Effects of Film Interfaces on the Properties of Poly-Si Grown by the Metal-Induced Technique for Solar Cell Applications
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EFFECTS OF FILM INTERFACES ON THE PROPERTIES OF POLY-SI GROWN BY THE METAL-INDUCED TECHNIQUE FOR SOLAR CELL APPLICATIONS Chunhai Ji and Wayne A. Anderson University at Buffalo, The State University of New York, Dept of Electrical Engineering, Buffalo, NY 14260
ABSTRACT In the metal-induced growth (MIG) process, the poly-Si layer hetero-epitaxially grows from a thin silicide layer, formed by reaction of a metal seed-layer and sputtered silicon, due to an extremely close lattice match between silicon and the metal silicide. The produced poly-Si has shown a promising device quality for photovoltaic applications. Recent results show that the interface of the silicide and poly-Si has a significant effect on the properties of the poly-Si which works as an active layer for photon absorption. In the study of the MIG process, two metals were used as a seedlayer, i.e. Ni and Co. Although CoSi2 has a larger lattice mismatch with Si (1.2%) than does NiSi2 (0.4%), the poly-Si growing from Co has a smoother interface between the poly-Si and silicide, while the one for the Ni seed-layer samples is rather rough. Backscattered XSEM shows that the Ni-contained phase extended into the Si layer by forming long spikes. This might cause crystal defects in the Si layer. The Auger depth profile also showed that the Ni atoms diffuse into the Si layer much more than does the Co. This kind of difference in interface structure causes the different properties of the poly-Si layer. X-ray diffraction (XRD) analysis on the Si layer showed that the Co seedlayer sample had a predominant growth orientation of (220) and the FWHM of 0.2°. The Ni seed-layer samples grew mainly in both and direction, with FWHM of 0.3° and 0.4°, respectively. By comparison, the poly-Si from the Co seed-layer had a higher carrier lifetime of 0.458µs compared to 0.305µs from Ni. INTRODUCTION Thin-film crystalline Si materials are likely to replace the bulk crystalline Si in many areas which include photovoltaic applications. Physically, a thin-film Si cell offers reduced bulk recombination leading to lower dark current, higher Voc and high fill factor. It also offers potential for low-cost cells, lightweight photovoltaics, lower energy consumption for device fabrication and potential for flexible solar cells [1]. Normally, increasing the poly-Si grain-size tends to increase the solar cell efficiency. Methods of grain enhancement of poly-Si include solid-phase crystallization, zonemelting recrystallization, laser-induced recrystallization and metal-induced crystallization (MIC) [1]. The major limitation of the MIC process is the high metal concentration in the grain boundaries of poly-Si which cause the shunting problem. In our metal-induced growth process, the sputtering deposition of Si was carried-out at relatively low temperature (525°C~625°C) on a previously evaporated Ni or Co metal layer. At elevated temperature, these two metals first react with the Si atoms to form NiSi2 or CoSi2. Thereafter, the Si tends to form poly-Si by heteroepitaxially growing on the
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