We have measured the differential resistance of lateral Nb contacts to a two-dimensional electron gas (2DEG) in an InP/${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructure. The I-V curves show strong deviations from the frequently used model developed by Blonder, Tinkham, and Klapwijk. In all samples the maximum of conductance at about eV=${\mathrm{\Delta }}_0$ is damped and shifted to lower voltages. Depending on the surface cleaning procedure two different regimes are observed. We will present two models that allow one to interpret the conductance mechanisms. The parameters used in the models are within a realistic range given by characteristic material values. In the case of wet chemically cleaned samples the 2DEG is assumed to be in the clean limit. To describe the measurement results of these samples we assume a proximity effect in a Nb oxide layer (N) located between the Nb (S) and the 2DEG causing the shift of the conductance maximum. Pair-breaking processes in this SN electrode are responsible for the damping of this maximum. Additionally we include the proximity effect between the electrode and the 2DEG in our model. When the semiconductor surface is cleaned by Ar ions, the 2DEG is damaged at the surface. For this case we have shown that an additional voltage drop occurs in this disturbed part of the 2DEG and that the inelastic scattering in the SN electrode is stronger than in the case of the wet chemically cleaned samples. © 1996 The American Physical Society.

• Received 11 June 1996

DOI:https://doi.org/10.1103/PhysRevB.54.17018