Science.gov

Sample records for dislocation density gan

  1. High-resistance GaN epilayers with low dislocation density via growth mode modification

    NASA Astrophysics Data System (ADS)

    Xu, Z. Y.; Xu, F. J.; Wang, J. M.; Lu, L.; Yang, Z. J.; Wang, X. Q.; Shen, B.

    2016-09-01

    High-resistance GaN with low dislocation density adopting growth mode modification has been investigated by metalorganic chemical vapor deposition. The sheet resistance of the order of 1016 Ω/sq has been achieved at room temperature by diminishing the oxygen impurity level close to the substrate with an AlN blocking layer. Attributed to this method which offers more freedom to tailor the growth mode, a three-dimensional (3D) growth process is introduced by adjusting the growth pressure and temperature at the initial stage of the GaN epitaxy to improve the crystalline quality. The large 3D GaN grains formed during this period roughen the surface, and the following coalescence of the GaN grains causes threading dislocations bending, which finally remarkably reduces the dislocation density.

  2. Core properties and mobility of the basal screw dislocation in wurtzite GaN: a density functional theory study

    NASA Astrophysics Data System (ADS)

    Belabbas, I.; Chen, J.; Heggie, M. I.; Latham, C. D.; Rayson, M. J.; Briddon, P. R.; Nouet, G.

    2016-10-01

    We have performed first principles simulations, based on density functional theory (DFT), to investigate the core properties of the basal a -type screw dislocation in wurtzite gallium nitride. Our calculations demonstrate that the fully coordinated shuffle core configuration is the most energetically favourable. The calculated electronic structure of the a -type screw dislocation was found to exhibit exclusively shallow gap states which are not associated with any extended metallization. This may explain why a -type screw dislocations are less detrimental to the performance of GaN based electronic devices than c -type screw dislocations.

  3. Ultralow threading dislocation density in GaN epilayer on near-strain-free GaN compliant buffer layer and its applications in hetero-epitaxial LEDs.

    PubMed

    Shih, Huan-Yu; Shiojiri, Makoto; Chen, Ching-Hsiang; Yu, Sheng-Fu; Ko, Chung-Ting; Yang, Jer-Ren; Lin, Ray-Ming; Chen, Miin-Jang

    2015-09-02

    High threading dislocation (TD) density in GaN-based devices is a long unresolved problem because of the large lattice mismatch between GaN and the substrate, which causes a major obstacle for the further improvement of next-generation high-efficiency solid-state lighting and high-power electronics. Here, we report InGaN/GaN LEDs with ultralow TD density and improved efficiency on a sapphire substrate, on which a near strain-free GaN compliant buffer layer was grown by remote plasma atomic layer deposition. This "compliant" buffer layer is capable of relaxing strain due to the absorption of misfit dislocations in a region within ~10 nm from the interface, leading to a high-quality overlying GaN epilayer with an unusual TD density as low as 2.2 × 10(5) cm(-2). In addition, this GaN compliant buffer layer exhibits excellent uniformity up to a 6" wafer, revealing a promising means to realize large-area GaN hetero-epitaxy for efficient LEDs and high-power transistors.

  4. Ultralow threading dislocation density in GaN epilayer on near-strain-free GaN compliant buffer layer and its applications in hetero-epitaxial LEDs

    PubMed Central

    Shih, Huan-Yu; Shiojiri, Makoto; Chen, Ching-Hsiang; Yu, Sheng-Fu; Ko, Chung-Ting; Yang, Jer-Ren; Lin, Ray-Ming; Chen, Miin-Jang

    2015-01-01

    High threading dislocation (TD) density in GaN-based devices is a long unresolved problem because of the large lattice mismatch between GaN and the substrate, which causes a major obstacle for the further improvement of next-generation high-efficiency solid-state lighting and high-power electronics. Here, we report InGaN/GaN LEDs with ultralow TD density and improved efficiency on a sapphire substrate, on which a near strain-free GaN compliant buffer layer was grown by remote plasma atomic layer deposition. This “compliant” buffer layer is capable of relaxing strain due to the absorption of misfit dislocations in a region within ~10 nm from the interface, leading to a high-quality overlying GaN epilayer with an unusual TD density as low as 2.2 × 105 cm−2. In addition, this GaN compliant buffer layer exhibits excellent uniformity up to a 6” wafer, revealing a promising means to realize large-area GaN hetero-epitaxy for efficient LEDs and high-power transistors. PMID:26329829

  5. 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. PMID:20397703

  6. Low dislocation density InAlN/AlN/GaN heterostructures grown on GaN substrates and the effects on gate leakage characteristics

    NASA Astrophysics Data System (ADS)

    Kotani, Junji; Yamada, Atsushi; Ishiguro, Tetsuro; Tomabechi, Shuichi; Nakamura, Norikazu

    2016-04-01

    This paper reports on the electrical characterization of Ni/Au Schottky diodes fabricated on InAlN high-electron-mobility transistor (HEMT) structures grown on low dislocation density free-standing GaN substrates. InAlN HEMT structures were grown on sapphire and GaN substrates by metal-organic vapor phase epitaxy, and the effects of threading dislocation density on the leakage characteristics of Ni/Au Schottky diodes were investigated. Threading dislocation densities were determined to be 1.8 × 104 cm-2 and 1.2 × 109 cm-2 by the cathodoluminescence measurement for the HEMT structures grown on GaN and sapphire substrates, respectively. Leakage characteristics of Ni/Au Schottky diodes were compared between the two samples, and a reduction of the leakage current of about three to four orders of magnitude was observed in the forward bias region. For the high reverse bias region, however, no significant improvement was confirmed. We believe that the leakage current in the low bias region is governed by a dislocation-related Frenkel-Poole emission, and the leakage current in the high reverse bias region originates from field emission due to the large internal electric field in the InAlN barrier layer. Our results demonstrated that the reduction of dislocation density is effective in reducing leakage current in the low bias region. At the same time, it was also revealed that another approach will be needed, for instance, band modulation by impurity doping and insertion of insulating layers beneath the gate electrodes for a substantial reduction of the gate leakage current.

  7. Dislocation core structures in Si-doped GaN

    SciTech Connect

    Rhode, S. L. Fu, W. Y.; Sahonta, S.-L.; Kappers, M. J.; Humphreys, C. J.; Horton, M. K.; Pennycook, T. J.; Dusane, R. O.; Moram, M. A.

    2015-12-14

    Aberration-corrected scanning transmission electron microscopy was used to investigate the core structures of threading dislocations in plan-view geometry of GaN films with a range of Si-doping levels and dislocation densities ranging between (5 ± 1) × 10{sup 8} and (10 ± 1) × 10{sup 9} cm{sup −2}. All a-type (edge) dislocation core structures in all samples formed 5/7-atom ring core structures, whereas all (a + c)-type (mixed) dislocations formed either double 5/6-atom, dissociated 7/4/8/4/9-atom, or dissociated 7/4/8/4/8/4/9-atom core structures. This shows that Si-doping does not affect threading dislocation core structures in GaN. However, electron beam damage at 300 keV produces 4-atom ring structures for (a + c)-type cores in Si-doped GaN.

  8. 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

    2016-02-01

    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.

  9. Dislocation luminescence in GaN single crystals under nanoindentation.

    PubMed

    Huang, Jun; Xu, Ke; Fan, Ying Min; Wang, Jian Feng; Zhang, Ji Cai; Ren, Guo Qiang

    2014-01-01

    This work presents an experimental study on the dislocation luminescence in GaN by nanoindentation, cathodoluminescence, and Raman. The dislocation luminescence peaking at 3.12 eV exhibits a series of special properties in the cathodoluminescence measurements, and it completely disappears after annealing at 500°C. Raman spectroscopy shows evidence for existence of vacancies in the indented region. A comprehensive investigation encompassing cathodoluminescence, Raman, and annealing experiments allow the assignment of dislocation luminescence to conduction-band-acceptor transition involving Ga vacancies. The nanoscale plasticity of GaN can be better understood by considering the dislocation luminescence mechanism.

  10. Dislocation confinement in the growth of Na flux GaN on metalorganic chemical vapor deposition-GaN

    SciTech Connect

    Takeuchi, S. Asazu, H.; Nakamura, Y.; Sakai, A.; Imanishi, M.; Imade, M.; Mori, Y.

    2015-12-28

    We have demonstrated a GaN growth technique in the Na flux method to confine c-, (a+c)-, and a-type dislocations around the interface between a Na flux GaN crystal and a GaN layer grown by metalorganic chemical vapor deposition (MOCVD) on a (0001) sapphire substrate. Transmission electron microscopy (TEM) clearly revealed detailed interface structures and dislocation behaviors that reduced the density of vertically aligned dislocations threading to the Na flux GaN surface. Submicron-scale voids were formed at the interface above the dislocations with a c component in MOCVD-GaN, while no such voids were formed above the a-type dislocations. The penetration of the dislocations with a c component into Na flux GaN was, in most cases, effectively blocked by the presence of the voids. Although some dislocations with a c component in the MOCVD-GaN penetrated into the Na flux GaN, their propagation direction changed laterally through the voids. On the other hand, the a-type dislocations propagated laterally and collectively near the interface, when these dislocations in the MOCVD-GaN penetrated into the Na flux GaN. These results indicated that the dislocation propagation behavior was highly sensitive to the type of dislocation, but all types of dislocations were confined to within several micrometers region of the Na flux GaN from the interface. The cause of void formation, the role of voids in controlling the dislocation behavior, and the mechanism of lateral and collective dislocation propagation are discussed on the basis of TEM results.

  11. Nanostructural engineering of nitride nucleation layers for GaN substrate dislocation reduction.

    SciTech Connect

    Koleske, Daniel David; Lee, Stephen Roger; Lemp, Thomas Kerr; Coltrin, Michael Elliott; Cross, Karen Charlene; Thaler, Gerald

    2009-07-01

    With no lattice matched substrate available, sapphire continues as the substrate of choice for GaN growth, because of its reasonable cost and the extensive prior experience using it as a substrate for GaN. Surprisingly, the high dislocation density does not appear to limit UV and blue LED light intensity. However, dislocations may limit green LED light intensity and LED lifetime, especially as LEDs are pushed to higher current density for high end solid state lighting sources. To improve the performance for these higher current density LEDs, simple growth-enabled reductions in dislocation density would be highly prized. GaN nucleation layers (NLs) are not commonly thought of as an application of nano-structural engineering; yet, these layers evolve during the growth process to produce self-assembled, nanometer-scale structures. Continued growth on these nuclei ultimately leads to a fully coalesced film, and we show in this research program that their initial density is correlated to the GaN dislocation density. In this 18 month program, we developed MOCVD growth methods to reduce GaN dislocation densities on sapphire from 5 x 10{sup 8} cm{sup -2} using our standard delay recovery growth technique to 1 x 10{sup 8} cm{sup -2} using an ultra-low nucleation density technique. For this research, we firmly established a correlation between the GaN nucleation thickness, the resulting nucleation density after annealing, and dislocation density of full GaN films grown on these nucleation layers. We developed methods to reduce the nuclei density while still maintaining the ability to fully coalesce the GaN films. Ways were sought to improve the GaN nuclei orientation by improving the sapphire surface smoothness by annealing prior to the NL growth. Methods to eliminate the formation of additional nuclei once the majority of GaN nuclei were developed using a silicon nitride treatment prior to the deposition of the nucleation layer. Nucleation layer thickness was determined

  12. Radiation enhanced basal plane dislocation glide in GaN

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

    A movement of basal plane segments of dislocations in GaN films grown by epitaxial lateral overgrowth under low energy electron beam irradiation (LEEBI) was studied by the electron beam induced current (EBIC) method. Only a small fraction of the basal plane dislocation segments were susceptible to irradiation and the movement was limited to relatively short distances. The effect is explained by the radiation enhanced dislocation glide (REDG) in the structure with strong pinning. A dislocation velocity under LEEBI with a beam current lower than 1 nA was estimated as about 10 nm/s. The results assuming the REDG for prismatic plane dislocations were presented.

  13. Electron scattering due to threading edge dislocations in n-type wurtzite GaN

    NASA Astrophysics Data System (ADS)

    You, Jeong Ho; Lu, Jun-Qiang; Johnson, H. T.

    2006-02-01

    The effect of electrically active VGa-ON threading edge dislocations on drift and Hall mobilities in n-type epitaxial wurtzite (WZ) GaN is investigated theoretically. The charge distribution along the dislocation core is first obtained by means of a density-functional theory atomistic calculation; the two N atoms near the missing Ga atom at the dislocation core are found to be electron acceptors. An accurate analytical expression for dislocation electrostatic strength is then derived for the case of up to -2q charge per structural unit of the threading dislocation core. This strength factor is determined by minimizing the total increase of free energy per site of the partially charged dislocation line. Two different models of scattering potentials for charged dislocation lines are then used to determine the dislocation effect on in-plane electron mobility, and closed-form solutions for the dislocation contribution to drift and Hall mobilities are derived for the more accurate potential. By estimating the effects of other scattering mechanisms, the total mobility is then compared with available experimental data. It is found that for free-carrier concentrations higher than 1016 cm-3, reducing dislocation density below ndis=108 cm-2 has little beneficial effect on total mobility for typical WZ GaN samples.

  14. Structure and electronic properties of mixed (a + c) dislocation cores in GaN

    SciTech Connect

    Horton, M. K.; Rhode, S. L.; Moram, M. A.

    2014-08-14

    Classical atomistic models and atomic-resolution scanning transmission electron microscopy studies of GaN films reveal that mixed (a + c)-type dislocations have multiple different core structures, including a dissociated structure consisting of a planar fault on one of the (12{sup ¯}10) planes terminated by two different partial dislocations. Density functional theory calculations show that all cores introduce localized states into the band gap, which affects device performance.

  15. Low dislocation GaN via defect-filtering, self-assembled SiO2-sphere layers.

    SciTech Connect

    Wang, George T.; Li, Qiming

    2009-09-01

    The III-nitride (AlGaInN) materials system forms the foundation for white solid-state lighting, the adoption of which could significantly reduce U.S. energy needs. While the growth of GaN-based devices relies on heteroepitaxy on foreign substrates, the heteroepitaxial layers possess a high density of dislocations due to poor lattice and thermal expansion match. These high dislocation densities have been correlated with reduced internal quantum efficiency and lifetimes for GaN-based LEDs. Here, we demonstrate an inexpensive method for dislocation reduction in GaN grown on sapphire and silicon substrates. This technique, which requires no lithographic patterning, GaN is selectively grown through self-assembled layers of silica microspheres which act to filter out dislocations. Using this method, the threading dislocation density for GaN on sapphire was reduced from 3.3 x 10{sup 9} cm{sup -2} to 4.0 x 10{sup 7} cm{sup -2}, and from the 10{sup 10} cm{sup -2} range to {approx}6.0 x 10{sup 7} cm{sup -2} for GaN on Si(111). This large reduction in dislocation density is attributed to a dislocation blocking and bending by the unique interface between GaN and silica microspheres.

  16. Synchrotron radiation x-ray topography and defect selective etching analysis of threading dislocations in GaN

    SciTech Connect

    Sintonen, Sakari Suihkonen, Sami; Jussila, Henri; Tuomi, Turkka O.; Lipsanen, Harri; Rudziński, Mariusz; Knetzger, Michael; Meissner, Elke; Danilewsky, Andreas

    2014-08-28

    The crystal quality of bulk GaN crystals is continuously improving due to advances in GaN growth techniques. Defect characterization of the GaN substrates by conventional methods is impeded by the very low dislocation density and a large scale defect analysis method is needed. White beam synchrotron radiation x-ray topography (SR-XRT) is a rapid and non-destructive technique for dislocation analysis on a large scale. In this study, the defect structure of an ammonothermal c-plane GaN substrate was recorded using SR-XRT and the image contrast caused by the dislocation induced microstrain was simulated. The simulations and experimental observations agree excellently and the SR-XRT image contrasts of mixed and screw dislocations were determined. Apart from a few exceptions, defect selective etching measurements were shown to correspond one to one with the SR-XRT results.

  17. Movement of basal plane dislocations in GaN during electron beam irradiation

    SciTech Connect

    Yakimov, E. B.; Vergeles, P. S.; Polyakov, A. Y.; Lee, In-Hwan; Pearton, S. J.

    2015-03-30

    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 be moved by irradiation and only until they meet the latter pinning sites.

  18. Role of oxygen at screw dislocations in GaN.

    PubMed

    Arslan, I; Browning, N D

    2003-10-17

    Here we report the first direct atomic scale experimental observations of oxygen segregation to screw dislocations in GaN using correlated techniques in the scanning transmission electron microscope. The amount of oxygen present in each of the three distinct types of screw dislocation core is found to depend on the evolution and structure of the core, and thus gives rise to a varying concentration of localized states in the band gap. Contrary to previous theoretical predictions, the substitution of oxygen for nitrogen is observed to extend over many monolayers for the open core dislocation. PMID:14611410

  19. Screw dislocation-driven epitaxial solution growth of ZnO nanowires seeded by dislocations in GaN substrates.

    PubMed

    Morin, Stephen A; Jin, Song

    2010-09-01

    In the current examples of dislocation-driven nanowire growth, the screw dislocations that propagate one-dimensional growth originate from spontaneously formed highly defective "seed" crystals. Here we intentionally utilize screw dislocations from defect-rich gallium nitride (GaN) thin films to propagate dislocation-driven growth, demonstrating epitaxial growth of zinc oxide (ZnO) nanowires directly from aqueous solution. Atomic force microscopy confirms screw dislocations are present on the native GaN surface and ZnO nanowires grow directly from dislocation etch pits of heavily etched GaN surfaces. Furthermore, transmission electron microscopy confirms the existence of axial dislocations. Eshelby twist in the resulting ZnO nanowires was confirmed using bright-/dark-field imaging and twist contour analysis. These results further confirm the connection between dislocation source and nanowire growth. This may eventually lead to defect engineering strategies for rationally designed catalyst-free dislocation-driven nanowire growth for specific applications.

  20. Reliability of AlGaN/GaN high electron mobility transistors on low dislocation density bulk GaN substrate: Implications of surface step edges

    SciTech Connect

    Killat, N. E-mail: Martin.Kuball@bristol.ac.uk; Montes Bajo, M.; Kuball, M. E-mail: Martin.Kuball@bristol.ac.uk; Paskova, T.; Evans, K. R.; Leach, J.; Li, X.; Özgür, Ü.; Morkoç, H.; Chabak, K. D.; Crespo, A.; Gillespie, J. K.; Fitch, R.; Kossler, M.; Walker, D. E.; Trejo, M.; Via, G. D.; Blevins, J. D.

    2013-11-04

    To enable gaining insight into degradation mechanisms of AlGaN/GaN high electron mobility transistors, devices grown on a low-dislocation-density bulk-GaN substrate were studied. Gate leakage current and electroluminescence (EL) monitoring revealed a progressive appearance of EL spots during off-state stress which signify the generation of gate current leakage paths. Atomic force microscopy evidenced the formation of semiconductor surface pits at the failure location, which corresponds to the interaction region of the gate contact edge and the edges of surface steps.

  1. Characterization of dislocations in GaN layer grown on 4-inch Si(111) with AlGaN/AlN strained layer superlattices

    NASA Astrophysics Data System (ADS)

    Sugawara, Yoshihiro; Ishikawa, Yukari; Watanabe, Arata; Miyoshi, Makoto; Egawa, Takashi

    2016-05-01

    Dislocations in a GaN layer grown on 4-in. Si(111) with AlGaN/AlN strained layer superlattices using a horizontal metal–organic chemical vapor deposition system were characterized by transmission electron microscopy and scanning transmission electron microscopy. Pure screw dislocations were not found in the observed area but mixed and edge dislocations were found. The dislocation density in the GaN layer decreased from the bottom (∼2 × 1010 cm‑2) to the top (∼6 × 109 cm‑2). Some dislocations were inclined from the c-axis, and half-loop dislocations were observed in the GaN layer. Plan-view weak-beam dark-field analysis indicated that the dislocation inclination was caused by climb and glide motions.

  2. Formation of helical dislocations in ammonothermal GaN substrate by heat treatment

    NASA Astrophysics Data System (ADS)

    Horibuchi, Kayo; Yamaguchi, Satoshi; Kimoto, Yasuji; Nishikawa, Koichi; Kachi, Tetsu

    2016-03-01

    GaN substrate produced by the basic ammonothermal method and an epitaxial layer on the substrate was evaluated using synchrotron radiation x-ray topography and transmission electron microscopy. We revealed that the threading dislocations present in the GaN substrate are deformed into helical dislocations and the generation of the voids by heat treatment in the substrate for the first observation in the GaN crystal. These phenomena are formed by the interactions between the dislocations and vacancies. The helical dislocation was formed in the substrate region, and not in the epitaxial layer region. Furthermore, the evaluation of the influence of the dislocations on the leakage current of Schottky barrier diodes fabricated on the epitaxial layer is discussed. The dislocations did not affect the leakage current characteristics of the epitaxial layer. Our results suggest that the deformation of dislocations in the GaN substrate does not adversely affect the epitaxial layer.

  3. 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

    2015-02-01

    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.

  4. Mass transport, faceting and behavior of dislocations in GaN

    SciTech Connect

    Nitta, S.; Kashima, T.; Kariya, M.; Yukawa, Y.; Yamaguchi, S.; Amano, H.; Akasaki, I.

    2000-07-01

    The behavior of threading dislocations during mass transport of GaN was investigated in detail by transmission electron microscopy. Mass transport occurred at the surface. Therefore, growing species are supplied from the in-plane direction. The behavior of threading dislocations was found to be strongly affected by the mass transport process as well as the high crystallographic anisotropy of the surface energy of the facets particular to GaN.

  5. Tracking the subsurface path of dislocations in GaN using scanning tunneling microscopy

    NASA Astrophysics Data System (ADS)

    Weidlich, P. H.; Schnedler, M.; Portz, V.; Eisele, H.; Strauß, U.; Dunin-Borkowski, R. E.; Ebert, Ph.

    2015-07-01

    A methodology for the determination of the subsurface line direction of dislocations using scanning tunneling microscopy (STM) images is presented. The depth of the dislocation core is derived from an analysis of the displacement field measured by STM. The methodology is illustrated for dislocations at GaN( 10 1 ¯ 0 ) cleavage surfaces. It is found that the dislocation line bends toward the surface, changing from predominantly edge-type to more screw-type character, when approaching the intersection point. Simultaneously, the total displacement detectable at the surface increases due to a preferred relaxation towards the surface.

  6. Repulsive interactions between dislocations and overgrown v-shaped defects in epitaxial GaN layers

    NASA Astrophysics Data System (ADS)

    Weidlich, P. H.; Schnedler, M.; Eisele, H.; Dunin-Borkowski, R. E.; Ebert, Ph.

    2013-09-01

    The spatial distribution and the projected line directions of dislocations intersecting a cross-sectional (101¯0) cleavage plane of a GaN(0001) epitaxial layer is mapped using scanning tunneling microscopy. The data is correlated with the spatial positions of v-shaped defects. The dislocations are found to be bent away from the inclined semipolar facets of v-shaped defects, due to a strain-induced repulsive interaction. The dislocation distribution is characterized by agglomerations and intersecting bundles of dislocations with parallel projected line directions, stabilized by many body effects in the repulsive strain interactions.

  7. Modification of dislocation behavior in GaN overgrown on engineered AlN film-on-bulk Si substrate

    NASA Astrophysics Data System (ADS)

    Tungare, Mihir; Weng, Xiaojun; Leathersich, Jeffrey M.; Suvarna, Puneet; Redwing, Joan M.; (Shadi) Shahedipour-Sandvik, F.

    2013-04-01

    The changes that the AlN buffer and Si substrate undergo at each stage of our substrate engineering process, previously shown to lead to a simultaneous and substantial reduction in film crack density and dislocation density in overgrown GaN, are presented. Evidence of ion-implantation assisted grain reorientation for AlN islands coupled with physical isolation from the bulk Si substrate prove to be the dominating driving forces. This is further emphasized with x-ray diffraction analysis that demonstrates a reduction in the in-plane lattice constant of AlN from 3.148 Å to 3.113 Å and a relative change in rotation of AlN islands by 0.135° with regard to the Si substrate after substrate engineering. Misfit dislocations at the AlN-Si interface and disorder that is normally associated with formation of amorphous SiNx at this interface are considered to be two of the major contributors to dislocation nucleation within overgrown GaN. Following our technique, the disappearance of disorder at the AlN-Si interface is observed. Extensive ellipsometry and transmission electron microscopy suggests that larger AlN islands with a smoother surface morphology could further reduce the dislocation density below that previously reported. A 1.2 μm GaN layer deposited on an AlN buffer with larger islands and smoother morphology exhibits a 14× reduction in surface pit density after undergoing the ion-implantation assisted substrate modification technique.

  8. 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.

  9. Depth dependence of defect density and stress in GaN grown on SiC

    SciTech Connect

    Faleev, N.; Temkin, H.; Ahmad, I.; Holtz, M.; Melnik, Yu.

    2005-12-15

    We report high resolution x-ray diffraction studies of the relaxation of elastic strain in GaN grown on SiC(0001). The GaN layers were grown with thickness ranging from 0.29 to 30 {mu}m. High level of residual elastic strain was found in thin (0.29 to 0.73 {mu}m thick) GaN layers. This correlates with low density of threading screw dislocations of 1-2x10{sup 7} cm{sup -2}, observed in a surface layer formed over a defective nucleation layer. Stress was found to be very close to what is expected from thermal expansion mismatch between the GaN and SiC. A model based on generation and diffusion of point defects accounts for these observations.

  10. Dislocation Reduction and Stress Relaxation of GaN and InGaN Multiple Quantum Wells with Improved Performance via Serpentine Channel Patterned Mask.

    PubMed

    Ji, Qingbin; Li, Lei; Zhang, Wei; Wang, Jia; Liu, Peichi; Xie, Yahong; Yan, Tongxing; Yang, Wei; Chen, Weihua; Hu, Xiaodong

    2016-08-24

    The existence of high threading dislocation density (TDD) in GaN-based epilayers is a long unsolved problem, which hinders further applications of defect-sensitive GaN-based devices. Multiple-modulation of epitaxial lateral overgrowth (ELOG) is used to achieve high-quality GaN template on a novel serpentine channel patterned sapphire substrate (SCPSS). The dislocation blocking brought by the serpentine channel patterned mask, coupled with repeated dislocation bending, can reduce the dislocation density to a yet-to-be-optimized level of ∼2 × 10(5) to 2 × 10(6) cm(-2). About 80% area utilization rate of GaN with low TDD and stress relaxation is obtained. The periodical variations of dislocation density, optical properties and residual stress in GaN-based epilayers on SCPSS are analyzed. The quantum efficiency of InGaN/GaN multiple quantum wells (MQWs) on it can be increased by 52% compared with the conventional sapphire substrate. The reduced nonradiative recombination centers, the enhanced carrier localization, and the suppressed quantum confined Stark effect, are the main determinants of improved luminous performance in MQWs on SCPSS. This developed ELOG on serpentine shaped mask needs no interruption and regrowth, which can be a promising candidate for the heteroepitaxy of semipolar/nonpolar GaN and GaAs with high quality.

  11. Dislocation Reduction and Stress Relaxation of GaN and InGaN Multiple Quantum Wells with Improved Performance via Serpentine Channel Patterned Mask.

    PubMed

    Ji, Qingbin; Li, Lei; Zhang, Wei; Wang, Jia; Liu, Peichi; Xie, Yahong; Yan, Tongxing; Yang, Wei; Chen, Weihua; Hu, Xiaodong

    2016-08-24

    The existence of high threading dislocation density (TDD) in GaN-based epilayers is a long unsolved problem, which hinders further applications of defect-sensitive GaN-based devices. Multiple-modulation of epitaxial lateral overgrowth (ELOG) is used to achieve high-quality GaN template on a novel serpentine channel patterned sapphire substrate (SCPSS). The dislocation blocking brought by the serpentine channel patterned mask, coupled with repeated dislocation bending, can reduce the dislocation density to a yet-to-be-optimized level of ∼2 × 10(5) to 2 × 10(6) cm(-2). About 80% area utilization rate of GaN with low TDD and stress relaxation is obtained. The periodical variations of dislocation density, optical properties and residual stress in GaN-based epilayers on SCPSS are analyzed. The quantum efficiency of InGaN/GaN multiple quantum wells (MQWs) on it can be increased by 52% compared with the conventional sapphire substrate. The reduced nonradiative recombination centers, the enhanced carrier localization, and the suppressed quantum confined Stark effect, are the main determinants of improved luminous performance in MQWs on SCPSS. This developed ELOG on serpentine shaped mask needs no interruption and regrowth, which can be a promising candidate for the heteroepitaxy of semipolar/nonpolar GaN and GaAs with high quality. PMID:27484167

  12. Characterization of Stress Relaxation, Dislocations and Crystallographic Tilt Via X-ray Microdiffraction in GaN (0001) Layers Grown by Maskless Pendeo-Epitaxy

    SciTech Connect

    Barabash, Rozaliya; Ice, Gene E; Liu, Wenjun; Einfeldt, S.; Hommel, D.; Roskowski, A. M.; Davis, R. F.

    2005-01-01

    Intrinsic stresses due to lattice mismatch and high densities of threading dislocations and extrinsic stresses resulting from the mismatch in the coefficients of thermal expansion are present in almost all III-Nitride heterostructures. Stress relaxation in the GaN layers occurs in conventional and in pendeo-epitaxial films via the formation of additional misfit dislocations, domain boundaries, elastic strain and wing tilt. Polychromatic X-ray microdiffraction, high resolution monochromatic X-ray diffraction and finite element simulations have been used to determine the distribution of strain, dislocations, sub-boundaries and crystallographic wing tilt in uncoalesced and coalesced GaN layers grown by maskless pendeo-epitaxy. An important parameter was the width-to-height ratio of the etched columns of GaN from which the lateral growth of the wings occurred. The strain and tilt across the stripes increased with the width-to-height ratio. Tilt boundaries formed in the uncoalesced GaN layers at the column/wing interfaces for samples with a large ratio. Sharper tilt boundaries were observed at the interfaces formed by the coalescence of two laterally growing wings. The wings tilted upward during cooling to room temperature for both the uncoalesced and the coalesced GaN layers. It was determined that finite element simulations that account for extrinsic stress relaxation can explain the experimental results for uncoalesced GaN layers. Relaxation of both extrinsic and intrinsic stress components in the coalesced GaN layers contribute to the observed wing tilt and the formation of sub-boundaries.

  13. Characterization of Stress Relaxation, Dislocations and Crystallographic Tilt Via X-ray Microdiffraction in GaN (0001) Layers Grown by Maskless Pendeo-Epitaxy

    SciTech Connect

    Barabash, R.I.; Ice, G.E.; Liu, W.; Einfeldt, S.; Hommel, D.; Roskowski, A.M.; Davis, R.F.

    2010-06-25

    Intrinsic stresses due to lattice mismatch and high densities of threading dislocations and extrinsic stresses resulting from the mismatch in the coefficients of thermal expansion are present in almost all III-Nitride heterostructures. Stress relaxation in the GaN layers occurs in conventional and in pendeo-epitaxial films via the formation of additional misfit dislocations, domain boundaries, elastic strain and wing tilt. Polychromatic X-ray microdiffraction, high resolution monochromatic X-ray diffraction and finite element simulations have been used to determine the distribution of strain, dislocations, sub-boundaries and crystallographic wing tilt in uncoalesced and coalesced GaN layers grown by maskless pendeo-epitaxy. An important parameter was the width-to-height ratio of the etched columns of GaN from which the lateral growth of the wings occurred. The strain and tilt across the stripes increased with the width-to-height ratio. Tilt boundaries formed in the uncoalesced GaN layers at the column/wing interfaces for samples with a large ratio. Sharper tilt boundaries were observed at the interfaces formed by the coalescence of two laterally growing wings. The wings tilted upward during cooling to room temperature for both the uncoalesced and the coalesced GaN layers. It was determined that finite element simulations that account for extrinsic stress relaxation can explain the experimental results for uncoalesced GaN layers. Relaxation of both extrinsic and intrinsic stress components in the coalesced GaN layers contribute to the observed wing tilt and the formation of sub-boundaries.

  14. Method of growing GaN films with a low density of structural defects using an interlayer

    DOEpatents

    Bourret-Courchesne, Edith D.

    2003-01-01

    A dramatic reduction of the dislocation density in GaN was obtained by insertion of a single thin interlayer grown at an intermediate temperature (IT-IL) after the growth of an initial grown at high temperature. A description of the growth process is presented with characterization results aimed at understanding the mechanisms of reduction in dislocation density. A large percentage of the threading dislocations present in the first GaN epilayer are found to bend near the interlayer and do not propagate into the top layer which grows at higher temperature in a lateral growth mode. TEM studies show that the mechanisms of dislocation reduction are similar to those described for the epitaxial lateral overgrowth process, however a notable difference is the absence of coalescence boundaries.

  15. Microstructural properties and dislocation evolution on a GaN grown on patterned sapphire substrate: A transmission electron microscopy study

    NASA Astrophysics Data System (ADS)

    Kim, Y. H.; Ruh, H.; Noh, Y. K.; Kim, M. D.; Oh, J. E.

    2010-03-01

    The microstructural properties of a GaN layer grown on a patterned sapphire substrate (PSS) were studied in detail using transmission electron microscope techniques to determine dislocation and growth behaviors. Regular and uniform recrystallized GaN islands were observed on the protruding pattern. On a flat sapphire surface, the crystallographic orientation relationship of ⟨1¯21¯0⟩GaN on FS//⟨11¯00⟩sapphire and {11¯01}GaN on FS//{12¯13}sapphire existed between the GaN and the substrate. On the other hand, the orientation relationship of ⟨1¯21¯0⟩GaN layer//⟨1¯21¯0⟩GaN island on IS//⟨11¯00⟩sapphire and {11¯01}GaN layer//{0002}GaN island on IS//{12¯13}sapphire was confirmed among the GaN layer, the recrystallized GaN islands on an inclined sapphire surface and the PSS. The flat surface among the protruding patterns began to fill rapidly with GaN. Then, the GaN gradually overgrew the protruding pattern and coalesced near the summit as the growth time increased. The generation of threading dislocations was observed in the vicinity of the coalescence points near the top of the protruding patterns.

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

    PubMed

    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 ∼10(11) 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

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

    PubMed

    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 ∼10(11) 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

  18. Dislocation-density function dynamics - An all-dislocation, full-dynamics approach for modeling intensive dislocation structures

    NASA Astrophysics Data System (ADS)

    Leung, H. S.; Ngan, A. H. W.

    2016-06-01

    It has long been recognized that a successful strategy for computational plasticity will have to bridge across the meso scale in which the interactions of high quantities of dislocations dominate. In this work, a new meso-scale scheme based on the full dynamics of dislocation-density functions is proposed. In this scheme, the evolution of the dislocation-density functions is derived from a coarse-graining procedure which clearly defines the relationship between the discrete-line and density representations of the dislocation microstructure. Full dynamics of the dislocation-density functions are considered based on an "all-dislocation" concept in which statistically stored dislocations are preserved and treated in the same way as geometrically necessary dislocations. Elastic interactions between dislocations in a 3D space are treated in accordance with Mura's formula for eigen stress. Dislocation generation is considered as a consequence of dislocations to maintain their connectivity, and a special scheme is devised for this purpose. The model is applied to simulate a number of intensive microstructures involving discrete dislocation events, including loop expansion and shrinkage under applied and self stress, dipole annihilation, and Orowan looping. The scheme can also handle high densities of dislocations present in extensive microstructures.

  19. The dissociation of the [a + c] dislocation in GaN

    NASA Astrophysics Data System (ADS)

    Hirsch, P. B.; Lozano, J. G.; Rhode, S.; Horton, M. K.; Moram, M. A.; Zhang, S.; Kappers, M. J.; Humphreys, C. J.; Yasuhara, A.; Okunishi, E.; Nellist, P. D.

    2013-09-01

    In 1952, Cottrell proposed that the Lomer dislocation formed by interaction of two glide dislocations in a fcc crystal could transform into an immobile dislocation by dissociation into partial dislocations bounding a stacking fault, causing a block to further slip, a concept important in work hardening. Evidence for this and many other dissociations has been provided by the diffraction contrast technique and in particular by its 'weak beam' variant, which has a resolution limit of about 15 angstroms. Today, with the advent of aberration corrected microscopes it is possible to determine directly the atomic structure of the faults and partial dislocations. In this paper, we describe the results of a study using the High Angle Angular Dark Field technique (HAADF) in an aberration corrected scanning transmission electron microscope (STEM), of the dissociation of the [a + c] dislocations in (0 0 0 1) GaN films grown on sapphire substrates. The dislocations are found to be inclined with respect to the c-axis, but optical sectioning imaging methods allow the structure to be determined and an estimate of the tilt angle to be made. The dislocations are found to be dissociated by climb and glide on the a plane and the STEM results, and structural arguments supported by theoretical calculations suggest that the dissociation reaction is [a + c] = 1/2[a + c] + 1/2[a + c] + fault. The structure of the fault is found to be similar to that identified in 1965 by Drum in AlN but frequently modified by steps due to kinks in the inclined dislocation.

  20. High dislocation density of tin induced by electric current

    SciTech Connect

    Liao, Yi-Han; Liang, Chien-Lung; Lin, Kwang-Lung; Wu, Albert T.

    2015-12-15

    A dislocation density of as high as 10{sup 17} /m{sup 2} in a tin strip, as revealed by high resolution transmission electron microscope, was induced by current stressing at 6.5 x 10{sup 3} A/ cm{sup 2}. The dislocations exist in terms of dislocation line, dislocation loop, and dislocation aggregates. Electron Backscattered Diffraction images reflect that the high dislocation density induced the formation of low deflection angle subgrains, high deflection angle Widmanstätten grains, and recrystallization. The recrystallization gave rise to grain refining.

  1. The Use of Ultrasound to Measure Dislocation Density

    NASA Astrophysics Data System (ADS)

    Barra, Felipe; Espinoza-González, Rodrigo; Fernández, Henry; Lund, Fernando; Maurel, Agnès; Pagneux, Vincent

    2015-08-01

    Dislocations are at the heart of the plastic behavior of materials yet they are very difficult to probe experimentally. Lack of a practical nonintrusive measuring tool for, say, dislocation density, seriously hampers modeling efforts, as there is little guidance from data in the form of quantitative measurements, as opposed to visualizations. Dislocation density can be measured using transmission electron microscopy (TEM) and x-ray diffraction (XRD). TEM can directly show the strain field around dislocations, which allows for the counting of the number of dislocations in a micrograph. This procedure is very laborious and local, since samples have to be very small and thin, and is difficult to apply when dislocation densities are high. XRD relies on the broadening of diffraction peaks induced by the loss of crystalline order induced by the dislocations. This broadening can be very small, and finding the dislocation density involves unknown parameters that have to be fitted with the data. Both methods, but especially TEM, are intrusive, in the sense that samples must be especially treated, mechanically and chemically. A nonintrusive method to measure dislocation density would be desirable. This paper reviews recent developments in the theoretical treatment of the interaction of an elastic wave with dislocations that have led to formulae that relate dislocation density to quantities that can be measured with samples of cm size. Experimental results that use resonant ultrasound spectroscopy supporting this assertion are reported, and the outlook for the development of a practical, nonintrusive, method to measure dislocation density is discussed.

  2. CONDENSED MATTER: STRUCTURE, THERMAL AND MECHANICAL PROPERTIES: Influence of dislocations in the GaN layer on the electrical properties of an AlGaN/GaN heterostructure

    NASA Astrophysics Data System (ADS)

    Gao, Zhi-Yuan; Hao, Yue; Zhang, Jin-Cheng; Li, Pei-Xian; Gu, Wen-Ping

    2009-11-01

    This paper reports on a comparative study of the spatial distributions of the electrical, optical, and structural properties in an AlGaN/GaN heterostructure. Edge dislocation density in the GaN template layer is shown to decrease in the regions of the wafer where the heterostructure sheet resistance increases and the GaN photoluminescence band-edge energy peak shifts to a high wavelength. This phenomenon is found to be attributed to the local compressive strain surrounding edge dislocation, which will generate a local piezoelectric polarization field in the GaN layer in the opposite direction to the piezoelectric polarization field in the AlGaN layer and thus help to increase the two-dimensional electron gas concentration.

  3. X-ray determination of threading dislocation densities in GaN/Al{sub 2}O{sub 3}(0001) films grown by metalorganic vapor phase epitaxy

    SciTech Connect

    Kopp, Viktor S. Kaganer, Vladimir M.; Baidakova, Marina V.; Lundin, Wsevolod V.; Nikolaev, Andrey E.; Verkhovtceva, Elena V.; Yagovkina, Maria A.; Cherkashin, Nikolay

    2014-02-21

    Densities of a- and a+c-type threading dislocations for a series of GaN films grown in different modes by metalorganic vapor phase epitaxy are determined from the x-ray diffraction profiles in skew geometry. The reciprocal space maps are also studied. Theory of x-ray scattering from crystals with dislocations is extended in order to take into account contribution from both threading and misfit dislocations. The broadening of the reciprocal space maps along the surface normal and the rotation of the intensity distribution ellipse is attributed to misfit dislocations at the interface. We find that the presence of a sharp AlN/GaN interface leads to an ordering of misfit dislocations and reduces strain inhomogeneity in GaN films.

  4. 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.

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

    NASA Astrophysics Data System (ADS)

    Liu, W.; Carlin, J.-F.; Grandjean, N.; Deveaud, B.; Jacopin, G.

    2016-07-01

    We investigate the dynamics of donor bound excitons (D°XA) 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°XA 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 of ˜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.

  6. 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.

  7. Self-induced GaN nanowire growth: surface density determination

    NASA Astrophysics Data System (ADS)

    Koryakin, A. A.; Repetun, L.; Sibirev, N. V.; Dubrovskii, V. G.

    2016-08-01

    A new numerical approach for the determination of the GaN nanowire surface density on an AlN/Si substrate as a function of the growth time and gallium flux is presented. Within this approach, the GaN island solid-like coalescence and island-nanowire transition are modeled by the Monte-Carlo method. We show the importance of taking into consideration the island coalescence for explaining that the maximum of GaN island surface density is several times larger than the maximum of GaN nanowire surface density. Also, we find that the nanowire surface density decreases with an increase of the gallium flux.

  8. Reactivity of pyrites and dislocation density

    SciTech Connect

    Pollack, S.S.; Martello, D.V.; Diehl, J.R.; Tamilia, J.V. ); Graham, R.A. )

    1991-01-01

    Highly reactive coal pyrites and unstable museum specimens are easily distinguished from the stable pyrites by the growth of white crystals that cover samples exposed to room atmosphere for short periods of time. Continued exposure to the atmosphere will eventually cause the specimens to fall apart. The term rotten pyrite has been applied to museum specimens that fall apart in this way. SEM studies show that reactive (rotten) pyrites contain between 100 and 10,000 times more dislocations than stable pyrites. Shock-loading of a stable pyrite to 7.5 GPa and 17 GPa increased its reactivity by a factor of two, probably caused by an increase in the number of imperfections. However, shock-loading at 22 GPa decreased the reactivity of pyrite because the imperfections produced at the higher pressure were removed during annealing the sample received at the higher temperature. Although there was a factor of six difference between the most and least reactive shocked MCB (commercial pyrite) samples, shock-loading did not increase the reactivity of the MCB pyrite to that of the Queensland coal pyrite. The results in hand show that while shock-loading produces sufficient imperfections to increase the reactivity of pyrites, there is insufficient data to show that imperfections are the main reason why some coal pyrites are highly reactive. 9 refs., 1 fig., 1 tab.

  9. Single crystal plasticity by modeling dislocation density rate behavior

    SciTech Connect

    Hansen, Benjamin L; Bronkhorst, Curt; Beyerlein, Irene; Cerreta, E. K.; Dennis-Koller, Darcie

    2010-12-23

    The goal of this work is to formulate a constitutive model for the deformation of metals over a wide range of strain rates. Damage and failure of materials frequently occurs at a variety of deformation rates within the same sample. The present state of the art in single crystal constitutive models relies on thermally-activated models which are believed to become less reliable for problems exceeding strain rates of 10{sup 4} s{sup -1}. This talk presents work in which we extend the applicability of the single crystal model to the strain rate region where dislocation drag is believed to dominate. The elastic model includes effects from volumetric change and pressure sensitive moduli. The plastic model transitions from the low-rate thermally-activated regime to the high-rate drag dominated regime. The direct use of dislocation density as a state parameter gives a measurable physical mechanism to strain hardening. Dislocation densities are separated according to type and given a systematic set of interactions rates adaptable by type. The form of the constitutive model is motivated by previously published dislocation dynamics work which articulated important behaviors unique to high-rate response in fcc systems. The proposed material model incorporates thermal coupling. The hardening model tracks the varying dislocation population with respect to each slip plane and computes the slip resistance based on those values. Comparisons can be made between the responses of single crystals and polycrystals at a variety of strain rates. The material model is fit to copper.

  10. 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.

  11. Core structures analyses of (a+c)-edge dislocations in wurtzite GaN through atomistic simulations and Peierls–Nabarro model

    SciTech Connect

    Chen, Cheng; Meng, Fanchao; Song, Jun

    2015-05-21

    The core structures and slip characteristics of (a+c)-edge dislocations on pyramidal planes in wurtzite GaN were investigated employing molecular dynamics simulations. Multiple stable core configurations are identified for dislocations along the glide and shuffle planes. The corresponding generalized-stacking-fault energy (GSFE) curves for the glide and shuffle slips are calculated. The GSFE curves, combined with the Peierls–Nabarro model, demonstrate that the shuffle slip is favored over the glide slip given the markedly lower Peierls energy and stress of the shuffle slip. Our findings also indicate that in general slip motions for (a+c)-edge dislocations are only possible at elevated temperature, and the necessity of further studies of thermally activated processes to better understand the dynamics of (a+c) dislocations in GaN.

  12. Dislocation

    MedlinePlus

    Joint dislocation ... It may be hard to tell a dislocated joint from a broken bone . Both are emergencies that ... to repair a ligament that tears when the joint is dislocated is needed. Injuries to nerves and ...

  13. Comment on ``The effects of Si doping on dislocation movement and tensile stress in GaN films'' [J. Appl. Phys. 109, 073509 (2011)

    NASA Astrophysics Data System (ADS)

    Dadgar, A.; Krost, A.

    2011-11-01

    In the publication by Moram et al. [J. Appl. Phys. 109, 073509 (2011)], some statements were made which disagree with the measurements presented by the authors of the article. In particular, silicon doping is claimed to suppress dislocation movement in GaN epitaxy hampering stress reduction during growth. We show that the data indeed prove the opposite, in agreement with prior publications.

  14. Edge dislocation induced self-assembly of InGaN nano-flower on GaN by metal organic vapor phase epitaxy

    NASA Astrophysics Data System (ADS)

    Zhao, Wei; Wang, Lai; Wang, Jiaxing; Hao, Zhibiao; Luo, Yi

    2011-07-01

    InGaN nano-flowers are self-assembled on GaN by metal organic vapor phase epitaxy. Scanning electron microscopy and transmission electron microscopy photos show the nano-flower structure is formed through InGaN quantum dots aggregating around the exposure site of the edge dislocation extending to the surface. Calculation on the strain states indicates that the edge dislocation can generate lateral tensile and compressive strain regions on the surface, but the screw dislocation cannot. And the tensile strain regions are corresponding to the shape of the nano-flower. This is attributed to that the tensile GaN lattices on surface are easy to attract adatoms to form InGaN quantum dots.

  15. Identifying dislocations and stacking faults in GaN films by scanning transmission electron microscopy

    NASA Astrophysics Data System (ADS)

    Su, X. J.; Niu, M. T.; Zeng, X. H.; Huang, J.; Zhang, J. C.; Zhang, J. P.; Wang, J. F.; Xu, K.

    2016-08-01

    The application of annular bright field (ABF) and medium-angle annular dark field (MAADF) scanning transmission electron microscopy (STEM) imaging to crystalline defect analysis has been extended to dislocations and stacking faults (SFs). Dislocations and SFs have been imaged under zone-axis and two-beam diffraction conditions. Comparing to conventional two-beam diffraction contrast images, the ABF and MAADF images of dislocations and SFs not only are complementary and symmetrical with their peaks at dislocation core and SFs plane, but also show similar extinction phenomenon. It is demonstrated that conventional TEM rules for diffraction contrast, i.e. g · b and g · R invisibility criteria remain applicable. The contrast mechanism and extinction of dislocation and SFs in ABF and MAADF STEM are illuminated by zero-order Laue zone Kikuchi diffraction.

  16. Density Functional Theory for Green Chemical Catalyst Supported on S-Terminated GaN(0001)

    NASA Astrophysics Data System (ADS)

    Yokoyama, Mami; Tsukamoto, Shiro; Ishii, Akira

    2011-12-01

    A novel function of nitried-based semiconductor is successfully developed for organic synthesis, in which palladium supported on the surface of S-terminated GaN(0001) serves as a unique green chemical catalyst. In this study we determined the structure of Pd-catalyst supported on S-terminated GaN(0001) surface by means of the density functional theory (DFT) within a Local Density Approximation (LDA). The important role of S on the case of GaN substrate is to make the number of the valence electron to be close to 0, it happened same way for GaAs substrate.

  17. 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; Mandadapu, Kranthi

    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

  18. Dislocation density evolution during high pressure torsion of a nanocrystalline Ni-Fe alloy

    SciTech Connect

    Li, Hongqi; Wang, Y B; Ho, J C; Cao, Y; Liao, X Z; Ringer, S P; Zhu, Y T; Zhao, Y H; Lavernia, E J

    2009-01-01

    High-pressure torsion (HPT) induced dislocation density evolution in a nanocrystalline Ni-20wt.%Fe alloy was investigated using X-ray diffraction and transmission electron microscopy. Results suggest that the dislocation density evolution is different from that in coarse-grained materials. An HPT process first reduces the dislocation density within nanocrystalline grains and produces a large number of dislocations located at small-angle sub grain boundaries that are formed via grain rotation and coalescence. Continuing the deformation process eliminates the sub grain boundaries but significantly increases the dislocation density in grains. This phenomenon provides an explanation of the mechanical behavior of some nanostructured materials.

  19. Dislocations

    MedlinePlus

    Dislocations are joint injuries that force the ends of your bones out of position. The cause is often a fall or a ... one, seek medical attention. Treatment depends on which joint you dislocate and the severity of the injury. It might include manipulations to reposition your bones, ...

  20. Fabrication of low-density GaN/AlN quantum dots via GaN thermal decomposition in MOCVD.

    PubMed

    Zhang, Jin; Li, Senlin; Xiong, Hui; Tian, Wu; Li, Yang; Fang, Yanyan; Wu, Zhihao; Dai, Jiangnan; Xu, Jintong; Li, Xiangyang; Chen, Changqing

    2014-01-01

    With an appropriate high anneal temperature under H2 atmosphere, GaN quantum dots (QDs) have been fabricated via GaN thermal decomposition in metal organic chemical vapor deposition (MOCVD). Based on the characterization of atomic force microscopy (AFM), the obtained GaN QDs show good size distribution and have a low density of 2.4 × 10(8) cm(-2). X-ray photoelectron spectroscopy (XPS) analysis demonstrates that the GaN QDs were formed without Ga droplets by thermal decomposition of GaN.

  1. 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.

  2. Threading dislocation-governed degradation in crystal quality of heteroepitaxial materials: The case of InAlN nearly lattice-matched to GaN

    NASA Astrophysics Data System (ADS)

    Chen, Z. T.; Fujita, K.; Ichikawa, J.; Egawa, T.

    2012-03-01

    The crystal qualities of InAlN nearly lattice-matched (LM) to GaN with different thicknesses have been investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that the crystal quality of InAlN starts to degrade when the InAlN thickness exceeds several hundred nanometers, forming a structure consisting of two sub-layers with one sub-layer coherent to GaN and the other being degraded. Moreover, the degradation was found to be governed by the threading dislocations (TDs) propagation from the underlying GaN layer, rather than by the misfit strain between InAlN and GaN. Based on TEM observations, the growth evolution of the two-sub-layer structure is proposed, which is different from those conventional mechanisms of crystal-quality degradation in heteroepitaxial material. The results of InAlN nearly LM to GaN are suggested to be helpful in understanding the growths of lattice-mismatched systems of other In-contained III-nitrides, including InGaN/GaN.

  3. Determination of edge and screw dislocation density in single crystals of high-purity iron

    SciTech Connect

    Park, Y.K.; Waber, J.T.; Snead, C.L. Jr.

    1985-01-01

    The trapping of positrons in dislocation-associated traps has been studied and the density of traps has been demonstrated to be in close agreement with the density of dislocations determined by TEM and etch-pit measurements on the same specimens. The specific trapping rates were determined.

  4. Density-dependent electron transport and precise modeling of GaN high electron mobility transistors

    NASA Astrophysics Data System (ADS)

    Bajaj, Sanyam; Shoron, Omor F.; Park, Pil Sung; Krishnamoorthy, Sriram; Akyol, Fatih; Hung, Ting-Hsiang; Reza, Shahed; Chumbes, Eduardo M.; Khurgin, Jacob; Rajan, Siddharth

    2015-10-01

    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 × 107 cm/s at a low sheet charge density of 7.8 × 1011 cm-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 shows 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.

  5. Density-dependent electron transport and precise modeling of GaN high electron mobility transistors

    SciTech Connect

    Bajaj, Sanyam Shoron, Omor F.; Park, Pil Sung; Krishnamoorthy, Sriram; Akyol, Fatih; Hung, Ting-Hsiang; Reza, Shahed; Chumbes, Eduardo M.; Khurgin, Jacob; Rajan, Siddharth

    2015-10-12

    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 shows 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.

  6. Depth profile characterization technique for electron density in GaN films by infrared reflectance spectroscopy

    NASA Astrophysics Data System (ADS)

    Kamijoh, Takaaki; Ma, Bei; Morita, Ken; Ishitani, Yoshihiro

    2016-05-01

    Infrared reflectance spectroscopy is a noncontact measurement method for carrier density and mobility. In this article, the model determination procedure of layer-type nonuniform electron distribution is investigated, since the spectrum fitting hitherto has been conducted on the basis of a multilayer model defined in advance. A simplified case of a high-electron-density GaN layer embedded in a GaN matrix is mainly studied. The following procedure is found to be applicable. The first step is the determination of the high-density layer position in the vicinity of the surface, in the middle region, or in the vicinity of the interface. This is followed by the specification of the sheet electron density and the layer thickness of the high-density region. It is found that this procedure is also applicable to the characterization of two-dimensional electron gases in the vicinity of AlGaN/GaN heterointerfaces.

  7. 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.

  8. High Efficiency m-plane LEDs on Low Defect Density Bulk GaN Substrates

    SciTech Connect

    David, Aurelien

    2012-10-15

    Solid-state lighting is a key technology for reduction of energy consumption in the US and worldwide. In principle, by replacing standard incandescent bulbs and other light sources with sources based on light-emitting diodes (LEDs), ultimate energy efficiency can be achieved. The efficiency of LEDs has improved tremendously over the past two decades, however further progress is required for solid- state lighting to reach its full potential. The ability of an LED at converting electricity to light is quantified by its internal quantum efficiency (IQE). The material of choice for visible LEDs is Gallium Nitride (GaN), which is at the basis of blue-emitting LEDs. A key factor limiting the performance of GaN LEDs is the so-called efficiency droop, whereby the IQE of the LED decreases significantly at high current density. Despite decades of research, efficiency droop remains a major issue. Since high-current operation is necessary for practical lighting applications, reducing droop is a major challenge for the scientific community and the LED industry. Our approach to solving the droop issue is the use of newly available low-defect-density bulk GaN non-polar substrates. In contrast to the standard foreign substrates (sapphire, silicon carbide, silicon) used in the industry, we have employed native bulk GaN substrates with very low defect density, thus ensuring exquisite material quality and high IQE. Whereas all commercial LEDs are grown along the c-plane crystal direction of GaN, we have used m-plane non-polar substrates; these drastically modify the physical properties of the LED and enable a reduction of droop. With this approach, we have demonstrated very high IQE performance and low droop. Our results focused on violet and blue LEDs. For these, we have demonstrated very high peak IQEs and current droops of 6% and 10% respectively (up to a high current density of 200A.cm-2). All these results were obtained under electrical operation. These high IQE and low droop

  9. Regional density functional theory for crystal growth in GaN

    NASA Astrophysics Data System (ADS)

    Nakamura, Koichi; Hayashi, Tensei; Tachibana, Akitomo; Matsumoto, Koh

    2000-12-01

    We have applied the regional density functional theory to the study of the electronic characteristics in the Ga and N adsorption process of crystal growth in gallium nitride (GaN) with the surface orientations of (0 0 0 1) and (0 0 0 1¯) . Potential energy curves along the Ga and N adsorption process were obtained for some adsorption sites by the first-principle density functional calculation using the periodic boundary model. The Ga-chemical potentials for the GaN(0 0 0 1) and GaN (0 0 0 1¯) surfaces have been derived, and we have observed the clear dependency of crystal growth in GaN on the surface orientation. Furthermore, we have calculated the regional electronic numbers and energies for the regions partitioned by the interface. The regional electronic energies are stabilized in consequence of the electron transfer as the adatom approaches the surface. It is concluded that an electron accumulation in the vicinity of the stable position of the adatom plays an important role in crystal growth.

  10. The measurement of threading dislocation densities in semiconductor crystals by X-ray diffraction

    NASA Astrophysics Data System (ADS)

    Ayers, J. E.

    1994-01-01

    The measurement of threading dislocation densities in heteroepitaxial semiconductor layers is important for the development of injection lasers, microwave transistors, and also the integration of devices in disimilar semiconductors. Dislocation density measurements have been made by destructive techniques such as etching and by transmission electron microscopy (TEM). However, X-ray rocking curves provide non-destructive measurements of dislocation densities with accuracy equal to crystallographic etches or TEM. The theory of this technique has been described by Gay, Hirsch, and Kelly [P. Gay, P.B. Hirsch, and A. Kelly, Acta Met. 1 (1953) 315] and Hordon and Averbach [M.J. Hordon and B.L. Averbach, Acta Met. 9 (1961) 237], for the case of highly dislocated metal crystals. In this paper, the theory of dislocation density measurement from rocking curves is extended to the case of (001) zinc-blende semiconductors. It is shown that the measurement of several ( hkl) rocking curve widths with a particular X-ray wavelength allows the calculation of the dislocation density by two independent techniques, thus allowing for a check of self-consistency. It is shown that for the case of epitaxial GaAs on Si(001), dislocation densities determined by these two methods are in good agreement.

  11. Growth of low-defect-density nonpolar a-plane GaN on r-plane sapphire using pulse NH3 interrupted etching.

    PubMed

    Son, Ji-Su; Honda, Yoshio; Amano, Hiroshi

    2014-02-10

    Nonpolar a-plane (11-20) GaN (a-GaN) layers with low overall defect density and high crystalline quality were grown on r-plane sapphire substrates using etched a-GaN. The a-GaN layer was etched by pulse NH3 interrupted etching. Subsequently, a 2-µm-thick Si-doped a-GaN layer was regrown on the etched a-GaN layer. A fully coalescent n-type a-GaN layer with a low threading dislocation density (~7.5 × 10(8) cm(-2)) and a low basal stacking fault density (~1.8 × 10(5) cm(-1)) was obtained. Compared with a planar sample, the full width at half maximum of the (11-20) X-ray rocking curve was significantly decreased to 518 arcsec along the c-axis direction and 562 arcsec along the m-axis direction.

  12. GaN-Ready Aluminum Nitride Substrates for Cost-Effective, Very Low Dislocation Density III-Nitride LED's

    SciTech Connect

    Sandra Schujman; Leo Schowalter

    2010-10-15

    The objective of this project was to develop and then demonstrate the efficacy of a costeffective approach for a low defect density substrate on which AlInGaN LEDs can be fabricated. The efficacy of this “GaN-ready” substrate would then be tested by growing high efficiency, long lifetime InxGa1-xN blue LEDs. The approach used to meet the project objectives was to start with low dislocation density AlN single-crystal substrates and grow graded AlxGa1-xN layers on top. Pseudomorphic AlxGa1-xN epitaxial layers grown on bulk AlN substrates were used to fabricate light emitting diodes and demonstrate better device performance as a result of the low defect density in these layers when benched marked against state-of-the-art LEDs fabricated on sapphire substrates. The pseudomorphic LEDs showed excellent output powers compared to similar wavelength devices grown on sapphire substrates, with lifetimes exceeding 10,000 hours (which was the longest time that could reliably be estimated). In addition, high internal quantum efficiencies were demonstrated at high driving current densities even though the external quantum efficiencies were low due to poor photon extraction. Unfortunately, these pseudomorphic LEDs require high Al content so they emit in the ultraviolet. Sapphire based LEDs typically have threading dislocation densities (TDD) > 108 cm-2 while the pseudomorphic LEDs have TDD ≤ 105 cm-2. The resulting TDD, when grading the AlxGa1-xN layer all the way to pure GaN to produce a “GaN-ready” substrate, has varied between the mid 108 down to the 106 cm-2. These inconsistencies are not well understood. Finally, an approach to improve the LED structures on AlN substrates for light extraction efficiency was developed by thinning and roughening the substrate.

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

    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-V measurements that yield mobility values as high as ≈4453 cm2/V s and the carrier concentration of ≈1.1 × 1017 cm-3, which are unprecedented in the literature for comparable InN NR diameters.

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

    SciTech Connect

    Miyajima, Yoji; Okubo, Satoshi; Abe, Hiroki; Okumura, Hiroki; Fujii, Toshiyuki; Onaka, Susumu; Kato, Masaharu

    2015-06-15

    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.

  15. Geometrically necessary dislocation densities in olivine obtained using high-angular resolution electron backscatter diffraction.

    PubMed

    Wallis, David; Hansen, Lars N; Ben Britton, T; Wilkinson, Angus J

    2016-09-01

    Dislocations in geological minerals are fundamental to the creep processes that control large-scale geodynamic phenomena. However, techniques to quantify their densities, distributions, and types over critical subgrain to polycrystal length scales are limited. The recent advent of high-angular resolution electron backscatter diffraction (HR-EBSD), based on diffraction pattern cross-correlation, offers a powerful new approach that has been utilised to analyse dislocation densities in the materials sciences. In particular, HR-EBSD yields significantly better angular resolution (<0.01°) than conventional EBSD (~0.5°), allowing very low dislocation densities to be analysed. We develop the application of HR-EBSD to olivine, the dominant mineral in Earth's upper mantle by testing (1) different inversion methods for estimating geometrically necessary dislocation (GND) densities, (2) the sensitivity of the method under a range of data acquisition settings, and (3) the ability of the technique to resolve a variety of olivine dislocation structures. The relatively low crystal symmetry (orthorhombic) and few slip systems in olivine result in well constrained GND density estimates. The GND density noise floor is inversely proportional to map step size, such that datasets can be optimised for analysing either short wavelength, high density structures (e.g. subgrain boundaries) or long wavelength, low amplitude orientation gradients. Comparison to conventional images of decorated dislocations demonstrates that HR-EBSD can characterise the dislocation distribution and reveal additional structure not captured by the decoration technique. HR-EBSD therefore provides a highly effective method for analysing dislocations in olivine and determining their role in accommodating macroscopic deformation. PMID:27337604

  16. Stochastically forced dislocation density distribution in plastic deformation.

    PubMed

    Chattopadhyay, Amit K; Aifantis, Elias C

    2016-08-01

    The dynamical evolution of dislocations in plastically deformed metals is controlled by both deterministic factors arising out of applied loads and stochastic effects appearing due to fluctuations of internal stress. Such types of stochastic dislocation processes and the associated spatially inhomogeneous modes lead to randomness in the observed deformation structure. Previous studies have analyzed the role of randomness in such textural evolution, but none of these models have considered the impact of a finite decay time (all previous models assumed instantaneous relaxation which is "unphysical") of the stochastic perturbations in the overall dynamics of the system. The present article bridges this knowledge gap by introducing a colored noise in the form of an Ornstein-Uhlenbeck noise in the analysis of a class of linear and nonlinear Wiener and Ornstein-Uhlenbeck processes that these structural dislocation dynamics could be mapped on to. Based on an analysis of the relevant Fokker-Planck model, our results show that linear Wiener processes remain unaffected by the second time scale in the problem, but all nonlinear processes, both the Wiener type and Ornstein-Uhlenbeck type, scale as a function of the noise decay time τ. The results are expected to ramify existing experimental observations and inspire new numerical and laboratory tests to gain further insight into the competition between deterministic and random effects in modeling plastically deformed samples. PMID:27627278

  17. Stochastically forced dislocation density distribution in plastic deformation

    NASA Astrophysics Data System (ADS)

    Chattopadhyay, Amit K.; Aifantis, Elias C.

    2016-08-01

    The dynamical evolution of dislocations in plastically deformed metals is controlled by both deterministic factors arising out of applied loads and stochastic effects appearing due to fluctuations of internal stress. Such types of stochastic dislocation processes and the associated spatially inhomogeneous modes lead to randomness in the observed deformation structure. Previous studies have analyzed the role of randomness in such textural evolution, but none of these models have considered the impact of a finite decay time (all previous models assumed instantaneous relaxation which is "unphysical") of the stochastic perturbations in the overall dynamics of the system. The present article bridges this knowledge gap by introducing a colored noise in the form of an Ornstein-Uhlenbeck noise in the analysis of a class of linear and nonlinear Wiener and Ornstein-Uhlenbeck processes that these structural dislocation dynamics could be mapped on to. Based on an analysis of the relevant Fokker-Planck model, our results show that linear Wiener processes remain unaffected by the second time scale in the problem, but all nonlinear processes, both the Wiener type and Ornstein-Uhlenbeck type, scale as a function of the noise decay time τ . The results are expected to ramify existing experimental observations and inspire new numerical and laboratory tests to gain further insight into the competition between deterministic and random effects in modeling plastically deformed samples.

  18. High voltage and high current density vertical GaN power diodes

    DOE PAGES

    Fischer, A. J.; Dickerson, J. R.; Armstrong, A. M.; Moseley, M. W.; Crawford, M. H.; King, M. P.; Allerman, A. A.; Kaplar, R. J.; van Heukelom, M. S.; Wierer, J. J.

    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.cm2. Diodes achieved a forward current of 1 A for on-wafer, DC measurements, corresponding to a current density > 1.4 kA/cm2. 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.

  19. High voltage and high current density vertical GaN power diodes

    SciTech Connect

    Fischer, A. J.; Dickerson, J. R.; Armstrong, A. M.; Moseley, M. W.; Crawford, M. H.; King, M. P.; Allerman, A. A.; Kaplar, R. J.; van Heukelom, M. S.; Wierer, J. J.

    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.cm2. Diodes achieved a forward current of 1 A for on-wafer, DC measurements, corresponding to a current density > 1.4 kA/cm2. 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.

  20. Influence of the Dislocation Density on the Performance of Heteroepitaxial Indium Phosphide Solar Cells

    NASA Technical Reports Server (NTRS)

    Jain, R. K.; Flood, D. J.

    1991-01-01

    Calculations are made to study the dependence of heteroepitaxial InP solar-cell efficiency on dislocation density. Effects of surface recombination velocity and cell emitter thickness are considered. Calculated results are compared with the available experimental results on representative InP solar cells. It is shown that heteroepitaxial InP cells with over 20 percent AM0 efficiency could be fabricated if dislocations are reduced to less than 100,000/sq cm.

  1. 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

    SciTech Connect

    Schmidt, Gordon Berger, Christoph; Veit, Peter; Metzner, Sebastian; Bertram, Frank; Bläsing, Jürgen; Dadgar, Armin; Strittmatter, André; Christen, Jürgen; Callsen, Gordon; Kalinowski, Stefan; Hoffmann, Axel

    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 at zero time delay.

  2. Dislocation Density-Based Constitutive Model for the Mechanical Behavior of Irradiated Cu

    SciTech Connect

    Arsenlis, A; Wirth, B D; Rhee, M

    2003-04-10

    Performance degradation of structural steels in nuclear environments results from the development of a high number density of nanometer scale defects. The defects observed in copper-based alloys are composed of vacancy clusters in the form of stacking fault tetrahedra and/or prismatic dislocation loops, which impede dislocation glide and are evidenced in macroscopic uniaxial stress-strain curves as increased yield strengths, decreased total strain to failure, decreased work hardening and the appearance of a distinct upper yield point above a critical defect concentration (neutron dose). In this paper, we describe the development of an internal state variable model for the mechanical behavior of materials subject to these environments. This model has been developed within an information-passing multiscale materials modeling framework, in which molecular dynamics simulations of dislocation--radiation defect interactions, inform the final coarse-grained continuum model. The plasticity model includes mechanisms for dislocation density growth and multiplication and for radiation defect density evolution with dislocation interaction. The general behavior of the constitutive (single material point) model shows that as the defect density increases, the initial yield point increases and the initial strain hardening decreases. The final coarse-grained model is implemented into a finite element framework and used to simulate the behavior of tensile specimens with varying levels of irradiation induced material damage. The simulation results compare favorably with the experimentally observed mechanical properties of irradiated materials in terms of their increased strength, decreased hardening, and decreased ductility with increasing irradiation dose.

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

    SciTech Connect

    Medvedev, O. S. 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 stacking fault in a split-dislocation core are considered to be the most probable mechanism of the observed phenomenon.

  4. High-resolution three-dimensional imaging of dislocations.

    PubMed

    Barnard, J S; Sharp, J; Tong, J R; Midgley, P A

    2006-07-21

    Dislocations and their interactions govern the properties of many materials, ranging from work hardening in metals to device pathology in semiconductor laser diodes. However, conventional electron micrographs are simply two-dimensional projections of three-dimensional (3D) structures, and even stereo microscopy cannot reveal the true 3D complexity of defect structures. Here, we describe an electron tomographic method that yields 3D reconstructions of dislocation networks with a spatial resolution three orders of magnitude better than previous work. We illustrate the method's success with a study of dislocations in a GaN epilayer, where dislocation densities of 1010 per square centimeter are common.

  5. Electron spin relaxation dynamics in GaN: influence of temperature, doping density, and crystal orientation

    NASA Astrophysics Data System (ADS)

    Buß, J. H.; Rudolph, J.; Starosielec, S.; Schaefer, A.; Semond, F.; Hägele, D.

    2013-03-01

    We present a systematic study of electron spin relaxation in wurtzite GaN. Fast relaxation is caused by a Rashba effective magnetic field that linearly depends on the electron momentum k. The field prevents spin lifetimes to exceed 50 ps at room temperature and is the origin of an anisotropic spin relaxation tensor that we evidence by magnetic field dependent magneto-optical pump-probe measurements. In addition, the spin lifetime depends - as compared to GaAs - weaker on temperature and doping density. We give a fully analytical description of both effects based on D'yakonov-Perel' theory that describes our results quantitatively without any fitting parameter.

  6. Nanoscale lateral epitaxial overgrowth of GaN on Si (111)

    SciTech Connect

    Zang, K.Y.; Wang, Y.D.; Chua, S.J.; Wang, L.S.

    2005-11-07

    We demonstrate that GaN can selectively grow by metalorganic chemical vapor deposition into the pores and laterally over the nanoscale patterned SiO{sub 2} mask on a template of GaN/AlN/Si. The nanoporous SiO{sub 2} on GaN surface with pore diameter of approximately 65 nm and pore spacing of 110 nm was created by inductively coupled plasma etching using anodic aluminum oxide template as a mask. Cross-section transmission electron microscopy shows that the threading-dislocation density was largely reduced in this nanoepitaxial lateral overgrowth region. Dislocations parallel to the interface are the dominant type of dislocations in the overgrown layer of GaN. A large number of the threading dislocations were filtered by the nanoscale mask, which leads to the dramatic reduction of the threading dislocations during the growth within the nano-openings. More importantly, due to the nanoscale size of the mask area, the very fast coalescence and subsequent lateral overgrowth of GaN force the threading dislocations to bend to the basal plane within the first 50 nm of the film thickness. The structure of overgrown GaN is a truncated hexagonal pyramid which is covered with six {l_brace}1101{r_brace} side facets and (0001) top surface depending on the growth conditions.

  7. Effect of strain rate and dislocation density on the twinning behavior in tantalum

    NASA Astrophysics Data System (ADS)

    Florando, Jeffrey N.; El-Dasher, Bassem S.; Chen, Changqiang; Swift, Damian C.; Barton, Nathan R.; McNaney, James M.; Ramesh, K. T.; Hemker, Kevin J.; Kumar, Mukul

    2016-04-01

    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 103/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 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.

  8. Measuring Depth-dependent Dislocation Densities and Elastic Strains in an Indented Ni-based Superalloy

    SciTech Connect

    Barabash, O.M.; Santella, M.; Barabash, R.I.; Ice, G.E.; Tischler, J.

    2011-12-14

    The indentation-induced elastic-plastic zone in an IN 740 Ni-based superalloy was studied by three-dimensional (3-D) x-ray microdiffraction and electron back scattering diffraction (EBSD). Large lattice reorientations and the formation of geometrically necessary dislocations are observed in the area with a radius of {approx}75 {mu}m. A residual compression zone is found close to the indent edge. An elastic-plastic transition is observed at {approx}20 {mu}m from the indent edge. Depth dependent dislocation densities are determined at different distances from the indent edge.

  9. Use of high-density vitreous substitutes in the removal of posteriorly dislocated lenses or intraocular lenses.

    PubMed

    Liu, K R; Peyman, G A; Chen, M S; Chang, K B

    1991-09-01

    We successfully used high-density vitreous substitutes to remove four dislocated crystalline lenses and three dislocated intraocular lenses (IOLs) from the vitreous cavity. Fluorosilicone oil (300 cs) was used to remove the four lenses and one of the IOLs. Perfluorophenanthrene (Vitreon) was used to remove the other two dislocated IOLs. The dislocated lenses or IOLs were floated by the high-density vitreous substitutes and removed safely without damaging the retina; there were no intraoperative complications. Postoperative residual fluorosilicone oil droplets were found in one case.

  10. Peierls potential of screw dislocations in bcc transition metals: Predictions from density functional theory

    SciTech Connect

    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 results 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.

  11. Charge accumulation at a threading edge dislocation in gallium nitride

    SciTech Connect

    Leung, K.; Wright, A.F.; Stechel, E.B.

    1999-04-01

    We have performed Monte Carlo calculations to determine the charge accumulation on threading edge dislocations in GaN as a function of the dislocation density and background dopant density. Four possible core structures have been examined, each of which produces defect levels in the gap and may therefore act as electron or hole traps. Our results indicate that charge accumulation, and the resulting electrostatic interactions, can change the relative stabilities of the different core structures. Structures having Ga and N vacancies at the dislocation core are predicted to be stable under nitrogen-rich and gallium-rich growth conditions, respectively. Due to dopant depletion at high dislocation density and the multitude of charge states, the line charge exhibits complex crossover behavior as the dopant and dislocation densities vary. {copyright} {ital 1999 American Institute of Physics.}

  12. High breakdown single-crystal GaN p-n diodes by molecular beam epitaxy

    SciTech Connect

    Qi, Meng; Zhao, Yuning; Yan, Xiaodong; Li, Guowang; Verma, Jai; Fay, Patrick; Nomoto, Kazuki; Zhu, Mingda; Hu, Zongyang; Protasenko, Vladimir; Song, Bo; Xing, Huili Grace; Jena, Debdeep; Bader, Samuel

    2015-12-07

    Molecular beam epitaxy grown GaN p-n vertical diodes are demonstrated on single-crystal GaN substrates. A low leakage current <3 nA/cm{sup 2} is obtained with reverse bias voltage up to −20 V. With a 400 nm thick n-drift region, an on-resistance of 0.23 mΩ cm{sup 2} is achieved, with a breakdown voltage corresponding to a peak electric field of ∼3.1 MV/cm in GaN. Single-crystal GaN substrates with very low dislocation densities enable the low leakage current and the high breakdown field in the diodes, showing significant potential for MBE growth to attain near-intrinsic performance when the density of dislocations is low.

  13. High breakdown single-crystal GaN p-n diodes by molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Qi, Meng; Nomoto, Kazuki; Zhu, Mingda; Hu, Zongyang; Zhao, Yuning; Protasenko, Vladimir; Song, Bo; Yan, Xiaodong; Li, Guowang; Verma, Jai; Bader, Samuel; Fay, Patrick; Xing, Huili Grace; Jena, Debdeep

    2015-12-01

    Molecular beam epitaxy grown GaN p-n vertical diodes are demonstrated on single-crystal GaN substrates. A low leakage current <3 nA/cm2 is obtained with reverse bias voltage up to -20 V. With a 400 nm thick n-drift region, an on-resistance of 0.23 mΩ cm2 is achieved, with a breakdown voltage corresponding to a peak electric field of ˜3.1 MV/cm in GaN. Single-crystal GaN substrates with very low dislocation densities enable the low leakage current and the high breakdown field in the diodes, showing significant potential for MBE growth to attain near-intrinsic performance when the density of dislocations is low.

  14. High-quality, 2-inch-diameter m-plane GaN substrates grown by hydride vapor phase epitaxy on acidic ammonothermal seeds

    NASA Astrophysics Data System (ADS)

    Tsukada, Yusuke; Enatsu, Yuuki; Kubo, Shuichi; Ikeda, Hirotaka; Kurihara, Kaori; Matsumoto, Hajime; Nagao, Satoru; Mikawa, Yutaka; Fujito, Kenji

    2016-05-01

    In this paper, we discusse the origin of basal-plane stacking faults (BSFs) generated in the homoepitaxial hydride vapor phase epitaxy (HVPE) growth of m-plane gallium nitride (GaN). We investigated the effects of seed quality, especially dislocation density, on BSF generation during homoepitaxy. The results clearly identify basal-plane dislocation in the seed as a cause of BSF generation. We realized high-quality m-plane GaN substrates with a 2-in. diameter using HVPE on low-dislocation-density m-plane seeds.

  15. 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.

  16. Development of patterned sapphire substrate and the application to the growth of non-polar and semi-polar GaN for light-emitting diodes

    NASA Astrophysics Data System (ADS)

    Tadatomo, Kazuyuki; Okada, Narihito

    2011-03-01

    The light-emitting diodes (LEDs) with high external quantum efficiency (EQE) are usually fabricated on the patterned sapphire substrate (PSS). The PSS reduces the dislocation density in the GaN layer and enhances the light extraction efficiency (LEE) from the LED chip by scattering the light confined in GaN layer attributed to the critical angle between GaN (n=2.4) and sapphire substrate (n=1.7) (or air (n=1.0)). On the other hand, non-polar GaN and semipolar GaN are attracted much attention to eliminate the quantum confined Stark effect (QCSE). Recently, we have developed novel technology to grow non-polar or semi-polar GaN on the PSS with high quality and large diameter by metal-organic vapor phase epitaxy (MOVPE). For example, m-plane GaN grown on a-plane PSS and {112 (see manuscript)} plane GaN grown on r-plane PSS. The growth of c-plane GaN from the c-plane-like sidewall of the r-plane PSS results in {112 (see manuscript)} GaN on the r-plane PSS. The full widths at half maximum of X-ray rocking curves (FWHM-XRC) of the {112(see manuscript)} GaN along the azimuths parallel and perpendicular to the c-direction were 533 and 260 arcsec, respectively. Dislocation density of the GaN was approximately 2×108 cm-2. These non-polar and semi-polar GaN are expected to be suitable for novel GaN substrate or GaN template for LEDs.

  17. A molecular dynamics study of dislocation density generation and plastic relaxation during shock of single crystal Cu

    NASA Astrophysics Data System (ADS)

    Sichani, Mehrdad M.; Spearot, Douglas E.

    2016-07-01

    The molecular dynamics simulation method is used to investigate the dependence of crystal orientation and shock wave strength on dislocation density evolution in single crystal Cu. Four different shock directions <100>, <110>, <111>, and <321> are selected to study the role of crystal orientation on dislocation generation immediately behind the shock front and plastic relaxation as the system reaches the hydrostatic state. Dislocation density evolution is analyzed for particle velocities between the Hugoniot elastic limit ( up H E L ) for each orientation up to a maximum of 1.5 km/s. Generally, dislocation density increases with increasing particle velocity for all shock orientations. Plastic relaxation for shock in the <110>, <111>, and <321> directions is primarily due to a reduction in the Shockley partial dislocation density. In addition, plastic anisotropy between these orientations is less apparent at particle velocities above 1.1 km/s. In contrast, plastic relaxation is limited for shock in the <100> orientation. This is partially due to the emergence of sessile stair-rod dislocations with Burgers vectors of 1/3<100> and 1/6<110>. The nucleation of 1/6<110> dislocations at lower particle velocities is mainly due to the reaction between Shockley partial dislocations and twin boundaries. On the other hand, for the particle velocities above 1.1 km/s, the nucleation of 1/3<100> dislocations is predominantly due to reaction between Shockley partial dislocations at stacking fault intersections. Both mechanisms promote greater dislocation densities after relaxation for shock pressures above 34 GPa compared to the other three shock orientations.

  18. Characterization and density control of GaN nanodots on Si (111) by droplet epitaxy using plasma-assisted molecular beam epitaxy

    PubMed Central

    2014-01-01

    In this report, self-organized GaN nanodots have been grown on Si (111) by droplet epitaxy method, and their density can be controlled from 1.1 × 1010 to 1.1 × 1011 cm-2 by various growth parameters, such as substrate temperatures for Ga droplet formation, the pre-nitridation treatment of Si substrate, the nitridation duration for GaN crystallization, and in situ annealing after GaN formation. Based on the characterization of in situ RHEED, we can observe the surface condition of Si and the formation of GaN nanodots on Si. The surface nitridaiton treatment at 600°C provides a-SiNx layer which makes higher density of GaN nanodots. Crystal GaN nanodots can be observed by the HRTEM. The surface composition of GaN nanodots can be analyzed by SPEM and μ-XPS with a synchrotron x-ray source. We can find GaN nanodots form by droplet epitaxy and then in situ annealing make higher-degree nitridation of GaN nanodots. PMID:25593560

  19. Characterization and density control of GaN nanodots on Si (111) by droplet epitaxy using plasma-assisted molecular beam epitaxy.

    PubMed

    Yu, Ing-Song; Chang, Chun-Pu; Yang, Chung-Pei; Lin, Chun-Ting; Ma, Yuan-Ron; Chen, Chun-Chi

    2014-01-01

    In this report, self-organized GaN nanodots have been grown on Si (111) by droplet epitaxy method, and their density can be controlled from 1.1 × 10(10) to 1.1 × 10(11) cm(-2) by various growth parameters, such as substrate temperatures for Ga droplet formation, the pre-nitridation treatment of Si substrate, the nitridation duration for GaN crystallization, and in situ annealing after GaN formation. Based on the characterization of in situ RHEED, we can observe the surface condition of Si and the formation of GaN nanodots on Si. The surface nitridaiton treatment at 600°C provides a-SiNx layer which makes higher density of GaN nanodots. Crystal GaN nanodots can be observed by the HRTEM. The surface composition of GaN nanodots can be analyzed by SPEM and μ-XPS with a synchrotron x-ray source. We can find GaN nanodots form by droplet epitaxy and then in situ annealing make higher-degree nitridation of GaN nanodots.

  20. GaAsP solar cells on GaP/Si with low threading dislocation density

    NASA Astrophysics Data System (ADS)

    Yaung, Kevin Nay; Vaisman, Michelle; Lang, Jordan; Lee, Minjoo Larry

    2016-07-01

    GaAsP on Si tandem cells represent a promising path towards achieving high efficiency while leveraging the Si solar knowledge base and low-cost infrastructure. However, dislocation densities exceeding 108 cm-2 in GaAsP cells on Si have historically hampered the efficiency of such approaches. Here, we report the achievement of low threading dislocation density values of 4.0-4.6 × 106 cm-2 in GaAsP solar cells on GaP/Si, comparable with more established metamorphic solar cells on GaAs. Our GaAsP solar cells on GaP/Si exhibit high open-circuit voltage and quantum efficiency, allowing them to significantly surpass the power conversion efficiency of previous devices. The results in this work show a realistic path towards dual-junction GaAsP on Si cells with efficiencies exceeding 30%.

  1. Effect of dislocation density on thermal boundary conductance across GaSb/GaAs interfaces

    SciTech Connect

    Hopkins, Patrick E.; Duda, John C.; Clark, Stephen P.; Hains, Christopher P.; Rotter, Thomas J.; Balakrishnan, Ganesh; Phinney, Leslie M.

    2011-04-18

    We report on the thermal boundary conductance across structurally-variant GaSb/GaAs interfaces characterized by different dislocations densities, as well as variably-rough Al/GaSb interfaces. The GaSb/GaAs structures are epitaxially grown using both interfacial misfit (IMF) and non-IMF techniques. We measure the thermal boundary conductance from 100 to 450 K with time-domain thermoreflectance. The thermal boundary conductance across the GaSb/GaAs interfaces decreases with increasing strain dislocation density. We develop a model for interfacial transport at structurally-variant interfaces in which phonon propagation and scattering parallels photon attenuation. We find that this model describes the measured thermal boundary conductances well.

  2. 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.

  3. Surface roughness and dislocation density in InP/InGaAs layers

    NASA Astrophysics Data System (ADS)

    Masson, Denis P.; Laframboise, Sylvain

    2004-12-01

    A subtle roughening of the surface of a buried 60 nm InGaAs epitaxial layer was detected using a combination of sample cleaving, selective chemical etching and Field Emission Scanning Electron Microscopy (FESEM). In our technology, InGaAs is the photo-absorbing layer of Metal Organic Chemical Vapor Deposition (MOCVD) grown layers used in the monolithic integration of active photo detectors and a passive mux/demux. Conventional Photo-Luminescence (PL) and X-Ray Diffraction (XRD) techniques used to monitor and optimize the growth of epitaxial layers did not show this microscopic surface roughness. The appearance of roughness in the InGaAs layer was linked to very large changes in the dislocation density of the layers grown over the rough surface. Increases of up to three orders of magnitude in the Etch Pit Density (EPD from 104 to 107 cm-2) were revealed using a standard Huber Etch. The Huber Etch also showed the preferred formation of "pairs" of dislocations threading out from a common point on the rough InGaAs surface. Changes in growth conditions resulted in the complete elimination of roughness and of excessive dislocation densities

  4. 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.

  5. Step-Free GaN Hexagons Grown by Selective-Area Metalorganic Vapor Phase Epitaxy

    NASA Astrophysics Data System (ADS)

    Akasaka, Tetsuya; Kobayashi, Yasuyuki; Kasu, Makoto

    2009-09-01

    Selective-area metalorganic vapor phase epitaxy of GaN has been investigated using the optimized growth conditions for the layer (Frank-van der Merwe) growth and GaN-template substrates with low dislocation density. The surface of a GaN hexagon with 16-µm diameter has a single wide terrace over almost the whole area (step-free surface), when there are no screw-type dislocations in the finite area. Step-free GaN hexagons grew in the two-dimensional nucleus growth mode and had approximately an eight times lower growth rate than that of a GaN film grown in the step-flow mode under the growth conditions used in this study.

  6. Determination of mass density, dielectric, elastic, and piezoelectric constants of bulk GaN crystal.

    PubMed

    Soluch, Waldemar; Brzozowski, Ernest; Lysakowska, Magdalena; Sadura, Jolanta

    2011-11-01

    Mass density, dielectric, elastic, and piezoelectric constants of bulk GaN crystal were determined. Mass density was obtained from the measured ratio of mass to volume of a cuboid. The dielectric constants were determined from the measured capacitances of an interdigital transducer (IDT) deposited on a Z-cut plate and from a parallel plate capacitor fabricated from this plate. The elastic and piezoelectric constants were determined by comparing the measured and calculated SAW velocities and electromechanical coupling coefficients on the Z- and X-cut plates. The following new constants were obtained: mass density p = 5986 kg/m(3); relative dielectric constants (at constant strain S) ε(S)(11)/ε(0) = 8.6 and ε(S)(11)/ε(0) = 10.5, where ε(0) is a dielectric constant of free space; elastic constants (at constant electric field E) C(E)(11) = 349.7, C(E)(12) = 128.1, C(E)(13) = 129.4, C(E)(33) = 430.3, and C(E)(44) = 96.5 GPa; and piezoelectric constants e(33) = 0.84, e(31) = -0.47, and e(15) = -0.41 C/m(2).

  7. Static Recovery Modeling of Dislocation Density in a Cold Rolled Clad Aluminum Alloy

    NASA Astrophysics Data System (ADS)

    Penlington, Alex

    Clad alloys feature one or more different alloys bonded to the outside of a core alloy, with non-equilibrium, interalloy interfaces. There is limited understanding of the recovery and recrystallization behaviour of cold rolled clad aluminum alloys. In order to optimize the properties of such alloys, new heat treatment processes may be required that differ from what is used for the monolithic alloys. This study examines the recovery behaviour of a cold rolled Novelis Fusion(TM) alloy containing an AA6XXX core with an AA3003 cladding on one side. The bond between alloys appears microscopically discrete and continuous, but has a 30 microm wide chemical gradient. The as-deformed structure at the interalloy region consists of pancaked sub-grains with dislocations at the misorientation boundaries and a lower density organized within the more open interiors. X-ray line broadening was used to extract the dislocation density from the interalloy region and an equivalently deformed AA6XXX following static annealing using a modified Williamson-Hall analysis. This analysis assumed that Gaussian broadening contributions in a pseudo-Voigt function corresponded only to strain from dislocations. The kinetics of the dislocation density evolution to recrystallization were studied isothermally at 2 minute intervals, and isochronally at 175 and 205°C. The data fit the Nes model, in which the interalloy region recovered faster than AA6XXX at 175°C, but was slower at 205°C. This was most likely caused by change in texture and chemistry within this region such as over-aging of AA6XXX . Simulation of a continuous annealing and self homogenization process both with and without pre-recovery indicates a detectable, though small change in the texture and grain size in the interalloy region.

  8. Influence of vicinal sapphire substrate on the properties of N-polar GaN films grown by metal-organic chemical vapor deposition

    SciTech Connect

    Lin, Zhiyu; Zhang, Jincheng Xu, Shengrui; Chen, Zhibin; Yang, Shuangyong; Tian, Kun; Hao, Yue; Su, Xujun; Shi, Xuefang

    2014-08-25

    The influence of vicinal sapphire substrates on the growth of N-polar GaN films by metal-organic chemical vapor deposition is investigated. Smooth GaN films without hexagonal surface feature are obtained on vicinal substrate. Transmission electron microscope results reveal that basal-plane stacking faults are formed in GaN on vicinal substrate, leading to a reduction in threading dislocation density. Furthermore, it has been found that there is a weaker yellow luminescence in GaN on vicinal substrate than that on (0001) substrate, which might be explained by the different trends of the carbon impurity incorporation.

  9. Dislocation density-based finite element method modeling of ultrasonic consolidation

    NASA Astrophysics Data System (ADS)

    Pal, Deepankar

    A dislocation density-based constitutive model has been developed and implemented into a crystal plasticity quasi-static finite element framework. This approach captures the statistical evolution of dislocation structures and grain fragmentation at the bonding interface when sufficient boundary conditions pertaining to the Ultrasonic Consolidation (UC) process are prescribed. The hardening is incorporated using statistically stored and geometrically necessary dislocation densities (SSDs and GNDs), which are dislocation analogs of isotropic and kinematic hardening, respectively. Since the macroscopic global boundary conditions during UC involves cyclic sinosuidal simple shear loading along with constant normal pressure, the cross slip mechanism has been included in the evolution equation for SSDs. The inclusion of cross slip promotes slip irreversibility, dislocation storage, and hence, cyclic hardening during the UC. The GND considers strain-gradient and thus renders the model size-dependent. The model is calibrated using experimental data from published refereed literature for simple shear deformation of single crystalline pure aluminum alloy and uniaxial tension of polycrystalline Aluminum 3003-H18 alloy. The model also incorporates various local and global effects such as (1) friction, (2) thermal softening, (3) acoustic softening, (4) surface texture of the sonotrode and initial mating surfaces, and (6) presence of oxide-scale at the mating surfaces, which further contribute significantly specifically to the grain substructure evolution at the interface and to the anisotropic bulk deformation away from the interface during UC in general. The model results have been predicted for Al-3003 alloy undergoing UC. A good agreement between the experimental and simulated results has been observed for the evolution of linear weld density and anisotropic global strengths macroscopically. Similarly, microscopic observations such as embrittlement due to grain substructure

  10. MOCVD growth of N-polar GaN on on-axis sapphire substrate: Impact of AlN nucleation layer on GaN surface hillock density

    NASA Astrophysics Data System (ADS)

    Marini, Jonathan; Leathersich, Jeffrey; Mahaboob, Isra; Bulmer, John; Newman, Neil; (Shadi) Shahedipour-Sandvik, F.

    2016-05-01

    We report on the impact of growth conditions on surface hillock density of N-polar GaN grown on nominally on-axis (0001) sapphire substrate by metal organic chemical vapor deposition (MOCVD). Large reduction in hillock density was achieved by implementation of an optimized high temperature AlN nucleation layer and use of indium surfactant in GaN overgrowth. A reduction by more than a factor of five in hillock density from 1000 to 170 hillocks/cm-2 was achieved as a result. Crystal quality and surface morphology of the resultant GaN films were characterized by high resolution x-ray diffraction and atomic force microscopy and found to be relatively unaffected by the buffer conditions. It is also shown that the density of smaller surface features is unaffected by AlN buffer conditions.

  11. Highly transparent ammonothermal bulk GaN substrates

    SciTech Connect

    Jiang, WK; Ehrentraut, D; Downey, BC; Kamber, DS; Pakalapati, RT; Do Yoo, H; D'Evelyn, MP

    2014-10-01

    A novel apparatus has been employed to grow ammonothermal (0001) gallium nitride (GaN) with diameters up to 2 in The crystals have been characterized by x-ray diffraction rocking-curve (XRC) analysis, optical and scanning electron microscopy (SEM), cathodoluminescence (CL), and optical spectroscopy. High crystallinity GaN with FWHM values about 20-50 arcsec and dislocation densities below 1 x 10(5) cm(-2) have been obtained. High optical transmission was achieved with an optical absorption coefficient below 1 cm(-1) at a wavelength of 450 nm. (C) 2014 Elsevier B.V. All rights reserved.

  12. High-Density Plasma-Induced Etch Damage of GaN

    SciTech Connect

    Baca, A.G.; Han, J.; Lester, L.F.; Pearton, S.J.; Ren, F.; Shul, R.J.; Willison, C.G.; Zhang, L.; Zolper, J.C.

    1999-04-29

    Anisotropic, smooth etching of the group-III nitrides has been reported at relatively high rates in high-density plasma etch systems. However, such etch results are often obtained under high de-bias andlor high plasma flux conditions where plasma induced damage can be significant. Despite the fact that the group-III nitrides have higher bonding energies than more conventional III-V compounds, plasma-induced etch damage is still a concern. Attempts to minimize such damage by reducing the ion energy or increasing the chemical activity in the plasma often result in a loss of etch rate or anisotropy which significantly limits critical dimensions and reduces the utility of the process for device applications requiring vertical etch profiles. It is therefore necessary to develop plasma etch processes which couple anisotropy for critical dimension and sidewall profile control and high etch rates with low-damage for optimum device performance. In this study we report changes in sheet resistance and contact resistance for n- and p-type GaN samples exposed to an Ar inductively coupled plasma (ICP). In general, plasma-induced damage was more sensitive to ion bombardment energies as compared to plasma flux. In addition, p-GaN was typically more sensitive to plasma-induced damage as compared to n-GaN.

  13. 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; Ramos, Kyle James

    2016-09-20

    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

  14. Selective area growth of high-density GaN nanowire arrays on Si(111) using thin AlN seeding layers

    NASA Astrophysics Data System (ADS)

    Wu, C. H.; Lee, P. Y.; Chen, K. Y.; Tseng, Y. T.; Wang, Y. L.; Cheng, K. Y.

    2016-11-01

    Selective area growth (SAG) of high-density (2.5×109 cm-2) GaN nanowires (NWs) on Si(111) substrate by plasma-assisted molecular beam epitaxy is presented. The effects of morphology and thickness of the AlN seeding layer on the quality of SAG GaN NWs are investigated. A thin AlN seeding layer of 30 nm thick with a surface roughness of less than 0.5 nm is suitable for high quality SAG GaN NWs growth. High-density AlN nanopedestal arrays used as seeds for SAG GaN NWs are fabricated from thin AlN seeding layers using soft nanoimprint lithography. By adjusting the growth temperature and Ga/N flux ratio, hexagonal shaped SAG GaN NWs are realized. The quality of SAG GaN NWs is evaluated by low temperature photoluminescence (PL) measurements. Three major groups of PL peaks at 3.47, 3.45, and 3.41 eV are identified. The peak at 3.471 eV is related to the neutral donor-bound exciton emission, and the 3.41 eV broadband emission is attributed to stacking faults or structural defects. The 3.45 eV peak is identified as the emission due to exciton recombination at polar inversion domain boundaries of NWs.

  15. Amplified spontaneous emission of phonons as a likely mechanism for density-dependent velocity saturation in GaN transistors

    NASA Astrophysics Data System (ADS)

    Khurgin, Jacob B.; Bajaj, Sanyam; Rajan, Siddharth

    2016-09-01

    We show that density-dependent velocity saturation in a GaN high electron mobility transistor (HEMT) can be related to the stimulated emission of longitudinal optical (LO) phonons. As the drift velocity of electrons increases, the drift of the Fermi distribution in reciprocal space results in population inversion and gain for the LO phonons. Once this gain reaches a threshold value, the avalanche-like increase in LO phonon emission causes a rapid loss of electron energy and momentum and leads to drift velocity saturation. Our simple model correctly predicts both the general trend of decreasing saturation velocity with increasing electron density, and the measured experimental values of saturation.

  16. Improved performance of GaN metal-semiconductor-metal ultraviolet detectors by depositing SiO2 nanoparticles on a GaN surface

    NASA Astrophysics Data System (ADS)

    Sun, Xiaojuan; Li, Dabing; Jiang, Hong; Li, Zhiming; Song, Hang; Chen, Yiren; Miao, Guoqing

    2011-03-01

    GaN metal-semiconductor-metal (MSM) ultraviolet detectors were investigated by depositing different density of SiO2 nanoparticles (SNPs) on the GaN. It was shown that the dark current of the detectors with SNPs was more than one order of magnitude lower than that without SNPs and the peak responsivity was enhanced after deposition of the SNPs. Atomic force microscopy observations indicated that the SNPs usually formed at the termination of screw and mixed dislocations, and further current-voltage measurements showed that the leakage of the Schottky contact for the GaN MSM detector decreased with deposited the SNPs. Moreover, the leakage obeyed the Frenkel-Poole emission model, which meant that the mechanism for improving the performance is the SNPs passivation of the dislocations followed by the reduction in the dark current.

  17. Defect Reduction via Selective Lateral Epitaxy of GaN on an Innovative Masked Structure with Serpentine Channels

    NASA Astrophysics Data System (ADS)

    Li, Lei; Liu, Justin P. C.; Liu, Lei; Li, Ding; Wang, Lei; Wan, Chenghao; Chen, Weihua; Yang, Zhijian; Xie, Yahong; Hu, Xiaodong; Zhang, Guoyi

    2012-05-01

    We demonstrated an innovative lateral epitaxy method to grow c-plane GaN film using serpentine masked structures, which simplified the entire fabrication process with only one single epitaxial growth step and could efficiently block the threading dislocations. The microstructural and optical properties of GaN indicated that the crystalline quality was effectively improved. Unlike the conventional epitaxial lateral overgrowth (ELOG) or the double ELOG method, the presented serpentine masked structure needs no regrowth process for obtaining low-defect-density GaN materials, and is promising for growing high-performance III-nitride-based devices including laser diodes (LDs), power transistors, and light-emitting diodes (LEDs).

  18. Growth of high quality GaN layer on carbon nanotube-graphene network structure as intermediate layer

    NASA Astrophysics Data System (ADS)

    Seo, Taeo Hoon; Park, Ah Hyun; Park, Sungchan; Kim, Myung Jong; Suh, Eun-Kyung

    2015-03-01

    In general, high-quality GaN layers are synthesized on low-temperature (LT) GaN buffer layer on a single crystal sapphire substrate. However, large differences in fundamental properties such as lattice constants and thermal expansion coefficients between GaN layer and sapphire substrate generate high density of threading dislocation (TD) that leads to deterioration of optical and structural properties. Graphene has been attracting much attention due to its excellent physical properties However, direct epitaxial growth of GaN film onto graphene layer on substrates is not easily accessible due to the lack of chemical reactivity on graphene which consisted of C-C bond of sp2 hexagonally arranged carbon atoms with no dangling bonds. In this work, an intermediate layer for the GaN growth on sapphire substrate was constructed by inserting carbon nanotubes and graphene hybrid structure (CGH) Optical and structural properties of GaN layer grown on CGH were compared with those of GaN layer directly grown on sapphire CNTs act as nucleation sites and play a crucial role in the growth of single crystal high-quality GaN on graphene layer. Also, graphene film acts as a mask for epitaxial lateral overgrowth of GaN layer, which can effectively reduce TD density. A grant from the Korea Institute of Science and Technology (KIST) institutional program.

  19. Electron mobility limited by scattering from threading dislocation lines within gallium nitride

    NASA Astrophysics Data System (ADS)

    Mohammad Alavi, Seyed; Bagani, Erfan

    2016-03-01

    Theoretical as well as experimental studies in the literature suggest that defect sites associated with the threading dislocation lines within n-type gallium nitride (GaN) act to trap free electrons from the bulk of this semiconductor material. As a result, the core of the threading dislocation lines become negatively charged. The charge accumulated along the core of a threading dislocation line should be screened by a charge of opposite polarity and equal in absolute value per unit length along the dislocation line. In the present work, we model this screened charge buildup along the threading dislocation lines by two concentric space-charge cylinders. Quantum mechanical theory of scattering in cylindrical coordinates is then employed in order to numerically compute the electron mobility limited by scattering from the charged threading dislocation lines. The dependence of the computed electron mobility on the dislocation line density and on the amount of charge accumulated per unit length along the core of the dislocation lines is also investigated in this work. Our computed electron mobility results are compared with results from existing calculations of the GaN dislocation scattering limited electron mobility in the literature.

  20. Reduced threading dislocation densities in high-T/N-rich grown InN films by plasma-assisted molecular beam epitaxy

    SciTech Connect

    Loitsch, Bernhard; Schuster, Fabian; Stutzmann, Martin; Koblmueller, Gregor

    2013-02-04

    We explore the effect of growth kinetics on the structural properties of In-polar InN films on GaN templates grown near the thermal dissociation limit by plasma-assisted molecular beam epitaxy. Unlike the common growth temperature limit (T Almost-Equal-To 500 Degree-Sign C) for In-polar InN grown under In-rich conditions, slightly N-rich conditions are demonstrated to shift the available growth temperature window to much higher temperatures (by >50 Degree-Sign C). InN films grown in this high-T/N-rich regime show significantly reduced off-axis X-ray diffraction rocking curve peak widths and record low threading dislocation densities (TDD {approx} 4 Multiplication-Sign 10{sup 9} cm{sup -2}) even for film thicknesses <1 {mu}m, as compared to state of the art In-rich growth. The reduction of TDD is attributed to more effective TD inclination and annihilation under N-rich growth, delineating prospective routes for improved InN-based materials.

  1. Nanoscale size dependence parameters on lattice thermal conductivity of Wurtzite GaN nanowires

    SciTech Connect

    Mamand, S.M.; Omar, M.S.; Muhammad, A.J.

    2012-05-15

    Graphical abstract: Temperature dependence of calculated lattice thermal conductivity of Wurtzite GaN nanowires. Highlights: Black-Right-Pointing-Pointer A modified Callaway model is used to calculate lattice thermal conductivity of Wurtzite GaN nanowires. Black-Right-Pointing-Pointer A direct method is used to calculate phonon group velocity for these nanowires. Black-Right-Pointing-Pointer 3-Gruneisen parameter, surface roughness, and dislocations are successfully investigated. Black-Right-Pointing-Pointer Dislocation densities are decreases with the decrease of wires diameter. -- Abstract: A detailed calculation of lattice thermal conductivity of freestanding Wurtzite GaN nanowires with diameter ranging from 97 to 160 nm in the temperature range 2-300 K, was performed using a modified Callaway model. Both longitudinal and transverse modes are taken into account explicitly in the model. A method is used to calculate the Debye and phonon group velocities for different nanowire diameters from their related melting points. Effect of Gruneisen parameter, surface roughness, and dislocations as structure dependent parameters are successfully used to correlate the calculated values of lattice thermal conductivity to that of the experimentally measured curves. It was observed that Gruneisen parameter will decrease with decreasing nanowire diameters. Scattering of phonons is assumed to be by nanowire boundaries, imperfections, dislocations, electrons, and other phonons via both normal and Umklapp processes. Phonon confinement and size effects as well as the role of dislocation in limiting thermal conductivity are investigated. At high temperatures and for dislocation densities greater than 10{sup 14} m{sup -2} the lattice thermal conductivity would be limited by dislocation density, but for dislocation densities less than 10{sup 14} m{sup -2}, lattice thermal conductivity would be independent of that.

  2. Change in equilibrium position of misfit dislocations at the GaN/sapphire interface by Si-ion implantation into sapphire—I. Microstructural characterization

    SciTech Connect

    Lee, Sung Bo Han, Heung Nam Lee, Dong Nyung; Ju, Jin-Woo; Kim, Young-Min; Yoo, Seung Jo; Kim, Jin-Gyu

    2015-07-15

    Much research has been done to reduce dislocation densities for the growth of GaN on sapphire, but has paid little attention to the elastic behavior at the GaN/sapphire interface. In this study, we have examined effects of the addition of Si to a sapphire substrate on its elastic property and on the growth of GaN deposit. Si atoms are added to a c-plane sapphire substrate by ion implantation. The ion implantation results in scratches on the surface, and concomitantly, inhomogeneous distribution of Si. The scratch regions contain a higher concentration of Si than other regions of the sapphire substrate surface, high-temperature GaN being poorly grown there. However, high-temperature GaN is normally grown in the other regions. The GaN overlayer in the normally-grown regions is observed to have a lower TD density than the deposit on the bare sapphire substrate (with no Si accommodated). As compared with the film on an untreated, bare sapphire, the cathodoluminescence defect density decreases by 60 % for the GaN layer normally deposited on the Si-ion implanted sapphire. As confirmed by a strain mapping technique by transmission electron microscopy (geometric phase analysis), the addition of Si in the normally deposited regions forms a surface layer in the sapphire elastically more compliant than the GaN overlayer. The results suggest that the layer can largely absorb the misfit strain at the interface, which produces the overlayer with a lower defect density. Our results highlight a direct correlation between threading-dislocation density in GaN deposits and the elastic behavior at the GaN/sapphire interface, opening up a new pathway to reduce threading-dislocation density in GaN deposits.

  3. Nanoheteroepitaxy of GaN on AlN/Si(111) nanorods fabricated by nanosphere lithography

    NASA Astrophysics Data System (ADS)

    Lee, Donghyun; Shin, In-Su; Jin, Lu; Kim, Donghyun; Park, Yongjo; Yoon, Euijoon

    2016-06-01

    Nanoheteroepitaxy (NHE) of GaN on an AlN/Si(111) nanorod structure was investigated by metal-organic chemical vapor deposition. Silica nanosphere lithography was employed to fabricate a periodic hexagonal nanorod array with a narrow gap of 30 nm between the nanorods. We were successful in obtaining a fully coalesced GaN film on the AlN/Si(111) nanorod structure. Transmission electron microscopy revealed that threading dislocation (TD) bending and termination by stacking faults occurred near the interface between GaN and the AlN/Si(111) nanorods, resulting in the reduction of TD density for the NHE GaN layer. The full width at half-maximum of the X-ray rocking curve for (102) plane of the NHE GaN was found to decrease down to 728 arcsec from 1005 arcsec for the GaN layer on a planar AlN/Si(111) substrate, indicating that the crystalline quality of the NHE GaN was improved. Also, micro-Raman measurement showed that tensile stress in the NHE GaN layer was reduced significantly as much as 70% by introducing air voids between the nanorods.

  4. Curvature and bow of bulk GaN substrates

    NASA Astrophysics Data System (ADS)

    Foronda, Humberto M.; Romanov, Alexey E.; Young, Erin C.; Roberston, Christian A.; Beltz, Glenn E.; Speck, James S.

    2016-07-01

    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 substrates 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.

  5. X-ray and neutron diffraction measurements of dislocation density and subgrain size in a friction stir welded aluminum alloy

    SciTech Connect

    Claussen, Bjorn; Woo, Wanchuck; Zhili, Feng; Edward, Kenik; Ungar, Tamas

    2009-01-01

    The dislocation density and subgrain size were determined in the base material and friction-stir welds of 6061-T6 aluminum alloy. High-resolution X-ray diffraction measurement was performed in the base material. The result of the line profile analysis of the X-ray diffraction peak shows that the dislocation density is about 4.5 x 10{sup 14} m{sup 02} and the subgrain size is about 200 nm. Meanwhile, neutron diffraction measurements have been performed to observe the diffraction peaks during friction-stir welding (FSW). The deep penetration capability of the neutron enables us to measure the peaks from the midplane of the Al plate underneath the tool shoulder of the friction-stir welds. The peak broadening analysis result using the Williamson-Hall method shows the dislocation density of about 3.2 x 10{sup 15} m{sup -2} and subgrain size of about 160 nm. The significant increase of the dislocation density is likely due to the severe plastic deformation during FSW. This study provides an insight into understanding the transient behavior of the microstructure under severe thermomechanical deformation.

  6. Patterning of GaN in high-density Cl{sub 2}- and BCl{sub 3}-based plasmas

    SciTech Connect

    Shul, R.J.; Briggs, R.D.; Han, J.; Pearton, S.J.; Lee, J.W.; Vartuli, C.B.; Killeen, K.P.; Ludowise, M.J.

    1997-05-01

    Fabrication of group-III nitride electronic and photonic devices relies heavily on the ability to pattern features with anisotropic profiles, smooth surface morphologies, etch rates often exceeding 1 {micro}m/min, and a low degree of plasma-induced damage. Patterning these materials has been especially difficult due to their high bond energies and their relatively inert chemical nature as compared to other compound semiconductors. However, high-density plasma etching has been an effective patterning technique due to ion fluxes which are 2 to 4 orders of magnitude higher than conventional RIE systems. GaN etch rates as high as {approximately}1.3 {micro}m/min have been reported in ECR generated ICl plasmas at {minus}150 V dc-bias. In this study, the authors report high-density GaN etch results for ECR- and ICP-generated plasmas as a function of Cl{sub 2}- and BCl{sub 3}-based plasma chemistries.

  7. Polarity control of GaN grown on pulsed-laser-deposited AlN/GaN template by metalorganic vapor phase epitaxy

    NASA Astrophysics Data System (ADS)

    Yoo, Jinyeop; Shojiki, Kanako; Tanikawa, Tomoyuki; Kuboya, Shigeyuki; Hanada, Takashi; Katayama, Ryuji; Matsuoka, Takashi

    2016-05-01

    We report on the polarity control of GaN regrown on pulsed-laser-deposition-grown N-polar AlN on a metalorganic-vapor-phase-epitaxy-grown Ga-polar GaN template. The polarity of the regrown GaN, which was confirmed using aqueous KOH solutions, can be inverted from that of AlN by inserting a low-temperature GaN (LT-GaN) buffer layer. We hypothetically ascribe the Ga-polarity selection of GaN on the LT-GaN buffer layer to the mixed polarity of LT-GaN grains and higher growth rate of the Ga-polar grain, which covers up the N-polar grain during the initial stage of the high-temperature growth. The X-ray rocking curve analysis revealed that the edge-dislocation density in the N-polar regrown GaN is 5 to 8 times smaller than that in the Ga-polar regrown GaN. N-polar GaN grows directly on N-polar AlN at higher temperatures. Therefore, nucleus islands grow larger than those of LT-GaN and the area fraction of coalescence boundaries between islands, where edge dislocations emerge, becomes smaller.

  8. Characterization of 100 mm Diameter 4H-Silicon Carbide CrystalsWith Extremely Low Basal Plane Dislocation Density

    SciTech Connect

    M Dudley; N Zhang; Y Zhang; B Raghothamachar; S Byrappa; G Choi; E Drachev; M Loboda

    2011-12-31

    Synchrotron White Beam X-ray Topography (SWBXT) studies are presented of basal plane dislocation (BPD) configurations and behavior in a new generation of 100mm diameter, 4H-SiC wafers with extremely low BPD densities (3-4 x 10{sup 2} cm{sup -2}). The conversion of non-screw oriented, glissile BPDs into sessile threading edge dislocations (TEDs) is observed to provide pinning points for the operation of single ended Frank-Read sources. In some regions, once converted TEDs are observed to re-convert back into BPDs in a repetitive process which provides multiple BPD pinning points.

  9. Gamma Prime Precipitation, Dislocation Densities, and TiN in Creep-Exposed Inconel 617 Alloy

    NASA Astrophysics Data System (ADS)

    Krishna, Ram; Atkinson, Helen V.; Hainsworth, Sarah V.; Gill, Simon P.

    2016-01-01

    Inconel 617 is a solid-solution-strengthened Ni-based superalloy with a small amount of gamma prime (γ') present. Here, samples are examined in the as-received condition and after creep exposure at 923 K (650 °C) for 574 hours and 45,000 hours and at 973 K (700 °C) for 4000 hours. The stress levels are intermediate (estimated, respectively, as of the order of 350, 275, and 200 MPa) and at levels of interest for the future operation of power plant. The hardness of the specimens has been measured in the gage length and the head. TEM thin foils have been obtained to quantify dislocation densities (3.5 × 1013 for the as-received, 5.0 × 1014, 5.9 × 1014, and 3.5 × 1014 lines/m2 for the creep-exposed specimens, respectively). There are no previous data in the literature for dislocation densities in this alloy after creep exposure. There is some evidence from the dislocation densities that for the creep-exposed samples, the higher hardness in the gage length in comparison with the creep test specimen head is due to work hardening rather than any other effect. Carbon replicas have been used to extract gamma prime precipitates. The morphology of γ' precipitates in the `as-received' condition was spheroidal with an average diameter of 18 nm. The morphology of these particles does not change with creep exposure but the size increases to 30 nm after 574 hours at 923 K (650 °C) but with little coarsening in 45,000 hours. At 973 K (700 °C) 4000 hours, the average gamma prime size is 32 nm. In the TEM images of the replicas, the particles overlap, and therefore, a methodology has been developed to estimate the volume fraction of gamma prime in the alloy given the carbon replica film thickness. The results are 5.8 vol pct in the as-received and then 2.9, 3.2, and 3.4 vol pct, respectively, for the creep-exposed specimens. The results are compared with predictions from thermodynamic analysis given the alloy compositions. Thermodynamic prediction shows that nitrogen

  10. 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

    SciTech Connect

    Dinara, Syed Mukulika Jana, Sanjay Kr.; Mukhopadhyay, Partha; Ghosh, Saptarsi; Bhattacharya, Sekhar; Biswas, Dhrubes

    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 and 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.

  11. 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.

  12. Micro-strain, dislocation density and surface chemical state analysis of multication thin films

    NASA Astrophysics Data System (ADS)

    Jayaram, P.; Pradyumnan, P. P.; Karazhanov, S. Zh.

    2016-11-01

    Multication complex metal oxide thin films are rapidly expanding the class of materials with many technologically important applications. Herein this work, the surface of the pulsed laser deposited thin films of Zn2SnO4 and multinary compounds obtained by substitution/co-substitution of Sn4+ with In3+ and Ga3+ are studied by X-ray photoelectron emission spectroscopy (X-PES) method. Peaks corresponding to the elements of Zn, Sn, Ga, In and O on the film surface has been identified and contribution of the elements has been studied by the computer aided surface analysis (CASA) software. Binding energies, full-width at half maximum (FWHM), spin-orbit splitting energies, asymmetric peak-shape fitting parameters and quantification of elements in the films are discussed. Studies of structural properties of the films by x-ray diffraction (XRD) technique showed inverse spinel type lattice with preferential orientation. Micro-strain, dislocation density and crystallite sizes in the film surface have been estimated.

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

    NASA Astrophysics Data System (ADS)

    Tendille, Florian; Martin, Denis; Vennéguès, Philippe; Grandjean, Nicolas; De Mierry, Philippe

    2016-08-01

    Semipolar GaN crystal stripes larger than 100 μm with dislocation densities below 5 × 106 cm-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 allow for even more efficient III-nitride based devices.

  14. Dislocation density analyses of multi-crystalline silicon during the directional solidification process with bottom grooved furnace

    NASA Astrophysics Data System (ADS)

    Karuppasamy, P.; Srinivasan, M.; Aravinth, K.; Ramasamy, P.

    2016-05-01

    A transient global model was used to investigate the effect of bottom grooved furnace upon the directional solidification (DS) process of multicrystalline silicon (mc-Si). The computations were carried out on a 2D axisymmetric model using the finite volume method. The temperature distribution, crystal-melt (c-m) interface and dislocation density were simulated. The modified heat exchanger block system was used for controlling the temperature gradient at the bottom of the crucible. The obtained results shows convex shape of the c-m interface. The dislocation density was reduced while using the bottom grooved furnace. This work was carried out for the different groove of radius 30 and 60 mm of the heat exchanger block.

  15. 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.

  16. Structural properties of maskless epitaxial lateral overgrown MOCVD GaN layers on Si (1 1 1) substrates

    NASA Astrophysics Data System (ADS)

    Haffouz, S.; Grzegorczyk, A.; Hageman, P. R.; Vennégu≐s, P.; van der Drift, E. W. J. M.; Larsen, P. K.

    2003-02-01

    We report on the maskless epitaxial lateral overgrowth of GaN on structured Si (1 1 1) substrates and on its structural properties using transmission electron microscopy and photo-electrochemical (PEC) etching techniques. The structured silicon substrates are achieved using photolithography and dry etching; 4-μm-deep holes of 1.5 μm in diameter, each separated by 2.5 μm, are etched in the (1 1 1) Si surface. The growth process is started by depositing a 10-nm-thick AlN buffer layer at 850°C and then followed by the growth at high temperature (1170°C) of the GaN epilayer. The deposition of GaN takes place first on the Si (1 1 1) surface covered with AlN in between the holes, i.e. no deposits are formed in the holes. During the growth the GaN layer extends vertically and laterally over the holes until complete coalescence. Transmission electron microscopy shows that regions over the holes only contain dislocations in the basal plane resulting from the bending of dislocations nucleated at the Si/AlN interface and at the coalescence boundary between the two laterally overgrown layers. This results in a drastic decrease of dislocation density in these areas of the films. The revelation of dislocations is also achieved by PEC etching technique.

  17. 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.

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

    PubMed Central

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

    2016-01-01

    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. PMID:26861595

  19. Pendeo-epitaxial growth and characterization of GaN and related materials on 6H-SiC(0001) and Si(111) substrates

    SciTech Connect

    Davis, R.F.; Gehrke, T.; Linthicum, K.J.; Zheleva, T.S.; Rajagopal, P.; Zorman, C.A.; Mehregany, M.

    2000-07-01

    Discrete and coalesced monocrystalline GaN and Al{sub x}Ga{sub 1{minus}x}N layers grown via Pendeo-epitaxy (PE) [1] originated from side walls of GaN seed structures containing SiN{sub x} top masks have been grown via organometallic vapor phase deposition on GaN/AlN/6H-SiC(0001) and GaN(0001)/AlN(0001)/3C-SiC(111)/Si(111) substrates. Scanning and transmission electron microscopies were used to evaluate the external microstructures and the distribution of dislocations, respectively. The dislocation densities in the PE grown films was reduced by at least five orders of magnitude relative to the initial GaN seed layers. Tilting in the coalesced GaN epilayers was observed via X-ray diffraction. A tilt of 0.2{degree} was confined to areas of mask overgrowth; however, no tilting was observed in the material suspended above the SiC substrate. The strong, low-temperature PL band-edge peak at 3.45 eV with a FWHM of 17 meV was comparable to that observed in PE GaN films grown on 6H-SiC(0001). The band-edge in the GaN grown on AlN(0001)/SiC(111)Si(111) substrates was shifted to a lower energy by 10 meV, indicative of a greater tensile stress.

  20. A novel unified dislocation density-based model for hot deformation behavior of a nickel-based superalloy under dynamic recrystallization conditions

    NASA Astrophysics Data System (ADS)

    Lin, Y. C.; Wen, Dong-Xu; Chen, Ming-Song; Chen, Xiao-Min

    2016-09-01

    In this study, a novel unified dislocation density-based model is presented for characterizing hot deformation behaviors in a nickel-based superalloy under dynamic recrystallization (DRX) conditions. In the Kocks-Mecking model, a new softening item is proposed to represent the impacts of DRX behavior on dislocation density evolution. The grain size evolution and DRX kinetics are incorporated into the developed model. Material parameters of the developed model are calibrated by a derivative-free method of MATLAB software. Comparisons between experimental and predicted results confirm that the developed unified dislocation density-based model can nicely reproduce hot deformation behavior, DRX kinetics, and grain size evolution in wide scope of initial grain size, strain rate, and deformation temperature. Moreover, the developed unified dislocation density-based model is well employed to analyze the time-variant forming processes of the studied superalloy.

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

    NASA Astrophysics Data System (ADS)

    Zhou, Hong; Du, Yuchen; Ye, Peide D.

    2016-05-01

    Herein, we report on achieving ultra-high electron density (exceeding 1014 cm-2) in a GaN bulk material device by ionic liquid gating, through the application of atomic layer deposition (ALD) of Al2O3 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 105 at room temperature. An ultra-high electron density exceeding 1014 cm-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 reduction 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.

  2. 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.

  3. The Effect of Grain Size and Dislocation Density on the Tensile Properties of Ni-SiCNP Composites During Annealing

    NASA Astrophysics Data System (ADS)

    Yang, Chao; Huang, Hefei; Thorogood, Gordon James; Jiang, Li; Ye, Xiangxi; Li, Zhijun; Zhou, Xingtai

    2016-03-01

    The grain size refinement, enhancement of mechanical properties, and static recrystallization behavior of metallic nickel-silicon carbide nano-particle (Ni-3wt.%SiCNP) composites, milled for times ranging from 8 to 48 h have been examined. One set of Ni-SiCNP composite samples were annealed at 300 °C for 250 h, while the other set of samples were maintained at room temperature for control purposes (reference). The electron backscatter diffraction results indicate that the grain size of the annealed Ni-SiCNP composite was refined due to grain restructuring during static recrystallization. The x-ray diffraction results indicate that low-temperature annealing effectively reduced the density of dislocations; this can be explained by the dislocation pile-up model. Additionally, the tensile tests indicated that the annealed Ni-SiCNP composite had a significant increase in strength due to an increase of the Hall-Petch strengthening effect with a slight increase in the total elongation. The decrease of dislocation pile-up in the grain interiors and the increase in grain boundary sliding are assumed to be the main mechanisms at play. The relationship between the microstructural evolution and the variation of tensile properties is examined in this study.

  4. Microstructure of GaN epitaxy on SiC using AlN buffer layers

    SciTech Connect

    Ponce, F.A.; Krusor, B.S.; Major, J.S. Jr.; Plano, W.E.; Welch, D.F.

    1995-07-17

    The crystalline structure of GaN epilayers on (0001) SiC substrates has been studied using x-ray diffraction and transmission microscopy. The films were grown by metalorganic chemical vapor deposition, using AlN buffer layers. X-ray diffraction measurements show negligible strain in the epilayer, and a long-range variation in orientation. Transmission electron lattice images show that the AlN buffer layer consists of small crystallites. The nature of the buffer layer and its interfaces with the substrate and the GaN film is discussed. The defect structure of the GaN film away from the substrate consists mostly of threading dislocations with a density of {similar_to}10{sup 9} cm{sup {minus}2}. {copyright} {ital 1995} {ital American} {ital Institute} {ital of} {ital Physics}.

  5. Effect of buffer layer growth temperature on epitaxial GaN films deposited by magnetron sputtering

    SciTech Connect

    Mohanta, P.; Singh, D.; Kumar, R.; Ganguli, T.; Srinivasa, R. S.; Major, S. S.

    2012-06-05

    Epitaxial GaN films were deposited by reactive sputtering of a GaAs target in 100 % nitrogen at 700 deg. C on ZnO buffer layers grown at different substrate temperatures over sapphire substrates. High resolution X-ray diffraction measurements and the corresponding analysis show that the growth temperature of buffer layers significantly affects the micro-structural parameters of GaN epilayer, such as lateral coherence length, tilt and twist, while the vertical coherence length remains unaffected. The optimum substrate temperature for buffer layer growth has been found to be 300 deg. C. High epitaxial quality GaN film grown on such a buffer layer exhibited micro strain of 1.8x10{sup -4} along with screw and edge type dislocation densities of 7.87x10{sup 9} and 1.16x10{sup 11}, respectively.

  6. Global analysis of GaN growth using a solution technique

    NASA Astrophysics Data System (ADS)

    Kashiwagi, D.; Gejo, R.; Kangawa, Y.; Liu, L.; Kawamura, F.; Mori, Y.; Sasaki, T.; Kakimoto, K.

    2008-04-01

    The solution growth technique is one of the key methods for fabricating gallium nitride (GaN) wafers with small dislocation density. Since the growth rate of GaN using the solution technique is small, the key issue of the technique is to enhance the growth rate of the crystal. We studied how nitrogen is transferred from the surface of the flux to the interface between the top of the flux and the crystal in a muffle furnace using a global model that includes radiative, convective and conductive heat and mass transfer, including nitrogen transfer. The average growth rate of GaN increased when the temperature difference between the furnace wall and a crucible wall became large. This phenomenon is based on mixing of the flux due to natural convection.

  7. 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.

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

    SciTech Connect

    Kyle, Erin C. H. Kaun, Stephen W.; Burke, Peter G.; Wu, Feng; Speck, James S.; Wu, Yuh-Renn

    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. Optimized 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.

  9. Transformation of c-oriented nanowall network to a flat morphology in GaN films on c-plane sapphire

    SciTech Connect

    Kesaria, Manoj; Shetty, Satish; Cohen, P.I.; Shivaprasad, S.M.

    2011-11-15

    Highlights: {yields} High quality wurtzite structures GaN nanowall network formed on c-plane sapphire. {yields} Tapering of nanowalls at the apex cause electron confinement effects. {yields} Temperature dependent transformation of the six fold nanowall network to a flat morphology. {yields} Growth kinetics is influenced by adatom diffusion, interactions and bonding for GaN layer. -- Abstract: The work significantly optimizes growth parameters for nanostructured and flat GaN film in the 480-830 {sup o}C temperature range. The growth of ordered, high quality GaN nanowall hexagonal honeycomb like network on c-plane sapphire under nitrogen rich (N/Ga ratio of 100) conditions at temperatures below 700 {sup o}C is demonstrated. The walls are c-oriented wurtzite structures 200 nm wide at base and taper to 10 nm at apex, manifesting electron confinement effects to tune optoelectronic properties. For substrate temperatures above 700 {sup o}C the nanowalls thicken to a flat morphology with a dislocation density of 10{sup 10}/cm{sup 2}. The role of misfit dislocations in the GaN overlayer evolution is discussed in terms of growth kinetics being influenced by adatom diffusion, interactions and bonding at different temperatures. The GaN films are characterized by reflection high energy electron diffraction (RHEED), field emission scanning electron (FESEM), high resolution X-ray diffraction (HRXRD) and cathodoluminescence (CL).

  10. 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.

  11. Hydride vapor phase GaN films with reduced density of residual electrons and deep traps

    SciTech Connect

    Polyakov, A. Y.; Smirnov, N. B.; Govorkov, A. V.; Yugova, T. G.; Cox, H.; Helava, H.; Makarov, Yu.; Usikov, A. S.

    2014-05-14

    Electrical properties and deep electron and hole traps spectra are compared for undoped n-GaN films grown by hydride vapor phase epitaxy (HVPE) in the regular process (standard HVPE samples) and in HVPE process optimized for decreasing the concentration of residual donor impurities (improved HVPE samples). It is shown that the residual donor density can be reduced by optimization from ∼10{sup 17} cm{sup −3} to (2–5) × 10{sup 14} cm{sup −3}. The density of deep hole traps and deep electron traps decreases with decreased donor density, so that the concentration of deep hole traps in the improved samples is reduced to ∼5 × 10{sup 13} cm{sup −3} versus 2.9 × 10{sup 16} cm{sup −3} in the standard samples, with a similar decrease in the electron traps concentration.

  12. Ab initio density functional theory study of non-polar (101{sup ¯}0), (112{sup ¯}0) and semipolar (202{sup ¯}1) GaN surfaces

    SciTech Connect

    Mutombo, P.; Romanyuk, O.

    2014-05-28

    The atomic structures of non-polar GaN(101{sup ¯}0), (112{sup ¯}0) and semipolar GaN(202{sup ¯}1), (202{sup ¯}1{sup ¯}) surfaces were studied using ab initio calculations within density functional theory. The bulk-like truncated (1 × 1) structure with buckled Ga-N or Ga-Ga dimers was found stable on the non-polar GaN(101{sup ¯}0) surface in agreement with previous works. Ga-N heterodimers were found energetically stable on the GaN(112{sup ¯}0)-(1 × 1) surface. The formation of vacancies and substitution site defects was found unfavorable for non-polar GaN surfaces. Semipolar GaN(202{sup ¯}1)-(1 × 1) surface unit cells consist of non-polar (101{sup ¯}0) and semipolar (101{sup ¯}1) nano-facets. The (101{sup ¯}1) nano-facets consist of two-fold coordinated atoms, which form N-N dimers within a (2 × 1) surface unit cell on a GaN(202{sup ¯}1) surface. Dimers are not formed on the GaN(202{sup ¯}1{sup ¯}) surface. The stability of the surfaces with single (101{sup ¯}0) or (101{sup ¯}1) nano-facets was analyzed. A single non-polar (101{sup ¯}0)-(1 × 1) nano-facet was found stable on the GaN(202{sup ¯}1) surface, but unstable on the GaN(202{sup ¯}1{sup ¯}) surface. A single (101{sup ¯}1) nano-facet was found unstable. Semipolar GaN surfaces with (202{sup ¯}1) and (202{sup ¯}1{sup ¯}) polarity can be stabilized with a Ga overlayer at Ga-rich experimental conditions.

  13. Ferromagnetism in GaN: Gd: A density functional theory study

    SciTech Connect

    Stevenson, Cynthia; Stevenson, Cynthia

    2008-02-04

    First principle calculations of the electronic structure and magnetic interaction of GaN:Gd have been performed within the Generalized Gradient Approximation (GGA) of the density functional theory (DFT) with the on-site Coulomb energy U taken into account (also referred to as GGA+U). The ferromagnetic p-d coupling is found to be over two orders of magnitude larger than the s-d exchange coupling. The experimental colossal magnetic moments and room temperature ferromagnetism in GaN:Gd reported recently are explained by the interaction of Gd 4f spins via p-d coupling involving holes introduced by intrinsic defects such as Ga vacancies.

  14. A nitrogen doped low-dislocation density free-standing single crystal diamond plate fabricated by a lift-off process

    SciTech Connect

    Mokuno, Yoshiaki Kato, Yukako; Tsubouchi, Nobuteru; Chayahara, Akiyoshi; Yamada, Hideaki; Shikata, Shinichi

    2014-06-23

    A nitrogen-doped single crystal diamond plate with a low dislocation density is fabricated by chemical vapor deposition (CVD) from a high pressure high temperature synthetic type IIa seed substrate by ion implantation and lift-off processes. To avoid sub-surface damage, the seed surface was subjected to deep ion beam etching. In addition, we introduced a nitrogen flow during the CVD step to grow low-strain diamond at a relatively high growth rate. This resulted in a plate with low birefringence and a dislocation density as low as 400 cm{sup −2}, which is the lowest reported value for a lift-off plate. Reproducing this lift-off process may allow mass-production of single crystal CVD diamond plates with low dislocation density and consistent quality.

  15. Reduction of Crosshatch Roughness and Threading Dislocation Density in Metamorphic GaInP Buffers and GaInAs Solar Cells

    SciTech Connect

    France, R. M.; Geisz, J. F.; Steiner, M. A.; To, B.; Romero, M. J.; Olavarria, W. J.; King, R. R.

    2012-05-15

    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 surface 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.

  16. Structural Defects in Laterally Overgrown GaN Layers Grown onNon-polar Substrates

    SciTech Connect

    Liliental-Weber, Z.; Ni, X.; Morkoc, H.

    2007-02-14

    Transmission electron microscopy was used to study defects in lateral epitaxial layers of GaN which were overgrown on a template of a-plane (11{und 2}0) GaN grown on (1{und 1}02) r-plane Al2O3. A high density of basal stacking faults is formed in these layers because the c-planes of wurtzite structure are arranged along the growth direction. Density of these faults is decreasing at least by two orders of magnitude lower in the wings compared to the seed areas. Prismatic stacking faults and threading dislocations are also observed, but their densities drastically decrease in the wings. The wings grow with opposite polarities and the Ga-wing width is at least 6 times larger than N-wing and coalescence is rather difficult. Some tilt and twist was detected using Large Angle Convergent Beam Electron Diffraction.

  17. Laser diodes with 353 nm wavelength enabled by reduced-dislocation-density AlGaN templates

    DOE PAGES

    Crawford, Mary H.; Allerman, Andrew A.; Armstrong, Andrew M.; Smith, Michael L.; Cross, Karen C.

    2015-10-30

    We fabricated optically pumped and electrically injected ultraviolet (UV) lasers on reduced-threading-dislocation-density (reduced-TDD) AlGaN templates. The overgrowth of sub-micron-wide mesas in the Al0.32Ga0.68N templates enabled a tenfold reduction in TDD, to (2–3) × 108 cm–2. Optical pumping of AlGaN hetero-structures grown on the reduced-TDD templates yielded a low lasing threshold of 34 kW/cm2 at 346 nm. Room-temperature pulsed operation of laser diodes at 353 nm was demonstrated, with a threshold of 22.5 kA/cm2. Furthermore, reduced-TDD templates have been developed across the entire range of AlGaN compositions, presenting a promising approach for extending laser diodes into the deep UV.

  18. Laser diodes with 353 nm wavelength enabled by reduced-dislocation-density AlGaN templates

    SciTech Connect

    Crawford, Mary H.; Allerman, Andrew A.; Armstrong, Andrew M.; Smith, Michael L.; Cross, Karen C.

    2015-10-30

    We fabricated optically pumped and electrically injected ultraviolet (UV) lasers on reduced-threading-dislocation-density (reduced-TDD) AlGaN templates. The overgrowth of sub-micron-wide mesas in the Al0.32Ga0.68N templates enabled a tenfold reduction in TDD, to (2–3) × 108 cm–2. Optical pumping of AlGaN hetero-structures grown on the reduced-TDD templates yielded a low lasing threshold of 34 kW/cm2 at 346 nm. Room-temperature pulsed operation of laser diodes at 353 nm was demonstrated, with a threshold of 22.5 kA/cm2. Furthermore, reduced-TDD templates have been developed across the entire range of AlGaN compositions, presenting a promising approach for extending laser diodes into the deep UV.

  19. Laser diodes with 353 nm wavelength enabled by reduced-dislocation-density AlGaN templates

    NASA Astrophysics Data System (ADS)

    Crawford, Mary H.; Allerman, Andrew A.; Armstrong, Andrew M.; Smith, Michael L.; Cross, Karen C.

    2015-11-01

    We fabricated optically pumped and electrically injected ultraviolet (UV) lasers on reduced-threading-dislocation-density (reduced-TDD) AlGaN templates. The overgrowth of sub-micron-wide mesas in the Al0.32Ga0.68N templates enabled a tenfold reduction in TDD, to (2-3) × 108 cm-2. Optical pumping of AlGaN hetero-structures grown on the reduced-TDD templates yielded a low lasing threshold of 34 kW/cm2 at 346 nm. Room-temperature pulsed operation of laser diodes at 353 nm was demonstrated, with a threshold of 22.5 kA/cm2. Reduced-TDD templates have been developed across the entire range of AlGaN compositions, presenting a promising approach for extending laser diodes into the deep UV.

  20. HVPE homoepitaxial growth of high quality bulk GaN using acid wet etching method and its mechanism analysis

    NASA Astrophysics Data System (ADS)

    Liu, Nanliu; Cheng, Yutian; Wu, Jiejun; Li, Xingbin; Yu, Tongjun; Xiong, Huan; Li, Wenhui; Chen, Jiao; Zhang, Guoyi

    2016-11-01

    In this paper, crack-free 2-inch bulk GaN wafer with the thickness up to 3 mm was obtained by HVPE homoepitaxy. A new method of acid wet etching was used to pre-treat GaN substrate before re-growth. The formation of the mesh-like subsurface crack and interface layer were found to be suppressed between the re-growth layer and as-grown GaN substrate. EDS and time varied contact angle measurement proved that chemical etching would decrease the oxygen related surface adsorption and increase atoms diffusion length during HVPE homoepitaxial growth. Moreover, Morphology, Low temperature photoluminescence measurements indicated a reduction in stress of wet etching treated as-grown GaN substrate due to etching effect on its N face. High quality bulk GaN with the dislocation density of 1×106 cm-2 was achieved by using wet etching and HVPE multiple re-growth. It would offer a simple method to obtain bulk GaN with thicker layer and high quality.

  1. High Quality, Low Cost Ammonothermal Bulk GaN Substrates

    SciTech Connect

    Ehrentraut, D; Pakalapati, RT; Kamber, DS; Jiang, WK; Pocius, DW; Downey, BC; McLaurin, M; D'Evelyn, MP

    2013-12-18

    Ammonothermal GaN growth using a novel apparatus has been performed on c-plane, m-plane, and semipolar seed crystals with diameters between 5 mm and 2 in. to thicknesses of 0.5-3 mm. The highest growth rates are greater than 40 mu m/h and rates in the 10-30 mu m/h range are routinely observed for all orientations. These values are 5-100x larger than those achieved by conventional ammonothermal GaN growth. The crystals have been characterized by X-ray diffraction rocking-curve (XRC) analysis, optical and scanning electron microscopy (SEM), cathodoluminescence (CL), optical spectroscopy, and capacitance-voltage measurements. The crystallinity of the grown crystals is similar to or better than that of the seed crystals, with FWHM values of about 20-100 arcsec and dislocation densities of 1 x 10(5)-5 x 10(6) cm(-2). Dislocation densities below 10(4) cm(-2) are observed in laterally-grown crystals. Epitaxial InGaN quantum well structures have been successfully grown on ammonothermal wafers. (C) 2013 The Japan Society of Applied Physics

  2. Neutron Diffraction Measurement of Residual Stresses, Dislocation Density and Texture in Zr-bonded U-10Mo ''Mini'' Fuel Foils and Plates

    SciTech Connect

    Brown, Donald W.; Okuniewski, M. A.; Sisneros, Thomas A.; Clausen, Bjorn; Moore, G. A.; Balogh, L

    2014-08-07

    Aluminum clad monolithic uranium 10 weight percent molybdenum (U-10Mo) fuel plates are being considered for conversion of several research and test nuclear reactors from high-enriched to low-enriched uranium fuel due to the inherently high density of fissile material. Comprehensive neutron diffraction measurements of the evolution 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 HIP procedure significantly reduces 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 stresses in the clad fuel plate do not depend strongly on the final processing step of the bare foil prior to HIP bonding. Rather, the residual stresses are dominated by the thermal expansion mismatch of the constituent materials of the fuel plate.

  3. Coseismic density redistribution of the Earth interior based on the spherical dislocation theory and comparison to GRACE data

    NASA Astrophysics Data System (ADS)

    Xu, Changyi; Sun, Wenke; Fu, Guangyu; Dong, Jie

    2015-04-01

    Coseismic deformation produces sudden changes in the Earth's layered density structure due to the volume and internal topography changes, which can disturb global gravitational field. Such gravitational perturbations have been detected by the gravity space mission data (Han et al., 2006; Heki and Matsuo, 2010; Zhou et al., 2011). Han et al. (2006) discussed the gravity changes produced by the density changes related to the crustal dilatation produce by the 2004 Sumatra earthquake (Mw 9.0). But he neglected the gravity changes due to the internal topography changes, and the adopted Earth model is the simple half space media. Cambiotti et al. (2011) also discussed the gravity changes due to coseismic volume changes based on the normal mode summation, in which he took the point source as the fault model. However, the maximum coseismic changes occur in the vicinity of the fault, if the point source is adopted to conduct the near-field computation, there are many errors in the results. In this work, we present a method to compute the coseismic density changes in term of volumetric dilatation and internal topography changes based on the elastic dislocation theory. Using this computing scheme, the modelling density changes can be compared directly with the GRACE-observed ones. Combined with the finite fault model, we conduct the case study of the 2004 Sumatra earthquake (Mw 9.3) and the 2011 Tohoku-Oki earthquake (Mw 9.0). Then we compare the modelling results to the GRACE-derived surface density changes given as the equivalent water height (EWH). The comparison reveals some interesting details about the pattern and behavior of the internal density redistribution due to earthquakes at the subduction zone.

  4. 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.; Clausen, Bjorn; Moore, Glenn A.; Balogh, Levente

    2016-09-22

    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

  5. 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.

  6. Comparison of AlGaN p-i-n ultraviolet avalanche photodiodes grown on free-standing GaN and sapphire substrates

    NASA Astrophysics Data System (ADS)

    Kim, Jeomoh; Ji, Mi-Hee; Detchprohm, Theeradetch; Dupuis, Russell D.; Ryou, Jae-Hyun; Sood, Ashok K.; Dhar, Nibir D.; Lewis, Jay

    2015-12-01

    We compare the performance characteristics of Al0.05Ga0.95N UV avalanche photodiodes (APDs) grown on different substrates. UV-APDs grown on a free-standing GaN substrate show lower dark-current densities for all fabricated mesa sizes than similar UV-APDs grown on a GaN/sapphire template. In addition, a stable avalanche gain higher than 5 × 105 and a significant increase in the responsivity of UV-APDs grown on a free-standing GaN substrate are observed. We believe that the high crystalline quality of Al0.05Ga0.95N UV-APDs grown on a free-standing GaN substrate with low dislocation density is responsible for the observed low leakage currents, high performance characteirstics, and reliability of the devices.

  7. Dislocation-related trap levels in nitride-based light emitting diodes

    SciTech Connect

    Venturi, Giulia; Castaldini, Antonio; Cavallini, Anna

    2014-05-26

    Deep level transient spectroscopy was performed on InGaN/GaN multiple quantum well light emitting diodes (LEDs) in order to determine the effect of the dislocation density on the deep intragap electronic levels. The LEDs were grown by metalorganic vapor phase epitaxy on GaN templates with a high dislocation density of 8 × 10{sup 9} cm{sup −2} and a low dislocation density of 3 × 10{sup 8} cm{sup −2}. Three trapping levels for electrons were revealed, named A, A1, and B, with energies E{sub A} ≈ 0.04 eV, E{sub A1} ≈ 0.13 eV, and E{sub B} ≈ 0.54 eV, respectively. The trapping level A has a much higher concentration in the LEDs grown on the template with a high density of dislocations. The logarithmic dependence of the peak amplitude on the bias pulse width for traps A and A1 identifies the defects responsible for these traps as associated with linearly arranged defects. We conclude that traps A and A1 are dislocation-related intragap energy levels.

  8. Effects of dislocation density and sample-size on plastic yielding at the nanoscale: a Weibull-like framework.

    PubMed

    Rinaldi, Antonio

    2011-11-01

    Micro-compression tests have demonstrated that plastic yielding in nanoscale pillars is the result of the fine interplay between the sample-size (chiefly the diameter D) and the density of bulk dislocations ρ. The power-law scaling typical of the nanoscale stems from a source-limited regime, which depends on both these sample parameters. Based on the experimental and theoretical results available in the literature, this paper offers a perspective about the joint effect of D and ρ on the yield stress in any plastic regime, promoting also a schematic graphical map of it. In the sample-size dependent regime, such dependence is cast mathematically into a first order Weibull-type theory, where the power-law scaling the power exponent β and the modulus m of an approximate (unimodal) Weibull distribution of source-strengths can be related by a simple inverse proportionality. As a corollary, the scaling exponent β may not be a universal number, as speculated in the literature. In this context, the discussion opens the alternative possibility of more general (multimodal) source-strength distributions, which could produce more complex and realistic strengthening patterns than the single power-law usually assumed. The paper re-examines our own experimental data, as well as results of Bei et al. (2008) on Mo-alloy pillars, especially for the sake of emphasizing the significance of a sudden increase in sample response scatter as a warning signal of an incipient source-limited regime.

  9. Effects of dislocation density and sample-size on plastic yielding at the nanoscale: a Weibull-like framework

    NASA Astrophysics Data System (ADS)

    Rinaldi, Antonio

    2011-11-01

    Micro-compression tests have demonstrated that plastic yielding in nanoscale pillars is the result of the fine interplay between the sample-size (chiefly the diameter D) and the density of bulk dislocations ρ. The power-law scaling typical of the nanoscale stems from a source-limited regime, which depends on both these sample parameters. Based on the experimental and theoretical results available in the literature, this paper offers a perspective about the joint effect of D and ρ on the yield stress in any plastic regime, promoting also a schematic graphical map of it. In the sample-size dependent regime, such dependence is cast mathematically into a first order Weibull-type theory, where the power-law scaling the power exponent β and the modulus m of an approximate (unimodal) Weibull distribution of source-strengths can be related by a simple inverse proportionality. As a corollary, the scaling exponent β may not be a universal number, as speculated in the literature. In this context, the discussion opens the alternative possibility of more general (multimodal) source-strength distributions, which could produce more complex and realistic strengthening patterns than the single power-law usually assumed. The paper re-examines our own experimental data, as well as results of Bei et al. (2008) on Mo-alloy pillars, especially for the sake of emphasizing the significance of a sudden increase in sample response scatter as a warning signal of an incipient source-limited regime.

  10. Study of methods to increase cluster/dislocation loop densities in electrodes

    NASA Astrophysics Data System (ADS)

    Yang, Xiaoling; Miley, George H.

    2009-03-01

    Recent research has developed a technique for imbedding ultra-high density deuterium ``clusters'' (50 to 100 atoms per cluster) in various metals such as Palladium (Pd), Beryllium (Be) and Lithium (Li). It was found the thermally dehydrogenated PdHx retained the clusters and exhibited up to 12 percent lower resistance compared to the virginal Pd samplesootnotetextA. G. Lipson, et al. Phys. Solid State. 39 (1997) 1891. SQUID measurements showed that in Pd these condensed matter clusters approach metallic conditions, exhibiting superconducting propertiesootnotetextA. Lipson, et al. Phys. Rev. B 72, 212507 (2005ootnotetextA. G. Lipson, et al. Phys. Lett. A 339, (2005) 414-423. If the fabrication methods under study are successful, a large packing fraction of nuclear reactive clusters can be developed in the electrodes by electrolyte or high pressure gas loading. This will provide a much higher low-energy-nuclear- reaction (LENR) rate than achieved with earlier electrodeootnotetextCastano, C.H., et al. Proc. ICCF-9, Beijing, China 19-24 May, 2002..

  11. 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

    NASA Astrophysics Data System (ADS)

    Min, Jung-Wook; Hwang, Hyeong-Yong; Kang, Eun-Kyu; Park, Kwangwook; Kim, Ci-Hyun; Lee, Dong-Seon; Jho, Young-Dahl; Bae, Si-Young; Lee, Yong-Tak

    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 other 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.

  12. 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

    NASA Astrophysics Data System (ADS)

    Min, Jung-Wook; Hwang, Hyeong-Yong; Kang, Eun-Kyu; Park, Kwangwook; Kim, Ci-Hyun; Lee, Dong-Seon; Jho, Young-Dahl; Bae, Si-Young; Lee, Yong-Tak

    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 other 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.

  13. Dependence of adhesion strength between GaN LEDs and sapphire substrate on power density of UV laser irradiation

    NASA Astrophysics Data System (ADS)

    Park, Junsu; Sin, Young-Gwan; Kim, Jae-Hyun; Kim, Jaegu

    2016-10-01

    Selective laser lift-off (SLLO) is an innovative technology used to manufacture and repair micro-light-emitting diode (LED) displays. In SLLO, laser is irradiated to selectively separate micro-LED devices from a transparent sapphire substrate. The light source used is an ultraviolet (UV) laser with a wavelength of 266 nm, pulse duration of 20 ns, and repetition rate of 30 kHz. Controlled adhesion between a LED and the substrate is key for a SLLO process with high yield and reliability. This study examined the fundamental relationship between adhesion and laser irradiation. Two competing mechanisms affect adhesion at the irradiated interface between the GaN LED and sapphire substrate: Ga precipitation caused by the thermal decomposition of GaN and roughened interface caused by thermal damage on the sapphire. The competition between these two mechanisms leads to a non-trivial SLLO condition that needs optimization. This study helps understand the SLLO process, and accelerate the development of a process for manufacturing micro-LED displays via SLLO for future applications.

  14. X-ray studies of structure defects in recycled aluminum alloy during equal channel angular extrusion: Dislocation density and stored energy

    NASA Astrophysics Data System (ADS)

    Makhlouf, T.; Njah, N.

    2012-02-01

    A recycled aluminum alloy was severely deformed by Equal Channel Angular Extrusion (ECAE) using the processing route Bc, to study the evolution of the micro structure and associated changes of mechanical properties. XRD patterns were analyzed to determine crystallite size, lattice strain and lattice parameter. As a result, ultrafine grains material has been obtained with microstructure showing a mixture of highly strained crystallites. High density of dislocations is achieved as a result of severe plastic deformation through the die. This can explain an important decrease of the lattice parameter observed after the first passage. Changes in mechanical behavior are also reveled after ECAE. This is due to strain hardening. Differential Scanning Calorimetry (DSC) results show overlapped exothermic peaks indicating the succession of the recovery, recrystallization and grain growth phenomena's. Moreover, we demonstrate that the energy stored during ECAE can be related to the dislocation density introduced by severe plastic deformation.

  15. 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

  16. Supersonic Dislocation Bursts in Silicon

    DOE PAGES

    Hahn, E. N.; Zhao, S.; Bringa, E. M.; Meyers, M. A.

    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

  17. Deep GaN etching by inductively coupled plasma and induced surface defects

    SciTech Connect

    Ladroue, J.; Meritan, A.; Boufnichel, M.; Lefaucheux, P.; Ranson, P.; Dussart, R.

    2010-09-15

    GaN etching was studied in Cl{sub 2}/Ar plasmas as a function of process parameters. In addition, for a better understanding of the etching mechanisms, Langmuir probe measurements and optical emission spectroscopy were carried out. Etch rate was found to depend strongly on bias power. After optimization, an etch rate greater than 1000 nm/min was achieved. A second part of this work is dedicated to the etched surface defects. An original method to estimate GaN dislocation density and to localize nanopipes in the material is presented. Columnar defects could also appear with impurities in the etching reactor. The authors also present a possible formation mechanism of those columnar defects.

  18. Nondestructive Characterization of Residual Threading Dislocation Density in HgCdTe Layers Grown on CdZnTe by Liquid-Phase Epitaxy

    NASA Astrophysics Data System (ADS)

    Fourreau, Y.; Pantzas, K.; Patriarche, G.; Destefanis, V.

    2016-09-01

    The performance of mercury cadmium telluride (MCT)-based infrared (IR) focal-plane arrays is closely related to the crystalline perfection of the HgCdTe thin film. In this work, Te-rich, (111)B-oriented HgCdTe epilayers grown by liquid-phase epitaxy on CdZnTe substrates have been studied. Surface atomic steps are shown on as-grown MCT materials using atomic force microscopy (AFM) and white-light interferometry (WLI), suggesting step-flow growth. Locally, quasiperfect surface spirals are also evidenced. A demonstration is given that these spirals are related to the emergence of almost pure screw threading dislocations. A nondestructive and quantitative technique to measure the threading dislocation density is proposed. The technique consists of counting the surface spirals on the as-grown MCT surface from images obtained by either AFM or WLI measurements. The benefits and drawbacks of both destructive—chemical etching of HgCdTe dislocations—and nondestructive surface imaging techniques are compared. The nature of defects is also discussed. Finally, state-of-the-art threading dislocation densities in the low 104 cm-2 range are evidenced by both etch pit density (EPD) and surface imaging measurements.

  19. High-breakdown-voltage pn-junction diodes on GaN substrates

    NASA Astrophysics Data System (ADS)

    Yoshizumi, Yusuke; Hashimoto, Shin; Tanabe, Tatsuya; Kiyama, Makoto

    2007-01-01

    GaN pn-junction diodes have been grown on GaN and sapphire substrates by metalorganic vapor phase epitaxy and their electrical characteristics have been studied. For the diode on the GaN substrate, the reverse leakage current is lower and the breakdown voltage VB is higher than those on the sapphire substrate owing to the lower dislocation density. The breakdown voltage is further improved with decreasing Mg concentration in p-GaN layers. Analysis of the depletion-layer capacitance of pn diodes has revealed that the Mg acceptors are fully ionized in the depletion layer. By optimizing the growth conditions, the diodes on GaN substrates show extremely low leakage current and the ideal hard breakdown at -925 V. The breakdown field is estimated to be 3.27 MV/cm. The specific on-resistance RON of 6.3 mΩ cm 2 is obtained, leading to the figure of merit, VB2/R, of 136 MW/cm 2.

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

    SciTech Connect

    Lotsari, A.; Kehagias, Th.; Katsikini, M.; Arvanitidis, J.; Ves, S.; Komninou, Ph.; Dimitrakopulos, G. P.; Tsiakatouras, G.; Tsagaraki, K.; Georgakilas, A.; Christofilos, D.

    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, 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.

  1. High optical and structural quality of GaN epilayers grown on (2{sup ¯}01) β-Ga{sub 2}O{sub 3}

    SciTech Connect

    Muhammed, M. M.; Roqan, I. S.; Peres, M.; Franco, N.; Lorenz, K.; Yamashita, Y.; Morishima, Y.; Sato, S.; Kuramata, A.

    2014-07-28

    Producing highly efficient GaN-based optoelectronic devices has been a challenge for a long time due to the large lattice mismatch between III-nitride materials and the most common substrates, which causes a high density of threading dislocations. Therefore, it is essential to obtain alternative substrates with small lattice mismatches, appropriate structural, thermal and electrical properties, and a competitive price. Our results show that (2{sup ¯}01) oriented β-Ga{sub 2}O{sub 3} has the potential to be used as a transparent and conductive substrate for GaN-growth. Photoluminescence spectra of thick GaN layers grown on (2{sup ¯}01) oriented β-Ga{sub 2}O{sub 3} are found to be dominated by intense bandedge emission. Atomic force microscopy studies show a modest threading dislocation density of ∼10{sup 8 }cm{sup −2}. X-ray diffraction studies show the high quality of the single-phase wurtzite GaN thin film on (2{sup ¯}01) β-Ga{sub 2}O{sub 3} with in-plane epitaxial orientation relationships between the β-Ga{sub 2}O{sub 3} and the GaN thin film defined by (010) β-Ga{sub 2}O{sub 3} || (112{sup ¯}0) GaN and (2{sup ¯}01) β-Ga{sub 2}O{sub 3} || (0001) GaN leading to a lattice mismatch of ∼4.7%. Complementary Raman spectroscopy indicates that the quality of the GaN epilayer is high.

  2. Coupling the Phase Field Method for diffusive transformations with dislocation density-based crystal plasticity: Application to Ni-based superalloys

    NASA Astrophysics Data System (ADS)

    Cottura, M.; Appolaire, B.; Finel, A.; Le Bouar, Y.

    2016-09-01

    A phase field model is coupled to strain gradient crystal plasticity based on dislocation densities. The resulting model includes anisotropic plasticity and the size-dependence of plastic activity, required when plasticity is confined in region below few microns in size. These two features are important for handling microstructure evolutions during diffusive phase transformations that involve plastic deformation occurring in confined areas such as Ni-based superalloys undergoing rafting. The model also uses a storage-recovery law for the evolution of the dislocation density of each glide system and a hardening matrix to account for the short-range interactions between dislocations. First, it is shown that the unstable modes during the morphological destabilization of a growing misfitting circular precipitate are selected by the anisotropy of plasticity. Then, the rafting of γ‧ precipitates in a Ni-based superalloy is investigated during [100] creep loadings. Our model includes most of the important physical phenomena accounted for during the microstructure evolution, such as the presence of different crystallographic γ‧ variants, their misfit with the γ matrix, the elastic inhomogeneity and anisotropy, the hardening, anisotropy and viscosity of plasticity. In agreement with experiments, the model predicts that rafting proceeds perpendicularly to the tensile loading axis and it is shown that plasticity slows down significantly the evolution of the rafts.

  3. NpN-GaN/InxGa1-xN/GaN heterojunction bipolar transistor on free-standing GaN substrate

    NASA Astrophysics Data System (ADS)

    Lochner, Zachary; Jin Kim, Hee; Lee, Yi-Che; Zhang, Yun; Choi, Suk; Shen, Shyh-Chiang; Doug Yoder, P.; Ryou, Jae-Hyun; Dupuis, Russell D.

    2011-11-01

    Data and analysis are presented for NpN-GaN/InGaN/GaN double-heterojunction bipolar transistors (HBTs) grown and fabricated on a free-standing GaN (FS-GaN) substrate in comparison to that on a sapphire substrate to investigate the effect of dislocations in III-nitride HBT epitaxial structures. The performance characteristics of HBTs on FS-GaN exhibit a maximum collector current density of ˜12.3 kA/cm2, dc current gain of ˜90, and maximum differential gain of ˜120 without surface passivation, representing a substantial improvement over similar devices grown on sapphire. This is attributed to the reduction in threading dislocation density afforded by using a homoepitaxial growth on a high-crystalline-quality substrate. The minority carrier diffusion length increases significantly owing to not only a mitigated carrier trap effect via fewer dislocations, but also possibly reduced microscopic localized states.

  4. Effects of nano-void density, size and spatial population on thermal conductivity: a case study of GaN crystal.

    PubMed

    Zhou, X W; Jones, R E

    2012-08-15

    The thermal conductivity of a crystal is sensitive to the presence of surfaces and nanoscale defects. While this opens tremendous opportunities to tailor thermal conductivity, true 'phonon engineering' of nanocrystals for a specific electronic or thermoelectric application can only be achieved when the dependence of thermal conductivity on the defect density, size and spatial population is understood and quantified. Unfortunately, experimental studies of the effects of nanoscale defects are quite challenging. While molecular dynamics simulations are effective in calculating thermal conductivity, the defect density range that can be explored with feasible computing resources is unrealistically high. As a result, previous work has not generated a fully detailed understanding of the dependence of thermal conductivity on nanoscale defects. Using GaN as an example, we have combined a physically motivated analytical model and highly converged large-scale molecular dynamics simulations to study the effects of defects on thermal conductivity. An analytical expression for thermal conductivity as a function of void density, size, and population has been derived and corroborated with the model, simulations, and experiments.

  5. Growth and characteristics of self-assembly defect-free GaN surface islands by molecular beam epitaxy.

    PubMed

    Hsu, Kuang-Yuan; Wang, Cheng-Yu; Liu, Chuan-Pu

    2011-04-01

    GaN surface nano-islands of high crystal quality, without any dislocations or other extended defects, are grown on a c-plane sapphire substrate by plasma-assisted molecular beam epitaxy. Nano-island growth requires special conditions in terms of V/III ratio and substrate temperature, distinct from either film or nanocolumn growth. The insertion of a nitrided Ga layer can effectively improve the uniformity of the nano-islands in both shape and size. The islands are well faced truncated pyramids with island size ranged from 30 to 110 nm, and height ranged from 30 to 55 nm. On, the other hand, the density and facet of the GaN surface islands would be affected by the growth conditions. An increase of the V/III ratio from 30 to 40 led to an increase in density from 1.4 x 10(9) to 4.3 x 10(9) cm(-2) and an evolution from {1-21-1} facets to {1-21-2} facets. The GaN layers containing the surface islands can moderate the compressive strain due to the lattice and thermal mismatch between GaN and c-sapphire. Conductive atomic force microscopy shows that the off-axis sidewall facets are more electrically active than those at the island center. The formation of the GaN surface islands is strongly induced by the Ehrlich-Schwoebel barrier effect of preexisting islands grown in the early growth stage. GaN surface islands are ideal templates for growing nano-devices.

  6. Defect reduction in (112_O) a-plane GaN by two-stage epitaxiallateral overgrowth

    SciTech Connect

    Ni, X.; Ozgur, U.; Fu, Y.; Biyikli, N.; Xie, J.; Baski, A.A.; Morkoc, H.; Liliental-Weber, Z.

    2006-10-20

    In the epitaxial lateral overgrowth (ELO) of (11{bar 2}0) a-plane GaN, the uneven growth rates of two opposing wings, Ga- and N-wings, makes the coalescence of two neighboring wings more difficult than that in c-plane GaN. We report a two-stage growth method to get uniformly coalesced epitaxial lateral overgrown a-plane GaN using metalorganic chemical vapor deposition (MOCVD) by employing relatively lower growth temperature in the first step followed by enhanced lateral growth in the second. Using this method, the height differences between Ga-polar and N-polar wings at the coalescence front could be reduced, thereby making the coalescence of two wings much easier. Transmission electron microscopy (TEM) showed that the threading dislocation density in the wing areas was 1.0x10{sup 8}cm{sup -2}, more than two orders of magnitude lower than that in the window areas (4.2x10{sup 10}cm{sup -2}). However, high density of basal stacking faults of 1.2x104 cm-1 was still observed in the wing areas as compared to c-plane GaN. Atomic force microscopy and photoluminescence measurements on the coalesced ELO a-GaN sample also indicated improved material quality.

  7. Threading dislocation reduction in III-V films: Theoretical modeling and experimental methods

    NASA Astrophysics Data System (ADS)

    Mathis, Sheila Kathleen

    Heteroepitaxy remains the most efficient and successful way to integrate materials with the same crystal structure but different lattice constants and optoelectronic properties. The purpose of this work is to understand the processes of strain relaxation and subsequent threading dislocation (TD) reduction through modeling and experiment. The interpretation of these data is made according to a previously published model that is based on dislocation reactions. 1,2 These reactions are the sole mechanism for threading dislocation reduction in III--V epitaxial films.3 Gallium nitride, with no available substrate even close to the lattice-matched condition, has a unique microstructure that develops as a result of initial island growth. Dislocation densities on the order of 109--10 10/cm2 are routinely measured in GaN grown on sapphire. Dislocation reduction in these hexagonal films is extremely slow, and it is shown in Chapter II that film thicknesses on the order of a substrate thickness are required to reduce threading dislocation densities to a low (10 6/cm2) level. A model is developed to treat the reduction of TDs in (0001)-oriented films that explains the non-saturating TD density in GaN. Screw dislocation behavior is shown to strongly affect the dislocation density falloff with thickness. Threading dislocation reduction in low-temperature-grown GaAs (250--350°C) and InGaAs was known to be more efficient than in high-temperature-grown GaAs (580°C). Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to study the microstructure of GaAs grown on InP substrates at temperatures between 250 and 580°C to determine the mechanism by which TD reduction was enhanced. While a high level of arsenic antisite defects [ASGa] may affect the TD density at temperatures below 300°C, they do not account for the improved dislocation reduction. When dislocations are generated at high temperature, the TD density on the (111)A and (111)B planes is asymmetric

  8. Control of stress and threading dislocation density in the thick GaN/AlN buffer layers grown on Si (111) substrates by low- temperature MBE

    NASA Astrophysics Data System (ADS)

    Zolotukhin, D.; Nechaev, D.; Kuznetsova, N.; Ratnikov, V.; Rouvimov, S.; Jmerik, V.; Ivanov, S.

    2016-08-01

    We report on successful growth by plasma-assisted molecular beam epitaxy on a Si(111) substrate crack-free GaN/AlN buffer layers with a thickness more than 1 μm. The layers fabricated at relatively low growth temperature of 780°C have at room temperature the residual compressive stress of -97 MPa. Intrinsic stress evolution during the GaN growth was monitored in situ with a multi-beam optical system. Strong dependence of a stress relaxation ratio in the growing layer vs growth temperature was observed. The best-quality crack-free layers with TDs density of ∼⃒109 cm-2 and roughly zero bowing were obtained in the sample with sharp 2D-GaN/2D-AlN interface.

  9. Synchrotron white beam topography characterization of physical vapor transport grown AlN and ammonothermal GaN

    NASA Astrophysics Data System (ADS)

    Raghothamachar, Balaji; Vetter, William M.; Dudley, Michael; Dalmau, Rafael; Schlesser, Raoul; Sitar, Zlatko; Michaels, Emily; Kolis, Joseph W.

    2002-12-01

    Structural defects in AlN single crystals grown by the sublimation method and GaN single crystals grown by the ammonothermal method are characterized by synchrotron white-beam X-ray topography in conjunction with optical microscopy. AlN platelets are either of (1 1 2¯ 0) or (0 0 0 1) type depending on the growth conditions. Dislocation densities of the order of 10 3 cm -2 or lower are observed in some crystals. X-ray topographs reveal the presence of growth sector boundaries, inclusions, and growth dislocations that indicate slight impurity contamination. The 2H crystal structure of GaN single crystals obtained by the ammonothermal method was verified by Laue X-ray pattern analysis. GaN crystals grown are of the order of 1 mm in size and are either (0 0 0 1) platelets or [0 0 0 1] prismatic needles. Generally, prismatic needles are characterized by lower degree of mosaicity than (0 0 0 1) platelets.

  10. Low defect large area semi-polar (112) GaN grown on patterned (113) silicon

    PubMed Central

    Pristovsek, Markus; Han, Yisong; Zhu, Tongtong; Frentrup, Martin; Kappers, Menno J; Humphreys, Colin J; Kozlowski, Grzegorz; Maaskant, Pleun; Corbett, Brian

    2015-01-01

    We report on the growth of semi-polar GaN (112) templates on patterned Si (113) substrates. Trenches were etched in Si (113) using KOH to expose Si {111} sidewalls. Subsequently an AlN layer to prevent meltback etching, an AlGaN layer for stress management, and finally two GaN layers were deposited. Total thicknesses up to 5 m were realised without cracks in the layer. Transmission electron microscopy showed that most dislocations propagate along [0001] direction and hence can be covered by overgrowth from the next trench. The defect densities were below and stacking fault densities less than 100 cm . These numbers are similar to reports on patterned r-plane sapphire. Typical X-ray full width at half maximum (FHWM) were 500” for the asymmetric (00.6) and 450” for the (11.2) reflection. These FHWMs were 50 % broader than reported for patterned r-plane sapphire which is attributed to different defect structures and total thicknesses. The surface roughness shows strong variation on templates. For the final surface roughness the roughness of the sidewalls of the GaN ridges at the time of coalescence are critical. PMID:26212392

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

    NASA Astrophysics Data System (ADS)

    Zhang, Y.; Bai, J.; Hou, Y.; Smith, R. M.; Yu, X.; Gong, Y.; Wang, T.

    2016-02-01

    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 SiO2 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.

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

    SciTech Connect

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

    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 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.

  13. Influence of Oxygen Partial Pressure on Opto-Electrical Properties, Crystallite Size and Dislocation Density of Sn Doped In_2O_3 Nanostructures

    NASA Astrophysics Data System (ADS)

    Soleimanian, Vishtasb; Ghasemi Varnamkhasti, Mohsen

    2016-10-01

    In this research, high-quality Sn doped indium oxide (ITO) thin films were grown on glass slide substrates using an electron beam evaporation method. Vacuum chamber partial pressure was changed and the electro-optical as well as the microstructure parameters were investigated. The microstructure of prepared films was evaluated by x-ray diffraction analysis in terms of crystallite size and dislocation density. It was found that the best results [high transparency (88%) over the visible wavelength region, low sheet resistance of 12.8 Ω /square, the optical band gap of 3.76 eV, crystallite size of 49.5 nm and dislocation density of 1.42 × 10^{14} m^{-2}] were achieved for the sample produced at a partial pressure of 1 × 10^{-4} mbar. Therefore, one can successfully control the physical properties of ITO films by varying the oxygen content of the evaporation system. The correlation between the band gap and carrier concentration in addition to the average crystallite size of films was also established.

  14. Influence of Oxygen Partial Pressure on Opto-Electrical Properties, Crystallite Size and Dislocation Density of Sn Doped In_2 O_3 Nanostructures

    NASA Astrophysics Data System (ADS)

    Soleimanian, Vishtasb; Ghasemi Varnamkhasti, Mohsen

    2016-06-01

    In this research, high-quality Sn doped indium oxide (ITO) thin films were grown on glass slide substrates using an electron beam evaporation method. Vacuum chamber partial pressure was changed and the electro-optical as well as the microstructure parameters were investigated. The microstructure of prepared films was evaluated by x-ray diffraction analysis in terms of crystallite size and dislocation density. It was found that the best results [high transparency (88%) over the visible wavelength region, low sheet resistance of 12.8 Ω /square, the optical band gap of 3.76 eV, crystallite size of 49.5 nm and dislocation density of 1.42 × 10^{14} m^{-2} ] were achieved for the sample produced at a partial pressure of 1 × 10^{-4} mbar. Therefore, one can successfully control the physical properties of ITO films by varying the oxygen content of the evaporation system. The correlation between the band gap and carrier concentration in addition to the average crystallite size of films was also established.

  15. Growth of InP bulk crystals by VGF: A comparative study of dislocation density and numerical stress analysis

    SciTech Connect

    Zemke, D.; Leister, H.J.; Mueller, G.

    1996-12-31

    The properties of electronic and optoelectronic devices produced on top of InP wafers are strongly affected by crystallographic defects, especially dislocations present in the InP substrate crystals. The potential of the VGF process is analyzed by using a flat bottom crucible for the growth of InP crystals with 2-inch diameter. Results of numerical simulations are used to design a set-up which can be run in a LEC facility. The EPD {approx} 3 {center_dot} 10{sup 3} cm{sup {minus}2} of the grown crystals as in accordance with calculated results based on an analysis of the thermal stress occurring during growth.

  16. Electrical properties of undoped GaN films grown by maskless epitaxial lateral overgrowth

    NASA Astrophysics Data System (ADS)

    Polyakov, A. Y.; Jeon, Dae-Woo; Lee, In-Hwan; Smirnov, N. B.; Govorkov, A. V.; Kozhukhova, E. A.; Yakimov, E. B.

    2013-02-01

    Electrical properties, deep traps spectra, microcathodoluminescence (MCL) spectra measurements, MCL imaging, and electron beam induced current (EBIC) imaging were performed for undoped GaN films grown by metalorganic chemical vapor deposition using maskless epitaxial lateral overgrowth on basal plane sapphire. The films showed a low dislocation density of ˜108 cm-2 in the laterally overgrown wings and an order of magnitude higher dislocation density in vertical growth seed regions, as determined by MCL and EBIC imaging. The polarity of EBIC signal measurements and the room temperature capacitance-voltage characteristics suggested that the high-dislocation-density seed regions were high-resistivity p-type, with the Fermi level pinned near Ev + 0.4 eV, as determined by admittance spectroscopy. The wing regions were n-type, with low residual donor concentration of some 1014 cm-3 near the surface. The donor concentration further decreased upon movement towards the sapphire substrate. Some possible explanations of the observed effects are discussed.

  17. Determination of the type of stacking faults in single-crystal high-purity diamond with a low dislocation density of <50 cm-2 by synchrotron X-ray topography

    NASA Astrophysics Data System (ADS)

    Masuya, Satoshi; Hanada, Kenji; Uematsu, Takumi; Moribayashi, Tomoya; Sumiya, Hitoshi; Kasu, Makoto

    2016-04-01

    The properties of stacking faults in a single-crystal high-purity diamond with a very low dislocation density of <50 cm-2 and a very low impurity concentration of <0.1 ppm were investigated by synchrotron X-ray topography. We found stacking faults on the {111} plane and determined the fault vector f of the stacking faults to be \\textbf{f} = a/3< 111> on the basis of the f · g extinction criteria. Furthermore, we have found that the partial dislocations are of the Shockley type on the basis of the b · g extinction criteria. Consequently, we concluded that the stacking faults are of the Shockley type and formed because of the decomposition of dislocations with \\textbf{b} = a/2< 1\\bar{1}0> into dislocations with \\textbf{b} = a/6< 2\\bar{1}1> and a/6< 1\\bar{2}\\bar{1}> .

  18. Thermal-mechanical modeling of single crystal AlN and GaN

    NASA Astrophysics Data System (ADS)

    Karvanirabori, Payman

    In this work, thermal-mechanical models are being developed, based on underlying micromechanical behavior of III-nitride single crystals at growth temperatures, for use in process design. A crystal plasticity model that is capable of capturing the underlying mechanisms of dislocation motion, multiplication, and interactions in wurtzite structure (hexagonal) crystals is defined to accurately model the elastic-plastic behavior of GaN and AlN crystals at elevated temperatures. The model for AlN is extended from relations developed for GaN based on available experimental data. Algorithms for integrating the constitutive model and computing the consistent tangent modulus are formulated, and the material model is implemented into a crystal plasticity finite element framework. Finite element models of crystal growth for different processing conditions are simulated. The simulation predicts cracking and dislocation defect density in order to improve the yield and reduce the manufacturing cost of high quality III-nitride semiconductors. Furthermore, the resulting simulation capability can be used in conjunction with relevant experiments to backout key thermal-mechanical material properties at high temperatures.

  19. Vertical GaN power diodes with a bilayer edge termination

    SciTech Connect

    Dickerson, Jeramy R.; Allerman, Andrew A.; Bryant, Benjamin N.; Fischer, Arthur J.; King, Michael P.; Moseley, Michael W.; Armstrong, Andrew M.; Kaplar, Robert J.; Kizilyalli, Isik C.; Aktas, Ozgur; Wierer, Jr., Jonathan J.

    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 (104 - 105 cm-2) GaN substrate, and has a 15-μm-thick n-type drift layer with a free carrier concentration of 5 × 1015 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) 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.

  20. Vertical GaN power diodes with a bilayer edge termination

    DOE PAGES

    Dickerson, Jeramy R.; Allerman, Andrew A.; Bryant, Benjamin N.; Fischer, Arthur J.; King, Michael P.; Moseley, Michael W.; Armstrong, Andrew M.; Kaplar, Robert J.; Kizilyalli, Isik C.; Aktas, Ozgur; et al

    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 (104 - 105 cm-2) GaN substrate, and has a 15-μm-thick n-type drift layer with a free carrier concentration of 5 × 1015 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) layer near the n-type driftmore » 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

  1. High uniform growth of 4-inch GaN wafer via flow field optimization by HVPE

    NASA Astrophysics Data System (ADS)

    Cheng, Yutian; Liu, Peng; Wu, Jiejun; Xiang, Yong; Chen, Xinjuan; Ji, Cheng; Yu, Tongjun; Zhang, Guoyi

    2016-07-01

    The uniformity of flow field inner the reactor plays a crucial role for hydride vapor phase epitaxy (HVPE) crystal growth and its more important for large scale substrate. A new nozzle structure was designed by adding a push and dilution (PD) gas pipe in the center of gas channels for a 4-inch HVPE (PD-HVPE) system. Experimental results showed that the thickness inhomogeneity of 46 μm 4-inch GaN layer could reach ±1.8% by optimizing PD gas, greatly improved from ±14% grown with conventional nozzle. The simulations of the internal flow field were consistent with our experiment, and the enhancement in uniformity should be attributed to the redistribution of GaCl and NH3 upon the wafer induced by PD pipe. The full width at half maximum (FWHM) of X-ray diffraction rocking curves for the 4-inch GaN film were about 224 and 200 arcsec for (002) and (102) reflection. The dislocation density of as-grown GaN was about 6.4×107 cm-2.

  2. Surface morphology of GaN: Flat versus vicinal surfaces

    SciTech Connect

    Xie, M.H.; Seutter, S.M.; Zheng, L.X.; Cheung, S.H.; Ng, Y.F.; Wu, H.; Tong, S.Y.

    2000-07-01

    The surface morphology of GaN films grown by molecular beam epitaxy (MBE) is investigated by scanning tunneling microscopy (STM). A comparison is made between flat and vicinal surfaces. The wurtzite structure of GaN leads to special morphological features such as step pairing and triangularly shaped islands. Spiral mounds due to growth at screw threading dislocations are dominant on flat surfaces, whereas for vicinal GaN, the surfaces show no spiral mound but evenly spaced steps. This observation suggests an effective suppression of screw threading dislocations in the vicinal films. This finding is confirmed by transmission electron microscopy (TEM) studies. Continued growth of the vicinal surface leads to step bunching that is attributed to the effect of electromigration.

  3. Scanning reflection electron microscopy study of surface defects in GaN films formed by epitaxial lateral overgrowth

    NASA Astrophysics Data System (ADS)

    Watanabe, Heiji; Kuroda, Naotaka; Sunakawa, Haruo; Usui, Akira

    2000-09-01

    We have used scanning reflection electron microscopy (SREM) to detect surface defects in GaN films formed by facet-initiated epitaxial lateral overgrowth. SREM revealed individual threading dislocations and single atomic steps on the GaN surface, and provided images of crystallographic tilting near the surfaces. We found that one of the two tilted GaN crystals in the overgrown areas became dominant and that the surface changed to a single domain after 50-μm-thick GaN deposition. Our SREM results also showed that the deposition of thick (over 100 μm) GaN films significantly improves the crystallographic structures of the overgrown regions, and reduces the threading dislocations in the GaN films.

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

    SciTech Connect

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

    2012-10-01

    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.

  5. Tuning the polarization-induced free hole density in nanowires graded from GaN to AlN

    SciTech Connect

    Golam Sarwar, A. T. M.; Carnevale, Santino D.; Kent, Thomas F.; Yang, Fan; McComb, David W.; Myers, Roberto C.

    2015-01-19

    We report a systematic study of p-type polarization-induced doping in graded AlGaN nanowire light emitting diodes grown on silicon wafers by plasma-assisted molecular beam epitaxy. The composition gradient in the p-type base is varied in a set of samples from 0.7%Al/nm to 4.95%Al/nm corresponding to negative bound polarization charge densities of 2.2 × 10{sup 18 }cm{sup −3} to 1.6 × 10{sup 19 }cm{sup −3}. Capacitance measurements and energy band modeling reveal that for gradients greater than or equal to 1.30%Al/nm, the deep donor concentration is negligible and free hole concentrations roughly equal to the bound polarization charge density are achieved up to 1.6 × 10{sup 19 }cm{sup −3} at a gradient of 4.95%Al/nm. Accurate grading lengths in the p- and n-side of the pn-junction are extracted from scanning transmission electron microscopy images and are used to support energy band calculation and capacitance modeling. These results demonstrate the robust nature of p-type polarization doping in nanowires and put an upper bound on the magnitude of deep donor compensation.

  6. GaN hexagonal pyramids formed by a photo-assisted chemical etching method

    NASA Astrophysics Data System (ADS)

    Zhang, Shi-Ying; Xiu, Xiang-Qian; Hua, Xue-Mei; Xie, Zi-Li; Liu, Bin; Chen, Peng; Han, Ping; Lu, Hai; Zhang, Rong; Zheng, You-Dou

    2014-05-01

    A series of experiments were conducted to systematically study the effects of etching conditions on GaN by a convenient photo-assisted chemical (PAC) etching method. The solution concentration has an evident influence on the surface morphology of GaN and the optimal solution concentrations for GaN hexagonal pyramids have been identified. GaN with hexagonal pyramids have higher crystal quality and tensile strain relaxation compared with as-grown GaN. A detailed analysis about evolution of the size, density and optical property of GaN hexagonal pyramids is described as a function of light intensity. The intensity of photoluminescence spectra of GaN etched with hexagonal pyramids significantly increases compared to that of as-grown GaN due to multiple scattering events, high quality GaN with pyramids and the Bragg effect.

  7. Nanoheteroepitaxial growth of GaN on Si nanopillar arrays

    NASA Astrophysics Data System (ADS)

    Hersee, S. D.; Sun, X. Y.; Wang, X.; Fairchild, M. N.; Liang, J.; Xu, J.

    2005-06-01

    Nanoheteroepitaxial growth of GaN by metal-organic chemical-vapor deposition on dense arrays of (111) Si nanopillars has been investigated. Scanning electron microscopy, cross-sectional transmission electron microscopy, and electron-diffraction analysis of 0.15-μm-thick GaN layers indicate single-crystal films. Most of the mismatch defects were in-plane stacking faults and the threading dislocation concentration was <108cm-2 at the interface and decreased away from the interface. High-resolution transmission electron microscopy indicated that grain-boundary defects could heal and were followed by high quality, single-crystal GaN. Facetted voids were also present at the GaN /Si interface and are believed to be an additional strain-energy reduction mechanism. The unusual defect behavior in these samples appears to be related to the high compliance of the nanopillar silicon substrate.

  8. Oxygen in GaN.

    NASA Astrophysics Data System (ADS)

    van de Walle, Chris G.; Neugebauer, Jörg

    1997-03-01

    Oxygen is commonly present during epitaxial growth of GaN. We have proposed that unintentional incorporation of O, as well as Si, is responsible for the frequently observed n-type conductivity in as-grown GaN. Here we present results from comprehensive density-functional-pseudopotential studies of GaN:O under pressure, and of O interactions with native defects and dopant impurities. We find that the O donor undergoes a DX-like transition under pressure: a large outward relaxation introduces a deep level in the band gap. This behavior explains the carrier freezeout in GaN under pressure.^1 Si donors do not exhibit the transition, consistent with experiment. Results for these impurities in AlGaN will also be discussed. We have also investigated the interaction between O and native defects. Most notably we find a large binding energy between O and the gallium vacancy (V_Ga), which we have proposed to be the source of the yellow luminescence. Finally, we have studied the interaction between O and Mg acceptors. The incorporation of the O donor is significantly enhanced in Mg-doped material. In addition, we calculate a binding energy of 0.6 eV for Mg-O complexes. The presence of O during growth can thus be detrimental to p-type GaN. ^1 C. Wetzel et al., Proc. ICPS-23 (World Scientific, Singapore, 1996), p. 2929.

  9. Knee Dislocations

    PubMed Central

    Schenck, Robert C.; Richter, Dustin L.; Wascher, Daniel C.

    2014-01-01

    Background: Traumatic knee dislocation is becoming more prevalent because of improved recognition and increased exposure to high-energy trauma, but long-term results are lacking. Purpose: To present 2 cases with minimum 20-year follow-up and a review of the literature to illustrate some of the fundamental principles in the management of the dislocated knee. Study Design: Review and case reports. Methods: Two patients with knee dislocations who underwent multiligamentous knee reconstruction were reviewed, with a minimum 20-year follow-up. These patients were brought back for a clinical evaluation using both subjective and objective measures. Subjective measures include the following scales: Lysholm, Tegner activity, visual analog scale (VAS), Short Form–36 (SF-36), International Knee Documentation Committee (IKDC), and a psychosocial questionnaire. Objective measures included ligamentous examination, radiographic evaluation (including Telos stress radiographs), and physical therapy assessment of function and stability. Results: The mean follow-up was 22 years. One patient had a vascular injury requiring repair prior to ligament reconstruction. The average assessment scores were as follows: SF-36 physical health, 52; SF-36 mental health, 59; Lysholm, 92; IKDC, 86.5; VAS involved, 10.5 mm; and VAS uninvolved, 2.5 mm. Both patients had excellent stability and were functioning at high levels of activity for their age (eg, hiking, skydiving). Both patients had radiographic signs of arthritis, which lowered 1 subject’s IKDC score to “C.” Conclusion: Knee dislocations have rare long-term excellent results, and most intermediate-term studies show fair to good functional results. By following fundamental principles in the management of a dislocated knee, patients can be given the opportunity to function at high levels. Hopefully, continued advances in the evaluation and treatment of knee dislocations will improve the long-term outcomes for these patients in the

  10. Orthodox etching of HVPE-grown GaN

    SciTech Connect

    Weyher, J.L.; Lazar, S.; Macht, L.; Liliental-Weber, Z.; Molnar,R.J.; Muller, S.; Nowak, G.; Grzegory, I.

    2006-08-10

    Orthodox etching of HVPE-grown GaN in molten eutectic of KOH + NaOH (E etch) and in hot sulfuric and phosphoric acids (HH etch) is discussed in detail. Three size grades of pits are formed by the preferential E etching at the outcrops of threading dislocations on the Ga-polar surface of GaN. Using transmission electron microscopy (TEM) as the calibration tool it is shown that the largest pits are formed on screw, intermediate on mixed and the smallest on edge dislocations. This sequence of size does not follow the sequence of the Burgers values (and thus the magnitude of the elastic energy) of corresponding dislocations. This discrepancy is explained taking into account the effect of decoration of dislocations, the degree of which is expected to be different depending on the lattice deformation around the dislocations, i.e. on the edge component of the Burgers vector. It is argued that the large scatter of optimal etching temperatures required for revealing all three types of dislocations in HVPE-grown samples from different sources also depends upon the energetic status of dislocations. The role of kinetics for reliability of etching in both etches is discussed and the way of optimization of the etching parameters is shown.

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

    SciTech Connect

    Armstrong, Andrew M.; Aubry, Sylvie; Shaner, Eric Arthur; Siegal, Michael P.; Li, Qiming; Jones, Reese E.; Westover, Tyler; Wang, George T.; Zhou, Xiao Wang; Talin, Albert Alec; Bogart, Katherine Huderle Andersen; Harris, C. Thomas; Huang, Jian Yu

    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 as 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

  12. Chemical exfoliation and optical characterization of threading-dislocation-free gallium-nitride ultrathin nanomembranes

    NASA Astrophysics Data System (ADS)

    ElAfandy, Rami T.; Majid, Mohammed A.; Ng, Tien Khee; Zhao, Lan; Cha, Dongkyu; Ooi, Boon S.

    2014-11-01

    Semiconductor nanostructures have generated tremendous scientific interests as well as practical applications stemming from the engineering of low dimensional physics phenomena. Unlike 0D and 1D nanostructures, such as quantum dots and nanowires, respectively, 2D structures, such as nanomembranes, are unrivalled in their scalability for high yield manufacture and are less challenging in handling with the current transfer techniques. Furthermore, due to their planar geometry, nanomembranes are compatible with the current complementary metal oxide semiconductor (CMOS) technology. Due to these superior characteristics, there are currently different techniques in exfoliating nanomembranes with different crystallinities, thicknesses and compositions. In this work we demonstrate a new facile technique of exfoliating gallium nitride (GaN) nanomembranes with novel features, namely with the non-radiative cores of their threading-dislocations (TDs) being etched away. The exfoliation process is based on engineering the gallium vacancy (VGa) density during the GaN epitaxial growth with subsequent preferential etching. Based on scanning and transmission electron microscopies, as well as micro-photoluminescence measurements, a model is proposed to uncover the physical processes underlying the formation of the nanomembranes. Raman measurements are also performed to reveal the internal strain within the nanomembranes. After transferring these freely suspended 25 nm thin GaN nanomembranes to other substrates, we demonstrate the temperature dependence of their bandgap by photoluminescence technique, in order to shed light on the internal carrier dynamics.

  13. Structural properties of free-standing 50 mm diameter GaN waferswith (101_0) orientation grown on LiAlO2

    SciTech Connect

    Jasinski, Jacek; Liliental-Weber, Zuzanna; Maruska, Herbert-Paul; Chai, Bruce H.; Hill, David W.; Chou, Mitch M.C.; Gallagher, John J.; Brown, Stephen

    2005-09-27

    (10{und 1}0) GaN wafers grown on (100) face of {gamma}-LiAlO{sub 2} were studied using transmission electron microscopy. Despite good lattice matching in this heteroepitaxial system, high densities of planar structural defects in the form of stacking faults on the basal plane and networks of boundaries located on prism planes inclined to the layer/substrate interface were present in these GaN layers. In addition, significant numbers of threading dislocations were observed. High-resolution electron microscopy indicates that stacking faults present on the basal plane in these layers are of low-energy intrinsic I1type. This is consistent with diffraction contrast experiments.

  14. Temperature dependence of threading dislocation density in In 0.2Ga 0.8As layers grown on GaAs substrates by metalorganic-vapor phase epitaxy

    NASA Astrophysics Data System (ADS)

    Takano, Y.; Kobayashi, K.; Shirakata, S.; Umezawa, M.; Fuke, S.

    2005-08-01

    In 0.2Ga 0.8As layers were grown on GaAs substrates with graded buffer layers by metalorganic vapor phase epitaxy at 370-630 °C. Good surface morphology with a crosshatch pattern (CHP) was obtained at 600 and 630 °C. Transmission electron microscopy (TEM) observation confirmed that the cap layers had a threading dislocation density of between 1.3 and 2.0×10 6 cm -2. At 500 °C, a layer showed a rough surface morphology. Phase separation was revealed by TEM. The threading dislocation density was over 1×10 7 cm -2 at 500 °C. Good surface morphology with a CHP was obtained at 430, 450 and 480 °C. A layer grown at 430 °C showed the lowest threading dislocation density of 2.5×10 5 cm -2. Low temperature growth was effective for lowering the threading dislocation density in the cap layers.

  15. Effect of Threading Dislocations on the Quality Factor of InGaN/GaN Microdisk Cavities

    PubMed Central

    2014-01-01

    In spite of the theoretical advantages associated with nitride microcavities, the quality factors of devices with embedded indium gallium nitride (InGaN) or gallium nitride (GaN) optical emitters still remain low. In this work we identify threading dislocations (TDs) as a major limitation to the fabrication of high quality factor devices in the nitrides. We report on the use of cathodoluminescence (CL) to identify individual TD positions within microdisk lasers containing either InGaN quantum wells or quantum dots. Using CL to accurately count the number, and map the position, of dislocations within several individual cavities, we have found a clear correlation between the density of defects in the high-field region of a microdisk and its corresponding quality factor (Q). We discuss possible mechanisms associated with defects, photon scattering, and absorption, which could be responsible for degraded device performance. PMID:25839048

  16. Dislocation Detection Through Harmonic Generation

    NASA Astrophysics Data System (ADS)

    Reinhardt, B. T.; Kropf, M.; Boudraeu, K.; Guers, M. J.; Tittmann, B. R.

    2010-02-01

    A fundamental goal of ultrasonic nondestructive evaluation is to characterize material defects before failure. During material fatigue, dislocations tend to nucleate, becoming sources of stress concentration. Eventually, cracks start to form and lead to material failure. Recent research has indicated that nonlinear harmonic generation can be used to distinguish between materials of high and low dislocation densities. This research reports nonlinear harmonic generation measurements to distinguish between those areas of high and low dislocation densities in copper bars. The copper bars were subjected to flexural fatigue. Periodic scans were taken in order to track dislocation development during the fatigue life of the material. We show that this technique provides improved early detection for critical components of failure.

  17. Investigation of the Dynamics of a Screw Dislocation in Copper

    NASA Astrophysics Data System (ADS)

    Kolupaeva, S. N.; Petelina, Yu. P.; Polosukhin, K. A.; Petelin, A. E.

    2015-08-01

    A modification of the mathematical model of forming the crystallographic shear band is proposed in which the strength of elastic interaction between all dislocations of the forming dislocation pileups is taken into account in addition to the Peach-Keller force; lattice, impurity, and dislocation friction; linear tension; viscous braking; and intensity of generation of point defects behind kinks. The model is used to investigate the influence of the dislocation density on the time characteristics of the formation of dislocation loops in copper.

  18. Electronic and optical characteristics of an m-plane GaN single crystal grown by hydride vapor phase epitaxy on a GaN seed synthesized by the ammonothermal method using an acidic mineralizer

    NASA Astrophysics Data System (ADS)

    Kojima, Kazunobu; Tsukada, Yusuke; Furukawa, Erika; Saito, Makoto; Mikawa, Yutaka; Kubo, Shuichi; Ikeda, Hirotaka; Fujito, Kenji; Uedono, Akira; Chichibu, Shigefusa F.

    2016-05-01

    Fundamental electronic and optical properties of a low-resistivity m-plane GaN single crystal, which was grown by hydride vapor phase epitaxy on a bulk GaN seed crystal synthesized by the ammonothermal method in supercritical ammonia using an acidic mineralizer, were investigated. The threading dislocation and basal-plane staking-fault densities of the crystal were around 104 cm-2 and less than 100 cm-1, respectively. Oxygen doping achieved a high electron concentration of 4 × 1018 cm-3 at room temperature. Accordingly, a photoluminescence (PL) band originating from the recombination of hot carriers was observed at low temperatures, even under weak excitation conditions. The simultaneous realization of low-level incorporation of Ga vacancies (VGa) less than 1016 cm-3 was confirmed by using the positron annihilation technique. Consistent with our long-standing claim that VGa complexes are the major nonradiative recombination centers in GaN, the fast-component PL lifetime of the near-band-edge emission at room temperature longer than 2 ns was achieved.

  19. 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. PMID:27351723

  20. 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.

  1. Dislocation patterning in a two-dimensional continuum theory of dislocations

    NASA Astrophysics Data System (ADS)

    Groma, István; Zaiser, Michael; Ispánovity, Péter Dusán

    2016-06-01

    Understanding the spontaneous emergence of dislocation patterns during plastic deformation is a long standing challenge in dislocation theory. During the past decades several phenomenological continuum models of dislocation patterning were proposed, but few of them (if any) are derived from microscopic considerations through systematic and controlled averaging procedures. In this paper we present a two-dimensional continuum theory that is obtained by systematic averaging of the equations of motion of discrete dislocations. It is shown that in the evolution equations of the dislocation densities diffusionlike terms neglected in earlier considerations play a crucial role in the length scale selection of the dislocation density fluctuations. It is also shown that the formulated continuum theory can be derived from an averaged energy functional using the framework of phase field theories. However, in order to account for the flow stress one has in that case to introduce a nontrivial dislocation mobility function, which proves to be crucial for the instability leading to patterning.

  2. Surface morphology of homoepitaxial c-plane GaN: Hillocks and ridges

    NASA Astrophysics Data System (ADS)

    Oehler, F.; Zhu, T.; Rhode, S.; Kappers, M. J.; Humphreys, C. J.; Oliver, R. A.

    2013-11-01

    We investigated the properties of a GaN epilayer grown by metalorganic vapour phase epitaxy on a c-plane bulk GaN substrate obtained by ammonothermal growth. X-ray diffraction measurements showed that the epilayer and substrate were fully relaxed, had a miscut angle of 0.3±0.05° towards m and had omega rocking curve width values of 20-30 arcsec, limited by the instrumental broadening. Scanning capacitance microscopy data of the sample in cross-section indicated that the substrate had n-type conductivity with a carrier concentration of at least 1019 cm-3. Combined optical Nomarski microscopy, atomic-force microscopy and scanning electron microscope-cathodoluminescence studies showed the presence of large hexagonal pyramids on the surface, each associated with one or two dislocations with a screw-component threading from the substrate. This observation leads us to calculate a lower limit of the threading dislocation density of 3×102 cm-2. We predict that the formation of such hexagonal hillocks during epitaxy can be avoided with a slightly larger miscut angle of 0.4° or 0.5°. Another type of defect observed were ridge-like surface structures with narrow arrays of edge-type threading defects with a local density of 109 cm-2. However, the absence of threading defects below the regrowth interface at a ridge suggested that this type of structure is linked to (polishing) damage to the substrate surface and is therefore rated as an avoidable problem.

  3. 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.

  4. Point defects as a test ground for the local density approximation +U theory: Mn, Fe, and V{sub Ga} in GaN

    SciTech Connect

    Volnianska, O.; Zakrzewski, T.; Boguslawski, P.

    2014-09-21

    Electronic structure of the Mn and Fe ions and of the gallium vacancy V{sub Ga} in GaN was analysed within the GGA + U approach. First, the +U term was treated as a free parameter, and applied to p(N), d(Mn), and d(Fe). The band gap of GaN is reproduced for U(N) ≈ 4 eV. The electronic structure of defect states was found to be more sensitive to the value of U than that of the bulk states. Both the magnitude and the sign of the U-induced energy shifts of levels depend on occupancies, and thus on the defect charge state. The energy shifts also depend on the hybridization between defect and host states, and thus are different for different level symmetries. In the case of V{sub Ga}, these effects lead to stabilization of spin polarization and the “negative-U{sub eff}” behavior. The values of Us were also calculated using the linear response approach, which gives U(Fe) ≈ U(Mn) ≈ 4 eV. This reproduces well the results of previous hybrid functionals calculations. However, the best agreement with the experimental data is obtained for vanishing or even negative U(Fe) and U(Mn)

  5. Initial dislocation structure and dynamic dislocation multiplication in Mo single crystals

    SciTech Connect

    Hsiung, L M; Lassila, D H

    2000-03-22

    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 of 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.

  6. First-principles study of d0 ferromagnetism in alkali-metal doped GaN

    NASA Astrophysics Data System (ADS)

    Zhang, Yong

    2016-08-01

    The d0 ferromagnetism in GaN has been studied based on density functional theory. Our results show that GaN with sufficient hole become spin-polarized. Alkali-metal doping can introduce holes in GaN. Among them, both of Li- and Na-doping induce ferromagnetism in GaN and Na-doped GaN behaves as half-metallic ferromagnet. Moreover, at a growth temperature of 2000 K under N-rich condition, both concentrations can exceed 18%, which is sufficient to produce detectable macroscopic magnetism in GaN. The Curie temperature of Li- and Na-doped GaN is estimated to be 304 and 740 K, respectively, which are well above room temperature.

  7. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

    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.

  9. 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

  10. 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

    2016-01-01

    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 × 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. PMID:27412372

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

    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.

  12. Quantifying dislocation microstructure evolution and cyclic hardening in fatigued face-centered cubic single crystals

    NASA Astrophysics Data System (ADS)

    Hussein, Ahmed M.; El-Awady, Jaafar A.

    2016-06-01

    Discrete dislocation dynamics simulations were performed to investigate the dislocation microstructure evolution and cyclic hardening during the early stages of fatigue loading in nickel single crystals. The effects of the crystal size and initial dislocation densities on both the mechanical response and the evolution of dislocation microstructure were quantified. Crystals having an initial dislocation density of 1012 m-2 and diameter less than 2.0 μm do not show any dislocation density multiplication or cyclic hardening. In contrast, crystals having the same initial dislocation density and diameters larger than 2.0 μm show a significant dislocation density accumulation in the form of dislocation cell-like structures, even after only a few number of loading cycles. This dislocation density accumulation was also accompanied by considerable cyclic hardening. The dislocation cell size and its wall thickness increase with increasing crystal size. With increasing dislocation density the critical crystal size, at which dislocation cell-structures form, decreases. The information theoretic entropy is utilized as a metric to quantify the extent of dislocation patterning and the formation and evolution of dislocation cell structures over time. Cross-slip was found to play a dominant role in the dislocation cell-structure formation. Further insights on the mechanisms contributing to the observed behavior are presented and discussed.

  13. Crystallographically tilted and partially strain relaxed GaN grown on inclined {111} facets etched on Si(100) substrate

    NASA Astrophysics Data System (ADS)

    Ansah Antwi, K. K.; Soh, C. B.; Wee, Q.; Tan, Rayson J. N.; Yang, P.; Tan, H. R.; Sun, L. F.; Shen, Z. X.; Chua, S. J.

    2013-12-01

    High resolution X-ray diffractometry (HR-XRD), Photoluminescence, Raman spectroscopy, and Transmission electron microscope measurements are reported for GaN deposited on a conventional Si(111) substrate and on the {111} facets etched on a Si(100) substrate. HR-XRD reciprocal space mappings showed that the GaN(0002) plane is tilted by about 0.63° ± 0.02° away from the exposed Si{111} growth surface for GaN deposited on the patterned Si(100) substrate, while no observable tilt existed between the GaN(0002) and Si(111) planes for GaN deposited on the conventional Si(111) substrate. The ratio of integrated intensities of the yellow to near band edge (NBE) luminescence (IYL/INBE) was determined to be about one order of magnitude lower in the case of GaN deposited on the patterned Si(100) substrate compared with GaN deposited on the conventional Si(111) substrate. The Raman E2(high) optical phonon mode at 565.224 ± 0.001 cm-1 with a narrow full width at half maximum of 1.526 ± 0.002 cm-1 was measured, for GaN deposited on the patterned Si(100) indicating high material quality. GaN deposition within the trench etched on the Si(100) substrate occurred via diffusion and mass-transport limited mechanism. This resulted in a differential GaN layer thickness from the top (i.e., 1.8 μm) of the trench to the bottom (i.e., 0.3 μm) of the trench. Mixed-type dislocation constituted about 80% of the total dislocations in the GaN grown on the inclined Si{111} surface etched on Si(100).

  14. Crystallographically tilted and partially strain relaxed GaN grown on inclined (111) facets etched on Si(100) substrate

    SciTech Connect

    Ansah Antwi, K. K.; Soh, C. B.; Wee, Q.; Tan, Rayson J. N.; Tan, H. R.; Yang, P.; Sun, L. F.; Shen, Z. X.; Chua, S. J.

    2013-12-28

    High resolution X-ray diffractometry (HR-XRD), Photoluminescence, Raman spectroscopy, and Transmission electron microscope measurements are reported for GaN deposited on a conventional Si(111) substrate and on the (111) facets etched on a Si(100) substrate. HR-XRD reciprocal space mappings showed that the GaN(0002) plane is tilted by about 0.63° ± 0.02° away from the exposed Si(111) growth surface for GaN deposited on the patterned Si(100) substrate, while no observable tilt existed between the GaN(0002) and Si(111) planes for GaN deposited on the conventional Si(111) substrate. The ratio of integrated intensities of the yellow to near band edge (NBE) luminescence (I{sub YL}/I{sub NBE}) was determined to be about one order of magnitude lower in the case of GaN deposited on the patterned Si(100) substrate compared with GaN deposited on the conventional Si(111) substrate. The Raman E{sub 2}(high) optical phonon mode at 565.224 ± 0.001 cm{sup −1} with a narrow full width at half maximum of 1.526 ± 0.002 cm{sup −1} was measured, for GaN deposited on the patterned Si(100) indicating high material quality. GaN deposition within the trench etched on the Si(100) substrate occurred via diffusion and mass-transport limited mechanism. This resulted in a differential GaN layer thickness from the top (i.e., 1.8 μm) of the trench to the bottom (i.e., 0.3 μm) of the trench. Mixed-type dislocation constituted about 80% of the total dislocations in the GaN grown on the inclined Si(111) surface etched on Si(100)

  15. Epitaxy of GaN Nanowires on Graphene.

    PubMed

    Kumaresan, Vishnuvarthan; Largeau, Ludovic; Madouri, Ali; Glas, Frank; Zhang, Hezhi; Oehler, Fabrice; Cavanna, Antonella; Babichev, Andrey; Travers, Laurent; Gogneau, Noelle; Tchernycheva, Maria; Harmand, Jean-Christophe

    2016-08-10

    Epitaxial growth of GaN nanowires on graphene is demonstrated using molecular beam epitaxy without any catalyst or intermediate layer. Growth is highly selective with respect to silica on which the graphene flakes, grown by chemical vapor deposition, are transferred. The nanowires grow vertically along their c-axis and we observe a unique epitaxial relationship with the ⟨21̅1̅0⟩ directions of the wurtzite GaN lattice parallel to the directions of the carbon zigzag chains. Remarkably, the nanowire density and height decrease with increasing number of graphene layers underneath. We attribute this effect to strain and we propose a model for the nanowire density variation. The GaN nanowires are defect-free and they present good optical properties. This demonstrates that graphene layers transferred on amorphous carrier substrates is a promising alternative to bulk crystalline substrates for the epitaxial growth of high quality GaN nanostructures.

  16. Epitaxy of GaN Nanowires on Graphene.

    PubMed

    Kumaresan, Vishnuvarthan; Largeau, Ludovic; Madouri, Ali; Glas, Frank; Zhang, Hezhi; Oehler, Fabrice; Cavanna, Antonella; Babichev, Andrey; Travers, Laurent; Gogneau, Noelle; Tchernycheva, Maria; Harmand, Jean-Christophe

    2016-08-10

    Epitaxial growth of GaN nanowires on graphene is demonstrated using molecular beam epitaxy without any catalyst or intermediate layer. Growth is highly selective with respect to silica on which the graphene flakes, grown by chemical vapor deposition, are transferred. The nanowires grow vertically along their c-axis and we observe a unique epitaxial relationship with the ⟨21̅1̅0⟩ directions of the wurtzite GaN lattice parallel to the directions of the carbon zigzag chains. Remarkably, the nanowire density and height decrease with increasing number of graphene layers underneath. We attribute this effect to strain and we propose a model for the nanowire density variation. The GaN nanowires are defect-free and they present good optical properties. This demonstrates that graphene layers transferred on amorphous carrier substrates is a promising alternative to bulk crystalline substrates for the epitaxial growth of high quality GaN nanostructures. PMID:27414518

  17. Lateral subtalar dislocation.

    PubMed

    Sharda, Praveen; DuFosse, Julian

    2008-07-01

    Subtalar dislocations are rare in routine orthopedic practice. While many of these dislocations are a result of high-energy injuries such as fall from a height or traffic accidents, it is not uncommon for patients to present after slipping down a few stairs. Two types of dislocation have been described, medial and lateral. The type of dislocation is described according to the position of the foot. In lateral subtalar dislocation the head of talus is found medially and the calcaneus is dislocated laterally. The navicular may lie dorsolateral to the talus. The reverse is true of lateral dislocation. Medial dislocation has been referred to as "basketball foot" due to its preponderance in basketball players.4 The deciding factor is the inverted or everted position of the foot when the force is dissipated through the weak talonavicular and talocalcaneal ligaments. This article presents a case of an adult with lateral subtalar dislocation following a fall.

  18. Enhancing the field emission properties of Se-doped GaN nanowires.

    PubMed

    Li, Enling; Wu, Guishuang; Cui, Zhen; Ma, Deming; Shi, Wei; Wang, Xiaolin

    2016-07-01

    Pure and Se-doped GaN nanowires (NWs) are synthesized on Pt-coated Si(111) substrates via chemical vapor deposition. The GaN NWs exhibit a uniform density with an average diameter of 20-120 nm. The structure of the NWs is wurtzite hexagonal, and the growth direction is along [0001]. Field emission measurements show that the Se-doped GaN NWs possess a low turn-on field (2.9 V μm(-1)) compared with the pure GaN NWs (7.0 V μm(-1)). In addition, density functional theory calculations indicate that the donor states near the Fermi level are mainly formed through the hybridization between Se 4p and N 2p orbitals and that the Fermi level move towards the vacuum level. Consequently, the work functions of Se-doped GaN NWs are lower than those of pure GaN NWs.

  19. Enhancing the field emission properties of Se-doped GaN nanowires

    NASA Astrophysics Data System (ADS)

    Li, Enling; Wu, Guishuang; Cui, Zhen; Ma, Deming; Shi, Wei; Wang, Xiaolin

    2016-07-01

    Pure and Se-doped GaN nanowires (NWs) are synthesized on Pt-coated Si(111) substrates via chemical vapor deposition. The GaN NWs exhibit a uniform density with an average diameter of 20–120 nm. The structure of the NWs is wurtzite hexagonal, and the growth direction is along [0001]. Field emission measurements show that the Se-doped GaN NWs possess a low turn-on field (2.9 V μm‑1) compared with the pure GaN NWs (7.0 V μm‑1). In addition, density functional theory calculations indicate that the donor states near the Fermi level are mainly formed through the hybridization between Se 4p and N 2p orbitals and that the Fermi level move towards the vacuum level. Consequently, the work functions of Se-doped GaN NWs are lower than those of pure GaN NWs.

  20. Dislocation motion and instability

    NASA Astrophysics Data System (ADS)

    Zhu, Yichao; Chapman, Stephen Jonathan; Acharya, Amit

    2013-08-01

    The Peach-Koehler expression for the stress generated by a single (non-planar) curvilinear dislocation is evaluated to calculate the dislocation self stress. This is combined with a law of motion to give the self-induced motion of a general dislocation curve. A stability analysis of a rectilinear, uniformly translating dislocation is then performed. The dislocation is found to be susceptible to a helical instability, with the maximum growth rate occurring when the dislocation is almost, but not exactly, pure screw. The non-linear evolution of the instability is determined numerically, and implications for slip band formation and non-Schmid behavior in yielding are discussed.

  1. Modeling hydrogen transport by dislocations

    NASA Astrophysics Data System (ADS)

    Dadfarnia, Mohsen; Martin, May L.; Nagao, Akihide; Sofronis, Petros; Robertson, Ian M.

    2015-05-01

    Recent experimental studies of the microstructure beneath fracture surfaces of specimens fractured in the presence of high concentrations of hydrogen suggest that the dislocation structure and hydrogen transported by mobile dislocations play important roles in establishing the local conditions that promote failure. The experiments demonstrate that hydrogen is responsible for the copious plasticity in large volumes of material before the onset of fracture and further afield from a crack tip. A revised model for hydrogen transport that accounts for hydrogen carried by dislocations along with stress driven diffusion and trapping at other microstructural defects is proposed. With the use of this new model, numerical simulation results for transient hydrogen profiles in the neighborhood of a crack tip are presented. Based on hydrogen-enhanced dislocation mobility and density, the results indicate that dislocation transport can contribute to the elevation of the local hydrogen concentrations ahead of the crack to levels above those predicted by the classical diffusion model and to distributions that extend farther afield.

  2. Effect of various buffer-layer structures on the material quality and dislocation density of high composition Al/sub x/Ga/sub 1/. sqrt. /sub x/As laser material grown by metalorganic chemical vapor deposition

    SciTech Connect

    Givens, M.E.; Coleman, J.J.; Zmudzinski, C.A.; Bryan, R.P.; Emanuel, M.A.; Miller, L.M.

    1988-05-15

    The effect of various types of buffer layers on the generation and propagation of dislocations in epitaxial layers of high composition (x = 0.85) Al/sub x/Ga/sub 1/..sqrt../sub x/As grown by metalorganic chemical vapor deposition (MOCVD) on horizontal Bridgman (HB) and liquid-encapsulated Czochralski (LEC) substrates is examined. Bulk epilayers of high composition (x = 0.85) Al/sub x/Ga/sub 1/..sqrt../sub x/As and graded-barrier quantum-well laser structures with confining layers of the same composition were grown simultaneously on high-qualitylow etch-pit density (EPD) HB substrates and comparatively lower qualityhigh EPD LEC substrates with one of four types of compositionally graded andor superlattice buffer-layer structures. The bulk material was characterized by delineation and measurement of surface EPD and the observation of overall surface morphology. Data are also presented on the device characteristics of graded-barrier quantum-well laser diodes grown with these same buffer layers in order to determine the correlation between dislocation density and laser threshold current. The various buffer-layer structures were seen to be effective in reducing the defect density and improving the surface morphology of high composition epilayers grown on both HB and LEC substrates. The threshold-current density of the laser diodes, however, was independent of both the type of prelayer andor substrate utilized

  3. Lateral growth of GaN by liquid phase electroepitaxy using mesa-shaped substrate

    NASA Astrophysics Data System (ADS)

    Kambayashi, Daisuke; Takakura, Hiroyuki; Tomita, Masafumi; Iwakawa, Muneki; Mizuno, Yosuke; Maruyama, Takahiro; Naritsuka, Shigeya

    2016-10-01

    GaN microchannel epitaxy (MCE) was performed using a mesa-shaped substrate and liquid phase electroepitaxy. A flat and wide MCE layer was successfully obtained with a rectangular shape, which is formed by ±c-planes on both the top and bottom surfaces. MCE growth proceeded mainly in the lateral direction by the formation of these planes. Cathodoluminescence measurements showed that the laterally grown layers were almost free of dislocations, and that the dislocations in the mesa areas were confined by the vertical sides of the mesas. In the case of inclined sides, the dislocations would be expected to bend and spread into the laterally grown areas.

  4. Microstructure of GaN Grown on (111) Si by MOCVD

    SciTech Connect

    Fleming, J.G.; Follstaedt, D.M.; Han, J.; Provencio, P.

    1998-12-17

    Gallium nitride was grown on (111) Si by MOCVD by depositing an AIN buffer at 108O"C and then GaN at 1060 {degrees}C. The 2.2pm layer cracked along {1-100} planes upon cooling to room temperature, but remained adherent. We were able to examine the microstructure of material between cracks with TEM. The character and arrangement of dislocation are much like those of GaN grown on Al{sub 2}O{sub 3}: -2/3 pure edge and - 1/3 mixed (edge + screw), arranged in boundaries around domains of GaN that are slightly disoriented with respect to neighboring material. The 30 nm AIN buffer is continuous, indicating that AIN wets the Si, in contrast to GaN on Al{sub 2}O{sub 3}.

  5. Temporomandibular joint dislocation

    PubMed Central

    Sharma, Naresh Kumar; Singh, Akhilesh Kumar; Pandey, Arun; Verma, Vishal; Singh, Shreya

    2015-01-01

    Temporomandibular joint (TMJ) dislocation is an uncommon but debilitating condition of the facial skeleton. The condition may be acute or chronic. Acute TMJ dislocation is common in clinical practice and can be managed easily with manual reduction. Chronic recurrent TMJ dislocation is a challenging situation to manage. In this article, we discuss the comprehensive review of the different treatment modalities in managing TMJ dislocation. PMID:26668447

  6. Thermal stress induced dislocation distribution in directional solidification of Si for PV application

    NASA Astrophysics Data System (ADS)

    Jiptner, Karolin; Gao, Bing; Harada, Hirofumi; Miyamura, Yoshiji; Fukuzawa, Masayuki; Kakimoto, Koichi; Sekiguchi, Takashi

    2014-12-01

    This paper presents the limitation of the cast technique for silicon growth and the obstacle to reduce the dislocation density below 103 cm-2. The thermal stress induced dislocation density, independent of other dislocation sources, is determined and the result suggests that local dislocation densities as high as 104 cm-2 are readily introduced alone in the cooling period of the crystal growth. Areas of high residual strain and dislocation densities are identified and presented. The experimental results are correlated with numerical simulation based on a three-dimensional Haasen-Alexander-Sumino (HAS) model. The dislocation introduction is caused by an activation of different slip systems in different ingot areas.

  7. Statistics of dislocation pinning at localized obstacles

    NASA Astrophysics Data System (ADS)

    Dutta, A.; Bhattacharya, M.; Barat, P.

    2014-10-01

    Pinning of dislocations at nanosized obstacles like precipitates, voids, and bubbles is a crucial mechanism in the context of phenomena like hardening and creep. The interaction between such an obstacle and a dislocation is often studied at fundamental level by means of analytical tools, atomistic simulations, and finite element methods. Nevertheless, the information extracted from such studies cannot be utilized to its maximum extent on account of insufficient information about the underlying statistics of this process comprising a large number of dislocations and obstacles in a system. Here, we propose a new statistical approach, where the statistics of pinning of dislocations by idealized spherical obstacles is explored by taking into account the generalized size-distribution of the obstacles along with the dislocation density within a three-dimensional framework. Starting with a minimal set of material parameters, the framework employs the method of geometrical statistics with a few simple assumptions compatible with the real physical scenario. The application of this approach, in combination with the knowledge of fundamental dislocation-obstacle interactions, has successfully been demonstrated for dislocation pinning at nanovoids in neutron irradiated type 316-stainless steel in regard to the non-conservative motion of dislocations. An interesting phenomenon of transition from rare pinning to multiple pinning regimes with increasing irradiation temperature is revealed.

  8. Statistics of dislocation pinning at localized obstacles

    SciTech Connect

    Dutta, A.; Bhattacharya, M. Barat, P.

    2014-10-14

    Pinning of dislocations at nanosized obstacles like precipitates, voids, and bubbles is a crucial mechanism in the context of phenomena like hardening and creep. The interaction between such an obstacle and a dislocation is often studied at fundamental level by means of analytical tools, atomistic simulations, and finite element methods. Nevertheless, the information extracted from such studies cannot be utilized to its maximum extent on account of insufficient information about the underlying statistics of this process comprising a large number of dislocations and obstacles in a system. Here, we propose a new statistical approach, where the statistics of pinning of dislocations by idealized spherical obstacles is explored by taking into account the generalized size-distribution of the obstacles along with the dislocation density within a three-dimensional framework. Starting with a minimal set of material parameters, the framework employs the method of geometrical statistics with a few simple assumptions compatible with the real physical scenario. The application of this approach, in combination with the knowledge of fundamental dislocation-obstacle interactions, has successfully been demonstrated for dislocation pinning at nanovoids in neutron irradiated type 316-stainless steel in regard to the non-conservative motion of dislocations. An interesting phenomenon of transition from rare pinning to multiple pinning regimes with increasing irradiation temperature is revealed.

  9. Defect reduction in (11-20) a-plane GaN by two step epitaxiallateral overgrowth

    SciTech Connect

    Ni, X.; Ozgur, U.; Fu, Y.; Biyikii, N.; Morkoc, H.; Liliental-Weber, Z.

    2006-11-25

    We report a two-step growth method to obtain uniformly coalesced epitaxial lateral overgrown a-plane GaN by metal-organic chemical vapor deposition (MOCVD). By obtaining a large wing height to width aspect ratio in the first step followed by enhanced lateral growth in the second step via controlling the growth temperature, we reduced the tilt angle between the advancing Ga-polar and N-polar wings for improved properties. Transmission electron microscopy (TEM) showed that the threading dislocation density in the wing area was 1.0 x 10{sup 8}cm{sup -2}, more than two orders of magnitude lower than that in the window area (4.2 x 10{sup 10} cm{sup -2}). However, a high density of basal stacking faults, 1.2 x 10{sup 4} cm{sup -1}, was still observed in the wing area. Near field scanning optical microscopy (NSOM) at room temperature revealed that the luminescence was mainly from the wing regions with very little contribution from the windows and meeting fronts. These observations suggest that due to significant reduction of threading dislocations radiative recombination is enhanced in the wings.

  10. Spin and phase relaxation dynamics in GaN and GaN/AlGaN quantum wells (Presentation Recording)

    NASA Astrophysics Data System (ADS)

    Gallart, Mathieu; Ziegler, Marc; Hönerlage, Bernd H.; Gilliot, Pierre; Feltin, Eric; Carlin, Jean-François; Butté, Raphaël.; Grandjean, Nicolas

    2015-09-01

    By performing time-resolved optical non-degenerate pump-probe experiments, we study the relaxation dynamics of spin-polarized excitons in wurtzite epitaxial GaN and in nitride nanostructures. Those materials are indeed promising candidates for spintronic applications because of their weak spin-orbit coupling and large exciton binding energy (~ 17 meV and ~ 26meV in bulk GaN, respectively). In epilayers, we show that the high density of dislocations increases dramatically the spin relaxation of electrons and holes through the defect assisted Elliott-Yafet mechanism. That makes the exciton dephasing time very short. In high quality GaN/AlGaN quantum wells, both the exciton-spin lifetime S and the exciton dephasing-time T2 were determined via pump-probe spectroscopy using polarized laser pulses and time-resolved four wave-mixing experiments. The evolution of both quantities with temperature shows that spin relaxation occurs in the motional narrowing regime up to 80 K. Above this threshold, the thermal energy becomes large enough for excitons to escape from the QW. Such measurements demonstrate that GaN-based heterostructures can reach a very high degree of control that was previously mostly restricted to conventional III-V semiconductors and more specifically to the arsenide family.

  11. The influence of the dislocation distribution heterogeneity degree on the formation of a non-misoriented dislocation cell substructures in f.c.c. metals

    NASA Astrophysics Data System (ADS)

    Cherepanov, D. N.; Selivanikova, O. V.; Matveev, M. V.

    2016-06-01

    Dislocation loops emitted by Frank-Reed source during crossing dislocations of the non-coplanar slip systems are accumulates jogs on the own dislocation line, resulting in the deceleration of the segments of dislocation loops with high jog density. As a result, bending around of the slowed segments the formation of dynamic dipoles in the shear zone occurs. In the present paper we consider formation mechanism of non-misoriented dislocation cell substructure during plastic deformation of f.c.c. metals and conclude that the increase in the degree heterogeneity of dislocation distribution leads to an increase in the jog density and reduce the mean value of arm dynamic dipoles.

  12. Two-step epitaxial lateral overgrowth of a-plane GaN by MOCVD

    NASA Astrophysics Data System (ADS)

    Ni, X.; Özgür, Ü.; Morkoç, H.; Baski, A. A.; Liliental-Weber, Z.; Everitt, H. O.

    2007-02-01

    We report on growth and characterization of epitaxial lateral overgrown (ELO) (11 II -0) a-plane GaN by metalorganic chemical vapor deposition (MOCVD). The ELO samples were grown using a SiO II striped mask pattern consisting of 4 μm wide open windows and 10 μm or 20 μm wide SiO II stripes. Different growth rates in Ga- and N-wings along with the wing tilt create a major obstacle for achieving a fully coalesced flat surface in ELO-GaN. To address this problem we have employed a two-step growth method that is able to provide a high height to width aspect ratio in the first growth step followed by enhanced lateral growth in the second step by controlling the growth temperature. Depending on the growth conditions, lateral growth rate of the wings with Ga-polarity were from 2 to 5 times larger than that of the N-polarity wings. We investigated the effects of growth parameters on wing tilt, which was observed to be ~0.25° from the Kikuchi lines using large angle convergent beam electron diffraction (LACBED) and accompanied by some twist (0.09°) between the two opposite wings. Transmission electron microscopy (TEM) results showed that the threading dislocation density in the resulting fully coalesced overgrown GaN was reduced from 4.2×10 10 cm -2 in the window area to 1.0×10 8 cm -2 in the wing area, and that the wing areas contained relatively high density of basal stacking faults, 1.2×10 4 cm -1. The recombination of carriers/excitons localized at stacking faults was evident in far-field near bandedge photoluminescence (PL) measured at 10 K. Moreover, atomic force microscopy (AFM) measurements revealed two orders of magnitude higher density of surface pits in window than in wing regions, which could be decorating dislocation termination on surface. Time-resolved PL measurements for the a-plane ELO-GaN samples revealed biexponential decays. The recombination times were significantly increased ( τ I =80 ps and τ II =250 ps) compared to the standard a

  13. Analysis of dislocation pile-ups using a dislocation-based continuum theory

    NASA Astrophysics Data System (ADS)

    Schulz, K.; Dickel, D.; Schmitt, S.; Sandfeld, S.; Weygand, D.; Gumbsch, P.

    2014-03-01

    The increasing demand for materials with well-defined microstructure, accompanied by the advancing miniaturization of devices, is the reason for the growing interest in physically motivated, dislocation-based continuum theories of plasticity. In recent years, various advanced continuum theories have been introduced, which are able to described the motion of straight and curved dislocation lines. The focus of this paper is the question of how to include fundamental properties of discrete dislocations during their motion and interaction in a continuum dislocation dynamics (CDD) theory. In our CDD model, we obtain elastic interaction stresses for the bundles of dislocations by a mean-field stress, which represents long-range stress components, and a short range corrective stress component, which represents the gradients of the local dislocation density. The attracting and repelling behavior of bundles of straight dislocations of the same and opposite sign are analyzed. Furthermore, considering different dislocation pile-up systems, we show that the CDD formulation can solve various fundamental problems of micro-plasticity. To obtain a mesh size independent formulation (which is a prerequisite for further application of the theory to more complex situations), we propose a discretization dependent scaling of the short range interaction stress. CDD results are compared to analytical solutions and benchmark data obtained from discrete dislocation simulations.

  14. 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.

  15. Intrinsic degradation mechanism of nearly lattice-matched InAlN layers grown on GaN substrates

    NASA Astrophysics Data System (ADS)

    Perillat-Merceroz, Guillaume; Cosendey, Gatien; Carlin, Jean-François; Butté, Raphaël; Grandjean, Nicolas

    2013-02-01

    Thanks to its high refractive index contrast, band gap, and polarization mismatch compared to GaN, In0.17Al0.83N layers lattice-matched to GaN are an attractive solution for applications such as distributed Bragg reflectors, ultraviolet light-emitting diodes, or high electron mobility transistors. In order to study the structural degradation mechanism of InAlN layers with increasing thickness, we performed metalorganic vapor phase epitaxy of InAlN layers of thicknesses ranging from 2 to 500 nm, on free-standing (0001) GaN substrates with a low density of threading dislocations, for In compositions of 13.5% (layers under tensile strain), and 19.7% (layers under compressive strain). In both cases, a surface morphology with hillocks is initially observed, followed by the appearance of V-defects. We propose that those hillocks arise due to kinetic roughening, and that V-defects subsequently appear beyond a critical hillock size. It is seen that the critical thickness for the appearance of V-defects increases together with the surface diffusion length either by increasing the temperature or the In flux because of a surfactant effect. In thick InAlN layers, a better (worse) In incorporation occurring on the concave (convex) shape surfaces of the V-defects is observed leading to a top phase-separated InAlN layer lying on the initial homogeneous InAlN layer after V-defects coalescence. It is suggested that similar mechanisms could be responsible for the degradation of thick InGaN layers.

  16. 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.

  17. Dislocation characterization in cold rolled stainless steel using nonlinear ultrasonic techniques: A comprehensive model

    NASA Astrophysics Data System (ADS)

    Zhang, Jianfeng; Xuan, Fu-zhen; Xiang, Yanxun

    2013-09-01

    Both dislocation density and character in cold rolled stainless steel cause the change of acoustic nonlinearity. An analytical model considering the different oscillating motion of edge and screw dislocations is presented for the generation of ultrasonic harmonic wave during the process of multiplication and motion of dislocation. Results reveal that the edge dislocation induces stronger acoustic nonlinearity response than screw dislocation. The new model is certified by the application to the cold rolled stainless steel.

  18. Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods

    NASA Astrophysics Data System (ADS)

    Conroy, M.; Li, H.; Kusch, G.; Zhao, C.; Ooi, B.; Edwards, P. R.; Martin, R. W.; Holmes, J. D.; Parbrook, P. J.

    2016-05-01

    We report a method of growing site controlled InGaN multiple quantum discs (QDs) at uniform wafer scale on coalescence free ultra-high density (>80%) nanorod templates by metal organic chemical vapour deposition (MOCVD). The dislocation and coalescence free nature of the GaN space filling nanorod arrays eliminates the well-known emission problems seen in InGaN based visible light sources that these types of crystallographic defects cause. Correlative scanning transmission electron microscopy (STEM), energy-dispersive X-ray (EDX) mapping and cathodoluminescence (CL) hyperspectral imaging illustrates the controlled site selection of the red, yellow and green (RYG) emission at these nano tips. This article reveals that the nanorod tips' broad emission in the RYG visible range is in fact achieved by manipulating the InGaN QD's confinement dimensions, rather than significantly increasing the In%. This article details the easily controlled method of manipulating the QDs dimensions producing high crystal quality InGaN without complicated growth conditions needed for strain relaxation and alloy compositional changes seen for bulk planar GaN templates.We report a method of growing site controlled InGaN multiple quantum discs (QDs) at uniform wafer scale on coalescence free ultra-high density (>80%) nanorod templates by metal organic chemical vapour deposition (MOCVD). The dislocation and coalescence free nature of the GaN space filling nanorod arrays eliminates the well-known emission problems seen in InGaN based visible light sources that these types of crystallographic defects cause. Correlative scanning transmission electron microscopy (STEM), energy-dispersive X-ray (EDX) mapping and cathodoluminescence (CL) hyperspectral imaging illustrates the controlled site selection of the red, yellow and green (RYG) emission at these nano tips. This article reveals that the nanorod tips' broad emission in the RYG visible range is in fact achieved by manipulating the InGaN QD

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

    NASA Astrophysics Data System (ADS)

    Aseev, P.; Gačević, Ž.; Torres-Pardo, A.; González-Calbet, J. M.; Calleja, E.

    2016-06-01

    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 propagation 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.

  20. Thermodynamic forces in single crystals with dislocations

    NASA Astrophysics Data System (ADS)

    Van Goethem, Nicolas

    2014-06-01

    A simple model for the evolution of macroscopic dislocation regions in a single crystal is presented. This model relies on maximal dissipation principle within Kröner's geometric description of the dislocated crystal. Mathematical methods and tools from shape optimization theory provide equilibrium relations at the dislocation front, similarly to previous work achieved on damage modelling (J Comput Phys 33(16):5010-5044, 2011). The deformation state variable is the incompatible strain as related to the dislocation density tensor by a relation involving the Ricci curvature of the crystal underlying elastic metric. The time evolution of the model variables follows from a novel interpretation of the Einstein-Hilbert flow in terms of dislocation microstructure energy. This flow is interpreted as the dissipation of non-conservative dislocations, due to the climb mechanism, modelled by an average effect of mesoscopic dislocations moving normal to their glide planes by adding or removing points defects. The model equations are a fourth-order tensor parabolic equation involving the operator "incompatibility," here appearing as a tensorial counterpart of the scalar Laplacian. This work encompasses and generalizes results previously announced (C R Acad Sci Paris Ser I 349:923-927, 2011), with in addition a series of physical interpretations to give a meaning to the newly introduced concepts.

  1. Bilateral Anterior Shoulder Dislocation

    PubMed Central

    Siu, Yuk Chuen; Lui, Tun Hing

    2014-01-01

    Introduction: Unilateral anterior shoulder dislocation is one of the most common problems encountered in orthopedic practice. However, simultaneous bilateral anterior dislocation of the shoulders is quite rare. Case Presentation: We report a case of a 75-year-old woman presented with simultaneous bilateral anterior shoulder dislocation following a trauma, complicated with a traction injury to the posterior cord of the brachial plexus. Conclusions: Bilateral anterior shoulder dislocation is very rare. The excessive traction force during closed reduction may lead to nerve palsy. Clear documentation of neurovascular status and adequate imaging before and after a reduction should be performed. PMID:25685749

  2. Initial stages of misfit stress relaxation through the formation of prismatic dislocation loops in GaN-Ga2O3 composite nanostructures

    NASA Astrophysics Data System (ADS)

    Gutkin, M. Yu.; Smirnov, A. M.

    2016-08-01

    The initial stages of misfit stress relaxation through the formation of rectangular prismatic dislocation loops in model composite nanostructures have been considered. The nanostructures are either spherical or cylindrical GaN shells grown on solid or hollow β-Ga2O3 cores or planar thin GaN films on β-Ga2O3 substrates. Three characteristic configurations of prismatic dislocation loops, namely, square loops, loops elongated along the GaN/Ga2O3 interface, and loops elongated along the normal to the GaN/Ga2O3 interface, have been analyzed. The generation of prismatic dislocation loops from the interface into the bulk of the GaN shell (film), from the free surface into the GaN shell (film), and from the interface into the β-Ga2O3 core (substrate) has been investigated. It has been shown that, for the minimum known estimate of the lattice misfit (2.6%) in some of the considered nanostructures, no any prismatic dislocation loops can be generated. If the generation of prismatic dislocation loops is possible, then in all the considered nanostructures, the energetically more favorable case corresponds to prismatic dislocation loops elongated along the GaN/Ga2O3 interfaces, and the more preferred generation of prismatic dislocation loops occurs from the GaN free surface. The GaN/Ga2O3 nanostructures that are the most and least resistant to the formation of prismatic dislocation loops have been determined. It has been found that, among the considered nanostructures, the planar two-layer GaN/Ga2O3 plate is the most resistant to the generation of prismatic dislocation loops, which is explained by the action of an alternative mechanism for the relaxation of misfit stresses due to the bending of the plate. The least resistant nanostructure is the planar three-layer GaN/Ga2O3/GaN plate, in which GaN films have an identical thickness and which itself as a whole does not undergo bending. The critical thicknesses of the GaN shells (films), which must be exceeded to ensure the

  3. Using the kinetic Wulff plot to design and control nonpolar and semipolar GaN heteroepitaxy

    NASA Astrophysics Data System (ADS)

    Leung, Benjamin; Sun, Qian; Yerino, Christopher D.; Han, Jung; Coltrin, Michael E.

    2012-02-01

    For nonpolar and semipolar orientations of GaN heteroepitaxially grown on sapphire substrates, the development of growth procedures to improve surface morphology and microstructure has been driven in a largely empirical way. This work attempts to comprehensively link the intrinsic properties of GaN faceted growth, across orientations, in order to understand, design and control growth methods for nonpolar (1 1 2 0) GaN and semipolar (1 1 2 2) GaN on foreign substrates. This is done by constructing a comprehensive series of kinetic Wulff plots (or v-plots) by monitoring the advances of convex and concave facets in selective area growth. A methodology is developed to apply the experimentally determined v-plots to the interpretation and design of evolution dynamics in nucleation and island coalescence. This methodology offers a cohesive and rational model for GaN heteroepitaxy along polar, nonpolar and semipolar orientations, and is broadly extensible to the heteroepitaxy of other materials. We demonstrate furthermore that the control of morphological evolution, based on invoking a detailed knowledge of the v-plots, holds a key to the reduction of microstructural defects through effective bending of dislocations and blocking of stacking faults. The status and outlook of semipolar and nonpolar GaN growth on sapphire substrates will be presented.

  4. Dislocation electrical conductivity of synthetic diamond films

    SciTech Connect

    Samsonenko, S. N. Samsonenko, N. D.

    2009-05-15

    A relationship between the electric resistance of single-crystal homoepitaxial and polycrystalline diamond films and their internal structure has been investigated. It is established that the electrical conductivity of undoped homoepitaxial and polycrystalline diamond films is directly related to the dislocation density in them. A relation linking the resistivity {rho} ({approx}10{sup 13}-10{sup 15} {omega} cm) with the dislocation density {gamma} ({approx}10{sup 14}-4 x 10{sup 16} m{sup -2}) is obtained. The character of this correlation is similar for both groups of homoepitaxial and polycrystalline diamond films. Thin ({approx}1-8 {mu}m) homoepitaxial and polycrystalline diamond films with small-angle dislocation boundaries between mosaic blocks exhibit dislocation conductivity. The activation energy of dislocation acceptor centers was calculated from the temperature dependence of the conductivity and was found to be {approx}0.3 eV. The conduction of thick diamond films (h > 10 {mu}m) with the resistivity {rho} {approx} 10{sup 8} {omega} cm is determined by the conduction of intercrystallite boundaries, which have a nondiamond hydrogenated structure. The electronic properties of the diamond films are compared with those of natural semiconductor diamonds of types IIb and Ic, in which dislocation acceptor centers have activation energies in the range 0.2-0.35 eV and are responsible for hole conduction.

  5. Study of defect management in the growth of semipolar (11-22) GaN on patterned sapphire

    NASA Astrophysics Data System (ADS)

    Vennéguès, P.; Tendille, F.; De Mierry, P.

    2015-08-01

    This work describes, using mainly transmission electron microscopy as an investigation tool, the nature and behaviour of the crystalline defects which are present in (11-22) semipolar GaN films grown epitaxially on patterned r-sapphire substrates using a 3 step growth process. The microstructure at these different growth stages is described. The independent 3D-crystallites nucleated on the substrate surface contain threading dislocations resulting from the epitaxy on c-sapphire facets and basal stacking faults (BSFs), mainly in the  -c-wings. These defects are concentrated in a few hundred nanometre wide stripe-like regions emerging on the top facet of the islands. By a careful choice of the growth conditions, these defective regions may be overgrown by defect-free material, blocking their propagation towards the coalesced surface. However, when the 3D crystals coalesce, new dislocations together with very few BSFs are created at the coalescence boundaries. These coalescence defects propagate to the surface of the films in (0001) planes. In summary, the control of the nucleation and propagation of the crystalline defects allows obtaining large area semipolar films with very low defect densities: 7   ×   107 cm-2 for TDs and 70 cm-1 for BSFs.

  6. Electron density window for best frequency performance, lowest phase noise and slowest degradation of GaN heterostructure field-effect transistors

    NASA Astrophysics Data System (ADS)

    Matulionis, Arvydas

    2013-07-01

    The problems in the realm of nitride heterostructure field-effect transistors (HFETs) are discussed in terms of a novel fluctuation-dissipation-based approach impelled by a recent demonstration of strong correlation of hot-electron fluctuations with frequency performance and degradation of the devices. The correlation has its genesis in the dissipation of the LO-mode heat accumulated by the non-equilibrium longitudinal optical phonons (hot phonons) confined in the channel that hosts the high-density hot-electron gas subjected to a high electric field. The LO-mode heat causes additional scattering of hot electrons and facilitates defect formation in a different manner than the conventional heat contained mainly in the acoustic phonon mode. We treat the heat dissipation problem in terms of the hot-phonon lifetime responsible for the conversion of the non-migrant hot phonons into migrant acoustic modes and other vibrations. The lifetime is measured over a wide range of electron density and supplied electric power. The optimal conditions for the dissipation of the LO-mode heat are associated with the plasmon-assisted disintegration of hot phonons. Signatures of plasmons are experimentally resolved in fluctuations, dissipation, hot-electron transport, transistor frequency performance, transistor phase noise and transistor reliability. In particular, a slower degradation and a faster operation of GaN-based HFETs take place inside the electron density window where the resonant plasmon-assisted ultrafast dissipation of the LO-mode heat comes into play. A novel heterostructure design for the possible improvement of HFET performance is proposed, implemented and tested.

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

    NASA Astrophysics Data System (ADS)

    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.

    2016-05-01

    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.

  8. 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.

  9. Structural effects of field emission from GaN nanofilms on SiC substrates

    SciTech Connect

    Chen, Cheng-Cheng; Wang, Ru-Zhi Zhu, Man-Kang; Yan, Hui; Liu, Peng; Wang, Bi-Ben

    2014-04-21

    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 of 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.

  10. 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.

  11. Traumatic proximal tibiofibular dislocation.

    PubMed

    Burgos, J; Alvarez-Montero, R; Gonzalez-Herranz, P; Rapariz, J M

    1997-01-01

    Proximal tibiofibular dislocation is an exceptional lesion. Rarer still is its presentation in childhood. We describe the clinical case of a 6-year-old boy, the victim of a road accident. He had a tibiofibular dislocation associated with a metaphyseal fracture of the tibia.

  12. Parallel Dislocation Simulator

    2006-10-30

    ParaDiS is software capable of simulating the motion, evolution, and interaction of dislocation networks in single crystals using massively parallel computer architectures. The software is capable of outputting the stress-strain response of a single crystal whose plastic deformation is controlled by the dislocation processes.

  13. Spatially resolved and orientation dependent Raman mapping of epitaxial lateral overgrowth nonpolar a-plane GaN on r-plane sapphire

    PubMed Central

    Jiang, Teng; Xu, Sheng-rui; Zhang, Jin-cheng; Xie, Yong; Hao, Yue

    2016-01-01

    Uncoalesced a-plane GaN epitaxial lateral overgrowth (ELO) structures have been synthesized along two mask stripe orientations on a-plane GaN template by MOCVD. The morphology of two ELO GaN structures is performed by Scanning electronic microscopy. The anisotropy of crystalline quality and stress are investigated by micro-Raman spectroscopy. According to the Raman mapping spectra, the variations on the intensity, peak shift and the full width at half maximum (FWHM) of GaN E2 (high) peak indicate that the crystalline quality improvement occurs in the window region of the GaN stripes along [0001], which is caused by the dislocations bending towards the sidewalls. Conversely, the wing regions have better quality with less stress as the dislocations propagated upwards when the GaN stripes are along []. Spatial cathodoluminescence mapping results further support the explanation for the different dislocation growth mechanisms in the ELO processes with two different mask stripe orientations. PMID:26821824

  14. Spatially resolved and orientation dependent Raman mapping of epitaxial lateral overgrowth nonpolar a-plane GaN on r-plane sapphire.

    PubMed

    Jiang, Teng; Xu, Sheng-Rui; Zhang, Jin-Cheng; Xie, Yong; Hao, Yue

    2016-01-29

    Uncoalesced a-plane GaN epitaxial lateral overgrowth (ELO) structures have been synthesized along two mask stripe orientations on a-plane GaN template by MOCVD. The morphology of two ELO GaN structures is performed by Scanning electronic microscopy. The anisotropy of crystalline quality and stress are investigated by micro-Raman spectroscopy. According to the Raman mapping spectra, the variations on the intensity, peak shift and the full width at half maximum (FWHM) of GaN E2 (high) peak indicate that the crystalline quality improvement occurs in the window region of the GaN stripes along [0001], which is caused by the dislocations bending towards the sidewalls. Conversely, the wing regions have better quality with less stress as the dislocations propagated upwards when the GaN stripes are along []. Spatial cathodoluminescence mapping results further support the explanation for the different dislocation growth mechanisms in the ELO processes with two different mask stripe orientations.

  15. Electromechanical simulations of dislocations

    NASA Astrophysics Data System (ADS)

    Skiba, Oxana; Gracie, Robert; Potapenko, Stanislav

    2013-04-01

    Improving the reliability of micro-electronic devices depends in part on developing a more in-depth understanding of dislocations because dislocations are barriers to charge carriers. To this end, the quasi-static simulation of discrete dislocations dynamics in materials under mechanical and electrical loads is presented. The simulations are based on the extended finite element method, where dislocations are modelled as internal discontinuities. The strong and weak forms of the boundary value problem for the coupled system are presented. The computation of the Peach-Koehler force using the J-integral is discussed. Examples to illustrate the accuracy of the simulations are presented. The motion of the network of the dislocations under different electrical and mechanical loads is simulated. It was shown that even in weak piezoelectric materials the effect of the electric field on plastic behaviour is significant.

  16. Strain Relief Analysis of InN Quantum Dots Grown on GaN

    PubMed Central

    2007-01-01

    We present a study by transmission electron microscopy (TEM) of the strain state of individual InN quantum dots (QDs) grown on GaN substrates. Moiré fringe and high resolution TEM analyses showed that the QDs are almost fully relaxed due to the generation of a 60° misfit dislocation network at the InN/GaN interface. By applying the Geometric Phase Algorithm to plan-view high-resolution micrographs, we show that this network consists of three essentially non-interacting sets of misfit dislocations lying along the directions. Close to the edge of the QD, the dislocations curve to meet the surface and form a network of threading dislocations surrounding the system. PMID:21794190

  17. Investigation of AlN films grown by molecular beam epitaxy on vicinal Si(111) as templates for GaN quantum dots

    SciTech Connect

    Benaissa, M.; Vennegues, P.; Tottereau, O.; Nguyen, L.; Semond, F.

    2006-12-04

    The use of AlN epitaxial films deposited on vicinal Si(111) as templates for the growth of GaN quantum dots is investigated by transmission electron microscopy and atomic force microscopy. It is found that the substrate vicinality induces both a slight tilt of the AlN (0001) direction with respect to the [111] direction and a step bunching mechanism. As a consequence, a dislocation dragging behavior is observed giving rise to dislocation-free areas well suited for the nucleation of GaN quantum dots.

  18. Meandering of overgrown v-shaped defects in epitaxial GaN layers

    NASA Astrophysics Data System (ADS)

    Weidlich, P. H.; Schnedler, M.; Portz, V.; Eisele, H.; Dunin-Borkowski, R. E.; Ebert, Ph.

    2014-07-01

    The meandering of v-shaped defects in GaN(0001) epitaxial layers is investigated by cross-sectional scanning tunneling microscopy. The spatial position of v-shaped defects is mapped on ( 10 1 ¯ 0 ) cleavage planes using a dopant modulation, which traces the overgrown growth front. Strong lateral displacements of the apex of the v-shaped defects are observed. The lateral displacements are suggested to be induced by the meandering of threading dislocations present in the v-shaped defects. The meandering of the dislocation is attributed to interactions with inhomogeneous strain fields.

  19. Continuum dynamics of the formation, migration and dissociation of self-locked dislocation structures on parallel slip planes

    NASA Astrophysics Data System (ADS)

    Zhu, Yichao; Niu, Xiaohua; Xiang, Yang

    2016-11-01

    In continuum models of dislocations, proper formulations of short-range elastic interactions of dislocations are crucial for capturing various types of dislocation patterns formed in crystalline materials. In this article, the continuum dynamics of straight dislocations distributed on two parallel slip planes is modelled through upscaling the underlying discrete dislocation dynamics. Two continuum velocity field quantities are introduced to facilitate the discrete-to-continuum transition. The first one is the local migration velocity of dislocation ensembles which is found fully independent of the short-range dislocation correlations. The second one is the decoupling velocity of dislocation pairs controlled by a threshold stress value, which is proposed to be the effective flow stress for single slip systems. Compared to the almost ubiquitously adopted Taylor relationship, the derived flow stress formula exhibits two features that are more consistent with the underlying discrete dislocation dynamics: (i) the flow stress increases with the in-plane component of the dislocation density only up to a certain value, hence the derived formula admits a minimum inter-dislocation distance within slip planes; (ii) the flow stress smoothly transits to zero when all dislocations become geometrically necessary dislocations. A regime under which inhomogeneities in dislocation density grow is identified, and is further validated through comparison with discrete dislocation dynamical simulation results. Based on the findings in this article and in our previous works, a general strategy for incorporating short-range dislocation correlations into continuum models of dislocations is proposed.

  20. Probing the character of ultra-fast dislocations.

    PubMed

    Ruestes, C J; Bringa, E M; Rudd, R E; Remington, B A; Remington, T P; Meyers, M A

    2015-01-01

    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 to 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. The simulated configuration enables a new class of experiments to probe average dislocation velocity at very high applied shear stress.

  1. Probing the character of ultra-fast dislocations

    NASA Astrophysics Data System (ADS)

    Ruestes, C. J.; Bringa, E. M.; Rudd, R. E.; Remington, B. A.; Remington, T. P.; Meyers, M. A.

    2015-11-01

    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 to 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. The simulated configuration enables a new class of experiments to probe average dislocation velocity at very high applied shear stress.

  2. Probing the character of ultra-fast dislocations

    PubMed Central

    Ruestes, C. J.; Bringa, E. M.; Rudd, R. E.; Remington, B. A.; Remington, T. P.; Meyers, M. A.

    2015-01-01

    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 to 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. The simulated configuration enables a new class of experiments to probe average dislocation velocity at very high applied shear stress. PMID:26592764

  3. Probing the character of ultra-fast dislocations

    DOE PAGES

    Rudd, R. E.; Ruestes, C. J.; Bringa, E. M.; Remington, B. A.; Remington, T. P.; Meyers, M. A.

    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

  4. Probing the character of ultra-fast dislocations

    SciTech Connect

    Rudd, R. E.; Ruestes, C. J.; Bringa, E. M.; Remington, B. A.; Remington, T. P.; Meyers, M. A.

    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 to 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.

  5. 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.

  6. Pattern formation in a minimal model of continuum dislocation plasticity

    NASA Astrophysics Data System (ADS)

    Sandfeld, Stefan; Zaiser, Michael

    2015-09-01

    The spontaneous emergence of heterogeneous dislocation patterns is a conspicuous feature of plastic deformation and strain hardening of crystalline solids. Despite long-standing efforts in the materials science and physics of defect communities, there is no general consensus regarding the physical mechanism which leads to the formation of dislocation patterns. In order to establish the fundamental mechanism, we formulate an extremely simplified, minimal model to investigate the formation of patterns based on the continuum theory of fluxes of curved dislocations. We demonstrate that strain hardening as embodied in a Taylor-type dislocation density dependence of the flow stress, in conjunction with the structure of the kinematic equations that govern dislocation motion under the action of external stresses, is already sufficient for the formation of dislocation patterns that are consistent with the principle of similitude.

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

    SciTech Connect

    Lee, Sung Bo Han, Heung Nam; Kim, Young-Min

    2015-07-15

    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 substitution of Si for Al.

  8. First-principle study on electronic structure and optical properties of GaN nanowires with different cross-sections

    NASA Astrophysics Data System (ADS)

    Kong, Yike; Liu, Lei; Xia, Sihao; Wang, Honggang; Wang, Meishan

    2016-08-01

    This paper explores the properties of intrinsic gallium nitride (GaN) nanowires (NWs) in terms of formation energy, band structure, density of state (DOS) and optical properties with plane-wave ultrasoft pseudopotential method based on first-principles. Results show that after relaxation, N atoms of the outer layers move outwards, while Ga atoms move inwards, and the relaxation of surface atomic structure appears less obvious with the increasing cross-sectional area. Comparing different cross-sections of GaN NWs, it is found that the formation energy decreases and the stability goes stronger with the increasing size. With the increasing cross-section, the bandgap is decreased. Moreover, through comparative investigation in optical properties between GaN NWs and bulk GaN, a valuable phenomenom is found that the static dielectric constants of GaN NWs are notably lower, which contributes remarkably to the excellent absorbing performance of GaN NWs.

  9. Wafer-scale crack-free AlGaN on GaN through two-step selective-area growth for optically pumped stimulated emission

    NASA Astrophysics Data System (ADS)

    Ko, Young-Ho; Bae, Sung-Bum; Kim, Sung-Bock; Kim, Dong Churl; Leem, Young Ahn; Cho, Yong-Hoon; Nam, Eun-Soo

    2016-07-01

    Crack-free AlGaN template has been successfully grown over entire 2-in. wafer by using 2-step selective-area growth (SAG). The GaN truncated structure was obtained by vertical growth mode with low growth temperature. AlGaN of second step was grown under lateral growth mode. Low pressure enhanced the relative ratio of lateral to vertical growth rate as well as absolute overall growth rate. High V/III ratio was favorable for lateral growth mode. Crack-free planar AlGaN was obtained under low pressure of 30 Torr and high V/III ratio of 4400. The AlGaN was crack-free over entire 2-in. wafer and had quite uniform Al-mole fraction. The dislocation density of the AlGaN with 20% Al-composition was as low as ~7.6×108 /cm2, measured by cathodoluminescence. GaN/AlGaN multi-quantum well (MQW) with cladding and waveguide layers were grown on the crack-free AlGaN template with low dislocation density. It was confirmed that the MQW on the AlGaN template emitted the stimulated emission at 355.5 nm through optical pumping experiment. The AlGaN obtained by 2-step SAG would provide high crystal quality for highly-efficient optoelectronic devices as well as the ultraviolet laser diode.

  10. Medial subtalar dislocation.

    PubMed

    Kinik, H; Oktay, O; Arikan, M; Mergen, E

    1999-01-01

    We report a medial subtalar dislocation without fracture in an eighteen year old male injured during basketball game. He was successfully treated with closed reduction and cast immobilization. At one year follow-up he was symptomless.

  11. Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods.

    PubMed

    Conroy, M; Li, H; Kusch, G; Zhao, C; Ooi, B; Edwards, P R; Martin, R W; Holmes, J D; Parbrook, P J

    2016-06-01

    We report a method of growing site controlled InGaN multiple quantum discs (QDs) at uniform wafer scale on coalescence free ultra-high density (>80%) nanorod templates by metal organic chemical vapour deposition (MOCVD). The dislocation and coalescence free nature of the GaN space filling nanorod arrays eliminates the well-known emission problems seen in InGaN based visible light sources that these types of crystallographic defects cause. Correlative scanning transmission electron microscopy (STEM), energy-dispersive X-ray (EDX) mapping and cathodoluminescence (CL) hyperspectral imaging illustrates the controlled site selection of the red, yellow and green (RYG) emission at these nano tips. This article reveals that the nanorod tips' broad emission in the RYG visible range is in fact achieved by manipulating the InGaN QD's confinement dimensions, rather than significantly increasing the In%. This article details the easily controlled method of manipulating the QDs dimensions producing high crystal quality InGaN without complicated growth conditions needed for strain relaxation and alloy compositional changes seen for bulk planar GaN templates. PMID:27174084

  12. Acute traumatic patellar dislocation.

    PubMed

    Duthon, V B

    2015-02-01

    Inaugural traumatic patellar dislocation is most often due to trauma sustained during physical or sports activity. Two-thirds of acute patellar dislocations occur in young active patients (less than 20 years old). Non-contact knee sprain in flexion and valgus is the leading mechanism in patellar dislocation, accounting for as many as 93% of all cases. The strong displacement of the patella tears the medial stabilizing structures, and notably the medial patellofemoral ligament (MPFL), which is almost always injured in acute patellar dislocation, most frequently at its femoral attachment. Lateral patellar glide can be assessed with the knee in extension or 20° flexion. Displacement by more than 50% of the patellar width is considered abnormal and may induce apprehension. Plain X-ray and CT are mandatory to diagnose bony risk factors for patellar dislocation, such as trochlear dysplasia or increased tibial tubercle-trochlear groove distance (TT-TG), and plan correction. MRI gives information on cartilage and capsulo-ligamentous status for treatment planning: free bodies or osteochondral fracture have to be treated surgically. If patellar dislocation occurs in an anatomically normal knee and osteochondral fracture is ruled out on MRI, non-operative treatment is usually recommended.

  13. A partly-contacted epitaxial lateral overgrowth method applied to GaN material

    PubMed Central

    Xiao, Ming; Zhang, Jincheng; Duan, Xiaoling; Shan, Hengsheng; Yu, Ting; Ning, Jing; Hao, Yue

    2016-01-01

    We have discussed a new crystal epitaxial lateral overgrowth (ELO) method, partly-contacted ELO (PC-ELO) method, of which the overgrowth layer partly-contacts with underlying seed layer. The passage also illustrates special mask structures with and without lithography and provides three essential conditions to achieve the PC-ELO method. What is remarkable in PC-ELO method is that the tilt angle of overgrowth stripes could be eliminated by contacting with seed layer. Moreover, we report an improved monolayer microsphere mask method without lithography of PC-ELO method, which was used to grow GaN. From the results of scanning electron microscopy, cathodoluminescence, x-ray diffraction (XRD), transmission electron microscopy, and atomic force microscope (AFM), overgrowth layer shows no tilt angle relative to the seed layer and high quality coalescence front (with average linear dislocation density <6.4 × 103 cm−1). Wing stripes peak splitting of the XRD rocking curve due to tilt is no longer detectable. After coalescence, surface steps of AFM show rare discontinuities due to the low misorientation of the overgrowth regions. PMID:27033154

  14. Lateral epitaxial overgrowth of GaN on Si(111). Progress report

    SciTech Connect

    1998-09-01

    The lateral epitaxial overgrowth of GaN on Si(111) substrates was achieved using an extension of the standard LEO process on GaN/Al{sub 2}O{sub 3} substrates, and the reduction of the dislocation density was demonstrated by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The growth on the Si(111) substrate was initiated with the deposition of a thin AlN buffer layer to avoid the formation of potentially detrimental silicon nitride at the interface. The wafers were then patterned with a SiO{sub 2} layer in which 5 micron wide opening separated by 35 microns were etched using buffered HF. After reloading the samples in the MOCVD chamber, the LEO growth was performed using the authors` standard parameters. There are a few unresolved issues concerning the effect of the AlN buffer thickness and its chemical compatibility with the SiO{sub 2} mask layer, but after a basic optimization they were able to obtain 5 microns of lateral overgrowth with smooth sidewalls in a reproducible manner. They are currently investigating the use of mask materials other than SiO{sub 2} to achieve LEO on Si(111) over a wider range of process parameters.

  15. GaN etching in BCl{sub 3}Cl{sub 2} plasmas

    SciTech Connect

    Shul, R.J.; Ashby, C.I.H.; Willison, C.G.; Zhang, L.; Han, J.; Bridges, M.M.; Pearton, S.J.; Lee, J.W.; Lester, L.F.

    1998-04-01

    GaN etching can be affected by a wide variety of parameters including plasma chemistry and plasma density. Chlorine-based plasmas have been the most widely used plasma chemistries to etch GaN due to the high volatility of the GaCl{sub 3} and NCl etch products. The source of Cl and the addition of secondary gases can dramatically influence the etch characteristics primarily due to their effect on the concentration of reactive Cl generated in the plasma. In addition, high-density plasma etch systems have yielded high quality etching of GaN due to plasma densities which are 2 to 4 orders of magnitude higher than reactive ion etch (RIE) plasma systems. The high plasma densities enhance the bond breaking efficiency of the GaN, the formation of volatile etch products, and the sputter desorption of the etch products from the surface. In this study, the authors report GaN etch results for a high-density inductively coupled plasma (ICP) as a function of BCl{sub 3}:Cl{sub 2} flow ratio, dc-bias, chamber-pressure, and ICP source power. GaN etch rates ranging from {approximately}100 {angstrom}/min to > 8,000 {angstrom}/min were obtained with smooth etch morphology and anisotropic profiles.

  16. Effect of residual stress on the microstructure of GaN epitaxial films grown by pulsed laser deposition

    NASA Astrophysics Data System (ADS)

    Wang, Haiyan; Wang, Wenliang; Yang, Weijia; Zhu, Yunnong; Lin, Zhiting; Li, Guoqiang

    2016-04-01

    The stress-free GaN epitaxial films have been directly grown by pulsed laser deposition (PLD) at 850 °C, and the effect of different stress on the microstructure of as-grown GaN epitaxial films has been explored in detail. The as-grown stress-free GaN epitaxial films exhibit very smooth surface without any particles and grains, which is confirmed by the smallest surface root-mean-square roughness of 2.3 nm measured by atomic force microscopy. In addition, they also have relatively high crystalline quality, which is proved by the small full-width at half maximum values of GaN(0002) and GaN (10 1 bar 2) X-ray rocking curves as 0.27° and 0.68°, respectively. However, when the growth temperature is lower or higher than 850 °C, internal or thermal stress would be increased in as-grown GaN epitaxial films. To release the larger stress, a great number of dislocations are generated. Many irregular particulates, hexagonal GaN gains and pits are therefore produced on the films surface, and the crystalline quality is greatly reduced consequently. This work has demonstrated the direct growth of stress-free GaN epitaxial films with excellent surface morphology and high crystalline quality by PLD, and presented a comprehensive study on the origins and the effect of stress in GaN layer. It is instructional to achieve high-quality nitride films by PLD, and shows great potential and broad prospect for the further development of high-performance GaN-based devices.

  17. Structural and morphological properties of GaN buffer layers grown by ammonia molecular beam epitaxy on SiC substrates for AlGaN/GaN high electron mobility transistors

    SciTech Connect

    Corrion, A. L.; Poblenz, C.; Wu, F.; Speck, J. S.

    2008-05-01

    The impact of growth conditions on the surface morphology and structural properties of ammonia molecular beam epitaxy GaN buffers layers on SiC substrates was investigated. The threading dislocation (TD) density was found to decrease with decreasing NH{sub 3}:Ga flux ratio, which corresponded to an increase in surface roughness and reduction in residual compressive lattice mismatch stress. Furthermore, the dislocation density and compressive stress decreased for increasing buffer thickness. TD inclination was proposed to account for these observations. Optimized surface morphologies were realized at high NH{sub 3}:Ga flux ratios and were characterized by monolayer-high steps, spiral hillocks, and pyramidal mounds, with rms roughness of {approx}1.0 nm over 2x2 {mu}m{sup 2} atomic force microscopy images. Smooth surface morphologies were realized over a large range of growth temperatures and fluxes, and growth rates of up to 1 {mu}m/h were achieved. TD densities in the buffers as low as 3x10{sup 9} cm{sup -2} were demonstrated. These buffers were highly insulating and were used in recently reported AlGaN/GaN HEMTs with power densities of >11 W/mm at 4 and 10 GHz.

  18. Mesoscale modeling of dislocations in molecular crystals

    NASA Astrophysics Data System (ADS)

    Lei, Lei; Koslowski, Marisol

    2011-02-01

    Understanding the inelastic deformation of molecular crystals is of fundamental importance to the modeling of the processing of drugs in the pharmaceutical industry as well as to the initiation of detonation in high energy density materials. In this work, we present dislocation dynamics simulations of the deformation of two molecular crystals of interest in the pharmaceutical industry, sucrose and paracetamol. The simulations calculate the yield stress of sucrose and paracetamol in good agreement with experimental observation and predict the anisotropy in the mechanical response observed in these materials. Our results show that dislocation dynamics is an effective tool to study plastic deformation in molecular crystals.

  19. Geometry of dislocated de Broglie waves

    SciTech Connect

    Holland, P.R.

    1987-04-01

    The geometrical structures implicit in the de Broglie waves associated with a relativistic charged scalar quantum mechanical particle in an external field are analyzed by employing the ray concept of the causal interpretation. It is shown how an osculating Finslerian metric tensor, a torsion tensor, and a tetrad field define respectively the strain, the dislocation density, and the Burgers vector in the natural state of the wave, which is a non-Riemannian space of distant parallelism. A quantum torque determined by the quantum potential is introduced and the example of a screw dislocated wave is discussed.

  20. Including dislocation flux in a continuum crystal plasticity model to produce size scale effects

    SciTech Connect

    Becker, R; Arsenlis, A; Bulatov, V V; Parks, D M

    2004-02-13

    A novel model has been developed to capture size scale and gradient effects within the context of continuum crystal plasticity by explicitly incorporating details of dislocation transport, coupling dislocation transport to slip, evolving spatial distributions of dislocations consistent with the flux, and capturing the interactions among various dislocation populations. Dislocation flux and density are treated as nodal degrees of freedom in the finite element model, and they are determined as part of the global system of equations. The creation, annihilation and flux of dislocations between elements are related by transport equations. Crystallographic slip is coupled to the dislocation flux and the stress state. The resultant gradients in dislocation density and local lattice rotations are analyzed for geometrically necessary and statistically stored dislocation contents that contribute to strength and hardening. Grain boundaries are treated as surfaces where dislocation flux is restricted depending on the relative orientations of the neighboring grains. Numerical results show different behavior near free surfaces and non-deforming surfaces resulting from differing levels of dislocation transmission. Simulations also show development of dislocation pile-ups at grain boundaries and an increase in flow strength reminiscent of the Hall-Petch model. The dislocation patterns have a characteristic size independent of the numerical discretization.

  1. Discrete dislocation dynamics study of strained-layer relaxation.

    PubMed

    Schwarz, K W

    2003-10-01

    Numerical simulations are performed to follow the evolution of an initial density of dislocation loops in an infinite strained layer to the point where the dislocations have stopped moving. Several unexpected results are obtained. First, many of the threading arms are either annihilated or prematurely immobilized by hardening interactions such as jogging and junction formation. Second, the remaining dislocation arms are eventually trapped by stress fluctuations that arise more from local overrelaxation than from the blocking mechanisms usually considered. Third, the degree of relaxation that can be attained depends strongly on the initial density of threading arms. PMID:14611535

  2. Discrete Dislocation Dynamics Study of Strained-Layer Relaxation

    NASA Astrophysics Data System (ADS)

    Schwarz, K. W.

    2003-10-01

    Numerical simulations are performed to follow the evolution of an initial density of dislocation loops in an infinite strained layer to the point where the dislocations have stopped moving. Several unexpected results are obtained. First, many of the threading arms are either annihilated or prematurely immobilized by hardening interactions such as jogging and junction formation. Second, the remaining dislocation arms are eventually trapped by stress fluctuations that arise more from local overrelaxation than from the blocking mechanisms usually considered. Third, the degree of relaxation that can be attained depends strongly on the initial density of threading arms.

  3. Effective mobility of dislocations from systematic coarse-graining

    NASA Astrophysics Data System (ADS)

    Kooiman, M.; Hütter, M.; Geers, MGD

    2015-06-01

    The dynamics of large amounts of dislocations governs the plastic response of crystalline materials. In this contribution we discuss the relation between the mobility of discrete dislocations and the resulting flow rule for coarse-grained dislocation densities. The mobilities used in literature on these levels are quite different, for example in terms of their intrinsic the stress dependence. To establish the relation across the scales, we have derived the macroscopic evolution equations of dislocation densities from the equations of motion of individual dislocations by means of systematic coarse-graining. From this, we can identify a memory kernel relating the driving force and the flux of dislocations. This kernel can be considered as an effective macroscopic mobility with two contributions; a direct contribution related to the overdamped motion of individual dislocations, and an emergent contribution that arises from time correlations of fluctuations in the Peach-Koehler force. Scaling analysis shows that the latter contribution is dominant for dislocations in metals at room temperature. We also discuss several concerns related to the separation of timescales.

  4. Design, fabrication and characterising of 100 W GaN HEMT for Ku-band application

    NASA Astrophysics Data System (ADS)

    Chunjiang, Ren; Shichang, Zhong; Yuchao, Li; Zhonghui, Li; Yuechan, Kong; Tangsheng, Chen

    2016-08-01

    Ku-band GaN power transistor with output power over 100 W under the pulsed operation mode is presented. A high temperature A1N nucleation together with an Fe doped GaN buffer was introduced for the developed GaN HEMT. The AlGaN/GaN hetero-structure deposited on 3 inch SiC substrate exhibited a 2DEG hall mobility and density of ˜2100 cm2/(V·s) and 1.0 × 1013 cm-2, respectively, at room temperature. Dual field plates were introduced to the designed 0.25 μm GaN HEMT and the source connected field plate was optimized for minimizing the peak field plate near the drain side of the gate, while maintaining excellent power gain performance for Ku-band application. The load-pull measurement at 14 GHz showed a power density of 5.2 W/mm for the fabricated 400 μm gate periphery GaN HEMT operated at a drain bias of 28 V. A Ku-band internally matched GaN power transistor was developed with two 10.8 mm gate periphery GaN HEMT chips combined. The GaN power transistor exhibited an output power of 102 W at 13.3 GHz and 32 V operating voltage under pulsed operation mode with a pulse width of 100 μs and duty cycle of 10%. The associated power gain and power added efficiency were 9.2 dB and 48%, respectively. To the best of the authors' knowledge, the PAE is the highest for Ku-band GaN power transistor with over 100 W output power.

  5. Roma Gans: Still Writing at 95.

    ERIC Educational Resources Information Center

    Sullivan, Joanna

    1991-01-01

    Recounts discussions with reading educator Roma Gans over a 25-year period. Presents Gans' views about reading, teachers, her family, and her years at Teachers College, Columbia. Notes that Gans has seen the teaching of reading come full circle since her first teaching assignment in 1919. (RS)

  6. Nucleus and Spiral Growth of N-face GaN(0001) Obtained by Selective-Area Metalorganic Vapor Phase Epitaxy

    NASA Astrophysics Data System (ADS)

    Lin, Chia-Hung; Akasaka, Tetsuya; Yamamoto, Hideki

    2013-03-01

    Nucleus and spiral growth mechanisms of N-face GaN(0001) were studied in selective-area metalorganic vapor phase epitaxy. An almost step-free N-face GaN surface is obtained by nucleus growth within a selective area without screw-type dislocations, while growth spirals are induced by the spiral growth mode when screw-type dislocations exist. The growth mechanism of N-face GaN is consistently explained by a theoretical analysis [W. K. Burton, N. Cabrera, and F. C. Frank: Philos. Trans. R. Soc. London, Ser. A 243 (1951) 299]. The step and activation energies are estimated to be 2.1 J/m2 and 2.15 eV, respectively.

  7. The effect of length scale on the determination of geometrically necessary dislocations via EBSD continuum dislocation microscopy.

    PubMed

    Ruggles, T J; Rampton, T M; Khosravani, A; Fullwood, D T

    2016-05-01

    Electron backscatter diffraction (EBSD) dislocation microscopy is an important, emerging field in metals characterization. Currently, calculation of geometrically necessary dislocation (GND) density is problematic because it has been shown to depend on the step size of the EBSD scan used to investigate the sample. This paper models the change in calculated GND density as a function of step size statistically. The model provides selection criteria for EBSD step size as well as an estimate of the total dislocation content. Evaluation of a heterogeneously deformed tantalum specimen is used to asses the method. PMID:26986021

  8. Differences and similarities between structural properties of GaN grown by different growth methods

    SciTech Connect

    Liliental-Weber, Z.; Jasinski, J.; Washburn, J.

    2002-08-01

    In this paper defects formed in GaN grown by different methods are reviewed. The crystal growth direction and growth rate play important roles. For bulk crystals grown under high pressure the highest growth rates are for planes perpendicular to the c-axis. Only planar defects formed on c-planes are observed in these crystals. There are no threading dislocations or nanotubes in the c-direction. However, polarity of the growth direction plays a role in the surface roughness and the distribution of planar defects. For growth of homo-epitaxial and hetero-epitaxial layers the growth is forced to take place in the much slower c-direction. As a result defects related to the purity of constituents used for growth are formed such as nanotubes and pinholes. In addition threading dislocations and dislocations that accommodate lattice and thermal expansion mismatch are formed.

  9. Electrical properties of dislocations in III-Nitrides

    SciTech Connect

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

    2014-02-21

    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.

  10. 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.

  11. The relationship between strain geometry and geometrically necessary dislocations

    NASA Astrophysics Data System (ADS)

    Hansen, Lars; Wallis, David

    2016-04-01

    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.

  12. Phonon Drag Dislocations at High Pressures

    SciTech Connect

    Wolfer, W.G.

    1999-10-19

    Phonon drag on dislocations is the dominant process which determines the flow stress of metals at elevated temperatures and at very high plastic deformation rates. The dependence of the phonon drag on pressure or density is derived using a Mie-Grueneisen equation of state. The phonon drag is shown to increase nearly linearly with temperature but to decrease with density or pressure. Numerical results are presented for its variation for shock-loaded copper and aluminum. In these cases, density and temperature increase simultaneously, resulting in a more modest net increase in the dislocation drag coefficient. Nevertheless, phonon drag increases by more than an order of magnitude during shock deformations which approach melting. Since the dependencies of elastic moduli and of the phonon drag coefficient on pressure and temperature are fundamentally different, the effect of pressure on the constitutive law for plastic deformation can not simply be accounted for by its effect on the elastic shear modulus.

  13. Dislocation Substructure in the Cold-Rolled Ni-20 Mass Pct Cr Alloy Analyzed by X-ray Diffraction, Positron Annihilation Lifetime, and Transmission Electron Microscopy

    NASA Astrophysics Data System (ADS)

    Yonemura, Mitsuharu; Inoue, Koji

    2016-08-01

    The systematic change in the dislocation density and characteristics that develop under cold rolling as a simulated deformation was studied in order to examine the fundamental behavior of dislocations in terms of the dislocation substructure formation. In particular, the dislocation density was quantified by X-ray line profile analysis (XLPA), which is effective for quantifying the dislocation density and character; positron annihilation lifetime (PAL), which is sensitive to vacancy-type lattice defects; the Bailey-Hirsch equation from the hardness (Hv); and transmission electron microscopy (TEM). The strain dependency of the dislocation density analyzed by XLPA, PAL, TEM, and Hv showed a similar tendency with an increase in the dislocation. In particular, the dislocation density by XLPA had good agreement with the results of TEM at low strain levels and with PAL at high strain levels. As a result, a combination of these techniques successfully showed the behavior of the dislocation substructure.

  14. Behavior of dislocations in silicon

    SciTech Connect

    Sumino, Koji

    1995-08-01

    A review is given of dynamic behavior of dislocations in silicon on the basis of works of the author`s group. Topics taken up are generation, motion and multiplication of dislocations as affected by oxygen impurities and immobilization of dislocations due to impurity reaction.

  15. Congenital hip dislocation (image)

    MedlinePlus

    ... by a blow, fall, or other trauma, a dislocation can also occur from birth. The cause is unknown but genetic factors may play a role. Problems resulting from very mild developmental dysplasia of the hip may not become apparent until the person is ...

  16. Dislocated Worker Project.

    ERIC Educational Resources Information Center

    1988

    Due to the severe economic decline in the automobile manufacturing industry in southeastern Michigan, a Dislocated Workers Program has been developed through the partnership of the Flint Area Chamber of Commerce, three community colleges, the National Center for Research in Vocational Education, the Michigan State Department of Education, the…

  17. Dorsal radiocarpal fracture dislocation.

    PubMed

    Tanzer, T L; Horne, J G

    1980-11-01

    A case of a rare radiocarpal fracture dislocation in a 17-year-old girl, with persisting loss of radiocarpal joint space following reduction under hematoma block, is described. The wrist joint was exposed, and two osteochondral fragments were rotated 90 degrees and secured with 2.7-mm AO screws. Satisfactory healing followed 3 months postinjury.

  18. Annealing effects on polycrystalline GaN using nitrogen and ammonia ambients

    NASA Astrophysics Data System (ADS)

    Ariff, A.; Zainal, N.; Hassan, Z.

    2016-09-01

    This paper describes effects of using post-annealing treatment in different conditions on the properties of polycrystalline GaN layer grown on m-plane sapphire substrate by electron beam (e-beam) evaporator. Without annealing, GaN surface was found to have a low RMS roughness with agglomeration of GaN grains in a specific direction and the sample consisted of gallium oxide (Ga2O3) material. When the post-annealing treatment was carried out in N2 ambient at 650 °C, initial re-crystallization of the GaN grains was observed while the evidence of Ga2O3 almost disappeared. As the NH3 annealing was conducted at 950 °C, more effect of re-crystallization occurred but with less grains coalescence. Three dominant XRD peaks of GaN in (10 1 bar 0) , (0002) and (10 1 bar 1) orientations were evident. Near band edge (NBE) related emission in GaN was also observed. The significant improvement was attributed to simultaneous recrystallization and effective reduction of N deficiency density. The post-annealing in a mixture of N2 and NH3 ambient at 950 °C was also conducted, but has limited the effectiveness of the N atoms to incorporate on the GaN layer due to 'clouding' effect by the inert N2 gas. Further increase in the annealing temperature at 980 °C and 1100 °C, respectively caused severe deteriorations of the structural and optical properties of the GaN layer. Overall, this work demonstrated initial potential in improving polycrystalline GaN material in simple and inexpensive manner.

  19. 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.

  20. Current mapping of nonpolar a-plane and polar c-plane GaN films by conductive atomic force microscopy

    NASA Astrophysics Data System (ADS)

    Xu, Shengrui; Jiang, Teng; Lin, Zhiyu; Zhao, Ying; Yang, Linan; Zhang, Jincheng; Li, Peixian; Hao, Yue

    2016-10-01

    Nonpolar (11-20) a-plane GaN and polar (0001) c-plane GaN films have been grown by metal organic chemical vapor deposition on r-plane (1-102) and c-plane (0001) sapphire substrates, respectively. Conductive atomic force microscopy (C-AFM) has been used to investigate the local conductivity of the films. C-AFM shows enhanced current conduction within the etch pits of c-plane GaN and triangular pits of a-plane GaN. The results indicate that the off-axis planes are more electrically active than c-plane and a-plane. Surprisingly, the C-AFM values in triangular pit of the a-plane GaN are much smaller than that in etch pits of the c-plane GaN. The dislocations type related current leakage mechanism is revealed for polar c-plane and nonpolar a-plane GaN films.

  1. Enhancing the field emission properties of Se-doped GaN nanowires.

    PubMed

    Li, Enling; Wu, Guishuang; Cui, Zhen; Ma, Deming; Shi, Wei; Wang, Xiaolin

    2016-07-01

    Pure and Se-doped GaN nanowires (NWs) are synthesized on Pt-coated Si(111) substrates via chemical vapor deposition. The GaN NWs exhibit a uniform density with an average diameter of 20-120 nm. The structure of the NWs is wurtzite hexagonal, and the growth direction is along [0001]. Field emission measurements show that the Se-doped GaN NWs possess a low turn-on field (2.9 V μm(-1)) compared with the pure GaN NWs (7.0 V μm(-1)). In addition, density functional theory calculations indicate that the donor states near the Fermi level are mainly formed through the hybridization between Se 4p and N 2p orbitals and that the Fermi level move towards the vacuum level. Consequently, the work functions of Se-doped GaN NWs are lower than those of pure GaN NWs. PMID:27197556

  2. Spatial organization of plastic deformation in single crystals with different structure of slip dislocation

    SciTech Connect

    Kunitsyna, T. S.; Teplyakova, L. A. Koneva, N. A.; Poltaranin, M. A.

    2015-10-27

    It is established that different structure of slip dislocation at the end of the linear hardening stage results in different distribution of dislocation charges in the volume of a single crystal. In the alloy with a near atomic order the slip of single dislocations leads to formation of planar structures—layers with the excess density of dislocations. In the alloy with long-range atomic order the slip of superdislocations brings the formation of the system of parallel rod-like charged dislocation linking.

  3. Dynamic phases, pinning, and pattern formation for driven dislocation assemblies

    DOE PAGES

    Zhou, Caizhi; Reichhardt, Charles; Olson Reichhardt, Cynthia J.; Beyerlein, Irene J.

    2015-01-23

    We examine driven dislocation assemblies and show that they can exhibit a set of dynamical phases remarkably similar to those of driven systems with quenched disorder such as vortices in superconductors, magnetic domain walls, and charge density wave materials. These phases include pinned-jammed, fluctuating, and dynamically ordered states, and each produces distinct dislocation patterns as well as specific features in the noise fluctuations and transport properties. Lastly, our work suggests that many of the results established for systems with quenched disorder undergoing plastic depinning transitions can be applied to dislocation systems, providing a new approach for understanding pattern formation andmore » dynamics in these systems.« less

  4. Dynamic phases, pinning, and pattern formation for driven dislocation assemblies

    SciTech Connect

    Zhou, Caizhi; Reichhardt, Charles; Olson Reichhardt, Cynthia J.; Beyerlein, Irene J.

    2015-01-23

    We examine driven dislocation assemblies and show that they can exhibit a set of dynamical phases remarkably similar to those of driven systems with quenched disorder such as vortices in superconductors, magnetic domain walls, and charge density wave materials. These phases include pinned-jammed, fluctuating, and dynamically ordered states, and each produces distinct dislocation patterns as well as specific features in the noise fluctuations and transport properties. Lastly, our work suggests that many of the results established for systems with quenched disorder undergoing plastic depinning transitions can be applied to dislocation systems, providing a new approach for understanding pattern formation and dynamics in these systems.

  5. Propagation of misfit dislocations from buffer/Si interface into Si

    DOEpatents

    Liliental-Weber, Zuzanna; Maltez, Rogerio Luis; Morkoc, Hadis; Xie, Jinqiao

    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.

  6. Dislocation Multiplication in the Early Stage of Deformation in Mo Single Crystals

    SciTech Connect

    Hsiung, L.; Lassila, D.H.

    2000-03-02

    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 after 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.

  7. Strength and Dislocation Structure Evolution of Small Metals under Vibrations

    NASA Astrophysics Data System (ADS)

    Ngan, Alfonso

    2015-03-01

    It is well-known that ultrasonic vibration can soften metals, and this phenomenon has been widely exploited in industrial applications concerning metal forming and bonding. In this work, we explore the effects of a superimposed small oscillatory load on metal plasticity, from the nano- to macro-size range, and from audible to ultrasonic frequency ranges. Macroscopic and nano-indentation were performed on aluminum, copper and molybdenum, and the results show that the simultaneous application of oscillatory stresses can lower the hardness of these samples. More interestingly, EBSD and TEM observations show that subgrain formation and reduction in dislocation density generally occurred when stress oscillations were applied. These findings point to an important knowledge gap in metal plasticity - the existing understanding of ultrasound softening in terms of the vibrations either imposing additional stress waves to augment the quasi-static applied load, or heating up the metal, whereas the metal's intrinsic deformation resistance or dislocation interactive processes are assumed unaltered by the ultrasound, is proven wrong by the present results. Furthermore, in the case of nanoindentation, the Continuous Stiffness Measurement technique for contact stiffness measurement assumes that the imposed signal-carrier oscillations do not intrinsically alter the material properties of the specimen, and again, the present results prove that this can be wrong. To understand the enhanced subgrain formation and dislocation annihilation, Discrete Dislocation Dynamics (DDD) simulations were carried out and these show that when an oscillatory stress is superimposed on a quasi-static applied stress, reversals of motion of dislocations may occur, and these allow the dislocations to revisit repeatedly suitable configurations for annihilation. DDD, however, was unable to predict the observed subgrain formation presumably because the number of dislocations that can be handled is not large

  8. Dislocation Starvation and Exhaustion Hardening in Mo-alloy Nanofibers

    SciTech Connect

    Chisholm, Claire; Bei, Hongbin; Lowry, M. B.; Oh, Jason; Asif, S.A. Syed; Warren, O.; Shan, Zhiwei; George, Easo P; Minor, Andrew

    2012-01-01

    The evolution of defects in Mo alloy nanofibers with initial dislocation densities ranging from 0 to 1.6 1014 m2 were studied using an in situ push-to-pull device in conjunction with a nanoindenter in a transmission electron microscope. Digital image correlation was used to determine stress and strain in local areas of deformation. When they had no initial dislocations the Mo alloy nanofibers suffered sudden catastrophic elongation following elastic deformation to ultrahigh stresses. At the other extreme fibers with a high dislocation density underwent sustained homogeneous deformation after yielding at much lower stresses. Between these two extremes nanofibers with intermediate dislocation densities demonstrated a clear exhaustion hardening behavior, where the progressive exhaustion of dislocations and dislocation sources increases the stress required to drive plasticity. This is consistent with the idea that mechanical size effects ( smaller is stronger ) are due to the fact that nanostructures usually have fewer defects that can operate at lower stresses. By monitoring the evolution of stress locally we find that exhaustion hardening causes the stress in the nanofibers to surpass the critical stress predicted for self-multiplication, supporting a plasticity mechanism that has been hypothesized to account for the rapid strain softening observed in nanoscale bcc materials at high stresses.

  9. Growth regimes during homoepitaxial growth of GaN by ammonia molecular beam epitaxy

    SciTech Connect

    Corrion, A. L.; Wu, F.; Speck, J. S.

    2012-09-01

    c-plane GaN films were grown by ammonia molecular beam epitaxy on metal-organic chemical vapor deposition templates for a wide range of NH{sub 3}:Ga flux ratios and growth temperatures, and the resulting films were characterized using atomic force microscopy, reflection high-energy electron diffraction, and transmission electron microscopy. Three distinct nitrogen-rich growth regimes - unstable layer-by-layer, quasi-stable step flow, and dislocation-mediated pitting - were identified based on the growth mode and film properties. In addition, step flow growth was observed under conditions of gallium droplet accumulation. The results indicate the existence of two regimes for step-flow growth of GaN by ammonia MBE - both gallium-rich and nitrogen-rich. Growth mode instabilities and mound formation were observed and are discussed in the context of a step-edge energy barrier to adatom diffusion over a terrace.

  10. Growth and characterization of horizontal GaN wires on silicon

    SciTech Connect

    Zou, Xinbo; May Lau, Kei; Lu, Xing; Lucas, Ryan; Kuech, Thomas F.; Choi, Jonathan W.; Gopalan, Padma

    2014-06-30

    We report the growth of in-plane GaN wires on silicon by metalorganic chemical vapor deposition. Triangular-shaped GaN microwires with semi-polar sidewalls are observed to grow on top of a GaN/Si template patterned with nano-porous SiO{sub 2}. With a length-to-thickness ratio ∼200, the GaN wires are well aligned along the three equivalent 〈 112{sup ¯}0 〉 directions. Micro-Raman measurements indicate negligible stress and a low defect density inside the wires. Stacking faults were found to be the only defect type in the GaN wire by cross-sectional transmission electron microscopy. The GaN wires exhibited high conductivity, and the resistivity was 20–30 mΩ cm, regardless of the wire thickness. With proper heterostructure and doping design, these highly aligned GaN wires are promising for photonic and electronic applications monolithically integrated on silicon.

  11. Congenital Dislocation of the Hip

    PubMed Central

    Premi, J. M.

    1976-01-01

    The implications of a diagnosis of congenital dislocation of the hip and the importance of the role of the family physician in early detection and treatment are identified. A review of the salient clinical features of congenital dislocation of the hip is undertaken. The results of a survey carried out in the author's practice on an unusual incidence of congenital dislocated hip are reviewed. PMID:21308053

  12. Period-doubling reconstructions of semiconductor partial dislocations

    SciTech Connect

    Park, Ji -Sang; Huang, Bing; Wei, Su -Huai; Kang, Joongoo; McMahon, William E.

    2015-09-18

    Atomic-scale understanding and control of dislocation cores is of great technological importance, because they act as recombination centers for charge carriers in optoelectronic devices. Using hybrid density-functional calculations, we present period-doubling reconstructions of a 90° partial dislocation in GaAs, for which the periodicity of like-atom dimers along the dislocation line varies from one to two, to four dimers. The electronic properties of a dislocation change drastically with each period doubling. The dimers in the single-period dislocation are able to interact, to form a dispersive one-dimensional band with deep-gap states. However, the inter-dimer interaction for the double-period dislocation becomes significantly reduced; hence, it is free of mid-gap states. The Ga core undergoes a further period-doubling transition to a quadruple-period reconstruction induced by the formation of small hole polarons. Lastly, the competition between these dislocation phases suggests a new passivation strategy via population manipulation of the detrimental single-period phase.

  13. Period-doubling reconstructions of semiconductor partial dislocations

    DOE PAGES

    Park, Ji -Sang; Huang, Bing; Wei, Su -Huai; Kang, Joongoo; McMahon, William E.

    2015-09-18

    Atomic-scale understanding and control of dislocation cores is of great technological importance, because they act as recombination centers for charge carriers in optoelectronic devices. Using hybrid density-functional calculations, we present period-doubling reconstructions of a 90° partial dislocation in GaAs, for which the periodicity of like-atom dimers along the dislocation line varies from one to two, to four dimers. The electronic properties of a dislocation change drastically with each period doubling. The dimers in the single-period dislocation are able to interact, to form a dispersive one-dimensional band with deep-gap states. However, the inter-dimer interaction for the double-period dislocation becomes significantly reduced;more » hence, it is free of mid-gap states. The Ga core undergoes a further period-doubling transition to a quadruple-period reconstruction induced by the formation of small hole polarons. Lastly, the competition between these dislocation phases suggests a new passivation strategy via population manipulation of the detrimental single-period phase.« less

  14. On the electronic properties of a single dislocation

    SciTech Connect

    Reiche, Manfred Erfurth, Wilfried; Pippel, Eckhard; Sklarek, Kornelia; Blumtritt, Horst; Haehnel, Angelika; Kittler, Martin

    2014-05-21

    A detailed knowledge of the electronic properties of individual dislocations is necessary for next generation nanodevices. Dislocations are fundamental crystal defects controlling the growth of different nanostructures (nanowires) or appear during device processing. We present a method to record electric properties of single dislocations in thin silicon layers. Results of measurements on single screw dislocations are shown for the first time. Assuming a cross-section area of the dislocation core of about 1 nm{sup 2}, the current density through a single dislocation is J = 3.8 × 10{sup 12} A/cm{sup 2} corresponding to a resistivity of ρ ≅ 1 × 10{sup −8} Ω cm. This is about eight orders of magnitude lower than the surrounding silicon matrix. The reason of the supermetallic behavior is the high strain in the cores of the dissociated dislocations modifying the local band structure resulting in high conductive carrier channels along defect cores.

  15. Status of GaN HEMT performance and reliability

    NASA Astrophysics Data System (ADS)

    Green, Daniel S.; Brown, J. D.; Vetury, R.; Lee, S.; Gibb, S. R.; Krishnamurthy, K.; Poulton, M. J.; Martin, J.; Shealy, J. B.

    2008-02-01

    This report will focus on the status of GaN HEMT based amplifier technology development at RFMD. This technology is based around GaN on semi-insulating SiC substrates for optimal thermal performance. RFMD's 0.5μm gate technology features high performance advanced field plate structures, including a unit power cell producing high gain (21dB), high power density (3-5W/mm at 28V) and high efficiency (65-70 percent) at cellular frequencies. We will report on transistor and module performance relevant to applications ranging from high power, high bandwidth amplifiers, to switches and ICs for radar, electronic warfare, cellular infrastructure and homeland security. Additionally, we will report on reliability results that demonstrate capability for dependable, high voltage operation.

  16. Dislocation and fracture-dislocation of the carpometacarpal joints.

    PubMed

    Jebson, P J; Engber, W D; Lange, R H

    1994-02-01

    Dislocations and fracture-dislocations of the carpometacarpal joints are rare. Diagnosis requires a high index of suspicion, careful examination, and appropriate radiography. Treatment is controversial and is based upon the injury pattern and the surgeon's experience and preference. This article reviews the pertinent anatomy, mechanism of injury, evaluation, and treatment of patients with carpometacarpal joint injuries.

  17. Structure and electronic properties of GaN tubelike clusters and single-walled GaN nanotubes

    NASA Astrophysics Data System (ADS)

    Liu, Liren; Zou, Yanbo; Zhu, Hengjiang

    2015-06-01

    Extensive studies of the geometric structures, stabilities and electronic properties of gallium nitride (GaN)n tubelike clusters and single-walled GaN nanotubes (GaNNTs) were carried out using density-functional theory (DFT) calculations. A family of stable tubelike structures with Ga-N alternating arrangement was observed when n≥8 and their structural units (four-membered rings (4MRs) and six-membered rings (6MRs)) obey the general developing formula. The size-dependent properties of the frontier molecular orbital surfaces explain why the long and stable tubelike clusters can be obtained successfully. They also illustrate the reason why GaNNTs can be synthesized experimentally. Our results also reveal that the single-walled GaNNTs, which as semiconductors with a large bandgap, can be prepared by using the proper assembly of tubelike clusters.

  18. Anelasticity of GaN Epitaxial Layer in GaN LED

    NASA Astrophysics Data System (ADS)

    Chung, C. C.; Yang, C. T.; Liu, C. Y.

    2016-10-01

    In this work, the anelasticity of the GaN layer in the GaN light-emitting-diode device was studied. The present results show that the forward-voltage of GaN LED increases with time, as the GaN light-emitting-diode was maintained at a constant temperature of 100 °C. We found that the increase of the forward-voltage with time attributes to the delay-response of the piezoelectric fields (internal electrical fields in GaN LED device). And, the delay-response of the internal electrical fields with time is caused by the anelasticity (time-dependent strain) of the GaN layer. Therefore, using the correlation of strain-piezoelectric-forward voltage, a plot of thermal strain of the GaN layer against time can be obtained by measuring the forward-voltage of the studied GaN LED against time. With the curves of the thermal strain of GaN epi-layers versus time, the anelasticity of the GaN compound can be studied. The key anelasticity parameter, characteristic relaxation time, of the GaN is defined to be 2623.76 min in this work.

  19. Atomistic Simulation of Dislocation-Defect Interactions in Cu

    SciTech Connect

    Wirth, B D; Bulatov, V V; Diaz de la Rubia, T

    2001-01-01

    The mechanisms of dislocation-defect interactions are of practical importance for developing quantitative structure-property relationships, mechanistic understanding of plastic flow localization and predictive models of mechanical behavior in metals under irradiation. In copper and other face centered cubic metals, high-energy particle irradiation produces hardening and shear localization. Post-irradiation microstructural examination in Cu reveals that irradiation has produced a high number density of nanometer sized stacking fault tetrahedra. Thus, the resultant irradiation hardening and shear localization is commonly attributed to the interaction between stacking fault tetrahedra and mobile dislocations, although the mechanism of this interaction is unknown. In this work, we present a comprehensive molecular dynamics simulation study that characterizes the interaction and fate of moving dislocations with stacking fault tetrahedra in Cu using an EAM interatomic potential. This work is intended to produce atomistic input into dislocation dynamics simulations of plastic flow localization in irradiated materials.

  20. Dislocation Interactions with Voids and Helium Bubbles in FCC Metals

    SciTech Connect

    Robertson, I; Robach, J; Wirth, B; Young, J

    2003-11-18

    The formation of a high number density of helium bubbles in FCC metals irradiated within the fusion energy environment is well established. Yet, the role of helium bubbles in radiation hardening and mechanical property degradation of these steels remains an outstanding issue. In this paper, we present the results of a combined molecular dynamics simulation and in-situ straining transmission electron microscopy study, which investigates the interaction mechanisms between glissile dislocations and nanometer-sized helium bubbles. The molecular dynamics simulations, which directly account for dislocation core effects through semi-empirical interatomic potentials, provide fundamental insight into the effect of helium bubble size and internal gas pressure on the dislocation/bubble interaction and bypass mechanisms. The combination of simulation and in-situ straining experiments provides a powerful approach to determine the atomic to microscopic mechanisms of dislocation-helium bubble interactions, which govern the mechanical response of metals irradiated within the fusion environment.

  1. Dislocation processes and deformation twinning in nanocrystalline Al.

    SciTech Connect

    Yamakov, V.; Wolf, D.; Phillpot, S.R.; Gleiter, H.

    2002-01-29

    Using a recently developed, massively parallel molecular-dynamics (MD) code for the simulation of polycrystal plasticity, we analyze for the case of nanocrystalline Al the complex interplay among various dislocation and grain-boundary processes during low-temperature deformation. A unique aspect of this work, arising from our ability to deform to rather large plastic strains and to consider a rather large grain size, is the observation of deformation under very high grain-boundary and dislocation densities, i.e., in a deformation regime where they compete on an equal footing. We are thus able to identify the intra- and intergranular dislocation and grain-boundary processes responsible for the extensive deformation twinning observed in our simulations. This illustrates the ability of this type of simulations to capture novel atomic-level insights into the underlying deformation mechanisms not presently possible experimentally. smaller grain size, mobile dislocations must be nucleated from other sources, such as the GBs or grain junctions.

  2. 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.

  3. In Situ Observation on Dislocation-Controlled Sublimation of Mg Nanoparticles.

    PubMed

    Yu, Qian; Mao, Min-Min; Li, Qing-Jie; Fu, Xiao-Qian; Tian, He; Li, Ji-Xue; Mao, Scott X; Zhang, Ze

    2016-02-10

    Sublimation is an important endothermic phase transition in which the atoms break away from their neighbors in the crystal lattice and are removed into the gas phase. Such debonding process may be significantly influenced by dislocations, the crystal defect that changes the bonding environment of local atoms. By performing systematic defects characterization and in situ transmission electron microscopy (TEM) tests on a core--shell MgO-Mg system, which enables us to "modulate" the internal dislocation density, we investigated the role of dislocations on materials' sublimation with particular focus on the sublimation kinetics and mechanism. It was observed that the sublimation rate increases significantly with dislocation density. As the density of screw dislocations is high, the intersection of screw dislocation spirals creates a large number of monatomic ledges, resulting in a "liquid-like" motion of solid-gas interface, which significantly deviates from the theoretically predicted sublimation plane. Our calculation based on density functional theory demonstrated that the remarkable change of sublimation rate with dislocation density is due to the dramatic reduction in binding energy of the monatomic ledges. This study provides direct observation to improve our understanding on this fundamental phase transition as well as to shed light on tuning materials' sublimation by "engineering" dislocation density in applications. PMID:26799861

  4. Misfit dislocations in epitaxy

    NASA Astrophysics Data System (ADS)

    van der Merwe, Jan H.

    2002-08-01

    This article on epitaxy highlights the following: the definition and some historical milestones; the introduction by Frenkel and Kontorowa (FK) of a truncated Fourier series to model the periodic interaction at crystalline interfaces; the invention by Frank and van der Merwe (FvdM)—using the FK model—of (interfacial) misfit dislocations as an important mechanism in accommodating misfit at epilayer-substrate interfaces; the generalization of the FvdM theory to multilayers; the application of the parabolic model by Jesser and van der Merwe to describe, for growing multilayers and superlattices, the impact of Fourier coefficients in the realization of epitaxial orientations and the stability of modes of misfit accommodation; the involvement of intralayer interaction in the latter—all features that impact on the attainment of perfection in crystallinity of thin films, a property that is so vital in the fabrication of useful uniformly thick epilayers (uniformity being another technological requirement), which also depends on misfit accommodation through the interfacial energy that function strongly in the criterion for growth modes, proposed by Bauer; and the ingenious application of the Volterra model by Matthews and others to describe misfit accommodation by dislocations in growing epilayers.

  5. CONDENSED MATTER: STRUCTURE, THERMAL AND MECHANICAL PROPERTIES: The effect of single AlGaN interlayer on the structural properties of GaN epilayers grown on Si (111) substrates

    NASA Astrophysics Data System (ADS)

    Wu, Yu-Xin; Zhu, Jian-Jun; Zhao, De-Gang; Liu, Zong-Shun; Jiang, De-Sheng; Zhang, Shu-Ming; Wang, Yu-Tian; Wang, Hui; Chen, Gui-Feng; Yang, Hui

    2009-10-01

    High-quality and nearly crack-free GaN epitaxial layer was obtained by inserting a single AlGaN interlayer between GaN epilayer and high-temperature AlN buffer layer on Si (111) substrate by metalorganic chemical vapor deposition. This paper investigates the effect of AlGaN interlayer on the structural properties of the resulting GaN epilayer. It confirms from the optical microscopy and Raman scattering spectroscopy that the AlGaN interlayer has a remarkable effect on introducing relative compressive strain to the top GaN layer and preventing the formation of cracks. X-ray diffraction and transmission electron microscopy analysis reveal that a significant reduction in both screw and edge threading dislocations is achieved in GaN epilayer by the insertion of AlGaN interlayer. The process of threading dislocation reduction in both AlGaN interlayer and GaN epilayer is demonstrated.

  6. On the influence of dislocation walls in CdTe:Cl

    NASA Astrophysics Data System (ADS)

    Venturi, Giulia; Castaldini, Antonio; Gros d'Aillon, Eric; Buis, Camille; Verger, Loick; Cavallini, Anna

    2016-03-01

    Studies were performed on two types of chlorine-compensated cadmium telluride crystals with a different density of native dislocations walls. The crystals were investigated by current-voltage measurements, photo-induced current transient spectroscopy and absorption measurements, in the view of investigating the influence of the density of dislocation walls on their charge transport properties and electronic levels scheme. It results that a higher density of dislocation walls increases the dark current in CdTe. To the contrary, the optical absorption properties do not seem to be influenced by the presence of dislocation walls. The PICTS measurements demonstrated that a lower density of dislocation walls provides a higher concentration of compensation-related defects and a lower concentration of the defects responsible for the peaks observed at high temperature, possibly associated to donor-pair complexes.

  7. Continuum representation of systems of dislocation lines: A general method for deriving closed-form evolution equations

    NASA Astrophysics Data System (ADS)

    Monavari, Mehran; Sandfeld, Stefan; Zaiser, Michael

    2016-10-01

    Plasticity is governed by the evolution of, in general anisotropic, systems of dislocations. We seek to faithfully represent this evolution in terms of density-like variables which average over the discrete dislocation microstructure. Starting from T. Hochrainer's continuum theory of dislocations (CDD), we introduce a methodology based on the 'Maximum Information Entropy Principle' (MIEP) for deriving closed-form evolution equations for dislocation density measures of different order. These equations provide an optimum representation of the kinematic properties of systems of curved and connected dislocation lines with the information contained in a given set of density measures. The performance of the derived equations is benchmarked against other models proposed in the literature, using discrete dislocation dynamics simulations as a reference. As a benchmark problem we study dislocations moving in a highly heterogeneous, persistent slip-band-like geometry. We demonstrate that excellent agreement with discrete simulations can be obtained in terms of a very small number of averaged dislocation fields containing information about the edge and screw components of the total and excess (geometrically necessary) dislocation densities. From these the full dislocation orientation distribution which emerges as dislocations move through a channel-wall structure can be faithfully reconstructed.

  8. Hole mediated magnetism in Mn-doped GaN nanowires

    NASA Astrophysics Data System (ADS)

    Zhang, Xiu-Wen; Li, Jingbo; Chang, Kai; Li, Shu-Shen; Xia, Jian-Bai

    2011-04-01

    The hole-mediated magnetism in Mn-doped GaN nanowires is investigated using the k .p method and the mean-field model. The Curie temperature (TC) as a function of the hole density p can be explained based on the calculated band structure of the nanowires. For low Mn concentration, TC vs. p shows many peaks stem from the peaks of the one-dimensional density of states. When the Mn concentration is increased, TC is enhanced, and the peaks of TC versus p are fully merged by the thermal distribution of the holes in the valence band. It is found that the Curie temperature in Mn-doped GaN wire can be higher than room temperature, in agreement with experiment [Song et al., J. Phys.: Condens. Matter 17, 5073 (2005)]. The ferromagnetism in Mn-doped GaN wire is slightly anisotropic due to the small spin-orbit coupling.

  9. Molecular beam epitaxy of single crystalline GaN nanowires on a flexible Ti foil

    NASA Astrophysics Data System (ADS)

    Calabrese, Gabriele; Corfdir, Pierre; Gao, Guanhui; Pfüller, Carsten; Trampert, Achim; Brandt, Oliver; Geelhaar, Lutz; Fernández-Garrido, Sergio

    2016-05-01

    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 pave the way for the fabrication of flexible optoelectronic devices based on GaN nanowires on metal foils.

  10. GaN High Power Devices

    SciTech Connect

    PEARTON,S.J.; REN,F.; ZHANG,A.P.; DANG,G.; CAO,X.A.; LEE,K.P.; CHO,H.; GILA,B.P.; JOHNSON,J.W.; MONIER,C.; ABERNATHY,C.R.; HAN,JUNG; BACA,ALBERT G.; CHYI,J.-I.; LEE,C.-M.; NEE,T.-E.; CHUO,C.-C.; CHI,G.C.; CHU,S.N.G.

    2000-07-17

    A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers, GaN/AlGaN heterojunction bipolar transistors, GaN heterostructure and metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.

  11. Terahertz electromodulation spectroscopy of electron transport in GaN

    SciTech Connect

    Engelbrecht, S. G.; Arend, T. R.; Kersting, R.; Zhu, T.; Kappers, M. J.

    2015-03-02

    Time-resolved terahertz (THz) electromodulation spectroscopy is applied to investigate the high-frequency transport of electrons in gallium nitride at different doping concentrations and densities of threading dislocations. At THz frequencies, all structures reveal Drude transport. The analysis of the spectral response provides the fundamental transport properties, such as the electron scattering time and the electrons' conductivity effective mass. We observe the expected impact of ionized-impurity scattering and that scattering at threading dislocations only marginally affects the high-frequency mobility.

  12. Superclimb of Dislocations in Solid 4He

    NASA Astrophysics Data System (ADS)

    Kuklov, Anatoly

    2011-03-01

    Edge dislocation with superfluid core can perform superclimb -- non-conservative motion (climb) assisted by superflow along its core. Such dislocation, with Burgers vector along the C-axis, has been found in ab initio simulations of hcp solid 4. Uniform network of superclimbing dislocations can induce isochoric compressibility which is finite (in contrast to ideal solid where it vanishes) and, practically, independent of the network density. Here N is total number of atoms and is chemical potential. Such giant response has been observed by Ray and Hallock during superfluid flow events through solid He4. Study of superclimbing dislocation within the model of Granato-Lücke string, subjected to Peierls potential and to vanishing bias by , has found that exhibits wide peak in the intermediate range of temperatures (T) - above some determined by Peierls energy and below above which superfluidity of the core essentially vanishes. Non-Luttinger type behavior characterized by K scaling as some power χ of dislocation length is observed in the wide peak region. Biasing superclimbing dislocation by finite μ (due to a contact with liquid through vycor electrodes,) can induce core roughening caused by thermally assisted tunneling of jog-antijog pairs through the barrier produced by combination of Peierls potential and the bias. The threshold for this effect scales as with some power a~ 1.7. The roughening is found to be hysteretic below some temperature Thyst , with TR determining temperature of thermal roughening, He exhibits strong and narrow resonant peak leading to a dip in the core superfluid sound velocity. This mechanism is proposed as an explanation for a strong and narrow dip observed in critical superflow rate. It is found that the dip characteristics are sensitive to the bias by μ and, therefore, this can be used as a test for the proposed mechanism. It is also predicted that the dip depth at given μc ~ 1 /La should be periodic in χ with the period T . This

  13. 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.

  14. Irradiation swelling behavior and its dependence on temperature, dose rate and dislocation structure evolution

    SciTech Connect

    Surh, M P; Sturgeon, J B; Wolfer, W G

    2004-01-16

    The microstructural evolution of high purity steel under irradiation is modeled including a dislocation density that evolves simultaneously with void nucleation and growth. The predicted void swelling trends versus temperature, flux, and time are compared to experiment and to earlier calculations with a fixed dislocation density. The behavior is further analyzed within a simplified picture of segregation of irradiation defects to microstructural sinks. Agreement with experimental swelling behavior improves when dislocations co-evolve with the void content versus simulations with a fixed dislocation density. The time-dependent dislocation content dictates the rate of void nucleation and shapes the overall void size distribution so as to give steady swelling behavior over long times.

  15. Piezo-generator integrating a vertical array of GaN nanowires

    NASA Astrophysics Data System (ADS)

    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.

    2016-08-01

    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.

  16. 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. PMID:27363777

  17. Piezo-generator integrating a vertical array of GaN nanowires

    NASA Astrophysics Data System (ADS)

    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.

    2016-08-01

    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.

  18. 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.

  19. Ion irradiation induced disappearance of dislocations in a nickel-based alloy

    NASA Astrophysics Data System (ADS)

    Chen, H. C.; Li, D. H.; Lui, R. D.; Huang, H. F.; Li, J. J.; Lei, G. H.; Huang, Q.; Bao, L. M.; Yan, L.; Zhou, X. T.; Zhu, Z. Y.

    2016-06-01

    Under Xe ion irradiation, the microstructural evolution of a nickel based alloy, Hastelloy N (US N10003), was studied. The intrinsic dislocations are decorated with irradiation induced interstitial loops and/or clusters. Moreover, the intrinsic dislocations density reduces as the irradiation damage increases. The disappearance of the intrinsic dislocations is ascribed to the dislocations climb to the free surface by the absorption of interstitials under the ion irradiation. Moreover, the in situ annealing experiment reveals that the small interstitial loops and/or clusters induced by the ion irradiation are stable below 600 °C.

  20. Moving Dislocations in Disordered Alloys.

    SciTech Connect

    Marian, J; Caro, A

    2006-11-18

    Using atomistic simulations of dislocation motion in Ni and Ni-Au alloys we report a detailed study of the mobility function as a function of stress, temperature and alloy composition. We analyze the results in terms of analytic models of phonon radiation and their selection rules for phonon excitation. We find a remarkable agreement between the location of the cusps in the {sigma}-v relation and the velocity of waves propagating in the direction of dislocation motion. We identify and characterize three regimes of dissipation whose boundaries are essentially determined by the direction of motion of the dislocation, rather than by its screw or edge character.

  1. Dislocation after total knee arthroplasty.

    PubMed

    Wazir, N N; Shan, Y; Mukundala, V V; Gunalan, R

    2007-05-01

    Two cases of dislocation of total knee arthroplasty presented to us within the same week. The first patient is a 71-year-old woman who underwent bilateral primary total knee arthroplasty. The left knee dislocated three weeks after the surgery. Due to failure of conservative measures, she underwent revision total knee arthroplasty. The other patient is a 72-year-old woman presenting ten years after primary total knee arthroplasty, with a traumatic dislocation of the knee joint. She was treated as an outpatient with closed manipulative reduction.

  2. Buckling of dislocation in graphene

    NASA Astrophysics Data System (ADS)

    Yao, Yin; Wang, Shaofeng; Bai, Jianhui; Wang, Rui

    2016-10-01

    The buckling of dislocation in graphene is discussed through the lattice theory of dislocation and elastic theory. The approximate solution of the buckling is obtained based on the inner stress distribution caused by different structure of dislocations and is proved to be suitable by the simulation. The position of the highest buckling is predicted to be at the vertex of the pentagon far away from the heptagon. The buckling is strongly influenced by the internal stress and the distance between the extrusive area and stretching area, as well as the critical stress σc. The SW defect is proved to be unbuckled due to its strong interaction between extrusion and stretching.

  3. Dislocation motion in {gamma} TiAl by in situ straining experiments in the HVEM

    SciTech Connect

    Messerschmidt, U.; Bartsch, M.; Haeussler, D.; Hattenhauer, R.; Aindow, M.; Jones, I.P.

    1995-08-01

    Micro-tensile specimens of coarse-grained Ti52at%Al crystals have been deformed in situ in a high voltage electron microscope at room temperature. In addition to some twinning, ``simple`` 1/2{l_angle}110] dislocations as well as super dislocations were moving, with the simple dislocations prevailing even if their orientation factor is lower than that of the super dislocations. Both types of dislocations are pinned, probably by small precipitates having a distance along the dislocations of about 100 nm. The precipitates consist most probably of Al{sub 2}O{sub 3}. Under stress, the dislocations bow out between the obstacles. The bowing is stronger for 1/2{l_angle}110] dislocations. An effective stress of about 41 MPa is estimated from their curvature. The kinematic behavior of the dislocations is in accord with precipitation hardening. The dislocations are generated by the double-cross slip mechanism. Their density within the slip bands corresponds to a long-range internal stress of about 40 MPa. These data are consistent with the flow stress of PST crystals in the easy orientation, taken from the literature.

  4. 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.

  5. 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.

  6. In situ transmission electron microscopy observation of dislocation motion in 9Cr steel at elevated temperatures: influence of shear stress on dislocation behavior.

    PubMed

    Yamada, Susumu; Sakai, Takayuki

    2014-12-01

    To elucidate high-temperature plastic deformation (creep) mechanism in materials, it is essential to observe dislocation motion under tensile loading. There are many reports on in situ transmission electron microscopy (TEM) observations in the literature; however, the relationship between the dislocation motion and shear stress in 9Cr steel is still not clear. In this study, in order to evaluate this relationship quantitatively, in situ TEM observations were carried out in conjunction with finite element method (FEM) analysis. A tensile test sample was strained at an elevated temperature (903 K) inside a transmission electron microscope, and the stress distribution in the strained sample was analyzed by FEM. The dislocation behavior was clearly found to depend on the shear stress. At a shear stress of 66 MPa, both the dislocation velocity and mobile dislocation density were low. However, a high shear stress level of 95 MPa caused a noticeable increase in the dislocation velocity and mobile dislocation density. Furthermore, in this article, we discuss the dependence of the dislocation behavior on stress. The results presented here also indicate that the relationship between the microstructure and the strength of materials can be revealed by the methods used in this work. PMID:25298228

  7. Growth behavior of GaN nanowires on c-plane sapphire substrate by applying various catalysts

    NASA Astrophysics Data System (ADS)

    Kuppulingam, B.; Bhalerao, G. M.; Singh, Shubra; Baskar, K.

    2016-07-01

    Systematic reaction has been used to control the vapor-liquid-solid growth of gallium nitride nanowires (NWs) using different catalysts. GaN NWs were grown using Cu, Au, Pd/Au alloy catalysts on c-plane sapphire substrate. XRD and Raman analysis revealed the crystalline wurtzite phase of GaN synthesized at 900 °C. High density GaN NWs were studied using SEM and HRTEM. Elemental composition and impurities were analyzed by EDX. Diameter of individual NW, grown using Au catalyst is found to be ~50 nm. The diameter of NWs grown with the help of Cu catalyst was found to be ˜65 nm, whereas with Pd/Au catalyst, the diameter was about 100-200 nm. NBE emission observed from PL spectra for Cu catalyst (377 nm), Au catalyst (372 nm) as well as Pd/Au catalyst (385 nm) growth of GaN NWs respectively has been presented and discussed.

  8. Detailed formation processes of stable dislocations in graphene

    NASA Astrophysics Data System (ADS)

    Lee, Gun-Do; Yoon, Euijoon; He, Kuang; Robertson, Alex W.; Warner, Jamie H.

    2014-11-01

    We use time-dependent HRTEM to reveal that stable dislocation pairs in graphene are formed from an initial complex multi-vacancy cluster that undergoes multiple bond rotations and adatom incorporation. In the process, it is found that the transformation from the formed complex multi-vacancy cluster can proceed without the increase of vacancy because many atoms and dimers are not only evaporated but also actively adsorbed. In tight-binding molecular dynamics simulations, it is confirmed that adatoms play an important role in the reconstruction of non-hexagonal rings into hexagonal rings. From density functional theory calculations, it is also found from simulations that there is a favorable distance between two dislocations pointing away from each other (i.e. formed from atom loss). For dislocation pairs pointing away from each other, the hillock-basin structure is more stable than the hillock-hillock structure for dislocation pairs pointing away from each other (i.e. formed from atom loss).We use time-dependent HRTEM to reveal that stable dislocation pairs in graphene are formed from an initial complex multi-vacancy cluster that undergoes multiple bond rotations and adatom incorporation. In the process, it is found that the transformation from the formed complex multi-vacancy cluster can proceed without the increase of vacancy because many atoms and dimers are not only evaporated but also actively adsorbed. In tight-binding molecular dynamics simulations, it is confirmed that adatoms play an important role in the reconstruction of non-hexagonal rings into hexagonal rings. From density functional theory calculations, it is also found from simulations that there is a favorable distance between two dislocations pointing away from each other (i.e. formed from atom loss). For dislocation pairs pointing away from each other, the hillock-basin structure is more stable than the hillock-hillock structure for dislocation pairs pointing away from each other (i.e. formed from

  9. Lattice location of deuterium in plasma and gas charged Mg doped GaN

    SciTech Connect

    Wampler, W.R.; Barbour, J.C.; Seager, C.H.; Myers, S.M. Jr.; Wright, A.F.; Han, J.

    1999-12-02

    The authors have used ion channeling to examine the lattice configuration of deuterium in Mg doped GaN grown by MOCVD. The deuterium is introduced both by exposure to deuterium gas and to ECR plasmas. A density functional approach including lattice relaxation, was used to calculate total energies for various locations and charge states of hydrogen in the wurtzite Mg doped GaN lattice. Computer simulations of channeling yields were used to compare results of channeling measurements with calculated yields for various predicted deuterium lattice configurations.

  10. Microstructural evolution in H ion induced splitting of freestanding GaN

    SciTech Connect

    Moutanabbir, O.; Scholz, R.; Senz, S.; Goesele, U.; Chicoine, M.; Schiettekatte, F.; Suesskraut, F.; Krause-Rehberg, R.

    2008-07-21

    We investigated the microstructural transformations during hydrogen ion-induced splitting of GaN thin layers. Cross-sectional transmission electron microscopy and positron annihilation spectroscopy data show that the implanted region is decorated with a high density of 1-2 nm bubbles resulting from vacancy clustering during implantation. These nanobubbles persist up to 450 deg. C. Ion channeling data show a strong dechanneling enhancement in this temperature range tentatively attributed to strain-induced lattice distortion. The dechanneling level decreases following the formation of plateletlike structures at 475 deg. C. Extended internal surfaces develop around 550 deg. C leading to the exfoliation of GaN thin layer.

  11. Nucleation and Growth of GaN on GaAs (001) Substrates

    SciTech Connect

    Drummond, Timothy J.; Hafich, Michael J.; Heller, Edwin J.; Lee, Stephen R.; Liliental-Weber, Zuzanna; Ruvimov, Sergei; Sullivan, John P.

    1999-05-03

    The nucleation of GaN thin films on GaAs is investigated for growth at 620 "C. An rf plasma cell is used to generate chemically active nitrogen from N2. An arsenic flux is used in the first eight monolayer of nitride growth to enhance nucleation of the cubic phase. Subsequent growth does not require an As flux to preserve the cubic phase. The nucleation of smooth interfaces and GaN films with low stacking fault densities is dependent upon relative concentrations of active nitrogen species in the plasma and on the nitrogen to gallium flux ratio.

  12. Polar properties of a hexagonally bonded GaN sheet under biaxial compression

    NASA Astrophysics Data System (ADS)

    Gao, Yanlin; Yayama, Tomoe; Okada, Susumu

    2016-09-01

    Using the density functional theory, we study the geometric and electronic structures of a GaN sheet possessing a honeycomb network. The sheet preserves the planar conformation under an equilibrium lattice constant of 3.2 Å, and has a semiconducting electronic structure with an indirect band gap of 2.28 eV. The biaxial compressive strain causes structural buckling, leading to polarization normal to the atomic layer. An external electric field normal to the layer also induces structural buckling with a height proportional to the field strength. The polarity of the buckled GaN sheet is tunable by attaching H atoms on Ga and N atoms.

  13. Individual GaN nanowires exhibit strong piezoelectricity in 3D.

    PubMed

    Minary-Jolandan, Majid; Bernal, Rodrigo A; Kuljanishvili, Irma; Parpoil, Victor; Espinosa, Horacio D

    2012-02-01

    Semiconductor GaN NWs are promising components in next generation nano- and optoelectronic systems. In addition to their direct band gap, they exhibit piezoelectricity, which renders them particularly attractive in energy harvesting applications for self-powered devices. Nanowires are often considered as one-dimensional nanostructures; however, the electromechanical coupling leads to a third rank tensor that for wurtzite crystals (GaN NWs) possesses three independent coefficients, d(33), d(13), and d(15). Therefore, the full piezoelectric characterization of individual GaN NWs requires application of electric fields in different directions and measurements of associated displacements on the order of several picometers. In this Letter, we present an experimental approach based on scanning probe microscopy to directly quantify the three-dimensional piezoelectric response of individual GaN NWs. Experimental results reveal that GaN NWs exhibit strong piezoelectricity in three dimensions, with up to six times the effect in bulk. Based on finite element modeling, this finding has major implication on the design of energy harvesting systems exhibiting unprecedented levels of power density production. The presented method is applicable to other piezoelectric NW materials as well as wires manufactured along different crystallographic orientations.

  14. Analysis of leakage current mechanisms in Pt/Au Schottky contact on Ga-polarity GaN by Frenkel-Poole emission and deep level studies

    SciTech Connect

    Rao, Peta Koteswara; Park, Byungguon; Lee, Sang-Tae; Noh, Young-Kyun; Kim, Moon-Deock; Oh, Jae-Eung

    2011-07-01

    We report the Frenkel-Poole emission in Pt/Au Schottky contact on Ga-polarity GaN grown by molecular beam epitaxy using current-voltage-temperature (I-V-T) characteristics in the temperature ranging from 200 K to 375 K. Using thermionic emission model, the estimated Schottky barrier height is 0.49 eV at 200 K and 0.83 eV at 375 K, respectively, and it is observed that the barrier height increases with increase in temperature. The extracted emission barrier height ({phi}{sub t}) for Ga-polarity GaN Schottky diode by Frenkel-Poole theory is about 0.15 eV. Deep level transient spectroscopy study shows a deep level with activation energy of 0.44 eV, having capture cross-section 6.09 x 10{sup -14} cm{sup 2}, which is located between the metal and semiconductor interface, and trap nature is most probably associated with dislocations in Ga-polarity GaN. The analysis of I-V-T characteristics represents that the leakage current is due to effects of electrical field and temperature on the emission of electron from a trap state near the metal-semiconductor interface into continuum states associated with conductive dislocations in Ga-polarity GaN Schottky diode.

  15. Chlorine-based plasma etching of GaN

    SciTech Connect

    Shul, R.J.; Briggs, R.D.; Pearton, S.J.; Vartuli, C.B.; Abernathy, C.R.; Lee, J.W.; Constantine, C.; Baratt, C.

    1997-02-01

    The wide band gap group-III nitride materials continue to generate interest in the semiconductor community with the fabrication of green, blue, and ultraviolet light emitting diodes (LEDs), blue lasers, and high temperature transistors. Realization of more advanced devices requires pattern transfer processes which are well controlled, smooth, highly anisotropic and have etch rates exceeding 0.5 {micro}m/min. The utilization of high-density chlorine-based plasmas including electron cyclotron resonance (ECR) and inductively coupled plasma (ICP) systems has resulted in improved GaN etch quality over more conventional reactive ion etch (RIE) systems.

  16. H enhancement of N vacancy migration in GaN.

    SciTech Connect

    Wixom, Ryan R.; Wright, Alan Francis

    2005-06-01

    We have used density functional theory to investigate diffusion of V{sub N}{sup +} in the presence of H{sup +}. Optimal migration pathways were determined using the climbing image nudged elastic band and directed dimer methods. Our calculations indicate that the rate-limiting barrier for VN{sub N}{sup +} migration will be reduced by 0.58 eV by interplay with H{sup +}, which will enhance migration by more than an order of magnitude at typical GaN growth temperatures.

  17. Perilunar carpal dislocations treatment outcome.

    PubMed

    Gagała, Jacek; Tarczyńska, Marta; Kosior, Piotr

    2006-06-30

    Background. The aim of the study was to analyze late outcomes of perilunar carpal dislocations, depending on the type of the injury, time of the diagnosis and the treatment methods. Material and methods. The material is constituted by 37 patients treated in our department between 1981-2004 because of perilunar dislocation. In group were 2 women and 35 men, aged 19-56 (mean 31 years). All patients were asked for control visit. DASH and Mayo score were used to evaluate the outcome. Range of wrist motion, its stability, grip strength and X-ray pictures were analyzed. Results. Better follow-up results were observed in persons with early diagnosed dislocations of the wrist. The best outcomes were observed in group with perilunar early diagnosed dislocations, which were treated by open reduction. Posttraumatic wrist instability often was diagnosed in patients with dislocation of lunar bone and late-diagnosed transscaphoid perilunar carpal dislocations. Conclusions. The data we obtained show, that the consequences of late-diagnosed and late-treated injuries of the wrist are instability, pain, decrease in range of motion and hand skills. PMID:17592406

  18. Dislocation sources in ordered intermetallics

    SciTech Connect

    Yoo, M.H.; Appel, F.; Wagner, R.; Mecking, H.

    1996-09-01

    An overview on the current understanding of dislocation sources and multiplication mechanisms is made for ordered intermetallic alloys of the L1{sub 2}, B2, and D0{sub 19} structures. In L1{sub 2} alloys, a large disparity of edge/screw segments in their relative mobility reduces the efficiency of a Frank-Read Type multiplication mechanism. In Fe-40%Al of the B2 structure, a variety of dislocation sources are available for <111> slip, including ones resulting from condensation of thermal vacancies. In NiAl with the relatively high APB energy, <100> dislocations may result from the dislocation decomposition reactions, the prismatic punching out from inclusion particles, and/or steps and coated layers of the surface. Internal interfaces often provide sites for dislocation multiplication, e.g., grain boundaries, sub-boundaries in Ni{sub 3}Ga, NiAl and Ti{sub 3}Al, and antiphase domain boundaries in Ti{sub 3}Al. As for the crack tip as a dislocation source, extended SISFs trailed by super-Shockley partials emanating form the cracks in Ni{sub 3}Al and Co{sub 3}Ti are discussed in view of a possible toughening mechanism.

  19. High internal quantum efficiency ultraviolet to green luminescence peaks from pseudomorphic m-plane Al{sub 1−x}In{sub x}N epilayers grown on a low defect density m-plane freestanding GaN substrate

    SciTech Connect

    Chichibu, S. F. Hazu, K.; Furusawa, K.; Ishikawa, Y.; Onuma, T.; Ohtomo, T.; Ikeda, H.; Fujito, K.

    2014-12-07

    Structural and optical qualities of half-a-μm-thick m-plane Al{sub 1−x}In{sub x}N epilayers grown by metalorganic vapor phase epitaxy were remarkably improved via coherent growth on a low defect density m-plane freestanding GaN substrate prepared by hydride vapor phase epitaxy. All the epilayers unexceptionally suffer from uniaxial or biaxial anisotropic in-plane stress. However, full-width at half-maximum values of the x-ray ω-rocking curves were nearly unchanged as the underlayer values being 80 ∼ 150 arc sec for (101{sup ¯}0) and (101{sup ¯}2) diffractions with both 〈0001〉 and 〈112{sup ¯}0〉 azimuths, as long as pseudomorphic structure was maintained. Such Al{sub 1−x}In{sub x}N epilayers commonly exhibited a broad but predominant luminescence peak in ultraviolet (x ≤ 0.14) to green (x = 0.30) wavelengths. Its equivalent value of the internal quantum efficiency at room temperature was as high as 67% for x = 0.14 and 44% for x = 0.30. Because its high-energy cutoff commonly converged with the bandgap energy, the emission peak is assigned to originate from the extended near-band-edge states with strong carrier localization.

  20. High Voltage GaN Schottky Rectifiers

    SciTech Connect

    CAO,X.A.; CHO,H.; CHU,S.N.G.; CHUO,C.-C.; CHYI,J.-I.; DANG,G.T.; HAN,JUNG; LEE,C.-M.; PEARTON,S.J.; REN,F.; WILSON,R.G.; ZHANG,A.P.

    1999-10-25

    Mesa and planar GaN Schottky diode rectifiers with reverse breakdown voltages (V{sub RB}) up to 550V and >2000V, respectively, have been fabricated. The on-state resistance, R{sub ON}, was 6m{Omega}{center_dot} cm{sup 2} and 0.8{Omega}cm{sup 2}, respectively, producing figure-of-merit values for (V{sub RB}){sup 2}/R{sub ON} in the range 5-48 MW{center_dot}cm{sup -2}. At low biases the reverse leakage current was proportional to the size of the rectifying contact perimeter, while at high biases the current was proportional to the area of this contact. These results suggest that at low reverse biases, the leakage is dominated by the surface component, while at higher biases the bulk component dominates. On-state voltages were 3.5V for the 550V diodes and {ge}15 for the 2kV diodes. Reverse recovery times were <0.2{micro}sec for devices switched from a forward current density of {approx}500A{center_dot}cm{sup -2} to a reverse bias of 100V.

  1. Dislocation related droop in InGaN/GaN light emitting diodes investigated via cathodoluminescence

    SciTech Connect

    Pozina, Galia; Ciechonski, Rafal; Bi, Zhaoxia; Samuelson, Lars; Monemar, Bo

    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 increase 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.

  2. 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).

  3. Mechanical properties of nanoporous GaN and its application for separation and transfer of GaN thin films.

    PubMed

    Huang, Shanjin; Zhang, Yu; Leung, Benjamin; Yuan, Ge; Wang, Gang; Jiang, Hao; Fan, Yingmin; Sun, Qian; Wang, Jianfeng; Xu, Ke; Han, Jung

    2013-11-13

    Nanoporous (NP) gallium nitride (GaN) as a new class of GaN material has many interesting properties that the conventional GaN material does not have. In this paper, we focus on the mechanical properties of NP GaN, and the detailed physical mechanism of porous GaN in the application of liftoff. A decrease in elastic modulus and hardness was identified in NP GaN compared to the conventional GaN film. The promising application of NP GaN as release layers in the mechanical liftoff of GaN thin films and devices was systematically studied. A phase diagram was generated to correlate the initial NP GaN profiles with the as-overgrown morphologies of the NP structures. The fracture toughness of the NP GaN release layer was studied in terms of the voided-space-ratio. It is shown that the transformed morphologies and fracture toughness of the NP GaN layer after overgrowth strongly depends on the initial porosity of NP GaN templates. The mechanical separation and transfer of a GaN film over a 2 in. wafer was demonstrated, which proves that this technique is useful in practical applications.

  4. Thermal boundary conductance between Al films and GaN nanowires investigated with molecular dynamics.

    PubMed

    Zhou, Xiao-Wang; Jones, Reese E; Hopkins, Patrick E; Beechem, Thomas E

    2014-05-28

    GaN nanowires are being pursued for optoelectronic and high-power applications. In either use, increases in operating temperature reduce both performance and reliability making it imperative to minimize thermal resistances. Since interfaces significantly influence the thermal response of nanosystems, the thermal boundary resistance between GaN nanowires and metal contacts has major significance. In response, we have performed systematic molecular dynamics simulations to study the thermal boundary conductance between GaN nanowires and Al films as a function of nanowire dimensions, packing density, and the depth the nanowire is embedded into the metal contact. At low packing densities, the apparent Kapitza conductance between GaN nanowires and an aluminum film is shown to be larger than when contact is made between films of these same materials. This enhancement decreases toward the film-film limit, however, as the packing density increases. For densely packed nanowires, maximizing the Kapitza conductance can be achieved by embedding the nanowires into the films, as the conductance is found to be proportional to the total contact area.

  5. A Theoretical Study of Lithium-Doped Gallium Clusters by Density Functional Theory

    NASA Astrophysics Data System (ADS)

    Şentürk, Şükrü; Ekincioğlu, Yavuz

    2012-05-01

    The geometrical structures, stabilities, and electronic properties of GanLi (n = 1 - 13) clusters were investigated within the density functional theory (DFT). The impurity lithium atom enhances the stability of GanLi (n = 1 - 13) clusters, especially GanLi (n = 9 - 13) compared to Gan(n = 9 - 14), that is at either apex position or side position. The dissociation energy, second-order energy differences, and the energy gaps between highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) indicate that the Ga7Li, Ga9Li, and Ga11Li clusters are more stable within the studied cluster range. Moreover, the variation of the average bond length of Ga-Li is due to the surface effect, and the binding strength increases resulting from the increase of charge amount.

  6. Electronic selection rules controlling dislocation glide in bcc metals.

    PubMed

    Jones, Travis E; Eberhart, Mark E; Clougherty, Dennis P; Woodward, Chris

    2008-08-22

    The validity of the structure-property relationships governing the low-temperature deformation behavior of many bcc metals was brought into question with recent ab initio density functional studies of isolated screw dislocations in Mo and Ta. These relationships were semiclassical in nature, having grown from atomistic investigations of the deformation properties of the group V and VI transition metals. We find that the correct form for these structure-property relationships is fully quantum mechanical, involving the coupling of electronic states with the strain field at the core of long a/<2111> screw dislocations. PMID:18764636

  7. Electronic Selection Rules Controlling Dislocation Glide in bcc Metals

    NASA Astrophysics Data System (ADS)

    Jones, Travis E.; Eberhart, Mark E.; Clougherty, Dennis P.; Woodward, Chris

    2008-08-01

    The validity of the structure-property relationships governing the low-temperature deformation behavior of many bcc metals was brought into question with recent ab initio density functional studies of isolated screw dislocations in Mo and Ta. These relationships were semiclassical in nature, having grown from atomistic investigations of the deformation properties of the group V and VI transition metals. We find that the correct form for these structure-property relationships is fully quantum mechanical, involving the coupling of electronic states with the strain field at the core of long a/2⟨111⟩ screw dislocations.

  8. Materials physics and device development for improved efficiency of GaN HEMT high power amplifiers.

    SciTech Connect

    Kurtz, Steven Ross; Follstaedt, David Martin; Wright, Alan Francis; Baca, Albert G.; Briggs, Ronald D.; Provencio, Paula Polyak; Missert, Nancy A.; Allerman, Andrew Alan; Marsh, Phil F.; Koleske, Daniel David; Lee, Stephen Roger; Shul, Randy John; Seager, Carleton Hoover; Tigges, Christopher P.

    2005-12-01

    GaN-based microwave power amplifiers have been identified as critical components in Sandia's next generation micro-Synthetic-Aperture-Radar (SAR) operating at X-band and Ku-band (10-18 GHz). To miniaturize SAR, GaN-based amplifiers are necessary to replace bulky traveling wave tubes. Specifically, for micro-SAR development, highly reliable GaN high electron mobility transistors (HEMTs), which have delivered a factor of 10 times improvement in power performance compared to GaAs, need to be developed. Despite the great promise of GaN HEMTs, problems associated with nitride materials growth currently limit gain, linearity, power-added-efficiency, reproducibility, and reliability. These material quality issues are primarily due to heteroepitaxial growth of GaN on lattice mismatched substrates. Because SiC provides the best lattice match and thermal conductivity, SiC is currently the substrate of choice for GaN-based microwave amplifiers. Obviously for GaN-based HEMTs to fully realize their tremendous promise, several challenges related to GaN heteroepitaxy on SiC must be solved. For this LDRD, we conducted a concerted effort to resolve materials issues through in-depth research on GaN/AlGaN growth on SiC. Repeatable growth processes were developed which enabled basic studies of these device layers as well as full fabrication of microwave amplifiers. Detailed studies of the GaN and AlGaN growth of SiC were conducted and techniques to measure the structural and electrical properties of the layers were developed. Problems that limit device performance were investigated, including electron traps, dislocations, the quality of semi-insulating GaN, the GaN/AlGaN interface roughness, and surface pinning of the AlGaN gate. Surface charge was reduced by developing silicon nitride passivation. Constant feedback between material properties, physical understanding, and device performance enabled rapid progress which eventually led to the successful fabrication of state of the art

  9. In-situ TEM observation of dislocation evolution in Kr-irradiated UO2 single crystal

    SciTech Connect

    Lingfeng He; Mahima Gupta; Clarissa A. Yablinsky; Jian Gan; Marquis A. Kirk; Xian-Ming Bai; Janne Pakarinen; Todd R. Allen

    2013-11-01

    In-situ transmission electron microscopy (TEM) observation of UO2 single crystal irradiated with Kr ions at high temperatures was conducted to understand the dislocation evolution due to high-energy radiation. The dislocation evolution in UO2 single crystal is shown to occur as nucleation and growth of dislocation loops at low-irradiation doses, followed by transformation to extended dislocation segments and networks at high doses, as well as shrinkage and annihilation of some loops and dislocations due to high temperature annealing. Generally the trends of dislocation evolution in UO2 are similar under Kr irradiation at different ion energies and temperatures (150 keV at 600 degrees C and 1 MeV at 800 degrees C) used in this work, although the specific dislocation loop size and density are quite different. Interstitial-type dislocation loops with Burgers vector along <110> were observed in the Kr-irradiated UO2.The irradiated specimens were denuded of dislocation loops near the surface.

  10. Growth kinetics of AlN and GaN films grown by molecular beam epitaxy on R-plane sapphire substrates

    SciTech Connect

    Chandrasekaran, R.; Moustakas, T. D.; Ozcan, A. S.; Ludwig, K. F.; Zhou, L.; Smith, David J.

    2010-08-15

    This paper reports the growth by molecular beam epitaxy of AlN and GaN thin films on R-plane sapphire substrates. Contrary to previous findings that GaN grows with its (1120) A-plane parallel to the (1102) R-plane of sapphire, our results indicate that the crystallographic orientation of the III-nitride films is strongly dependent on the kinetic conditions of growth for the GaN or AlN buffer layers. Thus, group III-rich conditions for growth of either GaN or AlN buffers result in nitride films having (1120) planes parallel to the sapphire surface, and basal-plane stacking faults parallel to the growth direction. The growth of these buffers under N-rich conditions instead leads to nitride films with (1126) planes parallel to the sapphire surface, with inclined c-plane stacking faults that often terminate threading dislocations. Moreover, electron microscope observations indicate that slight miscut ({approx}0.5 deg. ) of the R-plane sapphire substrate almost completely suppresses the formation of twinning defects in the (1126) GaN films.

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

    SciTech Connect

    Deng, Hui; Endo, Katsuyoshi; Yamamura, Kazuya

    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 in 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.

  12. Effect of growth temperature on defects in epitaxial GaN film grown by plasma assisted molecular beam epitaxy

    SciTech Connect

    Kushvaha, S. S. Pal, P.; Shukla, A. K.; Joshi, Amish G.; Gupta, Govind; Kumar, M.; Singh, S.; Gupta, Bipin K.; Haranath, D.

    2014-02-15

    We report the effect of growth temperature on defect states of GaN epitaxial layers grown on 3.5 μm thick GaN epi-layer on sapphire (0001) substrates using plasma assisted molecular beam epitaxy. The GaN samples grown at three different substrate temperatures at 730, 740 and 750 °C were characterized using atomic force microscopy and photoluminescence spectroscopy. The atomic force microscopy images of these samples show the presence of small surface and large hexagonal pits on the GaN film surfaces. The surface defect density of high temperature grown sample is smaller (4.0 × 10{sup 8} cm{sup −2} at 750 °C) than that of the low temperature grown sample (1.1 × 10{sup 9} cm{sup −2} at 730 °C). A correlation between growth temperature and concentration of deep centre defect states from photoluminescence spectra is also presented. The GaN film grown at 750 °C exhibits the lowest defect concentration which confirms that the growth temperature strongly influences the surface morphology and affects the optical properties of the GaN epitaxial films.

  13. Effect of GaAs substrate orientation on the growth kinetic of GaN layer grown by MOVPE

    NASA Astrophysics Data System (ADS)

    Laifi, J.; Chaaben, N.; Bouazizi, H.; Fourati, N.; Zerrouki, C.; El Gmili, Y.; Bchetnia, A.; Salvestrini, J. P.; El Jani, B.

    2016-06-01

    We have investigated the kinetic growth of low temperature GaN nucleation layers (LT-GaN) grown on GaAs substrates with different crystalline orientations. GaN nucleation layers were grown by metal organic vapor phase epitaxy (MOVPE) in a temperature range of 500-600 °C on oriented (001), (113), (112) and (111) GaAs substrates. The growth was in-situ monitored by laser reflectometry (LR). Using an optical model, including time-dependent surface roughness and growth rate profiles, simulations were performed to best approach the experimental reflectivity curves. Results are discussed and correlated with ex-situ analyses, such as atomic force microscopy (AFM) and UV-visible reflectance (SR). We show that the GaN nucleation layers growth results the formation of GaN islands whose density and size vary greatly with both growth temperature and substrate orientation. Arrhenius plots of the growth rate for each substrate give values of activation energy varying from 0.20 eV for the (001) orientation to 0.35 eV for the (113) orientation. Using cathodoluminescence (CL), we also show that high temperature (800-900 °C) GaN layers grown on top of the low temperature (550 °C) GaN nucleation layers, grown themselves on the GaAs substrates with different orientations, exhibit cubic or hexagonal phase depending on both growth temperature and substrate orientation.

  14. Congenital dislocation of the knee.

    PubMed

    Ko, J Y; Shih, C H; Wenger, D R

    1999-01-01

    Between February 1988 and June 1995, 24 congenital dislocations of the knee joints (17 patients) were reduced with closed methods including immediate reduction, serial casting, or traction in patients from 10 min to 26 days old. At an average follow-up of 4 years and 10 months, an excellent or good result was achieved if there were no associated anomalies. Fair or poor results were the result of delayed treatment or associated musculoskeletal anomalies including arthrogryposis multiplex congenita or Larsen's syndrome. Routine check of the hip dislocation is suggested. Diagnosis with manual testing was difficult, and other methods such as radiography or sonography were suggested in combination to detect hip dysplasia. The dislocated knee should be reduced before treating the hip dislocation. Concomitant treatment of the congenital dislocation of the knee and the hip with Pavlik harness provided satisfactory results. When late, progressive, genu valgus deformity occurred because of global instability of the knee and asymmetric physeal growth, reconstruction of the medial structures of the knee and prolonged bracing provided good results. PMID:10088699

  15. Dislocations and other topological oddities

    NASA Astrophysics Data System (ADS)

    Pieranski, Pawel

    2016-03-01

    We will show that the book Dislocations by Jacques Friedel, published half a century ago, can still be recommended, in agreement with the author's intention, as a textbook "for research students at University and for students at engineering schools as well as for research engineers". Indeed, today dislocations are known to occur not only in solid crystals but also in many other systems discovered more recently such as colloidal crystals or liquid crystals having periodic structures. Moreover, the concept of dislocations is an excellent starting point for lectures on topological defects occurring in systems equipped with order parameters resulting from broken symmetries: disclinations in nematic or hexatic liquid crystals, dispirations in chiral smectics or disorientations in lyotropic liquid crystals. The discussion of dislocations in Blue Phases will give us an opportunity to call on mind Sir Charles Frank, friend of Jacques Friedel since his Bristol years, who called these ephemeral mesophases "topological oddities". Being made of networks of disclinations, Blue Phases are similar to Twist Grain Boundary (TGB) smectic phases, which are made of networks of screw dislocations and whose existence was predicted by de Gennes in 1972 on the basis of the analogy between smectics and superconductors. We will stress that the book by Jacques Friedel contains seeds of this analogy.

  16. Dislocation dynamical approach to force fluctuations in nanoindentation experiments

    NASA Astrophysics Data System (ADS)

    Ananthakrishna, G.; Katti, Rohit; K, Srikanth

    2014-09-01

    We develop an approach that combines the power of nonlinear dynamics with the evolution equations for the mobile and immobile dislocation densities and force to explain force fluctuations in nanoindentation experiments. The model includes nucleation, multiplication, and propagation thresholds for mobile dislocations, and other well known dislocation transformation mechanisms. The model predicts all the generic features of nanoindentation such as the Hertzian elastic branch followed by several force drops of decreasing magnitudes, and residual plasticity after unloading. The stress corresponding to the elastic force maximum is close to the yield stress of an ideal solid. The predicted values for all the quantities are close to those reported by experiments. Our model allows us to address the indentation-size effect including the ambiguity in defining the hardness in the force drop dominated regime. At large indentation depths, the hardness remains nearly constant with a marginal decreasing trend.

  17. Quantitative observations of dislocation mechanisms in gamma TiAl

    SciTech Connect

    Viguier, B.; Cieslar, M.; Martin, J.L.; Hemker, K.J.

    1995-08-01

    Quantitative TEM observations have been made on a series of gamma Ti{sub 47}Al{sub 51}Mn{sub 2} polycrystals that were deformed at different temperatures. Special attention has been given to determining the statistical variation of defect densities that occur at the different temperatures. The results, which are in good agreement with mechanical testing, indicate that three different mechanisms control deformation in this alloy: lattice friction and the formation of faulted dipoles at low temperatures, the pinning of ordinary dislocations at intermediate temperatures, and the bowing and looping of dislocations at high temperatures. The anomalous flow strength of this alloy has been found to be related to the intrinsic pinning of ordinary dislocations. Details of this pinning and subsequent unzipping process, which are the basis for the new local-pinning-unzipping (LPU) models, are outlined within.

  18. Single crystalline Sc2O3/Y2O3 heterostructures as novel engineered buffer approach for GaN integration on Si (111)

    NASA Astrophysics Data System (ADS)

    Tarnawska, L.; Giussani, A.; Zaumseil, P.; Schubert, M. A.; Paszkiewicz, R.; Brandt, O.; Storck, P.; Schroeder, T.

    2010-09-01

    The preparation of GaN virtual substrates on Si wafers via buffer layers is intensively pursued for high power/high frequency electronics as well as optoelectronics applications. Here, GaN is integrated on the Si platform by a novel engineered bilayer oxide buffer, namely, Sc2O3/Y2O3, which gradually reduces the lattice misfit of ˜-17% between GaN and Si. Single crystalline GaN(0001)/Sc2O3(111)/Y2O3(111)/Si(111) heterostructures were prepared by molecular beam epitaxy and characterized ex situ by various techniques. Laboratory-based x-ray diffraction shows that the epitaxial Sc2O3 grows fully relaxed on the Y2O3/Si(111) support, creating a high quality template for subsequent GaN overgrowth. The high structural quality of the Sc2O3 film is demonstrated by the fact that the concentration of extended planar defects in the preferred {111} slip planes is below the detection limit of synchrotron based diffuse x-ray scattering studies. Transmission electron microscopy (TEM) analysis reveal that the full relaxation of the -7% lattice misfit between the isomorphic oxides is achieved by a network of misfit dislocations at the Sc2O3/Y2O3 interface. X-ray reflectivity and TEM prove that closed epitaxial GaN layers as thin as 30 nm can be grown on these templates. Finally, the GaN thin film quality is studied using a detailed Williamson-Hall analysis.

  19. Theoretical study of kinks on screw dislocation in silicon

    NASA Astrophysics Data System (ADS)

    Pizzagalli, L.; Pedersen, A.; Arnaldsson, A.; Jónsson, H.; Beauchamp, P.

    2008-02-01

    Theoretical calculations of the structure, formation, and migration of kinks on a nondissociated screw dislocation in silicon have been carried out using density functional theory calculations as well as calculations based on interatomic potential functions. The results show that the structure of a single kink is characterized by a narrow core and highly stretched bonds between some of the atoms. The formation energy of a single kink ranges from 0.9to1.36eV , and is of the same order as that for kinks on partial dislocations. However, the kinks migrate almost freely along the line of an undissociated dislocation unlike what is found for partial dislocations. The effect of stress has also been investigated in order to compare with previous silicon deformation experiments which have been carried out at low temperature and high stress. The energy barrier associated with the formation of a stable kink pair becomes as low as 0.65eV for an applied stress on the order of 1GPa , indicating that displacements of screw dislocations likely occur via thermally activated formation of kink pairs at room temperature.

  20. ''The Incubation Period for Void Swelling and its Dependence on Temperature, Dose Rate, and Dislocation Structure Evolution''

    SciTech Connect

    Surh, M P; Sturgeon, J B; Wolfer, W G

    2002-06-13

    Void swelling in structural materials used for nuclear reactors is characterized by an incubation period whose duration largely determines the usefulness of the material for core components. Significant evolution of the dislocation and void microstructures that control radiation-induced swelling can occur during this period. Thus, a theory of incubation must treat time-dependent void nucleation in combination with dislocation evolution, in which the sink strengths of voids and dislocations change in concert. We present theoretical results for void nucleation and growth including the time-dependent, self-consistent coupling of point defect concentrations to the evolution of both void populations and dislocation density. Simulations show that the incubation radiation dose is a strong function of the starting dislocation density and of the dislocation bias factors for vacancy and interstitial absorption. Irradiation dose rate and temperature also affect the duration of incubation. The results are in general agreement with experiment for high purity metals.

  1. Ultrafast carrier dynamics in GaN nanorods

    SciTech Connect

    Yang, Chi-Yuan; Chia, Chih-Ta; Chen, Hung-Ying; Gwo, Shangjr; Lin, Kung-Hsuan

    2014-11-24

    We present ultrafast time-resolved optical spectroscopy on GaN nanorods at room temperature. The studied GaN nanorods, with diameters of ∼50 nm and lengths of ∼400 nm, were grown on the silicon substrate. After femtosecond optical pulses excited carriers in the GaN nanorods, the carriers thermalized within a few picoseconds. Subsequently, the electrons are trapped by the surface states on the order of 20 ps. After the surface electric field was reformed in the GaN nanorods, we found the lifetime of the residue carriers in GaN nanorods is longer than 1.7 ns at room temperature, while the lifetime of carriers in GaN thin film is typically a few hundred picoseconds. Our findings indicate that GaN nanorods have higher electrical quality compared with GaN thin film.

  2. GaN Haeckelite Single-Layered Nanostructures: Monolayer and Nanotubes

    PubMed Central

    Camacho-Mojica, Dulce C.; López-Urías, Florentino

    2015-01-01

    Nowadays, III-V semiconductors are interesting candidate materials for the tailoring of two dimensional (2D) graphene-like structures. These new 2D materials have attracted profound interest opening the possibility to find semiconductor materials with unexplored properties. First-principles density functional theory calculations are performed in order to investigate the electronic properties of GaN planar and nanotube morphologies based on Haeckelite structures (containing octagonal and square membered rings). Optimized geometries, band-structures, phonon dispersion, binding energies, transmission electron microscopy images simulations, x-ray diffraction patterns, charge densities, and electronic band gaps are calculated. We demonstrated that GaN Haeckelite structures are stable exhibiting a semiconducting behavior with an indirect band gap. Furthermore, it was found that GaN Haeckelite nanotubes are semiconductor with a band gap nature (direct or indirect) that depends of the nanotube´s chirality and diameter. In addition, it was demonstrated that surface passivation and the interaction with hydrazine, water, ammonia, and carbon monoxide molecules can change the band-gap nature. Our results are compared with the corresponding GaN hexagonal honeycomb structures. PMID:26658148

  3. 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.

  4. White x-ray microdiffraction analysis of defects, strain and tilts in a free standing GaN film.

    SciTech Connect

    Barabash, R. I.; Ice, G. E.; Haskell, B. A.; Nakamura, S.; Speck, J. S.; Liu, W.; X-Ray Science Division; ORNL; Univ. of California

    2008-01-01

    A novel white-beam microdiffraction analysis of defects, strains and tilts in a free standing m-plane GaN film grown via hydride vapor phase epitaxy is presented. It is shown that misfit dislocations are grouped within cell boundaries creating local lattice rotations (tilts) between the growing cells. Distribution of lattice rotations in the film is not homogeneous. Regions of large rotations are separated by low rotations regions. The dominating rotation axis is parallel [11{bar 2}0] direction. High in plane shear stress component is observed along [0001].

  5. Irreducible dislocation of the knee.

    PubMed

    Schaefer, R A; Bellafiore, V A; Corzatt, R D

    1999-11-01

    Irreducible knee dislocation is a rare injury. This case report describes a knee dislocation in a 39-year-old male U.S. Army noncommissioned officer who was injured while playing in a softball game. Arthroscopy showed the medial collateral ligament and capsule to be locked in the intercondylar notch, covering the medial femoral condyle. Arthrotomy and open reduction were required. Staged posterior cruciate ligament reconstruction using patellar tendon autograft was later performed. Review of the magnetic resonance imaging scan showed the irreducible lesion. The diagnostic clinical and radiographic features of this unusual injury are described.

  6. Bipolar dislocation of the clavicle.

    PubMed

    Jiang, Wei; Gao, Shu-Guang; Li, Yu-Sheng; Lei, Guang-Hua

    2012-11-01

    Bipolar dislocation of the clavicle at acromioclavicular and sternoclavicular joint is an uncommon traumatic injury. The conservative treatments adopted in the past is associated with redislocation dysfunction and deformity. A 41 years old lady with bipolar dislocation of right shoulder is treated surgically by open reduction and internal fixation by oblique T-plate at sternoclavicular joint and Kirschner wire stabilization at acromioclavicular joint. The patient showed satisfactory recovery with full range of motion of the right shoulder and normal muscular strength. The case reported in view of rarity and at 2 years followup. PMID:23325981

  7. Dislocation nonlinearity and nonlinear wave processes in polycrystals with dislocations

    NASA Astrophysics Data System (ADS)

    Nazarov, V. E.

    2016-09-01

    Based on the modification of the linear part of the Granato-Lücke dislocation theory of absorption, the equation of state of polycrystalline solids with dissipative and reactive nonlinearity has been derived. The nonlinear effects of the interaction and self-action of longitudinal elastic waves in such media have been theoretically studied.

  8. GaN Nanowire Devices: Fabrication and Characterization

    NASA Astrophysics Data System (ADS)

    Scott, Reum

    The development of microelectronics in the last 25 years has been characterized by an exponential increase of the bit density in integrated circuits (ICs) with time. Scaling solid-state devices improves cost, performance, and power; as such, it is of particular interest for companies, who gain a market advantage with the latest technology. As a result, the microelectronics industry has driven transistor feature size scaling from 10 μm to ~30 nm during the past 40 years. This trend has persisted for 40 years due to optimization, new processing techniques, device structures, and materials. But when noting processor speeds from the 1970's to 2009 and then again in 2010, the implication would be that the trend has ceased. To address the challenge of shrinking the integrated circuit (IC), current research is centered on identifying new materials and devices that can supplement and/or potentially supplant it. Bottom-up methods tailor nanoscale building blocks---atoms, molecules, quantum dots, and nanowires (NWs)---to be used to overcome these limitations. The Group IIIA nitrides (InN, AlN, and GaN) possess appealing properties such as a direct band gap spanning the whole solar spectrum, high saturation velocity, and high breakdown electric field. As a result nanostructures and nanodevices made from GaN and related nitrides are suitable candidates for efficient nanoscale UV/ visible light emitters, detectors, and gas sensors. To produce devices with such small structures new fabrication methods must be implemented. Devices composed of GaN nanowires were fabricated using photolithography and electron beam lithography. The IV characteristics of these devices were noted under different illuminations and the current tripled from 4.8*10-7 A to 1.59*10 -6 A under UV light which persisted for at least 5hrs.

  9. TEM studies of laterally overgrown GaN layers grown on non-polarsubstrates

    SciTech Connect

    Liliental-Weber, Z.; Ni, X.; Morkoc, H.

    2006-01-05

    Transmission electron microscopy (TEM) was used to study pendeo-epitaxial GaN layers grown on polar and non-polar 4H SiC substrates. The structural quality of the overgrown layers was evaluated using a number of TEM methods. Growth of pendeo-epitaxial layers on polar substrates leads to better structural quality of the overgrown areas, however edge-on dislocations are found at the meeting fronts of two wings. Some misorientation between the 'seed' area and wing area was detected by Convergent Beam Electron Diffraction. Growth of pendeo-epitaxial layers on non-polar substrates is more difficult. Two wings on the opposite site of the seed area grow in two different polar directions with different growth rates. Most dislocations in a wing grown with Ga polarity are 10 times wider than wings grown with N-polarity making coalescence of these layers difficult. Most dislocations in a wing grown with Ga polarity bend in a direction parallel to the substrate, but some of them also propagate to the sample surface. Stacking faults formed on the c-plane and prismatic plane occasionally were found. Some misorientation between the wings and seed was detected using Large Angle Convergent Beam Diffraction.

  10. Midfit dislocation generation mechanisms in InGaAs/GaAs heterostructures

    NASA Technical Reports Server (NTRS)

    Kui, J.; Jesser, W. A.; Jones, S. H.

    1994-01-01

    An experimental investigation of misfit dislocation generation mechanisms at an InGaAs/GaAs heterointerface is reported. InGaAs epitaxial layers were grown by low-pressure oragnometallic vapor-phase epitaxy on patterned and unpatterned GaAs substrate having etch-pit densities (EPD) of 200, 1400, and 10,000 cm(exp -2). After epitaxial growth, the samples were annealed at temperatures between 650 and 750 C, and analyzed by optical and transmission electron microscopy. For the range of substrate EPD studied, it was found that the substrate EPD controls the onset of misfit dislocation generation for low-temperature epitaxy (less than 600 C) on unpatterned substrates. When epilayers were annealed at 750 C, the density of misfit dislocations was independent of the substrate EPD. These studies also show that the dominant misfit dislocation generation mechanism for films grown on patterned substrates is nucleation at the growth-mesa edge. The density of preexisting threading dislocations has little influence on misfit dislocation generation for films selectively deposited within 100 x 100 sq micrometer growth windows. For selective heteroepitaxy, misfit dislocation generation strongly depends on the crystallographic orientation of the growth-mesa edge.

  11. Defect-induced incompatability of elastic strains: dislocations within the Landau theory of martensitic phase transformations

    SciTech Connect

    Groger, Roman1; Lockman, Turab; Saxena, Avadh

    2008-01-01

    In dislocation-free martensites the components of the elastic strain tensor are constrained by the Saint-Venant compatibility condition which guarantees continuity of the body during external loading. However, in dislocated materials the plastic part of the distortion tensor introduces a displacement mismatch that is removed by elastic relaxation. The elastic strains are then no longer compatible in the sense of the Saint-Venant law and the ensuing incompatibility tensor is shown to be proportional to the gradients of the Nye dislocation density tensor. We demonstrate that the presence of this incompatibility gives rise to an additional long-range contribution in the inhomogeneous part of the Landau energy functional and to the corresponding stress fields. Competition among the local and long-range interactions results in frustration in the evolving order parameter (elastic) texture. We show how the Peach-Koehler forces and stress fields for any distribution of dislocations in arbitrarily anisotropic media can be calculated and employed in a Fokker-Planck dynamics for the dislocation density. This approach represents a self-consistent scheme that yields the evolutions of both the order parameter field and the continuous dislocation density. We illustrate our method by studying the effects of dislocations on microstructure, particularly twinned domain walls, in an Fe-Pd alloy undergoing a martensitic transformation.

  12. Gravitational effects of process-induced dislocations in silicon. [during thermal cycling

    NASA Technical Reports Server (NTRS)

    Porter, W. A.; Parker, D. L.

    1974-01-01

    Matters pertaining to semiconductor device fabrication were studied in terms of the influence of gravity on the production of dislocations in silicon wafers during thermal cycling in a controlled ambient where no impurities are present and oxidation is minimal. Both n-type and p-type silicon wafers having a diameter of 1.25 in to 1.5 in, with fixed orientation and resistivity values, were used. The surface dislocation densities were measured quantitatively by the Sirtl etch technique. The results show two significant features of the plastic flow phenomenon as it is related to gravitational stress: (1) the density of dislocations generated during a given thermal cycle is directly related to the duration of the cycle; and (2) the duration of the thermal cycle required to produce a given dislocation density is inversely related to the equilibrium temperature. Analysis of the results indicates that gravitational stress is instrumental in process-induced defect generation.

  13. Drift of dislocation tripoles under ultrasound influence.

    PubMed

    Murzaev, R T; Bachurin, D V; Nazarov, A A

    2016-01-01

    Numerical simulations of dynamics of different stable dislocation tripoles under influence of monochromatic standing sound wave were performed. The basic conditions necessary for the drift and mutual rearrangements between dislocation structures were investigated. The dependence of the drift velocity of the dislocation tripoles as a function of the frequency and amplitude of the external influence was obtained. The results of the work can be useful in analysis of motion and self-organization of dislocation structure under ultrasound influence. PMID:26278625

  14. Dislocation microstructures and strain-gradient plasticity with one active slip plane

    NASA Astrophysics Data System (ADS)

    Conti, Sergio; Garroni, Adriana; Müller, Stefan

    2016-08-01

    We study dislocation networks in the plane using the vectorial phase-field model introduced by Ortiz and coworkers, in the limit of small lattice spacing. We show that, in a scaling regime where the total length of the dislocations is large, the phase field model reduces to a simpler model of the strain-gradient type. The limiting model contains a term describing the three-dimensional elastic energy and a strain-gradient term describing the energy of the geometrically necessary dislocations, characterized by the tangential gradient of the slip. The energy density appearing in the strain-gradient term is determined by the solution of a cell problem, which depends on the line tension energy of dislocations. In the case of cubic crystals with isotropic elasticity our model shows that complex microstructures may form in which dislocations with different Burgers vector and orientation react with each other to reduce the total self-energy.

  15. Simultaneous double interphalangeal dislocation in one finger.

    PubMed

    Takami, H; Takahashi, S; Ando, M

    2000-01-01

    Isolated dislocation of the proximal or distal interphalangeal joint of a finger is common, but simultaneous dislocation of both joints is rare. Three cases of simultaneous dislocations of both interphalangeal joints in the same finger are reported. Closed reduction was easily achieved in all cases.

  16. Dislocated Workers: Neighbors, Friends, Relatives.

    ERIC Educational Resources Information Center

    Blessington, Robert

    1989-01-01

    Worker dislocation is a major problem in Wisconsin. Services needed include provision of coping mechanisms, job search methods, and retraining. A partnership between the state and the Wisconsin American Federation of Labor-Congress of Industrial Organizations was created to provide these services and referrals, as well as preventive training…

  17. Financial Dislocations among Divorcing Families.

    ERIC Educational Resources Information Center

    Little, Marilyn J.

    Extensive economic changes may be brought about by divorce. In an attempt to demonstrate that the degree of financial dislocation following divorce depends on three factors--custody arrangements, wife's employment, and social class--data on men's and women's employment, income, and support payments were gathered for 222 divorcing families. Total…

  18. Inversion domains in GaN grown on sapphire

    SciTech Connect

    Romano, L.T.; Northrup, J.E.; OKeefe, M.A.

    1996-10-01

    Planar defects observed in GaN films grown on (0001) sapphire have been identified as inversion domain boundaries (IDBs) by a combination of high resolution transmission electron microscopy, multiple dark field imaging, and convergent beam electron diffraction techniques. Films grown by molecular beam epitaxy (MBE), metalorganic vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE) were investigated and all were found to contain IDBs. The IDBs in the MBE and HVPE films extended from the interface to the film surface and formed columnar domains that ranged in width from 3 to 20 nm in the MBE films and up to 100 nm in the HVPE films. For the films investigated, the MBE films had the highest density, and the MOCVD films had the lowest density of IDBs. The nucleation of inversion domains (IDs) may result from step-related inhomogeneities of the GaN/sapphire interface. {copyright} {ital 1996 American Institute of Physics.}

  19. Ge doping of GaN beyond the Mott transition

    NASA Astrophysics Data System (ADS)

    Ajay, A.; Schörmann, J.; Jiménez-Rodriguez, M.; Lim, C. B.; Walther, F.; Rohnke, M.; Mouton, I.; Amichi, L.; Bougerol, C.; Den Hertog, M. I.; Eickhoff, M.; Monroy, E.

    2016-11-01

    We present a study of germanium as n-type dopant in wurtzite GaN films grown by plasma-assisted molecular-beam epitaxy, reaching carrier concentrations of up to 6.7  ×  1020 cm‑3 at 300 K, well beyond the Mott density. The Ge concentration and free carrier density were found to scale linearly with the Ge flux in the studied range. All the GaN:Ge layers present smooth surface morphology with atomic terraces, without trace of pits or cracks, and the mosaicity of the samples has no noticeable dependence on the Ge concentration. The variation of the GaN:Ge band gap with the carrier concentration is consistent with theoretical calculations of the band gap renormalization due to electron–electron and electron–ion interaction, and Burstein–Moss effect.

  20. First-principle natural band alignment of GaN / dilute-As GaNAs alloy

    SciTech Connect

    Tan, Chee-Keong Tansu, Nelson

    2015-01-15

    Density functional theory (DFT) calculations with the local density approximation (LDA) functional are employed to investigate the band alignment of dilute-As GaNAs alloys with respect to the GaN alloy. Conduction and valence band positions of dilute-As GaNAs alloy with respect to the GaN alloy on an absolute energy scale are determined from the combination of bulk and surface DFT calculations. The resulting GaN / GaNAs conduction to valence band offset ratio is found as approximately 5:95. Our theoretical finding is in good agreement with experimental observation, indicating the upward movements of valence band at low-As content dilute-As GaNAs are mainly responsible for the drastic reduction of the GaN energy band gap. In addition, type-I band alignment of GaN / GaNAs is suggested as a reasonable approach for future device implementation with dilute-As GaNAs quantum well, and possible type-II quantum well active region can be formed by using InGaN / dilute-As GaNAs heterostructure.

  1. Uniaxial strain effects on the optoelectronic properties of GaN nanowires

    NASA Astrophysics Data System (ADS)

    Xia, Sihao; Liu, Lei; Kong, Yike; Wang, Meishan

    2016-09-01

    Considering the importance of strain engineering on semiconductors, GaN nanowires under uniaxial compression deformation and stretch deformation are researched using first principle calculations with density functional theory. It is found that the deformation will destroy the stability of the nanowires except a weak stretch. The compression deformation is more difficult than the stretch deformation. Besides, the work function of the nanowires is reduced under increasing compression while that under increasing stretch is reversed. With increasing diameter, the band gaps of the nanowires gradually exhibit a linear decreasing relation as the elongation of uniaxial length of GaN nanowires. With increasing compression, the band gaps change from direct to indirect. The optical calculations exhibit a redshift for the imaginary part of dielectric function. This study demonstrates strain engineering can effectively adjust the optoelectronic characteristics of GaN nanowire. Moderate compression, which induces a lower work function with a direct band gap, can improve the photoemission performance of GaN nanowires.

  2. GaN m -plane: Atomic structure, surface bands, and optical response

    NASA Astrophysics Data System (ADS)

    Landmann, M.; Rauls, E.; Schmidt, W. Â. G.; Neumann, M. Â. D.; Speiser, E.; Esser, N.

    2015-01-01

    Density-functional-theory calculations are combined with many-body perturbation theory in order to elucidate the geometry, electronic, and optical properties of the w z -GaN (1 1 ¯00 ) surface, i.e., the so-called m -plane. The optical absorption and reflection anisotropy related to electronic transitions between surface states are identified by comparison with measured data covering transition energies from 2.4 up to 5.4 eV. Our results show a surface relaxation mechanism consistent with the electron counting rule that causes a moderate buckling of the GaN surface dimers and gives rise to two distinct surface states: The doubly occupied N dangling bonds form a surface band that is resonant with the GaN valence-band edge at the center of the Brillouin zone, whereas the empty Ga dangling bonds occur within the GaN band gap closely following the dispersion of the conduction-band edge. These two states contribute strongly to the formation of surface excitons that redshift the optical absorption with respect to the bulk optical response. The surface optical absorption i.e., the excitonic onset below the bulk band gap followed by a broad absorption band at higher energies related to the dispersion of the surface band structure, is calculated in agreement with the experimental data.

  3. Epitaxial GaN films by hyperthermal ion-beam nitridation of Ga droplets

    SciTech Connect

    Gerlach, J. W.; Ivanov, T.; Neumann, L.; Hoeche, Th.; Hirsch, D.; Rauschenbach, B.

    2012-06-01

    Epitaxial GaN film formation on bare 6H-SiC(0001) substrates via the process of transformation of Ga droplets into a thin GaN film by applying hyperthermal nitrogen ions is investigated. Pre-deposited Ga atoms in well defined amounts form large droplets on the substrate surface which are subsequently nitridated at a substrate temperature of 630 Degree-Sign C by a low-energy nitrogen ion beam from a constricted glow-discharge ion source. The Ga deposition and ion-beam nitridation process steps are monitored in situ by reflection high-energy electron diffraction. Ex situ characterization by x-ray diffraction and reflectivity techniques, Rutherford backscattering spectrometry, and electron microscopy shows that the thickness of the resulting GaN films depends on the various amounts of pre-deposited gallium. The films are epitaxial to the substrate, exhibit a mosaic like, smooth surface topography and consist of coalesced large domains of low defect density. Possible transport mechanisms of reactive nitrogen species during hyperthermal nitridation are discussed and the formation of GaN films by an ion-beam assisted process is explained.

  4. GaN nano-pyramid arrays as an efficient photoelectrode for solar water splitting

    NASA Astrophysics Data System (ADS)

    Hou, Y.; Yu, X.; Syed, Z. Ahmed; Shen, S.; Bai, J.; Wang, T.

    2016-11-01

    A prototype photoelectrode has been fabricated using a GaN nano-pyramid array structure grown on a cost-effective Si (111) substrate, demonstrating a significant improvement in performance of solar-powered water splitting compared with any planar GaN photoelectrode. Such a nano-pyramid structure leads to enhanced optical absorption as a result of a multi-scattering process which can effectively produce a reduction in reflectance. A simulation based on a finite-difference time-domain approach indicates that the nano-pyramid architecture enables incident light to be concentrated within the nano-pyramids as a result of micro-cavity effects, further enhancing optical absorption. Furthermore, the shape of the nano-pyramid further facilitates the photo-generated carrier transportation by enhancing a hole-transfer efficiency. All these features as a result of the nano-pyramid configuration lead to a large photocurrent of 1 mA cm‑2 under an illumination density of 200 mW cm‑2, with a peak incident photon-to-current conversion efficiency of 46.5% at ∼365 nm, around the band edge emission wavelength of GaN. The results presented are expected to pave the way for the fabrication of GaN based photoelectrodes with a high energy conversion efficiency of solar powered water splitting.

  5. Piezotronic Effect in Polarity-Controlled GaN Nanowires.

    PubMed

    Zhao, Zhenfu; Pu, Xiong; Han, Changbao; Du, Chunhua; Li, Linxuan; Jiang, Chunyan; Hu, Weiguo; Wang, Zhong Lin

    2015-08-25

    Using high-quality and polarity-controlled GaN nanowires (NWs), we studied the piezotronic effect in crystal orientation defined wurtzite structures. By applying a normal compressive force on c-plane GaN NWs with an atomic force microscopy tip, the Schottky barrier between the Pt tip and GaN can be effectively tuned by the piezotronic effect. In contrast, the normal compressive force cannot change the electron transport characteristics in m-plane GaN NWs whose piezoelectric polarization axis is turned in the transverse direction. This observation provided solid evidence for clarifying the difference between the piezotronic effect and the piezoresistive effect. We further demonstrated a high sensitivity of the m-plane GaN piezotronic transistor to collect the transverse force. The integration of c-plane GaN and m-plane GaN indicates an overall response to an external force in any direction.

  6. Solute drag on perfect and extended dislocations

    NASA Astrophysics Data System (ADS)

    Sills, R. B.; Cai, W.

    2016-04-01

    The drag force exerted on a moving dislocation by a field of mobile solutes is studied in the steady state. The drag force is numerically calculated as a function of the dislocation velocity for both perfect and extended dislocations. The sensitivity of the non-dimensionalized force-velocity curve to the various controlling parameters is assessed, and an approximate analytical force-velocity expression is given. A non-dimensional parameter S characterizing the strength of the solute-dislocation interaction, the background solute fraction ?, and the dislocation character angle ?, are found to have the strongest influence on the force-velocity curve. Within the model considered here, a perfect screw dislocation experiences no solute drag, but an extended screw dislocation experiences a non-zero drag force that is about 10 to 30% of the drag on an extended edge dislocation. The solutes can change the spacing between the Shockley partials in both stationary and moving extended dislocations, even when the stacking fault energy remains unaltered. Under certain conditions, the solutes destabilize an extended dislocation by either collapsing it into a perfect dislocation or causing the partials to separate unboundedly. It is proposed that the latter instability may lead to the formation of large faulted areas and deformation twins in low stacking fault energy materials containing solutes, consistent with experimental observations of copper and stainless steel containing hydrogen.

  7. Theory of interacting dislocations on cylinders

    NASA Astrophysics Data System (ADS)

    Amir, Ariel; Paulose, Jayson; Nelson, David R.

    2013-04-01

    We study the mechanics and statistical physics of dislocations interacting on cylinders, motivated by the elongation of rod-shaped bacterial cell walls and cylindrical assemblies of colloidal particles subject to external stresses. The interaction energy and forces between dislocations are solved analytically, and analyzed asymptotically. The results of continuum elastic theory agree well with numerical simulations on finite lattices even for relatively small systems. Isolated dislocations on a cylinder act like grain boundaries. With colloidal crystals in mind, we show that saddle points are created by a Peach-Koehler force on the dislocations in the circumferential direction, causing dislocation pairs to unbind. The thermal nucleation rate of dislocation unbinding is calculated, for an arbitrary mobility tensor and external stress, including the case of a twist-induced Peach-Koehler force along the cylinder axis. Surprisingly rich phenomena arise for dislocations on cylinders, despite their vanishing Gaussian curvature.

  8. Dislocation accumulation at large plastic strains -- An approach to the theoretical strength of materials

    SciTech Connect

    Embury, J.D. |; Han, K.

    1999-04-01

    The usual method of introducing engineers to the concept of dislocations and their role in plastic flow is to compare an estimate of the theoretical strength of solid (of order {micro}/30 where {micro} is the shear modulus) and the observed strength of either single crystals ({mu}/10{sup 4}) or practical engineering material such as structural steels where the yield stress in shear is of order {mu}/10{sup 3}. However, if one considers the problem in reverse, one can consider the accumulation of dislocations as an important mechanism by which one can produce engineering materials in which the strength level approaches the theoretical strength. If one assumes that the flow stress can be expressed in terms of te mean free path between stored dislocations or as the square root of the global dislocation density, then one can see the influence of dislocation density in a diagrammatic form. It is clear that the strengthening by dislocation accumulation due to large imposed plastic strains represents an important approach both to the development of new, potentially valuable, engineering materials and an important area of basic understanding in terms of the mechanical response of materials close to their theoretical strength. Thus, this article will survey some of the factors which influence dislocation accumulation at large strains and the consequences of such accumulation processes.

  9. 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.

  10. Response properties of axion insulators and Weyl semimetals driven by screw dislocations and dynamical axion strings

    NASA Astrophysics Data System (ADS)

    You, Yizhi; Cho, Gil Young; Hughes, Taylor L.

    2016-08-01

    In this paper, we investigate the theory of dynamical axion strings emerging from chiral symmetry breaking in three-dimensional Weyl semimetals. The chiral symmetry is spontaneously broken by a charge density wave (CDW) order which opens an energy gap and converts the Weyl semimetal into an axion insulator. Indeed, the phase fluctuations of the CDW order parameter act as a dynamical axion field θ (x ⃗,t ) and couple to electromagnetic field via Lθ=θ/(x ⃗,t ) 32 π2 ɛσ τ ν μFσ τFν μ. Additionally, when the axion insulator is coupled to deformations of the background geometry/strain fields via torsional defects, e.g., screw dislocations, there is interesting interplay between the crystal dislocations and dynamical axion strings. For example, the screw dislocation traps axial charge, and there is a Berry phase accumulation when an axion string (which carries axial flux) is braided with a screw dislocation. In addition, a cubic coupling between the axial current and the geometry fields is nonvanishing and indicates a Berry phase accumulation during a particular three-loop braiding procedure where a dislocation loop is braided with another dislocation and they are both threaded by an axion string. We also observe a chiral magnetic effect induced by a screw dislocation density in the absence of a nodal energy imbalance between Weyl points and describe an additional chiral geometric effect and a geometric Witten effect.

  11. Modelling dislocation cores in MgSiO3 perovskite

    NASA Astrophysics Data System (ADS)

    Cordier, P.; Carrez, P.; Gouriet, K.; Kraych, A.

    2012-12-01

    MgSiO3 perovskite is the most abundant mineral of the Earth's lower mantle (i.e. between 700 and 2900 km depth) and accounts for half of Earth's mass. At lower mantle pressures (25-135 GPa) MgSiO3 crystallises in a distorted (orthorhombic) perovskite structure (described in the following using the Pbnm space group). In this structure, SiO6 octahedra are tilted with tilt angles increasing with increasing pressure. Since it is very difficult to perform deformation experiments under the extreme P, T conditions of the lower mantle, little is known about plastic deformation of MgSiO3 perovskite and its slip systems are still a matter of debate. To overcome this difficulty, we model dislocation core structures in this mineral taking into account the influence of pressure. In this study, we focus on dislocation core structures of dislocations with [100] and [010] Burgers vectors (which derive from <110> Burgers vectors of the underlying pseudo-cubic structure). Atomistic calculations are performed using pair-wise potentials as implemented in the LAMMPS code. The choice of potentials was initially validated by comparing generalized stacking fault (GSF) energies to similar calculations performed with the density functional theory (DFT). The core structures of screw dislocations are calculated using two independent methods. The first one is based on Peierls-Nabarro-Galerkin simulations involving GSF as an input. Direct calculations have also bee performed using cluster approach. It turns out that screw dislocations with [100] Burgers vector are characterised by a core mostly spread in the (010) plane. The core exhibits two edge-sharing octahedra in a configuration very similar to that modelled in SrTiO3 cubic perovskite. The structure of [010] screw dislocations is more complex with dissociation into two, non-collinear partial dislocations with a significant non-screw component. Both dislocations exhibit high Peierls stresses. This illustrates the effect of orthorhombic

  12. Large thermoelectric figure of merit for three-dimensional topological Anderson insulators via line dislocation engineering

    NASA Astrophysics Data System (ADS)

    Tretiakov, O. A.; Abanov, Ar.; Murakami, Shuichi; Sinova, Jairo

    2010-08-01

    We study the thermoelectric properties of three-dimensional topological Anderson insulators with line dislocations. We show that at high densities of dislocations the thermoelectric figure of merit ZT can be dominated by one-dimensional topologically protected conducting states channeled through the lattice screw dislocations in the topological insulator materials with a nonzero time-reversal-invariant momentum such as Bi0.9Sb0.1. When the chemical potential does not exceed much the mobility edge the ZT at room temperatures can reach large values, much higher than unity for reasonable parameters, hence making this system a strong candidate for applications in heat management of nanodevices.

  13. Direct imaging of boron segregation at dislocations in B:diamond heteroepitaxial films

    NASA Astrophysics Data System (ADS)

    Turner, S.; Idrissi, H.; Sartori, A. F.; Korneychuck, S.; Lu, Y.-G.; Verbeeck, J.; Schreck, M.; van Tendeloo, G.

    2016-01-01

    A thin film of heavily B-doped diamond has been grown epitaxially by microwave plasma chemical vapor deposition on an undoped diamond layer, on top of a Ir/YSZ/Si(001) substrate stack, to study the boron segregation and boron environment at the dislocations present in the film. The density and nature of the dislocations were investigated by conventional and weak-beam dark-field transmission electron microscopy techniques, revealing the presence of two types of dislocations: edge and mixed-type 45° dislocations. The presence and distribution of B in the sample was studied using annular dark-field scanning transmission electron microscopy and spatially resolved electron energy-loss spectroscopy. Using these techniques, a segregation of B at the dislocations in the film is evidenced, which is shown to be intermittent along the dislocation. A single edge-type dislocation was selected to study the distribution of the boron surrounding the dislocation core. By imaging this defect at atomic resolution, the boron is revealed to segregate towards the tensile strain field surrounding the edge-type dislocations. An investigation of the fine structure of the B-K edge at the dislocation core shows that the boron is partially substitutionally incorporated into the diamond lattice and partially present in a lower coordination (sp2-like hybridization).A thin film of heavily B-doped diamond has been grown epitaxially by microwave plasma chemical vapor deposition on an undoped diamond layer, on top of a Ir/YSZ/Si(001) substrate stack, to study the boron segregation and boron environment at the dislocations present in the film. The density and nature of the dislocations were investigated by conventional and weak-beam dark-field transmission electron microscopy techniques, revealing the presence of two types of dislocations: edge and mixed-type 45° dislocations. The presence and distribution of B in the sample was studied using annular dark-field scanning transmission electron

  14. Boundary Conditions for Dislocation Dynamics Simulations and Stage 0 of BCC Metals at Low Temperature

    SciTech Connect

    Tang, M; Kubin, L P

    2001-05-01

    In order to study the dislocation density evolution of body centered cubic (bcc) crystals at low temperature by dislocation dynamics (DD) simulations, we investigated carefully three different boundary conditions (BC) for DD, i.e., the quasi-free surface BC, the flux-balanced BC, and the periodic BC. The latter two BCs can account for the dislocation loss from the boundary of the finite simulation box. PBC can also eliminate the influence of surfaces and improve the line connectivity. We have found that the PBC provides a convenient and effective boundary condition for DD simulations and have applied it to the study of dislocation density evolution of bcc metals during stage 0 deformation at low temperature.

  15. 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.

  16. Effect of initial growth on the quality of GaN on patterned sapphire substrate with ex situ physical vapor deposition AlN seed layer

    NASA Astrophysics Data System (ADS)

    Wang, Hongbo; Daigo, Yoshiaki; Seino, Takuya; Ishibashi, Sotaro; Sugiyama, Masakazu

    2016-10-01

    GaN epitaxy was explored on a cone-patterned sapphire substrate with an ex situ AlN seed layer prepared by physical vapor deposition (PVD). The effect of initial growth on the quality of the GaN epilayer was investigated using both ex situ PVD-AlN seed layers with various thicknesses and various deposition parameters such as temperature and reactor pressure in metal-organic vapor-phase epitaxy (MOVPE). It was found that the quality of GaN is insensitive to both the thickness of the ex situ PVD-AlN seed layer and the MOVPE growth conditions. A high-quality GaN film was realized, as indicated by room-temperature CL mapping (dark spot density of 1.6 × 108 cm-2), on a patterned sapphire substrate with a wide growth condition window by simply employing an ex situ PVD-AlN seed layer.

  17. Surfactant assisted growth of MgO films on GaN

    SciTech Connect

    Paisley, E. A.; Shelton, T. C.; Collazo, R.; Sitar, Z.; Maria, J.-P.; Christen, H. M.; Biegalski, M. D.; Mita, S.

    2012-08-27

    Thin epitaxial films of <111> oriented MgO on [0001]-oriented GaN were grown by molecular beam epitaxy and pulsed laser deposition using the assistance of a vapor phase surfactant. In both cases, surfactant incorporation enabled layer-by-layer growth and a smooth terminal surface by stabilizing the {l_brace}111{r_brace} rocksalt facet. Metal-insulator-semiconductor capacitor structures were fabricated on n-type GaN. A comparison of leakage current density for conventional and surfactant-assisted growth reveals a nearly 100 Multiplication-Sign reduction in leakage current density for the surfactant-assisted samples. These data verify numerous predictions regarding the role of H-termination in regulating the habit of rocksalt crystals.

  18. Enhanced performance of InGaN/GaN multiple-quantum-well light-emitting diodes grown on nanoporous GaN layers.

    PubMed

    Lee, Kwang Jae; Kim, Sang-Jo; Kim, Jae-Joon; Hwang, Kyungwook; Kim, Sung-Tae; Park, Seong-Ju

    2014-06-30

    We demonstrate the high efficiency of InGaN/GaN multiple quantum wells (MQWs) light-emitting diode (LED) grown on the electrochemically etched nanoporous (NP) GaN. The photoluminescence (PL) and Raman spectra show that the LEDs with NP GaN have a strong carrier localization effect resulting from the relaxed strain and reduced defect density in MQWs. Also, the finite-difference time-domain (FDTD) simulation shows that the light extraction efficiency (LEE) is increased by light scattering effect by nanopores. The output power of LED with NP GaN is increased up to 123.1% at 20 mA, compared to that of LED without NP GaN. The outstanding performance of LEDs with NP GaN is attributed to the increased internal quantum efficiency (IQE) by the carrier localization in the indium-rich clusters, low defect density in MQWs, and increased LEE owing to the light scattering in NP GaN.

  19. Numerical analysis on the origin of thickness unevenness and formation of pits at GaN thin film grown by HVPE

    NASA Astrophysics Data System (ADS)

    Han, Xue-Feng; Lee, Jae-Hak; Lee, Yoo-Jin; Song, Jae-Ho; Yi, Kyung-Woo

    2016-09-01

    In this study, we propose a 3D model for analyzing the fluid flow, mass fractions of reacting gases, GaN deposition thickness distribution and V/III ratio distribution at the GaN deposition surface in the multi-susceptor HVPE equipment. The GaN thin film is grown in the multi-susceptor HVPE equipment at 1213 K and 1 bar. The deposition thickness distribution from the calculation has been compared with the experimental results. Moreover, the standard deviations of deposition thickness of the films achieved from calculations and experiments have been compared. Besides, in the calculation results, we found that the V/III ratio at the GaN deposition surface increased from the center to the periphery and from low susceptor to high susceptor. Our calculation results have also been verified by 3D measuring laser microscope observation of the surface morphology of the GaN thin film. In according with the calculation results, the density of the pits also decreases from the center to the periphery as well as from low susceptor to high susceptor, demonstrating that the pit density at the surface of the GaN thin films could be reduced when the V/III ratio is increased.

  20. 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

    SciTech Connect

    Chowdhury, Subhra; Biswas, Dhrubes

    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 performed 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.

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

    SciTech Connect

    Bengoechea-Encabo, A.; Albert, S.; Barbagini, F.; Sanchez-Garcia, M. A.; Calleja, E.; Trampert, A.

    2013-12-09

    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.

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

    DOE PAGES

    Lin, Yong; Leung, Benjamin; Li, Qiming; Figiel, Jeffrey J.; Wang, George T.

    2015-07-14

    In this study, ammonia-based molecular beam epitaxy (NH3-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 NH3-MBE. Compared to GaN nanowires grown by Ni-catalyzed metal–organic chemical vapor deposition, the NH3-MBE grown GaN nanowires show more than an ordermore » of magnitude increase in band-edge to yellow luminescence intensity ratio, as measured by cathodoluminescence, indicating improved microstructural and optical properties.« less

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

    SciTech Connect

    Lin, Yong; Leung, Benjamin; Li, Qiming; Figiel, Jeffrey J.; Wang, George T.

    2015-07-14

    In this study, ammonia-based molecular beam epitaxy (NH3-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 NH3-MBE. Compared to GaN nanowires grown by Ni-catalyzed metal–organic chemical vapor deposition, the NH3-MBE grown GaN nanowires show more than an order of magnitude increase in band-edge to yellow luminescence intensity ratio, as measured by cathodoluminescence, indicating improved microstructural and optical properties.

  4. Rode's iterative calculation of surface optical phonon scattering limited electron mobility in N-polar GaN devices

    SciTech Connect

    Ghosh, Krishnendu Singisetti, Uttam

    2015-02-14

    N-polar GaN channel mobility is important for high frequency device applications. Here, we report theoretical calculations on the surface optical (SO) phonon scattering rate of two-dimensional electron gas (2DEG) in N-polar GaN quantum well channels with high-k dielectrics. Rode's iterative calculation is used to predict the scattering rate and mobility. Coupling of the GaN plasmon modes with the SO modes is taken into account and dynamic screening is employed under linear polarization response. The effect of SO phonons on 2DEG mobility was found to be small at >5 nm channel thickness. However, the SO mobility in 3 nm N-polar GaN channels with HfO{sub 2} and ZrO{sub 2} high-k dielectrics is low and limits the total mobility. The SO scattering for SiN dielectric on GaN was found to be negligible due to its high SO phonon energy. Using Al{sub 2}O{sub 3}, the SO phonon scattering does not affect mobility significantly only except the case when the channel is too thin with a low 2DEG density.

  5. Dislocation of the knee: imaging findings.

    PubMed

    Shearer, Damon; Lomasney, Laurie; Pierce, Kenneth

    2010-01-01

    Dislocations of the knee are relatively uncommon injuries. However, the incidence of this injury appears to be increasing. Knee dislocations are most often high velocity blunt injuries, with motor vehicle accidents being a frequent etiology. Other causes include falls from height, athletic injuries, farming and industrial accidents, and even low velocity mechanisms such as a misstep into a hole. Likewise, minor trauma in the morbidly obese is increasingly recognized as a mechanism of knee dislocation. Multiple forms of dislocation exist, with the common factor being disruption of the tibiofemoral articulation. Dislocation can occur in a variety of directions depending on the mechanism of injury. The most common dislocation is anterior, which may be seen in hyperextension injuries such as martial arts kicking. The "dashboard injury" of motor vehicle accidents can result in a posterior dislocation of the knee. Lateral and rotary dislocations are less common. Knee dislocation is more commonly diagnosed in men, with a mean age of 23 to 31 years old. This is the very patient population encountered by Special Operations Forces (SOF) healthcare providers. Given the mechanisms of injury noted above, it is reasonable to conclude that knee dislocations may be seen in a young, active SOF patient population, particularly those engaged in parachuting, fast-roping/rappelling, driving at high speeds during military operations, and mixed martial arts. PMID:20306414

  6. Three-dimensional formulation of dislocation climb

    NASA Astrophysics Data System (ADS)

    Gu, Yejun; Xiang, Yang; Quek, Siu Sin; Srolovitz, David J.

    2015-10-01

    We derive a Green's function formulation for the climb of curved dislocations and multiple dislocations in three-dimensions. In this new dislocation climb formulation, the dislocation climb velocity is determined from the Peach-Koehler force on dislocations through vacancy diffusion in a non-local manner. The long-range contribution to the dislocation climb velocity is associated with vacancy diffusion rather than from the climb component of the well-known, long-range elastic effects captured in the Peach-Koehler force. Both long-range effects are important in determining the climb velocity of dislocations. Analytical and numerical examples show that the widely used local climb formula, based on straight infinite dislocations, is not generally applicable, except for a small set of special cases. We also present a numerical discretization method of this Green's function formulation appropriate for implementation in discrete dislocation dynamics (DDD) simulations. In DDD implementations, the long-range Peach-Koehler force is calculated as is commonly done, then a linear system is solved for the climb velocity using these forces. This is also done within the same order of computational cost as existing discrete dislocation dynamics methods.

  7. Improved performance of GaN based light emitting diodes with ex-situ sputtered AlN nucleation layers

    NASA Astrophysics Data System (ADS)

    Chen, Shuo-Wei; Li, Heng; Lu, Tien-Chang

    2016-04-01

    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 shows the potential and constrain in applying ex-situ PVD AlN nucleation layers to fabricate high quality GaN crystals in various optoelectronics.

  8. Hidden surface states at non-polar GaN (101xAF0) facets: Intrinsic pinning of nanowires

    NASA Astrophysics Data System (ADS)

    Lymperakis, L.; Weidlich, P. H.; Eisele, H.; Schnedler, M.; Nys, J.-P.; Grandidier, B.; Stiévenard, D.; Dunin-Borkowski, R. E.; Neugebauer, J.; Ebert, Ph.

    2013-10-01

    We investigate the electronic structure of the GaN(101¯0) prototype surface for GaN nanowire sidewalls. We find a paradoxical situation that a surface state at all k points in the bandgap cannot be probed by conventional scanning tunneling microscopy, due to a dispersion characterized by a steep minimum with low density of states (DOS) and an extremely flat maximum with high DOS. Based on an analysis of the decay behavior into the vacuum, we identify experimentally the surface state minimum 0.6 ± 0.2 eV below the bulk conduction band in the gap. Hence, GaN nanowires with clean (101¯0) sidewall facets are intrinsically pinned.

  9. Confocal microscopic analysis of optical crosstalk in GaN micro-pixel light-emitting diodes

    NASA Astrophysics Data System (ADS)

    Li, K. H.; Cheung, Y. F.; Cheung, W. S.; Choi, H. W.

    2015-10-01

    The optical crosstalk phenomenon in GaN micro-pixel light-emitting diodes (LED) has been investigated by confocal microscopy. Depth-resolved confocal emission images indicate light channeling along the GaN and sapphire layers as the source of crosstalk. Thin-film micro-pixel devices are proposed, whereby the light-trapping sapphire layers are removed by laser lift-off. Optical crosstalk is significantly reduced but not eliminated due to the remaining GaN layer. Another design involving micro-pixels which are completely isolated is further proposed; such devices exhibited low-noise and enhanced optical performances, which are important attributes for high-density micro-pixel LED applications including micro-displays and multi-channel optical communications.

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

    SciTech Connect

    Uren, Michael J.; Cäsar, Markus; Kuball, Martin; Gajda, Mark A.

    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 with 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.

  11. Molecular simulations and lattice dynamics determination of Stillinger-Weber GaN thermal conductivity

    SciTech Connect

    Liang, Zhi; Jain, Ankit; McGaughey, Alan J. H.; Keblinski, Pawel

    2015-09-28

    The bulk thermal conductivity of Stillinger-Weber (SW) wurtzite GaN in the [0001] direction at a temperature of 300 K is calculated using equilibrium molecular dynamics (EMD), non-equilibrium MD (NEMD), and lattice dynamics (LD) methods. While the NEMD method predicts a thermal conductivity of 166 ± 11 W/m·K, both the EMD and LD methods predict thermal conductivities that are an order of magnitude greater. We attribute the discrepancy to significant contributions to thermal conductivity from long-mean free path phonons. We propose that the Grüneisen parameter for low-frequency phonons is a good predictor of the severity of the size effects in NEMD thermal conductivity prediction. For weakly anharmonic crystals characterized by small Grüneisen parameters, accurate determination of thermal conductivity by NEMD is computationally impractical. The simulation results also indicate the GaN SW potential, which was originally developed for studying the atomic-level structure of dislocations, is not suitable for prediction of its thermal conductivity.

  12. Characterization of vertical GaN p-n diodes and junction field-effect transistors on bulk GaN down to cryogenic temperatures

    NASA Astrophysics Data System (ADS)

    Kizilyalli, I. C.; Aktas, O.

    2015-12-01

    There is great interest in wide-bandgap semiconductor devices and most recently in vertical GaN structures for power electronic applications such as power supplies, solar inverters and motor drives. In this paper the temperature-dependent electrical behavior of vertical GaN p-n diodes and vertical junction field-effect transistors fabricated on bulk GaN substrates of low defect density (104 to 106 cm-2) is described. Homoepitaxial MOCVD growth of GaN on its native substrate and the ability to control the doping in the drift layers in GaN have allowed the realization of vertical device architectures with drift layer thicknesses of 6 to 40 μm and net carrier electron concentrations as low as 1 × 1015 cm-3. This parameter range is suitable for applications requiring breakdown voltages of 1.2 kV to 5 kV. Mg, which is used as a p-type dopant in GaN, is a relatively deep acceptor (E A ≈ 0.18 eV) and susceptible to freeze-out at temperatures below 200 K. The loss of holes in p-GaN has a deleterious effect on p-n junction behavior, p-GaN contacts and channel control in junction field-effect transistors at temperatures below 200 K. Impact ionization-based avalanche breakdown (BV > 1200 V) in GaN p-n junctions is characterized between 77 K and 423 K for the first time. At higher temperatures the p-n junction breakdown voltage improves due to increased phonon scattering. A positive temperature coefficient in the breakdown voltage is demonstrated down to 77 K; however, the device breakdown characteristics are not as abrupt at temperatures below 200 K. On the other hand, contact resistance to p-GaN is reduced dramatically above room temperature, improving the overall device performance in GaN p-n diodes in all cases except where the n-type drift region resistance dominates the total forward resistance. In this case, the electron mobility can be deconvolved and is found to decrease with T -3/2, consistent with a phonon scattering model. Also, normally-on vertical junction

  13. Enhanced functionality in GaN and SiC devices by using novel processing

    NASA Astrophysics Data System (ADS)

    Pearton, S. J.; Abernathy, C. R.; Gila, B. P.; Ren, F.; Zavada, J. M.; Park, Y. D.

    2004-11-01

    Some examples of recent advances in enhancing or adding functionality to GaN and SiC devices through the use of novel processing techniques are discussed. The first example is the use of ion implantation to incorporate transition metals such as Mn, Cr and Co at atomic percent levels in the wide bandgap semiconductors to produce room temperature ferromagnetism. A discussion is given of the phase space within which single-phase material can be obtained and the requirements for demonstrating the presence of a true dilute magnetic semiconductor. The ability to make GaN and SiC ferromagnetic leads to the possibility of magnetic devices with gain, spin FETs operating at low voltages and spin polarized light emitters. The second example is the use of novel oxides such as Sc 2O 3 and MgO as gate dielectrics or surface passivants on GaN. True inversion behavior has been demonstrated in gated MOS-GaN diodes with implanted n-regions supplying the minority carriers need for inversion. These oxide layers also effectively mitigate current collapse in AlGaN/GaN HEMTs through their passivation of surface states in the gate-drain region. The third example is the use of laser drilling to make through-wafer via holes in SiC, sapphire and GaN. The ablation rate is sufficiently high that this maskless, serial process appears capable of achieving similar throughput to the more conventional approach of plasma etching of vias. The fourth example is the use of either ungated AlGaN/GaN HEMTs or simple GaN and SiC Schottky diodes as sensors for chemicals, biogens, radiation, combustion gases or strain. The sensitivity of either the channel carrier density or the barrier height to changes in surface condition make these materials systems ideal for compact robust sensors capable of operating at elevated temperatures.

  14. Study of GaP single crystal layers grown on GaN by MOCVD

    SciTech Connect

    Li, Shuti; Liu, Chao; Ye, Guoguang; Xiao, Guowei; Zhou, Yugang; Su, Jun; Fan, Guanghan; Zhang, Yong; Liang, Fubo; Zheng, Shuwen

    2011-11-15

    Highlights: {yields} We investigated the growth of GaP layers on GaN by MOCVD. {yields} A single crystal GaP layer could be grown on GaN. {yields} The V/III ratio played an important role to improve GaP layer quality. {yields} The GaP:Mg layer with hole concentration of 4.2 x 10{sup 18} cm{sup -3} was obtained. -- Abstract: The performance of GaN based devices could possibly be improved by utilizing the good p-type properties of GaP layer and it provides the possibility of the integration of InAlGaN and AlGaInP materials to produce new devices, if high quality GaP compounds can be grown on III-nitride compounds. In this paper, the growth of GaP layers on GaN by metalorganic chemical vapor deposition (MOCVD) has been investigated. The results show that the GaP low temperature buffer layer can provide a high density of nucleation sites for high temperature GaP growth. Using a 40 nm thick GaP buffer layer, a single crystal GaP layer, whose full-width at half-maximum of the (1 1 1) plane measured by double crystal X-ray diffraction is 580'', can be grown on GaN. The V/III ratio plays an important role in the GaP layer growth and an appropriate V/III ratio can improve the quality of GaP layer. The GaP:Mg layer with hole carrier concentration of 4.2 x 10{sup 18} cm{sup -3} has been obtained.

  15. GaN based nanorods for solid state lighting

    SciTech Connect

    Li Shunfeng; Waag, Andreas

    2012-04-01

    In recent years, GaN nanorods are emerging as a very promising novel route toward devices for nano-optoelectronics and nano-photonics. In particular, core-shell light emitting devices are thought to be a breakthrough development in solid state lighting, nanorod based LEDs have many potential advantages as compared to their 2 D thin film counterparts. In this paper, we review the recent developments of GaN nanorod growth, characterization, and related device applications based on GaN nanorods. The initial work on GaN nanorod growth focused on catalyst-assisted and catalyst-free statistical growth. The growth condition and growth mechanisms were extensively investigated and discussed. Doping of GaN nanorods, especially p-doping, was found to significantly influence the morphology of GaN nanorods. The large surface of 3 D GaN nanorods induces new optical and electrical properties, which normally can be neglected in layered structures. Recently, more controlled selective area growth of GaN nanorods was realized using patterned substrates both by metalorganic chemical vapor deposition (MOCVD) and by molecular beam epitaxy (MBE). Advanced structures, for example, photonic crystals and DBRs are meanwhile integrated in GaN nanorod structures. Based on the work of growth and characterization of GaN nanorods, GaN nanoLEDs were reported by several groups with different growth and processing methods. Core/shell nanoLED structures were also demonstrated, which could be potentially useful for future high efficient LED structures. In this paper, we will discuss recent developments in GaN nanorod technology, focusing on the potential advantages, but also discussing problems and open questions, which may impose obstacles during the future development of a GaN nanorod based LED technology.

  16. 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.

  17. Direct growth of GaN layer on carbon nanotube-graphene hybrid structure and its application for light emitting diodes.

    PubMed

    Seo, Tae Hoon; Park, Ah Hyun; Park, Sungchan; Kim, Yong Hwan; Lee, Gun Hee; Kim, Myung Jong; Jeong, Mun Seok; Lee, Young Hee; Hahn, Yoon-Bong; Suh, Eun-Kyung

    2015-01-01

    We report the growth of high-quality GaN layer on single-walled carbon nanotubes (SWCNTs) and graphene hybrid structure (CGH) as intermediate layer between GaN and sapphire substrate by metal-organic chemical vapor deposition (MOCVD) and fabrication of light emitting diodes (LEDs) using them. The SWCNTs on graphene act as nucleation seeds, resulting in the formation of kink bonds along SWCNTs with the basal plane of the substrate. In the x-ray diffraction, Raman and photoluminescence spectra, high crystalline quality of GaN layer grown on CGH/sapphire was observed due to the reduced threading dislocation and efficient relaxation of residual compressive strain caused by lateral overgrowth process. When applied to the LED structure, the current-voltage characteristics and electroluminescence (EL) performance exhibit that blue LEDs fabricated on CGH/sapphire well-operate at high injection currents and uniformly emit over the whole emission area. We expect that CGH can be applied for the epitaxial growth of GaN on various substrates such as Si and MgO, which can be a great advantage in electrical and thermal properties of optical devices fabricated on them. PMID:25597492

  18. Nucleation, propagation, electronic levels and elimination of misfit dislocations in III-V semiconductor interfaces. Final report

    SciTech Connect

    Watson, G.P.; Matragrano, M.

    1995-03-01

    This report discusses the following topics: strained layer defects; the structural and electronic characteristics of misfit dislocations; requirements for the growth of high quality, low defect density InGaAs strained epitaxial layers; the isolation and nucleation of misfit dislocations in strained epitaxial layers grown on patterned, ion-damaged GaAs; the effect of pattern substrate trench depth on misfit dislocation density; the thermal stability of lattice mismatched InGaAs grown on patterned GaAs; misfit dislocations in ZnSe strained epitaxial layers grown on patterned GaAs; and the measurement of deep level states caused by misfit dislocations in InGaAs/GaAs grown on patterned GaAs substrates.

  19. Mobility Laws in Dislocation Dynamics Simulations

    SciTech Connect

    Cai, W; Bulatov, V V

    2003-10-21

    Prediction of the plastic deformation behavior of single crystals based on the collective dynamics of dislocations has been a challenge for computational materials science for a number of years. The difficulty lies in the inability of existing dislocation dynamics (DD) codes to handle a sufficiently large number of dislocation lines, to establish a statistically representative model of crystal plasticity. A new massively-parallel DD code is developed that is capable of modeling million-dislocation systems by employing thousands of processors. We discuss an important ingredient of this code--the mobility laws dictating the behavior of individual dislocations. They are materials input for DD simulations and are constructed based on the understanding of dislocation motion at the atomistic level.

  20. Line-Profile Analysis Combined with Texture Analysis for Characterizing Dislocation Distribution in Texture Components of Cold-Rolled Copper Sheets

    NASA Astrophysics Data System (ADS)

    Satoh, Kozue; Sato, Shigeo; Yamanaka, Kenta; Suzuki, Shigeru; Chiba, Akihiko; Wagatsuma, Kazuaki

    2016-08-01

    We described a newly developed characterization technique that dislocation density could be individually determined for each texture component of plastically deformed metals by combining the line-profile analysis with the texture analysis by using X-ray diffraction. This method was applied to major texture components of cube, copper, and brass evolved in cold-rolled copper sheets. The Warren-Averbach procedure using two diffraction peaks was used for estimating the dislocation density. An increase in the dislocation density with the rolling reduction was evaluated for individual texture components. Although the individual texture components underwent the different slip paths, the dislocation densities in these texture components were almost comparable; however, the non-texture component was shown to have a higher dislocation density than the texture components. The recovery and recrystallization proceeded preferentially in the non-texture component.

  1. An atomistically validated continuum model for strain relaxation and misfit dislocation formation

    NASA Astrophysics Data System (ADS)

    Zhou, X. W.; Ward, D. K.; Zimmerman, J. A.; Cruz-Campa, J. L.; Zubia, D.; Martin, J. E.; van Swol, F.

    2016-06-01

    In this paper, molecular dynamics (MD) calculations have been used to examine the physics behind continuum models of misfit dislocation formation and to assess the limitations and consequences of approximations made within these models. Without compromising the physics of misfit dislocations below a surface, our MD calculations consider arrays of dislocation dipoles constituting a mirror imaged "surface". This allows use of periodic boundary conditions to create a direct correspondence between atomistic and continuum representations of dislocations, which would be difficult to achieve with free surfaces. Additionally, by using long-time averages of system properties, we have essentially reduced the errors of atomistic simulations of large systems to "zero". This enables us to deterministically compare atomistic and continuum calculations. Our work results in a robust approach that uses atomistic simulation to accurately calculate dislocation core radius and energy without the continuum boundary conditions typically assumed in the past, and the novel insight that continuum misfit dislocation models can be inaccurate when incorrect definitions of dislocation spacing and Burgers vector in lattice-mismatched systems are used. We show that when these insights are properly incorporated into the continuum model, the resulting energy density expression of the lattice-mismatched systems is essentially indistinguishable from the MD results.

  2. Dislocation Strengthening without Ductility Trade-off in Metastable Austenitic Steels

    PubMed Central

    Liu, Jiabin; Jin, Yongbin; Fang, Xiaoyang; Chen, Chenxu; Feng, Qiong; Liu, Xiaowei; Chen, Yuzeng; Suo, Tao; Zhao, Feng; Huang, Tianlin; Wang, Hongtao; Wang, Xi; Fang, Youtong; Wei, Yujie; Meng, Liang; Lu, Jian; Yang, Wei

    2016-01-01

    Strength and ductility are mutually exclusive if they are manifested as consequence of the coupling between strengthening and toughening mechanisms. One notable example is dislocation strengthening in metals, which invariably leads to reduced ductility. However, this trend is averted in metastable austenitic steels. A one-step thermal mechanical treatment (TMT), i.e. hot rolling, can effectively enhance the yielding strength of the metastable austenitic steel from 322 ± 18 MPa to 675 ± 15 MPa, while retaining both the formability and hardenability. It is noted that no boundaries are introduced in the optimized TMT process and all strengthening effect originates from dislocations with inherited thermal stability. The success of this method relies on the decoupled strengthening and toughening mechanisms in metastable austenitic steels, in which yield strength is controlled by initial dislocation density while ductility is retained by the capability to nucleate new dislocations to carry plastic deformation. Especially, the simplicity in processing enables scaling and industrial applications to meet the challenging requirements of emissions reduction. On the other hand, the complexity in the underlying mechanism of dislocation strengthening in this case may shed light on a different route of material strengthening by stimulating dislocation activities, rather than impeding motion of dislocations. PMID:27739481

  3. Dislocation Strengthening without Ductility Trade-off in Metastable Austenitic Steels

    NASA Astrophysics Data System (ADS)

    Liu, Jiabin; Jin, Yongbin; Fang, Xiaoyang; Chen, Chenxu; Feng, Qiong; Liu, Xiaowei; Chen, Yuzeng; Suo, Tao; Zhao, Feng; Huang, Tianlin; Wang, Hongtao; Wang, Xi; Fang, Youtong; Wei, Yujie; Meng, Liang; Lu, Jian; Yang, Wei

    2016-10-01

    Strength and ductility are mutually exclusive if they are manifested as consequence of the coupling between strengthening and toughening mechanisms. One notable example is dislocation strengthening in metals, which invariably leads to reduced ductility. However, this trend is averted in metastable austenitic steels. A one-step thermal mechanical treatment (TMT), i.e. hot rolling, can effectively enhance the yielding strength of the metastable austenitic steel from 322 ± 18 MPa to 675 ± 15 MPa, while retaining both the formability and hardenability. It is noted that no boundaries are introduced in the optimized TMT process and all strengthening effect originates from dislocations with inherited thermal stability. The success of this method relies on the decoupled strengthening and toughening mechanisms in metastable austenitic steels, in which yield strength is controlled by initial dislocation density while ductility is retained by the capability to nucleate new dislocations to carry plastic deformation. Especially, the simplicity in processing enables scaling and industrial applications to meet the challenging requirements of emissions reduction. On the other hand, the complexity in the underlying mechanism of dislocation strengthening in this case may shed light on a different route of material strengthening by stimulating dislocation activities, rather than impeding motion of dislocations.

  4. Worker Dislocation. Case Studies of Causes and Cures.

    ERIC Educational Resources Information Center

    Cook, Robert F., Ed.

    Case studies were made of the following dislocated worker programs: Cummins Engine Company Dislocated Worker Project; GM-UAW Metropolitan Pontiac Retraining and Employment Program; Minnesota Iron Range Dislocated Worker Project; Missouri Dislocated Worker Program Job Search Assistance, Inc.; Hillsborough, North Carolina, Dislocated Worker Project;…

  5. Nanoscale anisotropic plastic deformation in single crystal GaN.

    PubMed

    Huang, Jun; Xu, Ke; Fan, Ying Min; Niu, Mu Tong; Zeng, Xiong Hui; Wang, Jian Feng; Yang, Hui

    2012-01-01

    Elasto-plastic mechanical deformation behaviors of c-plane (0001) and nonpolar GaN single crystals are studied using nanoindentation, cathodoluminescence, and transmission electron microscopy. Nanoindentation tests show that c-plane GaN is less susceptible to plastic deformation and has higher hardness and Young's modulus than the nonpolar GaN. Cathodoluminescence and transmission electron microscopy characterizations of indent-induced plastic deformation reveal that there are two primary slip systems for the c-plane GaN, while there is only one most favorable slip system for the nonplane GaN. We suggest that the anisotropic elasto-plastic mechanical properties of GaN are relative to its anisotropic plastic deformation behavior.PACS: 62.20.fq; 81.05.Ea; 61.72.Lk.

  6. 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.

  7. 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

  8. Robust atomistic calculation of dislocation line tension

    NASA Astrophysics Data System (ADS)

    Szajewski, B. A.; Pavia, F.; Curtin, W. A.

    2015-12-01

    The line tension Γ of a dislocation is an important and fundamental property ubiquitous to continuum scale models of metal plasticity. However, the precise value of Γ in a given material has proven difficult to assess, with literature values encompassing a wide range. Here results from a multiscale simulation and robust analysis of the dislocation line tension, for dislocation bow-out between pinning points, are presented for two widely-used interatomic potentials for Al. A central part of the analysis involves an effective Peierls stress applicable to curved dislocation structures that markedly differs from that of perfectly straight dislocations but is required to describe the bow-out both in loading and unloading. The line tensions for the two interatomic potentials are similar and provide robust numerical values for Al. Most importantly, the atomic results show notable differences with singular anisotropic elastic dislocation theory in that (i) the coefficient of the \\text{ln}(L) scaling with dislocation length L differs and (ii) the ratio of screw to edge line tension is smaller than predicted by anisotropic elasticity. These differences are attributed to local dislocation core interactions that remain beyond the scope of elasticity theory. The many differing literature values for Γ are attributed to various approximations and inaccuracies in previous approaches. The results here indicate that continuum line dislocation models, based on elasticity theory and various core-cut-off assumptions, may be fundamentally unable to reproduce full atomistic results, thus hampering the detailed predictive ability of such continuum models.

  9. Microdiffraction Analysis of Hierarchical Dislocation Organization

    SciTech Connect

    Barabash, R.I.; Ice, G.E.

    2007-12-19

    This article describes how x-ray microdiffraction is influenced by the number, kind, and organization of dislocations. Particular attention is placed on micro-Laue diffraction, where polychromatic x-rays are diffracted into characteristic Laue patterns that are sensitive to the dislocation content and arrangement. Diffraction is considered for various stages of plastic deformation. For early stages of plastic deformation with random dislocation spacing, the intensity in reciprocal space is redistributed about Laue spots with a length scale proportional to the number of dislocations within the sample volume and with a characteristic shape that depends on the kinds of dislocations and the momentum transfer vector. Unpaired dislocations that contribute to lattice rotations cause the largest redistribution of scattered intensity. In later stages of plastic deformation, strong interactions between individual dislocations cause them to organize into correlated arrangements. Here again, xray diffraction Laue spots are broadened in proportion to the number of excess (unpaired) dislocations inside the wall and to the total number of unpaired walls, but the broadening can be discontinuous. With microdiffraction it is possible to quantitatively test models of dislocation organization.

  10. Enabling Strain Hardening Simulations with Dislocation Dynamics

    SciTech Connect

    Arsenlis, A; Cai, W

    2006-12-20

    Numerical algorithms for discrete dislocation dynamics simulations are investigated for the purpose of enabling strain hardening simulations of single crystals on massively parallel computers. The algorithms investigated include the /(N) calculation of forces, the equations of motion, time integration, adaptive mesh refinement, the treatment of dislocation core reactions, and the dynamic distribution of work on parallel computers. A simulation integrating all of these algorithmic elements using the Parallel Dislocation Simulator (ParaDiS) code is performed to understand their behavior in concert, and evaluate the overall numerical performance of dislocation dynamics simulations and their ability to accumulate percents of plastic strain.

  11. Congenital dislocation of the patella - clinical case.

    PubMed

    Miguel Sá, Pedro; Raposo, Filipa; Santos Carvalho, Manuel; Alegrete, Nuno; Coutinho, Jorge; Costa, Gilberto

    2016-01-01

    Congenital patellar dislocation is a rare condition in which the patella is permanently dislocated and cannot be reduced manually. The patella develops normally as a sesamoid bone of the femur. This congenital dislocation results from failure of the internal rotation of the myotome that forms the femur, quadriceps muscle and extensor apparatus. It usually manifests immediately after birth, although in some rare cases, the diagnosis may be delayed until adolescence or adulthood. Early diagnosis is important, thereby allowing surgical correction and avoiding late sequelae, including early degenerative changes in the knee. A case of permanent dislocation of the patella is presented here, in a female child aged seven years.

  12. Multiscale modeling of dislocation-precipitate interactions in Fe: From molecular dynamics to discrete dislocations.

    PubMed

    Lehtinen, Arttu; Granberg, Fredric; Laurson, Lasse; Nordlund, Kai; Alava, Mikko J

    2016-01-01

    The stress-driven motion of dislocations in crystalline solids, and thus the ensuing plastic deformation process, is greatly influenced by the presence or absence of various pointlike defects such as precipitates or solute atoms. These defects act as obstacles for dislocation motion and hence affect the mechanical properties of the material. Here we combine molecular dynamics studies with three-dimensional discrete dislocation dynamics simulations in order to model the interaction between different kinds of precipitates and a 1/2〈111〉{110} edge dislocation in BCC iron. We have implemented immobile spherical precipitates into the ParaDis discrete dislocation dynamics code, with the dislocations interacting with the precipitates via a Gaussian potential, generating a normal force acting on the dislocation segments. The parameters used in the discrete dislocation dynamics simulations for the precipitate potential, the dislocation mobility, shear modulus, and dislocation core energy are obtained from molecular dynamics simulations. We compare the critical stresses needed to unpin the dislocation from the precipitate in molecular dynamics and discrete dislocation dynamics simulations in order to fit the two methods together and discuss the variety of the relevant pinning and depinning mechanisms. PMID:26871192

  13. Atomistic simulations of dislocation-interface interactions in the Cu-Ni multilayer system

    NASA Astrophysics Data System (ADS)

    Rao, S. I.; Hazzledine, P. M.

    2000-09-01

    , alter the barrier strengths by three mechanisms: firstly, they reduce the density of van der Merwe misfit dislocations, secondly, they enhance the Koehler barrier by altering the elastic constants of both Cu and Ni and, thirdly, non-glide stress components change the core structure of gliding dislocations, thereby altering the Koehler barrier. Overall, the barrier strength of (111) interfaces is independent of the wavelength of the multilayer and about 0.02mu up to the wavelength of uc, the coherence wavelength limit. At Cu(001)-Ni(001) interfaces the total barrier strength decreases from a value of 0.02mu at long wavelengths (u , X) to about 0.01mu at u = uc, as considered by Rao et al. in 1995 in their yield stress model for Cu-Ni multilayered structures. Slip-plane misorientations provide powerful barriers to slip transmission. Even at a (111) twinned interface in a coherent Cu-Ni multilayer, screw dislocations cross-slip on to the interface rather than into Ni because the stacking-fault energy at the interface is lower than in Ni. The blocking strength of the same interface to 60° dislocations (which must leave a step and a residual dislocation in the boundary) is very large, 0.03-0.04mu.

  14. Self-catalyzed anisotropic growth of GaN spirals

    NASA Astrophysics Data System (ADS)

    Patsha, Avinash; Sahoo, Prasana; Dhara, S.; Tyagi, A. K.

    2012-06-01

    GaN spirals with homogeneous size are grown using chemical-vapor-deposition technique in a self catalytic process. Raman and photoluminescence (PL) studies reveal wurtzite GaN phase. Nucleation of GaN sphere takes place with the agglomeration Ga clusters and simultaneous reaction with NH3. A growth mechanism involving diffusion limited aggregation process initiating supersaturation and subsequent neck formation along with possible role of thermodynamic fluctuation in different crystalline facets of GaN, is described for the anisotropic spiral structures. Temperature dependent PL spectra show strong excitonic emissions along with the presence of free-to-bound transition.

  15. Later Leaders in Education: Roma Gans--Teacher of Teachers.

    ERIC Educational Resources Information Center

    Almy, Millie

    1990-01-01

    Retired teacher Roma Gans is described in terms of her early life and education, early teaching experience, teaching experience at Teachers College, publishing experience, citizenship, and retirement. (DG)

  16. Dislocation mechanism for transformation between cubic ice Ic and hexagonal ice Ih

    NASA Astrophysics Data System (ADS)

    Hondoh, T.

    2015-11-01

    Cubic ice Ic is metastable, yet can form by the freezing of supercooled water, vapour deposition at low temperatures and by depressurizing high-pressure forms of ice. Its structure differs from that of common hexagonal ice Ih in the order its molecular layers are stacked. This stacking order, however, typically has considerable disorder; that is, not purely cubic, but alternating in hexagonal and cubic layers. In time, stacking-disordered ice gradually decreases in cubicity (fraction having cubic structure), transforming to hexagonal ice. But, how does this disorder originate and how does it transform to hexagonal ice? Here we use numerical data on dislocations in hexagonal ice Ih to show that (1) stacking-disordered ice (or Ic) can be viewed as fine-grained polycrystalline ice with a high density of extended dislocations, each a widely extended stacking fault bounded by partial dislocations, and (2) the transformation from ice Ic to Ih is caused by the reaction and motion of these partial dislocations. Moreover, the stacking disorder may be in either a higher stored energy state consisting of a sub-boundary network arrangement of partial dislocations bounding stacking faults, or a lower stored energy state consisting of a grain structure with a high density of stacking faults, but without bounding partial dislocations. Each state transforms to Ih differently, with a duration to fully transform that strongly depends on temperature and crystal grain size. The results are consistent with the observed transformation rates, transformation temperatures and wide range in heat of transformation.

  17. A computationally efficient hybrid 2D/3D thin film dislocation model

    NASA Astrophysics Data System (ADS)

    Sarrafan, Siavash

    Substantial research has been devoted to attempting to understand how dislocation structures evolve and how they affect device properties. However, current dislocation simulation methods are only able to model highly idealized systems accurately. The three-dimensional discrete dislocation dynamics models, in particular, are too computationally intensive for modelling high dislocation densities and their resultant deformations that are observed in some real applications. In this thesis, we propose a novel method to exploit the quasi-two-dimensional nature of three-dimensional dislocation loops in a thin film to model their behaviors. For most film configurations, simulation performance can be greatly enhanced by implementing a hybrid two-dimensional/three-dimensional model without losing significant fidelity. In this technique, misfits stress fields are modeled by superposing multiple two-dimensional models. Threads are modeled with a more traditional three-dimensional implementation as they move through the misfit stress field. Using this innovative technique, much higher strains and/or dislocation densities could be studied.

  18. In situ observation of dislocation nucleation and escape in a submicrometre aluminium single crystal

    NASA Astrophysics Data System (ADS)

    Oh, Sang Ho; Legros, Marc; Kiener, Daniel; Dehm, Gerhard

    2009-02-01

    `Smaller is stronger' does not hold true only for nanocrystalline materials but also for single crystals. It is argued that this effect is caused by geometrical constraints on the nucleation and motion of dislocations in submicrometre-sized crystals. Here, we report the first in situ transmission electron microscopy tensile tests of a submicrometre aluminium single crystal that are capable of providing direct insight into source-controlled dislocation plasticity in a submicrometre crystal. Single-ended sources emit dislocations that escape the crystal before being able to multiply. As dislocation nucleation and loss rates are counterbalanced at about 0.2 events per second, the dislocation density remains statistically constant throughout the deformation at strain rates of about 10-4s-1. However, a sudden increase in strain rate to 10-3s-1 causes a noticeable surge in dislocation density as the nucleation rate outweighs the loss rate. This observation indicates that the deformation of submicrometre crystals is strain-rate sensitive.

  19. Infrared absorption of hydrogen-related defects in ammonothermal GaN

    NASA Astrophysics Data System (ADS)

    Suihkonen, Sami; Pimputkar, Siddha; Speck, James S.; Nakamura, Shuji

    2016-05-01

    Polarization controlled Fourier transform infrared (FTIR) absorption measurements were performed on a high quality m-plane ammonothermal GaN crystal grown using basic chemistry. The polarization dependence of characteristic absorption peaks of hydrogen-related defects at 3000-3500 cm-1 was used to identify and determine the bond orientation of hydrogenated defect complexes in the GaN lattice. Majority of hydrogen was found to be bonded in gallium vacancy complexes decorated with one to three hydrogen atoms (VGa-H1,2,3) but also hydrogenated oxygen defect complexes, hydrogen in bond-center sites, and lattice direction independent absorption were observed. Absorption peak intensity was used to determine a total hydrogenated VGa density of approximately 4 × 1018 cm-3, with main contribution from VGa-H1,2. Also, a significant concentration of electrically passive VGa-H3 was detected. The high density of hydrogenated defects is expected to have a strong effect on the structural, optical, and electrical properties of ammonothermal GaN crystals.

  20. Quantum dislocations in solid Helium-4

    NASA Astrophysics Data System (ADS)

    Aleinikava, Darya

    In this thesis the following problems on properties of solid 4He are considered: (i) the role of long-range interactions in suppression of dislocation roughening at T = 0; (ii) the combined effect of 3He impurities and Peierls potential on shear modulus softening; (iii) the dislocation superclimb and its connection to the phenomenon of "giant isochoric compressibility"; (iv) non-linear dislocation response to the applied stress and stress-induces dislocation roughening as a I-order phase transition in 1D at finite temperature. First we investigate the effect of long-range interactions on the state of edge dislocation at T = 0. Such interactions are induced by elastic forces of the solid. We found that quantum roughening transition of a dislocation at T = 0 is completely suppressed by arbitrarily small long-range interactions between kinks. A heuristic argument is presented and the result has been verified by numerical Monte-Carlo simulations using Worm Algorithm in J-current model. It was shown that the Peierls potential plays a crucial role in explaining the elastic properties of dislocations, namely shear modulus softening phenomenon. The crossover from T = 0 to finite temperatures leads to intrinsic softening of the shear modulus and is solely controlled by kink typical energy. It was demonstrated that the mechanism, involving only the binding of 3He impurities to the dislocations, requires an unrealistically high concentrations of defects (or impurities) in order to explain the shear modulus phenomenon and therefore an inclusion of Peierls potential in consideration is required. Superclimbing dislocations, that is the edge dislocations with the superfluidity along the core, were investigated. The theoretical prediction that superclimb is responsible for the phenomenon of "giant isochoric compressibility" was confirmed by Monte-Carlo simulations. It was demonstrated that the isochoric compressibility is suppressed at low temperatures. The dependence of

  1. The core structure and pseudo-magnetic field of the dislocation in graphene

    NASA Astrophysics Data System (ADS)

    Wang, Shaofeng; Wang, Rui

    2013-10-01

    An analytical theory is presented for the dislocation (pentagon-heptagon pair) in graphene. In order to fully determine the nonlinear interaction which is responsible for the appearance of dislocation, a γ-potential is proposed and calculated from the electron-density functional theory. The improved dislocation equation, which takes into account the change in space between the mismatched chains, is solved by the variational principle. The strain and stress, in particular the change in the bond length and bond angle at the dislocation core, are obtained explicitly and verified by the first-principle calculation. The pseudo-magnetic field felt by electrons at the band edge is investigated including contribution from topology as well as deformation.

  2. Electrical wind force-driven and dislocation-templated amorphization in phase-change nanowires.

    PubMed

    Nam, Sung-Wook; Chung, Hee-Suk; Lo, Yu Chieh; Qi, Liang; Li, Ju; Lu, Ye; Johnson, A T Charlie; Jung, Yeonwoong; Nukala, Pavan; Agarwal, Ritesh

    2012-06-22

    Phase-change materials undergo rapid and reversible crystalline-to-amorphous structural transformation and are being used for nonvolatile memory devices. However, the transformation mechanism remains poorly understood. We have studied the effect of electrical pulses on the crystalline-to-amorphous phase change in a single-crystalline Ge(2)Sb(2)Te(5) (GST) nanowire memory device by in situ transmission electron microscopy. We show that electrical pulses produce dislocations in crystalline GST, which become mobile and glide in the direction of hole-carrier motion. The continuous increase in the density of dislocations moving unidirectionally in the material leads to dislocation jamming, which eventually induces the crystalline-to-amorphous phase change with a sharp interface spanning the entire nanowire cross section. The dislocation-templated amorphization explains the large on/off resistance ratio of the device.

  3. Misfit accommodation and dislocations in heteroepitaxial semiconductor layers: II-VI compounds on GaAs

    NASA Astrophysics Data System (ADS)

    Patriarche, G.; Rivière, J. P.; Castaing, J.

    1993-06-01

    We suggest a model for the nucleation and expansion of dislocations which accommodate the parameter misfit of an epitaxial layer on a substrate, applied, in this work, to a II-VI compound on GaAs. We examine in particular the dislocations threading through the layer, which must be kept as low as possible in density. Nous proposons un mécanisme de germination et de développement des dislocations permettant de compenser l'écart de paramètres d'une couche épitaxiée sur un substrat, par exemple dans notre cas, un composé II-VI sur GaAs. On porte une attention particulière aux dislocations résiduelles dans la couche dont on cherche à minimiser la densité.

  4. Atomic-scale details of dislocation - stacking fault tetrahedra interaction.

    SciTech Connect

    Osetskiy, Yury N; Stoller, Roger E; Rodney, David; Bacon, David J

    2005-01-01

    Stacking fault tetrahedra (SFTs) are formed during irradiation of fcc. metals and alloys with low stacking fault energy. The high number density of SFTs observed suggests that they should contribute to radiation-induced hardening and, therefore, be taken into account when estimating mechanical property changes of irradiated materials. The key issue is to describe the interaction between a moving dislocation and an individual SFT, which is characterized by a small physical scale of about 100 nm. In this paper we present results of an atomistic simulation of edge and screw dislocations interacting with small SFTs at different temperatures and strain rates and present mechanisms which can explain the formation of defect-free channels observed experimentally.

  5. Interstitial iron impurities at cores of dissociated dislocations in silicon

    NASA Astrophysics Data System (ADS)

    Ziebarth, Benedikt; Mrovec, Matous; Elsässer, Christian; Gumbsch, Peter

    2015-11-01

    Dislocations play an important role in semiconductor devices made of crystalline silicon (Si). They are known to be strongly performance-limiting defects in solar cell applications, since they act as preferred segregation sites for metallic impurities. In this work we investigate the segregation of iron (Fe) to the cores of the 30° and 90° partial dislocations in Si using atomistic calculations based on first-principles density functional theory. Our simulations show that interstitial Fe impurities segregate readily to all investigated cores and the driving force for the segregation increases with impurity concentration. Moreover, our analysis of the electronic structure reveals the existence of deep defect levels within the band gap that can be related to experimental observations by deep-level transient spectroscopy.

  6. Measurement of probability distributions for internal stresses in dislocated crystals

    SciTech Connect

    Wilkinson, Angus J.; Tarleton, Edmund; Vilalta-Clemente, Arantxa; Collins, David M.; Jiang, Jun; Britton, T. Benjamin

    2014-11-03

    Here, we analyse residual stress distributions obtained from various crystal systems using high resolution electron backscatter diffraction (EBSD) measurements. Histograms showing stress probability distributions exhibit tails extending to very high stress levels. We demonstrate that these extreme stress values are consistent with the functional form that should be expected for dislocated crystals. Analysis initially developed by Groma and co-workers for X-ray line profile analysis and based on the so-called “restricted second moment of the probability distribution” can be used to estimate the total dislocation density. The generality of the results are illustrated by application to three quite different systems, namely, face centred cubic Cu deformed in uniaxial tension, a body centred cubic steel deformed to larger strain by cold rolling, and hexagonal InAlN layers grown on misfitting sapphire and silicon carbide substrates.

  7. Direct experimental determination of the spontaneous polarization of GaN

    NASA Astrophysics Data System (ADS)

    Lähnemann, Jonas; Brandt, Oliver; Jahn, Uwe; Pfüller, Carsten; Roder, Claudia; Dogan, Pinar; Grosse, Frank; Belabbes, Abderrezak; Bechstedt, Friedhelm; Trampert, Achim; Geelhaar, Lutz

    2012-08-01

    We present a universal approach for determining the spontaneous polarization Psp of a wurtzite semiconductor from the emission energies of excitons bound to the different types of stacking faults in these crystals. Employing microphotoluminescence and cathodoluminescence spectroscopy, we observe emission lines from the intrinsic and extrinsic stacking faults in strain-free GaN microcrystals. By treating the polarization sheet charges associated with these stacking faults as a plate capacitor, Psp can be obtained from the observed transition energies with no additional assumptions. Self-consistent Poisson-Schrödinger calculations, aided by the microscopic electrostatic potential computed using density-functional theory, lead to nearly identical values for Psp. Our recommended value for Psp of GaN is -0.022±0.007 C/m2.

  8. Spectroscopic study of semipolar ( 11 2 ¯ 2 ) -HVPE GaN exhibiting high oxygen incorporation

    NASA Astrophysics Data System (ADS)

    Schustek, Philipp; Hocker, Matthias; Klein, Martin; Simon, Ulrich; Scholz, Ferdinand; Thonke, Klaus

    2014-10-01

    Spatially resolved luminescence and Raman spectroscopy investigations are applied to a series of ( 11 2 ¯ 2 ) -GaN samples grown by hydride vapor phase epitaxy (HVPE) grown over an initial layer deposited by metal organic vapor phase epitaxy on patterned sapphire substrates. Whereas these two differently grown GaN layers are crystallographically homogeneous, they differ largely in their doping level due to high unintentional oxygen uptake in the HVPE layer. This high doping shows up in luminescence spectra, which can be explained by a free-electron recombination band for which an analytical model considering the Burstein-Moss shift, conduction band tailing, and the bandgap renormalization is included. Secondary ion mass spectrometry, Raman spectroscopy, and Hall measurements concordantly determine the electron density to be above 1019 cm-3. In addition, the strain state is assessed by Raman spectroscopy and compared to a finite element analysis.

  9. Strain modulating half-metallicity of semifluorinated GaN nanosheets

    NASA Astrophysics Data System (ADS)

    Xiao, Meixia; Ao, Zhimin; Xu, Tianhan; He, Cheng; Song, Haiyang; Wang, Lei

    2016-06-01

    Strain-dependent half-metallicity of two-bilayer GaN nanosheets (NSs) with fluorinated Ga atoms is studied using density-functional theory. Our results demonstrate that the band gaps in spin-up states and half-metallic gaps vary with biaxial strain and uniaxial compressive strain along the zigzag direction, while the metallic behaviors in spin-down states remain regardless of strain. However, biaxial strain has a better effect on the half-metallicity. Semifluorinated GaN NSs may undergo a structural phase transition from wurtzite to graphite-like phase at high biaxial tension. Therefore, biaxial strain tuning half-metallicity efficiently could provide a viable route to GaN-based spintronic nanodevices.

  10. Elastic constants of GaN between 10 and 305 K

    NASA Astrophysics Data System (ADS)

    Adachi, K.; Ogi, H.; Nagakubo, A.; Nakamura, N.; Hirao, M.; Imade, M.; Yoshimura, M.; Mori, Y.

    2016-06-01

    Using the antenna-transmission resonant ultrasound spectroscopy, we measured the elastic constants of GaN between 10 and 305 K using 72 resonance frequencies. The mode Grüneisen parameter is determined from temperature dependence of each elastic constant, which is larger along the c axis than along the a axis, showing anisotropy in lattice anharmonicity. The zero-temperature elastic constants, determined using the Einstein-oscillator model, yield the Debye characteristic temperature of 636 K. The ab-initio calculation is carried out for deducing the elastic constants, and comparison between calculations and measurements at 0 K reveals that the local-density-approximation potential is preferable for theoretically evaluating characteristics of GaN. The theoretical calculation also supports the anisotropy in lattice anharmonicity.

  11. Vacancy complexes induce long-range ferromagnetism in GaN

    SciTech Connect

    Zhang, Zhenkui; Schwingenschlögl, Udo E-mail: Iman.Roqan@kaust.edu.sa; Roqan, Iman S. E-mail: Iman.Roqan@kaust.edu.sa

    2014-11-14

    By means of density functional theory, we argue that ferromagnetism in GaN can be induced by vacancy complexes. Spin polarization originates from the charge compensation between neutral N and Ga vacancies. Defect formation energy calculations predict that a vacancy complex of two positively charged N vacancies and one doubly negative Ga vacancy is likely to form. This defect complex induces a net moment of 1 μ{sub B}, which is localized around the negative Ga center and exhibits pronounced in-plane ferromagnetic coupling. In contrast to simple Ga vacancy induced ferromagnetism, the proposed picture is in line with the fact that N vacancies have a low formation energy. Formation energies indicate mutual stabilization of the intrinsic defects in GaN.

  12. Spectroscopic study of semipolar (112{sup ¯}2)-HVPE GaN exhibiting high oxygen incorporation

    SciTech Connect

    Schustek, Philipp; Hocker, Matthias; Thonke, Klaus; Klein, Martin; Scholz, Ferdinand; Simon, Ulrich

    2014-10-28

    Spatially resolved luminescence and Raman spectroscopy investigations are applied to a series of (112{sup ¯}2)-GaN samples grown by hydride vapor phase epitaxy (HVPE) grown over an initial layer deposited by metal organic vapor phase epitaxy on patterned sapphire substrates. Whereas these two differently grown GaN layers are crystallographically homogeneous, they differ largely in their doping level due to high unintentional oxygen uptake in the HVPE layer. This high doping shows up in luminescence spectra, which can be explained by a free-electron recombination band for which an analytical model considering the Burstein-Moss shift, conduction band tailing, and the bandgap renormalization is included. Secondary ion mass spectrometry, Raman spectroscopy, and Hall measurements concordantly determine the electron density to be above 10{sup 19 }cm{sup −3}. In addition, the strain state is assessed by Raman spectroscopy and compared to a finite element analysis.

  13. 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.

  14. Photoreflectance study on the surface states of n-type GaN

    NASA Astrophysics Data System (ADS)

    Liu, W.; Li, M. F.; Chua, S. J.; Akutsu, N.; Matsumoto, K.

    1999-05-01

    Photoreflectance spectroscopy was used to study n-type GaN epilayers with doping concentrations ranging from 0268-1242/14/5/004/img8 to 0268-1242/14/5/004/img9. The relative change of reflectance signal 0268-1242/14/5/004/img10 was investigated as a function of doping concentration 0268-1242/14/5/004/img11. The value of 0268-1242/14/5/004/img12 was found to be proportional to 0268-1242/14/5/004/img13 with a slope of 0268-1242/14/5/004/img14. This result can be explained by a simple model based on an assumption that low-density surface states exist in the surface of GaN. The result may clarify certain issues in the existing Schottky barrier experiments.

  15. Nonpolar GaN grown on Si by hydride vapor phase epitaxy using anodized Al nanomask

    NASA Astrophysics Data System (ADS)

    Polyakov, A. Y.; Markov, A. V.; Mezhennyi, M. V.; Govorkov, A. V.; Pavlov, V. F.; Smirnov, N. B.; Donskov, A. A.; D'yakonov, L. I.; Kozlova, Y. P.; Malakhov, S. S.; Yugova, T. G.; Osinsky, V. I.; Gorokh, G. G.; Lyahova, N. N.; Mityukhlyaev, V. B.; Pearton, S. J.

    2009-01-01

    GaN growth by the hydride vapor phase technique on (100) Si substrates masked by porous Al anodic oxide is described. The masks were prepared by vacuum deposition of Al with subsequent anodic oxidation in dilute sorrel acid. The grown GaN layer is nonpolar, with (112¯0) a-orientation and a full width at half maximum of the (112¯0) reflection below 500 arc sec and showing small anisotropy. This result is comparable with the results obtained for a-GaN growth using selective epitaxy or advanced buffer growth routines. Microcathodoluminescence spectra of the grown films confirm a low density of stacking faults. Possible growth mechanisms are discussed.

  16. Dislocated interests and climate change

    NASA Astrophysics Data System (ADS)

    Davis, Steven J.; Diffenbaugh, Noah

    2016-06-01

    The predicted effects of climate change on surface temperatures are now emergent and quantifiable. The recent letter by Hansen and Sato (2016 Environ. Res. Lett. 11 034009) adds to a growing number of studies showing that warming over the past four decades has shifted the distribution of temperatures higher almost everywhere, with the largest relative effects on summer temperatures in developing regions such as Africa, South America, southeast Asia, and the Middle East (e.g., Diffenbaugh and Scherer 2011 Clim. Change 107 615-24 Anderson 2011 Clim. Change 108 581; Mahlstein et al 2012 Geophys. Res. Lett. 39 L21711). Hansen and Sato emphasize that although these regions are warming disproportionately, their role in causing climate change—measured by cumulative historical CO2 emissions produced—is small compared to the US and Europe, where the relative change in temperatures has been less. This spatial and temporal mismatch of climate change impacts and the burning of fossil fuels is a critical dislocation of interests that, as the authors note, has ‘substantial implications for global energy and climate policies.’ Here, we place Hansen and Sato’s ‘national responsibilities’ into a broader conceptual framework of problematically dislocated interests, and briefly discuss the related challenges for global climate mitigation efforts.

  17. Dislocated interests and climate change

    NASA Astrophysics Data System (ADS)

    Davis, Steven J.; Diffenbaugh, Noah

    2016-06-01

    The predicted effects of climate change on surface temperatures are now emergent and quantifiable. The recent letter by Hansen and Sato (2016 Environ. Res. Lett. 11 034009) adds to a growing number of studies showing that warming over the past four decades has shifted the distribution of temperatures higher almost everywhere, with the largest relative effects on summer temperatures in developing regions such as Africa, South America, southeast Asia, and the Middle East (e.g., Diffenbaugh and Scherer 2011 Clim. Change 107 615–24 Anderson 2011 Clim. Change 108 581; Mahlstein et al 2012 Geophys. Res. Lett. 39 L21711). Hansen and Sato emphasize that although these regions are warming disproportionately, their role in causing climate change—measured by cumulative historical CO2 emissions produced—is small compared to the US and Europe, where the relative change in temperatures has been less. This spatial and temporal mismatch of climate change impacts and the burning of fossil fuels is a critical dislocation of interests that, as the authors note, has ‘substantial implications for global energy and climate policies.’ Here, we place Hansen and Sato’s ‘national responsibilities’ into a broader conceptual framework of problematically dislocated interests, and briefly discuss the related challenges for global climate mitigation efforts.

  18. 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.

  19. 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.

  20. Optical properties of ultra-thin (< 30 nm) GaN layers on c-sapphire substrates with different initial growth conditions measured by surface-plasmon enhanced Raman scattering.

    PubMed

    Kim, Ho-Jong; Kim, Tae-Soo; Lee, Jin-Gyu; Song, Jung Hoon

    2014-11-01

    We have carried out surface-plasmon enhanced Raman spectroscopy (SERS) on 30 nm-thick GaN samples grown at various temperatures, in order to investigate the properties of ultra thin GaN films on sapphire. We found that the properties, such as the strain and the free-carrier density of the thin layers, were sensitively affected by the growth temperatures. Our results show that SERS, by selectively enhancing the Raman signal near the surface, can be a very useful technique to investigate the optical properties of ultra-thin GaN films and their initial growth mode.