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 pressure Pₒ₍₂₎ as a parameter, and (ii) reactive magnetron sputtering (RMS) with substrate temperature T[sub s] as 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 hardness H. The RTE WOₓ films deposited at high Pₒ₍₂₎ were amorphous and porous, while H depended appreciably on normalized penetration depth h[sub D] (indentation depth/film thickness) due to the closing of the pores at the point of indentation. Decrease in Pₒ₍₂₎ from 10 to 2 × 10⁻³ retort led to smaller porosity, weaker h[sub D] dependence of H, and higher average H (measured at h[sub D] ≈ 0.2 to 0.3, for example). The RMS WOₓ film deposited at room temperature was amorphous and denser than all RTE films. The rise in substrate temperature T[sub s] first densified the film structure (up to 110 °C) and then induced crystallization with larger grain size for T[sub s] ≥ 300 °C. Correspondingly, the h D dependence of H became weaker. In particular, H of the RMS sample deposited at 110 °C showed a peak at h[sub D] slightly above 1 owing to pileup at the contact point of indentation. For higher T[sub s], pileup occurred at shallower h[sub D] and the average H (measured at h[sub D] ≈ 0.2 to 0.3, for example) rose, accompanied by the increase of grain size.