  We reported here the results of the calculations of wavevector dispersion of
oscillations frequencies, $\omega'(k)$, and damping $\omega''(k)$ of the
surface plasmon phonon polaritons (\mbox{SPPhP}) for the heavy-doped GaN
sample. We showed that $\omega'(k)$- dependence consists of the three branches
with the specific anticrossing behavior due to the interaction of surface
plasmon polariton (SPP) with surface phonon polariton(SPhP). The strong
renormalization of the damping $\omega''(k)$ in the vicinity of the
anticrossing region was found. The obtained dispersions of the $\omega'(k)$ and
$\omega''(k)$ were applied for the analytical analysis of exact electrodynamic
simulation of the resonant behavior of the reflectivity spectrum of the n-GaN
grating.
