HCl-based halide vapor phase epitaxy and selective-area HCl gas etching of (-112) β-Ga₂O₃

We propose (-112) and crystallographically equivalent (1-1-2), (-1-12), and (11-2) planes as the fundamental crystal orientations for β-Ga₂O₃studies. These planes correspond to the {100} planes of the slightly distorted face-centered-cubic oxygen sublattice in β-Ga₂O₃and therefore represent one of the primary crystallographic planes. On this basis, we have demonstrated both homoepitaxial growth and HCl gas etching on (-112) β-Ga₂O₃substrates using an HCl-based halide vapor phase epitaxy system in order to clarify both the growth and etching characteristics on the (-112) plane. 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 unintentional 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.