Submicron Ni/Au/Ge contacts to an AlGaAs/GaAs two-dimensional electron gas with resistance less than 400 Ω

We report on the fabrication and performance of Ni/Au/Ge ohmic contacts to an AlGaAs/GaAs two-dimensional gas (2DEG) with area ≤1μm² yielding contact resistance below 400 Ω. Galvanic connection to a two-dimensional electron gas (2DEG) in a AlGaAs/GaAs heterostructure is often achieved by annealing a metal stack of Ni/Au/Ge. Our study focuses on understanding the conditions necessary to produce very small contacts (area ≤1μm²) with low resistance. We utilize low temperature electrical measurements supplemented by transmission electron microscopy to gain insight into the structure-properties relationship. We report on the role factors including crystallographic orientation, contact geometry, surface preparation, and spatial distribution of metallic elements after rapid thermal annealing. Our data suggests that the spatial distribution and chemical concentration of germanium plays a critical role in contact formation. We also report significant crystallographic anisotropy in the quality of ohmic contacts in our heterostructures. We report on the ultra-low temperature (T=10mK) and high magnetic field (B=8 Tesla) behavior of our submicron contacts.