin, A.; Kaiser, U.

2009-02-01

This paper presents an x-ray study of GaN, which is grown on nominally undoped and oxygen-doped AlN nucleation layers on sapphire substrates by metal organic vapor phase epitaxy. Without additional oxygen doping a trimodal nucleation distribution of AlN is observed leading to inhomogeneous in-plane strain fields, whereas in oxygen-doped layers a homogeneous distribution of nucleation centers is observed. In both types of nucleation layers extremely sharp correlation peaks occur in transverse ω-scans which are attributed to a high density of edge-type dislocations having an in-plane Burgers vector. The correlation peaks are still visible in the (0002) ω-scans of 500 nm GaN which might mislead an observer to conclude incorrectly that there exists an extremely high structural quality. For the undoped nucleation layers depth-sensitive measurements in grazing incidence geometry reveal a strong thickness dependence of the lattice parameter a, whereas no such dependence is observed for doped samples. For oxygen-doped nucleation layers, in cross-sectional transmission electron microscopy images a high density of stacking faults parallel to the substrate surface is found in contrast to undoped nucleation layers where a high density of threading dislocations is visible. GaN of 2.5 μm grown on top of 25 nm AlN nucleation layers with an additional in situ SiN mask show full widths at half maximum of 160″ and 190″ in (0002) and (10-10) high-resolution x-ray diffraction ω-scans, respectively.

Control of epitaxial defects for optimal AlGaN/GaN HEMT performance and reliability

NASA Astrophysics Data System (ADS)

Green, D. S.; Gibb, S. R.; Hosse, B.; Vetury, R.; Grider, D. E.; Smart, J. A.

2004-12-01

High-quality GaN epitaxy continues to be challenged by the lack of matched substrates. Threading dislocations that result from heteroepitaxy are responsible for leakage currents, trapping effects, and may adversely affect device reliability. We have studied the impact of AlN nucleation conditions on the density and character of threading dislocations on SiC substrates. Variation of the nucleation temperature, V/III ratio, and thickness are seen to have a dramatic effect on the balance between edge, screw and mixed character dislocation densities. Electrical and structural properties have been assessed by AFM and XRD on a material level and through DC and RF performance at the device level. The ratio between dislocation characteristics has been established primarily through comparison of symmetric and asymmetric XRD rocking curve widths. The effect of each dislocation type on leakage current, RF power and reliability at 2 GHz, the targeted band for cell phone infrastructure applications, is discussed.

Deep levels in as-grown and electron-irradiated n-type GaN studied by deep level transient spectroscopy and minority carrier transient spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Duc, Tran Thien; School of Engineering Physics, Hanoi University of Science and Technology, 1 Dai Co Viet Road, Hanoi; Pozina, Galia

2016-03-07

Development of high performance GaN-based devices is strongly dependent on the possibility to control and understand defects in material. Important information about deep level defects is obtained by deep level transient spectroscopy and minority carrier transient spectroscopy on as-grown and electron irradiated n-type bulk GaN with low threading dislocation density produced by halide vapor phase epitaxy. One hole trap labelled H1 (E{sub V} + 0.34 eV) has been detected on as-grown GaN sample. After 2 MeV electron irradiation, the concentration of H1 increases and at fluences higher than 5 × 10{sup 14 }cm{sup −2}, a second hole trap labelled H2 is observed. Simultaneously, the concentration of twomore » electron traps, labelled T1 (E{sub C} – 0.12 eV) and T2 (E{sub C} – 0.23 eV), increases. By studying the increase of the defect concentration versus electron irradiation fluence, the introduction rate of T1 and T2 using 2 MeV- electrons was determined to be 7 × 10{sup −3 }cm{sup −1} and 0.9 cm{sup −1}, respectively. Due to the low introduction rate of T1, it is suggested that the defect is associated with a complex. The high introduction rate of trap H1 and T2 suggests that the defects are associated with primary intrinsic defects or complexes. Some deep levels previously observed in irradiated GaN layers with higher threading dislocation densities are not detected in present investigation. It is therefore suggested that the absent traps may be related to primary defects segregated around dislocations.« less

Abbreviated epitaxial growth mode (AGM) method for reducing cost and improving quality of LEDs and lasers

DOEpatents

Tansu, Nelson; Chan, Helen M; Vinci, Richard P; Ee, Yik-Khoon; Biser, Jeffrey

2013-09-24

The use of an abbreviated GaN growth mode on nano-patterned AGOG sapphire substrates, which utilizes a process of using 15 nm low temperature GaN buffer and bypassing etch-back and recovery processes during epitaxy, enables the growth of high-quality GaN template on nano-patterned AGOG sapphire. The GaN template grown on nano-patterned AGOG sapphire by employing abbreviated growth mode has two orders of magnitude lower threading dislocation density than that of conventional GaN template grown on planar sapphire. The use of abbreviated growth mode also leads to significant reduction in cost of the epitaxy. The growths and characteristics of InGaN quantum wells (QWs) light emitting diodes (LEDs) on both templates were compared. The InGaN QWs LEDs grown on the nano-patterned AGOG sapphire demonstrated at least a 24% enhancement of output power enhancement over that of LEDs grown on conventional GaN templates.

Local electronic and optical behaviors of a-plane GaN grown via epitaxial lateral overgrowth

NASA Astrophysics Data System (ADS)

Moore, J. C.; Kasliwal, V.; Baski, A. A.; Ni, X.; Özgür, Ü.; Morkoç, H.

2007-01-01

Conductive atomic force microscopy and near-field optical microscopy (NSOM) were used to study the morphology, conduction, and optical properties of a-plane GaN films grown via epitaxial lateral overgrowth (ELO) by metal organic chemical vapor deposition. The AFM images for the coalesced ELO films show undulations, where the window regions appear as depressions with a high density of surface pits. At reverse bias below 12V, very low uniform conduction (2pA) is seen in the window regions. Above 20V, a lower-quality sample shows localized sites inside the window regions with significant leakage, indicating a correlation between the presence of surface pits and leakage sites. Room temperature NSOM studies explicitly showed enhanced optical quality in the wing regions of the overgrown GaN due to a reduced density of dislocations, with the wings and the windows clearly discernible from near-field photoluminescence mapping.

In situ chemical functionalization of gallium nitride with phosphonic acid derivatives during etching.

PubMed

Wilkins, Stewart J; Greenough, Michelle; Arellano, Consuelo; Paskova, Tania; Ivanisevic, Albena

2014-03-04

In situ functionalization of polar (c plane) and nonpolar (a plane) gallium nitride (GaN) was performed by adding (3-bromopropyl) phosphonic acid or propyl phosphonic acid to a phosphoric acid etch. The target was to modulate the emission properties and oxide formation of GaN, which was explored through surface characterization with atomic force microscopy, X-ray photoelectron spectroscopy, photoluminescence (PL), inductively coupled plasma-mass spectrometry, and water contact angle. The use of (3-bromopropyl) phosphonic acid and propyl phosphonic acid in phosphoric acid demonstrated lower amounts of gallium oxide formation and greater hydrophobicity for both sample sets, while also improving PL emission of polar GaN samples. In addition to crystal orientation, growth-related factors such as defect density in bulk GaN versus thin GaN films residing on sapphire substrates were investigated as well as their responses to in situ functionalization. Thin nonpolar GaN layers were the most sensitive to etching treatments due in part to higher defect densities (stacking faults and threading dislocations), which accounts for large surface depressions. High-quality GaN (both free-standing bulk polar and bulk nonpolar) demonstrated increased sensitivity to oxide formation. Room-temperature PL stands out as an excellent technique to identify nonradiative recombination as observed in the spectra of heteroepitaxially grown GaN samples. The chemical methods applied to tune optical and physical properties of GaN provide a quantitative framework for future novel chemical and biochemical sensor development.

Cathodoluminescence Study on Spatial Luminescence Properties of InN/GaN Multiple Quantum Wells Consisting of 1-Monolayer-Thick InN Wells/GaN Matrix

NASA Astrophysics Data System (ADS)

Hwang, E. S.; Che, S. B.; Saito, H.; Wang, X.; Ishitani, Y.; Yoshikawa, A.

2008-05-01

Spatially resolved luminescence properties of InN/GaN multiple quantum wells (MQWs) consisting of nominally one monolayer (1-ML)-thick InN QWs embedded in a GaN matrix are studied by cross-sectional and plan-view cathodoluminescence measurements. First it is confirmed that the dominant emission peaks observed at around 390 nm to 430 nm in the MQWs samples are attributed to the effects of inserting ˜1-ML-thick InN wells in the GaN matrix, resulting in efficient localization of GaN excitons at InN QWs. Furthermore, it is revealed that the detailed structure of the MQWs, such as the thickness distribution and interface sharpness, is very sensitive to the presence of surface defects such as hillocks around screw-component threading dislocations, resulting in different emission wavelengths/energies. This is because the epitaxy process for depositing such thin InN wells is seriously affected by the atomic-level surface structures/properties of the growth front. It will be concluded that it is necessary to use lower dislocation density GaN bulk templates to obtain much higher structural quality InN/GaN MQWs good enough for characterizing their optical properties.

Investigation on thermodynamics of ion-slicing of GaN and heterogeneously integrating high-quality GaN films on CMOS compatible Si(100) substrates.

PubMed

Huang, Kai; Jia, Qi; You, Tiangui; Zhang, Runchun; Lin, Jiajie; Zhang, Shibin; Zhou, Min; Zhang, Bo; Yu, Wenjie; Ou, Xin; Wang, Xi

2017-11-08

Die-to-wafer heterogeneous integration of single-crystalline GaN film with CMOS compatible Si(100) substrate using the ion-cutting technique has been demonstrated. The thermodynamics of GaN surface blistering is in-situ investigated via a thermal-stage optical microscopy, which indicates that the large activation energy (2.5 eV) and low H ions utilization ratio (~6%) might result in the extremely high H fluence required for the ion-slicing of GaN. The crystalline quality, surface topography and the microstructure of the GaN films are characterized in detail. The full width at half maximum (FWHM) for GaN (002) X-ray rocking curves is as low as 163 arcsec, corresponding to a density of threading dislocation of 5 × 10 7  cm -2 . Different evolution of the implantation-induced damage was observed and a relationship between the damage evolution and implantation-induced damage is demonstrated. This work would be beneficial to understand the mechanism of ion-slicing of GaN and to provide a platform for the hybrid integration of GaN devices with standard Si CMOS process.

Optical and Structural Properties of Microcrystalline GaN on an Amorphous Substrate Prepared by a Combination of Molecular Beam Epitaxy and Metal-Organic Chemical Vapor Deposition

DOE Office of Scientific and Technical Information (OSTI.GOV)

Min, Jung-Wook; Hwang, Hyeong-Yong; Kang, Eun-Kyu

2016-05-01

Microscale platelet-shaped GaN grains were grown on amorphous substrates by a combined epitaxial growth method of molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). First, MBE GaN was grown on an amorphous substrate as a pre-orienting layer and its structural properties were investigated. Second, MOCVD grown GaN samples using the different growth techniques of planar and selective area growth (SAG) were comparatively investigated by transmission electron microscopy (TEM), cathodoluminescence (CL), and photoluminescence (PL). In MOCVD planar GaN, strong bound exciton peaks dominated despite the high density of the threading dislocations (TDs). In MOCVD SAG GaN, on the othermore » hand, TDs were clearly reduced with bending, but basal stacking fault (BSF) PL peaks were observed at 3.42 eV. The combined epitaxial method not only provides a deep understanding of the growth behavior but also suggests an alternative approach for the growth of GaN on amorphous substances.« less

High-quality III-nitride films on conductive, transparent (2̅01)-oriented β-Ga2O3 using a GaN buffer layer

PubMed Central

Muhammed, M. M.; Roldan, M. A.; Yamashita, Y.; Sahonta, S.-L.; Ajia, I. A.; Iizuka, K.; Kuramata, A.; Humphreys, C. J.; Roqan, I. S.

2016-01-01

We demonstrate the high structural and optical properties of InxGa1−xN epilayers (0 ≤ x ≤ 23) grown on conductive and transparent (01)-oriented β-Ga2O3 substrates using a low-temperature GaN buffer layer rather than AlN buffer layer, which enhances the quality and stability of the crystals compared to those grown on (100)-oriented β-Ga2O3. Raman maps show that the 2″ wafer is relaxed and uniform. Transmission electron microscopy (TEM) reveals that the dislocation density reduces considerably (~4.8 × 107 cm−2) at the grain centers. High-resolution TEM analysis demonstrates that most dislocations emerge at an angle with respect to the c-axis, whereas dislocations of the opposite phase form a loop and annihilate each other. The dislocation behavior is due to irregular (01) β-Ga2O3 surface at the interface and distorted buffer layer, followed by relaxed GaN epilayer. Photoluminescence results confirm high optical quality and time-resolved spectroscopy shows that the recombination is governed by bound excitons. We find that a low root-mean-square average (≤1.5 nm) of InxGa1−xN epilayers can be achieved with high optical quality of InxGa1−xN epilayers. We reveal that (01)-oriented β-Ga2O3 substrate has a strong potential for use in large-scale high-quality vertical light emitting device design. PMID:27412372

High-quality III-nitride films on conductive, transparent (2̅01)-oriented β-Ga2O3 using a GaN buffer layer.

PubMed

Muhammed, M M; Roldan, M A; Yamashita, Y; Sahonta, S-L; Ajia, I A; Iizuka, K; Kuramata, A; Humphreys, C J; Roqan, I S

2016-07-14

We demonstrate the high structural and optical properties of InxGa1-xN epilayers (0 ≤ x ≤ 23) grown on conductive and transparent (01)-oriented β-Ga2O3 substrates using a low-temperature GaN buffer layer rather than AlN buffer layer, which enhances the quality and stability of the crystals compared to those grown on (100)-oriented β-Ga2O3. Raman maps show that the 2″ wafer is relaxed and uniform. Transmission electron microscopy (TEM) reveals that the dislocation density reduces considerably (~4.8 × 10(7) cm(-2)) at the grain centers. High-resolution TEM analysis demonstrates that most dislocations emerge at an angle with respect to the c-axis, whereas dislocations of the opposite phase form a loop and annihilate each other. The dislocation behavior is due to irregular (01) β-Ga2O3 surface at the interface and distorted buffer layer, followed by relaxed GaN epilayer. Photoluminescence results confirm high optical quality and time-resolved spectroscopy shows that the recombination is governed by bound excitons. We find that a low root-mean-square average (≤1.5 nm) of InxGa1-xN epilayers can be achieved with high optical quality of InxGa1-xN epilayers. We reveal that (01)-oriented β-Ga2O3 substrate has a strong potential for use in large-scale high-quality vertical light emitting device design.

Improvement of optical quality of semipolar (11 2 ¯ 2 ) GaN on m-plane sapphire by in-situ epitaxial lateral overgrowth

NASA Astrophysics Data System (ADS)

Monavarian, Morteza; Izyumskaya, Natalia; Müller, Marcus; Metzner, Sebastian; Veit, Peter; Can, Nuri; Das, Saikat; Özgür, Ümit; Bertram, Frank; Christen, Jürgen; Morkoç, Hadis; Avrutin, Vitaliy

2016-04-01

Among the major obstacles for development of non-polar and semipolar GaN structures on foreign substrates are stacking faults which deteriorate the structural and optical quality of the material. In this work, an in-situ SiNx nano-network has been employed to achieve high quality heteroepitaxial semipolar (11 2 ¯ 2 ) GaN on m-plane sapphire with reduced stacking fault density. This approach involves in-situ deposition of a porous SiNx interlayer on GaN that serves as a nano-mask for the subsequent growth, which starts in the nanometer-sized pores (window regions) and then progresses laterally as well, as in the case of conventional epitaxial lateral overgrowth (ELO). The inserted SiNx nano-mask effectively prevents the propagation of defects, such as dislocations and stacking faults, in the growth direction and thus reduces their density in the overgrown layers. The resulting semipolar (11 2 ¯ 2 ) GaN layers exhibit relatively smooth surface morphology and improved optical properties (PL intensity enhanced by a factor of 5 and carrier lifetimes by 35% to 85% compared to the reference semipolar (11 2 ¯ 2 ) GaN layer) which approach to those of the c-plane in-situ nano-ELO GaN reference and, therefore, holds promise for light emitting and detecting devices.

Low-Angle-Incidence Microchannel Epitaxy of a-Plane GaN Grown by Ammonia-Based Metal-Organic Molecular Beam Epitaxy

NASA Astrophysics Data System (ADS)

Lin, Chia-Hung; Uchiyama, Shota; Maruyama, Takahiro; Naritsuka, Shigeya

2012-04-01

Low-angle-incidence microchannel epitaxy (LAIMCE) of a-plane GaN was performed using ammonia-based metal-organic molecular beam epitaxy to obtain wide and thin lateral overgrowth over a SiO2 mask. Trimethylgallium (TMG) was supplied perpendicular to the openings cut in the mask with a low incident angle of 5° relative to the substrate plane. The [NH3]/[TMG] ratio (R) dependence of GaN LAIMCE was optimized by varying R from 5 to 30. A wide lateral overgrowth of 3.7 µm with a dislocation density below the transmission electron microscope detection limit was obtained at R=15 for a thickness of 520 nm.

Three-Dimensional Hetero-Integration of Faceted GaN on Si Pillars for Efficient Light Energy Conversion Devices.

PubMed

Kim, Dong Rip; Lee, Chi Hwan; Cho, In Sun; Jang, Hanmin; Jeon, Min Soo; Zheng, Xiaolin

2017-07-25

An important pathway for cost-effective light energy conversion devices, such as solar cells and light emitting diodes, is to integrate III-V (e.g., GaN) materials on Si substrates. Such integration first necessitates growth of high crystalline III-V materials on Si, which has been the focus of many studies. However, the integration also requires that the final III-V/Si structure has a high light energy conversion efficiency. To accomplish these twin goals, we use single-crystalline microsized Si pillars as a seed layer to first grow faceted Si structures, which are then used for the heteroepitaxial growth of faceted GaN films. These faceted GaN films on Si have high crystallinity, and their threading dislocation density is similar to that of GaN grown on sapphire. In addition, the final faceted GaN/Si structure has great light absorption and extraction characteristics, leading to improved performance for GaN-on-Si light energy conversion devices.

Controlled growth of heteroepitaxial zinc oxide nanostructures on gallium nitride.

PubMed

Kong, Bo Hyun; Kim, Dong Chan; Mohanta, Sanjay Kumar; Han, Won Suk; Cho, Hyung Koun; Hong, Chang-Hee; Kim, Hyung Gu

2009-07-01

ZnO epitaxial layers were grown on GaN underlying films by metalorganic chemical vapor deposition at various temperatures. An increase in growth temperature led to morphological changes from a smooth film with hexagonal-shaped surface pits to honeycomb-like nanostructures with deep hollow, and additionally resulted in a decrease in dislocation density in the interfacial layers. The reduced dislocation density at the higher growth temperature was attributed to an increase in the size of the critical nucleus and the low nucleation density at the initial stage. The shifts in the peak positions in the X-ray diffraction and photoluminescence were also observed in the samples grown at different temperatures, and were caused by the variation of residual strains after the complete coalescence of the nuclei.

Efficient reduction of defects in (1120) non-polar and (1122) semi-polar GaN grown on nanorod templates

NASA Astrophysics Data System (ADS)

Bai, J.; Gong, Y.; Xing, K.; Yu, X.; Wang, T.

2013-03-01

(1120) non-polar and (1122) semi-polar GaNs with a low defect density have been achieved by means of an overgrowth on nanorod templates, where a quick coalescence with a thickness even below 1 μm occurs. On-axis and off-axis X-ray rocking curve measurements have shown a massive reduction in the linewidth for our overgrown GaN in comparison with standard GaN films grown on sapphire substrates. Transmission electron microscope observation demonstrates that the overgrowth on the nanorod templates takes advantage of an omni-directional growth around the sidewalls of the nanostructures. The dislocations redirect in basal planes during the overgrowth, leading to their annihilation and termination at voids formed due to a large lateral growth rate. In the non-polar GaN, the priority <0001> lateral growth from vertical sidewalls of nanorods allows basal plane stacking faults (BSFs) to be blocked in the nanorod gaps; while for semi-polar GaN, the propagation of BSFs starts to be impeded when the growth front is changed to be along inclined <0001> direction above the nanorods.

Improving optical performance of GaN nanowires grown by selective area growth homoepitaxy: Influence of substrate and nanowire dimensions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aseev, P., E-mail: pavel.aseev@isom.upm.es, E-mail: gacevic@isom.upm.es; Gačević, Ž., E-mail: pavel.aseev@isom.upm.es, E-mail: gacevic@isom.upm.es; Calleja, E.

2016-06-20

Series of GaN nanowires (NW) with controlled diameters (160–500 nm) and heights (420–1100 nm) were homoepitaxially grown on three different templates: GaN/Si(111), GaN/AlN/Si(111), and GaN/sapphire(0001). Transmission electron microscopy reveals a strong influence of the NW diameter on dislocation filtering effect, whereas photoluminescence measurements further relate this effect to the GaN NWs near-bandgap emission efficiency. Although the templates' quality has some effects on the GaN NWs optical and structural properties, the NW diameter reduction drives the dislocation filtering effect to the point where a poor GaN template quality becomes negligible. Thus, by a proper optimization of the homoepitaxial GaN NWs growth, the propagationmore » of dislocations into the NWs can be greatly prevented, leading to an exceptional crystal quality and a total dominance of the near-bandgap emission over sub-bandgap, defect-related lines, such as basal stacking faults and so called unknown exciton (UX) emission. In addition, a correlation between the presence of polarity inversion domain boundaries and the UX emission lines around 3.45 eV is established.« less

Characterisation of defects in p-GaN by admittance spectroscopy

NASA Astrophysics Data System (ADS)

Elsherif, O. S.; Vernon-Parry, K. D.; Evans-Freeman, J. H.; Airey, R. J.; Kappers, M.; Humphreys, C. J.

2012-08-01

Mg-doped GaN films have been grown on (0 0 0 1) sapphire using metal organic vapour phase epitaxy. Use of different buffer layer strategies caused the threading dislocation density (TDD) in the GaN to be either approximately 2×109 cm-2 or 1×1010 cm-2. Frequency-dependent capacitance and conductance measurements at temperatures up to 450 K have been used to study the electronic states associated with the Mg doping, and to determine how these are affected by the TDD. Admittance spectroscopy of the films finds a single impurity-related acceptor level with an activation energy of 160±10 meV for [Mg] of about 1×1019 cm-3, and 120±10 eV as the Mg precursor flux decreased. This level is thought to be associated with the Mg acceptor state. The TDD has no discernible effect on the trap detected by admittance spectroscopy. We compare these results with cathodoluminescence measurements reported in the literature, which reveal that most threading dislocations are non-radiative recombination centres, and discuss possible reasons why our admittance spectroscopy have not detected electrically active defects associated with threading dislocations.

Local electronic and optical behavior of ELO a-plane GaN

NASA Astrophysics Data System (ADS)

Baski, A. A.; Moore, J. C.; Ozgur, U.; Kasliwal, V.; Ni, X.; Morkoc, H.

2007-03-01

Conductive atomic force microscopy (CAFM) and near-field optical microscopy (NSOM) were used to study a-plane GaN films grown via epitaxial lateral overgrowth (ELO). The ELO films were prepared by metal organic chemical vapor deposition on a patterned SiO2 layer with 4-μm wide windows, which was deposited on a GaN template grown on r-plane sapphire. The window regions of the coalesced ELO films appear as depressions with a high density of surface pits. At reverse bias below 12 V, very low uniform conduction (2 pA) is seen in the window regions. Above 20 V, a lower-quality sample shows localized sites inside the window regions with significant leakage, indicating a correlation between the presence of surface pits and leakage sites. Room temperature NSOM studies also suggest a greater density of surface terminated dislocations in the window regions, while wing regions explicitly show enhanced optical quality of the overgrown GaN. The combination of CAFM and NSOM data therefore indicates a correlation between the presence of surface pits, localized reverse-bias current leakage, and low PL intensity in the window regions.

Gradual tilting of crystallographic orientation and configuration of dislocations in GaN selectively grown by vapour phase epitaxy methods

PubMed

Kuwan; Tsukamoto; Taki; Horibuchi; Oki; Kawaguchi; Shibata; Sawaki; Hiramatsu

2000-01-01

Cross-sectional transmission electron microscope (TEM) observation was performed for selectively grown gallium nitride (GaN) in order to examine the dependence of GaN microstructure on the growth conditions. The GaN films were grown by hydride vapour phase epitaxy (HVPE) or metalorganic vapour phase epitaxy (MOVPE) on GaN covered with a patterned mask. Thin foil specimens for TEM observation were prepared with focused ion beam (FIB) machining apparatus. It was demonstrated that the c-axis of GaN grown over the terrace of the mask tilts towards the centre of the terrace when the GaN is grown in a carrier gas of N2. The wider terrace results in a larger tilting angle if other growth conditions are identical. The tilting is attributed to 'horizontal dislocations' (HDs) generated during the overgrowth of GaN on the mask terrace. The HDs in HVPE-GaN have a semi-loop shape and are tangled with one another, while those in MOVPE-GaN are straight and lined up to form low-angle grain boundaries.

Correlation between dislocations and leakage current of p-n diodes on a free-standing GaN substrate

NASA Astrophysics Data System (ADS)

Usami, Shigeyoshi; Ando, Yuto; Tanaka, Atsushi; Nagamatsu, Kentaro; Deki, Manato; Kushimoto, Maki; Nitta, Shugo; Honda, Yoshio; Amano, Hiroshi; Sugawara, Yoshihiro; Yao, Yong-Zhao; Ishikawa, Yukari

2018-04-01

Dislocations that cause a reverse leakage current in vertical p-n diodes on a GaN free-standing substrate were investigated. Under a high reverse bias, dot-like leakage spots were observed using an emission microscope. Subsequent cathodoluminescence (CL) observations revealed that the leakage spots coincided with part of the CL dark spots, indicating that some types of dislocation cause reverse leakage. When etch pits were formed on the dislocations by KOH etching, three sizes of etch pits were obtained (large, medium, and small). Among these etch pits, only the medium pits coincided with leakage spots. Additionally, transmission electron microscopy observations revealed that pure screw dislocations are present under the leakage spots. The results revealed that 1c pure screw dislocations are related to the reverse leakage in vertical p-n diodes.

Si Complies with GaN to Overcome Thermal Mismatches for the Heteroepitaxy of Thick GaN on Si.

PubMed

Tanaka, Atsunori; Choi, Woojin; Chen, Renjie; Dayeh, Shadi A

2017-10-01

Heteroepitaxial growth of lattice mismatched materials has advanced through the epitaxy of thin coherently strained layers, the strain sharing in virtual and nanoscale substrates, and the growth of thick films with intermediate strain-relaxed buffer layers. However, the thermal mismatch is not completely resolved in highly mismatched systems such as in GaN-on-Si. Here, geometrical effects and surface faceting to dilate thermal stresses at the surface of selectively grown epitaxial GaN layers on Si are exploited. The growth of thick (19 µm), crack-free, and pure GaN layers on Si with the lowest threading dislocation density of 1.1 × 10 7 cm -2 achieved to date in GaN-on-Si is demonstrated. With these advances, the first vertical GaN metal-insulator-semiconductor field-effect transistors on Si substrates with low leakage currents and high on/off ratios paving the way for a cost-effective high power device paradigm on an Si CMOS platform are demonstrated. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

GaN nanostructure design for optimal dislocation filtering

NASA Astrophysics Data System (ADS)

Liang, Zhiwen; Colby, Robert; Wildeson, Isaac H.; Ewoldt, David A.; Sands, Timothy D.; Stach, Eric A.; García, R. Edwin

2010-10-01

The effect of image forces in GaN pyramidal nanorod structures is investigated to develop dislocation-free light emitting diodes (LEDs). A model based on the eigenstrain method and nonlocal stress is developed to demonstrate that the pyramidal nanorod efficiently ejects dislocations out of the structure. Two possible regimes of filtering behavior are found: (1) cap-dominated and (2) base-dominated. The cap-dominated regime is shown to be the more effective filtering mechanism. Optimal ranges of fabrication parameters that favor a dislocation-free LED are predicted and corroborated by resorting to available experimental evidence. The filtering probability is summarized as a function of practical processing parameters: the nanorod radius and height. The results suggest an optimal nanorod geometry with a radius of ˜50b (26 nm) and a height of ˜125b (65 nm), in which b is the magnitude of the Burgers vector for the GaN system studied. A filtering probability of greater than 95% is predicted for the optimal geometry.

AlGaN/GaN High Electron Mobility Transistor Grown and Fabricated on ZrTi Metallic Alloy Buffer Layers

DOE PAGES

Ren, Fan; Pearton, Stephen J.; Ahn, Shihyun; ...

2017-09-26

AlGaN/GaN high electron mobility transistors (HEMTs) were demonstrated for structures grown on ZrTi metallic alloy buffer layers, which provided lattice matching of the in-plane lattice parameter (“a-parameter”) to hexagonal GaN. The quality of the GaN buffer layer and HEMT structure were confirmed with X-ray 2θ and rocking scans as well as cross-section transmission electron microscopy (TEM) images. The X-ray 2θ scans showed full widths at half maximum (FWHM) of 0.06°, 0.05° and 0.08° for ZrTi alloy, GaN buffer layer, and the entire HEMT structure, respectively. TEM of the lower section of the HEMT structure containing the GaN buffer layer andmore » the AlN/ZrTi/AlN stack on the Si substrate showed that it was important to grow AlN on the top of ZrTi prior to growing the GaN buffer layer. Finally, the estimated threading dislocation (TD) density in the GaN channel layer of the HEMT structure was in the 10 8 cm -2 range.« less

Efficient reduction of defects in (1120) non-polar and (1122) semi-polar GaN grown on nanorod templates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bai, J.; Gong, Y.; Xing, K.

2013-03-11

(1120) non-polar and (1122) semi-polar GaNs with a low defect density have been achieved by means of an overgrowth on nanorod templates, where a quick coalescence with a thickness even below 1 {mu}m occurs. On-axis and off-axis X-ray rocking curve measurements have shown a massive reduction in the linewidth for our overgrown GaN in comparison with standard GaN films grown on sapphire substrates. Transmission electron microscope observation demonstrates that the overgrowth on the nanorod templates takes advantage of an omni-directional growth around the sidewalls of the nanostructures. The dislocations redirect in basal planes during the overgrowth, leading to their annihilationmore » and termination at voids formed due to a large lateral growth rate. In the non-polar GaN, the priority <0001> lateral growth from vertical sidewalls of nanorods allows basal plane stacking faults (BSFs) to be blocked in the nanorod gaps; while for semi-polar GaN, the propagation of BSFs starts to be impeded when the growth front is changed to be along inclined <0001> direction above the nanorods.« less

III-nitrides on oxygen- and zinc-face ZnO substrates

NASA Astrophysics Data System (ADS)

Namkoong, Gon; Burnham, Shawn; Lee, Kyoung-Keun; Trybus, Elaissa; Doolittle, W. Alan; Losurdo, Maria; Capezzuto, Pio; Bruno, Giovanni; Nemeth, Bill; Nause, Jeff

2005-10-01

The characteristics of III-nitrides grown on zinc- and oxygen-face ZnO by plasma-assisted molecular beam epitaxy were investigated. The reflection high-energy electron diffraction pattern indicates formation of a cubic phase at the interface between III-nitride and both Zn- and O-face ZnO. The polarity indicates that Zn-face ZnO leads to a single polarity, while O-face ZnO forms mixed polarity of III-nitrides. Furthermore, by using a vicinal ZnO substrate, the terrace-step growth of GaN was realized with a reduction by two orders of magnitude in the dislocation-related etch pit density to ˜108cm-2, while a dislocation density of ˜1010cm-2 was obtained on the on-axis ZnO substrates.

Comparative study on hydrostatic strain, stress and dislocation density of Al{sub 0.3}Ga{sub 0.7}N/GaN heterostructure before and after a-Si{sub 3}N{sub 4} passivation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dinara, Syed Mukulika, E-mail: smdinara.iit@gmail.com; Jana, Sanjay Kr.; Mukhopadhyay, Partha

2015-08-28

The hydrostatic strain, stress and dislocation densities were comparatively analyzed before and after passivation of amorphous silicon nitride (a-Si{sub 3}N{sub 4}) layer on Al{sub 0.3}Ga{sub 0.7}N/GaN heterostructure by nondestructive high resolution x-ray diffraction (HRXRD) technique. The crystalline quality, in-plane and out-of plane strain were evaluated from triple-axis (TA) (ω-2θ) diffraction profile across the (002) reflection plane and double-axis (DA) (ω-2θ) glancing incidence (GI) diffraction profile across (105) reflection plane. The hydrostatic strain and stress of Al{sub 0.3}Ga{sub 0.7}N barrier layer were increased significantly after passivation and both are tensile in nature. The dislocation density of GaN was also analyzed andmore » no significant change was observed after passivation of the heterostructure. The crystalline quality was not degraded after passivation on the heterostructure confirmed by the full-width-half-maximum (FWHM) analysis.« less

Evaluation of AlGaN/GaN high electron mobility transistors grown on ZrTi buffer layers with sapphire substrates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ren, Fan; Pearton, Stephen J.; Ahn, Shihyun

Here, AlGaN/GaN high electron mobility transistors (HEMTs) have been grown on sapphire substrates, using ZrTi buffer layers to provide in-plane lattice-matching to hexagonal GaN. X-ray diffraction (XRD) as well as cross-section transmission electron microscopy (TEM) were used to assess the quality of the HEMT structure. The XRD 2θ scans showed full-width-at-half-maximum values of 0.16°, 0.07°, and 0.08° for ZrTi alloy, GaN buffer layer, and the entire HEMT structure, respectively. TEM studies of the GaN buffer layer and the AlN/ZrTi/AlN stack showed the importance of growing thin AlN buffer layers on the ZrTi layer prior to growth of the GaN buffermore » layer. The density of threading dislocations in the GaN channel layer of the HEMT structure was estimated to be in the 10 8 cm –2 range. The HEMT device exhibited a saturation drain current density of 820 mA/mm, and the channel of the fabricated HEMTs could be well modulated. A cutoff frequency (f T) of 8.9 GHz and a maximum frequency of oscillation (f max) of 17.3 GHz were achieved for HEMTs with gate dimensions of 1 × 200 μm.« less

Evaluation of AlGaN/GaN high electron mobility transistors grown on ZrTi buffer layers with sapphire substrates

DOE PAGES

Ren, Fan; Pearton, Stephen J.; Ahn, Shihyun; ...

2016-09-21

Here, AlGaN/GaN high electron mobility transistors (HEMTs) have been grown on sapphire substrates, using ZrTi buffer layers to provide in-plane lattice-matching to hexagonal GaN. X-ray diffraction (XRD) as well as cross-section transmission electron microscopy (TEM) were used to assess the quality of the HEMT structure. The XRD 2θ scans showed full-width-at-half-maximum values of 0.16°, 0.07°, and 0.08° for ZrTi alloy, GaN buffer layer, and the entire HEMT structure, respectively. TEM studies of the GaN buffer layer and the AlN/ZrTi/AlN stack showed the importance of growing thin AlN buffer layers on the ZrTi layer prior to growth of the GaN buffermore » layer. The density of threading dislocations in the GaN channel layer of the HEMT structure was estimated to be in the 10 8 cm –2 range. The HEMT device exhibited a saturation drain current density of 820 mA/mm, and the channel of the fabricated HEMTs could be well modulated. A cutoff frequency (f T) of 8.9 GHz and a maximum frequency of oscillation (f max) of 17.3 GHz were achieved for HEMTs with gate dimensions of 1 × 200 μm.« less

Electrical current flow at conductive nanowires formed in GaN thin films by a dislocation template technique

NASA Astrophysics Data System (ADS)

Amma, Shin-ichi; Tokumoto, Yuki; Edagawa, Keiichi; Shibata, Naoya; Mizoguchi, Teruyasu; Yamamoto, Takahisa; Ikuhara, Yuichi

2010-05-01

Conductive nanowires were fabricated in GaN thin film by selectively doping of Al along threading dislocations. Electrical current flow localized at the nanowires was directly measured by a contact mode atomic force microscope. The current flow at the nanowires was considered to be Frenkel-Poole emission mode, suggesting the existence of the deep acceptor level along the nanowires as a possible cause of the current flow. The results obtained in this study show the possibility for fabricating nanowires using pipe-diffusion at dislocations in solid thin films.

Characteristics of Mg-doped and In-Mg co-doped p-type GaN epitaxial layers grown by metal organic chemical vapour deposition

NASA Astrophysics Data System (ADS)

Chung, S. J.; Senthil Kumar, M.; Lee, Y. S.; Suh, E.-K.; An, M. H.

2010-05-01

Mg-doped and In-Mg co-doped p-type GaN epilayers were grown using the metal organic chemical vapour deposition technique. The effect of In co-doping on the physical properties of p-GaN layer was examined by high resolution x-ray diffraction (HRXRD), transmission electron microscopy (TEM), Hall effect, photoluminescence (PL) and persistent photoconductivity (PPC) at room temperature. An improved crystalline quality and a reduction in threading dislocation density are evidenced upon In doping in p-GaN from HRXRD and TEM images. Hole conductivity, mobility and carrier density also significantly improved by In co-doping. PL studies of the In-Mg co-doped sample revealed that the peak position is blue shifted to 3.2 eV from 2.95 eV of conventional p-GaN and the PL intensity is increased by about 25%. In addition, In co-doping significantly reduced the PPC effect in p-type GaN layers. The improved electrical and optical properties are believed to be associated with the active participation of isolated Mg impurities.

Growth study of self-assembled GaN nanocolumns on silica glass by plasma assisted molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Liudi Mulyo, Andreas; Konno, Yuta; Nilsen, Julie S.; van Helvoort, Antonius T. J.; Fimland, Bjørn-Ove; Weman, Helge; Kishino, Katsumi

2017-12-01

We demonstrate GaN nanocolumn growth on fused silica glass by plasma-assisted molecular beam epitaxy. The effect of the substrate temperature, Ga flux and N2 flow rate on the structural and optical properties are studied. At optimum growth conditions, GaN nanocolumns are vertically aligned and well separated with an average diameter, height and density of 72 nm, 1.2 μm and 1.6 × 109 cm-2, respectively. The nanocolumns exhibit wurtzite crystal structure with no threading dislocations, stacking faults or twinning and grow in the [0 0 0 1] direction. At the interface adjacent to the glass, there is a few atom layers thick intermediate phase with ABC stacking order (zinc blende). Photoluminescence measurements evidence intense and narrow excitonic emissions, along with the absence of any defect-related zinc blende and yellow luminescence emission.

Evolution and characteristics of GaN nanowires produced via maskless reactive ion etching.

PubMed

Haab, Anna; Mikulics, Martin; Sutter, Eli; Jin, Jiehong; Stoica, Toma; Kardynal, Beata; Rieger, Torsten; Grützmacher, Detlev; Hardtdegen, Hilde

2014-06-27

The formation of nanowires (NWs) by reactive ion etching (RIE) of maskless GaN layers was investigated. The morphological, structural and optical characteristics of the NWs were studied and compared to those of the layer they evolve from. It is shown that the NWs are the result of a defect selective etching process. The evolution of density and length with etching time is discussed. Densely packed NWs with a length of more than 1 μm and a diameter of ∼60 nm were obtained by RIE of a ∼2.5 μm thick GaN layer. The NWs are predominantly free of threading dislocations and show an improvement of optical properties compared to their layer counterpart. The production of NWs via a top down process on non-masked group III-nitride layers is assessed to be very promising for photovoltaic applications.

Detection and modeling of leakage current in AlGaN-based deep ultraviolet light-emitting diodes

DOE PAGES

Moseley, Michael William; Allerman, Andrew A.; Crawford, Mary H.; ...

2015-03-01

Current-voltage (IV) characteristics of two AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) with differing densities of open-core threading dislocations (nanopipes) are analyzed. A three-diode circuit is simulated to emulate the IV characteristics of the DUV-LEDs, but is only able to accurately model the lower leakage current, lower nanopipe density DUV-LED. It was found that current leakage through the nanopipes in these structures is rectifying, despite nanopipes being previously established as inherently n-type. Using defect-sensitive etching, the nanopipes are revealed to terminate within the p-type GaN capping layer of the DUV-LEDs. The circuit model is modified to account for another p-nmore » junction between the n-type nanopipes and the p-type GaN, and an excellent fit to the IV characteristics of the leaky DUV-LED is achieved.« less

Alignment control and atomically-scaled heteroepitaxial interface study of GaN nanowires.

PubMed

Liu, Qingyun; Liu, Baodan; Yang, Wenjin; Yang, Bing; Zhang, Xinglai; Labbé, Christophe; Portier, Xavier; An, Vladimir; Jiang, Xin

2017-04-20

Well-aligned GaN nanowires are promising candidates for building high-performance optoelectronic nanodevices. In this work, we demonstrate the epitaxial growth of well-aligned GaN nanowires on a [0001]-oriented sapphire substrate in a simple catalyst-assisted chemical vapor deposition process and their alignment control. It is found that the ammonia flux plays a key role in dominating the initial nucleation of GaN nanocrystals and their orientation. Typically, significant improvement of the GaN nanowire alignment can be realized at a low NH 3 flow rate. X-ray diffraction and cross-sectional scanning electron microscopy studies further verified the preferential orientation of GaN nanowires along the [0001] direction. The growth mechanism of GaN nanowire arrays is also well studied based on cross-sectional high-resolution transmission electron microscopy (HRTEM) characterization and it is observed that GaN nanowires have good epitaxial growth on the sapphire substrate following the crystallographic relationship between (0001) GaN ∥(0001) sapphire and (101[combining macron]0) GaN ∥(112[combining macron]0) sapphire . Most importantly, periodic misfit dislocations are also experimentally observed in the interface region due to the large lattice mismatch between the GaN nanowire and the sapphire substrate, and the formation of such dislocations will favor the release of structural strain in GaN nanowires. HRTEM analysis also finds the existence of "type I" stacking faults and voids inside the GaN nanowires. Optical investigation suggests that the GaN nanowire arrays have strong emission in the UV range, suggesting their crystalline nature and chemical purity. The achievement of aligned GaN nanowires will further promote the wide applications of GaN nanostructures toward diverse high-performance optoelectronic nanodevices including nano-LEDs, photovoltaic cells, photodetectors etc.

Exciton dynamics at a single dislocation in GaN probed by picosecond time-resolved cathodoluminescence

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, W., E-mail: we.liu@epfl.ch, E-mail: gwenole.jacopin@epfl.ch; Carlin, J.-F.; Grandjean, N.

2016-07-25

We investigate the dynamics of donor bound excitons (D°X{sub A}) at T = 10 K around an isolated single edge dislocation in homoepitaxial GaN, using a picosecond time-resolved cathodoluminescence (TR-CL) setup with high temporal and spatial resolutions. An ∼ 1.3 meV dipole-like energy shift of D°X{sub A} is observed around the dislocation, induced by the local strain fields. By simultaneously recording the variations of both the exciton lifetime and the CL intensity across the dislocation, we directly assess the dynamics of excitons around the defect. Our observations are well reproduced by a diffusion model. It allows us to deduce an exciton diffusion length ofmore » ∼24 nm as well as an effective area of the dislocation with a radius of ∼95 nm, where the recombination can be regarded as entirely non-radiative.« less

Vertical GaN power diodes with a bilayer edge termination

DOE PAGES

Dickerson, Jeramy R.; Allerman, Andrew A.; Bryant, Benjamin N.; ...

2015-12-07

Vertical GaN power diodes with a bilayer edge termination (ET) are demonstrated. The GaN p-n junction is formed on a low threading dislocation defect density (10 4 - 10 5 cm -2) GaN substrate, and has a 15-μm-thick n-type drift layer with a free carrier concentration of 5 × 10 15 cm -3. The ET structure is formed by N implantation into the p+-GaN epilayer just outside the p-type contact to create compensating defects. The implant defect profile may be approximated by a bilayer structure consisting of a fully compensated layer near the surface, followed by a 90% compensated (p)more » layer near the n-type drift region. These devices exhibit avalanche breakdown as high as 2.6 kV at room temperature. In addition simulations show that the ET created by implantation is an effective way to laterally distribute the electric field over a large area. This increases the voltage at which impact ionization occurs and leads to the observed higher breakdown voltages.« less

Grouped and Multistep Nanoheteroepitaxy: Toward High-Quality GaN on Quasi-Periodic Nano-Mask.

PubMed

Feng, Xiaohui; Yu, Tongjun; Wei, Yang; Ji, Cheng; Cheng, Yutian; Zong, Hua; Wang, Kun; Yang, Zhijian; Kang, Xiangning; Zhang, Guoyi; Fan, Shoushan

2016-07-20

A novel nanoheteroepitaxy method, namely, the grouped and multistep nanoheteroepitaxy (GM-NHE), is proposed to attain a high-quality gallium nitride (GaN) epilayer by metal-organic vapor phase epitaxy. This method combines the effects of sub-100 nm nucleation and multistep lateral growth by using a low-cost but unique carbon nanotube mask, which consists of nanoscale growth windows with a quasi-periodic 2D fill factor. It is found that GM-NHE can facilely reduce threading dislocation density (TDD) and modulate residual stress on foreign substrate without any regrowth. As a result, high-quality GaN epilayer is produced with homogeneously low TDD of 4.51 × 10(7) cm(-2) and 2D-modulated stress, and the performance of the subsequent 410 nm near-ultraviolet light-emitting diode is greatly boosted. In this way, with the facile fabrication of nanomask and the one-off epitaxy procedure, GaN epilayer is prominently improved with the assistance of nanotechnology, which demonstrates great application potential for high-efficiency TDD-sensitive optoelectronic and electronic devices.

Recombination-related properties of a-screw dislocations in GaN: A combined CL, EBIC, TEM study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Medvedev, O. S., E-mail: o.s.medvedev@spbu.ru; Mikhailovskii, V. Yu.; IRC for Nanotechnology, Research Park, St.-Petersburg State University

2016-06-17

Cathodoluminescence (CL), electron beam current (EBIC) and transmission electron microscopy (TEM) techniques have been applied to investigate recombination properties and structure of freshly introduced dislocations in low-ohmic GaN crystals. It was confirmed that the only a-screw dislocations exhibited an intense characteristic dislocation-related luminescence (DRL) which persisted up to room temperature and was red-shifted by about 0.3 eV with respect to the band gap energy not only in HVPE but also in MOCVD grown samples. EBIC contrast of the dislocations was found to be temperature independent indicating that the dislocation-related recombination level is situated below 200 meV with respect of conductionmore » band minimum. With the increasing of the magnification of the dislocation TEM cross-sectional images they were found to disappear, probably, due to the recombination enhanced dislocation glide (REDG) under electron beam exposure which was immediately observed in CL investigations on a large scale. The stacking fault ribbon in the core of dissociated a-screw dislocation which form a quantum well for electrons was proposed to play an important role both in DRL spectrum formation and in REDG.« less

High-quality vertical light emitting diodes fabrication by mechanical lift-off technique

NASA Astrophysics Data System (ADS)

Tu, Po-Min; Hsu, Shih-Chieh; Chang, Chun-Yen

2011-10-01

We report the fabrication of mechanical lift-off high quality thin GaN with Hexagonal Inversed Pyramid (HIP) structures for vertical light emitting diodes (V-LEDs). The HIP structures were formed at the GaN/sapphire substrate interface under high temperature during KOH wet etching process. The average threading dislocation density (TDD) was estimated by transmission electron microscopy (TEM) and found the reduction from 2×109 to 1×108 cm-2. Raman spectroscopy analysis revealed that the compressive stress of GaN epilayer was effectively relieved in the thin-GaN LED with HIP structures. Finally, the mechanical lift-off process is claimed to be successful by using the HIP structures as a sacrificial layer during wafer bonding process.

Continuum elastic theory for dynamics of surfaces and interfaces

NASA Astrophysics Data System (ADS)

Pykhtin, Michael V.

This thesis is divided into three parts, different by problems they deal with, but similar by underlying assumptions (crystals are treated as classical elastic anisotropic media) and methods of solving (vibrational Green's functions). (i) In the first part we compute the density of vibrational modes for a vicinal Ni(977) surface. In the spectrum we find new step induced modes which are compared with recently reported experimental data for Ni(977) surface obtained by inelastic atom scattering. (ii) In the second part we study damping of low-frequency adsorbate vibrations via resonant coupling to the substrate phonons. Our theory provides a general expression for the vibrational damping rate which can be applied to widely varying coverages and arbitrary overlayer structures. The damping rates predicted by our theory for CO on Cu(100) are in excellent quantitative agreement with available experimental data. (iii) In the third part we develop a theory for the density of vibrational modes at the surface of a thin film of one anisotropic solid an on top of the other. We compute the density of modes for a GaN film on a sapphire substrate for a wide range of wavevector and frequency, and obtain dispersion maps which contain waves trapped between the surface of the film and the interface. Two families of the trapped modes were observed: Love waves and generalized Lamb waves. We also study the effect of threading edge dislocations (majority of defects in the GaN film) on the trapped modes. At the experimental dislocation density the effect is negligible.

Leakage current analysis for dislocations in Na-flux GaN bulk single crystals by conductive atomic force microscopy

NASA Astrophysics Data System (ADS)

Hamachi, T.; Takeuchi, S.; Tohei, T.; Imanishi, M.; Imade, M.; Mori, Y.; Sakai, A.

2018-04-01

The mechanisms associated with electrical conduction through individual threading dislocations (TDs) in a Na-flux GaN crystal grown with a multipoint-seed-GaN technique were investigated by conductive atomic force microscopy (C-AFM). To focus on individual TDs, dislocation-related etch pits (DREPs) were formed on the Na-flux GaN surface by wet chemical etching, after which microscopic Pt electrodes were locally fabricated on the DREPs to form conformal contacts to the Na-flux GaN crystal, using electron beam assisted deposition. The C-AFM data clearly demonstrate that the leakage current flows through the individual TD sites. It is also evident that the leakage current and the electrical conduction mechanism vary significantly based on the area within the Na-flux GaN crystal where the TDs are formed. These regions include the c-growth sector (cGS) in which the GaN grows in the [0001 ] direction on top of the point-seed with a c-plane growth front, the facet-growth sector (FGS) in which the GaN grows with {10 1 ¯ 1 } facets on the side of the cGS, the boundary region between the cGS and FGS (BR), and the coalescence boundary region between FGSs (CBR). The local current-voltage (I-V) characteristics of the specimen demonstrate space charge limited current conduction and conduction related to band-like trap states associated with TDs in the FGS, BR, and CBR. A detailed analysis of the I-V data indicates that the electrical conduction through TDs in the cGS may proceed via the Poole-Frenkel emission mechanism.

Impact of defects on the electrical transport, optical properties and failure mechanisms of GaN nanowires.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Armstrong, Andrew M.; Aubry, Sylvie; Shaner, Eric Arthur

2010-09-01

We present the results of a three year LDRD project that focused on understanding the impact of defects on the electrical, optical and thermal properties of GaN-based nanowires (NWs). We describe the development and application of a host of experimental techniques to quantify and understand the physics of defects and thermal transport in GaN NWs. We also present the development of analytical models and computational studies of thermal conductivity in GaN NWs. Finally, we present an atomistic model for GaN NW electrical breakdown supported with experimental evidence. GaN-based nanowires are attractive for applications requiring compact, high-current density devices such asmore » ultraviolet laser arrays. Understanding GaN nanowire failure at high-current density is crucial to developing nanowire (NW) devices. Nanowire device failure is likely more complex than thin film due to the prominence of surface effects and enhanced interaction among point defects. Understanding the impact of surfaces and point defects on nanowire thermal and electrical transport is the first step toward rational control and mitigation of device failure mechanisms. However, investigating defects in GaN NWs is extremely challenging because conventional defect spectroscopy techniques are unsuitable for wide-bandgap nanostructures. To understand NW breakdown, the influence of pre-existing and emergent defects during high current stress on NW properties will be investigated. Acute sensitivity of NW thermal conductivity to point-defect density is expected due to the lack of threading dislocation (TD) gettering sites, and enhanced phonon-surface scattering further inhibits thermal transport. Excess defect creation during Joule heating could further degrade thermal conductivity, producing a viscous cycle culminating in catastrophic breakdown. To investigate these issues, a unique combination of electron microscopy, scanning luminescence and photoconductivity implemented at the nanoscale will be used in concert with sophisticated molecular-dynamics calculations of surface and defect-mediated NW thermal transport. This proposal seeks to elucidate long standing material science questions for GaN while addressing issues critical to realizing reliable GaN NW devices.« less

Defect reduction in GaN on dome-shaped patterned-sapphire substrates

NASA Astrophysics Data System (ADS)

Chen, Po-Hsun; Su, Vin-Cent; Wu, Shang-Hsuan; Lin, Ray-Ming; Kuan, Chieh-Hsiung

2018-02-01

This paper demonstrates the behavior of defect reduction in un-doped GaN (u-GaN) grown on a commercial dome-shaped patterned-sapphire substrate (CDPSS). Residual strain inside the u-GaN grown on the CDPSS have been investigated as well. As verified by the experimentally measured data, the limited growth rate of the u-GaN on the sidewall of the CDPSS enhances the lateral growth of the GaN on the trench region while increasing the growth time. This subsequently contributes to improve the crystalline quality of the GaN on the CDPSS. The more prominent dislocations occur in the u-GaN epilayers on the CDPSS after reaching the summit of the accumulated strain inside the epilayers. Such prominent bent dislocations improve their blocking abilities, followed by the achievement of the better crystalline quality for the growth of the u-GaN on the CDPSS.

Effect of V/III ratio on the surface morphology and electrical properties of m-plane (10 1 bar 0) GaN homoepitaxial layers

NASA Astrophysics Data System (ADS)

Barry, Ousmane I.; Tanaka, Atsushi; Nagamatsu, Kentaro; Bae, Si-Young; Lekhal, Kaddour; Matsushita, Junya; Deki, Manato; Nitta, Shugo; Honda, Yoshio; Amano, Hiroshi

2017-06-01

We have investigated the effect of V/III ratio on the surface morphology, impurity concentration and electrical properties of m-plane (10 1 bar 0) Gallium Nitride (GaN) homoepitaxial layers. Four-sided pyramidal hillocks are observed on the nominally on-axis m-plane GaN films. Hillocks sizes relatively increase by increasing the V/III ratio. All facets of pyramidal hillocks exhibit well-defined step-terrace features. Secondary ion mass spectrometry depth profiles reveal that carbon impurities decrease by increasing the V/III ratio while the lowest oxygen content is found at an optimized V/III ratio of 900. Vertical Schottky barrier diodes fabricated on the m-GaN samples were characterized. Low leakage current densities of the order of 10-10 A/cm2 at -5 V are obtained at the optimum V/III ratio. Oxygen impurities and screw-component dislocations around hillocks are found to have more detrimental impact on the leakage current mechanism.

Defect reduction in overgrown semi-polar (11-22) GaN on a regularly arrayed micro-rod array template

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Y.; Bai, J.; Hou, Y.

2016-02-15

We demonstrate a great improvement in the crystal quality of our semi-polar (11-22) GaN overgrown on regularly arrayed micro-rod templates fabricated using a combination of industry-matched photolithography and dry-etching techniques. As a result of our micro-rod configuration specially designed, an intrinsic issue on the anisotropic growth rate which is a great challenge in conventional overgrowth technique for semi-polar GaN has been resolved. Transmission electron microscopy measurements show a different mechanism of defect reduction from conventional overgrowth techniques and also demonstrate major advantages of our approach. The dislocations existing in the GaN micro-rods are effectively blocked by both a SiO{sub 2}more » mask on the top of each GaN micro-rod and lateral growth along the c-direction, where the growth rate along the c-direction is faster than that along any other direction. Basal stacking faults (BSFs) are also effectively impeded, leading to a distribution of BSF-free regions periodically spaced by BSF regions along the [-1-123] direction, in which high and low BSF density areas further show a periodic distribution along the [1-100] direction. Furthermore, a defect reduction model is proposed for further improvement in the crystalline quality of overgrown (11-22) GaN on sapphire.« less

The K 2S 2O 8-KOH photoetching system for GaN

NASA Astrophysics Data System (ADS)

Weyher, J. L.; Tichelaar, F. D.; van Dorp, D. H.; Kelly, J. J.; Khachapuridze, A.

2010-09-01

A recently developed photoetching system for n-type GaN, a KOH solution containing the strong oxidizing agent potassium peroxydisulphate (K 2S 2O 8), was studied in detail. By careful selection of the etching parameters, such as the ratio of components and the hydrodynamics, two distinct modes were defined: defect-selective etching (denoted by KSO-D) and polishing (KSO-P). Both photoetching methods can be used under open-circuit (electroless) conditions. Well-defined dislocation-related etch whiskers are formed during KSO-D etching. All types of dislocations are revealed, and this was confirmed by cross-sectional TEM examination of the etched samples. Extended electrically active defects are also clearly revealed. The known relationship between etch rate and carrier concentration for photoetching of GaN in KOH solutions was confirmed for KSO-D etch using Raman measurements. It is shown that during KSO-P etching diffusion is the rate-limiting step, i.e. this etch is suitable for polishing of GaN. Some constraints of the KSO etching system for GaN are discussed and peculiar etch features, so far not understood, are described.

Cross Slip of Dislocation Loops in GaN Under Shear

DTIC Science & Technology

2014-03-01

methodology 2.1 Discrete dislocation dynamic ( DDD ) simula- tions In this work, we employ a modified version of the ParaDiS code [15, 16]. First a...plane. 4 Conclusions The cross slip mechanisms of different dislocation loops have been studied via DDD simulations using the type <a> active

Self-assembled InN quantum dots on side facets of GaN nanowires

NASA Astrophysics Data System (ADS)

Bi, Zhaoxia; Ek, Martin; Stankevic, Tomas; Colvin, Jovana; Hjort, Martin; Lindgren, David; Lenrick, Filip; Johansson, Jonas; Wallenberg, L. Reine; Timm, Rainer; Feidenhans'l, Robert; Mikkelsen, Anders; Borgström, Magnus T.; Gustafsson, Anders; Ohlsson, B. Jonas; Monemar, Bo; Samuelson, Lars

2018-04-01

Self-assembled, atomic diffusion controlled growth of InN quantum dots was realized on the side facets of dislocation-free and c-oriented GaN nanowires having a hexagonal cross-section. The nanowires were synthesized by selective area metal organic vapor phase epitaxy. A 3 Å thick InN wetting layer was observed after growth, on top of which the InN quantum dots formed, indicating self-assembly in the Stranski-Krastanow growth mode. We found that the InN quantum dots can be tuned to nucleate either preferentially at the edges between GaN nanowire side facets, or directly on the side facets by tuning the adatom migration by controlling the precursor supersaturation and growth temperature. Structural characterization by transmission electron microscopy and reciprocal space mapping show that the InN quantum dots are close to be fully relaxed (residual strain below 1%) and that the c-planes of the InN quantum dots are tilted with respect to the GaN core. The strain relaxes mainly by the formation of misfit dislocations, observed with a periodicity of 3.2 nm at the InN and GaN hetero-interface. The misfit dislocations introduce I1 type stacking faults (…ABABCBC…) in the InN quantum dots. Photoluminescence investigations of the InN quantum dots show that the emissions shift to higher energy with reduced quantum dot size, which we attribute to increased quantum confinement.

Room-temperature continuous-wave electrically injected InGaN-based laser directly grown on Si

NASA Astrophysics Data System (ADS)

Sun, Yi; Zhou, Kun; Sun, Qian; Liu, Jianping; Feng, Meixin; Li, Zengcheng; Zhou, Yu; Zhang, Liqun; Li, Deyao; Zhang, Shuming; Ikeda, Masao; Liu, Sheng; Yang, Hui

2016-09-01

Silicon photonics would greatly benefit from efficient, visible on-chip light sources that are electrically driven at room temperature. To fully utilize the benefits of large-scale, low-cost manufacturing foundries, it is highly desirable to grow direct bandgap III-V semiconductor lasers directly on Si. Here, we report the demonstration of a blue-violet (413 nm) InGaN-based laser diode grown directly on Si that operates under continuous-wave current injection at room temperature, with a threshold current density of 4.7 kA cm-2. The heteroepitaxial growth of GaN on Si is confronted with a large mismatch in both the lattice constant and the coefficient of thermal expansion, often resulting in a high density of defects and even microcrack networks. By inserting an Al-composition step-graded AlN/AlGaN multilayer buffer between the Si and GaN, we have not only successfully eliminated crack formation, but also effectively reduced the dislocation density. The result is the realization of a blue-violet InGaN-based laser on Si.

Electron mobility of self-assembled and dislocation free InN nanorods grown on GaN nano wall network template

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tangi, Malleswararao; De, Arpan; Ghatak, Jay

2016-05-28

A kinetically controlled two-step growth process for the formation of an array of dislocation free high mobility InN nanorods (NRs) on GaN nanowall network (NWN) by Molecular Beam Epitaxy is demonstrated here. The epitaxial GaN NWN is formed on c-sapphire under nitrogen rich conditions, and then changing the source from Ga to In at appropriate substrate temperature yields the nucleation of a self assembled spontaneous m-plane side faceted-InN NR. By HRTEM, the NRs are shown to be dislocation-free and have a low band gap value of 0.65 eV. Hall measurements are carried out on a single InN NR along with J-Vmore » measurements that yield mobility values as high as ≈4453 cm{sup 2}/V s and the carrier concentration of ≈1.1 × 10{sup 17} cm{sup −3}, which are unprecedented in the literature for comparable InN NR diameters.« less

Stress in (Al, Ga)N heterostructures grown on 6H-SiC and Si substrates byplasma-assisted molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Koshelev, O. A.; Nechaev, D. V.; Sitnikova, A. A.; Ratnikov, V. V.; Ivanov, S. V.; Jmerik, V. N.

2017-11-01

The paper describes experimental results on low temperature plasma-assisted molecular beam epitaxy of GaN/AlN heterostructures on both 6H-SiC and Si(111) substrates. We demonstrate that application of migration enhanced epitaxy and metal-modulated epitaxy for growth of AlN nucleation and buffer layers lowers the screw and edge(total)threading dislocation (TD) densities down to 1.7·108 and 2·109 cm-2, respectively, in a 2.8-μm-thick GaN buffer layer grown atop of AlN/6H-SiC. The screw and total TD densities of 1.2·109 and 7.4·109 cm-2, respectively, were achieved in a 1-μm-thickGaN/AlNheterostructure on Si(111). Stress generation and relaxation in GaN/AlN heterostructures were investigated by using multi-beam optical stress sensor (MOSS) to achieve zero substrate curvature at room temperature. It is demonstrated that a 1-μm-thick GaN/AlN buffer layer grown by PA MBE provides planar substrate morphology in the case of growth on Si substrates whereas 5-μm-thick GaN buffer layers have to be used to achieve the same when growing on 6H-SiC substrates.

Epitaxy of Polar Oxides and Semiconductors

NASA Astrophysics Data System (ADS)

Shelton, Christopher Tyrel

Integrating polar oxide materials with wide-bandgap nitride semiconductors offers the possibility of a tunable 2D carrier gas (2DCG) - provided defect densities are low and interfaces are abrupt. This dissertation investigates a portion of the synthesis science necessary to produce a "semiconductor-grade" interface between these highly dissimilar materials. A significant portion of this work is aligned with efforts to engineer a step-free GaN substrate to produce single in-plane oriented rocksalt oxide films. Initially, we explore the homoepitaxial MOCVD growth conditions necessary to produce highquality GaN films on ammonothermally grown substrates. Ammono substrates are only recently available for purchase and are the market leader in low-dislocation density material. Their novelty requires development of an understanding of morphology trade-offs in processing space. This includes preservation of the epi-polished surface in aggressive MOCVD environments and an understanding of the kinetic barriers affecting growth morphologies. Based on several factors, it was determined that GaN exhibits an 'uphill' diffusion bias that may likely be ascribed to a positive Ehrlich-Schwoebel (ES) barrier. This barrier should have a stabilizing effect against step-bunching but, for many growth conditions, regular step bunching was observed. One possible explanation for the step-bunching instability is the presence of impurities. Experimentally, conditions which incorporate more carbon into GaN homoepitaxial layers are correlated with step-bunching while conditions that suppress carbon produce bilayer stepped morphologies. These observations lead us to the conclusion that GaN homoepitaxial morphology is a competition between impurity induced step-bunching and a stabilizing diffusion bias due to a positive ES barrier. Application of the aforementioned homoepitaxial growth techniques to discrete substrate regions using selected- and confined area epitaxy (SAE,CAE) produces some remarkable surface morphologies. This work represents the first effort to extend SAE and CAE to true bulk single-crystal GaN substrates. By carefully controlling supersaturation during growth it is possible to prepare confined areas with a range of step densities, including surfaces that are entirely step-free. Single terrace GaN mesas up to 100 m in size have been observed, however the potential exists, due to the extremely low dislocation density of the substrate, to further extend the dimensions of these regions. Step-free GaN templates are ideal substrates for rocksalt heteroepitaxy and solve a long-standing challenge related to the integration of cubic and hexagonal materials. It has been previously observed that the origin of the two in-plane orientations in rocksalts grown on III-nitrides is a consequence of the stepped GaN surface. By using a substrate that is effectively step-free across a 100 m region, it is possible to prepare a rocksalt // GaN film with a single in-plane orientation. Heterojunctions of this type are disclination defect free and highly crystalline. The ability to locally prepare a single orientation rocksalt film, coupled with commensurate 2D layer-by-layer growth techniques, allows growth, for the first time, of a truly 'semiconductor-grade' oxide-nitride interface. To study the transport properties of oxide-nitride heterostructures, a series of experiments on standard GaN // sapphire template layers were conducted. Devices that allowed contact to the buried oxide-nitride interface were prepared and characterized using low-temperature Hall measurements. Although a high mobility 2DEG was not observed in these samples, a conduction path at the lattice matched Mg0.52Ca0.48O // GaN interface did appear. If confirmed, this finding could represent the first evidence for interfacial polar coupling between an oxide and a nitride. Overcoming the significant symmetry, chemistry and bonding environment barriers to forming a structurally perfect oxide-nitride interface has required many innovations and the development of several new technologies. It is encouraging, however, that a path around such challenging obstacles exists. We believe the techniques and findings presented in this work are general to many systems, offering hope for a future 'materials functionality' based integration strategy. (Abstract shortened by ProQuest.).

Effect of a Cooling Step Treatment on a High-Voltage GaN LED During ICP Dry Etching

NASA Astrophysics Data System (ADS)

Lin, Yen-Sheng; Hsiao, Sheng-Yu; Tseng, Chun-Lung; Shen, Ching-Hsing; Chiang, Jung-Sheng

2017-02-01

In this study, a lower dislocation density for a GaN surface and a reduced current path are observed at the interface of a SiO2 isolation sidewall, using high-resolution transmission electron microscopy. This is grown using a 3-min cooling step treatment during inductivity coupled plasma dry etching. The lower forward voltage is measured, the leakage current decreases from 53nA to 32nA, and the maximum output power increases from 354.8 W to 357.2 W for an input current of 30 mA. The microstructure and the optoelectronic properties of high-voltage light-emitting-diodes is proven to be affected by the cooling step treatment, which allows enough time to release the thermal energy of the SiO2 isolation well.

Molecular dynamics studies of InGaN growth on nonpolar (11 2 \\xAF0 ) GaN surfaces

NASA Astrophysics Data System (ADS)

Chu, K.; Gruber, J.; Zhou, X. W.; Jones, R. E.; Lee, S. R.; Tucker, G. J.

2018-01-01

We have performed direct molecular dynamics (MD) simulations of heteroepitaxial vapor deposition of I nxG a1 -xN films on nonpolar (11 2 ¯0 ) wurtzite-GaN surfaces to investigate strain relaxation by misfit-dislocation formation. The simulated growth is conducted on an atypically large scale by sequentially injecting nearly a million individual vapor-phase atoms towards a fixed GaN substrate. We apply time-and-position-dependent boundary constraints to affect the appropriate environments for the vapor phase, the near-surface solid phase, and the bulklike regions of the growing layer. The simulations employ a newly optimized Stillinger-Weber In-Ga-N system interatomic potential wherein multiple binary and ternary structures are included in the underlying density-functional theory and experimental training sets to improve the treatment of the In-Ga-N related interactions. To examine the effect of growth conditions, we study a matrix of 63 different MD-growth simulations spanning seven I nxG a1 -xN -alloy compositions ranging from x =0.0 to x =0.8 and nine growth temperatures above half the simulated melt temperature. We found a composition dependent temperature range where all kinetically trapped defects were eliminated, leaving only quasiequilibrium misfit and threading dislocations present in the simulated films. Based on the MD results obtained in this temperature range, we observe the formation of interfacial misfit and threading dislocation arrays with morphologies strikingly close to those seen in experiments. In addition, we compare the MD-observed thickness-dependent onset of misfit-dislocation formation to continuum-elasticity-theory models of the critical thickness and find reasonably good agreement. Finally, we use the three-dimensional atomistic details uniquely available in the MD-growth histories to directly observe the nucleation of dislocations at surface pits in the evolving free surface.

Hall-effect measurements of metalorganic vapor-phase epitaxy-grown p-type homoepitaxial GaN layers with various Mg concentrations

NASA Astrophysics Data System (ADS)

Horita, Masahiro; Takashima, Shinya; Tanaka, Ryo; Matsuyama, Hideaki; Ueno, Katsunori; Edo, Masaharu; Suda, Jun

2016-05-01

Mg-doped p-type gallium nitride (GaN) layers with doping concentrations in the range from 6.5 × 1016 cm-3 (lightly doped) to 3.8 × 1019 cm-3 (heavily doped) were investigated by Hall-effect measurement for the analysis of hole concentration and mobility. p-GaN was homoepitaxially grown on a GaN free-standing substrate by metalorganic vapor-phase epitaxy. The threading dislocation density of the p-GaN was 4 × 106 cm-2 measured by cathodoluminescence mapping. Hall-effect measurements of p-GaN were carried out at a temperature in the range from 160 to 450 K. A low compensation ratio of less than 1% was revealed. We also obtained the depth of the Mg acceptor level of 235 meV considering the lowering effect by the Coulomb potential of ionized acceptors. The hole mobilities of 33 cm2 V-1 s-1 at 300 K and 72 cm2 V-1 s-1 at 200 K were observed in lightly doped p-GaN.

Effect of threading defects on InGaN /GaN multiple quantum well light emitting diodes

NASA Astrophysics Data System (ADS)

Ferdous, M. S.; Wang, X.; Fairchild, M. N.; Hersee, S. D.

2007-12-01

Photoelectrochemical etching was used to measure the threading defect (TD) density in InGaN multiple quantum well light-emitting diodes (LEDs) fabricated from commercial quality epitaxial wafers. The TD density was measured in the LED active region and then correlated with the previously measured characteristics of these LEDs. It was found that the reverse leakage current increased exponentially with TD density. The temperature dependence of this dislocation-related leakage current was consistent with a hopping mechanism at low reverse-bias voltage and Poole-Frenkel emission at higher reverse-bias voltage. The peak intensity and spectral width of the LED electroluminescence were found to be only weakly dependent on TD density for the measured TD range of 1×107-2×108cm-2.

Structural properties and defects of GaN crystals grown at ultra-high pressures: A molecular dynamics simulation

NASA Astrophysics Data System (ADS)

Gao, Tinghong; Li, Yidan; Xie, Quan; Tian, Zean; Chen, Qian; Liang, Yongchao; Ren, Lei; Hu, Xuechen

2018-01-01

The growth of GaN crystals at different pressures was studied by molecular dynamics simulation employing the Stillinger-Weber potential, and their structural properties and defects were characterized using the radial distribution function, the Voronoi polyhedron index method, and a suitable visualization technology. Crystal structures formed at 0, 1, 5, 10, and 20 GPa featured an overwhelming number of <4 0 0 0> Voronoi polyhedra, whereas amorphous structures comprising numerous disordered polyhedra were produced at 50 GPa. During quenching, coherent twin boundaries were easily formed between zinc-blende and wurtzite crystal structures in GaN. Notably, point defects usually appeared at low pressure, whereas dislocations were observed at high pressure, since the simultaneous growth of two crystal grains with different crystal orientations and their boundary expansion was hindered in the latter case, resulting in the formation of a dislocation between these grains.

The influence of AlN interlayers on the microstructural and electrical properties of p-type AlGaN/GaN superlattices grown on GaN/sapphire templates

NASA Astrophysics Data System (ADS)

Li, Lei; Liu, Lei; Wang, Lei; Li, Ding; Song, Jie; Liu, Ningyang; Chen, Weihua; Wang, Yuzhou; Yang, Zhijian; Hu, Xiaodong

2012-09-01

AlN with different thicknesses were grown as interlayers (ILs) between GaN and p-type Al0.15Ga0.85N/GaN superlattices (SLs) by metal organic vapor phase epitaxy (MOVPE). It was found that the edge-type threading dislocation density (TDD) increased gradually from the minimum of 2.5×109 cm-2 without AlN IL to the maximum of 1×1010 cm-2 at an AlN thickness of 20 nm, while the screw-type TDD remained almost unchanged due to the interface-related TD suppression and regeneration mechanism. We obtained that the edge-type dislocations acted as acceptors in p-type Al x Ga1- x N/GaN SLs, through the comparison of the edge-type TDD and hole concentration with different thicknesses of AlN IL. The Mg activation energy was significantly decreased from 153 to 70 meV with a 10-nm AlN IL, which was attributed to the strain modulation between AlGaN barrier and GaN well. The large activation efficiency, together with the TDs, led to the enhanced hole concentration. The variation trend of Hall mobility was also observed, which originated from the scattering at TDs.

Enhanced optical output power of InGaN/GaN light-emitting diodes grown on a silicon (111) substrate with a nanoporous GaN layer.

PubMed

Lee, Kwang Jae; Chun, Jaeyi; Kim, Sang-Jo; Oh, Semi; Ha, Chang-Soo; Park, Jung-Won; Lee, Seung-Jae; Song, Jae-Chul; Baek, Jong Hyeob; Park, Seong-Ju

2016-03-07

We report the growth of InGaN/GaN multiple quantum wells blue light-emitting diodes (LEDs) on a silicon (111) substrate with an embedded nanoporous (NP) GaN layer. The NP GaN layer is fabricated by electrochemical etching of n-type GaN on the silicon substrate. The crystalline quality of crack-free GaN grown on the NP GaN layer is remarkably improved and the residual tensile stress is also decreased. The optical output power is increased by 120% at an injection current of 20 mA compared with that of conventional LEDs without a NP GaN layer. The large enhancement of optical output power is attributed to the reduction of threading dislocation, effective scattering of light in the LED, and the suppression of light propagation into the silicon substrate by the NP GaN layer.

The Peculiarities of Strain Relaxation in GaN/AlN Superlattices Grown on Vicinal GaN (0001) Substrate: Comparative XRD and AFM Study.

PubMed

Kuchuk, Andrian V; Kryvyi, Serhii; Lytvyn, Petro M; Li, Shibin; Kladko, Vasyl P; Ware, Morgan E; Mazur, Yuriy I; Safryuk, Nadiia V; Stanchu, Hryhorii V; Belyaev, Alexander E; Salamo, Gregory J

2016-12-01

Superlattices (SLs) consisting of symmetric layers of GaN and AlN have been investigated. Detailed X-ray diffraction and reflectivity measurements demonstrate that the relaxation of built-up strain in the films generally increases with an increasing number of repetitions; however, an apparent relaxation for subcritical thickness SLs is explained through the accumulation of Nagai tilt at each interface of the SL. Additional atomic force microscopy measurements reveal surface pit densities which appear to correlate with the amount of residual strain in the films along with the appearance of cracks for SLs which have exceeded the critical thickness for plastic relaxation. These results indicate a total SL thickness beyond which growth may be limited for the formation of high-quality coherent crystal structures; however, they may indicate a growth window for the reduction of threading dislocations by controlled relaxation of the epilayers.

Change in equilibrium position of misfit dislocations at the GaN/sapphire interface by Si-ion implantation into sapphire. II. Electron energy loss spectroscopic study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Sung Bo, E-mail: bolee@snu.ac.kr; Han, Heung Nam, E-mail: hnhan@snu.ac.kr; Kim, Young-Min

In Part I, we have shown that the addition of Si into sapphire by ion implantationmakes the sapphire substrate elastically softer than for the undoped sapphire. The more compliant layer of the Si-implanted sapphire substrate can absorb the misfit stress at the GaN/sapphire interface, which produces a lower threading-dislocation density in the GaN overlayer. Here in Part II, based on experimental results by electron energy loss spectroscopy and a first-principle molecular orbital calculation in the literature, we suggest that the softening effect of Si results from a reduction of ionic bonding strength in sapphire (α-Al{sub 2}O{sub 3}) with the substitutionmore » of Si for Al.« less

Growth of high-quality InGaN/GaN LED structures on (1 1 1) Si substrates with internal quantum efficiency exceeding 50%

NASA Astrophysics Data System (ADS)

Lee, JaeWon; Tak, Youngjo; Kim, Jun-Youn; Hong, Hyun-Gi; Chae, Suhee; Min, Bokki; Jeong, Hyungsu; Yoo, Jinwoo; Kim, Jong-Ryeol; Park, Youngsoo

2011-01-01

GaN-based light-emitting-diodes (LEDs) on (1 1 1) Si substrates with internal quantum efficiency (IQE) exceeding 50% have been successfully grown by metal organic vapor phase epitaxy (MOVPE). 3.5 μm thick crack-free GaN epitaxial layers were grown on the Si substrates by the re-growth method on patterned templates. Series of step-graded Al xGa 1- xN epitaxial layers were used as the buffer layers to compensate thermal tensile stresses produced during the post-growth cooling process as well as to reduce the density of threading dislocations (TDs) generated due to the lattice mismatches between III-nitride layers and the silicon substrates. The light-emitting region consisted of 1.8 μm thick n-GaN, 3 periods of InGaN/GaN superlattice, InGaN/GaN multiple quantum wells (MQWs) designed for a peak wavelength of about 455 nm, an electron blocking layer (EBL), and p-GaN. The full-widths at half-maximum (FWHM) of (0 0 0 2) and (1 0 -1 2) ω-rocking curves of the GaN epitaxial layers were 410 and 560 arcsec, respectively. Cross-sectional transmission electron microscopy (TEM) investigation revealed that the propagation of the threading dislocations was mostly limited to the interface between the last Al xGa 1- xN buffer and n-GaN layers. The density of the threading dislocations induced pits of n-GaN, as estimated by atomic force microscopy (AFM), was about 5.5×10 8 cm -2. Temperature dependent photoluminescence (PL) measurements with a relative intensity integration method were carried out to estimate the internal quantum efficiency (IQE) of the light-emitting structures grown on Si, which reached up to 55%.

Inclined dislocation arrays in AlGaN/AlGaN quantum well structures emitting at 290 nm

NASA Astrophysics Data System (ADS)

Chang, T. Y.; Moram, M. A.; McAleese, C.; Kappers, M. J.; Humphreys, C. J.

2010-12-01

We report on the structural and optical properties of deep ultraviolet emitting AlGaN/AlGaN multiple quantum wells (MQWs) grown on (0001) sapphire by metal-organic vapor phase epitaxy using two different buffer layer structures, one containing a thin (1 μm) AlN layer combined with a GaN interlayer and the other a thick (4 μm) AlN layer. Transmission electron microscopy analysis of both structures showed inclined arrays of dislocations running through the AlGaN layers at an angle of ˜30°, originating at bunched steps at the AlN surface and terminating at bunched steps at the surface of the MQW structure. In all layers, these inclined dislocation arrays are surrounded by AlGaN with a relatively higher Ga content, consistent with plan-view cathodoluminescence maps in which the bunched surface steps are associated with longer emission wavelengths. The structure with the 4 μm-thick AlN buffer layer had a dislocation density lower by a factor of 2 (at (1.7±0.1)×109 cm-2) compared to the structure with the 1 μm thick AlN buffer layer, despite the presence of the inclined dislocation arrays.

Responsivity drop due to conductance modulation in GaN metal-semiconductor-metal Schottky based UV photodetectors on Si(111)

NASA Astrophysics Data System (ADS)

Ravikiran, L.; Radhakrishnan, K.; Dharmarasu, N.; Agrawal, M.; Wang, Zilong; Bruno, Annalisa; Soci, Cesare; Lihuang, Tng; Kian Siong, Ang

2016-09-01

GaN Schottky metal-semiconductor-metal (MSM) UV photodetectors were fabricated on a 600 nm thick GaN layer, grown on 100 mm Si (111) substrate using an ammonia-MBE growth technique. In this report, the effect of device dimensions, applied bias and input power on the linearity of the GaN Schottky-based MSM photodetectors on Si substrate were investigated. Devices with larger interdigitated spacing, ‘S’ of 9.0 μm between the fingers resulted in good linearity and flat responsivity characteristics as a function of input power with an external quantum efficiency (EQE) of ˜33% at an applied bias of 15 V and an input power of 0.8 W m-2. With the decrease of ‘S’ to 3.0 μm, the EQE was found to increase to ˜97%. However, devices showed non linearity and drop in responsivity from flatness at higher input power. Moreover, the position of dropping from flatter responsivity was found to shift to lower powers with increased bias. The drop in the responsivity was attributed to the modulation of conductance in the MSM due to the trapping of electrons at the dislocations, resulting in the formation of depletion regions around them. In devices with lower ‘S’, both the image force reduction and the enhanced collection efficiency increased the photocurrent as well as the charging of the dislocations. This resulted in the increased depletion regions around the dislocations leading to the modulation of conductance and non-linearity.

Hall-effect measurements of metalorganic vapor-phase epitaxy-grown p-type homoepitaxial GaN layers with various Mg concentrations

NASA Astrophysics Data System (ADS)

Horita, Masahiro; Takashima, Shinya; Tanaka, Ryo; Matsuyama, Hideaki; Ueno, Katsunori; Edo, Masaharu; Takahashi, Tokio; Shimizu, Mitsuaki; Suda, Jun

2017-03-01

Mg-doped p-type gallium nitride (GaN) layers with doping concentrations in the range from 6.5 × 1016 cm-3 (lightly doped) to 3.8 × 1019 cm-3 (heavily doped) were investigated by Hall-effect measurement for the analysis of hole concentration and mobility. p-GaN was homoepitaxially grown on a GaN free-standing substrate by metalorganic vapor-phase epitaxy. The threading dislocation density of p-GaN was 4 × 106 cm-2 measured by cathodoluminescence mapping. Hall-effect measurements of p-GaN were carried out at a temperature in the range from 130 to 450 K. For the lightly doped p-GaN, the acceptor concentration of 7.0 × 1016 cm-3 and the donor concentration of 3.2 × 1016 cm-3 were obtained, where the compensation ratio was 46%. We also obtained the depth of the Mg acceptor level to be 220 meV. The hole mobilities of 86, 31, 14 cm2 V-1 s-1 at 200, 300, 400 K, respectively, were observed in the lightly doped p-GaN.

Identification of the primary compensating defect level responsible for determining blocking voltage of vertical GaN power diodes

DOE PAGES

King, M. P.; Kaplar, R. J.; Dickerson, J. R.; ...

2016-10-31

Electrical performance and characterization of deep levels in vertical GaN P-i-N diodes grown on low threading dislocation density (~10 4 –10 6 cm –2) bulk GaN substrates are investigated. The lightly doped n drift region of these devices is observed to be highly compensated by several prominent deep levels detected using deep level optical spectroscopy at E c-2.13, 2.92, and 3.2 eV. A combination of steady-state photocapacitance and lighted capacitance-voltage profiling indicates the concentrations of these deep levels to be N t = 3 × 10 12, 2 × 10 15, and 5 × 10 14 cm –3, respectively. Themore » E c-2.92 eV level is observed to be the primary compensating defect in as-grown n-type metal-organic chemical vapor deposition GaN, indicating this level acts as a limiting factor for achieving controllably low doping. The device blocking voltage should increase if compensating defects reduce the free carrier concentration of the n drift region. Understanding the incorporation of as-grown and native defects in thick n-GaN is essential for enabling large V BD in the next-generation wide-bandgap power semiconductor devices. Furthermore, controlling the as-grown defects induced by epitaxial growth conditions is critical to achieve blocking voltage capability above 5 kV.« less

Comparative study of GaN-based ultraviolet LEDs grown on different-sized patterned sapphire substrates with sputtered AlN nucleation layer

NASA Astrophysics Data System (ADS)

Zhou, Shengjun; Hu, Hongpo; Liu, Xingtong; Liu, Mengling; Ding, Xinghuo; Gui, Chengqun; Liu, Sheng; Guo, L. Jay

2017-11-01

GaN-based ultraviolet-light-emitting diodes (UV LEDs) with 375 nm emission were grown on different-sized patterned sapphire substrates (PSSs) with ex situ 15-nm-thick sputtered AlN nucleation layers by metal-organic chemical vapor deposition (MOCVD). It was observed through in situ optical reflectance monitoring that the transition time from a three-dimensional (3D) island to a two-dimensional (2D) coalescence was prolonged when GaN was grown on a larger PSS, owing to a much longer lateral growth time of GaN. The full widths at half-maximum (FWHMs) of symmetric GaN(002) and asymmetric GaN(102) X-ray diffraction (XRD) rocking curves decreased as the PSS size increased. By cross-sectional transmission electron microscopy (TEM) analysis, it was found that the threading dislocation (TD) density in UV LEDs decreased with increasing pattern size and fill factor of the PSS, thereby resulting in a marked improvement in internal quantum efficiency (IQE). Finite-difference time-domain (FDTD) simulations quantitatively demonstrated a progressive decrease in light extraction efficiency (LEE) as the PSS size increased. However, owing to the significantly reduced TD density in InGaN/AlInGaN multiple quantum wells (MQWs) and thus improved IQE, the light output power of the UV LED grown on a large PSS with a fill factor of 0.71 was 131.8% higher than that of the UV LED grown on a small PSS with a fill factor of 0.4, albeit the UV LED grown on a large PSS exhibited a much lower LEE.

Ultraviolet light-absorbing and emitting diodes consisting of a p-type transparent-semiconducting NiO film deposited on an n-type GaN homoepitaxial layer

NASA Astrophysics Data System (ADS)

Nakai, Hiroshi; Sugiyama, Mutsumi; Chichibu, Shigefusa F.

2017-05-01

Gallium nitride (GaN) and related (Al,Ga,In)N alloys provide practical benefits in the production of light-emitting diodes (LEDs) and laser diodes operating in ultraviolet (UV) to green wavelength regions. However, obtaining low resistivity p-type AlN or AlGaN of large bandgap energies (Eg) is a critical issue in fabricating UV and deep UV-LEDs. NiO is a promising candidate for useful p-type transparent-semiconducting films because its Eg is 4.0 eV and it can be doped into p-type conductivity of sufficiently low resistivity. By using these technologies, heterogeneous junction diodes consisting of a p-type transparent-semiconducting polycrystalline NiO film on an n-type single crystalline GaN epilayer on a low threading-dislocation density, free-standing GaN substrate were fabricated. The NiO film was deposited by using the conventional RF-sputtering method, and the GaN homoepitaxial layer was grown by metalorganic vapor phase epitaxy. They exhibited a significant photovoltaic effect under UV light and also exhibited an electroluminescence peak at 3.26 eV under forward-biased conditions. From the conduction and valence band (EV) discontinuities, the NiO/GaN heterointerface is assigned to form a staggered-type (TYPE-II) band alignment with the EV of NiO higher by 2.0 eV than that of GaN. A rectifying property that is consistent with the proposed band diagram was observed in the current-voltage characteristics. These results indicate that polycrystalline NiO functions as a hole-extracting and injecting layer of UV optoelectronic devices.

Highly repeatable room temperature negative differential resistance in AlN/GaN resonant tunneling diodes grown by molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Growden, Tyler A.; Fakhimi, Parastou; Berger, Paul R., E-mail: pberger@ieee.org

AlN/GaN resonant tunneling diodes grown on low dislocation density semi-insulating bulk GaN substrates via plasma-assisted molecular-beam epitaxy are reported. The devices were fabricated using a six mask level, fully isolated process. Stable room temperature negative differential resistance (NDR) was observed across the entire sample. The NDR exhibited no hysteresis, background light sensitivity, or degradation of any kind after more than 1000 continuous up-and-down voltage sweeps. The sample exhibited a ∼90% yield of operational devices which routinely displayed an average peak current density of 2.7 kA/cm{sup 2} and a peak-to-valley current ratio of ≈1.15 across different sizes.

The investigation of stress in freestanding GaN crystals grown from Si substrates by HVPE.

PubMed

Lee, Moonsang; Mikulik, Dmitry; Yang, Mino; Park, Sungsoo

2017-08-17

We investigate the stress evolution of 400 µm-thick freestanding GaN crystals grown from Si substrates by hydride vapour phase epitaxy (HVPE) and the in situ removal of Si substrates. The stress generated in growing GaN can be tuned by varying the thickness of the MOCVD AlGaN/AlN buffer layers. Micro Raman analysis shows the presence of slight tensile stress in the freestanding GaN crystals and no stress accumulation in HVPE GaN layers during the growth. Additionally, it is demonstrated that the residual tensile stress in HVPE GaN is caused only by elastic stress arising from the crystal quality difference between Ga- and N-face GaN. TEM analysis revealed that the dislocations in freestanding GaN crystals have high inclination angles that are attributed to the stress relaxation of the crystals. We believe that the understanding and characterization on the structural properties of the freestanding GaN crystals will help us to use these crystals for high-performance opto-electronic devices.

Strain-free bulk-like GaN grown by hydride-vapor-phase-epitaxy on two-step epitaxial lateral overgrown GaN template

NASA Astrophysics Data System (ADS)

Gogova, D.; Kasic, A.; Larsson, H.; Hemmingsson, C.; Monemar, B.; Tuomisto, F.; Saarinen, K.; Dobos, L.; Pécz, B.; Gibart, P.; Beaumont, B.

2004-07-01

Crack-free bulk-like GaN with high crystalline quality has been obtained by hydride-vapor-phase-epitaxy (HVPE) growth on a two-step epitaxial lateral overgrown GaN template on sapphire. During the cooling down stage, the as-grown 270-μm-thick GaN layer was self-separated from the sapphire substrate. Plan-view transmission electron microscopy images show the dislocation density of the free-standing HVPE-GaN to be ˜2.5×107 cm-2 on the Ga-polar face. A low Ga vacancy related defect concentration of about 8×1015 cm-3 is extracted from positron annihilation spectroscopy data. The residual stress and the crystalline quality of the material are studied by two complementary techniques. Low-temperature photoluminescence spectra show the main neutral donor bound exciton line to be composed of a doublet structure at 3.4715 (3.4712) eV and 3.4721 (3.4718) eV for the Ga- (N-) polar face with the higher-energy component dominating. These line positions suggest virtually strain-free material on both surfaces with high crystalline quality as indicated by the small full width at half maximum values of the donor bound exciton lines. The E1(TO) phonon mode position measured at 558.52 cm-1 (Ga face) by infrared spectroscopic ellipsometry confirms the small residual stress in the material, which is hence well suited to act as a lattice-constant and thermal-expansion-coefficient matched substrate for further homoepitaxy, as needed for high-quality III-nitride device applications.

Growth Optimization of Metal-polar III-Nitride High-electron-mobility Transistor Structures by Molecular Beam Epitaxy

NASA Astrophysics Data System (ADS)

Kaun, Stephen William

GaN-based high-electron-mobility transistors (HEMTs) will play an important role in the next generation of high-frequency amplifiers and power-switching devices. Since parasitic conduction (leakage) through the GaN buffer layer and (Al,Ga,In)N barrier reduces the efficiency of operation, HEMT performance hinges on the epitaxial quality of these layers. Increasing the sheet charge density and mobility of the two-dimensional electron gas (2DEG) is also essential for reducing the channel resistance and improving output. The growth conditions applied in plasma-assisted molecular beam epitaxy (PAMBE) and ammonia-based molecular beam epitaxy (NH3-MBE) that result in high-quality metal-polar HEMT structures are described. The effects of threading dislocations on the gate leakage and channel conductivity of AlGaN/GaN HEMTs were studied in detail. For this purpose, a series of HEMT structures were grown on GaN templates with threading dislocation densities (TDDs) that spanned three orders of magnitude. There was a clear trend of reduced gate leakage with reduced TDD for HEMTs grown by Ga-rich PAMBE; however, a reduction in TDD also entailed an increase in buffer leakage. By reducing the unintentionally doped (UID) GaN buffer thickness and including an AlGaN back barrier, a HEMT regrown by Ga-rich PAMBE on low-TDD free-standing (FS) GaN (~5 x 107 cm-2 TDD) yielded a three-terminal breakdown voltage greater than 50 V and a power output (power-added efficiency) of 6.7 W/mm (50 %) at 4 GHz with a 40 V drain bias. High TDD was then shown to severely degrade the 2DEG mobility of AlxGa1-xN/GaN (x = 0.24, 0.12, 0.06) and AlGaN/AlN/GaN heterostructures grown by Ga-rich PAMBE. By regrowing on low-TDD FS GaN and including a 2.5 nm AlN interlayer, an Al0.24Ga0.76N/AlN/GaN heterostructure achieved a room temperature (RT) 2DEG sheet resistance of 169 Ω/□. As evidenced by atom probe tomography, the AlN interlayer grown by Ga-rich PAMBE was pure with abrupt interfaces. The pure AlN interlayer greatly reduced alloy-related scattering. When AlGaN/AlN/GaN heterostructures were grown by NH3-MBE at 820 °C, the 2DEG sheet density was lower than expected. These AlN interlayers were shown to have a significant concentration of Ga impurities by atom probe tomography. The source of these impurities was most likely the decomposition of the underlying GaN layers, as reduction of the growth temperature below 750 °C yielded a much lower concentration of Ga impurities. Flux optimization and application of an In surfactant was necessary to reduce the interface roughness in AlGaN/AlN/GaN heterostructures grown by NH3-MBE at low temperature, yielding sheet resistances below 300 Ω/□. The growth of InAlN/(GaN)/(AlN)/GaN heterostructures with lattice-matched In0.17Al0.83N barriers by N-rich PAMBE is also described. Through flux optimization, the columnar microstructure previously observed in N-rich PAMBE-grown InAlN layers was eliminated. By including a 3 nm AlN interlayer and 2 nm GaN interlayer, an In0.17Al0.83N/GaN/AlN/GaN heterostructure regrown on low-TDD FS GaN achieved an exceptionally low RT 2DEG sheet resistance of 145 Ω/□.

Understanding of surface pit formation mechanism of GaN grown in MOCVD based on local thermodynamic equilibrium assumption

NASA Astrophysics Data System (ADS)

Zhi-Yuan, Gao; Xiao-Wei, Xue; Jiang-Jiang, Li; Xun, Wang; Yan-Hui, Xing; Bi-Feng, Cui; De-Shu, Zou

2016-06-01

Frank’s theory describes that a screw dislocation will produce a pit on the surface, and has been evidenced in many material systems including GaN. However, the size of the pit calculated from the theory deviates significantly from experimental result. Through a careful observation of the variations of surface pits and local surface morphology with growing temperature and V/III ratio for c-plane GaN, we believe that Frank’s model is valid only in a small local surface area where thermodynamic equilibrium state can be assumed to stay the same. If the kinetic process is too vigorous or too slow to reach a balance, the local equilibrium range will be too small for the center and edge of the screw dislocation spiral to be kept in the same equilibrium state. When the curvature at the center of the dislocation core reaches the critical value 1/r 0, at the edge of the spiral, the accelerating rate of the curvature may not fall to zero, so the pit cannot reach a stationary shape and will keep enlarging under the control of minimization of surface energy to result in a large-sized surface pit. Project supported by the National Natural Science Foundation of China (Grant Nos. 11204009 and 61204011) and the Beijing Municipal Natural Science Foundation, China (Grant No. 4142005).

Homogeneous AlGaN/GaN superlattices grown on free-standing (1100) GaN substrates by plasma-assisted molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shao, Jiayi; Malis, Oana; Physics Department, Purdue University, West Lafayette, Indiana 47907

Two-dimensional and homogeneous growth of m-plane AlGaN by plasma-assisted molecular beam epitaxy has been realized on free-standing (1100) GaN substrates by implementing high metal-to-nitrogen (III/N) flux ratio. AlN island nucleation, often reported for m-plane AlGaN under nitrogen-rich growth conditions, is suppressed at high III/N flux ratio, highlighting the important role of growth kinetics for adatom incorporation. The homogeneity and microstructure of m-plane AlGaN/GaN superlattices are assessed via a combination of scanning transmission electron microscopy and high resolution transmission electron microscopy (TEM). The predominant defects identified in dark field TEM characterization are short basal plane stacking faults (SFs) bounded by eithermore » Frank-Shockley or Frank partial dislocations. In particular, the linear density of SFs is approximately 5 × 10{sup −5} cm{sup −1}, and the length of SFs is less than 15 nm.« less

Lattice disorder produced in GaN by He-ion implantation

NASA Astrophysics Data System (ADS)

Han, Yi; Peng, Jinxin; Li, Bingsheng; Wang, Zhiguang; Wei, Kongfang; Shen, Tielong; Sun, Jianrong; Zhang, Limin; Yao, Cunfeng; Gao, Ning; Gao, Xing; Pang, Lilong; Zhu, Yabin; Chang, Hailong; Cui, Minghuan; Luo, Peng; Sheng, Yanbin; Zhang, Hongpeng; Zhang, Li; Fang, Xuesong; Zhao, Sixiang; Jin, Jin; Huang, Yuxuan; Liu, Chao; Tai, Pengfei; Wang, Dong; He, Wenhao

2017-09-01

The lattice disorders induced by He-ion implantation in GaN epitaxial films to fluences of 2 × 1016, 5 × 1016 and 1 × 1017 cm-2 at room temperature (RT) have been investigated by a combination of Raman spectroscopy, high-resolution X-ray diffraction (HRXRD), nano-indentation, and transmission electron microscopy (TEM). The experimental results present that Raman intensity decreases with increasing fluence. Raman frequency "red shift" occurs after He-ion implantation. Strain increases with increasing fluence. The hardness of the highly damaged layer increases monotonically with increasing fluence. Microstructural results demonstrate that the width of the damage band and the number density of observed dislocation loops increases with increasing fluence. High-resolution TEM images exhibit that He-ion implantation lead to the formation of planar defects and most of the lattice defects are of interstitial-type basal loops. The relationships of Raman intensity, lattice strain, swelling and hardness with He-implantation-induced lattice disorders are discussed.

Surface morphology of Al0.3Ga0.7N/Al2O3-high electron mobility transistor structure.

PubMed

Cörekçi, S; Usanmaz, D; Tekeli, Z; Cakmak, M; Ozçelik, S; Ozbay, E

2008-02-01

We present surface properties of buffer films (AIN and GaN) and Al0.3Gao.zN/Al2O3-High Electron Mobility Transistor (HEMT) structures with/without AIN interlayer grown on High Temperature (HT)-AIN buffer/Al2O3 substrate and Al2O3 substrate. We have found that the GaN surface morphology is step-flow in character and the density of dislocations was about 10(8)-10(9) cm(-2). The AFM measurements also exhibited that the presence of atomic steps with large lateral step dimension and the surface of samples was smooth. The lateral step sizes are in the range of 100-250 nm. The typical rms values of HEMT structures were found as 0.27, 0.30, and 0.70 nm. HT-AIN buffer layer can have a significant impact on the surface morphology of Al0.3Ga0.7N/Al2O3-HEMT structures.

Mocvd Growth of Group-III Nitrides on Silicon Carbide: From Thin Films to Atomically Thin Layers

NASA Astrophysics Data System (ADS)

Al Balushi, Zakaria Y.

Group-III nitride semiconductors (AlN, GaN, InN and their alloys) are considered one of the most important class of materials for electronic and optoelectronic devices. This is not limited to the blue light-emitting diode (LED) used for efficient solid-state lighting, but other applications as well, such as solar cells, radar and a variety of high frequency power electronics, which are all prime examples of the technological importance of nitride based wide bandgap semiconductors in our daily lives. The goal of this dissertation work was to explore and establish new growth schemes to improve the structural and optical properties of thick to atomically thin films of group-III nitrides grown by metalorganic chemical vapor deposition (MOCVD) on SiC substrates for future novel devices. The first research focus of this dissertation was on the growth of indium gallium nitride (InGaN). This wide bandgap semiconductor has attracted much research attention as an active layer in LEDs and recently as an absorber material for solar cells. InGaN has superior material properties for solar cells due to its wavelength absorption tunability that nearly covers the entire solar spectrum. This can be achieved by controlling the indium content in thick grown material. Thick InGaN films are also of interest as strain reducing based layers for deep-green and red light emitters. The growth of thick films of InGaN is, however, hindered by several combined problems. This includes poor incorporation of indium in alloys, high density of structural and morphological defects, as well as challenges associated with the segregation of indium in thick films. Overcoming some of these material challenges is essential in order integrate thick InGaN films into future optoelectronics. Therefore, this dissertation research investigated the growth mechanism of InGaN layers grown in the N-polar direction by MOCVD as a route to improve the structural and optical properties of thick InGaN films. The growth of N-polar InGaN by MOCVD is challenging. These challenges arise from the lack of available native substrates suitable for N-polar film growth. As a result, InGaN layers are conventionally grown in the III-polar direction (i.e. III-polar InGaN) and typically grow under considerable amounts of stress on III-polar GaN base layers. While the structure-property relations of thin III-polar InGaN layers have been widely studied in quantum well structures, insight into the growth of thick films and N-polar InGaN layers have been limited. Therefore, this dissertation research compared the growth of both thick III-polar and N-polar InGaN films grown on optimized GaN base layers. III-polar InGaN films were rough and exhibited a high density of V-pits, while the growth of thick N-polar InGaN films showed improved structural quality and low surface roughness. The results of this dissertation work thereby provide an alternative route to the fabrication of thick InGaN films for potential use in solar cells as well as strain reducing schemes for deep-green and red light emitters. Moreover, this dissertation investigated stress relaxation in thick N-polar films using in situ reflectivity and curvature measurements. The results showed that stress relaxation in N-polar InGaN significantly differed from III-polar InGaN due to the absence of V-pits and it was hypothesized that plastic relaxation in N-polar InGaN could occur by dislocation glide, which typically is kinetically limited at such low growth temperatures required for InGaN. The second part of this dissertation research work focused on buffer free growth of GaN directly on SiC and on epitaxial graphene produced on SiC for potential vertical devices. The studies presented in this dissertation work on the growth of GaN directly on SiC compared the stress evolution of GaN films grown with and without an AlN buffer layer. Films grown directly on SiC showed reduced threading dislocation densities and improved surface roughness when compared to the growth of GaN on an AlN buffer layer. The dislocations in the GaN films grown di

Electrical properties of dislocations in III-Nitrides

NASA Astrophysics Data System (ADS)

Cavalcoli, D.; Minj, A.; Pandey, S.; Cavallini, A.

2014-02-01

Research on GaN, AlN, InN (III-N) and their alloys is achieving new heights due their high potential applications in photonics and electronics. III-N semiconductors are mostly grown epitaxially on sapphire, and due to the large lattice mismatch and the differences in the thermal expansion coefficients, the structures usually contain many threading dislocations (TDs). While their structural properties have been widely investigated, their electrical characteristics and their role in the transport properties of the devices are still debated. In the present contribution we will show conductive AFM studies of TDs in GaN and Al/In GaN ternary alloys to evidence the role of strain, different surface polarity and composition on their electrical properties. Local I-V curves measured at TDs allowed us to clarify their role in the macroscopic electrical properties (leakage current, mobilities) of III-N based devices. Samples obtained by different growers (AIXTRON, III-V Lab) were studied. The comparison between the results obtained in the different alloys allowed us to understand the role of In and Al on the TDs electrical properties.

Natural substrate lift-off technique for vertical light-emitting diodes

NASA Astrophysics Data System (ADS)

Lee, Chia-Yu; Lan, Yu-Pin; Tu, Po-Min; Hsu, Shih-Chieh; Lin, Chien-Chung; Kuo, Hao-Chung; Chi, Gou-Chung; Chang, Chun-Yen

2014-04-01

Hexagonal inverted pyramid (HIP) structures and the natural substrate lift-off (NSLO) technique were demonstrated on a GaN-based vertical light-emitting diode (VLED). The HIP structures were formed at the interface between GaN and the sapphire substrate by molten KOH wet etching. The threading dislocation density (TDD) estimated by transmission electron microscopy (TEM) was reduced to 1 × 108 cm-2. Raman spectroscopy indicated that the compressive strain from the bottom GaN/sapphire was effectively released through the HIP structure. With the adoption of the HIP structure and NSLO, the light output power and yield performance of leakage current could be further improved.

Cantilever epitaxial process

DOEpatents

Ashby, Carol I.; Follstaedt, David M.; Mitchell, Christine C.; Han, Jung

2003-07-29

A process of growing a material on a substrate, particularly growing a Group II-VI or Group III-V material, by a vapor-phase growth technique where the growth process eliminates the need for utilization of a mask or removal of the substrate from the reactor at any time during the processing. A nucleation layer is first grown upon which a middle layer is grown to provide surfaces for subsequent lateral cantilever growth. The lateral growth rate is controlled by altering the reactor temperature, pressure, reactant concentrations or reactant flow rates. Semiconductor materials, such as GaN, can be produced with dislocation densities less than 10.sup.7 /cm.sup.2.

Polarity-inverted lateral overgrowth and selective wet-etching and regrowth (PILOSWER) of GaN.

PubMed

Jang, Dongsoo; Jue, Miyeon; Kim, Donghoi; Kim, Hwa Seob; Lee, Hyunkyu; Kim, Chinkyo

2018-03-07

On an SiO 2 -patterned c-plane sapphire substrate, GaN domains were grown with their polarity controlled in accordance with the pattern. While N-polar GaN was grown on hexagonally arranged circular openings, Ga-polar GaN was laterally overgrown on mask regions due to polarity inversion occurring at the boundary of the circular openings. After etching of N-polar GaN on the circular openings by H 3 PO 4 , this template was coated with 40-nm Si by sputtering and was slightly etched by KOH. After slight etching, a thin layer of Si left on the circular openings of sapphire,but not on GaN, was oxidized during thermal annealing and served as a dielectric mask during subsequent regrowth. Thus, the subsequent growth of GaN was made only on the existing Ga-polar GaN domains, not on the circular openings of the sapphire substrate. Transmission electron microscopy analysis revealed no sign of threading dislocations in this film. This approach may help fabricating an unholed and merged GaN film physically attached to but epitaxially separated from the SiO 2 -patterned sapphire.

DOE Office of Scientific and Technical Information (OSTI.GOV)

King, M. P.; Kaplar, R. J.; Dickerson, J. R.

Electrical performance and characterization of deep levels in vertical GaN P-i-N diodes grown on low threading dislocation density (~10 4 –10 6 cm –2) bulk GaN substrates are investigated. The lightly doped n drift region of these devices is observed to be highly compensated by several prominent deep levels detected using deep level optical spectroscopy at E c-2.13, 2.92, and 3.2 eV. A combination of steady-state photocapacitance and lighted capacitance-voltage profiling indicates the concentrations of these deep levels to be N t = 3 × 10 12, 2 × 10 15, and 5 × 10 14 cm –3, respectively. Themore » E c-2.92 eV level is observed to be the primary compensating defect in as-grown n-type metal-organic chemical vapor deposition GaN, indicating this level acts as a limiting factor for achieving controllably low doping. The device blocking voltage should increase if compensating defects reduce the free carrier concentration of the n drift region. Understanding the incorporation of as-grown and native defects in thick n-GaN is essential for enabling large V BD in the next-generation wide-bandgap power semiconductor devices. Furthermore, controlling the as-grown defects induced by epitaxial growth conditions is critical to achieve blocking voltage capability above 5 kV.« less

Atomic-scale and pit-free flattening of GaN by combination of plasma pretreatment and time-controlled chemical mechanical polishing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Deng, Hui; Endo, Katsuyoshi; Yamamura, Kazuya, E-mail: yamamura@upst.eng.osaka-u.ac.jp

2015-08-03

Chemical mechanical polishing (CMP) combined with atmospheric-pressure plasma pretreatment was applied to a GaN (0001) substrate. The irradiation of a CF{sub 4}-containing plasma was proven to be very useful for modifying the surface of GaN. When CMP was conducted on a plasma-irradiated surface, a modified layer of GaF{sub 3} acted as a protective layer on GaN by preventing the formation of etch pits. Within a short duration (8 min) of CMP using a commercially available CeO{sub 2} slurry, an atomically flat surface with a root mean square (rms) roughness of 0.11 nm was obtained. Moreover, etch pits, which are inevitably introduced inmore » conventional CMP, could not be observed at the dislocation sites on the polished GaN surface. It was revealed that CMP combined with the plasma pretreatment was very effective for obtaining a pit-free and atomically flat GaN surface.« less

Large electron capture-cross-section of the major nonradiative recombination centers in Mg-doped GaN epilayers grown on a GaN substrate

NASA Astrophysics Data System (ADS)

Chichibu, S. F.; Shima, K.; Kojima, K.; Takashima, S.; Edo, M.; Ueno, K.; Ishibashi, S.; Uedono, A.

2018-05-01

Complementary time-resolved photoluminescence and positron annihilation measurements were carried out at room temperature on Mg-doped p-type GaN homoepitaxial films for identifying the origin and estimating the electron capture-cross-section ( σ n ) of the major nonradiative recombination centers (NRCs). To eliminate any influence by threading dislocations, free-standing GaN substrates were used. In Mg-doped p-type GaN, defect complexes composed of a Ga-vacancy (VGa) and multiple N-vacancies (VNs), namely, VGa(VN)2 [or even VGa(VN)3], are identified as the major intrinsic NRCs. Different from the case of 4H-SiC, atomic structures of intrinsic NRCs in p-type and n-type GaN are different: VGaVN divacancies are the major NRCs in n-type GaN. The σ n value approximately the middle of 10-13 cm2 is obtained for VGa(VN)n, which is larger than the hole capture-cross-section (σp = 7 × 10-14 cm2) of VGaVN in n-type GaN. Combined with larger thermal velocity of an electron, minority carrier lifetime in Mg-doped GaN becomes much shorter than that of n-type GaN.

Total photoelectron yield spectroscopy of energy distribution of electronic states density at GaN surface and SiO2/GaN interface

NASA Astrophysics Data System (ADS)

Ohta, Akio; Truyen, Nguyen Xuan; Fujimura, Nobuyuki; Ikeda, Mitsuhisa; Makihara, Katsunori; Miyazaki, Seiichi

2018-06-01

The energy distribution of the electronic state density of wet-cleaned epitaxial GaN surfaces and SiO2/GaN structures has been studied by total photoelectron yield spectroscopy (PYS). By X-ray photoelectron spectroscopy (XPS) analysis, the energy band diagram for a wet-cleaned epitaxial GaN surface such as the energy level of the valence band top and electron affinity has been determined to obtain a better understanding of the measured PYS signals. The electronic state density of GaN surface with different carrier concentrations in the energy region corresponding to the GaN bandgap has been evaluated. Also, the interface defect state density of SiO2/GaN structures was also estimated by not only PYS analysis but also capacitance–voltage (C–V) characteristics. We have demonstrated that PYS analysis enables the evaluation of defect state density filled with electrons at the SiO2/GaN interface in the energy region corresponding to the GaN midgap, which is difficult to estimate by C–V measurement of MOS capacitors.

Direct evidence of single quantum dot emission from GaN islands formed at threading dislocations using nanoscale cathodoluminescence: A source of single photons in the ultraviolet

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schmidt, Gordon, E-mail: Gordon.Schmidt@ovgu.de; Berger, Christoph; Veit, Peter

2015-06-22

Intense emission from GaN islands embedded in AlN resulting from GaN/AlN quantum well growth is directly resolved by performing cathodoluminescence spectroscopy in a scanning transmission electron microscope. Line widths down to 440 μeV are measured in a wavelength region between 220 and 310 nm confirming quantum dot like electronic properties in the islands. These quantum dot states can be structurally correlated to islands of slightly enlarged thicknesses of the GaN/AlN quantum well layer preferentially formed in vicinity to dislocations. The quantum dot states exhibit single photon emission in Hanbury Brown-Twiss experiments with a clear antibunching in the second order correlation function atmore » zero time delay.« less

Nucleation mechanisms of epitaxial GaN nanowires: Origin of their self-induced formation and initial radius

NASA Astrophysics Data System (ADS)

Consonni, V.; Knelangen, M.; Geelhaar, L.; Trampert, A.; Riechert, H.

2010-02-01

The formation mechanisms of epitaxial GaN nanowires grown within a self-induced approach by molecular-beam epitaxy have been investigated at the onset of the nucleation process by combining in situ reflection high-energy electron-diffraction measurements and ex situ high-resolution transmission electron microscopy imaging. It is shown that the self-induced growth of GaN nanowires on the AlN buffer layer is initially governed by the nucleation of dislocation-free coherent islands. These coherent islands develop through a series of shape transitions from spherical caps through truncated to full pyramids in order to elastically relieve the lattice-mismatch-induced strain. A strong correlation between the subsequent process of plastic relaxation and the final shape transition from full pyramids toward the very first nanowires is found. The experimental critical radius at which the misfit dislocation nucleates is in very good agreement with the theoretical critical radius for the formation of the misfit dislocation in full pyramids, showing that the plastic relaxation process does take place within full pyramids: this critical size corresponds to the initial radius of the very first nanowires. We associate the plastic relaxation of the lattice-mismatch-induced strain occurring within full pyramids with a drastic change in their total free energy: this gives rise to a driving force for the shape transition toward the very first nanowires, which is mainly due to the anisotropy of surface energy.

Impact of thermal treatment on the optical performance of InGaN/GaN light emitting diodes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Meneghini, Matteo, E-mail: matteo.meneghini@dei.unipd.it; Meneghesso, Gaudenzio; Zanoni, Enrico

2015-10-15

This paper describes a detailed analysis of the effects of high temperatures on the optical performance and structural characteristics of GaN-based LED structures with a high threading dislocation density. Results show that, as a consequence of storage at 900 °C in N{sub 2} atmosphere, the samples exhibit: (i) an increase in the efficiency of GaN and quantum-well luminescence, well correlated to an increase in carrier lifetime; (ii) a decrease in the parasitic luminescence peaks related to Mg acceptors, which is correlated to the reduction in the concentration of Mg in the p-type region, detected by Secondary Ion Mass Spectroscopy (SIMS);more » (iii) a diffusion of acceptor (Mg) atoms to the quantum well region; (iv) a reduction in the yield of Rutherford Backscattering Spectrometry (RBS)-channeling measurements, possibly due to a partial re-arrangement of the dislocations, which is supposed to be correlated to the increase in radiative efficiency (see (i))« less

Impact of thermal treatment on the optical performance of InGaN/GaN light emitting diodes

NASA Astrophysics Data System (ADS)

Meneghini, Matteo; Zhu, Dandan; Humphreys, Colin J.; Berti, Marina; Gasparotto, Andrea; Cesca, Tiziana; Vinattieri, Anna; Bogani, Franco; Meneghesso, Gaudenzio; Zanoni, Enrico

2015-10-01

This paper describes a detailed analysis of the effects of high temperatures on the optical performance and structural characteristics of GaN-based LED structures with a high threading dislocation density. Results show that, as a consequence of storage at 900 °C in N2 atmosphere, the samples exhibit: (i) an increase in the efficiency of GaN and quantum-well luminescence, well correlated to an increase in carrier lifetime; (ii) a decrease in the parasitic luminescence peaks related to Mg acceptors, which is correlated to the reduction in the concentration of Mg in the p-type region, detected by Secondary Ion Mass Spectroscopy (SIMS); (iii) a diffusion of acceptor (Mg) atoms to the quantum well region; (iv) a reduction in the yield of Rutherford Backscattering Spectrometry (RBS)-channeling measurements, possibly due to a partial re-arrangement of the dislocations, which is supposed to be correlated to the increase in radiative efficiency (see (i)).

Effect of defects on the electrical/optical performance of gallium nitride based junction devices

NASA Astrophysics Data System (ADS)

Ferdous, Mohammad Shahriar

Commercial GaN based electronic and optoelectronic devices possess a high density (107-109 cm-2) of threading dislocations (TDs) because of the large mismatch in the lattice constant and the thermal expansion coefficient between the epitaxial layer structure and the substrate. In spite of these dislocations, high brightness light emitting diodes (LEDs) utilizing InGaN or AlGaN multiple quantum wells (MQWs) and with an external quantum efficiency of more than 40%, have already been achieved. This high external quantum efficiency in the presence of a high density of dislocations has been explained by carrier localization induced by indium fluctuations in the quantum well. TDs have been found to increase the reverse leakage current in InGaN based LEDs and to shorten the operating lifetime of InGaN MQW/GaN/AlGaN laser diodes. Thus it is important that the TD density is further reduced. It remains unclear how the TDs interact with the device to cause the effects mentioned above, hence the careful and precise characterization of threading defects and their effects on the electrical and optical performances of InGaN/GaN MQW LEDs is needed. This investigation will be useful not only from the point of view of device optimization but also to develop a clear understanding of the physical processes associated with TDs and especially with their effect on leakage current. We have employed photoelectrochemical (PEC) etching to accurately measure the dislocation density initially in home-grown GaN-based epitaxial structures and recently in InGaN/GaN MQW LEDs fabricated from commercial grade epitaxial structures that were supplied by our industrial collaborators. Measuring the electrical and electroluminescence (EL) characteristics of these devices has revealed correlations between some aspects of the LED behavior and the TD density, and promises to allow a deeper understanding of the role of threading dislocations to be elucidated. We observed that the LED reverse leakage current increased exponentially, and electroluminescence intensity decreased by 22%, as the TD density in the LEDs increased from 1.7 x 107 cm-2 to 2 x 108 cm-2. Forward voltage remained almost constant with the increase of TD density. A model of carrier conduction via hopping through defect related states, was found to provide an excellent fit to the experimental I-V data and provides a useful basis for understanding carrier conduction in the presence of TDs.

Ideal Channel Field Effect Transistors

DTIC Science & Technology

2010-03-01

well as on /?-GaAs/w-GaAs homojunctions grown by molecular beam epitaxy (MBE). The diode I-Vs at reverse bias are plotted below. The measured breakdown...transistors and composite channel InAlAs/InGaAs/lnP/InAlAs high electron mobility transistors ( HEMTs ), which have taken the full advantage of the matched...result in a large number of dislocations in GaAs films epitaxially grown on wurtzite GaN. In this work, we have successfully integrated GaAs with GaN

Continuous-Flow MOVPE of Ga-Polar GaN Column Arrays and Core-Shell LED Structures

NASA Astrophysics Data System (ADS)

Wang, Xue; Li, Shunfeng; Mohajerani, Matin Sadat; Ledig, Johannes; Wehmann, Hergo-Heinrich; Mandl, Martin; Strassburg, Martin; Steegmüller, Ulrich; Jahn, Uwe; Lähnemann, Jonas; Riechert, Henning; Griffiths, Ian; Cherns, David; Waag, Andreas

2013-06-01

Arrays of dislocation free uniform Ga-polar GaN columns have been realized on patterned SiOx/GaN/sapphire templates by metal organic vapor phase epitaxy using a continuous growth mode. The key parameters and the physical principles of growth of Ga-polar GaN three-dimensional columns are identified, and their potential for manipulating the growth process is discussed. High aspect ratio columns have been achieved using silane during the growth, leading to n-type columns. The vertical growth rate increases with increasing silane flow. In a core-shell columnar LED structure, the shells of InGaN/GaN multi quantum wells and p-GaN have been realized on a core of n-doped GaN column. Cathodoluminescence gives insight into the inner structure of these core-shell LED structures.

Improvement in crystal quality and optical properties of n-type GaN employing nano-scale SiO2 patterned n-type GaN substrate.

PubMed

Jo, Min Sung; Sadasivam, Karthikeyan Giri; Tawfik, Wael Z; Yang, Seung Bea; Lee, Jung Ju; Ha, Jun Seok; Moon, Young Boo; Ryu, Sang Wan; Lee, June Key

2013-01-01

n-type GaN epitaxial layers were regrown on the patterned n-type GaN substrate (PNS) with different size of silicon dioxide (SiO2) nano dots to improve the crystal quality and optical properties. PNS with SiO2 nano dots promotes epitaxial lateral overgrowth (ELOG) for defect reduction and also acts as a light scattering point. Transmission electron microscopy (TEM) analysis suggested that PNS with SiO2 nano dots have superior crystalline properties. Hall measurements indicated that incrementing values in electron mobility were clear indication of reduction in threading dislocation and it was confirmed by TEM analysis. Photoluminescence (PL) intensity was enhanced by 2.0 times and 3.1 times for 1-step and 2-step PNS, respectively.

In vacancies in InN grown by plasma-assisted molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Reurings, Floris; Tuomisto, Filip; Gallinat, Chad S.; Koblmüller, Gregor; Speck, James S.

2010-12-01

The authors have applied positron annihilation spectroscopy to study the effect of different growth conditions on vacancy formation in In- and N-polar InN grown by plasma-assisted molecular beam epitaxy. The results suggest that the structural quality of the material and limited diffusion of surface adatoms during growth dictate the In vacancy formation in low electron-density undoped epitaxial InN, while growth conditions and thermodynamics have a less important role, contrary to what is observed in, e.g., GaN. Furthermore, the results imply that in high quality InN, the electron mobility is likely limited not by ionized point defect scattering, but rather by threading dislocations.

Intrinsic luminescence and core structure of freshly introduced a-screw dislocations in n-GaN

NASA Astrophysics Data System (ADS)

Medvedev, O.; Vyvenko, O.; Ubyivovk, E.; Shapenkov, S.; Bondarenko, A.; Saring, P.; Seibt, M.

2018-04-01

Dislocations introduced by the scratching or by the indentation of the basal and prismatic surfaces of low-ohmic unintentionally n-type doped GaN crystals were investigated by means of cathodoluminescence and transmission electron microscopy (TEM). A strong luminescence of straight segments of a-screw dislocations was observed in the temperature range of 70-420 K. The spectrum of dislocation related luminescence (DRL) consisted of a doublet of narrow lines red shifted by about 0.3 eV with respect to the band gap. TEM revealed dissociated character of the screw dislocations and the formation of extended nodes at their intersection. From the analysis of the DRL spectral doublet temperature, power and strain dependences DRL was ascribed to direct and indirect excitons bound by 1D quantum wells formed by partials and stacking fault (SF) ribbon of dissociated screw dislocation.

Studies on the effect of ammonia flow rate induced defects in gallium nitride grown by MOCVD

NASA Astrophysics Data System (ADS)

Suresh, S.; Lourdudoss, S.; Landgren, G.; Baskar, K.

2010-10-01

Gallium nitride (GaN) epitaxial layers were grown with different V/III ratios by varying the ammonia (NH 3) flow rate, keeping the flow rate of the other precursor, trimethylgallium (TMG), constant, in an MOCVD system. X-ray rocking curve widths of a (1 0 2) reflection increase with an increase in V/III ratio while the (0 0 2) rocking curve widths decrease. The dislocation density was found to increase with an increase in ammonia flow rate, as determined by hot-wet chemical etching and atomic force microscopy. 77 K photoluminescence studies show near band emission at 3.49 eV and yellow luminescence peaking at 2.2 eV. The yellow luminescence (YL) intensity decreases with an increase in V/III ratio. Positron annihilation spectroscopy studies show that the concentration of Ga-like vacancies increases with an increase in ammonia flow rate. This study confirms that the yellow luminescence in the GaN arises due to deep levels formed by gallium vacancies decorated with oxygen atoms.

Structures and optical properties of \\text{H}_{2}^{+} -implanted GaN epi-layers

NASA Astrophysics Data System (ADS)

Li, B. S.; Wang, Z. G.

2015-06-01

The implantation damage build-up and optical properties of GaN epitaxial films under \\text{H}2+ ion implantation have been investigated by a combination of Rutherford backscattering in channeling geometry, Raman spectroscopy, UV-visible spectroscopy and transmission electron microscopy. GaN epitaxial films were implanted with 134 keV \\text{H}2+ ions to doses ranging from 3.75   ×   1016 to 1.75   ×   1017 \\text{H}2+  cm-2 at room temperature or the same dose of 1.5   ×   1017 \\text{H}2+  cm-2 at room temperature, 573 and 723 K. The dependence of lattice disorder induced by \\text{H}2+ -implantation on the ion dose can be divided into a three-step damage process. A strong influence of the H concentration on the defect accumulation is discussed. The decrease in relative Ga disorder induced by \\text{H}2+ -implantation is linear with increasing implantation temperature. The absorption coefficient of GaN epitaxial films increases with increasing ion dose, leading to the decrease in Raman scattering spectra of Ga-N vibration. With increasing implantation doses up to 5   ×   1016 \\text{H}2+  cm-2, nanoscale hydrogen bubbles are observed in the H deposition peak region. Interstitial-type dislocation loops are observed in the damaged layer located near the damage peak region, and the geometry of the dislocation loops produced by H implantation is analyzed. The surface layer is almost free of lattice disorder induced by \\text{H}2+ -implantation.

The management of stress in MOCVD-grown InGaN/GaN LED multilayer structures on Si(1 1 1) substrates

NASA Astrophysics Data System (ADS)

Jiang, Quanzhong; Allsopp, Duncan W. E.; Bowen, Chris R.; Wang, Wang N.

2013-09-01

The tensile stress in light-emitting diode (LED)-on-Si(1 1 1) multilayer structures must be reduced so that it does not compromise the multiple quantum well emission wavelength uniformity and structural stability. In this paper it is shown for non-optimized LED structures grown on Si(1 1 1) substrates that both emission wavelength uniformity and structural stability can be achieved within the same growth process. In order to gain a deeper understanding of the stress distribution within such a structure, cross-sectional Raman and photo-luminescence spectroscopy techniques were developed. It is observed that for a Si:GaN layer grown on a low-temperature (LT) AlN intermediate layer there is a decrease in compressive stress with increasing Si:GaN layer thickness during MOCVD growth which leads to a high level of tensile stress in the upper part of the layer. This may lead to the development of cracks during cooling to room temperature. Such a phenomenon may be associated with annihilation of defects such as dislocations. Therefore, a reduction of dislocation intensity should take place at the early stage of GaN growth on an AlN or AlGaN layer in order to reduce a build up of tensile stress with thickness. Furthermore, it is also shown that a prolonged three dimensional GaN island growth on a LT AlN interlayer for the reduction of dislocations may result in a reduction in the compressive stress in the resulting GaN layer.

Effect of 60Co γ-irradiation on the nature of electronic transport in heavily doped n-type GaN based Schottky photodetectors

NASA Astrophysics Data System (ADS)

Chatterjee, Abhishek; Khamari, Shailesh K.; Porwal, S.; Kher, S.; Sharma, T. K.

2018-04-01

GaN Schottky photodetectors are fabricated on heavily doped n-type GaN epitaxial layers grown by the hydride vapour phase epitaxy technique. The effect of 60Co γ-radiation on the electronic transport in GaN epilayers and Schottky detectors is studied. In contrast to earlier observations, a steady rise in the carrier concentration with increasing irradiation dose is clearly seen. By considering a two layer model, the contribution of interfacial dislocations in carrier transport is isolated from that of the bulk layer for both the pristine and irradiated samples. The bulk carrier concentration is fitted by using the charge balance equation which indicates that no new electrically active defects are generated by γ-radiation even at 500 kGy dose. The irradiation induced rise in the bulk carrier concentration is attributed to the activation of native Si impurities that are already present in an electrically inert form in the pristine sample. Further, the rise in interfacial contribution in the carrier concentration is governed by the enhanced rate of formation of nitrogen vacancies by irradiation, which leads to a larger diffusion of oxygen impurities. A large value of the characteristic tunnelling energy for both the pristine and irradiated Au/Ni/GaN Schottky devices confirms that the dislocation-assisted tunnelling dominates the low temperature current transport even after irradiation. The advantage of higher displacement energy and larger bandgap of GaN as compared to GaAs is evident from the change in leakage current after irradiation. Further, a fast recovery of the photoresponse of GaN photodetectors after irradiation signifies their compatibility to operate in high radiation zones. The results presented here are found to be crucial in understanding the interaction of 60Co γ-irradiation with n+-GaN epilayers.

On the luminescence of freshly introduced a-screw dislocations in low-resistance GaN

DOE Office of Scientific and Technical Information (OSTI.GOV)

Medvedev, O. S., E-mail: o.s.medvedev@spbu.ru; Vyvenko, O. F.; Bondarenko, A. S.

2015-09-15

Using scanning electron microscopy in the cathodoluminescence mode, it is shown that straight segments of a-screw dislocations introduced by plastic deformation at room temperature into unintentionally doped low-resistance gallium nitride luminesce in the spectral range 3.1–3.2 eV at 70 K. The spectral composition of dislocation luminescence shows a fine doublet structure with a component width of ∼15 meV and splitting of ∼30 meV, accompanied by LO-phonon replicas. Luminescent screw dislocations move upon exposure to an electron beam and at low temperatures, but retain immobility for a long time without external excitation. Optical transitions involving the quantum-well states of a stackingmore » fault in a split-dislocation core are considered to be the most probable mechanism of the observed phenomenon.« less

Elimination of trench defects and V-pits from InGaN/GaN structures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Smalc-Koziorowska, Julita; Grzanka, Ewa; Czernecki, Robert

2015-03-09

The microstructural evolution of InGaN/GaN multiple quantum wells grown by metalorganic chemical vapor phase epitaxy was studied as a function of the growth temperature of the GaN quantum barriers (QBs). We observed the formation of basal stacking faults (BSFs) in GaN QBs grown at low temperature. The presence of BSFs terminated by stacking mismatch boundaries (SMBs) leads to the opening of the structure at the surface into a V-shaped trench loop. This trench may form above an SMB, thereby terminating the BSF, or above a junction between the SMB and a subsequent BSF. Fewer BSFs and thus fewer trench defectsmore » were observed in GaN QBs grown at temperatures higher than 830 °C. Further increase in the growth temperature of the GaN QBs led to the suppression of the threading dislocation opening into V-pits.« less

Defect structure of epitaxial layers of III nitrides as determined by analyzing the shape of X-ray diffraction peaks

NASA Astrophysics Data System (ADS)

Kyutt, R. T.

2017-04-01

The shape of X-ray diffraction epitaxial layers with high dislocation densities has been studied experimentally. Measurements with an X-ray diffractometer were performed in double- and triple-crystal setups with both Cu K α and Mo K α radiation. Epitaxial layers (GaN, AlN, AlGaN, ZnO, etc.) with different degrees of structural perfection grown by various methods on sapphire, silicon, and silicon carbide substrates have been examined. The layer thickness varied in the range of 0.5-30 μm. It has been found that the center part of peaks is well approximated by the Voigt function with different Lorentz fractions, while the wing intensity drops faster and may be represented by a power function (with the index that varies from one structure to another). A well-marked dependence on the ordering of dislocations was observed. The drop in intensity in the majority of structures with a regular system and regular threading dislocations was close to the theoretically predicted law Δθ-3; the intensity in films with a chaotic distribution decreased much faster. The dependence of the peak shape on the order of reflection, the diffraction geometry, and the epitaxial layer thickness was also examined.

Intensity Distribution of the Three-Wave Diffraction from Dislocation Epitaxial Layers in the Reciprocal Space

NASA Astrophysics Data System (ADS)

Kyutt, R. N.

2018-04-01

The three-wave X-ray diffraction in strongly disordered epitaxial layers of GaN and ZnO is experimentally investigated. The charts of the intensity distribution in the reciprocal space are plotted in coordinates q θ and q ϕ for the most intensive three-wave combination (1010)/(1011) by means of subsequent θ- and ϕ-scanning. A nontrivial shape of the θ-sections of these contours at a distance from the ϕ center of reflection is revealed; it is different for different samples. For the θ-curves at the center of reflection, we observed a common peak that may be approximated by the Voigt function with a power-low decrease in the intensity at the wings; the decrease law (from-4.5 to-5.0) is found to be considerably greater than that for the similar curves of two-wave diffraction and not depending on the dislocation density and distribution in layers. In some films we observed a coarse-block structure; in addition, it follows from the distribution in the reciprocal space that these blocks are turned with respect to each other around a normal to the surface, which allows us to suggest the existence of low-angle boundaries between them, consisting exclusively of edge dislocations.

Visualization and automatic detection of defect distribution in GaN atomic structure from sampling Moiré phase.

PubMed

Wang, Qinghua; Ri, Shien; Tsuda, Hiroshi; Kodera, Masako; Suguro, Kyoichi; Miyashita, Naoto

2017-09-19

Quantitative detection of defects in atomic structures is of great significance to evaluating product quality and exploring quality improvement process. In this study, a Fourier transform filtered sampling Moire technique was proposed to visualize and detect defects in atomic arrays in a large field of view. Defect distributions, defect numbers and defect densities could be visually and quantitatively determined from a single atomic structure image at low cost. The effectiveness of the proposed technique was verified from numerical simulations. As an application, the dislocation distributions in a GaN/AlGaN atomic structure in two directions were magnified and displayed in Moire phase maps, and defect locations and densities were detected automatically. The proposed technique is able to provide valuable references to material scientists and engineers by checking the effect of various treatments for defect reduction. © 2017 IOP Publishing Ltd.

Luminescence properties of defects in GaN

NASA Astrophysics Data System (ADS)

Reshchikov, Michael A.; Morkoç, Hadis

2005-03-01

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The point defects include native isolated defects (vacancies, interstitial, and antisites), intentional or unintentional impurities, as well as complexes involving different combinations of the isolated defects. Further improvements in device performance and longevity hinge on an in-depth understanding of point defects and their reduction. In this review a comprehensive and critical analysis of point defects in GaN, particularly their manifestation in luminescence, is presented. In addition to a comprehensive analysis of native point defects, the signatures of intentionally and unintentionally introduced impurities are addressed. The review discusses in detail the characteristics and the origin of the major luminescence bands including the ultraviolet, blue, green, yellow, and red bands in undoped GaN. The effects of important group-II impurities, such as Zn and Mg on the photoluminescence of GaN, are treated in detail. Similarly, but to a lesser extent, the effects of other impurities, such as C, Si, H, O, Be, Mn, Cd, etc., on the luminescence properties of GaN are also reviewed. Further, atypical luminescence lines which are tentatively attributed to the surface and structural defects are discussed. The effect of surfaces and surface preparation, particularly wet and dry etching, exposure to UV light in vacuum or controlled gas ambient, annealing, and ion implantation on the characteristics of the defect-related emissions is described.

Self-Healing Thermal Annealing: Surface Morphological Restructuring Control of GaN Nanorods

DOE Office of Scientific and Technical Information (OSTI.GOV)

Conroy, Michele; Li, Haoning; Zubialevich, Vitaly Z.

With advances in nanolithography and dry etching, top-down methods of nanostructuring have become a widely used tool for improving the efficiency of optoelectronics. These nano dimensions can offer various benefits to the device performance in terms of light extraction and efficiency, but often at the expense of emission color quality. Broadening of the target emission peak and unwanted yellow luminescence are characteristic defect-related effects due to the ion beam etching damage, particularly for III–N based materials. In this article we focus on GaN based nanorods, showing that through thermal annealing the surface roughness and deformities of the crystal structure canmore » be “self-healed”. Correlative electron microscopy and atomic force microscopy show the change from spherical nanorods to faceted hexagonal structures, revealing the temperature-dependent surface morphology faceting evolution. The faceted nanorods were shown to be strain- and defect-free by cathodoluminescence hyperspectral imaging, micro-Raman, and transmission electron microscopy (TEM). In-situ TEM thermal annealing experiments allowed for real time observation of dislocation movements and surface restructuring observed in ex-situ annealing TEM sampling. This thermal annealing investigation gives new insight into the redistribution path of GaN material and dislocation movement post growth, allowing for improved understanding and in turn advances in optoelectronic device processing of compound semiconductors.« less

Interface science of virtual GaN substrates on Si(111) via Sc2O3/Y2O3 buffers: Experiment and theory

NASA Astrophysics Data System (ADS)

Tarnawska, L.; Dabrowski, J.; Grzela, T.; Lehmann, M.; Niermann, T.; Paszkiewicz, R.; Storck, P.; Schroeder, T.

2013-06-01

The final film quality of GaN on foreign substrates is known to crucially depend on the initial GaN interface and nucleation characteristics. To shed light on these characteristics of recently pioneered virtual, hexagonal GaN(0001) substrates on Si(111) via step graded Sc2O3(111)/Y2O3(111) buffers, a complex GaN(0001)/Sc2O3(111) interface structure model and the initial nucleation scenario is derived from a combined experimental (reflection high energy electron diffraction and X-ray photoelectron spectroscopy) and theoretical ab initio study. It is shown that the GaN/Sc2O3 interface chemistry is determined by a N-Ga-O-Sc atomic arrangement leading to N-polar GaN films. However, the atomic GaN(0001)/Sc2O3(111) interface configuration is complex and local perturbations might be at the origin of Ga-polar inversion domains in the mainly N-polar GaN films. The initial growth of GaN on Sc2O3 is characterized by an ultrathin N-Ga-O-Sc wetting layer which carries tensile strain and relaxes with increasing thickness. Further GaN deposition results in the formation of 3D islands which fully relax before island coalescence occurs. The implications of the GaN/Sc2O3 interface configuration, the 3D nucleation growth mode, and the coalescence process of misaligned islands are discussed with respect to the defect characteristics (inversion domains, cubic inclusions, threading dislocations) of the final GaN layer.

Thermal conductivity anisotropy in nanostructures and nanostructured materials

NASA Astrophysics Data System (ADS)

Termentzidis, Konstantinos

2018-03-01

Thermal conductivity anisotropy is a subject for both fundamental and application interests. The anisotropy can be induced either by van der Waals forces in bulk systems or by nanostructuration. Here, we will examine four cases in which thermal anisotropy has been observed: (i) Si/Ge superlattices which exhibit higher thermal anisotropy between in-plane and cross-plane directions for the case of smooth interfaces, (ii) amorphous/crystalline superlattices with much higher anisotropy than the crystalline/crystalline superlattices and which can reach a factor of six when the amorphous fraction increases, (iii) the impact of the density of edge and screw dislocations on the thermal anisotropy of defected GaN, and (iv) the influence of the growth direction of Bi2Te3 nanowires on thermal conductivity.

Molecular dynamics studies of defect formation during heteroepitaxial growth of InGaN alloys on (0001) GaN surfaces

DOE PAGES

Gruber, J.; Zhou, X. W.; Jones, R. E.; ...

2017-05-15

Here, we investigate the formation of extended defects during molecular-dynamics (MD) simulations of GaN and InGaN growth on (0001) and (11more » $$\\bar{2}$$0) wurtzite-GaN surfaces. The simulated growths are conducted on an atypically large scale by sequentially injecting nearly a million individual vapor-phase atoms towards a fixed GaN surface; we apply time-and-position-dependent boundary constraints that vary the ensemble treatments of the vapor-phase, the near-surface solid-phase, and the bulk-like regions of the growing layer. The simulations employ newly optimized Stillinger-Weber In-Ga-N-system potentials, wherein multiple binary and ternary structures are included in the underlying density-functional-theory training sets, allowing improved treatment of In-Ga-related atomic interactions. To examine the effect of growth conditions, we study a matrix of >30 different MD-growth simulations for a range of InxGa1-xN-alloy compositions (0 ≤ x ≤ 0.4) and homologous growth temperatures [0.50 ≤ T/T* m(x) ≤ 0.90], where T* m(x) is the simulated melting point. Growths conducted on polar (0001) GaN substrates exhibit the formation of various extended defects including stacking faults/polymorphism, associated domain boundaries, surface roughness, dislocations, and voids. In contrast, selected growths conducted on semi-polar (11$$\\bar{2}$$0) GaN, where the wurtzite-phase stacking sequence is revealed at the surface, exhibit the formation of far fewer stacking faults. We discuss variations in the defect formation with the MD growth conditions, and we compare the resulting simulated films to existing experimental observations in InGaN/GaN. Finally, while the palette of defects observed by MD closely resembles those observed in the past experiments, further work is needed to achieve truly predictive large-scale simulations of InGaN/GaN crystal growth using MD methodologies.« less

Molecular dynamics studies of defect formation during heteroepitaxial growth of InGaN alloys on (0001) GaN surfaces

NASA Astrophysics Data System (ADS)

Gruber, J.; Zhou, X. W.; Jones, R. E.; Lee, S. R.; Tucker, G. J.

2017-05-01

We investigate the formation of extended defects during molecular-dynamics (MD) simulations of GaN and InGaN growth on (0001) and ( 11 2 ¯ 0 ) wurtzite-GaN surfaces. The simulated growths are conducted on an atypically large scale by sequentially injecting nearly a million individual vapor-phase atoms towards a fixed GaN surface; we apply time-and-position-dependent boundary constraints that vary the ensemble treatments of the vapor-phase, the near-surface solid-phase, and the bulk-like regions of the growing layer. The simulations employ newly optimized Stillinger-Weber In-Ga-N-system potentials, wherein multiple binary and ternary structures are included in the underlying density-functional-theory training sets, allowing improved treatment of In-Ga-related atomic interactions. To examine the effect of growth conditions, we study a matrix of >30 different MD-growth simulations for a range of InxGa1-xN-alloy compositions (0 ≤ x ≤ 0.4) and homologous growth temperatures [0.50 ≤ T/T*m(x) ≤ 0.90], where T*m(x) is the simulated melting point. Growths conducted on polar (0001) GaN substrates exhibit the formation of various extended defects including stacking faults/polymorphism, associated domain boundaries, surface roughness, dislocations, and voids. In contrast, selected growths conducted on semi-polar ( 11 2 ¯ 0 ) GaN, where the wurtzite-phase stacking sequence is revealed at the surface, exhibit the formation of far fewer stacking faults. We discuss variations in the defect formation with the MD growth conditions, and we compare the resulting simulated films to existing experimental observations in InGaN/GaN. While the palette of defects observed by MD closely resembles those observed in the past experiments, further work is needed to achieve truly predictive large-scale simulations of InGaN/GaN crystal growth using MD methodologies.

Molecular dynamics studies of defect formation during heteroepitaxial growth of InGaN alloys on (0001) GaN surfaces.

PubMed

Gruber, J; Zhou, X W; Jones, R E; Lee, S R; Tucker, G J

2017-05-21

We investigate the formation of extended defects during molecular-dynamics (MD) simulations of GaN and InGaN growth on (0001) and ([Formula: see text]) wurtzite-GaN surfaces. The simulated growths are conducted on an atypically large scale by sequentially injecting nearly a million individual vapor-phase atoms towards a fixed GaN surface; we apply time-and-position-dependent boundary constraints that vary the ensemble treatments of the vapor-phase, the near-surface solid-phase, and the bulk-like regions of the growing layer. The simulations employ newly optimized Stillinger-Weber In-Ga-N-system potentials, wherein multiple binary and ternary structures are included in the underlying density-functional-theory training sets, allowing improved treatment of In-Ga-related atomic interactions. To examine the effect of growth conditions, we study a matrix of >30 different MD-growth simulations for a range of In x Ga 1-x N-alloy compositions (0 ≤  x  ≤ 0.4) and homologous growth temperatures [0.50 ≤  T/T * m ( x ) ≤ 0.90], where T * m ( x ) is the simulated melting point. Growths conducted on polar (0001) GaN substrates exhibit the formation of various extended defects including stacking faults/polymorphism, associated domain boundaries, surface roughness, dislocations, and voids. In contrast, selected growths conducted on semi-polar ([Formula: see text]) GaN, where the wurtzite-phase stacking sequence is revealed at the surface, exhibit the formation of far fewer stacking faults. We discuss variations in the defect formation with the MD growth conditions, and we compare the resulting simulated films to existing experimental observations in InGaN/GaN. While the palette of defects observed by MD closely resembles those observed in the past experiments, further work is needed to achieve truly predictive large-scale simulations of InGaN/GaN crystal growth using MD methodologies.

Electrical characterisation of defects in wide bandgap semiconductors

NASA Astrophysics Data System (ADS)

Elsherif, Osama S.

Defects usually have a very large influence on the semiconductor material properties and hence on fabricated electronic devices. The nature and properties of defects in semiconducting materials can be investigated by applying electrical characterization techniques such as thermal admittance spectroscopy (TAS), deep level transient spectroscopy (DLTS) and high resolution Laplace-DLTS measurements. This dissertation presents the electrical characterisation of two different wide bandgap semiconducting materials (polycrystalline diamond and GaN) which have both recently attracted a great deal of attention because of their potential applications in the fields of power electronics and optoelectronics. Raman spectroscopy, I-V and C-V measurements were carried out as supporting experiments for the above investigations. The first part of this work focuses on studying the effect of B concentration on the electronic states in polycrystalline diamond thin films grown on silicon by the hot filament chemical vapour deposition method. A combination of high-resolution LDLTS and direct-capture cross-section measurements was used to investigate whether the deep electronic states present in the layers originated from point or extended defects. There was good agreement between data on deep electronic levels obtained from DLTS and TAS experiments. A number of hole traps have been detected; the majority of these levels show an unusual dependence of the DLTS signal on the fill pulse duration which is interpreted as possibly the levels are part of extended defects within the grain boundaries. In contrast, a defect level found in a more highly doped film, with an activation energy of -0.37 eV, exhibited behaviour characteristic of an isolated point defect, which we attribute to B-related centres in the bulk diamond, away from the dislocations. The second part of this thesis presents electrical measurements carried out at temperatures up to 450 K in order to study the electronic states associated with Mg in Mg-doped GaN films grown on sapphire by metalorganic vapour phase epitaxy, and to determine how these are affected by the threading dislocation density (TDD). Two different buffer layer schemes between the film and the sapphire substrate were used, giving rise to different TDDs in the GaN. Admittance spectroscopy of the films finds a single impurity-related acceptor level. It is observed in theses experiments that admittance spectroscopy detects no traps that can be attributed to extended defects, despite the fact that the dislocations are well-known to be active recombination centres. This unexpected finding is discussed in detail.

Electrical characterisation of defects in wide bandgap semiconductors

NASA Astrophysics Data System (ADS)

Elsherif, Osama S.

Defects usually have a very large influence on the semiconductor material properties and hence on fabricated electronic devices. The nature and properties of defects in semiconducting materials can be investigated by applying electrical characterization techniques such as thermal admittance spectroscopy (TAS), deep level transient spectroscopy (DLTS) and high resolution Laplace-DLTS measurements. This dissertation presents the electrical characterisation of two different wide bandgap semiconducting materials (polycrystalline diamond and GaN) which have both recently attracted a great deal of attention because of their potential applications in the fields of power electronics and optoelectronics. Raman spectroscopy, I-V and C-V measurements were carried out as supporting experiments for the above investigations.The first part of this work focuses on studying the effect of B concentration on the electronic states in polycrystalline diamond thin films grown on silicon by the hot filament chemical vapour deposition method. A combination of high-resolution LDLTS and direct-capture cross-section measurements was used to investigate whether the deep electronic states present in the layers originated from point or extended defects. There was good agreement between data on deep electronic levels obtained from DLTS and TAS experiments. A number of hole traps have been detected; the majority of these levels show an unusual dependence of the DLTS signal on the fill pulse duration which is interpreted as possibly the levels are part of extended defects within the grain boundaries. In contrast, a defect level found in a more highly doped film, with an activation energy of -0.37 eV, exhibited behaviour characteristic of an isolated point defect, which we attribute to B-related centres in the bulk diamond, away from the dislocations.The second part of this thesis presents electrical measurements carried out at temperatures up to 450 K in order to study the electronic states associated with Mg in Mg-doped GaN films grown on sapphire by metalorganic vapour phase epitaxy, and to determine how these are affected by the threading dislocation density (TDD). Two different buffer layer schemes between the film and the sapphire substrate were used, giving rise to different TDDs in the GaN. Admittance spectroscopy of the films finds a single impurity-related acceptor level. It is observed in theses experiments that admittance spectroscopy detects no traps that can be attributed to extended defects, despite the fact that the dislocations are well-known to be active recombination centres. This unexpected finding is discussed in detail.

Low-voltage cross-sectional EBIC for characterisation of GaN-based light emitting devices.

PubMed

Moldovan, Grigore; Kazemian, Payam; Edwards, Paul R; Ong, Vincent K S; Kurniawan, Oka; Humphreys, Colin J

2007-01-01

Electron beam induced current (EBIC) characterisation can provide detailed information on the influence of crystalline defects on the diffusion and recombination of minority carriers in semiconductors. New developments are required for GaN light emitting devices, which need a cross-sectional approach to provide access to their complex multi-layered structures. A sample preparation approach based on low-voltage Ar ion milling is proposed here and shown to produce a flat cross-section with very limited surface recombination, which enables low-voltage high resolution EBIC characterisation. Dark defects are observed in EBIC images and correlation with cathodoluminescence images identify them as threading dislocations. Emphasis is placed on one-dimensional quantification which is used to show that junction delineation with very good spatial resolution can be achieved, revealing significant roughening of this GaN p-n junction. Furthermore, longer minority carrier diffusion lengths along the c-axis are found at dislocation sites, in both p-GaN and the multi-quantum well (MQW) region. This is attributed to gettering of point defects at threading dislocations in p-GaN and higher escape rate from quantum wells at dislocation sites in the MQW region, respectively. These developments show considerable promise for the use of low-voltage cross-sectional EBIC in the characterisation of point and extended defects in GaN-based devices and it is suggested that this technique will be particularly useful for degradation analysis.

Epitaxial growth and characterization of approximately 300-nm-thick AlInN films nearly lattice-matched to c-plane GaN grown on sapphire

NASA Astrophysics Data System (ADS)

Miyoshi, Makoto; Yamanaka, Mizuki; Egawa, Takashi; Takeuchi, Tetsuya

2018-05-01

AlInN epitaxial films with film thicknesses up to approximately 300 nm were grown nearly lattice-matched to a c-plane GaN-on-sapphire template by metalorganic chemical vapor deposition. The AlInN films showed relative good crystal qualities and flat surfaces, despite the existence of surface pits connected to dislocations in the underlying GaN film. The refractive index derived in this study agreed well with a previously reported result obtained over the whole visible wavelength region. The extinction coefficient spectrum exhibited a clear absorption edge, and the bandgap energy for AlInN nearly lattice-matched to GaN was determined to be approximately 4.0 eV.

Understanding luminescence properties of grain boundaries in GaN thin films and their atomistic origin

NASA Astrophysics Data System (ADS)

Yoo, Hyobin; Yoon, Sangmoon; Chung, Kunook; Kang, Seoung-Hun; Kwon, Young-Kyun; Yi, Gyu-Chul; Kim, Miyoung

2018-03-01

We report our findings on the optical properties of grain boundaries in GaN films grown on graphene layers and discuss their atomistic origin. We combine electron backscatter diffraction with cathodoluminescence to directly correlate the structural defects with their optical properties, enabling the high-precision local luminescence measurement of the grain boundaries in GaN films. To further understand the atomistic origin of the luminescence properties, we carefully probed atomic core structures of the grain boundaries by exploiting aberration-corrected scanning transmission electron microscopy. The atomic core structures of grain boundaries show different ordering behaviors compared with those observed previously in threading dislocations. Energetics of the grain boundary core structures and their correlation with electronic structures were studied by first principles calculation.

Strain relaxation in (0001) AlN/GaN heterostructures

NASA Astrophysics Data System (ADS)

Bourret, Alain; Adelmann, Christoph; Daudin, Bruno; Rouvière, Jean-Luc; Feuillet, Guy; Mula, Guido

2001-06-01

The strain-relaxation phenomena during the early stages of plasma-assisted molecular-beam epitaxy growth of lattice-mismatched wurtzite (0001) AlN/GaN heterostructures have been studied by real-time recording of the in situ reflection high-energy electron diffraction (RHEED), ex situ transmission electron microscopy (TEM), and atomic-force microscopy. A pseudo-two-dimensional layer-by-layer growth is observed at substrate temperatures of 640-660 °C, as evidenced by RHEED and TEM. However, the variation of the in-plane lattice parameter during growth and after growth has been found to be complex. Three steps have been seen during the deposition of lattice-mismatched AlN and GaN layers: they were interpreted as the succession of the formation of flat platelets, 3-6 monolayers high (0.8-1.5 nm) and 10-20 nm in diameter, their partial coalescence, and gradual dislocation introduction. Platelet formation leads to elastic relaxation as high as 1.8%, i.e., a considerable part of the AlN/GaN lattice mismatch of 2.4%, and can be reversible. Platelets are always observed during the initial stages of growth and are almost insensitive to the metal/N ratio. In contrast, platelet coalescence and dislocation introduction are very dependent on the metal/N ratio: no coalescence occurs and the dislocation introduction rate is higher under N-rich conditions. In all cases, the misfit dislocation density, as measured by the irreversible relaxation, is initially of the order of 7×1011 cm-2 and decreases exponentially with the layer thickness. These results are interpreted in the framework of a model that emphasizes the important role of the flat platelets for dislocation nucleation.

Impact of varying buffer thickness generated strain and threading dislocations on the formation of plasma assisted MBE grown ultra-thin AlGaN/GaN heterostructure on silicon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chowdhury, Subhra, E-mail: subhra1109@gmail.com; Biswas, Dhrubes; Department of E and E C E, Indian Institute of Technology Kharagpur, Kharagpur 721302

2015-05-15

Plasma-assisted molecular beam epitaxy (PAMBE) growth of ultra-thin Al{sub 0.2}Ga{sub 0.8}N/GaN heterostructures on Si(111) substrate with three buffer thickness (600 nm/400 nm/200 nm) have been reported. An unique growth process has been developed that supports lower temperature epitaxy of GaN buffer which minimizes thermally generated tensile strain through appropriate nitridation and AlN initiated epitaxy for achieving high quality GaN buffer which supports such ultra-thin heterostructures in the range of 10-15Å. It is followed by investigations of role of buffer thickness on formation of ultra-thin Al{sub 0.2}Ga{sub 0.8}N/GaN heterostructure, in terms of stress-strain and threading dislocation (TD). Structural characterization were performedmore » by High-Resolution X-Ray Diffraction (HRXRD), room-temperature Photoluminescence (RT-PL), High Resolution Transmission Electron Microscopy (HRTEM) and Atomic Force Microscopy (AFM). Analysis revealed increasing biaxial tensile stress of 0.6918 ± 0.04, 1.1084, 1.1814 GPa in heterostructures with decreasing buffer thickness of 600, 400, 200 nm respectively which are summed up with residual tensile strain causing red-shift in RT-PL peak. Also, increasing buffer thickness drastically reduced TD density from the order 10{sup 10} cm{sup −2} to 10{sup 8} cm{sup −2}. Surface morphology through AFM leads to decrease of pits and root mean square value with increasing buffer thickness which are resulted due to reduction of combined effect of strain and TDs.« less

GaN nanowires with pentagon shape cross-section by ammonia-source molecular beam epitaxy

DOE PAGES

Lin, Yong; Leung, Benjamin; Li, Qiming; ...

2015-07-14

In this study, ammonia-based molecular beam epitaxy (NH 3-MBE) was used to grow catalyst-assisted GaN nanowires on (11¯02) r-plane sapphire substrates. Dislocation free [112¯0] oriented nanowires are formed with pentagon shape cross-section, instead of the usual triangular shape facet configuration. Specifically, the cross-section is the result of the additional two nonpolar {101¯0} side facets, which appear due to a decrease in relative growth rate of the {101¯0} facets to the {101¯1} and {101¯1} facets under the growth regime in NH 3-MBE. Compared to GaN nanowires grown by Ni-catalyzed metal–organic chemical vapor deposition, the NH 3-MBE grown GaN nanowires show moremore » than an order of magnitude increase in band-edge to yellow luminescence intensity ratio, as measured by cathodoluminescence, indicating improved microstructural and optical properties.« less

High thermal stability of abrupt SiO2/GaN interface with low interface state density

NASA Astrophysics Data System (ADS)

Truyen, Nguyen Xuan; Taoka, Noriyuki; Ohta, Akio; Makihara, Katsunori; Yamada, Hisashi; Takahashi, Tokio; Ikeda, Mitsuhisa; Shimizu, Mitsuaki; Miyazaki, Seiichi

2018-04-01

The effects of postdeposition annealing (PDA) on the interface properties of a SiO2/GaN structure formed by remote oxygen plasma-enhanced chemical vapor deposition (RP-CVD) were systematically investigated. X-ray photoelectron spectroscopy clarified that PDA in the temperature range from 600 to 800 °C has almost no effects on the chemical bonding features at the SiO2/GaN interface, and that positive charges exist at the interface, the density of which can be reduced by PDA at 800 °C. The capacitance-voltage (C-V) and current density-SiO2 electric field characteristics of the GaN MOS capacitors also confirmed the reduction in interface state density (D it) and the improvement in the breakdown property of the SiO2 film after PDA at 800 °C. Consequently, a high thermal stability of the SiO2/GaN structure with a low fixed charge density and a low D it formed by RP-CVD was demonstrated. This is quite informative for realizing highly robust GaN power devices.

Propagation of misfit dislocations from buffer/Si interface into Si

DOEpatents

Liliental-Weber, Zuzanna [El Sobrante, CA; Maltez, Rogerio Luis [Porto Alegre, BR; Morkoc, Hadis [Richmond, VA; Xie, Jinqiao [Raleigh, VA

2011-08-30

Misfit dislocations are redirected from the buffer/Si interface and propagated to the Si substrate due to the formation of bubbles in the substrate. The buffer layer growth process is generally a thermal process that also accomplishes annealing of the Si substrate so that bubbles of the implanted ion species are formed in the Si at an appropriate distance from the buffer/Si interface so that the bubbles will not migrate to the Si surface during annealing, but are close enough to the interface so that a strain field around the bubbles will be sensed by dislocations at the buffer/Si interface and dislocations are attracted by the strain field caused by the bubbles and move into the Si substrate instead of into the buffer epi-layer. Fabrication of improved integrated devices based on GaN and Si, such as continuous wave (CW) lasers and light emitting diodes, at reduced cost is thereby enabled.

Proton irradiation effects on minority carrier diffusion length and defect introduction in homoepitaxial and heteroepitaxial n-GaN

NASA Astrophysics Data System (ADS)

Collins, K. C.; Armstrong, A. M.; Allerman, A. A.; Vizkelethy, G.; Van Deusen, S. B.; Léonard, F.; Talin, A. A.

2017-12-01

Inherent advantages of wide bandgap materials make GaN-based devices attractive for power electronics and applications in radiation environments. Recent advances in the availability of wafer-scale, bulk GaN substrates have enabled the production of high quality, low defect density GaN devices, but fundamental studies of carrier transport and radiation hardness in such devices are lacking. Here, we report measurements of the hole diffusion length in low threading dislocation density (TDD), homoepitaxial n-GaN, and high TDD heteroepitaxial n-GaN Schottky diodes before and after irradiation with 2.5 MeV protons at fluences of 4-6 × 1013 protons/cm2. We also characterize the specimens before and after irradiation using electron beam-induced-current (EBIC) imaging, cathodoluminescence, deep level optical spectroscopy (DLOS), steady-state photocapacitance, and lighted capacitance-voltage (LCV) techniques. We observe a substantial reduction in the hole diffusion length following irradiation (50%-55%) and the introduction of electrically active defects which could be attributed to gallium vacancies and associated complexes (VGa-related), carbon impurities (C-related), and gallium interstitials (Gai). EBIC imaging suggests long-range migration and clustering of radiation-induced point defects over distances of ˜500 nm, which suggests mobile Gai. Following irradiation, DLOS and LCV reveal the introduction of a prominent optical energy level at 1.9 eV below the conduction band edge, consistent with the introduction of Gai.

Metalorganic chemical vapor deposition of gallium nitride on sacrificial substrates

NASA Astrophysics Data System (ADS)

Fenwick, William Edward

GaN-based light emitting diodes (LEDs) face several challenges if the technology is to continue to make a significant impact in general illumination, and on technology that has become known as solid state lighting (SSL). Two of the most pressing challenges for the continued penetration of SSL into traditional lighting applications are efficacy and total lumens from the device, and their related cost. The development of alternative substrate technologies is a promising avenue toward addressing both of these challenges, as both GaN-based device technology and the associated metalorganic chemical vapor deposition (MOCVD) technology are already relatively mature technologies with a well-understood cost base. Zinc oxide (ZnO) and silicon (Si) are among the most promising alternative substrates for GaN epitaxy. These substrates offer the ability to access both higher efficacy and lumen devices (ZnO) at a much reduced cost. This work focuses on the development of MOCVD growth processes to yield high quality GaN-based materials and devices on both ZnO and Si. ZnO is a promising substrate for growth of low defect-density GaN because of its similar lattice constant and thermal expansion coefficient. The major hurdles for GaN growth on ZnO are the instability of the substrate in a hydrogen atmosphere, which is typical of nitride growth conditions, and the inter-diffusion of zinc and oxygen from the substrate into the GaN-based epitaxial layer. A process was developed for the MOCVD growth of GaN and InxGa 1-xN on ZnO that attempted to address these issues. The structural and optical properties of these films were studied using various techniques. X-ray diffraction (XRD) showed the growth of wurtzite GaN on ZnO, and room-temperature photoluminescence (RT-PL) showed near band-edge luminescence from the GaN and InxGa1-xN layers. However, high zinc and oxygen concentrations due to interdiffusion near the ZnO substrate remained an issue; therefore, the diffusion of zinc and oxygen into the subsequent GaN layer was studied in more detail. Several approaches were investigated---for example, transition layers such as Al2O3 and Al xGa1-xN/GaN---to minimize diffusion of these impurities into the GaN layer. Silicon, due to its prevalence, is the most promising material for the development of an inexpensive, large-area substrate technology. The challenge in MOCVD growth of GaN on Si is the tensile strain induced by the lattice and thermal mismatch between GaN and Si and the formation of anti-phase boundaries. Typical approaches to solve these problems involve complicated and multiple buffer layer structures, which lead to relatively slow growth rates. In this work, a thin atomic layer deposition (ALD)-grown Al2O3 interlayer was employed to relieve strain and increase material quality while also simplifying the growth process. While some residual strain was still observed in the GaN material by XRD and PL, the use of this oxide interlayer leads to an improvement in thin film quality as seen by a reduction in both crack density (<1 mm-2) on ALD-Al2O3/Si) and screw dislocation density (from 3x109cm-2 on bare Si to 2x108cm-2 on ALD-Al 2O3/Si) in the GaN films. A side-by-side comparison of GaN-based multiple quantum well LEDs grown on sapphire and on Al2O3/Si shows similar performance characteristic for both device structures. A redshift in peak emission wavelength was also observed on silicon compared to sapphire, and this is attributed to higher indium content due to the slight tensile strain in the layers on silicon. IQE of the devices on silicon is ˜32% as measured by LT-PL, compared to ˜37% on sapphire, but this difference can be assigned to the difference in indium compositions. These results show a great promise toward an inexpensive, large-area, silicon-based substrate technology for MOCVD growth of the next generation of GaN-based optoelectronic devices for SSL and other applications.

Impact of extended defects on optical properties of (1-101)GaN grown on patterned Si

NASA Astrophysics Data System (ADS)

Okur, S.; Izyumskaya, N.; Zhang, F.; Avrutin, V.; Metzner, S.; Karbaum, C.; Bertram, F.; Christen, J.; Morkoç, H.; Özgür, Ü.

2014-03-01

The optical quality of semipolar (1 101)GaN layers was explored by time- and polarization-resolved photoluminescence spectroscopy. High intensity bandedge emission was observed in +c-wing regions of the stripes as a result of better structural quality, while -c-wing regions were found to be of poorer optical quality due to basal plane and prismatic stacking faults (BSFs and PSFs) in addition to a high density of TDs. The high optical quality region formed on the +cwings was evidenced also from the much slower biexponential PL decays (0.22 ns and 1.70 ns) and an order of magnitude smaller amplitude ratio of the fast decay (nonradiative origin) to the slow decay component (radiative origin) compared to the -c-wing regions. In regard to defect-related emission, decay times for the BSF and PSF emission lines at 25 K (~ 0.80 ns and ~ 3.5 ns, respectively) were independent of the excitation density within the range employed (5 - 420 W/cm2), and much longer than that for the donor bound excitons (0.13 ns at 5 W/cm2 and 0.22 ns at 420 W/cm2). It was also found that the emission from BSFs had lower polarization degree (0.22) than that from donor bound excitons (0.35). The diminution of the polarization degree when photogenerated carriers recombine within the BSFs is another indication of the negative effects of stacking faults on the optical quality of the semipolar (1101)GaN. In addition, spatial distribution of defects in semipolar (1101)-oriented InGaN active region layers grown on stripe patterned Si substrates was investigated using near-field scanning optical microscopy. The optical quality of -c- wing regions was found to be worse compared to +c-wing regions due to the presence of higher density of stacking faults and threading dislocations. The emission from the +c-wings was very bright and relatively uniform across the sample, which is indicative of a homogeneous In distribution.

Characterization of nonpolar a-plane GaN epi-layers grown on high-density patterned r-plane sapphire substrates

NASA Astrophysics Data System (ADS)

Jinno, Daiki; Otsuki, Shunya; Sugimori, Shogo; Daicho, Hisayoshi; Iwaya, Motoaki; Takeuchi, Tetsuya; Kamiyama, Satoshi; Akasaki, Isamu

2018-02-01

To reduce the number of threading dislocations (TDs) in nonpolar a-plane GaN (a-GaN) epi-layers grown on flat r-plane sapphire substrates (r-FSS), we investigated the effects on the crystalline quality of the a-GaN epi-layers of high-density patterned r-plane sapphire substrates (r-HPSS), the patterns of which were placed at intervals of several hundred nanometers. Two types of r-HPSS, the patterns of which had diameters and heights on the order of several hundred nanometers (r-NHPSS) or several micrometers (r-MHPSS), were prepared with conventional r-FSS. The effect of these r-HPSS on the a-GaN epi-layers was demonstrated by evaluating the surface morphology and the crystalline quality of the epi-layers. The surfaces of the a-GaN epi-layer grown on r-FSS and r-NHPSS were pit-free and mirror-like, whereas the surface of the a-GaN epi-layer grown on r-MHPSS was very rough due to the large, irregular GaN islands that grew on the patterns, mainly at the initial growth stage. The crystalline quality of the a-GaN epi-layer grown on r-NHPSS was better than that of the a-GaN epi-layer grown on r-FSS. We confirmed that there were fewer TDs in the a-GaN epi-layer grown on r-NHPSS than there were in the a-GaN epi-layer grown on r-FSS. The TDs propagating to the surface in a-GaN epi-layer grown on r-NHPSS were mainly generated on the flat sapphire regions between the patterns. Interestingly, it was also found that the TDs that propagated to the surface concentrated with a periodic pitch along the c-axis direction. The TD densities of a-GaN epi-layers grown on r-FSS and r-NHPSS were estimated to be approximately 5.0 × 1010 and 1.5 × 109 cm-2, respectively. This knowledge will contribute to the further development of a-GaN epi-layers for high-performance devices.

Cross-stacked carbon nanotubes assisted self-separation of free-standing GaN substrates by hydride vapor phase epitaxy

NASA Astrophysics Data System (ADS)

Wei, Tongbo; Yang, Jiankun; Wei, Yang; Huo, Ziqiang; Ji, Xiaoli; Zhang, Yun; Wang, Junxi; Li, Jinmin; Fan, Shoushan

2016-06-01

We report a novel method to fabricate high quality 2-inch freestanding GaN substrate grown on cross-stacked carbon nanotubes (CSCNTs) coated sapphire by hydride vapor phase epitaxy (HVPE). As nanoscale masks, these CSCNTs can help weaken the interface connection and release the compressive stress by forming voids during fast coalescence and also block the propagation of threading dislocations (TDs). During the cool-down process, thermal stress-induced cracks are initiated at the CSCNTs interface with the help of air voids and propagated all over the films which leads to full self-separation of FS-GaN substrate. Raman and photoluminescence spectra further reveal the stress relief and crystalline improvement of GaN with CSCNTs. It is expected that the efficient, low cost and mass-producible technique may enable new applications for CNTs in nitride optoelectronic fields.

Mechanical responses of a-axis GaN nanowires under axial loads

NASA Astrophysics Data System (ADS)

Wang, R. J.; Wang, C. Y.; Feng, Y. T.; Tang, Chun

2018-03-01

Gallium nitride (GaN) nanowires (NWs) hold technological significance as functional components in emergent nano-piezotronics. However, the examination of their mechanical responses, especially the mechanistic understanding of behavior beyond elasticity (at failure) remains limited due to the constraints of in situ experimentation. We therefore performed simulations of the molecular dynamics (MD) of the mechanical behavior of [1\\bar{2}10]-oriented GaN NWs subjected to tension or compression loading until failure. The mechanical properties and critical deformation processes are characterized in relation to NW sizes and loading conditions. Detailed examinations revealed that the failure mechanisms are size-dependent and controlled by the dislocation mobility on shuffle-set pyramidal planes. The size dependence of the elastic behavior is also examined in terms of the surface structure determined modification of Young’s modulus. In addition, a comparison with c-axis NWs is made to show how size-effect trends vary with the growth orientation of NWs.

Cross-stacked carbon nanotubes assisted self-separation of free-standing GaN substrates by hydride vapor phase epitaxy.

PubMed

Wei, Tongbo; Yang, Jiankun; Wei, Yang; Huo, Ziqiang; Ji, Xiaoli; Zhang, Yun; Wang, Junxi; Li, Jinmin; Fan, Shoushan

2016-06-24

We report a novel method to fabricate high quality 2-inch freestanding GaN substrate grown on cross-stacked carbon nanotubes (CSCNTs) coated sapphire by hydride vapor phase epitaxy (HVPE). As nanoscale masks, these CSCNTs can help weaken the interface connection and release the compressive stress by forming voids during fast coalescence and also block the propagation of threading dislocations (TDs). During the cool-down process, thermal stress-induced cracks are initiated at the CSCNTs interface with the help of air voids and propagated all over the films which leads to full self-separation of FS-GaN substrate. Raman and photoluminescence spectra further reveal the stress relief and crystalline improvement of GaN with CSCNTs. It is expected that the efficient, low cost and mass-producible technique may enable new applications for CNTs in nitride optoelectronic fields.

Cross-stacked carbon nanotubes assisted self-separation of free-standing GaN substrates by hydride vapor phase epitaxy

PubMed Central

Wei, Tongbo; Yang, Jiankun; Wei, Yang; Huo, Ziqiang; Ji, Xiaoli; Zhang, Yun; Wang, Junxi; Li, Jinmin; Fan, Shoushan

2016-01-01

We report a novel method to fabricate high quality 2-inch freestanding GaN substrate grown on cross-stacked carbon nanotubes (CSCNTs) coated sapphire by hydride vapor phase epitaxy (HVPE). As nanoscale masks, these CSCNTs can help weaken the interface connection and release the compressive stress by forming voids during fast coalescence and also block the propagation of threading dislocations (TDs). During the cool-down process, thermal stress-induced cracks are initiated at the CSCNTs interface with the help of air voids and propagated all over the films which leads to full self-separation of FS-GaN substrate. Raman and photoluminescence spectra further reveal the stress relief and crystalline improvement of GaN with CSCNTs. It is expected that the efficient, low cost and mass-producible technique may enable new applications for CNTs in nitride optoelectronic fields. PMID:27340030

Mechanisms of Loss in Internal Quantum Efficiency in III-Nitride-based Blue-and Green-Light Emitting Diodes

NASA Astrophysics Data System (ADS)

Huang, Li

The overarching goals of the research conducted for this dissertation have been to understand the scientific reasons for the losses in the internal quantum efficiency (IQE) in Group III-nitride-based blue and especially green light-emitting diodes (LEDs) containing a multi-quantum well (MQW) active region and to simultaneously develop LED epitaxial structures to ameliorate these losses. The p-type AlGaN EBL was determined to be both mandatory and effective in the prevention of electron overflow from the MQW region into the p-type cladding layer and the resultant lowering of the IQE. The overflow phenomenon was partially due to the low concentration (˜ 5 x 1017 cm-3) and mobility (˜ 10 cm2/(V•s)) of the holes injected into the active region. Electroluminescence (EL) studies of LEDs without an EBL revealed a dominant emission from donor-acceptor pair recombination in the p-type GaN layer. The incorporation of a 90 nm compositionally graded In0-0.1 Ga1-0.9N buffer layer between each MQW and n-GaN cladding layer grown on an Al/SiC substrate resulted in an increase in the luminescence intensity and a blue-shift in the emission wavelength, as observed in photoluminescence (PL) spectra. The graded InGaN buffer layer reduced the stress and thus the piezoelectric field across the MQW; this improved the electron/hole overlap that, in turn, resulted in an enhanced radiative recombination rate and an increase in efficiency. A direct correlation was observed between an increase in the IQE measured in temperature-dependent PL (TDPL) and an increase in the roughness of all the upper InGaN QW/GaN barrier interfaces, as determined using cross-sectional transmission electron microscopy of the MQW. These results agreed in general with the average surface roughness values of the pit-free region on the top GaN barrier determined via atomic force microscopy and the average roughness values of all the interfaces in the MQW calculated from the FWHM of the emission peak in the PL spectra acquired at 10 K for LED structures grown on both SiC and GaN substrates. This improvement occurred as a result of carrier localization at the rougher interfaces that, in turn, resulted in shorter carrier lifetimes and faster decay rates, as determined using time-resolved PL. The peak current densities determined from the curves of external quantum efficiency as a function of current density calculated from EL spectra acquired from a set of LEDs having 3 QWs, 5 QWs, and 6 QWs were 63 A/cm2, 78 A/cm2 and 78 A/cm2, respectively. These data indicated that the minority carrier (holes) in our powered devices penetrated into at least the 4th QW from the top p-type cladding layer. The peak emission from these LEDs occurred at 522 nm. The hole density decreased with distance away from the top p-type layer. Finally, a new process route was developed in this research for the epitaxial deposition of GaN(0001) thin films on chemo-mechanically polished GaN(0001) substrates. The latter possessed threading dislocations (TDs) having a density of the order of 5 x 107 cm-2, predominantly edge in character and oriented along [0001]. Step-flow-controlled growth of the films was achieved; thus, no additional TDs were generated at the film/substrate interface. The density of V-defects in InGaN films and in subsequently grown MQWs containing In0.26Ga0.74N wells grown on the GaN substrates was also reduced to within an order of 107 cm -2. The density of the latter defects was determined to be a function of both the density of the TDs and the growth temperature when the latter was > 900 °C. (Abstract shortened by UMI.)

Gallium nitride vertical power devices on foreign substrates: a review and outlook

NASA Astrophysics Data System (ADS)

Zhang, Yuhao; Dadgar, Armin; Palacios, Tomás

2018-07-01

Vertical gallium nitride (GaN) power devices have attracted increased attention due to their superior high-voltage and high-current capacity as well as easier thermal management than lateral GaN high electron mobility transistors. Vertical GaN devices are promising candidates for next-generation power electronics in electric vehicles, data centers, smart grids and renewable energy process. The use of low-cost foreign substrates such as silicon (Si) substrates, instead of the expensive free-standing GaN substrates, could greatly trim material cost and enable large-diameter wafer processing while maintaining high device performance. This review illustrates recent progress in material epitaxy, device design, device physics and processing technologies for the development of vertical GaN power devices on low-cost foreign substrates. Although the device technologies are still at the early stage of development, state-of-the-art vertical GaN-on-Si power diodes have already shown superior Baliga’s figure of merit than commercial SiC and Si power devices at the voltage classes beyond 600 V. Furthermore, we unveil the design space of vertical GaN power devices on native and different foreign substrates, from the analysis of the impact of dislocation and defects on device performance. We conclude by identifying the application space, current challenges and exciting research opportunities in this very dynamic research field.

Structural characterization of bulk GaN crystals grown under high hydrostatic pressure

NASA Astrophysics Data System (ADS)

Liliental-Weber, Zuzanna; Kisielowski, C.; Ruvimov, S.; Chen, Y.; Washburn, J.; Grzegory, I.; Bockowski, M.; Jun, J.; Porowski, S.

1996-09-01

This paper describes TEM characterization of bulk GaN crystals grown at 1500-1800Kin the form of plates from a solution of atomic nitrogen in liquid gallium under high nitrogen pressure (up to 20 kbars). The x-ray rocking curves for these crystals were in the range of 20-30 arc-sec. The plate thickness along the c axis was about 100 times smaller than the nonpolar growth directions. A substantial difference in material quality was observed on the opposite sides of the plates normal to the c direction. On one side the surface was atomically flat, while on the other side the surface was rough, with pyramidal features up to 100 nm high. The polarity of the crystals was determined using convergent-beam electron diffraction. The results showed that, regarding the long bond between Ga and N along the c-axis, Ga atoms were found to be closer to the flat side of the crystal, while N atoms were found to be closer to the rough side. Near the rough side, within 1/10 to 1/4 of the plate thickness, there was a high density of planar defects (stacking faults and dislocation loops decorated by Ga/void precipitates). A model explaining the defect formation is proposed.

Growth kinetics and island evolution during double-pulsed molecular beam epitaxy of InN

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kraus, A.; Hein, C.; Bremers, H.

The kinetic processes of InN growth using alternating source fluxes with sub-monolayer In pulses in plasma-assisted molecular beam epitaxy have been investigated. Growth at various temperatures reveals the existence of two growth regimes. While growth at low temperatures is solely governed by surface diffusion, a combination of decomposition, desorption, and diffusion becomes decisive at growth temperatures of 470 °C and above. At this critical temperature, the surface morphology changes from a grainy structure to a structure made of huge islands. The formation of those islands is attributed to the development of an indium adlayer, which can be observed via reflection highmore » energy electron diffraction monitoring. Based on the growth experiments conducted at temperatures below T{sub Growth} = 470 °C, an activation energy for diffusion of 0.54 ± 0.02 eV has been determined from the decreasing InN island density. A comparison between growth on metalorganic vapor phase epitaxy GaN templates and pseudo bulk GaN indicates that step edges and dislocations are favorable nucleation sites. Based on the results, we developed a growth model, which describes the main mechanisms of the growth.« less

Estimation of dislocations density and distribution of dislocations during ECAP-Conform process

NASA Astrophysics Data System (ADS)

Derakhshan, Jaber Fakhimi; Parsa, Mohammad Habibi; Ayati, Vahid; Jafarian, Hamidreza

2018-01-01

Dislocation density of coarse grain aluminum AA1100 alloy (140 µm) that was severely deformed by Equal Channel Angular Pressing-Conform (ECAP-Conform) are studied at various stages of the process by electron backscattering diffraction (EBSD) method. The geometrically necessary dislocations (GNDs) density and statistically stored dislocations (SSDs) densities were estimate. Then the total dislocations densities are calculated and the dislocation distributions are presented as the contour maps. Estimated average dislocations density for annealed of about 2×1012 m-2 increases to 4×1013 m-2 at the middle of the groove (135° from the entrance), and they reach to 6.4×1013 m-2 at the end of groove just before ECAP region. Calculated average dislocations density for one pass severely deformed Al sample reached to 6.2×1014 m-2. At micrometer scale the behavior of metals especially mechanical properties largely depend on the dislocation density and dislocation distribution. So, yield stresses at different conditions were estimated based on the calculated dislocation densities. Then estimated yield stresses were compared with experimental results and good agreements were found. Although grain size of material did not clearly change, yield stress shown intensive increase due to the development of cell structure. A considerable increase in dislocations density in this process is a good justification for forming subgrains and cell structures during process which it can be reason of increasing in yield stress.

High-fluence ion implantation in silicon carbide for fabrication of a compliant substrate

NASA Astrophysics Data System (ADS)

Lioubtchenko, Mikhail

GaN and related nitrides are promising materials for applications as UV/blue light emitters and in high-power, high-temperature electonic devices. Unfortunately, the vast potential of these materials cannot be realized effectively due to a large density of threading dislocations, arising from large lattice mismatch between GaN and utilized substrates. Therefore, a new approach to the heteroepitaxial growth is desirable, and a compliant substrate might help to remedy the situation. A modified model for the compliant substrate consisting of the compliant membrane glued to a thick handling substrate by a soft layer was proposed. We have chosen 6H-SiC as a starting substrate and ion implantation as a means of creating a buried layer. High fluence ion implantation of different species in 6H-SiC was performed at elevated temperatures and damage removal/accumulation was studied. It was found that temperatures around 1600°C are necessary to successfully recrystallize the radiation damage for Ti, Ga, Si and C implantations, but no damage removal was monitored for In implantation. In order to minimize the damage produced during ion implantation, it was decided to employ a multistep process in which each implantation step was followed by annealing. This approach was realized for 125 keV Ti++ and 300 keV Ga+ implantations up to a total dose of 1.8 x 1017 cm--2. Ti-implanted substrates were shown to retain good quality in the top layer, whereas Ga implantation preserves the quality of the near-surface region only at lower doses. The implanted species concentration was monitored after each step using Rutherford Backscattering (RBS). GaN films were grown on the prepared substrates and a control SiC sample by MOCVD. TEM and photoluminescence measurements have demonstrated that the quality of GaN films improves upon growth on compliant substrates.

Crystallographic tilt and in-plane anisotropies of an a-plane InGaN/GaN layered structure grown by MOCVD on r-plane sapphire using a ZnO buffer

NASA Astrophysics Data System (ADS)

Liu, H. F.; Liu, W.; Guo, S.; Chi, D. Z.

2016-03-01

High-resolution x-ray diffraction (HRXRD) was used to investigate the crystallographic tilts and structural anisotropies in epitaxial nonpolar a-plane InGaN/GaN grown by metal-organic chemical vapor deposition on r-plane sapphire using a ZnO buffer. The substrate had an unintentional miscut of 0.14° towards its [-4 2 2 3] axis. However, HRXRD revealed a tilt of 0.26° (0.20°) between the ZnO (GaN) (11-20) and the Al2O3 (1-102) atomic planes, with the (11-20) axis of ZnO (GaN) tilted towards its c-axis, which has a difference of 163° in azimuth from that of the substrate’s miscut. Excess broadenings in the GaN/ZnO (11-20) rocking curves (RCs) were observed along its c-axis. Specific analyses revealed that partial dislocations and anisotropic in-plane strains, rather than surface-related effects, wafer curvature or stacking faults, are the dominant factors for the structural anisotropy. The orientation of the partial dislocations is most likely affected by the miscut of the substrate, e.g. via tilting of the misfit dislocation gliding planes created during island coalescences. Their Burgers vector components in the growth direction, in turn, gave rise to crystallographic tilts in the same direction as that of the excess RC-broadenings.

Growth and electrical transport properties of InGaN/GaN heterostructures grown by PAMBE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sinha, Neeraj; Department of Materials Science, Gulbarga University, Gulbarga 585106; Roul, Basanta, E-mail: basantaroul@gmail.com

2015-01-15

Highlights: • InGaN thin films were grown on GaN template by PAMBE. • InGaN films were characterized by HRXRD, SEM and PL and Raman spectroscopy. • The indium incorporation in single phase InGaN films was found to be 23%. • The I–V characteristic of the InGaN/GaN heterojunction shows rectifying behavior. • Log–log plot of the I–V characteristics indicates the presence of SCLC mechanism. - Abstract: InGaN epitaxial films were grown on GaN template by plasma-assisted molecular beam epitaxy. The composition of indium incorporation in single phase InGaN film was found to be 23%. The band gap energy of single phasemore » InGaN was found to be ∼2.48 eV. The current–voltage (I–V) characteristic of InGaN/GaN heterojunction was found to be rectifying behavior which shows the presence of Schottky barrier at the interface. Log–log plot of the I–V characteristics under forward bias indicates the current conduction mechanism is dominated by space charge limited current mechanism at higher applied voltage, which is usually caused due to the presence of trapping centers. The room temperature barrier height and the ideality factor of the Schottky junction were found to 0.76 eV and 4.9 respectively. The non-ideality of the Schottky junction may be due to the presence of high pit density and dislocation density in InGaN film.« less

Proton irradiation effects on minority carrier diffusion length and defect introduction in homoepitaxial and heteroepitaxial n-GaN [Proton irradiation effects on minority carrier diffusion length and defect introduction in homoepitaxial n-GaN

DOE Office of Scientific and Technical Information (OSTI.GOV)

Collins, K. C.; Armstrong, Andrew M.; Allerman, Andrew A.

Here, inherent advantages of wide bandgap materials make GaN-based devices attractive for power electronics and applications in radiation environments. Recent advances in the availability of wafer-scale, bulk GaN substrates have enabled the production of high quality, low defect density GaN devices, but fundamental studies of carrier transport and radiation hardness in such devices are lacking. Here, we report measurements of the hole diffusion length in low threading dislocation density (TDD), homoepitaxial n-GaN, and high TDD heteroepitaxial n-GaN Schottky diodes before and after irradiation with 2.5 MeV protons at fluences of 4–6 × 10 13 protons/cm 2. We also characterize themore » specimens before and after irradiation using electron beam-induced-current (EBIC) imaging, cathodoluminescence, deep level optical spectroscopy (DLOS), steady-state photocapacitance, and lighted capacitance-voltage (LCV) techniques. We observe a substantial reduction in the hole diffusion length following irradiation (50%–55%) and the introduction of electrically active defects which could be attributed to gallium vacancies and associated complexes (V Ga-related), carbon impurities (C-related), and gallium interstitials (Ga i). EBIC imaging suggests long-range migration and clustering of radiation-induced point defects over distances of ~500 nm, which suggests mobile Ga i. Following irradiation, DLOS and LCV reveal the introduction of a prominent optical energy level at 1.9 eV below the conduction band edge, consistent with the introduction of Ga i.« less

Proton irradiation effects on minority carrier diffusion length and defect introduction in homoepitaxial and heteroepitaxial n-GaN [Proton irradiation effects on minority carrier diffusion length and defect introduction in homoepitaxial n-GaN

DOE PAGES

Collins, K. C.; Armstrong, Andrew M.; Allerman, Andrew A.; ...

2017-12-21

Here, inherent advantages of wide bandgap materials make GaN-based devices attractive for power electronics and applications in radiation environments. Recent advances in the availability of wafer-scale, bulk GaN substrates have enabled the production of high quality, low defect density GaN devices, but fundamental studies of carrier transport and radiation hardness in such devices are lacking. Here, we report measurements of the hole diffusion length in low threading dislocation density (TDD), homoepitaxial n-GaN, and high TDD heteroepitaxial n-GaN Schottky diodes before and after irradiation with 2.5 MeV protons at fluences of 4–6 × 10 13 protons/cm 2. We also characterize themore » specimens before and after irradiation using electron beam-induced-current (EBIC) imaging, cathodoluminescence, deep level optical spectroscopy (DLOS), steady-state photocapacitance, and lighted capacitance-voltage (LCV) techniques. We observe a substantial reduction in the hole diffusion length following irradiation (50%–55%) and the introduction of electrically active defects which could be attributed to gallium vacancies and associated complexes (V Ga-related), carbon impurities (C-related), and gallium interstitials (Ga i). EBIC imaging suggests long-range migration and clustering of radiation-induced point defects over distances of ~500 nm, which suggests mobile Ga i. Following irradiation, DLOS and LCV reveal the introduction of a prominent optical energy level at 1.9 eV below the conduction band edge, consistent with the introduction of Ga i.« less

Low-dislocation-density epitatial layers grown by defect filtering by self-assembled layers of spheres

DOEpatents

Wang, George T.; Li, Qiming

2013-04-23

A method for growing low-dislocation-density material atop a layer of the material with an initially higher dislocation density using a monolayer of spheroidal particles to bend and redirect or directly block vertically propagating threading dislocations, thereby enabling growth and coalescence to form a very-low-dislocation-density surface of the material, and the structures made by this method.

Nanoselective area growth of GaN by metalorganic vapor phase epitaxy on 4H-SiC using epitaxial graphene as a mask

NASA Astrophysics Data System (ADS)

Puybaret, Renaud; Patriarche, Gilles; Jordan, Matthew B.; Sundaram, Suresh; El Gmili, Youssef; Salvestrini, Jean-Paul; Voss, Paul L.; de Heer, Walt A.; Berger, Claire; Ougazzaden, Abdallah

2016-03-01

We report the growth of high-quality triangular GaN nanomesas, 30-nm thick, on the C-face of 4H-SiC using nanoselective area growth (NSAG) with patterned epitaxial graphene grown on SiC as an embedded mask. NSAG alleviates the problems of defects in heteroepitaxy, and the high mobility graphene film could readily provide the back low-dissipative electrode in GaN-based optoelectronic devices. A 5-8 graphene-layer film is first grown on the C-face of 4H-SiC by confinement-controlled sublimation of silicon carbide. Graphene is then patterned and arrays of 75-nm-wide openings are etched in graphene revealing the SiC substrate. A 30-nm-thick GaN is subsequently grown by metal organic vapor phase epitaxy. GaN nanomesas grow epitaxially with perfect selectivity on SiC, in the openings patterned through graphene. The up-or-down orientation of the mesas on SiC, their triangular faceting, and cross-sectional scanning transmission electron microscopy show that they are biphasic. The core is a zinc blende monocrystal surrounded with single-crystal wurtzite. The GaN crystalline nanomesas have no threading dislocations or V-pits. This NSAG process potentially leads to integration of high-quality III-nitrides on the wafer scalable epitaxial graphene/silicon carbide platform.

Nanoselective area growth of GaN by metalorganic vapor phase epitaxy on 4H-SiC using epitaxial graphene as a mask

DOE Office of Scientific and Technical Information (OSTI.GOV)

Puybaret, Renaud; Jordan, Matthew B.; Voss, Paul L.

We report the growth of high-quality triangular GaN nanomesas, 30-nm thick, on the C-face of 4H-SiC using nanoselective area growth (NSAG) with patterned epitaxial graphene grown on SiC as an embedded mask. NSAG alleviates the problems of defects in heteroepitaxy, and the high mobility graphene film could readily provide the back low-dissipative electrode in GaN-based optoelectronic devices. A 5–8 graphene-layer film is first grown on the C-face of 4H-SiC by confinement-controlled sublimation of silicon carbide. Graphene is then patterned and arrays of 75-nm-wide openings are etched in graphene revealing the SiC substrate. A 30-nm-thick GaN is subsequently grown by metalmore » organic vapor phase epitaxy. GaN nanomesas grow epitaxially with perfect selectivity on SiC, in the openings patterned through graphene. The up-or-down orientation of the mesas on SiC, their triangular faceting, and cross-sectional scanning transmission electron microscopy show that they are biphasic. The core is a zinc blende monocrystal surrounded with single-crystal wurtzite. The GaN crystalline nanomesas have no threading dislocations or V-pits. This NSAG process potentially leads to integration of high-quality III-nitrides on the wafer scalable epitaxial graphene/silicon carbide platform.« less

Impacts of oxidants in atomic layer deposition method on Al2O3/GaN interface properties

NASA Astrophysics Data System (ADS)

Taoka, Noriyuki; Kubo, Toshiharu; Yamada, Toshikazu; Egawa, Takashi; Shimizu, Mitsuaki

2018-01-01

The electrical interface properties of GaN metal-oxide-semiconductor (MOS) capacitors with an Al2O3 gate insulator formed by atomic layer deposition method using three kinds of oxidants were investigated by the capacitance-voltage technique, Terman method, and conductance method. We found that O3 and the alternate supply of H2O and O3 (AS-HO) are effective for reducing the interface trap density (D it) at the energy range of 0.15 to 0.30 eV taking from the conduction band minimum. On the other hand, we found that surface potential fluctuation (σs) induced by interface charges for the AS-HO oxidant is much larger than that for a Si MOS capacitor with a SiO2 layer formed by chemical vapor deposition despite the small D it values for the AS-HO oxidant compared with the Si MOS capacitor. This means that the total charged center density including the fixed charge density, charged slow trap density, and charged interface trap density for the GaN MOS capacitor is higher than that for the Si MOS capacitor. Therefore, σs has to be reduced to improve the performances and reliability of GaN devices with the Al2O3/GaN interfaces.

The metalorganic chemical vapor deposition of III-V nitrides for optoelectronic device applications

NASA Astrophysics Data System (ADS)

Grudowski, Paul Alexander

Nitride-based light-emitting diodes (LEDs) and laser diodes are important for large-area LED displays, flat-panel displays, traffic signals, and optical data storage, due to their characteristic ultraviolet and visible light emission. However, much of the research and development addressing material related problems is recent. The room-temperature continuous wave (CW) operation of nitride-based laser diodes remains a major milestone because the material quality requirements for these devices are extremely high. This study investigates nitride material development by the metalorganic chemical vapor deposition (MOCVD) and characterization of GaN, AlGaN, and InGaN, and by qualifying these materials with fabricated devices. The ultimate goal was to develop a working laser diode. The nitride epitaxial films were characterized by 300K Hall effect, x-ray diffraction (XRD), photoluminescence (PL), cathodoluminescence (CL), secondary ion mass spectroscopy (SIMS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). GaN grown heteroepitaxially on (0001) sapphire substrates was first optimized. A low-temperature GaN nucleation layer was developed that gave subsequent high-temperature GaN layers with low background carrier concentrations (n < 1×10sp{17}\\ cmsp{-3}). Intentional p-type hole concentrations up to 2× 10sp{18} cmsp{-3} and n-type electron concentrations up to 1× 10sp{19} cmsp{-3} were achieved at 300K with magnesium and silicon, respectively. The ternary alloy Insb{x}Gasb{1-x}N was grown with indium compositions up to x = 0.25. These films exhibited strong and narrow 300K PL bandedge peaks. Multiple-quantum-well structures with Insb{0.13}Gasb{0.87}N wells and Insb{0.03}Gasb{0.97}N barriers were grown and gave enhanced PL intensity compared to single InGaN layers. Modulation-doped MQW's produced enhanced PL intensity compared to uniformly-doped MQW's. 300K photopumping experiments produced stimulated emission from a five-period MQW. Light-emitting device structures comprised of InGaN MQW active regions and p-type and n-type GaN contact layers and AlGaN confinement layers were grown and fabricated. LED's showed bright emission at a wavelength of 400 nm. While optically pumped lasers were demonstrated, no injection lasing action was achieved in these devices. GaN grown by selective area lateral epitaxial overgrowth (SALEO) has reduced dislocation defect density and, therefore, may prove to be a promising substrate for nearly defect-free device structures. Plan-view and cross-sectional CL was used to compare spatial inhomogeneities in the bandedge luminescence.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bajaj, Sanyam, E-mail: bajaj.10@osu.edu; Shoron, Omor F.; Park, Pil Sung

We report on the direct measurement of two-dimensional sheet charge density dependence of electron transport in AlGaN/GaN high electron mobility transistors (HEMTs). Pulsed IV measurements established increasing electron velocities with decreasing sheet charge densities, resulting in saturation velocity of 1.9 × 10{sup 7 }cm/s at a low sheet charge density of 7.8 × 10{sup 11 }cm{sup −2}. An optical phonon emission-based electron velocity model for GaN is also presented. It accommodates stimulated longitudinal optical (LO) phonon emission which clamps the electron velocity with strong electron-phonon interaction and long LO phonon lifetime in GaN. A comparison with the measured density-dependent saturation velocity showsmore » that it captures the dependence rather well. Finally, the experimental result is applied in TCAD-based device simulator to predict DC and small signal characteristics of a reported GaN HEMT. Good agreement between the simulated and reported experimental results validated the measurement presented in this report and established accurate modeling of GaN HEMTs.« less

Conductivity based on selective etch for GaN devices and applications thereof

DOEpatents

Zhang, Yu; Sun, Qian; Han, Jung

2015-12-08

This invention relates to methods of generating NP gallium nitride (GaN) across large areas (>1 cm.sup.2) with controlled pore diameters, pore density, and porosity. Also disclosed are methods of generating novel optoelectronic devices based on porous GaN. Additionally a layer transfer scheme to separate and create free-standing crystalline GaN thin layers is disclosed that enables a new device manufacturing paradigm involving substrate recycling. Other disclosed embodiments of this invention relate to fabrication of GaN based nanocrystals and the use of NP GaN electrodes for electrolysis, water splitting, or photosynthetic process applications.

Dislocation density evolution in the process of high-temperature treatment and creep of EK-181 steel

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vershinina, Tatyana, E-mail: vershinina@bsu.edu.ru

2017-03-15

X-ray diffraction has been used to study the dislocation structure in ferrite-martensite high-chromium steel EK-181 in the states after heat treatment and high-temperature creep. The influence of heat treatment and stress on evolution of lath martensite structure was investigated by and electron back-scattered diffraction. The effect of nitrogen content on the total dislocation density, fraction of edge and screw dislocation segments are analyzed. - Highlights: •Fraction of edge dislocation in quenched state depends on nitrogen concentration. •Nitrogen affects the character of dislocation structure evolution during annealing. •Edge dislocations fraction influences on dislocation density after aging and creep.

Dislocation Density Reduction in Cadmium Telluride and Mercury Cadmium Telluride Grown on Silicon Using Thermal Cycle Annealing

NASA Astrophysics Data System (ADS)

Farrell, Stuart Bennett

Mercury Cadmium Telluride (HgCdTe) is a material of great importance for infrared focal plane array applications. In order to produce large format detector arrays this material needs to be grown on a large area substrate, with silicon being the most mature substrate, it is the optimal choice for large format arrays. To help mitigate the effect of the lattice mismatch between the two materials, cadmium telluride (CdTe) is used as a buffer layer. The CdTe itself has nearly the same lattice mismatch (19.3%) to silicon, but due to the technological advantages it offers and compatibility with HgCdTe, it is the best buffer layer choice. The lattice mismatch between HgCdTe/CdTe and the silicon substrate leads to the formation of dislocations at densities in the mid 106 to low 107 cm-2 range in the epilayers. Such a high dislocation density greatly effects detector device performance quantities such as operability and sensitivity. Hence, the dislocation density should be brought down by at least an order of magnitude by adopting novel in situ and ex situ material processing techniques. In this work, in situ and ex situ thermal cycle annealing (TCA) methods have been used to decrease dislocation density in CdTe and HgCdTe. During the molecular beam epitaxial (MBE) growth of the CdTe buffer layer, the growth was interrupted and the layer was subjected to an annealing cycle within the growth chamber under tellurium overpressure. During the annealing cycle the temperature is raised to beyond the growth temperature (290 → 550 °C) and then allowed to cool before resuming growth again. This process was repeated several times during the growth. After growth, a portion of the material was subjected to a dislocation decoration etch in order to count the etch pit density (EPD) which has a direct correspondence with the dislocation density in the crystal. The crystalline quality was also characterized by x-ray diffraction rocking curves and photoluminescence. The in situ TCA resulted in almost a two order of magnitude reduction in the dislocation density, and factor of two reduction in the full width at half maximum of the x-ray rocking curves. Photoluminescence also suggested a decrease in the number of dislocations present in the material. This decrease is attributed to the movement of the dislocations during the annealing cycles and their subsequent interaction and annihilation. To decrease the dislocation density in HgCdTe layers grown on CdTe/Si composite substrates, ex situ TCA has been performed in a sealed quartz ampoule under a mercury overpressure in a conventional clam-shell furnace. The reduction in the dislocation density has been studied as a function of growth/annealing parameters such as the initial (as grown) dislocation density, buffer layer quality, Hg overpressure, annealing temperature, annealing duration, and the number of annealing cycles. It was found that the primary parameters that affect dislocation density reduction are the annealing temperature and the number of annealing cycles. Some secondary affects were observed by varying the duration spent at the maximum annealing temperature. Parameters such as the initial dislocation density and buffer layer quality did not play a significant role in dislocation reduction. Though no correlation between Hg overpressure and dislocation density was found, it did play a vital role in maintaining the quality of the surface. By using the ex situ TCA, a dislocation density of 1 x 106 cm-2 could be reliably and consistently achieved in HgCdTe layers that had a starting density ranging from 0.5 -- 3 x 107 cm-2. Examination of the annealing parameters revealed an exponential decay in the dislocation density as a function of increasing number of annealing cycles. In addition, a similar exponential decay was observed between the dislocation density and the annealing temperature. The decrease in the dislocation density is once again attributed to moving dislocations that interact and annihilate. This behavior was modeled using a second order reaction equation. It was found that the results of the model closely agreed with the experimental values for a wide range of annealing temperatures and number of annealing cycles.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Cheng-Cheng; Wang, Ru-Zhi, E-mail: wrz@bjut.edu.cn; Zhu, Man-Kang

GaN nanofilms (NFs) with different structures are grown on SiC substrates by pulsed laser deposition under different conditions. The synthesized GaN NFs are studied by X-ray diffraction, field-emission (FE) scanning electron microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. The GaN NFs are composed of diversified GaN nanoparticles with a diameter of 9–38 nm, thickness of 10–50 nm, and roughness of 0.22–13.03 nm. FE from the GaN NFs is structure dependent, which is explained by stress changing the band gap of the NFs. By structure modulation, the turn-on field of GaN NFs can be as low as 0.66 V/μm at a current density ofmore » 1 μA/cm{sup 2}, with a current density of up to 1.1 mA/cm{sup 2} at a field of 4.18 V/μm. Fowler-Nordheim curves of some samples contain multiple straight lines, which originate from the structural change and diversification of GaN nanoparticles under an applied field. Overall, our results suggest that GaN NFs with excellent FE properties can be prepared on SiC substrates, which provides a new route to fabricate high-efficiency FE nanodevices.« less

Prediction of dislocation generation during Bridgman growth of GaAs crystals

NASA Technical Reports Server (NTRS)

Tsai, C. T.; Yao, M. W.; Chait, Arnon

1992-01-01

Dislocation densities are generated in GaAs single crystals due to the excessive thermal stresses induced by temperature variations during growth. A viscoplastic material model for GaAs, which takes into account the movement and multiplication of dislocations in the plastic deformation, is developed according to Haasen's theory. The dislocation density is expressed as an internal state variable in this dynamic viscoplastic model. The deformation process is a nonlinear function of stress, strain rate, dislocation density and temperature. The dislocation density in the GaAs crystal during vertical Bridgman growth is calculated using a nonlinear finite element model. The dislocation multiplication in GaAs crystals for several temperature fields obtained from thermal modeling of both the GTE GaAs experimental data and artificially designed data are investigated.

Prediction of dislocation generation during Bridgman growth of GaAs crystals

NASA Astrophysics Data System (ADS)

Tsai, C. T.; Yao, M. W.; Chait, Arnon

1992-11-01

Dislocation densities are generated in GaAs single crystals due to the excessive thermal stresses induced by temperature variations during growth. A viscoplastic material model for GaAs, which takes into account the movement and multiplication of dislocations in the plastic deformation, is developed according to Haasen's theory. The dislocation density is expressed as an internal state variable in this dynamic viscoplastic model. The deformation process is a nonlinear function of stress, strain rate, dislocation density and temperature. The dislocation density in the GaAs crystal during vertical Bridgman growth is calculated using a nonlinear finite element model. The dislocation multiplication in GaAs crystals for several temperature fields obtained from thermal modeling of both the GTE GaAs experimental data and artificially designed data are investigated.

Oxygen adsorption and incorporation at irradiated GaN(0001) and GaN(0001¯) surfaces: First-principles density-functional calculations

NASA Astrophysics Data System (ADS)

Sun, Qiang; Selloni, Annabella; Myers, T. H.; Doolittle, W. Alan

2006-11-01

Density functional theory calculations of oxygen adsorption and incorporation at the polar GaN(0001) and GaN(0001¯) surfaces have been carried out to explain the experimentally observed reduced oxygen concentration in GaN samples grown by molecular beam epitaxy in the presence of high energy (˜10keV) electron beam irradiation [Myers , J. Vac. Sci. Technol. B 18, 2295 (2000)]. Using a model in which the effect of the irradiation is to excite electrons from the valence to the conduction band, we find that both the energy cost of incorporating oxygen impurities in deeper layers and the oxygen adatom diffusion barriers are significantly reduced in the presence of the excitation. The latter effect leads to a higher probability for two O adatoms to recombine and desorb, and thus to a reduced oxygen concentration in the irradiated samples, consistent with experimental observations.

Photoluminescence Study of Gallium Nitride Thin Films Obtained by Infrared Close Space Vapor Transport.

PubMed

Santana, Guillermo; de Melo, Osvaldo; Aguilar-Hernández, Jorge; Mendoza-Pérez, Rogelio; Monroy, B Marel; Escamilla-Esquivel, Adolfo; López-López, Máximo; de Moure, Francisco; Hernández, Luis A; Contreras-Puente, Gerardo

2013-03-15

Photoluminescence (PL) studies in GaN thin films grown by infrared close space vapor transport (CSVT-IR) in vacuum are presented in this work. The growth of GaN thin films was done on a variety of substrates like silicon, sapphire and fused silica. Room temperature PL spectra of all the GaN films show near band-edge emission (NBE) and a broad blue and green luminescence (BL, GL), which can be seen with the naked eye in a bright room. The sample grown by infrared CSVT on the silicon substrate shows several emission peaks from 2.4 to 3.22 eV with a pronounced red shift with respect to the band gap energy. The sample grown on sapphire shows strong and broad ultraviolet emission peaks (UVL) centered at 3.19 eV and it exhibits a red shift of NBE. The PL spectrum of GaN films deposited on fused silica exhibited a unique and strong blue-green emission peak centered at 2.38 eV. The presence of yellow and green luminescence in all samples is related to native defects in the structure such as dislocations in GaN and/or the presence of amorphous phases. We analyze the material quality that can be obtained by CSVT-IR in vacuum, which is a high yield technique with simple equipment set-up, in terms of the PL results obtained in each case.

Photoluminescence Study of Gallium Nitride Thin Films Obtained by Infrared Close Space Vapor Transport

PubMed Central

Santana, Guillermo; de Melo, Osvaldo; Aguilar-Hernández, Jorge; Mendoza-Pérez, Rogelio; Monroy, B. Marel; Escamilla-Esquivel, Adolfo; López-López, Máximo; de Moure, Francisco; Hernández, Luis A.; Contreras-Puente, Gerardo

2013-01-01

Photoluminescence (PL) studies in GaN thin films grown by infrared close space vapor transport (CSVT-IR) in vacuum are presented in this work. The growth of GaN thin films was done on a variety of substrates like silicon, sapphire and fused silica. Room temperature PL spectra of all the GaN films show near band-edge emission (NBE) and a broad blue and green luminescence (BL, GL), which can be seen with the naked eye in a bright room. The sample grown by infrared CSVT on the silicon substrate shows several emission peaks from 2.4 to 3.22 eV with a pronounced red shift with respect to the band gap energy. The sample grown on sapphire shows strong and broad ultraviolet emission peaks (UVL) centered at 3.19 eV and it exhibits a red shift of NBE. The PL spectrum of GaN films deposited on fused silica exhibited a unique and strong blue-green emission peak centered at 2.38 eV. The presence of yellow and green luminescence in all samples is related to native defects in the structure such as dislocations in GaN and/or the presence of amorphous phases. We analyze the material quality that can be obtained by CSVT-IR in vacuum, which is a high yield technique with simple equipment set-up, in terms of the PL results obtained in each case. PMID:28809356

AB INITIO Investigations of the Magnetism in Diluted Magnetic Semiconductor Fe-DOPED GaN

NASA Astrophysics Data System (ADS)

Cheng, Jie; Zhou, Jing; Xu, Wei; Dong, Peng

2014-01-01

In this paper, we present a first principle investigation on Fe-doped GaN with wurtzite and zinc-blend structure using full potential density functional calculations. Data point out that the magnetic behavior of Fe-doped GaN system is strongly dependent on Fe doping configurations. In agreement with the experimental reports, and independently by doping, antiferromagnetism occurs in the zinc-blend structure, while in the wurtzite structure ferromagnetism depends on the Fe doping configurations. Detailed analyses combined with density of state calculations support the assignment that the ferromagnetism is closely related to the impurity band at the origin of the hybridization of Fe 3d and N 2p states in the Fe-doped GaN of wurtzite phase.

Effects of doping impurity and growth orientation on dislocation generation in GaAs crystals grown from the melt: A qualitative finite-element study

NASA Astrophysics Data System (ADS)

Zhu, X. A.; Tsai, C. T.

2000-09-01

Dislocations in gallium arsenide (GaAs) crystals are generated by excessive thermal stresses induced during the crystal growth process. The presence of dislocations has adverse effects on the performance and reliability of the GaAs-based devices. It is well known that dislocation density can be significantly reduced by doping impurity atoms into a GaAs crystal during its growth process. A viscoplastic constitutive equation that couples the microscopic dislocation density with the macroscopic plastic deformation is employed in a crystallographic finite element model for calculating the dislocation density generated in the GaAs crystal during its growth process. The dislocation density is considered as an internal state variable and the drag stress caused by doping impurity is included in this constitutive equation. A GaAs crystal grown by the vertical Bridgman process is adopted as an example to study the influences of doping impurity and growth orientation on dislocation generation. The calculated results show that doping impurity can significantly reduce the dislocation density generated in the crystal. The level of reduction is also influenced by the growth orientation during the crystal growth process.

Model for threading dislocations in metamorphic tandem solar cells on GaAs (001) substrates

NASA Astrophysics Data System (ADS)

Song, Yifei; Kujofsa, Tedi; Ayers, John E.

2018-02-01

We present an approximate model for the threading dislocations in III-V heterostructures and have applied this model to study the defect behavior in metamorphic triple-junction solar cells. This model represents a new approach in which the coefficient for second-order threading dislocation annihilation and coalescence reactions is considered to be determined by the length of misfit dislocations, LMD, in the structure, and we therefore refer to it as the LMD model. On the basis of this model we have compared the average threading dislocation densities in the active layers of triple junction solar cells using linearly-graded buffers of varying thicknesses as well as S-graded (complementary error function) buffers with varying thicknesses and standard deviation parameters. We have shown that the threading dislocation densities in the active regions of metamorphic tandem solar cells depend not only on the thicknesses of the buffer layers but on their compositional grading profiles. The use of S-graded buffer layers instead of linear buffers resulted in lower threading dislocation densities. Moreover, the threading dislocation densities depended strongly on the standard deviation parameters used in the S-graded buffers, with smaller values providing lower threading dislocation densities.

Hydrogen incorporation in high hole density GaN:Mg

NASA Astrophysics Data System (ADS)

Zvanut, M. E.; Uprety, Y.; Dashdorj, J.; Moseley, M.; Doolittle, W. Alan

2011-03-01

We investigate hydrogen passivation in heavily doped p-type GaN using electron paramagnetic resonance (EPR) spectroscopy. Samples include both conventionally grown GaN (1019 cm-3 Mg, 1017 cm-3 holes) and films grown by metal modulation epitaxy (MME), which yielded higher Mg (1- 4 x 1020 cm-3) and hole (1- 40 x 1018 cm-3) densities than found in conventionally grown GaN. The Mg acceptor signal is monitored throughout 30 minute annealing steps in N2 :H2 (92%:7%)) and subsequently pure N2 . N2 :H2 heat treatments of the lower hole density films begin to reduce the Mg EPR intensity at 750 o C, but quench the signal in high hole density films at 600 o C. Revival of the signal by subsequent N2 annealing occurs at 800 o C for the low hole density material and 600 o C in MME GaN. The present work highlights chemical differences between heavily Mg doped and lower doped films; however, it is unclear whether the difference is due to changes in hydrogen-Mg complex formation or hydrogen diffusion. The work at UAB is supported by the NSF.

Ionic liquid gating on atomic layer deposition passivated GaN: Ultra-high electron density induced high drain current and low contact resistance

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhou, Hong; Du, Yuchen; Ye, Peide D., E-mail: yep@purdue.edu

2016-05-16

Herein, we report on achieving ultra-high electron density (exceeding 10{sup 14 }cm{sup −2}) in a GaN bulk material device by ionic liquid gating, through the application of atomic layer deposition (ALD) of Al{sub 2}O{sub 3} to passivate the GaN surface. Output characteristics demonstrate a maximum drain current of 1.47 A/mm, the highest reported among all bulk GaN field-effect transistors, with an on/off ratio of 10{sup 5} at room temperature. An ultra-high electron density exceeding 10{sup 14 }cm{sup −2} accumulated at the surface is confirmed via Hall-effect measurement and transfer length measurement. In addition to the ultra-high electron density, we also observe a reductionmore » of the contact resistance due to the narrowing of the Schottky barrier width on the contacts. Taking advantage of the ALD surface passivation and ionic liquid gating technique, this work provides a route to study the field-effect and carrier transport properties of conventional semiconductors in unprecedented ultra-high charge density regions.« less

Advanced processing of gallium nitride and gallium nitride-based devices: Ultra-high temperature annealing and implantation incorporation

NASA Astrophysics Data System (ADS)

Yu, Haijiang

This dissertation is focused on three fields: ultra-high temperature annealing of GaN, activation of implanted GaN and the implantation incorporation into AlGaN/GaN HEMT processing, with an aim to increase the performance, manufacturability and reliability of AlGaN/GaN HEMTs. First, the ultra high temperature (around 1500°C) annealing of MOCVD grown GaN on sapphire has been studied, and a thermally induced threading dislocation (TD) motion and reaction are reported. Using a rapid thermal annealing (RTA) approach capable of heating 2 inch wafers to around 1500°C with 100 bar N2 over-pressure, evidence of dislocation motion was first observed in transmission electron microscopy (TEM) micrographs of both planar and patterned GaN films protected by an AIN capping layer. An associated decrease in x-ray rocking curve (XRC) full-width-half-maximum (FWHM) was also observed for both the symmetric and asymmetric scans. After annealing, the AIN capping layer remained intact, and optical measurements showed no degradation of the opto-electronic properties of the films. Then activation annealing of Si implants in MOCVD grown GaN has been studied for use in ohmic contacts. Si was implanted in semi-insulating GaN at 100 keV with doses from 5 x 1014 cm-2 to 1.5 x 1016 cm-2. Rapid thermal annealing at 1500°C with 100 bar N2 over-pressure was used for dopant activation, resulting in a minimum sheet resistance of 13.9 O/square for a dose of 7 x 1015 cm-2. Secondary ion mass spectroscopy measurements showed a post-activation broadening of the dopant concentration peak by 20 nm (at half the maximum), while X-Ray triple axis o-2theta scans indicated nearly complete implant damage recovery. Transfer length method measurements of the resistance of Ti/Al/Ni/Au contacts to activated GaN:Si (5 x 1015 cm-2 at 100 keV) indicated lowest contact resistances of 0.07 Omm and 0.02 Omm for as-deposited and subsequently annealed contacts, respectively. Finally, the incorporation of Si implantation into AlGaN/GaN high electron mobility transistor processing has been first demonstrated. An ultra-high temperature (1500°C) rapid thermal annealing technique was developed for the activation of Si dopants implanted in the source and drain. In comparison to control devices processed by conventional fabrication, the implanted device with nonalloyed ohmic contact showed comparable device performance with a contact resistance of 0.4 Omm Imax 730 mA/mm ft/f max; 26/62 GHz and power 3.4 W/mm on sapphire. These early results demonstrate the feasibility of implantation incorporation into GaN based device processing as well as the potential to increase yield, reproducibility and reliability in AlGaN/GaN HEMTs.

Optical waveguiding properties into porous gallium nitride structures investigated by prism coupling technique

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alshehri, Bandar; Dogheche, Elhadj, E-mail: elhadj.dogheche@univ-valenciennes.fr; Lee, Seung-Min

2014-08-04

In order to modulate the refractive index and the birefringence of Gallium Nitride (GaN), we have developed a chemical etching method to perform porous structures. The aim of this research is to demonstrate that optical properties of GaN can be tuned by controlling the pores density. GaN films are prepared on sapphire by metal organic chemical vapor deposition and the microstructure is characterized by transmission electron microscopy, and scanning electron microscope analysis. Optical waveguide experiment is demonstrated here to determine the key properties as the ordinary (n{sub 0}) and extraordinary (n{sub e}) refractive indices of etched structures. We report heremore » the dispersion of refractive index for porous GaN and compare it to the bulk material. We observe that the refractive index decreases when the porous density p is increased: results obtained at 0.975 μm have shown that the ordinary index n{sub 0} is 2.293 for a bulk layer and n{sub 0} is 2.285 for a pores density of 20%. This value corresponds to GaN layer with a pore size of 30 nm and inter-distance of 100 nm. The control of the refractive index into GaN is therefore fundamental for the design of active and passive optical devices.« less

Characterization of GaN nanowires grown on PSi, PZnO and PGaN on Si (111) substrates by thermal evaporation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shekari, Leila; Hassan, Haslan Abu; Thahab, Sabah M.

2012-06-20

In this research, we used an easy and inexpensive method to synthesize highly crystalline GaN nanowires (NWs); on different substrates such as porous silicon (PSi), porous zinc oxide (PZnO) and porous gallium nitride (PGaN) on Si (111) wafer by thermal evaporation using commercial GaN powder without any catalyst. Micro structural studies by scanning electron microscopy and transmission electron microscope measurements reveal the role of different substrates in the morphology, nucleation and alignment of the GaN nanowires. The degree of alignment of the synthesized nanowires does not depend on the lattice mismatch between wires and their substrates. Further structural and opticalmore » characterizations were performed using high resolution X-ray diffraction and energy-dispersive X-ray spectroscopy. Results indicate that the nanowires are of single-crystal hexagonal GaN. The quality and density of grown GaN nanowires for different substrates are highly dependent on the lattice mismatch between the nanowires and their substrates and also on the size of the porosity of the substrates. Nanowires grown on PGaN have the best quality and highest density as compared to nanowires on other substrates. By using three kinds of porous substrates, we are able to study the increase in the alignment and density of the nanowires.« less

Dislocation density evolution of AA 7020-T6 investigated by in-situ synchrotron diffraction under tensile load

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhong, Z.Y., E-mail: zhengye.zhong@hzg.de; Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, D-21502 Geesthacht; Brokmeier, H.-G.

2015-10-15

The dislocation density evolution along the loading axis of a textured AA 7020-T6 aluminum alloy during uniaxial tension was investigated by in-situ synchrotron diffraction. The highly parallel synchrotron beam at the High Energy Materials Science beamline P07 in PETRA III, DESY, offers excellent conditions to separate different influences for line broadening from which micro-strains are obtained using the modified Williamson–Hall method which is also for defect density investigations. During tensile loading the dislocation density evolution was documented from the as-received material (initial micro-strain state) to the relaxation of the strains during elastic deformation. After yield, the increasing rate of dislocationmore » density growth was relatively fast till half-way between yield and UTS. After that, the rate started to decrease and the dislocation density fluctuated as the elongation increased due to the generation and annihilation of dislocations. When dislocation generation is dominant, the correlation between the flow stress and dislocation density satisfies the Taylor equation. Besides, a method to correct the thickness effect on peak broadening is developed in the present study. - Highlights: • In-situ synchrotron diffraction was applied to characterize peak broadening. • Dislocation evolution along the loading axis during uniaxial tension was investigated. • A method to correct the sample thickness effect on peak broadening was developed. • Dislocation density and flow stress satisfy the Taylor equation at a certain range. • The texture before load and after sample fracture was analyzed.« less

Collaborative Research and Development (CR&D). Task Order 0036: Physical and Chemical Processes of Operating Electronic Devices

DTIC Science & Technology

2006-09-01

actually seen. A. Hierro , … S. A. Ringel et al., Phys. Stat. Sol (b) 228, 937 (2001). Ohio State U. Use DLTS and DLOS (Deep Level Optical Spectroscopy...to threading dislocations. Also see A. Hierro et al., APL 76, 3064 (2000), where traps at EC-ET=0.58-0.62, 1.35, 2.57-2.64, 3.22eV are seen in GaN

Silicon Based Colloidal Quantum Dot and Nanotube Lasers

DTIC Science & Technology

2013-03-01

carrier density is theoretically and experimentally derived to be inversely proportional to the diameter; (b) demonstration of InGaN/ GaN light emitting...diodes and GaN single nanowire photonic crystal laser on silicon characterized by a lasing transition at λ=371.3 nm with a linewidth of 0.55 nm. The...derived to be inversely proportional to the diameter; (b) demonstration of InGaN/ GaN light emitting diodes and GaN single nanowire photonic crystal

Determination of carrier diffusion length in GaN

NASA Astrophysics Data System (ADS)

Hafiz, Shopan; Zhang, Fan; Monavarian, Morteza; Avrutin, Vitaliy; Morkoç, Hadis; Özgür, Ümit; Metzner, Sebastian; Bertram, Frank; Christen, Jürgen; Gil, Bernard

2015-01-01

Diffusion lengths of photo-excited carriers along the c-direction were determined from photoluminescence (PL) and cross-sectional cathodoluminescence (CL) measurements in p- and n-type GaN epitaxial layers grown on c-plane sapphire by metal-organic chemical vapor deposition. The investigated samples incorporate a 6 nm thick In0.15Ga0.85N active layer capped with either 500 nm p-GaN or 1500 nm n-GaN. The top GaN layers were etched in steps and PL from the InGaN active region and the underlying layers was monitored as a function of the top GaN thickness upon photo-generation near the surface region by above bandgap excitation. Taking into consideration the absorption in the top GaN layer as well as active and underlying layers, the diffusion lengths at 295 K and at 15 K were measured to be 93 ± 7 nm and 70 ± 7 nm for Mg-doped p-type GaN and 432 ± 30 nm and 316 ± 30 nm for unintentionally doped n-type GaN, respectively, at photogenerated carrier densities of 4.2 × 1018 cm-3 using PL spectroscopy. CL measurements of the unintentionally doped n-type GaN layer at much lower carrier densities of 1017 cm-3 revealed a longer diffusion length of 525 ± 11 nm at 6 K.

Density of dislocations in CdHgTe heteroepitaxial structures on GaAs(013) and Si(013) substrates

NASA Astrophysics Data System (ADS)

Sidorov, Yu. G.; Yakushev, M. V.; Varavin, V. S.; Kolesnikov, A. V.; Trukhanov, E. M.; Sabinina, I. V.; Loshkarev, I. D.

2015-11-01

Epitaxial layers of Cd x Hg1- x Te (MCT) on GaAs(013) and Si(013) substrates were grown by molecular beam epitaxy. The introduction of ZnTe and CdTe intermediate layers into the structures made it possible to retain the orientation close to that of the substrate in MCT epitaxial layers despite the large mismatch between the lattice parameters. The structures were investigated using X-ray diffraction and transmission electron microscopy. The dislocation families predominantly removing the mismatch between the lattice parameters were found. Transmission electron microscopy revealed Γ-shaped misfit dislocations (MDs), which facilitated the annihilation of threading dislocations. The angles of rotation of the lattice due to the formation of networks of misfit dislocations were measured. It was shown that the density of threading dislocations in the active region of photodiodes is primarily determined by the network of misfit dislocations formed in the MCT/CdTe heterojunction. A decrease in the density of threading dislocations in the MCT film was achieved by cyclic annealing under conditions of the maximally facilitated nonconservative motion of dislocations. The dislocation density was determined from the etch pits.

Modeling of dislocation dynamics in germanium Czochralski growth

NASA Astrophysics Data System (ADS)

Artemyev, V. V.; Smirnov, A. D.; Kalaev, V. V.; Mamedov, V. M.; Sidko, A. P.; Podkopaev, O. I.; Kravtsova, E. D.; Shimansky, A. F.

2017-06-01

Obtaining very high-purity germanium crystals with low dislocation density is a practically difficult problem, which requires knowledge and experience in growth processes. Dislocation density is one of the most important parameters defining the quality of germanium crystal. In this paper, we have performed experimental study of dislocation density during 4-in. germanium crystal growth using the Czochralski method and comprehensive unsteady modeling of the same crystal growth processes, taking into account global heat transfer, melt flow and melt/crystal interface shape evolution. Thermal stresses in the crystal and their relaxation with generation of dislocations within the Alexander-Haasen model have been calculated simultaneously with crystallization dynamics. Comparison to experimental data showed reasonable agreement for the temperature, interface shape and dislocation density in the crystal between calculation and experiment.

Initial leakage current paths in the vertical-type GaN-on-GaN Schottky barrier diodes

NASA Astrophysics Data System (ADS)

Sang, Liwen; Ren, Bing; Sumiya, Masatomo; Liao, Meiyong; Koide, Yasuo; Tanaka, Atsushi; Cho, Yujin; Harada, Yoshitomo; Nabatame, Toshihide; Sekiguchi, Takashi; Usami, Shigeyoshi; Honda, Yoshio; Amano, Hiroshi

2017-09-01

Electrical characteristics of leakage current paths in vertical-type n-GaN Schottky barrier diodes (SBDs) on free-standing GaN substrates are investigated by using photon emission microscopy (PEM). The PEM mapping shows that the initial failure of the SBD devices at low voltages is due to the leakage current paths from polygonal pits in the GaN epilayers. It is observed that these polygonal pits originate from carbon impurity accumulation to the dislocations with a screw-type component by microstructure analysis. For the SBD without polygonal pits, no initial failure is observed and the first leakage appeals at the edge of electrodes as a result of electric field concentration. The mechanism of leakage at pits is explained in terms of trap assisted tunneling through fitting current-voltage characteristics.

Mechanical, Thermodynamic and Electronic Properties of Wurtzite and Zinc-Blende GaN Crystals.

PubMed

Qin, Hongbo; Luan, Xinghe; Feng, Chuang; Yang, Daoguo; Zhang, Guoqi

2017-12-12

For the limitation of experimental methods in crystal characterization, in this study, the mechanical, thermodynamic and electronic properties of wurtzite and zinc-blende GaN crystals were investigated by first-principles calculations based on density functional theory. Firstly, bulk moduli, shear moduli, elastic moduli and Poisson's ratios of the two GaN polycrystals were calculated using Voigt and Hill approximations, and the results show wurtzite GaN has larger shear and elastic moduli and exhibits more obvious brittleness. Moreover, both wurtzite and zinc-blende GaN monocrystals present obvious mechanical anisotropic behavior. For wurtzite GaN monocrystal, the maximum and minimum elastic moduli are located at orientations [001] and <111>, respectively, while they are in the orientations <111> and <100> for zinc-blende GaN monocrystal, respectively. Compared to the elastic modulus, the shear moduli of the two GaN monocrystals have completely opposite direction dependences. However, different from elastic and shear moduli, the bulk moduli of the two monocrystals are nearly isotropic, especially for the zinc-blende GaN. Besides, in the wurtzite GaN, Poisson's ratios at the planes containing [001] axis are anisotropic, and the maximum value is 0.31 which is located at the directions vertical to [001] axis. For zinc-blende GaN, Poisson's ratios at planes (100) and (111) are isotropic, while the Poisson's ratio at plane (110) exhibits dramatically anisotropic phenomenon. Additionally, the calculated Debye temperatures of wurtzite and zinc-blende GaN are 641.8 and 620.2 K, respectively. At 300 K, the calculated heat capacities of wurtzite and zinc-blende are 33.6 and 33.5 J mol -1 K -1 , respectively. Finally, the band gap is located at the G point for the two crystals, and the band gaps of wurtzite and zinc-blende GaN are 3.62 eV and 3.06 eV, respectively. At the G point, the lowest energy of conduction band in the wurtzite GaN is larger, resulting in a wider band gap. Densities of states in the orbital hybridization between Ga and N atoms of wurtzite GaN are much higher, indicating more electrons participate in forming Ga-N ionic bonds in the wurtzite GaN.

Mechanical, Thermodynamic and Electronic Properties of Wurtzite and Zinc-Blende GaN Crystals

PubMed Central

Luan, Xinghe; Feng, Chuang; Yang, Daoguo; Zhang, Guoqi

2017-01-01

For the limitation of experimental methods in crystal characterization, in this study, the mechanical, thermodynamic and electronic properties of wurtzite and zinc-blende GaN crystals were investigated by first-principles calculations based on density functional theory. Firstly, bulk moduli, shear moduli, elastic moduli and Poisson’s ratios of the two GaN polycrystals were calculated using Voigt and Hill approximations, and the results show wurtzite GaN has larger shear and elastic moduli and exhibits more obvious brittleness. Moreover, both wurtzite and zinc-blende GaN monocrystals present obvious mechanical anisotropic behavior. For wurtzite GaN monocrystal, the maximum and minimum elastic moduli are located at orientations [001] and <111>, respectively, while they are in the orientations <111> and <100> for zinc-blende GaN monocrystal, respectively. Compared to the elastic modulus, the shear moduli of the two GaN monocrystals have completely opposite direction dependences. However, different from elastic and shear moduli, the bulk moduli of the two monocrystals are nearly isotropic, especially for the zinc-blende GaN. Besides, in the wurtzite GaN, Poisson’s ratios at the planes containing [001] axis are anisotropic, and the maximum value is 0.31 which is located at the directions vertical to [001] axis. For zinc-blende GaN, Poisson’s ratios at planes (100) and (111) are isotropic, while the Poisson’s ratio at plane (110) exhibits dramatically anisotropic phenomenon. Additionally, the calculated Debye temperatures of wurtzite and zinc-blende GaN are 641.8 and 620.2 K, respectively. At 300 K, the calculated heat capacities of wurtzite and zinc-blende are 33.6 and 33.5 J mol−1 K−1, respectively. Finally, the band gap is located at the G point for the two crystals, and the band gaps of wurtzite and zinc-blende GaN are 3.62 eV and 3.06 eV, respectively. At the G point, the lowest energy of conduction band in the wurtzite GaN is larger, resulting in a wider band gap. Densities of states in the orbital hybridization between Ga and N atoms of wurtzite GaN are much higher, indicating more electrons participate in forming Ga-N ionic bonds in the wurtzite GaN. PMID:29231902

Probing defect states in polycrystalline GaN grown on Si(111) by sub-bandgap laser-excited scanning tunneling spectroscopy

NASA Astrophysics Data System (ADS)

Hsiao, F.-M.; Schnedler, M.; Portz, V.; Huang, Y.-C.; Huang, B.-C.; Shih, M.-C.; Chang, C.-W.; Tu, L.-W.; Eisele, H.; Dunin-Borkowski, R. E.; Ebert, Ph.; Chiu, Y.-P.

2017-01-01

We demonstrate the potential of sub-bandgap laser-excited cross-sectional scanning tunneling microscopy and spectroscopy to investigate the presence of defect states in semiconductors. The characterization method is illustrated on GaN layers grown on Si(111) substrates without intentional buffer layers. According to high-resolution transmission electron microscopy and cathodoluminescence spectroscopy, the GaN layers consist of nanoscale wurtzite and zincblende crystallites with varying crystal orientations and hence contain high defect state densities. In order to discriminate between band-to-band excitation and defect state excitations, we use sub-bandgap laser excitation. We probe a clear increase in the tunnel current at positive sample voltages during sub-bandgap laser illumination for the GaN layer with high defect density, but no effect is found for high quality GaN epitaxial layers. This demonstrates the excitation of free charge carriers at defect states. Thus, sub-bandgap laser-excited scanning tunneling spectroscopy is a powerful complimentary characterization tool for defect states.

RBS/Channeling Studies of Swift Heavy Ion Irradiated GaN Layers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sathish, N.; Dhamodaran, S.; Pathak, A. P.

2009-03-10

Epitaxial GaN layers grown by MOCVD on c-plane sapphire substrates were irradiated with 150 MeV Ag ions at a fluence of 5x10{sup 12} ions/cm{sup 2}. Samples used in this study are 2 {mu}m thick GaN layers, with and without a thin AlN cap-layer. Energy dependent RBS/Channeling measurements have been carried out on both irradiated and unirradiated samples for defects characterization. Observed results are compared and correlated with previous HRXRD, AFM and optical studies. The {chi}{sub min} values for unirradiated samples show very high value and the calculated defect densities are of the order of 10{sup 10} cm{sup -2} as expectedmore » in these samples. Effects of irradiation on these samples are different as initial samples had different defect densities. Epitaxial reconstruction of GaN buffer layer has been attributed to the observed changes, which are generally grown to reduce the strain between GaN and Sapphire.« less

Size effects under homogeneous deformation of single crystals: A discrete dislocation analysis

NASA Astrophysics Data System (ADS)

Guruprasad, P. J.; Benzerga, A. A.

Mechanism-based discrete dislocation plasticity is used to investigate the effect of size on micron scale crystal plasticity under conditions of macroscopically homogeneous deformation. Long-range interactions among dislocations are naturally incorporated through elasticity. Constitutive rules are used which account for key short-range dislocation interactions. These include junction formation and dynamic source and obstacle creation. Two-dimensional calculations are carried out which can handle high dislocation densities and large strains up to 0.1. The focus is laid on the effect of dimensional constraints on plastic flow and hardening processes. Specimen dimensions ranging from hundreds of nanometers to tens of microns are considered. Our findings show a strong size-dependence of flow strength and work-hardening rate at the micron scale. Taylor-like hardening is shown to be insufficient as a rationale for the flow stress scaling with specimen dimensions. The predicted size effect is associated with the emergence, at sufficient resolution, of a signed dislocation density. Heuristic correlations between macroscopic flow stress and macroscopic measures of dislocation density are sought. Most accurate among those is a correlation based on two state variables: the total dislocation density and an effective, scale-dependent measure of signed density.

Evolution of Dislocation Density During Tensile Deformation of BH220 Steel at Different Pre-strain Conditions

NASA Astrophysics Data System (ADS)

Seth, Prem Prakash; Das, A.; Bar, H. N.; Sivaprasad, S.; Basu, A.; Dutta, K.

2015-07-01

Tensile behavior of BH220 steel with different pre-strain conditions (2 and 8%) followed by bake hardening was studied at different strain rates (0.001 and 0.1/s). Dislocation densities of the deformed specimens were successfully estimated from x-ray diffraction profile analysis using the modified Williamson-Hall equation. The results indicate that other than 2% pre-strain the dislocation density increases with increase in pre-strain level as well as with strain rate. The decrease in the dislocation density in 2% pre-strain condition without any drop in strength value is attributed to the characteristic dislocation feature formed during pre-straining.

Threading Dislocations in InGaAs/GaAs (001) Buffer Layers for Metamorphic High Electron Mobility Transistors

NASA Astrophysics Data System (ADS)

Song, Yifei; Kujofsa, Tedi; Ayers, John E.

2018-07-01

In order to evaluate various buffer layers for metamorphic devices, threading dislocation densities have been calculated for uniform composition In x Ga1- x As device layers deposited on GaAs (001) substrates with an intermediate graded buffer layer using the L MD model, where L MD is the average length of misfit dislocations. On this basis, we compare the relative effectiveness of buffer layers with linear, exponential, and S-graded compositional profiles. In the case of a 2 μm thick buffer layer linear grading results in higher threading dislocation densities in the device layer compared to either exponential or S-grading. When exponential grading is used, lower threading dislocation densities are obtained with a smaller length constant. In the S-graded case, lower threading dislocation densities result when a smaller standard deviation parameter is used. As the buffer layer thickness is decreased from 2 μm to 0.1 μm all of the above effects are diminished, and the absolute threading dislocation densities increase.

Structural and optical properties of II-VI and III-V compound semiconductors

NASA Astrophysics Data System (ADS)

Huang, Jingyi

This dissertation is on the study of structural and optical properties of some III-V and II-VI compound semiconductors. The first part of this dissertation is a study of the deformation mechanisms associated with nanoindentation and nanoscratching of InP, GaN, and ZnO crystals. The second part is an investigation of some fundamental issues regarding compositional fluctuations and microstructure in GaInNAs and InAlN alloys. In the first part, the microstructure of (001) InP scratched in an atomic force microscope with a small diamond tip has been studied as a function of applied normal force and crystalline direction in order to understand at the nanometer scale the deformation mechanisms in the zinc-blende structure. TEM images show deeper dislocation propagation for scratches along <110> compared to <100>. High strain fields were observed in <100> scratches, indicating hardening due to locking of dislocations gliding on different slip planes. Reverse plastic flow have been observed in <110> scratches in the form of pop-up events that result from recovery of stored elastic strain. In a separate study, nanoindentation-induced plastic deformation has been studied in c-, a-, and m-plane ZnO single crystals and c-plane GaN respectively, to study the deformation mechanism in wurtzite hexagonal structures. TEM results reveal that the prime deformation mechanism is slip on basal planes and in some cases, on pyramidal planes, and strain built up along particular directions. No evidence of phase transformation or cracking was observed in both materials. CL imaging reveals quenching of near band-edge emission by dislocations. In the second part, compositional inhomogeneity in quaternary GaInNAs and ternary InAlN alloys has been studied using TEM. It is shown that exposure to antimony during growth of GaInNAs results in uniform chemical composition in the epilayer, as antimony suppresses the surface mobility of adatoms that otherwise leads to two-dimensional growth and elemental segregation. In a separate study, compositional instability is observed in lattice-matched InAlN films grown on GaN, for growth beyond a certain thickness. Beyond 200 nm of thickness, two sub-layers with different indium content are observed, the top one with lower indium content.

Multiphysical simulation analysis of the dislocation structure in germanium single crystals

NASA Astrophysics Data System (ADS)

Podkopaev, O. I.; Artemyev, V. V.; Smirnov, A. D.; Mamedov, V. M.; Sid'ko, A. P.; Kalaev, V. V.; Kravtsova, E. D.; Shimanskii, A. F.

2016-09-01

To grow high-quality germanium crystals is one of the most important problems of growth industry. The dislocation density is an important parameter of the quality of single crystals. The dislocation densities in germanium crystals 100 mm in diameter, which have various shapes of the side surface and are grown by the Czochralski technique, are experimentally measured. The crystal growth is numerically simulated using heat-transfer and hydrodynamics models and the Alexander-Haasen dislocation model in terms of the CGSim software package. A comparison of the experimental and calculated dislocation densities shows that the dislocation model can be applied to study lattice defects in germanium crystals and to improve their quality.

Determination of dislocation density by electron backscatter diffraction and X-ray line profile analysis in ferrous lath martensite

DOE Office of Scientific and Technical Information (OSTI.GOV)

Berecz, Tibor, E-mail: berecz@eik.bme.hu; Jenei, Péter, E-mail: jenei@metal.elte.hu; Csóré, András, E-mail: csorean@gmail.com

2016-03-15

The microstructure and the dislocation density in as-quenched ferrous lath martensite were studied by different methods. The blocks, packets and variants formed due to martensitic transformation were identified and their sizes were determined by electron backscatter diffraction (EBSD). Concomitant transmission electron microscopy (TEM) investigation revealed that the laths contain subgrains with the size between 50 and 100 nm. A novel evaluation procedure of EBSD images was elaborated for the determination of the density and the space distribution of geometrically necessary dislocations from the misorientation distribution. The total dislocation density obtained by X-ray diffraction line profile analysis was in good agreementmore » with the value determined by EBSD, indicating that the majority of dislocations formed due to martensitic transformation during quenching are geometrically necessary dislocations.« less

Work Hardening, Dislocation Structure, and Load Partitioning in Lath Martensite Determined by In Situ Neutron Diffraction Line Profile Analysis

NASA Astrophysics Data System (ADS)

Harjo, Stefanus; Kawasaki, Takuro; Tomota, Yo; Gong, Wu; Aizawa, Kazuya; Tichy, Geza; Shi, Zengmin; Ungár, Tamas

2017-09-01

A lath martensite steel containing 0.22 mass pct carbon was analyzed in situ during tensile deformation by high-resolution time-of-flight neutron diffraction to clarify the large work-hardening behavior at the beginning of plastic deformation. The diffraction peaks in plastically deformed states exhibit asymmetries as the reflection of redistributions of the stress and dislocation densities/arrangements in two lath packets: soft packet, where the dislocation glides are favorable, and hard packet, where they are unfavorable. The dislocation density was as high as 1015 m-2 in the as-heat-treated state. During tensile straining, the load and dislocation density became different between the two lath packets. The dislocation character and arrangement varied in the hard packet but hardly changed in the soft packet. In the hard packet, dislocations that were mainly screw-type in the as-heat-treated state became primarily edge-type and rearranged towards a dipole character related to constructing cell walls. The hard packet played an important role in the work hardening in martensite, which could be understood by considering the increase in dislocation density along with the change in dislocation arrangement.

A statistical analysis of the elastic distortion and dislocation density fields in deformed crystals

DOE PAGES

Mohamed, Mamdouh S.; Larson, Bennett C.; Tischler, Jonathan Z.; ...

2015-05-18

The statistical properties of the elastic distortion fields of dislocations in deforming crystals are investigated using the method of discrete dislocation dynamics to simulate dislocation structures and dislocation density evolution under tensile loading. Probability distribution functions (PDF) and pair correlation functions (PCF) of the simulated internal elastic strains and lattice rotations are generated for tensile strain levels up to 0.85%. The PDFs of simulated lattice rotation are compared with sub-micrometer resolution three-dimensional X-ray microscopy measurements of rotation magnitudes and deformation length scales in 1.0% and 2.3% compression strained Cu single crystals to explore the linkage between experiment and the theoreticalmore » analysis. The statistical properties of the deformation simulations are analyzed through determinations of the Nye and Kr ner dislocation density tensors. The significance of the magnitudes and the length scales of the elastic strain and the rotation parts of dislocation density tensors are demonstrated, and their relevance to understanding the fundamental aspects of deformation is discussed.« less

Unravelling the physics of size-dependent dislocation-mediated plasticity

NASA Astrophysics Data System (ADS)

El-Awady, Jaafar A.

2015-01-01

Size-affected dislocation-mediated plasticity is important in a wide range of materials and technologies. Here we develop a generalized size-dependent dislocation-based model that predicts strength as a function of crystal/grain size and the dislocation density. Three-dimensional (3D) discrete dislocation dynamics (DDD) simulations reveal the existence of a well-defined relationship between strength and dislocation microstructure at all length scales for both single crystals and polycrystalline materials. The results predict a transition from dislocation-source strengthening to forest-dominated strengthening at a size-dependent critical dislocation density. It is also shown that the Hall-Petch relationship can be physically interpreted by coupling with an appropriate kinetic equation of the evolution of the dislocation density in polycrystals. The model is shown to be in remarkable agreement with experiments. This work presents a micro-mechanistic framework to predict and interpret strength size-scale effects, and provides an avenue towards performing multiscale simulations without ad hoc assumptions.

High voltage and high current density vertical GaN power diodes

DOE PAGES

Fischer, A. J.; Dickerson, J. R.; Armstrong, A. M.; ...

2016-01-01

We report on the realization of a GaN high voltage vertical p-n diode operating at > 3.9 kV breakdown with a specific on-resistance < 0.9 mΩ.cm 2. Diodes achieved a forward current of 1 A for on-wafer, DC measurements, corresponding to a current density > 1.4 kA/cm 2. An effective critical electric field of 3.9 MV/cm was estimated for the devices from analysis of the forward and reverse current-voltage characteristics. Furthermore this suggests that the fundamental limit to the GaN critical electric field is significantly greater than previously believed.

GaN MOSFET with Boron Trichloride-Based Dry Recess Process

NASA Astrophysics Data System (ADS)

Jiang, Y.; Wang, Q. P.; Tamai, K.; Miyashita, T.; Motoyama, S.; Wang, D. J.; Ao, J. P.; Ohno, Y.

2013-06-01

The dry recessed-gate GaN metal-oxide-semiconductor field-effect transistors (MOSFETs) on AlGaN/GaN heterostructure using boron trichloride (BCl3) as etching gas were fabricated and characterized. Etching with different etching power was conducted. Devices with silicon tetrachloride (SiCl4) etching gas were also prepared for comparison. Field-effect mobility and interface state density were extracted from current-voltage (I-V) characteristics. GaN MOSFETs on AlGaN/GaN heterostructure with BCl3 based dry recess achieved a high maximum electron mobility of 141.5 cm2V-1s-1 and a low interface state density.

Buffer transport mechanisms in intentionally carbon doped GaN heterojunction field effect transistors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Uren, Michael J.; Cäsar, Markus; Kuball, Martin

2014-06-30

Temperature dependent pulsed and ramped substrate bias measurements are used to develop a detailed understanding of the vertical carrier transport in the buffer layers in a carbon doped GaN power heterojunction field effect transistor. Carbon doped GaN and multiple layers of AlGaN alloy are used in these devices to deliver an insulating and strain relieved buffer with high breakdown voltage capability. However, understanding of the detailed physical mechanism for its operation is still lacking. At the lowest electric fields (<10 MV/m), charge redistribution within the C doped layer is shown to occur by hole conduction in the valence band withmore » activation energy 0.86 eV. At higher fields, leakage between the two-dimensional electron gas and the buffer dominates occurring by a Poole-Frenkel mechanism with activation energy ∼0.65 eV, presumably along threading dislocations. At higher fields still, the strain relief buffer starts to conduct by a field dependent process. Balancing the onset of these leakage mechanisms is essential to allow the build-up of positive rather than negative space charge, and thus minimize bulk-related current-collapse in these devices.« less

Ultrafast carrier dynamics in a p-type GaN wafer under different carrier distributions

NASA Astrophysics Data System (ADS)

Fang, Yu; Yang, Junyi; Yang, Yong; Wu, Xingzhi; Xiao, Zhengguo; Zhou, Feng; Song, Yinglin

2016-02-01

The dependence of the carrier distribution on photoexcited carrier dynamics in a p-type Mg-doped GaN (GaN:Mg) wafer were systematically measured by femtosecond transient absorption (TA) spectroscopy. The homogeneity of the carrier distribution was modified by tuning the wavelength of the UV pulse excitation around the band gap of GaN:Mg. The TA kinetics appeared to be biexponential for all carrier distributions, and only the slower component decayed faster as the inhomogeneity of the carrier distribution increased. It was concluded that the faster component (50-70 ps) corresponded to the trap process of holes by the Mg acceptors, and the slower component (150-600 ps) corresponded to the combination of non-radiative surface recombination and intrinsic carrier recombination via dislocations. Moreover, the slower component increased gradually with the incident fluence due to the saturation of surface states.

Size dependence of yield strength simulated by a dislocation-density function dynamics approach

NASA Astrophysics Data System (ADS)

Leung, P. S. S.; Leung, H. S.; Cheng, B.; Ngan, A. H. W.

2015-04-01

The size dependence of the strength of nano- and micron-sized crystals is studied using a new simulation approach in which the dynamics of the density functions of dislocations are modeled. Since any quantity of dislocations can be represented by a density, this approach can handle large systems containing large quantities of dislocations, which may handicap discrete dislocation dynamics schemes due to the excessive computation time involved. For this reason, pillar sizes spanning a large range, from the sub-micron to micron regimes, can be simulated. The simulation results reveal the power-law relationship between strength and specimen size up to a certain size, beyond which the strength varies much more slowly with size. For specimens smaller than ∼4000b, their strength is found to be controlled by the dislocation depletion condition, in which the total dislocation density remains almost constant throughout the loading process. In specimens larger than ∼4000b, the initial dislocation distribution is of critical importance since the presence of dislocation entanglements is found to obstruct deformation in the neighboring regions within a distance of ∼2000b. This length scale suggests that the effects of dense dislocation clusters are greater in intermediate-sized specimens (e.g. 4000b and 8000b) than in larger specimens (e.g. 16 000b), according to the weakest-link concept.

Comparison of dislocation density tensor fields derived from discrete dislocation dynamics and crystal plasticity simulations of torsion

DOE PAGES

Jones, Reese E.; Zimmerman, Jonathan A.; Po, Giacomo; ...

2016-02-01

Accurate simulation of the plastic deformation of ductile metals is important to the design of structures and components to performance and failure criteria. Many techniques exist that address the length scales relevant to deformation processes, including dislocation dynamics (DD), which models the interaction and evolution of discrete dislocation line segments, and crystal plasticity (CP), which incorporates the crystalline nature and restricted motion of dislocations into a higher scale continuous field framework. While these two methods are conceptually related, there have been only nominal efforts focused at the global material response that use DD-generated information to enhance the fidelity of CPmore » models. To ascertain to what degree the predictions of CP are consistent with those of DD, we compare their global and microstructural response in a number of deformation modes. After using nominally homogeneous compression and shear deformation dislocation dynamics simulations to calibrate crystal plasticity ow rule parameters, we compare not only the system-level stress-strain response of prismatic wires in torsion but also the resulting geometrically necessary dislocation density fields. To establish a connection between explicit description of dislocations and the continuum assumed with crystal plasticity simulations we ascertain the minimum length-scale at which meaningful dislocation density fields appear. Furthermore, our results show that, for the case of torsion, that the two material models can produce comparable spatial dislocation density distributions.« less

GaN and ZnO nanostructures

NASA Astrophysics Data System (ADS)

Fündling, Sönke; Sökmen, Ünsal; Behrends, Arne; Al-Suleiman, Mohamed Aid Mansur; Merzsch, Stephan; Li, Shunfeng; Bakin, Andrey; Wehmann, Hergo-Heinrich; Waag, Andreas; Lähnemann, Jonas; Jahn, Uwe; Trampert, Achim; Riechert, Henning

2010-07-01

GaN and ZnO are both wide band gap semiconductors with interesting properties concerning optoelectronic and sensor device applications. Due to the lack or the high costs of native substrates, alternatives like sapphire, silicon, or silicon carbide are taken, but the resulting lattice and thermal mismatches lead to increased defect densities which reduce the material quality. In contrast, nanostructures with high aspect ratio have lower defect densities as compared to layers. In this work, we give an overview on our results achieved on both ZnO as well as GaN based nanorods. ZnO nanostructures were grown by a wet chemical approach as well as by VPT on different substrates - even on flexible polymers. To compare the growth results we analyzed the structures by XRD and PL and show possible device applications. The GaN nano- and microstructures were grown by metal organic vapor phase epitaxy either in a self- organized process or by selective area growth for a better control of shape and material composition. Finally we take a look onto possible device applications, presenting our attempts, e.g., to build LEDs based on GaN nanostructures.

On the laser lift-off of lightly doped micrometer-thick n-GaN films from substrates via the absorption of IR radiation in sapphire

DOE Office of Scientific and Technical Information (OSTI.GOV)

Voronenkov, V. V.; Virko, M. V.; Kogotkov, V. S.

The intense absorption of CO{sub 2} laser radiation in sapphire is used to separate GaN films from GaN templates on sapphire. Scanning of the sapphire substrate by the laser leads to the thermal dissociation of GaN at the GaN/sapphire interface and to the detachment of GaN films from the sapphire. The threshold density of the laser energy at which n-GaN started to dissociate is 1.6 ± 0.5 J/cm{sup 2}. The mechanical-stress distribution and the surface morphology of GaN films and sapphire substrates before and after laser lift-off are studied by Raman spectroscopy, atomic-force microscopy, and scanning electron microscopy. A verticalmore » Schottky diode with a forward current density of 100 A/cm{sup 2} at a voltage of 2 V and a maximum reverse voltage of 150 V is fabricated on the basis of a 9-μm-thick detached n-GaN film.« less

Correlation between border traps and exposed surface properties in gate recessed normally-off Al2O3/GaN MOSFET

NASA Astrophysics Data System (ADS)

Yin, Ruiyuan; Li, Yue; Sun, Yu; Wen, Cheng P.; Hao, Yilong; Wang, Maojun

2018-06-01

We report the effect of the gate recess process and the surface of as-etched GaN on the gate oxide quality and first reveal the correlation between border traps and exposed surface properties in normally-off Al2O3/GaN MOSFET. The inductively coupled plasma (ICP) dry etching gate recess with large damage presents a rough and active surface that is prone to form detrimental GaxO validated by atomic force microscopy and X-ray photoelectron spectroscopy. Lower drain current noise spectral density of the 1/f form and less dispersive ac transconductance are observed in GaN MOSFETs fabricated with oxygen assisted wet etching compared with devices based on ICP dry etching. One decade lower density of border traps is extracted in devices with wet etching according to the carrier number fluctuation model, which is consistent with the result from the ac transconductance method. Both methods show that the density of border traps is skewed towards the interface, indicating that GaxO is of higher trap density than the bulk gate oxide. GaxO located close to the interface is the major location of border traps. The damage-free oxidation assisted wet etching gate recess technique presents a relatively smooth and stable surface, resulting in lower border trap density, which would lead to better MOS channel quality and improved device reliability.

Gallium hydride vapor phase epitaxy of GaN nanowires

PubMed Central

2011-01-01

Straight GaN nanowires (NWs) with diameters of 50 nm, lengths up to 10 μm and a hexagonal wurtzite crystal structure have been grown at 900°C on 0.5 nm Au/Si(001) via the reaction of Ga with NH3 and N2:H2, where the H2 content was varied between 10 and 100%. The growth of high-quality GaN NWs depends critically on the thickness of Au and Ga vapor pressure while no deposition occurs on plain Si(001). Increasing the H2 content leads to an increase in the growth rate, a reduction in the areal density of the GaN NWs and a suppression of the underlying amorphous (α)-like GaN layer which occurs without H2. The increase in growth rate with H2 content is a direct consequence of the reaction of Ga with H2 which leads to the formation of Ga hydride that reacts efficiently with NH3 at the top of the GaN NWs. Moreover, the reduction in the areal density of the GaN NWs and suppression of the α-like GaN layer is attributed to the reaction of H2 with Ga in the immediate vicinity of the Au NPs. Finally, the incorporation of H2 leads to a significant improvement in the near band edge photoluminescence through a suppression of the non-radiative recombination via surface states which become passivated not only via H2, but also via a reduction of O2-related defects. PMID:21711801

Gallium hydride vapor phase epitaxy of GaN nanowires.

PubMed

Zervos, Matthew; Othonos, Andreas

2011-03-28

Straight GaN nanowires (NWs) with diameters of 50 nm, lengths up to 10 μm and a hexagonal wurtzite crystal structure have been grown at 900°C on 0.5 nm Au/Si(001) via the reaction of Ga with NH3 and N2:H2, where the H2 content was varied between 10 and 100%. The growth of high-quality GaN NWs depends critically on the thickness of Au and Ga vapor pressure while no deposition occurs on plain Si(001). Increasing the H2 content leads to an increase in the growth rate, a reduction in the areal density of the GaN NWs and a suppression of the underlying amorphous (α)-like GaN layer which occurs without H2. The increase in growth rate with H2 content is a direct consequence of the reaction of Ga with H2 which leads to the formation of Ga hydride that reacts efficiently with NH3 at the top of the GaN NWs. Moreover, the reduction in the areal density of the GaN NWs and suppression of the α-like GaN layer is attributed to the reaction of H2 with Ga in the immediate vicinity of the Au NPs. Finally, the incorporation of H2 leads to a significant improvement in the near band edge photoluminescence through a suppression of the non-radiative recombination via surface states which become passivated not only via H2, but also via a reduction of O2-related defects.

Microstructurally Based Prediction of High Strain Failure Modes in Crystalline Solids

DTIC Science & Technology

2016-07-05

SECURITY CLASSIFICATION OF: New three-dimensional dislocation-density based crystalline plasticity formulations was used with grain-boundary (GB...Army Research Office P.O. Box 12211 Research Triangle Park, NC 27709-2211 High strain-rate; failure, crsytalline plasticity , dislocation-density...Solids Report Title New three-dimensional dislocation-density based crystalline plasticity formulations was used with grain-boundary (GB) kinematic

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gibert, Ivan, E-mail: gibert1993@mail.ru; Kiseleva, Svetlana, E-mail: kisielieva1946@mail.ru; Popova, Natalya, E-mail: natalya-popova-44@mail.ru

The investigation of excess dislocation density accumulation in the deformed polycrystalline austenitic steel was carried out using transmission electron microscopy (TEM). The distributions of the excess dislocation density in the grains of the deformed austenitic steel with different bending types were obtained and plotted. It was established that in the austenitic polycrystalline steel at the deformation degrees ε = 14 and 25 % the distributions of the excess dislocation density are multimodal. In both cases the grain with compound bending is more stressed. The values of the average excess dislocation density in the grains with the compound and simple bendingmore » are less at ε = 25 % than that at ε = 14 %. This is explained by a significant relaxation of the internal stresses in steel with the increase of the deformation degree from 14 % to 25 %. The increase of the number of twinning systems and the material volume fraction covered by twinning leads to the internal stress relaxation and consequently to the increase of the excess dislocation density. The presence of microtwins in the deformed material has an influence on the distribution of the excess dislocation density. In the deformed polycrystalline austenitic steel the number of grains with compound bending is increased with the increase of the plastic deformation degree.« less

Selective-area nanoheteroepitaxy for light emitting diode (LED) applications

NASA Astrophysics Data System (ADS)

Wildeson, Isaac H.

Over 20% of the electricity in the United States is consumed for lighting, and the majority of this energy is wasted as heat during the lighting process. A solid-state (or light emitting diode (LED)-based) light source has the potential of saving the United States billions of dollars in electricity and reducing megatons of global CO2 emissions annually. While white light LEDs are currently on the market with efficiencies that are superior to incandescent and fluorescent light sources, their high up-front cost is inhibiting mass adoption. One reason for the high cost is the inefficiency of green and amber LEDs that can used to make white light. The inefficiency of green and amber LEDs results in more of these chips being required, and thus a higher cost. Improvements in the performance of green and amber LEDs is also required in order to realize the full potential of solid-state lighting. Nanoheteroepitaxy is an interesting route towards achieving efficient green and amber LEDs as it resolves major challenges that are currently plaguing III-nitride LEDs such as high dislocation densities and limited active region critical thicknesses. A method for fabricating III-nitride nanopyramid LEDs is presented that employs conventional processing used in industry. The present document begins with an overview of the current challenges in III-nitride LEDs and the benefits of nanoheteroepitaxy. A process for controlled selective-area growth of nanopyramid LEDs by organometallic vapor phase epitaxy has been developed throughout the course of this work. Dielectric templates used for the selective-area growth are patterned by two methods, namely porous anodic alumina and electron-beam lithography. The dielectric templates serve as efficient dislocation filters; however, planar defects are initiated during lower temperature growth on the nanopyramids. The quantum wells outline six semipolar planes that form each hexagonal pyramid. Quantum wells grown on these semipolar planes generate built-in electric fields with magnitudes that are one-tenth those on the polar c-plane with the same (In,Ga)N composition. The lateral strain relaxation innate in the nanoheterostructures allows greater coherent InN incorporation in the nanopyramids as compared to thin-film heterostructures, as confirmed by electroluminescence and transmission electron microscopy. In addition to applications for light emitting diodes, selective area growth of GaN nanostructures is also important for biological and sensing applications. A process for fabricating porous GaN nanorods is presented that also relies on selective-area organometallic vapor phase epitaxy. The nanopore walls are primarily outlined by nonpolar planes, and the diameter of the nanopore can be controlled by the diameter of the opening in the dielectric template and the growth time. The lining of the nanopore walls is comprised of crystalline GaN, which makes these structures interesting for sensing, electrical and optical applications.

Statistical analysis of dislocations and dislocation boundaries from EBSD data.

PubMed

Moussa, C; Bernacki, M; Besnard, R; Bozzolo, N

2017-08-01

Electron BackScatter Diffraction (EBSD) is often used for semi-quantitative analysis of dislocations in metals. In general, disorientation is used to assess Geometrically Necessary Dislocations (GNDs) densities. In the present paper, we demonstrate that the use of disorientation can lead to inaccurate results. For example, using the disorientation leads to different GND density in recrystallized grains which cannot be physically justified. The use of disorientation gradients allows accounting for measurement noise and leads to more accurate results. Misorientation gradient is then used to analyze dislocations boundaries following the same principle applied on TEM data before. In previous papers, dislocations boundaries were defined as Geometrically Necessary Boundaries (GNBs) and Incidental Dislocation Boundaries (IDBs). It has been demonstrated in the past, through transmission electron microscopy data, that the probability density distribution of the disorientation of IDBs and GNBs can be described with a linear combination of two Rayleigh functions. Such function can also describe the probability density of disorientation gradient obtained through EBSD data as reported in this paper. This opens the route for determining IDBs and GNBs probability density distribution functions separately from EBSD data, with an increased statistical relevance as compared to TEM data. The method is applied on deformed Tantalum where grains exhibit dislocation boundaries, as observed using electron channeling contrast imaging. Copyright © 2017 Elsevier B.V. All rights reserved.

Growth of Gallium Nitride Nanowires: A Study Using In Situ Transmission Electron Microscopy

NASA Astrophysics Data System (ADS)

Diaz Rivas, Rosa Estela

Owing to their special characteristics, group III-Nitride semiconductors have attracted special attention for their application in a wide range of optoelectronic devices. Of particular interest are their direct and wide band gaps that span from ultraviolet to the infrared wavelengths. In addition, their stronger bonds relative to the other compound semiconductors makes them thermally more stable, which provides devices with longer life time. However, the lattice mismatch between these semiconductors and their substrates cause the as-grown films to have high dislocation densities, reducing the life time of devices that contain these materials. One possible solution for this problem is to substitute single crystal semiconductor nanowires for epitaxial films. Due to their dimensionality, semiconductor nanowires typically have stress-free surfaces and better physical properties. In order to employ semiconductor nanowires as building blocks for nanoscale devices, a precise control of the nanowires' crystallinity, morphology, and chemistry is necessary. This control can be achieved by first developing a deeper understanding of the processes involved in the synthesis of nanowires, and then by determining the effects of temperature and pressure on their growth. This dissertation focuses on understanding of the growth processes involved in the formation of GaN nanowires. Nucleation and growth events were observed in situ and controlled in real-time using an environmental transmission electron microscope. These observations provide a satisfactory elucidation of the underlying growth mechanism during the formation of GaN nanowires. Nucleation of these nanowires appears to follow the vapor-liquid-solid mechanism. However, nanowire growth is found to follow both the vapor-liquid-solid and vapor-solid-solid mechanisms. Direct evidence of the effects of III/V ratio on nanowire growth is also reported, which provides important information for tailoring the synthesis of GaN nanowires. These findings suggest in situ electron microscopy is a powerful tool to understand the growth of GaN nanowires and also that these experimental approach can be extended to study other binary semiconductor compound such as GaP, GaAs, and InP, or even ternary compounds such as InGaN. However, further experimental work is required to fully elucidate the kinetic effects on the growth process. A better control of the growth parameters is also recommended.

Interface Trap Density Reduction for Al2O3/GaN (0001) Interfaces by Oxidizing Surface Preparation prior to Atomic Layer Deposition.

PubMed

Zhernokletov, Dmitry M; Negara, Muhammad A; Long, Rathnait D; Aloni, Shaul; Nordlund, Dennis; McIntyre, Paul C

2015-06-17

We correlate interfacial defect state densities with the chemical composition of the Al2O3/GaN interface in metal-oxide-semiconductor (MOS) structures using synchrotron photoelectron emission spectroscopy (PES), cathodoluminescence and high-temperature capacitance-voltage measurements. The influence of the wet chemical pretreatments involving (1) HCl+HF etching or (2) NH4OH(aq) exposure prior to atomic layer deposition (ALD) of Al2O3 were investigated on n-type GaN (0001) substrates. Prior to ALD, PES analysis of the NH4OH(aq) treated surface shows a greater Ga2O3 component compared to either HCl+HF treated or as-received surfaces. The lowest surface concentration of oxygen species is detected on the acid etched surface, whereas the NH4OH treated sample reveals the lowest carbon surface concentration. Both surface pretreatments improve electrical characteristics of MOS capacitors compared to untreated samples by reducing the Al2O3/GaN interface state density. The lowest interfacial trap density at energies in the upper band gap is detected for samples pretreated with NH4OH. These results are consistent with cathodoluminescence data indicating that the NH4OH treated samples show the strongest band edge emission compared to as-received and acid etched samples. PES results indicate that the combination of reduced carbon contamination while maintaining a Ga2O3 interfacial layer by NH4OH(aq) exposure prior to ALD results in fewer interface traps after Al2O3 deposition on the GaN substrate.

One-step fabrication of porous GaN crystal membrane and its application in energy storage

NASA Astrophysics Data System (ADS)

Zhang, Lei; Wang, Shouzhi; Shao, Yongliang; Wu, Yongzhong; Sun, Changlong; Huo, Qin; Zhang, Baoguo; Hu, Haixiao; Hao, Xiaopeng

2017-03-01

Single-crystal gallium nitride (GaN) membranes have great potential for a variety of applications. However, fabrication of single-crystalline GaN membranes remains a challenge owing to its chemical inertness and mechanical hardness. This study prepares large-area, free-standing, and single-crystalline porous GaN membranes using a one-step high-temperature annealing technique for the first time. A promising separation model is proposed through a comprehensive study that combines thermodynamic theories analysis and experiments. Porous GaN crystal membrane is processed into supercapacitors, which exhibit stable cycling life, high-rate capability, and ultrahigh power density, to complete proof-of-concept demonstration of new energy storage application. Our results contribute to the study of GaN crystal membranes into a new stage related to the elelctrochemical energy storage application.

Study on GaN nanostructures: Growth and the suppression of the yellow emission

NASA Astrophysics Data System (ADS)

Wang, Ting; Chen, Fei; Ji, Xiaohong; Zhang, Qinyuan

2018-07-01

GaN nanostructures were synthesized via a simple chemical vapor deposition using Ga2O3 and NH3 as precursors. Structural and morphological properties were systematically characterized by field emission scanning electron microscopy, X-ray diffractometer, transmission electron microscopy, and Raman spectroscopy. The configuration of GaN nanostructures was found to be strongly dependent on the growth temperature and the NH3 flow rate. Photoluminescence analysis revealed that all the fabricated GaN NSs exhibited a strong ultra-violet emission (∼364 nm), and the yellow emission of GaN nanorods can be suppressed at appropriate III/V ratio. The suppression of the yellow emission was attributed to the low density of surface or the VGa defect. The work demonstrates that the GaN nanostructures have potential applications in the optoelectronic and nanoelectronic devices.

Bandgap engineering of GaN nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ming, Bang-Ming; Yan, Hui; Wang, Ru-Zhi, E-mail: wrz@bjut.edu.cn, E-mail: yamcy@csrc.ac.cn

2016-05-15

Bandgap engineering has been a powerful technique for manipulating the electronic and optical properties of semiconductors. In this work, a systematic investigation of the electronic properties of [0001] GaN nanowires was carried out using the density functional based tight-binding method (DFTB). We studied the effects of geometric structure and uniaxial strain on the electronic properties of GaN nanowires with diameters ranging from 0.8 to 10 nm. Our results show that the band gap of GaN nanowires depends linearly on both the surface to volume ratio (S/V) and tensile strain. The band gap of GaN nanowires increases linearly with S/V, whilemore » it decreases linearly with increasing tensile strain. These linear relationships provide an effect way in designing GaN nanowires for their applications in novel nano-devices.« less

Modeling dislocation generation in high pressure Czochralski growth of indium phosphide single crystals

NASA Astrophysics Data System (ADS)

Pendurti, Srinivas

InP is an important material for opto-electronic and high speed electronics applications. Its main use today is as the substrate material for epitaxy to produce GaInAsP lasers. The present technology for growing bulk InP is the high pressure Czochralski process. Bulk InP grown through this technique suffers from presence of a high density of line defects or dislocations, which are produced by thermal stresses the material goes through during its growth in the high temperature furnace. Modeling of these thermal stresses and the resulting plastic deformation, giving rise to dislocation densities, entails simulation of the entire thermal history of the crystal during its growth in the furnace, and studying the deformation of the crystal through suitable visco-plastic constitutive equations. Accordingly, a suitable visco-plastic model for deformation of InP was constructed, integrated with the ABAQUS finite element code, and verified through experimental data for uniaxial constant strain rate deformation tests available in literature. This was then coupled with a computation fluid dynamics model, predicting the entire temperature history in the furnace during crystal growth, to study the plastic deformation and dislocation density evolution in the crystal during growth. Growth in a variety of conditions was simulated and those conditions that generate minimum dislocation density identified. Macroscopic controllable parameters that affect the dislocation densities the most, have also been delineated. It was found that the strength of gas convection in the Czochralski furnace has the strongest effect on the dislocation densities in the fully grown crystal. Comparison of the simulated dislocation densities on wafers, with experimentally recorded etch pit profiles on as-grown crystals was reasonable. Finally some limitations in the work are discussed and avenues for future work identified.

Reduction of threading dislocation density in SiGe epilayer on Si (0 0 1) by lateral growth liquid-phase epitaxy

NASA Astrophysics Data System (ADS)

O'Reilly, Andrew J.; Quitoriano, Nathaniel J.

2018-02-01

Si0.973Ge0.027 epilayers were grown on a Si (0 0 1) substrate by a lateral liquid-phase epitaxy (LLPE) technique. The lateral growth mechanism favoured the glide of misfit dislocations and inhibited the nucleation of new dislocations by maintaining the thickness less than the critical thicknesses for dislocation nucleation and greater than the critical thickness for glide. This promoted the formation of an array of long misfit dislocations parallel to the [1 1 0] growth direction and reduced the threading dislocation density to 103 cm-2, two orders of magnitude lower than the seed area with an isotropic misfit dislocation network.

A New Selective Area Lateral Epitaxy Approach for Depositing a-Plane GaN over r-Plane Sapphire

NASA Astrophysics Data System (ADS)

Chen, Changqing; Zhang, Jianping; Yang, Jinwei; Adivarahan, Vinod; Rai, Shiva; Wu, Shuai; Wang, Hongmei; Sun, Wenhong; Su, Ming; Gong, Zheng; Kuokstis, Edmundas; Gaevski, Mikhail; Khan, Muhammad Asif

2003-07-01

We report a new epitaxy procedure for growing extremely low defect density a-plane GaN films over r-plane sapphire. By combining selective area growth through a SiO2 mask opening to produce high height to width aspect ratio a-plane GaN pillars and lateral epitaxy from their c-plane facets, we obtained fully coalesced a-plane GaN films. The excellent structural, optical and electrical characteristics of these selective area lateral epitaxy (SALE) deposited films make them ideal for high efficiency III-N electronic and optoelectronic devices.

Near Field Scanning Optical Microscopy (NSOM) of Nano Devices

DTIC Science & Technology

2008-12-01

FEATURES OF GaN NANOWIRES Gallium Nitride (GaN) nanowires are semiconductor wires of great interest lately for its some of its unique properties. These...via chemical vapour deposition (CVD) [19] or even with gas source molecular beam epitaxy (MBE) [20] The GaN nanowires growth techniques will not be...Denlinger, and Peidong Yang, Crystallographic alignment of high-density gallium nitride nanowire arrays, Nature Materials, Issue 3 Vol 8, pg 524

Poole-Frenkel effect on electrical characterization of Al-doped ZnO films deposited on p-type GaN

DOE Office of Scientific and Technical Information (OSTI.GOV)

Huang, Bohr-Ran; Liao, Chung-Chi; Ke, Wen-Cheng, E-mail: wcke@saturn.yzu.edu.tw

2014-03-21

This paper presents the electrical properties of Al-doped ZnO (AZO) films directly grown on two types of p-type GaN thin films. The low-pressure p-GaN thin films (LP-p-GaN) exhibited structural properties of high-density edge-type threading dislocations (TDs) and compensated defects (i.e., nitrogen vacancy). Compared with high-pressure p-GaN thin films (HP-p-GaN), X-ray photoemission spectroscopy of Ga 3d core levels indicated that the surface Fermi-level shifted toward the higher binding-energy side by approximately 0.7 eV. The high-density edge-type TDs and compensated defects enabled surface Fermi-level shifting above the intrinsic Fermi-level, causing the surface of LP-p-GaN thin films to invert to n-type semiconductor. A highlymore » nonlinear increase in leakage current regarding reverse-bias voltage was observed for AZO/LP-p-GaN. The theoretical fits for the reverse-bias voltage region indicated that the field-assisted thermal ionization of carriers from defect associated traps, which is known as the Poole-Frenkel effect, dominated the I-V behavior of AZO/LP-p-GaN. The fitting result estimated the trap energy level at 0.62 eV below the conduction band edge. In addition, the optical band gap increased from 3.50 eV for as-deposited AZO films to 3.62 eV for 300 °C annealed AZO films because of the increased carrier concentration. The increasing Fermi-level of the 300 °C annealed AZO films enabled the carrier transport to move across the interface into the LP-p-GaN thin films without any thermal activated energy. Thus, the Ohmic behavior of AZO contact can be achieved directly on the low-pressure p-GaN films at room temperature.« less

Poole-Frenkel effect on electrical characterization of Al-doped ZnO films deposited on p-type GaN

NASA Astrophysics Data System (ADS)

Huang, Bohr-Ran; Liao, Chung-Chi; Ke, Wen-Cheng; Chang, Yuan-Ching; Huang, Hao-Ping; Chen, Nai-Chuan

2014-03-01

This paper presents the electrical properties of Al-doped ZnO (AZO) films directly grown on two types of p-type GaN thin films. The low-pressure p-GaN thin films (LP-p-GaN) exhibited structural properties of high-density edge-type threading dislocations (TDs) and compensated defects (i.e., nitrogen vacancy). Compared with high-pressure p-GaN thin films (HP-p-GaN), X-ray photoemission spectroscopy of Ga 3d core levels indicated that the surface Fermi-level shifted toward the higher binding-energy side by approximately 0.7 eV. The high-density edge-type TDs and compensated defects enabled surface Fermi-level shifting above the intrinsic Fermi-level, causing the surface of LP-p-GaN thin films to invert to n-type semiconductor. A highly nonlinear increase in leakage current regarding reverse-bias voltage was observed for AZO/LP-p-GaN. The theoretical fits for the reverse-bias voltage region indicated that the field-assisted thermal ionization of carriers from defect associated traps, which is known as the Poole-Frenkel effect, dominated the I-V behavior of AZO/LP-p-GaN. The fitting result estimated the trap energy level at 0.62 eV below the conduction band edge. In addition, the optical band gap increased from 3.50 eV for as-deposited AZO films to 3.62 eV for 300 °C annealed AZO films because of the increased carrier concentration. The increasing Fermi-level of the 300 °C annealed AZO films enabled the carrier transport to move across the interface into the LP-p-GaN thin films without any thermal activated energy. Thus, the Ohmic behavior of AZO contact can be achieved directly on the low-pressure p-GaN films at room temperature.

Selective area growth and characterization of GaN nanocolumns, with and without an InGaN insertion, on semi-polar (11–22) GaN templates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bengoechea-Encabo, A.; Albert, S.; Barbagini, F.

The aim of this work is the selective area growth (SAG) of GaN nanocolumns, with and without an InGaN insertion, by molecular beam epitaxyon semi-polar (11–22) GaN templates. The high density of stacking faults present in the template is strongly reduced after SAG. A dominant sharp photoluminescence emission at 3.473 eV points to high quality strain-free material. When embedding an InGaN insertion into the ordered GaN nanostructures, very homogeneous optical properties are observed, with two emissions originating from different regions of each nanostructure, most likely related to different In contents on different crystallographic planes.

The application of water coupled nonlinear ultrasonics to quantify the dislocation density in aluminum 1100

NASA Astrophysics Data System (ADS)

Mostavi, Amir; Tehrani, N.; Kamali, N.; Ozevin, D.; Chi, S. W.; Indacochea, J. E.

2017-02-01

This article investigates water coupled nonlinear ultrasonic method to measure the dislocation density in aluminum 1100 specimens. The different levels of dislocation densities are introduced to the samples by applying different levels of plastic strains by tensile loading. The ultrasonic testing includes 2.25 MHz transducer as transmitter and 5.0 MHz transducer as receiver in an immersion tank. The results of immersion experiments are compared with oil-coupled experiments. While water has significant nonlinearity within itself, the immersion ultrasound results agree with the literature of oil coupled ultrasound results of the specimens that the nonlinearity coefficient increases with the increase of dislocation density in aluminum.

Ductile failure initiation and evolution in porous polycrystalline aggregates due to interfacial effects

NASA Astrophysics Data System (ADS)

Ashmawi, Waeil Muhammad Al-Anwar

New analytical and computational formulations have been developed for the investigation of micro structurally induced ductile failure mechanisms in porous polycrystalline aggregates with low and high (CSL) angle grain-boundaries (GBs). A multiple-slip rate-dependent crystalline constitutive formulation that is coupled to the evolution of mobile and immobile dislocation densities, a new internal porosity formulation for void nucleation and growth, and specialized computational schemes have been developed to obtain a detailed understanding of the multi-scale interrelated physical mechanisms that result in ductile failure in polycrystalline materials. Comprehensive transmission and pile-up mechanisms have also been introduced to investigate dislocation-density impedance and slip-rate incompatibility at the GBs. The interrelated effects of GB orientation, mobile and immobile dislocation densities, strain hardening, geometrical softening, localized plastic strains, and dislocation-density transmission and blockage on void growth, interaction, and coalescence have been studied. Criteria have been developed to identify and monitor the initiation and development of potential dislocation-density activity sites adjacent to GB regions. These interactions play an important role in the formation of GB pile-up and transmission regions. The effects of GB structure and orientation on ductile failure have been accounted for by the development of GB interfacial kinematic conditions that account for a multitude of dislocation-density interactions with GBs, such as full and partial transmission, impedance, blockage, and absorption. Pile-ups and transmission regions are identified and monitored as the deformation and failure evolve. These kinematic conditions are linked to the initiation and evolution of failure modes by the development of a new internal porosity evolution formulation that accounts for void nucleation and growth. The internal porosity relation is coupled with the proposed dislocation-density based crystalline constitutive formulation, the interfacial GB dislocation-density interaction models, and the specialized computational schemes to obtain detailed predictions of the behavior of aggregates with explicit voids that have different orientations and combinations of sizes, shapes, and spacings. Results from the present study indicate that material failure is a competition between different interrelated effects, such as stress triaxiality, accumulated plastic shear strain, temperature, dislocation density concentration, and grain and GB crystallographic orientations. For all void arrangements, as the void size is increased, specimen necking is diffuse and failure is concentrated in the ligament regions. Furthermore, there are more dislocation-density activity sites for potential transmission and pile-ups at the GBs. Failure is concentrated along the void peripheries and within intervoid ligaments. It has been shown that the evolution of the mobile dislocation density saturation curves, and their saturation rate are directly related to the aggregate response. Nucleation and growth for all void distributions have occurred in regions of maximum dislocation density and along preferred crystallographic orientations. Spatial distributions of porosity, accumulated plastic strains, and pressure have been obtained to further elucidate how these parameters evolve and affect void to void interaction in critical ligament and localized regions as a function of intervoid spacing and nominal strains. These failure predictions can be also used to identify intergranular and transgranular failure propagation. The present study underscores the importance of using dislocation-density based multiple-slip crystalline constitutive formulations and GB interfacial mechanisms that are consistent with experimental observations and results to accurately characterize the microstructural evolution of deformation and failure modes on a length scale that is commensurate with the material competition between the inherent strengthening and softening mechanisms of crystalline systems.

Edge Stabilized Ribbon (ESR); Stress, Dislocation Density and Electronic Performance

NASA Technical Reports Server (NTRS)

Sachs, E. M.

1984-01-01

The edge stabilized ribbon (ESR) silicon ribbon was grown in widths of 1, 2.2 and 4.0 inches at speeds ranging from .6 to 7 in/min, which result in ribbon thicknesses of 5 to 400 microns. One of the primary problems remaining in ESR growth is that of thermally induced mechanical stresses. This problem is manifested as ribbon with a high degree of residual stress or as ribbon with buckled ribbon. Thermal stresses result in a high dislocation density in the grown material, resulting in compromised electronic performance. Improvements in ribbon flatness were accomplished by modification of the ribbon cooling profile. Ribbon flatness and other experimental observations of ESR ribbon are discussed. Laser scanner measurements show a good correlation between diffusion length and dislocation density which indicates that the high dislocation densities are the primary cause of the poor current performance of ESR materials. Dislocation densities were reduced and improved electronic performance resulted. Laser scanner data on new and old material are presented.

Neutron diffraction measurement of residual stresses, dislocation density and texture in Zr-bonded U-10Mo “mini” fuel foils and plates

DOE PAGES

Brown, Donald William; Okuniewski, Maria A.; Sisneros, Thomas A.; ...

2016-12-01

Here, Al clad U-10Mo fuel plates are being considered for conversion of several research reactors from high-enriched to low-enriched U fuel. Neutron diffraction measurements of the textures, residual phase stresses, and dislocation densities in the individual phases of the mini-foils throughout several processing steps and following hot-isostatic pressing to the Al cladding, have been completed. Recovery and recrystallization of the bare U-10Mo fuel foil, as indicated by the dislocation density and texture, are observed depending on the state of the material prior to annealing and the duration and temperature of the annealing process. In general, the cladding procedure significantly reducesmore » the dislocation density, but the final state of the clad plate, both texture and dislocation density, depends strongly on the final processing step of the fuel foil. In contrast, the residual stress state of the final plate is dominated by the thermal expansion mismatch of the constituent materials.« less

Neutron diffraction measurement of residual stresses, dislocation density and texture in Zr-bonded U-10Mo “mini” fuel foils and plates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brown, Donald William; Okuniewski, Maria A.; Sisneros, Thomas A.

Here, Al clad U-10Mo fuel plates are being considered for conversion of several research reactors from high-enriched to low-enriched U fuel. Neutron diffraction measurements of the textures, residual phase stresses, and dislocation densities in the individual phases of the mini-foils throughout several processing steps and following hot-isostatic pressing to the Al cladding, have been completed. Recovery and recrystallization of the bare U-10Mo fuel foil, as indicated by the dislocation density and texture, are observed depending on the state of the material prior to annealing and the duration and temperature of the annealing process. In general, the cladding procedure significantly reducesmore » the dislocation density, but the final state of the clad plate, both texture and dislocation density, depends strongly on the final processing step of the fuel foil. In contrast, the residual stress state of the final plate is dominated by the thermal expansion mismatch of the constituent materials.« less

Dislocation and Structural Studies at Metal-Metallic Glass Interface at Low Temperature

NASA Astrophysics Data System (ADS)

Gupta, Pradeep; Yedla, Natraj

2017-12-01

In this paper, molecular dynamics (MD) simulation deformation studies on the Al (metal)-Cu50Zr50 (metallic glass) model interface is carried out based on cohesive zone model. The interface is subjected to mode-I loading at a strain rate of 109 s-1 and temperature of 100 K. The dislocations reactions and evolution of dislocation densities during the deformation have been investigated. Atomic interactions between Al, Cu and Zr atoms are modeled using EAM (embedded atom method) potential, and a timestep of 0.002 ps is used for performing the MD simulations. A circular crack and rectangular notch are introduced at the interface to investigate the effect on the deformation behavior and fracture. Further, scale size effect is also investigated. The structural changes and evolution of dislocation density are also examined. It is found that the dominant deformation mechanism is by Shockley partial dislocation nucleation. Amorphization is observed in the Al regions close to the interface and occurs at a lower strain in the presence of a crack. The total dislocation density is found to be maximum after the first yield in both the perfect and defect interface models and is highest in the case of perfect interface with a density of 6.31 × 1017 m-2. In the perfect and circular crack defect interface models, it is observed that the fraction of Shockley partial dislocation density decreases, whereas that of strain rod dislocations increases with increase in strain.

Mechanical annealing under low-amplitude cyclic loading in micropillars

NASA Astrophysics Data System (ADS)

Cui, Yi-nan; Liu, Zhan-li; Wang, Zhang-jie; Zhuang, Zhuo

2016-04-01

Mechanical annealing has been demonstrated to be an effective method for decreasing the overall dislocation density in submicron single crystal. However, simultaneously significant shape change always unexpectedly happens under extremely high monotonic loading to drive the pre-existing dislocations out of the free surfaces. In the present work, through in situ TEM experiments it is found that cyclic loading with low stress amplitude can drive most dislocations out of the submicron sample with virtually little change of the shape. The underlying dislocation mechanism is revealed by carrying out discrete dislocation dynamic (DDD) simulations. The simulation results indicate that the dislocation density decreases within cycles, while the accumulated plastic strain is small. By comparing the evolution of dislocation junction under monotonic, cyclic and relaxation deformation, the cumulative irreversible slip is found to be the key factor of promoting junction destruction and dislocation annihilation at free surface under low-amplitude cyclic loading condition. By introducing this mechanics into dislocation density evolution equations, the critical conditions for mechanical annealing under cyclic and monotonic loadings are discussed. Low-amplitude cyclic loading which strengthens the single crystal without seriously disturbing the structure has the potential applications in the manufacture of defect-free nano-devices.

Strengthening and toughening mechanisms in low-c microalloyed martensitic steel as influenced by austenite conditioning

NASA Astrophysics Data System (ADS)

Kennett, Shane C.

Three low-carbon ASTM A514 microalloyed steels were used to assess the effects of austenite conditioning on the microstructure and mechanical properties of martensite. A range of prior austenite grain sizes with and without thermomechanical processing were produced in a Gleeble RTM 3500 and direct-quenched. Samples in the as-quenched, low temperature tempered, and high temperature tempered conditions were studied. The microstructure was characterized with scanning electron microscopy, electron backscattered diffraction, transmission electron microscopy, and x-ray diffraction. The uniaxial tensile properties and Charpy V-notch properties were measured and compared with the microstructural features (prior austenite grain size, packet size, block size, lath boundaries, and dislocation density). For the equiaxed prior austenite grain conditions, prior austenite grain size refinement decreases the packet size, decreases the block size, and increases the dislocation density of as-quenched martensite. However, after high temperature tempering the dislocation density decreases with prior austenite grain size refinement. Thermomechanical processing increases the low angle substructure, increases the dislocation density, and decreases the block size of as-quenched martensite. The dislocation density increase and block size refinement is sensitive to the austenite grain size before ausforming. The larger prior austenite grain size conditions have a larger increase in dislocation density, but the small prior austenite grain size conditions have the largest refinement in block size. Additionally, for the large prior austenite grain size conditions, the packet size increases with thermomechanical processing. The strength of martensite is often related to an effective grain size or carbon concentration. For the current work, it was concluded that the strength of martensite is primarily controlled by the dislocation density and dislocation substructure; which is related to a grain size and carbon concentration. In the microyielding regime, the strength and work hardening is related to the motion of unpinned dislocation segments. However, with tensile strain, a dislocation cell structure is developed and the flow strength (greater than 1% offset) is controlled by the dislocation density following a Taylor hardening model, thereby ruling out any grain size effects on the flow strength. Additionally, it is proposed that lath boundaries contribute to strength. It is shown that the strength differences associated with thermomechanically processed steels can be fully accounted for by dislocation density differences and the effect of lath boundaries. The low temperature ductile to brittle transition of martensite is controlled by the martensite block size, packet size, and prior austenite grain size. However, the effect of block size is likely small in comparison. The ductile to brittle transition temperature is best correlated to the inverse square root of the martensite packet size because large crack deflections are typical at packet boundaries.

GaN: From three- to two-dimensional single-layer crystal and its multilayer van der Waals solids

NASA Astrophysics Data System (ADS)

Onen, A.; Kecik, D.; Durgun, E.; Ciraci, S.

2016-02-01

Three-dimensional (3D) GaN is a III-V compound semiconductor with potential optoelectronic applications. In this paper, starting from 3D GaN in wurtzite and zinc-blende structures, we investigated the mechanical, electronic, and optical properties of the 2D single-layer honeycomb structure of GaN (g -GaN ) and its bilayer, trilayer, and multilayer van der Waals solids using density-functional theory. Based on high-temperature ab initio molecular-dynamics calculations, we first showed that g -GaN can remain stable at high temperature. Then we performed a comparative study to reveal how the physical properties vary with dimensionality. While 3D GaN is a direct-band-gap semiconductor, g -GaN in two dimensions has a relatively wider indirect band gap. Moreover, 2D g -GaN displays a higher Poisson ratio and slightly less charge transfer from cation to anion. In two dimensions, the optical-absorption spectra of 3D crystalline phases are modified dramatically, and their absorption onset energy is blueshifted. We also showed that the physical properties predicted for freestanding g -GaN are preserved when g -GaN is grown on metallic as well as semiconducting substrates. In particular, 3D layered blue phosphorus, being nearly lattice-matched to g -GaN , is found to be an excellent substrate for growing g -GaN . Bilayer, trilayer, and van der Waals crystals can be constructed by a special stacking sequence of g -GaN , and they can display electronic and optical properties that can be controlled by the number of g -GaN layers. In particular, their fundamental band gap decreases and changes from indirect to direct with an increasing number of g -GaN layers.

Systematic study on dynamic atomic layer epitaxy of InN on/in +c-GaN matrix and fabrication of fine-structure InN/GaN quantum wells: Role of high growth temperature

NASA Astrophysics Data System (ADS)

Yoshikawa, Akihiko; Kusakabe, Kazuhide; Hashimoto, Naoki; Hwang, Eun-Sook; Imai, Daichi; Itoi, Takaomi

2016-12-01

The growth kinetics and properties of nominally 1-ML (monolayer)-thick InN wells on/in +c-GaN matrix fabricated using dynamic atomic layer epitaxy (D-ALEp) by plasma-assisted molecular beam epitaxy were systematically studied, with particular attention given to the effects of growth temperature. Attention was also given to how and where the ˜1-ML-thick InN layers were frozen or embedded on/in the +c-GaN matrix. The D-ALEp of InN on GaN was a two-stage process; in the 1st stage, an "In+N" bilayer/monolayer was formed on the GaN surface, while in the 2nd, this was capped by a GaN barrier layer. Each process was monitored in-situ using spectroscopic ellipsometry. The target growth temperature was above 620 °C and much higher than the upper critical epitaxy temperature of InN (˜500 °C). The "In+N" bilayer/monolayer tended to be an incommensurate phase, and the growth of InN layers was possible only when they were capped with a GaN layer. The InN layers could be coherently inserted into the GaN matrix under self-organizing and self-limiting epitaxy modes. The growth temperature was the most dominant growth parameter on both the growth process and the structure of the InN layers. Reflecting the inherent growth behavior of D-ALEp grown InN on/in +c-GaN at high growth temperature, the embedded InN layers in the GaN matrix were basically not full-ML in coverage, and the thickness of sheet-island-like InN layers was essentially either 1-ML or 2-ML. It was found that these InN layers tended to be frozen at the step edges on the GaN and around screw-type threading dislocations. The InN wells formed type-I band line-up heterostructures with GaN barriers, with exciton localization energies of about 300 and 500 meV at 15 K for the 1-ML and 2-ML InN wells, respectively.

Atomic layer deposition TiO 2-Al 2O 3 stack: An improved gate dielectric on Ga-polar GaN metal oxide semiconductor capacitors

DOE PAGES

Wei, Daming; Edgar, James H.; Briggs, Dayrl P.; ...

2014-10-15

This research focuses on the benefits and properties of TiO 2-Al 2O 3 nano-stack thin films deposited on Ga 2O 3/GaN by plasma-assisted atomic layer deposition (PA-ALD) for gate dielectric development. This combination of materials achieved a high dielectric constant, a low leakage current, and a low interface trap density. Correlations were sought between the films’ structure, composition, and electrical properties. The gate dielectrics were approximately 15 nm thick and contained 5.1 nm TiO 2, 7.1 nm Al 2O 3 and 2 nm Ga 2O 3 as determined by spectroscopic ellipsometry. The interface carbon concentration, as measured by x-ray photoelectronmore » spectroscopy (XPS) depth profile, was negligible for GaN pretreated by thermal oxidation in O 2 for 30 minutes at 850°C. The RMS roughness slightly increased after thermal oxidation and remained the same after ALD of the nano-stack, as determined by atomic force microscopy. The dielectric constant of TiO 2-Al 2O 3 on Ga2O3/GaN was increased to 12.5 compared to that of pure Al 2O 3 (8~9) on GaN. In addition, the nano-stack's capacitance-voltage (C-V) hysteresis was small, with a total trap density of 8.74 × 10 11 cm -2. The gate leakage current density (J=2.81× 10 -8 A/cm 2) was low at +1 V gate bias. These results demonstrate the promising potential of plasma ALD deposited TiO 2/Al 2O 3 for serving as the gate oxide on Ga 2O 3/GaN based MOS devices.« less

Atomic layer deposition TiO 2-Al 2O 3 stack: An improved gate dielectric on Ga-polar GaN metal oxide semiconductor capacitors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wei, Daming; Edgar, James H.; Briggs, Dayrl P.

This research focuses on the benefits and properties of TiO 2-Al 2O 3 nano-stack thin films deposited on Ga 2O 3/GaN by plasma-assisted atomic layer deposition (PA-ALD) for gate dielectric development. This combination of materials achieved a high dielectric constant, a low leakage current, and a low interface trap density. Correlations were sought between the films’ structure, composition, and electrical properties. The gate dielectrics were approximately 15 nm thick and contained 5.1 nm TiO 2, 7.1 nm Al 2O 3 and 2 nm Ga 2O 3 as determined by spectroscopic ellipsometry. The interface carbon concentration, as measured by x-ray photoelectronmore » spectroscopy (XPS) depth profile, was negligible for GaN pretreated by thermal oxidation in O 2 for 30 minutes at 850°C. The RMS roughness slightly increased after thermal oxidation and remained the same after ALD of the nano-stack, as determined by atomic force microscopy. The dielectric constant of TiO 2-Al 2O 3 on Ga2O3/GaN was increased to 12.5 compared to that of pure Al 2O 3 (8~9) on GaN. In addition, the nano-stack's capacitance-voltage (C-V) hysteresis was small, with a total trap density of 8.74 × 10 11 cm -2. The gate leakage current density (J=2.81× 10 -8 A/cm 2) was low at +1 V gate bias. These results demonstrate the promising potential of plasma ALD deposited TiO 2/Al 2O 3 for serving as the gate oxide on Ga 2O 3/GaN based MOS devices.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Calabrese, Gabriele, E-mail: calabrese@pdi-berlin.de; Corfdir, Pierre; Gao, Guanhui

We demonstrate the self-assembled growth of vertically aligned GaN nanowire ensembles on a flexible Ti foil by plasma-assisted molecular beam epitaxy. The analysis of single nanowires by transmission electron microscopy reveals that they are single crystalline. Low-temperature photoluminescence spectroscopy demonstrates that in comparison to standard GaN nanowires grown on Si, the nanowires prepared on the Ti foil exhibit an equivalent crystalline perfection, a higher density of basal-plane stacking faults, but a reduced density of inversion domain boundaries. The room-temperature photoluminescence spectrum of the nanowire ensemble is not influenced or degraded by the bending of the substrate. The present results pavemore » the way for the fabrication of flexible optoelectronic devices based on GaN nanowires on metal foils.« less

Effect of p-GaN layer doping on the photoresponse of GaN-based p-i-n ultraviolet photodetectors

NASA Astrophysics Data System (ADS)

Wang, Jun; Guo, Jin; Xie, Feng; Wang, Wanjun; Wang, Guosheng; Wu, Haoran; Wang, Tanglin; Song, Man

2015-08-01

We report on two-dimensional (2D) numerical simulations of photoresponse characteristics for GaN based p-i-n ultraviolet (UV) photodetectors. Effects of doping density of p-GaN layer on the photoresponse have been investigated. In order to accurately simulate the device performance, the theoretical calculation includes doping-dependent mobility degradation by Arora model and high field saturation model. Theoretical modeling shows that the doping density of p- GaN layer can significantly affect the photoresponse of GaN based p-i-n UV photodetectors, especially at schottky contact. We have to make a suitable choice of the doping in the device design according to the simulation results.

Evolution of the substructure of a novel 12% Cr steel under creep conditions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yadav, Surya Deo, E-mail: surya.yadav@tugraz.at; Kalácska, Szilvia, E-mail: kalacska@metal.elte.hu; Dománková, Mária, E-mail: maria.domankova@stuba.sk

2016-05-15

In this work we study the microstruture evolution of a newly developed 12% Cr martensitic/ferritic steel in as-received condition and after creep at 650 °C under 130 MPa and 80 MPa. The microstructure is described as consisting of mobile dislocations, dipole dislocations, boundary dislocations, precipitates, lath boundaries, block boundaries, packet boundaries and prior austenitic grain boundaries. The material is characterized employing light optical microscopy (LOM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). TEM is used to characterize the dislocations (mobile + dipole) inside the subgrains and XRD measurements are used tomore » the characterize mobile dislocations. Based on the subgrain boundary misorientations obtained from EBSD measurements, the boundary dislocation density is estimated. The total dislocation density is estimated for the as-received and crept conditions adding the mobile, boundary and dipole dislocation densities. Additionally, the subgrain size is estimated from the EBSD measurements. In this publication we propose the use of three characterization techniques TEM, XRD and EBSD as necessary to characterize all type of dislocations and quantify the total dislocation densty in martensitic/ferritic steels. - Highlights: • Creep properties of a novel 12% Cr steel alloyed with Ta • Experimental characterization of different types of dislocations: mobile, dipole and boundary • Characterization and interpretation of the substructure evolution using unique combination of TEM, XRD and EBSD.« less

Development of a Photoelectrochemical Etch Process to Enable Heterogeneous Substrate Integration of Epitaxial III-Nitride Semiconductors

DTIC Science & Technology

2017-12-01

Chung, Stephen Kelley, Kimberley Olver, Blair C. Connelly, Anand V. Sampath, and Meredith L. Reed Sensors and Electron Devices Directorate, ARL...nitride [GaN], indium nitride, and corresponding ternary alloys) provide a basis for a variety of electronic and photonic devices across several...and driven by an electron beam irradiation, which leads to high carrier densities. This necessitates the transfer/removal of the GaN substrate (or GaN

Effect of substrate nitridation temperature on the persistent photoconductivity of unintentionally-doped GaN layer grown by PAMBE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Prakash, Nisha, E-mail: prakasnisha@gmail.com; Barvat, Arun; Anand, Kritika

2016-05-23

The surface roughness and defect density of GaN epitaxial layers grown on c-plane sapphire substrate are investigated and found to be dependent on nitridation temperature. GaN epitaxial layers grown after nitridation of sapphire at 200°C have a higher defect density and higher surface roughness compared to the GaN layers grown at 646°C nitridation as confirmed by atomic force microscopy (AFM). The persistent photoconductivity (PPC) was observed in both samples and it was found to be decreasing with decreasing temperature in the range 150-300°C due to long carrier lifetime and high electron mobility at low temperature. The photoresponse of the GaNmore » films grown in this study exhibit improved PPC due to their better surface morphology at 646°C nitrided sample. The point defects or extended microstructure defects limits the photocarrier lifetime and electron mobility at 200°C nitrided sample.« less

Impact of oxygen precursor flow on the forward bias behavior of MOCVD-Al2O3 dielectrics grown on GaN

NASA Astrophysics Data System (ADS)

Chan, Silvia H.; Bisi, Davide; Liu, Xiang; Yeluri, Ramya; Tahhan, Maher; Keller, Stacia; DenBaars, Steven P.; Meneghini, Matteo; Mishra, Umesh K.

2017-11-01

This paper investigates the effects of the oxygen precursor flow supplied during metalorganic chemical vapor deposition (MOCVD) of Al2O3 films on the forward bias behavior of Al2O3/GaN metal-oxide-semiconductor capacitors. The low oxygen flow (100 sccm) delivered during the in situ growth of Al2O3 on GaN resulted in films that exhibited a stable capacitance under forward stress, a lower density of stress-generated negative fixed charges, and a higher dielectric breakdown strength compared to Al2O3 films grown under high oxygen flow (480 sccm). The low oxygen grown Al2O3 dielectrics exhibited lower gate current transients in stress/recovery measurements, providing evidence of a reduced density of trap states near the GaN conduction band and an enhanced robustness under accumulated gate stress. This work reveals oxygen flow variance in MOCVD to be a strategy for controlling the dielectric properties and performance.

GaAsP/InGaP HBTs grown epitaxially on Si substrates: Effect of dislocation density on DC current gain

NASA Astrophysics Data System (ADS)

Heidelberger, Christopher; Fitzgerald, Eugene A.

2018-04-01

Heterojunction bipolar transistors (HBTs) with GaAs0.825P0.175 bases and collectors and In0.40Ga0.60P emitters were integrated monolithically onto Si substrates. The HBT structures were grown epitaxially on Si via metalorganic chemical vapor deposition, using SiGe compositionally graded buffers to accommodate the lattice mismatch while maintaining threading dislocation density at an acceptable level (˜3 × 106 cm-2). GaAs0.825P0.175 is used as an active material instead of GaAs because of its higher bandgap (increased breakdown voltage) and closer lattice constant to Si. Misfit dislocation density in the active device layers, measured by electron-beam-induced current, was reduced by making iterative changes to the epitaxial structure. This optimized process culminated in a GaAs0.825P0.175/In0.40Ga0.60P HBT grown on Si with a DC current gain of 156. By considering the various GaAsP/InGaP HBTs grown on Si substrates alongside several control devices grown on GaAs substrates, a wide range of threading dislocation densities and misfit dislocation densities in the active layers could be correlated with HBT current gain. The effect of threading dislocations on current gain was moderated by the reduction in minority carrier lifetime in the base region, in agreement with existing models for GaAs light-emitting diodes and photovoltaic cells. Current gain was shown to be extremely sensitive to misfit dislocations in the active layers of the HBT—much more sensitive than to threading dislocations. We develop a model for this relationship where increased base current is mediated by Fermi level pinning near misfit dislocations.

Dislocation density of pure copper processed by accumulative roll bonding and equal-channel angular pressing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Miyajima, Yoji, E-mail: miyajima.y.ab@m.titech.ac.jp; Okubo, Satoshi; Abe, Hiroki

The dislocation density of pure copper fabricated by two severe plastic deformation (SPD) processes, i.e., accumulative roll bonding and equal-channel angular pressing, was evaluated using scanning transmission electron microscopy/transmission electron microscopy observations. The dislocation density drastically increased from ~ 10{sup 13} m{sup −} {sup 2} to about 5 × 10{sup 14} m{sup −} {sup 2}, and then saturated, for both SPD processes.

Interfacial Structure and Chemistry of GaN on Ge(111)

NASA Astrophysics Data System (ADS)

Zhang, Siyuan; Zhang, Yucheng; Cui, Ying; Freysoldt, Christoph; Neugebauer, Jörg; Lieten, Ruben R.; Barnard, Jonathan S.; Humphreys, Colin J.

2013-12-01

The interface of GaN grown on Ge(111) by plasma-assisted molecular beam epitaxy is resolved by aberration corrected scanning transmission electron microscopy. A novel interfacial structure with a 5∶4 closely spaced atomic bilayer is observed that explains why the interface is flat, crystalline, and free of GeNx. Density functional theory based total energy calculations show that the interface bilayer contains Ge and Ga atoms, with no N atoms. The 5∶4 bilayer at the interface has a lower energy than a direct stacking of GaN on Ge(111) and enables the 5∶4 lattice-matching growth of GaN.

Impact of Group-II Acceptors on the Electrical and Optical Properties of GaN

NASA Astrophysics Data System (ADS)

Lyons, John L.; Janotti, Anderson; Van de Walle, Chris G.

2013-08-01

We explore the properties of group-II acceptors in GaN by performing hybrid density functional calculations. We find that MgGa gives rise to hole localization in zinc-blende GaN, similar to the behavior in the wurtzite phase. Alternative acceptor impurities, such as Zn and Be, also lead to localized holes in wurtzite GaN, and their ionization energies are larger than that of Mg. All these group-II acceptors also cause large lattice distortions in their neutral charge state, which in turn lead to deep and broad luminescence signals. We explore the consequences of these results for p-type doping.

Leakage current reduction of vertical GaN junction barrier Schottky diodes using dual-anode process

NASA Astrophysics Data System (ADS)

Hayashida, Tetsuro; Nanjo, Takuma; Furukawa, Akihiko; Watahiki, Tatsuro; Yamamuka, Mikio

2018-04-01

The origin of the leakage current of a trench-type vertical GaN diode was discussed. We found that the edge of p-GaN is the main leakage spot. To reduce the reverse leakage current at the edge of p-GaN, a dual-anode process was proposed. As a result, the reverse blocking voltage defined at the leakage current density of 1 mA/cm2 of a vertical GaN junction barrier Schottky (JBS) diode was improved from 780 to 1,190 V, which is the highest value ever reported for vertical GaN Schottky barrier diodes (SBDs).

Droplet heteroepitaxy of zinc-blende vs. wurtzite GaN quantum dots

NASA Astrophysics Data System (ADS)

Reese, C.; Jeon, S.; Hill, T.; Jones, C.; Shusterman, S.; Yacoby, Y.; Clarke, R.; Deng, H.; Goldman, Rs

We have developed a GaN droplet heteroepitaxy process based upon plasma-assisted molecular-beam epitaxy. Using various surface treatments and Ga deposition parameters, we have demonstrated polycrystalline, zinc-blende (ZB), and wurtzite (WZ) GaN quantum dots (QDs) on Si(001), r-Al2O3, Si(111), and c-GaN substrates. For the polar substrates (i.e. Si(111) and c-GaN), high-resolution transmission electron microscopy and coherent Bragg rod analysis reveals the formation of coherent WZ GaN QDs with nitridation-temperature-dependent sizes and densities. For the non-polar substrates (i.e. Si(001) and r-Al2O3) , QDs with strong near-band photoluminescence emission are observed and ZB GaN QD growth on Si(001) is demonstrated for the first time.

Microstructural comparison of the kinematics of discrete and continuum dislocations models

NASA Astrophysics Data System (ADS)

Sandfeld, Stefan; Po, Giacomo

2015-12-01

The Continuum Dislocation Dynamics (CDD) theory and the Discrete Dislocation Dynamics (DDD) method are compared based on concise mathematical formulations of the coarse graining of discrete data. A numerical tool for converting from a discrete to a continuum representation of a given dislocation configuration is developed, which allows to directly compare both simulation approaches based on continuum quantities (e.g. scalar density, geometrically necessary densities, mean curvature). Investigating the evolution of selected dislocation configurations within analytically given velocity fields for both DDD and CDD reveals that CDD contains a surprising number of important microstructural details.

Geometrically Necessary Dislocation Density Evolution in Interstitial Free Steel at Small Plastic Strains

NASA Astrophysics Data System (ADS)

Kundu, Amrita; Field, David P.

2018-06-01

Measurement of geometrically necessary dislocation (GND) density using electron backscatter diffraction (EBSD) has become rather common place in modern metallurgical research. The utility of this measure as an indicator of the expected flow behavior of the material is not obvious. Incorporation of total dislocation density into the Taylor equation relating flow stress to dislocation density is generally accepted, but this does not automatically extend to a similar relationship for the GND density. This is discussed in the present work using classical equations for isotropic metal plasticity in a rather straight-forward theoretical framework. This investigation examines the development of GND structure in a commercially produced interstitial free steel subject to tensile deformation. Quantification of GND density was carried out using conventional EBSD at various strain levels on the surface of a standard dog-bone-shaped tensile specimen. There is linear increase of the average GND density with imposed macroscopic strain. This is in agreement with the established framework.

A dislocation density-based continuum model of the anisotropic shock response of single crystal α-cyclotrimethylene trinitramine

NASA Astrophysics Data System (ADS)

Luscher, D. J.; Addessio, F. L.; Cawkwell, M. J.; Ramos, K. J.

2017-01-01

We have developed a model for the finite deformation thermomechanical response of α-cyclotrimethylene trinitramine (RDX). Our model accounts for nonlinear thermoelastic lattice deformation through a free energy-based equation of state developed by Cawkwell et al. (2016) in combination with temperature and pressure dependent elastic constants, as well as dislocation-mediated plastic slip on a set of slip systems motivated by experimental observation. The kinetics of crystal plasticity are modeled using the Orowan equation relating slip rate to dislocation density and the dislocation velocity developed by Austin and McDowell (2011), which naturally accounts for transition from thermally activated to dislocation drag limited regimes. Evolution of dislocation density is specified in terms of local ordinary differential equations reflecting dislocation-dislocation interactions. This paper presents details of the theory and parameterization of the model, followed by discussion of simulations of flyer plate impact experiments. Impact conditions explored within this combined simulation and experimental effort span shock pressures ranging from 1 to 3 GPa for four crystallographic orientations and multiple specimen thicknesses. Simulation results generated using this model are shown to be in strong agreement with velocimetry measurements from the corresponding plate impact experiments. Finally, simulation results are used to motivate conclusions about the nature of dislocation-mediated plasticity in RDX.

Characterization of Deep and Shallow Levels in GaN

NASA Astrophysics Data System (ADS)

Wessels, Bruce

1997-03-01

The role of native defects and impurities in compensating n-type GaN was investigated. From the observed dependence of carrier concentration on dopant partial pressure the compensating acceptor in n-type material is attributed to the triply charged gallium vacancy. This is consistent with recent calculations on defect stability using density functional theory. The interaction of hydrogen and point defects in GaN was also investigated using FTIR. The role of these defects in compensation will be discussed.

Effect of strain rate and dislocation density on the twinning behavior in Tantalum

DOE PAGES

Florando, Jeffrey N.; El-Dasher, Bassem S.; Chen, Changqiang; ...

2016-04-28

The conditions which affect twinning in tantalum have been investigated across a range of strain rates and initial dislocation densities. Tantalum samples were subjected to a range of strain rates, from 10 –4/s to 10 3/s under uniaxial stress conditions, and under laser-induced shock-loading conditions. In this study, twinning was observed at 77K at strain rates from 1/s to 103/s, and during laser-induced shock experiments. The effect of the initial dislocation density, which was imparted by deforming the material to different amounts of pre-strain, was also studied, and it was shown that twinning is suppressed after a given amount ofmore » pre-strain, even as the global stress continues to increase. These results indicate that the conditions for twinning cannot be represented solely by a critical global stress value, but are also dependent on the evolution of the dislocation density. Additionally, the analysis shows that if twinning is initiated, the nucleated twins may continue to grow as a function of strain, even as the dislocation density continues to increase.« less

Characteristics of dislocation structure in creep deformed lamellar tial alloy within primary regime

NASA Astrophysics Data System (ADS)

Cho, H. S.; Nam, Soo W.

1999-06-01

In this investigation, dislocations of a lamellar TiAl alloy are analyzed after creeping in the primary range at 800°C/200MPa in order to interpret their mobility It was found that the dislocation density in γ-laths decreased as the creep deformation proceeds within primary creep regime Schmid factor analysis suggests that the creep deformation in the early stage of the primary creep regime is controlled by the gliding of some of the initial dislocations which have a high enough Schmid factor As the creep deformation progressed, those dislocations with high Schmid factors slip preferentially to be annihilated at the α-γ interface For further continuous deformation, dislocation generation is required, and for this, α-phase is transformed to γ-phase in order to generate new dislocations A slow dislocation generation process by phase transformation of α-phase compared with the absorbing rate to sinks is responsible for the decreasing dislocation density as the creep strain increases

Vacancy-type defects in Al2O3/GaN structure probed by monoenergetic positron beams

NASA Astrophysics Data System (ADS)

Uedono, Akira; Nabatame, Toshihide; Egger, Werner; Koschine, Tönjes; Hugenschmidt, Christoph; Dickmann, Marcel; Sumiya, Masatomo; Ishibashi, Shoji

2018-04-01

Defects in the Al2O3(25 nm)/GaN structure were probed by using monoenergetic positron beams. Al2O3 films were deposited on GaN by atomic layer deposition at 300 °C. Temperature treatment above 800 °C leads to the introduction of vacancy-type defects in GaN due to outdiffusion of atoms from GaN into Al2O3. The width of the damaged region was determined to be 40-50 nm from the Al2O3/GaN interface, and some of the vacancies were identified to act as electron trapping centers. In the Al2O3 film before and after annealing treatment at 300-900 °C, open spaces with three different sizes were found to coexist. The density of medium-sized open spaces started to decrease above 800 °C, which was associated with the interaction between GaN and Al2O3. Effects of the electron trapping/detrapping processes of interface states on the flat band voltage and the defects in GaN were also discussed.

Understanding the Growth Mechanism of GaN Epitaxial Layers on Mechanically Exfoliated Graphite

NASA Astrophysics Data System (ADS)

Li, Tianbao; Liu, Chenyang; Zhang, Zhe; Yu, Bin; Dong, Hailiang; Jia, Wei; Jia, Zhigang; Yu, Chunyan; Gan, Lin; Xu, Bingshe; Jiang, Haiwei

2018-04-01

The growth mechanism of GaN epitaxial layers on mechanically exfoliated graphite is explained in detail based on classic nucleation theory. The number of defects on the graphite surface can be increased via O-plasma treatment, leading to increased nucleation density on the graphite surface. The addition of elemental Al can effectively improve the nucleation rate, which can promote the formation of dense nucleation layers and the lateral growth of GaN epitaxial layers. The surface morphologies of the nucleation layers, annealed layers and epitaxial layers were characterized by field-emission scanning electron microscopy, where the evolution of the surface morphology coincided with a 3D-to-2D growth mechanism. High-resolution transmission electron microscopy was used to characterize the microstructure of GaN. Fast Fourier transform diffraction patterns showed that cubic phase (zinc-blend structure) GaN grains were obtained using conventional GaN nucleation layers, while the hexagonal phase (wurtzite structure) GaN films were formed using AlGaN nucleation layers. Our work opens new avenues for using highly oriented pyrolytic graphite as a substrate to fabricate transferable optoelectronic devices.

Understanding the Growth Mechanism of GaN Epitaxial Layers on Mechanically Exfoliated Graphite.

PubMed

Li, Tianbao; Liu, Chenyang; Zhang, Zhe; Yu, Bin; Dong, Hailiang; Jia, Wei; Jia, Zhigang; Yu, Chunyan; Gan, Lin; Xu, Bingshe; Jiang, Haiwei

2018-04-27

The growth mechanism of GaN epitaxial layers on mechanically exfoliated graphite is explained in detail based on classic nucleation theory. The number of defects on the graphite surface can be increased via O-plasma treatment, leading to increased nucleation density on the graphite surface. The addition of elemental Al can effectively improve the nucleation rate, which can promote the formation of dense nucleation layers and the lateral growth of GaN epitaxial layers. The surface morphologies of the nucleation layers, annealed layers and epitaxial layers were characterized by field-emission scanning electron microscopy, where the evolution of the surface morphology coincided with a 3D-to-2D growth mechanism. High-resolution transmission electron microscopy was used to characterize the microstructure of GaN. Fast Fourier transform diffraction patterns showed that cubic phase (zinc-blend structure) GaN grains were obtained using conventional GaN nucleation layers, while the hexagonal phase (wurtzite structure) GaN films were formed using AlGaN nucleation layers. Our work opens new avenues for using highly oriented pyrolytic graphite as a substrate to fabricate transferable optoelectronic devices.

Influence of basal-plane dislocation structures on expansion of single Shockley-type stacking faults in forward-current degradation of 4H-SiC p-i-n diodes

NASA Astrophysics Data System (ADS)

Hayashi, Shohei; Yamashita, Tamotsu; Senzaki, Junji; Miyazato, Masaki; Ryo, Mina; Miyajima, Masaaki; Kato, Tomohisa; Yonezawa, Yoshiyuki; Kojima, Kazutoshi; Okumura, Hajime

2018-04-01

The origin of expanded single Shockley-type stacking faults in forward-current degradation of 4H-SiC p-i-n diodes was investigated by the stress-current test. At a stress-current density lower than 25 A cm-2, triangular stacking faults were formed from basal-plane dislocations in the epitaxial layer. At a stress-current density higher than 350 A cm-2, both triangular and long-zone-shaped stacking faults were formed from basal-plane dislocations that converted into threading edge dislocations near the interface between the epitaxial layer and the substrate. In addition, the conversion depth of basal-plane dislocations that expanded into the stacking fault was inside the substrate deeper than the interface. These results indicate that the conversion depth of basal-plane dislocations strongly affects the threshold stress-current density at which the expansion of stacking faults occurs.

Carrier concentration dependence of donor activation energy in n-type GaN epilayers grown on Si (1 1 1) by plasma-assisted MBE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kumar, Mahesh; Central Research Laboratory, Bharat Electronics, Bangalore 560 013; Bhat, Thirumaleshwara N.

Highlights: ► The n-type GaN layers were grown by plasma-assisted molecular beam epitaxy. ► The optical characteristics of a donor level in Si-doped GaN were studied. ► Activation energy of a Si-related donor was estimated from temperature dependent PL measurements. ► PL peak positions, FWHM of PL and activation energies are found to be proportional to the cube root of carrier density. ► The involvement of donor levels is supported by the temperature-dependent electron concentration measurements. -- Abstract: The n-type GaN layers were grown by plasma-assisted MBE and either intentionally doped with Si or unintentionally doped. The optical characteristics ofmore » a donor level in Si-doped, GaN were studied in terms of photoluminescence (PL) spectroscopy as a function of electron concentration. Temperature dependent PL measurements allowed us to estimate the activation energy of a Si-related donor from temperature-induced decay of PL intensity. PL peak positions, full width at half maximum of PL and activation energies are found to be proportional to the cube root of carrier density. The involvement of donor levels is supported by the temperature-dependent electron concentration measurements.« less

Modeling and optimal designs for dislocation and radiation tolerant single and multijunction solar cells

NASA Astrophysics Data System (ADS)

Mehrotra, A.; Alemu, A.; Freundlich, A.

2011-02-01

Crystalline defects (e.g. dislocations or grain boundaries) as well as electron and proton induced defects cause reduction of minority carrier diffusion length which in turn results in degradation of efficiency of solar cells. Hetro-epitaxial or metamorphic III-V devices with low dislocation density have high BOL efficiencies but electron-proton radiation causes degradation in EOL efficiencies. By optimizing the device design (emitter-base thickness, doping) we can obtain highly dislocated metamorphic devices that are radiation resistant. Here we have modeled III-V single and multi junction solar cells using drift and diffusion equations considering experimental III-V material parameters, dislocation density, 1 Mev equivalent electron radiation doses, thicknesses and doping concentration. Thinner device thickness leads to increment in EOL efficiency of high dislocation density solar cells. By optimizing device design we can obtain nearly same EOL efficiencies from high dislocation solar cells than from defect free III-V multijunction solar cells. As example defect free GaAs solar cell after optimization gives 11.2% EOL efficiency (under typical 5x1015cm-2 1 MeV electron fluence) while a GaAs solar cell with high dislocation density (108 cm-2) after optimization gives 10.6% EOL efficiency. The approach provides an additional degree of freedom in the design of high efficiency space cells and could in turn be used to relax the need for thick defect filtering buffer in metamorphic devices.

Zinc-blende MnN bilayer formation on the GaN(111) surface

NASA Astrophysics Data System (ADS)

Gutierrez-Ojeda, S. J.; Guerrero-Sánchez, J.; Garcia-Diaz, R.; Ramirez-Torres, A.; Takeuchi, Noboru; H. Cocoletzi, Gregorio

2017-07-01

Atomic layers of manganese nitride, deposited on the cubic gallium nitride (111) surface, are investigated using spin polarized periodic density functional theory calculations. The adsorption of a manganese atom has been evaluated at different high symmetry sites. Incorporation into the GaN substrate by replacing gallium atoms drives the formation of a site in which the displaced Ga atom forms bonds with Ga atoms at the surface. This energetically favorable configuration shows a ferromagnetic alignment. Surface formation energy calculations demonstrate that when a full Mn ML is incorporated into the GaN structure, a Ga ML on top of a MnN bilayer may be formed for very Ga-rich conditions. On the other hand, when a full Mn ML is deposited on top of the nitrogen terminated surface, an epitaxial MnN bilayer is formed with antiferromagnetic characteristics. Density of states and partial density of states are reported to show the antiferromagnetic alignment in both structures. This behavior is mainly induced by the Mn-d orbitals.

Method to reduce dislocation density in silicon using stress

DOEpatents

Buonassisi, Anthony; Bertoni, Mariana; Argon, Ali; Castellanos, Sergio; Fecych, Alexandria; Powell, Douglas; Vogl, Michelle

2013-03-05

A crystalline material structure with reduced dislocation density and method of producing same is provided. The crystalline material structure is annealed at temperatures above the brittle-to-ductile transition temperature of the crystalline material structure. One or more stress elements are formed on the crystalline material structure so as to annihilate dislocations or to move them into less harmful locations.

Electron spin relaxation in two polymorphic structures of GaN

NASA Astrophysics Data System (ADS)

Kang, Nam Lyong

2015-03-01

The relaxation process of electron spin in systems of electrons interacting with piezoelectric deformation phonons that are mediated through spin-orbit interactions was interpreted from a microscopic point of view using the formula for the electron spin relaxation times derived by a projection-reduction method. The electron spin relaxation times in two polymorphic structures of GaN were calculated. The piezoelectric material constant for the wurtzite structure obtained by a comparison with a previously reported experimental result was {{P}pe}=1.5 × {{10}29} eV {{m}-1}. The temperature and magnetic field dependence of the relaxation times for both wurtzite and zinc-blende structures were similar, but the relaxation times in zinc-blende GaN were smaller and decreased more rapidly with increasing temperature and magnetic field than that in wurtzite GaN. This study also showed that the electron spin relaxation for wurtzite GaN at low density could be explained by the Elliot-Yafet process but not for zinc-blende GaN in the metallic regime.

The formation mechanisms of surface nanocrystallites in β-type biomedical TiNbZrFe alloy by surface mechanical attrition treatment

NASA Astrophysics Data System (ADS)

Jin, Lei; Cui, Wenfang; Song, Xiu; Zhou, Lian

2015-08-01

A nanostructured surface layer was successfully performed on a biomedical β-type TiNbZrFe alloy by surface mechanical attrition treatment (SMAT). The results reveal that the surface layer along the depth from treated surface to strain-free matrix could be divided into an outer nanocrystalline layer (0-30 μm), a high-density dislocation region (30-200 μm) and an inner region with low-density dislocations and twins (200-700 μm) when the surface was treated for 60 min. The microhardness of the surface layer is enhanced and increases with increasing treatment time. Although the {1 1 2} <1 1 1> twin coordinates the deformations with dislocations, this coordination only occurs in the low strain area and cannot affect the nanocrystalline formation. The self-nanocrystallization of TiNbZrFe alloy is mainly attributed to dislocation movements. First, the dislocations start to move and easily form dislocation bands along certain crystal directions; then, multiple slips of dislocations gradually form dislocation tangles; after that, high-density dislocation tangles increases, which divides primary grains into many small domain areas. As high strain energies accumulate on the interfaces among these areas, the lattice rotation can be driven between the adjacent small domain areas, finally resulting in a large number of nanocrystalline regions with low or large angle grain boundaries.

Materials properties and dislocation dynamics in InAsP compositionally graded buffers on InP substrates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jandl, Adam, E-mail: jandl@mit.edu; Bulsara, Mayank T.; Fitzgerald, Eugene A.

The properties of InAs{sub x}P{sub 1−x} compositionally graded buffers grown by metal organic chemical vapor deposition are investigated. We report the effects of strain gradient (ε/thickness), growth temperature, and strain initiation sequence (gradual or abrupt strain introduction) on threading dislocation density, surface roughness, epi-layer relaxation, and tilt. We find that gradual introduction of strain causes increased dislocation densities (>10{sup 6}/cm{sup 2}) and tilt of the epi-layer (>0.1°). A method of abrupt strain initiation is proposed which can result in dislocation densities as low as 1.01 × 10{sup 5} cm{sup −2} for films graded from the InP lattice constant to InAs{sub 0.15}P{sub 0.85}.more » A model for a two-energy level dislocation nucleation system is proposed based on our results.« less

Computational issues in the simulation of two-dimensional discrete dislocation mechanics

NASA Astrophysics Data System (ADS)

Segurado, J.; LLorca, J.; Romero, I.

2007-06-01

The effect of the integration time step and the introduction of a cut-off velocity for the dislocation motion was analysed in discrete dislocation dynamics (DD) simulations of a single crystal microbeam. Two loading modes, bending and uniaxial tension, were examined. It was found that a longer integration time step led to a progressive increment of the oscillations in the numerical solution, which would eventually diverge. This problem could be corrected in the simulations carried out in bending by introducing a cut-off velocity for the dislocation motion. This strategy (long integration times and a cut-off velocity for the dislocation motion) did not recover, however, the solution computed with very short time steps in uniaxial tension: the dislocation density was overestimated and the dislocation patterns modified. The different response to the same numerical algorithm was explained in terms of the nature of the dislocations generated in each case: geometrically necessary in bending and statistically stored in tension. The evolution of the dislocation density in the former was controlled by the plastic curvature of the beam and was independent of the details of the simulations. On the contrary, the steady-state dislocation density in tension was determined by the balance between nucleation of dislocations and those which are annihilated or which exit the beam. Changes in the DD imposed by the cut-off velocity altered this equilibrium and the solution. These results point to the need for detailed analyses of the accuracy and stability of the dislocation dynamic simulations to ensure that the results obtained are not fundamentally affected by the numerical strategies used to solve this complex problem.

A continuum theory of edge dislocations

NASA Astrophysics Data System (ADS)

Berdichevsky, V. L.

2017-09-01

Continuum theory of dislocation aims to describe the behavior of large ensembles of dislocations. This task is far from completion, and, most likely, does not have a "universal solution", which is applicable to any dislocation ensemble. In this regards it is important to have guiding lines set by benchmark cases, where the transition from a discrete set of dislocations to a continuum description is made rigorously. Two such cases have been considered recently: equilibrium of dislocation walls and screw dislocations in beams. In this paper one more case is studied, equilibrium of a large set of 2D edge dislocations placed randomly in a 2D bounded region. The major characteristic of interest is energy of dislocation ensemble, because it determines the structure of continuum equations. The homogenized energy functional is obtained for the periodic dislocation ensembles with a random contents of the periodic cell. Parameters of the periodic structure can change slowly on distances of order of the size of periodic cells. The energy functional is obtained by the variational-asymptotic method. Equilibrium positions are local minima of energy. It is confirmed the earlier assertion that energy density of the system is the sum of elastic energy of averaged elastic strains and microstructure energy, which is elastic energy of the neutralized dislocation system, i.e. the dislocation system placed in a constant dislocation density field making the averaged dislocation density zero. The computation of energy is reduced to solution of a variational cell problem. This problem is solved analytically. The solution is used to investigate stability of simple dislocation arrays, i.e. arrays with one dislocation in the periodic cell. The relations obtained yield two outcomes: First, there is a state parameter of the system, dislocation polarization; averaged stresses affect only dislocation polarization and cannot change other characteristics of the system. Second, the structure of dislocation phase space is strikingly simple. Dislocation phase space is split in a family of subspaces corresponding to constant values of dislocation polarizations; in each equipolarization subspace there are many local minima of energy; for zero external stresses the system is stuck in a local minimum of energy; for non-zero slowly changing external stress, dislocation polarization evolves, while the system moves over local energy minima of equipolarization subspaces. Such a simple picture of dislocation dynamics is due to the presence of two time scales, slow evolution of dislocation polarization and fast motion of the system over local minima of energy. The existence of two time scales is justified for a neutral system of edge dislocations.

Microstructural investigation of plastically deformed Ti{sub 20}Zr{sub 20}Hf{sub 20}Nb{sub 20}Ta{sub 20} high entropy alloy by X-ray diffraction and transmission electron microscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dirras, G., E-mail: dirras@univ-paris13.fr; Gubicza, J.; Heczel, A.

2015-10-15

The microstructure evolution in body-centered cubic (bcc) Ti{sub 20}Zr{sub 20}Hf{sub 20}Nb{sub 20}Ta{sub 20} high entropy alloy during quasi-static compression test was studied by X-ray line profile analysis (XLPA) and transmission electron microscopy (TEM). The average lattice constant and other important parameters of the microstructure such as the mean crystallite size, the dislocation density and the edge/screw character of dislocations were determined by XLPA. The elastic anisotropy factor required for XLPA procedure was determined by nanoindentation. XLPA shows that the crystallite size decreased while the dislocation density increased with strain during compression, and their values reached about 39 nm and 15more » × 10{sup 14} m{sup −2}, respectively, at a plastic strain of ~ 20%. It was revealed that with increasing strain the dislocation character became more screw. This can be explained by the reduced mobility of screw dislocations compared to edge dislocations in bcc structures. These observations are in line with TEM investigations. The development of dislocation density during compression was related to the yield strength evolution. - Highlights: • Ti{sub 20}Zr{sub 20}Hf{sub 20}Nb{sub 20}Ta{sub 20} high entropy alloy was processed by arc-melting. • The mechanical was evaluated by RT compression test. • The microstructure evolution was studied by XLPA and TEM. • With increasing strain the dislocation character became more screw. • The yield strength was related to the development of the dislocation density.« less

Effect of strain rate and dislocation density on the twinning behavior in tantalum

DOE Office of Scientific and Technical Information (OSTI.GOV)

Florando, Jeffrey N., E-mail: florando1@llnl.gov; Swift, Damian C.; Barton, Nathan R.

2016-04-15

The conditions which affect twinning in tantalum have been investigated across a range of strain rates and initial dislocation densities. Tantalum samples were subjected to a range of strain rates, from 10{sup −4}/s to 10{sup 3}/s under uniaxial stress conditions, and under laser-induced shock-loading conditions. In this study, twinning was observed at 77 K at strain rates from 1/s to 10{sup 3}/s, and during laser-induced shock experiments. The effect of the initial dislocation density, which was imparted by deforming the material to different amounts of pre-strain, was also studied, and it was shown that twinning is suppressed after a givenmore » amount of pre-strain, even as the global stress continues to increase. These results indicate that the conditions for twinning cannot be represented solely by a critical global stress value, but are also dependent on the evolution of the dislocation density. In addition, the analysis shows that if twinning is initiated, the nucleated twins may continue to grow as a function of strain, even as the dislocation density continues to increase.« less

Computational modelling of mesoscale dislocation patterning and plastic deformation of single crystals

NASA Astrophysics Data System (ADS)

Xia, Shengxu; El-Azab, Anter

2015-07-01

We present a continuum dislocation dynamics model that predicts the formation of dislocation cell structure in single crystals at low strains. The model features a set of kinetic equations of the curl type that govern the space and time evolution of the dislocation density in the crystal. These kinetic equations are coupled to stress equilibrium and deformation kinematics using the eigenstrain approach. A custom finite element method has been developed to solve the coupled system of equations of dislocation kinetics and crystal mechanics. The results show that, in general, dislocations self-organize in patterns under their mutual interactions. However, the famous dislocation cell structure has been found to form only when cross slip is implemented in the model. Cross slip is also found to lower the yield point, increase the hardening rate, and sustain an increase in the dislocation density over the hardening regime. Analysis of the cell structure evolution reveals that the average cell size decreases with the applied stress, which is consistent with the similitude principle.

Direct bonding of gallium nitride to silicon carbide: Physical, and electrical characterization

NASA Astrophysics Data System (ADS)

Lee, Jaeseob

The direct bonding method is applied to the GaN/SiC system, and the processing conditions for successful direct bonding are clarified. Direct bonding of GaN/SiC is achieved at 900°C. The direct bonding of GaN to Si-face SiC is very dependent on the choice of chemical treatments, but the bonding of GaN to C-face SiC is less dependent on surface preparation. If a native oxide is present when the bonded interface is prepared, the current through the interface is decreased, which is attributed to an energy barrier due to the presence of charged interface states. TEM images indicate 10nm spaced dislocations at the interface for the GaN/SiC (Si-face), and ˜6nm for the GaN/SiC (C-face), which form to accommodate the lattice mismatch (3.4%) and twist (1˜2°) and tilt misfit (0.2° for Si-face SiC and 3° for C-face SiC). In some regions (˜30%) an amorphous oxide layer forms at the interface, which is attributed to inadequate surface preparation prior to bonding. The strain of the GaN film with a Ga/C interface was ˜0.1%, tensile strain, and that of GaN with a Ga/Si interface was ˜0.2%, tensile strain. Our analysis indicates that the GaN/SiC thermal misfit dominates the strain of the GaN after bonding. The electrical characteristics of n-p GaN/SiC heterojunctions display diode ideality factors, saturation currents, energy barrier heights, and band offsets of 1.5 +/- 0.1, 10-13 A/cm 2, 0.75 +/- 0.10 eV, and DeltaEC = 0.87 +/- 0.10 eV for the Ga/Si interface and 1.2 +/- 0.1, 10 -16 A/cm2, 0.56 +/- 0.10 eV, and Delta EC = 0.46 +/- 0.10 eV for the Ga/C interface.

Electronic and Optical Properties of Two-Dimensional GaN from First-Principles.

PubMed

Sanders, Nocona; Bayerl, Dylan; Shi, Guangsha; Mengle, Kelsey A; Kioupakis, Emmanouil

2017-12-13

Gallium nitride (GaN) is an important commercial semiconductor for solid-state lighting applications. Atomically thin GaN, a recently synthesized two-dimensional material, is of particular interest because the extreme quantum confinement enables additional control of its light-emitting properties. We performed first-principles calculations based on density functional and many-body perturbation theory to investigate the electronic, optical, and excitonic properties of monolayer and bilayer two-dimensional (2D) GaN as a function of strain. Our results demonstrate that light emission from monolayer 2D GaN is blueshifted into the deep ultraviolet range, which is promising for sterilization and water-purification applications. Light emission from bilayer 2D GaN occurs at a similar wavelength to its bulk counterpart due to the cancellation of the effect of quantum confinement on the optical gap by the quantum-confined Stark shift. Polarized light emission at room temperature is possible via uniaxial in-plane strain, which is desirable for energy-efficient display applications. We compare the electronic and optical properties of freestanding two-dimensional GaN to atomically thin GaN wells embedded within AlN barriers in order to understand how the functional properties are influenced by the presence of barriers. Our results provide microscopic understanding of the electronic and optical characteristics of GaN at the few-layer regime.

Improvement of efficiency in graphene/gallium nitride nanowire on Silicon photoelectrode for overall water splitting

NASA Astrophysics Data System (ADS)

Bae, Hyojung; Rho, Hokyun; Min, Jung-Wook; Lee, Yong-Tak; Lee, Sang Hyun; Fujii, Katsushi; Lee, Hyo-Jong; Ha, Jun-Seok

2017-11-01

Gallium nitride (GaN) nanowires are one of the most promising photoelectrode materials due to their high stability in acidic and basic electrolytes, and tunable band edge potentials. In this study, GaN nanowire arrays (GaN NWs) were prepared by molecular beam epitaxy (MBE); their large surface area enhanced the solar to hydrogen conversion efficiency. More significantly, graphene was grown by chemical vapor deposition (CVD), which enhanced the electron transfer between NWs for water splitting and protected the GaN NW surface. Structural characterizations of the prepared composite were performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photocurrent density of Gr/GaN NWs exhibited a two-fold increase over pristine GaN NWs and sustained water splitting up to 70 min. These improvements may accelerate possible applications for hydrogen generation with high solar to hydrogen conversion efficiency.

First-Principle Study of the Optical Properties of Dilute-P GaN1-xPx Alloys.

PubMed

Borovac, Damir; Tan, Chee-Keong; Tansu, Nelson

2018-04-16

An investigation on the optical properties of dilute-P GaN 1-x P x alloys by First-Principle Density Functional Theory (DFT) methods is presented, for phosphorus (P) content varying from 0% up to 12.5%. Findings on the imaginary and real part of the dielectric function are analyzed and the results are compared with previously reported theoretical works on GaN. The complex refractive index, normal-incidence reflectivity and birefringence are presented and a difference in the refractive index in the visible regime between GaN and GaNP alloys of ~0.3 can be engineered by adding minute amounts of phosphorus, indicating strong potential for refractive index tunability. The optical properties of the GaN 1-x P x alloys indicate their strong potential for implementation in various III-nitride-based photonic waveguide applications and Distributed Bragg Reflectors (DBR).

Tuning electronic and magnetic properties of GaN nanosheets by surface modifications and nanosheet thickness.

PubMed

Xiao, Meixia; Yao, Tingzhen; Ao, Zhimin; Wei, Peng; Wang, Danghui; Song, Haiyang

2015-04-14

Density-functional theory calculations are performed to investigate the effects of surface modifications and nanosheet thickness on the electronic and magnetic properties of gallium nitride (GaN) nanosheets (NSs). Unlike the bare GaN NSs terminating with polar surfaces, the systems with hydrogenated Ga (H-GaN), fluorinated Ga (F-GaN), and chlorinated Ga (Cl-GaN) preserve their initial wurtzite structures and exhibit ferromagnetic states. The abovementioned three different decorations on Ga atoms are energetically more favorable for thicker GaN NSs. Moreover, as the thickness increases, H-GaN and F-GaN NSs undergo semiconductor to metal and half-metal to metal transition, respectively, while Cl-GaN NSs remain completely metallic. The predicted diverse and tunable electronic and magnetic properties highlight the potential of GaN NSs for novel electronic and spintronic nanodevices.

High purity low dislocation GaAs single crystals

NASA Technical Reports Server (NTRS)

Chen, R. T.; Holmes, D. E.; Kirkpatrick, C. G.

1982-01-01

Recent advances in GaAs bulk crystal growth using the LEC (liquid encapsulated Czochralski) technique are described. The dependence of the background impurity concentration and the dislocation density distribution on the materials synthesis and growth conditions were investigated. Background impurity concentrations as low as 4 x 10 to the 15th power were observed in undoped LEC GaAs. The dislocation density in selected regions of individual ingots was very low, below the 3000 cm .3000/sq cm threshold. The average dislocation density over a large annular ring on the wafers fell below the 10000/sq cm level for 3 inch diameter ingots. The diameter control during the program advanced to a diameter variation along a 3 inch ingot less than 2 mm.

Microstructure of calcite deformed by high-pressure torsion: An X-ray line profile study

NASA Astrophysics Data System (ADS)

Schuster, Roman; Schafler, Erhard; Schell, Norbert; Kunz, Martin; Abart, Rainer

2017-11-01

Calcite aggregates were deformed to high strain using high-pressure torsion and applying confining pressures of 1-6 GPa and temperatures between room temperature and 450 °C. The run products were characterized by X-ray diffraction, and key microstructural parameters were extracted employing X-ray line profile analysis. The dominant slip system was determined as r { 10 1 bar 4 } ⟨ 2 bar 021 ⟩ with edge dislocation character. The resulting dislocation density and the size of the coherently scattering domains (CSD) exhibit a systematic dependence on the P-T conditions of deformation. While high pressure generally impedes recovery through reducing point defect mobility, the picture is complicated by pressure-induced phase transformations in the CaCO3 system. Transition from the calcite stability field to those of the high-pressure polymorphs CaCO3-II, CaCO3-III and CaCO3-IIIb leads to a change of the microstructural evolution with deformation. At 450 °C and pressures within the calcite stability field, dislocation densities and CSD sizes saturate at shear strains exceeding 10 in agreement with earlier studies at lower pressures. In the stability field of CaCO3-II, the dislocation density exhibits a more complex behavior. Furthermore, at a given strain and strain rate, the dislocation density increases and the CSD size decreases with increasing pressure within the stability fields of either calcite or of the high-pressure polymorphs. There is, however, a jump from high dislocation densities and small CSDs in the upper pressure region of the calcite stability field to lower dislocation densities and larger CSDs in the low-pressure region of the CaCO3-II stability field. This jump is more pronounced at higher temperatures and less so at room temperature. The pressure influence on the deformation-induced evolution of dislocation densities implies that pressure variations may change the rheology of carbonate rocks. In particular, a weakening is expected to occur at the transition from the calcite to the CaCO3-II stability field, if aragonite does not form.

Highly resistive C-doped hydride vapor phase epitaxy-GaN grown on ammonothermally crystallized GaN seeds

NASA Astrophysics Data System (ADS)

Iwinska, Malgorzata; Piotrzkowski, Ryszard; Litwin-Staszewska, Elzbieta; Sochacki, Tomasz; Amilusik, Mikolaj; Fijalkowski, Michal; Lucznik, Boleslaw; Bockowski, Michal

2017-01-01

GaN crystals were grown by hydride vapor phase epitaxy (HVPE) and doped with C. The seeds were high-structural-quality ammonothermally crystallized GaN. The grown crystals were highly resistive at 296 K and of high structural quality. High-temperature Hall effect measurements revealed p-type conductivity and a deep acceptor level in the material with an activation energy of 1 eV. This is in good agreement with density functional theory calculations based on hybrid functionals as presented by the Van de Walle group. They obtained an ionization energy of 0.9 eV when C was substituted for N in GaN and acted as a deep acceptor.

Polar semiconductor heterojunction structure energy band diagram considerations

NASA Astrophysics Data System (ADS)

Lin, Shuxun; Wen, Cheng P.; Wang, Maojun; Hao, Yilong

2016-03-01

The unique nature of built-in electric field induced positive/negative charge pairs of polar semiconductor heterojunction structure has led to a more realistic device model for hexagonal III-nitride HEMT. In this modeling approach, the distribution of charge carriers is dictated by the electrostatic potential profile instead of Femi statistics. The proposed device model is found suitable to explain peculiar properties of GaN HEMT structures, including: (1) Discrepancy in measured conventional linear transmission line model (LTLM) sheet resistance and contactless sheet resistance of GaN HEMT with thin barrier layer. (2) Below bandgap radiation from forward biased Nickel Schottky barrier diode on GaN HEMT structure. (3) GaN HEMT barrier layer doping has negligible effect on transistor channel sheet charge density.

Spin polarized first principles study of Mn doped gallium nitride monolayer nanosheet

NASA Astrophysics Data System (ADS)

Sharma, Venus; Kaur, Sumandeep; Srivastava, Sunita; Kumar, Tankeshwar

2017-05-01

The structural, electronic and magnetic properties of gallium nitride nanosheet (GaNs) doped with Mn atoms have been studied using spin polarized density functional theory. The binding energy per atom, Energy Band gap, Fermi energy, magnetic moment, electric dipole moment have been found. The doped nanosheet is found to be more stable than pure GaN monolayer nanosheet. Adsorption of Mn atom has been done at four different sites on GaNs which affects the fermi level position. It is found that depending on the doping site, Mn can behave both like p-type semiconductor and also as n-type semiconductor. Also, it is ascertained that Mn doped GaNs (GaNs-Mn) exhibits ferromagnetic behavior.

Radiation Tolerant Interfaces: Influence of Local Stoichiometry at the Misfit Dislocation on Radiation Damage Resistance of Metal/Oxide Interfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shutthanandan, Vaithiyalingam; Choudhury, Samrat; Manandhar, Sandeep

To understand how variations in interface properties such as misfit-dislocation density and local chemistry affect radiation-induced defect absorption and recombination, we have explored a model system of CrxV1-x alloy epitaxial films deposited on MgO single crystals. By controlling film composition, the lattice mismatch with MgO was adjusted so that the misfit-dislocation density varies at the interface. These interfaces were exposed to irradiation and in situ results show that the film with a semi-coherent interface (Cr) withstands irradiation while V film, which has similar semi-coherent interface like Cr, showed the largest damage. Theoretical calculations indicate that, unlike at metal/metal interfaces, themore » misfit dislocation density does not dominate radiation damage tolerance at metal/oxide interfaces. Rather, the stoichiometry, and the precise location of the misfit-dislocation density relative to the interface, drives defect behavior. Together, these results demonstrate the sensitivity of defect recombination to interfacial chemistry and provide new avenues for engineering radiation-tolerant nanomaterials.« less

Stoichiometry effect on the irradiation response in the microstructure of zirconium carbides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Young Yang; Wei-Yang Lo; Clayton Dickerson

2014-11-01

Zone-refined ultra high pure ZrC with five C/Zr ratios ranging from 0.84 to 1.17 was irradiated using a 2 MeV proton beam at 1125 C. The stoichiometry effect on the irradiation response of ZrC microstructure was examined using transmission electron microscopy following the irradiation. The irradiated microstructures generally feature a high density of perfect dislocation loops particularly at away from the graphite precipitates, and the C/Zr ratio shows a notable effect on the size and density of dislocation loops. The dislocation loops are identified as interstitial type perfect loops, and it was indirectly proved that the dislocation loop core likelymore » consists of carbon atoms. Graphite precipitates that form with excess carbon in the super-stoichiometric ZrC are detrimental, and the dramatic increases in the size of and density of dislocation loops in the vicinity of graphite precipitates in ZrC phase were observed. Irradiationinduced faceted voids were only observed in ZrC0.95, which is attributed to the pre-existing dislocation lines as biased sinks for vacancies.« less

The Effects of Prior Cold Work on the Shock Response of Copper

NASA Astrophysics Data System (ADS)

Millett, J. C. F.; Higgins, D. L.; Chapman, D. J.; Whiteman, G.; Jones, I. P.; Chiu, Y.-L.

2018-04-01

A series of experiments have been performed to probe the effects of dislocation density on the shock response of copper. The shear strength immediately behind the shock front has been measured using embedded manganin stress gauges, whilst the post shock microstructural and mechanical response has been monitored via one-dimensional recovery experiments. Material in the half hard (high dislocation density) condition was shown to have both a higher shear strength and higher rate of change of shear strength with impact stress than its annealed (low dislocation density) counterpart. Microstructural analysis showed a much higher dislocation density in the half hard material compared to the annealed after shock loading, whilst post shock mechanical examination showed a significant degree of hardening in the annealed state with reduced, but still significant amount in the half hard state, thus showing a correlation between temporally resolved stress gauge measurements and post shock microstructural and mechanical properties.

Dislocation evolution in 316 L stainless steel during multiaxial ratchetting deformation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dong Yawei; Kang Guozheng, E-mail: guozhengkang@yahoo.com.cn; Liu Yujie

2012-03-15

Dislocation patterns and their evolutions in 316 L stainless steel during the multiaxial ratchetting deformation were observed by transmission electron microscopy (TEM). The microscopic observations indicate that the dislocation evolution presented during the multiaxial ratchetting with four kinds of multiaxial loading paths is similar to that in the uniaxial case [G. Z. Kang et al., Mater Sci Eng A 527 (2010) 5952]. That is, dislocation networks and dislocation tangles are formed quickly by the multiple-slip and cross-slip of dislocation activated by applied multiaxial stress; and then polarized patterns such as dislocation walls and elongated incipient dislocation cells are formed atmore » the last stage of multiaxial ratchetting. The dislocation patterns evolve more quickly from the modes at low dislocation density to the ones at high density during the multiaxial ratchetting than that in the uniaxial case, and some traces of multiple-slip are observed in the multiaxial ones. The dislocation evolution during the multiaxial ratchetting deformation is summarized by comparing the observed dislocation patterns with those presented in the multiaxial strain-controlled and symmetrical stress-controlled cyclic tests. The multiaxial ratchetting of 316 L stainless steel can be microscopically and qualitatively explained by the observed evolution of dislocation patterns. - Highlights: Black-Right-Pointing-Pointer Dislocation patterns change from lines and nets to tangles, walls and cells. Black-Right-Pointing-Pointer Dislocation patterns evolve quicker in the multiaxial case. Black-Right-Pointing-Pointer Aligned dislocation arrays and some traces of multiple slips are observed. Black-Right-Pointing-Pointer Heterogeneous dislocation patterns result in the multiaxial ratchetting.« less

Effects of strain and thickness on the electronic and optical behaviors of two-dimensional hexagonal gallium nitride

NASA Astrophysics Data System (ADS)

Behzad, Somayeh

2017-06-01

The full potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory has been used to study effects of strain and thickness on the electronic and optical properties of two-dimensional GaN. The band gap of monolayer and bilayer GaN under compressive in-plane strain change from indirect to direct with bond length shortening. Also, the semiconductor to semimetal transition occurs for monolayer and bilayer GaN under in-plane tensile strain with bond length elongation. It is found that the tensile and compressive strains cause the red and blue shifts in the optical spectra, respectively, for both monolayer and bilayer GaN. Applying the perpendicular strain on the bilayer GaN by decreasing the inter layer distance leads to the shift of valence band maximum towards the Γ point in the band structure and shift of peak positions and variation of peak intensities in ε2(ω) spectrum. The results show that the n-layer GaN has an indirect band gap for n < 16. The results suggest that monolayer and multilayer GaN are good candidates for application in optoelectronics and flexible electronics.

Polarity control of GaN epitaxial films grown on LiGaO2(001) substrates and its mechanism.

PubMed

Zheng, Yulin; Wang, Wenliang; Li, Xiaochan; Li, Yuan; Huang, Liegen; Li, Guoqiang

2017-08-16

The polarity of GaN epitaxial films grown on LiGaO 2 (001) substrates by pulsed laser deposition has been well controlled. It is experimentally proved that the GaN epitaxial films grown on nitrided LiGaO 2 (001) substrates reveal Ga-polarity, while the GaN epitaxial films grown on non-nitrided LiGaO 2 (001) substrates show N-polarity. The growth mechanisms for these two cases are systematically studied by first-principles calculations based on density functional theory. Theoretical calculation presents that the adsorption of a Ga atom preferentially occurs at the center of three N atoms stacked on the nitrided LiGaO 2 (001) substrates, which leads to the formation of Ga-polarity GaN. Whereas the adsorption of a Ga atom preferentially deposits at the top of a N atom stacked on the non-nitrided LiGaO 2 (001) substrates, which results in the formation of N-polarity GaN. This work of controlling the polarity of GaN epitaxial films is of paramount importance for the fabrication of group-III nitride devices for various applications.

Reduction of Crosshatch Roughness and Threading Dislocation Density in Metamorphic GaInP Buffers and GaInAs Solar Cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

France, R. M.; Geisz, J. F.; Steiner, M. A.

Surface crosshatch roughness typically develops during the growth of lattice-mismatched compositionally graded buffers and can limit misfit dislocation glide. In this study, the crosshatch roughness during growth of a compressive GaInP/GaAs graded buffer is reduced by increasing the phosphine partial pressure throughout the metamorphic growth. Changes in the average misfit dislocation length are qualitatively determined by characterizing the threading defect density and residual strain. The decrease of crosshatch roughness leads to an increase in the average misfit dislocation glide length, indicating that the surface roughness is limiting dislocation glide. Growth rate is also analyzed as a method to reduce surfacemore » crosshatch roughness and increase glide length, but has a more complicated relationship with glide kinetics. Using knowledge gained from these experiments, high quality inverted GaInAs 1 eV solar cells are grown on a GaInP compositionally graded buffer with reduced roughness and threading dislocation density. The open circuit voltage is only 0.38 V lower than the bandgap potential at a short circuit current density of 15 mA/cm{sup 2}, suggesting that there is very little loss due to the lattice mismatch.« less

Initial dislocation structure and dynamic dislocation multiplication in Mo single crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hsiung, L M; Lassila, D H

Initial dislocation structure in annealed high-purity Mo single crystals and deformation substructure in a crystal subjected to 1% compression have been examined and studied in order to investigate dislocation multiplication mechanisms in the early stages of plastic deformation. The initial dislocation density is in a range of 10{sup 6} {approx} 10{sup 7} cm{sup -2}, and the dislocation structure is found to contain many grown-in superjogs along dislocation lines. The dislocation density increases to a range of 10{sup 8} {approx} 10{sup 9} cm{sup -2}, and the average jog height is also found to increase after compressing for a total strain ofmore » 1%. It is proposed that the preexisting jogged screw dislocations can act as (multiple) dislocation multiplication sources when deformed under quasi-static conditions. Both the jog height and length of link segment (between jogs) can increase by stress-induced jog coalescence, which takes place via the lateral migration (drift) of superjogs driven by unbalanced line-tension partials acting on link segments of unequal lengths. Applied shear stress begins to push each link segment to precede dislocation multiplication when link length and jog height are greater than critical lengths. This dynamic dislocation multiplication source is subsequently verified by direct simulations of dislocation dynamics under stress to be crucial in the early stages of plastic deformation in Mo single crystals.« less

AlGaN/GaN heterostructures with an AlGaN layer grown directly on reactive-ion-etched GaN showing a high electron mobility (>1300 cm2 V-1 s-1)

NASA Astrophysics Data System (ADS)

Yamamoto, Akio; Makino, Shinya; Kanatani, Keito; Kuzuhara, Masaaki

2018-04-01

In this study, the metal-organic-vapor-phase-epitaxial growth behavior and electrical properties of AlGaN/GaN structures prepared by the growth of an AlGaN layer on a reactive-ion-etched (RIE) GaN surface without regrown GaN layers were investigated. The annealing of RIE-GaN surfaces in NH3 + H2 atmosphere, employed immediately before AlGaN growth, was a key process in obtaining a clean GaN surface for AlGaN growth, that is, in obtaining an electron mobility as high as 1350 cm2 V-1 s-1 in a fabricated AlGaN/RIE-GaN structure. High-electron-mobility transistors (HEMTs) were successfully fabricated with AlGaN/RIE-GaN wafers. With decreasing density of dotlike defects observed on the surfaces of AlGaN/RIE-GaN wafers, both two-dimensional electron gas properties of AlGaN/RIE-GaN structures and DC characteristics of HEMTs were markedly improved. Since dotlike defect density was markedly dependent on RIE lot, rather than on growth lot, surface contaminations of GaN during RIE were believed to be responsible for the formation of dotlike defects and, therefore, for the inferior electrical properties.

Metalorganic vapor phase epitaxy of AlN on sapphire with low etch pit density

NASA Astrophysics Data System (ADS)

Koleske, D. D.; Figiel, J. J.; Alliman, D. L.; Gunning, B. P.; Kempisty, J. M.; Creighton, J. R.; Mishima, A.; Ikenaga, K.

2017-06-01

Using metalorganic vapor phase epitaxy, methods were developed to achieve AlN films on sapphire with low etch pit density (EPD). Key to this achievement was using the same AlN growth recipe and only varying the pre-growth conditioning of the quartz-ware. After AlN growth, the quartz-ware was removed from the growth chamber and either exposed to room air or moved into the N2 purged glove box and exposed to H2O vapor. After the quartz-ware was exposed to room air or H2O, the AlN film growth was found to be more reproducible, resulting in films with (0002) and (10-12) x-ray diffraction (XRD) rocking curve linewidths of 200 and 500 arc sec, respectively, and EPDs < 100 cm-2. The EPD was found to correlate with (0002) linewidths, suggesting that the etch pits are associated with open core screw dislocations similar to GaN films. Once reproducible AlN conditions were established using the H2O pre-treatment, it was found that even small doses of trimethylaluminum (TMAl)/NH3 on the quartz-ware surfaces generated AlN films with higher EPDs. The presence of these residual TMAl/NH3-derived coatings in metalorganic vapor phase epitaxy (MOVPE) systems and their impact on the sapphire surface during heating might explain why reproducible growth of AlN on sapphire is difficult.

A dislocation density-based continuum model of the anisotropic shock response of single crystal α-cyclotrimethylene trinitramine

DOE PAGES

Luscher, Darby Jon; Addessio, Francis L.; Cawkwell, Marc Jon; ...

2017-01-01

Here, we have developed a model for the finite deformation thermomechanical response of α-cyclotrimethylene trinitramine (RDX). Our model accounts for nonlinear thermoelastic lattice deformation through a free energy-based equation of state developed by Cawkwell et al. (2016) in combination with temperature and pressure dependent elastic constants, as well as dislocation-mediated plastic slip on a set of slip systems motivated by experimental observation. The kinetics of crystal plasticity are modeled using the Orowan equation relating slip rate to dislocation density and the dislocation velocity developed by Austin and McDowell (2011), which naturally accounts for transition from thermally activated to dislocation dragmore » limited regimes. Evolution of dislocation density is specified in terms of local ordinary differential equations reflecting dislocation–dislocation interactions. This paper presents details of the theory and parameterization of the model, followed by discussion of simulations of flyer plate impact experiments. Impact conditions explored within this combined simulation and experimental effort span shock pressures ranging from 1 to 3 GPa for four crystallographic orientations and multiple specimen thicknesses. Simulation results generated using this model are shown to be in strong agreement with velocimetry measurements from the corresponding plate impact experiments. Finally, simulation results are used to motivate conclusions about the nature of dislocation-mediated plasticity in RDX.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Khurgin, Jacob B., E-mail: jakek@jhu.edu; Bajaj, Sanyam; Rajan, Siddharth

Longitudinal optical (LO) phonons in GaN generated in the channel of high electron mobility transistors (HEMT) are shown to undergo nearly elastic scattering via collisions with hot electrons. The net result of these collisions is the diffusion of LO phonons in the Brillouin zone causing reduction of phonon and electron temperatures. This previously unexplored diffusion mechanism explicates how an increase in electron density causes reduction of the apparent lifetime of LO phonons, obtained from the time resolved Raman studies and microwave noise measurements, while the actual decay rate of the LO phonons remains unaffected by the carrier density. Therefore, themore » saturation velocity in GaN HEMT steadily declines with increased carrier density, in a qualitative agreement with experimental results.« less

Study on the Electronic Transport Properties of Zigzag GaN Nanotubes

NASA Astrophysics Data System (ADS)

Li, Enling; Wang, Xiqiang; Hou, Liping; Zhao, Danna; Dai, Yuanbin; Wang, Xuewen

2011-02-01

The electronic transport properties of zigzag GaN nanotubes (n, 0) (4 <= n <= 9) have been calculated using the density functional theory and non-equilibrium Green's functions method. Firstly, the density functional theory (DFT) is used to optimize and calculate the electronic structure of GaNNTs (n, 0) (4<=n<=9). Secondly, DFT and non-equilibrium Green function (NEGF) method are also used to predict the electronic transport properties of GaNNTs two-probe system. The results showed: there is a corresponding relation between the electronic transport properties and the valley of state density of each GaNNT. In addition, the volt-ampere curve of GaNNT is approximately linear.

Four Terminal Gallium Nitride MOSFETs

NASA Astrophysics Data System (ADS)

Veety, Matthew Thomas

All reported gallium nitride (GaN) transistors to date have been three-terminal devices with source, drain, and gate electrodes. In the case of GaN MOSFETs, this leaves the bulk of the device at a floating potential which can impact device threshold voltage. In more traditional silicon-based MOSFET fabrication a bulk contact can be made on the back side of the silicon wafer. For GaN grown on sapphire substrates, however, this is not possible and an alternate, front-side bulk contact must be investigated. GaN is a III-V, wide band gap semiconductor that as promising material parameters for use in high frequency and high power applications. Possible applications are in the 1 to 10 GHz frequency band and power inverters for next generation grid solid state transformers and inverters. GaN has seen significant academic and commercial research for use in Heterojunction Field Effect Transistors (HFETs). These devices however are depletion-mode, meaning the device is considered "on" at zero gate bias. A MOSFET structure allows for enhancement mode operation, which is normally off. This mode is preferrable in high power applications as the device has lower off-state power consumption and is easier to implement in circuits. Proper surface passivation of seminconductor surface interface states is an important processing step for any device. Preliminary research on surface treatments using GaN wet etches and depletion-mode GaN devices utilizing this process are discussed. Devices pretreated with potassium pursulfate prior to gate dielectric deposition show significant device improvements. This process can be applied to any current GaN FET. Enhancement-mode GaN MOSFETs were fabricated on magnesium doped p-type Wurtzite gallium nitride grown by Metal Organic Chemical Vapor Deposition (MOCVD) on c-plane sapphire substrates. Devices utilized ion implant source and drain which was activated under NH3 overpressure in MOCVD. Also, devices were fabricated with a SiO2 gate dielectric and metal gate. Preliminary devices exhibited high GaN-oxide interface state density, Dit, on the order of 1013 cm-2· eV-1. Additional experiments and device fabrication was focused on improving device performance through optimization of the ion implantation activation anneal as well as incorporation of a bulk p-type ohmic contact and migration to a thicker, lower defect density, HVPE-grown template substrate. The first reported MOSFET on HVPE grown GaN substrates (templates) is reported with peak measured drain current of 1.05 mA/mm and a normalized transconductance of 57 muS/mm. Fabricated devices exhibited large (greater than 1 muA) source-to-drain junction leakage which is attributed to low activated doping density in the MOCVD-grown p-type bulk. MOSFETs fabricated on template substrates show more than twice the measured drain current as similar devices fabricated on traditional MOCVD GaN on sapphire substrates for the same bias conditions. Also, template MOSFETs have decreased gate leakage which allowed for a much greater range of operation. This performance increase is attributed to a more than doubled effective channel mobility on template GaN MOSFETs due to decreased crystal defect scattering when compared to a MOCVD-grown GaN-on-sapphire MOSFET. Fabricated MOSFETs also exhibit decreased interface state density with lower bound of 2.2x1011 cm-2·eV-1 when compared to prelimary MOSFETs. This decrease is associated with the use of a sacrificial oxide cap during source/drain activation. Suggested work for continued research is also presented which includes experiments to improve source/drain ion implantation profile, utilization of selective area growth for the active area, improved n- and p-type ohmic contact resistance and investigation of alternate oxides.

Native defects in GaN: a hybrid functional study

NASA Astrophysics Data System (ADS)

Diallo, Ibrahima Castillo; Demchenko, Denis

Intrinsic defects play an important role in the performance of GaN-based devices. We present hybrid density functional calculations of the electronic and possible optical properties of interstitial N (Ni-Ni) , N antisite (NGa) , interstitial Ga (Gai) , Ga antisite (GaN) , Ga vacancy (VGa) , N vacancy (VN) and Ga-N divacancies (VGaVN) in GaN. Our results show that the vacancies display relatively low formation energies in certain samples, whereas antisites and interstitials are energetically less favorable. However, interstitials can be created by electron irradiation. For instance, in 2.5 MeV electron-irradiated GaN samples, a strong correlation between the frequently observed photoluminescence (PL) band centered around 0.85 eV accompanied with a rich phonon sideband of ~0.88 eV and the theoretical optical behavior of interstitial Ga is discussed. N vacancies are found to likely contribute to the experimentally obtained green luminescence band (GL2) peaking at 2.24 eV in high-resistivity undoped and Mg-doped GaN. National Science Foundation (DMR-1410125) and the Thomas F. and Kate Miller Jeffress Memorial Trust.

High current density 2D/3D MoS2/GaN Esaki tunnel diodes

NASA Astrophysics Data System (ADS)

Krishnamoorthy, Sriram; Lee, Edwin W.; Lee, Choong Hee; Zhang, Yuewei; McCulloch, William D.; Johnson, Jared M.; Hwang, Jinwoo; Wu, Yiying; Rajan, Siddharth

2016-10-01

The integration of two-dimensional materials such as transition metal dichalcogenides with bulk semiconductors offer interesting opportunities for 2D/3D heterojunction-based device structures without any constraints of lattice matching. By exploiting the favorable band alignment at the GaN/MoS2 heterojunction, an Esaki interband tunnel diode is demonstrated by transferring large area Nb-doped, p-type MoS2 onto heavily n-doped GaN. A peak current density of 446 A/cm2 with repeatable room temperature negative differential resistance, peak to valley current ratio of 1.2, and minimal hysteresis was measured in the MoS2/GaN non-epitaxial tunnel diode. A high current density of 1 kA/cm2 was measured in the Zener mode (reverse bias) at -1 V bias. The GaN/MoS2 tunnel junction was also modeled by treating MoS2 as a bulk semiconductor, and the electrostatics at the 2D/3D interface was found to be crucial in explaining the experimentally observed device characteristics.

The role of surface roughness on dislocation bending and stress evolution in low mobility AlGaN films during growth

NASA Astrophysics Data System (ADS)

Bardhan, Abheek; Mohan, Nagaboopathy; Chandrasekar, Hareesh; Ghosh, Priyadarshini; Sridhara Rao, D. V.; Raghavan, Srinivasan

2018-04-01

The bending and interaction of threading dislocations are essential to reduce their density for applications involving III-nitrides. Bending of dislocation lines also relaxes the compressive growth stress that is essential to prevent cracking on cooling down due to tensile thermal expansion mismatch stress while growing on Si substrates. It is shown in this work that surface roughness plays a key role in dislocation bending. Dislocations only bend and relax compressive stresses when the lines intersect a smooth surface. These films then crack. In rough films, dislocation lines which terminate at the bottom of the valleys remain straight. Compressive stresses are not relaxed and the films are relatively crack-free. The reasons for this difference are discussed in this work along with the implications on simultaneously meeting the requirements of films being smooth, crack free and having low defect density for device applications.

Effect of annealing temperature on the thermal stress and dislocation density of mc-Si ingot grown by DS process for solar cell application

NASA Astrophysics Data System (ADS)

Sanmugavel, S.; Srinivasan, M.; Aravinth, K.; Ramasamy, P.

2018-04-01

90% of the solar industries are using crystalline silicon. Cost wise the multi-crystalline silicon solar cells are better compared to mono crystalline silicon. But because of the presence of grain boundaries, dislocations and impurities, the efficiency of the multi-crystalline silicon solar cells is lower than that of mono crystalline silicon solar cells. By reducing the defect and dislocation we can achieve high conversion efficiency. The velocity of dislocation motion increases with stress. By annealing the grown ingot at proper temperature we can decrease the stress and dislocation. Our simulation results show that the value of stress and dislocation density is decreased by annealing the grown ingot at 1400K and the input parameters can be implemented in real system to grow a better mc-Si ingot for energy harvesting applications.

Theoretical study for heterojunction surface of NEA GaN photocathode dispensed with Cs activation

NASA Astrophysics Data System (ADS)

Xia, Sihao; Liu, Lei; Wang, Honggang; Wang, Meishan; Kong, Yike

2016-09-01

For the disadvantages of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, new-type NEA GaN photocathodes with heterojunction surface dispensed with Cs activation are investigated based on first-principle study with density functional theory. Through the growth of an ultrathin n-type GaN cap layer on p-type GaN emission layer, a p-n heterojunction is formed on the surface. According to the calculation results, it is found that Si atoms tend to replace Ga atoms to result in an n-type doped cap layer which contributes to the decreasing of work function. After the growth of n-type GaN cap layer, the atom structure near the p-type emission layer is changed while that away from the surface has no obvious variations. By analyzing the E-Mulliken charge distribution of emission surface with and without cap layer, it is found that the positive charge of Ga and Mg atoms in the emission layer decrease caused by the cap layer, while the negative charge of N atom increases. The conduction band moves downwards after the growth of cap layer. Si atom produces donor levels around the valence band maximum. The absorption coefficient of GaN emission layer decreases and the reflectivity increases caused by n-type GaN cap layer.

Modeling of dislocation channel width evolution in irradiated metals

DOE PAGES

Doyle, Peter J.; Benensky, Kelsa M.; Zinkle, Steven J.

2017-11-08

Defect-free dislocation channel formation has been reported to promote plastic instability during tensile testing via localized plastic flow, leading to a distinct loss of ductility and strain hardening in many low-temperature irradiated materials. In order to study the underlying mechanisms governing dislocation channel width and formation, the channel formation process is modeled via a simple stochastic dislocation-jog process dependent upon grain size, defect cluster density, and defect size. Dislocations traverse a field of defect clusters and jog stochastically upon defect interaction, forming channels of low defect-density. And based upon prior molecular dynamics (MD) simulations and in-situ experimental transmission electron microscopymore » (TEM) observations, each dislocation encounter with a dislocation loop or stacking fault tetrahedron (SFT) is assumed to cause complete absorption of the defect cluster, prompting the dislocation to jog up or down by a distance equal to half the defect cluster diameter. Channels are predicted to form rapidly and are comparable to reported TEM measurements for many materials. Predicted channel widths are found to be most strongly dependent on mean defect size and correlated well with a power law dependence on defect diameter and density, and distance from the dislocation source. Due to the dependence of modeled channel width on defect diameter and density, maximum channel width is predicted to slowly increase as accumulated dose increases. The relatively weak predicted dependence of channel formation width with distance, in accordance with a diffusion analogy, implies that after only a few microns from the source, most channels observed via TEM analyses may not appear to vary with distance because of limitations in the field-of-view to a few microns. Furthermore, examinations of the effect of the so-called “source-broadening” mechanism of channel formation showed that its effect is simply to add a minimum thickness to the channel without affecting channel dependence on the given parameters.« less

The Weighted Burgers Vector: a new quantity for constraining dislocation densities and types using electron backscatter diffraction on 2D sections through crystalline materials.

PubMed

Wheeler, J; Mariani, E; Piazolo, S; Prior, D J; Trimby, P; Drury, M R

2009-03-01

The Weighted Burgers Vector (WBV) is defined here as the sum, over all types of dislocations, of [(density of intersections of dislocation lines with a map) x (Burgers vector)]. Here we show that it can be calculated, for any crystal system, solely from orientation gradients in a map view, unlike the full dislocation density tensor, which requires gradients in the third dimension. No assumption is made about gradients in the third dimension and they may be non-zero. The only assumption involved is that elastic strains are small so the lattice distortion is entirely due to dislocations. Orientation gradients can be estimated from gridded orientation measurements obtained by EBSD mapping, so the WBV can be calculated as a vector field on an EBSD map. The magnitude of the WBV gives a lower bound on the magnitude of the dislocation density tensor when that magnitude is defined in a coordinate invariant way. The direction of the WBV can constrain the types of Burgers vectors of geometrically necessary dislocations present in the microstructure, most clearly when it is broken down in terms of lattice vectors. The WBV has three advantages over other measures of local lattice distortion: it is a vector and hence carries more information than a scalar quantity, it has an explicit mathematical link to the individual Burgers vectors of dislocations and, since it is derived via tensor calculus, it is not dependent on the map coordinate system. If a sub-grain wall is included in the WBV calculation, the magnitude of the WBV becomes dependent on the step size but its direction still carries information on the Burgers vectors in the wall. The net Burgers vector content of dislocations intersecting an area of a map can be simply calculated by an integration round the edge of that area, a method which is fast and complements point-by-point WBV calculations.

Modeling of dislocation channel width evolution in irradiated metals

NASA Astrophysics Data System (ADS)

Doyle, Peter J.; Benensky, Kelsa M.; Zinkle, Steven J.

2018-02-01

Defect-free dislocation channel formation has been reported to promote plastic instability during tensile testing via localized plastic flow, leading to a distinct loss of ductility and strain hardening in many low-temperature irradiated materials. In order to study the underlying mechanisms governing dislocation channel width and formation, the channel formation process is modeled via a simple stochastic dislocation-jog process dependent upon grain size, defect cluster density, and defect size. Dislocations traverse a field of defect clusters and jog stochastically upon defect interaction, forming channels of low defect-density. Based upon prior molecular dynamics (MD) simulations and in-situ experimental transmission electron microscopy (TEM) observations, each dislocation encounter with a dislocation loop or stacking fault tetrahedron (SFT) is assumed to cause complete absorption of the defect cluster, prompting the dislocation to jog up or down by a distance equal to half the defect cluster diameter. Channels are predicted to form rapidly and are comparable to reported TEM measurements for many materials. Predicted channel widths are found to be most strongly dependent on mean defect size and correlated well with a power law dependence on defect diameter and density, and distance from the dislocation source. Due to the dependence of modeled channel width on defect diameter and density, maximum channel width is predicted to slowly increase as accumulated dose increases. The relatively weak predicted dependence of channel formation width with distance, in accordance with a diffusion analogy, implies that after only a few microns from the source, most channels observed via TEM analyses may not appear to vary with distance because of limitations in the field-of-view to a few microns. Further, examinations of the effect of the so-called "source-broadening" mechanism of channel formation showed that its effect is simply to add a minimum thickness to the channel without affecting channel dependence on the given parameters.

Modeling of dislocation channel width evolution in irradiated metals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Doyle, Peter J.; Benensky, Kelsa M.; Zinkle, Steven J.

Defect-free dislocation channel formation has been reported to promote plastic instability during tensile testing via localized plastic flow, leading to a distinct loss of ductility and strain hardening in many low-temperature irradiated materials. In order to study the underlying mechanisms governing dislocation channel width and formation, the channel formation process is modeled via a simple stochastic dislocation-jog process dependent upon grain size, defect cluster density, and defect size. Dislocations traverse a field of defect clusters and jog stochastically upon defect interaction, forming channels of low defect-density. And based upon prior molecular dynamics (MD) simulations and in-situ experimental transmission electron microscopymore » (TEM) observations, each dislocation encounter with a dislocation loop or stacking fault tetrahedron (SFT) is assumed to cause complete absorption of the defect cluster, prompting the dislocation to jog up or down by a distance equal to half the defect cluster diameter. Channels are predicted to form rapidly and are comparable to reported TEM measurements for many materials. Predicted channel widths are found to be most strongly dependent on mean defect size and correlated well with a power law dependence on defect diameter and density, and distance from the dislocation source. Due to the dependence of modeled channel width on defect diameter and density, maximum channel width is predicted to slowly increase as accumulated dose increases. The relatively weak predicted dependence of channel formation width with distance, in accordance with a diffusion analogy, implies that after only a few microns from the source, most channels observed via TEM analyses may not appear to vary with distance because of limitations in the field-of-view to a few microns. Furthermore, examinations of the effect of the so-called “source-broadening” mechanism of channel formation showed that its effect is simply to add a minimum thickness to the channel without affecting channel dependence on the given parameters.« less

Effect of dislocations on properties of heteroepitaxial InP solar cells

NASA Technical Reports Server (NTRS)

Weinberg, I.; Swartz, C. K.; Curtis, H. B.; Brinker, D. J.; Jenkins, P.; Faur, M.

1991-01-01

The apparently unrelated phenomena of temperature dependency, carrier removal and photoluminescence are shown to be affected by the high dislocation densities present in heteroepitaxial InP solar cells. Using homoepitaxial InP cells as a baseline, it is found that the relatively high dislocation densities present in heteroepitaxial InP/GaAs cells lead to increased volumes of dVoc/dt and carrier removal rate and substantial decreases in photoluminescence spectral intensities. With respect to dVoc/dt, the observed effect is attributed to the tendency of dislocations to reduce Voc. Although the basic cause for the observed increase in carrier removal rate is unclear, it is speculated that the decreased photoluminescence intensity is attributable to defect levels introduced by dislocations in the heteroepitaxial cells.

Defect structure of high temperature hydride vapor phase epitaxy-grown epitaxial (0 0 0 1) AlN/sapphire using growth mode modification process

NASA Astrophysics Data System (ADS)

Su, Xujun; Zhang, Jicai; Huang, Jun; Zhang, Jinping; Wang, Jianfeng; Xu, Ke

2017-06-01

Defect structures were investigated by transmission electron microscopy for AlN/sapphire (0 0 0 1) epilayers grown by high temperature hydride vapor phase epitaxy using a growth mode modification process. The defect structures, including threading dislocations, inversion domains, and voids, were analyzed by diffraction contrast, high-resolution imaging, and convergent beam diffraction. AlN film growth was initiated at 1450 °C with high V/III ratio for 8 min. This was followed by low V/III ratio growth for 12 min. The near-interfacial region shows a high density of threading dislocations and inversion domains. Most of these dislocations have Burgers vector b = 1/3〈1 1 2 0〉 and were reduced with the formation of dislocation loops. In the middle range 400 nm < h < 2 μm, dislocations gradually aggregated and reduced to ∼109 cm-2. The inversion domains have a shuttle-like shape with staggered boundaries that deviate by ∼ ±5° from the c axis. Above 2 μm thickness, the film consists of isolated threading dislocations with a total density of 8 × 108 cm-2. Most of threading dislocations are either pure edge or mixed dislocations. The threading dislocation reduction in these films is associated with dislocation loops formation and dislocation aggregation-interaction during island growth with high V/III ratio.

Piezoelectric effect on the thermal conductivity of monolayer gallium nitride

NASA Astrophysics Data System (ADS)

Zhang, Jin

2018-01-01

Using molecular dynamics and density functional theory simulations, in this work, we find that the heat transport property of the monolayer gallium nitride (GaN) can be efficiently tailored by external electric field due to its unique piezoelectric characteristic. As the monolayer GaN possesses different piezoelectric properties in armchair and zigzag directions, different effects of the external electric field on thermal conductivity are observed when it is applied in the armchair and zigzag directions. Our further study reveals that due to the elastoelectric effect in the monolayer GaN, the external electric field changes the Young's modulus and therefore changes the phonon group velocity. Also, due to the inverse piezoelectric effect, the applied electric field induces in-plane stress in the monolayer GaN subject to a length constraint, which results in the change in the lattice anharmonicity and therefore affects the phonon mean free path. Furthermore, for relatively long GaN monolayers, the in-plane stress may trigger the buckling instability, which can significantly reduce the phonon mean free path.

The Calculation Study of Electronic Properties of Doped RE (Eu, Er and Tm)-GaN using Density Functional Theory

NASA Astrophysics Data System (ADS)

Zaharo, Aflah; Purqon, Acep

2017-07-01

The calculation of the structure and electronic properties of Rare Earth (RE) at the wurtzite Gallium Nitride (GaN) based on DFT has completed. GGA approximation used for exchange correlation and Ultra soft pseudo potential too. The stability structure of GaN is seen that difference lattice parameter 11% lower than another calculation and experiment result. It is shown the stability structure GaN have direct band gap energy on Gamma point hexagonal lattice Brillouin zone. The width Eg is 2.6 eV. When one atom Ga is substituted with one atom RE, the bond length is change 12 % longest. An in good agreement with theoretical doping RE concentration increases, the edge of energy level shifted towards to make the band gap narrow which is allow the optical transitions and help to improve the optical performance of GaN. The RE doped GaN is potentially applicable for various color of LED with lower energy consumption and potentially energy saving application

Carrier confinement effects of InxGa1-xN/GaN multi quantum disks with GaN surface barriers grown in GaN nanorods

NASA Astrophysics Data System (ADS)

Park, Youngsin; Chan, Christopher C. S.; Taylor, Robert A.; Kim, Nammee; Jo, Yongcheol; Lee, Seung W.; Yang, Woochul; Im, Hyunsik

2018-04-01

Structural and optical properties of InxGa1-xN/GaN multi quantum disks (QDisks) grown on GaN nanorods by molecular beam epitaxy have been investigated by transmission electron microscopy and micro-photoluminescence (PL) spectroscopy. Two types of InGaN QDisks were grown: a pseudo-3D confined InGaN pillar-type QDisks embedded in GaN nanorods; and QDisks in flanged cone type GaN nanorods. The PL emission peak and excitation dependent PL behavior of the pillar-type Qdisks differ greatly from those of the flanged cone type QDisks. Time resolved PL was carried out to probe the differences in charge carrier dynamics. The results suggest that by constraining the formation of InGaN QDisks within the centre of the nanorod, carriers are restricted from migrating to the surface, decreasing the surface recombination at high carrier densities.

Less strained and more efficient GaN light-emitting diodes with embedded silica hollow nanospheres

PubMed Central

Kim, Jonghak; Woo, Heeje; Joo, Kisu; Tae, Sungwon; Park, Jinsub; Moon, Daeyoung; Park, Sung Hyun; Jang, Junghwan; Cho, Yigil; Park, Jucheol; Yuh, Hwankuk; Lee, Gun-Do; Choi, In-Suk; Nanishi, Yasushi; Han, Heung Nam; Char, Kookheon; Yoon, Euijoon

2013-01-01

Light-emitting diodes (LEDs) become an attractive alternative to conventional light sources due to high efficiency and long lifetime. However, different material properties between GaN and sapphire cause several problems such as high defect density in GaN, serious wafer bowing, particularly in large-area wafers, and poor light extraction of GaN-based LEDs. Here, we suggest a new growth strategy for high efficiency LEDs by incorporating silica hollow nanospheres (S-HNS). In this strategy, S-HNSs were introduced as a monolayer on a sapphire substrate and the subsequent growth of GaN by metalorganic chemical vapor deposition results in improved crystal quality due to nano-scale lateral epitaxial overgrowth. Moreover, well-defined voids embedded at the GaN/sapphire interface help scatter lights effectively for improved light extraction, and reduce wafer bowing due to partial alleviation of compressive stress in GaN. The incorporation of S-HNS into LEDs is thus quite advantageous in achieving high efficiency LEDs for solid-state lighting. PMID:24220259

Current conduction mechanism and electrical break-down in InN grown on GaN

NASA Astrophysics Data System (ADS)

Kuzmik, J.; Fleury, C.; Adikimenakis, A.; Gregušová, D.; Ťapajna, M.; Dobročka, E.; Haščík, Š.; Kučera, M.; Kúdela, R.; Androulidaki, M.; Pogany, D.; Georgakilas, A.

2017-06-01

Current conduction mechanism, including electron mobility, electron drift velocity (vd) and electrical break-down have been investigated in a 0.5 μm-thick (0001) InN layer grown by molecular-beam epitaxy on a GaN/sapphire template. Electron mobility (μ) of 1040 cm2/Vs and a free electron concentration (n) of 2.1 × 1018 cm-3 were measured at room temperature with only a limited change down to 20 K, suggesting scattering on dislocations and ionized impurities. Photoluminescence spectra and high-resolution X-ray diffraction correlated with the Hall experiment showing an emission peak at 0.69 eV, a full-width half-maximum of 30 meV, and a dislocation density Ndis ˜ 5.6 × 1010 cm-2. Current-voltage (I-V) characterization was done in a pulsed (10 ns-width) mode on InN resistors prepared by plasma processing and Ohmic contacts evaporation. Resistors with a different channel length ranging from 4 to 15.8 μm obeyed the Ohm law up to an electric field intensity Eknee ˜ 22 kV/cm, when vd ≥ 2.5 × 105 m/s. For higher E, I-V curves were nonlinear and evolved with time. Light emission with a photon energy > 0.7 eV has been observed already at modest Erad of ˜ 8.3 kV/cm and consequently, a trap-assisted interband tunneling was suggested to play a role. At Eknee ˜ 22 kV/cm, we assumed electron emission from traps, with a positive feed-back for the current enhancement. Catastrophic break-down appeared at E ˜ 25 kV/cm. Reduction of Ndis was suggested to fully exploit InN unique prospects for future high-frequency devices.

Impurity distribution and microstructure of Ga-doped ZnO films grown by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Kvit, A. V.; Yankovich, A. B.; Avrutin, V.; Liu, H.; Izyumskaya, N.; Özgür, Ü.; Morkoç, H.; Voyles, P. M.

2012-12-01

We report microstructural characterization of heavily Ga-doped ZnO (GZO) thin films on GaN and sapphire by aberration-corrected scanning transmission electron microscopy. Growth under oxygen-rich and metal-rich growth conditions leads to changes in the GZO polarity and different extended defects. For GZO layers on sapphire, the primary extended defects are voids, inversion domain boundaries, and low-angle grain boundaries. Ga doping of ZnO grown under metal-rich conditions causes a switch from pure oxygen polarity to mixed oxygen and zinc polarity in small domains. Electron energy loss spectroscopy and energy dispersive spectroscopy spectrum imaging show that Ga is homogeneous, but other residual impurities tend to accumulate at the GZO surface and at extended defects. GZO grown on GaN on c-plane sapphire has Zn polarity and no voids. There are misfit dislocations at the interfaces between GZO and an undoped ZnO buffer layer and at the buffer/GaN interface. Low-angle grain boundaries are the only threading microstructural defects. The potential effects of different extended defects and impurity distributions on free carrier scattering are discussed.

Polychromatic Microdiffraction Analysis of Defect Self-Organization in Shock Deformed Single Crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Barabash, Rozaliya; Ice, Gene E; Liu, Wenjun

A spatially resolved X-ray diffraction method - with a submicron 3D resolution together with SEM and OIM analysis are applied to understand the arrangements of voids, geometrically necessary dislocations and strain gradient distributions in samples of Al (1 2 3) and Cu (0 0 1) single crystals shocked to incipient spallation fracture. We describe how geometrically necessary dislocations and the effective strain gradient alter white beam Laue patterns of the shocked materials. Several distinct structural zones are observed at different depths under the impact surface. The density of geometrically necessary dislocations (GNDs) is extremely high near the impact and backmore » surface of the shock recovered crystals. The spall region is characterized by a large density of mesoscale voids and GNDs. The spall region is separated from the impact and back surfaces by compressed regions with high total dislocation density but lower GNDs density. Self-organization of shear bands is observed in the shock recovered Cu single crystal.« less

Fermi energy control of vacancy coalescence and dislocation density in melt-grown GaAs

NASA Technical Reports Server (NTRS)

Lagowski, J.; Gatos, H. C.; Lin, D. G.; Aoyama, T.

1984-01-01

A striking effect of the Fermi energy on the dislocation density in melt-grown GaAs has been discovered. Thus, a shift of the Fermi energy from 0.1 eV above to 0.2 eV below its intrinsic value (at high temperature, i.e., near 1100 K) increases the dislocation density by as much as five orders of magnitude. The Fermi energy shift was brought about by n-type and p-type doping at a level of about 10 to the 17th per cu cm (under conditions of optimum partial pressure of As, i.e., under optimum melt stoichiometry). This effect must be associated with the fact that the Fermi energy controls the charge state of vacancies (i.e., the occupancy of the associated electronic states) which in turn must control their tendency to coalesce and thus the dislocation density. It appears most likely that gallium vacancies are the critical species.

First principles study of size and external electric field effects on the atomic and electronic properties of gallium nitride nanostructures

NASA Astrophysics Data System (ADS)

Yilmaz, Hulusi

A comprehensive density functional theory study of atomic and the electronic properties of wurtzite gallium nitride (GaN) nanostructures with different sizes and shapes is presented and the effect of external electric field on these properties is examined. We show that the atomic and electronic properties of [101¯0] facet single-crystal GaN nanotubes (quasi-1D), nanowires (1D) and nanolayers (2D) are mainly determined by the surface to volume ratio. The shape dependent quantum confinement and strain effects on the atomic and electronic properties of these GaN nanostructures are found to be negligible. Based on this similarity between the atomic and electronic properties of the small size GaN nanostructures, we calculated the atomic and electronic properties of the practical size (28.1 A wall thickness) single-crystal GaN nanotubes through computational much economical GaN nanoslabs (nanolayers). Our results show that, regardless of diameter, hydrogen saturated single-crystal GaN tubes with the wall thickness of 28.1 A are energetically stable and they have a noticeably larger band gap with respect to the band gap of bulk GaN. The band gap of unsaturated single-crystal GaN tubes, on the other hand, is always smaller than the band gap of the wurtzite bulk GaN. In a separate study, we show that a transverse electric field induces a homojunction across the diameter of initially semiconducting GaN single-crystal nanotubes and nanowires. The homojunction arises due to the decreased energy of the electronic states in the higher potential region with respect to the energy of those states in the lower potential region under the transverse electric field. Calculations on single-crystal GaN nanotubes and nanowires of different diameter and wall thickness show that the threshold electric field required for the semiconductor-homojunction induction increases with increasing wall thickness and decreases significantly with increasing diameter.

Dislocation Multiplication in the Early Stage of Deformation in Mo Single Crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hsiung, L.; Lassila, D.H.

Initial dislocation structure in annealed high-purity Mo single crystals and deformation substructure in a crystal subjected to 1% compression have been examined and studied using transmission electron microscopy (TEM) techniques in order to investigate dislocation multiplication mechanisms in the early stage of plastic deformation. The initial dislocation density is in a range of 10{sup 6} {approx} 10{sup 7} cm{sup -2}, and the dislocation structure is found to contain many grown-in superjogs along dislocation lines. The dislocation density increases to a range of 10{sup 8} {approx} 10{sup 9} cm{sup -2}, and the average jog height is also found to increase aftermore » compressing for a total strain of 1%. It is proposed that the preexisting jogged screw dislocations can act as (multiple) dislocation multiplication sources when deformed under quasi-static conditions. The jog height can increase by stress-induced jog coalescence, which takes place via the lateral migration (drift) of superjogs driven by unbalanced line-tension partials acting on link segments of unequal lengths. The coalescence of superjogs results in an increase of both link length and jog height. Applied shear stress begins to push each link segment to precede dislocation multiplication when link length and jog height are greater than critical lengths. This ''dynamic'' dislocation multiplication source is suggested to be crucial for the dislocation multiplication in the early stage of plastic deformation in Mo.« less

Estimating ice particle scattering properties using a modified Rayleigh-Gans approximation

NASA Astrophysics Data System (ADS)

Lu, Yinghui; Clothiaux, Eugene E.; Aydin, Kültegin; Verlinde, Johannes

2014-09-01

A modification to the Rayleigh-Gans approximation is made that includes self-interactions between different parts of an ice crystal, which both improves the accuracy of the Rayleigh-Gans approximation and extends its applicability to polarization-dependent parameters. This modified Rayleigh-Gans approximation is both efficient and reasonably accurate for particles with at least one dimension much smaller than the wavelength (e.g., dendrites at millimeter or longer wavelengths) or particles with sparse structures (e.g., low-density aggregates). Relative to the Generalized Multiparticle Mie method, backscattering reflectivities at horizontal transmit and receive polarization (HH) (ZHH) computed with this modified Rayleigh-Gans approach are about 3 dB more accurate than with the traditional Rayleigh-Gans approximation. For realistic particle size distributions and pristine ice crystals the modified Rayleigh-Gans approach agrees with the Generalized Multiparticle Mie method to within 0.5 dB for ZHH whereas for the polarimetric radar observables differential reflectivity (ZDR) and specific differential phase (KDP) agreement is generally within 0.7 dB and 13%, respectively. Compared to the A-DDA code, the modified Rayleigh-Gans approximation is several to tens of times faster if scattering properties for different incident angles and particle orientations are calculated. These accuracies and computational efficiencies are sufficient to make this modified Rayleigh-Gans approach a viable alternative to the Rayleigh-Gans approximation in some applications such as millimeter to centimeter wavelength radars and to other methods that assume simpler, less accurate shapes for ice crystals. This method should not be used on materials with dielectric properties much different from ice and on compact particles much larger than the wavelength.

Microstructural changes in Beta-silicon nitride grains upon crystallizing the grain-boundary glass

NASA Technical Reports Server (NTRS)

Lee, William E.; Hilmas, Gregory E.; Lange, F. F. (Editor)

1991-01-01

Crystallizing the grain boundary glass of a liquid phase sintered Si3N4 ceramic for 2 h or less at 1500 C led to formation of gamma Y2Si2O7. After 5 h at 1500 C, the gamma Y2Si2O7 had transformed to beta Y2Si2O7 with a concurrent dramatic increase in dislocation density within beta Si3N4 grains. Reasons for the increased dislocation density is discussed. Annealing for 20 h at 1500 C reduced dislocation densities to the levels found in as-sintered materials.

The structure of crystallographic damage in GaN formed during rare earth ion implantation with and without an ultrathin AlN capping layer

NASA Astrophysics Data System (ADS)

Gloux, F.; Ruterana, P.; Wojtowicz, T.; Lorenz, K.; Alves, E.

2006-10-01

The crystallographic nature of the damage created in GaN implanted by rare earth ions at 300 keV and room temperature has been investigated by transmission electron microscopy versus the fluence, from 7×10 13 to 2×10 16 at/cm 2, using Er, Eu or Tm ions. The density of point defect clusters was seen to increase with the fluence. From about 3×10 15 at/cm 2, a highly disordered 'nanocrystalline layer' (NL) appears on the GaN surface. Its structure exhibits a mixture of voids and misoriented nanocrystallites. Basal stacking faults (BSFs) of I 1, E and I 2 types have been noticed from the lowest fluence, they are I 1 in the majority. Their density increases and saturates when the NL is observed. Many prismatic stacking faults (PSFs) with Drum atomic configuration have been identified. The I 1 BSFs are shown to propagate easily through GaN by folding from basal to prismatic planes thanks to the PSFs. When implanting through a 10 nm AlN cap, the NL threshold goes up to about 3×10 16 at/cm 2. The AlN cap plays a protective role against the dissociation of the GaN up to the highest fluences. The flat surface after implantation and the absence of SFs in the AlN cap indicate its high resistance to the damage formation.

Effects of GaN/AlGaN/Sputtered AlN nucleation layers on performance of GaN-based ultraviolet light-emitting diodes

PubMed Central

Hu, Hongpo; Zhou, Shengjun; Liu, Xingtong; Gao, Yilin; Gui, Chengqun; Liu, Sheng

2017-01-01

We report on the demonstration of GaN-based ultraviolet light-emitting diodes (UV LEDs) emitting at 375 nm grown on patterned sapphire substrate (PSS) with in-situ low temperature GaN/AlGaN nucleation layers (NLs) and ex-situ sputtered AlN NL. The threading dislocation (TD) densities in GaN-based UV LEDs with GaN/AlGaN/sputtered AlN NLs were determined by high-resolution X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM), which revealed that the TD density in UV LED with AlGaN NL was the highest, whereas that in UV LED with sputtered AlN NL was the lowest. The light output power (LOP) of UV LED with AlGaN NL was 18.2% higher than that of UV LED with GaN NL owing to a decrease in the absorption of 375 nm UV light in the AlGaN NL with a larger bandgap. Using a sputtered AlN NL instead of the AlGaN NL, the LOP of UV LED was further enhanced by 11.3%, which is attributed to reduced TD density in InGaN/AlInGaN active region. In the sputtered AlN thickness range of 10–25 nm, the LOP of UV LED with 15-nm-thick sputtered AlN NL was the highest, revealing that optimum thickness of the sputtered AlN NL is around 15 nm. PMID:28294166

Molecular-beam heteroepitaxial growth and characterization of wide-band-gap semiconductor films and devices

NASA Astrophysics Data System (ADS)

Piquette, Eric Charles

The thesis consists of two parts. Part I describes work on the molecular beam epitaxial (MBE) growth of GaN, AlN, and AlxGa 1-xN alloys, as well as efforts in the initial technical development and demonstration of nitride-based high power electronic devices. The major issues pertaining to MBE growth are discussed, including special requirements of the growth system, substrates, film nucleation, n - and p-type doping, and the dependence of film quality on growth parameters. The GaN films were characterized by a variety of methods, including high resolution x-ray diffraction, photoluminescence, and Hall effect measurement. It is found that the film polarity and extended defect density as well as quality of photoluminescence and electrical transport properties depend crucially on how the nitride layer is nucleated on the substrate and how the subsequent film surface morphology evolves, which can be controlled by the growth conditions. A technique is proposed and demonstrated that utilizes the control of morphology evolution to reduce defect density and improve the structural quality of MBE GaN films. In addition to growth, the design and processing of high voltage GaN Schottky diodes is presented, as well as an experimental study of sputter-deposited ohmic and rectifying metal contacts to GaN. Simple models for high power devices, based on materials properties such as minority carrier diffusion length and critical electric breakdown field, are used to estimate the voltage standoff capability, current carrying capacity, and maximum operating frequency of unipolar and bipolar GaN power devices. The materials and transport properties of GaN pertinent to high power device design were measured experimentally. High voltage Schottky rectifiers were fabricated which verify the impressive electric breakdown field of GaN (2--5 MV/cm). Electron beam induced current (EBIC) experiments were also conducted to measure the minority carrier diffusion length for both electrons and holes in GaN. Part II of the thesis describes studies of the MBE growth of ZnS and investigations of ZnS/GaN fight emitting heterojunctions which show promise for application as blue and green light emitters. Zinc sulfide layers doped with Ag and Al were grown by MBE on sapphire, GaAs, and GaN substrates and characterized by x-ray diffraction and photoluminescence. Preliminary current-voltage and electroluminescence results are presented for a processed ZnS:Al,Ag/GaN:Mg prototype blue light emitting device.

Heterogeneous dislocation loop formation near grain boundaries in a neutron-irradiated commercial FeCrAl alloy

NASA Astrophysics Data System (ADS)

Field, Kevin G.; Briggs, Samuel A.; Hu, Xunxiang; Yamamoto, Yukinori; Howard, Richard H.; Sridharan, Kumar

2017-01-01

FeCrAl alloys are an attractive class of materials for nuclear power applications because of their increased environmental compatibility compared with more traditional nuclear materials. Preliminary studies into the radiation tolerance of FeCrAl alloys under accelerated neutron testing between 300 and 400 °C have shown post-irradiation microstructures containing dislocation loops and a Cr-rich α‧ phase. Although these initial studies established the post-irradiation microstructures, there was little to no focus on understanding the influence of pre-irradiation microstructures on this response. In this study, a well-annealed commercial FeCrAl alloy, Alkrothal 720, was neutron irradiated to 1.8 displacements per atom (dpa) at 382 °C and then the effect of random high-angle grain boundaries on the spatial distribution and size of a〈100〉 dislocation loops, a/2〈111〉 dislocation loops, and black dot damage was analyzed using on-zone scanning transmission electron microscopy. Results showed a clear heterogeneous dislocation loop formation with a/2〈111〉 dislocation loops showing an increased number density and size, black dot damage showing a significant number density decrease, and a〈100〉 dislocation loops exhibiting an increased size in the vicinity of the grain boundary. These results suggest the importance of the pre-irradiation microstructure and, specifically, defect sink density spacing to the radiation tolerance of FeCrAl alloys.

Ab-initio study of boron incorporation and compositional limits at GaN and AlN (0001) surfaces

NASA Astrophysics Data System (ADS)

Lymperakis, L.

2018-06-01

Density functional theory calculations are employed to investigate B incorporation at the GaN(0001) and AlN(0001) surfaces. It is found that under typical metal-organic chemical vapor deposition (MOCVD) and metal rich molecular beam epitaxy (MBE) conditions, the maximum B contents at the surfaces are in the order of 3% for GaN and 15% for AlN. Under MBE N-rich growth conditions the calculations reveal a rehybridization enhanced solubility mechanism that dominates at the surface. This mechanism offers a promising route to kinetically stabilize B contents above the bulk solubility limit and as high as 25%.

Doping Induced Structural Stability and Electronic Properties of GaN Nanotubes

PubMed Central

Khan, Mohammad Irfan; Tyagi, Neha; Swaroop Khare, Purnima

2014-01-01

The present paper discusses the effect of manganese doping on the structural stability and electronic band gap of chiral (2, 1), armchair (3, 3), and zigzag ((6, 0) and (10, 0)) single walled GaN nanotube by using density functional theory based Atomistix Toolkit (ATK) Virtual NanoLab (VNL). The structural stability has been analyzed in terms of minimum ground state total energy, binding, and formation energy. As an effect of Mn doping (1–4 atoms), all the GaN nanotubes taken into consideration show semiconducting to metallic transition first and after certain level of Mn doping changes its trend. PMID:24707225

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nguyen, Thao; Luscher, D. J.; Wilkerson, J. W.

We developed a framework for dislocation-based viscoplasticity and dynamic ductile failure to model high strain rate deformation and damage in single crystals. The rate-dependence of the crystal plasticity formulation is based on the physics of relativistic dislocation kinetics suited for extremely high strain rates. The damage evolution is based on the dynamics of void growth, which are governed by both micro-inertia as well as dislocation kinetics and dislocation substructure evolution. Furthermore, an averaging scheme is proposed in order to approximate the evolution of the dislocation substructure in both the macroscale as well as its spatial distribution at the microscale. Inmore » addition, a concept of a single equivalent dislocation density that effectively captures the collective influence of dislocation density on all active slip systems is proposed here. Together, these concepts and approximations enable the use of semi-analytic solutions for void growth dynamics developed in [J. Wilkerson and K. Ramesh. A dynamic void growth model governed by dislocation kinetics. J. Mech. Phys. Solids, 70:262–280, 2014.], which greatly reduce the computational overhead that would otherwise be required. The resulting homogenized framework has been implemented into a commercially available finite element package, and a validation study against a suite of direct numerical simulations was carried out.« less

A dislocation-based crystal plasticity framework for dynamic ductile failure of single crystals

NASA Astrophysics Data System (ADS)

Nguyen, Thao; Luscher, D. J.; Wilkerson, J. W.

2017-11-01

A framework for dislocation-based viscoplasticity and dynamic ductile failure has been developed to model high strain rate deformation and damage in single crystals. The rate-dependence of the crystal plasticity formulation is based on the physics of relativistic dislocation kinetics suited for extremely high strain rates. The damage evolution is based on the dynamics of void growth, which are governed by both micro-inertia as well as dislocation kinetics and dislocation substructure evolution. An averaging scheme is proposed in order to approximate the evolution of the dislocation substructure in both the macroscale as well as its spatial distribution at the microscale. Additionally, a concept of a single equivalent dislocation density that effectively captures the collective influence of dislocation density on all active slip systems is proposed here. Together, these concepts and approximations enable the use of semi-analytic solutions for void growth dynamics developed in (Wilkerson and Ramesh, 2014), which greatly reduce the computational overhead that would otherwise be required. The resulting homogenized framework has been implemented into a commercially available finite element package, and a validation study against a suite of direct numerical simulations was carried out.

A dislocation-based crystal plasticity framework for dynamic ductile failure of single crystals

DOE PAGES

Nguyen, Thao; Luscher, D. J.; Wilkerson, J. W.

2017-08-02

We developed a framework for dislocation-based viscoplasticity and dynamic ductile failure to model high strain rate deformation and damage in single crystals. The rate-dependence of the crystal plasticity formulation is based on the physics of relativistic dislocation kinetics suited for extremely high strain rates. The damage evolution is based on the dynamics of void growth, which are governed by both micro-inertia as well as dislocation kinetics and dislocation substructure evolution. Furthermore, an averaging scheme is proposed in order to approximate the evolution of the dislocation substructure in both the macroscale as well as its spatial distribution at the microscale. Inmore » addition, a concept of a single equivalent dislocation density that effectively captures the collective influence of dislocation density on all active slip systems is proposed here. Together, these concepts and approximations enable the use of semi-analytic solutions for void growth dynamics developed in [J. Wilkerson and K. Ramesh. A dynamic void growth model governed by dislocation kinetics. J. Mech. Phys. Solids, 70:262–280, 2014.], which greatly reduce the computational overhead that would otherwise be required. The resulting homogenized framework has been implemented into a commercially available finite element package, and a validation study against a suite of direct numerical simulations was carried out.« less

Investigation of dislocation cluster evolution during directional solidification of multicrystalline silicon

NASA Astrophysics Data System (ADS)

Oriwol, Daniel; Trempa, Matthias; Sylla, Lamine; Leipner, Hartmut S.

2017-04-01

Dislocation clusters are the main crystal defects in multicrystalline silicon and are detrimental for solar cell efficiency. They were formed during the silicon ingot casting due to the relaxation of strain energy. The evolution of the dislocation clusters was studied by means of automated analysing tools of the standard wafer and cell production giving information about the cluster development as a function of the ingot height. Due to the observation of the whole wafer surface the point of view is of macroscopic nature. It was found that the dislocations tend to build clusters of high density which usually expand in diameter as a function of ingot height. According to their structure the dislocation clusters can be divided into light and dense clusters. The appearance of both types shows a clear dependence on the orientation of the grain growth direction. Additionally, a process of annihilation of dislocation clusters during the crystallization has been observed. To complement the macroscopic description, the dislocation clusters were also investigates by TEM. It is shown that the dislocations within the subgrain boundaries are closely arranged. Distances of 40-30 nm were found. These results lead to the conclusion that the dislocation density within the cluster structure is impossible to quantify by means of etch pit counting.

Coupling continuum dislocation transport with crystal plasticity for application to shock loading conditions

DOE PAGES

Luscher, Darby Jon; Mayeur, Jason Rhea; Mourad, Hashem Mohamed; ...

2015-08-05

Here, we have developed a multi-physics modeling approach that couples continuum dislocation transport, nonlinear thermoelasticity, crystal plasticity, and consistent internal stress and deformation fields to simulate the single-crystal response of materials under extreme dynamic conditions. Dislocation transport is modeled by enforcing dislocation conservation at a slip-system level through the solution of advection-diffusion equations. Nonlinear thermoelasticity provides a thermodynamically consistent equation of state to relate stress (including pressure), temperature, energy densities, and dissipation. Crystal plasticity is coupled to dislocation transport via Orowan's expression where the constitutive description makes use of recent advances in dislocation velocity theories applicable under extreme loading conditions.more » The configuration of geometrically necessary dislocation density gives rise to an internal stress field that can either inhibit or accentuate the flow of dislocations. An internal strain field associated with the internal stress field contributes to the kinematic decomposition of the overall deformation. The paper describes each theoretical component of the framework, key aspects of the constitutive theory, and some details of a one-dimensional implementation. Results from single-crystal copper plate impact simulations are discussed in order to highlight the role of dislocation transport and pile-up in shock loading regimes. The main conclusions of the paper reinforce the utility of the modeling approach to shock problems.« less

Quantification of dislocation nucleation stress in TiN through high-resolution in situ indentation experiments and first principles calculations

DOE PAGES

Li, N.; Yadav, S. K.; Liu, X. -Y.; ...

2015-11-05

Using the in situ indentation of TiN in a high-resolution transmission electron microscope, the nucleation of full as well as partial dislocations has been observed from {001} and {111} surfaces, respectively. The critical elastic strains associated with the nucleation of the dislocations were analyzed from the recorded atomic displacements, and the nucleation stresses corresponding to the measured critical strains were computed using density functional theory. The resolved shear stress was estimated to be 13.8 GPa for the partial dislocation 1/6 <110> {111} and 6.7 GPa for the full dislocation ½ <110> {110}. Moreover, such an approach of quantifying nucleation stressesmore » for defects via in situ high-resolution experiment coupled with density functional theory calculation may be applied to other unit processes.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Meimei; Wang, Leyun; Almer, Jonathan D.

Deformation processes in Grade 91 (Fe–9%Cr–1%Mo–V,Nb) and Grade 92 (Fe–9%Cr–0.5%Mo–2%W–V,Nb) ferritic–martensitic steels were investigated at temperatures between 20 and 650 °C using high-energy synchrotron X-ray diffraction with in situ thermal–mechanical loading. The change of the dislocation density with strain was quantified by X-ray diffraction line profile analysis complemented by transmission electron microscopy measurements. The relationship between dislocation density and strain during uniform deformation was described by a dislocation model, and two critical materials parameters, namely dislocation mean free path and dynamic recovery coefficient, were determined as a function of temperature. Effects of alloy chemistry, thermal–mechanical treatment and temperature on themore » tensile deformation process in Grade 91 and Grade 92 steels can be well understood by the dislocation evolution behavior.« less

Three-dimensional evaluation of gettering ability for oxygen atoms at small-angle tilt boundaries in Czochralski-grown silicon crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ohno, Yutaka, E-mail: yutakaohno@imr.tohoku.ac.jp; Inoue, Kaihei; Fujiwara, Kozo

2015-06-22

Three-dimensional distribution of oxygen atoms at small-angle tilt boundaries (SATBs) in Czochralski-grown p-type silicon ingots was investigated by atom probe tomography combined with transmission electron microscopy. Oxygen gettering along edge dislocations composing SATBs, post crystal growth, was observed. The gettering ability of SATBs would depend both on the dislocation strain and on the dislocation density. Oxygen atoms would agglomerate in the atomic sites under the tensile hydrostatic stress larger than about 2.0 GPa induced by the dislocations. It was suggested that the density of the atomic sites, depending on the tilt angle of SATBs, determined the gettering ability of SATBs.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Guangming; Zhou, Zhangjian; Mo, Kun

An application of high-energy wide angle synchrotron X-ray diffraction to investigate the tensile deformation of 9Cr ferritic/martensitic (F/M) ODS steel is presented. With tensile loading and in-situ Xray exposure, the lattice strain development of matrix was determined. The lattice strain was found to decrease with increasing temperature, and the difference in Young's modulus of six different reflections at different temperatures reveals the temperature dependence of elastic anisotropy. The mean internal stress was calculated and compared with the applied stress, showing that the strengthening factor increased with increasing temperature, indicating that the oxide nanoparticles have a good strengthening impact at highmore » temperature. The dislocation density and character were also measured during tensile deformation. The dislocation density decreased with increasing of temperature due to the greater mobility of dislocation at high temperature. The dislocation character was determined by best-fit methods for different dislocation average contrasts with various levels of uncertainty. The results shows edge type dislocations dominate the plastic strain at room temperature (RT) and 300 C, while the screw type dislocations dominate at 600 C. The dominance of edge character in 9Cr F/M ODS steels at RT and 300 C is likely due to the pinning effect of nanoparticles for higher mobile edge dislocations when compared with screw dislocations, while the stronger screw type of dislocation structure at 600 C may be explained by the activated cross slip of screw segments.« less

Strain relaxation of thick (11–22) semipolar InGaN layer for long wavelength nitride-based device

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Jaehwan; Min, Daehong; Jang, Jongjin

2014-10-28

In this study, the properties of thick stress-relaxed (11–22) semipolar InGaN layers were investigated. Owing to the inclination of growth orientation, misfit dislocations (MDs) occurred at the heterointerface when the strain state of the (11–22) semipolar InGaN layers reached the critical point. We found that unlike InGaN layers based on polar and nonpolar growth orientations, the surface morphologies of the stress-relaxed (11–22) semipolar InGaN layers did not differ from each other and were similar to the morphology of the underlying GaN layer. In addition, misfit strain across the whole InGaN layer was gradually relaxed by MD formation at the heterointerface.more » To minimize the effect of surface roughness and defects in GaN layers on the InGaN layer, we conducted further investigation on a thick (11–22) semipolar InGaN layer grown on an epitaxial lateral overgrown GaN template. We found that the lateral indium composition across the whole stress-relaxed InGaN layer was almost uniform. Therefore, thick stress-relaxed (11–22) semipolar InGaN layers are suitable candidates for use as underlying layers in long-wavelength devices, as they can be used to control strain accumulation in the heterostructure active region without additional influence of surface roughness.« less

Dislocation substructure of mantle-derived olivine as revealed by selective chemical etching and transmission electron microscopy

USGS Publications Warehouse

Kirby, S.H.; Wegner, M.W.

1978-01-01

Cleaved and mechanically polished surfaces of olivine from peridotite xenoliths from San Carlos, Arizona, were chemically etched using the techniques of Wegner and Christie (1974). Dislocation etch pits are produced on all surface orientations and they tend to be preferentially aligned along the traces of subgrain boundaries, which are approximately parallel to (100), (010), and (001). Shallow channels were also produced on (010) surfaces and represent dislocations near the surface that are etched out along their lengths. The dislocation etch channel loops are often concentric, and emanate from (100) subgrain boundaries, which suggests that dislocation sources are in the boundaries. Data on subgrain misorientation and dislocation line orientation and arguments based on subgrain boundary energy minimization are used to characterize the dislocation structures of the subgrain boundaries. (010) subgrain boundaries are of the twist type, composed of networks of [100] and [001] screw dislocations. Both (100) and (001) subgrain boundaries are tilt walls composed of arrays of edge dislocation with Burgers vectors b=[100] and [001], respectively. The inferred slip systems are {001} ???100???, {100} ???001???, and {010} ???100??? in order of diminishing importance. Exploratory transmission electron microscopy is in accord with these identifications. The flow stresses associated with the development of the subgrain structure are estimated from the densities of free dislocations and from the subgrain dimensions. Inferred stresses range from 35 to 75 bars using the free dislocation densities and 20 to 100 bars using the subgrain sizes. ?? 1978 Springer-Verlag.

Effect of dislocations on the open-circuit voltage, short-circuit current and efficiency of heteroepitaxial indium phosphide solar cells

NASA Technical Reports Server (NTRS)

Jain, Raj K.; Flood, Dennis J.

1990-01-01

Excellent radiation resistance of indium phosphide solar cells makes them a promising candidate for space power applications, but the present high cost of starting substrates may inhibit their large scale use. Thin film indium phosphide cells grown on Si or GaAs substrates have exhibited low efficiencies, because of the generation and propagation of large number of dislocations. Dislocation densities were calculated and its influence on the open circuit voltage, short circuit current, and efficiency of heteroepitaxial indium phosphide cells was studied using the PC-1D. Dislocations act as predominant recombination centers and are required to be controlled by proper transition layers and improved growth techniques. It is shown that heteroepitaxial grown cells could achieve efficiencies in excess of 18 percent AMO by controlling the number of dislocations. The effect of emitter thickness and surface recombination velocity on the cell performance parameters vs. dislocation density is also studied.

Dislocation related droop in InGaN/GaN light emitting diodes investigated via cathodoluminescence

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pozina, Galia; Ciechonski, Rafal; Bi, Zhaoxia

2015-12-21

Today's energy saving solutions for general illumination rely on efficient white light emitting diodes (LEDs). However, the output efficiency droop experienced in InGaN based LEDs with increasing current injection is a serious limitation factor for future development of bright white LEDs. We show using cathodoluminescence (CL) spatial mapping at different electron beam currents that threading dislocations are active as nonradiative recombination centers only at high injection conditions. At low current, the dislocations are inactive in carrier recombination due to local potentials, but these potentials are screened by carriers at higher injection levels. In CL images, this corresponds to the increasemore » of the dark contrast around dislocations with the injection (excitation) density and can be linked with droop related to the threading dislocations. Our data indicate that reduction of droop in the future efficient white LED can be achieved via a drastic reduction of the dislocation density by using, for example, bulk native substrates.« less

Evolution of stress and microstructure in silicon-doped aluminum gallium nitride thin films

NASA Astrophysics Data System (ADS)

Manning, Ian C.

The present work examines the effects of the Si incorporation on the stress evolution of AlxGa1-xN thin films deposited using metalorganic chemical vapor deposition. Specifically, tensile stress generation was evaluated using an in situ wafer curvature measurement technique, and correlated with the inclination of edge-type threading dislocations observed with transmission electron microscopy (TEM). This microstructural process had been theorized to relax compressive strain with increasing film thickness by expanding the missing planes of atoms associated with the dislocations. Prior work regarded dislocation bending as being the result of an effective climb mechanism. In a preliminary investigation, the accuracy of the model derived to quantify the strain induced by dislocation inclination was tested. The relevant parameters were measured to calculate a theoretical stress gradient, which was compared with the gradient as extract from experimental stress data. The predicted value was found to overestimate the measured value. It was also confirmed during the preliminary investigation that Si incorporation alone was sufficient to initiate dislocation bending. The overestimation of the stress gradient yielded by the prediction of the model was then addressed by exploring the effects of dislocation annihilation and fusion reactions occurring during film growth. Si-doped Al0.42Ga 0.58N layers exhibiting inclined threading dislocations were grown to different thicknesses. The dislocation density at the surface of each sample was then measured using plan-view TEM, and was found to be inversely proportional to the thickness. As the original model assumed a constant dislocation density, applying the correction for its reduction yielded a better prediction of the stress evolution. In an attempt to extend the predictive capabilities of the model beyond the single composition examined above, and to better understand the interaction of Si with the host AlxGa1-xN lattice, several sets of AlxGa1-xN films were grown, each with a unique composition. The Si doping level was varied within each set. It was determined that the dominant influence on tensile strain generation is in fact the initial dislocation density, which increased with increasing Al content as observed with plan-view TEM. This was expounded in a series of modeling examples. In addition, threading dislocation inclination was studied in nominally undoped and Si-doped Al xGa1-xN grown under conditions of tensile stress to isolate the influence of Si from that of compressive stress, which had also been found to induce dislocation bending. The effects due to Si and compressive stress were found not to combine as expected, based on a stochastic model of dislocation jog formation that had been developed in prior work to describe the inclination mechanism. Having confirmed the strong, direct relationship between the initial dislocation density and the degree of tensile stress generated in the Al xGa1-xN epilayers during growth, an effort was made to demonstrate the advantage that might be gained by using AlN substrates rather than SiC. In principle, AlN provides a growth surface that inhibits defect formation due to its close similarity to AlxGa1-xN lattice structure and chemistry, particularly at high Al mole fractions. Threading dislocation densities were reduced by an order of magnitude in comparison with samples grown on SiC, with a corresponding reduction in the stress gradient arising from dislocation inclination. (Abstract shortened by UMI.)

Effect of Temper Condition on Stress Relaxation Behavior of an Aluminum Copper Lithium Alloy

NASA Astrophysics Data System (ADS)

Mishra, Sumeet; Beura, Vikrant Kumar; Singh, Amit; Yadava, Manasij; Nayan, Niraj

2018-07-01

Deformation behavior of an Al-Cu-Li alloy in different temper conditions (solutionized and T8) is investigated using stress relaxation tests. Fundamental parameters such as the apparent and physical activation volume, strain rate sensitivity, effective stress, and exhaustion rate of mobile dislocation density are determined from single and multiple relaxation tests. It was found that dislocation-dislocation interaction controls the kinetics of plastic deformation in the solutionized sample, whereas dislocation-precipitate interaction is the overriding factor in the presence of T1 precipitates. The apparent activation volume was found to be significantly lower in the presence of T1 precipitates compared with solutionized samples. Strain rate sensitivity and effective stress were found to be higher in the presence of T1 precipitates. In addition, multiple relaxation tests showed that irrespective of microstructural features (solutes, semi-coherent precipitates), the mobile dislocation density reduces during the relaxation period. Further evidence regarding reduction in mobile dislocation density is obtained from uniaxial tensile tests carried out after stress relaxation tests, where both solutionized and T8 samples show an increase in strength. Additional discussion on relaxation strain is included to provide a complete overview regarding the time-dependent deformation behavior of the Al-Cu-Li alloy in different temper conditions.

Effect of Temper Condition on Stress Relaxation Behavior of an Aluminum Copper Lithium Alloy

NASA Astrophysics Data System (ADS)

Mishra, Sumeet; Beura, Vikrant Kumar; Singh, Amit; Yadava, Manasij; Nayan, Niraj

2018-04-01

Deformation behavior of an Al-Cu-Li alloy in different temper conditions (solutionized and T8) is investigated using stress relaxation tests. Fundamental parameters such as the apparent and physical activation volume, strain rate sensitivity, effective stress, and exhaustion rate of mobile dislocation density are determined from single and multiple relaxation tests. It was found that dislocation-dislocation interaction controls the kinetics of plastic deformation in the solutionized sample, whereas dislocation-precipitate interaction is the overriding factor in the presence of T1 precipitates. The apparent activation volume was found to be significantly lower in the presence of T1 precipitates compared with solutionized samples. Strain rate sensitivity and effective stress were found to be higher in the presence of T1 precipitates. In addition, multiple relaxation tests showed that irrespective of microstructural features (solutes, semi-coherent precipitates), the mobile dislocation density reduces during the relaxation period. Further evidence regarding reduction in mobile dislocation density is obtained from uniaxial tensile tests carried out after stress relaxation tests, where both solutionized and T8 samples show an increase in strength. Additional discussion on relaxation strain is included to provide a complete overview regarding the time-dependent deformation behavior of the Al-Cu-Li alloy in different temper conditions.

Low-temperature preparation of GaN-SiO2 interfaces with low defect density. II. Remote plasma-assisted oxidation of GaN and nitrogen incorporation

NASA Astrophysics Data System (ADS)

Bae, Choelhwyi; Lucovsky, Gerald

2004-11-01

Low-temperature remote plasma-assisted oxidation and nitridation processes for interface formation and passivation have been extended from Si and SiC to GaN. The initial oxidation kinetics and chemical composition of thin interfacial oxide were determined from analysis of on-line Auger electron spectroscopy features associated with Ga, N, and O. The plasma-assisted oxidation process is self-limiting with power-law kinetics similar to those for the plasma-assisted oxidation of Si and SiC. Oxidation using O2/He plasma forms nearly pure GaOx, and oxidation using 1% N2O in N2 forms GaOxNy with small nitrogen content, ~4-7 at. %. The interface and dielectric layer quality was investigated using fabricated GaN metal-oxide-semiconductor capacitors. The lowest density of interface states was achieved with a two-step plasma-assisted oxidation and nitridation process before SiO2 deposition.

Thick nonpolar m-plane and semipolar (10 1 ̅ 1 ̅) GaN on an ammonothermal seed by tri-halide vapor-phase epitaxy using GaCl3

NASA Astrophysics Data System (ADS)

Iso, Kenji; Matsuda, Karen; Takekawa, Nao; Hikida, Kazuhiro; Hayashida, Naoto; Murakami, Hisashi; Koukitu, Akinori

2017-03-01

GaN layers of thickness 0.5-1.3 mm were grown at 1280 °C at a growth rate of 95-275 μm/h by tri-halide vapor-phase epitaxy on nonpolar m-plane (10 1 ̅ 0) and semipolar (10 1 ̅ 1 ̅) ammonothermal GaN substrates. For nonpolar m-plane (10 1 ̅ 0) with a -5° off-angle, the full widths at half maximum (FWHMs) of X-ray rocking curves (XRCs) and the basal plane stacking fault (BSF) density increased from 50 to 178″ and from 4.8×101 to 1.0×103 cm-1, respectively, upon increasing the growth rate from 115 to 245 μm/h. On the other hand, the XRC-FWHM and the BSF density for semipolar (10 1 ̅ 1 ̅) grown at 275 μm/h were as small as 28″ and 8.3×101 cm-1, respectively.

Ab initio electronic structure calculations for metallic intermediate band formation in photovoltaic materials

NASA Astrophysics Data System (ADS)

Wahnón, P.; Tablero, C.

2002-04-01

A metallic isolated band in the middle of the band gap of several III-V semiconductors has been predicted as photovoltaic materials with the possibility of providing substantially enhanced efficiencies. We have investigated the electronic band structures and lattice constants of GanAsmM and GanPmM with M=Sc, Ti, V, and Cr, to identify whether this isolated band is likely to exist by means of accurate calculations. For this task, we use the SIESTA program, an ab initio periodic density-functional method, fully self consistent in the local-density approximation. Norm-conserving, nonlocal pseudopotentials and confined linear combination of atomic orbitals have been used. We have carried out a case study of GanAsmTi and GanPmTi energy-band structure including analyses of the effect of the basis set, fine k-point mesh to ensure numerical convergence, structural parameters, and generalized gradient approximation for exchange and correlation corrections. We find the isolated intermediate band when one Ti atom replaces the position of one As (or P) atom in the crystal structure. For this kind of compound we show that the intermediate band relative position inside the band gap and width are sensitive to the dynamic relaxation of the crystal and the size of the basis set.

Combined angle-resolved X-ray photoelectron spectroscopy, density functional theory and kinetic study of nitridation of gallium arsenide

NASA Astrophysics Data System (ADS)

Mehdi, H.; Monier, G.; Hoggan, P. E.; Bideux, L.; Robert-Goumet, C.; Dubrovskii, V. G.

2018-01-01

The high density of interface and surface states that cause the strong Fermi pinning observed on GaAs surfaces can be reduced by depositing GaN ultra-thin films on GaAs. To further improve this passivation, it is necessary to investigate the nitridation phenomena by identifying the distinct steps occurring during the process and to understand and quantify the growth kinetics of GaAs nitridation under different conditions. Nitridation of the cleaned GaAs substrate was performed using N2 plasma source. Two approaches have been combined. Firstly, an AR-XPS (Angle Resolved X-ray Photoelectron Spectroscopy) study is carried out to determine the chemical environments of the Ga, As and N atoms and the composition depth profile of the GaN thin film which allow us to summarize the nitridation process in three steps. Moreover, the temperature and time treatment have been investigated and show a significant impact on the formation of the GaN layer. The second approach is a refined growth kinetic model which better describes the GaN growth as a function of the nitridation time. This model clarifies the exchange mechanism of arsenic with nitrogen atoms at the GaN/GaAs interface and the phenomenon of quasi-saturation of the process observed experimentally.

High-power AlGaInN lasers for Blu-ray disc system

NASA Astrophysics Data System (ADS)

Takeya, Motonubu; Ikeda, Shinroh; Sasaki, Tomomi; Fujimoto, Tsuyoshi; Ohfuji, Yoshio; Mizuno, Takashi; Oikawa, Kenji; Yabuki, Yoshifumi; Uchida, Shiro; Ikeda, Masao

2003-07-01

This paper describes an improved laser structure for AlGaInN based blue-violet lasers (BV-LDs). The design realizes a small beam divergence angle perpendicular to the junction plane and high characteristic temperature wihtout significant increase in threshold current density (Jth) by optimizing the position of the Mg-doped layer and introducing an undoped AlGaN layer between the active layer and the Mg-doped electron-blocking layer. The mean time to failure (MTTF) of devices based on this design was found to be closely related to the dislocation density of ELO-GaN basal layer. Under 50 mW CW operation at 70°C, a MTTF of over 5000 h was realized whenthe dark spot density (indicative of dislocation density) is less than ~5×106 cm-2. Power consumption under 50mW CW operation at 70°C was approximately 0.33 W, independent of the dislocation density.

Continuum dislocation-density based models for the dynamic shock response of single-crystal and polycrystalline materials

NASA Astrophysics Data System (ADS)

Luscher, Darby

2017-06-01

The dynamic thermomechanical responses of polycrystalline materials under shock loading are often dominated by the interaction of defects and interfaces. For example, polymer-bonded explosives (PBX) can initiate under weak shock impacts whose energy, if distributed homogeneously throughout the material, translates to temperature increases that are insufficient to drive the rapid chemistry observed. In such cases, heterogeneous thermomechanical interactions at the mesoscale (i.e. between single-crystal and macroscale) lead to the formation of localized hot spots. Within metals, a prescribed deformation associated with a shock wave may be accommodated by crystallographic slip, provided a sufficient population of mobile dislocations is available. However, if the deformation rate is large enough, there may be an insufficient number of freely mobile dislocations. In these cases, additional dislocations may be nucleated, or alternate mechanisms (e.g. twinning, damage) activated in order to accommodate the deformation. Direct numerical simulation at the mesoscale offers insight into these physical processes that can be invaluable to the development of macroscale constitutive theories, if the mesoscale models adequately represent the anisotropic nonlinear thermomechanical response of individual crystals and their interfaces. This talk will briefly outline a continuum mesoscale modeling framework founded upon local and nonlocal variations of dislocation-density based crystal plasticity theory. The nonlocal theory couples continuum dislocation transport with the local theory. In the latter, dislocation transport is modeled by enforcing dislocation conservation at a slip-system level through the solution of advection-diffusion equations. The configuration of geometrically necessary dislocation density gives rise to a back-stress that inhibits or accentuates the flow of dislocations. Development of the local theory and application to modeling the explosive molecular crystal RDX and polycrystalline PBX will be discussed. The talk will also emphasize recent implementation of the coupled nonlocal model into a 3D shock hydrocode and simulation results for the dynamic response of polycrystalline copper in two and three dimensions.

Computational analysis of heat transfer, thermal stress and dislocation density during resistively Czochralski growth of germanium single crystal

NASA Astrophysics Data System (ADS)

Tavakoli, Mohammad Hossein; Renani, Elahe Kabiri; Honarmandnia, Mohtaram; Ezheiyan, Mahdi

2018-02-01

In this paper, a set of numerical simulations of fluid flow, temperature gradient, thermal stress and dislocation density for a Czochralski setup used to grow IR optical-grade Ge single crystal have been done for different stages of the growth process. A two-dimensional steady state finite element method has been applied for all calculations. The obtained numerical results reveal that the thermal field, thermal stress and dislocation structure are mainly dependent on the crystal height, heat radiation and gas flow in the growth system.

Improved performance of GaN based light emitting diodes with ex-situ sputtered AlN nucleation layers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Shuo-Wei; Epistar Corporation, Hsinchu 300, Taiwan; Li, Heng

The crystal quality, electrical and optical properties of GaN based light emitting diodes (LEDs) with ex-situ sputtered physical vapor deposition (PVD) aluminum nitride (AlN) nucleation layers were investigated. It was found that the crystal quality in terms of defect density and x-ray diffraction linewidth was greatly improved in comparison to LEDs with in-situ low temperature GaN nucleation layer. The light output power was 3.7% increased and the reverse bias voltage of leakage current was twice on LEDs with ex-situ PVD AlN nucleation layers. However, larger compressive strain was discovered in LEDs with ex-situ PVD AlN nucleation layers. The study showsmore » the potential and constrain in applying ex-situ PVD AlN nucleation layers to fabricate high quality GaN crystals in various optoelectronics.« less

Investigations of the Optical Properties of GaNAs Alloys by First-Principle.

PubMed

Borovac, Damir; Tan, Chee-Keong; Tansu, Nelson

2017-12-11

We present a Density Functional Theory (DFT) analysis of the optical properties of dilute-As GaN 1-x As x alloys with arsenic (As) content ranging from 0% up to 12.5%. The real and imaginary parts of the dielectric function are investigated, and the results are compared to experimental and theoretical values for GaN. The analysis extends to present the complex refractive index and the normal-incidence reflectivity. The refractive index difference between GaN and GaNAs alloys can be engineered to be up to ~0.35 in the visible regime by inserting relatively low amounts of As-content into the GaN system. Thus, the analysis elucidates on the birefringence of the dilute-As GaNAs alloys and comparison to other experimentally characterized III-nitride systems is drawn. Our findings indicate the potential of GaNAs alloys for III-nitride based waveguide and photonic circuit design applications.

Substrate misorientation induced strong increase in the hole concentration in Mg doped GaN grown by metalorganic vapor phase epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Suski, T.; Litwin-Staszewska, E.; Piotrzkowski, R.

We demonstrate that relatively small GaN substrate misorientation can strongly change hole carrier concentration in Mg doped GaN layers grown by metalorganic vapor phase epitaxy. In this work intentionally misoriented GaN substrates (up to 2 deg. with respect to ideal <0001> plane) were employed. An increase in the hole carrier concentration to the level above 10{sup 18} cm{sup -3} and a decrease in GaN:Mg resistivity below 1 {omega} cm were achieved. Using secondary ion mass spectroscopy we found that Mg incorporation does not change with varying misorientation angle. This finding suggests that the compensation rate, i.e., a decrease in unintentionalmore » donor density, is responsible for the observed increase in the hole concentration. Analysis of the temperature dependence of electrical transport confirms this interpretation.« less

Comparing electrical characteristics of in situ and ex situ Al2O3/GaN interfaces formed by metalorganic chemical vapor deposition

NASA Astrophysics Data System (ADS)

Chan, Silvia H.; Bisi, Davide; Tahhan, Maher; Gupta, Chirag; DenBaars, Steven P.; Keller, Stacia; Zanoni, Enrico; Mishra, Umesh K.

2018-04-01

Al2O3/n-GaN MOS-capacitors grown by metalorganic chemical vapor deposition with in-situ- and ex-situ-formed Al2O3/GaN interfaces were characterized. Capacitors grown entirely in situ exhibited ˜4 × 1012 cm-2 fewer positive fixed charges and up to ˜1 × 1013 cm-2 eV-1 lower interface-state density near the band-edge than did capacitors with ex situ oxides. When in situ Al2O3/GaN interfaces were reformed via the insertion of a 10-nm-thick GaN layer, devices exhibited behavior between the in situ and ex situ limits. These results illustrate the extent to which an in-situ-formed dielectric/GaN gate stack improves the interface quality and breakdown performance.

Optical and structural characteristics of high indium content InGaN/GaN multi-quantum wells with varying GaN cap layer thickness

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, J.; Zhao, D. G., E-mail: dgzhao@red.semi.ac.cn; Jiang, D. S.

2015-02-07

The optical and structural properties of InGaN/GaN multi-quantum wells (MQWs) with different thicknesses of low temperature grown GaN cap layers are investigated. It is found that the MQW emission energy red-shifts and the peak intensity decreases with increasing GaN cap layer thickness, which may be partly caused by increased floating indium atoms accumulated at quantum well (QW) surface. They will result in the increased interface roughness, higher defect density, and even lead to a thermal degradation of QW layers. An extra growth interruption introduced before the growth of GaN cap layer can help with evaporating the floating indium atoms, andmore » therefore is an effective method to improve the optical properties of high indium content InGaN/GaN MQWs.« less

Numerical analysis of thermal stress and dislocation density distributions in large size multi-crystalline silicon ingots during the seeded growth process

NASA Astrophysics Data System (ADS)

Nguyen, Thi Hoai Thu; Chen, Jyh-Chen; Hu, Chieh; Chen, Chun-Hung; Huang, Yen-Hao; Lin, Huang-Wei; Yu, Andy; Hsu, Bruce

2017-06-01

In this study, a global transient numerical simulation of silicon growth from the beginning of the solidification process until the end of the cooling process is carried out modeling the growth of an 800 kg ingot in an industrial seeded directional solidification furnace. The standard furnace is modified by the addition of insulating blocks in the hot zone. The simulation results show that there is a significant decrease in the thermal stress and dislocation density in the modified model as compared to the standard one (a maximal decrease of 23% and 75% along the center line of ingot for thermal stress and dislocation density, respectively). This modification reduces the heating power consumption for solidification of the silicon melt by about 17% and shortens the growth time by about 2.5 h. Moreover, it is found that adjusting the operating conditions of modified model to obtain the lower growth rate during the early stages of the solidification process can lower dislocation density and total heater power.

P-type surface effects for thickness variation of 2um and 4um of n-type layer in GaN LED

NASA Astrophysics Data System (ADS)

Halim, N. S. A. Abdul; Wahid, M. H. A.; Hambali, N. A. M. Ahmad; Rashid, S.; Ramli, M. M.; Shahimin, M. M.

2017-09-01

The internal quantum efficiency of III-Nitrides group, GaN light-emitting diode (LED) has been considerably limited due to the insufficient hole injection and this is caused by the lack of performance p-type doping and low hole mobility. The low hole mobility makes the hole less energetic, thus reduced the performance operation of GaN LED itself. The internal quantum efficiency of GaN-based LED with surface roughness (texture) can be changed by texture size, density, and thickness of GaN film or by the combined effects of surface shape and thickness of GaN film. Besides, due to lack of p-type GaN, attempts to look forward the potential of GaN LED relied on the thickness of n-type layer and surface shape of p-type GaN layer. This work investigates the characteristics of GaN LED with undoped n-GaN layer of different thickness and the surface shape of p-type layer. The LEDs performance is significantly altered by modifying the thickness and shape. Enhancement of n-GaN layer has led to the annihilation of electrical conductivity of the chip. Different surface geometry governs the emission rate extensively. Internal quantum efficiency is also predominantly affected by the geometry of n-GaN layer which subjected to the current spreading. It is recorded that the IQE droop can be minimized by varying the thickness of the active layer without amplifying the forward voltage. Optimum forward voltage (I-V), total emission rate relationship with the injected current and internal quantum efficiency (IQE) for 2,4 µm on four different surfaces of p-type layer are also reported in this paper.

Detection of deep-level defects and reduced carrier concentration in Mg-ion-implanted GaN before high-temperature annealing

NASA Astrophysics Data System (ADS)

Akazawa, Masamichi; Yokota, Naoshige; Uetake, Kei

2018-02-01

We report experimental results for the detection of deep-level defects in GaN after Mg ion implantation before high-temperature annealing. The n-type GaN samples were grown on GaN free-standing substrates by metalorganic vapor phase epitaxy. Mg ions were implanted at 50 keV with a small dosage of 1.5×1011 cm-2, which did not change the conduction type of the n-GaN. By depositing Al2O3 and a Ni/Au electrode onto the implanted n-GaN, metal-oxide-semiconductor (MOS) diodes were fabricated and tested. The measured capacitance-voltage (C-V) characteristics showed a particular behavior with a plateau region and a region with an anomalously steep slope. Fitting to the experimental C-V curves by simulation showed the existence of deep-level defects and a reduction of the carrier concentration near the GaN surface. By annealing at 800oC, the density of the deep-level defects was reduced and the carrier concentration partially recovered.

Growth and characterization of GaN nanostructures under various ammoniating time with fabricated Schottky gas sensor based on Si substrate

NASA Astrophysics Data System (ADS)

Abdullah, Q. N.; Ahmed, A. R.; Ali, A. M.; Yam, F. K.; Hassan, Z.; Bououdina, M.; Almessiere, M. A.

2018-05-01

This paper presents the investigation of the influence of the ammoniating time of GaN nanowires (NWs) on the crystalline structure, surface morphology, and optical characteristics. Morphological analysis indicates the growth of good quality and high density of NWs with diameters around 50 nm and lengths up to tens of microns after ammoniating for 30 min. Structural analysis shows that GaN NWs have a typical hexagonal wurtzite crystal structure. Raman spectroscopy confirms the formation of GaN compound with the presence of compressive stress. Photoluminescence (PL) measurements revealed two band emissions, an UV and a broad visible emission. Hydrogen sensor was subsequently fabricated by depositing Pt Schottky contact onto GaN NWs film. The sensor response was measured at various H2 concentrations ranged from 200 up to 1200 ppm at room temperature. It was found that the response increases significantly for low H2 concentration (200-300 ppm) to reach about 50% then increases smoothly to reach 60% at 1200 ppm. The as-fabricated sensor possesses higher performances as compared to similar devices reported in the literature.

Study of the effects of GaN buffer layer quality on the dc characteristics of AlGaN/GaN high electron mobility transistors

DOE PAGES

Ahn, Shihyun; Zhu, Weidi; Dong, Chen; ...

2015-04-21

Here we studied the effect of buffer layer quality on dc characteristics of AlGaN/GaN high electron mobility (HEMTs). AlGaN/GaN HEMT structures with 2 and 5 μm GaN buffer layers on sapphire substrates from two different vendors with the same Al concentration of AlGaN were used. The defect densities of HEMT structures with 2 and 5 μm GaN buffer layer were 7 × 10 9 and 5 × 10 8 cm ₋2, respectively, as measured by transmission electron microscopy. There was little difference in drain saturation current or in transfer characteristics in HEMTs on these two types of buffer. However, theremore » was no dispersion observed on the nonpassivated HEMTs with 5 μm GaN buffer layer for gate-lag pulsed measurement at 100 kHz, which was in sharp contrast to the 71% drain current reduction for the HEMT with 2 μm GaN buffer layer.« less

High quality InP-on-Si for solar cell applications

NASA Technical Reports Server (NTRS)

Shellenbarger, Zane A.; Goodwin, Thomas A.; Collins, Sandra R.; Dinetta, Louis C.

1994-01-01

InP on Si solar cells combine the low-cost and high-strength of Si with the high efficiency and radiation tolerance of InP. The main obstacle in the growth of single crystal InP-on-Si is the high residual strain and high dislocation density of the heteroepitaxial InP films. The dislocations result from the large differences in lattice constant and thermal expansion mismatch of InP and Si. Adjusting the size and geometry of the growth area is one possible method of addressing this problem. In this work, we conducted a material quality study of liquid phase epitaxy overgrowth layers on selective area InP grown by a proprietary vapor phase epitaxy technique on Si. The relationship between growth area and dislocation density was quantified using etch pit density measurements. Material quality of the InP on Si improved both with reduced growth area and increased aspect ratio (length/width) of the selective area. Areas with etch pit density as low as 1.6 x 10(exp 4) sq cm were obtained. Assuming dislocation density is an order of magnitude greater than etch pit density, solar cells made with this material could achieve the maximum theoretical efficiency of 23% at AMO. Etch pit density dependence on the orientation of the selective areas on the substrate was also studied.

Orbitally driven low thermal conductivity of monolayer gallium nitride (GaN) with planar honeycomb structure: a comparative study.

PubMed

Qin, Zhenzhen; Qin, Guangzhao; Zuo, Xu; Xiong, Zhihua; Hu, Ming

2017-03-23

Two-dimensional (2D) materials with graphene as a representative have been intensively studied for a long time. Recently, monolayer gallium nitride (ML GaN) with honeycomb structure was successfully fabricated in experiments, generating enormous research interest for its promising applications in nano- and opto-electronics. Considering all these applications are inevitably involved with thermal transport, systematic investigation of the phonon transport properties of 2D GaN is in demand. In this paper, by solving the Boltzmann transport equation (BTE) based on first-principles calculations, we performed a comprehensive study of the phonon transport properties of ML GaN, with detailed comparison to bulk GaN, 2D graphene, silicene and ML BN with similar honeycomb structure. Considering the similar planar structure of ML GaN to graphene, it is quite intriguing to find that the thermal conductivity (κ) of ML GaN (14.93 W mK -1 ) is more than two orders of magnitude lower than that of graphene and is even lower than that of silicene with a buckled structure. Systematic analysis is performed based on the study of the contribution from phonon branches, comparison among the mode level phonon group velocity and lifetime, the detailed process and channels of phonon-phonon scattering, and phonon anharmonicity with potential energy well. We found that, different from graphene and ML BN, the phonon-phonon scattering selection rule in 2D GaN is slightly broken by the lowered symmetry due to the large difference in the atomic radius and mass between Ga and N atoms. Further deep insight is gained from the electronic structure. Resulting from the special sp orbital hybridization mediated by the Ga-d orbital in ML GaN, the strongly polarized Ga-N bond, localized charge density, and its inhomogeneous distribution induce large phonon anharmonicity and lead to the intrinsic low κ of ML GaN. The orbitally driven low κ of ML GaN unraveled in this work would make 2D GaN prospective for applications in energy conversion such as thermoelectrics. Our study offers fundamental understanding of phonon transport in ML GaN within the framework of BTE and further electronic structure, which will enrich the studies of nanoscale phonon transport in 2D materials and shed light on further studies.

Competing charge density wave and antiferromagnetism of metallic atom wires in GaN(10 1 ¯ ) and ZnO(10 1 ¯ )

NASA Astrophysics Data System (ADS)

Kang, Yoon-Gu; Kim, Sun-Woo; Cho, Jun-Hyung

2017-12-01

Low-dimensional electron systems often show a delicate interplay between electron-phonon and electron-electron interactions, giving rise to interesting quantum phases such as the charge density wave (CDW) and magnetism. Using the density-functional theory (DFT) calculations with the semilocal and hybrid exchange-correlation functionals as well as the exact-exchange plus correlation in the random-phase approximation (EX + cRPA), we systematically investigate the ground state of the metallic atom wires containing dangling-bond (DB) electrons, fabricated by partially hydrogenating the GaN(10 1 ¯0 ) and ZnO(10 1 ¯0 ) surfaces. We find that the CDW or antiferromagnetic (AFM) order has an electronic energy gain due to a band-gap opening, thereby being more stabilized compared to the metallic state. Our semilocal DFT calculation predicts that both DB wires in GaN(10 1 ¯0 ) and ZnO(10 1 ¯0 ) have the same CDW ground state, whereas the hybrid DFT and EX + cRPA calculations predict the AFM ground state for the former DB wire and the CDW ground state for the latter one. It is revealed that more localized Ga DB electrons in GaN(10 1 ¯0 ) prefer the AFM order, while less localized Zn DB electrons in ZnO(10 1 ¯0 ) the CDW formation. Our findings demonstrate that the drastically different ground states are competing in the DB wires created on the two representative compound semiconductor surfaces.

Characterization of Dislocations in Semiconductor Heterostructures Using X-ray Rocking Curve Pendellösung

NASA Astrophysics Data System (ADS)

Althowibi, Fahad A.; Ayers, John E.

2018-02-01

In this work we investigated the dislocation-dependent behavior of Pendellösung fringes from two types of semiconductor heterostructures: a uniform-composition InGaAs epitaxial layer grown on a GaAs (001) substrate with an intermediate step-graded InGaAs buffer, and an InGaAs/InAlAs high electron mobility transistor grown on an InP (001) substrate. Dynamical x-ray diffraction simulations were carried out in the 004, 115,135, and 117 geometry, assuming Cu kα1 incident radiation, for both structures. The dislocation density strongly affects the intensities and widths of Pendellösung fringes, and we have established quantitative relationships which will allow characterization of the dislocation density.

Dual nature of acceptors in GaN and ZnO: The curious case of the shallow MgGa deep state

NASA Astrophysics Data System (ADS)

Lany, Stephan; Zunger, Alex

2010-04-01

Employing a Koopmans corrected density functional method, we find that the metal-site acceptors Mg, Be, and Zn in GaN and Li in ZnO bind holes in deep levels that are largely localized at single anion ligand atoms. In addition to this deep ground state (DGS), we observe an effective-masslike delocalized state that can exist as a short lived shallow transient state (STS). The Mg dopant in GaN represents the unique case where the ionization energy of the localized deep level exceeds only slightly that of the shallow effective-mass acceptor, which explains why Mg works so exceptionally well as an acceptor dopant.

Enhanced ultraviolet photoconductivity in porous GaN prepared by metal-assisted electroless etching

NASA Astrophysics Data System (ADS)

Guo, X. Y.; Williamson, T. L.; Bohn, P. W.

2006-10-01

The ultraviolet photoconductivity of porous GaN (PGaN) produced by Pt-assisted electroless etching has been investigated. The photoresponse of PGaN prepared from highly doped GaN ( n>1018 cm) shows enhanced ( 15×) magnitude and faster decay of persistent photoconductivity relative to bulk crystalline (CGaN), suggesting advantages for PGaN in photodetector applications. A space charge model for changes in photoconductivity is used to explain these observations. Heightened defect density in the etched material plays an important role in the enhanced photoconductivity in PGaN. Flux-dependent optical quenching (OQ) behavior, linked to the presence of metastable states, is also observed in PGaN as in CGaN.

Position-controlled MOVPE growth and electro-optical characterization of core-shell InGaN/GaN microrod LEDs

NASA Astrophysics Data System (ADS)

Schimpke, Tilman; Lugauer, H.-J.; Avramescu, A.; Varghese, T.; Koller, A.; Hartmann, J.; Ledig, J.; Waag, A.; Strassburg, M.

2016-03-01

Today's InGaN-based white LEDs still suffer from a significant efficiency reduction at elevated current densities, the so-called "Droop". Core-shell microrods, with quantum wells (QWs) covering their entire surface, enable a tremendous increase in active area scaling with the rod's aspect ratio. Enlarging the active area on a given footprint area is a viable and cost effective route to mitigate the droop by effectively reducing the local current density. Microrods were grown in a large volume metal-organic vapor phase epitaxy (MOVPE) reactor on GaN-on-sapphire substrates with a thin, patterned SiO2 mask for position control. Out of the mask openings, pencil-shaped n-doped GaN microrod cores were grown under conditions favoring 3D growth. In a second growth step, these cores are covered with a shell containing a quantum well and a p-n junction to form LED structures. The emission from the QWs on the different facets was studied using resonant temperature-dependent photoluminescence (PL) and cathodoluminescence (CL) measurements. The crystal quality of the structures was investigated by transmission electron microscopy (TEM) showing the absence of extended defects like threading dislocations in the 3D core. In order to fabricate LED chips, dedicated processes were developed to accommodate for the special requirements of the 3D geometry. The electrical and optical properties of ensembles of tens of thousands microrods connected in parallel are discussed.

TEM study of 〈110〉-type 35.26° dislocations specially induced by polishing of SrTiO₃ single crystals.

PubMed

Jin, L; Guo, X; Jia, C L

2013-11-01

The dislocations created by mechanical polishing of SrTiO₃ (100) single crystals were investigated by means of transmission electron microscopy (TEM) techniques combined with scanning TEM (STEM) techniques. A high density of dislocations was observed in the surface layer with a thickness of about 5 μm. These dislocations were found to be straight and highly aligned along the 〈111〉 directions. In most cases they appear in pairs or as a bundle. The nature of the dislocations was determined as mixed 〈110〉-type with the line vector t=〈111〉. They are 〈110〉-type 35.26° dislocations. The isolated 〈110〉-type 35.26° dislocations possess a compact core structure with a core spreading of ~0.5 nm. Dissociation of the dislocation occurs on the {1−10} glide plane, leading to the formation of two b=a/2〈110〉 partials separated by a stacking fault. The separation of the two partials was estimated to be 2.53 ± 0.32 nm based on a cross-correlation analysis of atomic-resolution images. Our results provide a solid experimental evidence for this special type of dislocation in SrTiO₃. The high density of straight and highly 〈111〉-orientated dislocations is expected to have an important influence on the anisotropy in electrical and mass transport properties. © 2013 Elsevier B.V. All rights reserved.

Piezo-generator integrating a vertical array of GaN nanowires.

PubMed

Jamond, N; Chrétien, P; Houzé, F; Lu, L; Largeau, L; Maugain, O; Travers, L; Harmand, J C; Glas, F; Lefeuvre, E; Tchernycheva, M; Gogneau, N

2016-08-12

We demonstrate the first piezo-generator integrating a vertical array of GaN nanowires (NWs). We perform a systematic multi-scale analysis, going from single wire properties to macroscopic device fabrication and characterization, which allows us to establish for GaN NWs the relationship between the material properties and the piezo-generation, and to propose an efficient piezo-generator design. The piezo-conversion of individual MBE-grown p-doped GaN NWs in a dense array is assessed by atomic force microscopy (AFM) equipped with a Resiscope module yielding an average output voltage of 228 ± 120 mV and a maximum value of 350 mV generated per NW. In the case of p-doped GaN NWs, the piezo-generation is achieved when a positive piezo-potential is created inside the nanostructures, i.e. when the NWs are submitted to compressive deformation. The understanding of the piezo-generation mechanism in our GaN NWs, gained from AFM analyses, is applied to design a piezo-generator operated under compressive strain. The device consists of NW arrays of several square millimeters in size embedded into spin-on glass with a Schottky contact for rectification and collection of piezo-generated carriers. The generator delivers a maximum power density of ∼12.7 mW cm(-3). This value sets the new state of the art for piezo-generators based on GaN NWs and more generally on nitride NWs, and offers promising prospects for the use of GaN NWs as high-efficiency ultra-compact energy harvesters.

The relationship between strain geometry and geometrically necessary dislocations

NASA Astrophysics Data System (ADS)

Hansen, Lars; Wallis, David

2016-04-01

The kinematics of past deformations are often a primary goal in structural analyses of strained rocks. Details of the strain geometry, in particular, can help distinguish hypotheses about large-scale tectonic phenomena. Microstructural indicators of strain geometry have been heavily utilized to investigate large-scale kinematics. However, many of the existing techniques require structures for which the initial morphology is known, and those structures must undergo the same deformation as imposed macroscopically. Many deformed rocks do not exhibit such convenient features, and therefore the strain geometry is often difficult (if not impossible) to ascertain. Alternatively, crystallographic textures contain information about the strain geometry, but the influence of strain geometry can be difficult to separate from other environmental factors that might affect slip system activity and therefore the textural evolution. Here we explore the ability for geometrically necessary dislocations to record information about the deformation geometry. It is well known that crystallographic slip due to the motion of dislocations yields macroscopic plastic strain, and the mathematics are established to relate dislocation glide on multiple slip systems to the strain tensor of a crystal. This theoretical description generally assumes that dislocations propagate across the entire crystal. However, at any point during the deformation, dislocations are present that have not fully transected the crystal, existing either as free dislocations or as dislocations organized into substructures like subgrain boundaries. These dislocations can remain in the lattice after deformation if the crystal is quenched sufficiently fast, and we hypothesize that this residual dislocation population can be linked to the plastic strain geometry in a quantitative manner. To test this hypothesis, we use high-resolution electron backscatter diffraction to measure lattice curvatures in experimentally deformed single crystals and aggregates of olivine for which the strain geometry is known. Tested geometries include constrictional strain, flattening strain, and plane strain. We use measured lattice curvatures to calculate the densities and spatial distributions of geometrically necessary dislocations. Dislocation densities are calculated for each of the major dislocation types in olivine. These densities are then used to estimate the plastic strain geometry under the assumption that the population of geometrically necessary dislocations accurately represents the relative activity of different dislocations during deformation. Our initial results demonstrate compelling relationships between the imposed strain geometry and the calculated plastic strain geometry. In addition, the calculated plastic strain geometry is linked to the distribution of crystallographic orientations, giving insight into the nature of plastic anisotropy in textured olivine aggregates. We present this technique as a new microstructural tool for assessing the kinematic history of deformed rocks.

Recycling process for recovery of gallium from GaN an e-waste of LED industry through ball milling, annealing and leaching

DOE Office of Scientific and Technical Information (OSTI.GOV)

Swain, Basudev, E-mail: swain@iae.re.kr; Mishra, Chinmayee; Kang, Leeseung

Waste dust generated during manufacturing of LED contains significant amounts of gallium and indium, needs suitable treatment and can be an important resource for recovery. The LED industry waste dust contains primarily gallium as GaN. Leaching followed by purification technology is the green and clean technology. To develop treatment and recycling technology of these GaN bearing e-waste, leaching is the primary stage. In our current investigation possible process for treatment and quantitative leaching of gallium and indium from the GaN bearing e-waste or waste of LED industry dust has been developed. To recycle the waste and quantitative leaching of gallium,more » two different process flow sheets have been proposed. In one, process first the GaN of the waste the LED industry dust was leached at the optimum condition. Subsequently, the leach residue was mixed with Na{sub 2}CO{sub 3}, ball milled followed by annealing, again leached to recover gallium. In the second process, the waste LED industry dust was mixed with Na{sub 2}CO{sub 3}, after ball milling and annealing, followed acidic leaching. Without pretreatment, the gallium leaching was only 4.91 w/w % using 4 M HCl, 100 °C and pulp density of 20 g/L. After mechano-chemical processing, both these processes achieved 73.68 w/w % of gallium leaching at their optimum condition. The developed process can treat and recycle any e-waste containing GaN through ball milling, annealing and leaching. - Highlights: • Simplest process for treatment of GaN an LED industry waste developed. • The process developed recovers gallium from waste LED waste dust. • Thermal analysis and phase properties of GaN to Ga{sub 2}O{sub 3} and GaN to NaGaO{sub 2} revealed. • Solid-state chemistry involved in this process reported. • Quantitative leaching of the GaN was achieved.« less

Interaction of 〈1 0 0〉 dislocation loops with dislocations studied by dislocation dynamics in α-iron

NASA Astrophysics Data System (ADS)

Shi, X. J.; Dupuy, L.; Devincre, B.; Terentyev, D.; Vincent, L.

2015-05-01

Interstitial dislocation loops with Burgers vector of 〈1 0 0〉 type are formed in α-iron under neutron or heavy ion irradiation. As the density and size of these loops increase with radiation dose and temperature, these defects are thought to play a key role in hardening and subsequent embrittlement of iron-based steels. The aim of the present work is to study the pinning strength of the loops on mobile dislocations. Prior to run massive Dislocation Dynamics (DD) simulations involving experimentally representative array of radiation defects and dislocations, the DD code and its parameterization are validated by comparing the individual loop-dislocation reactions with those obtained from direct atomistic Molecular Dynamics (MD) simulations. Several loop-dislocation reaction mechanisms are successfully reproduced as well as the values of the unpinning stress to detach mobile dislocations from the defects.

Effect of copper on the recombination activity of extended defects in silicon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Feklisova, O. V., E-mail: feklisov@iptm.ru; Yakimov, E. B.

2015-06-15

The effect of copper atoms introduced by high-temperature diffusion on the recombination properties of dislocations and dislocation trails in p-type single-crystal silicon is studied by the electron-beam-induced current technique. It is shown that, in contrast to dislocations, dislocation trails exhibit an increase in recombination activity after the introduction of copper. Bright contrast appearance in the vicinity of dislocation trails is detected after the diffusion of copper and quenching of the samples. The contrast depends on the defect density in these trails.

Optimization of hetero-epitaxial growth for the threading dislocation density reduction of germanium epilayers

NASA Astrophysics Data System (ADS)

Chong, Haining; Wang, Zhewei; Chen, Chaonan; Xu, Zemin; Wu, Ke; Wu, Lan; Xu, Bo; Ye, Hui

2018-04-01

In order to suppress dislocation generation, we develop a "three-step growth" method to heteroepitaxy low dislocation density germanium (Ge) layers on silicon with the MBE process. The method is composed of 3 growth steps: low temperature (LT) seed layer, LT-HT intermediate layer as well as high temperature (HT) epilayer, successively. Threading dislocation density (TDD) of epitaxial Ge layers is measured as low as 1.4 × 106 cm-2 by optimizing the growth parameters. The results of Raman spectrum showed that the internal strain of heteroepitaxial Ge layers is tensile and homogeneous. During the growth of LT-HT intermediate layer, TDD reduction can be obtained by lowering the temperature ramping rate, and high rate deposition maintains smooth surface morphology in Ge epilayer. A mechanism based on thermodynamics is used to explain the TDD and surface morphological dependence on temperature ramping rate and deposition rate. Furthermore, we demonstrate that the Ge layer obtained can provide an excellent platform for III-V materials integrated on Si.

High purity, low dislocation GaAs single crystals

NASA Technical Reports Server (NTRS)

Chen, R. T.; Holmes, D. E.; Kirkpatrick, C. G.

1983-01-01

Liquid encapsulated Czochralski crystal growth techniques for producing undoped, high resistivity, low dislocation material suitable for device applications is described. Technique development resulted in reduction of dislocation densities in 3 inch GaAs crystals. Control over the melt stoichiometry was determined to be of critical importance for the reduction of twinning and polycrystallinity during growth.

Supersonic Dislocation Bursts in Silicon

DOE PAGES

Hahn, E. N.; Zhao, S.; Bringa, E. M.; ...

2016-06-06

Dislocations are the primary agents of permanent deformation in crystalline solids. Since the theoretical prediction of supersonic dislocations over half a century ago, there is a dearth of experimental evidence supporting their existence. Here we use non-equilibrium molecular dynamics simulations of shocked silicon to reveal transient supersonic partial dislocation motion at approximately 15 km/s, faster than any previous in-silico observation. Homogeneous dislocation nucleation occurs near the shock front and supersonic dislocation motion lasts just fractions of picoseconds before the dislocations catch the shock front and decelerate back to the elastic wave speed. Applying a modified analytical equation for dislocation evolutionmore » we successfully predict a dislocation density of 1.5 x 10(12) cm(-2) within the shocked volume, in agreement with the present simulations and realistic in regards to prior and on-going recovery experiments in silicon.« less

Supersonic Dislocation Bursts in Silicon

PubMed Central

Hahn, E. N.; Zhao, S.; Bringa, E. M.; Meyers, M. A.

2016-01-01

Dislocations are the primary agents of permanent deformation in crystalline solids. Since the theoretical prediction of supersonic dislocations over half a century ago, there is a dearth of experimental evidence supporting their existence. Here we use non-equilibrium molecular dynamics simulations of shocked silicon to reveal transient supersonic partial dislocation motion at approximately 15 km/s, faster than any previous in-silico observation. Homogeneous dislocation nucleation occurs near the shock front and supersonic dislocation motion lasts just fractions of picoseconds before the dislocations catch the shock front and decelerate back to the elastic wave speed. Applying a modified analytical equation for dislocation evolution we successfully predict a dislocation density of 1.5 × 1012 cm−2 within the shocked volume, in agreement with the present simulations and realistic in regards to prior and on-going recovery experiments in silicon. PMID:27264746

Supersonic Dislocation Bursts in Silicon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hahn, E. N.; Zhao, S.; Bringa, E. M.

Dislocations are the primary agents of permanent deformation in crystalline solids. Since the theoretical prediction of supersonic dislocations over half a century ago, there is a dearth of experimental evidence supporting their existence. Here we use non-equilibrium molecular dynamics simulations of shocked silicon to reveal transient supersonic partial dislocation motion at approximately 15 km/s, faster than any previous in-silico observation. Homogeneous dislocation nucleation occurs near the shock front and supersonic dislocation motion lasts just fractions of picoseconds before the dislocations catch the shock front and decelerate back to the elastic wave speed. Applying a modified analytical equation for dislocation evolutionmore » we successfully predict a dislocation density of 1.5 x 10(12) cm(-2) within the shocked volume, in agreement with the present simulations and realistic in regards to prior and on-going recovery experiments in silicon.« less

The equivalence between dislocation pile-ups and cracks

NASA Technical Reports Server (NTRS)

Liu, H. W.; Gao, Q.

1990-01-01

Cracks and dislocation pile-ups are equivalent to each other. In this paper, the physical equivalence between cracks and pile-ups is delineated, and the relationshps between crack-extension force, force on the leading dislocation, stress-intensity factor, and dislocation density are reviewed and summarized. These relations make it possible to extend quantitatively the recent advances in the concepts and practices of fracture mechanics to the studies of microfractures and microplastic deformations.

Band gap renormalization and Burstein-Moss effect in silicon- and germanium-doped wurtzite GaN up to 1020 cm-3

NASA Astrophysics Data System (ADS)

Feneberg, Martin; Osterburg, Sarah; Lange, Karsten; Lidig, Christian; Garke, Bernd; Goldhahn, Rüdiger; Richter, Eberhard; Netzel, Carsten; Neumann, Maciej D.; Esser, Norbert; Fritze, Stephanie; Witte, Hartmut; Bläsing, Jürgen; Dadgar, Armin; Krost, Alois

2014-08-01

The interplay between band gap renormalization and band filling (Burstein-Moss effect) in n-type wurtzite GaN is investigated. For a wide range of electron concentrations up to 1.6×1020cm-3 spectroscopic ellipsometry and photoluminescence were used to determine the dependence of the band gap energy and the Fermi edge on electron density. The band gap renormalization is the dominating effect up to an electron density of about 9×1018cm-3; at higher values the Burstein-Moss effect is stronger. Exciton screening, the Mott transition, and formation of Mahan excitons are discussed. A quantitative understanding of the near gap transition energies on electron density is obtained. Higher energy features in the dielectric functions up to 10eV are not influenced by band gap renormalization.

High-density convergent plasma sputtering device for a liquid metal target using an unheated glass plate

NASA Astrophysics Data System (ADS)

Motomura, T.; Tabaru, T.

2018-06-01

A high-density convergent plasma sputtering device has been developed for a liquid metal target, using an unheated glass plate. The convergent magnetic field lines, which are produced by an external solenoid coil and a permanent magnet positioned behind the liquid metal target, effectively transport high-density plasmas near the target. In this study, a liquid gallium target was sputtered with nitrogen plasmas, without additive gas required for depositing gallium nitride films on the unheated substrates. The deposition rate of the GaN film was estimated at ˜13 nm/min at a gas pressure of 0.2 Pa. A strong diffraction peak was observed along the GaN (002) axis, with the use of an unheated glass plate and a target-substrate distance of ˜45 mm.

Ab initio calculations on the initial stages of GaN and ZnO growth on lattice-matched ScAlMgO4 (0001) substrates

NASA Astrophysics Data System (ADS)

Guo, Yao; Wang, Yanfei; Li, Chengbo; Li, Xianchang; Niu, Yongsheng; Hou, Shaogang

2016-12-01

The initial stages of GaN and ZnO epitaxial growth on lattice-matched ScAlMgO4 substrates have been investigated by ab initio calculation. The geometrical parameters and electronic structure of ScAlMgO4 bulk and (0001) surface have been investigated by density-functional first-principles study. The effects of different surface terminations have been examined through surface energy and relaxation calculations. The O-Mg-O termination is more favorable than other terminations by comparing the calculated surface energies. It should be accepted as the appropriate surface structure in subsequent calculation. The initial stages of GaN and ZnO epitaxial growths are discussed based on the adsorption and diffusion of the adatoms on reconstructed ScAlMgO4 (0001) surface. According to theoretical characterizations, N adatom on the surface is more stable than Ga. O adatom is more favorable than Zn. These observations lead to the formation of GaN and ZnO epilayer and explain experimentally-confirmed in-plane alignment mechanisms of GaN and ZnO on ScAlMgO4 substrates. Furthermore, the polarity of GaN and ZnO surfaces on ScAlMgO4 (0001) at the initial growth stage have been explored by ab initio calculation. Theoretical studies indicate that the predominant growths of Ga-polar GaN and Zn-polar ZnO are determined by the initial growth stage.

AlGaN materials for semiconductor sensors and emitters in 200- to 365-nm range

NASA Astrophysics Data System (ADS)

Usikov, Alexander S.; Shapvalova, Elizaveta V.; Melnik, Yuri V.; Ivantsov, Vladimir A.; Dmitriev, Vladimir A.; Collins, Charles J.; Sampath, Anand V.; Garrett, Gregory A.; Shen, Paul H.; Wraback, Michael

2004-12-01

In this paper we report on the fabrication and characterization of GaN, AlGaN, and AlN layers grown by hydride vapor phase epitaxy (HVPE). The layers were grown on 2-inch and 4-inch sapphire and 2-inch silicon carbide substrates. Thickness of the GaN layers was varied from 2 to 80 microns. Surface roughness, Rms, for the smoothest GaN layers was less than 0.5 nm, as measured by AFM using 10 μm x 10 μm scans. Background Nd-Na concentration for undoped GaN layers was less than 1x1016 cm-3. For n-type GaN layers doped with Si, concentration Nd-Na was controlled from 1016 to 1019 cm-3. P-type GaN layers were fabricated using Mg doping with concentration Na-Nd ranging from 4x1016 to 3x1018 cm-3, for various samples. Zn doping also resulted in p-type GaN formation with concnetration ND-NA in the 1017 cm-3 range. UV transmission, photoluminescence, and crystal structure of AlGaN layers with AlN concentration up to 85 mole.% were studied. Dependence of optical band gap on AlGaN alloy composition was measured for the whole composition range. Thick (up to 75 microns) crack-free AlN layers were grown on SiC substrates. Etch pit density for such thick AlN layers was in the 107 cm-2 range.

Growth kinetics for temperature-controlled atomic layer deposition of GaN using trimethylgallium and remote-plasma-excited NH3

NASA Astrophysics Data System (ADS)

Pansila, P.; Kanomata, K.; Miura, M.; Ahmmad, B.; Kubota, S.; Hirose, F.

2015-12-01

Fundamental surface reactions in the atomic layer deposition of GaN with trimethylgallium (TMG) and plasma-excited NH3 are investigated by multiple-internal-reflection infrared absorption spectroscopy (MIR-IRAS) at surface temperatures varying from room temperature (RT) to 400 °C. It is found that TMG is saturated at RT on GaN surfaces when the TMG exposure exceeds 8 × 104 Langmuir (L), where 1 L corresponds to 1.33 × 10-4 Pa s (or 1.0 × 10-6 Torr s), and its saturation density reaches the maximum value at RT. Nitridation with the plasma-excited NH3 on the TMG-saturated GaN surface is investigated by X-ray photoelectron spectroscopy (XPS). The nitridation becomes effective at surface temperatures in excess of 100 °C. The reaction models of TMG adsorption and nitridation on the GaN surface are proposed in this paper. Based on the surface analysis, a temperature-controlled ALD process consisting of RT-TMG adsorption and nitridation at 115 °C is examined, where the growth per cycle of 0.045 nm/cycle is confirmed. XPS analysis indicates that all N atoms are bonded as GaN. Atomic force microscopy indicates an average roughness of 0.23 nm. We discuss the reaction mechanism of GaN ALD in the low-temperature region at around 115 °C with TMG and plasma-excited NH3.

Single-crystal sapphire microstructure for high-resolution synchrotron X-ray monochromators

DOE PAGES

Asadchikov, Victor E.; Butashin, Andrey V.; Buzmakov, Alexey V.; ...

2016-03-22

We report on the growth and characterization of several sapphire single crystals for the purpose of x-ray optics applications. Structural defects were studied by means of laboratory double-crystal X-ray diffractometry and white beam synchrotron-radiation topography. The investigations confirmed that the main defect types are dislocations. The best quality crystal was grown using the Kyropoulos technique with a dislocation density of 10 2-10 3 cm -2 and a small area with approximately 2*2 mm 2 did not show dislocation contrast in many reflections and has suitable quality for application as a backscattering monochromator. As a result, a clear correlation between growthmore » rate and dislocation density is observed, though growth rate is not the only parameter impacting the quality.« less

Gradient Plasticity Model and its Implementation into MARMOT

DOE Office of Scientific and Technical Information (OSTI.GOV)

Barker, Erin I.; Li, Dongsheng; Zbib, Hussein M.

2013-08-01

The influence of strain gradient on deformation behavior of nuclear structural materials, such as boby centered cubic (bcc) iron alloys has been investigated. We have developed and implemented a dislocation based strain gradient crystal plasticity material model. A mesoscale crystal plasticity model for inelastic deformation of metallic material, bcc steel, has been developed and implemented numerically. Continuum Dislocation Dynamics (CDD) with a novel constitutive law based on dislocation density evolution mechanisms was developed to investigate the deformation behaviors of single crystals, as well as polycrystalline materials by coupling CDD and crystal plasticity (CP). The dislocation density evolution law in thismore » model is mechanism-based, with parameters measured from experiments or simulated with lower-length scale models, not an empirical law with parameters back-fitted from the flow curves.« less

Enhancing elastic stress relaxation in SiGe/Si heterostructures by Si pillar necking

NASA Astrophysics Data System (ADS)

Isa, F.; Salvalaglio, M.; Arroyo Rojas Dasilva, Y.; Jung, A.; Isella, G.; Erni, R.; Timotijevic, B.; Niedermann, P.; Gröning, P.; Montalenti, F.; von Känel, H.

2016-10-01

We demonstrate that the elastic stress relaxation mechanism in micrometre-sized, highly mismatched heterostructures may be enhanced by employing patterned substrates in the form of necked pillars, resulting in a significant reduction of the dislocation density. Compositionally graded Si1-xGex crystals were grown by low energy plasma enhanced chemical vapour deposition, resulting in tens of micrometres tall, three-dimensional heterostructures. The patterned Si(001) substrates consist of micrometre-sized Si pillars either with the vertical {110} or isotropically under-etched sidewalls resulting in narrow necks. The structural properties of these heterostructures were investigated by defect etching and transmission electron microscopy. We show that the dislocation density, and hence the competition between elastic and plastic stress relaxation, is highly influenced by the shape of the substrate necks and their proximity to the mismatched epitaxial material. The SiGe dislocation density increases monotonically with the crystal width but is significantly reduced by the substrate under-etching. The drop in dislocation density is interpreted as a direct effect of the enhanced compliance of the under-etched Si pillars, as confirmed by the three-dimensional finite element method simulations of the elastic energy distribution.

Metal viscoplasticity with two-temperature thermodynamics and two dislocation densities

NASA Astrophysics Data System (ADS)

Roy Chowdhury, Shubhankar; Kar, Gurudas; Roy, Debasish; Reddy, J. N.

2018-03-01

Posed within the two-temperature theory of non-equilibrium thermodynamics, we propose a model for thermoviscoplastic deformation in metals. We incorporate the dynamics of dislocation densities-mobile and forest—that play the role of internal state variables in the formulation. The description based on two temperatures appears naturally when one recognizes that the thermodynamic system undergoing viscoplastic deformation is composed of two weakly interacting subsystems, viz. a kinetic-vibrational subsystem of the vibrating atomic lattices and a configurational subsystem of the slower degrees of freedom relating to defect motion, each with its own temperature. Starting with a basic model that involves only homogeneous deformation, a three-dimensional model for inhomogeneous viscoplasticity applicable to finite deformation is charted out in an overstress driven viscoplastic deformation framework. The model shows how the coupled evolutions of mobile and forest dislocation densities, which are critically influenced by the dynamics of configurational temperature, govern the strength and ductility of the metal. Unlike most contemporary models, the current proposal also affords a prediction of certain finer details as observed in the experimental data on stress-strain behaviour of metals and this in turn enhances the understanding of the evolving and interacting dislocation densities.

Demonstration of a High Open-Circuit Voltage GaN Betavoltaic Microbattery

NASA Astrophysics Data System (ADS)

Cheng, Zai-Jun; San, Hai-Sheng; Chen, Xu-Yuan; Liu, Bo; Feng, Zhi-Hong

2011-07-01

A high open-circuit voltage betavoltaic microbattery based on a GaN p-i-n diode is demonstrated. Under the irradiation of a 4×4 mm2 planar solid 63Ni source with an activity of 2 mCi, the open-circuit voltage Voc of the fabricated single 2×2mm2 cell reaches as high as 1.62 V, the short-circuit current density Jsc is measured to be 16nA/cm2. The microbattery has a fill factor of 55%, and the energy conversion efficiency of beta radiation into electricity reaches to 1.13%. The results suggest that GaN is a highly promising potential candidate for long-life betavoltaic microbatteries used as power supplies for microelectromechanical system devices.

Backward diodes using heavily Mg-doped GaN growth by ammonia molecular-beam epitaxy

NASA Astrophysics Data System (ADS)

Okumura, Hironori; Martin, Denis; Malinverni, Marco; Grandjean, Nicolas

2016-02-01

We grew heavily Mg-doped GaN using ammonia molecular-beam epitaxy. The use of low growth temperature (740 °C) allows decreasing the incorporation of donor-like defects (<3 × 1017 cm-3) responsible for p-type doping compensation. As a result, a net acceptor concentration of 7 × 1019 cm-3 was achieved, and the hole concentration measured by Hall effect was as high as 2 × 1019 cm-3 at room temperature. Using such a high Mg doping level, we fabricated GaN backward diodes without polarization-assisted tunneling. The backward diodes exhibited a tunneling-current density of 225 A/cm2 at a reverse bias of -1 V at room temperature.

Effect of hexagonal hillock on luminescence characteristic of multiple quantum wells structure

NASA Astrophysics Data System (ADS)

Du, Jinjuan; Xu, Shengrui; Li, Peixian; Zhang, Jincheng; Zhao, Ying; Peng, Ruoshi; Fan, Xiaomeng; Hao, Yue

2018-04-01

GaN based ultraviolet multiple quantum well structures grown on a c-plane sapphire substrate by metal organic chemical deposition showed a microstructure with a large amount of huge hexagonal hillocks. The polarity of the sample is confirmed by etching with sodium hydroxide solution. The luminous intensity distribution of a typical hexagonal hillock was investigated by the phototluminescent mapping and the luminous intensity at hillock top regions was found to be 15 times higher than that of the regions around hillocks. The reduction of dislocations, the decreasing of the quantum confirmed stack effect caused by semipolar plane and the inclination of the sidewalls of the hexagonal hillock were responsible for the enhancement of luminous intensity.

Semiconductor Film Grown on a Circular Substrate: Predictive Modeling of Lattice-Misfit Stresses

NASA Astrophysics Data System (ADS)

Suhir, E.; Nicolics, J.; Khatibi, G.; Lederer, M.

2016-03-01

An effective and physically meaningful analytical predictive model is developed for the evaluation the lattice-misfit stresses (LMS) in a semiconductor film grown on a circular substrate (wafer). The two-dimensional (plane-stress) theory-of-elasticity approximation (TEA) is employed in the analysis. The addressed stresses include the interfacial shearing stress, responsible for the occurrence and growth of dislocations, as well as for possible delaminations and the cohesive strength of a buffering material, if any. Normal radial and circumferential (tangential) stresses acting in the film cross-sections and responsible for its short- and long-term strength (fracture toughness) are also addressed. The analysis is geared to the GaN technology.

Optimization design on breakdown voltage of AlGaN/GaN high-electron mobility transistor

NASA Astrophysics Data System (ADS)

Yang, Liu; Changchun, Chai; Chunlei, Shi; Qingyang, Fan; Yuqian, Liu

2016-12-01

Simulations are carried out to explore the possibility of achieving high breakdown voltage of GaN HEMT (high-electron mobility transistor). GaN cap layers with gradual increase in the doping concentration from 2 × 1016 to 5 × 1019 cm-3 of N-type and P-type cap are investigated, respectively. Simulation results show that HEMT with P-doped GaN cap layer shows more potential to achieve higher breakdown voltage than N-doped GaN cap layer under the same doping concentration. This is because the ionized net negative space charges in P-GaN cap layer could modulate the surface electric field which makes more contribution to RESURF effect. Furthermore, a novel GaN/AlGaN/GaN HEMT with P-doped GaN buried layer in GaN buffer between gate and drain electrode is proposed. It shows enhanced performance. The breakdown voltage of the proposed structure is 640 V which is increased by 12% in comparison to UID (un-intentionally doped) GaN/AlGaN/GaN HEMT. We calculated and analyzed the distribution of electrons' density. It is found that the depleted region is wider and electric field maximum value is induced at the left edge of buried layer. So the novel structure with P-doped GaN buried layer embedded in GaN buffer has the better improving characteristics of the power devices. Project supported by the National Basic Research Program of China (No. 2014CB339900) and the Open Fund of Key Laboratory of Complex Electromagnetic Environment Science and Technology, China Academy of Engineering Physics (No. 2015-0214.XY.K).

Identification of the spatial location of deep trap states in AlGaN/GaN heterostructures by surface photovoltage spectroscopy

NASA Astrophysics Data System (ADS)

Jana, Dipankar; Porwal, S.; Sharma, T. K.

2017-12-01

Spatial and spectral origin of deep level defects in molecular beam epitaxy grown AlGaN/GaN heterostructures are investigated by using surface photovoltage spectroscopy (SPS) and pump-probe SPS techniques. A deep trap center ∼1 eV above the valence band is observed in SPS measurements which is correlated with the yellow luminescence feature in GaN. Capture of electrons and holes is resolved by performing temperature dependent SPS and pump-probe SPS measurements. It is found that the deep trap states are distributed throughout the sample while their dominance in SPS spectra depends on the density, occupation probability of deep trap states and the background electron density of GaN channel layer. Dynamics of deep trap states associated with GaN channel layer is investigated by performing frequency dependent photoluminescence (PL) and SPS measurements. A time constant of few millisecond is estimated for the deep defects which might limit the dynamic performance of AlGaN/GaN based devices.

A numerical spectral approach to solve the dislocation density transport equation

NASA Astrophysics Data System (ADS)

Djaka, K. S.; Taupin, V.; Berbenni, S.; Fressengeas, C.

2015-09-01

A numerical spectral approach is developed to solve in a fast, stable and accurate fashion, the quasi-linear hyperbolic transport equation governing the spatio-temporal evolution of the dislocation density tensor in the mechanics of dislocation fields. The approach relies on using the Fast Fourier Transform algorithm. Low-pass spectral filters are employed to control both the high frequency Gibbs oscillations inherent to the Fourier method and the fast-growing numerical instabilities resulting from the hyperbolic nature of the transport equation. The numerical scheme is validated by comparison with an exact solution in the 1D case corresponding to dislocation dipole annihilation. The expansion and annihilation of dislocation loops in 2D and 3D settings are also produced and compared with finite element approximations. The spectral solutions are shown to be stable, more accurate for low Courant numbers and much less computation time-consuming than the finite element technique based on an explicit Galerkin-least squares scheme.

Electrical Current Leakage and Open-Core Threading Dislocations in AlGaN-Based Deep Ultraviolet Light-Emitting Diodes.

DOE PAGES

Moseley, Michael William; Allerman, Andrew A.; Crawford, Mary H.; ...

2014-08-04

Electrical current transport through leakage paths in AlGaN-based deep ultraviolet (DUV) lightemitting diodes (LEDs) and their effect on LED performance are investigated. Open-core threading dislocations, or nanopipes, are found to conduct current through nominally insulating Al0.7Ga0.3N layers and limit the performance of DUV-LEDs. A defect-sensitive phosphoric acid etch reveals these opencore threading dislocations in the form of large, micron-scale hexagonal etch pits visible with optical microscopy, while closed-core screw-, edge-, and mixed-type threading dislocations are represented by smaller and more numerous nanometer-scale pits visible by atomic-force microscopy. The electrical and optical performances of DUV-LEDs fabricated on similar Si-doped Al0.7Ga0.3N templatesmore » are found to have a strong correlation to the density of these nanopipes, despite their small fraction (<0.1% in this study) of the total density of threading dislocations.« less

BInGaN alloys nearly lattice-matched to GaN for high-power high-efficiency visible LEDs

NASA Astrophysics Data System (ADS)

Williams, Logan; Kioupakis, Emmanouil

2017-11-01

InGaN-based visible light-emitting diodes (LEDs) find commercial applications for solid-state lighting and displays, but lattice mismatch limits the thickness of InGaN quantum wells that can be grown on GaN with high crystalline quality. Since narrower wells operate at a higher carrier density for a given current density, they increase the fraction of carriers lost to Auger recombination and lower the efficiency. The incorporation of boron, a smaller group-III element, into InGaN alloys is a promising method to eliminate the lattice mismatch and realize high-power, high-efficiency visible LEDs with thick active regions. In this work, we apply predictive calculations based on hybrid density functional theory to investigate the thermodynamic, structural, and electronic properties of BInGaN alloys. Our results show that BInGaN alloys with a B:In ratio of 2:3 are better lattice matched to GaN compared to InGaN and, for indium fractions less than 0.2, nearly lattice matched. Deviations from Vegard's law appear as bowing of the in-plane lattice constant with respect to composition. Our thermodynamics calculations demonstrate that the solubility of boron is higher in InGaN than in pure GaN. Varying the Ga mole fraction while keeping the B:In ratio constant enables the adjustment of the (direct) gap in the 1.75-3.39 eV range, which covers the entire visible spectrum. Holes are strongly localized in non-bonded N 2p states caused by local bond planarization near boron atoms. Our results indicate that BInGaN alloys are promising for fabricating nitride heterostructures with thick active regions for high-power, high-efficiency LEDs.

In-situ NC-AFM measurements of high quality AlN(0001) layers grown at low growth rate on 4H-SiC(0001) and Si(111) substrates using ammonia molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chaumeton, Florian, E-mail: florian.chaumeton@cemes.fr; Gauthier, Sébastien, E-mail: gauthier@cemes.fr; Martrou, David, E-mail: david.martrou@cemes.fr

Nitride wide-band-gap semiconductors are used to make high power electronic devices or efficient light sources. The performance of GaN-based devices is directly linked to the initial AlN buffer layer. During the last twenty years of research on nitride growth, only few information on the AlN surface quality have been obtained, mainly by ex-situ characterization techniques. Thanks to a Non Contact Atomic Force Microscope (NC-AFM) connected under ultra high vacuum (UHV) to a dedicated molecular beam epitaxy (MBE) chamber, the surface of AlN(0001) thin films grown on Si(111) and 4H-SiC(0001) substrates has been characterized. These experiments give access to a quantitativemore » determination of the density of screw and edge dislocations at the surface. The layers were also characterized by ex-situ SEM to observe the largest defects such as relaxation dislocations and hillocks. The influence of the growth parameters (substrate temperature, growth speed, III/V ratio) and of the initial substrate preparation on the dislocation density was also investigated. On Si(111), the large in-plane lattice mismatch with AlN(0001) (19%) induces a high dislocation density ranging from 6 to 12×10{sup 10}/cm{sup 2} depending on the growth conditions. On 4H-SiC(0001) (1% mismatch with AlN(0001)), the dislocation density decreases to less than 10{sup 10}/cm{sup 2}, but hillocks appear, depending on the initial SiC(0001) reconstruction. The use of a very low growth rate of 10 nm/h at the beginning of the growth process allows to decrease the dislocation density below 2 × 10{sup 9}/cm{sup 2}.« less

Heterogeneous dislocation loop formation near grain boundaries in a neutron-irradiated commercial FeCrAl alloy

DOE PAGES

Field, Kevin G.; Briggs, Samuel A.; Hu, Xunxiang; ...

2016-11-01

FeCrAl alloys are an attractive materials class for nuclear power applications due to their increased environmental compatibility over more traditional nuclear materials. Preliminary studies into the radiation tolerance of FeCrAl alloys under accelerated neutron testing between 300-400 °C have shown post-irradiation microstructures containing dislocation loops and Cr-rich ' phase. Although these initial works established the post-irradiation microstructures, little to no focus was applied towards the influence of pre-irradiation microstructures on this response. Here, a well annealed commercial FeCrAl alloy, Alkrothal 720, was neutron irradiated to 1.8 dpa at 382 °C and then the role of random high angle grain boundariesmore » on the spatial distribution and size of dislocation loops, dislocation loops, and black dot damage was analyzed using on-zone scanning transmission electron microscopy. Results showed a clear heterogeneous dislocation loop formation with dislocation loops showing an increased number density and size, black dot damage showing a significant number density decrease, and an increased size of dislocation loops in the vicinity directly adjacent to the grain boundary. Lastly, these results suggest the importance of the pre-irradiation microstructure on the radiation tolerance of FeCrAl alloys.« less

FOREWORD: The physics of III-V nitrides The physics of III-V nitrides

NASA Astrophysics Data System (ADS)

Ridley, B. K.

2009-04-01

The evolution of semiconductor physics is driven by the increasing sophistication of the art of crystal growing and fabrication techniques. From Ge at the birth of the transistor, possibly the purest material ever grown, through Si, the work-horse of the crystal revolution, to the III-Vs, whose optical properties opened up a second front, namely, optoelectronics. Crystal growth with monolayer control gave us quantum wells, superlattices, quantum wires and quantum dots, along with the quantum Hall effect and quantized resistance. The potential for high-power devices triggered interest in the III-V nitrides with their large bandgaps. The nitrides mostly crystallize in the hexagonal form, and this has introduced the phenomenon of spontaneous polarization into mainstream semiconductor physics. Its effect manifests itself in huge electric fields in heterostructures like AlGaN/GaN which, in turn, causes the induction of substantial electron populations in the channel of a HFET without the need for doping. High-power microwave transistors have been successfully fabricated, even though there are features associated with spontaneous polarization that still needs clarifying. Another strange effect is the large electron population on the surface of InN. The lack of a suitable substrate for growing GaN has meant that the dislocation density is higher than we would wish, but that situation is expected to steadily improve. Given the current interest in the physics of nitrides, it is natural to come across a special issue devoted to this topic. The difficulty presented by the surface layer in InN in the attempt to measure transport properties is discussed in the paper by King et al. A property that can affect transport is the lifetime of optical phonons and its dependence on electron density. Measurements of phonon lifetime in InN are reported by Tsen and Ferry, and in GaN channels, via the measure of hot-electron fluctuations, by Matulionis. The dependence on electron density is thought to be associated with the coupling of plasma and phonon modes, and this is discussed by Dyson. The intrinsic cause of phonon decay is the anharmonic interaction involving three phonons, and this process is described for zinc blende BN and hexagonal AlN, GaN and InN by Srivastava. The principal electron scattering mechanism at room temperature is associated with the interaction with polar optical modes. At high fields, transfer to the upper conduction-band valleys can take place and this involves the deformation-potential interaction. Deformation potentials have been derived by Yamakawa et al for GaN, and they have been incorporated into a cellular Monte Carlo simulation to describe high-field transport. In high-power devices, thermal as well as electronic transport is important. The thermal conductivity of the substrate of devices is a vital factor, and the possible use of AlN ceramics is discussed by AlShaikhi and Srivastarva. A striking device based on a zinc blende superlattice is the quantum cascade laser. Exploiting intersubband transitions in the AlN/GaN superlattice for the high-speed detection of infrared light is described by Hofstetter et al, clearly a first step towards a nitride based quantum cascade laser. In bulk material the displacement of As by N that transforms GaAs to GaN produces a huge change in properties. Adding a small amount of N to GaAs might be expected to produce a gradual more-or-less linear shift towards the properties of GaN, but this turns out to be far from the case. The strange properties of dilute nitrides have intrigued many workers in recent years. Its curious bandstructure suggested that hot-electron transport could exhibit a negative differential resistance, and a report on this topic can be found in the article by Patane et al A comprehensive study of transport of electrons and holes in dilute nitride/GaAs quantum wells is reported by Sun et al. An unusual new device—a spin filter—is presaged by the work of Zhao et al on spin-dependent recombination, controllable by adjusting the N content. Answers to a number of problems presented by the physics of III-V nitrides are to be found in the articles composing this edition, but there are still many that need clarifying. That clarification will have to await the future work that will form the contents of a future special edition.

Surface sensitization mechanism on negative electron affinity p-GaN nanowires

NASA Astrophysics Data System (ADS)

Diao, Yu; Liu, Lei; Xia, Sihao; Feng, Shu; Lu, Feifei

2018-03-01

The surface sensitization is the key to prepare negative electron affinity photocathode. The thesis emphasizes on the study of surface sensitization mechanism of p-type doping GaN nanowires utilizing first principles based on density function theory. The adsorption energy, work function, dipole moment, geometry structure, electronic structure and optical properties of Mg-doped GaN nanowires surfaces with various coverages of Cs atoms are investigated. The GaN nanowire with Mg doped in core position is taken as the sensitization base. At the initial stage of sensitization, the best adsorption site for Cs atom on GaN nanowire surface is BN, the bridge site of two adjacent N atoms. Surface sensitization generates a p-type internal surface with an n-type surface state, introducing a band bending region which can help reduce surface barrier and work function. With increasing Cs coverage, work functions decrease monotonously and the "Cs-kill" phenomenon disappears. For Cs coverage of 0.75 ML and 1 ML, the corresponding sensitization systems reach negative electron affinity state. Through surface sensitization, the absorption curves are red shifted and the absorption coefficient is cut down. All theoretical calculations can guide the design of negative electron affinity Mg doped GaN nanowires photocathode.

Hybrid GaN LED with capillary-bonded II-VI MQW color-converting membrane for visible light communications

NASA Astrophysics Data System (ADS)

Santos, Joao M. M.; Jones, Brynmor E.; Schlosser, Peter J.; Watson, Scott; Herrnsdorf, Johannes; Guilhabert, Benoit; McKendry, Jonathan J. D.; De Jesus, Joel; Garcia, Thor A.; Tamargo, Maria C.; Kelly, Anthony E.; Hastie, Jennifer E.; Laurand, Nicolas; Dawson, Martin D.

2015-03-01

The rapid emergence of gallium-nitride (GaN) light-emitting diodes (LEDs) for solid-state lighting has created a timely opportunity for optical communications using visible light. One important challenge to address this opportunity is to extend the wavelength coverage of GaN LEDs without compromising their modulation properties. Here, a hybrid source for emission at 540 nm consisting of a 450 nm GaN micro-sized LED (micro-LED) with a micron-thick ZnCdSe/ZnCdMgSe multi-quantum-well color-converting membrane is reported. The membrane is liquid-capillary-bonded directly onto the sapphire window of the micro-LED for full hybridization. At an injection current of 100 mA, the color-converted power was found to be 37 μW. At this same current, the -3 dB optical modulation bandwidth of the bare GaN and hybrid micro-LEDs were 79 and 51 MHz, respectively. The intrinsic bandwidth of the color-converting membrane was found to be power-density independent over the range of the micro-LED operation at 145 MHz, which corresponds to a mean carrier lifetime of 1.9 ns.

III-nitrate ultraviolet photonic materials: epitaxial growth, optical and electrical properties, and applications

NASA Astrophysics Data System (ADS)

Lin, Jingyu; Jiang, Hongxing

2003-07-01

This paper summarizes some of the recent advances made by our group on the growth, characterization and applications of AlGaN alloys with high Al contents. Recently, our group has achieved highly conductive n-type AlxGa1-xN for x as high as 0.7 (a resistivity value as low as 0.15 ohm-cm has been achieved). Prior to this, only insulating AlxGa1-xN (x > 0.5) can be obtained. Our success is largely attributed to our unique capability for monitoring the optical qualities of these layers -- the development of the world's first (and presently only) deep UV picosecond time-resolved optical spectroscopy system for probing the optical properties of III-nitrides [photoluminescence (PL), electro-luminescence (EL), etc.] with a time-resolution of a few ps and wavelength down to deep UV (down to 195 nm). Our time- resolved PL results have shown that we must fill in the localization states (caused by alloy fluctuation) by doping before conduction could occur. The density of states of localization states is about 1018/cm3 in this system. It was also shown that AlxGa1-xN alloys could be made n-type for x up to 1 (pure AlN). Time-resolved photoluminescence (PL) studies carried out on these materials have revealed that Si-doping reduces the effect of carrier localization in AlxGa1-xN alloys and a sharp drop in carrier localization energy as well as a sharp increase in conductivity occurs when the Si doping concentration increases to above 1 x 1018 cm-3. For the Mg-doped AlxGa1-xN alloys, p-type conduction was achieved for x up to 0.27. The Mg acceptor activation energy as a function of Al content has been deduced. Mg-δ-doping in GaN and AlGaN epilayers has been investigated. We have demonstrated that δ-doping significantly suppresses the dislocation density, enhances the p-type conduction, and reduces the non-radiative recombination centers in GaN and AlGaN. AlN epilayers with high optical qualities have also been grown on sapphire substrates. Very efficient band-edge PL emission lines have been observed for the first time with above bandgap deep UV laser excitation. We have shown that the thermal quenching of the PL emission intensity is much less severe in AlN than in GaN and the optical quality of AlN can be as good as GaN. From the low temperature (10 K) emission spectra, as well as the temperature dependence of the recombination lifetime and the PL emission intensity, the binding energies of the bound excitons and free excitons in AlN were deduced to be around 16 meV and 80 meV, respectively. From this, the energy bandgap of AlN epilayers grown on sapphire was found to be around 6.11 eV at 10 K. The observed large free exciton binding energy implies that excitons in AlN are extremely robust entities. This together with other well-known physical properties of AlN may considerably expand future prospects for the application of III-nitride materials.

Special quasiordered structures: Role of short-range order in the semiconductor alloy (GaN) 1 -x(ZnO) x

NASA Astrophysics Data System (ADS)

Liu, Jian; Fernández-Serra, Maria V.; Allen, Philip B.

2016-02-01

This paper studies short-range order (SRO) in the semiconductor alloy (GaN) 1 -x(ZnO) x. Monte Carlo simulations performed on a density functional theory (DFT)-based cluster expansion model show that the heterovalent alloys exhibit strong SRO because of the energetic preference for the valence-matched nearest-neighbor Ga-N and Zn-O pairs. To represent the SRO-related structural correlations, we introduce the concept of special quasiordered structure (SQoS). Subsequent DFT calculations reveal the dramatic influence of SRO on the atomic, electronic, and vibrational properties of the (GaN) 1 -x(ZnO) x alloy. Due to the enhanced statistical presence of the energetically unfavored Zn-N bonds with the strong Zn 3 d -N 2 p repulsion, the disordered alloys exhibit much larger lattice bowing and band-gap reduction than those of the short-range ordered alloys. Lattice vibrational entropy tilts the alloy toward less SRO.

Microplastic flow in SIC/AL composites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shi, N.; Arsenault, R.J.

Experimentally it has been determined that if a composite containing a reinforcement which has a different (in general lower) thermal coefficient of expansion as compared to the matrix, then upon cooling from the processing or annealing temperature, plastic relaxation of the misfit strain will occur. Also, experimentally it has been shown that as the size of the reinforcement is increased, i.e., from small spheres to large spheres, there is a decrease in the summation of the effective plastic strain in the matrix. In other words there is a decrease in the average dislocation density in the matrix. However, if themore » shape of the reinforcement is changed from spherical to short fiber to continuous filament, then the dislocation density increases. This experimental data is obtained at a constant volume fraction. A very simple model of plastic relaxation based on prismatic punching of dislocations from the interface can account for the decrease in the dislocation density with an increase reinforcement size, and the increase in dislocation density when changing the shape from a sphere to a continuous filament. A FEM analysis of the shape factor is also capable of predicting the correct trend. However, at present the continuum mechanics methods that have been investigated can not predict the size dependence. A simple model to explain the size effect in Al{sub 2}O{sub 3}/NiAl composites based on the deformation characteristics of NiAl will be discussed.« less

Geometric contribution leading to anomalous estimation of two-dimensional electron gas density in GaN based heterostructures

NASA Astrophysics Data System (ADS)

Upadhyay, Bhanu B.; Jha, Jaya; Takhar, Kuldeep; Ganguly, Swaroop; Saha, Dipankar

2018-05-01

We have observed that the estimation of two-dimensional electron gas density is dependent on the device geometry. The geometric contribution leads to the anomalous estimation of the GaN based heterostructure properties. The observed discrepancy is found to originate from the anomalous area dependent capacitance of GaN based Schottky diodes, which is an integral part of the high electron mobility transistors. The areal capacitance density is found to increase for smaller radii Schottky diodes, contrary to a constant as expected intuitively. The capacitance is found to follow a second order polynomial on the radius of all the bias voltages and frequencies considered here. In addition to the quadratic dependency corresponding to the areal component, the linear dependency indicates a peripheral component. It is further observed that the peripheral to areal contribution is inversely proportional to the radius confirming the periphery as the location of the additional capacitance. The peripheral component is found to be frequency dependent and tends to saturate to a lower value for measurements at a high frequency. In addition, the peripheral component is found to vanish when the surface is passivated by a combination of N2 and O2 plasma treatments. The cumulative surface state density per unit length of the perimeter of the Schottky diodes as obtained by the integrated response over the distance between the ohmic and Schottky contacts is found to be 2.75 × 1010 cm-1.

Dislocation mechanisms in stressed crystals with surface effects

NASA Astrophysics Data System (ADS)

Wu, Chi-Chin; Crone, Joshua; Munday, Lynn; Discrete Dislocation Dynamics Team

2014-03-01

Understanding dislocation properties in stressed crystals is the key for important processes in materials science, including the strengthening of metals and the stress relaxation during the growth of hetero-epitaxial structures. Despite existing experimental approaches and theories, many dislocation mechanisms with surface effects still remain elusive in experiments. Even though discrete dislocation dynamics (DDD) simulations are commonly employed to study dislocations, few demonstrate sufficient computational capabilities for massive dislocations with the combined effects of surfaces and stresses. Utilizing the Army's newly developed FED3 code, a DDD computation code coupled with finite elements, this work presents several dislocation mechanisms near different types of surfaces in finite domains. Our simulation models include dislocations in a bended metallic cantilever beam, near voids in stressed metals, as well as threading and misfit dislocations in as-grown semiconductor epitaxial layers and their quantitative inter-correlations to stress relaxation and surface instability. Our studies provide not only detailed physics of individual dislocation mechanisms, but also important collective dislocation properties such as dislocation densities and strain-stress profiles and their interactions with surfaces.

Cyclic deformation leads to defect healing and strengthening of small-volume metal crystals

DOE PAGES

Wang, Zhang-Jie; Li, Qing-Jie; Cui, Yi-Nan; ...

2015-10-19

When microscopic and macroscopic specimens of metals are subjected to cyclic loading, the creation, interaction, and accumulation of defects lead to damage, cracking, and failure. We demonstrate that when aluminum single crystals of submicrometer dimensions are subjected to low-amplitude cyclic deformation at room temperature, the density of preexisting dislocation lines and loops can be dramatically reduced with virtually no change of the overall sample geometry and essentially no permanent plastic strain. Furthermore, this “cyclic healing” of the metal crystal leads to significant strengthening through dramatic reductions in dislocation density, in distinct contrast to conventional cyclic strain hardening mechanisms arising frommore » increases in dislocation density and interactions among defects in microcrystalline and macrocrystalline metals and alloys. Our real-time, in situ transmission electron microscopy observations of tensile tests reveal that pinned dislocation lines undergo shakedown during cyclic straining, with the extent of dislocation unpinning dependent on the amplitude, sequence, and number of strain cycles. Those unpinned mobile dislocations moving close enough to the free surface of the thin specimens as a result of such repeated straining are then further attracted to the surface by image forces that facilitate their egress from the crystal. Our results point to a versatile pathway for controlled mechanical annealing and defect engineering in submicrometer-sized metal crystals, thereby obviating the need for thermal annealing or significant plastic deformation that could cause change in shape and/or dimensions of the specimen.« less

Cyclic deformation leads to defect healing and strengthening of small-volume metal crystals

PubMed Central

Wang, Zhang-Jie; Li, Qing-Jie; Cui, Yi-Nan; Liu, Zhan-Li; Ma, Evan; Li, Ju; Sun, Jun; Zhuang, Zhuo; Dao, Ming; Shan, Zhi-Wei; Suresh, Subra

2015-01-01

When microscopic and macroscopic specimens of metals are subjected to cyclic loading, the creation, interaction, and accumulation of defects lead to damage, cracking, and failure. Here we demonstrate that when aluminum single crystals of submicrometer dimensions are subjected to low-amplitude cyclic deformation at room temperature, the density of preexisting dislocation lines and loops can be dramatically reduced with virtually no change of the overall sample geometry and essentially no permanent plastic strain. This “cyclic healing” of the metal crystal leads to significant strengthening through dramatic reductions in dislocation density, in distinct contrast to conventional cyclic strain hardening mechanisms arising from increases in dislocation density and interactions among defects in microcrystalline and macrocrystalline metals and alloys. Our real-time, in situ transmission electron microscopy observations of tensile tests reveal that pinned dislocation lines undergo shakedown during cyclic straining, with the extent of dislocation unpinning dependent on the amplitude, sequence, and number of strain cycles. Those unpinned mobile dislocations moving close enough to the free surface of the thin specimens as a result of such repeated straining are then further attracted to the surface by image forces that facilitate their egress from the crystal. These results point to a versatile pathway for controlled mechanical annealing and defect engineering in submicrometer-sized metal crystals, thereby obviating the need for thermal annealing or significant plastic deformation that could cause change in shape and/or dimensions of the specimen. PMID:26483463

Design of Alpha Voltaic Power Source Using Americium 241 (241Am) and Diamond with a Power Density of 10 mW/cm3

DTIC Science & Technology

2017-10-19

GaN) was calculated and compared . Alpha-voltaic energy converters were designed in diamond and GaN based on the energy deposition calculations...Example Power Source Two example device designs are calculated and compared . A diamond device containing 2 charge collection regions (Schottky and p...ARL-TR-8189 ● OCT 2017 US Army Research Laboratory Design of Alpha-Voltaic Power Source Using Americium-241 (241Am) and Diamond

DFT applied to the study of carbon-doped zinc-blende (cubic) GaN

NASA Astrophysics Data System (ADS)

Espitia R, M. J.; Ortega-López, C.; Rodríguez Martínez, J. A.

2016-08-01

Employing first principles within the framework of density functional theory, the structural properties, electronic structure, and magnetism of C-doped zincblende (cubic) GaN were investigated. The calculations were carried out using the pseudopotential method, employed exactly as implemented in Quantum ESPRESSO code. For GaC0.0625N0.9375 concentration, a metallic behavior was found. This metallic property comes from the hybridization and polarization of C-2p states and their neighboring N-2p and G-4p states.

Vertical GaN Devices for Power Electronics in Extreme Environments

DTIC Science & Technology

2016-03-31

electronics applications. In this paper vertical p-n diodes and transistors fabricated on pseudo bulk low defect density (104 to 106 cm-2) GaN substrates are...holes in p-GaN has deleterious effect on p-n junction behavior (Fig. 2), p-GaN contacts, and channel control in junction field-effect transistors at...and transistors ) utilizing p-n junctions are suitable for most practical applications including automotive (210K < T < 423K) but may have limitations

Computational study of dislocation based mechanisms in FCC materials

NASA Astrophysics Data System (ADS)

Yellakara, Ranga Nikhil

Understanding the relationships between microstructures and properties of materials is a key to developing new materials with more suitable qualities or employing the appropriate materials in special uses. In the present world of material research, the main focus is on microstructural control to cost-effectively enhance properties and meet performance specifications. This present work is directed towards improving the fundamental understanding of the microscale deformation mechanisms and mechanical behavior of metallic alloys, particularly focusing on face centered cubic (FCC) structured metals through a unique computational methodology called three-dimensional dislocation dynamics (3D-DD). In these simulations, the equations of motion for dislocations are mathematically solved to determine the evolution and interaction of dislocations. Microstructure details and stress-strain curves are a direct observation in the simulation and can be used to validate experimental results. The effect of initial dislocation microstructure on the yield strength has been studied. It has been shown that dislocation density based crystal plasticity formulations only work when dislocation densities/numbers are sufficiently large so that a statistically accurate description of the microstructure can be obtainable. The evolution of the flow stress for grain sizes ranging from 0.5 to 10 mum under uniaxial tension was simulated using an improvised model by integrating dislocation pile-up mechanism at grain boundaries has been performed. This study showed that for a same initial dislocation density, the Hall--Petch relationship holds well at small grain sizes (0.5--2 mum), beyond which the yield strength remains constant as the grain size increases. Various dislocation-particle interaction mechanisms have been introduced and investigations were made on their effect on the uniaxial tensile properties. These studies suggested that increase in particle volume fraction and decrease in particle size has contributed to the strength of these alloys. This work has been successful of capturing complex dislocation mechanisms that involves interactions with particles during the deformation of particle hardened FCC alloys. Finally, the DD model has been extended into studying the cyclic behavior of FCC metallic alloys. This study showed that the strength as well as the cyclic hardening increases due to grain refinement and increase in particle volume fraction. It also showed that the cyclic deformation of ultra-fine grained (UFG) material have undergone cyclic softening at all plastic strain amplitudes. The results provided very useful quantitative information for developing future fatigue models.

Reducing dislocations in semiconductors utilizing repeated thermal cycling during multistage epitaxial growth

DOEpatents

Fan, John C. C.; Tsaur, Bor-Yeu; Gale, Ronald P.; Davis, Frances M.

1992-02-25

Dislocation densities are reduced in growing semiconductors from the vapor phase by employing a technique of interrupting growth, cooling the layer so far deposited, and then repeating the process until a high quality active top layer is achieved. The method of interrupted growth, coupled with thermal cycling, permits dislocations to be trapped in the initial stages of epitaxial growth.

Reducing dislocations in semiconductors utilizing repeated thermal cycling during multistage epitaxial growth

DOEpatents

Fan, John C. C.; Tsaur, Bor-Yeu; Gale, Ronald P.; Davis, Frances M.

1986-12-30

Dislocation densities are reduced in growing semiconductors from the vapor phase by employing a technique of interrupting growth, cooling the layer so far deposited, and then repeating the process until a high quality active top layer is achieved. The method of interrupted growth, coupled with thermal cycling, permits dislocations to be trapped in the initial stages of epitaxial growth.

Electron microscopy observations of radiation damage in irradiated and annealed tungsten

NASA Astrophysics Data System (ADS)

Grzonka, J.; Ciupiński, Ł.; Smalc-Koziorowska, J.; Ogorodnikova, O. V.; Mayer, M.; Kurzydłowski, K. J.

2014-12-01

In the present work tungsten samples were irradiated with W6+ ions with a kinetic energy of 20 MeV in order to simulate radiation damage by fast neutrons. Two samples with cumulative damage of 2.3 and 6.36 displacements per atom were produced. The scanning transmission electron microscopy investigations were carried out in order to determine structure changes resulting from the irradiation. The evolution of the damage with post implantation annealing in the temperature range 673-1100 K was also assessed. Damage profiles were studied at cross-sections. Scanning transmission electron microscopy studies of the lamellae after annealing revealed aggregation of defects and rearrangement as well as partial healing of dislocations at higher temperatures. The results confirm the higher density of radiation-induced dislocations in the near surface area of the sample (1.8 * 1014 m-2) in comparison with a deeper damage area (1.5 * 1014 m-2). Significant decrease of dislocation density was observed after annealing with a concurrent growth of dislocation loops. Transmission electron microscopy analyses show that the dislocation loops are perfect dislocations with the Burgers vectors of b = ½[ 1 1 1].

Neutral and charged gallium clusters: structures, physical properties and implications for the melting features

NASA Astrophysics Data System (ADS)

Núñez, Sara; López, José M.; Aguado, Andrés

2012-09-01

We report the putative Global Minimum (GM) structures and electronic properties of GaN+, GaN and GaN- clusters with N = 13-37 atoms, obtained from first-principles density functional theory structural optimizations. The calculations include spin polarization and employ an exchange-correlation functional which accounts for van der Waals dispersion interactions (vdW-DFT). We find a wide diversity of structural motifs within the located GM, including decahedral, polyicosahedral, polytetrahedral and layered structures. The GM structures are also extremely sensitive to the number of electrons in the cluster, so that the structures of neutral and charged clusters differ for most sizes. The main magic numbers (clusters with an enhanced stability) are identified and interpreted in terms of electronic and geometric shell closings. The theoretical results are consistent with experimental abundance mass spectra of GaN+ and with photoelectron spectra of GaN-. The size dependence of the latent heats of melting, the shape of the heat capacity peaks, and the temperature dependence of the collision cross-sections, all measured for GaN+ clusters, are properly interpreted in terms of the calculated cohesive energies, spectra of configurational excitations, and cluster shapes, respectively. The transition from ``non-melter'' to ``magic-melter'' behaviour, experimentally observed between Ga30+ and Ga31+, is traced back to a strong geometry change. Finally, the higher-than-bulk melting temperatures of gallium clusters are correlated with a more typically metallic behaviour of the clusters as compared to the bulk, contrary to previous theoretical claims.We report the putative Global Minimum (GM) structures and electronic properties of GaN+, GaN and GaN- clusters with N = 13-37 atoms, obtained from first-principles density functional theory structural optimizations. The calculations include spin polarization and employ an exchange-correlation functional which accounts for van der Waals dispersion interactions (vdW-DFT). We find a wide diversity of structural motifs within the located GM, including decahedral, polyicosahedral, polytetrahedral and layered structures. The GM structures are also extremely sensitive to the number of electrons in the cluster, so that the structures of neutral and charged clusters differ for most sizes. The main magic numbers (clusters with an enhanced stability) are identified and interpreted in terms of electronic and geometric shell closings. The theoretical results are consistent with experimental abundance mass spectra of GaN+ and with photoelectron spectra of GaN-. The size dependence of the latent heats of melting, the shape of the heat capacity peaks, and the temperature dependence of the collision cross-sections, all measured for GaN+ clusters, are properly interpreted in terms of the calculated cohesive energies, spectra of configurational excitations, and cluster shapes, respectively. The transition from ``non-melter'' to ``magic-melter'' behaviour, experimentally observed between Ga30+ and Ga31+, is traced back to a strong geometry change. Finally, the higher-than-bulk melting temperatures of gallium clusters are correlated with a more typically metallic behaviour of the clusters as compared to the bulk, contrary to previous theoretical claims. Electronic supplementary information (ESI) available: Atomic coordinates (in xyz format and Å units) and point group symmetries for the global minimum structures reported in this paper. See DOI: 10.1039/c2nr31222k

Single versus successive pop-in modes in nanoindentation tests of single crystals

DOE PAGES

Xia, Yuzhi; Gao, Yanfei; Pharr, George M.; ...

2016-05-24

From recent nanoindentation experiments, two types of pop-in modes have been identified: a single pop-in with a large displacement excursion, or a number of pop-ins with comparable and small displacement excursions. Theoretical analyses are developed here to study the roles played by indenter tip radius, pre-existing defect density, heterogeneous nucleation source type, and lattice resistance on the pop-in modes. The evolution of dislocation structures in earlier pop-ins provides input to modeling a stochastic, heterogeneous mechanism that may be responsible for the subsequent pop-ins. It is found that when the first pop-in occurs near theoretical shear stress, the pop-in mode ismore » determined by the lattice resistance and tip radius. When the first pop-in occurs at low shear stress, whether the successive pop-in mode occurs depends on how the heterogeneous dislocation nucleation source density increases as compared to the increase of the total dislocation density. Lastly, the above transitions are found to correlate well with the ratio of indenter tip radius to the mean spacing of dislocation nucleation sources.« less

Effects of Wavelength and Defect Density on the Efficiency of (In,Ga)N-Based Light-Emitting Diodes

NASA Astrophysics Data System (ADS)

Pristovsek, Markus; Bao, An; Oliver, Rachel A.; Badcock, Tom; Ali, Muhammad; Shields, Andrew

2017-06-01

We measure the electroluminescence of light-emitting diodes (LEDs) on substrates with low dislocation densities (LDD) at 106 cm-2 and low 108 cm-2 , and compare them to LEDs on substrates with high dislocation densities (HDD) closer to 1010 cm-2 . The external quantum efficiencies (EQEs) are fitted using the A B C model with and without localization. The nonradiative-recombination (NR) coefficient A is constant for HDD LEDs, indicating that the NR is dominated by dislocations at all wavelengths. However, A strongly increases for LDD LEDs by a factor of 20 when increasing the emission wavelength from 440 to 540 nm. We attribute this to an increased density of point defects due to the lower growth temperatures used for longer wavelengths. The radiative recombination coefficient B follows the squared wave-function overlap for all samples. Using the observed coefficients, we calculate the peak efficiency as a function of the wavelength. For HDD LEDs the change of wave-function overlap (i.e., B ) is sufficient to reduce the EQE as observed, while for LDD LEDs also the NR coefficient A must increase to explain the observed EQEs. Thus, reducing NR is important to improving the EQEs of green LEDs, but this cannot be achieved solely by reducing the dislocation density: point defects must also be addressed.

Correlations between critical current density, j{sub c}, critical temperature, T{sub c}, and structural quality of Y{sub 1}B{sub 2}Cu{sub 3}O{sub 7-x} thin superconducting films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chrzanowski, J.; Xing, W.B.; Atlan, D.

1994-12-31

Correlations between critical current density (j{sub c}) critical temperature (T{sub c}) and the density of edge dislocations and nonuniform strain have been observed in YBCO thin films deposited by pulsed laser ablation on (001) LaAlO{sub 3} single crystals. Distinct maxima in j{sub c} as a function of the linewidths of the (00{ell}) Bragg reflections and as a function of the mosaic spread have been found in the epitaxial films. These maxima in j{sub c} indicate that the magnetic flux lines, in films of structural quality approaching that of single crystals, are insufficiently pinned which results in a decreased critical currentmore » density. T{sub c} increased monotonically with improving crystalline quality and approached a value characteristic of a pure single crystal. A strong correlation between j{sub c} and the density of edge dislocations N{sub D} was found. At the maximum of the critical current density the density of edge dislocations was estimated to be N{sub D}{approximately}1-2 x 10{sup 9}/cm{sup 2}.« less

Correlations between critical current density, j(sub c), critical temperature, T(sub c),and structural quality of Y1B2Cu3O(7-x) thin superconducting films

NASA Technical Reports Server (NTRS)

Chrzanowski, J.; Xing, W. B.; Atlan, D.; Irwin, J. C.; Heinrich, B.; Cragg, R. A.; Zhou, H.; Angus, V.; Habib, F.; Fife, A. A.

1995-01-01

Correlations between critical current density (j(sub c)) critical temperature (T(sub c)) and the density of edge dislocations and nonuniform strain have been observed in YBCO thin films deposited by pulsed laser ablation on (001) LaAlO3 single crystals. Distinct maxima in j(sub c) as a function of the linewidths of the (00 l) Bragg reflections and as a function of the mosaic spread have been found in the epitaxial films. These maxima in j(sub c) indicate that the magnetic flux lines, in films of structural quality approachingthat of single crystals, are insufficiently pinned which results in a decreased critical current density. T(sub c) increased monotonically with improving crystalline quality and approached a value characteristic of a pure single crystal. A strong correlation between j(sub c) and the density of edge dislocations ND was found. At the maximum of the critical current density the density of edge dislocations was estimated to be N(sub D) approximately 1-2 x 10(exp 9)/sq cm.

Quantitative analysis of dislocation arrangements induced by electromigration in a passivated Al (0.5 wt % Cu) interconnect

NASA Astrophysics Data System (ADS)

Barabash, R. I.; Ice, G. E.; Tamura, N.; Valek, B. C.; Bravman, J. C.; Spolenak, R.; Patel, J. R.

2003-05-01

Electromigration during accelerated testing can induce plastic deformation in apparently undamaged Al interconnect lines as recently revealed by white beam scanning x-ray microdiffraction. In the present article, we provide a first quantitative analysis of the dislocation structure generated in individual micron-sized Al grains during an in situ electromigration experiment. Laue reflections from individual interconnect grains show pronounced streaking during the early stages of electromigration. We demonstrate that the evolution of the dislocation structure during electromigration is highly inhomogeneous and results in the formation of unpaired randomly distributed dislocations as well as geometrically necessary dislocation boundaries. Approximately half of all unpaired dislocations are grouped within the walls. The misorientation created by each boundary and density of unpaired individual dislocations is determined. The origin of the observed plastic deformation is considered in view of the constraints for dislocation arrangements under the applied electric field during electromigration.

Substrate impact on the low-temperature growth of GaN thin films by plasma-assisted atomic layer deposition

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kizir, Seda; Haider, Ali; Biyikli, Necmi, E-mail: biyikli@unam.bilkent.edu.tr

2016-07-15

Gallium nitride (GaN) thin films were grown on Si (100), Si (111), and c-plane sapphire substrates at 200 °C via hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD) using GaEt{sub 3} and N{sub 2}/H{sub 2} plasma as group-III and V precursors, respectively. The main aim of the study was to investigate the impact of substrate on the material properties of low-temperature ALD-grown GaN layers. Structural, chemical, and optical characterizations were carried out in order to evaluate and compare film quality of GaN on different substrates. X-ray reflectivity measurements showed film density values of 5.70, 5.74, and 5.54 g/cm{sup 3} for GaN grown on Simore » (100), Si (111), and sapphire, respectively. Grazing incidence x-ray diffraction measurements exhibited hexagonal wurtzite structure in all HCPA-ALD grown GaN samples. However, dominant diffraction peak for GaN films grown on Si and sapphire substrates were detected differently as (002) and (103), respectively. X-ray diffraction gonio scans measured from GaN grown on c-plane sapphire primarily showed (002) orientation. All samples exhibited similar refractive index values (∼2.17 at 632 nm) with 2–3 at. % of oxygen impurity existing within the bulk of the films. The grain size was calculated as ∼9–10 nm for GaN grown on Si (100) and Si (111) samples while it was ∼5 nm for GaN/sapphire sample. Root-mean-square surface roughness values found as 0.68, 0.76, and 1.83 nm for GaN deposited on Si (100), Si (111), and sapphire, respectively. Another significant difference observed between the samples was the film growth per cycle: GaN/sapphire sample showed a considerable higher thickness value when compared with GaN/Si samples, which might be attributed to a possibly more-efficient nitridation and faster nucleation of sapphire surface.« less

Dislocation dynamics simulations of interactions between gliding dislocations and radiation induced prismatic loops in zirconium

NASA Astrophysics Data System (ADS)

Drouet, Julie; Dupuy, Laurent; Onimus, Fabien; Mompiou, Frédéric; Perusin, Simon; Ambard, Antoine

2014-06-01

The mechanical behavior of Pressurized Water Reactor fuel cladding tubes made of zirconium alloys is strongly affected by neutron irradiation due to the high density of radiation induced dislocation loops. In order to investigate the interaction mechanisms between gliding dislocations and loops in zirconium, a new nodal dislocation dynamics code, adapted to Hexagonal Close Packed metals, has been used. Various configurations have been systematically computed considering different glide planes, basal or prismatic, and different characters, edge or screw, for gliding dislocations with -type Burgers vectors. Simulations show various interaction mechanisms such as (i) absorption of a loop on an edge dislocation leading to the formation of a double super-jog, (ii) creation of a helical turn, on a screw dislocation, that acts as a strong pinning point or (iii) sweeping of a loop by a gliding dislocation. It is shown that the clearing of loops is more favorable when the dislocation glides in the basal plane than in the prismatic plane explaining the easy dislocation channeling in the basal plane observed after neutron irradiation by transmission electron microscopy.

Probing the character of ultra-fast dislocations

DOE PAGES

Rudd, R. E.; Ruestes, C. J.; Bringa, E. M.; ...

2015-11-23

Plasticity is often controlled by dislocation motion, which was first measured for low pressure, low strain rate conditions decades ago. However, many applications require knowledge of dislocation motion at high stress conditions where the data are sparse, and come from indirect measurements dominated by the effect of dislocation density rather than velocity. Here we make predictions based on atomistic simulations that form the basis for a new approach to measure dislocation velocities directly at extreme conditions using three steps: create prismatic dislocation loops in a near-surface region using nanoindentation, drive the dislocations with a shockwave, and use electron microscopy tomore » determine how far the dislocations moved and thus their velocity at extreme stress and strain rate conditions. We report on atomistic simulations of tantalum that make detailed predictions of dislocation flow, and find that the approach is feasible and can uncover an exciting range of phenomena, such as transonic dislocations and a novel form of loop stretching. Furthermore, the simulated configuration enables a new class of experiments to probe average dislocation velocity at very high applied shear stress.« less

Electron and hole stability in GaN and ZnO.

PubMed

Walsh, Aron; Catlow, C Richard A; Miskufova, Martina; Sokol, Alexey A

2011-08-24

We assess the thermodynamic doping limits of GaN and ZnO on the basis of point defect calculations performed using the embedded cluster approach and employing a hybrid non-local density functional for the quantum mechanical region. Within this approach we have calculated a staggered (type-II) valence band alignment between the two materials, with the N 2p states contributing to the lower ionization potential of GaN. With respect to the stability of free electron and hole carriers, redox reactions resulting in charge compensation by ionic defects are found to be largely endothermic (unfavourable) for electrons and exothermic (favourable) for holes, which is consistent with the efficacy of electron conduction in these materials. Approaches for overcoming these fundamental thermodynamic limits are discussed. © 2011 IOP Publishing Ltd

Ion channeling study of defects in compound crystals using Monte Carlo simulations

NASA Astrophysics Data System (ADS)

Turos, A.; Jozwik, P.; Nowicki, L.; Sathish, N.

2014-08-01

Ion channeling is a well-established technique for determination of structural properties of crystalline materials. Defect depth profiles have been usually determined basing on the two-beam model developed by Bøgh (1968) [1]. As long as the main research interest was focused on single element crystals it was considered as sufficiently accurate. New challenge emerged with growing technological importance of compound single crystals and epitaxial heterostructures. Overlap of partial spectra due to different sublattices and formation of complicated defect structures makes the two beam method hardly applicable. The solution is provided by Monte Carlo computer simulations. Our paper reviews principal aspects of this approach and the recent developments in the McChasy simulation code. The latter made it possible to distinguish between randomly displaced atoms (RDA) and extended defects (dislocations, loops, etc.). Hence, complex defect structures can be characterized by the relative content of these two components. The next refinement of the code consists of detailed parameterization of dislocations and dislocation loops. Defect profiles for variety of compound crystals (GaN, ZnO, SrTiO3) have been measured and evaluated using the McChasy code. Damage accumulation curves for RDA and extended defects revealed non monotonous defect buildup with some characteristic steps. Transition to each stage is governed by the different driving force. As shown by the complementary high resolution XRD measurements lattice strain plays here the crucial role and can be correlated with the concentration of extended defects.

Peierls potential of screw dislocations in bcc transition metals: Predictions from density functional theory

DOE Office of Scientific and Technical Information (OSTI.GOV)

Weinberger, Christopher R.; Tucker, Garritt J.; Foiles, Stephen M.

2013-02-01

It is well known that screw dislocation motion dominates the plastic deformation in body-centered-cubic metals at low temperatures. The nature of the nonplanar structure of screw dislocations gives rise to high lattice friction, which results in strong temperature and strain rate dependence of plastic flow. Thus the nature of the Peierls potential, which is responsible for the high lattice resistance, is an important physical property of the material. However, current empirical potentials give a complicated picture of the Peierls potential. Here, we investigate the nature of the Peierls potential using density functional theory in the bcc transition metals. The resultsmore » show that the shape of the Peierls potential is sinusoidal for every material investigated. Furthermore, we show that the magnitude of the potential scales strongly with the energy per unit length of the screw dislocation in the material.« less

Doping and compensation in Al-rich AlGaN grown on single crystal AlN and sapphire by MOCVD

NASA Astrophysics Data System (ADS)

Bryan, Isaac; Bryan, Zachary; Washiyama, Shun; Reddy, Pramod; Gaddy, Benjamin; Sarkar, Biplab; Breckenridge, M. Hayden; Guo, Qiang; Bobea, Milena; Tweedie, James; Mita, Seiji; Irving, Douglas; Collazo, Ramon; Sitar, Zlatko

2018-02-01

In order to understand the influence of dislocations on doping and compensation in Al-rich AlGaN, thin films were grown by metal organic chemical vapor deposition (MOCVD) on different templates on sapphire and low dislocation density single crystalline AlN. AlGaN grown on AlN exhibited the highest conductivity, carrier concentration, and mobility for any doping concentration due to low threading dislocation related compensation and reduced self-compensation. The onset of self-compensation, i.e., the "knee behavior" in conductivity, was found to depend only on the chemical potential of silicon, strongly indicating the cation vacancy complex with Si as the source of self-compensation. However, the magnitude of self-compensation was found to increase with an increase in dislocation density, and consequently, AlGaN grown on AlN substrates demonstrated higher conductivity over the entire doping range.

Shear-Coupled Grain Growth and Texture Development in a Nanocrystalline Ni-Fe Alloy during Cold Rolling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Li; Ungár, Tamás; Toth, Laszlo S.

The evolution of texture, grain size, grain shape, dislocation and twin density has been determined by synchrotron X-ray diffraction and line profile analysis in a nanocrystalline Ni- Fe alloy after cold rolling along different directions related to the initial fiber and the long axis of grains. The texture evolution has been simulated by the Taylor-type relaxed constraints viscoplastic polycrystal model. The simulations were based on the activity of partial dislocations in correlation with the experimental results of dislocation density determination. The concept of stress-induced shear-coupling is supported and strengthened by both the texture simulations and the experimentally determined evolution ofmore » the microstructure parameters. Grain-growth and texture evolution are shown to proceed by the shear-coupling mechanism supported by dislocation activity as long as the grain size is not smaller than about 20 nm.« less

The low thermal gradient CZ technique as a way of growing of dislocation-free germanium crystals

NASA Astrophysics Data System (ADS)

Moskovskih, V. A.; Kasimkin, P. V.; Shlegel, V. N.; Vasiliev, Y. V.; Gridchin, V. A.; Podkopaev, O. I.

2014-09-01

This paper considers the possibility of growth of dislocation-free germanium single crystals. This is achieved by reducing the temperature gradients at the level of 1 K/cm and lower. Single germanium crystals 45-48 mm in diameter with a dislocation density of 102 cm-2 were grown by a Low Thermal Gradient Czochralski technique (LTG CZ).

Influence of cold work on electrochemical behavior of 316L ASS in PEMFC environment

NASA Astrophysics Data System (ADS)

Tandon, Vipin; Patil, Awanikumar P.; Rathod, Ramesh C.; Shukla, Sourabh

2018-02-01

The influence of cold work (CW) on electrochemical behavior of 316L ASS in PEMFC (0.5M H2SO4 + 2 ppm HF at 70 °C) environment was investigated by microstructural observations, x-ray diffraction (XRD), polarization, electrochemical impedance spectroscopy (EIS) and Mott-Schottky (M-S) techniques. The XRD is used to analyze the increase in dislocation density and formation of α‧-martensite with increasing CW degree. The EIS is used to find out the effect of substrate dislocation density on the film resistance. The EIS result show that with increasing CW, the diameter of depressed semi-circular arc and consequently film resistance decreased. This indicates the formation of highly disordered and porous film on CW. From PDP results, it is found that icrit, ip and icorr increased on increasing CW degree. Moreover, the direct relationship was drawn from the dislocation density of the substrate to the defect density of the passive film from M-S technique.

Predicted lattice-misfit stresses in a gallium-nitride (GaN) film

NASA Astrophysics Data System (ADS)

Suhir, E.; Yi, S.

2017-02-01

Effective, easy-to-use and physically meaningful analytical predictive models are developed for the evaluation the lattice-misfit stresses (LMS) in a semiconductor film grown on a circular substrate (wafer). The two-dimensional (plane-stress) theory-of-elasticity approximation (TEA) is employed. First of all, the interfacial shearing stresses are evaluated. These stresses might lead to the occurrence and growth of dislocations, as well as to possible delaminations (adhesive strength of the assembly) and the elevated stress and strain in the buffering material, if any (cohesive strength of the assembly). Second of all, the normal radial and circumferential (tangential) stresses acting in the film cross-sections are determined. These stresses determine the short- and long-term strength (fracture toughness) of the film material. It is shown that while the normal stresses in the semiconductor film are independent of its thickness, the interfacial shearing stresses increase with an increase in the induced force (not stress!) acting in the film cross-sections, and that this force increases with an increase in the film thickness. This leads, for a thick enough film, to the occurrence, growth and propagation of dislocations. These start at the assembly ends and propagate, when the film thickness increases, inwards the structure. The TEA data are compared with the results obtained using a simplified strength-of-materials approach (SMA). This approach considers, instead of an actual circular assembly, an elongated bi-material rectangular strip of unit width and of finite length equal to the wafer diameter. The analysis, although applicable to any semiconductor crystal growth (SCG) technology is geared in this analysis to the Gallium-Nitride (GaN) technology. The numerical example is carried out for a GaN film grown on a Silicon Carbide (SiC) substrate. It is concluded that the SMA model is acceptable for understanding the physics of the state of stress and for the prediction of the normal stresses acting in the major mid-portion of the assembly. The SMA model underestimates, however, the maximum interfacial shearing stress at the assembly periphery, and, because of the very nature of the SMA, is unable to address the circumferential stress. This stress can be quite high at the circular boundary of the assembly. At the assembly edge the circumferential stress is as high as σθ = (2-ν1)σ1, i.e., by the factor of 2-ν1 higher than the normal stress, σ1, in the mid-portion of the film. In this formula, ν1 is Poisson's ratio of the film material.

Method for reducing or eliminating interface defects in mismatched semiconductor epilayers

DOEpatents

Fitzgerald, Jr., Eugene A.; Ast, Dieter G.

1992-01-01

The present invention and process relates to crystal lattice mismatched semiconductor composite having a first semiconductor layer and a second semiconductor growth layer deposited thereon to form an interface wherein the growth layer can be deposited at thicknesses in excess of the critical thickness, even up to about 10.times. critical thickness. Such composite has an interface which is substantially free of interface defects. For example, the size of the growth areas in a mismatched In.sub.0.05 Ga.sub.0.95 As/(001)GaAs interface was controlled by fabricating 2-.mu.m high pillars of various lateral geometries and lateral dimensions before the epitaxial deposition of 3500.ANG. of In.sub.0.05 Ga.sub.0.95 As. The linear dislocation density at the interface was reduced from >5000 dislocations/cm to about zero for 25-.mu.m lateral dimensions and to less than 800 dislocations/cm for lateral dimensions as large as 100 .mu.m. The fabricated pillars control the lateral dimensions of the growth layer and block the glide of misfit dislocations with the resultant decrease in dislocation density.

Method for reducing or eliminating interface defects in mismatched semiconductor eiplayers

DOEpatents

Fitzgerald, Jr., Eugene A.; Ast, Dieter G.

1991-01-01

The present invention and process relates to crystal lattice mismatched semiconductor composite having a first semiconductor layer and a second semiconductor growth layer deposited thereon to form an interface wherein the growth layer can be deposited at thicknesses in excess of the critical thickness, even up to about 10x critical thickness. Such composite has an interface which is substantially free of interface defects. For example, the size of the growth areas in a mismatched In.sub.0.05 Ga.sub.0.95 As/(001)GaAs interface was controlled by fabricating 2-.mu.m high pillars of various lateral geometries and lateral dimensions before the epitaxial deposition of 3500.ANG. of In.sub.0.05 Ga.sub.0.95 As. The linear dislocation density at the interface was reduced from >5000 dislocations/cm to about zero for 25-.mu.m lateral dimensions and to less than 800 dislocations/cm for lateral dimensions as large as 100 .mu.m. The fabricated pillars control the lateral dimensions of the growth layer and block the glide of misfit dislocations with the resultant decrease in dislocation density.

Method for reducing or eliminating interface defects in mismatched semiconductor epilayers

DOEpatents

Fitzgerald, E.A. Jr.; Ast, D.G.

1992-10-20

The present invention and process relates to crystal lattice mismatched semiconductor composite having a first semiconductor layer and a second semiconductor growth layer deposited thereon to form an interface wherein the growth layer can be deposited at thicknesses in excess of the critical thickness, even up to about 10[times] critical thickness. Such composite has an interface which is substantially free of interface defects. For example, the size of the growth areas in a mismatched In[sub 0.05]Ga[sub 0.95]As/(001)GaAs interface was controlled by fabricating 2-[mu]m high pillars of various lateral geometries and lateral dimensions before the epitaxial deposition of 3500 [angstrom] of In[sub 0.05]Ga[sub 0.95]As. The linear dislocation density at the interface was reduced from >5000 dislocations/cm to about zero for 25-[mu]m lateral dimensions and to less than 800 dislocations/cm for lateral dimensions as large as 100 [mu]m. The fabricated pillars control the lateral dimensions of the growth layer and block the glide of misfit dislocations with the resultant decrease in dislocation density. 7 figs.

The Role of Geometrically Necessary Dislocations in Cantilever Beam Bending Experiments of Single Crystals

PubMed Central

Husser, Edgar; Bargmann, Swantje

2017-01-01

The mechanical behavior of single crystalline, micro-sized copper is investigated in the context of cantilever beam bending experiments. Particular focus is on the role of geometrically necessary dislocations (GNDs) during bending-dominated load conditions and their impact on the characteristic bending size effect. Three different sample sizes are considered in this work with main variation in thickness. A gradient extended crystal plasticity model is presented and applied in a three-dimensional finite-element (FE) framework considering slip system-based edge and screw components of the dislocation density vector. The underlying mathematical model contains non-standard evolution equations for GNDs, crystal-specific interaction relations, and higher-order boundary conditions. Moreover, two element formulations are examined and compared with respect to size-independent as well as size-dependent bending behavior. The first formulation is based on a linear interpolation of the displacement and the GND density field together with a full integration scheme whereas the second is based on a mixed interpolation scheme. While the GND density fields are treated equivalently, the displacement field is interpolated quadratically in combination with a reduced integration scheme. Computational results indicate that GND storage in small cantilever beams strongly influences the evolution of statistically stored dislocations (SSDs) and, hence, the distribution of the total dislocation density. As a particular example, the mechanical bending behavior in the case of a physically motivated limitation of GND storage is studied. The resulting impact on the mechanical bending response as well as on the predicted size effect is analyzed. Obtained results are discussed and related to experimental findings from the literature. PMID:28772657

Electrical properties of grain boundaries and dislocations in crystalline silicon: Influence of impurity incorporation and hydrogenation

NASA Astrophysics Data System (ADS)

Park, Yongkook

This thesis examines the electrical properties of grain boundaries (GBs) and dislocations in crystalline silicon. The influence of impurity incorporation and hydrogenation on the electrical properties of grain boundaries , as well as the electrical activity of impurity decorated dislocations and the retention of impurities at dislocations at high temperatures have been investigated. The electrical properties of Si GB were examined by C-V, J-V , and capacitance transient methods using aluminum/Si(100)/Si(001) junctions. First, the density of states and the carrier capture cross-sections of the clean GB were evaluated by C-V/J-V analyses. The density of GB states was determined as 4.0x1012 cm-2eV -1. It was found that the states close to the valance band edge have relatively smaller hole capture cross sections than those at higher energy position, and electron capture cross sections are at least two or three orders larger than the corresponding hole capture cross sections. Secondly, the influence of iron contamination and hydrogenation following iron contamination on the electrical properties of (110)/(001) Si GB was characterized by a capacitance transient technique. Compared with the clean sample, iron contamination increased both the density of states by at least three times and the zero-bias barrier height by 70 meV, while reducing by two orders of magnitude the electron/hole capture cross-section ratio. Hydrogenation following iron contamination led to the reduction of the density of Fe-decorated GB states, which was increased to over 2x1013 cm-2eV-1 after iron contamination, to ˜1x1013 cm-2 eV-1 after hydrogenation treatment. The increased zero-bias GB energy barrier due to iron contamination was reversed as well by hydrogen treatment. The density of GB states before and after hydrogenation was evaluated by J-V, C-V and capacitance transient methods using gold/direct-silicon-bonded (DSB) (110) thin silicon top layer/(100) silicon substrate junctions. The GB potential energy barrier in thermal equilibrium was reduced by 70 meV. Whereas the clean sample had a density of GB states of ˜6x1012 cm-2eV-1 in the range of Ev+0.54˜0.64 eV, hydrogenation reduced the density of GB states to ˜9x1011 cm-2eV -1 in the range of Ev+0.56˜0.61 eV, which is about a seven-fold reduction from that of the clean sample. Segregation and thermal dissociation kinetics of hydrogen at a large-angle general GB in crystalline silicon have been investigated using deuterium as a readily identifiable isotope which duplicates hydrogen chemistry. Segregation or trapping of deuterium (hydrogen) introduced was found to take place at (110)/(001) Si GB. The segregation coefficient (k) of deuterium (hydrogen) at GB was determined as k≈24+/-3 at 100°C. Thermal dissociation of deuterium (hydrogen) from GB obeyed first-order kinetics with an activation energy of ˜1.62 eV. The electrical activities of dislocations in a SiGe/Si heterostructure were examined by deep level transient spectroscopy (DLTS) after iron contamination and phosphorous diffusion gettering. DLTS of iron contaminated samples revealed a peak at 210 K, which was assigned to individual iron atoms or very small (<2 nm) precipitates decorated along dislocations. Arrhenius plot of the 210 K peak yielded a hole capture cross section of 2.4x10-14 cm2 and an energy level of 0.42 eV above the valance band. DLTS of the iron contaminated sample revealed that 6x10 14 cm-3 of boron can more effectively trap interstitial iron at room temperature than the strain field/defect sites at 107 ˜108 cm-2 dislocations. Phosphorous diffusion experiments revealed that the gettering efficiency of iron impurities depends on the dislocation density. For regions of high dislocation density, phosphorous diffusion cannot remove all iron impurities decorated at dislocations, suggesting a strong binding of iron impurities at dislocation core defects.

Impact of GaN cap on charges in Al₂O₃/(GaN/)AlGaN/GaN metal-oxide-semiconductor heterostructures analyzed by means of capacitance measurements and simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ťapajna, M., E-mail: milan.tapajna@savba.sk; Jurkovič, M.; Válik, L.

2014-09-14

Oxide/semiconductor interface trap density (D{sub it}) and net charge of Al₂O₃/(GaN)/AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor (MOS-HEMT) structures with and without GaN cap were comparatively analyzed using comprehensive capacitance measurements and simulations. D{sub it} distribution was determined in full band gap of the barrier using combination of three complementary capacitance techniques. A remarkably higher D{sub it} (∼5–8 × 10¹²eV⁻¹ cm⁻²) was found at trap energies ranging from E C-0.5 to 1 eV for structure with GaN cap compared to that (D{sub it} ∼ 2–3 × 10¹²eV⁻¹ cm⁻²) where the GaN cap was selectively etched away. D{sub it} distributions were then used for simulation of capacitance-voltage characteristics. A good agreement betweenmore » experimental and simulated capacitance-voltage characteristics affected by interface traps suggests (i) that very high D{sub it} (>10¹³eV⁻¹ cm⁻²) close to the barrier conduction band edge hampers accumulation of free electron in the barrier layer and (ii) the higher D{sub it} centered about E C-0.6 eV can solely account for the increased C-V hysteresis observed for MOS-HEMT structure with GaN cap. Analysis of the threshold voltage dependence on Al₂O₃ thickness for both MOS-HEMT structures suggests that (i) positive charge, which compensates the surface polarization, is not necessarily formed during the growth of III-N heterostructure, and (ii) its density is similar to the total surface polarization charge of the GaN/AlGaN barrier, rather than surface polarization of the top GaN layer only. Some constraints for the positive surface compensating charge are discussed.« less

Radiation Tolerant Interfaces: Influence of Local Stoichiometry at the Misfit Dislocation on Radiation Damage Resistance of Metal/Oxide Interfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shutthanandan, Vaithiyalingam; Choudhury, Samrat; Manandhar, Sandeep

The interaction of radiation with materials controls the performance, reliability, and safety of many structures in nuclear power systems. Revolutionary improvements in radiation damage resistance may be attainable if methods can be found to manipulate interface properties to give optimal interface stability and point defect recombination capability. To understand how variations in interface properties such as misfit dislocation density and local chemistry affect radiation-induced defect absorption and recombination, a model system of metallic Cr xV 1-x (0 ≤ x ≤ 1) epitaxial films deposited on MgO(001) single crystal substrates has been explored in this paper. By controlling film composition, themore » lattice mismatch between the film and MgO is adjusted to vary the misfit dislocation density at the metal/oxide interface. The stability of these interfaces under various irradiation conditions is studied experimentally and theoretically. The results indicate that, unlike at metal/metal interfaces, the misfit dislocation density does not dominate radiation damage tolerance at metal/oxide interfaces. Rather, the stoichiometry and the location of the misfit dislocation extra half-plane (in the metal or the oxide) drive radiation-induced defect behavior. Finally, together, these results demonstrate the sensitivity of defect recombination to interfacial chemistry and provide new avenues for engineering radiation-tolerant nanomaterials for next-generation nuclear power plants.« less

Radiation Tolerant Interfaces: Influence of Local Stoichiometry at the Misfit Dislocation on Radiation Damage Resistance of Metal/Oxide Interfaces

DOE PAGES

Shutthanandan, Vaithiyalingam; Choudhury, Samrat; Manandhar, Sandeep; ...

2017-04-24

The interaction of radiation with materials controls the performance, reliability, and safety of many structures in nuclear power systems. Revolutionary improvements in radiation damage resistance may be attainable if methods can be found to manipulate interface properties to give optimal interface stability and point defect recombination capability. To understand how variations in interface properties such as misfit dislocation density and local chemistry affect radiation-induced defect absorption and recombination, a model system of metallic Cr xV 1-x (0 ≤ x ≤ 1) epitaxial films deposited on MgO(001) single crystal substrates has been explored in this paper. By controlling film composition, themore » lattice mismatch between the film and MgO is adjusted to vary the misfit dislocation density at the metal/oxide interface. The stability of these interfaces under various irradiation conditions is studied experimentally and theoretically. The results indicate that, unlike at metal/metal interfaces, the misfit dislocation density does not dominate radiation damage tolerance at metal/oxide interfaces. Rather, the stoichiometry and the location of the misfit dislocation extra half-plane (in the metal or the oxide) drive radiation-induced defect behavior. Finally, together, these results demonstrate the sensitivity of defect recombination to interfacial chemistry and provide new avenues for engineering radiation-tolerant nanomaterials for next-generation nuclear power plants.« less

Microstructural characterisation of proton irradiated niobium using X-ray diffraction technique

NASA Astrophysics Data System (ADS)

Dutta, Argha; Gayathri, N.; Neogy, S.; Mukherjee, P.

2018-04-01

The microstructural parameters in pure Nb, irradiated with 5 MeV proton beam have been evaluated as a function of dose using X-ray diffraction line profile analysis. In order to assess the microstructural changes in the homogeneous region and in the peak damage region of the damage energy deposition profile, X-ray diffraction patterns have been collected using two different geometries (Bragg-Brentano and parallel beam geometries). Different X-ray line profile analysis like Williamson-Hall (W-H) analysis, modified W-H analysis, double-Voigt analysis, modified Rietveld technique and convolutional multiple whole profile fitting have been employed to extract the microstructural parameters like coherent domain size, microstrain within the domain, dislocation density and arrangement of dislocations. The coherent domain size decreases drastically along with increase in microstrain and dislocation density in the first dose for both the geometries. With increasing dose, a decreasing trend in microstrain associated with decrease in dislocation density is observed for both the geometries. This is attributed to the formation of defect clusters due to irradiation which with increasing dose collapse to dislocation loops to minimise the strain in the matrix. This is corroborated with the observation of black dots and loops in the TEM images. No significant difference is observed in the trend of microstructural parameters between the homogeneous and peak damage region of the damage profile.

Epitaxial growth of GaN by radical-enhanced metalorganic chemical vapor deposition (REMOCVD) in the downflow of a very high frequency (VHF) N2/H2 excited plasma - effect of TMG flow rate and VHF power

NASA Astrophysics Data System (ADS)

Lu, Yi; Kondo, Hiroki; Ishikawa, Kenji; Oda, Osamu; Takeda, Keigo; Sekine, Makoto; Amano, Hiroshi; Hori, Masaru

2014-04-01

Gallium nitride (GaN) films have been grown by using our newly developed Radical-Enhanced Metalorganic Chemical Vapor Deposition (REMOCVD) system. This system has three features: (1) application of very high frequency (60 MHz) power in order to increase the plasma density, (2) introduction of H2 gas together with N2 gas in the plasma discharge region to generate not only nitrogen radicals but also active NHx molecules, and (3) radical supply under remote plasma arrangement with suppression of charged ions and photons by employing a Faraday cage. Using this new system, we have studied the effect of the trimethylgallium (TMG) source flow rate and of the plasma generation power on the GaN crystal quality by using scanning electron microscopy (SEM) and double crystal X-ray diffraction (XRD). We found that this REMOCVD allowed the growth of epitaxial GaN films of the wurtzite structure of (0001) orientation on sapphire substrates with a high growth rate of 0.42 μm/h at a low temperature of 800 °C. The present REMOCVD is a promising method for GaN growth at relatively low temperature and without using costly ammonia gas.

Direct spontaneous growth and interfacial structural properties of inclined GaN nanopillars on r-plane sapphire

DOE Office of Scientific and Technical Information (OSTI.GOV)

Adikimenakis, A.; Aretouli, K. E.; Tsagaraki, K.

2015-06-28

The spontaneous growth of GaN nanopillars (NPs) by direct plasma-assisted molecular beam epitaxy on nitridated r-plane sapphire substrates has been studied. The emanation of metal-polarity NPs from inside an a-plane nonpolar GaN film was found to depend on both the substrate nitridation and the growth conditions. The density of NPs increased with increasing the duration of the nitridation process and the power applied on the radio-frequency plasma source, as well as the III/V flux ratio, while variation of the first two parameters enhanced the roughness of the substrate's surface. Transmission electron microscopy (TEM) techniques were employed to reveal the structuralmore » characteristics of the NPs and their nucleation mechanism from steps on the sapphire surface and/or interfacial semipolar GaN nanocrystals. Lattice strain measurements showed a possible Al enrichment of the first 5–6 monolayers of the NPs. By combining cross-sectional and plan-view TEM observations, the three-dimensional model of the NPs was constructed. The orientation relationship and interfacial accommodation between the NPs and the nonpolar a-plane GaN film were also elucidated. The NPs exhibited strong and narrow excitonic emission, suggesting an excellent structural quality.« less

Dislocation pinning effects induced by nano-precipitates during warm laser shock peening: Dislocation dynamic simulation and experiments

NASA Astrophysics Data System (ADS)

Liao, Yiliang; Ye, Chang; Gao, Huang; Kim, Bong-Joong; Suslov, Sergey; Stach, Eric A.; Cheng, Gary J.

2011-07-01

Warm laser shock peening (WLSP) is a new high strain rate surface strengthening process that has been demonstrated to significantly improve the fatigue performance of metallic components. This improvement is mainly due to the interaction of dislocations with highly dense nanoscale precipitates, which are generated by dynamic precipitation during the WLSP process. In this paper, the dislocation pinning effects induced by the nanoscale precipitates during WLSP are systematically studied. Aluminum alloy 6061 and AISI 4140 steel are selected as the materials with which to conduct WLSP experiments. Multiscale discrete dislocation dynamics (MDDD) simulation is conducted in order to investigate the interaction of dislocations and precipitates during the shock wave propagation. The evolution of dislocation structures during the shock wave propagation is studied. The dislocation structures after WLSP are characterized via transmission electron microscopy and are compared with the results of the MDDD simulation. The results show that nano-precipitates facilitate the generation of highly dense and uniformly distributed dislocation structures. The dislocation pinning effect is strongly affected by the density, size, and space distribution of nano-precipitates.

Non-Micropipe Dislocations in 4H-SiC Devices: Electrical Properties and Device Technology Implications

NASA Technical Reports Server (NTRS)

Neudeck, Philip G.; Huang, Wei; Dudley, Michael; Fazi, Christian

1998-01-01

It is well-known that SiC wafer quality deficiencies are delaying the realization of outstandingly superior 4H-SiC power electronics. While efforts to date have centered on eradicating micropipes (i.e., hollow core super-screw dislocations with Burgers vectors greater than or equal to 2c), 4H-SiC wafers and epilayers also contain elementary screw dislocations (i.e., Burgers vector = 1c with no hollow core) in densities on the order of thousands per sq cm, nearly 100-fold micropipe densities. While not nearly as detrimental to SiC device performance as micropipes, it has recently been demonstrated that elementary screw dislocations somewhat degrade the reverse leakage and breakdown properties of 4H-SiC p(+)n diodes. Diodes containing elementary screw dislocations exhibited a 5% to 35% reduction in breakdown voltage, higher pre-breakdown reverse leakage current, softer reverse breakdown I-V knee, and microplasmic breakdown current filaments that were non-catastrophic as measured under high series resistance biasing. This paper details continuing experimental and theoretical investigations into the electrical properties of 4H-SiC elementary screw dislocations. The nonuniform breakdown behavior of 4H-SiC p'n junctions containing elementary screw dislocations exhibits interesting physical parallels with nonuniform breakdown phenomena previously observed in other semiconductor materials. Based upon experimentally observed dislocation-assisted breakdown, a re-assessment of well-known physical models relating power device reliability to junction breakdown has been undertaken for 4H-SiC. The potential impact of these elementary screw dislocation defects on the performance and reliability of various 4H-SiC device technologies being developed for high-power applications will be discussed.

Breakdown Degradation Associated with Elementary Screw Dislocations in 4H-SiC P(+)N Junction Rectifiers

NASA Technical Reports Server (NTRS)

Neudeck, P. G.; Huang, W.; Dudley, M.

1998-01-01

It is well-known that SiC wafer quality deficiencies are delaying the realization of outstandingly superior 4H-SiC power electronics. While efforts to date have centered on eradicating micropipes (i.e., hollow core super-screw dislocations with Burgers vector greater than 2c), 4H-SiC wafers and epilayers also contain elementary screw dislocations (i.e., Burgers vector = lc with no hollow core) in densities on the order of thousands per sq cm, nearly 100-fold micropipe densities. This paper describes an initial study into the impact of elementary screw dislocations on the reverse-bias current-voltage (I-V) characteristics of 4H-SiC p(+)n diodes. First, Synchrotron White Beam X-ray Topography (SWBXT) was employed to map the exact locations of elementary screw dislocations within small-area 4H-SiC p(+)n mesa diodes. Then the high-field reverse leakage and breakdown properties of these diodes were subsequently characterized on a probing station outfitted with a dark box and video camera. Most devices without screw dislocations exhibited excellent characteristics, with no detectable leakage current prior to breakdown, a sharp breakdown I-V knee, and no visible concentration of breakdown current. In contrast devices that contained at least one elementary screw dislocation exhibited a 5% to 35% reduction in breakdown voltage, a softer breakdown I-V knee, and visible microplasmas in which highly localized breakdown current was concentrated. The locations of observed breakdown microplasmas corresponded exactly to the locations of elementary screw dislocations identified by SWBXT mapping. While not as detrimental to SiC device performance as micropipes, the undesirable breakdown characteristics of elementary screw dislocations could nevertheless adversely affect the performance and reliability of 4H-SiC power devices.