Structural and optical characterization of ZnO thin films grown by plasma-assisted molecular beam epitaxy

High-quality ZnO thin films were grown by plasma-assisted molecular beam epitaxy (P-MBE) on Al₂O₃ (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 diffraction (XRD) spectra with the full-width at half maximum (FWHM) value 0.18°, and two peaks 1LO (579 cm^-1) and 2LO (1152 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 376 nm were obtained at room temperature, it proved that the ZnO thin films had high-quality but low density of defect.