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The growth of heteroepitaxially strained semiconductors at the nanoscale enables tailoring of material properties for enhanced device performance. For core/shell nanowires (NWs), theoretical predictions of the coherency limits and the implications they carry remain uncertain without proper identification of the mechanisms by which strains relax. We present here for the Ge/Si core/shell NW system the first experimental measurement of critical shell thickness for strain relaxation in a semiconductor NW heterostructure and the identification of the relaxation mechanisms. Axial and tangential strain relief is initiated by the formation of periodic a/2 ⟨110⟩ perfect dislocations via nucleation and glide on {111} slip-planes. Glide of dislocation segments is directly confirmed by real-time in situ transmission electron microscope observations and by dislocation dynamics simulations. Further shell growth leads to roughening and grain formation which provides additional strain relief. As a consequence of core/shell strain sharing in NWs, a 16 nm radius Ge NW with a 3 nm Si shell is shown to accommodate 3% coherent strain at equilibrium, a factor of 3 increase over the 1 nm equilibrium critical thickness for planar Si/Ge heteroepitaxial growth.

Ordering of misfit dislocation segments in concentric polygons at the base of SiGe epitaxial islands on Si(001) has been recently indicated by in situ Transmission Electron Microscope observation. In this paper we confirm the very regular spacing by Atomic Force Microscope and He-ion Microscope measurements of the footprint carved in the Si substrate by the plastic events. We explain the intriguing ordering, as obtained with no gliding rearrangements, by cyclic occurrence of the thermodynamic critical conditions for plastic events. Quantitative predictions by a fully analytical model, which includes the dependence on island shape and composition in the generation of misfit dislocations, matched very well experimental measurements.

We demonstrate dislocation engineering without oxide masks. By using finite element simulations we show how nanopatterning of Si substrates with {111} trenches provides anisotropic elastic relaxation in a SiGe film, generates preferential nucleation sites for dislocation loops, and allows for dislocation trapping, leaving wide areas free of threading dislocations. These predictions are confirmed by atomic force and transmission electron microscopy performed on overcritical Si0.7 Ge0.3 films. These were grown by molecular beam epitaxy on a Si(001) substrate patterned with periodic arrays of selectively etched {111}-terminated trenches. © 2011 American Institute of Physics.

We show that suitable pit-patterning of a Si(001) substrate can strongly influence the nucleation and the propagation of dislocations during epitaxial deposition of Si-rich Si1-xGex alloys, preferentially gettering misfit segments along pit rows. In particular, for a 250 nm layer deposited by molecular beam epitaxy at x(Ge) - 15%, extended film regions appear free of dislocations, by atomic force microscopy, as confirmed by transmission electron microscopy sampling. This result is quite general, as explained by dislocation dynamics simulations, which reveal the key role of the inhomogeneous distribution in stress produced by the pit-patterning.