Modified strain and elastic energy behavior of Ge islands formed on high-miscut Si(0 0 1) substrates

Abstract We investigate here the influence of Si substrate miscut on the strain and elastic energy of Ge islands. We show how the morphology, composition and the elastic energy for 4 and 13 monolayers (ML) Ge islands grown at 600 °C and 730 °C on vicinal Si(0 0 1) surfaces change with miscut angles ranging between 0° and 10°. Scanning Tunneling Microscopy is used to determine the island morphology. Resonant x-ray diffraction near the Ge-K absorption edge allows the determination of the Ge concentration as well as the elastic energy stored on such structures from their dependency on the lattice parameter. Simulations using the Finite Elements Method indicate that the enlargement of the SiGe broad peak retrieved from the x-ray diffraction measurements is actually caused by the asymmetrical faceting induced by large miscut angles. Such faceting has a strong effect on island density and elastic energy, producing differences that are proportional to those observed in conditions with distinct SiGe content

Nanoscale Mapping of the 3D Strain Tensor in a Germanium Quantum Well Hosting a Functional Spin Qubit Device

A strained Ge quantum well, grown on a SiGe/Si virtual substrate and hosting two electrostatically defined hole spin qubits, is nondestructively investigated by synchrotron-based scanning X-ray diffraction microscopy to determine all its Bravais lattice parameters. This allows rendering the three-dimensional spatial dependence of the six strain tensor components with a lateral resolution of approximately 50 nm. Two different spatial scales governing the strain field fluctuations in proximity of the qubits are observed at 1 μm, respectively. The short-ranged fluctuations have a typical bandwidth of 2 × 10-4 and can be quantitatively linked to the compressive stressing action of the metal electrodes defining the qubits. By finite element mechanical simulations, it is estimated that this strain fluctuation is increased up to 6 × 10-4 at cryogenic temperature. The longer-ranged fluctuations are of the 10-3 order and are associated with misfit dislocations in the plastically relaxed virtual substrate. From this, energy variations of the light and heavy-hole energy maxima of the order of several 100 μeV and 1 meV are calculated for electrodes and dislocations, respectively. These insights over material-related inhomogeneities may feed into further modeling for optimization and design of large-scale quantum processors manufactured using the mainstream Si-based microelectronics technology

Crystal Phase Transitions in the Shell of PbS CdS Core Shell Nanocrystals Influences Photoluminescence Intensity

ABSTRACT We reveal the existence of two different crystalline phases, i.e., the metastable rock salt and the equilibrium zinc blende phase within the CdS shell of PbS CdS core shell nanocrystals formed by cationic exchange. The chemical composition profile of the core shell nanocrystals with different dimensions is determined by means of anomalous small angle X ray scattering with subnanometer resolution and is compared to X ray diffraction analysis. We demonstrate that the photoluminescence emission of PbS nanocrystals can be drastically enhanced by the formation of a CdS shell. Especially, the ratio of the two crystalline phases in the shell significantly influences the photoluminescence enhancement. The highest emission was achieved for chemically pure CdS shells below 1 nm thickness with a dominant metastable rock salt phase fraction matching the crystal structure of the PbS core. The metastable phase fraction decreases with increasing shell thickness and increasing Exchange times. The photoluminescence intensity depicts a constant decrease with decreasing metastable rock salt phase fraction but Shows an abrupt drop for shells above 1.3 nm thickness. We relate this effect to two different transition mechanisms for changing from the metastable rock salt phase to the equilibrium zinc blende phase depending on the shell thicknes

Microdiffraction imaging—a suitable tool to characterize organic electronic devices

Tailoring device architecture and active film morphology is crucial for improving organic electronic devices. Therefore, knowledge about the local degree of crystallinity is indispensable to gain full control over device behavior and performance. In this article, we report on microdiffraction imaging as a new tool to characterize organic thin films on the sub-micron length scale. With this technique, which was developed at the ID01 beamline at the ESRF in Grenoble, a focused X-ray beam (300 nm diameter, 12.5 keV energy) is scanned over a sample. The beam size guarantees high resolution, while material and structure specificity is gained by the choice of Bragg condition.Here, we explore the possibilities of microdiffraction imaging on two different types of samples. First, we measure the crystallinity of a pentacene thin film, which is partially buried beneath thermally deposited gold electrodes and a second organic film of fullerene C$_{60}$. The data shows that the pentacene film structure is not impaired by the subsequent deposition and illustrates the potential of the technique to characterize artificial structures within fully functional electronic devices. Second, we investigate the local distribution of intrinsic polymorphism of pentacene thin films, which is very likely to have a substantial influence on electronic properties of organic electronic devices. An area of 40 μm by 40 μm is scanned under the Bragg conditions of the thin-film phase and the bulk phase of pentacene, respectively. To find a good compromise between beam footprint and signal intensity, third order Bragg condition is chosen. The scans show complementary signal distribution and hence demonstrate details of the crystalline structure with a lateral resolution defined by the beam footprint (300 nm by 3 μm). The findings highlight the range of applications of microdiffraction imaging in organic electronics, especially for organic field effect transistors and for organic solar cells

The catalytic activity of KMoCo carbon spheres for higher alcohols synthesis from syngas

Catalytic conversion of syngas to higher carbon number alcohols remains challenging till date owing to poor alcohols selectivity and limited understanding of catalytic active sites. In this study, we report bimetallic molybdenum-cobalt carbon spheres produced via in-situ hydrothermal reduction mediated by monosaccharides (glucose). The addition of potassium promoter (K/Mo = 0.14) during hydrothermal synthesis increased the concentration of high-valence molybdenum (MoO) and selectivity towards higher alcohols (28 %). The field emission-SEM and HR-TEM images revealed a spherical morphology, embedding nanocrystalline MoCo oxides. The resulting carbon spheres enabled C alcohols synthesis under mild operating conditions (270 ⁰C, 30 bar, and 3000 h)

Direct observation of large-area strain propagation on free-standing nanomembranes

Investigations on epitaxial nanostructures with size of tens of nanometers have been a challenging issue for techniques that present high strain sensitivity but restricted spatial resolution. This is the case of recently developed x-ray nanoprobe techniques. Despite its inherent nondestructive character, submicron x-ray spots have only been successfully applied to the study of individual nanostructures which are either strain free or present extremely mild spatial lattice parameter gradients. Such limitation, with an uttermost barrier given by the diffraction limit, leads to voxel or pixel sizes between 5 and 10 nm obtained in coherent diffraction imaging or ptychographic reconstructions of real-space objects. Whenever the strain field of a nanostructure is successfully reconstructed from reciprocal space measurements, it cannot vary considerably in short distances since this would induce diffraction peak broadening and cause abrupt phase variations, leading to convergence issues on reconstruction algorithms. Here we show how epitaxial systems with large lattice mismatch and appreciable interfacial strain can be identified and directly analyzed throughout their strain field propagation in nanometer-thin crystalline membrane platforms, using the InGaAs/GaAs Stranski-Krastanov system as a model. The strain-induced footprint becomes observable along a few microns if the membrane thickness is comparable to the nanostructure size. It is possible to retrieve both interfacial strain and nanostructure size by probing individual objects

Structural Mapping of Functional Ge Layers Grown on Graded SiGe Buffers for sub-10 nm CMOS Applications Using Advanced X-ray Nanodiffraction

We report a detailed advanced materials characterization study on 40 nm thick strained germanium (Ge) layers integrated on 300 mm Si(001) wafers via strain-relaxed silicon-germanium (SiGe) buffer layers. Fast-scanning X-ray microscopy is used to directly image structural inhomogeneities, lattice tilt, thickness, and strain of a functional Ge layer down to the sub-micrometer scale with a real space step size of 750 ?m. The structural study shows that the metastable Ge layer, pseudomorphically grown on Si0.3Ge0.7, exhibits an average compressive biaxial strain of ?1.27%. By applying a scan area of 100 × 100 ?m2, we observe microfluctuations of strain, lattice tilt, and thickness of ca. ±0.03%, ±0.05°, and ±0.8 nm, respectively. This study confirms the high materials homogeneity of the compressively strained Ge layer realized by the step-graded SiGe buffer approach on 300 mm Si wafers. This presents thus a promising materials science approach for advanced sub-10 nm complementary metal oxide-semiconductor applications based on strain-engineered Ge transistors to outperform current Si channel technologies

Unveiling Core–Shell Structure Formation in a Ni 3 Fe Nanoparticle with In Situ Multi-Bragg Coherent Diffraction Imaging

International audienceSolid-state reactions play a key role in materials science. The evolution of the structure of a single 350 nm Ni3Fe nanoparticle, i.e., its morphology (facets) as well as its deformation field, has been followed by applying multireflection Bragg coherent diffraction imaging. Through this approach, we unveiled a demixing process that occurs at high temperatures (600 °C) under an Ar atmosphere. This process leads to the gradual emergence of a highly strained core–shell structure, distinguished by two distinct lattice parameters with a difference of 0.4%. Concurrently, this transformation causes the facets to vanish, ultimately yielding a rounded core–shell nanoparticle. This final structure comprises a Ni3Fe core surrounded by a 40 nm Ni-rich outer shell due to preferential iron oxidation. Providing in situ 3D imaging of the lattice parameters at the nanometer scale while varying the temperature, this study─with the support of atomistic simulations─not only showcases the power of in situ multireflection BCDI but also provides valuable insights into the mechanisms at work during a solid-state reaction characterized by a core–shell transition