in situ Growth of and Proximity-induced Superconductivity in Al-Ge Quantum Well Heterostructures

<title>Abstract</title> <p> Planar Ge/SiGe heterostructures have emerged as a compelling platform for engineering and exploring topological superconductivity. However, an <italic>in situ</italic> epitaxial superconductor–semiconductor architecture made from this material system has yet to be established. Here, we demonstrate <italic>in situ</italic> molecular beam epitaxy (MBE) growth of Al-Ge quantum well (QW) heterostructures and show the proximity-induced superconductivity in the Ge QWs. The heterostructures are achieved via low-temperature Al deposition onto a 1-nm-Si-capped Ge QW, and the structural and elemental mapping analyses show that the as-grown Al film has an atomically sharp interface to the Si-capped Ge QW and a flat top surface. The heterostructures are employed to fabricate Josephson junction and superconducting quantum interference devices, and low-temperature transport measurements reveal excellent gate-tunable proximity-induced superconductivity in the Ge QWs. We anticipate that the Al-Ge QW heterostructures grown <italic>in situ</italic> via MBE would have great potential for the study of emerging superconducting phenomena, such as novel superconducting pairing, and topological quantum states, such as Majorana zero modes, in hole-based semiconductor-superconductor hybrid devices. </p>