Interface Response Function Research Articles

Molecular dynamics simulation of crystal growth in [formula omitted] (100) heterostructures

Pulsed laser-induced liquid phase epitaxy of SiGe alloy/pure Si substrate heterostructures, exposing both (100) and (111) orientations, has been studied by nonequilibrium molecular dynamics (NEMD) simulation techniques using Stillinger-Weber potential models for the two components. Solid-liquid interfacial morphology, the quality of the regrown materials and Ge redistribution at the interface were studied under both fast and slow recrystallization conditions, controlled by the substrate temperature. For the (100) orientation, a significantly non-planar solid-liquid interface was found for all Ge concentrations, with Ge atoms at the bottom of this interfacial “well”, causing solidification to be retarded in their vicinity. For the (111) orientation, in-plane defects with alternating structures of two five-membered rings and one eight-membered ring were found in the regrown material due to the unstable growth kinetics on the “back” side of the interface response function. At slow crystallization rates, only stacking defects were found in the regrown material, without in-plane defects. The Ge partition coefficient obtained from simulation at slow regrowth speeds is in good agreement with experimental measurements and also agrees well with the prediction of Aziz's continuous growth model if the Ge diffusivity at the solid-liquid interface and the width of the interface are used in the calculation of the “diffusive velocity”.

Non-Equilibrium Two-Dimensional Model of Excimer-Laser Melting and Solidification of Thin Si Films on SiO2

ABSTRACTWe have developed a two-dimensional numerical model of excimer-laser melting and solidification that properly takes into account the non-equilibrium and transient nature of the process. The model incorporates a novel explicit finite difference scheme for efficiently solving the heat conduction equation and an algorithm that incorporates the interface response function for properly simulating the evolution of phase domains. The model provides space- and time-resolved information regarding the thermal profile and phase domains from which nearly all of the important solidification details can be extracted (e.g., interface location, solidification velocity, interfacial undercooling, etc.). For the simple partial-melting-and-vertical-regrowth scenario, results from the model converge with the results from the well-established one-dimensional model. As a result of its two-dimensional and non-equilibrium formulation, which also respects the amorphous and inert nature of the underlying oxide surface, the model is unique in its capability for properly simulating those solidification scenarios that involve extensive lateral growth of solids, as for example those behind the super-lateral growth phenomenon and various artificially controlled super-lateral growth processes.

On the super lateral growth phenomenon observed in excimer laser-induced crystallization of thin Si films

This letter reports on the experimental findings, and provides a theoretical description of the super lateral growth (SLG) phenomenon observed in the pulsed laser-induced solidification of amorphous thin Si films on SiO2. Experimentally, we report and elaborate on the isolated single-crystal disk structure that is observed at the upper threshold of the SLG regime; the structure is revealed as an important microstructural feature for understanding the phenomenon. A theoretical discussion of the SLG phenomenon is provided in terms of the key factors that are suggested by our model—the interface response function of the solid, the nucleation kinetics of the solid, and a highly transient lateral-thermal profile near the solid-melt interface. Our model and analysis point out the important inadequacies associated with the vertical solidification rate/temperature gradient model, which is currently being utilized to explain the excimer laser crystallization of thin Si films on SiO2.

Coupled diffusional/displacive transformations: Part II. Solute trapping

An earlier theory for the nonequilibrium transformation of austenite in Fe-C alloys to partially supersaturated plates of ferrite is extended to include a recent solute trapping model by Aziz. The previous model yielded a relationship between interface velocity and supersaturation for a specified transformation temperature. However, a unique growth velocity was determined from a velocity maximization criterion that could not be justified. By including the Aziz solute trapping function as a third interface response function (in addition to functions describing the diffusion field velocity and interface mobility), it has been possible to obtain a more physically based unique solution to the growth velocity for a specified temperature. The new calculations on an Fe-C alloy indicate a smoothly increasing supersaturation during both nucleation and growth, as the transformation temperature is reduced. Both the nucleation and growth processes tend to become completely diffusionless only below the martensite start temperature.

Evolution of the Microstructure During Laser-Induced Amorphization of Si

AbstractThe interface growth dynamics of Si leading to laser-induced amorphization have been studied as a function of incident energy density and crystallographic orientation using tran-smission electron microscopy (TEM). Using lithographic and dry etching techniques, cross sectional TEM specimens across the entire melt region were produced. These samples allowed observation of the evolution of the amorphous thickness and crystal defect structure from shallow anmorphous Si (a-Si) formation near the melt threshold energy density to crystallized material (c-Si) at higher energy densities. For surfaces near [111] orientations, marked differences in the solidification behavior were observed as compared to [001] or [110] orientations. The transition to a-Si growth at the moving liquid/crystal interface is discussed in terms of interface response function kinetics and crystallization models. A growth instability model, arising from the shape of the interface response function, is proposed to explain the observed nmicrostructure.

Nonequilibrium molecular dynamic simulation of rapid melting and solidification

A new nonequilibrium molecular dynamics computer simulation method has been developed for the study of rapid interface kinetics. In this method a strong heat gradient is set up to simulate the action of a laser in a (100) f.c.c. crystalline substrate composed of Lennard-Jones atoms. The substrate is shown to melt partially and then to regrow epitaxially as a crystalline solid. The characteristics of properties such as the melt depth as a function of time, together with the interface temperature and interface velocity as a function of time, are shown to display all the qualitative features of experimental studies of the laser annealing of silicon. The construction of the interface response function (interface velocity versus temperature) is also shown.

Time-resolved x-ray study of Ge during pulsed laser melting

Nanosecond resolution time-resolved x-ray diffraction has been used to make the first measurements of the liquid-solid interface overheating and undercooling in germanium. The results show an orientation-dependent undercooling for growth on (111) and (001) faces. For both the (111) and (001) faces the velocity versus temperature interface response function has a slope of ∼5 K/(m/s), except for undercooling on the (111) face where the slope is 20 K/(m/s).

Dynamics of Rapid Solidification in Silicon

AbstractReal-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.

Dynamics of Rapid Solidification in Silicon

AbstractReal-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.