Reflectionless tunneling in planar Nb/GaAs hybrid junctions

Reflectionless tunneling was observed in Nb/GaAs superconductor/semiconductor junctions fabricated by a two-step procedure. First, periodic ?-doped layers were grown by molecular-beam epitaxy near the GaAs surface, followed by an As cap layer to protect the surface during ex situ transfer. Second, Nb was deposited by dc-magnetron sputtering onto a GaAs(001) 2×4 surface in situ after thermal desorption of the cap layer. The magnetotransport behavior of the resulting hybrid junctions was successfully analyzed within the random matrix theory of phase-coherent Andreev transport. The impact of junction morphology on reflectionless tunneling and the applicability of the fabrication technique to the realization of complex superconductor/semiconductor mesoscopic systems are discussed.