Surface Self-Diffusion by Vacancy Motion: Island Ripening on Cu(001)

Abstract

We have used scanning tunneling microscopy to study the Ostwald ripening of 2D islands of Cu grown on Cu(001). By considering the time dependence of the sizes of individual islands we have characterized the mechanisms for the ripening. Our result is unexpected for a simple metal surface: The flow of atoms from one island to another is limited by attachment-detachment kinetics at the island edges. To explain this result, we propose that the transport of atoms between islands occurs by vacancy, rather than by adatom, diffusion. [S0031-9007(97)04112-4] PACS numbers: 68.35.Fx, 61.16.Ch, 82.65.Dp It is relatively easy to catalog the many atomic processes possibly involved in surface self-diffusion. It is much harder to quantify them experimentally or to determine which particular processes govern surface evolution on a large length scale. A basic process to consider is an adatom diffusing on a terrace towards a step edge, where the adatom is incorporated. On a simple metal surface there are no obvious additional barriers for this incorporation if the adatom approaches the step from below: The barrier for diffusion on the terrace will be the same or larger than the barriers associated with incorporation into the step edge. It is thus generally believed, and has been observed in 2D island decay on Ag(111) [1] and Cu(111) [2], that the flow of atoms to or from step edges is limited by the rate at which adatoms diffuse on the terraces. Since it is often assumed that surface morphology equilibrates by surface steps exchanging adatoms, this belief is central to models of surface self-diffusion which are detailed enough to take surface steps into account [3]. In this paper we will show, however, that mass transfer between islands on Cu(001) is not diffusion limited over a range of temperatures: Rather it is limited by the attachment and detachment processes at step edges. We propose that this occurs because surface self-diffusion on Cu(001) occurs by vacancy, rather than by adatom, diffusion.To probe the mechanisms of diffusion between step edges we have used scanning tunneling microscopy (STM) to measure the time dependence of the sizes of 2D Cu islands grown on Cu(001). The deposited islands are out of equilibrium: Large islands grow at the expense of small islands to reduce the total step length and thus the free energy of the system. By studying this ripening process it is possible to extract quantitative information on the kinetics of mass transport at the surface: 2D island ripening provides a geometry which can be readily analyzed because the driving force for adatom diffusion and step motion (step tension and curvature) can be determined from the island shapes and sizes.In the standard theory [4-6] of island ripening by adatom motion, the time rate of change of the area of an island is determined by two kinetic processes: the diffusion of adatoms on the surrounding terrace towards or away from the island, and the transfer of atoms onto, or off, the island edge. The diffusion rate on the terraces is determined by ...

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