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.

AlGaN/GaN high electron mobility transistor grown on GaN template substrate by molecule beam epitaxy system

NASA Astrophysics Data System (ADS)

Tsai, Jenn-Kai; Chen, Y. L.; Gau, M. H.; Pang, W. Y.; Hsu, Y. C.; Lo, Ikai; Hsieh, C. H.

2008-03-01

In this study, AlGaN/GaN high electron mobility transistor (HEMT) structure was grow on GaN template substrate radio frequency plasma assisted molecular beam epitaxy (MBE) equipped with an EPI UNI-Bulb nitrogen plasma source. The undoped GaN template substrate was grown on c-sapphire substrate by metal organic vapor phase epitaxy system (MOPVD). After growth of MOVPE and MBE, the samples are characterized by double crystal X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (SEM), atomic force microscopy (AFM), and Hall effect measurements. We found that the RMS roughness of template substrate play the major role in got the high value of mobility on AlGaN/GaN HEMT. When the roughness was lower than 0.77 nm in a 25 μm x 25 μm area, the mobility of HEMT at the temperature of 77 K was over 10000 cm^2/Vs.

Surface cleaning for negative electron affinity GaN photocathode

NASA Astrophysics Data System (ADS)

Qiao, Jianliang; Yin, Yingpeng; Gao, Youtang; Niu, Jun; Qian, Yunsheng; Chang, Benkang

2012-10-01

In the preparation process for negative electron affinity (NEA) GaN photocathode, the surface cleanness is very important to activation, it influences the sensitivity and stability of NEA GaN photocathode. The traditional corrosion methods based on oxidizing and dissolving can't remove oxygen (O) and carbon (C) on GaN surface effectively. How to get an ideal atom clean surface is still an important question at present. The cleaning techniques for GaN photocathode was studied by using NEA photocathode activation system and XPS surface analysis system. The experiment sample is p-type GaN doped with Mg, doped concentration is 1.37×1017 cm-3, the transfer rate is 3.08 cm2/V-S, and the thickness of activation layer is 0.51 μm, the substrate is 300 μm thick sapphire. The sample was dealed with chemical cleaning depuration at first. And to get the atom clean surface, the vacuum heat cleaning process was needed. The methods of chemical cleaning and the vacuum heating cleaning were given in detail. According to the X-ray photoelectron spectroscopy of GaN surface after chemical cleaning and the vacuum degree curve of the activation chamber during the heat cleaning, the cleaning effect and the cleaning mechanism were discussed. After the effective chemical cleaning and the heating of 700 Centigrade degree about 20 minutes in ultrahigh vacuum system, the oxides and carbon contaminants on cathode surface can be removed effectively, and the ideal atom clean surface can be obtained. The purpose of heating depuration process is that not only to get the atom clean GaN surface, but also to guarantee the contents of Ga, N on GaN surface stabilize and to keep the system ultra-high vacuum degree. Because of the volatilization of oxide and carbon impurity on the cathode surface, the vacuum degree curve drops with the rising of temperature on the whole.

Study of the effects of GaN buffer layer quality on the dc characteristics of AlGaN/GaN high electron mobility transistors

DOE PAGES

Ahn, Shihyun; Zhu, Weidi; Dong, Chen; ...

2015-04-21

Here we studied the effect of buffer layer quality on dc characteristics of AlGaN/GaN high electron mobility (HEMTs). AlGaN/GaN HEMT structures with 2 and 5 μm GaN buffer layers on sapphire substrates from two different vendors with the same Al concentration of AlGaN were used. The defect densities of HEMT structures with 2 and 5 μm GaN buffer layer were 7 × 10 9 and 5 × 10 8 cm ₋2, respectively, as measured by transmission electron microscopy. There was little difference in drain saturation current or in transfer characteristics in HEMTs on these two types of buffer. However, theremore » was no dispersion observed on the nonpassivated HEMTs with 5 μm GaN buffer layer for gate-lag pulsed measurement at 100 kHz, which was in sharp contrast to the 71% drain current reduction for the HEMT with 2 μm GaN buffer layer.« less

Electron affinity and surface states of GaN m -plane facets: Implication for electronic self-passivation

NASA Astrophysics Data System (ADS)

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

2018-03-01

The electron affinity and surface states are of utmost importance for designing the potential landscape within (heterojunction) nanowires and hence for tuning conductivity and carrier lifetimes. Therefore, we determined for stoichiometric nonpolar GaN (10 1 ¯0 ) m -plane facets, i.e., the dominating sidewalls of GaN nanowires, the electron affinity to 4.06 ±0.07 eV and the energy of the empty Ga-derived surface state in the band gap to 0.99 ±0.08 eV below the conduction band minimum using scanning tunneling spectroscopy. These values imply that the potential landscape within GaN nanowires is defined by a surface state-induced Fermi-level pinning, creating an upward band bending at the sidewall facets, which provides an electronic passivation.

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.

Hot electron generation under large-signal radio frequency operation of GaN high-electron-mobility transistors

NASA Astrophysics Data System (ADS)

Latorre-Rey, Alvaro D.; Sabatti, Flavio F. M.; Albrecht, John D.; Saraniti, Marco

2017-07-01

In order to assess the underlying physical mechanisms of hot carrier-related degradation such as defect generation in millimeter-wave GaN power amplifiers, we have simulated the electron energy distribution function under large-signal radio frequency conditions in AlGaN/GaN high-electron-mobility transistors. Our results are obtained through a full band Monte Carlo particle-based simulator self-consistently coupled to a harmonic balance circuit solver. At lower frequency, simulations of a Class AB power amplifier at 10 GHz show that the peak hot electron generation is up to 43% lower under RF drive than it is under DC conditions, regardless of the input power or temperature of operation. However, at millimeter-wave operation up to 40 GHz, RF hot carrier generation reaches that from DC biasing and even exceeds it up to 75% as the amplifier is driven into compression. Increasing the temperature of operation also shows that degradation of DC and RF characteristics are tightly correlated and mainly caused by increased phonon scattering. The accurate determination of the electron energy mapping is demonstrated to be a powerful tool for the extraction of compact models used in lifetime and reliability analysis.

Determination of the electron-capture coefficients and the concentration of free electrons in GaN from time-resolved photoluminescence

PubMed Central

Reshchikov, M. A.; McNamara, J. D.; Toporkov, M.; Avrutin, V.; Morkoç, H.; Usikov, A.; Helava, H.; Makarov, Yu.

2016-01-01

Point defects in high-purity GaN layers grown by hydride vapor phase epitaxy are studied by steady-state and time-resolved photoluminescence (PL). The electron-capture coefficients for defects responsible for the dominant defect-related PL bands in this material are found. The capture coefficients for all the defects, except for the green luminescence (GL1) band, are independent of temperature. The electron-capture coefficient for the GL1 band significantly changes with temperature because the GL1 band is caused by an internal transition in the related defect, involving an excited state acting as a giant trap for electrons. By using the determined electron-capture coefficients, the concentration of free electrons can be found at different temperatures by a contactless method. A new classification system is suggested for defect-related PL bands in undoped GaN. PMID:27901025

Hot phonon effect on electron velocity saturation in GaN: A second look

NASA Astrophysics Data System (ADS)

Khurgin, Jacob; Ding, Yujie J.; Jena, Debdeep

2007-12-01

A theoretical model is developed for electron velocity saturation in high power GaN transistors. It is shown that electron velocity at high electric fields is reduced due to heating of electron gas since the high density of nonequilibrium LO phonons cannot efficiently transfer heat to the lattice. However, the resulting degradation of electron velocity is found to be weaker than previously reported. The results are compared with experimental data, and the ways to improve the efficiency of cooling the electron gas to increase the drift velocity are discussed.

High optical quality GaN nanopillar arrays

NASA Astrophysics Data System (ADS)

Wang, Y. D.; Chua, S. J.; Tripathy, S.; Sander, M. S.; Chen, P.; Fonstad, C. G.

2005-02-01

GaN nanopillar arrays have been fabricated by inductively coupled plasma etching of GaN films using anodic aluminum oxide film as an etch mask. The average diameter and length of these pillars are 60-65nm and 350-400nm, respectively. Ultraviolet microphotoluminescence measurements indicate high photoluminescence intensity and stress relaxation in these GaN nanopillars as compared to the starting epitaxial GaN films. Evidence of good crystalline quality is also observed by micro-Raman measurements, wherein a redshift of the E2high mode from GaN nanopillars suggests partial relaxation of the compressive strain. In addition, breakdown of the polarization selection rules led to the appearance of symmetry-forbidden and quasipolar modes.

Scatterings and Quantum Effects in (Al ,In )N /GaN Heterostructures for High-Power and High-Frequency Electronics

NASA Astrophysics Data System (ADS)

Wang, Leizhi; Yin, Ming; Khan, Asif; Muhtadi, Sakib; Asif, Fatima; Choi, Eun Sang; Datta, Timir

2018-02-01

Charge transport in the wide-band-gap (Al ,In )N /GaN heterostructures with high carrier density approximately 2 ×1013 cm-2 is investigated over a large range of temperature (270 mK ≤T ≤280 K ) and magnetic field (0 ≤B ≤18 T ). We observe the first evidence of weak localization in the two-dimensional electron gas in this system. From the Shubnikov-de Haas (SdH) oscillations a relatively light effective mass of 0.23 me is determined. Furthermore, the linear dependence with temperature (T <20 K ) of the inelastic scattering rate (τi-1∝T ) is attributed to the phase breaking by electron-electron scattering. Also in the same temperature range the less-than unit ratio of quantum lifetime to Hall transport time (τq/τt<1 ) is taken to signify the dominance of small-angle scattering. Above 20 K, with increasing temperature scattering changes from acoustic phonon to optical phonon scattering, resulting in a rapid decrease in carrier mobility and increase in sheet resistance. Suppression of such scatterings will lead to higher mobility and a way forward to high-power and high-frequency electronics.

Performance Analysis of GaN Capping Layer Thickness on GaN/AlGaN/GaN High Electron Mobility Transistors.

PubMed

Sharma, N; Periasamy, C; Chaturvedi, N

2018-07-01

In this paper, we present an investigation of the impact of GaN capping layer and AlGaN layer thickness on the two-dimensional (2D)-electron mobility and the carrier concentration which was formed close to the AlGaN/GaN buffer layer for Al0.25Ga0.75N/GaN and GaN/Al0.25Ga0.75N/GaN heterostructures deposited on sapphire substrates. The results of our analysis clearly indicate that expanding the GaN capping layer thickness from 1 nm to 100 nm prompts an increment in the electron concentration at hetero interface. As consequence of which drain current was additionally increments with GaN cap layer thicknesses, and eventually saturates at approximately 1.85 A/mm for capping layer thickness greater than 40 nm. Interestingly, for the same structure, the 2D-electron mobility, decrease monotonically with GaN capping layer thickness, and saturate at approximately 830 cm2/Vs for capping layer thickness greater than 50 nm. A device with a GaN cap layer didn't exhibit gate leakage current. Furthermore, it was observed that the carrier concentration was first decrease 1.03 × 1019/cm3 to 6.65 × 1018/cm3 with AlGaN Layer thickness from 5 to 10 nm and after that it increases with the AlGaN layer thickness from 10 to 30 nm. The same trend was followed for electric field distributions. Electron mobility decreases monotonically with AlGaN layer thickness. Highest electron mobility 1354 cm2/Vs were recorded for the AlGaN layer thickness of 5 nm. Results obtained are in good agreement with published experimental data.

A high efficiency C-band internally-matched harmonic tuning GaN power amplifier

NASA Astrophysics Data System (ADS)

Lu, Y.; Zhao, B. C.; Zheng, J. X.; Zhang, H. S.; Zheng, X. F.; Ma, X. H.; Hao, Y.; Ma, P. J.

2016-09-01

In this paper, a high efficiency C-band gallium nitride (GaN) internally-matched power amplifier (PA) is presented. This amplifier consists of 2-chips of self-developed GaN high-electron mobility transistors (HEMTs) with 16 mm total gate width on SiC substrate. New harmonic manipulation circuits are induced both in the input and output matching networks for high efficiency matching at fundamental and 2nd-harmonic frequency, respectively. The developed amplifier has achieved 72.1% power added efficiency (PAE) with 107.4 W output power at 5 GHz. To the best of our knowledge, this amplifier exhibits the highest PAE in C-band GaN HEMT amplifiers with over 100 W output power. Additionally, 1000 hours' aging test reveals high reliability for practical applications.

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

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

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

Enzymatic glucose detection using ZnO nanorods on the gate region of AlGaN /GaN high electron mobility transistors

NASA Astrophysics Data System (ADS)

Kang, B. S.; Wang, H. T.; Ren, F.; Pearton, S. J.; Morey, T. E.; Dennis, D. M.; Johnson, J. W.; Rajagopal, P.; Roberts, J. C.; Piner, E. L.; Linthicum, K. J.

2007-12-01

ZnO nanorod-gated AlGaN /GaN high electron mobility transistors (HEMTs) are demonstrated for the detection of glucose. A ZnO nanorod array was selectively grown on the gate area using low temperature hydrothermal decomposition to immobilize glucose oxidase (GOx). The one-dimensional ZnO nanorods provide a large effective surface area with high surface-to-volume ratio and provide a favorable environment for the immobilization of GOx. The AlGaN /GaN HEMT drain-source current showed a rapid response of less than 5s when target glucose in a buffer with a pH value of 7.4 was added to the GOx immobilized on the ZnO nanorod surface. We could detect a wide range of concentrations from 0.5nMto125μM. The sensor exhibited a linear range from 0.5nMto14.5μM and an experiment limit of detection of 0.5nM. This demonstrates the possibility of using AlGaN /GaN HEMTs for noninvasive exhaled breath condensate based glucose detection of diabetic application.

Fabrications and application of single crystalline GaN for high-performance deep UV photodetectors

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

Velazquez, R.; Rivera, M.; Feng, P., E-mail: p.feng@upr.edu

2016-08-15

High-quality single crystalline Gallium Nitride (GaN) semiconductor has been synthesized using molecule beam epitaxy (MBE) technique for development of high-performance deep ultraviolet (UV) photodetectors. Thickness of the films was estimated by using surface profile meter and scanning electron microscope. Electronic states and elemental composition of the films were obtained using Raman scattering spectroscopy. The orientation, crystal structure and phase purity of the films were examined using a Siemens x-ray diffractometer radiation. The surface microstructure was studied using high resolution scanning electron microscopy (SEM). Two types of metal pairs: Al-Al, Al-Cu or Cu-Cu were used for interdigital electrodes on GaN filmmore » in order to examine the Schottky properties of the GaN based photodetector. The characterizations of the fabricated prototype include the stability, responsivity, response and recovery times. Typical time dependent photoresponsivity by switching different UV light source on and off five times for each 240 seconds at a bias of 2V, respectively, have been obtained. The detector appears to be highly sensitive to various UV wavelengths of light with very stable baseline and repeatability. The obtained photoresponsivity was up to 354 mA/W at the bias 2V. Higher photoresponsivity could be obtained if higher bias was applied but it would unavoidably result in a higher dark current. Thermal effect on the fabricated GaN based prototype was discussed.« less

Ambient-temperature diffusion and gettering of Pt atoms in GaN with surface defect region under 60Co gamma or MeV electron irradiation

NASA Astrophysics Data System (ADS)

Hou, Ruixiang; Li, Lei; Fang, Xin; Xie, Ziang; Li, Shuti; Song, Weidong; Huang, Rong; Zhang, Jicai; Huang, Zengli; Li, Qiangjie; Xu, Wanjing; Fu, Engang; Qin, G. G.

2018-01-01

Generally, the diffusion and gettering of impurities in GaN needs high temperature. Calculated with the ambient-temperature extrapolation value of the high temperature diffusivity of Pt atoms in GaN reported in literature, the time required for Pt atoms diffusing 1 nm in GaN at ambient temperature is about 19 years. Therefore, the ambient-temperature diffusion and gettering of Pt atoms in GaN can hardly be observed. In this work, the ambient-temperature diffusion and gettering of Pt atoms in GaN is reported for the first time. It is demonstrated by use of secondary ion mass spectroscopy that in the condition of introducing a defect region on the GaN film surface by plasma, and subsequently, irradiated by 60Co gamma-ray or 3 MeV electrons, the ambient-temperature diffusion and gettering of Pt atoms in GaN can be detected. It is more obvious with larger irradiation dose and higher plasma power. With a similar surface defect region, the ambient-temperature diffusion and gettering of Pt atoms in GaN stimulated by 3 MeV electron irradiation is more marked than that stimulated by gamma irradiation. The physical mechanism of ambient-temperature diffusion and gettering of Pt atoms in a GaN film with a surface defect region stimulated by gamma or MeV electron irradiation is discussed.

Determination of axial and lateral exciton diffusion length in GaN by electron energy dependent cathodoluminescence

NASA Astrophysics Data System (ADS)

Hocker, Matthias; Maier, Pascal; Jerg, Lisa; Tischer, Ingo; Neusser, Gregor; Kranz, Christine; Pristovsek, Markus; Humphreys, Colin J.; Leute, Robert A. R.; Heinz, Dominik; Rettig, Oliver; Scholz, Ferdinand; Thonke, Klaus

2016-08-01

We demonstrate the application of low-temperature cathodoluminescence (CL) with high lateral, depth, and spectral resolution to determine both the lateral (i.e., perpendicular to the incident primary electron beam) and axial (i.e., parallel to the electron beam) diffusion length of excitons in semiconductor materials. The lateral diffusion length in GaN is investigated by the decrease of the GaN-related luminescence signal when approaching an interface to Ga(In)N based quantum well stripes. The axial diffusion length in GaN is evaluated from a comparison of the results of depth-resolved CL spectroscopy (DRCLS) measurements with predictions from Monte Carlo simulations on the size and shape of the excitation volume. The lateral diffusion length was found to be (95 ± 40) nm for nominally undoped GaN, and the axial exciton diffusion length was determined to be (150 ± 25) nm. The application of the DRCLS method is also presented on a semipolar (11 2 ¯ 2 ) sample, resulting in a value of (70 ± 10) nm in p-type GaN.

Electron and hole stability in GaN and ZnO.

PubMed

Walsh, Aron; Catlow, C Richard A; Miskufova, Martina; Sokol, Alexey A

2011-08-24

We assess the thermodynamic doping limits of GaN and ZnO on the basis of point defect calculations performed using the embedded cluster approach and employing a hybrid non-local density functional for the quantum mechanical region. Within this approach we have calculated a staggered (type-II) valence band alignment between the two materials, with the N 2p states contributing to the lower ionization potential of GaN. With respect to the stability of free electron and hole carriers, redox reactions resulting in charge compensation by ionic defects are found to be largely endothermic (unfavourable) for electrons and exothermic (favourable) for holes, which is consistent with the efficacy of electron conduction in these materials. Approaches for overcoming these fundamental thermodynamic limits are discussed. © 2011 IOP Publishing Ltd

Tuning electronic and magnetic properties of GaN nanosheets by surface modifications and nanosheet thickness.

PubMed

Xiao, Meixia; Yao, Tingzhen; Ao, Zhimin; Wei, Peng; Wang, Danghui; Song, Haiyang

2015-04-14

Density-functional theory calculations are performed to investigate the effects of surface modifications and nanosheet thickness on the electronic and magnetic properties of gallium nitride (GaN) nanosheets (NSs). Unlike the bare GaN NSs terminating with polar surfaces, the systems with hydrogenated Ga (H-GaN), fluorinated Ga (F-GaN), and chlorinated Ga (Cl-GaN) preserve their initial wurtzite structures and exhibit ferromagnetic states. The abovementioned three different decorations on Ga atoms are energetically more favorable for thicker GaN NSs. Moreover, as the thickness increases, H-GaN and F-GaN NSs undergo semiconductor to metal and half-metal to metal transition, respectively, while Cl-GaN NSs remain completely metallic. The predicted diverse and tunable electronic and magnetic properties highlight the potential of GaN NSs for novel electronic and spintronic nanodevices.

High-efficiency S-band harmonic tuning GaN amplifier

NASA Astrophysics Data System (ADS)

Cao, Meng-Yi; Zhang, Kai; Chen, Yong-He; Zhang, Jin-Cheng; Ma, Xiao-Hua; Hao, Yue

2014-03-01

In this paper, we present a high-efficiency S-band gallium nitride (GaN) power amplifier (PA). This amplifier is fabricated based on a self-developed GaN high-electron-mobility transistor (HEMT) with 10 mm gate width on SiC substrate. Harmonic manipulation circuits are presented in the amplifier. The matching networks consist of microstrip lines and discrete components. Open-circuited stub lines in both input and output are used to tune the 2nd harmonic wave and match the GaN HEMT to the highest efficiency condition. The developed amplifier delivers an output power of 48.5 dBm (~70 W) with a power-added efficiency (PAE) of 72.2% at 2 GHz in pulse condition. When operating at 1.8-2.2 GHz (20% relative bandwidth), the amplifier provides an output power higher than 48 dBm (~ 65 W), with a PAE over 70% and a power gain above 15 dB. When operating in continuous-wave (CW) operating conditions, the amplifier gives an output power over 46 dBm (40 W) with PAE beyond 60% over the whole operation frequency range.

Electron band bending of polar, semipolar and non-polar GaN surfaces

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

Bartoš, I.; Romanyuk, O., E-mail: romanyuk@fzu.cz; Houdkova, J.

2016-03-14

The magnitudes of the surface band bending have been determined by X-ray photoelectron spectroscopy for polar, semipolar, and non-polar surfaces of wurtzite GaN crystals. All surfaces have been prepared from crystalline GaN samples grown by the hydride-vapour phase epitaxy and separated from sapphire substrates. The Ga 3d core level peak shifts have been used for band bending determination. Small band bending magnitudes and also relatively small difference between the band bendings of the surfaces with opposite polarity have been found. These results point to the presence of electron surface states of different amounts and types on surfaces of different polaritymore » and confirm the important role of the electron surface states in compensation of the bound surface polarity charges in wurtzite GaN crystals.« less

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.

GaN transistors on Si for switching and high-frequency applications

NASA Astrophysics Data System (ADS)

Ueda, Tetsuzo; Ishida, Masahiro; Tanaka, Tsuyoshi; Ueda, Daisuke

2014-10-01

In this paper, recent advances of GaN transistors on Si for switching and high-frequency applications are reviewed. Novel epitaxial structures including superlattice interlayers grown by metal organic chemical vapor deposition (MOCVD) relieve the strain and eliminate the cracks in the GaN over large-diameter Si substrates up to 8 in. As a new device structure for high-power switching application, Gate Injection Transistors (GITs) with a p-AlGaN gate over an AlGaN/GaN heterostructure successfully achieve normally-off operations maintaining high drain currents and low on-state resistances. Note that the GITs on Si are free from current collapse up to 600 V, by which the drain current would be markedly reduced after the application of high drain voltages. Highly efficient operations of an inverter and DC-DC converters are presented as promising applications of GITs for power switching. The high efficiencies in an inverter, a resonant LLC converter, and a point-of-load (POL) converter demonstrate the superior potential of the GaN transistors on Si. As for high-frequency transistors, AlGaN/GaN heterojuction field-effect transistors (HFETs) on Si designed specifically for microwave and millimeter-wave frequencies demonstrate a sufficiently high output power at these frequencies. Output powers of 203 W at 2.5 GHz and 10.7 W at 26.5 GHz are achieved by the fabricated GaN transistors. These devices for switching and high-frequency applications are very promising as future energy-efficient electronics because of their inherent low fabrication cost and superior device performance.

Design of high breakdown voltage vertical GaN p-n diodes with high-K/low-K compound dielectric structure for power electronics applications

NASA Astrophysics Data System (ADS)

Du, Jiangfeng; Li, Zhenchao; Liu, Dong; Bai, Zhiyuan; Liu, Yang; Yu, Qi

2017-11-01

In this work, a vertical GaN p-n diode with a high-K/low-K compound dielectric structure (GaN CD-VGD) is proposed and designed to achieve a record high breakdown voltage (BV) with a low specific on-resistance (Ron,sp). By introducing compound dielectric structure, the electric field near the p-n junction interface is suppressed due to the effects of high-K passivation layer, and a new electric field peak is induced into the n-type drift region, because of a discontinuity of electrical field at the interface of high-K and low-K layer. Therefore the distribution of electric field in GaN p-n diode becomes more uniform and an enhancement of breakdown voltage can be achieved. Numerical simulations demonstrate that GaN CD-VGD with a BV of 10650 V and a Ron,sp of 14.3 mΩ cm2, resulting in a record high figure-of-merit of 8 GW/cm2.

Wedge-Shaped GaN Nanowalls: A Potential Candidate for Two-Dimensional Electronics and Spintronics

NASA Astrophysics Data System (ADS)

Deb, Swarup; Dhar, Subhabrata

Schrödingerand Poisson equations are solved self-consistently in order to obtain the potential and charge density distribution in n-type GaN nanowalls tapered along c-axis by different angles. The study shows two-dimensional (2D) quantum confinement of electrons in the central vertical plane of the wall for the entire range of tapering. Calculation of room temperature electron mobility in the 2D channel shows a steady decrease with the increase of the inclination angle of the side facets with respect to the base. However, it is interesting to note that the mobility remains to be much larger than that of bulk GaN even for the inclination angle of 65∘. The properties of high mobility and the vertical orientation of the 2DEG plane in this system can be exploited in fabricating highly conducting transparent interconnects and field effect transistors, which can lead to large scale integration of 2D devices in future.

Carbon doped GaN buffer layer using propane for high electron mobility transistor applications: Growth and device results

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

Li, X.; Nilsson, D.; Danielsson, Ö.

2015-12-28

The creation of a semi insulating (SI) buffer layer in AlGaN/GaN High Electron Mobility Transistor (HEMT) devices is crucial for preventing a current path beneath the two-dimensional electron gas (2DEG). In this investigation, we evaluate the use of a gaseous carbon gas precursor, propane, for creating a SI GaN buffer layer in a HEMT structure. The carbon doped profile, using propane gas, is a two stepped profile with a high carbon doping (1.5 × 10{sup 18 }cm{sup −3}) epitaxial layer closest to the substrate and a lower doped layer (3 × 10{sup 16 }cm{sup −3}) closest to the 2DEG channel. Secondary Ion Mass Spectrometry measurement showsmore » a uniform incorporation versus depth, and no memory effect from carbon doping can be seen. The high carbon doping (1.5 × 10{sup 18 }cm{sup −3}) does not influence the surface morphology, and a roughness root-mean-square value of 0.43 nm is obtained from Atomic Force Microscopy. High resolution X-ray diffraction measurements show very sharp peaks and no structural degradation can be seen related to the heavy carbon doped layer. HEMTs are fabricated and show an extremely low drain induced barrier lowering value of 0.1 mV/V, demonstrating an excellent buffer isolation. The carbon doped GaN buffer layer using propane gas is compared to samples using carbon from the trimethylgallium molecule, showing equally low leakage currents, demonstrating the capability of growing highly resistive buffer layers using a gaseous carbon source.« less

Pyramidal defects in highly Mg-doped GaN: atomic structure and influence on optoelectronic properties

NASA Astrophysics Data System (ADS)

Leroux, M.; Vennéguès, P.; Dalmasso, S.; de Mierry, P.; Lorenzini, P.; Damilano, B.; Beaumont, B.; Gibart, P.; Massies, J.

2004-07-01

A detailed transmission electron microscopy study is performed on the pyramidal inversion domains that appear in highly Mg-doped GaN grown by metalorganics vapor phase epitaxy or by the high-pressure, high-temperature method. From a comparison between high resolution images of the inversion domain boundaries and simulations using different atomic models, we conclude that both basal and inclined domain boundaries are likely formed of a monomolecular layer of the definite compound Mg{3}N{2}. We show that, due to their high concentration, the formation of these defects may account for auto-compensation in Mg-doped GaN. We also show that the local band bending induced by the polarity inversion due to these defects can be at the origin of the blue luminescence of highly Mg-doped GaN, always observed when nanometric pyramidal inversion domains are also present.

CO2 detection using polyethylenimine/starch functionalized AlGaN /GaN high electron mobility transistors

NASA Astrophysics Data System (ADS)

Chang, C. Y.; Kang, B. S.; Wang, H. T.; Ren, F.; Wang, Y. L.; Pearton, S. J.; Dennis, D. M.; Johnson, J. W.; Rajagopal, P.; Roberts, J. C.; Piner, E. L.; Linthicum, K. J.

2008-06-01

AlGaN /GaN high electron mobility transistors (HEMTs) functionalized with polyethylenimine/starch were used for detecting CO2 with a wide dynamic range of 0.9%-50% balanced with nitrogen at temperatures from 46to220°C. Higher detection sensitivity to CO2 gas was achieved at higher testing temperatures. At a fixed source-drain bias voltage of 0.5V, drain-source current of the functionalized HEMTs showed a sublinear correlation upon exposure to different CO2 concentrations at low temperature. The superlinear relationship was at high temperature. The sensor exhibited a reversible behavior and a repeatable current change of 32 and 47μA with the introduction of 28.57% and 37.5% CO2 at 108°C, respectively.

Vacancy-hydrogen complexes in ammonothermal GaN

NASA Astrophysics Data System (ADS)

Tuomisto, F.; Kuittinen, T.; Zając, M.; Doradziński, R.; Wasik, D.

2014-10-01

We have applied positron annihilation spectroscopy to study in-grown vacancy defects in bulk GaN crystals grown by the ammonothermal method. We observe a high concentration of Ga vacancy related defects in n-type samples with varying free electron and oxygen content. The positron lifetimes found in these samples suggest that the Ga vacancies are complexed with hydrogen impurities. The number of hydrogen atoms in each vacancy decreases with increasing free electron concentration and oxygen and hydrogen content. The local vibrational modes observed in infrared absorption support this conclusion. Growth of high-quality ammonothermal GaN single crystals with varying electron concentrations. Identification of defect complexes containing a Ga vacancy and 1 or more hydrogen atoms, and possibly O. These vacancy complexes provide a likely explanation for electrical compensation in ammonothermal GaN.

AlGaN/GaN heterostructures with an AlGaN layer grown directly on reactive-ion-etched GaN showing a high electron mobility (>1300 cm2 V-1 s-1)

NASA Astrophysics Data System (ADS)

Yamamoto, Akio; Makino, Shinya; Kanatani, Keito; Kuzuhara, Masaaki

2018-04-01

In this study, the metal-organic-vapor-phase-epitaxial growth behavior and electrical properties of AlGaN/GaN structures prepared by the growth of an AlGaN layer on a reactive-ion-etched (RIE) GaN surface without regrown GaN layers were investigated. The annealing of RIE-GaN surfaces in NH3 + H2 atmosphere, employed immediately before AlGaN growth, was a key process in obtaining a clean GaN surface for AlGaN growth, that is, in obtaining an electron mobility as high as 1350 cm2 V-1 s-1 in a fabricated AlGaN/RIE-GaN structure. High-electron-mobility transistors (HEMTs) were successfully fabricated with AlGaN/RIE-GaN wafers. With decreasing density of dotlike defects observed on the surfaces of AlGaN/RIE-GaN wafers, both two-dimensional electron gas properties of AlGaN/RIE-GaN structures and DC characteristics of HEMTs were markedly improved. Since dotlike defect density was markedly dependent on RIE lot, rather than on growth lot, surface contaminations of GaN during RIE were believed to be responsible for the formation of dotlike defects and, therefore, for the inferior electrical properties.

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.

Study on GaN buffer leakage current in AlGaN/GaN high electron mobility transistor structures grown by ammonia-molecular beam epitaxy on 100-mm Si(111)

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

Ravikiran, L.; Radhakrishnan, K., E-mail: ERADHA@e.ntu.edu.sg; Ng, G. I.

2015-06-28

The effect of carbon doping on the structural and electrical properties of GaN buffer layer of AlGaN/GaN high electron mobility transistor (HEMT) structures has been studied. In the undoped HEMT structures, oxygen was identified as the dominant impurity using secondary ion mass spectroscopy and photoluminescence (PL) measurements. In addition, a notable parallel conduction channel was identified in the GaN buffer at the interface. The AlGaN/GaN HEMT structures with carbon doped GaN buffer using a CBr{sub 4} beam equivalent pressure of 1.86 × 10{sup −7} mTorr showed a reduction in the buffer leakage current by two orders of magnitude. Carbon doped GaN buffersmore » also exhibited a slight increase in the crystalline tilt with some pits on the growth surface. PL and Raman measurements indicated only a partial compensation of donor states with carbon acceptors. However, AlGaN/GaN HEMT structures with carbon doped GaN buffer with 200 nm thick undoped GaN near the channel exhibited good 2DEG characteristics.« less

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.

Doping Induced Structural Stability and Electronic Properties of GaN Nanotubes

PubMed Central

Khan, Mohammad Irfan; Tyagi, Neha; Swaroop Khare, Purnima

2014-01-01

The present paper discusses the effect of manganese doping on the structural stability and electronic band gap of chiral (2, 1), armchair (3, 3), and zigzag ((6, 0) and (10, 0)) single walled GaN nanotube by using density functional theory based Atomistix Toolkit (ATK) Virtual NanoLab (VNL). The structural stability has been analyzed in terms of minimum ground state total energy, binding, and formation energy. As an effect of Mn doping (1–4 atoms), all the GaN nanotubes taken into consideration show semiconducting to metallic transition first and after certain level of Mn doping changes its trend. PMID:24707225

Spin-polarized electron emitter: Mn-doped GaN nanotubes and their arrays

NASA Astrophysics Data System (ADS)

Hao, Shaogang; Zhou, Gang; Wu, Jian; Duan, Wenhui; Gu, Bing-Lin

2004-03-01

The influences from the doping magnetic atom, Mn, on the geometry, electronic properties, and spin-polarization characteristics are demonstrated for open armchair gallium nitrogen (GaN) nanotubes and arrays by use of the first-principles calculations. The interaction between dangling bonds of Ga (Mn) and N atoms at the open-end promotes the self-close of the tube mouth and formation of a more stable open semicone top. Primarily owing to hybridization of Mn 3d and N 2p orbitals, one Mn atom introduces several impurity energy levels into the original energy gap, and the calculated magnetic moment is 4μB. The electron spin polarizations in the field emission are theoretically evaluated. We suggest that armchair open GaN nanotube arrays doped with a finite number of magnetic atoms may have application potential as the electron source of spintronic devices in the future.

Electron concentration in highly resistive GaN substrates co-doped with Si, C, and Fe

NASA Astrophysics Data System (ADS)

Tokuda, Hirokuni; Suzuki, Kosuke; Asubar, Joel T.; Kuzuhara, Masaaki

2018-07-01

Electron concentration in highly resistive GaN substrates with intentional iron (Fe) dopants as well as unintentionally incorporated silicon (Si) and carbon (C) dopants has been investigated. Si, C, and Fe atomic concentrations were 2 × 1017, 1 × 1016, and 1 × 1019 cm‑3, respectively as measured by secondary ion mass spectroscopy (SIMS). Temperature dependence of current–voltage (I–V) characteristics revealed that the resistivity (ρ) was 3.8 × 109 Ω cm at 300 K and monotonously decreased to 3.1 × 104 Ω cm at 570 K, giving an activation energy of 0.63 eV. Electron concentration (n) was modeled using analytical equation assuming three impurity levels of Si donor, C and Fe acceptors. The n of 5.0 × 107 and 3.1 × 1012 cm‑3 at 300 and 570 K, respectively, with an effective activation energy of 0.60 eV, were derived based on the model. These calculated electron concentration values are in good agreement with the experimental results. In addition, quantitatively analyzed results revealed that around 2 orders of magnitude reduction of n is expected by increasing doping concentration of Fe from 1.0 × 1018 to 1.0 × 1020 cm‑3.

Co-adsorption of water and oxygen on GaN: Effects of charge transfer and formation of electron depletion layer.

PubMed

Wang, Qi; Puntambekar, Ajinkya; Chakrapani, Vidhya

2017-09-14

Species from ambient atmosphere such as water and oxygen are known to affect electronic and optical properties of GaN, but the underlying mechanism is not clearly known. In this work, we show through careful measurement of electrical resistivity and photoluminescence intensity under various adsorbates that the presence of oxygen or water vapor alone is not sufficient to induce electron transfer to these species. Rather, the presence of both water and oxygen is necessary to induce electron transfer from GaN that leads to the formation of an electron depletion region on the surface. Exposure to acidic gases decreases n-type conductivity due to increased electron transfer from GaN, while basic gases increase n-type conductivity and PL intensity due to reduced charge transfer from GaN. These changes in the electrical and optical properties, as explained using a new electrochemical framework based on the phenomenon of surface transfer doping, suggest that gases interact with the semiconductor surface through electrochemical reactions occurring in an adsorbed water layer present on the surface.

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.

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

PubMed

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

2016-07-20

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

Native defects in GaN: a hybrid functional study

NASA Astrophysics Data System (ADS)

Diallo, Ibrahima Castillo; Demchenko, Denis

Intrinsic defects play an important role in the performance of GaN-based devices. We present hybrid density functional calculations of the electronic and possible optical properties of interstitial N (Ni-Ni) , N antisite (NGa) , interstitial Ga (Gai) , Ga antisite (GaN) , Ga vacancy (VGa) , N vacancy (VN) and Ga-N divacancies (VGaVN) in GaN. Our results show that the vacancies display relatively low formation energies in certain samples, whereas antisites and interstitials are energetically less favorable. However, interstitials can be created by electron irradiation. For instance, in 2.5 MeV electron-irradiated GaN samples, a strong correlation between the frequently observed photoluminescence (PL) band centered around 0.85 eV accompanied with a rich phonon sideband of ~0.88 eV and the theoretical optical behavior of interstitial Ga is discussed. N vacancies are found to likely contribute to the experimentally obtained green luminescence band (GL2) peaking at 2.24 eV in high-resistivity undoped and Mg-doped GaN. National Science Foundation (DMR-1410125) and the Thomas F. and Kate Miller Jeffress Memorial Trust.

Increase the threshold voltage of high voltage GaN transistors by low temperature atomic hydrogen treatment

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

Erofeev, E. V., E-mail: erofeev@micran.ru; Fedin, I. V.; Kutkov, I. V.

High-electron-mobility transistors (HEMTs) based on AlGaN/GaN epitaxial heterostructures are a promising element base for the fabrication of high voltage electronic devices of the next generation. This is caused by both the high mobility of charge carriers in the transistor channel and the high electric strength of the material, which makes it possible to attain high breakdown voltages. For use in high-power switches, normally off-mode GaN transistors operating under enhancement conditions are required. To fabricate normally off GaN transistors, one most frequently uses a subgate region based on magnesium-doped p-GaN. However, optimization of the p-GaN epitaxial-layer thickness and the doping levelmore » makes it possible to attain a threshold voltage of GaN transistors close to V{sub th} = +2 V. In this study, it is shown that the use of low temperature treatment in an atomic hydrogen flow for the p-GaN-based subgate region before the deposition of gate-metallization layers makes it possible to increase the transistor threshold voltage to V{sub th} = +3.5 V. The effects under observation can be caused by the formation of a dipole layer on the p-GaN surface induced by the effect of atomic hydrogen. The heat treatment of hydrogen-treated GaN transistors in a nitrogen environment at a temperature of T = 250°C for 12 h reveals no degradation of the transistor’s electrical parameters, which can be caused by the formation of a thermally stable dipole layer at the metal/p-GaN interface as a result of hydrogenation.« less

Bandgap engineering of GaN nanowires

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

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

2016-05-15

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

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.

Growth of hierarchical GaN nanowires for optoelectronic device applications

NASA Astrophysics Data System (ADS)

Raj, Rishabh; Vignesh, Veeramuthu; Ra, Yong-Ho; Nirmala, Rajkumar; Lee, Cheul-Ro; Navamathavan, Rangaswamy

2017-01-01

Gallium nitride nanostructures have been receiving considerable attention as building blocks for nanophotonic technologies due to their unique high aspect ratios, promising the realization of photonic and biological nanodevices such as blue light emitting diodes (LEDs), short-wavelength ultraviolet nanolasers, and nanofluidic biochemical sensors. We report on the growth of hierarchical GaN nanowires (NWs) by dynamically adjusting the growth parameters using the pulsed flow metal-organic chemical vapor deposition technique. We carried out two step growth processes to grow hierarchical GaN NWs. In the first step, the GaN NWs were grown at 950°C, and in the second, we suitably decreased the growth temperature to 630°C and 710°C to grow the hierarchical structures. The surface morphology and optical characterization of the grown GaN NWs were studied by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, photoluminescence, and cathodoluminescence measurements. These kinds of hierarchical GaN NWs are promising for allowing flat band quantum structures that are shown to improve the efficiency of LEDs.

Mechanism of radiative recombination in acceptor-doped bulk GaN crystals

NASA Astrophysics Data System (ADS)

Godlewski, M.; Suski, T.; Grzegory, I.; Porowski, S.; Bergman, J. P.; Chen, W. M.; Monemar, B.

1999-12-01

Optical and electrical properties of acceptor-doped bulk GaN crystals are discussed. Though introducing Zn and Ca to bulk GaN does not significantly change electron concentration, it results in the appearance of a blue photoluminescence band accompanying the relatively strong yellow band usually present. Highly resistive GaN : Mg crystals are obtained when high amount of Mg is introduced to the Ga melt during high-pressure synthesis. Change of electrical properties of Mg-doped bulk crystals is accompanied by the appearance of a strong blue emission of GaN similar to that in Ca- and Zn-doped crystals. Optically detected magnetic resonance investigations indicate a multi-band character of this blue emission and suggest possible mechanism of compensation in acceptor-doped bulk GaN.

Proximity Effects of Beryllium-Doped GaN Buffer Layers on the Electronic Properties of Epitaxial AlGaN/GaN Heterostructures

DTIC Science & Technology

2010-05-17

arranged by Prof. A. Zaslavsky Keywords: Gallium nitride High electron mobility transistor Molecular beam epitaxy Homoepitaxy Doping a b s t r a c t AlGaN...GaN/Be:GaN heterostructures have been grown by rf-plasma molecular beam epitaxy on free- standing semi-insulating GaN substrates, employing...hydride vapor phase epitaxy (HVPE) grown GaN sub- strates has enabled the growth by molecular beam epitaxy (MBE) of AlGaN/GaNHEMTswith significantly

Energetics of Mg incorporation at GaN(0001) and GaN(0001¯) surfaces

NASA Astrophysics Data System (ADS)

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

2006-04-01

By using density functional calculations in the generalized gradient approximation, we investigate the energetics of Mg adsorption and incorporation at GaN(0001) and GaN(0001¯) surfaces under various Ga and Mg coverage conditions as well as in presence of light or electron beam-induced electronic excitation. We find significant differences in Mg incorporation between Ga- and N-polar surfaces. Mg incorporation is easier at the Ga-polar surface, but high Mg coverages are found to cause important distortions which locally change the polarity from Ga to N polar. At the N-rich and moderately Ga-rich GaN(0001) surface, 0.25 ML of Mg substituting Ga in the top bilayer strongly reduce the surface diffusion barriers of Ga and N adatoms, in agreement with the surfactant effect observed in experiments. As the Mg coverage exceeds 0.5 ML, partial incorporation in the subsurface region (second bilayer) becomes favorable. A surface structure with 0.5 ML of incorporated Mg in the top bilayer and 0.25 ML in the second bilayer is found to be stable over a wide range of Ga chemical potential. At the Ga bilayer-terminated GaN(0001) surface, corresponding to Ga-rich conditions, configurations where Mg is incorporated in the interface region between the metallic Ga bilayer and the underlying GaN bilayer appear to be favored. At the N-polar surface, Mg is not incorporated under N-rich or moderately Ga-rich conditions, whereas incorporation in the adlayer may take place under Ga-rich conditions. In the presence of light or electron beam induced excitation, energy differences between Mg incorporated at the surface and in deeper layers are reduced so that the tendency toward surface segregation is also reduced.

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

PubMed

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

2017-12-13

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

Movement of basal plane dislocations in GaN during electron beam irradiation

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

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

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

Prospects for the application of GaN power devices in hybrid electric vehicle drive systems

NASA Astrophysics Data System (ADS)

Su, Ming; Chen, Chingchi; Rajan, Siddharth

2013-07-01

GaN, a wide bandgap semiconductor successfully implemented in optical and high-speed electronic devices, has gained momentum in recent years for power electronics applications. Along with rapid progress in material and device processing technologies, high-voltage transistors over 600 V have been reported by a number of teams worldwide. These advances make GaN highly attractive for the growing market of electrified vehicles, which currently employ bipolar silicon devices in the 600-1200 V class for the traction inverter. However, to capture this billion-dollar power market, GaN has to compete with existing IGBT products and deliver higher performance at comparable or lower cost. This paper reviews key achievements made by the GaN semiconductor industry, requirements of the automotive electric drive system and remaining challenges for GaN power devices to fit in the inverter application of hybrid vehicles.

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

PubMed Central

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

2015-01-01

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

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

PubMed

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

2015-09-02

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

Activation and evaluation of GaN photocathodes

NASA Astrophysics Data System (ADS)

Qian, Yunsheng; Chang, Benkang; Qiao, Jiangliang; Zhang, Yijun; Fu, Rongguo; Qiu, Yafeng

2009-09-01

Gallium Nitride (GaN) photocathodes are potentially attractive as UV detective materials and electron sources. Based on the activation and evaluation system for GaAs photocathode, which consists of ultra-high vacuum (UHV) activation chamber, multi-information measurement system, X-ray photoelectron spectroscopy (XPS), and ultraviolet ray photoelectron spectroscopy (UPS), the control and measurement system for the activation of UV photocathodes was developed. The developed system, which consists of Xenon lamp, monochromator with scanner, signal-processing module, power control unit of Cs and O source, A/D adapter, digital I/O card, computer and software, can control the activation of GaN photocathodes and measure on-line the spectral response curves of GaN photocathodes. GaN materials on sapphire substrate were grown by Metal-Organic Chemical Vapor Deposition (MOCVD) with p-type Mg doping. The GaN materials were activated by Cs-O. The spectral response and quantum efficiency (QE) were measured and calculated. The experiment results are discussed.

Study on photoemission surface of varied doping GaN photocathode

NASA Astrophysics Data System (ADS)

Qiao, Jianliang; Du, Ruijuan; Ding, Huan; Gao, Youtang; Chang, Benkang

2014-09-01

For varied doping GaN photocathode, from bulk to surface the doping concentrations are distributed from high to low. The varied doping GaN photocathode may produce directional inside electric field within the material, so the higher quantum efficiency can be obtained. The photoemission surface of varied doping GaN photocathode is very important to the high quantum efficiency, but the forming process of the surface state after Cs activation or Cs/O activation has been not known completely. Encircling the photoemission mechanism of varied GaN photocathode, considering the experiment phenomena during the activation and the successful activation results, the varied GaN photocathode surface model [GaN(Mg):Cs]:O-Cs after activation with cesium and oxygen was given. According to GaN photocathode activation process and the change of electronic affinity, the comparatively ideal NEA property can be achieved by Cs or Cs/O activation, and higher quantum efficiency can be obtained. The results show: The effective NEA characteristic of GaN can be gotten only by Cs. [GaN(Mg):Cs] dipoles form the first dipole layer, the positive end is toward the vacuum side. In the activation processing with Cs/O, the second dipole layer is formed by O-Cs dipoles, A O-Cs dipole includes one oxygen atom and two Cs atoms, and the positive end is also toward the vacuum side thus the escape of electrons can be promoted.

Study on the Electronic Transport Properties of Zigzag GaN Nanotubes

NASA Astrophysics Data System (ADS)

Li, Enling; Wang, Xiqiang; Hou, Liping; Zhao, Danna; Dai, Yuanbin; Wang, Xuewen

2011-02-01

The electronic transport properties of zigzag GaN nanotubes (n, 0) (4 <= n <= 9) have been calculated using the density functional theory and non-equilibrium Green's functions method. Firstly, the density functional theory (DFT) is used to optimize and calculate the electronic structure of GaNNTs (n, 0) (4<=n<=9). Secondly, DFT and non-equilibrium Green function (NEGF) method are also used to predict the electronic transport properties of GaNNTs two-probe system. The results showed: there is a corresponding relation between the electronic transport properties and the valley of state density of each GaNNT. In addition, the volt-ampere curve of GaNNT is approximately linear.

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

Potential-dependent recombination kinetics of photogenerated electrons in n- and p-type GaN photoelectrodes studied by time-resolved IR absorption spectroscopy.

PubMed

Yamakata, Akira; Yoshida, Masaaki; Kubota, Jun; Osawa, Masatoshi; Domen, Kazunari

2011-07-27

Recombination kinetics of photogenerated electrons in n-type and p-type GaN photoelectrodes active for H(2) and O(2) evolution, respectively, from water was examined by time-resolved IR absorption (TR-IR) spectroscopy. Illumination of a GaN film with UV pulse (355 nm and 6 ns in duration) gives transient interference spectra in both transmittance and reflection modes. Simulation shows that the interference spectra are caused by photogenerated electrons. We observed that recombination in the microsecond region is greatly affected by the applied potentials, the lifetime becoming longer at negative and positive potentials for n- and p-type GaN electrodes, respectively. There is a good correlation between potential dependence of the steady-state reaction efficiency and that of the number of surviving electrons in the millisecond region. We also performed potential jump measurement to examine the shift in Fermi level by photogenerated charge carriers. In the case of n-type GaN, the electrode potential jumps to the negative side by accumulation of electrons in the bulk. However, in the case of p-type GaN, the electrode potential first jumps to the negative side within 20 μs and gradually shifts to the positive side in a few milliseconds, while the number of charge carriers is constant at >0.2 ms. This two-step process is ascribed to electron transport from the bulk to the surface of GaN, because the electrode potential is sensitive to the number of electrons in the bulk. The results confirm that TR-IR combined with potential jump measurement provides useful information for understanding the behavior of charge carriers in photoelectrochemical systems.

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

NASA Astrophysics Data System (ADS)

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

2013-09-01

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

N-Face GaN Electronics for Heteroepitaxial and Bonded Structures

DTIC Science & Technology

2015-08-27

GaN ! ?" InGaAs’Channel’ InAlAs’ !!!!!S! !!!!!!D! !!!!G! Ga (In)N’Dri2 ’Region! Wafer* Bonded! Junc2on! !!!!!S...Gate InGaAs InAlAs (In, Ga )N Source GaN on Sapphire Aperture CBL WBI InGaN n-InGaAs InAlAs n+ GaN S D WBI...about. Polarization effects at the interface may need to be considered. For Ga -polar InGaN- GaN homojunctions,

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

NASA Astrophysics Data System (ADS)

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

2006-11-01

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

Modeling and Design of GaN High Electron Mobility Transistors and Hot Electron Transistors through Monte Carlo Particle-based Device Simulations

NASA Astrophysics Data System (ADS)

Soligo, Riccardo

In this work, the insight provided by our sophisticated Full Band Monte Carlo simulator is used to analyze the behavior of state-of-art devices like GaN High Electron Mobility Transistors and Hot Electron Transistors. Chapter 1 is dedicated to the description of the simulation tool used to obtain the results shown in this work. Moreover, a separate section is dedicated the set up of a procedure to validate to the tunneling algorithm recently implemented in the simulator. Chapter 2 introduces High Electron Mobility Transistors (HEMTs), state-of-art devices characterized by highly non linear transport phenomena that require the use of advanced simulation methods. The techniques for device modeling are described applied to a recent GaN-HEMT, and they are validated with experimental measurements. The main techniques characterization techniques are also described, including the original contribution provided by this work. Chapter 3 focuses on a popular technique to enhance HEMTs performance: the down-scaling of the device dimensions. In particular, this chapter is dedicated to lateral scaling and the calculation of a limiting cutoff frequency for a device of vanishing length. Finally, Chapter 4 and Chapter 5 describe the modeling of Hot Electron Transistors (HETs). The simulation approach is validated by matching the current characteristics with the experimental one before variations of the layouts are proposed to increase the current gain to values suitable for amplification. The frequency response of these layouts is calculated, and modeled by a small signal circuit. For this purpose, a method to directly calculate the capacitance is developed which provides a graphical picture of the capacitative phenomena that limit the frequency response in devices. In Chapter 5 the properties of the hot electrons are investigated for different injection energies, which are obtained by changing the layout of the emitter barrier. Moreover, the large signal characterization of the

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.

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.

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.

Electrical characteristics of high-power AlGaN-GaN high electron mobility transistors irradiated with protons and heavy ions

NASA Astrophysics Data System (ADS)

Sin, Yongkun; Bonsall, Jeremy; Lingley, Zachary; Brodie, Miles; Mason, Maribeth

2017-02-01

High electron mobility transistors (HEMTs) based on AlGaN-GaN hetero-structures are finding an increasing number of commercial and military applications that require high voltage, high power, and high efficiency operation. In recent years, leading GaN HEMT manufacturers have reported excellent RF power characteristics and encouraging reliability, but long-term reliability in the space environment still remains a major concern due to a large number of defects and traps present both in the bulk as well as at the surface, leading to undesirable characteristics including current collapse. Furthermore, degradation mechanisms in GaN HEMTs are still not well understood. Thus, reliability and radiation effects of GaN HEMTs should be studied before solid state power amplifiers (SSPAs) based on GaN HEMT technology are successfully deployed in space satellite systems. For the present study, we investigated electrical characteristics of high-power GaN HEMTs irradiated with protons and heavy ions under various irradiation and biasing conditions.

Photoemission stability of negative electron affinity GaN photocathode

NASA Astrophysics Data System (ADS)

Zhang, Junju; Wang, Xiaohui; Yang, Wenzheng; Tang, Weidong; Fu, Xiaoqian; Li, Biao; Chang, Benkang

2012-11-01

The stability for reflection-mode GaN photocathode has been investigated by monitoring the photocurrent and the spectral response at room temperature. We watch that the photocurrent of the cathode decays with time in the vacuum system, and compare the spectral response curves after activation and after degradation. The photocurrent decay mechanism for reflection-mode NEA GaN photocathode was studied by the surface model ?GaN (Mg) :Cs ?:O-Cs. The reduction of the effective dipole quantity, which is caused by harmful gases, is the key factor of the photocurrent reduction.

Vertical GaN Devices for Power Electronics in Extreme Environments

DTIC Science & Technology

2016-03-31

electronics applications. In this paper vertical p-n diodes and transistors fabricated on pseudo bulk low defect density (104 to 106 cm-2) GaN substrates are...holes in p-GaN has deleterious effect on p-n junction behavior (Fig. 2), p-GaN contacts, and channel control in junction field-effect transistors at...and transistors ) utilizing p-n junctions are suitable for most practical applications including automotive (210K < T < 423K) but may have limitations

Ultra High p-doping Material Research for GaN Based Light Emitters

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

Vladimir Dmitriev

2007-06-30

The main goal of the Project is to investigate doping mechanisms in p-type GaN and AlGaN and controllably fabricate ultra high doped p-GaN materials and epitaxial structures. Highly doped p-type GaN-based materials with low electrical resistivity and abrupt doping profiles are of great importance for efficient light emitters for solid state lighting (SSL) applications. Cost-effective hydride vapor phase epitaxial (HVPE) technology was proposed to investigate and develop p-GaN materials for SSL. High p-type doping is required to improve (i) carrier injection efficiency in light emitting p-n junctions that will result in increasing of light emitting efficiency, (ii) current spreading inmore » light emitting structures that will improve external quantum efficiency, and (iii) parameters of Ohmic contacts to reduce operating voltage and tolerate higher forward currents needed for the high output power operation of light emitters. Highly doped p-type GaN layers and AlGaN/GaN heterostructures with low electrical resistivity will lead to novel device and contact metallization designs for high-power high efficiency GaN-based light emitters. Overall, highly doped p-GaN is a key element to develop light emitting devices for the DOE SSL program. The project was focused on material research for highly doped p-type GaN materials and device structures for applications in high performance light emitters for general illumination P-GaN and p-AlGaN layers and multi-layer structures were grown by HVPE and investigated in terms of surface morphology and structure, doping concentrations and profiles, optical, electrical, and structural properties. Tasks of the project were successfully accomplished. Highly doped GaN materials with p-type conductivity were fabricated. As-grown GaN layers had concentration N{sub a}-N{sub d} as high as 3 x 10{sup 19} cm{sup -3}. Mechanisms of doping were investigated and results of material studies were reported at several International conferences

Flexible GaN for High Performance, Strainable Radio Frequency Devices (Postprint)

DTIC Science & Technology

2017-11-02

devices on van der Waals (vdW) layers has been facilitated by the recent avail - ability of high -quality atomically smooth BN and graphene epi- taxial...AFRL-RX-WP-JA-2017-0333 FLEXIBLE GaN FOR HIGH PERFORMANCE, STRAINABLE RADIO FREQUENCY DEVICES (POSTPRINT) Elizabeth A. Moore and Timothy...2. REPORT TYPE 3. DATES COVERED (From - To) 5 April 2017 Interim 8 September 2014 – 5 March 2017 4. TITLE AND SUBTITLE FLEXIBLE GaN FOR HIGH

High voltage and high current density vertical GaN power diodes

DOE PAGES

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

2016-01-01

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

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.

10kW TWT Transition to GaN IRE

DTIC Science & Technology

2015-03-31

tubes in high power radar and Electronic Warfare (EW) applications. GaN transistors, using evaluation boards, were tested and analyzed, supplementing...29 Appendix B. High Power Amplifier Testing Data...19 Figure 11. High Power RF Amplifier Test Set ............................................................................. 22 Figure 12

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.

The trap states in lightly Mg-doped GaN grown by MOVPE on a freestanding GaN substrate

NASA Astrophysics Data System (ADS)

Narita, Tetsuo; Tokuda, Yutaka; Kogiso, Tatsuya; Tomita, Kazuyoshi; Kachi, Tetsu

2018-04-01

We investigated traps in lightly Mg-doped (2 × 1017 cm-3) p-GaN fabricated by metalorganic vapor phase epitaxy (MOVPE) on a freestanding GaN substrate and the subsequent post-growth annealing, using deep level transient spectroscopy. We identified four hole traps with energy levels of EV + 0.46, 0.88, 1.0, and 1.3 eV and one electron trap at EC - 0.57 eV in a p-type GaN layer uniformly doped with magnesium (Mg). The Arrhenius plot of hole traps with the highest concentration (˜3 × 1016 cm-3) located at EV + 0.88 eV corresponded to those of hole traps ascribed to carbon on nitrogen sites in n-type GaN samples grown by MOVPE. In fact, the range of the hole trap concentrations at EV + 0.88 eV was close to the carbon concentration detected by secondary ion mass spectroscopy. Moreover, the electron trap at EC - 0.57 eV was also identical to the dominant electron traps commonly observed in n-type GaN. Together, these results suggest that the trap states in the lightly Mg-doped GaN grown by MOVPE show a strong similarity to those in n-type GaN, which can be explained by the Fermi level close to the conduction band minimum in pristine MOVPE grown samples due to existing residual donors and Mg-hydrogen complexes.

Unstable behaviour of normally-off GaN E-HEMT under short-circuit

NASA Astrophysics Data System (ADS)

Martínez, P. J.; Maset, E.; Sanchis-Kilders, E.; Esteve, V.; Jordán, J.; Bta Ejea, J.; Ferreres, A.

2018-04-01

The short-circuit capability of power switching devices plays an important role in fault detection and the protection of power circuits. In this work, an experimental study on the short-circuit (SC) capability of commercial 600 V Gallium Nitride enhancement-mode high-electron-mobility transistors (E-HEMT) is presented. A different failure mechanism has been identified for commercial p-doped GaN gate (p-GaN) HEMT and metal-insulator-semiconductor (MIS) HEMT. In addition to the well known thermal breakdown, a premature breakdown is shown on both GaN HEMTs, triggered by hot electron trapping at the surface, which demonstrates that current commercial GaN HEMTs has requirements for improving their SC ruggedness.

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

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.

BInGaN alloys nearly lattice-matched to GaN for high-power high-efficiency visible LEDs

NASA Astrophysics Data System (ADS)

Williams, Logan; Kioupakis, Emmanouil

2017-11-01

InGaN-based visible light-emitting diodes (LEDs) find commercial applications for solid-state lighting and displays, but lattice mismatch limits the thickness of InGaN quantum wells that can be grown on GaN with high crystalline quality. Since narrower wells operate at a higher carrier density for a given current density, they increase the fraction of carriers lost to Auger recombination and lower the efficiency. The incorporation of boron, a smaller group-III element, into InGaN alloys is a promising method to eliminate the lattice mismatch and realize high-power, high-efficiency visible LEDs with thick active regions. In this work, we apply predictive calculations based on hybrid density functional theory to investigate the thermodynamic, structural, and electronic properties of BInGaN alloys. Our results show that BInGaN alloys with a B:In ratio of 2:3 are better lattice matched to GaN compared to InGaN and, for indium fractions less than 0.2, nearly lattice matched. Deviations from Vegard's law appear as bowing of the in-plane lattice constant with respect to composition. Our thermodynamics calculations demonstrate that the solubility of boron is higher in InGaN than in pure GaN. Varying the Ga mole fraction while keeping the B:In ratio constant enables the adjustment of the (direct) gap in the 1.75-3.39 eV range, which covers the entire visible spectrum. Holes are strongly localized in non-bonded N 2p states caused by local bond planarization near boron atoms. Our results indicate that BInGaN alloys are promising for fabricating nitride heterostructures with thick active regions for high-power, high-efficiency LEDs.

The optimal thickness of a transmission-mode GaN photocathode

NASA Astrophysics Data System (ADS)

Wang, Xiao-Hui; Shi, Feng; Guo, Hui; Hu, Cang-Lu; Cheng, Hong-Chang; Chang, Ben-Kang; Ren, Ling; Du, Yu-Jie; Zhang, Jun-Ju

2012-08-01

A 150-nm-thick GaN photocathode with a Mg doping concentration of 1.6 × 1017 cm-3 is activated by Cs/O in an ultrahigh vacuum chamber, and a quantum efficiency (QE) curve of the negative electron affinity transmission-mode (t-mode) of the GaN photocathode is obtained. The maximum QE reaches 13.0% at 290 nm. According to the t-mode QE equation solved from the diffusion equation, the QE curve is fitted. From the fitting results, the electron escape probability is 0.32, the back-interface recombination velocity is 5 × 104 cm·s-1, and the electron diffusion length is 116 nm. Based on these parameters, the influence of GaN thickness on t-mode QE is simulated. The simulation shows that the optimal thickness of GaN is 90 nm, which is better than the 150-nm GaN.

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.

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.

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.

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.

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.

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.

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

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

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

2016-03-07

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

Injection Laser Using Rare Earth Doped GaN Thin Films for Visible and Infrared Applications

DTIC Science & Technology

2010-05-01

system is used to measure the conductivity of GaN thin films in Nanolab. Undoped GaN thin films are usually n-type conductive with the electron ...being investigated for potential applications in optical communication and displays, due to the wide and direct energy bandgap of GaN resulting in low...enhancement on Eu3+ PL intensity. The electrical properties of GaN:RE thin films were changed from high resistive to

Transient atomic behavior and surface kinetics of GaN

NASA Astrophysics Data System (ADS)

Moseley, Michael; Billingsley, Daniel; Henderson, Walter; Trybus, Elaissa; Doolittle, W. Alan

2009-07-01

An in-depth model for the transient behavior of metal atoms adsorbed on the surface of GaN is developed. This model is developed by qualitatively analyzing transient reflection high energy electron diffraction (RHEED) signals, which were recorded for a variety of growth conditions of GaN grown by molecular-beam epitaxy (MBE) using metal-modulated epitaxy (MME). Details such as the initial desorption of a nitrogen adlayer and the formation of the Ga monolayer, bilayer, and droplets are monitored using RHEED and related to Ga flux and shutter cycles. The suggested model increases the understanding of the surface kinetics of GaN, provides an indirect method of monitoring the kinetic evolution of these surfaces, and introduces a novel method of in situ growth rate determination.

Fabrication and characterization of GaN nanowire doubly clamped resonators

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

Maliakkal, Carina B., E-mail: carina@tifr.res.in; Mathew, John P.; Hatui, Nirupam

2015-09-21

Gallium nitride (GaN) nanowires (NWs) have been intensely researched as building blocks for nanoscale electronic and photonic device applications; however, the mechanical properties of GaN nanostructures have not been explored in detail. The rigidity, thermal stability, and piezoelectric properties of GaN make it an interesting candidate for nano-electromechanical systems. We have fabricated doubly clamped GaN NW electromechanical resonators on sapphire using electron beam lithography and estimated the Young's modulus of GaN from resonance frequency measurements. For wires of triangular cross section with side ∼90 nm, we obtained values for the Young's modulus to be about 218 and 691 GPa, which are ofmore » the same order of magnitude as the values reported for bulk GaN. We also discuss the role of residual strain in the nanowire on the resonant frequency and the orientation dependence of the Young's modulus in wurtzite crystals.« less

Droplet heteroepitaxy of zinc-blende vs. wurtzite GaN quantum dots

NASA Astrophysics Data System (ADS)

Reese, C.; Jeon, S.; Hill, T.; Jones, C.; Shusterman, S.; Yacoby, Y.; Clarke, R.; Deng, H.; Goldman, Rs

We have developed a GaN droplet heteroepitaxy process based upon plasma-assisted molecular-beam epitaxy. Using various surface treatments and Ga deposition parameters, we have demonstrated polycrystalline, zinc-blende (ZB), and wurtzite (WZ) GaN quantum dots (QDs) on Si(001), r-Al2O3, Si(111), and c-GaN substrates. For the polar substrates (i.e. Si(111) and c-GaN), high-resolution transmission electron microscopy and coherent Bragg rod analysis reveals the formation of coherent WZ GaN QDs with nitridation-temperature-dependent sizes and densities. For the non-polar substrates (i.e. Si(001) and r-Al2O3) , QDs with strong near-band photoluminescence emission are observed and ZB GaN QD growth on Si(001) is demonstrated for the first time.

Study of GaN nanowires converted from β-Ga2O3 and photoconduction in a single nanowire

NASA Astrophysics Data System (ADS)

Kumar, Mukesh; Kumar, Sudheer; Chauhan, Neha; Sakthi Kumar, D.; Kumar, Vikram; Singh, R.

2017-08-01

The formation of GaN nanowires from β-Ga2O3 nanowires and photoconduction in a fabricated single GaN nanowire device has been studied. Wurtzite phase GaN were formed from monoclinic β-Ga2O3 nanowires with or without catalyst particles at their tips. The formation of faceted nanostructures from catalyst droplets presented on a nanowire tip has been discussed. The nucleation of GaN phases in β-Ga2O3 nanowires and their subsequent growth due to interfacial strain energy has been examined using a high resolution transmission electron microscope. The high quality of the converted GaN nanowire is confirmed by fabricating single nanowire photoconducting devices which showed ultra high responsivity under ultra-violet illumination.

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.

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

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

Qi, Meng; Zhao, Yuning; Yan, Xiaodong

2015-12-07

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

Optimization design on breakdown voltage of AlGaN/GaN high-electron mobility transistor

NASA Astrophysics Data System (ADS)

Yang, Liu; Changchun, Chai; Chunlei, Shi; Qingyang, Fan; Yuqian, Liu

2016-12-01

Simulations are carried out to explore the possibility of achieving high breakdown voltage of GaN HEMT (high-electron mobility transistor). GaN cap layers with gradual increase in the doping concentration from 2 × 1016 to 5 × 1019 cm-3 of N-type and P-type cap are investigated, respectively. Simulation results show that HEMT with P-doped GaN cap layer shows more potential to achieve higher breakdown voltage than N-doped GaN cap layer under the same doping concentration. This is because the ionized net negative space charges in P-GaN cap layer could modulate the surface electric field which makes more contribution to RESURF effect. Furthermore, a novel GaN/AlGaN/GaN HEMT with P-doped GaN buried layer in GaN buffer between gate and drain electrode is proposed. It shows enhanced performance. The breakdown voltage of the proposed structure is 640 V which is increased by 12% in comparison to UID (un-intentionally doped) GaN/AlGaN/GaN HEMT. We calculated and analyzed the distribution of electrons' density. It is found that the depleted region is wider and electric field maximum value is induced at the left edge of buried layer. So the novel structure with P-doped GaN buried layer embedded in GaN buffer has the better improving characteristics of the power devices. Project supported by the National Basic Research Program of China (No. 2014CB339900) and the Open Fund of Key Laboratory of Complex Electromagnetic Environment Science and Technology, China Academy of Engineering Physics (No. 2015-0214.XY.K).

Self-Supporting GaN Nanowires/Graphite Paper: Novel High-Performance Flexible Supercapacitor Electrodes.

PubMed

Wang, Shouzhi; Sun, Changlong; Shao, Yongliang; Wu, Yongzhong; Zhang, Lei; Hao, Xiaopeng

2017-02-01

Flexible supercapacitors have attracted great interest as energy storage devices because of their promise in applications such as wearable and smart electronic devices. Herein, a novel flexible supercapacitor electrode based on gallium nitride nanowire (GaN NW)/graphite paper (GP) nanocomposites is reported. The outstanding electrical conductivities of the GaN NW (6.36 × 10 2 S m -1 ) and GP (7.5 × 10 4 S m -1 ) deliver a synergistically enhanced electrochemical performance that cannot be achieved by either of the components alone. The composite electrode exhibits excellent specific capacitance (237 mF cm -2 at 0.1 mA cm -2 ) and outstanding cycling performance (98% capacitance retention after 10 000 cycles). The flexible symmetric supercapacitor also manifests high energy and power densities (0.30 mW h cm -3 and 1000 mW cm -3 ). These findings demonstrate that the GaN/GP composite electrode has significant potential as a candidate for the flexible energy storage devices. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Effect of different electrolytes on porous GaN using photo-electrochemical etching

NASA Astrophysics Data System (ADS)

Al-Heuseen, K.; Hashim, M. R.; Ali, N. K.

2011-05-01

This article reports the properties and the behavior of GaN during the photoelectrochemical etching process using four different electrolytes. The measurements show that the porosity strongly depends on the electrolyte and highly affects the surface morphology of etched samples, which has been revealed by scanning electron microscopy (SEM) images. Peak intensity of the photoluminescence (PL) spectra of the porous GaN samples was observed to be enhanced and strongly depend on the electrolytes. Among the samples, there is a little difference in the peak position indicating that the change of porosity has little influence on the PL peak shift, while it highly affecting the peak intensity. Raman spectra of porous GaN under four different solution exhibit phonon mode E 2 (high), A 1 (LO), A 1 (TO) and E 2 (low). There was a red shift in E 2 (high) in all samples, indicating a relaxation of stress in the porous GaN surface with respect to the underlying single crystalline epitaxial GaN. Raman and PL intensities were high for samples etched in H 2SO 4:H 2O 2 and KOH followed by the samples etched in HF:HNO 3 and in HF:C 2H 5OH.

Local electronic and optical behavior of ELO a-plane GaN

NASA Astrophysics Data System (ADS)

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

2007-03-01

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

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.

Geometric contribution leading to anomalous estimation of two-dimensional electron gas density in GaN based heterostructures

NASA Astrophysics Data System (ADS)

Upadhyay, Bhanu B.; Jha, Jaya; Takhar, Kuldeep; Ganguly, Swaroop; Saha, Dipankar

2018-05-01

We have observed that the estimation of two-dimensional electron gas density is dependent on the device geometry. The geometric contribution leads to the anomalous estimation of the GaN based heterostructure properties. The observed discrepancy is found to originate from the anomalous area dependent capacitance of GaN based Schottky diodes, which is an integral part of the high electron mobility transistors. The areal capacitance density is found to increase for smaller radii Schottky diodes, contrary to a constant as expected intuitively. The capacitance is found to follow a second order polynomial on the radius of all the bias voltages and frequencies considered here. In addition to the quadratic dependency corresponding to the areal component, the linear dependency indicates a peripheral component. It is further observed that the peripheral to areal contribution is inversely proportional to the radius confirming the periphery as the location of the additional capacitance. The peripheral component is found to be frequency dependent and tends to saturate to a lower value for measurements at a high frequency. In addition, the peripheral component is found to vanish when the surface is passivated by a combination of N2 and O2 plasma treatments. The cumulative surface state density per unit length of the perimeter of the Schottky diodes as obtained by the integrated response over the distance between the ohmic and Schottky contacts is found to be 2.75 × 1010 cm-1.

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.

Atomic scale morphology, growth behaviour and electronic properties of semipolar {101[overline]3} GaN surfaces.

PubMed

Kioseoglou, J; Kalesaki, E; Lymperakis, L; Karakostas, Th; Komninou, Ph

2013-01-30

First-principles calculations relating to the atomic structure and electronic properties of {101[overline]3} GaN surfaces reveal significant differentiations between the two polarity orientations. The (101[overline]3) surface exhibits a remarkable morphological stability, stabilizing a metallic structure (Ga adlayer) over the entire range of the Ga chemical potential. In contrast, the semiconducting, cleaved surface is favoured on (101[overline]3[overline]) under extremely and moderately N-rich conditions, a Ga bilayer is stabilized under corresponding Ga-rich conditions and various transitions between metallic reconstructions take place in intermediate growth stoichiometries. Efficient growth schemes for smooth, two-dimensional GaN layers and the isolation of {101[overline]3} material from parasitic orientations are identified.

Dopant Adsorption and Incorporation at Irradiated GaN Surfaces

NASA Astrophysics Data System (ADS)

Sun, Qiang; Selloni, Annabella; Myers, Thomas; Doolittle, W. Alan

2006-03-01

Mg and O are two of the common dopants in GaN, but, in spite of extensive investigation, the atomic scale understanding of their adsorption and incorporation is still incomplete. In particular, high-energy electron irradiation, such as occurring during RHEED, has been reported to have an important effect on the incorporation of these impurities, but no study has addressed the detailed mechanisms of this effect yet. Here we use DFT calculations to study the adsorption and incorporation of Mg and O at the Ga- and N-polar GaN surfaces under various Ga, Mg and O coverage conditions as well as in presence of light or electron beam-induced electronic excitation. We find that the adsorption and incorporation of the two impurities have opposite surface polarity dependence: substitutional Mg prefers to incorporate at the GaN(0001) surface, while O prefers to adsorb and incorporate at the N-polar surface. In addition, our results indicate that in presence of light irradiation the tendency of Mg to surface-segregate is reduced. The O adsorption energy on the N-polar surface is also significantly reduced, consistent with the experimental observation of a much smaller concentration of oxygen in the irradiated samples.

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.

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.

Effect of screw threading dislocations and inverse domain boundaries in GaN on the shape of reciprocal-space maps.

PubMed

Barchuk, Mykhailo; Motylenko, Mykhaylo; Lukin, Gleb; Pätzold, Olf; Rafaja, David

2017-04-01

The microstructure of polar GaN layers, grown by upgraded high-temperature vapour phase epitaxy on [001]-oriented sapphire substrates, was studied by means of high-resolution X-ray diffraction and transmission electron microscopy. Systematic differences between reciprocal-space maps measured by X-ray diffraction and those which were simulated for different densities of threading dislocations revealed that threading dislocations are not the only microstructure defect in these GaN layers. Conventional dark-field transmission electron microscopy and convergent-beam electron diffraction detected vertical inversion domains as an additional microstructure feature. On a series of polar GaN layers with different proportions of threading dislocations and inversion domain boundaries, this contribution illustrates the capability and limitations of coplanar reciprocal-space mapping by X-ray diffraction to distinguish between these microstructure features.

Suppression of the self-heating effect in GaN HEMT by few-layer graphene heat spreading elements

NASA Astrophysics Data System (ADS)

Volcheck, V. S.; Stempitsky, V. R.

2017-11-01

Self-heating has an adverse effect on characteristics of gallium nitride (GaN) high electron mobility transistors (HEMTs). Various solutions to the problem have been proposed, however, a temperature rise due to dissipated electrical power still hinders the production of high power and high speed GaN devices. In this paper, thermal management of GaN HEMT via few-layer graphene (FLG) heat spreading elements is investigated. It is shown that integration of the FLG elements on top of the device structure considerably reduces the maximum temperature and improves the DC and small signal AC performance.

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

NASA Astrophysics Data System (ADS)

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

2003-07-01

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

p-type zinc-blende GaN on GaAs substrates

NASA Astrophysics Data System (ADS)

Lin, M. E.; Xue, G.; Zhou, G. L.; Greene, J. E.; Morkoç, H.

1993-08-01

We report p-type cubic GaN. The Mg-doped layers were grown on vicinal (100) GaAs substrates by plasma-enhanced molecular beam epitaxy. Thermally sublimed Mg was, with N2 carrier gas, fed into an electron-cyclotron resonance source. p-type zinc-blende-structure GaN films were achieved with hole mobilities as high as 39 cm2/V s at room temperature. The cubic nature of the films were confirmed by x-ray diffractometry. The depth profile of Mg was investigated by secondary ions mass spectroscopy.

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.

Pt-decorated GaN nanowires with significant improvement in H2 gas-sensing performance at room temperature.

PubMed

Abdullah, Q N; Yam, F K; Hassan, Z; Bououdina, M

2015-12-15

Superior sensitivity towards H2 gas was successfully achieved with Pt-decorated GaN nanowires (NWs) gas sensor. GaN NWs were fabricated via chemical vapor deposition (CVD) route. Morphology (field emission scanning electron microscopy and transmission electron microscopy) and crystal structure (high resolution X-ray diffraction) characterizations of the as-synthesized nanostructures demonstrated the formation of GaN NWs having a wurtzite structure, zigzaged shape and an average diameter of 30-166nm. The Pt-decorated GaN NWs sensor shows a high response of 250-2650% upon exposure to H2 gas concentration from 7 to 1000ppm respectively at room temperature (RT), and then increases to about 650-4100% when increasing the operating temperature up to 75°C. The gas-sensing measurements indicated that the Pt-decorated GaN NWs based sensor exhibited efficient detection of H2 at low concentration with excellent sensitivity, repeatability, and free hysteresis phenomena over a period of time of 100min. The large surface-to-volume ratio of GaN NWs and the catalytic activity of Pt metal are the most influential factors leading to the enhancement of H2 gas-sensing performances through the improvement of the interaction between the target molecules (H2) and the sensing NWs surface. The attractive low-cost, low power consumption and high-performance of the resultant decorated GaN NWs gas sensor assure their uppermost potential for H2 gas sensor working at low operating temperature. Copyright © 2015 Elsevier Inc. All rights reserved.

P-type doping of GaN

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

Wong, Raechelle Kimberly

2000-04-01

After implantation of As, As + Be, and As + Ga into GaN and annealing for short durations at temperatures as high as 1500 C, the GaN films remained highly resistive. It was apparent from c-RBS studies that although implantation damage did not create an amorphous layer in the GaN film, annealing at 1500 C did not provide enough energy to completely recover the radiation damage. Disorder recovered significantly after annealing at temperatures up to 1500 C, but not completely. From SIMS analysis, oxygen contamination in the AIN capping layer causes oxygen diffusion into the GaN film above 1400 C.more » The sapphire substrate (A1203) also decomposed and oxygen penetrated into the backside of the GaN layer above 1400 C. To prevent donor-like oxygen impurities from the capping layer and the substrate from contaminating the GaN film and compensating acceptors, post-implantation annealing should be done at temperatures below 1500 C. Oxygen in the cap could be reduced by growing the AIN cap on the GaN layer after the GaN growth run or by depositing the AIN layer in a ultra high vacuum (UHV) system post-growth to minimize residual oxygen and water contamination. With longer annealing times at 1400 C or at higher temperatures with a higher quality AIN, the implantation drainage may fully recover.« less

Demonstration of a High Open-Circuit Voltage GaN Betavoltaic Microbattery

NASA Astrophysics Data System (ADS)

Cheng, Zai-Jun; San, Hai-Sheng; Chen, Xu-Yuan; Liu, Bo; Feng, Zhi-Hong

2011-07-01

A high open-circuit voltage betavoltaic microbattery based on a GaN p-i-n diode is demonstrated. Under the irradiation of a 4×4 mm2 planar solid 63Ni source with an activity of 2 mCi, the open-circuit voltage Voc of the fabricated single 2×2mm2 cell reaches as high as 1.62 V, the short-circuit current density Jsc is measured to be 16nA/cm2. The microbattery has a fill factor of 55%, and the energy conversion efficiency of beta radiation into electricity reaches to 1.13%. The results suggest that GaN is a highly promising potential candidate for long-life betavoltaic microbatteries used as power supplies for microelectromechanical system devices.

High quality self-separated GaN crystal grown on a novel nanoporous template by HVPE.

PubMed

Huo, Qin; Shao, Yongliang; Wu, Yongzhong; Zhang, Baoguo; Hu, Haixiao; Hao, Xiaopeng

2018-02-16

In this study, a novel nanoporous template was obtained by a two-step etching process from MOCVD-GaN/Al 2 O 3 (MGA) with electrochemical etching sequentially followed by chemical wet etching. The twice-etched MOCVD-GaN/Al 2 O 3 (TEMGA) templates were utilized to grow GaN crystals by hydride vapor phase epitaxy (HVPE) method. The GaN crystals were separated spontaneously from the TEMGA template with the assistance of voids formed by the etched nanopores. Several techniques were utilized to characterize the quality of the free-standing GaN crystals obtained from the TEMGA template. Results showed that the quality of the as-obtained GaN crystals was improved obviously compared with those grown on the MGA. This convenient technique can be applied to grow high-quality free-standing GaN crystals.

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.

Reflection high energy electron diffraction study of nitrogen plasma interactions with a GaAs (100) surface

NASA Astrophysics Data System (ADS)

Hauenstein, R. J.; Collins, D. A.; Cai, X. P.; O'Steen, M. L.; McGill, T. C.

1995-05-01

Effect of a nitrogen electron-cyclotron-resonance (ECR) microwave plasma on near-surface composition, crystal structure, and morphology of the As-stabilized GaAs (100) surface is investigated with the use of digitally image-processed in situ reflection high energy electron diffraction. Nitridation is performed on molecular beam epitaxially (MBE) grown GaAs surfaces near 600 °C under typical conditions for ECR microwave plasma-assisted MBE growth of GaN films on GaAs. Brief plasma exposures (≊3-5 s) are shown to result in a specular, coherently strained, relatively stable, GaN film approximately one monolayer in thickness, which can be commensurately overgrown with GaAs while longer exposures (up to 1 min) result in incommensurate zincblende epitaxial GaN island structures. Specular and nonspecular film formations are explained in terms of N-for-As surface and subsurface anion exchange reactions, respectively. Commensurate growth of ultrathin buried GaN layers in GaAs is achieved.

Comparison of gate and drain current detection of hydrogen at room temperature with AlGaN /GaN high electron mobility transistors

NASA Astrophysics Data System (ADS)

Wang, Hung-Ta; Kang, B. S.; Ren, F.; Fitch, R. C.; Gillespie, J. K.; Moser, N.; Jessen, G.; Jenkins, T.; Dettmer, R.; Via, D.; Crespo, A.; Gila, B. P.; Abernathy, C. R.; Pearton, S. J.

2005-10-01

Pt-gated AlGaN /GaN high electron mobility transistors can be used as room-temperature hydrogen gas sensors at hydrogen concentrations as low as 100ppm. A comparison of the changes in drain and gate current-voltage (I-V) characteristics with the introduction of 500ppm H2 into the measurement ambient shows that monitoring the change in drain-source current provides a wider gate voltage operation range for maximum detection sensitivity and higher total current change than measuring the change in gate current. However, over a narrow gate voltage range, the relative sensitivity of detection by monitoring the gate current changes is up to an order of magnitude larger than that of drain-source current changes. In both cases, the changes are fully reversible in <2-3min at 25°C upon removal of the hydrogen from the ambient.

Luminescence properties of defects in GaN

NASA Astrophysics Data System (ADS)

Reshchikov, Michael A.; Morkoç, Hadis

2005-03-01

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

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

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

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

2015-10-07

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

Individual electron and hole localization in submonolayer InN quantum sheets embedded in GaN

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

Feix, F., E-mail: feix@pdi-berlin.de; Flissikowski, T.; Chèze, C.

2016-07-25

We investigate sub-monolayer InN quantum sheets embedded in GaN(0001) by temperature-dependent photoluminescence spectroscopy under both continuous-wave and pulsed excitation. Both the peak energy and the linewidth of the emission band associated with the quantum sheets exhibit an anomalous dependence on temperature indicative of carrier localization. Photoluminescence transients reveal a power law decay at low temperatures reflecting that the recombining electrons and holes occupy spatially separate, individual potential minima reminiscent of conventional (In,Ga)N(0001) quantum wells exhibiting the characteristic disorder of a random alloy. At elevated temperatures, carrier delocalization sets in and is accompanied by a thermally activated quenching of the emission.more » We ascribe the strong nonradiative recombination to extended states in the GaN barriers and confirm our assumption by a simple rate-equation model.« less

The photocatalytic properties of hollow (GaN)1-x(ZnO)x composite nanofibers synthesized by electrospinning

NASA Astrophysics Data System (ADS)

Wang, Ding; Zhang, Minglu; Zhuang, Huaijuan; Chen, Xu; Wang, Xianying; Zheng, Xuejun; Yang, Junhe

2017-02-01

(GaN)1-x(ZnO)x composite nanofibers with hollow structure were prepared by initial electrospinning, and the subsequent calcination and nitridation. The structure and morphology characteristics of samples were investigated by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The characterization results showed the phase transition from ZnGa2O4 to (GaN)1-x(ZnO)x solid-solution under ammonia atmosphere. The preparation conditions were explored and the optimum nitridation temperature and holding time are 750 °C and 2 h, respectively. The photocatalytic properties of (GaN)1-x(ZnO)x with different Ga:Zn atomic ratios were investigated by degrading Rhodamine B under the visible light irradiation. The photocatalytic activity sequence is (GaN)1-x(ZnO)x (Ga:Zn = 1:2) > (GaN)1-x(ZnO)x (Ga:Zn = 1:3) > ZnO nanofibers > (GaN)1-x(ZnO)x (Ga:Zn = 1:4) > (GaN)1-x(ZnO)x (Ga:Zn = 1:1). The photocatalytic mechanism of the (GaN)1-x(ZnO)x hollow nanofibers was further studied by UV-vis diffuse reflectance spectra. The excellent photocatalytic performance of (GaN)1-x(ZnO)x hollow nanofibers was attributed to the narrow band gap and high surface area of porous nanofibers with hollow structure.

GaN light-emitting device based on ionic liquid electrolyte

NASA Astrophysics Data System (ADS)

Hirai, Tomoaki; Sakanoue, Tomo; Takenobu, Taishi

2018-06-01

Ionic liquids (ILs) are attractive materials for fabricating unique hybrid devices based on electronics and electrochemistry; thus, IL-gated transistors and organic light-emitting devices of light-emitting electrochemical cells (LECs) are investigated for future low-voltage and high-performance devices. In LECs, voltage application induces the formation of electrochemically doped p–n homojunctions owing to ion rearrangements in composites of semiconductors and electrolytes, and achieves electron–hole recombination for light emission at the homojunctions. In this work, we applied this concept of IL-induced electrochemical doping to the fabrication of GaN-based light-emitting devices. We found that voltage application to the layered IL/GaN structure accumulated electrons on the GaN surface owing to ion rearrangements and improved the conductivity of GaN. The ion rearrangement also enabled holes to be injected by the strong electric field of electric double layers on hole injection contacts. This simultaneous injection of holes and electrons into GaN mediated by ions achieves light emission at a low voltage of around 3.4 V. The light emission from the simple IL/GaN structure indicates the usefulness of an electrochemical technique in generating light emission with great ease of fabrication.

Adsorption and desorption of hydrogen at nonpolar GaN (1 1 ¯ 00 ) surfaces: Kinetics and impact on surface vibrational and electronic properties

NASA Astrophysics Data System (ADS)

Lymperakis, L.; Neugebauer, J.; Himmerlich, M.; Krischok, S.; Rink, M.; Kröger, J.; Polyakov, V. M.

2017-05-01

The adsorption of hydrogen at nonpolar GaN (1 1 ¯00 ) surfaces and its impact on the electronic and vibrational properties is investigated using surface electron spectroscopy in combination with density functional theory (DFT) calculations. For the surface mediated dissociation of H2 and the subsequent adsorption of H, an energy barrier of 0.55 eV has to be overcome. The calculated kinetic surface phase diagram indicates that the reaction is kinetically hindered at low pressures and low temperatures. At higher temperatures ab initio thermodynamics show, that the H-free surface is energetically favored. To validate these theoretical predictions experiments at room temperature and under ultrahigh vacuum conditions were performed. They reveal that molecular hydrogen does not dissociatively adsorb at the GaN (1 1 ¯00 ) surface. Only activated atomic hydrogen atoms attach to the surface. At temperatures above 820 K, the attached hydrogen gets desorbed. The adsorbed hydrogen atoms saturate the dangling bonds of the gallium and nitrogen surface atoms and result in an inversion of the Ga-N surface dimer buckling. The signatures of the Ga-H and N-H vibrational modes on the H-covered surface have experimentally been identified and are in good agreement with the DFT calculations of the surface phonon modes. Both theory and experiment show that H adsorption results in a removal of occupied and unoccupied intragap electron states of the clean GaN (1 1 ¯00 ) surface and a reduction of the surface upward band bending by 0.4 eV. The latter mechanism largely reduces surface electron depletion.

Fine structure of the red luminescence band in undoped GaN

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

Reshchikov, M. A., E-mail: mreshchi@vcu.edu; Usikov, A.; Saint-Petersburg National Research University of Information Technologies, Mechanics and Optics, 49 Kronverkskiy Ave., 197101 Saint Petersburg

2014-01-20

Many point defects in GaN responsible for broad photoluminescence (PL) bands remain unidentified. Their presence in thick GaN layers grown by hydride vapor phase epitaxy (HVPE) detrimentally affects the material quality and may hinder the use of GaN in high-power electronic devices. One of the main PL bands in HVPE-grown GaN is the red luminescence (RL) band with a maximum at 1.8 eV. We observed the fine structure of this band with a zero-phonon line (ZPL) at 2.36 eV, which may help to identify the related defect. The shift of the ZPL with excitation intensity and the temperature-related transformation of the RLmore » band fine structure indicate that the RL band is caused by transitions from a shallow donor (at low temperature) or from the conduction band (above 50 K) to an unknown deep acceptor having an energy level 1.130 eV above the valence band.« less

Use of GaN as a Scintillating Ionizing Radiation Detector

NASA Astrophysics Data System (ADS)

Wensman, Johnathan; Guardala, Noel; Mathur, Veerendra; Alasagas, Leslie; Vanhoy, Jeffrey; Statham, John; Marron, Daniel; Millett, Marshall; Marsh, Jarrod; Currie, John; Price, Jack

2017-09-01

Gallium nitride (GaN) is a III/V direct bandgap semiconductor which has been used in light emitting diodes (LEDs) since the 1990s. Currently, due to a potential for increased efficiency, GaN is being investigated as a replacement for silicon in power electronics finding potential uses ranging from data centers to electric vehicles. In addition to LEDs and power electronics though, doped GaN can be used as a gamma insensitive fast neutron detector due to the direct band-gap, light propagation properties, and response to ionizing radiations. Investigation of GaN as a semiconductor scintillator for use in a radiation detection system involves mapping the response function of the detector crystal over a range of photon and neutron energies, and measurements of light generation in the GaN crystal due to proton, alpha, and nitrogen projectiles. In this presentation we discuss the measurements made to date, and plausible interpretations of the response functions. This work funded in part by the Naval Surface Warfare Center, Carderock Division In-house Laboratory Independent Research program.

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

NASA Astrophysics Data System (ADS)

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

2018-04-01

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

Study of GaN nanorods converted from β-Ga2O3

NASA Astrophysics Data System (ADS)

Li, Yuewen; Xiong, Zening; Zhang, Dongdong; Xiu, Xiangqian; Liu, Duo; Wang, Shuang; Hua, Xuemei; Xie, Zili; Tao, Tao; Liu, Bin; Chen, Peng; Zhang, Rong; Zheng, Youdou

2018-05-01

We report here high-quality β-Ga2O3 nanorods (NRs) grown on sapphire substrates by hydrothermal method. Ammoniating the β-Ga2O3 NRs results in strain-free wurtzite gallium nitride (GaN) NRs. It was shown by XRD and Raman spectroscopy that β-Ga2O3 was partially converted to GaN/β-Ga2O3 at 1000 °C and then completely converted to GaN NRs at 1050 °C, as confirmed by high-resolution transmission electron microscopy (HRTEM). There is no band-edge emission of β-Ga2O3 in the cathodoluminescence spectrum, and only a deep-level broad emission observed at 3.68-3.73 eV. The band edge emission (3.39 eV) of GaN NRs converted from β-Ga2O3 can also be observed.

Laser-induced local activation of Mg-doped GaN with a high lateral resolution for high power vertical devices

NASA Astrophysics Data System (ADS)

Kurose, Noriko; Matsumoto, Kota; Yamada, Fumihiko; Roffi, Teuku Muhammad; Kamiya, Itaru; Iwata, Naotaka; Aoyagi, Yoshinobu

2018-01-01

A method for laser-induced local p-type activation of an as-grown Mg-doped GaN sample with a high lateral resolution is developed for realizing high power vertical devices for the first time. As-grown Mg-doped GaN is converted to p-type GaN in a confined local area. The transition from an insulating to a p-type area is realized to take place within about 1-2 μm fine resolution. The results show that the technique can be applied in fabricating the devices such as vertical field effect transistors, vertical bipolar transistors and vertical Schottkey diode so on with a current confinement region using a p-type carrier-blocking layer formed by this technique.

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.

Highly resistive C-doped hydride vapor phase epitaxy-GaN grown on ammonothermally crystallized GaN seeds

NASA Astrophysics Data System (ADS)

Iwinska, Malgorzata; Piotrzkowski, Ryszard; Litwin-Staszewska, Elzbieta; Sochacki, Tomasz; Amilusik, Mikolaj; Fijalkowski, Michal; Lucznik, Boleslaw; Bockowski, Michal

2017-01-01

GaN crystals were grown by hydride vapor phase epitaxy (HVPE) and doped with C. The seeds were high-structural-quality ammonothermally crystallized GaN. The grown crystals were highly resistive at 296 K and of high structural quality. High-temperature Hall effect measurements revealed p-type conductivity and a deep acceptor level in the material with an activation energy of 1 eV. This is in good agreement with density functional theory calculations based on hybrid functionals as presented by the Van de Walle group. They obtained an ionization energy of 0.9 eV when C was substituted for N in GaN and acted as a deep acceptor.

Boron doped GaN and InN: Potential candidates for spintronics

NASA Astrophysics Data System (ADS)

Fan, S. W.; Huang, X. N.; Yao, K. L.

2017-02-01

The full potential linearized augmented plane wave method together with the Tran-Blaha modified Becke-Johnson potential is utilized to investigate the electronic structures and magnetism for boron doped GaN and InN. Calculations show the boron substituting nitrogen (BN defects) could induce the GaN and InN to be half-metallic ferromagnets. The magnetic moments mainly come from the BN defects, and each BN defect would produce the 2.00 μB total magnetic moment. The electronic structures indicate the carriers-mediated double exchange interaction plays a crucial role in forming the ferromagnetism. Positive chemical pair interactions imply the BN defects would form the homogeneous distribution in GaN and InN matrix. Moderate formation energies suggest that GaN and InN with BN defects could be fabricated experimentally.

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.

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

NASA Astrophysics Data System (ADS)

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

2018-07-01

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

Thermo-piezo-electro-mechanical simulation of AlGaN (aluminum gallium nitride) / GaN (gallium nitride) High Electron Mobility Transistors

NASA Astrophysics Data System (ADS)

Stevens, Lorin E.

Due to the current public demand of faster, more powerful, and more reliable electronic devices, research is prolific these days in the area of high electron mobility transistor (HEMT) devices. This is because of their usefulness in RF (radio frequency) and microwave power amplifier applications including microwave vacuum tubes, cellular and personal communications services, and widespread broadband access. Although electrical transistor research has been ongoing since its inception in 1947, the transistor itself continues to evolve and improve much in part because of the many driven researchers and scientists throughout the world who are pushing the limits of what modern electronic devices can do. The purpose of the research outlined in this paper was to better understand the mechanical stresses and strains that are present in a hybrid AlGaN (Aluminum Gallium Nitride) / GaN (Gallium Nitride) HEMT, while under electrically-active conditions. One of the main issues currently being researched in these devices is their reliability, or their consistent ability to function properly, when subjected to high-power conditions. The researchers of this mechanical study have performed a static (i.e. frequency-independent) reliability analysis using powerful multiphysics computer modeling/simulation to get a better idea of what can cause failure in these devices. Because HEMT transistors are so small (micro/nano-sized), obtaining experimental measurements of stresses and strains during the active operation of these devices is extremely challenging. Physical mechanisms that cause stress/strain in these structures include thermo-structural phenomena due to mismatch in both coefficient of thermal expansion (CTE) and mechanical stiffness between different materials, as well as stress/strain caused by "piezoelectric" effects (i.e. mechanical deformation caused by an electric field, and conversely voltage induced by mechanical stress) in the AlGaN and GaN device portions (both

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

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.

Thermal Conductivity and Large Isotope Effect in GaN from First Principles

DTIC Science & Technology

2012-08-28

August 2012) We present atomistic first principles results for the lattice thermal conductivity of GaN and compare them to those for GaP, GaAs, and GaSb ...weak scattering results from stiff atomic bonds and the large Ga to N mass ratio, which give phonons high frequencies and also a pronounced energy gap...66.70.f, 63.20.kg, 71.15.m Introduction.—Gallium nitride (GaN) is a wide band gap semiconductor and a promising candidate for use in opto- electronic

Fast Growth of GaN Epilayers via Laser-Assisted Metal-Organic Chemical Vapor Deposition for Ultraviolet Photodetector Applications.

PubMed

Rabiee Golgir, Hossein; Li, Da Wei; Keramatnejad, Kamran; Zou, Qi Ming; Xiao, Jun; Wang, Fei; Jiang, Lan; Silvain, Jean-François; Lu, Yong Feng

2017-06-28

In this study, we successfully developed a carbon dioxide (CO 2 )-laser-assisted metal-organic chemical vapor deposition (LMOCVD) approach to fast synthesis of high-quality gallium nitride (GaN) epilayers on Al 2 O 3 [sapphire(0001)] substrates. By employing a two-step growth procedure, high crystallinity and smooth GaN epilayers with a fast growth rate of 25.8 μm/h were obtained. The high crystallinity was confirmed by a combination of techniques, including X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and atomic force microscopy. By optimizing growth parameters, the ∼4.3-μm-thick GaN films grown at 990 °C for 10 min showed a smooth surface with a root-mean-square surface roughness of ∼1.9 nm and excellent thickness uniformity with sharp GaN/substrate interfaces. The full-width at half-maximum values of the GaN(0002) X-ray rocking curve of 313 arcsec and the GaN(101̅2) X-ray rocking curve of 390 arcsec further confirmed the high crystallinity of the GaN epilayers. We also fabricated ultraviolet (UV) photodetectors based on the as-grown GaN layers, which exhibited a high responsivity of 0.108 A W -1 at 367 nm and a fast response time of ∼125 ns, demonstrating its high optical quality with potential in optoelectronic applications. Our strategy thus provides a simple and cost-effective means toward fast and high-quality GaN heteroepitaxy growth suitable for fabricating high-performance GaN-based UV detectors.

Rogue waves lead to the instability in GaN semiconductors

PubMed Central

Yahia, M. E.; Tolba, R. E.; El-Bedwehy, N. A.; El-Labany, S. K.; Moslem, W. M.

2015-01-01

A new approach to understand the electron/hole interfaced plasma in GaN high electron mobility transistors (HEMTs). A quantum hydrodynamic model is constructed to include electrons/holes degenerate pressure, Bohm potential, and the exchange/correlation effect and then reduced to the nonlinear Schrödinger equation (NLSE). Numerical analysis of the latter predicts the rough (in)stability domains, which allow for the rogue waves to occur. Our results might give physical solution rather than the engineering one to the intrinsic problems in these high frequency/power transistors. PMID:26206731

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.

Electronic and optical properties of GaN under pressure: DFT calculations

NASA Astrophysics Data System (ADS)

Javaheri, Sahar; Boochani, Arash; Babaeipour, Manuchehr; Naderi, Sirvan

2017-12-01

Optical and electronic properties of ZB, RS and WZ structures of gallium nitride (GaN) are studied in equilibrium and under pressure using the first-principles calculation in the density functional theory (DFT) framework to obtain quantities like dielectric function, loss function, reflectance and absorption spectra, refractive index and their relation parameters. The electronic properties are studied using EV-GGA and GGA approximations and the results calculated by EV-GGA approximation were found to be much closer to the experimental results. The interband electron transitions are studied using the band structure and electron transition peaks in the imaginary part of the dielectric function; these transitions occur in three structures from N-2p orbital to Ga-4s and Ga-4p orbitals in the conduction band. Different optical properties of WZ structure were calculated in two polarization directions of (100) and (001) and the results were close to each other. Plasmon energy corresponding to the main peak of the energy-loss function in RS with the value of 26 eV was the highest one, which increased under pressure. In general, RS shows more different properties than WZ and ZB.

Carrier confinement effects of InxGa1-xN/GaN multi quantum disks with GaN surface barriers grown in GaN nanorods

NASA Astrophysics Data System (ADS)

Park, Youngsin; Chan, Christopher C. S.; Taylor, Robert A.; Kim, Nammee; Jo, Yongcheol; Lee, Seung W.; Yang, Woochul; Im, Hyunsik

2018-04-01

Structural and optical properties of InxGa1-xN/GaN multi quantum disks (QDisks) grown on GaN nanorods by molecular beam epitaxy have been investigated by transmission electron microscopy and micro-photoluminescence (PL) spectroscopy. Two types of InGaN QDisks were grown: a pseudo-3D confined InGaN pillar-type QDisks embedded in GaN nanorods; and QDisks in flanged cone type GaN nanorods. The PL emission peak and excitation dependent PL behavior of the pillar-type Qdisks differ greatly from those of the flanged cone type QDisks. Time resolved PL was carried out to probe the differences in charge carrier dynamics. The results suggest that by constraining the formation of InGaN QDisks within the centre of the nanorod, carriers are restricted from migrating to the surface, decreasing the surface recombination at high carrier densities.

High performance of Ga-doped ZnO transparent conductive layers using MOCVD for GaN LED applications.

PubMed

Horng, Ray-Hua; Shen, Kun-Ching; Yin, Chen-Yang; Huang, Chiung-Yi; Wuu, Dong-Sing

2013-06-17

High performance of Ga-doped ZnO (GZO) prepared using metalorganic chemical vapor deposition (MOCVD) was employed in GaN blue light-emitting diodes (LEDs) as transparent conductive layers (TCL). By the post-annealing process, the annealed 800°C GZO films exhibited a high transparency above 97% at wavelength of 450 nm. The contact resistance of GZO decreased with the annealing temperature increasing. It was attributed to the improvement of the GZO crystal quality, leading to an increase in electron concentration. It was also found that some Zn atom caused from the decomposition process diffused into the p-GaN surface of LED, which generated a stronger tunneling effect at the GZO/p-GaN interface and promoted the formation of ohmic contact. Moreover, contrast to the ITO-LED, a high light extraction efficiency of 77% was achieved in the GZO-LED at injection current of 20 mA. At 350 mA injection current, the output power of 256.51 mW of GZO-LEDs, corresponding to a 21.5% enhancement as compared to ITO-LEDs was obtained; results are promising for the development of GZO using the MOCVD technique for GaN LED applications.

Origins of Fermi-level pinning on GaN and InN polar and nonpolar surfaces

NASA Astrophysics Data System (ADS)

Segev, D.; Van de Walle, C. G.

2006-10-01

Using band structure and total energy methods, we study the atomic and electronic structures of the polar (+c and - c plane) and nonpolar (a and m plane) surfaces of GaN and InN. We identify two distinct microscopic origins for Fermi-level pinning on GaN and InN, depending on surface stoichiometry and surface polarity. At moderate Ga/N ratios unoccupied gallium dangling bonds pin the Fermi level on n-type GaN at 0.5 0.7 eV below the conduction-band minimum. Under highly Ga-rich conditions metallic Ga adlayers lead to Fermi-level pinning at 1.8 eV above the valence-band maximum. We also explain the source of the intrinsic electron accumulation that has been universally observed on polar InN surfaces. It is caused by In-In bonds leading to occupied surface states above the conduction-band minimum. We predict that such a charge accumulation will be absent on the nonpolar surfaces of InN, when prepared under specific conditions.

Synthesis and Raman scattering of GaN nanorings, nanoribbons and nanowires

NASA Astrophysics Data System (ADS)

Li, Z. J.; Chen, X. L.; Li, H. J.; Tu, Q. Y.; Yang, Z.; Xu, Y. P.; Hu, B. Q.

Low-dimensional GaN materials, including nanorings, nanoribbons and smooth nanowires have been synthesized by reacting gallium and ammonia using Ag particles as a catalyst on the substrate of MgO single crystals. They were characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). EDX, XRD indicated that the low-dimensional nanomaterials were wurtzite GaN. New features are found in Raman scatterings for these low-dimensional GaN materials, which are different from the previous observations of GaN materials.

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

NASA Astrophysics Data System (ADS)

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

2018-06-01

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

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

Characterization of vertical GaN p-n diodes and junction field-effect transistors on bulk GaN down to cryogenic temperatures

NASA Astrophysics Data System (ADS)

Kizilyalli, I. C.; Aktas, O.

2015-12-01

There is great interest in wide-bandgap semiconductor devices and most recently in vertical GaN structures for power electronic applications such as power supplies, solar inverters and motor drives. In this paper the temperature-dependent electrical behavior of vertical GaN p-n diodes and vertical junction field-effect transistors fabricated on bulk GaN substrates of low defect density (104 to 106 cm-2) is described. Homoepitaxial MOCVD growth of GaN on its native substrate and the ability to control the doping in the drift layers in GaN have allowed the realization of vertical device architectures with drift layer thicknesses of 6 to 40 μm and net carrier electron concentrations as low as 1 × 1015 cm-3. This parameter range is suitable for applications requiring breakdown voltages of 1.2 kV to 5 kV. Mg, which is used as a p-type dopant in GaN, is a relatively deep acceptor (E A ≈ 0.18 eV) and susceptible to freeze-out at temperatures below 200 K. The loss of holes in p-GaN has a deleterious effect on p-n junction behavior, p-GaN contacts and channel control in junction field-effect transistors at temperatures below 200 K. Impact ionization-based avalanche breakdown (BV > 1200 V) in GaN p-n junctions is characterized between 77 K and 423 K for the first time. At higher temperatures the p-n junction breakdown voltage improves due to increased phonon scattering. A positive temperature coefficient in the breakdown voltage is demonstrated down to 77 K; however, the device breakdown characteristics are not as abrupt at temperatures below 200 K. On the other hand, contact resistance to p-GaN is reduced dramatically above room temperature, improving the overall device performance in GaN p-n diodes in all cases except where the n-type drift region resistance dominates the total forward resistance. In this case, the electron mobility can be deconvolved and is found to decrease with T -3/2, consistent with a phonon scattering model. Also, normally-on vertical junction

GaN Initiative for Grid Applications (GIGA)

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

Turner, George

2015-07-03

For nearly 4 ½ years, MIT Lincoln Laboratory (MIT/LL) led a very successful, DoE-funded team effort to develop GaN-on-Si materials and devices, targeting high-voltage (>1 kV), high-power, cost-effective electronics for grid applications. This effort, called the GaN Initiative for Grid Applications (GIGA) program, was initially made up of MIT/LL, the MIT campus group of Prof. Tomas Palacios (MIT), and the industrial partner M/A Com Technology Solutions (MTS). Later in the program a 4th team member was added (IQE MA) to provide commercial-scale GaN-on-Si epitaxial materials. A basic premise of the GIGA program was that power electronics, for ubiquitous utilization -evenmore » for grid applications - should be closer in cost structure to more conventional Si-based power electronics. For a number of reasons, more established GaN-on-SiC or even SiC-based power electronics are not likely to reach theses cost structures, even in higher manufacturing volumes. An additional premise of the GIGA program was that the technical focus would be on materials and devices suitable for operating at voltages > 1 kV, even though there is also significant commercial interest in developing lower voltage (< 1 kV), cost effective GaN-on-Si devices for higher volume applications, like consumer products. Remarkable technical progress was made during the course of this program. Advances in materials included the growth of high-quality, crack-free epitaxial GaN layers on large-diameter Si substrates with thicknesses up to ~5 μm, overcoming significant challenges in lattice mismatch and thermal expansion differences between Si and GaN in the actual epitaxial growth process. Such thick epilayers are crucial for high voltage operation of lateral geometry devices such as Schottky barrier (SB) diodes and high electron mobility transistors (HEMTs). New “Normally-Off” device architectures were demonstrated – for safe operation of power electronics circuits. The trade-offs between lateral

The Formation and Characterization of GaN Hexagonal Pyramids

NASA Astrophysics Data System (ADS)

Zhang, Shi-Ying; Xiu, Xiang-Qian; Lin, Zeng-Qin; Hua, Xue-Mei; Xie, Zi-Li; Zhang, Rong; Zheng, You-Dou

2013-05-01

GaN with hexagonal pyramids is fabricated using the photo-assisted electroless chemical etching method. Defective areas of the GaN substrate are selectively etched in a mixed solution of KOH and K2S2O8 under ultraviolet illumination, producing submicron-sized pyramids. Hexagonal pyramids on the etched GaN with well-defined {101¯1¯} facets and very sharp tips are formed. High-resolution x-ray diffraction shows that etched GaN with pyramids has a higher crystal quality, and micro-Raman spectra reveal a tensile stress relaxation in GaN with pyramids compared with normal GaN. The cathodoluminescence intensity of GaN after etching is significantly increased by three times, which is attributed to the reduction in the internal reflection, high-quality GaN with pyramids and the Bragg effect.

The controlled growth of GaN microrods on Si(111) substrates by MOCVD

NASA Astrophysics Data System (ADS)

Foltynski, Bartosz; Garro, Nuria; Vallo, Martin; Finken, Matthias; Giesen, Christoph; Kalisch, Holger; Vescan, Andrei; Cantarero, Andrés; Heuken, Michael

2015-03-01

In this paper, a selective area growth (SAG) approach for growing GaN microrods on patterned SiNx/Si(111) substrates by metal-organic chemical vapor deposition (MOCVD) is studied. The surface morphology, optical and structural properties of vertical GaN microrods terminated by pyramidal shaped facets (six { 10 1 bar 1} planes) were characterized using scanning electron microscopy (SEM), room temperature photoluminescence (PL) and Raman spectroscopy, respectively. Measurements revealed high-quality GaN microcolumns grown with silane support. Characterized structures were grown nearly strain-free (central frequency of Raman peak of 567±1 cm-1) with crystal quality comparable to bulk crystals (FWHM=4.2±1 cm-1). Such GaN microrods might be used as a next-generation device concept for solid-state lighting (SSL) applications by realizing core-shell InGaN/GaN multi-quantum wells (MQWs) on the n-GaN rod base.

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

GaN MOSFET with Boron Trichloride-Based Dry Recess Process

NASA Astrophysics Data System (ADS)

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

2013-06-01

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

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

NASA Astrophysics Data System (ADS)

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

2018-04-01

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

Free-standing GaN grating couplers and rib waveguide for planar photonics at telecommunication wavelength

NASA Astrophysics Data System (ADS)

Liu, Qifa; Wang, Wei

2018-01-01

Gallium Nitride (GaN) free-standing planar photonic device at telecommunication wavelength based on GaN-on-silicon platform was presented. The free-standing structure was realized by particular double-side fabrication process, which combining GaN front patterning, Si substrate back releasing and GaN slab etching. The actual device parameters were identified via the physical characterizations employing scanning electron microscope (SEM), atomic force microscope (AFM) and reflectance spectra testing. High coupling efficiency and good light confinement properties of the gratings and rib waveguide at telecommunication wavelength range were verified by finite element method (FEM) simulation. This work illustrates the potential of new GaN photonic structure which will enable new functions for planar photonics in communication and sensing applications, and is favorable for the realization of integrated optical circuit.

Energy and momentum relaxation of electrons in bulk and 2D GaN

NASA Astrophysics Data System (ADS)

Zanato, D.; Balkan, N.; Hill, G.; Schaff, W. J.

2004-10-01

We present our experimental and theoretical studies regarding the energy and momentum relaxation of hot electrons in n-type bulk GaN and AlGaN/GaN HEMT structures. We determine the non-equilibrium temperatures and the energy relaxation rates in the steady state using the mobility mapping technique together with the power balance conditions as described by us elsewhere [N. Balkan, M.C. Arikan, S. Gokden, V. Tilak, B. Schaff, R.J. Shealy, J. Phys.: Condens. Matter 14 (2002) 3457]. We obtain the e-LO phonon scattering time of 8 fs and show that the power loss of electrons due to optical phonon emission agrees with the theoretical prediction. The drift velocity-field curves at high electric fields indicate that the drift velocity saturates at approximately 3×10 6 cm/s for the two-dimensional structure and 4×10 6 cm/s for the bulk material at 77 K. These values are much lower than those predicted by the existing theories. A critical analysis of the observations is given with a model taking into account of the non-drifting non-equilibrium phonon production.

Nano-scaled Pt/Ag/Ni/Au contacts on p-type GaN for low contact resistance and high reflectivity.

PubMed

Kwon, Y W; Ju, I C; Kim, S K; Choi, Y S; Kim, M H; Yoo, S H; Kang, D H; Sung, H K; Shin, K; Ko, C G

2011-07-01

We synthesized the vertical-structured LED (VLED) using nano-scaled Pt between p-type GaN and Ag-based reflector. The metallization scheme on p-type GaN for high reflectance and low was the nano-scaled Pt/Ag/Ni/Au. Nano-scaled Pt (5 A) on Ag/Ni/Au exhibited reasonably high reflectance of 86.2% at the wavelength of 460 nm due to high transmittance of light through nano-scaled Pt (5 A) onto Ag layer. Ohmic behavior of contact metal, Pt/Ag/Ni/Au, to p-type GaN was achieved using surface treatments of p-type GaN prior to the deposition of contact metals and the specific contact resistance was observed with decreasing Pt thickness of 5 A, resulting in 1.5 x 10(-4) ohms cm2. Forward voltages of Pt (5 A)/Ag/Ni contact to p-type GaN showed 4.19 V with the current injection of 350 mA. Output voltages with various thickness of Pt showed the highest value at the smallest thickness of Pt due to its high transmittance of light onto Ag, leading to high reflectance. Our results propose that nano-scaled Pt/Ag/Ni could act as a promising contact metal to p-type GaN for improving the performance of VLEDs.

Ultrafast Hot Carrier Dynamics in GaN and Its Impact on the Efficiency Droop.

PubMed

Jhalani, Vatsal A; Zhou, Jin-Jian; Bernardi, Marco

2017-08-09

GaN is a key material for lighting technology. Yet, the carrier transport and ultrafast dynamics that are central in GaN light-emitting devices are not completely understood. We present first-principles calculations of carrier dynamics in GaN, focusing on electron-phonon (e-ph) scattering and the cooling and nanoscale dynamics of hot carriers. We find that e-ph scattering is significantly faster for holes compared to electrons and that for hot carriers with an initial 0.5-1 eV excess energy, holes take a significantly shorter time (∼0.1 ps) to relax to the band edge compared to electrons, which take ∼1 ps. The asymmetry in the hot carrier dynamics is shown to originate from the valence band degeneracy, the heavier effective mass of holes compared to electrons, and the details of the coupling to different phonon modes in the valence and conduction bands. We show that the slow cooling of hot electrons and their long ballistic mean free paths (over 3 nm at room temperature) are a possible cause of efficiency droop in GaN light-emitting diodes. Taken together, our work sheds light on the ultrafast dynamics of hot carriers in GaN and the nanoscale origin of efficiency droop.

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 .

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

NASA Astrophysics Data System (ADS)

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

2015-12-01

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

High quality and uniformity GaN grown on 150 mm Si substrate using in-situ NH3 pulse flow cleaning process

NASA Astrophysics Data System (ADS)

Ji, Panfeng; Yang, Xuelin; Feng, Yuxia; Cheng, Jianpeng; Zhang, Jie; Hu, Anqi; Song, Chunyan; Wu, Shan; Shen, Jianfei; Tang, Jun; Tao, Chun; Pan, Yaobo; Wang, Xinqiang; Shen, Bo

2017-04-01

By using in-situ NH3 pulse flow cleaning method, we have achieved the repeated growth of high quality and uniformity GaN and AlGaN/GaN high electron mobility transistors (HEMTs) on 150 mm Si substrate. The two dimensional electron gas (2DEG) mobility is 2200 cm2/Vs with an electron density of 7.3 × 1012 cm-2. The sheet resistance is 305 ± 4 Ω/□ with ±1.3% variation. The achievement is attributed to the fact that this method can significantly remove the Al, Ga, etc. metal droplets coating on the post growth flow flange and reactor wall which are difficult to clean by normal bake process under H2 ambient.

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.

Radar Waveform Pulse Analysis Measurement System for High-Power GaN Amplifiers

NASA Technical Reports Server (NTRS)

Thrivikraman, Tushar; Perkovic-Martin, Dragana; Jenabi, Masud; Hoffman, James

2012-01-01

This work presents a measurement system to characterize the pulsed response of high-power GaN amplifiers for use in space-based SAR platforms that require very strict amplitude and phase stability. The measurement system is able to record and analyze data on three different time scales: fast, slow, and long, which allows for greater detail of the mechanisms that impact amplitude and phase stability. The system is fully automated through MATLAB, which offers both instrument control capability and in-situ data processing. To validate this system, a high-power GaN HEMT amplifier operated in saturation was characterized. The fast time results show that variations to the amplitude and phase are correlated to DC supply transients, while long time characteristics are correlated to temperature changes.

Design, fabrication, and performance analysis of GaN vertical electron transistors with a buried p/n junction

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

Yeluri, Ramya, E-mail: ramyay@ece.ucsb.edu; Lu, Jing; Keller, Stacia

2015-05-04

The Current Aperture Vertical Electron Transistor (CAVET) combines the high conductivity of the two dimensional electron gas channel at the AlGaN/GaN heterojunction with better field distribution offered by a vertical design. In this work, CAVETs with buried, conductive p-GaN layers as the current blocking layer are reported. The p-GaN layer was regrown by metalorganic chemical vapor deposition and the subsequent channel regrowth was done by ammonia molecular beam epitaxy to maintain the p-GaN conductivity. Transistors with high ON current (10.9 kA/cm{sup 2}) and low ON-resistance (0.4 mΩ cm{sup 2}) are demonstrated. Non-planar selective area regrowth is identified as the limiting factormore » to transistor breakdown, using planar and non-planar n/p/n structures. Planar n/p/n structures recorded an estimated electric field of 3.1 MV/cm, while non-planar structures showed a much lower breakdown voltage. Lowering the p-GaN regrowth temperature improved breakdown in the non-planar n/p/n structure. Combining high breakdown voltage with high current will enable GaN vertical transistors with high power densities.« less

The GaN trench gate MOSFET with floating islands: High breakdown voltage and improved BFOM

NASA Astrophysics Data System (ADS)

Shen, Lingyan; Müller, Stephan; Cheng, Xinhong; Zhang, Dongliang; Zheng, Li; Xu, Dawei; Yu, Yuehui; Meissner, Elke; Erlbacher, Tobias

2018-02-01

A novel GaN trench gate (TG) MOSFET with P-type floating islands (FLI) in drift region, which can suppress the electric field peak at bottom of gate trench during the blocking state and prevent premature breakdown in gate oxide, is proposed and investigated by TCAD simulations. The influence of thickness, position, doping concentration and length of the FLI on breakdown voltage (BV) and specific on-resistance (Ron_sp) is studied, providing useful guidelines for design of this new type of device. Using optimized parameters for the FLI, GaN FLI TG-MOSFET obtains a BV as high as 2464 V with a Ron_sp of 3.0 mΩ cm2. Compared to the conventional GaN TG-MOSFET with the same structure parameters, the Baliga figure of merit (BFOM) is enhanced by 150%, getting closer to theoretical limit for GaN devices.

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

NASA Astrophysics Data System (ADS)

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

2016-12-01

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

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.

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.

Wurtzite/zinc-blende electronic-band alignment in basal-plane stacking faults in semi-polar GaN

NASA Astrophysics Data System (ADS)

Monavarian, Morteza; Hafiz, Shopan; Izyumskaya, Natalia; Das, Saikat; Özgür, Ümit; Morkoç, Hadis; Avrutin, Vitaliy

2016-02-01

Heteroepitaxial semipolar and nonpolar GaN layers often suffer from high densities of extended defects including basal plane stacking faults (BSFs). BSFs which are considered as inclusions of cubic zinc-blende phase in wurtzite matrix act as quantum wells strongly affecting device performance. Band alignment in BSFs has been discussed as type of band alignment at the wurtzite/zinc blende interface governs the response in differential transmission; fast decay after the pulse followed by slow recovery due to spatial splitting of electrons and heavy holes for type- II band alignment in contrast to decay with no recovery in case of type I band alignment. Based on the results, band alignment is demonstrated to be of type II in zinc-blende segments in wurtzite matrix as in BSFs.

High hole mobility p-type GaN with low residual hydrogen concentration prepared by pulsed sputtering

NASA Astrophysics Data System (ADS)

Arakawa, Yasuaki; Ueno, Kohei; Kobayashi, Atsushi; Ohta, Jitsuo; Fujioka, Hiroshi

2016-08-01

We have grown Mg-doped GaN films with low residual hydrogen concentration using a low-temperature pulsed sputtering deposition (PSD) process. The growth system is inherently hydrogen-free, allowing us to obtain high-purity Mg-doped GaN films with residual hydrogen concentrations below 5 × 1016 cm-3, which is the detection limit of secondary ion mass spectroscopy. In the Mg profile, no memory effect or serious dopant diffusion was detected. The as-deposited Mg-doped GaN films showed clear p-type conductivity at room temperature (RT) without thermal activation. The GaN film doped with a low concentration of Mg (7.9 × 1017 cm-3) deposited by PSD showed hole mobilities of 34 and 62 cm2 V-1 s-1 at RT and 175 K, respectively, which are as high as those of films grown by a state-of-the-art metal-organic chemical vapor deposition apparatus. These results indicate that PSD is a powerful tool for the fabrication of GaN-based vertical power devices.

High Temperature Annealing of MBE-grown Mg-doped GaN

NASA Astrophysics Data System (ADS)

Contreras, S.; Konczewicz, L.; Peyre, H.; Juillaguet, S.; Khalfioui, M. Al; Matta, S.; Leroux, M.; Damilano, B.; Brault, J.

2017-06-01

In this report, are shown the results of high temperature resistivity and Hall Effect studies of Mg-doped GaN epilayers. The samples studied were grown on (0001) (c-plane) sapphire by molecular beam epitaxy and 0.5 μm GaN:Mg layers have been achieved on low temperature buffers of GaN (30 nm) and AlN ( 150 nm). The experiments were carried out in the temperature range from 300 K up to 900 K. Up to about 870 K a typical thermally activated conduction process has been observed with the activation energy value EA = 215 meV. However, for higher temperatures, an annealing effect is observed in all the investigated samples. The increase of the free carrier concentration as a function of time leads to an irreversible decrease of sample resistivity of more than 60%.

Breaking Through the Multi-Mesa-Channel Width Limited of Normally Off GaN HEMTs Through Modulation of the Via-Hole-Length.

PubMed

Chien, Cheng-Yen; Wu, Wen-Hsin; You, Yao-Hong; Lin, Jun-Huei; Lee, Chia-Yu; Hsu, Wen-Ching; Kuan, Chieh-Hsiung; Lin, Ray-Ming

2017-12-01

We present new normally off GaN high-electron-mobility transistors (HEMTs) that overcome the typical limitations in multi-mesa-channel (MMC) width through modulation of the via-hole-length to regulate the charge neutrality screen effect. We have prepared enhancement-mode (E-mode) GaN HEMTs having widths of up to 300 nm, based on an enhanced surface pinning effect. E-mode GaN HEMTs having MMC structures and widths as well as via-hole-lengths of 100 nm/2 μm and 300 nm/6 μm, respectively, exhibited positive threshold voltages (V th ) of 0.79 and 0.46 V, respectively. The on-resistances of the MMC and via-hole-length structures were lower than those of typical tri-gate nanoribbon GaN HEMTs. In addition, the devices not only achieved the E-mode but also improved the power performance of the GaN HEMTs and effectively mitigated the device thermal effect. We controlled the via-hole-length sidewall surface pinning effect to obtain the E-mode GaN HEMTs. Our findings suggest that via-hole-length normally off GaN HEMTs have great potential for use in next-generation power electronics.

Electrical transport and structural characterization of epitaxial monolayer MoS2 /n- and p-doped GaN vertical lattice-matched heterojunctions

NASA Astrophysics Data System (ADS)

Ruzmetov, D.; O'Regan, T.; Zhang, K.; Herzing, A.; Mazzoni, A.; Chin, M.; Huang, S.; Zhang, Z.; Burke, R.; Neupane, M.; Birdwell, Ag; Shah, P.; Crowne, F.; Kolmakov, A.; Leroy, B.; Robinson, J.; Davydov, A.; Ivanov, T.

We investigate vertical semiconductor junctions consisting of monolayer MoS2 that is epitaxially grown on n- and p-doped GaN crystals. Such a junction represents a building block for 2D/3D vertical semiconductor heterostructures. Epitaxial, lattice-matched growth of MoS2 on GaN is important to ensure high quality interfaces that are crucial for the efficient vertical transport. The MoS2/GaN junctions were characterized with cross-sectional and planar scanning transmission electron microscopy (STEM), scanning tunneling microscopy, and atomic force microscopy. The MoS2/GaN lattice mismatch is measured to be near 1% using STEM. The electrical transport in the out-of-plane direction across the MoS2/GaN junctions was measured using conductive atomic force microscopy and mechanical nano-probes inside a scanning electron microscope. Nano-disc metal contacts to MoS2 were fabricated by e-beam lithography and evaporation. The current-voltage curves of the vertical MoS2/GaN junctions exhibit rectification with opposite polarities for n-doped and p-doped GaN. The metal contact determines the general features of the current-voltage curves, and the MoS2 monolayer modifies the electrical transport across the contact/GaN interface.

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.

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

NASA Astrophysics Data System (ADS)

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

2017-01-01

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

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.

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.

A Compact Two-Stage 120 W GaN High Power Amplifier for SweepSAR Radar Systems

NASA Technical Reports Server (NTRS)

Thrivikraman, Tushar; Horst, Stephen; Price, Douglas; Hoffman, James; Veilleux, Louise

2014-01-01

This work presents the design and measured results of a fully integrated switched power two-stage GaN HEMT high-power amplifier (HPA) achieving 60% power-added efficiency at over 120Woutput power. This high-efficiency GaN HEMT HPA is an enabling technology for L-band SweepSAR interferometric instruments that enable frequent repeat intervals and high-resolution imagery. The L-band HPA was designed using space-qualified state-of-the-art GaN HEMT technology. The amplifier exhibits over 34 dB of power gain at 51 dBm of output power across an 80 MHz bandwidth. The HPA is divided into two stages, an 8 W driver stage and 120 W output stage. The amplifier is designed for pulsed operation, with a high-speed DC drain switch operating at the pulsed-repetition interval and settles within 200 ns. In addition to the electrical design, a thermally optimized package was designed, that allows for direct thermal radiation to maintain low-junction temperatures for the GaN parts maximizing long-term reliability. Lastly, real radar waveforms are characterized and analysis of amplitude and phase stability over temperature demonstrate ultra-stable operation over temperature using integrated bias compensation circuitry allowing less than 0.2 dB amplitude variation and 2 deg phase variation over a 70 C range.

Electronic structure of ytterbium-implanted GaN at ambient and high pressure: experimental and crystal field studies.

PubMed

Kaminska, A; Ma, C-G; Brik, M G; Kozanecki, A; Boćkowski, M; Alves, E; Suchocki, A

2012-03-07

The results of high-pressure low-temperature optical measurements in a diamond-anvil cell of bulk gallium nitride crystals implanted with ytterbium are reported in combination with crystal field calculations of the Yb(3+) energy levels. Crystal field analysis of splitting of the (2)F(7/2) and (2)F(5/2) states has been performed, with the aim of assigning all features of the experimental luminescence spectra. A thorough analysis of the pressure behavior of the Yb(3+) luminescence lines in GaN allowed the determination of the ambient-pressure positions and pressure dependence of the Yb(3+) energy levels in the trigonal crystal field as well as the pressure-induced changes of the spin-orbit coupling coefficient.

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.

Photoelectrochemical etching measurement of defect density in GaN grown by nanoheteroepitaxy

NASA Astrophysics Data System (ADS)

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

2006-05-01

The density of dislocations in n-type GaN was measured by photoelectrochemical etching. A 10× reduction in dislocation density was observed compared to planar GaN grown at the same time. Cross-sectional transmission electron microscopy studies indicate that defect reduction is due to the mutual cancellation of dislocations with equal and opposite Burger's vectors. The nanoheteroepitaxy sample exhibited significantly higher photoluminescence intensity and higher electron mobility than the planar reference sample.

Correlation of doping, structure, and carrier dynamics in a single GaN nanorod

NASA Astrophysics Data System (ADS)

Zhou, Xiang; Lu, Ming-Yen; Lu, Yu-Jung; Gwo, Shangjr; Gradečak, Silvija

2013-06-01

We report the nanoscale optical investigation of a single GaN p-n junction nanorod by cathodoluminescence (CL) in a scanning transmission electron microscope. CL emission characteristic of dopant-related transitions was correlated to doping and structural defect in the nanorod, and used to determine p-n junction position and minority carrier diffusion lengths of 650 nm and 165 nm for electrons and holes, respectively. Temperature-dependent CL study reveals an activation energy of 19 meV for non-radiative recombination in Mg-doped GaN nanorods. These results directly correlate doping, structure, carrier dynamics, and optical properties of GaN nanostructure, and provide insights for device design and fabrication.

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.

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.

High power blue laser diodes on semipolar (202¯1¯) GaN substrates

NASA Astrophysics Data System (ADS)

Pourhashemi, Seyed Arash

High power blue laser didoes (LDs), among other applications, show the promise of realizing efficient and reliable solid state lighting systems. Since first GaN optoelectronic devices were demonstrated in early 1990s, GaN LDs were traditionally fabricated on polar c-plane. However in recent years there has been a growing interest in nonpolar and semipolar planes. Nonpolar and semipolar devices offer the prospect of achieving higher efficiencies though elimination or reduction of polarization-related electric fields. In this project I investigated semipolar (202 ¯1 ¯) plane of GaN for blue LDs fabrication. Results include blue LD (Lambda=450 nm) with highest output power, differential quantum efficiency (?d) and external quantum efficiency (EQE) reported for a GaN LD on a semipolar plane to date. Output power of 2.52 W, etad=50% and EQE=39% were achieved in pulsed mode and output power of 1.71 W was achieved in true CW mode. Moreover, use of indium tin oxide (ITO) as cladding layer in order to reduce the thickness of Mg-doped p-GaN layer was investigated. Blue LDs with ITO cladding were demonstrated in this work with highest output power, etad and EQE reported for a GaN LD with transparent conducting oxide (TCO) cladding layer to date. The lack of any natural cleavage plane orthogonal to the in-plane projection of the c-axis on semipolar planes has made Cl2-based dry etch processes the most common way to form mirror facets for semipolar LDs. However, mirror facets fabricated by dry etching can be inclined or rough. For this work, mechanical polishing was used to form LD mirror facets. The dependence of output power on current did not change with repeated CW measurements, indicating that the polished facets did not degrade under high power CW operation. These results show that polished facets are a viable alternative to cleaved or etched facets for high power CW semipolar LDs.

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

PubMed

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

2016-02-10

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

Depth Profiles of Mg, Si, and Zn Implants in GaN by Trace Element Accelerator Mass Spectrometry

NASA Astrophysics Data System (ADS)

Ravi Prasad, G. V.; Pelicon, P.; Mitchell, L. J.; McDaniel, F. D.

2003-08-01

GaN is one of the most promising electronic materials for applications requiring high-power, high frequencies, or high-temperatures as well as opto-electronics in the blue to ultraviolet spectral region. We have recently measured depth profiles of Mg, Si, and Zn implants in GaN substrates by the TEAMS particle counting method for both matrix and trace elements, using a gas ionization chamber. Trace Element Accelerator Mass Spectrometry (TEAMS) is a combination of Secondary Ion Mass Spectrometry (SIMS) and Accelerator Mass Spectrometry (AMS) to measure trace elements at ppb levels. Negative ions from a SIMS like source are injected into a tandem accelerator. Molecular interferences inherent with the SIMS method are eliminated in the TEAMS method. Negative ion currents are extremely low with GaN as neither gallium nor nitrogen readily forms negative ions making the depth profile measurements more difficult. The energies of the measured ions are in the range of 4-8 MeV. A careful selection of mass/charge ratios of the detected ions combined with energy-loss behavior of the ions in the ionization chamber eliminated molecular interferences.

Structure and Composition of Isolated Core-Shell (In ,Ga )N /GaN Rods Based on Nanofocus X-Ray Diffraction and Scanning Transmission Electron Microscopy

NASA Astrophysics Data System (ADS)

Krause, Thilo; Hanke, Michael; Nicolai, Lars; Cheng, Zongzhe; Niehle, Michael; Trampert, Achim; Kahnt, Maik; Falkenberg, Gerald; Schroer, Christian G.; Hartmann, Jana; Zhou, Hao; Wehmann, Hergo-Heinrich; Waag, Andreas

2017-02-01

Nanofocus x-ray diffraction is used to investigate the structure and local strain field of an isolated (In ,Ga )N /GaN core-shell microrod. Because the high spatial resolution of the x-ray beam is only 80 ×90 nm2, we are able to investigate several distinct volumes on one individual side facet. Here, we find a drastic increase in thickness of the outer GaN shell along the rod height. Additionally, we performed high-angle annular dark-field scanning-transmission-electron-microscopy measurements on several rods from the same sample showing that (In,Ga)N double-quantum-well and GaN barrier thicknesses also increase strongly along the height. Moreover, plastic relaxation is observed in the top part of the rod. Based on the experimentally obtained structural parameters, we simulate the strain-induced deformation using the finite-element method, which serves as the input for subsequent kinematic scattering simulations. The simulations reveal a significant increase of elastic in-plane relaxation along the rod height. However, at a certain height, the occurrence of plastic relaxation yields a decrease of the elastic strain. Because of the experimentally obtained structural input for the finite-element simulations, we can exclude unknown structural influences on the strain distribution, and we are able to translate the elastic relaxation into an indium concentration which increases by a factor of 4 from the bottom to the height where plastic relaxation occurs.

High quality factor whispering gallery modes from self-assembled hexagonal GaN rods grown by metal-organic vapor phase epitaxy.

PubMed

Tessarek, C; Sarau, G; Kiometzis, M; Christiansen, S

2013-02-11

Self-assembled GaN rods were grown on sapphire by metal-organic vapor phase epitaxy using a simple two-step method that relies first on a nitridation step followed by GaN epitaxy. The mask-free rods formed without any additional catalyst. Most of the vertically aligned rods exhibit a regular hexagonal shape with sharp edges and smooth sidewall facets. Cathodo- and microphotoluminescence investigations were carried out on single GaN rods. Whispering gallery modes with quality factors greater than 4000 were measured demonstrating the high morphological and optical quality of the self-assembled GaN rods.

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.

Interfacial Structure and Chemistry of GaN on Ge(111)

NASA Astrophysics Data System (ADS)

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

2013-12-01

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

Exciton emission from bare and hybrid plasmonic GaN nanorods

NASA Astrophysics Data System (ADS)

Mohammadi, Fatemesadat; Kunert, Gerd; Hommel, Detlef; Ge, Jingxuan; Duscher, Gerd; Schmitzer, Heidrun; Wagner, Hans Peter

We study the exciton emission of hybrid gold nanoparticle/Alq3 (aluminiumquinoline)/wurtzite GaN nanorods. GaN nanorods of 1.5 μm length and 250 nm diameter were grown by plasma assisted MBE. Hybrid GaN nanorods were synthesized by organic molecular beam deposition. Temperature and power dependent time integrated (TI) and time resolved (TR) photoluminescence (PL) measurements were performed on bare and hybrid structures. Bare nanorods show donor (D0,X) and acceptor bound (A0,X) exciton emission at 3.473 eV and at 3.463 eV, respectively. TR-PL trace modeling reveal lifetimes of 240 ps and 1.4 ns for the (D0,X) and (A0,X) transition. 10 nm gold coated GaN nanorods show a significant PL quenching and (D0,X) lifetime shortening which is tentatively attributed to impact ionization of (D0,X) due to hot electron injection from the gold nanoparticles. This is supported by electron energy loss spectroscopy that shows a redshift of a midgap state transition indicating a reduction of a preexisting band-bending at the nanorod surface due to positive charging of the gold nanoparticles. Inserting a nominally 5 nm thick Alq3 spacer between the nanorod and the gold reduces the PL quenching and lifetime shortening. Plasmonic nanorods with a 30 nm thick Alq3 spacer reveal lifetimes which are nearly identical to uncoated GaN nanorods.

Propagation of THz acoustic wave packets in GaN at room temperature

NASA Astrophysics Data System (ADS)

Maznev, A. A.; Hung, T.-C.; Yao, Y.-T.; Chou, T.-H.; Gandhi, J. S.; Lindsay, L.; Shin, H. D.; Stokes, D. W.; Forrest, R. L.; Bensaoula, A.; Sun, C.-K.; Nelson, K. A.

2018-02-01

We use femtosecond laser pulses to generate coherent longitudinal acoustic phonons at frequencies of 1-1.4 THz and study their propagation in GaN-based structures at room temperature. Two InGaN-GaN multiple-quantum-well (MQW) structures separated by a 2.3 μm-thick GaN spacer are used to simultaneously generate phonon wave packets with a central frequency determined by the period of the MQW and detect them after passing through the spacer. The measurements provide lower bounds for phonon lifetimes in GaN, which are still significantly lower than those from first principles predictions. The material Q-factor at 1 THz is found to be at least as high as 900. The measurements also demonstrate a partial specular reflection from the free surface of GaN at 1.4 THz. This work shows the potential of laser-based methods for THz range phonon spectroscopy and the promise for extending the viable frequency range of GaN-based acousto-electronic devices.

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.

Anisotropic carrier mobility in buckled two-dimensional GaN.

PubMed

Tong, Lijia; He, Junjie; Yang, Min; Chen, Zheng; Zhang, Jing; Lu, Yanli; Zhao, Ziyuan

2017-08-30

Developing nanoelectronic engineering requires two-dimensional (2d) materials with both usable carrier mobility and proper large band-gap. In this study, we present a detailed theoretical investigation of the intrinsic carrier mobilities of buckled 2d GaN. This buckled 2d GaN is accessed by hydrofluorination (FGaNH) and hydrogenation (HGaNH). We predict that the anisotropic carrier mobilities of buckled 2d GaN can exceed those of 2d MoS 2 and can be altered by an alterable surface chemical bond (convert from a Ga-F-Ga bond of FGaNH to a Ga-H bond of HGaNH). Moreover, converting FGaNH to HGaNH can significantly suppress hole mobility (even close to zero) and result in a transition from a p-type-like semiconductor (FGaNH) to an n-type-like semiconductor (HGaNH). These features make buckled 2d GaN a promising candidate for application in future conductivity-adjustable electronics.

Vacancy-type defects in Al2O3/GaN structure probed by monoenergetic positron beams

NASA Astrophysics Data System (ADS)

Uedono, Akira; Nabatame, Toshihide; Egger, Werner; Koschine, Tönjes; Hugenschmidt, Christoph; Dickmann, Marcel; Sumiya, Masatomo; Ishibashi, Shoji

2018-04-01

Defects in the Al2O3(25 nm)/GaN structure were probed by using monoenergetic positron beams. Al2O3 films were deposited on GaN by atomic layer deposition at 300 °C. Temperature treatment above 800 °C leads to the introduction of vacancy-type defects in GaN due to outdiffusion of atoms from GaN into Al2O3. The width of the damaged region was determined to be 40-50 nm from the Al2O3/GaN interface, and some of the vacancies were identified to act as electron trapping centers. In the Al2O3 film before and after annealing treatment at 300-900 °C, open spaces with three different sizes were found to coexist. The density of medium-sized open spaces started to decrease above 800 °C, which was associated with the interaction between GaN and Al2O3. Effects of the electron trapping/detrapping processes of interface states on the flat band voltage and the defects in GaN were also discussed.

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

Surfactant effect of gallium during molecular-beam epitaxy of GaN on AlN (0001)

NASA Astrophysics Data System (ADS)

Mula, Guido; Adelmann, C.; Moehl, S.; Oullier, J.; Daudin, B.

2001-11-01

We study the adsorption of Ga on (0001) GaN surfaces by reflection high-energy electron diffraction. It is shown that a dynamically stable Ga bilayer can be formed on the GaN surface for appropriate Ga fluxes and substrate temperatures. The influence of the presence of this Ga film on the growth mode of GaN on AlN(0001) by plasma-assisted molecular-beam epitaxy is studied. It is demonstrated that under nearly stoichiometric and N-rich conditions, the GaN layer relaxes elastically during the first stages of epitaxy. At high temperatures the growth follows a Stranski-Krastanov mode, whereas at lower temperatures kinetically formed flat platelets are observed. Under Ga-rich conditions-where a Ga bilayer is rapidly formed due to excess Ga accumulating on the surface-the growth follows a Frank-van der Merwe layer-by-layer mode at any growth temperature and no initial elastic relaxation occurs. Hence, it is concluded that excess Ga acts as a surfactant, effectively suppressing both Stranski-Krastanov islanding and platelet formation. It is further demonstrated that the Stranski-Krastanov transition is in competition with elastic relaxation by platelets, and it is only observed when relaxation by platelets is inefficient. As a result, a growth mode phase diagram is outlined for the growth of GaN on AlN(0001).

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.

Imaging the Impact of Proton Irradiation on Edge Terminations in Vertical GaN pin Diodes

DOE PAGES

Collins, Kimberlee C.; King, Michael P.; Dickerson, Jeramy R.; ...

2017-05-29

Devices based on GaN have shown great promise for high power electronics, including their potential use as radiation tolerant components. An important step to realizing high power diodes is the design and implementation of an edge termination to mitigate field crowding, which can lead to premature breakdown. However, little is known about the effects of radiation on edge termination functionality. We experimentally examine the effects of proton irradiation on multiple field ring edge terminations in high power vertical GaN pin diodes using in operando imaging with electron beam induced current (EBIC). We find that exposure to proton irradiation influences fieldmore » spreading in the edge termination as well as carrier transport near the anode. By using depth-dependent EBIC measurements of hole diffusion length in homoepitaxial n-GaN we demonstrate that the carrier transport effect is due to a reduction in hole diffusion length following proton irradiation.« less

Imaging the Impact of Proton Irradiation on Edge Terminations in Vertical GaN pin Diodes

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

Collins, Kimberlee C.; King, Michael P.; Dickerson, Jeramy R.

Devices based on GaN have shown great promise for high power electronics, including their potential use as radiation tolerant components. An important step to realizing high power diodes is the design and implementation of an edge termination to mitigate field crowding, which can lead to premature breakdown. However, little is known about the effects of radiation on edge termination functionality. We experimentally examine the effects of proton irradiation on multiple field ring edge terminations in high power vertical GaN pin diodes using in operando imaging with electron beam induced current (EBIC). We find that exposure to proton irradiation influences fieldmore » spreading in the edge termination as well as carrier transport near the anode. By using depth-dependent EBIC measurements of hole diffusion length in homoepitaxial n-GaN we demonstrate that the carrier transport effect is due to a reduction in hole diffusion length following proton irradiation.« less

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

NASA Astrophysics Data System (ADS)

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

2004-07-01

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

High-Efficiency Visible Transmitting Polarizations Devices Based on the GaN Metasurface.

PubMed

Guo, Zhongyi; Xu, Haisheng; Guo, Kai; Shen, Fei; Zhou, Hongping; Zhou, Qingfeng; Gao, Jun; Yin, Zhiping

2018-05-15

Metasurfaces are capable of tailoring the amplitude, phase, and polarization of incident light to design various polarization devices. Here, we propose a metasurface based on the novel dielectric material gallium nitride (GaN) to realize high-efficiency modulation for both of the orthogonal linear polarizations simultaneously in the visible range. Both modulated transmitted phases of the orthogonal linear polarizations can almost span the whole 2π range by tailoring geometric sizes of the GaN nanobricks, while maintaining high values of transmission (almost all over 90%). At the wavelength of 530 nm, we designed and realized the beam splitter and the focusing lenses successfully. To further prove that our proposed method is suitable for arbitrary orthogonal linear polarization, we also designed a three-dimensional (3D) metalens that can simultaneously focus the X -, Y -, 45°, and 135° linear polarizations on spatially symmetric positions, which can be applied to the linear polarization measurement. Our work provides a possible method to achieve high-efficiency multifunctional optical devices in visible light by extending the modulating dimensions.

GaN grown on nano-patterned sapphire substrates

NASA Astrophysics Data System (ADS)

Jing, Kong; Meixin, Feng; Jin, Cai; Hui, Wang; Huaibing, Wang; Hui, Yang

2015-04-01

High-quality gallium nitride (GaN) film was grown on nano-patterned sapphire substrates (NPSS) and investigated using XRD and SEM. It was found that the optimum thickness of the GaN buffer layer on the NPSS is 15 nm, which is thinner than that on micro-patterned sapphire substrates (MPSS). An interesting phenomenon was observed for GaN film grown on NPSS:GaN mainly grows on the trench regions and little grows on the sidewalls of the patterns at the initial growth stage, which is dramatically different from GaN grown on MPSS. In addition, the electrical and optical properties of LEDs grown on NPSS were characterized. Project supported by the Suzhou Nanojoin Photonics Co., Ltd and the High-Tech Achievements Transformation of Jiangsu Province, China (No.BA2012010).

Structure guided GANs

NASA Astrophysics Data System (ADS)

Cao, Feidao; Zhao, Huaici; Liu, Pengfei

2017-11-01

Generative adversarial networks (GANs) has achieved success in many fields. However, there are some samples generated by many GAN-based works, whose structure is ambiguous. In this work, we propose Structure Guided GANs that introduce structural similar into GANs to overcome the problem. In order to achieve our goal, we introduce an encoder and a decoder into a generator to design a new generator and take real samples as part of the input of a generator. And we modify the loss function of the generator accordingly. By comparison with WGAN, experimental results show that our proposed method overcomes largely sample structure ambiguous and can generate higher quality samples.

Growth behavior and growth rate dependency in LEDs performance for Mg-doped a-plane GaN

NASA Astrophysics Data System (ADS)

Song, Keun-Man; Kim, Jong-Min; Lee, Dong-Hun; Shin, Chan-Soo; Ko, Chul-Gi; Kong, Bo-Hyun; Cho, Hyung-Koun; Yoon, Dae-Ho

2011-07-01

We investigated the influence of growth rate of Mg-doped a-plane GaN on the surface morphological and electrical properties, and the characteristics of InGaN-based nonpolar LEDs. Mg-doped a-plane GaN layers were grown on r-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and cathode luminescence (CL) analysis exhibited that the surface morphology changed from stripe features with large triangular pits to rough and rugged surface with small asymmetric V-shape pits, as the growth rate increased. The Mg incorporation into a-plane GaN layers increased with increasing growth rate of Mg-doped a-plane GaN, while the activation efficiency of Mg dopants decreased in a-plane GaN. Additionally, it was found that operation voltage at 20 mA decreased in characteristics of LEDs, as the growth rate of Mg-doped a-plane GaN decreased. Meanwhile, the EL intensity of LEDs with p-GaN layers grown at higher growth rate was improved compared to that of LEDs with p-GaN layers grown at lower growth rate. Such an increase of EL intensity is attributed to the rougher surface morphology with increasing growth rate of Mg-doped a-plane GaN.

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

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

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

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

2016-05-23

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

Magneto-ballistic transport in GaN nanowires

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

Santoruvo, Giovanni, E-mail: giovanni.santoruvo@epfl.ch; Allain, Adrien; Ovchinnikov, Dmitry

2016-09-05

The ballistic filtering property of nanoscale crosses was used to investigate the effect of perpendicular magnetic fields on the ballistic transport of electrons on wide band-gap GaN heterostructures. The straight scattering-less trajectory of electrons was modified by a perpendicular magnetic field which produced a strong non-linear behavior in the measured output voltage of the ballistic filters and allowed the observation of semi-classical and quantum effects, such as quenching of the Hall resistance and manifestation of the last plateau, in excellent agreement with the theoretical predictions. A large measured phase coherence length of 190 nm allowed the observation of universal quantum fluctuationsmore » and weak localization of electrons due to quantum interference up to ∼25 K. This work also reveals the prospect of wide band-gap GaN semiconductors as a platform for basic transport and quantum studies, whose properties allow the investigation of ballistic transport and quantum phenomena at much larger voltages and temperatures than in other semiconductors.« less

First Principles Electronic Structure of Mn doped GaAs, GaP, and GaN Semiconductors

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

Schulthess, Thomas C; Temmerman, Walter M; Szotek, Zdzislawa

We present first-principles electronic structure calculations of Mn doped III-V semiconductors based on the local spin-density approximation (LSDA) as well as the self-interaction corrected local spin density method (SIC-LSD). We find that it is crucial to use a self-interaction free approach to properly describe the electronic ground state. The SIC-LSD calculations predict the proper electronic ground state configuration for Mn in GaAs, GaP, and GaN. Excellent quantitative agreement with experiment is found for magnetic moment and p-d exchange in (GaMn)As. These results allow us to validate commonly used models for magnetic semiconductors. Furthermore, we discuss the delicate problem of extractingmore » binding energies of localized levels from density functional theory calculations. We propose three approaches to take into account final state effects to estimate the binding energies of the Mn-d levels in GaAs. We find good agreement between computed values and estimates from photoemisison experiments.« less

Ku and K band GaN High Power Amplifier MMICs

DTIC Science & Technology

2017-03-20

end Ku-band HPA operates from 13 to 14.5 GHz and delivers 48 Watts of output power with 43% PAE. A high-end Ku-band HPA operates from 15.5 to 18 GHz and...delivers 25 Watts of output power with 45% PAE. A K-band HPA operates from 19.5 to 22 GHz and delivers 18 Watts of output power with 29% PAE...15.5 and 18 GHz. The circuit is a three-stage reactively-matched amplifier. A photograph of a fabricated high-end Ku-band GaN HPA is shown as an

High active nitrogen flux growth of GaN by plasma assisted molecular beam epitaxy

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

McSkimming, Brian M., E-mail: mcskimming@engineering.ucsb.edu; Speck, James S.; Chaix, Catherine

2015-09-15

In the present study, the authors report on a modified Riber radio frequency (RF) nitrogen plasma source that provides active nitrogen fluxes more than 30 times higher than those commonly used for plasma assisted molecular beam epitaxy (PAMBE) growth of gallium nitride (GaN) and thus a significantly higher growth rate than has been previously reported. GaN films were grown using N{sub 2} gas flow rates between 5 and 25 sccm while varying the plasma source's RF forward power from 200 to 600 W. The highest growth rate, and therefore the highest active nitrogen flux, achieved was ∼7.6 μm/h. For optimized growth conditions,more » the surfaces displayed a clear step-terrace structure with an average RMS roughness (3 × 3 μm) on the order of 1 nm. Secondary ion mass spectroscopy impurity analysis demonstrates oxygen and hydrogen incorporation of 1 × 10{sup 16} and ∼5 × 10{sup 17}, respectively. In addition, the authors have achieved PAMBE growth of GaN at a substrate temperature more than 150 °C greater than our standard Ga rich GaN growth regime and ∼100 °C greater than any previously reported PAMBE growth of GaN. This growth temperature corresponds to GaN decomposition in vacuum of more than 20 nm/min; a regime previously unattainable with conventional nitrogen plasma sources. Arrhenius analysis of the decomposition rate shows that samples with a flux ratio below stoichiometry have an activation energy greater than decomposition of GaN in vacuum while samples grown at or above stoichiometry have decreased activation energy. The activation energy of decomposition for GaN in vacuum was previously determined to be ∼3.1 eV. For a Ga/N flux ratio of ∼1.5, this activation energy was found to be ∼2.8 eV, while for a Ga/N flux ratio of ∼0.5, it was found to be ∼7.9 eV.« less

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

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

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

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

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

DOE PAGES

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

2016-09-21

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

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.

Group-III nitride based high electron mobility transistor (HEMT) with barrier/spacer layer

DOEpatents

Chavarkar, Prashant; Smorchkova, Ioulia P.; Keller, Stacia; Mishra, Umesh; Walukiewicz, Wladyslaw; Wu, Yifeng

2005-02-01

A Group III nitride based high electron mobility transistors (HEMT) is disclosed that provides improved high frequency performance. One embodiment of the HEMT comprises a GaN buffer layer, with an Al.sub.y Ga.sub.1-y N (y=1 or y 1) layer on the GaN buffer layer. An Al.sub.x Ga.sub.1-x N (0.ltoreq.x.ltoreq.0.5) barrier layer on to the Al.sub.y Ga.sub.1-y N layer, opposite the GaN buffer layer, Al.sub.y Ga.sub.1-y N layer having a higher Al concentration than that of the Al.sub.x Ga.sub.1-x N barrier layer. A preferred Al.sub.y Ga.sub.1-y N layer has y=1 or y.about.1 and a preferred Al.sub.x Ga.sub.1-x N barrier layer has 0.ltoreq.x.ltoreq.0.5. A 2DEG forms at the interface between the GaN buffer layer and the Al.sub.y Ga.sub.1-y N layer. Respective source, drain and gate contacts are formed on the Al.sub.x Ga.sub.1-x N barrier layer. The HEMT can also comprising a substrate adjacent to the buffer layer, opposite the Al.sub.y Ga.sub.1-y N layer and a nucleation layer between the Al.sub.x Ga.sub.1-x N buffer layer and the substrate.

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.

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).

GaN and ZnO nanostructures

NASA Astrophysics Data System (ADS)

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

2010-07-01

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

The nature of catalyst particles and growth mechanisms of GaN nanowires grown by Ni-assisted metal-organic chemical vapor deposition.

PubMed

Weng, Xiaojun; Burke, Robert A; Redwing, Joan M

2009-02-25

The structure and chemistry of the catalyst particles that terminate GaN nanowires grown by Ni-assisted metal-organic chemical vapor deposition were investigated using a combination of electron diffraction, high-resolution transmission electron microscopy, and x-ray energy dispersive spectrometry. The crystal symmetry, lattice parameter, and chemical composition obtained reveal that the catalyst particles are Ni(3)Ga with an ordered L 1(2) structure. The results suggest that the catalyst is a solid particle during growth and therefore favor a vapor-solid-solid mechanism for the growth of GaN nanowires under these conditions.

Structural and electronic properties of the V-V compounds isoelectronic to GaN and isostructural to gray arsenic

NASA Astrophysics Data System (ADS)

Yang, Zhao; Han, Dan; Chen, Guohong; Chen, Shiyou

2018-03-01

The III-V binary compound semiconductors such as GaN, GaP, InN and InP have extensive applications in various optoelectronic, microwave and power-electronic devices. Using first-principles calculation, we systematically studied the structural and electronic properties of the V-V binary compounds (BiN, BiP, SbN and SbP) that are isoelectronic to GaN, GaP, InN and InP if Bi and Sb are in the +3 valence state. Interestingly, we found that the ground-state structures of BiP, SbN and SbP have the R-3m symmetry and are isostructural to the layered structure of gray arsenic, whereas BiN prefers a different ground-state structure with the C2 symmetry. Electronic structure calculations showed that the bulk BiN is a narrow bandgap semiconductor for its bandgap is about 0.2 eV. In contrast, BiP, SbN and SbP are metallic. The layered ground-state structure of the V-V binary compounds motivates us to study the electronic properties of their few-layer structures. As the structure becomes monolayer, their bandgaps increase significantly and are all in the range from about 1 eV to 1.7 eV, which are comparative to the bandgap of the monolayer gray arsenic. The monolayer BiP, SbN and SbP have indirect bandgaps, and they show a semiconductor-metal transition as the number of layers increase. Interestingly, the monolayer BiP has the largest splitting (350 meV) of the CBM valley, and thus may have potential application in novel spintronics and valleytronics devices.

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.

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

PubMed

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

2018-03-07

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

Resistivity control of unintentionally doped GaN films

NASA Astrophysics Data System (ADS)

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

2005-05-01

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

Kinetics of optically excited charge carriers at the GaN surface: Influence of catalytic Pt nanostructures

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

Winnerl, Andrea, E-mail: andrea.winnerl@wsi.tum.de; Pereira, Rui N.; Stutzmann, Martin

2015-10-21

In this work, we use GaN with different deposited Pt nanostructures as a controllable model system to investigate the kinetics of photo-generated charge carriers in hybrid photocatalysts. We combine conductance and contact potential difference measurements to investigate the influence of Pt on the processes involved in the capture and decay of photo-generated charge carriers at and close to the GaN surface. We found that in the presence of Pt nanostructures the photo-excitation processes are similar to those found in Pt free GaN. However, in GaN with Pt nanostructures, photo-generated holes are preferentially trapped in surface states of the GaN coveredmore » with Pt and/or in electronic states of the Pt and lead to an accumulation of positive charge there, whereas negative charge is accumulated in localized states in a shallow defect band of the GaN covered with Pt. This preferential accumulation of photo-generated electrons close to the surface is responsible for a dramatic acceleration of the turn-off charge transfer kinetics and a stronger dependence of the surface photovoltage on light intensity when compared to a Pt free GaN surface. Our study shows that in hybrid photocatalysts, the metal nanostructures induce a spatially inhomogeneous surface band bending of the semiconductor that promotes a lateral drift of photogenerated charges towards the catalytic nanostructures.« less

Ultrafast, superhigh gain visible-blind UV detector and optical logic gates based on nonpolar a-axial GaN nanowire

NASA Astrophysics Data System (ADS)

Wang, Xingfu; Zhang, Yong; Chen, Xinman; He, Miao; Liu, Chao; Yin, Yian; Zou, Xianshao; Li, Shuti

2014-09-01

Nonpolar a-axial GaN nanowire (NW) was first used to construct the MSM (metal-semiconductor-metal) symmetrical Schottky contact device for application as visible-blind ultraviolet (UV) detector. Without any surface or composition modifications, the fabricated device demonstrated a superior performance through a combination of its high sensitivity (up to 104 A W-1) and EQE value (up to 105), as well as ultrafast (<26 ms) response speed, which indicates that a balance between the photocurrent gain and the response speed has been achieved. Based on its excellent photoresponse performance, an optical logic AND gate and OR gate have been demonstrated for performing photo-electronic coupled logic devices by further integrating the fabricated GaN NW detectors, which logically convert optical signals to electrical signals in real time. These results indicate the possibility of using a nonpolar a-axial GaN NW not only as a high performance UV detector, but also as a stable optical logic device, both in light-wave communications and for future memory storage.Nonpolar a-axial GaN nanowire (NW) was first used to construct the MSM (metal-semiconductor-metal) symmetrical Schottky contact device for application as visible-blind ultraviolet (UV) detector. Without any surface or composition modifications, the fabricated device demonstrated a superior performance through a combination of its high sensitivity (up to 104 A W-1) and EQE value (up to 105), as well as ultrafast (<26 ms) response speed, which indicates that a balance between the photocurrent gain and the response speed has been achieved. Based on its excellent photoresponse performance, an optical logic AND gate and OR gate have been demonstrated for performing photo-electronic coupled logic devices by further integrating the fabricated GaN NW detectors, which logically convert optical signals to electrical signals in real time. These results indicate the possibility of using a nonpolar a-axial GaN NW not only as a high

Characterization of Pb-Doped GaN Thin Films Grown by Thermionic Vacuum Arc

NASA Astrophysics Data System (ADS)

Özen, Soner; Pat, Suat; Korkmaz, Şadan

2018-03-01

Undoped and lead (Pb)-doped gallium nitride (GaN) thin films have been deposited by a thermionic vacuum arc (TVA) method. Glass and polyethylene terephthalate were selected as optically transparent substrates. The structural, optical, morphological, and electrical properties of the deposited thin films were investigated. These physical properties were interpreted by comparison with related analysis methods. The crystalline structure of the deposited GaN thin films was hexagonal wurtzite. The optical bandgap energy of the GaN and Pb-doped GaN thin films was found to be 3.45 eV and 3.47 eV, respectively. The surface properties of the deposited thin films were imaged using atomic force microscopy and field-emission scanning electron microscopy, revealing a nanostructured, homogeneous, and granular surface structure. These results confirm that the TVA method is an alternative layer deposition system for Pb-doped GaN thin films.

Hybrid device based on GaN nanoneedles and MEH-PPV/PEDOT:PSS polymer

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

Shin, Min Jeong; Gwon, Dong-Oh; Lee, Chan-Mi

2015-08-15

Highlights: • A hybrid device was demonstrated by using MEH-PPV, PEDOT:PSS, and GaN nanoneedles. • I–V curve of the hybrid device showed its rectification behaviour, similar to a diode. • EL peak originated by the different potential barriers at MEH-PPV and GaN interface. - Abstract: A hybrid device that combines the properties of organic and inorganic semiconductors was fabricated and studied. It incorporated poly[2-methoxy-5-(2-ethylhexyloxy)- 1,4-phenylenevinylene] (MEH-PPV) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as organic polymers and GaN nanoneedles as an inorganic semiconductor. Layers of the two polymers were spin coated on to the GaN nanoneedles. The one peak in the electroluminescence spectrum originatedmore » from the MEH-PPV layer owing to the different potential barriers of electrons and holes at its interface with the GaN nanoneedles. However, the photoluminescence spectrum showed peaks due to both GaN nanoneedles and MEH-PPV. Such hybrid structures, suitably developed, might be able to improve the efficiency of optoelectronic devices.« less

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

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

Takeuchi, S., E-mail: takeuchi@ee.es.osaka-u.ac.jp; Asazu, H.; Nakamura, Y.

2015-12-28

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

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

PubMed

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

2013-01-01

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

Electronic states and optical properties of single donor in GaN conical quantum dot with spherical edge

NASA Astrophysics Data System (ADS)

El Aouami, A.; Feddi, E.; El-Yadri, M.; Aghoutane, N.; Dujardin, F.; Duque, C. A.; Phuc, Huynh Vinh

2018-02-01

In this paper we present a theoretical investigation of quantum confinement effects on the electron and single donor states in GaN conical quantum dot with spherical edge. In the framework of the effective mass approximation, the Schrödinger equations of electron and donor have been solved analytically in an infinite potential barrier model. Our calculations show that the energies of electron and donor impurity are affected by the two characteristic parameters of the structure which are the angle Ω and the radial dimension R. We show that, despite the fact that the reduction of the two parameters Ω and R leads to the same confinement effects, the energy remains very sensitive to the variation of the radial part than the variation of the angular part. The analysis of the photoionization cross-section corresponding to optical transitions between the conduction band and the first donor energy level shows clearly that the reduction of the radius R causes a shift in resonance peaks towards the high energies. On the other hand, the optical transitions between 1 s - 1 p , 1 p - 1 d and 1 p - 2 s show that the increment of the conical aperture Ω (or reduction of R) implies a displacement of the excitation energy to higher energies.

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.

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

NASA Astrophysics Data System (ADS)

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

2017-02-01

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

Growth of GaN on Sapphire via Low-Temperature Deposited Buffer Layer and Realization of p-Type GaN by Mg Doping Followed by Low-Energy Electron Beam Irradiation

NASA Astrophysics Data System (ADS)

Amano, Hiroshi

2015-12-01

This is a personal history of one of the Japanese researchers engaged in developing a method for growing GaN on a sapphire substrate, paving the way for the realization of smart television and display systems using blue LEDs. The most important work was done in the mid- to late 80s. The background to the author's work and the process by which the technology enabling the growth of GaN and the realization of p-type GaN was established are reviewed.

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

Fabrication of high reflectivity nanoporous distributed Bragg reflectors by controlled electrochemical etching of GaN

NASA Astrophysics Data System (ADS)

Lee, Seung-Min; Kang, Jin-Ho; Lee, June Key; Ryu, Sang-Wan

2016-09-01

The nanoporous medium is a valuable feature of optical devices because of its variable optical refractive index with porosity. One important application is in a GaN-based vertical cavity surface emitting laser having a distributed Bragg reflector (DBR) composed of alternating nanoporous and bulk GaNs. However, optimization of the fabrication process for high reflectivity DBRs having wellcontrolled high reflection bands has not been studied yet. We used electrochemical etching to study the fabrication process of a nanoporous GaN DBR and analyzed the relationship between the morphology and optical reflectivity. Several electrolytes were examined for the formation of the optimized nanoporous structure. A highly reflective DBRs having reflectivity of ~100% were obtained over a wide wavelength range of 450-750 nm. Porosification of semiconductors into nanoporous layers could provide a high reflectivity DBR due to controlled index-contrast, which would be advantages for the construction of a high-Q optical cavity.

Orbitally driven low thermal conductivity of monolayer gallium nitride (GaN) with planar honeycomb structure: a comparative study.

PubMed

Qin, Zhenzhen; Qin, Guangzhao; Zuo, Xu; Xiong, Zhihua; Hu, Ming

2017-03-23

Two-dimensional (2D) materials with graphene as a representative have been intensively studied for a long time. Recently, monolayer gallium nitride (ML GaN) with honeycomb structure was successfully fabricated in experiments, generating enormous research interest for its promising applications in nano- and opto-electronics. Considering all these applications are inevitably involved with thermal transport, systematic investigation of the phonon transport properties of 2D GaN is in demand. In this paper, by solving the Boltzmann transport equation (BTE) based on first-principles calculations, we performed a comprehensive study of the phonon transport properties of ML GaN, with detailed comparison to bulk GaN, 2D graphene, silicene and ML BN with similar honeycomb structure. Considering the similar planar structure of ML GaN to graphene, it is quite intriguing to find that the thermal conductivity (κ) of ML GaN (14.93 W mK -1 ) is more than two orders of magnitude lower than that of graphene and is even lower than that of silicene with a buckled structure. Systematic analysis is performed based on the study of the contribution from phonon branches, comparison among the mode level phonon group velocity and lifetime, the detailed process and channels of phonon-phonon scattering, and phonon anharmonicity with potential energy well. We found that, different from graphene and ML BN, the phonon-phonon scattering selection rule in 2D GaN is slightly broken by the lowered symmetry due to the large difference in the atomic radius and mass between Ga and N atoms. Further deep insight is gained from the electronic structure. Resulting from the special sp orbital hybridization mediated by the Ga-d orbital in ML GaN, the strongly polarized Ga-N bond, localized charge density, and its inhomogeneous distribution induce large phonon anharmonicity and lead to the intrinsic low κ of ML GaN. The orbitally driven low κ of ML GaN unraveled in this work would make 2D GaN prospective for

Effect of photocatalytic oxidation technology on GaN CMP

NASA Astrophysics Data System (ADS)

Wang, Jie; Wang, Tongqing; Pan, Guoshun; Lu, Xinchun

2016-01-01

GaN is so hard and so chemically inert that it is difficult to obtain a high material removal rate (MRR) in the chemical mechanical polishing (CMP) process. This paper discusses the application of photocatalytic oxidation technology in GaN planarization. Three N-type semiconductor particles (TiO2, SnO2, and Fe2O3) are used as catalysts and added to the H2O2-SiO2-based slurry. By optical excitation, highly reactive photoinduced holes are produced on the surface of the particles, which can oxidize OH- and H2O absorbed on the surface of the catalysts; therefore, more OH* will be generated. As a result, GaN MRRs in an H2O2-SiO2-based polishing system combined with catalysts are improved significantly, especially when using TiO2, the MRR of which is 122 nm/h. The X-ray photoelectron spectroscopy (XPS) analysis shows the variation trend of chemical composition on the GaN surface after polishing, revealing the planarization process. Besides, the effect of pH on photocatalytic oxidation combined with TiO2 is analyzed deeply. Furthermore, the physical model of GaN CMP combined with photocatalytic oxidation technology is proposed to describe the removal mechanism of GaN.

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

NASA Astrophysics Data System (ADS)

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

2007-01-01

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

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

Fast and slow UV-photoresponse in n-type GaN

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

Rocha, R.; Koynov, S.; Brogueira, P.

1999-07-01

The photocurrent decay in n-type GaN films prepared by low-pressure chemical vapor deposition (LPCVD) was measured in the ms-to-s time range using steady-state UV light and in the {micro}s time regime using short high-power pulses from higher harmonics of a Nd:YAG laser. A power law time dependence is observed with exponents ranging from {minus}0.1 to {minus}0.3, which is an indication of a broad distribution of trapping states inside the band gap. Combining Hall effect results and the magnitude of the initial slope of the photocurrent decay they estimate a mobility-lifetime product of 2.1 x 10{sup {minus}4} cm{sup 2}/V for photogeneratedmore » electrons at times below a few {micro}s. Slow transients might be a handicap for applications of GaN in UV detectors.« less

Comparison of stress states in GaN films grown on different substrates: Langasite, sapphire and silicon

NASA Astrophysics Data System (ADS)

Park, Byung-Guon; Saravana Kumar, R.; Moon, Mee-Lim; Kim, Moon-Deock; Kang, Tae-Won; Yang, Woo-Chul; Kim, Song-Gang

2015-09-01

We demonstrate the evolution of GaN films on novel langasite (LGS) substrate by plasma-assisted molecular beam epitaxy, and assessed the quality of grown GaN film by comparing the experimental results obtained using LGS, sapphire and silicon (Si) substrates. To study the substrate effect, X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and photoluminescence (PL) spectra were used to characterize the microstructure and stress states in GaN films. Wet etching of GaN films in KOH solution revealed that the films deposited on GaN/LGS, AlN/sapphire and AlN/Si substrates possess Ga-polarity, while the film deposited on GaN/sapphire possess N-polarity. XRD, Raman and PL analysis demonstrated that a compressive stress exist in the films grown on GaN/LGS, AlN/sapphire, and GaN/sapphire substrates, while a tensile stress appears on AlN/Si substrate. Comparative analysis showed the growth of nearly stress-free GaN films on LGS substrate due to the very small lattice mismatch ( 3.2%) and thermal expansion coefficient difference ( 7.5%). The results presented here will hopefully provide a new framework for the further development of high performance III-nitride-related devices using GaN/LGS heteroepitaxy.

Structural characterization of bulk GaN crystals grown under high hydrostatic pressure

NASA Astrophysics Data System (ADS)

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

1996-09-01

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

Excited states of neutral donor bound excitons in GaN

NASA Astrophysics Data System (ADS)

Callsen, G.; Kure, T.; Wagner, M. R.; Butté, R.; Grandjean, N.

2018-06-01

We investigate the excited states of a neutral donor bound exciton (D0X) in bulk GaN by means of high-resolution, polychromatic photoluminescence excitation (PLE) spectroscopy. The optically most prominent donor in our sample is silicon accompanied by only a minor contribution of oxygen—the key for an unambiguous assignment of excited states. Consequently, we can observe a multitude of Si0X-related excitation channels with linewidths down to 200 μeV. Two groups of excitation channels are identified, belonging either to rotational-vibrational or electronic excited states of the hole in the Si0X complex. Such identification is achieved by modeling the excited states based on the equations of motion for a Kratzer potential, taking into account the particularly large anisotropy of effective hole masses in GaN. Furthermore, several ground- and excited states of the exciton-polaritons and the dominant bound exciton are observed in the photoluminescence (PL) and PLE spectra, facilitating an estimate of the associated complex binding energies. Our data clearly show that great care must be taken if only PL spectra of D0X centers in GaN are analyzed. Every PL feature we observe at higher emission energies with regard to the Si0X ground state corresponds to an excited state. Hence, any unambiguous peak identification renders PLE spectra highly valuable, as important spectral features are obscured in common PL spectra. Here, GaN represents a particular case among the wide-bandgap, wurtzite semiconductors, as comparably low localization energies for common D0X centers are usually paired with large emission linewidths and the prominent optical signature of exciton-polaritons, making the sole analysis of PL spectra a challenging task.

ZnO nanorod arrays and direct wire bonding on GaN surfaces for rapid fabrication of antireflective, high-temperature ultraviolet sensors

NASA Astrophysics Data System (ADS)

So, Hongyun; Senesky, Debbie G.

2016-11-01

Rapid, cost-effective, and simple fabrication/packaging of microscale gallium nitride (GaN) ultraviolet (UV) sensors are demonstrated using zinc oxide nanorod arrays (ZnO NRAs) as an antireflective layer and direct bonding of aluminum wires to the GaN surface. The presence of the ZnO NRAs on the GaN surface significantly reduced the reflectance to less than 1% in the UV and 4% in the visible light region. As a result, the devices fabricated with ZnO NRAs and mechanically stable aluminum bonding wires (pull strength of 3-5 gf) showed higher sensitivity (136.3% at room temperature and 148.2% increase at 250 °C) when compared with devices with bare (uncoated) GaN surfaces. In addition, the devices demonstrated reliable operation at high temperatures up to 300 °C, supporting the feasibility of simple and cost-effective UV sensors operating with higher sensitivity in high-temperature conditions, such as in combustion, downhole, and space exploration applications.

GaN Nanowire Devices: Fabrication and Characterization

NASA Astrophysics Data System (ADS)

Scott, Reum

The development of microelectronics in the last 25 years has been characterized by an exponential increase of the bit density in integrated circuits (ICs) with time. Scaling solid-state devices improves cost, performance, and power; as such, it is of particular interest for companies, who gain a market advantage with the latest technology. As a result, the microelectronics industry has driven transistor feature size scaling from 10 μm to ~30 nm during the past 40 years. This trend has persisted for 40 years due to optimization, new processing techniques, device structures, and materials. But when noting processor speeds from the 1970's to 2009 and then again in 2010, the implication would be that the trend has ceased. To address the challenge of shrinking the integrated circuit (IC), current research is centered on identifying new materials and devices that can supplement and/or potentially supplant it. Bottom-up methods tailor nanoscale building blocks---atoms, molecules, quantum dots, and nanowires (NWs)---to be used to overcome these limitations. The Group IIIA nitrides (InN, AlN, and GaN) possess appealing properties such as a direct band gap spanning the whole solar spectrum, high saturation velocity, and high breakdown electric field. As a result nanostructures and nanodevices made from GaN and related nitrides are suitable candidates for efficient nanoscale UV/ visible light emitters, detectors, and gas sensors. To produce devices with such small structures new fabrication methods must be implemented. Devices composed of GaN nanowires were fabricated using photolithography and electron beam lithography. The IV characteristics of these devices were noted under different illuminations and the current tripled from 4.8*10-7 A to 1.59*10 -6 A under UV light which persisted for at least 5hrs.

Effects of GaN interlayer on the transport properties of lattice-matched AlInN/AlN/GaN heterostructures

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

Wu, F.; Gao, K. H., E-mail: khgao@tju.edu.cn; Li, Z. Q.

2015-04-21

We study the effects of GaN interlayer on the transport properties of two-dimensional electron gases confined in lattice-matched AlInN/AlN/GaN heterostructures. It is found that the Hall mobility is evidently enhanced when an additional ultrathin GaN interlayer is introduced between AlInN and AlN layers. The enhancement of the Hall mobility is especially remarkable at low temperature. The high Hall mobility results in a low sheet resistance of 23 Ω/◻ at 2 K. Meanwhile, Shubnikov-de Haas oscillations (SdH) are also remarkably enhanced due to the existence of GaN interlayer. The enhancement of the SdH oscillations is related to the larger quantum mobility μ{sub q}more » owing to the suppression of the interface roughness, alloy disorder, and ionized impurity scatterings by the GaN interlayer.« less

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.

A Hybrid Multi-gate Model of a Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT) Device Incorporating GaN-substrate Thermal Boundary Resistance

DTIC Science & Technology

2012-10-01

right by a pitch (P) and subsequently summed to provide a multi-gate superimposed temperature distribution ( TMG (x)). An example is shown in figure...temperature rise over the coolant, or the difference between the centerline multi gate junction temperature on the upper surface ( TMG ,GaN(0)) of the GaN...TC coolant temperature (°C) TCP(x) cold plate temperature distribution (°C) TGaN(x,y) temperature distribution within GaN (°C) TMG (x) multiple gate

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.

Growth diagram of N-face GaN (0001{sup ¯}) grown at high rate by plasma-assisted molecular beam epitaxy

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

Okumura, Hironori, E-mail: okumura@engineering.ucsb.edu; McSkimming, Brian M.; Speck, James S.

2014-01-06

N-face GaN was grown on free-standing GaN (0001{sup ¯}) substrates at a growth rate of 1.5 μm/h using plasma-assisted molecular beam epitaxy. Difference in growth rate between (0001{sup ¯}) and (0001) oriented GaN depends on nitrogen plasma power, and the (0001{sup ¯}) oriented GaN had only 70% of the growth rate of the (0001) oriented GaN at 300 W. Unintentional impurity concentrations of silicon, carbon, and oxygen were 2 × 10{sup 15}, 2 × 10{sup 16}, and 7 × 10{sup 16} cm{sup −3}, respectively. A growth diagram was constructed that shows the dependence of the growth modes on the difference in the Ga and active nitrogen flux, Φ{sub Ga} − Φ{submore » N*}, and the growth temperature. At high Φ{sub Ga} − Φ{sub N*} (Φ{sub Ga} ≫ Φ{sub N*}), two-dimensional (step-flow and layer-by-layer) growth modes were realized. High growth temperature (780 °C) expanded the growth window of the two-dimensional growth modes, achieving a surface with rms roughness of 0.48 nm without Ga droplets.« less

Effect of annealing time and NH3 flow on GaN films deposited on amorphous SiO2 by MOCVD

NASA Astrophysics Data System (ADS)

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

2018-05-01

GaN polycrystalline films were successfully grown on amorphous SiO2 by metal-organic chemical vapour deposition to fabricate transferable devices using inorganic films. Field-emission scanning electron microscopy images show that by prolonging the annealing time, re-evaporation is enhanced, which reduced the uniformity of the nucleation layer and GaN films. X-ray diffraction patterns indicate that the decomposition rate of the nucleation layer increases when the annealing flow rate of NH3 is 500 sccm, which makes the unstable plane and amorphous domains decompose rapidly, thereby improving the crystallinity of the GaN films. Photoluminescence spectra also indicate the presence of fewer defects when the annealing flow rate of NH3 is 500 sccm. The excellent crystal structure of the GaN films grown under optimized conditions was revealed by transmission electron microscopy analysis. More importantly, the crystal structure and orientation of GaN grown on SiO2 are the same as that of GaN grown on conventional sapphire substrate when a buffer layer is used. This work can aid in the development of transferable devices using GaN films.

Epitaxial growth of GaN by radical-enhanced metalorganic chemical vapor deposition (REMOCVD) in the downflow of a very high frequency (VHF) N2/H2 excited plasma - effect of TMG flow rate and VHF power

NASA Astrophysics Data System (ADS)

Lu, Yi; Kondo, Hiroki; Ishikawa, Kenji; Oda, Osamu; Takeda, Keigo; Sekine, Makoto; Amano, Hiroshi; Hori, Masaru

2014-04-01

Gallium nitride (GaN) films have been grown by using our newly developed Radical-Enhanced Metalorganic Chemical Vapor Deposition (REMOCVD) system. This system has three features: (1) application of very high frequency (60 MHz) power in order to increase the plasma density, (2) introduction of H2 gas together with N2 gas in the plasma discharge region to generate not only nitrogen radicals but also active NHx molecules, and (3) radical supply under remote plasma arrangement with suppression of charged ions and photons by employing a Faraday cage. Using this new system, we have studied the effect of the trimethylgallium (TMG) source flow rate and of the plasma generation power on the GaN crystal quality by using scanning electron microscopy (SEM) and double crystal X-ray diffraction (XRD). We found that this REMOCVD allowed the growth of epitaxial GaN films of the wurtzite structure of (0001) orientation on sapphire substrates with a high growth rate of 0.42 μm/h at a low temperature of 800 °C. The present REMOCVD is a promising method for GaN growth at relatively low temperature and without using costly ammonia gas.

Adsorption and diffusion of Ga and N adatoms on GaN surfaces: Comparing the effects of Ga coverage and electronic excitation

NASA Astrophysics Data System (ADS)

Takeuchi, Noboru; Selloni, Annabella; Myers, T. H.; Doolittle, A.

2005-09-01

We present density-functional-theory calculations of the binding and diffusion of Ga and N adatoms on GaN (0001) and (000-1) surfaces under different conditions, including stoichiometric and Ga-rich surfaces, as well as in the presence of electron-hole (e-h) pairs induced by light- or electron-beam irradiation. We find that both Ga-rich conditions and electronic excitations cause a significant reduction of the adatom diffusion barriers, as required to improve the quality of the material. However, the two effects are nonadditive, as the influence of e-h pairs are found to be less important for the more metallic situations.

High-quality nonpolar a-plane GaN epitaxial films grown on r-plane sapphire substrates by the combination of pulsed laser deposition and metal–organic chemical vapor deposition

NASA Astrophysics Data System (ADS)

Yang, Weijia; Zhang, Zichen; Wang, Wenliang; Zheng, Yulin; Wang, Haiyan; Li, Guoqiang

2018-05-01

High-quality a-plane GaN epitaxial films have been grown on r-plane sapphire substrates by the combination of pulsed laser deposition (PLD) and metal–organic chemical vapor deposition (MOCVD). PLD is employed to epitaxial growth of a-plane GaN templates on r-plane sapphire substrates, and then MOCVD is used. The nonpolar a-plane GaN epitaxial films with relatively small thickness (2.9 µm) show high quality, with the full-width at half-maximum values of GaN(11\\bar{2}0) along [1\\bar{1}00] direction and GaN(10\\bar{1}1) of 0.11 and 0.30°, and a root-mean-square surface roughness of 1.7 nm. This result is equivalent to the quality of the films grown by MOCVD with a thickness of 10 µm. This work provides a new and effective approach for achieving high-quality nonpolar a-plane GaN epitaxial films on r-plane sapphire substrates.

Catalytic activity of enzymes immobilized on AlGaN /GaN solution gate field-effect transistors

NASA Astrophysics Data System (ADS)

Baur, B.; Howgate, J.; von Ribbeck, H.-G.; Gawlina, Y.; Bandalo, V.; Steinhoff, G.; Stutzmann, M.; Eickhoff, M.

2006-10-01

Enzyme-modified field-effect transistors (EnFETs) were prepared by immobilization of penicillinase on AlGaN /GaN solution gate field-effect transistors. The influence of the immobilization process on enzyme functionality was analyzed by comparing covalent immobilization and physisorption. Covalent immobilization by Schiff base formation on GaN surfaces modified with an aminopropyltriethoxysilane monolayer exhibits high reproducibility with respect to the enzyme/substrate affinity. Reductive amination of the Schiff base bonds to secondary amines significantly increases the stability of the enzyme layer. Electronic characterization of the EnFET response to penicillin G indicates that covalent immobilization leads to the formation of an enzyme (sub)monolayer.

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

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

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

2016-05-01

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

Growth of GaN Layers on Sapphire by Low-Temperature-Deposited Buffer Layers and Realization of p-type GaN by Magesium Doping and Electron Beam Irradiation (Nobel Lecture).

PubMed

Amano, Hiroshi

2015-06-26

This Review is a personal reflection on the research that led to the development of a method for growing gallium nitride (GaN) on a sapphire substrate. The results paved the way for the development of smart display systems using blue LEDs. The most important work was done in the mid to late 80s. The background to the author's work and the process by which the technology that enables the growth of GaN and the realization of p-type GaN was established are reviewed. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fuchs-Kliewer phonons of H-covered and clean GaN(1 1 bar 00)

NASA Astrophysics Data System (ADS)

Rink, M.; Himmerlich, M.; Krischok, S.; Kröger, J.

2018-01-01

Inelastic electron scattering is used to study surface phonon polaritons on H-covered and clean GaN(1 1 bar 00) surfaces. The Fuchs-Kliewer phonon of GaN(1 1 bar 00) -H gives rise to characteristic signatures of its single and multiple excitation in specular electron energy loss spectra. The loss intensities for multi-phonon scattering processes decrease according to a Poisson distribution. Vibrational spectra of this surface are invariant on the time scale of days reflecting its chemical passivation by the H layer. In contrast, vibrational spectra of pristine GaN(1 1 bar 00) are subject to a pronounced temporal evolution where spectroscopic weight is gradually shifted towards the multiple excitation of the Fuchs-Kliewer phonon. As a consequence, the monotonous decrease of the cross section for multiple quantum excitation as observed for the H-covered surface is not applicable. This remarkable effect is particularly strong in spectra acquired at low primary energies of incident electrons, which hints at processes occurring in the very surface region. Scenarios that may contribute to these observations are discussed.

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.

Observation and discussion of avalanche electroluminescence in GaN p-n diodes offering a breakdown electric field of 3 MV cm‑1

NASA Astrophysics Data System (ADS)

Mandal, S.; Kanathila, M. B.; Pynn, C. D.; Li, W.; Gao, J.; Margalith, T.; Laurent, M. A.; Chowdhury, S.

2018-06-01

We report on the first observation of avalanche electroluminescence resulting from band-to-band recombination (BTBR) of electron hole pairs at the breakdown limit of Gallium Nitride p-n diodes grown homo-epitaxially on single crystalline GaN substrates. The diodes demonstrated a near ideal breakdown electric field of 3 MV cm‑1 with electroluminescence (EL) demonstrating sharp peaks of emission energies near and at the band gap of GaN. The high critical electric field, near the material limit of GaN, was achieved by generating a smooth curved mesa edge with low plasma damage, using etch engineering without any use of field termination. The superior material quality was critical for such a near-ideal performance. An electric field of 3 MV cm‑1 recorded at the breakdown resulted in impact ionization, confirmed by a positive temperature dependence of the breakdown voltage. The spectral data provided evidence of BTBR of electron hole pairs that were generated by avalanche carrier multiplication in the depletion region.

Results From Cs Activated GaN Photocathode Development for MCP Detector Systems at GSFC

NASA Technical Reports Server (NTRS)

Norton, Tim; Woodgate, Bruce; Stock, Joe; Hilton, George; Ulmer, Mel; Aslam, Shahid; Vispute, R. D.

2003-01-01

We describe the development of high quantum efficiency W photocathodes for use in large area two dimensional microchannel plate based detector arrays to enable new W space astronomy missions. Future W missions will require improvements in detector sensitivity, which has the most leverage for cost-effective improvements in overall telescope/instrument sensitivity. We use new materials such as p-doped GaN, AIGaN, ZnMgO, Sic and diamond. We have currently obtained QE values > 40 % at 185 nm with Cesiated GaN, and hope to demonstrate higher values in the future. By using controlled internal fields and nano-structuring of the surfaces, we plan to provide field emission assistance for photoelectrons while maintaining their energy distinction from dark noise electrons. We will transfer these methods from GaN to ZnMgO, a new family of wide band-gap materials more compatible with microchannel plates. We also are exploring technical parameters such as doping profiles, internal and external field strengths, angle of incidence, field emission assistance, surface preparation, etc.

Photo-induced water oxidation at the aqueous GaN (101¯0) interface: Deprotonation kinetics of the first proton-coupled electron-transfer step

DOE PAGES

Ertem, Mehmed Z.; Kharche, Neerav; Batista, Victor S.; ...

2015-03-12

Photoeclectrochemical water splitting plays a key role in a promising path to the carbon-neutral generation of solar fuels. Wurzite GaN and its alloys ( e.g., GaN/ZnO and InGaN) are demonstrated photocatalysts for water oxidation, and they can drive the overall water splitting reaction when coupled with co-catalysts for proton reduction. In the present work, we investigate the water oxidation mechanism on the prototypical GaN (101¯0) surface using a combined ab initio molecular dynamics and molecular cluster model approach taking into account the role of water dissociation and hydrogen bonding within the first solvation shell of the hydroxylated surface. The investigationmore » of free-energy changes for the four proton-coupled electron-transfer (PCET) steps of the water oxidation mechanism shows that the first PCET step for the conversion of –Ga-OH to –Ga-O˙⁻ requires the highest energy input. We further examine the sequential PCETs, with the proton transfer (PT) following the electron transfer (ET), and find that photo-generated holes localize on surface –NH sites is thermodynamically more favorable than –OH sites. However, proton transfer from –OH sites with subsequent localization of holes on oxygen atoms is kinetically favored owing to hydrogen bonding interactions at the GaN (101¯0)–water interface. We find that the deprotonation of surface –OH sites is the limiting factor for the generation of reactive oxyl radical ion intermediates and consequently for water oxidation.« less

4 Gbps direct modulation of 450 nm GaN laser for high-speed visible light communication.

PubMed

Lee, Changmin; Zhang, Chong; Cantore, Michael; Farrell, Robert M; Oh, Sang Ho; Margalith, Tal; Speck, James S; Nakamura, Shuji; Bowers, John E; DenBaars, Steven P

2015-06-15

We demonstrate high-speed data transmission with a commercial high power GaN laser diode at 450 nm. 2.6 GHz bandwidth was achieved at an injection current of 500 mA using a high-speed visible light communication setup. Record high 4 Gbps free-space data transmission rate was achieved at room temperature.

Triple-twin domains in Mg doped GaN wurtzite nanowires: structural and electronic properties of this zinc-blende-like stacking

NASA Astrophysics Data System (ADS)

Arbiol, Jordi; Estradé, Sònia; Prades, Joan D.; Cirera, Albert; Furtmayr, Florian; Stark, Christoph; Laufer, Andreas; Stutzmann, Martin; Eickhoff, Martin; Gass, Mhairi H.; Bleloch, Andrew L.; Peiró, Francesca; Morante, Joan R.

2009-04-01

We report on the effect of Mg doping on the properties of GaN nanowires grown by plasma assisted molecular beam epitaxy. The most significant feature is the presence of triple-twin domains, the density of which increases with increasing Mg concentration. The resulting high concentration of misplaced atoms gives rise to local changes in the crystal structure equivalent to the insertion of three non-relaxed zinc-blende (ZB) atomic cells, which result in quantum wells along the wurtzite (WZ) nanowire growth axis. High resolution electron energy loss spectra were obtained exactly on the twinned (zinc-blende) and wurtzite planes. These atomically resolved measurements, which allow us to identify modifications in the local density of states, revealed changes in the band to band electronic transition energy from 3.4 eV for wurtzite to 3.2 eV in the twinned lattice regions. These results are in good agreement with specific ab initio atomistic simulations and demonstrate that the redshift observed in previous photoluminescence analyses is directly related to the presence of these zinc-blende domains, opening up new possibilities for band-structure engineering.

Optical design of GaN nanowire arrays for photocatalytic applications

NASA Astrophysics Data System (ADS)

Winnerl, Julia; Hudeczek, Richard; Stutzmann, Martin

2018-05-01

GaN nanowire (NW) arrays are interesting candidates for photocatalytic applications due to their high surface-to-volume ratio and their waveguide character. The integration of GaN NW arrays on GaN-based light emitting diodes (LEDs), serving as a platform for electrically driven NW-based photocatalytic devices, enables an efficient coupling of the light from the planar LED to the GaN NWs. Here, we present a numerical study of the influence of the NW geometries, i.e., the NW diameter, length, and period, and the illumination wavelength on the transmission of GaN NW arrays on transparent substrates. A detailed numerical analysis reveals that the transmission characteristics for large periods are determined by the waveguide character of the single NW, whereas for dense GaN NW arrays inter-wire coupling and diffraction effects originating from the periodic arrangement of the GaN NWs dominate the transmission. The numerically simulated results are confirmed by experimental transmission measurements. We also investigate the influence of a dielectric NW shell and of the surrounding medium on the transmission characteristics of a GaN NW array.

The possibly important role played by Ga{sub 2}O{sub 3} during the activation of GaN photocathode

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

Fu, Xiaoqian, E-mail: ise-fuxq@ujn.edu.cn, E-mail: 214808748@qq.com; Institute of Electronic Engineering and Optoelectronic Technology, Nanjing University of Science and Technology, Nanjing 210094; Wang, Honggang

2015-08-14

Three different chemical solutions are used to remove the possible contamination on GaN surface, while Ga{sub 2}O{sub 3} is still found at the surface. After thermal annealing at 710 °C in the ultrahigh vacuum (UHV) chamber and activated with Cs/O, all the GaN samples are successfully activated to the effective negative electron affinity (NEA) photocathodes. Among all samples, the GaN sample with the highest content of Ga{sub 2}O{sub 3} after chemical cleaning obtains the highest quantum efficiency. By analyzing the property of Ga{sub 2}O{sub 3}, the surface processing results, and electron affinity variations during Cs and Cs/O{sub 2} deposition on GaNmore » of other groups, it is suggested that before the adsorption of Cs, Ga{sub 2}O{sub 3} is not completely removed from GaN surface in our samples, which will combine with Cs and lead to a large decrease in electron affinity. Furthermore, the effective NEA is formed for GaN photocathode, along with the surface downward band bending. Based on this assumption, a new dipole model Ga{sub 2}O{sub 3}-Cs is suggested, and the experimental effects are explained and discussed.« less

Simultaneous measurement of temperature, stress, and electric field in GaN HEMTs with micro-Raman spectroscopy.

PubMed

Bagnall, Kevin R; Moore, Elizabeth A; Badescu, Stefan C; Zhang, Lenan; Wang, Evelyn N

2017-11-01

As semiconductor devices based on silicon reach their intrinsic material limits, compound semiconductors, such as gallium nitride (GaN), are gaining increasing interest for high performance, solid-state transistor applications. Unfortunately, higher voltage, current, and/or power levels in GaN high electron mobility transistors (HEMTs) often result in elevated device temperatures, degraded performance, and shorter lifetimes. Although micro-Raman spectroscopy has become one of the most popular techniques for measuring localized temperature rise in GaN HEMTs for reliability assessment, decoupling the effects of temperature, mechanical stress, and electric field on the optical phonon frequencies measured by micro-Raman spectroscopy is challenging. In this work, we demonstrate the simultaneous measurement of temperature rise, inverse piezoelectric stress, thermoelastic stress, and vertical electric field via micro-Raman spectroscopy from the shifts of the E 2 (high), A 1 longitudinal optical (LO), and E 2 (low) optical phonon frequencies in wurtzite GaN. We also validate experimentally that the pinched OFF state as the unpowered reference accurately measures the temperature rise by removing the effect of the vertical electric field on the Raman spectrum and that the vertical electric field is approximately the same whether the channel is open or closed. Our experimental results are in good quantitative agreement with a 3D electro-thermo-mechanical model of the HEMT we tested and indicate that the GaN buffer acts as a semi-insulating, p-type material due to the presence of deep acceptors in the lower half of the bandgap. This implementation of micro-Raman spectroscopy offers an exciting opportunity to simultaneously probe thermal, mechanical, and electrical phenomena in semiconductor devices under bias, providing unique insight into the complex physics that describes device behavior and reliability. Although GaN HEMTs have been specifically used in this study to

Contributed Review: Experimental characterization of inverse piezoelectric strain in GaN HEMTs via micro-Raman spectroscopy

NASA Astrophysics Data System (ADS)

Bagnall, Kevin R.; Wang, Evelyn N.

2016-06-01

Micro-Raman thermography is one of the most popular techniques for measuring local temperature rise in gallium nitride (GaN) high electron mobility transistors with high spatial and temporal resolution. However, accurate temperature measurements based on changes in the Stokes peak positions of the GaN epitaxial layers require properly accounting for the stress and/or strain induced by the inverse piezoelectric effect. It is common practice to use the pinched OFF state as the unpowered reference for temperature measurements because the vertical electric field in the GaN buffer that induces inverse piezoelectric stress/strain is relatively independent of the gate bias. Although this approach has yielded temperature measurements that agree with those derived from the Stokes/anti-Stokes ratio and thermal models, there has been significant difficulty in quantifying the mechanical state of the GaN buffer in the pinched OFF state from changes in the Raman spectra. In this paper, we review the experimental technique of micro-Raman thermography and derive expressions for the detailed dependence of the Raman peak positions on strain, stress, and electric field components in wurtzite GaN. We also use a combination of semiconductor device modeling and electro-mechanical modeling to predict the stress and strain induced by the inverse piezoelectric effect. Based on the insights gained from our electro-mechanical model and the best values of material properties in the literature, we analyze changes in the E2 high and A1 (LO) Raman peaks and demonstrate that there are major quantitative discrepancies between measured and modeled values of inverse piezoelectric stress and strain. We examine many of the hypotheses offered in the literature for these discrepancies but conclude that none of them satisfactorily resolves these discrepancies. Further research is needed to determine whether the electric field components could be affecting the phonon frequencies apart from the inverse

Simultaneous measurement of temperature, stress, and electric field in GaN HEMTs with micro-Raman spectroscopy

NASA Astrophysics Data System (ADS)

Bagnall, Kevin R.; Moore, Elizabeth A.; Badescu, Stefan C.; Zhang, Lenan; Wang, Evelyn N.

2017-11-01

As semiconductor devices based on silicon reach their intrinsic material limits, compound semiconductors, such as gallium nitride (GaN), are gaining increasing interest for high performance, solid-state transistor applications. Unfortunately, higher voltage, current, and/or power levels in GaN high electron mobility transistors (HEMTs) often result in elevated device temperatures, degraded performance, and shorter lifetimes. Although micro-Raman spectroscopy has become one of the most popular techniques for measuring localized temperature rise in GaN HEMTs for reliability assessment, decoupling the effects of temperature, mechanical stress, and electric field on the optical phonon frequencies measured by micro-Raman spectroscopy is challenging. In this work, we demonstrate the simultaneous measurement of temperature rise, inverse piezoelectric stress, thermoelastic stress, and vertical electric field via micro-Raman spectroscopy from the shifts of the E2 (high), A1 longitudinal optical (LO), and E2 (low) optical phonon frequencies in wurtzite GaN. We also validate experimentally that the pinched OFF state as the unpowered reference accurately measures the temperature rise by removing the effect of the vertical electric field on the Raman spectrum and that the vertical electric field is approximately the same whether the channel is open or closed. Our experimental results are in good quantitative agreement with a 3D electro-thermo-mechanical model of the HEMT we tested and indicate that the GaN buffer acts as a semi-insulating, p-type material due to the presence of deep acceptors in the lower half of the bandgap. This implementation of micro-Raman spectroscopy offers an exciting opportunity to simultaneously probe thermal, mechanical, and electrical phenomena in semiconductor devices under bias, providing unique insight into the complex physics that describes device behavior and reliability. Although GaN HEMTs have been specifically used in this study to

Contributed Review: Experimental characterization of inverse piezoelectric strain in GaN HEMTs via micro-Raman spectroscopy

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

Bagnall, Kevin R.; Wang, Evelyn N.

2016-06-15

Micro-Raman thermography is one of the most popular techniques for measuring local temperature rise in gallium nitride (GaN) high electron mobility transistors with high spatial and temporal resolution. However, accurate temperature measurements based on changes in the Stokes peak positions of the GaN epitaxial layers require properly accounting for the stress and/or strain induced by the inverse piezoelectric effect. It is common practice to use the pinched OFF state as the unpowered reference for temperature measurements because the vertical electric field in the GaN buffer that induces inverse piezoelectric stress/strain is relatively independent of the gate bias. Although this approachmore » has yielded temperature measurements that agree with those derived from the Stokes/anti-Stokes ratio and thermal models, there has been significant difficulty in quantifying the mechanical state of the GaN buffer in the pinched OFF state from changes in the Raman spectra. In this paper, we review the experimental technique of micro-Raman thermography and derive expressions for the detailed dependence of the Raman peak positions on strain, stress, and electric field components in wurtzite GaN. We also use a combination of semiconductor device modeling and electro-mechanical modeling to predict the stress and strain induced by the inverse piezoelectric effect. Based on the insights gained from our electro-mechanical model and the best values of material properties in the literature, we analyze changes in the E{sub 2} high and A{sub 1} (LO) Raman peaks and demonstrate that there are major quantitative discrepancies between measured and modeled values of inverse piezoelectric stress and strain. We examine many of the hypotheses offered in the literature for these discrepancies but conclude that none of them satisfactorily resolves these discrepancies. Further research is needed to determine whether the electric field components could be affecting the phonon frequencies apart

Detection of deep-level defects and reduced carrier concentration in Mg-ion-implanted GaN before high-temperature annealing

NASA Astrophysics Data System (ADS)

Akazawa, Masamichi; Yokota, Naoshige; Uetake, Kei

2018-02-01

We report experimental results for the detection of deep-level defects in GaN after Mg ion implantation before high-temperature annealing. The n-type GaN samples were grown on GaN free-standing substrates by metalorganic vapor phase epitaxy. Mg ions were implanted at 50 keV with a small dosage of 1.5×1011 cm-2, which did not change the conduction type of the n-GaN. By depositing Al2O3 and a Ni/Au electrode onto the implanted n-GaN, metal-oxide-semiconductor (MOS) diodes were fabricated and tested. The measured capacitance-voltage (C-V) characteristics showed a particular behavior with a plateau region and a region with an anomalously steep slope. Fitting to the experimental C-V curves by simulation showed the existence of deep-level defects and a reduction of the carrier concentration near the GaN surface. By annealing at 800oC, the density of the deep-level defects was reduced and the carrier concentration partially recovered.

Effects of radiation and temperature on gallium nitride (GaN) metal-semiconductor-metal ultraviolet photodetectors

NASA Astrophysics Data System (ADS)

Chiamori, Heather C.; Angadi, Chetan; Suria, Ateeq; Shankar, Ashwin; Hou, Minmin; Bhattacharya, Sharmila; Senesky, Debbie G.

2014-06-01

The development of radiation-hardened, temperature-tolerant materials, sensors and electronics will enable lightweight space sub-systems (reduced packaging requirements) with increased operation lifetimes in extreme harsh environments such as those encountered during space exploration. Gallium nitride (GaN) is a ceramic, semiconductor material stable within high-radiation, high-temperature and chemically corrosive environments due to its wide bandgap (3.4 eV). These material properties can be leveraged for ultraviolet (UV) wavelength photodetection. In this paper, current results of GaN metal-semiconductor-metal (MSM) UV photodetectors behavior after irradiation up to 50 krad and temperatures of 15°C to 150°C is presented. These initial results indicate that GaN-based sensors can provide robust operation within extreme harsh environments. Future directions for GaN-based photodetector technology for down-hole, automotive and space exploration applications are also discussed.

Ultraviolet GaN photodetectors on Si via oxide buffer heterostructures with integrated short period oxide-based distributed Bragg reflectors and leakage suppressing metal-oxide-semiconductor contacts

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

Szyszka, A., E-mail: szyszka@ihp-microelectronics.com, E-mail: adam.szyszka@pwr.wroc.pl; Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, Janiszewskiego 11/17, 50-372 Wroclaw; Lupina, L.

2014-08-28

Based on a novel double step oxide buffer heterostructure approach for GaN integration on Si, we present an optimized Metal-Semiconductor-Metal (MSM)-based Ultraviolet (UV) GaN photodetector system with integrated short-period (oxide/Si) Distributed Bragg Reflector (DBR) and leakage suppressing Metal-Oxide-Semiconductor (MOS) electrode contacts. In terms of structural properties, it is demonstrated by in-situ reflection high energy electron diffraction and transmission electron microscopy-energy dispersive x-ray studies that the DBR heterostructure layers grow with high thickness homogeneity and sharp interface structures sufficient for UV applications; only minor Si diffusion into the Y{sub 2}O{sub 3} films is detected under the applied thermal growth budget. Asmore » revealed by comparative high resolution x-ray diffraction studies on GaN/oxide buffer/Si systems with and without DBR systems, the final GaN layer structure quality is not significantly influenced by the growth of the integrated DBR heterostructure. In terms of optoelectronic properties, it is demonstrated that—with respect to the basic GaN/oxide/Si system without DBR—the insertion of (a) the DBR heterostructures and (b) dark current suppressing MOS contacts enhances the photoresponsivity below the GaN band-gap related UV cut-off energy by almost up to two orders of magnitude. Given the in-situ oxide passivation capability of grown GaN surfaces and the one order of magnitude lower number of superlattice layers in case of higher refractive index contrast (oxide/Si) systems with respect to classical III-N DBR superlattices, virtual GaN substrates on Si via functional oxide buffer systems are thus a promising robust approach for future GaN-based UV detector technologies.« less

Low p-type contact resistance by field-emission tunneling in highly Mg-doped GaN

NASA Astrophysics Data System (ADS)

Okumura, Hironori; Martin, Denis; Grandjean, Nicolas

2016-12-01

Mg-doped GaN with a net acceptor concentration (NA-ND) in the high 1019 cm-3 range was grown using ammonia molecular-beam epitaxy. Electrical properties of NiO contact on this heavily doped p-type GaN were investigated. A potential-barrier height of 0.24 eV was extracted from the relationship between NA-ND and the specific contact resistivity (ρc). We found that there is an optimum NA-ND value of 5 × 1019 cm-3 for which ρc is as low as 2 × 10-5 Ω cm2. This low ρc is ascribed to hole tunneling through the potential barrier at the NiO/p+-GaN interface, which is well accounted for by the field-emission model.

Stable and High Piezoelectric Output of GaN Nanowire-Based Lead-Free Piezoelectric Nanogenerator by Suppression of Internal Screening.

PubMed

Johar, Muhammad Ali; Hassan, Mostafa Afifi; Waseem, Aadil; Ha, Jun-Seok; Lee, June Key; Ryu, Sang-Wan

2018-06-14

A piezoelectric nanogenerator (PNG) that is based on c-axis GaN nanowires is fabricated on flexible substrate. In this regard, c-axis GaN nanowires were grown on GaN substrate using the vapor-liquid-solid (VLS) technique by metal organic chemical vapor deposition. Further, Polydimethylsiloxane (PDMS) was coated on nanowire-arrays then PDMS matrix embedded with GaN nanowire-arrays was transferred on Si-rubber substrate. The piezoelectric performance of nanowire-based flexible PNG was measured, while the device was actuated using a cyclic stretching-releasing agitation mechanism that was driven by a linear motor. The piezoelectric output was measured as a function of actuation frequency ranging from 1 Hz to 10 Hz and a linear tendency was observed for piezoelectric output current, while the output voltages remained constant. A maximum of piezoelectric open circuit voltages and short circuit current were measured 15.4 V and 85.6 nA, respectively. In order to evaluate the feasibility of our flexible PNG for real application, a long term stability test was performed for 20,000 cycles and the device performance was degraded by less than 18%. The underlying reason for the high piezoelectric output was attributed to the reduced free carriers inside nanowires due to surface Fermi-level pinning and insulating metal-dielectric-semiconductor interface, respectively; the former reduced the free carrier screening radially while latter reduced longitudinally. The flexibility and the high aspect ratio of GaN nanowire were the responsible factors for higher stability. Such higher piezoelectric output and the novel design make our device more promising for the diverse range of real applications.

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.

Polarization engineered enhancement mode GaN HEMT: Design and investigation

NASA Astrophysics Data System (ADS)

Verma, Sumit; Loan, Sajad A.; Alharbi, Abdullah G.

2018-07-01

In this paper, we propose and perform the experimentally calibrated simulation of a novel structure of a GaN/AlGaN high electron mobility transistor (HEMT). The novelty of the structure is the realization of enhancement mode operation by employing polarization engineering approach. In the proposed polarization engineered HEMT (PE-HEMT) a buried Aluminum Nitride (AlN) box is employed in the GaN layer just below the gate. The AlN box creates a two-dimensional hole gas (2DHG) at the GaN/AlN interface, which creates a conduction band barrier in the path of the already existing two-dimensional electron gas (2DEG) at GaN/AlGaN. Therefore, there is no direct path between the source and drain regions at zero gate voltage due to the barrier created by AIN and the device is initially OFF, an enhancement mode operation. A two dimensional (2D) calibrated simulation study of proposed PE-HEMT shows that the device has a threshold voltage (Vth) of 2.3 V. The PE-HEMT also reduces the electron spillover and thus improves the breakdown voltage by 108% as compared to conventional HEMT. The thermal analysis of the GaN PE-HEMT shows that a hot zone occurs on the drain side gate edge. It has been observed that the drain current in the PE-HEMT structure can be improved by 157% by using AlN heat sink.

Materials Design of the Codoping for the Fabrication of Low-Resistivity p-Type ZnSe and GaN by ab-initio Electronic Structure Calculation

NASA Astrophysics Data System (ADS)

Katayama-Yoshida, H.; Yamamoto, T.

1997-08-01

We propose an effective doping method, the codoping (doping with n- and p-type dopants at the same time) method, for the fabrication of low-resistivity p-type ZnSe and GaN with wide-band-gap based upon ab-initio electronic band structure calculations. p-type doping eminently leads to an increase in the electrostatic energy, called the Madelung energy, which shifts the Se 4p levels for p-type doped ZnSe and the N 2p levels for p-type doped GaN materials towards higher energy regions. This leads to a destabilization of ionic charge distributions in p-type ZnSe and p-type GaN crystals, resulting in the self-compensation of anion intrinsic defects. For ZnSe crystals, we propose the codoping of n-type In donors at Zn sites and p-type N acceptors at Se sites based on the calculation. In addition, we propose the codoping of n-type Si-donors at Ga sites (n-type O donors at N sites) and p-type Be- or Mg acceptors at Ga sites. The codoping decreases the Madelung energy and leads to an increase in the net acceptor carrier density.

Delta-doping optimization for high quality p-type GaN

NASA Astrophysics Data System (ADS)

Bayram, C.; Pau, J. L.; McClintock, R.; Razeghi, M.

2008-10-01

Delta (δ -) doping is studied in order to achieve high quality p-type GaN. Atomic force microscopy, x-ray diffraction, photoluminescence, and Hall measurements are performed on the samples to optimize the δ-doping characteristics. The effect of annealing on the electrical, optical, and structural quality is also investigated for different δ-doping parameters. Optimized pulsing conditions result in layers with hole concentrations near 1018 cm-3 and superior crystal quality compared to conventional p-GaN. This material improvement is achieved thanks to the reduction in the Mg activation energy and self-compensation effects in δ-doped p-GaN.

Theoretical study for heterojunction surface of NEA GaN photocathode dispensed with Cs activation

NASA Astrophysics Data System (ADS)

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

2016-09-01

For the disadvantages of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, new-type NEA GaN photocathodes with heterojunction surface dispensed with Cs activation are investigated based on first-principle study with density functional theory. Through the growth of an ultrathin n-type GaN cap layer on p-type GaN emission layer, a p-n heterojunction is formed on the surface. According to the calculation results, it is found that Si atoms tend to replace Ga atoms to result in an n-type doped cap layer which contributes to the decreasing of work function. After the growth of n-type GaN cap layer, the atom structure near the p-type emission layer is changed while that away from the surface has no obvious variations. By analyzing the E-Mulliken charge distribution of emission surface with and without cap layer, it is found that the positive charge of Ga and Mg atoms in the emission layer decrease caused by the cap layer, while the negative charge of N atom increases. The conduction band moves downwards after the growth of cap layer. Si atom produces donor levels around the valence band maximum. The absorption coefficient of GaN emission layer decreases and the reflectivity increases caused by n-type GaN cap layer.

Synthesis and optical properties of Eu 3+ and Tb 3+ doped GaN nanocrystallite powders

NASA Astrophysics Data System (ADS)

Nyk, M.; Kudrawiec, R.; Strek, W.; Misiewicz, J.

2006-05-01

The GaN nanocrystallite powders obtained by thermal decomposition of pure and doped gallium nitrate followed by nitridation with ammonia are investigated in this paper. The evolution of the phase composition, structure and morphology was studied. The average size of GaN nanocrystallites estimated from the broadening of XRD diffraction peaks was found to be ˜9-21 nm. The photoluminescence and cathodoluminescence properties of pure and Eu 3+ and Tb 3+ doped GaN nanocrystallites were measured and analyzed. A strong emission related to f-f electron transition in Eu and Tb ions has been observed. In addition, a red/yellow emission related to a recombination in the GaN nanocrystalline grains has been observed. It has been shown that this emission strongly depends on the excitation source.

Electro-Thermo-Mechanical Transient Modeling of Stress Development in AlGaN/GaN High Electron Mobility Transistors (HEMTs) (Postprint)

DTIC Science & Technology

2014-02-01

Applied Drain Voltage Ids Drain-to-Source current MPa Megapascals σxx x-Component of Stress INTRODUCTION Gallium nitride (GaN) based high electron...the thermodynamic model to obtain the current densities within a semiconductor device. In doing so, it is possible to determine the electric

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

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

Koleske, Daniel David; Lee, Stephen Roger; Lemp, Thomas Kerr

2009-07-01

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

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

NASA Astrophysics Data System (ADS)

Kaun, Stephen William

GaN-based high-electron-mobility transistors (HEMTs) will play an important role in the next generation of high-frequency amplifiers and power-switching devices. Since parasitic conduction (leakage) through the GaN buffer layer and (Al,Ga,In)N barrier reduces the efficiency of operation, HEMT performance hinges on the epitaxial quality of these layers. Increasing the sheet charge density and mobility of the two-dimensional electron gas (2DEG) is also essential for reducing the channel resistance and improving output. The growth conditions applied in plasma-assisted molecular beam epitaxy (PAMBE) and ammonia-based molecular beam epitaxy (NH3-MBE) that result in high-quality metal-polar HEMT structures are described. The effects of threading dislocations on the gate leakage and channel conductivity of AlGaN/GaN HEMTs were studied in detail. For this purpose, a series of HEMT structures were grown on GaN templates with threading dislocation densities (TDDs) that spanned three orders of magnitude. There was a clear trend of reduced gate leakage with reduced TDD for HEMTs grown by Ga-rich PAMBE; however, a reduction in TDD also entailed an increase in buffer leakage. By reducing the unintentionally doped (UID) GaN buffer thickness and including an AlGaN back barrier, a HEMT regrown by Ga-rich PAMBE on low-TDD free-standing (FS) GaN (~5 x 107 cm-2 TDD) yielded a three-terminal breakdown voltage greater than 50 V and a power output (power-added efficiency) of 6.7 W/mm (50 %) at 4 GHz with a 40 V drain bias. High TDD was then shown to severely degrade the 2DEG mobility of AlxGa1-xN/GaN (x = 0.24, 0.12, 0.06) and AlGaN/AlN/GaN heterostructures grown by Ga-rich PAMBE. By regrowing on low-TDD FS GaN and including a 2.5 nm AlN interlayer, an Al0.24Ga0.76N/AlN/GaN heterostructure achieved a room temperature (RT) 2DEG sheet resistance of 169 Ω/□. As evidenced by atom probe tomography, the AlN interlayer grown by Ga-rich PAMBE was pure with abrupt interfaces. The pure Al

Metal-interconnection-free integration of InGaN/GaN light emitting diodes with AlGaN/GaN high electron mobility transistors

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

Liu, Chao; Cai, Yuefei; Liu, Zhaojun

2015-05-04

We report a metal-interconnection-free integration scheme for InGaN/GaN light emitting diodes (LEDs) and AlGaN/GaN high electron mobility transistors (HEMTs) by combining selective epi removal (SER) and selective epitaxial growth (SEG) techniques. SER of HEMT epi was carried out first to expose the bottom unintentionally doped GaN buffer and the sidewall GaN channel. A LED structure was regrown in the SER region with the bottom n-type GaN layer (n-electrode of the LED) connected to the HEMTs laterally, enabling monolithic integration of the HEMTs and LEDs (HEMT-LED) without metal-interconnection. In addition to saving substrate real estate, minimal interface resistance between the regrownmore » n-type GaN and the HEMT channel is a significant improvement over metal-interconnection. Furthermore, excellent off-state leakage characteristics of the driving transistor can also be guaranteed in such an integration scheme.« less

First-Principle Electronic Properties of Dilute-P GaN(1-x)P(x) Alloy for Visible Light Emitters.

PubMed

Tan, Chee-Keong; Borovac, Damir; Sun, Wei; Tansu, Nelson

2016-04-14

A study on the electronic properties of the dilute-P GaN(1-x)P(x)alloy using First-Principle Density Functional Theory (DFT) calculations is presented. Our results indicate a band gap energy coverage from 3.645 eV to 2.697 eV, with P-content varying from 0% to 12.5% respectively. In addition, through line fitting of calculated and experimental data, a bowing parameter of 9.5 ± 0.5 eV was obtained. The effective masses for electrons and holes are analyzed, as well as the split-off energy parameters where findings indicate minimal interband Auger recombination. The alloy also possesses the direct energy band gap property, indicating its strong potential as a candidate for future photonic device applications.

Formation of spherical-shaped GaN and InN quantum dots on curved SiN/Si surface.

PubMed

Choi, Ilgyu; Lee, Hyunjoong; Lee, Cheul-Ro; Jeong, Kwang-Un; Kim, Jin Soo

2018-08-03

This paper reports the formation of GaN and InN quantum dots (QDs) with symmetric spherical shapes, grown on SiN/Si(111). Spherical QDs are grown by modulating initial growth behavior via gallium and indium droplets functioning as nucleation sites for QDs. Field-emission scanning electron microscope (FE-SEM) images show that GaN and InN QDs are formed on curved SiN/Si(111) instead of on a flat surface similar to balls on a latex mattress. This is considerably different from the structural properties of In(Ga)As QDs grown on GaAs or InP. In addition, considering the shape of the other III-V semiconductor QDs, the QDs in this study are very close to the ideal shape of zero-dimensional nanostructures. Transmission-electron microscope images show the formation of symmetric GaN and InN QDs with a round shape, agreeing well with the FE-SEM results. Compared to other III-V semiconductor QDs, the unique structural properties of Si-based GaN and InN QDs are strongly related to the modulation in the initial nucleation characteristics due to the presence of droplets, the degree of lattice mismatch between GaN or InN and SiN/Si(111), and the melt-back etching phenomenon.

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)

Design of high breakdown voltage GaN vertical HFETs with p-GaN buried buffer layers for power switching applications

NASA Astrophysics Data System (ADS)

Du, Jiangfeng; Liu, Dong; Zhao, Ziqi; Bai, Zhiyuan; Li, Liang; Mo, Jianghui; Yu, Qi

2015-07-01

To achieve a high breakdown voltage, a GaN vertical heterostructure field effect transistor with p-GaN buried layers (PBL-VHFET) is proposed in this paper. The breakdown voltage of this GaN-based PBL-VHFET could be improved significantly by the optimizing thickness of p-GaN buried layers and doping concentration in PBL. When the GaN buffer layer thickness is 15 μm, the thickness, length and p-doping concentration of PBL are 0.3 μm, 2.7 μm, and 3 × 1017 cm-3, respectively. Simulation results show that the breakdown voltage and on-resistance of the device with two p-GaN buried layers are 3022 V and 3.13 mΩ cm2, respectively. The average breakdown electric field would reach as high as 201.5 V/μm. Compared with the typical GaN vertical heterostructure FETs without PBL, both of breakdown voltage and average breakdown electric field of device are increased more than 50%.

Imaging TiO2 nanoparticles on GaN nanowires with electrostatic force microscopy

NASA Astrophysics Data System (ADS)

Xie, Ting; Wen, Baomei; Liu, Guannan; Guo, Shiqi; Motayed, Abhishek; Murphy, Thomas; Gomez, R. D.

Gallium nitride (GaN) nanowires that are functionalized with metal-oxides nanoparticles have been explored extensively for gas sensing applications in the past few years. These sensors have several advantages over conventional schemes, including miniature size, low-power consumption and fast response and recovery times. The morphology of the oxide functionalization layer is critical to achieve faster response and recovery times, with the optimal size distribution of nanoparticles being in the range of 10 to 30 nm. However, it is challenging to characterize these nanoparticles on GaN nanowires using common techniques such as scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. Here, we demonstrate electrostatic force microscopy in combination with atomic force microscopy as a non-destructive technique for morphological characterization of the dispersed TiO2 nanoparticles on GaN nanowires. We also discuss the applicability of this method to other material systems with a proposed tip-surface capacitor model. This project was sponsored through N5 Sensors and the Maryland Industrial Partnerships (MIPS, #5418).

Optimization of ion-atomic beam source for deposition of GaN ultrathin films.

PubMed

Mach, Jindřich; Šamořil, Tomáš; Kolíbal, Miroslav; Zlámal, Jakub; Voborny, Stanislav; Bartošík, Miroslav; Šikola, Tomáš

2014-08-01

We describe the optimization and application of an ion-atomic beam source for ion-beam-assisted deposition of ultrathin films in ultrahigh vacuum. The device combines an effusion cell and electron-impact ion beam source to produce ultra-low energy (20-200 eV) ion beams and thermal atomic beams simultaneously. The source was equipped with a focusing system of electrostatic electrodes increasing the maximum nitrogen ion current density in the beam of a diameter of ≈15 mm by one order of magnitude (j ≈ 1000 nA/cm(2)). Hence, a successful growth of GaN ultrathin films on Si(111) 7 × 7 substrate surfaces at reasonable times and temperatures significantly lower (RT, 300 °C) than in conventional metalorganic chemical vapor deposition technologies (≈1000 °C) was achieved. The chemical composition of these films was characterized in situ by X-ray Photoelectron Spectroscopy and morphology ex situ using Scanning Electron Microscopy. It has been shown that the morphology of GaN layers strongly depends on the relative Ga-N bond concentration in the layers.

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

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

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

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

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

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

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

2016-02-15

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

Basic ammonothermal GaN growth in molybdenum capsules

NASA Astrophysics Data System (ADS)

Pimputkar, S.; Speck, J. S.; Nakamura, S.

2016-12-01

Single crystal, bulk gallium nitride (GaN) crystals were grown using the basic ammonothermal method in a high purity growth environment created using a non-hermetically sealed molybdenum (Mo) capsule and compared to growths performed in a similarly designed silver (Ag) capsule and capsule-free René 41 autoclave. Secondary ion mass spectrometry (SIMS) analysis revealed transition metal free (<1×1017 cm-3) GaN crystals. Anomalously low oxygen concentrations ((2-6)×1018 cm-3) were measured in a {0001} seeded crystal boule grown using a Mo capsule, despite higher source material oxygen concentrations ((1-5)×1019 cm-3) suggesting that molybdenum (or molybdenum nitrides) may act to getter oxygen under certain conditions. Total system pressure profiles from growth runs in a Mo capsule system were comparable to those without a capsule, with pressures peaking within 2 days and slowly decaying due to hydrogen diffusional losses. Measured Mo capsule GaN growth rates were comparable to un-optimized growth rates in capsule-free systems and appreciably slower than in Ag-capsule systems. Crystal quality replicated that of the GaN seed crystals for all capsule conditions, with high quality growth occurring on the (0001) Ga-face. Optical absorption and impurity concentration characterization suggests reduced concentrations of hydrogenated gallium vacancies (VGa-Hx).

High-Temperature Growth of GaN and Al x Ga1- x N via Ammonia-Based Metalorganic Molecular-Beam Epitaxy

NASA Astrophysics Data System (ADS)

Billingsley, Daniel; Henderson, Walter; Doolittle, W. Alan

2010-05-01

The effect of high-temperature growth on the crystalline quality and surface morphology of GaN and Al x Ga1- x N grown by ammonia-based metalorganic molecular-beam epitaxy (NH3-MOMBE) has been investigated as a means of producing atomically smooth films suitable for device structures. The effects of V/III ratio on the growth rate and surface morphology are described herein. The crystalline quality of both GaN and AlGaN was found to mimic that of the GaN templates, with (002) x-ray diffraction (XRD) full-widths at half- maximum (FWHMs) of ~350 arcsec. Nitrogen-rich growth conditions have been found to provide optimal surface morphologies with a root-mean-square (RMS) roughness of ~0.8 nm, yet excessive N-rich environments have been found to reduce the growth rate and result in the formation of faceted surface pitting. AlGaN exhibits a decreased growth rate, as compared with GaN, due to increased N recombination as a result of the increased pyrolysis of NH3 in the presence of Al. AlGaN films grown directly on GaN templates exhibited Pendellösung x-ray fringes, indicating an abrupt interface and a planar AlGaN film. AlGaN films grown for this study resulted in an optimal RMS roughness of ~0.85 nm with visible atomic steps.

Wafer-scale Fabrication of Non-Polar Mesoporous GaN Distributed Bragg Reflectors via Electrochemical Porosification.

PubMed

Zhu, Tongtong; Liu, Yingjun; Ding, Tao; Fu, Wai Yuen; Jarman, John; Ren, Christopher Xiang; Kumar, R Vasant; Oliver, Rachel A

2017-03-27

Distributed Bragg reflectors (DBRs) are essential components for the development of optoelectronic devices. For many device applications, it is highly desirable to achieve not only high reflectivity and low absorption, but also good conductivity to allow effective electrical injection of charges. Here, we demonstrate the wafer-scale fabrication of highly reflective and conductive non-polar gallium nitride (GaN) DBRs, consisting of perfectly lattice-matched non-polar (11-20) GaN and mesoporous GaN layers that are obtained by a facile one-step electrochemical etching method without any extra processing steps. The GaN/mesoporous GaN DBRs exhibit high peak reflectivities (>96%) across the entire visible spectrum and wide spectral stop-band widths (full-width at half-maximum >80 nm), while preserving the material quality and showing good electrical conductivity. Such mesoporous GaN DBRs thus provide a promising and scalable platform for high performance GaN-based optoelectronic, photonic, and quantum photonic devices.

Electron-phonon relaxation and excited electron distribution in gallium nitride

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

Zhukov, V. P.; Donostia International Physics Center; Tyuterev, V. G., E-mail: valtyut00@mail.ru

2016-08-28

We develop a theory of energy relaxation in semiconductors and insulators highly excited by the long-acting external irradiation. We derive the equation for the non-equilibrium distribution function of excited electrons. The solution for this function breaks up into the sum of two contributions. The low-energy contribution is concentrated in a narrow range near the bottom of the conduction band. It has the typical form of a Fermi distribution with an effective temperature and chemical potential. The effective temperature and chemical potential in this low-energy term are determined by the intensity of carriers' generation, the speed of electron-phonon relaxation, rates ofmore » inter-band recombination, and electron capture on the defects. In addition, there is a substantial high-energy correction. This high-energy “tail” largely covers the conduction band. The shape of the high-energy “tail” strongly depends on the rate of electron-phonon relaxation but does not depend on the rates of recombination and trapping. We apply the theory to the calculation of a non-equilibrium distribution of electrons in an irradiated GaN. Probabilities of optical excitations from the valence to conduction band and electron-phonon coupling probabilities in GaN were calculated by the density functional perturbation theory. Our calculation of both parts of distribution function in gallium nitride shows that when the speed of the electron-phonon scattering is comparable with the rate of recombination and trapping then the contribution of the non-Fermi “tail” is comparable with that of the low-energy Fermi-like component. So the high-energy contribution can essentially affect the charge transport in the irradiated and highly doped semiconductors.« less

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.

High speed visible light communication using blue GaN laser diodes

NASA Astrophysics Data System (ADS)

Watson, S.; Viola, S.; Giuliano, G.; Najda, S. P.; Perlin, P.; Suski, T.; Marona, L.; Leszczyński, M.; Wisniewski, P.; Czernecki, R.; Targowski, G.; Watson, M. A.; White, H.; Rowe, D.; Laycock, L.; Kelly, A. E.

2016-10-01

GaN-based laser diodes have been developed over the last 20 years making them desirable for many security and defence applications, in particular, free space laser communications. Unlike their LED counterparts, laser diodes are not limited by their carrier lifetime which makes them attractive for high speed communication, whether in free space, through fiber or underwater. Gigabit data transmission can be achieved in free space by modulating the visible light from the laser with a pseudo-random bit sequence (PRBS), with recent results approaching 5 Gbit/s error free data transmission. By exploiting the low-loss in the blue part of the spectrum through water, data transmission experiments have also been conducted to show rates of 2.5 Gbit/s underwater. Different water types have been tested to monitor the effect of scattering and to see how this affects the overall transmission rate and distance. This is of great interest for communication with unmanned underwater vehicles (UUV) as the current method using acoustics is much slower and vulnerable to interception. These types of laser diodes can typically reach 50-100 mW of power which increases the length at which the data can be transmitted. This distance could be further improved by making use of high power laser arrays. Highly uniform GaN substrates with low defectivity allow individually addressable laser bars to be fabricated. This could ultimately increase optical power levels to 4 W for a 20-emitter array. Overall, the development of GaN laser diodes will play an important part in free space optical communications and will be vital in the advancement of security and defence applications.

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

PubMed

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

2016-07-14

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

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.

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.

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

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

Tangi, Malleswararao; De, Arpan; Ghatak, Jay

2016-05-28

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

Enhanced mobility in vertically scaled N-polar high-electron-mobility transistors using GaN/InGaN composite channels

NASA Astrophysics Data System (ADS)

Li, Haoran; Wienecke, Steven; Romanczyk, Brian; Ahmadi, Elaheh; Guidry, Matthew; Zheng, Xun; Keller, Stacia; Mishra, Umesh K.

2018-02-01

A GaN/InGaN composite channel design for vertically scaled N-polar high-electron-mobility transistor (HEMT) structures is proposed and demonstrated by metal-organic chemical vapor deposition. In a conventional N-polar HEMT structure, as the channel thickness (tch) decreases, the sheet charge density (ns) decreases, the electric field in the channel increases, and the centroid of the two-dimensional electron gas (2DEG) moves towards the back-barrier/channel interface, resulting in stronger scattering and lower electron mobility (μ). In this study, a thin InGaN layer was introduced in-between the channel and the AlGaN cap to increase the 2DEG density and reduce the electric field in the channel and therefore increase the electron mobility. The dependence of μ on the InGaN thickness (tInGaN) and the indium composition (xIn) was investigated for different channel thicknesses. With optimized tInGaN and xIn, significant improvements in electron mobility were observed. For a 6 nm channel HEMT structure, the electron mobility increased from 606 to 1141 cm2/(V.s) when the 6 nm thick pure GaN channel was replaced by the 4 nm GaN/2 nm In0.1Ga0.9N composite channel.

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

Topical Review: Development of overgrown semi-polar GaN for high efficiency green/yellow emission

NASA Astrophysics Data System (ADS)

Wang, T.

2016-09-01

The most successful example of large lattice-mismatched epitaxial growth of semiconductors is the growth of III-nitrides on sapphire, leading to the award of the Nobel Prize in 2014 and great success in developing InGaN-based blue emitters. However, the majority of achievements in the field of III-nitride optoelectronics are mainly limited to polar GaN grown on c-plane (0001) sapphire. This polar orientation poses a number of fundamental issues, such as reduced quantum efficiency, efficiency droop, green and yellow gap in wavelength coverage, etc. To date, it is still a great challenge to develop longer wavelength devices such as green and yellow emitters. One clear way forward would be to grow III-nitride device structures along a semi-/non-polar direction, in particular, a semi-polar orientation, which potentially leads to both enhanced indium incorporation into GaN and reduced quantum confined Stark effects. This review presents recent progress on developing semi-polar GaN overgrowth technologies on sapphire or Si substrates, the two kinds of major substrates which are cost-effective and thus industry-compatible, and also demonstrates the latest achievements on electrically injected InGaN emitters with long emission wavelengths up to and including amber on overgrown semi-polar GaN. Finally, this review presents a summary and outlook on further developments for semi-polar GaN based optoelectronics.

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.

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.

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.

Chemical lift-off of (11-22) semipolar GaN using periodic triangular cavities

NASA Astrophysics Data System (ADS)

Jeon, Dae-Woo; Lee, Seung-Jae; Jeong, Tak; Baek, Jong Hyeob; Park, Jae-Woo; Jang, Lee-Woon; Kim, Myoung; Lee, In-Hwan; Ju, Jin-Woo

2012-01-01

Chemical lift-off of (11-22) semipolar GaN using triangular cavities was investigated. The (11-22) semipolar GaN was grown using epitaxial lateral overgrowth by metal-organic chemical vapor deposition on m-plane sapphire, in such a way as to keep N terminated surface of c-plane GaN exposed in the cavities. After regrowing 300 μm thick (11-22) semipolar GaN by hydride vapor phase epitaxy for a free-standing (11-22) semipolar GaN substrate, the triangular cavities of the templates were chemically etched in molten KOH. The (000-2) plane in the triangular cavities can be etched in the [0002] direction with the high lateral etching rate of 196 μm/min. The resulting free-standing (11-22) semipolar GaN substrate was confirmed to be strain-free by the Raman analysis.

Photo-induced changes of the surface band bending in GaN: Influence of growth technique, doping and polarity

NASA Astrophysics Data System (ADS)

Winnerl, Andrea; Pereira, Rui N.; Stutzmann, Martin

2017-05-01

In this work, we use conductance and contact potential difference photo-transient data to study the influence of the growth technique, doping, and crystal polarity on the kinetics of photo-generated charges in GaN. We found that the processes, and corresponding time scales, involved in the decay of charge carriers generated at and close to the GaN surface via photo-excitation are notably independent of the growth technique, doping (n- and p-types), and also crystal polarity. Hence, the transfer of photo-generated charges from band states back to surface states proceeds always by hopping via shallow defect states in the space-charge region (SCR) close to the surface. Concerning the charge carrier photo-generation kinetics, we observe considerable differences between samples grown with different techniques. While for GaN grown by metal-organic chemical vapor deposition, the accumulation of photo-conduction electrons results mainly from a combined trapping-hopping process (slow), where photo-generated electrons hop via shallow defect states to the conduction band (CB), in hydride vapor phase epitaxy and molecular beam epitaxy materials, a faster direct process involving electron transfer via CB states is also present. The time scales of both processes are quite insensitive to the doping level and crystal polarity. However, these processes become irrelevant for very high doping levels (both n- and p-types), where the width of the SCR is much smaller than the photon penetration depth, and therefore, most charge carriers are generated outside the SCR.

Electric field dynamics in nitride structures containing quaternary alloy (Al, In, Ga)N

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

Borysiuk, J., E-mail: jolanta.borysiuk@ifpan.edu.pl; Faculty of Physics, University of Warsaw, Pasteura 5, 02-093 Warsaw; Sakowski, K.

2016-07-07

Molecular beam epitaxy growth and basic physical properties of quaternary AlInGaN layers, sufficiently thick for construction of electron blocking layers (EBL), embedded in ternary InGaN layers are presented. Transmission electron microscopy (TEM) measurement revealed good crystallographic structure and compositional uniformity of the quaternary layers contained in other nitride layers, which are typical for construction of nitride based devices. The AlInGaN layer was epitaxially compatible to InGaN matrix, strained, and no strain related dislocation creation was observed. The strain penetrated for limited depth, below 3 nm, even for relatively high content of indium (7%). For lower indium content (0.6%), the strain wasmore » below the detection limit by TEM strain analysis. The structures containing quaternary AlInGaN layers were studied by time dependent photoluminescence (PL) at different temperatures and excitation powers. It was shown that PL spectra contain three peaks: high energy donor bound exciton peak from the bulk GaN (DX GaN) and the two peaks (A and B) from InGaN layers. No emission from quaternary AlInGaN layers was observed. An accumulation of electrons on the EBL interface in high-In sample and formation of 2D electron gas (2DEG) was detected. The dynamics of 2DEG was studied by time resolved luminescence revealing strong dependence of emission energy on the 2DEG concentration. Theoretical calculations as well as power-dependence and temperature-dependence analysis showed the importance of electric field inside the structure. At the interface, the field was screened by carriers and could be changed by illumination. From these measurements, the dynamics of electric field was described as the discharge of carriers accumulated on the EBL.« less

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.

Mg doping of GaN by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Lieten, R. R.; Motsnyi, V.; Zhang, L.; Cheng, K.; Leys, M.; Degroote, S.; Buchowicz, G.; Dubon, O.; Borghs, G.

2011-04-01

We present a systematic study on the influence of growth conditions on the incorporation and activation of Mg in GaN layers grown by plasma-assisted molecular beam epitaxy. We show that high quality p-type GaN layers can be obtained on GaN-on-silicon templates. The Mg incorporation and the electrical properties have been investigated as a function of growth temperature, Ga : N flux ratio and Mg : Ga flux ratio. It was found that the incorporation of Mg and the electrical properties are highly sensitive to the Ga : N flux ratio. The highest hole mobility and lowest resistivity were achieved for slightly Ga-rich conditions. In addition to an optimal Ga : N ratio, an optimum Mg : Ga flux ratio was also observed at around 1%. We observed a clear Mg flux window for p-type doping of GaN : 0.31% < Mg : Ga < 5.0%. A lowest resistivity of 0.98 Ω cm was obtained for optimized growth conditions. The p-type GaN layer then showed a hole concentration of 4.3 × 1017 cm-3 and a mobility of 15 cm2 V-1 s-1. Temperature-dependent Hall effect measurements indicate an acceptor depth in these samples of 100 meV for a hole concentration of 5.5 × 1017 cm-3. The corresponding Mg concentration is 5 × 1019 cm-3, indicating approximately 1% activation at room temperature. In addition to continuous growth of Mg-doped GaN layers we also investigated different modulated growth procedures. We show that a modulated growth procedure has only limited influence on Mg doping at a growth temperature of 800 °C or higher. This result is thus in contrast to previously reported GaN : Mg doping at much lower growth temperatures of 500 °C.

Curvature and bow of bulk GaN substrates

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

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

2016-07-21

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

Emission dynamics of hybrid plasmonic gold/organic GaN nanorods

NASA Astrophysics Data System (ADS)

Mohammadi, F.; Schmitzer, H.; Kunert, G.; Hommel, D.; Ge, J.; Duscher, G.; Langbein, W.; Wagner, H. P.

2017-12-01

We studied the emission of bare and aluminum quinoline (Alq3)/gold coated wurtzite GaN nanorods by temperature- and intensity-dependent time-integrated and time-resolved photoluminescence (PL). The GaN nanorods of ˜1.5 μm length and ˜250 nm diameter were grown by plasma-assisted molecular beam epitaxy. Gold/Alq3 coated GaN nanorods were synthesized by organic molecular beam deposition. The near band-edge and donor-acceptor pair luminescence was investigated in bare GaN nanorods and compared with multilevel model calculations providing the dynamical parameters for electron-hole pairs, excitons, impurity bound excitons, donors and acceptors. Subsequently, the influence of a 10 nm gold coating without and with an Alq3 spacer layer was studied and the experimental results were analyzed with the multilevel model. Without a spacer layer, a significant PL quenching and lifetime reduction of the near band-edge emission is found. The behavior is attributed to surface band-bending and Förster energy transfer from excitons to surface plasmons in the gold layer. Inserting a 5 nm Alq3 spacer layer reduces the PL quenching and lifetime reduction which is consistent with a reduced band-bending and Förster energy transfer. Increasing the spacer layer to 30 nm results in lifetimes which are similar to uncoated structures, showing a significantly decreased influence of the gold coating on the excitonic dynamics.

Emission dynamics of hybrid plasmonic gold/organic GaN nanorods.

PubMed

Mohammadi, F; Schmitzer, H; Kunert, G; Hommel, D; Ge, J; Duscher, G; Langbein, W; Wagner, H P

2017-12-15

We studied the emission of bare and aluminum quinoline (Alq 3 )/gold coated wurtzite GaN nanorods by temperature- and intensity-dependent time-integrated and time-resolved photoluminescence (PL). The GaN nanorods of ∼1.5 μm length and ∼250 nm diameter were grown by plasma-assisted molecular beam epitaxy. Gold/Alq 3 coated GaN nanorods were synthesized by organic molecular beam deposition. The near band-edge and donor-acceptor pair luminescence was investigated in bare GaN nanorods and compared with multilevel model calculations providing the dynamical parameters for electron-hole pairs, excitons, impurity bound excitons, donors and acceptors. Subsequently, the influence of a 10 nm gold coating without and with an Alq 3 spacer layer was studied and the experimental results were analyzed with the multilevel model. Without a spacer layer, a significant PL quenching and lifetime reduction of the near band-edge emission is found. The behavior is attributed to surface band-bending and Förster energy transfer from excitons to surface plasmons in the gold layer. Inserting a 5 nm Alq 3 spacer layer reduces the PL quenching and lifetime reduction which is consistent with a reduced band-bending and Förster energy transfer. Increasing the spacer layer to 30 nm results in lifetimes which are similar to uncoated structures, showing a significantly decreased influence of the gold coating on the excitonic dynamics.

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

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

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

2014-06-07

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

GaN nanophosphors for white-light applications

NASA Astrophysics Data System (ADS)

Kumar, Mirgender; Singh, V. P.; Dubey, Sarvesh; Suh, Youngsuk; Park, Si-Hyun

2018-01-01

GaN nanoparticles (NPs) were synthesized by carbothermal reduction combined with nitridation, using Ga2O3 powder and graphitic carbon nitride (g-C3N4) as precursors. Characterization of the NPs was performed by X-ray diffraction, scanning electron microscopy, and room-temperature photoluminescence measurements. X-ray photoelectron spectroscopy was also performed to detect the chemical states of the different species. A universal yellow luminescence (YL) band was observed from complexes of Ga vacancies with O anti-sites and of O anti-sites with C. Further increments in the C content were observed with continued growth and induced an additional blue luminescence (BL) band. Tuning of the YL and BL bands resulted in white-light emission under certain experimental conditions, thus offering a new way of employing GaN nanophosphors for solid-state white lighting. Calculations of the correlated color temperature and color-quality scale parameters confirmed the utility of the experimental process for different applications.

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.

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.

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

NASA Astrophysics Data System (ADS)

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

2017-03-01

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

Impact of substrate off-angle on the m-plane GaN Schottky diodes

NASA Astrophysics Data System (ADS)

Yamada, Hisashi; Chonan, Hiroshi; Takahashi, Tokio; Shimizu, Mitsuaki

2018-04-01

We investigated the effects of the substrate off-angle on the m-plane GaN Schottky diodes. GaN epitaxial layers were grown by metal-organic chemical vapor deposition on m-plane GaN substrates having an off-angle of 0.1, 1.1, 1.7, or 5.1° toward [000\\bar{1}]. The surface of the GaN epitaxial layers on the 0.1°-off substrate consisted of pyramidal hillocks and contained oxygen (>1017 cm-3) and carbon (>1016 cm-3) impurities. The residual carbon and oxygen impurities decreased to <1016 cm-3 when the off-angle of the m-plane GaN substrate was increased. The leakage current of the 0.1°-off m-plane GaN Schottky diodes originated from the +c facet of the pyramidal hillocks. The leakage current was efficiently suppressed through the use of an off-angle that was observed to be greater than 1.1°. The off-angle of the m-plane GaN substrate is critical in obtaining high-performance Schottky diodes.

Design of an Ultra-Efficient GaN High Power Amplifier for Radar Front-Ends Using Active Harmonic Load-Pull

NASA Technical Reports Server (NTRS)

Thrivikraman, Tushar; Hoffman, James

2012-01-01

This work presents a new measurement technique, mixed-signal active harmonic load-pull (MSALP) developed by Anterverta-mw in partnership with Maury Microwave, that allows for wide-band ultra-high efficiency amplifiers to be designed using GaN technology. An overview of the theory behind active load-pull is presented and why load-pull is important for high-power device characterization. In addition, an example procedure is presented that outlines a methodology for amplifier design using this measurement system. Lastly, measured results of a 10W GaN amplifier are presented. This work aims to highlight the benefit of using this sophisticated measurement systems for to optimize amplifier design for real radar waveforms that in turn will simplify implementation of space-based radar systems

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.

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.

A compensating point defect in carbon-doped GaN substrates studied with electron paramagnetic resonance spectroscopy

NASA Astrophysics Data System (ADS)

Willoughby, W. R.; Zvanut, M. E.; Paudel, Subash; Iwinska, M.; Sochacki, T.; Bockowski, M.

2018-04-01

Electron paramagnetic resonance (EPR) spectroscopy was used to investigate a type of point defect present in 1019 cm-3 carbon-doped GaN substrates grown by hydride vapor phase epitaxy. A broad, isotropic resonance at g ˜ 1.987 was observed at 3.5 K, and the EPR intensity increased with illumination at energies greater than 2.75 eV and decreased with photon energies greater than 0.95 eV. The latter is consistent with a deep level of 0.95 eV above the valence band maximum and implies that the associated defect likely participates in donor compensation. The ionization energy for this defect is close to the predicted value for the (-/0) transition level of CN and transition levels associated with Ga vacancies such as VGa and VGa-ON-2H.

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

NASA Astrophysics Data System (ADS)

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

2016-03-01

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

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

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

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

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

An investigation of GaN thin films on AlN on sapphire substrate by sol-gel spin coating method

NASA Astrophysics Data System (ADS)

Amin, Nur Fahana Mohd; Ng, Sha Shiong

2017-12-01

In this research, the gallium nitride (GaN) thin films were deposited on aluminium nitride on sapphire (AlN/Al2O3) substrate by sol-gel spin coating method. Simple ethanol-based precursor with the addition of diethanolamine solution was used. The structural and morphology properties of synthesized GaN thin films were characterized by using X-ray Diffraction, Field-Emission Scanning Electron Microscopy and Atomic Force Microscopy. While the elemental compositions and the lattice vibrational properties of the films were investigated by means of the Energy Dispersive X-ray spectroscopy and Raman spectroscopy. All the results revealed that the wurtzite structure GaN thin films with GaN(002) preferred orientation and smooth surface morphology were successfully grown on AlN/Al2O3 substrate by using inexpensive and simplified sol-gel spin coating technique. The sol-gel spin coated GaN thin film with lowest oxygen content was also achieved.FESEM images show that GaN thin films with uniform and packed grains were formed. Based on the obtained results, it can be concluded that wurtzite structure GaN thin films were successfully deposited on AlN/Al2O3 substrate.

Piezo-generator integrating a vertical array of GaN nanowires.

PubMed

Jamond, N; Chrétien, P; Houzé, F; Lu, L; Largeau, L; Maugain, O; Travers, L; Harmand, J C; Glas, F; Lefeuvre, E; Tchernycheva, M; Gogneau, N

2016-08-12

We demonstrate the first piezo-generator integrating a vertical array of GaN nanowires (NWs). We perform a systematic multi-scale analysis, going from single wire properties to macroscopic device fabrication and characterization, which allows us to establish for GaN NWs the relationship between the material properties and the piezo-generation, and to propose an efficient piezo-generator design. The piezo-conversion of individual MBE-grown p-doped GaN NWs in a dense array is assessed by atomic force microscopy (AFM) equipped with a Resiscope module yielding an average output voltage of 228 ± 120 mV and a maximum value of 350 mV generated per NW. In the case of p-doped GaN NWs, the piezo-generation is achieved when a positive piezo-potential is created inside the nanostructures, i.e. when the NWs are submitted to compressive deformation. The understanding of the piezo-generation mechanism in our GaN NWs, gained from AFM analyses, is applied to design a piezo-generator operated under compressive strain. The device consists of NW arrays of several square millimeters in size embedded into spin-on glass with a Schottky contact for rectification and collection of piezo-generated carriers. The generator delivers a maximum power density of ∼12.7 mW cm(-3). This value sets the new state of the art for piezo-generators based on GaN NWs and more generally on nitride NWs, and offers promising prospects for the use of GaN NWs as high-efficiency ultra-compact energy harvesters.

The MOVPE growth mechanism of catalyst-free self-organized GaN columns in H2 and N2 carrier gases

NASA Astrophysics Data System (ADS)

Wang, Xue; Jahn, Uwe; Ledig, Johannes; Wehmann, Hergo-H.; Mandl, Martin; Straßburg, Martin; Waag, Andreas

2013-12-01

Columnar structures of III-V semiconductors recently attract considerable attention because of their potential applications in novel optoelectronic and electronic devices. In the present study, the mechanisms for the growth of catalyst-free self-organized GaN columns on sapphire substrate by metal organic vapor phase epitaxy have been thoroughly investigated. The growth behaviours are strongly affected by the choice of carrier gas. If pure nitrogen is used, Ga droplets are able to accumulate on the top of columns during growth, and they are converted into a high quality GaN layer during the cool down phase due to nitridation. Hydrogen as the carrier gas can improve the optical quality of the overall GaN columns substantially, and in addition increase the vertical growth rate. In this case, no indication of Ga droplets could be detected. Furthermore, silane doping during the growth promotes the vertical growth in both cases either pure nitrogen or pure hydrogen as the carrier gas.

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

PubMed Central

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

2016-01-01

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

How localized acceptors limit p-type conductivity in GaN

NASA Astrophysics Data System (ADS)

Lyons, John L.

2013-03-01

Despite the impressive development of GaN as an optoelectronic material, p-type conductivity is still limited. Only a single acceptor impurity, magnesium, is known to lead to p-type GaN. But Mg is far from a well-behaved acceptor. Hydrogen is known to passivate Mg, necessitating a post-growth anneal for acceptor activation. In addition, the ionization energy is quite large (~ 200 meV in GaN), meaning only a few percent of Mg acceptors are ionized at room temperature. Thus, hole conductivity is limited, and high concentrations of Mg are required to achieve moderately p-type GaN. Other acceptor impurities have not proven to be effective p-type dopants, for reasons that are still unresolved. Using advanced first-principles calculations based on a hybrid functional, we investigate the electrical and optical properties of the isolated Mg acceptor and its complexes with hydrogen in GaN, InN, and AlN.[2] We employ a technique that overcomes the band-gap-problem of traditional density functional theory, and allows for quantitative predictions of acceptor ionization energies and optical transition energies. Our results allow us to explain the deep or shallow nature of the Mg acceptor and its relation to the optical signals observed in Mg-doped GaN. We also revisit the properties of other group-II acceptors in GaN. We find that all cation-site acceptors show behavior similar to MgGa, and lead to highly localized holes. The ZnGa and BeGa acceptors have ionization energies that are even larger than that of Mg, making them ineffective dopants. All acceptors cause large lattice distortions in their neutral charge state, in turn leading to deep, broad luminescence signals that can serve as a means of experimentally verifying the deep nature of these acceptors. This work was performed in collaboration with Audrius Alkauskas, Anderson Janotti, and Chris G. Van de Walle. It was supported by the NSF and by the Solid State Lighting and Energy Center at UCSB.

Structural and magnetic properties of ultra-thin Fe films on metal-organic chemical vapour deposited GaN(0001)

NASA Astrophysics Data System (ADS)

Kim, Jun-Young; Ionescu, Adrian; Mansell, Rhodri; Farrer, Ian; Oehler, Fabrice; Kinane, Christy J.; Cooper, Joshaniel F. K.; Steinke, Nina-Juliane; Langridge, Sean; Stankiewicz, Romuald; Humphreys, Colin J.; Cowburn, Russell P.; Holmes, Stuart N.; Barnes, Crispin H. W.

2017-01-01

Structural and magnetic properties of 1-10 nm thick Fe films deposited on GaN(0001) were investigated. In-situ reflecting high energy electron diffraction images indicated a α-Fe(110)/GaN(0001) growth of the 3D Volmer-Weber type. The α-Fe(110) X-ray diffraction peak showed a 1° full-width at half-maximum, indicating ≈20 nm grain sizes. A significant reduction in Fe atomic moment from its bulk value was observed for films thinner than 4 nm. Both GaN/Fe interface roughness and Fe film coercivity increased with Fe thickness, indicating a possible deterioration of Fe crystalline quality. Magnetic anisotropy was mainly uniaxial for all films while hexagonal anisotropies appeared for thicknesses higher than 3.7 nm.

GaN microwires as optical microcavities: whispering gallery modes Vs Fabry-Perot modes.

PubMed

Coulon, Pierre-Marie; Hugues, Maxime; Alloing, Blandine; Beraudo, Emmanuel; Leroux, Mathieu; Zuniga-Perez, Jesus

2012-08-13

GaN microwires grown by metalorganic vapour phase epitaxy and with radii typically on the order of 1-5 micrometers exhibit a number of resonances in their photoluminescence spectra. These resonances include whispering gallery modes and transverse Fabry-Perot modes. A detailed spectroscopic study by polarization-resolved microphotoluminescence, in combination with electron microscopy images, has enabled to differentiate both kinds of modes and determined their main spectral properties. Finally, the dispersion of the ordinary and extraordinary refractive indices of strain-free GaN in the visible-UV range has been obtained thanks to the numerical simulation of the observed modes.

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

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.

Simultaneously Enhancing Light Emission and Suppressing Efficiency Droop in GaN Microwire-Based Ultraviolet Light-Emitting Diode by the Piezo-Phototronic Effect.

PubMed

Wang, Xingfu; Peng, Wenbo; Yu, Ruomeng; Zou, Haiyang; Dai, Yejing; Zi, Yunlong; Wu, Changsheng; Li, Shuti; Wang, Zhong Lin

2017-06-14

Achievement of p-n homojuncted GaN enables the birth of III-nitride light emitters. Owing to the wurtzite-structure of GaN, piezoelectric polarization charges present at the interface can effectively control/tune the optoelectric behaviors of local charge-carriers (i.e., the piezo-phototronic effect). Here, we demonstrate the significantly enhanced light-output efficiency and suppressed efficiency droop in GaN microwire (MW)-based p-n junction ultraviolet light-emitting diode (UV LED) by the piezo-phototronic effect. By applying a -0.12% static compressive strain perpendicular to the p-n junction interface, the relative external quantum efficiency of the LED is enhanced by over 600%. Furthermore, efficiency droop is markedly reduced from 46.6% to 7.5% and corresponding droop onset current density shifts from 10 to 26.7 A cm -2 . Enhanced electrons confinement and improved holes injection efficiency by the piezo-phototronic effect are revealed and theoretically confirmed as the physical mechanisms. This study offers an unconventional path to develop high efficiency, strong brightness and high power III-nitride light sources.

A comparative study on magnetism in Zn-doped AlN and GaN from first-principles

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

Xu, Liang; Wang, Lingling, E-mail: llwang@hnu.edu.cn, E-mail: xiaowenzhi@hnu.edu.cn; Huang, Weiqing

2014-09-14

First-principles calculations have been used to comparatively investigate electronic and magnetic properties of Zn-doped AlN and GaN. A total magnetic moment of 1.0 μ B{sub B} induced by Zn is found in AlN, but not in GaN. Analyses show that the origin of spontaneous polarization not only depend on the localized atomic orbitals of N and sufficient hole concentration, but also the relative intensity of the covalency of matrix. The relatively stronger covalent character of GaN with respect to AlN impedes forming local magnetic moment in GaN matrix. Our study offers a fresh sight of spontaneous spin polarization in d⁰more » magnetism. The much stronger ferromagnetic coupling in c-plane of AlN means that it is feasible to realize long-range ferromagnetic order via monolayer delta-doping. This can apply to other wide band-gap semiconductors in wurtzite structure.« less

High-voltage vertical GaN Schottky diode enabled by low-carbon metal-organic chemical vapor deposition growth

NASA Astrophysics Data System (ADS)

Cao, Y.; Chu, R.; Li, R.; Chen, M.; Chang, R.; Hughes, B.

2016-02-01

Vertical GaN Schottky barrier diode (SBD) structures were grown by metal-organic chemical vapor deposition on free-standing GaN substrates. The carbon doping effect on SBD performance was studied by adjusting the growth conditions and spanning the carbon doping concentration between ≤3 × 1015 cm-3 and 3 × 1019 cm-3. Using the optimized growth conditions that resulted in the lowest carbon incorporation, a vertical GaN SBD with a 6-μm drift layer was fabricated. A low turn-on voltage of 0.77 V with a breakdown voltage over 800 V was obtained from the device.

Surface Interaction Kinetics of GaAs (100) Surfaces Under Electron Cyclotron Resonance Nitrogen Plasma Exposure

NASA Astrophysics Data System (ADS)

O'Steen, M. L.; Hauenstein, R. J.; Bandić, Z. Z.; Feenstra, R. M.; Hwang, S. J.; McGill, T. C.

1996-03-01

GaN is a robust semiconducting material offering a large, direct bandgap appropriate for use in blue-green to UV light emitting diodes and laser diodes. Attainment of device quality GaN has been difficult due to the lack of substrate materials that are suitably matched to the unusually small lattice parameter of GaN. To better control heteroepitaxial growth quality, a fundamental study of the initial stages of GaN growth by Electron Cyclotron Resonance Nitrogen Plasma-Assisted Molecular Beam Epitaxy (ECR-MBE) has been performed. The effect of an ECR Nitrogen plasma on a GaAs (100) surface is examined through time resolved reflection high energy electron diffraction, high resolution x-ray diffraction, and cross-sectional scanning tunneling microscopy. Fully commensurate GaN_yAs_1-y/GaAs heterostructures involving ultrathin GaN_yAs_1-y layers are obtained, and thermally activated microscopic growth processes are identified and quantitatively characterized through the aid of a specially developed kinetic model. The implications for ECR-MBE growth of GaN/GaAs mutilayers is discussed.

Codoping characteristics of Zn with Mg in GaN

NASA Astrophysics Data System (ADS)

Kim, K. S.; Han, M. S.; Yang, G. M.; Youn, C. J.; Lee, H. J.; Cho, H. K.; Lee, J. Y.

2000-08-01

The doping characteristics of Mg-Zn codoped GaN films grown by metalorganic chemical vapor deposition are investigated. By means of the concept of Mg-Zn codoping technique, we have grown p-GaN showing a low electrical resistivity (0.72 Ω cm) and a high hole concentration (8.5×1017cm-3) without structural degradation of the film. It is thought that the codoping of Zn atoms with Mg raises the Mg activation ratio by reducing the hydrogen solubility in p-GaN. In addition, the measured specific contact resistance of Mg-Zn codoped GaN film is 5.0×10-4 Ω cm2, which is one order of magnitude lower than that of Mg doped only GaN film (1.9×10-3 Ω cm2).

Advances in Hydrogen, Carbon Dioxide, and Hydrocarbon Gas Sensor Technology Using GaN and ZnO-Based Devices

PubMed Central

Anderson, Travis; Ren, Fan; Pearton, Stephen; Kang, Byoung Sam; Wang, Hung-Ta; Chang, Chih-Yang; Lin, Jenshan

2009-01-01

In this paper, we review our recent results in developing gas sensors for hydrogen using various device structures, including ZnO nanowires and GaN High Electron Mobility Transistors (HEMTs). ZnO nanowires are particularly interesting because they have a large surface area to volume ratio, which will improve sensitivity, and because they operate at low current levels, will have low power requirements in a sensor module. GaN-based devices offer the advantage of the HEMT structure, high temperature operation, and simple integration with existing fabrication technology and sensing systems. Improvements in sensitivity, recoverability, and reliability are presented. Also reported are demonstrations of detection of other gases, including CO2 and C2H4 using functionalized GaN HEMTs. This is critical for the development of lab-on-a-chip type systems and can provide a significant advance towards a market-ready sensor application. PMID:22408548

White emission from non-planar InGaN/GaN MQW LEDs grown on GaN template with truncated hexagonal pyramids.

PubMed

Lee, Ming-Lun; Yeh, Yu-Hsiang; Tu, Shang-Ju; Chen, P C; Lai, Wei-Chih; Sheu, Jinn-Kong

2015-04-06

Non-planar InGaN/GaN multiple quantum well (MQW) structures are grown on a GaN template with truncated hexagonal pyramids (THPs) featuring c-plane and r-plane surfaces. The THP array is formed by the regrowth of the GaN layer on a selective-area Si-implanted GaN template. Transmission electron microscopy shows that the InGaN/GaN epitaxial layers regrown on the THPs exhibit different growth rates and indium compositions of the InGaN layer between the c-plane and r-plane surfaces. Consequently, InGaN/GaN MQW light-emitting diodes grown on the GaN THP array emit multiple wavelengths approaching near white light.

Axial p-n junction and space charge limited current in single GaN nanowire.

PubMed

Fang, Zhihua; Donatini, Fabrice; Daudin, Bruno; Pernot, Julien

2018-01-05

The electrical characterizations of individual basic GaN nanostructures, such as axial nanowire (NW) p-n junctions, are becoming indispensable and crucial for the fully controlled realization of GaN NW based devices. In this study, electron beam induced current (EBIC) measurements were performed on two single axial GaN p-n junction NWs grown by plasma-assisted molecular beam epitaxy. I-V characteristics revealed that both ohmic and space charge limited current (SCLC) regimes occur in GaN p-n junction NW. Thanks to an improved contact process, both the electric field induced by the p-n junction and the SCLC in the p-part of GaN NW were disclosed and delineated by EBIC signals under different biases. Analyzing the EBIC profiles in the vicinity of the p-n junction under 0 V and reverse bias, we deduced a depletion width in the range of 116-125 nm. Following our previous work, the acceptor N a doping level was estimated to be 2-3 × 10 17 at cm -3 assuming a donor level N d of 2-3 × 10 18 at cm -3 . The hole diffusion length in n-GaN was determined to be 75 nm for NW #1 and 43 nm for NW #2, demonstrating a low surface recombination velocity at the m-plane facet of n-GaN NW. Under forward bias, EBIC imaging visualized the electric field induced by the SCLC close to p-side contact, in agreement with unusual SCLC previously reported in GaN NWs.

Axial p-n junction and space charge limited current in single GaN nanowire

NASA Astrophysics Data System (ADS)

Fang, Zhihua; Donatini, Fabrice; Daudin, Bruno; Pernot, Julien

2018-01-01

The electrical characterizations of individual basic GaN nanostructures, such as axial nanowire (NW) p-n junctions, are becoming indispensable and crucial for the fully controlled realization of GaN NW based devices. In this study, electron beam induced current (EBIC) measurements were performed on two single axial GaN p-n junction NWs grown by plasma-assisted molecular beam epitaxy. I-V characteristics revealed that both ohmic and space charge limited current (SCLC) regimes occur in GaN p-n junction NW. Thanks to an improved contact process, both the electric field induced by the p-n junction and the SCLC in the p-part of GaN NW were disclosed and delineated by EBIC signals under different biases. Analyzing the EBIC profiles in the vicinity of the p-n junction under 0 V and reverse bias, we deduced a depletion width in the range of 116-125 nm. Following our previous work, the acceptor N a doping level was estimated to be 2-3 × 1017 at cm-3 assuming a donor level N d of 2-3 × 1018 at cm-3. The hole diffusion length in n-GaN was determined to be 75 nm for NW #1 and 43 nm for NW #2, demonstrating a low surface recombination velocity at the m-plane facet of n-GaN NW. Under forward bias, EBIC imaging visualized the electric field induced by the SCLC close to p-side contact, in agreement with unusual SCLC previously reported in GaN NWs.

Controlled growth of ordered nanopore arrays in GaN.

PubMed

Wildeson, Isaac H; Ewoldt, David A; Colby, Robert; Stach, Eric A; Sands, Timothy D

2011-02-09

High-quality, ordered nanopores in semiconductors are attractive for numerous biological, electrical, and optical applications. Here, GaN nanorods with continuous pores running axially through their centers were grown by organometallic vapor phase epitaxy. The porous nanorods nucleate on an underlying (0001)-oriented GaN film through openings in a SiN(x) template that are milled by a focused ion beam, allowing direct placement of porous nanorods. Nanopores with diameters ranging from 20-155 nm were synthesized with crystalline sidewalls.

Defect formation in MeV H+ implanted GaN and 4H-SiC investigated by cross-sectional Raman spectroscopy

NASA Astrophysics Data System (ADS)

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

2017-09-01

Cross-sectional Raman spectroscopy is used to characterize the defect formation and the defect recovery in MeV H+ implanted bulk GaN and 4H-SiC in the high energy MeV ion-cut process. The Raman intensity decreases but the forbidden modes are activated at the damage region, and the intensity decrease is proportional to the damage level. The Raman spectrum is quite sensitive to detect the damage recovery after annealing. The main peak intensity increases and the forbidden mode disappears in both annealed GaN and 4H-SiC samples. The Raman spectra of GaN samples annealed at different temperatures suggest that higher annealing temperature is more efficient for damage recovery. While, the Raman spectra of SiC indicate that higher implantation temperature results in heavier lattice damage and other polytype clusters might be generated by high annealing temperature in the annealed SiC samples. The cross-sectional Raman spectroscopy is a straightforward method to characterize lattice damage and damage recovery in high energy ion-cut process. It can serve as a fast supplementary measurement technique to Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA) and transmission electron microscope (TEM) for the defect characterizations.

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

NASA Astrophysics Data System (ADS)

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

2013-03-01

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

GaN Schottky diodes with single-crystal aluminum barriers grown by plasma-assisted molecular beam epitaxy

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

Tseng, H. Y.; Yang, W. C.; Lee, P. Y.

2016-08-22

GaN-based Schottky barrier diodes (SBDs) with single-crystal Al barriers grown by plasma-assisted molecular beam epitaxy are fabricated. Examined using in-situ reflection high-energy electron diffractions, ex-situ high-resolution x-ray diffractions, and high-resolution transmission electron microscopy, it is determined that epitaxial Al grows with its [111] axis coincident with the [0001] axis of the GaN substrate without rotation. In fabricated SBDs, a 0.2 V barrier height enhancement and 2 orders of magnitude reduction in leakage current are observed in single crystal Al/GaN SBDs compared to conventional thermal deposited Al/GaN SBDs. The strain induced piezoelectric field is determined to be the major source of themore » observed device performance enhancements.« less

Less strained and more efficient GaN light-emitting diodes with embedded silica hollow nanospheres

PubMed Central

Kim, Jonghak; Woo, Heeje; Joo, Kisu; Tae, Sungwon; Park, Jinsub; Moon, Daeyoung; Park, Sung Hyun; Jang, Junghwan; Cho, Yigil; Park, Jucheol; Yuh, Hwankuk; Lee, Gun-Do; Choi, In-Suk; Nanishi, Yasushi; Han, Heung Nam; Char, Kookheon; Yoon, Euijoon

2013-01-01

Light-emitting diodes (LEDs) become an attractive alternative to conventional light sources due to high efficiency and long lifetime. However, different material properties between GaN and sapphire cause several problems such as high defect density in GaN, serious wafer bowing, particularly in large-area wafers, and poor light extraction of GaN-based LEDs. Here, we suggest a new growth strategy for high efficiency LEDs by incorporating silica hollow nanospheres (S-HNS). In this strategy, S-HNSs were introduced as a monolayer on a sapphire substrate and the subsequent growth of GaN by metalorganic chemical vapor deposition results in improved crystal quality due to nano-scale lateral epitaxial overgrowth. Moreover, well-defined voids embedded at the GaN/sapphire interface help scatter lights effectively for improved light extraction, and reduce wafer bowing due to partial alleviation of compressive stress in GaN. The incorporation of S-HNS into LEDs is thus quite advantageous in achieving high efficiency LEDs for solid-state lighting. PMID:24220259

Catalyst and processing effects on metal-assisted chemical etching for the production of highly porous GaN

NASA Astrophysics Data System (ADS)

Geng, Xuewen; Duan, Barrett K.; Grismer, Dane A.; Zhao, Liancheng; Bohn, Paul W.

2013-06-01

Metal-assisted chemical etching is a facile method to produce micro-/nanostructures in the near-surface region of gallium nitride (GaN) and other semiconductors. Detailed studies of the production of porous GaN (PGaN) using different metal catalysts and GaN doping conditions have been performed in order to understand the mechanism by which metal-assisted chemical etching is accomplished in GaN. Patterned catalysts show increasing metal-assisted chemical etching activity to n-GaN in the order Ag < Au < Ir < Pt. In addition, the catalytic behavior of continuous films is compared to discontinuous island films. Continuous metal films strongly shield the surface, hindering metal-assisted chemical etching, an effect which can be overcome by using discontinuous films or increasing the irradiance of the light source. With increasing etch time or irradiance, PGaN morphologies change from uniform porous structures to ridge and valley structures. The doping type plays an important role, with metal-assisted chemical etching activity increasing in the order p-GaN < intrinsic GaN < n-GaN. Both the catalyst identity and the doping type effects are explained by the work functions and the related band offsets that affect the metal-assisted chemical etching process through a combination of different barriers to hole injection and the formation of hole accumulation/depletion layers at the metal-semiconductor interface.

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

Optical and structural characteristics of high indium content InGaN/GaN multi-quantum wells with varying GaN cap layer thickness

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

Yang, J.; Zhao, D. G., E-mail: dgzhao@red.semi.ac.cn; Jiang, D. S.

2015-02-07

The optical and structural properties of InGaN/GaN multi-quantum wells (MQWs) with different thicknesses of low temperature grown GaN cap layers are investigated. It is found that the MQW emission energy red-shifts and the peak intensity decreases with increasing GaN cap layer thickness, which may be partly caused by increased floating indium atoms accumulated at quantum well (QW) surface. They will result in the increased interface roughness, higher defect density, and even lead to a thermal degradation of QW layers. An extra growth interruption introduced before the growth of GaN cap layer can help with evaporating the floating indium atoms, andmore » therefore is an effective method to improve the optical properties of high indium content InGaN/GaN MQWs.« less

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

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

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

2015-01-12

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

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

NASA Astrophysics Data System (ADS)

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

2018-01-01

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

Thermal stability of isolated and complexed Ga vacancies in GaN bulk crystals

NASA Astrophysics Data System (ADS)

Saarinen, K.; Suski, T.; Grzegory, I.; Look, D. C.

2001-12-01

We have applied positron annihilation spectroscopy to show that 2-MeV electron irradiation at 300 K creates primary Ga vacancies in GaN with an introduction rate of 1 cm-1. The Ga vacancies recover in long-range migration processes at 500-600 K with an estimated migration energy of 1.5 (2) eV. Since the native Ga vacancies in as-grown GaN survive up to much higher temperatures (1300-1500 K), we conclude that they are stabilized by forming complexes with oxygen impurities. The estimated binding energy of 2.2 (4) eV of such complexes is in good agreement with the results of theoretical calculations.

Photoluminescence enhancement from GaN by beryllium doping

NASA Astrophysics Data System (ADS)

García-Gutiérrez, R.; Ramos-Carrazco, A.; Berman-Mendoza, D.; Hirata, G. A.; Contreras, O. E.; Barboza-Flores, M.

2016-10-01

High quality Be-doped (Be = 0.19 at.%) GaN powder has been grown by reacting high purity Ga diluted alloys (Be-Ga) with ultra high purity ammonia in a horizontal quartz tube reactor at 1200 °C. An initial low-temperature treatment to dissolve ammonia into the Ga melt produced GaN powders with 100% reaction efficiency. Doping was achieved by dissolving beryllium into the gallium metal. The powders synthesized by this method regularly consist of two particle size distributions: large hollow columns with lengths between 5 and 10 μm and small platelets in a range of diameters among 1 and 3 μm. The GaN:Be powders present a high quality polycrystalline profile with preferential growth on the [10 1 bar 1] plane, observed by means of X-ray diffraction. The three characteristics growth planes of the GaN crystalline phase were found by using high resolution TEM microscopy. The optical enhancing of the emission in the GaN powder is attributed to defects created with the beryllium doping. The room temperature photoluminescence emission spectra of GaN:Be powders, revealed the presence of beryllium on a shoulder peak at 3.39 eV and an unusual Y6 emission at 3.32eV related to surface donor-acceptor pairs. Also, a donor-acceptor-pair transition at 3.17 eV and a phonon replica transition at 3.1 eV were observed at low temperature (10 K). The well-known yellow luminescence band coming from defects was observed in both spectra at room and low temperature. Cathodoluminescence emission from GaN:Be powders presents two main peaks associated with an ultraviolet band emission and the yellow emission known from defects. To study the trapping levels related with the defects formed in the GaN:Be, thermoluminescence glow curves were obtained using UV and β radiation in the range of 50 and 150 °C.

Correlation between mobility collapse and carbon impurities in Si-doped GaN grown by low pressure metalorganic chemical vapor deposition

NASA Astrophysics Data System (ADS)

Kaess, Felix; Mita, Seiji; Xie, Jingqiao; Reddy, Pramod; Klump, Andrew; Hernandez-Balderrama, Luis H.; Washiyama, Shun; Franke, Alexander; Kirste, Ronny; Hoffmann, Axel; Collazo, Ramón; Sitar, Zlatko

2016-09-01

In the low doping range below 1 × 1017 cm-3, carbon was identified as the main defect attributing to the sudden reduction of the electron mobility, the electron mobility collapse, in n-type GaN grown by low pressure metalorganic chemical vapor deposition. Secondary ion mass spectroscopy has been performed in conjunction with C concentration and the thermodynamic Ga supersaturation model. By controlling the ammonia flow rate, the input partial pressure of Ga precursor, and the diluent gas within the Ga supersaturation model, the C concentration in Si-doped GaN was controllable from 6 × 1019 cm-3 to values as low as 2 × 1015 cm-3. It was found that the electron mobility collapsed as a function of free carrier concentration, once the Si concentration closely approached the C concentration. Lowering the C concentration to the order of 1015 cm-3 by optimizing Ga supersaturation achieved controllable free carrier concentrations down to 5 × 1015 cm-3 with a peak electron mobility of 820 cm2/V s without observing the mobility collapse. The highest electron mobility of 1170 cm2/V s was obtained even in metalorganic vapor deposition-grown GaN on sapphire substrates by optimizing growth parameters in terms of Ga supersaturation to reduce the C concentration.

Growth optimization and characterization of GaN epilayers on multifaceted (111) surfaces etched on Si(100) substrates

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

Ansah-Antwi, KwaDwo Konadu, E-mail: kakadee@gmail.com; Chua, Soo Jin; Department of Electrical and Computer Engineering, National University of Singapore, E4-5-45, 4 Engineering Drive 3, Singapore 117576

2015-11-15

The four nearest Si(111) multifaceted sidewalls were exposed inside an array of 3 μm-wide square holes patterned on an Si(100) substrate, and this patterned Si(100) substrate was used as a substrate for the deposition of a gallium nitride (GaN) epilayer. Subsequently the effect that the growth pressure, the etched-hole profiles, and the etched-hole arrangement had upon the quality of the as-grown GaN was investigated. The coalescence of the as-grown GaN epilayer on the exposed Si(111) facets was observed to be enhanced with reduced growth pressure from 120 to 90 Torr. A larger Si(001) plane area at the bottom of the etched holesmore » resulted in bidirectional GaN domains, which resulted in poor material quality. The bidirectional GaN domains were observed as two sets of six peaks via a high-resolution x-ray diffraction phi scan of the GaN(10-11) reflection. It was also shown that a triangular array of etched holes was more desirable than square arrays of etched holes for the growth high-quality and continuous GaN films.« less

A Microstructural Comparison of the Initial Growth of AIN and GaN Layers on Basal Plane Sapphire and SiC Substrates by Low Pressure Metalorganic Chemical Vapor Depositon

NASA Technical Reports Server (NTRS)

George, T.; Pike, W. T.; Khan, M. A.; Kuznia, J. N.; Chang-Chien, P.

1994-01-01

The initial growth by low pressure metalorganic chemical vapor deposition and subsequent thermal annealing of AIN and GaN epitaxial layers on SiC and sapphire substrates is examined using high resolution transmission electron microscopy and atomic force microscopy.

Physical origin of the incubation time of self-induced GaN nanowires

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

Consonni, V.; Trampert, A.; Geelhaar, L.

2011-07-18

The nucleation process of self-induced GaN nanowires grown by molecular beam epitaxy has been investigated by reflection high-energy electron diffraction measurements. It is found that stable nuclei in the form of spherical cap-shaped islands develop only after an incubation time that is strongly dependent upon the growth conditions. Its evolution with the growth temperature and gallium rate has been described within standard island nucleation theory, revealing a nucleation energy of 4.9 {+-} 0.1 eV and a very small nucleus critical size. The consideration of the incubation time is critical for the control of the nanowire morphology.

Propagation of THz acoustic wave packets in GaN at room temperature

DOE PAGES

Maznev, A. A.; Hung, T.-C.; Yao, Y.-T.; ...

2018-02-05

We use femtosecond laser pulses to generate coherent longitudinal acoustic phonons at frequencies of 1–1.4 THz and study their propagation in GaN-based structures at room temperature. Two InGaN-GaN multiple-quantum-well (MQW) structures separated by a 2.3 μm-thick GaN spacer are used to simultaneously generate phonon wave packets with a central frequency determined by the period of the MQW and detect them after passing through the spacer. The measurements provide lower bounds for phonon lifetimes in GaN, which are still significantly lower than those from first principles predictions. The material Q-factor at 1 THz is found to be at least as highmore » as 900. The measurements also demonstrate a partial specular reflection from the free surface of GaN at 1.4 THz. This work shows the potential of laser-based methods for THz range phonon spectroscopy and the promise for extending the viable frequency range of GaN-based acousto-electronic devices.« less

Propagation of THz acoustic wave packets in GaN at room temperature

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

Maznev, A. A.; Hung, T.-C.; Yao, Y.-T.

We use femtosecond laser pulses to generate coherent longitudinal acoustic phonons at frequencies of 1–1.4 THz and study their propagation in GaN-based structures at room temperature. Two InGaN-GaN multiple-quantum-well (MQW) structures separated by a 2.3 μm-thick GaN spacer are used to simultaneously generate phonon wave packets with a central frequency determined by the period of the MQW and detect them after passing through the spacer. The measurements provide lower bounds for phonon lifetimes in GaN, which are still significantly lower than those from first principles predictions. The material Q-factor at 1 THz is found to be at least as highmore » as 900. The measurements also demonstrate a partial specular reflection from the free surface of GaN at 1.4 THz. This work shows the potential of laser-based methods for THz range phonon spectroscopy and the promise for extending the viable frequency range of GaN-based acousto-electronic devices.« less

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

PubMed

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

2008-02-01

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

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

Light Modulation and Water Splitting Enhancement Using a Composite Porous GaN Structure.

PubMed

Yang, Chao; Xi, Xin; Yu, Zhiguo; Cao, Haicheng; Li, Jing; Lin, Shan; Ma, Zhanhong; Zhao, Lixia

2018-02-14

On the basis of the laterally porous GaN, we designed and fabricated a composite porous GaN structure with both well-ordered lateral and vertical holes. Compared to the plane GaN, the composite porous GaN structure with the combination of the vertical holes can help to reduce UV reflectance and increase the saturation photocurrent during water splitting by a factor of ∼4.5. Furthermore, we investigated the underlying mechanism for the enhancement of the water splitting performance using a finite-difference time-domain method. The results show that the well-ordered vertical holes can not only help to open the embedded pore channels to the electrolyte at both sides and reduce the migration distance of the gas bubbles during the water splitting reactions but also help to modulate the light field. Using this composite porous GaN structure, most of the incident light can be modulated and trapped into the nanoholes, and thus the electric fields localized in the lateral pores can increase dramatically as a result of the strong optical coupling. Our findings pave a new way to develop GaN photoelectrodes for highly efficient solar water splitting.

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.

High current density 2D/3D MoS2/GaN Esaki tunnel diodes

NASA Astrophysics Data System (ADS)

Krishnamoorthy, Sriram; Lee, Edwin W.; Lee, Choong Hee; Zhang, Yuewei; McCulloch, William D.; Johnson, Jared M.; Hwang, Jinwoo; Wu, Yiying; Rajan, Siddharth

2016-10-01

The integration of two-dimensional materials such as transition metal dichalcogenides with bulk semiconductors offer interesting opportunities for 2D/3D heterojunction-based device structures without any constraints of lattice matching. By exploiting the favorable band alignment at the GaN/MoS2 heterojunction, an Esaki interband tunnel diode is demonstrated by transferring large area Nb-doped, p-type MoS2 onto heavily n-doped GaN. A peak current density of 446 A/cm2 with repeatable room temperature negative differential resistance, peak to valley current ratio of 1.2, and minimal hysteresis was measured in the MoS2/GaN non-epitaxial tunnel diode. A high current density of 1 kA/cm2 was measured in the Zener mode (reverse bias) at -1 V bias. The GaN/MoS2 tunnel junction was also modeled by treating MoS2 as a bulk semiconductor, and the electrostatics at the 2D/3D interface was found to be crucial in explaining the experimentally observed device characteristics.

In-situ transport and microstructural evolution in GaN Schottky diodes and epilayers exposed to swift heavy ion irradiation

NASA Astrophysics Data System (ADS)

Kumar, Ashish; Singh, R.; Kumar, Parmod; Singh, Udai B.; Asokan, K.; Karaseov, Platon A.; Titov, Andrei I.; Kanjilal, D.

2018-04-01

A systematic investigation of radiation hardness of Schottky barrier diodes and GaN epitaxial layers is carried out by employing in-situ electrical resistivity and cross sectional transmission electron microscopy (XTEM) microstructure measurements. The change in the current transport mechanism of Au/n-GaN Schottky barrier diodes due to irradiation is reported. The role of irradiation temperature and ion type was also investigated. Creation of damage is studied in low and medium electron energy loss regimes by selecting different ions, Ag (200 MeV) and O (100 MeV) at various fluences at two irradiation temperatures (80 K and 300 K). GaN resistivity increases up to 6 orders of magnitude under heavy Ag ions. Light O ion irradiation has a much lower influence on sheet resistance. The presence of isolated defect clusters in irradiated GaN epilayers is evident in XTEM investigation which is explained on the basis of the thermal spike model.

ARM MJO Investigation Experiment on Gan Island (AMIE-Gan) Science Plan

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

Long, CL; Del Genio, A; Deng, M

2011-04-11

The overarching campaign, which includes the ARM Mobile Facility 2 (AMF2) deployment in conjunction with the Dynamics of the Madden-Julian Oscillation (DYNAMO) and the Cooperative Indian Ocean experiment on intraseasonal variability in the Year 2011 (CINDY2011) campaigns, is designed to test several current hypotheses regarding the mechanisms responsible for Madden-Julian Oscillation (MJO) initiation and propagation in the Indian Ocean area. The synergy between the proposed AMF2 deployment with DYNAMO/CINDY2011, and the corresponding funded experiment on Manus, combine for an overarching ARM MJO Investigation Experiment (AMIE) with two components: AMF2 on Gan Island in the Indian Ocean (AMIE-Gan), where the MJOmore » initiates and starts its eastward propagation; and the ARM Manus site (AMIE-Manus), which is in the general area where the MJO usually starts to weaken in climate models. AMIE-Gan will provide measurements of particular interest to Atmospheric System Research (ASR) researchers relevant to improving the representation of MJO initiation in climate models. The framework of DYNAMO/CINDY2011 includes two proposed island-based sites and two ship-based locations forming a square pattern with sonde profiles and scanning precipitation and cloud radars at both island and ship sites. These data will be used to produce a Variational Analysis data set coinciding with the one produced for AMIE-Manus. The synergy between AMIE-Manus and AMIE-Gan will allow studies of the initiation, propagation, and evolution of the convective cloud population within the framework of the MJO. As with AMIE-Manus, AMIE-Gan/DYNAMO also includes a significant modeling component geared toward improving the representation of MJO initiation and propagation in climate and forecast models. This campaign involves the deployment of the second, marine-capable, AMF; all of the included measurement systems; and especially the scanning and vertically pointing radars. The campaign will include

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

Mechanism of high-fluence proton induced electrical degradation in AlGaN/GaN high-electron-mobility transistors

NASA Astrophysics Data System (ADS)

Lei, Zhifeng; Guo, Hongxia; Tang, Minghua; Peng, Chao; Zhang, Zhangang; Huang, Yun; En, Yunfei

2018-07-01

The effects of displacement damage induced by 3 and 6 MeV protons in AlGaN/GaN high-electron-mobility transistors (HEMTs) are investigated. For the 6 MeV protons at a dose of 5 × 1014 cm‑2, a 12% decrease in saturation current, a 3.8% decrease in the peak transconductance, a 0.3 V positive shift of the threshold voltage, and a three-to fourfold decrease in reverse gate leakage current are observed compared with the pre-irradiation values. The main degradation mechanism is considered to be the generation of deep trap states in the band gap, which remove electrons and reduce the carrier mobility in a two-dimensional electron gas (2DEG). Both the carrier removal rate and negatively charged trap density can be extracted, which shows that about 3500 proton injections lead to one carrier removal. Proton fluence and energy are found to be two key parameters that affect the degradation characteristics of irradiated GaN HEMTs.

Determination of carrier diffusion length in GaN

NASA Astrophysics Data System (ADS)

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

2015-01-01

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

UMA/GAN network architecture analysis

NASA Astrophysics Data System (ADS)

Yang, Liang; Li, Wensheng; Deng, Chunjian; Lv, Yi

2009-07-01

This paper is to critically analyze the architecture of UMA which is one of Fix Mobile Convergence (FMC) solutions, and also included by the third generation partnership project(3GPP). In UMA/GAN network architecture, UMA Network Controller (UNC) is the key equipment which connects with cellular core network and mobile station (MS). UMA network could be easily integrated into the existing cellular networks without influencing mobile core network, and could provides high-quality mobile services with preferentially priced indoor voice and data usage. This helps to improve subscriber's experience. On the other hand, UMA/GAN architecture helps to integrate other radio technique into cellular network which includes WiFi, Bluetooth, and WiMax and so on. This offers the traditional mobile operators an opportunity to integrate WiMax technique into cellular network. In the end of this article, we also give an analysis of potential influence on the cellular core networks ,which is pulled by UMA network.

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.

Alternating InGaN barriers with GaN barriers for enhancing optical performance in InGaN light-emitting diodes

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

Yang, Yujue; Zeng, Yiping, E-mail: ypzeng@semi.ac.cn

2015-01-21

InGaN-based light-emitting diodes (LEDs) with some specific designs on the quantum barrier layers by alternating InGaN barriers with GaN barriers are proposed and studied numerically. In the proposed structure, simulation results show that the carriers are widely dispersed in the multi-quantum well active region, and the radiative recombination rate is efficiently improved and the electron leakage is suppressed accordingly, due to the appropriate band engineering. The internal quantum efficiency and light-output power are thus markedly enhanced and the efficiency droop is smaller, compared to the original structures with GaN barriers or InGaN barriers. Moreover, the gradually decrease of indium compositionmore » in the alternating quantum barriers can further promote the LED performance because of the more uniform carrier distribution, which provides us a simple but highly effective approach for high-performance LED applications.« less

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.

Carrier concentration dependence of donor activation energy in n-type GaN epilayers grown on Si (1 1 1) by plasma-assisted MBE

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

Kumar, Mahesh; Central Research Laboratory, Bharat Electronics, Bangalore 560 013; Bhat, Thirumaleshwara N.

Highlights: ► The n-type GaN layers were grown by plasma-assisted molecular beam epitaxy. ► The optical characteristics of a donor level in Si-doped GaN were studied. ► Activation energy of a Si-related donor was estimated from temperature dependent PL measurements. ► PL peak positions, FWHM of PL and activation energies are found to be proportional to the cube root of carrier density. ► The involvement of donor levels is supported by the temperature-dependent electron concentration measurements. -- Abstract: The n-type GaN layers were grown by plasma-assisted MBE and either intentionally doped with Si or unintentionally doped. The optical characteristics ofmore » a donor level in Si-doped, GaN were studied in terms of photoluminescence (PL) spectroscopy as a function of electron concentration. Temperature dependent PL measurements allowed us to estimate the activation energy of a Si-related donor from temperature-induced decay of PL intensity. PL peak positions, full width at half maximum of PL and activation energies are found to be proportional to the cube root of carrier density. The involvement of donor levels is supported by the temperature-dependent electron concentration measurements.« less