Effect of extended defects on AlGaN QDs for electron-pumped UV-emitters

We study the origin of bimodal emission in AlGaN/AlN QD superlattices displaying high internal quantum efficiency (around 50%) in the 230-300 nm spectral range. The secondary emission at longer wavelengths is linked to the presence of cone-like defects starting at the first AlN buffer/superlattice interface and propagating vertically. These defects are associated with a dislocation that produces strong shear strain, which favors the formation of 30{\deg} faceted pits. The cone-like structures present Ga enrichment at the boundary facets and larger QDs within the defect. The bimodality is attributed to the differing dot size/composition within the defects and at the defect boundaries, which is confirmed by the correlation of microscopy results and Schr\"odinger-Poisson calculations

Quantitative Scanning Transmission Electron Microscopy–High-Angle-Annular Dark-Field Study of the Structure of Pseudo-2D Sb2Te3 Films Grown by (Quasi) Van der Waals Epitaxy

peer reviewedScanning transmission electron microscopy (STEM) techniques are used to improve the understanding of out-of-plane oriented Sb2Te3 thin films deposited by sputtering on SiO2 and Si substrates. Nanobeam precession electron diffraction, energy-dispersive X-ray spectroscopy, and high-angle-annular dark-field imaging show that the presence of 1–2 atomic planes of Te on top of the substrate is a crucial factor for successful growth of such films, which can be achieved by optimizing cosputtering of Te and Sb2Te3 targets. The formation of an actual van der Waals (vdW) gap between the substrate and the first Sb2Te3 quintuple layer allows for vdW epitaxy. This gap is larger than those separating Te planes in the pseudo-2D Sb2Te3 structure. HAADF image analysis provides detailed information on the atomic arrangement such as interplanar distances, vdW gaps, and Debye–Waller coefficients, all these with a few pm precision. For the anisotropic atomic displacements, a new methodology is introduced based on the statistical analysis of atomic column positions that provides information on the low-frequency phonon modes. Ab initio calculations are used to support our results. Overall, this study provides quantitative STEM tools particularly well suited for nonperiodic pseudo-2D materials, such as Sb2Te3/GeTe superlattices

AlGaN/AlN Stranski–Krastanov Quantum Dots for Highly Efficient Electron Beam-Pumped Emitters: The Role of Miniaturization and Composition to Attain Far UV-C Emission

Conventional ultraviolet lamps for disinfection emit radiation in the 255–270 nm range, which poses a high risk of causing cancer and cataracts. To address these concerns, solid-state UV-C sources emitting below 240 nm are attractive as a safe and sustainable disinfection solution for occupied spaces. This article delves into the extension of AlxGa1–xN/AlN quantum dot (QD) technology toward the far UV-C range. The structural and optical impact of increasing the Al content in the QDs through the increase of the Al flux and eventual correction of the Ga flux, and the effect of extreme miniaturization of the QDs achieved by reducing their growth time, are explored. The internal quantum efficiency of self-assembled QDs, grown with a metal/N ratio ranging from 0.5 to 0.8, remains around 50% regardless of the Al content (up to 65%) or emission wavelength (300–230 nm). However, QDs emitting below 270 nm display bimodal luminescence due to inhomogeneous in-plane emission caused by fluctuations in the QD shape linked to extended defects. Reducing the QD size exacerbates this bimodality without altering the emission wavelength. Power efficiencies under electron beam pumping range from 0.4 to 1%, with the potential for improvement through surface treatments enhancing light extraction efficiency

Thin layer etching of low-k SiCO spacer using hydrogen ion implantation followed by hydrofluoric acid

International audienceSpacer etching realization is considered today as one of the most critical processes for the fully depleted silicon on insulator devices realization. The challenge arises from the fact that low-k spacer needs to be introduced to improve device performances. In this paper, a new approach to etch the low-k SiCO film was investigated. This approach is based on the SiCO film modification by H-2 plasma followed by a removal step of this modified layer using hydrofluoric acid- (HF) based wet cleaning. It has been demonstrated that SiCO layer modification depth is controlled by plasma parameters like bias voltage. The SiCO film modification induced by H-2 plasma has been well understood by x-ray photoelectron spectroscopy, infrared spectroscopy analyses in transmission or in multi-internal reflection. It has been demonstrated that the H-2 plasma exposure converts the SiCO film in volume into an SiOxHy film, explaining the higher damaged film removal rate when dipped in liquid phase HF. Finally, the compatibility of the new etch approach with the SiCO low-k film has been demonstrated on pattern structures

Evaluation and Characterization of a Potential CO2 Storage Site in the South Adriatic Offshore

International audienceThe Southern Adriatic Sea is one of the five prospective areas for CO2 storage beingevaluated under the FP7 European Sitechar project. The potential reservoir identified in theinvestigated area is represented by a carbonate formation (Scaglia Formation – Late Cretaceous).This paper shows the site characterization applied to one of the structures identified in the carbonatestorage system of the South Adriatic offshore. The interpretation and analysis of seismic andborehole data allowed the construction of a 3D geological static model on both regional and localscales. Dynamic modeling was applied, adopting a sensitivity approach ( i.e. fault transmissivity,petrophysical properties of the caprock and reservoir, and different stress regimes). Coupled fluidflow and geomechanical simulation was applied to investigate the potential risk of leakage inducedby mechanical solicitation on the faults occurring in the investigated area

Gradient-based and wavelet-based compressed sensing approaches for highly undersampled tomographic datasets

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Effect of Extended Defects on AlGaN Quantum Dots for Electron-Pumped Ultraviolet Emitters

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Nanocomposites of chalcogenide phase-change materials: from C-doping of thin films to advanced multilayers

International audienceEngineering of chalcogenide phase-change materials at the nanoscale is required to improve the performances of ultimate size memory devices and reduce their power consumption. Amorphous C-doped GeTe thin films and innovative multilayers consisting of periodic stacks of a few nm thick GeTe, or Ge$_2$Sb$_2$Te$_5$, and C layers with a thickness between 0.5 and 2 nm are deposited by magnetron sputtering at room temperature. The phase-change material is then crystallized by heat treatment. In C-doped GeTe films, the phase separation of C and GeTe during the GeTe crystallization leads to the spontaneous formation of a nanocomposite, consisting of amorphous C located at the grain boundaries of GeTe crystallites, but the resulting nanostructure is highly disordered. In contrast, the deposition of multilayers allows control of the nanostructure and the interfaces between the phase-change material and the C phase. Transmission electron microscopy and X-ray diffraction at room temperature and as a function of temperature during annealing show that the multilayer structure is maintained after crystallization of the phase-change material, even when the thickness of the C layer is as low as 0.5 nm. GeTe and Ge$_2$Sb$_2$Te$_5$ crystallites are anisotropic, their size in the direction perpendicular to the layers being determined by the design of the multilayer. The crystallisation temperature of the GeTe and Ge2Sb2Te5 layers depends on the structure of the stack, revealing scaling and stress effects. The results presented show that GeTe/C and Ge$_2$Sb$_2$Te$_5$/C MLs are promising for applications in memory devices and also in photonic and thermoelectric devices

Impact of strain on Si and Sn incorporation in (Si)GeSn alloys by STEM analyses

International audienceThe structural properties of CVD-grown (Si)GeSn heterostructures were assessed thanks to scanning transmission electron microscopy at the nanometer scale. Quantitative energy dispersive x-ray (EDX) spectroscopy together with precession electron diffraction and geometrical phase analysis (GPA) were performed to probe the chemical and structural properties of the different layers. Results presented in this paper demonstrated the advantages of a multilayer structure, with successive layers grown at decreasing temperatures in order to gradually accommodate the in-plane lattice parameter and incorporate more and more Sn into the stack. It was shown how the GeSn emissive layer could be manufactured with low plastic deformation and a high relaxation rate, necessary for better light emission performances. SiGeSn alloys used as confinement barriers around the emissive layer were also investigated. For such thin layers, we showed the importance of the starting lattice parameter (SLP) prior to the growth on their composition. Indeed, higher SLPs resulted, for the very same process conditions, into higher Sn contents and lower Si contents. The interest in combining EDX, which was accurate enough to detect slight chemical concentration variations, and GPA, for local strain analyses, was clearly demonstrated. Present results will be very useful to predict and control the bandgap and structural quality of (Si)GeSn materials and, in turn, device properties

Quantitative Scanning Transmission Electron Microscopy–High‐Angle‐Annular Dark‐Field Study of the Structure of Pseudo‐2D Sb 2 Te 3 Films Grown by (Quasi) Van der Waals Epitaxy

International audienceScanning transmission electron microscopy (STEM) techniques are used to improve the understanding of out‐of‐plane oriented Sb 2 Te 3 thin films deposited by sputtering on SiO 2 and Si substrates. Nanobeam precession electron diffraction, energy‐dispersive X‐ray spectroscopy, and high‐angle‐annular dark‐field imaging show that the presence of 1–2 atomic planes of Te on top of the substrate is a crucial factor for successful growth of such films, which can be achieved by optimizing cosputtering of Te and Sb 2 Te 3 targets. The formation of an actual van der Waals (vdW) gap between the substrate and the first Sb 2 Te 3 quintuple layer allows for vdW epitaxy. This gap is larger than those separating Te planes in the pseudo‐2D Sb 2 Te 3 structure. HAADF image analysis provides detailed information on the atomic arrangement such as interplanar distances, vdW gaps, and Debye–Waller coefficients, all these with a few pm precision. For the anisotropic atomic displacements, a new methodology is introduced based on the statistical analysis of atomic column positions that provides information on the low‐frequency phonon modes. Ab initio calculations are used to support our results. Overall, this study provides quantitative STEM tools particularly well suited for nonperiodic pseudo‐2D materials, such as Sb 2 Te 3 /GeTe superlattices