Critical point analysis of dielectric constant in ZnO thin films on different electronic environments

Abstract

We study the effects of different electronic environments on excitonic properties of ZnO thin films. The thin films were deposited on Si and SiO2 substrates using DC-unbalanced magnetron sputtering at room temperature. Optical properties of ZnO were measured by spectroscopic ellipsometry with the energy range from 1.2 to 6.5 eV. Spectroscopic ellipsometry data were modelled by Tauc-Lorentz and Gaussian oscillators to obtain the complex dielectric function. To extract the excitonic properties, the dielectric function was analysed by critical point line shapes. The results show that the excitonic states of ZnO/SiO2 are stronger and shifted to the lower energy, as compared to ZnO/Si. We find that electron transfer occurs from Si substrate to the ZnO films. The electron transfer then reduces the binding energy of exciton in ZnO/Si system owing to electronic blocking effect. Our results reveal that the different electronic environments affect the excitonic properties of ZnO, which is important for optoelectronic applications.