by tuning oxygen stoichiometry*

Thermally driven metal-insulator transitions (MITs) in vanadium oxides (VO_X) have been of special interest in fundamental physics and oxide electronics. The existence of multi-valence states of vanadium leads to a complicated vanadium-oxygen phase diagram. Moreover, for thin films, finite size or strain effects can alter the physical and thermodynamic properties of the V-O system. Therefore, precise oxygen stoichiometry control in these oxide films remains an outstanding issue and a clear phase stability diagram has not yet been found. We demonstrate a gas evolution technique to precisely modify the oxygen stoichiometry in VO_X thin films grown on sapphire substrates. By carefully tuning the temperature-oxygen partial pressure-time relationship, controlled phase evolution between VO₂, V₂O₃ and Magnéli phases (V_nO_2n-1) was achieved, along with the detailed characterization of their electrical transport and structural properties. Moreover, we show that high quality films, with well-defined MITs can be synthesized this way.