  Following significant progress in the visualization and characterization of
Majorana end modes in hybrid systems of semiconducting nanowires and
superconducting islands, much attention is devoted to the investigation of the
electronic structure at the buried interface between the semiconductor and the
superconductor. The properties of that interface and the structure of the
electronic wavefunctions that occupy it determine the functionality and the
topological nature of the superconducting state induced therein. Here we study
this buried interface by performing spectroscopic mappings of superconducting
aluminum islands epitaxially grown in-situ on indium arsenide nanowires. We
find unexpected robustness of the hybrid system as the direct contact with the
aluminum islands does not lead to any change in the chemical potential of the
nanowires, nor does it induce a significant band bending in their vicinity. We
attribute this to the presence of surface states bound to the facets of the
nanowire. Such surface states, that are present also in bare nanowires prior to
aluminum deposition, pin the Fermi-level thus rendering the nanowires resilient
to surface perturbations. The aluminum islands further display Coulomb blockade
gaps and peaks that signify the formation of a resistive tunneling barrier at
the InAs-Al interface. At low energies we identify a potential energy barrier
that further suppresses the transmittance through the interface. A
corresponding barrier exists in bare semiconductors between surface states and
the accumulation layer, induced to maintain charge neutrality. Our observations
elucidate the delicate interplay between the resistive nature of the InAs-Al
interface and the ability to proximitize superconductivity and tune the
chemical potential in semiconductor-superconductor hybrid nanowires.
