Ohmic, superconducting, shallow AuGe/Nb contacts to GaAs

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Abstract
An emerging new class of superconductor-semiconductor devices requires Ohmic superconducting contacts which do not diffuse deeply into a semiconductor. We developed a contact to GaAs based on ∼150 Å of AuGe covered with ∼2000 Å of Nb annealed in reducing atmosphere at 390–420 °C for 1–5 s. The resulting contact has linear I-V characteristics at all temperatures down to 4.2 K, resistivity of ∼2×10−6 Ω cm2 (∼0.1 Ω mm), and superconducting transition temperature Tc ≳8 K. Secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM) studies revealed very shallow penetration of the active dopant Ge into GaAs; the upper limit for the thickness of the doped layer with Ge concentration over 1017 cm−3 is estimated as 200–300 Å. Gold, a nonactive dopant (deep level) drops to below 1018 cm−3 within the same distance, with the possible tail extending further. Morphology and uniformity of a contact, as revealed in TEM and optical microscopy, is good, owing to the Nb overlayer which prevents AuGe from ‘‘balling up’’ upon melting. These contacts can be used in low-temperature devices and conventional devices based on GaAs.