Impurities In Silicon Research Articles

A bright future for silicon in quantum technologies

Silicon is the most widely used material in microelectronic devices; integration of atomic impurities in silicon via doping during growth or ion implant is now widely used as it allows to form conventional transistors. Exploiting all the knowledge accumulated over the last 60 years in the context of the second quantum revolution that is now underway would help accelerate the commercialization of quantum technologies. Several works have already reported that silicon can be an optically active material with point-like defects emitting below the Si bandgap, both in ensemble emission and absorption in natural Si as well as in isotopically purified 28Si, even under electrical pumping. Very recently, the detection of individual impurities in silicon opened the door for further exploitation of this indirect bandgap material to applications in quantum technologies, including single photon emission at near-infrared frequency, matching the telecommunication band and optical detection of individual spins. Here, we describe the current state-of-the-art and discuss the forthcoming challenges and goals toward a reliable exploitation of these solid-state quantum-emitters in the context of quantum technologies. In particular, we examine opportunities, issues, and challenges in controlling defect formation and localization, extrinsic effects, and integration of optical devices.

(Invited) Metallic Impurity Control in Silicon Processing

The performances of a few techniques for the analysis of metal contamination in silicon are reviewed. The chemical dissolution of a polysilicon gettering layer associated to ICP-MS (Inductively Coupled Plasma - Mass Spectroscopy) analysis, TXRF (Total Reflection X-Ray Fliuorescence) Recombination lifetime measurement techniques, DLTS (Deep Level Transient Spectroscopy) and micro-photoluminescence analyses are compared. A few case studies are discussed to test the ability of these techniques in detecting contaminants with different properties as silicon impurities. The results reported in this paper show that it is not possible to define a unique recipe that can be applied in all cases. Different approaches are required depending, on one hand, on the contaminant diffusivity and solubility, and, on the other hand, on the process step to be monitored. Finally, we show that the device itself can be a sensitive monitor of metal contamination.

Reverse Column Flotation of Ultrafine Magnetite Mixture with Fine Glass Beads Enhanced by Fine Bubbles

Magnetite ores are among the most important sources of iron, which is in high demand in the global economy. Metallurgical properties of the magnetite concentrate significantly depend on impurities of silicon- and aluminum-bearing minerals. These impurities have to be separated from ore by magnetic separation and flotation techniques. Reverse column flotation is one of the methods applied for reducing the content of impurities in magnetite concentrate. This method allows recovering impurities from ore in the froth product (flotation tailings). However, the efficiency of this method significantly decreases with the decrease in particle size. As previously demonstrated, the effectiveness of fine particle column flotation can be increased if, before feeding the pulp into a column, fine bubbles are introduced into the pulp and the pulp is then passed through a tubular reactor. The major purpose of this study was to define the effectiveness of the reverse column flotation performance of ultrafine magnetite from the mixture with fine glass beads (ballotini) when, before the pulp is fed into the flotation column, it is mixed with fine air-in-water dispersion, and the mixture is then passed through a tubular flotation reactor (TFR). The obtained experimental findings allowed the definition of the optimal conditions of the mixture treatment in TFR that ensured high concentrate grade and iron recovery for the initial iron content in the mixture of 63.76%. These conditions were defined as follows: treatment time of the mixture in the TFR—7.5 s; average flow shear rate inside the TFR—1000 s−1; volume dosage of fine bubbles per solid mass unit—0.032 or 0.21 mL/g. At the fine bubble dosage of 0.032 mL/g, the iron recovery and the concentrate grade were, respectively, 88.1% and 68.3% Fe, and at the dosage of 0.21 mL/g, the iron recovery reached 89.4% for the concentrate grade of 68.7% Fe.

Impurity Gettering in Polycrystalline‐Silicon Based Passivating Contacts—The Role of Oxide Stoichiometry and Pinholes

AbstractPolycrystalline‐silicon/oxide (poly‐Si/SiOx) passivating contacts for high efficiency solar cells exhibit excellent surface passivation, carrier selectivity, and impurity gettering effects. However, the ultrathin SiOx interlayer can act as a diffusion barrier for metal impurities and this potentially slows down the overall gettering rate of the poly‐Si/SiOx structures. Herein, the factors that determine the blocking effects of the SiOx interlayers are identified and investigated by examining two general types of the SiOx interlayers: 1.3 nm ultrathin tunneling SiOx with negligible pinholes and 2.5 nm SiOx with thermally created pinholes. Iron is used as tracer impurity in silicon to quantify the gettering rate. By fitting the experimental gettering kinetics by a diffusion‐limited segregation gettering model, the blocking effects of the SiOx interlayers are quantified by a transport parameter. Both the oxide stoichiometry and pinhole density affect the effective transport of iron through SiOx interlayers. The oxide stoichiometry depends strongly on the oxidation method, while the pinhole density is affected by the activation temperature, doping concentration, doping technique, and possibly the dopant type as well. To enable a fast gettering process during typical high‐temperature formation of the poly‐Si/SiOx structures, a SiOx interlayer that is less stoichiometric or with a higher pinhole density is preferred.

Removal of Silicon, Aluminum and Phosphorus Impurities from Low-grade Iron Ore by Reverse Froth Flotation and Alkaline Roasting

Removal of Silicon, Aluminum and Phosphorus Impurities from Low-grade Iron Ore by Reverse Froth Flotation and Alkaline Roasting

Impurity-impurity interaction during the growth of UMG-Si-based mc-Si

This article investigates the relationship between the chemical composition and electrophysical properties of p- and n-type multicrystalline silicon ingots based on metallurgical silicon with a purity of 99.99 at.%. In particular, the role of impurity-impurity interactions in the production of multisilicon by the Bridgman vertical method is evaluated in order to identify approaches to controlling this process effectively. The phase equilibrium calculations in the “silicon–all impurities” and “silicon-impurity-oxygen” systems were carried out based on the Gibbs energy minimization in the Selector software package. The study investigates the rank correlations of the concentrations of various impurities with each other, as well as with the specified electrical resistivity (SER) and the lifetime of nonequilibrium charge carriers (NCC) in the direction of crystal growth. Pair correlations of the element distribution profiles were considered based on the role of the main factor represented by the ratio of individual impurity solubilities in solid or liquid silicon (k0), as well as from the standpoint of direct interaction between two elements. It was found that the k0 value for two individual impurities in silicon does not automatically lead to the pair correlation of their distribution profiles in the ingot. A significant effect on the distribution profiles of impurities in multisilicon with k0→0 has the factor of binding some part of the impurity into such a form that this impurity can be incorporated easily into a growing crystal. Binding may be induced by the interaction of the impurity in the melt with the oxygen background, its segregation at the grain boundaries, and its capture by the crystallization front in the composition of the liquid inclusion. Significant correlations of impurity distribution profiles in the ingot were demonstrated by the pairs whose elements interact without the formation of chemical compounds in the 25–1413 °C temperature range. The conducted phase equilibrium calculations for the “silicon–all impurities” system revealed the possibility of forming the VB2, TiB2, ZrB2, and MgTiO4 solid phases in the melt.

Framework for Donor-Qubit Spatial Metrology in Silicon with Depths Approaching the Bulk Limit

Impurities in silicon are fundamental to a variety of modern nanoscale technologies working in both classical and quantum regimes of operation. The aggressive miniaturization of electronic devices has reduced their size to the nanometer scale, where the exact count and positioning of a few impurity atoms dictates their overall operation and performance. In the emerging area of quantum hardware development, single-impurity spins in silicon form excellent qubits and identifying their exact locations is important to engineer two-qubit interactions for high-fidelity quantum operations and the associated quantum control systems. This work reports a theoretical framework for the spatial metrology of single-impurity atoms in silicon with exact atomic precision for impurity depths approaching the bulk limit. The application of a carefully designed electric field pulls the impurity wave functions toward the surface, leading to spatially resolved scanning tunneling microscope images of electronic states exhibiting features that distinctly depend on the exact locations of the impurity atoms beneath the silicon surface. After verification of the developed metrology technique for individual atom positions, we train a machine-learning algorithm that can autonomously perform the metrology with high throughput in the presence of noise commensurate with experimental measurements. A future experimental implementation of the established capability for impurity-atom characterization is anticipated to play an important role in the design of a wide range of electronic and quantum devices.

Compositional partitioning during early stages of oxidation of a uranium-molybdenum alloy

Compositional partitioning during uranium alloy oxidation was studied via complementary ex situ - in situ atom probe tomography. Nanoscopic volumes of uranium- 22 at. % molybdenum were exposed to air at room temperature/atmospheric pressure for 30–60 minutes (ex situ), and 300 ∘C - 10−5 mbar O2 gas for 2–5 minutes in a chemical reaction chamber attached to an atom probe system (in situ). For all environmental conditions, a hypostoichiometric uranium oxide is formed. Reaction fronts are observed at oxide/metal, oxide/hydride, and outer oxide/environment interfaces. Results reveal Mo redistributes across the oxide/metal interface, with a tendency for enrichment in the outer oxide. The formation of a hydrogen-rich subsurface layer between the oxide and base alloy accompanies oxidation in both air and oxygen gas environments. Carbon and silicon impurity elements also redistribute to the outer oxide, contributing to oxide film composition.

Thermal Activation of Valley-Orbit States of Neutral Magnesium in Silicon

Interstitial magnesium acts as a moderately deep double donor in silicon, and is relatively easily introduced by diffusion. Unlike the case of the chalcogen double donors, parameters of the even-parity valley-orbit excited states 1s(T2) and 1s(E) have remained elusive. Here we report on further study of these states in neutral magnesium through temperature dependence absorption measurements. The results demonstrate thermal activation from the ground state 1s(A1) to the valley-orbit states, as observed by transitions from the thermally populated levels to the odd-parity states 2p0 and 2p±. Analysis of the data makes it possible to determine the thermal activation energies of transitions from the donor ground state to 1s(T2) and 1s(E) levels, as well as the binding energies of an electron with the valley-orbit excited states. Keywords: magnesium impurity in silicon, deep center, optical spectroscopy.

Исследования комплексного дефектообразования в кремнии, легированном никелем

The behavior of nickel and hydrogen impurities in silicon has been studied by deep-level transient spectroscopy (DLTS) and Raman spectroscopy. It is shown that the vibrations of Ni and H atoms in Si are in the range of 925-985 cm-1. The DLTS spectra in n - and p -type silicon doped with Ni atoms exhibit two deep levels with energies Ev +0.17 eV and Ec -0.42 eV. It was found that during chemical etching in Si doped with Ni, various hydrogen-bonded defect complexes are formed, the energies of which are Ec -0.18 eV, Ec -0.54 eV, Ev +0.26 eV, and Ev +0.55 eV.

Evaluation of the Use of Burkolderia Caribensis Bacteria for the Reduction of Phosphorus Content in Iron Ore Particles

With the growing industrialization, there is a constant increase in the world demand for iron ore, thus implying an increase in the volume of mining, resulting in the extraction of ores with higher levels of silicon, phosphorus and sulfur impurities. Therefore, it is highly relevant to use beneficiation techniques that allow the use of iron ores with high levels of impurities, and it is known that a widely used alternative is to perform a pre-treatment of iron ore particles in order to reach the limit of acceptable phosphorus before application in steel production. Therefore, this study aims to evaluate the pre-treatment process of iron ore particles aiming at reducing the phosphorus content using the bacterial strain burkolderia caribensis in contaminated iron ore samples. Through the rotatable central composite design technique, it was possible to evaluate the best conditions for the use of the bacterial strain to reduce the phosphorus content in the particles. The results indicated that the developed process has relevant applicability. The results contribute to a better understanding of the use of the bacterial strain as a pretreatment process to reduce the phosphorus content of iron ore particles and also to the development of new cleaner technologies.

Hydrometallurgical process development to recycle valuable metals from spent SCR deNOX catalyst

Spent catalyst, containing vanadium and tungsten oxide in a TiO2 glass fiber matrix, pose a risk of environmental contamination due to the high toxicity of its metal oxides if leached into the soil when disposed in landfills. Due to the increasing demand of metals and the continuous depletion of primary resources there is an growing necessity for recycling and reprocessing of spent catalysts and other secondary metal sources for environmental and economical reasons. Study of spent SCR catalyst soda roasting process with dissolved NaOH compared with the usual NaOH dry roasting and its influence in the subsequent water leaching. After optimization, the ideal parameters are roasting using a 0.4 ratio of NaOH/spent SCR catalyst in solution for 2 h at 973 K and de-ionized water leaching for 30 min, at 298 K with a pulp density of 30%. The research results show an important reduction of the roasting temperature and leaching time during the processing of spent SCR catalyst obtaining a 95.4% W and 80.2% V leaching efficiency liquor. Silicon compounds are one of the main impurities leached alongside the valuable metals and in this work, the silicon compounds leached are reduced significantly with the aim of avoiding the de-silication post-processing of the leach liquor. The main advantage of the proposed process is the increase of the leaching efficiency of vanadium and tungsten with a minimization of silicon impurities in a shorter time regardless of the leaching temperature.

Flotation separation of quartz from gypsum using benzyl quaternary ammonium salt as collector

Phosphogypsum is a kind of industrial by-product, which could be used as building materials instead of natural gypsum after removing impurities. Flotation is an effective method to remove the silicon-containing impurity quartz in the phosphogypsum. In this work, a surfactant tetradecyl dimethyl benzyl ammonium chloride (TDBAC) was first used as the collector of quartz to be separated from gypsum. The flotation behaviors of quartz and gypsum were investigated by micro-flotation experiments. The flotation mechanisms were analyzed through Zeta potential testing, Fourier Transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Micro-flotation results show that TDBAC exhibited outstanding collecting capacity and selectivity for quartz over the wide pH range of 2.5–9.5. FTIR results declared that TDBAC could be successfully adsorbed on the surface quartz. The analysis results of zeta potential and XPS indicate that the interaction mechanisms of TDBAC with quartz were involved electrostatic attraction and hydrogen-bond interaction, whereas the adsorption onto gypsum surface was only by the weak electrostatic attraction and hydrogen-bond interaction. The results of this work showed that TDBAC is expected to be an effective collector of quartz for removing silicon impurity from the industrial phosphogypsum.

Морфологические, структурные и оптические свойства микропорошков диоксида титана

The work is devoted to a comprehensive experimental study of the morphology, structure and optical properties of titanium dioxide micropowders with several of modern analytical equipment. The method for obtaining photoluminescence (PL) and Raman scattering (RS) for titanium dioxide micropowders in microresonator cuvette (photon traps) was described. The powders consisting of close-packed particles in the shape of spherical particles of specified sizes (29-63 μm) were studied. In the photon traps, a certain mode is implemented, which is associated of trapping of the exciting radiation inside of device. It is shown that, at room temperature, intense photoluminescence in titanium dioxide micropowders (2.91 eV) was observed in excitation by the second optical harmonic (λ exc = 255.3 nm) of a copper vapor laser. It has been established that in micropowders of titanium dioxide micropowders in photon traps, is possible to observe the phenomenon of combination opalescence, which leads to the sharp (5–6 orders) increasing in the RS intensity in an ultradispersed medium. The high conversion efficiency of the exciting radiation into the RS signal is explained by the large value of the total path that the exciting radiation photon travels in the dispersed medium in the photon trap. It was found that an insignificant amount of silicon and aluminum impurities is present in titanium dioxide micropowders that not affect to formation of the photoluminescence band in the region of 2.91 eV. The developed method for recording PL and RS opens up wide possibilities for recording weak signals of secondary radiation that is important for inorganic and organic substances, as well as for creating small-sized laser analyzers of chemical compounds which is necessary for solving many practical problems.

Etched-And-Regrown GaN P-N Diodes with Low-Defect Interfaces Prepared by In Situ TBCl Etching.

The ability to form pristine interfaces after etching and regrowth of GaN is a prerequisite for epitaxial selective area doping, which in turn is needed for the formation of lateral PN junctions and advanced device architectures. In this work, we report the electrical properties of etched-and-regrown GaN PN diodes using an in situ Cl-based precursor, tertiary butylchloride (TBCl). We demonstrated a regrowth diode with I-V characteristics approaching that from a continuously grown reference diode. The sources of unintentional contamination from the silicon (Si) impurity and the mediating effect of Si during the TBCl etching are also investigated in this study. This work points to the potential of in situ TBCl etching toward the realization of GaN lateral PN junctions.

Electropurification of Reusable CaCl2-Based Molten Salt for Silicon Electrorefining

Electrorefining in molten salt is a promising solar-grade silicon production process due to its low-cost, energy-saving and eco-friendly characteristics. After the silicon electrorefining process, the impurities in the metallurgical-grade silicon are induced into the salt. However, the salt has to be ultrapure and the impurity concentration in molten salt must be strictly controlled to produce high-purity silicon. This work electropurified the eutectic CaCl2-CaF2 molten salt after the silicon electrorefining process. The cyclic voltammetry is used to identify the impurity level of the salt. The single-potential and multi-potential electropurification process have been conducted and compared. The multi-potential electropurification process is proven to be more efficient in impurity removal. After the multi-potential electropurification process, the salt can be regenerated to its original impurity level, indicating a reusable high-purity salt for solar-grade silicon electrorefining.

Simultaneous Observation of Silicon and Boron Impurity Behaviors in the Core Region of a Mid-Density LHD Plasma

Simultaneous Observation of Silicon and Boron Impurity Behaviors in the Core Region of a Mid-Density LHD Plasma

Influence of silicon on the structure and properties of high-strength cast aluminum alloy AZ5NF

The phase composition and the mechanical properties of the AZ5NF with various contents of silicon impurities alloy were investigated by both calculation and experimental methods. For evaluation of the alloy phase composition and the effect of the chemical composition, CALPHAD method integrated in Thermo-Calc software was applied. Further scanning electron microscopy technique a significant increase of the volume fraction of Mg2Si phase with an increase of silicon content from 0.1% to 0.3%. It is shown that in case of silicon content higher than 0,2 wt.% Mg2Si phase starts forming during equilibrium crystallization. This phase forms as the thin layers at the grain boundaries. This leads to brittle intergranular fracture and significant reduction of the alloy ductility both in the as-cast state and in T6 temper. Based on this limits for the silicon content in AZ5NF have been proven. It is shown that dissolution of Mg2Si phase (full or partial depending on silicon content) requires solid solution treatment. Following quenching and artificial ageing provide high strength and ductility.

Single Indium Atoms and Few-Atom Indium Clusters Anchored onto Graphene via Silicon Heteroatoms.

Single atoms and few-atom nanoclusters are of high interest in catalysis and plasmonics, but pathways for their fabrication and placement remain scarce. We report here the self-assembly of room-temperature-stable single indium (In) atoms and few-atom In clusters (2–6 atoms) that are anchored to substitutional silicon (Si) impurity atoms in suspended monolayer graphene membranes. Using atomically resolved scanning transmission electron microscopy (STEM), we find that the symmetry of the In structures is critically determined by the three- or fourfold coordination of the Si “anchors”. All structures are produced without electron-beam induced materials modification. In turn, when activated by electron beam irradiation in the STEM, we observe in situ the formation, restructuring, and translation of the Si-anchored In structures. Our results on In–Si-graphene provide a materials system for controlled self-assembly and heteroatomic anchoring of single atoms and few-atom nanoclusters on graphene.

Effects of Strontium Content on the Microstructure and Ionic Conductivity of Samarium-Doped Ceria

Due to its high oxygen ion conductivity at elevated temperatures, samarium-doped ceria (SDC) is a very promising material for application in solid state electrochemical devices and especially in the electrolytes of solid oxide fuel cells. Several prior studies have reported a further improvement in the ionic conductivity of SDC on doping with small amounts of strontium. It is suggested that strontium acts as a sintering aid—improving the microstructure of SDC—and as a scavenger of silicon impurities, decreasing its tendency to form resistive phases at grain boundaries. However, because of the range of preparation methods and the resulting differences in microstructure and silicon levels, some inconsistencies exist in the literature. Furthermore, the effect of strontium on the intrinsic (bulk) conductivity of SDC is not often discussed. To address these issues, a systematic, combined microstructural and conductivity study has been performed on a compositional series with a range of strontium contents, Ce0.8−xSm0.2SrxO2−δ (x = 0, 0.002, 0.005, 0.01, 0.02, 0.03, 0.04). A low temperature synthesis affording products with low silicon was employed. Total bulk and grain boundary conductivity data were obtained over a wide temperature range. Increasing strontium content caused a general decrease in total and intrinsic conductivity, but there was an improvement in grain boundary conductivity at the lowest strontium levels. These results were interpreted by reference to the microstructures using, among other parameters, the blocking, and normalised blocking, factors.