Atomic-Scale Determination of Misfit Dislocation Loops at Metal-Metal Interfaces

The growth of one monolayer of Au on Ni(111) is shown to lead to an ordered array of misfit dislocation loops in the underlying Ni(111) surface. The signature of these loops is observed by scanning tunneling microscopy, and atomistic simulations are used to relate the observed surface structure to that of the buried interface. The new interface structure is different from normal misfit dislocation structures in three respects: (i) it forms already during growth of a single Au monolayer, (ii) it forms in the substrate and not in the overlayer, and (iii) it is controlled by the interface energy rather than by the strain in the two phases. The structure of the interphase between two different materials is determined by a detailed balance between the structure of the two bulk phases and the interatomic interactions at the interface. As early as 1949 it was suggested by Frank and van der Merwe (1) that the stress induced by the mismatch in the interface may be relieved through the formation of misfit dislocations, that is, regions with large changes in the interatomic distances separating domains with more normal bond lengths. These misfit dislocations affect not only the adhesive properties, and hence the strength of the interface, but also the electronic properties. A large number of electron- microscopy studies of interface structures have described misfit dislocations on the micron scale (2), but the detailed atomic structure is much harder to obtain. Recent scanning tunneling microscopy (STM) studies have observed the formation of misfit dislocations during the growth of one metal on another. For Ag on Pt(111) (3) and Cu on Ru(0001) (4), it is found that initially a pseudomorphically strained first layer is formed, followed by subsequent layers in which the strain is relieved either unidirectionally (1D domain walls) or more isotropically in two-dimensional (2D) dislocation networks in the overlayer. Similar observations have been published for EuTe on PbTe(111), although the thickness of the films was considerably larger in this case (5). These systems nicely illustrate the concept of a critical layer thickness, beyond which the strain in a pseudomorphic overlayer is so large that it must be relieved through the formation of misfit dislocations (2). In this Letter, we present a new scenario for the matching between two metals with large lattice mismatch (16%). We show that the growth of a single monolayer of Au on Ni(111) results in an ordered array of triangular misfit dislocation loops in the underlying Ni surface. The signature of these loops is revealed in STM images of the surface structure, and atomistic simulations within the effective-medium theory relate this to the atomic structure at the buried interface. For this system, the misfit dislocations are formed in the substrate and not in the overlayer as previously observed for other systems. We furthermore conclude that the origin of the novel misfit dislocation structure is closely related to the large lattice misfit and the strong Au-Ni interactions at the