Fluorine doping: A feasible solution to enhancing the conductivity of high-resistance wide bandgap Mg0.51Zn0.49O active components

N-type doping of high-resistance wide bandgap semiconductors, wurtzite high-Mg-content MgxZn1-xO for instance, has always been a fundamental application-motivated research issue. Herein, we report a solution to enhancing the conductivity of high-resistance Mg0.51Zn0.49O active components, which has been reliably achieved by fluorine doping via radio-frequency plasma assisted molecular beam epitaxial growth. Fluorine dopants were demonstrated to be effective donors in Mg0.51Zn0.49O single crystal film having a solar-blind 4.43 eV bandgap, with an average concentration of 1.0E19 F/cm3.The dramatically increased carrier concentration (2.85E17 cm-3 vs ~1014 cm-3) and decreased resistivity (129 ohm.cm vs ~10E6 ohm cm) indicate that the electrical properties of semi-insulating Mg0.51Zn0.49O film can be delicately regulated by F doping. Interestingly, two donor levels (17 meV and 74 meV) associated with F were revealed by temperature-dependent Hall measurements. A Schottky type metal-semiconductor-metal ultraviolet photodetector manifests a remarkably enhanced photocurrent, two orders of magnitude higher than that of the undoped counterpart. The responsivity is greatly enhanced from 0.34 mA/W to 52 mA/W under 10 V bias. The detectivity increases from 1.89E9 cm Hz1/2/W to 3.58eE10 cm Hz1/2/W under 10 V bias at room temperature.These results exhibit F doping serves as a promising pathway for improving the performance of high-Mg-content MgxZn1-xO-based devices.Comment: 8 page

Le Kouri : race bovine du lac Tchad. I. Introduction générale à son étude zootechnique et biochimique : origines et écologie de la race

Les auteurs présentent de façon détaillée la race des taurins Kouris, bovins du lac Tchad. Cette première étude est l'introduction pour l'ensemble des travaux qui visent à éclairer les divers aspects zootechniques et biochimiques de cette race, connue depuis longtemps déjà et qui est étroitement liée au milieu aquatique du lac Tchad. Le but poursuivi est de ne pas laisser disparaître un maillon, sans doute essentiel, dans la phylogénie des races bovines et de tirer profit rapidement des propriétés les plus intéressantes de la race : production laitière, production de viande de qualité et propriétés des métis de 1re ou de 2e génération. Après l'étude écologique de l'aire géographique, le type caractéristique est décrit ainsi que ses propriétés économiques. Le mode d'élevage et la pathologie sont largement détaillés. De nombreuses raisons plaident actuellement pour le maintien et la sélection des bovins Kouris purs. (Résumé d'auteur

Changing vacancy balance in ZnO by tuning synthesis between zinc/oxygen lean conditions

The nature of intrinsic defects in ZnO films grown by metal organic vapor phase epitaxy was studied by positron annihilation and photoluminescence spectroscopy techniques. The supply of Zn and O during the film synthesis was varied by applying different growth temperatures (325–485 °C), affecting decomposition of the metal organic precursors. The microscopic identification of vacancy complexes was derived from a systematic variation in the defect balance in accordance with Zn/O supply trends.Peer reviewe

Two vicious circles contributing to a diagnostic delay for tuberculosis patients in Arkhangelsk

Setting: Delay in tuberculosis (TB) diagnosis increases the infectious pool in the community and the risk of development of resistance of mycobacteria, which results in an increased number of deaths. Objective: To describe patients’ and doctors’ perceptions of diagnostic delay in TB patients in the Arkhangelsk region and to develop a substantive model to better understand the mechanisms of how these delays are linked to each other. Design: A grounded theory approach was used to study the phenomenon of diagnostic delay. Patients with TB diagnostic delay and doctors–phthisiatricians were interviewed. Results: A model named ‘sickness trajectory in health-seeking behaviour among tuberculosis patients’ was developed and included two core categories describing two vicious circles of diagnostic delay in patients with TB: ‘limited awareness of the importance to contact the health system’ and ‘limited resources of the health system’ and the categories: ‘factors influencing health-seeking behaviour’ and ‘factors influencing the health system effectiveness’. Men were more likely to report patient delay, while women were more likely to report health system delay. Conclusions: To involve people in early medical examinations, it is necessary to increase alertness on TB among patients and to improve health systems in the districts

Vacancy clustering and acceptor activation in nitrogen-implanted ZnO

The role of vacancy clustering and acceptor activation on resistivity evolution in N ion-implanted n-type hydrothermally grown bulk ZnO has been investigated by positron annihilation spectroscopy, resistivity measurements, and chemical profiling. Room temperature 220 keV N implantation using doses in the low 10 exp 15 cm exp −2 range induces small and big vacancy clusters containing at least 2 and 3–4 Zn vacancies, respectively. The small clusters are present already in as-implanted samples and remain stable up to 1000°C with no significant effect on the resistivity evolution. In contrast, formation of the big clusters at 600°C is associated with a significant increase in the free electron concentration attributed to gettering of amphoteric Li impurities by these clusters. Further annealing at 800°C results in a dramatic decrease in the free electron concentration correlated with activation of 10 exp 16–10 exp 17cm exp −3 acceptors likely to be N and/or Li related. The samples remain n type, however, and further annealing at 1000°C results in passivation of the acceptor states while the big clusters dissociate.Peer reviewe

Back-illuminated Si photocathode:a combined experimental and theoretical study for photocatalytic hydrogen evolution

We present the first experimental demonstration of c-Si based photocathode for hydrogen production under back-illumination with theoretical model study.</p

Thermal Conductivity of Double Polymorph Ga2O3 Structures

Recently discovered double gamma/beta ({\gamma}/\b{eta}) polymorph Ga2O3 structures constitute a class of novel materials providing an option to modulate functional properties across interfaces without changing chemical compositions of materials, in contrast to that in conventional heterostructures. In this work, for the first time, we investigate thermal transport in such homo-interface structures as an example of their physical properties. Specifically, the cross-plane thermal conductivity (k) was measured by femtosecond laser-based time-domain thermoreflectance with MHz modulation rates, effectively obtaining depth profiles of the thermal conductivity across the {\gamma}/\b{eta}-Ga2O3 structures. In this way, the thermal conductivity of {\gamma}-Ga2O3 k=1.84{\div}2.11 W m-1K-1 was found to be independent of the initial \b{eta}-substrates orientations, in accordance with the cubic spinel structure of the {\gamma}-phase and consistently with the molecular dynamics simulation data. In its turn, the thermal conductivity of monoclinic \b{eta}-Ga2O3 showed a distinct anisotropy, with values ranging from 10 W m-1K-1 for [201] to 20 Wm-1K-1 for [010] orientations. Thus, for double {\gamma}/\b{eta} Ga2O3 polymorph structures formed on [010] \b{eta}-substrates, there is an order of magnitude difference in thermal conductivity across the {\gamma}/\b{eta} interface, which potentially can be exploited in thermal energy conversion applications

Cross-Sectional Carrier Lifetime Profiling and Deep Level Monitoring in Silicon Carbide Films Exhibiting Variable Carbon Vacancy Concentrations

The carrier lifetime control over 150 μm thick 4H-SiC epitaxial layers via thermal generation and annihilation of carbon vacancy (VC) related Z1/2 lifetime killer sites is reported. The defect developments upon typical SiC processing steps, such as high- and moderate-temperature anneals in the presence of a carbon cap, are monitored by combining electrical characterization techniques capable of VC depth-profiling, capacitance–voltage (CV) and deep-level transient spectroscopy (DLTS), with a novel all-optical approach of cross-sectional carrier lifetime profiling across 4H-SiC epilayer/substrate based on imaging time-resolved photoluminescence (TRPL) spectroscopy in orthogonal pump-probe geometry, which readily exposes in-depth efficacy of defect reduction and surface recombination effects. The lifetime control is realized by initial high-temperature treatment (1800 °C) to increase VC concentration to ≈1013 cm−3 level followed by a moderate-temperature (1500 °C) post-annealing of variable duration under C-rich thermodynamic equilibrium conditions. The post-annealing carried out for 5 h in effect eliminates VC throughout the entire ultra-thick epilayer. The reduction of VC-related Z1/2 sites is proven by a significant lifetime increase from 0.8 to 2.5 μs. The upper limit of lifetimes in terms of carrier surface leakage and the presence of other nonradiative recombination centers besides Z1/2, possibly related to residual impurities such as boron are discussed.publishedVersio

Crystallization Instead of Amorphization in Collision Cascades in Gallium Oxide

Disordering of solids typically leads to amorphization, but polymorph transitions, facilitated by favorable atomic rearrangements, may temporarily help to maintain long-range periodicity in the solid state. In far-from-equilibrium situations, such as atomic collision cascades, these rearrangements may not necessarily follow a thermodynamically gainful path, but may be kinetically limited. In this Letter, we focused on such crystallization instead of amorphization in collision cascades in gallium oxide (\ce{Ga2O3}). We determined the disorder threshold for irreversible $\beta$-to-$\gamma$ polymorph transition and explained why it results in elevating energy to that of the $\gamma$-polymorph, which exhibits the highest polymorph energy in the system below the amorphous state. Specifically, we demonstrate that upon reaching the disorder transition threshold, the \ce{Ga}-sublattice kinetically favors transitioning to the $\gamma$-like configuration, requiring significantly less migration for \ce{Ga} atoms to reach the lattice sites during post-cascade processes. As such, our data provide a consistent explanation of this remarkable phenomenon and can serve as a toolbox for predictive multi-polymorph fabrication.Comment: 6 pages, 4 figures, under revie