Random stopping power and energy straggling of O ions into amorphous Si target

In the present contribution, we report results on random stopping power and range straggling of ¹⁶O ions into amorphous Si target. The measurements were performed in a 300 keV–13.5 MeV energy interval by using the Rutherford backscattering technique together with a marker system. The present stopping results were compared with the TRIM predictions based on the Ziegler, Biersack and Littmark calculations, the theoretical values being always higher than the experimental ones. On the other hand, the calculated straggling data are compared with the Bohr predictions. For low energies, the calculated values over-estimate the experimental ones. For energies larger than 2 MeV, the experimental results became larger than the Bohr predictions. However, with increasing energies, the experimental results approach the Bohr values, and above 12 MeV a quite good theoretical experimental agreement was found.     

Electronic stopping power of swift heavy ions in carbon

The mean stopping power of He, Be, C and Al in carbon has been studied in a continuous energy ranging from 100 to 800 keV per nucleon using a modified time of flight-energy elastic recoil detection analysis set-up. In order to eliminate the calibration problems of Si detectors associated with heavy ions, the time of flight section was used to measure recoil energy both before entering and after passing through the stopping foil. Consequently, the energy dependence of the stopping power was achieved with high precision. The effect of the foil thickness on the mean stopping power was investigated by tilting the stopping foil at different angles to the incoming particles to increase the effective foil thickness up to 22%. The stopping curves obtained at different tilting angles show a similar energy dependence with scattering less than 0.8% from each other, and no obvious thickness dependence is observed. Both the stopping power and the energy dependence are in good agreement with the literature data. Comparing with semi-empirical SRIM prediction, considerable discrepancies up to 6%, 10% and 8% in stopping values for Be, C and Al, respectively, are observed.     

Recent results in stopping power for positive ions, and some critical comments

An overview is given of developments in the field of stopping power during the last few years, with particular emphasis on cases that seem to require further work. We discuss experimental results showing that stopping power for low energy protons, in metals and insulators, is not necessarily proportional to velocity. We mention that there is no discrepancy between the mean ionization potential derived from optical data or from stopping power. The problem of the stopping power of liquid water is discussed, considering both experiment and theory, and we find that recent experimental results and one of the theories are apparently too low. We discuss various types of theories for stopping power. We stress that in analyzing bulk backscattering spectra for stopping power, a low energy limit of validity must be stated. Using statistical analysis of experimental data for heavy ions in elemental solids, we find that the ATIMA and SRIM programs and the Hubert table are best in different regions of specific energy.     

Pair production and bremsstrahlung contributions to the stopping of relativistic heavy ions

We examine the energy loss to electron–positron pairs and bremsstrahlung for relativistic heavy ions penetrating matter. Pair production is the dominant source of loss at extreme projectile energies, and already at energies of a few hundred GeV/u it may add several per cent to the stopping power. An analytical formula for the pair-production contribution to the average energy loss is produced. In bremsstrahlung coherent action of the constituents of each collision partner is required in order to keep the nuclei from breaking up. This limits the emission substantially at high energies compared to the emission in collisions between pointlike non-composite but otherwise similar objects. A simple formula for the bremsstrahlung loss is presented. We conclude with remarks on the statistics of the energy-loss processes.     

Determination of the electronic energy loss of light ions in a silicon lattice by using the transmission ion channeling method

Charged particle activation analysis (CPAA) is able to analyze light elements such as carbon and oxygen at trace levels in semiconductor materials. This technique requires the knowledge of the stopping powers of these materials for channeled ions. The electronic energy loss for ions entering the crystal lattice in a random direction is well established. The electronic energy losses for protons, deuterons, ³He⁺ and ⁴He⁺ ions entering a 3.6 μm thick silicon single crystal along the 1 0 0 direction were measured by using the transmission of particles technique. Data obtained were compared with those obtained by other authors using theoretical and experimental methods.     

Mean excitation energy for the stopping power of light elements

We have evaluated the mean excitation energy or I value for Coulomb excitations by swift charged particles passing through carbon, aluminum and silicon. A self-consistent Kramers–Kronig analysis was used to treat X-ray optical spectra now available from synchrotron light sources allowing us to carry out Bethe’s original program of evaluating I from the observed dielectric response. We find that the K and L shell are the dominant contributors to I in these light elements and that the contribution of valence electrons is relatively small, primarily because of their low binding energy. The optical data indicate that Si and Al have nearly equal I values, in contrast to Bloch’s Thomas–Fermi result, I ∝ Z. The optically based I values for C and Al are in excellent agreement with experiment. However, the dielectric-response I value for Si is 164 ± 2 eV, at variance with the commonly quoted value of 173 ± 3 eV derived from stopping-power measurements.     

Fundamental effects and non-linear Si detector response

Non-linearity in the energy response of a Si p-i-n charged particle detector has been studied for incident particles with Z₁ between 3 and 26, and energies between 0.1 and 0.7 MeV per nucleon. Although the data closely followed a straight line relations, fitting of the data to a third order polynomial revealed that the response exhibited a persistent curvature that acted to reduce the energy interval spanned by a channel as the energy increased. The curvature increased as Z₁ increased from 4 to 8 and then systematically decreased. The curvature is larger and has the opposite energy dependence to the stopping in a dead entrance window and the energy deposited in non-ionising processes within the active layer. The plasma recombination dependence on the average stopping along the plasma column may account for the reduction in curvature as Z₁ increases from 9 to 25 but cannot explain the net effect. The low-energy increase in energy channel span, which has also been reported by others, might be associated with electron excitation in resonant and direct classical quasi-elastic collisions for low-energy ions, or less likely, electronic non-linearity's associated with Z₁ and energy dependent time structure in the current pulse from the detector. Simple interpolation of the window-loss corrected polynomial coefficients is the best approach if the calibration for Z₁ cannot be established directly.     

Molecular dynamics simulation of fluorine ion etching of silicon

Formation of precursors and desorption of etching products by fluorine ion irradiation were studied using molecular dynamics (MD) simulation. When F atoms impact sequentially on a Si substrate, a mixed layer of F and Si atoms is formed on the surface. When the incident energy is below 30 eV, the fluorine coverage reaches steady state after 1.0×10¹⁶ atoms/cm² irradiation. The ratio of F to Si in the mixed layer is about 1:1. At an incident energy of 15 eV, the mixed layer at steady state consists of 4.5 ML of fluorine with a depth of 20 Å and the main etching products are SiF₃ and SiF₄. At 30 eV incident energy, the mixed layer at steady state is 6.5 ML with a depth of 40 Å and the main etching products are SiF₂ and SiF₃. When F atoms were irradiated onto a Si substrate heated at 1000 K, a significant reduction of F coverage was observed due to diffusion of F atoms.     

用次级束流对硅望远镜的时间和能量定标

利用９５ＭｅＶ／ｕ的１６Ｏ束产生的次级束流对半导体硅探测器进行了时间和能量定标。实验结果表明，由于次级束流提供的次级粒子种类多，飞行距离长，因此，能给探测器带来非常精确的定标。     

Stopping power of Mylar for heavy ions up to copper

The stopping powers of Mylar for several heavy ions covering Z=11 to 29 in the energy range 0.3 to 2.3 MeV/n have been measured using the elastic recoil detection technique and twin detector system. The technique provided a unique method to generate a variety of variable energy ion species utilizing a fixed energy 140 MeV Ag¹³⁺ primary beam from the Pelletron accelerator facility at Nuclear Science Center, New Delhi, India. Most of these measurements are new. The experimentally measured stopping power values have been compared with those calculated using LSS theory, Ziegler et al. formulation and Northcliffe and Schilling tabulations. Merits and demerits of these formulations are highlighted. Stopping power calculations using the Hubert et al. formulation have been extended successfully beyond its recommended range of validity, i.e. 2.5–500 MeV/n down to energies as low as 0.5 MeV/n.     

Valence structure effects in the stopping of swift ions

The valence structure of a material may affect the stopping of swift charged particles primarily via Z₂ structure, atom–molecule differences, gas–solid differences and metal–insulator differences. These material effects have a common physical origin and can therefore be considered from a unified point of view. Theoretical arguments focus on the effect of binding and orbital motion of the target electrons as well as projectile screening and Barkas–Andersen effect. Generally, valence effects depend on the atomic number, charge state and velocity of the projectile. Reference is made to recent calculations on the basis of binary stopping theory as well as experimental findings.     

Electronic stopping power of hydrogen in KCl at the stopping maximum and at very low energies

The electronic energy loss of hydrogen ions in KCl was investigated in a wide energy range. Thin films of KCl were evaporated on an Au/Si substrate. Rutherford Backscattering Spectrometry (RBS) was performed with protons and deuterons at energies from 30 to 400 keV/nucleon. At lower energies experiments were performed by Time-Of-Flight Low energy ion scattering (TOF-LEIS) again with proton and deuteron projectiles. Experimental results are compared to calculated/tabulated values for the electronic energy loss. Whereas at energies beyond the stopping maximum very good agreement is found, at lower ion energies discrepancies between experiment and calculations increase. At very low ion velocities the extrapolated stopping cross section ε predicts vanishing electronic energy loss at energies below 100 eV/nucleon.     

Spatial and energy distributions of the fragments resulting from the dissociation of swift molecular ions in solids

We have simulated the spatial evolution and energy loss of the fragments that result when swift molecular ions dissociate inside solid targets. In our calculations we have considered that these fragments undergo the following interactions: Coulomb repulsion (among like charged particles), stopping and wake forces (due to electronic excitations induced in the target), and nuclear scattering (with the target nuclei). We study the case of silicon targets irradiated with boron molecular or atomic ions; our results show that the main differences in the energy and spatial distributions of molecular fragments or atomic ions appear at shallow regions, and these tend to disappear at deeper depths.     

Exploring reciprocity as a tool in low-energy electronic stopping

A computer code has been developed to explore reciprocity, i.e. the equivalence of electronic stopping cross sections for the ion-target pair A in B with that for B in A in the regime around and below the Bohr speed. With the aim of establishing reciprocity as a tool for identifying reliable experimental data and interpolating between them, we have studied stopping cross sections involving carbon, aluminium and gold either as projectile ions or as target materials. The case of carbon - where numerous data are available - is used to illustrate in detail various options to establish a body of credible data. Aluminium and gold serve to illustrate the case where these elements mostly are found as target materials in available data.     

Electronic stopping powers for heavy ions in niobium and tantalum pentoxides

Electronic energy loss is the fundamental mechanism accountable for the response of materials to swift heavy ions that drives many new applications. Due to experimental difficulties in preparing and handling compound targets for energy-loss measurements, stopping data in compounds are limited. The electronic stopping power of He, Li, Be and O ions in self-supporting foils of niobium and tantalum pentoxides (Nb₂O₅ and Ta₂O₅) have been measured in transmission over a continuous range of energies. The measured stopping powers are compared with theoretical predictions based on the SRIM (Stopping and Range of Ions in Matter) code. In general, the predicted values are in reasonable agreement with the experimental data. However, significant deviations are observed in some cases, particularly around the stopping maximum.     

Deposition of polycrystalline Si thin films on glass substrates by direct negative Si ion beam deposition

Polycrystalline Si (Poly-Si) thin films were deposited on a glass substrate by direct negative Si (Si⁻) ion beam deposition. The glass substrate temperature was kept constant at 500 °C for all depositions. Prior to deposition, the ion energy spread and ion-to-atom arrival ratio were evaluated as a function of the ion beam energy.The Si⁻ ion energy spread was less than 10% regardless of the ion energy, while the ion-to-atom arrival ratio increased proportionally from 1.3 to 1.6 according to the ion beam energy.Atomic force microscopy images showed that a relatively rough surface was obtained at 50 eV of Si⁻ ion energy and it is also concluded that the Si⁻ ion beam irradiation at 50 eV is effective to deposit Si thin film with small grains as shown in Fig. 3.     

Optimization of large amorphous silicon and silica structures for molecular dynamics simulations of energetic impacts

A practical method to create optimized amorphous silicon and silica structures for molecular dynamics simulations is developed and tested. The method is based on the Wooten, Winer, and Weaire algorithm and combination of small optimized blocks to larger structures. The method makes possible to perform simulations of either very large cluster hypervelocity impacts on amorphous targets or small displacements induced by low energy ion impacts in silicon.     

The phase effect of electronic stopping power

1IntroductionWhenthechargedparticlewithenergyofEo(kcV5EOSMeV/u)passesthroughtheprojectedrange,ds,insolids,itwillloseitsenergybyinelasticcollisionwithtargetatomandcausethehostatomtobeexcited,ionizedanddisplaced.Theamount(ds/coso)(dE/dx)ofionenergyisdepositedonthepath,whichisnotonlycorrelativetotheenergyandeffectivechargeofprojectilesandtheangleofionincidencebutalsoassociatedwiththemeanionizationenergy,electronicdensityselementalcompositionandsolidstructureoftarget.[']Inthepresent,basingonthe…     

Optical contrast formation in amorphous silicon carbide with high-energy focused ion beams

Thin films (d ～ 1 μm) of hydrogenated amorphous silicon carbide (a-Si_1?xC_x:H), deposited by RF reactive magnetron sputtering with different carbon content x, have been implanted with high fluences (Φ = 1016–1017 cm^?2) of high-energy (E = 0.2–1 MeV) He⁺ ions as the implant species. The induced structural modification of the implanted material results in a considerable change of its optical properties, best manifested by a significant shift of the optical absorption edge to lower photon energies as obtained from photo-thermal-deflection spectroscopy (PDS) data. This shift is accompanied by a remarkable increase of the absorption coefficient over one order of magnitude (photo-darkening effect) in the measured photon energy range (0.6–3.8 eV), depending on the ion fluence, energy and carbon content of the films. These effects could be attributed both to additional defect introduction and increased graphitization, as confirmed by Raman spectroscopy and infra-red (IR) optical transmission measurements. The optical contrast thus obtained (between implanted and unimplanted film material) could be made use of in the area of high-density optical data storage using focused high-energy He⁺ ion beams.