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.