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

High-quality thin Nb and NbN films (60-100 Å) are grown on (100) n⁺-InAs (n= 10¹⁹ 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 × 10¹⁹ - 2 × 10¹⁹ 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.