Influence of the Decoration by Dislocations on Grain Boundary Passivation by Hydrogen in Silicon
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INFLUENCE OF THE DECORATION BY DISLOCATIONS ON GRAIN BOUNDARY PASSIVATION BY HYDROGEN IN SILICON L. AMMOR, G. MATHIAN AND S. MARTINUZZI Laboratoire de Photo~lectricit6 des Semi-conducteurs, Facultc des Sciences et Techniques de Saint-J6r~me, Universit6 d'AixMarseille III, F-13397 Marseille Cedex 13 ABSTRACT In large grained polycrystalline silicon, the recombination activity of G.B.'s and their passivation by hydrogen is found to be dependent on the decoration by dislocations. Dislocations appear to be preferential paths for in-diffusion, at a depth of a few hundreds of pim's. Similar enhancements of diffusion and passivation exist in grains around dislocations. INTRODUCTION It is actually admitted that hydrogen is able to passivate bulk recombination centres in polycrystalline silicon wafers and solar cells, and consequently to increase minority carrier diffusion lengths, photocurrent and photovoltage. Several technics can be used to hydrogenate the samples like anneals in plasmas, low energy ion implantations and anneals in gas flows [1-5]. This last method was used in spite of the fact that hydrogen cannot be adsorbed at the surface of silicon single crystals, because it appeared that the emergences at the surface of dislocations and grain boundaries (G.B.'s) could constitute adsorption sites and diffusion pipes
[5,6].
In the materials and in the cells, it was verified that extended crystallographic defects, like G.B.'s and dislocations are passivated, and that point defects or impurities can also be neutralized [4,8). Consequently the materials are improved because the interfacial recombination velocity S of G.B.'s and the recombination activity of intragrain defects are reduced. Generally, papers devoted to the passivation of defects by hydrogen conclude that the penetration depth and consequently passivation is limited to a few tens of um's below the surface. However it was suggested that dislocations favour drastically the in-diffusion of hydrogen [7]. As they constitute the most active intragrain defects [5], and as the values of S are high 5 ( o10 cm.s 1 ) at decorated G.B.'s [9], dislocations play a key role in the recombination and passivation phenomena. In the present paper we show by means of light beam induced current (LBIC) scans that in polycrystalline silicon, the depth and efficiency of passivation is enhanced at G.B.'s provided they are decorated by dislocations, and that a similar effect is observed in grains, around dislocations. EXPERIMENTAL The investigated materials were P-type CZ bicrystals (Z 9 and 2 13 ; boron c6ncentration z 1016 cm- 3 ; interstitial Mat. Res. Soc. Symp. Proc. Vol. 106. ý 1988 Materials Research Society
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oxygen concentration 1oile•1018 cm-3), and large grained P-type wafers cut out of cast ingots (Wacker ; Polyx ; thickness 400 Ym ; grain size >1 mm ; boron concentration • 10 1 6 cm- 3 ," interstitial oxygen concentration O.l < 5x1017 cm3; substitutional carbon concentration ICsl 5xi01 7 cm-3 ). Samples of 2 x 1 cm2 were cut from photocells made by PHOTOWATT by me
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