The Crystalline Quality of Epitaxial Si Layers Solution Grown on Polycrystalline Si Substrates
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andVII, G.Wagner* A.Voigt*, W.Dorsch*, H.P.Strunk* M.Albrecht*, B.Steiner**, Th.Bergmann, Universitait Erlangen-NOmberg, Institut ffir Werkstoffwissenschaften, Lehrstuhl -Mikrocharakterisierung-, Cauerstr. 6, D-91058 Erlangen, F.R.G. Institut ffir Kristallzaichtung Berlin, Rudower Chaussee 6, D-12489 Berlin, F.R.G. ABSTRACT
We investigate the crystalline and electrical quality of thin layers epitaxially grown on polycrystalline substrates from metallic solution by the method of electron beam induced current, transmission electron microscopy and etching experiments. We observe a reduced recombination strength of dislocations and small angle grain boundaries, i.e. an improved electrical quality of the epitaxial layer compared to the substrate. The improved quality can be attributed (i) to an altered structure of grain boundaries and dislocations and (ii) to a reduced defect density in the epitaxial layer. 1. INTRODUCTION
For solar cell applications thin polycrystalline films (10-50 gim thickness) on a low-cost substrate like silicon sheet material [1], glass [2,3] or ceramics are a promising alternative to thick single crystalline or polycrystalline wafer material. Growth of a layer on substrates such as glass or ceramics require a seed layer, wich is ingeneral polycrystalline and thus is characterized by a large number of grain boundaries and dislocations. However photovoltaic applications require a sufficiently large grain size and a low intragrain defect density in such films, in order to assure a minority carrier diffusion length comparable with the thickness of the silicon layers. Growth of an epitaxial layer from a solution near thermodynamical equilibrium on a cheap polycrystalline substrate can reduce the defect density as compared to that of the substrate and thus be used to optimize the electrical parameters such as minority carrier diflbsion length and open circuit voltage for solar cell applications. As an example, we investigate the growth of Si layers from In solution on a multicrystalline cast Si-substrate. Werner et al. have shown that it is possible to produce high efficiency solar cells from these layers [1]. In this work we focus on the correlation between the structural and electrical properties of these layers. Therefore we compare the electrical activity of identical dislocations and grain boundaries in the epitaxial layer to that of the substrate by the electron beam induced current (EBIC) method and relate these measurements to transmission electron microscopy (TEM) of the relevant defects. We perform etching experiments to compare dislocation densities of the epilayer to those of the substrate. 2. EXPERIMENTAL LPE growth The samples have been grown in a horizontal solution growth system. The epitaxy apparatus 889 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
essentially consists of a graphite slider boat mounted inside a quartz tube which is flowed through by palladium-purified hydrogen and heated by a temperature controlled furnace. The solute indium is saturated b
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