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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.