Structure Analysis of Co Epitaxial Thin Films Grown on Al2O3 Single-Crystal Substrates

Co thin films were prepared on Al2O3 single-crystal substrates with (0001) and (11-20) orientations by ultra-high vacuum molecular beam epitaxy. Effects of substrate temperature and substrate orientation on the film structure were investigated. Co epitaxial thin films with close-packed planes parallel to the substrate surfaces are obtained on Al2O3(0001) and Al2O3(11-20) substrates at 50-500 °C and 300-500 °C, respectively. The films grown on Al2O3(0001) substrates at temperatures lower than 100 °C consist of fcc-Co(111) crystal. With increasing the substrate temperature, hcp-Co(0001) crystal begins to coexist with the fcc crystal and the volume ratio of hcp to fcc crystal increases. The films grown at temperatures higher than 300 °C consist primarily of hcp crystal coexisting with a small volume of fcc crystal. On the contrary, the Co thin films grown on Al2O3(11-20) substrates at temperatures higher than 300 °C consist of only the hcp crystal. The in-plane lattice strains of these Co epitaxial films are larger than the out-of-plane lattice strains due to the accommodation of lattice mismatches between the films and the substrates. The film strain decreases as the substrate temperature increases.