Energetics of cubic and hexagonal phases in Mn-doped GaN : First-principles pseudopotential calculations

NASA Astrophysics Data System (ADS)

Choi, Eun-Ae; Kang, Joongoo; Chang, K. J.

2006-12-01

We perform first-principles pseudopotential calculations to study the influence of Mn doping on the stability of two polytypes, wurtzite and zinc-blende, in GaN . In Mn δ -doped GaN and GaMnN alloys, we find similar critical concentrations of the Mn ions for stabilizing the zinc-blende phase against the wurtzite phase. Using a slab geometry of hexagonal lattices, we find that it is energetically unfavorable to form inversion domains with Mn exposure, in contrast to Mg doping. At the initial stage of epitaxial growth, a stacking fault that leads to the cubic bonds can be generated with the Mn exposure to the Ga-polar surface. However, the influence of the Mn δ -doped layer on the formation of the cubic phase is only effective for GaN layers deposited up to two monolayers. We find that the Mn ions are energetically more stable on the growth front than in the bulk, indicating that these ions act as a surfactant. Thus it is possible to grow cubic GaN if the Mn ions are periodically supplied or diffuse out from the Mn δ -doped layer to the growth front during the growth process.

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.

Two-dimensional X-ray diffraction and transmission electron microscopy study on the effect of magnetron sputtering atmosphere on GaN/SiC interface and gallium nitride thin film crystal structure

NASA Astrophysics Data System (ADS)

Shen, Huaxiang; Zhu, Guo-Zhen; Botton, Gianluigi A.; Kitai, Adrian

2015-03-01

The growth mechanisms of high quality GaN thin films on 6H-SiC by sputtering were investigated by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The XRD θ-2θ scans show that high quality ( 0002 ) oriented GaN was deposited on 6H-SiC by reactive magnetron sputtering. Pole figures obtained by 2D-XRD clarify that GaN thin films are dominated by ( 0002 ) oriented wurtzite GaN and { 111 } oriented zinc-blende GaN. A thin amorphous silicon oxide layer on SiC surfaces observed by STEM plays a critical role in terms of the orientation information transfer from the substrate to the GaN epilayer. The addition of H2 into Ar and/or N2 during sputtering can reduce the thickness of the amorphous layer. Moreover, adding 5% H2 into Ar can facilitate a phase transformation from amorphous to crystalline in the silicon oxide layer and eliminate the unwanted { 3 3 ¯ 02 } orientation in the GaN thin film. Fiber texture GaN thin films can be grown by adding 10% H2 into N2 due to the complex reaction between H2 and N2.

Selected Growth of Cubic and Hexagonal GaN Epitaxial Films on Polar MgO(111)

NASA Astrophysics Data System (ADS)

Lazarov, V. K.; Zimmerman, J.; Cheung, S. H.; Li, L.; Weinert, M.; Gajdardziska-Josifovska, M.

2005-06-01

Selected molecular beam epitaxy of zinc blende (111) or wurtzite (0001) GaN films on polar MgO(111) is achieved depending on whether N or Ga is deposited first. The cubic stacking is enabled by nitrogen-induced polar surface stabilization, which yields a metallic MgO(111)-(1×1)-ON surface. High-resolution transmission electron microscopy and density functional theory studies indicate that the atomically abrupt semiconducting GaN(111)/MgO(111) interface has a Mg-O-N-Ga stacking, where the N atom is bonded to O at a top site. This specific atomic arrangement at the interface allows the cubic stacking to more effectively screen the substrate and film electric dipole moment than the hexagonal stacking, thus stabilizing the zinc blende phase even though the wurtzite phase is the ground state in the bulk.

Biosensing operations based on whispering-gallery-mode optical cavities in single 1.0-µm diameter hexagonal GaN microdisks grown by radio-frequency plasma-assisted molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Kouno, Tetsuya; Sakai, Masaru; Kishino, Katsumi; Hara, Kazuhiko

2016-05-01

Biosensing operations based on a whispering-gallery-mode optical cavity in a single hexagonal GaN microdisk of approximately 1.0 µm diameter were demonstrated here. The sharp resonant peak in the photoluminescence spectrum obtained from the microdisk in aqueous sucrose solution redshifts with a change in sucrose concentration. The results indicate that an extremely small microdisk could be used as an optical transducer for sensing sugar, namely, as a biosensor. Furthermore, we investigate the relationship between the diameter of the microdisk and the sensitivity of the biosensor.

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

Wu, Ruikang; Hu, Run, E-mail: hurun@hust.edu.cn, E-mail: luoxb@hust.edu.cn; Luo, Xiaobing, E-mail: hurun@hust.edu.cn, E-mail: luoxb@hust.edu.cn

In this study, we developed a first-principle-based full-dispersion Monte Carlo simulation method to study the anisotropic phonon transport in wurtzite GaN thin film. The input data of thermal properties in MC simulations were calculated based on the first-principle method. The anisotropy of thermal conductivity in bulk wurtzite GaN is found to be strengthened by isotopic scatterings and reduced temperature, and the anisotropy reaches 40.08% for natural bulk GaN at 100 K. With the GaN thin film thickness decreasing, the anisotropy of the out-of-plane thermal conductivity is heavily reduced due to both the ballistic transport and the less importance of the low-frequencymore » phonons with anisotropic group velocities. On the contrary, it is observed that the in-plane thermal conductivity anisotropy of the GaN thin film is strengthened by reducing the film thickness. And the anisotropy reaches 35.63% when the natural GaN thin film thickness reduces to 50 nm at 300 K with the degree of specularity being zero. The anisotropy is also improved by increasing the surface roughness of the GaN thin film.« less

Group III nitride semiconductors for short wavelength light-emitting devices

NASA Astrophysics Data System (ADS)

Orton, J. W.; Foxon, C. T.

1998-01-01

The group III nitrides (AlN, GaN and InN) represent an important trio of semiconductors because of their direct band gaps which span the range 1.95-6.2 eV, including the whole of the visible region and extending well out into the ultraviolet (UV) range. They form a complete series of ternary alloys which, in principle, makes available any band gap within this range and the fact that they also generate efficient luminescence has been the main driving force for their recent technological development. High brightness visible light-emitting diodes (LEDs) are now commercially available, a development which has transformed the market for LED-based full colour displays and which has opened the way to many other applications, such as in traffic lights and efficient low voltage, flat panel white light sources. Continuously operating UV laser diodes have also been demonstrated in the laboratory, exciting tremendous interest for high-density optical storage systems, UV lithography and projection displays. In a remarkably short space of time, the nitrides have therefore caught up with and, in some ways, surpassed the wide band gap II-VI compounds (ZnCdSSe) as materials for short wavelength optoelectronic devices. The purpose of this paper is to review these developments and to provide essential background material in the form of the structural, electronic and optical properties of the nitrides, relevant to these applications. We have been guided by the fact that the devices so far available are based on the binary compound GaN (which is relatively well developed at the present time), together with the ternary alloys AlGaN and InGaN, containing modest amounts of Al or In. We therefore concentrate, to a considerable extent, on the properties of GaN, then introduce those of the alloys as appropriate, emphasizing their use in the formation of the heterostructures employed in devices. The nitrides crystallize preferentially in the hexagonal wurtzite structure and devices have so far been based on this material so the majority of our paper is concerned with it, however, the cubic, zinc blende form is known for all three compounds, and cubic GaN has been the subject of sufficient work to merit a brief account in its own right. There is significant interest based on possible technological advantages, such as easier doping, easier cleaving (for laser facets) and easier contacting. It also appears, at present, that the cubic form gives higher electron and hole mobilities than the hexagonal form. The dominant hexagonal structure is similar to that found in a number of II-VI compounds such as CdS and they can therefore be taken as role models. In particular, the lower symmetry gives rise to three separate valence bands at the zone centre and exciton spectra associated with each of these have been reported by many workers for GaN. Interpretation is complicated by the presence of strain in many samples due to the fact that most material consists of epitaxial thin films grown on non-lattice-matched substrates (bulk GaN crystals not being widely available). However, much progress has been made in understanding the physics of these films and we discuss the current position with regard to band gaps, effective masses, exciton binding energies, phonon energies, dielectric constants, etc. Apart from a lack of knowledge of the anticipated valence band anisotropy, it can be said that GaN is now rather well documented. Less detail is available for AlN or InN and we make no attempt to provide similar data for them. The structure of the paper is based on a historical introduction, followed by a brief account of the various crystal growth methods used to produce bulk GaN and epitaxial films of GaN and the ternary alloys. This is then followed by an account of the structural properties of hexagonal GaN as measured by x-ray diffraction and electron microscopy, phonon properties from infrared and Raman spectroscopy, electrical properties, with emphasis on n- and p-type doping, and optical properties, measured mainly by photoluminescence. A brief comparative account of cubic GaN properties follows. Discussion of alloy properties in the context of their use in quantum well and superlattice structures forms an introduction to the device sections which close the paper. These include details of the technology necessary for etching, contacting and forming laser facets, as an introduction to recent results on LEDs and laser diodes. Having described the current position, we speculate briefly on likely future developments.

Growth of GaN micro/nanolaser arrays by chemical vapor deposition.

PubMed

Liu, Haitao; Zhang, Hanlu; Dong, Lin; Zhang, Yingjiu; Pan, Caofeng

2016-09-02

Optically pumped ultraviolet lasing at room temperature based on GaN microwire arrays with Fabry-Perot cavities is demonstrated. GaN microwires have been grown perpendicularly on c-GaN/sapphire substrates through simple catalyst-free chemical vapor deposition. The GaN microwires are [0001] oriented single-crystal structures with hexagonal cross sections, each with a diameter of ∼1 μm and a length of ∼15 μm. A possible growth mechanism of the vertical GaN microwire arrays is proposed. Furthermore, we report room-temperature lasing in optically pumped GaN microwire arrays based on the Fabry-Perot cavity. Photoluminescence spectra exhibit lasing typically at 372 nm with an excitation threshold of 410 kW cm(-2). The result indicates that these aligned GaN microwire arrays may offer promising prospects for ultraviolet-emitting micro/nanodevices.

GaN thin films growth and their application in photocatalytic removal of sulforhodamine B from aqueous solution under UV pulsed laser irradiation.

PubMed

Gondal, Mohammed A; Chang, Xiao F; Yamani, Zain H; Yang, Guo F; Ji, Guang B

2011-01-01

Single-crystalline Gallium Nitride (GaN) thin films were fabricated and grown by metal organic chemical vapor deposition (MOCVD) method on c-plane sapphire substrates and then characterized by high resolution-X-ray diffraction (HR-XRD) and photoluminescence (PL) measurements. The photocatalytic decomposition of Sulforhodamine B (SRB) molecules on GaN thin films was investigated under 355 nm pulsed UV laser irradiation. The results demonstrate that as-grown GaN thin films exhibited efficient degradation of SRB molecules and exhibited an excellent photocatalytic-activity-stability under UV pulsed laser exposure.

Lattice distortions in GaN on sapphire using the CBED-HOLZ technique.

PubMed

Sridhara Rao, D V; McLaughlin, K; Kappers, M J; Humphreys, C J

2009-09-01

The convergent beam electron diffraction (CBED) methodology was developed to investigate the lattice distortions in wurtzite gallium nitride (GaN) from a single zone-axis pattern. The methodology enabled quantitative measurements of lattice distortions (alpha, beta, gamma and c) in transmission electron microscope (TEM) specimens of a GaN film grown on (0,0,0,1) sapphire by metal-organic vapour-phase epitaxy. The CBED patterns were obtained at different distances from the GaN/sapphire interface. The results show that GaN is triclinic above the interface with an increased lattice parameter c. At 0.85 microm from the interface, alpha=90 degrees , beta=8905 degrees and gamma=11966 degrees . The GaN lattice relaxes steadily back to hexagonal further away from the sapphire substrate. The GaN distortions are mainly confined to the initial stages of growth involving the growth and the coalescence of 3D GaN islands.

Synthesis and excellent field emission properties of three-dimensional branched GaN nanowire homostructures

NASA Astrophysics Data System (ADS)

Li, Enling; Sun, Lihe; Cui, Zhen; Ma, Deming; Shi, Wei; Wang, Xiaolin

2016-10-01

Three-dimensional branched GaN nanowire homostructures have been synthesized on the Si substrate via a two-step approach by chemical vapor deposition. Structural characterization reveals that the single crystal GaN nanowire trunks have hexagonal wurtzite characteristics and grow along the [0001] direction, while the homoepitaxial single crystal branches grow in a radial direction from the six-sided surfaces of the trunks. The field emission measurements demonstrate that the branched GaN nanowire homostructures have excellent field emission properties, with low turn-on field at 2.35 V/μm, a high field enhancement factor of 2938, and long emission current stability. This indicates that the present branched GaN nanowire homostructures will become valuable for practical field emission applications.

Bright Room-Temperature Single-Photon Emission from Defects in Gallium Nitride.

PubMed

Berhane, Amanuel M; Jeong, Kwang-Yong; Bodrog, Zoltán; Fiedler, Saskia; Schröder, Tim; Triviño, Noelia Vico; Palacios, Tomás; Gali, Adam; Toth, Milos; Englund, Dirk; Aharonovich, Igor

2017-03-01

Room-temperature quantum emitters in gallium nitride (GaN) are reported. The emitters originate from cubic inclusions in hexagonal lattice and exhibit narrowband luminescence in the red spectral range. The sources are found in different GaN substrates, and therefore are promising for scalable quantum technologies. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synthesis of GaN by high-pressure ammonolysis of gallium triiodide

NASA Astrophysics Data System (ADS)

Purdy, Andrew P.; Case, Sean; Muratore, Nicole

2003-05-01

The ammonothermal conversion of GaI 3 to both cubic (zinc-blende) and hexagonal GaN was explored in detail. Gallium triiodide, anhydrous NH 3, and in some cases CuI or LiI co-mineralizers, were sealed in quartz tubes and heated in a pressurized autoclave from 300°C to 515°C. At hot-zone temperatures above 430°C, a deposit of mostly c-GaN collects in the upper portion of the tube, and deposits of phase-pure c-GaN were reliably produced on a 50-60 mg scale when CuI co-mineralizer was added. Crystal morphologies of these microcrystalline c-GaN products are highly dependent on growth conditions and range from triangular prisms to triangular plates, dendritic crystals, and irregular particles. Hexagonal GaN products were either in the form of microrods or micron sized prisms. Nanorods, of presumably h-GaN, also formed in some reactions in low yields, intermixed with microcrystalline c-GaN products.

A Rapid Method for Deposition of Sn-Doped GaN Thin Films on Glass and Polyethylene Terephthalate Substrates

NASA Astrophysics Data System (ADS)

Pat, Suat; Özen, Soner; Korkmaz, Şadan

2018-01-01

We report the influence of Sn doping on microstructure, surface, and optical properties of GaN thin films deposited on glass and polyethylene terephthalate (PET) substrate. Sn-doped GaN thin films have been deposited by thermionic vacuum arc (TVA) at low temperature. TVA is a rapid deposition technology for thin film growth. Surface and optical properties of the thin films were presented. Grain size, height distribution, roughness values were determined. Grain sizes were calculated as 20 nm and 13 nm for glass and PET substrates, respectively. Nano crystalline forms were shown by field emission scanning electron microscopy. Optical band gap values were determined by optical methods and photoluminescence measurement. The optical band gap values of Sn doped GaN on glass and PET were determined to be approximately ˜3.40 eV and ˜3.47 eV, respectively. As a result, TVA is a rapid and low temperature deposition technology for the Sn doped GaN deposited on glass and PET substrate.

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

Bolat, S., E-mail: bolat@ee.bilkent.edu.tr, E-mail: aokyay@ee.bilkent.edu.tr; Tekcan, B.; UNAM, National Nanotechnology Research Center, Bilkent University, Ankara 06800

We report GaN thin film transistors (TFT) with a thermal budget below 250 °C. GaN thin films are grown at 200 °C by hollow cathode plasma-assisted atomic layer deposition (HCPA-ALD). HCPA-ALD-based GaN thin films are found to have a polycrystalline wurtzite structure with an average crystallite size of 9.3 nm. TFTs with bottom gate configuration are fabricated with HCPA-ALD grown GaN channel layers. Fabricated TFTs exhibit n-type field effect characteristics. N-channel GaN TFTs demonstrated on-to-off ratios (I{sub ON}/I{sub OFF}) of 10{sup 3} and sub-threshold swing of 3.3 V/decade. The entire TFT device fabrication process temperature is below 250 °C, which is the lowest process temperaturemore » reported for GaN based transistors, so far.« less

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.

Ion Beam Assisted Deposition of Thin Epitaxial GaN Films.

PubMed

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

2017-06-23

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

Fabrication and properties of ZnO/GaN heterostructure nanocolumnar thin film on Si (111) substrate

PubMed Central

2013-01-01

Zinc oxide thin films have been obtained on bare and GaN buffer layer decorated Si (111) substrates by pulsed laser deposition (PLD), respectively. GaN buffer layer was achieved by a two-step method. The structure, surface morphology, composition, and optical properties of these thin films were investigated by X-ray diffraction, field emission scanning electron microscopy, infrared absorption spectra, and photoluminiscence (PL) spectra, respectively. Scanning electron microscopy images indicate that the flower-like grains were presented on the surface of ZnO thin films grown on GaN/Si (111) substrate, while the ZnO thin films grown on Si (111) substrate show the morphology of inclination column. PL spectrum reveals that the ultraviolet emission efficiency of ZnO thin film on GaN buffer layer is high, and the defect emission of ZnO thin film derived from Zni and Vo is low. The results demonstrate that the existence of GaN buffer layer can greatly improve the ZnO thin film on the Si (111) substrate by PLD techniques. PMID:23448090

Fabrication and properties of ZnO/GaN heterostructure nanocolumnar thin film on Si (111) substrate.

PubMed

Wei, Xianqi; Zhao, Ranran; Shao, Minghui; Xu, Xijin; Huang, Jinzhao

2013-02-28

Zinc oxide thin films have been obtained on bare and GaN buffer layer decorated Si (111) substrates by pulsed laser deposition (PLD), respectively. GaN buffer layer was achieved by a two-step method. The structure, surface morphology, composition, and optical properties of these thin films were investigated by X-ray diffraction, field emission scanning electron microscopy, infrared absorption spectra, and photoluminiscence (PL) spectra, respectively. Scanning electron microscopy images indicate that the flower-like grains were presented on the surface of ZnO thin films grown on GaN/Si (111) substrate, while the ZnO thin films grown on Si (111) substrate show the morphology of inclination column. PL spectrum reveals that the ultraviolet emission efficiency of ZnO thin film on GaN buffer layer is high, and the defect emission of ZnO thin film derived from Zni and Vo is low. The results demonstrate that the existence of GaN buffer layer can greatly improve the ZnO thin film on the Si (111) substrate by PLD techniques.

Thermoelectric properties of the 3C, 2H, 4H, and 6H polytypes of the wide-band-gap semiconductors SiC, GaN, and ZnO

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

Huang, Zheng; Lü, Tie-Yu; Wang, Hui-Qiong

We have investigated the thermoelectric properties of the 3C, 2H, 4H, and 6H polytypes of the wide-band-gap(n-type) semiconductors SiC, GaN, and ZnO based on first-principles calculations and Boltzmann transport theory. Our results show that the thermoelectric performance increases from 3C to 6H, 4H, and 2H structures with an increase of hexagonality for SiC. However, for GaN and ZnO, their power factors show a very weak dependence on the polytype. Detailed analysis of the thermoelectric properties with respect to temperature and carrier concentration of 4H-SiC, 2H-GaN, and 2H-ZnO shows that the figure of merit of these three compounds increases with temperature,more » indicating the promising potential applications of these thermoelectric materials at high temperature. The significant difference of the polytype-dependent thermoelectric properties among SiC, GaN, and ZnO might be related to the competition between covalency and ionicity in these semiconductors. Our calculations may provide a new way to enhance the thermoelectric properties of wide-band-gap semiconductors through atomic structure design, especially hexagonality design for SiC.« less

Coupling of phonons with excitons bound to different donors and acceptors in hexagonal GaN

NASA Astrophysics Data System (ADS)

Korona, K. P.; Wysmoek, A.; Kuhl, J.; Kamiska, M.; Baranowski, J. M.; Look, D. C.; Park, S. S.

2006-06-01

Time-resolved measurements of GaN with different donors (oxygen or silicon) and acceptors (zinc or magnesium) showed pronounced bound exciton lines and their phonon replicas. The analysis included three phonon modes characteristic for the wurtzite (hexagonal) phase: A1(LO), E1(TO) and E2H. It was shown that relative amplitudes of replicas depended upon the chemical nature of the defects that the bind excitons. The replicas were stronger for acceptor- than for donor-related features. Huang-Rhys factors S = 0.06 +/- 0.02 and S = 0.025 +/- 0.01, were found for the A0X and the D0X LO replicas, respectively. A significant difference in phonon coupling to silicon and oxygen donor bound excitons has been observed.

Hexagonal Nanopyramidal Prisms of Nearly Intrinsic InN on Patterned GaN Nanowire Arrays

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

Golam Sarwar, A. T. M.; Leung, Benjamin; Wang, George T.

By using multiple growth steps that separate the nucleation and growth processes, we show that nearly intrinsic InN single nanocrystals of high optical quality can be formed on patterned GaN nanowire arrays by molecular beam epitaxy. The InN nanostructures form into well-defined hexagonal prisms with pyramidal tops. Micro-photoluminescence (μ-PL) is carried out at low temperature (LT: 28.2 K) and room temperature (RT: 285 K) to gauge the relative material quality of the InN nanostructures. Nanopyramidal prisms grown using a three-step growth method are found to show superior quantum efficiency. In conclusion, excitation and temperature dependent μ-PL demonstrates the very highmore » quality and nearly intrinsic nature of the ordered InN nanostructure arrays.« less

Hexagonal Nanopyramidal Prisms of Nearly Intrinsic InN on Patterned GaN Nanowire Arrays

DOE PAGES

Golam Sarwar, A. T. M.; Leung, Benjamin; Wang, George T.; ...

2018-01-04

By using multiple growth steps that separate the nucleation and growth processes, we show that nearly intrinsic InN single nanocrystals of high optical quality can be formed on patterned GaN nanowire arrays by molecular beam epitaxy. The InN nanostructures form into well-defined hexagonal prisms with pyramidal tops. Micro-photoluminescence (μ-PL) is carried out at low temperature (LT: 28.2 K) and room temperature (RT: 285 K) to gauge the relative material quality of the InN nanostructures. Nanopyramidal prisms grown using a three-step growth method are found to show superior quantum efficiency. In conclusion, excitation and temperature dependent μ-PL demonstrates the very highmore » quality and nearly intrinsic nature of the ordered InN nanostructure arrays.« less

The effects of GaN nanocolumn arrays and thin SixNy buffer layers on the morphology of GaN layers grown by plasma-assisted molecular beam epitaxy on Si(111) substrates

NASA Astrophysics Data System (ADS)

Shubina, K. Yu; Pirogov, E. V.; Mizerov, A. M.; Nikitina, E. V.; Bouravleuv, A. D.

2018-03-01

The effects of GaN nanocolumn arrays and a thin SixNy layer, used as buffer layers, on the morphology of GaN epitaxial layers are investigated. Two types of samples with different buffer layers were synthesized by PA-MBE. The morphology of the samples was characterized by SEM. The crystalline quality of the samples was assessed by XRD. The possibility of synthesis of continuous crystalline GaN layers on Si(111) substrates without the addition of other materials such as aluminum nitride was demonstrated.

Novel activation process for Mg-implanted GaN

NASA Astrophysics Data System (ADS)

Hashimoto, Shin; Nakamura, Takao; Honda, Yoshio; Amano, Hiroshi

2014-02-01

A novel activation process for Mg-implanted GaN was demonstrated. As opposed to the conventional thermal annealing process, an H2/NH3 alternating supply annealing process achieved better optical activation, stronger near-ultraviolet luminescence and weaker yellow luminescence in the photoluminescence spectroscopy. After this process, small hexagonal hillocks were observed on the surface, which indicated that crystal regrowth was induced by this process, consisting of decomposition of GaN by H2 supplies and re-crystallization by NH3 supplies. It was revealed that the implanted Mg could easily be located at the activation site by means of crystal regrowth by this process.

Understanding the Growth Mechanism of GaN Epitaxial Layers on Mechanically Exfoliated Graphite

NASA Astrophysics Data System (ADS)

Li, Tianbao; Liu, Chenyang; Zhang, Zhe; Yu, Bin; Dong, Hailiang; Jia, Wei; Jia, Zhigang; Yu, Chunyan; Gan, Lin; Xu, Bingshe; Jiang, Haiwei

2018-04-01

The growth mechanism of GaN epitaxial layers on mechanically exfoliated graphite is explained in detail based on classic nucleation theory. The number of defects on the graphite surface can be increased via O-plasma treatment, leading to increased nucleation density on the graphite surface. The addition of elemental Al can effectively improve the nucleation rate, which can promote the formation of dense nucleation layers and the lateral growth of GaN epitaxial layers. The surface morphologies of the nucleation layers, annealed layers and epitaxial layers were characterized by field-emission scanning electron microscopy, where the evolution of the surface morphology coincided with a 3D-to-2D growth mechanism. High-resolution transmission electron microscopy was used to characterize the microstructure of GaN. Fast Fourier transform diffraction patterns showed that cubic phase (zinc-blend structure) GaN grains were obtained using conventional GaN nucleation layers, while the hexagonal phase (wurtzite structure) GaN films were formed using AlGaN nucleation layers. Our work opens new avenues for using highly oriented pyrolytic graphite as a substrate to fabricate transferable optoelectronic devices.

Understanding the Growth Mechanism of GaN Epitaxial Layers on Mechanically Exfoliated Graphite.

PubMed

Li, Tianbao; Liu, Chenyang; Zhang, Zhe; Yu, Bin; Dong, Hailiang; Jia, Wei; Jia, Zhigang; Yu, Chunyan; Gan, Lin; Xu, Bingshe; Jiang, Haiwei

2018-04-27

The growth mechanism of GaN epitaxial layers on mechanically exfoliated graphite is explained in detail based on classic nucleation theory. The number of defects on the graphite surface can be increased via O-plasma treatment, leading to increased nucleation density on the graphite surface. The addition of elemental Al can effectively improve the nucleation rate, which can promote the formation of dense nucleation layers and the lateral growth of GaN epitaxial layers. The surface morphologies of the nucleation layers, annealed layers and epitaxial layers were characterized by field-emission scanning electron microscopy, where the evolution of the surface morphology coincided with a 3D-to-2D growth mechanism. High-resolution transmission electron microscopy was used to characterize the microstructure of GaN. Fast Fourier transform diffraction patterns showed that cubic phase (zinc-blend structure) GaN grains were obtained using conventional GaN nucleation layers, while the hexagonal phase (wurtzite structure) GaN films were formed using AlGaN nucleation layers. Our work opens new avenues for using highly oriented pyrolytic graphite as a substrate to fabricate transferable optoelectronic devices.

Structural and optical studies of GaN pn-junction with AlN buffer layer grown on Si (111) by RF plasma enhanced MBE

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

Yusoff, Mohd Zaki Mohd; Hassan, Zainuriah; Woei, Chin Che

2012-06-29

GaN pn-junction grown on silicon substrates have been the focus in a number of recent reports and further effort is still necessary to improve its crystalline quality for practical applications. GaN has the high n-type background carrier concentration resulting from native defects commonly thought to be nitrogen vacancies. In this work, we present the growth of pn-junction of GaN on Si (111) substrate using RF plasma-enhanced molecular beam epitaxy (MBE). Both of the layers show uniformity with an average thickness of 0.709 {mu}m and 0.095 {mu}m for GaN and AlN layers, respectively. The XRD spectra indicate that no sign ofmore » cubic phase of GaN are found, so it is confirmed that the sample possessed hexagonal structure. It was found that all the allowed Raman optical phonon modes of GaN, i.e. the E2 (low), E1 (high) and A1 (LO) are clearly visible.« less

Effect of hexagonal hillock on luminescence characteristic of multiple quantum wells structure

NASA Astrophysics Data System (ADS)

Du, Jinjuan; Xu, Shengrui; Li, Peixian; Zhang, Jincheng; Zhao, Ying; Peng, Ruoshi; Fan, Xiaomeng; Hao, Yue

2018-04-01

GaN based ultraviolet multiple quantum well structures grown on a c-plane sapphire substrate by metal organic chemical deposition showed a microstructure with a large amount of huge hexagonal hillocks. The polarity of the sample is confirmed by etching with sodium hydroxide solution. The luminous intensity distribution of a typical hexagonal hillock was investigated by the phototluminescent mapping and the luminous intensity at hillock top regions was found to be 15 times higher than that of the regions around hillocks. The reduction of dislocations, the decreasing of the quantum confirmed stack effect caused by semipolar plane and the inclination of the sidewalls of the hexagonal hillock were responsible for the enhancement of luminous intensity.

Large-roll growth of 25-inch hexagonal BN monolayer film for self-release buffer layer of free-standing GaN wafer

NASA Astrophysics Data System (ADS)

Wu, Chenping; Soomro, Abdul Majid; Sun, Feipeng; Wang, Huachun; Huang, Youyang; Wu, Jiejun; Liu, Chuan; Yang, Xiaodong; Gao, Na; Chen, Xiaohong; Kang, Junyong; Cai, Duanjun

2016-10-01

Hexagonal boron nitride (h-BN) is known as promising 2D material with a wide band-gap (~6 eV). However, the growth size of h-BN film is strongly limited by the size of reaction chamber. Here, we demonstrate the large-roll synthesis of monolayer and controllable sub-monolayer h-BN film on wound Cu foil by low pressure chemical vapor deposition (LPCVD) method. By winding the Cu foil substrate into mainspring shape supported by a multi-prong quartz fork, the reactor size limit could be overcome by extending the substrate area to a continuous 2D curl of plane inward. An extremely large-size monolayer h-BN film has been achieved over 25 inches in a 1.2” tube. The optical band gap of h-BN monolayer was determined to be 6.0 eV. The h-BN film was uniformly transferred onto 2” GaN or 4” Si wafer surfaces as a release buffer layer. By HVPE method, overgrowth of thick GaN wafer over 200 μm has been achieved free of residual strain, which could provide high quality homo-epitaxial substrate.

X-ray probe of GaN thin films grown on InGaN compliant substrates

NASA Astrophysics Data System (ADS)

Xu, Xiaoqing; Li, Yang; Liu, Jianming; Wei, Hongyuan; Liu, Xianglin; Yang, Shaoyan; Wang, Zhanguo; Wang, Huanhua

2013-04-01

GaN thin films grown on InGaN compliant substrates were characterized by several X-ray technologies: X-ray reciprocal space mapping (RSM), grazing incidence X-ray diffraction (GIXRD), and X-ray photoemission spectrum (XPS). Narrow Lorentz broadening and stress free state were observed for GaN grown on InGaN compliant substrate, while mosaic structure and large tensile stress were observed at the presence of residual indium atoms. RSM disclosed the mosaicity, and the GIXRD was conducted to investigate the depth dependences of crystal quality and strain states. XPS depth profile of indium contents indicated that residual indium atoms deteriorated the crystal quality of GaN not only by producing lattice mismatch at the interface of InGaN and GaN but also by diffusing into GaN overlayers. Accordingly, two solutions were proposed to improve the efficiency of self-patterned lateral epitaxial overgrowth method. This research goes a further step in resolving the urgent substrate problem in GaN fabrication.

The Calculation Study of Electronic Properties of Doped RE (Eu, Er and Tm)-GaN using Density Functional Theory

NASA Astrophysics Data System (ADS)

Zaharo, Aflah; Purqon, Acep

2017-07-01

The calculation of the structure and electronic properties of Rare Earth (RE) at the wurtzite Gallium Nitride (GaN) based on DFT has completed. GGA approximation used for exchange correlation and Ultra soft pseudo potential too. The stability structure of GaN is seen that difference lattice parameter 11% lower than another calculation and experiment result. It is shown the stability structure GaN have direct band gap energy on Gamma point hexagonal lattice Brillouin zone. The width Eg is 2.6 eV. When one atom Ga is substituted with one atom RE, the bond length is change 12 % longest. An in good agreement with theoretical doping RE concentration increases, the edge of energy level shifted towards to make the band gap narrow which is allow the optical transitions and help to improve the optical performance of GaN. The RE doped GaN is potentially applicable for various color of LED with lower energy consumption and potentially energy saving application

Effects of strain and thickness on the electronic and optical behaviors of two-dimensional hexagonal gallium nitride

NASA Astrophysics Data System (ADS)

Behzad, Somayeh

2017-06-01

The full potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory has been used to study effects of strain and thickness on the electronic and optical properties of two-dimensional GaN. The band gap of monolayer and bilayer GaN under compressive in-plane strain change from indirect to direct with bond length shortening. Also, the semiconductor to semimetal transition occurs for monolayer and bilayer GaN under in-plane tensile strain with bond length elongation. It is found that the tensile and compressive strains cause the red and blue shifts in the optical spectra, respectively, for both monolayer and bilayer GaN. Applying the perpendicular strain on the bilayer GaN by decreasing the inter layer distance leads to the shift of valence band maximum towards the Γ point in the band structure and shift of peak positions and variation of peak intensities in ε2(ω) spectrum. The results show that the n-layer GaN has an indirect band gap for n < 16. The results suggest that monolayer and multilayer GaN are good candidates for application in optoelectronics and flexible electronics.

Gallium nitride microcavities formed by photoenhanced wet oxidation

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

Peng, L.-H.; Lu, C.-Y.; Wu, W.-H.

We report the formation of gallium nitride (GaN) microcavities by manipulating a photoenhanced oxidation rate difference between the polar and nonpolar crystallographic planes of GaN. When immersed in a buffered acetic (CH{sub 3}COOH) electrolyte of pH{approx}6.2 at room temperature, it is shown that the photo-oxidation can proceed at a rate that is one order of magnitude slower on the nonpolar plane of {l_brace}1100{r_brace}{sub GaN} than on the polar plane of {l_brace}0001{r_brace}{sub GaN} due to the reduced surface field action. Gallium nitride microcavities bounded by optically smooth {l_brace}1100{r_brace} and {l_brace}1103{r_brace} facets can thus be preferentially formed on the c-plane sapphire substratemore » after dissolving the oxide layer. The optical properties of these GaN hexagonal cavities reveal characteristic peaks of whispering gallery modes in resonance with the GaN band edge emission spectrum. A typical cavity Q factor of 10{sup 3} is observed in these GaN microcavities due to a reduced optical scattering loss in the wet chemical reaction process.« less

Phase transformation of molecular beam epitaxy-grown nanometer-thick Gd₂O₃ and Y₂O₃ on GaN.

PubMed

Chang, Wen-Hsin; Wu, Shao-Yun; Lee, Chih-Hsun; Lai, Te-Yang; Lee, Yi-Jun; Chang, Pen; Hsu, Chia-Hung; Huang, Tsung-Shiew; Kwo, J Raynien; Hong, Minghwei

2013-02-01

High quality nanometer-thick Gd₂O₃ and Y₂O₃ (rare-earth oxide, R₂O₃) films have been epitaxially grown on GaN (0001) substrate by molecular beam epitaxy (MBE). The R₂O₃ epi-layers exhibit remarkable thermal stability at 1100 °C, uniformity, and highly structural perfection. Structural investigation was carried out by in situ reflection high energy electron diffraction (RHEED) and ex-situ X-ray diffraction (XRD) with synchrotron radiation. In the initial stage of epitaxial growth, the R₂O₃ layers have a hexagonal phase with the epitaxial relationship of R₂O₃ (0001)(H)<1120>(H)//GaN(0001)(H)<1120>(H). With the increase in R₂O₃ film thickness, the structure of the R₂O₃ films changes from single domain hexagonal phase to monoclinic phase with six different rotational domains, following the R₂O₃ (201)(M)[020](M)//GaN(0001)(H)<1120>(H) orientational relationship. The structural details and fingerprints of hexagonal and monoclinic phase Gd₂O₃ films have also been examined by using electron energy loss spectroscopy (EELS). Approximate 3-4 nm is the critical thickness for the structural phase transition depending on the composing rare earth element.

Systematic Study on the Self-Assembled Hexagonal Au Voids, Nano-Clusters and Nanoparticles on GaN (0001).

PubMed

Pandey, Puran; Sui, Mao; Li, Ming-Yu; Zhang, Quanzhen; Kim, Eun-Soo; Lee, Jihoon

2015-01-01

Au nano-clusters and nanoparticles (NPs) have been widely utilized in various electronic, optoelectronic, and bio-medical applications due to their great potentials. The size, density and configuration of Au NPs play a vital role in the performance of these devices. In this paper, we present a systematic study on the self-assembled hexagonal Au voids, nano-clusters and NPs fabricated on GaN (0001) by the variation of annealing temperature and deposition amount. At relatively low annealing temperatures between 400 and 600°C, the fabrication of hexagonal shaped Au voids and Au nano-clusters are observed and discussed based on the diffusion limited aggregation model. The size and density of voids and nano-clusters can systematically be controlled. The self-assembled Au NPs are fabricated at comparatively high temperatures from 650 to 800°C based on the Volmer-Weber growth model and also the size and density can be tuned accordingly. The results are symmetrically analyzed and discussed in conjunction with the diffusion theory and thermodynamics by utilizing AFM and SEM images, EDS maps and spectra, FFT power spectra, cross-sectional line-profiles and size and density plots.

Systematic Study on the Self-Assembled Hexagonal Au Voids, Nano-Clusters and Nanoparticles on GaN (0001)

PubMed Central

Pandey, Puran; Sui, Mao; Li, Ming-Yu; Zhang, Quanzhen; Kim, Eun-Soo; Lee, Jihoon

2015-01-01

Au nano-clusters and nanoparticles (NPs) have been widely utilized in various electronic, optoelectronic, and bio-medical applications due to their great potentials. The size, density and configuration of Au NPs play a vital role in the performance of these devices. In this paper, we present a systematic study on the self-assembled hexagonal Au voids, nano-clusters and NPs fabricated on GaN (0001) by the variation of annealing temperature and deposition amount. At relatively low annealing temperatures between 400 and 600°C, the fabrication of hexagonal shaped Au voids and Au nano-clusters are observed and discussed based on the diffusion limited aggregation model. The size and density of voids and nano-clusters can systematically be controlled. The self-assembled Au NPs are fabricated at comparatively high temperatures from 650 to 800°C based on the Volmer-Weber growth model and also the size and density can be tuned accordingly. The results are symmetrically analyzed and discussed in conjunction with the diffusion theory and thermodynamics by utilizing AFM and SEM images, EDS maps and spectra, FFT power spectra, cross-sectional line-profiles and size and density plots. PMID:26285135

Preparation of p-type GaN-doped SnO2 thin films by e-beam evaporation and their applications in p-n junction

NASA Astrophysics Data System (ADS)

Lv, Shuliang; Zhou, Yawei; Xu, Wenwu; Mao, Wenfeng; Wang, Lingtao; Liu, Yong; He, Chunqing

2018-01-01

Various transparent GaN-doped SnO2 thin films were deposited on glass substrates by e-beam evaporation using GaN:SnO2 targets of different GaN weight ratios. It is interesting to find that carrier polarity of the thin films was converted from n-type to p-type with increasing GaN ratio higher than 15 wt.%. The n-p transition in GaN-doped SnO2 thin films was explained for the formation of GaSn and NO with increasing GaN doping level in the films, which was identified by Hall measurement and XPS analysis. A transparent thin film p-n junction was successfully fabricated by depositing p-type GaN:SnO2 thin film on SnO2 thin film, and a low leakage current (6.2 × 10-5 A at -4 V) and a low turn-on voltage of 1.69 V were obtained for the p-n junction.

Luminescence studies of laser MBE grown GaN on ZnO nanostructures

NASA Astrophysics Data System (ADS)

Dewan, Sheetal; Tomar, Monika; Kapoor, Ashok K.; Tandon, R. P.; Gupta, Vinay

2017-08-01

GaN films have been successfully fabricated using Laser Molecular Beam Epitaxy (LMBE) technique on bare c-plane sapphire substrate and ZnO nanostructures (NS) decorated Si (100) substrates. The ZnO nanostructures were grown on Si (100) substrate using high pressure assisted Pulsed laser deposition technique in inert gas ambience. Discrete nanostructured morphology of ZnO was obtained using the PLD growth on Si substrates. Photoluminescence studies performed on the prepared GaN/Sapphire and GaN/ZnO-NS/Si systems, revealed a significant PL enhancement in case of GaN/ZnO-NS/Si system compared to the former. The hexagonal nucleation sites provided by the ZnO nanostructures strategically enhanced the emission of GaN film grown by Laser MBE Technique at relatively lower temperature of 700°C. The obtained results are attractive for the realization of highly luminescent GaN films on Si substrate for photonic devices.

Harmonic surface acoustic waves on gallium nitride thin films.

PubMed

Justice, Joshua; Lee, Kyoungnae; Korakakis, D

2012-08-01

SAW devices operating at the fundamental frequency and the 5th, 7th, 9th, and 11th harmonics have been designed, fabricated, and measured. Devices were fabricated on GaN thin films on sapphire substrates, which were grown via metal organic vapor phase epitaxy (MOVPE). Operating frequencies of 230, 962, 1338, 1720, and 2100 MHz were achieved with devices that had a fundamental wavelength, lambda0 = 20 μm. Gigahertz operation is realized with relatively large interdigital transducers that do not require complicated submicrometer fabrication techniques. SAW devices fabricated on the GaN/sapphire bilayer have an anisotropic propagation when the wavelength is longer than the GaN film thickness. It is shown that for GaN thin films, where kh(GaN) > 10 (k = 2pi/lambda and h(GaN) = GaN film thickness), effects of the substrate on the SAW propagation are eliminated. Bulk mode suppression at harmonic operation is also demonstrated.

Design and simulation of GaN based Schottky betavoltaic nuclear micro-battery.

PubMed

San, Haisheng; Yao, Shulin; Wang, Xiang; Cheng, Zaijun; Chen, Xuyuan

2013-10-01

The current paper presents a theoretical analysis of Ni-63 nuclear micro-battery based on a wide-band gap semiconductor GaN thin-film covered with thin Ni/Au films to form Schottky barrier for carrier separation. The total energy deposition in GaN was calculated using Monte Carlo methods by taking into account the full beta spectral energy, which provided an optimal design on Schottky barrier width. The calculated results show that an 8 μm thick Schottky barrier can collect about 95% of the incident beta particle energy. Considering the actual limitations of current GaN growth technique, a Fe-doped compensation technique by MOCVD method can be used to realize the n-type GaN with a carrier concentration of 1×10(15) cm(-3), by which a GaN based Schottky betavoltaic micro-battery can achieve an energy conversion efficiency of 2.25% based on the theoretical calculations of semiconductor device physics. Copyright © 2013 Elsevier Ltd. All rights reserved.

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

NASA Astrophysics Data System (ADS)

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

2013-06-01

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

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

DOE PAGES

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

2017-09-26

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

Optical properties of C-doped bulk GaN wafers grown by halide vapor phase epitaxy

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

Khromov, S.; Hemmingsson, C.; Monemar, B.

2014-12-14

Freestanding bulk C-doped GaN wafers grown by halide vapor phase epitaxy are studied by optical spectroscopy and electron microscopy. Significant changes of the near band gap (NBG) emission as well as an enhancement of yellow luminescence have been found with increasing C doping from 5 × 10{sup 16} cm{sup −3} to 6 × 10{sup 17} cm{sup −3}. Cathodoluminescence mapping reveals hexagonal domain structures (pits) with high oxygen concentrations formed during the growth. NBG emission within the pits even at high C concentration is dominated by a rather broad line at ∼3.47 eV typical for n-type GaN. In the area without pits,more » quenching of the donor bound exciton (DBE) spectrum at moderate C doping levels of 1–2 × 10{sup 17} cm{sup −3} is observed along with the appearance of two acceptor bound exciton lines typical for Mg-doped GaN. The DBE ionization due to local electric fields in compensated GaN may explain the transformation of the NBG emission.« less

Growth and characterization of GaN nanostructures under various ammoniating time with fabricated Schottky gas sensor based on Si substrate

NASA Astrophysics Data System (ADS)

Abdullah, Q. N.; Ahmed, A. R.; Ali, A. M.; Yam, F. K.; Hassan, Z.; Bououdina, M.; Almessiere, M. A.

2018-05-01

This paper presents the investigation of the influence of the ammoniating time of GaN nanowires (NWs) on the crystalline structure, surface morphology, and optical characteristics. Morphological analysis indicates the growth of good quality and high density of NWs with diameters around 50 nm and lengths up to tens of microns after ammoniating for 30 min. Structural analysis shows that GaN NWs have a typical hexagonal wurtzite crystal structure. Raman spectroscopy confirms the formation of GaN compound with the presence of compressive stress. Photoluminescence (PL) measurements revealed two band emissions, an UV and a broad visible emission. Hydrogen sensor was subsequently fabricated by depositing Pt Schottky contact onto GaN NWs film. The sensor response was measured at various H2 concentrations ranged from 200 up to 1200 ppm at room temperature. It was found that the response increases significantly for low H2 concentration (200-300 ppm) to reach about 50% then increases smoothly to reach 60% at 1200 ppm. The as-fabricated sensor possesses higher performances as compared to similar devices reported in the literature.

Zn-dopant dependent defect evolution in GaN nanowires

NASA Astrophysics Data System (ADS)

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

2015-10-01

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

Electronic and optical device applications of hollow cathode plasma assisted atomic layer deposition based GaN thin films

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

Bolat, Sami, E-mail: bolat@ee.bilkent.edu.tr; Tekcan, Burak; Ozgit-Akgun, Cagla

2015-01-15

Electronic and optoelectronic devices, namely, thin film transistors (TFTs) and metal–semiconductor–metal (MSM) photodetectors, based on GaN films grown by hollow cathode plasma-assisted atomic layer deposition (PA-ALD) are demonstrated. Resistivity of GaN thin films and metal-GaN contact resistance are investigated as a function of annealing temperature. Effect of the plasma gas and postmetallization annealing on the performances of the TFTs as well as the effect of the annealing on the performance of MSM photodetectors are studied. Dark current to voltage and responsivity behavior of MSM devices are investigated as well. TFTs with the N{sub 2}/H{sub 2} PA-ALD based GaN channels aremore » observed to have improved stability and transfer characteristics with respect to NH{sub 3} PA-ALD based transistors. Dark current of the MSM photodetectors is suppressed strongly after high-temperature annealing in N{sub 2}:H{sub 2} ambient.« less

STIR: Novel Electronic States by Gating Strongly Correlated Materials

DTIC Science & Technology

2016-03-01

plan built on my group’s recent demonstration of electrolyte gating in Strontium Titanate, using an atomically thin hexagonal Boron Nitride barrier to...demonstration of electrolyte gating in Strontium Titanate, using an atomically thin hexagonal Boron Nitride barrier to prevent disorder and chemical...techniques and learned to apply thin hexagonal Boron Nitride to single crystals of materials expected to show some of the most exciting correlated

Enhanced piezoelectric output of NiO/nanoporous GaN by suppression of internal carrier screening

NASA Astrophysics Data System (ADS)

Waseem, Aadil; Jeong, Dae Kyung; Johar, Muhammad Ali; Kang, Jin-Ho; Ha, Jun-Seok; Key Lee, June; Ryu, Sang-Wan

2018-06-01

The efficiency of piezoelectric nanogenerators (PNGs) significantly depends on the free carrier concentration of semiconductors. In the presence of a mechanical stress, piezoelectric charges are generated at both ends of the PNG, which are rapidly screened by the free carriers. The screening effect rapidly decreases the piezoelectric output within fractions of a second. In this study, the piezoelectric outputs of bulk- and nanoporous GaN-based heterojunction PNGs are compared. GaN thin films were epitaxially grown on sapphire substrates using metal organic chemical vapor deposition. Nanoporous GaN was fabricated using electrochemical etching, depleted of free carriers owing to the surface Fermi-level pinning. A highly resistive NiO thin film was deposited on bulk- and nanoporous GaN using radio frequency magnetron sputter. The NiO/nanoporous GaN PNG (NPNG) under a periodic compressive stress of 4 MPa exhibited an output voltage and current of 0.32 V and 1.48 μA cm‑2, respectively. The output voltage and current of the NiO/thin film-GaN PNG (TPNG) were three and five times smaller than those of the NPNG, respectively. Therefore, the high-resistivity of NiO and nanoporous GaN depleted by the Fermi-level pinning are advantageous and provide a better piezoelectric performance of the NPNG, compared with that of the TPNG.

Electronic and Optical Properties of Two-Dimensional GaN from First-Principles.

PubMed

Sanders, Nocona; Bayerl, Dylan; Shi, Guangsha; Mengle, Kelsey A; Kioupakis, Emmanouil

2017-12-13

Gallium nitride (GaN) is an important commercial semiconductor for solid-state lighting applications. Atomically thin GaN, a recently synthesized two-dimensional material, is of particular interest because the extreme quantum confinement enables additional control of its light-emitting properties. We performed first-principles calculations based on density functional and many-body perturbation theory to investigate the electronic, optical, and excitonic properties of monolayer and bilayer two-dimensional (2D) GaN as a function of strain. Our results demonstrate that light emission from monolayer 2D GaN is blueshifted into the deep ultraviolet range, which is promising for sterilization and water-purification applications. Light emission from bilayer 2D GaN occurs at a similar wavelength to its bulk counterpart due to the cancellation of the effect of quantum confinement on the optical gap by the quantum-confined Stark shift. Polarized light emission at room temperature is possible via uniaxial in-plane strain, which is desirable for energy-efficient display applications. We compare the electronic and optical properties of freestanding two-dimensional GaN to atomically thin GaN wells embedded within AlN barriers in order to understand how the functional properties are influenced by the presence of barriers. Our results provide microscopic understanding of the electronic and optical characteristics of GaN at the few-layer regime.

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.

Mechanics of graded glass composites and zinc oxide thin films grown at 90 degrees Celsius in water

NASA Astrophysics Data System (ADS)

Fillery, Scott Pierson

2007-06-01

The purpose of this research was to study the mechanical stability of two different material systems. The glass laminate system, exhibiting a threshold strength when placed under an applied load and ZnO thin films grown on GaN buffered Al2O3 substrates, exhibiting variations in film stability with changes to the Lateral Epitaxial Overgrowth architecture. The glass laminates were fabricated to contain periodic thin layers containing biaxial compressive stresses using ion exchange treatments to create residual compressive stresses at the surface of soda lime silicate glass sheets. Wafer direct bonding of the ion exchanged glass sheets resulted in the fabrication of glass laminates with thin layers of compressive stress adjacent to the glass interfaces. The threshold flexural strength of the ion exchanged glass laminates was determined to be 112 MPa after the introduction of indentation cracks with indent loads ranging from 1kg to 5kg and the laminates were found to exhibit a threshold strength, i.e., a stress below which failure will not occur. Contrary to similar ceramic laminates where cracks either propagate across the compressive layer or bifurcate within the compressive layer, the cracks in the glass laminates were deflected along the interface between the bonded sheets. ZnO films were grown on (0001) GaN buffered Al2O3 substrates by aqueous solution routes at 90°C. The films were found to buckle under compressive residual stresses at film thicknesses greater than 4mum. Lateral epitaxial overgrowth techniques using hexagonal hole arrays showed an increasing film stability with larger array spacing, resulting in film thicknesses up to 92mum. Stress determinations using Raman spectroscopy indicated that stress relaxation at the free surface during film growth played a major role in film stability. Investigations using Finite Element Analysis and Raman spectroscopy demonstrated that the strain energy within the film/substrate system decreased with increasing array spacing. ZnO films grown on III-nitride LED devices for use as transparent conducting layers showed intrinsic n-type doping, high transparency and adequate electrical contact resistance, resulting in linear light output with forward bias current and improved light extraction.

Improved growth of GaN layers on ultra thin silicon nitride/Si (1 1 1) by RF-MBE

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

Kumar, Mahesh; Roul, Basanta; Central Research Laboratory, Bharat Electronics, Bangalore 560013

High-quality GaN epilayers were grown on Si (1 1 1) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN filmmore » grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth.« less

General control of transition-metal-doped GaN nanowire growth: toward understanding the mechanism of dopant incorporation.

PubMed

Stamplecoskie, Kevin G; Ju, Ling; Farvid, Shokouh S; Radovanovic, Pavle V

2008-09-01

We report the first synthesis and characterization of cobalt- and chromium-doped GaN nanowires (NWs), and compare them to manganese-doped GaN NWs. Samples were synthesized by chemical vapor deposition method, using cobalt(II) chloride and chromium(III) chloride as dopant precursors. For all three impurity dopants hexagonal, triangular, and rectangular NWs were observed. The fraction of NWs having a particular morphology depends on the initial concentration of the dopant precursors. While all three dopant ions have the identical effect on GaN NW growth and faceting, Co and Cr are incorporated at much lower concentrations than Mn. These findings suggest that the doping mechanism involves binding of the transition-metal intermediates to specific NW facets, inhibiting their growth and causing a change in the NW morphology. We discuss the doping concentrations of Mn, Co, and Cr in terms of differences in their crystal-field stabilization energies (DeltaCFSE) in their gas-phase intermediates and in substitutionally doped GaN NWs. Using iron(III) chloride and cobalt(II) acetate as dopant precursors we show that the doping concentration dependence on DeltaCFSE allows for the prediction of achievable doping concentrations for different dopant ions in GaN NWs, and for a rational choice of a suitable dopant-ion precursor. This work further demonstrates a general and rational control of GaN NW growth using transition-metal impurities.

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.

NH3-free growth of GaN nanostructure on n-Si (1 1 1) substrate using a conventional thermal evaporation technique

NASA Astrophysics Data System (ADS)

Saron, K. M. A.; Hashim, M. R.; Farrukh, M. A.

2012-06-01

We have investigated the influence of carrier gas on grown gallium nitride (GaN) epitaxial layers deposited on n-Si (1 1 1) by a physical vapour deposition (PVD) via thermal evaporation of GaN powder at 1150 °C. The GaN nanostructures were grown at a temperature of 1050 °C for 60 min under various gases (N2, H2 mixed with N2, and Ar2) with absence of NH3. The morphology, structure, and optical properties (SEM) images showed that the morphology of GaN displayed various shapes of nanostructured depending on the type of carrier gas. X-ray diffraction (XRD) pattern showed that the GaN polycrystalline reveals a wurtzite-hexagonal structure with [0 0 1] crystal orientation. Raman spectra exhibited a red shift in peaks of E2 (high) as a result of tensile stress. Photoluminescence (PL) measurements showed two band emissions aside from the UV emission. The ultraviolet band gap of GaN nanostructure displayed a red shift as compared with the bulk GaN; this might be attributed to an increase in the defect and stress present in the GaN nanostructure. In addition, the observed blue and green-yellow emissions indicated defects due to the N vacancy and C impurity of the supplied gas. These results clearly indicated that the carrier gas, similar to the growth temperature, is one of the important parameters to control the quality of thermal evaporation (TE)-GaN epilayers.

Large Area Stress Distribution in Crystalline Materials Calculated from Lattice Deformation Identified by Electron Backscatter Diffraction

NASA Astrophysics Data System (ADS)

Shao, Yongliang; Zhang, Lei; Hao, Xiaopeng; Wu, Yongzhong; Dai, Yuanbin; Tian, Yuan; Huo, Qin

2014-08-01

We report a method to obtain the stress of crystalline materials directly from lattice deformation by Hooke's law. The lattice deformation was calculated using the crystallographic orientations obtained from electron backscatter diffraction (EBSD) technology. The stress distribution over a large area was obtained efficiently and accurately using this method. Wurtzite structure gallium nitride (GaN) crystal was used as the example of a hexagonal crystal system. With this method, the stress distribution of a GaN crystal was obtained. Raman spectroscopy was used to verify the stress distribution. The cause of the stress distribution found in the GaN crystal was discussed from theoretical analysis and EBSD data. Other properties related to lattice deformation, such as piezoelectricity, can also be analyzed by this novel approach based on EBSD data.

Large area stress distribution in crystalline materials calculated from lattice deformation identified by electron backscatter diffraction.

PubMed

Shao, Yongliang; Zhang, Lei; Hao, Xiaopeng; Wu, Yongzhong; Dai, Yuanbin; Tian, Yuan; Huo, Qin

2014-08-05

We report a method to obtain the stress of crystalline materials directly from lattice deformation by Hooke's law. The lattice deformation was calculated using the crystallographic orientations obtained from electron backscatter diffraction (EBSD) technology. The stress distribution over a large area was obtained efficiently and accurately using this method. Wurtzite structure gallium nitride (GaN) crystal was used as the example of a hexagonal crystal system. With this method, the stress distribution of a GaN crystal was obtained. Raman spectroscopy was used to verify the stress distribution. The cause of the stress distribution found in the GaN crystal was discussed from theoretical analysis and EBSD data. Other properties related to lattice deformation, such as piezoelectricity, can also be analyzed by this novel approach based on EBSD data.

Large Area Stress Distribution in Crystalline Materials Calculated from Lattice Deformation Identified by Electron Backscatter Diffraction

PubMed Central

Shao, Yongliang; Zhang, Lei; Hao, Xiaopeng; Wu, Yongzhong; Dai, Yuanbin; Tian, Yuan; Huo, Qin

2014-01-01

We report a method to obtain the stress of crystalline materials directly from lattice deformation by Hooke's law. The lattice deformation was calculated using the crystallographic orientations obtained from electron backscatter diffraction (EBSD) technology. The stress distribution over a large area was obtained efficiently and accurately using this method. Wurtzite structure gallium nitride (GaN) crystal was used as the example of a hexagonal crystal system. With this method, the stress distribution of a GaN crystal was obtained. Raman spectroscopy was used to verify the stress distribution. The cause of the stress distribution found in the GaN crystal was discussed from theoretical analysis and EBSD data. Other properties related to lattice deformation, such as piezoelectricity, can also be analyzed by this novel approach based on EBSD data. PMID:25091314

Vertically Oriented Growth of GaN Nanorods on Si Using Graphene as an Atomically Thin Buffer Layer.

PubMed

Heilmann, Martin; Munshi, A Mazid; Sarau, George; Göbelt, Manuela; Tessarek, Christian; Fauske, Vidar T; van Helvoort, Antonius T J; Yang, Jianfeng; Latzel, Michael; Hoffmann, Björn; Conibeer, Gavin; Weman, Helge; Christiansen, Silke

2016-06-08

The monolithic integration of wurtzite GaN on Si via metal-organic vapor phase epitaxy is strongly hampered by lattice and thermal mismatch as well as meltback etching. This study presents single-layer graphene as an atomically thin buffer layer for c-axis-oriented growth of vertically aligned GaN nanorods mediated by nanometer-sized AlGaN nucleation islands. Nanostructures of similar morphology are demonstrated on graphene-covered Si(111) as well as Si(100). High crystal and optical quality of the nanorods are evidenced through scanning transmission electron microscopy, micro-Raman, and cathodoluminescence measurements supported by finite-difference time-domain simulations. Current-voltage characteristics revealed high vertical conduction of the as-grown GaN nanorods through the Si substrates. These findings are substantial to advance the integration of GaN-based devices on any substrates of choice that sustains the GaN growth temperatures, thereby permitting novel designs of GaN-based heterojunction device concepts.

Role of an ultra-thin AlN/GaN superlattice interlayer on the strain engineering of GaN films grown on Si(110) and Si(111) substrates by plasma-assisted molecular beam epitaxy

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

Shen, X. Q.; Takahashi, T.; Matsuhata, H.

2013-12-02

We investigate the role of an ultra-thin AlN/GaN superlattice interlayer (SL-IL) on the strain engineering of the GaN films grown on Si(110) and Si(111) substrates by plasma-assisted molecular beam epitaxy. It is found that micro-cracks limitted only at the SL-IL position are naturally generated. These micro-cracks play an important role in relaxing the tensile strain caused by the difference of the coefficient of thermal expansion between GaN and Si and keeping the residual strain in the crack-free GaN epilayers resulted from the SL-IL during the growth. The mechanism understanding of the strain modulation by the SL-IL in the GaN epilayersmore » grown on Si substrates makes it possible to design new heterostructures of III-nitrides for optic and electronic device applications.« less

Role of an ultra-thin AlN/GaN superlattice interlayer on the strain engineering of GaN films grown on Si(110) and Si(111) substrates by plasma-assisted molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Shen, X. Q.; Takahashi, T.; Rong, X.; Chen, G.; Wang, X. Q.; Shen, B.; Matsuhata, H.; Ide, T.; Shimizu, M.

2013-12-01

We investigate the role of an ultra-thin AlN/GaN superlattice interlayer (SL-IL) on the strain engineering of the GaN films grown on Si(110) and Si(111) substrates by plasma-assisted molecular beam epitaxy. It is found that micro-cracks limitted only at the SL-IL position are naturally generated. These micro-cracks play an important role in relaxing the tensile strain caused by the difference of the coefficient of thermal expansion between GaN and Si and keeping the residual strain in the crack-free GaN epilayers resulted from the SL-IL during the growth. The mechanism understanding of the strain modulation by the SL-IL in the GaN epilayers grown on Si substrates makes it possible to design new heterostructures of III-nitrides for optic and electronic device applications.

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

PubMed

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

2014-03-04

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

Low-cost growth of magnesium doped gallium nitride thin films by sol-gel spin coating method

NASA Astrophysics Data System (ADS)

Amin, N. Mohd; Ng, S. S.

2018-01-01

Low-cost sol-gel spin coating growth of magnesium (Mg) doped gallium nitride (GaN) thin films with different concentrations of Mg was reported. The effects of the Mg concentration on the structural, surface morphology, elemental compositions, lattice vibrational, and electrical properties of the deposited films were investigated. X-ray diffraction results show that the Mg-doped samples have wurtzite structure with preferred orientation of GaN(002). The crystallite size decreases and the surface of the films with pits/pores were formed, while the crystalline quality of the films degraded as the Mg concentration increases from 2% to 6. %. All the Raman active phonon modes of the wurtzite GaN were observed while a broad peak attributed to the Mg-related lattice vibrational mode was detected at 669 cm-1. Hall effect results show that the resistivity of the thin films decreases while the hole concentration and hall mobility of thin films increases as the concentration of the Mg increases.

Luminescence from defects in GaN

NASA Astrophysics Data System (ADS)

Reshchikov, M. A.; Morkoç, H.

2006-04-01

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

Gallium hydride vapor phase epitaxy of GaN nanowires

PubMed Central

2011-01-01

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

Gallium hydride vapor phase epitaxy of GaN nanowires.

PubMed

Zervos, Matthew; Othonos, Andreas

2011-03-28

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

Influence of ammonia flow rate for improving properties of polycrystalline GaN

NASA Astrophysics Data System (ADS)

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

2018-06-01

Post-annealing treatment in ammonia ambient is widely accepted for GaN material, but less works have been done to investigate the influence of the ammonia (NH3) flow rate for reducing the N-deficiency as well as improving the quality of the material. In this work, we investigated the influence of NH3 flow rate at 1, 2, 3, and 4 slm in improving properties of a ∼1 μm thick polycrystalline GaN layer. Our simulation work suggested that the uniformity of temperature and pressure gradient of the NH3 gas did not lead to the reduction of N-deficiency of the polycrystalline GaN layer. Instead, it was found that the mitigation of the N-deficiency was strongly influenced by the fluid velocity of the NH3 gas, which had passed over the layer. Either at lower or higher fluid velocity, the chance for the active N atoms to incorporate into the GaN lattice structure was low. Therefore, the N-deficiency on the polycrystalline GaN layer could not be minimized under these conditions. As measured by EDX, the N atoms incorporation was the most effective when the NH3 flow rate at 3 slm, suggesting the flow rate significantly improved the N-deficiency of the polycrystalline GaN layer. Furthermore, it favored the formation of larger hexagonal faceted grains, with the smallest FWHM of XRD peaks from the GaN diffractions in (10 1 bar 0), (0002) and (10 1 bar 1) orientations, while allowing the polycrystalline GaN layer to show sharp and intense emissions peak of NBE in a PL spectrum.

Broad visible emission from GaN nanowires grown on n-Si (1 1 1) substrate by PVD for solar cell application

NASA Astrophysics Data System (ADS)

Saron, K. M. A.; Hashim, M. R.

2013-04-01

Nanostructured gallium nitrides (GaNs) were grown on a catalyst-free Si (1 1 1) substrates using physical vapor deposition via thermal evaporation of GaN powder at 1150 °C in the absence of NH3 gas for different deposition time. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometer (EDX) results indicated that the growth of GaN nanostructure varies with deposition time. Both X-ray diffraction (XRD) patterns and Raman spectra reveals a hexagonal GaN with wurtzite structure. Photoluminescence (PL) showed that the UV emission was suppressed, and the visible band emission was enhanced with increasing deposition time. Enhancement of visible band emission from the GaN NWs is due to the increasement of deep level states, which was resulted from growth process. Current-voltage (IV) characteristics of GaN/Si heterostructure were measured and good rectifying behavior was observed for this photodiode (PD). The forward current under illumination was almost three times than that in the dark current at +5 V. Responsivity of the photodetector was 10.5 A/W at range from 350 nm to 500 nm, which rapidly increased to 13.6 A/W at 700 nm. We found that the fabricated photodiode PD has an infra-red (IR) photoresponse behavior. The analysis of optical and electrical properties indications that the grown GaN in the absent of NH3 is a promising optical material and has potential applications in photo voltage solar cell.

Growth and characterization of GaN thin film on Si substrate by thermionic vacuum arc (TVA)

NASA Astrophysics Data System (ADS)

Kundakçı, Mutlu; Mantarcı, Asim; Erdoğan, Erman

2017-01-01

Gallium nitride (GaN) is an attractive material with a wide-direct band gap (3.4 eV) and is one of the significant III-nitride materials, with many advantageous device applications such as high electron mobility transistors, lasers, sensors, LEDs, detectors, and solar cells, and has found applications in optoelectronic devices. GaN could also be useful for industrial research in the future. Chemical vapor deposition (CVD), molecular beam epitaxy (MBE), sputter, and pulsed laser deposition (PLD) are some of the methods used to fabricate GaN thin film. In this research, a GaN thin film grown on a silicon substrate using the thermionic vacuum arc (TVA) technique has been extensively studied. Fast deposition, short production time, homogeneity, and uniform nanostructure with low roughness can be seen as some of the merits of this method. The growth of the GaN was conducted at an operating pressure of 1× {{10}-6} \\text{Torr} , a plasma current 0.6 \\text{A} and for a very short period of time of 40 s. For the characterization process, scanning electron microscopy (SEM) was conducted to determine the structure and surface morphology of the material. Energy dispersive x-ray spectroscopy (EDX) was used to comprehend the elemental analysis characterization of the film. X-ray diffraction (XRD) was used to analyze the structure of the film. Raman measurements were taken to investigate the phonon modes of the material. The morphological properties of the material were analyzed in detail by atomic force microscopy (AFM).

Epitaxial Gd2O3 on GaN and AlGaN: a potential candidate for metal oxide semiconductor based transistors on Si for high power application

NASA Astrophysics Data System (ADS)

Ghosh, Kankat; Das, S.; Khiangte, K. R.; Choudhury, N.; Laha, Apurba

2017-11-01

We report structural and electrical properties of hexagonal Gd2O3 grown epitaxially on GaN/Si (1 1 1) and AlGaN/GaN/Si(1 1 1) virtual substrates. GaN and AlGaN/GaN heterostructures were grown on Si(1 1 1) substrates by plasma assisted molecular beam epitaxy (PA-MBE), whereas the Gd2O3 layer was grown by the pulsed laser ablation (PLA) technique. Initial structural characterizations show that Gd2O3 grown on III-nitride layers by PLA, exhibit a hexagonal structure with an epitaxial relationship as {{≤ft[ 0 0 0 1 \\right]}G{{d2}{{O}3}}}||{{≤ft[ 0 0 0 1 \\right]}GaN} and {{≤ft[ 1 \\bar{1} 0 0 \\right]}G{{d2}{{O}3}}}||{{≤ft[ 1 \\bar{1} 0 0 \\right]}GaN} . X-ray photoelectron measurements of the valence bands revealed that Gd2O3 exhibits band offsets of 0.97 eV and 0.4 eV, for GaN and Al0.3Ga0.7N, respectively. Electrical measurements such as capacitance-voltage and leakage current characteristics further confirm that epi-Gd2O3 on III-nitrides could be a potential candidate for future metal-oxide-semiconductor (MOS)-based transistors also for high power applications in radio frequency range.

Energy bands and acceptor binding energies of GaN

NASA Astrophysics Data System (ADS)

Xia, Jian-Bai; Cheah, K. W.; Wang, Xiao-Liang; Sun, Dian-Zhao; Kong, Mei-Ying

1999-04-01

The energy bands of zinc-blende and wurtzite GaN are calculated with the empirical pseudopotential method, and the pseudopotential parameters for Ga and N atoms are given. The calculated energy bands are in agreement with those obtained by the ab initio method. The effective-mass theory for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN for both structures are given. The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor are 24 and 142 meV for the zinc-blende structure, 20 and 131, and 97 meV for the wurtzite structure, respectively, which are consistent with recent experimental results. It is proposed that there are two kinds of acceptor in wurtzite GaN. One kind is the general acceptor such as C, which substitutes N, which satisfies the effective-mass theory. The other kind of acceptor includes Mg, Zn, Cd, etc., the binding energy of these acceptors is deviated from that given by the effective-mass theory. In this report, wurtzite GaN is grown by the molecular-beam epitaxy method, and the photoluminescence spectra were measured. Three main peaks are assigned to the donor-acceptor transitions from two kinds of acceptors. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material.

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

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

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

2012-06-20

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

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

NASA Astrophysics Data System (ADS)

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

2010-05-01

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

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

PubMed

Hayashi, Hiroaki; Konno, Yuta; Kishino, Katsumi

2016-02-05

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

Epitaxial hexagonal materials on IBAD-textured substrates

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

Matias, Vladimir; Yung, Christopher

2017-08-15

A multilayer structure including a hexagonal epitaxial layer, such as GaN or other group III-nitride (III-N) semiconductors, a <111> oriented textured layer, and a non-single crystal substrate, and methods for making the same. The textured layer has a crystalline alignment preferably formed by the ion-beam assisted deposition (IBAD) texturing process and can be biaxially aligned. The in-plane crystalline texture of the textured layer is sufficiently low to allow growth of high quality hexagonal material, but can still be significantly greater than the required in-plane crystalline texture of the hexagonal material. The IBAD process enables low-cost, large-area, flexible metal foil substratesmore » to be used as potential alternatives to single-crystal sapphire and silicon for manufacture of electronic devices, enabling scaled-up roll-to-roll, sheet-to-sheet, or similar fabrication processes to be used. The user is able to choose a substrate for its mechanical and thermal properties, such as how well its coefficient of thermal expansion matches that of the hexagonal epitaxial layer, while choosing a textured layer that more closely lattice matches that layer.« less

Epitaxy of Zn{sub 2}TiO{sub 4} (1 1 1) thin films on GaN (0 0 1)

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

Hsiao, Chu-Yun; Wu, Jhih-Cheng; Shih, Chuan-Feng, E-mail: cfshih@mail.ncku.edu.tw

2013-03-15

Highlights: ► High-permittivity spinel Zn{sub 2}TiO{sub 4} thin films were grown on GaN (0 0 1) by sputtering. ► Oxygen atmosphere and post heat-treatment annealing effectively enhanced epitaxy. ► The epitaxial Zn{sub 2}TiO{sub 4} modifies the dielectric properties of ceramic oxide. - Abstract: High-permittivity spinel Zn{sub 2}TiO{sub 4} thin films were grown on GaN (0 0 1) by rf-sputtering. Grazing-angle, powder, and pole-figure X-ray diffractometries (XRD) were performed to identify the crystallinity and the preferred orientation of the Zn{sub 2}TiO{sub 4} films. Lattice image at the Zn{sub 2}TiO{sub 4} (1 1 1)/GaN (0 0 1) interface was obtained by high-resolutionmore » transmission-electron microscopy (HR-TEM). An oxygen atmosphere in sputtering and post heat-treatment using rapid thermal annealing effectively enhanced the epitaxy. The epitaxial relationship was determined from the XRD and HR-TEM results: (111){sub Zn{sub 2TiO{sub 4}}}||(001){sub GaN}, (202{sup ¯}){sub Zn{sub 2TiO{sub 4}}}||(110){sub GaN},and[21{sup ¯}1{sup ¯}]{sub Zn{sub 2TiO{sub 4}}}||[01{sup ¯}10]{sub GaN}. Finally, the relative permittivity, interfacial trap density and the flat-band voltage of the Zn{sub 2}TiO{sub 4} based capacitor were ∼18.9, 8.38 × 10{sup 11} eV{sup −1} cm{sup −2}, and 1.1 V, respectively, indicating the potential applications of the Zn{sub 2}TiO{sub 4} thin film to the GaN-based metal-oxide-semiconductor capacitor.« less

Effect of strain on gallium nitride and gallium indium arsenide nitride growth and doping

NASA Astrophysics Data System (ADS)

G. S., Sudhir

GaN and the related (Al,In)N materials are currently used in manufacturing optoelectronic and electronic devices. However, the efficiency of these devices is limited due to lack of high structural quality and of low resistive p-type GaN. The GaN thin films are under strain during growth due to the large lattice mismatch, thermal expansion difference, and low growth temperature. Developing a better understanding of the effect of strain on the properties of thin films is important in furthering our knowledge of thin film growth affecting the performance of III-nitride based devices. Pulsed laser deposition was used to grow thin films of AlN and GaN on sapphire substrates. It is shown that the structure and surface morphology of layers are controlled by the nitrogen partial pressure during the growth. Through these nitrogen pressure related effects, thin films with microstructure ranging from crystalline to amorphous can be produced. A minimal surface root mean square roughness of 0.7 nm for amorphous AlN is obtained which compares well with the substrate roughness of 0.5 nm. Incorporation of impurities changes the lattice constants of thin films of GaN deposited on basal plane sapphire by molecular beam epitaxy. Both Mg (1017 cm-3) and Zn (3 x 10 20 cm-3) doping were found to expand the c lattice parameter by +0.38 x 10-2 and +0.62 x 10 -2, respectively. Oxygen up to concentrations 9 x 10 21 cm-3 is shown to replace nitrogen in GaN thin films reducing the c parameter only by a small amount. Incorporation of Si leads to a large decrease of the c parameter, which can not be attributed to the different size of Ga and Si. It is suggested that doping alters the film stoichiometry by a predicted Fermi level dependence of defect formation energies and thereby, lattice parameters and stress. A proper buffer layer design is shown to increase the incorporation of Mg by two orders of magnitude Finally, the balance of lattice parameter change caused by dopant and native point defects with strain contributed by growth condition leads to high mobility p-type GaN thin films. Incorporation of N in thin films of GaAsN and GaInAsN on GaAs was studied by molecular beam epitaxy. X-ray diffraction results indicated that the amount of N in GaAs increased with the power to plasma source and the slower growth rate, but was not affected by the growth temperature. Photoluminescence (PL) results showed a drastic narrowing of the bandgap with increased N incorporation. High pressure measurements showed the pressure coefficient of the absorption edge of 4 mum thick GaInAsN layer to be unusually small (51 meV/GPa). Also, the temperature-induced shift of the edge is reduced by 50% compared to that of GaAs. Based on the results of the detailed materials characterization, optimized p-GaAs/i-GaInAsN/n-GaAs structures were grown for I mum photo-detectors. The device characteristics of the prototype devices are presented.

GaN Micromechanical Resonators with Meshed Metal Bottom Electrode.

PubMed

Ansari, Azadeh; Liu, Che-Yu; Lin, Chien-Chung; Kuo, Hao-Chung; Ku, Pei-Cheng; Rais-Zadeh, Mina

2015-03-17

This work describes a novel architecture to realize high-performance gallium nitride (GaN) bulk acoustic wave (BAW) resonators. The method is based on the growth of a thick GaN layer on a metal electrode grid. The fabrication process starts with the growth of a thin GaN buffer layer on a Si (111) substrate. The GaN buffer layer is patterned and trenches are made and refilled with sputtered tungsten (W)/silicon dioxide (SiO₂) forming passivated metal electrode grids. GaN is then regrown, nucleating from the exposed GaN seed layer and coalescing to form a thick GaN device layer. A metal electrode can be deposited and patterned on top of the GaN layer. This method enables vertical piezoelectric actuation of the GaN layer using its largest piezoelectric coefficient ( d 33 ) for thickness-mode resonance. Having a bottom electrode also results in a higher coupling coefficient, useful for the implementation of acoustic filters. Growth of GaN on Si enables releasing the device from the frontside using isotropic xenon difluoride (XeF₂) etch and therefore eliminating the need for backside lithography and etching.

Interfacing epitaxial oxides to gallium nitride

NASA Astrophysics Data System (ADS)

Losego, Mark Daniel

Molecular beam epitaxy (MBE) is lauded for its ability to control thin film material structures at the atomic level. This precision of control can improve performance of microelectronic devices and cultivate the development of novel device structures. This thesis explores the utility of MBE for designing interfaces between oxide epilayers and the wide band gap semiconductor gallium nitride (GaN). The allure of wide gap semiconductor microelectronics (like GaN, 3.4 eV) is their ability to operate at higher frequencies, higher powers, and higher temperatures than current semiconductor platforms. Heterostructures between ferroelectric oxides and GaN are also of interest for studying the interaction between GaN's fixed polarization and the ferroelectric's switchable polarization. Two major obstacles to successful integration of oxides with GaN are: (1) interfacial trap states; and (2) small electronic band offsets across the oxide/nitride interface due to the semiconductor's large band gap. For this thesis, epitaxial rocksalt oxide interfacial layers (˜8 eV band gap) are investigated as possible solutions to overcoming the challenges facing oxide integration with GaN. The cubic close-packed structure of rocksalt oxides forms a suitable epitaxial interface with the hexagonal close-packed wurtzite lattice of GaN. Three rocksalt oxide compounds are investigated in this thesis: MgO, CaO, and YbO. All are found to have a (111) MO || (0001) GaN; <1 10> MO || <11 20> GaN epitaxial relationship. Development of the epilayer microstructure is dominated by the high-energy polar growth surface (drives 3D nucleation) and the interfacial symmetry, which permits the formation of twin boundaries. Using STEM, strain relief for these ionicly bonded epilayers is observed to occur through disorder within the initial monolayer of growth. All rocksalt oxides demonstrate chemical stability with GaN to >1000°C. Concurrent MBE deposition of MgO and CaO is known to form complete solid solutions. By controlling the composition of these alloys, the oxide's lattice parameter can be engineered to match GaN and reduce interfacial state density. Compositional control is a universal challenge to oxide MBE, and the MgO-CaO system (MCO) is further complicated by magnesium's high volatility and the lack of a thermodynamically stable phase. Through a detailed investigation of MgO's deposition rate and subsequent impact on MCO composition, the process space for achieving lattice-matched compositions to GaN are fully mapped. Lattice-matched compositions are demonstrated to have the narrowest off-axis rocking curve widths ever reported for an epitaxial oxide deposited directly on GaN (0.7° in φ-circle for 200 reflection). Epitaxial deposition of the ferroelectric (Ba,Sr)TiO3 by hot RF sputtering on GaN surfaces is also demonstrated. Simple MOS capacitors are fabricated from epitaxial rocksalt oxides and (Ba,Sr)TiO3 layers deposited on n-GaN substrates. Current-voltage measurements reveal that BST epilayers have 5 orders of magnitude higher current leakage than rocksalt epilayers. This higher leakage is attributed to the smaller band offset expected at this interface; modeling confirms that electronic transport occurs by Schottky emission. In contrast, current transport across the rocksalt oxide/GaN interface occurs by Frenkel-Poole emission and can be reduced with pre-deposition surface treatments. Finally, through this work, it is realized that the integration of oxides with III-nitrides requires an appreciation of many different fields of research including materials science, surface science, and electrical engineering. By recognizing the importance that each of these fields play in designing oxide/III-nitride interfaces, this thesis has the opportunity to explore other related phenomena including accessing metastable phases through MBE (ytterbium monoxide), spinodal decomposition in metastable alloys (MCO), how polar surfaces grown by MBE compensate their bound surface charge, room temperature epitaxy, and the use of surface modification to achieve selective epitaxial deposition (SeEDed growth).

Preparation and evaluation of Mn3GaN1-x thin films with controlled N compositions

NASA Astrophysics Data System (ADS)

Ishino, Sunao; So, Jongmin; Goto, Hirotaka; Hajiri, Tetsuya; Asano, Hidefumi

2018-05-01

Thin films of antiperovskite Mn3GaN1-x were grown on MgO (001) substrates by reactive magnetron sputtering, and their structural, magnetic, and magneto-optical properties were systematically investigated. It was found that the combination of the deposition rate and the N2 gas partial pressure could produce epitaxial films with a wide range of N composition (N-deficiency) and resulting c/a values (0.93 - 1.0). While the films with c/a = 0.992 - 1.0 were antiferromagnetic, the films with c/a = 0.93 - 0.989 showed perpendicular magnetic anisotropy (PMA) with the maximum PMA energy up to 1.5×106 erg/cm3. Systematic dependences of the energy spectra of the polar Kerr signals on the c/a ratio were observed, and the Kerr ellipticity was as large as 2.4 deg. at 1.9 eV for perpendicularly magnetized ferromagnetic thin films with c/a = 0.975. These results highlight that the tetragonal distortion plays an important role in magnetic and magneto-optical properties of Mn3GaN1-x thin films.

Hydrogen-surfactant-assisted coherent growth of GaN on ZnO substrate

NASA Astrophysics Data System (ADS)

Zhang, Jingzhao; Zhang, Yiou; Tse, Kinfai; Zhu, Junyi

2018-01-01

Heterostructures of wurtzite based devices have attracted great research interest because of the tremendous success of GaN in light emitting diodes (LED) industry. High-quality GaN thin films on inexpensive and lattice matched ZnO substrates are both commercially and technologically desirable. Intrinsic wetting conditions, however, forbid such heterostructures as the energy of ZnO polar surfaces is much lower than that of GaN polar surfaces, resulting in 3D growth mode and poor crystal quality. Based on first-principles calculations, we propose the use of surfactant hydrogen to dramatically alter the growth mode of the heterostructures. Stable H-involved surface configurations and interfaces are investigated with the help of our newly developed modelling techniques. The temperature and chemical potential dependence of our proposed strategy, which is critical in experiments, is predicted by applying the experimental Gibbs free energy of H2. Our thermodynamic wetting condition analysis is a crucial step for the growth of GaN on ZnO, and we find that introducing H will not degrade the stability of ZnO substrate. This approach will allow the growth of high-quality GaN thin films on ZnO substrates. We believe that our new strategy may reduce the manufactory cost, improve the crystal quality, and improve the efficiency of GaN-based devices.

Multicycle rapid thermal annealing optimization of Mg-implanted GaN: Evolution of surface, optical, and structural properties

NASA Astrophysics Data System (ADS)

Greenlee, Jordan D.; Feigelson, Boris N.; Anderson, Travis J.; Tadjer, Marko J.; Hite, Jennifer K.; Mastro, Michael A.; Eddy, Charles R.; Hobart, Karl D.; Kub, Francis J.

2014-08-01

The first step of a multi-cycle rapid thermal annealing process was systematically studied. The surface, structure, and optical properties of Mg implanted GaN thin films annealed at temperatures ranging from 900 to 1200 °C were investigated by Raman spectroscopy, photoluminescence, UV-visible spectroscopy, atomic force microscopy, and Nomarski microscopy. The GaN thin films are capped with two layers of in-situ metal organic chemical vapor deposition -grown AlN and annealed in 24 bar of N2 overpressure to avoid GaN decomposition. The crystal quality of the GaN improves with increasing annealing temperature as confirmed by UV-visible spectroscopy and the full widths at half maximums of the E2 and A1 (LO) Raman modes. The crystal quality of films annealed above 1100 °C exceeds the quality of the as-grown films. At 1200 °C, Mg is optically activated, which is determined by photoluminescence measurements. However, at 1200 °C, the GaN begins to decompose as evidenced by pit formation on the surface of the samples. Therefore, it was determined that the optimal temperature for the first step in a multi-cycle rapid thermal anneal process should be conducted at 1150 °C due to crystal quality and surface morphology considerations.

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

PubMed

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

2013-03-15

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

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

PubMed Central

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

2013-01-01

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

Nitrogen-Polar (000 1 ¯ ) GaN Grown on c-Plane Sapphire with a High-Temperature AlN Buffer.

PubMed

Song, Jie; Han, Jung

2017-03-02

We demonstrate growing nitrogen-polar (N-polar) GaN epilayer on c-plane sapphire using a thin AlN buffer layer by metalorganic chemical vapor deposition. We have studied the influence of the AlN buffer layer on the polarity, crystalline quality, and surface morphology of the GaN epilayer and found that the growth temperature of the AlN buffer layer played a critical role in the growth of the GaN epilayer. The low growth temperature of the AlN buffer results in gallium-polar GaN. Even a nitridation process has been conducted. High growth temperature for an AlN buffer layer is required to achieve pure N-polarity, high crystalline quality, and smooth surface morphology for a GaN epilayer.

Incorporation of Mg in Free-Standing HVPE GaN Substrates

NASA Astrophysics Data System (ADS)

Zvanut, M. E.; Dashdorj, J.; Freitas, J. A.; Glaser, E. R.; Willoughby, W. R.; Leach, J. H.; Udwary, K.

2016-06-01

Mg, the only effective p-type dopant for nitrides, is well studied in thin films due to the important role of the impurity in light-emitting diodes and high-power electronics. However, there are few reports of Mg in thick free-standing GaN substrates. Here, we demonstrate successful incorporation of Mg into GaN grown by hydride vapor-phase epitaxy (HVPE) using metallic Mg as the doping source. The concentration of Mg obtained from four separate growth runs ranged between 1016 cm-3 and 1019 cm-3. Raman spectroscopy and x-ray diffraction revealed that Mg did not induce stress or perturb the crystalline quality of the HVPE GaN substrates. Photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopies were performed to investigate the types of point defects in the crystals. The near-band-edge excitonic and shallow donor-shallow acceptor radiative recombination processes involving shallow Mg acceptors were prominent in the PL spectrum of a sample doped to 3 × 1018 cm-3, while the EPR signal was also thought to represent a shallow Mg acceptor. Detection of this signal reflects minimization of nonuniform strain obtained in the thick free-standing HVPE GaN compared with heteroepitaxial thin films.

High Quality, Low Cost Bulk Gallium Nitride Substrates Grown by the Electrochemical Solution Growth Method

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

Seacrist, Michael

The objective of this project was to develop the Electrochemical Solution Growth (ESG) method conceived / patented at Sandia National Laboratory into a commercially viable bulk gallium nitride (GaN) growth process that can be scaled to low cost, high quality, and large area GaN wafer substrate manufacturing. The goal was to advance the ESG growth technology by demonstrating rotating seed growth at the lab scale and then transitioning process to prototype commercial system, while validating the GaN material and electronic / optical device quality. The desired outcome of the project is a prototype commercial process for US-based manufacturing of highmore » quality, large area, and lower cost GaN substrates that can drive widespread deployment of energy efficient GaN-based power electronic and optical devices. In year 1 of the project (Sept 2012 – Dec 2013) the overall objective was to demonstrate crystalline GaN growth > 100um on a GaN seed crystal. The development plan included tasks to demonstrate and implement a method for purifying reagent grade salts, develop the reactor 1 process for rotating seed Electrochemical Solution Growth (ESG) of GaN, grow and characterize ESG GaN films, develop a fluid flow and reaction chemistry model for GaN film growth, and design / build an improved growth reactor capable of scaling to 50mm seed diameter. The first year’s project objectives were met in some task areas including salt purification, film characterization, modeling, and reactor 2 design / fabrication. However, the key project objective of the growth of a crystalline GaN film on the seed template was not achieved. Amorphous film growth on the order of a few tenths of a micron has been detected with a film composition including Ga and N, plus several other impurities originating from the process solution and hardware. The presence of these impurities, particularly the oxygen, has inhibited the demonstration of crystalline GaN film growth on the seed template. However, the presence of both Ga and N at the growth surface indicates that the reactor hardware physics is all functioning properly; achieving film growth is a matter of controlling the chemistry at the interface. The impurities originating from the hardware are expected to be straightforward to eliminate. Activities were defined for an extension of budget period 1 to eliminate the undesired impurities originating from the reactor hardware and interfering with crystalline GaN film growth. The budget period 1 extension was negotiated during the 1st half of 2014. The budget period 1 extension spanned approximately from August 2014 to August 2015. The project objective for this extension period was to demonstrate at least 0.5um crystalline GaN film on a GaN seed in the lab scale reactor. The focus of the budget 1 extension period from August 2014 to August 2015 was to eliminate oxygen contamination interference with GaN film growth. The team procured the highest purity lowest oxygen salt for testing. Low oxygen crucible materials such as silicon carbide were installed and evaluated in the laboratory reactor. Growth experiments were performed with high purity salt, high purity hardware, and optimized oxide removal from the seed surface. Experiments were characterized with methods including UV inspection, profilometry, x-ray diffraction (XRD) to determine crystalline structure, optical and scanning electron microscopy, photoluminescence, x-ray photon spectroscopy (XPS), transmission electron microscopy (TEM), and secondary ion mass spectroscopy (SIMS). Despite successfully integrating the low oxygen materials in the laboratory reactor, the goal of depositing 0.5um of crystalline GaN on the MOCVD GaN seed was not met. Very thin (ca. 10nm) cubic phase GaN deposition was observed on the hexagonal MOCVD GaN seeds. But there was a competing etching reaction which was also observed and thought to be related to the presence of metallic lithium, a byproduct of the LiCl-KCl salt used as the process medium. The etching reaction could potentially be addressed by alternate salts not containing lithium, but would necessitate starting all over on the reactor and process design. Further, controlling the reaction of Ga and N in the bulk salt to favor deposition on the seed has proved to be very difficult and unlikely to be solved within the scope of this project in a manner consistent with the original objective for wafer or crystal scale thickness for GaN deposition on a GaN seed. Upon completion of the budget 1 extension period in August 2015 the project partners and DOE agreed to stop work on the project.« less

Single layers and multilayers of GaN and AlN in square-octagon structure: Stability, electronic properties, and functionalization

NASA Astrophysics Data System (ADS)

Gürbüz, E.; Cahangirov, S.; Durgun, E.; Ciraci, S.

2017-11-01

Further to planar single-layer hexagonal structures, GaN and AlN can also form free-standing, single-layer structures constructed from squares and octagons. We performed an extensive analysis of dynamical and thermal stability of these structures in terms of ab initio finite-temperature molecular dynamics and phonon calculations together with the analysis of Raman and infrared active modes. These single-layer square-octagon structures of GaN and AlN display directional mechanical properties and have wide, indirect fundamental band gaps, which are smaller than their hexagonal counterparts. These density functional theory band gaps, however, increase and become wider upon correction. Under uniaxial and biaxial tensile strain, the fundamental band gaps decrease and can be closed. The electronic and magnetic properties of these single-layer structures can be modified by adsorption of various adatoms, or by creating neutral cation-anion vacancies. The single-layer structures attain magnetic moment by selected adatoms and neutral vacancies. In particular, localized gap states are strongly dependent on the type of vacancy. The energetics, binding, and resulting electronic structure of bilayer, trilayer, and three-dimensional (3D) layered structures constructed by stacking the single layers are affected by vertical chemical bonds between adjacent layers. In addition to van der Waals interaction, these weak vertical bonds induce buckling in planar geometry and enhance their binding, leading to the formation of stable 3D layered structures. In this respect, these multilayers are intermediate between van der Waals solids and wurtzite crystals, offering a wide range of tunability.

Kinetic Monte Carlo simulations of GaN homoepitaxy on c- and m-plane surfaces

DOE PAGES

Xu, Dongwei; Zapol, Peter; Stephenson, G. Brian; ...

2017-04-12

The surface orientation can have profound effects on the atomic-scale processes of crystal growth and is essential to such technologies as GaN-based light-emitting diodes and high-power electronics. We investigate the dependence of homoepitaxial growth mechanisms on the surface orientation of a hexagonal crystal using kinetic Monte Carlo simulations. To model GaN metal-organic vapor phase epitaxy, in which N species are supplied in excess, only Ga atoms on a hexagonal close-packed (HCP) lattice are considered. The results are thus potentially applicable to any HCP material. Growth behaviors on c-plane (0001) and m-plane (011¯0) surfaces are compared. We present a reciprocal spacemore » analysis of the surface morphology, which allows extraction of growth mode boundaries and direct comparison with surface X-ray diffraction experiments. For each orientation, we map the boundaries between 3-dimensional, layer-by-layer, and step flow growth modes as a function of temperature and growth rate. Two models for surface diffusion are used, which produce different effective Ehrlich-Schwoebel step-edge barriers and different adatom diffusion anisotropies on m-plane surfaces. Simulation results in agreement with observed GaN island morphologies and growth mode boundaries are obtained. These indicate that anisotropy of step edge energy, rather than adatom diffusion, is responsible for the elongated islands observed on m-plane surfaces. As a result, island nucleation spacing obeys a power-law dependence on growth rate, with exponents of –0.24 and –0.29 for the m- and c-plane, respectively.« less

Kinetic Monte Carlo simulations of GaN homoepitaxy on c- and m-plane surfaces

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

Xu, Dongwei; Zapol, Peter; Stephenson, G. Brian

The surface orientation can have profound effects on the atomic-scale processes of crystal growth and is essential to such technologies as GaN-based light-emitting diodes and high-power electronics. We investigate the dependence of homoepitaxial growth mechanisms on the surface orientation of a hexagonal crystal using kinetic Monte Carlo simulations. To model GaN metal-organic vapor phase epitaxy, in which N species are supplied in excess, only Ga atoms on a hexagonal close-packed (HCP) lattice are considered. The results are thus potentially applicable to any HCP material. Growth behaviors on c-plane (0001) and m-plane (011¯0) surfaces are compared. We present a reciprocal spacemore » analysis of the surface morphology, which allows extraction of growth mode boundaries and direct comparison with surface X-ray diffraction experiments. For each orientation, we map the boundaries between 3-dimensional, layer-by-layer, and step flow growth modes as a function of temperature and growth rate. Two models for surface diffusion are used, which produce different effective Ehrlich-Schwoebel step-edge barriers and different adatom diffusion anisotropies on m-plane surfaces. Simulation results in agreement with observed GaN island morphologies and growth mode boundaries are obtained. These indicate that anisotropy of step edge energy, rather than adatom diffusion, is responsible for the elongated islands observed on m-plane surfaces. As a result, island nucleation spacing obeys a power-law dependence on growth rate, with exponents of –0.24 and –0.29 for the m- and c-plane, respectively.« less

Nonlinear optical effects in semi-polar GaN micro-cavity emitter

NASA Astrophysics Data System (ADS)

Butler, Sween; Jiang, Hongxing; Lin, Jingyu; Neogi, Arup

Nonlinear optical (NLO) response of low dimensional emitters is of current interest because of the need for active elements in photonic applications. NLO effects in a selectively grown array of semi-polar GaN microcavity structures offer a promising route toward devices for integrated optical circuitry in optoelectronics and photonics field. Localized spatial excitation of a single hexagonal GaN microcavity with semipolar facets formed by selective area growth was optimized for nonlinear optical light generation due to second harmonic generation (SHG) and multi-photon luminescence(MPL). Multi-photon transition induced by tightly focused femtosecond NIR incident field results in ultra-violet and yellow luminescence for excitations above and below half bandgap energy, whereas SHG was observed for below half bandgap energy. We show that color and coherence of the light generation from the emitter can be controlled by selective onset of the nonlinear process which depends not only on the incident laser energy and intensity but also on the geometry of the microcavity. Quasi-WGM like modes were observed for off-resonant excitations from the GaN microcavity resulting in enhanced SHG. The directionality of MPL and SHG will be presented as a function of the pump polarization.

GaN nanowire arrays with nonpolar sidewalls for vertically integrated field-effect transistors

NASA Astrophysics Data System (ADS)

Yu, Feng; Yao, Shengbo; Römer, Friedhard; Witzigmann, Bernd; Schimpke, Tilman; Strassburg, Martin; Bakin, Andrey; Schumacher, Hans Werner; Peiner, Erwin; Suryo Wasisto, Hutomo; Waag, Andreas

2017-03-01

Vertically aligned gallium nitride (GaN) nanowire (NW) arrays have attracted a lot of attention because of their potential for novel devices in the fields of optoelectronics and nanoelectronics. In this work, GaN NW arrays have been designed and fabricated by combining suitable nanomachining processes including dry and wet etching. After inductively coupled plasma dry reactive ion etching, the GaN NWs are subsequently treated in wet chemical etching using AZ400K developer (i.e., with an activation energy of 0.69 ± 0.02 eV and a Cr mask) to form hexagonal and smooth a-plane sidewalls. Etching experiments using potassium hydroxide (KOH) water solution reveal that the sidewall orientation preference depends on etchant concentration. A model concerning surface bonding configuration on crystallography facets has been proposed to understand the anisotropic wet etching mechanism. Finally, NW array-based vertical field-effect transistors with wrap-gated structure have been fabricated. A device composed of 99 NWs exhibits enhancement mode operation with a threshold voltage of 1.5 V, a superior electrostatic control, and a high current output of >10 mA, which prevail potential applications in next-generation power switches and high-temperature digital circuits.

GaN nanowire arrays with nonpolar sidewalls for vertically integrated field-effect transistors.

PubMed

Yu, Feng; Yao, Shengbo; Römer, Friedhard; Witzigmann, Bernd; Schimpke, Tilman; Strassburg, Martin; Bakin, Andrey; Schumacher, Hans Werner; Peiner, Erwin; Wasisto, Hutomo Suryo; Waag, Andreas

2017-03-03

Vertically aligned gallium nitride (GaN) nanowire (NW) arrays have attracted a lot of attention because of their potential for novel devices in the fields of optoelectronics and nanoelectronics. In this work, GaN NW arrays have been designed and fabricated by combining suitable nanomachining processes including dry and wet etching. After inductively coupled plasma dry reactive ion etching, the GaN NWs are subsequently treated in wet chemical etching using AZ400K developer (i.e., with an activation energy of 0.69 ± 0.02 eV and a Cr mask) to form hexagonal and smooth a-plane sidewalls. Etching experiments using potassium hydroxide (KOH) water solution reveal that the sidewall orientation preference depends on etchant concentration. A model concerning surface bonding configuration on crystallography facets has been proposed to understand the anisotropic wet etching mechanism. Finally, NW array-based vertical field-effect transistors with wrap-gated structure have been fabricated. A device composed of 99 NWs exhibits enhancement mode operation with a threshold voltage of 1.5 V, a superior electrostatic control, and a high current output of >10 mA, which prevail potential applications in next-generation power switches and high-temperature digital circuits.

Multicycle rapid thermal annealing optimization of Mg-implanted GaN: Evolution of surface, optical, and structural properties

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

Greenlee, Jordan D., E-mail: jordan.greenlee.ctr@nrl.navy.mil; Feigelson, Boris N.; Anderson, Travis J.

2014-08-14

The first step of a multi-cycle rapid thermal annealing process was systematically studied. The surface, structure, and optical properties of Mg implanted GaN thin films annealed at temperatures ranging from 900 to 1200 °C were investigated by Raman spectroscopy, photoluminescence, UV-visible spectroscopy, atomic force microscopy, and Nomarski microscopy. The GaN thin films are capped with two layers of in-situ metal organic chemical vapor deposition -grown AlN and annealed in 24 bar of N{sub 2} overpressure to avoid GaN decomposition. The crystal quality of the GaN improves with increasing annealing temperature as confirmed by UV-visible spectroscopy and the full widths at halfmore » maximums of the E{sub 2} and A{sub 1} (LO) Raman modes. The crystal quality of films annealed above 1100 °C exceeds the quality of the as-grown films. At 1200 °C, Mg is optically activated, which is determined by photoluminescence measurements. However, at 1200 °C, the GaN begins to decompose as evidenced by pit formation on the surface of the samples. Therefore, it was determined that the optimal temperature for the first step in a multi-cycle rapid thermal anneal process should be conducted at 1150 °C due to crystal quality and surface morphology considerations.« less

Non-polar a-plane ZnO films grown on r-Al2O3 substrates using GaN buffer layers

NASA Astrophysics Data System (ADS)

Xu, C. X.; Chen, W.; Pan, X. H.; Chen, S. S.; Ye, Z. Z.; Huang, J. Y.

2016-09-01

In this work, GaN buffer layer has been used to grow non-polar a-plane ZnO films by laser-assisted and plasma-assisted molecular beam epitaxy. The thickness of GaN buffer layer ranges from ∼3 to 12 nm. The GaN buffer thickness effect on the properties of a-plane ZnO thin films is carefully investigated. The results show that the surface morphology, crystal quality and optical properties of a-plane ZnO films are strongly correlated with the thickness of GaN buffer layer. It was found that with 6 nm GaN buffer layer, a-plane ZnO films display the best crystal quality with X-ray diffraction rocking curve full-width at half-maximum of only 161 arcsec for the (101) reflection.

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

NASA Astrophysics Data System (ADS)

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

2012-06-01

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

Ultra-thin ohmic contacts for p-type nitride light emitting devices

DOEpatents

Raffetto, Mark; Bharathan, Jayesh; Haberern, Kevin; Bergmann, Michael; Emerson, David; Ibbetson, James; Li, Ting

2014-06-24

A flip-chip semiconductor based Light Emitting Device (LED) can include an n-type semiconductor substrate and an n-type GaN epi-layer on the substrate. A p-type GaN epi-layer can be on the n-type GaN epi-layer and a metal ohmic contact p-electrode can be on the p-type GaN epi-layer, where the metal ohmic contact p-electrode can have an average thickness less than about 25 .ANG.. A reflector can be on the metal ohmic contact p-electrode and a metal stack can be on the reflector. An n-electrode can be on the substrate opposite the n-type GaN epi-layer and a bonding pad can be on the n-electrode.

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

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

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

1997-08-01

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

Free and bound excitons in thin wurtzite GaN layers on sapphire

NASA Astrophysics Data System (ADS)

Merz, C.; Kunzer, M.; Kaufmann, U.; Akasaki, I.; Amano, H.

1996-05-01

Free and bound excitons have been studied by photoluminescence in thin (0268-1242/11/5/010/img8) wurtzite-undoped GaN, n-type GaN:Si as well as p-type GaN:Mg and GaN:Zn layers grown by metal-organic chemical vapour phase deposition (MOCVD). An accurate value for the free A exciton binding energy and an estimate for the isotropically averaged hole mass of the uppermost 0268-1242/11/5/010/img9 valence band are deduced from the data on undoped samples. The acceptor-doped samples reveal recombination lines which are attributed to excitons bound to 0268-1242/11/5/010/img10 and 0268-1242/11/5/010/img11 respectively. These lines are spectrally clearly separated and the exciton localization energies are in line with Haynes' rule. Whenever a comparison is possible, it is found that the exciton lines in these thin MOCVD layers are ultraviolet-shifted by 20 to 25 meV as compared to quasi-bulk (0268-1242/11/5/010/img12) samples. This effect is interpreted in terms of the compressive hydrostatic stress component which thin GaN layers experience when grown on sapphire with an AlN buffer layer.

Advanced Technologies for Heterodyne Radio Astronomy Instrumentation - Part1 By A. Pavolotsky, and Advanced Technologies for Heterodyne Radio Astronomy Instrumentation - Part2 By V. Desmaris

NASA Astrophysics Data System (ADS)

Desmaris, Vincent

2018-01-01

We present the advanced micro/nano technological engineering at the atomic level producing state-of-the-art epitaxial NbN thin-films on GaN buffer layers. Furthermore, we report the outstanding performance of the hot electron bolometers fabricated on epitaxial NbN thin films on GaN buffer layers. Finally we present advanced passive devices such as waveguide hybrids, IF hybrids and combiners for the realization of heterodyne THz receivers.

Characterization of remote O2-plasma-enhanced CVD SiO2/GaN(0001) structure using photoemission measurements

NASA Astrophysics Data System (ADS)

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

2018-01-01

The control of chemical composition and bonding features at a SiO2/GaN interface is a key to realizing high-performance GaN power devices. In this study, an ∼5.2-nm-thick SiO2 film has been deposited on an epitaxial GaN(0001) surface by remote O2-plasma-enhanced chemical vapor deposition (O2-RPCVD) using SiH4 and Ar/O2 mixture gases at a substrate temperature of 500 °C. The depth profile of chemical structures and electronic defects of the O2-RPCVD SiO2/GaN structures has been evaluated from a combination of SiO2 thinning examined by X-ray photoelectron spectroscopy (XPS) and the total photoelectron yield spectroscopy (PYS) measurements. As a highlight, we found that O2-RPCVD is effective for fabricating an abrupt SiO2/GaN interface.

Spin injection in epitaxial MnGa(111)/GaN(0001) heterostructures

NASA Astrophysics Data System (ADS)

Zube, Christian; Malindretos, Joerg; Watschke, Lars; Zamani, Reza R.; Disterheft, David; Ulbrich, Rainer G.; Rizzi, Angela; Iza, Michael; Keller, Stacia; DenBaars, Steven P.

2018-01-01

Ferromagnetic MnGa(111) layers were grown on GaN(0001) by molecular beam epitaxy. MnGa/GaN Schottky diodes with a doping level of around n = 7 × 1018 cm-3 were fabricated to achieve single step tunneling across the metal/semiconductor junction. Below the GaN layer, a thin InGaN quantum well served as optical spin detector ("spin-LED"). For electron spin injection from MnGa into GaN and subsequent spin transport through a 45 nm (70 nm) thick GaN layer, we observe a circular polarization of 0.3% (0.2%) in the electroluminescence at 80 K. Interface mixing, spin polarization losses during electrical transport in the GaN layer, and spin relaxation in the InGaN quantum well are discussed in relation with the low value of the optically detected spin polarization.

AlGaN/GaN field effect transistors for power electronics—Effect of finite GaN layer thickness on thermal characteristics

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

Hodges, C., E-mail: chris.hodges@bristol.ac.uk; Anaya Calvo, J.; Kuball, M.

2013-11-11

AlGaN/GaN heterostructure field effect transistors with a 150 nm thick GaN channel within stacked Al{sub x}Ga{sub 1−x}N layers were investigated using Raman thermography. By fitting a thermal simulation to the measured temperatures, the thermal conductivity of the GaN channel was determined to be 60 W m{sup −1} K{sup −1}, over 50% less than typical GaN epilayers, causing an increased peak channel temperature. This agrees with a nanoscale model. A low thermal conductivity AlGaN buffer means the GaN spreads heat; its properties are important for device thermal characteristics. When designing power devices with thin GaN layers, as well as electrical considerations, the reducedmore » channel thermal conductivity must be considered.« less

Analysis of light extraction efficiency enhancement for thin-film-flip-chip InGaN quantum wells light-emitting diodes with GaN micro-domes.

PubMed

Zhao, Peng; Zhao, Hongping

2012-09-10

The enhancement of light extraction efficiency for thin-film flip-chip (TFFC) InGaN quantum wells (QWs) light-emitting diodes (LEDs) with GaN micro-domes on n-GaN layer was studied. The light extraction efficiency of TFFC InGaN QWs LEDs with GaN micro-domes were calculated and compared to that of the conventional TFFC InGaN QWs LEDs with flat surface. The three dimensional finite difference time domain (3D-FDTD) method was used to calculate the light extraction efficiency for the InGaN QWs LEDs emitting at 460nm and 550 nm, respectively. The effects of the GaN micro-dome feature size and the p-GaN layer thickness on the light extraction efficiency were studied systematically. Studies indicate that the p-GaN layer thickness is critical for optimizing the TFFC LED light extraction efficiency. Significant enhancement of the light extraction efficiency (2.5-2.7 times for λ(peak) = 460nm and 2.7-2.8 times for λ(peak) = 550nm) is achievable from TFFC InGaN QWs LEDs with optimized GaN micro-dome diameter and height.

Plasma-assisted Molecular Beam Epitaxy of N-polar InAlN-barrier High-electron-mobility Transistors.

PubMed

Hardy, Matthew T; Storm, David F; Katzer, D Scott; Downey, Brian P; Nepal, Neeraj; Meyer, David J

2016-11-24

Plasma-assisted molecular beam epitaxy is well suited for the epitaxial growth of III-nitride thin films and heterostructures with smooth, abrupt interfaces required for high-quality high-electron-mobility transistors (HEMTs). A procedure is presented for the growth of N-polar InAlN HEMTs, including wafer preparation and growth of buffer layers, the InAlN barrier layer, AlN and GaN interlayers and the GaN channel. Critical issues at each step of the process are identified, such as avoiding Ga accumulation in the GaN buffer, the role of temperature on InAlN compositional homogeneity, and the use of Ga flux during the AlN interlayer and the interrupt prior to GaN channel growth. Compositionally homogeneous N-polar InAlN thin films are demonstrated with surface root-mean-squared roughness as low as 0.19 nm and InAlN-based HEMT structures are reported having mobility as high as 1,750 cm 2 /V∙sec for devices with a sheet charge density of 1.7 x 10 13 cm -2 .

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

NASA Astrophysics Data System (ADS)

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

1998-08-01

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

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

PubMed

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

2000-01-01

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

Increase in the diffusion length of minority carriers in Al{sub x}Ga{sub 1–x}N alloys ({sub x} = 0–0.1) fabricated by ammonia molecular beam epitaxy

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

Malin, T. V., E-mail: mal-tv@mail.ru; Gilinsky, A. M.; Mansurov, V. G.

2015-10-15

The room-temperature diffusion length of minority carriers in n-Al{sub 0.1}Ga{sub 0.9}N layers grown by ammonia molecular beam epitaxy on sapphire (0001) substrates used in structures for ultraviolet photodetectors is studied. Measurements were performed using the spectral dependence of the photocurrent recorded in a built-in p–n junction for thin samples and using the induced electron-current procedure for films up to 2 µm thick. The results show that the hole diffusion length in n-AlGaN films is 120–150 nm, which is larger than in GaN films grown under similar growth conditions by a factor of 3–4. This result can be associated with themore » larger lateral sizes characteristic of hexagonal columns in AlGaN layers grown by molecular beam epitaxy. No increase in the hole diffusion length is observed for thicker films.« less

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

Moutanabbir, O.; Scholz, R.; Senz, S.

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.

High brightness nonpolar a-plane (11-20) GaN light-emitting diodes

NASA Astrophysics Data System (ADS)

Jung, Sukkoo; Chang, Younghak; Bang, Kyu-Hyun; Kim, Hyung-Gu; Choi, Yoon-Ho; Hwang, Sung-Min; Baik, Kwang Hyeon

2012-02-01

We report on high brightness nonpolar a-plane InGaN/GaN LEDs using patterned lateral overgrowth (PLOG) epitaxy. High crystal-quality and smooth surfaces for a-plane GaN (a-GaN) films were achieved using PLOG with an array of hexagonal SiO2 patterns. The XRC FWHMs of as-grown PLOG a-GaN films were found to be 414 and 317 arcsec (450 and 455 arcsec for planar a-GaN films) along the c-axis and m-axis directions, respectively. Plan-view CL clearly reveals the periodic hexagonal patterns with higher band edge emission intensity, implying that the luminescence properties of a-GaN films lying above the SiO2 mask are improved. The light output powers of a-InGaN/GaN PLOG LEDs were measured to be 7.5 mW and 20 mW at drive currents of 20 mA and 100 mA, respectively. A negligible blue-shift was observed in the peak emission wavelength with increasing drive current up to 100 mA, indicating that there are no strong internal fields in nonpolar a-InGaN/GaN LEDs. We believe that nonpolar a-plane InGaN/GaN LEDs hold promise for efficient nitride emitters if the growth conditions are further optimized.

Lateral polarity control of III-nitride thin film and application in GaN Schottky barrier diode

NASA Astrophysics Data System (ADS)

Li, Junmei; Guo, Wei; Sheikhi, Moheb; Li, Hongwei; Bo, Baoxue; Ye, Jichun

2018-05-01

N-polar and III-polar GaN and AlN epitaxial thin films grown side by side on single sapphire substrate was reported. Surface morphology, wet etching susceptibility and bi-axial strain conditions were investigated and the polarity control scheme was utilized in the fabrication of Schottky barrier diode where ohmic contact and Schottky contact were deposited on N-polar domains and Ga-polar domains, respectively. The influence of N-polarity on on-state resistivity and I–V characteristic was discussed, demonstrating that lateral polarity structure of GaN and AlN can be widely used in new designs of optoelectronic and electronic devices. Project partially supported by the National Key Research and Development Program of China (No. 2016YFB0400802), the National Natural Science Foundation of China (No. 61704176), and the Open project of Zhejiang Key Laboratory for Advanced Microelectronic Intelligent Systems and Applications (No. ZJUAMIS1704).

Growth kinetics and characterizations of gallium nitride thin films by remote PECVD

NASA Technical Reports Server (NTRS)

Choi, S. W.; Bachmann, K. J.; Lucovsky, G.

1993-01-01

Thin films of GaN have been deposited at relatively low growth temperatures by remote plasma-enhanced chemical-vapor deposition (RPECVD), using a plasma excited NH3, and trimethylgallium (TMG), injected downstream from the plasma. The activation energy for GaN growth has been tentatively assigned to the dissociation of NH groups as the primary N-atom precursors in the surface reaction with adsorbed TMG, or TMG fragments. At high He flow rates, an abrupt increase in the growth rate is observed and corresponds to a change in the reaction mechanism attributed to the formation of atomic N. XRD reveals an increased tendency to ordered growth in the (0001) direction with increasing growth temperature, He flow rate, and RF plasma power. IR spectra show the fundamental lattice mode of GaN at 530 cm without evidence for vibrational modes of hydrocarbon groups.

Metal-organic chemical vapor deposition of N-polar InN quantum dots and thin films on vicinal GaN

NASA Astrophysics Data System (ADS)

Lund, Cory; Catalano, Massimo; Wang, Luhua; Wurm, Christian; Mates, Thomas; Kim, Moon; Nakamura, Shuji; DenBaars, Steven P.; Mishra, Umesh K.; Keller, Stacia

2018-02-01

N-polar InN layers were deposited using MOCVD on GaN-on-sapphire templates which were miscut 4° towards the GaN m-direction. For thin layers, quantum dot-like features were spontaneously formed to relieve the strain between the InN and GaN layers. As the thickness was increased, the dots elongated along the step direction before growing outward perpendicular to the step direction and coalescing to form a complete InN layer. XRD reciprocal space maps indicated that the InN films relaxed upon quantum dot formation after nominally 1 nm thick growth, resulting in 5-7 nm tall dots with diameters around 20-50 nm. For thicker layers above 10 nm, high electron mobilities of up to 706 cm2/V s were measured using Hall effect measurements indicating high quality layers.

Effect of SiC buffer layer on GaN growth on Si via PA-MBE

NASA Astrophysics Data System (ADS)

Kukushkin, S. A.; Mizerov, A. M.; Osipov, A. V.; Redkov, A. V.; Telyatnik, R. S.; Timoshnev, S. N.

2017-11-01

The study is devoted to comparison of GaN thin films grown on SiC/Si substrates made by the method of atoms substitution with the films grown directly on Si substrates. The growth was performed in a single process via plasma assisted molecular beam epitaxy. The samples were studied via optical microscopy, Raman spectroscopy, ellipsometry, and a comparison of their characteristics was made. Using chemical etching in KOH, the polarity of GaN films grown on SiC/Si and Si substrates was determined.

GaN growth via HVPE on SiC/Si substrates: growth mechanisms

NASA Astrophysics Data System (ADS)

Sharofidinov, Sh Sh; Redkov, A. V.; Osipov, A. V.; Kukushkin, S. A.

2017-11-01

The article focuses on the study of GaN thin film growth via chloride epitaxy on SiC/Si hybrid substrate. SiC buffer layer was grown by a method of substitution of atoms, which allows one to reduce impact of mechanical stress therein on subsequent growth of III-nitride films. It is shown, that change in GaN growth conditions leads to change in its growth mechanism. Three mechanisms: epitaxial, spiral and stepwise growth are considered and mechanical stresses are estimated via Raman spectroscopy.

A novel technique based on a plasma focus device for nano-porous gallium nitride formation on P-type silicon

NASA Astrophysics Data System (ADS)

Sharifi Malvajerdi, S.; Salar Elahi, A.; Habibi, M.

2017-04-01

A new deposition formation was observed with a Mather-type Plasma Focus Device (MPFD). MPFD was unitized to fabricate porous Gallium Nitride (GaN) on p-type Silicon (Si) substrate with a (100) crystal orientation for the first time in a deposition process. GaN was deposited on Si with 4 and 7 shots. The samples were subjected to a 3 phase annealing procedure. First, the semiconductors were annealed in the PFD with nitrogen plasma shots after their deposition. Second, a thermal chemical vapor deposition annealed the samples for 1 h at 1050 °C by nitrogen gas at a pressure of 1 Pa. Finally, an electric furnace annealed the samples for 1 h at 1150 °C with continuous flow of nitrogen. Porous GaN structures were observed by Field emission scanning electron microscopy and atomic force microscopy. Furthermore, X-Ray diffraction analysis was carried out to determine the crystallinity of GaN after the samples were annealed. Energy-Dispersive X-Ray Spectroscopy indicated the amount of gallium, nitrogen, and oxygen due to the self-oxidation of the samples. Photoluminescence spectroscopy revealed emissions at 2.94 eV and 3.39 eV, which shows that hexagonal wurtzite crystal structures were formed.

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

NASA Astrophysics Data System (ADS)

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

2018-04-01

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

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.

Fabrication of stable, wide-bandgap thin films of Mg, Zn and O

DOEpatents

Katiyar, Ram S.; Bhattacharya, Pijush; Das, Rasmi R.

2006-07-25

A stable, wide-bandgap (approximately 6 eV) ZnO/MgO multilayer thin film is fabricated using pulsed-laser deposition on c-plane Al₂O₃ substrates. Layers of ZnO alternate with layers of MgO. The thickness of MgO is a constant of approximately 1 nm; the thicknesses of ZnO layers vary from approximately 0.75 to 2.5 nm. Abrupt structural transitions from hexagonal to cubic phase follow a decrease in the thickness of ZnO sublayers within this range. The band gap of the thin films is also influenced by the crystalline structure of multilayer stacks. Thin films with hexagonal and cubic structure have band-gap values of 3.5 and 6 eV, respectively. In the hexagonal phase, Mg content of the films is approximately 40%; in the cubic phase Mg content is approximately 60%. The thin films are stable and their structural and optical properties are unaffected by annealing at 750.degree. C.

Tailoring the light absorption of Ag-PZT thin films by controlling the growth of hexagonal- and cubic-phase Ag nanoparticles

NASA Astrophysics Data System (ADS)

Hu, Tao; Wang, Zongrong; Ma, Ning; Du, Piyi

2017-12-01

PbZr0.52Ti0.48O3 thin films containing hexagonal and cubic Ag nanoparticles (Ag NPs) of various sizes were prepared using the sol-gel technique. During the aging process, Ag ions were photo-reduced to form hexagonal Ag NPs. These NPs were uniform in size, and their uniformity was maintained in the thin films during the heat treatment process. Both the total volume and average size of the hexagonal Ag NPs increased with an increasing Ag ion concentration from 0.02 to 0.08 mol l-1. Meanwhile, the remaining Ag ions were reduced to form unstable Ag-Pb alloy particles with Pb ions during the early heating stage. During subsequent heat treatment, these alloys decomposed to form cubic Ag NPs in the thin films. The absorption range of the thin films, quantified as the full width at half maximum in the ultraviolet-visible absorption spectrum, expanded from 6.3 × 1013 Hz (390-425 nm) to 8.4 × 1013 Hz (383-429 nm) as the Ag NPs/PZT ratio increased from 0.2 to 0.8. This work provides an effective way to broaden the absorption range and enhance the optical properties of such films.

Piezo-phototronic effect on electroluminescence properties of p-type GaN thin films.

PubMed

Hu, Youfan; Zhang, Yan; Lin, Long; Ding, Yong; Zhu, Guang; Wang, Zhong Lin

2012-07-11

We present that the electroluminescence (EL) properties of Mg-doped p-type GaN thin films can be tuned by the piezo-phototronic effect via adjusting the minority carrier injection efficiency at the metal-semiconductor (M-S) interface by strain induced polarization charges. The device is a metal-semiconductor-metal structure of indium tin oxide (ITO)-GaN-ITO. Under different straining conditions, the changing trend of the transport properties of GaN films can be divided into two types, corresponding to the different c-axis orientations of the films. An extreme value was observed for the integral EL intensity under certain applied strain due to the adjusted minority carrier injection efficiency by piezoelectric charges introduced at the M-S interface. The external quantum efficiency of the blue EL at 430 nm was changed by 5.84% under different straining conditions, which is 1 order of magnitude larger than the change of the green peak at 540 nm. The results indicate that the piezo-phototronic effect has a larger impact on the shallow acceptor states related EL process than on the one related to the deep acceptor states in p-type GaN films. This study has great significance on the practical applications of GaN in optoelectronic devices under a working environment where mechanical deformation is unavoidable such as for flexible/printable light emitting diodes.

Effect of ZnO seed layer on the morphology and optical properties of ZnO nanorods grown on GaN buffer layers

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

Nandi, R., E-mail: rajunandi@iitb.ac.in; Mohan, S., E-mail: rajunandi@iitb.ac.in; Major, S. S.

2014-04-24

ZnO nanorods were grown by chemical bath deposition on sputtered, polycrystalline GaN buffer layers with and without ZnO seed layer. Scanning electron microscopy and X-ray diffraction show that the ZnO nanorods on GaN buffer layers are not vertically well aligned. Photoluminescence spectrum of ZnO nanorods grown on GaN buffer layer, however exhibits a much stronger near-band-edge emission and negligible defect emission, compared to the nanorods grown on ZnO buffer layer. These features are attributed to gallium incorporation at the ZnO-GaN interface. The introduction of a thin (25 nm) ZnO seed layer on GaN buffer layer significantly improves the morphology andmore » vertical alignment of ZnO-NRs without sacrificing the high optical quality of ZnO nanorods on GaN buffer layer. The presence of a thick (200 nm) ZnO seed layer completely masks the effect of the underlying GaN buffer layer on the morphology and optical properties of nanorods.« less

Growth of semiconducting GaN hollow spheres and nanotubes with very thin shells via a controllable liquid gallium-gas interface chemical reaction.

PubMed

Yin, Long-Wei; Bando, Yoshio; Li, Mu-Sen; Golberg, Dmitri

2005-11-01

An in situ liquid gallium-gas interface chemical reaction route has been developed to synthesize semiconducting hollow GaN nanospheres with very small shell size by carefully controlling the synthesis temperature and the ammonia reaction gas partial pressure. In this process the gallium droplet does not act as a catalyst but rather as a reactant and a template for the formation of hollow GaN structures. The diameter of the synthesized hollow GaN spheres is typically 20-25 nm and the shell thickness is 3.5-4.5 nm. The GaN nanotubes obtained at higher synthesis temperatures have a length of several hundreds of nanometers and a wall thickness of 3.5-5.0 nm. Both the hollow GaN spheres and nanotubes are polycrystalline and are composed of very fine GaN nanocrystalline particles with a diameter of 3.0-3.5 nm. The room-temperature photoluminescence (PL) spectra for the synthesized hollow GaN spheres and nanotubes, which have a narrow size distribution, display a sharp, blue-shifted band-edge emission peak at 3.52 eV (352 nm) due to quantum size effects.

Melioration of Optical and Electrical Performance of Ga-N Codoped ZnO Thin Films

NASA Astrophysics Data System (ADS)

Narayanan, Nripasree; Deepak, N. K.

2018-06-01

Transparent and conducting p-type zinc oxide (ZnO) thin films doped with gallium (Ga) and nitrogen (N) simultaneously were deposited on glass substrates by spray pyrolysis technique. Phase composition analysis by X-ray diffraction confirmed the polycrystallinity of the films with pure ZnO phase. Energy dispersive X-ray analysis showed excellent incorporation of N in the ZnO matrix by means of codoping. The optical transmittance of N monodoped film was poor but got improved with Ga-N codoping and also resulted in the enhancement of optical energy gap. Hole concentration increased with codoping and consequently, lower resistivity and high stability were obtained.

Catalytic Activation of Mg-Doped GaN by Hydrogen Desorption Using Different Metal Thin Layers

NASA Astrophysics Data System (ADS)

Wei, Tongbo; Wang, Junxi; Liu, Naixin; Lu, Hongxi; Zeng, Yiping; Wang, Guohong; Li, Jinmin

2010-10-01

The annealing of Mg-doped GaN with Pt and Mo layers has been found to effectively improve the hole concentration of such material by more than 2 times as high as those in the same material without metal. Compared with the Ni and Mo catalysts, Pt showed good activation effect for hydrogen desorption and ohmic contact to the Ni/Au electrode. Despite the weak hydrogen desorption, Mo did not diffuse into the GaN epilayer in the annealing process, thus suppressing the carrier compensation phenomenon with respect to Ni and Pt depositions, which resulted in the high activation of Mg acceptors. For the GaN activated with the Ni, Pt, and Mo layers, the blue emission became dominant, followed by a clear peak redshift and the degradation of photoluminescence signal when compared with that of GaN without metal.

GaN based nanorods for solid state lighting

NASA Astrophysics Data System (ADS)

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.

Novel approach for III-N on Si (111) templates fabrication by low-temperature PA MBE using porous Si layer

NASA Astrophysics Data System (ADS)

Zolotukhin, D.; Seredin, P.; Lenshin, A.; Goloshchapov, D.; Mizerov, A.

2017-11-01

We report on successful growth of GaN nanorods by low-temperature plasma-assisted molecular beam epitaxy on a Si(111) substrate with and without preformed thin porous Si layer (por-Si). The deposited GaN initially forms islands which act as a seed for the wires. Porous structure of the por-Si layer helps to control nucleation islands sizes and achieve homogeneous distribution of the nanorods diameters. In addition 850 nm-thick crack-free GaN layer was formed on Si(111) substrate with preformed por-Si layer.

Hole-mediated stabilization of cubic GaN.

PubMed

Dalpian, Gustavo M; Wei, Su-Huai

2004-11-19

We propose here a new approach to stabilize the cubic zinc-blende (ZB) phase by incorporation of impurities into a compound that has a hexagonal wurtzite (WZ) ground state. For GaN, we suggest that this can be achieved by adding 3d acceptors such as Zn, Mn, or Cu because the p-d repulsion between the 3d impurity levels and the valence band maximum is larger in the ZB phase than in the WZ phase. This makes the top of the valence states of the ZB structure higher than that of the WZ structure. As holes are created at the top of the valence states by the impurities, it will cost less energy for the holes to be created in the ZB structure, thus stabilizing this phase. Our first-principles total energy calculations confirm this novel idea.

Metalorganic chemical vapor deposition growth of high-mobility AlGaN/AlN/GaN heterostructures on GaN templates and native GaN substrates

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

Chen, Jr-Tai, E-mail: jrche@ifm.liu.se; Hsu, Chih-Wei; Forsberg, Urban

2015-02-28

Severe surface decomposition of semi-insulating (SI) GaN templates occurred in high-temperature H{sub 2} atmosphere prior to epitaxial growth in a metalorganic chemical vapor deposition system. A two-step heating process with a surface stabilization technique was developed to preserve the GaN template surface. Utilizing the optimized heating process, a high two-dimensional electron gas mobility ∼2000 cm{sup 2}/V·s was obtained in a thin AlGaN/AlN/GaN heterostructure with an only 100-nm-thick GaN spacer layer homoepitaxially grown on the GaN template. This technique was also demonstrated viable for native GaN substrates to stabilize the surface facilitating two-dimensional growth of GaN layers. Very high residual silicon andmore » oxygen concentrations were found up to ∼1 × 10{sup 20 }cm{sup −3} at the interface between the GaN epilayer and the native GaN substrate. Capacitance-voltage measurements confirmed that the residual carbon doping controlled by growth conditions of the GaN epilayer can be used to successfully compensate the donor-like impurities. State-of-the-art structural properties of a high-mobility AlGaN/AlN/GaN heterostructure was then realized on a 1 × 1 cm{sup 2} SI native GaN substrate; the full width at half maximum of the X-ray rocking curves of the GaN (002) and (102) peaks are only 21 and 14 arc sec, respectively. The surface morphology of the heterostructure shows uniform parallel bilayer steps, and no morphological defects were noticeable over the entire epi-wafer.« less

Direct evidence of recombination between electrons in InGaN quantum discs and holes in p-type GaN.

PubMed

Sun, Xiaoxiao; Wang, Xinqiang; Wang, Ping; Wang, Tao; Sheng, Bowen; Zheng, Xiantong; Li, Mo; Zhang, Jian; Yang, Xuelin; Xu, Fujun; Ge, Weikun; Shen, Bo

2017-11-27

Intense emission from an InGaN quantum disc (QDisc) embedded in a GaN nanowire p-n junction is directly resolved by performing cathodoluminescence spectroscopy. The luminescence observed from the p-type GaN region is exclusively dominated by the emission at 380 nm, which has been usually reported as the emission from Mg induced impurity bands. Here, we confirm that the robust emission from 380 nm is actually not due to the Mg induced impurity bands, but rather due to being the recombination between electrons in the QDisc and holes in the p-type GaN. This identification helps to get a better understanding of the confused luminescence from nanowires with thin QDiscs embedded for fabricating electrically driven single photon emitters.

In0.15Ga0.85N visible-light metal-semiconductor-metal photodetector with GaN interlayers deposited by pulsed NH3

NASA Astrophysics Data System (ADS)

Wang, Hongxia; Zhang, Xiaohan; Wang, Hailong; Lv, Zesheng; Li, Yongxian; Li, Bin; Yan, Huan; Qiu, Xinjia; Jiang, Hao

2018-05-01

InGaN visible-light metal-semiconductor-metal photodetectors with GaN interlayers deposited by pulsed NH3 were fabricated and characterized. By periodically inserting the GaN thin interlayers, the surface morphology of InGaN active layer is improved and the phase separation is suppressed. At 5 V bias, the dark current reduced from 7.0 × 10-11 A to 7.0 × 10-13 A by inserting the interlayers. A peak responsivity of 85.0 mA/W was measured at 420 nm and 5 V bias, corresponding to an external quantum efficiency of 25.1%. The insertion of GaN interlayers also lead to a sharper spectral response cutoff.

Coaxial metal-oxide-semiconductor (MOS) Au/Ga2O3/GaN nanowires.

PubMed

Hsieh, Chin-Hua; Chang, Mu-Tung; Chien, Yu-Jen; Chou, Li-Jen; Chen, Lih-Juann; Chen, Chii-Dong

2008-10-01

Coaxial metal-oxide-semiconductor (MOS) Au-Ga2O3-GaN heterostructure nanowires were successfully fabricated by an in situ two-step process. The Au-Ga2O3 core-shell nanowires were first synthesized by the reaction of Ga powder, a mediated Au thin layer, and a SiO2 substrate at 800 degrees C. Subsequently, these core-shell nanowires were nitridized in ambient ammonia to form a GaN coating layer at 600 degrees C. The GaN shell is a single crystal, an atomic flat interface between the oxide and semiconductor that ensures that the high quality of the MOS device is achieved. These novel 1D nitride-based MOS nanowires may have promise as building blocks to the future nitride-based vertical nanodevices.

GaN membrane MSM ultraviolet photodetectors

NASA Astrophysics Data System (ADS)

Muller, A.; Konstantinidis, G.; Kostopoulos, A.; Dragoman, M.; Neculoiu, D.; Androulidaki, M.; Kayambaki, M.; Vasilache, D.; Buiculescu, C.; Petrini, I.

2006-12-01

GaN exhibits unique physical properties, which make this material very attractive for wide range of applications and among them ultraviolet detection. For the first time a MSM type UV photodetector structure was manufactured on a 2.2 μm. thick GaN membrane obtained using micromachining techniques. The low unintentionally doped GaN layer structure was grown by MOCVD on high resistivity (ρ>10kΩcm) <111> oriented silicon wafers, 500μm thick. The epitaxially grown layers include a thin AlN layer in order to reduce the stress in the GaN layer and avoid cracking. Conventional contact lithography, e-gun Ni/Au (10nm /200nm) evaporation and lift-off techniques were used to define the interdigitated Schottky metalization on the top of the wafer. Ten digits with a width of 1μm and a length of 100μm were defined for each electrode. The distance between the digits was also 1μm. After the backside lapping of the wafer to a thickness of approximately 150μm, a 400nm thick Al layer was patterned and deposited on the backside, to be used as mask for the selective reactive ion etching of silicon. The backside mask, for the membrane formation, was patterned using double side alignment techniques and silicon was etched down to the 2.2μm thin GaN layer using SF 6 plasma. A very low dark current (30ρA at 3V) was obtained. Optical responsivity measurements were performed at 1.5V. A maximum responsivity of 18mA/W was obtained at a wavelength of 370nm. This value is very good and can be further improved using transparent contacts for the interdigitated structure.

Synthesis of p-type GaN nanowires.

PubMed

Kim, Sung Wook; Park, Youn Ho; Kim, Ilsoo; Park, Tae-Eon; Kwon, Byoung Wook; Choi, Won Kook; Choi, Heon-Jin

2013-09-21

GaN has been utilized in optoelectronics for two decades. However, p-type doping still remains crucial for realization of high performance GaN optoelectronics. Though Mg has been used as a p-dopant, its efficiency is low due to the formation of Mg-H complexes and/or structural defects in the course of doping. As a potential alternative p-type dopant, Cu has been recognized as an acceptor impurity for GaN. Herein, we report the fabrication of Cu-doped GaN nanowires (Cu:GaN NWs) and their p-type characteristics. The NWs were grown vertically via a vapor-liquid-solid (VLS) mechanism using a Au/Ni catalyst. Electrical characterization using a nanowire-field effect transistor (NW-FET) showed that the NWs exhibited n-type characteristics. However, with further annealing, the NWs showed p-type characteristics. A homo-junction structure (consisting of annealed Cu:GaN NW/n-type GaN thin film) exhibited p-n junction characteristics. A hybrid organic light emitting diode (OLED) employing the annealed Cu:GaN NWs as a hole injection layer (HIL) also demonstrated current injected luminescence. These results suggest that Cu can be used as a p-type dopant for GaN NWs.

Selective-area catalyst-free MBE growth of GaN nanowires using a patterned oxide layer.

PubMed

Schumann, T; Gotschke, T; Limbach, F; Stoica, T; Calarco, R

2011-03-04

GaN nanowires (NWs) were grown selectively in holes of a patterned silicon oxide mask, by rf-plasma-assisted molecular beam epitaxy (PAMBE), without any metal catalyst. The oxide was deposited on a thin AlN buffer layer previously grown on a Si(111) substrate. Regular arrays of holes in the oxide layer were obtained using standard e-beam lithography. The selectivity of growth has been studied varying the substrate temperature, gallium beam equivalent pressure and patterning layout. Adjusting the growth parameters, GaN NWs can be selectively grown in the holes of the patterned oxide with complete suppression of the parasitic growth in between the holes. The occupation probability of a hole with a single or multiple NWs depends strongly on its diameter. The selectively grown GaN NWs have one common crystallographic orientation with respect to the Si(111) substrate via the AlN buffer layer, as proven by x-ray diffraction (XRD) measurements. Based on the experimental data, we present a schematic model of the GaN NW formation in which a GaN pedestal is initially grown in the hole.

Process for producing fine and ultrafine filament superconductor wire

DOEpatents

Kanithi, H.C.

1992-02-18

A process for producing a superconductor wire made up of a large number of round monofilament rods is provided for, comprising assembling a multiplicity of round monofilaments inside each of a multiplicity of thin wall hexagonal tubes and then assembling a number of said thin wall hexagonal tubes within an extrusion can and subsequently consolidating, extruding and drawing the entire assembly down to the desired wire size. 8 figs.

Process for producing fine and ultrafine filament superconductor wire

DOEpatents

Kanithi, Hem C.

1992-01-01

A process for producing a superconductor wire made up of a large number of round monofilament rods is provided for, comprising assembling a multiplicity of round monofilaments inside each of a multiplicity of thin wall hexagonal tubes and then assembling a number of said thin wall hexagonal tubes within an extrusion can and subsequently consolidating, extruding and drawing the entire assembly down to the desired wire size.

GaN ultraviolet p-i-n photodetectors with enhanced deep ultraviolet quantum efficiency

NASA Astrophysics Data System (ADS)

Wang, Guosheng; Xie, Feng; Wang, Jun; Guo, Jin

2017-10-01

GaN ultraviolet (UV) p-i-n photodetectors (PDs) with a thin p-AlGaN/GaN contact layer are designed and fabricated. The PD exhibits a low dark current density of˜7 nA/cm2 under -5 V, and a zero-bias peak responsivity of ˜0.16 A/W at 360 nm, which corresponds to a maximum quantum efficiency of 55%. It is found that, in the wavelength range between 250 and 365 nm, the PD with thin p-AlGaN/GaN contact layer exhibits enhanced quantum efficiency especially in a deep-UV wavelength range, than that of the control PD with conventional thin p-GaN contact layer. The improved quantum efficiency of the PD with thin p-AlGaN/GaN contact layer in the deep-UV wavelength range is mainly attributed to minority carrier reflecting properties of thin p-AlGaN/GaN heterojunction which could reduce the surface recombination loss of photon-generated carriers and improve light current collection efficiency.

Combined angle-resolved X-ray photoelectron spectroscopy, density functional theory and kinetic study of nitridation of gallium arsenide

NASA Astrophysics Data System (ADS)

Mehdi, H.; Monier, G.; Hoggan, P. E.; Bideux, L.; Robert-Goumet, C.; Dubrovskii, V. G.

2018-01-01

The high density of interface and surface states that cause the strong Fermi pinning observed on GaAs surfaces can be reduced by depositing GaN ultra-thin films on GaAs. To further improve this passivation, it is necessary to investigate the nitridation phenomena by identifying the distinct steps occurring during the process and to understand and quantify the growth kinetics of GaAs nitridation under different conditions. Nitridation of the cleaned GaAs substrate was performed using N2 plasma source. Two approaches have been combined. Firstly, an AR-XPS (Angle Resolved X-ray Photoelectron Spectroscopy) study is carried out to determine the chemical environments of the Ga, As and N atoms and the composition depth profile of the GaN thin film which allow us to summarize the nitridation process in three steps. Moreover, the temperature and time treatment have been investigated and show a significant impact on the formation of the GaN layer. The second approach is a refined growth kinetic model which better describes the GaN growth as a function of the nitridation time. This model clarifies the exchange mechanism of arsenic with nitrogen atoms at the GaN/GaAs interface and the phenomenon of quasi-saturation of the process observed experimentally.

Measurement of second order susceptibilities of GaN and AlGaN

NASA Astrophysics Data System (ADS)

Sanford, N. A.; Davydov, A. V.; Tsvetkov, D. V.; Dmitriev, A. V.; Keller, S.; Mishra, U. K.; DenBaars, S. P.; Park, S. S.; Han, J. Y.; Molnar, R. J.

2005-03-01

Rotational Maker fringes, scaled with respect to χ11(2) of crystalline quartz, were used to determine the second order susceptibilities χ31(2) and χ33(2) for samples of thin AlxGa1-xN films, a thicker GaN film, and a free-standing GaN platelets. The pump wavelength was 1064nm. The AlxGa1-xN samples, ranging in thickness from roughly 0.5to4.4μm, were grown by metalorganic chemical vapor deposition (MOCVD) and hydride vapor-phase epitaxy (HVPE) on (0001) sapphire substrates. The Al mole fractions x were 0, 0.419, 0.507, 0.618, 0.660, and 0.666, for the MOCVD-grown samples, and x =0, 0.279, 0.363, and 0.593 for the HVPE-grown samples. An additional HVPE-grown GaN sample ˜70μm thick was also examined. The free-standing bulk GaN platelets consisted of an HVPE grown film ˜226μm thick removed from its growth substrate, and a crystal ˜160μm thick grown by high-pressure techniques. For the AlxGa1-xN samples, the magnitudes of χ31(2) and χ33(2) decrease roughly linearly with increasing x and extrapolate to ˜0 for x =1. Furthermore, the constraint expected for a perfect wurtzite structure, namely χ33(2)=-2χ31(2), was seldom observed, and the samples with x =0.660 and x =0.666 showed χ31(2) and χ33(2) having the same sign. These results are consistent with the theoretical studies of nonlinear susceptibilities for AlN and GaN performed by Chen et al. [Appl. Phys. Lett. 66, 1129 (1995)]. The thicker bulk GaN samples displayed a complex superposition of high- and low-frequency Maker fringes due to the multiple-pass interference of the pump and second-harmonic generation beams, and the nonlinear coefficients were approximately consistent with those measured for the thin-film GaN sample.

Crystallographic alignment of high-density gallium nitride nanowire arrays.

PubMed

Kuykendall, Tevye; Pauzauskie, Peter J; Zhang, Yanfeng; Goldberger, Joshua; Sirbuly, Donald; Denlinger, Jonathan; Yang, Peidong

2004-08-01

Single-crystalline, one-dimensional semiconductor nanostructures are considered to be one of the critical building blocks for nanoscale optoelectronics. Elucidation of the vapour-liquid-solid growth mechanism has already enabled precise control over nanowire position and size, yet to date, no reports have demonstrated the ability to choose from different crystallographic growth directions of a nanowire array. Control over the nanowire growth direction is extremely desirable, in that anisotropic parameters such as thermal and electrical conductivity, index of refraction, piezoelectric polarization, and bandgap may be used to tune the physical properties of nanowires made from a given material. Here we demonstrate the use of metal-organic chemical vapour deposition (MOCVD) and appropriate substrate selection to control the crystallographic growth directions of high-density arrays of gallium nitride nanowires with distinct geometric and physical properties. Epitaxial growth of wurtzite gallium nitride on (100) gamma-LiAlO(2) and (111) MgO single-crystal substrates resulted in the selective growth of nanowires in the orthogonal [1\\[Evec]0] and [001] directions, exhibiting triangular and hexagonal cross-sections and drastically different optical emission. The MOCVD process is entirely compatible with the current GaN thin-film technology, which would lead to easy scale-up and device integration.

Enhanced light extraction from free-standing InGaN/GaN light emitters using bio-inspired backside surface structuring.

PubMed

Pynn, Christopher D; Chan, Lesley; Lora Gonzalez, Federico; Berry, Alex; Hwang, David; Wu, Haoyang; Margalith, Tal; Morse, Daniel E; DenBaars, Steven P; Gordon, Michael J

2017-07-10

Light extraction from InGaN/GaN-based multiple-quantum-well (MQW) light emitters is enhanced using a simple, scalable, and reproducible method to create hexagonally close-packed conical nano- and micro-scale features on the backside outcoupling surface. Colloidal lithography via Langmuir-Blodgett dip-coating using silica masks (d = 170-2530 nm) and Cl 2 /N 2 -based plasma etching produced features with aspect ratios of 3:1 on devices grown on semipolar GaN substrates. InGaN/GaN MQW structures were optically pumped at 266 nm and light extraction enhancement was quantified using angle-resolved photoluminescence. A 4.8-fold overall enhancement in light extraction (9-fold at normal incidence) relative to a flat outcoupling surface was achieved using a feature pitch of 2530 nm. This performance is on par with current photoelectrochemical (PEC) nitrogen-face roughening methods, which positions the technique as a strong alternative for backside structuring of c-plane devices. Also, because colloidal lithography functions independently of GaN crystal orientation, it is applicable to semipolar and nonpolar GaN devices, for which PEC roughening is ineffective.

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

Zhou, Kun; Ikeda, Masao, E-mail: mikeda2013@sinano.ac.cn, E-mail: jpliu2010@sinano.ac.cn; Liu, Jianping, E-mail: mikeda2013@sinano.ac.cn, E-mail: jpliu2010@sinano.ac.cn

The efficiency droop of InGaN/GaN(InGaN) multiple quantum well (MQW) light emitting diodes (LEDs) with thin quantum barriers (QB) is studied. With thin GaN QB (3 nm–6 nm thickness), the efficiency droop is not improved, which indicates that hole transport cannot be significantly enhanced by the thin GaN QBs. On the contrary, the efficiency droop was remarkably reduced by using a InGaN staircase QB (InGaN SC-QB) MQWs structure where InGaN SC-QBs lower the transport energy barrier of holes. The efficiency droop ratio was as low as 3.3% up to 200 A/cm{sup 2} for the InGaN SC-QB LED. By using monitoring QW with longer wavelengthmore » we observe a much uniform carrier distribution in the InGaN SC-QB LEDs, which reveals the mechanism of improvement in the efficiency droop.« less

RBS/Channeling Studies of Swift Heavy Ion Irradiated GaN Layers

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

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

2009-03-10

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

Ultrathin silicon oxynitride layer on GaN for dangling-bond-free GaN/insulator interface.

PubMed

Nishio, Kengo; Yayama, Tomoe; Miyazaki, Takehide; Taoka, Noriyuki; Shimizu, Mitsuaki

2018-01-23

Despite the scientific and technological importance of removing interface dangling bonds, even an ideal model of a dangling-bond-free interface between GaN and an insulator has not been known. The formation of an atomically thin ordered buffer layer between crystalline GaN and amorphous SiO 2 would be a key to synthesize a dangling-bond-free GaN/SiO 2 interface. Here, we predict that a silicon oxynitride (Si 4 O 5 N 3 ) layer can epitaxially grow on a GaN(0001) surface without creating dangling bonds at the interface. Our ab initio calculations show that the GaN/Si 4 O 5 N 3 structure is more stable than silicon-oxide-terminated GaN(0001) surfaces. The electronic properties of the GaN/Si 4 O 5 N 3 structure can be tuned by modifying the chemical components near the interface. We also propose a possible approach to experimentally synthesize the GaN/Si 4 O 5 N 3 structure.

The controlled growth of GaN nanowires.

PubMed

Hersee, Stephen D; Sun, Xinyu; Wang, Xin

2006-08-01

This paper reports a scalable process for the growth of high-quality GaN nanowires and uniform nanowire arrays in which the position and diameter of each nanowire is precisely controlled. The approach is based on conventional metalorganic chemical vapor deposition using regular precursors and requires no additional metal catalyst. The location, orientation, and diameter of each GaN nanowire are controlled using a thin, selective growth mask that is patterned by interferometric lithography. It was found that use of a pulsed MOCVD process allowed the nanowire diameter to remain constant after the nanowires had emerged from the selective growth mask. Vertical GaN nanowire growth rates in excess of 2 mum/h were measured, while remarkably the diameter of each nanowire remained constant over the entire (micrometer) length of the nanowires. The paper reports transmission electron microscopy and photoluminescence data.

Hexagon solar power panel

NASA Technical Reports Server (NTRS)

Rubin, I. (Inventor)

1978-01-01

A solar energy panel support is described upon which silicon cells are arrayed. The cells are wafer thin and of two geometrical types, both of the same area and electrical rating, namely hexagon cells and hourglass cells. The hourglass cells are composites of half hexagons. A near perfect nesting relationship of the cells achieves a high density packing whereby optimum energy production per panel area is achieved.

Non-Reciprocal on Wafer Microwave Devices

DTIC Science & Technology

2015-05-27

filter uses a barium hexagonal ferrite film incorporated into the dielectric layer of a microstrip transmission line. The zero-field operational...Fal,, Robert E. Camley. Millimeter wave phase shifter based on ferromagnetic resonancein a hexagonal barium ferrite thin film, Applied Physics...materials for on-wafer microwave devices concentrated on barium hexagonal ferrite (BaM) films grown on Si because these material is a good candidate

Hexagon solar power panel

DOEpatents

Rubin, Irwin

1978-01-01

A solar energy panel comprises a support upon which silicon cells are arrayed. The cells are wafer thin and of two geometrical types, both of the same area and electrical rating, namely hexagon cells and hourglass cells. The hourglass cells are composites of half hexagons. A near perfect nesting relationship of the cells achieves a high density packing whereby optimum energy production per panel area is achieved.

Self-assembled growth and structural analysis of inclined GaN nanorods on nanoimprinted m-sapphire using catalyst-free metal-organic chemical vapor deposition

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

Lee, Kyuseung; Chae, Sooryong; Jang, Jongjin

2016-04-15

In this study, self-assembled inclined (1-10-3)-oriented GaN nanorods (NRs) were grown on nanoimprinted (10-10) m-sapphire substrates using catalyst-free metal-organic chemical vapor deposition. According to X-ray phi-scans, the inclined GaN NRs were tilted at an angle of ∼57.5° to the [10-10]{sub sapp} direction. Specifically, the GaN NRs grew in a single inclined direction to the [11-20]{sub sapp}. Uni-directionally inclined NRs were formed through the one-sided (10-11)-faceted growth of the interfacial a-GaN plane layer. It was confirmed that a thin layer of a-GaN was formed on r-facet nanogrooves of the m-sapphire substrate by nitridation. The interfacial a-GaN nucleation affected both the inclinedmore » angle and the growth direction of the inclined GaN NRs. Using X-ray diffraction and selective area electron diffraction, the epitaxial relationship between the inclined (1-10-3) GaN NRs and interfacial a-GaN layer on m-sapphire substrates was systematically investigated. Moreover, the inclined GaN NRs were observed to be mostly free of stacking fault-related defects using high-resolution transmission electron microscopy.« less

Growth of β-Ga2O3 and GaN nanowires on GaN for photoelectrochemical hydrogen generation.

PubMed

Hwang, Jih-Shang; Liu, Tai-Yan; Chattopadhyay, Surjit; Hsu, Geng-Ming; Basilio, Antonio M; Chen, Han-Wei; Hsu, Yu-Kuei; Tu, Wen-Hsun; Lin, Yan-Gu; Chen, Kuei-Hsien; Li, Chien-Cheng; Wang, Sheng-Bo; Chen, Hsin-Yi; Chen, Li-Chyong

2013-02-08

Enhanced photoelectrochemical (PEC) performances of Ga(2)O(3) and GaN nanowires (NWs) grown in situ from GaN were demonstrated. The PEC conversion efficiencies of Ga(2)O(3) and GaN NWs have been shown to be 0.906% and 1.09% respectively, in contrast to their 0.581% GaN thin film counterpart under similar experimental conditions. A low crystallinity buffer layer between the grown NWs and the substrate was found to be detrimental to the PEC performance, but the layer can be avoided at suitable growth conditions. A band bending at the surface of the GaN NWs generates an electric field that drives the photogenerated electrons and holes away from each other, preventing recombination, and was found to be responsible for the enhanced PEC performance. The enhanced PEC efficiency of the Ga(2)O(3) NWs is aided by the optical absorption through a defect band centered 3.3 eV above the valence band of Ga(2)O(3). These findings are believed to have opened up possibilities for enabling visible absorption, either by tailoring ion doping into wide bandgap Ga(2)O(3) NWs, or by incorporation of indium to form InGaN NWs.

Substrate nitridation induced modulations in transport properties of wurtzite GaN/p-Si (100) heterojunctions grown by molecular beam epitaxy

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

Bhat, Thirumaleshwara N.; Rajpalke, Mohana K.; Krupanidhi, S. B.

Phase pure wurtzite GaN films were grown on Si (100) substrates by introducing a silicon nitride layer followed by low temperature GaN growth as buffer layers. GaN films grown directly on Si (100) were found to be phase mixtured, containing both cubic ({beta}) and hexagonal ({alpha}) modifications. The x-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy studies reveal that the significant enhancement in the structural as well as in the optical properties of GaN films grown with silicon nitride buffer layer grown at 800 deg. C when compared to the samples grown in the absence of silicon nitridemore » buffer layer and with silicon nitride buffer layer grown at 600 deg. C. Core-level photoelectron spectroscopy of Si{sub x}N{sub y} layers reveals the sources for superior qualities of GaN epilayers grown with the high temperature substrate nitridation process. The discussion has been carried out on the typical inverted rectification behavior exhibited by n-GaN/p-Si heterojunctions. Considerable modulation in the transport mechanism was observed with the nitridation conditions. The heterojunction fabricated with the sample of substrate nitridation at high temperature exhibited superior rectifying nature with reduced trap concentrations. Lowest ideality factors ({approx}1.5) were observed in the heterojunctions grown with high temperature substrate nitridation which is attributed to the recombination tunneling at the space charge region transport mechanism at lower voltages and at higher voltages space charge limited current conduction is the dominating transport mechanism. Whereas, thermally generated carrier tunneling and recombination tunneling are the dominating transport mechanisms in the heterojunctions grown without substrate nitridation and low temperature substrate nitridation, respectively.« less

Metal organic vapour-phase epitaxy growth of GaN wires on Si (111) for light-emitting diode applications

PubMed Central

2013-01-01

GaN wires are grown on a Si (111) substrate by metal organic vapour-phase epitaxy on a thin deposited AlN blanket and through a thin SiNx layer formed spontaneously at the AlN/Si interface. N-doped wires are used as templates for the growth of core-shell InGaN/GaN multiple quantum wells coated by a p-doped shell. Standing single-wire heterostructures are connected using a metallic tip and a Si substrate backside contact, and the electroluminescence at room temperature and forward bias is demonstrated at 420 nm. This result points out the feasibility of lower cost nitride-based wires for light-emitting diode applications. PMID:23391377

Liquid phase deposition synthesis of hexagonal molybdenum trioxide thin films

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

Deki, Shigehito; Beleke, Alexis Bienvenu; Kotani, Yuki

2009-09-15

Hexagonal molybdenum trioxide thin films with good crystallinity and high purity have been fabricated by the liquid phase deposition (LPD) technique using molybdic acid (H{sub 2}MoO{sub 4}) dissolved in 2.82% hydrofluoric acid (HF) and H{sub 3}BO{sub 3} as precursors. The crystal was found to belong to a hexagonal hydrate system MoO{sub 3}.nH{sub 2}O (napprox0.56). The unit cell lattice parameters are a=10.651 A, c=3.725 A and V=365.997 A{sup 3}. Scanning electron microscope (SEM) images of the as-deposited samples showed well-shaped hexagonal rods nuclei that grew and where the amount increased with increase in reaction time. X-ray photon electron spectroscopy (XPS) spectramore » showed a Gaussian shape of the doublet of Mo 3d core level, indicating the presence of Mo{sup 6+} oxidation state in the deposited films. The deposited films exhibited an electrochromic behavior by lithium intercalation and deintercalation, which resulted in coloration and bleaching of the film. Upon dehydration at about 450 deg. C, the hexagonal MoO{sub 3}.nH{sub 2}O was transformed into the thermodynamically stable orthorhombic phase. - Abstract: SEM photograph of typical h-MoO{sub 3}.nH{sub 2}O thin film nuclei obtained after 36 h at 40 deg. C by the LPD method. Display Omitted« less

Synthesis of gallium nitride nanostructures by nitridation of electrochemically deposited gallium oxide on silicon substrate.

PubMed

Ghazali, Norizzawati Mohd; Yasui, Kanji; Hashim, Abdul Manaf

2014-01-01

Gallium nitride (GaN) nanostructures were successfully synthesized by the nitridation of the electrochemically deposited gallium oxide (Ga2O3) through the utilization of a so-called ammoniating process. Ga2O3 nanostructures were firstly deposited on Si substrate by a simple two-terminal electrochemical technique at a constant current density of 0.15 A/cm(2) using a mixture of Ga2O3, HCl, NH4OH and H2O for 2 h. Then, the deposited Ga2O3 sample was ammoniated in a horizontal quartz tube single zone furnace at various ammoniating times and temperatures. The complete nitridation of Ga2O3 nanostructures at temperatures of 850°C and below was not observed even the ammoniating time was kept up to 45 min. After the ammoniating process at temperature of 900°C for 15 min, several prominent diffraction peaks correspond to hexagonal GaN (h-GaN) planes were detected, while no diffraction peak of Ga2O3 structure was detected, suggesting a complete transformation of Ga2O3 to GaN. Thus, temperature seems to be a key parameter in a nitridation process where the deoxidization rate of Ga2O3 to generate gaseous Ga2O increase with temperature. The growth mechanism for the transformation of Ga2O3 to GaN was proposed and discussed. It was found that a complete transformation can not be realized without a complete deoxidization of Ga2O3. A significant change of morphological structures takes place after a complete transformation of Ga2O3 to GaN where the original nanorod structures of Ga2O3 diminish, and a new nanowire-like GaN structures appear. These results show that the presented method seems to be promising in producing high-quality h-GaN nanostructures on Si.

Synthesis of gallium nitride nanostructures by nitridation of electrochemically deposited gallium oxide on silicon substrate

PubMed Central

2014-01-01

Gallium nitride (GaN) nanostructures were successfully synthesized by the nitridation of the electrochemically deposited gallium oxide (Ga2O3) through the utilization of a so-called ammoniating process. Ga2O3 nanostructures were firstly deposited on Si substrate by a simple two-terminal electrochemical technique at a constant current density of 0.15 A/cm2 using a mixture of Ga2O3, HCl, NH4OH and H2O for 2 h. Then, the deposited Ga2O3 sample was ammoniated in a horizontal quartz tube single zone furnace at various ammoniating times and temperatures. The complete nitridation of Ga2O3 nanostructures at temperatures of 850°C and below was not observed even the ammoniating time was kept up to 45 min. After the ammoniating process at temperature of 900°C for 15 min, several prominent diffraction peaks correspond to hexagonal GaN (h-GaN) planes were detected, while no diffraction peak of Ga2O3 structure was detected, suggesting a complete transformation of Ga2O3 to GaN. Thus, temperature seems to be a key parameter in a nitridation process where the deoxidization rate of Ga2O3 to generate gaseous Ga2O increase with temperature. The growth mechanism for the transformation of Ga2O3 to GaN was proposed and discussed. It was found that a complete transformation can not be realized without a complete deoxidization of Ga2O3. A significant change of morphological structures takes place after a complete transformation of Ga2O3 to GaN where the original nanorod structures of Ga2O3 diminish, and a new nanowire-like GaN structures appear. These results show that the presented method seems to be promising in producing high-quality h-GaN nanostructures on Si. PMID:25593562

Thermal annealing effect on structural and thermoelectric properties of hexagonal Bi2Te3 nanoplate thin films by drop-casting technique

NASA Astrophysics Data System (ADS)

Hosokawa, Yuichi; Wada, Kodai; Tanaka, Masaki; Tomita, Koji; Takashiri, Masayuki

2018-02-01

High-purity hexagonal bismuth telluride (Bi2Te3) nanoplates were prepared by a solvothermal synthesis method, followed by the fabrication of nanoplate thin films by the drop-casting technique. The Bi2Te3 nanoplates exhibited a single-crystalline phase with a rhombohedral crystal structure. The nanoplates had a flat surface with edge sizes ranging from 500 to 2000 nm (average size of 1000 nm) and a thickness of less than 50 nm. The resulting Bi2Te3 nanoplate thin films were composed of well-aligned hexagonal nanoplates along the surface direction with an approximate film thickness of 40 µm. To tightly connect the nanoplates together within the thin films, thermal annealing was performed at different temperatures. We found that the thermoelectric properties, especially the Seebeck coefficient, were very sensitive to the annealing temperature. Finally, the optimum annealing temperature was determined to be 250 °C and the Seebeck coefficient and power factor were -300 µV/K and 3.5 µW/(cm·K2), respectively.

Strain engineering of atomic and electronic structures of few-monolayer-thick GaN

NASA Astrophysics Data System (ADS)

Kolobov, A. V.; Fons, P.; Saito, Y.; Tominaga, J.; Hyot, B.; André, B.

2017-07-01

Two-dimensional (2D) semiconductors possess the potential to ultimately minimize the size of devices and concomitantly drastically reduce the corresponding energy consumption. In addition, materials in their atomic-scale limit often possess properties different from their bulk counterparts paving the way to conceptually novel devices. While graphene and 2D transition-metal dichalcogenides remain the most studied materials, significant interest also exists in the fabrication of atomically thin structures from traditionally 3D semiconductors such as GaN. While in the monolayer limit GaN possesses a graphenelike structure and an indirect band gap, it was recently demonstrated that few-layer GaN acquires a Haeckelite structure in the direction of growth with an effectively direct gap. In this work, we demonstrate the possibility of strain engineering of the atomic and electronic structure of few-monolayer-thick GaN structures, which opens new avenues for their practical application in flexible nanoelectronics and nano-optoelectronics. Our simulations further suggest that due to the weak van der Waals-like interaction between a substrate and an overlayer, the use of a MoS2 substrate may be a promising route to fabricate few-monolayer Haeckelite GaN experimentally.

Elimination of surface band bending on N-polar InN with thin GaN capping

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

Kuzmík, J., E-mail: Jan.Kuzmik@savba.sk; Haščík, Š.; Kučera, M.

2015-11-09

0.5–1 μm thick InN (0001) films grown by molecular-beam epitaxy with N- or In-polarity are investigated for the presence of native oxide, surface energy band bending, and effects introduced by 2 to 4 monolayers of GaN capping. Ex situ angle-resolved x-ray photo-electron spectroscopy is used to construct near-surface (GaN)/InN energy profiles, which is combined with deconvolution of In3d signal to trace the presence of InN native oxide for different types of polarity and capping. Downwards surface energy band bending was observed on bare samples with native oxide, regardless of the polarity. It was found that the In-polar InN surface is mostmore » readily oxidized, however, with only slightly less band bending if compared with the N-polar sample. On the other hand, InN surface oxidation was effectively mitigated by GaN capping. Still, as confirmed by ultra-violet photo-electron spectroscopy and by energy band diagram calculations, thin GaN cap layer may provide negative piezoelectric polarization charge at the GaN/InN hetero-interface of the N-polar sample, in addition to the passivation effect. These effects raised the band diagram up by about 0.65 eV, reaching a flat-band profile.« less

Isotype InGaN/GaN heterobarrier diodes by ammonia molecular beam epitaxy

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

Fireman, Micha N.; Browne, David A.; Speck, James S.

The design of isotype InGaN/GaN heterobarrier diode structures grown by ammonia molecular beam epitaxy is presented. On the (0001) Ga-polar plane, a structure consisting of a surface n{sup +} GaN contact layer, followed by a thin InGaN layer, followed by a thick unintentionally doped (UID) GaN layer, and atop a buried n{sup +} GaN contact layer induces a large conduction band barrier via a depleted UID GaN layer. Suppression of reverse and subthreshold current in such isotype barrier devices under applied bias depends on the quality of this composite layer polarization. Sample series were grown under fixed InGaN growth conditionsmore » that varied either the UID GaN NH{sub 3} flow rate or the UID GaN thickness, and under fixed UID GaN growth conditions that varied InGaN growth conditions. Decreases in subthreshold current and reverse bias current were measured for thicker UID GaN layers and increasing InGaN growth rates. Temperature-dependent analysis indicated that although extracted barrier heights were lower than those predicted by 1D Schrödinger Poisson simulations (0.9 eV–1.4 eV for In compositions from 10% to 15%), optimized growth conditions increased the extracted barrier height from ∼11% to nearly 85% of the simulated values. Potential subthreshold mechanisms are discussed, along with those growth factors which might affect their prevalence.« less

GaN microring waveguide resonators bonded to silicon substrate by a two-step polymer process.

PubMed

Hashida, Ryohei; Sasaki, Takashi; Hane, Kazuhiro

2018-03-20

Using a polymer bonding technique, GaN microring waveguide resonators were fabricated on a Si substrate for future hybrid integration of GaN and Si photonic devices. The designed GaN microring consisted of a rib waveguide having a core of 510 nm in thickness, 1000 nm in width, and a clad of 240 nm in thickness. A GaN crystalline layer of 1000 nm in thickness was grown on a Si(111) substrate by metal organic chemical vapor deposition using a buffer layer of 300 nm in thickness for the compensation of lattice constant mismatch between GaN and Si crystals. The GaN/Si wafer was bonded to a Si(100) wafer by a two-step polymer process to prevent it from trapping air bubbles. The bonded GaN layer was thinned from the backside by a fast atom beam etching to remove the buffer layer and to generate the rib waveguides. The transmission characteristics of the GaN microring waveguide resonators were measured. The losses of the straight waveguides were measured to be 4.0±1.7  dB/mm around a wavelength of 1.55 μm. The microring radii ranged from 30 to 60 μm, where the measured free-spectral ranges varied from 2.58 to 5.30 nm. The quality factors of the microring waveguide resonators were from 1710 to 2820.

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

PubMed

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

2017-10-01

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

Inversion by metalorganic chemical vapor deposition from N- to Ga-polar gallium nitride and its application to multiple quantum well light-emitting diodes

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

Hosalli, A. M.; Van Den Broeck, D. M.; Bedair, S. M.

2013-12-02

We demonstrate a metalorganic chemical vapor deposition growth approach for inverting N-polar to Ga-polar GaN by using a thin inversion layer grown with high Mg flux. The introduction of this inversion layer allowed us to grow p-GaN films on N-polar GaN thin film. We have studied the dependence of hole concentration, surface morphology, and degree of polarity inversion for the inverted Ga-polar surface on the thickness of the inversion layer. We then use this approach to grow a light emitting diode structure which has the MQW active region grown on the advantageous N-polar surface and the p-layer grown on themore » inverted Ga-polar surface.« less

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

NASA Astrophysics Data System (ADS)

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

2012-04-01

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

Single n-GaN microwire/p-Silicon thin film heterojunction light-emitting diode.

PubMed

Ahn, Jaehui; Mastro, Michael A; Klein, Paul B; Hite, Jennifer K; Feigelson, Boris; Eddy, Charles R; Kim, Jihyun

2011-10-24

The emission and waveguiding properties of individual GaN microwires as well as devices based on an n-GaN microwire/p-Si (100) junction were studied for relevance in optoelectronics and optical circuits. Pulsed photoluminescence of the GaN microwire excited in the transverse or longitudinal direction demonstrated gain. These n-type GaN microwires were positioned mechanically or by dielectrophoretic force onto pre-patterned electrodes on a p-type Si (100) substrate. Electroluminescence from this p-n point junction was characteristic of a heterostructure light-emitting diode. Additionally, waveguiding was observed along the length of the microwire for light originating from photoluminescence as well as from electroluminescence generated at the p-n junction. © 2011 Optical Society of America

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

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

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

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

Structural and optical inhomogeneities of Fe doped GaN grown by hydride vapor phase epitaxy

NASA Astrophysics Data System (ADS)

Malguth, E.; Hoffmann, A.; Phillips, M. R.

2008-12-01

We present the results of cathodoluminescence experiments on a set of Fe doped GaN samples with Fe concentrations of 5×1017, 1×1018, 1×1019, and 2×1020 cm-3. These specimens were grown by hydride vapor phase epitaxy with different concentrations of Fe. The introduction of Fe is found to promote the formation of structurally inhomogeneous regions of increased donor concentration. We detect a tendency of these regions to form hexagonal pits at the surface. The locally increased carrier concentration leads to enhanced emission from the band edge and the internal T41(G)-A61(S) transition of Fe3+. In these areas, the luminescence forms a finely structured highly symmetric pattern, which is attributed to defect migration along strain-field lines. Fe doping is found to quench the yellow defect luminescence band and to enhance the blue luminescence band due to the lowering of the Fermi level and the formation of point defects, respectively.

Structural and optical characterization of ZnO/Mg(x)Zn(1-x)O multiple quantum wells based random laser diodes.

PubMed

Jiang, Qike; Zheng, He; Wang, Jianbo; Long, Hao; Fang, Guojia

2012-12-01

Two kinds of laser diodes (LDs) comprised of ZnO/Mg(x)Zn(1-x)O (ZnO/MZO) multiple quantum wells (MQWs) grown on GaN (MQWs/GaN) and Si (MQWs/Si) substrates, respectively, have been constructed. The LD with MQWs/GaN exhibits ultraviolet random lasing under electrical excitation, while that with MQWs/Si does not. In the MQWs/Si, ZnO/MZO MQWs consist of nanoscaled crystallites, and the MZO layers undergo a phase separation of cubic MgO and hexagonal ZnO. Moreover, the Mg atom predominantly locates in the MZO layers along with a significant aggregation at the ZnO/MZO interfaces; in sharp contrast, the ZnO/MZO MQWs in the MQWs/GaN show a well-crystallized structure with epitaxial relationships among GaN, MZO, and ZnO. Notably, Mg is observed to diffuse into the ZnO well layers. The structure-optical property relationship of these two LDs is further discussed.

Polarity in GaN and ZnO: Theory, measurement, growth, and devices

NASA Astrophysics Data System (ADS)

Zúñiga-Pérez, Jesús; Consonni, Vincent; Lymperakis, Liverios; Kong, Xiang; Trampert, Achim; Fernández-Garrido, Sergio; Brandt, Oliver; Renevier, Hubert; Keller, Stacia; Hestroffer, Karine; Wagner, Markus R.; Reparaz, Juan Sebastián; Akyol, Fatih; Rajan, Siddharth; Rennesson, Stéphanie; Palacios, Tomás; Feuillet, Guy

2016-12-01

The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade.

Effect of aluminium doping on structural and optical properties of ZnO thin films by sol-gel method

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

Vijayaprasath, G.; Murugan, R.; Ravi, G., E-mail: raviganesa@rediffmail.com, E-mail: gravicrc@gmail.com

2015-06-24

We systematically investigated the structural, morphological and optical properties of 0.05 mol % Al doped ZnO (Al:ZnO) thin films deposited on glass substrates by sol-gel spin coating method. The influences of Al doping in ZnO thin films are characterized by Powder X-ray diffraction study. ZnO and Al:ZnO thin films have showed hexagonal wurtzite structure without any secondary phase in c-axis (002) orientation. The SEM images also proved the hexagonal rod like morphologies for both films. All the films exhibited transmittance of 70-80% in the visible range up to 800 nm and cut-off wavelength observed at ∼390 nm corresponding to the fundamental absorption ofmore » ZnO. The band gap of the ZnO thin films slightly widened with the Al doping. The photoluminescence properties have been studied for Al: ZnO thin films and the results are presented in detail.« less

A charge inverter for III-nitride light-emitting diodes

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

Zhang, Zi-Hui, E-mail: zh.zhang@hebut.edu.cn, E-mail: wbi@hebut.edu.cn, E-mail: volkan@stanfordalumni.org, E-mail: sunxw@sustc.edu.cn; Zhang, Yonghui; Bi, Wengang, E-mail: zh.zhang@hebut.edu.cn, E-mail: wbi@hebut.edu.cn, E-mail: volkan@stanfordalumni.org, E-mail: sunxw@sustc.edu.cn

In this work, we propose a charge inverter that substantially increases the hole injection efficiency for InGaN/GaN light-emitting diodes (LEDs). The charge inverter consists of a metal/electrode, an insulator, and a semiconductor, making an Electrode-Insulator-Semiconductor (EIS) structure, which is formed by depositing an extremely thin SiO{sub 2} insulator layer on the p{sup +}-GaN surface of a LED structure before growing the p-electrode. When the LED is forward-biased, a weak inversion layer can be obtained at the interface between the p{sup +}-GaN and SiO{sub 2} insulator. The weak inversion region can shorten the carrier tunnel distance. Meanwhile, the smaller dielectric constantmore » of the thin SiO{sub 2} layer increases the local electric field within the tunnel region, and this is effective in promoting the hole transport from the p-electrode into the p{sup +}-GaN layer. Due to the improved hole injection, the external quantum efficiency is increased by 20% at 20 mA for the 350 × 350 μm{sup 2} LED chip. Thus, the proposed EIS holds great promise for high efficiency LEDs.« less

Integrated thin film Si fluorescence sensor coupled with a GaN microLED for microfluidic point-of-care testing

NASA Astrophysics Data System (ADS)

Robbins, Hannah; Sumitomo, Keiko; Tsujimura, Noriyuki; Kamei, Toshihiro

2018-02-01

An integrated fluorescence sensor consisting of a SiO2/Ta2O5 multilayer optical interference filter and hydrogenated amorphous silicon (a-Si:H) pin photodiode was coupled with a GaN microLED to construct a compact fluorescence detection module for point-of-care microfluidic biochemical analysis. The combination of the small size of the GaN microLED and asymmetric microlens resulted in a focal spot diameter of the excitation light of approximately 200 µm. The limit of detection of the sensor was as high as 36 nM for fluorescein solution flowing in a 100 µm deep microfluidic channel because of the lack of directionality of the LED light. Nevertheless, we used the GaN microLED coupled with the a-Si:H fluorescence sensor to successfully detect fluorescence from a streptavidin R-phycoerythrin conjugate that bound to biotinylated antibody-coated microbeads trapped by the barrier in the microfluidic channel.

The Search for Effective p-Type Material in GaN-Based Devices: Past, Present, and Future

NASA Astrophysics Data System (ADS)

Juday, Reid; Fischer, Alec; Ponce, Fernando; Dupuis, Russell

2009-10-01

In the continued drive towards viable, large-scale solid state lighting, GaN and its alloys with In and Al have risen to the forefront of current research. Regardless of GaN's success in LEDs and laser diodes, certain technological obstacles have remained. Since the beginning of GaN fabrication, the ability to reliably and effectively create p-type material has been a major concern. Mg is the most widely used and successful acceptor in GaN and appears to behave even more favorably in InxGa1-xN with small values of x (< 0.1). It is commonly accepted, however, that Mg-H complexes form during growth, inhibiting hole formation. This talk will focus on comparing the techniques most commonly used to activate p-GaN, such as thermal annealing and low-energy electron beam irradiation in a scanning electron microscope, as well as the properties of low-indium content p-type InGaN thin films.

Effect of growth temperature on the epitaxial growth of ZnO on GaN by ALD

NASA Astrophysics Data System (ADS)

Särkijärvi, Suvi; Sintonen, Sakari; Tuomisto, Filip; Bosund, Markus; Suihkonen, Sami; Lipsanen, Harri

2014-07-01

We report on the epitaxial growth of ZnO on GaN template by atomic layer deposition (ALD). Diethylzinc (DEZn) and water vapour (H2O) were used as precursors. The structure and the quality of the grown ZnO layers were studied with scanning electron microscope (SEM), X-ray diffraction (XRD), photoluminescence (PL) measurements and positron annihilation spectroscopy. The ZnO films were confirmed epitaxial, and the film quality was found to improve with increasing deposition temperature in the vicinity of the threshold temperature of two dimensional growth. We conclude that high quality ZnO thin films can be grown by ALD. Interestingly only separate Zn-vacancies were observed in the films, although ZnO thin films typically contain fairly high density of surface pits and vacancy clusters.

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

NASA Astrophysics Data System (ADS)

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

2018-03-01

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

MOVPE growth of violet GaN LEDs on β-Ga2O3 substrates

NASA Astrophysics Data System (ADS)

Li, Ding; Hoffmann, Veit; Richter, Eberhard; Tessaro, Thomas; Galazka, Zbigniew; Weyers, Markus; Tränkle, Günther

2017-11-01

We report that a H2-free atmosphere is essential for the initial stage of metalorganic vapour phase epitaxy (MOVPE) growth of GaN on β-Ga2O3 to prevent the surface from damage. A simple growth method is proposed that can easily transfer established GaN growth recipes from sapphire to β-Ga2O3 with both (-2 0 1) and (1 0 0) orientations. This method features a thin AlN nucleation layer grown below 900 °C in N2 atmosphere to protect the surface of β-Ga2O3 from deterioration during further growth under the H2 atmosphere. Based on this, we demonstrate working violet vertical light emitting diodes (VLEDs) on n-conductive β-Ga2O3 substrates.

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.

Refractive index of erbium doped GaN thin films

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

Alajlouni, S.; Sun, Z. Y.; Li, J.

2014-08-25

GaN is an excellent host for erbium (Er) to provide optical emission in the technologically important as well as eye-safe 1540 nm wavelength window. Er doped GaN (GaN:Er) epilayers were synthesized on c-plane sapphire substrates using metal organic chemical vapor deposition. By employing a pulsed growth scheme, the crystalline quality of GaN:Er epilayers was significantly improved over those obtained by conventional growth method of continuous flow of reaction precursors. X-ray diffraction rocking curve linewidths of less than 300 arc sec were achieved for the GaN (0002) diffraction peak, which is comparable to the typical results of undoped high quality GaN epilayers andmore » represents a major improvement over previously reported results for GaN:Er. Spectroscopic ellipsometry was used to determine the refractive index of the GaN:Er epilayers in the 1540 nm wavelength window and a linear dependence on Er concentration was found. The observed refractive index increase with Er incorporation and the improved crystalline quality of the GaN:Er epilayers indicate that low loss GaN:Er optical waveguiding structures are feasible.« less

Cubic GaN quantum dots embedded in zinc-blende AlN microdisks

NASA Astrophysics Data System (ADS)

Bürger, M.; Kemper, R. M.; Bader, C. A.; Ruth, M.; Declair, S.; Meier, C.; Förstner, J.; As, D. J.

2013-09-01

Microresonators containing quantum dots find application in devices like single photon emitters for quantum information technology as well as low threshold laser devices. We demonstrate the fabrication of 60 nm thin zinc-blende AlN microdisks including cubic GaN quantum dots using dry chemical etching techniques. Scanning electron microscopy analysis reveals the morphology with smooth surfaces of the microdisks. Micro-photoluminescence measurements exhibit optically active quantum dots. Furthermore this is the first report of resonator modes in the emission spectrum of a cubic AlN microdisk.

Effect of growth parameters on crystallinity and properties of ZnO films grown by plasma assisted MOCVD

NASA Astrophysics Data System (ADS)

Losurdo, M.; Giangregorio, M. M.; Sacchetti, A.; Capezzuto, P.; Bruno, G.; Malandrino, G.; Fragalà, I. L.

2007-07-01

Thin films of ZnO have been grown by plasma assisted metal-organic chemical vapour deposition (PA-MOCVD) using a 13.56 MHz O 2 plasma and the Zn(TTA)•tmed (HTTA=2-thenoyltrifluoroacetone, TMED=N,N,N',N'-tetramethylethylendiamine) precursor. The effects of growth parameters such as the plasma activation, the substrate, the surface temperature, and the ratio of fluxes of precursors on the structure, morphology, and optical and electrical properties of ZnO thin films have been studied. Under a very low plasma power of 20 W, c-axis oriented hexagonal ZnO thin films are grown on hexagonal sapphire (0001), cubic Si(001) and amorphous quartz substrates. The substrate temperature mainly controls grain size.

Comparison of the physical, chemical and electrical properties of ALD Al 2 O 3 on c- and m- plane GaN: Comparison of the physical, chemical and electrical properties of ALD Al 2 O 3 on c- and m- plane GaN

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

Wei, D.; Hossain, T.; Nepal, N.

2014-02-01

Our study compares the physical, chemical and electrical properties of Al 2O 3 thin films deposited on gallium polar c- and nonpolar m -plane GaN substrates by atomic layer deposition (ALD). Correlations were sought between the film's structure, composition, and electrical properties. The thickness of the Al 2O 3 films was 19.2 nm as determined from a Si witness sample by spectroscopic ellipsometry. We measured the gate dielectric was slightly aluminum-rich (Al:O=1:1.3) from X-ray photoelectron spectroscopy (XPS) depth profile, and the oxide-semiconductor interface carbon concentration was lower on c -plane GaN. The oxide's surface morphology was similar on both substrates,more » but was smoothest on c -plane GaN as determined by atomic force microscopy (AFM). Circular capacitors (50-300 μm diameter) with Ni/Au (20/100 nm) metal contacts on top of the oxide were created by standard photolithography and e-beam evaporation methods to form metal-oxide-semiconductor capacitors (MOSCAPs). Moreover, the alumina deposited on c -plane GaN showed less hysteresis (0.15 V) than on m -plane GaN (0.24 V) in capacitance-voltage (CV) characteristics, consistent with its better quality of this dielectric as evidenced by negligible carbon contamination and smooth oxide surface. These results demonstrate the promising potential of ALD Al 2O 3 on c -plane GaN, but further optimization of ALD is required to realize the best properties of Al 2O 3 on m -plane GaN.« less

Room temperature chemical bath deposition of cadmium selenide, cadmium sulfide and cadmium sulfoselenide thin films with novel nanostructures

NASA Astrophysics Data System (ADS)

VanderHyde, Cephas A.; Sartale, S. D.; Patil, Jayant M.; Ghoderao, Karuna P.; Sawant, Jitendra P.; Kale, Rohidas B.

2015-10-01

A simple, convenient and low cost chemical synthesis route has been used to deposit nanostructured cadmium sulfide, selenide and sulfoselenide thin films at room temperature. The films were deposited on glass substrates, using cadmium acetate as cadmium ion and sodium selenosulfate/thiourea as a selenium/sulfur ion sources. Aqueous ammonia was used as a complex reagent and also to adjust the pH of the final solution. The as-deposited films were uniform, well adherent to the glass substrate, specularly reflective and red/yellow in color depending on selenium and sulfur composition. The X-ray diffraction pattern of deposited cadmium selenide thin film revealed the nanocrystalline nature with cubic phase; cadmium sulfide revealed mixture of cubic along with hexagonal phase and cadmium sulfoselenide thin film were grown with purely hexagonal phase. The morphological observations revealed the growth and formation of interesting one, two and three-dimensional nanostructures. The band gap of thin films was calculated and the results are reported.

Epitaxial growth of aligned AlGalnN nanowires by metal-organic chemical vapor deposition

DOEpatents

Han, Jung; Su, Jie

2008-08-05

Highly ordered and aligned epitaxy of III-Nitride nanowires is demonstrated in this work. <1010> M-axis is identified as a preferential nanowire growth direction through a detailed study of GaN/AlN trunk/branch nanostructures by transmission electron microscopy. Crystallographic selectivity can be used to achieve spatial and orientational control of nanowire growth. Vertically aligned (Al)GaN nanowires are prepared on M-plane AlN substrates. Horizontally ordered nanowires, extending from the M-plane sidewalls of GaN hexagonal mesas or islands demonstrate new opportunities for self-aligned nanowire devices, interconnects, and networks.

Properties of Mg and Zn acceptors in MOVPE GaN as studied by optically detected magnetic resonance

NASA Astrophysics Data System (ADS)

Kunzer, M.; Baur, J.; Kaufmann, U.; Schneider, J.; Amano, H.; Akasaki, I.

1997-02-01

We have studied the photoluminescence (PL) and optically detected magnetic resonance (ODMR) of undoped, n-doped and p-doped thin wurtzite GaN layers grown by metal-organic chemical vapor deposition on sapphire substrates. The ODMR data obtained for undoped. Mg-doped and Zn-doped GaN layers provide an insight into the recombination mechanisms responsible for the broad yellow (2.25 eV), the violet (3.15 eV) and the blue (2.8 eV) PL bands, respectively. The ODMR results for Mg and Zn also show that these acceptors do not behave effective mass like and indicate that the acceptor hole is mainly localized in the nearest neighbor shell surrounding the acceptor core. In addition concentration effects in heavily doped GaN:Mg have been studied.

Zero lattice mismatch and twin-free single crystalline ScN buffer layers for GaN growth on silicon

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

Lupina, L.; Zoellner, M. H.; Dietrich, B.

2015-11-16

We report the growth of thin ScN layers deposited by plasma-assisted molecular beam epitaxy on Sc{sub 2}O{sub 3}/Y{sub 2}O{sub 3}/Si(111) substrates. Using x-ray diffraction, Raman spectroscopy, and transmission electron microscopy, we find that ScN films grown at 600 °C are single crystalline, twin-free with rock-salt crystal structure, and exhibit a direct optical band gap of 2.2 eV. A high degree of crystalline perfection and a very good lattice matching between ScN and GaN (misfit < 0.1%) makes the ScN/Sc{sub 2}O{sub 3}/Y{sub 2}O{sub 3} buffer system a very promising template for the growth of high quality GaN layers on silicon.

An atomistic mechanism study of GaN step-flow growth in vicinal m-plane orientations

DOE PAGES

Liu, Zhun; Wang, Ru-Zhi; Zapol, Peter

2016-10-12

We present elucidation of homoepitaxial growth mechanisms on vicinal non-polar surfaces of GaN that is highly important for gaining an understanding of and control thin film surface morphology and properties. Using first-principles calculations, we study the step-flow growth in m-plane GaN based on atomic row nucleation and kink propagation kinetics. Ga–N dimer adsorption onto the m-plane is energetically more favorable than that of Ga and N isolated adatoms. Therefore, we have treated the dimers as the dominant growth species attached to the step edges. By calculating the free energies of sequentially attached Ga–N dimers, we have elucidated that the a-stepmore » edge kink growth proceeds by parallel attachment rather than by across the step edge approach. We found a series of favorable configurations of kink propagation and calculated the free energy and nucleation barriers for kink evolution on five types of step edges (a, +c, -c, +a + c, and -a - c). By changing the chemical potential μGa and the excess chemical potential Δμ, the growth velocities at the five types of edges are controlled by the corresponding kink pair nucleation barrier E* in their free energy profiles. To explore the kink-flow growth instability observed at different Ga/N flux ratios, calculations of kink pairs on the incompact -c and +c-step edges are further performed to study their formation energies. Variations of these step edge morphologies with a tuned chemical environment are consistent with previous experimental observations, including stable diagonal ±a ± c-direction steps. In conclusion, our work provides a first-principles approach to explore step growth and surface morphology of the vicinal m-plane GaN, which is applicable to analyze and control the step-flow growth of other binary thin films.« less

High In-content InGaN nano-pyramids: Tuning crystal homogeneity by optimized nucleation of GaN seeds

NASA Astrophysics Data System (ADS)

Bi, Zhaoxia; Gustafsson, Anders; Lenrick, Filip; Lindgren, David; Hultin, Olof; Wallenberg, L. Reine; Ohlsson, B. Jonas; Monemar, Bo; Samuelson, Lars

2018-01-01

Uniform arrays of submicron hexagonal InGaN pyramids with high morphological and material homogeneity, reaching an indium composition of 20%, are presented in this work. The pyramids were grown by selective area metal-organic vapor phase epitaxy and nucleated from small openings in a SiN mask. The growth selectivity was accurately controlled with diffusion lengths of the gallium and indium species, more than 1 μm on the SiN surface. High material homogeneity of the pyramids was achieved by inserting a precisely formed GaN pyramidal seed prior to InGaN growth, leading to the growth of well-shaped InGaN pyramids delimited by six equivalent {" separators="| 10 1 ¯ 1 } facets. Further analysis reveals a variation in the indium composition to be mediated by competing InGaN growth on two types of crystal planes, {" separators="| 10 1 ¯ 1 } and (0001). Typically, the InGaN growth on {" separators="| 10 1 ¯ 1 } planes is much slower than on the (0001) plane. The formation of the (0001) plane and the growth of InGaN on it were found to be dependent on the morphology of the GaN seeds. We propose growth of InGaN pyramids seeded by {" separators="| 10 1 ¯ 1 }-faceted GaN pyramids as a mean to avoid InGaN material grown on the otherwise formed (0001) plane, leading to a significant reduction of variations in the indium composition in the InGaN pyramids. The InGaN pyramids in this work can be used as a high-quality template for optoelectronic devices having indium-rich active layers, with a potential of reaching green, yellow, and red emissions for LEDs.

Kinetic instability of AlGaN alloys during MBE growth under metal-rich conditions on m-plane GaN miscut towards the -c axis

NASA Astrophysics Data System (ADS)

Shirazi-HD, M.; Diaz, R. E.; Nguyen, T.; Jian, J.; Gardner, G. C.; Wang, H.; Manfra, M. J.; Malis, O.

2018-04-01

AlxGa1-xN layers with Al-composition above 0.6 (0.6 < x < 0.9) grown under metal-rich conditions by plasma-assisted molecular beam epitaxy on m-plane GaN miscut towards the -c axis are kinetically unstable. Even under excess Ga flux, the effective growth rate of AlGaN is drastically reduced, likely due to suppression of Ga-N dimer incorporation. The defect structure generated during these growth conditions is studied with energy dispersive x-ray spectroscopy scanning transmission electron microscopy as a function of Al flux. The AlGaN growth results in the formation of thin Al(Ga)N layers with Al-composition higher than expected and lower Al-composition AlGaN islands. The AlGaN islands have a flat top and are elongated along the c-axis (i.e., stripe-like shape). Possible mechanisms for the observed experimental results are discussed. Our data are consistent with a model in which Al-N dimers promote release of Ga-N dimers from the m-plane surface.

Quantitative secondary ion mass spectrometric analysis of secondary ion polarity in GaN films implanted with oxygen

NASA Astrophysics Data System (ADS)

Hashiguchi, Minako; Sakaguchi, Isao; Adachi, Yutaka; Ohashi, Naoki

2016-10-01

Quantitative analyses of N and O ions in GaN thin films implanted with oxygen ions (16O+) were conducted by secondary ion mass spectrometry (SIMS). Positive (CsM+) and negative secondary ions extracted by Cs+ primary ion bombardment were analyzed for oxygen quantitative analysis. The oxygen depth profiles were obtained using two types of primary ion beams: a Gaussian-type beam and a broad spot beam. The oxygen peak concentrations in GaN samples were from 3.2 × 1019 to 7.0 × 1021 atoms/cm3. The depth profiles show equivalent depth resolutions in the two analyses. The intensity of negative oxygen ions was approximately two orders of magnitude higher than that of positive ions. In contrast, the O/N intensity ratio measured using CsM+ molecular ions was close to the calculated atomic density ratio, indicating that the SIMS depth profiling using CsM+ ions is much more effective for the measurements of O and N ions in heavy O-implanted GaN than that using negative ions.

The fabrication of white light-emitting diodes using the n-ZnO/NiO/p-GaN heterojunction with enhanced luminescence.

PubMed

Abbasi, Mazhar Ali; Ibupoto, Zafar Hussain; Hussain, Mushtaque; Nur, Omer; Willander, Magnus

2013-07-13

Cheap and efficient white light-emitting diodes (LEDs) are of great interest due to the energy crisis all over the world. Herein, we have developed heterojunction LEDs based on the well-aligned ZnO nanorods and nanotubes on the p-type GaN with the insertion of the NiO buffer layer that showed enhancement in the light emission. Scanning electron microscopy have well demonstrated the arrays of the ZnO nanorods and the proper etching into the nanotubes. X-ray diffraction study describes the wurtzite crystal structure array of ZnO nanorods with the involvement of GaN at the (002) peak. The cathodoluminescence spectra represent strong and broad visible emission peaks compared to the UV emission and a weak peak at 425 nm which is originated from GaN. Electroluminescence study has shown highly improved luminescence response for the LEDs fabricated with NiO buffer layer compared to that without NiO layer. Introducing a sandwich-thin layer of NiO between the n-type ZnO and the p-type GaN will possibly block the injection of electrons from the ZnO to the GaN. Moreover, the presence of NiO buffer layer might create the confinement effect.

Polarity determination of polar and semipolar (112¯2) InN and GaN layers by valence band photoemission spectroscopy

NASA Astrophysics Data System (ADS)

Skuridina, D.; Dinh, D. V.; Lacroix, B.; Ruterana, P.; Hoffmann, M.; Sitar, Z.; Pristovsek, M.; Kneissl, M.; Vogt, P.

2013-11-01

We demonstrate that the polarity of polar (0001), (0001¯) and semipolar (112¯2) InN and GaN thin layers can be determined by valence band X-ray photoemission spectroscopy (XPS). The polarity of the layers has been confirmed by wet etching and convergent beam electron diffraction. Unlike these two techniques, XPS is a non-destructive method and unaffected by surface oxidation or roughness. Different intensities of the valence band states in spectra recorded by using AlKα X-ray radiation are observed for N-polar and group-III-polar layers. The highest intensity of the valence band state at ≈3.5 eV for InN and ≈5.2 eV for GaN correlates with the group-III polarity, while the highest intensity at ≈6.7 eV for InN and ≈9.5 eV for GaN correlates with the N-polarity. The difference between the peaks for the group-III- and N-polar orientations was found to be statistically significant at the 0.05 significance level. The polarity of semipolar (112¯2) InN and GaN layers can be determined by recording valence band photoelectrons emitted along the [000 ± 1] direction.

Red Light Emitting Schottky Diodes on p-TYPE GaN/AlN/Si(111) Substrate

NASA Astrophysics Data System (ADS)

Chuah, L. S.; Hassan, Z.; Abu Hassan, H.

High quality GaN layers doped with Mg were grown on Si(111) substrates using high temperature AlN as buffer layer by radio-frequency molecular beam epitaxy. From the Hall measurements, fairly uniform high hole concentration as high as (4-5) × 1020 cm-3 throughout the GaN was achieved. The fabrication of the device is very simple. Nickel ohmic contacts and Schottky contacts using indium were fabricated on Mg-doped p-GaN films. The light emission has been obtained from these thin film electroluminescent devices. Thin film electroluminescent devices were operated under direct current bias. Schottky and ohmic contacts used as cathode and anode were employed in these investigations. Alternatively, two Schottky contacts could be probed as cathode and anode. Thin film electroluminescent devices were able to emit light. However, electrical and optical differences could be observed from the two different probing methods. The red light color could be observed when the potential between the electrodes was increased gradually under forward bias of 8 V at room temperature. Electrical properties of these thin film electroluminescent devices were characterized by current-voltage (I-V) system, the heights of barriers determined from the I-V measurements were found to be related to the electroluminescence.

Anisotropy of the nitrogen conduction states in the group III nitrides studied by polarized x-ray absorption

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

Lawniczak-Jablonska, K.; Liliental-Weber, Z.; Gullikson, E.M.

1997-04-01

Group III nitrides (AlN, GaN, and InN) consist of the semiconductors which appear recently as a basic materials for optoelectronic devices active in the visible/ultraviolet spectrum as well as high-temperature and high-power microelectronic devices. However, understanding of the basic physical properties leading to application is still not satisfactory. One of the reasons consists in unsufficient knowledge of the band structure of the considered semiconductors. Several theoretical studies of III-nitrides band structure have been published but relatively few experimental studies have been carried out, particularly with respect to their conduction band structure. This motivated the authors to examine the conduction bandmore » structure projected onto p-states of the nitrogen atoms for AlN, GaN and InN. An additional advantage of their studies is the availability of the studied nitrides in two structures, hexagonal (wurtzite) and cubic (zincblende). This offers an opportunity to gain information about the role of the anisotropy of electronic band states in determining various physical properties.« less

Substrate temperature influence on the properties of GaN thin films grown by hollow-cathode plasma-assisted atomic layer deposition

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

Alevli, Mustafa, E-mail: mustafaalevli@marmara.edu.tr; Gungor, Neşe; Haider, Ali

2016-01-15

Gallium nitride films were grown by hollow cathode plasma-assisted atomic layer deposition using triethylgallium and N{sub 2}/H{sub 2} plasma. An optimized recipe for GaN film was developed, and the effect of substrate temperature was studied in both self-limiting growth window and thermal decomposition-limited growth region. With increased substrate temperature, film crystallinity improved, and the optical band edge decreased from 3.60 to 3.52 eV. The refractive index and reflectivity in Reststrahlen band increased with the substrate temperature. Compressive strain is observed for both samples, and the surface roughness is observed to increase with the substrate temperature. Despite these temperature dependent material properties,more » the chemical composition, E{sub 1}(TO), phonon position, and crystalline phases present in the GaN film were relatively independent from growth temperature.« less

Multilayer porous structures of HVPE and MOCVD grown GaN for photonic applications

NASA Astrophysics Data System (ADS)

Braniste, T.; Ciers, Joachim; Monaico, Ed.; Martin, D.; Carlin, J.-F.; Ursaki, V. V.; Sergentu, V. V.; Tiginyanu, I. M.; Grandjean, N.

2017-02-01

In this paper we report on a comparative study of electrochemical processes for the preparation of multilayer porous structures in hydride vapor phase epitaxy (HVPE) and metal organic chemical vapor phase deposition (MOCVD) grown GaN. It was found that in HVPE-grown GaN, multilayer porous structures are obtained due to self-organization processes leading to a fine modulation of doping during the crystal growth. However, these processes are not totally under control. Multilayer porous structures with a controlled design have been produced by optimizing the technological process of electrochemical etching in MOCVD-grown samples, consisting of five pairs of thin layers with alternating-doping profiles. The samples have been characterized by SEM imaging, photoluminescence spectroscopy, and micro-reflectivity measurements, accompanied by transfer matrix analysis and simulations by a method developed for the calculation of optical reflection spectra. We demonstrate the applicability of the produced structures for the design of Bragg reflectors.

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

Hsu, Chih-Hao; Yue, Kan; Wang, Jing

Controlling self-assembled nanostructures in thin films allows the bottom-up fabrication of ordered nanoscale patterns. Here we report the unique thickness-dependent phase behavior in thin films of a bolaform-like giant surfactant, which consists of butyl- and hydroxyl-functionalized polyhedral oligomeric silsesquioxane (BPOSS and DPOSS) cages telechelically located at the chain ends of a polystyrene (PS) chain with 28 repeating monomers on average. In the bulk, BPOSS-PS28-DPOSS forms a double gyroid (DG) phase. Both grazing incidence small angle X-ray scattering and transmission electron microscopy techniques are combined to elucidate the thin film structures. Interestingly, films with thicknesses thinner than 200 nm exhibit anmore » irreversible phase transition from hexagonal perforated layer (HPL) to compressed hexagonally packed cylinders (c-HEX) at 130 °C, while films with thickness larger than 200 nm show an irreversible transition from HPL to DG at 200 °C. The thickness-controlled transition pathway suggests possibilities to obtain diverse patterns via thin film self-assembly.« less

Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: A first-principles perspective to recent synthesis

NASA Astrophysics Data System (ADS)

Kecik, D.; Onen, A.; Konuk, M.; Gürbüz, E.; Ersan, F.; Cahangirov, S.; Aktürk, E.; Durgun, E.; Ciraci, S.

2018-03-01

Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed.

Effect of oxygen partial pressure and VO2 content on hexagonal WO3 thin films synthesized by pulsed laser deposition technique

NASA Astrophysics Data System (ADS)

Kaushal, Ajay; Kaur, Davinder

2011-06-01

We report on the effect of oxygen partial pressure and vacuum annealing on structural and optical properties of pulsed laser-deposited nanocrystalline WO3 thin films. XRD results show the hexagonal phase of deposited WO3 thin films. The crystallite size was observed to increase with increase in oxygen partial pressure. Vacuum annealing changed the transparent as-deposited WO3 thin film to deep shade of blue color which increases the optical absorption of the film. The origin of this blue color could be due to the presence of oxygen vacancies associated with tungsten ions in lower oxidation states. In addition, the effects of VO2 content on structural, electrochemical, and optical properties of (WO3)1- x (VO2) x nanocomposite thin films have also been systematically investigated. Cyclic voltammogram exhibits a modification with the appearance of an extra cathodic peak for VO2-WO3 thin film electrode with higher VO2 content ( x ≥ 0.2). Increase of VO2 content in (WO3)1- x (VO2) x films leads to red shift in optical band gap.

MOCVD growth of gallium nitride with indium surfactant

NASA Astrophysics Data System (ADS)

Won, Dong Jin

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

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

NASA Astrophysics Data System (ADS)

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

2014-04-01

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

Self-organization of palladium nanoislands on GaN and AlxGa1-xN/GaN heterostructures

NASA Astrophysics Data System (ADS)

Stafiniak, Andrzej; Szymański, Tomasz; Paszkiewicz, Regina

2017-12-01

We report on systematic study on the dewetting process of thin Pd layer and self-organized Pd nano-islands on SiO2, GaN and AlxGa1-xN/GaN heterostructures with various Al content. The influence of factors such as the thickness of metal layer, type of top layer of AlGaN/GaN heterostructures, temperature and time of annealing process on the dimensions, shapes and density of Pd islands was analyzed. Comparing the behavior of self-organization of Pd islands on Al0.25Ga0.75N/GaN and SiO2 we can conclude that solid-state dewetting process on SiO2 occures much faster than on Al0.25Ga0.75N. For substrates with SiO2 this process requires less energy and can arise for thicker layer. On the Al0.25Ga0.75N surface the islands take more crystalline shape which is probably due to surface reconstruction of Pd-Ga alloy thin layer on interface. For thin metal layer the coalescence of islands into larger islands similar to Ostwald ripening mechanism was observed. Greater surface roughness of AlxGa1-xN/GaN heterostructures with higher Al content causes an increase of surface density of islands and the reduction of their sizes which improves the roundness. In case of GaN and AlxGa1-xN layers with Al content lower than 20%, the surface degradation caused by annealing process was observed. Probably, this is due to the decomposition of layers with gallium droplet formation on catalytic metal islands.

Silicon-on-insulator with hybrid orientations for heterogeneous integration of GaN on Si (100) substrate

NASA Astrophysics Data System (ADS)

Zhang, Runchun; Zhao, Beiji; Huang, Kai; You, Tiangui; Jia, Qi; Lin, Jiajie; Zhang, Shibin; Yan, Youquan; Yi, Ailun; Zhou, Min; Ou, Xin

2018-05-01

Heterogeneous integration of materials pave a new way for the development of the microsystem with miniaturization and complex functionalities. Two types of hybrid silicon on insulator (SOI) structures, i.e., Si (100)-on-Si (111) and Si (111)-on-Si (100), were prepared by the smart-cut technique, which is consist of ion-slicing and wafer bonding. The precise calculation of the lattice strain of the transferred films without the epitaxial matching relationship to the substrate was demonstrated based on X-ray diffraction (XRD) measurements. The XRD and Raman measurement results suggest that the transferred films possess single crystalline quality. With a chemical mechanical polishing (CMP) process, the surface roughness of the transferred thin films can be reduced from 5.57 nm to 0.30 nm. The 4-inch GaN thin film epitaxially grown on the as-prepared hybrid SOI of Si (111)-on-Si (100) by metalorganic chemical vapor deposition (MOCVD) is of improved quality with a full width at half maximum (FWHM) of 672.54 arcsec extracted from the XRD rocking curve and small surface roughness of 0.40 nm. The wafer-scale GaN on Si (111)-on-Si (100) can serve as a potential platform for the one chip integration of GaN-based high electron mobility transistors (HEMT) or photonics with the Si (100)-based complementary metal oxide semiconductor (CMOS).

Polarized edge emission from GaN-based light-emitting diodes sandwiched by dielectric/metal hybrid reflectors

NASA Astrophysics Data System (ADS)

Yan, L. J.; Sheu, J. K.; Huang, F. W.; Lee, M. L.

2010-12-01

Edge-emitting c-plane GaN/sapphire-based light-emitting diodes (LEDs) sandwiched by two dielectric/metal hybrid reflectors on both sapphire and GaN surfaces were studied to determine their light emission polarization. The hybrid reflectors comprised dielectric multiple thin films and a metal layer. The metal layers of Au or Ag used in this study were designed to enhance the polarization ratio from S-polarization (transverse electric wave, TE) to P-polarization (transverse magnetic wave, TM). The two sets of optimized dielectric multi thin films served as matching layers for wide-angle incident light on both sapphire and GaN surfaces. To determine which reflector scheme would achieve a higher polarization ratio, simulations of the reflectance at the hybrid reflectors on sapphire (or GaN) interface were performed before the fabrication of experimental LEDs. Compared with conventional c-plane InGaN/GaN/sapphire LEDs without dielectric/metal hybrid reflectors, the experimental LEDs exhibited higher polarization ratio (ITE-max/ITM-max) with r=2.174 (˜3.37 dB) at a wavelength of 460 nm. In contrast, the original polarized light (without dielectric/metal hybrid reflectors) was partially contributed (r=1.398) by C-HH or C-LH (C band to the heavy-hole sub-band or C band to the crystal-field split-off sub-band) transitions along the a-plane or m-plane direction.

Magnetic tunnel junctions with monolayer hexagonal boron nitride tunnel barriers

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

Piquemal-Banci, M.; Galceran, R.; Bouzehouane, K.

We report on the integration of atomically thin 2D insulating hexagonal boron nitride (h-BN) tunnel barriers into Co/h-BN/Fe magnetic tunnel junctions (MTJs). The h-BN monolayer is directly grown by chemical vapor deposition on Fe. The Conductive Tip Atomic Force Microscopy (CT-AFM) measurements reveal the homogeneity of the tunnel behavior of our h-BN layers. As expected for tunneling, the resistance depends exponentially on the number of h-BN layers. The h-BN monolayer properties are also characterized through integration into complete MTJ devices. A Tunnel Magnetoresistance of up to 6% is observed for a MTJ based on a single atomically thin h-BN layer.

Stress-Induced Cubic-to-Hexagonal Phase Transformation in Perovskite Nanothin Films.

PubMed

Cao, Shi-Gu; Li, Yunsong; Wu, Hong-Hui; Wang, Jie; Huang, Baoling; Zhang, Tong-Yi

2017-08-09

The strong coupling between crystal structure and mechanical deformation can stabilize low-symmetry phases from high-symmetry phases or induce novel phase transformation in oxide thin films. Stress-induced structural phase transformation in oxide thin films has drawn more and more attention due to its significant influence on the functionalities of the materials. Here, we discovered experimentally a novel stress-induced cubic-to-hexagonal phase transformation in the perovskite nanothin films of barium titanate (BaTiO 3 ) with a special thermomechanical treatment (TMT), where BaTiO 3 nanothin films under various stresses are annealed at temperature of 575 °C. Both high-resolution transmission electron microscopy and Raman spectroscopy show a higher density of hexagonal phase in the perovskite thin film under higher tensile stress. Both X-ray photoelectron spectroscopy and electron energy loss spectroscopy does not detect any change in the valence state of Ti atoms, thereby excluding the mechanism of oxygen vacancy induced cubic-to-hexagonal (c-to-h) phase transformation. First-principles calculations show that the c-to-h phase transformation can be completed by lattice shear at elevated temperature, which is consistent with the experimental observation. The applied bending plus the residual tensile stress produces shear stress in the nanothin film. The thermal energy at the elevated temperature assists the shear stress to overcome the energy barriers during the c-to-h phase transformation. The stress-induced phase transformation in perovskite nanothin films with TMT provides materials scientists and engineers a novel approach to tailor nano/microstructures and properties of ferroelectric materials.

Polarization-Engineered Ga-Face GaN-Based Heterostructures for Normally-Off Heterostructure Field-Effect Transistors

NASA Astrophysics Data System (ADS)

Kim, Hyeongnam; Nath, Digbijoy; Rajan, Siddharth; Lu, Wu

2013-01-01

Polarization-engineered Ga-face GaN-based heterostructures with a GaN cap layer and an AlGaN/ p-GaN back barrier have been designed for normally-off field-effect transistors (FETs). The simulation results show that an unintentionally doped GaN cap and p-GaN layer in the buffer primarily deplete electrons in the channel and the Al0.2Ga0.8N back barrier helps to pinch off the channel. Experimentally, we have demonstrated a normally-off GaN-based field-effect transistor on the designed GaN cap/Al0.3Ga0.7N/GaN channel/Al0.2Ga0.8N/ p-GaN/GaN heterostructure. A positive threshold voltage of 0.2 V and maximum transconductance of 2.6 mS/mm were achieved for 80- μm-long gate devices. The device fabrication process does not require a dry etching process for gate recessing, while highly selective etching of the GaN cap against a very thin Al0.3GaN0.7N top barrier has to be performed to create a two-dimensional electron gas for both the ohmic and access regions. A self-aligned, selective etch of the GaN cap in the access region is introduced, using the gate metal as an etch mask. The absence of gate recess etching is promising for uniform and repeatable threshold voltage control in normally-off AlGaN/GaN heterostructure FETs for power switching applications.

Growth of different phases and morphological features of MnS thin films by chemical bath deposition: Effect of deposition parameters and annealing

NASA Astrophysics Data System (ADS)

Hannachi, Amira; Maghraoui-Meherzi, Hager

2017-03-01

Manganese sulfide thin films have been deposited on glass slides by chemical bath deposition (CBD) method. The effects of preparative parameters such as deposition time, bath temperature, concentration of precursors, multi-layer deposition, different source of manganese, different complexing agent and thermal annealing on structural and morphological film properties have been investigated. The prepared thin films have been characterized using the X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). It exhibit the metastable forms of MnS, the hexagonal γ-MnS wurtzite phase with preferential orientation in the (002) plane or the cubic β-MnS zinc blende with preferential orientation in the (200) plane. Microstructural studies revealed the formation of MnS crystals with different morphologies, such as hexagons, spheres, cubes or flowers like.

More Efficient Power Conversion for EVs: Gallium-Nitride Advanced Power Semiconductor and Packaging

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

None

2010-02-01

Broad Funding Opportunity Announcement Project: Delphi is developing power converters that are smaller and more energy efficient, reliable, and cost-effective than current power converters. Power converters rely on power transistors which act like a very precisely controlled on-off switch, controlling the electrical energy flowing through an electrical circuit. Most power transistors today use silicon (Si) semiconductors. However, Delphi is using semiconductors made with a thin layer of gallium-nitride (GaN) applied on top of the more conventional Si material. The GaN layer increases the energy efficiency of the power transistor and also enables the transistor to operate at much higher temperatures,more » voltages, and power-density levels compared to its Si counterpart. Delphi is packaging these high-performance GaN semiconductors with advanced electrical connections and a cooling system that extracts waste heat from both sides of the device to further increase the device’s efficiency and allow more electrical current to flow through it. When combined with other electronic components on a circuit board, Delphi’s GaN power transistor package will help improve the overall performance and cost-effectiveness of HEVs and EVs.« less

Tailoring of Perpendicular Magnetic Anisotropy in Dy13Fe87 Thin Films with Hexagonal Antidot Lattice Nanostructure

PubMed Central

Vega, Victor; Ibabe, Angel; Jaafar, Miriam; Asenjo, Agustina

2018-01-01

In this article, the magnetic properties of hexagonally ordered antidot arrays made of Dy13Fe87 alloy are studied and compared with corresponding ones of continuous thin films with the same compositions and thicknesses, varying between 20 nm and 50 nm. Both samples, the continuous thin films and antidot arrays, were prepared by high vacuum e-beam evaporation of the alloy on the top-surface of glass and hexagonally self-ordered nanoporous alumina templates, which serve as substrates, respectively. By using a highly sensitive magneto-optical Kerr effect (MOKE) and vibrating sample magnetometer (VSM) measurements an interesting phenomenon has been observed, consisting in the easy magnetization axis transfer from a purely in-plane (INP) magnetic anisotropy to out-of-plane (OOP) magnetization. For the 30 nm film thickness we have measured the volume hysteresis loops by VSM with the easy magnetization axis lying along the OOP direction. Using magnetic force microscopy measurements (MFM), there is strong evidence to suggest that the formation of magnetic domains with OOP magnetization occurs in this sample. This phenomenon can be of high interest for the development of novel magnetic and magneto-optic perpendicular recording patterned media based on template-assisted deposition techniques. PMID:29642476

Tailoring of Perpendicular Magnetic Anisotropy in Dy13Fe87 Thin Films with Hexagonal Antidot Lattice Nanostructure.

PubMed

Salaheldeen, Mohamed; Vega, Victor; Ibabe, Angel; Jaafar, Miriam; Asenjo, Agustina; Fernandez, Agustin; Prida, Victor M

2018-04-08

In this article, the magnetic properties of hexagonally ordered antidot arrays made of Dy 13 Fe 87 alloy are studied and compared with corresponding ones of continuous thin films with the same compositions and thicknesses, varying between 20 nm and 50 nm. Both samples, the continuous thin films and antidot arrays, were prepared by high vacuum e-beam evaporation of the alloy on the top-surface of glass and hexagonally self-ordered nanoporous alumina templates, which serve as substrates, respectively. By using a highly sensitive magneto-optical Kerr effect (MOKE) and vibrating sample magnetometer (VSM) measurements an interesting phenomenon has been observed, consisting in the easy magnetization axis transfer from a purely in-plane (INP) magnetic anisotropy to out-of-plane (OOP) magnetization. For the 30 nm film thickness we have measured the volume hysteresis loops by VSM with the easy magnetization axis lying along the OOP direction. Using magnetic force microscopy measurements (MFM), there is strong evidence to suggest that the formation of magnetic domains with OOP magnetization occurs in this sample. This phenomenon can be of high interest for the development of novel magnetic and magneto-optic perpendicular recording patterned media based on template-assisted deposition techniques.

Nature of exciton transitions in hexagonal boron nitride

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

Li, J.; Cao, X. K.; Lin, J. Y.

2016-03-21

In contrast to other III-nitride semiconductors GaN and AlN, the intrinsic (or free) exciton transition in hexagonal boron nitride (h-BN) consists of rather complex fine spectral features (resolved into six sharp emission peaks) and the origin of which is still unclear. Here, the free exciton transition (FX) in h-BN bulk crystals synthesized by a solution method at atmospheric pressure has been probed by deep UV time-resolved photoluminescence (PL) spectroscopy. Based on the separations between the energy peak positions of the FX emission lines, the identical PL decay kinetics among different FX emission lines, and the known phonon modes in h-BN,more » we suggest that there is only one principal emission line corresponding to the direct intrinsic FX transition in h-BN, whereas all other fine features are a result of phonon-assisted transitions. The identified phonon modes are all associated with the center of the Brillouin zone. Our results offer a simple picture for the understanding of the fundamental exciton transitions in h-BN.« less

Atomically Thin Hexagonal Boron Nitride Nanofilm for Cu Protection: The Importance of Film Perfection.

PubMed

Khan, Majharul Haque; Jamali, Sina S; Lyalin, Andrey; Molino, Paul J; Jiang, Lei; Liu, Hua Kun; Taketsugu, Tetsuya; Huang, Zhenguo

2017-01-01

Outstanding protection of Cu by high-quality boron nitride nanofilm (BNNF) 1-2 atomic layers thick in salt water is observed, while defective BNNF accelerates the reaction of Cu toward water. The chemical stability, insulating nature, and impermeability of ions through the BN hexagons render BNNF a great choice for atomic-scale protection. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Method of manufacture of atomically thin boron nitride

DOEpatents

Zettl, Alexander K

2013-08-06

The present invention provides a method of fabricating at least one single layer hexagonal boron nitride (h-BN). In an exemplary embodiment, the method includes (1) suspending at least one multilayer boron nitride across a gap of a support structure and (2) performing a reactive ion etch upon the multilayer boron nitride to produce the single layer hexagonal boron nitride suspended across the gap of the support structure. The present invention also provides a method of fabricating single layer hexagonal boron nitride. In an exemplary embodiment, the method includes (1) providing multilayer boron nitride suspended across a gap of a support structure and (2) performing a reactive ion etch upon the multilayer boron nitride to produce the single layer hexagonal boron nitride suspended across the gap of the support structure.

MOCVD growth and characterization of gallium nitride and gallium antimonide nanowires

NASA Astrophysics Data System (ADS)

Burke, Robert Alan

Group-III nitride and group-III antimonide thin films have been used for years in optoelectronic, high-speed applications, and high power/high temperature applications such as light emitting diodes (LEDs), microwave power devices, and thermovoltaics. In recent years, nanowires have gained interest due to the ability to take advantage of their geometry for increased light absorption and the synthesis of radial heterostructures. Several growth techniques have been explored for the growth of GaN and GaSb nanowires. Metal-organic chemical vapor deposition (MOCVD) is of particular interest due to its use in the commercial growth and fabrication of GaN-based and GaSb-based devices. The first part of this thesis focused on addressing several key issues related to the growth of GaN nanowires by MOCVD. Preliminary studies investigated the effect of growth conditions on GaN nanowire formation in a hot wall MOCVD reactor. A computational fluid dynamics-based model was developed to predict the gas phase velocity, temperature and concentration profiles in the reactor. The results demonstrate a strong dependence of GaN nanowire growth on substrate position within the reactor which is due to the rapid reaction and depletion of precursors near the gas inlet of the reactor. Ni-catalyzed GaN nanowire growth was observed to occur over the temperature range of 800-900°C, which is significantly lower than typical GaN thin film temperatures. The nanowires, however, exhibited a tapered diameter due to thin film deposition which occurred simultaneously with nanowire growth. Based on the low growth temperatures, TEM characterization was carried out to investigate the nature of the catalyst. Through these studies, the catalyst was found to consist of Ni3Ga, indicating the presence of a vapor-solid-solid growth mechanism. In an attempt to improve the nanowire growth selectivity, GeCl4 was added during growth resulting in a drastic increase in nanowire density and a reduction in the tapering of the nanowires. Upon further inspection with TEM, the nanowires were found to consist of two morphologies: smooth nanowires and serrated nanowires. The smooth nanowires were found to consist of the wurtzite crystal structure, while the serrated nanowires were determined to have a wurtzite core with zinc blende faceted islands protruding from the wurtzite core. The second half of this thesis focused on the growth and characterization of GaSb nanowires. An extensive amount of work has been carried out on GaSb thin films, however only a few reports exist on GaSb nanowire growth. As a result, it was necessarily to complete a systematic study to determine a growth window for GaSb nanowires. A narrow range of growth conditions were found for Au-catalyzed GaSb nanowire growth. Vertically oriented nanowires were observed over a pressure range of 150-300 Torr depending on the substrate. Based on these findings, additional characterization was carried out to investigate the structural properties of the nanowires along with chemical analysis of the catalyst to determine the nature of the catalyst as a function of the growth conditions. The catalyst was found to consist of Ga, Sb, and Au consistent with that expected for vapor-liquidsolid growth, however the concentrations varied depending on the growth conditions and nanowire sample. For one set of nanowires, the seed particle contained a Au-Sb solid solution (1-15 at.% Sb). For the other set of nanowires, the particle consisted of an AuSb2 grain and an AuGa or Au2Ga grain that resulted in the formation of a bicrystalline nanowire. Photoluminescence measurements were also obtained on these samples and compared to the thin film literature. Samples grown on Si (111) were found to possess good optical properties, while samples grown on sapphire substrates were dominated by native defect transitions. The optical quality of the nanowire sample was also found to have a significant dependence on the V/III ratio.

Oriented Y-type hexagonal ferrite thin films prepared by chemical solution deposition

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

Buršík, J., E-mail: bursik@iic.cas.cz; Kužel, R.; Knížek, K.

2013-07-15

Thin films of Ba{sub 2}Zn{sub 2}Fe{sub 12}O{sub 22} (Y) hexaferrite were prepared through the chemical solution deposition method on SrTiO{sub 3}(1 1 1) (ST) single crystal substrates using epitaxial SrFe{sub 12}O{sub 19} (M) hexaferrite thin layer as a seed template layer. The process of crystallization was mainly investigated by means of X-ray diffraction and atomic force microscopy. A detailed inspection revealed that growth of seed layer starts through the break-up of initially continuous film into isolated grains with expressive shape anisotropy and hexagonal habit. The vital parameters of the seed layer, i.e. thickness, substrate coverage, crystallization conditions and temperature rampmore » were optimized with the aim to obtain epitaxially crystallized Y phase. X-ray diffraction Pole figure measurements and Φ scans reveal perfect parallel in-plane alignment of SrTiO{sub 3} substrate and both hexaferrite phases. - Graphical abstract: XRD pole figure and AFM patterns of Ba{sub 2}Zn{sub 2}Fe{sub 12}O{sub 22} thin film epitaxially grown on SrTiO{sub 3}(1 1 1) single crystal using seeding layer templating. - Highlights: • Single phase Y-type hexagonal ferrite thin films were prepared by CSD method. • Seed M layer breaks into isolated single crystal islands and serves as a template. • Large seed grains grow by consuming the grains within the bulk of recoated film. • We explained the observed orientation relation of epitaxial domains. • Epitaxial growth on SrTiO{sub 3}(1 1 1) with relation (0 0 1){sub M,Y}//(1 1 1){sub ST}+[1 0 0]{sub M,Y}//[2 −1 −1]{sub ST}.« less

Thermal conductivity of ultra-thin chemical vapor deposited hexagonal boron nitride films

NASA Astrophysics Data System (ADS)

Alam, M. T.; Bresnehan, M. S.; Robinson, J. A.; Haque, M. A.

2014-01-01

Thermal conductivity of freestanding 10 nm and 20 nm thick chemical vapor deposited hexagonal boron nitride films was measured using both steady state and transient techniques. The measured value for both thicknesses, about 100 ± 10 W m-1 K-1, is lower than the bulk basal plane value (390 W m-1 K-1) due to the imperfections in the specimen microstructure. Impressively, this value is still 100 times higher than conventional dielectrics. Considering scalability and ease of integration, hexagonal boron nitride grown over large area is an excellent candidate for thermal management in two dimensional materials-based nanoelectronics.

Defect-related photoluminescence in Mg-doped GaN nanostructures

NASA Astrophysics Data System (ADS)

Reshchikov, M. A.; Shahedipour-Sandvik, F.; Messer, B. J.; Jindal, V.; Tripathi, N.; Tungare, M.

2009-12-01

Thin film of GaN:Mg, pyramidal GaN:Mg on GaN, sapphire and AlN substrates were grown in a MOCVD system under same growth conditions and at the same time. In samples with Mg-doped GaN pyramids on GaN:Si template a strong ultraviolet (UVL) band with few phonon replicas dominated at low temperature and was attributed to transitions from shallow donors to shallow Mg acceptor. In samples grown on sapphire and AlN substrates the UVL band appeared as a structureless band with the maximum at about 3.25 eV. There is a possibility that the structureless UVL band and the UVL band with phonon structure have different origin. In addition to the UVL band, the blue luminescence (BL) band peaking at 2.9 eV was observed in samples representing GaN:Mg pyramids on GaN:Si substrate. It is preliminary attributed to transitions from shallow donors to Zn acceptor in GaN:Si substrate.

Low-temperature preparation of GaN-SiO2 interfaces with low defect density. II. Remote plasma-assisted oxidation of GaN and nitrogen incorporation

NASA Astrophysics Data System (ADS)

Bae, Choelhwyi; Lucovsky, Gerald

2004-11-01

Low-temperature remote plasma-assisted oxidation and nitridation processes for interface formation and passivation have been extended from Si and SiC to GaN. The initial oxidation kinetics and chemical composition of thin interfacial oxide were determined from analysis of on-line Auger electron spectroscopy features associated with Ga, N, and O. The plasma-assisted oxidation process is self-limiting with power-law kinetics similar to those for the plasma-assisted oxidation of Si and SiC. Oxidation using O2/He plasma forms nearly pure GaOx, and oxidation using 1% N2O in N2 forms GaOxNy with small nitrogen content, ~4-7 at. %. The interface and dielectric layer quality was investigated using fabricated GaN metal-oxide-semiconductor capacitors. The lowest density of interface states was achieved with a two-step plasma-assisted oxidation and nitridation process before SiO2 deposition.

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

Sabah, Fayroz A., E-mail: fayroz-arif@yahoo.com; Department of Electrical Engineering, College of Engineering, Al-Mustansiriya University, Baghdad; Ahmed, Naser M., E-mail: naser@usm.my

The copper sulphide (CuS) thin films were grown with good adhesion by spray pyrolysis deposition (SPD) on Ti, ITO and glass substrates at 200 °C. The distance between nozzle and substrate is 30 cm. The composition was prepared by mixing copper chloride CuCl{sub 2}.2H{sub 2}O as a source of Cu{sup 2+} and sodium thiosulfate Na{sub 2}S{sub 2}O{sub 3}.5H{sub 2}O as a source of and S{sup 2−}. Two concentrations (0.2 and 0.4 M) were used for each CuCl{sub 2} and Na{sub 2}S{sub 2}O{sub 3} to be prepared and then sprayed (20 ml). The process was started by spraying the solution formore » 3 seconds and after 10 seconds the cycle was repeated until the solution was sprayed completely on the hot substrates. The structural characteristics were studied using X-ray diffraction; they showed covellite CuS hexagonal crystal structure for 0.2 M concentration, and covellite CuS hexagonal crystal structure with two small peaks of chalcocite Cu{sub 2}S hexagonal crystal structure for 0.4 M concentration. Also the surface and electrical characteristics were investigated using Field Emission Scanning Electron Microscopy (FESEM) and current source device, respectively. The surface study for the CuS thin films showed nanorods to be established for 0.2 M concentration and mix of nanorods and nanoplates for 0.4 M concentration. The electrical study showed ohmic behavior and low resistivity for these films. Hall Effect was measured for these thin films, it showed that all samples of CuS are p- type thin films and ensured that the resistivity for thin films of 0.2 M concentration was lower than that of 0.4 M concentration; and for the two concentrations CuS thin film deposited on ITO had the lowest resistivity. This leads to the result that the conductivity was high for CuS thin film deposited on ITO substrate, and the conductivity of the three thin films of 0.2 M concentration was higher than that of 0.4 M concentration.« less

PHASEGO: A toolkit for automatic calculation and plot of phase diagram

NASA Astrophysics Data System (ADS)

Liu, Zhong-Li

2015-06-01

The PHASEGO package extracts the Helmholtz free energy from the phonon density of states obtained by the first-principles calculations. With the help of equation of states fitting, it reduces the Gibbs free energy as a function of pressure/temperature at fixed temperature/pressure. Based on the quasi-harmonic approximation (QHA), it calculates the possible phase boundaries among all the structures of interest and finally plots the phase diagram automatically. For the single phase analysis, PHASEGO can numerically derive many properties, such as the thermal expansion coefficients, the bulk moduli, the heat capacities, the thermal pressures, the Hugoniot pressure-volume-temperature relations, the Grüneisen parameters, and the Debye temperatures. In order to check its ability of phase transition analysis, I present here two examples: semiconductor GaN and metallic Fe. In the case of GaN, PHASEGO automatically determined and plotted the phase boundaries among the provided zinc blende (ZB), wurtzite (WZ) and rocksalt (RS) structures. In the case of Fe, the results indicate that at high temperature the electronic thermal excitation free energy corrections considerably alter the phase boundaries among the body-centered cubic (bcc), face-centered cubic (fcc) and hexagonal close-packed (hcp) structures.

Controlling the optical properties of monocrystalline 3C-SiC heteroepitaxially grown on silicon at low temperatures

NASA Astrophysics Data System (ADS)

Colston, Gerard; Myronov, Maksym

2017-11-01

Cubic silicon carbide (3C-SiC) offers an alternative wide bandgap semiconductor to conventional materials such as hexagonal silicon carbide (4H-SiC) or gallium nitride (GaN) for the detection of UV light and can offer a closely lattice matched virtual substrate for subsequent GaN heteroepitaxy. As 3C-SiC can be heteroepitaxially grown on silicon (Si) substrates its optical properties can be manipulated by controlling the thickness and doping concentrations. The optical properties of 3C-SiC epilayers have been characterized by measuring the transmission of light through suspended membranes. Decreasing the thickness of the 3C-SiC epilayers is shown to shift the absorbance edge to lower wavelengths, a result of the indirect bandgap nature of silicon carbide. This property, among others, can be exploited to fabricate very low-cost, tuneable 3C-SiC based UV photodetectors. This study investigates the effect of thickness and doping concentration on the optical properties of 3C-SiC epilayers grown at low temperatures by a standard Si based growth process. The results demonstrate the potential photonic applications of 3C-SiC and its heterogeneous integration into the Si industry.

Specific peptide for functionalization of GaN

NASA Astrophysics Data System (ADS)

Estephan, E.; Larroque, C.; Cloitre, T.; Cuisinier, F. J. G.; Gergely, C.

2008-04-01

Nanobiotechnology aims to exploit biomolecular recognition and self-assembly capabilities for integrating advanced materials into medicine and biology. However frequent problems are encountered at the interface of substrate-biological molecule, as the direct physical adsorption of biological molecules is dependent of unpredictable non-specific interactions with the surface, often causing their denaturation. Therefore, a proper functionalization of the substrate should avoid a loss of biological activity. In this work we address the functionalization of the semiconductor GaN (0001) for biosensing applications. The basic interest of using III-V class semiconductors is their good light emitting properties and a fair chemical stability that allows various applications of these materials. The technology chosen to elaborate GaN-specific peptides is the combinatorial phage-display method, a biological screening procedure based on affinity selection. An M13 bacteriophage library has been used to screen 10 10 different peptides against the GaN (0001) surface to finally isolate one specific peptide. The preferential attachment of the biotinylated selected peptide onto the GaN (0001), in close proximity to a surface of different chemical and structural composition has been demonstrated by fluorescence microscopy. Further physicochemical studies have been initiated to evaluate the semiconductor-peptide interface and understand the details in the specific recognition of peptides for semiconductor substrates. Fourier Transform Infrared spectroscopy in Attenuated Total Reflection mode (FTIR-ATR) has been employed to prove the presence of peptides on the surface. Our Atomic Force Microscopy (AFM) studies on the morphology of the GaN surface after functionalization revealed a total surface coverage by a very thin, homogeneous peptide layer. Due to its good biocompatibility, functionalized GaN devices might evolve in a new class of implantable biosensors for medical applications.

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

DOE PAGES

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

2014-10-15

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

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

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

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

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

Hexagonal quartz resonator

DOEpatents

Peters, Roswell D. M.

1982-01-01

A generally flat, relatively thin AT-cut piezoelectric resonator element structured to minimize the force-frequency effect when mounted and energized in a housing. The resonator is in the form of an equilateral hexagon with the X crystallographic axis of the crystal passing through one set of opposing corners with mounting being effected at an adjacent set of corners respectively .+-.60.degree. away from the X axis which thereby results in a substantially zero frequency shift of the operating frequency.

Self-Assembled Soft Optical Negative Index Materials

DTIC Science & Technology

2008-08-05

within the MURI indicated that anodization of aluminum films provides hexagonal nano-hole arrays, which, when backfilled with e.g. silver via...bath determine pore size and spacing. Then AAO is removed with chromic and phosphoric acid at 70°C for 6 hrs. A 2nd anodization results in hexagonal...array of pores. Anodization time sets membrane thickness. Pores widened in acid such as phosphoric acid. The barrier layer is thinned by gradually

Structural and optical properties of InGaN-GaN nanowire heterostructures grown by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Limbach, F.; Gotschke, T.; Stoica, T.; Calarco, R.; Sutter, E.; Ciston, J.; Cusco, R.; Artus, L.; Kremling, S.; Höfling, S.; Worschech, L.; Grützmacher, D.

2011-01-01

InGaN/GaN nanowire (NW) heterostructures grown by plasma assisted molecular beam epitaxy were studied in comparison to their GaN and InGaN counterparts. The InGaN/GaN heterostructure NWs are composed of a GaN NW, a thin InGaN shell, and a multifaceted InGaN cap wrapping the top part of the GaN NW. High-resolution transmission electron microscopy (HRTEM) images taken from different parts of a InGaN/GaN NW show a wurtzite structure of the GaN core and the epitaxial InGaN shell around it, while additional crystallographic domains are observed whithin the InGaN cap region. Large changes in the lattice parameter along the wire, from pure GaN to higher In concentration demonstrate the successful growth of a complex InGaN/GaN NW heterostructure. Photoluminescence (PL) spectra of these heterostructure NW ensembles show rather broad and intense emission peak at 2.1 eV. However, μ-PL spectra measured on single NWs reveal a reduced broadening of the visible luminescence. The analysis of the longitudinal optical phonon Raman peak position and its shape reveal a variation in the In content between 20% and 30%, in agreement with the values estimated by PL and HRTEM investigations. The reported studies are important for understanding of the growth and properties of NW heterostructures suitable for applications in optoelectronics and photovoltaics.

Nanoscale morphology and optical property evolution of Pt nanostructures on GaN (0 0 0 1) by the systematic control of annealing temperature and duration with various Pt thickness

NASA Astrophysics Data System (ADS)

Kunwar, Sundar; Pandey, Puran; Sui, Mao; Zhang, Quanzhen; Li, Ming-Yu; Lee, Jihoon

2017-06-01

By the controlled fabrication of Pt nanostructures, various surface morphology dependent electronic, catalytic and optical properties can be exploited for a wide range of applications. In this paper, the evolution of Pt nanostructures on GaN (0 0 0 1) by the solid-state dewetting of Pt thin films is investigated. Controlling the annealing temperature, time and film thickness allows us to fabricate distinct size, density and configurations of Pt nanostructures. For 10 nm Pt thickness, tiny voids and Pt hillocks up to 550 °C, extensive void expansion and Pt nanostructure evolution between 600 °C-750 °C and finally Pt nanostructures assisted nanoholes penetration on GaN surface above 800 °C are demonstrated. Furthermore, comparatively elongated Pt nanostructures and NHs are resulted with 20 nm Pt thickness and voids growth and connected Pt nanostructure are formed by annealing duration control. The transformation of Pt films to nanostructures is governed by the surface diffusion, Rayleigh instability, Volmer-Weber growth and energy minimization mechanism whereas NHs penetration is commenced by the decomposition of GaN, Pt-Ga alloying and nitrogen desorption at high temperature. In addition, the optical characteristic of Pt nanostructures on GaN (0 0 0 1) by reflectance, photoluminescence (PL) and Raman spectroscopy demonstrate the surface morphology dependent spectral response.

Diffusion of Mg dopant in metal-organic vapor-phase epitaxy grown GaN and AlxGa1-xN

NASA Astrophysics Data System (ADS)

Köhler, K.; Gutt, R.; Wiegert, J.; Kirste, L.

2013-02-01

Diffusion of the p-type dopant Mg in GaN and AlxGa1-xN which is accompanied by segregation and affected by transient effects in metal-organic vapor-phase epitaxy reactors is investigated. We have grown 110 nm thick Mg doped GaN and Al0.1Ga0.9N layers on top of undoped GaN and Al0.1Ga0.9N layers, respectively, in a temperature range between 925 °C and 1050 °C where we placed special emphasis on the lower temperature limit without diffusion to allow separation of Mg transients, diffusion, and segregation. Hereby, AlxGa1-xN layers enable monitoring of the resolution limit by secondary ion mass spectrometry analyses for the respective samples; therefore, thin AlxGa1-xN marker layers are incorporated in the thick GaN layers. We found an upper limit of 1.25 × 1019 cm-3 for diffusing Mg atoms in both sample types. Owing to the marked influence of Mg segregation in Al0.1Ga0.9N, diffusion is only seen by using a GaN cap on top of the Al0.1Ga0.9N layer sequence. Diffusion in Al0.1Ga0.9N is shown to be increased by about 25%-30% compared to GaN. Post growth annealing experiments under conditions equivalent to those used for growth of the Mg doped samples showed negligible diffusion. Comparing the results to well established findings on other doped III-V compounds, diffusion is explained by an interstitial-substitutional mechanism with a diffusion coefficient, which is concentration dependent. Analysis of the temperature dependent diffusivity revealed an activation energy of 5.0 eV for GaN:Mg and 5.2 eV for Al0.1Ga0.9N:Mg.

Effects of Mn Ion Implantation on XPS Spectroscopy of GaN Thin Films

NASA Astrophysics Data System (ADS)

Majid, Abdul; Ahmad, Naeem; Rizwan, Muhammad; Khan, Salah Ud-Din; Ali, Fekri Abdulraqeb Ahmed; Zhu, Jianjun

2018-02-01

Gallium nitride (GaN) thin film was deposited onto a sapphire substrate and then implanted with 250 keV Mn ions at two different doses of 2 × 1016 ions/cm2 and 5 × 1016 ions/cm2. The as-grown and post-implantation-thermally-annealed samples were studied in detail using x-ray photoelectron spectroscopy (XPS). The XPS peaks of Ga 3 d, Ga 2 p, N 1 s, Mn 2 p and C 1 s were recorded in addition to a full survey of the samples. The doublet peaks of Ga 2 p for pure GaN were observed blue-shifted when compared with elemental Ga, and appeared further shifted to higher energies for the implanted samples. These observations point to changes in the bonds and the chemical environment of the host as a result of ion implantation. The results revealed broadening of the N 1 s peak after implantation, which is interpreted in terms of the presence of N-Mn bonds in addition to N-Ga bonds. The XPS spectra of Mn 2 p recorded for ion-implanted samples indicated splitting of Mn 2 p 1/2 and Mn 2 p 3/2 peaks higher than that for metallic Mn, which helps rule out the possibility of clustering and points to substitutional doping of Mn. These observations provide a framework that sheds light on the local environment of the material for understanding the mechanism of magnetic exchange interactions in Mn:GaN based diluted magnetic semiconductors.

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

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

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

2010-09-01

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

Electronic Band Structure Tuning of Highly-Mismatched-Alloys for Energy Conversion Applications

NASA Astrophysics Data System (ADS)

Ting, Min

Highly-mismatched alloys: ZnO1-xTe x and GaN1-xSb x are discussed within the context of finding the suitable material for a cost-effective Si-based tandem solar cell (SBTSC). SBTSC is an attractive concept for breaking through the energy conversion efficiency theoretical limit of a single junction solar cell. Combining with a material of 1.8 eV band gap, SBTSC can theoretically achieve energy conversion efficiency > 45%. ZnO and GaN are wide band gap semiconductors. Alloying Te in ZnO and alloying Sb in GaN result in large band gap reduction to < 2 eV from 3.3 eV and 3.4 eV respectively. The band gap reduction is majorly achieved by the upward shift of valence band (VB). Incorporating Te in ZnO modifies the VB of ZnO through the valence-band anticrossing (VBAC) interaction between localized Te states and ZnO VB delocalized states, which forms a Te-derived VB at 1 eV above the host VB. Similar band structure modification is resulted from alloying Sb in GaN. Zn1-xTex and GaN 1-xSbx thin films are synthesized across the whole composition range by pulsed laser deposition (PLD) and low temperature molecular beam epitaxy (LT-MBE) respectively. The electronic band edges of these alloys are measured by synchrotron X-ray absorption, emission, and the X-ray photoelectron spectroscopies. Modeling the optical absorption coefficient with the band anticrossing (BAC) model revealed that the Te and Sb defect levels to be at 0.99 eV and 1.2 eV above the VB of ZnO and GaN respectively. Electrically, Zn1-xTex is readily n-type conductive and GaN1-xSbx is strongly p-type conductive. A heterojunction device of p-type GaN 0.93Sb0.07 with n-type ZnO0.77Te0.93 upper cell (band gap at 1.8 eV) on Si bottom cell is proposed as a promising SBTSC device.

Bi2Te3 thin hexagonal nanoplatelets: Synthesis and its characterization studies

NASA Astrophysics Data System (ADS)

Vinoth, S.; Balaganapathi, T.; KaniAmuthan, B.; Arun, T.; Muthuselvam, I. Panneer; Chou, Fang-Cheng; Thilakan, P.

2017-08-01

Solvothermal synthesis and optimization of pure Bismuth telluride (Bi2Te3) hexagonal nanoplatelets was carried out from Bismuth Oxide (Bi2O3) and Tellurium dioxide (TeO2). XRD measurements revealed a sensitive change in crystallization behaviour in correlation with variation in Te/Bi stoichiometry identified through the exchange in intensities between (10 10 ̅) and (110) peaks. Further, Energy Dispersive X-ray (EDAX) analysis revealed the variation in Te/Bi ratio with respect to autoclave temperature. Field emission scanning electron Microscope (FESEM) and the high resolution transmission electron Microscope (HRTEM) studies show the complete growth of hexagonal nanoplatelets at 200 °C. Confocal Micro-Raman measurements revealed the occurrence of symmetry breaking in the synthesized hexagonal nanoplatelets. The electrical conductivity and the activation energy were recorded as 6.01×10-3 S/m and 0.042 eV respectively. Highest maximum absolute value of Seebeck coefficient of -355 μV/K was obtained for the hexagonal nanoplatelets.

Insight into the Near-Conduction Band States at the Crystallized Interface between GaN and SiN x Grown by Low-Pressure Chemical Vapor Deposition.

PubMed

Liu, Xinyu; Wang, Xinhua; Zhang, Yange; Wei, Ke; Zheng, Yingkui; Kang, Xuanwu; Jiang, Haojie; Li, Junfeng; Wang, Wenwu; Wu, Xuebang; Wang, Xianping; Huang, Sen

2018-06-12

Constant-capacitance deep-level transient Fourier spectroscopy is utilized to characterize the interface between a GaN epitaxial layer and a SiN x passivation layer grown by low-pressure chemical vapor deposition (LPCVD). A near-conduction band (NCB) state E LP ( E C - E T = 60 meV) featuring a very small capture cross section of 1.5 × 10 -20 cm -2 was detected at 70 K at the LPCVD-SiN x /GaN interface. A partially crystallized Si 2 N 2 O thin layer was detected at the interface by high-resolution transmission electron microscopy. Based on first-principles calculations of crystallized Si 2 N 2 O/GaN slabs, it was confirmed that the NCB state E LP mainly originates from the strong interactions between the dangling bonds of gallium and its vicinal atoms near the interface. The partially crystallized Si 2 N 2 O interfacial layer might also give rise to the very small capture cross section of the E LP owing to the smaller lattice mismatch between the Si 2 N 2 O and GaN epitaxial layer and a larger mean free path of the electron in the crystallized portion compared with an amorphous interfacial layer.

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

NASA Astrophysics Data System (ADS)

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

2008-05-01

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

Observation of phonon-polaritons in thin flakes of hexagonal boron nitride on gold

NASA Astrophysics Data System (ADS)

Ciano, C.; Giliberti, V.; Ortolani, M.; Baldassarre, L.

2018-04-01

Hexagonal Boron Nitride (hBN) is a layered van der Waals material able to sustain hyperbolic phonon-polaritons within its mid-infrared reststrahlen bands. We study the effect of a metallic substrate adjacent to hBN flakes on the polariton dispersion and on the standing wave patterns in nanostructures by means of mid-infrared nanospectroscopy and nanoimaging. We exploit the gold-coated tip apex for atomic force microscopy to launch polaritons in thin hBN flakes. The photo-thermal induced mechanical resonance is used to detect the amplitude profile of polariton standing waves with a lateral resolution of 30 nm. We observe the polariton excitation spectra on hBN flakes as thin as 4 nm, thanks to the infrared field enhancement in the nanogap between the gold-coated tip apex and an ultraflat gold substrate. The data indicate no major effect of remote screening of the free electrons in gold on the phonon-polariton excitation that appears robust also against geometrical imperfections.

Hexagonal quartz resonator

DOEpatents

Peters, R.D.M.

1982-11-02

A generally flat, relatively thin AT-cut piezoelectric resonator element structured to minimize the force-frequency effect when mounted and energized in a housing. The resonator is in the form of an equilateral hexagon with the X crystallographic axis of the crystal passing through one set of opposing corners with mounting being effected at an adjacent set of corners respectively [+-]60[degree] away from the X axis which thereby results in a substantially zero frequency shift of the operating frequency. 3 figs.

Ga flux dependence of Er-doped GaN luminescent thin films

NASA Astrophysics Data System (ADS)

Lee, D. S.; Steckl, A. J.

2002-02-01

Er-doped GaN thin films have been grown on (111) Si substrates with various Ga fluxes in a radio frequency plasma molecular beam epitaxy system. Visible photoluminescence (PL) and electroluminescence (EL) emission at 537/558 nm and infrared (IR) PL emission at 1.5 μm from GaN:Er films exhibited strong dependence on the Ga flux. Both visible and IR PL and visible EL increase with the Ga flux up to the stoichiometric growth condition, as determined by growth rate saturation. Beyond this condition, all luminescence levels abruptly dropped to the detection limit with increasing Ga flux. The Er concentration, measured by secondary ion mass spectroscopy and Rutherford backscattering, decreases with increasing Ga flux under N-rich growth conditions and remains constant above the stoichiometric growth condition. X-ray diffraction indicated that the crystalline quality of the GaN:Er film was improved with increasing Ga flux up to stoichiometric growth condition and then saturated. Er ions in the films grown under N-rich conditions appear much more optically active than those in the films grown under Ga-rich conditions.

Improving hole injection efficiency by manipulating the hole transport mechanism through p-type electron blocking layer engineering.

PubMed

Zhang, Zi-Hui; Ju, Zhengang; Liu, Wei; Tan, Swee Tiam; Ji, Yun; Kyaw, Zabu; Zhang, Xueliang; Hasanov, Namig; Sun, Xiao Wei; Demir, Hilmi Volkan

2014-04-15

The p-type AlGaN electron blocking layer (EBL) is widely used in InGaN/GaN light-emitting diodes (LEDs) for electron overflow suppression. However, a typical EBL also reduces the hole injection efficiency, because holes have to climb over the energy barrier generated at the p-AlGaN/p-GaN interface before entering the quantum wells. In this work, to address this problem, we report the enhancement of hole injection efficiency by manipulating the hole transport mechanism through insertion of a thin GaN layer of 1 nm into the p-AlGaN EBL and propose an AlGaN/GaN/AlGaN-type EBL outperforming conventional AlGaN EBLs. Here, the position of the inserted thin GaN layer relative to the p-GaN region is found to be the key to enhancing the hole injection efficiency. InGaN/GaN LEDs with the proposed p-type AlGaN/GaN/AlGaN EBL have demonstrated substantially higher optical output power and external quantum efficiency.

A Comprehensive Review of Semiconductor Ultraviolet Photodetectors: From Thin Film to One-Dimensional Nanostructures

PubMed Central

Sang, Liwen; Liao, Meiyong; Sumiya, Masatomo

2013-01-01

Ultraviolet (UV) photodetectors have drawn extensive attention owing to their applications in industrial, environmental and even biological fields. Compared to UV-enhanced Si photodetectors, a new generation of wide bandgap semiconductors, such as (Al, In) GaN, diamond, and SiC, have the advantages of high responsivity, high thermal stability, robust radiation hardness and high response speed. On the other hand, one-dimensional (1D) nanostructure semiconductors with a wide bandgap, such as β-Ga2O3, GaN, ZnO, or other metal-oxide nanostructures, also show their potential for high-efficiency UV photodetection. In some cases such as flame detection, high-temperature thermally stable detectors with high performance are required. This article provides a comprehensive review on the state-of-the-art research activities in the UV photodetection field, including not only semiconductor thin films, but also 1D nanostructured materials, which are attracting more and more attention in the detection field. A special focus is given on the thermal stability of the developed devices, which is one of the key characteristics for the real applications. PMID:23945739

Magnetic anisotropy in antiferromagnetic hexagonal MnTe

NASA Astrophysics Data System (ADS)

Kriegner, D.; Reichlova, H.; Grenzer, J.; Schmidt, W.; Ressouche, E.; Godinho, J.; Wagner, T.; Martin, S. Y.; Shick, A. B.; Volobuev, V. V.; Springholz, G.; Holý, V.; Wunderlich, J.; Jungwirth, T.; Výborný, K.

2017-12-01

Antiferromagnetic hexagonal MnTe is a promising material for spintronic devices relying on the control of antiferromagnetic domain orientations. Here we report on neutron diffraction, magnetotransport, and magnetometry experiments on semiconducting epitaxial MnTe thin films together with density functional theory (DFT) calculations of the magnetic anisotropies. The easy axes of the magnetic moments within the hexagonal basal plane are determined to be along 〈1 1 ¯00 〉 directions. The spin-flop transition and concomitant repopulation of domains in strong magnetic fields is observed. Using epitaxially induced strain the onset of the spin-flop transition changes from ˜2 to ˜0.5 T for films grown on InP and SrF2 substrates, respectively.

Nonlinear optical characterization of ZnS thin film synthesized by chemical spray pyrolysis method

NASA Astrophysics Data System (ADS)

G, Sreeja V.; V, Sabitha P.; Anila, E. I.; R, Reshmi; John, Manu Punnan; Radhakrishnan, P.

2014-10-01

ZnS thin film was prepared by Chemical Spray Pyrolysis (CSP) method. The sample was characterized by X-ray diffraction method and Z scan technique. XRD pattern showed that ZnS thin film has hexagonal structure with an average size of about 5.6nm. The nonlinear optical properties of ZnS thin film was studied by open aperture Z-Scan technique using Q-switched Nd-Yag Laser at 532nm. The Z-scan plot showed that the investigated ZnS thin film has saturable absorption behavior. The nonlinear absorption coefficient and saturation intensity were also estimated.

Investigation of hexagonal boron nitride as an atomically thin corrosion passivation coating in aqueous solution.

PubMed

Zhang, Jing; Yang, Yingchao; Lou, Jun

2016-09-09

Hexagonal boron nitride (h-BN) atomic layers were utilized as a passivation coating in this study. A large-area continuous h-BN thin film was grown on nickel foil using a chemical vapor deposition method and then transferred onto sputtered copper as a corrosion passivation coating. The corrosion passivation performance in a Na2SO4 solution of bare and coated copper was investigated by electrochemical methods including cyclic voltammetry (CV), Tafel polarization and electrochemical impedance spectroscopy (EIS). CV and Tafel analysis indicate that the h-BN coating could effectively suppress the anodic dissolution of copper. The EIS fitting result suggests that defects are the dominant leakage source on h-BN films, and improved anti-corrosion performances could be achieved by further passivating these defects.

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

NASA Astrophysics Data System (ADS)

Zhou, Wei

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

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

NASA Astrophysics Data System (ADS)

Wildeson, Isaac H.

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

Metalorganic chemical vapor deposition of gallium nitride on sacrificial substrates

NASA Astrophysics Data System (ADS)

Fenwick, William Edward

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

Superconductivity and tunneling-junctions in epitaxial Nb2N/AlN/GaN heterojunctions

NASA Astrophysics Data System (ADS)

Yan, Rusen; Han, Yimo; Khalsa, Guru; Vishwanath, Suresh; Katzer, Scott; Nepal, Neeraj; Downey, Brian; Muller, David; Meyer, David; Xing, Grace; Jena, Debdeep; ECE Collaboration; AEP Collaboration; MSE Collaboration; NRL Collaboration

We have discovered that ultrathin highly crystalline Nb2N layers grown epitaxially (by MBE) on SiC and integrated with AlN and GaN heterostructures are high-quality superconductors with transition temperatures from 9-13 K. The out-of-plane critical magnetic fields are found to be 14 Tesla range, and the critical current density is 4*1E5 A/cm2 at 5 K. Preliminary in-plane magnetotransport measurements on 4 nm thin films indicate a significantly high critical magnetic field exceeding 40 T. Since Nb2N superconducting layers can be epitaxially integrated with GaN, AlN, and AlGaN, we also demonstrate Nb2N superconductivity in a layer located beneath an N-polar GaN high-electron-mobility transistor (HEMT) heterostructure that uses a 2DEG channel as a microwave amplifier; such a demonstration illustrates the potential emergence of a new paradigm where an all-epitaxial III-N/Nb2N platform could serve as the basis for microwave qubits to power quantum computation as well as quantum communications.

Quantum state engineering with ultra-short-period (AlN)m/(GaN)n superlattices for narrowband deep-ultraviolet detection.

PubMed

Gao, Na; Lin, Wei; Chen, Xue; Huang, Kai; Li, Shuping; Li, Jinchai; Chen, Hangyang; Yang, Xu; Ji, Li; Yu, Edward T; Kang, Junyong

2014-12-21

Ultra-short-period (AlN)m/(GaN)n superlattices with tunable well and barrier atomic layer numbers were grown by metal-organic vapour phase epitaxy, and employed to demonstrate narrowband deep ultraviolet photodetection. High-resolution transmission electron microscopy and X-ray reciprocal space mapping confirm that superlattices containing well-defined, coherently strained GaN and AlN layers as thin as two atomic layers (∼ 0.5 nm) were grown. Theoretical and experimental results demonstrate that an optical absorption band as narrow as 9 nm (210 meV) at deep-ultraviolet wavelengths can be produced, and is attributable to interband transitions between quantum states along the [0001] direction in ultrathin GaN atomic layers isolated by AlN barriers. The absorption wavelength can be precisely engineered by adjusting the thickness of the GaN atomic layers because of the quantum confinement effect. These results represent a major advance towards the realization of wavelength selectable and narrowband photodetectors in the deep-ultraviolet region without any additional optical filters.

Ring resonator optical modes in InGaN/GaN structures grown on micro-cone-patterned sapphire substrates

NASA Astrophysics Data System (ADS)

Kazanov, D. R.; Pozina, G.; Jmerik, V. N.; Shubina, T. V.

2018-03-01

Molecular beam epitaxy (MBE) of III-nitride compounds on specially prepared cone-shaped patterned substrates is being actively developed nowadays, especially for nanophotonic applications. This type of substrates enables the successful growth of hexagonal nanorods (NRs). The insertion of an active quantum-sized region of InGaN inside a GaN NR allows us to enhance the rate of optical transitions by coupling them with resonant optical modes in the NR. However, we have observed the enhancement of emission not only from the NR but also around the circumference region of the cone-shaped base. We have studied this specific feature and demonstrated its impact on the output signal.

Growth of Ferromagnetic Epitaxial Film of Hexagonal FeGe on (111) Ge Surface

NASA Astrophysics Data System (ADS)

Kumar, Dushyant; Joshi, P. C.; Hossain, Z.; Budhani, R. C.

2014-03-01

The realization of semiconductors showing ferromagnetic order at easily accessible temperatures has been of interest due to their potential use in spintronic devices where long spin life times are of key interest. We have realized the growth of FeGe thin films on Ge (111) wafers using pulsed laser deposition (PLD). The stoichiometric and single phase FeGe target used in PLD chamber has been made by arc melting. A typical θ-2 θ diffraction spectra performed on 40 nm thick FeGe film suggests the stabilization of β-Ni2In (B82-type) hexagonal phase with an epitaxial orientation of (0001)FeGe ||(111)Ge and [11-20]FeGe ||[-110]Ge. SEM images shows a granular structure with the formation of very large grains of about 100 to 500 nm in lateral dimension. The magnetization vs. temperature data taken from SQUID reveal the TC of ~ 270K. Since, PLD technique makes it easier to stabilize the B82 (Ni2In) hexagonal phase in thin FeGe films, this work opens opportunities to reinvestigate many conflicting results on various properties of the FeGe system.

Raman and IR-ATR spectroscopy studies of heteroepitaxial structures with a GaN:C top layer

NASA Astrophysics Data System (ADS)

Cerqueira, M. F.; Vieira, L. G.; Alves, A.; Correia, R.; Huber, M.; Andreev, A.; Bonanni, A.; Vasilevskiy, M. I.

2017-09-01

This work, motivated by the technologically important task of determination of carbon dopant location in the GaN crystal lattice, employed Raman spectroscopy, with both resonant and non-resonant excitation, and infrared (IR) spectroscopy, in the attenuated total reflection (ATR) configuration, to study lattice vibration modes in a set of carbon-doped GaN (GaN:C) epilayers grown by metalorganic vapour phase epitaxy. We analyse Raman and IR-ATR spectra from the point of view of possible effects of the carbon doping, namely: (i) local vibration mode of C atoms in the nitrogen sublattice (whose frequency we theoretically estimate as 768 cm-1 using an isotope defect model), and (ii) shift in the positions of longitudinal modes owing to the phonon-plasmon coupling. We find only indirect hints of the doping effect on the resonant Raman spectra. However, we show theoretically and confirm experimentally that the IR-ATR spectroscopy can be a much more sensitive tool for this purpose, at least for the considered structures. A weak perturbation of the dielectric function of GaN:C, caused by the substitutional carbon impurity, is shown to produce a measurable dip in the ATR reflectivity spectra at  ≈770 cm-1 for both p- and s-polarizations. Moreover, it influences a specific (guided-wave type) mode observed at  ≈737 cm-1, originating from the GaN layer, which appears in the narrow frequency window where the real parts of the two components of the dielectric tensor of the hexagonal crystal have opposite signs. This interpretation is supported by our modelling of the whole multilayer structure, using a transfer matrix formalism.

Deep traps in n-type GaN epilayers grown by plasma assisted molecular beam epitaxy

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

Kamyczek, P.; Placzek-Popko, E.; Zielony, E.

2014-01-14

In this study, we present the results of investigations on Schottky Au-GaN diodes by means of conventional DLTS and Laplace DLTS methods within the temperature range of 77 K–350 K. Undoped GaN layers were grown using the plasma-assisted molecular beam epitaxy technique on commercial GaN/sapphire templates. The quality of the epilayers was studied by micro-Raman spectroscopy (μ-RS) which proved the hexagonal phase and good crystallinity of GaN epilayers as well as a slight strain. The photoluminescence spectrum confirmed a high crystal quality by intense excitonic emission but it also exhibited a blue emission band of low intensity. DLTS signal spectra revealed themore » presence of four majority traps: two high-temperature and two low-temperature peaks. Using the Laplace DLTS method and Arrhenius plots, the apparent activation energy and capture cross sections were obtained. For two high-temperature majority traps, they were equal to E{sub 1} = 0.65 eV, σ{sub 1} = 8.2 × 10{sup −16} cm{sup 2} and E{sub 2} = 0.58 eV, σ{sub 2} = 2.6 × 10{sup −15} cm{sup 2} whereas for the two low-temperature majority traps they were equal to E{sub 3} = 0.18 eV, σ{sub 3} = 9.7 × 10{sup −18} cm{sup 2} and E{sub 4} = 0.13 eV, σ{sub 4} = 9.2 × 10{sup −18} cm{sup 2}. The possible origin of the traps is discussed and the results are compared with data reported elsewhere.« less

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

NASA Astrophysics Data System (ADS)

Al Balushi, Zakaria Y.

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

Size effects in the thermal conductivity of gallium oxide (β-Ga{sub 2}O{sub 3}) films grown via open-atmosphere annealing of gallium nitride

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

Szwejkowski, Chester J.; Giri, Ashutosh; Donovan, Brian F.

2015-02-28

Gallium nitride (GaN) is a widely used semiconductor for high frequency and high power devices due to of its unique electrical properties: a wide band gap, high breakdown field, and high electron mobility. However, thermal management has become a limiting factor regarding efficiency, lifetime, and advancement of GaN devices and GaN-based applications. In this work, we study the thermal conductivity of beta-phase gallium oxide (β-Ga{sub 2}O{sub 3}) thin films, a component of typical gate oxides used in such devices. We use time domain thermoreflectance to measure the thermal conductivity of a variety of polycrystalline β-Ga{sub 2}O{sub 3} films of differentmore » thicknesses grown via open atmosphere annealing of the surfaces of GaN films on sapphire substrates. We show that the measured effective thermal conductivity of these β-Ga{sub 2}O{sub 3} films can span 1.5 orders of magnitude, increasing with an increased film thickness, which is indicative of the relatively large intrinsic thermal conductivity of the β-Ga{sub 2}O{sub 3} grown via this technique (8.8 ± 3.4 W m{sup −1} K{sup −1}) and large mean free paths compared to typical gate dielectrics commonly used in GaN device contacts. By conducting time domain thermoreflectance (TDTR) measurements with different metal transducers (Al, Au, and Au with a Ti wetting layer), we attribute this variation in effective thermal conductivity to a combination of size effects in the β-Ga{sub 2}O{sub 3} film resulting from phonon scattering at the β-Ga{sub 2}O{sub 3}/GaN interface and thermal transport across the β-Ga{sub 2}O{sub 3}/GaN interface. The measured thermal properties of open atmosphere-grown β-Ga{sub 2}O{sub 3} and its interface with GaN set the stage for thermal engineering of gate contacts in high frequency GaN-based devices.« less

An enhancement of photoluminescence property of Ag doped La2O3 thin films at room temperature

NASA Astrophysics Data System (ADS)

Jbeli, R.; Boukhachem, A.; Ben Jemaa, I.; Mahdhi, N.; Saadallah, F.; Elhouichet, H.; Alleg, S.; Amlouk, M.; Ezzaouïa, H.

2017-09-01

Metal transition doped oxide thin films or nanocomposites have recently emerged at the forefront of potentials research. With the focus mainly on efficiency, the aspect of stability against optical irradiation of such materials has so far not been thoroughly addressed. This work covers the synthesis of silver doped lanthanum oxide thin films (La2O3:Ag) which have been prepared by the spray pyrolysis technique on glass substrates at 460 °C. Then, Ag thin films were grown on lanthanum oxide thin films by thermal evaporation. The present work aims to reach the synthesis of La2O3:Ag thin films using both the spray pyrolysis and thermal evaporation techniques. First, X-ray diffraction analysis shows that undoped and Ag doped films crystallize in a mixture of hexagonal and cubic phase with crystallites oriented along (001) direction. Raman spectroscopy shows the bands positions corresponding to hexagonal and cubic phases. On the other hand, an attempt regarding their optical properties has been carried out by means of photoluminescence measurements. Second, from electrical conductivity measurements, the activation energy decreases from 1.42 to 1.09 eV with the increase of annealing time and the charge carriers are following the CBH model as dominant charge transport mechanism. Finally, the annealing time influences the surface wettability property and transforms La2O3 character from hydrophobic (θ > 90°) to hydrophilic (θ < 90°).

An enhancement of photoluminescence property of Ag doped La2O3 thin films at room temperature.

PubMed

Jbeli, R; Boukhachem, A; Ben Jemaa, I; Mahdhi, N; Saadallah, F; Elhouichet, H; Alleg, S; Amlouk, M; Ezzaouïa, H

2017-09-05

Metal transition doped oxide thin films or nanocomposites have recently emerged at the forefront of potentials research. With the focus mainly on efficiency, the aspect of stability against optical irradiation of such materials has so far not been thoroughly addressed. This work covers the synthesis of silver doped lanthanum oxide thin films (La 2 O 3 :Ag) which have been prepared by the spray pyrolysis technique on glass substrates at 460°C. Then, Ag thin films were grown on lanthanum oxide thin films by thermal evaporation. The present work aims to reach the synthesis of La 2 O 3 :Ag thin films using both the spray pyrolysis and thermal evaporation techniques. First, X-ray diffraction analysis shows that undoped and Ag doped films crystallize in a mixture of hexagonal and cubic phase with crystallites oriented along (001) direction. Raman spectroscopy shows the bands positions corresponding to hexagonal and cubic phases. On the other hand, an attempt regarding their optical properties has been carried out by means of photoluminescence measurements. Second, from electrical conductivity measurements, the activation energy decreases from 1.42 to 1.09eV with the increase of annealing time and the charge carriers are following the CBH model as dominant charge transport mechanism. Finally, the annealing time influences the surface wettability property and transforms La 2 O 3 character from hydrophobic (θ>90°) to hydrophilic (θ<90°). Copyright © 2017 Elsevier B.V. All rights reserved.

Indium Doped Zinc Oxide Thin Films Deposited by Ultrasonic Chemical Spray Technique, Starting from Zinc Acetylacetonate and Indium Chloride

PubMed Central

Biswal, Rajesh; Maldonado, Arturo; Vega-Pérez, Jaime; Acosta, Dwight Roberto; Olvera, María De La Luz

2014-01-01

The physical characteristics of ultrasonically sprayed indium-doped zinc oxide (ZnO:In) thin films, with electrical resistivity as low as 3.42 × 10−3 Ω·cm and high optical transmittance, in the visible range, of 50%–70% is presented. Zinc acetylacetonate and indium chloride were used as the organometallic zinc precursor and the doping source, respectively, achieving ZnO:In thin films with growth rate in the order of 100 nm/min. The effects of both indium concentration and the substrate temperature on the structural, morphological, optical, and electrical characteristics were measured. All the films were polycrystalline, fitting well with hexagonal wurtzite type ZnO. A switching in preferential growth, from (002) to (101) planes for indium doped samples were observed. The surface morphology of the films showed a change from hexagonal slices to triangle shaped grains as the indium concentration increases. Potential applications as transparent conductive electrodes based on the resulting low electrical resistance and high optical transparency of the studied samples are considered. PMID:28788118

Indium Doped Zinc Oxide Thin Films Deposited by Ultrasonic Chemical Spray Technique, Starting from Zinc Acetylacetonate and Indium Chloride.

PubMed

Biswal, Rajesh; Maldonado, Arturo; Vega-Pérez, Jaime; Acosta, Dwight Roberto; De La Luz Olvera, María

2014-07-04

The physical characteristics of ultrasonically sprayed indium-doped zinc oxide (ZnO:In) thin films, with electrical resistivity as low as 3.42 × 10 -3 Ω·cm and high optical transmittance, in the visible range, of 50%-70% is presented. Zinc acetylacetonate and indium chloride were used as the organometallic zinc precursor and the doping source, respectively, achieving ZnO:In thin films with growth rate in the order of 100 nm/min. The effects of both indium concentration and the substrate temperature on the structural, morphological, optical, and electrical characteristics were measured. All the films were polycrystalline, fitting well with hexagonal wurtzite type ZnO. A switching in preferential growth, from (002) to (101) planes for indium doped samples were observed. The surface morphology of the films showed a change from hexagonal slices to triangle shaped grains as the indium concentration increases. Potential applications as transparent conductive electrodes based on the resulting low electrical resistance and high optical transparency of the studied samples are considered.

Pseudomorphic Semiconducting Heterostructures from Combinations of AlN, GaN and Selected SiC Polytypes: Theoretical Advancement and its Coordination with Experimental Studies of Nucleation, Growth, Characterization and Device Development

DTIC Science & Technology

1994-06-01

simultaneously expluiting the favorable characteristics of these materials include the thin film deposition of both pseudomorphic beterostructure and alloys ...diagram proposed by Zangvil and Ruh [10] shows a flat miscibility gap at =1900*C between -20 and 80 wt % AIN. Above this temperature, a 2H solid solution...was reported from >20 wt % AIN. For .20 wt % AIN, 8 I I solutions and two phase mixtures of 6H, 4H, and 2H were observed. Thin film solid solutions

Mechanisms of the micro-crack generation in an ultra-thin AlN/GaN superlattice structure grown on Si(110) substrates by metalorganic chemical vapor deposition

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

Shen, X. Q., E-mail: xq-shen@aist.go.jp; Takahashi, T.; Ide, T.

2015-09-28

We investigate the generation mechanisms of micro-cracks (MCs) in an ultra-thin AlN/GaN superlattice (SL) structure grown on Si(110) substrates by metalorganic chemical vapor deposition. The SL is intended to be used as an interlayer (IL) for relaxing tensile stress and obtaining high-quality crack-free GaN grown on Si substrates. It is found that the MCs can be generated by two different mechanisms, where large mismatches of the lattice constant (LC) and the coefficient of thermal expansion (CTE) play key roles in the issue. Different MC configurations (low-density and high-density MCs) are observed, which are considered to be formed during the differentmore » growth stages (SL growth and cooling down processes) due to the LC and the CTE effects. In-situ and ex-situ experimental results support the mechanism interpretations of the MCs generation. The mechanism understanding makes it possible to optimize the SL IL structure for growing high-quality crack-free GaN films on Si substrates for optical and electronic device applications.« less

Onset of hexagons in surface-tension-driven Benard convection

NASA Technical Reports Server (NTRS)

Schatz, Michael F.; Vanhook, Stephen J.; Swift, John B.; Mccormick, William D.; Swinney, Harry L.

1994-01-01

High resolution laboratory experiments with large aspect ratio are being conducted for thin fluid layers heated from below and bounded from above by a free surface. The fluid depths are chosen sufficiently small (less than 0.06 cm) so that surface tension is the dominant driving mechanisms; the Rayleigh number is less than 5 for the results reported here. Shadowgraph visualization reveals that the primary instability leading to hexagons is slightly hysteretic (approximately 1 percent). Preliminary measurements of the convection amplitude using infrared imaging are also presented.

Corrosion resistance of monolayer hexagonal boron nitride on copper

PubMed Central

Mahvash, F.; Eissa, S.; Bordjiba, T.; Tavares, A. C.; Szkopek, T.; Siaj, M.

2017-01-01

Hexagonal boron nitride (hBN) is a layered material with high thermal and chemical stability ideal for ultrathin corrosion resistant coatings. Here, we report the corrosion resistance of Cu with hBN grown by chemical vapor deposition (CVD). Cyclic voltammetry measurements reveal that hBN layers inhibit Cu corrosion and oxygen reduction. We find that CVD grown hBN reduces the Cu corrosion rate by one order of magnitude compared to bare Cu, suggesting that this ultrathin layer can be employed as an atomically thin corrosion-inhibition coating. PMID:28191822

Epitaxy of Polar Oxides and Semiconductors

NASA Astrophysics Data System (ADS)

Shelton, Christopher Tyrel

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

Highly crystalline MoS{sub 2} thin films grown by pulsed laser deposition

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

Serrao, Claudy R.; You, Long; Gadgil, Sushant

2015-02-02

Highly crystalline thin films of MoS{sub 2} were prepared over large area by pulsed laser deposition down to a single monolayer on Al{sub 2}O{sub 3} (0001), GaN (0001), and SiC-6H (0001) substrates. X-ray diffraction and selected area electron diffraction studies show that the films are quasi-epitaxial with good out-of-plane texture. In addition, the thin films were observed to be highly crystalline with rocking curve full width half maxima of 0.01°, smooth with a RMS roughness of 0.27 nm, and uniform in thickness based on Raman spectroscopy. From transport measurements, the as-grown films were found to be p-type.

CdS thin films prepared by continuous wave Nd:YAG laser

NASA Astrophysics Data System (ADS)

Wang, H.; Tenpas, Eric W.; Vuong, Khanh D.; Williams, James A.; Schuesselbauer, E.; Bernstein, R.; Fagan, J. G.; Wang, Xing W.

1995-08-01

We report new results on continuous wave Nd:YAG laser deposition of cadmium sulfide thin films. Substrates were soda-lime silicate glass, silica glass, silicon, and copper coated formvar sheets. As deposited films were mixtures of cubic and hexagonal phases, with two different grain sizes. As revealed by SEM micrographs, films had smooth surface morphology. As revealed by TEM analysis, grain sizes were extremely small.

Preparation of thin hexagonal highly-ordered anodic aluminum oxide (AAO) template onto silicon substrate and growth ZnO nanorod arrays by electrodeposition

NASA Astrophysics Data System (ADS)

Chahrour, Khaled M.; Ahmed, Naser M.; Hashim, M. R.; Elfadill, Nezar G.; Qaeed, M. A.; Bououdina, M.

2014-12-01

In this study, anodic aluminum oxide (AAO) templates of Aluminum thin films onto Ti-coated silicon substrates were prepared for growth of nanostructure materials. Hexagonally highly ordered thin AAO templates were fabricated under controllable conditions by using a two-step anodization. The obtained thin AAO templates were approximately 70 nm in pore diameter and 250 nm in length with 110 nm interpore distances within an area of 3 cm2. The difference between first and second anodization was investigated in details by in situ monitoring of current-time curve. A bottom barrier layer of the AAO templates was removed during dropping the voltage in the last period of the anodization process followed by a wet etching using phosphoric acid (5 wt%) for several minutes at ambient temperature. As an application, Zn nanorod arrays embedded in anodic alumina (AAO) template were fabricated by electrodeposition. Oxygen was used to oxidize the electrodeposited Zn nanorods in the AAO template at 700 °C. The morphology, structure and photoluminescence properties of ZnO/AAO assembly were analyzed using Field-emission scanning electron microscope (FESEM), Energy dispersive X-ray spectroscopy (EDX), Atomic force microscope (AFM), X-ray diffraction (XRD) and photoluminescence (PL).

Efficient broad color luminescence from InGaN/GaN single quantum-well nanocolumn crystals on Si (111) substrate

NASA Astrophysics Data System (ADS)

Zhang, Wei; Zhang, Xuehua; Wang, Yongjin; Hu, Fangren

2017-10-01

Nanocolumn InGaN/GaN single quantum well crystals were deposited on Si (111) substrate with nitrified Ga dots as buffer layer. Transmission electron microscopy image shows the crystals' diameter of 100-130 nm and length of about 900 nm. Nanoscale spatial phase separation of cubic and hexagonal GaN was observed by selective area electron diffraction on the quantum well layer. Raman spectrum of the quantum well crystals proved that the crystals were fully relaxed. Room temperature photoluminescence from 450 to 750 nm and full width at half maximum of about 420 meV indicate broad color luminescence covering blue, green, yellow and red emission, which is helpful for the fabrication of tunable optoelectronic devices and colorful light emitting diodes.

Controlled growth of heteroepitaxial zinc oxide nanostructures on gallium nitride.

PubMed

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

2009-07-01

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

Synthesis of nanocrystalline ZnO thin films by electron beam evaporation

NASA Astrophysics Data System (ADS)

Kondkar, V.; Rukade, D.; Bhattacharyya, V.

2018-05-01

Nanocrystalline ZnO thin films have potential for applications in variety of optoelectronic devices. In the present study, nanocrystalline thin films of ZnO are grown on fused silica substrate using electron beam (e-beam) evaporation technique. Phase identification is carried out using Glancing angle X-ray diffraction (GAXRD) and Raman spectroscopy. Ultraviolet-Visible (UV-Vis) spectroscopic analysis is carried out to calculate energy band gap of the ZnO film. Surface morphology of the film is investigated using atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM). Highly quality nanocrystalline thin films of hexagonal wurtzite ZnO are synthesized using e-beam evaporation technique.

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

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

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

2015-05-15

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

Surfaces and Interfaces of Magnetoelectric Oxide Systems

NASA Astrophysics Data System (ADS)

Cao, Shi

Magnetoelectric materials Cr2O3, hexagonal LuFeO 3 and YbFeO3 are studied in this thesis. The surface of chromia (Cr2O3) has a surface electronic structure distinct from the bulk. Our work shows that placing a Cr2O3 single crystal into a single domain state will result in net Cr2O 3 spin polarization at the boundary, even in the presence of a gold overlayer. From the Cr 2p3/2 X-ray magnetic circular dichroism signal, there is clear evidence of interface polarization with overlayers of both Pd and Pt on chromia. Cobalt thin films on Cr2O3(0001) show larger magnetic contrast in magnetic force microscopy indicating enhancement of perpendicular anisotropy induced by Cr2O3. The interfacial charge transfer between mechanically exfoliated few-layer graphene and Cr2O3(0001) surfaces has been investigated showing hole doping of few-layer graphene. Density functional theory calculations furthermore confirm the p-type nature of the graphene on top of chromia, and suggest that the chromia is able to induce a significant carrier spin polarization in the graphene layer. The surface termination and the nominal valence states for hexagonal LuFeO3 thin films were characterized. The stable surface terminates in a Fe-O layer. This is consistent wit the results of density functional calculations. The structural transition at about 1000 °C, from the hexagonal to the orthorhombic phase of LuFeO3, has been investigated in thin films of LuFeO3. The electronic structure for the conduction bands of both hexagonal and orthorhombic LuFeO3 thin films have been measured. Dramatic differences in both the spectral features and the linear dichroism are observed. We have also studied the ferrimagnetism in h-YbFeO3 by measuring the magnetization of Fe and Yb separately. The results directly show antialignment of magnetization of Yb and Fe ions in h-YbFeO3 at low temperature, with an exchange field on Yb of about 17 kOe. All ferrimagnets, by default, are magnetoelectrics. These findings directly demonstrate that ferrimagnetic order exists in h-YbFeO3.

Structural and optical properties of InGaN--GaN nanowire heterostructures grown by molecular beam epitaxy

DOE PAGES

Limbach, F.; Gotschke, T.; Stoica, T.; ...

2011-01-01

InGaN/GaN nanowire (NW) heterostructures grown by plasma assisted molecular beam epitaxy were studied in comparison to their GaN and InGaN counterparts. The InGaN/GaN heterostructure NWs are composed of a GaN NW, a thin InGaN shell, and a multifaceted InGaN cap wrapping the top part of the GaN NW. High-resolution transmission electron microscopy (HRTEM) images taken from different parts of a InGaN/GaN NW show a wurtzite structure of the GaN core and the epitaxial InGaN shell around it, while additional crystallographic domains are observed whithin the InGaN cap region. Large changes in the lattice parameter along the wire, from pure GaNmore » to higher In concentration demonstrate the successful growth of a complex InGaN/GaN NW heterostructure. Photoluminescence (PL) spectra of these heterostructure NW ensembles show rather broad and intense emission peak at 2.1 eV. However, μ-PL spectra measured on single NWs reveal a reduced broadening of the visible luminescence. The analysis of the longitudinal optical phonon Raman peak position and its shape reveal a variation in the In content between 20% and 30%, in agreement with the values estimated by PL and HRTEM investigations. The reported studies are important for understanding of the growth and properties of NW heterostructures suitable for applications in optoelectronics and photovoltaics.« less

Microstructural response of InGaN to swift heavy ion irradiation

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

Zhang, L. M.; Jiang, W.; Fadanelli, R. C.

2016-12-01

A monocrystalline In0.18Ga0.82N film of ~275 nm in thickness grown on a GaN/Al 2O 3 substrate was irradiated with 290 MeV 238U 32+ ions to a fluence of 1.2 x 12 cm -2 at room temperature. The irradiated sample was characterized using helium ion microscopy (HIM), Rutherford backscattering spectrometry under ion-channeling conditions (RBS/C), and high-resolution x-ray diffraction (HRXRD). The irradiation leads to formation of ion tracks throughout the thin In 0.18Ga 0.82N film and the 3.0 µm thick GaN buffer layer. The mean diameter of the tracks in In 0.18Ga 0.82N is ~9 nm, as determined by HIM examination. Combinationmore » of the HIM and RBS/C data suggests that the material in the track is likely to be highly disordered or fully amorphized, in contrast to a crystalline structure within the ion track in GaN. Lattice relaxation in In0.18Ga0.82N and a distribution of d-spacing of the (0002) planes in GaN with lattice expansion are observed after irradiation.« less

Insertion of NiO electron blocking layer in fabrication of GaN-organic heterostructures

NASA Astrophysics Data System (ADS)

Li, Junmei; Guo, Wei; Jiang, Jie'an; Gao, Pingqi; Bo, Baoxue; Ye, Jichun

2018-03-01

We report the fabrication of a NiO thin film on top of an n-type GaN epitaxial layer. The electron-blocking capability of NiO in a hybrid organic/inorganic heterostructure consisting of n-GaN/NiO/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is discussed. Surface morphology, crystallography orientation, bandgap, and fermi level information of NiO films were investigated in detail. A rectifying property consistent with the proposed band diagram was observed in the current-voltage measurement. Theoretical analysis also demonstrated the effective electron blocking due to band alignment and a more balanced carrier distribution inside the GaN region with NiO inserted into the n-GaN/PEDOT:PSS heterostructure. This work provides a promising approach to the fabrication of high-efficiency hybrid optoelectronic devices.

Planar micro- and nano-patterning of GaN light-emitting diodes: Guidelines and limitations

NASA Astrophysics Data System (ADS)

Herrnsdorf, Johannes; Xie, Enyuan; Watson, Ian M.; Laurand, Nicolas; Dawson, Martin D.

2014-02-01

The emission area of GaN light-emitting diodes can be patterned by etch-free current aperturing methods which exploit the thin and highly resistive nature of the p-doped layer in these devices. Here, the fundamental underlying electrical and optical aspects of high-resolution current aperturing are investigated theoretically. The most critical parameter for the possible resolution is the thickness d of the p-GaN layer, but the interplay of p-GaN resistivity and electrical junction characteristics is also important. A spatial resolution of 1.59d can in principle be achieved, corresponding to about 300 nm in typical epitaxial structures. Furthermore, the emission from such a small emitter will spread by about 600 nm while propagating through the p-GaN. Both values can be reduced by reducing d.

Epitaxial Hexagonal Ferrites for Millimeter Wave Tunable Filters.

DTIC Science & Technology

1982-12-13

form of thin films or slabs, the LPE format should be particularly suitable. Another potential advantage of the LPE format is that the insulating...flux (solvent). In effect, this emulates the successful LPE garnet (YIG) technology which employs this flux. In contrast to garnets , Pb atoms can be...member for a workshop entitled "Application of Garnet and Ferrite Thin Films to Microwave Devices." The principal investigator also attended the 6th

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

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

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

2014-03-21

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

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

NASA Astrophysics Data System (ADS)

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

2014-03-01

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

Fabrication and characterization of hexagonally patterned quasi-1D ZnO nanowire arrays

PubMed Central

2014-01-01

Quasi-one-dimensional (quasi-1D) ZnO nanowire arrays with hexagonal pattern have been successfully synthesized via the vapor transport process without any metal catalyst. By utilizing polystyrene microsphere self-assembled monolayer, sol–gel-derived ZnO thin films were used as the periodic nucleation sites for the growth of ZnO nanowires. High-quality quasi-1D ZnO nanowires were grown from nucleation sites, and the original hexagonal periodicity is well-preserved. According to the experimental results, the vapor transport solid condensation mechanism was proposed, in which the sol–gel-derived ZnO film acting as a seed layer for nucleation. This simple method provides a favorable way to form quasi-1D ZnO nanostructures applicable to diverse fields such as two-dimensional photonic crystal, nanolaser, sensor arrays, and other optoelectronic devices. PMID:24521308

Large Excitonic Reflectivity of Monolayer MoSe2 Encapsulated in Hexagonal Boron Nitride

NASA Astrophysics Data System (ADS)

Scuri, Giovanni; Zhou, You; High, Alexander A.; Wild, Dominik S.; Shu, Chi; De Greve, Kristiaan; Jauregui, Luis A.; Taniguchi, Takashi; Watanabe, Kenji; Kim, Philip; Lukin, Mikhail D.; Park, Hongkun

2018-01-01

We demonstrate that a single layer of MoSe2 encapsulated by hexagonal boron nitride can act as an electrically switchable mirror at cryogenic temperatures, reflecting up to 85% of incident light at the excitonic resonance. This high reflectance is a direct consequence of the excellent coherence properties of excitons in this atomically thin semiconductor. We show that the MoSe2 monolayer exhibits power-and wavelength-dependent nonlinearities that stem from exciton-based lattice heating in the case of continuous-wave excitation and exciton-exciton interactions when fast, pulsed laser excitation is used.

Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal Ferrite Thin Films via Strain Engineering.

PubMed

Han, Hyeon; Kim, Donghoon; Chu, Kanghyun; Park, Jucheol; Nam, Sang Yeol; Heo, Seungyang; Yang, Chan-Ho; Jang, Hyun Myung

2018-01-17

Ferroelectric photovoltaics (FPVs) are being extensively investigated by virtue of switchable photovoltaic responses and anomalously high photovoltages of ∼10 4 V. However, FPVs suffer from extremely low photocurrents due to their wide band gaps (E g ). Here, we present a promising FPV based on hexagonal YbFeO 3 (h-YbFO) thin-film heterostructure by exploiting its narrow E g . More importantly, we demonstrate enhanced FPV effects by suitably exploiting the substrate-induced film strain in these h-YbFO-based photovoltaics. A compressive-strained h-YbFO/Pt/MgO heterojunction device shows ∼3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/Al 2 O 3 device. We have shown that the enhanced photovoltaic efficiency mainly stems from the enhanced photon absorption over a wide range of the photon energy, coupled with the enhanced polarization under a compressive strain. Density functional theory studies indicate that the compressive strain reduces E g substantially and enhances the strength of d-d transitions. This study will set a new standard for determining substrates toward thin-film photovoltaics and optoelectronic devices.

Direct Observation of the BCC (100) Plane in Thin Films of Sphere-forming Diblock Copolymers

NASA Astrophysics Data System (ADS)

Ji, Shengxiang; Nagpal, Umang; Liao, Wen; de Pablo, Juan; Nealey, Paul

2010-03-01

In sphere-forming diblock copolymers, periodic arrays of spheres are arranged in a body-centred cubic (BCC) lattice structure in bulk. However, in thin films different surface morphologies were observed as a function of the film thickness, and the transition from the hexagonal array to the BCC (110) arrangement of spheres on film surfaces was located with respect to the increase of the film thickness. Here we report the first direct observation of the BCC (100) plane in thin films of poly (styrene-b-methyl methacrylate) diblock copolymers on homogeneous substrates. By balancing the surface energies of both blocks, the lower energy BCC (100) plane corresponding to a square arrangement of half spheres, formed on film surfaces when the film thickness was commensurate with the spacing, L100, between (100) planes or greater than 2 L100. A hexagonal arrangement of spheres was only observed when the thickness was less than 2 L100 and incommensurate with 1 L100. Monte Carlo (MC) simulation confirmed our experimental observation and was used to investigate the transition of the arrangement of spheres as a function of the film thickness.

Microstructural study of Mg-doped p-type GaN: Correlation between high-resolution electron microscopy and Raman spectroscopy

NASA Astrophysics Data System (ADS)

Tsen, S.-C. Y.; Smith, David J.; Tsen, K. T.; Kim, W.; Morkoç, H.

1997-12-01

A series of Mg-doped GaN films (˜1-1.3 μm) grown by reactive molecular beam epitaxy at substrate temperatures of 750 and 800 °C has been studied by high-resolution electron microscopy (HREM) and Raman spectroscopy. Stacking defects parallel to the substrate surface were observed in samples grown on sapphire substrates at 750 °C with AlN buffer layers (60-70 nm) at low Mg concentration. A transition region with mixed zinc-blende cubic (c) and wurtzite hexagonal (h) phases having the relative orientations of (111)c//(00.1)h and (11¯0)c//(10.0)h was observed for increased Mg concentration. The top surfaces of highly doped samples were rough and assumed a completely zinc-blende phase with some inclined stacking faults. Samples grown with a Mg cell temperature of 350 °C and high doping levels were highly disordered with many small crystals having inclined stacking faults, microtwins, and defective wurtzite and zinc-blende phases. Correlation between HREM and Raman scattering results points towards the presence of compressive lattice distortion along the growth direction which might be attributable to structural defects. The films grown at 800 °C had better quality with less observable defects and less yellow luminescence than samples grown at 750 °C.

Efficient room temperature hydrogen sensor based on UV-activated ZnO nano-network

NASA Astrophysics Data System (ADS)

Kumar, Mohit; Kumar, Rahul; Rajamani, Saravanan; Ranwa, Sapana; Fanetti, Mattia; Valant, Matjaz; Kumar, Mahesh

2017-09-01

Room temperature hydrogen sensors were fabricated from Au embedded ZnO nano-networks using a 30 mW GaN ultraviolet LED. The Au-decorated ZnO nano-networks were deposited on a SiO2/Si substrate by a chemical vapour deposition process. X-ray diffraction (XRD) spectrum analysis revealed a hexagonal wurtzite structure of ZnO and presence of Au. The ZnO nanoparticles were interconnected, forming nano-network structures. Au nanoparticles were uniformly distributed on ZnO surfaces, as confirmed by FESEM imaging. Interdigitated electrodes (IDEs) were fabricated on the ZnO nano-networks using optical lithography. Sensor performances were measured with and without UV illumination, at room temperate, with concentrations of hydrogen varying from 5 ppm to 1%. The sensor response was found to be ˜21.5% under UV illumination and 0% without UV at room temperature for low hydrogen concentration of 5 ppm. The UV-photoactivated mode enhanced the adsorption of photo-induced O- and O2- ions, and the d-band electron transition from the Au nanoparticles to ZnO—which increased the chemisorbed reaction between hydrogen and oxygen. The sensor response was also measured at 150 °C (without UV illumination) and found to be ˜18% at 5 ppm. Energy efficient low cost hydrogen sensors can be designed and fabricated with the combination of GaN UV LEDs and ZnO nanostructures.

Synthesis and characterizations of nanoscale single crystal GaN grown by ion assisted gas source MBE

NASA Astrophysics Data System (ADS)

Cui, Bentao; Cohen, P. I.

2004-03-01

Nanoscale patterns could be induced by ion bombardment [1, 2]. In this study, an in-situ real time light scattering technique, combined with Reflection High Energy Electron Diffraction (RHEED), were used to study the surface morphology evolution during the ion beam assisted growth of GaN in a gas source MBE system. Ga was provided by a thermal effusion cell. Ammonia was used as the nitrogen source. A hot-filament Kaufman ion source was used to supply sub-KeV ion beams. Sapphire and MOCVD GaN templates were used as the substrates. A custom-designed Desorption Mass Spectrometer (DMS) was used to calibrate the growth temperature and determine the growth rate. Before growing GaN, the sapphire substrates were pretreated in an ion flux and then annealed for cleaning. The sapphire surface was then nitrided in ammonia at 1100K for about 10 min. After nitridation, a thin GaN buffer layer was prepared by a sequence of adsorption and annealing steps. During the growth, the short-range surface morphology and film quality were monitored in situ by RHEED. In a real-time way, the long-range surface morphology was monitored in-situ by light scattering technique. Photodiode array detector and CCD camera were used to record the reflected light scattering intensity and spectra profile respectively. Periodical patterns, such as ripple, have been observed during ion bombardment on GaN with or without growth. A linear theory (from Bradley and Harper 1988 [3]) has been modified to explain the dependence of ripple wavelength on ion species and ion energy. Partially supported by the National Science Foundation and the Air Force Office of Scientific Research. [1]. J. Erlebacher, M. J. Aziz, E. Chason, M. B. Sinclair, and J. A. Floro, Phys. Rev. Lett. 82, 2330 (1998); J. Erlebacher, M. J. Aziz, E. Chason, M. B. Sinclair, and J. A. Floro, Phys. Rev. Lett. 84, 5800 (2000). [2]. S. Facsko, T. Dekorsy, C. Koerdt, C. Trappe, H. Kurz, A. Vogt et al.. Science 285, 1551 (1999). [3]. R. M. Bradley and J. M. E. Harper, J. Vac. Sci. Technol. A 6, 2390 (1988).

Formation of Indium-Doped Zinc Oxide Thin Films Using Ultrasonic Spray Pyrolysis: The Importance of the Water Content in the Aerosol Solution and the Substrate Temperature for Enhancing Electrical Transport.

PubMed

Biswal, Rajesh; Castañeda, Luis; Moctezuma, Rosario; Vega-Pérez, Jaime; Olvera, María De La Luz; Maldonado, Arturo

2012-03-12

Indium doped zinc oxide [ZnO:In] thin films have been deposited at 430°C on soda-lime glass substrates by the chemical spray technique, starting from zinc acetate and indium acetate. Pulverization of the solution was done by ultrasonic excitation. The variations in the electrical, structural, optical, and morphological characteristics of ZnO:In thin films, as a function of both the water content in the starting solution and the substrate temperature, were studied. The electrical resistivity of ZnO:In thin films is not significantly affected with the increase in the water content, up to 200 mL/L; further increase in water content causes an increase in the resistivity of the films. All films show a polycrystalline character, fitting well with the hexagonal ZnO wurtzite-type structure. No preferential growth in samples deposited with the lowest water content was observed, whereas an increase in water content gave rise to a (002) growth. The surface morphology of the films shows a consistency with structure results, as non-geometrical shaped round grains were observed in the case of films deposited with the lowest water content, whereas hexagonal slices, with a wide size distribution were observed in the other cases. In addition, films deposited with the highest water content show a narrow size distribution.

Validity of Vegard’s rule for Al1-xInxN (0.08  <  x  <  0.28) thin films grown on GaN templates

NASA Astrophysics Data System (ADS)

Magalhães, S.; Franco, N.; Watson, I. M.; Martin, R. W.; O'Donnell, K. P.; Schenk, H. P. D.; Tang, F.; Sadler, T. C.; Kappers, M. J.; Oliver, R. A.; Monteiro, T.; Martin, T. L.; Bagot, P. A. J.; Moody, M. P.; Alves, E.; Lorenz, K.

2017-05-01

In this work, comparative x-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) measurements allow a comprehensive characterization of Al1-xInxN thin films grown on GaN. Within the limits of experimental accuracy, and in the compositional range 0.08  <  x  <  0.28, the lattice parameters of the alloys generally obey Vegard’s rule, varying linearly with the InN fraction. Results are also consistent with the small deviation from linear behaviour suggested by Darakchieva et al (2008 Appl. Phys. Lett. 93 261908). However, unintentional incorporation of Ga, revealed by atom probe tomography (APT) at levels below the detection limit for RBS, may also affect the lattice parameters. Furthermore, in certain samples the compositions determined by XRD and RBS differ significantly. This fact, which was interpreted in earlier publications as an indication of a deviation from Vegard’s rule, may rather be ascribed to the influence of defects or impurities on the lattice parameters of the alloy. The wide-ranging set of Al1-xInxN films studied allowed furthermore a detailed investigation of the composition leading to lattice-matching of Al1-xInxN/GaN bilayers.

Tunable magnetic and transport properties of Mn3Ga thin films on Ta/Ru seed layer

NASA Astrophysics Data System (ADS)

Hu, Fang; Xu, Guizhou; You, Yurong; Zhang, Zhi; Xu, Zhan; Gong, Yuanyuan; Liu, Er; Zhang, Hongguo; Liu, Enke; Wang, Wenhong; Xu, Feng

2018-03-01

Hexagonal D019-type Mn3Z alloys that possess large anomalous and topological-like Hall effects have attracted much attention due to their great potential in antiferromagnetic spintronic devices. Herein, we report the preparation of Mn3Ga films in both tetragonal and hexagonal phases with a tuned Ta/Ru seed layer on a thermally oxidized Si substrate. Large coercivity together with large anomalous Hall resistivity is found in the Ta-only sample with a mixed tetragonal phase. By increasing the thickness of the Ru layer, the tetragonal phase gradually disappears and a relatively pure hexagonal phase is obtained in the Ta(5)/Ru(30) buffered sample. Further magnetic and transport measurements revealed that the anomalous Hall conductivity nearly vanishes in the pure hexagonal sample, while an abnormal asymmetric hump structure emerges in the low field region. The extracted additional Hall term is robust in a large temperature range and presents a sign reversal above 200 K. The abnormal Hall properties are proposed to be closely related to the frustrated spin structure of D019 Mn3Ga.

Growth, Structural, Electronic, and Magnetic Characterization of GaN, CrN, Fe Islands on CrN, and Fe/CrN Bilayer Thin Films

NASA Astrophysics Data System (ADS)

Alam, Khan

As a part of my Ph.D research, initially I was involved in construction and calibration of an ultra-high vacuum thin film facility, and later on I studied structural, electronic, and magnetic properties of GaN, CrN, Fe/CrN bilayers, and Fe islands on CrN thin films. All of these films were grown by molecular beam epitaxy and characterized with a variety of state-of-the-art techniques including variable temperature reflection high energy electron diffraction, low temperature scanning tunneling microscopy and spectroscopy, variable temperature vibrating sample magnetometry, variable temperature neutron diffraction and reflectometry, variable temperature x-ray diffraction, x-ray reflectometry, Rutherford backscattering, Auger electron spectroscopy, and cross-sectional tunneling electron microscopy. The experimental results are furthermore understood by comparing with numerical calculations using generalized gradient approximation, local density approximation with Hubbard correction, Refl1D, and data analysis and visual environment program. In my first research project, I studied Ga gas adatoms on GaN surfaces. We discovered frozen-out gallium gas adatoms on atomically smooth c(6x12) GaN(0001¯) surface using low temperature scanning tunneling microscopy. We identified adsorption sites of the Ga adatoms on c(6x12) reconstructed surface. Their bonding is determined by measuring low unoccupied molecular orbital level. Absorption sites of the Ga gas adatoms on centered 6x12 are identified, and their asymmetric absorption on the chiral domains is investigated. In second project, I investigated magneto-structural phase transition in chromium nitride (CrN) thin films. The CrN thin films are grown by molecular beam epitaxy. Structural and magnetic transition are studied using variable temperature reflection high energy electron diffraction and variable temperature neutron diffraction. We observed a structural phase transition at the surface at 277+/-2 K, and a sharp, first-order magnetic phase transition from paramagnetic (room temperature) to antiferromagnetic (low temperature) at 280+/-3 K. Our experiments suggest that the structural transition in CrN thin films occur in out-of-plane direction, and epitaxial constraints suppress the in-plane transition; therefore, the low temperature crystal structure of CrN is tetragonal. This new model explains our structural and magnetic data at low temperatures, but it is different than the previously published orthorhombic model. In third project, I studied exchange bias and exchange spring effect in MBE grown Fe/CrN bilayer thin films. We grew Fe/CrN bilayer thin films on MgO(001) substrate by molecular beam epitaxy, and studied them using variable temperature vibrating sample magnetometry, polarized neutron reflectometry, x-ray reflectivity, and cross-sectional transmission electron microscopy. We observed exchange bias and exchange spring effect in all bilayer thin films. We studied the relationship of exchange bias, blocking temperature, and coercivity with Fe and CrN layers thicknesses. We used polarized neutron beam reflectometry to see if spins at Fe/CrN interface are pinned. We found a thin ferromagnetically ordered CrN layer at the interface. In my final project, I studied growth of submonolayer Fe islands on CrN thin films. These films are prepared in two stages: first, a CrN layer is grown by MBE and then a submonolayer Fe is deposited at room temperature from a carefully degassed e-beam evaporator. The films are studied at liquid helium temperature using low temperature scanning tunneling microscopy and spectroscopy. Islands are seen in STM images, after the Fe deposition, at the edges as well as at the center of atomically flat CrN terraces. However, numerical calculations performed by our collaborator Ponce-P'erez from Benem'erita Universidad Aut'onoma de Puebla show that the Fe islands are energetically unstable on the surface. The Fe atoms substitute Cr atoms in the surface layer and the Cr atoms comes out and form islands. In order to find out elemental composition of the islands, we attempted to map local density of state by measuring differential conductance spectra as a function of bias voltage using LT-STS. We observed three characteristically different spectra; one in the CrN substrate and two in the islands. The CrN substrate curve has a "U" shape near Fermi level and a peak at ≈ 105 mV. The islands spectra show Kondo-like resonances at Fermi level; some islands produce a peak whereas others produce a dip the dI/dV curves near Fermi level. Further investigations are needed to determine the origin of the peak and dip in the island curves, as well as to find the composition of the islands.

Monolithic photonic integrated circuit with a GaN-based bent waveguide

NASA Astrophysics Data System (ADS)

Cai, Wei; Qin, Chuan; Zhang, Shuai; Yuan, Jialei; Zhang, Fenghua; Wang, Yongjin

2018-06-01

Integration of a transmitter, waveguide and receiver into a single chip can generate a multicomponent system with multiple functionalities. Here, we fabricate and characterize a GaN-based photonic integrated circuit (PIC) on a GaN-on-silicon platform. With removal of the silicon and back wafer thinning of the epitaxial film, ultrathin membrane-type devices and highly confined suspended GaN waveguides were formed. Two suspended-membrane InGaN/GaN multiple-quantum-well diodes (MQW-diodes) served as an MQW light-emitting diode (MQW-LED) to emit light and an MQW photodiode (MQW-PD) to sense light. The optical interconnects between the MQW-LED and MQW-PD were achieved using the GaN bent waveguide. The GaN-based PIC consisting of an MQW-LED, waveguides and an MQW-PD forms an in-plane light communication system with a data transmission rate of 70 Mbps.

Au-Doped Indium Tin Oxide Ohmic Contacts to p-Type GaN

NASA Astrophysics Data System (ADS)

Guo, H.; Andagana, H. B.; Cao, X. A.

2010-05-01

Indium tin oxide (ITO) thin films doped with Au, Ni, or Pt (3.5 at.% to 10.5 at.%) were deposited on p-GaN epilayers (Mg ~4 × 1019 cm-3) using direct-current (DC) sputter codeposition. It was found that undoped ITO con- tacts to p-GaN exhibited leaky Schottky behavior, whereas the incorporation of a small amount of Au (3.5 at.% to 10.5 at.%) significantly improved their ohmic characteristics. Compared with standard Ni/ITO contacts, the Au-doped ITO contacts had a similar specific contact resistance in the low 10-2 Ω cm-2 range, but were more stable above 600°C and more transparent at blue wavelengths. These results provide support for the use of Au-doped ITO ohmic contact to p-type GaN in high-brightness blue light-emitting diodes.

Study of Electrical Contacts and Devices in Advanced Semiconductors

NASA Technical Reports Server (NTRS)

Hall, H. P.; Das, K.; Alterovitz, Samuel (Technical Monitor)

2001-01-01

Research conducted at Tuskegee University concentrates on electrical contacts to GaN films and their characterization with the objective of understanding contact formation and realizing low-resistance metal contacts. Contact properties are known to be strongly related to surface preparation. It appears that the as-received material had a thin oxide film on the surface of the GaN film. Various cleaning treatments were employed in order to render the surface contamination free and removal of the oxide film. Metal films were then deposited by e-beam evaporation. Electrical characteristics of these contacts indicated that the optimal treatment was an organic solvent cleaning followed by etching in buffered oxide solution. Contacts established with Al were observed to be ohmic in nature, whereas Au, Cr, Ti, and Pt exhibit rectifying contacts. Platinum contacts were almost ideal as shown by an ideality factor of 1.02.

Correct implementation of polarization constants in wurtzite materials and impact on III-nitrides

DOE PAGES

Dreyer, Cyrus E.; Janotti, Anderson; Van de Walle, Chris G.; ...

2016-06-20

Here, accurate values for polarization discontinuities between pyroelectric materials are critical for understanding and designing the electronic properties of heterostructures. For wurtzite materials, the zincblende structure has been used in the literature as a reference to determine the effective spontaneous polarization constants. We show that, because the zincblende structure has a nonzero formal polarization, this method results in a spurious contribution to the spontaneous polarization differences between materials. In addition, we address the correct choice of "improper" versus "proper" piezoelectric constants. For the technologically important III-nitride materials GaN, AlN, and InN, we determine polarization discontinuities using a consistent reference basedmore » on the layered hexagonal structure and the correct choice of piezoelectric constants, and discuss the results in light of available experimental data.« less

Investigation of Mild Steel Thin-Wall Tubes in Unfilled and Foam-Filled Triangle, Square, and Hexagonal Cross Sections Under Compression Load

NASA Astrophysics Data System (ADS)

Rajak, Dipen Kumar; Kumaraswamidhas, L. A.; Das, S.

2018-02-01

This study has examined proposed structures with mild steel-reinforced LM30 aluminum (Al) alloy having diversely unfilled and 10 wt.% SiCp composite foam-filled tubes for improving axial compression performance. This class of material has novel physical, mechanical, and electrical properties along with low density. In the present experiment, Al alloy foams were prepared by the melt route technique using metal hydride powder as a foaming agent. Crash energy phenomena for diverse unfilled and foam-filled in mild steel thin-wall tubes (triangular, square and hexagonal) were studied as well. Compression deformation investigation was conducted at strain rates of 0.001-0.1/s for evaluating specific energy absorption (SEA) under axial loading conditions. The results were examined to measure plateau stress, maximum densification strain, and deformation mechanism of the materials. Specific energy absorption and total energy absorption capacities of the unfilled and filled sections were determined from the compressive stress-strain curves, which were then compared with each other.

Noncollinear antiferromagnetic Mn3Sn films

NASA Astrophysics Data System (ADS)

Markou, A.; Taylor, J. M.; Kalache, A.; Werner, P.; Parkin, S. S. P.; Felser, C.

2018-05-01

Noncollinear hexagonal antiferromagnets with almost zero net magnetization were recently shown to demonstrate giant anomalous Hall effect. Here, we present the structural and magnetic properties of noncollinear antiferromagnetic Mn3Sn thin films heteroepitaxially grown on Y:ZrO2 (111) substrates with a Ru underlayer. The Mn3Sn films were crystallized in the hexagonal D 019 structure with c -axis preferred (0001) crystal orientation. The Mn3Sn films are discontinuous, forming large islands of approximately 400 nm in width, but are chemical homogeneous and characterized by near perfect heteroepitaxy. Furthermore, the thin films show weak ferromagnetism with an in-plane uncompensated magnetization of M =34 kA/m and coercivity of μ0Hc=4.0 mT at room temperature. Additionally, the exchange bias effect was studied in Mn3Sn /Py bilayers. Exchange bias fields up to μ0HEB=12.6 mT can be achieved at 5 K. These results show Mn3Sn films to be an attractive material for applications in antiferromagnetic spintronics.

Spatiotemporal patterns in reaction-diffusion system and in a vibrated granular bed

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

Swinney, H.L.; Lee, K.J.; McCormick, W.D.

Experiments on a quasi-two-dimensional reaction-diffusion system reveal transitions from a uniform state to stationary hexagonal, striped, and rhombic spatial patterns. For other reactor conditions lamellae and self-replicating spot patterns are observed. These patterns form in continuously fed thin gel reactors that can be maintained indefinitely in well-defined nonequilibrium states. Reaction-diffusion models with two chemical species yield patterns similar to those observed in the experiments. Pattern formation is also being examined in vertically oscillated thin granular layers (typically 3-30 particle diameters deep). For small acceleration amplitudes, a granular layer is flat, but above a well-defined critical acceleration amplitude, spatial patterns spontaneouslymore » form. Disordered time-dependent granular patterns are observed as well as regular patterns of squares, stripes, and hexagons. A one-dimensional model consisting of a completely inelastic ball colliding with a sinusoidally oscillating platform provides a semi-quantitative description of most of the observed bifurcations between the different spatiotemporal regimes.« less

Millimeter wave complementary metal-oxide-semiconductor on-chip hexagonal nano-ferrite circulator

NASA Astrophysics Data System (ADS)

Chao, Liu; Oukacha, Hassan; Fu, Enjin; Koomson, Valencia Joyner; Afsar, Mohammed N.

2015-05-01

Hexagonal ferrites such as M-type BaFe12O19 and SrFe12O19 have strong uniaxial anisotropic magnetic field and remanent magnetism. The nano-sized ferrite powder exhibits high compatibility and processability in composite material. New magnetic devices using the M-type ferrite materials can work in the tens of GHz frequency range from microwave to millimeter wave without the application of strong external magnetic field. The micro- and nano-sized hexagonal ferrite can be conveniently utilized to fabricate magnetic components integrated in CMOS integrated circuits as thin as several micrometers. The micro-fabrication method of such nano ferrite device is presented in this paper. A circulator working at 60 GHz is designed and integrated into the commercial CMOS process. The circulator exhibits distinct circulation properties in the frequency range from 56 GHz to 58 GHz.

Synthesis and annealing study of RF sputtered ZnO thin film

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

Singh, Shushant Kumar, E-mail: singhshushant86@gmail.com; Sharma, Himanshu; Singhal, R.

2016-05-23

In this paper, we have investigated the annealing effect on optical and structural properties of ZnO thin films, synthesized by RF magnetron sputtering. ZnO thin films were deposited on glass and silicon substrates simultaneously at a substrate temperature of 300 °C using Argon gas in sputtering chamber. Thickness of as deposited ZnO thin film was found to be ~155 nm, calculated by Rutherford backscattering spectroscopy (RBS). These films were annealed at 400 °C and 500 °C temperature in the continuous flow of oxygen gas for 1 hour in tube furnace. X-ray diffraction analysis confirmed the formation of hexagonal wurtzite structuremore » of ZnO thin film along the c-axis (002) orientation. Transmittance of thin films was increased with increasing the annealing temperature estimated by UV-visible transmission spectroscopy. Quality and texture of the thin films were improved with annealing temperature, estimated by Raman spectroscopy.« less

A study on micro-structural and optical parameters of InxSe1-x thin film

NASA Astrophysics Data System (ADS)

Patel, P. B.; Desai, H. N.; Dhimmar, J. M.; Modi, B. P.

2018-04-01

Thin film of Indium Selenide (InSe) has been deposited by thermal evaporation technique onto pre cleaned glass substrate under high vacuum condition. The micro-structural and optical properties of InxSe1-x (x = 0.6, 1-x = 0.4) thin film have been characterized by X-ray diffractrometer (XRD) and UV-Visible spectrophotometer. The XRD spectra showed that InSe thin film has single phase hexagonal structure with preferred orientation along (1 1 0) direction. The micro-structural parameters (crystallite size, lattice strain, dislocation density, domain population) for InSe thin film have been calculated using XRD spectra. The optical parameters (absorption, transmittance, reflectance, energy band gap, Urbach energy) of InSe thin film have been evaluated from absorption spectra. The direct energy band gap and Urbach energy of InSe thin film is found to be 1.90 eV and 235 meV respectively.

Highly mismatched GaN1-x Sb x alloys: synthesis, structure and electronic properties

NASA Astrophysics Data System (ADS)

Yu, K. M.; Sarney, W. L.; Novikov, S. V.; Segercrantz, N.; Ting, M.; Shaw, M.; Svensson, S. P.; Martin, R. W.; Walukiewicz, W.; Foxon, C. T.

2016-08-01

Highly mismatched alloys (HMAs) is a class of semiconductor alloys whose constituents are distinctly different in terms of size, ionicity and/or electronegativity. Electronic properties of the alloys deviate significantly from an interpolation scheme based on small deviations from the virtual crystal approximation. Most of the HMAs were only studied in a dilute composition limit. Recent advances in understanding of the semiconductor synthesis processes allowed growth of thin films of HMAs under non-equilibrium conditions. Thus reducing the growth temperature allowed synthesis of group III-N-V HMAs over almost the entire composition range. This paper focuses on the GaN x Sb1-x HMA which has been suggested as a potential material for solar water dissociation devices. Here we review our recent work on the synthesis, structural and optical characterization of GaN1-x Sb x HMA. Theoretical modeling studies on its electronic structure based on the band anticrossing (BAC) model are also reviewed. In particular we discuss the effects of growth temperature, Ga flux and Sb flux on the incorporation of Sb, film microstructure and optical properties of the alloys. Results obtained from two separate MBE growths are directly compared. Our work demonstrates that a large range of direct bandgap energies from 3.4 eV to below 1.0 eV can be achieved for this alloy grown at low temperature. We show that the electronic band structure of GaN1-x Sb x HMA over the entire composition range is well described by a modified BAC model which includes the dependence of the host matrix band edges as well as the BAC model coupling parameters on composition. We emphasize that the modified BAC model of the electronic band structure developed for the full composition of GaN x Sb1-x is general and is applicable to any HMA.

APCVD hexagonal boron nitride thin films for passive near-junction thermal management of electronics

NASA Astrophysics Data System (ADS)

KC, Pratik; Rai, Amit; Ashton, Taylor S.; Moore, Arden L.

2017-12-01

The ability of graphene to serve as an ultrathin heat spreader has been previously demonstrated with impressive results. However, graphene is electrically conductive, making its use in contact with electronic devices problematic from a reliability and integration perspective. As an alternative, hexagonal boron nitride (h-BN) is a similarly structured material with large in-plane thermal conductivity but which possesses a wide band gap, thereby giving it potential to be utilized for directing contact, near-junction thermal management of electronics without shorting or the need for an insulating intermediate layer. In this work, the viability of using large area, continuous h-BN thin films as direct contact, near-junction heat spreaders for electronic devices is experimentally evaluated. Thin films of h-BN several square millimeters in size were synthesized via an atmospheric pressure chemical vapor deposition (APCVD) method that is both simple and scalable. These were subsequently transferred onto a microfabricated test device that simulated a multigate transistor while also allowing for measurements of the device temperature at various locations via precision resistance thermometry. Results showed that these large-area h-BN films with thicknesses of 77-125 nm are indeed capable of significantly lowering microdevice temperatures, with the best sample showing the presence of the h-BN thin film reduced the effective thermal resistance by 15.9% ± 4.6% compared to a bare microdevice at the same power density. Finally, finite element simulations of these experiments were utilized to estimate the thermal conductivity of the h-BN thin films and identify means by which further heat spreading performance gains could be attained.

Fabrication of WS2/GaN p-n Junction by Wafer-Scale WS2 Thin Film Transfer

PubMed Central

Yu, Yang; Fong, Patrick W. K.; Wang, Shifeng; Surya, Charles

2016-01-01

High quality wafer-scale free-standing WS2 grown by van der Waals rheotaxy (vdWR) using Ni as a texture promoting layer is reported. The microstructure of vdWR grown WS2 was significantly modified from mixture of crystallites with their c-axes both parallel to (type I) and perpendicular to (type II) the substrate to large type II crystallites. Wafer-scale transfer of vdWR grown WS2 onto different substrates by an etching-free technique was demonstrated for the first time that utilized the hydrophobic property of WS2 and hydrophilic property of sapphire. Our results show that vdWR is a reliable technique to obtain type-II textured crystallites in WS2, which is the key factor for the wafer-scale etching-free transfer. The transferred films were found to be free of observable wrinkles, cracks, or polymer residues. High quality p-n junctions fabricated by room-temperature transfer of the p-type WS2 onto an n-type GaN was demonstrated with a small leakage current density of 29.6 μA/cm2 at −1 V which shows superior performances compared to the directly grown WS2/GaN heterojunctions. PMID:27897210

Absolute surface energy calculations of Wurtzite (0001)/(000-1): a study of ZnO and GaN

NASA Astrophysics Data System (ADS)

Zhang, Jingzhao; Zhang, Yiou; Tse, Kinfai; Deng, Bei; Xu, Hu; Zhu, Junyi

The accurate absolute surface energies of (0001)/(000-1) surfaces of wurtzite structures are crucial in determining the thin film growth mode of important energy materials. However, the surface energies still remain to be solved due to the intrinsic difficulty of calculating dangling bond energy of asymmetrically bonded surface atoms. We used a pseudo-hydrogen passivation method to estimate the dangling bond energy and calculate the polar surfaces of ZnO and GaN. The calculations were based on the pseudo chemical potentials obtained from a set of tetrahedral clusters or simple pseudo-molecules, using density functional theory approaches, for both GGA and HSE. And the surface energies of (0001)/(000-1) surfaces of wurtzite ZnO and GaN we obtained showed relatively high self-consistencies. A wedge structure calculation with a new bottom surface passivation scheme of group I and group VII elements was also proposed and performed to show converged absolute surface energy of wurtzite ZnO polar surfaces. Part of the computing resources was provided by the High Performance Cluster Computing Centre, Hong Kong Baptist University. This work was supported by the start-up funding and direct Grant with the Project code of 4053134 at CUHK.

Fabrication of WS2/GaN p-n Junction by Wafer-Scale WS2 Thin Film Transfer.

PubMed

Yu, Yang; Fong, Patrick W K; Wang, Shifeng; Surya, Charles

2016-11-29

High quality wafer-scale free-standing WS 2 grown by van der Waals rheotaxy (vdWR) using Ni as a texture promoting layer is reported. The microstructure of vdWR grown WS 2 was significantly modified from mixture of crystallites with their c-axes both parallel to (type I) and perpendicular to (type II) the substrate to large type II crystallites. Wafer-scale transfer of vdWR grown WS 2 onto different substrates by an etching-free technique was demonstrated for the first time that utilized the hydrophobic property of WS 2 and hydrophilic property of sapphire. Our results show that vdWR is a reliable technique to obtain type-II textured crystallites in WS 2 , which is the key factor for the wafer-scale etching-free transfer. The transferred films were found to be free of observable wrinkles, cracks, or polymer residues. High quality p-n junctions fabricated by room-temperature transfer of the p-type WS 2 onto an n-type GaN was demonstrated with a small leakage current density of 29.6 μA/cm 2 at -1 V which shows superior performances compared to the directly grown WS 2 /GaN heterojunctions.

Micro-Raman investigations of InN-GaN core-shell nanowires on Si (111) substrate

NASA Astrophysics Data System (ADS)

Sangeetha, P.; Jeganathan, K.; Ramakrishnan, V.

2013-06-01

The electron-phonon interactions in InN-GaN core-shell nanowires grown by plasma assisted- molecular beam epitaxy (MBE) on Si (111) substrate have been analysed using micro-Raman spectroscopic technique with the excitation wavelength of 633, 488 and 325 nm. The Raman scattering at 633 nm reveals the characteristic E2 (high) and A1 (LO) phonon mode of InN core at 490 and 590 cm-1 respectively and E2 (high) phonon mode of GaN shell at 573 cm-1. The free carrier concentration of InN core is found to be low in the order ˜ 1016 cm-3 due to the screening of charge carriers by thin GaN shell. Diameter of InN core evaluated using the spatial correlation model is consistent with the transmission electron microscopic measurement of ˜15 nm. The phonon-life time of core-shell nanowire structure is estimated to be ˜0.4 ps. The micro-Raman mapping and its corresponding localised spectra for 325 nm excitation exhibit intense E2 (high) phonon mode of GaN shell at 573 cm-1 as the decrease of laser interaction length and the signal intensity is quenched at the voids due to high spacing of NWs.

Pre-stressed thermal protection systems

NASA Technical Reports Server (NTRS)

Dunn, T. J. (Inventor)

1984-01-01

A hexagonal protective and high temperature resistant system for the Space Shuttle Orbiter consists of a multiplicity of pockets formed by hexagonally oriented spacer bars secured on the vehicle substructure. A packing of low density insulating batt material 18 in each pocket, and a thin protective panel of laterally resilient advanced carbon-carbon material surmounting the peripherals bars and packing. Each panel has three stepped or offset lips on contiguous edges. At the center of each pocket is a fully insulated stanchion secured to and connecting the substructure and panel for flexing the panel toward the substructure and thereby prestressing the panel and forcing the panel edges firmly against the spacer bars.

Germanium layers grown by zone thermal crystallization from a discrete liquid source

NASA Astrophysics Data System (ADS)

Yatsenko, A. N.; Chebotarev, S. N.; Lozovskii, V. N.; Mohamed, A. A. A.; Erimeev, G. A.; Goncharova, L. M.; Varnavskaya, A. A.

2017-11-01

It is proposed and investigated a method for growing thin uniform germanium layers onto large silicon substrates. The technique uses the hexagonally arranged local sources filled with liquid germanium. Germanium evaporates on very close substrate and in these conditions the residual gases vapor pressure highly reduces. It is shown that to achieve uniformity of the deposited layer better than 97% the critical thickness of the vacuum zone must be equal to l cr = 1.2 mm for a hexagonal arranged system of round local sources with the radius of r = 0.75 mm and the distance between the sources of h = 0.5 mm.

Dual Mode Thin Film Bulk Acoustic Resonators (FBARs) Based on AlN, ZnO and GaN Films with Tilted c-Axis Orientation

DTIC Science & Technology

2010-01-01

TERMS MEMS , acoustic wave devices, acoustic wave sensors Qing-Ming Wang University of Pittsburgh 123 University Place University Club Pittsburgh, PA...resonators,” Proc. SPIE Vol. 6223, 62230I, Micro ( MEMS ) and Nanotechnologies for Space Applications; Thomas George, Zhong-Yang Cheng; Eds. (May...microelectromechanical resonators has been recognized as a technological challenge in the current microelectronics and MEMS development. The

Single-Crystalline, Nanoporous Gallium Nitride Films With Fine Tuning of Pore Size for Stem Cell Engineering.

PubMed

Han, Lin; Zhou, Jing; Sun, Yubing; Zhang, Yu; Han, Jung; Fu, Jianping; Fan, Rong

2014-11-01

Single-crystalline nanoporous gallium nitride (GaN) thin films were fabricated with the pore size readily tunable in 20-100 nm. Uniform adhesion and spreading of human mesenchymal stem cells (hMSCs) seeded on these thin films peak on the surface with pore size of 30 nm. Substantial cell elongation emerges as pore size increases to ∼80 nm. The osteogenic differentiation of hMSCs occurs preferentially on the films with 30 nm sized nanopores, which is correlated with the optimum condition for cell spreading, which suggests that adhesion, spreading, and stem cell differentiation are interlinked and might be coregulated by nanotopography.

Terahertz time-domain measurement of non-Drude conductivity in silver nanowire thin films for transparent electrode applications

NASA Astrophysics Data System (ADS)

Kim, Jaeseok; Maeng, Inhee; Jung, Jongwook; Song, Hyunjoon; Son, Joo-Hiuk; Kim, Kilsuk; Lee, Jaeik; Kim, Chul-Hong; Chae, Geesung; Jun, Myungchul; Hwang, YongKee; Jeong Lee, Su; Myoung, Jae-Min; Choi, Hyunyong

2013-01-01

We have investigated the complex conductivity of silver nanowire thin films using terahertz time-domain spectroscopy. Maxwell-Garnett effective medium theory, which accounts for the effective complex conductivity of silver nanowires, is presented in detail theoretically and experimentally. The conductivity of nanowires exhibits a characteristic non-Drude response in which the applied terahertz field is polarized in the longitudinal nanowire direction. The non-Drude responses of the silver nanowires are explained by the Gans approximation and the Drude-Smith model, and both agree well with the experimental data. Our results provide a basis for further explorations of charge carrier dynamics in nanowire-based transparent electrode applications.

Investigations of electromagnetic scattering by columnar ice crystals

NASA Technical Reports Server (NTRS)

Weil, H.; Senior, T. B. A.

1976-01-01

An integral equation approach was developed to determine the scattering and absorption of electromagnetic radiation by thin walled cylinders of arbitrary cross-section and refractive index. Based on this method, extensive numerical data was presented at infrared wavelengths for hollow hexagonal cross section cylinders which simulate columnar sheath ice crystals.

Growth of wide-bandgap nitride semiconductors by MBE

NASA Astrophysics Data System (ADS)

Moustakas, T. D.

2002-08-01

This paper reviews progress in the heteroepitaxial growth of Ill-Nitride semiconductors. The growth of wurtzite and zinc-blende allotropic forms of GaN on various substrates with hexagonal and cubic symmetry respectively were discussed. In particular we addressed the growth on the various faces of sapphire, 6H-SiC and (001) Si. It has been shown that the kinetics of growth by plasma-MBE or ammonia-MBE are different. Specifically, in plasma-assisted MBE smooth films are obtained under group-III rich conditions of growth. On the other hand in ammonia-MBE smooth films are obtained under nitrogen rich conditions of growth. High quality films were obtained on 6H-SiC without the employment of any buffer. The various nucleation steps used to improve the two dimensional growth as well as to control the film polarity were discussed. The n- and p-doping of GaN were addressed. The concept of increasing the solubility of Mg in GaN by simultaneously bombarding the surface of the growing film with a flux of electrons (co-doping GaN with Mg and electrons) was discussed. The influence of the strength of Al-N, Ga-N and In-N bonds on the kinetics of growth of nitride alloys was pointed out. Specifically, it was shown that in both the nitrogen-rich and group-III rich growth regimes, the incorporation probability of aluminum is unity for the investigated temperature range of 750-800° C. On the other hand the incorporation probability of gallium is constant but less than unity only in the nitrogen-rich regime of growth. In the group-III regime the incorporation probability of gallium decreases monotonically with the total group-III flux, due to the competition with aluminum for the available active nitrogen. Alloy phenomena such as phase separation and atomic ordering and the influence of these phenomena to the optical properties were addressed. InGaN alloys are thermodynamically unstable against phase separation. At compositions above 30% they tend to undergo partial phase separation. Furthermore, InGaN alloys were found to undergo 1x1 monolayer cation ordering. AlGaN alloys do not show evidence of phase separation but they were found to undergo multiple type of superlattice ordering. Under nitrogen-rich growth conditions they show one monolayer periodicity, while under group-III rich growth it was found that the structure is a superposition of a seven monolayer and twelve monolayer superlattices. Finally, the growth of heterostructures and MQWs and the use of the MBE method for the fabrication of optical, electronic and electromechanical devices were discussed.

Electron density and currents of AlN/GaN high electron mobility transistors with thin GaN/AlN buffer layer

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

Bairamis, A.; Zervos, Ch.; Georgakilas, A., E-mail: alexandr@physics.uoc.gr

2014-09-15

AlN/GaN high electron mobility transistor (HEMT) structures with thin GaN/AlN buffer layer have been analyzed theoretically and experimentally, and the effects of the AlN barrier and GaN buffer layer thicknesses on two-dimensional electron gas (2DEG) density and transport properties have been evaluated. HEMT structures consisting of [300 nm GaN/ 200 nm AlN] buffer layer on sapphire were grown by plasma-assisted molecular beam epitaxy and exhibited a remarkable agreement with the theoretical calculations, suggesting a negligible influence of the crystalline defects that increase near the heteroepitaxial interface. The 2DEG density varied from 6.8 × 10{sup 12} to 2.1 × 10{sup 13} cm{sup −2} as themore » AlN barrier thickness increased from 2.2 to 4.5 nm, while a 4.5 nm AlN barrier would result to 3.1 × 10{sup 13} cm{sup −2} on a GaN buffer layer. The 3.0 nm AlN barrier structure exhibited the highest 2DEG mobility of 900 cm{sup 2}/Vs for a density of 1.3 × 10{sup 13} cm{sup −2}. The results were also confirmed by the performance of 1 μm gate-length transistors. The scaling of AlN barrier thickness from 1.5 nm to 4.5 nm could modify the drain-source saturation current, for zero gate-source voltage, from zero (normally off condition) to 0.63 A/mm. The maximum drain-source current was 1.1 A/mm for AlN barrier thickness of 3.0 nm and 3.7 nm, and the maximum extrinsic transconductance was 320 mS/mm for 3.0 nm AlN barrier.« less

Formation of Indium-Doped Zinc Oxide Thin Films Using Ultrasonic Spray Pyrolysis: The Importance of the Water Content in the Aerosol Solution and the Substrate Temperature for Enhancing Electrical Transport

PubMed Central

Biswal, Rajesh; Castañeda, Luis; Moctezuma, Rosario; Vega-Pérez, Jaime; De La Luz Olvera, María; Maldonado, Arturo

2012-01-01

Indium doped zinc oxide [ZnO:In] thin films have been deposited at 430°C on soda-lime glass substrates by the chemical spray technique, starting from zinc acetate and indium acetate. Pulverization of the solution was done by ultrasonic excitation. The variations in the electrical, structural, optical, and morphological characteristics of ZnO:In thin films, as a function of both the water content in the starting solution and the substrate temperature, were studied. The electrical resistivity of ZnO:In thin films is not significantly affected with the increase in the water content, up to 200 mL/L; further increase in water content causes an increase in the resistivity of the films. All films show a polycrystalline character, fitting well with the hexagonal ZnO wurtzite-type structure. No preferential growth in samples deposited with the lowest water content was observed, whereas an increase in water content gave rise to a (002) growth. The surface morphology of the films shows a consistency with structure results, as non-geometrical shaped round grains were observed in the case of films deposited with the lowest water content, whereas hexagonal slices, with a wide size distribution were observed in the other cases. In addition, films deposited with the highest water content show a narrow size distribution. PMID:28817056

The properties of plasma-enhanced atomic layer deposition (ALD) ZnO thin films and comparison with thermal ALD

NASA Astrophysics Data System (ADS)

Kim, Doyoung; Kang, Hyemin; Kim, Jae-Min; Kim, Hyungjun

2011-02-01

Zinc oxide (ZnO) thin films were prepared by plasma-enhanced atomic layer deposition (PE-ALD) using oxygen plasma as a reactant and the properties were compared with those of thermal atomic layer deposition (TH-ALD) ZnO thin films. While hexagonal wurzite phase with preferential (0 0 2) orientation was obtained for both cases, significant differences were observed in various aspects of film properties including resistivity values between these two techniques. Photoluminescence (PL) measurements have shown that high resistivity of PE-ALD ZnO thin films is due to the oxygen interstitials at low growth temperature of 200 °C, whose amount decreases with increasing growth temperature. Thin film transistors (TFT) using TH- and PE-ALD ZnO as an active layer were also fabricated and the device properties were evaluated comparatively.

MEASUREMENT OF PHONON TRANSPORT IN GaN-ON-SiC AND GaN-ON-DIAMOND HIGH ELECTRON MOBILITY TRANSISTOR (HEMT) DEVICES

DTIC Science & Technology

2017-10-16

DARPA) or the U.S. Government. Report contains color. 14. ABSTRACT The objective of this project is to experimentally study the transient non ...the Metal Thin Film in TDTR ........................................ 14 4.3 Experimental Observation of the Frequency Filtering Effect...scale of the device layers and the high density of interfaces, non -diffusive heat conduction plays a critical role in thermal transport of GaN devices

Near Field Imaging of Gallium Nitride Nanowires for Characterization of Minority Carrier Diffusion

DTIC Science & Technology

2009-12-01

diffusion length in nanowires is critical to potential applications in solar cells , spectroscopic sensing, and/or lasers and light emitting diodes (LED...technique has been successfully demonstrated with thin film solar cell materials [4, 5]. In these experiments, the diffusion length was measured using a...minority carrier diffusion length . This technique has been used in the near-field collection mode to image the diffusion of holes in n-type GaN

Semiconductor light-emitting devices having concave microstructures providing improved light extraction efficiency and method for producing same

DOEpatents

Tansu, Nelson; Gilchrist, James F; Ee, Yik-Khoon; Kumnorkaew, Pisist

2013-11-19

A conventional semiconductor LED is modified to include a microlens layer over its light-emitting surface. The LED may have an active layer including at least one quantum well layer of InGaN and GaN. The microlens layer includes a plurality of concave microstructures that cause light rays emanating from the LED to diffuse outwardly, leading to an increase in the light extraction efficiency of the LED. The concave microstructures may be arranged in a substantially uniform array, such as a close-packed hexagonal array. The microlens layer is preferably constructed of curable material, such as polydimethylsiloxane (PDMS), and is formed by soft-lithography imprinting by contacting fluid material of the microlens layer with a template bearing a monolayer of homogeneous microsphere crystals, to cause concave impressions, and then curing the material to fix the concave microstructures in the microlens layer and provide relatively uniform surface roughness.

Graded bandgap perovskite solar cells.

PubMed

Ergen, Onur; Gilbert, S Matt; Pham, Thang; Turner, Sally J; Tan, Mark Tian Zhi; Worsley, Marcus A; Zettl, Alex

2017-05-01

Organic-inorganic halide perovskite materials have emerged as attractive alternatives to conventional solar cell building blocks. Their high light absorption coefficients and long diffusion lengths suggest high power conversion efficiencies, and indeed perovskite-based single bandgap and tandem solar cell designs have yielded impressive performances. One approach to further enhance solar spectrum utilization is the graded bandgap, but this has not been previously achieved for perovskites. In this study, we demonstrate graded bandgap perovskite solar cells with steady-state conversion efficiencies averaging 18.4%, with a best of 21.7%, all without reflective coatings. An analysis of the experimental data yields high fill factors of ∼75% and high short-circuit current densities up to 42.1 mA cm -2 . The cells are based on an architecture of two perovskite layers (CH 3 NH 3 SnI 3 and CH 3 NH 3 PbI 3-x Br x ), incorporating GaN, monolayer hexagonal boron nitride, and graphene aerogel.

MOCVD growth and study of thin films of indium nitride

NASA Astrophysics Data System (ADS)

Jain, Abhishek

This thesis is focused on a study of MOCVD growth of InN with the goal of providing new information on the effects of growth conditions and buffer/substrate materials on InN film properties. Initial studies, using both (111) Si and (0001) sapphire substrates, identified an optimum growth temperature window of 540--560°C for the formation of stable InN films. When attempting to grow InN films on sapphire with thicknesses greater than approximately 150 nanometers using an AlN buffer layer, the InN films were observed to delaminate from the buffer/substrate at growth temperature. The combined effect of compressive stress due to high lattice mismatch between InN and AlN (˜14%) and tensile stress due to grain coalescence along with the relatively weak bond strength of InN compared to GaN and AlN, is believed to cause the InN film to crack along the interface and delaminate. To further investigate the effect of the buffer layer on InN growth, studies were carried out using GaN films grown on sapphire as the growth template. Recent MBE results had indicated a significant difference in the thermal stability and growth mode of In-polar and N-polar InN, with improved properties reported for N-polar material grown on N-polar GaN. MOCVD growth of N-polar GaN is very difficult; consequently, all of the results reported in the literature for InN growth on GaN were likely carried out on Ga-polar material resulting in films with a high surface roughness. By utilizing N-polar and Ga-polar GaN films, it was possible to produce N-polar and In-polar InN films by MOCVD, as determined by convergent beam electron diffraction (CBED) analysis. Furthermore, the polarity was found to dramatically alter the surface roughness and growth mode of the InN films with enhanced lateral growth and reduced surface roughness obtained for N-polar InN. A qualitative model was proposed to explain the different growth mechanisms observed for In-polar and N-polar InN. In spite of the improvements in surface morphology obtained with growth of N-polar InN, delamination at the InN/GaN interface was still observed in these films, and was also present in In-polar InN samples. Attempts were made to further reduce the lattice mismatch and improve the adhesion between InN and GaN by using a compositionally graded InGaN buffer layer. The fabrication of InGaN over its entire composition range is challenging since the optimal growth parameters window for InGaN varies with composition and film quality is strongly dependent on temperature and precursor flow rates. The structural properties of the InN films grown on the graded InGaN layers were comparable to films grown directly on GaN, however, the film adhesion was significantly improved with no evidence of interfacial cracks between the InN and GaN. These preliminary results indicate that graded InGaN layers can be used to improve the adhesion of InN on both Ga-polar and N-polar GaN, however, further work is needed to develop graded InGaN buffer layers or constant composition InGaN interlayers with improved structural properties for InN growth. (Abstract shortened by UMI.)

Evidence for Defect-Mediated Tunneling in Hexagonal Boron Nitride-Based Junctions.

PubMed

Chandni, U; Watanabe, K; Taniguchi, T; Eisenstein, J P

2015-11-11

We investigate electron tunneling through atomically thin layers of hexagonal boron nitride (hBN). Metal (Cr/Au) and semimetal (graphite) counter-electrodes are employed. While the direct tunneling resistance increases nearly exponentially with barrier thickness as expected, the thicker junctions also exhibit clear signatures of Coulomb blockade, including strong suppression of the tunnel current around zero bias and step-like features in the current at larger biases. The voltage separation of these steps suggests that single-electron charging of nanometer-scale defects in the hBN barrier layer are responsible for these signatures. We find that annealing the metal-hBN-metal junctions removes these defects and the Coulomb blockade signatures in the tunneling current.

Crystalline phase transformation of colloidal cadmium sulfide nanocrystals

NASA Astrophysics Data System (ADS)

Ghali, M.; Eissa, A. M.; Mosaad, M. M.

2017-03-01

In this paper, we give a microscopic view concerning influence of the growth conditions on the physical properties of nanocrystals (NCs) thin films made of CdS, prepared using chemical bath deposition CBD technique. We show a crystalline phase transformation of CdS NCs from hexagonal wurtzite (W) structure to cubic zincblende (ZB) when the growth conditions change, particularly the solution pH values. This effect was confirmed using X-ray diffraction (XRD), transmission electron microscopy (TEM), optical absorption and photoluminescence (PL) measurements. The optical absorption spectra allow calculation of the bandgap value, Eg, where significant increase ˜200 meV in the CdS bandgap when transforming from Hexagonal to Cubic phase was found.

Long-Range Order in Cylindrical Block Copolymer Thin Films using Graphoepitaxy

NASA Astrophysics Data System (ADS)

Dadmun, Mark; Lowndes, Doug

2005-03-01

A block copolymer thin film of polystyrene and methyl methacrylate on a neutral surface forms cylinders that are removed by UV exposure and rinsing, creating a template for the deposition of nickel nanodots, which are subsequently used to grow carbon nanofibers. Additionally, graphoepitaxy is used to successfully induce long-range order in hexagonally packed cylinders. The kinetics of the ordering process, limitations on this technique, and specific sample preparation conditions for successful transfer of the porous template to the deposited nanodots will also be discussed

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

Marimuthu, T.; Anandhan, N., E-mail: anandhan-kn@rediffmail.com; Mummoorthi, M.

Zinc oxide (ZnO) and zinc oxide/eosin yellow (ZnO/EY) thin films were potentiostatically deposited onto fluorine doped tin oxide (FTO) glass substrate. Effect of eosin yellow dye on structural, morphological and optical properties was studied. X-ray diffraction patterns, micro Raman spectra and photoluminescence (PL) spectra reveal hexagonal wurtzite structure with less atomic defects in 101 plane orientation of the ZnO/EY film. Scanning electron microscopy (SEM) images show flower for ZnO and porous like structure for ZnO/EY thin film, respectively. DSSC was constructed and evaluated by measuring the current density verses voltage curve.

Effect of composition on SILAR deposited CdxZn1-xS thin films

NASA Astrophysics Data System (ADS)

Ashith V., K.; Gowrish Rao, K.

2018-04-01

In the group of II-VI compound semiconductor, cadmium zinc sulphide (CdxZn1-xS) thin films have broad application in photovoltaic, optoelectronic devices etc. For heterojunction aspects, CdxZn1-xS thin film can be used as heterojunction partner for CdTe as the absorber layer. In this work, CdZnS thin films prepared on glass substrates by Successive Ion Layer Adsorption and Reaction (SILAR) method by varying the composition. The XRD patterns of deposited films showed polycrystalline with the hexagonal phase. The crystallite size of the films was estimated from W-H plot. The bond length of the film varied w.r.to the composition of the CdxZn1-xS films. The urbach energy of the films was calcualted from absorbance data.

A study on the enhancement of opto-electronic properties of CdS thin films: seed-assisted fabrication

NASA Astrophysics Data System (ADS)

Kumarage, W. G. C.; Wijesundera, R. P.; Seneviratne, V. A.; Jayalath, C. P.; Dassanayake, B. S.

2017-04-01

A novel method of fabricating chemical bath deposited CdS thin films (CBD-CdS) by using electrodeposited CdS (ED-CdS) as a seed layer is reported. The resulting thin, compact, uniform and adherent seed-assisted CdS films (ED/CBD-CdS) show enhanced effective surface area compared to both ED-CdS and CBD-CdS. The phase of these CdS films was determined to be hexagonal. The fabricated ED/CBD-CdS films show higher photoelectrochemical (PEC) cell efficiency than either ED-CdS and CBD-CdS thin films. Carrier concentration and flat band potential values for ED/CBD-CdS systems are also found to be superior compared to both ED-CdS and CBD-CdS systems.

Swift heavy ion induced modifications in optical and electrical properties of cadmium selenide thin films

NASA Astrophysics Data System (ADS)

Choudhary, Ritika; Chauhan, Rishi Pal

2017-07-01

The modification in various properties of thin films using high energetic ion beam is an exciting area of basic and applied research in semiconductors. In the present investigations, cadmium selenide (CdSe) thin films were deposited on ITO substrate using electrodeposition technique. To study the swift heavy ion (SHI) induced effects, the deposited thin films were irradiated with 120 MeV heavy Ag9+ ions using pelletron accelerator facility at IUAC, New Delhi, India. Structural phase transformation in CdSe thin film from metastable cubic phase to stable hexagonal phase was observed after irradiation leading to decrease in the band gap from 2.47 eV to 2.12 eV. The phase transformation was analyzed through X-ray diffraction patterns. During SHI irradiation, Generation of high temperature and pressure by thermal spike along the trajectory of incident ions in the thin films might be responsible for modification in the properties of thin films.[Figure not available: see fulltext.

Structural and morphological study on ZnO:Al thin films grown using DC magnetron sputtering

NASA Astrophysics Data System (ADS)

Astuti, B.; Sugianto; Mahmudah, S. N.; Zannah, R.; Putra, N. M. D.; Marwoto, P.; Aryanto, D.; Wibowo, E.

2018-03-01

ZnO doped Al (ZnO:Al ) thin film was deposited on corning glass substrate using DC magnetron sputtering method. Depositon process of the ZnO:Al thin films was kept constant at plasma power, deposition temperature and deposition time are 40 watt, 400°C and 2 hours, respectivelly. Furthermore, for annealing process has been done on the variation of oxygen pressure are 0, 50, and 100 mTorr. X-ray diffraction (XRD), and SEM was used to characterize ZnO:Al thin film was obtained. Based on XRD characterization results of the ZnO:Al thin film shows that deposited thin film has a hexagonal structure with the dominant diffraction peak at according to the orientation of the (002) plane and (101). Finally, the crystal structure of the ZnO:Al thin films that improves with an increasing the oxygen pressure at annealing process up to 100 mTorr and its revealed by narrow FWHM value and also with dense crystal structure.

Growth and characterization of EDTA assisted CBD-CdS

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

Kumarage, W. G. C.; Wijesundera, R. P.; Seneviratne, V. A.

2017-10-01

Chemical bath deposition of CdS (CBD-CdS) thin films with the assistance of a cationic surfactant, ethylenediamine tetraacetic acid (EDTA), is reported in this work. Also the EDTA treated CdS thin films are compared with that of conventional CBD-CdS. Fabricated thin, compact, uniform and adherent EDTA treated CdS films show enhanced effective surface area and roughness compared to conventional CBD-CdS. The grazing incidence x-ray diffraction analysis shows all the fabricated CdS films are hexagonally crystallized. EDTA-treated CdS films show excellent photo activity compared to conventional CBDCdS. The flat band potential (Vfb) value was found to be tunable with EDTA concentration.

Control of excitons in multi-layer van der Waals heterostructures

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

Calman, E. V., E-mail: ecalman@gmail.com; Dorow, C. J.; Fogler, M. M.

2016-03-07

We report an experimental study of excitons in a double quantum well van der Waals heterostructure made of atomically thin layers of MoS{sub 2} and hexagonal boron nitride. The emission of neutral and charged excitons is controlled by gate voltage, temperature, and both the helicity and the power of optical excitation.

Magnetic properties of epitaxial hexagonal HoFeO3 thin films

NASA Astrophysics Data System (ADS)

Wang, Xiao; Xiao, Zhuyun; Xu, Xiaoshan; Wang, Wenbin; Keavney, David; Liu, Yaohua; Cheng, X. M.

2014-03-01

Multiferroic materials exhibit multiple ferroic orders simultaneously and thus have great potential applications in information technology, sensing and actuation. Epitaxial hexagonal HoFeO3 (h-HFO) films are very promising candidates as multiferroic materials with room temperature ferromagnetism, because magnetic Ho3+ ions are expected to have stronger exchange interactions with Fe3+ ions than the well-studied h-LuFeO3 films. We report study of magnetic properties of epitaxial h-HFO thin films deposited using laser molecular beam epitaxy on Yttria-stabilized zirconia (YSZ) substrates. X-ray diffraction measurements confirmed the epitaxial registry and six-fold symmetry of the film. Temperature dependence of magnetization of the film measured by a Quantum Design SQUID magnetometer shows dominating paramagnetic characteristic. Element specific x-ray magnetic circular dichroism measurements performed at beamline 4-ID-C of the Advanced Photon Source show a ferromagnetic ordering of Fe and an exchange coupling between Ho3+ and Fe3+ ions. Work at BMC is supported by NSF Career award (DMR 1053854). Work at ANL is supported by US-DOE, Office of Science, BES (No. DE-AC02-06CH11357).

Investigation of ZrO x /ZrC-ZrN/Zr thin-film structural evolution and their degradation using X-ray diffraction and Raman spectrometry

NASA Astrophysics Data System (ADS)

Usmani, B.; Vijay, V.; Chhibber, R.; Dixit, A.

2016-11-01

The thin-film structures of DC/FR magnetron-sputtered ZrO x /ZrC-ZrN/Zr tandem solar-selective coatings are investigated using X-ray diffraction and room-temperature Raman spectroscopic measurements. These studies suggest that the major contribution is coming from h-ZrN0.28, c-ZrC, h-Zr3C2 crystallographic phases in ZrN-ZrC absorber layer, in conjunction with mixed ZrO x crystallographic phases. The change in structure for thermally annealed samples has been examined and observed that cubic and hexagonal ZrO x phase converted partially into tetragonal and monoclinic ZrO x phases, whereas hexagonal and cubic ZrN phases, from absorber layer, have not been observed for these thermally treated samples in air. These studies suggest that thermal treatment may lead to the loss of ZrN phase in absorber, degrading the thermal response for the desired wavelength range in open ambient conditions in contrast to vacuum conditions.

Fabrication of non-hexagonal close packed colloidal array on a substrate by transfer

NASA Astrophysics Data System (ADS)

Banik, Meneka; Mukherjee, Rabibrata

Self-organized colloidal arrays find application in fabrication of solar cells with advanced light management strategies. We report a simple spincoating based approach for fabricating two dimensional colloidal crystals with hexagonal and non-hexagonal close packed assembly on flat and nanopatterned substrates. The non-HCP arrays were fabricated by spin coating the particles onto soft lithographically fabricated substrates. The substrate patterns impose directionality to the particles by confining them within the grooves. We have developed a technique by which the HCP and non-HCP arrays can be transferred to any surface. For this purpose the colloidal arrays were fabricated on a UV degradable PMMA layer, resulting in transfer of the particles on UV exposure. This allows the colloidal structures to be transported across substrates irrespective of their surface energy, wettability or morphology. Since the particles are transferred without exposing it to any kind of chemical or thermal environment, it can be utilized for placing particles on top of thin film solar cells for improving their absorption efficiency.

Alleviation of efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes with trapezoidal quantum barriers

NASA Astrophysics Data System (ADS)

Kim, Sang-Jo; Lee, Kwang Jae; Park, Seong-Ju

2018-06-01

We numerically investigated the effects of trapezoidal quantum barriers (QBs) on efficiency droop in InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs). Simulations showed that the electrostatic field in QWs of LEDs with trapezoidal barriers is reduced because of the reduced sheet charge density at the QW-QB interface caused by the thin GaN layer in trapezoidal QBs. Additionally, the InGaN grading region in trapezoidal QBs suppresses hot carrier transport and this enhances efficient carrier injection into the QWs. The electroluminescence intensity of an LED with trapezoidal QBs is increased by 10.2% and 6.7% at 245 A cm‑2 when compared with the intensities of LEDs with square-type GaN barriers and multilayer barriers, respectively. The internal quantum efficiency (IQE) droop of an LED with trapezoidal QBs is 16% at 300 A cm‑2, while LEDs with square-type GaN barriers and multilayer barriers have IQE droop of 31% and 24%, respectively. This IQE droop alleviation in LEDs with trapezoidal QBs is attributed to the reduced energy band bending, efficient hole injection, and more uniform hole distribution in the MQWs that results from reduction of the piezoelectric field by the trapezoidal QBs. These results indicate that the trapezoidal QB in MQWs is promising for enhanced efficiency in high-power GaN-based LEDs.

Study of the characteristics current-voltage and capacitance-voltage in nitride GaAs Schottky diode

NASA Astrophysics Data System (ADS)

Rabehi, Abdelaziz; Amrani, Mohamed; Benamara, Zineb; Akkal, Boudali; Hatem-Kacha, Arslane; Robert-Goumet, Christine; Monier, Guillaume; Gruzza, Bernard

2015-10-01

This article reports the study of Au/GaN/GaAs Schottky diodes, where the thin GaN film is prepared by nitridation of GaAs substrates with thicknesses of 0.7 and 0.8 nm. The resulting GaN sample with thickness 0.8 nm is then treated with an annealing operation (heating to 620 °C) to improve the current transport. The current-voltage (I-V) and capacitance-voltage (C-V) of the Au/GaN/GaAs structures were investigated at room temperature. In fact, the I-V characteristics show that the annealed sample has low series resistance (Rs) and ideality factor (n) (63 Ω, 2.27 respectively) when compared to the values obtained in the untreated sample (1.83 kΩ, 3.31 respectively). The formation of the GaN layer on the gallium arsenide surface is investigated through calculation of the interface state density NSS with and without the presence of series resistance Rs. The value of the interface state density NSS(E) close to the mid-gap was estimated to be in the order of 4.7×1012 cm-2 eV-1 and 1.02× 1013 cm-2 eV-1 with and without the annealing operation, respectively. However, nitridation with the annealing operation at 620 °C improves the electrical properties of the resultant Schottky diode.

The investigation of Al0.29Ga0.71N/GaN/AlN and AlN/GaN/AlN thin films grown on Si (111) by RF plasma-assisted MBE

NASA Astrophysics Data System (ADS)

Yusoff, Mohd Zaki Mohd; Mahyuddin, Azzafeerah; Hassan, Zainuriah; Hassan, Haslan Abu; Abdullah, Mat Johar

2012-06-01

Recently, gallium nitride (GaN) and its related compounds involving Al and In have attracted much attention because of their potential to be used as high-efficiency UV light emitting devices, and as high frequency and high power electronic devices. Consequently, the growth and physics of GaN-based materials have attracted remarkable scientific attention. In this work, the growth and characterization of epitaxial Al0.29Ga0.71N and AlN layers grown on Si (111) by RF-plasma assisted molecular beam epitaxy (MBE) are described. The Al mole fraction was derived from the HR-XRD symmetric rocking curve (RC) ω/2θ scans of (0002) plane as x = 0.29. For AlN/GaN/AlN sample, the maximum Raman intensity at 521.53 cm-1 is attributed to crystalline silicon. It was found that the allowed Raman optical phonon mode of GaN, the E1 (high) is clearly visible, which is located at 570.74 cm-1. Photoluminscence (PL) spectrums of both samples have shown sharp and intense band edge emission of GaN without the existence of yellow emission band, showing good crystal quality of the samples have been successfully grown on Si substrate.

Algan/Gan Hemt By Magnetron Sputtering System

NASA Astrophysics Data System (ADS)

Garcia Perez, Roman

In this thesis, the growth of the semiconductor materials AlGaN and GaN is achieved by magnetron sputtering for the fabrication of High Electron Mobility Transistors (HEMTs). The study of the deposited nitrides is conducted by spectroscopy, diffraction, and submicron scale microscope methods. The preparation of the materials is performed using different parameters in terms of power, pressure, temperature, gas, and time. Silicon (Si) and Sapphire (Al2O3) wafers are used as substrates. The chemical composition and surface topography of the samples are analyzed to calculate the materials atomic percentages and to observe the devices surface. The instruments used for the semiconductors characterization are X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Atomic Force Microscope (AFM). The project focused its attention on the reduction of impurities during the deposition, the controlled thicknesses of the thin-films, the atomic configuration of the alloy AlxGa1-xN, and the uniformity of the surfaces.

Effects of Si-doping on magnetic properties of Ga1-xCrxN

NASA Astrophysics Data System (ADS)

Zhou, Zhongpo; Yang, Zongxian; Liu, Chang

2015-01-01

Ga1-xCrxN thin films with and without the Si doping have been prepared by molecular beam epitaxy. The samples have been investigated by X-ray diffraction, X-ray photoemission spectroscopy, photoluminescence, optical absorption spectra and magnetic measurements. It has been confirmed that for the undoped samples Cr in GaN is predominantly trivalent when substituting for Ga and that the Cr 3d state appears within the band gap of GaN. In Si doped specimens the upward shifts of the chemical potential are observed, and the electrons supplied by the Si doping are trapped at Cr sites forming Cr2+. As a result, the Si doping effects show an increase of the Curie temperature, and a reduction of the saturation magnetization in the Ga1-xCrxN:Si samples. The significant effect on the ferromagnetism with Si doping in Ga1-xCrxN is explained by the percolation theory of bound magnetic polarons.

Fracture and buckling of piezoelectric nanowires subject to an electric field

NASA Astrophysics Data System (ADS)

Zhang, Jin; Wang, Chengyuan; Adhikari, Sondipon

2013-11-01

Fracture and buckling are major failure modes of thin and long nanowires (NWs), which could be affected significantly by an electric field when piezoelectricity is involved in the NWs. This paper aims to examine the issue based on the molecular dynamics simulations, where the gallium nitride (GaN) NWs are taken as an example. The results show that the influence of the electric field is strong for the fracture and the critical buckling strains, detectable for the fracture strength but almost negligible for the critical buckling stress. In addition, the reversed effects are achieved for the fracture and the critical buckling strains. Subsequently, the Timoshenko beam model is utilized to account for the effect of the electric field on the axial buckling of the GaN NWs, where nonlocal effect is observed and characterized by the nonlocal coefficient e0a=1.1 nm. The results show that the fracture and buckling of piezoelectric NWs can be controlled by applying an electric field.

FOREWORD: The physics of III-V nitrides The physics of III-V nitrides

NASA Astrophysics Data System (ADS)

Ridley, B. K.

2009-04-01

The evolution of semiconductor physics is driven by the increasing sophistication of the art of crystal growing and fabrication techniques. From Ge at the birth of the transistor, possibly the purest material ever grown, through Si, the work-horse of the crystal revolution, to the III-Vs, whose optical properties opened up a second front, namely, optoelectronics. Crystal growth with monolayer control gave us quantum wells, superlattices, quantum wires and quantum dots, along with the quantum Hall effect and quantized resistance. The potential for high-power devices triggered interest in the III-V nitrides with their large bandgaps. The nitrides mostly crystallize in the hexagonal form, and this has introduced the phenomenon of spontaneous polarization into mainstream semiconductor physics. Its effect manifests itself in huge electric fields in heterostructures like AlGaN/GaN which, in turn, causes the induction of substantial electron populations in the channel of a HFET without the need for doping. High-power microwave transistors have been successfully fabricated, even though there are features associated with spontaneous polarization that still needs clarifying. Another strange effect is the large electron population on the surface of InN. The lack of a suitable substrate for growing GaN has meant that the dislocation density is higher than we would wish, but that situation is expected to steadily improve. Given the current interest in the physics of nitrides, it is natural to come across a special issue devoted to this topic. The difficulty presented by the surface layer in InN in the attempt to measure transport properties is discussed in the paper by King et al. A property that can affect transport is the lifetime of optical phonons and its dependence on electron density. Measurements of phonon lifetime in InN are reported by Tsen and Ferry, and in GaN channels, via the measure of hot-electron fluctuations, by Matulionis. The dependence on electron density is thought to be associated with the coupling of plasma and phonon modes, and this is discussed by Dyson. The intrinsic cause of phonon decay is the anharmonic interaction involving three phonons, and this process is described for zinc blende BN and hexagonal AlN, GaN and InN by Srivastava. The principal electron scattering mechanism at room temperature is associated with the interaction with polar optical modes. At high fields, transfer to the upper conduction-band valleys can take place and this involves the deformation-potential interaction. Deformation potentials have been derived by Yamakawa et al for GaN, and they have been incorporated into a cellular Monte Carlo simulation to describe high-field transport. In high-power devices, thermal as well as electronic transport is important. The thermal conductivity of the substrate of devices is a vital factor, and the possible use of AlN ceramics is discussed by AlShaikhi and Srivastarva. A striking device based on a zinc blende superlattice is the quantum cascade laser. Exploiting intersubband transitions in the AlN/GaN superlattice for the high-speed detection of infrared light is described by Hofstetter et al, clearly a first step towards a nitride based quantum cascade laser. In bulk material the displacement of As by N that transforms GaAs to GaN produces a huge change in properties. Adding a small amount of N to GaAs might be expected to produce a gradual more-or-less linear shift towards the properties of GaN, but this turns out to be far from the case. The strange properties of dilute nitrides have intrigued many workers in recent years. Its curious bandstructure suggested that hot-electron transport could exhibit a negative differential resistance, and a report on this topic can be found in the article by Patane et al A comprehensive study of transport of electrons and holes in dilute nitride/GaAs quantum wells is reported by Sun et al. An unusual new device—a spin filter—is presaged by the work of Zhao et al on spin-dependent recombination, controllable by adjusting the N content. Answers to a number of problems presented by the physics of III-V nitrides are to be found in the articles composing this edition, but there are still many that need clarifying. That clarification will have to await the future work that will form the contents of a future special edition.

Investigation of layered structure SAW devices fabricated using low temperature grown AlN thin film on GaN/sapphire.

PubMed

Lin, Hui-Feng; Wu, Chun-Te; Chien, Wei-Cheng; Chen, Sheng-Wen; Kao, Hui-Ling; Chyi, Jen-Inn; Chen, Jyh-Shin

2005-05-01

Epitaxial AlN films have been grown on GaN/sapphire using helicon sputtering at 300 degrees C. The surface acoustic wave (SAW) filters fabricated on AlN/GaN/sapphire exhibit more superior characteristics than those made on GaN/sapphire. This composite structure of AlN on GaN may bring about the development of high-frequency components, which integrate and use their semiconducting, optoelectronic, and piezoelectric properties.

Process for growing epitaxial gallium nitride and composite wafers

DOEpatents

Weber, Eicke R.; Subramanya, Sudhir G.; Kim, Yihwan; Kruger, Joachim

2003-05-13

A novel growth procedure to grow epitaxial Group III metal nitride thin films on lattice-mismatched substrates is proposed. Demonstrated are the quality improvement of epitaxial GaN layers using a pure metallic Ga buffer layer on c-plane sapphire substrate. X-ray rocking curve results indicate that the layers had excellent structural properties. The electron Hall mobility increases to an outstandingly high value of .mu.>400 cm.sup.2 /Vs for an electron background concentration of 4.times.10.sup.17 cm.sup.-3.

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

NASA Astrophysics Data System (ADS)

Kishino, Katsumi; Ishizawa, Shunsuke

2015-06-01

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

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

Hannachi, Amira, E-mail: amira.hannachi88@gmail.com; Maghraoui-Meherzi, Hager

Manganese sulfide thin films have been deposited on glass slides by chemical bath deposition (CBD) method. The effects of preparative parameters such as deposition time, bath temperature, concentration of precursors, multi-layer deposition, different source of manganese, different complexing agent and thermal annealing on structural and morphological film properties have been investigated. The prepared thin films have been characterized using the X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). It exhibit the metastable forms of MnS, the hexagonal γ-MnS wurtzite phase with preferential orientation in the (002) plane or the cubic β-MnS zinc blende with preferentialmore » orientation in the (200) plane. Microstructural studies revealed the formation of MnS crystals with different morphologies, such as hexagons, spheres, cubes or flowers like. - Graphical Abstract: We report the preparation of different phases of manganese sulfide thin films (γ, β and α-MnS) by chemical bath deposition method. The effects of deposition parameters such as deposition time and temperature, concentrations of precursors and multi-layer deposition on MnS thin films structure and morphology were investigated. The influence of thermal annealing under nitrogen atmosphere at different temperature on MnS properties was also studied. Different manganese precursors as well as different complexing agent were also used. - Highlights: • γ and β-MnS films were deposited on substrate using the chemical bath deposition. • The effect of deposition parameters on MnS film properties has been investigated. • Multi-layer deposition was also studied to increase film thickness. • The effect of annealing under N{sub 2} at different temperature was investigated.« less

Multiferroic RMnO3 thin films

NASA Astrophysics Data System (ADS)

Fontcuberta, Josep

2015-03-01

Multiferroic materials have received an astonishing attention in the last decades due to expectations that potential coupling between distinct ferroic orders could inspire new applications and new device concepts. As a result, a new knowledge on coupling mechanisms and materials science has dramatically emerged. Multiferroic RMnO3 perovskites are central to this progress, providing a suitable platform to tailor spin-spin and spin-lattice interactions. With views towards applications, the development of thin films of multiferroic materials have also progressed enormously and nowadays thin-film manganites are available, with properties mimicking those of bulk compounds. Here we review achievements on the growth of hexagonal and orthorhombic RMnO3 epitaxial thin films and the characterization of their magnetic and ferroelectric properties, we discuss some challenging issues, and we suggest some guidelines for future research and developments. En ce qui concerne les applications, le développement de films minces de matériaux multiferroïques a aussi énormément progressé, et de nos jours des films minces de manganites avec des propriétés similaires à celles des matériaux massifs existent. Nous passons en revue ici les résultats obtenus dans le domaine de la croissance de couches minces épitaxiés de RMnO3 hexagonal et orthorhombique et de la caractérisation de leurs propriétés magnétiques et ferroélectriques. Nous discutons certains enjeux et proposons quelques idées pour des recherches et développements futurs.

X-ray diffraction analysis of residual stresses in textured ZnO thin films

NASA Astrophysics Data System (ADS)

Dobročka, E.; Novák, P.; Búc, D.; Harmatha, L.; Murín, J.

2017-02-01

Residual stresses are commonly generated in thin films during the deposition process and can influence the film properties. Among a number of techniques developed for stress analysis, X-ray diffraction methods, especially the grazing incidence set-up, are of special importance due to their capability to analyze the stresses in very thin layers as well as to investigate the depth variation of the stresses. In this contribution a method combining multiple {hkl} and multiple χ modes of X-ray diffraction stress analysis in grazing incidence set-up is used for the measurement of residual stress in strongly textured ZnO thin films. The method improves the precision of the stress evaluation in textured samples. Because the measurements are performed at very low incidence angles, the effect of refraction of X-rays on the measured stress is analyzed in details for the general case of non-coplanar geometry. It is shown that this effect cannot be neglected if the angle of incidence approaches the critical angle. The X-ray stress factors are calculated for hexagonal fiber-textured ZnO for the Reuss model of grain-interaction and the effect of texture on the stress factors is analyzed. The texture in the layer is modelled by Gaussian distribution function. Numerical results indicate that in the process of stress evaluation the Reuss model can be replaced by much simpler crystallite group method if the standard deviation of Gaussian describing the texture is less than 6°. The results can be adapted for fiber-textured films of various hexagonal materials.

Lithium ion intercalation in thin crystals of hexagonal TaSe2 gated by a polymer electrolyte

NASA Astrophysics Data System (ADS)

Wu, Yueshen; Lian, Hailong; He, Jiaming; Liu, Jinyu; Wang, Shun; Xing, Hui; Mao, Zhiqiang; Liu, Ying

2018-01-01

Ionic liquid gating has been used to modify the properties of layered transition metal dichalcogenides (TMDCs), including two-dimensional (2D) crystals of TMDCs used extensively recently in the device work, which has led to observations of properties not seen in the bulk. The main effect comes from the electrostatic gating due to the strong electric field at the interface. In addition, ionic liquid gating also leads to ion intercalation when the ion size of the gate electrolyte is small compared to the interlayer spacing of TMDCs. However, the microscopic processes of ion intercalation have rarely been explored in layered TMDCs. Here, we employed a technique combining photolithography device fabrication and electrical transport measurements on the thin crystals of hexagonal TaSe2 using multiple channel devices gated by a polymer electrolyte LiClO4/Polyethylene oxide (PEO). The gate voltage and time dependent source-drain resistances of these thin crystals were used to obtain information on the intercalation process, the effect of ion intercalation, and the correlation between the ion occupation of allowed interstitial sites and the device characteristics. We found a gate voltage controlled modulation of the charge density waves and a scattering rate of charge carriers. Our work suggests that ion intercalation can be a useful tool for layered materials engineering and 2D crystal device design.

Direct growth of nanocrystalline hexagonal boron nitride films on dielectric substrates

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

Tay, Roland Yingjie; Temasek Laboratories@NTU, 50 Nanyang Avenue, Singapore, Singapore 639798; Tsang, Siu Hon

Atomically thin hexagonal-boron nitride (h-BN) films are primarily synthesized through chemical vapor deposition (CVD) on various catalytic transition metal substrates. In this work, a single-step metal-catalyst-free approach to obtain few- to multi-layer nanocrystalline h-BN (NCBN) directly on amorphous SiO{sub 2}/Si and quartz substrates is demonstrated. The as-grown thin films are continuous and smooth with no observable pinholes or wrinkles across the entire deposited substrate as inspected using optical and atomic force microscopy. The starting layers of NCBN orient itself parallel to the substrate, initiating the growth of the textured thin film. Formation of NCBN is due to the random andmore » uncontrolled nucleation of h-BN on the dielectric substrate surface with no epitaxial relation, unlike on metal surfaces. The crystallite size is ∼25 nm as determined by Raman spectroscopy. Transmission electron microscopy shows that the NCBN formed sheets of multi-stacked layers with controllable thickness from ∼2 to 25 nm. The absence of transfer process in this technique avoids any additional degradation, such as wrinkles, tears or folding and residues on the film which are detrimental to device performance. This work provides a wider perspective of CVD-grown h-BN and presents a viable route towards large-scale manufacturing of h-BN substrates and for coating applications.« less

Synthesis and energy applications of oriented metal oxide nanoporous films

NASA Astrophysics Data System (ADS)

Wu, Qingliu

This dissertation mainly addresses the synthesis of well-ordered mesoporous titania thin films by dip coating with PEO-PPO-PEO triblock copolymer surfactant template P123. Because P123 is composed of poly(ethylene oxide) [PEO] and poly(propylene oxide) [PPO] blocks, concentrations of ingredients are adjusted to tune the films' wall thickness, pore size and mesophase. Structural changes are consistent with partitioning of species among PEO blocks, PPO blocks, and the PEO/PPO interface. Titanates localize near PEO and increase wall thickness (by 5 nm to 7 nm). Depending on aging temperature, PPG either swells the PPO cores (when it is hydrophobic) or introduces large (>200 nm) voids (when it is hydrophilic but phase separates during heating). 1-butanol localizes at the PEO/PPO interface to favor a 3D hexagonal mesostructure. In another approach, anodizing Ti foils yields vertically aligned titania nanotubes arrays with exceptional stabilities as anodes in lithium ion batteries; they maintain capacities of 130-230 mAhg-1 over 200 cycles. No microstructural changes are induced by battery cycling and good electrical contact is maintained. A diffusion induced stress model suggests that thin-walled nanotubes arrays should be stable under testing conditions, and that ordered hexagonal columnar pore arrays should have both high charge/discharge rates and low stress development. KEY WORDS: materials synthesis, porous, thin film, alternative energy, self-assembly

Method of making a silicon nanowire device

DOEpatents

None, None

2017-05-23

There is provided an electronic device and a method for its manufacture. The device comprises an elongate silicon nanowire less than 0.5 .mu.m in cross-sectional dimensions and having a hexagonal cross-sectional shape due to annealing-induced energy relaxation. The method, in examples, includes thinning the nanowire through iterative oxidation and etching of the oxidized portion.

Optical Sensing And Imaging Opportunities

DTIC Science & Technology

2016-02-12

Functional Materials Workshops, supported by AFOSR.Potentially Useful New Research Areas.- Plasmonics - Infrared antennae- IV-VI (lead salt) Infrared Photo...Potentially Useful New Research Areas. - Plasmonics - Infrared antennae - IV-VI (lead salt) Infrared Photo Detectors and Focal Plane Arrays...Hexagonal Ferrite Thin Films for Q-Band Signal Processing Devices Plasmonics New techniques for transmitting optical signals through nano-scale

Structural, electrical, optical and magnetic properties of NiO/ZnO thin films

NASA Astrophysics Data System (ADS)

Sushmitha, V.; Maragatham, V.; Raj, P. Deepak; Sridharan, M.

2018-02-01

Nickel oxide/Zinc oxide (NiO/ZnO) thin films have been deposited onto thoroughly cleaned glass substrates by reactive direct current (DC) magnetron sputtering technique and subsequently annealed at 300 °C for 3 h in vacuum. The NiO/ZnO thin films were then studied for their structural, optical and electrical properties. X-ray diffraction (XRD) pattern of ZnO and NiO showed the diffraction planes corresponding to hexagonal and cubic phase respectively. The optical properties showed that with the increase in the deposition time of NiO the energy band gap varied between 3.1 to 3.24 eV. Hence, by changing the deposition time of NiO the tuning of band gap and conductivity were achieved. The magnetic studies revealed the diamagnetic nature of the NiO/ZnO thin films.

Nucleation Control for Large, Single Crystalline Domains of Monolayer Hexagonal Boron Nitride via Si-Doped Fe Catalysts

PubMed Central

2015-01-01

The scalable chemical vapor deposition of monolayer hexagonal boron nitride (h-BN) single crystals, with lateral dimensions of ∼0.3 mm, and of continuous h-BN monolayer films with large domain sizes (>25 μm) is demonstrated via an admixture of Si to Fe catalyst films. A simple thin-film Fe/SiO2/Si catalyst system is used to show that controlled Si diffusion into the Fe catalyst allows exclusive nucleation of monolayer h-BN with very low nucleation densities upon exposure to undiluted borazine. Our systematic in situ and ex situ characterization of this catalyst system establishes a basis for further rational catalyst design for compound 2D materials. PMID:25664483

ZnO-based transparent conductive thin films via sonicated-assisted sol-gel technique

NASA Astrophysics Data System (ADS)

Malek, M. F.; Mamat, M. H.; Ismail, A. S.; Yusoff, M. M.; Mohamed, R.; Rusop, M.

2018-05-01

We report on the growth of Al-doped ZnO (AZO) thin films onto Corning 7740 glass substrates via sonicated-assisted sol-gel technique. The influence of Al dopant on crystallisation behavior, optical and electrical properties of AZO films has been systematically investigated. All films are polycrystalline with a hexagonal wurtzite structure with a preferential orientation according to the direction <002>. All films exhibit a transmittance above than 80-90 % along the visible range up to 800 nm and a sharp absorption onset below 400 nm corresponding to the fundamental absorption edge of ZnO.

Fabrication of thin bulk ceramics for microwave circulator applications

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

Ings, J.B.; Simmins, J.J.; May, J.L.

1995-09-01

Planer MMIC circulator applications require the production of thin, flat garnet, spinel, and hexagonal ferrite circulator elements. Fabrication of cira 250 {mu}m circulator elements was done by tape casting and roll compaction. For the garnet, tape cast gave equivalent results to roll compaction. For the spinel and hexaferrite materials, which undergo magnetic flocculation, roll compaction was found to be the preferred fabrication method. Roll compacted lithium ferrite resulted in higher densities and lower {triangle}H and tan{delta} than did the tape case material. Roll compacted barium hexaferrite resulted in higher densities and remanent magnetization than did the tape cast material.

Preparation of freestanding GaN wafer by hydride vapor phase epitaxy on porous silicon

NASA Astrophysics Data System (ADS)

Wu, Xian; Li, Peng; Liang, Renrong; Xiao, Lei; Xu, Jun; Wang, Jing

2018-05-01

A freestanding GaN wafer was prepared on porous Si (111) substrate using hydride vapor phase epitaxy (HVPE). To avoid undesirable effects of the porous surface on the crystallinity of the GaN, a GaN seed layer was first grown on the Si (111) bare wafer. A pattern with many apertures was fabricated in the GaN seed layer using lithography and etching processes. A porous layer was formed in the Si substrate immediately adjacent to the GaN seed layer by an anodic etching process. A 500-μm-thick GaN film was then grown on the patterned GaN seed layer using HVPE. The GaN film was separated from the Si substrate through the formation of cracks in the porous layer caused by thermal mismatch stress during the cooling stage of the HVPE. Finally, the GaN film was polished to obtain a freestanding GaN wafer.

Role of the ganSPQAB Operon in Degradation of Galactan by Bacillus subtilis.

PubMed

Watzlawick, Hildegard; Morabbi Heravi, Kambiz; Altenbuchner, Josef

2016-10-15

Bacillus subtilis possesses different enzymes for the utilization of plant cell wall polysaccharides. This includes a gene cluster containing galactan degradation genes (ganA and ganB), two transporter component genes (ganQ and ganP), and the sugar-binding lipoprotein-encoding gene ganS (previously known as cycB). These genes form an operon that is regulated by GanR. The degradation of galactan by B. subtilis begins with the activity of extracellular GanB. GanB is an endo-β-1,4-galactanase and is a member of glycoside hydrolase (GH) family 53. This enzyme was active on high-molecular-weight arabinose-free galactan and mainly produced galactotetraose as well as galactotriose and galactobiose. These galacto-oligosaccharides may enter the cell via the GanQP transmembrane proteins of the galactan ABC transporter. The specificity of the galactan ABC transporter depends on the sugar-binding lipoprotein, GanS. Purified GanS was shown to bind galactotetraose and galactotriose using thermal shift assay. The energy for this transport is provided by MsmX, an ATP-binding protein. The transported galacto-oligosaccharides are further degraded by GanA. GanA is a β-galactosidase that belongs to GH family 42. The GanA enzyme was able to hydrolyze short-chain β-1,4-galacto-oligosaccharides as well as synthetic β-galactopyranosides into galactose. Thermal shift assay as well as electrophoretic mobility shift assay demonstrated that galactobiose is the inducer of the galactan operon regulated by GanR. DNase I footprinting revealed that the GanR protein binds to an operator overlapping the -35 box of the σ(A)-type promoter of Pgan, which is located upstream of ganS IMPORTANCE: Bacillus subtilis is a Gram-positive soil bacterium that utilizes different types of carbohydrates, such as pectin, as carbon sources. So far, most of the pectin degradation systems and enzymes have been thoroughly studied in B. subtilis Nevertheless, the B. subtilis utilization system of galactan, which is found as the side chain of the rhamnogalacturonan type I complex in pectin, has remained partially studied. Here, we investigated the galactan utilization system consisting of the ganSPQAB operon and its regulator ganR This study improves our knowledge of the carbohydrate degradation systems of B. subtilis, especially the pectin degradation systems. Moreover, the galactan-degrading enzymes may be exploited for the production of galacto-oligosaccharides, which are used as prebiotic substances in the food industry. Copyright © 2016, American Society for Microbiology. All Rights Reserved.

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

NASA Astrophysics Data System (ADS)

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

2016-12-01

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

Autonomic nervous system involvement in the giant axonal neuropathy (GAN) KO mouse: implications for human disease.

PubMed

Armao, Diane; Bailey, Rachel M; Bouldin, Thomas W; Kim, Yongbaek; Gray, Steven J

2016-08-01

Giant axonal neuropathy (GAN) is an inherited severe sensorimotor neuropathy. The aim of this research was to investigate the neuropathologic features and clinical autonomic nervous system (ANS) phenotype in two GAN knockout (KO) mouse models. Little is known about ANS involvement in GAN in humans, but autonomic signs and symptoms are commonly reported in early childhood. Routine histology and immunohistochemistry was performed on GAN KO mouse specimens taken at various ages. Enteric dysfunction was assessed by quantifying the frequency, weight, and water content of defecation in GAN KO mice. Histological examination of the enteric, parasympathetic and sympathetic ANS of GAN KO mice revealed pronounced and widespread neuronal perikaryal intermediate filament inclusions. These neuronal inclusions served as an easily identifiable, early marker of GAN in young GAN KO mice. Functional studies identified an age-dependent alteration in fecal weight and defecation frequency in GAN KO mice. For the first time in the GAN KO mouse model, we described the early, pronounced and widespread neuropathologic features involving the ANS. In addition, we provided evidence for a clinical autonomic phenotype in GAN KO mice, reflected in abnormal gastrointestinal function. These findings in GAN KO mice suggest that consideration should be given to ANS involvement in human GAN, especially when considering treatments and patient care.

Structure evolution of zinc oxide thin films deposited by unbalance DC magnetron sputtering

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

Aryanto, Didik, E-mail: didi027@lipi.go.id; Materials Research Group, Physics Department, Universitas Negeri Semarang, Gunungpati, Semarang 50229 Jawa Tengah; Marwoto, Putut

Zinc oxide (ZnO) thin films are deposited on corning glass substrates using unbalanced DC magnetron sputtering. The effect of growth temperature on surface morphology and crystallographic orientation of ZnO thin film is studied using atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. The surface morphology and crystallographic orientation of ZnO thin film are transformed against the increasing of growth temperature. The mean grain size of film and the surface roughness are inversely and directly proportional towards the growth temperature from room temperature to 300 °C, respectively. The smaller grain size and finer roughness of ZnO thin film are obtainedmore » at growth temperature of 400 °C. The result of AFM analysis is in good agreement with the result of XRD analysis. ZnO thin films deposited in a series of growth temperatures have hexagonal wurtzite polycrystalline structures and they exhibit transformations in the crystallographic orientation. The results in this study reveal that the growth temperature strongly influences the surface morphology and crystallographic orientation of ZnO thin film.« less

High Piezoelectric Conversion Properties of Axial InGaN/GaN Nanowires.

PubMed

Jegenyes, Nikoletta; Morassi, Martina; Chrétien, Pascal; Travers, Laurent; Lu, Lu; Julien, Francois H; Tchernycheva, Maria; Houzé, Frédéric; Gogneau, Noelle

2018-05-25

We demonstrate for the first time the efficient mechanical-electrical conversion properties of InGaN/GaN nanowires (NWs). Using an atomic force microscope equipped with a modified Resiscope module, we analyse the piezoelectric energy generation of GaN NWs and demonstrate an important enhancement when integrating in their volume a thick In-rich InGaN insertion. The piezoelectric response of InGaN/GaN NWs can be tuned as a function of the InGaN insertion thickness and position in the NW volume. The energy harvesting is favoured by the presence of a PtSi/GaN Schottky diode which allows to efficiently collect the piezo-charges generated by InGaN/GaN NWs. Average output voltages up to 330 ± 70 mV and a maximum value of 470 mV per NW has been measured for nanostructures integrating 70 nm-thick InGaN insertion capped with a thin GaN top layer. This latter value establishes an increase of about 35% of the piezo-conversion capacity in comparison with binary p-doped GaN NWs. Based on the measured output signals, we estimate that one layer of dense InGaN/GaN-based NW can generate a maximum output power density of about 3.3 W/cm². These results settle the new state-of-the-art for piezo-generation from GaN-based NWs and offer a promising perspective for extending the performances of the piezoelectric sources.

The Effect of Sunlight in Parenchyma Pith Cells Diameter of Manihot esculenta

NASA Astrophysics Data System (ADS)

Susanti, D.; Aziz, D. N.; Astuti, W.; Nuraeni, E.

2017-03-01

Sunlight is one of the factors that effect on the grow of a plant. Manihot esculenta is one of the plants that easily found in Indonesia because its role as staple food. The aim of this research is to know the correlation between sunlight the grow of parenchyma pith cells diameter of Manihot esculenta. Independent variable in this research is sunlight, and dependent variable is the parenchyma pith cells diameter of Manihot esculenta. Data was collected is in qualitative and quantitative form. Qualitative data gotten gained by morphology observation. The parenchyma pith cells of Manihot esculenta that is affected by sunlight in 1310 x 10 Lux, morphologically has hexagon, cell walls thick, solid state, and regular composition. Meanwhile, the parenchyma pith cells that has less sunlight (363 x 10 Lux) has a hexagon shape, thin cell walls thin, soft state, and irregular composition. Qualitative data suported by quantitative data. The size of parenchyma pith cells diameter that is affected by sunlight in 1310 x 10 Lux 96,4 µm. While, the stem parenchyma pith cells diameter empulur that has less sunlight (363 x 10 Lux) is 129,8 µm.

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.

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

PubMed

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

2017-04-20

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

Synthesis of ZnO thin film by sol-gel spin coating technique for H2S gas sensing application

NASA Astrophysics Data System (ADS)

Nimbalkar, Amol R.; Patil, Maruti G.

2017-12-01

In this present work, zinc oxide (ZnO) thin film synthesized by a simple sol-gel spin coating technique. The structural, morphology, compositional, microstructural, optical, electrical and gas sensing properties of the film were studied by using XRD, FESEM, EDS, XPS, HRTEM, Raman, FTIR and UV-vis techniques. The ZnO thin film shows hexagonal wurtzite structure with a porous structured morphology. Gas sensing performance of synthesized ZnO thin film was tested initially for H2S gas at different operating temperatures as well as concentrations. The maximum gas response is achieved towards H2S gas at 300 °C operating temperature, at 100 ppm gas concentration as compared to other gases like CH3OH, Cl2, NH3, LPG, CH3COCH3, and C2H5OH with a good stability.

Size-tunable and monodisperse Tm³⁺/Gd³⁺-doped hexagonal NaYbF₄ nanoparticles with engineered efficient near infrared-to-near infrared upconversion for in vivo imaging.

PubMed

Damasco, Jossana A; Chen, Guanying; Shao, Wei; Ågren, Hans; Huang, Haoyuan; Song, Wentao; Lovell, Jonathan F; Prasad, Paras N

2014-08-27

Hexagonal NaYbF4:Tm(3+) upconversion nanoparticles hold promise for use in high contrast near-infrared-to-near-infrared (NIR-to-NIR) in vitro and in vivo bioimaging. However, significant hurdles remain in their preparation and control of their morphology and size, as well as in enhancement of their upconversion efficiency. Here, we describe a systematic approach to produce highly controlled hexagonal NaYbF4:Tm(3+) nanoparticles with superior upconversion. We found that doping appropriate concentrations of trivalent gadolinium (Gd(3+)) can convert NaYbF4:Tm(3+) 0.5% nanoparticles with cubic phase and irregular shape into highly monodisperse NaYbF4:Tm(3+) 0.5% nanoplates or nanospheres in a pure hexagonal-phase and of tunable size. The intensity and the lifetime of the upconverted NIR luminescence at 800 nm exhibit a direct dependence on the size distribution of the resulting nanoparticles, being ascribed to the varied surface-to-volume ratios determined by the different nanoparticle size. Epitaxial growth of a thin NaYF4 shell layer of ∼2 nm on the ∼22 nm core of hexagonal NaYbF4:Gd(3+) 30%/Tm(3+) 0.5% nanoparticles resulted in a dramatic 350 fold NIR upconversion efficiency enhancement, because of effective suppression of surface-related quenching mechanisms. In vivo NIR-to-NIR upconversion imaging was demonstrated using a dispersion of phospholipid-polyethylene glycol (DSPE-PEG)-coated core/shell nanoparticles in phosphate buffered saline.

Size-Tunable and Monodisperse Tm3+/Gd3+-Doped Hexagonal NaYbF4 Nanoparticles with Engineered Efficient Near Infrared-to-Near Infrared Upconversion for In Vivo Imaging

PubMed Central

2015-01-01

Hexagonal NaYbF4:Tm3+ upconversion nanoparticles hold promise for use in high contrast near-infrared-to-near-infrared (NIR-to-NIR) in vitro and in vivo bioimaging. However, significant hurdles remain in their preparation and control of their morphology and size, as well as in enhancement of their upconversion efficiency. Here, we describe a systematic approach to produce highly controlled hexagonal NaYbF4:Tm3+ nanoparticles with superior upconversion. We found that doping appropriate concentrations of trivalent gadolinium (Gd3+) can convert NaYbF4:Tm3+ 0.5% nanoparticles with cubic phase and irregular shape into highly monodisperse NaYbF4:Tm3+ 0.5% nanoplates or nanospheres in a pure hexagonal-phase and of tunable size. The intensity and the lifetime of the upconverted NIR luminescence at 800 nm exhibit a direct dependence on the size distribution of the resulting nanoparticles, being ascribed to the varied surface-to-volume ratios determined by the different nanoparticle size. Epitaxial growth of a thin NaYF4 shell layer of ∼2 nm on the ∼22 nm core of hexagonal NaYbF4:Gd3+ 30%/Tm3+ 0.5% nanoparticles resulted in a dramatic 350 fold NIR upconversion efficiency enhancement, because of effective suppression of surface-related quenching mechanisms. In vivo NIR-to-NIR upconversion imaging was demonstrated using a dispersion of phospholipid-polyethylene glycol (DSPE-PEG)-coated core/shell nanoparticles in phosphate buffered saline. PMID:25027118

International Symposium on Epidemic Hemorrhagic Fever (Hemorrhagic Fever with Renal Syndrome) Held in Wuhan, Hubei, China on 31 October - 2 November 1988

DTIC Science & Technology

1989-10-01

15 Song Gan Prophylaxis of Hemorrhagic Fever With Renal Syndrome, Development of Inactivated Cell Culture Vaccine Against HFRS 11:45 END EPIDEMIOLOGYt...Intraveinous Ribavirin or Placebo 11:15 Yang Zhi-cheng Effect of Ribavirin on the White Blood Cell System and the Platelet of the Patients With EHF...from R. nortegicus ias madc by Hlung T. et al (5. A). It %sas a ers brilliant piecc of %;ork. because by observing thin-sections of infected cells in

Deposition of undoped and Al doped ZnO thin films using RF magnetron sputtering and study of their structural, optical and electrical properties

NASA Astrophysics Data System (ADS)

Parvathy Venu, M.; Shrisha B., V.; Balakrishna, K. M.; Naik, K. Gopalakrishna

2017-05-01

Undoped ZnO and Al doped ZnO thin films were deposited on glass and p-Si(100) substrates by RF magnetron sputtering technique at room temperature using homemade targets. ZnO target containing 5 at% of Al2O3 as doping source was used for the growth of Al doped ZnO thin films. XRD revealed that the films have hexagonal wurtzite structure with high crystallinity. Morphology and chemical composition of the films have been indicated by FESEM and EDAX studies. A blue shift of the band gap energy and higher optical transmittance has been observed in the case of Al doped ZnO (ZnO:Al) thin films with respect to the ZnO thin films. The as deposited films on p-Si were used to fabricate n-ZnO/p-Si(100) and n-ZnO:Al/p-Si(100) heterojunction diodes and their room temperature current-voltage characteristics were studied.

Enhanced photo response of mesoporous nanostructured CdS thin film via electrospray aerosol deposition technique

NASA Astrophysics Data System (ADS)

Logu, T.; Soundarrajan, P.; Sankarasubramanian, K.; Sethuraman, K.

2018-04-01

In this work, a high crystalline and mesoporous nanostructured cadmium sulfide (CdS) thin film was successfully grown on the FTO substrates using facile Electrospray Aerosol Deposition (ESAD) technique. The structural, optical, morphological and electrical properties of CdS thin film have been systematically examined. CdS thin film exhibits the hexagonal wurtzite crystal structure with polycrystalline nature. The optical band gap energy of the prepared film was estimated from the Tauc plot and is 2.43 eV. The SEM and AFM images show that the well-interconnected CdS nanoparticles gives mesoporous like morphology. The fine aerosol generated from the ESAD process induces the alteration in the surface morphological structure of deposited CdS film that consequences in enhanced electrical and photo-physical properties. The photoconductivity of the sample has been studied which demonstrates significant photo current. The present study predicts that mesoporous nanostructured CdS thin film would be given a special interest for optoelectronic applications.

Electrical and optical properties of p-type codoped ZnO thin films prepared by spin coating technique

NASA Astrophysics Data System (ADS)

Pathak, Trilok Kumar; Kumar, Vinod; Swart, H. C.; Purohit, L. P.

2016-03-01

Undoped, doped and codoped ZnO thin films were synthesized on glass substrates using a spin coating technique. Zinc acetate dihydrate, ammonium acetate and aluminum nitrate were used as precursor for zinc, nitrogen and aluminum, respectively. X-ray diffraction shows that the thin films have a hexagonal wurtzite structure for the undoped, doped and co-doped ZnO. The transmittance of the films was above 80% and the band gap of the film varied from 3.20 eV to 3.24 eV for undoped and doped ZnO. An energy band diagram to describe the photoluminescence from the thin films was also constructed. This diagram includes the various defect levels and possible quasi-Fermi levels. A minimum resistivity of 0.0834 Ω-cm was obtained for the N and Al codoped ZnO thin films with p-type carrier conductivity. These ZnO films can be used as a window layer in solar cells and in UV lasers.

Indium hexagonal island as seed-layer to boost a-axis orientation of AlN thin films

NASA Astrophysics Data System (ADS)

Redjdal, N.; Salah, H.; Azzaz, M.; Menari, H.; Manseri, A.; Guedouar, B.; Garcia-Sanchez, A.; Chérif, S. M.

2018-06-01

Highly a-axis oriented aluminum nitride films have been grown on Indium coated (100) Si substrate by DC reactive magnetron sputtering. It is shown that In incorporated layer improve the extent of preferential growth along (100) axis and form dense AlN films with uniform surface and large grains, devoid of micro-cracks. As revealed by SEM cross section images, AlN structure consists of oriented columnar grains perpendicular to the Si surface, while AlN/In structure results in uniformely tilted column. SEM images also revealed the presence of In hexagonal islands persistent throughout the entire growth. Micro -Raman spectroscopy of the surface and the cross section of the AlN/In grown films evidenced their high degree of homogeneity and cristallinity.

Efficiency of Launching Highly Confined Polaritons by Infrared Light Incident on a Hyperbolic Material.

PubMed

Dai, Siyuan; Ma, Qiong; Yang, Yafang; Rosenfeld, Jeremy; Goldflam, Michael D; McLeod, Alex; Sun, Zhiyuan; Andersen, Trond I; Fei, Zhe; Liu, Mengkun; Shao, Yinming; Watanabe, Kenji; Taniguchi, Takashi; Thiemens, Mark; Keilmann, Fritz; Jarillo-Herrero, Pablo; Fogler, Michael M; Basov, D N

2017-09-13

We investigated phonon-polaritons in hexagonal boron nitride-a naturally hyperbolic van der Waals material-by means of the scattering-type scanning near-field optical microscopy. Real-space nanoimages we have obtained detail how the polaritons are launched when the light incident on a thin hexagonal boron nitride slab is scattered by various intrinsic and extrinsic inhomogeneities, including sample edges, metallic nanodisks deposited on its top surface, random defects, and surface impurities. The scanned tip of the near-field microscope is itself a polariton launcher whose efficiency proves to be superior to all the other types of polariton launchers we studied. Our work may inform future development of polaritonic nanodevices as well as fundamental studies of collective modes in van der Waals materials.

Synthesis of atomically thin hexagonal boron nitride films on nickel foils by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Nakhaie, S.; Wofford, J. M.; Schumann, T.; Jahn, U.; Ramsteiner, M.; Hanke, M.; Lopes, J. M. J.; Riechert, H.

2015-05-01

Hexagonal boron nitride (h-BN) is a layered two-dimensional material with properties that make it promising as a dielectric in various applications. We report the growth of h-BN films on Ni foils from elemental B and N using molecular beam epitaxy. The presence of crystalline h-BN over the entire substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used to examine the morphology and continuity of the synthesized films. A scanning electron microscopy study of films obtained using shorter depositions offers insight into the nucleation and growth behavior of h-BN on the Ni substrate. The morphology of h-BN was found to evolve from dendritic, star-shaped islands to larger, smooth triangular ones with increasing growth temperature.

Magnetic and electronic properties of SrMnO3 thin films

NASA Astrophysics Data System (ADS)

Mandal, Arup Kumar; Panchal, Gyanendra; Choudhary, R. J.; Phase, D. M.

2018-05-01

Single phase hexagonal bulk SrMnO3 (SMO) was prepared by solid state route and it was used for depositing thin films by pulsed laser deposition (PLD) technique on single crystalline (100) oriented SrTiO3 (STO) substrate. X-ray diffraction shows that the thin film is deposited in cubic SrMnO3 phase. From X-ray absorption at the Mn L edge we observed the mixed valency of Mn (Mn3+& Mn4+) due to strain induced by the lattice mismatching between SMO and STO. Due to this mixed valency of Mn ion in SMO film, the ferromagnetic nature is observed at lower temperature because of double exchange. After post annealing with very low oxygen partial pressure, magnetic and electronic property of SMO films are effectively modified.

Structural, optical and photoelectric properties of sprayed CdS thin films

NASA Astrophysics Data System (ADS)

Chandel, Tarun; Dwivedi, Shailendra Kumar; Zaman, M. Burhanuz; Rajaram, P.

2018-05-01

In this study, CdS thin films were grown via a facile spray pyrolysis technique. The crystalline phase, morphological, compositional and optical properties of the CdS thin films have been studied using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and UV-vis absorption spectroscopy, respectively. XRD patterns show that the grown CdS films crystallized in the hexagonal structure. Scanning electron microscopy (SEM) study shows that the surfaces of the films are smooth and are uniformly covered with nanoparticles. EDAX results reveal that the grown films have good stochiometry. UV-vis spectroscopy shows that the grown films have transparency above 80% over the entire visible region. The photo-electric response of the CdS films grown on glass substrates has been observed.

Anomalous Photovoltaic Response of Graphene-on-GaN Schottky Photodiodes.

PubMed

Lee, Jae Hyung; Lee, Won Woo; Yang, Dong Won; Chang, Won Jun; Kwon, Sun Sang; Park, Won Il

2018-04-25

Graphene has attracted great attention as an alternative to conventional metallic or transparent conducting electrodes. Despite its similarities with conventional electrodes, recent studies have shown that a single-atom layer of graphene possesses unique characteristics, such as a tunable work function and transparencies for electric potential, reactivity, and wetting. Nevertheless, a systematic analysis of graphene and semiconductor junction characteristics has not yet been carried out. Here, we report the photoresponse characteristics of graphene-on-GaN Schottky junction photodiodes (Gr-GaN SJPDs), showing a typical rectifying behavior and distinct photovoltaic and photoelectric responses. Following the initial abrupt response to UV illumination, the Gr-GaN SJPDs exhibited a distinct difference in photocarrier dynamics depending on the applied bias voltage, which is characterized by either a negative or positive change in photocurrent with time. We propose underlying mechanisms for the anomalous photocarrier dynamics based on the interplay between electrostatic molecular interactions over the one-atom-thick graphene and GaN junction and trapped photocarriers at the defect states in the GaN thin film.

Room temperature microwave oscillations in GaN/AlN resonant tunneling diodes with peak current densities up to 220 kA/cm2

NASA Astrophysics Data System (ADS)

Encomendero, Jimy; Yan, Rusen; Verma, Amit; Islam, S. M.; Protasenko, Vladimir; Rouvimov, Sergei; Fay, Patrick; Jena, Debdeep; Xing, Huili Grace

2018-03-01

We report the generation of room temperature microwave oscillations from GaN/AlN resonant tunneling diodes, which exhibit record-high peak current densities. The tunneling heterostructure grown by molecular beam epitaxy on freestanding GaN substrates comprises a thin GaN quantum well embedded between two AlN tunneling barriers. The room temperature current-voltage characteristics exhibit a record-high maximum peak current density of ˜220 kA/cm2. When biased within the negative differential conductance region, microwave oscillations are measured with a fundamental frequency of ˜0.94 GHz, generating an output power of ˜3.0 μW. Both the fundamental frequency and the output power of the oscillator are limited by the external biasing circuit. Using a small-signal equivalent circuit model, the maximum intrinsic frequency of oscillation for these diodes is predicted to be ˜200 GHz. This work represents a significant step towards microwave power generation enabled by resonant tunneling transport, an ultra-fast process that goes beyond the limitations of current III-Nitride high electron mobility transistors.

Impurity distribution and microstructure of Ga-doped ZnO films grown by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Kvit, A. V.; Yankovich, A. B.; Avrutin, V.; Liu, H.; Izyumskaya, N.; Özgür, Ü.; Morkoç, H.; Voyles, P. M.

2012-12-01

We report microstructural characterization of heavily Ga-doped ZnO (GZO) thin films on GaN and sapphire by aberration-corrected scanning transmission electron microscopy. Growth under oxygen-rich and metal-rich growth conditions leads to changes in the GZO polarity and different extended defects. For GZO layers on sapphire, the primary extended defects are voids, inversion domain boundaries, and low-angle grain boundaries. Ga doping of ZnO grown under metal-rich conditions causes a switch from pure oxygen polarity to mixed oxygen and zinc polarity in small domains. Electron energy loss spectroscopy and energy dispersive spectroscopy spectrum imaging show that Ga is homogeneous, but other residual impurities tend to accumulate at the GZO surface and at extended defects. GZO grown on GaN on c-plane sapphire has Zn polarity and no voids. There are misfit dislocations at the interfaces between GZO and an undoped ZnO buffer layer and at the buffer/GaN interface. Low-angle grain boundaries are the only threading microstructural defects. The potential effects of different extended defects and impurity distributions on free carrier scattering are discussed.

Self-terminated etching of GaN with a high selectivity over AlGaN under inductively coupled Cl2/N2/O2 plasma with a low-energy ion bombardment

NASA Astrophysics Data System (ADS)

Zhong, Yaozong; Zhou, Yu; Gao, Hongwei; Dai, Shujun; He, Junlei; Feng, Meixin; Sun, Qian; Zhang, Jijun; Zhao, Yanfei; DingSun, An; Yang, Hui

2017-10-01

Etching of GaN/AlGaN heterostructure by O-containing inductively coupled Cl2/N2 plasma with a low-energy ion bombardment can be self-terminated at the surface of the AlGaN layer. The estimated etching rates of GaN and AlGaN were 42 and 0.6 nm/min, respectively, giving a selective etching ratio of 70:1. To study the mechanism of the etching self-termination, detailed characterization and analyses were carried out, including X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS). It was found that in the presence of oxygen, the top surface of the AlGaN layer was converted into a thin film of (Al,Ga)Ox with a high bonding energy, which effectively prevented the underlying atoms from a further etching, resulting in a nearly self-terminated etching. This technique enables a uniform and reproducible fabrication process for enhancement-mode high electron mobility transistors with a p-GaN gate.

Luminescence emission from Al0.3Ga0.7N/GaN multi quantum disc core/shell nanowire: Numerical approach

NASA Astrophysics Data System (ADS)

Namvari, E.; Shojaei, S.; Asgari, A.

2017-09-01

In this work, a numerical approach to investigate the room temperature luminescence emission from core/shell nanowire is presented where GaN quantum discs (QDiscs), periodically distributed in AlxGa1-xN nanowire, is considered as core and AlxGa1-xN as shell. Thin disc shaped (Ring shaped) n-doped region has been placed at the GaN/ AlxGa1-xN (AlxGa1-xN /air) interface in AlxGa1-xN region in axial (radial) directions. To obtain energy levels and related wavefunctions, self-consistent procedure has been employed to solve Schrodinger-Poisson equations with considering the spontaneous and piezoelectric polarization. Then luminescence spectrum is studied in details to recognize the parameters influent in luminescence. The results show that the amount of doping, size of QDiscs and theirs numbers have remarkable effects on the band to band luminescence emission. Our numerical calculations gives some insights into the luminescence emission of core/shell nanowire and exhibits a useful tool to analyze findings in experiments.

Development of Intrathecal AAV9 Gene Therapy for Giant Axonal Neuropathy.

PubMed

Bailey, Rachel M; Armao, Diane; Nagabhushan Kalburgi, Sahana; Gray, Steven J

2018-06-15

An NIH-sponsored phase I clinical trial is underway to test a potential treatment for giant axonal neuropathy (GAN) using viral-mediated GAN gene replacement (https://clinicaltrials.gov/ct2/show/NCT02362438). This trial marks the first instance of intrathecal (IT) adeno-associated viral (AAV) gene transfer in humans. GAN is a rare pediatric neurodegenerative disorder caused by autosomal recessive loss-of-function mutations in the GAN gene, which encodes the gigaxonin protein. Gigaxonin is involved in the regulation, turnover, and degradation of intermediate filaments (IFs). The pathologic signature of GAN is giant axonal swellings filled with disorganized accumulations of IFs. Herein, we describe the development and characterization of the AAV vector carrying a normal copy of the human GAN transgene (AAV9/JeT-GAN) currently employed in the clinical trial. Treatment with AAV/JeT-GAN restored the normal configuration of IFs in patient fibroblasts within days in cell culture and by 4 weeks in GAN KO mice. IT delivery of AAV9/JeT-GAN in aged GAN KO mice preserved sciatic nerve ultrastructure, reduced neuronal IF accumulations and attenuated rotarod dysfunction. This strategy conferred sustained wild-type gigaxonin expression across the PNS and CNS for at least 1 year in mice. These results support the clinical evaluation of AAV9/JeT-GAN for potential therapeutic outcomes and treatment for GAN patients.

Effect of different thickness h-BN coatings on interface shear strength of quartz fiber reinforced Sisbnd Osbnd Csbnd N composite

NASA Astrophysics Data System (ADS)

Wang, Shubin; Zheng, Yu

2014-02-01

Hexagonal boron nitride (h-BN) coatings with different thickness were prepared on quartz fibers to improve mechanical properties of quartz fiber reinforced Sisbnd Osbnd Csbnd N composite. Scanning electron microscopy (SEM), push-out test and single edge notched beam (SENB) in three point bending test were employed to study morphology, interface shear strength and fracture toughness of the composite. The results showed that h-BN coatings changed the crack growth direction and weaken the interface shear strength efficiently. When the h-BN coating was 308.2 nm, the interface shear strength was about 5.2 MPa, which was about one-quarter of that of the sample without h-BN coatings. After the heating process for obtaining composite, the h-BN nanometer-sized grains would grow up to micron-sized hexagonal grains. Different thickness h-BN coatings had different structure. When the coatings were relatively thin, the hexagonal grains were single layer structure, and when the coatings were thicker, the hexagonal grains were multiple layer structure. This multiple layer interface phase would consume more power of cracks, thus interface shear strength of the composite decreased steadily with the increasing of h-BN coatings thickness. When the coating thickness was 238.8 nm, KIC reaches the peak value 3.8 MPa m1/2, which was more than two times of that of composites without h-BN coatings.

Defect mediated van der Waals epitaxy of hexagonal boron nitride on graphene

NASA Astrophysics Data System (ADS)

Heilmann, M.; Bashouti, M.; Riechert, H.; Lopes, J. M. J.

2018-04-01

Van der Waals heterostructures comprising of hexagonal boron nitride and graphene are promising building blocks for novel two-dimensional devices such as atomically thin transistors or capacitors. However, demonstrators of those devices have been so far mostly fabricated by mechanical assembly, a non-scalable and time-consuming method, where transfer processes can contaminate the surfaces. Here, we investigate a direct growth process for the fabrication of insulating hexagonal boron nitride on high quality epitaxial graphene using plasma assisted molecular beam epitaxy. Samples were grown at varying temperatures and times and studied using atomic force microscopy, revealing a growth process limited by desorption at high temperatures. Nucleation was mostly commencing from morphological defects in epitaxial graphene, such as step edges or wrinkles. Raman spectroscopy combined with x-ray photoelectron measurements confirm the formation of hexagonal boron nitride and prove the resilience of graphene against the nitrogen plasma used during the growth process. The electrical properties and defects in the heterostructures were studied with high lateral resolution by tunneling current and Kelvin probe force measurements. This correlated approach revealed a nucleation apart from morphological defects in epitaxial graphene, which is mediated by point defects. The presented results help understanding the nucleation and growth behavior during van der Waals epitaxy of 2D materials, and point out a route for a scalable production of van der Waals heterostructures.

Hole polarons and p -type doping in boron nitride polymorphs

NASA Astrophysics Data System (ADS)

Weston, L.; Wickramaratne, D.; Van de Walle, C. G.

2017-09-01

Boron nitride polymorphs hold great promise for integration into electronic and optoelectronic devices requiring ultrawide band gaps. We use first-principles calculations to examine the prospects for p -type doping of hexagonal (h -BN ), wurtzite (w z -BN ), and cubic (c -BN ) boron nitride. Group-IV elements (C, Si) substituting on the N site result in a deep acceptor, as the atomic levels of the impurity species lie above the BN valence-band maximum. On the other hand, group-II elements (Be, Mg) substituting on the B site do not give impurity states in the band gap; however, these dopants lead to the formation of small hole polarons. The tendency for polaron formation is far more pronounced in h -BN compared to w z -BN or c -BN . Despite forming small hole polarons, Be acceptors enable p -type doping, with ionization energies of 0.31 eV for w z -BN and 0.24 eV for c -BN ; these values are comparable to the Mg ionization energy in GaN.

Systematic approach to developing empirical interatomic potentials for III-N semiconductors

NASA Astrophysics Data System (ADS)

Ito, Tomonori; Akiyama, Toru; Nakamura, Kohji

2016-05-01

A systematic approach to the derivation of empirical interatomic potentials is developed for III-N semiconductors with the aid of ab initio calculations. The parameter values of empirical potential based on bond order potential are determined by reproducing the cohesive energy differences among 3-fold coordinated hexagonal, 4-fold coordinated zinc blende, wurtzite, and 6-fold coordinated rocksalt structures in BN, AlN, GaN, and InN. The bond order p is successfully introduced as a function of the coordination number Z in the form of p = a exp(-bZn ) if Z ≤ 4 and p = (4/Z)α if Z ≥ 4 in empirical interatomic potential. Moreover, the energy difference between wurtzite and zinc blende structures can be successfully evaluated by considering interaction beyond the second-nearest neighbors as a function of ionicity. This approach is feasible for developing empirical interatomic potentials applicable to a system consisting of poorly coordinated atoms at surfaces and interfaces including nanostructures.

InGaN/GaN light-emitting diode having direct hole injection plugs and its high-current operation.

PubMed

Kim, Sungjoon; Cho, Seongjae; Jeong, Jaedeok; Kim, Sungjun; Hwang, Sungmin; Kim, Garam; Yoon, Sukho; Park, Byung-Gook

2017-03-20

The light-emitting diode (LED) with an improved hole injection and straightforward process integration is proposed. p-type GaN direct hole injection plugs (DHIPs) are formed on locally etched multiple-quantum wells (MQWs) by epitaxial lateral overgrowth (ELO) method. We confirm that the optical output power is increased up to 23.2% at an operating current density of 100 A/cm². Furthermore, in order to identify the origin of improvement in optical performance, the transient light decay time and light intensity distribution characteristics were analyzed on the DHIP LED devices. Through the calculation of the electroluminescence (EL) decay time, internal quantum efficiency (IQE) is extracted along with the recombination parameters, which reveals that the DHIPs have a significant effect on enhancement of radiative recombination and reduction of efficiency droop. Furthermore, the mapping PL reveals that the DHIP LED also has a potential to improve the light extraction efficiency by hexagonal pyramid shaped DHIPs.

Characterization of Non-Polar ZnO Layers with Positron Annihilation Spectroscopy

NASA Astrophysics Data System (ADS)

Zubiaga, A.; Tuomisto, F.; Zúñiga-Pérez, J.; Muñoz-San José, V.

2008-11-01

We applied positron annihilation spectroscopy to study the effect of growth polarity on the vacancy defects in ZnO grown by metal-organic vapor phase deposition on sapphire. Both c-plane and a-plane ZnO layers were measured, and Zn vacancies were identified as the dominant defects detected by positrons. The results are qualitatively similar to those of earlier experiments in GaN. The Zn vacancy concentration decreases in c-plane ZnO by almost one order of magnitude (from high 1017 cm-3 to low 1017 cm-3) when the layer thickness is increased from 0.5 to 2 μm. Interestingly, in a-plane ZnO the Zn vacancy concentration is constant at a level of about 2×1017 cm-3 in all the samples with thicknesses varying from 0.6 to 2.4 μm. The anisotropy of the Doppler broadening of the annihilation radiation parallel and perpendicular to the hexagonal c-axis was also measured.

Blueish green photoluminescence from nitrided GaAs(100) surfaces

NASA Astrophysics Data System (ADS)

Shimaoka, Goro; Udagawa, Takashi

1999-04-01

Optical and structural studies were made on the Si-doped (100)GaAs surfaces nitrided at a temperature between 650° and 750°C for 15 min in the flowing NH 3 gas. The wavelength of photoluminescence (PL) spectra were observed to be shortened from 820 nm of the GaAs nitrided at 650°C with increasing nitridation temperature. Blueish green PL with wavelengths of approx. 490 nm and 470 nm were emitted from the nitrided surfaces at 700° and 750°C, respectively. Results of AES and SIMS indicated that the surfaces are nitrided as GaAs 1- xN x, (0< x≤1) alloy layer, and the nitrided region also tended to increase as the temperature raised. High-resolution transmission electron microscopic (HRTEM), transmission electron diffraction (TED) and energy dispersive X-ray (EDX) results showed that films peeled off from the nitrided surfaces consisted mainly of hexagonal, wurtzite-type gallium nitride (GaN) with stacking faults and microtwins.

Nanoair-bridged lateral overgrowth of GaN on ordered nanoporous GaN template

NASA Astrophysics Data System (ADS)

Wang, Y. D.; Zang, K. Y.; Chua, S. J.; Tripathy, S.; Chen, P.; Fonstad, C. G.

2005-12-01

We report the growth of high-quality GaN epilayers on an ordered nanoporous GaN template by metalorganic chemical vapor deposition. The nanopores in GaN template were created by inductively coupled plasma etching using anodic aluminum oxide film as an etch mask. The average pore diameter and interpore distance is about 65 and 110nm, respectively. Subsequent overgrowth of GaN first begins at the GaN crystallite surface between the pores, and then air-bridge-mediated lateral overgrowth leads to the formation of the continuous layer. Microphotoluminescence and micro-Raman measurements show improved optical properties and significant strain relaxation in the overgrown layer when compared to GaN layer of same thickness simultaneously grown on sapphire without any template. Similar to conventional epitaxial lateral overgrown GaN, such overgrown GaN on a nanopatterned surface would also serve as a template for the growth of ultraviolet-visible light-emitting III-nitride devices.

Chemical bath deposited and dip coating deposited CuS thin films - Structure, Raman spectroscopy and surface study

NASA Astrophysics Data System (ADS)

Tailor, Jiten P.; Khimani, Ankurkumar J.; Chaki, Sunil H.

2018-05-01

The crystal structure, Raman spectroscopy and surface microtopography study on as-deposited CuS thin films were carried out. Thin films deposited by two techniques of solution growth were studied. The thin films used in the present study were deposited by chemical bath deposition (CBD) and dip coating deposition techniques. The X-ray diffraction (XRD) analysis of both the as-deposited thin films showed that both the films possess covellite phase of CuS and hexagonal unit cell structure. The determined lattice parameters of both the films are in agreement with the standard JCPDS as well as reported data. The crystallite size determined by Scherrer's equation and Hall-Williamsons relation using XRD data for both the as-deposited thin films showed that the respective values were in agreement with each other. The ambient Raman spectroscopy of both the as-deposited thin films showed major emission peaks at 474 cm-1 and a minor emmision peaks at 265 cm-1. The observed Raman peaks matched with the covellite phase of CuS. The atomic force microscopy of both the as-deposited thin films surfaces showed dip coating thin film to be less rough compared to CBD deposited thin film. All the obtained results are presented and deliberated in details.

Room temperature rubbing for few-layer two-dimensional thin flakes directly on flexible polymer substrates

PubMed Central

Yu, Yan; Jiang, Shenglin; Zhou, Wenli; Miao, Xiangshui; Zeng, Yike; Zhang, Guangzu; Liu, Sisi

2013-01-01

The functional layers of few-layer two-dimensional (2-D) thin flakes on flexible polymers for stretchable applications have attracted much interest. However, most fabrication methods are “indirect” processes that require transfer steps. Moreover, previously reported “transfer-free” methods are only suitable for graphene and not for other few-layer 2-D thin flakes. Here, a friction based room temperature rubbing method is proposed for fabricating different types of few-layer 2-D thin flakes (graphene, hexagonal boron nitride (h-BN), molybdenum disulphide (MoS2), and tungsten disulphide (WS2)) on flexible polymer substrates. Commercial 2-D raw materials (graphite, h-BN, MoS2, and WS2) that contain thousands of atom layers were used. After several minutes, different types of few-layer 2-D thin flakes were fabricated directly on the flexible polymer substrates by rubbing procedures at room temperature and without any transfer step. These few-layer 2-D thin flakes strongly adhere to the flexible polymer substrates. This strong adhesion is beneficial for future applications. PMID:24045289

Effect of temperature on NH3 sensing by ZnO: Mg thin film grown by radio frequency magnetron sputtering technique

NASA Astrophysics Data System (ADS)

Vinoth, E.; Gopalakrishnan, N.

2018-04-01

Undoped and Mg doped (at l0 mol %) ZnO thin films have been grown on glass substrates by using the RF magnetron sputtering. The structural properties of the fabricated thin films were studied by X-ray diffraction analysis and it was found hexagonal wurtzite phase and preferential orientation along (002) of both films. Green Band Emission peaks in the Photoluminescence spectra confirm the structural defects such as oxygen vacancies (Vo) in the films. Uniform distribution of spherical shape morphology of grains observed in the both films by FESEM. However, the growth of grains was found in the Mg doped thin film. The temperature dependent ammonia sensing is done by the indigenously made gas sensing setup. The gas response of the both films was increased as the temperature increases, attains maximum at 75° C and then decreases. Response and recovery time measurementswere donefor boththe films and it shows the fast response time and quick recovery for doped thin film compared to the pure ZnO thin film.

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

Laskowski, Lukasz, E-mail: lukasz.laskowski@kik.pcz.pl; Laskowska, Magdalena, E-mail: magdalena.laskowska@onet.pl; Jelonkiewicz, Jerzy, E-mail: jerzy.jelonkiewicz@kik.pcz.pl

The SBA-15 silica thin films containing copper ions anchored inside channels via propyl phosphonate groups are investigated. Such materials were prepared in the form of thin films, with hexagonally arranged pores, laying rectilinear to the substrate surface. However, in the case of our thin films, their free standing form allowed for additional research possibilities, that are not obtainable for typical thin films on a substrate. The structural properties of the samples were investigated by X-ray reflectometry, atomic force microscopy (AFM) and transmission electron microscopy (TEM). The molecular structure was examined by Raman spectroscopy supported by numerical simulations. Magnetic measurements (SQUIDmore » magnetometry and EPR spectroscopy) showed weak antiferromagnetic interactions between active units inside silica channels. Consequently, the pores arrangement was determined and the process of copper ions anchoring by propyl phosphonate groups was verified in unambiguous way. Moreover, the type of interactions between magnetic atoms was determined. - Highlights: • Functionalized free-standing SBA-15 thin films were synthesized for a first time. • Thin films synthesis procedure was described in details. • Structural properties of the films were thoroughly investigated and presented. • Magnetic properties of the novel material was investigated and presented.« less

Nanorods on surface of GaN-based thin-film LEDs deposited by post-annealing after photo-assisted chemical etching

NASA Astrophysics Data System (ADS)

Chen, Lung-Chien; Lin, Wun-Wei; Liu, Te-Yu

2017-01-01

This study investigates the optoelectronic characteristics of gallium nitride (GaN)-based thin-film light-emitting diodes (TF-LEDs) that are formed by a two-step transfer process that involves wet etching and post-annealing. In the two-step transfer process, GaN LEDs were stripped from sapphire substrates by the laser lift-off (LLO) method using a KrF laser and then transferred onto ceramic substrates. Ga-K nanorods were formed on the surface of the GaN-based TF-LEDs following photo-assisted chemical etching and photo-enhanced post-annealing at 100 °C for 1 min. As a result, the light output power of GaN-based TF-LEDs with wet etching and post-annealing was over 72% more than that of LEDs that did not undergo these treatments.

Ordering kinetics in two-dimensional hexagonal pattern of cylinder-forming PS-b -PMMA block copolymer thin films: Dependence on the segregation strength

NASA Astrophysics Data System (ADS)

Seguini, Gabriele; Zanenga, Fabio; Laus, Michele; Perego, Michele

2018-05-01

This paper reports the experimental determination of the growth exponents and activation enthalpies for the ordering process of standing cylinder-forming all-organic polystyrene-block-poly (methyl methacrylate) block copolymer (BCP) thin films as a function of the BCP degree of polymerization (N). The maximum growth exponent of 1/3 is observed for the BCP with the lowest N at the border of the order-disorder transition. Both the growth exponents and the activation enthalpies exponentially decrease with the BCP segregation strength (χN) following the same path of the diffusivity.

Chemically stabilized epitaxial wurtzite-BN thin film

NASA Astrophysics Data System (ADS)

Vishal, Badri; Singh, Rajendra; Chaturvedi, Abhishek; Sharma, Ankit; Sreedhara, M. B.; Sahu, Rajib; Bhat, Usha; Ramamurty, Upadrasta; Datta, Ranjan

2018-03-01

We report on the chemically stabilized epitaxial w-BN thin film grown on c-plane sapphire by pulsed laser deposition under slow kinetic condition. Traces of no other allotropes such as cubic (c) or hexagonal (h) BN phases are present. Sapphire substrate plays a significant role in stabilizing the metastable w-BN from h-BN target under unusual PLD growth condition involving low temperature and pressure and is explained based on density functional theory calculation. The hardness and the elastic modulus of the w-BN film are 37 & 339 GPa, respectively measured by indentation along <0001> direction. The results are extremely promising in advancing the microelectronic and mechanical tooling industry.

Electromagnetic scattering and absorption by thin walled dielectric cylinders with application to ice crystals

NASA Technical Reports Server (NTRS)

Senior, T. B. A.; Weil, H.

1977-01-01

Important in the atmospheric heat balance are the reflection, transmission, and absorption of visible and infrared radiation by clouds and polluted atmospheres. Integral equations are derived to evaluate the scattering and absorption of electromagnetic radiation from thin cylindrical dielectric shells of arbitrary cross section when irradiated by a plane wave of any polarization incident in a plane perpendicular to the generators. Application of the method to infinitely long hexagonal cylinders has yielded numerical scattering and absorption data which simulate columnar sheath ice crystals. It is found that the numerical procedures are economical for cylinders having perimeters less than approximately fifteen free-space wavelengths.

Defect free C-axis oriented zinc oxide (ZnO) films grown at room temperature using RF magnetron sputtering

NASA Astrophysics Data System (ADS)

Kunj, Saurabh; Sreenivas, K.

2016-05-01

Radio frequency Magnetron sputtering technique was employed to fabricate ZnO thin films on quartz substrate at room temperature. The effect of varying oxygen to argon (O2/Ar) gas ratio on the structural and photoluminescence properties of the film is analyzed.X-ray diffraction (XRD) spectra reveals the formation of hexagonal wurtzite structured ZnO thin films with preferred orientation along (002) plane. Photoluminescence (PL) characterization reveals the preparation of highly crystalline films exhibiting intense Ultraviolet (UV) emission with negligible amount of defects as indicated by the absence of Deep Level Emission (DLE) in the PL spectra.

Defect free C-axis oriented zinc oxide (ZnO) films grown at room temperature using RF magnetron sputtering

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

Kunj, Saurabh, E-mail: saurabhkunj22@gmail.com; Sreenivas, K.

2016-05-23

Radio frequency Magnetron sputtering technique was employed to fabricate ZnO thin films on quartz substrate at room temperature. The effect of varying oxygen to argon (O{sub 2}/Ar) gas ratio on the structural and photoluminescence properties of the film is analyzed.X-ray diffraction (XRD) spectra reveals the formation of hexagonal wurtzite structured ZnO thin films with preferred orientation along (002) plane. Photoluminescence (PL) characterization reveals the preparation of highly crystalline films exhibiting intense Ultraviolet (UV) emission with negligible amount of defects as indicated by the absence of Deep Level Emission (DLE) in the PL spectra.

Two-dimensional flexible nanoelectronics

NASA Astrophysics Data System (ADS)

Akinwande, Deji; Petrone, Nicholas; Hone, James

2014-12-01

2014/2015 represents the tenth anniversary of modern graphene research. Over this decade, graphene has proven to be attractive for thin-film transistors owing to its remarkable electronic, optical, mechanical and thermal properties. Even its major drawback--zero bandgap--has resulted in something positive: a resurgence of interest in two-dimensional semiconductors, such as dichalcogenides and buckled nanomaterials with sizeable bandgaps. With the discovery of hexagonal boron nitride as an ideal dielectric, the materials are now in place to advance integrated flexible nanoelectronics, which uniquely take advantage of the unmatched portfolio of properties of two-dimensional crystals, beyond the capability of conventional thin films for ubiquitous flexible systems.

Two-dimensional flexible nanoelectronics.

PubMed

Akinwande, Deji; Petrone, Nicholas; Hone, James

2014-12-17

2014/2015 represents the tenth anniversary of modern graphene research. Over this decade, graphene has proven to be attractive for thin-film transistors owing to its remarkable electronic, optical, mechanical and thermal properties. Even its major drawback--zero bandgap--has resulted in something positive: a resurgence of interest in two-dimensional semiconductors, such as dichalcogenides and buckled nanomaterials with sizeable bandgaps. With the discovery of hexagonal boron nitride as an ideal dielectric, the materials are now in place to advance integrated flexible nanoelectronics, which uniquely take advantage of the unmatched portfolio of properties of two-dimensional crystals, beyond the capability of conventional thin films for ubiquitous flexible systems.

Synthesis of thin films in boron-carbon-nitrogen ternary system by microwave plasma enhanced chemical vapor deposition

NASA Astrophysics Data System (ADS)

Kukreja, Ratandeep Singh

The Boron Carbon Nitorgen (B-C-N) ternary system includes materials with exceptional properties such as wide band gap, excellent thermal conductivity, high bulk modulus, extreme hardness and transparency in the optical and UV range that find application in most fields ranging from micro-electronics, bio-sensors, and cutting tools to materials for space age technology. Interesting materials that belong to the B-C-N ternary system include Carbon nano-tubes, Boron Carbide, Boron Carbon Nitride (B-CN), hexagonal Boron Nitride ( h-BN), cubic Boron Nitride (c-BN), Diamond and beta Carbon Nitride (beta-C3N4). Synthesis of these materials requires precisely controlled and energetically favorable conditions. Chemical vapor deposition is widely used technique for deposition of thin films of ceramics, metals and metal-organic compounds. Microwave plasma enhanced chemical vapor deposition (MPECVD) is especially interesting because of its ability to deposit materials that are meta-stable under the deposition conditions, for e.g. diamond. In the present study, attempt has been made to synthesize beta-carbon nitride (beta-C3N4) and cubic-Boron Nitride (c-BN) thin films by MPECVD. Also included is the investigation of dependence of residual stress and thermal conductivity of the diamond thin films, deposited by MPECVD, on substrate pre-treatment and deposition temperature. Si incorporated CNx thin films are synthesized and characterized while attempting to deposit beta-C3N4 thin films on Si substrates using Methane (CH4), Nitrogen (N2), and Hydrogen (H2). It is shown that the composition and morphology of Si incorporated CNx thin film can be tailored by controlling the sequence of introduction of the precursor gases in the plasma chamber. Greater than 100mum size hexagonal crystals of N-Si-C are deposited when Nitrogen precursor is introduced first while agglomerates of nano-meter range graphitic needles of C-Si-N are deposited when Carbon precursor is introduced first in the deposition chamber. Hexagonal -- BN thin films are successfully deposited using Diborane (B2H6) (5% in H2), Ammonia (NH3) and H2 as precursor gases in the conventional MPECVD mode with and without the negative DC bias. The quality of h-BN in the films improved with pressure and when NH3 used as the first precursor gas in the deposition chamber. c-BN thin films are successfully deposited using Boron-Trifluoride (BF3) (10% in Argon (Ar)), N2, H2, Ar and Helium (He) gases in the electron cyclotron resonance (ECR) mode of the MPECVD system with negative DC bias. Up-to 66% c-BN in the films is achieved under deposition conditions of lower gas flow rates and higher deposition pressures than that reported in the literature for film deposited by ECR-MPECVD. It is shown that the percentage c-BN in the films correlates with the deposition pressure, BF3/H2 ratio and, negative DC bias during nucleation and growth. Diamond thin films are deposited using 60%Ar, 39% H2 and, 1%CH4 at 600°C, 700°C and 800°C substrate temperatures, measured by an IR pyrometer, on Si substrates pre-treated with 3-6nm diamond sol and 20-40mum diamond slurry. Raman spectroscopy, FTIR, X-Ray diffraction (XRD) and, photo-thermal reflectivity methods are used to characterize the thin films. Residual stresses observed for the diamond thin films deposited in this study are tensile in nature and increased with deposition temperature. Better quality diamond films with lower residual stresses are obtained for films deposited on Si substrate pre-treated with 3-6nm diamond sol. Preliminary results on thermal conductivity, k, suggest that k is directly dependent on the deposition temperature and independent of substrate pre-treatment signifying that the nano-seeding technique can be used to replace conventional surface activation technique for diamond seeding where needed.

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

NASA Astrophysics Data System (ADS)

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

2016-12-01

The growth kinetics of nominally one-monolayer (˜1-ML)-thick InN wells on/in the +c-GaN matrix fabricated using dynamic atomic layer epitaxy (D-ALEp) by plasma-assisted molecular beam epitaxy were systematically studied, with particular attention given to the impacts of excess In atoms and/or In droplets at a high growth temperature of 650 °C. Even at a constant growth temperature of 650 °C, the thickness of the sheet-island-like InN-well layers could be controlled/varied from 1-ML to 2-ML owing to the effect of excess In atoms and/or In droplets accumulated during growth. The possible growth mechanism is discussed based on the ring-shaped bright cathodoluminescence emissions introduced along the circumference of the In droplets during growth. The effective thermal stability of N atoms below the bilayer adsorbed In atoms was increased by the presence of In droplets, resulting in the freezing of 2-ML-thick InN wells into the GaN matrix. It therefore became possible to study the difference between the emission properties of 1-ML and 2-ML-thick InN wells/GaN matrix quantum wells (QWs) having similar GaN matrix crystallinity grown at the same temperature. InN/GaN QW-samples grown under widely different In + N* supply conditions characteristically separated into two groups with distinctive emission-peak wavelengths originating from 1-ML and 2-ML-thick InN wells embedded in the GaN matrix. Reflecting the growth mechanism inherent to the D-ALEp of InN on/in the +c-GaN matrix at high temperature, either 1-ML or 2-ML-thick "binary" InN well layers tended to be frozen into the GaN matrix rather InGaN random ternary-alloys. Both the structural quality and uniformity of the 1-ML InN well sample were better than those of the 2-ML InN well sample, essentially owing to the quite thin critical thickness of around 1-ML arising from the large lattice mismatch of InN and GaN.

Lattice Matched Iii-V IV Semiconductor Heterostructures: Metalorganic Chemical Vapor Deposition and Remote Plasma Enhanced Chemical Vapor Deposition.

NASA Astrophysics Data System (ADS)

Choi, Sungwoo

1992-01-01

This thesis describes the growth and characterization of wide gap III-V compound semiconductors such as aluminum gallium arsenide (Al_{rm x} Ga_{rm 1-x}As), gallium nitride (GaN), and gallium phosphide (GaP), deposited by the metalorganic chemical vapor deposition (MOCVD) and remote plasma enhanced chemical vapor deposition (Remote PECVD). In the first part of the thesis, the optimization of GaAs and Al_{rm x}Ga _{rm 1-x}As hetero -epitaxial layers on Ge substrates is described in the context of the application in the construction of cascade solar cells. The emphasis on this study is on the trade-offs in the choice of the temperature related to increasing interdiffusion/autodoping and increasing perfection of the epilayer with increasing temperature. The structural, chemical, optical, and electrical properties of the heterostructures are characterized by x-ray rocking curve measurement, scanning electron microscopy (SEM), electron beam induced current (EBIC), cross-sectional transmission electron microscopy (X-TEM), Raman spectroscopy, secondary ion mass spectrometry (SIMS), and steady-state and time-resolved photoluminescence (PL). Based on the results of this work the optimum growth temperature is 720^circC. The second part of the thesis describes the growth of GaN and GaP layers on silicon and sapphire substrates and the homoepitaxy of GaP by remote PECVD. I have designed and built an ultra high vacuum (UHV) deposition system which includes: the gas supply system, the pumping system, the deposition chamber, the load-lock chamber, and the waste disposal system. The work on the deposition of GaN on Si and sapphire focuses onto the understanding of the growth kinetics. In addition, Auger electron spectroscopy (AES) for surface analysis, x-ray diffraction methods and microscopic analyses using SEM and TEM for structural characterization, infrared (IR) and ultraviolet (UV) absorption measurements for optical characterization, and electrical characterization results on the GaN films are presented. In the deposition GaP thin films by remote PECVD, trimethylgallium and in-situ generated phosphine precursors are employed as source gases which permits homo- and heteroepitaxial growth as substrate temperature of 590-620^ circC. Also, the growth kinetics of gallium phosphide is discussed. As in the case of GaN, the surface, structural, chemical, optical, and electrical properties are characterized and the results are discussed.

A DFT study on NEA GaN photocathode with an ultrathin n-type Si-doped GaN cap layer

NASA Astrophysics Data System (ADS)

Xia, Sihao; Liu, Lei; Kong, Yike; Diao, Yu

2016-10-01

Due to the drawbacks of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, a new-type NEA GaN photocathodes with heterojunction surface dispense with Cs activation are proposed. This structure can be obtained through the coverage of an ultrathin n-type Si-doped GaN cap layer on the p-type Mg-doped GaN emission layer. The influences of the cap layer on the photocathode are calculated using DFT. This study indicates that the n-type cap layer can promote the photoemission characteristics of GaN photocathode and demonstrates the probability of the preparation of a NEA GaN photocathode with an n-type cap layer.

Viability and proliferation of endothelial cells upon exposure to GaN nanoparticles.

PubMed

Braniste, Tudor; Tiginyanu, Ion; Horvath, Tibor; Raevschi, Simion; Cebotari, Serghei; Lux, Marco; Haverich, Axel; Hilfiker, Andres

2016-01-01

Nanotechnology is a rapidly growing and promising field of interest in medicine; however, nanoparticle-cell interactions are not yet fully understood. The goal of this work was to examine the interaction between endothelial cells and gallium nitride (GaN) semiconductor nanoparticles. Cellular viability, adhesion, proliferation, and uptake of nanoparticles by endothelial cells were investigated. The effect of free GaN nanoparticles versus the effect of growing endothelial cells on GaN functionalized surfaces was examined. To functionalize surfaces with GaN, GaN nanoparticles were synthesized on a sacrificial layer of zinc oxide (ZnO) nanoparticles using hydride vapor phase epitaxy. The uptake of GaN nanoparticles by porcine endothelial cells was strongly dependent upon whether they were fixed to the substrate surface or free floating in the medium. The endothelial cells grown on surfaces functionalized with GaN nanoparticles demonstrated excellent adhesion and proliferation, suggesting good biocompatibility of the nanostructured GaN.

Electrical contact of wurtzite GaN mircrodisks on p-type GaN template

NASA Astrophysics Data System (ADS)

Tsai, Cheng-Da; Lo, Ikai; Wang, Ying-Chieh; Hsu, Yu-Chi; Shih, Cheng-Hung; Pang, Wen-Yuan; You, Shuo-Ting; Hu, Chia-Hsuan; Chou, Mitch M. C.; Yang, Chen-Chi; Lin, Yu-Chiao

2015-03-01

We developed a back processing to fabricate a secure electrical contact of wurtzite GaN microdisk on a transparent p-type GaN template with the orientation, [10-10]disk // [10-10]template. GaN microdisks were grown on LiAlO2 substrate by using plasma-assisted molecular beam epitaxy. In the further study, we analyzed the TEM specimen of a sample with annealed GaN microdisk/p-typed GaN template by selection area diffraction (SAD) to confirm the alignment of the microdisks with the template at the interface. From the I-V measurements performed on the samples, we obtained a threshold voltage of ~ 5.9 V for the current passing through the GaN microdisks with a resistance of ~ 45 K Ω. The electrical contact can be applied to the nanometer-scaled GaN light-emitting diode.

Growth and characterizations of various GaN nanostructures on C-plane sapphire using laser MBE

NASA Astrophysics Data System (ADS)

Ch., Ramesh; Tyagi, P.; Maurya, K. K.; Kumar, M. Senthil; Kushvaha, S. S.

2017-05-01

We have grown various GaN nanostructures such as three-dimensional islands, nanowalls and nanocolumns on c-plane sapphire substrates using laser assisted molecular beam epitaxy (LMBE) system. The shape of the GaN nanostructures was controlled by using different nucleation surfaces such as bare and nitridated sapphire with GaN or AlN buffer layers. The structural and surface morphological properties of grown GaN nanostructures were characterized by ex-situ high resolution x-ray diffraction, Raman spectroscopy and field emission scanning electron microscopy. The symmetric x-ray rocking curve along GaN (0002) plane shows that the GaN grown on pre-nitridated sapphire with GaN or AlN buffer layer possesses good crystalline quality compared to sapphire without nitridation. The Raman spectroscopy measurements revealed the wurtzite phase for all the GaN nanostructures grown on c-sapphire.

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

NASA Astrophysics Data System (ADS)

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

2015-08-01

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

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

NASA Astrophysics Data System (ADS)

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

2018-06-01

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

Characterizations of GaN film growth by ECR plasma chemical vapor deposition

NASA Astrophysics Data System (ADS)

Fu, Silie; Chen, Junfang; Zhang, Hongbin; Guo, Chaofen; Li, Wei; Zhao, Wenfen

2009-06-01

The electron cyclotron resonance plasma-enhanced metalorganic chemical vapor deposition technology (ECR-MOPECVD) is adopted to grow GaN films on (0 0 0 1) α-Al2O3 substrate. The gas sources are pure N2 and trimethylgallium (TMG). Optical emission spectroscopy (OES) and thermodynamic analysis of GaN growth are applied to understand the GaN growth process. The OES of ECR plasma shows that TMG is significantly dissociated in ECR plasma. Reactants N and Ga in the plasma, obtained easily under the self-heating condition, are essential for the GaN growth. They contribute to the realization of GaN film growth at a relatively low temperature. The thermodynamic study shows that the driving force for the GaN growth is high when N2:TMG>1. Furthermore, higher N2:TMG flow ratio makes the GaN growth easier. Finally, X-ray diffraction, photoluminescence, and atomic force microscope are applied to investigate crystal quality, morphology, and roughness of the GaN films. The results demonstrate that the ECR-MOPECVD technology is favorable for depositing GaN films at low temperatures.

Direct growth of freestanding GaN on C-face SiC by HVPE.

PubMed

Tian, Yuan; Shao, Yongliang; Wu, Yongzhong; Hao, Xiaopeng; Zhang, Lei; Dai, Yuanbin; Huo, Qin

2015-06-02

In this work, high quality GaN crystal was successfully grown on C-face 6H-SiC by HVPE using a two steps growth process. Due to the small interaction stress between the GaN and the SiC substrate, the GaN was self-separated from the SiC substrate even with a small thickness of about 100 μm. Moreover, the SiC substrate was excellent without damage after the whole process so that it can be repeatedly used in the GaN growth. Hot phosphoric acid etching (at 240 °C for 30 min) was employed to identify the polarity of the GaN layer. According to the etching results, the obtained layer was Ga-polar GaN. High-resolution X-ray diffraction (HRXRD) and electron backscatter diffraction (EBSD) were done to characterize the quality of the freestanding GaN. The Raman measurements showed that the freestanding GaN film grown on the C-face 6H-SiC was stress-free. The optical properties of the freestanding GaN layer were determined by photoluminescence (PL) spectra.

Atomic force microscopy studies of homoepitaxial GaN layers grown on GaN template by laser MBE

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

Choudhary, B. S.; Rajasthan Technical University, Rawatbhata Road, Kota 324010; Singh, A.

We have grown homoepitaxial GaN films on metal organic chemical vapor deposition (MOCVD) grown 3.5 µm thick GaN on sapphire (0001) substrate (GaN template) using an ultra-high vacuum (UHV) laser assisted molecular beam epitaxy (LMBE) system. The GaN films were grown by laser ablating a polycrystalline solid GaN target in the presence of active r.f. nitrogen plasma. The influence of laser repetition rates (10-30 Hz) on the surface morphology of homoepitaxial GaN layers have been studied using atomic force microscopy. It was found that GaN layer grown at 10 Hz shows a smooth surface with uniform grain size compared to the rough surfacemore » with irregular shape grains obtained at 30 Hz. The variation of surface roughness of the homoepitaxial GaN layer with and without wet chemical etching has been also studied and it was observed that the roughness of the film decreased after wet etching due to the curved structure/rough surface.« less

Berkovich Nanoindentation on AlN Thin Films.

PubMed

Jian, Sheng-Rui; Chen, Guo-Ju; Lin, Ting-Chun

2010-03-31

Berkovich nanoindentation-induced mechanical deformation mechanisms of AlN thin films have been investigated by using atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM) techniques. AlN thin films are deposited on the metal-organic chemical-vapor deposition (MOCVD) derived Si-doped (2 × 1017 cm-3) GaN template by using the helicon sputtering system. The XTEM samples were prepared by means of focused ion beam (FIB) milling to accurately position the cross-section of the nanoindented area. The hardness and Young's modulus of AlN thin films were measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) option. The obtained values of the hardness and Young's modulus are 22 and 332 GPa, respectively. The XTEM images taken in the vicinity regions just underneath the indenter tip revealed that the multiple "pop-ins" observed in the load-displacement curve during loading are due primarily to the activities of dislocation nucleation and propagation. The absence of discontinuities in the unloading segments of load-displacement curve suggests that no pressure-induced phase transition was involved. Results obtained in this study may also have technological implications for estimating possible mechanical damages induced by the fabrication processes of making the AlN-based devices.

Synergistic effect of indium and gallium co-doping on the properties of RF sputtered ZnO thin films

NASA Astrophysics Data System (ADS)

Shaheera, M.; Girija, K. G.; Kaur, Manmeet; Geetha, V.; Debnath, A. K.; Karri, Malvika; Thota, Manoj Kumar; Vatsa, R. K.; Muthe, K. P.; Gadkari, S. C.

2018-04-01

ZnO thin films were synthesized using RF magnetron sputtering, with simultaneous incorporation of Indium (In) and Gallium (Ga). The structural, optical, chemical composition and surface morphology of the pure and co-doped (IGZO) thin films were characterized by X-Ray diffraction (XRD), UV-visible spectroscopy, Field Emission Scanning Electron Microscopy (FESEM), and Raman spectroscopy. XRD revealed that these films were oriented along c-axis with hexagonal wurtzite structure. The (002) diffraction peak in the co-doped sample was observed at 33.76° with a slight shift towards lower 2θ values as compared to pure ZnO. The surface morphology of the two thin films was observed to differ. For pure ZnO films, round grains were observed and for IGZO thin films round as well as rod type grains were observed. All thin films synthesized show excellent optical properties with more than 90% transmission in the visible region and band gap of the films is observed to decrease with co-doping. The co doping of In and Ga is therefore expected to provide a broad range optical and physical properties of ZnO thin films for a variety of optoelectronic applications.

Structural and Optical Properties of Cd 1- x Se x Thin Films Deposited by Electron Beam Evaporation Technique

NASA Astrophysics Data System (ADS)

Tripathi, Ravishankar Nath; Verma, Aneet Kumar; Rahul, Vishwakarma, S. R.

2011-10-01

Cadmium selenide (CdSe) thin films deposited by means of electron beam evaporation technique under high vacuum ˜10 -5 torr on ultrasonically cleaned glass substrate. Using stating materials of various compositions of cadmium and selenium using formula Cd 1- x Se x where x is orbitory constant having value 0.20≤ x ≤0.40 here we take less value of x for the creation of anion vacancy in thin films. In present work the structural properties have been studies using XRD technique and found that starting materials and thin films both are polycrystalline in nature having hexagonal structure. Here we study the effect of composition ratio Cd/Se in starting material and its prepared thin films on its grain size and lattice parameter. From the analysis of X-Ray diffractogram found that lattice parameter and grain size both are decreases with increasing Cd/Se ratio in thin films as well as in starting material the preferred orientation in thin films along (100) plane. The surface morphology was studied using SEM characterization and found that films are smooth and homogeneous. The films have been analysed for optical band gap and absorbed a direct band gap.

Doping induced c-axis oriented growth of transparent ZnO thin film

NASA Astrophysics Data System (ADS)

Mistry, Bhaumik V.; Joshi, U. S.

2018-04-01

c-Axis oriented In doped ZnO (IZO) transparent conducting thin films were optimized on glass substrate using sol gel spin coating method. The Indium content in ZnO was varied systematically and the structural parameters were studied. Along with the crystallographic properties, the optoelectronic and electrical properties of IZO thin films were investigated in detail. The IZO thin films revealed hexagonal wurtzite structure. It was found that In doping in ZnO promotes the c-axis oriented growth of the thin films deposited on amorphous substrate. The particle size of the IZO films were increase as doping content increases from 2% to 5%. The 2% In doped ZnO film show electrical resistivity of 0.11 Ω cm, which is far better than the reported value for ZnO thin film. Better than 75% average optical transmission was estimated in the wavelength range from 400-800 nm. Systematic variartions in the electron concentration and band gap was observed with increasing In doping. Note worthy finding is that, with suitable amount of In doping improves not only transparency and conductivity but also improves the preferred orientation of the oxide thin film.

Analysis of NiAlTa precipitates in beta-NiAl + 2 at. pct Ta alloy

NASA Technical Reports Server (NTRS)

Pathare, V.; Michal, G. M.; Vedula, K.; Nathal, M. V.

1987-01-01

Results are reported from experiments performed to identify the precipitates, and their orientation in the matrix, in a beta-NiAl alloy containing 2 at. pct. Ta after undergoing creep test at 1300 K. Test specimens formed by extruding hot powders were compressed at 1300 K for about 50 hr at a strain rate averaging 6/1 million per sec. The specimens were then thinned and examined under an electron microscope and by X-ray diffractometry. An intermetallic NiAlTa compound with a hexagonal Cl4 structure appeared as second phase precipitates in the samples, exhibiting plate-like shapes and a habit plane close to (012). The prism planes of the hexagonal NiAlTa precipitates paralleled the closest packed planes in the cubic beta-NiAl matrix.

Heat pipe nuclear reactor for space power

NASA Technical Reports Server (NTRS)

Koening, D. R.

1976-01-01

A heat-pipe-cooled nuclear reactor has been designed to provide 3.2 MWth to an out-of-core thermionic conversion system. The reactor is a fast reactor designed to operate at a nominal heat-pipe temperature of 1675 K. Each reactor fuel element consists of a hexagonal molybdenum block which is bonded along its axis to one end of a molybdenum/lithium-vapor heat pipe. The block is perforated with an array of longitudinal holes which are loaded with UO2 pellets. The heat pipe transfers heat directly to a string of six thermionic converters which are bonded along the other end of the heat pipe. An assembly of 90 such fuel elements forms a hexagonal core. The core is surrounded by a thermal radiation shield, a thin thermal neutron absorber, and a BeO reflector containing boron-loaded control drums.

Viability and proliferation of endothelial cells upon exposure to GaN nanoparticles

PubMed Central

Braniste, Tudor; Tiginyanu, Ion; Horvath, Tibor; Raevschi, Simion; Cebotari, Serghei; Lux, Marco; Haverich, Axel

2016-01-01

Summary Nanotechnology is a rapidly growing and promising field of interest in medicine; however, nanoparticle–cell interactions are not yet fully understood. The goal of this work was to examine the interaction between endothelial cells and gallium nitride (GaN) semiconductor nanoparticles. Cellular viability, adhesion, proliferation, and uptake of nanoparticles by endothelial cells were investigated. The effect of free GaN nanoparticles versus the effect of growing endothelial cells on GaN functionalized surfaces was examined. To functionalize surfaces with GaN, GaN nanoparticles were synthesized on a sacrificial layer of zinc oxide (ZnO) nanoparticles using hydride vapor phase epitaxy. The uptake of GaN nanoparticles by porcine endothelial cells was strongly dependent upon whether they were fixed to the substrate surface or free floating in the medium. The endothelial cells grown on surfaces functionalized with GaN nanoparticles demonstrated excellent adhesion and proliferation, suggesting good biocompatibility of the nanostructured GaN. PMID:27826507

Growth process for gallium nitride porous nanorods

DOEpatents

Wildeson, Isaac Harshman; Sands, Timothy David

2015-03-24

A GaN nanorod and formation method. Formation includes providing a substrate having a GaN film, depositing SiN.sub.x on the GaN film, etching a growth opening through the SiN.sub.x and into the GaN film, growing a GaN nanorod through the growth opening, the nanorod having a nanopore running substantially through its centerline. Focused ion beam etching can be used. The growing can be done using organometallic vapor phase epitaxy. The nanopore diameter can be controlled using the growth opening diameter or the growing step duration. The GaN nanorods can be removed from the substrate. The SiN.sub.x layer can be removed after the growing step. A SiO.sub.x template can be formed on the GaN film and the GaN can be grown to cover the SiO.sub.x template before depositing SiN.sub.x on the GaN film. The SiO.sub.x template can be removed after growing the nanorods.

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

PubMed

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

2017-12-12

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

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

PubMed Central

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

2017-01-01

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

Growth of ternary CdxZn1-xO thin films in oxygen ambient using pulsed laser deposition

NASA Astrophysics Data System (ADS)

Sharma, Sugandha; Saini, Basant; Kaur, Ravinder; Gupta, Vinay; Tomar, Monika; Kapoor, Avinashi

2018-05-01

This study reports the growth of cadmium alloyed zinc (CdxZn1-xO) oxide thin films using pulsed laser deposition. The films are deposited on Corning glass substrates at different oxygen pressures of 5, 20, and 40 mTorr. High resolution X-ray diffraction studies reveal mixed phase (hexagonal and cubic) for films deposited at 20 and 40 mTorr, while a cubic phase for film deposited at 5 mTorr pressure. Optical transmittance studies indicate red-shifting of transmission edge as oxygen pressure decreases to 5 mTorr from 20 mTorr, hinting at a possible increase in cadmium content in thin films. Minimum band gap energy is obtained at growth pressure of 5 mTorr. Resistivity measurements have been performed using Hall effect measurement set up at 298 K.

Controlled growth of hexagonal gold nanostructures during thermally induced self-assembling on Ge(001) surface

PubMed Central

Jany, B. R.; Gauquelin, N.; Willhammar, T.; Nikiel, M.; van den Bos, K. H. W.; Janas, A.; Szajna, K.; Verbeeck, J.; Van Aert, S.; Van Tendeloo, G.; Krok, F.

2017-01-01

Nano-sized gold has become an important material in various fields of science and technology, where control over the size and crystallography is desired to tailor the functionality. Gold crystallizes in the face-centered cubic (fcc) phase, and its hexagonal closed packed (hcp) structure is a very unusual and rare phase. Stable Au hcp phase has been reported to form in nanoparticles at the tips of some Ge nanowires. It has also recently been synthesized in the form of thin graphene-supported sheets which are unstable under electron beam irradiation. Here, we show that stable hcp Au 3D nanostructures with well-defined crystallographic orientation and size can be systematically created in a process of thermally induced self-assembly of thin Au layer on Ge(001) monocrystal. The Au hcp crystallite is present in each Au nanostructure and has been characterized by different electron microscopy techniques. We report that a careful heat treatment above the eutectic melting temperature and a controlled cooling is required to form the hcp phase of Au on a Ge single crystal. This new method gives scientific prospects to obtain stable Au hcp phase for future applications in a rather simple manner as well as redefine the phase diagram of Gold with Germanium. PMID:28195226

Controlled growth of hexagonal gold nanostructures during thermally induced self-assembling on Ge(001) surface

NASA Astrophysics Data System (ADS)

Jany, B. R.; Gauquelin, N.; Willhammar, T.; Nikiel, M.; van den Bos, K. H. W.; Janas, A.; Szajna, K.; Verbeeck, J.; van Aert, S.; van Tendeloo, G.; Krok, F.

2017-02-01

Nano-sized gold has become an important material in various fields of science and technology, where control over the size and crystallography is desired to tailor the functionality. Gold crystallizes in the face-centered cubic (fcc) phase, and its hexagonal closed packed (hcp) structure is a very unusual and rare phase. Stable Au hcp phase has been reported to form in nanoparticles at the tips of some Ge nanowires. It has also recently been synthesized in the form of thin graphene-supported sheets which are unstable under electron beam irradiation. Here, we show that stable hcp Au 3D nanostructures with well-defined crystallographic orientation and size can be systematically created in a process of thermally induced self-assembly of thin Au layer on Ge(001) monocrystal. The Au hcp crystallite is present in each Au nanostructure and has been characterized by different electron microscopy techniques. We report that a careful heat treatment above the eutectic melting temperature and a controlled cooling is required to form the hcp phase of Au on a Ge single crystal. This new method gives scientific prospects to obtain stable Au hcp phase for future applications in a rather simple manner as well as redefine the phase diagram of Gold with Germanium.

Low resistivity and low compensation ratio Ga-doped ZnO films grown by plasma-assisted molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Chen, Cheng-Yu; Hsiao, Li-Han; Chyi, Jen-Inn

2015-09-01

In this study, Ga-doped ZnO (GZO) thin films were deposited on GaN templates by using plasma-assisted molecular beam epitaxy. To obtain low resistivity GZO films, in-situ post-annealing under Zn overpressure was carried out to avoid the generation of acceptor-liked Zn vacancies. The resultant films showed optical transparency over 95% in the visible spectral range. By reducing the acceptor-like defects, GZO films with compensation ratio near 0.4 and resistivity simultaneously lower than 1×10-4 Ω cm have been successfully demonstrated.

Kinetic-limited etching of magnesium doping nitrogen polar GaN in potassium hydroxide solution

NASA Astrophysics Data System (ADS)

Jiang, Junyan; Zhang, Yuantao; Chi, Chen; Yang, Fan; Li, Pengchong; Zhao, Degang; Zhang, Baolin; Du, Guotong

2016-01-01

KOH based wet etchings were performed on both undoped and Mg-doped N-polar GaN films grown by metal-organic chemical vapor deposition. It is found that the etching rate for Mg-doped N-polar GaN gets slow obviously compared with undoped N-polar GaN. X-ray photoelectron spectroscopy analysis proved that Mg oxide formed on N-polar GaN surface is insoluble in KOH solution so that kinetic-limited etching occurs as the etching process goes on. The etching process model of Mg-doped N-polar GaN in KOH solution is tentatively purposed using a simplified ideal atomic configuration. Raman spectroscopy analysis reveals that Mg doping can induce tensile strain in N-polar GaN films. Meanwhile, p-type N-polar GaN film with a hole concentration of 2.4 ÿ 1017 cm⿿3 was obtained by optimizing bis-cyclopentadienyl magnesium flow rates.

GaN epitaxial layers grown on multilayer graphene by MOCVD

NASA Astrophysics Data System (ADS)

Li, Tianbao; Liu, Chenyang; Zhang, Zhe; Yu, Bin; Dong, Hailiang; Jia, Wei; Jia, Zhigang; Yu, Chunyan; Gan, Lin; Xu, Bingshe

2018-04-01

In this study, GaN epitaxial layers were successfully deposited on a multilayer graphene (MLG) by using metal-organic chemical vapor deposition (MOCVD). Highly crystalline orientations of the GaN films were confirmed through electron backscatter diffraction (EBSD). An epitaxial relationship between GaN films and MLG is unambiguously established by transmission electron microscope (TEM) analysis. The Raman spectra was used to analyze the internal stress of GaN films, and the spectrum shows residual tensile stress in the GaN films. Moreover, the results of the TEM analysis and Raman spectra indicate that the high quality of the MLG substrate is maintained even after the growth of the GaN film. This high-quality MLG makes it possible to easily remove epitaxial layers from the supporting substrate by micro-mechanical exfoliation technology. This work can aid in the development of transferable devices using GaN films.

The origin of the residual conductivity of GaN films on ferroelectric materials

NASA Astrophysics Data System (ADS)

Lee, Kyoung-Keun; Cai, Zhuhua; Ziemer, Katherine; Doolittle, William Alan

2009-08-01

In this paper, the origin of the conductivity of GaN films grown on ferroelectric materials was investigated using XPS, AES, and XRD analysis tools. Depth profiles confirmed the existence of impurities in the GaN film originating from the substrates. Bonding energy analysis from XPS and AES verified that oxygen impurities from the substrates were the dominant origin of the conductivity of the GaN film. Furthermore, Ga-rich GaN films have a greater chance of enhancing diffusion of lithium oxide from the substrates, resulting in more substrate phase separation and a wider inter-mixed region confirmed by XRD. Therefore, the direct GaN film growth on ferroelectric materials causes impurity diffusion from the substrates, resulting in highly conductive GaN films. Future work needs to develop non-conductive buffer layers for impurity suppression in order to obtain highly resistive GaN films.

Electrical properties of polycrystalline GaN films functionalized with cysteine and stabilization of GaN nanoparticles in aqueous media.

PubMed

Arízaga, Gregorio Guadalupe Carbajal; Oviedo, Mariana J; López, Oscar Edel Contreras

2012-10-01

GaN was synthesized onto sapphire substrates by chemical vapor deposition, reacting gallium, ammonium chloride and ammonia. The polycrystalline films were immersed in glycine, aspartic acid and cysteine solutions. Cysteine chemisorbed onto GaN films produced detectable changes in conductivity, mobility and Hall coefficient indicating that GaN is capable of detecting and reacting with thiolate groups, which was confirmed by X-ray photoelectron spectroscopy. The Cys-GaN film solution was adjusted to pH 10, upon which the GaN nanoparticles were transferred to the aqueous phase forming a suspension stable for seven days. The alkaline colloid was then further adjusted down to pH 3 retaining stability for three days. The GaN colloid obtained represents a suitable medium to study GaN properties for biological applications. Copyright © 2012 Elsevier B.V. All rights reserved.

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.

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

PubMed

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

2016-03-07

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

High-quality GaN epitaxially grown on Si substrate with serpentine channels

NASA Astrophysics Data System (ADS)

Wei, Tiantian; Zong, Hua; Jiang, Shengxiang; Yang, Yue; Liao, Hui; Xie, Yahong; Wang, Wenjie; Li, Junze; Tang, Jun; Hu, Xiaodong

2018-06-01

A novel serpentine-channeled mask was introduced to Si substrate for low-dislocation GaN epitaxial growth and the fully coalesced GaN film on the masked Si substrate was achieved for the first time. Compared with the epitaxial lateral overgrowth (ELOG) growth method, this innovative mask only requires one-step epitaxial growth of GaN which has only one high-dislocation region per mask opening. This new growth method can effectively reduce dislocation density, thus improving the quality of GaN significantly. High-quality GaN with low dislocation density ∼2.4 × 107 cm-2 was obtained, which accounted for about eighty percent of the GaN film in area. This innovative technique is promising for the growth of high-quality GaN templates and the subsequent fabrication of high-performance GaN-based devices like transistors, laser diodes (LDs), and light-emitting diodes (LEDs) on Si substrate.

Structural and chemical characterization of terbia thin films grown on hexagonally close packed metal substrates

NASA Astrophysics Data System (ADS)

Cartas, William

Rare earth oxides (REOs) exhibit favorable catalytic performance for a diverse set of chemical transformations, including both partial and complete oxidation reactions. I will discuss our efforts to develop thin film systems of terbia for model surface science investigations of a REO that is effectively reducible, and which is thus expected to promote complete oxidation chemistry of adsorbed species. The growth of terbia on Cu(111) is shown to produce a complex surface that exhibits multiple phases of the oxide as well as exposed substrate. Growing the film on Pt(111) results in more uniform, single phase, and closed film. We used low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to characterize the structural properties of terbia thin films grown on Pt(111) in ultrahigh vacuum (UHV) using physical vapor deposition. We find that the REO grows as a high quality Tb2O 3(111) film, and adopts oxygen-deficient fluorite structures wherein the metal cations form a hexagonal lattice in registry with the Pt(111) substrate, while oxygen vacancies are randomly distributed within the film. The Tb 2O3(111) films are thermally stable when heated to 1000 K in UHV. LEED and STM show that a fraction of the Tb2O3 forms hexagonal islands when first deposited, and further depositions typically result in three dimensional growth of the film. The Tb2O3 (111) / Pt(111) system produces a coincidence structure, seen very clearly in LEED images. We have also found that Tb2O3(111) films can be oxidized in UHV by exposure to plasma-generated atomic oxygen beams. The oxidized films have an estimated TbO2 stoichiometry and decompose to Tb2O3 during heating, with O2 desorption starting at about 500 K. Terbia films oxidized at 90 K show a weakly bound state of oxygen that is likely chemisorbed. Temperature programmed reaction spectroscopy (TPRS) studies using methanol show that increased oxygen in the film does not modify the chemical selectivity of the film; however, the increased oxygen content does increase the activity of the film toward methanol dehydrogenation. We have found that when methanol is adsorbed onto the terbia-Pt(111) system, it reacts to form formaldehyde and water and reduces the surface. The development of high-quality terbia thin films on Pt(111) provides new opportunities to investigate oxidation chemistry on an REO that has distinct reduction and oxidation properties.

Effects of bacteria on CdS thin films used in technological devices

NASA Astrophysics Data System (ADS)

Alpdoğan, S.; Adıgüzel, A. O.; Sahan, B.; Tunçer, M.; Metin Gubur, H.

2017-04-01

Cadmium sulfide (CdS) thin films were fabricated on glass substrates by the chemical bath deposition method at 70 {}^\\circ \\text{C} considering deposition times ranging from 2 h to 5 h. The optical band gaps of CdS thin films were found to be in the 2.42-2.37 eV range. CdS thin films had uniform spherical nano-size grains which had polycrystalline, hexagonal and cubic phases. The films had a characteristic electrical resistivity of the order of {{10}5} Ω \\text{cm} and n-type conductivity at room condition. CdS thin films were incubated in cultures of B.domonas aeruginosa and Staphylococcus aureus, which exist abundantly in the environment, and form biofilms. SEM images showed that S. aureus and K. pneumonia were detected significantly on the film surfaces with a few of P. aeruginosa and B. subtilis cells attached. CdS thin film surface exhibits relatively good resistance to the colonization of P. aeruginosa and B. subtilis. Optical results showed that the band gap of CdS thin films which interacted with the bacteria is 2.42 \\text{eV} . The crystal structure and electrical properties of CdS thin films were not affected by bacterial adhesion. The antimicrobial effect of CdS nanoparticles was different for different bacterial strains.

Nanorods on surface of GaN-based thin-film LEDs deposited by post-annealing after photo-assisted chemical etching.

PubMed

Chen, Lung-Chien; Lin, Wun-Wei; Liu, Te-Yu

2017-12-01

This study investigates the optoelectronic characteristics of gallium nitride (GaN)-based thin-film light-emitting diodes (TF-LEDs) that are formed by a two-step transfer process that involves wet etching and post-annealing. In the two-step transfer process, GaN LEDs were stripped from sapphire substrates by the laser lift-off (LLO) method using a KrF laser and then transferred onto ceramic substrates. Ga-K nanorods were formed on the surface of the GaN-based TF-LEDs following photo-assisted chemical etching and photo-enhanced post-annealing at 100 °C for 1 min. As a result, the light output power of GaN-based TF-LEDs with wet etching and post-annealing was over 72% more than that of LEDs that did not undergo these treatments.

Nanowire growth from the viewpoint of the thin film polylayer growth theory

NASA Astrophysics Data System (ADS)

Kashchiev, Dimo

2018-03-01

The theory of polylayer growth of thin solid films is employed for description of the growth kinetics of single-crystal nanowires. Expressions are derived for the dependences of the height h and radius r of a given nanowire on time t, as well as for the h(r) dependence. These dependences are applicable immediately after the nanowire nucleation on the substrate and thus include the period during which the nucleated nanowire changes its shape from that of cap to that of column. The analysis shows that the nanowire cap-to-column shape transition is continuous and makes it possible to kinetically define the nanowire shape-transition radius by means of the nanowire radial and axial growth rates. The obtained h(t), r(t) and h(r) dependences are found to provide a good description of available experimental data for growth of self-nucleated GaN nanowires by the vapor-solid mechanism.

Sum-Frequency Generation from a Thin Cylindrical Layer

NASA Astrophysics Data System (ADS)

Shamyna, A. A.; Kapshai, V. N.

2018-01-01

In the Rayleigh-Gans-Debye approximation, we have solved the problem of the sum-frequency generation by two plane elliptically polarized electromagnetic waves from the surface of a dielectric particle of a cylindrical shape that is coated by a thin layer possessing nonlinear optical properties. The formulas that describe the sum-frequency field have been presented in the tensor and vector forms for the second-order nonlinear dielectric susceptibility tensor, which was chosen in the general form, containing chiral components. Expressions describing the sum-frequency field from the cylindrical particle ends have been obtained for the case of a nonlinear layer possessing chiral properties. Three-dimensional directivity patterns of the sum-frequency radiation have been analyzed for different combinations of parameters (angles of incidence, degrees of ellipticity, orientations of polarization ellipses, cylindrical particle dimensions). The mathematical properties of the spatial distribution functions of the sum-frequency field, which characterize the symmetry of directivity patterns, have been revealed.

Deep-level traps in lightly Si-doped n-GaN on free-standing m-oriented GaN substrates

NASA Astrophysics Data System (ADS)

Yamada, H.; Chonan, H.; Takahashi, T.; Yamada, T.; Shimizu, M.

2018-04-01

In this study, we investigated the deep-level traps in Si-doped GaN epitaxial layers by metal-organic chemical vapor deposition on c-oriented and m-oriented free-standing GaN substrates. The c-oriented and m-oriented epitaxial layers, grown at a temperature of 1000 °C and V/III ratio of 1000, contained carbon atomic concentrations of 1.7×1016 and 4.0×1015 cm-3, respectively. A hole trap was observed at about 0.89 eV above the valence band maximum by minority carrier transient spectroscopy. The trap concentrations in the c-oriented and m-oriented GaN epitaxial layers were consistent with the carbon atomic concentrations from secondary ion mass spectroscopy and the yellow luminescence intensity at 2.21 eV from photoluminescence. The trap concentrations in the m-oriented GaN epitaxial layers were lower than those in the c-oriented GaN. Two electron traps, 0.24 and 0.61 eV below the conduction band (EC) minimum, were observed in the c-oriented GaN epitaxial layer. In contrast, the m-oriented GaN epitaxial layer was free from the electron trap at EC - 0.24 eV, and the trap concentration at EC - 0.61 eV in the m-oriented GaN epitaxial layer was lower than that in the c-oriented GaN epitaxial layer. The m-oriented GaN epitaxial layer exhibited fewer hole and electron traps compared to the c-oriented GaN epitaxial layers.

Effect of Si, Mg, and Mg Zn doping on structural properties of a GaN layer grown by metalorganic chemical vapor deposition

NASA Astrophysics Data System (ADS)

Cho, H. K.; Lee, J. Y.; Kim, K. S.; Yang, G. M.

2001-12-01

We have studied the structural properties of undoped, Si-doped, Mg-doped, and Mg-Zn codoped GaN using high-resolution X-ray diffraction (HRXRD) and transmission electron microscopy. When compared with undoped GaN, the dislocation density at the surface of the GaN layer decreases with Si doping and increases with Mg doping. In addition, we observed a reduction of dislocation density by codoping with Zn atoms in the Mg-doped GaN layer. The full width at half maximum of HRXRD shows that Si doping and Mg-Zn codoping improve the structural quality of the GaN layer as compared with undoped and Mg-doped GaN, respectively.

Formation of gallium nitride templates and freestanding substrates by hydride vapor phase epitaxy for homoepitaxial growth of III-nitride devices

NASA Astrophysics Data System (ADS)

Williams, Adrian Daniel

Gallium nitride (GaN) is a direct wide band gap semiconductor currently under heavy development worldwide due to interest in its applications in ultra-violet optoelectronics, power electronics, devices operating in harsh environments (high temperature or corrorsive), etc. While a number of devices have been demonstrated with this material and its related alloys, the unavailability of GaN substrates is seen as one of the current major bottlenecks to both material quality and device performance. This dissertation is concerned with the synthesis of high quality GaN substrates by the hydride vapor phase epitaxy method (HVPE). In this work, the flow of growth precursors in a home-built HVPE reactor was modeled by the Navier-Stokes equation and solved by finite element analysis to promote uniformity of GaN on 2'' sapphire substrates. Kinetics of growth was studied and various regimes of growth were identified to establish a methodology for HVPE GaN growth, independent of reactor geometry. GaN templates as well as bulk substrates were fabricated in this work. Realization of freestanding GaN substrates was achieved through discovery of a natural stress-induced method of separating bulk GaN from sapphire via mechanical failure of a low-temperature GaN buffer layer. Such a process eliminates the need for pre- or post-processing of sapphire substrates, as is currently the standard. Stress in GaN-on-sapphire is discussed, with the dominant contributor identified as thermal stress due to thermal expansion coefficient mismatch between the two materials. This thermal stress is analyzed using Stoney's equation and conditions for crack-free growth of thick GaN substrates were identified. An etch-back process for planarizing GaN templates was also developed and successfully applied to rough GaN templates. The planarization of GaN has been mainly addressed by chemo-mechanical polishing (CMP) methods in the literature, with notable shortcomings including the inability to effectively planarize gallium-polar GaN, the preferred growth plane for devices. The process developed in this work bypasses the constraints of CMP, allowing for the planarization of all surfaces of GaN irrespective of crystal orientation. The GaN samples grown for this dissertation were studied by various techniques to characterize their structural, optical, and electrical properties.

Synthesis and characterization of spin-coated ZnS thin films

NASA Astrophysics Data System (ADS)

Zaman, M. Burhanuz; Chandel, Tarun; Dehury, Kshetramohan; Rajaram, P.

2018-05-01

In this paper, we report synthesis of ZnS thin films using a sol-gel method. A unique aprotic solvent, dimethlysulphoxide (DMSO) has been used to obtain a homogeneous ZnS gel. Zinc acetate and thiourea were used as the precursor sources for Zn and S, respectively, to deposit nanocrystalline ZnS thin films. Optical, structural and morphological properties of the films were studied. Optical studies reveal high transmittance of the samples over the entire visible region. The energy band gap (Eg) for the ZnS thin films is found to be about 3.6 eV which matches with that of bulk ZnS. The interference fringes in transmissions spectrum show the high quality of synthesized samples. Strong photoluminescence peak in the UV region makes the films suitable for optoelectronic applications. X-ray diffraction studies reveal that sol-gel derived ZnS thin films are polycrystalline in nature with hexagonal structure. SEM studies confirmed that the ZnS films show smooth and uniform grains morphology having size in 20-25 nm range. The EDAX studies confirmed that the films are nearly stoichiometric.

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

PubMed

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

2017-08-17

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

Edge-on dislocation loop in anisotropic hcp zirconium thin foil

NASA Astrophysics Data System (ADS)

Wu, Wenwang; Xia, Re; Qian, Guian; Xu, Shucai; Zhang, Jinhuan

2015-10-01

Edge-on dislocation loops with 〈 a 〉 -type and 〈 c 〉 -type of Burgers vectors can be formed on prismatic or basel habit planes of hexagonal close-packed (hcp) zirconium alloys during in-situ ion irradiation and neutron irradiation experiments. In this work, an anisotropic image stress method was employed to analyze the free surface effects of dislocation loops within hcp Zr thin foils. Calculation results demonstrate that image stress has a remarkable effect on the distortion fields of dislocation loops within infinite medium, and the image energy becomes remarkable when dislocation loops are situated close to the free surfaces. Moreover, image forces of the 1 / 2 〈 0001 〉 (0001) dislocation loop within (0001) thin foil is much stronger than that of the 1 / 3 〈 11 2 bar 0 〉 (11 2 bar 0) dislocation loop within (11 2 bar 0) thin foil of identical geometrical configurations. Finally, image stress effect on the physical behaviors of loops during in-situ ion irradiation experiments is discussed.

Raman spectroscopy of ZnMnO thin films grown by pulsed laser deposition

NASA Astrophysics Data System (ADS)

Orozco, S.; Riascos, H.; Duque, S.

2016-02-01

ZnMnO thin films were grown by Pulsed Laser Deposition (PLD) technique onto Silicon (100) substrates at different growth conditions. Thin films were deposited varying Mn concentration, substrate temperature and oxygen pressure. ZnMnO samples were analysed by using Raman Spectroscopy that shows a red shift for all vibration modes. Raman spectra revealed that nanostructure of thin films was the same of ZnO bulk, wurzite hexagonal structure. The structural disorder was manifested in the line width and shape variations of E2(high) and E2(low) modes located in 99 and 434cm-1 respectively, which may be due to the incorporation of Mn ions inside the ZnO crystal lattice. Around 570cm-1 was found a peak associated to E1(LO) vibration mode of ZnO. 272cm-1 suggest intrinsic host lattice defects. Additional mode centred at about 520cm-1 can be overlap of Si and Mn modes.

Vapor-solid growth of one-dimensional layer-structured gallium sulfide nanostructures.

PubMed

Shen, Guozhen; Chen, Di; Chen, Po-Chiang; Zhou, Chongwu

2009-05-26

Gallium sulfide (GaS) is a wide direct bandgap semiconductor with uniform layered structure used in photoelectric devices, electrical sensors, and nonlinear optical applications. We report here the controlled synthesis of various high-quality one-dimensional GaS nanostructures (thin nanowires, nanobelts, and zigzag nanobelts) as well as other kinds of GaS products (microbelts, hexagonal microplates, and GaS/Ga(2)O(3) heterostructured nanobelts) via a simple vapor-solid method. The morphology and structures of the products can be easily controlled by substrate temperature and evaporation source. Optical properties of GaS thin nanowires and nanobelts were investigated and both show an emission band centered at 580 nm.

Temperature dependent growth of GaN nanowires using CVD technique

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

Kumar, Mukesh, E-mail: mukeshjihrnp@gmail.com; Singh, R.; Kumar, Vikram

2016-05-23

Growth of GaN nanowires have been carried out on sapphire substrates with Au as a catalyst using chemical vapour deposition technique. GaN nanowires growth have been studied with the experimental parameter as growth temperature. Diameter of grown GaN nanowires are in the range of 50 nm to 100 nm while the nanowire length depends on growth temperature. Morphology of the GaN nanowires have been studied by scanning electron microscopy. Crystalline nature has been observed by XRD patterns. Optical properties of grown GaN nanowires have been investigated by photoluminescence spectra.

Dislocation-induced nanoparticle decoration on a GaN nanowire.

PubMed

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

2015-02-04

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

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

NASA Astrophysics Data System (ADS)

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

2018-05-01

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

Cubic crystalline erbium oxide growth on GaN(0001) by atomic layer deposition

NASA Astrophysics Data System (ADS)

Chen, Pei-Yu; Posadas, Agham B.; Kwon, Sunah; Wang, Qingxiao; Kim, Moon J.; Demkov, Alexander A.; Ekerdt, John G.

2017-12-01

Growth of crystalline Er2O3, a rare earth sesquioxide, on GaN(0001) is described. Ex situ HCl and NH4OH solutions and an in situ N2 plasma are used to remove impurities on the GaN surface and result in a Ga/N stoichiometry of 1.02. Using atomic layer deposition with erbium tris(isopropylcyclopentadienyl) [Er(iPrCp)3] and water, crystalline cubic Er2O3 (C-Er2O3) is grown on GaN at 250 °C. The orientation relationships between the C-Er2O3 film and the GaN substrate are C-Er2O3(222) ǁ GaN(0001), C-Er2O3⟨-440⟩ ǁ GaN ⟨11-20⟩, and C-Er2O3⟨-211⟩ ǁ GaN ⟨1-100⟩. Scanning transmission electron microscopy and electron energy loss spectroscopy are used to examine the microstructure of C-Er2O3 and its interface with GaN. With post-deposition annealing at 600 °C, a thicker interfacial layer is observed, and two transition layers, crystalline GaNwOz and crystalline GaErxOy, are found between GaN and C-Er2O3. The tensile strain in the C-Er2O3 film is studied with x-ray diffraction by changes in both out-of-plane and in-plane d-spacing. Fully relaxed C-Er2O3 films on GaN are obtained when the film thickness is around 13 nm. Additionally, a valence band offset of 0.7 eV and a conduction band offset of 1.2 eV are obtained using x-ray photoelectron spectroscopy.

Quantitative Scanning Transmission Electron Microscopy of Electronic and Nanostructured Materials

NASA Astrophysics Data System (ADS)

Yankovich, Andrew B.

Electronic and nanostructured materials have been investigated using advanced scanning transmission electron microscopy (STEM) techniques. The first topic is the microstructure of Ga and Sb-doped ZnO. Ga-doped ZnO is a candidate transparent conducting oxide material. The microstructure of GZO thin films grown by MBE under different growth conditions and different substrates were examined using various electron microscopy (EM) techniques. The microstructure, prevalent defects, and polarity in these films strongly depend on the growth conditions and substrate. Sb-doped ZnO nanowires have been shown to be the first route to stable p-type ZnO. Using Z-contrast STEM, I have showed that an unusual microstructure of Sb-decorated head-to-head inversion domain boundaries and internal voids contain all the Sb in the nanowires and cause the p-type conduction. InGaN thin films and InGaN / GaN quantum wells (QW) for light emitting diodes are the second topic. Low-dose Z-contrast STEM, PACBED, and EDS on InGaN QW LED structures grown by MOCVD show no evidence for nanoscale composition variations, contradicting previous reports. In addition, a new extended defect in GaN and InGaN was discovered. The defect consists of a faceted pyramid-shaped void that produces a threading dislocation along the [0001] growth direction, and is likely caused by carbon contamination during growth. Non-rigid registration (NRR) and high-precision STEM of nanoparticles is the final topic. NRR is a new image processing technique that corrects distortions arising from the serial nature of STEM acquisition that previously limited the precision of locating atomic columns and counting the number of atoms in images. NRR was used to demonstrate sub-picometer precision in STEM images of single crystal Si and GaN, the best achieved in EM. NRR was used to measure the atomic surface structure of Pt nanoacatalysts and Au nanoparticles, which revealed new bond length variation phenomenon of surface atoms. In addition, NRR allowed for measuring the 3D atomic structure of the nanoparticles with less than 1 atom uncertainty, a long-standing problem in EM. Finally, NRR was adapted to EDS spectrum images, significantly enhancing the signal to noise ratio and resolution of an EDS spectrum image of Ca-doped NdTiO3 compared to conventional methods.

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

NASA Astrophysics Data System (ADS)

Huang, Jingyi

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

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)

On the solubility of gallium nitride in supercritical ammonia-sodium solutions

NASA Astrophysics Data System (ADS)

Griffiths, Steven; Pimputkar, Siddha; Speck, James S.; Nakamura, Shuji

2016-12-01

Due to the disparity between observed gallium nitride (GaN) growth under conditions for which literature reports normal solubility, GaN solubility in supercritical NH3-Na containing solutions was re-evaluated. Isothermal gravimetric experiments on polycrystalline GaN were performed in the temperature range (T =415-650 °C) for which retrograde growth of GaN routinely occurs (P ≈ 200 MPa, molar NH3:Na fill ratio =20:1). Two previously-unreported error contributions to the gravimetric determination of GaN solubility were identified: Ga-alloying of exposed Ni-containing components, and the presence of a dense, Ga-absorbing Na-rich, second phase under these conditions. Due to the inability to measure Ga-alloying of the exposed autoclave wall for each experiment, considerable scatter was introduced in the refined GaN solubility curve. No clear dependence of GaN solubility on temperature was resolvable, while most solubility values were determined to be within a band of 0.03-0.10 mol% GaN, normalized by fill NH3.

Edge enhanced growth induced shape transition in the formation of GaN nanowall network

NASA Astrophysics Data System (ADS)

Nayak, Sanjay; Kumar, Rajendra; Shivaprasad, S. M.

2018-01-01

We address the mechanism of early stages of growth and shape transition of the unique nanowall network (NwN) of GaN by experimentally monitoring its morphological evolution and complementing it by first-principles calculations. Using atomic force and scanning electron microscopy, we observe the formation of oval shaped islands at very early stages of the growth which later transformed into tetrahedron shaped (3 faced pyramid) islands. These tetrahedron shaped islands further grow anisotropically along their edges of the (20 2 ¯ 1) facets to form the wall-like structure as the growth proceeds. The mechanism of this crystal growth is discussed in light of surface free energies of the different surfaces, adsorption energy, and diffusion barrier of Ga ad-atoms on the (20 2 ¯ 1) facets. By first-principles calculations, we find that the diffusion barrier of ad-atoms reduces with decreasing width of facets and is responsible for the anisotropic growth leading to the formation of NwN. This study suggests that formation of NwN is an archetype example of structure dependent attachment kinetic instability induced shape transition in thin film growth.

Use of a bilayer lattice-matched AlInGaN barrier for improving the channel carrier confinement of enhancement-mode AlInGaN/GaN hetero-structure field-effect transistors

NASA Astrophysics Data System (ADS)

Rahbardar Mojaver, Hassan; Gosselin, Jean-Lou; Valizadeh, Pouya

2017-06-01

A quaternary lattice-matched layer structure based on employing a bilayer barrier for improving the carrier confinement in the channel of enhancement-mode metal-face c-plane wurtzite AlInGaN/GaN hetero-structure field effect transistors (HFETs) is for the first time proposed. Using the commercial self-consistent Poisson-Schrödinger solver Nextnano, electronic properties of the proposed hetero-structure, including the sheet charge density and carrier confinement on the GaN side of the hetero-interface, are evaluated. Based on these evaluations, it is shown that while the proposed layer structure substantially improves the carrier confinement in the GaN channel layer, it also upholds the merits of employing a lattice-matched barrier towards achieving an enhancement-mode operation (i.e., in the absence of the piezoelectric effect). According to these simulations, in terms of maintaining the required positive threshold-voltage for the enhancement-mode operation, it is also shown that the proposed layer structure substantially outperforms the quaternary AlInGaN/GaN HFETs employing a thin AlN spacer layer.

Microstructural response of InGaN to swift heavy ion irradiation

NASA Astrophysics Data System (ADS)

Zhang, L. M.; Jiang, W.; Fadanelli, R. C.; Ai, W. S.; Peng, J. X.; Wang, T. S.; Zhang, C. H.

2016-12-01

A monocrystalline In0.18Ga0.82N film of ∼275 nm in thickness grown on a GaN/Al2O3 substrate was irradiated with 290 MeV 238U32+ ions to a fluence of 1.2 × 1012 cm-2 at room temperature. The irradiated sample was characterized using helium ion microscopy (HIM), Rutherford backscattering spectrometry under ion-channeling conditions (RBS/C), and high-resolution X-ray diffraction (HRXRD). The irradiation leads to formation of ion tracks throughout the thin In0.18Ga0.82N film and the 3.0 μm thick GaN buffer layer. The mean diameter of the tracks in In0.18Ga0.82N is ∼9 nm, as determined by HIM examination. Combination of the HIM and RBS/C data suggests that the In0.18Ga0.82N material in the track is likely to be highly disordered or fully amorphized. The irradiation induced lattice relaxation in In0.18Ga0.82N and a distribution of d-spacing of the (0 0 0 2) planes in GaN with lattice expansion are observed by HRXRD.

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.

Study of spin-ordering and spin-reorientation transitions in hexagonal manganites through Raman spectroscopy

PubMed Central

Chen, Xiang-Bai; Hien, Nguyen Thi Minh; Han, Kiok; Nam, Ji-Yeon; Huyen, Nguyen Thi; Shin, Seong-Il; Wang, Xueyun; Cheong, S. W.; Lee, D.; Noh, T. W.; Sung, N. H.; Cho, B. K.; Yang, In-Sang

2015-01-01

Spin-wave (magnon) scattering, when clearly observed by Raman spectroscopy, can be simple and powerful for studying magnetic phase transitions. In this paper, we present how to observe magnon scattering clearly by Raman spectroscopy, then apply the Raman method to study spin-ordering and spin-reorientation transitions of hexagonal manganite single crystal and thin films and compare directly with the results of magnetization measurements. Our results show that by choosing strong resonance condition and appropriate polarization configuration, magnon scattering can be clearly observed, and the temperature dependence of magnon scattering can be simple and powerful quantity for investigating spin-ordering as well as spin-reorientation transitions. Especially, the Raman method would be very helpful for investigating the weak spin-reorientation transitions by selectively probing the magnons in the Mn3+ sublattices, while leaving out the strong effects of paramagnetic moments of the rare earth ions. PMID:26300075

Effect of hexagonal patterned arrays and defect geometry on the critical current of superconducting films

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

Sadovskyy, I. A.; Wang, Y. L.; Xiao, Z. -L.

Understanding the effect of pinning on the vortex dynamics in superconductors is a key factor towards controlling critical current values. Large-scale simulations of vortex dynamics can provide a rational approach to achieve this goal. Here, we use the time-dependent Ginzburg-Landau equations to study thin superconducting films with artificially created pinning centers arranged periodically in hexagonal lattices. We calculate the critical current density for various geometries of the pinning centers—varying their size, strength, and density. Furthermore, we shed light upon the influence of pattern distortion on the magnetic-field-dependent critical current. We compare our result directly with available experimental measurements on patternedmore » molybdenum-germanium films, obtaining good agreement. In conclusion, our results give important systematic insights into the mechanisms of pinning in these artificial pinning landscapes and open a path for tailoring superconducting films with desired critical current behavior.« less

Singlet and triplet trions in WS2 monolayer encapsulated in hexagonal boron nitride.

PubMed

Vaclavkova, D; Wyzula, J; Nogajewski, K; Bartos, M; Slobodeniuk, A O; Faugeras, C; Potemski, M; Molas, M R

2018-08-10

Embedding a WS 2 monolayer in flakes of hexagonal boron nitride allowed us to resolve and study the photoluminescence response due to both singlet and triplet states of negatively charged excitons (trions) in this atomically thin semiconductor. The energy separation between the singlet and triplet states has been found to be relatively small reflecting rather weak effects of the electron-electron exchange interaction for the trion triplet in a WS 2 monolayer, which involves two electrons with the same spin but from different valleys. Polarization-resolved experiments demonstrate that the helicity of the excitation light is better preserved in the emission spectrum of the triplet trion than in that of the singlet trion. Finally, the singlet (intravalley) trions are found to be observable even at ambient conditions whereas the emission due to the triplet (intervalley) trions is only efficient at low temperatures.

Fabrication of hexagonal star-shaped and ring-shaped patterns arrays by Mie resonance sphere-lens-lithography

NASA Astrophysics Data System (ADS)

Liu, Xianchao; Wang, Jun; Li, Ling; Gou, Jun; Zheng, Jie; Huang, Zehua; Pan, Rui

2018-05-01

Mie resonance sphere-lens-lithography has proved to be a good candidate for fabrication of large-area tunable surface nanopattern arrays. Different patterns on photoresist surface are obtained theoretically by adjusting optical coupling among neighboring spheres with different gap sizes. The effect of light reflection from the substrate on the pattern produced on the photoresist with a thin thickness is also discussed. Sub-micron hexagonal star-shaped and ring-shaped patterns arrays are achieved with close-packed spheres arrays and spheres arrays with big gaps, respectively. Changing of star-shaped vertices is induced by different polarization of illumination. Experimental results agree well with the simulation. By using smaller resonance spheres, sub-400 nm star-shaped and ring-shaped patterns can be realized. These tunable patterns are different from results of previous reports and have enriched pattern morphology fabricated by sphere-lens-lithography, which can find application in biosensor and optic devices.

Effect of hexagonal patterned arrays and defect geometry on the critical current of superconducting films

NASA Astrophysics Data System (ADS)

Sadovskyy, I. A.; Wang, Y. L.; Xiao, Z.-L.; Kwok, W.-K.; Glatz, A.

2017-02-01

Understanding the effect of pinning on the vortex dynamics in superconductors is a key factor towards controlling critical current values. Large-scale simulations of vortex dynamics can provide a rational approach to achieve this goal. Here, we use the time-dependent Ginzburg-Landau equations to study thin superconducting films with artificially created pinning centers arranged periodically in hexagonal lattices. We calculate the critical current density for various geometries of the pinning centers—varying their size, strength, and density. Furthermore, we shed light upon the influence of pattern distortion on the magnetic-field-dependent critical current. We compare our result directly with available experimental measurements on patterned molybdenum-germanium films, obtaining good agreement. Our results give important systematic insights into the mechanisms of pinning in these artificial pinning landscapes and open a path for tailoring superconducting films with desired critical current behavior.

Effect of hexagonal patterned arrays and defect geometry on the critical current of superconducting films

NASA Astrophysics Data System (ADS)

Sadovskyy, Ivan; Wang, Yonglei; Xiao, Zhili; Kwok, Wai-Kwong; Glatz, Andreas

Understanding the effect of pinning on the vortex dynamics in superconductors is a key factor towards controlling critical current values. Large-scale simulations of vortex dynamics can provide a rational approach to achieve this goal. Here, we use the time-dependent Ginzburg-Landau equations to study thin superconducting films with artificially created pinning centers arranged periodically in hexagonal lattices. We calculate the critical current density for various geometries of the pinning centers - varying their size, strength, and density. Furthermore, we shed light upon the influence of pattern distortion on the magnetic field dependent critical current. We compare our result directly with available experimental measurements on patterned molybdenum-germanium films, obtaining good agreement. Our results give important systematic insights into the mechanisms of pinning in these artificial pinning landscapes and open a path for tailoring superconducting films with desired critical current behavior.

Effect of hexagonal patterned arrays and defect geometry on the critical current of superconducting films

DOE PAGES

Sadovskyy, I. A.; Wang, Y. L.; Xiao, Z. -L.; ...

2017-02-07

Understanding the effect of pinning on the vortex dynamics in superconductors is a key factor towards controlling critical current values. Large-scale simulations of vortex dynamics can provide a rational approach to achieve this goal. Here, we use the time-dependent Ginzburg-Landau equations to study thin superconducting films with artificially created pinning centers arranged periodically in hexagonal lattices. We calculate the critical current density for various geometries of the pinning centers—varying their size, strength, and density. Furthermore, we shed light upon the influence of pattern distortion on the magnetic-field-dependent critical current. We compare our result directly with available experimental measurements on patternedmore » molybdenum-germanium films, obtaining good agreement. In conclusion, our results give important systematic insights into the mechanisms of pinning in these artificial pinning landscapes and open a path for tailoring superconducting films with desired critical current behavior.« less

Hexagonal photonic crystal waveguide based on barium titanate thin films

NASA Astrophysics Data System (ADS)

Li, Jianheng; Liu, Zhifu; Wessels, Bruce W.; Tu, Yongming; Ho, Seng-Tiong; Joshi-Imre, Alexandra; Ocola, Leonidas E.

2011-03-01

The simulation, fabrication and measurement of nonlinear photonic crystals (PhCs) with hexagonal symmetry in epitaxial BaTiO3 were investigated. The optical transmission properties of a PhC were simulated by a 2-D finite-difference time domain (FDTD) method. A complete bandgap exists for both the TE and TM optical modes. The fabricated PhC has a well-defined stop band over the spectral region of 1525 to 1575 nm. A microcavity structure was also fabricated by incorporation of a line defect in the PhC. Transmission of the microcavity structure over the spectral region from 1456 to 1584nm shows a well-defined 5 nm wide window at 1495nm. Simulations indicate that the phase velocity matched PhC microcavity device of 0.5 mm long can potentially serve as modulator with a 3 dB bandwidth of 4 THz.

Synthesis of ZnO Hexagonal Micro Discs on Glass Substrates Using the Spray Pyrolysis Technique

NASA Astrophysics Data System (ADS)

Ikhmayies, Shadia J.; Zbib, Mohamad B.

2017-07-01

Zinc oxide (ZnO) is an important transparent conducting oxide of potential use in solar cells, electronics, photoelectronics, and sensors. In this work ZnO micro discs were synthesized in thin film form on glass substrates using the low cost spray pyrolysis method. The films were prepared from a precursor solution of ZnCl2 in distilled water at a substrate temperature of 300 ± 5°C. The as-synthesized samples were analyzed with x-ray diffraction, scanning electron microscopy, and x-ray energy dispersive spectroscopy (EDS). The morphology of the films showed randomly distributed micro discs of hexagonal shape. The EDS reports showed that the films contained Cl and Fe. Size analysis was performed using ImageJ software, where the average diameter was found to be 4.8 ± 0.9 μm, and the average thickness was found to be 254 ± 43 nm.

Valence band offset of β-Ga2O3/wurtzite GaN heterostructure measured by X-ray photoelectron spectroscopy.

PubMed

Wei, Wei; Qin, Zhixin; Fan, Shunfei; Li, Zhiwei; Shi, Kai; Zhu, Qinsheng; Zhang, Guoyi

2012-10-10

A sample of the β-Ga2O3/wurtzite GaN heterostructure has been grown by dry thermal oxidation of GaN on a sapphire substrate. X-ray diffraction measurements show that the β-Ga2O3 layer was formed epitaxially on GaN. The valence band offset of the β-Ga2O3/wurtzite GaN heterostructure is measured by X-ray photoelectron spectroscopy. It is demonstrated that the valence band of the β-Ga2O3/GaN structure is 1.40 ± 0.08 eV.

Valence band offset of β-Ga2O3/wurtzite GaN heterostructure measured by X-ray photoelectron spectroscopy

PubMed Central

2012-01-01

A sample of the β-Ga2O3/wurtzite GaN heterostructure has been grown by dry thermal oxidation of GaN on a sapphire substrate. X-ray diffraction measurements show that the β-Ga2O3 layer was formed epitaxially on GaN. The valence band offset of the β-Ga2O3/wurtzite GaN heterostructure is measured by X-ray photoelectron spectroscopy. It is demonstrated that the valence band of the β-Ga2O3/GaN structure is 1.40 ± 0.08 eV. PMID:23046910

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

PubMed

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

2017-11-08

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

The GAN Exonuclease or the Flap Endonuclease Fen1 and RNase HII Are Necessary for Viability of Thermococcus kodakarensis.

PubMed

Burkhart, Brett W; Cubonova, Lubomira; Heider, Margaret R; Kelman, Zvi; Reeve, John N; Santangelo, Thomas J

2017-07-01

Many aspects of and factors required for DNA replication are conserved across all three domains of life, but there are some significant differences surrounding lagging-strand synthesis. In Archaea , a 5'-to-3' exonuclease, related to both bacterial RecJ and eukaryotic Cdc45, that associates with the replisome specifically through interactions with GINS was identified and designated GAN (for G INS- a ssociated n uclease). Despite the presence of a well-characterized flap endonuclease (Fen1), it was hypothesized that GAN might participate in primer removal during Okazaki fragment maturation, and as a Cdc45 homologue, GAN might also be a structural component of an archaeal CMG (Cdc45, MCM, and GINS) replication complex. We demonstrate here that, individually, either Fen1 or GAN can be deleted, with no discernible effects on viability and growth. However, deletion of both Fen1 and GAN was not possible, consistent with both enzymes catalyzing the same step in primer removal from Okazaki fragments in vivo RNase HII has also been proposed to participate in primer processing during Okazaki fragment maturation. Strains with both Fen1 and RNase HII deleted grew well. GAN activity is therefore sufficient for viability in the absence of both RNase HII and Fen1, but it was not possible to construct a strain with both RNase HII and GAN deleted. Fen1 alone is therefore insufficient for viability in the absence of both RNase HII and GAN. The ability to delete GAN demonstrates that GAN is not required for the activation or stability of the archaeal MCM replicative helicase. IMPORTANCE The mechanisms used to remove primer sequences from Okazaki fragments during lagging-strand DNA replication differ in the biological domains. Bacteria use the exonuclease activity of DNA polymerase I, whereas eukaryotes and archaea encode a flap endonuclease (Fen1) that cleaves displaced primer sequences. RNase HII and the GINS-associated exonuclease GAN have also been hypothesized to assist in primer removal in Archaea Here we demonstrate that in Thermococcus kodakarensis , either Fen1 or GAN activity is sufficient for viability. Furthermore, GAN can support growth in the absence of both Fen1 and RNase HII, but Fen1 and RNase HII are required for viability in the absence of GAN. Copyright © 2017 American Society for Microbiology.

Evaluation of Ceramic Honeycomb Core Compression Behavior at Room Temperature

NASA Technical Reports Server (NTRS)

Bird, Richard K.; Lapointe, Thomas S.

2013-01-01

Room temperature flatwise compression tests were conducted on two varieties of ceramic honeycomb core specimens that have potential for high-temperature structural applications. One set of specimens was fabricated using strips of a commercially-available thin-gage "ceramic paper" sheet molded into a hexagonal core configuration. The other set was fabricated by machining honeycomb core directly from a commercially available rigid insulation tile material. This paper summarizes the results from these tests.

Synthesis and characterization of ZnO thin films

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

Anilkumar, T. S., E-mail: anil24march@gmail.com; Girija, M. L., E-mail: girija.ml.grt1@gmail.com; Venkatesh, J., E-mail: phph9502@yahoo.com

2016-05-06

Zinc oxide (ZnO) Thin films were deposited on glass substrate using Spin coating method. Zinc acetate dehydrate, Carbinol and Mono-ethanolamine were used as the precursor, solvent and stabilizer respectively to prepare ZnO Thin-films. The molar ratio of Monoethanolamine to Zinc acetate was maintained as approximately 1. The thickness of the films was determined by Interference technique. The optical properties of the films were studied by UV Vis-Spectrophotometer. From transmittance and absorbance curve, the energy band gap of ZnO is found out. Electrical Conductivity measurements of ZnO are carried out by two probe method and Activation energy for the electrical conductivitymore » of ZnO are found out. The crystal structure and orientation of the films were analyzed by XRD. The XRD patterns show that the ZnO films are polycrystalline with wurtzite hexagonal structure.« less

All 2D, high mobility, flexible, transparent thin film transistor

DOEpatents

Das, Saptarshi; Sumant, Anirudha V.; Roelofs, Andreas

2017-01-17

A two-dimensional thin film transistor and a method for manufacturing a two-dimensional thin film transistor includes layering a semiconducting channel material on a substrate, providing a first electrode material on top of the semiconducting channel material, patterning a source metal electrode and a drain metal electrode at opposite ends of the semiconducting channel material from the first electrode material, opening a window between the source metal electrode and the drain metal electrode, removing the first electrode material from the window located above the semiconducting channel material providing a gate dielectric above the semiconducting channel material, and providing a top gate above the gate dielectric, the top gate formed from a second electrode material. The semiconducting channel material is made of tungsten diselenide, the first electrode material and the second electrode material are made of graphene, and the gate dielectric is made of hexagonal boron nitride.

[Thin filament elasticity and its role in the muscle contraction].

PubMed

Skubiszak, L

2006-01-01

The available experimental methods do not allow one to establish unambiguously the molecular structural events during muscle contraction. To resolve the existing controversies, I have devised an unconventional original computer program. The new approach allows the reconstruction of the hexagonal lattice of the sarcomere for different muscle states and verification of the structure by comparison of the calculated Fourier spectra with the real diffraction patterns. Previously, by the use of this approach, the real structure of a myosin filament from vertebrate striated muscle has been reconstructed (http://zope.ibib.waw.pl/pspk). In this work, a reconstruction for the thin filament is presented for three states: relaxed, after activation, and during contraction. Good consistency of the calculated Fourier spectra with the real diffraction patterns available in the literature suggests that the thin filament, due to flexibility, plays an active part in muscle contraction, as myosin cross-bridges do.

Effect of atmospheric-pressure plasma treatment on the adhesion properties of a thin adhesive layer in a selective transfer process

NASA Astrophysics Data System (ADS)

Yoon, Min-Ah; Kim, Chan; Hur, Min; Kang, Woo Seok; Kim, Jaegu; Kim, Jae-Hyun; Lee, Hak-Joo; Kim, Kwang-Seop

2018-01-01

The adhesion between a stamp and thin film devices is crucial for their transfer on a flexible substrate. In this paper, a thin adhesive silicone layer on the stamp was treated by atmospheric pressure plasma to locally control the adhesion strength for the selective transfer. The adhesion strength of the silicone layer was significantly reduced after the plasma treatment, while its surface energy was increased. To understand the inconsistency between the adhesion strength and surface energy changes, the surface properties of the silicone layer were characterized using nanoindentation and X-ray photoelectron spectroscopy. These techniques revealed that a thin, hard, silica-like layer had formed on the surface from plasma-enhanced oxidation. This layer played an important role in decreasing the contact area and increasing the interfacial slippage, resulting in decreased adhesion. As a practical application, the transfer process was demonstrated on GaN LEDs that had been previously delaminated by a laser lift-off (LLO) process. Although the LEDs were not transferred onto the treated adhesive layer due to the reduced adhesion, the untreated adhesive layer could readily pick up the LEDs. It is expected that this simple method of controlling the adhesion of a stamp with a thin adhesive layer would enable a continuous, selective and large-scale roll-to-roll selective transfer process and thereby advance the development of flexible, stretchable and wearable electronics.

A new approach to epitaxially grow high-quality GaN films on Si substrates: the combination of MBE and PLD.

PubMed

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

2016-04-22

High-quality GaN epitaxial films have been grown on Si substrates with Al buffer layer by the combination of molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) technologies. MBE is used to grow Al buffer layer at first, and then PLD is deployed to grow GaN epitaxial films on the Al buffer layer. The surface morphology, crystalline quality, and interfacial property of as-grown GaN epitaxial films on Si substrates are studied systematically. The as-grown ~300 nm-thick GaN epitaxial films grown at 850 °C with ~30 nm-thick Al buffer layer on Si substrates show high crystalline quality with the full-width at half-maximum (FWHM) for GaN(0002) and GaN(102) X-ray rocking curves of 0.45° and 0.61°, respectively; very flat GaN surface with the root-mean-square surface roughness of 2.5 nm; as well as the sharp and abrupt GaN/AlGaN/Al/Si hetero-interfaces. Furthermore, the corresponding growth mechanism of GaN epitaxial films grown on Si substrates with Al buffer layer by the combination of MBE and PLD is hence studied in depth. This work provides a novel and simple approach for the epitaxial growth of high-quality GaN epitaxial films on Si substrates.