Adatom Clusters Research Articles

Atomic layer growth on Al(111) by ion bombardment

Instead of the expected erosion morphology composed of craters, rare gas ion bombardment of the Al(111) surface is found to cause initial surface growth of several atomic layers. This phenomenon is observed for Ne+, Ar+, and Xe+ at all temperatures at which bombardment induces morphological surface changes and for ion energies down to a few hundred eV. The effect is interpreted on the basis of a thermal spike induced separation of damage into subsurface vacancy clusters and surface adatom clusters.

Theory of scanning tunneling spectroscopy of a magnetic adatom on a metallic surface

A comprehensive theory is presented for the voltage, temperature, and spatial dependence of the tunneling current between a scanning tunneling microscope (STM) tip and a metallic surface with an individual magnetic adatom. Modeling the adatom by a nondegenerate Anderson impurity, a general expression is derived for a weak tunneling current in terms of the dressed impurity Green's function, the impurity-free surface Green's function, and the tunneling matrix elements. This generalizes Fano's analysis to the interacting case. The differential-conductance line shapes seen in recent STM experiments with the tip directly over the magnetic adatom are reproduced within our model, as is the rapid decay, $\ensuremath{\sim}10$ \AA{}, of the low-bias structure as one moves the tip away from the adatom. With our simple model for the electronic structure of the surface, there is no dip in the differential conductance at approximately one lattice spacing from the magnetic adatom, but rather we see a resonant enhancement. The formalism for tunneling into small clusters of magnetic adatoms is developed.

Formation and stability of large step-free areas on Si(001) and Si(111)

We have created arrays of step-free regions on Si(111) and Si(001) starting with fabricated square ridge structures and using high-temperature annealing. The conditions for the formation of step-free regions are different on the two surfaces due to differences in the temperature dependences of the surface structures and different diffusion characteristics. The step-free regions are unstable against the formation of circular vacancy pits due to the supersaturation of vacancies at large distances from atomic steps and the invasion of pre-existing steps from the surrounding barriers. The effects of electromigration on the behavior of the sublimation pits are different on the two surfaces. On Si(001) the details of the step arrays depend markedly on whether direct or alternating current heating is used. With direct-current heating the pits are no longer concentric. The effect can be explained by electromigration and the anisotropic mobility of adatoms on the reconstructed Si(001) surface. The major type of point defect on the terraces at high temperature can be inferred from the observation of vacancy or adatom clusters after quenching.

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.

Dynamics and stability of nanostructures on metal surfaces

Ultrathin metal films consisting of two-dimensional clusters are typically unstable: the cluster ensemble has the tendency to reduce its total free energy via Ostwald ripening or dynamic coalescence of mobile clusters. In this paper we give an overview of recent model experiments addressing these coarsening mechanisms. The experiments have been performed using STM on ensembles consisting of adatom or vacancy clusters with typical diameters in the nanometer range on fcc(111)-metal surfaces. Agreement with and deviations from conventional theories are discussed.

Cs/GaAs(100) surface: Two-dimensional metal or Hubbard insulator?

The evolution of the characteristic electron energy loss spectra accompanying the deposition of cesium on GaAs(100) surfaces with various superstructural reconstructions is studied experimentally. It is shown that the appearance of loss peaks in the GaAs band gap for Cs coverages θCs>0.5 monolayers is due to resonances in the longitudinal and transverse polarizability of two-dimensional metallic clusters of adatoms.

Combined atom probe and STM study of tip-substrate interactions

A combination of an imaging atom probe (field ion microscope with a time-of-flight mass spectrometer) and a scanning tunneling microscope (STM), which are integrated in the UHV-system, has been used to study the tip-substrate interactions after electrical contact and mechanical indentation. After “jump-to-contact” experiments with tungsten tips on a gold substrate ad-atom clusters has been observed in the FIM-images. The imaged clusters can be identified by time-of-flight mass analysis as Au and give evidence for material transfer in sub-monolayer range. The image contrast in the micrographs gives almost no reference to induced lattice defects after this contact experiments. In additional indentation-experiments clearly induced plastic deformation of the tip crystallite can be observed as well as material transfer of several monolayers. The FIM micrographs after field desorption of consecutive atomic layers show the features of these deformations typically into a depth up to 20–25 atomic layers.

Field ion microscopy of platinum adatoms deposited on a thin Al 2O 3 film on NiAl(1 1 0)

Aiming to contribute to a characterisation of the surface diffusion and clustering of individual metal adatoms on a well-characterised oxide film the feasibility of FIM investigations is demonstrated for the catalytically relevant system Pt/Al 2O 3/NiAl(1 1 0). A thin aluminium oxide film was grown on a [1 1 0] oriented NiAl field emitter through dissociative oxygen adsorption followed by heating cycles. The film was characterised using FEM, FES and neon FIM. Field desorption sequences revealed that the film prepared on NiAl(1 1 0) was approximately two layers thick. Below 80 K the film was stable at field-strengths applied for neon FIM. The presence of an ordered structure was suggested by FEM and FIM pattern. FIM of Pt adatoms was achieved at 79 K and below best image field strength using neon as the imaging gas. From preliminary displacement observations the surface diffusion activation energy of individual Pt adatoms was estimated and ranges between 0.4 and 0.5 eV. After three heating cycles to a maximum temperature of 230 K an ordered arrangement of Pt adatoms was noticed in the vicinity of a presumable point defect.

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.

Local density of states of adatom clusters on the Mo(110) surface

In this study, we have measured both the energy distributions of field emitted electrons and the energy distributions of field desorbed He ions from molybdenum clusters adsorbed on a Mo(110) surface. The purpose of these experiments was to obtain the local density of states of the microstructure.

Structure and stability of metastable transition metal clusters on W(011)

The structure of clusters of rhodium and palladium atoms formed on W(011) by deposition below and above the adatom migration temperature, as well as upon subsequent annealing of the films in the range T an=100–1000 K, is studied by conventional and video low-energy electron diffraction (LEED) for various submonolayer coverages. Films deposited below the single-adatom migration temperature show a predominantly random distribution of adatoms over the adsorption sites with some signs of order, evidencing mobility anomalous for single adatoms. Upon annealing, the structure of the films acquires long-range order and undergoes a set of irreversible phase transitions. Two types of adatom cluster are found: one-dimensional densely packed chains aligned along the substrate 〈111〉 directions and two-dimensional densely packed islands. The parameters of the cluster structure, such as interatomic and interchain spacing, chain length and temperature range of stability, are derived from LEED intensity profiles. The cluster structures and phase transitions are discussed in terms of indirect interactions between adatoms. The work function changes are measured with the contact potential difference method as a function of coverage and annealing temperature. The correlation between work function changes and phase transitions is determined.

LEED holography applied to a complex superstructure: A direct view of the adatom cluster on SiC(111)-(3×3)

For the example of the SiC(111)-(3x3) reconstruction we show that a holographic interpretation of discrete Low Energy Electron Diffraction (LEED) spot intensities arising from ordered, large unit cell superstructures can give direct access to the local geometry of a cluster around an elevated atom, provided there is only one such prominent atom per surface unit cell. By comparing the holographic images obtained from experimental and calculated data we illuminate validity, current limits and possible shortcomings of the method. In particular, we show that periodic vacancies such as cornerholes may inhibit the correct detection of the atomic positions. By contrast, the extra diffraction intensity due to slight substrate reconstructions, as for example buckling, seems to have negligible influence on the images. Due to the spatial information depth of the method the stacking of the cluster can be imaged down to the fourth layer. Finally, it is demonstrated how this structural knowledge of the adcluster geometry can be used to guide the dynamical intensity analysis subsequent to the holographic reconstruction and necessary to retrieve the full unit cell structure.

First-Principles Study of SI(lll) Homoepitaxy

AbstractEpitaxial growth on the Si(111) surface is studied using first-principles total-energy pseudopotential calculations. The energetics of added Si atoms essentially determines epitaxial growth modes under different growth conditions (surface temperature, Si flux rate, and surface step density). We have determined the surface adatom diffusion barriers and cluster formation energies; we use these microscopic energy parameters to address the possibilities for macroscopic morphological evolution of the surface under different conditions.

Fluctuations of Step Edges: Revelations About Atomic Processes Underlying Surface Mass Transport

AbstractExperimental advances in recent years make possible quantitative observations of step-edge fluctuations. By applying a capillary-wave analysis to these fluctuations, one can extract characteristic times, from which one learns about the mass-transport mechanisms that underlie the motion as well as the associated kinetic coefficients [1-3]. The latter do not require a priori insight about the microscopic energy barriers and can be applied to situations away from equilibrium. We have studied a large number of limiting cases and, by means of a unified formalism, the crossover between many of these cases[4]. Monte Carlo simulations have been used to corroborate these ideas. We have considered both isolated steps and vicinal surfaces; illustrations will be drawn from noble-metal systems, though semiconductors have also been studied. Attachment asymmetries associated with Ehrlich-Schwoebel barriers play a role in this behavior. We have adapted the formalism for nearly straight steps to nearly circular steps in order to describe the Brownian motion of single-layer clusters of adatoms or vacancies on metal surfaces, again in concert with active experimental activity [3,5]. We are investigating the role of external influences, particularly electromigration, on the fluctuations.

The Effect of Deposition Method on Growth Morphology - Comparison of Molecular Beam Epitaxy, Ion Beam Assisted Deposition and Sputter Deposition

AbstractThin films created by the deposition or under influence of atoms with hyperthermal energies (E = 100 -104eV) exhibit properties which differ in many respects from those of films created by deposition of atoms with thermal energy. The morphologies of thin Pt-films deposited on Pt(111) under otherwise identical deposition conditions by molecular beam epitaxy (MBE), ion beam assisted deposition (IBAD) and sputter deposition (SD) differ in film structure size, island shapes and film roughness. The different film structure sizes are unambiguously traced back to two different island formation mechanisms inherent to these deposition methods. While in MBE the islands result from nucleation in a supersaturated adatom gas, in IBAD and SD they result by direct or indirect creation of adatom clusters as a consequence of single impacts of energetic atoms present in the depositing particle flux. The differences in film roughness are not only due to the different island formation mechanisms, but seem to be closely related to the different step edge structures at the growth front.

Computer simulation of surface diffusion of copper, silver and gold

The binding energies to copper, silver and gold (111) surfaces of self-atom clusters have been calculated. The activation energies of motion of these ad-atom clusters, vacancies and divacancies on copper, silver and gold (111) surface, and of the conversion of ad-atom clusters on (111) and (100) have been calculated by use ofn-body embedded atom potentials and molecular dynamics.

Antiphase boundaries on low-energy-ion bombarded Ge(001)

Surface vacancy and adatom clusters have been created on Ge(001) by bombarding the surface with 800 eV argon ions at various substrate temperatures ranging from room temperature to 600 K. The vacancies preferentially annihilate at the ends rather than at the sides of the dimer rows, resulting in monolayer deep vacancy islands which are elongated in a direction of the dimer rows of the upper terrace. As vacancy islands nucleate and expand, the dimer rows in neighbouring vacancy islands need not, in general, align with each other. An antiphase boundary will develop if two growing vacancy islands meet, but their internal dimer rows are not in the same registry. In contrast to Si(001), where only one type of antiphase boundary is found, we have found three different types of antiphase boundaries on Ge(001). Higher dose (> several monolayers) room temperature ion bombardment followed by annealing at temperatures in the range 400–500 K results in a surface which contains a high density of valleys. In addition to the preference for the annihilation of dimer vacancies at descending versus ascending steps we also suggest that the development of antiphase boundaries drives the roughening of this surface. Finally, several atomic rearrangement events, which might be induced by the tunneling process, are observed after low-dose ion bombardment at room temperature.

Modeling of Ir adatoms on Ir surfaces.

We used the embedded-atom method potential to study the structures, adsorption energies, binding energies, migration paths, and energy barriers of the Ir adatom and small clusters on fcc Ir (100), (110), and (111) surfaces. We found that the barrier for single-adatom diffusion is lowest on the (111) surface, higher on the (110) surface, and highest on the (100) surface. The exchange mechanisms of adatom diffusion on (100) and (110) surfaces are energetically favored. On all three Ir surfaces, ${\mathrm{Ir}}_{2}$ dimers with nearest-neighbor spacing are the most stable. On the (110) surface, the ${\mathrm{Ir}}_{2}$ dimer diffuses collectively along the 〈110〉 channel, while motion perpendicular to the channel walls is achieved by successive one-atom and correlated jumps. On (111) surface, the ${\mathrm{Ir}}_{2}$ dimer diffuses in a zigzag motion on hcp and fcc sites without breaking into two single atoms. On the (100) surface, diffusion of the ${\mathrm{Ir}}_{2}$ dimer is achieved by successive one-atom exchange with the substrate atom accompanying by a 90\ifmmode^\circ\else\textdegree\fi{} rotation of the ${\mathrm{Ir}}_{2}$ dimer. This mechanism has a surprisingly low activation energy of 0.65 eV, which is 0.14 eV lower than the energy for single adatom exchange on the (100) surface. Trimers were found to have a one-dimensional (1D) structure on (100) and (110) surfaces, and a 2D structure on the (111) surface. The observed abrupt drop of the diffusion barrier of tetramer, ${\mathit{I}}_{{\ensuremath{\gamma}}_{4}}$ on the Ir (111) surface was confirmed theoretically. \textcopyright{} 1996 The American Physical Society.

Influence of multiatom interactions on the shapes and energetics of two-dimensional homoepitaxial clusters on close-packed metallic surfaces.

We describe a scheme (BSM for ``bond saturation model'') for modeling the energetics of adatom clusters on close-packed metallic surfaces. In the BSM, atoms interact via coordination dependent nearest-neighbor bonds. We show that the BSM yields a detailed understanding of the relationship between shape and energy for two-dimensional (2D) homoepitaxial adatom clusters on close-packed metallic surfaces. In particular, we advance a simple rule for predicting the binding energies of small clusters based on the moments of their coordination distributions, derive a useful expression for the binding energy of a 2D cluster of arbitrary geometry, and study the role played by coordination dependent bonding in determining step energies for adatom islands. Our analysis of step energies provides insight into why the two distinct types of monatomic steps on Pt(111) have similar free energies as observed by Michely and Comsa. While our numerical results are for Pt(111), we expect our conclusions to hold for any chemically similar system. \textcopyright{} 1996 The American Physical Society.