Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy

We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency of the QD three-dimensional confinement in circumventing the potential fluctuations related to structural defects or alloy inhomogeneity. PL excitation measurements demonstrate efficient carrier transfer from the wetting layer to the QDs in the GaN/AlN system, even for low QD densities (~1010 cm-3). In the case of InGaN/GaN QDs, transport losses in the GaN barriers cannot be discarded, but an upper limit to these losses of 15% is deduced from PL measurements as a function of the excitation wavelength

Nanostructure and strain properties of core-shell GaAs/AlGaAs nanowires

GaAs/AlGaAs core–shell nanowires (NWs) were grown on Si(111) by Ga-assisted molecular beam epitaxy via the vapor–liquid–solid mechanism. High-resolution and scanning transmission electron microscopy observations showed that NWs were predominantly zinc-blende single crystals of hexagonal shape, grown along the [111] direction. GaAs core NWs emerged from the Si surface and subsequently, the NW growth front advanced by a continuous sequence of (111) rotational twins, while the AlGaAs shell lattice was perfectly aligned with the core lattice. Occasionally, single or multiple stacking faults induced wurtzite structure NW pockets. The AlGaAs shell occupied at least half of the NW's projected diameter, while the average Al content of the shell, estimated by energy dispersive x-ray analysis, was x = 0.35. Furthermore, molecular dynamics simulations of hexagonal cross-section NW slices, under a new parametrization of the Tersoff interatomic potential for AlAs, showed increased atom relaxation at the hexagon vertices of the shell. This, in conjunction with the compressively strained Al0.35Ga0.65As shell close to the GaAs core, can trigger a kinetic surface mechanism that could drive Al adatoms to accumulate at the relaxed sites of the shell, namely along the diagonals of the shell's hexagon. Moreover, the absence of long-range stresses in the GaAs/Al0.35Ga0.65As core–shell system may account for a highly stable heterostructure. The latter was consolidated by temperature-dependent photoluminescence spectroscopy

Tem study of twin boundary dislocations in cadmium

SCOPUS: ar.jinfo:eu-repo/semantics/publishe

INTERFACE JUNCTIONS AND SIMMETRY

Des jonctions triples de haute symétrie sont examinées par MET dans le silicium. Les propriétés de symétrie des grains et de leurs interfaces sont étudiées sur base du modèle de CSL. Des jonctions de flexions suivant ˂110˃ de type Σ(3,3,9), Σ(3,9,27a), et Σ(3,27a,81d) sont examinées ; elles contiennent toutes une Σ=3 avec interface {111}. La relation des interfaces avec les CSL et la symétrie des jonctions triples est analysée en details.High symmetry triple junctions in polysilicon observed by TEM are presented. The symmetry properties of their grains and interfaces are studied with the use of the CSL model. The examples concern triple junctions of ˂110˃ tilt boundaries with the CSL configurations of Σ(3,3,9), Σ(3,9,27a) and Σ(3,27a,81d). In all the cases the Σ=3 twin with interface the {111} plane is at least one of the components. The connection of the interfaces with the CSLs and the symmetry of the triple junction is analytically discussed

A DISLOCATION MECHANISM FOR THE GROWTH OF TWINS IN DEFORMED HCP METALS

Des macles mécaniques ont été examinées par MET dans les métaux hexagonaux comme Zn, Cd, Zr, Ti. En particulier, les joints contenant des dislocations intrinsèques ont été examinés. Les caractéristiques de ces dislocations ont été étudiées à partir de leur contraste et par diffraction électronique. Il est déduit que dans les cas du Zn et du Cd, les macles mécaniques croissent par le mouvement de dislocations dont les vecteurs de Burgers sont déduits des caractéristiques des macles mécaniques. Dans le Zr et le Ti des familles de dislocations intrinsèques parallèles à la direction de coincidence sont observées.Deformation twins in hexagonal metals are examined by TEM. The materials studied are Zn, Cd, Zr, Ti. The observations are focussed on the twin boundaries that contain intrinsic dislocation structures. The characteristics of these dislocations are analysed using contrast and electron diffraction techniques. It is deduced that in Zn and Cd deformation twinning proceeds by a dislocation mechanism in which the glide of the dislocation as well as its Burgers vector are connected with the crystallographic elements of the twinning mechanism. In Zr and Ti parallel arrays of intrinsic dislocations, with lines along coincident direction in both grains, are also observed

Dislocation core investigation by geometric phase analysis and the dislocation density tensor

The dislocation density tensor can be employed to \u27localize\u27 the dislocation core. We present a method for obtaining experimentally the dislocation core location and size from high resolution transmission electron microscopy observations using geometrical phase analysis. Experimental results are then compared with the results of atomistic simulations, and we also evaluate the applicability of continuum elasticity and its modifications for describing these results. The analysis is performed for partial dislocations in wurtzite GaN. Higher-order gradient elasticity theory is found to be in good agreement with the experimental observations. © 2008 IOP Publishing Ltd

Probing the structural role of Cr in stabilized tannery wastes with X ray absorption fine structure spectroscopy

The effective stabilization of tannery sludge wastes is explored using X Ray Absorption Fine Structure XAFS spectroscopies. Solidification of the Cr rich waste was realized via vitrification of the incinerated sludge with silica and flux agents. It is demonstrated that the effective reduction of Cr VI and the structural role of Cr are strongly modulated by the chemical composition of the waste. Eskolaite microcrystallites are embedded in the silica matrix of all vitrified samples and the extent of microcrystalline formation is strongly related to the glass basicity. Both Cr VI and Cr III species are identified, corresponding to Cr VI O4 glass formers and Cr III O6 network modifiers. The toxic Cr VI prevails only in the glasses with the highest basicity index and lowest waste content, nevertheless it is safely incorporated and immobilized in the silica matrix. However, the detected abundance of Cr VI increases glass basicity and as a result, glass polymerization is hindered. Thermal treatment, a process that leads to glass ceramics transforms almost all Cr VI to Cr III , while eskolaite formation is promoted concurrently. Nevertheless, microcrystalline growth proceeds mainly via depletion of Cr III from the silica matrix and not from the reduced Cr VI ; yet, Cr removal from the glass matrix does not impair the chemical stability of the devitrified product

Strain and elastic constants of GaN and InN

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