Robin Nicholas

Abstract:

The external quantum efficiency of p-GaSb/n-GaAs heterojunction TPV cells has been enhanced by using a thin n-GaSb buffer layer between the GaAs and the p-GaSb to offset the p-n junction from the heterojunction. Cells with the p-n junction displaced from the heterojunction showed a significant improvement in photoresponse and open circuit voltage (V-OC). Short circuit current (I-SC) was also increased by addition of a thin n-GaSb layer (similar to100nm) but little or no improvement resulted if the n-GaSb layer thickness was further increased. However, short circuit current was significantly improved by growth on 3degrees off-axis substrates. Samples using both these techniques achieved efficiencies of around 70% that of a GaSb homojunction. All samples were grown by MOVPE at temperatures of 590degreesC.

Abstract:

We review recent results from magneto-transport studies of an InAs/GaSb based electron-hole system. In high magnetic field, the system shows two types of overall behaviour depending on whether the effective occupancy nu(c) - nu(h) = nu(eff) is zero or finite. When nu(e) - nu(h) is finite the resistivity has a metallic temperature dependence and the Hall resistance is quantized at a finite value. When nu(e) - nu(h) = 0 however, the resistivity shows an insulating temperature dependence. Since the condition nu(e) - nu(h) = nu(eff) for a given value of nu(eff) can be achieved with different values of the individual occupancies, the system can oscillate between insulating and metallic behaviour with magnetic field.In the insulating states where nu(e) - nu(h) - 0, the system exhibits novel behaviour, where the Hall resistance becomes symmetric under field reversal, and fluctuates reproducibly with magnetic field. Geometry dependence measurements show that in this state the sample interior becomes extremely insulating, and conduction is dominated by the mesa edge. The transport is one dimensional, and the chirality usually associated with the Hall effect is lost.Current driven breakdown is studied for quantum Hall states where nu(e) - nu(h) is finite. There is a strong dependence of the breakdown behaviour on channel width and the relative electron and hole densities. This suggests a strong involvement of the sample interior in stark contrast to the insulating states. (C) 2003 Published by Elsevier B.V.

Abstract:

We report a study of novel structures in which an InSb quantum dot (QD) layer in GaSb is coupled to a nearby narrow InAs quantum well (QW). Magnetic fields have been used to probe the photoluminescence (PL) of the dots and the wells separately and in the coupled structures. We have found that the introduction of the dot-well coupling leads to a dramatic suppression of the diamagnetic shift of the exciton when the magnetic field is oriented parallel to the well plane. The observed shift is 1-2 meV with an applied field of 17 T. In contrast, when the field is perpendicular to the layer plane, there is little difference in the response of the coupled structures compared to that of the quantum wells on their own, suggesting that the behaviour is dominated by the motion of the electrons in the wells.

Abstract:

We perform local anodic oxidation (LAO) on III-V semiconductor surfaces with the help of an atomic force microscope (AFM). Regular arrays of oxide dots are created by applying a voltage in the range 10-20 V to a conducting tip. We succeed in drawing dots with diameters of 30nm or less and heights of 4-6nm, allowing for lattice periods as small as 75nm. With an anodisation time of 100-250ms per dot, the creation of large patterns is possible provided the drift of the instrument is corrected for.