Deposition of Polysilicon Films by Hot-Wire CVD at Low Temperatures for Photovoltaic Applications
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ABSTRACT Polysilicon (poly-Si) thin films have been obtained using hot-wire chemical vapor deposition (H-WCVD) from silane-hydrogen mixtures. The films were prepared at low substrate temperatures (down to 200 C) and at very high deposition rates (up to 40 A/s). They showed good crystalline properties and no amorphous phases were detected. The films can also be efficiently doped by adding diborane or phosphine to gas phase. In this paper, an overview of the properties of the poly-Si films, intrinsic and p and n-doped, deposited at our laboratory by HWCVD is presented and discussed. The properties of the material and the features of the deposition technique which are interesting for their application in photovoltaics are emphasized.
INTRODUCTION Thin film silicon based solar cells are obtained from a large variety of deposition techniques, which produce materials from amorphous to polycrystalline silicon. The latter can present a variety of structures, with grain sizes ranging from few nm to hundred /Am. The large grain poly-Si has been used to produce solar cells with efficiencies exceeding 10%, by using a high temperature process [1]. In addition to the well known a-Si:H solar cells, cells based on nm size poly-Si have been obtained with low temperature processes, presenting efficiencies near 4% [2]. The search for technological processes allowing the deposition of larger crystals at low temperatures is a clue to the development of high efficiency and low cost silicon solar cells. The large crystal size helps to increase the mobility of the carriers and the low temperature process allows films to deposit on cheap substrates. The hot-wire chemical vapor deposition (HWCVD) technique has been used to deposit high quality hydrogenated amorphous silicon at low temperature [3]. The high deposition rate, together with the improvement of the electronic properties and stability of the films obtained, make this technique very attractive in comparison to the well established glow discharge technique. Besides these good achievements, the HWCVD has appeared recently as a technique that allows the preparation of polycrystalline silicon films in the micrometer crystal size at moderate substrate temperature and with high deposition rates [4,5]. In this paper we present the morphological, structural, electrical and optical properties of intrinsic and doped samples deposited in our laboratory by the HWCVD technique. According to these properties, the possibility of using this technique for depositing solar cells is discussed.
63 Mat. Res. Soc. Symp. Proc. Vol. 377 0 1995 Materials Research Society
EXPERIMENTAL PROCEDURES Several poly-Si films were prepared in a HWCVD reactor described elsewhere [5]. Dissociation of the gas was achieved by a tungsten filament (1 mm thick and 150 mm long, folded to cover the whole sample surface), which was heated by a 300 W AC power (50 Hz). Its temperature in these experiments was set to 1600*C as measured by an optical pyrometer. The distance from the filament to the substrates (d) was varied b
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