Exciton-polariton behaviour in bulk and polyerystalline ZnO

摘要

We report detailed reflectance studies of the exciton-polariton structure of thin film polyerystalline ZnO and comparison with bulk crystal behaviour. Near-normal incidence reflectance spectra of these samples are fitted using a two-band dielectric response function. Our data show that the reflectance data in polyerystalline ZnO differ substantially from the bulk material, with Fabry-Perot oscillations at energies below the transverse A exciton and above the longitudinal B exciton in the films. In the strong interaction regime between these energies no evidence is seen of the normally rapid oscillations associated with the anomalous waves. We demonstrate that the strong interaction of the damped exciton with the photon leads to polaritons in this region with substantial damping such that the Fabry-Perot modes are eliminated. Good qualitative agreement is achieved between the model and data. The importance of the polariton model in understanding the reflectance data of polyerystalline material is clearly seen.