The deposition and microstructure of amorphous tungsten oxide films by sputtering

Amorphous tungsten oxide film under different oxygen flow rates were deposited by direct current sputtering. The deposition process was monitored by the Langmuir probe and optical emission spectrometer. From the voltage change at target and all plasma parameters, the deposited films under low oxygen flow rate (5 sccm) are metal-rich tungsten oxides. The films were completely oxidized under higher oxygen flow rate (10-20 sccm). The color of films is also changed from dark blue to transparent accordingly. We analyzed the deposited films by XRD, SEM, EDS and XPS confirmed that the compositions change of deposited films. More interestingly, the XPS reveals the existence of inter-valance state W5+ in all sample films besides the commonly recognized W6+ and W4+ states. This may be accredited to the incomplete bonding between tungsten and oxygen due to the amorphous structures of films. The color change of deposited films examined by the UV-Vis-NIR spectroscopy indicates that the optical band gap is widened and absorbance reduced for films deposited under high oxygen flow rate. These results together indicate that WO3 films with compositions between metal-rich and full oxide are easier for chemical insertion of electrons and ions to achieve better electrochromic functions.