Collision Cascades In Metals Research Articles

Modeling hot-electron generation induced by electron promotion in atomic collision cascades in metals

We present a computer simulation model for the investigation of electron promotion processes in atomic collision cascades in metals. The model combines molecular dynamics and molecular orbital calculations to describe the formation of hot electrons in close atomic collisions. We apply this model to a set of collision cascades initiated by the impact of a 5-keV silver atom onto an Ag(111) surface. The calculations show that about 15% of the bombarding energy originally introduced into the solid is dissipated into the generation of hot electrons in close collisions. Furthermore, we find that the nascent excitation energy spectrum closely resembles a power law $f({E}_{\text{exc}})\ensuremath{\propto}{E}_{\text{exc}}^{\ensuremath{-}\ensuremath{\delta}}$ with exponents $\ensuremath{\delta}\ensuremath{\approx}2--3$.

Predicting secondary ion formation in molecular dynamics simulations of sputtering

We present a model to incorporate the excitation and ionization of sputtered particles into molecular dynamics simulations of ion bombardment-induced collision cascades in metals. The kinetic excitation of the solid is described by electronic friction experienced by all moving particles and electron promotion in close atomic collisions. Transport of the resulting excitation energy is treated by a nonlinear diffusion equation. The resulting space- and time-dependent electron temperature is then introduced into a rate equation model describing the ionization of ejected particles. This way, secondary ion formation is described by assigning an individual ionization probability to every sputtered atom. Averaging over the entire flux, this allows the prediction of measurable quantities like integral or spectral ionization probabilities as well as velocity spectra of secondary ions.

Low-energy electronic excitation in atomic collision cascades: A nonlinear transport model

A computer simulation model is presented which allows one to incorporate low-energy electronic excitation into the molecular dynamics computer simulation of atomic collision cascades in metals. The model treats the electronic energy losses experienced by all moving atoms as a source term for electronic excitation energy, which is assumed to spread around the original point of excitation with a diffusivity D. In order to acknowledge i the large temperature gradients and ii the local lattice disorder within the cascade volume, the electronic heat diffusivity is allowed to vary as a function of space and time, thus leading to a strongly nonlinear diffusion of electronic excitation energy. The corresponding diffusion equation is developed and numerically solved for an exemplary collision cascade initiated by the impact of a 5 keV silver atom onto an Ag111 surface. It is shown that electron surface temperatures of several thousand kelvin can be reached at times after the impact at which most emission of surface particles occurs. This excitation may therefore influence the ionization or excitation of such sputtered species.

The effects of one-dimensional glide on the reaction kinetics of interstitial clusters

Collision cascades in metals produce small interstitial clusters and perfect dislocation loops that glide in thermally activated one-dimensional (1D) random walks. These gliding defects can change their Burgers vectors by thermal activation or by interactions with other defects. Their migration is therefore `mixed 1D/3D migration' along a 3D path consisting of 1D segments. The defect reaction kinetics under mixed 1D/3D diffusion are different from pure 1D diffusion and pure 3D diffusion, both of which can be formulated within analytical rate theory models of microstructure evolution under irradiation. Atomic-scale kinetic Monte Carlo (kMC) defect migration simulations are used to investigate the effects of mixed 1D/3D migration on defect reaction kinetics as a guide for implementing mixed 1D/3D migration into the analytical rate theory. The functional dependence of the sink strength on the size and concentration of sinks under mixed 1D/3D migration is shown to lie between that for pure 1D and pure 3D migration and varies with L, the average distance between direction changes of the gliding defects. It is shown that the sink strength in simulations for spherical sinks of radius R under mixed 1D/3D migration for values of L greater than R can be approximated by an expression that varies directly as R 2. For small L, the form of the transition from mixed 1D/3D to pure 3D diffusion as L decreases is demonstrated in the simulations, the results of which can be used in the future development of an analytical expression describing this transition region.

Collision cascades in metals and semiconductors: defect creation and interface behavior

Using molecular dynamics simulations of collision cascades, we examine point defect and defect cluster formation mechanisms in metals and semiconductors. In metals we find that the primary mechanism causing separation of interstitials and vacancies is the pushing of vacancies toward the cascade center during the cooling phase of the cascade. We further describe how the isolation of a part of the liquid formed in the cascade can lead to the formation of interstitial clusters in metals. By comparing ballistically similar pairs of metals and semiconductors like Al and Si and Cu and Ge, we deduce how the cascade behavior depends on the nature of interatomic bonding and crystal structure. We also find that close to sharp interfaces of metals with different melting points the `vacancy push' mechanism can lead to most vacancies being pushed to one of the materials, and an asymmetry in the impurity introduction over the interface owing to an inverse Kirkendall effect.

A kinetic Monte Carlo study of mixed 1D/3D defect migration

Defect clusters form readily in collision cascades in metals, and some of the self-interstitial atom clusters form as crowdion clusters that diffuse by one-dimensional migration along a close-packed direction. Defect interactions and thermal fluctuations can cause the direction of one-dimensional migration to change, resulting in a mixed one-dimensional/ three-dimensional migration. Kinetic Monte Carlo computer simulations applied to model systems are used to investigate the effects of one-dimensional, three-dimensional and mixed one-dimensional/ three-dimensional migration on defect reaction kinetics. The functional relationships between the sink strength, the size of sinks and the average distance between direction changes during mixed one-dimensional/three-dimensional migration are explored.

Thermalization of sparse collision cascades in metals: simultaneous relaxation of nonequilibrium electron and ion distributions

Thermalization of ion bombardment induced large and sparse cascades in metals is studied. We introduce a model where nonequilibrium electron and ion distributions relax simultaneously, and coupling between the thermal ions and valence electrons allows heat exchange between electronic and ionic systems. This model takes into account the competing time scales of the relaxation processes and the heat outdiffusion from the cascade zone. Nearly free conduction electron bands are assumed, which restricts the applicability of the model to simple metals only. It is shown that in cascades with an extent of about 10 nm or more, a considerable part of the energy, residing initially in the electronic system, may reappear in the ionic system in the cascade zone. Ions in cascades with radius larger than 20 nm take up nearly all of the energy available in the cascade zone. Due to this energy transfer, the ionic system may experience appreciable additional heating in large cascades.

On the structure of irradiation-induced collision cascades in metals as a function of recoil energy and crystal structure

Abstract The binary collision code MARLOWE is used to study cascade sizes, defect densities and subcascade configurations in f.c.c., b.c.c. and h.c.p. metals as a function of recoil energy up to 1 MeV. The threshold energies for the formation of subcascades are determined using a definition based on the defect configuration after the collisional phase of the cascade. A computational method was devised for identifying subcascades, and it was used to determine the number and spacing of subcascades as a function of recoil energy. The results are presented to illustrate the effect of recoil energy, atomic mass density and crystal structure on cascade volume and vacancy density after the collisional phase of the cascade, and on the number of subcascades and the subcascade spacing. The cascade morphology in the collisional phase sets the initial conditions for defect production and clustering during the development of each cascade, as well as influencing the global evolution of the microstructure. Comparisons with experimental observations illustrate fundamental differences in cascade structure for cascades of high and low energy density.

High energy ion beam mixing in dense collision cascades

Atomic mixing of tracer atoms in dense collision cascades in metals is calculated. The model used in the calculations describes the atomic transport from the initial collisional phase to the late thermalized stage. The collisional mixing is calculated by Monte Carlo simulation and a thermal spike model is used to describe the late phase of the cascade. The cooling of a thermal spike is described by coupled heat conduction equations for conduction electrons and lattice. Atomic transport takes place above the glass transition temperature of a metallic matrix and the transport is treated as diffusion in liquids. The heat exchange between conduction electrons and lattice allows rapid quenching of thermal spikes in metals with high d-electron density. Mass and heat transport coefficients are calculated on the basis of elementary solid state models, accordingly; the model for ion beam mixing contains no adjustable parameters Calculations for metal markers in Ni, Cu, Ag, Pd, Au and Pt are in agreement with experimental results, when the cohesive energy of marker atoms is sufficiently similar to that of matrix atoms. The model predicts a pronounced temperature dependence for the high-energy ion-beam mixing in Cu, Ag and Au.

Energy spectra of electronic excitations in collision cascades

Abstract The energy distribution of excited electrons and the electronic energy density produced by collision cascades in metals is discussed. The value of the excitation energy density at the surface of aluminum are numerically estimated. It is shown that the energy density at the Fermi surface can reach fairly high values above 1020 eV/cm3 which correspond to temperatures above 103 K when thermalized.

Emission of atoms and electrons from high-density collision cascades in metals

Particle emission from metals subjected to energetic heavy ion bombardment is briefly reviewed. In the case of atom emission, emphasis is placed on the number (yield) and the molecular state (cluster) of ejected particles. The importance of near-surface sub-cascades is stressed. Electron emission is discussed with respect to the influence of bombardment with composite projectiles. It shows a strictly linear behaviour, as evidenced by an electron yield determined solely by the number and velocity of the atoms in the incident molecule/cluster. Thermionic emission as well as effects of molecular disintegration are concluded to be insignificant under the bombardment conditions considered here.

No enhanced electron emission from high-density atomic collision cascades in metals

Ion-induced secondary electron emission was studied for vanadium and niobium cluster ions at incident energies between 12.5 and 25 keV. The number of electrons ejected per incident atomic particle was found to be solely a function of the velocity, being independent of the number of atoms in the cluster. In spite of the extremely high energy densities created by cluster ion impact, there is no enhanced electron emission from collision spikes. This is discussed in terms of energy coupling between the system of lattice atoms and that of the electrons in the solid.