The intrasubband transition rates of bound carriers interacting with piezoelectric and polar optical phonons in a bulk GaN lattice are presented, considering the anisotropy of the uniaxial wurtzite structure. The directional dependence of the excitonic Bohr radius has a measurable effect for both types of piezoelectric and polar optical phonon scattering. In the latter case, the anisotropy of the uniaxial crystal induces exciton scattering involving both strongly mixed transverse-like and longitudinal-like polar optical phonons. By taking into account the angular frequency dispersion of the zone centre polar optical phonons, we can relate the anisotropy of the excitonic scattering rate to a particular orientation of the transverse-like phonon wave vector with respect to the c-axis. The same effect is not as pronounced in the case of the longitudinal-like polar optical phonon emission due to the much lower anisotropy of the longitudinal-like phonon frequency branch.

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