Si Flux Research Articles

Twin-free thermal laser epitaxy of Si on sapphire

The heteroepitaxial growth of silicon (Si) is essential for modern electronics. Our study investigates the potential of thermal laser epitaxy (TLE) for Si epitaxy. A systematic study on the evaporation behavior of Si during TLE identifies and addresses the causes of notable flux rate fluctuations, resulting in a Si flux with ±0.3% stability over time. We also demonstrate heteroepitaxy of Si on c-plane sapphire substrates via TLE. High-temperature substrate preparation combined with deposition at a substrate temperature of 1000 °C produced high-quality epitaxial Si (111) films without twin domains.

What drives the heterogeneous interdiffusion in the Li-Si interfacial region of Si anodes: the Li flux or the Si flux?

The electrochemical reaction in silicon (Si) electrode, accompanying with tremendous volume expansion, causes rapid capacity fade of Li-ion batteries. The Li-ion concentration gradient and structural distribution uniformity influence the inhomogeneous expansion, and the kinetic mechanism of lithiation and interfacial morphology evolvement remains debated. The present study focuses on the dynamics of Li-Si interdiffusion at Si/Li interfaces with various Si-facet orientations and phases using a machine-learning potential. We find that the Si flux from bulk Si to Li-Si interface regions controls the length of Li-Si interdiffusion region. The lithiation length in different Si/Li interface systems exhibits the order of amorphous-Si > crystalline-Si(110) > crystalline-Si(100) > crystalline-Si(111), which agrees with the experimental trend. Our atomic simulations further reveal that the key factor determining the Li-Si interdiffusion is the difference of on-site Si atomic energies between the bulk Si and the Li-Si interface regions. We propose that the large interdiffusion extent is due to a low thermodynamics barrier. Our findings provide insights for the development of high-performance Si anode materials.

Volcanic and climatic impacts on silicon abundance in shale: Implications for the expansion of Permo-Carboniferous terrestrial plants in North China

Silicon (Si) is known as an important regulator of the global carbon cycle via its effect on silicate weathering in continents and phytoplankton productivity in the oceans. It is also beneficial to plants by improving their resistance to biotic and abiotic stresses. Detailed analysis of geological record of Si in sediments underpins our understanding of ecosystem evolution over geological time. Here, we unravel the mystery of low Si content in Permo-Carboniferous shales from the central North China and link the terrestrial Si fluxes to the thriving forest ecosystems at that time. Provenance fluctuations as indicated by Zr/TiO2 are well explained by episodic volcanic input which are identified by elevated Zr/TiO2 and lowered V/Al2O3 and Cr/Al2O3. CIA and Ga/Rb and K2O/Al2O3 ratios imply cyclic changes of climate in an overall warm and humid condition. The covariation between the volcanic indicators and climatic proxies indicates that the volcanism forced the climate cycles. Despite the felsic sources (high Zr/TiO2 and Th/Sc ratios and low Zr/Sc ratios), the Si-poor shales are universal in Permo-Carboniferous across the North China Block. The enhanced continental weathering of mineral silicates and the hydrolysis of volcanic materials released silica into the environment and led to the eventual deposition of Si-poor shales. The shales lacking in Si tend to have larger TOC contents except for the most distal shales. It implies that the sufficient supply of dissolved-Si in environments may benefit the expansion of terrestrial plants in Permo-Carboniferous and therefore the accumulation of organic matter in the shales. This work provides new insight into feedback linking volcanism, climates, terrestrial dissolved silica flux, and terrestrial ecosystems in Permo-Carboniferous North China Block.

Alkaline mineral addition to anoxic to hypoxic Baltic Sea sediments as a potentially efficient CO2-removal technique

Recent studies have begun to explore the potential of enhanced benthic weathering (EBW) in the Baltic Sea as a measure for climate change mitigation. To augment the understanding of EBW under seasonally changing conditions, this study aims to investigate weathering processes under anoxia to hypoxia in corrosive bottom waters, which reflect late summer conditions in the Baltic Sea. Dunite and calcite were added to sediment cores retrieved from Eckernförde Bay (Western Baltic Sea) with a constant flow-through of deoxygenated, CO2-enriched Baltic Sea bottom water. The addition of both materials increased benthic alkalinity release by 2.94 μmol cm−2 d−1 (calcite) and 1.12 μmol cm−2 d−1 (dunite), compared to the unamended control experiment. These excess fluxes are significantly higher than those obtained under winter conditions. The comparison with bottom water oxygen concentrations emphasizes that highest fluxes of alkalinity were associated with anoxic phases of the experiment. An increase in Ca and Si fluxes showed that the enhanced alkalinity fluxes could be attributed to calcite and dunite weathering. First order rate constants calculated based on these data were close to rates published in previous studies conducted under different conditions. This highlights the suitability of these proxies for mineral dissolution and justifies the use of these rate constants in modeling studies investigating EBW in the Baltic Sea and areas with similar chemical conditions. Generally stable pH profiles over the course of the experiment, together with the fact that the added minerals remained on the sediment surface, suggest that corrosive bottom waters were the main driving factor for the dissolution of the added minerals. These factors have important implications for the choice of mineral and timing for EBW as a possible marine carbon dioxide removal method in seasonally hypoxic to anoxic regions of the Baltic Sea.

The Role of Coastal Yedoma Deposits and Continental Shelf Sediments in the Arctic Ocean Silicon Cycle

AbstractThe availability of silicon (Si) in the ocean plays an important role in regulating biogeochemical and ecological processes. The Si budget of the Arctic Ocean appears balanced, with inputs equivalent to outputs, though it is unclear how a changing climate might aggravate this balance. In this study, we focus on Si cycling in Arctic coastal areas and continental shelf sediments to better constrain the Arctic Ocean Si budget. We provide the first estimate of amorphous Si (ASi) loading from erosion of coastal Yedoma deposits (30–90 Gmol yr−1), demonstrating comparable rates to particulate Si loading from rivers (10–90 Gmol yr−1). We found a positive relationship between surface sediment ASi and organic matter content on continental shelves. Combining these values with published Arctic shelf sediment properties and burial rates we estimate 70 Gmol Si yr−1 is buried on Arctic continental shelves, equivalent to 4.5% of all Si inputs to the Arctic Ocean. Sediment dissolved Si fluxes increased with distance from river mouths along cruise transects of shelf regions influenced by major rivers in the Laptev and East Siberian seas. On an annual basis, we estimate that Arctic shelf sediments recycle approximately up to twice as much DSi (680 Gmol Si) as is loaded from rivers (340–500 Gmol Si).

Benthic contribution to seasonal silica budgets in two macrotidal estuaries in North-Western France

The paper aims to build seasonal silica budgets in two macrotidal estuaries, the Elorn and Aulne estuaries of the Bay of Brest (North-Western France), based on modeling and measurements, in order to increase our understanding of the silica (Si) cycle at land-sea interfaces. A diagenetic model was developed to quantify benthic Si fluxes, e.g. aSiO2 deposition fluxes that are difficult to assess through direct measurements. Sediment cores were also seasonally sampled at six stations to provide data essential to parametrize and validate the model. Vertical profiles of porosity, burrowing depth, biodiffusive coefficients, concentrations of amorphous silica (aSiO2) and silicic acid (Si(OH)4 and the proportion of reactive aSiO2 were measured. The results show that sites sampled along the Elorn and Aulne estuaries constitute significant net Si deposition areas (1-4.5 mmol Si m-2 d-1), particularly in the upstream during winter and in midstream and downstream during summer. Year round, reprecipitation is negligible (< 3%) while burial accounts for the retention of ~ 30-80% of deposited aSiO2. In winter, burial dominates the benthic Si budget. As surface-integrated benthic Si fluxes are low compared to riverine aSiO2 fluxes, the Si export to coastal waters is high (93%) during winter. In contrast, in summer, burial accounts for 38% of river Si fluxes, and Si(OH)4 flux from the sediment is high as a result of enhanced benthic recycling and bioirrigation. Internal estuarine processes, e.g., benthic and pelagic primary production, dissolution and benthic Si fluxes, surpass river fluxes in magnitude during summer. Overall, we conclude that the Elorn and Aulne macrotidal estuaries are efficient filters of Si, retaining about 4-38% of river Si fluxes, and even 6-67% when accounting for retention in intertidal marshes, but with massive exports occurring during winter floods.

Long‐Term Changes in Concentration and Yield of Riverine Dissolved Silicon From the Poles to the Tropics

AbstractRiverine exports of silicon (Si) influence global carbon cycling through the growth of marine diatoms, which account for ∼25% of global primary production. Climate change will likely alter river Si exports in biome‐specific ways due to interacting shifts in chemical weathering rates, hydrologic connectivity, and metabolic processes in aquatic and terrestrial systems. Nonetheless, factors driving long‐term changes in Si exports remain unexplored at local, regional, and global scales. We evaluated how concentrations and yields of dissolved Si (DSi) changed over the last several decades of rapid climate warming using long‐term data sets from 60 rivers and streams spanning the globe (e.g., Antarctic, tropical, temperate, boreal, alpine, Arctic systems). We show that widespread changes in river DSi concentration and yield have occurred, with the most substantial shifts occurring in alpine and polar regions. The magnitude and direction of trends varied within and among biomes, were most strongly associated with differences in land cover, and were often independent of changes in river discharge. These findings indicate that there are likely diverse mechanisms driving change in river Si biogeochemistry that span the land‐water interface, which may include glacial melt, changes in terrestrial vegetation, and river productivity. Finally, trends were often stronger in months outside of the growing season, particularly in temperate and boreal systems, demonstrating a potentially important role of shifting seasonality for the flux of Si from rivers. Our results have implications for the timing and magnitude of silica processing in rivers and its delivery to global oceans.

Interannual variability (2000–2013) of mesopelagic and bathypelagic particle fluxes in relation to variable sea ice cover in the eastern Fram Strait

The Fram Strait connects the Atlantic and Arctic Oceans and is a key conduit for sea ice advected southward by the Transpolar Drift and northward inflow of warm Atlantic Waters. Continued sea ice decline and “Atlantification” are expected to influence pelagic–benthic coupling in the Fram Strait and Arctic as a whole. However, interannual variability and the impact of changing ice conditions on deepwater particle fluxes in the Arctic remain poorly characterized. Here, we present long-term sediment trap records (2000–2013) from mesopelagic (200 m) and bathypelagic (2,300 m) depths at two locations (HGIV and HGN) in the Fram Strait subjected to variable ice conditions. Sediment trap catchment areas were estimated and combined with remote sensing data and a high-resolution model to determine the ice cover, chlorophyll concentration, and prevailing stratification regimes. Surface chlorophyll increased between 2000 and 2013, but there was no corresponding increase in POC flux, suggesting a shift in the efficiency of the biological carbon pump. A decrease in particulate biogenic Si flux, %opal, Si:POC, and Si:PIC at mesopelagic depths indicates a shift away from diatom-dominated export as a feasible explanation. Biogenic components accounted for 72% ± 16% of mass flux at 200 m, but were reduced to 34% ± 11% at 2,300 m, substituted by a residual (lithogenic) material. Total mass fluxes of biogenic components, including POC, were higher in the bathypelagic. Biomarkers and ∂13C values suggest both lateral advection and ice-rafted material contribute to benthic carbon input, although constraining their precise contribution remains challenging. The decadal time series was used to describe two end-members of catchment area conditions representing the maximum temperatures of Atlantic inflow water in 2005 at HGIV and high ice coverage and a meltwater stratification regime at HGN in 2007. Despite similar chlorophyll concentrations, bathypelagic POC flux, Si flux, Si:POC, and Si:PIC were higher and POC:PIC was lower in the high-ice/meltwater regime. Our findings suggest that ice concentration and associated meltwater regimes cause higher diatom flux. It is possible this will increase in the future Arctic as meltwater regimes increase, but it is likely to be a transient feature that will disappear when no ice remains.

Decoupling of Barium and Silicon at the Congo River‐Dominated Southeast Atlantic Margin: Insights From Combined Barium and Silicon Isotopes

AbstractThe correlation between concentrations of dissolved barium (dBa) and silicon (dSi) in the modern ocean supports the use of Ba as a paleoceanographic proxy. However, the mechanisms behind their linkage and the exact processes controlling oceanic Ba cycling remain enigmatic. To discern the extent to which this association arises from biogeochemical processes versus physical mixing, we examine the behavior of Ba and Si at the Congo River‐dominated Southeast Atlantic margin where active biological processes and large boundary inputs override the large‐scale ocean circulation. Here we present the first combined measurements of dissolved stable Ba (δ138Ba) and Si (δ30Si) isotopes as well as Ba and Si fluxes estimated based on 228Ra from the Congo River mouth to the northern Angola Basin. In the surface waters, river‐borne particle desorption or dissolution and shelf inputs lead to non‐conservative additions of both dBa and dSi to the Congo‐shelf‐zone, with the Ba flux increasing more strongly than that of Si across the shelf. In the epipelagic and mesopelagic layers, Ba and Si are decoupled likely due to different depths of in situ barite precipitation and biogenic silica production. In the deep waters of the northern Angola Basin, we observe large enrichment of dBa, likely originating from high benthic inputs from the Congo deep‐sea fan sediments. Our results reveal different mechanisms controlling the biogeochemical cycling of Ba and Si and highlight a strong margin influence on marine Ba cycling. Their close association across the global ocean must therefore mainly be a consequence of the large‐scale ocean circulation.

Silicon pools, fluxes and the potential benefits of a silicon soil amendment in a nitrogen-enriched tidal marsh restoration

Tidal marshes are important sites of silicon (Si) transformation, where dissolved Si (DSi) taken up by macrophytic vegetation and algal species is converted to biogenic silica (BSi), which can accumulate in the soil, be recycled within the marsh, or be exported to adjacent coastal waters. The role of restored and created tidal marshes in these processes is not well understood, nor is the impact of nutrient enrichment at either the plant or ecosystem level. Here, Si fluxes were examined to develop a Si mass balance in a nitrogen (N)-enriched marsh created with fine-grained dredged material from the Chesapeake Bay, United States. In addition, the effectiveness of Si soil amendments to ameliorate the negative effects of excess nitrogen on Spartina alterniflora was examined through laboratory and field experiments. Silicon was exported to the estuary as DSi (49 g m−2 y−1) and BSi (35 g m−2y−1) in stoichiometric excess of nitrogen and phosphorus. Rapid recycling of Si within both marsh and the tidal creeks appeared to be important in the transformation of Si and export from the marsh. Enhanced macrophyte SiO2 tissue concentrations were observed in the field experiment, with end-of-season mean values of 2.20–2.69% SiO2 in controls and 2.49–3.24% SiO2 in amended plots, among the highest reported for S. alterniflora; however, improved plant fitness was not detected in either experiment. Thus, tidal marshes created with a fine-grained, N-rich dredged material appear to function as a rich source of Si to the restored marsh and local estuarine environment, an overlooked ecosystem service. Soil Si amendments, however, did not appear likely to alleviate N-induced stress in S. alterniflora.

Goethite affects phytolith dissolution through clay particle aggregation and pH regulation

Formed in plant tissues as fine silica particles, phytoliths are deposited within plant debris in soils where they can dissolve and feed silicon (Si) fluxes to the biosphere and hydrosphere. Yet, soil phytoliths can be protected from dissolution by entrapment in aggregates, and thus be withdrawn from the global silica cycle. Among clay-sized minerals interacting in stable aggregates, goethite and kaolinite are ubiquitous in soils. We analyzed the impact of goethite-mediated aggregation on the release of aqueous Si from phytoliths entrapped in microaggregates containing organic matter and kaolinite, with quartz and goethite at variable concentrations, simulating a soil sequence with increasing contents of clay and iron oxide, as it can commonly occur in terrestrial ecosystems. The microaggregates stored sizable amount of organic carbon, the release of which decreased with increasing goethite concentration. Aqueous Si was assessed through a kinetic extraction with dilute CaCl2, while pH, aluminum (Al) and germanium (Ge) concentrations were measured in the CaCl2 extracts. Our experimental data showed that phytoliths were the source of aqueous Si. They also showed that the process of phytolith dissolution prevailed over Si adsorption on goethite. As mediated by goethite, aggregation protected phytoliths from dissolution at goethite concentrations above 20 g kg−1. Increasing goethite concentration enhanced aggregation on the one hand, but increased pH from 5.5 to 7.5 on the other. Thus, while aggregation significantly reduced the release of aqueous Si by 1.7- to 3-fold at a given pH, the increase in pH enhanced it. Overall, at common soil solution pH (5–6), aggregation reduced Si release by 2–3 times. Thus, preservation of phytoliths in aggregates can be widespread in well-aerated soils and more effective than Si adsorption on secondary oxides in retaining Si.

Biogenic silica cycling in the Skagerrak

Dissolved silicate (H4SiO4) is essential for the formation of the opaline skeletal structures of diatoms and other siliceous plankton. A fraction of particulate biogenic silica (bSi) formed in surface waters sinks to the seabed, where it either dissolves and returns to the water column or is permanently buried. Global silica budgets are still poorly constrained since data on benthic bSi cycling are lacking, especially on continental margins. This study describes benthic bSi cycling in the Skagerrak, a sedimentary depocenter for particles from the North Sea. Biogenic silica burial fluxes, benthic H4SiO4 fluxes to the water column and bSi burial efficiencies are reported for nine stations by evaluating data from in-situ benthic landers and sediment cores with a diagenetic reaction-transport model. The model simulates bSi contents and H4SiO4 concentrations at all sites using a novel power law to describe bSi dissolution kinetics with a small number of adjustable parameters. Our results show that, on average, 1100 mmol m-2 yr-1 of bSi rains down to the Skagerrak basin seafloor, of which 50% is released back to overlying waters, with the remainder being buried. Biogenic silica cycling in the Skagerrak is generally consistent with previously reported global trends, showing higher Si fluxes and burial efficiencies than deep-sea sites and similar values compared to other continental margins. A significant finding of this work is a molar bSi-to-organic carbon burial ratio of 0.22 in Skagerrak sediments, which is distinctively lower compared to other continental margins. We suggest that the continuous dissolution of bSi in suspended sediments transported over long distances from the North Sea leads to the apparent decoupling between bSi and organic carbon in Skagerrak sediments.

Benthic Dissolved Silicon and Iron Cycling at Glaciated Patagonian Fjord Heads.

Glacier meltwater supplies silicon (Si) and iron (Fe) sourced from weathered bedrock to downstream ecosystems. However, the extent to which these nutrients reach the ocean is regulated by the nature of the benthic cycling of dissolved Si and Fe within fjord systems, given the rapid deposition of reactive particulate fractions at fjord heads. Here, we examine the benthic cycling of the two nutrients at four Patagonian fjord heads through geochemical analyses of sediment pore waters, including Si and Fe isotopes (δ30Si and δ56Fe), and reaction-transport modeling for Si. A high diffusive flux of dissolved Fe from the fjord sediments (up to 0.02mmolm-2day-1) compared to open ocean sediments (typically <0.001mmolm-2day-1) is supported by both reductive and non-reductive dissolution of glacially-sourced reactive Fe phases, as reflected by the range of pore water δ56Fe (-2.7 to +0.8‰). In contrast, the diffusive flux of dissolved Si from the fjord sediments (0.02-0.05mmolm-2day-1) is relatively low (typical ocean values are >0.1mmolm-2day-1). High pore water δ30Si (up to +3.3‰) observed near the Fe(II)-Fe(III) redox boundary is likely associated with the removal of dissolved Si by Fe(III) mineral phases, which, together with high sedimentation rates, contribute to the low diffusive flux of Si at the sampled sites. Our results suggest that early diagenesis promotes the release of dissolved Fe, yet suppresses the release of dissolved Si at glaciated fjord heads, which has significant implications for understanding the downstream transport of these nutrients along fjord systems.

Atmospheric input of silicon to the China adjacent seas: Non-negligible contributions from anthropogenic sources

Atmospheric deposition is an important source of exogenous Si in the oceans. As a typical crustal element, Si in the atmosphere emitted from anthropogenic sources is ignored. In this study, the atmospheric dry deposition of anthropogenic Si to China adjacent seas was calculated using WRF–CMAQ in January and July 2019 to investigate the contribution of anthropogenic Si to the oceans. Si emitted from 13 anthropogenic sources in China, the Korean Peninsula, Japan, and marine ships was considered. Emissions of anthropogenic Si in January and July 2019 were 30.2 and 22.0 Gg, respectively. The highest Si emissions were concentrated over eastern China, e.g. Beijing–Tianjin–Hebei region, Shandong province, Yangtze river delta area (0.2–21.3 ng m−2 s−1), while the lowest emissions were in northwestern China (< 5.2 ng m−2 s−1). Among the Bohai (BS), Yellow (YS), and East China seas (ECS), dry deposition fluxes over the southern YS were highest (4.6–16.8 μg m−2 d−1), and those over the ECS were lowest (0.2–7.7 μg m−2 d−1). During pollution episodes, the outflow of polluted air masses from the continent caused a 10-fold increase in Si deposition compared with clear days. The relative contribution of continental anthropogenic emissions and ship combustion varied significantly in two seasons. In winter, deposition from continental anthropogenic emissions to total anthropogenic Si deposition was higher than 96 %. While in summer, the contributions from ship combustion increased obviously, accounting for 10–38 %. Deposition flux of dissolved Si from anthropogenic sources over China adjacent seas was about 4–38 % of that of dissolved mineral dust Si. The annual Si depositions from atmospheric anthropogenic sources to the Si fluxes from rivers to the China adjacent seas were 0.03 %–2.8 %. The marine primary productivity in the BS, YS, and ECS caused by atmospheric anthropogenic dissolved Si deposition were 1.3, 1.2, and 0.7 mg C m−2 a−1, respectively.

Substitutional Carbon Incorporation in SiGeC/Si Heterostructures: Influence of Silicon Precursors

The incorporation of germanium and carbon by RP-CVD epitaxy in silicon brings lots of interest in silicon devices thanks to the variety of properties that can be addressed: band gap or lattice parameter engineering, dopant diffusion reduction, chemical properties, optical properties...For all the applications requiring SiGeC materials, carbon atoms must be incorporated in fully substitutional site (Cs) despite its low bulk solubility into the Si and SiGe lattice (3x1017 at.cm-3 at the Si melting point). Beyond a total carbon concentration (depending on process parameter), carbon atoms are also incorporated into interstitial sites (Ci). These Ci atoms usually form extended defects such as clusters or SiC precipitates which are harmful for devices performances.Solubility limits of carbon can be extended into the metastable domain by optimizing the epitaxy process. Indeed, it is well established that low temperature and high growth rate are favorable to obtain high Cs atoms incorporation [1].This work compares silane (SiH4) and disilane (Si2H6) precursors coupled with germane (GeH4) and methylsilane (SiH3CH3 or MS) to process low temperature thin film of SiGeC with the highest amount of Cs without Ci.First, the SiGe growth kinetics using silane and disilane have been studied. The temperature range investigated varied from 500 °C to 600 °C at low pressure. Using similar atomic fluxes of Si, and with all the other parameters constant, a higher growth rate and lower germanium content has been observed with disilane compared to silane. For instance, there is a factor of 1.85 at 550°C. The germanium concentration was varying from 30.4 to 22.3 % and from 19.4 to 15.7 % for silane and disilane, respectively. To have a better understanding about this kinetics difference, an impoverishment rate or theoretical yield, of the gas phase, in reactive species has been calculated. The hypothesis is that the impoverishment rate is linked to the reactivity of a molecule, and thus related to the sticking probability of a molecule onto a same surface [2].The impoverishment rate is defined as the ratio between the number of moles of Si (or Ge) deposited per minute (nDeposited) and the number of moles of silicon precursor (or germanium) injected per minute(nInjected) in the epitaxy reactor: αPrecursor = nDeposited/nInjected. The impoverishment rate of the disilane molecule increases much more rapidly than the silane molecule as the temperature increases (Fig.1a). Disilane is more consumed than silane which is relevant of the higher reactivity of disilane compared to silane.Moreover, the impoverishment of germane increases more rapidly with disilane than with silane (Fig.1b). The GeH4 contribution to epitaxy is more significant using disilane than silane. The impoverishment rate of precursors according to the germane flow will be presented later.Then, silane and disilane precursors have been compared with respect to the C incorporation at 550 °C. Ge concentration was fixed to 20 % with both silicon precursors. This leads to a growth rate of SiGe equal to 2.9 and 14.1 nm.min-1 using silane and disilane, respectively. XRD has been used to gain access to the “apparent” Ge concentration in Si1-x-yGexCy layers (Fig.2(a), (b)). Carbon atoms, much smaller than silicon and germanium, compensate the compressive strain induced by germanium in the SiGe layers. It then yields smaller Ge concentration in XRD than the real Ge concentration. The carbon in substitutional position can therefore be extracted using this coefficient: 1 % of carbon compensate 12 % of Ge [3].Using disilane, the substitutional carbon concentration linearly increases with the MS flow over the whole studied range (Fig.3). However, using silane, a deviation from the linearity is observed (above 0.7 %). In the linearity part of the curves, all the carbon incorporated are in substitutional site (as the total amount of carbon atoms increasing linearly with the MS flow [4]). When a non-linearity occurs, C atoms started to be incorporated in interstitial position also. SIMS measurement will be performed to confirm that. At 550 °C, the growth rate of SiGe is more important with disilane precursor due its higher reactivity than silane. Its leads to a better substitutional carbon incorporation. Indeed, using disilane, up to 1.15 % of fully substitutional carbon can be reached while using silane, only 0.7 % is reached.[1] V. Loup et al., J. Vac. Sci. Technol. B 21(1), Jan/Feb 2003.[2] D.J. Robbins et al., Journal of Applied Physics 69, 3729 (1991).[3] D. De Salvador et al., Phys. Rev. B, vol. 61, pp. 13005, 2000.[4] V. Loup et al., J. Vac. Sci. Technol. B 20(3), May/Jun 2002. Figure 1

Tuning 3C-SiC(100)/Si(100) Heterostructure Interface Quality

We report a systematic spectroscopic and structural investigation of 3C-silicon carbide (3C-SiC) films grown on Si(100)-(2 × 1) by codeposition of C60 molecules and Si atoms in ultrahigh-vacuum conditions. This work focuses on reducing the macroscopic defects formed at the interface in Si-SiC heterojunctions. A wide range of parameters influence the growth process, including the substrate deposition temperature, the relative effusion fluxes of C60 and Si, and the clean Si(100) surface order. By adjusting the Si and C60 deposition rates, it is possible to reduce the Si atom diffusion from the substrate and control both the surface morphology and the specific formation of the C-rich c(2 × 2) or Si-rich 3 × 2 ordered surfaces. Our results show that the growth of 3C-SiC on flat and good quality Si substrates is a crucial and necessary starting point to obtaining good quality 3C-SiC/Si interfaces with a minimum number of defects.

Peatland degradation in Asia threatens the biodiversity of testate amoebae (Protozoa) with consequences for protozoic silicon cycling

Anthropogenic peatland degradation is a global threat. As peatlands store large amounts of carbon (C) their potential for mitigation of climate change has been emphasized recently. Global C cycling is linked to silicon (Si) fluxes from the continents into the oceans. These fluxes in turn are driven by biosilicification, the incorporation of inorganic Si into living organisms, in terrestrial ecosystems. Biosilicification by testate amoeba (TA) communities and its potential for Si cycling has been highlighted since the beginning of the 21st century. However, the effects of peatland degradation on TA biodiversity and corresponding protozoic biosilicification on a continental scale remained unresolved so far. We show that TA biodiversity in Asian peatlands is strongly affected by the grade of human impact. This biodiversity decline was accompanied by an unexpected increase in protozoic biosilicification. Our findings provide new insights into the interactions between the biodiversity of soil microorganisms and biogeochemical Si cycling.

Aggregation reduces the release of bioavailable silicon from allophane and phytolith

Phytoliths form in plant tissues as fine, silt-sized amorphous silica particles. Once deposited in soils by plant debris, they can dissolve and feed silicon (Si) fluxes to the biosphere and hydrosphere, enhancing the positive effects of Si on plant health and carbon fixation by marine diatoms. In soils, microaggregates (<250 μm) may entrap phytoliths and protect them from dissolution. Here we aimed at analyzing the role of aggregation in protecting phytoliths and delaying the release of bioavailable Si. Synthetic 32-days aged aggregates were composed of (gkg−1): organic matter (50), aluminosilicate mineral (370), iron (Fe) oxide (60), quartz (500) and rice phytolith (20). Models with either amorphous (Am) or crystalline (Cryst) aluminosilicates and Fe-oxides were tested. The Am model included allophane and ferrihydrite while the Cryst model included kaolinite and goethite. Aggregates were visualized by Scanning and Transmission Electron Microscopy. They were compared to individual compounds and unaggregated mixtures through kinetic Na2CO3 and CaCl2 extraction assessing the pools of phytoliths and bioavailable Si, respectively. Aluminum (Al) and germanium (Ge) concentrations, and pH were measured in the CaCl2 extracts. CaCl2 extractable Si decreased in the order (gkg−1): Am mixture (3.72) > Am aggregate (0.95) > Cryst mixture (0.48) > Cryst aggregate (0.39). Aggregation thus reduced Si release by 3.9- and 1.2-fold in the amorphous and crystalline model, respectively. The Si/Al and Ge/Si atomic ratios showed that allophane and phytolith were the main sources of bioavailable Si in the amorphous and crystalline model, respectively. In contrast to the crystalline model (pH5.0–7.8), the acidic medium in the amorphous model (pH3.7–4.9) enhanced allophane dissolution. Aggregation thus protected both allophane and phytolith from dissolution, and reduced the release of bioavailable Si, the source of which depended on the component stability and pH.

Assessment of the influence of bamboo expansion on Si pools and fluxes in a disturbed subtropical evergreen broadleaved forest

Silicon (Si) pools and fluxes are affected by plant community shifts in forest ecosystems. Moso bamboo (Phyllostachys pubescens) is a Si accumulating plant, and its expansion exerts various ecological effects in colonized forest ecosystems. However, it is unclear how bamboo expansion might alter the biogeochemical Si cycle. To estimate the consequences of bamboo expansion on Si availability and biochemical cycling in subtropical areas, we selected bamboo-pure forest (BPF), bamboo-broadleaved mixed forest (BMF), and adjacent secondary evergreen broadleaved forest (EBF) stands in an area of bamboo expansion and compared Si pools and fluxes of plants in each forest type. We found that the content of the soluble and exchangeable Si in soils of BPF was lower than EBF (0.18 g kg−1 versus 0.25 g kg−1), that is, bamboo expansion decreased the soluble and exchangeable Si fraction, whereas there was no significant difference among BPF, BMF and EBF. In addition, the plant Si pool in BPF (285 g Si m−2) was 32.6 % lower than EBF (428 g Si m−2), and there was a shift in Si allocation from aboveground to belowground with conversion from EBF to BPF, largely attributed to abundant roots in BPF. We found that bamboo expansion accelerated the biogeochemical Si cycle to some extent, with higher production of phytoliths and quicker turnover of fine roots in BPF than EBF. We also highlight that bamboo expansion accelerated the uptake and return of Si, and promoted the release and dissolution of phytoliths. These results have implications for assessing the impacts of vegetation shifts on the biogeochemical Si cycle.

Optimization of tungsten-doped high density carbon target in inertial confinement fusion

A tungsten (W)-doped high density carbon (HDC) target is a promising design for an ignition target in inertial confinement fusion (ICF). The influence of silicon (Si) and W on the transmission of M-band x-ray has been studied by experiment. With a radiation temperature of ∼200 eV, the transmitted M-band x-ray (1.6–4.4 keV) flux and spectrum of Si or W-buried HDC sample were measured by M-XRDs and TGS, respectively. The thin layer of Si or W was buried at two different depths (2.1 μm and 11 μm). Results show that M-band transmission flux of Si-buried HDC sample decreases with buried depth (bd). However, bd does not influence that of the W-buried HDC sample. The one-dimensional simulation result is consistent with the experimental result. In the 1D simulation, the Au M-band (2–5 keV) transmission flux of Si-buried HDC also decreases with bd. However, the Au M-band transmission flux of W-buried HDC increases at first and then decreases. This is mainly due to the different characteristics of Si and W opacity. Especially as the peak radiation temperature reaches 260 eV, W can still absorb the M-band x-ray efficiently as it is buried near the radiation source. Based on these studies, an optimized W-doped HDC target with low doped fraction and mass has been proposed in this paper.