Binary Collision Approximation Research Articles

Reflection of hydrogen and deuterium atoms from the beryllium, carbon, tungsten surfaces

Particle reflection coefficients for scattering of hydrogen and deuterium atoms from amorphous beryllium, carbon and tungsten were obtained, which are of interest for thermonuclear reactor physics. For the case of deuterium scattering from tungsten the data were also calculated for polycrystalline and crystalline target. The calculations were carried out by two methods: by modeling the trajectories of the incident particles and by using the binary collision approximation. Interaction potentials between hydrogen and helium atoms and the selected materials were calculated in the scope of the density function theory using program DMol for choosing wave functions. The dependence of the reflection coefficient RN on the potential well depth was found. The results demonstrate a good agreement with the available experimental values.

Pressure effects on the vibrational and rotational relaxation of vibrationally excited OH (ν, J) in an argon bath.

Quasi-classical molecular dynamics simulations were used to study the energy relaxation of an initially non-rotating, vibrationally excited (ν = 4) hydroxyl radical (OH) in an Ar bath at 300 K and at high pressures from 50 atm to 400 atm. A Morse oscillator potential represented the OH, and two sets of interaction potentials were used based on whether the Ar-H potential was a Buckingham (Exp6) or a Lennard-Jones (LJ) potential. The vibrational and rotational energies were monitored for 25 000-90 000 ps for Exp6 trajectories and 5000 ps for LJ trajectories. Comparisons to measured vibrational relaxation rates show that Exp6 rates are superior. Simulated initial vibrational relaxation rates are linearly proportional to pressure, implying no effect of high-pressure breakdown in the isolated binary collision approximation. The vibrational decay curves upward from single-exponential decay. A model based on transition rates that exponentially depend on the anharmonic energy gap between vibrational levels fits the vibrational decay well at all pressures, suggesting that anharmonicity is a major cause of the curvature. Due to the competition of vibration-to-rotation energy transfer and bath gas relaxation, the rotational energy overshoots and then relaxes to its thermal value. Approximate models with adjustable rates for this competition successfully reproduced the rotational results. These models show that a large fraction of the vibrational energy loss is initially converted to rotational energy but that fraction decreases rapidly as the vibrational energy content of OH decreases. While simulated rates change dramatically between Exp6 and LJ potentials, the mechanisms remain the same.

Simulation of Particle Scattering at Amorphous and Polycrystalline Targets

A procedure for simulating the scattering of atomic particles at amorphous and crystalline targets is described in the binary collision approximation. The influence of thermal vibrations, choice of the potential, and the inclusion of inelastic energy losses are analyzed using the simulation of hydrogen-atom scattering at a tungsten surface as an example. The coefficients of reflection from crystalline, polycrystalline, and amorphous surfaces are compared.

Ion-induced nanopatterning of silicon: Toward a predictive model

We review recent progress toward the development of predictive models of ion-induced pattern formation on room-temperature silicon, with a particular emphasis on efforts to eliminate fit parameters in the linear regime by means of experimental measurements or atomistic simulations. Analytical approaches considered include “mechanistic” models of the impact-induced collision cascade, the Crater Function Framework, and continuum treatments of ion-induced stress and viscous flow. Parameter evaluation methods include molecular dynamics and binary collision approximation simulations, as well as wafer curvature measurements and grazing incidence small-angle x-ray scattering. Mathematical detail is provided in the context of key results from pattern formation theory, which are also briefly summarized.

Atomic insight into concurrent He, D, and T sputtering and near-surface implantation of 3C-SiC crystallographic surfaces

Abstract We present results from atomistic simulations of sputtering and near-surface implantation of concurrent He, D, and T bombardment of cubic silicon carbide (3C-SiC). This is achieved by first establishing a many-body interatomic potential parameter set to treat interactions of He and hydrogenic species in 3C-SiC informed by ab-initio calculations. To obtain sputtering yields we perform both classical molecular dynamics and binary collision approximation simulations for normal incident particles having energies ranging from 25 to 800 eV. We find that due to differences in species surface binding energy of various crystallographic surfaces in 3C-SiC, the sputtering yield of Si is significantly less than that of C, but sputtering yields show limited sensitivity to crystallographic surface orientation. An exception to this occurs when the terminating crystallographic surface plane is more rich in Si rather than C, resulting in comparable sputtering yields of Si and C. The influence of temperature on sputtering is explored and shows limited effect. Finally, the nature of implanted He, D, and T within 3C-SiC surfaces is investigated to understand implantation profiles and stability of defects.

Multi-model quantification of defects in irradiated lithium titanate

Lithium based ceramics (Li2TiO3, Li4SiO4 etc.) in the packed pebble form, used in breeder blankets to breed tritium, are promising candidates for future fusion reactors. Neutron induced trap sites in the ceramic breeder affect the diffusion and inventory of tritium and thereby influence the quantity of tritium recovered. In the present work, a multi model framework has been employed to quantify the number of defects, which can be used to evaluate trap-sites, due to neutron irradiation in porous Li2TiO3 pebbles. This framework is based on a three stage calculation viz. (i) radiation transport, (ii) obtaining the primary knock-on atom (PKA) energy spectrum and (iii) quantifying the ensuing damage by collision cascade simulations. The output of one stage is coupled to the next stage and finally the defect distribution is obtained. We use radiation transport calculations to obtain the neutron spectrum in ceramic pebbles. This is used as an input to SPECTER, a code that uses evaluated cross-sections, to obtain the Primary Knock-on Atom (PKA) energy spectrum. The PKA energy spectrum is then used to find the number and distribution of vacancies and displacements in porous Li2TiO3 pebbles using binary collision approximation (BCA) based Monte Carlo simulations. The vacancy distribution and the number of vacancies in Li2TiO3 have been evaluated for the ITER neutron source using the above frame work. The PKA energy spectrum of Li, Ti and O PKAs, have been obtained using SPECTER, and their average energies are 680, 120 and 320 keV respectively. The collision cascade simulations shows the expected Bragg peak for the number of defects created and has a range of 6–8 microns for O and Li PKA respectively and a fraction of a micron for Ti PKA. The total number of displaced atoms which is equal to the number of vacancies, at the end of the collision cascades, obtained using the above framework has also been used to estimate the displacements per atom (dpa) in Li2TiO3 and this shows a good agreement with the dpa calculated using the NRT model.

Large current ion beam polishing and characterization of mechanically finished titanium alloy (Ti6Al4V) surface

Ti6Al4V samples with a surface roughness of about 0.2 μm were polished by a high fluence ion beam source, and influences of the ion current, energy and incident angle on the surface roughness of Ti6Al4V were studied. The as-polished surfaces were characterized by laser scanning confocal microscopy (LSCM), atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS). The LSCM images revealed that the density of pits in the original surface decreased greatly after the ion polishing process. In addition, the surface roughness calculated from these images proved that the mean value reduced to 53 nm with an ion energy, current and incident angle of 600 eV, 130 mA and 75°, respectively. The smoothing effect was also confirmed by the power spectral density (PSD) functions calculated from the AFM data. GIXRD patterns indicated that the Ti6Al4V samples mainly contain the hexagonal close packed (HCP) α-Ti, no recognizable peaks corresponding to the body centered cubic (BCC) β-Ti were observed. Furthermore, after the ion beam polishing process, the phase of all the as-polished samples kept steady. The XPS results revealed that C, O, Ti, Al and V were present in both the original as well as the polished samples. In addition, due to the highly reactive characteristics of Ti, Al and V elements, a higher signal for oxygen can also be observed on the outermost surface of the ion beam polished samples. The polishing process of Ti6Al4V surface bombarded by Ar ion beam is also simulated by binary collision approximation (BCA) code SDTrimSP, the effects of ion beam energy and incident angle on the sputtering yield were investigated. The results showed that the sputtering yield of Ti, Al and V atoms increased almost linearly with the ion energy. However, as for the influence of ion incident angle, the sputtering yield first increased gradually from 0° and peaked at about 65°, then decreased rapidly because the incident angle is too large for Ar ion to penetrate into the surface of Ti6Al4V sample.

Quick calculation of damage for ion irradiation: implementation in Iradina and comparisons to SRIM

Binary collision approximation (BCA) calculation allows for two types of damage calculation: full cascade and quick calculations. Full cascade mode describes fully the cascades while in quick calculations, only the trajectory of the ion is followed and effective formulas give an estimation of the damage resulting from each collision of the ion. We implement quick calculation of damage in the Iradina code both for elemental and multi-component solids. Good agreement is obtained with SRIM. We show that quick calculations are unphysical in multi-component systems. The choice between full cascade and quick calculations is discussed. We advise to favour full cascade over quick calculation because it is more grounded physically and applicable to all materials. Quick calculations remain a good option for pure solids in the case of actual quantitative comparisons with neutron irradiations simulations in which damage levels are estimated with the NRT (Norgett-Robinson and Torrens) formulas.

The sputtering yield of crystalline Si(1 0 0) surface by monoatomic and diatomic nitrogen ions impact

We have investigated the sputtering rate of thin crystalline silicon film forming silicon-on-insulator structure impinged by atomic and molecular nitrogen ions with medium energies of 25, 50, and 100 keV/(N atom) at several incident angles of 0, 30, 45, and 60°. The thickness of the surface silicon layer after ion irradiation was quantitatively estimated by Rutherford backscattering spectroscopy using 2.56-MeV 10B2+ ions with a grazing angle geometry to accurately analyze the thickness decrement in several nanometers for ultrathin Si layers. The sputtering rates were compared with a full-cascade TRIM simulation based on a binary collision approximation. We found that the sputtering yields of monoatomic N ion with higher energies (>50 keV/N) took significantly higher than that of diatomic N2 ion, probably suggesting that a wake-like effect of relatively fast NN molecular ions surviving for a long period due to a considerably strong triple bond. In addition, the observed sputtering yields for all present conditions were a few times smaller than that estimated by the full-cascade TRIM calculation using typical lattice parameters. We quantitatively discuss the details of sputtering phenomenon of crystalline silicon by nitrogen ions at medium energy region.

Defects in swift heavy ion irradiated n-4H-SiC

We have used confocal Raman spectroscopy, atomic force microscopy (AFM), Binary collision approximations and Deep level transient spectroscopy (DLTS) to study the defects introduced in n-type 4H-SiC by 167 MeV Xe26+ ions (swift heavy ions (SHIs)). Moderately doped epitaxial layers were irradiated with SHIs to a fluence of 5 × 1011 cm−2 at room temperature. Raman spectroscopy was used to investigate the effects of irradiation on the crystal structure. Raman intensity reduced after irradiation but the overall bond structure was conserved. Cluster spectra from confocal Raman spectroscopy showed a damage impact that was consistent with SRIM simulations. AFM showed that the incident radiation resulted in elongated protrusions. The virgin samples contained the E0.09, E0.12, E0.15 and E0.65 as the only electrically active defects. After irradiation the E0.40 and E0.71 defects were introduced.

Computational study on interatomic potentials to dynamic effects in rainbow scattering under axial channeling at Al(0 0 1) surface

The interatomic potentials between projectiles and target atoms have a strong influence on the rainbow angles corresponding to pronounced peaks in the angular distributions of scattered projectiles with slight angle, attributed to rainbow scattering (RS), under axial surface channeling conditions. Schüller and Winter (SW) (2009) have measured the dependence of rainbow angles on normal energies to the target surface of projectile energy in RS of reactive N atomic projectile under <1 0 0> axial channeling conditions at a Al(0 0 1) metal surface with projectile energies of 10–70 keV. The SW’s data were analyzed by computer simulations using the ACOCT code based on the binary collision approximation with interatomic pair potentials. The agreement between the ACOCT results using the ZBL pair potential with only a repulsive part and the experimental ones was found in RS for projectile energy of 70 keV. The experimental results included the dynamic effects indicating that rainbow angles increase with decreasing projectile energies at low normal energies were roughly reproduced by the ACOCT simulations for the HF-like pair potentials with an attractive part depending on projectile energies.

Site-controlled formation of single Si nanocrystals in a buried SiO2 matrix using ion beam mixing.

For future nanoelectronic devices – such as room-temperature single electron transistors – the site-controlled formation of single Si nanocrystals (NCs) is a crucial prerequisite. Here, we report an approach to fabricate single Si NCs via medium-energy Si+ or Ne+ ion beam mixing of Si into a buried SiO2 layer followed by thermally activated phase separation. Binary collision approximation and kinetic Monte Carlo methods are conducted to gain atomistic insight into the influence of relevant experimental parameters on the Si NC formation process. Energy-filtered transmission electron microscopy is performed to obtain quantitative values on the Si NC size and distribution in dependence of the layer stack geometry, ion fluence and thermal budget. Employing a focused Ne+ beam from a helium ion microscope, we demonstrate site-controlled self-assembly of single Si NCs. Line irradiation with a fluence of 3000 Ne+/nm2 and a line width of 4 nm leads to the formation of a chain of Si NCs, and a single NC with 2.2 nm diameter is subsequently isolated and visualized in a few nanometer thin lamella prepared by a focused ion beam (FIB). The Si NC is centered between the SiO2 layers and perpendicular to the incident Ne+ beam.

Calculation of Self-Sputtering Yield under Ion Bombardment of Solids: Computer Simulation and Theory

We developed a computer simulation program for investigation of solid self-sputtering under ion bombardment. The program is based on binary collision approximation, truncated Coulomb potential, and the model of local inelastic losses proportional to energy. As a result of simulation, the self-sputtering yield is calculated as a function of atomic number and ion energy. To verify the cascade simulation code, we created a self-sputtering theory for the case of hard sphere potential. Simulation results show good agreement with both experimental data and theory.

A model of defect cluster creation in fragmented cascades in metals based on morphological analysis

The impacts of ions and neutrons in metals cause cascades of atomic collisions that expand and shrink, leaving microstructure defect debris, i.e. interstitial or vacancy clusters or loops of different sizes. In De Backer et al (2016 Europhys. Lett. 115 26001), we described a method to detect the first morphological transition, i.e. the cascade fragmentation in subcascades, and a model of primary damage combining the binary collision approximation and molecular dynamics (MD). In this paper including W, Fe, Be, Zr and 20 other metals, we demonstrate that the fragmentation energy increases with the atomic number and decreases with the atomic density following a unique power law. Above the fragmentation energy, the cascade morphology can be characterized by the cross pair correlation functions of the multitype point pattern formed by the subcascades. We derive the numbers of pairs of subcascades and observed that they follow broken power laws. The energy where the power law breaks indicates the second morphological transition when cascades are formed by branches decorated by chaplets of small subcascades. The subcascade interaction is introduced in our model of primary damage by adding pairwise terms. Using statistics obtained on hundreds of MD cascades in Fe, we demonstrate that the interaction of subcascades increases the proportion of large clusters in the damage created by high energy cascades. Finally, we predict the primary damage of 500 keV Fe ion in Fe and obtain cluster size distributions when large statistics of MD cascades are not feasible.

A combined BCA-MD method with adaptive volume to simulate high-energy atomic-collision cascades in solids under irradiation

We present a method combining the Binary Collision Approximation (BCA) and Molecular Dynamics (MD) with adaptive volume to simulate high-energy collision cascades in solids under irradiation. The results obtained with the BCA-MD method with adaptive volume are in very good agreement with those obtained with full MD calculations. The BCA-MD predicts very well in a broad range of Primary Knock-on Atom (PKA) energies the different phases of evolution of the cascade, the number of Frenkel pairs, the fraction of point defects in clusters and their size distribution. The BCA-MD method proposed here achieves a considerable acceleration of MD simulations with a speedup factor that increases with the PKA energy. For a PKA energy of 80 keV in Fe, the speedup measured is higher than 30. For a PKA energy of 0.5 MeV in Fe, by comparison with MD simulations performed by other authors, we estimate that the BCA-MD accelerates MD simulations by a factor 312.

THE COLLIDING PARTICLES MASS RATIO INFLUENCE ON PECULIARITIES OF LOW-ENERGY ION NEAR-SURFACE IMPLANTATION AT CHANNELING CONDITIONS

In the present work the peculiarities of ion implantation and colliding particles mass ratio influence on the ranges, energy loss and profiles of distribution for 1−5 keV P+ ions channelling in Si(110) and SiC(110) at normal incidence, and 1 keV Be+ and Se+ ions in GaAs(100), as well as 5 keV Ar+ and Kr+ on Cu(001) surface at glancing incidence are carried out by computer simulation in binary collision approximation. It is shown that for paraxial part of a beam the main contribution to the total energy loss comes from inelastic ones. It has been established that the energy loss of ions transmitted through thin crystal and depth profile distributions depend on width of the channel and mass ratio of colliding atoms. It was shown that at grazing surface channeling conditions the main peak of the implanted depth distributions is considerably shallow, the range for Se+ ions is shallower and the half-width of profile for these ions is narrow than that for Be+ ions. The results allow one to select the optimum for implanted depth distributions with demanded shape at narrow near-surface area of crystals obtaining.

Improved atomic displacement cross-sections for proton irradiation of aluminium, iron, copper, and tungsten at energies up to 10 GeV

Abstract Displacement cross-sections for an advanced assessment of radiation damage rates were obtained for a number of structural materials irradiated with protons at energies from threshold up to 10 GeV. The proposed calculation method utilises an athermal recombination-corrected dpa model with corrections obtained from simulations using the binary collision approximation model. Justification of the method was performed using available measured and systematics data.

Molecular dynamics studies of ion beam implantation and patterning of silicon: Effect of noble gas cluster formation

The use of energetic ion beams to induce nanopattern formation at surfaces has been well studied both experimentally and theoretically. However, the influence on morphological evolution of the implanted species themselves remains little understood, particularly in the case when the incident ion species does not interact chemically with the target material. In this work, MD simulation results are presented for cumulative ion bombardment of Si to a fluence of $3\ifmmode\times\else\texttimes\fi{}1{0}^{15}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$ or more for a range of incident ion energies (20--1000 eV), angles ($0\ensuremath{-}{85}^{\ensuremath{\circ}}$), and species (Ne, Ar, Kr, Xe). For most cases, the implanted ions are observed to form gas clusters or bubbles beneath the surface as the fluence increases. The implantation and cluster formation decrease in magnitude with increasing ion incidence angle, and remain fairly similar for the heavier-than-Si species (Ar, Kr, and Xe). However, the implantation and cluster formation are much more prominent for Ne irradiation. As the fluence continues to increase beyond $\ensuremath{\sim}1{0}^{15}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$, the gas clusters begin to become exposed to the vacuum as the Si layers trapping the gas atoms are eroded by the incident ions. The exposed gas clusters then degas very rapidly, leading to disruption at the surface and to viscous material flow of Si into the void left behind. Comparison to dynamic binary collision approximation (BCA) simulations indicates that cluster formation and degassing contributes to a wide distribution of single-impact emission yields of implanted ions, contrary to intuitive expectations based on BCA simulations. Notably, the increased size and frequency of many-atom implanted ion emission events contributes to a much lower concentration of the implanted species than is otherwise expected from BCA simulations. Additionally, this cluster degassing phenomenon is conjectured to provide a potential ``antipatterning'' mechanism by disrupting or destroying nanopattern ``seeds'' at the surface. This could provide an additional mechanism to improve model predictions of critical angles for patterning transitions, and may also provide at least a partial explanation for the difficulty of obtaining patterns on Ne-bombarded Si surfaces.

Solar wind sputtering of wollastonite as a lunar analogue material – Comparisons between experiments and simulations

The sputtering of wollastonite (CaSiO3) by solar wind-relevant ions has been investigated experimentally and the results are compared to the binary collision approximation (BCA) codes SDTrimSP and SRIM-2013. Absolute sputtering yields are presented for Ar projectiles as a function of ion impact energy, charge state and impact angle as well as for solar wind H projectiles as a function of impact angle. Erosion of wollastonite by singly charged Ar ions is dominated by kinetic sputtering. The absolute magnitude of the sputtering yield and its dependence on the projectile impact angle can be well described by SDTrimSP as long as the actual sample composition is used in the simulation. SRIM-2013 largely overestimates the yield especially at grazing impact angles. For higher Ar charge states, the measured yield is strongly enhanced due to potential sputtering. Sputtering yields under solar wind-relevant H+ bombardment are smaller by two orders of magnitude compared to Ar. Our experimental yields also show a less pronounced angular dependence than predicted by both BCA programs, probably due to H implantation in the sample. Based on our experimental findings and extrapolations to other solar wind ions by using SDTrimSP, we present a model for the complete solar wind sputtering of a flat wollastonite surface as a function of projectile ion impact angle, which predicts a sputtering yield of 1.29 atomic mass units per solar wind ion for normal impact. We find that mostly He and some heavier ions increase the sputtering yield by more than a factor of two as compared to bombardment with only H+ ions.

Molecular dynamics studies of low-energy atomic hydrogen cumulative bombardment on tungsten surface

Applicability of tungsten (W) in divertor regions which are exposed to high flux hydrogen (H) isotope with the energy below 100 eV, is concerned. In this study, the cumulative bombardment of low-energy H atoms on W surface was investigated by molecular dynamics (MD) simulations. For the sake of comparison, the reflection and the retention of the incident H atoms were also calculated and analyzed by the Monte Carlo (MC) method based on the binary collision (BC) approximation and the analytical method based on the bipartition model (BPM). It was found that in addition to the reflection of the incident H atoms, some of the H atoms can implant into the substrate and penetrate from the bottom surface of the substrate due to the channeling effect. The channeling effect mainly contributes to the difference of the distribution of H projectiles, since the structure of substrate in MD is crystal while that in the other two methods is usually homogeneous amorphous. As expected, the point defects (including sputtering atoms) were not observed in all simulations since the energy projectile is below the threshold displacement energy.