Molecular beam epitaxy of high mobility near surface Ge quantum wells for quantum computing

High mobility strained Ge quantum wells (QWs) have been of interest for superconducting-semiconducting Josephson junctions (JJs) with applications in voltage-tunable superconducting qubits. These JJs require the mean free path of the QW to exceed 100 nm, and have been fabricated with ex-situ superconducting contacts. Epitaxial superconducting contacts and an extremely shallow QW are desireable for superconductor/semiconductor JJs because of their high interface transparency and consistency.

 

              In this work, we investigate the growth of Ge QWs with in situ deposited aluminum contacts on float zone silicon substrates with MBE. Ge QWs with the top spacer thickness varying from 7-nm to 22-nm are studied. Low temperature mobilities exceeding 120,000 cm²/Vs are obtained in Ge QWs with a 22-nm top spacer, representing the highest mobility for MBE-grown Ge QWs. Modeling the density dependent mobility indicates that surface scattering is the limiting scattering mechanism in shallow QWs, while interface roughness scattering limits the performance of deeper quantum wells. The tradeoffs between mobility, mean free path, and interface transparency as the depth of the Ge QW from the surface decreases will be discussed. These findings demonstrate MBE-grown Ge QWs as an emerging platform for quantum computing.