Optical properties of ultrathin metal oxide and metal films: Effect of free electrons, quantum confinement and localized surface plasmon resonance
Li, Xiao Dong
Date of Issue2016
School of Electrical and Electronic Engineering
Knowledge of optical properties of a material is crucial for design and optimization of the nanoscale devices. In this thesis, a study of the effects of free electrons, quantum confinement and localized surface plasmon resonance (LSPR) on optical properties of metal oxide and metal thin films is presented. Ultrathin metal oxide thin films including undoped ZnO, Al-doped ZnO (AZO) and amorphous indium gallium zinc oxide (a-IGZO) were deposited using RF magnetron sputtering technique. Ultrathin Au films were deposited using electron beam evaporation technique. Optical properties including dielectric function, optical constant and band gap energy of the thin films were investigated with spectroscopic ellipsometry (SE) based on various optical dispersion models. The effect of free electrons on optical properties of AZO and a-IGZO thin films has been investigated. The comparison study of the free electrons between the undoped ZnO and AZO shows that free electrons could greatly suppress the excitonic absorption near the band edge and result in an expansion of band gap due to the free electrons effect. It has been observed that thermal annealing at temperature range of 400-700°C could greatly change the free electrons concentration of the AZO films, which directly influences the free electrons absorption and band edge absorption. The thickness dependence of optical properties of a-IGZO films has also been observed. The evolution of the dielectric function is shown due to the change of free electrons concentration in the films. The effect of quantum confinement on optical properties of ZnO and a-IGZO thin films has been investigated. The ultrathin ZnO film exhibits a significant reduction in dielectric function and an expansion of band gap with decreasing film thickness. The evolution of the dielectric function is shown related to the changes in the interband absorption, discrete-exciton absorption, and continuum-exciton absorption, which can be attributed to the quantum confinement effect on both the band gap and exciton binding energies. Thickness dependence of optical properties of a-IGZO thin films has also been observed. The evolution of the dielectric function is explained as the net result of both the free electrons effect and quantum confinement effect. The effects of LSPR and quantum confinement on optical properties of ultrathin Au films have been investigated. A significant evolution of the LSPR, interband transition energies, plasma energy and conductivity with the film thickness is observed. The evolution is attributed to the changes in the size, shape and spacing of the self-assembled Au nanoparticles (NPs) as well as the aggregation of the Au NPs in the Au films.