The study of semiconducting nanowires is one of the most rapidly growing research areas in materials science and nanotechnology. Like carbon nanotubes, nanowires represent an excellent test-bed of how the quantum effects determine their geometric and electronic structure and, especially, their electron transport properties.
With the continuous miniaturization of electronic devices the search for building blocks of future molecular electronics application have attracted a great interest and nanowires seem to be one of the most promising alternative. The great advantage that they present over carbon nanotubes is that they are invariantly semiconducting, while, as it is well known, carbon nanotubes are metallic or semiconducting depending on their chirality that, to date, cannot be controlled at growth time. This is an essential feature for the design and realization of nanoelectronics applications, which require basic components that are semiconductors. Hence, not only nanowires can be used as interconnections between different components, but they can also directly operate as the active region of the device and several examples of (nano)transistors have been demonstrated.
However, an exhaustive physical understanding of many fundamental issues, such as the influence of the structure on the conducting properties, the role of surface conduction or defect scattering, is still lacking. A crucial mechanism like the contribution of extrinsic carriers (those provided by dopants) to the conduction, for instance, is far to be fully understood in systems of low dimensionality and nanoscopic sizes like nanowires and basic questions such as the radial localization of impurities, their ionization energy in the confined nanowire geometry, and their ability to trap states, are still largely unanswered. The difficulties inherent to the density functional theory to calculate excitation energies are a serious difficulty to address part of these questions.
The role of theoretical modeling and simulation is extraordinarily important, since many of the properties of nanowires only difficultly can be accessed experimentally. As a matter of fact, although the structure of the most common nanowires is reasonably well-know, thanks to several theoretical studies and experimental works, relatively little is known about the structure and the electronic properties in less conventional, but equally interesting situations, such as (i) heterostructures, lattice mismatch, strained structures, (ii) growth / deposition on surfaces (iii) stability, diffusion and electronic properties of defects and (iv) spin-transport in presence of magnetic impurities/leads. All these situations have the highest interest, and they are far to be satisfactorily understood.
Concerning silicon nanowires in particular, it must be underlined that wires grown along most of the crystallographic orientations exhibit a direct band-gap, while it is well-know that in bulk silicon is indirect. This circumstance opens a brand new research line which, obviously, has already attracted the interests of a large part of the nanoelectronics community: the realization of silicon-based optics and optoelectronics applications.
(Technical University of Denmark, Lyngby)