Robin Nicholas

Abstract:

Thermodynamic arguments are presented for the formation of atomic order in heteroepitaxially grown semiconductor quantum dots. From thermodynamics several significant properties of these systems can be derived, such as an enhanced critical temperature of the disorder-order transition, the possible co-existence of differently ordered domains of varying size and orientation, the possible existence of structures that have not been observed before in semiconductors, the occurrence of atomic order over time, and the occurrence of short range order when the growth proceeds at low temperatures. Transmission electron microscopy results support these predictions. Finally, we speculate on the cause for the observed increase in life time of (In, Ga)As/GaS quantum dot lasers [H-Y. Liu et al., Appl. Phys. Lett. 79,2868 (2001)].

Abstract:

We present a magneto-optical study of biexcitons confined within ZnS quantum wells at fields Lip to 57 T. We have measured the luminescence energy splitting of these excitonic systems and find that the results contrast with the conventional picture of biexcitons in a magnetic field in which the luminescence splitting is entirely determined by the splitting (g-factor) of the exciton. Our results suggest that biexcitons are much more complex than previously thought and that the additional confinement and Coulomb interaction within the quantum wells must be taken into account. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

The excitonic properties of cubic ZnS quantum wells in ZnMgS are studied by reflectivity and magneto-optics. A remarkable improvement in the quality of the samples grown by MBE on Gal? substrates has allowed the observation of heavy and light hole exciton transitions with values for the FWHM as narrow as 5 meV The 2s state of the heavy hole exciton is identified and exciton binding energies as high as 55 meV are deduced, indicating that for quantum wells narrower than 3.5 nm the exciton-LO phonon scattering can be suppressed. Zeeman splittings; of the order of 10 meV for both the light and heavy hole exciton transitions appear in magneto-reflectivity spectra in magnetic fields up to 54 T. Large light hole exciton g-values of the order of four for all quantum wells are obtained due to the light hole being the uppermost valence band in these tensile strained quantum wells. A strong reduction in the diamagnetic shifts for narrow wells is observed due to increasing quantum confinement.

Abstract:

Magnetotransport measurements have been performed in strongly coupled two-dimensional electron-hole gases in an InAs Gash heterostructure in the presence of antidot potentials of different strengths. We have observed commensurability features in the magnetoresistance that we attribute to high mobility electrons. It is proposed that they can be observed due to magnetic breakdown, despite the fact that the electron-hole mixing is found to play an important role. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

We report a study of cyclotron resonance (CR) in a bilayer-bipolar InAs/GaSb structure and the influence of the spatial separation between the electrons and holes. At low magnetic fields we find that the cyclotron mass of the electrons can double when the electrons and holes have a very small separation. At higher cyclotron energies we find that the CR lines are split by a series of resonances which are due to interband coupling, and which also increase rapidly in magnitude when the electron-hole separation is decreased. At very high fields, in the quantum limit a new series of couplings is observed, with the resonances being split by energies in the range of 3-12 meV. (C) 2002 Elsevier Science B.V. All rights reserved.