  In this article, we estimate hydrostatic stress developed in gallium ion
implanted gallium nitride epitaxial layers using Raman measurements. We have
calculated deformation potential constants for $E_2$(high) mode in these
epi-layers. The presence of a polar phonon-plasmon coupling in these systems
has also been demonstrated. In as-implanted samples, with an increase in
implantation fluence, we have observed disorder-activated Raman scattering.
