Structure And Optical Properties Of ZnO-SiO₂ Composite Thin Films

Zinc oxide (ZnO) is a wide band gap (～3.3 eV) semiconductor with a hexagonal wurtzite structure and an excellent photoelectric characteristics. Silicon oxide (SiO₂) is an ideal optic transparence medium material with stable chemistry character. So the work of embedding ZnO in SiO₂ material and investigating the influence of size effects on its microstructure and luminescence characters is important for putting the material into practical use. In this dissertation, pulsed-laser deposition (PLD) was used to prepare ZnO-SiO₂ nanocomposite thin films. We mainly investigate the effects of SiO₂ content and annealing temperatures on the microstructure and luminescence properties. In addition, the production of ZnO and SiO₂ at high temperature, zinc silicate (Zn₂SiO₄), is taken as the thin-film medium and the luminescence characteristics of rare earth element, europium ion (Eu³⁺), are discussed. The main results are as follows:1. ZnO-SiO₂ composite thin films have been obtained by PLD on Si (110) substrate with different SiO₂ concentrations in oxygen atmosphere at room temperature. Grain size and shape can be observed in the composite thin films with low SiO₂ contents in SEM images, while no signals when SiO₂ contents at or over 20%. At the same time, the ultraviolet fluorescence intensity of the composite films and the energy gap both increase with SiO₂ content.2. Annealing was performed on the composite thin film with SiO₂ content of 40% at different temperature, and the microstructure of the thin film was characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). The results show that β-Zn₂SiO₄ particles can be detected in the film annealed at 700℃ and willemite Zn₂SiO₄ forms at 900℃. As can be seen from the photoluminescence results, ultraviolet emission intensity of the sample annealed at 700℃ remarkably increases as compared with the as-prepared sample, while the intensity decreases slightly at higher annealing temperature. On the other hand, visible light intensity decreases with increasing the annealing temperature and almost disappears at 900°C.3. The photoluminescence properties for Eu³⁺ in zinc silicate thin film are studied. The sample displays emission peaks at 590 nm, 610 nm, 650 nm and 720 nm, originating from ⁵D₀→ ⁷F₁, ⁵D₀→7F₂, ⁵D₀→⁷ F₃ and ⁵D₀→⁷F₄ transition, respectively. The transition between ⁵D₀→⁷F₂ at 610 nm plays a dominant role.