Superconducting phase of TixOy thin films grown by molecular beam epitaxy

We investigate the complex relationship between the growth conditions and the structural and transport properties of TixOy thin films grown by molecular beam epitaxy. Transport properties ranging from metallicity to superconductivity and insulating states are stabilized by effectively tuning the O/Ti ratio via the Ti flux rate and the O partial pressure P-Ox for films grown on (0001)-Al2O3 substrates at 850 degrees C. A cubic c-TiO1 +/-delta buffer layer is formed for low O/Ti ratios, while a corundum cr-Ti2O3 layer is formed under higher-oxidizing conditions. Metallicity is observed for c-TiO1-delta buffer layers. The superconducting gamma-Ti3O5 Magneli phase is found to nucleate on a c-TiO1-delta buffer for intermediate P-Ox conditions, and an insulator-superconducting transition is observed at 4.5 K (T-C(onset)= 6K) for 85 nm thick films. Strain relaxation of gamma-Ti3O5 occurs with increasing film thickness and correlates with a thickness-dependent increase in T-C observed for TixOy thin films.