High Rates and Very Low Temperature Fabrication of Polycrystalline Silicon From Fluorinated Source GAS and Their Transpo
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T. Kamiya, K. Nakahata, K. Ro, C. M. Fortmann and I. Shimizu The Graduate School, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, JAPAN ABSTRACT
Low temperature (50-300'C) growth of polycrystalline silicon (poly-Si) by very high frequency (100MHz) glow-discharge plasma enhanced CVD using SiF 4 and H2 mixtures was studied. The poly-Si microstructure was strongly affected by the SiF 4/H2 gas flow ratio. For example, either (220) or (400) preferentially oriented films were prepared by appropriate SiF 4/H2 ratio selection. The addition of small SiH 4 flows to the SiF 4/H2 mixtures could be used to increase the growth rate while the SiF4/H2 continued to control the film structures such as preferential orientation. Highly crystalline films were grown at a growth rate of 0.52nm/s using SiF4/H2/SiH 4 flow rates of 30/90/2.Osccm (respectively). However, at higher SiH 4 flows amorphous films were deposited. Under the small SiF 4/H2 ratio condition, highly crystallized poly-Si was grown at temperatures as low as 50'C. N/i/Pt Schottky diode solar cells were prepared using these poly-Si for both the n- and the i-layers. These solar cells exhibited good performance; for example, open circuit voltages over 0.32V. N-i-p solar cell results are very promising with 6.2% of conversion efficiency being achieved in the initial trials. INTRODUCTION
Previously Ishihara et al. [ 1] reported that by using of SiF 4 and 1-2 gas mixtures and a remote-type microwave plasma (RT-MW) polycrystalline silicon (poly-Si) thin films could be grown on glass substrates at temperatures below 400'C. These poly-Si films showed excellent electronic transport with a high photoconductivity and high electron mobility (-20cm 2/Vs measured at room temperature). This excellent transport was attributed to well passivated dangling bonds and large grain sizes (>100nm).[2,3] Despite the low temperature growth, high growth rate (>1 nm/s) was attained. Therefore, a favorable chemical reaction among hydrogen, halogen and the growth surface was suggested for the SiF 4/H2 reaction system. In addition by selecting deposition parameters, in particular, the SiF 4/H2 ratio [R], preferentially oriented films could be deposited. For example, (220) preferentially oriented films were grown with [R]
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