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The development of Ge-rich GeSbTe (GGST) alloys significantly enhanced the high-temperature stability required for Phase-Change Memory technology. Previous studies on Ge enrichment in GeSbTe (GST) materials with Sb-over-Te ratio lower than one (Sb / Te < 1) highlighted the segregation into cubic Ge and cubic GST phases. Such a segregated cubic GST phase is metastable and presents a polycrystalline structure with disordered grain boundaries that could lead to structural relaxation and then to drift phenomena. In this work, using resistivity measurements, Raman spectroscopy, and in situ x-ray diffraction analyses, we demonstrate for the first time to our knowledge that GGST with Sb/Te higher than one (Sb / Te > --> 1) upon annealing leads to the direct formation of a GST hexagonal phase featuring a high growth speed, bypassing the cubic metastable phase. Combined with Ge enrichment, the increased value of the activation energy of the nucleation of Sb / Te > --> 1 GGST alloys ensures a high stability of the amorphous phase. Finally, nitrogen introduction further stabilizes the system against the crystallization, without compromising the high crystalline growth speed and the formation of the stable GST hexagonal phase in alloys with Sb / Te > --> 1. These results demonstrate the possibility to tune the crystalline structure of the segregated phases in Ge-rich GeSbTe alloys, combining the stability at high temperature of the amorphous phase with the high crystallization speed and uniformity (with larger grains) of a targeted GST phase. [ABSTRACT FROM AUTHOR]

Use of an anomalous x-ray diffuse scattering method to analyze the lattice distortions and short-range order (SRO) in an electron-type superconductor yields a set of fitted SRO parameters that indicate the development of Ce-Ce atom pairs along the (001) direction. The Ce free domains display an average size of approximately 20 angstroms that is greater than the coherence length of the superconductors and hence function as good superconductors. Increase in doping beyond a specific level causes the displacive and chemical defects in the superconductors to decrease the superconductivity.

In this work, we investigate the local atomic structure of defect-free homogeneous and self-organized core-shell structure nanowires by means of X-ray Absorption Fine Structure (XAFS) Spectroscopy at the In LIII and K edges and Multiwavelength Anomalous Diffraction. The results are interpreted by comparison of the experimental data with X-ray absorption calculations carried out with ab initio structural models. Extended-XAFS data analysis at In K-edge shows an anisotropic In distribution in the second nearest neighbors pointing out to a deviation from randomness in In distribution for the core-shell sample. [ABSTRACT FROM AUTHOR]

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]

Medium energy ion scattering (MEIS) has been used to measure at the monolayer scale the strain profile of self-organized GaN quantum dots grown on (11–20) or a-plane AlN by molecular-beam epitaxy. By confronting the MEIS results with a structural analysis carried out by atomic force microscopy, it is established that the strain profile is anisotropic, i.e., fully elastic along [1–100] and a combination of plastic and elastic along [0001]. High resolution transmission electron microscopy measurements reveal the presence of misfit dislocations with 1/2 [0001] Burgers vector, consistent with MEIS data. [ABSTRACT FROM AUTHOR]

The strain state of the [1100] GaN quantum dots and quantum wires is studied by a combination of multiwavelength anomalous diffraction and diffraction anomalous fine structure under grazing incidence. Based on high resolution transmission electron microscopy, the three components of the anisotropic deformation 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.

The reflection high-energy electron diffraction (RHEED) is used to monitor the strain of GaN/AlN quantum dots (QDs) grown by molecular beam epitaxy. The dot ripening under vacuum has induced a progressive in and out-of-plane strain relaxation that is attributed to the size increase of the dots.