The Study On Structure And Optical Properties Of Nanocrystalline ZnO Thin Films

Zinc oxide (ZnO) is an interesting wide band gap (3.3 eV) semiconductor material with a binding energy of 60 meV. It makes more attention to the ultraviolet (UV) optoelectronic devices. Up to now, the visible emission and ultraviolet lasing emission of ZnO have been the subject of much research. Although many groups put forward different mechanism of visible luminescence from ZnO, a suitable mechanism is very difficult to establish. Of course, to carry out ultraviolet lasing emission is what one expects to do. In order to study these two questions, high quality nanocrystalline ZnO thin films were prepared by thermal oxidation of ZnS thin films, which were deposited by using low pressure metalorganic chemical vapor deposition technique. The X-ray diffraction patterns and Raman spectra indicated that ZnS fully transformed into ZnO with a polycrystalline hexagonal wurtzite structure when ZnS thin films were oxidized at annealing temperature of above 700 oC in an oxygen ambient. The photoluminescence (PL) spectra of the ZnO thin films showed that the strongest ultraviolet photoluminescence (PL) at 380 nm was observed at annealing temperature of 900 oC, while the visible photoluminescence was barely observed. The effect of annealing temperature on its photoluminescence was discussed in detail in this thesis. The ultraviolet emission with a low energy tail resulted from the overlapping of free exciton emission and bound exciton emission. We use the laser output (320 nm, 200 fs) of Optical Parametric Amplifier (OPA) in an active passive mode-locked femtosecond Ti-Sapphire laser operating at a repetition rate of 1kHz as a exciting resource to develop optically pumped stimulated emission of ZnO thin films. When rectangular stripe laser irradiates thin films, optical resonant cavity is naturally formed between two nanocrystallites along with the rectangular laser stripe and planar weveguide confines the light scattering. We observed the ultraviolet stimulated emission of nanocrystalline ZnO thin films on SiO2 substrate. In the discussion of visible luminescence mechanism, in order to prove that the oxygen vacancies or defects distribute on the surface of nanocrystallites, we presented to prepare the ZnO thin films with doped Mn and studied the photoluminescence of ZnO:Mn. Together with luminescence spectra, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonant (EPR), core-shell structure of nanocrystalline ZnO was put forward. In this thesis, we put forward that Vo** and [Vo*, electron] or [Vo**, two electrons] complex are luminescent centers of visible luninescnece. Passivation on the surface of nanocrystalline ZnO was responsible for an energy potential high enough to prevent surface states trapping the electrons or holes photogenerated, it should block the pathway to form the luminescence centers as the Vo** and [Vo*, electron] or [Vo**, two electrons] complex. A new visible luminescence mechanism was presented.