Phase identification of vanadium oxide thin films prepared by atomic layer deposition using X-ray absorption spectroscopy

The chemical and local structures of vanadium oxide (VO(x)) thin films prepared by atomic layer deposition (ALD) were investigated by soft X-ray absorption spectroscopy. It is shown that the as-deposited film was a mixture of VO(2) and V(2)O(5) in disordered form, while the chemistry changed significantly after heat treatment, subject to the different gas environment. Forming gas (95% N(2) + 5% H(2)) annealing resulted in a VO(2) composition, consisting mostly of the VO(2) (B) phase with small amount of the VO(2) (M) phase, whereas O(2) annealing resulted in the V(2)O(5) phase. An X-ray circular magnetic dichroism study further revealed the absence of ferromagnetic ordering, confirming the absence of oxygen vacancies despite the reduction of V ions in VO(2) (V(4+)) with respect to the precursor used in the ALD (V(5+)). This implies that the prevalence of VO(2) in the ALD films cannot be attributed to a simple oxygen-deficiency-related reduction scheme but should be explained by the metastability of the local VO(2) structures.