D. V. Davydov

Continuous measurements of methane from a tower network over Siberia

We have been conducting continuous measurements of Methane (sCH4) concentration from an expanding network of towers (JR-STATION: Japan–Russia Siberian Tall Tower Inland Observation Network) located in taiga, steppe and wetland biomes of Siberia since 2004. High daytime means (>2000 ppb) observed simultaneously at several towers during winter, together with in situ weather data and NCEP/NCAR reanalysis data, indicate that high pressure systems caused CH4 accumulation at subcontinental scale due to the widespread formation of an inversion layer. Daytime means sometimes exceeded 2000 ppb, particularly in the summer of 2007 when temperature and precipitation rates were anomalously high over West Siberia, which implies that CH4 emission from wetlands were exceptionally high in 2007. Many hot spots detected by MODIS in the summer of 2007 illustrate that the contribution of biomass burning also cannot be neglected. Daytime mean CH4 concentrations from the Siberian tower sites were generally higher than CH4 values reported at NOAA coastal sites in the same latitudinal zone, and the difference in concentrations between two sets of sites was reproduced with a coupled Eulerian–Lagrangian transport model. Simulations of emissions from different CH4 sources suggested that the major contributor to variation switched from wetlands during summer to fossil fuel during winter.

Electrical characteristics of p-3C-SiC/n-6H-SiC heterojunctions grown by sublimation epitaxy on 6H-SiC substrates

Abstract Sublimation epitaxy in a vacuum (SEV) has been used to obtain p-3C–SiC/n-6H–SiC heteroepitaxial structures. Results of a study of epilayers (X-ray diffraction analysis, scanning electron microscopy involving secondary electrons and electron beam induced current modes) and diode structures on their base ( I – V and C – V characteristics, electroluminescence spectra, DLTS) are presented. Band discontinuities are determined and a band diagram of the p-3C–SiC/n-6H–SiC heterostructure is constructed.

Radiation hardness of wide-gap semiconductors (using the example of silicon carbide)

Results obtained in studying the effect of ionizing radiation on epitaxial layers and devices based on silicon carbide (SiC) are considered. It is shown that, in investigations of wide-gap semiconductors (WGS), account should be taken of how the rate of removal of mobile charge carriers—the standard parameter in determining the radiation hardness of a material—depends on temperature. The use of data obtained only at room temperature may lead to an incorrect assessment of the radiation hardness of WGS. A conclusion is made that the WGS properties combine, on the one hand, high radiation hardness of high-temperature devices based on these semiconductors and, on the other, the possibility of effective radiation-induced doping (e.g., for obtaining semi-insulating local regions in a material at room temperature).

Annual variation of CH4 emissions from the middle taiga in West Siberian Lowland (2005–2009): a case of high CH4 flux and precipitation rate in the summer of 2007

ABSTRACT We described continuous measurements of CH4 and CO2 concentration obtained at two sites placed in the middle taiga, Karasevoe (KRS) and Demyanskoe (DEM), in West Siberian Lowland (WSL) from 2005 to 2009. Although both CH4 and CO2 accumulation (ΔCH4 and ΔCO2) during night-time at KRS in June and July 2007 showed an anomalously high concentration, higher ratios of ΔCH4/ΔCO2 compared with those in other years indicated that a considerably higher CH4 flux occurred relative to the CO2 flux. The daily CH4 flux calculated with the ratio of ΔCH4/ΔCO2 and terrestrial biosphere CO2 flux from an ecosystem model showed a maximum in July at the both sites. Although anomalously high flux was observed in June and July 2007 at KRS, only a small flux variation was observed at DEM. The high regional CH4 flux in June and July 2007 at KRS was reproduced using a process-based ecosystem model, Vegetation Integrative Simulator for Trace gases (VISIT), in response to high water table depth caused by the anomalously high precipitation during the summer of 2007.

Observation of localized centers with anomalous behavior in light-emitting heterostructures with multiple InGaN/GaN quantum wells

For the first time, using a complex of admittance spectroscopy, light-emitting heterostructures with InGaN/GaN multiple quantum wells were studied in a wide temperature range of 6–300 K. Three peaks are found in the conductance spectra; these peaks correspond to emission of charge carriers from the quantum wells and point defects distributed in the semiconductor bulk. Two low-temperature peaks possess an anomalous behavior, specifically, the peak with a low value of apparent activation energy (17 meV) is shifted to higher temperatures compared with the higher-energy peak (30 meV). The latter is attributed to a bulk defect having anomalously large capture cross section σn = 1.5 × 10−11 cm2.

Radiation defects in n-4H-SiC irradiated with 8-MeV protons

Capacitance methods and electron spin resonance (ESR) were applied to study deep centers in n-6H-SiC irradiated with 8 MeV protons. Schottky diodes and p-n structures grown by sublimation epitaxy or commercially produced by CREE Inc. (United States) were used. The type of the irradiation-induced centers is independent of the material fabrication technology and the kind of charged particles used. Irradiation results in an increase in the total concentration of donor centers. The possible structure of the centers is suggested on the basis of data on defect annealing and ESR.

High-efficiency (49%) and high-power photovoltaic cells based on gallium antimonide

High-efficiency GaSb-based photovoltaic cells designed for conversion of high-power laser radiation and infrared radiation of emitters heated by concentrated solar radiation are fabricated and studied. The maximum efficiency of conversion of the radiation with λ = 1680 nm was 49% at the photocurrent density of 50–100 A/cm2 for the fabricated photovoltaic cells. The methods for reducing the losses at ohmic contacts to p-and n-GaSb are investigated. The minimum values of the specific resistance, (1–3) × 10−6 Θ cm2, of contact to p-GaSb with the doping level of 1020 cm−3 were obtained using the Ti/Pt/Au contact system. The minimum values of the specific contact resistance were (1–3) × 10−6 Θ cm2 in the case of n-GaSb with the doping level of 2 × 1018 cm−3 if the Au(Ge)/Ni/Au and Au/Ni/Au contact systems are used.

Capacitance measurements for diodes in the case of strong dependence of the diode-base series resistance on the applied voltage

Epitaxial-diffused 6H-SiC diodes incorporating a high-resistivity interlayer in the base were studied; the resistance of this interlayer varied when the forward-bias voltage was applied. It is shown that, in spite of the absence of direct indications of the effects of the series resistance (the capacitance is independent of frequency and the value of capacitive cutoff voltage is small), the capacitance measurements for such structures may be incorrect.

6H-SiC epilayers as nuclear particle detectors

Schottky diodes based on the n-n+ 6H-SiC epilayers grown by sublimation epitaxy and also the layers produced by CREE company (USA) were used as detectors of α-particles of spontaneous decay. Since the thickness of n-layers was smaller than the range of the particles, geometrical parameters of the experiment differed from conventional ones; in the latter case, a particle is brought to rest in the region of electric field in the detector. The calculated and experimental data were compared to study the special features of transport of nonequilibrium charge under the conditions of complete and partial depletion of the structure. It is shown that characteristics of the material that govern the transport of charge carriers can be deduced from the analysis of the behavior of the signal amplitude and the shape of the pulse-height spectrum in relation to bias voltage applied to the Schottky diode. It follows from the results that the present-day sublimation-grown SiC layers are suitable for use as the basis for fabrication of nuclear particle detectors.

Perspectives of Use of Diagnostic Mirrors with Transparent Protection Layer in Burning Plasma Experiments

We evaluate using of metal mirrors over‐coated with transparent protection layer for the in‐vessel diagnostic systems in reactor‐grade fusion devices. Ideally, these should satisfy the contradictory demands of high reflectivity and small rate degradation when being bombarded by CX atoms. The serious threat to the performance of diagnostic mirrors is surface contamination with carbon‐based material eroded from carbon tiles. Via coupling the protective layer to a bulk mirror we can mitigate the deposit infiuence on the reflectance spectra. The regards are given to survivability in plasma environment of protected coated metallic mirrors.