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

Abstract: WO₃ and NiO-WO₃ thin films of various thicknesses were deposited on anAl₂O₃-Si (alumina-silicon) substrate using high vacuum thermal evaporation. Afterannealing at 500oC for 30 minutes in air, the crystallanity and surface morphology of WO₃and NiO-WO₃ thin films were investigated using X-ray diffraction (XRD) and ScanningElectron Microscopy (SEM). It is observed that the WO₃ thin films were resulted in cracksbetween the polycrystalline grains and the grain growth was increased with increasingthickness causing deteriorated sensing characteristics of the films. On the other hand, anoptimum deposition of NiO on WO₃ thin film has inhibited the grain growth and improvedthe sensitivity of the films. The inhibition is limited to a certain thickness of WO₃ and NiOcontent (mol %) of inclusion and below or above this limitation the grain growth could notbe suppressed. Moreover, the deposition sequence of NiO and WO₃ is also playing asignificant role in controlling the grain growth. A probable mechanism for the control ofgrain growth and improving the sensing property has been discussed.