Microscopic Aspects of Thin Metal Film Epitaxial Growth on Metallic Substrates

Abstract

Crystal Growth I Metals I Nonequilibrium Phenomena I Nucleation I SurfacesMicroscopic processes in epitaxial growth of thin metal films on metal substrates are discussed in the light of recent scanning tunneling microscopy data. Details on the nature and role of different atomic processes, though too fast for direct observation, can be inferred from the characteristic development of the film morphology with increasing coverages and its modification upon varying the deposition rate and temperature. It is shown that for deposition at room temperature or moderately increased temperatures film growth and the resulting film morphology are dominated by kinetic effects, and that film growth proceeds via nucleation and growth of 2D islands. -At room temperature, where adatoms are sufficiently mobile to allow efficient intralayer mass transport, film growth begins by nucleation of supercritical clusters in a supersaturated 2D gas of mobile metal adatoms. These clusters can grow into 2D islands. The correlation between adatom mobility, substrate temperature and deposition rate on the one hand and island density on the other are demonstrated. The shape of the growing islands can vary widely, from dendritic to compact island forms. The different shapes are shown to result from kinetic limitations, specifically from a limited adatom mobility along island edges. -For kinetically controlled film growth in the multilayer regime the film morphology is determined by the amount of interlayer mass transport from higher to lower layer levels. 2D layer growth prevails for practically unhindered interlayer mass transport, while multilayer or 3D growth dominates where mass transport is strongly hindered or even absent. Different processes and kinetic effects determining this interlayer transport are discussed, in particular the effect of modifications in the adatom potential at island or step edges. Only for deposition at or annealing to higher temperatures the film morphologies resemble those expected from the thermodynamically controlled growth modes. In agreement with expectations layer-by-layer growth is found for homoepitaxial growth, layer-plus island growth with one or two layers critical thickness, respectively, for the heteroepitaxial growth systems investigated.

Microscopic view of epitaxial metal growth: Nucleation and aggregation

“…Trigonal networks of the type just described are found in a number of epitaxial systems. Examples for metals are Na adsorption on Au(1 1 1) [262], Ni on Ru(0 0 0 1) [151], or Cu on the same surface [257,263]. There are also examples of regular dislocation networks in semiconductors, for example the b-phase of Ga on Ge (1 1 1) [264,265], or surfactant mediated growth of Ge on Si(1 1 1) [266].…”

Nucleation Kinetics on Inhomogeneous Substrates: Al/Au(111)

“…Our model system, Al͞Au͑111͒-͑ Molecular beam epitaxy of semiconductors and metals is frequently governed by the growth kinetics being determined by the competition between the adatom diffusion rates and the deposition flux [1]. Recent scanning tunneling microscopy (STM) studies established the link between the activation energies of the related atomic displacement processes and the resulting island densities, shapes, and film morphologies [2].…”

Growth of Cr on Cu(001) studied by scanning tunneling microscopy

“…No diffusion along the island edges would lead to the formation of dendritic islands, while a high diffusion rate produces more compact islands. 21 The type of growth displayed at 285 K is an intermediary case between fractal and compact island formation. The transition between these two kinds is the source of some disagreement in the literature and has been the subject of recent theoretical calculations.…”