Onset of plastic relaxation in the growth of Ge on Si(001) at low temperatures: Atomic-scale microscopy and dislocation modeling

""\\"We present a synergic experimental and theoretical investigation of the plastic relaxation onset in low-temperature Ge growth on Si(001). High-resolution transmission electron microscopy reveals that misfit is released by pairs of coupled 60° dislocations. Atomic resolution proved to be key in distinguishing pairs from single 90°dislocations because of the revealed small intra-pair dislocation distance (even less than 1 nm). By exploiting dislocation theory and molecular dynamics simulations, we demonstrate that the observed pairing naturally occurs as a result of the mutual interactions between the two dislocations. In particular, analytical models show that the stress field arising in a thin film when a dislocation segment lies at the interface with the substrate determines the most favored nucleation site for a new (complementary) dislocation that leads, after migration, to the coupling with the first in a stable position. At the growth temperature, further motion or recombination due to atomic scale effects is excluded by classical molecular dynamics simulations. A clear picture of the early stages in the strain relaxation emerges, gliding out of the interface and\\\\\\\/or short-range climbing (as typically produced by annealing or higher temperature steps, but not taking place under the present growth conditions) being required to transform pairs into a single edge dislocations. The present results also offer answers to the long-held puzzling question about the mechanism originating 90° dislocation in high mismatch Ge\\\\\\\/Si systems. \\""