ε-Ga₂O₃ Grown on c-Plane Sapphire by MOCVD with a Multistep Growth Process

Gallium oxide (Ga2O3) has especially become popular because of its established applications in semiconductors. Of five polymorphs, monoclinic β-Ga2O3 is the most thermodynamically stable phase. However, orthorhombic Ga2O3 (also known as ε-Ga2O3 or κ-Ga2O3) is gaining increasing interest due to its high lattice symmetry and peculiar ferroelectricity. Although the structural approach for estimating Ga2O3 has been studied both theoretically and experimentally, ε-Ga2O3 and κ-Ga2O3 are still confused. In this study, ε-Ga2O3 epilayers are grown on c-plane sapphire by metal–organic chemical vapor deposition with a multistep growth process. A thin annealed ε-Ga2O3 buffer layer is grown in the first step. The sequent growth steps with slow, fast, or combination of slow then fast growth rate significantly influence the quality of epilayers compared with that of directly grown Ga2O3. Through a detailed transmission electron microscopy (TEM) characterization of these Ga2O3 epilayers, the structural relationship between orthorhombic κ-Ga2O3 and hexagonal ε-Ga2O3 is elucidated. A series of first-principles density functional theory calculations are also carried out to confirm the argument.