Thickness dependence of infrared lattice absorption and excitonic absorption in ZnO layers on Si and SiO2 grown by atomic layer deposition

Abstract

Using spectroscopic ellipsometry from the midinfrared (0.03 eV) to the deep ultraviolet (6.5 eV), the authors determined the thickness dependence of the dielectric function for ZnO thin layers (5–50 nm) on Si and quartz in comparison to bulk ZnO. They observed a small blueshift of the band gap (∼80 meV) in thin ZnO layers due to quantum confinement, which is consistent with a simple effective mass theory in an infinite potential well. There is a drastic reduction in the excitonic effects near the bandgap, especially for thin ZnO on Si, which not only affects the excitonic absorption peak but also lowers the high-frequency dielectric constant by up to 40%. No significant change of the phonon parameters (except an increased broadening) in thin ZnO layers was found.