Variation of the amorphous carbon density under the action of 10–500 eV carbon atoms

The process of variation of the density of an amorphous carbon sample under the action of carbon atoms with energies in the range from 10 to 500 eV was numerically modeled using the methods of molecular dynamics. It is shown that a maximum densification of the substrate takes place for energies of bombarding carbon atoms in the interval of 30–40 eV. The results of calculations are compared to experimental data.     

Controlled growth of Co nanofilms on Si(100) by ion-beam deposition

This paper examines the effect of ion-beam sputtering conditions on the nucleation of Co nanofilms on Si(100). The argon ion energy is shown to play a key role in determining the sputtering process. Sputtering a cobalt target with argon ions less than 0.8 keV in energy produces granular layers. The cobalt layers grown at Ar⁺ ion energies above 1.2 keV are continuous even in the nucleation stage. The layers 1.2 to 2 nm in thickness have high resistivity and are comparable in magnetic properties to bulk material. The high-energy component of the total flux of cobalt atoms ejected from the target plays an important role in the initial stages of deposition, especially at argon ion energies from 1.2 to 2.2 keV. In the nucleation stage, the cobalt atoms have a finite penetration depth in the silicon substrate, where they give up energy which facilitates the formation of a continuous layer in the initial stage of the process.     

Effect of surface morphology of metallic zinc films deposited by ion beam sputter deposition on the formation of ZnO nanowires

Metallic zinc film with various surface roughnesses was deposited on Si (100) substrates by ion beam sputter deposition utilizing beam energies at 8, 12 and 16 keV. The surface roughness of the metallic zinc film increased as ion beam energy increased and was found to act as a crucial factor for the formation of ZnO nanowires by subsequent thermal oxidation. ZnO nanowires with diameters of ∼30 nm and average length of ∼1 μm were obtained from 12 to 16 keV ion beam deposited samples while no ZnO nanowires were found on 8 keV ion beam deposited samples. Photoluminescence study of ZnO nanowires exhibits a strong UV emission at 377.2 nm (3.287 eV) with a full-width at half maximum of 95.0 meV and negligible defect related deep level emission. The ZnO nanowires are grown along the [110] direction and the growth mechanism is likely due to a solid state based-up diffusion process. Field-emission measurement shows a turn-on field of 7.9 MV/m and a field enhancement factor β of 691 is achieved.     

Effect of surface morphology of metallic zinc films deposited by ion beam sputter deposition on the formation of ZnO nanowires

Metallic zinc film with various surface roughnesses was deposited on Si (100) substrates by ion beam sputter deposition utilizing beam energies at 8, 12 and 16 keV. The surface roughness of the metallic zinc film increased as ion beam energy increased and was found to act as a crucial factor for the formation of ZnO nanowires by subsequent thermal oxidation. ZnO nanowires with diameters of ∼30 nm and average length of ∼1 μm were obtained from 12 to 16 keV ion beam deposited samples while no ZnO nanowires were found on 8 keV ion beam deposited samples. Photoluminescence study of ZnO nanowires exhibits a strong UV emission at 377.2 nm (3.287 eV) with a full-width at half maximum of 95.0 meV and negligible defect related deep level emission. The ZnO nanowires are grown along the [110] direction and the growth mechanism is likely due to a solid state based-up diffusion process. Field-emission measurement shows a turn-on field of 7.9 MV/m and a field enhancement factor β of 691 is achieved.     

Controlled growth of Co nanofilms on Si(100) by ion-beam sputtering

The effect of process conditions on the properties of cobalt films grown on silicon by ion-beam sputtering is analyzed from the nucleation stage to film thicknesses corresponding to the properties of bulk material. The argon ion energy is shown to play a central role in determining the sputtering process. Sputtering a cobalt target with argon ions less than 0.8 keV in energy produces granular layers. The cobalt layers grown at argon ion energies above 1.2 keV are continuous even in the nucleation stage. The layers 1.2 to 2 nm in thickness have high resistivity and are comparable in magnetic properties to bulk material. The high-energy component of the total flux of cobalt atoms ejected from the target plays an important role in the initial stages of deposition, especially at argon ion energies from 1.2 to 2.2 keV. In the nucleation stage, the energy deposited by cobalt atoms in the silicon substrate facilitates the formation of a continuous layer in the initial stage of the process.     

Effect of SiO2 substrate on the irradiation-assisted manipulation of supported graphene: a molecular dynamics study

The irradiation effects in graphene supported by SiO(2) substrate including defect production and implantation efficiency are investigated using the molecular dynamics (MD) method with empirical potentials. We show that the irradiation damage in supported graphene comes from two aspects: the direct damage induced by the incident ions and the indirect damage resulting from backscattered particles and sputtered atoms from the substrate. In contrast with the damage in suspended graphene, we find that the indirect damage is dominant in supported graphene at high energies. As a result, enhanced irradiation damage in supported graphene is observed when the incident energy is above 5?keV for Ar and 3?keV for Si. The direct damage probability at all energies, even the total damage probability at low energies, in supported graphene is always much lower than that in suspended graphene because of the higher threshold displacement energy of carbon atoms. In addition, we demonstrate the striking finding that it is possible to dope graphene with sputtered atoms from the substrate and the implantation probability is considerable at optimal energies. Our results indicate that the substrate is an important factor in the process of ion-irradiation-assisted engineering of the properties of graphene.     

分子动力学模拟不同入射能量的SiF2与SiC的相互作用

采用分子动力学模拟方法研究了入射能量对SiF2与SiC样品表面相互作用的影响。本次模拟选择的入射初始能量分别为0.3,1,5,10和25 eV。模拟结果显示SiF2分解率与Si和F原子的沉积率有密切的关系。沉积的Si和F原子在SiC表面形成一层SixFy薄膜。随入射能量的增加,薄膜厚度先增加后减小,薄膜中Si-Si键密度增大。构成薄膜的主要成分SiFx(x=1~4)中主要是SiF和SiF2,随入射能量的增加,薄膜成分由SiF2向SiF转变。     

Numerical estimates for energy of sputtered target atoms and reflected Ar neutrals in sputter processes

During the sputtering process in Ar gas, the sputtered target atoms and the reflected Ar neutrals from the target have much higher energy than the background gas. In this study, the Thompson distribution and an updated Meyer model based on the elastic energy transfer between two colliding particles were used to obtain energy distributions and average energies for the sputtered metal atoms and the reflected Ar neutrals. An energy dependent elastic collision cross section was incorporated into Meyer’s model and a thermalization criterion based on power balance was used. Under typical sputtering conditions (0.5 mTorr and 1000 K Ar, 400 eV incident Ar ion), the model predictions indicate that for Cu, Ti and Ta targets, the sputtered metal atoms have initial average energies from 15 to 22 eV and thermalize with the background Ar gas between 10 and 20 collisions. The reflected Ar neutrals thermalize after about 10 collisions. Depending on the number of collisions, the energy dependent mean free path values of the sputtered metal atoms range from 300 to 100 cm while the mean free path values for the reflected Ar neutrals range from 200 to 100 cm.     

Principles and applications of ion beam techniques for the analysis of solids and thin films

This paper reviews in principle and by examples how a collimated mono-energetic and mono-atomic beam incident on a target provides information on its structure and composition when the energy of the back-scattered beam atoms, or of the particles generated by nuclear reactions, is analyzed. Examples are selected with particular emphasis on thin films and Si technology. For convenience, we define three different energy ranges of the incident beam (low energies from 1 to 6 keV, medium energies from 100 to 500 keV and high energies from 1 to 2 MeV) and discuss each range separately, according to the following table of contents.     

Molecular dynamics simulations of CF3 etching of SiC

Molecular dynamics simulations were performed to investigate CF₃ continuously bombarding SiC surfaces with energies of 100, 150 and 200 eV at normal incidence and room temperature. The simulated results show that the etching rates of Si and C atoms increase linearly with the incident energy. The etch rate of Si atoms is much more than that of C atoms. A carbon-rich surface layer is observed which is in good agreement with experiments. Under bombarding by CF₃, an F-containing reaction layer is formed through which Si and C atoms are removed. In reaction layer, SiF and CF species are dominant. The formation mechanisms of ejected products are discussed. In etching products, SiF₃ is dominant. It is found that etching of C atoms in SiC is controlled by physical sputtering, while etching of Si atoms in SiC is controlled by chemical sputtering.     

Cluster size dependence and yield linearity in cluster bombardment simulations of benzene

Cluster bombardment of a molecular solid, benzene, is modeled using molecular dynamics simulations in order to investigate the effect of projectile cluster size and incident energy on the resulting yield. Using the mesoscale energy deposition footprint (MEDF) model, we are able to model large projectiles with incident energies from 5 to 140 keV and predict trends in ejection yield. The highest ejection yield at 5 keV was observed at C 20 and C 60, but shifts toward larger clusters for higher energies. These trends are explained in terms of the MEDF model. For these projectiles, all of the incident energy is deposited in the near-surface region, which is optimal for the projectile energy to contribute to the ejection yield. Because the energy is deposited in the optimal position for contributing to the ejection process, the yields increase linearly with incident energy with a slope that is nearly independent of the cluster size.     

Energy deposition control during cluster bombardment: a molecular dynamics view

Molecular dynamics simulations are performed to model C60 and Au3 bombardment of a molecular solid, benzene, in order to understand the energy deposition placement as a function of incident kinetic energy and incident angle. Full simulations are performed for 5 keV projectiles, and the yields are calculated. For higher energies, 20 and 40 keV, the mesoscale energy deposition footprint model is employed to predict trends in yield. The damage accumulation is discussed in relationship to the region where energy is deposited to the sample. The simulations show that the most favorable conditions for increasing the ejection yield and decreasing the damage accumulation are when most of the projectile energy is deposited in the near-surface region. For molecular organic solids, grazing angles are the best choice for achieving these conditions.     

Density functional theory study of alloy element interstitials in Al

Density Functional Theory calculations were used to study Mg, Si, Cr, Mn, Fe, Co, Ni, and Cu interstitial configurations in Al. Energies of these elements in (100) dumbbell and octahedral configurations were determined. Results show that it is energetically favourable for metal alloying element atoms to replace Al selfinterstitials if the alloying atoms are smaller than the Al atoms, as expected. The system energy can thus be decreased by up to 2 eV. The difference between the energies of (100) dumbbell and octahedral configurations is only a few tenth eV for the alloys with metallic alloying elements. For Si, the difference can be up to 0.9 eV. This exceptional behavior of Si is most likely due to its angularly dependent bonding characteristics. Short ab-initio Molecular Dynamics simulations were performed on Mg and Si interstitials to allow these systems to evolve into different interstitial configurations rather than just the (100) dumbbell and octahedral configurations. For Si an alternative configuration with tetrahedral-like coordination was found. Consequences of the calculation results for radiation-induced segregation are discussed.     

Molecular dynamics simulations of ion bombardment processes

Abstract

An improved molecular dynamics technique that allows reduction of the computation time required in ion bombardment simulations is presented. This technique has been used to study the influence of the target temperature and structure on the argon sputtering of silicon. Molecular dynamics simulations of l keV Ar+ ion bombardment of silicon were carried out for several types of sample: (100) crystalline at 0 K, (100) crystalline at 300 K, and amorphous at 300 K. The yield of the sputtering process and the energy distribution of the sputtered atoms have been obtained. These results show that the sputtering process depends on the target surface binding energy which, in turn, is very sensitive to the structure of the sample surface.     

Molecular dynamics modeling of microstructure evolution during growth of amorphous carbon films

Summary Classical molecular dynamics simulations, using Brenner's bond-order interatomic potential model, is used to study the bonding microstructure formation during quench from liquid and during growth on a diamond surface. For a 64-atom quench simulation we find 56 sp³- and 8 sp²-bonded carbon atoms, in qualitative agreement with tight-binding simulations. The growth of amorphous carbon films was simulated by depositing carbon and hydrogen atoms onto a diamond surface at energies up to 100 eV The simulated films are amorphous with a maximal density near the deposition energies (20–40 eV) used to grow films on magnetic disks. Lower deposition energies yield open graphitic structures, while much higher deposition energies cause the surface to ablate, leading to a poorly defined interface. The hardness calculated from the densest simulated films is about twice that found experimentally.     

Precursor gas effect on the mechanical and optical properties of ion beam deposited diamond-like carbon films

The effect of precursor gases on the diamond-like carbon (DLC) film deposition was investigated by the direct ion beam deposition method. DLC films were deposited using methane and benzene as the precursor gases. Ion energies for the deposition range from 100 to 700 eV were achieved by adjusting the beam voltage. The residual stresses, refractive indices and optical band gaps were compared at the same ion energy. We observed significant differences in residual stress and optical properties between these films. As in r.f. plasma-assisted CVD, the residual stresses of the films deposited from benzene show a characteristic behaviour of lower ion energy deposition than those deposited from methane. The present observations are discussed in terms of the difference in ion energy per carbon atom at the growth surface. We also observed that the Ar addition effect on the residual stress is strongly dependent on the precursor gases.     

Ion-activated interface adsorption of oxygen during silver deposition on silicon

The process of ion-activated oxygen adsorption on silicon has been investigated using an experimental concept with simultaneous deposition of silver films. Auger electron spectroscopy in combination with sputter depth profiling is subsequently performed to determine the amount of oxygen adsorbed at the Ag---Si interface. Noble gas ions (⁴He⁺, ²⁰Ne⁺ and ⁴⁰Ar⁺)with energies between 50 and 175 keV were used, and it was found that for substrate temperatures of 300–700 K the oxygen adsorption depends strongly on ion mass, ion energy and ion flux density. For flux densities of 5 × 10¹¹ cm⁻² s⁻¹ or less, adsorption dominates and, in particular, for light-ion bombardment the majority of adsorbed oxygen atoms form chemical bonds with the silicon surface atoms (Si---O). However, for heavy ions, physical sputtering starts to compete and limits the effective rate of adsorption. At sufficiently high ion fluxes the adsorption starts to decrease, and for all ions and energies used in this work it is found that, if the electronic energy deposition density exceeds a critical value of about 1.2 × 10²¹ eV cm⁻² s⁻¹, dissociation of the Si---O bonds prevails with a corresponding loss in the adsorbed oxygen quantity.     

Surface modification and chemical sputtering of graphite induced by low-energy atomic and molecular deuterium ions

The surface morphology, and chemical/structural modifications induced during chemical sputtering of ATJ graphite by low-energy (<200 eV/D) deuterium atomic and molecular ions are explored by Scanning Electron Microscopy (SEM), Raman and Auger Electron Spectroscopy (AES) diagnostics. At the lowest impact energies, the ion range may become less than the probe depth of Raman and AES spectroscopy diagnostics. We show that such diagnostics are still useful probes at these energies. As demonstration, we used these surface diagnostics to confirm the characteristic changes of surface texture, increased amorphization, enhanced surface reactivity to impurity species, and increased sp³ content that low-energy deuterium ion bombardment to steady-state chemical sputtering conditions produces. To put these studies into proper context, we also present new chemical sputtering yields for methane production of ATJ graphite at room temperature by impact of D₂⁺ in the energy range 10-250 eV/D, and by impact of D⁺ and D₃⁺ at 30 eV/D and 125 eV/D, obtained using a Quadrupole Mass Spectroscopy (QMS) approach. Below 100 eV/D, the methane production in ATJ graphite is larger than that in HOPG by a factor of ∼2. In the energy range 10-60 eV/D, the methane production yield is almost independent of energy and then decreases with increasing ion energies. The results are in good agreement with recent molecular dynamics simulations.     

X-ray photoelectron spectroscopic studies of partially coked HZSM-5 catalysts

XPS studies of partially coked HZSM-5 catalysts have been carried out. As a result of the coke deposition Si(2s) and O(1s) binding energies were decreased by 1.0 eV due to the electron charge transfer from the coke to the framework atoms. The electron energy loss peaks were observed for Si(2s) of coked HZSM-5 catalyst. The results suggest that the coke was deposited on SiO₄ tetrahedra.     

Effect of ion beam etching on metal-Si and metal-GaAs barriers

A hot filament ion source was used in this investigation. The ion beam acceleration system consisted of two graphite grids for extraction of a 1 mA cm⁻² ion beam density of energy between 500 and 1000 eV. A very low energy ion beam [50–200 eV] with the same current density was obtained by using a 〈100〉 single-crystal silicon grid.I-V and C-V measurements of AlSi Schottky diodes pre-cleaned with a 100 eV ion beam before metal evaporation showed no change in the diode barrier height suggesting minimal surface damage (a barrier lowering). Ion etching of the Si surface at an energy higher than 100 eV leads to lowering of the barrier, with a tendency to saturation at a given energy. The induced damage in Si can be completely annealed at 700°C (as indicated by both I-V and C-V measurements) before metal evaporation.The GaAs surface is more sensitive to ion bombardment than Si. Only AlGaAs Schottky diodes pre-cleaned with a 50 eV ion beam did not change the barrier height. Any other higher ion energy drastically lowers the potential barrier of the metal-GaAs system.