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Semiconductor-superconductor hybrid systems have outstanding potential foremerging high-performance nanoelectronics and quantum devices. However,critical to their successful application is the fabrication of high-quality andreproducible semiconductor-superconductor interfaces. Here, we realize andmeasure axial Al-Ge-Al nanowire heterostructures with atomically preciseinterfaces, enwrapped by an ultrathin epitaxial Si layer further denoted asAl-Ge/Si-Al nanowire heterostructures. The heterostructures were synthesized bya thermally induced exchange reaction of single-crystalline Ge/Si core/shellnanowires and lithographically defined Al contact pads. Applying thisheterostructure formation scheme enables self-aligned quasi one-dimensionalcrystalline Al leads contacting ultrascaled Ge/Si segments with contacttransparencies greater than 96%. Integration into back-gated field-effectdevices and continuous scaling beyond lithographic limitations allows us toexploit the full potential of the highly transparent contacts to the 1D holegas at the Ge-Si interface. This leads to the observation of ballistictransport as well as quantum confinement effects up to temperatures of 150 K.Low-temperature measurements reveal proximity-induced superconductivity in theGe/Si core/shell nanowires. The realization of a Josephson field-effecttransistor allows us to study the subgap structure caused by multiple Andreevreflections. Most importantly, the absence of a quantum dot regime indicates ahard superconducting gap originating from the highly transparent contacts tothe 1D hole gas, which is potentially interesting for the study of Majoranazero modes. Moreover, underlining the importance of the proposed thermallyinduced Al-Ge/Si-Al heterostructure formation technique, our system couldcontribute to the development of key components of quantum computing such asgatemon or transmon qubits

Highly transparent contacts to the 1D hole gas in ultra-scaled Ge/Si core/shell nanowires