Amorphous WO3−x thin films with color characteristics tuned by the oxygen vacancies created during reactive DC sputtering

Abstract

Tungsten oxides are interesting for a variety of applications due to their versatile optoelectronic characteristics, which can be tuned changing the composition and/or the crystalline structure. Coloration due to sub-bandgap absorption is often achieved by ion intercalation or doping in WO3:M films (with M = H+, Li+, Na+, etc. introducing extra electrons), but a more direct way is creating charged oxygen vacancies (VO+ and/or VO 2+) in sub-stoichiometric WO3-x forms. Here, amorphous WO3-x thin films are obtained by reactive DC sputtering of a pure W target, on unheated glass substrates, changing the oxygen to argon pressures ratio. The control of intrinsic defects (oxygen vacancies and tungsten valence states) by the oxygen partial pressure allows tuning the morphology, sub-bandgap absorption and carrier density in these WO3-x films, as it is proven by Raman spectroscopy, atomic force microscopy, optical spectrophotometry and Hall effect measurements.