  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.
