Direct observation of strain relaxation in iron layers on W(110) by time-resolved STM

Time-resolved,in-situ-applied STM has been used to study the epitaxial growth of iron on W(110) at room temperature. By this way, sequences of STM images show directly the atomistics of the growth process on the surface. The first layer of iron on W(110) grows pseudomorphically without a preferred growth direction. Beginning with the second layer, the islands grow anisotropically with preferred growth in the [001]-direction. The generation of an ordered two-dimensional dislocation network starts at a coverage of 1.4 pseudomorphic monolayers to relax the misfit of 9.4%. A direct correlation of the creation of misfit dislocations in the second layer and the nucleation of the third-layer islands was found. Together with the onset of strain relaxation, the growth mode abruptly changes from layer-by-layer to statistical growth. A quantitative statistical analysis of the data allows to exactly determine the onset of relaxation, the vertical location of the dislocation lines, and the lateral extension of an island that is necessary to induce the formation of dislocations.