MeV Heavy Ions Research Articles

Ion-beam mixing of metal-insulator multilayers: a promising technique for the formation of metallic nanophases

The presence of metallic nanoclusters in an insulating matrix leads to non-linear optical properties with potential applications in optoelectronics. Ion implantation is currently used to produce such a composite material with limitations inherent to the implantation process, particularly concerning the doped thickness and the homogeneity of the transformed layer. Here we show that high-energy ion-beam mixing can be applied as an alternative ion beam technique to form metallic nanoclusters without the drawbacks of direct ion implantation. Thin SiO 2-metal multilayers were irradiated at room temperature with MeV heavy ions in order to produce an homogeneous SiO 2 layer containing metallic nanoclusters over the whole sample thickness. The present works deals with the mechanism leading to the formation of metallic nanophases by ion-beam mixing.

Crater formation due to grazing incidence C 60 cluster ion impacts on mica: a tapping-mode scanning force microscopy study

First tapping-mode scanning force microscopy (TM-SFM) observations of swift-heavy-ion-induced topographical craters on muscovite mica are presented. The gigantic cratered hillocks were induced by surface-grazing 20.4 MeV C 60 (fullerene) ions from the Orsay tandem MP accelerator. The operating parameters of TM-SFM were varied in order to show that the observed features were indeed of topographical nature. The greater dimensions of these hillocks compared to those observed with normally-incident MeV C 60 ions further support the occurrence of a radial expansion in the material around each ion track. The great dimensions, and also the formation of a topographical crater on each hillock, are attributed to the greater energy deposition by the C 60 ion impact compared to that of MeV heavy atomic ions.

Formation of metallic nanophases in insulators by high-energy ion-beam mixing

Composite materials consisting of nanometer-sized metallic clusters embedded in a dielectric matrix exhibit interesting optical properties with potential applications to non-linear optics and optoelectronics. Ion implantation appears to be a very promising technique to generate such clusters, but it suffers redhibitory limitations mostly owing to the thickness and homogeneity of the doped material. This paper presents a new methodology, also based on ion-beam processing, used to overcome these drawbacks: high-energy ion-beam mixing. Alternated metallic and insulator multilayers are irradiated with MeV heavy ions at room temperature. The amount of mixing and the homogeneity of the mixed layer are determined by PBS experiments performed in situ during the mixing process. The formation of metallic nanoclusters is characterized by X-ray, electron microscopy and optical absorption experiments.

Radical formation in the radiolysis of solid alanine by heavy ions

Radical formation in solid α-alanine irradiated with 175-MeV Ar 8+ and 460-MeV Ar 13+, 220-MeV C 5+, and 350-MeV Ne 8+ ions were studied by the ESR method. The radical yield (number of radicals per incident ion) is constant below the critical fluence of about 10 10 ions cm −2 for the Ar ions, 10 11 ions cm −2 for the C ion, and 5 × 10 11ions cm −2 for the Ne ion. Above the critical fluence, the yield decreases with increasing ion fluence. G-value of the radical formation was obtained from the constant yield at the low fluences. The G-value is not a simple function of LET. This is ascribed to the difference in lateral dose distribution of ion tracks. Assuming a simple cylindrical shape of the ion tracks, the average dose in and the radius of the ion tracks were estimated from the G-values. The radius is 8–25 nm, which is larger than the radius of 2–5 nm for 0.5–3 MeV H + and He + ion-irradiations. The fluence-yield curves were simulated with the cylindrical tracks and with the dose-yield relationship for the radical formation in γ-irradiated alanine. The simulated curves agree well with the experimental ones. With the cylindrical model of ion tracks, the variation of the radical yield at the high fluences can be estimated for solid alanine irradiated with several hundreds of MeV heavy ions.

Low-mass secondary-ion ejection from molecular solids by MeV heavy ions: Radial velocity distributions.

Secondary ions sputtered in individual MeV ion impacts are analyzed in a high resolution time-of-flight mass spectrometer. The initial radial velocity distributions of low mass (up to m/z\ensuremath{\approxeq}300 u) positive and negative secondary ions, sputtered from carbon-containing molecular solids (polymers, bioorganic molecules, and fullerene targets) are investigated. The first (〈v〉) and second (〈${\mathit{v}}^{2}$〉) moments of the velocity distributions vary systematically with the atomic composition of secondary ions of the type ${\mathrm{C}}_{\mathit{n}}$${\mathrm{H}}_{\mathit{m}}^{+}$, ${\mathrm{C}}_{\mathit{n}}$${\mathrm{F}}_{\mathit{m}}^{+}$, ${\mathrm{C}}_{\mathit{n}}$${\mathrm{H}}_{\mathit{m}}$${\mathrm{F}}^{+}$, and ${\mathrm{C}}_{\mathit{n}}$${\mathrm{H}}_{\mathit{m}}$${\mathrm{O}}^{+}$. Positive ions formed from extensive fragmentation-rearrangement of the original molecular structure (e.g., ${\mathrm{C}}_{\mathit{n}}^{+}$) tend to be ejected towards the MeV ion trajectory (positive mean radial velocity) and to have the largest 〈${\mathit{v}}^{2}$〉. Saturated species (e.g., ${\mathrm{C}}_{\mathit{nX}2\mathit{n}}$, X=F, H) tend to have smaller 〈${\mathit{v}}^{2}$〉 and negative 〈v〉. These effects are weaker as the stopping power of the primary ions is decreased and are not observed for negative ions. The observed effects demonstrate a correlation between chemical composition of an ion and its formation and ejection processes. The chemical transformations and the processes leading to ion formation and ejection are functions of both the density and the gradient of the deposited energy at a particular position from the track center. This interconnection results in a regular dependence of the properties of ejected ions (e.g., momentum received) on their chemical composition. The correlation of the momentum imparted to the fragment ions with the geometry of impact indicates that such species are predominantly ejected in a nonevaporative process. \textcopyright{} 1996 The American Physical Society.

Energy loss of MeV heavy ions in carbon

A novel technique, using secondary forward recoil ions generated in thin standard targets by energetic very heavy primany ion beam, has been employed to measure energy loss of several ion species (Z 1 = 8 to 29) covering energies between 0.1 and 1.0 MeV/u. Energy variation was simp y affected by changing the detector angle between 35° to 70°, avording the dominant elastic scattering of the primary beam. A twin detector permitted simultaneous measurement of both unabsorted and absorbed spectra. The experimental data on stopping power are compared with evaluated values using LSS theory and TRIM. A better understanding of the validity of available theoretical evaluations has become possible, with a need for higher precision in experimental data in some cases.

Damage in metals from MeV heavy ions

Despite the long-held belief that MeV-ion initiated damage in metals arises only from nuclear recoils, recently it has been convincingly established that at high levels of ion energy loss to electronic excitation (∼ 1 keV/Å) the response of some metals is significatly influenced. In this paper I present a simple model of how this occurs that is in good agreement with the extensive experimental data available for damage produced by GeV heavy ions in Fe. The parameters of the model behave in a reasonable fashion for other metals where these effects are observed (Ti, Zr, Co, Nb, and Pt), which suggests that more detailed measurements on these materials could provide a sseful check on the model's generality.

Damaging of C60 films by MeV heavy ions

Thin films of C60 (99.99% purity) have been irradiated with 55 MeV 127I10+ ions in a fluence range from 2.5 × 109 to 3 × 1012 ions/cm2. Two methods have been employed to assess the modifications induced by the MeV ion irradiation: in situ plasma desorption mass spectrometry analysis and off-line micro-Raman spectroscopy. The yields of secondary low mass carbon cluster ions and intact C60 ions have been determined at different fluences. Damage cross-sections for the C60 molecules have been extracted from the ion yield curves as a function of MeV ion fluence. The secondary ion spectra of the C60 targets also show peaks corresponding to larger fullerenes with masses at least up to 2500 u. The evolution of the ion yields of these higher mass carbon clusters gives an evidence that they are not originally present in the target. They may be formed by coalescence reactions of C60 molecules as a result of an individual MeV ion impact.

Surface swelling of sapphire induced by MeV and GeV heavy ions

α-Al 2O 3 single crystals were bombarded with MeV xenon ions from 10 15 to 10 17 ions cm −2 and GeV uranium ions from 10 11 to 10 13 ions cm −2 to study the surface swelling of sapphire at 77 and 300 K due to atomic collision processes (Xe) and electronic energy loss processes in the 20–45 keV/nm regime (U). The induced damage was studied by channeling Rutherford backscattering. Surface swelling was measured with a profilometer. The step height induced by nuclear cascades of MeV xenon increases with the ion fluence and saturates. With GeV uranium, an electronic stopping power threshold for surface swelling was observed and the step height increased with the damage for d E/d x higher than this threshold.

Generation of point defects in crystalline silicon by MeV heavy ions: Dose rate and temperature dependence.

Si(100) samples have been implanted with low doses (10 7 -10 9 cm -2 ) of MeV 76 Ge and 120 Sn ions. Deep level transient spectroscopy was used for sample analysis, and the generation or vacancy-related point defects is round to increase with increasing implantation temperature and to decrease with increasing ion dose rate. These results are in direct contrast to that for damage buildup at high doses (≥10 12 cm -2 ), and the effect is attributed to rapidly diffusing Si self-interstitials which overlap and annihilate vacancies created in adjacent ion tracks

Convoy electrons produced at glancing-angle incidence of MeV heavy ions on crystal surfaces

Convoy electrons produced at glancing-angle incidence of MeV H, He, Li and C ions on a SnTe(001) surface are studied. Acceleration of the convoy electrons is observed. The acceleration increases with increasing atomic number of the projectile ion and with decreasing ion energy, in harmony with the surface-wake-acceleration model. The acceleration of convoy electrons by the surface wake is calculated with a classical-trajectory Monte Carlo simulation. The calculated results reproduce the characteristic features of the observed convoy electrons showing the validity of the surface-wake-acceleration model.

Heavy ion RBS analysis of strained layer superlattices

The extension of Rutherford backscattering spectrometry (RBS) to heavier mass projectiles has several advantages over conventional RBS. To understand the interaction of heavy ions with solids properly, a systematic study of the energy straggling of MeV heavy ions has led to an empirical formula for this term. This formula, along with the stopping power, lateral spread and multiple scattering, predicts the energy width for a wide range of energies, projectiles and targets, allowing the development of realistic computer simulation for the energy spectra. This model has been verified by measurements of the depth resolution of 4 and 6 MeV C projectiles using Si/Si 1− x Ge x /Si strained layer superlattices (SLSs) samples.

MICRO PIXE ANALYSIS USING AN X-RAY FROM INCIDENT IONS

A possibility of using an X-ray from incident ions in micro PIXE measurement with MeV heavy ions was discussed. To reveal the feature, a Cu-Ti plate was analyzed by a 5 MeV silicon and a 5 MeV nickel ion microprobe and the PIXE-mapping images of the X-rays from the incident ions were compared with the normal PIXE-mapping images of the X-rays from the target. These images indirectly reflected the distribution of a specific element with good statistics.

High-resolution transmission electron microscopy study of the radiation damage defects in high temperature superconductors

High resolution transmission electron microscopy (HREM) reveals that MeV heavy ions like Au generate radiation damage defects, a few nanometers in size, in surface regions (about 100 Å) of the superconductors. Much larger defects have been found in the deeper regions of the superconductor sample, which is believed to be due to the larger damage cascade created by the slower ions after their electronic energy loss along the track. Monte Carlo simulation (trim) shows the same result in damage size as the HREM study. Light ions, like protons, generate mostly point defects, which are not visible to current electron microscopy. These point defects, although they can pin the flux, are not ideal for flux pinning, due to their small sizes.

Enhanced depth and mass resolution with HIRBS

The extension of Rutherford backscattering spectrometry (RBS) to heavier mass projectiles (HIRBS) has been limited, as these projectiles cause much more radiation damage in the detectors and curtail their lifetime. Despite this limitation interest in the use of heavier projectiles continues as there are several significant benefits which can accrue from their use. To properly understand the interaction of heavy ions with solids a systematic study of the energy loss and straggling of MeV heavy ions has been conducted and an empirical expression for these terms has been obtained. This expression has allowed the development of a realistic computer simulation which accurately predicts the energy spectra for a wide range of energies, projectiles and targets. In parallel with that study, measurements of the depth resolution of Si/Ge multilayer films using 4–6 MeV C projectiles have been used to verify the simulation.

Pulse height response of Si surface barrier detectors to 5–70 MeV heavy ions

An extensive series of pulse height measurements have been performed in partially depleted Si surface barrier detectors, using various heavy ions (Li, B, C, O, Al and Cl), at energies between 5 and 70 MeV. After correcting for the small energy loss of the incident ions in traversing the gold surface barrier layer of the detector and for the residual nuclear stopping, the resulting pulse heights per MeV for the various heavy ions were found to be up to 2.5% larger than for the 241Am (5.486 MeV) alpha particle. This increase, although significant, is smaller than had been anticipated from an extrapolation of the earlier study of H, He and Li pulse heights by Lennard et al. [Nucl. Instr. and Meth. A248 (1986) 454, and B24/25 (1987) 1035.] A new method of analysis of pulse height data, which significantly reduces the uncertainties associated with the dead layer energy loss and nuclear stopping corrections, was used in order to determine directly the variation of the average energy for electron-hole pair creation with increasing particle stopping power in Si. The dependence of the detector energy resolution (FWHM) on the energy and atomic number of the particle has also been measured in order to provide a suitable data base for selecting the optimum ion beams for various backscattering analysis problems.

CALCULATION OF THE MEAN PROJECT RANGE OF MeV HEAVY IONS IN SOLID TARGETS

Based on Biersack's angular diffusion model, an effective method was established for calculating the mean project range of MeV heavy ions in solid targets. The calculated values are compared with recent experimental data on the mean project range for 1.0 MeV In+, Xe+, Ta+ and 1.0-2.0 MeV Pb+ The maximum difference-between present calculated values and experimental data is less than 8%, but the maximum differences between experimental data and calculated values by TRIM 86 and PRAL are 23% and 22%, respectively- The results show that the present calculation procedure is sucsessful for predicting the mean projected range on 1.0-2.0 MeV such heavy ions as In+, Xe+, Ta+ and Pb+ implanted in solid target (Si).

Radiation annealing in Ni and Cu by heavy ion irradiation

Stage I recovery in Ni and Cu irradiated with various ions has been measured using electrical resistivity changes. The fraction of its recovery can be scaled by the logarithm of PKA median energy or the electronic stopping power. A new formulation describing the defect production and radiation annealing has been proposed. Radiation annealing of pre-doped stage-I defects by 100 MeV I-ion irradiations has been studied in Ni and Cu through the defect production rate measurements and using this formulation. The anomalous reduction of stage I recovery and the large cross sections for radiation annealing have been found in Ni irradiated with ∼ 100 MeV heavy ions; they are interpreted as due to the electron excitation by irradiating ions and the subsequent energy transfer from excited electrons to lattice atoms.

Three-dimensional microanalysis using a focused MeV oxygen ion beam

A focused MeV ion beam line for nearly any element has been developed by combining a focusing system with a beam line of a tandem-type accelerator. A minimum beam spot size of 5.6 μm × 8.0 μm has been obtained for 2 MeV O + ions. This system has realized ion beam processing such as maskless MeV ion implantation and ion beam microanalysis by Rutherford backscattering (RBS) using heavy ions. RBS-mapping measurements of two- or three-dimensional structures with MeV heavy ions were demonstrated.

Single-particle techniques

The high radiation damage of MeV heavy ions does not only destroy targets, it can also be used to produce microstructures in an unusually wide range of materials by a combination of irradiation and chemistry. Unlike any other type of radiation used for microlithography, already a single ion is able to produce a microstructure, usually an etch cone of various opening angles. If one is able to control the position, the length and the shape of an etched nuclear track, this method may be a valuable complement to the already existing microlithographic tools. Unfortunately microlithography with heavy ions does not work the usual way: The use of irradiation masks is hampered by the fact that masks of sufficient thickness cannot be produced economically and, if they could be produced, every ion scattered in the mask could give rise to a defect. Therefore we decided to avoid the use of masks and to write microstructures directly by a heavy ion microbeam which has the additional advantage that it can be almost free of scattered particles. To compensate for the economic disadvantage of that slow writing method, we started using single precisely aimed ions, because there are microstructures which can exclusively be produced that way. As it turned out later, other applications in microelectronics and biology may also profit from single particle techniques and the tools developed for single-particle techniques may be useful for a much wider range of applications.