Poly-Si Thin Film Formation Using a Novel Low Thermal Budget Process
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Poly-Si Thin Film Formation Using a Novel Low Thermal Budget Process Minghao Zhu1, Yue Kuo1, Chen-Han Lin1 and Qi Wang2 1 Thin Film Nano & Microelectronics Research Laboratory, Texas A&M University, College Station, TX 77843-3122, U.S.A. 2 National Renewable Energy Laboratory, Golden, Colorado 80401, U.S.A.
ABSTRACT Polycrystalline silicon thin films were formed from the amorphous silicon thin film by the pulsed rapid thermal annealing process enhanced with a thin nickel seed layer through the vertical crystallization mechanism. In this paper, authors presented the results on the material properties of the crystallized film. The dopant and film thickness effects were also investigated. It has been demonstrated that a 2 m thick amorphous silicon n+-i-p+ diode structure could be transformed into polycrystalline stack with a 4-pulse 1 sec 850°C heating and 5 sec cooling cycle process. INTRODUCTION Thin film amorphous silicon (a-Si:H) solar cells have advantages over the single crystal silicon solar cells, such as requiring a very small amount of composing materials, low temperature fabrication process, unlimited supply of low-cost materials, and large-area substrate capability [1,2]. Compared with a-Si:H thin film solar cells, the polycrystalline silicon (poly-Si) thin film cells have additional advantages of high conversion efficiency and improved stability [3]. J.-D Hwang et al. reported a high conversion efficiency poly-Si solar cell formed from a-Si without passivation or antireflection layers, with a conversion efficiency of 10.4% [4]. However, the fabrication of the poly-Si solar cells is limited to processes that take a long time or require a high temperature, such as chemical vapor deposition (CVD), solid phase crystallization, or metal induced crystallization [5-10]. A novel, low thermal-budget pulsed rapid thermal annealing (PRTA) process enhanced with a Ni seed layer has been introduced to transform the a-Si thin film into a poly-Si thin film in the horizontal direction, i.e., parallel to the substrate surface [11]. Recently, this method has been used to form the poly-Si film in the vertical growth mechanism, i.e., perpendicular to the substrate surface [1]. In this paper, detailed material properties and process effects on this kind of poly-Si thin film have been investigated. EXPERIMENTAL The intrinsic or doped a-Si:H thin film was deposited in a PECVD system, which has the parallel-plate electrode arrangement driven by a 13.56 MHz RF generator, on the corning 1737 glass substrate pre-coated with the sputter deposited Mo (150 nm) and Ni (1nm) films. The intrinsic a-Si:H film was deposited at a rate of 6 nm/min from SiH4 50 sccm, 80 W, 150 mT at 250°C. The n+ film was deposited at a rate of 4 nm/min from SiH4/PH3 (6.88% in H2)/H2 10 sccm /10 sccm/1,000 sccm, 500 W, 750 mT at 250°C. The p+ film was deposited at a rate of 10 nm/min from SiH4/B2H6 (2% in H2)/H2 10 sccm /20sccm/1,000 sccm, 500W, 750mT at 250°C.
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The n+/intrinsic/p+ a-Si:H thin film stack was deposited sequentially in one-pump down in the
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