Epitaxial Josephson junctions for superconducting qubits made by wafer-bonding*

State-of-the-art transmons most often utilize Al/AlO x /Al Josephson junctions (JJs) even though the
amorphous AlO x is known to be defective and lossy. A scalable process to realize epitaxial JJs with low
loss barrier dielectrics could improve qubit performance beyond state-of-the-art; however, challenges in
materials integration have not yet been overcome. In this work, a wafer-bonding technique is used to
realize epitaxial JJ structures utilizing conventional semiconductors for the JJ dielectric. Semiconductors
and epitaxial superconductors are grown by molecular beam epitaxy on III-V semiconductor substrates
and bonded to low-loss substrates. Selective etching is used to remove the III-V substrate followed by
surface cleaning and superconductor regrowth, resulting in epitaxial Al/Semiconductor/Al tri-layers on
low loss substrates.