Isotopic Effect in Ion-Induced Re-Emission of Hydrogen Implanted into Graphite

The re-emission of H and D atoms, implanted into graphite, induced by 1.5 MeV He⁺bombardment has been studied by means of the elastic recoil detection (ERD) technique by changing the initial peak concentration of H and D, the temperature and the He⁺ ion flux. The experimental re-emission profiles were analyzed solving analytically the mass balance equations. The whole re-emission profiles are shown to be excellently reproduced in terms of the ion- induced detrapping (σd), trapping (or retrapping) (Σ_T ) and local molecular recombination (k) between an activated (free) hydrogen atom and a trapped one and that the effective detrapping cross sections Σ_dK/Σ_T for H and D are determined as the best fitting parameter to be 2.9xl0-¹⁸ and 7.7x 10-¹⁹ cm², respectively. It is also shown from the experimental data on little dependence of the re-emission profile on the He⁺ ion flux that the local molecular recombination plays a major role in the ion-induced re-emission. Furthermore, the difference between the effective detrapping cross sections for H and D is reasonably explained in terms of isotopic difference in the values of K for H and D.     

High dose He+ bombardment of niobium at 800° tO 1400°C

In proposed fusion reactors, the first wall will be bombarded by low energy D⁺, T⁺ and He⁺ ions to very high doses. Room temperature irradiations with 9 keV He⁺ (Roth et al. 1975) showed an initial phase of blistering between 0.05 and 0.25 C/ cm², and a final state of considerable surface roughness at 8 to 112 C/cm²(7 × 10²⁰He⁺/cm²). The present irradiation experiments with 6 keV He⁺ are carried out with polycrystalline Nb foils at temperatures between 800 and 1400°C in order to study the influence of He mobility and of Nb surface diffusion during irradiation. The applied doses range from 2.5 × 10¹⁷He⁺/cm² (blistering) to 5 × 10²⁰He⁺/cm² which corresponds to at least several weeks of reactor operation. The resulting changes of the niobium surface structures are observed by scanning electron microscopy and are pictorially presented in this paper. Mainly, sponge-like open structures are seen to develop at high doses, with increasing physical dimensions at higher temperatures.     

Helium and hydrogen re-emission during implantation of molybdenum,vanadium and stainless steel

He re-emission has been measured during 300 keV He⁺ implantation from 400°C to 1200°C in V and Mo between ?170°C and 700°C in 316 SS. H re-emission has been measured during 150 keV H⁺ implantation of Mo, V, and 316 SS between ?90°C and 115 °C. In general the He re-emission is low until a critical dose is reached, at which time the re-emission abruptly rises to higher values. Both the critical dose and re-emission are strongly temperature dependent. The H re-emission generally rises smoothly to an equilibrium value which is both temperature and material dependent. Only at the lowest temperature does the H re-emission in Mo bear a qualitative resemblance to He re-emission.     

The influence of temperature on the blister behavior of vanadium and binary V Ti-alloys during 200- and 2000-keV helium irradiation

The formation and growth of gas bubbles is one of the major problems for the integrity of the first wall in a fusion reactor. The helium-induced surface damage as well as the helium-induced bulk damage beneath the surface has been investigated by scanning and transmission electron microscopy techniques. Samples of vanadium, V-3 wt.%-Ti and V-20 wt.%-Ti were implanted with 200-keV and 2000-keV helium ions at temperatures between 450 and 700° C to fluences above the threshold dose for blistering (2 × 10¹⁷He⁺/cm²). The dependence of surface damage phenomena of blistering, exfoliation and perforation on dose, temperature, ion energy and yield strength is discussed. The results can be explained by the helium behavior in the bulk. This behavior is found to be characterized by the formation of large gas bubbles grown by a coalescence process and by the formation of small helium clusters at high dose implantation.     

Surface deformations and gas escape process studied by quasi-simultaneous multiple-energy He irradiation

Single crystal silicon wafer covered by Al foil of thickness 5 μm was irradiated quasi-simultaneously by multiple-energy He ions up to the fluence level of $\text{3.3 × 10}{}^{19}$ ions/cm². The implantation was performed by 3.5 MeV ⁴He⁺ through a moving Al absorber foil in such a way that a nearly uniform He distribution was obtained extending in both Si and Al to depth intervals of $\text{1.9 × 10}{}^{19}$ Si/cm² and of $\text{1.5 × 10}{}^{19}$ Al/cm², respectively. During irradiation the evolution of the He concentration-depth profile was studied in situ by 3 MeV proton RBS analysis. It was found that the He concentration in the Al cover foil, after reaching a maximum value of 30 at.% began to decrease. This accelerated re-emission process was initiated near the inter-boundary surface and extended inward gradually. No significant He escape was observed from the Si up to the applied dose, so at the end of the implantation a He concentraton of 80 at.% has been reached.Exposing both inter-boundary surfaces to SEM, flaking from numerous spots was observed on the Al but not on the Si where only one crater was found. After mechanically breaking the implanted Si wafer and Al cover foil it could be seen on the fracture surfaces that regions containing a large amount of He acquire a sponge-like structure. Channels and large cavities were also observed in this region of the broken Si produced as cracks of interconnected He bubbles. The appearance of the flaking processes clearly demonstrates that even with such a depth distribution the He implanted in the materials in spite of re-emission may reach the critical value for blistering or flaking. The critical concentrations required for the observed surface deformation together with the thickness of the flaked layers evaluated from RBS and SEM observations are discussed.     

Light particle irradiation effects in Si nanocrystals

Si nanocrystals, formed by Si ion implantation into SiO₂ layers and subsequent annealing at 1150°C, were irradiated at room temperature either with He⁺ions at energies of 30 or 130 keV, or with 400 keV electrons. Transmission electron microscopy (TEM) and photoluminescence (PL) studies were performed. TEM experiments revealed that the Si nanocrystals were ultimately amorphized (for example at ion doses ∼10¹⁶ He cm⁻²) and could not be recrystallized by annealing up to 775°C. This contrasts with previous results on bulk Si, in which electron- and very light ion-irradiation never led to amorphization. Visible photoluminescence, usually ascribed to quantum-size effects in the Si nanocrystals, was found to decrease and vanish after He⁺ ion doses as low as 3 × 10¹²–3 × 10¹³ He cm⁻² (which produce about 1 displacement per nanocrystal). This PL decrease is due to defect-induced non-radiative recombination centers, possibly situated at the Si nanocrystal/SiO₂ interface, and the pre-irradiation PL is restored by a 600°C anneal.     

The effects of gas transport properties on blister formation in He+-implanted glass

Blister formation in He⁺-implanted glasses is correlated with the measured helium gas diffusivity. A scries of glasses with diffusivities from ～3 × 10^?7 to ～5 × 10^?12cm²sec^?1 was implanted under nearly identical conditions with 150 keV He⁺ ions at a flux of 15 μA cm^?2 and a nominal sample temperature of 110°C. Glasses with D less than ～1 × 10^?9cm²sec^?1 were fully blistered, whereas those with D greater than ～3 × 10^?8cm²sec^?1 showed no surface deformation. Glasses with diffusivities between ～3 × 10^?8 and ～1 × 10^?9cm²sec^?1 had local regions with low density coverage of relatively large blisters. The critical concentration of implanted helium, estimated by comparing experimental data with results from a simple theoretical model, is ^{～1 × 1019 cm?3}, consistent with high pressure solubility measurements. Reemission data at low fluence are qualitatively in agreement with analytical calculations. Implications for CTR technology are discussed.     

Graphite surface erosion by 100 keV helium and hydrogen bombardment

Surface erosion in pyrolytic graphite by 100 keV ⁴He⁺ and 200 keV H⁺₂ ion bombardment has been observed by scanning electron microscopy. The particle fluence ranged from 1 × 10¹⁷ to 5 × 10¹⁸ particles/cm². Although the surface is eroded at 1 × 10¹⁷ particles/cm² in helium bombardment, it is not eroded so heavily even at 5 × 10¹⁷ particles/cm² in hydrogen bombardment. In helium bombardment flaking is significantly observed at 1 × 10¹⁸ particles/cm², and a cone structure appears at 5 × 10¹⁸ particles/cm², which is produced after the first cover flakes off completely. In hydrogen bombardment at 1 × 10¹⁸ particles/cm², many circular blisters are formed which are sputtered off at 5 × 10¹⁸ particles/cm². The surface roughness of the target also affects the erosion.     

Isothermal Re-Emission of Hydrogen Implanted into Isotropic Graphite

Isothermal re-emission of hydrogen from graphite implanted with 5 keV H₂ ⁺ ion beam up to saturation (3×10¹⁸/cm²) at room temperature has been studied by means of the elastic recoil detection (ERD) technique at temperatures of 450, 500, 550 and 600°C. It is found that the concentration of retained hydrogen decreases rapidly in the beginning and then decreases very gradually as the annealing time increases.

The re-emission profiles have been analyzed taking into account local molecular recombination between activated hydrogen atoms and that between an activated hydrogen atom and a trapped one together with retrapping of the activated hydrogen atom. It is shown that the re-emission of hydrogen by isothermal annealing occurs mainly due to the former type of local molecular recombination and that the activation energy of the thermally activated detrapping rate constant is 0.50±0.04eV. Moreover, it is shown that an analytical expression for the re-emission profile reproduces reasonably well the observed thermal desorption spectra.     

Measurement of the depth distribution of light impurities in first-wall materials: He in Nb

We have measured the concentrations and depth profiles of implanted helium in niobium by a method demonstrated previously with hydrogen and lithium in copper. The three targets, bombarded at room temperature with 10 keV He⁺ at doses of 0.01, 0.16 and 0.98 C/cm⁺, were respectively: unblistered; covered with circular blisters; and marked with “microrelief”, without blisters. The corresponding doses retained in the metal were 0.0076, 0.039 and 0.052 C/cm² (i.e.≈3 × 10²² He atoms/cm³) with a 10% normalization uncertainty. The profile shapes did not change much: in particular we did not observe, as the dose increased, an accumulation near the surface, which is receding by erosion (sputtering, blistering). These results show that a mechanism of helium loss starts operating at a dose ?0.16 C/cm², i.e. before the bursting of blisters (if they burst at all), and it is most effective near the surface.     

Depth distribution and migration of helium in vanadium at elevated temperatures

The depth profiles and retention behavior of ⁴He implanted at 80 keV into vanadium have been investigated in cold rolled bulk and foil samples. The specimens were implanted at temperatures of 100, 400, and 800° C and at fluences up to 1 × 10¹⁸He/cm². Helium depth distributions were found to be Gaussian for the 100° C implants but exhibited a double peak appearance for some of the highest fluence 400° C and 800° C implants. In polished samples blistering and flaking occurred at both 100° C and 400° C implant temperatures for fluences greater than 5 × 10¹⁷He/cm², but little of the implanted helium appeared to have been released at a result of the exfoliation. Conversely, most of the implanted helium was released from 800 C implant areas though no significant blistering or surface perforation was observed. Blistering was observed to occur only on polished samples, through the release behavior was similar to that of the unpolished specimens on which no blistering was observed at any temperature or fluence. The surface condition of samples polished by different techniques was found to exert an influence on the temperature dependence of release and on the character of the blister topography.     

Vacancy-type defects near Al surface studied by slow positron annihilation spectroscopy before and after He implantation

The vacancy-type defects He_nV_m near Al surface before and after He⁺ implantation and their evolutions with annealing temperatures and aging time have been investigated by mono-energy slow positron annihilation spectroscopy (SPAS) with S parameters. The results show that many vacancies are produced during the sample preparation process, which can be re-occupied by Al atoms during annealing, Al⁺ and MeV He⁺ implantation. S parameters denote the concentration and size of He_nV_m clusters induced by He⁺ implantation in Al. The higher fluence of He implanted, the larger S parameters will be, indicating more He_nV_m clusters produced. S parameters decrease with the increase of annealing temperatures until the fastest change temperature, and then an opposite or minor change occurs depending on the fluence of He implanted in Al, showing that the concentration and size of He_nV_m clusters will vary with the annealing temperatures. Aged at RT for some time, the concentration and mean size of He_nV_m clusters in Al will get smaller and larger, respectively, resulting in the decrease of S parameters with the aging time. In conclusions, the evolution of vacancy-type defects He_nV_m near Al surface after He⁺ implantation depends on the annealing temperatures, He concentration and aging time.     

Study of 3He+-bombarded palladium using nuclear magnetic resonance techniques

The nuclear magnetic resonance (NMR) from ion-implanted ³He atoms has been observed in ³He⁺-bombarded palladium. Two 1 μm palladium thin films, one on each side of a copper foil substrate, were bombarded at 75°C with 75 and 140 keV ³He⁺-ions to a fluence of $\text{～10}{}^{18}{}^3\text{He}{}^{\mathrm{+}}\text{/}\text{cm}{}^2$ for each energy. Spin-lattice ($\text{T}{}_1$) and spin-spin ($\text{T}{}_2$) relaxation times were measured at 10, 20, and 35 MHz in the temperature range 1–4K. The ³He nuclear relaxation data indicate that the implanted atoms after the 75°C bombardment are situated in small clusters within the Pd thin films.     

Charge exchange of medium energy H and He ions emerging from solid surfaces

Charge exchange of medium energy H and He ions emerging from clean solid surfaces is studied extensively using a toroidal electrostatic analyzer with an excellent energy resolution. The charge distributions of He ions scattered from sub-monolayers near a surface are non-equilibrated, resulting in a surface peak even for poly-crystal solids. By solving simultaneous rate equations numerically, we derive electron capture and loss cross sections for Ni and Au surfaces. Based on a free electron gas model, non-equilibrated He⁺ fractions dependent on emerging angle reveals uniform electronic surfaces for metals and corrugated surfaces for Si and graphite with covalent bonds. It is also found that equilibrium charge fractions of H⁺ are independent of surface materials (Z₂) and in contrast equilibrium He⁺ fractions depend pronouncedly on Z₂. The data obtained are compared with semi-empirical formulas.     

Synthesis of Amino Acids by 60Co Gamma Irradiation

Measurements of creep by the indentation with Knoop indenter were performed for sintered UO₂ pellets before and after irradiation and a non-irradiated single crystal with an applied load of 50 g. Room temperature creep of non-irradiated sinters included grain boundary sliding. The creep at the lowest dose (7.8×10¹⁴ fiss./cm³) also involved the grain boundary sliding, while no creep was observed after irradiation of 2.9×10¹⁶ fiss./cm³. Irradiated sinters at a dose between 1.4×10¹⁶ and 1×10¹⁸ fiss./cm³ yielded not only primary creep without the grain boundary sliding but also secondary creep, in which cracks were formed when a dose was less than 4.8×10¹⁷ fiss./cm³. At a dose of 1×10¹⁸ fiss./cm³ the enhanced secondary creep independent of the temperature between 100 and 150°C was observed.     

Matrix effects in SIMS analysis of high-dose boron implanted silicon wafers

Secondary ion emission from ¹¹B⁺ implanted silicon wafers with dose of 1 × 10¹²−5 × 10¹⁶ cm⁻² has been investigated. Experiments were performed using O₂⁺ primary ions with an impact energy of 8.0 keV and an incident angle of 39° from the surface normal. The emission of ¹¹B⁺ is enhanced and ²⁸Si⁺ is suppressed at the peak region of boron profile for the high-dose sample, such as at doses ⩾ 5 × 10¹⁵ cm⁻² (peak concentration ∼5 × 10²⁰ cm⁻³). The secondary ion energy distribution of ¹¹B⁺ is broadened and ²⁸Si⁺ is sharpened with increasing the boron concentration. The mechanisms of these phenomena are also considered.     

Optical and electrical characterizations of Mn doped p-type β-FeSi2

β-FeSi₂ has attracted increasing attention as a promising material for optoelectronic and thermoelectronic devices due to a high optical absorption coefficient (α) of about 10⁵ cm⁻¹ near 1.0 eV and its chemical stability at higher temperatures. For the future practical use of this material in devices, the control of each electrical conductivity type and the improvement of the material quality are highly required. Although unintentionally doped β-FeSi₂ layers formed on n-type Si(1 0 0) by the conventional electron-beam deposition (EBD) have typically shown n-type conductivity, the p-type β-FeSi₂ layers were formed by the introduction of Mn impurity using ion-implantation at room temperature (RT) and subsequent annealing procedures. In this study, we aimed to make p-type β-FeSi₂ by implantation of ⁵⁵Mn⁺ ions into EBD-grown n-type β-FeSi₂ layers/n-Si, where ⁵⁵Mn⁺ ions were implanted at two different temperatures (T_sub) of RT and 250°C using an energy and a dose of 300 keV and 2.68 × 10¹⁵ cm⁻², respectively. Their optical and electrical properties, which ought to be affected by implantation and annealing temperatures (T_a2), were investigated by Raman scattering, optical transmittance, reflectance and van der Pauw measurements. The results showed that the ⁵⁵Mn⁺ doping with T_sub=RT and higher thermal annealing at T_a2=900°C produced p-type layers of good quality with maximum hole mobility of 454.5 cm²/Vs at about 65 K.     

The solid film lubrication by carbon ion implantation into α-Al2O3

Improvement in tribological performance by C⁺110 keV implantation can be achieved by having a more graphite-like carbon structure on Al₂O₃. It was shown that fracture toughness and critical peeling load increased for a fluence of 5 × 10¹⁷C⁺/cm² because of residual compression stress and amorphism of surface. The testing in a different implantation dose indicated that the friction and wear mechanism in Optimol fretting wear machine (SRV) was a combination of surface structure and its abrasive wear. Raman shift shows that the amorphous graphite with 5 × 10¹⁷–1 × 10¹⁸ C⁺/cm² implantation dose was formed on Al₂O₃ surface, so that it reduced friction coefficient and wear of Al₂O₃, also it is noticed that the failure of lubrication due to graphite-like film wear is much earlier in the implantation sample with 1 × 10¹⁷C⁺/cm² dose.     

Effects of BF2+ implantation on the strain-relaxation of pseudomorphic metastable Ge0.06Si0.94 alloy layers

Metastable pseudomorphic Ge_0.06Si_0.94 alloy layers grown by molecular beam epitaxy (MBE) on Si (1 0 0) substrates were implanted at room temperature by 70 keV BF₂⁺ ions with three different doses of 3 × 10¹³, 1 × 10¹⁴, and 2.5 × 10¹⁴ cm⁻². The implanted samples were subsequently annealed at 800°C and 900°C for 30 min in a vacuum tube furnace. Observed by MeV ⁴He channeling spectrometry, the sample implanted at a dose of 2.5 × 10¹⁴ BF₂⁺ cm⁻² is amorphized from surface to a depth of about 90 nm among all as-implanted samples. Crystalline degradation and strain-relaxation of post-annealed Ge_0.06Si_0.94 samples become pronounced as the dose increases. Only the samples implanted at 3 × 10¹³ cm⁻² do not visibly degrade nor relax during anneal at 800°C . In the leakage current measurements, no serious leakage is found in most of the samples except for one which is annealed at 800°C for 30 min after implantation to a dose of 2.5 × 10¹⁴ cm⁻². It is concluded that such a low dose of 3 × 10¹³ BF₂⁺ cm⁻² can be doped by implantation to conserve intrinsic strain of the pseudomorphic GeSi, while for high dose regime to meet the strain-relaxation, annealing at high temperatures over 900°C is necessary to prevent serious leakages from occuring near relaxed GeSi/Si interfaces.     

Energy Dependence of MeV He+ Ion-Induced Re-Emission of Hydrogen Isotopes Implanted into Graphite

The energy dependence of MeV He⁺ ion-induced re-emission of hydrogen isotopes (H and D) implanted into graphite has been measured by means of the elastic recoil detection (ERD) technique in order to clarify the collision process for the ion-induced detrapping. The experimental re-emission profiles have been analyzed by solving the mass balance equations, in which the ion-induced detrapping cross section σ _d and the rate constants of the retrapping Σ ^T and local molecular recombination K between an activated hydrogen atom and a trapped one are taken into account. The values of σ _d and K/Σ _T have been determined from the best-fit analytical solution to the experimental re-emission profiles. It has been found that the average values of σ _d and K/Σ _T for H are twice as large as those for D, which is the so-called isotope effect.

It has been shown that the experimental values of σ _d and their energy dependence agree well with the theoretical ones, which are calculated using the power-law approximations for Thomas-Fermi potential, on the assumption that the ion-induced detrapping of hydrogen isotopes takes place due to elastic displacement collisions with energetic carbon recoils produced by incident MeV He⁺ ions.