Papers by Keyword: Kinetic Monte Carlo

Abstract: Kinetic Monte Carlo simulations are a useful tool to predict and analyze the ionic conductivity in crystalline materials. We present here the basic functionalities and capabilities of our recently published Monte Carlo software for solid state ionics called MOCASSIN, exemplified by simulations of several model systems and real materials. We address the simulation of tracer correlation factors for various structures, the correlation in systems with complex migration mechanisms like interstitialcy or vehicle transport, and the impact of defect interactions on ionic conductivity. Simulations of real materials include a review of oxygen vacancy migration in doped ceria, oxygen interstitial migration in La-rich melilites, and proton conduction in acceptor doped fully hydrated barium zirconate. The results reveal the impact of defect interactions on the ionic conductivity and the importance of the defect distribution. Combinations of these effects can lead to unexpected transport behavior in solid state ionic materials, especially for multiple mobile species. Kinetic Monte Carlo simulations are therefore useful to interpret experimental data which shows unexpected behavior regarding the dependence on temperature and composition.

Abstract: The coalescence of the fullerenes encapsulated in the host single-walled carbon nanotube (SWNT) with bend junction is explored theoretically by energy driven kinetic Monte Carlo (EDKMC) method. Despite the lower productivity of successful coalescence (with clear identified chiralitys), there is still a possibility to form the inner tube with bend junction which can copy the separated pentagon and heptagon from the host tube exactly with the chiralitys at the two sides clearly identified. The statistic to ~20 successfully coalesced inner tubes with bend junctions shows that the chiral angle differences (CAD) between the two sides > 20 ^o, which is determined by the minimization of the formation energies of the junctions. Therefore, the chirality distribution of the inner tube may be effectively narrowed by tuning the CAD of the bend host tube, which may provide an alternative way to the application of specific control to the chiralitys.

Abstract: In this paper we use three dimensional kinetic Monte Carlo simulations on super-lattices to study the hetero-polytypical growth of cubic silicon carbide polytype (3C-SiC) on hexagonal 6H-SiC step-bunched substrates with miscuts towards the <11-20> and <1-100> directions. We find that the preferential 3C conversion observed on <1-100> misoriented substrates could be due to a different step-to-island interaction which enhances island stability and expansion in this specific direction. For this reason 3-4° degrees off step-bunched 6H substrates with miscut towards the <1-100> direction should be the best choice for the stable and reproducible hetero-polytypical growth of high quality cubic epitaxial films.

Abstract: Triple-defect formation in B2-ordered binary A-B intermetallic compounds results fromthe asymmetry between the formation energies of A- and B-antisite defects. Chemical disorderingin such systems is strictly correlated with vacancy formation, which is the reason for usually veryhigh vacancy concentration. Consequently, Kinetic Monte Carlo (KMC) simulation of processes occurringin the triple-defect systems and controlled by atomic migration via vacancy mechanism mustinvolve complete vacancy thermodynamics – i.e. the simulated system must contain the equilibriumtemperature-dependent number of vacancies. The fully consistent approach based on two differentMonte Carlo techniques has been applied in the present study. The AB intermetallic was modelled withan Ising-type Hamiltonian and KMC simulated for “order-order” kinetics with temperature-dependentequilibrium number of vacancies previously determined by means of Semi Grand Canonical MonteCarlo (SGCMC) simulations. The procedure required in addition the determination of saddle -pointenergies assigned to particular atomic jumps to nn vacancies. Their values were estimated in relationto the nn pair-interaction energies with reference to Molecular Statics simulations performed for NiAlsystem with EAM energetics. The results elucidated the role of triple-defect formation as the atomisticscaleorigin of the experimentally observed surprisingly low rate of the “order-order” kinetics in bulkNiAl.

Abstract: In situ monitoring of surface processes and understanding of growth processes are important in achieving precise control of crystal growth. Therefore, many surface monitoring techniques are used during crystal growth by molecular beam epitaxy (MBE). The most popular is reflection high-energy electron diffraction (RHEED) and photoemission current which provides information on the morphology during the growing surface. The photoemission oscillation technique has been successfully used in situ to monitor the growth of materials and to control the thickness as well as the roughness of the deposited layer. In this paper, we report results of atomic scale simulations used to study the dynamics of homoepitaxial growth of GaAs(001) β2(2x4) reconstructed surface and, in particular, the RHEED oscillations of the photoemission current.

Abstract: Several methods have been introduced to study and simulate homoepitaxial growth of III-V materials. GaAs (001) surface has widely been used in the last three decades due both to its importance as substrate and for characterization of epitaxial growth. In this paper, we firstly study the initial stage of homoepitaxial growth on a GaAs (001) β2(2x4) reconstructed surface using As2 . The simulation was carried out with Kinetic Monte Carlo simulations including the zinc blend structure β2 (2x4) reconstruction of GaAs surface. Then we discus results of the homoepitaxy GaAs on GaAs particularly morphological evolution of the two dimensional islands and observations were made in real-time at the growth temperature using reflection high energy electron diffraction (RHEED) and roughness morphology.

Abstract: This paper introduces a octree search Kinetic Monte Carlo(OS-KMC) that combines the topological requirements for representing an evolving multivalued surface using minimal memory and an efficient search algorithm for the realization of the KMC time evolution. In addition, the data structure for OS-KMC also provides a nature way to generate hexahedral element meshes for the integration between simulator and performance analysis tool. The density of mesh grid and the refinement of the hexahedral element can be controlled in this octree based mesh generation method. In simulating surface morphology during wet etching and micro structure formed by composite MEMS processes, the octree search KMC shows good simulation results with better calculation performance. The octree structure enables the Monte Carlo solutions for large scale problems with complex dynamic surfaces.

Abstract: In this study, the role of solute segregation on the strength and the evolution behavior of dislocation junctions is studied by utilizing kinetic Monte Carlo and 3D dislocation dynamics simulations. The different solute concentrations and the character of the junctions are all included in the simulations in an effort to make a parametric investigation. The results indicate that solute segregation can lead to both strengthening and weakening behavior depending upon the evolution of the dislocation junctions.

Abstract: A theoretical and atomistic study of diffusion and stability of a pure element hollow nanosphere and nanotube is performed. The shrinkage via the vacancy mechanism of these hollow nano-objects is described analytically. Using Gibbs-Thomson boundary conditions an exact solution of the kinetic equation in quasi steady-state at the linear approximation is obtained. The collapse time as a function of the geometrical sizes of the hollow nano-objects is determined. Kinetic Monte Carlo simulation of the shrinkage of these nano-objects is performed: it confirms the predictions of the analytical analysis. Next, molecular dynamics simulation in combination with the embedded atom method is used to investigate diffusion by the vacancy mechanism in a Pd hollow nanosphere and nanotube. It is found that the diffusion coefficient in a Pd hollow nanosphere and nanotube is larger near the inner and external surfaces compared with the middle part of a nanoshell. The molecular dynamics results provide quite a strong but indirect argument that a real pure element hollow nanosphere and nanotube may not shrink as readily via the vacancy mechanism as compared with the predictions of the analytical analysis and kinetic Monte Carlo simulations.

Abstract: This paper describes the computer simulation of irradiation growths induced by neutron irradiations in zirconium using a combination of Molecular Dynamics (MD) and Kinetic Monte Carlo (KMC) methods. First, we performed the MD simulation of the displacement cascade on a defect cluster to study the interaction between the defect cluster and the displacement cascade. The MD simulations provide a lot of information on the amount of the defect production and the subsequent morphological change in the defect cluster. The results are used to make simple models that describe the nature of the displacement cascade overlap on the defect clusters. The models are then implemented into the KMC simulation code to extend the length- and time-scale of the simulation, which allows us to evaluate directly the defect cluster accumulations during a long-term irradiation. The irradiation growth strain resulting from the defect cluster accumulations is simply evaluated, and compared to an available experimental data. The comparison suggests that the displacement cascade overlap plays an important role on the irradiation growth, and, consequently, the KMC method with the simple models must be appropriate for the simulations of the irradiation growth.