  In most single-photon defect emitters, such as those in SiC and diamond,
interaction with low-energy acoustic phonons determines the temperature
dependence of the decoherence rate and the resulting broadening of the ZPL with
the temperature obeys a power law. GaN hosts bright and stable single-photon
emitters in the 600 nm to 700 nm wavelength range with strong ZPLs even at room
temperature. In this work, we study the temperature dependence of the ZPL
spectra of GaN SPEs integrated with solid immersion lenses with the goal of
understanding the relevant decoherence mechanisms. At temperatures below ~50 K,
the ZPL lineshape is found to be Gaussian and the ZPL linewidth is temperature
independent and dominated by spectral diffusion. Above ~50 K, the linewidth
increases monotonically with the temperature and the lineshape evolves into a
Lorentzian. Quite remarkably, the temperature dependence of the linewidth does
not follow a power law. We propose a model in which decoherence caused by
absorption/emission of optical phonons in an elastic Raman process determines
the temperature dependence of the lineshape and the linewidth. Our model
explains the temperature dependence of the ZPL linewidth and lineshape in the
entire 10 K to 270 K temperature range explored in this work. The ~19 meV
optical phonon energy extracted by fitting the model to the data matches
remarkably well the ~18 meV zone center energy of the lowest optical phonon
band (E2(low)) in GaN. Our work sheds light on the mechanisms responsible for
linewidth broadening in GaN SPEs. Since a low energy optical phonon band
(E2(low)) is a feature of most group III-V nitrides with a wurtzite crystal
structure, including hBN and AlN, we expect our proposed mechanism to play an
important role in defect emitters in these materials as well.
