Understanding the efficacy of Cu in creating oxygen vacancies and temperature dependent electrical transport in solution processed Cu:ZnO thin films

Abstract

In this investigation, chemically synthesized 5% copper doped ZnO (Cu:ZnO) thin films were deposited onto ITO/Glass using sol-gel spin coating method. Firstly, the effects of inclusion of Cu doping on the electronic properties of ZnO were studied using Density Functional Theory (DFT) based first principle calculations. The comparative energy band (E-k) and orbital projections of density of states profiles were extensively analysed to understand the electronic activities of Cu induced defect states in Cu:ZnO. X-ray photoelectron spectroscopy (XPS) analysis was performed to confirm the oxidation states of Zn, Cu and O and also confirm the existence of oxygen vacancy oriented defects in the Cu:ZnO films. This analysis also indicates that Cu existed in +2 valence state. The experimentally obtained results were compared with the theoretical analysis and both were closely matched. Furthermore, extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) characterizations were also employed to study the role of oxygen vacancies, ions and defect states in Cu:ZnO, which play a pivotal role in the operation of resistive switching devices. XANES and EXAFS have confirmed that Cu conveniently replaced the Zn atoms within the crystal lattice in the deposited films. To investigate the temperature dependent charge transport mechanism, the resistivity of the Cu:ZnO films was measured in the temperature range of 10–300 K. It was ascertained that above 115 K, thermally activated type of conduction governs the charge carrier transport, whereas Mott's variable range hopping type of conduction mechanism is prevalent below 40 K.