Identification of prismatic slip bands in 4H SiC boules grown by physical vapor transport

Transmission electron microscopy (TEM) and KOH etching have been used to study the dislocation structure of 4H SiC wafers grown by physical vapor transport. A new type of threading dislocation arrays was observed. Rows of etch pits corresponding to dislocation arrays were observed in vicinity of micropipes, misoriented grains and polytypic inclusions at the periphery of the boules and extended along the directions. Plan view conventional and high resolution TEM showed that the arrays consisted of dislocations threading along the c-axis with Burgers vectors having edge components of the a/3 type. The Burgers vectors were parallel to the corresponding arrays. The dislocation arrays were interpreted as slip bands formed by dislocation glide in the prismatic slip system of hexagonal SiC during post-growth cooling.     

Effect of nitridation on crystallinity of GaN grown on GaAs by MBE

GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. Films are evaluated by X-ray diffraction and the dependence of crystalline quality on the nitridation temperature is studied. It is demonstrated that nitridation has to be performed at low-temperature to achieve c-oriented α-GaN. Higher nitridation temperature promotes formation of mis-oriented domains and β-GaN inclusions     

Growth, structure, and morphology of TiO2 films deposited by molecular beam epitaxy in pure ozone ambients

TiO₂ films were grown using a reactive molecular beam epitaxy system equipped with high-temperature effusion cells as sources for Ti and an ozone distillation system as a source for O. The growth mode, characterized in-situ by reflection high-energy electron diffraction (RHEED), as well as the phase assemblage, structural quality, and surface morphology, characterized ex-situ by X-ray diffraction and atomic force microscopy (AFM), depended on the choice of substrate, growth temperature, and ozone flux. Films deposited on (1 0 0) surfaces of SrTiO₃, (La_0.27Sr_0.73)(Al_0.65Ta_0.35)O₃, and LaAlO₃ grew as (0 0 1)-oriented anatase. Both RHEED and AFM indicated that smoother surfaces were observed for those grown at higher ozone fluxes. Moreover, while RHEED patterns indicated that anatase films grown at higher temperatures were smoother, AFM images showed presence of large inclusions in these films.     

Three-dimensional canopy fuel loading predicted using upward and downward sensing LiDAR systems

We calibrated upward sensing profiling and downward sensing scanning LiDAR systems to estimates of canopy fuel loading developed from field plots and allometric equations, and then used the LiDAR datasets to predict canopy bulk density (CBD) and crown fuel weight (CFW) in wildfire prone stands in the New Jersey Pinelands. LiDAR-derived height profiles were also generated in 1-m layers, and regressed on CBD estimates calculated for 1-m layers from field plots to predict three-dimensional canopy fuel loading. We then produced maps of canopy fuel metrics for three 9 km² forested areas in the Pinelands.Correlations for standard LiDAR-derived parameters between the two LiDAR systems were all highly significant, with correlation coefficients ranging between 0.82 and 0.98. Stepwise linear regression models developed from the profiling LiDAR data predicted maximum CBD and CFW (r² = 0.94 and 0.92) better than those developed from the scanning LiDAR data (r² = 0.83 and 0.71, respectively). A single regression for the prediction of CBD at all canopy layers had r² values of 0.93 and 0.82 for the profiling and scanning datasets, respectively. Individual bin regressions for predicting CBD at each canopy height layer were also highly significant at most canopy heights, with r² values for each layer ranging between 0.36 and 0.89, and 0.44 and 0.99 for the profiling and scanning datasets, respectively. Relationships were poorest mid-canopy, where highest average values and highest variability in fuel loading occurred. Fit of data to Gaussian distributions of canopy height profiles facilitated a simpler expression of these parameters for analysis and mapping purposes, with overall r² values of 0.86 and 0.92 for the profiling and scanning LiDAR datasets, respectively. Our research demonstrates that LiDAR data can be used to generate accurate, three-dimensional representations of canopy structure and fuel loading at high spatial resolution by linking 1-m return height profiles to biometric estimates from field plots.     

A study of vacancies and vacancy pair defects in 4H SiC grown by halide chemical vapor deposition

The understanding of the structure and associated defect level of a point defect in SiC is important because the material is to be used both as a semiconductor and semi-insulator. Production of the latter is achieved by compensation of unavoidable impurities using defects that require more energy for ionization than the unintentional donors or acceptors. The purpose of the present work is to measure the defect energy level of one center in high resistivity 4H SiC using photo-induced electron paramagnetic resonance (photo-EPR). The center is identified as SI-5, an EPR signal that others have attributed to the negative charge state of the carbon vacancy carbon antisite pair, $V_C C_{Si}^ - .$ The photo-threshold for detection of the signal is 0.75 eV in samples with resistivity activation energy, E_a, of 0.5 eV or smaller. For samples with larger E_a, SI-5 is detected only after irradiation with photon energy greater than 2.5 eV. The results suggest that $V_C C_{Si}^{ - - / - }$ is 0.75 eV below the conduction band edge and that the negative to neutral level is within 0.1 eV of $V_C C_{Si}^{- - / -} .$      

Structural organization of virulence-associated plasmids of Yersinia pestis

The complete nucleotide sequence and gene organization of the three virulence plasmids from Yersinia pestis KIM5 were determined. Plasmid pPCP1 (9,610 bp) has a GC content of 45.3% and encodes two previously known virulence factors, an associated protein, and a single copy of IS100. Plasmid pCD1 (70,504 bp) has a GC content of 44.8%. It is known to encode a number of essential virulence determinants, regulatory functions, and a multiprotein secretory system comprising the low-calcium response stimulation that is shared with the other two Yersinia species pathogenic for humans (Y. pseudotuberculosis and Y. enterocolitica). A new pseudogene, which occurs as an intact gene in the Y. enterocolitica and Y. pseudotuberculosis-derived analogues, was found in pCD1. It corresponds to that encoding the lipoprotein YlpA. Several intact and partial insertion sequences and/or transposons were also found in pCD1, as well as six putative structural genes with high homology to proteins of unknown function in other yersiniae. The sequences of the genes involved in the replication of pCD1 are highly homologous to those of the cognate plasmids in Y. pseudotuberculosis and Y. enterocolitica, but their localization within the plasmid differs markedly from those of the latter. Plasmid pMT1 (100,984 bp) has a GC content of 50.2%. It possesses two copies of IS100, which are located 25 kb apart and in opposite orientations. Adjacent to one of these IS100 inserts is a partial copy of IS285. A single copy of an IS200-like element (recently named IS1541) was also located in pMT1. In addition to 5 previously described genes, such as murine toxin, capsule antigen, capsule anchoring protein, etc., 30 homologues to genes of several bacterial species were found in this plasmid, and another 44 open reading frames without homology to any known or hypothetical protein in the databases were predicted.     

Pannexins and Connexins: Their Relevance for Oocyte Developmental Competence

The oocyte is the major determinant of embryo developmental competence in all mammalian species. Although fundamental advances have been generated in the field of reproductive medicine and assisted reproductive technologies in the past three decades, researchers and clinicians are still trying to elucidate molecular factors and pathways, which could be pivotal for the oocyte’s developmental competence. The cell-to-cell and cell-to-matrix communications are crucial not only for oocytes but also for multicellular organisms in general. This latter mentioned communication is among others possibly due to the Connexin and Pannexin families of large-pore forming channels. Pannexins belong to a protein group of ATP-release channels, therefore of high importance for the oocyte due to its requirements of high energy supply. An increasing body of studies on Pannexins provided evidence that these channels not only play a role during physiological processes of an oocyte but also during pathological circumstances which could lead to the development of diseases or infertility. Connexins are proteins that form membrane channels and gap-junctions, and more precisely, these proteins enable the exchange of some ions and molecules, and therefore they do play a fundamental role in the communication between the oocyte and accompanying cells. Herein, the role of Pannexins and Connexins for the processes of oogenesis, folliculogenesis, oocyte maturation and fertilization will be discussed and, at the end of this review, Pannexin and Connexin related pathologies and their impact on the developmental competence of oocytes will be provided.     

Electronic Instabilities Leading to Electroformation of Binary Metal Oxide‐based Resistive Switches

Oxide‐based resistive switching devices are a leading contender for the next generation memories. Before use, each device has to go through a conditioning process called electroformation which has been suggested to be initiated by the accumulation of oxygen vacancies. Here, experimental evidence is presented which shows that both Ta₂O_5‐x‐ and TiO_2‐x‐based crossbar devices, exhibit characteristic electronic instability leading to a reversible constriction of the current flow to a narrow filament prior to permanent change. Thus, it is asserted, electroformation is initiated through purely electronic and reversible events, to be followed later by structural changes in the material, like oxygen vacancy redistribution. Furthermore, the electronic instability responsible for electroformation also gives rise to negative differential resistance (NDR) and that this characteristics appears to involve two distinct mechanisms: a thermal one in which Joule heating causes resistance to decrease as current increases and a second electronic mechanism that appears not to require Joule heating for NDR. Using a combination of thermometry and thermal modeling, a self‐consistent temperature and filament radius as a function of power are found for the 5 μm cross‐bar devices. In the thermal NDR regime, the filament appears to be ～500 nm in diameter and has a peak temperature of ～300 °C, while in the adiabatic regime, the estimated filament diameter is much smaller (<50 nm) and the maximum temperature inside it exceeds 800 °C.