Molecular dynamics simulations of large fluorine cluster impact on silicon with supersonic velocity

The etching process by very large reactive gas cluster impact was investigated by molecular dynamics (MD) simulations. Fluorine-molecule clusters with the size up to 100,000 atoms (50,000 F₂ molecules) were irradiated on silicon (1 0 0) targets at supersonic velocity regime (0.1-1 eV/atom, 1.0-3.2 km/s). The MD simulations revealed that the existence of threshold energy-per-atom around 0.3 eV/atom (1.75 km/s) to cause surface deformation and enhancement of Si desorption. When the incident energy-per-atom is less than the threshold, the incident cluster breaks up itself on the target without surface deformation. The fluorine molecules in the cluster spread in the lateral direction along the target surface, and some part of them decompose and adsorbs on the target to form silicon fluoride composites. On the other hand, the clusters penetrate the surface of silicon target when the energy-per-atom is larger than 0.3 eV/atom. In these collisional processes, the target surface is deformed to create shallow crater shape. The incident fluorine molecules are preferentially concentrated at the bottom of the crater, which resulted in high desorption yield of silicon as in the form of SiF₂, SiF₃ and SiF₄.     

Molecular dynamics simulations of argon cluster impacts on a nickel film surface

In this study we probe the surface phenomena that occur on nickel thin films after argon cluster impacts by performing several simulations using various energies. The simulations are carried out based on a molecular dynamics (MD) approach. The argon cluster consists of 353 atoms with energies ranging from 1 keV to 3.0 keV. The simulation results show that when the incident energy is 1 keV, the surface retains its smoothness after impact although a slight thermal effect appears near the surface beneath the impact area. Increasing the argon cluster energy to 2 keV causes the atoms in the film to shift slightly under impact and a small hillock appears on the film surface after impact. When the cluster energy increases to 3 keV, a hemispherical crater will appear on the film surface after impact. In addition, a shock wave is generated within the film due to the impact, which propagates toward to the substrate in a hemispherical shape. These shock wave related phenomena are difficult to probe experimentally on an atomic level however molecular dynamics simulations are a suitable tool for investigating the shock wave phenomena in thin film.     

Study of density effect of large gas cluster impact by molecular dynamics simulations

Large gas cluster impacts cause unique surface modification effects because a large number of target atoms are moved simultaneously due to high-density particle collisions between cluster and surface atoms. Molecular dynamics (MD) simulations of large gas cluster impacts on solid targets were carried out in order to investigate the effect of high-density irradiation with a cluster ion beam from the viewpoint of crater formation and sputtering. An Ar cluster with the size of 2000 was accelerated with 20 keV (10 eV for each constituent atom) and irradiated on a Si(1 0 0) solid target consisting of 2 000 000 atoms. The radius of the Ar cluster was scaled by ranging from 2.3 nm (corresponding to the solid state of Ar) to 9.2 nm (64× lower density than solid state). When the Ar cluster was as dense as solid state, the incident cluster penetrated the target surface and generated crater-like damage. On the other hand, as the cluster radius increased and the irradiation particle density decreased, the depth of crater caused by cluster impact was reduced. MD results also revealed that crater depth was mainly dominated by the horizontal scaling rather than vertical scaling. A high sputtering yield of more than several tens of Si atoms per impact was observed with clusters of 4-20× lower volume density than solid state.     

Structural study of Co and Au nanoclusters landed onto Cu

The process in which nanoclusters of Co and Au at low energy (<1 eV/atom) impinge on a substrate of Cu (0 0 1) has been studied by molecular-dynamics (MD). Particularly our interest is focussed on the crystalline structure of the clusters and on whether the epitaxy is achieved. The atomic interactions are calculated by means of a many-body potential based on the second momentum approximation of tight-binding scheme.The size of the clusters ranges from a few tens to a few hundreds of atoms. Techniques such as the analysis of grains, the calculation of epitaxy factor and the common neighbour analysis (CNA) are methods of analysis, used in this work, capable to determine the crystalline orientation and resemblance of the clusters to the substrate once they have been deposited on it. The number of atoms with fcc structure is explicitly calculated. The high values found, for large clusters, corroborate that part of the original structure of the cluster remains intact.The results obtained can be summarized as follows: whenever epitaxy requires an expansion of the cluster structure to fit the substrate, the process is more effective than the opposite in which a compression is required. The size of the cluster plays an opposite role in the epitaxial process, i.e. the effectiveness to get epitaxy diminishes for large sizes.     

Investigation of cascade-induced re-solution from nanometer sized coherent precipitates in dilute Fe–Cu alloys

Molecular dynamics (MD) simulations have been used to investigate the re-solution of copper atoms from coherent, nanometer-sized copper precipitates in a body-centered cubic iron matrix. The molecular dynamics simulations used Finnis–Sinclair type interatomic potentials to describe the Fe–Cu system. Precipitate diameters of 1, 3 and 5 nm were studied, with primary knock-on atom (PKA) from 1 to 100 keV, although the majority of the cascade simulations and analysis of solute re-solution were performed for cascades of 10 or 20 keV. The simulation results provide an assessment of the re-solution on a per-atom basis as a function of precipitate size, cascade location and energy. Smaller sized precipitates, with a larger surface to volume ratio, experienced larger re-solution on a per-atom basis than larger precipitates. Re-solution was observed to occur predominantly in the initial ballistic stages of the cascades when atomic collisions occur at high kinetic energy. A minimum PKA energy of around 1 keV was required to produce re-solution, and the amount of re-solution appears to saturate for PKA energies above approximately 10 keV, indicating that the MD results are representative of the energy range of interest. A model for prompt, cascade induced solute atom re-solution has been derived, following the approach used to describe fission gas bubble re-solution, and the parameters for describing copper atom re-solution are provided.     

Nanoscale metal-silicide films prepared by surfactant sputtering and analyzed by RBS

Surfactant sputtering has been applied to modify the surface structure of Si substrates and to produce ultrathin metal-silicide films with nickel and platinum surfactants, utilizing the steady state coverage of a Si-substrate surface with surfactant atoms simultaneously during sputter erosion by combined ion irradiation and surfactant atom deposition. Si (1 0 0) substrates were eroded using 5 keV Xe-ions and 10–30 keV Ar ions at incident angles of 65° and 70° with fluences of up to 2 × 10¹⁸/cm² under continuous sputter deposition of platinum and nickel from targets irradiated simultaneously by the same ion beam. These surfactant atoms form metal-silicides in the surface near region and strongly modify the substrate sputter yield and the surface nanostructure. Atomic force microscopy and scanning electron microscopy were carried out to observe a transition of surface topography from ripple to relief patterns, granular patterns or smooth surfaces. The Si and metal sputter yield as function of the steady state metal coverage were determined by combination of Rutherford-backscattering spectroscopy (RBS) and profilometry. The composition and the depth distributions of metal-silicide films were analyzed via high-resolution RBS and transmission electron microscopy. We show that RBS results in comparison with SRIM and TRIDYN sputter yield simulations allows us to identify the silicide surface structure on the nanometer scale.     

Cluster size effect on reactive sputtering by fluorine cluster impact using molecular dynamics simulation

The mechanism of high-yield sputtering induced by reactive cluster impact was investigated using molecular dynamics (MD) simulations. Various sizes of fluorine clusters were radiated on clean silicon surface. At an incident energy of 1 eV/atom, F atom and F₂ molecule are only adsorbed on the surface and sputtering of Si atom does not occur. However, fluorine cluster, which consists of more than several tens molecules causes sputtering. In this case, most of Si atoms are sputtered as fluorinated material such as SiF_x. This effect is due to the fact that cluster impact induces high-density particle and energy deposition, which enhances both formation of precursors and desorption of etching products. The deposition of atoms and energy becomes denser as the incident cluster size increases, so that larger clusters have shown higher sputtering yield.     

High-speed processing with Cl2 cluster ion beam

Cluster ion beam processes can produce high rate sputtering with low damage compared with monomer ion beam processes. Cl₂ cluster ion beams with different size distributions were generated with controlling the ionization conditions. Size distributions were measured using the time-of-flight (TOF) method. Si substrates and SiO₂ films were irradiated with the Cl₂ cluster ions at acceleration energies of 10–30 keV and the etching ratio of Si/SiO₂ was investigated. The sputtering yield increased with acceleration energy and was a few thousand times higher than that of Ar monomer ions. The sputtering yield of Cl₂ cluster ions was about 4400 atoms/ion at 30 keV acceleration energy. The etching ratio of Si/SiO₂ was above eight at acceleration energies in the range 10–30 keV. Thus, SiO₂ can be used as a mask for irradiation with Cl₂ cluster ion beam, which is an advantage for semiconductor processing. In order to keep high sputtering yield and high etching ratio, the cluster size needs to be sufficiently large and size control is important.     

Surface segregation of Ti atoms during NiTi alloy sputtering

The paper addresses NiTi alloy sputtering by 9 keV He and Ar ions and discusses the experiment performed by V.S. Chernysh et al. about 10 years ago. The binary collision simulation has been applied to extract the concentrations of surface Ni and Ti atoms from the experimental data. The results of simulations favor segregation of Ti for both He and Ar ion bombardment. The effect of non-symmetric surface collisions (Ti on Ni and Ni on Ti) was found to be negligible. A pronounced effect of the interatomic (target–target) potential is noted.     

Temporal development of sputtered atom distributions from Au(1 1 1) targets following bombardment with 100 keV/atom Au2 ions

Recently Bouneau et al. measured the angular and energy distributions of negative Au_n (n=2–7) ions emitted from gold targets following bombardment with swift gold cluster projectiles. They found that the energy distributions could be fitted with a spike-like model, and that the angular distributions were independent of the azimuthal emission angle and relatively strongly forward directed. We have used MD simulations to investigate the temporal development of energy and angular distributions of sputtered atoms from Au(1 1 1) targets following bombardment with 100 keV/atom Au₂ ions. Our results show that during the very early stages of the collision cascade the energy distribution of sputtered atoms is described well by the linear cascade model. Essentially all high energy sputtered atoms are emitted during this phase of the collision cascade. However, the energy distributions of atoms sputtered after 0.5 ps were typical of emission from a thermal spike and could be fitted well with a Sigmund–Claussen model. The polar angle distributions of sputtered atoms were strongly forward directed early in the collision cascade, but became less forward directed as the thermal spike developed.     

Repulsive potentials for low energy projectiles incident on copper surfaces

Interatomic potentials that are relevant for low energy (<2 keV) projectiles (X = He, Cl, Ar, Cu) incident on the Cu(1 0 0) and Cu(1 1 1) surfaces (represented by Cu₁₀ clusters), and on isolated Cu atoms, have been calculated from first principles using density functional theory. For energies above 10–20 eV, the diatomic X–Cu potentials provide acceptable approximations (typically within 1–2 eV) to the corresponding potentials for the surface environment.     

Damage analysis of benzene induced by keV fullerene bombardment

Molecular dynamics computer simulations have been used to investigate the damage of a benzene crystal induced by 5 keV C₂₀, C₆₀, C₁₂₀ and C₁₈₀ fullerene bombardment. The sputtering yield, the mass distributions, and the depth distributions of ejected organic molecules are analyzed as a function of the size of the projectile. The results indicate that all impinging clusters lead to the creation of almost hemispherical craters, and the process of crater formation only slightly depends on the size of the fullerene projectile. The total sputtering yield as well as the efficiency of molecular fragmentation are the largest for 5 keV C₂₀, and decrease with the size of the projectile. Most of the molecules damaged by the projectile impact are ejected into the vacuum during cluster irradiation. Similar behavior does not occur during atomic bombardment where a large portion of fragmented benzene molecules remain inside the crystal after projectile impact. This “cleaning up” effect may explain why secondary ion mass spectrometry (SIMS) analysis of some organic samples with cluster projectiles can produce significantly less accumulated damage compared to analysis performed with atomic ion beams.     

Interaction of 〈1 0 0〉 loops with Carbon atoms and 〈1 0 0〉 dislocations in BCC Fe: An atomistic study

We apply atomistic simulations using the so called ‘metallic-covalent bonding’ interatomic model for the Fe–C system to study mobility of 〈1 0 0〉 interstitial dislocation loops, known to form in Fe and Fe-based ferritic alloys under irradiation, and their interaction with Carbon atoms. Carbon atoms represent an effective trap for the 〈1 0 0〉 loops with a binding energy of the order of 1 eV. The mobility of the loops is studied using the dislocation – loop drag model. From this model the activation parameters are identified and discussed.     

Study of modifications in Lexan polycarbonate induced by swift O6+ ion irradiation

Swift Heavy Ion (SHI) irradiation of the polymeric materials modifies their physico-chemical properties. Lexan polycarbonate films were irradiated with 95 MeV oxygen ions to the fluences of 10¹⁰, 10¹¹, 10¹², 10¹³ and 2 × 10¹³ ions/cm². Characterization of optical, chemical, electrical and structural modifications were carried out by UV–Vis spectroscopy, FTIR spectroscopy, Dielectric measurements and X-ray Diffraction. A shift in the optical absorption edge towards the red end of the spectrum was observed with the increase in ion fluence. The optical band gap (E_g), calculated from the absorption edge of the UV–Vis spectra of these films in 200–800 nm region varied from 4.12 eV to 2.34 eV for virgin and irradiated samples. The cluster size varied in a range of 69–215 carbon atoms per cluster. In FTIR spectra, appreciable modification in terms of breaking of the cleavaged C–O bond of carbonate and formation of phenolic O–H bond was observed on irradiation. A rapidly decreasing trend in dielectric constant is observed at lower frequencies. The dielectric constant increases with fluence. It is observed that the loss factor increases moderately with fluence and it may be due to scissoring of polymer chains, resulting in an increase in free radicals. A sharp increase in A.C. conductivity in pristine as well as in irradiated samples is observed with frequency and is attributed to scissoring of polymer chains. XRD analyses show significant change in crystallinity with fluence. A decrease of ～9.02% in crystallite size of irradiated sample at the fluence of 2 × 10¹³ ions/cm² is observed.     

Sputtering of thin benzene films by large noble gas clusters

Molecular dynamics computer simulations have been employed to investigate the sputtering process of a benzene (C₆H₆) monolayer deposited on Ag{1 1 1} induced by an impact of slow clusters composed of large number of noble gas atoms. The sputtering yield, surface modifications, and the kinetic energy distributions of ejected species have been analyzed as a function of the cluster size and the binding energy of benzene to the Ag substrate. It is shown that high- and low-energy components can be identified in the kinetic energy distributions of ejected molecules. The mechanistic analysis of calculated trajectories reveals that high-energy molecules are emitted by direct interaction with projectile atoms that are backreflected from the metal substrate. Most of the molecules are ejected by this process. Low-energy molecules are predominantly emitted by a recovering action of the substrate deformed by the impact of a massive cluster. The increase of the binding energy leads to attenuation of both high- and low-energy ejection channels. However, low-energy ejection is particularly sensitive to the variation of this parameter. The area of the molecular overlayer sputtered by the projectile impact is large and increases with the cluster size and the kinetic energy of the projectile. Also the size and the shape of this area are sensitive to the changes of the binding energy. The radius of the sputtered region decreases, and its shape changes from almost circular to a ring-like zone when the binding energy is increased. Some predictions about the perspectives of the application of large clusters in the organic secondary ion mass spectrometry are discussed.     

Formation of germanium nanoparticles in silica glass studied by optical absorption and X-ray absorption fine structure analysis

We examined the relation between the 3.1 eV emission band and local structure for Ge⁺ implanted silica glass by means of photoluminescence, optical and X-ray absorption spectroscopies. In the 2 × 10¹⁵ cm^?2 implanted sample, a new emission band around 2.7 eV was observed, the origin of which was assigned to the B_2α oxygen deficient center and/or small Si clusters in silica. When the Ge⁺ fluence exceeded 2 × 10¹⁶ cm^?2, a sharp and intense 3.1 eV emission band replaced the 2.7 eV band. We found that the intense 3.1 eV PL occurred by the prolonged X-ray irradiation onto the 2 × 10¹⁵ cm^?2 implanted sample. UV–vis absorption and XAFS spectroscopies suggested that the aggregation of atomically dispersed tetravalent (Ge(IV)) atoms into Ge(0) clusters of ～1 nm exhibit strongly correlation with the generation of the 3.1 eV PL. Such nano- and/or subnano-size Ge(0) clusters formed by the X-ray radiation were oxidized and decomposed again to the isolated Ge(IV) atoms, while those produced by the higher Ge⁺ fluence were stable against the exposure to air.     

MD simulation of atomic displacements in pure metals and metallic bilayers during low energy ion bombardment at 0 K

Molecular Dynamics (MD) simulations of 100 eV Ar ion bombardment of (1 0 0) Ni and Al crystals, of bilayer crystals consisting of an Al (or Ni) layer on a (1 0 0) Ni (or Al) substrate, as well as pseudo-isotope bilayer crystals have been performed at 0 K using tight-binding potentials. For these systems sputtering yields, energy deposition with depth, atomic relocations, production of ad-atoms, depth distributions of vacancies and interstitials per ion impact were studied for times up to 7 ps. We observed that both the mean square displacement of atoms and defect production (vacancies, interstitials and ad-atoms) are larger in pure Al than in pure Ni. In addition, we observed for the bilayer systems Al/Ni and Ni/Al a high number of the near surface atomic relocations; especially ion bombardment induced exchange processes between atoms of the 1st (Al or Ni) and of the 2nd (substrate) layer. Potential energy calculations indicate that such relocations between the 1st and the 2nd layer are in both bilayer crystals energetically favourable. Both Al/Ni and Ni/Al bilayers show considerable higher production of ad-atoms as the pure Al and Ni targets. Typically ad-atoms are from the first layer, but in the Ni/Al bilayer system we found a substantial amount of Al ad-atoms from the 2nd layer (first Al layer). They contribute more to the ad-atom number than 1st layer Ni atoms. The mean square displacement of atoms in Al/Ni and Ni/Al crystals increases considerably during the thermal stages of the cascade evolution while it is almost constant in the case of the pure Al and Ni. Finally we observed that the maximum kinetic energies of atoms in the cascade volume after 4 ps are lower in the Ni and Al/Ni crystals than in Al and Ni/Al crystals, reflecting the lower cohesive energy of Al as compared to Ni. Calculations with pseudo-isotope bilayer crystals were performed to elucidate the influence of mass or potential on the observed effects.     

A proposed nuclear nanoprobe with Angstrom resolution

An idea is presented for an entirely new point-projection nuclear microscope that theoretically would have sub-nanometer resolution, approaching one Angstrom. The concept involves using a Kalbitzer super tip as a source of low energy (～100 eV) He or H ions that would be transmitted through a molecular sample (e.g. buckyball, carbon nanotube, graphene sheet, DNA molecule, etc.) placed very near the tip (～1–100 nm), and then projected onto a microchannel plate (MCP) screen placed ～1 m from the tip. Small angle scattering of the incident ions with atoms in the sample result in the development of shadow cones with an increase in scattered ion intensity at the critical cone angle. The enhanced intensity patterns formed at multiple intersections of cone perimeters are called “threads”. The shadow cones and threads are projected onto a suitable low-energy ion position sensitive detector to create a “shadow or thread image” of the sample. Such point-projection microscopes have no aberrations that affect the image, and the magnification would be of the order 1 m/10 nm, or 1.E8! The feasibility of this scheme is currently being studied theoretically using a deterministic atomic scattering model and a Monte Carlo molecular dynamics code. The results of these calculations indicate that only very low energy (60–120 eV) incident hydrogen ions can be used to avoid displacing atoms in the sample. At these energies, the critical cone angles are quite large and it would be necessary to position the molecular samples very close to the tip (down to only 1 nm) so that the projected ion maps are interpretable as distinct threads. Projecting distinct shadows is not supported by the scattering physics.     

Determination of the sticking coefficient of energetic hydrocarbon molecules by molecular dynamics

The sticking coefficient of hydrocarbon species is a key quantity that influences the growth process of amorphous hydrocarbon layers. To extend the very limited database for low impact energies, classical molecular dynamics simulations were performed, determining the sticking coefficients of CH_x (x = 0 … 4) with kinetic energies between 5 and 100 eV. Similar simulations are performed with hydrogen substituted by deuterium. Additionally, analytical formulas are presented that fit the data very well and can be used to interpolate the simulation results.     

MD simulations to evaluate effects of applied tensile strain on irradiation-induced defect production at various PKA energies

Molecular Dynamics (MD) simulations were conducted to investigate the influence of applied tensile strain on defect production during cascade damages at various Primary Knock-on Atom (PKA) energies of 1–30 keV. When 1% strain was applied, the number of surviving defects increased at PKA energies higher than 5 keV, although they did not increase at 1 keV. The rate of increase by strain application was higher with higher PKA energy, and attained the maximum at 20 keV PKA energy with a subsequent gradual decrease at 30 keV PKA energy The cluster size, mostly affected by strain, was larger with higher PKA energy, although clusters with fewer than seven interstitials did not increase in number at any PKA energy.