Thickness-Dependent Optical Constants and Annealed Phase Transitions of Ultrathin ZnO Films

The thickness-dependent optical constants and annealed phase transitions of atomic-layer-deposited ZnO ultrathin films with a thickness of less than 50 nm have been demonstrated by spectroscopic ellipsometry. The thickness dependence of refractive index and extinction coefficient was discussed, and the mechanisms were given in the molecule level based on previous reports. Furthermore, the optical properties of ZnO ultrathin films varied with annealing temperatures, and the phase transition was found at high annealing temperature. The thickness of the ultrathin films decreased obviously, and the refractive index of the ultrathin films changed a lot after annealing at high temperature while Zn₂SiO₄ formed at a temperature above 800 °C. The low phase transition temperature of Zn₂SiO₄ may be due to the ultrathin scale effect. What’s more, photoluminescence spectra showed the annealing effect on ultrathin films and the enhanced defects luminescence were observed. We believe that these investigations will help improved understanding of essential physical chemistry and optoelectronic devices based on ultrathin oxide films for optical and photoelectric applications.