Effects of Deposition Parameters on the Structure and Photovoltaic Performance of Si Thin Films by Metal Induced Growth
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Effects of Deposition Parameters on the Structure and Photovoltaic Performance of Si Thin Films by Metal Induced Growth Peter T. Mersich1, Shubhranshu Verma1, Wayne A. Anderson1, and Rossman F. Giese Jr.2 1 Department of Electrical Engineering, University at Buffalo, State University of New York, Buffalo, NY 14260, U.S.A. 2 Department of Geology, University at Buffalo, State University of New York, Buffalo, NY 14260, U.S.A.
ABSTRACT A metal-induced growth (MIG) process was employed to deposit thin films of microcrystalline silicon (µc-Si) for solar cell applications. Due to different grain orientations of the crystals, the absorption coefficient of µc-Si is about 10 times higher than the absorption coefficient of single crystalline Si. The properties of the Si film were investigated resulting from variations in several parameters. A range of Ni and Co thicknesses were examined from 7.5 nm to 60 nm including combinations of the two, while the dc sputtering power was stepped up from 150 W to 225 W. The structure of the resulting film was studied using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD). SEM of the film revealed that 5 hr of Si deposition at 150 W yields a film thickness of 6.5 µm and a maximum grain size of about 0.6 µm. EDS data showed that at the middle of the Si film the atomic percentage of the Si was 99.17%. XRD data showed that the dominant crystal orientation is {220}. To characterize the photovoltaic properties of the µc-Si, Schottky photodiodes were fabricated. Ni alone as the seed layer resulted in ohmic behavior. With Co only, MIG formed a rectifying contact with open-circuit voltage (Voc). The combination of Co layered over Ni formed better thin films and gave a Voc of 0.24 V and short-circuit current density (Jsc) of 5.0 mA/cm2 since the Co prevents Ni contamination of the top of the grown Si layer. INTRODUCTION Amid the escalating energy crisis, there has been greatly expanded interest in establishing a viable, low-cost approach to solar cells [1]. While traditional wafer-based crystalline silicon solar cells are able to achieve relatively high efficiencies, this is easily outweighed by the cost of manufacturing. Therefore, many have turned to thin film solar cells [1]. Thin films not only have the advantage of much cheaper fabrication but also utilize far less material. This is made possible through the use of µc-Si, which has much higher optical absorption when compared to single crystal silicon due to different crystal orientations in the grains [2]. However, the effects of grain boundaries must then be taken into account, where larger grain sizes are desirable to minimize the adverse effects [3]. Metal-induced growth (MIG) is a method used to epitaxially grow thin layers of µc-Si [4]. Metal-induced crystallization (MIC) uses the two step process of depositing amorphous Si
and later converting it to crystal form [5]. MIG has the advantage of a one step process to form the µc-Si and simultaneously form the back ohmi
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