The Use of Seed Layers in Hot Wire Chemical Vapor Deposition of Microcrystalline Silicon Films
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The Use of Seed Layers in Hot Wire Chemical Vapor Deposition of Microcrystalline Silicon Films G.A. Zaharias1, A.H. Mahan, R.E.I. Schropp2, Y. Xu, D.L Williamson3, M.M. Al-Jassim, M.J. Romero and L.M. Gedvilas National Renewable Energy Laboratory (NREL), Golden, CO 80401, USA 1 Dept. Chem. Engineering, Stanford University, Stanford, CA 94305, USA 2 Utrecht Univ., Debye Institute, Physics of Devices, 3508 TA Utrecht, The Netherlands 3 Dept. of Physics, Colo. School of Mines, Golden, CO 80401, USA ABSTRACT The effect of thin seed layers on the subsequent growth of thick, high growth rate bulk µc-Si is investigated by XRD, SEM and cross sectional TEM. All layers were deposited by hotwire chemical vapor deposition (HWCVD). When the seed layer as observed by XRD is highly crystalline, made by using high H2 dilution (H2:SiH4 100:1), the amorphous incubation layer typical of µc-Si growth is virtually eliminated. Furthermore, with this seed layer, bulk layer deposition conditions that would otherwise produce highly amorphous material enable a composite film with significant crystallinity. When the seed layer is predominantly amorphous, made using a much lower H2 dilution (10:1), there is evidence that very small crystallites, undetected by XRD, still facilitate immediate nucleation and enable the formation of larger grains in the subsequent bulk layer. In concurrence with other HWCVD results, lowering the filament temperature results in significant improvements in film compactness, photoresponse and grain size, while maintaining significant crystallinity. Such films have been incorporated into high deposition rate solar cells. INTRODUCTION In the quest for efficient and inexpensive materials for solar cells, microcrystalline silicon (µc-Si) has become the focus of much research, showing particular promise as a low bandgap absorber in both single and multi-junction cells. Single junction cell efficiencies have recently exceeded 10% [1], and tandem devices, with one intrinsic layer a-Si:H and the other µc-Si, allow for absorption of a greater % of the solar spectrum than with each layer used individually. In the latter case, µc-Si is an attractive alternative to a-SiGe:H, possessing greater stability under sunlight and the ability to be deposited without using highly toxic germane gas. Two aspects of this material, however, make its incorporation into a successful commercial device structure technologically challenging. First, as thick absorber layers are needed for efficient light collection, deposition rate (Rd) issues assume an increased importance. In particular, most of the previously mentioned single junction devices have been deposited at very low Rd, thus limiting potential device throughput in factories. And second, unless the evolutionary growth of this layer is carefully understood and controlled, devices exhibiting poor performance can quite easily be generated. In particular, it is well known that non-epitaxial growth of µc-Si usually contains an amorphous incubation layer. Such a layer exhibits inferior charge t
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