Structural, optical and electrochromic properties of RF magnetron sputtered WO3 thin films

Abstract

Thin films of tungsten trioxide (WO3) have been prepared by RF reactive magnetron sputtering of tungsten target at different substrate temperatures in the range 303–673K and at fixed oxygen partial pressure of 6×10−2Pa and sputter pressure of 4Pa. The effect of substrate temperature on the structural, morphological, optical and electrochromic properties of WO3 films was systematically studied. The films formed at 303K were of X-ray amorphous, while those deposited at substrate temperatures ≥473K were crystallized into orthorhombic phase WO3. The crystallite size of the films increased from 17 to 24nm with increase of substrate temperature from 473 to 673K. Raman studies confirmed that the presence of O–W–O and W=O bonds in WO3 films. The surface morphology of the films was significantly varied with substrate temperature. The optical transmittance data revealed that the optical band gap increased from 3.08 to 3.48eV and refractive index increased from 2.18 to 2.26 with increase of substrate temperature from 303 to 673K respectively. The WO3 films formed at substrate temperature of 473K exhibited better optical transmittance modulation of 40% between colored and bleached state with a color efficiency of 33.8cm2/C and diffusion coefficient of 1.85×10−11cm2/s.