Optical functions of nanocrystalline ZnO containing voids and doped with Ga

Abstract

The pulsed laser deposited nanocrystalline ZnO films doped by Ga up to six weight percent are studied by X-ray difraction and generalized spectro-ellipsometry. We report substantial atomic structure modification of heavy Ga-doped ZnO resulted and a concentration dependent increase of inter-planar distance. Measured dielectric function spectra show strong blue-shift of the samples studied. Equilibrium atomic configurations and electron energy structure of ZnO containing defects (voids and Ga impurities) are studied by the density functional theory (DFT) and generalized gradient approximation (GGA). Atomic geometries are obtained from the total energy minimization method. Optical functions are calculated within the random phase approximation including the quasi-particles corrections and plasma excitation effects. We report energetically favorable paths of the voids growth and aggregation in ZnO. Comparative analysis of experimental and theoretical results indicate that measured blue-shift in ZnO:Ga substantially exceeds the Burstein-Moss shift as used in many previous work to interpret concentration dependence of optical functions in heavy doped ZnO. We demonstrate that additional mechanisms, such as structural and alloying effect, should be involved for quantitative interpretation of optics of the nano-crystalline heavy-doped ZnO films.