Glow Discharge Produce Amorphous SiC:H Thin Films
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GUM DISCHAR
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US SiC:H THIN FILMS
N.T. TRAN, D.J. OLSEN, R.I. PATEL and K.A. EPSTEIN Electronic and Information Sector Laboratories, St. Paul, MN 55144
3M
Company,
ABSTRACT Amorphous hydrogenated silicon carbide alloy films (a-SiC:H) were produced by the decomposition of methane and silane in a glow discharge deposition system. A deposition rate of +13 sý/sec was achieved for good quality films. Amorphous SiC:H films of p type and n type of a band gap of 1.86 eV and 1.8 eV and the activation energy of 0.4 eV+ and 0.23 eV, respectively were obtained. Results show that p type and n type a-SiC:H films can be good window layers and good diffusion barriers for indium in polyimide/metal/n-i-p(p-i-n)/ITO amorphous silicon solar cells.
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Amorphous hydrogenated silicon carbide alloy films (a-SiC:H) are of interest as an example of tetrahedrally bonded amorphous films; and their current application to amorphous silicon solar cells [1-2]. The material is useful for the window layer because of its excellent optical transpasency, combined with its good photoconductivity. An 11% efficiency for 1 cm areý has been reported by several laboratories for p-i-n structure with p a-SiC:H film as a window layer on glass covered with transparent electrically conductive film [3-4]. Amorphous SiC:H films can be fabricated by the plasma deposition of silane and hydrocarbon gas mixtures [5-6]. The optical and electrical properties and the chemical bonding structure of the films depend strongly upon both carbon sources and deposition conditions. Hamakawa and his co-workers [7] reported that the methane based a-SiC:H films exhibit better window material properties than ethylene based a-SiC:H films. In this report, we have conducted a series of experiments on the electrical and optical properties of a-SiC:H films which were prepared by the decomposition of methane (CH4 ) and silane (SiH ). The films are intended as a window layer and indium diffusion barrnr in both p-i-n and n-i-p amorphous silicon solar cells on polyimide substrate.
Deposition of a-SiC:H from silane and methane were done in a RF glow deposition system. Details of the0 system have been described elsewhere [8]. The substrate temperature was 250 C. Films were prepared at RF power inputs between 5 - 15 watts and pressures between 0.5 +- 1.0 +Torr. For the intrinsic films, pure SiH and CH were used; for p and n a-SiC:H films, B H mixed in He and PH3 In He wera used. Composition of carbon in a-SiC:H fHlis was controlled by varying the methane fraction in the gas mixture. Amorphous SiC:H films were deposited on Corning glass substrates for optical and electrical measurements and on crystal Si for infrared measurements. The film thicknesses were obtained from interference fringes in the visible and with Dektak II stylus profilometer. The Swanepoel method [9] was used to calculate the refractive index n, the extinction coefficient k, and the absorption coefficiT}ý a. Optical band gaps were obtained by extrapolation of a plot of (aE) versus E where E is the
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