Robin Nicholas

Abstract:

We investigate de and pulsed magneto-transport in InAs/GaSb superlattices, at low temperatures. High electric and magnetic fields up to 14.5T are applied parallel to the superlattice growth direction. In the low electric and magnetic field regimes eve observe a temperature dependent threshold to conduction. At higher biases the superlattice exhibits a large magneto-resistance and in the quantum limit we observe conduction peaks, which satisfy the Stark-cyclotron-resonance condition. We have performed eight band ii p calculations, which show that the miniband width in this system is strongly k dependent. (C) 2001 Elsevier Science B.V. All rights reserved.

Abstract:

Self-assembled InSb quantum dots (QDs) were grown by metal-organic vapour phase epitaxy (MOVPE) in a GaSb matrix. Atomic force microscopy (AFM), conventional diffraction contrast transmission electron microscopy (CTEM), high resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) were used for the assessment of the QDs. Reductions in the (III)/v, ratios and growth rates resulted in a change of the morphology of the InSb islands from hillocks without facets, and a low level of order to dumbbell shaped islands with distinct facets and a higher level of order. (C) 2001 Published by Elsevier Science B.V.

Abstract:

A reordered valence band in GaN/AlxGa1-xN quantum wells with respect to GaN epilayers has been found as a result of the observation of an enhanced g factor (g* ∼ 3) in magnetoluminescence spectra in fields up to 55 T. This has been caused by a reversal of the states in the strained AlxGa1-xN barriers thus giving different barrier heights for the different quantum well hole states. From k-p calculations in the quasicubic approximation, a change in the valence band ordering will account for the observed values for the g factors. We have also observed the well-width dependence of the in-plane extent of the excitonic wave function from which we infer an increase in the exciton binding energy with the reduction of the well width in general agreement with theoretical calculations of Bigenwald et al. [Phys. Status Solidi B 216, 371 (1999)] that use a variational approach in the envelope function formalism that includes the effect of the electric field in the wells.

Abstract:

InAs/GaSb heterojunctions form bipolar 2-D layers due to the overlapping conduction and valence bands. Such systems have generated considerable interest recently due to the possibilities of gap formation by both excitonic and single particle interactions. The quantum Hall effect in this system shows a digital sequence oscillating from 0-1-0 conductance quanta and the diagonal resistivity, p. shows re-entrant insulating behaviour due to the formation of a total gap in the energy spectrum. The origin of the insulating behaviour is thought to be due to the formation of unusual counter propagating edge states where the electron and hole system interact to form closed loops which generate localised states. Cyclotron resonance on this system is found to show an increased effective mass when the electron and hole states approach each other physically and at high magnetic fields the resonances show large splittings which can be strongly temperature dependent.

Abstract:

We report a strong enhancement and transformation of the Fermi edge singularity at high magnetic fields in a series of doped GaAs-Al33%Ga67%As quantum wells with width varying from 68 Angstrom to 150 Angstrom with carrier densities similar to4 (.) 10(11)cm(-2). We explain this behavior in terms of a resonance process due to collective excitations of the 2D system created by interband absorption. Such a collective excitation or "Mahan" trion occurs whenever a pair of electrons with a total momentum equal to zero becomes bound by the Coulomb attraction to a valence band hole.