SEM Analysis of the Nucleation of Tin Oxide Films on Glass Surfaces
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ABSTRACT Fluorine-doped tin oxide films were produced by atmospheric pressure chemical vapor deposition (APCVD) from the reaction of tin tetrachloride, water, and difluoroethane. Tin oxide films deposited on glass substrates containing high sodium concentrations tended to be of lower quality with higher resistivities than films deposited on low sodium glass. Scanning electron microscopy (SEM) was used to study the differences in film nucleation on the high and low sodium containing glass substrates. Early in the nucleation process, individual grains of tin oxide crystals arc observed to be distributed discontinuously over the substrates. As the deposition time was increased the number of nucleation sites increased and the individual grains became larger. The number of nucleation sites was consistently higher on the high sodium glass. However, numerous inclusions of sodium chloride crystals were observed by EDX in the tin oxide films deposited on the high sodium glass. SEM cross sections revealed how the film growth converted from the grains, which initially nucleated on the glass surface, to columns in the direction perpendicular to the substrate surface. Results for tin oxide grown on sodiumcontaining glass over-coated with silica are also be presented.
INTRODUCTION Transparent conducting thin films are important materials in the fabrication of heat reflecting (i.e. low E ) windows and as transparent electrodes in thin film solar cells, liquid crystal displays, and electrochromic devices. Tin oxide doped with either antimony or fluorine has become an important material used in these applications. The properties of tin oxide films have been reviewed by Jarzebski and Marlon 11-31 and transparent conductors by Chopra et al 141 Properties of fluorine-doped tin oxide films produced by atmospheric pressure chemical vapor deposition (APCVD) from tetramethyltin have been recently described 151. In the present study, the nucleation properties of fluorine-doped tin oxide films on a low sodium containing glass (Corning 7059 glass) and a high sodium containing glass (microscope slides) were study by SEM. The fluorine-doped tin oxide films were deposited by APCVD from the reaction of tin tetrachloride, water, and difluoroethane. The use of difluoroethane as a doping source for tin oxide has been previously noted 161. This reaction is particularly useful because each reactant is inexpensive and relatively nontoxic.
EXPERIMENTAL The fluorine-doped tin oxide coatings were deposited on Corning 7059 glass (low sodium glass) and on microscope slide glass (high sodium 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 futmace heated radiatively by resistance heaters, a motor-driven inconel sample holder, and a reaction zone in which gases were introduced and exhausted (Figure 1). The substrates were positioned on the sample holder and transported to a preheat zone. The substrates were allowed to come to thermal equilibr
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