Strain distribution in nitride quantum dot multilayers

Nitride quantum dots (QD’s) grown in the wurtzite phase present a strong vertical ordering along the (0001) direction when they are stacked in multilayers. This alignment results from a minimum of the elastic energy density at the surface of the AlN capping layer induced by the buried GaN dot underneath. The aim of this work is to investigate the strain distribution in a quantum dot multilayer using high-resolution transmission electron microscopy and anomalous grazing incidence x-ray diffraction. This x-ray method is based on the strong sensitivity of the elastic scattering cross section to the Ga compounds for energies in the vicinity of the Ga absorption edge. It is observed that uncapped GaN dots are almost completely relaxed, while embedded quantum dots are compressively strained. In addition, a modulation of the in-plane lattice parameter in the AlN spacer layer is clearly identified, induced by the QD’s on the surrounding matrix which causes the vertical alignment.