Bias-assisted photoelectrochemical planarization of GaN (0001) with impurity concentration distribution

To planarize semiconductor materials such as gallium nitride (GaN) and silicon carbide with high efficiency, we developed a polarization method that combines ultraviolet irradiation and an abrasive-free polishing method known as catalyst-referred etching (CARE). In this method, the substrate surface is photoelectrochemically oxidized, thus improving the removal rate. Accordingly, an atomically well-ordered surface was obtained at a removal rate 100 times higher than that of the conventional CARE method without ultraviolet irradiation. However, in some cases, for GaN substrates with a high oxygen impurity concentration area, the oxidation rate is nonuniform on the substrate surface, resulting in the formation of a rough surface. In this study, we propose the application of a positive bias to the GaN substrate to suppress the oxidation rate fluctuation. In the positive bias state, the width of the depletion layer generated at the interface of GaN and the etchant becomes uniform on the entire surface regardless of crystallographic fluctuation, thereby achieving a uniform oxidation rate. When only 3.0 V was applied, the oxidation rate was uniform; thus, a flat GaN surface without the footprint originating from crystallographic fluctuations was obtained.