Luminescence Properties Of ZnO And V/ Al-doped ZnO Thin Films

ZnO is an important candidate for future high power, high frequency, highdensity UV light-emission devices, due to outstanding ultraviolet (UV) luminescentproperties as it has an wide band-gap (3.37 eV) and a large exciton band energy (60meV). ZnO has, which makes it and attracts great attention and hot study passion ofresearchers and scientists. In recent years, great improvements have been made ontheoretical and experimental research of ZnO films, but challenges still remain, suchas understanding light- emission mechanism, improving the technique of film growth,improving the property and stability of prepared films and improving the luminescentproperties of ZnO films. Among various synthetic methods of ZnO thin films, sol-gelmethod is widely used due to its advantages of low growing temperature, rich andcheap raw materials, convenience to control the chemical composition and easinessfor scaling up production.In this thesis, pure and V, A1 doped ZnO thin films were synthesized by sol-gelmethod, and the crystal structures and luminescent properties are analyzed. Theeffects of growth technique, post-treatment and dopants on the structural andluminescent properties of ZnO films were deeply discussed. Optimum synthetic.parameters for ZnO films were presented. We discussed the light-emission mechanismin detail.The optical properties of prepared ZnO films were closely concerned with crystalquality and defects density. It was also found that post-annealing improved the crystalquality of ZnO films, but at the same time, too high annealing temperature enlargedthe amount of defects, which in turn increased the emission intensity relating todefects. The optimum annealing temperature was shown to be about 550℃.Remarkably, the introduction of oxygen in annealing process made the greenemission intensity decrease while the UV intensity increase, compared with that of thesample annealed in air. This result was different from the study by other groups. We thought, the green emission caused by defects is closely related to the density of V_oand Zn_i, in other words, it is related to chemical composition of ZnO crystal.V-doping decreased the crystal quality of ZnO film and induced the centers offluorescence quenching into the sample, which made the intensities of both greenpeak and UV peak decrease greatly. But when the doped samples were annealed under800℃, the crystal quality became better than those undoped ZnO samples. At thesame time, it displayed contrary optical properties to that of undoped ZnO samples.Namely, the intensity of UV peak was higher than that of green peak, which indicatedV-doping decrease the defects that cause green peak and therefore restrained greenlight emission. On the other hand, high temperature annealing reduced the centers offluorescence quenching caused by V-doping and increased the efficiency ofrecombination radiation, therefore, improved UV emission efficiency. The work onAl-doped ZnO film grown on Si and quartz substrates showed different substrates anddifferent doping densities have important effects on crystal quality and luminescentproperties.Investigation of Al-doped ZnO films on Si and quartz substrates shows thatsubstrates and doping concentrations play important roles on crystal quality andluminescent properties.The luminescent and surface photo-voltage properties of ZnO nanocrystals werealso simply studied in this thesis.