On the Thermo-Optical Interactions at the Band-Edge in ZnO Thin Films

The study of the band edge in UV-Vis absorption spectra has impacted fundamental understanding of semiconductor physics as well as advances in technological application. An analytical approach based on the derivative of the absorption spectra was used to explore the nature of the near band edge (NBE) of ZnO thin films grown via the sputtering technique. It was found that the NBE is composed of a Gaussian where the width and peak position were employed to model the electron-phonon (e-p) interaction and defect characteristics of the film. These characteristics were studied via transmission experiments in the temperature range of 77 K to 532 K. The as-grown film was found to exhibit a very weak e-p coupling relative to the static contribution of defects. Upon successive controlled annealing of the ZnO film, up to 800 C, the defect component diminished and the phonon contribution became dominant. X-ray diffraction and imaging studies agree with these results. The defects in the ZnO films are discussed in terms of structural inhomogeneities and Zn interstitials which are prevalent in ZnO films grown via sputtering.