  The ultra-wide bandgap of diamond distinguishes it from other semiconductors,
in that all known defects have deep energy levels that are inactive at room
temperature. Here, we present the effect of deep defects on the mechanical
energy dissipation of single-crystal diamond experimentally and theoretically
up to 973 K. Energy dissipation is found to increase with temperature and
exhibits local maxima due to the interaction between phonons and deep defects
activated at specific temperatures. A two-level model with deep energies is
proposed to well explain the energy dissipation at elevated temperatures. It is
evident that the removal of boron impurities can substantially increase the
quality factor of room-temperature diamond mechanical resonators. The
deep-energy nature of nitrogen bestows single-crystal diamond with outstanding
low-intrinsic energy dissipation in mechanical resonators at room temperature
or above.
