Optical and sensing properties of Al doped ZnO nanocrystalline thin films prepared by spray pyrolysis

Abstract

Nanocrystalline undoped and aluminum doped ZnO films are deposited by spray pyrolysis method. All film samples show polycrystalline nature and hexagonal wurtzite structure of ZnO. Crystallite size decreases with increase in dopant concentration. The lattice constant-a of undoped ZnO thin film is greater than doped samples whereas c-lattice constant is maximum for 3at.% Al doping. Bond length decreases whereas dislocation density increases in doped samples. Doping decreases the transmission of the films and the optical band gap of ZnO is also reduced from 3.28 to 3.18 eV. SEM shows porous spot-like structures which change to web of needle-like structures when dopant is introduced. Surface morphology of the films further evolves to open ends of tubular structures as the doping is increased further. Undoped ZnO shows maximum sensitivity at 400°C for higher concentration of CO2 whereas aluminium doped samples show maximum sensitivity either at 350°C or below for all CO2 concentrations.