Low Filament Temperature Deposition of a-Si:H by Catalytic Chemical Vapor Deposition
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LOW FILAMENT TEMPERATURE DEPOSITION OF a-Si:H BY CATALYTIC CHEMICAL VAPOR DEPOSITION P. Brogueira*, S. Grebner**, F. Wang**, R. Schwarz**, V. Chut and J.P. Conde* * Instituto Superior T6cnico, Department of Physics, 1096 Lisboa Codex, Portugal "**Physics Department - E-16, Technical University of Munich, D-8046 Garching, Germany t INESC, Rua Alves Redol 9, 1000 Lisboa, Portugal
ABSTRACT Hydrogenated amorphous silicon,a-Si:1, is deposited from silane (SilH 4 ) and hydrogen (H 2 ) using a tungsten wire at low filament temperatures (Tl, = 1200 'C) by catalytic chemical vapor deposition. The deposition rate increases monotonically with the depositions pressures and shows a maximum at an H2 : Silt4 flow ratio of unity. Vanishingly small deposition rates were observed for silane-only depositions and for 112-to-SiH 4 flow ratios of 2.5 and above. The optoelectronic properties show complex dependence on substrate temperature (Tu~b). Three intervals of T8 ,b with distinct optoelectronic were observed: as T8 ub increases from 180 to 220 'C, the optical bandgap, ETauc increases from 1.9 to 2.4 eV, the dark conductivity,ad, decreases from 1010 to 10-15 - 1 cm-' and the photoconductivity, ah, decreases from 10-- to 10`0 Q'-cm-' (region (i)). As Tu~b increases from 220 to 250 °C, E[Tac decreases to 1.8 eV and the photosensitivity, aphl/ad decreases to - 1 due to an increase of both ad and aCph(region (ii)). Throughout these two regions, the photoconductivity Y factor remains between 0.6 and 0.9 and the activation energy of the dark conductivity, E",0 d, remains between 0.7 and 0.9 eV. Above 250 'C, the aph and ad remain approximately constant at 10-4 I-'cm-' and y decreases to below 0.5 and Ea,,d to - 0.3 eV.
INTRODUCTION Amorphous silicon-based thin-film semiconductors are attractive materials for use in large-area devices such as, solar cells, flat-panel displays and image scanners. The most common method of film preparation has been rf glow discharge. a-Si:H films prepared by rf glow discharge show undesirable metastable effects and low growth rates. Furthermore, the complexity of the plasma reactions severely limits the modeling of a-Si:H deposition in rf glow discharge reactors. Novel deposition methods which show improved film stability, higher deposition rates, and simpler modeling have been proposed and are actively being studied. One such method is the hot-wire (also known as catalytic-CVD or Cat-CVD) technique in which the feed gases break down on the surface of a heated wire. The resulting fragments act as precursors of possible gas-phase reactions and thin-film depositions. Cat-CVD was first introduced by Wiesmann et al.[1] in 1979, but the films obtained were of poor quality and no further attempts to improve the quality of the films were made at the time. Only in the late 1980's, the method was rediscovered by Matsumura[2, 3], Doyle et al.[4] and Mahan et al.[5].These authors reported the production of high-quality a-Si:Hfilms deposited using filament temperatures, T1 it, in the range of 1400 - 1900 'C, with depo
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