Molecular simulations of solid-liquid interfacial tension of silicon

Molecular dynamics simulation of the solidification process of multicrystalline silicon from homogeneous nucleation to grain coarsening

Based on the Stillinger–Weber potential, molecular dynamics simulations of the solidification processes of multicrystalline silicon were carried out.

Reactive Wetting Controlled by Very Small Vertical Temperature Gradients in a Chemical Transport Mini Reactor

Reactive wetting of quartz by silicon is an ideal model system for the understanding and control of reactive wetting effects. Very slow and well controlled reactive spreading of a small silicon droplet on quartz can be achieved and observed in a new type of chemical transport mini reactor equipped with a video microscope setup operating in the melting point vicinity of silicon at Tm=1685 K. The spreading velocities are investigated in dependence of a very small temperature gradient that causes a slight chemical gradient and induces a slow oxygen transport through the system. The oxygen source is a SiO2 substrate at the temperature TA. From here the oxygen is transported upwards through the liquid silicon droplet and is released to the gas phase in form of SiO-molecules. The SiO-molecules are transported towards a cooler SiO2 substrate at TB where SiO2-nano whiskers are formed via a vapor–liquid–solid-growth mechanism. In this way the oxygen drain is located in the gas phase above the droplet. The source reaction drives the reactive spreading process. By reducing the temperature difference ΔT=TB−TA<0 a very small chemical potential difference ΔμO=μOB−μOA<0 of the oxygen between the sessile droplet at position A and the whiskers at position B can be created and adjusted. The velocity of the moving triple line is then measured in dependence of ΔμO and expressed by a power law. The results are explained in terms of an irreversible thermodynamic model that couples the dynamics of the triple line to an ongoing solid state reaction in the underlying substrate, where a thin solid silicon suboxide gradient layer forms between substrate and melt. By fitting the data to the model formula one can determine reactive contributions to the tensions at the triple line and investigate these quantities in the limit ΔμO→−0.

Silicon-wall interfacial free energy via thermodynamics integration

We compute the interfacial free energy of a silicon system in contact with flat and structured walls by molecular dynamics simulation. The thermodynamics integration method, previously applied to Lennard-Jones potentials [R. Benjamin and J. Horbach, J. Chem. Phys. 137, 044707 (2012)], has been extended and implemented in Tersoff potentials with two-body and three-body interactions taken into consideration. The thermodynamic integration scheme includes two steps. In the first step, the bulk Tersoff system is reversibly transformed to a state where it interacts with a structureless flat wall, and in a second step, the flat structureless wall is reversibly transformed into an atomistic SiO₂ wall. Interfacial energies for liquid silicon-wall interfaces and crystal silicon-wall interfaces have been calculated. The calculated interfacial energies have been employed to predict the nucleation mechanisms in a slab of liquid silicon confined by two walls and compared with MD simulation results.

Melting and superheating in solids with volume shrinkage at melting: a molecular dynamics study of silicon

The thermodynamics of homogeneous melting in superheated crystalline solids with volume shrinkage at melting is investigated using extensive molecular dynamics simulation in conjunction with a classical nucleation theory. A liquid-solid co-existing model is established to overcome the difficult in observing liquid phase formation in a superheated Si crystal. We found that melting is governed by two major factors, the volume change induced strain energy and the curvature of the interface between the liquid and the solid phases. The driving force for melting in superheating regime is lowered by the additional strain energy that restricts homogeneous nucleation of a liquid phase till temperature rises above the normal melting point, thus causing superheating. However, due to the abnormal behavior in the compressibility of the silicon liquid in the superheating regime, the degree of superheating in terms of the liquid nucleation gap becomes significantly reduced. More potential complications caused by the change of the atomic bonding in Si at melting are discussed.

Surface-induced crystallization in supercooled tetrahedral liquids

Surfaces have long been known to have an intricate role in solid-liquid phase transformations. Whereas melting is often observed to originate at surfaces, freezing usually starts in the bulk, and only a few systems have been reported to exhibit signatures of surface-induced crystallization. These include assembly of chain-like molecules, some liquid metals and alloys and silicate glasses. Here, we report direct computational evidence of surface-induced nucleation in supercooled liquid silicon and germanium, and we illustrate the crucial role of free surfaces in the freezing process of tetrahedral liquids exhibiting a negative slope of their melting lines (dT/dP|coexist<0). Our molecular dynamics simulations show that the presence of free surfaces may enhance the nucleation rates by several orders of magnitude with respect to those found in the bulk. Our findings provide insight, at the atomistic level, into the nucleation mechanism of widely used semiconductors, and support the hypothesis of surface-induced crystallization in other tetrahedrally coordinated systems, in particular water in the atmosphere.

Solid–liquid transitions of sodium chloride at high pressures

We investigate solid-liquid transitions in NaCl at high pressures using molecular dynamics simulations, including the melting curve and superheating/supercooling as well as solid-solid and liquid-liquid transitions. The first-order B1-B2 (NaCl-CsCl type) transition in solid is observed at high pressures besides continuous liquid structure transitions, which are largely analogous to the B1-B2 transition in solid. The equilibrium melting temperatures (T(m)) up to megabar pressure are obtained from the solid-liquid coexistence technique and the superheating-supercooling hysteresis method. Lindemann's vibrational and Born's mechanical instabilities are found upon melting. The Lindemann frequency is calculated from the vibrational density of states. The Lindemann parameter (fractional root-mean-squared displacement) increases with pressure and approaches a constant asymptotically, similar to the Lennard-Jones system. However, the Lindemann melting relation holds for both B1 and B2 phases to high accuracy as for the Lennard-Jonesium. The B1 and B2 NaCl solids can be superheated by 0.18T(m) and 0.24T(m), and the NaCl liquid, supercooled by 0.22T(m) and 0.32T(m), respectively, at heating or cooling rates of 1 K/s and 1 K/ps. The amount of maximum superheating or supercooling and its weak pressure dependence observed for NaCl are in accord with experiments on alkali halides and with simulations on the Lennard-Jones system and Al.