Transport properties of a quantum dot modulation-doped field-effect transistor

• Main Author: Beattie, N. S.
• Published: University of Cambridge 2005
• Subjects: 537.622
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596501

This dissertation reports on the properties of a two-dimensional electron system in the presence of charge which is localised in the close vicinity. This is realised in a GaAs/Al_0.33Ga_0.67As semiconductor heterostructure containing a layer of InAs self-assembled quantum dots. A very important and unique feature of this material system is that the population of excess electrons stored in the quantum dots can be reduced via recombination with photo-excited holes. It may also be restored via the application of an appropriate front gate voltage. The presence of an AlGaAs barrier layer ensures that a population of electrons in the quantum dots is persistent. The results of experiments which are performed over a wide range of carrier concentrations in the 2DEG are presented, yielding insight into the transport mechanisms in both the diffusive and strongly localised regimes. In the diffusive transport regime, a treatment of the quantum dots as remote Coulomb scattering centres with a variable charge occupancy, allows a measurement of the contribution to the scattering exclusively from the dots themselves. When the quantum dots are large, the treatment of the dots as scattering centres breaks down and strong localisation is observed. In this insulating regime a transition from variable range hopping to weak localisation can be observed as a consequence of removing electrons from the quantum dots. Samples which are fabricated from this system and are of a length scale which is comparable to the average hopping length, are sensitive to single-photons. By removing an electron from an individual quantum dot, an incident photon modulates the transmission through a tunnelling barrier which is created by the repulsive potential from this dot. This results in a detectable conductance change. The statistics of such events reveal the transport mechanisms in a mescoscopic sample over three orders of magnitude in conductance as the localisation potential is tuned site-by-site, during a single experiment.