Optical properties of high-quality ZnO thin films grown by a sputtering method

We have investigated optical properties of high-quality ZnO thin films grown by a sputtering method. By introducing a low-temperature buffer layer of ZnO, the lattice-mismatched strain is relaxed and the crystallinity is improved remarkably. In the absorption spectrum at 10 K, the absorption peaks of the A and B excitons are observed. In the photoluminescence (PL) spectrum, the free-exciton PL is observed and the defect-related PL is negligibly weak. These results clearly indicate high crystallinity of the film. Furthermore, under high-density excitation conditions, a PL band originating from an inelastic scattering process of excitons, the so-called P emission, is observed in the ZnO thin film.