Quasicrystalline Phase of Silicon on Glass
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Quasicrystalline Phase of Silicon on Glass
A. R. Middya1,2 and Kartik Ghosh3 1 3
2
Department of Physics, Syracuse University, Syracuse, NY 13244 - 1130
Physics, Astronomy and Materials Science Department, Missouri State University, Springfield, MO 65804
Present address: Silicon Solar, Inc., Fremont, CA 94536
ABSTRACT In this paper, we report new phase of crystalline silicon, quasicrystalline silicon thin-film on glass substrate. The surface topography of these films reveal simultaneous existence of sixfold and fivefold symmetry. We found an array of quasi-unit cell in 2-D that formed quasicrystalline solid. This is first time demonstration of quasicrystalline for single element, silicon (Si). Raman spectra suggests that we found crystalline silicon structure on glass substrate that is not singlecrystal silicon (c-Si) but very close to c-Si.
INTRODUCTION Polycrystalline silicon thin-film is an important semiconductor for application in solar cells as well as microelectronics [1]. Hot wire chemical vapor deposition (hot-wire CVD) technique is a promising technique for developing hydrogenated amorphous silicon (a-Si:H) and polycrystalline silicon (poly-Si) thin-film on glass and other foreign substrates [2, 3]. The hotwire CVD technique became popular in past 15 years in the field of poly-Si thin-film because these films show much improved electronic properties (high doping activation) as well as inactive grain boundaries [4]. This is one of the wonders of condensed matter physics that people succeeded to develop polycrystalline solid having no grain boundary (GB) by hot-wire CVD technique. In this technique, process gases like silane (SiH4) and hydrogen (H2) are decomposed by heated filament (tungsten or tantalum) maintained at 1400°C to 1800°C and thereby, produces Si radicals and high density of atomic hydrogen (H). It was possible to have diamond thin-film at laboratory as low as 650°C substrate temperature because catalytic decomposition of high density of atomic hydrogen (H) from molecular hydrogen (H2). Scientists and technologists took advantage of large abundance of atomic hydrogen (H) to produce insulators and low-k dielectrics having better optical properties [4]. In this work, we shall report on development of quasicrystalline phase of silicon on glass substrate at low temperature (250°C) by hot-wire CVD technique.
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EXPERIMENT In this report, the hot-wire CVD system was built within a RF PECVD system (Oxford Instruments). The tungsten filament was mounted on a rectangular ceramic frame, shown below [Fig. 1(a)]. The glass or other substrates are placed at the center of bottom rectangular ceramic frame.
Fig. 1(a): Ceramic filament holder.
Fig. 1(b): Confinement of light within parallel plate.
The distance between top frame (filament plane) and bottom frame can be adjusted with spacer. The filament temperature is measured by optical pyrometer (Lead & Thrope). The deposition conditions during preparation of polycrystalline and quasicrystalline silicon samples, are as follows: silane (SiH4) and
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