Search

Native defects and ion-implantation induced defects in GaN were studied by means of positron annihilation. Measurements of Doppler broadening spectra of the annihilation radiation for GaN layers grown on Si substrates showed that optically active vacancy-type defects were formed in the layers. Charge transition of the defects due to electron capture occurred when the layers were irradiated by photons with energy above 2.7 eV. It was found that Ti deposition and subsequent annealing introduced vacancy clusters. We also characterized vacancy-type defects in Mg-implanted GaN. The major defect species of vacancies introduced by Mg-implantation was a complex between Ga-vacancy (VGa) and nitrogen vacancies (VNs). After annealing above 1000C, these defects started to agglomerate, and the major defect species became (VGa)2 coupled with VNs. Through this work, we have demonstrated that positron annihilation spectroscopy is a powerful tool for characterizing vacancy-type defects in GaN for power devices applications.

The properties of materials involving polymer–solid interfaces are determined not only by the properties of the constituents but also by the interfacial region of the polymer near the solid, the so-called interphase. Here we report on positron annihilation lifetime spectroscopy (PALS) measurements of the free volume near the interface with a focused positron beam. The measurements were performed at an interface between a Teflon AF 1600 film and a 30 nm thick SiN membrane from the backside of a Si substrate which was locally etched away to expose the ultrathin SiN membrane. We were able to focus the positron beam onto the exposed region and to probe the SiN–Teflon AF 1600 interfacial region with low implantation energies, resulting in very narrow implantation profiles and excellent depth resolution. We found a pronounced decrease in orthopositronium lifetime close the interface. On the basis of modeling, we could estimate the interphase width and the average density increase in the interfacial region as ∼1...

Slow positron beam and optical absorption measurements are carried out to study structural defects and positronium formation in 40 keV B(+)-implanted polymethylmethacrylate (B:PMMA) with ion doses from 6.25 × 10(14) to 5.0 × 10(16) ions/cm(2). Detailed depth-selective information on defects in implanted samples was obtained by measuring of Doppler broadening of positron annihilation γ rays as a function of incident positron energy and these experimental results were compared with SRIM (stopping and range of ions in matter) simulation results. Two general processes, appearance of free radicals at lower ion doses (10(16) ions/cm(2)) and carbonization at higher ion doses (10(16) ions/cm(2)), are considered from the Doppler S-E and W-E dependences in the framework of the concept of defects formation during radiation damage of polymer structure. Probabilities of ortho-positronium (o-Ps) formation are analyzed using S-W plot and slow positron annihilation lifetime measurements. Dose dependence of o-Ps lifetime τ3 and intensity I3 at the incident positron energy of 2.15 keV correlates well with the dose dependence of S-parameter and seems to account for the existence of the expected two processes, i.e., scission of polymer chains and appearance of free radicals preceding the aggregation of the clusters resulting in the formation of network of conjugated bonds at lower ion doses and carbonization at higher ion doses. The increase of optical absorption observed with increasing ion implantation dose also suggests a formation of carbonaceous phase in the ion-irradiated PMMA.

Commercial composite membranes for water purification were studied by means of the variable-energy positron annihilation γ-ray technique. The positron incident energy E dependence of R parameter (3γ/2γ ratio) analyzed from positron annihilation γ-ray spectra revealed the layer structure consisting of a dense top layer and a porous substrate for the membranes. Thickness of the dense layer for each membrane was evaluated from the R(E) based on a positron implantation stopping profile.

Positron annihilation gamma energy distribution, lifetime spectroscopy and time-of-flight method were used to study surfactant-templated mesoporous silica films deposited on glass. The lifetime depth profiling was correlated to Doppler broadening and 3γ annihilation fraction measurements to determine the annihilation characteristics inside the films. A set of consistent fingerprints for positronium annihilation, o-Ps reemission into vacuum, and pore size was directly determined. The lifetime measurements were performed in reflection mode with a specially designed lifetime spectrometer mounted on a slow positron beam system. The intensity of the 142 ns vacuum lifetime component was recorded as a function of the energy of the positron beam. In a film with high porosity a reemission efficiency of as high as 40 % was found at low positron energy. Positron lifetime in samples capped by a thin silica layer was used to determine the pore size. The energy of the reemitted o-Ps fraction was measured by a time-of-flight detector, mounted on the same system, allowing determination of both o-Ps re-emission efficiency and energy in the same sample. We demonstrate the potential of the simultaneous use of different positron annihilation techniques in the study of thin porous films.

The impact of TiN deposition on thin high-k (HfO2 and HfSiON) films formed on Si substrates was studied using monoenergetic positron beams. For the predeposition sample, the positrons implanted into Si diffuse toward the high-k/Si interface under the influence of the electric field, suggesting the presence of negative charges in the high-k films. After TiN was deposited on HfO2, positive charges were introduced at the TiN/HfO2 interface, which were associated with the incorporation of nitride into HfO2, resulting in the formation of positively charged oxygen vacancies (VOs). From the isochronal annealing experiments for TiN/HfSiON/Si, it was found that positively charged defects (such as VO) were introduced into HfSiON after annealing at 700–900 °C. These defects were introduced by the interaction between TiN and HfSiON, resulting in the formation of polycrystalline TiO2 at the interface. The positively charged defects were annealed out at 1100 °C, but the dielectric properties of HfSiON degraded.

Thin Hf0.3Al0.7Ox films fabricated by an atomic-layer-deposition technique were characterized using monoenergetic positron beams. From the measurements of the Doppler broadening spectra of annihilation radiation and the lifetime spectra of positrons for 3- and 7-nm-thick HfAlOx films deposited on SiON(0.9 nm)/Si, it was found that positrons in the HfAlOx films annihilate from the trapped state by open spaces that exist intrinsically in their amorphous structure. The line-shape parameter S and the positron lifetime corresponding to the HfAlOx films decreased with increasing oxygen content (0.004–1%) in the annealing atmosphere. This fact was attributed to the shrinkage of the open spaces due to the change in the matrix structure of amorphous HfAlOx. The increase in oxygen content was also found to decrease the transient current of n+-polycrystalline-Si/HfAlOx/SiON/Si. The decrease in the mean size of the open spaces and the resultant hardening of the matrix structure are considered to suppress the dielectric polarization of HfAlOx.

Silicon (1 0 0) wafers have been implanted with 1×10 15 He + or Ar + ions/cm 2 and the vacancy (V)–He or –Ar complexes induced have been investigated with positron lifetime and coincidence Doppler broadening measurements using a positron beam. He + ion implantation into Si gives rise to formation of V 2 He complexes, which are transformed into V x He y ( x – y =1) complexes by annealing at 300°C. The amorphous phase induced by Ar irradiation of Si is transformed into a crystalline phase around 700°C. The crystallization accompanies formation of V–Ar complexes with six vacant sites, and these complexes are found to be stable above 800°C. These results have proved that positrons are very useful probes to study the chemical state of vacancy-inert gas atoms complexes.

The positron annihilation lifetimes by slow positron beam are measured in the thin Fe films, which are grown under the hydrogen pressures 5.0×10 −10 and 4.5×10 −7 Torr, in positron-incident energies 2 and 3 keV. We have suggested experimentally the presence of hydrogen-induced vacancies in thin Fe films.

Oxygen-related defects in Separation-by-IMplanted OXygen (SIMOX) wafers and their annealing behavior are studied by variable-energy positron annihilation spectroscopy. After implantation of 180 keV oxygen into Si substrates at a dose of 4×1017 cm−2, two kinds of vacancy-type defects are observed by positrons, i.e. vacancy clusters and vacancy-oxygen complexes. Their sizes are larger than and close to V6, respectively. According to the annealing behavior of the vacancy cluster, there are three annealing stages. A detailed discussion of the annealing behavior of the two defects is presented.

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 (

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.

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 (

MeV implantation of gold ions into MgO(100) followed by annealing is a method to form gold nanoparticles for obtaining modified optical properties. We show from variable-energy positron spectroscopy that clusters of 2 Mg and 2 O vacancies (v{sub 4}) are attached to the gold nanoparticle surfaces within the projected range (R{sub p}) . We also find that v{sub 4} vacancy clusters are created at depths less than R{sub p} , and extend into the region greater than R{sub p} due to damage induced by knock-on collisions. (c) 1999 The American Physical Society.

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 have developed an apparatus for time-of-flight positron annihilation induced Auger electron spectroscopy (TOF-PAES) at an intense, slow positron beamline of the Electrotechnical Laboratory. We carried out TOF-PAES analysis for the adsorption of oxygen on Si(100). The results demonstrate that PAES can yield useful and unique information on adsorbates on top of surfaces.

Positron has various unique properties, e.g., positive charge, annihilation with electrons, emission of annihilation gamma ray, positronium formation, etc. These unique properties provide us with new information on surface and subsurface regions. In recent years, generation techniques of intense slow-positron beams have been developed and those beams have been applied to various surface studies. In this article, we briefly introduce slow-positron generation techniques and measurement techniques for surface studies.

Gate oxide layers grown by dry and wet oxidation on 4H-SiC C(0001)- and Si(0001)-faces have been measured by positron annihilation spectroscopy. The incident positron energy, which corresponds to the peak of Doppler-broadening W-parameter curves, was chosen to characterize the SiC/oxide interfaces. Positron trapping sites with lifetimes in the range of 0.16–0.32 ns were detected, and the lifetimes for the C(0001) face were longer than those for the Si(0001) face. Age–momentum correlation measurements revealed that the origin of these trapping sites is neither the SiC bulk nor the SiC vacancies, suggesting that defective oxide layers exist at the interface.

Positron lifetimes in TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane) and κ-(BEDT-TTF)2Cu(SCN)2 (BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene) single crystals have been measured utilizing a pulsed variable-energy positron beam. The bulk positron lifetimes are 338 and 357 ps, respectively. The incident positron energy dependence of the lifetime is rather weak for both materials. Theoretical simulations have been also performed with several descriptions of the electronic wave functions and the electron-positron correlation. Results are compared with each other and the experiments.

An apparatus for high-resolution positron-annihilation induced Auger electron spectroscopy (PAES) has been constructed at the intense slow positron beam line of the Electrotechnical Laboratory. The system uses a short pulsed positron beam (∼ ns width) and the time-of-flight (TOF) method for the energy analysis of the Auger electrons. In order to improve the energy resolution, an electrically isolated flight-tube is installed between a sample and a detector, and the electrons are retarded by biasing the flight tube. This method makes it possible to measure high-resolution PAES spectra with high efficiency.

An apparatus for positron-annihilation-induced Auger electron spectroscopy has been constructed at the intense slow positron beam-line of the Electrotechnical Laboratory. The energy of Auger electrons is measured with a time-of-flight technique. The combination of inhomogeneous magnetic field and pulsed positron beam enables us to measure positron-annihilation-induced Auger electron spectra with high resolution (ΔE/E∼ 0.1), with low background, and with high count rate (∼ 10 min/spectrum).

Positron lifetime measurements were carried out on molecular crystals of C60, TTF-TCNQ, and (BEDT-TTF)2Cu(NCS)2 utilizing a pulsed variable-energy positron beam. Each lifetime spectrum is well described by a single component. Their values are 390, 338, and 357 ps, respectively. In addition, positron lifetimes were calculated theoretically. The electronic density was obtained by the superposed-neutral-atom model or quantum-chemical molecular-orbital calculations with various basis sets. The lifetime enhancement due to the positron–electron correlation was taken into account through an enhancement factor as a function of the high-frequency dielectric constant in the local density approximation or a gradient correction in the generalized gradient approximation. The results are compared with the measured lifetimes.

Undoped ZnO single crystals were implanted with multiple-energy N + ions ranging from 50 to 380 keV with doses from 10 12 to 10 14 cm −2 .P os itron annihilation measurements show that vacancy defects are introduced in the implanted layers. The concentration of the vacancy defects increases with increasing ion dose. The annealing behaviour of the defects can be divided into four stages, which correspond to the formation and recovery of large vacancy clusters and the formation and disappearance of vacancy–impurity complexes, respectively. All the implantation-induced defects are removed by annealing at 1200 ◦ C. Cathodoluminescence measurements show that the ion-implantationinduced defects act as nonradiative recombination centres to suppress the ultraviolet (UV) emission. After annealing, these defects disappear gradually and the UV emission reappears, which coincides with positron annihilation measurements. Hall measurements reveal that after N + implantation, the ZnO layer still shows n-type conductivity.

Relaxation dynamics of ion damage in fused quartz is investigated by our newly developed pump–probe technique combining energetic ions (pump) with slow positrons (probe). This method enables determination of time-resolved positron lifetime. We study the time-dependent relaxation of ion damage, by analyzing the intensity variation in the ortho-positronium lifetime component associated with irradiation damage. For irradiation with 160 keV He ions in the temperature range of 300–573 K, the positron annihilation lifetime spectra are obtained as a function of time after ion irradiation. We observe that the relaxation time of ion damage is strongly influenced by specimen temperatures; the relaxation time constant is approximately 400 ms at room temperature (300 K) and becomes smaller with an increasing temperature. Analysis for the effect of temperature on damage accumulation reveals that the activation energy for thermal annealing of the observed damage is approximately 0.1 eV.

Diffusion of ortho-positronium ($o$-Ps) in mesoporous silica films with different pore morphologies and its emission from the surfaces were studied using the Ps time-of-flight method and positron annihilation $\ensuremath{\gamma}$-ray energy spectroscopy. Probabilities of $o$-Ps formation are almost the same in the studied films, however, the emission intensity of $o$-Ps from the film with much larger cagelike pores is significantly less than that from the film with much smaller tubelike pores, where Ps diffusion between cages is feasible by tunneling. A simple strategy is suggested to use mesoporous silica film for the generation of dense enough Ps for many-positronium experiments.

A positron pulsing system for an intense positron beam generated by an electron linac is reported. The pulsing system generates an intense pulsed positron beam of variable energy and variable pulse period. The pulsed positron beam is used as a non-destructive probe for various material research. In this paper, we also discuss applications of the pulsed positron beam: positron lifetime spectroscopy, age-momentum correlation spectroscopy, positronium time-of-flight measurement, and positron annihilation-induced Auger electron spectroscopy with a time-of-flight technique.

A technique for investigating atomic-scale defects and/or nanometer (sub-nanometer)-order pores near the surface of samples mounted in air (without vacuum) by positron annihilation spectroscopy (PAS) has been developed. The method relies on the extraction of slow positron beams from the vacuum chamber to air through a thin SiN membrane vacuum window. Using a positron beam with an injection energy of 2.6 keV and a vacuum window with a thickness of 30 nm, samples mounted in air can be investigated by PAS to a depth of ~100 nm.

Monte Carlo simulations of slow positron production via energetic electron interaction with a solid target have been performed. The aim of the simulations was to determine the expected slow positron beam intensity from a low energy, high current electron accelerator. By simulating (a) the fast positron production from a tantalum electron-positron converter and (b) the positron depth deposition profile in a tungsten moderator, the slow positron production probability per incident electron was estimated. Normalizing the calculated result to the measured slow positron yield at the present AIST LINAC the expected slow positron yield as a function of energy was determined. For an electron beam energy of 5 MeV (10 MeV) and current 240 $\mu$A (30 $\mu$A) production of a slow positron beam of intensity 5 $\times$ 10$^{6}$ s$^{-1}$ is predicted. The simulation also calculates the average energy deposited in the converter per electron, allowing an estimate of the beam heating at a given electron energy and current. For low energy, high-current operation the maximum obtainable positron beam intensity will be limited by this beam heating., Comment: 11 pages, 15 figures, submitted to Review of Scientific Instruments

Ion-beam-induced crystallization in silicon preamorphized by Ge-ion implantation was studied by combined means of Rutherford backscattering and channeling, and positron annihilation. The epitaxial regrowth of amorphous surface layers in a 〈100〉 Si substrate has been studied with irradiation of 400-keV ${\mathrm{Ar}}^{+}$ ions at the temperature of 400 \ifmmode^\circ\else\textdegree\fi{}C. The ion-beam-induced epitaxy was found to result in a drastic increase in the positron lifetime to a maximum value of 400 psec in the recrystallized silicon layer. It is demonstrated that vacancy migration is promoted during the epitaxial recrystallization to form defect complexes like trivacancies and/or quadrivacancies.

Defects in the separation by implanted oxygen (SIMOX) wafers were studied by monoenergetic positron beams. For the as-implanted specimen, vacancies introduced by ion implantation were found to form vacancy-oxygen complexes. After high temperature annealing, a drastic increase in the formation probability of positronium was observed. This suggests an introduction of open-spaces by the formation of an amorphous SiO2. The present investigation shows that positrons provide a nondestructive probe for the detection of defects in SIMOX wafers and microstructures of SiO2.

RBS channeling analysis and positron annihilation measurements were used to study ion beam induced crystallization in silicon. The epitaxial regrowth of amorphous surface layers in 〈100〉 oriented Si substrate has been performed under irradiation with 400 keV Ar + ions at the temperature of 400°C. A slow positron pulsing system was used to measure lifetime spectra in the recrystallised near-surface region, and the lifetime was found to increase by 40–70 ps, compared to the as-amorphized state, whereas RBS channeling shows a crystalline recovery from the amorphous structure. The results indicate that vacancy migration and clustering are promoted during the epitaxial recrystallization.

The authors detect vacancy clusters at Au nanoparticle surfaces using a combination of positron lifetime spectroscopy, 1- detector, and 2-detector measurements of Doppler broadening of annihilation radiation. Gold nanoparticles are formed by MeV implantation of gold ions into MgO (100) followed by annealing. Clusters of two Mg and two O vacancies (v{sub 4}) are attached to the gold nanoparticle surfaces within the projected range (R{sub p}).

An enhanced positronium (Ps) formation in low-temperature polymers has been widely observed. The additional positronium formation is due to shallow trapped electrons in them. Positron annihilation lifetime spectroscopy was applied to investigate the Ps formation in a series of polymers, such as low-density polyethylene, ethylene-methyl methacrylate copolymers with various methyl methacrylate contents, and pure poly(methyl methacrylate) at low temperature. An analysis of the experimental data based on simple kinetic equations enables one to understand the Ps formation mechanism in polymers during low-temperature positron annihilation experiments as functions of the temperature and elapsed time. Good fittings of the experimental data were obtained. The fitting parameters seemed to show clear physical meanings.

ZnO crystals were implanted with 20–80 keV hydrogen ions up to a total dose of 4.4×1015 cm−2. Positron lifetime and Doppler broadening of annihilation radiation measurements show introduction of zinc vacancy-related defects after implantation. These vacancies are found to be filled with hydrogen atoms. After isochronal annealing at 200–500 °C, the vacancies agglomerate into hydrogen bubbles. Further annealing at 600–700 °C causes release of hydrogen out of the bubbles, leaving a large amount of microvoids. These microvoids are annealed out at high temperature of 1000 °C. Raman spectroscopy for the implanted sample shows the enhancement of vibration modes at about 575 cm−1, which indicates introduction of oxygen vacancies. These oxygen vacancies disappear at temperatures of 600–700 °C, which is supposed to contribute to the hydrogen bubble formation. Cathodoluminescence measurements reveal that hydrogen ions also passivate deep level emission centers before their release from the sample, leading to the improvement of the UV emission.

Polycrystalline tungsten foil annealed at successively higher temperature up to $\ensuremath{\sim}2300\mathrm{K}$ has been investigated for slow positron moderation yield. LINAC-based intense positron beam lifetime studies have revealed that reemitted slow positron and positronium fractions gradually improve upon high-temperature annealing. So as to correlate the presence of defects and chemical impurities with the improvement in slow positron yield, positron annihilation, Auger electron spectroscopy, and secondary ion mass spectroscopy (SIMS), studies have been carried out on virgin and high-temperature $(\ensuremath{\sim}2300\mathrm{K})$ annealed W foils. The positron beam S parameter shows a large value throughout the sample depth corresponding to the virgin sample, while it is lower for annealed samples. This indicates that, as compared to virgin sample, the annealed W sample has a lower concentration of vacancylike defects. Auger studies revealed that in virgin state the surface is fully contaminated with carbon, while the annealed foil shows prominent W peaks. Corroborative SIMS concentration profiles have indicated that the carbon content is much lower in an annealed sample over a large depth region. From these studies, it is concluded that improvement in slow positron yield upon high-temperature annealing is obtained due to the removal of the surface tungsten-carbide layer as well as carbon-vacancy complexes present throughout the sample depth.

Undoped ZnO single crystals were implanted with aluminum ions up to a dose of 1015Al+/cm2. Vacancy defects in the implanted layers were detected using positron lifetime and Doppler broadening measurements with slow positron beams. It shows that vacancy clusters, which are close to the size of V8, are generated by implantation. Postimplantation annealing shows that the Doppler broadening S parameter increases in the temperature range from 200°C to 600°C suggesting further agglomeration of vacancy clusters to voids. Detailed analyses of Doppler broadening spectra show formation of positronium after 600°C annealing of the implanted samples with doses higher than 1014Al+/cm2. Positron lifetime measurements further suggest that the void diameter is about 0.8 nm. The voids disappear and the vacancy concentration reaches the detection limit after annealing at 600–900°C. Hall measurement shows that the implanted Al+ ions are fully activated with improved carrier mobility after final annealing. Cathodoluminescence measurements show that the ultraviolet luminescence is much stronger than the unimplanted state. These findings also suggest that the electrical and optical properties of ZnO become much better by Al+ implantation and subsequent annealing.