Rf Plasma-assisted Molecular Beam Epitaxial Growth Of High Quality Zno Thin Films

The main task of this paper is to investigate the growth of high quality single crystalline Zinc Oxide (ZnO) thin film on sapphire (0001; α-Al₂O₃) substrate via radio frequency plasma-assisted molecular beam epitaxy (rf-MBE). The work will lay a foundation for application of novel semiconductor material, ZnO, in the field of photoelectronic devices.The relation between Zn beam flux and film quality was investigated. During the growth of ZnO film samples on sapphire (0001) substrates, the RF power was set at 350W, O₂ flux was 1.5 SCCM, Zn beam flux was ranged from 10^-8 to 10^-6 mbar. It can be observed in Photoluminescence (PL) spectra that with the change of Zn beam flux, the UV emitting at about 376nm which attributes to free exciton recombination was increased, while the deep energy level emitting causing by impurity and defects was restrained. This result confirms that our method is available for growth of high quality ZnO film.High quality ZnO film was grown on sapphire (0001) substrate with optimized experimental parameter. Besides sapphire <006> peak, only a <002> peak was observed in the X-ray diffraction (XRD) spectra with the full-width at half maximum (FWHM) value of 0.18°. Three peaks at 1LO (575cm^-1), 2LO (1149cm^-1) , and 3LO (1724cm^-1) were detected in the resonance Raman scattering spectra at room temperature. The results above indicated that ZnO films had single orientation of c axis and high-quality of wurtzite crystalline structure. The absorption peaks of free-exciton and exciton-LO phonon appeared in the absorption spectra, which confirmed that the exciton state in the ZnO films were stable at room temperature. From the photoluminescence spectra, no defect-related deep energy level emission were observed, only a remarkable free-exciton emission located at 376nm were obtained at room temperature, it proved that the ZnO films had high-quality but low density of defect.