Molecular dynamics studies of superlattice growth phenomena using the embedded atom method

Detta är en avhandling från Linköping : Linköpings universitet

Sammanfattning: This thesis is about the simulation of phenomena related to the growth of superlattices (SL). SLs have attracted considerable attentions in recent years due to their many interesting qualities. Work on the growth and characterisation of SLs has been performed in the Thin Film group at the department of Physics and Measurement Technology at Linkoping university. Investigations, in conjunction with the experimental work, have been carried out in the Theoretical Physics group. The work presented in this thesis has been part of that effort. The studied structures consisted of molybdenum (Mo), and tungsten (W), two metals with a very small lattice mismatch, thus forming a good model system. The simulations have been performed using Molecular Dynamics (MD) and the Embedded Atom Method (EAM). In MD, the classical equations of motions are solved numerically for a many-particle system. The behaviour of the studied system is governed by the used interaction potential. The EAM is a semi-empirical pair-functional interaction potential based on Density Functional Theory. It incorprates many-body effects in a natural way while retaining computational efficiency. The impact of backscattered atoms of the sputter gas, Ar or Kr, on the growth surface of the superlattice has been studied. The effects are dominated by the exact impact point of the atom on the surface unit cell, and the mass ratio between the gas atom and the metal atoms in the surface layer. Among the studiedphenomena are energy accomodation coefficients, energy dissipation, types and amount of defects generated by the impact, and dynamics as well as mechanisms of defect generation,Further studies of the detailed structure and dynamics of a single interstitial in an otherwise defect-free lattice showed that the interstitial is accommodated into the lattice through small rearrangements of the surrounding atoms. The resulting configuration is a two-dimensional dumbbell, which does not move. Movement of the defect is performed through the transformation of the dumbbell into a classical one-dimensional crowdion, which then moves through transformation into other, larger, crowdions.The results are compared with the classical model of diffusion through hopping between lattice sites. Through suitable simplifications, the classical model is retained. Diffusion velocities and the activation energy for the process were also computed. 

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