Spatial distribution of defects in ion-implanted and annealed Si: The RP/2 effect

Gettering of metal impurities in ion-implanted Si occurs midway between the surface and the projected ion range, R_P, after annealing at temperatures in the range of 700–1000°C and vanishes at higher temperatures. This phenomenon, called the R_P/2 effect, seems to be a common feature of ion-implanted and annealed Si. The gettering ability of the damage at R_P/2 is commensurate with or may exceed that of the damage at R_P. The defects around R_P/2 acting as gettering sites have not yet been identified by other analysis techniques. They are formed after ion implantation in the process of defect evolution during annealing and, probably, consist of small complexes of intrinsic defects (vacancies or/and self-interstitials).     

Effects of environment on annealing behavior of In+ implanted c-axis sapphire

Single crystal samples of 〈0001〉α-Al₂O₃ were implanted with 360 keV indium ions of doses of 1 × 10¹⁶ and 3 × 10¹⁶ ions/cm², at room temperature (RT). The implanted samples were annealed isothermally in air or in flowing high purity argon ambient at 900°C for 2 or 12 h. The damage and thermal annealing were evaluated using Rutherford Backscattering Spectrometry and Channeling (RBS-C) and X-ray Diffraction (XRD). Amorphization of sapphire was not observed, despite damage energy densities up to 105 dpa (displacements per atom obtained from TRIM code calculation) for the Al sublattice, which indicated that self-annealing was severe during In ion implantation of sapphire at RT. RBS-C measurement revealed differences in the annealing characteristics of the implanted indium ions that depended on the annealing environment. The XRD spectrum indicated the presence of the In₂O₃ phase in the subsurface region of the sample after annealing in air.     

Thermal Recovery of Radiation Defects and Microstructural Change in Irradiated UO2 Fuels

Abstract

Thermal recovery of radiation defects and microstructural change in UO₂ fuels irradiated under LWR conditions (burnup: 25 and 44 GWd/t) have been studied after annealing at temperature range of 450-1,800°C by X-ray diffractometry and transmission electron microscopy (TEM). The lattice parameter of as-irradiated fuels increase with higher burnup, which was mainly due to the accumulation of fission induced point defects. The lattice parameter for both fuels began to recover around 450-650°C with one stage and was almost completely recovered by annealing at 850°C for 5 h. Based on the recovery of broadening of X-ray reflections and TEM observations, defect clusters of dislocations and small intragranular bubbles began to recover around 1.150–1,450°C. Complete recovery of the defect clusters, however, was not found even after annealing at 1,800°C for 5h. The effect of irradiation temperature on microstructural change of sub-grain structure in high burnup fuels was assessed from the experimental results.     

Positron beam and Raman analysis of hydrogen plasma treated and annealed Cz-Si

Positron beam annihilation Doppler-broadening (PADB) and Raman studies were carried out on H-plasma treated and annealed p-type Cz-Si. Samples were treated by a 13.56 MHz H-plasma for 2 h at RT (∼30 °C) and 250 °C, respectively. Annealing was done in air between 100 and 600 °C for annealing times between 10 and 480 min. The Raman spectra of the RT treated samples show no (or only very weak) H₂-molecule signals, in contrast to the samples H-plasma treated at 250 °C. Raman intensity changes as a function of temperature are observed, which are attributed to the evolution of voids or platelets. The positron annihilation data correlate with these results, since annealing of the RT plasma treated samples reduces the Doppler-broadening S-values with increasing temperature and/or annealing time. Together with an increasing positron diffusion length this suggests that defects acting as positron trapping centers are (partially) annealed out. For the sample plasma treated at 250 °C the following depth dependent behavior of the Doppler parameters is found. Below 100 nm depth the Doppler parameters follow the same trend as those of the RT treated samples. In a zone between 100 and 200 nm depth the S parameter strongly increases after annealing at 600 °C. This is attributed to the formation of positronium (Ps), indicative for the presence of nano-cavities capable of trapping molecular hydrogen in this region.     

Dwell-time dependence of the defect accumulation in focused ion beam synthesis of CoSi2

Cobalt disilicide microstructures were formed by 70 keV Co²⁺ focused ion beam implantation into Si(1 1 1) at substrate temperatures of about 400°C and a subsequent two step annealing (600°C, 60 min and 1000°C, 30 min in N₂). It was found that the CoSi₂ layer quality strongly depends on the pixel dwell time and the implantation temperature. Only for properly chosen parameters continuous CoSi₂ layers could be obtained. Scanning electron microscopy and Rutherford backscattering/channelling investigations were carried out combined with a special preparation technique for structures which are smaller than the analysing beam. The quality of the CoSi₂ layers which is correlated to the damage was investigated as a function of dwell-time (1–250 μs) and target temperature (355–415°C). The results show that the irradiation damage increases with the dwell-time. The Si top layer was amorphized for longer dwell-times although the substrate temperature was always above the critical temperature for amorphization of about 270°C according to the model of Morehead and Crowder. For the high current density of a focused ion beam (1–10 A/cm²) the damage creation rate is higher than the rate of dynamic annealing.     

Stability of defects created by high fluence helium implantation in silicon

Stability of extended defects created by high fluence helium implantation (50 keV, 5 × 10¹⁶ cm⁻²) from room temperature to 800 °C has been studied using transmission electron microscopy. Our results clearly show that the cavities behave as good sinks for interstitial type defects generated during ion implantation, leading in some cases to the cavity dissolution. A three-dimensional “phase diagram” related to the formation and evolution of interstitial-type defects is also proposed. It is plotted in terms of quantity of damage, annealing time and implantation temperature.     

Electrically active defects in BF2+ implanted and germanium preamorphized silicon

Ultra-shallow p⁺-n junctions have been formed using 15 keV/10¹⁵ cm⁻² BF₂⁺ implantation into both Ge⁺-preamorphized and crystalline 〈1 0 0〉 silicon substrates. Rapid thermal annealing (RTA) for 15 s at 950°C was used for dopant electrical activation and implantation damage gettering. The electrically active defects present in these samples were characterized using Deep Level Transient Spectroscopy (DLTS) and isothermal transient capacitance (ΔC(t, T)). Two electron traps were detected in the upper half of the band gap at, respectively, E_c - 0.20 eV and E_c - 0.45 eV. They are shown to be related to Ge⁺ implantation-induced damage. On the other hand, BF₂⁺ implantation along with RTA give rise to a depth distributed energy continuum which lies within the forbidden gap between E_c - 0.13 eV and E_c - 0.36 eV. From isothermal transient capacitance (ΔC(t, T)), reliable damage concentration profiles were derived. They revealed that preamorphization induces not only defects in the regrown silicon layer but also a relatively high concentration of electrically active defects as deep as 3.5 μm into the bulk.     

Damage accumulation and annealing in 6H–SiC irradiated with Si+

Damage accumulation and annealing in 6H-silicon carbide (α-SiC) single crystals have been studied in situ using 2.0 MeV He⁺ RBS in a 〈0 0 0 1〉-axial channeling geometry (RBS/C). The damage was induced by 550 keV Si⁺ ion implantation (30° off normal) at a temperature of −110°C, and the damage recovery was investigated by subsequent isochronal annealing (20 min) over the temperature range from −110°C to 900°C. At ion fluences below 7.5 × 10¹³ Si⁺/cm² (0.04 dpa in the damage peak), only point defects appear to be created. Furthermore, the defects on the Si sublattice can be completely recovered by thermal annealing at room temperature (RT), and recovery of defects on the C sublattice is suggested. At higher fluences, amorphization occurs; however, partial damage recovery at RT is still observed, even at a fluence of 6.6 × 10¹⁴ Si⁺/cm² (0.35 dpa in the damage peak) where a buried amorphous layer is produced. At an ion fluence of 6.0 × 10¹⁵ Si⁺/cm² (−90°C), an amorphous layer is created from the surface to a depth of 0.6 μm. Because of recovery processes at the buried crystalline–amorphous interface, the apparent thickness of this amorphous layer decreases slightly (<10%) with increasing temperature over the range from −90°C to 600°C.     

Effects of hydrogen implantation into GaN

Proton implantation in GaN is found to reduce the free carrier density through two mechanisms – first, by creating electron and hole traps at around E_C − 0.8 eV and E_V + 0.9 eV that lead to compensation in both n- and p-type material, and second, by leading to formation of (AH)° complexes, where A is any acceptor (Mg, Ca, Zn, Be, Cd). The former mechanism is useful in creating high resistivity regions for device isolation, whereas the latter produces unintentional acceptor passivation that is detrimental to device performance. The strong affinity of hydrogen for acceptors leads to markedly different redistribution behavior for implanted H⁺ in n- and p-GaN due to the chemical reaction to form neutral complexes in the latter. The acceptors may be reactivated by simple annealing at ⩾600°C, or by electron injection at 25–150°C that produces debonding of the (AH)° centers. Implanted hydrogen is also strongly attracted to regions of strain in heterostructure samples during annealing, leading to pile-up at epi–epi and epi–substrate interfaces. IR spectroscopy shows that implanted hydrogen also decorates V_Ga defects in undoped and n-GaN.     

Formation of thin silicon nitride layers on Si by low energy N2+ ion bombardment

The formation of nitride layers in silicon due to low-energy implantation of nitrogen in a wide range of ion bombardment parameters (energy E and angle of incidence θ) and for different temperatures of subsequent annealing (T) has been studied using Auger Electron Spectroscopy (AES), Secondary Ion Mass Spectrometry (SIMS) and Fourier Transform InfraRed Spectroscopy (FTIRS). Bombardment at angles θ < 40° produces an amorphous layer of stoichiometric Si₃N₄ the thickness of which depends on implantation energy and incidence angle. Annealing of the samples at 1000°C produces layers with rather sharp interfaces.     

Critical current density changes in irradiated Nb3Sn

Söll recently modeled changes in the current-carrying capacity of superconducting Nb₃Sn after irradiation. As the dose increases, the critical current density (J_c) generally increases, reaches a maximum, and decreases. The model relates the maximum J_c for different types of irradiations to the integrated damage energy (E_D) that the irradiating particles transfer to the lattice. Earlier, Söll et al. related cirtical-temperature (T_c) decreases in irradiated Nb₃Sn to E_d, which appears more reasonable since T_c is a measure of disorder (or replacements) accompanying defect production, and the final defect configuration (or displacements) are less important. The annealing temperature for the disorder (~700°C) exceeds any of the irradiation temperatures (T_IRR); therefore, T_IRR is unimportant in the T_c experiments. However, different defect structures exhibit considerably different flux pinning and the J_c model does not consider different spatial variations of the defects during production or migration and agglomeration of the defects during high-T_IRR experiments, both of which affect flux pinning. The lack of the model to take into account the physics of the damage is the subject of this paper. Arguments are presented why the defect configurations should be considered, and recently published data is presented that conflict with the conclusions of this damage-energy model.     

Effect of Grain Size on Microstructural Change and Damage Recovery in UO2 Fuels Irradiated to 23 GWd/t

The effect of grain size on microstructural change and damage recovery in U0₂ fuels was studied by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The as-irradiated lattice parameter of the standard fuel (grain size: 16/μm) was larger than that of the large-grained fuel (43 μm), indicating a larger number of fission-induced point defects in the lattice of the former fuel. This tendency was in contrast to previously reported results for low burnup fuels below 1 GWd/t. The lattice dilation in the present high burnup fuels was mainly due to the accumulation of vacancies. The lattice parameter of both fuels began to recover around an irradiation temperature of 450~650°C, and both had a complete recovery at 850°C. On annealing at high temperatures of 1,450~ 1,800°C, the bubble diameter in the standard fuel was larger than that in the large-grained fuel. This indicated that vacancy diffusion from the grain boundaries plays an important role during bubble coarsening at high temperatures.     

Internal Friction Measurements on Neutron- Irradiated Fe-3.48% Ni Alloy

The Snoek peak and the strain-amplitude dependence of internal friction were measured in order to gain further information on the behavior of interstitial solute atoms in Fe-3.48%Ni alloy which had been neutron-irradiated and annealed. This alloy contained 37 ppm N and 7 ppm C.

Interstitial solute N and C atoms were trapped by irradiation-induced defects and released by annealing at above 250°C. The Snoek peak and the slops of Granato-Lücke plots which latter is proportional to 1/L_c (L_c being the dislocation loop length between impurity atoms) increased with annealing temperature.

The binding energy of interstitial solute N atoms to dislocations in the irradiated and annealed specimen was obtained from the strain-amplitude dependent internal friction. The value was about 16, 500 cal/mol.     

Homogeneously size distributed Ge nanoclusters embedded in SiO2 layers produced by ion beam synthesis

500 nm SiO₂ layers were implanted with 450 keV (F=3 × 10¹⁶ at./cm²) and 230 keV (F=1.8 × 10¹⁶ at./cm²) Ge ions at room temperature to obtain an almost constant Ge concentration of about 2.5 at.% in the insulating layer. Subsequently, the specimens were annealed at temperatures between 500°C and 1200°C for 30 min in a dry N₂ ambient atmosphere. Cross-sectional TEM analysis reveal homogeneously distributed Ge nanoclusters arranged in a broad band within the SiO₂ layer. Their mean cluster size varies between 2.0 and 6.5 nm depending on the annealing conditions. Cluster-free regions are always observed close to the surface of the specimens independent of the annealing process, whereas a narrow Ge nanocluster band appears at the SiO₂/Si interface at high annealing temperatures, e.g. ⩾1000°C. The atomic Ge redistribution due to the annealing treatment was investigated with a scanning TEM energy dispersive X-ray system and Rutherford back scattering (RBS).     

The U0.8Pu0.2C-W system

A phase diagram of the U_0.8Pu_0.2C-W system has been established based on X-ray diffraction measurements and metallographic observations. A peritectic four-phase reaction has been found to occur at 2100 ± 40°C:

The peritectic point is close to U_0.8Pu_0.2C-27 wt.%W. Composition of the peritectic liquid is near 48(U + Pu)/18W/34C (at %) by analogy with the UC-W system. The liquid phase transforms, during rapid cooling, to a metastable mixture [(U, Pu)C + (U, Pu)metal + W] with accompanied retention of (U, Pu)WC_1.75. The (U, Pu)WC_1.75 phase crystallizes in a UWC_1.75-type monoclinic structure. Essentially single-phase (U_0.8Pu_0.2)WC_1.75 has been produced by sufficient annealing of the arc-melted specimen, with lattice parameters: a₀ = 5.6257 ± 0.0008 Å, b₀= 3.2498 ± 0.0005Å, c₀ = 11.623 ± 0.002 Å, β = 109.61 ± 0.01°. This compound perhaps melts incongruently as:

The terminal solid solubility limit of tungsten in U_0.8Pu_0.2C increases from about 0.8 wt%W at 1500°C to a maximum of about 3.2 wt%W at 2100°C. The partial molar heat of solution of tungsten in U_0.8Pu_0.2C is approximately 14.4 kcal/mole. Un diagramme de phase du système U_0,8Pu_0,2C-W a étée'tabli en se basant sur les mesures par diffraction aux rayons X et par examen métallographique. Une réaction péritectique a quatre phases a été observée à 2100 ± 40°C:

     

Effects of rare earth oxides and UO2 + x on the structure and the mechanical properties of Zircaloy

Tension test specimens of Zircaloy have been annealed with simulated fission products, as CeO₂, La₂O₃, Nd₂O₃, Y₂O₃ or mixtures of these rare earth oxides and UO_{2 + x} at 350°C up to 10 000 hours and at 500 or 700°C up to 2000 hours. The long term effects have been studied by tension tests, scanning electron microscopy, X-ray diffraction and metallography. Annealing of Zircaloy at 700°C with rare earth oxides generally leads to total embrittlement. There exists a gradation of efficacy which becomes obvious when the results of the annealing series at 500°C are compared. Rare earth oxides in mixtures with UO_{2 + x} cause improportional intense reductions of ductility. The structural characteristics of specimens lead to thermodynamic considerations of the probable reaction mechanism.     

Etude de l'alliage mulberry [U-7,5 Nb-2,5 Zr (% ponderaux)]. diagramme de transformation en refroidissement continu-structures et proprietes mecaniques

We have studied by dilatometry, microscopy, X-ray and electron diffraction, the phase transformations generated by controlled cooling from 900° C of the “Mulberry” alloy (U-7.5 wt%Zr). For slow cooling rates, three types of decomposition of the 7 phase appear successively: (i) Between 600°C and 550°C, grain boundaries and inclusion serve as nucleation sites for a cellular reaction, $\text{γ→α+γ}$; (ii) between 550°C and 400°C, a continuous transformation of the matrix generates an a phase in the form of Widmanstätten plates, and (iii) below 400°C, the γ phase gives rise to an α″ phase which has an ultrafine structure only imperfectly resolved by TEM. On rapid cooling, a tetragonal phase, γ₀, appears. Quenching generates the γ_S phase derived from γ, with a structure crystallizing in 143m which has been described by YakeL The various methods of investigation used have allowed us to obtain heating and cooling curves of the alloy and thus to determine phase fields. In general terms, the structures formed are similar to those obtained by isothermal annealing below 600° C. Only the γ_S phase has useful mechanical properties, notably good ductibility. The other phases are characterised by extreme brittleness.     

Modelling high-temperature co-implantation of N+ and Al+ ions in silicon carbide: the effect of stress on the implant and damage distributions

This work is an initial attempt to model the fundamental processes that occur when SiC is implanted at elevated substrate temperatures T_i (200°–800°) with high doses of N⁺ and Al⁺ ions to synthesise buried layers of (SiC)_1 − x(AlN)_x. The theoretical treatment has involved ballistic calculation of the implant and damage profiles by means of computer codes (TRIRS and DYTRIRS) specifically developed for modelling complex, multi-elemental targets. The influence of the mechanical stress induced the by implanted ions has been taken into account by adding a special term to the differential equations describing the evolution of the implant and damage distributions. Results from the simulations have been correlated with data obtained by Rutherford backscattering spectrometry/ion channelling (RBS/C). The theoretical approach described has enabled one to determine the interaction energies of the interstitials with the internal stress field as well as the role of stress on the defect distribution.     

Chemical aspects of iodine-induced stress corrosion cracking failure of zircaloy-4 tubing above 50°C

The chemical environment associated with iodine-induced SCC failure of Zircaloy-4 tubing above 500°C has been characterized. At the critical iodine concentrations which result in SCC initiation and propagation, most of the iodine is present as condensed zirconium subiodides (I/Zr ? 0.4). Only a small part of the iodine remains in the gas phase as ZrI₄. The gaseous ZrI₄ is probably responsible for crack initiation and propagation. The critical ZrI₄ pressures for SCC failure have been estimated in zircaloy/iodine reaction experiments performed with unstressed zircaloy tube specimens. These pressures were confirmed in additional creep rupture tests conducted under controlled ZrI₄ partial pressure conditions. The estimated critical ZrI₄ pressure above which low-ductility SCC failure of the zircaloy tubing always occurs, independent of time-to-failure, varies between 0.005 bar at 550°C and 0.043 bar at 800°C. Below the critical values, however, a rather wide range of ZrI₄ pressures is associated with the onset of the SCC, especially at temperatures below 800°C. A comparison of the experimental results with available thermochemical data in the Zr-I system indicates that the main reaction involved during crack propagation is chemisorption of iodine-containing species on the fresh zircaloy surfaces created by metal straining at the crack tip.     

Studies of Ceramic Fuels with the Use of Fission Gas Release Loop, (I)

Fission gas release from a UO²-graphite mixture was studied during irradiation with the use of the Fission Gas Release Loop in the JRR-3 reactor. The release rates of fission krypton and xenon increased proportionally with neutron flux (6×10¹⁰–6×10¹² n/cm²·sec) and exponentially with temperature (400°–1,000°C). A burst of fission gas was observed when the specimen was abruptly heated to a higher temperature. These results can be explained by a mechanism whereby fission gas is trapped in defects created in graphite by fission fragments and released through annealing of the defects.