Capacitance and Tunnelling Studies on GaAs/AlGaAs based Electron-Hole Bilayers

Abstract

This thesis studies two dimensional electron-hole bilayers in close spatial proximity. The charge layers are produced in an MBE grown p-i-n heterostructure and are separated by an insulating barrier. The width of the barrier determines the interaction strength between the two 2D layers. At high interaction strength (i.e. small barrier width), the formation of excitons is expected. In addition, theory predicts the Bose-Einstein condensation of the excitons at sufficiently low temperatures. In order to study the effect of the barrier width on the device behaviour, capacitance and conductance measurements were performed. Here, an applied DC bias pushes the charges from the doping layers towards the barrier and a rise in capacitance can be observed as soon as the two charge systems form on each side of the barrier. Experiments on weakly interacting charge systems (i.e. a barrier width of 20 nm) showed no tunnelling or leakage even at high bias voltages. Further measurements with a magnetic field showed the formation of Landau levels. This suggest that the charge layers have a 2D character. An increase in the interaction strength (i.e. reducing the barrier width to 10 nm) did not change the observed behaviour. However, the reduced barrier width, combined with the introduction of a quantum well (QW) adjacent to the barrier on the p-doped side, led to a drastic change in device response. In this case, oscillations in capacitance and differential conductance as a function of bias voltage were observed. These oscillations are surprisingly regular, and their amplitude can exceed the geometric capacitance. It is shown that this behaviour is consistent with resonant tunnelling (RT) of electrons being the origin of the oscillations. Further experiments have also shown that the oscillation period scales with the inverse of the QW width and that a sufficiently large parallel B-field suppresses the oscillations completely. This is in agreement with the resonant tunnelling picture. Interestingly, adding a quantum well on the n-side of the barrier did not change the character of the oscillations. Therefore, it can be assumed that the phenomenon is caused by electron tunnelling rather than the tunnelling of holes. An intermediate interaction strength (i.e. barrier width of 15 nm) did not show presence the of the RT-oscillations, but other interesting phenomena, e.g. hysteretic bias response or atypical features in a Landau fan. This does not align with the resonant tunnelling picture and needs to be investigated in future studies. This work is hoped to be a precursor for further research on electron-hole bilayer systems in close spatial proximity.