Improved quality of GaN epilayer grown on porous SiC substrate by in situ H2 pre-treatment

GaN epilayers were grown on the Si-terminated (0001) 6H-SiC substrates pre-treated by in situ H₂ in metal organic chemical vapor deposition system. It was found that in situ H₂ treatment brought a porous SiC surface. The influence of H₂ pre-treatment conditions on SiC surface was carefully investigated. Moreover, our experiment demonstrated that the H₂ pre-treatment can distinctly influence the GaN basic characteristics.     

Improvement of the quality of GaN epilayer by combining a SiNx interlayer and changed GaN growth mode

GaN epilayers with porous SiN_x interlayer and changed growth modes were grown by metal–organic chemical vapor deposition on c-plane sapphire substrates. Comparing with GaN epilayer grown by ordinary method, the crystalline qualities were significantly improved. The improvement was attributed to the reduction of the density of threading dislocations causing by over-growth process combining with delayed coalescence of individual GaN islands. The influence of the deposition and annealing of nucleation layer on the GaN regrowth was also discussed.     

Improvements of epitaxial quality and stress state of GaN grown on SiC by in situ SiN<Subscript>x</Subscript> interlayer

In this study, 4.5 μm thick GaN films with graded Al_xGa_1?xN/AlN buffer and SiN_x interlayer were prepared on 6H–SiC substrates by metal–organic chemical vapor deposition. To determine the effects of SiN_x interlayer on epitaxial quality and stress state of GaN films, a series of comparative experiments were carried out by changing the deposition time and the insert location of SiN_x interlayer. By optimizing growth conditions of SiN_x interlayer, the full width at half maximum values of \( (0002) \) and \( (10\bar{1}2) \) rocking curves of GaN films were improved to 142 and 170 arcsec, respectively. A crack-free GaN film with a small root-mean-squared roughness of 0.21 ± 0.02 nm was achieved. Simultaneously, the reduction in threading dislocation density of GaN films was confirmed by using wet etching method. In addition, stress values in GaN films were investigated by Raman and low-temperature photoluminescence spectra, which indicated that the lower tensile stress in GaN film, the higher the film’s crystallinity.     

Improving the quality of GaN epilayer by preparing a novel patterned sapphire substrate

GaN epilayer was grown on a new polyhedral patterned sapphire substrate (new PSS) by metal–organic chemical vapor deposition. The new PSS was prepared by combining the dry etching technique and wet etching technique. The X-ray diffraction indicated that the full width at half maximum values of (0002) and ( $10\overline{1}2$ ) diffraction peaks in the GaN epilayer grown on the new PSS were evidently smaller than that in the GaN epilayer grown on the normal treated PSS. The improvement of GaN quality was attributed to the reduction of threading dislocations (TDs) in GaN epilayer, and the mechanism of the reduction of TDs was analyzed. The influence of the new PSS on the optical properties as well as the residual stress in GaN epilayer was also discussed.     

MOCVD growth of GaN layer on InN interlayer and relaxation of residual strain

100 nm InN layer was grown on sapphire c-plane using a metal-organic chemical vapor deposition (MOCVD) system. Low temperature (LT) GaN layer was grown on InN layer to protect InN layer from direct exposure to hydrogen flow during high temperature (HT) GaN growth and/or abrupt decomposition. Subsequently, thick HT GaN layer (2.5 μm thick) was grown at 1000 °C on LT GaN/InN/sapphire template. Microstructure of epilayer-substrate interface was investigated by transmission electron microscopy (TEM). From the high angle annular dark field TEM image, the growth of columnar structured LT GaN and HT GaN with good crystallinity was observed. Though thickness of InN interlayer is assumed to be about 100 nm based on growth rate, it was not clearly shown in TEM image due to the InN decomposition. The lattice parameters of GaN layers were measured by XRD measurement, which shows that InN interlayer reduces the compressive strain in GaN layer. The relaxation of compressive strain in GaN layer was also confirmed by photoluminescence (PL) measurement. As shown in the PL spectra, red shift of GaN band edge peak was observed, which indicates the reduction of compressive strain in GaN epilayer.     

Strain analysis of a GaN epilayer grown on a c-plane sapphire substrate with different growth times

The strain analysis of a GaN epilayer with different growth times on a c-plane sapphire substrate via a two-step growth method, using low-pressure, metalorganic chemical vapor deposition, was conducted based on the precise measurement of the lattice parameters, using high-resolution X-ray diffraction. The high-temperature growth time was changed at a fixed growth condition. The c- and a-lattice parameters were measured, followed by the out-of-plane and in-plane strains. Then, the biaxial and hydrostatic components were extracted from the total strain values obtained, and were discussed in this paper as functions of GaN growth time.     

Wing tilt investigations on GaN epilayer grown on maskless grooved sapphire by MOCVD

GaN epilayer is grown on maskless periodically grooved sapphire by metal organic chemical vapor deposition (MOCVD) in this article. Wing tilt is detected by high resolution X-ray rocking curve. Inhomogeneous deformations between the wing and mesa regions are found by atomic force microscopy (AFM) characterization. The stress distribution is investigated using finite-element simulations. Inhomogeneous stress distribution in the mesa and wing regions is shown, which is also confirmed by micro-Raman spectroscopy. The results show that the wing tilt in GaN layers grown on maskless periodically grooved sapphire mainly originates from the different deformations for mesa and wing region caused by inhomogeneous stress distribution.     

Influence of silane flow on MOVPE grown GaN on sapphire substrate by an in situ SiN treatment

In this paper, the influence of silane flow on metalorganic vapour phase epitaxy (MOVPE) grown GaN on sapphire substrate by an in situ SiN treatment has been investigated. A flow of 10 sccm with treatment duration of 120 s appears to be the optimal value and improves the crystal quality. The dislocation density, determined by atomic force microscopic (AFM), is as low as 5×10⁸ cm⁻². A reduction of I₂ full width at half maximum (FWHM) to 4 meV and an increase of both BE/YL and BE/DAP intensity ratio are also obtained.     

Improvement of GaN epilayer by gradient layer method with molecular-beam epitaxy

We demonstrated a molecular beam epitaxy method to resolve the dilemma between structural and morphological quality in growth of the GaN epilayer. A gradient buffer layer was grown in such a way that the N/Ga ratio was gradually changed from nitrogen-rich to gallium-rich. The GaN epitaxial layer was then grown on the gradient buffer layer. In the X-ray diffraction analysis of GaN(002) rocking curves, we found that the full width at half-maximum was improved from 531.69″ to 59.43″ for the sample with a gradient buffer layer as compared to a purely gallium-rich grown sample. Atomic force microscopy analysis showed that the root-mean-square roughness of the surface was improved from 18.28 nm to 1.62 nm over an area of 5 × 5 μm² with respect to a purely nitrogen-rich grown sample. Raman scattering showed the presence of a slightly tilted plane in the gradient layer. Furthermore we showed that the gradient layer can also slash the strain force caused by either Ga-rich GaN epitaxial layer or AlN buffer layer.     

The structure of GaN layers grown on SiC and sapphire by molecular beam epitaxy

A comparative study of the defects at the interfaces and inside the layers was carried out in GaN/AlN epitaxial layers on SiC and sapphire. Whereas surface cleaning of the sapphire substrates is rather standardised now, the SiC substrates cleaning is still to optimise conditions, as the high densities of defects inside the epitaxial layers cannot be explained solely by the 3.54% lattice mismatch. The investigated specimens were grown by molecular beam epitaxy (MBE), either assisted by electron cyclotron resonance or an NH₃ gas source system to provide atomic nitrogen. Assuming that MBE is a growth technique more or less close to equilibrium, the observed defects are interpreted and a growth mechanism, for GaN layers on the stepped (0001) SiC and sapphire surfaces, is proposed.     

Structure and stimulated emission of a high-quality zinc oxide epilayer grown by atomic layer deposition on the sapphire substrate

A high-quality ZnO epilayer was grown on the (0001) sapphire substrate by atomic layer deposition (ALD) and followed by high-temperature rapid thermal annealing (RTA). The layer-by-layer growth and low deposition temperature of ALD, as well as the RTA treatment, prevent the formation of columnar structures in the ZnO epilayer. A distorted ZnO layer at the ZnO/sapphire interface, which relaxes the misfit in ZnO leads to a low threading dislocation density in the ZnO epilayer. Optically-pumped stimulated emission was achieved with a low-threshold intensity of 153 kW/cm² at room temperature. The good crystalline quality and low-threshold stimulated emission indicate that the ALD approach is appropriate for preparing high-quality ZnO epilayers.     

SiC缓冲层用于改善硅基氮化镓薄膜的质量研究

用脉冲激光沉积法在硅衬底上沉积GaN薄膜,为了减小Si衬底与GaN薄膜之间的热失配和晶格失配引入SiC缓冲层.脉冲激光沉积后的GaN薄膜是非晶结构,将样品在氨气氛围中在950℃下退火15min.得到结晶的GaN薄膜.并用X 射线衍射、原子力显微镜、傅立叶红外吸收谱、光致发光谱研究了SiC缓冲层对GaN薄膜的结晶、形貌和光学性质的影响.     

Studies on growth mechanism of HgCdTe epilayer on Si grown by HWE

In this paper, the mechanism of Hg_1−xCd_xTe/Si heterojunction grown by HWE (Hot Well Epitaxy) was studied. Opitical characterizations were shown with FTIR, the composition x = 0.39 was deduced by using MIR transmittance, the absorbing peak at 319.4 cm⁻¹ was measured by FIR transmittance, 319.4 cm⁻¹ confirmed the existence of Si–Te bond of at Si/HgCdTe interfacial layer. The I-V characteristics at both room temperature and 77 K of HgCdTe (n-type)/Si (p-type) heterojunction show that the good p-n heterojunction properties was obtained by using HWE. XRD study confirmed the formation of (111) oriented HgCdTe on (211) Si. Morphology of a cross section observed using EPMA indicates the columnar growth of HgCdTe. An analysis of interfcial layer by EPMA indicated presence of three layers composed of Si + Te, Si + Te + Hg and Si + Te + Cd + Hg. Among them, the most important one is the first layer. The problem of lattice mismatch and the difference of thermal expansion coefficient between Si and CdTe or HgTe may be improved by formation of Si–Te stable chemical bond through bybridization orbital bonding between Si and Te. The second and third layers are formed by evaporation-interdiffusion. Formation of the whole interfacial layer provides the appetency for the growth of (111) Hg_1−xCd_xTe epilayer on (211) Si substrate.     

Field emission performance of SiC nanowires directly grown on graphite substrate

Lawn-like SiC nanowire arrays were successfully synthesized on graphite substrates by thermal evaporation of silicon powders at high temperature. The morphology, microstructure and composition of the nanowires were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The product grown on graphite substrates was hexagonal prism-shaped single-crystal 3C-SiC nanowires with high aspect ratio. Planar defects, such as microtwins and stacking faults were observed in SiC nanowires. Field emission measurements of the SiC nanowires grown on graphite substrate showed a very low threshold field of 2.1 V μm⁻¹, high brightness and stable field emission performance.     

Study of the thermal step signal of GaN grown on porous silicon substrate by MOVPE

In this work, we report the electric investigation of thin gallium nitride films by the thermal step method (TSM). The space charge dynamics was studied using the thermal step method with applied negative step (ΔT = −30 °C). The experimental TSM current indicates the presence of two peaks indicating the presence of two depletion widths in the silicon substrate and the GaN epilayer. The comparison between the theoretical and the measured TSM current indicates a good agreement for times less than 2 s. The divergence for times above 2 s is probably due to a thermoelectric current similar to Seebeck effect, due to the majority charge carriers’ contribution.     

Structural and photoluminescence study of thin GaN film grown on silicon substrate by metalorganic chemical vapor deposition

GaN epitaxial layer was grown on Si(111) substrate by metalorganic chemical vapor deposition (MOCVD). The structure consists of 50 nm thick high-temperature grown AlN buffer layer, 150 nm thick AlGaN layer, 30 nm low-temperature grown AlN layer, 300 nm GaN layer, 50 nm AlGaN superlattice layer, followed by 100 nm GaN epitaxial layer. The low-temperature AlN interlayer and AlGaN superlattice layer were inserted as the defect-blocking layers in the MOCVD grown sample to eliminate the dislocations and improve the structural and optical properties of the GaN layer. The dislocation density at the top surface was decreased to ∼ 2.8 × 10⁹/cm². The optical quality was considerably improved. The photoluminescence emission at 3.42-3.45 eV is attributed to the recombination of free hole-to-donor electron. The observed 3.30 eV emission peak is assigned to be donor-acceptor transition with two longitudinal optical phonon side bands. The relationship of the peak energy and the temperature is discussed.     

新型硅基GaN外延材料的热应力模拟

采用有限元方法，通过ANSYS软件模拟了体硅衬底上和SOI衬底上生长的GaN外延膜从1100℃的生长温度降到20℃的热应力变化情况。模拟结果表明SOI衬底作为一种柔性衬底，能有效减少异质外延的晶格失配，但是单从热失配的角度，由于引入了热膨胀系数（CET）更小的埋层SiO2，SOI衬底会使得外延层热应力略有增大。为了降低外延层中的热应力，我们结合微机电系统（MEMS）的制造工艺，用深反应离子刻蚀（DRIE）的方法，借助于SOI材料自停止刻蚀的优势，将衬底硅和埋氧去除，使得SOI的超薄顶层硅部分悬空，形成一种新型的SOI衬底。模拟结果表明，这种新型SOI衬底可以将GaN外延层中的热应力降低20％左右。     

Stress reduction in GaN films on (111) silicon-on-insulator substrate grown by metal-organic chemical vapor deposition

The GaN film was grown on the (111) silicon-on-insulator (SOI) substrate by metal-organic chemical vapor deposition and then annealed in the deposition chamber. A multiple beam optical stress sensor was used for the in-situ stress measurement, and X-ray diffraction (XRD) and Raman spectroscopy were used for the characterization of GaN film. Comparing the characterization results of the GaN films on the bulk silicon and SOI substrates, we can see that the Raman spectra show the 3.0 cm^− 1 frequency shift of E₂(TO), and the full width at half maximum of XRD rocking curves for GaN (0002) decrease from 954 arc sec to 472 arc sec. The results show that the SOI substrates can reduce the tensile stress in the GaN film and improve the crystalline quality. The annealing process is helpful for the stress reduction of the GaN film. The SOI substrate with the thin top silicon film is more effective than the thick top silicon film SOI substrate for the stress reduction.     

Mechanism of misfit stress relaxation during epitaxial growth of GaN on porous SiC substrates

A decrease in the density of threading dislocations has been observed during the epitaxial growth of GaN layers on porous silicon carbide (PSC) substrates by means of chloride hydride vapor phase epitaxy. It is established that, in the early growth stage, the substrate is capable of redistributing stresses in the growing heterostructure, which leads to relaxation of the lattice misfit stresses via generation of a superlattice of planar defects. In the subsequent growth stage, these defects prevent the propagation of threading dislocations. Owing to this phenomenon, 1-μm-thick GaN layers on PSC can be obtained with a density of dislocations reduced by two orders of magnitude as compared to epilayers of the same thickness grown on nonporous substrates.