Biaxial Texturing of Inorganic Photovoltaic Thin Films Using Low Energy Ion Beam Irradiation During Growth
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Biaxial Texturing of Inorganic Photovoltaic Thin Films Using Low Energy Ion Beam Irradiation During Growth James R. Groves 1,2, Garrett J. Hayes 1, Joel B. Li 4, Raymond F. DePaula 2, Robert H. Hammond 3, Alberto Salleo 1 and Bruce M. Clemens 1 1 Department of Materials Science and Engineering, Stanford University 2 Superconductivity Technology Center, Los Alamos National Laboratory 3 Geballe Laboratory for Advanced Materials 4 Department of Electrical Engineering, Stanford University
ABSTRACT We describe our efforts to control the grain boundary alignment in polycrystalline thin films of silicon by using a biaxially textured template layer of CaF2 for photovoltaic device applications. We have chosen CaF2 as a candidate material due to its close lattice match with silicon and its suitability as an ion beam assisted deposition (IBAD) material. We show that the CaF2 aligns biaxially at a thickness of ~10 nm and, with the addition of an epitaxial CaF2 layer, has an in-plane texture of ~15°. Deposition of a subsequent layer of Si aligns on the template layer with an in-plane texture of 10.8°. The additional improvement of in-plane texture is similar to the behavior observed in more fully characterized IBAD materials systems. A germanium buffer layer is used to assist in the epitaxial deposition of Si on CaF2 template layers and single crystal substrates. These experiments confirm that an IBAD template can be used to biaxially orient polycrystalline Si. INTRODUCTION Solar cell efficiency is a strong function of minority carrier lifetime, since photogenerated carriers that recombine before reaching the p-n junction do not contribute to photocurrent. Grain boundaries in polycrystalline silicon films provide electron traps that act as recombination centers that reduce minority carrier lifetimes [1]. This recombination is a function of the grain boundary structure. In particular, the high dislocation density of high angle grain boundaries result in higher recombination rate than low angle grain boundaries. Dimitriadis et al. showed that the effective carrier lifetime increases as the dislocation density decreases [2], and in an elegant experiment using electron beam induced current contrast ratios in polycrystalline silicon films, Seifert et al. showed that recombination is a strong function of grain boundary defect density [3]. Grain boundaries can be described as having both out-of-plane and in-plane misorientation known as tilt and twist, respectively. Both types of misorientation result in defect densities that lead to recombination. The degree of tilt and twist in a thin film grain boundary population reflects the crystallographic texture of the film. Biaxial texture, which has a preferred crystallographic direction for both out-of-plane and in-
plane directions, can decrease both twist and tilt misorientation between grains. One way to develop biaxial texture is application of an ion beam during the initial stages of nucleation of a thin film. This ion beam assisted deposition (IBAD) process uses a low
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