Nanowires are structures with very small dimensions (few 10 to few 100 nm) in two directions, but longer (typically few micrometer) in the third one. We are more specifically dealing with epitaxially grown nanowires on a substrate, in contrast to wires fabricated by etching or more general by lithographic techniques.
fabrication of nanowires
The wires we are investigating are grown in a self-assembled fashion mainly using the so-called vapor-liquid-solid technique. This essentially follows the scheme:
Small seed particles are deposited or created on a semiconductor surface. This may be Au aerosols or Au droplets formed by heating a Au film, which dewets the surface. This can also be seed particles formed during deposition of material (e.g. In or Sb) onto the semiconductor surface, see below.
A flow of gases containing the materials for the nanowires is directed onto the surface, which is held at high temoperatures. This can be compounds like metal-organic molecules (in Metal-Organic Chemical Vapour deposition) or single atoms in molecular beam epitaxy. The materials partly dissolve in the seed particle, until it is saturated with the different materials.
upon further deposition, material is crystallized out of the seed particle. Since nucleation energies are often lower at the contact interface to the substrate, a small column, the nanowire, is growing below the seed particle.
By changing the composition of the precursor materials, heterostructures along the wire axis can be grown, or by changing growth conditions, the wires can be wrapped into a shell of different material.
The structural properties of nanowires are the topic of our research. This starts with basic properties like material composition, which can be inferred from the lattice parameters, but comprises also the crystal structure (III-V compound semiconductor nanowires are often crystallizing in a hexagonal crystal lattice, which does not occur for bulk crystals or 2D epitaxial films), and complex strain distributions within heterostructures.
We are engaged in several projects using nanowires, among them the AMON-Ra project, where nanowires are used for solar cells with improved efficiencies, the Green-Si project, where wires are used for thermoelectric devices, but we are also collaborating with several groups around the globe for basic science studies on the details of nanowire growth mode, details of unit cell distortions, or to link the structural properties to the resulting band structures and band alignments, which is important for optical and electronic properties of nanowires, which can be rather different from the same materials grown as epitaxial layers.