RESUMO
Using high temperature solid state reaction method, ZnO and Ga2O3 with high purity as raw materials, different ZnGa2O4 samples were prepared at different molar ratios of raw materials and different temperatures. After excitation of the ZnGa2O4 samples by 254 nm UV lamps, obvious long lasting luminescence was detected for the first time, which showed two new long lasting luminescence peaks at 505 nm and 690 nm, respectively. And the relative intensity of the two peaks was effected by the preparation conditions such as molar ratio of the two raw materials and the sintering temperature. Less ZnO or higher temperature will strengthen the relative intensity of the 505 nm peak, while more ZnO or lower temperature will strengthen the relative intensity of the 690 nm peak. The origin of the two peaks was discussed based on some corresponding documents, and the conclusion is that the 505 nm peak comes from the 2E(A)-->4A2 transition of Ga3+ in a relatively distorted octahedral after some Zn2+ are substituted by Ga3+; and the 690 nm peak comes from the V0*--O2- transition after the singly charged ion oxygen vacancies appeare in the octahedral structure. The reasons why the preparing conditions can affect the relative peak intensity of the two peaks were also discussed. Less ZnO will cause Zn2O vacancies in the structure and more Ga3+ will occupy the Zn2+ positions, then will form distorted octahedral, and then the transition from energy level 2E(A) to (4)A2 of Ga3+ will cause the 505 nm peak to be dominant. On the other hand. too much ZnO will form oxygen vacancies, which will cause the 690 nm peak to be dominant. Higher temperature will cause more evaporation of ZnO and then relative less ZnO, and lower temperature will cause less evaporation of ZnO and then more oxygen vacancies. These conclusions are corresponding with the origin of the two peaks discussed before.
RESUMO
High-quality ZnO thin films were grown by plasma-assisted molecular beam epitaxy (P-MBE) on Al2 O3 (0001) substrate with a low temperature ZnO buffer layer. Structural and optical characterization were studied for ZnO thin films. Only a peak at (0002) were observed in the X-ray reflectivity (XRD) spectra with the full-width at half maximum (FWHM) value 0.18 degrees, and two peaks 1LO (579 cm(-1)) and 2LO (1 152 cm(-1)) were detected in the resonance Raman scattering spectra at room temperature. These results indicated that ZnO thin films had single orientation of c axis and high-quality of crystal wurtzite structure. The absorption of free-exciton and exciton-LO phonon appeared in the absorption spectra, which confirmed that the exciton state in the ZnO thin films were stable even at room temperature. And the energy spacing between these two peaks is 71.2 meV, corresponds to the longitudinal optical phonon energy of 71 meV of ZnO. Besides, from the photoluminescence spectra, no defect-related deep emission were observed, but just a remarkable free-exciton emission located at 376nm were obtained at room temperature, it proved that the ZnO thin films had high-quality but low density of defect.