Dislocations work not only as recombination sites for photogenerated carriers but also shunts of PN junctions of solar cells.  Therefore, reduction of dislocation density is demanded to improve the conversion efficiency of solar cells.  Crucibles used in conventional casting methods by unidirectional solidification constrain expansion of Si crystal caused by the solidification of the Si melt during whole processes of crystal growth, then, dislocations are introduced in Si crystals by stress.  Once dislocations are generated, they increase in subsequent unidirectional growth process.  This occurs also in the case of mono-like crystal which investigated actively these days.  Recently, we proposed a noncontact crucible method [1-3], which enables reduction of stress due to constraint by crucible.  In this method, nucleation occurs on the surface of a Si melt using seed crystals, and crystals grow inside the Si melt without touching the crucible walls.  Then, the ingots continue to grow while being slowly pulled upward maintaining the front of crystal growth in the low-temperature region in the Si melt.

  In the present study, growth morphology, defects, electrical properties and their relations in the p-type multicrystalline Si ingots grown by noncontact crucible method were studied with orientation analysis by electron back scattering diffraction patterns (EBSP), observation of etch-pit densities (EPD) and lifetime measurements.  From Si ingots grown by noncontact crucible method, transverse and vertical cross-sections near Si seed were obtained.  The transverse structures were radially-grown multicrystals.  Such multicrystallization was thought to be aftereffects of Si₃N₄ particles which originally coated on the silica crucible.  Almost grain boundaries in the transverse cross-section were Σ3 (84 % in the present sample) which should be electrically inactive.  However, random grain boundaries (13 % in the present sample) caused the generation of dislocations, which increased along radial growth.  On the other hand, in the case of that floating Si₃N₄ particles were not observed during growth of ingots, such ingots consisted of some large grains and dislocation density was low.

[1] K. Nakajima, R. Murai, K. Morishita, K. Kutsukake, N. Usami, J. Crystal Growth, 344, pp. 6-11, 2012.

[2] K. Nakajima, K. Morishita, R. Murai, and K. Kutsukake, J. Crystal Growth, 355, pp. 38-45, 2012.

[3] K. Nakajima, R. Murai, K. Morishita, and K. Kutsukake, J. Crystal Growth, accepted.

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1. Improvement of femtosecond laser-induced nucleation technique for proteins: Studies of effect of gel-solution interface
2. Growth of  Si large single bulk crystals for solar cells using small crucibles with a given diameter by the Noncontact Crucible method