An Expanding Thermal Plasma for Deposition of a-Si:H
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R.J. SEVERENS, G.J.H. BRUSSAARD, H.J.M. VERHOEVEN, M.C.M. VAN DE SANDEN AND D.C. SCHRAM Department of Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands ABSTRACT A remote argon/hydrogen plasma is used to deposit amorphous hydrogenated silicon. The plasma is generated in a DC thermal arc (typical operating conditions 0.5 bar, 5 kW) and expands into a low pressure chamber (20 Pa) thus creating a plasma jet with a typical flow velocity of 103 m/s. Pure silane is injected into the jet immediately after the nozzle, in a typical flow mixture of Ar:H 2 :SiH 4=55:10:6 scc/s. The electron temperature in the jet is low (typ. 0.3 eV): silane radicals are thought to be produced mainly by hydrogen abstraction, but also by a sequence of dissociative charge exchange and consecutive dissociative recombination. In-situ ellipsometry yields refractive indices of 3.6-4.2 at 632.8 nm and growth rates of 1020 nm/s. FuIR analysis yields a hydrogen content of 9-25 at.% and refractive indices of 2.7-3.3 in the infrared. The SiH density decreases with increasing hydrogen content, whereas the SiH 2 density increases. Above 11 at.%, the majority of hydrogen is bonded in the SiH 2 configuration. The optical bandgap remains constant at approximately 1.72 eV. The photoconductivity is of the order 10-6 (92cm)"1 and the photoresponse 106.
INTRODUCTION In cost reduction of amorphous silicon based large area applications such as photovoltaic cells, upscaling is a major issue. A new approach gaining much attention is roll-to-roll deposition on flexible substrates [1], where a high deposition rate is desired. Furthermore, Ganguly and Matsuda have argued that at elevated substrate temperatures (>350 'C), the defect density decreases with increasing deposition rate [2]. As their data only streches to a rate of 2 nm/s, it would be interesting to investigate if the observed trend holds for even higher rates. The method proposed in this contribution is based on convective transport of radicals instead of diffusive, and has been used succesfully for fast deposition of diamond-like coatings [3]. Plasma production and deposition are geometrically separated, and in that sense it is related to the silicon deposition method described by Lucovsky et. al. using an inductively coupled plasma source [4,5] or the microwave cavity plasma source described by Johnson et. al. [6,7]: remote plasma enhanced chemical vapour deposition.
EXPERIMENTAL SETUP The setup consists of a DC thermal arc plasma source and a low-pressure chamber (Fig. 1). A substrate holder, on which c-Si and Corning glass 7059 samples of approximately 2.5x2.5 cm 2 are attached, is mounted on a chuck in the deposition chamber through a load lock. Temperature is monitored using a thermocouple inserted in a dummy substrate mounted on the chuck. 33 Mat. Res. Soc. Symp. Proc. Vol. 377 ©1995 Materials Research Society
Cascaded arc
Substrate \
Loadlock
•'-Silane
trgon+Hydrogen S\',
Detector
Fig. 1. Expanding plasma deposition setup Plasma Source The cascaded ar
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