Fabrication of tall-mesa Ge/SiGe, voltage tunable Josephson junctions

Planar Josephson junctions compatible with high-Q quantum circuits are fabricated with strained Ge quantum wells grown by Molecular Beam Epitaxy and fabricated into a 3-μm tall mesa structure. Junctions with lengths of 50-200 nm are formed by selective removal the Al layer using a two-step etch comprised of a Cl-based dry etch and a wet etch that is selective to SiGe. By optimizing the etching parameters, a unform junction etch with less than 10-nm of broadening is achieved. For integration into highly coherent superconducting circuits, it is necessary to isolate the lossy epitaxial layers from the passive quantum circuit components. Towards this end, a mesa is formed by etching through the full 3-μm SiGe growth, down to the low-loss Float Zone (FZ) Si substrate. Chlorine and Fluorine based etches are investigated and optimized for etch sidewall uniformity throughout the different compositions of the growth structure to ensure electrode continuity up the mesa sidewall.