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AND IMPROVED MICROCRYSTALLINE ELECTRON CYCLOTRON RESONANCE

Young J. Song and Wayne A. Anderson, State University of New York at Buffalo, Dept. of Electrical Engineering, Amherst, NY 14260 ABSTRACT Amorphous (a-Si) and microcrystalline silicon (jtc-Si) are widely used in photovoltaics and thin film transistors. These products suffer from instability and less than desired electrical properties. We have utilized photon-assisted electron cyclotron resonance chemical vapor deposition (PA-ECRCVD) resulting in great improvement in both areas. For example, PAECRCVD compared with conventional ECR-CVD gives carrier lifetime of 1.35 4ts compared to 0.17 ls, and photovoltaic solar cell efficiency of 10 % compared to 5.9 %. Moreover, the PAECRCVD cell only degraded to 9.8 % compared to the ECR-CVD cell degradation to 5.5 %, under long term exposure to tungsten lamp illumination. In addition, PA-ECRCVD gives much enhanced crystallinity in lic-Si as revealed by atomic force microscopy and Raman spectroscopy. INTRODUCTION Innumerable efforts to improve the quality of hydrogenated amorphous (a-SifH) and microcrystalline silicon (ltc-Si) films have introduced various growth techniques other than standard rf-driven plasma enhanced chemical vapor deposition (PECVD). Since the hydrogen related profile in the films (e.g. hydrogen content and hydrogen-silcon chemical bonding) plays a key role in electro-optical and structural properties as well as stability of the films [1-2], most of the growth techniques have been developed to provide an optimal hydrogen configuration. Among the techniques, hot wire chemical vapor deposition (HWCVD) [3], which utilizes a thermal decomposition of silane (Si0 4 ), and high density plasma deposition techniques, such as very high frequency plasma CVD (VHF-CVVD) [4] and electron cyclotron resonance CVD (ECRCVD) [5] have been known to provide better control of hydrogen evolution in the film. The silicon film deposited by these methods normally reveals superior properties due to better microstructure and lower hydrogen content. In this paper, we have introduced a photon-assist (PA) process into the ECR-CVD for the purpose of further control of hydrogen because the PA process during the deposition may modify both gas phase in the plasma and chemical reaction mechanism in the growth zone. We have studied the effects of the PA process both on a-Si:H and ltc-Si films, in terms of electro-optical, structural and photovoltaic properties as well as stability. EXPERIMENT All silicon films used in this work were deposited by microwave (2.45 GHz) ECR-CVD both with and without the PA process. Its schematic diagram is described elsewhere [6]. The reactant gas was a 2% SiH4/He mixture, while a minimum amount of Ar was used for plasma generation. The deposition of the a-Si:H film was done without H 2 dilution at a substrate temperature of 250 'C, a chamber pressure of 10 mTorr, an input power of 400 W, and a deposition rate of 70 - 80 A/min. Meanwhile, the /ic-Si film was grown with H2 dilution (Ri = 0.33, where RH = H 2/