Robin Nicholas

Abstract:

We report studies of sequential resonant tunneling in GaAs/AlGaAs multi-quantum well p-i-n structures in the presence of parallel magnetic fields up to 15 T. New features are observed which are attributed to inter-Landau level resonances involving a change of Landau index of +1 and +2. In one of the samples the tunnelling resonances saturate and shift with the magnetic field. This result may be related to the similar effects observed in coupled two-dimensional electron gases at high magnetic fields.

Abstract:

We report measurements of magneto-reflectivity in ZnTe/Zn1-xMnxTe superlattices in magnetic fields up to 45 Tesla. From an analysis of the Zeeman splitting, we investigate the change of band alignment with field and the band offset ratio. A crossing of the 1 s exciton transitions from the ZnTe buffer layer and the 1 s heavy hole sigma(+) exciton of the superlattice is observed, providing unambiguous evidence of a band alignment change from type I to type II. The excitonic energy levels for both type I and type II band structure are calculated using a variational method. This model fits the experimental data very well at high field for both sigma(+) and sigma(-) transitions. A conduction band offset ratio of Delta E(c)/Delta E(g) = 0.72 +/- 0.04 is deduced.

Abstract:

A series of compressively strained G(0.44)In(0.56)P/(Al0.7Ga0.3)(0.52)In-0.48 P multiple quantum well structures have been studied in magnetic fields up to 45 T. Laudau levels from several excitomic transitions have been observed in reflectivity spectra, allowing an accurate determination of the carrier masses and excitonic binding energies. It is shown that while the observed masses are lower than those seen in unstained GaInP/AlGaInP for wide wells, as the width decreases the mass in the strained samples increases sharply.

Abstract:

The quantum Hall effect has been measured in an intrinsic semimetallic superlattice with almost equal electron and hole densities. Giant oscillations are seen in the Hall resistivity, and at high fields and pressures a zero-resistance state is observed where the Hall resistivity goes to zero due to compensation of the Hall fields of the two carrier types. This behaviour is interpreted in terms of the limiting properties of rho(xx) and rho(xy)