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MRS BULLETIN/JULY 1994

most fundamental steps in the growth of crystalline materials and epitaxial overlayers. To develop realistic models of materials growth processes, it is necessary to determine quantitatively the relative energetics of surface diffusion and cluster nucleation processes. Although recent developments in scanning tunneling microscopy have led to preliminary investigations of single-atom surface diffusion,3 our current understanding of metal atom diffusion on metal surfaces is still based primarily on studies with the FIM. By following well-established procedures, the FIM can be used to track the motion of an individual atom as it migrates across a perfectly defined crystal surface and to determine its activation energy of surface diffusion.4 The experimental procedures used in FIM surface diffusion studies were developed in the mid-1960s.5 During the following years, researchers have examined

the migration of single adatoms and small clusters for a wide variety of metal-metal combinations.4 For many systems, these studies have reinforced our intuitive notions about metal atom diffusion and cluster nucleation—i.e., that atom motion takes place by a series of "hops" across the surface and that simple pair interactions between neighboring atoms can explain cluster nucleation. However, more and more, FIM studies are yielding results counter to this intuitive picture. This and the following section discuss how the FIM has contributed to our changing views on the fundamental aspects of surface diffusion and cluster nucleation. One of the biggest surprises to result from FIM studies of single-atom diffusion was the discovery that adatoms may migrate across a surface by a mechanism other than conventional hopping. For certain combinations of metal adatoms and substrate surfaces, adatoms on top of the surface find it energetically favorable to push neighboring atoms out from the substrate and take their place in the top layer of surface atoms. The displaced surface atom is then left to continue the migration process. It is now established that these "exchange" or "substitutional" displacements, first observed in FIM and atom probe investigations of single-atom diffusion on the corrugated (110) surfaces of Pt and Ir,6'7 also take place on atomically smooth fcc(100) surfaces. The exchange mechanism for diffusion on fcc(100) surfaces is indicated schematically in Figure I.8 An adatom resting in a fourfold hollow begins the displacement by moving down toward a surface atom. At the same time the adatom moves down,

[100] Figure 1. Schematic illustration of the exchange process on fcc(1OO) surfaces. In (a), an adatom resides in a fourfold hollow. In (b), the adatom has moved down toward the surface and pushed a surface atom up. In (c), the adatom takes the place of the surface atom, leaving the displaced atom to continue the migration process. Displacements are along (100) directions.

35

Atomic-Level Studies of Processes on Metal Surfaces

the surface atom moves up out of its lattice position. At the saddl