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The strain state of the [formula] GaN quantum dots and quantum wires has been studied by a combination of multiwavelength anomalous diffraction and diffraction anomalous fine structure under grazing incidence. The three components of the anisotropic deformation have been independently determined. On one hand, a weak residual strain state along the c-axis, similar for quantum dots and quantum wires, is found. On the other hand, along the [1120] direction, quantum wires appear to be more relaxed than quantum dots. Based on high resolution transmission electron microscopy, this is assigned to elastic strain relaxation of GaN governed by either misfit dislocation formation or three-dimensional islanding, depending on the anisotropic strain state of the [1100] AlN buffer layer. [ABSTRACT FROM AUTHOR]

Reflection high-energy electron diffraction (RHEED) was used to monitor the strain of GaN/AlN quantum dots (QDs) grown by molecular beam epitaxy. Quantitative and absolute values of the in- and out-of-plane strains of the QDs were determined and compared to reference values, obtained by x-ray diffraction measurements. A very good agreement was found between RHEED and x-ray measurements. The growth and thermal ripening of the dots were analyzed. A progressive strain relaxation was observed during the ripening stage, suggesting a morphology evolution of the dots. [ABSTRACT FROM AUTHOR]

Presents diffraction anomalous fine-structure spectroscopy at beamline at the European Synchrotron Radiation Facility. Use of resonant elastic X-rays scattering as an atomic, shell and site-selective probe; Electronic structure; Long-range-ordered crystallographic structure; Complex atomic scattering factor.

Abstract: We report on the study of strain, size and composition of small-size encapsulated semiconductor nanostructures. We show that the partial structure factor of As atoms in InAs stick-like nanostructures (quantum sticks), embedded in InP, can be directly extracted from grazing incidence anomalous X-ray diffraction maps at the As K-edge. We have recovered the average height and strain of the islands and determined their composition. The average height of the quantum sticks (QSs), as deduced from the width of the structure factor profile is 2.54nm. The InAs out of plane deformation, relative to InP, is equal to 6.1%. Fixed-Q anomalous diffraction spectra, measured at the As K-edge, in grazing incidence provide clear evidence of pure InAs QSs. This is confirmed by the analysis of the diffraction anomalous fine structure (DAFS) that also gives a direct way to recover the strain accomodation inside the quantum sticks. Finite difference method calculations reproduce well the diffraction data. Chemical mixing at interfaces is at most 1ML. This paper shows that ultimate application of anomalous diffraction and DAFS together with reciprocal space maps is a powerful method to sudy the structural properties of nanostructures. [Copyright &y& Elsevier]

Abstract: 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 (30−30) Bragg reflection of AlN at several energies above and below the Ga K-edge, allow to extract the structure factor F A 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. [Copyright &y& Elsevier]