Optical bandgap and stress variations induced by the formation of latent tracks in GaN under swift heavy ion irradiation

In this work, GaN epilayers, grown on (0 0 0 1) sapphire substrate, were studied under swift heavy ion irradiation with a broad variety of projectiles at different energies. Several characterization techniques including transmission electron microscopy, optical absorption spectroscopy and Raman scattering spectroscopy, employed for different irradiation conditions, allowed the identification and the attribution of the origin of the observed modifications. It has been established that for projectiles presenting an electronic stopping power higher than a threshold of 17 keV.nm−1, there was the formation of disordered latent tracks on the ion paths. We have shown that these tracks become more continuous and are visible until a higher depth with the increase of the projectile electronic stopping power. We have also highlighted that the latent tracks induce the appearance of a biaxial stress of the order of some GPa and strongly modify the optical bandgap values. Contrary to this effect of electronic energy loss, the atomic displacements generated by nuclear energy loss process did not imply significant biaxial stress and optical bandgap closing.