Investigation of AuNiGe-based superconducting ohmic contacts and hydrodynamic transport effects in GaAs/AlGaAs-based two-dimensional electron gases

This thesis describes an investigation of AuNiGe-based ohmic contacts as well as a series of differential resistance and magnetic field measurements performed on GaAs/AlGaAs-based two-dimensional electron gases (2DEGs) alongside all the required background information.
The aim of the AuNiGe ohmic contact study was to discover the cause of the superconductivity occurring below 1 K. The result is a list of the most probable superconducting compounds, likely AuAl or AuGe-based but there could be a combination of several compounds. The main compound identified, Au₇Ga₂, is not a superconductor and no other compound was found in sufficient amounts to cause bulk superconductivity. As a result, it is possible that the observed superconductivity could be the result of a percolating network existing throughout the ohmic contact. If true, then a much more granular TEM study is required. The interface between the ohmic contact and the semiconductor was also found to be very inhomogeneous and the existence of the 2DEG below it is questionable.

The differential resistance and magnetic field measurements performed on 2DEG Hall bars of various widths showed that applying a magnetic field to a narrow Hall bar recovers the Bloch-Grüneisen transition observed in wider Hall bars. Without a magnetic field the behaviour is hidden by hydrodynamic effects which only occur in narrow Hall bars due to the interplay between the width of the Hall bar and the electron-electron scattering length. Primary thermometry based on cross-correlated Johnson noise was also performed to establish that the transition was the Bloch Grüneisen transition; with all wide Hall bars measured having a transition temperature in line with the Bloch Grüneisen temperature.