Ohmic, superconducting, shallow AuGe/Nb contacts to gaas

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 T_c ≳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.