Reducing Carbon Concentration Research Articles

Physical insight of thin AlGaN back barrier for millimeter-wave high voltage AlN/GaN on SiC HEMTs

In this work, physical mechanisms underlying carbon-doped buffer combined with an AlGaN back-barrier layer are investigated in state-of-the-art millimeter-wave AlN/GaN transistors. We have fabricated devices with and without the insertion of a thin AlGaN back-barrier layer with reduced carbon concentration to analyze the improvement resulting from this buffer architecture. More specifically, the impact of the Al mole fraction into the back-barrier, carbon doping in the buffer, and channel thickness on 100 nm gate length device performance has been studied. It appears that a 150 nm undoped GaN channel followed by a highly carbon-doped GaN buffer results in good electron confinement at the expense of a high current collapse. On the other hand, an Al mole fraction of 25% in the AlGaN back barrier layer coupled with a 150 nm undoped GaN channel provides excellent electron confinement, resulting not only in a low DIBL under high electric field but also low current collapse. Calibrated on experimental devices, TCAD simulations reveal that the electric field penetration inside the GaN buffer is prevented owing to a strong polarization from the back barrier when the Al-content is high enough. That is why, the electron confinement is superior for the 25% Al mole fraction in the back barrier along with reduced current collapse. As a result, careful engineering of the carbon concentration together with the undoped GaN channel thickness is crucial to achieve robust devices, which can, thus, deliver high device performance with superior voltage operation while using short gate lengths.

Novel application of aerobic granular biofilm systems for treating nitrate-polluted groundwater at low temperature: Microbial community and performance

An aerobic granular biofilm process was operated in a sequential batch reactor for treating nitrate-pollute groundwater at 6 ºC, which was fed with a progressively reduced carbon concentration to understand the nitrate removal efficacy in cold regions by denitrifying microorganisms. The evaluation of the performance reflected a quick granulation promoted by hydraulic stress and a high organic loading rate (OLR). The granules were dense and compact and kept stable conformation until the end of operation, despite the low temperature. Nitrate removal in the stage with high OLR was close to 100%, while the stage with the lowest OLR reached efficiencies higher than 87% in steady-state, suppose a successful advantage against other biological processes for treating drinking water. The bacterial community in the mature granules were colonized by cold-adapted microorganisms such as Holophagaceae and Candidatus Microthrix. Methanospirillum and Bathyarchaeota were the most abundant archaeal phylotypes, which showed a more stable consortium and Nectriaceae and Trichosporonaceae were the most dominant fungal phylotypes for all stages. The qPCR revealed that the crucial role was played by Fungi and Bacteria, while Archaea lost abundance in granular biomass, despite extreme conditions. Aerobic granular sludge is a robust biological technology able to remove nitrate in oligotrophic waters bodies.

Effect of low-temperature tempering on confined precipitation and mechanical properties of carburised steels

Surface-carburised steels are widely used in gears and bearings. The mechanical properties of the surface layers are crucial to improve contact fatigue and anti-wear performance. In this study, the effect of low-temperature tempering on confined precipitation and mechanical properties across surface layers in a high-alloy carburising steel (18Cr2Ni4W) were investigated. The steel was tempered at 240 °C with different tempering durations. Microstructure changes, including confined precipitation, and local mechanical properties were analysed layer-by-layer. During tempering, the morphology and size of martensite remained unchanged while confined precipitation occurred in martensitic matrix; coherent ε phase precipitates formed after 1.5 h, they then transformed to less coherent η phase within 3 h and subsequently evolved to non-coherent θ phase after 3 h of tempering. After tempering for 1.5 h, the strength and ductility of the surface layers reach a maximum value, and elongation of the surface layers with high carbon content is comparable to that of the core with low carbon content. It is revealed that the coherent ε precipitates are mainly responsible for the increase in strength; whilst ductility of the martensitic matrix increases due to a reduced carbon concentration, avoiding tempering brittleness. The results are valuable to the optimisation of surface properties of carburised steels using lower temperature tempering.

(Invited) Effect of Low Carbon Concentration on Bulk Lifetime of Silicon Crystal

Introduction Increasing the bulk lifetime of silicon crystals is crucial for improving insulated-gate bipolar transistor (IGBT) properties such as on-resistance. In order to achieve a longer bulk lifetime, carbon and oxygen impurities in silicon crystals should be reduced. This is because these impurities strongly affect the formation behavior of oxygen precipitates, which is a major cause of decrease in the bulk lifetime [1]. Floating-zone (FZ) silicon is mainly used for high-power devices because the oxygen concentration in FZ silicon is significantly lower than that in magnetic-field-induced Czochralski (MCZ) silicon. However, FZ silicon growth process encounters difficulties regarding mass productivity, because it is difficult to grow crystals with a diameter larger than 8 inches. In order to meet the growing demand for high-power IGBTs, high-quality MCZ silicon with large crystal diameter needs to be developed as a replacement of FZ silicon. We focused on the reduction of carbon impurities in MCZ silicon, which act as heterogeneous nucleation sites for oxygen precipitates. In our recent study, we demonstrated the growth of 8-inch-diameter MCZ silicon crystals with a carbon concentration less than 1.0E14 atoms/cm3, which is significantly lower than that in FZ silicon crystals [2]. In this study, the relationship between ultralow carbon concentration (<1.0E14 atoms/cm3) and the bulk lifetime was investigated in order to clarify the impact of carbon impurities on the bulk lifetime of MCZ silicon crystals. Experimental The bulk lifetime of MCZ silicon and FZ silicon crystals was measured by the direct-current photoconductive decay (PCD) method designated by ASTM F28-75[3] in order to avoid the influence of surface recombination. The carrier concentrations of these samples were approximately 1.0E12 atoms/cm3. To analyze the effect of carbon concentration on the bulk lifetime, MCZ crystals having a carbon concentration from 1.0E13 to 1.0E15 atoms/cm3, which was measured by photoluminescence (PL) spectroscopy [4], were used. Furthermore, to investigate the dependence of the oxygen concentration on the bulk lifetime, MCZ and FZ crystals with a wide range of oxygen concentration from 1.0E15 to 1.0E18 atoms/cm3were used. Results and Discussion The relationship between the carbon concentration and the bulk lifetime of FZ and MCZ silicon crystals is shown in Figure 1. The bulk lifetime of MCZ silicon dramatically increased with a decrease in the carbon concentration. This indicates that the nuclei formation of oxygen precipitates was suppressed during crystal growth with reduced carbon concentration. Furthermore, when the carbon concentration was less than 1.0E14 atoms/cm3, the dependence of the oxygen concentration on the bulk lifetime was negligible. Therefore, we demonstrated that MCZ silicon crystal with a carbon concentration of less than 1.0E14 atoms/cm3 exhibits a longer bulk lifetime than FZ silicon crystal. This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI) of Japan. References S. Kishino, Y. Matsushita, M. Kanamori and I. Iizuka, Jpn. J. Appl. Phys. 21 (1982) 1-12.Y. Nagai, K. Kashima, S. Nakagawa and M. Higasa, Solid State Phenomena 242 (2015) 3-9.ASTM Designation: F28-75(Reapproved 1981) 195.S. Nakagawa, K. Kashima, M. Tajima, Proceedings of the Forum on the Science and Technology of Silicon Materials 2010 (2010) 326. Figure 1

In situ excimer laser irradiation as cleaning tool for solid phase epitaxy of laser crystallized polycrystalline silicon thin films

This study demonstrates an innovative approach to clean a polycrystalline seed layer surface for solid phase epitaxy of amorphous silicon (a-Si). The excimer laser cleaning (ELC) makes use of in situ excimer laser irradiation during the first stages of a-Si deposition leading to melting of the as deposited a-Si and parts of the seed layer. The increased diffusion in liquid silicon allows for “smearing out” of surface contamination species. After liquid phase epitaxy is finished, further a-Si is deposited and crystallized by solid phase epitaxy. Thanks to the “smearing out,” the interface between the crystalline and amorphous silicon exhibits less contamination locally. SIMS measurements demonstrate a reduced carbon concentration at the seed layer surface after ELC. Numerical simulations, taking into account heat transfer and temperature dependent diffusion, support the reduction of carbon concentration. The simulations agree very well with experimental results. To get optimal solid phase growth conditions, the a-Si deposition temperature has to be below 200 °C. Above 200 °C deposition temperature, defective growth occurs resulting in poor crystallinity or even in nonepitaxial growth.

An experimental approach to compare carbon sequestration rate of four species of legume used as biological façade

Vegetation played an important role in reducing carbon concentration of the environment. Instead of planting horizontally, we attempted to look at planting leguminous plant vertically as biological facade. In tropical climate as in Malaysia, public sensitivity in vertical planting is mostly for aesthetic purposes. This research is focused on selecting vertically grown plant from the point of carbon uptake. We investigated the physiological performance especially the rate of C sequestration on potted plants of four legume species, which are Pisum sativum, Vigna unguiculata sesquipedalis, Psophocarpus tetrogonobulus and Phaseolus vulgaris. The research is carried out in an urban area of Penang Island (5°21’20.39”N, 100°17’30.03”E). Data were collected during non-rainy season where the temperature was about 32.5°C to 37.5°C. Three potted plants represented each species of legume. Seedlings were grown in shed with fewer than 50% daylight for the first month, thereafter transferred to open sunlight after maturation. The potted plants received direct evening sunlight from 1 pm to 5 pm. The photosynthetic assimilation rates were taken when the plants were four months old. Vigna unguiculata sesquipedalis (long yard bean) has recorded the highest average photosynthetic assimilation rate of 13.275 μmole m–2 s–1.

Effect of deposition condition and UV-ozone post-treatment on work function of DC magnetron sputtered AZO thin films

Highly transparent and conductive aluminum-doped zinc oxide (AZO) thin films were successfully deposited on glass by DC magnetron sputtering. The effects of Al-doping content (1–4 at.%), substrate temperature (250–350 °C) and ultraviolet (UV)-ozone treatment time (0–20 min) on AZO film work function were systematically investigated. It was revealed that the AZO work function increased with the decrease of Al content, while it was almost insensitive to the substrate temperature. Moreover, the UV-ozone treatment significantly improved AZO work function, while the post-treatment time had no effect on AZO work function. Two factors, [OZnAl]/([Zn] + 1.5[Al]) ratio and carbon contamination on film surface, affected the AZO work function. It was suggested that AZO films with high [OZnAl]/([Zn] + 1.5[Al]) ratio and reduced carbon concentration, possessed high work function.

Dependence of aluminum-doped zinc oxide work function on surface cleaning method as studied by ultraviolet and X-ray photoelectron spectroscopies

The dependence of aluminum-doped zinc oxide (AZO) work function on surface treatment, i.e. acetone solvent cleaning and ultraviolet (UV)-ozone cleaning, was studied by ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. Comparing different cleaning methods, UV-ozone treated AZO achieved relative higher work function of 4.26eV; whereas acetone solvent treated AZO had relative lower work function of 3.94eV. Two factors, stoichiometry ratio of [OZnAl]/[Zn]+1.5[Al] and carbon contamination on AZO surface, affected the work function and, moreover, the stoichiometry ratio was supposed to be the controlling factor. It was concluded that AZO with high stoichiometry ratio and reduced carbon concentration possessed high work function.

Intersubband transition at 1.55 μm in AlN/GaN multiple quantum wells by metal organic vapor phase epitaxy using the pulse injection method at 770 °C

Intersubband transition (ISBT) at 1.55 μm in AlN/GaN multi quantum wells (MQWs) was realized by metal organic vapor phase epitaxy (MOVPE) using the pulse injection (PI) method to grow GaN well layers at 770 °C. It was shown that a main factor for shifting ISBT wavelength to shorter region to cover 1.55 μm and improving ISBT properties of MQWs is the growth temperature of MQWs. Best structural and ISBT properties are observed at low growth temperature of 770 °C in this study. Carbon incorporation level in GaN layer grown by the PI method (PI-GaN) showed one order smaller value compared with that by the conventional continuous method. Moreover, further decrease in growth temperature to 770 °C did not show significant increase in carbon incorporation in PI-GaN layer. It clearly indicates that the PI method is very effective in reducing carbon concentration in GaN layer, especially at low temperature region. The low carbon concentration of 4×10 18 cm −3 released by the PI method was indispensable for realizing enough carrier concentration of 1.6×10 19 cm −3 to achieve strong ISBT at 1.55 μm.

Energy Security, Economic Development and Global Warming: Addressing Short and Long Term Challenges

Energy security, economic development and averting global warming are conflicting objectives in a fossil fuel economy. In the long run, sustainable development requires a shift to renewable energy sources. In the short run the climate change problem requires swift action2 and different strategies. There is no “silver bullet” and a combination of technologies and strategies will be required to meet the Intergovernmental Panel on Climate Change (IPCC) requirements.3 The article develops one example based on ‘negative carbon’ using air capture of carbon dioxide for storage into geological sites and solid materials. When driven by energy produced by carbon-neutral sources (such as Concentrated Solar Power or CSP) this approach can co-produce electricity while reducing carbon concentration in the atmosphere (Jones 2008, 2008b, Chichilnisky 2008, Chichilnisky and Eisenberger, 2009, Chichilnisky and Eisenberger et. al., 2009). While providing additional energy the process makes fossil power plants net carbon sinks, however changing entirely the relationship between the three problems: in the short run it advances energy security and economic development while averting climate change. In the long run, the approach accelerates the transition to renewable sources of energy and is compatible with sustainable development. The article addresses short and long run challenges with this capability in the context of the economic incentives provided by the carbon market of the UN Kyoto Protocol, whose functioning is analyzed. We explore the implications for extending Kyoto’s Clean Development Mechanism (CDM) in a way that benefits most the developing nations in Latin America and Africa, and the global transition from a fossil to a renewable economy.

Energy Security, Economic Development and Global Warming: Addressing Short and Long Term Challenges (Seguridad Energética, Desarrollo Económico Y Calentamiento Global: Desafíos De Corto y Largo Plazo) (Spanish)

Energy security, economic development and averting global warming are conflicting objectives in a fossil fuel economy. In the long run, sustainable development requires a shift to renewable energy sources. In the short run the climate change problem requires swift action and different strategies. There is no “silver bullet” and a combination of technologies and strategies will be required to meet the Intergovernmental Panel on Climate Change (IPCC) requirements. The article develops one example based on ‘negative carbon’ using air capture of carbon dioxide for storage into geological sites and solid materials. When driven by energy produced by carbon-neutral sources (such as Concentrated Solar Power or CSP) this approach can co-produce electricity while reducing carbon concentration in the atmosphere (Jones 2008, 2008b, Chichilnisky 2008, Chichilnisky and Eisenberger, 2009, Chichilnisky and Eisenberger et. al., 2009). While providing additional energy the process makes fossil power plants net carbon sinks, however changing entirely the relationship between the three problems: in the short run it advances energy security and economic development while averting climate change. In the long run, the approach accelerates the transition to renewable sources of energy and is compatible with sustainable development. The article addresses short and long run challenges with this capability in the context of the economic incentives provided by the carbon market of the UN Kyoto Protocol, whose functioning is analyzed. We explore the implications for extending Kyoto’s Clean Development Mechanism (CDM) in a way that benefits most the developing nations in Latin America and Africa, and the global transition from a fossil to a renewable economy.

Energy security, economic development and global warming: addressing short and long term challenges

Energy security, economic development and averting global warming are conflicting objectives in a fossil fuel economy. In the long run, sustainable development requires a shift to renewable energy sources. In the short run it requires swift action (IPCC) and different strategies 1 . The article analyses a negative carbon process to co-produce electricity while reducing carbon concentration in the atmosphere (Jones, 2008, 2009; Chichilnisky, 2008b; Chichilnisky and Eisenberger, 2009; Eisenberger et al., 2009). While providing additional energy the process makes fossil power plants net carbon sinks. The article addresses short and long run challenges with this capability in the context of the economic incentives provided by the carbon market of the UN Kyoto Protocol, created by one of the authors in 1997 (Chichilnisky, 1993, 1996; Chichilnisky and Heal, 1994, 1995; Chichilnisky and Sheeran, 2009; Pagnamenta, 2009). We propose extending Kyoto's clean development mechanism (CDM) in a way that benefits Latin America and Africa (Chichilnisky and Heal, 1999; Chichilnisky, 1996), and analyse the global transition from a fossil to a renewable economy.

Alloy design of creep resistant 9Cr steel using a dispersion of nano-sized carbonitrides

In order to improve creep strength of tempered martensitic 9Cr steel at 650 °C, alloy design for a dispersion of fine and thermally stable precipitate particles has been investigated for 9Cr–3W–3Co–VNb steel and 9Cr–2W–VNbTi steel. Creep tests were carried out at 650 °C for up to about 10 000 h. By reducing carbon concentration below 0.02%, a dispersion of nano-size MX nitrides of vanadium and niobium along boundaries as well as in the matrix is achieved in 9Cr–3W–3Co–VNb steel with 0.05% nitrogen. This gives rise to excellent creep strength at 650 °C, as shown by much higher creep rupture strength than conventional 9Cr steels P92 and T91. Excess addition of nitrogen, 0.07 and 0.1%, promotes the coarsening of MX nitrides and the formation of Z-phase during creep. A dispersion of nano-size TiC carbides is also achieved in 9Cr–2W–VNb steel containing 0.05% Ti by high-temperature normalizing at 1300 °C followed by tempering. This effectively decreases minimum creep rate and improves creep life.

Serrated yielding in 9Cr–1Mo ferritic steel

Tensile tests were performed on specimens in quenched and tempered and thermally aged conditions over a wide temperature range (300–873 K) to assess the occurrence of serrated flow, a manifestation of dynamic strain aging (DSA), in 9Cr–1Mo ferritic steel, with an emphasis on the influence of prior thermal aging on serrated yielding. The alloy exhibited jerky/serrated flow in the load–elongation curves at intermediate temperatures. Types A, B, and C serrations were observed, depending on the test temperature and applied strain rate. The apparent activation energy of 83 kJ mol-1 measured for serrated flow suggests that diffusion of an interstitial solute such as carbon is responsible for dynamic strain aging in 9Cr–1Mo steel. Prior thermal aging at 793 K for 5000 h and at 873 K for 1000 and 5000 h resulted in a significant decrease in the height of serrations, i.e. the magnitude of the stress drop, as well as an increase in the critical strain for the onset of serrations. Both of these observations indicate reduced propensity to DSA as a result of increased precipitate sinks as well as a reduced carbon concentration in solid solution owing to an increased density of carbides in the thermally aged conditions. Reduced propensity to DSA resulted in a significant reduction in the strength values at intermediate temperatures.

Analysis of coccal cell formation by Campylobacter jejuni using continuous culture techniques, and the importance of oxidative stress

Campylobacter jejuni is an important human gastrointestinal pathogen. In hostile environments it may adapt its physiology to prolong survival, potentially including the adoption of a viable, non-culturable form and a change to coccal cell morphology. By independently controlling the individual parameters of continuous cultures of Camp. jejuni (e.g. pH, nutrient limitation, growth rate, etc.), coccal cell formation was shown to be elicited only be high oxygen tension in conjunction with reduced carbon concentration. Electron microscopy revealed degradative changes in these cells. This occurred as a transient response over 48 h coincident with a large reduction in maximum growth rate and viable count. Kinetic analysis of the biomass reduction of the cultures demonstrated that significant underlying growth was maintained, with the subsequent selection of a more oxygen-resistant population of cells and reversion to spiral morphology. Coccal cells appear to be predominantly a degenerate form of Camp. jejuni resulting from oxidative damage. While some of these coccal cells may recover, the more interesting population of cells is probably that which retains, or regains, spiral morphology during adaptation to oxidative stress.

Growth control of GaAs using short-pulse supersonic beam epitaxy

We report the growth behavior and doping properties of GaAs short-pulse supersonic beam epitaxy (SSBE). Various GaAs films have been grown by injecting short-pulse supersonic beams of different pulsewidths generated from a pure gas of trimethylgallium (TMG) and a mixed gas of 1% TMG + 99% H 2 maintained respectively at a source pressure of 40 and 400 Torr. A growth rate of 1 10 monolayer per pulse to just one monolayer per pulse (pulse controlled layer-by-layer growth) was readily achieved by adjusting the pulsewidth of the supersonic beam. It is emphasized that the pulse controlled layer-by-layer growth achieved by the SSBE method provides a growth rate of 1 μm/h which is much higher than that obtained by conventional ALE (atomic layer epitaxy). The incorporated carbon concentration and the corresponding hole concentration in the grown films decrease when pure TMG was used from 1.6 × 10 20 to 3.3 × 10 18cm −3 with decreasing TMG pulsewidth from 100 to 5 ms. This indicates that the impurity concentration, and therefore the electrical properties of films grown by SSBE can be easily controlled by simply adjusting the pulsewidth of the supersonic beam of the source materials. Moreover, a dramatically reduced carbon concentration down to 6.5 × 10 17cm −3 (which is two orders of magnitude lower than that obtained by conventional CBE using TMG) and a significantly improved hole mobility were achieved when the TMG molecules were accelerated to a high kinetic energy of 3.9 eV by producing supersonic beams from a mixed gas of 1% TMG + 99% H 2. These results indicate that short-pulse supersonic beam epitaxy is a very promising technique to achieve highly controlled growth properties.

Mechanical stength of Czochralski silicon crystals with carbon concentrations from 1014 to 1016 cm−3

Using Czochralski-grown silicon crystals with carbon concentrations from 1014 to 1016 cm−3, we studied the influence of oxygen precipitates on the mechanical strength of silicon crystals. Reducing carbon concentration from 1016 to 1014 cm−3 suppressed oxygen precipitation and thus, improved the strength. Carbon concentration reduction from 1015 to 1014 cm−3 did not, however, decrease oxygen precipitation as dramatically as reduction from 1016 to 1015 cm−3. Carbon impurities below 1014 cm−3 no longer affect oxygen precipitation. Therefore, we conclude that reducing carbon concentration below 1014 cm−3 no longer improves the mechanical strength of silicon crystals.

Effects of Volume Fraction and Stability of Retained Austenite on Ductility of TRIP-aided Dual-phase Steels.

The effects of silicon and manganese contents on volume fraction and stability of retained austenite particles in 0.2C-(1.0-2.5)Si-(1.0-2.5)Mn (mass%) TRIP-aided dual-phase steels were investigated. In addition, the relationships between above retained austenite parameters and ductility at room and moderate temperatures were discussed through studies on strain-induced transformation behavior of retained austenite.As increasing the silicon and manganese contents except for 2.5 mass% manganese steel, the initial volume fraction of retained austenite increased with accompanied by reducing carbon concentration in retained austenite. It was found that the ductilities of these steels became maximum at a given temperature between 23 and 175°C, i.e., a peak temperature. The peak temperature was concluded to agree well with the temperature at which the strain-induced transformation of retained austenite was suppressed moderately for each steel. Moreover, the peak temperature Tp (°C) was related to estimated martensite-start temperature Ms (°C) of the retained austenite as Tp=3.04Ms+187. Strength-ductility balance, i.e., the product of tensile strength and total elongation, at the peak temperature linearly increased with an increase in the initial volume fraction of retained austenite.

Solution growth, structure and composition of boron carbide crystals

Boron carbide crystals, with dimensions up to several millimeters on a side and typically twinned, have been prepared by slow cooling in molten copper, palladium and platinum solutions. Despite the wide single-phase region of boron carbides prepared by solid state reactions, the lattice parameters and Raman spectra of these crystals indicate that their composition is always near 20 at.% C (“B 4C”), independent of B:C ratio in solution or the solvent employed. Thermodynamic analysis shows that the relative instability of pure β-rhombohedral boron allows the formation of low carbon boron carbides prepared by solid state techniques. However, the reduced chemical potential of boron in these metal solutions limits the composition of the solution-grown boron carbide crystals to near B 4C. X-ray structural refinements of these B 4C crystals are consistent with a model of B 11C icosahedra and CBC chains. The lattice constants, bond lengths and thermal parameters of the solution-grown B 4C crystals show remarkably little variation between samples. Crystals utilized in a previous study have larger unit cells, suggesting a reduced carbon concentration of 15–16 at.%, and thermal factors consistent with expectations of a mixture of CBB and CBC chains at this composition.