Dendrite growth processes of silicon and germanium from highly undercooled melts
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I. INTRODUCTION
THE crystal growth behavior of a semiconductor from a highly undercooled melt has been attracting a great deal of attention, because microstructure, growth velocity, and crystal structure are expected to depend heavily on the bulk undercooling. Highly undercooled melts can be obtained with a good reproducibility, and the solidification processes can be directly analyzed using various containerless methods such as electromagnetic (EM) levitation,[1] electrostatic levitation,[2] or aeroacoustic levitation.[3] A few investigations of these undercooling experiments have been carried out, especially for metals.[4,5] Studies on the solidification of undercooled semiconductors are still at an early stage, because it is difficult for the EM levitator to levitate materials with a low electrical conductivity. Several interesting results were, however, reported on the growth velocities of pure Ge and Ge-based alloys by Li et. al., in which the crystal growth behavior was classified into three categories of lateral, continuous, and rapid growth at low, moderate, and high undercooling values, respectively, and the transition was observed as a drastic increase in the growth velocity.[6] A complete explanation of the phenomenon based only on the current dendrite growth theories for metallic systems is inadequate. Moreover, it would be possible to produce a spherical semiconductor using this levitator. Since the spherical semiconductor has a larger utilizable surface area than a wafer semiconductor, the spherical single crystal may have varied fields of industrial application as a simple alternative to Si wafer processing. Therefore, it would be very desirable to reveal the dependence of crystallinity on the growth velocity and the undercooling by studying the solidification of the spherical levitating sample. In this work, the growth behavior on recalescence from highly undercooled Si and Ge was investigated by measuring the growth velocities and observing the microstructures in TOMOTSUGU AOYAMA, Graduate Student, YUZURU TAKAMURA, Research Associate, and KAZUHIKO KURIBAYASHI, Professor, are with the Institute of Space and Astronautical Science, Kanagawa 229-8510, Japan. Manuscript submitted August 4, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
a wide range of undercoolings. “Recalescence” defines the phenomenon in which an undercooled melt is abruptly heated, at most, to the melting temperature by releasing of its latent heat while solidifying. When the solidification rate is great enough, as compared to the heat-extraction rate, an adiabatic condition is assumed on this recalescence. The results of measured velocities were compared to previous studies on Ge and to growth theories for metals. II. EXPERIMENTAL PROCEDURE Figure 1 shows a photograph and a schematic illustration of the apparatus. The chamber was evacuated to 1023 Pa by a turbo molecular pump and filled with 99.999 pct pure argon gas. Undoped Si or Ge of 99.999 pct purity was used as a source material. The sample was placed on a sintered boron n
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