A-plane free-standing GaN was grown on a-plane GaN templates by HVPE. A-plane GaN templates were grown on
r-plane sapphire by MOCVD with multilayer high-low-high temperature AlN buffer layers. A regrowth method was used
for growing GaN through HVPE. First, GaN was grown on a-plane GaN templates, followed by separating the a-plane
GaN film from r-plane sapphire using LLO. Then, the GaN films were regrown using HVPE. The resulting free-standing
GaN contained some voids, which causes to release the stress.
Although it is known that GaN tend to decompose in hydrogen environments, there has been few investigations in
hydrogen etch of GaN. This study performs a systematic research on hydrogen etch of GaN under various pressures. It is
observed that hydrogen atoms initially etch into GaN to form pinholes. Dislocations are usually the preferred places for
initial hydrogen etch, but not all etched holes result from dislocations. When etched at low pressure and high temperature,
deep vertical holes extending several microns can be formed by the hydrogen etch. However, when etch is performed at
high pressure, apparent lateral etch are observed under the initial holes, leading to bollard-like GaN posts. From this
systematic study, a model has been proposed to explain the vertical and the lateral etching mechanisms. With the
established model, a sequential etch of GaN in hydrogen under varying pressure has been designed to successfully
maintain a smooth GaN front surface, but to etch the underlying GaN to form a porous cave structure. Thick GaN films
are then overgrown on such GaN layers with the hydride vapor phase epitaxy technology. It is demonstrated that the
overgrown GaN thick films can self-separate from the underlying Al2O3 substrates.
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