An analysis of the formation of bulk amorphous silicon from the melt
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I.
INTRODUCTION
THE relative stability of three phases of silicon is shown in the free energy diagram of Figure 1. The crystalline phase (c-Si) is a semiconductor with a diamond cubic structure, with each of the atoms tetrahedrally coordinated. The structure of the liquid phase (l-Si) is similar to that of a liquid metal; the coordination number of the atoms and the density are considerably higher than in c-Si. The amorphous phase (a-Si) is a semiconductor with a continuous random network structure, with all atoms tetrahedrally coordinated; its density is close to that of c-Si, and the absence of translational symmetry in its structure is achieved by a distortion of the bond angles (rms deviation of 57 deg from the ideal angle of 109.5 deg). The free energy of the amorphous phase is calculated[1,2] from the measured heat of crystallization (Hac)[3,4,5] and from a model-based estimate of the configurational entropy of the random network.[6] Figure 1 shows a-Si as a metastable phase, which melts at a temperature (Tal) below that of the stable crystalline phase (Tcl). The most accurate calculation gives[5] Tal 5 Tcl 2 245 K 5 1440 K, in agreement with the thermal arrest observed in transient conductivity measurements during pulsed laser heating of a-Si thin films.[7,8] Amorphous Si is not the glassy phase that would form* continuously from *Glassy Si has not been observed. The failure of l-Si to form a glass is similar to the behavior of other pure metals. Glass formation is forestalled by the nucleation of the crystalline or the amorphous phase. Preservation of the glassy phase, if formed at all, may also be difficult in very pure systems with collision-limited crystallization kinetics.
the liquid when sufficiently undercooled. Instead, it is a
YAN SHAO, formerly Graduate Student, Division of Engineering and Applied Sciences, Harvard University, is Senior Scientist, Eaton Corporation, Beverly, MA 01915. FRANS SPAEPEN, Professor, and DAVID TURNBULL, Professor Emeritus, are with the Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138. This article is based on a presentation made in the ‘‘Structure and Properties of Bulk Amorphous Alloys’’ Symposium as part of the 1997 Annual Meeting of TMS at Orlando, Florida, February 10–11, 1997, under the auspices of the TMS-EMPMD/SMD Alloy Phases and MDMD Solidification Committees, the ASM-MSD Thermodynamics and Phase Equilibria, and Atomic Transport Committees, and sponsorship by the Lawrence Livermore National Laboratory and the Los Alamos National Laboratory. METALLURGICAL AND MATERIALS TRANSACTIONS A
separate phase that undergoes a first-order phase transition at Tal. Direct evidence for the first-order nature of the a-Si → c-Si transition includes (1) the arrest of the interface position and the attendant absorption of latent heat observed in pulsed laser melting of an a-Si layer on a c-Si substrate; [7,8] (2) the occurrence of solute rejection ahead of a moving a-l interface;[9] and (3) the observation of morphological instabilities of t
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