FINITE ELEMENT ANALYSIS OF THE INTERFACE SHAPE IN THE GROWTH OF RAD SILICON RIBBONS

The RAD growth process has been conceived in order to achieve thin and flat silicon sheets readily usable for the fabrication of low-cost solar cells. It is based on the crystallization of silicon films of equal thickness on either side of a carbon ribbon which is pulled upwards and vertically through a silicon melt. The carbon ribbon used in this process has a primary function which is to shape and stabilize the meniscus rising from the melt. It also has a definite influence on the shape of the freezing isotherm. This latter aspect is the purpose of this article. It is shown that the carbon ribbon eventually favours the formation of an extended growth front whose concavity is oriented downwards. This property was established on the basis of a two-dimensional finite-element thermal analysis. The predictions of the calculations are fairly well corroborated by experimental observations regarding the growth front shape and the relationship between key growth factors, e.g., pull rate and ribbon thickness, despite the simplifying assumptions made. The inclination of the growth front on the horizontal makes the RAD process intermediate between vertical and horizontal ribbon growth processes in terms of (i) the extraction of the latent heat of solidification and (ii) the built-in residual stresses in the silicon layers.