Binary Collision Approximation Research Articles

On the use of SRIM for calculating vacancy production: Quick calculation and full-cascade options

The vacancy production behavior for a broad range of incident ion-target combinations were examined by using both the full cascade (F-C) and quick calculation of damage (Q-C) options of Stopping and Range of Ions in Matter (SRIM-2013) code. Vacancy production for each option was evaluated using both the convenient vacancy text file in SRIM, and by manual manipulation of the energy transfer, ionization, and phonon text file data to calculate the depth-dependent damage energy for the Norgett-Robinson-Torrens (NRT) defect production model. The F-C vacancy text file method was observed to overpredict vacancy production by a factor of ~1.1 to 4 times when compared to values calculated using the F-C damage energy method or either of the SRIM Q-C methods. Based on our extensive data set obtained for 142 unique bombarding ion-target combinations, we show that the SRIM F-C vacancy text file method should not be used for vacancy production calculations. This error appears to be due to mischaracterization within the SRIM F-C option of some near-threshold replacement events as vacancies instead of replacements. Use of the latest SRIM stopping powers in the SRIM F-C approach provides a better calculation of electronic and nuclear stopping compared to the Lindhard stopping power analytical approximation used to calculate the damage energy of recoil atoms in the SRIM Q-C option, and therefore SRIM F-C approach is deemed to provide the best accuracy for vacancy production (within the binary collision approximation) as long as the damage energy method is used. Alternately, the SRIM Q-C option using either the vacancy.txt or damage energy method provides relatively fast calculation speeds with moderate quantitative differences from the F-C damage energy results (~0 to ~ 30% for the investigated ion-target cases).

Sputtering of surfaces by ion irradiation: A comparison of molecular dynamics and binary collision approximation models to laboratory measurements

We compare various sputtering simulation methods to experimental results in both the low energy (<1 keV) and high energy (≥1 keV) impact regimes for argon ions impacting a pure copper substrate at normal incidence. Our results indicate that for high energy impacts, both binary collision approximation (BCA) and molecular dynamics methods can be used to generate reasonable predictions for the yield and energy distribution of the sputtered atoms. We also find reasonable agreement between the theoretical and experimental results down to impact energies of 600 eV. However, at 200 eV impact energies, significant discrepancies appear between the experimental and theoretical ejecta energy distributions in the peak position, the width of the energy distribution, and the magnitude of the high energy tail. These discrepancies appear to arise from the experimental results being only for atoms sputtered normal to the substrate surface, whereas the theoretical results are integrated over all 2π solid angles above the surface. Using the BCA code SDTrimSP and limiting the results to only atoms emitted within ±15° of the surface normal brings theory and experiment into reasonable agreement. These results suggest that for low energy impacts, the energy distribution of sputtered atoms is highly dependent on the emission angle of the ejecta.

A study of the formation of isotopically pure 28Si layers for quantum computers using conventional ion implantation

We have investigated the use of conventional ion implantation to fabricate enriched 28Si layers for use in quantum computers. The final compositions of samples enriched using ultra-low energy (ULE) (800 eV and 2 keV) and low energy (20 keV) 28Si implants of varying fluences (1 × 1016–3.8 × 1017 cm−2) using two different implanters were measured using channelled Rutherford Backscattering Spectroscopy (RBS). The dynamic, binary collision approximation program TRIDYN was used to model the implantation profiles to guide the analysis of the RBS spectra. It was found that ULE implants achieved high 28Si enrichment levels but were heavily contaminated with oxygen due to poor vacuum in the implanter wafer end station. It was shown that oxidation could be reduced by using an accelerator with an end station with better vacuum and increasing the implant energy to 20 keV. However, TRIDYN simulations predict that the best 28Si enrichment levels that could be achieved under these conditions would saturate at ∼99.2% due to self-sputtering. We modelled a range of conditions with TRIDYN and so recommend low energies (<3 keV), ultra-high vacuum implantation for high 28Si enrichment (>99.9%) with the lowest possible fluences (∼5–10 × 1017 cm−2).

Inelastic energy loss of Ar ions scattered Al2O3 surface under grazing incidence

In this paper presents the investigation of the surface Al2O3 by ion scattering spectroscopy. The trajectory of small angle scattered ions calculated by the method of binary collision approximation. It was found dependence of inelastic energy loss and trajectories of scattered ions. The value of inelastic energy loss scattered ions almost depend to the angle of incidence and the geometrical parameters of surface semichannels.

A linked-scale coupled model of mass erosion and redistribution in plasma-exposed micro-foam surfaces

Surface evolution due to exposure to harsh environments is of importance in many scientific and technological applications. In plasma-exposed materials, the surface receives charged particles from the plasma, leading to a series of processes that drive the system far from equilibrium and may lead to the severe deterioration of the surface properties. Although surface morphological changes are driven by atomic collisions taking place over picoseconds and nanometers, these processes result in mass loss and redistribution of matter over much larger length and time scales. This necessitates a multi-scale approach capable of capturing the range of processes linking primary atomic collision events with engineering-level surface geometry changes. In this paper, we develop a computational model to simulate the morphological evolution and effective erosion rate of micro-architected tungsten foams during low-energy plasma ion bombardment. The model acts on several length scales, with the energy and angular dependence of the sputtering yield of flat tungsten surfaces determined using the SRIM code based on the binary collision approximation. This information is introduced into a low-fluence, short-term Monte Carlo raytracing model, and further into a high-fluence, long-term particle transport model. In the latter, material particles representing billions of atoms are described in a digitized 3-D representation of the foam structure from X-ray tomography data, and are sputtered off and redeposited using the atomistic information. We show that the redeposition of sputtered atoms leads to partial self-healing in the bottom layers with a sharp reduction in the sputtering coefficient of low density foams, roughly 25% of the solid W value. This is in qualitative agreement with recent experiments on low porosity W structures. At high fluence, the foam structure degrades considerably as there are fewer ligaments available to recapture sputtered atoms and, consequently, the sputtering rate increases again.

Understanding the role of energetic particles during the growth of TiO2 thin films by reactive magnetron sputtering through multi-scale Monte Carlo simulations and experimental deposition

In this paper, a previously established 3D multi-scale simulation chain of plasma deposition process, based on a combination of a direct simulation Monte Carlo (gas phase) algorithm and a kinetic Monte Carlo (kMC) (film growth) code, is improved by the addition of a particle-in-cell Monte Carlo collision algorithm in order to take into account and clarify the role of charged particles. The kinetic Monte Carlo code is also extended with a binary collision approximation algorithm to handle charged particles. This modelling strategy is successfully applied to the growth of TiO2 thin films by means of reactive magnetron sputtering. In order to highlight the effects of negative oxygen ions, two substrate locations are selected: one in the median plane of the targets and another one off the median plane. The model efficiently predicts the densities and fluxes of both charged and neutral particles towards the substrate. Typical results such as particle densities, the discharge current density and ion flux onto the target, and the various substrate locations are calculated. The angular distribution and energy distribution of all involved particles are sampled at these very same substrate locations and the nanoscale modelling (NASCAM) code, implementing the kMC approach, uses these results to explain the morphology of the experimentally deposited coatings. The changes throughout the transition from metallic deposition to stoichiometric TiO2 of the columnar structure of the deposited films is explained by the suppression of the atom diffusion on the growing film due to Ti oxidation. Moreover, the high-energy negative atomic oxygen ions originating from the targets are identified as the origin of the abnormally low inclination of the columnar structure experimentally observed for the oxide mode coatings. Measurements of the normalized energy flux (energy per deposited atom) are experimentally investigated to support and highlight the important role of energetic particles during film growth.

Ultrathin Co films with Pt and Au covers—magnetic and structural properties driven by Ga+ ion irradiation

The X/Co 3 nm/Y (where X, Y = Au, Pt) trilayers with as deposited in-plane magnetization alignment were irradiated with 30 keV Ga+ ions in the wide range of ion fluence. The samples were investigated by means of complementary techniques: magneto-optical magnetometry and spectroscopy (in the photon energy range from 1.2 eV to 4.5 eV), magnetic force microscopy, positron annihilation spectroscopy, x-ray diffraction and reflectivity. Difference in miscibility of interface atoms is clearly manifested in various intermixing extent at Co/Pt and Co/Au interfaces and consequently in magnetic properties of the irradiated trilayers. Low irradiation fluence (∼1014 ions cm−2) leads to ∼1 nm interfaces broadening without visible surface etching for all samples, which is related with a distinct drop of magnetic anisotropy. However, the high irradiation fluence (∼5 × 1015 ions cm−2) results in enhanced interface broadening and significant surface etching (∼5 nm) partially removing also Co atoms. Tensile strains (up to 0.5%) were developed in the cover layers. The tensile strain, layers intermixing and the creation of Co–Pt(Au) alloys with different composition formed by irradiation are correlated with the increase of magnetic anisotropy. Moreover it was observed that substitution of Au instead of Pt (as a cap or buffer layer) results in substantial increase of perpendicular magnetic anisotropy. Maximal increase of magnetooptical parameters was observed for Pt/Co/Pt layer. Irradiation induced changes of concentration profiles are revealed using magnetooptical spectra, x-ray reflectivity spectra and simulations with use of binary collision approximation.

Measurements of the energy distribution of W atoms sputtered by low energy Ar ions using high-resolution Doppler spectroscopy

Emission spectra of neural tungsten (W) sputtered by impact of argon (Ar) ions in a weakly magnetized (<0.1 T) Ar plasma were measured using a high resolution spectrometer at normal incidence angle to the surface. The measurements were performed for the mono-energetic impact energies between 70 and 150 eV using the neutral tungsten (W I) line at 4982.593 Å. The line shape of this line was simulated using a Doppler-shifted emission model to determine the energy distribution. Additional broadening mechanisms were taken into account: instrumental broadening, Zeeman effect and finally the photon or light reflectance at the W surface.The obtained energy distribution was found in a very good agreement with the Thompson distribution, even though deviations for lower impact energies are observed, e.g. the high-energy tail of sputtered particles demonstrates a faster drop compared to 1/E 2 at energies below 100 eV. Moreover, the standard cosine (Knudsen cosine law) distribution provides a rather good description of emission spectra in the energy range of study. Finally, the energy distribution was also compared with simulations carried out with the binary collision approximation (BCA) based Monte-Carlo code SDTrimSP. It shows a marginally worse description at low energies and better description of the high energy tail compared to the Thompson one. Furthermore, the model was used to determine in-situ the degree of light reflection at the W surface. The results are in excellent agreement with the literature data.

Effect of the Nanoscale Surface Relief on the Sputtering of Amorphous Carbon

In the binary collision approximation, the computer simulation of amorphous-carbon sputtering under bombardment with Ar ions with an energy of 1—10 keV is carried out with inclusion of the surface nanorelief of sinusoidal form. The angular characteristics of the target sputtering yield are obtained for different relief parameters. The simulation results are compared with analytical estimates and Monte Carlo calculations based on Sigmund sputtering theory. The simulation does not confirm a sharp increase in the sputtering yield of the undulating surface with increasing relief amplitude, which was predicted theoretically (Makeev, Barabasi). Significant discrepancies in the angular dependences of the sputtering yield are also observed. The calculation results are compared with existing experimental data.

Monte Carlo simulations of MgO and complex oxide protective thin layers bombarded with noble-gas ion in plasma discharge devices

The lifespan of plasma discharge devices is strongly influenced by the deterioration characteristics of MgO protective layer deposited on the dielectric covering the electrode. In order to attain both lower driving voltage and higher luminous efficiency in these devices, different complex metal oxides protective layers have been extensively studied as alternatives to MgO. However, the interaction between energetic ions and fast neutral atoms with the protective layer can produce serious damages to it and their nearby components. In this paper, we study the ion beam bombardment for several protective layers by low-energy noble gas with various ion incidence angles by using the Monte Carlo simulation. On the basis of the binary collision approximation using SRIM-2013, different parameters are discussed for instance backscattering yield, retained dose, sputtering yield, number of vacancies, and ion range of MgO, (Mg,Ca)O, (Mg,Sr)O, (Mg,Ba)O and (Mg,Ca,Sr)O. From our results, the retained dose, sputtering yield and number of vacancies of complex metal oxide protective layers are lower than that of MgO. Moreover, the backscattering yield increases by increasing the incident angles and it is highest for complex metal oxide protective layers.

Quantitative investigation of surface structure and interatomic potential with impact-collision ion scattering spectroscopy

Helium ion beam interactions with materials have important implications for magnetic confinement fusion, material modification, and helium ion microscopy. These interactions depend on the precise physics of how helium ions channel into the materials, which can vary greatly based on the local crystalline orientation. In this work, we performed a dedicated experiment to investigate helium ion channeling in a well-characterized tungsten single crystal. Time-of-flight impact-collision ion scattering spectroscopy was used to obtain multi-angle maps of the backscattering intensity for 3 keV He+ → W(111). We found that the backscattering intensity profile arising from helium ion channeling could be well described by a shadow cone analysis. This analysis revealed that subsurface W atoms as deep as the ninth monolayer contributed to the backscattering intensity profile. Binary collision approximation simulations were performed with MARLOWE to model the experimental maps with sufficient accuracy to allow for quantitative comparisons using reliability factors. These quantitative comparisons were applied to investigate how the W lattice structure and He–W interatomic potential affect the multi-angle maps.

New developments in the simulation of Rutherford backscattering spectrometry in channeling mode using arbitrary atom structures

As Rutherford backscattering spectrometry in channeling mode (RBS/C) is an efficient technique for characterizing crystallographic defects, its computational simulation has drawn attention over the past several decades. Recently, a RBS/C simulation code based on the binary collision approximation called Rutherford backscattering simulation in arbitrary defective crystals has been suggested and successfully applied to predict the RBS/C spectra from different damaged materials, whose structures were generated in high-dose ion irradiation atomistic simulations. In the present paper, we introduce new developments improving the flexibility of the developed software and its applicability to different types of materials. More precisely, we modified the algorithm describing the slowdown process of backscattered ions, added fitting parameters in the collision partner search routine, modified the routine taking into account target atom thermal vibrations and provided new descriptions of the ion beam divergence. As an example, the effect of the modifications on simulated RBS/C spectra is shown for an 〈011〉-oriented UO2 crystal analyzed with a 3.085 MeV He2+ ion beam. Some of these changes proved necessary to achieve satisfying agreement between simulations and experimental data. Similar observation was made for 〈001〉-oriented Si and 〈001〉-oriented GaAs crystals analyzed with a 1.4 MeV He+ ion beam. In these simulations, the modifications have also resulted in good agreement with experiment.

Structural and chemical evolution of Au-silica core\u2013shell nanoparticles during 20\xa0keV helium ion irradiation: a comparison between experiment and simulation

Au-silica core–shell nanoparticles have been irradiated with 20 keV He+ ions up to a maximum fluence of 4.7 × 1017 ions/cm2. The nanoscale structural and crystallographic evolution induced by He+ ion irradiation was followed at various stages using Transmission Electron Microscopy (TEM). During irradiation satellite Au clusters are formed around the main Au core, which remained crystalline even after the maximum He+ ion fluence. The spherical silica shell deformed into a hemisphere due to He+ ion irradiation. Three dimensional Monte-Carlo simulations, based on the binary collision approximation, have been performed on stacked infinite layers and an individual particle. The stacked layers results show that the He+ beam interacts with most of the nanoparticle and Au migrates in the direction of beam incidence agreeing with experimental findings. The individual particle results match the experiment in terms of the volume which is sputtered away however additional mechanisms, not included in the simulations, are present in the experiment during the satellite formation and silica shell deformation. These results show the ability for 20 keV He+ ions to be used for the modification of nanostructures. Furthermore, these results contribute to a quantitative understanding of the dynamic evolution of materials observed using microscopy techniques based on He+ ions.

Models and regressions to describe primary damage in silicon carbide

Silicon carbide (SiC) and SiC/SiC composites are important candidate materials for use in the nuclear industry. Coarse grain models are the only tools capable of modelling defect accumulation under different irradiation conditions at a realistic time and length scale. The core of any such model is the so-called “source term”, which is described by the primary damage. In the present work, classical molecular dynamics (MD), binary collision approximation (BCA) and NRT model are applied to describe collision cascades in 3C-SiC with primary knock-on atom (PKA) energy in the range 1–100 keV. As such, BCA and NRT are benchmarked against MD. Particular care was taken to account for electronic stopping and the use of a threshold displacement energy consistent with density functional theory and experiment. Models and regressions are developed to characterize the primary damage in terms of number of stable Frenkel pairs and their cluster size distribution, anti-sites, and defect type. As such, an accurate cascade database is developed with simple descriptors. One of the main results shows that the defect cluster size distribution follows the geometric distribution rather than a power law.

Simulation of very-low energy alkali ion (≤ 10 KeV) induced effects on Al2O3 micro flakes

Objectives: To simulate the Monte-carlo simulation of irradiation of alkali ions (sodium) having very low energy (5 keV and 10 keV) on aluminum oxide micro flakes. Methods/Statistical analysis: We have utilized a simulation process namely SRIM (Stopping of ion ranges in matter), which is based on the binary collision approximation technique. We have fixed our target as an aluminum oxide in the layered structure having a thickness about 65 nm. We have incorporated two different types of ion energy as the input parameters which are normally incident on the targets. We have analyzed ion distributions, recoil distributions, and further ionizations. Findings: The projected average range for 10 keV is significantly found to be higher, almost double than that of 5 keV. The reason behind this increment is due to the high penetration depth because of higher energy. The straggling of 10 keV is higher than that of 5 keV, which is evident from the recoil distribution where the cascade collision has created a large volume of vacancies, which is very high for higher energy. Application/Improvements: This simulation helps us to gather a rich amount of information regarding ion-induced defects, which is highly essential for experiments on aluminum oxide micro flakes. The surface modification after this low ion energy bombardment leads to low detrimental effects which may modify the wetting properties of these flakes Keywords: Montecarlo simulation; SRIM; Aluminium oxide micro flakes; ion induced defects; BCA

A Survey of the Analytical Methods of Proton-NIEL Calculations in Silicon and Germanium

The nonionizing energy loss (NIEL) concept has been used for many years for qualitative estimation of the damage induced by radiation (particles and gamma rays). Different approaches, based on the physics of radiation interaction with the nuclear and electronic subsystems of the target atoms, are used for calculating NIEL values. The simplest model used is the binary collision approximation (BCA), which is applicable for energies larger than the threshold energy for Frenkel pair creation. In this article, we analyze the dependence of the calculated NIEL values (in silicon and germanium) on the basic interaction characteristics: the differential cross section (DCS) of energy transferred to the recoiling atoms and the partition factor between ionization and ion displacement. We estimate the differences between existing approaches and the possible scatter of the NIEL data for Coulomb, elastic, and inelastic nuclear scatterings. We also present new partition factors based on fits to experimental data. The contribution of low-energy cascade zones of displaced atoms (pockets) to the total damage is estimated using results of molecular dynamics (MD) calculation. We find for silicon a total increase of NIEL by 30%–40% relative to the original (BCA) NIEL value. The role of phonon excitations in the subthreshold cascades on the damage value is discussed.

Influence of the Potential-Well Depth on the Reflection of Deuterium Atoms from the Surface of Tungsten

The dependence of the reflection coefficient on the form of the atom—surface interaction potential for the D—W system was found. Potentials with different potential well depths were considered. Calculations were carried out by simulating projectile trajectories and in the binary collision approximation.

Sputtering of nanostructured tungsten and comparison to modelling with TRI3DYN

He-induced nanostructured tungsten (so-called W-fuzz) was bombarded with Ar ions under 60° and the dynamic erosion behaviour was experimentally investigated. By using a highly sensitive quartz-crystal-microbalance technique in a particle catcher configuration, the sputtered particles distribution of W-fuzz could be evaluated. In contrast to a flat sample, where sputtered particles are emitted primarily in forward direction, we find that W-fuzz samples emit sputtered particles preferably in backward direction (i.e. in the direction of the incident ion beam). After continuous Ar irradiation of a W-fuzz sample the distribution approaches that of a flat sample. In addition to experimental data we also show modelling results obtained with a state-of-the-art Monte-Carlo (MC) binary collision approximation (BCA) code TRI3DYN in full 3D. Surface morphology changes as monitored by SEM as well as the dynamic sputtering behaviour can be well reproduced by the full 3D MC-BCA code.

Simulation of Single Particle Displacement Damage in Si₁₋ₓGeₓ Alloys—Interaction of Primary Particles With the Material and Generation of the Damage Structure

Primary simulations of neutron interactions are performed on Si1− x Ge x alloys with a Monte Carlo (MC) code using the binary collision approximation (BCA). Then, a statistical study of the collision cascades development in Si0.8Ge0.2, Si0.7Ge0.3, and Si0.5Ge0.5 is carried out using molecular dynamics (MD), starting from both Si and Ge primary knock-on atoms (PKAs) of 1, 5, and 10 keV. The well-known Stillinger–Weber (SW) MD potential, which can be used to study Si, Ge, and Si1− x Ge x , is coupled to the Ziegler–Biersack–Littmark (ZBL) universal potential to better describe the collisions between atoms. To account for the stopping power of the electrons, the two-temperature model (TTM) is combined with MD. Similar studies are performed on pure Si and pure Ge in order to be able to compare our Si–Ge alloys damaged structures with reference materials. Moreover, data obtained by TTM-MD on Si, Ge, and Si1− x Ge x are compared with collision cascades statistical data from MC codes.

Defect and density evolution under high-fluence ion irradiation of Si/SiO2 heterostructures

We present molecular dynamics simulations of atomic mixing over a $\mathrm{Si}/{\mathrm{SiO}}_{2}$ heterostructure interface, induced by focused ${\mathrm{Ne}}^{+}$ and broad ${\mathrm{Si}}^{+}$ ion-beam irradiations, using a speed-up scheme that significantly reduces the relaxation time of the cascading recoils. To assess the qualitative reliance of the chosen method, two different potential models for Si--O, Si--Si, and O--O interactions were used: the Stillinger-Weber--like Watanabe-Samela potential and the Tersoff-like Munetoh potential. Furthermore, the molecular dynamics simulations were assessed by simulating a similar case, at a total fluence of $1\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, with the binary collision approximation. The same general atomic density profile distributions were achieved with both models; however, the binary collision approach showed shallower penetration of Si into the ${\mathrm{SiO}}_{2}$ layer. Coordination analysis of the molecular dynamics results provides strong evidence that ion mixing at high fluences leads to coordination defects, which will affect the electronic properties of the structures unless removed with annealing.