Enhancement of band-edge photoluminescence of bulk ZnO single crystals coated with alkali halide

We have studied coating effects of bulk ZnO crystal by means of photoluminescence (PL) measurements. Bulk crystal surface of ZnO is once covered by the melt of alkali halide at high temperature and is coated with thin alkali halide crystal. At low temperature, band-edge PL intensity of the ZnO crystal coated with KCl or KI is about 300 times larger than that of a polished crystal. In the no-coating ZnO crystal treated by the same thermal conditions, the enhancement of the PL intensity is only about 20 times larger. The enhancement of band-edge PL also occurs considerably at room temperature. The experimental results including the PL spectra in the visible light region, suggesting the qualitative improvement of ZnO surface by the coating, are discussed in terms of the incorporation of halogen ions supplied from alkali halide into oxygen vacancies.