SiGe Raman spectra vs. local clustering/anticlustering : Percolation scheme and ab initio calculations

We formalize within the percolation scheme, that operates along the linear chain approximation, namely at one dimension (1D), an intrinsic ability behind Raman scattering to achieve a quantitative insight into local clustering or anticlustering in an alloy, using SiGe as a case study. For doing so, we derive general expressions of the individual fractions of the six SiGe percolation-type oscillators [1(Ge-Ge), 3(Si-Ge), 2(Si-Si)], which monitor directly the Raman intensities, via a relevant order parameter k. This is introduced by adapting to the 1D oscillators of the SiGe diamond version of the 1D percolation scheme, namely along a fully consistent 1D treatment, the approach originally used by Verleur and Barker for the three-dimensional (3D) oscillators of their 1D cluster scheme applying to zincblende alloys [H.W. Verleur and A.S. Barker, Phys. Rev. 149, 715 (1966)], a somehow problematic one in fact, due to its 3D vs. 1D ambivalence. Predictive k-dependent intensity interplays between the SiGe (50 at.%Si) Raman lines are confronted with existing experimental data and with ab initio Raman spectra obtained by using large (32 atom) disordered supercells matching the required k values, with special attention to the Si-Ge triplet and to the Si-Si doublet, respectively.Comment: 20 pages, 6 figure

Theory of reactions between hydrogen and group-III acceptors in silicon

The thermodynamics of several reactions involving atomic and molecular hydrogen with group-III acceptors is investigated. The results provide a first-principles-level account of thermally- and carrier-activated processes involving these species. Acceptor-hydrogen pairing is revisited as well. We present a refined physicochemical picture of long-range migration, compensation effects, and short-range reactions, leading to fully passivated $\equiv\textrm{Si-H}\cdots X\equiv$ structures, where $X$ is a group-III acceptor element. The formation and dissociation of acceptor-H and acceptor-H$_{2}$ complexes is considered in the context of Light and elevated Temperature Induced Degradation (LeTID) of silicon-based solar cells. Besides explaining observed trends and answering several fundamental questions regarding the properties of acceptor-hydrogen pairing, we find that the BH$_{2}$ complex is a by-product along the reaction of H$_{2}$ molecules with boron toward the formation of BH pairs (along with subtraction of free holes). The calculated changes in Helmholtz free energies upon the considered defect reactions, as well as activation barriers for BH$_{2}$ formation/dissociation (close to $\sim1$ eV) are compatible with the experimentally determined activation energies of degradation/recovery rates of Si:B-based cells during LeTID. Dihydrogenated acceptors heavier than boron are anticipated to be effective-mass-like shallow donors, and therefore, unlikely to show similar non-radiative recombination activity

Silicon carbide diodes for neutron detection

In the last two decades we have assisted to a rush towards finding a He3-replacing technology capable of detecting neutrons emitted from fissile isotopes. The demand stems from applications like nuclear war-head screening or preventing illicit traffic of radiological materials. Semiconductor detectors stand among the stronger contenders, particularly those based on materials possessing a wide band gap like silicon carbide. We review the workings of SiC-based neutron detectors, along with several issues related to material properties, device fabrication and testing. The paper summarizes the experimental and theoretical work carried out within the E-SiCure project, co-funded by the NATO SPS Programme. Among the achievements, we have the development of successful Schottky barrier based detectors and the identification of the main carrier life-time-limiting defects in the SiC active areas, either already present in pristine devices or introduced upon exposure to radiation fields. The physical processes involved in neutron detection are described. Material properties as well as issues related to epitaxial growth and device fabrication are addressed. The presence of defects in as-grown material, as well as those introduced by ionizing radiation are reported. We finally describe several experiments carried out at the Jozef Stefan Institute TRIGA Mark II reactor (Ljubljana, Slovenia), where a set of SiC-based neutron detectors were tested, some of which being equipped with a thermal neutron converter layer. We show that despite the existence of large room for improvement, Schottky barrier diodes based on state-of-the-art 4H-SiC are closing the gap regarding the sensitivity offered by gas-based and that of semiconductor detectors

Double negatively charged carbon vacancy at the h- and k-sites in 4H-SiC: Combined Laplace-DLTS and DFT study

We present results from combined Laplace-Deep Level Transient Spectroscopy (Laplace-DLTS) and density functional theory studies of the carbon vacancy (VC) in n-type 4H-SiC. Using Laplace-DLTS, we were able to distinguish two previously unresolved sub-lattice-inequivalent emissions, causing the broad Z1/2 peak at 290K that is commonly observed by conventional DLTS in n-type 4H-SiC. This peak has two components with activation energies for electron emission of 0.58 eV and 0.65 eV. We compared these results with the acceptor levels of VC obtained by means of hybrid density functional supercell calculations. The calculations support the assignment of the Z1/2 signal to a superposition of emission peaks from double negatively charged VC defects. Taking into account the measured and calculated energy levels, the calculated relative stability of VC in hexagonal (h) and cubic (k) lattice sites, as well as the observed relative amplitude of the Laplace-DLTS peaks, we assign Z1 and Z2 to VC(h) and VC(k), respectively. We also present the preliminary results of DLTS and Laplace-DLTS measurements on deep level defects (ET1 and ET2) introduced by fast neutron irradiation and He ion implantation in 4H-SiC. The origin of ET1 and ET2 is still unclear

Measurement of the cosmic ray spectrum above 4×10184{\times}10^{18} eV using inclined events detected with the Pierre Auger Observatory

A measurement of the cosmic-ray spectrum for energies exceeding $4{\times}10^{18}$ eV is presented, which is based on the analysis of showers with zenith angles greater than $60^{\circ}$ detected with the Pierre Auger Observatory between 1 January 2004 and 31 December 2013. The measured spectrum confirms a flux suppression at the highest energies. Above $5.3{\times}10^{18}$ eV, the "ankle", the flux can be described by a power law $E^{-\gamma}$ with index $\gamma=2.70 \pm 0.02 \,\text{(stat)} \pm 0.1\,\text{(sys)}$ followed by a smooth suppression region. For the energy ($E_\text{s}$) at which the spectral flux has fallen to one-half of its extrapolated value in the absence of suppression, we find $E_\text{s}=(5.12\pm0.25\,\text{(stat)}^{+1.0}_{-1.2}\,\text{(sys)}){\times}10^{19}$ eV.Comment: Replaced with published version. Added journal reference and DO

Consistent patterns of common species across tropical tree communities

Trees structure the Earth’s most biodiverse ecosystem, tropical forests. The vast number of tree species presents a formidable challenge to understanding these forests, including their response to environmental change, as very little is known about most tropical tree species. A focus on the common species may circumvent this challenge. Here we investigate abundance patterns of common tree species using inventory data on 1,003,805 trees with trunk diameters of at least 10 cm across 1,568 locations1,2,3,4,5,6 in closed-canopy, structurally intact old-growth tropical forests in Africa, Amazonia and Southeast Asia. We estimate that 2.2%, 2.2% and 2.3% of species comprise 50% of the tropical trees in these regions, respectively. Extrapolating across all closed-canopy tropical forests, we estimate that just 1,053 species comprise half of Earth’s 800 billion tropical trees with trunk diameters of at least 10 cm. Despite differing biogeographic, climatic and anthropogenic histories7, we find notably consistent patterns of common species and species abundance distributions across the continents. This suggests that fundamental mechanisms of tree community assembly may apply to all tropical forests. Resampling analyses show that the most common species are likely to belong to a manageable list of known species, enabling targeted efforts to understand their ecology. Although they do not detract from the importance of rare species, our results open new opportunities to understand the world’s most diverse forests, including modelling their response to environmental change, by focusing on the common species that constitute the majority of their trees.Publisher PDFPeer reviewe

Clonal chromosomal mosaicism and loss of chromosome Y in elderly men increase vulnerability for SARS-CoV-2

The pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2, COVID-19) had an estimated overall case fatality ratio of 1.38% (pre-vaccination), being 53% higher in males and increasing exponentially with age. Among 9578 individuals diagnosed with COVID-19 in the SCOURGE study, we found 133 cases (1.42%) with detectable clonal mosaicism for chromosome alterations (mCA) and 226 males (5.08%) with acquired loss of chromosome Y (LOY). Individuals with clonal mosaic events (mCA and/or LOY) showed a 54% increase in the risk of COVID-19 lethality. LOY is associated with transcriptomic biomarkers of immune dysfunction, pro-coagulation activity and cardiovascular risk. Interferon-induced genes involved in the initial immune response to SARS-CoV-2 are also down-regulated in LOY. Thus, mCA and LOY underlie at least part of the sex-biased severity and mortality of COVID-19 in aging patients. Given its potential therapeutic and prognostic relevance, evaluation of clonal mosaicism should be implemented as biomarker of COVID-19 severity in elderly people. Among 9578 individuals diagnosed with COVID-19 in the SCOURGE study, individuals with clonal mosaic events (clonal mosaicism for chromosome alterations and/or loss of chromosome Y) showed an increased risk of COVID-19 lethality

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data