Investigation of Z-shaped Photoluminescence of Exciton in ZnO Single-crystal Microtubes

The photoluminescence properties of ZnO single-crystal microtubes grown on Si substrate by hydrothermal method were investigated. An anomalous Z-shaped red-shift was found in the photoluminescence spectra, implying a new temperature switch characteristics. These results are considered to be from the competition among three kinds of excitons, which are localized excitons due to trapping in the intrinsic impurities and interface states at low temperatures; localized excitons and bound from thermally activated and transferred into bound states at intermediate temperatures; and, free excitons and unknown exciton complexes by thermal energy (kT) transfer part of the bound excitons into free excitons at high temperatures. As a result, free excitonsand interface states-excitons strongly overlaped.