Arvaidas Galdikas

Study of the oxygen diffusion on three-way catalysts: a kinetic model

A computer model was developed to take into account all the phenomena that can occur in 18O/16O isotopic exchange over Pt/CeZrOx materials: adsorption/desorption on the metal, surface and bulk O diffusion. Discriminating each step of the exchange process is no longer necessary: kinetic parameters and O diffusivity can be calculated in a single experiment.

Surface topography development and ion mixing in the study of depth profiling of multilayered structures

The depth profiling in combination with sputtering of multilayered structures is considered by the proposed models. In order to emphasize the changes in the surface composition produced by the processes of surface topography development and ion mixing during profiling two different models are proposed and considered separately. The first one includes the processes of surface roughening as a result of sputtering and smoothing of rough surface as a result of surface atom migration (no mixing in the bulk of sample). The second model includes the sequential removal of surface atoms (no surface roughening) and ion mixing in the bulk of the sample. The dependencies of the main characteristics such as the amplitude and the shape of the peaks of the oscillations of the surface concentration, surface roughness and depth resolution on sputtering time and ion beam parameters are calculated and analyzed. The influence of the ion energy, angle of incidence, ion flux and substrate temperature to the depth profile of multilayered structure are studied.

A phenomenological study of the initial stages of film growth

The kinetics of island nucleation, coalescence and growth during deposition of Au atoms on amorphous carbon are studied experimentally and the obtained results are analysed using rate equations. Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM), grazing-incidence small angle scattering (GISAXS) and atomic force microscopy (AFM) techniques are used to measure the quantity of deposited Au, density, size and average height of the islands as a function of time. Parameters used in rate equations are deduced from a quantitative comparison between calculated and experimental evolution of the above dependencies. Suggested rate equations take into account the adsorption rate on a bare substrate as well as on existing islands, the mobility of adatoms, possible redistribution of atoms between islands and bare substrate and coalescence of islands. The rate equations are used to study the kinetics of the coverage of a carbon substrate by gold atoms and the kinetics of the island density. It is shown that experimental and calculated dependencies are in agreement if the mobility of islands is included.

Development of the microstructure of sputter deposited gold on amorphous carbon

Abstract The deposition of sputtered gold atoms on the substrate of amorphous carbon was investigated using transmission electron microscope, atomic force microscope and grazing-incidence small-angle scattering techniques. It was found from Rutherford back-scattering that, in the range of deposited gold 10 15 –1.5 × 10 16 cm −2 , the condensation rate is constant and does not depend on the fractional coverage which exponentially increases versus time. It it shown that the majority of the gold atoms arriving onto the substrate are collected by the growing clusters. The experimentally measured average height of gold clusters only slightly increases with the amount of deposited gold and is always larger than the radius of clusters, indicating a quite cylindrical shape.

Redistribution of components in the altered layer formed by preferential sputtering

Abstract A model for the redistribution of atoms in the altered layer formed by ion bombardment and based on the processes of preferential sputtering and mutual diffusion is considered. The calculated results are compared with experimental results for two- and three-component alloys. It is concluded that preferential sputtering and mutual diffusion are the prevailing processes determining the redistribution of atoms in the altered layer which occurs after variation of the parameters of the ion beam. It is shown that an increase in sputtering rate results in a redistribution of atoms in the altered layer, leading to a temporary increase in the depleted component and a temporary decrease in the enriched component at the surface.

Simulation of dry etching through a mask

Abstract The two-dimensional profiles of etched grooves have been calculated as a function of the size of masks and the ion flux of bombarding ions. The model includes processes of adsorption of chemically active species arriving from plasma, heterogeneous chemical reactions (formation volatile compounds on the surface), thermal desorption, physical sputtering, activation of surface atoms by the bombarding ions and ion beam activated desorption. Special interest is concentrated on the etching anisotropy, lateral etching and aspect ratio with dependence on the size of mask. It is shown that ion irradiation changes etching kinetics and etching anisotropy. The main role of ions is related to the activation of heterogeneous reactions. It is shown that the profiles of the etched grooves can be modified by changing irradiation conditions and geometry of masks. Calculated results are qualitatively in good correspondence with ones obtained experimentally.

Elementary processes in thin film formation stimulated by high energy ion irradiation

Abstract The elementary processes (such as adsorption from residual gases, desorption, sputtering, redistribution of the film and substrate atoms) occuring during ion beam activated growth of a metallic film on a semiconductor substrate are considered in the proposed phenomenological model. The probabilities of the above processes are included in an equation of balance describing the i th sort atoms concentration variation in the first monolayer. Ion activated adsorption is treated for the interaction of oxygen atoms with ion activated surface atoms. It is assumed that activation of atoms is only in the first monolayer, arising from energy transfer from incident ions to atoms in the first monolayer. The AgSi interface adsorbed oxygen amount (from residual gases) for experimental dependences on the Ar + , Ne + , He + ion energy (50–175 keV) and ion flux density were extrapolated using the phenomenological model. It was shown that at ion flux densities below 10 12 cm −2 s −1 adsorption dominates strongly. The increase of the amount of activated atoms with the electronic energy deposition growth allows the conclusion that electronic energy losses are essential in the process of ion activated oxygen adsorption and the ion activated atoms may be interpreted as the atoms with excited extranuclear electrons. According to the calculations, ion activated adsorption is expected if the ratio of ion and oxygen flux densities is less than 0.025.

Analysis of Component Depth Profiles at the Interface of Ti6242 Alloy and SiC, SiN Coatings after High Temperature Oxidation in Air

We have analyzed the interfacial elemental depth profile evolution after high temperature isothermal oxidation of SixCy and SixNy protective coatings deposited by dynamic ion mixing on a Ti6242 alloy (Ti-6Al-2Sn-4Zr-2Mo). Isothermal oxidation tests have been carried out at 600°C during 100 hours in air. We have observed a non-monotonic depth distribution of zirconium in GDOES and SIMS depth profiles after oxidation of SiC/Ti6242 and SiN/Ti6242 and we propose a kinetic model based on rate equations for analyzing the results. It is shown by modeling that a non-monotonic depth profile of zirconium occurs because zirconium from the Ti6242 alloy forms a zirconium oxide compound. As a result, the atomic concentration of zirconium decreases at the interface which induces a diffusion flux of zirconium from the bulk to the interface. This process leads to the increase of the total amount of zirconium at the film interface and thus formation of a non-monotonic depth profile.