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Molecular dynamics simulation of the effects of the samples' temperature on Ar + interactions with SiC surface
Abstract
Molecular dynamics simulations were performed to investigate the effects of surface temperature on Ar + interactions with the SiC surface. The simulation results show that the number of Si atoms removed increases with increasing surface temperature, while the number of C atoms removed slightly changes. The sputtering yield of Si atoms is greater than that of C atoms. It is found that most of Si and C atoms removed come from the surface region. After modified by Ar + ions, a Si-rich amorphous layer is formed.
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The detailed research of the B4C and SiC sputtering differential characteristics under Ar+ and He+-ions irradiation with the energies 0.5–20 keV has been carried out using both CASNEW and like-TRIM Monte Carlo codes. The major difference between these codes is the following: the dependence of the total interaction cross-section on energy is considered by CASNEW. In its turn it influences upon the collision probability with species and the free flight path. The partial and total yields as well as the angle and energy spectra, ion and point defect depth distributions have been calculated. As it was noticed both CASNEW and TRIM codes give close results at low energies. Differences grow with energy and are more for SiC than for B4C. In both cases the preferred sputtering of the heavier component is observed.
The results of the reactive ion etching of GaN films on sapphire and bulk 4H–SiC in the fluorine containing plasmas are presented. CF4/Ar chemistry for etching of GaN as well as CF4/Ar and CF4/O2 chemistries for etching of 4H–SiC are used. Etch rates up to ∼500 Å/min for GaN and ∼1800 Å/min for 4H–SiC were obtained. The influence of pressure and gas flow rates on the etch rate and surface morphology were investigated. Stylus profilometry was used to measure the etch profiles, whereas AFM and SEM measurements provided us with information on the quality of the etched surfaces. Additionally, SIMS measurements were carried out to determine the influence of ion bombardment on the exposed surfaces and the possibility of their implantation.
The use of periodic boundary conditions in molecular-dynamics simulations leads to the microcanonical EVNMG ensemble of Ray and Zhang [J.R. Ray, H. Zhang, Phys. Rev. E 59 (1999) 4781] in which the total linear momentum M and the generator of infinitesimal Galilean boosts G are conserved quantities in addition to the total energy E, the volume V, and the number of particles N of the system. We find that the invariance of G should be of importance in the analysis of ensemble averages of quantities that only depend on the spatial coordinate r. As an application we study the density profile of an isolated system of hard disks with periodic boundary conditions in the absence of external forces. We find that the periodic boundary conditions give rise to an anomalous inhomogeneity in the density profile of the system. This inhomogeneity is only relevant for systems with a very small number of disks and is related to the conservation of the center of mass coordinates.
Light ion sputtering yields have been determined for the fusion-relevant low Z materials Be, SiC, TiC and B4C in the temperature range 20–1100°C. SiC, TiC and B4C show practically no change in the sputtering yield at elevated temperatures. These materials can be used also as protective coatings for graphite.Iron evaporated on graphite can also serve as an protective layer. At the investigated temperature of 600°C the iron diffuses into the graphite and therefore such a layer does not totally suppress chemical sputtering. At an Fe bulk concentration of about 15 at-% the reduction is about a factor of 4.The sputtering yield of Be does not show a change with temperature for ion energies above 1 keV. At temperatures above 700°C thermal evaporation is the dominant weight loss process. For ion energies below 1 keV the sputtering yields increase somewhat with temperature. This is attributed to an oxide layer and to the diffusion of Be through this layer at temperatures above 300°C.
In this paper, we provide an overview of the use of molecular dynamics for simulations involving energetic particles (Ar, F, and CFx) interacting with silicon surfaces. The groups (including our own) that have performed this work are seeking to advance the fundamental understanding of plasma interactions at surfaces. Although this paper restricts itself largely to the systems bracketed above, the approach and general mechanisms involved are applicable to a much wider range of systems. Proper description of plasma-related systems generally requires a large number of atoms in order to correctly characterize the interactions. Consequently, the bulk of the present work, and the main focus of the text, is based on classical molecular dynamics. In MD simulations, one of the most critical considerations is the selection of the interatomic potential. For simulations involving silicon etching, the choice is typically made between the Stillinger-Weber and the Tersoff-Brenner potentials. An outline of the two potentials is given, including efforts that have been made to improve and optimize the potentials and their parameters. Subsequently, we focus on some of the practical details involved in establishing the simulation process and outline how various parameters (e.g. heat bath, relaxation time and cell size) influence the simulation results. These sections deal with the influences of the heat bath (application time, rising time), the time-step and total integration time of molecular trajectories, the relaxation of the sample (during and post-etching) and the sample size. The approach is essentially pedagogical in nature, and may be of interest to those less familiar with the techniques. To illustrate the type of results that can be produced we present a case study for 100 eV CF3+ interacting with a Si(100)-2x1 surface at different sample temperatures (100-800 K). The simulations reveal details of the change in etch rate, the F-turnover and the standing coverage of functional groups as a function of the temperature. Our primary interest is in studies with relevance for plasma-surface interactions. We discuss the general mechanisms that are most important in plasma-surface interactions and give an overview of some of the wide range of results that have been produced for various systems. The results presented illustrate that careful consideration must be given to the precise configuration of the plasma system. Numerous factors, including the chemical species, the energy and chemical mix of the incident particles and the surface composition and structure can play a crucial role in determining the net outcome of the interaction.
The in-depth concentration of Ar atoms in Si surface after Ar+ ion sputtering was investigated using medium-energy ion spectroscopy (MEIS) and dynamic Monte Carlo simulation. The primary Ar+ ion energy was 0.5, 1 and 3 keV, and the primary Ar+ ion beam direction was varied from surface normal to glancing angle. In the case of the surface normal incidence, the MEIS result shows that the maximum atomic concentration of Ar atoms increases from 6% at the depth of 2 nm to 16% at the depth of 3 nm as the primary ion energy increases from 0.5 to 3 keV. However, in the case of the incident angle of 80°, that is 2.5% at the depth of 1.5 nm when the primary energy is 3 keV, in-depth Ar distribution cannot be observable when the primary ion energy is 0.5 keV. Dynamic Monte Carlo simulation reproduced the in-depth concentration distribution of Ar atoms quantitatively.