Reduction of parasitic reaction in high-temperature AlN growth by jet stream gas flow metal–organic vapor phase epitaxy

AlGaN-based deep ultraviolet light-emitting diodes (LEDs) have a wide range of applications such as medical diagnostics, gas sensing, and water sterilization. Metal–organic vapor phase epitaxy (MOVPE) method is used for the growth of all-in-one structures, including doped layer and thin multilayers, using metal–organic and gas source raw materials for semiconductor devices. For AlN growth with high crystalline quality, high temperature is necessary to promote the surface migration of Al atoms and Al-free radicals. However, increase in temperature generates parasitic gas-phase prereactions such as adduct formation. In this work, AlN growth at 1500 °C by a stable vapor phase reaction has been achieved by jet stream gas flow metal–organic vapor phase epitaxy. The AlN growth rate increases with gas flow velocity and saturates at ~ 10 m/s at room temperature. Moreover, it is constant at an ammonia flow rate at a V/III ratio from 50 to 220. These results demonstrate the reduction in adduct formation, which is a typical issue with the vapor phase reaction between triethylaluminum and ammonia. The developed method provides the in-plane uniformity of AlN thickness within 5%, a low concentration of unintentionally doped impurities, smooth surface, and decrease in dislocation density because of the suppression of parasitic reactions

Stargazer: Long-Term and Multiregional Measurement of Timing/ Geolocation-Based Cloaking

Malicious hosts have come to play a significant and varied role in today's cyber attacks. Some of these hosts are equipped with a technique called cloaking, which discriminates between access from potential victims and others and then returns malicious content only to potential victims. This is a serious threat because it can evade detection by security vendors and researchers and cause serious damage. As such, cloaking is being extensively investigated, especially for phishing sites. We are currently engaged in a long-term cloaking study of a broader range of threats. In the present study, we implemented Stargazer, which actively monitors malicious hosts and detects geographic and temporal cloaking, and collected 30,359,410 observations between November 2019 and February 2022 for 18,397 targets from 13 sites where our sensors are installed. Our analysis confirmed that cloaking techniques are widely abused, i.e., not only in the context of specific threats such as phishing. This includes geographic and time-based cloaking, which is difficult to detect with single-site or one-shot observations. Furthermore, we found that malicious hosts that perform cloaking include those that survive for relatively long periods of time, and those whose contents are not present in VirusTotal. This suggests that it is not easy to observe and analyze the cloaking malicious hosts with existing technologies. The results of this study have deepened our understanding of various types of cloaking, including geographic and temporal ones, and will help in the development of future cloaking detection methods

分子系統地理学に生態ニッチモデリングがもたらす新展開と課題

Understanding species distribution patterns is a long-standing challenge in biodiversity assessments and spatiotemporal shifts in these patterns has recently been the focus of many studies within the fields of palaeoecology and phylogeography. Due to recent advances in ecological niche models （ENM）, it is now possible to reconstruct past species distributions, thus ENM combined with genetic-based inference has the potential to provide new insights in phylogeography, which were not possible using traditional approaches, by taking into account the spatiotemporal dynamics of distribution ranges, niche shifts, demographic events, and local adaptations. In this review paper, we give overviews of several topics, including: 1） the introduction of recent advances in ENM, as well as the problems associated with traditional approaches of reconstructing past species distributions in palaeoecology; 2） niche shifts along temporal scales; 3） the inference of past demographic history based on genetic data and ENM; and 4） the estimation of the distribution of genetic variation in relation to local adaptation. Although we have reached a new stage of phylogeography incorporating ENM, some limitations remain. Thus, we discuss how to apply the current tools to advance the study and application of phylogeography, as well as the need for further development within the field

Hayabusa2’s superior solar conjunction mission operations: planning and post-operation results

Abstract In late 2018, the asteroid Ryugu was in the Sun’s shadow during the superior solar conjunction phase. As the Sun-Earth-Ryugu angle decreased to below 3°, the Hayabusa2 spacecraft experienced 21 days of planned blackout in the Earth-probe communication link. This was the first time a spacecraft had experienced solar conjunction while hovering around a minor body. For the safety of the spacecraft, a low energy transfer trajectory named Ayu was designed in the Hill reference frame to increase its altitude from 20 to 110 km. The trajectory was planned with the newly developed optNEAR tool and validated with real time data. This article shows the results of the conjunction operation, from planning to flight data.</jats:p

On the origin and evolution of the asteroid Ryugu: A comprehensive geochemical perspective

Presented here are the observations and interpretations from a comprehensive analysis of 16 representative particles returned from the C-type asteroid Ryugu by the Hayabusa2 mission. On average Ryugu particles consist of 50% phyllosilicate matrix, 41% porosity and 9% minor phases, including organic matter. The abundances of 70 elements from the particles are in close agreement with those of CI chondrites. Bulk Ryugu particles show higher δ18O, Δ17O, and ε54Cr values than CI chondrites. As such, Ryugu sampled the most primitive and least-thermally processed protosolar nebula reservoirs. Such a finding is consistent with multi-scale H-C-N isotopic compositions that are compatible with an origin for Ryugu organic matter within both the protosolar nebula and the interstellar medium. The analytical data obtained here, suggests that complex soluble organic matter formed during aqueous alteration on the Ryugu progenitor planetesimal (several 10’s of km), <2.6 Myr after CAI formation. Subsequently, the Ryugu progenitor planetesimal was fragmented and evolved into the current asteroid Ryugu through sublimation

Influx of nitrogen-rich material from the outer Solar System indicated by iron nitride in Ryugu samples

Large amounts of nitrogen compounds, such as ammonium salts, may be stored in icy bodies and comets, but the transport of these nitrogen-bearing solids into the near-Earth region is not well understood. Here, we report the discovery of iron nitride on magnetite grains from the surface of the near-Earth C-type carbonaceous asteroid Ryugu, suggesting inorganic nitrogen fixation. Micrometeoroid impacts and solar wind irradiation may have caused the selective loss of volatile species from major iron-bearing minerals to form the metallic iron. Iron nitride is a product of nitridation of the iron metal by impacts of micrometeoroids that have higher nitrogen contents than the CI chondrites. The impactors are probably primitive materials with origins in the nitrogen-rich reservoirs in the outer Solar System. Our observation implies that the amount of nitrogen available for planetary formation and prebiotic reactions in the inner Solar System is greater than previously recognized

A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu

Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe3+ to Fe2+ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss

Four‐dimensional‐STEM analysis of the phyllosilicate‐rich matrix of Ryugu samples

Ryugu asteroid grains brought back to the Earth by the Hayabusa2 space mission are pristine samples containing hydrated minerals and organic compounds. Here, we investigate the mineralogy of their phyllosilicate-rich matrix with four-dimensional scanning transmission electron microscopy (4D-STEM). We have identified and mapped the mineral phases at the nanometer scale (serpentine, smectite, pyrrhotite), observed the presence of Ni-bearing pyrrhotite, and identified the serpentine polymorph as lizardite, in agreement with the reported aqueous alteration history of Ryugu. Furthermore, we have mapped the d-spacings of smectite and observed a broad distribution of values, ranging from 1 to 2 nm, with an average d-spacing of 1.24 nm, indicating significant heterogeneity within the sample. Such d-spacing variability could be the result of either the presence of organic matter trapped in the interlayers or the influence of various geochemical conditions at the submicrometer scale, suggestive of a range of organic compounds and/or changes in smectite crystal chemistry

A pristine record of outer Solar System materials from asteroid Ryugu’s returned sample

Volatile and organic-rich C-type asteroids may have been one of the main sources of Earth’s water. Our best insight into their chemistry is currently provided by carbonaceous chondritic meteorites, but the meteorite record is biased: only the strongest types survive atmospheric entry and are then modified by interaction with the terrestrial environment. Here we present the results of a detailed bulk and microanalytical study of pristine Ryugu particles, brought to Earth by the Hayabusa2 spacecraft. Ryugu particles display a close compositional match with the chemically unfractionated, but aqueously altered, CI (Ivuna-type) chondrites, which are widely used as a proxy for the bulk Solar System composition. The sample shows an intricate spatial relationship between aliphatic-rich organics and phyllosilicates and indicates maximum temperatures of ~30 °C during aqueous alteration. We find that heavy hydrogen and nitrogen abundances are consistent with an outer Solar System origin. Ryugu particles are the most uncontaminated and unfractionated extraterrestrial materials studied so far, and provide the best available match to the bulk Solar System composition