Helium Bombardment Research Articles

Clustering of Silver Nanoclusters Embedded in Soda Lime Glasses Using Ionic Exchange and Helium Ion Bombardment

Silver nanocluster precipitation in AR Schott glass by subsequently silver ionic exchange and helium bombardment was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV-visible optical absorption. Helium ion bombardment was used to induce local defects in the matrix and to promote the growth of the silver nano-aggregates. The typical implantation depth was estimated at 1 μm by Trim simulation. SEM investigations give us the concentration profile of the exchanged samples and the maximum depth. Optical absorption was performed to visualize the effect of the He+ fluence on the ion exchanged sample spectra and compared to the Drude model varying size and matrix refractive index. TEM was used to evaluate the distribution size of the silver nanoparticles, their structure by diffraction pattern, size, and shape and to correlate it to the experimental and theoretical absorption curves. The TEM observations prove that we are in a confinement regime with a particle size below the mean free path of the silver bulk metal.

Electrical characterization of Fe-doped semi-insulating InP after helium bombardment at different implant temperatures

The effect of helium ion bombardment into semi-insulating (SI) InP at room temperature (RT) and 200 °C has been investigated with various ion doses in the range of 4×1012 to 1×1016/cm2 at 600 keV. The sheet resistivity of SI InP decreases by at least an order of magnitude (∼1×107 Ω/□) after RT helium bombardment at different doses. In the case of the 200 °C implant, the sheet resistivity decreases by less than an order of magnitude. We infer that the hot implant creates less donor defects as compared to RT implants. Samples irradiated with a helium dose of 1×1014/cm2 at RT were annealed from 100 °C to 800 °C. The sheet resistivity remains constant up to an annealing temperature of 500 °C but decreases in the temperature range of 600 °C–650 °C and is increased by annealing to higher temperatures. The removal of defects varies with annealing temperature. These results can be used to choose the right implant conditions before the InP substrate becomes conductive for effective electrical isolation of In-based devices.

Electrical isolation of n-type InP layers by helium implantation at variable substrate temperatures

The electrical isolation in n-type InP layers produced by helium bombardment was investigated for substrate temperatures: room temperature (RT), 100 and 200 °C. Semi-insulating InP wafers of (1 0 0) orientation with Fe doping were implanted with multi-energy 28 Si atoms to create a flat dopant distribution. A uniform damage density was formed within the conductive layer by 2×10 14 cm −2 helium implantation at 600 keV to isolate the structure. Hall effect and resistivity measurements were performed in order to study the evolution of the sheet resistivity in n-type InP layers implanted at three different temperatures and the stability of the formed isolation during post implantation annealing. An isolation resistance of more than 10 6 Ω/□ was achieved for as-implanted material. The samples were annealed in the range 100–800 °C and an optimum isolation of ∼10 7 Ω/□ was achieved for samples implanted at either RT or elevated temperatures after annealing at 400 °C. Annealing at higher temperatures returned the resistivity to a value close to that of the starting material and this recovery of conductivity was similar for all three cases. The isolated regions exhibited good stability to heat treatment up to 500 °C and again this annealing window for the thermal stability of the obtained isolation was the same for all cases irrespective of the irradiation temperature. Based on the obtained data we infer that for this particular isolation scheme, the implantation fluence is at its threshold. This provides optimum isolation and quite broad thermally stable region due to the formation of such defects during implantation, which are stable to higher annealing temperatures. This data also suggests that for this scheme, the implant temperature is an insensitive parameter. The magnitude of isolation and its characteristic curves in this case are very much similar to p-type InP, which is known to behave differently from n-type InP. This feature makes the data technologically relevant and may have some ramifications for the device engineering industry.

Review of recent works in development and evaluation of high-Z plasma facing materials

This paper reviews the recent activities and results in development and evaluation of new high-Z materials focusing on tungsten and its alloys. VPS-W coating on graphite and CFC showed superior high heat load properties by inserting sophisticated multilayer diffusion barriers (W/Re) between tungsten and substrate. For successful application it is necessary to know the temperatures for recrystallization and interdiffusion of rhenium and tungsten, which may degrade bonding properties. Sputtering by plasma impurities and damage by energetic charge-exchange neutrals must be considered. Retention of hydrogen isotopes strongly depends on material grade and production process. Dense bubbles and dislocations formed by helium bombardment may act as effective traps for hydrogen. It is anticipated that long range diffusion of helium induces embrittlement. Neutron irradiation is one of the major concerns for tungsten materials in fusion reactor application. The tungsten materials developed so far perform poorly under neutron irradiation and their brittle nature could limit their application as a fusion material. Further efforts are needed to improve ductility while keeping other desirable properties such as high thermal conductivity.

Secondary-ions from atomic collision processes in solid surfaces

Using time-of-flight technique (TOF) we investigated the secondary ion emission from an uncleaned carbon surface induced by fast helium bombardment near the maximum of electronic stopping power ( dE dx ) e . We examined the production of secondary ions dependend on projectile properties only, as specific projectile energy ( 50 keV/u ≤ E P m P ≤ 500 keV/u ) and incident charge state (0 ≤ q i ≤ 2), but not on target properties, as surface conditions. The “pre-equilibrium stopping power” plays a major role in secondary ion production in the examinated projectile velocity regime.

Ion-induced release of deuterium from co-deposits by high energy helium bombardment

Ion-induced release of deuterium from co-deposits by high energy helium bombardment

Ion-induced release of deuterium from co-deposits by high energy helium bombardment

Ion-induced release of deuterium from thick and thin co-deposits formed on plasma facing surfaces in a tokamak and from thick hydrogenated carbon films by MeV 3He + beams was studied. The layers of different thickness and D content, up to 2 × 10 19 cm −2, built on graphite, CFC, Ni, Ti, V and Mo under different operation conditions (i.e. heating mode or wall composition in the machine) were irradiated with 3He ions in the MeV range, 0.8–1.5 MeV and a total fluence exceeding 1 × 10 17 He + cm −2. The D content was determined by nuclear reaction analysis (NRA) and the films, prior and after the irradiation, were also examined by RBS and EDS. The amount released was found to be dependent on the layer elemental composition: 55–60% from the carbon and boron containing films and only 10% from those containing silicon. Effective detrapping cross-sections have been calculated on the basis of the results obtained.

The role of surface microstructure in the sputtering of graphite

Extensive exposure to tokamak plasmas may result in significant alterations to the surface microstructure of graphite plasma-facing components. A change in microstructure from a commercial isotropic graphite to an amorphous carbon film may produce a significant change in the total sputtering yield and the level of plasma contamination. To investigate this sensitivity to surface microstructure, sputtering experiments on a variety of graphites with various surface structures were performed using the ion–surface interaction system (ISIS).1 ISIS is a computerized ion beam sputtering system equipped with twin quartz crystal microbalances capable of simultaneously monitoring both sputtering and redeposition of the beam target material. ISIS was used to obtain sputtering data on two orientations of pyrolytic graphite at seven energies between 100 eV and 10 keV. Helium bombardment perpendicular to the prism plane produced yields 2 to 7 times higher than on the basal plane. Proton bombardment perpendicular to the prism plane produced yields 45% higher than those on the basal plane. Amorphous graphite films produced from Poco AXF-5Q and Union Carbide ATJ graphites using an argon radio-frequency (rf) plasma discharge were also irradiated. Sputtering yields on the amorphous films were as much as 50% to an order of magnitude higher than those measured on commercial bulk samples. Pre and post-irradiation scanning electron microscopy of selected targets was performed to monitor surface microstructure. A structural mechanism responsible for the magnitude of physical sputtering is suggested, and an effective surface binding energy is introduced to quantify this structural dependence.

Sputtering properties of redeposited graphite surfaces

Sputtering yields for carbon redeposited films, put down in the presence of high neutral hydrogen and helium background concentrations, have been measured. The data were obtained using the ion-surface interaction system (ISIS) [1] which is an ion beam sputtering system capable of creating redeposited films and measuring sputtering yields. Yields were determined by calibrated collection of a portion of the sputtered material onto a quartz-crystal-microbalance. Incident ion beam energies ranged from 100 eV up to 10 keV. Sputtering yields for hydrogen and helium bombardment of redeposited films created in ISIS from targets of Union Carbide ATJ graphite are reported. In addition, yields obtained from ISIS proton and deuteron bombardment of Poco AXF-5Q graphite surfaces previously modified in PISCES [2] are also presented. Measurements of sputtering yields from pristine, bulk samples are reported for comparison. Hydrogen sputtering yields from redeposited films generated in ISIS are 2.5 times higher than those of pristine ATJ at an incident energy of 100 eV. Above 200 eV, the hydrogen yields are a factor of 1.4 higher for the redeposited material. Helium yields are 34 times greater for redeposited films at 100 eV and remain as much as 10 times greater above 500 eV. Curve-fits to the data, obtained by incorporating an effective surface binding energy for the redeposited film into a semi-empirical yield expression, are also presented.

Helium implantation effects in hard hydrogenated carbon layers

Hard amorphous films of a-C:D were deposited by a rf glow discharge in CD4. The implantation of 40-keV 3He+ ions in these films was studied as a function of the helium fluence at temperatures between 100 K and room temperature. The trapped amounts of helium and the deuterium losses in the layer were measured in situ using nuclear reaction analysis with 0.5-MeV D+ and 1-MeV 3He+ beams, respectively, and subsequent ex-situ elastic-recoil detection analysis. A transient helium retention appears in the carbonized layers, occurring only at temperatures below 200 K. Above a critical fluence which depends on temperature and ion flux, outdiffusion of He is observed which is ascribed to the formation of diffusion channels by radiation damage. The deuterium depletion induced by helium bombardment below 200 K sets on in correlation with the helium outdiffusion, but is otherwise independent of the helium trapping.

Ellipsometric and Rutherford backscattering characterization of low-energy hydrogen-, helium-, neon-, and argon-bombarded silicon

Ellipsometry and Rutherford backscattering/channeling were used to study silicon surfaces modified by low-energy hydrogen, helium, neon, and argon ion bombardment. Ellipsometry was found to be a very sensitive technique for characterization of these samples, able in some cases to simultaneously distinguish the ion identity, incident energy, and total dose experienced by the material. A comparison of the ellipsometric results for the different cases indicates qualitative differences in the physical modifications induced in the silicon surface region under bombardment by different ions. Comparison of the ellipsometric results with Rutherford backscattering/channeling data yielded estimates of the complex refractive index of the silicon layer modified by hydrogen and helium bombardment, showing a general decrease in the refractive index and an increase in the optical extinction coefficient upon bombardment. It was possible to explain these changes, at least qualitatively, by using the Bruggeman effective medium approximation to estimate the volume composition of this layer subject to certain simplifying assumptions. One result of this was the development of the exact solution for the three-component Bruggeman analysis; these equations are derived in the Appendix.

Morphology of micro-cavities in nickel during Helium bombardment and post-irradiation annealing

Abstract Changes in sizes and morphology of small cavities in nickel irradiated by 20 keV helium ions at 700°C were investigated during irradiation and on annealing after that by means of transmission electron microscopy. During irradiation, the first observable cavities were in the form of spheres, which changed to cubes bounded by {100} planes and finally to cuboctahedra by the growth of {111} facets from cube corners. On annealing at 700°C after an irradiation of medium dose, cubic cavities changed to octahedra, slightly increasing their volumes, and at 775°C they changed to roundish cubes, remarkably increasing their volumes. The morphological changes during and after irradiation were explained by the effect of the internal gas pressure.

Soft X-rays from neon implanted in aluminium under MeV helium bombardment

Neon gas samples and aluminium samples implanted with 25 keV Ne ions have been irradiated with 2.8 MeV 4He ions. Neon K α 1,2 X-rays spectra from ionization show the presence of wide band structures in implanted metals. Two components are present, a 2 eV wide band attributed to the valence band of overpressurized liquid (or solid) neon and a peak attributed to the presence of gas bubbles. Measurements show the effect of radiation damage. Soft X-ray spectroscopy is presented as a new tool for the study of the physical state of rare gases implanted into metals.

Analysis of near-surface tritium in materials by elastic recoil detection under MeV energy helium bombardment

The method of elastic recoil detection, using 1.5–2.2 MeV energy 4He beams, has been used for the first time to detect tritium. Test samples were fabricated by thermal sorption of tritium into thin (5 μg cm −2) Ti films and by implantation of low energy (10 keV) HT + ions into amorphized silicon and oxide targets. Distributions of surface and subsurface tritium has been profiled to a depth of a few tenths of a micron, with a depth resolution of 10 to 20 nm and a sensitivity better than 10 15 atoms cm −1.

Ion beam induced epitaxial crystallisation of silicon

It is shown that epitaxial crystallisation of amorphous silicon layers can be induced by irradiating samples with energetic light ions at temperatures in the range 673–723 K. This epitaxial process is shown not to be a consequence of macroscopic beam heating. Under certain irradiation conditions Rutherford backscattering/channeling analysis suggests that the regrown silicon is nearly perfect single crystal but TEM analysis, while confirming epitaxial growth, reveals the presence of extended defects which result from the elevated temperature helium bombardment. Evidence is presented to support the view that ion beam induced epitaxy is a consequence of mobile, ion irradiation induced defects created at, or near, the crystal-amorphous interface.

High resistivity in InP by helium bombardment

Helium implants over a fluence range from 1011 to 1016 ions/cm2, reproducibly form high resistivity regions in both p- and n-type InP. Average resistivities of greater than 109 Ω cm for p-type InP and of 103 Ω cm for n-type InP are reported. Results are presented of a Monte Carlo simulation of helium bombardment into the compound target InP that yields the mean projected range and the range straggling.

Surface structure of silicon carbide irradiated with helium ions with monoenergy and continuous energy distributions

Silicon carbide was irradiated at room temperature and 600 °C with helium ions with monoenergy (3–100 keV) and continuous energy distribution ranging from 0.5 keV to 20 keV up to a total dose of 1×1019 ions/cm2 using an ion accelerator controlled by a microcomputer. For room temperature irradiation, porous structures appear on the surface of samples irradiated to dose of ∼5×1018 ions/cm2, and the damaged layer is easily removed from underlying undamaged layer by ultrasonic vibrations. On the other hand, for target temperature of 600 °C, such porous surface structures are not observed, which is due to thermal annealing. It has been shown that the surface deformation depends strongly not only on the implantation profile, but also on irradiation modes of helium ions. Flaking is completely avoided by prebombardment of helium ions with continuous energy distribution, which deliberately produces a pathway for gas release. In order to simulate wall erosion due to helium bombardment, silicon carbide was also irradiated with a Maxwellian distribution. No blistering or flaking is observed for an average energy of 3 keV.

On the low-temperature nucleation and growth of bubbles by helium bombardment of metals

A model for the nucleation and growth of helium bubbles at low temperatures is described. The model characteristics are based on recently published results of helium desorption experiments and on atomistic calculations, which indicate that mass transport is achieved by punching out and migration of self interstitials. Simulations are performed up to a helium content of 30 at % resulting in the prediction of the bubble density, bubble volume, fraction of helium in bubbles and point defect concentration. The results show good agreement with experiment and a possible mechanism of blistering is discussed.

He pumping in the presence of an H beam

In a tokamak, one of the major questions concerning the applicability of passive pumping of helium ions accelerated across the plasma sheath involves the effect on helium retention of the concurrent bombardment of the surface with D/T ions. We have investigated helium trapping and critical fluence behavior during simultaneous 2 keV hydrogen and 4 keV helium bombardment in nickel and beryllium. In nickel, the helium trapping behavior appears unaffected by simultaneous H bombardment, reaching saturation trapping levels of ~1.0 × 10 17 helium cm −2. In contrast, similar measurements on beryllium indicate that the simultaneous hydrogen bombardment significantly reduces He trapping.

Blistering of molybdenum-base alloy TZM under helium ion bombardment

Polycrystalline TZM alloy samples have been irradiated at room temperature with helium ions of energy from 75–350 keV in high vacuum. Irradiation parameters such as ion energy, total dose, and ion flux were varied, and the surfaces were examined in a scanning electron microscope. The critical dose for blister formation was found to lie between 4 and 7×1017 ions cm−2 and is independent of the beam flux in the range 6.7×1013–2×1015 ions cm−2 s−1. However, within this flux range the average blister diameter increases with the flux and the blister number density, and the exfoliation maximizes at an intermediate flux. Blister-cover thickness for TZM is found to be less than that for molybdenum, although the average blister diameter for the two materials differs only at the highest energy used. When the TZM surface is successively bombarded with varying ion energies, blistering is reduced compared to monoenergetic helium bombardment, but this reduction is not as substantial as for pure molybdenum.