【International Papers】Heteroepitaxial Growth of α-Ga₂O₃ by MOCVD on a-, m-, r-, and c-Planes of Sapphire

      Researchers from the University of Bristol have published a dissertation titled "Heteroepitaxial Growth of α-Ga₂O₃ by MOCVD on a-, m-, r-, and c-Planes of Sapphire" in Crystal Growth & Design.

Background

      Corundum α-Ga₂O₃ (space group R3̅c) boasts the widest bandgap of all Ga₂O₃ polymorphs (5.1–5.3 eV) and is isostructural with α-Al₂O₃, commonly known as sapphire. This permits not only the growth of high-quality domain-matched α-Ga₂O₃ layers on economical substrates (sapphire being available at significantly lower cost than native Ga₂O₃ substrates used for high-quality β-Ga₂O₃ growth) but also extensive bandgap engineering by alloying with Al over the full composition range from x = 0 to 1 in α-(Al_xGa_1–x)₂O₃, leading to a bandgap range of 5.1–8.8 eV. Such compositional control is generally more difficult in the monoclinic phase, where β-(Al_xGa_1–x)₂O₃ films typically suffer from local segregation of Al and Ga (e.g., on (001) and (2̅01) β-Ga₂O₃ substrates) or phase segregation of β- and γ-Ga₂O₃ (e.g., on (010) β-Ga₂O₃ substrates) toward high Al content. Up to 99% Al composition β-(AlGa)₂O₃ films have been achieved on (100) β-Ga₂O₃ substrates, but the film quality degrades with increasing Al content. In contrast, α-(AlGa)₂O₃ films do not degrade or even improve in crystallinity toward Al-rich regimes. Finally, it is worth highlighting that, similar to β-Ga₂O₃, n-type conductivity of α-Ga₂O₃ films can be achieved by Si, Sn, or Ge doping, with controllable carrier density over the range of 10¹⁷–10¹⁹ cm^–3 and electron mobility up to 98.7 cm² V^–1 s^–1 having been reported. These qualities make α-Ga₂O₃ an attractive material for the fabrication of high-breakdown power devices like Schottky diodes, field-effect transistors, and optoelectronic devices such as solar blind photodetectors.

Abstract

      Ga₂O₃ thin films were deposited simultaneously on (112̅0) a-plane, (101̅0) m-plane, (0001) c-plane, and (011̅2) r-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-Ga₂O₃ nucleation was found to be preferential over α-Ga₂O₃ on sapphire orientations with <11̅00> α-Al₂O₃ present (c- and a-planes) when grown under the same conditions. In contrast, α-Ga₂O₃ is preferred during the initial stages of growth on the r- and m-plane, although suppression of island growth is required to prevent the formation of inclined facets on which β-Ga₂O₃ might nucleate. Transmission electron microscopy (TEM) provided a direct confirmation of this growth for r-plane substrates. Classical nucleation theory was applied to rationalize these observations and guide the search for the growth window of α-Ga₂O₃. As a result, decreasing the VI/III ratio and increasing the TEGa flow rate were found to be effective in realizing phase-pure α-Ga₂O₃ on a-plane sapphire by MOCVD with good structural quality (62 arcsec full width half-maxima of X-ray rocking curve), though the equivalent growth on c-plane substrates yielded mixed-phase β- and κ-Ga₂O₃─another metastable phase of Ga₂O₃, instead. Growth on the m-plane resulted in the smoothest surface morphology and thickest phase-pure α-Ga₂O₃ film, indicating that it is the most promising substrate orientation for future device manufacturing.