  The loss of amorphous hydrogenated silicon nitride (a-SiN$_{x}$:H) is
measured at 30 mK and 5 GHz using a superconducting LC resonator down to
energies where a single-photon is stored, and analyzed with an independent
two-level system (TLS) defect model. Each a-SiN$_{x}$:H film was deposited with
different concentrations of hydrogen impurities. We find that quantum-regime
dielectric loss tangent $\tan\delta_{0}$ in a-SiN$_{x}$:H is strongly
correlated with N-H impurities, including NH$_{2}$. By slightly reducing $x$ we
are able to reduce $\tan\delta_0$ by approximately a factor of 50, where the
best films show $\tan\delta_0$ $\simeq$ 3 $\times$ 10$^{-5}$.
