||ZnO thin film is a suitable material for the phosphor layer of green emission of the electroluminescence (EL) device. Therefore, the luminescence mechanism of green emission of ZnO thin film is a key issue to be investigated.|
In this thesis, ZnO thin films are deposited on SiO2/Si substrates using sol-gel method and sputtering technology, and then post-annealed by a rapid thermal annealing (RTA) process under various annealing temperatures (200℃~900℃) and atmospheres (vacuum, ambient atmosphere and oxygen). The physical and photoluminescence (PL) characteristics were first discussed. Secondly, the relationship between the chemical composition and the PL properties were also investigated to figure out the dominant luminescent center of ZnO thin film. Finally, ZnO thin film was applied as the phosphor layer of AC thin film EL device and the characteristics were discussed.
According to the experimental results of ZnO thin film prepared using sol-gel method and RTA process, the XRD patterns show a preferred (002) orientation after annealing. The grain size became larger with the increasing annealing temperature. From PL measurement, two ultraviolet (UV) luminescence bands were obtained, and the intensity became stronger when the annealing temperature was increased. The strongest UV light emission appeared at annealing temperature of 900℃ in oxygen. The X-ray photoelectron spectrum (XPS) demonstrated that a more stoichiometric ZnO thin film was obtained upon annealing in oxygen and more excitons were generated from the radiative recombination carriers consistently, and resulted in the strong UV emission. However, no green emission was obtained from ZnO thin film prepared by sol-gel method.
The XRD patterns also exit an excellent preferred (002) orientation of ZnO thin film deposited using sputtering and RTA process. The grain size of ZnO thin film annealed at 200℃~500℃ increased with the increasing annealing temperature, and then exhibited a melting state with the temperature of 600℃~700℃. A large and complete grain was observed at the temperature of 900℃. The PL spectrum illustrated that a stronger UV emission intensity appeared at annealing temperature of 500℃. On the other hand, the green light emission could be obtained as ZnO films were annealed above 500℃ and reached a maximum intensity at 900℃. Based on the XPS analysis, the O1s peak of ZnO film revealed that the concentration of oxygen vacancy increased with the annealing temperature from 600℃ to 900℃ under an ambient atmosphere. The PL results demonstrated that the intensity of green light emission at 523nm also increased with temperature. Under various annealing atmospheres, the analyses of PL indicated that only one emission peak (523nm) was obtained, indicating that only one class of defect was responsible for the green luminescence. The green light emission was strongest and the concentration of oxygen vacancies was highest when the ZnO film was annealed in ambient atmosphere at 900℃.
According to the experimental results manifested above, room temperature was used to deposit films to increase the ratio of Zn atoms inside the thin film when using sputtering technique to deposit ZnO thin film. With the modulation of the annealing parameters, stronger green light emission could be obtained. The luminescence mechanism of the emission of green light from a ZnO thin film is associated primarily with oxygen vacancies. In addition, only UV light emission of ZnO thin film prepared using sol-gel method was obtained because of the better stoichiometry.