Improvement Of Ultraviolet Emission Performance Of ZnO Nanocrystalline Thin Films By Buffer Layers And Capping Layers

Zinc oxide (ZnO) is an importantâ…¡-â…¥group semiconductor material with high chemical and thermal stability. At room temperature, ZnO has a wide band gap of 3.37 eV and a large exciton binding energy of 60 meâ…¤. Such a large exciton binding energy makes ZnO materials have good excitonic emission performance at room temperature even at higher temperatures. Therefore, ZnO materials are the ideal materials which are used to fabricate the next-generation short-wavelength light-emitting devices such as ultraviolet light-emitting diodes, ultraviolet lasers, etc. So far, although many ZnO nanostructured materials have been prepared and widely investigated, ZnO thin films are still the ZnO materials which are studied most deeply and widely. As for the application of ZnO thin films in ultraviolet light-emitting devices, how to improve their ultraviolet emission performance and how to fabricate the ZnO thin films used in industrial production are the things that people always widely concern. Just in such a background, we have carried out some related research work.(1) Effects of TiO2 buffer layer and SiO2 buffer layer on the structures and ultraviolet emission performance of ZnO thin films deposited on Si substrates were investigated. It is well-known to all that the single-crystal Si is one of the most important ZnO thin film substrates. However, there are two problems when one prepares ZnO thin films on bare Si substrates. On the one hand, when one deposites ZnO thin films on Si substrates at relatively high temperature or anneals ZnO thin films grown on Si substrates at high temperature, the Si atoms on the substrate surface will "capture" oxygen atoms from ZnO thin films; as a result, many oxygen vacancies emerge in ZnO thin films. On the other hand, the lattice mismatch and thermal expansion mismatch is relatively large between ZnO and Si. These factors will all deteriorate the crystalline quality of ZnO thin films and reduce their ultraviolet emission efficiency. The results of this work show that TiO2 buffer layer and SiO2 buffer layer can effectively improve the crystallinity and ultraviolet emission performance of ZnO thin films grown on Si substrates. It is attributed to TiO2 buffer layer and SiO2 buffer layer solving the above mentioned problems.(2) Effect of TiO2 capping layer on the ultraviolet emission performance of ZnO thin films was investigated. The ZnO thin films capped by TiO2 nanoparticles were prepared by electron beam evaporation. Through the fluorescence resonance energy transfer between ZnO thin film and TiO2 nanoparticles, the ultraviolet emission intensity of ZnO thin film is greatly enhanced. Furthermore, the experimental results also show that appropriate annealing treatment is very important for TiO2-ZnO thin films to obtain the best ultraviolet emission performance.(3) Effect of film thickness on the structural and optical properties of ZnO thin films derived from sol-gel method was investigated. The experimental results show that with the increase of film thickness, the crystalline quality of ZnO thin film is gradually improved and the ultraviolet emission performance is gradually enhanced, and the growth mode of ZnO grains changes from vertical growth to lateral growth.(4) Nanocone ZnO thin films were prepared by E-beam evaporation and rapid high-temperature annealing treatment, and effect of aging time on the photoluminescence behavior of these films was studied. The experimental results show that with the prolonging of aging time, the intensity of green emission is gradually decreased and the intensiy of ultraviolet emission is somewhat increased for the non-sealed sample. These experimental results support the viewpoint that the green emission of ZnO originates from oxygen vacancies.These research results are meaningful and worthful for deeply understanding the properties of ZnO thin films and developing ultraviolet light-emitting devices based on ZnO thin films.