  Phonons confined in mechanical resonators can be coupled to a variety of
quantum systems and are expected to be applied to hybrid quantum systems.
Diamond surface acoustic wave (SAW) devices are capable of high efficiency in
phonon interaction with color centers in diamond. The temperature dependence of
the quality factor is crucial for inferring the governing mechanism of coupling
efficiency between phonons and color centers in diamond. In this paper, we
report on the temperature dependence of the quality factor of an AlN/diamond
SAW device from room temperature to 5 K. The temperature dependence of the
quality factor and resonant frequency suggests that the mechanism of SAW
dissipation in the AlN/diamond SAW resonator at 5 GHz is the phonon-phonon
scattering in the Akheiser region, and that further cooling can be expected to
improve the quality factor. This result provides a crucial guideline for the
future design of AlN/diamond SAW devices.
