Research on HVPE Growth, Stress and Crystallographic Orientation of GaN Crystal
|Keywords||HVPE GaN EBSD etching stress|
GaN crystal material has a high electron mobility and thermal conductivity, high chemical stability. So it can be widely used in photoelectric devices, high frenquece microwave devices. Due to the lack of GaN single crystal substrate material, devices are mostly epitaxy on foreign substrate, such as sapphire and SiC crystals. There are large lattice mismatch and thermal expansion coefficient mismatch between GaN and substrate. In this case the devices have a larger residual stress and dislocation density after cooling from the growth temperature. In order to solve these problems, free-standing GaN crystal materials need to be grown. Hydride vapor phase epitaxy (HVPE) method growing GaN single crystal is considered to be a suitable way to obtain GaN single crystal substrate materials because of its high growth rate, relatively simple crystal growth system and the low cost of growth sources.In this work we grow GaN crystals by HVPE method and optimized the growth parameter. The patterned substrate and etching process are used to reduce the dislocation density and residual stress of HVPE growth GaN and obtain free-standing GaN crystal. The growth modes on these substrates are researched. EBSD technology is used to analyze the heteroepitaxy system crystallographic orientation relationship between GaN and substrate. The lattice mismatch and stress are calculated from the crystallographic orientation. The mainly research achievements are as follows:1. The influence of GaCl carrier gas flow rate was studied to obtain optimal condition for high crystalline quality GaN film. The1.3slm GaCl carrier gas flow rate sample with low FWHM of both (0002) and (10-12) reflection was demonstrated to have high crystalline quality and low dislocation density. Room temperature photoluminescence results showed a strong band-edge emission (NBE) peak with a FWHM of20-30meV which was detected in each sample. GaN films grown by high GaCl carrier gas flow rate had a very weak yellow luminescence (YL) peak at2.26-2.29eV. The HVPE GaN films grown with1.3slm GaCl carrier gas flow rate were found to have the lowest the residual compressive stress0.36GPa, which corresponded to the wave number of E2Raman mode at567.7cm-1. The surface morphology of all films showed a clear step flow growth model in AFM images. The carrier concentration and mobility of these GaN crystals are obtained by fitting of the A1(LO) Raman peak which is a LOCP mode.2. Hexagonal etch pits formed in the MOCVD grown GaN on sapphire after being etched in hot phosphoric acids. With long etching time the etch pits reached the sapphire substrate and kept extending etching time the etch pits connected with each other to form large irregularly shaped etch pits. There were some pyramid structures in the N-polar face of GaN clustered around the etch pits reached the sapphire substrate. These pyramids have twelve facets. The crystallographic plane indexes of the facets were identified as (4,-1,-3,-4) and (3,1,-4,-4) according to the symmetry of wurtzite structure GaN. This kind of structures reduced the contact area between epitaxial GaN and sapphire substrate and was beneficial to self-separated process. This wet etching method provided a possible facile method to obtain free-standing GaN using HVPE growth.3. GaN crystal was grown using a MOCVD-GaN/sapphire substrate with hexagonally distributed SiO2masks. GaN epitaxial grown on the maskless area underwent lateral overgrowth and coalesced on the top of the masks. Voids were formed on the masks because of this growth mode. The orientation relationship was examined by EBSD Kikuchi diffraction patterns and pole figures of GaN and sapphire. The cross-sectional plane was1-210for GaN and10-10for sapphire, respectively. The in-plane orientation relationship between the two materials constitutes an angular rotation of30°. The lattice mismatch calculated from the orientation relationship and the lattice parameters was16.1%and13.8%between GaN and the sapphire substrate, according to different evaluation methods. As a result of the large lattice mismatch, the disorientation of GaN was largest at the interface and decreased as the GaN grew. The crystallographic orientation approached the ideal growth direction as the GaN was growing. The EBSD mapping analysis also confirmed this result. The Raman spectrum measurements confirmed reduced residual stress values in the HVPE grown GaN on the substrate with masks. 4. The method of calculating stress in the crystal materials directly from the deformation of lattice identified by EBSD was provided. The stress of crystal materials at each mapping point was obtained from EBSD by this method. Stress distribution in large area was obtained efficiently and exactly by this method. Wurtzite structure GaN crystals grown by HVPE on foreign substrate was used as the example of hexagonal crystal system. Stress obtained from Raman spectroscopy confirmed the distribution identified by EBSD. We think that the stress distribution of other crystal materials also can be calculated by this method depends on changing the form of elasticity tensor. Other properties related to the lattice deformation were also analyzed by this way.5. The GaN crystal HVPE grown on the6H-SiC substrate was researched by EBSD. The cross-sectional plane was10-10for GaN and6H-SiC substrate. It was different from the in-plane orientation relationship of sapphire/GaN heteroepitaxy system. The lattice mismatch calculated from the orientation relationship and the lattice parameters was3.7%and3.5%between GaN and the sapphire substrate, according to different evaluation methods. The lattice mismatch was much smaller than the GaN on sapphire substrate. The use of6H-SiC substrate reduced the dislocation density and residual stress values in the HVPE grown GaN. The small angle grain boundary of sapphire was also researched by the EBSD.