Grain Boundary Carrier Scattering in ZnO Thin Films: a Study by Temperature-Dependent Charge Carrier Transport Measurements

In the present investigation, the effects of annealing in oxygen atmosphere on the electrical and optical properties of pulsed direct-current (DC) magnetron-sputtered ZnO thin films have been studied. With annealing, the electrical conductivity was found to increase from 2.3 S cm to 123 S cm. The optical transparency was also found to improve from 83% to 90%. The improvement in the electrical properties with annealing is attributed to the reduction of the grain boundary potential from 34 meV to 8 meV. X-ray photoelectron spectroscopy (XPS) measurements revealed oxygen deficiency in as-deposited films, whereas adequate incorporation of oxygen was observed in the annealed films. Ultraviolet photoelectron spectroscopy (UPS) measurements showed a shift of +0.8 eV in the Fermi level with annealing. This shift indicates significant changes in the electrical conductivity with annealing, which is due to an increase in the carrier concentration.