Doping of Vacuum Deposited Zinc Oxide Thin Films

Abstract

The main objective of present work is to study the doping of vacuum evaporated zinc oxide (ZnO) thin films. Initially, optimum conditions required to obtain good quality of undoped ZnO thin films is determined by depositing films using two evaporation sources (boats) namely molybdenum and tungsten and annealing them under different conditions. Compositional analysis of films showed incorporation of boat atoms in to ZnO thin films prepared using molybdenum as well as tungsten boat. A considerable reduction in atomic percentage incorporated boat atoms on annealing was observed in both cases. ZnO thin films obtained under optimum conditions were found to be amorphous in nature with good combination of visible region transmittance of up to 90% and room temperature conductivity of 92 Ω−1cm−1. XPS analysis has shown that the film is approximately stoichiometric with slight oxygen deficiency. From the measurements of activation energy it is observed that ZnO thin film is having two donor levels below the conduction band. Further, ZnO films are doped with third group dopants to improve their n-type conductivity. Investigation has been carried out to know the optimum percentage of dopnat to be added to retain the transmittance of the film. Role of third group elements as n-type dopants in the form of pure metals and metal oxides is studied by doping the film with indium and indium oxide separately. It is observed that contribution of indium oxide dopants is more than indium dopants in improving the conductivity of films. ZnO films were then doped with other two third group oxides, namely gallium oxide and aluminum oxide. From structural, optical and electrical properties of these oxide doped ZnO films it is found that all films have smooth surface with visible region transparency of above 90% and significantly high room temperature conductivity of the order of 103 Ω−1cm−1, which are well suited for the application of transparent electrodes.