Dielectric Loss of Boron-Based Dielectrics on Niobium Resonators

Advanced solid-state quantum bits (qubits) are likely to require a variety of dielectrics for wiring crossovers, substrates, and Josephson junctions. Microwave superconducting resonators are an excellent tool for measuring the internal dielectric loss of materials. We report the dielectric loss of boron-based dielectric films using a microwave coplanar waveguide (CPW) resonator with heterostructure geometry. Power-dependent internal quality factors of magnetron-sputtered boron carbide (\(\hbox {B}_{{4}}\hbox {C}\)) and boron nitride (BN) were measured and are compared to silicon oxide (\(\hbox {SiO}_{{2}}\)), a common material used in wiring crossovers. The internal dielectric loss due to two-level systems for \(\hbox {B}_{{4}}\hbox {C}\), and BN is less than silicon dioxide (\(\hbox {SiO}_{{2}}\)), which demonstrates the existence of low-loss sputtered materials. We also found that niobium (Nb) CPW resonators suffer a decrease in internal quality factor after deposition of \(\hbox {B}_{{4}}\hbox {C}\) at temperatures above 150 \(^{\circ }\hbox {C}\). This result is consistent with the idea that the oxidation of the surface of the superconducting metal can contribute to loss in a device.