Surface Morphology and Sensing Property of NiO-WO3 Thin Films Prepared by Thermal Evaporation

WO3 and NiO-WO3 thin films of various thicknesses were deposited on an Al2O3-Si (alumina-silicon) substrate using high vacuum thermal evaporation. After annealing at 500C for 30 minutes in air, the crystallanity and surface morphology of WO3 and NiO-WO3 thin films were investigated using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). It is observed that the WO3 thin films were resulted in cracks between the polycrystalline grains and the grain growth was increased with increasing thickness causing deteriorated sensing characteristics of the films. On the other hand, an optimum deposition of NiO on WO3 thin film has inhibited the grain growth and improved the sensitivity of the films. The inhibition is limited to a certain thickness of WO3 and NiO content (mol %) of inclusion and below or above this limitation the grain growth could not be suppressed. Moreover, the deposition sequence of NiO and WO3 is also playing a significant role in controlling the grain growth. A probable mechanism for the control of grain growth and improving the sensing property has been discussed.