Optically induced strain relaxation in anisotropically strained M-plane GaN films

We study the anisotropic in-plane strain in M-plane GaN films by photoreflectance, photoluminescence, Raman, and time-resolved pump-and-probe spectroscopy. We find that a highly strained film partially relaxes, if it is pumped by an intense optical pulse. The strain relaxation can be observed by a shift of the E 2 -Raman line to lower energies and by a shift of the fundamental interband transition energies in the photoreflectance spectra. The photoluminescence intensity of the exposed areas is significantly reduced as compared to the one for areas, which have not been exposed to the intense optical pulse. This suggests that the strain relaxation is connected to the introduction of defects, which can act as non-radiative recombination centers.