GaN quantum dots (QDs) in AIN matrix were grown by molecular beam epitaxy on silicon (111) substrates using NH3 as nitrogen precursor. QDs were formed under growth interruption by two-dimensional-three-dimensional strain induced transition. Their size was controlled by the GaN nominal thickness deposited onto relaxed AlN. QD shapes and densities were determined by transmission electron microscopy and atomic force microscopy experiments. We observe intense room temperature visible photoluminescence (PL) varying from orange to violet depending on the QD size. This is the consequence of the strong built-in electric field present in wurtzite nitride heterostructures. For stacked QD layers,the PL intensity increases with the number of QD planes allowing us to determine the QD absorption coefficient.