Dynamics of Rapid Solidification in Silicon
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DYNAMICS OF RAPID SOLIDIFICATION IN SILICON+ P. S. Peercy*, Michael 0. Thompson**, and J. Y. Tsao** *Sandia National Laboratories Albuquerque, New Mexico 87185 **Department of Materials Science Cornell University, Ithaca, New York ABSTRACT Real-time techniques were used to study rapid melt and solidification dynamics in silicon. In crystalline Si, the interface response function was characterized and found to be asymmetric for large deviations from the melting temperature, which will require reevaluation of conventional transition state treatments of melt and solidification. In amorphous Si, the mechanism of explosive crystallization was studied. The explosive transformation is mediated by a buried liquid layer, and detailed measurements have led to the suggestion that polycrystalline Si nucleates at the moving liquid-amorphous interface. For certain conditions, this process could yield fine-grained polycrystalline Si; for other conditions it permits epitaxial regrowth from the underlying crystalline Si for maximum melt thickness much less than the original amorphous layer thickness. I. INTRODUCTION When an absorbing solid is irradiated with a laser pulse, energy is initially absorbed by the electronic system. A variety of studies have shown that this energy is transferred to the lattice on picosecond time scales [1]. As a result, for pulse durations > 1 ps, the electronic system is in thermal equilibrium with the lattice, and the net effect is to heat the irradiated region of the solid. With sufficient absorbed energy, the temperature of the near-surface region can exceed the melting temperature. Under normal conditions, melt initiates at the surface and propagates into the solid. Melt propagation continues as long as the interface temperature exceeds the melting temperature. Once the interface cools to the melting temperature, there is no thermodynamic driving force for continued melting and the liquidsolid interface stops. Upon further cooling of the interface by thermal conduction, the interface reverses direction and returns toward the surface. The well-defined and controllable temperature gradients afforded by laser-induced surface melting permit the dynamics of melt and solidification to be examined over an unprecedented range of velocities [2]. Such studies have generated considerable interest both because of the fundamental insights they provide on the basic melt and solidification processes and because the achievement of steep temperature gradients permit the formation of novel nonequilibrium systems, including metastable phases and supersaturated solutions [3]. Detailed understanding of the kinetics at a moving liquid-solid interface requires knowledge of three quantities: instantaneous interface velocity, interface temperature, and composition of the liquid and solid phases at the interface. For elemental systems, and thus fixed compositions, experimentally the most important quantity is the solidification velocity. This velocity ultimately determines the resulting phases and microstructures. I
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