Progress in the emergent field of topological superconductivity relies on
synthesis of new material combinations, combining superconductivity, low
density, and spin-orbit coupling (SOC). For example, theory [1-4] indicates
that the interface between a one-dimensional (1D) semiconductor (Sm) with
strong SOC and a superconductor (S) hosts Majorana modes with nontrivial
topological properties [5-8]. Recently, epitaxial growth of Al on InAs
nanowires was shown to yield a high quality S-Sm system with uniformly
transparent interfaces [9] and a hard induced gap, indicted by strongly
suppressed sub gap tunneling conductance [10]. Here we report the realization
of a two-dimensional (2D) InAs/InGaAs heterostructure with epitaxial Al,
yielding a planar S-Sm system with structural and transport characteristics as
good as the epitaxial wires. The realization of 2D epitaxial S-Sm systems
represent a significant advance over wires, allowing extended networks via
top-down processing. Among numerous potential applications, this new material
system can serve as a platform for complex networks of topological
superconductors with gate-controlled Majorana zero modes [1-4]. We demonstrate
gateable Josephson junctions and a highly transparent 2D S-Sm interface based
on the product of excess current and normal state resistance
