Oxygen Effects on Plastic Flow During Growth of Dendritic Web Si
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OXYGEN EFFECTS ON PLASTIC FLOW DURING GROWTH OF DENDRITIC WEB Si J. A. SPITZNAGEL, R. G. SEIDENSTICKER, S. Y. LIEN AND R. H. HOPKINS Westinghouse R&D Center, 1310 Beulah Road, Pittsburgh, PA 15235 ABSTRACT The possible effects of oxygen on plastic flow of Si ribbons during growth by the, dendritic web process are described. Cooling rates preclude homogeneous nucleation and growth of detectable SiO precipitates. Calculated resolved shear stresses on f111 slip systems arising from thermoelastic strains exhibit periodic oscillations in sign and magnitude. The dwell times at elevated temperatures corresponding to zero or low stress periods accompanying such sign reversals are shown to be sufficiently long to permit decoration and locking of dislocations by oxygen atoms. INTRODUCTION Dendritic web Si is a single crystal ribbon formed by the freezing of a thin liquid film drawn up between two dendrites [1]. A thermal gradient in the ribbon is established by a series of lids and shields placed above the susceptor holding the quartz crucible. Stresses in the web, arising mainly from nonlinearities in the temperature profile as the ribbon moves up through the lid and shields, can cause buckling or dislocation generation and plastic flow in the crystal. The webs, typically 3 to 6 cm wide and approximately 150 pm thick, exhibit purity rivaling that of Czochralski Si. OxygT9 cogcentrations are also similar to CZ material and are on the order of 10 /cm . High efficiency photovoltaic devices have been fabricated from such crystals. Since web crystals contain large concentrations of oxygen, they should exhibit mechanical property effects similar to those observed in CZ Si. Although the thickness of the web is comparable with the extent of outdiffusion zones introduced in CZ wafers during normal device processing, the rapid pull rates (1-3 cm/min) inhibit oxygen loss from the (111) faces of the ribbon. Thus, phenomena such as the locking of glide dislocations by oxygen atoms [2] and the possible introduction of dislocation sources through punching of prismatic loops around SiOx precipitates [3,4] must be addressed in modeling plastic flow in the dendritic web. Dislocation (etch pit) densities measured 2 in wPb crystals grown in different lid and shield configurations range from 10 cm- to approximately 5 x 10 cm- . Etch pits are often aligned along directions on the (111) faces of the crystal indicating they originated from slip processes during growth, and dislocation densities appear to correlate with residual stresses in the web. Dislocation effects on minority carrier diffusion length are less clear although impurity gettering, similar to that observed in suitably heat-treated CZ Si, is thought to be important [5]. APPROACH Analysis of plastic flow and related oxygen effects in the Si web is complicated by the presence of twin planes parallel to the ribbon face [1], crystal symmetries which result in simultaneous multiple slip on several slip systems, and the lack of suitable tensile and compressive test data at temperatures ab
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