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][2][3][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][6][7][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 subgap 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][2][3][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.The recent focus on topological states in solid state systems has revealed new directions in condensed matter physics with potential applications in topological quantum information [11,12]. In an exciting development, it was realized one could readily engineering an effective one-dimensional (1D) spinless superconductor using the proximity effect from conventional superconductors (Al, Nb) in nanowires with strong SOC (InAs, InSb), and that Majorana zero modes would naturally emerge at the ends of the wire [1][2][3][4]. First experiments on nanowires grown by chemical vapor deposition (CVD) revealed striking evidence of Majorana zero modes states [13][14][15][16][17][18]. In order to eventually move beyond demonstrations of braiding [19][20][21], to larger-scale Majorana networks [22] it is likely that a top-down patterning approach will be needed. Molecular beam epitaxy (MBE) growth of large-area 2D SSm systems can form the basis for such an approach, but to date have not been available.Narrow bandgap semiconductors such as InAs and InSb are natural choices for the Sm component due to large g factors and strong SOC, which are important for the stability of an emergent topological phase in S-Sm heterostructures, with the topological gap proportional to the SOC strength [23]. There are, however, significant challenges in growing high quality quantum wells in these systems. The lack of insulating latticematched substrates and difficulty in device fabrication, compared to well-developed GaAs material system, has restricted their use in mesoscopic devices. Nevertheless, it has long been known [24] that surface level pinning in InAs could allow for fabrication of transparent contact to superconduct...

We investigate zero-bias conductance peaks that arise from coalescing subgap Andreev states, consistent with emerging Majorana zero modes, in hybrid semiconductor-superconductor wires defined in a two-dimensional InAs/Al heterostructure using top-down lithography and gating. The measurements indicate a hard superconducting gap, ballistic tunneling contact, and in-plane critical fields up to 3 T. Top-down lithography allows complex geometries, branched structures, and straightforward scaling to multicomponent devices compared to structures made from assembled nanowires.