Electric forces on a confined ad vacancy island

The passage of an electric current in a material can cause a biased mass transport at its surface. This migration phenomenon is intimately related to the microscopic details of atomic processes of diffusion and attachment/detachment at step edges. Using low energy electron microscopy we have examined in operando under an electric current the migration of Si(111)-1×1 monoatomic ad vacancy islands confined on Si(111)-7×7 terraces. The islands move opposite to the current direction, with velocity increasing with the radius. We show that the kinetics of attachment/detachment of atoms at step edges and the migration of ad atoms on terraces are in competition. The effective valence of Si ad atoms is 3.4±0.6 and the kinetic length of attachment-detachment is about 500 nm. The analysis of the islands shape reveals that the electric current biases significantly the kinetic rate of mass transfers at step edges modifying the overall island shape.