Positron Study On WO_x Gas Sensitivity Materials And ZnO/Metal/ZnO Multilayer Films

This dissertation presents studies on WOx gas sensitivity materials and ZnO/Metal/ZnO multilayer films by positron annihilation techniques. Some interesting results were discussed. Meanwhile, a design of transparent conductive ZnO/Metal/ZnO multilayer films was done to obtain high transmittance of wavelength from 400nm to 700nm.1. Positron study on WOx gas sensitivity materialsTungsten oxide (WOx) nanostructures were prepared by a hot filament chemical vapor deposition (HFCVD) system and the temperature of the hot tungsten filaments were changed by steps of degrees. The morphology and average growth rate were indicated by scanning electron microscopy which showed that the morphology was highly related to filaments temperature (Tf) and the distance between filaments and polished Si (100) substrates (df). The influence of Tf on the crystalline nature was studied by X-ray diffraction and Raman spectroscopy. The evolution of stoichiometry and types of defects was indicated by X-ray photoelectron spectroscopy and slow positron implantation spectroscopy. When Tf was up to 1750℃, tungsten oxide nanostructure was synthesized. A turning point of Tf was found that at this point the crystallinity and stoichiometry natures were the best. As Tf increased to 2100℃or df decreased, the film crystallinity decreased, correspondingly the component ratio of stoichiometry WO3 decreased and lots of vacancy agglomerates presented. In order to develop the chemical phase from substoichiometry to stoichiometry, the oxygen gas concentration in the mixture gas during deposition should be raised to an appropriate level.2. Positron study on ZnO/Metal/ZnO multilayer filmsTransparent conductive ZnO/Cu/ZnO and ZnO/Al/ZnO multilayers were prepared by pulsed laser deposition. The morphology and crystalline natures of the samples were measured by scanning electron microscopy and X-ray diffraction. The samples were also analyzed by slow positron implantation spectroscopy (SPIS). The evolution of S-E curves for samples with lower thickness of middle metal is different with others after annealing in the air. Crystalline nature, interdiffusion of metal and ZnO, and agglomeration of metal layer change the electrical and optical properties of the multilayers. The thickness of each layers and suitable post annealing can be adjusted to obtain low resistance and high transmittance multilayers.3. Design of transparent conductive ZnO/Metal/ZnO multilayer films and its anti-reflextion and anti-polarization coatingsBoth anti-polarization and antireflection coatings were designed and optimized by means of inhomogeneous coatings theory to obtain the ZnO/Metal/ZnO multilayer systems of low resistance and high transmittance of wavelength from 400nm to 700nm. The thicknesses of each layer and antireflection coatings have been optimized to improve the optical properties. Ag is found out to be the best choice of the middle metal layer. The transmittance of optimized MgF2/ZnO/Ag/ZnO is over 98% with incident angle of 0°. After adding only 6 layers antireflection coatings, both p-transmittance and s-transmittance of Antireflection coatings/ZnO(30nm)/Ag/ZnO(30nm) are over 84% in all visible range with incident angle of 70°. The design result indicated that the transmittance has been increased greatly.This design has an important meaning to the broad application of transparent conductive ZnO/Metal/ZnO multilayers for incidence of broad angle range.