Self-assembled GaSb/GaAs quantum dots and quantum rings
Atomic resolution images of self-assembled GaSb/GaAs nanostructures
grown in Lancaster. The upper image (a) is a quantum dot, whilst the
lower image (b) is a quantum ring.
(Images supplied by TU Eindhoven.)
The overwhelming majority of research on self-assembled quantum dots (QDs) is on type-I QDs which confine both electrons and holes (positively-charged carriers generated in semiconductors by the absence of electrons). The physics of such QDs is principally determined only by the confinement. Much more interesting, perhaps, are type-II QDs which confine either electrons or holes, but not both. In these nanostructures the physics is very strongly dependent on the Coulomb interaction between the confined and free carrier. Most of our work is on GaSb/GaAs type-II quantum dots, which are much more like 'artificial atoms' than typical QDs, consisting of a positively-charged 'nucleus' (the dot) with a cloud of electrons bound to it.
On the fundamental physics side, we are interested in studying how the optical properties of the dot vary with the number of (photoexcited) electrons and holes. It turns out that sometimes increasing the carrier density increases the attractive interaction between electrons and holes, and sometimes it decreases it (screening), depending on, for example, subtle changes in dot morphology. This interplay between electron-hole and electron-electron interaction means that there is tremendous scope for fine-tuning the physics of these systems. Intriguingly, it also turns out that when GaSb/GaAs are grown by molecular beam epitaxy (as ours are), it is very easy to generate high quality quantum rings, which has interesting quantum-mechanical implications related to the topology.
On the applications side the primary motivation is the exploitation of the very deep hole confining potential of GaSb/GaAs QDs (several hundred meV) as the basis for novel charge-based memories that will be 10x faster than DRAM while consuming 10x less power. This work is undertaken in cooperation with University of Duisburg-Essen, Technical University of Berlin and Technical University of Eindhoven in the framework of the European project QD2D.
Acknowledgements: This work is supported by the Engineering and Physical Sciences Research Council [grant number EP/H006419] in the framework of the QD2D project, the Royal Society - Brian Mercer Feasibility Award and QinetiQ.