Rapid adatom island decay on Cu(111): a kinetic Monte Carlo simulation study
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Rapid adatom island decay on Cu(111): a kinetic Monte Carlo simulation study Mats I. Larsson Department of Physics, Universitetsgatan 1, Karlstad University, SE-65188 Karlstad, Sweden ABSTRACT Kinetic Monte Carlo (KMC) simulations are used to investigate the recent scanning tunneling microscopy (STM) measurements of fast decaying adatom islands on Cu(111). The KMC model is a full diffusion bond-counting model including nearest neighbor as well as second-nearest neighbor interactions. For encounters between steps in adjacent atomic layers of an island it is demonstrated that a moderately reduced activation energy for interlayer adatom transport is enough to obtain correspondence between simulations and experiments, provided that the one-dimensional Ehrlich-Schwoebel barrier for corner transitions is reduced to zero. The results presented in this report are interesting because they demonstrate that stepedge crossing by simple adatom hopping is sufficient to explain the rapid island-decay mechanism.
INTRODUCTION Fast decay of adatom islands and mounds on Cu(111) was recently observed by means of scanning tunneling microscopy (STM) measurements [1,2]. For this system a dramatic increase in the island decay rate was discovered when a small two-dimensional (2D) island migrating on top of a larger one encounters the boundary of the larger island. Thereafter rapid top island decay was also reported for Ag(111) in two recent publications [3,4]. The phenomenon for Cu(111) was first explained with a "quantum confinement effect" [5]. One year later, it was shown that this effect could not explain the Ag(111) behavior [3]. The real reason why the decay dynamics change was proposed to be the onset of concerted downward mass transport [6]. Thereafter it was suggested that concerted events play an important role, for both Ag(111) and Cu(111) [4]. Concerted motion can not be directly considered in the present kinetic Monte Carlo (KMC) model; nevertheless it will be shown that such concerted events are not really needed to explain the fast-decay dynamics. The intention of this paper is to use KMC simulations to study the dynamics of biatomic high Cu islands on Cu(111). In particular, the effect of a step-step interaction induced reduction of the interlayer potential-energy barriers will be investigated in conjunction with the influence of reduced activation energies for step-edge diffusion along the island boundaries.
KINETIC MONTE CARLO MODEL The kinetic Monte Carlo model used in this work is a full-diffusion bond-counting model. It is a slightly modified version of the model described in reference 7. The hopping rate ν(ni,nf) = ν0 exp(-E(ni,nf)/kT)
(1)
O1.3.1
is dependent on both the initial coordination number ni (i.e. the number of nearest neighbors before a hop) and the final coordination number nf (i.e. the number of nearest neighbors after a hop). Only adatom occupation of fcc lattice sites is modeled and the attempt frequency ν0 = 3×1012 s-1 is tentatively chosen for an atomic hop in one of the six possible hop directio
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