Effect of Twin Growth on Unidirectional Solidification Control of Multicrystal Silicon for Solar Cells

Abstract

The solidification microstructure and crystal orientation have been investigated for solar cell grade high purity multicrystal silicon through a unidirectional solidification technique. The mechanism of the twin growth on a reentrant corner has been also discussed. A columnar structure is observed at solidification velocities of 1.25-30 mm/s and positive temperature gradient of 20 K/cm in the rod-like silicon specimens in an electric resistance furnace. In the solidification velocity range of 1.25-2.5 mm/s, the grain size enlarges as solidification progresses. Furthermore, large columnar grains contain many twin boundaries. However, the average grain size decreases as the solidification velocity increases and above the critical velocity around 40 mm/s, equiaxed structure appears at the central part of specimens. Therefore, molten silicon must be solidified at the velocity below 2.5 mm/s where twins are always introduced into grains to obtain large columnar crystal grains. The undercooling for directional growth is less than 4 K in the solidification velocity range of 1.25-30 mm/s. A model of two-dimensional nucleation on the reentrant corner was established, and the critical nucleus could be estimated to be 70 to 80% of the radius of the general two-dimensional nucleus. The nucleation undercooling on the surface containing twins also decreased to 70% of the general undercooling. The reduction of the critical radius and undercooling on the reentrant corner could eventually influence on the priority growth direction and the enlargement of the grain size.

{1 1 1} facet growth laws and grain competition during silicon crystallization

“…The result of this approach is that a high undercooling (several degrees) is needed in the case of growth on {111} silicon facets (solid black line Figure 1) which is not what is observed during experiments [15].…”

The grain–grain–liquid triple phase line during solidification of multi-crystalline silicon

“…The horizontal arrow, for a gradient of 20 K m −1 , represents Miyahara's experiments [27], in which the temperature of the solid-liquid interface has been measured with a thermocouple, as a function of the growth rate. For a low growth rate the interface undercooling was negligible, showing a mainly rough interface, and it increased up to 3 K for large velocities, showing a mainly faceted interface.…”

Some remarks on the undercooling of the Si(1 1 1) facet and the “Monte Carlo modeling of silicon crystal growth” by Kirk M. Beatty & Kenneth A. Jackson, J. Crystal Growth 211 (2000) 13

“…[12], Ciszek [10], Brice (Ge) [11], Voronkov [9], Abe [7], Miyahara et al (multi-crystalline Si) [13]. The cyan curve crossing the data of Voronkov is given by Eq.…”