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

High angle boundaries formed by grain subdivision mechanisms

Morphological evolution during epitaxial thin film growth: Formation of 2D islands and 3D mounds

We describe the microscopic details of porosity formation during dealloying as illuminated by a kinetic Monte Carlo model incorporating site coordination-dependent surface diffusion of all alloy components, and site coordination-dependent dissolution of the less-noble atoms. Our simulation model reproduces the entire range of phenomena associated with selective dissolution. These phenomena include composition and geometric restrictions on dealloying (parting limits), a composition-dependent critical potential, a passivation regime, a regime of steady-state dissolution flux, and porosity formation. We find that an intrinsic critical potential exists as a well-defined threshold potential separating surface passivation and porosity formation behaviors, but this intrinsic critical potential typically sits at values well below the experimental measurements of the empirical critical potential. Finally, by detailed examination of the temperature dependence of the dissolution flux, and also of the types of surface sites contributing to the dissolution flux, we predict that the binding energy of a terrace atom may be straightforwardly extracted from analysis of experimental polarization curves.

https://iopscience.iop.org/article/10.1149/1.1784820/pdf

An Atomistic Description of Dealloying Porosity Evolution, the Critical Potential, and Rate-Limiting Behavior

The nucleation and growth of two-dimensional islands during the submonolayer stage of epitaxial growth is studied with kinetic Monte Carlo simulations and mean-field rate equations. Previous work on irreversible growth is extended to include relaxation of island shapes by edge diffusion. Island morphologies range from ramified structures at low temperatures to compact, polygonal shapes at higher temperatures. Using a self-consistent calculation of the rate coefficients, [ital quantitative] agreement is obtained between the solution to coupled, mean-field rate equations and the simulation results for average quantities. The island size distribution function is described by a single universal scaling function.'' The average island size is the only important scale for determining island densities. It is shown that the general form of the scaling [ital ansatz] applies to wider range of coverages than anticipated previously. This scaling form is combined with the solution to the rate equations to explore explicitly the [ital D]/[ital F] dependence of the number density of islands ([ital D] is the surface diffusion coefficient and [ital F] is the deposition flux).

Dynamics of irreversible island growth during submonolayer epitaxy.

Single atom high islands which form during the early stages (submonolayer) of epitaxial growth exhibit morphologies which range from ramified structures to compact polygonal shapes. For example, scanning tunneling microscopy images of Au on Ru(0001) grown at room temperature reveal fractal islands. Deposition of Co on the same substrate produces compact triangular islands [1]. Similarly, ramified islands are observed for Pt on Pt at low temperature, and compact islands are found for higher temperatures [2]. In this work the dependence of the island morphologies on externally controlled growth parameters is addressed. It is important to understand this dependence, since these islands form the template upon which all further growth proceeds [3]. The observation of ramified island growth has been understood [1] by comparison with the simplest \"hit and stick\" model of diffusion limited aggregation (DLA) [4]. In this model, a particle diffuses on the substrate to an island from far away and sticks at the first point of contact with the cluster. The fractal dimension for a DLA cluster is identical to that measured for islands of Au on Ru(0001) [1]. However, the growth of islands during epitaxy differs from DLA in two very important respects: (1) Atoms which diffuse to the island relax to a lower energy site, e.g. , by diffusion along the edge of the island (edge diffusion). As the rate of relaxation increases relative to the rate at which atoms diffuse Io the island, a morphological transition occurs from ramified island growth to compact island growth. For an isolated island this competition is well understood [5,6]. (2) The islands grow in the presence of other islands on the surface and, therefore, compete for the available diffusing monomers. The effects of this complication on the island morphology are not yet well understood. In fact, there is disagreement in recent literature [7—9] as to the dependence of the transition on the growth parameters. In this paper, the morphological transition is studied by considering the shape stability of a compact island. A linear stability analysis, similar to that applied to the study of the Mullins-Sekerka instability [10], is used to study the stability of a circular island with respect to infinitesimal morphological fluctuations. In contrast to a traditional stability analysis [5,11], the environment surrounding any particular island on the surface is changing with time during the growth process, due to the presence of the other islands. In this work, therefore, the stability analysis is combined with a selfconsistent mean-field theory which describes accurately average properties of the entire surface [12]. Based on this novel approach and the results of kinetic Monte Carlo (KMC) simulations a new scaling law is formulated, and the disagreements in the literature reconciled. During the growth process, a competition develops between the nucleation of new islands on the substrate and the capture of monomers by already existing islands. This competition leads to a simple relation between the average distance between islands 4, the deposition flux F, and the monomer diffusion constant D, namely,

G. S. Bales

Papers

Dynamics of irreversible island growth during submonolayer epitaxy.

Transition from compact to fractal islands during submonolayer epitaxial growth.