Hydrogen etch of GaN and its application to produce porous GaN caves

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.