Small Impurity Research Articles

Hydrogen isotope electromigration and impurity effect in vanadium

The effective charge of protium and tritium in the α-phase of vanadium has been measured by means of a stationary state electromigration experiment in a temperature region from 323 to 523 K. The obtained results have shown a definite mass dependence such as Z *(T)> Z *(H) in a lower temperature region and a tendency that the effective charge of H and T approaches a same value in a higher temperature region. By comparing this result with the data for protium and deuterium previously reported by other investigators, we have concluded a mass dependence according to the sequence Z *(T)> Z *(D)> Z *(H) in a lower temperature region. The origins of this mass dependence has been discussed by decomposing the effective charge into the direct and wind charges. The trapping effect due to a small zirconium impurity in the investigated vanadium specimen on the electromigration of hydrogen isotopes has been also examined and the trapping model has been discussed.

The low-temperature afterglow in N 2 –ε·CH 4 gas mixtures

The influence of the small methane impurities on the pure nitrogen afterglow was studied in DC flowing plasma reactor in wide range of methane concentrations (0.01–400 ppm) at different wall temperatures of the reactor observation part (77–300 K). The relative vibrational distributions of $${\text{N}}_{\text{2}} ({\text{C}}^{\text{3}} \Pi _u ){\text{ and N}}_{\text{2}}^{\text{ + }} ({\text{B}}^{\text{2}} \sum _{\text{u}}^{\text{ + }} )$$ states have been calculated from the recorded spectra in pure nitrogen. We observed strong quenching of the nitrogen pink afterglow at methane concentrations of a few ppm, however the pink afterglow intensity was growing up at the methane concentrations under 1 ppm. Simultaneously, the maximum pink afterglow intensity was observable at later decay times with the increase of the methane concentration. At low wall temperatures, especially at later decay times, we observed extremely high sensitivity of the pure nitrogen to the methane pretence in the discharge. Thus we are able to detect the methane concentrations in order of 0.01 ppm. We also observed the higher transitions of the CN violet system which are usually observed in the spectra of space emission sources. The method detecting hydrocarbon and fluorocarbon impurities in pure nitrogen is based on the results of the above mentioned experiments. We present first results of some experiments studying the polyhydrocarbon destruction rates measured by this new sensitive method. Finally, we designed also the simple kinetic model describing the processes during the afterglow in the N2–e·CH4 mixture.

Optical characteristics of a transverse discharge in neon at atmospheric pressure

We present the results of investigation into radiation of a pulsed transverse discharge in neon at a pressure of 10–200 kPa. Survey spectra of plasma radiation, time characteristics of radiation, and the effect of small impurities of water vapors and air on the optical characteristics of a neon plasma were studied. We show that at a pressure of residual gases at a level of 10 Pa intense OH*, NO*, and N * 2 bands are observed in radiation of the plasma of a nanosecond transverse discharge in Ne against the background of continuous plasma radiation, and in the spectral region with λ>400 nm radiation was observed on the Hβ 486.1 nm and NeI 585.3 nm lines, and (when P≥100 kPa) on the line at the 3s–3p-transitions of a Ne atom. The radiation intensity of the third continuum of neon increases with pressure and with energy contribution to plasma, with its maximum being located in the VUV spectral region (λ max <200 nm).

Large irreversible magnetization arising from the domain freezing in La 0.7Ca 0.3MnO 3 perovskite

Magnetisation and a.c. susceptibility were measured for porous and high density polycrystalline La 0.7Ca 0.3MnO 3 bulk samples synthesised by a conventional solid state reaction and a partial melting technique, respectively. A large irreversibility in the zero-field cooled and field-cooled magnetization was observed between 4.2 K and Curie temperature 270 K, for fields smaller than 200 Oe. Magnetization and a.c. susceptibility measurements gave a cusp at T cusp. At T cusp, a.c. susceptibility χ′ decreases with increasing frequency for the porous sample. The high density bulks having a sharp transition, but low T c (250 K) exhibited a similar behaviour with a large irreversible magnetization at 50 Oe and a cusp in a.c. susceptibility for fields up to 1 Tesla. With increasing d.c. field, the irreversibility in magnetization was gradually suppressed. Large irreversibility in d.c. magnetization was explained by magnetic domain freezing instead of spin glass state. The small impurity particles embedded in large grains act as a stronger pining centre than weak-link grain boundaries for domains.

Microstructure and mechanical properties of neutron irradiated beryllium

Microstructure and mechanical properties of the neutron irradiated beryllium were studied. Specimens were produced by various methods and had different grain sizes and impurity contents, and neutron irradiation was carried out with total fast neutron fluences of 1.3–4.3 × 10 21 n/cm 2 ( E > 1 MeV) at 327–616°C. Swelling was increased by high irradiation temperature, high fluence, and by small grain size and high impurity content. Obvious decreasing of the fracture stress was observed in small grain specimens which had many helium bubbles on the grain boundary. Decreasing of the fracture stress for small grain specimens was presumably caused by crack propagation on the grain boundaries which were weakened by helium bubbles.

Effect of Nd3+ doping upon ferroelectric properties of LaBGeO5 crystals

The results of experimental study of thermal, optical and dielectric properties of La1-x Nd x BGeO5 crystals with a small substitutional impurity of Nd3+ ions (x = 0.03; 0.05) are presented. It was proposed that Nd3+ ions can be considered as strong “random local transition temperature” — type defects which can affect the critical phenomena. The relevant effects were revealed from temperature dependences of the specific heat, birefringence and dielectric constant. The progressive smearing of the phase transitions was observed from ϵ(T) and Δn(T) dependences while Cp(T) behaviour near Tc gave a chance to compare the critical indexes for pure and dopped crystals.

Phonon effects in the thermoelectric power of impure metals

Using the quantum transport equations for interacting electrons and phonons we study the phonon effects in the thermoelectric transport in impure metals. The contributions of both equilibrium phonons (the diffusive part) and non-equilibrium phonons (the drag effect) are investigated. We show that the drag effect which dominates in the thermopower of pure samples is strongly suppressed even by a small impurity concentration owing to the inelastic electron-impurity scattering processes. As a result we find the form of the phonon drag thermopower of an impure metal taking into account both normal and Umklapp processes of the electron-phonon scattering.

Influence of structural disorder on the current-voltage characteristic of a quasi-one-dimensional tunnel junction

The influence of the subbarrier impurity scattering of tunneling electrons on the current-voltage characteristic of a quasi-one-dimensional insulator layer with weak structural disorder (a small impurity concentration) is considered in the one-electron approximation at T=0. An expansion in powers of the impurity concentration gives the form of the current-voltage characteristic and the conditions for small mesoscopic fluctuations of the static tunneling conductance of such a layer in the cases of resonant and nonresonant tunneling.

Tunneling spectroscopy around the boundary of a small impurity phase on the surface of 2H-NbSe 2

single crystal were studied at 4.2 K in an ultra-high vacuum using scanning tunneling microscopy. Small domains with \(\)×\(\) surface modulation were found adjacent to the flat 2H-type surface with the 3×3 intrinsic charge density wave (CDW). Using tunneling spectroscopy, density of states around the Fermi energy at the domains was found to be lower than that at the 2H-type surface. The \(\)×\(\) modulation is considered to originate from another type of CDW. The observed proximity of superconductivity of 2H-crystal to the highly modulated domain was very small. These results suggest that the crystal structure of the highly-modulated domain is the 4Hd polytype.

A New Approach based on Brownian Motion for theSimulation of Ultra‐Small Semiconductor Devices

We present a new approach to the simulation of ultra‐small semiconductor devices based on Brownian motion of the carriers described by the Langevin equation. It follows the trajectories of individual particles in real space but does not require the computational effort of a full Monte Carlo simulation. This method is particulary useful for modeling very small devices where individual impurities and carries must be considered as discrete entities, and where a molecular dynamics approach is used in a full‐scale three‐dimensional simulation. We show that this method gives the correct Maxwell‐Boltzmann distribution and that it provides a good description of transport through a short diode.

Neon gas target for the production of radioactive fluorine beams

A neon gas target has been developed to produce radioactive fluorine. Small CF4 impurities were added to the neon gas and the recovery efficiency of 18F-labeled CF4 has been measured as a function of the impurity level. Extraction efficiencies up to 90% have been obtained, which makes this technique to produce and extract radioactive fluorine from a production target a powerful method to generate intense radioactive 17F and 18F beams, using the 20Ne(p,α)17F and 20Ne(d,α)18F reactions, respectively.

Identification of Metal-Impurity Gettering Sites in Silicon Formed by Supersaturation Boron Implantation and Annealing Using HRTEM and STEM Micro Analysis

Future Si microelectronic devices will require increasingly stringent limits on transition-metal impurities. There is thus a need to develop new methods for impurity gettering that rely on gettering sites that are active for arbitrarily small impurity concentrations, below the characteristic solid solubility at which metal suicides precipitate. These sites must also be highly preferred relative to solution sites in the Si matrix even at elevated temperatures. One such method has been developed that is also expected to be compatible with front (device) side gettering enabling smaller diffusion lengths for lower processing temperatures. This method involves boron implantion in the front side to levels above the boron-saturation limit and annealing to generate the gettering sites. The gettering layer is introduced at a depth beneath the device zone through appropriate choice of implantation energy. SIMS compositional profiling shows transition metals are strongly gettered within the boron-supersaturated layer. The purpose of this study was to identify the structure and composition of the gettering sites within the boron-supersaturated layer.

Spherical particle imaging and glare-ring spectroscopy with backscattered light

The imaging of a dielectric microsphere with linearly polarized backscattered light is investigated theoretically. Analytical expressions for the electric field and the imaged light intensity distribution in two dimensions are obtained for an arbitrary backscatter geometry, including both telescopic and microscopic arrangements. These distributions are shown to be in many cases more sensitive to small variations in the refractive index of the imaged particle than those obtained by imaging in the off-axis scattering. In microscopic geometry, the sensitivity may be further enhanced by the secondary imaging of the glare rings formed by the backscattered peripheral rays. The possibility of using the backscatter imaging for the absorption spectroscopy of small impurities is discussed.

Spectral properties of three-dimensional quantum billiards with a pointlike scatterer

We examine the spectral properties of three-dimensional quantum billiards with a single pointlike scatterer inside. It is found that the spectrum shows chaotic (random-matrix-like) characteristics when the inverse of the formal strength {bar v}{sup {minus}1} is within a band whose width increases parabolically as a function of the energy. This implies that the spectrum becomes random-matrix-like at very high energy irrespective to the value of the formal strength. The predictions are confirmed by numerical experiments with a rectangular box. The findings for a pointlike scatterer are applied to the case for a small but finite-size impurity. We clarify the proper procedure for its zero-size limit which involves nontrivial divergence. The previously known results in one- and two-dimensional quantum billiards with small impurities inside are also reviewed from the present perspective. {copyright} {ital 1997} {ital The American Physical Society}

Electron microprobe analysis and synchrotron diffraction study of crystalline (Nd 0.5Y 0.5)Ba 2Cu 3O 7− δ

A melt-textured growth process, starting with the composite material (Nd 0.5Y 0.5)Ba 2Cu 3O 7−δ as feedstock, was used to grow large intergrowth crystals containing 97% (Nd/Y)Ba 2Cu 3Of 7−δ [(Nd/Y)123] and 3% (Nd/Y) 2BaCuO 5[(Nd/Y)211]. Electron microprobe analysis and synchrotron diffraction studies showed the detailed compositions to be Nd 0.55Y 0.43Ba 1.95Cu 3.00Sr 0.10O 7−δ and Nd 0.60Y 0.40Ba 2.00Cu 2.95O 6.76 for the (Nd/Y)123 phase and Nd 0.57Y 1.37Ba 0.99-Cu 1.00Sr 0.01O 5− x and Nd 0.65Y 1.35Ba 1.00Cu 1.00O 4.95 for the (Nd/Y)211 phase, respectively. The small strontium impurity detected arose from contamination of the high purity barium carbonate used to prepare the feedstock. The (Nd/Y)211 structure showed a significant variation in the occupancy of one of the rare-earth sites, while no evidence was found for the substitution of Nd 3+ or Y 3+ on the Ba 2+ site in either phase. These intergrowth crystals showed a significantly lower critical current density when compared with the unusually large value for the pure melt processed material which has been found to contain Nd 3+ substituting on the Ba 2+ sites.

Effect of the mass resolution of a multichannel atomic analyzer on the measured isotopic composition of hydrogen plasma

The isotopic composition of the ionic component of a plasma can be determined by analyzing streams of charge-transfer atoms registered by atomic particle analyzers as they leave the plasma. Since the mass resolution of an atomic analyzer is finite, difficulties often arise in the determinations of the concentration of a small impurity against the background of the main constituent of the plasma. This paper presents a method that can be used to estimate the size of the background signal and to introduce the necessary corrections when such measurements are carried out.

μ-Carbido Diporphyrinates and Diphthalocyaninates of Iron and Ruthenium

μ-Carbido diporphyrinates and diphthalocyaninates of general formula [{ Mp 2−}2(μ- C )] ( p 2− = tpp ( M = Fe ), oep ( Fe ), pc ( Fe , Ru ); H 2 tpp : 21H,23H-5,10,15,20-tetraphenylporphine; H 2 oep : 21H,23H-2,3,4,8,12,13,17,18-octaethylporphine; H 2 pc : 29H,31H-phthalocyanine) of formally Fe IV and Ru IV are prepared by a new and improved ‘one-pot’ synthesis. The corresponding chloro complexes of the tervalent metal ions react successively with potassium hydroxide in boiling 2-propanol and then with trichloromethane. Potassium hydroxide is proven to be a very versatile and powerful reductant in tetrapyrrolic chemistry. As evidenced from electron spin resonance and UV vis spectral measurements, the precursor is reduced primarily to an ate-complex of type [ M I p 2−]− of a formally monovalent metal ion. This active species is assumed to react with trichloromethane via a dichlorocarbene complex of type [ M II( CCl 2) p 2−] to yield the actual -carbido complex. [{ Feoep 2−}2(μ- C )] is crystallographically characterized. It is monoclinic, space group C12/c1 (15), with a = 18.279(3) Å, b = 15.005(2) Å, c = 23.392(7) Å, β = 107.12(2)°, Z = 4, R1 = 0.0773. The iron atom is displaced by 0.192(3) Å out of the centre ( Ct ) of the ( N p )4 plane toward the (μ- C ) atom. Average d ( Fe - N p ) is 1.986(5) Å; d ( Fe -(μ- C )) is 1.6638(9) Å. The Fe - C - Fe skeleton is linear (179.5(3)°). The two slightly waving porphyrinato cores are in a staggered conformation, the ( N p - Fe - Fe ″- N p ″) torsion angle being 21.0(3)°. Solutions of each μ-carbido complex in pyridine/dichloromethane show four distinct quasi-reversible redox processes in their differential-pulse voltammograms and these are assigned to the successive one-electron reduction and oxidation of the macrocyclic ligands. 13 C CP MAS NMR spectra indicate effective four-fold symmetry within the series of the μ-carbido complexes with isotropic shifts occurring at similar fields to those of the corresponding macrocyclic complex of a closed-shell metal ion. Resonances of the bridging carbon atom are not detected. A characteristic increase of line broadening within the series tpp 2− &gt; oep 2− &gt; pc 2− may be due to Fermi contact interactions with the strongly coupled low-spin M IV centres. The magnetic susceptibility studies show that the complexes all display non-zero μ values at 295 K increasing from pc 2− to tpp 2−. Mössbauer spectra confirm the low-spin Fe IV oxidation state for the iron centres. Isomer shift, δ, and quadrupole splitting, ΔE Q , for [{ Fepc 2−}2(μ- C )] and [{ Fetpp 2−}2(μ- C )] are identical to those previously reported. Data for [{ Feoep 2−}2(μ- C )] are essentially the same as for the pc and tpp complex. Thus the order of δ is tpp ≈ oep &gt; pc whilst that of Δ E Q is pc &gt;&gt; oep &gt; tpp . Small impurity lines are observed which help explain the magnetic data. UV vis/NIR spectra of the μ-carbido complexes show the characteristic π-π* transitions. These are shifted with respect to the corresponding mononuclear complexes to higher energy because of excitonic interactions. Vibrational spectra are discussed in detail ν as ( M - C - M ) (in cm−1) is at 937 ( M = Fe ; tpp ) &lt; 976 ( Fe / oep ) &lt; 997 ( Fe / pc ) &lt; 1050 ( Ru / pc ), ν s ( M - C - M ) (in cm−1 at 433 ( Fe / tpp ) &lt; 460 ( Fe / oep ) &lt; 477 ( Fe / pc ). Hence, valence force constants increase significantly in the order tpp &lt; oep &lt; pc . ν s ( Fe - C - Fe ) of [{ Fepc 2−}2(μ- C )] is selectively resonance Raman enhanced. As evidenced from the excitation profile a C → Fe charge transfer, not detected in the vis spectrum, is assumed to be present at 22 000 &gt; ν &gt; 25 000 cm −1.

Weak anisotropic impurity scattering in unconventional superconductors

The effect of weak anisotropic (momentum-dependent) impurity scattering in unconventional superconductors has been investigated. It is shown that the anisotropic scattering can lead either to a small reduction or a small enhancement of the isotropic pair-breaking effect. The influence of the anisotropy of the scattering potential becomes significant for the order parameters with large Fermi surface average values. In that case an unexpected enhancement (up to ∼10%) of the critical temperature T c over the critical temperature in the absence of impurities T c 0 is predicted for a small impurity concentration.

Strong segregation gettering of transition metals by implantation-formed cavities and boron-silicide precipitates in silicon

We have mechanistically and quantitatively characterized the binding of transition-metal impurities in Si to cavities formed by He implantation and to BSi precipitates resulting from B implantation. Both sinks are inferred to act by the segregation of metal atoms to pre-existing low-energy sites, namely surface chemisorption sites in the case of cavities and bulk solution sites in the case of the BSi phase. These gettering processes exhibit large binding energies, and they are predicted to remain active for arbitrarily small initial impurity concentrations as a result of the segregation mechanisms. Both appear promising for gettering in Si devices.

Characteristics of and issues regarding combined H-modes

Owing to the recent progress of `profile controls' utilizing the freedom of profiles of plasma parameters, various types of combined (advanced) H-modes have been obtained; PEP H-mode, high- H-mode, high- H-mode, high- H-mode, reversed shear H-mode, CH-mode, VH-mode, etc. These combined H-modes with high potential for confinement and stability are characterized by improved transport in the core region in addition to the edge or deep inward penetration of the edge confinement pedestal. Such additional improvements seem to be related to flow shear, magnetic shear, safety factor, density profile or . The improved confinement region can propagate from the core to the edge or from the edge to the core, or appear almost simultaneously over the whole radius. However, there are some critical issues to be solved concerning reactor conditions, that is, under electron heating at high edge density with small impurity accumulation in the steady-state. Most of the NB heated combined H-modes have been obtained with ion heating, where ion thermal diffusivity and particle diffusivity can be reduced to the neoclassical level. However electron thermal diffusivity has so far not been clearly reduced except in the H-mode and in the VH-mode. Therefore, improvement of electron transport is one of the main issues. The second issue is that some improved modes are accompanied by strong density peaking which may result in large impurity accumulation. The third issue is the difficulty in achieving the improved modes at a high edge density with the high particle recycling essential for a dense and cold divertor. The fourth issue is the stability at high in the steady state. It is not certain that pressure and current profiles including the bootstrap current can be sustained stably. The requirement of ELMs for heat and particle exhaust is also an important issue in stability design as concerns the extent of the second regime access for the high-n ballooning mode.