Surface morphologies, chemical compositions and luminescent properties of ZnO thin films flattened by ion milling procedure

Abstract

The luminescent properties of ZnO thin films have been developed to form a nanometric light source for an electron beam-excitation assisted optical microscope. Spatial uniformity of the electron-induced luminescence is important when using this luminescent ZnO film-based light source. In this work, ZnO thin film surfaces have been flattened using broad ion beam polishing with Ar+ ions at a grazing angle of incidence. Atomic force microscopy and electron microscopy confirmed transformation of needle-like ZnO surface structures into scale-like morphologies during ion irradiation. The root-square-mean roughness, corresponding to the height distribution decreased dramatically from 32 to 8 nm after 5 keV Ar+ ion irradiation for 60 min. X-ray photoelectron spectroscopy and Monte Carlo simulations revealed changes in the chemical state and lattice defects during ion irradiation. Positive shifts in the binding energies of the Zn 2p and O 1s spectra suggested preferential Zn atom sputtering and chemical composition change from oxygen-deficient to stoichiometric ZnO. The ultraviolet region luminescent intensity improved after ion irradiation for 60 min. Valence band electron redistribution was considered to influence the ZnO thin film’s luminescent properties. We propose that homogeneous luminescence over a specified area with flat surfaces can be accomplished by repeated short-duration ion irradiation.