Loss Mechanisms of Epitaxial NbN Coplanar Waveguide Resonators on an MgO Substrate

We investigated the loss mechanisms of superconducting circuits with epitaxially grown NbN(100) on MgO(100) substrates by measuring quarter-wave coplanar waveguide resonators with various strip and slot widths. The power dependence of the internal quality factor and the temperature dependence of the resonance frequency shift were evaluated in a dilution refrigerator. The measurement and analysis of the loss in these resonators indicate that the loss was dominated by a two-level system (TLS) loss and the TLS host materials were primarily distributed at the surface of the substrate.