  Grazing incidence anomalous x-ray scattering was used to monitor in situ the
molecular beam epitaxy growth of GaN/AlN quantum dots (QDs). The strain state
was studied by means of grazing incidence Multi-wavelength Anomalous Di raction
(MAD) in both the QDs and the AlN during the progressive coverage of QDs by AlN
monolayers. Vertical correlation in the position of the GaN QDs was also
studied by both grazing incidence MAD and anomalous Grazing Incidence Small
Angle Scattering (GISAXS) as a function of the number of GaN planes and of the
AlN spacer thickness. In a regime where the GaN QDs and the AlN capping are
mutually strain influenced, a vertical correlation in the position of QDs is
found with as a side-e ect an average increase in the QDs width.
