Relationship between the microstructure and nanoindentation hardness of thermally evaporated and magnetron-sputtered electrochromic tungsten oxide films

Tungsten oxide (WO_x) films were fabricated by (i) reactive thermal evaporation (RTE) at room temperature with oxygen ambient pressureP_{O₂}as a parameter, and (ii) reactive magnetron sputtering (RMS) with substrate temperatureT_sas a parameter. The film structure revealed by x-ray photoelectron spectroscopy, x-ray diffraction, density measurements, infrared absorption, and atomic force microscopy was correlated with the nanoindentation hardnessH. The RTE WO_xfilms deposited at highP_{o₂}were amorphous and porous, whileHdepended appreciably on normalized penetration depthh_D(indentation depth/film thickness) due to the closing of the pores at the point of indentation. Decrease inP_{o₂}from 10 to 2 × 10⁻³mtorr led to smaller porosity, weakerh_Ddependence ofH, and higher averageH(measured ath_D≈ 0.2 to 0.3, for example). The RMS WO_xfilm deposited at room temperature was amorphous and denser than all RTE films. The rise in substrate temperatureT_sfirst densified the film structure (up to 110 °C) and then induced crystallization with larger grain size forT_s≥ 300 °C. Correspondingly, theh_Ddependence ofHbecame weaker. In particular,Hof the RMS sample deposited at 110 °C showed a peak ath_Dslightly above 1 owing to pileup at the contact point of indentation. For higherT_s, pileup occurred at shallowerh_Dand the averageH(measured ath_D≈ 0.2 to 0.3, for example) rose, accompanied by the increase of grain size.