Electrical Properties Of GaN Films Research Articles

Improving Optical and Electrical Characteristics of GaN Films via 3D Island to 2D Growth Mode Transition Using Molecular Beam Epitaxy

Molecular beam epitaxy (MBE) is demonstrated as an excellent growth technique for growing a low-defect GaN channel layer, which is crucial for controlling vertical leakage current and improving breakdown voltage (BV) in GaN-based high-electron mobility transistors (HEMTs). The 3D islands to 2D growth mode transition approach was induced by modulating substrate growth temperature (Tsub), displaying an overall improvement in film quality. A comprehensive investigation was conducted into the effects of Tsub on surface morphologies, crystal quality, and the optical and electrical properties of GaN films. Optimal results were achieved with a strain-relaxed GaN film grown at 690 °C, exhibiting significantly improved surface characteristics (root-mean-square roughness, Rq = 0.3 nm) and impressively reduced edge dislocations. However, the film with the smoothest surface roughness, attributed to the effect of the Ga-rich condition, possessed a high surface pit density, negatively affecting optical and electrical properties. A reduction in defect-related yellow emission further confirmed the enhanced crystalline quality of MBE GaN films. The optimized GaN film demonstrated outstanding electrical properties with a BV of ~1450 V, surpassing that of MOCVD GaN (~1180 V). This research significantly contributes to the advancement of MBE GaN-based high electron mobility transistor (HEMT) applications by ensuring outstanding reliability.

Effect of laser fluence on the optoelectronic properties of nanostructured GaN/porous silicon prepared by pulsed laser deposition

In this study, the fabrication of nanostructured GaN/porous Si by pulsed laser deposition (PLD) was demonstrated. The porous silicon was prepared using laser-assisted electrochemical etching (LAECE). The structural, optical, and electrical properties of GaN films were investigated as a function of laser fluence. XRD studies revealed that the GaN films deposited on porous silicon were nanocrystalline, exhibiting a hexagonal wurtzite structure along the (100) plane. Spectroscopic property results revealed that the photoluminescence PL emission peaks of the gallium nitride over porous silicon (GaN/PSi) sample prepared at 795 mJ/mm2 were centered at 260 nm and 624 nm. According to topographical and morphological analyses, the deposited film consisted of spherical grains with an average diameter of 178.8 nm and a surface roughness of 50.61 nm. The surface of the prepared films exhibited a cauliflower-like morphology. The main figures of merit of the nanostructured GaN/P-Si photodetectors were studied in the spectral range of 350–850 nm. The responsivity, detectivity, and external quantum efficiency of the photodetector at 575 nm under − 3 V were 19.86 A/W, 8.9 × 1012 Jones, and 50.89%, respectively. Furthermore, the photodetector prepared at a laser fluence of 795 mJ/mm2 demonstrates a switching characteristic, where the rise time and fall time are measured to be 363 and 711 μs, respectively.

Leakage current reduction in n-GaN/p-Si (100) heterojunction solar cells

We report on the growth of n-GaN/p-Si heterojunction solar cells via thermal chemical vapor deposition on Si (100) substrates at different growth temperatures (900, 950, and 1000 °C). The influence of growth temperature on the morphological, structural, optical, and electrical properties of GaN films has been elucidated. Increasing the growth temperature was found to reduce the internal stress and improve the material's crystallinity as confirmed via x-ray diffraction and Raman spectroscopy analyses. The photoluminescence spectra exhibit strong near band edge peaks in the range between 375 and 366 nm, with the peak intensity increasing with increasing the growth temperature. The current–voltage (J–V) characteristics of the assembled heterojunction solar cells showed the reverse leakage current to decrease with increasing the growth temperature. Consequently, the solar cell fabricated using the films grown at 1000 °C exhibits higher conversion efficiency (8.17%) than those grown at 950 °C (5.15%) and 900 °C (2.48%), respectively. This work shows that the structural, optical, and photovoltaic properties of the grown n-GaN/p-Si heterojunction solar cell structures are strongly influenced by the growth temperature.

Electrical Characteristics and Deep Traps Spectra of Undoped GaN Films Grown on Si Using Different Strain-Relieving Buffer Types

Electrical properties of GaN films grown on Si by molecular beam epitaxy using various types of strain-relieving layers have been studied by means of Hall/van der Pauw measurements, capacitance-voltage profiling, admittance spectroscopy, and deep levels transient spectroscopy with electrical and optical injection. The electrical properties of all grown films were determined by relatively deep electron traps N1, N2, and N3 with aggregate concentration of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . Freezing out of these traps led to the films freezing out down to the depth corresponding to the nearest underlying heterointerface where a strong band bending caused a sharp nonuniformity of charge carriers concentration. For AlN or Al-rich AlGaN underlying films, this band bending could cause formation of hole sheet charge leading to apparent conductivity to appear p-type in Hall. Other deep traps detected in the grown films were N4 and N5 acceptors with levels near E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> - 0.6 eV, and hole traps H1 and H2 with levels near E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> + 0.9 eV. Possible consequences of the observed phenomena for designing the thick GaN stand-off films in power transistors are briefly discussed.

Structural and Electrical Properties of High C-Orientation GaN Films on Diamond Substrates

Preferred orientation GaN films are deposited on freestanding thick diamond films by electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition (ECR-PEMOCVD). The TMGa and N2 are applied as precursors and different N2 flux is used to achieve high quality GaN films. The influence of N2 flux on the properties of GaN films is systematically investigated by x-ray diffraction analysis (XRD), atomic force microscopy (AFM), electron probe microanalysis (EPMA) and Hall Effect Measurement (HL). The results show that the high quality GaN films deposited at the proper N2 flux display a fine structural and electrical property and the Ga/N atomic ratio plays an important role in the electrical property of GaN films.

Effects of thermal annealing treatment on the optical and the electrical properties of GaN films grown on Si substrates

The effects of rapid thermal annealing on the optical and the electrical properties in GaN epilayers grown on Si substrates were investigated by using cathodoluminescence (CL) and Hall effect measurements. From the two-dimensional CL image measurements, the distribution of defects embedded in the films was investigated before and after rapid thermal annealing. The penetration depth dependence of the CL spectrum demonstrated that the defect density become large in the deeper region away from the GaN surface.

Micro-Raman measurement of high-energy deuterium irradiated GaN

Micro-Raman spectroscopy was used to examine the changes in the structural and electrical properties of GaN films exposed to high-energy deuterium irradiation. The locations of the larger structurally damaged regions, at irradiation energy of 5MeV and 10MeV, were determined by monitoring the variations of the quasi-longitudinal optical phonon frequency. The empirical penetration depths after 5MeV and 10MeV deuterium irradiation were approximately 120 and [email protected], respectively, where the carrier concentrations was lower than that of the pre-irradiated samples. Defect clusters were generated in GaN crystal through collisions with lattice atoms, and acted as carrier traps. The carrier concentrations of the GaN sample after deuterium irradiation, which were derived from the quasi-longitudinal optical (QLO) Raman shifts, ranged from 5.5x10^1^5~9.0x10^1^5cm^-^3. The reduced free carrier concentration of the sample irradiated at 5MeV was recovered almost to its pre-irradiation values by annealing at 900^oC, whereas only partial recovery was observed in the sample irradiated at 10MeV. This study provides insights into the degradation mechanism caused by high-energy particle irradiation and the long-term reliability of GaN-based electronic devices.

Defect evolution and accompanied change of electrical properties during the GaN growth by metalorganic chemical vapor deposition

The defect evolution and its correlation with electrical properties of GaN films grown by metalorganic chemical vapor deposition are investigated. It is found that the dislocation density decreases gradually during the growth process, and the dislocation reduction rate in the island coalescence process is especially rapid. The changes in electron mobility of GaN with the increase of growth time are mainly dependent on the dislocations acting as scattering centers. Furthermore, the variation of carrier concentration in GaN may be related with the point defects and their clusters. The quality of GaN could be improved by suitably increasing the film thickness.

Effect of thickness on structural and electrical properties of GaN films grown on SiN-treated sapphire

GaN films were grown by metalorganic chemical vapor deposition (MOCVD) on silicon nitride (SiN)-treated c-plane sapphire substrate. Using in situ laser reflectometry, the growth was interrupted at the different stages of the film coalescence. By the help of high-resolution X-ray diffraction (HRXRD) and Hall effect measurements, the mosaicity, stress and electrical properties were investigated at all the growth stages. The results showed that tilt and twist mosaic drop with thickness increasing at different rates. During the coalescence process, interaction between tilt and twist mosaic increases and remain almost constant for fully coalesced layers. Hall effect measurements have shown that carrier concentration and mobility have the same sensitivity to the tilt and the twist mosaic. The a and c lattice parameters, as well as the in plane and out-of plane strains were determined. The room temperature compressive stress increases with increasing layer thickness and reaches a maximum level of −0.45 GPa. The effects of SiN treatment on the mosaicity and the stress evolution were discussed from a comparison with the properties of untreated GaN grown layer.

Epitaxial GaN Film Grown at Low Temperature by Hydrogen-Plasma Assisted MOCVD

A comparative study of the structural and electrical properties of GaN films grown by plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD) at 700°C, with and without AlN buffer layer is presented . Hydrogen plasma was used in addition to nitrogen plasma to produce GaN. The introduction of H-plasma is found to influence the properties of the films. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and Hall Effect measurements show that the sample with AlN buffer layer possesses a smoother and more homogenous morphological characteristics as well as a lower background electron and higher Hall mobility as compared to the sample without buffer layer. X-ray diffraction (XRD) reveals that hydrogenation is capable of producing the epitaxial GaN films at reduced temperatures with the full width at half maximum (FWHM) of the x-ray rocking (XRC) of GaN (0002) reflection was found to be 54.8 and 256 arcmin for samples with buffer layer and without buffer layer, respectively.

Characterization of polycrystalline GaN grown on silica glass substrates

The structural, optical, and electrical properties of GaN films grown on silica glass substrate by metalorganic chemical vapor deposition were studied. X-ray diffraction showed that the films were grown in hexagonal structure with a predominant (0 0 0 2) peak. A broad and strong band-edge emission and very weak yellow luminescence in photoluminescence (PL) spectra were observed. And the temperature dependence of the PL spectra was extensively studied. The thermal quenching activation energy was found to be very close to the donor activation energy determined from the temperature dependence of the carrier concentration. Longitudinal optical phonons were found to be responsible for the PL broadening above 100 K .

Micro-Raman scattering and microphotoluminescence of GaN thin films grown on sapphire by metal-organic chemical vapor deposition

Microscale optical spectroscopic probe techniques, mainly micro-Raman scattering (μ-RS) and microphotoluminescence (μ-PL) spectroscopy, are employed to study a variety of GaN thin film materials, undoped and Si and Mg doped, grown on c-sapphire substrates by low-pressure metal-organic chemical vapor deposition. The optical properties and correlated structural features of these epitaxial materials grown under different conditions are studied. Visible micro-Raman measurements show different mode behaviors under different laser incidence configurations. Raman line shape analysis through two sets of equations leads to the monitoring of the sample quality and stress, and the optical determination of electrical properties of GaN films. An approximate 3 cm - 1 shift in the E 1 (LO) mode frequency is observed across the GaN film from the sapphire/GaN interface to the GaN surface, indicating the spatial variation of strains along the film growth direction within a few micrometers thickness range. Combined Raman and PL spectra are obtained under UV excitation, and interesting high-order resonance Raman features are observed using a sensitive UV-Raman-PL microscope. A new way to characterize the p-type GaN is introduced. These fruitful results demonstrate the capabilities of μ-RS-PL technology for the research and development of GaN and other materials.

Raman scattering properties of GaN thin films grown on sapphire under visible and ultraviolet excitation

AbstractRaman scattering properties from various GaN thin films grown on sapphire by metalorganic chemical vapor deposition were studied. Visible micro‐Raman measurements show different mode behaviors with different laser incidence configurations. Raman lineshape analyses through two sets of equations led to the monitoring of the sample quality and stress and the optical determination of electrical properties of GaN films. An approximate 3 cm−1shift in theE1(LO) mode wavenumber is observed across the GaN film from the sapphire/GaN interface to the GaN surface, indicating the spatial variation of strains along the film growth direction within a few microns thickness range. Combined Raman and PL spectra were obtained under UV excitation, and interesting high‐order resonance Raman features were observed using a sensitive UV–Raman–photoluminescence microscope. A new way to characterize p‐type GaN is introduced. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Structural, optical and electrical properties of GaN films grown by metalorganic chemical vapor deposition on sapphire.

ABSTRACTProperties of GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on c-plane of sapphire have been investigated using atomic force microscopy (AFM), wet etching for defect investigation, transmission electron microscopy (TEM), high-resolution X-ray diffraction, Hall effect measurements and low-temperature photoluminescence (PL). Tapping-mode AFM images of the as-grown samples showed atomically smooth surfaces (rms roughness ≍ 0.2 nm) consisting of terraces separated by about 3Å bi-layer steps. Hot H3PO4 chemical etching was used to produce hexagonal-shaped etch pits at the surface defect sites as revealed by AFM imaging. The obtained etch pit densities (9×108 - 2 ×109 cm−2) were in agreement with the dislocation density found by plan-view and cross-sectional TEM observations. The full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve was about 4.8 and 3.9 arcmin for the symmetric (002) and asymmetric (104) directions, respectively. PL spectrum at 15 K demonstrated sharp peaks (FWHM ≍ 4 meV) in the excitonic region, which were attributed to free and bound excitons. The spectrum contained also weak PL bands with maxima at about 2.2, 2.9 and 3.27 eV, which have been attributed to three different acceptors.

Electrical characterization of isoelectronic In-doping effects in GaN films grown by metalorganic vapor phase epitaxy

Indium isoelectronic doping was found to have profound effects on electrical properties of GaN films grown by metalorganic chemical vapor deposition. When a small amount of In atoms was introduced into the epilayer, the ideality factor of n-GaN Schottky diode was improved from 1.20 to 1.06, and its calculated saturation current could be reduced by 2 orders of magnitude as compared to that of the undoped sample. Moreover, it is interesting to note that In isodoping can effectively suppress the formation of deep levels at 0.149 and 0.601 eV below the conduction band, with the 0.149 eV trap concentration even reduced to an undetected level. Our result indicates that the isoelectronic In-doping technique is a viable way to improve the GaN film quality.

Investigations of optical and electrical properties of In-doped GaN films grown by gas-source molecular beam epitaxy

Optical and electrical properties of GaN films grown on α-Al 2O 3(0 0 0 1) substrates by GSMBE using In-doping method were investigated. It was found that both of them were improved, compared to those of a nondoped GaN one. μ-PL results at 20 K indicated that In-doped films emit luminescence more uniformly than that of a nondoped one. Furthermore, Hall effect measurements at 300 K showed higher electron mobility of In-doped samples than that of a nondoped one. It is suggested that the presence of In during the growth of GaN films plays a role in reducing the number of structural imperfections to improve the optical and electrical properties.

Properties of Nanocrystalline Gallium Nitride Thin Films Grown on Monocrystalline Substrates and Buffer Layers

Properties of GaN thin films deposited by rf-magnetron sputtering on monocrystalline sapphire, Si, and MgO substrates as well as on ZnO, MgAl2O4, and AlN buffer layers were investigated. The X-ray structural and crystallochemical analysis of phase composition, structure parameters, texture parameters and crystallite size of GaN films was carried out. Luminescent and electrical properties of GaN films on various substrates and buffer layers are described.

Nucleation layer microstructure, grain size, and electrical properties in GaN grown on a-plane sapphire

We have investigated the microstructural origins of center-to-edge differences in the electrical properties of GaN films grown on a-plane sapphire via metal–organic vapor-phase epitaxy. Using cross-sectional transmission electron microscopy, we have observed that the grain size at the wafer edge is approximately 1 μm, whereas the grain size in the wafer center ranges from 0.1 to 0.5 μm. The smaller grain size at the wafer center is traced to a higher density of extended defects in the AlN nucleation layer: defects which, in turn, act as nucleation sites for GaN grain growth.

Scanning capacitance microscopy imaging of threading dislocations in GaN films grown on (0001) sapphire by metalorganic chemical vapor deposition

A combination of atomic force microscopy and scanning capacitance microscopy was used to investigate the relationship between the surface morphology and the near-surface electrical properties of GaN films grown on c-axis sapphire substrates by metalorganic chemical vapor deposition. Local regions surrounding the surface termination of threading dislocations displayed a reduced change in capacitance with applied voltage relative to regions that contained no dislocations. Capacitance–voltage characteristics obtained from these regions indicated the presence of negative charge in the vicinity of dislocations.

Se-doped GaN Films Grown by Low-Pressure Metalorganic Chemical Vapor Deposition

Se-doped GaN films are grown for the first time by low-pressure metalorganic chemical vapor deposition (LP-MOCVD), in which H2Se is used as the Se source gas. Effects of Se doping on electrical properties of GaN films are reported. Se atoms tend to out-diffuse to the surface of the GaN film at high temperature. The N atomic percentage is influenced by the incorporation of H2Se in the MOCVD process. The carrier concentration was found to be significantly affected by the surface defects which develop at high H2Se dosages. The highest free electron concentration obtained is about 1.5×1018 cm-3. Increasing the growth temperature from 1000° C to 1050° C reduces the maximum carrier concentration to about 7×1017 cm-3.