Formation of near-vertical trenches and fins with flat {-201} sidewalls on (512) β-Ga₂O₃ via TMAH wet etching

We achieved the formation of near-vertical trenches and fins with exceptionally flat {-201}-faceted sidewalls on (512) β-Ga₂O₃ substrates by crystallographic wet etching in 25-wt% tetramethylammonium hydroxide (TMAH). Since wet-etched {-201} surfaces exhibit atomically flat step-and-terrace morphologies, substrate orientations perpendicular to the {-201} plane and having high wet-etch rates are expected to enable smooth vertical trench/fin formation by wet etching alone. To identify such orientations, we measured the in-plane dependence of the side-etch rate on a (-201) substrate and found that side etching was most enhanced near the (512) and equivalent planes. We therefore investigated the wet-etch characteristics of (512) substrates. The planar etch rate increased exponentially with temperature, reaching 0.724 μm h^-1 at 90°C, indicating surface-reaction-limited behavior with an activation energy of 83.3 kJ mol^-1 (0.863 eV). Near-vertical etching was achieved only for etching windows aligned along the [1-92] direction, parallel to the intersection between the (-201) and (512) planes. The developed {-201}-faceted sidewalls were flat and slightly inclined from the (512) surface normal by approximately 1.8°, in good agreement with the crystallographically estimated angle of 1.70°, suggesting that perfectly vertical etching could be achieved using 1.70°-miscut (512) substrates.