Positron annihilation on the surfaces of SiO2 films thermally grown on single crystal of Cz-Si

Two-detector coincidence system and mono-energetic slow positron beam has been applied to measure the Doppler broadening spectra for single crystals of SiO₂, SiO₂ films with different thickness thermally grown on single crystal of Cz-Si, and single crystal of Si without oxide film. Oxygen is recognized as a peak at about 11.85 × 10⁻³m₀c on the ratio curves. The S parameters decrease with the increase of positron implantation energy for the single crystal of SiO₂ and Si without oxide film. However, for the thermally grown SiO₂-Si sample, the S parameters in near surface of the sample increase with positron implantation energy. It is due to the formation of silicon oxide at the surface, which lead to lower S value. S and W parameters vary with positron implantation depth indicate that the SiO₂-Si system consist of a surface layer, a SiO₂ layer, a SiO₂-Si interface layer and a semi-infinite Si substrate.     

Microdefects and 3d electrons in ordered B2-FeAl alloys investigated by positron annihilation techniques

Microdefects and 3d electrons in B2-FeAl alloys with different chemical composition, single crystal of Fe and cold-rolled Fe has been studied by positron lifetime and coincidence Doppler broadening spectroscopy. The coincidence Doppler broadening spectrum of the single crystal of Fe shows the highest 3d electron signal in the spectra of all tested samples. The 3d electron signal in the spectrum of Fe₅₀Al₅₀ alloy is much lower than that of the cold-rolled Fe. This indicates that some of the 3d electrons of Fe atoms and 3p electrons of Al atoms in B2-FeAl alloy are localized to form strong covalent bonds, thus decreasing the probability of positron annihilation with 3d electrons of Fe atoms. With the increase of Al content in B2-FeAl alloys, the 3d electron signal in the spectrum of the alloy decreases, while the open volume of defect increases.     

The influence of f-type function in positron-He/positron-H2 scattering with the Schwinger multichannel method

In this paper we present results for positron-Helium and positron-H₂ scattering with the inclusion of the f-type Cartesian Gaussian functions in our computer codes of the Schwinger multichannel method (SMC). The effects of this modification can be noticed in the integral cross-section for both studied targets, with our new curves being closer to the most recent experimental measurements. The inclusion of the f-type function in the scattering wave function expansion also helped us to obtain a better set of results with the SMC method for the annihilation parameter. Data for differential cross-section (DCS) for helium is presented as well as our improvement in the DCS data in the forward scattering angles for the hydrogen molecule.     

Positron lifetime calculations of defects in fusion irradiated beryllium

Numerical quantum-mechanical positron lifetime calculations were performed for mono-vacancies, di-vacancies, tri-vacancies and small nano-voids containing helium and hydrogen in neutron irradiated beryllium. Helium and hydrogen atoms in the sample after the irradiation are considered as atoms forming interstitial O-type loops. Spherical clusters of vacancies are included in the calculations as a reference. It was found that the presence of He and H atoms significantly changes the positron lifetime in irradiated beryllium. A correlation between the positron lifetime and mutual position of vacancies in nano-voids and interstitial loops was established.     

Molecular model for annihilation rates in positron complexes

The molecular approach for positron interaction with atoms is developed further. Potential energy curves for positron motion are obtained. Two procedures accounting for the nonadiabatic effective positron mass are introduced for calculating annihilation rate constants. The first one takes the bound-state energy eigenvalue as an input parameter. The second is a self-contained and self-consistent procedure. The methods are tested with quite different states of the small complexes HPs, e⁺He (electronic triplet) and e⁺Be (electronic singlet and triplet). For states yielding the positronium cluster, the annihilation rates are quite stable, irrespective of the accuracy in binding energies. For the e⁺Be states, annihilation rates are larger and more consistent with qualitative predictions than previously reported ones.     

Variation of optical band gap with radiation dose in PbO-B2O3 glasses

The effect of dose variation of γ-irradiation on optical band gap of PbO-B₂O₃ glasses have been studied in the wavelength region from 200 to 1200 nm. Absorption of glasses in near ultraviolet/visible have been used to calculate the optical mobility gap and width of tail before and after irradiation. The decrease in transmission due to irradiation indicates the formation of colour centers and structural changes in glass matrix. The optical spectrum has been measured before irradiation and in 50 Gy-50 kGy absorbed dose range.     

Formation of metastable conductive nanowire in a thiospinel compound CuIr2S4 induced by ion irradiation

We observed an increase in the conductivity of a thiospinel compound, CuIr₂S₄, induced by H⁺ and He⁺ irradiation with energies of 1-2 MeV. It was indicated that the metastable conductive phase was produced by electronic excitation due to the ion beam and this phase was similar to the X-ray-induced phase. Conductivity as a function of ion fluence was analyzed by a simple model where the ion-induced change occurred in a cylindrical region around an ion trajectory. The cross-sectional area of the cylinder was obtained by analyzing the conductivity as a function of ion fluence for each ion, and it was found that an impinging ion produced a nanowire in the conductive phase. In addition, the yield of the Ir dimer displacement, which was related to the increase in conductivity, was considerably high. The ion irradiation effect reported in this paper is unique with regard to the high yield and low linear energy transfer (LET) in the formation of the conductive-phase nanowire. Both these unique aspects could be ascribed to the low band-gap energy and strong electron-lattice interaction of this compound.     

Characterization of defect accumulation in neutron-irradiated Mo by positron annihilation spectroscopy

Positron annihilation lifetime spectroscopy measurements were performed on neutron-irradiated low carbon arc cast Mo. Irradiation took place in the high flux isotope reactor, Oak Ridge National Laboratory, at a temperature of 80 ± 10 °C. Neutron fluences ranged from 2 × 10²¹ to 8 × 10²⁴ n/m² (E > 0.1 MeV), corresponding to displacement damage levels in the range from 7.2 × 10⁻⁵ to 2.8 × 10⁻¹ displacements per atom (dpa). A high density of submicroscopic cavities was observed in the neutron-irradiated Mo and their size distributions were estimated. Cavities were detected even at a very low-dose of ∼10⁻⁴ dpa. The average size of the cavities did not change significantly with dose, in contrast to neutron-irradiated bcc Fe where cavity sizes increased with increasing dose. It is suggested that the in-cascade vacancy clustering may be significant in neutron-irradiated Mo, as predicted by molecular dynamics simulations.     

First temperature stage evolution of irradiation-induced defects in tungsten studied by positron annihilation spectroscopy

The behaviour of vacancy like implantation-induced defects created in the track region of 800 keV ³He ions in polycrystalline tungsten was studied by Doppler broadening spectroscopy as a function of annealing temperature. A slow positron beam, coupled with a Doppler broadening spectrometer, was used to measure the low- and high-momentum annihilation fractions, S and W, respectively, as a function of positron energy in tungsten samples implanted at different fluences from 10¹⁴ to 5 × 10¹⁶ cm⁻². The behaviour of the S(E), W(E) and S(W) plots with the annealing temperature clearly indicates that the irradiation-induced vacancy like defects begin to evolve between 523 and 573 K, whatever the implantation fluence. This first temperature stage evolution corresponds to the migration of the monovacancies created during implantation to form larger vacancy like defects of which depth profile is different from the initial radiation-induced defects one.     

Study of the behavior of 3d-shell electrons in binary Nd-Fe alloys

The behavior of 3d electrons in binary Nd-Fe alloys with different Nd content from 7 to 13 at.% has been studied by using positron coincidence Doppler broadening techniques. It has been found that the 3d electron signal in Nd₂Fe₁₇ alloy is relatively high as compared with other alloys. In Fe-rich Nd₂Fe₁₇ alloys, as the content of Fe increasing, the phase boundaries between α-Fe and Nd₂Fe₁₇ phases will increase, which gives rise to the decrease in the probability of positron annihilation with 3d electrons. In Nd-rich Nd₂Fe₁₇ alloys, with the decrease of Fe content, the d-d interactions are weakened, and the probability of positron annihilation with 3d electrons will decrease. The coercivity JH_c and remanence Jr of the Nd-Fe alloy increase with the d-d interaction.     

Z2 structure and gas-solid effect in the stopping of slow ions

A recent claim by Paul of a systematic gas-solid difference in stopping cross sections for ions such as nitrogen and oxygen in the velocity range v ? v₀ is studied on the basis of existing experimental data. We find that all existing data support the commonly known Z₂ structure which, by and large, follows the valence structure of the target material. Existing experimental evidence is not found to support a specific gas-solid difference in the velocity range under consideration. The possibility of such an effect due to a gas-solid difference in charge state is rejected on theoretical grounds. Data for compound gases and solids are found to be well described by the Bragg additivity rule.We have also studied nitrogen/helium and oxygen/helium stopping ratios which determine the so-called effective-charge ratio. Taking into account the scatter of experimental data, we do not find clear evidence against Northcliffe’s assumption of a stopping ratio independent of Z₂ and common for gases and solids in the considered velocity range, although the absolute value appears too high.     

Interface strain study of thin Lu2O3/Si using HRBS

The interface of thin Lu₂O₃ on silicon has been studied using high-resolution RBS (HRBS) for samples annealed at different temperatures. Thin rare earth metal oxides are of interest as candidates for next generation transistor gate dielectrics, due to their high-k values allowing for equivalent oxide thickness (EOT) of less than 1 nm. Among them, Lu₂O₃ has been found to have the highest lattice energy and largest band gap, making it a good candidate for an alternative high-k gate dielectric. HRBS depth profiling results have shown the existence of a thin (∼2 nm) transitional silicate layer beneath the Lu₂O₃ films. The thicknesses of the Lu₂O₃ films were found to be ∼8 nm and the films were determined to be non-crystalline. Angular scans were performed across the [1 1 0] and [1 1 1] axis along planar channels, and clear shifts in the channeling minimum indicate the presence of Si lattice strain at the silicate/Si interface.     

Frenkel pair accumulation induced crystallization of amorphous MgAl2O4

The epitaxial and homogeneous irradiation induced re-crystallization of amorphous MgAl₂O₄ was studied by means of continuous Frenkel pair accumulation in the molecular dynamics framework. Present results point out that the re-crystallization induced by Frenkel pair accumulation appears in both cases to be thermally enhanced but non diffusive. It is governed by a local rearrangement of each point defect in the homogeneous case, while spontaneous Frenkel pair recombination process in the crystalline part or at the interface drives the re-crystallization in the epitaxial case.     

Synthesis of nanodimensional TiO2 thin films using energetic ion beam

Nanophases of TiO₂ are achieved by irradiating polycrystalline thin films of TiO₂ by 100 MeV Au ion beam at varying fluence. The surface morphology of pristine and irradiated films is studied by atomic force microscopy (AFM). Phase of the film before and after irradiation is identified by glancing angle X-ray diffraction (GAXRD). The blue shift observed in UV-vis absorption edge of the irradiated films indicates nanostructure formation. Electron spin resonance (ESR) studies are carried out to identify defects created by the irradiation. The nanocrystallisation induced by SHI irradiation in polycrystalline thin films is studied.     

Characteristics of the electron-emission defects introduced in Si-SiO2 structures by MeV electron irradiation

Defects induced by high-energy electrons in Si-SiO₂ structure have been studied by the optically stimulated electron emission (OSEE) method. Si-SiO₂ structures with oxide thickness of 100 nm are irradiated with 23 MeV electrons for different durations. It is shown that most of the defects created by electron irradiation at the interface and in the oxide bulk are vacancies like E′-centers. Most of the photoemission activity changes are observed during low doses electron irradiation. Some uncharged defects like diamagnetic oxygen-deficient centers are also observed, together with E′-centers.     

Radiation damage evolution in ceramics

A review is presented of recent results on radiation damage production, defect accumulation and dynamic annealing in a number of ceramics, such as silicon carbide, zircon and zirconia. Under energetic particle irradiation, ceramics can undergo amorphization by the accumulation of point defects and defect clusters (silicon carbide) or direct impact amorphization (zircon). Ceramics that resist radiation-induced amorphization have mechanisms to dissipate the primary knock-on atom energy, such as replacement collision sequences that leave the lattice undisturbed and low-energy cation site exchange. The presence of engineered mobile defects, such as structural vacancies in stabilized zirconia, can dynamically anneal radiation damage. Thus, defect engineering is a promising strategy to design radiation tolerance for applications such as nuclear waste disposal.     

Photoluminescence and XEL in Y2O3:Eu phosphor

Eu-activated Y₂O₃ phosphors were prepared by combustion synthesis and also by precipitation techniques. Photoluminescence and X-ray excited luminescence of prepared Y₂O₃:Eu phosphor, under two different techniques were compared and reported in this paper. Y₂O₃:Eu³⁺ phosphor were prepared by precipitation technique followed by annealing at 900 °C. It gives cubic nature of the particle that may be more favourable for high lumen output. X-ray excited luminescence of Y₂O₃:Eu³⁺ phosphors also reported in this paper.     

Existence states of deuterium irradiated into LiAlO2

The existence states of deuterium in LiAlO₂ were analyzed by in situ IR absorption spectroscopy during irradiation with 3 keV at room temperature. Multiple IR absorption peaks that were related to O-D stretching vibrations were observed, mainly at 2650 cm⁻¹ (O-D_α), 2600 cm⁻¹ (O-D_β), and 2500 cm⁻¹ (O-D_γ). The O-D_α was assigned to the surface O-D. The O-D_β and O-D_γ were interpreted as two distinct O-D states for three candidates: O-D of substitutional D⁺ for Li⁺; O-D of substitutional D⁺ for Al³⁺; and O-D of interstitial D⁺. O-D_β was the dominant O-D state for deuterium irradiated into LiAlO₂, and had higher stability than O-D_γ. Heating after ion irradiation led to the desorption of D₂ and an increase in the intensity of O-D_β, which implies that some of the deuterium irradiated into LiAlO₂ exists in non-O-D states, such as D⁻ captured by F centers.     

Irradiation effect on dielectric properties and electrical conductivity of Au/SiO2/n-Si (MOS) structures

The Au/SiO₂/n-Si (MOS) structures were exposed to beta-ray irradiation to a total dose of 30 kGy at room temperature. Irradiation effect on dielectric properties of MOS structures were investigated using capacitance−voltage (C−V) and conductance−voltage (G/ω−V) characteristics. The C−V and G/ω−V measurements carried out in the frequency range from 1 kHz to 10 MHz and at various radiation doses, while the dc voltage was swept from positive bias to negative bias for MOS structures. The dielectric constant (ε′), dielectric loss (ε″), loss factor (tan δ) and ac electrical conductivity (σ_ac) were calculated from the C−V and G/ω−V measurements and plotted as a function of frequency at various radiation doses. A decrease in the ε′ and ε″ were observed when the irradiation dose increased. The decrease in the ε′ and ε″ of irradiated MOS structures in magnitude is explained on the basis of Maxwell−Wagner interfacial polarization. Also, the σ_ac is found to decrease with increasing radiation dose. In addition, the values of the tan δ decrease with increasing radiation dose and give a peak. From the experimental results, it is confirmed that the peak of loss tangent is due to the interaction between majority carriers and interface states which induced by radiation.     

Evaluation of melting point of UO2 by molecular dynamics simulation

The melting point of UO₂ has been evaluated by molecular dynamics simulation (MD) in terms of interatomic potential, pressure and Schottky defect concentration. The Born-Mayer-Huggins potentials with or without a Morse potential were explored in the present study. Two-phase simulation whose supercell at the initial state consisted of solid and liquid phases gave the melting point comparable to the experimental data using the potential proposed by Yakub. The heat of fusion was determined by the difference in enthalpy at the melting point. In addition, MD calculations showed that the melting point increased with pressure applied to the system. Thus, the Clausius-Clapeyron equation was verified. Furthermore, MD calculations clarified that an addition of Schottky defects, which generated the local disorder in the UO₂ crystal, lowered the melting point.