Observation of the superconducting proximity effect in Nb/InAs and NbNx/InAs by Raman scattering

High-quality thin Nb and NbN films (60-100 A) are grown on (100) n + -InAs (n = 10 1 9 cm - 3 ) substrates by dc-magnetron sputter deposition. Studies of the electronic properties of interfaces between the superconductor and the semiconductor are done by Raman scattering measurements. The superconducting proximity effect at superconductor-semiconductor interfaces is observed through its impact on inelastic light scattering intensities originating from the near-interface region of InAs. The InAs longitudinal optical phonon LO mode (237 cm - 1 ) and the plasmon-phonon coupled modes L_ (221 cm - 1 ) and L + (1100 to 1350 cm - 1 ), for n + =1×10 1 9 - 2×10 1 9 cm - 3 are measured. The intensity ratio of the LO mode (associated with the near-surface charge accumulation region, in InAs) to that of the L_ mode (associated with bulk InAs), is observed to increase by up to 40% below the superconducting transition temperature. This temperature-dependent change in light scattering properties is only observed with high quality superconducting films and when the superconductor and the semiconductor are in good electrical contact. A few possible mechanisms of the observed effect are proposed.