Organic mass spectrometry with low-energy projectiles

Molecular dynamics computer simulations have been employed to elucidate mechanisms responsible for uplifting of a monolayer of benzene and polystyrene molecules adsorbed on Ag{1 1 1} by low-energy atomic and cluster Ar projectiles. The sputtering yield and mass distributions of ejected particles are analyzed depending on the type and the kinetic energy of a projectile. It is shown that the relative contribution of intact molecules can be greatly enhanced if the kinetic energy of atomic projectile is reduced below 60 eV. At these energies, however, the efficiency of desorption is low and the ejection process is limited only to loosely bound molecules. Much better results can be obtained for cluster projectiles containing hundreds of Ar atoms with the incident energy of a few eV per atom. The impact of such particles leads to a gentle and very efficient removal of intact organic molecules originally adsorbed at the surface.     

Desorption of organic overlayers by Ga and C60 bombardment

This paper reviews our recent work on computer simulations of Ga and C₆₀ bombardment of thin organic overlayers deposited on metal substrate. A multilayer of benzene, a monolayer of PS4 on Ag{1 1 1} and a self-assembled monolayer of octanethiol molecules on Au{1 1 1} were irradiated with 15 keV monoatomic (Ga) and polyatomic (C₆₀) projectiles that are recognized as valuable sources for desorption of high mass particles in secondary ion and neutral mass spectrometry (SIMS/SNMS) experiments. The results indicate that the sputtering yield decreases with the increase of the binding energy and the average kinetic energy of parent molecules is shifted toward higher kinetic energy. Although the total sputtering yield of organic material is larger for 15 keV C₆₀, the impact of this projectile leads to a significant fragmentation of ejected species. As a result, the yield of the intact molecules is comparable for C₆₀ and Ga projectiles. Our results indicate that the chemical analysis of thin organic films performed by detection of sputtered neutrals will not benefit from the use of C₆₀ projectiles.     

Molecular dynamics simulations for nitridation of organic polymer surfaces due to hydrogen-nitrogen ion beam injections

Interaction of organic polymer surfaces with energetic reactive ions during etching processes by hydrogen-nitrogen plasmas has been investigated microscopically with the use of classical molecular dynamics (MD) simulations. Especially examined in the present study are interactions of atomic-nitrogen, molecular-nitrogen, or ammonia beams with a polyparaphenylene (PPP) surface and the resulting surface modification. It has been observed in the simulations that, when reactive atomic-nitrogen (N) beams are injected into a PPP surface, a carbon nitride layer with carbon-nitrogen bonds of higher bond orders (i.e., bonds containing π bonds) tend to be formed and it also acts as a source of carbon-nitride clusters for sputtered species. This observation is consistent with the fact that excessive supply of nitrogen to a carbon nitride film makes the film structurally weak as nitrogen atoms tend to break up carbon chains. On the other hands, when ammonia (NH₃) beams are injected, carbon-nitrogen single bonds (i.e., σ bonds only) are more likely to be formed since hydrogen atoms would efficiently terminate π bonds should they be formed.     

Sputtering of ice films by the bombardment of Ar ions

The process of sputtering ice films covered on Au(1 1 1) surface at normal incidence by Ar ions with an initial energy of E₀ = 300 eV and 700 eV has been investigated by the molecular dynamic simulation. The mass spectrum and kinetic energy distributions have been calculated. Our simulations clearly show that the mass spectrum contains peaks of the water molecules, water clusters and Au atoms. These results are of interest for the study of mass spectrometry in films and surfaces.     

Influence of the reactive N2 gas flow on the properties of rf-sputtered ZnO thin films

Nitrogen (N)-doped ZnO thin films were RF sputtered with different N₂ volume (ranging from 10% to 100%) on sapphire (001) substrates. The influence of N₂ vol.% on the properties of ZnO films was analyzed by various characterization techniques. The X-ray diffraction studies showed that the films grow along the preferential (002) crystallographic plane and the crystallinity varied with varying N₂ vol.%. The films sputtered with 25 vol.% N₂ showed better crystallinity. The transmittance was decreased with increasing N₂ volume until 25% and was almost constant above 25%. A maximum optical band gap (2.08 eV) obtained for 10 vol.% N₂ decreased with increasing N₂ volume to reach a minimum of 1.53 eV at 100%. The compositional analysis confirmed the incorporation of N into ZnO films, and its concentration increased with increasing N₂ volume to reach a maximum of ∼ 3.7 × 10²¹ atom/cm³ at 75% but then decreased slightly to 3.42 × 10²¹ atoms/cm³. The sign of Hall coefficient confirmed that the films sputtered with ≤ 25 vol.% N₂ possess p-type conductivity which changes to n-type for > 25 vol.% N₂.     

Transient hole formation during the growth of thin metal oxide layers

Using a quinternary variable charge molecular dynamics simulation technique, we have discovered a transient hole formation phenomenon during oxidation of thin aluminum layers on Ni₆₅Co₂₀Fe₁₅ substrates. Holes were found to first develop and expand at the earliest stage of the oxidation. These holes then shrank and finally disappeared as oxidation further proceeded. Thermodynamic analysis of the hole healing indicated that it is accompanied by a significant decrease in system potential energy. This suggests that the effect is largely driven by thermodynamics and is less related to the flux shadowing or kinetically introduced island coalescence. The simulations provide insights for the growth of dielectric tunnel barrier layers with reduced layer thicknesses.     

Molecular dynamics study of cluster impact on the (0 0 1) and (1 1 0) surfaces of fcc metals

Molecular dynamics simulations of the impact deposition of metal clusters on fcc metal surfaces are presented. Two-dimensional elongated islands are formed when the incident cluster travels parallel to the surface. For perpendicular incidence the results of the impact event are very sensitive to the relative cohesive properties of the cluster and substrate atoms.     

Surface self-diffusion of adatom on Pt cluster with truncated octahedron structure

Surface diffusion of single Pt adatom on Pt cluster with truncated octahedron structure is investigated through a combination of molecular dynamics and nudged elastic band method. Using an embedded atom method to describe the atomic interactions, the minimum energy paths are determined and the energy barriers for adatom diffusion across and along step are evaluated. The diffusion of adatom crossing step edge between {111} and {100} facets has a surprisingly low barrier of 0.03 eV, which is 0.12 eV lower than the barrier for adatom diffusion from {111} to neighboring {111} facet. Owing to the small barrier of adatom diffusion across the step edge between {111} and {100} facets, the diffusion of adatom along the step edge cannot occur. The molecular dynamics simulations at low temperatures also support these results. Our results show that mass transport will prefer step with {100} microfacet and the Pt clusters can have only {111} facets in epitaxial growth.     

Simulations of nanoindentation in a thin amorphous metal film

Nanoindentation was simulated in a two dimensional model metallic glass thin film using molecular dynamics. Strain localization was observed in simulations where the system was sufficiently structurally relaxed prior to deformation. Indentation simulations utilized an atomic indenter that adhered to the surface or a frictionless indenter. Boundary conditions were varied to constrain the film or allow the film to relax in-plane to examine the effect on shear band formation. The most constrained system, i.e. that with the atomic indenter and the constrained boundaries exhibited the highest hardness.     

Molecular dynamics investigation to indicate facet effects on coalescence processes of two copper clusters with different structures

Molecular dynamics simulations have been conducted to investigate facet effects on coalescence processes of Cu₅₅ and Cu₄₂₉ clusters respectively having icosahedral (Ih) and faced center cubic (FCC) geometries. It is shown that taking into account initial positions of the two clusters, structural changes of two coalescing clusters present different patterns. Simulation results establish the pathway of the structural evolution during coalescing by using shrinkage factors, average energy per atom, mean square displacements as well as atom packing configurations. The coalescence process can be separated into three stages including an approaching stage, a coalescing stage, and a coalesced stage. In the four coalescence processes, the structural transformations mainly occur in the Cu₅₅ clusters. The simulations show that the contact crystallization orientation plays an important role in the coalescing processes and resultant structures.     

Evolution of texture of CeO2 thin film buffer layers prepared by ion-assisted deposition

Evolution of texture in CeO₂ thin films was studied using biased magnetron sputtering and ion beam assisted magnetron sputtering. Films deposited onto polycrystalline Hastelloy metal substrates by biased magnetron sputtering develop preferential (002) growth as the energy of the ions is increased from zero to above 100 eV. For ion beam assisted magnetron sputtering (magnetron IBAD), with the ion beam directed at 55° to the substrate normal, the evolution of biaxial alignment is controlled by the ion beam energy and the ion/atom arrival rate ratio. Ion beam energies >200 eV and ion/atom ratios >0.3 lead to perfect biaxial alignment with one pole aligned along the ion beam direction. Epitaxial growth of CeO₂ films was observed for MgO(001) substrates at 750°C without any ion assistance, and on yttria-stabilised zirconia (001) buffer layers at room temperature and a bias of −80 V.     

Molecular dynamics simulations of ionized cluster beam deposition: case of study of aluminum

In order to understand interactions between energetic Al clusters and Al(1 0 0) surfaces, we investigated the cluster-size dependence of the maximum substrate temperature T_max and of the time t_max within which this temperature is reached. We considered two cases: (1) clusters with the same total energy E_T and (2) clusters with the same energy per atom. The temperature T_max changed linearly with both the energy per atom and the total energy of the cluster. When E_T was constant and the cluster size increased, the time t_max approached a constant value because energy was rapidly transferred to the substrate by correlated collisions. As the size and energy of clusters increase, such correlated collisions play a progressively important role in their interactions with surfaces.     

Strain analysis of photocatalytic TiO2 thin films on polymer substrates

Titania (TiO₂) thin films have been deposited on polymer sheets by magnetron sputtering at room temperature. Previous X-ray diffraction experiments revealed, for a wide range of deposition parameters, that the as-deposited titania thin films are predominantly amorphous; however, Raman scattering experiments revealed small traces of crystalline phases. The photocatalytic behaviour of the titania coatings was determined by combined ultra-violet (UV) irradiation and absorption measurements of a chosen dye (pollutants) in the presence of this catalyst. In order to assess the mechanical behaviour of the as-sputtered films, the film/substrate composite system was loaded unidirectionally using a tensile testing machine. As the system was stretched, cracks transverse to the loading direction developed in the film. The number of cracks increased as the applied strain increased, thus the relation between the measured crack density and the applied strain has been used to characterize the film strength and has also been correlated with the film photocatalytic efficiency. As a result of moderate fissuring on the titania film, it was found that for strain deformations up to 5% the photocatalytic activity is enhanced due to the exposure of more catalyst surface area for the pollutant to be adsorbed and subsequently dissociated upon UV illumination.     

Structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm Si (111) by ammonia molecular beam epitaxy

The structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm diameter Si (111) substrate by ammonia molecular beam epitaxy have been reported. High resolution X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy and secondary ion mass spectroscopy have been used to study the influence of AlN thickness and AlGaN growth temperature on the quality of GaN. GaN grown on thicker AlN showed reduced dislocation density and lesser tensile strain. Three-dimensional growth regime was observed for GaN grown at lower AlGaN growth temperature while higher AlGaN growth temperature resulted in two-dimensional growth mode. The dislocation bending and looping at the AlGaN/AlN interface was found to have significant influence on the dislocation density and strain in the GaN layer. The evolution and interaction of threading dislocations play a major role in determining the quality and the strain states of GaN.     

Molecular dynamics in semiconductor physics

Molecular dynamics, which is a powerful method for studying both structural and dynamic properties of condensed matter, is employed as a “microscope for the motion of atoms”. Information about the motion of individual atoms that is difficult to obtain experimentally can be obtained using this method. Parallel computers with tera-flop speed and tera-byte memory will make it possible to perform hundreds of million particle simulations using empirical potential. Moreover, the macroscopic phenomena analyzed by continuum theory can be obtained by coarse-graining the atomic-level information provided by this approach. When considering the problem of data analysis, examples in semiconductor physics such as the implantation process are provided.     

Modelling of boron nitride: Atomic scale simulations on thin film growth

Molecular-dynamics simulations on ion-beam deposition of boron nitride are presented. A realistic Tersoff-like potential energy functional for boron nitride, which was specially fitted to ab initio-data, has been used. The impact of energetic boron and nitrogen atoms on a c-BN target is simulated with energies ranging from 10 to 600 eV. The structural analysis of the grown films shows that a loose, dominantly sp²-bonded structure arises at high ion flux. In no case the formation of a sp³-bonded phase is observed, but the obtained films partially reveal textured basal planes as found in experiment. Two different growth regimes are identified for ion energies above and below 100 eV.     

Morphology and electronic structure of thin 3,4,9,10-perylenetetracarboxylic dianhydride layers on Si(001)

The growth of 3,4,9,10-perylenetetracarboxylic dianhydride on Si(001) was examined in the light of varying flux of impinging molecules. Using atomic force microscopy and synchrotron radiation photoelectron spectroscopy Vollmer-Weber growth mode was observed on a wide range of growth rates. The island size initially decreases rapidly with growth rate, for the low growth rate reaches a minimum, and then gradually increases. Polarization dependent photoemission indicates that the orientation of the molecules within the islands remains flat on the substrate.     

Anisotropic crystallite size analysis of textured nanocrystalline silicon thin films probed by X-ray diffraction

A newly developed X-ray technique is used, which is able to quantitatively combine texture, structure, anisotropic crystallite shape and film thickness analyses of nanocrystalline silicon films. The films are grown by reactive magnetron sputtering in a plasma mixture of H₂ and Ar onto amorphous SiO₂ and single-crystal (100)-Si substrates. Whatever the used substrate, preferred orientations are observed with texture strengths around 2-3 times a random distribution, with a tendency to achieve lower strengths for films grown on SiO₂ substrates. As a global trend, anisotropic shapes and textures are correlated with longest crystallite sizes along the 〈111〉 direction but absence of 〈111〉 oriented crystallites. Cell parameters are systematically observed larger than the value for bulk silicon, by approximately 0.005-0.015 Å.