MICROSCOPIC ASPECTS OF THIN METAL-FILM EPITAXIAL-GROWTH ON METALLIC SUBSTRATES

Microscopic processes in epitaxial growth of thin metal films on metal substrates are discussed in the light of recent scanning tunneling mi croscopy data. Details on the nature and role of different atomic proc esses, though too fast for direct observation, can be inferred from th e characteristic development of the film morphology with increasing co verages and its modification upon varying the deposition rate and temp erature. It is shown that for deposition at room temperature or modera tely increased temperatures film growth and the resulting film morphol ogy are dominated by kinetic effects, and that film growth proceeds vi a nucleation and growth of 2D islands. - At room temperature, where ad atoms are sufficiently mobile to allow efficient intralayer mass trans port, 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 t emperature and deposition rate on the one hand and island density on t he other are demonstrated. The shape of the growing islands can vary w idely, from dendritic to compact island forms. The different shapes ar e shown to result from kinetic limitations, specifically from a limite d adatom mobility along island edges. - For kinetically controlled fil m growth in the multilayer regime the film morphology is determined by the amount of interlayer mass transport from higher to lower layer le vels. 2D layer growth prevails for practically unhindered interlayer m ass transport, while multilayer or 3D growth dominates where mass tran sport is strongly hindered or even absent. Different processes and kin etic effects determining this interlayer transport are discussed, in p articular the effect of modifications in the adatom potential at islan d or step edges. Only for deposition at or annealing to higher tempera tures the film morphologies resemble those expected from the thermodyn amically 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 th e heteroepitaxial growth systems investigated.