Noble Gas Ions Research Articles

Sputtering Yields of CaO, SrO, and BaO by Monochromatic Noble Gas Ion Bombardment

Although MgO is widely used for barrier coating in the current generation of commercial plasma display panel (PDP) cells, other alkaline earth oxides have been studied as alternatives to MgO because some of them have lower discharge breakdown voltages for PDP cells, which would increase energy efficiency of the cells. On the other hand, the resistance against physical sputtering is another critical parameter for barrier coating. In this work, sputtering yields of CaO, SrO, and BaO by monochromatic He+, Ar+, or Kr+ ion beams of normal incidence have been obtained experimentally at relatively low incident energies by a mass-selected ion beam system. Despite the large differences in mass among the target materials, sputtering yields are found to be similar in magnitude among them for a given incident energy. It has been also found that sputtering yields depend weekly on the mass of incident species among Ne+, Ar+, Kr+, and Xe+ ions.

Nanoscale mechanisms of surface stress and morphology evolution in FCC metals under noble-gas ion bombardments

Here, we uncover three new nanoplasticity mechanisms, operating in highly stressed interstitial-rich regions in face-centred-cubic (FCC) metals, which are particularly important in understanding evolution of surface stress and morphology of a FCC metal under low-energy noble-gas ion bombardments. The first mechanism is the configurational motion of self-interstitials in subsonic scattering during ion bombardments. We have derived a stability criterion of self-interstitial scattering during ion embedding, which consistently predicts the possibility of vacancy- and interstitial-rich double-layer formation for various ion bombardments. The second mechanism is the growth by gliding of prismatic dislocation loops (PDLs) in a highly stressed interstitial-rich zone. This mechanism allows certain prismatic dislocations with their Burgers vectors parallel to the surface to grow in subway-glide mode (SGM) during ion bombardment. The SGM growth creates a large population of nanometre-sized prismatic dislocations beneath the surface. The third mechanism is the Burgers vector switching of a PDL that leads to unstable eruption of adatom islands during certain ion bombardments of FCC metals. We have also derived the driving force and kinetics for the growth by gliding of prismatic dislocations in an interstitial-rich environment as well as the criterion for Burgers vector switching, which consistently clarifies previously unexplainable experimental observations.

The study of graphite disordering using the temperature dependence of ion-induced electron emission

The temperature dependences of ion-induced electron emission yield γ(T) under 6–30 keV N2+, Ne+, Ar+ and 15 keV N+ high-fluence ion irradiation of polycrystalline graphite at normal ion incidence have been analyzed to trace the structure change dependence depending on irradiation temperature and the level of radiation damage v measured in dpa (displacements per atom). It has been found that, under irradiation at room temperature, the threshold value for graphite lattice disordering equals vd ≈ 60 dpa for noble gas ions (Ar+, Ne+) and νd ≈ 40 dpa for N2+.

Influence of nonstationary atomic mixing on depth resolution in sputter depth profiling

Atomic mixing is an important parameter in the quantitative description of sputter depth profiles by the mixing‐roughness‐information depth model. By changing the atomic mixing length from zero to a stationary state value, it is possible to simulate the transient zone at the beginning of sputter depth profiling with noble gas ions, which is important for shallow depth profiles. Accordingly, the depth profile is gradually broadened and the depth resolution value increases with sputtered depth. Applications of changing the atomic mixing length in the mixing‐roughness‐information depth model are shown to yield excellent fits for measured AES depth profiles of N+ implantation in Co films. Copyright © 2011 John Wiley & Sons, Ltd.

Development of intense high-energy noble gas ion beams from in-terminal ion injector of tandem accelerator using an ECR ion source

Abstract An ECRIS-based heavy ion injector was constructed in the high-voltage terminal of JAEA-Tokai Tandem Accelerator to develop new beam species of highly charged noble gas ions. This work was associated with a lot of development to operate the ion source on the 20UR Pelletron high voltage terminal in high pressure SF 6 gas environment. Highly charged ions of N, O, Ne, Ar, Kr and Xe have been accelerated satisfactorily. Operating data integrated during many years long beam delivery service are summarized.

High energy excitations in ion-induced electron emission from AlF 3

Measurements of electron emission spectra from surfaces of aluminum fluoride impacted by keV noble gas ions show a high-energy structure, peaking around 7 eV that increases in intensity with ion energy. The shape of this structure, identified by Factor Analysis, is independent of the nature and the energy of the impinging ions. We discuss one electron, two electron and plasmon excitation mechanisms and conclude that the high-energy structure results from the autoionization of F − 2 p 4 nl n′l′ excited by electron promotion in close atomic collisions.

Mixing of Cr and Si atoms induced by noble gas ions irradiation of Cr/Si bilayers

Cr/Si bilayers were irradiated at room temperature with 120 keV Ar, 140 keV Kr and 350 keV Xe ions to fluences ranging from 10 15 to 2 × 10 16 ions/cm 2. The thickness of Cr layer evaporated on Si substrate was about 400 Å. Rutherford backscattering spectrometry (RBS) was used to investigate the atomic mixing induced at the Cr–Si interface as function of the incident ion mass and fluence. We observed that for the samples irradiated with Ar ions, RBS yields from both Cr layer and Si substrate are the same as before the irradiation. There is no mixing of Cr and Si atoms, even at the fluence of 2 × 10 16 ions/cm 2. For the samples irradiated with Kr ions, a slight broadening of the Cr and Si interfacial edges was produced from the fluence of 5 × 10 15 ions/cm 2. The broadening of the Cr and Si interfacial edges is more pronounced with Xe ions particularly to the fluence of 10 16 ions/cm 2. The interface broadening was found to depend linearly on the ion fluence and suggests that the mixing is like a diffusion controlled process. The experimental mixing rates were determined and compared with values predicted by ballistic and thermal spike models. Our experimental data were well reproduced by the thermal spikes model.

Evidence for a plasma core during acoustic cavitation in sulfuric acid.

Extreme temperatures and pressures are produced through, acoustic cavitation: the formation, growth and collapse of bubbles in a liquid irradiated with high intensity ultrasound. Single bubbles have generally been assumed to give higher temperature conditions than bubble clouds, but confirmation from the single bubble sonoluminescence (SBSL) emission spectra has been problematic because SBSL typically produces featureless emission spectra that reveal little about the intra-cavity physical conditions or chemical processes. Here definitive evidence of the existence of a hot, highly energetic plasma core during SBSL is presented. From a luminescing bubble in sulfuric acid, excited state to excited state emission lines are observed both from noble gas ions (Ar+, Kr+, and Xe+) and from neutral atoms (Ne, Ar, Kr, and Xe). The excited states responsible for these emission lines range from 8.3 eV (for Xe) to 37.1 eV (for Ar+) above the ground state. Observation of emission lines allows for identification of intra-cavity species responsible for light emission; the energy levels and bandshapes of the emitters indicate that the plasma generated during cavitation is comprised of highly energetic atomic and ionic species with Ne as large as 10×D21/cc. Ionization and plasma can also be created during multi-bubble sonoluminescence in sulfuric acid.

Making channeling visible: keV noble gas ion trails on Pt(111)

The impact of argon and xenon noble gas ions on Pt(111) in grazing incidence geometry are studied through direct comparison of scanning tunneling microscopy images and molecular dynamics simulations. The energy range investigated is 1–15 keV and the angles of incidence with respect to the surface normal are between 78.5° and 88°. The focus of the paper is on events where ions gently enter the crystal at steps and are guided in channels between the top most layers of the crystal. The trajectories of the subsurface channeled ions are visible as trails of surface damage. The mechanism of trail formation is analyzed using simulations and analytical theory. Significant differences between Xe+ and Ar+ projectiles in damage, in the onset energy of subsurface channeling as well as in ion energy dependence of trail length and appearance are traced back to the projectile and ion energy dependence of the stopping force. The asymmetry of damage production with respect to the ion trajectory direction is explained through the details of the channel shape and subchannel structure as calculated from the continuum approximation of the channel potential. Measured and simulated channel switching in directions normal and parallel to the surface as well as an increase of ions entering into channels from the perfect surface with increasing angles of incidence are discussed.

The possible role of anisotropy in kinetic electronic excitation of solids by particle bombardment

The kinetic excitation of a solid surface by impact of energetic particles is investigated by means of internal electron emission across a buried metal–insulator–metal (MIM) tunnel junction. By bombarding the top metal surface of such a device with keV noble gas ions, internal emission yields were determined as a function of projectile impact energy and angle of incidence with respect to the surface normal. In order to understand the observed impact angle dependence, we apply a modified formalism originally published to describe external electron emission. As a result, we find that the measured data can be explained by assuming the spatial distribution of excited electrons propagating towards the buried oxide interface to be strongly influenced by the projectile impact angle. A simple ballistic model assuming excited electrons generated by direct collisions with the projectile to preferably propagate along the direction of the original projectile motion, while electrons excited by scattering from moving recoils propagate isotropically, appears to describe the observed experimental data quite well.

Using a current method for measuring ion-induced electron emission from LiF

The interaction of ions with an insulating substrate leads to surface charging. For intense ion beams these charges are not sufficiently fast removed between successive ion impact events. As a result the trajectories of the incident beam and the electron emission yield may be altered in a hardly predictable way. We demonstrate, that by heating an alkali-halide sample to a sufficiently high temperature, this macroscopic charging can be avoided and a simple current method can be used to study electron emission from such a surface. Measured electron emission yields from a LiF(1 0 0) single crystal due to impact of singly and multiply charged heavy noble gas ions (argon and xenon) are presented. For argon projectiles our results compare well with previous data, which were obtained using a more sophisticated electron statistics method and six orders of magnitude less intense ion beams.

Calculation of Auger-neutralization probabilities for He +-ions in LEIS

In Low Energy Ion Scattering (LEIS), Auger-neutralization is an omnipresent charge exchange mechanism, especially when noble gas ions are used as projectiles, with a primary energy below the threshold energy, E th, for collision induced charge exchange processes (neutralization and reionization). Recent experiments revealed a significant dependence of the ion survival probability, P +, on the crystal plane, when He + ions are scattered from a metal surface. This is in contrast to the fact, that the neutralization probability in LEIS is usually assumed to be independent of the chemical environment of the collision partner (absence of matrix effects). In order to investigate this crystal effect, an existent theory on Auger-neutralization (based on a Linear Combination of Atomic Orbitals) is adapted to the LEIS geometry. With this model, Auger-neutralization rates are calculated for a Ag(1 1 0) surface. Trajectories for He particles scattered from this surface into different azimuth directions are obtained by means of Molecular Dynamics simulations. Subsequently, the ion survival probability is calculated and compared to measurements. Good agreement is obtained which gives confidence in the applicability of this model in the LEIS regime. Moreover, it was possible to obtain detailed information on the properties of the neutralization process.

Production of defects in hexagonal boron nitride monolayer under ion irradiation

Atomistic computer simulations based on analytical potentials are employed to investigate the response of a hexagonal boron nitride monolayer to irradiation with noble gas ions having energies from 35 eV up to 10 MeV. Probabilities for creating different types of defects are calculated as functions of ion energy and incidence angle, along with sputtering yields of boron and nitrogen atoms. The presented results can be used for the optimization of ion processing of single-layer and bulk hexagonal boron nitride samples and for predicting the evolution of the material in radiation hostile environments.

Electron–ion collision rates in atomic clusters irradiated by femtosecond laser pulses

In atomic clusters irradiated by femtosecond laser pulses, plasmas with high density and high temperature are created. The heating is mainly caused by inverse bremsstrahlung, i.e. determined by electron–ion collisions. In the description of the scattering of electrons on noble gas ions in such plasmas, it is important to account for the inner structure of the ions and the screening by the surrounding plasma medium which can be accomplished by using suitable model potentials. In the wide parameter range met in experiments, the Born approximation is not applicable. Instead, the electron–ion collision frequency is calculated on the basis of classical momentum transport cross sections. Results are presented for xenon, krypton and argon ions in different charge states. A comparison of these results to those for the scattering on Coulomb particles with the same charge shows an enhancement of the collision frequency. The Born approximation, however, leads to an overestimation.

On the effect of oxygen flooding on the detection of noble gas ions in a SIMS instrument

We have investigated the report by Desgranges and Pasquet (2004) [1] that O2 gas flooding in a secondary ion mass spectrometer could enhance by a factor of about ∼5 the Xe+ ion yield for a Xe implant in UO2 sputtered by O2+ primary ions. For a Xe implant in Si sputtered by O2+ primary ions and for Xe+ sputtering of silicon in steady state, O2 gas flooding reduced the Xe+ ion signal by a factor of about 2, presumably due to loss of Xe+ by resonant charge exchange with gas-phase oxygen molecules. The yield of a Kr co-implant in Si was unaffected by oxygen flooding. However, we demonstrate that for steady-state Ar+ sputtering of uranium, the Ar+ ion yield can be increased by a factor of ∼1.7 by oxygen flooding. This enhancement, predicted by a simple model to be a factor of √3, is attributed to a lowering of the neutral Ar emission velocity and hence increased post-ionization efficiency, caused by the suppression of argon bubble formation in uranium by oxidation of the near-surface region, and re-emission of argon by thermal effusion rather than in supersonic expansions from bursting high-pressure gas bubbles. We show that bubble formation, evidenced by the observation of Xe2+ clusters, also occurs for Xe+-bombarded uranium when the surface is sufficiently sputter-cleaned, and that the bubbles are similarly suppressed by oxygen flooding producing a factor of ∼√3 enhancement in the Xe+ signal.

Trails of Kilovolt Ions Created by Subsurface Channeling

Using scanning tunneling microscopy, we observe the damage trails produced by keV noble-gas ions incident at glancing angles onto Pt(111). Surface vacancies and adatoms aligned along the ion trajectory constitute the ion trails. Atomistic simulations reveal that these straight trails are produced by nuclear (elastic) collisions with surface layer atoms during subsurface channeling of the projectiles. In a small energy window around 5 keV, Xe+ ions create vacancy grooves that mark the ion trajectory with atomic precision. The asymmetry of the adatom production on the two sides of the projectile path is traced back to the asymmetry of the ion's subsurface channel.

Angular distributions and correlations in sequential two-photon atomic double ionization

Theory of the sequential two-photon double ionization of atoms in the XUV wavelength region is discussed in the context of recent experiments with the free-electron laser. Angular distributions and angular correlations of photoelectrons are analyzed within the statistical tensor formalism and examples are given for sequential two-photon double ionization of noble gases.

Subsurface excitations in a metal

We investigate internal hot carrier excitations in a Au thin film bombarded by hyperthermal and low energy alkali and noble gas ions. Excitations within the thin film of a metal-oxide-semiconductor device are measured revealing that ions whose velocities fall below the classical threshold given by the free-electron model of a metal still excite hot carriers. Excellent agreement between these results and a nonadiabatic model that accounts for the time-varying ion-surface interaction indicates that the measured excitations are due to semilocalized electrons near the metal surface.

Ion drift in a gas in an external electric field

A model of ion collisions with gas atoms is constructed that takes into account resonant ion-atom charge exchange, polarization interaction, and short-range (gas-kinetic) collisions. The cross sections for resonant charge exchange of the ions of noble gases, as well as rubidium, cesium, and mercury ions, with atoms are calculated based on the experimental data and the results of numerical simulations of ion collisions with parent gas atoms in a uniform electric field. The effect of the electric field, the temperature of the buffer gas atoms, and the percentage composition of the gas mixture on the ion velocity distribution function is investigated.

Oxygen radical and plasma damage of low-k organosilicate glass materials: Diffusion-controlled mechanism for carbon depletion

Fourier transform infrared (FTIR) analyses of low-k materials exposed to either oxygen radicals or to capacitively coupled O2 plasma indicate that carbon depletion from these materials is dominated by O radical diffusion. FTIR measurements of changes in absorbance related to silanol formation (3500 cm−1) and carbon depletion (2980 cm−1, 900–700 cm−1) exhibit a linear dependence on the square root of the exposure time. Diffusion is faster for a sample of higher porosity and interconnectedness (k=2.54) than for a sample with lower porosity (k=3.0). However, a sample with high porosity (k=2.57) but low interconnectedness (as measured by liquid diffusion) exhibits a high initial rate of carbon loss, followed by no further carbon loss at longer times. Further, pretreatment of k=3.0 material by 500 eV noble gas ions results in a sharp decrease in the rate of carbon loss upon subsequent exposure to oxygen radicals. The data indicate that the main mechanism of carbon depletion in organosilicate glass (OSG) materials during oxygen plasma exposure is loss due to a reaction front created by oxygen radicals diffusing through interconnected pores. Further, carbon depletion can be minimized for low-k OSG materials either by formation of high porosity/low interconnectedness samples, or by pretreatment by noble gas ion bombardment, which seals surface pores.