Fabrication of High Quality Poly-Si from Fluorinated Precursors
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FABRICATION OF HIGH QUALITY POLY-Si FROM FLUORINATED PRECURSORS Shin-ichi ISHIHARA, Deyan HE, Tetsuya AKASAKA, Yuzoh ARAKI, and Isamu SHIMIZU The Graduate School, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama-city, Japan
ABSTRACT Poly-Si thin films with grains 100--200 nm in dia. showing a highly ordered texture were grown from fluorinated precursors, SiFnHm (n+m=3), on a glass substrate at 300--400 'C with the aid of atomic hydrogen. According to the in situ observation by ellipsometry, the reconstruction was undergone in a solid phase stimulated by impinging atomic hydrogens within a thin layer of about 10 nm thick owing to the strong chemical interaction of the pair of H and F in Si-network. Both H and F were 3 3 released efficiently from the network to the levels of 2 x 1020 cm- and (2-5) x 1019 cmI respectively. Dangling bonds were also efficiently passivated down to 4 x 1016 cm-3 with hydrogens diffused through the network. P-doped films showing electrical conductivity of 10-2 S/cm (300 'K) with the activation energy of 0.24 eV was obtained by alternately repeating the deposition of thin layer and the treatment with atomic hydrogens.
INTRODUCTION Poly-Si thin films grown on glass substrate have opened up new possibilities for fabrication of large area electronic devices such as TFT for display devices, sensors and solar cells because of its carrier-mobilities faster than that of a-Si:H and the stable structure. Laser-annealing (LA) , rapid thermal annealing (RTA) or low temperature annealing (LTA) have been undertaken extensively to fabricate poly-Si thin films on glass substrates.[1],[2],[3] In recent years, the LA techniques have undergone a remarkable improvement and a FET exhibiting the mobility of 380 cm 2 Ns was successfully made of the poly-Si thin film grown on a glass.[4] With regard to the fabrication of solar cells, poly-Si thin films grown at rather low temperatures are expected to improve either the efficiency or the stability by using them in combination with a-Si:H thin film.[5] In actuality, however, these techniques based on the physical treatments, have some problems confronting the practical fabrication of devices, i.e., difficulties in making homogeneous films with a large area or in enlarging the production scale. As far as the production scale is concerned, there are more advantages in the techniques based on chemical processes similar to the fabrication of hydrogenated amorphous silicon. Some specific features have so far been found in the formation of crystalline silicon from fluorinated precursors, SiHnFm (n+m=3), under control over atomic hydrogens.[6] Under the existence of an excessive amount of hydrogens compared with that of the precursors, microcrystalline silicon (pc-Si) with a grain size of 20-50 nm in dia. on the average was made at 150 -- 350 *C. Either the grain size or the volume fraction of crystal was independent of the temperature, and the materials used as the substrate as well.[7] On the other hand, markedly different behavior was observed in the str
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