A.M.C. Pérez-Martín

Escape probability of the outermost sputtered atoms

Abstract For sake of comparison computer simulation (CS) and Monte Carlo (MC) codes have been developed and applied to the sputtering of a random target. The codes consider binary collisions and neglect inelastic energy losses. The escape probability of the outermost sputtered atoms is calculated by the two approaches. It turns out that, in this very shallow region, the mean free path should depend on depth and emerging angle, besides the fact that the azimuthal symmetry is broken. When the MC code is modified to account for these effects, a very good agreement is obtained with the results of the CS code.

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.

Effect of temperature on the bulk atomic relocation in low-energy collision cascades in silicon: A molecular dynamics study

Abstract The production of damage in a Si lattice by internally starting 100 eV self-recoils has been studied using a MD simulation. Different initial lattice temperatures below the Debye temperature for Si have been considered. The number of stable atomic displacements and the amount of atomic mixing increase with the initial target temperature. The increase with temperature of atomic mixing is nonlinear -appreciable changes take place between 300 and 500 K, while the difference between the amount of mixing corresponding to 0 and 300 K is negligibly small. The size of the cascade zone in which stable atomic displacements occur doubles itself for temperature changes between 0 and 300 K, with a value for 500 K lying in between. This nonmonotonic variation with the initial target temperature of the size of the cascade zone may have its origin in the correlation between the initial direction of motion of the starting recoil and the directions of thermal velocities of the neighbouring atoms around this recoil.

Angular distribution and escape probability of sputtered atoms

Abstract To quantify the importance of the role played by the inherent discontinuity due to the surface, depth of origin, angular and energy distributions of sputtered atoms are calculated, for various mass ratios, on the basis of a Monte Carlo simulation code. Calculations are performed, on a random target, assuming an initially isotropic and E0−2-like energy distribution. Surface effect is treated by a mean free path length which depends on depth and direction of the emerging atom, its effect being a decrement in the collision rate and a net average deflection towards the normal to the surface, this being more pronounced as the moving atom becomes closer to the surface. This surface effect plus the role of a planar surface barrier increase the escape probability to the peak below the surface position, particularly for light atoms. Mass dependence of the ejected atoms, with respect to the mass of the matrix, is also analyzed in terms of the angular and energy distributions of the ejected atoms, from a given depth. The transfer energy cross-section plays a role in the opposite direction to that of the angular scattering but differences between a light and a heavy atom can be found. From shallow regions, heavy atoms are more likely to be ejected, for glancing emerging directions but, the opposite happens from greater depths. Energy distribution shows clearly that heavy atoms get to the surface, from greater depths, after having spent more energy.

Atomic structure of Ni nanoclusters on Cu(001) surfaces

Depositions of Ni clusters on a Cu(001) surface have been simulated by molecular dynamics in order to produce magnetic nanostructures. Two arrangements of the atoms at the interface between the Ni clusters (a few monolayers) and the Cu substrate, overlapped and non-overlapped, have been analysed. The difference between Ni and Cu lattice parameters (2.6%) gives rise to strain at the interface, which is the cause of magnetoelastic anisotropy. We have focused our interest especially on matching effects. The bombardment energy was varied between 0 and 1 eV/atom. Differences in the nanocluster morphology due to this have been discussed. Lattice defects which develop in the deposited clusters have been analysed. Final atomic distances, especially mean changes in lattice parameters, have been quantified at the interface. A study of atomic mixing and of its influence in spacing between layers has been also accomplished.

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.

Energy spectra of reflected and sputtered particles in magnetron deposition systems

Abstract We have studied by the Monte Carlo (MC) simulation technique the transport through the plasma of sputtered particles and gas atoms reflected at the cathode in a magnetron system. The energy spectra and the total energy transported to the substrate by these two fluxes of particles are calculated. Results are presented for different cathode species and for various pressures of the discharge gas. The energy spectra of the reflected particles show a strong dependence on the cathode material and this may have an influence on the properties of the thin films produced.

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.

Elastic Properties of Co/Cu Nanocomposite Nanowires

The mechanical deformation of Co/Cu composite nanowires was simulated by molecular dynamics in a state of uniaxial tensile and compressive stress. The Young’s modulus and initial yield stress have been derived from the stress–strain curves at different conditions. For tensile strength, the effect of strain rate, volume/surface area ratio, temperature, and thickness ratio between Co and Cu sublayers was analyzed depending on the crystallographic orientations of the nanowires. At high values, the elastic modulus and yield stress depend on the strain rate; and some differences with the crystallographic orientation due to nonlinear effects appear. Both magnitudes diverge from the bulk values with decreasing the volume/surface area ratio, increasing in the case of 〈110〉 nanowires and decreasing for the other two directions. For 〈100〉 nanowires, grains undergo a crystallographic reorientation towards 〈111〉 and 〈110〉 directions. Besides, for these nanowires hexagonal close-packed atoms are preferably in the Co sublayer; and face-centered cubic atoms, in the Cu sublayer unlike nanowires in the other two directions. Plastic deformation takes place more easily in Cu sublayers. Nanowires show differences in the slip mechanism for 〈110〉 and 〈100〉 directions. In compression, the former system slips via both {111}〈112〉 and {111}〈110〉 dislocations; and the latter, only through {111}〈112〉 dislocations.