Monte Carlo simulation of ion beam-assisted solid phase epitaxy

Abstract

We have developed a computer simulation of ion beam-induced epitaxial crystallisation (IBIEC) based on the familiar kink-and-ledge model which describes thermally activated solid phase epitaxy. In the latter, kinks are assumed to form (via thermal activation) on ledges. Here, we assume that the deposited ion energy induces kink formation on the terraces as well as on the ledges. We then derive, with minimal restrictions on the growth laws, the interface morphology evolution. The latter is correlated to the growth speed, and an interpretation of the IBIEC temperature dependence is proposed. The power laws which characterise the simulated interface evolution are those of the Kardar-Parisi-Zhang universality class.     

Monte Carlo simulation of carbon fragmentation at high energies

The dominating decay channels of fragmenting carbon nuclei in high-energy proton-carbon collisions were studied by means of a Monte Carlo simulation. We study the influence of the Coulomb interaction on the energy spectra and two-fragment (energy and angular) correlations between the various fragments in the context of a statistical model. We show that the Coulomb interaction between the fragments strongly affects their energy and angular correlation functions. Qualitative predictions for the exclusive analysis of the various final states are given. Comparison with experiments of this type should soon be possible.     

Monte Carlo simulation studies of sidewall roughening during reactive ion etching

The evolution of sidewall roughness (SWR) during reactive ion etching (RIE) was simulated using a Monte Carlo method. It was discovered that the sidewall roughness established during an earlier etch period represents a historical archive of the moment during which the etch front passes by and is not affected by further etching. We also found that the behavior of SWR follows two distinct trends. At the initial stages, SWR increases with etch time, or its equivalent, the etch depth, but, beyond a certain etch depth, SWR of etched surfaces stabilizes and does not change with further etching. This is related to the change of the number of shadowing and reemitted particles as the etch depth increases. When the shadowing sticking coefficient decreases significantly, SWR increases beyond a certain critical depth due to reemission. Additionally, the noise also increases as the sticking coefficient decreases because of an increased number of reemitted particles. The simulated results support very well the assertions and mechanisms of the experimental findings and an etch model based on shadowing and first-order reemission effects at low RIE pressure. PACS 52.65.Pp; 52.77.Bn; 81.65.Cf     

Monte Carlo simulation of mictomagnets

Monte Carlo simulations of a binary alloy with impurity concentrations between 20 and 45 at.% have been carried out. The proportion of large clusters relative to that of small clusters increases with the number of MC diffusion steps as well as impurity concentration. Magnetic susceptibility peaks become more prominent and occur at higher temperatures with increasing impurity concentration. The different peaks in the susceptibility and specific heat curves seem to correspond to different sized clusters. A freezing model would explain the observed behaviour with the large clusters freezing first and the small clusters contributing to susceptibility (specific heat) peaks at lower temperatures.Contribution No. 153 from the Solid State and Structural Chemistry Unit     

Monte Carlo simulation of carbon monoxide,carbon dioxide and methane adsorption on activated carbon

In this study, the adsorption capacity of pure and activated carbon with regard to carbon monoxide (CO), carbon dioxide (CO₂) and methane (CH₄) gases at 298?K and pressure from 0.01 up to 2.0?MPa has been investigated computationally. Computational work refers to Monte Carlo (MC) simulation of each adsorbed gas on a graphite model with varying density of activation sites. The Grand Canonical Monte Carlo (GCMC) simulation technique was employed to obtain the uptake of each adsorbed gas by considering a graphite model of parallel sheets activated by carboxyl and hydroxyl groups, as observed experimentally. The simulation adsorption data for these gases within the examined carbon pore material are presented and discussed in terms of the adsorbate fluid molecular characteristics and corresponding interactions between adsorbate species and adsorbent material. We found that the simulated adsorption uptake of the examined graphite model under these conditions with regard to the aforementioned fluids increases in the order CO?<?CH₄?<?CO₂.     

Monte Carlo simulation for ultra-small MOS devices

This paper discusses advanced needs of Monte Carlo simulation approaches for MOS silicon devices scaled below 0.1 μ m channel length. For predictive simulation over a wide range of biases, it is necessary to provide the Monte Carlo procedure with tuning capabilities to adjust the mobility through calibration of the interface roughness scattering. This is accomplished by introducing a semi-empirical procedure with a physical elastic scattering rate and an inelastic rate with tunable strength. To resolve the role of hot carriers in relation to oxide interface damage, it is also important to realize fully bipolar MOS simulation, so that one can analyze the transport of impact-ionization generated carriers and secondary ionization. As the devices become quite small, three-dimensional simulation can be not only feasible, but also necessary to resolve the granularity of doping profiles and the complete carrier–carrier and carrier–ion interactions. Issues of device Monte Carlo implementation on parallel environments are discussed, and a practical approach for resolving the short-range forces of the charge–charge interaction in three dimensionals is described. Several examples and preliminary results are presented to illustrate the various issues.     

Monte Carlo simulation for radiative kaon decays

For high precision measurements of K decays, the presence of radiated photons cannot be neglected. The Monte Carlo simulations must include the radiative corrections in order to compute the correct event counting and for efficiency calculations. In this paper, a method for simulating such decays is briefly described. Arrival of the final proofs: 24 November 2005     

Fully parallel code for Monte Carlo simulation

We present a fully parallel version of Monte Carlo simulation of the Ising model using the Metropolis algorithm. In the 3-dimensional version the performance can be enhanced by a factor >20 in 16-bit word processors relative to other multispin codes. This factor could be further increased if implemented in 64-bit word computers.     

Efficient kinetic Monte Carlo simulation

This paper concerns kinetic Monte Carlo (KMC) algorithms that have a single-event execution time independent of the system size. Two methods are presented—one that combines the use of inverted-list data structures with rejection Monte Carlo and a second that combines inverted lists with the Marsaglia–Norman–Cannon algorithm. The resulting algorithms apply to models with rates that are determined by the local environment but are otherwise arbitrary, time-dependent and spatially heterogeneous. While especially useful for crystal growth simulation, the algorithms are presented from the point of view that KMC is the numerical task of simulating a single realization of a Markov process, allowing application to a broad range of areas where heterogeneous random walks are the dominate simulation cost.     

Monte Carlo simulation of the enantioseparation process

By means of Monte Carlo simulation, a study of enantioseparation by capillary electrophoresis has been carried out. A simplified system consisting of two enantiomers S (R) and a selector chiral C, which reacts with the enantiomers to form complexes RC (SC), has been considered. The dependence of Δμ$\Delta \mu$ (enantioseparation) with the concentration of chiral selector and with temperature have been analyzed by simulation. The effect of the binding constant and the charge of the complexes are also analyzed. The results are qualitatively satisfactory, despite the simplicity of the model.     

Monte Carlo simulation of exposure factor

In this paper, we simulate the exposure factor by a simple model of a free-air ionization chamber with the Monte Carlo programme Geant4. Special emphasis is placed on the discussion of the exposure factor related to parameters of the chamber model. The reason for the variation in exposure factor with incident ray energy is also analysed in terms of reaction cross section for different types of reactions. The obtained results indicate that our simulation is accurate in the calculation of the exposure factor and can serve as a reference in designing air ionization chambers.     

Monte Carlo simulation of nanometric cutting

Nanometric cutting of single-crystal materials at conventional cutting speeds (5?m?s^?1) is simulated for the first time using a new Monte Carlo method that is applicable to systems that are neither canonical nor microcanonical. This is accomplished by defining a local temperature in the cutting zone using the thermal analysis developed by Komanduri and Hou for conventional machining. Extension of this method to the nanometric regime permits an accurate estimate of the local temperature in cutting. This temperature is then employed in the Boltzmann probability distribution function that is used to determine the acceptance–rejection of Monte Carlo moves in the simulation. Since cutting speed is closely related to cutting temperature, the cutting speed enters the calculation via the thermal analysis equations. The method is applied to nanometric cutting of single-crystal aluminium with the crystal oriented in the (001) plane and cut in the [100] direction. Three positive rake cutting tools, namely 10°, 30° and 45°, are employed to investigate the effect of the rake angle on the forces, the specific energy and the nature of the chip formation. The method is evaluated by direct comparison with corresponding molecular dynamics simulations conducted under the same conditions.     

Monte Carlo simulation of liquid neopentane

Monte Carlo simulations are reported for liquid neopentane at 25°C and 0·592 g cm^-3 using three different potential models. These results are compared with X-ray diffraction data and simulations for other tetrahedral molecules.     

A Monte Carlo simulation of coagulation

A Monte Carlo simulation technique is described for the study of the coagulation of suspended particles. The method is computationally efficient since the particle trajectories are not used to determine coagulations. Instead, pairs of particles are assigned probabilities to coagulate and the evolution is computed as a stochastic Markov game. We also describe a simple analytic method to obtain the stationary distribution of sizes for the various mechanisms of relative particle motion. It is demonstrated that the simulation yields the correct stationary size distribution independent of initial condition.     

Monte Carlo simulation of aorta autofluorescence

Results of numerical simulation of autofluorescence of the aorta by the method of Monte Carlo are reported. Two states of the aorta, normal and with atherosclerotic lesions, are studied. A model of the studied tissue is developed on the basis of information about optical, morphological, and physico-chemical properties. It is shown that the data obtained by numerical Monte Carlo simulation are in good agreement with experimental results indicating adequacy of the developed model of the aorta autofluorescence.     

Metastable ion studies. XI. A Monte Carlo method for the simulation of consecutive metastable fragmentations

A method is presented for predicting the shape of the metastable peak resulting from two metastable fragmentations taking place consecutively in the first field-free region of the GEC/AEI MS902S mass spectrometer. Distributions of released kinetic energy for the pair of reactions: m₁⁺ → m₂⁺ + x; m₂⁺ + m₃⁺ + y, derived from their respective metastable peaks, were used in a Monte Carlo simulation of the two-stage consecutive metastable fragmentation: m₁⁺

(m₂⁺)

m₃⁺. Results are presented for the reactions C₂H₅N⁺ →C₂H₄N⁺ + H^-; C₂H₄N⁺ → C₂H₂N⁺ + H₂; C₂H₅N⁺ → C₂H₂N⁺ + H₂ + H^∓. The method canreadily be extended to multistage decompositions.     

X射线能谱测量的蒙特卡罗成像模拟

针对高能强流电子束轰击高Z靶产生的X射线的能谱测量问题,采用蒙特卡罗方法进行成像模拟研究。高能X射线能谱通常由对X射线经过衰减体的直穿透射率曲线进行解谱获得。设计了带多准直孔的截锥体模型,在单次模拟成像中获得完整的衰减透射率曲线,有效避免了散射光子对透射率曲线以及X射线能谱重建的影响。成像面采用非均匀划分网格计数,将大部分探测计数点集中于各准直孔出口处,保证关注区域模拟成像的精细度,同时减少总体计算时间。