We report on transmission electron microscopy and optical characterization of GaN dots embedded in an AlN matrix. GaN dots were grown by molecular beam epitaxy on top of an AlN layer deposited on (0001) sapphire. We found that the GaN dots (average diameter of 16 nm, average height of 4 nm) are coherently grown on AlN, but nucleate next to threading edge dislocations that were propagating in AlN. The presence of these adjacent dislocations does not inhibit the optical emission of the GaN dots: contrary to the photoluminescence of thick GaN layers or GaN/AlGaN quantum wells, the photoluminescence of these GaN dots is found to be temperature independent.

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