WET ETCHING OF P-GAN FOR CRYSTALLOGRAPHICALLY SMOOTH VERTICAL SIDEWALLS
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GaN vertical power devices have gained increased attention in recent years due to the advantages over lateral transistors in high breakdown voltage/high current applications as well as thermal performance. Trench etching is a key technology to achieve high-voltage trench-based vertical GaN devices; rough surfaces or sharp corners can lead to device preliminary breakdown due to the electric field crowding. The combination of inductively couple plasma and tetramethylammonium hydroxide (TMAH) etching is commonly utilized in fabrication of smooth and vertical gate planes to minimize surface state charge and achieve high channel mobility. Several groups have reported that TMAH can smooth the n-GaN or UID GaN etched surfaces to reveal m-planes, while negligible etching occurs for p-GaN due to the lack of holes at the surface. In this work, the etch process was found to be diffusion limited for GaN in TMAH. A reaction mechanism for the etch process is proposed. In addition, crystallographically smooth vertical p-GaN sidewalls were achieved by a novel wet etching technique: “hydrogen-passivation-assisted wet etching”. The vertical trench sidewall is preferable for the realization of damage-free high mobility normally-off channel in trench-based vertical GaN devices for high-voltage applications.