Microscopic view of epitaxial metal growth: nucleation and aggregation

Growth of thin films from atoms deposited from the gas phase is intrinsically a nonequilibrium phenomenon governed by a competition between kinetics and thermodynamics. Precise control of the growth and thus of the properties of deposited films becomes possible only after an understanding of this competition is achieved. Here, the atomic nature of the most important kinetic mechanisms of film growth is explored. These mechanisms include adatom diffusion on terraces, along steps, and around island corners; nucleation and dynamics of the stable nucleus; atom attachment to and detachment from terraces and islands; and interlayer mass transport. Ways to manipulate the growth kinetics in order to select a desired growth mode are briefly addressed.

https://science.sciencemag.org/content/sci/276/5311/377.full.pdf

Atomistic Processes in the Early Stages of Thin-Film Growth

Physical and chemical properties of bimetallic surfaces

The role of surface stress in reconstruction, epitaxial growth and stabilization of mesoscopic structures

We present a method for accelerating dynamic simulations of activated processes in solids. By raising the temperature, but allowing only those events that should occur at the original temperature, the time scale of a simulation is extended by orders of magnitude compared to ordinary molecular dynamics, while preserving the correct dynamics at the original temperature. The main assumption behind the method is harmonic transition state theory. Importantly, the method does not require any prior knowledge about the transition mechanisms. As an example, the method is applied to a study of surface diffusion, where concerted processes play a key role. In the example, times of hours are achieved at a temperature of 150 K.

/pdf/temperature-accelerated-dynamics-for-simulation-of-5dycu83bcm.pdf

Temperature-accelerated dynamics for simulation of infrequent events

The influence of strain on diffusion and nucleation has been studied by means of scanning tunneling microscopy and effective-medium theory for Ag self-diffusion on strained and unstrained (111) surfaces. Experimentally, the diffusion barrier is observed to be substantially lower on a pseudomorphic Ag monolayer on Pt(111), 60 meV, compared to that on Ag(111), 97 meV. The calculations show that this strong effect is due to the 4.2% compressive strain of the Ag monolayer on Pt. It is shown that in general isotropic two-dimensional strain as well as its relief via dislocations have a drastic effect on surface diffusion and nucleation in heteroepitaxy and are thus of significance for the film morphology in the kinetic growth regime.

/pdf/effect-of-strain-on-surface-diffusion-and-nucleation-59izvkxhqr.pdf

Effect of Strain on Surface-Diffusion and Nucleation

/pdf/island-shape-induced-transition-from-2d-to-3d-growth-for-pt-4vu71frxk5.pdf

Island shape-induced transition from 2D to 3D growth for Pt/Pt(111).

We present a critical view of the analysis of experimental island densities acquired as a function of temperature in terms of barriers and prefactors for tracer diffusion at surfaces. We investigate the achievable precision for methods ranging from simple application of scaling laws, via integration of mean-field rate equations within various approximations for the capture rates, to kinetic Monte Carlo simulations that account for the various island shapes realized on square and hexagonal lattices. The discussion of theoretical models will be accompanied by variable temperature STM data for the nucleation of Ag on a Pt~111! surface. We introduce experimental methods to test for dimer diffusion and dissociation, as well as for transient mobility of monomers. Density scaling is analyzed in the presence of post-deposition mobility and easy adatom attachment to islands and other monomers. From extended kinetic Monte Carlo simulations we establish density scaling for the various island shapes on square and hexagonal lattices for coverages up to percolation, which is particularly relevant for methods working in reciprocal space. @S0163-1829~99!01231-X#

/pdf/measuring-surface-diffusion-from-nucleation-island-densities-3fdaebku3d.pdf

Measuring surface diffusion from nucleation island densities

The growth of one monolayer of Au on Ni(111) is shown to lead to an ordered array of misfit dislocation loops in the underlying Ni(111) surface. The signature of these loops is observed by scanning tunneling microscopy, and atomistic simulations are used to relate the observed surface structure to that of the buried interface. The new interface structure is different from normal misfit dislocation structures in three respects: (i) it forms already during growth of a single Au monolayer, (ii) it forms in the substrate and not in the overlayer, and (iii) it is controlled by the interface energy rather than by the strain in the two phases.

/pdf/atomic-scale-determination-of-misfit-dislocation-loops-at-adr3av8xqc.pdf

Atomic-scale determination of misfit dislocation loops at metal-metal interfaces.

A set of density-functional calculations for clean and O-covered Al(111) are presented. At low O coverages the potential energy surface (PES) of chemisorbed O is investigated. The PES indicates large barriers (0.8 eV) against O diffusion and a large corrugation of the equilibrium O height over the Al(111) while only a moderate energy gain (5 eV per atom) is found upon ${\mathrm{O}}_{2}$ dissociation over the surface. The possible existence of ``hot'' O adatoms after ${\mathrm{O}}_{2}$ dissociation is discussed on the basis of the presented PES and existing dynamical simulations on model potentials. At high O coverages an attractive O-O interaction is identified together with an enhancement in the dipole moment induced per O atom. Finally, Tersoff-Hamann-type scanning tunneling microscopy (STM) topographs are derived based on the calculated one-electron wave functions and spectra. For the clean Al(111) a theoretical STM height corrugation compatible with the experimentally observed one is obtained if the tunneling conductance is assumed dominated by contributions from orbitals of atomic p character centered on the tip. For the O-covered Al(111) the theoretical topographs agree well with the observed ones.

/pdf/electronic-structure-total-energies-and-stm-images-of-clean-3m1rlwjosg.pdf

Joachim Jacobsen

Papers

Effect of Strain on Surface-Diffusion and Nucleation

Island shape-induced transition from 2D to 3D growth for Pt/Pt(111).

Measuring surface diffusion from nucleation island densities

Atomic-scale determination of misfit dislocation loops at metal-metal interfaces.

Electronic structure, total energies, and STM images of clean and oxygen-covered Al(111).