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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.

A comparative analysis of two popular spectroscopy techniques is conducted in a coaxial cylindrical capacitively coupled discharge designed for the plasma processing of superconducting radio frequency (SRF) cavities. The density of the metastable and resonant levels in Ar is measured in both Ar and Ar/Cl 2 discharges to properly characterize the unique discharge system and aid in the development of a cavity etching routine. The first method, deemed the “branching fraction method,” utilizes the sensitivity of photon reabsorption of radiative decay to measure the lower state (metastable and resonant) densities by taking ratios of spectral lines with a common upper level. This method has been gaining popularity as it does not require any a priori knowledge about the electron energy distribution. The second method is a tunable diode laser absorption spectroscopy technique that measures the thermal Doppler broadening of spectral lines, from which the neutral gas temperature and lower state density of the transition can be evaluated. The two methods were conducted in tandem, while external parameters that were empirically determined to be important to the etching mechanism of SRF cavities are varied. Relationships between the excited state densities and the external parameters are presented for both spectroscopy methods and conclusions about the effects of these parameters on the discharge are stated when appropriate.

Fluffy spherical structure NiCo2S4 decorated with cubic phase hexagonal nanosheets was successfully synthesized through a two-step solvothermal method for energy storage. The electrochemical measurements reveal its good reversibility together with its high rate ability (1354 F g−1 at 0.5 A g−1 and 1128 F g−1 at 10 A g−1). Positron annihilation techniques (positron annihilation lifetime spectra and Doppler broadening spectra) were conducted to investigate the defects and the electronic structure of the two-step synthesized NiCo2S4. The results showed high S/W ratio, which is consistent with the electrochemical results. An asymmetric supercapacitor (ASC) was assembled in a button cell using the two-step synthesized NiCo2S4 as the positive electrode material and commercially activated carbon as the negative electrode material. The ASC presents a high energy density of 30.8 W kg−1 at a power density of 352.2 W kg−1 and 15.5 W kg−1 at a power density of 6860 W kg−1, with excellent cycling stability (89.3% capacitance retention after 3000 cycles) and nearly 100% Coulombic efficiency.

Funding: European Union Horizon 2020 research and innovation program under Grant Agreement No. 636930 (iCspec). We report multi-species spectroscopy of hydrocarbons with a continuous wave external-cavity quantum cascade laser based spectrometer providing tunability from 6 to 11μm to measure direct absorption spectra of the first 7 alkanes and their mixtures. The gas spectra were acquired in the range from 1440 to 1480cm−1 at a reduced pressure of 50 mbar and at a temperature of 323 K. By linearization of the measured wavelengths with a custom-made highly temperature stable air spaced etalon, a high spectral accuracy of ±0.001cm−1 is achieved for the whole spectral range. The simultaneous high resolution of 0.001cm−1 yields spectra of unprecedented richness of detail for the heavier alkanes (C3–C5) and allows the discrimination of narrow spectral features for the lighter ones (C1–C2). Thereby, the measured spectra reveal the influences of collisional broadening effects among the measured species. Quantitative spectroscopic multi-species gas sensing relies on the comprehension of the extent of spectral broadening. Studying the spectral broadening in combination with highly accurate reference spectra is mandatory for highly sensitive and specific multi-species gas analyzers. The quantitative results that can be obtained with our approach are presented for an exemplary calibrated mixture of all 7 components and reveal an absolute accuracy below 0.5 vol. % for the determination of the mole fraction of each gas. Postprint Postprint

Thin film ferroelectric capacitors of composition Pb(Zr0.52Ti0.48)O3 were exposed to Fe3+ radiation (1011 to 1013 ions/cm2), and the change in the defect structure was investigated by Doppler broadening positron annihilation spectroscopy and other characterization techniques. As the radiation fluence increases, a systematic drop of the S parameter of the positron annihilation photopeak is observed and attributed to an increase in the Zr- and Ti-site related vacancies relative to the Pb-sites. The results demonstrate that the radiation has a more significant influence on the Zr- and Ti-sites relative to the Pb-sites. It is also observed that the S parameter of the Mn-doped samples is higher than the undoped counterparts. Coupled with ferroelectricity measurements and X-ray diffraction, the results suggest that the Mn dopant modifies the initial structure of the material and leads to a different functional response.Thin film ferroelectric capacitors of composition Pb(Zr0.52Ti0.48)O3 were exposed to Fe3+ radiation (1011 to 1013 ions/cm2), and the change in the defect structure was investigated by Doppler broadening positron annihilation spectroscopy and other characterization techniques. As the radiation fluence increases, a systematic drop of the S parameter of the positron annihilation photopeak is observed and attributed to an increase in the Zr- and Ti-site related vacancies relative to the Pb-sites. The results demonstrate that the radiation has a more significant influence on the Zr- and Ti-sites relative to the Pb-sites. It is also observed that the S parameter of the Mn-doped samples is higher than the undoped counterparts. Coupled with ferroelectricity measurements and X-ray diffraction, the results suggest that the Mn dopant modifies the initial structure of the material and leads to a different functional response.

Novel solar cell materials consisting of Si nanoparticles embedded in SiO2 layers have been studied using positron annihilation spectroscopy in Doppler broadening mode and photoluminescence. Two positron-trapping interface states are observed after high temperature annealing at 1100 °C. One of the states is attributed to the (SiO2/Si bulk) interface and the other to the interface between the Si nanoparticles and SiO2. A small reduction in positron trapping into these states is observed after annealing the samples in N2 atmosphere with 5% H2. Enhanced photoluminescence is also observed from the samples following this annealing step.

Various nominally undoped and Si-doped (InxGa1- x)2O3 thin films were grown by pulsed laser deposition in a continuous composition spread mode on c-plane α-sapphire and (100)-oriented MgO substrates. Positron annihilation spectroscopy in the Doppler broadening mode was used as the primary characterisation technique in order to investigate the effect of alloy composition and dopant atoms on the formation of vacancy-type defects. In the undoped samples, we observe a Ga2O3-like trend for low indium concentrations changing to In2O3-like behaviour along with the increase in the indium fraction. Increasing indium concentration is found to suppress defect formation in the undoped samples at [In] > 70 at. %. Si doping leads to positron saturation trapping in VIn-like defects, suggesting a vacancy concentration of at least mid-1018 cm-3 independent of the indium content.

We report detailed experimental results on the optical nutation effect in acetylene-filled hollow-core photonic crystal fibers (HC-PCFs). This coherent quantum optical effect is manifested by the appearance of attenuated oscillations in the profiles of transmitted step-like coherent optical pulses. The experiments used 15 ns optical pulses with peak powers up to ∼4 W and centered at 1530.37 nm, which corresponds to the acetylene (12C2H2) vibration-rotation resonance absorption line P9. The all-fiber cell made from a HC-PCF with internal diameter ∼10.3 μm contained a gas pressure of about 0.12 Torr. The Maxwell-Bloch equations are used to analyze the influence of the homogeneous relaxation, the inhomogeneous Doppler broadening of the absorption line, and the random orientation of the acetylene molecules. By comparing the experimental results with numerical simulations, we obtain the longitudinal and the transverse relaxation times to be close to 10 ns and find that the transition dipole moment of the acety...

Defects and their annealing behavior for low (2×1015/cm2) and high (1.7×1018/cm2) doses of 180 keV oxygen-implanted silicon have been investigated by the coincidence Doppler broadening (CDB) and lifetime measurements in variable-energy positron annihilation spectroscopy. In the low-dose sample, divacancies are induced throughout the entire implantation region. In the vacancy-oxygen coexisting region (300–500 nm depths), by raising the annealing temperature to 600 °C, vacancy–oxygen VxOy complexes with one vacant site are formed and, simultaneously, the migration of oxygen begins to takes place. In the vacancy-rich region (−200 nm depths), the evolution of simple vacancy clusters to V4 is mainly observed below 600 °C. From CDB and lifetime measurements, it has been proven that after annealing at 800 °C, the VxOy complexes are formed throughout the implanted region and they contain four vacant sites and a high ratio of y to x. On the other hand, high-dose implantation at 550 °C produces the VxOy complexes w...

The 3γ annihilation of orthopositronium and the Doppler broadening of the positron annihilation line have been measured by implanting low energy positrons in low dielectric constant (low-k) SiOCH films. The evolution and stability of film porosity with thermal treatments in the 400–900 °C temperature range has been studied. The films have been produced by plasma enhanced chemical vapor deposition and after annealing in N2 atmospheres at 480 °C have been treated in N2+He plasma. The minimum free volume of the pores in the as-produced samples has been estimated to correspond to that of a sphere with radius r=0.6 nm. The treatment in the N2 plasma was found to seal the pores up to 45 nm depth. Both the composition of the films (as obtained by Rutherford backscattering spectroscopy and elastic recoil detection analysis) and the chemical environment of the pores probed by positrons were found to be very stable up to 600 °C thermal treatment. Above such a temperature a reduction of the hydrogen content accompan...

Positron annihilation was used to probe vacancy-type defects in electroplated Cu films. Doppler broadening spectra of the annihilation radiation for Cu films deposited on samples with a Ta(20 nm)/SiO2(100 nm)/Si structure were measured with a monoenergetic positron beam. For an as-deposited Cu film, the line-shape parameter S measured 20 days after deposition was larger than that measured 1 day after deposition. The observed increase in the value of S was attributed to grain growth at room temperature and the corresponding increase in the fraction of positrons trapped by vacancy clusters in the grains. In isochronal annealing experiments, the value of S for an electroplated Cu film increased for annealing below 200 °C, suggesting agglomeration of vacancy-type defects (vacancy clusters). A decrease in the S value was observed for annealing above 300 °C, and this was attributed mainly to the decrease in the concentration of vacancy clusters. The annealing stages of the defects in electroplated Cu were found to agree with those for irradiated pure Cu.

The defect distributions in 11B-, 14N-, and 27Al-implanted epitaxial 4H–SiC are studied using monoenergetic positron beams. At least three types of defects are needed to account for the Doppler broadening annihilation spectra and two of the defects are tentatively identified as VSi, and VSiVC. By comparing the defect profiles extracted from the annihilation spectra to the chemical profiles determined by secondary ion mass spectrometry, and to the primary defect profiles obtained from binary collision approximation simulations, it is concluded that the defects found at depths considerably deeper than the projected range of the implanted ions mainly originate from deeply channeled ions.

Variable-energy positron annihilation spectroscopy (VEPAS) has been extensively applied to study defects in near-surface regions and buried interfaces, but there is an inherent limit for depth resolution due to broadening of the positron implantation profile. In order to overcome this limit and obtain optimum depth resolution, iterative chemical etching of the sample surface and VEPAS measurement are employed. This etch-and-measure technique is described in detail and the capabilities are illustrated by investigating the depth profile of defects in Si after B and P implantations with 2×1014/cm2 at 100 keV followed by annealing. Defect tails can be accurately examined and the extracted defect profile is proven to extend beyond the implanted ion range predicted by the Monte Carlo code TRIM. This behavior is more remarkable for P ion implantation than B, and the mass difference of the implanted ions is strongly related to it. No significant difference is recognized in the annealing behavior between B and P i...

We measured the Doppler broadening of positron 2γ annihilation and the probability of positron 3γ annihilation (I3γ) for paramagnetic defects (E′ and Pb) containing SiOx (1.5

Zinc oxide (ZnO) thin films grown on ScAlMgO4 substrates were characterized by means of positron annihilation. We measured Doppler broadening spectra of annihilation radiation and photoluminescence spectra for the ZnO films deposited by laser molecular-beam epitaxy and single-crystal ZnO. Although the lifetime of positrons in single-crystal ZnO was close to the lifetime of positrons annihilated from the free state, the diffusion length of positrons was shorter than that for typical defect-free materials. We attribute this to the scattering of positrons by native defects. For the ZnO films, we observed a correlation between the defects and the lifetime of bound exciton emissions τEx; the main defect species detected by positron annihilation was Zn vacancies or other related defects. Isochronal annealing at 750–850 °C was found to introduce additional vacancy-type defects into the film, although the value of τEx was scarcely changed by the annealing.

The sensitivity of positron annihilation spectroscopy (PAS) to process parameters associated with the formation of buried oxide layers in silicon (SIMOX, Separation by IMplantation of OXygen, structures) has been investigated and evaluated. Ion beam current density, implantation temperature, and ion dose and energy were varied over ranges used in standard and low-dose SIMOX formation. Whereas the positron (Doppler broadening) parameter for as-implanted samples was found to have no measurable sensitivity to low-energy ion current density, the results suggest that there is a small dependence on implantation temperature. More significantly, PAS is highly sensitive to ion dose (i.e., to changes in dose of a few percent), and to ion energy. An empirical parameter is proposed as a measure of the former. The results for as-implanted samples suggest that beam-based PAS is an ideal method for improved dose control, particularly during the formation of low-dose SIMOX. Measurements on fully-formed SIMOX samples are ...

The Doppler broadening of ion line profiles emitted by z-pinch plasma provides information about the thermalization of the implosion kinetic energy and the radiation efficiency of the pinch. Measurements of these line profiles are often complicated by source broadening in the instrument and opacity broadening of the emitted radiation. A high resolution concave crystal spectrometer in the Johann geometry was used to record the time averaged spectra of optically thin trace elements in the load. An imaging slit provided radially resolved but axially averaged spectra. The measurements indicate that lower ion temperatures (3–5 keV) are observed for Al wire loads on both the Saturn and Double EAGLE accelerators in the short current pulse mode (60–100 ns) than in the long pulse mode (125–225 ns) where values of 6.3–9.5 keV are observed. These values are smaller than those observed on Saturn by others. Furthermore, the wavelength at the line center of axially resolved ion line profiles on the DM-2 accelerator at ...

Metalorganic vapor-phase epitaxially grown GaN films have been studied using beam positron annihilation spectroscopy. Positron lifetime and Doppler broadening of annihilation radiation measurements indicate that defects, probably Ga vacancies, are reduced in concentration when films are doped with Mg, as previously observed by other workers. In lightly doped films a homogeneous layer with a minimum defect concentration is present from just below the surface, to a depth of around 0.1 μm. For heavily doped films, there is evidence of a further substantial decrease in the defect concentration following an anneal to 900 °C. The effects of impurities and defects are discussed.

For the most advanced multi-frame camera in the world, the shortest response time between two frames is no shorter than 1.5 ns. Thus, there is no effective way to diagnose the ultrafast sub-nanosecond dynamic of a microwave-driven plasma discharge in a single pulse. Different-length multi-sub-beam optical fibers, together with a spectrometer and an EMICCD camera, are proposed and designed to detect the nanosecond discharge spectra in a single pulse, like a real-time multi-frame spectral camera. This novel method could realize a time interval between two consecutive frames shorter than 0.1 ns by a length difference of 2 cm for sub-fibers, achieving the measurement of ultrafast plasma dynamics. Temporal evolution of electron density as well as energy of electrons and ions during nanosecond microwave discharge is further studied by de-convolving the Stark broadening and thermal Doppler broadening and by calculating the ratio of emission coefficients.

Microstructure evolution of three Al-Ag alloys with different Ag contents (1 wt. % Ag, 5 wt. % Ag, and 15 wt. % Ag) was studied by positron annihilation spectroscopy during the aging process. In situ measurements of the positron lifetime and Doppler broadening of annihilation radiation indicate the fast formation of Ag-rich clusters during natural aging of the alloys. The formation of Ag-rich clusters was further confirmed by coincidence Doppler broadening measurements. The Ag signal reflected by the Coincidence Doppler broadening spectrum increases with increasing Ag content and is further enhanced after subsequent artificial aging at 140 °C. This might be due to the increase in the size of Ag clusters. The temperature dependence of the Doppler broadening spectra between 10 K and 290 K was measured for the Al-Ag alloys after natural and artificial aging. Detrapping of positrons from Ag clusters with increasing temperature was observed for all the three Al-Ag alloys after natural aging and for the Al-1 wt...

The depth distributions of oxygen-related defects in separation-by-implanted oxygen wafers were determined from measurements of Doppler broadening spectra of the annihilation radiation. Vacany–oxygen complexes were introduced by implanting 180-keV oxygen at (2–6)×1017 cm−2 into Si substrates. Their sizes decreased below the subsurface region (

A molecular monolayer (octadecanethiol on gold) is compressed between a bronze ball and a sapphire prism. Vibrational spectra are obtained by optical sum-frequency generation before, during, and after a pressure cycle to 660 MPa (6.6 kbar). Both reversible and permanent effects are observed on the frequency, intensity, and shape of methyl C–H stretching resonances in the monolayer. Spectral broadening indicates the heterogeneity of chemical environments in the contact, while a pressure-dependent loss of orientational order attenuates the resonant signals. These results from a model system demonstrate that contact-modifying molecular films can be investigated spectroscopically.

Results of spectroscopic investigations of the plasma formed on the surface of a ferroelectric cathode upon the application of a driving pulse are presented. The ferroelectric plasma cathode was made of a solid solution of Sr, Ba, Ti, Nb, Pb, and O. Its front side was covered by Cu grounded strip electrodes. A driving pulse with an amplitude ≲18 kV and pulse duration of ∼400 ns was applied to the rear Cu disk electrode. A Jobin-Yvon 750M spectrometer was used for visible light dispersion. Spectral line profiles were obtained by a fast framing camera. It was shown that light is emitted from the excited ions and neutral atoms of Cu, Pb, Sr, Ba, Ti, and H within the first 50 ns after the beginning of the driving pulse. By analyzing the Doppler broadening of the observed spectral line profiles it was found that the ion and neutral atom temperature is ⩽0.8 eV. Analysis of the Stark broadening of the Hα and Hβ spectral lines showed the absence of a high (>1 kV/cm) electric field which could be developed at the ...

One- and two-detector Doppler broadening measurements performed on low (∼1014 to 1015 O+/cm2) and high dose (∼1017 to 1018 O+/cm2) oxygen-irradiated Si using variable-energy slow positrons are analyzed in terms of S and W parameters. After annealing the low-dose samples at 800 °C, large VxOy complexes are formed at depths around 400 nm. These complexes produce a clear-cut signature when the ratio of S to that of defect-free bulk Si is plotted. Similar behavior is found for samples irradiated with 2 and 4×1017 O+/cm2 and annealed at 1000 °C. After irradiation with 1.7×1018 O+/cm2 and anneal at 1350 °C a 170 nm thick almost-bulk-quality Si surface layer is formed on top of a 430 nm thick buried oxide layer. This method of preparation is called separation by implantation of oxygen. S−W measurements show that the surface layer contains electrically inactive VxOy complexes not seen by electron microscopy. A method is presented to decompose the Doppler broadening line shape into contributions of the bulk, surfa...

Defects in SiO2 (48 nm)/Si induced by 30 keV Er ion implantation were studied by positron annihilation. Depth-selective information on defects for samples implanted with doses of 3.0×1014 and 1.5×1015 Er/cm2 was obtained by a variable-energy positron beam by measuring Doppler broadening of positron annihilation γ rays as a function of incident positron energy. Comparison of the results by Doppler broadening with those by electron spin resonance after annealing indicates that the types of defects (which predominantly exist in the SiO2 layer) depend on implantation dose. The annealing temperature dependence of positron data is compared with that of the cathodoluminescence intensity at 1.54 μm, and the possible effect of defects on luminescence intensity is discussed.

The spectroscopic properties of P2O5–Y2O3–Er2O3 and P2O5–Li2O–Al2O3–Y2O3–Er2O3 ultraphosphate glasses have been investigated. The concentration quenching rates of Er3+ in these two systems are lower compared with those in other oxide glasses. The spectral broadening of the measured fluorescence of 83.33P2O5–(16.67−x)Er2O3–xY2O3 (x=0.1 and 16.67) was observed by increasing temperature. Meanwhile, the lifetime of the 4I13/2 level shows the temperature dependence with increasing temperature, ascribed to the possible thermalization of higher-lying sublevels of the 4I13/2 level with faster radiative rates and the effect of radiation trapping. At room temperature, the deformation of the emission spectrum line shape has been observed with the increase of Er2O3 concentration due to the reabsorption effect. The 1.5-μm-amplification characteristics of P2O5–Y2O3–Er2O3 ultraphosphate glasses have been predicted from the calculated gain spectra at different pump powers.

Vacancy-type defects introduced by the carburization of Si were studied by means of monoenergetic positron beams. Doppler broadening spectra of the annihilation radiation were measured for Si substrates with carbon films at temperatures between 298 and 1473 K. The line-shape parameter S, which corresponds to the annihilation of positrons near the interface between the carbon film and the Si substrate, started to increase above 1173 K. This was attributed to the trapping of positrons by vacancy-type defects introduced by carburization. The major species of the defects detected by positron annihilation was identified to be vacancy clusters in the Si substrate.

Depth distributions and species of defects were determined from measurements of Doppler broadening spectra of annihilation radiation and lifetime spectra of positrons for 6H-SiC implanted with 200 keV P+ at a dose of 1×1015 cm−2. The annealing behavior of an amorphous layer was divided into four stages. Stages I (100–500 °C) and II (500–1100 °C) were identified as the relaxation of amorphous networks and the agglomeration of open spaces owing to rearrangements of atoms, respectively. In states III (1100–1500 °C) and IV (1500–1700 °C), corresponding to the recrystallization of the amorphous layer, the mean size of the open volume of defects decreased with increasing annealing temperature; these defects were identified as open spaces adjacent to extended defects. Vacancy-type defects were found in the subsurface region (

The depth distributions and species of defects in Si on insulator (SOI) fabricated by low-dose oxygen implantation were determined from measurements of Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons. The temperature range for the annealing of the defects in the subsurface region (

The development of a sensitive, depth-tuneable probe, with mapping capability, for low-energy ion implantation dosimetry, is described. The technique is based on the dependence of the extent of Doppler broadening of the positron annihilation linewidth upon the concentration of open-volume defects produced by the implanted ions. By varying the incident positron energy one can ensure that most of the annihilation events occur near the damage peak. A series of diagnostic measurements have been performed which demonstrate that the positron response lies on an almost universal curve, independent of ion mass and energy. The probe is highly sensitive, with a dose threshold in the 10−9 cm−2 range for many implants, and has the potential to measure dose/dose uniformity to better than 1×1011 cm−2 in the 1011−1012 cm−2 range in run times of a few seconds. Considerations underlying the development of a practical instrument based on these findings are discussed.

Defects in p+-gate metal–oxide–semiconductor structures were probed using monoenergetic positron beams. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured for BF2+- or B+-implanted polycrystalline-Si(300 nm)/SiO2(4 nm)/Si specimens. The line-shape parameter, S, corresponding to the annihilation of positrons near SiO2 films was decreased by annealing treatments performed after ion implantation. This fact was attributed to the trapping of positrons by vacancy-oxygen complexes introduced by the boron diffusion in the SiO2 film. The introduction of such complexes was enhanced by incorporation of fluorine, but suppressed by doping of phosphorus into polycrystalline films.

We used Doppler broadening positron annihilation spectroscopy as a nanovoid characterization tool in the study of low dielectric constant (low-k) hydrogen-silsesquioxane (HSSQ) thin films. The high void sensitivity of this method, combined with depth-resolving capability, enables one to observe changes in the local electronic environment in a thin film. We established a correlation between the annihilation parameters and the dielectric properties for a series of samples subjected to various annealing conditions in nitrogen. Qualitative differences are found between the pore structures of isochronally and isothermally annealed films, suggesting a complex relationship between the film dielectric constant and pore size and pore density. Results showing changes in the chemical environment caused by exposure to various processing environments are also presented. First, the changes in the HSSQ films were determined after a year of exposure to ambient air in which the film properties changed with time due to wat...

Numerical simulations of radiation transport in cylindrical geometry are used to determine the effect of radially symmetric inhomogeneities. The focus of this study are inhomogeneities such as may be produced by radial cataphoresis or temperature gradients in cylindrical glow discharges, i.e., a quadratic profile of absorbing atoms which has a minimum on the axis of the cylinder. A propogator function analysis of the Holstein–Biberman equation and a Monte Carlo simulation of resonance photon scattering are simultaneously used to examine three limiting cases of interest: (i) a pure Doppler broadened atomic lineshape, (ii) a pure Lorentz atomic lineshape produced by foreign gas broadening, and (iii) a pure Lorentz atomic lineshape produced by resonance collision broadening. The fundamental mode distribution of excited atoms, the fundamental mode trapped decay rate, and the volume-averaged escape rate for a homogeneous production rate per unit volume are calculated for each of these cases. The trapped decay rates are found to change modestly (depending upon lineshape) as the degree of inhomogeneity is increased, if the volume integral of the absorbing atom density remains fixed. Correction factors for the fundamental mode trapped decay rate are reported.

The techniques of positron annihilation spectroscopy have been applied to the study of well-characterized reservoir rock samples. In this article we focus primarily on sandstone and carbonate samples. We report (a) measurements of the Doppler broadening (DB) parameters and mean lifetime values (〈τ〉) of dry samples; (b) measurement of the DB parameters as a function of temperature from room temperature to 200 °C; and (c) observation of the outgassing of heated samples simultaneously with the DB measurements. We interpret the measurements in terms of positron annihilation with both valence and core electrons and discuss how the DB parameters depend on the major chemical constituents (lithology) and to a lesser degree on the structure of the samples. The major result of this work is that the positron parameters such DB parameters (S, SW, and W) and mean lifetime values are shown to be reproducible and reliable bulk properties of well-characterized rock samples and can be used to categorize rocks of geophysic...

Positron lifetime and Doppler broadening spectroscopies were applied to investigate a porous silicon film subjected to heat treatment in an argon atmosphere. Heating between 300 and 500 °C increased the mass of the film by 17% due to oxygen uptake and the concentration of open volume defects associated with the formation of an oxide layer on the silicon nanocrystallites increased by a factor of 3. Between 600 and 1000 °C their concentration decreased gradually to 1/2 the original concentration. Doppler broadening results indicate two distinct electron momentum distributions, one arising from open volume defects and one from pickoff annihilation of positronium at the pore walls caused by electrons with an unexpectedly narrow momentum distribution.

Depth profiling studies of defects in CdS thin films using a low energy positron beam are reported. CdS films of 1 μm thickness on a glass substrate have been grown using the chemical bath deposition method and irradiated with 140 keV Ar ions to doses of 5×1014 and 5×1016 cm−2, respectively. The Doppler broadening line shape S parameter in as-grown and irradiated films has been monitored as a function of positron beam energy Ep. S vs Ep curves have been analyzed, using a positron diffusion model in terms of annihilations at surface, epithermal and bulk states. The S parameter at the surface (Ss) is found to be significantly higher than that in the bulk. This is understood as due to positron trapping at a large concentration of Cd vacancies in the surface region, which act as acceptors. The observed reduction of Ss with increasing Ar ion dose seems to indicate the formation of antisite defects in the surface region, brought about by irradiation. At sample depths corresponding to the peak damage layers, the...

Positron lifetime measurements and Doppler broadening spectroscopy using slow positrons were combined to investigate open-volume defects created by sawing wafers from GaAs ingots by a diamond saw cutter. It was found during step-by-step polishing that the depth distribution represents a wedgelike profile. The depth and the concentration of the defects introduced by the diamond saw depend on the advance of the saw blade. An isochronal annealing experiment was carried out to study the thermal stability of the observed defects. The positron lifetime of the dominant open-volume defect was estimated to be larger than 330 ps. It was concluded from this lifetime value and from the Doppler broadening parameters as well as from the annealing behavior that vacancy aggregates consisting of at least two vacancies are created by the sawing procedure. More extended defects such as microcracks were analyzed by scanning electron microscopy.

We report on the influence of the gas velocity on the discharge equilibrium in a low-pressure (1 Torr) microwave driven hydrogen plasma. We show that the gas velocity has a great influence on the dissociation degree, the key parameter being the residence time of the molecules in the plasma with respect to their effective dissociation time. The power balance (plasma length, electric field…) is substantially modified by the variations of the dissociation degree induced by the change in gas velocity. On the contrary, the gas temperature, determined by Doppler broadening of H atom Balmer lines, is not directly dependent on the gas velocity. Simple calculations confirm that the gas temperature is determined by local parameters and that heat is not axially transported. Surprisingly, dissociation of the H2 molecules does not seem to play any significant role for the heating of the gas.