Direct observation of the crystal-growth transition in undercooled silicon
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I. INTRODUCTION
COMPARING rapid solidification through melt substrate quenching with rapid solidification through highnucleation undercooling, the latter has the advantage that it can create new kinds of materials in bulk volume. Bulk amorphous alloys, metastable phases, quasicrystals, single crystals, and so on, have been obtained from deeply undercooled metal melts. Recently, more and more attention has been paid to the solidification behavior of undercooled semiconductor silicon and germanium. Three methods have been applied to investigate the solidification behavior of the undercooled semiconductors. The first is observation of the morphology of the cross section[1–5] or the surface[5–13] of undercooled samples after solidification by means of optical microscopy or scanning electron microscopy. The second is a measure of the growth rate of crystals using two photodiodes[3,6,10,11] or a high-speed camera.[12] The third is to record the recalescence interface of undercooled droplets during the recalescence process by using a high-speed camera.[12] The generally acknowledged view is that there should exist a transition of the growth mode in undercooled semiconductor silicon and germanium. However, direct evidence for the transition, such as a change in the morphology of the growing crystal during solidification, has not yet been reported. As is well known, the semiconductors silicon and germanium have a large latent heat of fusion. Therefore, the volume fraction of solid solidifying adiabatically during the recalescence process is very small. In other words, the temperature at which the majority of liquid solidifies is near the melting point. The growth feature that semiconductor silicon and germanium show at the initial high undercooling may be covered up after the solidification. So, it is very difficult ZENGYUN JIAN, Foreign Research Fellow, is with the Institute of Space and Astronautical Science, Kanagawa 229-8510, Japan. Contact e-mail: [email protected] KOSUKE NAGASHIO, Postdoctoral Student, is with Stanford University, Stanford, CA 9430S. KAZUHIKO KURIBAYASHI, Professor, Institute of Space and Astronautical Science, is also with CREST, Japan Science and Technology Corporation, Ibaraki 305-0047, Japan. Manuscript submitted April 5, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
to use the first method[1–13] to obtain the original features of growth during the initial undercooling. The second[12] and third[4,6,10–12] methods can give some information on the solidification behavior of the undercooled sample during the recalescence process. However, one could not obtain the real growth behavior of crystals unless the relationship between the recalescence interface and the crystalliquid interface has been perceived. For a long time, it has been believed that the recalescence interface is the crystalliquid interface, which is also the basis for measuring the crystal growth rate by using two photodiodes[4,6,10,11] or a high-speed camera.[12] However, it has not been ascertained that the recalescence inter
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