Robin Nicholas

Abstract:

CdSe quantum dots (QDs) in a ZnSe matrix, quasi-2D CdSe platelets in a (Zn,Mn)Se matrix, and InSb QDs in InAs and GaSb matrices are characterized by transmission electron microscopy (TEM) in the scanning electron-probe and the parallel-illumination modes. A phenomenological scheme of self-ordering of QDs from the literature is used as a tool for the classification of laterally self-ordered arrangements of these QDs and their predecessor islands. Modified growth conditions led in all studied cases to self-ordered QD and island arrangements of higher levels. CdSe and InSb rich agglomerates of varying sizes (approximately 5 nm to a few 100 nm) with different types of internal compositional modulation have been formed by self-ordering on an atomic scale, potentially forming a new type of QDs and suggesting that the self-ordering process may be of a dissipative nature.

Abstract:

The magneto-luminescence of GaN/AlGaN quantum wells in fields up to 52 T shows a field dependence that is strongly dependent on the well width. Strong redshifts are seen for the narrowest wells that are attributed to a Zeeman splitting. This is unexpected, since in bulk GaN epilayers the electron and hole y-factors of the lowest valence band cancel each other almost exactly. Therefore, we attribute this splitting to a reordering of the valence band due to the different band offsets caused by the strain and the aluminium component in the AlGaN barriers. The field dependence also gives information on the size of the exciton that has been converted into values for the exciton binding energy, and these values agree reasonably well with a theory that includes the presence of the electric field. (C) 2001 Elsevier Science B.V. All rights reserved.

Abstract:

Nano-agglomerates of In(Sb,As) in InAs, (In,Ga)Sb in GaSb, and (Cd,Zn,Mn)Se in (Zn,Mn)Se are classified by transmission electron microscopy. In scanning transmission electron microscopy, atomic resolution Z-contrast images reveal different modes of internal compositional modulation on the atomic length scale, resulting for all three material systems in nano-agglomerates of an appropriate size that may constitute a new type of quantum dot. For other nano-agglomerates of In(Sb,As) in InAs and (In,Ga)Sb in GaSb, we observed a second type of nanoscale ordering that results in nano-agglomerates with an internal compositional modulation on a length scale of a few nm. Both types of compositional modulation are discussed as having arisen from a rather long-term structural response to a combination of internal and external strains. © 2007 American Institute of Physics.

Abstract:

We examine the edge states picture as applied to the electron-hole system, and show that compensated quantum Hall states are fundamentally different to quantum Hall states of single carrier type systems. Measurements of their current driven breakdown are described, and we show that these states have very small breakdown currents. (C) 2001 Elsevier Science B.V. All rights reserved.

Abstract:

We find that an InAs/GaSb based electron-hole system exhibits insulating behaviour when the numbers of occupied electron and hole Landau levels are equal. In this insulating state, the Hall resistance becomes symmetric under field reversal, and both the Hall and longitudinal resistances display reproducible fluctuations. We propose a simple model based on edge states to account for these properties, and show that a comparison to the conductivity measured from a Corbino disc is consistent with the model. (C) 2001 Elsevier Science B.V. All rights reserved.