Photoluminescence studies of high-quality ZnO single crystals by hydrothermal method

Photoluminescence and reflectivity spectral measurement have been carried out between liquid helium (4.2 K) and room temperature to evaluate the high-quality ZnO wafers grown by hydrothermal method. The + c, -c and in surface of the ZnO wafer, illuminated by 325 nm ultraviolet light at 4.2 K, emitted the visible luminescence of pale green, pale yellow and pale green, respectively. The broad and weak emissions appeared at 2.43 eV in the + c and in surface, and at 2.29 eV in the -c surface. These peaks were caused by the cation atoms incorporated into the ZnO crystal during the crystal growth. The excitation density dependence of the PL spectra was also investigated, and it was found that no donor-acceptor pair emissions included in the PL spectra of the +c, -c and in surface. The thermal quenching process of the neutral donor-bound and free exciton emissions were observed in the temperature range of 13-300 K, and then the thermal activation energy of A-free exciton was estimated to be 63 and 66 meV in the + c and in surface, respectively. (c) 2006 Elsevier B.V. All rights reserved.