Tuning of work function of ZnO by doping and co-doping: An investigation using X-ray photoelectron spectroscopy

•Surface study of In and Al co-doped ZnO films by X-ray photoelectron spectroscopy.•Notable blueshift in the work function with change in codopant ratio.•XPS analysis revealed the activated dopant in thin films.•Demonstrated inverse relationship of work function with carrier density. The shift in the work function of the ZnO thin film upon varying codoping ratio has been investigated by X-ray photoelectron spectroscopy. A notable shift of 0.2 eV in the work function was achieved in the films when the Al:In doping ratio was changed from 0:10 to 10:0. The elemental composition of the dopants obtained from XPS analysis showed the presence of more amount of In dopant than Al in codoped ZnO films. In this way a clear understanding of the variation in electrical properties on dopant ratio is obtained. A decrease in work function was also observed with the increase in free carrier concentration when the codopant ratio is changed. The effects of Burstein moss, band narrowing, and band renormalization observed in the bandgap were explained by an upward and downward shift of valence band maxima of the corresponding thin films. The controllable work function of the codoped ZnO films by varying the doping ratio offers excellent potential advantages in optoelectronic devices.