Co γ-ray irradiation effects on dielectric characteristics of tin oxide films of different thicknesses on n-type Si(1 1 1) substrates

We have tried to determine the effects of ⁶⁰Co gamma irradiation on properties of Au/SnO₂/n-Si (MOS) structures such as dielectric constant (ε′), dielectric loss (ε″), tangent loss (tan δ) and ac conductivity (σ_ac). Three samples were fabricated with different deposition time. The samples were irradiated using a ⁶⁰Co γ-ray source irradiation with the total dose range of 0–500 kGy at room temperature. Capacitance and conductance (C–G–V) measurements were performed at a frequency of 500 kHz in the dark and at room temperature before and after irradiation. The experimental data were analyzed using complex permittivity and electric modulus. The values of ε′, ε″, tan δ and σ_ac showed a strong dependence on the applied voltage and irradiation dose. The dielectric properties of MOS structures have been found to be strongly influenced by the presence of dominant radiation-induced defects. Experimental results show that the interfacial polarization contributes to the improvement of dielectric properties of Au/SnO₂/n-Si (MOS) Schottky diodes.     

Ion beam enhanced etching of LiNbO3

Single crystals of z- and x-cut LiNbO₃ were irradiated at room temperature and 15 K using He⁺- and Ar⁺-ions with energies of 40 and 350 keV and ion fluences between 5 × 10¹² and 5 × 10¹⁶ cm⁻². The damage formation investigated with Rutherford backscattering spectrometry (RBS) channeling analysis depends on the irradiation temperature as well as the ion species. For instance, He⁺-irradiation of z-cut material at 300 K provokes complete amorphization at 2.0 dpa (displacements per target atom). In contrast, 0.4 dpa is sufficient to amorphize the LiNbO₃ in the case of Ar⁺-irradiation. Irradiation at 15 K reduces the number of displacements per atom necessary for amorphization. To study the etching behavior, 400 nm thick amorphous layers were generated via multiple irradiation with He⁺- and Ar⁺-ions of different energies and fluences. Etching was performed in a 3.6% hydrofluoric (HF) solution at 40 °C. Although the etching rate of the perfect crystal is negligible, that of the amorphized regions amounts to 80 nm min⁻¹. The influence of the ion species, the fluence, the irradiation temperature and subsequent thermal treatment on damage and etching of LiNbO₃ are discussed.     

XPS and optical studies of Xe-implanted and annealed YSZ single crystals

Xe⁺ ion implantation with 200 keV was completed at room temperature up to a fluence of 1 × 10¹⁷ ion/cm² in yttria-stabilized zirconia (YSZ) single crystals. Optical absorption and X-ray photoelectron spectroscopy (XPS) were used to characterize the changes of optical properties and charge state in the as-implanted and annealed crystals. A broad absorption band centered at 522 or 497 nm was observed in the optical absorption spectra of samples implanted with fluences of 1 × 10¹⁶ ion/cm² and 1 × 10¹⁷ ion/cm², respectively. These two absorption bands both disappeared due to recombination of color centers after annealing at 250 °C. XPS measurements showed two Gaussian components of O_1s spectrum assigned to Zr–O and Y–O, respectively, in YSZ single crystals. After ion implantation, these two peaks merged into a single peak with the increasing etching depth. However, this single peak split into two Gaussian components again after annealing at 250 °C. The concentration of Xe decreased drastically after annealing at 900 °C. And the XPS measurement barely detected the Xe. There was no change in the photoluminescence of YSZ single crystals with a fluence of 1 × 10¹⁷ ion/cm² after annealing up to 900 °C.     

Size dependent hcp-to-fcc transition temperature in Co nanoclusters obtained by ion implantation in silica

In this work we present in situ investigations on the increase of the hcp-to-fcc transition temperature for Co with respect to the bulk value (420 °C) when nanoclusters are considered. Starting from Co:SiO₂ composites obtained by ion implantation with average Co cluster size of about 5 nm, a transition temperature between 800 °C and 900 °C is found upon thermal annealing in vacuum by in situ transmission electron microscopy. Preliminary results on electron irradiation to promote the transition at lower temperatures are presented.     

Medium temperature carbon dioxide gas turbine reactor

A carbon dioxide (CO₂) gas turbine reactor with a partial pre-cooling cycle attains comparable cycle efficiencies of 45.8% at medium temperature of 650 °C and pressure of 7 MPa with a typical helium (He) gas turbine reactor of GT-MHR (47.7%) at high temperature of 850 °C. This higher efficiency is ascribed to: reduced compression work around the critical point of CO₂; and consideration of variation in CO₂ specific heat at constant pressure, C_p, with pressure and temperature into cycle configuration. Lowering temperature to 650 °C provides flexibility in choosing materials and eases maintenance through the lower diffusion leak rate of fission products from coated particle fuel by about two orders of magnitude. At medium temperature of 650 °C, less expensive corrosion resistant materials such as type 316 stainless steel are applicable and their performance in CO₂ have been proven during extensive operation in AGRs. In the previous study, the CO₂ cycle gas turbomachinery weight was estimated to be about one-fifth compared with He cycles. The proposed medium temperature CO₂ gas turbine reactor is expected to be an alternative solution to current high-temperature He gas turbine reactors.     

Enthalpy and heat capacity of LiAlO2 between 298 and 1700 K by drop calorimetry

The enthalpy of γ-LiAlO₂ was measured between 403 and 1673 K by isothermal drop calorimetry. The smoothed enthalpy curve between 298 and 1700 K results in H⁰(T) − H⁰(298 K)=−37 396 + 93.143 · T + 0.00557 · T² + 2 725 221 · T⁻¹ J/mol. The standard deviation is 2.2%. The heat capacity was derived by differentiation of the enthalpy curve. The value extrapolated to 298 K is C_p,298=(65.8 ± 2.0) J/K mol.     

Electrical and optical properties of Co ion implanted a-Si1 − xCx:H alloys

Low resistivity a-Si_1 − xC_x:H alloy films have been formed by high dose Co⁺ ion implantation. The influence of the carbon content of the films on the resistivity has been studied and the lowest values, of the order of 10 Ω/Sq, have been observed for the carbon free films. Even lower resistivities, a further reduction of up to 50%, have resulted from annealing at temperatures up to 500°C. Changes in the optical and structural properties of the implanted a-Si_1 − xC_x:H films have been studied by means of IR and Raman spectroscopy. Results show that the implantation produces considerable structural and chemical modifications. The formation of, and the transition to, a possible CoSi₂ phase has been observed by examining the IR and Raman spectra as a function of implant dose.     

Structural changes in anatase TiO2 thin films irradiated with high-energy heavy ions

In order to investigate possible structural changes due to high-density electronic excitation, anatase TiO₂ thin film specimens were irradiated with 230 MeV ¹³⁶Xe¹⁵⁺ ions and 200 MeV ¹⁹⁷Au¹³⁺ ions. X-ray diffraction (XRD) patterns were measured before and after irradiation. The intensity of the XRD peak assigned to the (0 0 4) planes of anatase TiO₂ decreases in an exponential manner as a function of ion-fluence. This result can be explained by the formation of the cylindrical damaged regions (i.e. ion tracks) with diameters of 9.6 and 16.3 nm for 230 MeV Xe and for 200 MeV Au ion irradiations, respectively. The difference in the track diameter between Xe ion irradiation and Au ion irradiation can be attributed to the difference in the electronic stopping power (and to the ion-velocity effect, if any). For 200 MeV Au ion irradiation, splitting of the (0 0 4) peak is observed. The original (0 0 4) TiO₂ peak remains in the same position, but the new peak shifts to higher angles as fluence increases.     

Optical behaviour of Er doped rutile by ion implantation

Rutile single crystals were implanted at room temperature with fluences of 5 × 10¹⁵ Er⁺/cm² ions with 150 keV energy. Rutherford backscattering/channeling along the 0 0 1 axis reveals complete amorphization of the implanted region. Photoluminescence reveals the presence of an optical centre close to the intra-ionic emission of Er³⁺ in the as-implanted samples. After annealing at 800 °C in air no changes were observed in the aligned RBS spectrum. On the contrary, annealing in reducing atmosphere (vacuum) induces the epitaxy of the damage layer. These results are unexpected, since for implantations of other ions under the same conditions, epitaxial recrystallization of the damage region occurs at this temperature. On the other hand, photoluminescence studies show the presence of new Er-related optical centres with high thermal stability in the samples annealed under oxidizing conditions. Annealing at 1000 °C in vacuum leads to the complete recrystallization of the damaged region. At this temperature a large fraction of Er segregates to the surface.     

Structural stability of Cu nanocrystals in SiO2 exposed to high-energy ion irradiation

Cu nanocrystals (NCs) were synthesized in SiO₂ by ion implantation and thermal annealing. Annealing at two different temperatures of 950 °C and 650 °C yielded two different nanocrystal size distributions with an average diameter of 8.1 and 2.5 nm, respectively. Subsequently the NCs were exposed to 5.0 MeV Sn³⁺ ion irradiation simultaneously with a thin Cu film as a bulk reference. The short-range atomic structure and average NC diameter was measured by means of extended X-ray absorption fine structure (EXAFS) spectroscopy and small angle X-ray scattering (SAXS), respectively. Consistent with the high regeneration rate of bulk elemental metals, no irradiation induced defects were observed for the reference, whereas the small NCs (2.5 nm) were dissolved as Cu monomers in the matrix. The latter was attributed to irradiation-induced mixing of Cu, Si and O based on dynamic binary collision simulations. For the large NCs (8.1 nm) only minor structural changes were observed upon irradiation, consistent with a more bulk-like pre-irradiation structure.     

Irradiation effects with 100 MeV Xe ions on optical properties of Al-doped ZnO films

Al-doped ZnO (AZO) films are known as n-type transparent semiconductors. We have investigated the effects of 100 MeV Xe ion irradiation on the optical and structural properties of AZO films, which were prepared on SiO₂ glass at 400 °C by using a RF-magnetron sputtering deposition method. We discuss relationships between these property modifications and the recent observations of the conductivity increase by ion irradiation. It is suggested that the band-gap modification has more close relation with the conductivity increase than the structural modification.     

Carbon ion-implanted optical waveguides in Nd:YLiF4 crystal: Refractive index profiles and thermal stability

We report on the optical planar waveguides in Nd:YLiF₄ laser crystals fabricated by 6.0 MeV C³⁺ ion implantation at doses of 1 × 10¹⁵ or 2.5 × 10¹⁵ ions/cm², respectively. The refractive index profiles, which are reconstructed according to the measured dark mode spectroscopy, show that the ordinary index had a positive change in the surface region, forming non-leaky waveguide structures. The extraordinary index is with a typical barrier-shaped distribution, which may be mainly due to the nuclear energy deposition of the incident ions into the substrate. In order to investigate the thermal stability of the waveguides, the samples are annealed at temperature of 200–300 °C in air. The results show that waveguide produced by higher-dose carbon implantation remains relatively stable with post-irradiation annealing treatment at 200 °C in air.     

Enthalpies of formation of U-, Th-, Ce-brannerite: implications for plutonium immobilization

Brannerite, ideally MTi₂O₆, (M=actinides, lanthanides and Ca) occurs in titanate-based ceramics proposed for the immobilization of plutonium. Standard enthalpies of formation, ΔH⁰_f at 298 K, for three brannerite compositions (kJ/mol): CeTi₂O₆ (−2948.8 ± 4.3), U_0.97Ti_2.03O₆ (−2977.9 ± 3.5) and ThTi₂O₆ (−3096.5 ± 4.3) were determined by high temperature oxide melt drop solution calorimetry at 975 K using 3Na₂O · 4MoO₃ solvent. The enthalpies of formation were also calculated from an oxide phase assemblage (ΔH⁰_f-ox at 298 K): MO₂ + 2TiO₂=MTi₂O₆. Only UTi₂O₆ is energetically stable with respect to an oxide assemblage: U_0.97Ti_2.03O₆ (ΔH⁰_f-ox=−7.7±2.8 kJ/mol). Both CeTi₂O₆ and ThTi₂O₆ are higher in enthalpy with respect to their oxide assemblages with (ΔH⁰_f-ox=+29.4±3.6 kJ/mol) and (ΔH⁰_f-ox=+19.4±1.6 kJ/mol) respectively. Thus, Ce- and Th-brannerite are entropy stabilized and are thermodynamically stable only at high temperature.     

Electrical conductivity increase of Al-doped ZnO films induced by high-energy-heavy ions

We have investigated the effects on electrical properties of Al-doped ZnO (AZO) semiconductor films induced by 90 MeV Ni, 100 MeV Xe and 200 MeV Xe ions. The AZO films with c-axis orientation on SiO₂-glass substrate were prepared by using a RF-sputter-deposition method at 400 °C. We find that the conductivity increases by two order of magnitude under high-energy-heavy ion irradiation, as has already been observed for 100 keV Ne ion irradiation. We also find that the efficiency of the conductivity enhancement, which is defined as the conductivity increment per a unit of ion fluence, scales super-linearly with the electronic stopping power (S_e). The carrier density and mobility for unirradiated and irradiated AZO films are presented.     

The crystal structure of oxides grown on Zr–20Nb alloy

Oxides that were grown on Zr–20Nb in water at 300°C for 3 d, or in air at 400°C for 2 h were characterized by analytical electron microscopy. In both oxides, a similar microstructure was observed and similar electron diffraction patterns and high resolution lattice images were obtained. Analyses of the results showed that the crystal structure of the oxides was identical to that of an incommensurate modulated Nb₂Zr_x−2O_2x+1 phase, with x ≈ 10.     

Surface morphology of the Ar ion-irradiated Ag/Si(1 1 1) system

In the present study, a 500 Å thin Ag film was deposited by thermal evaporation on 5% HF etched Si(1 1 1) substrate at a chamber pressure of 8×10⁻⁶ mbar. The films were irradiated with 100 keV Ar⁺ ions at room temperature (RT) and at elevated temperatures to a fluence of 1×10¹⁶ cm⁻² at a flux of 5.55×10¹² ions/cm²/s. Surface morphology of the Ar ion-irradiated Ag/Si(1 1 1) system was investigated using scanning electron microscopy (SEM). A percolation network pattern was observed when the film was irradiated at 200°C and 400°C. The fractal dimension of the percolated pattern was higher in the sample irradiated at 400°C compared to the one irradiated at 200°C. The percolation network is still observed in the film thermally annealed at 600°C with and without prior ion irradiation. The fractal dimension of the percolated pattern in the sample annealed at 600°C was lower than in the sample post-annealed (irradiated and then annealed) at 600°C. All these observations are explained in terms of self-diffusion of Ag atoms on the Si(1 1 1) substrate, inter-diffusion of Ag and Si and phase formations in Ag and Si due to Ar ion irradiation.     

Ion-induced electron emission – monitoring the structure transitions in graphite

The temperature dependence of ion-induced electron emission yield γ under 30 keV Ar⁺ ion impacts at incidence angles θ = 0−80° under dynamically steady-state conditions has been measured for polygranular graphite POCO-AXF-5Q. The fluencies were 10¹⁸–10¹⁹ ion/cm², the temperatures varied from the room temperature (RT) to 400 °C. The RHEED has shown that same diffraction patterns correspond to a high degree of disorder at RT. At high temperature (HT), some patterns have been found similar to those for the initial graphite surfaces. The dependence γ(T) has been found to be non-monotonic and for normal and near normal ion incidence manifests a step-like increase typical for a radiation induced phase transition. At oblique and grazing incidence (θ > 30°), a broad peak was found at T_p = 100 °C. An analysis based on the theory of kinetic ion-induced electron emission connects the behavior of γ(θ,T) to the dependence of both secondary electron path length λ and primary ion ionizing path length R_e on lattice structure that drastically changes due to damage annealing.     

Observation of micro-cavities in nickel during He and D irradiation and post-irradiation annealing

Changes in sizes and morphology of small cavities in nickel irradiated by 25 keV helium ions and 20 keV deuterons were investigated during irradiation and on annealing after irradiation by means of transmission electron microscopy. In the early stage of He⁺ irradiations at 600 and 700° C, roundish cubes appeared, gradually changed to octahedra. and, then, by the truncation of apexes, finally reached cubo-octahedra. Nucleation and growth behavior of cubic cavities in D⁺ irradiated nickel was different from the case of He⁺ irradiation. On annealing of the He⁺ irradiated specimen, only octahedral cavities showed marked growth, finally changing to roundish cubes at 750° C. Cavities of roundish cubes and cubo-octahedra did not grow nor change their shapes remarkably by the annealing. The cubic cavities formed by D⁺ irradiation at 360° C showed gradual shrinkage on annealing at 600° C and disappeared at 625° C. The changes of cavities during irradiation and on annealing were interpreted by the effect of the internal gas pressure.     

Influence of protecting gel film on oxidation of zirconium alloys

Thin oxide layers of Zr1Nb and Zry-4S on tubes, used for fuel cladding in light water reactors, which had been protected by thin gel films, were compared with oxide layers on unprotected specimens of the same kind, grown together in two batches in water at 360 °C for 21 and 42 days, respectively. The analysis of the I–V characteristics at constant temperatures up to 180 °C showed a strong decreasing resistivity with oxidation time of the gel-protected layers of the Zry-4S specimens, with indication of an energy dependent trap distribution, although the gel and the oxide alone showed evidence of single, but different, energy traps. The changes are believed to be due to diffusion of different trap centers in both directions. The Zr1Nb specimens retained their single energy trap behaviour also in the combination with the protecting gel film and resistivity dropping with increasing oxidation time.     

The excitation power density effect on the Si nanocrystals photoluminescence

In the present work we have studied the photoluminescence (PL) behavior from Si nanocrystals (NCs) as a function of the excitation power density and annealing time. The NCs were produced in a SiO₂ matrix by Si implantations from room temperature (RT) up to 700 °C, followed by post-annealing in N₂ atmosphere at high temperature. With this aim we have changed the excitation power density (from 2 × 10⁻³ W/cm² up to 15 W/cm²) and the annealing time (from 10 min up to 15 h). The strong PL signal, which at 15 W/cm² is composed by a single-peak structure (650–1000 nm) centered at around 780 nm, expands up to 1200 nm showing a two-peak structure when measured at 20 × 10⁻³ W/cm². The peak structure located at the short wavelength side is kept at 780 nm, while the second peak, starting at around 900 nm, redshifts and increases its intensity with the implantation temperature and annealing time. The effect of the annealing time on the PL spectra behavior measured at low excitation power agrees by the first time with the Si NC growth according to quantum confinement effects.