Formation of distorted rutile-type NbO₂, MoO₂, and WO₂ films by reactive sputtering

We report the film growth of distorted rutile-type NbO₂ (4d¹ system), MoO₂ (4d²), and WO₂ (5d²) by radio-frequency (RF) magnetron reactive sputtering. Through optimization of growth conditions for the three oxides on Al₂O₃(0001), we found that the increase/decrease in the RF power had an equivalent role to that of the decrease/increase in the oxygen ratio in the Ar–O₂ sputtering gas. X-ray photoelectron spectroscopy supported the d¹ electronic configuration of NbO₂ and d² electronic configurations of MoO₂ and WO₂. An electrical transport measurement confirmed that NbO₂ was insulating, while MoO₂ and WO₂ were metallic, consistent with the d-electron filling in the molecular orbital bonding band. The growth scheme presented in this study will be useful for valence control in various oxide thin films with a simple sputtering technique.