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

doi:10.1016/j.jmst.2020.11.036

Abstract

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 VO²⁺) in sub-stoichiometric WO_3-x forms. Here, amorphous WO_3-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 WO_3-x films, as it is proven by Raman spectroscopy, atomic force microscopy, optical spectrophotometry and Hall effect measurements.

Key words: Metal oxide, Stoichiometry, Optical absorption, Electrical conductivity

C. Guillén, J. Herrero. Amorphous WO_3-_x thin films with color characteristics tuned by the oxygen vacancies created during reactive DC sputtering[J]. J. Mater. Sci. Technol., 2021, 78: 223-228.

Figures/Tables 8

Fig. 1. Transmittance spectra corresponding to WO3-x thin films prepared at different oxygen partial pressures.

Fig. 2. Evolution of the O/W atomic ratio and the oxygen vacancies proportion in WO3-x films grown at different oxygen partial pressures.

Fig. 3. Raman spectra for the WO3-x layers prepared at different oxygen partial pressures.

Fig. 4. AFM images and representative line scans taken on the surface of WO3-x layers prepared at different oxygen pressures: a) Opp = 13 %, b) Opp = 11 %, c) Opp = 9 %.

Fig. 5. Absorption coefficients as a function of the radiation energy for WO3-x films grown at different oxygen partial pressures. The lines represent fits according to an indirect interband transition (αi) and the Brysin polaron model (αp).

Fig. 6. Parameters used to fit the sub-bandgap absorption to the Brysin polaron model, for WO3-x films prepared at different oxygen partial pressures.

Fig. 7. Electrical conductivity, carrier concentration and mobility for the WO3-x films grown at different oxygen partial pressures.

Fig. 8. Concentration of W5+ states calculated from sub-bandgap absorption in comparison with the maximum concentration of W5+ or W4+ states deduced from the oxygen vacancies.

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