Fluorine Impurities Research Articles

Application of secondary ion mass spectrometer for measuring the diffusion profiles in alkali-halide crystals

Depth profiles of magnesium, fluorine and oxygen impurities was examined in the surface layers of alkali-halide KBr crystals using method of secondary ion mass spectrometry. Samples of potassium bromide, coated with a surface film of magnesium fluoride were subjected to isothermal diffusion annealing in air at various times. It is shown that the diffusion of O ions occurs from the ambient atmosphere besides the diffusion of Mg and F ions during annealing of KBr crystals. Accurate estimation of the diffusion coefficients of cationic impurity Mg requires taking into account the possible interaction of this impurity and oxygen.

Optical and transport measurement and first-principles determination of the ScN band gap

The electronic structure of scandium nitride is determined by combining results from optical and electronic transport measurements with first-principles calculations. Hybrid functional Heyd-Scuseria-Ernzerhof (HSE06) calculations indicate a 0.92 eV indirect \ensuremath{\Gamma} to $X$ band gap and direct transition energies of 2.02 and 3.75 eV at $X$ and \ensuremath{\Gamma} points, respectively, while ${G}_{\mathrm{o}}{W}_{\mathrm{o}}$ and $G{W}_{\mathrm{o}}$ methods suggest 0.44--0.74 eV higher gap values. Epitaxial ScN(001) layers deposited on MgO(001) substrates by reactive sputtering exhibit degenerate $n\text{-type}$ semiconductor properties with a temperature-independent electron density that is varied from $N=1.12\ensuremath{-}12.8\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ using fluorine impurity doping. The direct optical gap increases linearly with $N$ from 2.18 to 2.70 eV, due to a Burstein-Moss effect. This strong dependence on $N$ is likely the cause for the large range (2.03--3.2 eV) of previously reported gap values. However, here extrapolation to $N=0$ yields $2.07\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ for the direct $X$ point transition of intrinsic ScN. A reflection peak at $3.80\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ is independent of $N$ and in perfect agreement with the HSE06-predicted peak at 3.79 eV, associated with a high joint density of states (DOS) near the \ensuremath{\Gamma} point. The electron mobility at 4 K is $100\ifmmode\pm\else\textpm\fi{}30\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{Vs}$ and decreases with temperature due to scattering at polar optical phonons with characteristic frequencies that decrease from 620 to $440\ifmmode\pm\else\textpm\fi{}30\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, with increasing $N$, due to free carrier screening. The transport and DOS electron effective mass, determined from measured intra- and interband transitions, respectively, are $0.40\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.16em}{0ex}}{m}_{\mathrm{o}}$ and $0.33\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.16em}{0ex}}{m}_{\mathrm{o}}$, in good agreement with the first-principles predictions of ${m}_{tr}=0.33\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}{m}_{\mathrm{o}}$ and ${m}_{\mathrm{DOS}}=0.43\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}{m}_{\mathrm{o}}$. The ScN refractive index increases with increasing $h\ensuremath{\nu}=1.0\ensuremath{-}2.0\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ from 2.6--3.1 based on optical measurements and from 2.8--3.2 based on the calculated dielectric function. An overall comparison of experiment and simulation indicates (i) an overestimation of band gaps by $\mathit{GW}$ methods, but (ii) excellent agreement with a deviation of \ensuremath{\le}0.05 eV for the hybrid functional and (iii) a value for the fundamental indirect gap of ScN of $0.92\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$.

DFT calculations of tin dioxide crystals containing heavily-doped fluorine

First-principles calculations based on the density functional theory (DFT) within the generalized gradient approximation have been used in the present research. Fluorine doping in the SnO 2 crystals has been carried out considering a number of different defect concentrations. Dopant influence upon structural, electronic and electrical properties of the tin dioxide has been discussed in detail. The system presents n-type electrical conductivity relating our work directly to a number of empirical studies in this area. An experimental fact that n-type conductivity tends to decrease at rather high fluorine impurity rates has been explained at the theoretical level.

Solvent extraction of vanadium from a stone coal acidic leach solution using D2EHPA/TBP: Continuous testing

This paper reports on continuous testing of a process for the recovery of vanadium from a H2SO4-HF solution, generated from the leaching of stone coal, by solvent extraction using a D2EHPA/TBP mixture as the extractant. Three unit processes are included: fully continuous countercurrent solvent extraction and stripping; oxidation and precipitation; and calcination. The solvent-extraction circuit was operated for 96h. With six stages of countercurrent extraction, 97.7% vanadium, 98.3% molybdenum, and 97.9% titanium were extracted, while the sodium, potassium, fluorine, and nickel impurities were barely extracted. Scrubbing removed co-extracted and aqueous-entrained zinc, magnesium, silicon, and arsenic impurities. Most co-extracted impurities were scrubbed in two stages. Using five stages of stripping, more than 99.8% of the vanadium was stripped, yielding a vanadium concentration in the loaded strip solution of 34.1g/L. Co-extracted molybdenum(VI), titanium(IV), and iron(III) remaining in the stripped organic phase were removed by a saturated sodium carbonate solution and the regenerated organic was reused for vanadium extraction. Vanadium was selectively precipitated with 98.5% efficiency from the loaded strip solution by oxidation and precipitation with ammonium sulfate. A high-purity V2O5 (99.61%) product was successfully produced by this flowsheet.

Oxygen vacancies versus fluorine at CeO2(111): a case of mistaken identity?

We propose a resolution to the puzzle presented by the surface defects observed with STM at the (111) surface facet of CeO 2 single crystals. In the seminal paper of Esch et al. [Science 309, 752 (2005)] they were identified with oxygen vacancies, but the observed behavior of these defects is inconsistent with the results of density functional theory (DFT) studies of oxygen vacancies in the literature. We resolve these inconsistencies via DFT calculations of the properties of both oxygen vacancies and fluorine impurities at CeO2(111), the latter having recently been shown to exist in high concentrations in single crystals from a widely used commercial source of such samples. We find that the simulated filled-state STM images of surface-layer oxygen vacancies and fluorine impurities are essentially identical, which would render problematic their experimental distinction by such images alone. However, we find that our theoretical results for the most stable location, mobility, and tendency to cluster, of fluorine impurities are consistent with experimental observations, in contrast to those for oxygen vacancies. Based on these results, we propose that the surface defects observed in STM experiments on CeO2 single crystals reported heretofore were not oxygen vacancies, but fluorine impurities. Since the similarity of the simulated STM images of the two defects is due primarily to the relative energies of the 2p states of oxygen and fluorine ions, this confusion might also occur for other oxides which have been either doped or contaminated with fluorine.

Asymmetric transport property of fluorinated graphene

Carrier transport properties of fluorinated graphene with various fluorination rates are presented. Onset of transition from insulating to metallic conduction is observed in dilute fluorinated graphene. Highly fluorinated graphene shows electron-hole asymmetry in transport properties and local resistivity maximum at the hole conduction region, which are presumably caused by the existence of resonant fluorine impurities. Drastic change of the asymmetric feature occurs after removing fluorine atoms and creating structural defects by thermal annealing. These results suggest that the type of impurities or defects in graphene is detectable by examining asymmetry in transport properties.

Oxygen doped SiC nanocrystals: first principles study of the optical properties

We have studied a typical spherical SiC nanocrystal with a diameter of 1.2 nm (Si43C44H76) using linear combination of atomic orbitals in combination with pesudopotential density functional calculation. The role of fluorine and oxygen impurities was investigated on the electronic and optical properties of the Si43C44H76 nanocrystal. Total energy calculations show that the fluorine doped Si43C44H76 nanocrystals are unstable. Oxygen doped Si43C44H76 have different binding energies in various substitutional and interstitial defects. The maximum binding energy of the oxygen at carbon substitutional defect is about -0.5 eV and at interstitial defect is -0.18 eV. The HOMO-LUMO gap of the pure Si43C44H76 is about 6.71 eV and after doping with oxygen changes on the order of 0.1 eV. Our studies show that the refractive index of the doped Si43C44H76 nanocrystal significantly dispersed in comparison with pure SiC nanocrystal especially at the range of 6 to 8 eV.

Effect of fluorine, nitrogen, and carbon impurities on the electronic and magnetic properties of WO3

Within electron density functional theory with the use of the Vienna ab-initio simulation package (VASP), the effect of the sp substitutional impurities of fluorine (n-type dopant), nitrogen, and carbon (p-type dopants) on the electronic and magnetic properties of tungsten trioxide WO3 is studied. It is established that these impurities induce the transformation of tungsten trioxide (nonmagnetic semiconductor) into nonmagnetic metal (WO3:F), magnetic semimetal (WO3:N), or magnetic metal (WO3:C) states.

Ab initio study of the formation of structural kinks in carbon chains

Our theoretical studies using methods of electron density functional theory show that hydrogen, fluorine, and oxygen impurities must be present in the chamber during the synthesis of linear-chain carbon to form and stabilize experimentally observed kinks in carbon chains. The kink in the carbon chain forms during the adsorption of impurity atoms onto the chain. It is shown that it is energetically advantageous for impurity atoms to form kinks rather than to occupy other adsorption positions. The optimal kink angle of carbon chains is determined. The stability of bent carbon chains is estimated as a function of the length of linear fragments. It is shown that the formation of kinks leads to that the formation of a narrow band gap near the Fermi level: EgH = 0.56 eV, EgF = 0.59 eV, and EgO = 1.04 eV.

Doslidzhennya metodamy yadernoho mikroanalizu poverkhnevykh nanoshariv struktury zoloto−kremniy

The Rutherford backscattering and particle-induced X-ray emission methods are used to study the surface layers in gold–silicon structures, the parameters of which govern the operational characteristics of electron devices constructed on their basis. The measurements are performed on a high-precision microanalytical unit “Nuclear scanning probe” recently put into operation at the “Spectrum” laboratory. The thicknesses of a gold layer sputtered onto the specimen surface were about 17 and 20 nm for two different specimens. The layer non-uniformity was less than 1.6 nm and did not exceed the experimental error. A substantial amount of the fluorine impurity was revealed under the gold layer in the gold–silicon interface layer. Probably, it may be remnants of fluorine remaining after the etching of the silicon surface in a mixture of acids that included the hydrofluoric one (HF). The amount of fluorine detected in a series of measurements was found to strongly correlate with the current of alpha-particles at the target surface during the spectrum measurement. Since the local heating of the target depends on the current, it is evident that the local diffusion rate of fluorine atoms varied over the target surface.

Improvement of a-Si:H solar cell performance by SiH4 purging treatment

Amorphous silicon (a-Si:H) thin film solar cells were prepared in a single chamber large area plasma enhanced chemical vapor deposition (PECVD) system. A purging process using silane (SiH4) gas was developed to remove the residual contaminations in the reactor after a nitrogen trifluoride (NF3) plasma dry cleaning process. Such a purging treatment leads to a clear improvement in initial fill factor (FF) and in efficiency of as-prepared a-Si:H solar cells. Secondary ion mass spectroscopy (SIMS) results demonstrate that fluorine impurity concentration [F] at the p-layer as well as p/i interface of solar cells reduces by more than one order of magnitude after this purging process. Additionally, high [F] is accompanied with high oxygen impurity concentration [O] which plays a great role in the solar cell performance. Low degradation rate of open circuit voltage (Voc) and fill factor (FF) of solar cells after a purging process after a 1000 h light soaking further illustrates an improvement in the material properties. Implanting such a purging process in the practical production line, about 2 W in power for a-Si:H solar modules (1.1 m × 1.3 m) are gained and meanwhile the champion solar module (1.1 m × 1.3 m) of stabilized power of 113 W with 160 nm thick intrinsic layer has been achieved.

Entangling Single Photons from Independently Tuned Semiconductor Nanoemitters

Quantum communication systems based on nanoscale semiconductor devices is challenged by inhomogeneities from device to device. We address this challenge using ZnMgSe/ZnSe quantum-well nanostructures with local laser-based heating to tune the emission of single impurity-bound exciton emitters in two separate devices. The matched emission in combination with photon bunching enables quantum interference from the devices and allows the postselection of polarization-entangled single photons. The ability to entangle single photons emitted from nanometer-sized sources separated by macroscopic distances provides an essential step for a solid-state realization of a large-scale quantum optical network. This paves the way toward measurement-based entanglement generation between remote electron spins localized at macroscopically separated fluorine impurities.

Low-threshold ZnSe microdisk laser based on fluorine impurity bound-exciton transitions

Impurity states in semiconductors, in which two long-lived ground states can be optically coupled to a single excited state, provide a powerful mechanism for applications including lasing without inversion, electromagnetically induced transparency, and optically addressable quantum memory for quantum information processing. We report low-threshold lasing from fluorine-doped ZnMgSe/ZnSe quantum wells in microdisk cavities. The lasing mechanism was studied by power-dependent photoluminescence spectroscopy. Lasing thresholds lower than 50Wcm−2 were observed and the fraction of spontaneous emission contributed to the lasing modes was about β=0.03–0.1.

Effect of synthesis parameters on the composition and properties of the product silica

The properties of two silica samples were studied; one sample precipitated by ammonia from a saturated (NH4)2SiF6 solution and the other washed out from the sublimate obtained by joint evaporation of (NH4)2SiF6 and SiO2. These silicas are fundamentally different compounds. Their chemical composition was determined. Evolution of samples during heating to 1000°C was interpreted using chemical analysis, IR spectroscopy, and X-ray powder diffraction. A possibility of removing fluorine and ammonia from test samples by heat and chemical treatment is demonstrated. Fluorine impurities in the form of fluoroammonium salts are removed completely during heating to 300–400°C; surface fluoride ions are removed only upon heating to 800°C.

F–B-codoping of anodized TiO 2 nanotubes using chemical vapor deposition

Chemical vapor deposition (CVD) was firstly used to simultaneously codope fluorine and boron into TiO 2 nanotubes anodized Ti in C 2H 2O 4 · 2H 2O + NH 4F electrolyte. F–B-codoping was successfully carried out by annealing the anodized TiO 2 nanotubes through CVD, as evidenced from XPS analysis. SEM images showed that the higher the annealing temperature, the greater structure damage of F–B-codoped sample. XRD results confirmed that annealing temperature had an influence on the phase structure and boron and fluorine impurities could retard anatase–rutile phase transition. F–B-codoped samples displayed remarkably strong absorption in both UV and visible range. Under visible-light irradiation, F–B-codoped samples showed the higher I ph and catalytic activity in methyl orange photoelectrodegradation than F-doped sample and B-doped sample. This showed a convincing evidence of F–B-codoping of TiO 2 had an obvious synergistic effect on the enhancement of photocurrents and photoelectrocatalytic activity.

Plasma-Enhanced Atomic Layer Deposition of TaN Thin Films Using Tantalum-Pentafluoride and N2 ∕ H2 ∕ Ar Plasma

TaN thin films were grown by plasma-enhanced atomic layer deposition using tantalum-pentafluoride as the metal precursor and plasma as the reducing agent at a temperature of . When the source pulse time exceeded , the thickness per cycle of TaN thin films was saturated at and the resistivity was about . Under this condition, the TaN thin films had a high density , which was very close to the theoretical value , and fluorine or hydrogen impurities were below detection limit. The resistivity and ratio of the TaN thin films increased with the plasma time, and did not saturate. In addition, increasing the ratio induced a sudden rise in the resistivity, which was related to the formation of a phase. This dielectric phase was reduced by lowering the ratio. In addition, when plasma post-treatment was applied to the deposition process, the resistivity of the TaN thin films was reduced to .

Investigation of excitons bound to fluorine donors in ZnSe

The electrical and optical properties of fluorine impurities in ZnSe have been studied. Bulk and delta-doped ZnSe:F layers were grown by molecular beam epitaxy. Electrical measurements reveal evidence that a majority of fluorine impurities act as shallow donors. In delta-doped samples, we find strong photoluminescence due to the radiative recombination of fluorine donor bound excitons. We measured a fluorescence lifetime of less than 250 ps and determined the inhomogeneous line width to be below 2 meV at liquid helium temperature.

Donor passivation in pseudomorphic-high electron mobility transistors due to plasma-incorporated fluorine impurities observed using x-ray photoemission spectroscopy

To investigate the effect of plasma-incorporated fluorine on Si donors in pseudomorphic-high electron mobility transistors (P-HEMTs), we used x-ray photoemission spectroscopy to analyze three layers near the Si δ-doped layer and the Si δ-doped layer itself, in which we previously found fluorine accumulation after post-thermal annealing following fluorocarbon-based plasma exposure. For this evaluation, we developed controllable and low-speed AlGaAs wet-chemical etching using citric-acid-based wet etchant. We used it to expose one of the layers to be analyzed: one 7.5 nm above the Si δ-doped layer, one 1.5 nm above it, the δ-doped layer itself, and one 1.5 nm below it. We found that the accumulated fluorine localized in the δ-doped layer and reacted with Si donors. This is apparently the main reason for the carrier passivation in the fluorocarbon-based plasma-exposed P-HEMTs.

Impurity transport in edge plasmas with application to liquid walls

The transport of impurity ions in a magnetically confined plasma is studied in the region between their origin at a material surface and the core plasma as defined by closed magnetic flux surfaces. The focus is physics understanding of the results of two-dimensional (2-D) transport modeling of the plasma and neutrals. A simple one-dimensional model is introduced to identify key processes and illustrate how such processes affect the core-edge impurity level. The 2-D simulation gives detailed results of scaling of the impurity level with parameters such as anomalous radial diffusion, hydrogen–plasma recycling, core power flux, and core-edge density. The results are obtained for a slab model of a tokamak, but by changing the magnetic connection length, scaling to other types of devices can be inferred. Lithium and fluorine impurities are considered explicitly as examples with low and moderate charge-state number, Z, for liquid wall materials; trends found for these cases provide guidance to the behavior of other impurities. The tolerable amount of impurity influx can be closely associated with the partial thermal collapse of the edge plasma. The results are used to provide a physics picture of previous results on the acceptable evaporative impurity flux from different types of liquid wall materials, and to show how these results can be expected to scale with parameters.

Mechanical and electrical properties of fluorine-containing niobate ferroelectric ceramics

The presence of fluorine impurity in alkali metal niobate ceramics sharply deteriorates both mechanical and electrical properties of these ferroelectric materials. This effect is related to the separation of a glassy phase and the formation of an inhomogeneous microstructure, these factors giving rise to local stresses in the structural elements. It is concluded that intact niobate ferroelectric articles can be made of technical-grade Nb2O5 only provided that the initial material contains a minimum amount of fluorine impurity (below 0.2 wt%).