Neutral Impurity Research Articles

γ-Radiation Stimulated Structural Transition of Monoclinic TlInS2to Hexagonal Phase

Temperature dependent dielectric function and conductivity, as well as room temperature photoconductivity have been investigated on monoclinic TlInS2 subjected to γ-radiation in a wide range of radiation doses. The structural changes have been detected by XRD performed after every stage of γ-irradiation. Frenkel pairs (vacancies and interstitial sulfur atoms) have been assumed to appear in TlInS2 at irradiation doses below 100 Mrad through well-known Klinger's "residual charged impurity assisted electrostatic" mechanism. These defects eventually lead to the formation of neutral impurity complexes responsible for the observed sharpening of the anomalies in dielectric function and conductivity at the temperature of the well-known ferroelectric phase transition in TlInS2. The last anomalies become smeared out at irradiation doses above 200 Mrad. Finally, the transformation of the monoclinic TlInS2 to the one with hexagonal structure has been witnessed in a thin surface layer at high fluencies (1000–1500 Mrad) of γ-radiation.

Minority-charge-carrier mobility at low injection level in semiconductors

From the kinetic equations, the distribution functions for majority and minority charge carriers are obtained at a low injection level. For describing the electron-hole collisions, the Landau collision integral is used. The carrier scattering at ionized or neutral impurity and at acoustic phonons is taken into account. The majority-carrier distribution function is presented in the analytical form. The minority-carrier mobility is calculated and analyzed, and the features of its behavior at low temperatures are revealed. It follows from the developed theory that the hole mobility in an n-type material increases with doping and neutral-impurity concentration. This effect is attributed to mutual charge-carrier collisions and different effective masses of different-sign carriers.

Effect of Li3+ heavy ion irradiation on the Mo doped In2O3 thin films prepared by spray pyrolysis technique

The high visible–near infrared transparent and high carrier mobility (μ) Mo doped (0.5 at%) indium oxide (IMO) films were deposited by the spray pyrolysis technique. The deposited films were irradiated by 50 MeV Li3+ ions with different fluences of 1 × 1011, 1 × 1012 and 1 × 1013 ions cm−2. X-ray diffraction analysis confirmed the cubic bixbyite structure of indium oxide. A fascinating feature is that the ion irradiation process has introduced a fraction of the molybdenum oxide phase. The μ of as-deposited IMO films is decreased from ∼122.4 to 93.3 cm2 V−1 s−1, following the ion irradiation. The theoretically calculated μ and carrier density values were correlated with those measured experimentally. The transport mechanism has been analysed based on the ionized and neutral impurity scattering centres. The average transmittance (400–2500 nm) of the as-deposited IMO films is decreased from 83% to 60% following irradiation.

Fluorine-doped ZnO thin films deposited by spray pyrolysis technique

Fluorine doped ZnO thin films (FZO) are prepared onto glass substrates at 350 °C by the chemical spray pyrolysis technique. X-ray diffraction spectra show a polycrystalline of ZnO (wurtzite structure) where the amount of fluorine doping affects to preferential orientation (002 plane along c -axis) and does not vary the lattice parameters. Therefore, F introduction in lattice is by the substitution of O -2 ions by F -1 ions. Any variation is observed in transmittance and reflectance measurements in 400–2000 nm wavelength range when samples present F dopant; they have transmittance around 80% in the near infrared and visible zones. The FZO films are degenerate and exhibit n -type electrical conductivity. The best resistivity and mobility are 7.6 × 10 -3 Ω cm and 3.77 cm 2 V -1 s -1 respectively. The calculated values of the mean free path are very small compared to the grain sizes calculated using XRD measurements. Therefore, we suggest that ionized impurity and/or neutral impurity scattering are the dominant scattering mechanisms in these films.

The FTIR studies of SnO 2:Sb(ATO) films deposited by spray pyrolysis

Thin films of SnO 2:Sb were prepared by ultrasonic spray pyrolysis method. The role of antimony in the SnO 2 is demonstrated by FTIR. It is found that at low Sb concentrations, the Sb ions prefer to substitute Sn 4+ in form of Sb 5+. While beyond a certain Sb concentration, Sb ions start to be reduced to Sb 3+, which have a negative effect on carrier concentration that, in turn, affects the reflectivity of SnO 2:Sb films. The increased disorder of SnO 2 at high Sb concentration is also shown by FTIR. The discussion of carrier scattering suggests that the ionized impurity and/or neutral impurity scattering are the dominant scattering mechanisms.

Polaronic Properties of an Ion in a Bose-Einstein Condensate in the Strong-Coupling Limit

Cold atoms can be used to model the Frohlich polaron Hamiltonian using an impurity in a condensate. To probe the strong-coupling regime (which remains elusive in the solid state) charged impurities can be better suited that neutral impurities. However, the ion–atom interaction leads to a stronger depletion of the condensate as compared to the depletion from interaction with neutral impurities, and a good correspondence between the Frohlich Hamiltonian and the cold atom system is only possible when the Bogoliubov approximation is valid. We use a strong-coupling variational approach to estimate an upper limit to the ion-atom coupling strength (expressed through a dimensionless coupling constant). The all-coupling path integral approach is used to estimate the values of the coupling constant that would need to be reached to probe the strong-coupling polaron physics. From these two approaches, we identify a regime where ions in condensates can model strong-coupling polarons and we calculate ground state properties in this regime.

Effect of divertor legs on neutral particle and impurity retention for a closed helical divertor configuration in the Large Helical Device

A closed helical divertor (CHD) has been designed for efficient particle control in the plasma periphery and for retaining neutral particles and impurity ions in the divertor region. The effect of impurity retention by divertor legs for the CHD configuration is investigated from the viewpoints of neutral impurity transport and force balance of impurity ions along magnetic field lines. A fully three-dimensional neutral particle transport simulation proves that the plasma on the divertor legs is effective for retaining neutral particles/impurities in the CHD region. A one-dimensional impurity ion transport analysis predicts that friction force by plasma flow from the main plasma sweep impurity ions toward the divertor plates even in high neutral density case in which a steep temperature gradient is formed. It shows that the CHD configuration is promising for enhancing LHD plasma performance by effective control of the neutral particles and the impurity ions in the plasma periphery.

Structural, optical, electrical properties and FTIR studies of fluorine doped SnO2 films deposited by spray pyrolysis

Thin films of SnO2:F were prepared by ultrasonic spray pyrolysis method. The effect of fluorine concentration on the structural, optical, and electrical properties of SnO2:F films was investigated. The X-ray diffraction results showed the preferred growth along (110). FTIR was employed to study the defects in SnO2 lattice. The evidence of oxygen vacancy and substitution of fluorine for oxygen in FTIR were investigated. It was found that at low doping levels, fluorine ions preferred to replace the oxygen in the lattice. While beyond a certain doping level, fluorine ions started to occupy interstitial site, which had a negative effect on carrier concentration that, in turn, affected the infrared reflectivity of SnO2:F films. The increased disorder of SnO2 at high doping levels was also shown by FTIR. The discussion of carrier scattering suggested that ionized impurity and/or neutral impurity scattering were the dominant scattering mechanisms in SnO2:F films.

On the Nature and Behavior of Li Atoms in Si: A First Principles Study

On the basis of density functional theory calculations, we present the bonding and dynamic behavior of Li atoms in crystalline Si and how the incorporation of Li atoms affects the structure and stability of the host Si lattice. Our calculations clearly evidence that the inserted Li atom energetically prefers a tetrahedral interstitial site while exhibiting a shallow donor level. Because of their positive ionization, the interactions between neutral Li impurities are repulsive, suggesting that they favorably remain isolated, rather than clustered. We also find that the charge transferred from neutral Li is largely localized within the first nearest Si atoms, thereby effectively screening the positively ionized Li. In addition, our electronic structure analysis highlights that the charge transfer leads to a significant weakening of nearby Si−Si bonds by filling the antibonding sp3 states of Si. The mobility of Li interstitials is also estimated in the neutral and positive charge states.

Functions of intrinsic magnetic field line structures on lithium ion transport in the LHD plasma periphery

Plasma discharge operation with lithium coating suggests that the lithium effectively control neutral particles in the plasma periphery, which can lead to improvement of plasma parameters. The effect of lithium coating on the large helical device (LHD) for a closed helical divertor configuration is discussed from viewpoints of neutral particle and impurity ion transport in the plasma periphery. It shows that the closed helical divertor configuration can enhance the neutral particle density in the divertor region, which is enough to achieve efficient particle control, and that it can effectively confine neutral lithium atoms near divertor plates. A one-dimensional impurity (lithium) ion transport analysis along magnetic field lines on divertor legs indicates that the friction force due to the plasma flow from the main plasma is dominant over the thermal force caused by the temperature gradient on the divertor legs, which prevents lithium ion contamination in the main plasma and excessive cooling of the plasma temperature in an ergodic layer. The analysis shows that the lithium coating is compatible with LHD plasma discharge operation for the closed helical divertor configuration.

Convergence study of neutral and charged defect formation energies in Si nanowires

We present a systematic study of the convergence with the supercell size of the defect formation energy of a substitutional Al impurity in a Si nanowire within density-functional theory. We discuss the case of neutral and charged impurities, highlighting the importance, in the presence of ionized defects and periodic boundary conditions, of including a Madelung-like correction to the total energy in order to get correct results, which cannot be obtained by the simple use of a large vacuum buffer layer or an extrapolation thereof.

Electron Scattering Mechanism of FTO Films Grown by Spray Pyrolysis Method

Transparent conducting thin films of fluorine-doped SnO2 (FTO) were deposited on glass substrates by spray pyrolysis. This work studies electron scattering mechanisms of the FTO films through analysis of the temperature-dependent mobility. The mechanisms of lattice vibration and dislocation scattering were not important in the degenerate FTO films. The mean free path was found to be considerably smaller than the grain size, suggesting that ionized impurity and/or neutral impurity scattering are the dominant scattering mechanisms

CE assay for simultaneous determination of charged and neutral impurities in dexamphetamine sulfate using a dual CD system

A CE assay for the simultaneous determination of charged and uncharged potential impurities (1S,2S-(+)-norpseudoephedrine, 1R,2S-(-)-norephedrine, phenylacetone and phenylacetone oxime) of dexamphetamine sulfate including the stereoisomer levoamphetamine was developed and validated. The optimized background electrolyte consisted of a 50 mM sodium phosphate buffer, pH 3.0, containing 80 mg/mL sulfobutylether-beta-CD and 25 mg/mL sulfated beta-CD. Separations were performed in 40.2/35 cm, 50 mum id fused-silica capillaries at a temperature of 20 degrees C and an applied voltage of -10 kV. 1R,2S-(-)-ephedrine was used as internal standard. The assay was validated in the range of 0.05-1.0% for the related substances and in the range of 0.05-5.0% for levoamphetamine. The LOD was 0.01-0.02% depending on the analyte. The assay also allowed the separation of the E,Z-stereoisomers of phenylacetone oxime. The effect of the degree of substitution of sulfobutylether-beta-CD was investigated. In commercial samples of dexamphetamine sulfate between 3.2 and 3.7% of levoamphetamine were found. Furthermore, phenylacetone and phenylacetone oxime could be observed at the LOD, indicating the synthetic origin of the investigated samples.

Ab initiocalculation of the binding energy of impurities in semiconductors: Application to Si nanowires

We discuss the binding energy E_b of impurities in semiconductors within density functional theory (DFT) and the GW approximation, focusing on donors in nanowires as an example. We show that DFT succeeds in the calculation of E_b from the Kohn-Sham (KS) hamiltonian of the ionized impurity, but fails in the calculation of E_b from the KS hamiltonian of the neutral impurity, as it misses most of the interaction of the bound electron with the surface polarization charges of the donor. We trace this deficiency back to the lack of screened exchange in the present functionals.

Scattering mechanisms and anomalous conductivity of heavily N-doped 3C-SiC in ultraviolet region

Using the first-principles density functional method, we investigate the band structures and conductivity spectra for N-doped 3C-SiC. It is found that conductivity peaks of heavily N-doped 3C-SiC are observed in the ultraviolet (UV), visible and infrared (IR) regions while the peaks can be only seen in the UV region for 3C-SiC. In the UV region, the conductivity peaks of 3C-SiC are obviously higher than those of N-doped 3C-SiC. According to the data of band structures, we calculate the ionized impurity scattering, inter-carrier scattering and neutral impurity scattering. The calculation results show that the scattering by incomplete ionization N to electrons and inter-carrier scattering have large effect on the conductive behavior of heavily N-doped 3C-SiC at room temperature. In the UV region, the conductivity of 3C-SiC depends on long-wavelength optical wave scattering, which has a longer relaxation time than that inter-carrier scattering and neutral scattering. This is the reason of anomalous conductivity of N-doped 3C-SiC in the UV region.

Lateral electric field effects on quantum size confinement in cylindrical quantum dot under parabolic potential

Within the effective mass approximation, we investigated theoretically the ground-state energy of a single particle and the binding energy of the neutral donor impurity (D 0) affected by a lateral electric field in a parabolic quantum dot (QD). The results show that the electron and the hole ground-state energy and the band to band transition energies shift to lower values (red shift) by increasing the field intensity. The quantum Stark shift (QSS) for the electron increases rapidly in the quasi spherical QD (QSQD) by increasing the lateral field, whereas for the hole it increases monotony. In the cylindrical QDs (CQDs), we found that the QSS for electron and hole increase monotonically. The quantum size, lateral electric field and impurity position effect on the binding energy of neutral donor (D 0) is studied. Unexpected behavior of D 0 in quantum well limit (QW), the binding energy of D 0 is increasing (blue shift) with increasing QD radius R at the presence of a lateral electric field. It appears that for a fixed size of the QD, the off-center binding energy decreases when the impurity ion is displaced from the center to the QD borders, while it is shifted to lower energy with increasing the field.

Fine Structure of Ultraviolet Photoluminescence of Tin Oxide Nanowires

Photoluminescence (PL) properties of tin oxide (SnO2) nanowires are studied in detail using high spectral resolution spectroscopy in a temperature range of 10-300 K. The nanowires have an average diameter of 86 nm. The high quality of the nanowires enables the observation of rich fine structures in the ultraviolet PL spectra at low temperatures. By carefully analyzing the temperature and excitation power dependent spectra, the following emissions are identified: recombination of donor-acceptor pairs, excitons bound to neutral and ionized donor impurities and optical transitions from free electrons to neutral acceptor impurities. Moreover, it is believed that the emission from recombination of free excitons is observed, which is unusual for SnO2, a dipole forbidden direct band gap semiconductor.

Interstitial diffusion under conditions of trapping of interstitial impurity atoms

Abstract The diffusion equation for nonequilibrium interstitial impurity atoms in the form convenient for numerical solution has been obtained. The proposed equation takes into account all different charge states of interstitial atoms and drift of all mobile charged species in the built-in electric field, although only the concentration of the neutral impurity interstitials must be obtained to solve the equation. The absorption of impurity interstitials by immobile sinks, mobile vacancies, and due to the passivation of dopant atoms is also taken into account. The derived effective coefficients that describe diffusion and trapping of impurity interstitials have the following characteristic features: (i) their dependences on the concentration of substitutionally dissolved dopant atoms are smooth and monotone; (ii) these concentration dependences are obtained in the form traditionally used for the approximation of effective diffusivity of substitutionally dissolved dopant atoms in processing-simulation codes. To illustrate the usability and efficiency of the derived equation for simulation of diffusion of nonequilibrium interstitial impurity atoms, the migration of deuterium interstitials in the silicon substrate doped with boron has been simulated. The calculated profile of deuterium concentration in the passivated region agrees well with the experimental data.

Signature Profiling and Classification of Illicit Heroin by GC‐MS Analysis of Acidic and Neutral Manufacturing Impurities

The illicit manufacture of heroin results in the formation of trace level acidic and neutral impurities. These impurities are detectable in illicit heroin and provide valuable information about the manufacturing process used. The isolation, derivatization, and semiquantitative analysis of neutral and acidic heroin manufacturing impurities by programmed temperature vaporizing injector-gas chromatography-mass spectrometry (PTV-GC-MS) is described. Trace acidic and neutral heroin impurities were isolated from basic fractions using liquid-liquid extraction. Extracted impurities were treated with N-Methyl-N-trimethylsilyltrifluoroacetamide followed by PTV-GC-MS analyses. Semiquantitative data were obtained using full scan mass spectrometry utilizing unique ions or ion combinations for 36 trace impurities found in crude and/or highly refined heroin samples. Minimum detection limits for acidic and neutral impurities were estimated to be at the 10(-7) level relative to total morphine. Over 500 authentic heroin samples from South America, Mexico, Southwest Asia, and Southeast Asia were analyzed. Classification of illicit heroin based on the presence or absence and relative amounts of acidic and neutral impurities is presented.

10.1007/s11453-008-3002-9

A quantitative model for near-surface redistribution of doping impurity in silicon in the course of proton-stimulated diffusion is developed for the first time. According to the model, the near-surface peak of the impurity concentration is caused by migration of neutral impurity—self-interstitial pairs to the surface with subsequent decomposition of these pairs and accumulation of the impurity at the silicon surface within a thin layer (referred to as δ-doped layer). The depletion and enhancement regions that are found deeper than the near-surface concentration peak are caused by expulsion of ionized impurity by an electric field from the near-surface region of the field penetration. The field appears due to the charge formed in the natural-oxide film at the silicon surface as a result of irradiation with protons. The diffusion-kinetic equations for the impurity, self-interstitials, vacancies, and impurity—self-interstitial pairs were solved numerically simultaneously with the Poisson equation. It is shown that the results of calculations are in quantitative agreement with experimental data on the proton-stimulated diffusion of boron impurity in the near-surface region of silicon.