Facile synthesis of vanadium oxide thin films by atomic layer deposition and post-annealing

We report the facile synthesis of vanadium oxide (VOx) thin films on sapphire substrates by atomic layer deposition (ALD) and post-annealing. Amorphous VOx thin films are grown by ALD employing tetrakis[ethylmethylamino]vanadium and ozone as precursors. Subsequent post-annealing of the as-grown VOx thin films at 500 °C in atmospheres of air, 1 Pa O2, and vacuum (10−4 Pa) could, respectively, result in the crystallization of V2O5, VO2, and V2O3 phases, as verified by x-ray diffraction and Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) reveals that the valence states of vanadium in the annealed thin films exhibit dramatic changes with the variation of annealing atmosphere toward the targeted phases. The VO2 thin film exhibits a sharp metal–insulator transition (MIT) near 340 K with a resistivity change exceeding three orders of magnitude. The V2O3 thin film features a characteristic MIT with obvious thermal hysteresis between the cooling and warming processes, whereas the V2O5 thin film is highly insulating. The spectral weight near the Fermi level revealed by the XPS in the valence band region coincides with the changes of VOx phases and consequent transport properties due to post-annealing. Our results demonstrate that the combination of ALD and post-annealing provides a facile method for the synthesis of multivalent VOx thin films for practical applications.