Not yet reviewed by Pith; the record is open.

Our study shows that deposited Ge and Si dielectric thin-films can exhibit low microwave losses at near single-photon powers and sub-Kelvin temperatures ($\approx$40 mK). This low loss enables their use in a wide range of devices, including low-loss coplanar, microstrip, and stripline resonators, as well as layers for device isolation, inter-wiring dielectrics, and passivation in microwave and Josephson junction circuit fabrication. We use coplanar microwave resonator structures with narrow trace widths of 2-16 $\mu \textrm{m}$ 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 $\approx 1 \mu \textrm{m}$ thick amorphous germanium (a-Ge) films deposited on Si (100) have a single photon loss tangent of $1-2\times10^{-6}$ and, $9 \mu \textrm{m}$-thick chemical vapor deposited (CVD) homoepitaxial Si has a single photon loss tangent of $0.6-2\times 10^{-5}$. Interface contamination limits the loss in these devices.