A Rhombic Dodecahedral Honeycomb Structure with Cation Vacancy Ordering in a !‑Ga2O3 Crystal

Crystal structure of a γ-Ga2O3 layer grown epitaxially on an MgO substrate by a vapor phase transport method was investigated by transmission electron micrscopy, electron diffraction, and scanning transmission electron microscopy with aberration correctors. Some forbidden reflections were excited in electron diffraction patterns by double reflection from the vicinity of the substrate interface. Anti-phase boundaries are observed in an atomic column imaging by the high angle annular dark field images. A structure model is proposed to explaine the experimental results. Cation vacancy ordering is introduced in the structure model to distort the γ-Ga2O3 crystal lattice for one axis and reduce the mismatch with the substrate. Some grains are formed and alter the directions to reduce the distortion for the other axis. The grains are stacked with {110} phase boundaries and form a rhombic dodecahedral honeycomb. The rhombic dodecahedral honeycomb structure model with cation vacancy ordering is stabilized by the lattice mismatch between the γ-Ga2O3 crystal and the MgO substrate, and it disappears at a depth of 170 nm from the interface.