ELSEVIER Physica B 203 (1994) 298-306 Quasiparticle transport and induced superconductivity in InAs-A1Sb quantum wells with Nb electrodes Herbert Kroemer a,c'd'*, Chanh Nguyen a'c' 1 Evelyn L. Hu a'~, Esther L. Yuh b'c, Mason Thomas a'c Ki C. Wong b'c aDepartment of Electrical and ComputerEngineering, bDepartment of Physics, and CQUEST, Center for Quantized Electronic Strucures, University of California, Santa Barbara, CA 93106, USA °Max Planck Institutfur Festki~rperforschung, Stuttgart, Germany Abstract Current transport through InAs A1Sb quantum wells contacted with superconducting Nb electrodes shows strong evidence for the presence of multiple Andreev reflections (AR's). The efficiency of the multiple AR process is greatly enhanced by the specular normal reflection of electrons at the backplane of the quantum well, thereby permitting multiple AR attempts. Superconductivity observed for sufficiently narrow inter-electrode gaps is interpreted as the result of phase-coherent multiple AR's. Series-connected multi-junction InAs-Nb arrays have been constructed by contacting the InAs-A1Sb quantum well with a periodic grating of superconducting Nb electrodes with sub-micrometer spacings. They showed superconductivity at sufficiently low temperatures, in one case as high as 4.2 K. Above the transition temperatures, strong precursors of the superconductivity were observed, in the form of dramatically enhanced zero-bias conductances, decreasing with increasing temperature, but larger by about a factor on the order 104 than the fluctuation-induced precursors of thin BCS films. Weak magnetic fields restored non-zero resistance values; the increase in resistance with increasing magnetic field contained a component periodic in the magnetic field, with a period corresponding to a flux per grating cell of only a fraction ( ~ ~½) of a conventional flux quantum. The observations are interpreted in terms of the formation of a flux cell superlattice. 1. Introduction and background It was pointed out already in 1980 by Clark et al. [1] that InAs is a particularly suitable semiconductor for transport through superconductor-semiconducto~ superconductor (SpSmSp) structures. The principal rea- * Corresponding author. 1Present address: Hughes Research Laboratories, Malibu, CA 90265, USA. son is that the Fermi level at metal-to-InAs interfaces is pinned inside the InAs conduction band, leading to con- tacts without significant Schottky barriers impeding the free coupling of electrons between the two materials. A second desirable property of InAs is that pure InAs exhibits higher electron mobilities than any other III-V compound except InSb. However, in most of the InAs- based SpSmSp work done so far, this mobility advantage could be utilized only to a limited extent, because the high electron concentration levels required for such work required high doping levels of the InAs, which drastically 0921-4526/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0921-4526(94)00345-9