Heteroepitaxial Growth of α‑GaO by MOCVD on ‑, ‑, ‑, and ‑Planes of Sapphire.

GaO thin films were deposited simultaneously on (112̅0) -plane, (101̅0) -plane, (0001) -plane, and (011̅2) -plane sapphire substrates using metal-organic chemical vapor deposition (MOCVD) and characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The different surface energy and strain conditions imposed by each sapphire plane make the choice of substrate orientation critical to the stabilization of the α-phase. β-GaO nucleation was found to be preferential over α-GaO on sapphire orientations with <11̅00> α-AlO present (c- and -planes) when grown under the same conditions. In contrast, α-GaO is preferred during the initial stages of growth on the - and -plane, although suppression of island growth is required to prevent the formation of inclined facets on which β-GaO might nucleate. Transmission electron microscopy (TEM) provided a direct confirmation of this growth for -plane substrates. Classical nucleation theory was applied to rationalize these observations and guide the search for the growth window of α-GaO. As a result, decreasing the VI/III ratio and increasing the TEGa flow rate were found to be effective in realizing phase-pure α-GaO on -plane sapphire by MOCVD with good structural quality (62 arcsec full width half-maxima of X-ray rocking curve), though the equivalent growth on -plane substrates yielded mixed-phase β- and κ-GaOanother metastable phase of GaO, instead. Growth on the -plane resulted in the smoothest surface morphology and thickest phase-pure α-GaO film, indicating that it is the most promising substrate orientation for future device manufacturing.