Supercooling of Silicon and Germanium after Laser Melting
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SUPERCOOLING OF SILICON AND GERMANIUM AFTER LASER MELTING S. R. Stiffler, Michael 0. Thompson and P. S. Peercy* Vaterials Science Dept., Cornell University, Ithaca, NY 14853 Sandia National Laboratories, Albuquerque, NM 87185
ABSTRACT Following pulsed laser melting, supercoolings of 505 K in Si and 430 K in Ge were observed prior to bulk nucleation. These large supercoolings are obtained because of the extremely high thermal quench which follows laser irradiation. Nucleation rates were estimated to be ~i029 events/m0/s. Assuming that homogeneous nucleation was achieved, surface energies are estimated to be 0.34 J/m2 for Si and 0.24 J/m' for Ge. These results are in reasonable agreement with traditional homogeneous nucleation experiments sensitive to rates of only -1010 events/ms/s. This laser melting technique is applicable to nucleation studies in a wide variety of materials.
INTRODUCTION: Phase transformations provide an interesting opportunity to observe the interaction between energies associated with interfaces and those associated with bulk materials. For instance, it is often observed that liquids may be cooled (supercooled [1]) considerably below their melting temperature prior to solidification. In such cases, solidification is limited by the initial formation of a crystalline nuclei, referred to as the nucleation process. Nucleation cannot occur until the energy associated with the bulk formation of solid is sufficient to overcome the energy required to create a surface between the crystallite and the remaining liquid. This initial nucleation is most often heterogeneous, occurring at either the walls of a container or from dispersed second phase particles in the liquid. If care is taken to avoid such heterogeneous nucleation sites, bulk metals such as Ni can be supercooled to roughly half the melting temperature before homogeneous nucleation and solidification occurs. Organic materials and alloys, indeed, may be cooled as liquids to room temperature forming glassy phases. The fundamental limit to such cooling experiments is the temperature at which the liquid is spontaneously unstable to formation of nuclei, referred to as the homogeneous nucleation temperature. Study of this temperature allows determination of the energy associated with the liquid-solid interface. Study of such nucleation processes has typically been done in droplet experiments, where volumes are -10-' mm' and homogeneous nucleation occurs on time scales of seconds [2-4]. These conditions correspond to nucleation rates of 106 - 1012 events/m'/s. Using known bulk values for the enthalpy (AH ) and temperature (Tm) of melting, the energy associated with the liquid-solid interface can be determined from measurements of the nucleation rate and temperature. The nucleation rate is normally assumed to increase so rapidly with supercooling that a unique nucleation temperature can be specified; for example, increasing the supercooling by 5 K increases the nucleation rate by approximately an order of magnitude. In this paper, we report measuremen
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