Morphologies of silicon crystals solidified on a chill plate
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Morphologies of Silicon Crystals Solidified on a Chill Plate R.P. LIU, D.M. HERLACH, M. VANDYOUSSEFI, and A.L. GREER Electromagnetically levitated liquid droplets of pure Si or a Si-Ge alloy were cooled to different temperatures and then dropped onto a chill plate of Cu. Droplet oscillations mark the solid/liquid interface during solidification and permit the different crystal morphologies of silicon to be observed on the quenched surface by scanning electron microscopy (SEM). A spherical morphology found on the quenched surface represents the initial stage of crystal growth. Further growth leads to octahedral crystals bounded by {111} faces near equilibrium and to other polyhedra or even faceted dendrites further from equilibrium. The spherical growth can be observed only when the initial melt undercooling is moderately high. The critical size at which spherical crystals start to develop dendritic growth is much bigger than that calculated from the Mullins and Sekerka model, and is bigger than the Coriell and Parker model when kinetic undercooling is taken into account.
I. INTRODUCTION
THE morphology of silicon crystals growing from a melt is of interest in situations ranging from casting of Al-Si alloys[1–5] to laser processing of silicon thin films.[6,7,8] In particular, the morphologies of the primary Si phase in Al-Si alloys, and the associated growth mechanisms, have long been studied[1,9–12] in relation to improving the microstructure and properties of castings. A series of morphologies was reported:[1] plate-shaped crystals of hexagonal form, octahedral equiaxed crystals, equiaxed crystals containing varying numbers of parallel twins, star-like crystals containing two to five radiating twin planes, and spherical crystals. These morphologies are readily observed for crystals in an alloy matrix. In contrast, traces of the growth morphology in pure silicon are more difficult to detect. Patterns on etched cross sections of bulk silicon suggest a strong trend toward predominance of octahedral crystals bounded by {111} facets,[13–16] but no direct evidence has been presented for this morphology. In connection with its use as a semiconductor, much attention was paid in the past century to purification and singlecrystal growth of silicon. More recently, research has focused on melt undercooling, crystal morphology, growth mechanisms, and metastable phase formation in undercooled melts, both for germanium[17–24] (which is isomorphous with silicon) and for silicon itself.[25–29] Deeply undercooled bulk melts of pure silicon have been obtained only in a few investigations using containerless processing techniques, which can reduce or eliminate contamination of the sample and heterogeneous nucleation on container walls. Containerless processing has been achieved by electromagnetic levitation,[26,27] electrostatic levitation,[28] and flux processing.[29] Early experiR.P. LIU, Professor, is with the Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangd
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