  Controlling the properties of semiconductor/metal interfaces is a powerful
method for designing functionality and improving the performance of electrical
devices. Recently semiconductor/superconductor hybrids have appeared as an
important example where the atomic scale uniformity of the interface plays a
key role for the quality of the induced superconducting gap. Here we present
epitaxial growth of semiconductor-metal core-shell nanowires by molecular beam
epitaxy, a method that provides a conceptually new route to controlled
electrical contacting of nanostructures and for designing devices for
specialized applications such as topological and gate-controlled
superconducting electronics. Our materials of choice, InAs/Al, are grown with
epitaxially matched single plane interfaces, and alternative
semiconductor/metal combinations allowing epitaxial interface matching in
nanowires are discussed. We formulate the grain growth kinetics of the metal
phase in general terms of continuum parameters and bicrystal symmetries. The
method realizes the ultimate limit of uniform interfaces and appears to solve
the soft-gap problem in superconducting hybrid structures.
