Strain state in GaN epilayers from optical experiments

The intrinsic optical properties of GaN thin epilayers grown on a sapphire substrate and by Mg-enhanced lateral overgrowth (ELO) are studied as a function of temperature (from 20 up to 250 K) by means of photoluminescence excitation spectroscopy. The comparison between the experimental and calculated results allows us to determine the strain state of the GaN layers. At low temperature $(T=20 K)$ the strain is stronger in the ELO epilayers than in the GaN layers grown on sapphire, but, contrary to the GaN layers grown on the sapphire substrate, the strain state remains constant in the overgrown GaN layers when temperature varies.