Electronic structure of CuCrO₂ thin films grown on Al₂O₃(001) by oxygen plasma assisted molecular beam epitaxy

Thin films of CuCrO₂ have been grown on Al₂O₃(001) substrates by oxygen plasma assisted molecular beam epitaxy. With a substrate temperature of 700 °C or 750 °C, the films showed an unanticipated (015) orientation but at a higher substrate temperature of 800 °C the expected basal (001) orientation predominates. The optical absorption spectrum of CuCrO₂ shows a direct allowed absorption onset at 3.18 eV together with a weak peak at 2.0 eV which is suppressed by Sn doping. This suggests that the low energy peak should be attributed to 3d→3d excitations associated with Cu²⁺ defect states rather than excitations localised on Cr³⁺. Valence band X-ray photoemission spectra of (001) and (015) oriented CuCrO₂ are compared with those obtained from polycrystalline samples.