High-quality epitaxial growth ofγ-alumina films onα-alumina sapphire induced by ion-beam bombardment

We report the formation of epitaxial γ-alumina thin films on α-alumina substrates induced by ion-beam bombardment. Single-crystal (0001) α-alumina was coated with 70-nm amorphous-alumina thin films and then bombarded with either 360-keV argon ions or 180-keV oxygen ions at 400, 500, and 600 ^∘{}C. Ion-channeling measurements showed a consistent minimum yield of 50% for the aluminum in the grown films. Crosssectional transmission-electron microscopy revealed the formation of γ-alumina epitaxially grown onto α-alumina with an orientation relationship [11̄0](111)γ∥[011̄0](0001)α. The epitaxy of γ-alumina was further confirmed by x-ray-diffraction φ scans. This study indicates that ion-beam bombardment at 400-600 ^∘{}C not only induces the amorphous-to-γ phase transformation but also effectively eliminates {111} twins of γ-alumina, which are normally observed after thermal annealing at 800-900 ^∘{}C.