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

Abstract

Tungsten oxide (WOx) films were fabricated by (i) reactive thermal evaporation (RTE) at room temperature with oxygen ambient pressurePO2as a parameter, and (ii) reactive magnetron sputtering (RMS) with substrate temperatureTsas 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 WOxfilms deposited at highPo2were amorphous and porous, whileHdepended appreciably on normalized penetration depthhD(indentation depth/film thickness) due to the closing of the pores at the point of indentation. Decrease inPo2from 10 to 2 × 10−3mtorr led to smaller porosity, weakerhDdependence ofH, and higher averageH(measured athD≈ 0.2 to 0.3, for example). The RMS WOxfilm deposited at room temperature was amorphous and denser than all RTE films. The rise in substrate temperatureTsfirst densified the film structure (up to 110 °C) and then induced crystallization with larger grain size forTs≥ 300 °C. Correspondingly, thehDdependence ofHbecame weaker. In particular,Hof the RMS sample deposited at 110 °C showed a peak athDslightly above 1 owing to pileup at the contact point of indentation. For higherTs, pileup occurred at shallowerhDand the averageH(measured athD≈ 0.2 to 0.3, for example) rose, accompanied by the increase of grain size.