Monoenergetic Positron Beam Research Articles

Annealing behaviors of open spaces and gas desorption in chemical vapor deposited SiO2 studied with monoenergetic positron beams

The annealing properties of open spaces in 90-nm-thick SiO2 deposited from tetraethylorthosilicate (TEOS) using plasma-enhanced chemical vapor deposition (PECVD) were studied with monoenergetic positron beams. From the lifetime of positronium (Ps) and an empirical model assuming a spherical open space, the mean diameter of open spaces was estimated to be 0.45 nm for PECVD-SiO2 before annealing. In the annealing temperature range below 350 °C, the size of the open spaces and their concentration increased as the temperature increased. Because initial water desorption from PECVD-SiO2 occurred in this temperature range, the observed increases in the size and concentration of spaces were attributed to the detrapping of water from such regions. Above 400 °C annealing, Ps formation was suppressed due to carrier traps introduced by the desorption of gas incorporated during TEOS decomposition. The size of the open spaces reached its maximum value (0.61 nm) after 800 °C annealing and started to decrease above 900 °C. After 1000 °C annealing, although the size of the spaces was close to that in thermally grown SiO2, their concentration remained low, which was attributed to residual impurities in the SiO2 network.

Vacancy-Type Defects and Oxygen Incorporation in NiAl for Advanced Interconnects Probed by Monoenergetic Positron Beams and Atom Probe Tomography

Vacancy-Type Defects and Oxygen Incorporation in NiAl for Advanced Interconnects Probed by Monoenergetic Positron Beams and Atom Probe Tomography

In-situ positron annihilation spectroscopy during tensile tests on Al alloys

We present a materials analysis method to enable in-situ Doppler broadening spectroscopy (DBS) of the 511 keV annihilation line in matter during tensile tests. This technique allows the correlation between the formation of lattice defects on an atomic scale and the macroscopic physical materials properties stress and strain. By implanting a monoenergetic positron beam into samples of AlMg3 (3.3535) and AlMgSi (3.3206) the onset of plastic deformation was clearly observed in the recorded annihilation spectra during the measurement of the corresponding stress–strain curves. The changes of the DBS spectra are attributed to positron annihilation in (open-volume) defects—predominantly vacancies elastically bound to dislocation lines—formed during plastic deformation. The elastic strain (Hook’s region), however, does not lead to changes in DBS spectra.

Impact of nanosecond laser annealing on vacancies in electroplated Cu films studied by monoenergetic positron beams

Positron annihilation was used to probe vacancy-type defects in electrodeposited Cu films after nanosecond pulse laser annealing. For the as-deposited Cu film, we identified the coexistence of two different vacancy-type defects, vacancy clusters (such as V16) and monovacancy-type defects, coupled with impurities. An enlargement in the vacancy size was observed after the laser annealing process. The size of these defects was estimated to be close to V30, and such defects could not be formed by conventional furnace annealing. After furnace annealing at 400 °C, the size of the larger vacancy clusters decreased, but that of the smaller vacancies increased. The observed change in the sizes of vacancies is considered to be related to interactions between vacancies and impurities. The depth profile of the defects varied by changing the laser energy density and the number of laser shots. The impact of laser annealing on the vacancy-type defects was observed even after furnace annealing at 800 °C. Because the presence of point defects in electroplated Cu directly correlates with electromigration and grain growth, the ability of laser annealing to introduce large vacancy clusters in the localized region shows the potential of nanosecond laser annealing as a low-thermal budget process tool for back-end-of-line materials.

Vacancy-type defects in AlInN/AlN/GaN structures probed by monoenergetic positron beam

Vacancy-type defects in AlInN(10 nm)/AlN(1–2 nm)/GaN were probed by using a positron annihilation technique. The crystal quality of the AlInN layer and atomic diffusion near heterointerfaces were also studied by x-ray diffraction reciprocal space mapping, transmission electron microscopy, and energy-dispersive x-ray spectroscopy. For an as-deposited sample without an AlN spacer layer (AlInN/GaN), Ga atoms diffused into the AlInN layer, and as a result, the concentration of Ga-vacancy-type defects in the GaN layer increased. The vacancy concentration was decreased by inserting the AlN layer, which was attributed to the suppression of out-diffusion of Ga from the GaN layer. The effect of the thickness of the AlN layer on the mobility of two-dimensional electron gas is discussed in terms of the introduction of vacancies into the channel region. The annealing behaviors of vacancies in the GaN layer and atomic exchange near heterointerfaces are also discussed.

Dosimetry of positrons from PET radiotracers using Monte Carlo simulations and measurements

Positrons, the anti-particle of electrons, are broadly used in medicine and materials science. Thus, understanding how free positrons interact with soft biological matter is important for medical diagnostic procedures as well as for safety. Most research in this regard focuses on positron transport in the context of image quality; however, scholars lack knowledge regarding radiation dose exposure. Too little attention has been paid to the full analysis of positron internal dosimetry.In this paper we examine the quality of positron transport using EGS5 Monte Carlo (MC) code, comparing it to GAMOS Monte Carlo code. To validate the suitability of EGS5 and GAMOS codes for positron dose evaluations, an experiment with a concentrate FDG syringe was performed. Dose rate was measured using a mobile MOSFET device and compared to MC simulation results obtained by EGS5 and GAMOS. In this study simulations of the transport of mono-energetic positron beams from 100 keV to 1.2 MeV were performed, in addition to two practical radionuclide isotopes through two different tissue layers: 5 mm and 0.07 mm thickness. The results of the simulations using ESG5 Monte Carlo code were compared to the GAMOS Monte Carlo code results. The simulations were performed to obtain detailed energy deposition from interactions of positrons and secondary electrons induced by positron sources along tissue layers.The EGS5 code dose results demonstrate good agreement with the MOSFET measured results. The GAMOS code results yielded a difference of more than twice the measured dose. Differences between the EGS5 and the GAMOS codes resulting from the tissue layer simulations are presented in this paper.

Vacancy-type defects in TiN/ZrO2/TiN capacitors probed by monoenergetic positron beams

TiN/ZrO2/TiN capacitors were fabricated by using a physical vapor deposition technique, and their microstructures were characterized using X-ray diffraction, a scanning transmission electron microscope, energy-dispersive X-ray spectroscopy, and monoenergetic positron beams. After the deposition of a 10-nm-thick ZrO2 layer on the top TiN layer, a defect-rich interlayer was formed in the ZrO2 layer. After post-deposition annealing at 550°C, the width of the interlayer expanded toward the center of the ZrO2 layer. The major vacancy-type defects in the ZrO2 layer were determined to be a Zr-vacancy coupled with oxygen vacancies. After annealing, the size of the vacancies increased, and it was close to the open spaces in amorphous high-k materials. The presence of such vacancy-type defects in the ZrO2 layer suggests that not only oxygen vacancies but also cation vacancies play an important role in the defect formation of the ZrO2 layer and affect its electrical properties. The introduction of the interlayer and defect-rich region was suppressed by annealing with the top TiN layer. The leakage current mechanism of the capacitor is discussed in terms of vacancy-type defects in the ZrO2 layer.

Open volume defects in ultra-thin TiO2 layers embedded in VMCO-like samples studied with positron annihilation spectroscopy

Positron annihilation signals from VMCO-like samples grown by atomic layer deposition at different temperatures are utilized for the characterization of differences in open volume defects in TiN/TiO2/a-Si heterostructures. Doppler and coincidence Doppler mode of positron annihilation spectroscopy combined with a monoenergetic positron beam were used for this study. Differences observed in the Doppler parameters indicate differences in the positron trapping states of the TiO2 epilayers grown at different temperatures. Furthermore, the coincidence-Doppler results show that these differences cannot be due to intermixing of the TiO2 and a-Si layers and formation of thin SiO2 layers at the interface during the growth process. The results indicate that the amount of open volume defects in the TiO2 layer of the VMCO-structure seems to increase with an increase in the growth temperature.

Dopant activation process in Mg-implanted GaN studied by monoenergetic positron beam

A process for activating Mg and its relationship with vacancy-type defects in Mg-implanted GaN were studied by positron annihilation spectroscopy. Mg+ ions were implanted with an energy of 10 keV, and the Mg concentration in the subsurface region (≤ 50 nm) was on the order of 1019 cm−3. After the Mg-implantation, N+ ions were implanted to provide a 300-nm-deep box profile with a N concentration of 6 × 1018 cm−3. From capacitance–voltage measurements, the sequential implantation of N was found to enhance the activation of Mg. For N-implanted GaN before annealing, the major defect species were determined to Ga-vacancy related defects such as divacancy. After annealing below 1000 °C, the clustering of vacancies was observed. Above 1200 °C annealing, however, the size of the vacancies started to decrease, which was due to recombinations of vacancy clusters and excess N atoms in the damaged region. The suppression of vacancy clustering by sequential N-implantation in Mg-implanted GaN was attributed to the origin of the enhancement of the Mg activation.

Low-temperature annealing behavior of defects in Mg-ion-implanted GaN studied using MOS diodes and monoenergetic positron beam

Mg ions were implanted into Si-doped (5 × 1017 cm–3) n-GaN at a dose of 1.5 × 1011 or 1.5 × 1012 cm–2. MOS diodes were used to characterize the implanted GaN after 300 °C annealing for 3 h and after additional 500 °C annealing for 3 min. Although capacitance–voltage (C–V) characteristics varied with the dosage, the effects of acceptor-like defects induced by ion implantation were observed in the C–V characteristics independently of dosage and annealing temperature. A defect level at approximately 0.25 eV below the conduction band edge was detected electrically. By positron annihilation spectroscopy, its origin was identified as a divacancy consisting of Ga and N vacancies. It was found that its density compared with that of as-implanted GaN decreased with 300 °C annealing, and further increased with 500 °C annealing. This phenomenon was explained on the basis of the difference between the diffusion barriers of possible point defects.

Effects of ultra-high-pressure annealing on characteristics of vacancies in Mg-implanted GaN studied using a monoenergetic positron beam

Vacancy-type defects in Mg-implanted GaN were probed by using a monoenergetic positron beam. Mg ions were implanted into GaN to obtain 0.3-μm-deep box profiles with Mg concentrations of 1 × 1019 cm−3. The major defect species in an as-implanted sample was determined to be Ga-vacancy related defects such as a complex between Ga and N vacancies. The sample was annealed under a nitrogen pressure of 1 GPa in a temperature range of 1000–1480 °C without a protective capping layer. Compared with the results for Mg-implanted GaN annealed with an AlN capping layer, the defect concentration was decreased by the cap-less annealing, suggesting that the surface of the sample was an effective sink for vacancies migrating toward the surface. Depth distributions of Mg after annealing above 1300 °C were influenced by the presence of residual vacancies at this temperature. Hydrogen atoms were unintentionally incorporated into the sample during annealing, and their diffusion properties were also affected by both vacancies and Mg.

Annealing behaviors of vacancy-type defects in AlN deposited by radio-frequency sputtering and metalorganic vapor phase epitaxy studied using monoenergetic positron beams

Vacancy-type defects in AlN films were probed by using monoenergetic positron beams. The AlN films were deposited on sapphire substrates by using a radio-frequency sputtering technique. Epitaxial films grown by metalorganic vapor phase epitaxy on the sputtered AlN films were also characterized. For the sputtered AlN, the major defect species was identified to be complexes between Al-vacancy and oxygen atoms located at nitrogen sites. Vacancy clusters were introduced after annealing at 1300 °C in the N2 atmosphere but their concentration decreased with a higher annealing temperature. The vacancy–oxygen complexes, however, still existed in the AlN film after annealing at 1700 °C. For the AlN epitaxial films, the concentration of vacancy clusters increased as the growth temperature increased up to 1300 °C but it decreased with the post-growth annealing at 1700 °C.

Voids and vacancy-type defects in SiO2/GaN structures probed by monoenergetic positron beams

Voids in SiO2 films deposited on GaN were probed by using monoenergetic positron beams. The films were fabricated on GaN substrates by using plasma-enhanced chemical vapor deposition. The size and density of the voids in the films increased up to an annealing temperature of 800 °C and then decreased at 1000 °C. The observed annealing behaviors of the voids were attributed to the desorption of impurities incorporated during the deposition process and the shrinkage of the Si–O matrix by high-temperature annealing. Vacancy-type defects were introduced into the GaN substrate after 1000 °C annealing in O2 atmosphere due to the diffusion of Ga from the substrate to the SiO2 film. No out-diffusion of Ga into the SiO2 film was observed for the annealing in N2 atmosphere. Thus, the observed out-diffusion of Ga was attributed to the enhanced oxidation of GaN during the annealing in O2 atmosphere. The diffusion of positrons implanted into the GaN substrate toward the SiO2 film was suppressed by annealing, suggesting a decrease in the negative charges in the SiO2 film or near the SiO2/GaN interface.

Annealing Behaviours of Open Spaces in Thin Al2O3 Films Deposited on Semiconductors Studied Using Monoenergetic Positron Beams

Annealing Behaviours of Open Spaces in Thin Al₂O₃ Films Deposited on Semiconductors Studied Using Monoenergetic Positron Beams

Positron Annihilation Spectroscopy in Material studies

Positron annihilation spectroscopy (PAS) is a method dedicated to detection of open-volume type of defects in materials. Nowadays, this technique is of a great interest due to the practical character of obtained results. New devices using monoenergetic positron beams are built. The paper presents the basics of PAS, a description of popular experimental techniques and their application on the basis of results obtained in the frame of collaboration between Joint Institute for Nuclear Research (JINR), Institute of Physics - Vietnamese Academy of Science and Technology (VAST) and Vietnam Atomic Energy Institute (Vinatom).. The positron beam with energy up to 35 keV and other apparatus for PAS studies are under operation in Dubna.

Annealing Behavior of Vacancy‐Type Defects in Mg‐ and H‐Implanted GaN Studied Using Monoenergetic Positron Beams

Vacancy‐type defects in Mg‐implanted GaN with and without hydrogen (H) implantation are probed by using monoenergetic positron beams. Mg+ and H+ ions are implanted into GaN(000) to obtain 0.1 and 0.7‐µm‐deep box profiles with Mg and H concentrations of 1 × 1019 and 2 × 1020 cm−3, respectively. For the as‐implanted samples, the major defect species is determined to be Ga‐vacancy (VGa) related defects such as VGa, divacancy (VGaVN), and their complexes with impurities. For Mg‐implanted samples, an agglomeration of vacancies starts at 800 °C annealing, leading to the formation of vacancy clusters such as (VGaVN)3. For the samples annealed above 1000 °C, the trapping rate of positrons by vacancies is increased by illumination of a He–Cd laser. This is attributed to the capture of photon‐excited electrons by the defects and their charge transition. For Mg‐ and H‐implanted samples, the hydrogenation of vacancy‐type defects starts after 800 °C annealing. Comparing with the annealing behavior of defects for the samples without H‐implantation, the clustering of vacancy‐type defects is suppressed, which can be attributed to the interaction between Mg, H, and vacancies.

Vacancy-type defects in GaN self-assembled nanowires probed using monoenergetic positron beam

Vacancy-type defects in GaN nanowires (NWs) and the trapping of electrons by the vacancies were studied by positron annihilation. Undoped, Si-, and Mg-doped GaN NWs were grown on Si substrates by plasma-assisted molecular beam epitaxy. The major species of vacancies in the undoped and Si-doped samples was identified as a complex between a Ga vacancy and impurities such as oxygen and hydrogen. For the Mg-doped samples, the trapping rate of positrons for such defects decreased with the increase in Mg concentration because of the downward shift of Fermi level position and a resultant shift of the vacancy charge states from neutral (negative) to positive. Under the illumination of a 325-nm He-Cd laser, positrons were found to be trapped by vacancy-type defects, which was attributed to the trapping of excited electrons by these defects.

Effect of Zn for Ni substitution on the properties of Nickel-Zinc ferrites as studied by low-energy implanted positrons

Abstract Samples of Nickel-Zinc ferrite, Ni1−xZnxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0), were synthesized by a solid state reaction method. The morphology of four of the samples (x = 0.0, 0.4, 0.6 and 1.0) was observed with 3D X-ray imaging. The distortion caused by the increase of zinc in the ferrite structure and the respective movement of the trivalent (Fe) and divalent (Ni) ions from the tetrahedral- to the octahedral-sites cause some vacancies which act as positron traps. Doppler broadening of annihilation radiation experiments have been performed with a variable energy monoenergetic positron beam. The positron results as a function of the energy/depth implantation are presented in this work.

Implantation profiles and depth distribution of slow positron beam simulated by Geant4 toolkit

The positron implantation profiles and depth distribution in metal, semiconductor, and polymer surfaces are simulated using the Monte-Carlo-method-based Geant4 toolkit. As per the slow positron beam technique, the monoenergetic positron beams (in the range from 1 to 35 keV) with 1.5 mm radius is injected into semi-infinite samples in the present work. The implantation profile of 3.1 keV positrons in Fe is in good agreement with the Makhovian profile. The mean depth and depth resolution exhibit a general negative correlation with the material densities and incident position energy, respectively, while the fixed peak energy of backscattered positrons exhibits a net correlation with atomic number Z. Furthermore, the implantation profiles present better z-axis resolution with increasing incident angle, and this effect is more prominent for low-density materials. The results can provide theoretical support for new measurement methods based on the slow positron beam technique, such as segment measurements and micro-beam scanning measurements.

Status of the PADME experiment

Among the theoretical models addressing the dark matter problem, the category based on a secluded sector is attracting increasing interest. The PADME experiment, at the Laboratori Nazionali di Frascati of INFN, is designed to be sensitive to the production of a low mass gauge boson A′ of a new U(1) symmetry holding for dark sector particles and weakly coupled to the Standard Model photon. The DAΦNE Beam-Test Facility at LNF will provide a high intensity, mono-energetic positron beam impinging on a low Z target. The PADME detector will measure with high precision the momentum of the photon, produced along with the A′ boson in e+e− → A′+γ annihilation in the target, thus allowing to measure the A′ mass as the missing mass in the final state. This technique, particularly useful in case of invisible decays of the A′ boson, will be exploited for the first time in a fixed target experiment. Simulation studies predict a sensitivity on the interaction strength (ϵ2 parameter) down to 10−6, in the mass region 1 MeV < MA′ < 23.7 MeV. In this work the physics potential, the experimental strategy and the status of readiness of the experiment will be reviewed.