Interface Velocity Transients During Melting of a-Si/C-Si Thin Films
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INTERFACE VELOCITY TRANSIENTS DURING MELTING OF a-Si/c-Si THIN FILMS
J.Y. TSAO*, M.J. AZIZ**, P.S. PEERCY* AND M.O. THOMPSON*** * Sandia National Laboratories, Albuquerque, NM 87185 MA 02138 Cambridge, ** Harvard University, NY 14853 Ithaca, ***Cornell University,
ABSTRACT We report transient conductance measurements of liquid/solid interface velocities during pulsed laser melting of amorphous Si (a-Si) films on crystalline Si (c-Si), and a more accurate, systematic procedure for analyzing these measurements than described in previous work [1]. From these analyses are extracted relations between the melting velocities of aSi and c-Si at a given interface temperature, and between the temperatures during steady-state melting of a-Si and c-Si at a given interface velocity.
INTRODUCTION How fast planar liquid/solid interfaces move in response to deviations from the equilibrium melting temperature T m is a basic question in materials science, whose answer is contained in velocity vs temperature interface response functions v(T). For highly viscous liquids, it has been found [2] that the response function scales with diffusive velocities D/a (D is 0 the atomic diffusivity, a0 is the lattice constant) and is locally "symmetric" (melting and freezing rates are the same for small but equal deviations above and below T ). The scaling and general shape of the response function have been more difficult to establish for non-viscous (most elemental) liquids. In Si, a system which has been studied intensively recently, transient conductance measurements [3] indicate that there is global asymmetry in that the melting and freezing behavior far from equilibrium are different. Previously, we [4] and others [5] have argued that this asymmetry can at least partially be understood as a natural outcome of transition-state theory, which predicts that far from equilibrium the melting rate should be "collision-limited" (limited only by the rate at which atoms collide with the interface), while the freezing rate should be "entropy-limited" (limited further by the entropy difference between liquid and solid) [6]. Differences between the densities of liquid and solid may also play a role [7]. Even if there is global asymmetry, though, we expect local symmetry. Recently, however, elegant x-ray diffraction measurements by Larson and coworkers [8] indicate that there may be a local asymmetry to the interface response function for Si. If so, the origin of this asymmetry is an extremely interesting theoretical problem. In this work, we present measurements of liquid/solid interface velocities during pulsed laser melting of amorphous Si (a-Si) films on crystalline Si (c-Si), and a systematic procedure for analyzing these measurements. The result of our measurements and analyses is that melting and freezing of c-Si is locally symmetric, in disagreement with the x-ray diffraction measurements.
Mat. Res. Soc. Symp. Proc. Vol. 100. 01988 Materials Research Society
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TRANSIENT CONDUCTANCE MEASUREMENTS real time by the transient Liquid layer thic
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