Superconductivity in AuNiGe Ohmic contacts to a GaAs-based high mobility two-dimensional electron gas

To cool a high mobility two-dimensional electron gas (2DEG) at a GaAs-AlGaAs heterojunction to milliKelvin temperatures, we have fabricated low resistance Ohmic contacts based on alloys of Au, Ni, and Ge. The Ohmic contacts have a typical contact resistance of R C ≈ 0.8 ω at 4.2 K, which drops to 0.2 ω below 0.9 K. Scanning electron microscope images establish that the contacts have the same inhomogeneous microstructure that has been observed in previous studies. Measurements of the contact resistance R C, the four-terminal resistance along the top of a single contact, and the vertical resistance RV all show that there is a superconductor in the Ohmic contact, which can be turned completely normal with a magnetic field of 0.15 T. We briefly discuss how this superconductivity may be affecting the electrical transport measurements of 2DEGs, especially how it may hinder the cooling of electrons in a 2DEG below 0.1 K.