Preparation And Optical Properties Of ZnO Nanoparticals Embedded In SiO₂ Thin Films By Thermal Evaporation Assisted Electron Beam Evaporation

Zinc oxide (ZnO) is a wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV) and good chemical stability, which allows high efficient ultraviolet (UV) exciton emission and wide applications to short-wavelength laser diodes and UV detectors. Most studies were performed on ZnO thin film devices. Recently, one dimensional hetero-sturcture nanodevices were also investigated. However, the electroluminescence of ZnO nanoparticles were rarely studied, which might have relatively higher effiencies than thin films and one dimensional nanodevices. Eunice S. P. Leong and Siu Fung Yu reported electrically pumped random ultraviolet lasers on the SiO₂ and ZnO nanoparticle composite thin films which were prepared by spin coating technologies.In this study, a more simple e-beam evaporation combined with thermal evaporation were performed to prepare high quality SiO₂-ZnO composite thin films followed by two step oxidization. Our study shows that the oxidation process is very slow when the annealing temperature is lower than 500oC due to the protection of SiO₂ matrix. However, the oxidation rate becomes quick when annealed at 600oC. Thus, the turning point of oxidation rate might between 500oC and 600o. The size distribution and the crystal quality become better, When ZnO nanoparticals react with the SiO₂ matrix and form Zn2SiO4. The formation temperature of Zn2SiO4 might between 700oC and 800oC. By calculating the intensity of 2LO and 1LO phonon line in the resonant Raman spectra, it is estimated that the average diameter is less than 15 nm. The photoluminescence spectra show that the ultraviolet emission is first enhanced then decreased, corresponding to the growth and reduction of the ZnO nanoparticals. Low temperature photoluminescence of SiO₂/ZnO nanopartical composite annealed at 500oC for 1 h may mainly come from bound exciton emission. By fitting the peak energy as a function of temperature, we obtained a E(0) of about 3.329 eV. The coupling coefficient between phonon and exciton obtained by fitting the FWHM as a function of temperature is much smaller than the reported value in 30-40 nm nanocrystal thin film.Moreover, we give a possible explanation on the result that the intensity and linetype of resonant Raman and photoluminescence spectra is independent of annealing temperature. The contribution on the spectra mainly comes from the surface layer of ZnO nanoparticals. Increasing the annealing time leads to the oxidation of deeply embeded metal Zn, which contributes less to the spectra.