Grazing incidence diffraction anomalous fine structure study of GaN/AlN quantum dots

We report on a general method that takes advantage of the full capability of anomalous diffraction and can be applied to the very interesting and challenging case of small size embedded nanostructures. We study GaN quantum dots (QDs) grown by MBE in the Modified-Stransky-Krastanow regime, and encapsulated by an AlN epilayer. We investigate the QDs strain and composition that are related to size, morphology, and cap layer thickness by means of anomalous diffraction in grazing incidence. The X-ray energy is tuned across the absorption Ga K-edge where the Ga atoms scattering power is strongly modified and diffraction becomes chemically selective, giving direct information on composition. hkl-scans close to the (3 0 −3 0) Bragg reflection of AlN at several energies above and below the Ga K-edge, allow to extract the structure factor FA of the Ga atoms which gives the in-plane lattice parameter, a, of GaN. Quantitative analysis of the anomalous edge lineshape and of the oscillatory extended region above the edge give information on the composition and the out-of-plane strain of the dots. A two-step covering mechanism of QDs By AlN is suggested.