Structures And Properties Of The ZnO Thin Films And Amorphous Nanorods Prepared By Pulsed Laser Deposition

ZnO is a II-VI compound semiconductor with a wide direct band gap of 3.3 eV at room temperature, a large exciton binding energy of 60meV, and a hexagonal wurtzite structure. Due to its excellent physical and chemical properties and the ability to integrate with other semiconductor materials, ZnO have many realized and potential applications such as surface acoustic wave devices,planar optical waveguides, transparent electrodes, ultraviolet photodetectors, piezoelectric devices, varistors, gas sensors, UV/violet/blue-LEDs (light emitting diodes) and -LDs (laser diodes), etc. In recent years, the studies and developments of ZnO films and naostructures have attracted great attention and interest from the researchers and the industries.Spectroscopic ellipsmotry(SE) was employed to characterize ZnO thin films prepared by pulsed laser deposition (PLD) on Si (100) substrates at various temperature of 400℃, 500℃, 600℃and 700℃. The refractive indices (n) and extinction coefficients (k) of the ZnO films were calculated in the spectral range of 400~800 nm at each deposition temperature by fitting the ellipsometic parameters based on a three-layers dispersion Cauchy model. It was found that the optical constants were significantly affected by the substrate temperature. Through analyzing the crystalline structures and surface morphologies of ZnO thin films grown at different substrate temperature by X-ray diffraction (XRD) and atomic force microscopy (AFM) respectively, the variation of the refractive index can be attributed to the changes of the packing density of the thin film. After comparing the results obtained at different grown temperature, it is suggested 600℃might be the optimum deposition temperature for growing dense ZnO films with high optical and film quality.In addition, ZnO/ZnS complex films have also been prepared by PLD. Scan electron microscopy (SEM) investigation revealed that a large number of one-dimensional nanorods were spontaneously formed on the surface of the films after putting them in air at room temperature for about one month. These nanorods were confirmed to be amorphous ZnO under high resolution transmission electron-microscopy (HRTEM) detection. A growth mechanism was proposed to interpreted the spontaneously growth of the amorphous nanorods based on the XRD and SEM measurements. Furthermore, a new emission peak located at 410 nm can be observed in the photoluminescence (PL) spectrum. The study on the amorphous ZnO nanorods can help us further understand the formation mechanism of one-dimensional amorphous semiconductors and open up new opportunities by utilizing the unique properties of this kind of materials.