J.J. Jiménez-Rodríguez

On the influence of atomic mixing on the evolution of ion-implantation profiles

Abstract Various processes contributing to the evolution of high-fluence implantation profiles in solids are considered within a general scheme. In particular the influence of atomic mixing is analysed. A Green function is derived which contains all the information on the instantaneous profile. In the Gaussian approximation, simple expressions for parameters which describe the ion-induced relocation effects on the depth profile are found. The contributions of recoil and cascade mixing are discussed. The relative significance of diffusive and collisional processes on the profiles can also be assessed.

Early stages of bump formation on the surface of ion-bombarded graphite

Abstract We investigate possible mechanisms for the formation of bumps, of atomic dimensions, in the surface layer of graphite under low-energy, single ion bombardment. We perform a Molecular Dynamics simulation and analyse the results in view of recent experimental data obtained with the Scanning Tunnelling Microscope. Only the early stages of bump formation are described. The depth profile of momentum deposited in the cascade is evaluated and used, in conjunction with the atomic density profile, to characterise interlayer distortions. The surface bumps appear under the presence of stresses developed in the near-surface layers of the target, due to defects created by the collision cascade. The bump originates around the positions of vacancies and interstitials, and is stabilised by the presence of interlayer interstitials. We have also produced a video film that helps in the visualisation of the time evolution of topographical features.

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.

Fluence dependent mixing of isotopic targets: a theoretical case study

Abstract The collisional mixing of two isotopic species in a Ge sample by 5 keV Ar bombardment is studied theoretically. The evolution of the target isotope concentrations with fluence is calculated, by solving numerically the balance equation with the help of the TUI code. Its analytical input quantities for the atomic relocation under ion bombardment are checked against the predictions of a Monte Carlo computer simulation. In steady state, the light isotope is found to be depleted up to depths of around 90 A, and enriched further inside. We compare the dependence of the preferential sputter enrichment on fluence with experimental data.

Atomic mixing of multi-component materials: the dilute limit

Abstract A formalism for the analysis of ion-induced desorption of overlayers is discussed. The approach is based on a linearization of the equations developed by Sigmund et al 1 for the sputtering of multi-component targets. The parameters entering the evolution equation for the concentration profiles are evaluated and commented upon for two atomic mixing mechanisms.

Non-linear differential equation for atomic mixing: 2. Numerical results

Abstract A discussion is given of a non-linear Fokker-Planck (diffusion-type) differential equation for atomic mixing, developed in an earlier paper by the authors. Some simplifications of the earlier theory are presented, together with a sign-correction. Numerical results are given, obtained by a recently-developed machine program, for the case of an initial uniform layer of silver on a silicon substrate, irradiated by a 45 keV beam of Ar + ions at normal incidence.

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.

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.