Zinc Oxide based Thin Films for Sensor Application

Abstract

The present work deals with the preparation of ZnO thin films doped with Cd [CdxZn1-xO (0-20 at. %)], In [InxZn1-xO (0-5 at. %)], Al [AlxZn1-xO (0-5 at. %)] and mixed oxides with copper [Cu1-xZnxO (0≤x≤1)], tin [ZnxSn1-xO (0≤x≤1)] on glass substrates using spray pyrolysis technique. The structural, morphological, compositional and optical properties were studied to understand the effect of different doping and its concentration with intension of using prepared thin films for gas sensor application. The XRD analysis revealed that doped ZnO thin films were having hexagonal wurtzite structure with preferred orientation along (002) direction. Slight shift in the (002) peak was also observed and it was dependent on doping concentration and radii of dopant. The SEM studies confirmed the formation of homogeneous and uniform thin films with granular morphology and surface features were reliant on doping concentration. The resistivity of undoped films were very high and it was decreasing with addition of dopant. Optimal doping concentration was found to be 10 at. % for cadmium and 3 at. % for indium and aluminium with >80% optical transmittance in visible region. The gas sensing properties were investigated at optimal temperature of 3500C for various volatile organic compounds like acetone, ethanol and methanol. The ZnO thin films with 10 at. % cadmium doping showed highest sensitivity of 50% for 1 ppm ethanol. XRD studies on prepared Cu1-xZnxO and ZnxSn1-xO thin films confirmed formation of composite and amorphous thin films respectively. Formation of Cu1- xZnxO composite thin film improves sensing property, but formation of amorphous phase in ZnxSn1-xO thin films do not contribute towards any improvement in sensitivity. Maximum electrical conductivity was achieved for Cu0.75Zn0.25O composite thin films, which also showed highest sensing property for VOCs. Cu0.75Zn0.25O thin films were selective towards ethanol and were capable of detecting 1 ppm of ethanol at operating temperature of 2900C. Studies gave better insight into behaviour of metal oxides thin films towards sensor application, which will help in improvement of device applicability. Furtherresearch is required to improve selectivity, stability and miniaturization of sensor material for hand held gas sensor and electronic nose application.