Adatom Production Research Articles

One-dimensional surface damage at grazing projectile incidence: linear vacancy chains on channeled planes

The onset of erosion pattern formation through exposure of the pristine {211} and {321} tungsten surface with 5keV He atoms at grazing incidence has been investigated at 21K by field ion microscopy. We demonstrate that the <111> linear chains of surface vacancies related to irradiation are created in addition to expected individual vacancies and two-dimensional vacancy clusters. The vacancy chains were ideally straight and did not displayed atomic-scale kinks. The asymmetry of the radiation adatom production on the two lateral sides of the <111> vacancy chains was revealed on the W {321} plane. Possible origins for the asymmetry of damage production with respect to the vacancy chains were analyzed in terms of the details of the energy landscape of interatomic interactions on this surface.

Trails of Kilovolt Ions Created by Subsurface Channeling

Using scanning tunneling microscopy, we observe the damage trails produced by keV noble-gas ions incident at glancing angles onto Pt(111). Surface vacancies and adatoms aligned along the ion trajectory constitute the ion trails. Atomistic simulations reveal that these straight trails are produced by nuclear (elastic) collisions with surface layer atoms during subsurface channeling of the projectiles. In a small energy window around 5 keV, Xe+ ions create vacancy grooves that mark the ion trajectory with atomic precision. The asymmetry of the adatom production on the two sides of the projectile path is traced back to the asymmetry of the ion's subsurface channel.

Mechanisms of pattern formation in grazing-incidence ion bombardment of Pt(111)

Ripple patterns forming on Pt(111) due to $5\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ ${\mathrm{Ar}}^{+}$ grazing-incidence ion bombardment were investigated by scanning tunneling microscopy in a broad temperature range from $100\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}720\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and for ion fluences up to $3\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.3em}{0ex}}\text{ions}∕{\mathrm{m}}^{2}$. A detailed morphological analysis together with molecular dynamics simulations of single ion impacts allow us to develop atomic scale models for the formation of these patterns. The large difference in step edge versus terrace damage is shown to be crucial for ripple formation under grazing incidence. The importance of distinct diffusion processes---step adatom generation at kinks and adatom lattice gas formation---for temperature dependent transitions in the surface morphology is highlighted. Surprisingly, ion bombardment effects like thermal spike induced adatom production and planar subsurface channeling are important for pattern ordering.

Substrate induced enhancement of atomic layer growth on Al(1 1 1): The effect of the mass anisotropy

The evolution of the thin-film morphology is studied by molecular dynamics simulations and we find a strong tendency of adatom island growth on the (1 1 1) surface of a thin Al overlayer placed on a heavy substrate (Pt(1 1 1)) when the system is subjected to low-energy Xe + irradiation. The large adatom yield of 10 2−10 3 is found for 5–10 keV rare gas ion impacts. We also find that the mass effect due to the small atomic mass ratio (large mass anisotropy) in the bilayer has a direct effect on the atomic layer growth on the surface. A mass anisotropy induced scattering of the light overlayer atoms from the heavy substrate contributes to the enhancement of adatom production. It has been found that the volume increase (density decrease) of the amorphous intermixed phase keeps the adatoms on the surface. The competition between cratering and atomic layer growth can also be seen: three events out of 10 leads to cratering (erosion) morphology at 6 keV ion energy. The substrate induced enhancement of atomic layer growth might be a promising tool for making arrays of nanodots as nanotemplates for nanofabrication.

Ion-irradiation-induced effects inSimGensuperlattices

We have studied the effect on ion irradiation in short period ${\mathrm{Si}}_{m}{\mathrm{Ge}}_{n}$ superlattices by using classical molecular dynamics. We have analyzed the degree of amorphization, mixing of layers, displacement lengths for atoms, etc., in order to find out what kind of effects collision cascades cause in the system. Particular interest was put on the effect of pseudomorphic strain. We found that the strain does not have any influence on the outcome in the bulk whereas on the surface there is a clear effect on the adatom production. Furthermore, we also found that there is an asymmetry in the impurity distribution in the layers that we concluded to be an atomic size effect.

Temperature dependent morphological evolution of Pt(1 1 1) by ion erosion: destabilization, phase coexistence and coarsening

A scanning tunneling microscopy investigation of the morphological evolution of Pt(1 1 1) upon removal of 0.2 to 680 atomic layers by 1 keV Xe + ion bombardment at normal incidence and at temperatures between 600 and 800 K is reported. The surface is found to become unstable in the temperature interval investigated. The destabilization of Pt(1 1 1) is most probably due to a step edge barrier for the attachment of a vacancy to an ascending step. Adatom production during ion bombardment enhances the destabilization of the surface. A qualitative change in roughness evolution is observed around 700 K, signified by an increase of the growth exponent from 0.40 to about 0.58. While below this temperature roughness buildup is due to pit formation, above 700 K it is initiated by remainders, left after vacancy island coalescence. The change in roughness evolution is caused by the onset of step atom detachment. For large removed amounts and above 700 K the surface exhibits an extended temperature regime of phase coexistence between a rough and a flat surface phase. The characteristic length scale of the surface morphology increases with the removed amount according to a power law with a temperature independent exponent of 0.28±0.02. This coarsening is due to step edge diffusion, rather than step atom detachment. The morphologies kinetically facet for the lower investigated temperatures. Possible origins for this slope selection are analyzed.

Multiple excitations and self-diffusion processes on and near the Cu(110) surface by molecular dynamics simulations

We present molecular dynamics simulation results concerning diffusion processes happening on and near the Cu(110) surface. We found that, in our simulations, Arrhenius-type interlayer diffusion between the atomic layers near the surface becomes important above a temperature T s=850 K, contributing significantly to the production of adatoms, most of which combine to form dimers or trimers. These diffusion processes follow the Meyer–Neldel rule with an activation energy of 0.11 eV, indicating that their driving force comes from multiple excitations. All of these phenomena act as precursors of the high diffusivity this surface exhibits at much higher temperatures.

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

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.

Collective effects in the adatom production by 4.5 keV rare-gas impacts on Pt(111): A low-temperature scanning tunnelling microscopy analysis

Abstract Using variable-temperature scanning tunnelling microscopy (20—700K) we analysed single Ne+, Ar+, Kr+ and Xe+ (4.5 eV) impacts on Pt(111). The adatom yield, the number distribution of adatoms created per impact, the radial distribution of the adatoms and the mean sizes of produced adatom and vacancy clusters were determined. Ne+ (4.5keV) impacts produce only isolated adatoms with a number distribution of adatoms per impact close to the Poisson distribution. The adatom yield is compatible with a linear cascade model. At an increased energy density corresponding to an Ar+ (4.5keV) impact, the adatom production mechanism changes. At this and higher energy densities, adatom clusters with number distributions much broader than the corresponding Poisson distribution are created. This and an increased ratio of adatom yield to sputter yield indicate that collective effects have to be taken into account to describe adatom production. Molecular dynamics simulations performed for Ne and Xe (4.5 keV) impacts ...

Collective effects in the adatom production by 4.5 keV rare-gas impacts on Pt(111): a low-temperature scanning tunnelling microscopy analysis

Abstract Using variable-temperature scanning tunnelling microscopy (20—700K) we analysed single Ne+, Ar+, Kr+ and Xe+ (4.5 eV) impacts on Pt(111). The adatom yield, the number distribution of adatoms created per impact, the radial distribution of the adatoms and the mean sizes of produced adatom and vacancy clusters were determined. Ne+ (4.5keV) impacts produce only isolated adatoms with a number distribution of adatoms per impact close to the Poisson distribution. The adatom yield is compatible with a linear cascade model. At an increased energy density corresponding to an Ar+ (4.5keV) impact, the adatom production mechanism changes. At this and higher energy densities, adatom clusters with number distributions much broader than the corresponding Poisson distribution are created. This and an increased ratio of adatom yield to sputter yield indicate that collective effects have to be taken into account to describe adatom production. Molecular dynamics simulations performed for Ne and Xe (4.5 keV) impacts on Pt(111) and presented in the accompanying paper reproduce the experimental trends and conclusions. Experiments with successively increasing fluence give evidence that an ion induced adatom mobility occurs at lower energy densities (Ne+ (4.5 keV) impacts). In addition, for Xe bombardment at higher energies (6.5 keV), vacancy cluster formation below the surface is found.

MD simulation of atomic displacements in pure metals and metallic bilayers during low energy ion bombardment at 0 K

Molecular Dynamics (MD) simulations of 100 eV Ar ion bombardment of (1 0 0) Ni and Al crystals, of bilayer crystals consisting of an Al (or Ni) layer on a (1 0 0) Ni (or Al) substrate, as well as pseudo-isotope bilayer crystals have been performed at 0 K using tight-binding potentials. For these systems sputtering yields, energy deposition with depth, atomic relocations, production of ad-atoms, depth distributions of vacancies and interstitials per ion impact were studied for times up to 7 ps. We observed that both the mean square displacement of atoms and defect production (vacancies, interstitials and ad-atoms) are larger in pure Al than in pure Ni. In addition, we observed for the bilayer systems Al/Ni and Ni/Al a high number of the near surface atomic relocations; especially ion bombardment induced exchange processes between atoms of the 1st (Al or Ni) and of the 2nd (substrate) layer. Potential energy calculations indicate that such relocations between the 1st and the 2nd layer are in both bilayer crystals energetically favourable. Both Al/Ni and Ni/Al bilayers show considerable higher production of ad-atoms as the pure Al and Ni targets. Typically ad-atoms are from the first layer, but in the Ni/Al bilayer system we found a substantial amount of Al ad-atoms from the 2nd layer (first Al layer). They contribute more to the ad-atom number than 1st layer Ni atoms. The mean square displacement of atoms in Al/Ni and Ni/Al crystals increases considerably during the thermal stages of the cascade evolution while it is almost constant in the case of the pure Al and Ni. Finally we observed that the maximum kinetic energies of atoms in the cascade volume after 4 ps are lower in the Ni and Al/Ni crystals than in Al and Ni/Al crystals, reflecting the lower cohesive energy of Al as compared to Ni. Calculations with pseudo-isotope bilayer crystals were performed to elucidate the influence of mass or potential on the observed effects.

Island growth in ion beam assisted metal-on-metal deposition modelled by rate equations

Island growth in ion beam assisted deposition (IBAD) is modelled by rate equations, where deposition, diffusion driven aggregation, defect and target adatom production and island fragmentation are taken into account. It is shown that compared with results obtained for a thermal deposition, deposition with ion bombardment induced island fragmentation causes the total island density to increase and average island size to decrease. The observed behaviour is in qualitative agreement with experimental findings. It is demonstrated how the appropriately scaled island size distributions depend crucially on the fragmentation process, but remain relatively unaffected by diffusion. Possibilities to use scaled distributions in recognizing experimentally the relative importance of different processes in IBAD are pointed out.

Ion scattering and scanning tunneling microscopy studies of stepped Cu surfaces

The structure and dynamics of stepped Cu(115) surfaces and their behaviour under oxygen adsorption was studied by ion scattering spectroscopy (ISS) and scanning tunneling microscopy (STM). Azimuthal ion intensity distributions show pronounced minima along the first layer atom rows which can be reproduced by MARLOWE simulations. Detailed analysis in the backscattering mode indicates relaxation of step edge atoms. Increasing the surface temperature leads to scattering into otherwise blocked regions of the azimuthal and polar intensity distributions that can be explained by the production of adatoms or kinks along the [1 1 0] step edges. This thermal roughening is observed above an onset temperature of about 390 K and is completely reversible. These results are supported by STM studies that show that regular monoatomic steps are rather stable at room temperature, whereas for double steps pronounced kink motion is observed with an activation energy of about 0.1 eV. Oxygen adsorption causes pinning of the steps and above temperatures of about 500 K it leads to strong faceting of the surface with (014), (104) and (113) faces.

Ion-beam induced generation of Cu adatoms on Cu(100)

Low-energy ion scattering was used to study on-beam induced adatom generation during irradiation of a Cu(100) surface with 6 keV Ne ions at a sample temperature of 60 K. It was found that the number of adatoms produced per incoming ion decreases from an average of 3.5 to a saturation level of 1.8 after prolonged irradiation. Two mechanisms are believed to contribute to the adatom generation: ion-beam induced atom-replacement sequences and diffusion of self-interstitial atoms to the surface. From the adatom production as a function of irradiation dose the relative contributions of the two mechanisms were determined.

Ion-beam induced generation of Cu adatoms on Cu(100)

Low-energy ion scattering was used to study on-beam induced adatom generation during irradiation of a Cu(100) surface with 6 keV Ne ions at a sample temperature of 60 K. It was found that the number of adatoms produced per incoming ion decreases from an average of 3.5 to a saturation level of 1.8 after prolonged irradiation. Two mechanisms are believed to contribute to the adatom generation: ion-beam induced atom-replacement sequences and diffusion of self-interstitial atoms to the surface. From the adatom production as a function of irradiation dose the relative contributions of the two mechanisms were determined.

Detailed Model for Ion-Beam Induced Adatom Generation

ABSTRACTIn earlier low-energy ion scattering experiments of 6 keV Ne ions on Cu(100), Cu adatoms were observed to appear on the surface. The number of produced adatoms per incoming ion was found to decrease to 50% of its initial value of 3.5 after prolonged irradiation. In this paper we present calculations of the adatom production as a function of irradiation dose. From a comparison with the experimental data, the trapping radius of a vacancy was determined to be 5.4 ± 0.9 Å, in agreement with values reported in the literature.