High-speed growth of FZ silicon for photovoltaics

Floating Zone (FZ) silicon can be grown with pull speeds comparably higher than for CZ silicon because the heat is dissipated by radiation into the colder surroundings. In 1955, Billig analyzed theoretically the heat balance of crystals growing under FZ-Iike conditions and computed approximately the maximum of the crystallization rate v(c) as a function of the crystal radius regardless of the crystal structure. In this work we have experimentally tested how fast FZ silicon crystals of diameters up to 150mm can grow dislocation-free. The main result is that typically at least 85% of the theoretical v(c) value after Billig can be reached before the crystal dislocates, e.g. v(c) = 3,8mm/min for a crystal of 125mm diameter. The deflection of the crystallization interface evaluated from the Lateral Photovoltage Scanning (LPS) striation pattern increases with v(c). When approaching the critical pull rate, the interface shows symmetry deviations indicating instability. Touching of non-molten core parts of the feed rod onto the growing interface could frequently be observed when the structure got lost. The minority carrier lifetime tau is a main quality parameter for silicon solar cells. Ciszec found for thin crystals, that tau increases with a faster pull rate. On the other hand for high growth rates, the segregation coefficients are enhanced for impurities with recombination activity, too. We observed a stable or weakly rising tau with v(c). After the sample surfaces were carefully passivated, reasonably high lifetimes could be measured.