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We show that two-dimensional Si/Ge nanostructures can be imaged with chemical sensitivity on the nanometer scale using a scanning tunneling microscope (STM). Using an atomic layer of Bi terminating the surface, we can distinguish between Si and Ge. The apparent height measured by the STM is ∼0.09 nm higher at areas consisting of Ge than on areas consisting of Si. This distinction between Si and Ge enabled us to fabricate and characterize two-dimensional Si/Ge nanostructures in a controlled way. Si/Ge nanostructures consisting of alternating Si and Ge rings having a width of 5–10 nm were grown around a 2D Si core island on a Si(111) substrate. The thickness of the Si and Ge rings is only one atomic layer (0.3 nm). Intermixing of Ge and Si is observed if the growth conditions are not chosen properly. The optimized growth conditions were obtained by lowering the temperature and decreasing the growth rate to prevent intermixing and nucleation of secondary islands. The present fabrication method is expected to be applied for nanostructured devices. [Copyright &y& Elsevier]

Rearrangement of two-dimensional Ge and Si islands after coarsening on a laterally strain modulated Ge surface covered with a monolayer of Bi is studied by scanning tunneling microscopy. Spatial ordering of the islands with respect to the modulations of the strain field shows that the binding energy of Si atoms at strained Si islands is, unexpectedly, higher than at unstrained islands. This behavior is explained by a strong compressive stress of the surfactant layer, which favors an expansion of the underlying surface structures and therefore stabilizes the tensile strained Si islands. It is shown that a Si mass transport towards more strained Si islands can be also favored by an increased binding energy of Si adatoms to the less strained parts of the Ge surface. [Copyright &y& Elsevier]

Abstract: Surface undulations induced by interfacial misfit dislocations in the Ge/Si(111) films grown by conventional molecular beam epitaxy and by surfactant-mediated epitaxy with Bi as a surfactant have been analyzed using scanning tunnelling microscopy and elasticity theory. A comparison of the experimentally measured undulation patterns with patterns calculated with elasticity theory leads to identification of the dislocations in both systems as 90° Shockley partial dislocations. Dislocations are primarily arranged into a triangular network in Bi-mediated growth, whereas in conventional epitaxy a strongly disordered honeycomb network prevails. The dislocation density in conventional epitaxy is found to be 30% smaller than in Bi-mediated growth, which is attributed to strong Si–Ge intermixing. [Copyright &y& Elsevier]

The tensile-strained Si(111) layers grown on top of Ge(111) substrates are studied by combining scanning tunneling microscopy, low-energy electron diffraction, and first-principles calculations. It is shown that the layers exhibit c(2×4) domains, which are separated by domain walls along ⟨110⟩ directions. A model structure for the c(2×4) domains is proposed, which shows low formation energy and good agreement with the experimental data. The results of our calculations suggest that Ge atoms are likely to replace Si atoms with dangling bonds on the surface (rest-atoms and adatoms), thus significantly lowering the surface energy and inducing the formation of domain walls. The experiments and calculations demonstrate that when surface strain changes from compressive to tensile, the (111) reconstruction converts from dimer-adatom-stacking fault-based to adatom-based structures. [ABSTRACT FROM AUTHOR]