Session B12: Focus Session: Structure and Dynamics of Metal Thin Films

Effects of shadowing and steering in oblique incidence epitaxial growth

Recently the fabrication of novel nanostructures by oblique deposition has drawn much attention due to their potential application in electronic and mechanical devices as well as the interesting morphologies observed in various experiments, such as nanorods, nanocolumns, and nanohelicoids. Unlike self-organization by misfit strain in heteroepitaxial growth, oblique deposition provides a relatively direct way of controlling surface structures of growing films. Recent experiments indicate that oblique incidence deposition can significantly alter materials properties such as surface roughness, magnetic anisotropy, optical transmittance, and porosity. After a review of these experimental results, we first show that a series of morphological transitions observed in oblique incidence Cu/Cu(100) growth near room temperature can be explained primarily by geometrical shadowing effects [1]. We then discuss the modifying effects of steering due to short-range and long-range attraction [2] as well as of substrate rotation on the surface morphology. Finally, we present the results of recent multiscale simulations of Cu/Cu(100) growth at lower temperature (T = 160 - 200 K) [3] as well as parallel accelerated dynamics and molecular dynamics simulations at very low temperature [4]. Based on these simulations we have been able to explain a number of recent intriguing but previously unexplained experimental results including the strong dependence of the surface morphology and roughening behavior on temperature as well as the development of compressive strain in metal thin film growth. \\[4pt] [1] Y. Shim and J. G. Amar, Phys. Rev. Lett. {\bf 98}, 046103 (2007).\\[0pt] [2] Y. Shim, V. Borovikov and J. G. Amar, Phys. Rev. B {\bf 77}, 235423 (2008).\\[0pt] [3] V. Borovikov, Y. Shim and J. G. Amar, Phys. Rev. B {\bf 76}, 241401(R) (2007).\\[0pt] [4] Y. Shim, V. Borovikov, B. P. Uberuaga, A. F. Voter, and J. G. Amar, Phys. Rev. Lett. {\bf 101}, 116101 (2008).   

Diffusion of two-dimensional Cu islets on Ag(111) studied with the Molecular Dynamics Method

Our molecular dynamics simulations (at 300, 500 and 700 K) of the diffusion of two-dimensional Cu$_{n}$ islets (1$\le $n$\le $9) on Ag(111) using many-body potentials yield an Arrhenius behavior. Concerted motion is seen to dominate the diffusion of smaller islets (2-4 atoms) whereas multiple-atom, shape-adjusting processes control the diffusion of the larger ones. Although the effective energy barrier scales with islet size, the barriers do not change considerably for islands containing 4 to 9 atoms (they are $\sim $ 220 $\pm $ 37 meV). While the diffusion barrier for Cu monomers on Ag(111) is higher than that on Cu(111) (both in experiment and theory), the situation reverses starting from the dimer. Our results for monomer and dimer are in excellent agreement with those extracted from experiments.$^{1}$ On comparing our results with those for Cu islets on Cu(111), we find that the comparatively large Ag-Ag bond-length sets the contrast between Cu monomer diffusion on Cu(111) and on Ag(111). The diffusivity of Cu dimer, however, is boosted on Ag(111) by the competition between optimizing the Cu-Cu and the Cu-Ag bonds. For larger islets (3-9 atoms) our results reveal several novel diffusion processes, including those in which an islet-atom climbs atop. $^{1 }$Morgenstern \textit{et al.} PRL\textbf{93}, 056102 (2005). Work supported by NSF-ITR 0428826.   

Atomic structure determination of the (001) surface of the semimetal Bi by STM and LEED

The Bi surfaces differ from the semimetal bulk due to the metallic surface states, induced by the symmetry breaking and strong spin-orbit interaction. All Bi surface states studied are spatially confined to the first layer. Bi(001) is a notable exception with deeply penetrating states, which could have a significant effect on the bulk properties of nanostructures. This work presents surface morphology observation by STM and atomic structure determination by LEED, which are expected to be closely related to the electronic properties. STM shows an unreconstructed surface and wide terraces with double-layer step heights of about 3.76 $\pm$ 0.02 \AA~. We also identify the short termination by obtaining unstable single step heights via special sputtering operations. In the LEED analysis, the termination with an intact bilayer also results in a much better agreement between calculated and measured intensities than the broken bilayer. Strong multilayer oscillatory relaxations (about 10\%) are found to reach deep into the fifth layer, which can be seen as the structural response to the unusually deep surface state penetration at this surface. The measured relaxations agree well with those from first-principles calculations.   

Thermal-stability of Pd-Cu surface alloys investigated at the nanometer-scale by LEEM-IV analysis

Pd-Cu(100) surface alloys are interesting as model systems for metal/metal epitaxy, as well as for their catalytic properties, and as coatings, e.g. for electromigration resistance. We employ the LEEM-IV technique, with 8.5 nm spatial resolution and submonolayer chemical sensitivity, to investigate Pd interdiffusion into the Cu(100) surface. The LEEM-IV technique is sensitive to the layer-by-layer composition down to the fourth subsurface layer. After annealing a 0.4 ML Pd surface alloy at around 540 K, some regions of the surface develop a Cu$_{3}$Pd structure, a familiar bulk alloy phase. In other regions, the surface Pd concentration becomes dilute due to Pd diffusion into the bulk. We estimate the thermal activation barrier to Pd diffusion from the surface alloy into Cu bulk to be 1.7$\pm $0.15 eV. The LEEM allows real-time, real-space, observation of the interdiffusion process, and the concurrent evolution of the surface structure, at the nanometer scale. Sandia is operated by Sandia Corp., a Lockheed Martin Company, for the U. S. DOE's NNSA under Contract No. DE-AC04-94AL85000. Work at UNH is funded by the NSF under Grant No. DMR-0134933.   

Bistability of Nanoscale Ag Islands on Anisotropic Si(111)-(4x1)-In Surface Stress Template

We present a combined experimental and theoretical study of stability of Ag nanoislands grown on Si(111)-(4x1)-In surface. Experiments show the existence of two stability regimes: a conventional regime at low temperature where only one island shape is stable, and an unconventional regime at room temperature (RT) where isotropic compact islands coexist with anisotropic elongated ones. First-principles calculations show the unusual bistability at RT arises from the fact that the Ag nanoislands are under anisotropic stress, further supported by a continuum model of island shape evolution as a function of island size.   

Impurities in Vacuum Deposition: Effect on Island Nucleation and Surface Morphologies

The effect of impurities on epitaxial growth in the submonolayer regime is studied using kinetic Monte Carlo simulations of a two-species solid-on-solid growth model. Both species are mobile, and attractive interactions among adatoms and between adatoms and impurities are incorporated. Impurities can be codeposited with the growing material or predeposited prior to growth. We discuss the peculiar morphologies observed in copper on copper deposition on vicinal surfaces, and argue that only the presence of impurities can explain all observed features.\footnote{A.BH.Hamouda et al., Phys.\ Rev.\ B 77, 245430 (2008)} We also investigate the effect on island nucleation using a recently developed approach based on capture zone distributions.\footnote{A.\ Pimpinelli \& T.L.\ Einstein, Phys.Rev.\ Lett.\ 99, 226102 (2007)}   

Interface evolution and cluster formation during de-wetting of thin solid films

Morphology evolution of solid thin films is investigated within a continuum interface dynamics model that incorporates both the interface relaxation and the long range de-wetting interactions. The model is used to explore the cluster formation phenomena seen on the surfaces of polymeric and metallic thin films. Via numerical simulations and analytic arguments, we obtain the scaling laws governing the coarsening growth of these clusters. These scaling laws are found to be super-universal at long time scales: They do not depend on the dimensionality of the film substrate and the nature of the long range de-wetting interactions, as documented by our numerical simulations on 1-d and 2-d substrates with de-wetting interactions of various forms. However, for the physically interesting 2-d substrates, the long range interactions introduce a distinct early time scaling behavior that persists over many decades of time and may be significant for the understanding of the current experimental phenomenology.   

First-passage time approach to kinetic Monte Carlo simulations of metal(100) growth

One of the difficulties in carrying out realistic kinetic Monte Carlo simulations is the existence of rapid, repetitive low-barrier processes which can dramatically slow down the simulation. For example, in metal(100) growth the rate for edge-diffusion can be very fast even at moderate temperatures, while the barriers for edge-detachment and corner rounding are relatively high. While one approach to this problem is to artificially reduce the rate of edge-diffusion, such an approach can significantly alter the thin-film evolution. To address this problem while still preserving the relative rates for all processes, we have developed a modified KMC method in which edge-diffusion and corner-rounding are treated using a first-passage time formalism, while the remaining processes are treated as in normal KMC. In simulations of an effective-medium theory (EMT) based model of Cu/Cu(100) growth at $T = 200$ K and above we find that a speed-up of several orders of magnitude is possible, without sacrificing accuracy. Preliminary results for Cu/Cu(100) growth at high temperatures will also be presented.   

Growth morphology of ultra-thin Ni films on Pd (100)

A series of thin Ni films with thicknesses between 0.2 ML to 13 ML were deposited on a Pd(100) substrate at room temperature (RT). Growth morphology was investigated using scanning tunneling microscopy (STM). The STM images indicate the existence of three different growth modes. Up to 6.5 ML, the films grow pseudomorphically, consistent with a face-centered tetragonal (fct) structure. From 6.5 ML to 10.5 ML a new interlayer distance of 1.0 $\pm$ 0.1 ${\AA}$ is established. The new structure is accompanied by the appearance of an arrangement of filaments on the top layer. These filaments are presumably related to a strain relief mechanism of the fct films. Finally above 10.5 ML the Ni films recover their face-centered cubic (fcc) lattice constants. The filaments evolve to form a net-like structure over the whole surface. Preliminary data indicates the magnetic properties of the layers are linked to the evolution of the film's structure.   

Dynamical LEED analysis of a quasicrystalline Cu film using periodic approximant structure models

Quasicrystalline surfaces pose a challenge to diffraction techniques since they have no periodicity. They do, however, have good long-range order and produce diffraction patterns with sharp peaks. Copper grows on the 5-fold surface of i-Al-Pd-Mn in a layer-by-layer mode. Although its atomic structure cannot be determined by STM, the diffraction pattern from a 5-layer thick film consists of sharp peaks and streaks, the location of which indicate that this film consists an aperiodic array of rows of periodically-spaced Cu atoms. We have applied the method of periodic approximants in a dynamical LEED analysis of the structure of this quasiperiodic copper film to determine the atomic structure of the Cu film. This analysis indicates that the Cu film has a distorted cubic structure that conforms to the quasiperiodic substrate structure, providing an example of periodic-aperiodic structure matching. This research is supported by NSF-DMR-0505160 and the Academy of Finland.   

Theoretical Study of Adsorbates-induced Restructuring of Pb flat-top mesas in the Quantum Regime

Based on first-principles total energy calculations, we study the adsorption and diffusion of a series of metal adsorbates (Fe, Co and Cs) on flat-top Pb mesas, focusing on their influence on the morphology of the mesas. We found that single Fe and Co atoms can easily dive into the subsurface interstitial sites by overcoming a small energy barrier upon deposition onto the mesa top. In contrast, Cs atoms are able to substitute first-layer Pb atoms via a place exchange process resulting in a surface alloy. This induces a dramatic change on the surface morphology of these mesas as observed in recent experiments. This morphological transformation is characterized by the emergence of Cs-decorated monolayer-high Pb islands which are predominately formed on quantum mechanically unstable regions of the Pb mesas. Connections with experimental observations in other related systems will also be discussed.   

Stress development and relaxation during early stages of oxidation of metals and alloys

It has been long recognized that oxidation of metals results in the generation of stresses and these stresses play an important role in shaping the microstructure of oxide films. However, the mechanism governing the stress development and relaxation during early stages of oxidation of metals and alloys is still to a significant degree unclear. Using a combination of in-situ ultra-high vacuum (UHV) transmission electron microscopy (TEM) and finite element method, we show how oxidation-induced stresses can be used to tailor the initial oxide formation during early-stage oxidation of Cu(100) and Cu-Au(100). From analysis of the observed morphological evolution of Cu$_{2}$O nanoislands, we establish a close relationship between the stresses generated from the oxide growth and the thermodynamic selection of nanoscale morphology of the oxide film. We expect that our results have implications for controlled production of novel oxide nano structures through controlling the oxidation-induced stresses via oxidation temperature or alloying.   

Studies of oxidation of the Cu(100) surface using low energy positrons.

Changes in the surface of an oxidized Cu(100) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the positron annihilation induced Cu M2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300$^{o}$ C. The intensity then decreases monotonically as the annealing temperature is increased to $\sim $600$^{o}$ C. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron-positron correlations effects. Possible explanation for the observed behavior of the intensity of positron annihilation induced Cu M2,3VV Auger peak with changes of the annealing temperature is proposed.