Importance of the additional step-edge barrier in determining film morphology during epitaxial growth

Abstract

A model of epitaxial growth based on steady-state assumptions is derived and shows that the decisive quantity determining the film morphology is the additional energy barrier at the step edges, i.e. , the barrier to descend the step minus the surface-diffusion barrier. The model allows this barrier to be directly determined from experimentally observed film morphologies. It is applied to homoepitaxy on Ag(111) and Pt (111) where the additional barriers amount to -150 and -165 meV, respectively. In addition, this model provides new understanding of more complex processes, such as the surfactant effect of Sb on Ag(111) and reentrant growth on Pt(111).There has been continued interest in epitaxial growth with the parallel goals of obtaining both a more fundamental understanding of the growth and improving control to obtain new materials with desirable properties. While homoepitaxial growth has limited prospects for the latter, its simple thermodynamics makes it useful for the former. In spite of the apparent triviality of homoepitaxial systems, recent experiments have show a surprising variety of behaviors, including reentrant layer-by-layer growth and the change in growth morphology with surface active species.Although it was immediately recognized that the energy barrier for adatoms to move over step edges and the shape of the island perimeter~l ag an important role in determining the film morphology, ' ' an analytic model capable of explicitly showing the relative importance of these factors has not been presented. Here we develop such a model which allows the energy barrier for adatoms to move over descending step edges to be calculated directly from STM (scanning tunneling microscopy) or LEEM (low-energy electron microscopy) observations. An accessible experimental determination of this decisive barrier is of obvious importance for learning to control epitaxial growth and for comparing results from simulations and first-principle calculations.We demonstrate the use of this model by analyzing homoepitaxy on Ag(111) and Pt(111) and obtain step edge barriers of 150 and 165 meV in excess of the terrace diffusion barriers, respectively. Additionally, this model is used to analyze the associated problems of the role of Sb additives on Ag(111) homoepitaxy and reentrant growth in Pt (111) homoepitaxy. ' The onset of vertical growth, i.e. , the nucleation of the second layer, can occur in three different stages of the firstlayer growth process: (i) in the transient regime where the first-layer island density builds up, (ii) in the region of saturated island density, or (iii) in the coverage domain above the onset of coalescence, where the density of first-layer islands decreases. The actual situation is largely determined by the amount of interlayer transport and hence by the relative size of the barrier for diffusing over island edges. In the absence of an additional repulsive barrier at island edges, secondlayer islands will not occur until the onset of first-layer island coalescence and layer-by-layer growth resul...

Spatially Resolved Chemistry on Bimetallic Surfaces

“…In an atomistic picture these processes are dominated by adatom migration over the free substrate surface (nucleation), migration along island edges (2D growth) and migration across island and step edges (vertical growth). The decisive parameters herefore are the preexponentials and activation energies for the respective migration steps (see [18,19] and references therein).…”

Bi induced layer-by-layer growth in the homoepitaxial growth of Fe on Fe (1 0 0)-c(2×2)O reconstruction surface

“…The second explanation is that Bi reduces the additional energy barrier at the step edge of islands E a : The reduced E a lowers the probability of nucleation on top of the island even if the island density does not change by the surfactant, leading to layer-by-layer growth. The reduced E a can be caused by either a decrease of E b or by an increase of E d : According to H. A. van der Vegt et al [15], the former is the case for the growth of Cu(1 0 0) [15] and Cu(1 1 1) [16] with In, and the latter which is local effect at the step edge is the case for the growth of Ag(1 1 1) with Sb [17][18][19]. When we deposited Bi on the Fe(1 0 0)-c(2 Â 2)O surface the RHEED intensities always decreased (not shown), whether the substrate temperature was 100 C or room temperature.…”

Overcoming doping bottleneck by using surfactant and strain

“…To effectively achieve certain materials design goals, surface active agents (surfactants) are often used [5][6][7]. Surfactants are elements that flow to the top of the surface during growth.…”