Misfit-dislocation-mediated heteroepitaxial island diffusion

a r t i c l e i n f o Scanning tunneling microscopy combined with molecular dynamics simulations reveals a dislocation-mediated island diffusion mechanism for Cu on Ag(111), a highly mismatched system. Cluster motion is tracked with atomic precision at multiple temperatures and diffusion barriers and prefactors are determined from direct measurements of hop rates. The barrier to nucleate a dislocation is sensitive to island size and shape, resulting in a non-monotonic size dependence of the diffusion barrier. Dislocations are key in the mechanical properties of solids by enabling crystalline materials to deform plastically when subjected to stress orders of magnitude lower than their theoretical critical shear stress [1]. They have also been shown to relieve stress in strained films, greatly affecting the growth mode [2–4], and have been predicted [5,6] but never experimentally implicated in adatom island diffusion. The majority of experimental studies of island diffusion have been limited to homoepitaxial systems where motion is usually a result of diffusion at steps, particularly for large islands (10 2 –10 3 atoms) [7–12]. In these cases, the barrier is insensitive to size, but diffusivity scales with size depending on the rate-limiting process [13–15]. In contrast, there have been numerous theoretical predictions [16–22] and a few experimental demonstrations [23–27] of non-trivial size dependencies of the diffusion barrier and/or prefactor for smaller homoepitaxial clusters (2–20 atoms). This work shows that dislocations in highly mismatched heteroepitaxial islands reduce barriers for islands of special sizes and shapes in the same way that they reduce the yield stress of bulk materials: by enabling slip to occur in a piecewise fashion. Using scanning tunneling microscopy (STM) and molecular dynamics (MD) simulations, we reveal a dislocation-mediated island diffusion mechanism for Cu on Ag(111). Simulations show that the lattice mismatch of ~ 12% favors dislocation nucleation in islands larger than tetramers, resulting in a non-trivial size dependence that is manifest in experiments where clusters containing up to 26 diffuse much faster than smaller clusters. The barriers for island sizes and shapes favoring this mechanism are lower than that of edge diffusion or Ostwald ripening, and cluster coalescence is the kinetically preferred coarsening pathway. Measurements were carried out in an Omicron LT-STM that can image at 4.5–300 K. The sample stage was enclosed by a cryogenical-ly-cooled metal shroud which kept the temperature regulated and the sample clean. The Ag(111) substrate was prepared in an adjoining chamber by evaporating …