Q -disease in epitaxial thin film niobium resonators from HF acid exposure

It is well known that the superconducting properties, such as Tc and rf loss, of high-purity bulk niobium can be degraded from unwanted hydrogen doping and the precipitation of niobium hydrides. Here, we present the impact of hydrogen fluoride (HF) based acid cleans on a single crystal niobium thin film, which was grown using molecular beam epitaxy on a c-plane sapphire wafer. After growth and processing, individual chips were subjected to different acid cleaning treatments using piranha, a mixture of sulfuric acid and hydrogen peroxide, and different exposures to HF. For exposure to piranha only, the Nb film’s dc residual resistivity ratio (RRR) was measured to be 41, the superconducting Tc = 9.2 K, and the internal quality factor Qi to be as large as 7.9 × 106 for ∼5 GHz quarter-wave coplanar waveguide resonators. After the Nb was exposed to buffered HF, an expansion of the Nb lattice was observed, a decrease of the dc RRR, and an increase in the microwave loss such that Qi approached 4 × 104 for a maximum 90-min exposure. The formation of 50 nm tall surface crystallites was also observed on the Nb after exposure to HF. These observations are consistent with doping of H into the Nb material, resulting in Q-disease, as previously observed in high-purity Nb material.