Hydrogen Dilution Effect on the Crystallinity of Silicon Films Grow by Hot Wire Cell Method
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ABSTRACT A new process, the Hot Wire Cell method, was developed and successfully used to grow polycrystalline silicon thin films at a low temperature and high growth rate. In the Hot Wire Cell method, reactant gases are decomposed as a result of reacting with a heated tungsten filament placed near to a substrate and polycrystalline silicon films can be deposited at a growth rate of 1.2nm/s without hydrogen dilution and 0.9nm/s with the use hydrogen dilution. The film crystallinity changed from amorphous to polycrystalline due to the addition of hydrogen, thus hydrogen dilution was effective for improving film crystallinity. Furthermore, we obtained (220) oriented polycrystalline silicon thin films with a 90% crystal fraction by the use of hydrogen dilution. These results showed that the Hot Wire Cell method is promising for the deposition of device-grade polycrystalline silicon films for photovoltaic applications. INTRODUCTION The Hot Wire Cell method is a new and very promising process for depositing polycrystalline silicon thin films for photovoltaic applications. In this process, a heated tungsten filament is used to induce the catalytic and/or pyrolytic dissociation of reactant gases such as SiH 4 and H2 [I] and produce atomic hydrogen as a reaction by-product, which then reacts with silane molecules to produce SiH 3 radicals. In this work, since the filament is positioned perpendicular to the substrate holder, the reactant gas is efficiently decomposed during its passage through the filament thus enhancing its decomposition rate. In the Hot Wire Cell method, atomic hydrogen is also produced as a result of the decomposition of the silane. We have previously obtained polycrystalline silicon thin films without hydrogen dilution and demonstrated that polycrystalline silicon films could be deposited at low temperatures of 175400'C at a relatively high growth rate of I.Onm/s[2]. In this study, we present our experimental results on the effect of filament temperature and hydrogen dilution on the growth rate and structural properties of polycrystalline silicon films. EXPERIMENT A schematic diagram of the deposition chamber incorporating the Hot Wire Cell is shown in Fig. 1. The Hot Wire Cell consists of gas inlet and a tungsten filament where the axis of the gas inlet into the chamber was perpendicular to the plane of the substrate holder. A tungsten wire with a diameter of 0.3mm was used as the filament. The filament, coiled with a diameter of 4mm and length of 15mm, was arranged to be parallel to the gas inlet and gas flow and the distance between the filament and the substrate was about 6cm. The temperature of the filament was measured using an optical pyrometer through a window of the chamber. The filament was heated by a D.C. power supply and the power kept constant throughout the experiment by P.ID. control. Thin films were deposited on Coming 7059glass substrate using silane or hydrogen diluted silane and in the case of hydrogen dilution, H2 gas was also passed through the heated filament. 531
Mat. Res. Soc. S
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