The Caf <sub> 2 </ Sub>-embedded Optical And Electrical Properties Of Zno Nanocrystals And F-passivated Zno Thin Films

Zinc oxide (ZnO), a semiconductor with a large exciton binding energy (60 meV) and wide band gap (3.37 eV), has attracted much attention because of its versatile physical properties, such as piezo-electrical, ferro-electrical, electro-optical and acousto-optical characteristics. The excellent luminescence properties of ZnO (the intense near band edge ultraviolet emission and the neglectable visible emission associated with deep level defects) are necessary for the wide use of ZnO. In the paper, the ZnO nanocrystals embedded in CaF2 matrix and the F-passivated nanocrystalline ZnO thin films are prepared to obtain the optimized luminescence properties of ZnO. Moreover, the electrical properties of ZnO:F films are also researched.High quality ZnO nanocrystals were fabricated by zinc ion implantation (160keV, 1×1017 ions/cm 2) into a CaF2(111) single crystal substrate followed by thermal annealing from 300℃ to 700℃. X-ray diffraction results showed that ZnO nanocrystals in CaF2 (111) substrate had a (002) preferred orientation. The average grain size was ranging from 14 to 19 nm corresponding to the annealing temperatures from 500℃ to 700℃. A very strong ultraviolet near-band-edge emission was observed from 372 to 379 nm. The emission intensity was enhanced and linewidth was narrowed as the annealing temperature increased. The commonly observed visible green emission associated with deep-level defects in ZnO was suppressed. Moreover, the emission of F-centers induced by zinc ion implantation was observed. The processes of electrons transfer between ZnO nanocrystals and F-centers were discussed in detail.The F-passivated nanocrystalline ZnO thin films were prepared by thermally oxidizing ZnF2 films, where the ZnF2 films were grown onto the Si and SiO2 substrate by electron beam evaporation technique. The x-ray diffraction and x-ray photoelectron spectroscopy measurements were used to study the phase change processes of ZnF2 versus the annealing temperature. When ZnF2 film was annealed at 400 oC for 30 minutes, the ZnO:F thin film with a polycrystalline hexagonal wurtzite structure was obtained. In the photoluminescence spectrum of the ZnO:F film, a strong near band edge ultraviolet emission located at 379nm with a narrow linewidth of 70 meV was observed, while commonly observed visible emission associated with deep level defects was quenched. The photoluminescence results demonstrated that the F passivation dramatically improved the luminescence characteristics of the ZnO films. The mechanisms of F passivation were proposed: (1) in the ZnO nanocrystals, the F atoms occupied the lattice sites of Vo* centers (the oxygen vacancies with one electron), thus, decreasing the number of Vo* centers; (2) on the surface of ZnO nanocrystals, F saturated the dangling bonds and passivated the defect states, as a result, the processes that the holes in the valence band are trapped into surface defects and return into nanocrystals to combine with Vo* to form Vo** centers (Vo*+hâ†'Vo**) were effectively restrained, the radiative combination centers (Vo**) correlated with the visible emission of ZnO decreased remarkably. Two main factors mentioned above resulted in the quenching of the visible emission.The electrical properties of F-doped ZnO films were measured by the Van der Pauw method. Low resistivity of 7.60×10-4Ω·cm, high carrier concentration of 2.0×1020cm-3 and Hall mobility of 41.1cm2V-1s-1 at room temperature for the ZnO:F film were obtained. F doping effects on the electrical and optical properties were investigated. 1) Due to the Burstein-Moss energy shift from the high carrier concentration, the optical band gap of ZnO was widened. 2) F-passivation decreased the scattering centers of the carriers and the height of the potential barriers at the grain boundaries, thus, increased the Hall mobility of the carriers.