Correlational Study of Structure and Superconducting Resonator Loss in Epitaxial Al on Si

One limiting mechanism of the internal quality factor of the superconducting resonators used in circuit quantum electrodynamics is the resonant interaction of two-level system (TLS) defects. The superconductor/substrate interface experiences the highest electric field energy in a coplanar waveguide and has been shown to be the most critical boundary for TLS loss.
In this study, the performance of the resonators fabricated from epitaxial aluminum (Al) on three different silicon (111) surface reconstructions (1×1, 7×7, and √3×√3) is investigated. The different surface reconstructions are known to modify the structural properties of overgrown aluminum showing polycrystalline, twinned single crystal, and pure single crystal Al films. Significant differences in the measured single photon quality factor are observed with mean quality factors ranging from 400k to over 800k. Connections between material structure and quality factor are investigated in detail.