Fluorination of the dimerized Si(100) surface studied by molecular-dynamics simulation

Simulation of the Irradiation Cascade Effect of 6H-SiC Based on Molecular Dynamics Principles

When semiconductor materials are exposed to radiation fields, cascade collision effects may form between the radiation particles in the radiation field and the lattice atoms in the target material, creating irradiation defects that can lead to degradation or failure of the performance of the device. In fact, 6H-SiC is one of the typical materials for third-generation broadband semiconductors and has been widely used in many areas of intense radiation, such as deep space exploration. In this paper, the irradiation cascade effect between irradiated particles of different energies in the radiation and lattice atoms in 6H-SiC target materials is simulated based on the molecular dynamics analysis method, and images of the microscopic trajectory evolution of PKA and SKA are obtained. The recombination rates of the Frenkel pairs were calculated at PKA energies of 1 keV, 2 keV, 5 keV, and 10 keV. The relationship between the number of defects, the spatial distribution pattern of defects, and the clustering of defects in the irradiation cascade effect of 6H-SiC materials with time and the energy of PKA are investigated. The results show that the clusters are dominated by vacant clusters and are mainly distributed near the trajectories of the SKA. The number and size of vacant clusters, the number of Frenkel pairs, and the intensity of cascade collisions of SKAs are positively correlated with the magnitude of the energy of the PKA. The recombination rate of Frenkel pairs is negatively correlated with the magnitude of the energy of PKA.

Influence of water contamination on the sputtering of silicon with low-energy argon ions investigated by molecular dynamics simulations

Focused ion beams (FIB) are a common tool in nanotechnology for surface analysis, sample preparation for electron microscopy and atom probe tomography, surface patterning, nanolithography, nanomachining, and nanoprinting. For many of these applications, a precise control of ion-beam-induced processes is essential. The effect of contaminations on these processes has not been thoroughly explored but can often be substantial, especially for ultralow impact energies in the sub-keV range. In this paper we investigate by molecular dynamics (MD) simulations how one of the most commonly found residual contaminations in vacuum chambers (i.e., water adsorbed on a silicon surface) influences sputtering by 100 eV argon ions. The incidence angle was changed from normal incidence to close to grazing incidence. For the simulation conditions used in this work, the adsorption of water favours the formation of defects in silicon by mixing hydrogen and oxygen atoms into the substrate. The sputtering yield of silicon is not significantly changed by the contamination, but the fraction of hydrogen and oxygen atoms that is sputtered largely depends on the incidence angle. This fraction is the largest for incidence angles between 70 and 80° defined with respect to the sample surface. Overall, it changes from 25% to 65%.

Rare event simulations reveal subtle key steps in aqueous silicate condensation

A replica exchange transition interface sampling (RETIS) study combined with Born–Oppenheimer molecular dynamics (BOMD) is used to investigate the dynamics, thermodynamics and the mechanism of the early stages of the silicate condensation process.

Molecular dynamics simulations of a chemical reaction; conditions for local equilibrium in a temperature gradient

We have examined a simple chemical reaction in a temperature gradient; 2F <==> F2. A mechanical model was used, based on Stillinger and Weber's 2- and 3-body potentials. Equilibrium and non-equilibrium molecular dynamics simulations showed that the chemical reaction is in local thermodynamic as well as in local chemical equilibrium (delta(r)G = 0) in the supercritical fluid, for temperature gradients up to 10(12) K m(-1). The reaction is thus diffusion-controlled. The velocity distributions of both components were everywhere close to being Maxwellian. The peak distributions were shifted slightly up or down from the average velocity of all particles. The shift depended on the magnitude of the temperature gradient. The results support the assumption that the entropy production of the reacting mixture can be written as a product sum of fluxes and forces. The temperature gradient promotes interdiffusion of components in the stationary state, a small reaction rate and an accumulation of the molecule in the cold region and the atom in the hot region.

Improved interatomic potentials for silicon–fluorine and silicon–chlorine

Improved sets of empirical interatomic potentials for silicon-fluorine and silicon-chlorine are presented. The Tersoff-Brenner potential form has been reparameterized using the density-functional theory (DFT) cluster calculations of Walch. Halogenated silicon cluster energetics computed with DFT are, on average, within several tenths of an eV of the energies of the corresponding clusters with the reparameterized empirical potential for both Si-F and Si-Cl. Using the reparameterized Tersoff-Brenner potentials, molecular-dynamics simulations of F and Cl atom exposure to undoped silicon surfaces are in excellent agreement with published data on etch probability, halogen coverage at steady state, and etch product distributions.

AB INITIO DYNAMICS OF SURFACE CHEMISTRY

▪ Abstract  We review the young field of ab initio molecular dynamics applied to molecule-surface reactions. The techniques of ab initio molecular dynamics include methods that use an analytic potential energy function fit to ab initio data and those that are fully ab initio. In this review, we focus on the insights provided by ab initio–based molecular dynamics that are currently unavailable from experimental studies and discuss current techniques and limitations. As an example of how different aspects of a problem can be tackled with state-of-the-art theoretical tools, we consider the well-studied case of H2 desorption and adsorption from the Si(100)-2 × 1 surface.

Molecular dynamics simulations of silicon-fluorine etching

Molecular dynamics simulations of the reactions between gaseous fluorine atoms and (SiFx)n adsorbates on the Si(100) - (2 x 1) surface are performed using the SW potential and compared to simulations with the WWC reparameterization of the SW potential. Theoretical and experimental work has demonstrated that the reactive fluorosilyl layer during silicon-fluorine etching is composed of tower-like adspecies of SiF, SiF2 and SiF3 groups. The objective of the simulations is to determine how the chemical composition, mechanism of formation, and energy distribution of the etched gas-phase products depend on the identity of the reacting adsorbate, the incident kinetic energy, and the parameterization of the potential energy function. Three reactions are simulated: F(g) + SiF3(a), F(g) + SiF2-SiF3(a), and F(g) + SiF2-SiF2-SiF3(a). SiF4 is the major product and Si2F6 and Si3F8 are minor products. In Si2F6 and Si3F8, the silicon-fluorine bond that is formed is stronger than the silicon-silicon bond in the molecule and, therefore, the majority of these products have enough energy to dissociate and will fragment before reaching the detector. An SN2-like mechanism is the primary mechanism responsible for the formation of SiF4, Si2F6, and Si3F8. In addition, at higher energies, the simulations have discovered a previously unknown mechanism for the formation of SiF4, which involves an insertion between a silicon-silicon bond. The results of the simulations with the two potentials differ quite substantially in their prediction of the reactivity of the adsorbates. The SW potential predicts a 2- to 3-eV lower energy threshold for reaction and a much higher reaction cross-section, especially for the SiF4 product. These results are explained in terms of the differences in the potential energy functions used to describe the silicon-fluorine interactions. In addition, the results are compared to experimental data on silicon-fluorine etching.