Doppler Broadening Research Articles

Point defects in BaSi2 thin films for photovoltaic applications studied by positron annihilation spectroscopy

Barium di-silicide (BaSi2) is a very promising absorber material for high-efficiency thin-film solar cells, due to its suitable bandgap, high light absorption coefficient, and long minority-carrier lifetime. In this study, we compare the nanostructure, layer composition, and point defects of BaSi2 thin films deposited by Radio Frequency (RF) sputtering, Thermal Evaporation (TE), and Molecular Beam Epitaxy (MBE), using Doppler Broadening Positron Annihilation Spectroscopy (DB-PAS) depth profiling, Raman spectroscopy, and x-ray diffraction. Our DB-PAS study on thermally annealed RF-sputter deposited and on TE-deposited BaSi2 layers, in a comparison with high quality BaSi2 films produced by MBE, points to the presence of vacancy-oxygen complexes and Si or Ba mono-vacancies, respectively, in the (poly)crystalline BaSi2 films. The degree of near-surface oxidation increases, going from MBE and TE to the industrially applicable RF-sputtered deposition synthesis. The use of a-Si capping layers on the thermally annealed RF-sputtered BaSi2 films leads to a clear reduction in sub-surface oxidation and improves the quality of the BaSi2 films, as judged from DB-PAS.

Elemental Characterization of the Topmost Atomic Layer of Surface Using Doppler Broadening Spectroscopy

Elemental Characterization of the Topmost Atomic Layer of Surface Using Doppler Broadening Spectroscopy

Magnetic quenching of positronium studied by positron annihilation lifetime and Doppler broadening measurements

Annihilation of positronium (Ps) in static magnetic fields is investigated by positron annihilation lifetime (PAL) and Doppler broadening (DB) measurements of γ-Al2O3 nanopowder. The properties of four eigenstates of Ps atoms (|1,1⟩, |1,−1⟩, |+⟩ (long-lived eigenstate mixed from |1,0⟩ and |0,0⟩ due to Zeeman effect), and |−⟩ (short-lived eigenstate mixed from |1,0⟩ and |0,0⟩ due to Zeeman effect)) were revealed from PAL and DB results. Thereafter, with magnetic field increasing from 0.105 to 0.760 T, clear and regular variation of Ps annihilation parameters are observed: (1) The mean lifetime of the three long-lived Ps states (|1,1⟩, |1,−1⟩, and |+⟩) shows clear quenching effect; (2) The formation probability of each Ps eigenstate (|1,1⟩, |1,−1⟩, |+⟩, and |−⟩) keeps unchanged; (3) The intensity of the narrow component (derived from multi-Gaussian fitting) of DB spectrum exhibits a gradual increase which is in good agreement with the theoretical calculation from PAL results (2γ decay of |+⟩ and |−⟩ states, and pick-off quenching of long-lived Ps states); (4) The intensity of the low energy region of DB spectrum is diminished by the decrease of 3γ decay of |+⟩ state. The quantitative correlations between magnetic field and o-Ps annihilation parameters will be helpful for the further understanding of complex Ps quenching effects, such as the effect mixed from orhto-para spin conversion and magnetic quenching.

Characterization of ion induced damage as a function of depth in proton irradiated pure Ti and Ti–6Al–4V

The inhomogeneous damage caused by ion irradiation (proton) as a function of depth has been characterized on pure Ti and Ti–6Al–4V at different doses. X-ray line profile analysis was used to evaluate the microstructural parameters such as domain size and microstrain within the domain. Although a systematic decrease in domain size as a function of depth is observed in both the materials, but the changes are less prominent in Ti–6Al–4V than in pure Ti. The increase in the value of S-parameters measured from Doppler Broadening Spectroscopy of Positron annihilation technique reveals higher concentration of open volume defects near Bragg peak (maximum damage region). Micro-hardness is also measured as a function of depth for both irradiated and unirradiated samples. The results obtained from microhardness measurements are found to corroborate with the findings from PAS and XRDLPA. Radiation induced segregation (RIS) which was observed in the Ti–6Al–4V at the highest dose 5E17p/cm2 is found to be limited near to the surface. Detailed X-ray analysis using Convolutional Multiple Whole Profile fitting of the XRD data from the surface of the irradiated Ti–6Al–4V samples could reveal the nature of the dislocations (i.e.<a> or <c> type).

Variation of Doppler Broadening in High-Temperature Bubbles Created in an ECR Plasma

Variation of Doppler Broadening in High-Temperature Bubbles Created in an ECR Plasma

Doppler line shape model for the W I 5d56s—5d56p transition at 400.9 nm in tokamaks

Tungsten (W) spectroscopy is known to be a useful tool for quantifying W fluxes in tokamaks. We aim to analyze how background plasma parameters influence the W spectral-line shape measured in the divertor and scrape-off layer and what information could be retrieved from such an analysis. To that end, a Monte Carlo model is developed to simulate the line shape of the 400.9 nm W I line emitted by physically sputtered W ions from plasma facing components. The influence of background plasma parameters (ion and electron temperature) and geometrical effects (detector positioning with respect to the eroded surface) are investigated. The effect of Zeeman splitting on the spectral line-shape is also explored. A set of requirements for the experimental set-up needed to measure the analyzed spectral line features with line peak shifts of the order of 10−2Å and peak widths in the range from 10−1Å to 10−2Å are proposed. These requirements are shown to be experimentally accessible, through the installation of specific high-resolution spectrometers not typically used on magnetic fusion devices.

Irradiation studies on a reactor pressure vessel steel using Fe+ ion

Reactor pressure vessel steel samples were irradiated to three different doses 10, 25 and 50 displacements per atom (dpa) at 300 °C (reactor operating temperature) with 1.76 MeV Fe+ ion and at the dose rate of ∼4.4 × 10−3 dpa s−1. Samples were examined by Electron Backscatter Diffraction (EBSD), Grazing Incidence x-ray Diffraction (GIXRD), depth dependent Doppler Broadening Spectroscopy (DBS) and nanoindentation before and after irradiation. EBSD showed an increase in local misorientation values suggesting an increase in geometrically necessary dislocation density. GIXRD showed an increase in overall dislocation density after irradiation. DBS results suggested a non-uniform vacancy-depth distribution with the highest damage at a depth (∼500 nm) which was consistent with the values from the Stopping and Range of Ions in Matter (SRIM) calculations. The mechanical response after irradiation was measured using nanoindentation. The nanohardness of the irradiated samples was higher than that of the unirradiated one and increased with the increase in dpa. GIXRD result showed that the dislocation density of 50 dpa irradiated sample was almost twice that of the unirradiated sample.

Ion irradiation induced changes in defects of iron thin films: Electron microscopy and positron annihilation spectroscopy

Single crystal Fe thin films (∼250 nm) were grown on MgO substrates and irradiated with 2.0 MeV Fe+ ions at 10 and 50 dpa, and the defect evolution was studied using high resolution Transmission Electron Microscopy (HR-TEM) and Doppler Broadening Positron Annihilation Spectroscopy (PAS). It was shown that irradiation induced or exacerbated a thin oxide layer at the outer interface and produced substantial Fe/Mg mixing at the film/substrate interface, particularly for the higher dose. Modeling of the PAS data allowed interpretation of the defect types at different distances from the Fe surface, and included several types of MgO substrate damage and annihilation condition changes, indicative of damage due to ballistic effects of the Fe atoms as well as chemical changes due to implantation and subsequent diffusion. This detailed PAS study compared with TEM and energy dispersive spectroscopy (EDS) provides significant insight into depth-dependent defect creation. These results will be useful for predicting defect creation in Fe-based materials under irradiation conditions, for extension to neutron irradiated structural materials.

Positron Annihilation Lifetime and Doppler Broadening Spectroscopy of polymers

Positron Annihilation Lifetime and Doppler Broadening spectroscopy of positron annihilation radiation are performed in four different polymers. The size of elementary free volume sites and their chemical environments are determined. It is shown that polymers with different groups of C–H, CO, and C–F can be distinguished by their orbital electron momentum spectrum. Correlation between parameters of the PALS and CDBS techniques in situations involving O-Ps formation at different chemical environments are clarified. The results indicate that both the S and W-parameters show a linear correlation with the size of elementary free volume sites of the polymers. Moreover, for polymers belonging to CO and C–F groups, the effect of positron trapping by polar groups on the probability of O-Ps formation is discussed.

Er3+ induced point defects in ZnO and impact of Li+/Na+/K+ on the vacancy defects in ZnO:Er studied by positron annihilation spectroscopy

The defects species in as-prepared ZnO, Er-doped, and co-doped (with Li/Na/K) nanocrystallites synthesized by combustion method were examined by Positron Annihilation Lifetime (PAL) and Doppler Broadening (DB) spectroscopy. The S and W parameters were derived from positron trapping of valence and core electrons extracted from the central and wing regions of DB spectra to identify the vacancy-type defects. For as-prepared Er-doped ZnO samples, the observed high value of S indicated that the vacancies and vacancy cluster defects were located at the grain surface and two-grain junction. With co-doping of Li/Na/K, the S value was decreased due to the dopant occupying the vacancy clusters. On annealing, the major recovery of such defects was observed with enormous decreasing S for Li co-doped sample. Positron lifetime variations signified the presence of defects compared with bulk ZnO. Er doping in ZnO evidenced a small increase of defects with a gradual decrease when co-doped with Li/Na/K indicating the co-dopant impurity occupied the vacancies in the grain boundary. The first lifetime component of 193 ps owing to the Zn-type vacancies in as-prepared ZnO were increased on Er doping, evidencing the replacement of the Zn2+ by Er3+ ion and thereby created point defects due to charge compensation. On annealing to 800 °C, Li co-doped ZnO:Er resulted to enormous reduction of second lifetime τ2, mean lifetime τave, and S value signifying the Li ion localised in Zn vacancy site. Annealed ZnO:Er + Na/K resulted in considerable intensification in defect concentrations than ZnO:Er + Li due to the vacancy migration to form as clusters. The defect type, defect modification, the influence of Er together with co-doped Li/Na/K in ZnO, and their effect on annealing are elucidated in detail from the positron annihilation characteristics.

Positron annihilation apectroscopy on colloidal CuIn1-xGaxSe2 semiconductor sanocrystals

Simple and low-cost hot-injection method was used to synthesize three samples of colloidal nanocrystals with general chemical formula CuIn1-xGaxSe2 (x=0.0, 0.6 and 0.82). X-ray diffraction (XRD) and transmittance electron microscopy (TEM) have been used to investigate the structural properties of the synthesized nanocrystals and proved their high crystallinity. Positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques were used to give more insights on the structural defects of the grown samples. The results of the positron lifetime measurements indicate that the shortest lifetime component (τ1) with intensity ≳ 97% is greater than the calculated positron bulk lifetime, 235-240 ps. This indicates that the concentrations of the vacancy-type defects for the as-synthesized CuIn1-xGaxSe2 samples are greater than the saturation trapping limit (1018 cm-3). Moreover, results of the PAL measurements and the theoretically calculated positron lifetimes indicate that Cu-Se, In-Se and/or Ga-Se are the dominant vacancy-type defect for all studied range of Ga. Also, the results show that the vacancy-type defect concentrations of samples with Ga content of 0.6 and 0.82 are reduced compared with that for zero-Ga content sample.

Giant negative and positive Goos–Hänchen shifts via Doppler broadening effect

The magnitude enhancement of the Goos–Hänchen shift (GHS) in reflected light (RL) is investigated. We consider a cavity in which a four-level atomic medium with a double lambda ()-type configuration is contained. By changing the relative phase of optical fields in the four-level atomic configuration, we obtain normal and anomalous dispersions. Similarly, the manipulation of the positive and negative GHS in the RL is explored via the control of the relative phase. In addition, we investigated the GHS in the RL with and without the Doppler broadening (DB) effect. Interestingly, giant GHS in the RL is revealed for the DB medium. The role of the Doppler width on the magnitude of the GHS in RL is also studied. We find that the magnitude of the negative and positive GHS in the RL increased via increasing the Doppler width.

Effect of dopant concentration and annealing of Yttrium doped CuO nanocrystallites studied by positron annihilation spectroscopy

In this paper, we report the solution combustion synthesized Yttrium doped CuO nanocrystallites studied by positron annihilation lifetime (PAL) and Doppler broadening (DB) spectroscopy. The lattice defect in Y doped CuO nanocrystallites is presented by the effect of dopant concentration and annealing temperature. X-ray diffraction measurements indicated monoclinic CuO with impurity phase (Y2O3) of Y at elevated temperature. Scanning Electron Microscopy observations showed the particles are more agglomerated upon annealing. The PAL study exhibited three-lifetime components (τ1, τ2, and τ3) wherein, as-prepared CuO exhibited an increased lifetime (τ1) of 212 ps compared to the reported bulk lifetime of 169 ps. The analysis confirmed the mixed contribution of the annihilations of trapped positrons in CuO bulk, point, and extended defects. The τ2 and τ3 are ascribed to the vacancy clusters and voids. Defects modification with doping concentration and annealing temperature are explained in detail. The average positron lifetime (τav) of all the samples increases with doping concentration and decreases with annealing temperature. Grain growth hindrance due to Y ion migration is presented effectively, signifying the higher average lifetime. Comparative analysis of the average lifetime with the extracted from the valance electron annihilations associated S parameter from DB and on the crystallite size variation with annealing is presented on the basis of defects formation and its curing mechanism. The defect structure change owing to charge compensation of Y in CuO is explained. The defect modification of doped and annealed CuO is explained well from the change in crystallite growth and surface defects.

A deformed Doppler Broadening Function considering the Tsallis speed distribution

The Doppler Broadening Phenomenon provides a good description for the effects of thermal agitation on the cross-section of the neutron-nucleus interaction and has its theoretical basis on Quantum Mechanics through the Single Level Breit-Wigner Formalism, as well as on Statistical Mechanics, with the Maxwell Boltzmann distribution. In this paper, the consequences of replacing the Maxwell Boltzmann speed distribution by the non-extensive Tsallis distribution in the evaluation of the absorption cross section in three-dimension is discussed. The Tsallis speed distribution introduces a deformation parameter q that measures the deviation from the Maxwellian speed distribution and, by taking the limit q→1, the deformation is removed. In this context, and considering the Tsallis distribution, an integral form is obtained for a deformed Doppler Broadening Function in the scope of the single-level Breit-Wigner formalism and Bethe-Placzek approximations. This deformed Doppler Broadening Function reproduces the well-established conventional Doppler Broadening Function at the limit when q→1 in which the deformation is removed. In order to study the range of values where the effect is significant, numerical tests were carried out considering several values for the q parameter.

Thermal Stability of Rolled Metastable Austenitic Stainless Steel 1.4307 Studied Using Positron Annihilation

Positron annihilation spectroscopy (PAS) was used in the study of cumulative isochronal and isothermal annealing of rolled 1.4307 (EN) stainless steel (SS). Due to different rolling temperatures, the SS samples varied in the α′-martensite volume fraction from 0.09 to 0.91. The measurements of positron annihilation characteristics, i.e., Doppler broadening (DB) of the annihilation line, showed a gradual annealing of vacancies in the temperature range between 200 °C and 400 °C, which indicated the first stage of recovery. This first stage of recovery did not change the microhardness. In the temperature range from 475 °C to 600 °C, a decrease in the microhardness and generation of new open volume defects accompanying the reversion of α′-martensite to austenite was observed. The amount of these defects correlated with the initial α′-martensite volume fraction. Their formation could be related to the volume contraction occurring during bcc/fcc transformation. The different chemical surroundings suggested that the positron trapping defects were associated with the metal carbide precipitates.

Thermal Doppler Broadening of Spectral Emissions in Space and Astrophysical Plasmas

Abstract The paper develops analytical modeling of thermal Doppler broadening of spectral profiles for particle populations described by kappa distributions, in the absence or presence of potential fields. The kappa distribution provides a straightforward replacement for the Maxwell distribution, that is, a generalization for describing systems characterized by local correlations among their particles, commonly found in space and astrophysical plasmas. The corresponding Voigt profiles are derived by convoluting the thermal and natural/collisional Lorentzian profiles. The kappa velocity distributions are employed to derive the thermal Doppler and Voigt profiles, while the kappa phase-space distributions in the presence of potential fields that depend on the position vector, are used to derive their respective differential profiles. We focus on attractive power-law potentials (oscillation-type, gravitational-type, and angular potentials), and study the variations of the produced Voigt differential profiles in detail. The developed formulations and guidelines provide a useful and statistically well-grounded “toolbox” for future reference in data analyses, simulations, analytical modeling, and theories of spectroscopy and related subjects of space and astrophysical plasmas.

Investigation of X-ray-induced Defects on Metals and Silicon by Using Coincidence Doppler Broadening Positron Annihilation Spectroscopy

The mechanical properties of Al, Ti, Fe, and Cu metals p-type Si, and n-type Si were investigated by using coincidence Doppler broadening (CDB) positron annihilation spectroscopy. The samples in this experiment were irradiated by using X-rays at generating powers for up to 9 kW. The data taken after the irradiation showed all the characteristic features predicted from defects with vacancies. The S parameter values of the metals were generally less than those of semiconductors such as p-type Si and n-type Si. The relationship between n-type Si and p-type Si were more affected when n-type Si rather than p-type Si was irradiated with X-rays.

Free Volume in Poly(ether imide) Membranes Measured by Positron Annihilation Lifetime Spectroscopy and Doppler Broadening of Annihilation Radiation

In order to characterize the free volume of a homologous series all-aromatic poly(ether imide) (PEI) membranes, two positron annihilation techniques were utilized: positron annihilation lifetime sp...

Point defect structure of La-doped SrTiO3 ceramics with colossal permittivity

Sr1-xLaxTiO3 (SLTO) ceramics with colossal permittivity were fabricated by conventional solid-state reaction method. The point defects of pure STO and SLTO ceramics were analyzed by Positron Annihilation Lifetime Spectroscopy (PALS) and Coincidence Doppler Broadening (CDB). The charge compensation mechanisms and dielectric properties of ceramics were investigated. The results indicated that the intrinsic defects in pure STO ceramics were mainly VTi″″. The charge compensation mechanism of SLTO ceramics was predominantly formation of VSr″. With increasing La content, εr of SLTO ceramics increased up to ∼70000 at room temperature. The results of first-principle calculations indicated that the colossal permittivity came from a sharp polarization increase caused by dipole structure of defects. tanδ of SLTO ceramics showed obvious Debye relaxation at high temperatures and the relaxation showed a multiple relaxation times derived from different kinds of polarization mechanism. The main polarization mechanism of SLTO ceramics gradually changed from ion displacement polarization to defect dipole polarization influenced by the concentration of La dopants.

Investigation of Helium Behavior in RAFM Steel by Positron Annihilation Doppler Broadening and Thermal Desorption Spectroscopy

The behavior of helium in reduced-activation ferritic/martensitic steels was investigated systematically with positron annihilation Doppler broadening measurement and thermal desorption spectroscopy. Specimens were irradiated with helium ions with different energies to various fluences at different temperatures. A threshold fluence was observed above which the rate of formation and growth of helium bubbles dramatically increased. Irradiation at higher temperature could suppress the formation and growth of HenVm clusters with low binding energies and enhance that of helium bubbles and HenVm clusters with high binding energies. Different changes of S parameters were observed in various depth after the irradiation temperature was increased from 523 K to 723 K. Irradiation of 18 keV-He+ enhanced the growth of HenVm clusters and helium bubbles compared with 100 keV-He+ irradiation. A possible mechanism is discussed.