Influence of the thermal annealing on the optical (PL and PLE) and XRR properties of GaN QD/AlN SL structures. (a) Temperature-dependent PL spectra of the excitonic recombination for a 10-period GaN QD/AlN SL structure before and after thermal annealing at 1200°C (full lines). Normalised RT PLE spectra monitored at the PL band maximum for the as-grown (line + closed symbols) and annealed (line + open symbols) samples. 14 K PL and PLE spectra of an AlN layer (dashed lines). (b) Specular X-ray reflection for the as-grown and annealed GaN QD/AlN SL structures. The XRR determined period thickness is shown in the graph for both samples.