Evolution with temperature of the strain state of GaN thin layers grown ondifferent substrates

The intrinsic optical properties of GaN thin layers grown on two different substrates - sapphire and ELOG (Mg-enhanced lateral overgrowth) layer - are compared by means of temperature dependent photoluminescence excitation sp ectroscopy. From the comparison between the experimental and calculated exc itonic energies, the strain state of each layer can be evaluated. At low te mperature (T = 20 K), the in-plane biaxial strain is larger in the overgrow n GaN layer than in the GaN layer grown on the sapphire substrate. We find that, contrary to the sapphire substrate: the strain state remains constant in the overgrown GaN layer when temperature varies. This is an indication that the ELOG layers can be used as substrates of good quality, allowing to obtain reproducible optical properties when temperature varies.