Ion Energy Range Research Articles

The influence of electron reflection/sticking coefficients at the electrodes on plasma parameters in particle-in-cell simulations of capacitive radio-frequency plasmas

Capacitive radio-frequency discharges are frequently used to process different materials. In these systems, plasma-surface interactions are known to affect the discharge by particle emission and reflection. In simulations, the corresponding surface coefficients are input parameters, which are often unknown and are, therefore, roughly estimated or ignored. Electron reflection at boundary surfaces is typically either neglected completely or the reflection coefficient, , where S is the sticking coefficient, is assumed to be small, independently of the surface material and its conditions, although it is known to cover a wide range depending on the material. Here, we systematically investigate the effect of changing ρ in particle-in-cell simulations on plasma parameters such as the plasma density, electric field, and ionization rates, in geometrically symmetric single- and dual-frequency discharges operated in argon at a fundamental frequency of 13.56 MHz and at pressures of 5 Pa–20 Pa. We find that the plasma density strongly depends on the reflection coefficient. High coefficients cause electric field reversals during sheath collapse at the electrodes and an enhanced generation of energetic electron beams during sheath expansion, which lead to additional ionization and higher plasma densities. Different reflection coefficients at both electrodes are found to induce a discharge asymmetry that leads to the generation of a DC self-bias and different mean ion energies at the electrodes. In dual-frequency discharges, the electrical generation of the DC self-bias as a function of the phase between two consecutive driving harmonics via the electrical asymmetry effect can be significantly enhanced by choosing electrode materials with different reflection coefficients. In this way the electrical control range of the mean ion energy via phase control is shifted to different energies.

Plasma-graphene interaction and its effects on nanoscale patterning

Scalable and precise nanopatterning of graphene is an essential step for graphene-based device fabrication. Hydrogen-plasma reactions have been shown to narrow graphene only from the edges, or to selectively produce circular or hexagonal holes in the basal plane of graphene, but the underlying plasma-graphene chemistry is unknown. Here, we study the hydrogen-plasma etching of monolayer graphene supported on $\mathrm{Si}{\mathrm{O}}_{2}$ substrates across the range of plasma ion energies using scale-bridging molecular dynamics (MD) simulations based on reactive force-field potential. Our results uncover distinct etching mechanisms, operative within narrow ion energy windows, which fully explain the differing plasma-graphene reactions observed experimentally. Specific ion energy ranges are demonstrated for stable isotropic ($\ensuremath{\sim}2\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$) versus anisotropic hole growth ($\ensuremath{\sim}20--30\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$) within the basal plane of graphene, as well as for pure edge etching of graphene ($\ensuremath{\sim}1\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$). Understanding the complex plasma-graphene chemistry opens up a means for controlled patterning of graphene nanostructures.

Energy Loss, Range and Straggling of Light Ions in Polystyrene Film

Energy loss of alpha particles of energy 5.48 MeV [Am-241], has measured in polystyrene (Ps) films at a pressure of 4.4-7 mbar. The experiment was carried out using the alpha ray spectrometer consisting of Silicon Surface Barrier Detector. The alpha source and the detector were kept at a fixed distance of 12mm in scattering chamber. The energy calibration of alpha particles in vacuum at a pressure of 4.4 mbar is 9.64KeV/channel. The value of energy loss found in polystyrene film is 3.418 MeV and the computed stopping power [SRIM 2008] for alpha particle in polystyrene film is 1.0600×102 MeV/mm. The nuclear, electronic energy loss and projected range as a function of projectile energy in the range 2-10 MeV for alpha particles in polystyrene film is also studied using SRIM. It was found that the stopping power of alpha particles in polystyrene film decreases exponentially as transmitted energy increases. Further the projected range of alpha particles increases exponentially as energy increases.

Inelastic processes inNa+-Ne,Na+-Ar,Ne+-Na, andAr+-Na collisions in the energy range 0.5–14 keV

Absolute cross sections for charge-exchange, ionization and excitation in Na$% ^{+}-$Ne and Na$^{+}-$Ar collisions were measured in the ion energy range $% 0.5-10$ keV using a refined version of a capacitor method, and collision and optical spectroscopy methods simultaneously in the same experimental set-up. Ionization cross sections for Ne$^{+}-$Na and Ar$^{+}-$Na collisions are measured at the energies of $2-14$ keV using a crossed-beam spectroscopy method. The experimental data and the schematic correlation diagrams are used to analyze and determine the mechanisms for these processes. For the charge-exchange process in Na$^{+}$ $-$Ar collisions two nonadiabatic regions are revealed and mechanisms responsible for these regions are explained. Structural peculiarity on the excitation function for the resonance lines of argon atoms in Na$^{+}$ $-$Ar collisions are observed and the possible mechanisms of this phenomenon are explored. The measured ionization cross sections for Na$^{+}-$Ne and Ne$^{+}-$Na collisions in conjunction with the Landau-Zener formula are used to determine the coupling matrix element and transition probability in a region of pseudo-crossing of the potential curves.

Determination of recombination radius in Si for binary collision approximation codes

Displacement damage caused by ions or neutrons in microelectronic devices can have significant effect on the performance of these devices. Therefore, it is important to predict not only the displacement damage profile, but also its magnitude precisely. Analytical methods and binary collision approximation codes working with amorphous targets use the concept of displacement energy, the energy that a lattice atom has to receive to create a permanent replacement. It was found that this “displacement energy” is direction dependent; it can range from 12 to 32eV in silicon. Obviously, this model fails in BCA codes that work with crystalline targets, such as Marlowe. Marlowe does not use displacement energy; instead, it uses lattice binding energy only and then pairs the interstitial atoms with vacancies. Then based on the configuration of the Frenkel pairs it classifies them as close, near, or distant pairs, and considers the distant pairs the permanent replacements. Unfortunately, this separation is an ad hoc assumption, and the results do not agree with molecular dynamics calculations. After irradiation, there is a prompt recombination of interstitials and vacancies if they are nearby, within a recombination radius. In order to implement this recombination radius in Marlowe, we used the comparison of MD and Marlowe calculation in a range of ion energies in single crystal silicon target. The calculations showed that a single recombination radius of ∼7.4Å in Marlowe for a range of ion energies gives an excellent agreement with MD.

Plasma induced sputtering yield and backscattering for ITER-relevant materials as calculated by TRIM

In this paper, we report an analytical model to estimate the variation of the yield (and hence the pressure) from sputtering, elastic collisions, and backscattering on a surface exposed to International Thermonuclear Experimental Reactor-relevant plasmas. To study the yield and the pressure exerted on the surfaces of carbon (C), silicon (Si) and tungsten (W) due to plasma exposure, the irradiation of H, Ar and Xe ions at normal incidence was considered. Transport and range of ions in matter simulations were done to calculate the backscattering/sputtering yields and the energies on the surfaces. The calculations are significant for the optimization of material facing the plasma in future fusion reactors where erosion and long term operation are key issues. The results show that yield increases with the increase of ion energy. The carbon surface is the least affected to exposure of the plasma. In the chosen ion energy range (0–1 keV), the simulations/calculations (for Ar ion incident on Si and W) are in good agreement with published experimental results.

Modeling of sputtering of the ice surfaces under impact of H+ ions: Redistribution of the h and o isotopes applied to the satellites of Jupiter

Using computer modeling, the current models for sputtering coefficient calculation of the ice surfaces under the impact of H+ ions are statistically analyzed and the sputtering coefficients and their confidence intervals are calculated over a wide range of ion energies. It was established that the approximation model (Fama et al., 2008) with a calculated confidence interval of ±20% is less sensitive to the variable parameters. The sputtering coefficients of water ice p = 0.94, T = 80 K) are calculated under the impact of H+ ions for energies from several eV to 10 keV and the results are verified with experimental data. The maximum sputtering coefficient is 0.9 H2O/ion at the energy of the incident H+ ions of 200 eV. The modeling of the sputtering coefficients dependence of H2O molecules from the ice surface temperature showed that they weakly vary in the temperature range of 40–100 K and increase with an increase in the surface temperature. At maximum of distribution under T = 40–100 K, the sputtering coefficient Y(E = 200 eV) is 0.9 H2O/ion at T = 200 K — 1.1 H2O/ion. The distribution by kinetic energy of the dispersed H2O molecules and H and O atoms are modeled for the energy of 1–100 keV of the incident H+ ions. These results may be applied for modeling of variable isotopic compositions of the exosphere of the Jupiter’s satellites in the course of sputtering. The calculated ratios of the sputtering coefficients of H, D, 18O, and 16O from the surface for the Jupiter’s satellites (Europe, Ganymede, Callisto) under the impact of H+ ions for energy from several eV to 10 keV are (1.7 ± 0.3) × 10−5 and 0.18 ± 0.03, respectively. These ratios are distinct from the initial isotope ratios on the surface of the Jupiter’s satellites. This significant difference may lead to the redistribution of isotopes on the surface of the Jupiter’s satellites. The variation in the D/H ratio on the surface of the Jupiter’s satellites depends on the flux density of the radiated ions.

Leakage current behavior of BiFeO3/BiMnO3 multilayer fabricated by pulsed laser deposition

The temperature dependence of the J–E characteristics of BiFeO3/BiMnO3 (BFO/BMO) multilayer thin film on LaNiO3/Si substrate was studied in the temperature range from 300K to 450K. In the light of different models the leakage behavior of BFO/BMO multilayer thin film was analyzed, which indicates the dominant leakage mechanism is Poole–Frenkel mechanism in the medium field region from 50 to 350kV/cm. With this Poole–Frenkel emission type conduction, the trap ionization energy range of ∼0.49 to 0.57eV was calculated. At higher applied electric fields the conduction is space charge limited where as Ohmic conduction is the dominant mechanism in the lower electric field region for all the temperatures in the given range. Such analysis of leakage current in multilayer thin film eases interface engineering of heterostructures for FeRAM applications.

Power supply and impedance matching to drive technological radio-frequency plasmas with customized voltage waveforms.

We present a novel radio-frequency (RF) power supply and impedance matching to drive technological plasmas with customized voltage waveforms. It is based on a system of phase-locked RF generators that output single frequency voltage waveforms corresponding to multiple consecutive harmonics of a fundamental frequency. These signals are matched individually and combined to drive a RF plasma. Electrical filters are used to prevent parasitic interactions between the matching branches. By adjusting the harmonics' phases and voltage amplitudes individually, any voltage waveform can be approximated as a customized finite Fourier series. This RF supply system is easily adaptable to any technological plasma for industrial applications and allows the commercial utilization of process optimization based on voltage waveform tailoring for the first time. Here, this system is tested on a capacitive discharge based on three consecutive harmonics of 13.56 MHz. According to the Electrical Asymmetry Effect, tuning the phases between the applied harmonics results in an electrical control of the DC self-bias and the mean ion energy at almost constant ion flux. A comparison with the reference case of an electrically asymmetric dual-frequency discharge reveals that the control range of the mean ion energy can be significantly enlarged by using more than two consecutive harmonics.

A compact broadband ion beam focusing device based on laser-driven megagauss thermoelectric magnetic fields.

Ultra-intense lasers can nowadays routinely accelerate kiloampere ion beams. These unique sources of particle beams could impact many societal (e.g., proton-therapy or fuel recycling) and fundamental (e.g., neutron probing) domains. However, this requires overcoming the beam angular divergence at the source. This has been attempted, either with large-scale conventional setups or with compact plasma techniques that however have the restriction of short (<1 mm) focusing distances or a chromatic behavior. Here, we show that exploiting laser-triggered, long-lasting (>50 ps), thermoelectric multi-megagauss surface magnetic (B)-fields, compact capturing, and focusing of a diverging laser-driven multi-MeV ion beam can be achieved over a wide range of ion energies in the limit of a 5° acceptance angle.

Nanostructure growth by helium plasma irradiation to tungsten in sputtering regime

The formation of nanostructure on tungsten (W) surface due to Helium (He) plasma irradiation can be harmful for fusion reactors. Up to now, W nanostructure growth was investigated mainly without sputtering. Under sputtering regime, nanostructure growth competes with erosion due to sputtering. In this study, the nanostructure growth was investigated in the linear divertor simulator NAGDIS-II at incident ion energy range of 200–500eV. The growth of nanostructures was investigated by experiments and calculations under the sputtering regime. With increasing incident ion energy, the thickness of nanostructured W layer saturated rapidly at a lower He fluence, resulting in thinner W nanostructured layer. The erosion rate of the top of the W nanostructured layer was obtained from the comparison with the numerical calculation.

Charge distributions and energy losses of ions in solids

The theoretical method is suggested for analysis of inelastic processes during the penetration of fast ions in solids. With the help of this method, we obtained charge exchange, ionization, and excitation cross sections, equilibrium and nonequilibrium charge distributions, and inelastic energy losses over wide ion energy range. The calculations, performed for nitrogen ions in celluloid, were compared with experimental data and the results of other authors.

Investigation of SOL plasma interaction with graphite PFC

Morphological changes of graphite plasma facing component (PFC) were investigated with various ion irradiation conditions which may change the sputtering yield and the scrape-off layer (SOL) plasma property. The experiments were carried out by using electron cyclotron resonance deuterium plasma with the conditions of electron density range of 2.5–3.5×1011cm−3, electron temperature range of 3.3–4.1eV, and ion energy range of 17–100eV. The graphite morphology was changed from plane to conical tip with increasing ion energy and dose. In addition, morphological changes increase the sputtering yield due to the increase of local angle of ion incidence. The inflow of hydrocarbon causes the increase of electron density and the decrease of electron temperature due to its higher dissociation and ionization rates than those of deuterium. This result suggests that the morphological changes of graphite should be considered in the SOL plasma interaction with graphite PFC.

Ionization of the quasimolecule HeH2+ formed in He+–H2 collisions

Ionization processes in He+–H2 collisions have been studied in the ion energy range of 2–10 keV using electron spectroscopy. Quasimolecular autoionization and direct ionization due to superpromotion of the diabatic term were found to dominate at lower (<5 keV) and higher (>5 keV) ion energies of the range under study, respectively. The parameters of the diabatic quasimolecular term responsible for direct ionization were deduced from the experimental data by solving the ‘inverse problem’, and the following features of this channel have been revealed: (i) it proceeds via intermediate resonant two-electron rearrangement: 1a′(2a′)2 → (1a′)23a′; (ii) in the subsequent superpromotion of the outer electron, the incident ion forms a quasimolecule with only one of the atoms of the target molecule. Apart from the structure associated with autoionization transitions in the helium atom, a structure associated with the decay of autoionization states of a hydrogen molecule was revealed, and the relevant energies, widths and line profiles were determined.

Cross sections for electron capture and excitation in collisions of Liq+ (q=1, 2, 3) with atomic hydrogen

The two-center atomic orbital close-coupling method is employed to study electron capture and excitation reactions in collisions of Liq+ (q = 1-3) ions with ground state atomic hydrogen in the ion energy range from 0.1 keV/u to 300 keV/u, where u is the atomic mass unit. The interaction of the active electron with the projectile ions (Li+, Li2+) is represented by a model potential. Total and state-selective cross sections for charge transfer and excitation processes are calculated and compared with data from other sources when available.

Electronic stopping power from first-principles calculations with account for core electron excitations and projectile ionization

We use Ehrenfest dynamics and time-dependent density functional theory to calculate electronic stopping power ${S}_{e}$ of energetic ions in graphitic targets from first principles. By treating core electrons as valence electrons within the projected augmented wave framework, we demonstrate that this approach provides an accurate description of ${S}_{e}$ for a wide range of ions and ion energies, even when not only valence, but also core electron excitations are essential. Our impact-parameter-dependent approach capable of describing the stopping of both low- and high-energy ions is a significant step forward in ${S}_{e}$ calculations, as it makes it possible to monitor projectile charge state during impacts, estimate contributions of core and valence electron excitations to ${S}_{e}$, and it gives a quantitative description of electronic stopping in the cross-over region for bulk solids and nanostructures from first principles.

Determination of thicknesses of oxide films grown on titanium under argon irradiation by spectroscopic ellipsometry

In this article we present a study of the oxidation of pure titanium bulk samples under argon ion irradiation at 500°C under rarefied air. In particular we follow the dependence of the oxide thickness as a function of the energy of argon ions. The novelty of this study consists in the range of ion energy explored, from 1 to 9MeV. Until very recently it was commonly accepted that metal surfaces were transparent to ion beams in this low energy range (few MeV), and no surface modifications were expected. In a previous paper by the authors of this work, the formation of shallow craters in the surface of titanium was reported as a result of argon ion bombardment with energies of 2, 4 and 9MeV under the same environmental conditions. We show here that around 3MeV the oxide growth is unexpectedly enhanced. We think that an interplay of electronic excitations and nuclear ballistic collisions could possibly explain this enhanced oxide growth. We have used spectroscopic visible ultraviolet ellipsometry and XPS to determine the thickness of the oxide layers and characterize their optical properties. From the optical properties of the oxides we observed that for ion energies below 3–4MeV the oxides show a dielectric-like behavior, whereas for ion energies above 3–4MeV the oxides show a metal-like behavior. These findings indicate also that ion bombardment in this energy range may change substantially the oxygen-to-titanium ratio in the oxide films grown under irradiation leading to the formation of titanium sub-oxides.

Influence of Light Ion Irradiation on the Current-Voltage Characteristics of Electrochemical Anodization of p-Type Silicon

The influence of ion irradiation on porous silicon formation of p-type silicon in dilute hydrofluoric acid has been examined over a wide range of ion fluence and energy. The current-voltage curves show a positive potential shift as the ion fluence is increased, a phenomenon that is explained in terms of a corresponding shift in the flatband potential. Furthermore an observed decrease in the magnitude of the Jps peak as the ion fluence is increased can be accounted for by the fact that the ion irradiation induced defects reduce boundary effects that are known to cause increased etching of the silicon's edges. At very high fluences sufficient defects are created in the end of range region to remove all the original acceptors in the p-type silicon. This causes drastic changes in the shape of the I-V curves, but electropolishing is still possible at potentials positive of flatband although at reduced rates.

Role of Ion Beam Irradiation and Annealing Effect on the Deposition of AlON Nanolayers by Using Plasma Focus Device

AlON nanolayers are synthesized on Al substrate by the irradiation of energetic nitrogen ions using plasma focusing. Samples are exposed to multiple (5, 10, 15, 20 and 25) focus shots. Ion energy and ion number density range from 80 keV to 1.4 MeV and 5.6×1019 m−3 to 1.3×1019 m−3, respectively. Moreover, the effect of continuous annealing (473 K and 523 K) on an AlN surface layer synthesized with 25 focus shots is also examined. The main features of the X-ray diffraction (XRD) patterns with increasing focus shots are: (i) variation in the crystallinity of AlN along (111), (200) and (311) planes, (ii) increasing average crystallite size of AlN (111) plane, and (iii) stress relaxation observed in AlN (111) and (200) planes. The crystallinity of AlN surface layer is comparatively better at 473 K annealing temperature. A broadened diffraction peak related to an aluminium oxide phase showing weak crystallinity is observed for 15 focus shots while non-bounded oxides are present in all other deposited layers. Raman and Fourier transform infrared spectroscopy (FTIR) analysis confirm the presence of AlN and Al2O3 for the surface layer annealed at 473 K temperature. Raman analysis shows that the overlapping of AlN and Al2O3 results in the development of residual stresses. Scanning electron microscope (SEM) results demonstrate that the formation of rounded grains (range from 20 nm to 200 nm) and variations in their microstructures features depend on the increasing number of focus shots. Decomposition of larger clusters into smaller ones is observed.

Ionization in simple quasimolecules: HeH22+ (He2+ + H2)

Energy spectra of electrons ejected into 3He2+–H2 collisions have been measured in the incident ion energy range 6–30 keV. Parameters of the quasimolecule formed during the collision are determined. The dominant role of the channel (2a′)2 → 1a′3a′ → 1a′ + e in the production of electrons at keV ion energies has been revealed. The structure associated with autoionization transitions in the projectile helium atoms shows enormously large broadening and shift to lower energies.