PROPERTIES OF LATERAL NB CONTACTS TO A 2-DIMENSIONAL ELECTRON-GAS IN AN IN0.77GA0.23AS INP HETEROSTRUCTURE/

We have measured the differential resistance of lateral Nb contacts to a two-dimensional electron gas (2DEG) in an InP/InxGa1-xAs heterostru cture. 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=Delta(0) is damped and shifted to l ower voltages. Depending on the surface cleaning procedure two differe nt regimes are observed. We will present two models that allow one to interpret the conductance mechanisms. The parameters used in the model s 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 s amples we assume a proximity effect in a Nb oxide layer (N) located be tween the Nb (S) and the 2DEG causing the shift of the conductance max imum. Pair-breaking processes in the SN electrode are responsible for the damping of this maximum. Additionally we include the proximity eff ect between the electrode and the 2DEG in our model. When the semicond uctor surface is cleaned by Ar ions, the 2DEG is damaged at the surfac e. For this case we have shown that an additional voltage drop occurs in this disturbed part of the 2DEG and that the inelastic scattering i n the SN electrode is stronger than in the case of the wet chemically cleaned samples.