Deposition of Fluorine Doped Tungsten Oxide Thin Films by Atmospheric Pressure Chemical Vapor Deposition
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DEPOSITION OF FLUORINE DOPED TUNGSTEN OXIDE THIN FILMS BY ATMOSPHERIC PRESSURE CHEMICAL VAPOR DEPOSITION JAMES W. PROSCIA*, CHARLES H. WLNTER**, GENE P. RECK**, AND GANG GANG WEN** *Ford Motor Company, Glass Division, Dearborn, MI 48120 "**Department of Chemistry, Wayne State University, Detroit, MI 48202. ABSTRACT Polycrystalline fluorine doped tungsten oxide thin films were deposited onto glass substrates by reacting tungsten hexafluoride, difluoroethane, and isopropanol. The reaction was performed in an atmospheric pressure chemical vapor deposition reactor at a temperature of approximately 400 oC. The electrical conductivity and spectral properties were suggestive of n-type substitutional doping in which oxygen is partially replaced by fluorine. Temperature dependent resistivity measurements were consistent with the presence of polarons. The films were blue in color with enhanced reflectance and absorbance in the IR. These optical characteristics are potentially useful in improving the solar control properties of glass. Scanning electron microscopy revealed a faceted surface morphology. The fluorine atom concentration decreased when the films were annealed in air. INTRODUCTION Doped metal oxides are important optical and electrical materials with thin film applications that include functioning as transparent electrodes in photovoltaic and electrochromic devices, low E glass (low emissivity glass), arnd coatings for solar screening. Common examples include indium oxide doped with tin (ITO);1 tin oxide doped with fluorine 2 or antimony; 3 zinc oxide doped with fluorine, 4 aluminum, 5 or boron 6 and tungsten oxide doped with lithium.7 In each of this examples, the useful properties occur because the dopant causes the material to acquire some metallic properties. It would be desirable if a metal oxide could be doped to a sufficient high level that near-infrared radiation is either absorbed or reflected while some visible transmission is preserved. Such a material would be useful in automotive glass applications and in the design of energy efficient architectural glass products. Neither tin oxide or ITO as capable of rejecting much of the near-IR. Tungsten oxide doped with lithium is capable of rejecting most of the near IR radiation. However, lithium is usually introduced electrochemically into tungsten oxide. In the present paper, the reaction of tungsten hexafluoride with isopropanol was used to produce polycrystalline tungsten oxide thin films in an atmospheric pressure chemical vapor deposition (APCVD) reactor. The fluorine atom concentration in the films was adjusted by varying the flow of difluoroethane (DFE). Tungsten oxide has previously been doped by fluorine. 8 However, thin films of fluorine doped tungsten oxide have not been previously described. EXPERIMENTAL The fluorine doped tungsten oxide thin film coatings were produced on Coming 7059 glass in a research APCVD reactor manufactured by BTU, 'International (North Billerica, Massachusetts). This reactor consists of a 197 cm long by 21 cm wide muffled f
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