Impact of rapid thermal annealing and hydrogenation on the doping concentration and carrier mobility in solid phase crys
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Impact of rapid thermal annealing and hydrogenation on the doping concentration and carrier mobility in solid phase crystallized poly-Si thin films A. Kumar, 1, 2 P.I. Widenborg, 1 2 H. Hidayat, 1, 2 Qiu Zixuan1 and A.G. Aberle1, 2 1. Solar Energy Research Institute of Singapore, National University of Singapore, Singapore 2. Department of Electrical and Computer Engineering, National University of Singapore, Singapore ABSTRACT The effect of the rapid thermal annealing (RTA) and hydrogenation step on the electronic properties of the n+ and p+ solid phase crystallized (SPC) poly-crystalline silicon (poly-Si) thin films was investigated using Hall effect measurements and four-point-probe measurements. Both the RTA and hydrogenation step were found to affect the electronic properties of doped poly-Si thin films. The RTA step was found to have the largest impact on the dopant activation and majority carrier mobility of the p+ SPC poly-Si thin films. A very high Hall mobility of 71 cm2/Vs for n+ poly-Si and 35 cm2/Vs for p+ poly-Si at the carrier concentration of 2×1019 cm-3 and 4.5×1019 cm-3, respectively, were obtained. INTRODUCTION With the increase in natural disasters and serious environmental problems such as global warming, it has been never more apt to look into alternative clean energy sources which are not detrimental to the environment. Out of the various renewable energy sources available, solar energy has attracted significant attention in the past two decades and has grown at a substantial rate of about 30-40% per annum. More than 90% of today’s photovoltaic market is dominated by crystalline silicon (c-Si) wafer solar cells [1]. However this technology is material and energy intensive, which acts as bottleneck in further cost reduction. High efficiency thin film module is an effective alternative technology. Thin-film polycrystalline silicon (poly-Si) is a promising semiconductor material for photovoltaic (PV) devices. The thin-film poly-Si PV technology was pioneered by Sanyo in the 1990s. The company achieved an efficiency of 9.2% for small-area (1 cm2) solar cells fabricated on metal substrates [2]. Pacific Solar (now CSG Solar) then developed poly-Si on glass and achieved a record efficiency of 10.4% for a 94-cm2 mini-module [3]. It is possible to improve the efficiency of poly-Si thin film solar cell modules further by optimizing the doping concentration and improving the thin film electronic properties such as majority carrier mobility and minority carrier diffusion length. There are various fabrication techniques to fabricate the poly-Si thin films, such as liquid phase crystallization [4], vapor phase processing [5] and laser induced crystallization from amorphous silicon [6]. However, these processes require high process temperatures and, hence, limit the substrate choice. In this work we investigate poly-Si thin film prepared by solid phase crystallization (SPC) [7] of hydrogenated amorphous silicon (a-Si:H). The SPC method has many features suited towards low cost fabrication process of solar ce
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