HIGH-QUALITY EPITAXIAL-GROWTH OF GAMMA-ALUMINA FILMS ON ALPHA-ALUMINASAPPHIRE INDUCED BY ION-BEAM BOMBARDMENT

We report the formation of epitaxial gamma-alumina thin films on alpha -alumina substrates induced by ion-beam bombardment. Single-crystal (0 001) alpha-alumina was coated with 70-nm amorphous-alumina thin films and then bombarded with either 360-keV argon ions or 180-keV oxygen io ns at 400, 500, and 600 degrees C. Ion-channeling measurements showed a consistent minimum yield of 50% for the aluminum in the grown films. Cross-sectional transmission-electron microscopy revealed the formati on of gamma-alumina epitaxially grown onto alpha-alumina with an orien tation relationship [1(1) over bar0$](111)gamma parallel to [<01(1)ove r bar 0>](0001) alpha. The epitaxy of gamma-alumina was further confir med by x-ray-diffraction phi scans. This study indicates that ion-beam bombardment at 400-600 degrees C not only induces the amorphous-to-ga mma phase transformation but also effectively eliminates {111} twins o f gamma-alumina, which are normally observed after thermal annealing a t 800-900 degrees C.