Carbon Impurity Concentration Research Articles

Influence of low-Z impurity on the stabilization of m/n = 2/1 tearing/locked modes in EAST

The impact of the low-Z impurity concentration on mode stabilization has been investigated in the EAST tokamak. A series of tearing modes (TMs) with multiple helicities are excited by the low-Z (carbon) impurity concentration, and the dominant mode structure features m/n = 2/1 magnetic islands that propagate in the electron diamagnetic drift direction (m and n are poloidal and toroidal mode numbers, respectively). The m/n = 2/1 locked modes (LMs) can be formed by the redistribution of low-Z impurity concentration, and are unlocked spontaneously due to the decrease in the impurity concentration, where the width of the magnetic islands can reach w ≈ 5 cm (w/a ≈ 0.1, a is the minor radius). The increase in the electromagnetic brake torque is the primary reason for the mode locking, and the ‘O’-point of the m/n = 2/1 magnetic islands is locked by the tungsten protector limiter (toroidal position: −0.4π ⩽ ϕ ⩽ −0.3π) with separation of Δϕ ≈ 0. The 3D asymmetric structure of the m/n = 2/1 magnetic islands is formed for the interaction with the tungsten protector limiter, and the electromagnetic interaction decreases dramatically for the separation of Δϕ ⩾ 0.2π. The mode excitation and locking mechanisms can be illustrated by the ‘hysteresis effect’ between the low-Z impurity concentration and the width of the m/n = 2/1 magnetic islands; namely, the growth of magnetic islands is modulated by the low-Z impurity concentration, and the rotation velocity is decelerated accordingly. However, the intrinsic mechanism for the unlocking of m/n = 2/1 LMs is complicated by considering the concentration of the low-Z impurity, and the possible unlocking mechanism is discussed. Therefore, understanding the relationship between the impurities and magnetic islands is more important for optimization of the control techniques (resonant magnetic perturbations → LMs, electron cyclotron resonant heating (ECRH) → neoclassical tearing mode (NTM), impurity seeding → major collapse, etc).

In Situ LID and Regeneration of PERC Solar Cells from Different Positions of a B-Doped Cz-Si Ingot

In order to investigate the light-induced-degradation (LID) and regeneration of industrial PERC solar cells made from different positions of silicon wafers in a silicon ingot, five groups of silicon wafers were cut from a commercial solar-grade boron-doped Czochralski silicon (Cz-Si) ingot from top to bottom with a certain distance and made into PERC solar cells by using the standard industrial process after measuring lifetimes of minority carriers and concentrations of boron, oxygen, carbon, and transition metal impurities. Then, the changes of their I - V characteristic parameters (efficiency η , open-circuit voltage V oc , short-circuit current I sc , and fill factor FF ) with time were in situ measured by using a solar cell I - V tester during the 1st LID (45°C, 1 sun, 12 h), regeneration (100°C, 1 sun, 24 h), and 2nd LID (45°C, 1 sun, 12 h). The results show that the LID and regeneration of the PERC solar cells are caused by the transition of B-O defects playing a dominant role together with the dissociation of Fe-B pairs playing a secondary role. The decay of η during the 1st LID is caused by the degradation of V oc , I sc , and FF , while the increase of η during the regeneration is mainly contributed by V oc and FF , and the decay of η during the 2nd LID is mainly induced by the degradation of I sc . After regeneration, the decay rate of η reduces from 4.43%–5.56% (relative) during the 1st LID to 0.33%–1.75% (relative) during the 2nd LID.

Исследование порошка титана, полученного методом СВС-гидрирования и дегидрирования в вакуумной печи

The method of SHS (self-propagating high-temperature synthesis) allows the efficient synthesis of titanium hydride. The article presents new results of experimental studies of titanium powders synthesized by the method of SHS hydrogenation and dehydrogenation in a vacuum furnace. Changes in the microstructure, phase and chemical composition during hydrogenation-dehydrogenation of a titanium sponge were studied. The titanium sponge was hydrogenated in a high-pressure SHS reactor at a hydrogen pressure of 3 MPa. The content of oxygen and carbon impurities decrease in the process of SHS hydrogenation was found. After hydrogenation, the sponge is a single-phase δ-hydride of titanium with a tetragonal lattice, the particles have a fragmentary shape. In the obtained titanium hydride, an increased hydrogen content of 4.64 wt. % was noted. The hydrogenated titanium sponge was mechanically crushed in a drum-ball mill to a particle size of 40 – 250 microns. Dehydrogenation of titanium hydride powder was carried out in a vacuum furnace at a temperature of 850 °C for 220 minutes. Titanium after dehydrogenation is a single-phase α-titanium powder with a hexagonal close packed lattice, the size and shape of the particles have not changed. The technological process under study provides the possibility of obtaining high-quality titanium powders of the necessary granulometric composition for various fields of powder metallurgy.

Observation of direct power deposition of ICRF in EAST plasma boundary

Waves in the ion-cyclotron range of frequencies (ICRF) used for plasma heating are known to interact strongly with the scrape-off layer (SOL) plasma, potentially limiting ICRF performance. We present the evidence of direct power deposition of ICRF in the Experimental Advanced Superconducting Tokamak (EAST) plasma boundary, with two ICRF antennas and 12 MW of ICRF source power in support of high-power and long-pulse operation. By applying Retarding Field Analyzer (RFA) diagnostic, a sharp rise in average collected ion parallel energy was obtained during the period of ICRF heating as the RFA probe stayed still at 2 mm behind the main limiter which indicates that a certain amount of power is deposited near the limiter instead of being absorbed by the main plasma. A significant increase of the target floating potential in electron temperature measured by Divertor Langmuir Probes arrays was observed. The wall temperature in the far SOL was also enhanced. As a consequence, an enhancement of the carbon and tungsten impurity concentration was observed both at the plasma boundary and core. Besides, plasma radiation, the impurity concentration of the boundary and core increased, and the temperature rise of the first wall was also observed, which are intuitive and powerful pieces of evidence of ICRF direct power deposition at the boundary.

High-resistivity nitrogen-polar GaN for GaN/AlGaN high electron mobility transistors by metalorganic chemical vapor deposition

Nitrogen-polar (N-polar) III-nitride has natural advantages in the fabrication of electronic devices. In this work, unintentionally doped N-polar GaN films were grown on sapphire substrates by metalorganic chemical vapor deposition. By optimizing the growth parameters including growth temperature, pressure, and V/III ratio, N-polar GaN with a relatively high sheet resistivity of 3.6 × 105 Ω/sq was achieved. The secondary ion mass spectroscopy and x-ray diffraction measurement results demonstrate that the increased carbon impurity concentration is primarily responsible for the high resistivity. Moreover, N-polar GaN/Al0.25Ga0.75N heterojunctions were deposited on the high-resistivity N-polar GaN template. An extremely high-density two-dimensional electron gas of up to 1.12 × 1013 cm−2 was realized at the interface between the Al0.25Ga0.75N and the GaN. It is reasonably believed that the experimental results obtained here are beneficial for the development of high-performance N-polar GaN-based electronic devices.

Study the influence mechanism of In/Ga ratio on InGaN waveguide layers during epitaxial growth

We have investigated the effect of In/Ga ratio on InGaN waveguide layers obtained by the epitaxial growth, and analyzed their physical mechanisms in detail. It is found that enhancing the In/Ga ratio by increasing the TMIn flux and reducing the TEGa flux can alleviate the decrease of In composition in InGaN waveguide. Moreover, the intensity of the yellow band of the photoluminescence is obviously reduced due to the reduction of the residual carbon impurity concentration. The smaller scattering of injected carriers by impurities and defects is beneficial for enhancing the hole concentration in the p-type layer. Meanwhile, the results of Raman spectroscopy clarify that InGaN waveguide layers grown with larger In/Ga ratio can achieve higher carrier mobility and lower resistivity. Therefore, properly increasing the In/Ga ratio during the growth can effectively improve the crystal quality of InGaN waveguide layers, which facilitates the obtaining of high-performance GaN-based laser diodes.

Monotonic variation in carbon-related defects with Fermi level in different conductive types of GaN

This study examined the photoluminescence (PL) of samples of GaN and found that the intensity of the peak of luminescence related to carbon impurities in p-GaN changed with the doping concentration of Mg. However, the results of a secondary ion mass spectrometry test showed that the concentration of carbon impurities did not change correspondingly. Moreover, we observed changes in the relative strength of the peak related to carbon impurities in the PL spectra of a series of samples of n-type conductive GaN. This suggests a connection between the behavior of carbon-related defects and the conductivity of GaN. The results show that the variation in carbon-related defects was monotonic. As the Fermi level approached the conductive band, carbon-related defects that generated higher-energy photonics became more dominant in the PL spectra in the series from p-type to n-type GaN.

Flow modulation metalorganic vapor phase epitaxy of GaN at temperatures below 600 ºC

Heterogeneous integration of materials systems for devices and circuits is becoming of increasing interest, particularly for the wide bandgap nitrides in both optoelectronic and electronic applications. However, typical high growth temperatures of GaN by metalorganic vapor phase epitaxy prevent the growth of GaN on wafers with temperature sensitive materials. This work presents a flow modulation epitaxy (FME) growth scheme which allows for step-flow growth of GaN at temperatures below 600 °C. The utilization of a pulsed growth scheme such as FME allows for the mitigation of challenges associated with limited motion of adatoms at lower growth temperatures. Factors such as temperature, precursor flow rate, cycle time, and film thickness were explored. Under optimized conditions, step-flow growth of GaN films at 550 °C was demonstrated. In addition to an improved surface morphology over layers grown in the conventional continuous growth mode, the carbon and oxygen residual impurity concentrations in the FME grown layers were significantly lower, with carbon and oxygen contents of 9 × 1017 cm−3 and 8 × 1016 cm−3, respectively. Although the growth rate of the low temperature growth was very slow, ∼ 0.006 Å/s, the flow modulation growth scheme developed here provides a pathway towards further integration of GaN with temperature sensitive materials.

Improved critical temperature of superconducting plasma-enhanced atomic layer deposition of niobium nitride thin films by thermal annealing

The efficiency of the superconducting radio frequency cavities composed of Nb required the deposition of thickness-controlled multilayer coatings of superconductor-insulator-superconductor (S-I-S) on the internal surfaces of the cavities. Herein, we report the plasma-enhanced atomic layer deposition of carbon-free NbN (50 µm thick), followed by a thermal treatment, to obtain the superconducting layer in the S-I-S structure. Using (tert-butylimido)-tris(diethylamino)-niobium as the niobium precursor and H2 and NH3 plasma as reactive gasses, the deposition and annealing parameters were optimized by studying their effects on the film properties (crystallinity, density, and composition). We demonstrated that the superconducting critical temperature (Tc) can be improved after thermal annealing up to 13.8 K, a value compatible with the targeted application. In addition to the expected densified layers and increased grain size, we observed a partial transformation of the oxide present in the as-deposited layer into niobium oxynitride, which could indicate the origin of the improvement of the superconducting properties. With low carbon and oxygen impurity concentrations in the films, this study contributes to the understanding of the relationship between the structure, composition, and superconductivity in NbN.

Carbon impurity concentrations in BaSnO3 films grown by molecular beam epitaxy using a tin oxide source

Achieving high carrier mobilities and low carrier densities has been one of the major challenges in the growth of BaSnO3 thin films. In this study, unintentional carbon impurities are investigated as one possible cause for poor electrical transport. Carbon concentrations in La-doped BaSnO3 films grown by molecular beam epitaxy using solid SnO2 sources are quantified using secondary ion mass spectroscopy. High carbon concentrations of mid-to-low 1018 atoms/cm3 are detected in samples grown from SnO2 powder and pellet sources, respectively. The carbon contamination is attributed to adsorbents present on the surface of the air exposed source material. Use of an oxygen plasma during growth does not reduce the amount of carbon detected in the films. Possible steps to reduce carbon contamination are discussed.

Fabrication and characterization of polymer‐derived high‐entropy carbide ceramic powders

AbstractPolymer‐derived ceramic (PDC) route has been widely used to fabricate various ceramics or ceramic‐matrix composites in recent years. However, the synthesis of high‐entropy ceramics via PDC route has rarely been reported until now. Herein, we successfully synthesized a class of high‐entropy carbides, namely (Hf0.25Nb0.25Zr0.25Ti0.25)C (HEC‐1), via PDC route. The polymer‐derived HEC‐1 ceramics consisted of numerous superfine particles with the average particle size ~800 nm. Meanwhile, they possessed a rock‐salt structure of metal carbides and high‐compositional uniformity from nanoscale to microscale. In addition, the as‐obtained HEC‐1 ceramics had a low oxygen impurity content of 0.51% and a low free carbon impurity content of 2.56%. This work will open up a new research field on the fabrication of high‐entropy ceramics or high‐entropy ceramic‐matrix composites via PDC route.

Study on the effects of growth rate on GaN films properties grown by plasma-assisted molecular beam epitaxy

Abstract This work systematically investigates the effects of growth rate on GaN films properties grown on thick GaN-on-sapphire templates by plasma-assisted molecular beam epitaxy (PA-MBE) under Ga-rich conditions without formation of large metal droplets. The nitrogen-limited growth rate is varied by modulating the nitrogen flows while maintaining the power of radio frequency plasma source constant, and the thickness of epitaxial GaN films is determined by Capacitance-Voltage and verified by second ion mass spectroscopy simultaneously. It is found that GaN films grown at high growth rate exhibit an improved crystal quality measured by X-ray diffraction, and display a rough surface morphology along with higher hillocks and clearer step terrace resolved by atomic force microscopy. Photoluminescence spectroscopy characterization indicates the full width at half maximum of near band emission peak and intensity of yellow band emission are decreased for GaN epilayers grown at high growth rate, corresponding to a reduced density of defect states and a lowered concentration of residual carbon impurity, respectively. The presented results suggest that the growth rate strong influences the structural, morphological and optical properties of GaN epitaxial films, and high growth rate facilitates realizing a better GaN film quality, giving a conducive insight for PA-MBE-grown III-nitride device-quality materials for electronic and optoelectronic applications.

Luminescence and dosimetric characteristics of nanocrystalline Al2O3:C synthesized by thermal plasma reactor

Nanocrystalline Al2O3:C has been synthesized by thermal plasma method. The sample was annealed at 1300 °C in air and vacuum. XRD studies show phase change from γ-Al2O3:C to α-Al2O3:C. Average particle sizes estimated from XRD peaks and TEM images show increase in the particle size from ~30 nm to ~ 67 nm. The UV–visible spectrum of the as-synthesized Al2O3:C shows absorption peaks at 209 and 255 nm confirming the formation of F and F+ centers. XPS analysis reveals that the concentration of carbon impurity remains intact in Al2O3 lattice. The TL glow curves were fitted by CGCD method. The glow curve of as-synthesized Al2O3:C found to have six peaks and that of annealed sample have four peaks. The changes in the glow curve structures of the as-synthesized and annealed phosphor may be attributed to the phase change of the crystal structure and/or different kinds of defects formed on annealing in air/vacuum.

Ab initiotypical medium theory of substitutional disorder

By merging single-site typical medium theory with density functional theory we introduce a self-consistent framework for electronic structure calculations of materials with substitutional disorder which takes into account Anderson localization. The scheme and details of the implementation are presented and applied to the hypothetical alloy Li$_{c}$Be$_{1-c}$, and the results are compared with those obtained with the coherent potential approximation. Furthermore we demonstrate that Anderson localization suppresses ferromagnetic order for a very low concentration of (i) carbon impurities substituting oxygen in MgO$_{1-c}$C$_{c}$, and (ii) manganese impurities substituting magnesium in Mg$_{1-c}$Mn$_c$O for the low-spin magnetic configuration.

Growth of Highly Crystalline GaN at High Growth Rate by Trihalide Vapor‐Phase Epitaxy

Free‐standing gallium nitride (GaN) wafers are being increasingly used for high‐power and high‐frequency electronic devices. However, these have to be produced at low costs while maintaining high quality. With the aim of reducing the cost of manufacturing GaN substrates, trihalide vapor‐phase epitaxy (THVPE) is investigated to explore the effect of growth temperatures up to 1400 °C. High growth rate and high crystal quality are achieved simultaneously using THVPE. High‐quality GaN crystals are obtained at the growth rate of more than 300 μm h−1. Crystal characteristics are confirmed via the full width at half maximum (FWHM) of the (002) plane, obtained by the X‐ray two‐crystal (XRC) method. Dark spot density (DSD) measured via cathode luminescence decreases to the level of seed substrate at growth temperatures above 1300 °C. The yellow emission band is not visible in the photoluminescence spectra of the 2 in.‐diameter sample, which is consistent with very small concentrations of carbon impurities in the crystal. In addition to the attainment of high‐purity materials, parasitic polycrystal growth around the wafer and the reactor wall is eliminated, which can improve the productivity of the THVPE, due to very little down time of the reactor.

Absorption of Hydrocarbons on Palladium Catalysts: From Simple Models Towards Machine Learning Analysis of X-ray Absorption Spectroscopy Data

Element selectivity and possibilities for in situ and operando applications make X-ray absorption spectroscopy a powerful tool for structural characterization of catalysts. While determination of coordination numbers and interatomic distances from extended spectral region is rather straightforward, analysis of X-ray absorption near-edge structure (XANES) spectra remains a highly debated and topical problem. The latter region of spectra is shaped depending on the local 3D geometry and electronic structure. However, there is no straightforward procedure for the unambiguous extraction of these parameters. This work gives a critical vision on the amount of information that can be practically extracted from Pd K-edge XANES spectra measured under in situ and operando conditions, in which adsorption of reactive molecules at the surface of palladium with further formation of subsurface and bulk palladium carbides are expected. We investigate how particle size, concentration of carbon impurities, and their distribution in the bulk and at the surface of palladium particles affect Pd K-edge XANES features and to which extend they should be implemented in the theoretical model to adequately reproduce experimental data. Then, we show how the step-by-step increasing the complexity of the theoretical model improves the agreement with experiment. Finally, we suggest a set of formal descriptors relevant to possible structural diversity and construct a library of theoretical spectra for machine-learning-based analysis of the experimental data.

Application of multiple correlation analysis method to modeling the physical properties of crystals (on the example of gallium arsenide)

The use of modern applied computer programs expands the possibility of multicomponent statistical analysis in materials science. The procedure for applying the method of multiple correlation and regression analysis for the study and modeling of multifactorial relationships of physical characteristics in crystalline structures is considered. The consideration is carried out using single crystals of undoped gallium arsenide as an example. The statistical analysis involved a complex of seven physical characteristics obtained by non-destructive methods for each of 32 points along the diameter of the crystal plate. The data array is investigated using multiple correlation analysis methods. A computational model of regression analysis is built. Based on it, using the programs Excel, STADIA and SPSS Statistics 17.0, statistical data processing and analytical study of the relationships of all characteristics are carried out. Regression relationships are obtained and analyzed in determining the concentration of the background carbon impurity, residual mechanical stresses, and the concentration of the background silicon impurity. The ability to correctly conduct multiple statistical analysis to model the properties of a GaAs crystal is established.New relationships between the parameters of the GaAs crystal are revealed. It is found that the concentration of the background silicon impurity is related to the vacancy composition of the crystal and the concentration of cents EL2. It is also found that there is no relationship between the silicon concentration and the value of residual mechanical stresses. These facts and the thermal conditions for the formation of point defects during the growth of a single crystal indicate the absence of a redistribution of background impurities during cooling of an undoped GaAs crystal.The use of the multiple regression analysis method in materials science allows not only to model multifactor bonds in binary crystals, but also to carry out stochastic modeling of factor systems of variable composition

Roles of carbon impurities and intrinsic nonradiative recombination centers on the carrier recombination processes of GaN crystals

The relationship between the concentration of carbon (C) impurity, [C], and quantum efficiency (QE) of radiation in n-type GaN crystals was clarified under photo-excitation conditions at room temperature. Since C acts as a trap for minority holes as well as a compensator for majority electrons, the external QE (EQE) and internal QE (IQE) values for the near-band-edge (NBE) emission showed a monotonic increase with decreasing [C], where NBE EQE (IQE) values are 0.02% (0.70%) and 0.53% (14.2%) for [C] = 2.0 × 1016 cm−3 and [C] = 4.0 × 1014 cm−3, respectively, under a cw photo-pumping density of 140 W cm−2.

Impurity levels in cerium oxide microspheres prepared by internal gelation sol–gel methods

Impurity levels were measured in cerium oxide microspheres fabricated by modified internal gelation sol–gel methods. A combination of glow discharge mass spectrometry, electron impact mass spectrometry, combustion gas analysis, and instrumental gas analysis were used to assess a wide range of potential elemental impurities. Low concentrations of carbon, nitrogen, and hydrogen impurities in microspheres showed a dependence on how microspheres were washed. Silicon impurities are believed to derive from silicone oil used during sol–gel processing. Spheres washed by the preferred approach had impurity levels below 100 ppm for all elements tested. For applications such as nuclear fuels, sol–gel methods could meet purity specifications as long as metal nitrate feed solutions of sufficient purity are used and microspheres are washed appropriately. In this study, cerium was used as a surrogate for plutonium-238, which is used in radioisotope power systems as a heat source, to determine whether carbon or other impurities were concentrated during internal gelation processing and remained after heat treatments. Analyses indicated low concentrations of impurities in cerium oxide microspheres after sintering steps that were well below documented limits for plutonium-238 oxide fuels. Modified washing methods, combined with a pressurized water treatment, resulted in sintered cerium oxide sol–gel microspheres with low impurity levels.

Effects of carbon impurity due to ICF capsule compression on the effective ignition parameters

Direct-drive spherical implosions can be used to access high-energy-density plasma conditions for inertial confinement fusion (ICF). In this paper, the effects of carbon impurity due to ICF capsule compression by a laser pulse on the process of $${\mathrm{D}}/^{3}{\mathrm{He}}$$ thermonuclear plasma burning are studied in an ICF cycle. The effect of carbon impurity concentrations on the effective ignition parameters, the plasma heating rate caused by energy deposition of reaction products, and energy loss rates in the process of burning plasma have also been analyzed in the ICF process. The results obtained show that during the ICF spherical implosions of the $${\mathrm{D}}/^{3}{\mathrm{He}}$$ fuel, the ignition temperature, $$T_{({\mathrm{ig}})}$$ , and the confinement parameter, $$\rho R$$ , in the hot spot are approximately equal to 58 keV and 5 g/cm $$^{2}$$ , respectively. These values are increased by the presence of impurity.