Structural changes of SiC under electron-beam irradiation: Temperature dependence

Electron-beam-irradiation effects on silicon carbide (SiC) was investigated as a function of the irradiated temperatures. Single crystalline 6H-SiC was irradiated with 300 kV electrons at temperatures ranging from −170 to 250 °C. It was found that amorphous SiC is induced at −170 °C and room temperature, while crystalline Si is formed at 250 °C with a high electron fluence. It is considered that preferential knock-on displacement of C atoms and damage recovery play an important role in the formation of the amorphous SiC and crystalline Si.     

Breakaway bubble growth during the annealing of helium bubbles in metals

Recent experimental studies on the annealing of helium implanted copper showed unusual surface pinholes which could not be explained using conventional bubble migration and coalescence theory. However, computer simulation results indicated that at some threshold, breakaway bubble swelling was taking place, thus giving a plausible picture of the experimental results. In the present study the computer simulations have been repeated for an initially uniform helium level in bulk material, allowing the threshold effects to be examined without any surface effects and any loss of helium. Assuming equilibrium bubble conditions, the results showed that when bubble coarsening reached an average of 20% swelling, breakaway swelling occurred. This phenomenon has been examined as a function of helium content and results extrapolated (with some assumptions) to other temperatures and metals. The implications for fusion reactor materials is briefly discussed.     

Effect of high temperature annealing on ferromagnetism induced by energetic ion irradiation in FeRh alloy

Effects of thermal annealing on ion-irradiation induced ferromagnetism of Fe-50at.%Rh bulk alloy and the related structural change were investigated by means of superconducting quantum interference device (SQUID) and extended X-ray absorption fine structure (EXAFS), respectively. Depending on the annealing temperature from 100 to 500 °C, the magnetization induced by 10 MeV iodine ion irradiation and the lattice structure of the alloy were remarkably changed. After 500 °C annealing, the magnetization and the lattice ordering of the alloy become similar to the states before the irradiation. The experimental result indicates that the thermal relaxation of irradiation-induced atomic disordering dominates the magnetic state of ion-irradiated Fe-50at.% Rh alloy.     

Mathematical models for tritium permeation analysis in liquid metal flows with helium bubbles

In LIBRETTO-2 test, evidence was obtained that helium bubbles nucleated and grew in the neutron irradiated PbLi probes. If such phenomenon occurs inside liquid metal (LM) breeding blanket channels, the study of its effect on tritium permeation and heat transfer in the near wall region will acquire utmost importance. The T4F research group has developed in the past a nucleation, growth and transport model for helium bubbles in LM flows, as well as a tritium transport model in such a multi-fluid system. In the present study, we are focused on the near-wall region analysis in order to obtain a wall function that allow reproducing the tritium permeation with coarse meshes and, hence, reduce the computational time. First, we perform some detailed CFD simulations of the near-wall region where bubbles might be attached. In these simulations, tritium diffusion processes as well as tritium recombination and dissociation are modelled. The analysis of such simulations allows us to further understand the complex phenomena and justify the use of simplified models. As a result, a new model for tritium transport across a LM–solid interface partially covered by helium bubbles is developed, implemented and validated. This simplified model can be seen as a wall function for the CFD simulation which substantially reduces computational time.     

氦在钛靶及Al2O3中的行为研究

报道了利用质子增强背散射和扫描电镜等方法，研究不同工艺制作的钛膜靶和不同纯度的Ａｌ２Ｏ３中注入的＾４Ｈｅ在室温下的行为。研究结果表明，离子束增强淀积新工艺制备的钛膜与基体的结合强度优于原来热蒸发镀制的钛靶；氦在Ａｌ２Ｏ３与在钛靶中具有不同的释放机制；钛靶的起泡注入剂量在４×１０＾１７－７×１０＾１７Ｈｅ／ｃｍ＾２之间，而且起泡剂量与泡破裂剂量相差不大；较之新工艺制备的钛靶，氦在Ａｌ２Ｏ３中更不易形     

Study of iodine diffusion in silicon carbide

Diffusion of iodine in 6H-SiC and polycrystalline CVD-SiC was investigated using Rutherford backscattering spectroscopy and electron microscopy. A fluence of 1 × 10¹⁶ cm⁻² of ¹²⁷I⁺ was implanted with an energy of 360 keV at room temperature, producing an amorphous surface layer of approximately 220 nm thickness. The implantation profile reached an atomic density of approximately 1.3% at the projected range of about 95 nm. Broadening of the implantation profile and iodine loss through the front surface during isochronal and isothermal vacuum annealing was determined. At a temperature of 1100 °C no iodine loss was observed after 120 h and a diffusion coefficient of less than 10⁻²¹ m² s⁻¹ was extracted from the analysis of profile widths. Relatively strong broadening occurred after 60 h annealing at 1200 °C with the iodine profile extending beyond 300 nm into the bulk, accompanied by a surprisingly modest iodine loss through the surface. Electron microscopic studies reveal a drastic restructuring of the surface region at this temperature, indicating possible chemical reactions between iodine and silicon carbide.     

Quantitative characterization of xenon bubbles in silicon: Correlation of bubble size with the damage generated during implantation

Making use of Fresnel fringe contrast under different focusing conditions in transmission electron microscopy (TEM), we present a detailed evaluation of the depth dependent size distribution of gas bubbles contained in a stationary profile of 40 keV Xe implanted in Si. Voids generated during sample preparation by ion milling were also characterized carefully. The largest bubbles, with mean and maximum sizes of 5 and 7 nm, respectively, were observed at depths <22 nm. However, the first 2 nm of the sample did not contain any bubbles. Towards the end of range the bubble size decreased rapidly. No bubbles were found beyond 45 nm (the minimum size of detectable bubbles was estimated to be about 1.8 nm). Some observations suggest that the bubbles were over-pressurized. The derived data could be converted to a depth dependence of the Xe concentration contained in bubbles, n_Xe,b. Comparison with the previously reported depth distribution of Xe measured by Rutherford backscattering spectrometry (RBS), n_Xe,b turned out to be depth dependent, with a maximum of ∼28% in the region of maximum bubble size. n_Xe,b is shown to correlate closely with the damage density generated during Xe implantation. The findings lead to a model of bubble formation which involves the idea that the redistribution and transport processes initiated by ion impact take place mostly during the lifetime of the collision cascade.     

Interaction of displacement cascade with helium bubbles in α-iron: Computer simulation

Molecular dynamics (MD) method has been performed to study the interaction of displacement cascade with He bubbles with two sets of potentials. The results show that the stability of He bubbles depends much on the initial He–vacancy (He/V) ratio and the recoil energy. For an initial He/V ratio of 3, the cascade leads to the increase in the number of vacancies in the He bubble and the decrease in the He/V ratio. For an initial He/V ratio of 0.5, the interaction of a cascade with the He/V bubble results in the decrease in the number of vacancies and the increase in the He/V ratio. For an initial He/V ratio of 1, the stability of the bubbles slightly depends on the primary knock-on atom (PKA) energy. Furthermore, a large number of self-interstitial atom clusters are formed after cascade collision for the He/V ratio of 3, while large vacancy clusters are observed for the He/V ratio of 0.5. However, some differences of defect production and clustering between the two sets of potentials are observed, which may be associated the formation energies of He–V clusters, the binding energies of vacancies and He atoms to the clusters and the probability of subcascade formation.     

Enhanced defects recombination in ion irradiated SiC

Point defects induced in SiC by ion irradiation show a recombination at temperatures as low as 320 K and this process is enhanced after running current density ranging from 80 to 120 A/cm².Ion irradiation induces in SiC the formation of different defect levels and low-temperature annealing changes their concentration. Some levels (S₀, S_x and S₂) show a recombination and simultaneously a new level (S₁) is formed.An enhanced recombination of defects is besides observed after running current in the diode at room temperature. The carriers introduction reduces the S₂ trap concentration, while the remaining levels are not modified. The recombination is negligible up to a current density of 50 A/cm² and increases at higher current density. The enhanced recombination of the S₂ trap occurs at 300 K, which otherwise requires a 400 K annealing temperature. The process can be related to the electron-hole recombination at the associated defect.     

Ion beam induced formation of nanocrystalline silicon in pulsed laser deposited SiOX thin films

Synthesis and structural studies of nanocrystalline silicon grown in pulsed laser deposited SiO_X films is reported. The effect of high energy heavy ion beam irradiation on these films is studied using 100 MeV Ag ions. The structural studies were carried out using micro Raman spectroscopy, GAXRD, FTIR, TEM, HRTEM, SAED and EDX. The occurrence of phase separation in non-stoichiometric silicon oxide by means of ion beam irradiation leading to the formation of silicon nanocrystals in the films is confirmed by the results. HRTEM results reveal the structure of silicon phase formed after ion beam treatment and the particle size can be controlled up to 2-3 nm. A detailed analysis by micro Raman and HRTEM studies suggest the presence of crystallite size distribution. The results of GAXRD and SAED confirm the formation of cubic phase of silicon with two different lattice parameters. The studies conclude that the size of the nanocrystals can be controlled by varying deposition and ion irradiation parameters.     

Temperature dependence of damage formation in Ag ion irradiated 4H-SiC

Rutherford backscattering spectrometry (RBS) in channelling mode was used to study the defect formation in silver (Ag) ion irradiated silicon carbide (SiC). The 4H-SiC samples were irradiated with 360 keV Ag ions at different temperatures (15, 295, 375, 475, 625 and 875 K) over a wide range of fluences (1×10¹¹ to , depending on the irradiation temperature). The results can be divided into two groups: (i) for irradiation temperatures between 15 and 475 K amorphisation of the implanted layers is reached for ion fluences between 7×10¹³ and . The over-all cross-section of defect production at very low ion fluences which comprises the formation of point defects and of amorphous clusters, is almost identical for all data sets measured in this temperature range. Differences in the damage evolution which occur at higher ion fluences, suggest that the relative contribution of amorphous clusters within single ion impacts in crystalline material decreases with rising temperature. (ii) For irradiations performed at 625 and 875 K no amorphisation is found for ion fluences as high as . With increasing ion fluence the defect concentration exhibits a distinctive plateau due to the balance between formation and recombination of point defects before increasing up to a saturation level well below amorphisation. For this final stage our results indicate a mixture of point defect clusters and extended defects most probably dislocations. A comparison with data from the literature suggests that the damage evolution for implantation at 625 and 875 K is strongly influenced by the mobility of vacancies starting at around 600 K.     

氦离子辐照烧结碳化硅损伤效应研究

利用中国科学院近代物理研究所320 kV高压平台提供的氦离子辐照烧结碳化硅,辐照温度从室温到1 000 ℃,辐照注量为1015~1017 cm-2。辐照完成后,进行退火处理,然后开展透射电子显微镜、拉曼光谱、纳米硬度和热导率测试。研究发现,烧结碳化硅中氦泡形核阈值注量低于单晶碳化硅。同时,氦泡形貌和尺寸与辐照温度、退火温度有关。另外,对辐照产生的晶格缺陷、元素偏析进行了研究。结果表明,辐照产生了大量的缺陷团簇,同时氦泡生长也会发射间隙子,在氦泡周围形成间隙型位错环。在晶界处,容易发生碳原子聚集。辐照导致材料先发生硬化而后发生软化,且热导率降低。     

注砷硅片低能电子束退火

本文报道了用低能大面积电子束处理注砷硅片的实验结果。由四探针和背散射、沟道效应测量结果表明，用本方法退火的样品具有电激活率高和砷原子再分布小的优点。     

Defect formation and annealing kinetics in ion irradiated carbon nanotube buckypapers

We studied the frequency shifts in G, D and D^* Raman modes in freestanding multiwall carbon nanotube buckypapers. Upon ion irradiation by 140 keV He⁺ or 3 MeV H⁺ ions, the intensity ratio of D–G modes linearly increases with fluence before amorphization. The ratio is used to quantitatively measure the level of disorder in the buckypaper. The study shows that, upon post-irradiation annealing, defect removal requires little energy addition in lightly damaged buckypaper, which is evidenced by an activation energy of 0.36 eV. Once amorphized, defect removal becomes very difficult. The D–G intensity ratio has no reduction in heavily damage sample after annealing up to 850 °C.     

Density functional study on helium and hydrogen interstitials in silicon carbide

Silicon carbide with various defects such as carbon or silicon vacancies, anti-sites, and helium or hydrogen interstitials are studied in detail by ab initio calculations. The energy minimized structures of vacancies and helium or hydrogen interstitials are investigated. The calculated electronic spectra explain well some of the important features of silicon carbide observed in experiments. The phonon spectra of silicon carbide with hydrogen interstitials between silicon and carbon bond are calculated, and the frequency of the hydrogen-carbon stretching mode is consistent with experiments.     

Coalescence of helium bubbles in aluminum

The formation and coalescence of helium bubbles in aluminum with annealing at 550°C is examined experimentally using electrical resistivity and electron microscopy. Results for the kinetics of coalescence are compared with those obtained by computer simulation in order to test models based upon various mechanisms for bubble motion. The computer method previously developed is extended from an ideal to a Van der Waals gas and effects due to the surface energy of aluminum are considered. Comparison of experimental and computer results indicates that the motion of helium bubbles in aluminum is not surface-diffusion limited and that it is consistent with the facet nucleation-limited mechanism originally proposed by Willertz and Shewmon.     

Electron-beam radial distribution analysis of irradiation-induced amorphous SiC

Advanced electron microscopy techniques have been employed to examine atomistic structures of ion-beam-induced amorphous silicon carbide (SiC). Single crystals of 4H-SiC were irradiated at a cryogenic temperature (120 K) with 300 keV Xe ions to a fluence of 10¹⁵ cm⁻². A continuous amorphous layer formed on the topmost layer of the SiC substrate was characterized by energy-filtering transmission electron microscopy in combination with imaging plate techniques. Atomic pair-distribution functions obtained by a quantitative analysis of energy-filtered electron diffraction patterns revealed that amorphous SiC networks contain heteronuclear Si–C bonds, as well as homonuclear Si–Si and C–C bonds, within the first coordination shell. The effects of inelastically-scattered electrons on atomic pair-distribution functions were discussed.     

Point defects reactions in ion irradiated SiC

The defects produced in 4H-SiC epitaxial layers by irradiation with 800 keV C⁺ were characterized by Low Temperature Photoluminescence. Ion beam irradiation induces the formation of some sharp lines in the wavelength range 428-441 nm of the photoluminescence spectra, that are typically known as “alphabet lines”. These photoluminescence features are due to the recombination of excitons at structural defects. The photoluminescence results allow to single out two groups of peaks: the P₁ lines (e-f-g) and the P₂ lines (a-b-c-d), that exhibit a different trend with the ion fluence. The P₁ group intensity increases with fluence and tends to reach a saturation value at high fluence. The P₂ group yield, instead, exhibits a threshold at low fluence and then increases toward a saturation. Subsequent UV-laser irradiation decreases the intensity of the P₂ lines related to a change in the structural configuration of the associated defects.     

Formation of Si/SiC multilayers by low-energy ion implantation and thermal annealing

Si/SiC multilayer systems for XUV reflection optics with a periodicity of 10-20 nm were produced by sequential deposition of Si and implantation of 1 keV ions. Only about 3% of the implanted carbon was transferred into the SiC, with a thin, 0.5-1 nm, buried SiC layer being formed. We investigated the effect of thermal annealing on further completion of the carbide layer. For the annealing we used a vacuum furnace, a rapid thermal annealing system in argon atmosphere, and a scanning e-beam, for different temperatures, heating rates, and annealing durations. Annealing to a temperature as low as 600 °C resulted in the formation of a 4.5 nm smooth, amorphous carbide layer in the carbon-implanted region. However, annealing at a higher temperature, 1000 °C, lead to the formation of a rough poly-crystalline carbide layer. The multilayers were characterized by grazing incidence X-ray reflectometry and cross section TEM.