Workfunction tuning of zinc oxide films by argon sputtering and oxygen plasma: an experimental and computational study

Abstract

Zinc oxide (ZnO) films were grown by radio frequency magnetron sputter deposition and the changes to its surface composition and workfunction induced by argon sputter cleaning and oxygen plasma treatments were characterized using x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and density functional theory modelling. Compared with a workfunction of 3.74 eV for the as-deposited ZnO films, a workfunction of 3.95 eV was obtained after Ar sputter cleaning and 4.21 eV after exposure to oxygen plasma. The data indicate that oxygen plasma treatment leads to a more negative ZnO surface. The dipole induced by this charge redistribution reinforces the original surface dipole, which results in an increase in the surface dipole moment and an increase in workfunction. The reverse is true for hydrocarbon contamination of ZnO surfaces. Excellent qualitative agreement between the experimental results and computational modelling was obtained. The results suggest that specific surface functionalization may be a viable method of controlling the workfunction of ZnO for use as the transparent conducting oxide in optoelectronic applications such as solar cells and organic light-emitting diodes.