HCl-based halide vapor phase epitaxy and selective-area HCl gas etching of (−112) β-Ga ₂ O ₃

Takayoshi Oshima, Yuichi Oshima

We propose (−112) and crystallographically equivalent (1−1−2), (−1−12), and (11−2) planes as fundamental crystal orientations for β-Ga2O3 studies. In homoepitaxy, the epilayer exhibited single crystallinity with tilt and twist dispersions comparable to those of the substrate and a step-and-terrace surface morphology with a root-mean-square roughness of 0.10–0.12 nm. Although slit-like pits whose sidewalls were vertically aligned (100) facets appeared on the surface, these pits were attributed to SiO2 nanomasks at the interface and are expected to be eliminated by improving pre-growth surface treatments or the initial growth process. Furthermore, the concentration of Cl impurities in the epilayer was as low as 1×1015 cm−3, which was significantly lower than 2×1016 cm−3 observed in the simultaneously grown (001)-oriented homoepitaxial layer. For HCl gas etching, selective-area etching was performed using a SiO2 mask with patterned windows. The etched structures clearly reflected the intrinsic crystal anisotropy. Side etching was minimized when the windows were aligned along the [02−1] direction due to the formation of exceptionally flat and vertical (100) facets, which possess the lowest surface energy density. Additionally, the vertical etch rate for the (−112) plane was approximately 50 times higher than the side etch rate for the (100) plane, enabling precise fabrication of high-aspect-ratio fins and trenches. These results—particularly the excellent surface smoothness achieved in homoepitaxy and the high-aspect-ratio patterning enabled by HCl-gas etching—demonstrate that the {−112} orientations are promising candidates for β-Ga2O3 studies.