GaN Thick Film Growth By HVPE Method

As thick GaN film can effectively reduce the dislocation density, thus the growth of thick GaN film has became a research hotspot in recent years. In this paper hydride vapor phase epitaxy (HVPE) method with faster growth rate had been adopted to grow GaN thick film, to improve the quality of GaN thick film, we used a buffer layer to reduce the misfit dislocations and the thermal stress, and the effect of buffer layer on GaN thick film was analyzed.In this paper the process conditions of ZnO buffer layer on sapphire substrate were firstly optimized. By analyzing the mechanism of ZnO thin film prepared by radio frequency magnetron sputtering, the process parameters of ZnO buffer layers had been studied separately such as sputtering power, sputtering pressure, and substrate temperature. Results show that the optimal sputtering conditions are:60W,2.0Pa,200℃Secondly, the process parameters of GaN thick film prepared by HVPE on ZnO buffer layer were studied. Based on the theory of GaN growth by HVPE, we investigated the effects of V/III ratio, carrier gas type and growth temperature on GaN thick film respectively. Differential interference contrast microscope (DIC), scanning electron microscope (SEM), X-ray diffraction (XRD), and photoluminescence (PL) were used to analyze the surface morphology, crystalline quality and optical properties of GaN. Research discovered that:(1) With low Ⅴ/Ⅲ ratio (low NH3flow rate), GaN is in lateral growth state, dislocation density is low and luminescence property is good, but there are a lot of cracks on the surface; with higher Ⅴ/Ⅲ ratio (high NH3flow rate), the growth state of GaN is vertical structure, there are many small pits on the surface, ZnO buffer layer is severely corroded by NH3, which made the impurities density in GaN is big, crystalline quality is bad, but the cracks greatly reduced for the defects released stress effectively.(2) Within the growth temperature (1010℃～1035℃), GaN sample under1020℃has the strongest c axis preferred orientation, the minimum FWHM value, less zinc impurity diffusion and less defects such as ON, VGa, thus has the high quality of crystal.(3) Carrier gas type mainly affects the surface morphology of GaN, when N2is as carrier gas, nuclear islands on GaN surface have small size and high density, and surface roughness is big; while, the mixed carrier gas of H2and N2can lead to large size and low density of nuclear islands, dislocations which are produced through island mergers are very few, and surface is smooth and flat.To further improve the quality of GaN, the effects of ZnO buffer layers with different thickness on the quality of GaN epitaxial layer were studied with other process parameters unchanging. Results show that the existence of ZnO buffer layer can effectively reduce the misfit dislocation and stress in GaN epitaxial layer, but ZnO buffer layer which is too thin or too thick will produce poor surface morphology and crystalline quality of GaN, FWHM value of GaN is bigger too; however, ZnO buffer layer with200nm is the most suitable for GaN growth, for the surface of GaN sample is smooth, FWHM is0.230°, with less impurities, and the crystalline quality is best.