Island shape controls magic-size effect for heteroepitaxial diffusion

Abstract

Lattice mismatch of Cu on Ag(111) produces fast diffusion for special "magic sizes" of islands. A sizeand shape-dependent reptation mechanism is responsible for low diffusion barriers. Initiating the reptation mechanism requires a suitable island shape, a property not considered in previous studies of 1D island chains and 2D closed-shell islands. Shape determines the dominant diffusion mechanism and leads to multiple clearly identifiable magic-size trends for diffusion depending on the number of atoms whose bonds are shortened during diffusion. PACS numbers: 68.35.Fx,68.35.bd,68.35.Gy,71.15.Pd 1 Control of thin-film morphology relies on understanding multiple ongoing processes during deposition and growth. In particular, diffusion of small atom clusters on surfaces play a critical role in thin film growth, especially in early stages. The diffusion kinetics of small islands in heteroepitaxial systems is less well understood than that of homoepitaxial diffusion, for which much experimental [1,2,3,4] and theoretical [5,6,7,8] work has been done. Strain is known to govern the mesoscale morphology in self-assembling systems [9]. While predictions about the role of size and misfit for small islands go back over a decade [10], only recent experiments have captured and quantified the rapid diffusion at "magic sizes" in the heteroepitaxial Cu/Ag (111)

Preferential nucleation and high mobility of linear Cu trimers on Ag(111)

“…Using low-temperature scanning tunneling microscopy (STM), we have previously shown that larger clusters have surprisingly low diffusion barriers and a novel dislocation mechanism that is related to the ~12% lattice mismatch for Cu on Ag(111) [25,26]. Here, we show that Cu trimers diffuse large distances following their formation.…”

“…The extremely low diffusion barrier for the linear trimer, combined with the preference for this structure during nucleation, make the trimer an extremely important contributor to mass transport for Cu on Ag(111). The effects of lattice mismatch are manifest in the observed directions of hopping and rotation that require intermediate steps involving trimers of mixed fcc-hcp character that shorten Cu-Cu bonds to relieve strain, in a one-dimensional analog of the misfit-dislocation mechanism responsible for the low diffusion barriers of larger Cu clusters on Ag(111) [25,26]. Because the novel diffusion phenomena for both small and large clusters of Cu on Ag(111) result from lattice mismatch, it is likely that similar behavior will occur in other heteroepitaxial systems.…”

“…1 for individual atoms, though it is possibly much lower than that. This low barrier is surprising in light of molecular dynamics simulations that predict a barrier of ~290 meV-prohibitively large to allow diffusion at 24 K [25,26,37]. In these simulations, the ground state of a Cu trimer on Ag(111) was a compact triangle with all atoms in fcc sites, and diffusion occurred in a concerted fashion.…”

“…Because the majority of cluster diffusion studies have focused on homoepitaxial systems, even less is known about the role of lattice mismatch in determining the properties of heteroepitaxial clusters despite predictions of interesting phenomena [19,20]. While the mobility of larger islands generally decreases with size, several cases of non-monotonic size dependencies [21][22][23][24][25][26][27][28][29][30][31][32] and, remarkably, cases where clusters have even higher mobility than individual atoms, have been reported [27][28][29][30][31][32].…”

Stochastic model for the epitaxial growth of two-dimensional islands in the submonolayer regime

“…To analyse the eect of the mobility of small islands on the growth of the large island we consider that islands diuse as clusters. The eects of cluster diusion on the island size distribution have been reviewed for the case of irreversible growth of compact islands on a 2D substrate, and several mechanisms for the diusion of large particles on solid surfaces were proposed [74][75][76][77][78][79]. In most cases, scaling arguments predict that the coecient of particle diusion decays as a power law with particle sizes n ∼ µ −…”