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 RC0.8Ω at 4.2 K, which drops to 0.2Ω 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 RC, 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.

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