Optical characterization of metal oxide thin films

Abstract

Optical properties including complex dielectric function and band gap of ZnO, indium gallium zinc oxide (IGZO) and Al-doped ZnO (AZO) thin films were studied with spectroscopic ellipsometry based on various optical dispersion models. ZnO thin film exhibits a slightly blue shift in both real and imaginary parts of complex dielectric function. The expansion of band gap of ZnO thin films could be attributed to the Burstein-Moss effect due to the increase of electron concentration in the thicker films. On the other hand, IGZO thin film shows a significant blue shift of band gap energy with increasing film thickness. The evolution of the band gap energy and the dielectric function are related to the changes of electron concentration in the films. Dielectric function of AZO thin film showed a blue shift under annealing temperatures < 500°C and a red shift > 500°C. The blue shift can be explained as the Burstein–Moss effect where the band gap increases with the increases of free electron concentrations in the films. It could be attributed to the changes of donor-like defect concentration such as oxygen vacancies in the films caused by annealing.