Javier Domínguez-Vázquez

A molecular dynamics study of an Au/Cu(001) interface

This work focuses on the analysis of atomic distances and deformations in an Au/Cu(001) metallic interface and on the calculation of the energy of this interface. We study the possible adaptation of the atomic distances at the interface of two crystals with a considerable difference between their lattice parameters, such as found in Au and Cu. These crystals have a misfit of 12.8% of such parameters. Hence, the growing thin film-substrate interface is strained. We show how the relaxation of different substrate-cluster structures (a few monolayers) takes place on an atomic scale. We find that pseudomorphic growth is only possible when the system is a Cu cluster on top of an Au substrate. In the opposite case, Au on a Cu substrate, the system relaxes generating a network of dislocations. In particular, mean changes in the lattice parameters at the interface are quantified. In addition, we carry out the energetic analysis of these systems, which is of great interest to describe local properties such as electrical conduction.

Molecular dynamics study of a Ni/Cu(001) interface

The Ni/Cu(001) metallic interface shows interesting magnetic properties due to the lattice misfit. Its components exhibit a misfit of 2.6% in their lattice parameters. Hence, the growing thin film–substrate interface is strained. We are interested exclusively in the solid phase formation effects; therefore, the growth kinetics effects will be avoided. This work is focused on the analysis of atomic distances and deformations in this system and on the calculation of the interface energy. It is shown how the stabilization on an atomic scale of different Ni nanocrystals set down on top of a large enough Cu(001) crystal is achieved. The adjustment between the lattice parameters of the Ni clusters on the Cu substrate is analysed. Specially, changes in the atomic distances at the interface are quantified. The main result is that the anisotropy of the structural matching causes a cubic lattice to become a tetragonal one. In addition, we carry out the energetic analysis of this interface.

Empirical approach for the interatomic potential of carbon

Abstract A new empirical potential for both diamond and graphite structures, inspired by the previously published work by Heggie [J. Phys. Condensed Matter 3 (1991) 3065; Carbon 30 (1992) 71], has been developed. This potential accounts for the weak attractive interaction between planes as well as for a direct interpolation between the sp2 and the sp3 orbital configurations. In order to check the validity of the proposed interatomic potential we study by molecular dynamics (MD) carbon structures (diamond and graphite) around the equilibrium and finally, the dynamic behaviour of a vacancy and an interstitial are also studied.

Relaxation of metals: A model based on MD calculations

Abstract The relaxation processes of copper associated with the presence of point defects such as vacancies and interstitials, have been analysed via Molecular Dynamics (MD). The accommodation of the atoms in the lattice is due to a tension field which arises from the presence of the above mentioned defects. These tension fields are calculated and expressed in terms of the effective displacement volume associated with the corresponding defect. Two main conclusions are: First, the finiteness of the tension fields, in contradiction with usual relaxation models applied in solving problems related to atomic mixing. Second, the relaxation process may be considered on average as isotropic except for very near to the surface, where the process is markedly anisotropic due to the surface itself. These results are summarised by means of an analytical expression, applicable in studies of solid relaxation and in other related problems of ion beam modification of materials.

A hybrid MC–MD calculation study

Abstract It is widely accepted that the binary collision approximation (BCA) is rather accurate in the high energy regime of a collision cascade while multiple interaction describes better the low energy and post collisional regimes. It will be therefore desirable to determine the grade of accuracy when both regimes are treated by their corresponding approximations in a single case study. A silicon sample self-bombarded by 2.5 keV ions has been studied. First of all, 1000 trajectories have been calculated by means of the molecular dynamic MD-TOPS code in order to have a reasonable statistic. These results will be considered as a reference when comparing with a second set of 1000 trajectories calculated within the BC approximation using the Monte Carlo MC-TOPS code in the 2.5–0.25 keV energy range and resuming the calculation in the low energy regime by MD. Input parameters required by MC calculations, such as the displacement energy, E d, bulk energy, E b, and surface binding energy, U, for silicon have b...

Ion beam mixing calculation of multilayer samples

Abstract Compositional changes induced by ion beam bombardment have been studied for a variety of multilayer structures by means of the computer code tops-tui , which has been developed to simulate the collisional transport in polyatomic targets. In this code, the relocation cross-sections are calculated via Monte Carlo simulation and are introduced into the set of integro-differential equations to be solved numerically. These equations describe the dynamic behaviour of the mixing processes in which sputtering and relaxation are also been included. Different multilayer structures based on Fe plus metals like Al, Ni and Zr are analysed. The distortion of the concentration profiles at the interfaces and the width of the regions which attain the nominal composition are investigated as a function of the fluence. The dependence of these magnitudes on the nuclear deposition energy and the effective volume ratios of the components is studied in more detail.

A three-dimensional relaxation model for calculation of atomic mixing and topography changes induces by ion beams

Abstract A simple model for three-dimensional material relaxation associated with atomic mixing is presented. The relaxation of the solid to accommodate the extra effective displacement volume Ω of an implanted or relocated atom is modelled by treating the surrounding solid as an incompressible medium. This leads to a tractable general formalism which can be used to predict implant distribution and changes in surface topography induced by ion beams, both in monatomic and multicomponent targets. The two-component case is discussed in detail.

Monte Carlo prediction of crater formation by single ion impact on solid surface

Abstract A method is presented for predicting the topography changes following the impact of one energetic ion on the plane surface of a monatomic amorphous solid. This is done in two stages. The first is a Monte Carlo calculation of the sputter yield and interior distribution relocated atoms, with no compensation for local departures from equilibrium density. In the second stage there is a systematic relaxation of the solid, in which the density returns to its previous constant value and a crater develops in the surface. Two alternative methods of carrying out stage two are compared. In the first the solid is subdivided into cells within which relaxation is carried out normal to the surface, as in previous one-dimensional studies. The second method treats the solid as a 3-dimensional incompressible medium. Both seem to reproduce quite well the main features found experimentally.

Simulation of ion beam induced atomic mixing of interfaces

Collisional atomic mixing at an interface induced by an ion beam is analysed by a dynamic Monte Carlo simulation. The target taken for the simulations is a metallic Zr(50 nm)/Ni bilayer bombarded with different ions and energies in order to compare with experimental results. We discuss the dependence of the mixing rate with different microscopic parameters like the deposited energy by the ion, the atomic flows through the interface and the mean range of recoils. Mixing rates in the bombardment of Zr(50 nm)/Ni and Ni(50 nm)/Zr bilayers are compared.

Molecular dynamics study of the relaxation processes induced by defects in metals

Abstract The aim of this work is to study the relaxation processes associated with the most common defects generated in solids by ion bombardment. Vacancies, self-interstitials, di-vacancies, di-self-interstitials and small clusters of vacancies and/or self-interstitials were generated in a microcrystal of copper, allowing the system defect + crystal to evolve dynamically. Elastic deformation and effective volumes were some of the physical magnitudes determined in this study. The depth dependence of the above-mentioned magnitudes was also investigated. The crystals had 13 × 13 × 13 cubic unit cells containing 9842 atoms. Prior to the generation of any defect, relaxation of the whole crystal at T =0 K was performed in order to isolate the relaxation processes being studied from relaxation processes such as surface relaxation or the displacement of the atoms to accommodate themselves exactly at the minima of the potential energy of the lattice. The tension fields generated in the solid by a single point defect (one interstitial or one vacancy) were calculated. Their corresponding effective volumes were also determined, showing a strong dependence on the distance to the point at which the defects were generated. Both vacancy and interstitial effective volumes tend to zero for large distances from the defect. For the vacancy, distances greater than 9 A are enough for the defect to produce a negligible deformation. The interstitial, however, requires distances of the order of 15 A to be absorbed by the lattice without significant displacement of the lattice atoms.