Substrate temperature: A critical parameter for the growth of microcrystalline silicon-carbon alloy thin films at low po
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Substrate temperature: A critical parameter for the growth of microcrystalline silicon-carbon alloy thin films at low power Arup Dasgupta, S. C. Saha, and Swati Raya) Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta-700 032, India
R. Carius ISI-PV, Forschungszentrum, D-52425 J¨ulich, Germany (Received 5 February 1998; accepted 27 January 1999)
P-type microcrystalline silicon-carbon alloy thin films have been prepared at low power by employing radio-frequency plasma-enhanced chemical vapor deposition (rf-PECVD) technique; judicious choice of deposition parameters is necessary. Substrate temperature has been observed to be the most critical parameter, while high hydrogen dilution is necessary but not a sufficient condition for obtaining crystallinity in silicon-carbon alloy thin films. Best microcrystallinity at moderate power density (78 mWycm2 ) has been obtained at a fairly low substrate temperature (180 ±C). The highest conductivity of 5.7 Scm21 of a boron-doped microcrystalline sample could be achieved. Incorporation of carbon in these films has been confirmed from x-ray photoelectron spectroscopic (XPS) studies. Carbon is, however, incorporated only in the amorphous phase while the crystallites are of silicon only as observed from Raman spectra.
I. INTRODUCTION
Boron-doped microcrystalline silicon-carbon alloy thin films (mc-Si : C : H) have wide optical gap and high electrical conductivity. These materials find application in various photovoltaic devices, namely as top or window layers1 and in one or more internal tunnel junctions of multijunction solar cells.2 It has been widely observed that mc-Si : C : H thin films can be grown at very high power densities (viz., .100 mWycm2 ) (undesirable for device applications), by conventional radio-frequency (13.56 MHz) plasma enhanced chemical vapor deposition (rf-PECVD) technique.3–5 Hamakawa et al.6 have reported fairly high conductivity for such films (,10 Scm21 ) using electron cyclotron resonance chemical vapor deposition (ECR-CVD) technique. Although this technique is free from radiation damage, it is not yet commercially viable because of problems like nonuniform deposition over a large area together with the requirements of a large number of parameters, such as microwave excitation mode, magnetic field, and substrate bias, which make the technique very complex. Recently, Fl¨ukiger et al.7 have reported conductivity ,1021 Scm21 prepared at a fairly low power density of ,37.6 mWycm2 in a PECVD system by employing a very high frequency of 70 MHz. Once, again, large area deposition is difficult in this technique owing to nonuniformity. We had also reported earlier highly conducting mc-Si : C : H thin films by producing submicron size crystallites in an amorphous matrix using a)
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J. Mater. Res., Vol. 14, No. 6, Jun 1999
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a rather low energy deposition process, namely pho
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