Coupling of single quantum dots to two-dimensional systems

Lead Research Organisation: Lancaster University
Department Name: Physics


The interaction between nano-objects of different dimensionality, e.g. electrostatic Coulomb-coupling of a zero-dimensional quantum dot (QD) to a two-dimensional (2D) system is of fundamental interest and of great relevance for charge-based memories. This interaction between a single QD and a 2D system shall be studied here. Innovative use of the complementary expertise of the partners will combine, for the first time, Sb-based QDs with a split-gate structure, which will allow the precise control of the charge-state of a single QD. Sb-based QDs have strong hole confinement yielding a potential retention time of many years at room temperature, enabling the analysis of the influence of charged QDs on a 2D system up to 300 K. In the mid-long term perspective, the results could be important for future generations of memories: knowledge of the interaction of a 2D system with a single QD might allow us to reach the ultimate limits of charged-based memories (e.g. Flash).


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Description Self-assembled quantum dots (QDs) are spontaneously formed when a few atomic monolayers of a semiconductor are deposited on a substrate that is crystal-lattice mismatched by more than about 5%. The most common example, InAs QDs in GaAs, is a type-I system, meaning that it confines negatively-charged electrons and their positively-charged counterparts, called holes. Potentially much more interesting are type-II QDs which confine only electrons or only holes. GaSb/GaAs QDs are the prime example of such a system: holes are very strongly confined, making them attractive as a novel material system for a new generation of memories based on III-V compound semiconductors.

This EPSRC project was conducted as part of a European NanoSci ERA+ project called QD2D, in cooperation with U. Duisberg-Essen, TU Berlin and TU Eindhoven, with further contributions from U. Cadiz and U. Warwick. Our understanding of GaSb/GaAs QDs has been enormously advanced by the project. Access to state-of-the-art cross-sectional scanning tunnelling microscopy (Eindhoven) and transmission electron microscopy (Warwick, Cadiz) has allowed us to study the morphology of the GaSb/GaAs QDs, and we now understand what growth conditions are needed to form quantum dots and quantum rings (QRs). We have shown that QRs are generally 'better' than QDs as ring formation relaxes the strain, allowing ten-fold stacked layers to be grown. We have also shown that the formation of GaSb rings allows the unconfined electron to sit in the GaAs at the centre of the ring, such that the electron-hole wave-function overlap is maximised and giving good optical properties. The proximity of the electron to the hole can even be tuned by changing the strain (via small changes in growth conditions), which directly affects the strength of the electron-hole interaction. We have experimentally and theoretically (Berlin) determined the ground-state localisation energy of holes in GaSb/GaAs QDs to be ~600 meV. This deep confining potential has a strong tendency to trap acceptors from unintentional background carbon doping in the sample, such that even in the dark, GaSb/GaAs QD/QRs are occupied by a few holes (typically 1 to 5). The large capacitive charging energy (~24 meV per hole) has allowed us to directly observe dot/ring occupancy in a simple photoluminescence experiment on very large ensembles, and we have a good understanding of how dot/ring occupancy varies with temperature (2 to 400 K) and incident laser excitation power.
Exploitation Route In terms of applications the focus of QD2D was charge-based memories. The results of the project have shown that the localisation energy of holes is insufficient for a non-volatile memory, even if the dots/rings were embedded in AlAs. However, there is still potential for GaSb/AlGaAs to be used in a semi-volatile RAM memory with fast access times, long refresh and non-destructive read (which we call SV-RAM). We have also shown that due to their good optical properties GaSb/GaAs QRs have strong potential for applications in photonics, e.g. in lasers, quantum information processing and solar cells. As a direct result of this project Lancaster became a World-leading centre for the growth and optical characterisation of self-assembled GaSb/GaAs quantum dots and quantum rings. This has stimulated research around the world, as evidenced by the workshop held at the end of the project which inlcuded speakers from mainland Europe, USA and Asia. The results of the project will be used at Lancaster and elsewhere to further investigate the unusual properties of these structures, with potential applications in a range of devices.
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Energy

Description Industrial co-funding for Doctoral Prize
Amount £6,500 (GBP)
Organisation IQE Europe Limited 
Sector Private
Country United Kingdom
Start 12/2014 
End 11/2015