Optimized oxygen deprived low temperature sputtered WO₃ thin films for crystalline structures

We report a detailed analysis on the effects of processing parameters for sputtered tungsten trioxide (WO₃) thin nanoscale films on their structural, vibrational and electrical properties. The research aims to understand the fundamental aspects of WO₃ sputtering at relatively low temperatures and in an oxygen deprived environment targeting applications of temperature and oxygen sensitive substrates. Structural analysis indicates that films deposited at room temperature, or substrate temperatures at or below 400 °C with low oxygen partial pressure are amorphous. Crystallization of the films was observed with distinct Raman peaks when the films were annealed at 300 °C or above using rapid thermal annealing for 10 min. Films revealed monoclinic phases of WO₃ with the presence of W-O-W stretching, bending and lattice vibrational modes in the Raman spectra. Interestingly, a change of transport behavior from insulating to semiconducting was observed for as deposited films on post annealing. Annealed films revealed stoichiometric WO₃ phases with no external defects detected. The present study adopts a route to intercalate WO₃ in a variety of applications from electrochromic coloration to a nanocrystalline thin film for electronic devices sensitive to higher temperatures and gas flow in the sputtering system.