Study On Preparation Of GaN Buffer Substrate And GaN Single Crystal Growth

GaN is a typical representative of the third generation of semiconductors.Its excellent physical and chemical properties are very attracting.Jt has a wide range of applications in high frequency and high power devices.However,most of the GaN-based devices currently in use are prepared by heteroepitaxial growth.The heteroepitaxial growing GaN crystals has a high dislocation density due to lattice mismatch and thermal mismatch between GaN and the substrate.The residual stress seriously affects the performance of GaN-based devices.The homoepitaxial growth of devices on GaN single crystals is the fundamental solution to this problem.Hydride Vapor Phase Epitaxy(HVPE)has the characteristics of relatively mild growth conditions,fast growth rate and the ability to grow large-sized GaN crystals,and is considered to be the most likely method for commercial growth of GaN crystals.At the same time,pre-processing the substrate to prepare a GaN buffer substrate is a key technology for growing high-quality GaN single crystals.In this paper,a variety of GaN buffer substrates were designed and fabricated by different methods.The reaction mechanism and optimal conditions for the preparation of GaN buffer substrates were determined.The growth of GaN single crystals was studied on the as-prepared buffer substrate by HYPE method.The main research works and results of the thesis are as follows:1.The corrosion behaviors of GaN under different conditions were explored.A porous buffer substrate was prepared and the nucleation mode of GaN crystal on porous substrate was determined.Different effects were compared for GaN materials reacting in different acid and alkali environments to form hexagonal etch pits and surface etch.The corrosion rates of GaN substrates in the lateral and longitudinal directions were compared under different reaction times,and the addition of auxiliary oxidizing etchants and ultraviolet light irradiation was realized.A porous buffer substrate was successfully prepared in the pure solution with a low degree of corrosion or difficulty,and the Raman test showed that the stress in the porous buffer substrate was reduced.Surface observation of the porous buffer substrate during different initial growth times As a result,it was found that the initial growth mode of the GaN crystal growing on the porous buffer substrate was a three-dimensional island growth mode,and the HRXRD test results showed that the quality of the GaN crystal grown on the porous buffer substrate was remarkably improved.2.The high temperature annealing process was optimized,and a high temperature annealing buffer substrate was prepared.A 2 inch self-exfoliating GaN single crystal was obtained.The high temperature annealing GaN buffer substrate was designed and fabricated utilizing the GaN pyrolysis characteristics.The buffer liner prepared at different annealing temperatures and times were found to have an annealing temperature of 1050-1100 ? and an annealing time of 15-30 min.The seed layer on the surface of the buffer substrate is completely continuous,and the seed layer and the sapphire substrate are weakly connected,and the structure is the most ideal for the growth of GaN crystals.The FWHM of the(002)and(102)plane was reduced by 60 arcsec and 146 arcsec,respectively.The internal residual stress of the GaN crystal was reduced by 0.42 GPa,and the optical quality was also greatly improved,indicating that a high-quality GaN buffer crystal can be used to grow high-quality GaN crystal.3.The double selective etching technique under hydrothermal conditions was developed.The hydrothermal buffer substrate was prepared.A 2 inch GaN single crystal with self-peeling effect and thickness of nearly 1 mm was grown by HVPE method.By means of the subcritical environment provided by hydrothermal method,the corrosion of GaN in low temperature and low concentration solution was successfully realized.At the same time,double selective corrosion of GaN substrate was realized,and corrosion was prepared by controlling the concentration of etching solution,reaction termperature and time.The degree of controllable hydrothermal buffer substrate,the results of hydrothermal buffer substrate prepared under different conditions showed that the buffer substrate prepared by hydrothermal reaction for 150 min in 35%H3PO4 at 150 ?.had the best effect.The HRXRD,Raman and PL tests of the HVPE grown 2-inch GaN single crystal on the buffer substrate showed that the crystal quality and optical quality were improved,and the residual stress in the crystal was reduced by 0.35 GPa,while XPS showed that the hydrothermal buffer substrate was not introduced.Additional impurities affect the growth of GaN single crystals,MSM structure detectors prepared from self-supporting GaN single crystals grown on hydrothermal buffer substrates have good optical response in the ultraviolet region,demonstrating their potential for fabrication.It is indicated that the hydrothermal buffer substrate prepared by double selective etching has a good application prospect in GaN crystal growth.4.Using a BCN nanosheet as the intercalation layer,a two dimensional material(2D)intercalation buffer substrate was prepared,and the growth mechanism of GaN crystal on this substrate was studied.The relationship between the degree of uniform dispersion of BCN nanosheets on the substrate and the solution viscosity and surface tension was studied.It was found that when PVP with a concentration of 0.25 g/ml was added as a viscosity agent,a 2D buffer substrate with uniformly dispersed BCN nanosheets was obtained.The quality of GaN single crystal grown by 2D intercalation buffer substrate prepared by coating different concentrations of BCN nanosheet solution was characterized.The buffer liner prepared when the concentration of BCN solution was 0.0025 mg/ml was found.The bottom surface widths of the(002)plane and the(102)plane of the GaN single crystal are 192 and 213 arcsec,respectively,the dislocation density is the smallest,the residual stress in the crystal is the lowest,and the optical quality is the best.Calculation results showed that the 2D intercalation substrate promoted the nucleation process of GaN crystals.EBSD test results showed that the 2D intercalation buffer substrate will generate a stress buffer region near the BCN nanosheet,which reduced the stress inside the grown GaN crystal,which was consistent with the Raman test results.TEM characterization showed that 2D material blocked the dislocation from extending inside the GaN crystal,thereby reducing the dislocation density and improving the crystal quality.