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We present an extensive investigation of the epitaxial growth of Au-assisted axial heterostructure nanowires composed of group IV and III-V materials and derive a model to explain the overall morphology of such wires. By analogy with 2D epitaxial growth, this model relates the wire morphology (i.e., whether it is kinked or straight) to the relationship of the interface energies between the two materials and the particle. This model suggests that, for any pair of materials, it should be easier to form a straight wire with one interface direction than the other, and we demonstrate this for the material combinations presented here. However, such factors as kinetics and the use of surfactants may permit the growth of straight double heterostructure nanowires. Finally, we demonstrate that branched nanowire heterostructures, also known as nanotrees, can be successfully explained by the same model.

Germanium nanowires grown by chemical vapor deposition exhibit a peculiar dopant incorporation mechanism. The dopant atoms, such as boron and phosphorus, get incorporated through the wire surface, a mechanism which limits the doping modulation along the wire length, and therefore the fabrication of more elaborate structures that combine both n- and p-type doping. Using a novel device design that circumvents these constraints, we demonstrate here a linear Ge nanowire p-n junction.

The formation of abrupt Si/Ge heterointerfaces in nanowires presents useful possibilities for bandgap engineering. We grow Si nanowires containing thick Ge layers and sub-1 nm thick Ge "quantum wells" and measure the interfacial strain fields using geometric phase analysis. Narrow Ge layers show radial compressive strains of several percent, while stress at the Si/Ge interface causes lattice rotation. High strains can be achieved in these heterostructures, but we show that they are unstable to interdiffusion. [ABSTRACT FROM AUTHOR]

Germanium nucleation on a silicon surface typically proceeds via spontaneous formation of nanoscopic islands, or quantum dots (QDs), at random locations. However, potential applications of epitaxial QDs, such as quantum cellular automata, require precise control of Ge island positions on Si in clusters with the individual islands' separation of tens of nanometers or less. Controlled Ge island placement with an inter-island separation down to 100 nm can be reliably obtained by depositing Ge onto a Si(001) surface modified with a low dose focused ion beam (FIB) pattern to create preferred nucleation sites for individual islands. Here we investigate QD self-assembly of multiple islands on single topographical features intended to obtain clusters of QDs with smaller separations. We observe formation of 50 nm clusters on a single location defined by the FIB patterning of a larger and shallower feature, where cluster formation is promoted by reduced surface diffusion. We also discuss cluster formation on single sites defined by residual trenches on a footprint of a larger island that has been desorbed. We propose that controlled placement of islands during the first deposition may result in greater control over subsequent cluster size and placement fidelity due to improved uniformity of target features. [ABSTRACT FROM AUTHOR]