Microstructure control of NiO-Doped WO3 thin films prepared by thermal evaporation

WO3 and NiO-WO3 thin films were deposited on Al2O3-Si (alumina-silicon) substrates by using high vacuum thermal evaporation. After annealing at 500 .deg. C for 30 minutes in air, the crystallinity and the surface morphology of the WO3 and the NiO-WO3 thin films were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The WO3 thin films were observed to have cracks between the large polycrystalline grains. The cracks and the grain growth can be inhibited by optimum deposition of NiO on WO3 thin films. The grain growth, however, could not be suppressed below or above the optimum content of NiO. Moreover, the deposition sequence of NiO and WO3 also played a significant role in controlling the grain growth. A probable mechanism for the control of grain growth is discussed.