O. Pierre-Louis1,2 and T. L. Einstein,1

We describe the steady states, fluctuations, dynamics, and instabilities of atom and of vacancy single-layer-height islands during electromigration. We emphasize the dependence on cluster size and on the mass-transport mechanism: periphery diffusion (PD), terrace diffusion (TD), or evaporation-condensation (EC), and the differences between atom and vacancy clusters. The cluster drift velocity has the size dependence of (cluster area) ´ (equilibrium cluster diffusion constant). For PD (unlike TD or EC), atom and vacancy islands drift in opposite directions, potentially resolving controversies regarding mechanisms of diffusion. We find circular steady states for PD and TD, and non-circular ones for EC vacancy islands. Analytical calculations are corroborated by both Monte Carlo simulations and numerical integration. For weak electromigration the cluster responds isotropically for TD or PD but not EC. In EC, clusters elongate perpendicular to the drift axis. In PD a morphological instability leads to cluster splitting. TD or EC induces a new instability for vacancy clusters above a threshold. Using Langevin formalism, we derive the non-equilibrium cluster diffusion constant and study morphological fluctuations. Electromigration can also alter attachment/ detachment probabilities; with novel consequences for cluster dynamics.Work supported by NSF MRSEC grant DMR 96-32521.