Si Impurities Research Articles

A Review of Slag Refining of Crude Silicon

Metallurgical refining of crude silicon for low-cost, mass production of solar silicon has been studied extensively over the past two decades. One of the methods attempted to target the deleterious impurities of Si is slag refining, where liquid Si or its alloys are treated with a flux. This article presents an overview of the impurity response to various slag treatment conditions for crude Si metal from both fundamental and practical perspectives, aiming to provide a basis for the design and application of more effective fluxes and alloys. It was found that removal of B and P through slag refining alone is not practical on an industrial scale as it would require a large volume of slag, however the removal percentage can be significantly increased if the silicon has been alloyed with a suitable metal.

Detection limit of carbon concentration measurement in Si for photoluminescence method after electron irradiation

We investigated the detection limit of carbon in Si for the photoluminescence (PL) method after electron irradiation. The detection limit was obtained from the intensity ratio of the G-line to the free exciton line with the G-line intensity twice as high as the noise level and was estimated at 4 × 1013 cm−3 under the measurement condition in accordance with the standard [JIS H0615] for quantification of donor and acceptor impurities in Si at 4.2 K. We showed that the limit is extendable down to 2 × 1013 cm−3 by optimizing the excitation power and sample temperature.

Recovery of vanadium and tungsten from waste selective catalytic reduction catalysts by K2CO3 roasting and water leaching followed by CaCl2 precipitation

Waste selective catalytic reduction (SCR) catalysts are potential environmental hazards. In this study, the recovery of vanadium and tungsten from waste SCR catalysts by K2CO3 roasting and water leaching was investigated. The roasting and leaching conditions were optimized: the leaching efficiencies of vanadium and tungsten were 91.19% and 85.36%, respectively, when 18 equivalents of K2CO3 were added to perform the roasting at 900 °C for 2 h, followed by leaching at 90 °C for 1 h. Notably, in the described conditions, the leaching rate of silicon was only 28.55%. Titanates, including K2Ti6O13 and KTi8O17, were also produced. Si removal was achieved in 85% efficiency adjusting the pH to 9.5, and the Si impurity thus isolated was composed of amorphous Si. Tungsten and vanadium were precipitated using CaCl2. At pH 10 and following the addition of 0.10 mol of H2O2 and 16 equivalents of CaCl2, the precipitating efficiencies of tungsten and vanadium were 96.89% and 99.65%, respectively. The overall yield of tungsten and vanadium was 82.71% and 90.87%, respectively.Graphic abstract

Reduction in leakage current through interface between Ga2O3 epitaxial layer and substrate by ion implantation doping of compensating impurities

Strong accumulation of Si impurities has been observed at a Ga2O3 epilayer/substrate interface. The highly Si-doped region around the interface typically becomes a current conduction path, causing buffer leakage for lateral Ga2O3 field-effect transistors (FETs). To overcome the drawback, we performed Mg- or Fe-ion implantation doping into a Ga2O3 substrate prior to the subsequent molecular-beam epitaxy growth to compensate the accumulated Si donors at the interface. The Mg implantation doping showed a minimal effect on reduction in the interface leakage, irrespective of its concentration. On the other hand, the Fe doping with a high density of 2 × 1019 cm−3 provided a significant decrease in the leakage and decent FET characteristics.

Raman scattering in heavily donor doped β -Ga2O3

β-Ga2O3 crystals doped with the donor impurities Si or Sn are investigated by Raman spectroscopy. In addition to the well-known intrinsic Raman allowed phonon modes, we discover several spectral features when the doping concentration exceeds the Mott criterion for the metal-insulator transition (∼ 3 × 1018 cm−3). The most prominent extra Raman peak at 255 cm−1 is an asymmetrically broadened one. It is due to single-particle, electronic excitations involving the impurity band formed by effective-mass-like (hydrogenic) shallow donors. A similar type of excitation is attributed to a Raman line at 675 cm−1 that appears below room temperature in Si doped samples and might be due to a non-hydrogenic donor. Furthermore, we observe four longitudinal phonon-plasmon coupled modes at 215 cm−1, 280 cm−1, 400 cm−1, and 560 cm−1 associated with infrared active phonon modes. They arise from inelastic light scattering via charge-density fluctuations.

A simulation model for controlling mass transfer of vapor silicon and volatile impurities with gas flow guidance during refining of silicon

The mass transfer behavior of vapor silicon (Si) and volatile impurities during refining of Si by electron beam melting is still unclear. In present work, with gas flow guidance, the mass transfer mechanism of vapor Si and volatile impurities, and corresponding heat transfer rule in the furnace chamber are investigated by simulation and experiment. Numerical simulation shows that vapor Si is preferentially transferred to the outlet, and temperature in the outlet is lower than the solidification temperature of vapor Si when gas flow is fixed at 0° and 1.16 m s−1 (optimal conditions). Experimental results show that volatile impurities are effectively removed (P and Al both are less than 0.01 ppmw in the refined Si ingot with >99% of removal efficiency). Moreover, volatile impurities flux positively increases with increase of vapor Si flux. Gas flow significantly gathers the vapor Si in the outlet, reinforcing evaporation kinetics of molten Si and volatile impurities, which demonstrates the numerical simulation model is feasible to control the mass transfer and evaporation kinetics. Gas flow guidance may be a promising way to improve volatile impurities removal from Si.

Atomic structures of Ti‐doped α‐Al2O3 Σ13 grain boundary with a small amount of Si impurity

AbstractDoping is a fundamental technique to control sintering of α‐Al2O3, and many types of elements have been used to control grain growth and material properties. Such dopants tend to be concentrated at the grain boundaries and modify grain boundary properties. It is therefore important to investigate the variation of grain boundary atomic structures induced by additives. However, in the sintering process it is difficult to prevent small amounts of unintentional impurity (such as Si), and the potential influence of additives and impurities on the grain boundary structure should be taken into account. Here we show that two types of grain boundary atomic structures are formed in a Ti‐doped Σ13 []/() α‐Al2O3, which has been determined by atomic‐resolution scanning transmission electron microscopy. Combining with atomic‐scale spectroscopic analyses, we elucidate that the local concentration of Si impurities significantly alters the grain boundary atomic structures and the valence state of Ti additives. The present results suggest that the subtle change in concentrations of both additives and impurity should have a strong impact on sintering processes and resultant properties in α‐Al2O3.

Study on the effective distribution coefficient of impurity separation in the preparation of high purity aluminum

Effective distribution coefficient (ke) plays an important role in zone melting purification but has never been systematically calculated as an important parameter. A novel analysis method for impurity separation separating from Al using determination of impurities (Si, Fe, Cu, Zn) content and calculation of ke has been investigated. Experimental results show that the impurity separation effect was significantly improved due to the ke closely approximating the equilibrium distribution coefficient (k0) via optimization of purification times and speed of the melting zone. When the times of purification were increased from 15 to 20 times, the ke values of Si, Fe, Cu, and Zn at the sample (No. 3) were 0.603, 0.702, 0.364, and 0.502, respectively. Correspondingly, the removal rate of impurities of Si, Fe, Cu, and Zn increased by 6.25%, 1%, 10.49%, and 27.58%, respectively. The effect on the ke of Cu and Fe is more significant at the sample (No. 3) when the melting zone speed is changed. The removal rates of Fe and Cu in the sample are 99.0% and 94.75% when the melting zone speed was 1/3 mm/min, and, correspondingly, the removal rates of Si and Zn were 81.7% and 88.51%, respectively. The Al content at sample position No. 3 is more than 999994.59 ppm when the sample was purified 15 times at a speed of 0.5 mm/min, which meets the standard of 5N high purity aluminum.

Microstructure-corrosion behaviour relationship of micro-alloyed Mg-0.5Zn alloy with the addition of Ca, Sr, Ag, In and Cu

The effects of micro-addition (0.2 wt.%) of Ca, Sr, Ag, In and Cu on the microstructure and corrosion properties of the as-cast Mg-0.5Zn alloy were systematically studied. It is found that the grain refinement efficiency and intermetallics forming ability differed greatly among these elements. Si impurity also played a vital role in the precipitates formation in all systems. It is generally observed that Mg-Zn-Ca possessed the highest corrosion resistance due to the refined grain size, limited intermetallics and uniform corrosion, whereas Mg-Zn-Cu showed the highest corrosion rate due to the coarse grains, intermetallics activity and severe pitting corrosion. With the exception of Mg-Zn-Cu, the corrosion performances of Mg-0.5Zn(-0.2X) alloys were comparable with the higher concentrated systems from literature, indicating the feasibility of these micro-alloying systems for biomedical applications.

Si-mediated reassembly of interfacially segregated Sc atoms in an Al–Cu–Sc alloy exposed to high-temperature creep

The formation of second-phase particles by solid-state precipitation is well-known as the most effective strengthening method for the light-weight aluminum (Al) alloys. Here, the interfacial evolutions from Sc solute segregation to Al3Sc precipitation at θ′-Al2Cu/matrix interfaces were investigated in an Al–Cu–Sc alloy crept at beyond 300 °C. By considering the trace Si impurity, it is proposed that Al3Sc precipitation preferentially occurs at the Si-concentrated interfacial locations, i.e. growth ledges. Two effects of interfacial Al3Sc precipitation on θ′-Al2Cu were demonstrated: (1) The Al3Sc precipitation was processed at the expense of the adjacent Sc segregation and allows localized θ′-Al2Cu decomposition, causing “precipitate splitting”. (2) The Al3Sc-enclosed growth ledges resultantly prevented θ′-Al2Cu from coarsening and equilibrium transformation to θ-phase at temperature as high as 400 °C. This work is expected to provide a precipitate-stabilizing strategy by interfacial architecture to maintain the closely spaced dispersion of precipitates and its strengthening contribution at elevated temperature.

Получение заданных электрофизических характеристик слоев In0,01Ga0,99As: (Zn и Si) солнечных элементов в процессе эпитаксии из газовой фазы

The paper presents the results of studying the electrophysical characteristics (conductivity and concentration of the main charge carriers) of In0.01Ga0.99As layers of the middle cascade and other structural parts of the space-based InGaP/InGaAs/Ge solar cell depending on the doping type and level specified during epitaxial growth from the gas phase by varying the supply parameters of the sources of Si and Zn impurities. The studies were performed using X-ray diffractometry, non-contact conductivity measurement, electrochemical C–V profilometry, van der Pau method (Hall effect). A linear dependence of the main charge carrier concentration in the layer on the fraction of the doping precursor in the gas mixture was confirmed. The proportionality coefficients were determined for silicon and its disilane precursor, for zinc and its dimethylzinc precursor. The results of the dopant distribution uniformity study are shown and discussed as well as the assumptions about the effects of the temperature field gradient and the stress state in the layer and substrate.

Electrical Resistivity Measurements of Fe‐Si With Implications for the Early Lunar Dynamo

AbstractThe effect of impurities on the electrical resistivity of Fe at core conditions is controversial due to the challenges of measuring transport properties of Fe alloys under high‐pressure and high‐temperature conditions. In this study we describe an innovative technique to make wires of Fe‐Si samples initially in powder form for measuring electrical resistivity. The electrical resistivity of Fe‐2, Fe‐8.5, and Fe‐17 wt%Si was measured at 3, 4, and 5 GPa at temperatures into the liquid state and compared to results of Fe‐4.5 wt%Si and Fe from prior studies. Isothermal electrical resistivity increases linearly with increasing Si content. Yet the effect of Si content on the magnitude of the electrical resistivity compared to that of Fe diminished as temperature increased at all experimental pressures. This implies the contribution to the electrical resistivity due to Si impurities is dependent on temperature, in disagreement with Matthiessen's rule. Thermal conductivity of Fe‐Si alloys calculated using the Wiedemann‐Franz law indicates a nonnegligible influence of the Si content on the thermal conductive properties of Fe‐Si alloys. The results are used to calculate the adiabatic heat flux of an Fe‐Si lunar core and date the end of the high magnetic field intensity era of the lunar dynamo to be in the range 3.32–3.80 Ga.

Requirements of epitaxially grown InGaAs channel layers for tunnel field-effect transistors

The requirements of the epitaxially grown InGaAs channel layers for tunnel field-effect transistors (TFETs) have been studied by a comparison between the experimental results and technology computer-aided design (TCAD) simulations. It is observed that the threshold voltage (Vth) of InGaAs TFETs with high In contents is shifted more toward the negative bias direction and the off-characteristics are degraded more than that expected in simulation results. These results are attributable to the increase in n-type effective carrier concentration (Nch) in the InGaAs and InAs channels, which can be caused by lattice relaxation because of the large lattice mismatch between the epitaxial layer and the substrate. The use of a thin channel layer with high In content InGaAs and/or InAs can be a solution for achieving high on-current and low off-current simultaneously. It is found that p-type doping in the InGaAs and InAs channels can mitigate the degradation of the cut-off properties. Moreover, it is observed that increased Nch by unintentional Si impurity atoms at the epitaxial layer and the substrate interface can affect the performance of TFETs with thin InGaAs channels, which can be suppressed by introducing an insertional undoped buffer layer. The impact of Nch is also studied for the short channel devices by TCAD simulations. The p-type doping in InGaAs channels can also mitigate short channel effects in comparison to the n-type doping. The carefully controlled epitaxial growth of InGaAs and InAs channel layers with Nch <1 × 1017 cm−3 is required to realize superior performance InGaAs TFETs.

Recycling of High-Purity Strontianite and Hematite from Strontium-Bearing Sludge.

Sr-bearing sludge is a hazardous waste that is commonly generated by nuclear power plants and mineral refining operations. In this work, Sr-bearing sludge was simulated and then cleanly recycled into high-purity strontianite with hematite nanoparticles as a byproduct via a novel hematite precipitation route. The sludge contained 26.1% Fe, 3.5% Sr, and Si impurities. After dissolution in 1.2 M nitric acid, the sludge was treated hydrothermally with the addition of glycol to precipitate Fe effectively. Without the addition of glycol, only 52% Fe was hydrothermally precipitated in the form of hematite aggregates. With the addition of glycol at the optimal Mglycol/Mnitrate molar ratio of 0.4, nearly 100% Fe was removed in the form of hematite nanoparticles with an average diameter of 50 nm, whereas over 98% of Sr was retained in the leachate. The generated hematite was highly purified with an Fe2O3 content of 95.23%. Sr was present at a high concentration of 3.9 g/L in the treated leachate and further precipitated in the form of strontianite with a purity of 96.8% through Na2CO3 addition. Tertiary butanol (TeB) exhibited a similar Fe removal rate as glycol even though its optimal MTeB/Mnitrate molar ratio was 0.1, which was approximately one-fourth the optimal Mglycol/Mnitrate molar ratio. Fe removal involved spontaneous Fe3+ hydrolysis under hydrothermal conditions and was promoted by increasing the pH of the redox reaction between nitrate and glycol and/or TeB. The method reported here successfully enabled the resource recycling of Sr-bearing sludge to generate high-purity strontianite and hematite products without producing any secondary waste.

Effects of deposition environment and temperature on photoluminescence, particle morphology, and crystal structure of pulsed laser deposited Ga2O3 thin films

Gallium oxide (Ga2O3) thin films were laser ablation deposited on Si(100) substrates in vacuum, argon, and oxygen (O2) at different substrate temperatures by using the pulsed laser deposition technique. X-ray diffraction patterns showed that the films were crystallized in a mixed phase of β-Ga2O3 and Ga(OH)3. Data from scanning electron microscopy and atomic force microscopy showed that the major influence in the deposition conditions on the photoluminescence (PL) intensity was through changes in the particle morphology and surface topography of the films. The surface morphology studied using a field emission scanning electron microscope showed that the films were made of nanoparticles of spherical and cubic shapes at lower and higher temperatures, respectively. The energy-dispersive x-ray spectroscopy spectra confirmed the presence of the major elements Ga and O, with C coming from atmospheric hydrocarbons and Si from Si impurity in Ga2O3 and the substrate. The Si peak intensity was found to increase with the deposition temperature. X-ray photoelectron spectroscopy further confirmed the presence of Ga, O, C, Si, and N on the surface of the films. The PL spectrum excited using a 325 nm He-Cd laser showed nanoparticle shape driven tunable broadband emissions in the wavelength range between 350 and 750 nm. The comparison of the PL intensities of the films deposited in different atmospheres shows that the film deposited in O2 has the highest PL intensity.

Probing Charge Transport and Background Doping in Metal‐Organic Chemical Vapor Deposition‐Grown (010) β‐Ga2O3

A new record‐high room‐temperature electron Hall mobility (μRT = 194 cm2 V−1 s−1 at n ≈ 8 × 1015 cm−3) for β‐Ga2O3 is demonstrated in the unintentionally doped thin film grown on (010) semi‐insulating substrate via metal‐organic chemical vapor deposition (MOCVD). A peak electron mobility of ≈9500 cm2 V−1 s−1 is achieved at 45 K. Further investigation on the transport properties indicates the existence of sheet charges near the epilayer/substrate interface. Si is identified as the primary contributor to the background carrier in both the epilayer and the interface, originating from both surface contamination and growth environment. The pregrowth hydrofluoric acid cleaning of the substrate leads to an obvious decrease in Si impurity both at the interface and in the epilayer. In addition, the effect of the MOCVD growth condition, particularly the chamber pressure, on the Si impurity incorporation is studied. A positive correlation between the background charge concentration and the MOCVD growth pressure is confirmed. It is noteworthy that in a β‐Ga2O3 film with very low bulk charge concentration, even a reduced sheet charge density plays an important role in the charge transport properties.

Influence of Surface Compositions on the Reactivity of Pyrite toward Aqueous U(VI).

Pyrite plays a significant role in governing the mobility of toxic uranium in an anaerobic environment via an oxidation-reduction process occurring at the mineral-water interface, but the factors influencing the reaction kinetics remain poorly understood. In this study, natural pyrites with different impurities (Pb, As, and Si) and different surface pretreatments were used to react with aqueous U(VI) from pH ∼3.0 to ∼9.5. Both aqueous and solid results indicated that freshly crushed pyrites, which do have more surface Fe2+/Fe3+ and S2- sites that were generated from breakage of Fe(S)-S bonds during ball milling, exhibited a much stronger reactivity than those treated with acid washing. Besides, U(VI) reduction which involves the possible intermediate U(V) and the formation of hyperstoichiometric UO2+x(s) was found to preferentially occur at Pb- and As-rich spots on the pyrite surface, suggesting that the incorporated impurities could act as reactive sites because of the generation of lattice defects and galena- and arsenopyrite-like local configurations. These reactive surface sites can be removed by acid washing, leaving a pyrite surface nearly inert toward aqueous U(VI). Thus, reactivity of pyrite toward U(VI) is largely governed by its surface compositions, which provides an insight into the chemical behavior of both pyrite and uranium in various environments.

Experimental Liquidus Studies of the ZnO-“CuO0.5” and ZnO-“CuO0.5”-SiO2 Liquidus in Equilibrium with Cu-Zn Metal

Phase equilibria in the ZnO-“CuO0.5”-SiO2 system have been investigated at 1403-1948 K (1130-1675 °C) for oxide liquid in equilibrium with Cu-Zn metal (> 99% Cu) and solid oxide phases: (a) tridymite or cristobalite SiO2; (b) willemite Zn2SiO4; (c) zincite ZnO; and (d) cuprite Cu2O. Two-liquid immiscibility range in the high-SiO2 slags has also been studied. High-temperature equilibration on primary phase (SiO2, Cu2O), inert metal (platinum-iridium wire), or ceramic (Al2O3 for high-Cu2O slags) substrates, followed by quenching and direct measurement of Zn, Cu, Si and possible impurities (Al, Pt, Ir) concentrations in the phases with the electron probe x-ray microanalysis (EPMA) has been used to accurately characterize the system in equilibrium with metal. All results are projected onto the ZnO-“CuO0.5”-SiO2 plane for presentation purposes.

Self-organization of various “phase-separated” nanostructures in a single chemical vapor deposition

Chemical vapor deposition (CVD) is one of the most versatile techniques for the controlled synthesis of functional nanomaterials. When multiple precursors are induced, the CVD process often gives rise to the growth of doped or alloy compounds. In this work, we demonstrate the self-assembly of a variety of ‘phase-separated’ functional nanostructures from a single CVD in the presence of various precursors. In specific, with silicon substrate and powder of Mn and SnTe as precursors, we achieved self-organized nanostructures including Si/SiOx core-shell nanowire heterostructures both with and without embedded manganese silicide particles, Mn11Si19 nanowires, and SnTe nanoplates. The Si/SiOx core-shell nanowires embedded with manganese silicide particles were grown along the direction of the crystalline Si via an Au-catalyzed vapor-liquid-solid process, in which the Si and Mn vapors were supplied from the heated silicon substrates and Mn powder, respectively. In contrast, direct vapor-solid deposition led to particle-free -oriented Si/SiOx core-shell nanowires and -oriented Mn11Si19 nanowires, a promising thermoelectric material. No Sn or Te impurities were detected in these nanostructures down to the experimental limit. Topological crystalline insulator SnTe nanoplates with dominant {100} and {111} facets were found to be free of Mn (and Si) impurities, although nanoparticles and nanowires containing Mn were found in the vicinity of the nanoplates. While multiple-channel transport was observed in the SnTe nanoplates, it may not be related to the topological surface states due to surface oxidation. Finally, we carried out thermodynamic analysis and density functional theory calculations to understand the ‘phase-separation’ phenomenon and further discuss general approaches to grow phase-pure samples when the precursors contain residual impurities.

Effect of Diatomic Silicon Molecular Impurities on the Luminescent Properties of Semiconductor Solid Solutions

One of the urgent problems of materials science in the field of photovoltaics is obtaining promising novel photoactive semiconductor materials with predetermined photoelectric parameters to provide an efficient manifestation of photovoltaic, thermovoltaic, and emitting effects in the visible and near-infrared regions of the radiation spectrum. In this aspect, it is of great practical and fundamental interest to clarify the effect of diatomic molecular impurities on the electrophysical and photoelectric properties of semiconductor materials, as well as to reveal the energy levels of impurity molecules depending on the parameters of the base material. For this purpose, several semiconductor substitutional solid solutions based on elemental semiconductors such as Si and Ge, as well as III–V isoperiodic binary compounds such as GaAs and GaP, and II–VI isoperiodic binary compounds such as ZnSe and ZnS with Si2 molecular impurities have been obtained and studied. (Si2)1 – x – y(Ge2)x(GaAs)y, (ZnSe)1 – x – y(Si2)x(GaP)y, (Si2)1 – x(GaP)x and (Si2)1 – x(ZnS)x solid solutions have been grown on silicon Si (111) substrates from a limited volume of tin and lead solution-melt by means of liquid-phase epitaxy. The grown epitaxial layers have a monocrystalline structure with mirror-like smooth surfaces. The photoluminescence spectrum of solid solutions has been studied. The spectrum covers the photon energy range from 1.18 to 1.55 eV for (GaAs)0.95(Ge2)0.05〈Si2〉, from 1.38 to 3.1 eV for (GaP)0.98(Si2)0.02, from 1.55 to 3.1 eV for (ZnSe)0.88(Si2)0.03(GaP)0.09, and from 1.55 to 3.2 eV for (ZnS)0.97(Si2)0.03. It has been found that Si2 molecules in the case of (GaAs)0.95(Ge2)0.05〈Si2〉 solid solution form deep impurity energy levels lying at 1.33 eV below the bottom of the conduction band, in the case of (GaP)0.98(Si2)0.02, it is 1.47 eV, in the case of (ZnSe)0,88(Si2)0,03(GaP), 1.67 eV, and in the case of (ZnS)0.97(Si2)0.03, 1.82 eV. It is shown that in solid solutions with Si2 molecular impurities, an increase in the dissociation energy of the covalent bond in Si–Si impurity molecules is observed with increasing band gap and with decreasing crystal lattice parameter of the base semiconductor. Change in the dissociation energy of the Si–Si covalent bond in the tetrahedral crystal lattice of different semiconductors is caused by a change in the length of the Si–Si covalent bond, as well as by a change in the hybridization of the atomic electron shells of Si2 impurity molecules and base material molecules. The studied (Si2)1 – x(GaP)x and (Si2)1 – x(ZnS)x solid solutions can be used as a photoactive material for the development of photoconverters operating in the visible and near-infrared regions of the radiation spectrum.