Showing papers by "Robert S Averback published in 1999"

Molecular dynamics investigations of surface damage produced by kiloelectronvolt self-bombardment of solids

Abstract: Molecular dynamics computer simulations were employed to study damage production mechanisms at solid surfaces during bombardment with kiloelectronvolt ions. Three separate mechanisms are identified: ballistic damage, viscous flow and microexplosions. Ballistic damage is created by the direct knock-on of atoms onto the surface as described within the binary collision approximation. Viscous flow refers to local melting and the forced flow of liquid onto the surface, and microexplosions occur when the high pressures in cascades lead to rupturing of the nearby surface. The relative importance of each mechanism depends on several parameters: atomic mass, melting temperature, atomic density, structure and atomic bonding of the target, and the mass and energy of the projectile. The simulations were performed for Pt. Au, Cu, Ni and Ge self-atom bombardment. Cascades in the interior of the targets were also examined to provide a comparison for the surface events. In addition several events of 4.5keV Ne an...

Coherent displacement of atoms during ion irradiation

Abstract: Ion irradiation is a common technique of materials processing, as well as being relevant to the radiation damage incurred in nuclear reactors. Early models of the effects of ion irradiation typically assumed that particles undergo two-body elastic collisions1, like billiard balls colliding in three dimensions. Later descriptions invoked such phenomena as localization of kinetic energy, thermalization and localized melting2,3,4. In all these descriptions, the displacement of atoms is chaotic in that slight variations in the ion's trajectory produce completely different, unpredictable sets of atomic displacements5. Here we report molecular-dynamics simulations of high-energy self-bombardment of copper and nickel, in which we see collective displacements of atoms. The high pressures developed in collision cascades centred well below the surface can cause a coherent displacement of thousands of atoms, over tens of atomic planes, in a shear-induced slip motion towards the surface. The mechanism leads to a significant increase in damage production near the surface, characterized by well-ordered islands of adsorbed atoms. Our findings suggest an explanation for some features of radiation damage, as well as for differences between ion and neutron irradiation6.

Burrowing of Co Nanoparticles on Clean Cu and Ag Surfaces

Abstract: Metal nanoparticles can display a unique behavior when deposited on substrates with a significantly lower surface energy. Co nanoparticles in the 10 nm size regime burrow into clean Cu(100) and Ag(100) substrates when deposited at 600 K and also assume the substrate orientation. Deposition at room temperature fails to show either burrowing or reorientation. Crucial in understanding these results are the capillary forces and surface tension associated with a nanoparticle: they must be high enough to drive atoms away from underneath the cluster.

Inverse Kirkendall mixing in collision cascades

Abstract: We study ion-beam mixing of metallic bilayer interfaces using classical molecular-dynamics simulations of 5 keV collision cascades in the vicinity of Co/Cu and Ni/Cu ~111! interfaces. We find that the production of vacancies and interface roughening is asymmetrical. On average, more Cu is introduced into the Co or Ni parts than vice versa, and more vacancies are produced in the Cu, indicative of an inverse Kirkendall effect in collision cascades. The effect is explained by the difference in melting points leading to different recrystallization rates of the two materials. @S0163-1829~99!11601-1# The forced mixing of atoms across interfaces is of both technological and scientific interest for ion implantation in layered structures, precipitation dissolution in reactor materials, and processing of materials by high strain conditions. Theoretical approaches to treating materials under such driving conditions has recently been reviewed by Martin and Bellon. 1 An as yet unknown element in these formulations, however, is an atomistic description of the mixing events at the interface. Such a description, specifying the configuration of mixed atoms and any defects that are created, is important as these details strongly influence the resulting microstructures. The kinetic roughening of interfaces is for instance expected to be sensitive to the average number of atoms

Recoils, flows and explosions: surface damage mechanisms in metals and semiconductors during 50 eV–50 keV ion bombardment

Abstract: We review some recent simulation results on mechanisms of damage production close to a surface during ion irradiation. The simulation work encompasses studies of several metals and semiconductors at irradiation energies ranging from a few tens of eVs to 50 keV. The results show that in dense metals the presence of a surface can dramatically enhance the damage production upto energies of at least 50 keV. The added damage is mostly in the form of vacancy clusters, which can extend quite deep, ∼10 nm, in the sample. In semiconductors, by contrast, the surface in general has little effect on the damage production in bulk.

Surface Defects and Bulk Defect Migration Produced by Ion Bombardment of Si(001)

Abstract: Variable-temperature scanning tunneling microscopy is used to characterize surface defects created by 4.5 keV He ion bombardment of Si(001) at 80--294 K; surface defects are created directly by ion bombardment and by diffusion of bulk defects to the surface. The heights and areal densities of adatoms, dimers, and adatom clusters at 80 and 130 K are approximately independent of temperature and in reasonable agreement with molecular dynamics calculations of adatom production. At 180 K, the areal density of these surface features is enhanced by a factor of $\ensuremath{\sim}3$. This experimental result is explained by the migration and surface trapping of bulk interstitials formed within $\ensuremath{\sim}2\mathrm{nm}$ of the surface.

Nanoscale Science, Engineering and Technology Research Directions

Abstract: This report describes important future research directions in nanoscale science, engineering and technology. It was prepared in connection with an anticipated national research initiative on nanotechnology for the twenty-first century. The research directions described are not expected to be inclusive but illustrate the wide range of research opportunities and challenges that could be undertaken through the national laboratories and their major national scientific user facilities with the support of universities and industry.

Freely migrating defects in ion-irradiated cu3au

Abstract: The efficiency of producing freely migrating vacancy defects in irradiated Cu3Au was examined using electrical resistivity measurements of radiation-induced ordering on highly perfect single-crystal films. Relative efficiencies for He, Ne, and Ar bombardments at different ion energy and specimen temperature were obtained. The ratio of the efficiencies of 0.6 MeV Ne to He increased with temperature from ∼0.25 at 340 K to a saturation value of ∼0.40 at 520 K. For Ar and He, the ratio increased from ∼0.11 at 360 K to ∼0.18 at 540 K. Estimates indicate that about half of all defects created in cascades are freely migrating.

Novel interactions of supported clusters: contact epitaxy

Abstract: The study of clusters of ‘model’ metal systems such as Cu and Ag provide a valuable route to explore critical issues in materials epitaxy. Our investigations have led to observations of novel interactions between supported metal clusters in both homo- and heteroepitaxial configurations. In the experiments, clusters of both Cu and Ag were produced by inert gas condensation and deposited on the clean (001)Cu surface under ultrahigh vacuum. Following deposition, the Cu clusters were observed to be of initially random orientation on the substrate surface, undergoing reorientation upon annealing by a mechanism involving sintering and grain growth. In the case of Ag clusters, the formation of a heteroepitaxial layer between the particle and substrate was observed upon initial contact. The phenomenon, which we call ‘contact epitaxy’, may be understood from molecular dynamics simulations of a ‘soft impact’ between the nanoparticle and substrate which indicate that the ordered layers form within picoseconds of impact. The experiments were performed in an ultrahigh vacuum transmission electron microscope equipped with an in-situ nanoparticle sputtering system.

Thermal and radiation-enhanced diffusion in Cu 3 Au

Abstract: Thermal and radiation-enhanced diffusion in thin films of the order-disorder alloy Cu{sub 3}Au have been analyzed above and below the transition temperature. It is demonstrated that thin films grown by molecular beam epitaxy are uniquely suited for such experiments as the surface provides a dominant and well-characterized sink for migrating defects. The analysis is applied to recent experiments. Quantitative predictions of radiation-enhanced diffusion agree closely with experimental values. Analysis of the tracer-impurity thermal diffusion experiments provide host diffusion coefficients in a temperature regime through the transition temperature which were heretofore unavailable. {copyright} {ital 1999} {ital The American Physical Society}

Glancing incidence diffuse X-ray scattering studies of implantation damage in Si

Abstract: Diffuse X-ray scattering (DXS) at glancing incidence is a potentially powerful means for elucidating damage structures in irradiated solids. Fundamental to the analysis of diffuse X-ray scattering data is a knowledge of the atomic displacement field around defects, which for implantation damage in crystals like Si has been difficult to obtain using analytical solutions of elastic continuum theory. We present a method for predicting the diffuse scattering pattern by calculating the displacement field around a defect using fully atomistic simulations and performing discrete sums for the scattering intensity. We apply the method to analyze experimental DXS results of defects produced by 4.5 keV He and 20 keV Ga irradiations of Si at temperatures of 100–300 K. The results show that the self-interstitial in ion-irradiated Si becomes mobile around 150 K, and that amorphization of silicon by light and medium-heavy projectiles occurs homogeneously through the buildup of interstitial clusters, and not within single cascade events.

Cluster 'contact epitaxy'- direct evidence for novel particle: substrate interactions

Abstract: In this paper we describe observations of novel interactions between clusters of Ag deposited on the clean (001) Cu surface. The experiments are analogous to those performed by Gleiter and co-workers in the 1970’s, where grain boundary orientations in particles of Cu and Ag supported on single crystal metal substrates were studied. Upon annealing close to the melting point, these particles (∼10-100μm in diameter) were found to rotate on the surface, forming low-energy grain boundary configurations with the substrate. The particles studied in our experiments are ∼104 times smaller, and show rather different behavior. In the case of Ag nanoparticles we have observed a novel phenomenon, which we call ‘contact epitaxy’, involving the formation of several monolayers of epitaxially oriented Ag at the Cu surface upon contact between this surface and the Ag cluster. The phenomenon may be understood from molecular dynamics simulations of the ‘soft impact’ between the nanoparticle and surface, which indicate that the ordered layers form within picoseconds of contact. We will discuss the mechanisms by which ‘contact epitaxy’ is believed to occur.

Molecular-dynamics study of the decomposition of an unstable Ni–Ag alloy

Abstract: In an attempt to understand how ballistically mixed alloys decompose under the influence of an ion-induced thermal spike, we perform model molecular-dynamics simulations in unstable random NixAg1−x alloys. We study the time dependence of decomposition as a function of the temperature, density and concentration in the alloy by monitoring the short-range order parameter and atom mobility. Mobility is diffusion-like; its temperature dependence is determined by activation energies of 2.6 (3.6) eV for Ag (Ni) atoms. The decomposition is strongly assisted by the reduced atom densities which are characteristic of thermal spikes. Decomposition works most efficiently in a mid-concentration range (x=0.5), since here the heat of mixing liberated by the decomposition process heats the system and enhances diffusion.

Recoil ranges in ion-induced collision cascades

Abstract: Using molecular-dynamics simulation, we study the recoil atom ranges in collision cascades as a function of the recoil energy. We focus on cascades initiated by 1 keV ions in Cu and Cu-related model metals, in which the cohesive energy has been changed with respect to that of Cu. At high energies, E>100 eV, the recoil range follows the prediction of linear cascade theory. At lower energies, the ranges are still considerable. In particular, we observe a 15% probability of atom relocation at the nominal displacement threshold; the relocation probability does not vanish even for 1 eV recoils. We attribute this feature to the moderate collision spike existing in the system. The cohesive energy of the material affects the ranges considerably; a decrease of the cohesive energy increases low-energy recoil ranges. On the other hand, the recoil density is only moderately influenced by the cohesive energy. We hence conclude that the strong mixing observed in ion-bombarded materials with small cohesive energy is due to the increased recoil ranges.

Surface smoothing upon deposition of nanoparticles on single crystalline substrates

Abstract: Surfaces with artificial roughness were generated by deposition of nanoparticles on single crystalline substrates. Nanoparticles with an average size ≈ 15 nm were produced by inert gas condensation and deposited in situ on the substrate mounted inside a modified ultra high vacuum (UHV) transmission electron microscope (TEM). We have investigated the smoothing behavior on annealing based on the difference in surface energies between cluster and substrate and their heat of mixing. The cluster substrate combination Co/Cu(100) was chosen as a model system in which the cluster has a significantly higher surface energy than the substrate. Upon deposition at 600 K, the clusters do not remain on the surface, but rather burrow into the substrate. This is confirmed by a detailed strain analysis of the particles. Nanoparticles in the system Ge/Si(100) in contrast have a lower surface energy than the substrate and are completely miscible. The particles assume the substrate orientation around 700 K. At 900 K coherent islands form which are arranged in clusters of 4 in the form of a square. The reason for this previously unobserved pattern is not yet understood.