Low microwave loss in deposited Si and Ge thin-film dielectrics at single-photon power and low temperatures

In this study, we show that deposited Ge and Si dielectric thin-films can exhibit low microwave losses at single-photon powers and sub-Kelvin temperatures (≈40 mK). This low loss enables their use in a wide range of devices, including coplanar, microstrip, and stripline resonators, as well as layers for device isolation, interwiring dielectrics, and passivation in microwave and Josephson junction circuit fabrication. We use coplanar microwave resonator structures with narrow trace widths and minimal over-etch to maximize the sensitivity of loss tangent measurements to the interface and properties of the deposited dielectrics, rather than to optimize the quality factor. In this configuration, thermally evaporated ≈1 µm thick amorphous germanium (a-Ge) films deposited on Si (100) have effective single-photon loss tangents of 4–5 × 10−6 and 9 μm-thick chemical vapor deposited homoepitaxial single-crystal Si has effective single-photon loss tangents of 4–14 × 10−6. Material characterization suggests that interface contamination could be the limiting factor for the loss.