Preferred grain orientations in silicon ribbons grown by the string ribbon and the edge-defined film-fed growth methods

Silicon ribbons for photovoltaic applications grown under typical industrial processing conditions by the String Ribbon and the Edge defined Film fed Growth (EFG) methods were quantitatively analyzed by newly developed scanning technologies with respect to the grain structure and orientation. As a result the grain structure consists typically of elongated grains with a < 2 1 1 > orientation nearly parallel to the growth direction and a {1 1 0} ribbon surface. These grains are mainly separated by Sigma 3 twin boundaries which are nearly perpendicular to the {1 1 0} ribbon surface. This result is found to be independent from the orientation of seed crystals and is in agreement with earlier studies on silicon ribbon growth. The experimental observations will be explained by a growth model which considers the surface energies of the growing grains and the need for undercooling in front of the phase boundary.