We present morphological, structural and optical properties of superlattices of MBE- grown self assembled GaN QDs in AlN. At low temperature the recombination of the smallest as-grown dots is dominated by a broad ultraviolet PL band with maximum at around 3.8 eV. When increasing the dot size, the main PL emission is shifted to lower energies with its maximum ranging between 2.9 eV and 2.7 eV in the studied samples. In both cases the QD recombination is detected up to RT. The grown samples were further submitted to thermal annealing treatments in nitrogen atmosphere at 1100^oC. After annealing the emission of the GaN QDs embedded in the AlN spacer layers was found to be substantially modified: for the samples with smaller and medium sized GaN QDs a high energy shift of the PL peak position was detected. For the sample with larger dots an opposite trend was observed. In this case a low energy shift of the main emission was measured after thermal annealing. The structural properties of the multilayers were analysed using X-ray diffraction (XRD), X-ray reflection (XRR) and Rutherford backscattering spectrometry (RBS) showing excellent crystalline quality and interface properties. XRR indicates that after annealing intermixing effects are enhanced, which was confirmed by scanning transmission electron microscopy measurements for one of the samples with small dots. The influence of the intermixing and possible strain relaxation effects on the PL of differently sized GaN QDs will be discussed.

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