Solid Silicon at the Melting Temperature is Crystalline
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SOLID SILICON AT THE MELTING TEMPERATURE IS CRYSTALLINE D. K. BIEGELSEN, R. J. NEMANICH, L. E. FENNELL, and R. A. STREET Xerox Palo Alto Research Center, Palo Alto, CA 94304 ABSTRACT Recently it has been proposed that solid silicon at the melting temperature is amorphous. There is no known case of a solid for which an amorphous structure is the equilibrium state. Silicon thin films on insulating substrates, when heated radiantly, melt inhomogeneously and provide an accessible high temperature system for a study of a solid coexisting with its melt. Using the intensity, energy distribution and polarization of Raman scattering from silicon lamellae, we have proved that the equilibrium phase is in fact crystalline. Furthermore, we give strong evidence that the solid regions have (1001 texture at Tm. INTRODUCTION There is no case of a material for which the thermodynamic equilibrium solid structure is known to be amorphous. Glasses are thought to be non-equilibrium states. Disordered materials such as alloys or superionic conductors still have long-range order as manifested by the existence of Bragg scattering peaks. For amorphous materials, the pair correlation function goes to zero beyond a short coherence length. Although no equilibriuum amorphous solids are known, there is, on the other hand, no proof that an amorphous state cannot, in fact, be a lower energy configuration than the crystal at finite temperatures. One might particularly expect to find such an amorphous phase for a solid at high temperatures where the entropic reduction in the total energy is large. Recently there has been a controversy concerning the relative magnitudes of the melting temperatures (or free energies) of crystalline and amorphous silicon.[1] It has been proposed[2] that the apparantly facetless morphology of silicon lamellae is evidence of an amorphous atomic configuration. In this paper we describe experiments which demonstrate that solid silicon at the melting temperature, Tm, is crystalline. When silicon is heated by a radiant flux melting occurs inhomogeneously.[3] An explanation for this phenomenon has been based on the notion that molten silicon is metallic and therefore more highly reflecting than the hot solid.[4] This leads to a natural negative feedback mechanism because the heating depends on the optical coupling. For powers between that sufficient to initiate melting, P 1, and that required to completely melt a region, P2 , the silicon phase-separates into domains of approximately uniform size and spacing (Fig. 1) so that the total absorbed power is fixed and the average surface temperature is Tm. The ratio of liquid to solid areas varies monotonically with the incident power between Pi and P2. For thin (here t = 0.5 /tm) silicon films on silica substrates, the solid domains are lamellae of -3 [Lm width surrounded by liquid. This configuration lends itself readily to Raman scattering from solid regions coexisting with liquid because the material is uniform in the direction of light propagation. Raman scattering is a conveni
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