Amorphous-to-Microcrystalline Silicon Transition in Hot-Wire Chemical Vapor Deposition
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ABSTRACT The conductivity and the structural properties of thin films deposited by Hot-Wire Chemical Vapor Deposition (HW-CVD) from silane and hydrogen at a substrate temperature of 220 'C are shown to be strongly dependent on the filament temperature, Tfil, and process pressure, p. Amorphous silicon films are obtained at low pressures, p < 3 x 10-2 Torr, for Tf il - 1900 0C and FH2 -= FSiH4 . At this T il, high deposition rates are observed, both with and without hydrogen dilution, and no silicon was deposited on the filaments. At Tfil - 1500 0C, a transition from a-Si:H for p > 0.3 Torr to microcrystalline silicon (/Lc-Si:H) for p _ 0.1 Torr occurs. In this temperature regime, silicon growth on the filaments is observed. pc-Si:H growth both without hydrogen dilution and also in very thin films (-, 0.05 1um) is achieved. Raman and X-Ray spectra give typical grain sizes of 10 - 20 nm, with a crystalline fraction higher than 50%. For both, Tfil - 1500 0C, p > 0.3 Torr and Tfil '- 1900 'C and p - 2.7 x 102 Torr, an increase of the crystalline fraction from 0 to - 30% is observed when the hydrogen dilution, FH2 /FsiH4 , increases from 1 to > 4.
INTRODUCTION Microcrystalline silicon, lic-Si:H, films have received considerable attention because of important properties such as their high electrical conductivity and high optical band gap as compared to a-Si:H[1, 2]. Microcrystalline silicon prepared by rf-glow discharge is usually deposited using high hydrogen dilution and high rf power[3]. In HW-CVD the feed gases are decomposed on the surface of a heated wire. The resulting fragments act as precursors of possible gas-phase reactions and thin-film deposition. The HW-CVD method has been successfully applied to the deposition of fic-Si:H[4] and polysilicon films[5, 6]. The influence of the silane flow rate[4] and T5 ,b[6] on the structural properties of the /c-Si:H have been studied. In this paper the influence of Tfil, p and hydrogen dilution on the conductivity, grain size and crystalline fraction are reported.
EXPERIMENTAL PROCEDURES A turbomolecular-pumped deposition system with a base pressure of 10' Torr is used. The axis of the gas inlet into the reactor is perpendicular to the plane of the substrate holder and to the plane of the filaments. Resistive heating is applied to the back of the substrate holder by a PID controller using the measured substrate temperature as reference. The substrate holder is also water cooled. The deposition temperature is monitored by a thermocouple sandwiched between two glass slides, placed near the substrate. A pure W 57 Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
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wire of 0.25 mm diameter and a total surface area of 1.5 cm 2 was used as the filament. Four filaments are coiled and placed 3 cm below the substrate and are re
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