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

Dong-Myong Na,Gwang-Pyo Choi,Yong-Jin Shin,L Satyanarayana,Jin Park

doi:10.3390/s5120519

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.

Highlights

Metal oxide thin films have become the focus of many studies in recent years in view of their electrical properties, stability at high temperature and durability
The inhibition is limited to a certain thickness of WO3 and NiO content of inclusion and below or above this limitation the grain growth could not be suppressed
We report the surface characteristics of WO3 and NiO-WO3 thin films deposited by thermal evaporation and their growing mechanism as a function of amount of NiO and WO3 thin film thickness in order to control the surface morphology and microstructure and to improve the sensing property

Summary

Introduction

Metal oxide thin films have become the focus of many studies in recent years in view of their electrical properties, stability at high temperature and durability. WO3 thin films used as sensing materials are typically amorphous or polycrystalline, and are usually treated by post deposited annealing [8]. The effect of annealing causes stoichiometry or microstructural changes that typically lead to a decrease in film conductivity [9]. In our earlier studies we have reported the microstructure control and electrical properties of thick film WO3 as a function of NiO doping, partial pressure of oxygen and concentration of NO2 [10]. We report the surface characteristics of WO3 and NiO-WO3 thin films deposited by thermal evaporation and their growing mechanism as a function of amount of NiO (mol%) and WO3 thin film thickness in order to control the surface morphology and microstructure and to improve the sensing property

Experimental

Results and discussion

Conclusions