Self-organized evolution of Ge/Si(001) into intersecting bundles of horizontal nanowires during annealing

We report the observation of large scale self-assembly of long horizontal nanowires into orthogonally oriented bundles, during in situ annealing of a few monolayers of Ge on Si(001). Results are interpreted in terms of a collective wave-propagation mechanism, previously suggested for interpreting ripple faceting on Ge/Si(1110) surfaces. Quantitative agreement between experiments and theory is found. The onset of the mechanism, the number of wires in the bundles, and their total density can be controlled by carefully tuning the growth parameters.