Characterization of structural and optical properties of Zinc Oxide thin films

Abstract

The objective of this thesis is to determine the optical properties of ZnO thin films with spectroscopic ellipsometry in a wide spectral region covering the both regimes of below and above the ZnO bandgap. Three dispersion models, namely, Sellmeier dispersion model, Cauchy model and Forouhi-Bloomer model, were used and evaluated to determine the optical constants of ZnO thin films below the energy band gap. On the other hand, two ellipsometric models, namely, two-phase model and three-phase point-by-point fit, were used to determine the optical constants above the band gap. Cauchy model is found to be the most suitable model for extracting the optical constants for photon energies below the ZnO bandgap while the point-by-point fit with the refractive index and extinction coefficient calculated from the two-phase model as the initial values can be used to reliably determine the optical constants for photon energies above the bandgap. Based on the above studies, the optical properties including optical constants, dielectric functions, reflectance, and absorption coefficient in the wavelength range of 250 - 1100 nm, covering both regions of below and above the bandgap, have been determined. Annealing effects on the optical properties and the structures of ZnO thin films have also been studied. It is observed that annealing causes a large reduction in the refractive index and the real part of the dielectric function. Atomic force microscopic study reveals that annealing changes the surface roughness, grain size and size distribution of the ZnO thin films significantly. The reduction in refractive index and dielectric function may be attributed to the increase of the volume fraction of voids in the thin films as a result of annealing.