Radio-frequency reactively sputtered VO_x thin films deposited at different oxygen flows

The effect of different oxygen flow rate on the properties of vanadium oxide thin films formed by radio-frequency, reactive sputtering deposition are investigated. The stoichiometry of the as-deposited films was investigated by Rutherford backscattering spectrometry. For high oxygen flows, films were mainly V₂O₅ while a lower oxide was obtained at the lowest flow. Forward recoil spectrometry indicated a significant hydrogen content in the thin films. X-ray diffraction showed that the films were amorphous. The electrochromism shown by such films upon lithium intercalation was also studied. Samples deposited at high oxygen flow were transparent/yellow, and showed a somewhat complicated electrochromic behavior. Upon lithium insertion up to 25 mC/cm², the films acted as cathodically coloring materials at wavelengths higher than 500 nm and as anodically coloring material at lower wavelengths. For the higher insertion levels, the opposite behavior was observed. Samples deposited at very low O₂ flows showed no electrochromic behavior.