Properties Of GaN Thin Films Research Articles

PECVD-prepared high-quality GaN films and their photoresponse properties

In this study, the high-quality GaN films are prepared by a simple, green and low-cost plasma enhanced chemical vapor deposition (PECVD) method at 950 ℃, with Ga<sub>2</sub>O<sub>3</sub> and N<sub>2</sub> serving as a gallium source and a nitrogen source, respectively. In order to improve the crystal quality of GaN films and ascertain the photoresponse mechanism of GaN films, the effect of the preparation temperature of GaN buffer layer on the crystal quality and photoelectric properties of GaN thin films are investigated. It is indicated that with the increase of the buffer temperature of GaN films, the crystal quality of GaN films first increases and then decreases, and the highest crystal quality is obtained at 875 ℃. When buffer layer temperature is 875 ℃, the calculated total dislocation density is 9.74 × 10<sup>9</sup> cm<sup>–2</sup>, and the carrier mobility is 0.713 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>. The crystal quality of GaN film after being annealed is improved. The total dislocation density of GaN film decreases to 7.38 × 10<sup>9</sup> cm<sup>–2</sup>, and the carrier mobility increases to 43.5 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>. The UV-Vis absorption spectrum results indicate that the optical band gap of GaN film is 3.35 eV. The scanning electron microscope (SEM) results indicate that GaN film (buffer layer temperature is 875 ℃) has smooth surface and compact structure. The Hall and X-ray photoelectron spectroscopy (XPS) results indicate that there are N vacancies, Ga vacancies or O doping in the GaN film, which act as deep level to capture photogenerated electrons and holes. With the bias increasing, the photoresponsivity of the GaN film photodetector gradually increases and then reaches a saturation value. This is due to the deep levels produced by vacancy or O doping. In addition, photocurrent response and recovery of GaN film are slow, which is also due to the deep levels formed by vacancy or O doping. At 5-V bias, the photoresponsivity of GaN film is 0.2 A/W, rise time is 15.4 s, and fall time is 24 s. Therefore, the high-quality GaN film prepared by the proposed green and low-cost PECVD method present a strong potential application in ultraviolet photodetector. The PECVD method developed by us provides a feasible way of preparing high-quality GaN films, and the understanding of the photoresponse mechanism of GaN films provides a theoretical basis for the wide application of GaN films.

Comparative study on the nanomechanical behavior and physical properties influenced by the epitaxial growth mechanisms of GaN thin films

The effect of carrier gas on nanomechanical properties of GaN thin films grown on (0001) sapphire substrates by metal–organic chemical vapor deposition (MOCVD) process, was studied by Berkovich nanoindentation. GaN/H2 and GaN/N2 films were deposited using hydrogen and nitrogen, as carrier gases, respectively. New method was developed to compute the contact stiffness versus the penetration depth using a single load–displacement curve. It was found that the hardness and Young’s modulus of GaN/H2 are lower than those of GaN/N2. Moreover, pop-in events were revealed for GaN/H2, in contrast for GaN/N2. Fracture was only manifested in the imprint of GaN/N2 due to its low fracture toughness. Besides, it was disclosed the high sensitivity of loading rate only for GaN/H2. The GaN/N2 sample experiences plastic strain relaxation, while GaN/H2 is a highly stressed sample; hence, it has less dislocation density compared to GaN/N2 sample. We demonstrated that the used carrier gas influences on GaN epitaxial growth process, which in return deeply affects the resulting dislocation density and dislocation plasticity mechanisms. These latter, tailor the nanomechanical properties of GaN samples and the indentation-induced deformation behavior.

Temperature Effect of van der Waals Epitaxial GaN Films on Pulse-Laser-Deposited 2D MoS2 Layer

Van der Waals epitaxial GaN thin films on c-sapphire substrates with a sp2-bonded two-dimensional (2D) MoS2 buffer layer, prepared by pulse laser deposition, were investigated. Low temperature plasma-assisted molecular beam epitaxy (MBE) was successfully employed for the deposition of uniform and ~5 nm GaN thin films on layered 2D MoS2 at different substrate temperatures of 500, 600 and 700 °C, respectively. The surface morphology, surface chemical composition, crystal microstructure, and optical properties of the GaN thin films were identified experimentally by using both in situ and ex situ characterizations. During the MBE growth with a higher substrate temperature, the increased surface migration of atoms contributed to a better formation of the GaN/MoS2 heteroepitaxial structure. Therefore, the crystallinity and optical properties of GaN thin films can obviously be enhanced via the high temperature growth. Likewise, the surface morphology of GaN films can achieve a smoother and more stable chemical composition. Finally, due to the van der Waals bonding, the exfoliation of the heterostructure GaN/MoS2 can also be conducted and investigated by transmission electron microscopy. The largest granular structure with good crystallinity of the GaN thin films can be observed in the case of the high-temperature growth at 700 °C.

Role of RF power on physical properties of RF magnetron sputtered GaN/p-Si(1 0 0) thin film

Abstract GaN thin films were deposited on p-Si(1 0 0) substrates using RF magnetron sputtering at various RF powers. Influence of RF power on morphological, optical and structural properties of GaN thin films were investigated and presented in detail. XRD results proved that the films were polycrystalline in structure with (1 0 0) and (1 1 0) planes of hexagonal GaN. It was found that increasing RF power led to deterioration of crystal structure of the films due to increased decomposition of GaN. Stress in GaN thin films was calculated from XRD measurements and the reasons for this stress were discussed. Furthermore, it was analyzed and interpreted whether the experimental measurement results support each other. E2 (high) optical phonon mode of hexagonal GaN was obtained from the analysis of Raman results. UV-Vis spectroscopy results showed that optical band gap of the films varied by changing RF power. The reasons of this variation were discussed. AFM study of the surfaces of the GaN thin films showed that some of them were grown in Stranski-Krastanov mode and others were grown in Frank-Van der Merwe mode. AFM measurements revealed almost homogeneous, nanostructured, low-roughness surface of the GaN thin films. SEM analysis evidenced agglomerations in some regions of surface of the films and their possible causes have been discussed. It has been inferred that morphological, optical, structural properties of GaN thin film can be changed by controlling RF power, making them a potential candidate for LED, solar cell, diode applications.

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

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.

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

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 7.1×108cm−2 for GaN on bare sapphire to 4.9×108cm−2 for GaN on a high-AR NPSS. In addition, the carrier mobility increases from 85cm2/Vs for GaN on bare sapphire to 199cm2/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.

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

Gallium nitride films were grown by hollow cathode plasma-assisted atomic layer deposition using triethylgallium and N2/H2 plasma. An optimized recipe for GaN film was developed, and the effect of substrate temperature was studied in both self-limiting growth window and thermal decomposition-limited growth region. With increased substrate temperature, film crystallinity improved, and the optical band edge decreased from 3.60 to 3.52 eV. The refractive index and reflectivity in Reststrahlen band increased with the substrate temperature. Compressive strain is observed for both samples, and the surface roughness is observed to increase with the substrate temperature. Despite these temperature dependent material properties, the chemical composition, E1(TO), phonon position, and crystalline phases present in the GaN film were relatively independent from growth temperature.

Effects of native defects on properties of low temperature grown, non-stoichiomtric gallium nitride

The properties of GaN thin films grown by molecular beam epitaxy at temperatures from 80 to 500 °C under a wide range of Ga:N flux ratios are studied. We found that at growth temperatures as low as ~80 °C, GaN films still have a polycrystalline, columnar morphology with c-axis preferred orientation. Soft x-ray absorption and emission and optical absorption measurements on Ga-rich samples suggest the presence of a partially occupied GaN antisite defect band located at ~1.2 eV below the conduction band minimum. P-type conductivity observed in this LTMBE Ga-rich GaN is consistent with transport within this partially occupied defect band.

Room-temperature thermoelectric properties of GaN thin films grown by metal organic chemical vapor deposition

The GaN thin films with different doping concentrations are grown by metal organic chemical vapor deposition. Carrier concentrations, mobilities and Seebeck coefficients of the GaN thin films are measured by Hall and Seebeck system at room temperature. The power factor and the thermoelectric figure of merit are calculated by experimental and theoretical data. The mobility and Seebeck coefficient of GaN thin film decrease with the increase of carrier concentration. The conductivity of GaN thin film increases with the increase of carrier concentration. The Seebeck coefficient of GaN thin film varies from 100 to 500 μV/K, depending on carrier concentration. The highest power factor is 4.72×10-4 W/mK2 when the carrier concentration is 1.60×1018 cm-3. The thermal conductivity of GaN thin film decreases with the increase of carrier concentration due to the increase of phonon scattering. The largest thermoelectric figure of merit of the GaN thin film at room temperature is 0.0025 when the carrier concentration is 1.60×1018 cm-3.

Temperature dependence of the point defect properties of GaN thin films studied by terahertz time-domain spectroscopy

The dielectric functions of GaN for the temperature and frequency ranges of 10-300 K and 0.3-1 THz are obtained using terahertz time-domain spectroscopy. It is found that there are oscillations of the dielectric functions at various temperatures. Physically, the oscillation behavior is attributed to the resonance states of the point defects in the material. Furthermore, the dielectric functions are well fitted by the combination of the simple Drude model together with the classical damped oscillator model. According to the values of the fitting parameters, the concentration and electron lifetime of the point defects for various temperatures are determined, and the temperature dependences of them are in accordance with the previously reported result. Therefore, terahertz time-domain spectroscopy can be considered as a promising technique for investigating the relevant characteristics of the point defects in semiconductor materials.

Microstructures of GaN Thin Films Grown on Graphene Layers

Plan-view and cross-sectional transmission electron microscopy images show the microstructural properties of GaN thin films grown on graphene layers, including dislocation types and density, crystalline orientation and grain boundaries. The roles of ZnO nanowalls and GaN intermediate layers in the heteroepitaxial growth of GaN on graphene, revealed by cross-sectional transmission electron microscopy, are also discussed.

Structural and optical properties of GaN thin films grown on Al2O3 substrates by MOCVD at different reactor pressures

GaN thin films grown by MOCVD on (0001) Al2O3 substrates at different growth pressures were characterized by field-emission scanning electron microscopy, atomic force microscopy, micro-Raman, and photoluminescence at room temperature. It was found that there is an optimum pressure of 76Torr at which the structural and optical properties of the GaN samples are superior. On the other hand samples grown at higher pressure exhibited hexagonal surface pits and surface spirals. The results showed that the growth pressure strongly influences the morphology, and significantly affects the structural and optical properties of the GaN epilayers.

One-step synthesis of GaN thin films on Si substrate by a convenient electrochemical technique at low temperature for different durations

This paper reports the synthesis and characterization of GaN thin films deposited on Si (1 1 1) substrate, using electrochemical deposition technique below room temperature for different durations. The effects of deposition duration on the morphology, structure and optical properties of GaN thin films are reported. The SEM images showed different surface morphologies of the grown structures on Si substrate for different duration times. The shape and the dimensions of the structures were also found to be dependent on the growth time. From XRD analysis the lattice constants, the average size of h-GaN crystals and the in-plane (along a-axis) and out of plane (along c-axis) strains were calculated. The effects of deposition duration on crystal parameters from XRD and Raman modes were investigated. Photoluminescence spectrum showed that the energy gaps of h-GaN and c-GaN were about 3.39 and 3.2 eV, respectively, at room temperature and the PL-intensity of h-GaN increased with deposition duration. The growth mechanism is also proposed.

Microbridge tests on gallium nitride thin films

In this work, we develop further the microbridge testing method by deriving a closed formula of deflection versus load, which is applied at an arbitrary position on the microbridge beam. Testing a single beam at various positions allows us to characterize simultaneouslyYoung's modulus and residual stress of the beam. The developed method was then used to characterize the mechanical properties of GaN thin films on patterned-Si (1 1 1) substrates grown by metal organic chemical vapor deposition (MOCVD). The microbridge samples were fabricated by using the microelectromechanical fabrication technique and tested with a nanoindentation system. Young's modulus and residual stress of the GaN films were determined to be 287 ± 190 GPa and 851 ± 155 MPa, respectively. In addition, alternative measurements of the residual stress, Young's modulus and hardness of the GaN films, were conducted with micro-Raman spectroscopy and the nanoindentation test, yielding the corresponding values of 847 ± 46 MPa, 269.0 ± 7.0 GPa and 17.8 ± 1.1 GPa.

Material Properties of GaN Films Grown on SiC/SOI Substrate

Material properties of GaN thin films grown on 3C-SiC/semiconductor-on-insulator (SOI) substrate, by metalorganic chemical vapor deposition technology, are studied by X-ray diffraction, photoluminescence and Raman scattering, with data indicating the high quality of GaN films. Our results have shown that SiC/SOI structures obtained by carbonization have the potential to serve as useful substrates for GaN growth.

Structural and optical properties of nonpolar GaN thin films

A correlation between the structural and optical properties of GaN thin films grown in the [112¯0] direction has been established using transmission electron microscopy and cathodoluminescence spectroscopy. The GaN films were grown on an r-plane sapphire substrate, and epitaxial lateral overgrowth was achieved using SiO2 masks. A comparison between the properties of GaN directly grown on sapphire and GaN laterally grown over the SiO2 mask is presented. The densities and dimensions of the stacking faults vary significantly with a high density of short faults in the window region and a much lower density of longer faults in the wing region. The low-temperature luminescence spectra consist of peaks at 3.465 and 3.41eV, corresponding to emission from donor-bound excitons and basal-plane stacking faults, respectively. A correlation between the structural defects and the light emission characteristics is presented.

Structural characterization and optoelectronic properties of GaN thin films on Si(111) substrates using pulsed laser deposition assisted by gas discharge

GaN films have been grown on Si(111) substrates with a thin AlN buffer layer using pulsed laser deposition (PLD) assisted by gas discharge. The crystalline quality, surface morphology and optoelectronic properties of the deposited films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) spectroscopy, and room-temperature Van der Pauw–Hall measurements. The influence of the deposition temperature in the range 637–1037 K on the crystallinity of GaN films, the laser incident energy in the range 150–250 mJ/pulse on the surface morphology and the optoelectronic properties were systematically studied. The XRD analysis shows that the crystalline quality of the GaN films improves with increasing deposition temperature to 937 K, but further increase of the deposition temperature to 1037 K leads to the degradation of the crystalline quality. AFM results show that the surface roughness of the GaN films can be decreased with increasing laser incident energy to 220 mJ/pulse. Further increase of the laser incident energy to 250 mJ/pulse leads to an increase in the surface roughness. The optoelectronic properties of GaN films were also improved by increasing the laser incident energy to 220 mJ/pulse. GaN films which have a n-type carrier concentration of 1.26×1017 cm-3 and a mobility of 158.1 cm2/Vs can be deposited at a substrate temperature of 937 K, a deposition pressure of 20 Pa and a laser incident energy of 220 mJ/pulse. Their room-temperature PL spectra exhibit a strong band-edge emission at 365 nm.

Photoluminescent and structural properties of GaN thin films obtained by radical-beam gettering epitaxy on porous GaAs (0 0 1)

Cubic GaN (c-GaN) layers were grown on porous GaAs (001) substrate by radical-beam gettring epitaxy. X-ray difraction and photoluminescence measurement indicate that the both single crystalline cubic GaN and hexagonal GaN was successfully grown on the porous GaAs (001) substrate.

Excitons and Electron–Hole Plasmas in Highly Excited GaN

We have studied photoluminescence (PL) properties of GaN thin films at low temperatures for various excitation intensities. Under weak excitation intensities, the free exciton line as well as the bound exciton line appear in the PL spectrum. With an increase of the excitation intensity, the M-line and the P-line due to inelastic exciton–exciton interactions are clearly observed. Under strong excitation intensities, electron–hole plasmas (EHP) emission appears at lower energy below the M-line. The radiative recombination processes of highly excited GaN are discussed.

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 & Sons, Ltd.