  Titanium nitride is an attractive material for a range of superconducting
quantum-circuit applications owing to its low microwave losses, high surface
inductance, and chemical stability. The physical properties and device
performance, nevertheless, depend strongly on the quality of the materials.
Here we focus on the highly crystalline and epitaxial titanium nitride thin
films deposited on sapphire substrates using magnetron sputtering at an
intermediate temperature (300$^{\circ}$C). We perform a set of systematic and
comprehensive material characterization to thoroughly understand the
structural, chemical, and transport properties. Microwave losses at low
temperatures are studied using patterned microwave resonators, where the best
internal quality factor in the single-photon regime is measured to be
$3.3\times 10^6$, and $> 1.0\times 10^7$ in the high-power regime. Adjusted
with the material filling factor of the resonators, the microwave loss-tangent
here compares well with the previously reported best values for superconducting
resonators. This work lays the foundation of using epitaxial titanium nitride
for low-loss superconducting quantum circuits.
