Order and disorder in the heteroepitaxy of semiconductor nanostructures

The heteroepitaxy of semiconductor pairs with a small lattice mismatch is a process of tremendous interest in materials science and technology. The principal mechanism of strain relief in these interfaces is the formation of three dimensional islands either directly on a bare substrate (Volmer-Weber growth mode) or following the formation of an initially flat wetting layer (Stranski-Krastanov growth mode). The elemental and strain inhomogeneities associated with these three dimensional islands may result into a confinement potential for electrons and/or holes, as in a standard quantum well. At variance with a standard quantum well, the confinement in these nanostructures (often referred to as 'quantum dots' (QDs)) occurs in all three spatial dimensions and over length scales comparable with the relevant De Broglie wavelength. This strong confinement may give rise to a discrete spectrum of charge carrier energy levels, as in an artificial atom. On the other hand the spectra of these nanostructures may be tuned with their physical and chemical properties, providing an enabling opportunity to design novel optical and electronic components. Epitaxial nanostructures are proposed as the building blocks of a variety of innovative applications, which may represent step-change solutions to many challenges in the fields of photonics and electronics, such as e.g. new possibilities to integrate versatile lasers and transistors in Information and Communication Technologies and to replace MOSFET devices with miniature components capable of sustaining the race to miniaturization of integrated circuits. Examples of possible applications include lasers, optical detectors, white-light sources, single-photon and entangled-pair sources, single electron transistors, quantum cellular automata, quantum bits, etc.