Ricardo Alberto Vidal

Chemical reactions at surfaces: titanium oxidation

Auger electron spectroscopy has been used to study the first stages of titanium oxidation. Titanium at room and high temperature, as well as thin films deposited over a metal substrate, were oxidized at low pressure. The authors found, using factor analysis, two diVerent regimes depending on the sample temperature. For temperatures below 200°C the oxidation is characterized by the presence of only one oxide phase: TiO 2 . At higher temperatures, the presence of Ti 2 O 3 is detected from the beginning of the process. For thin films [less than one monolayer (ML)] only TiO 2 was found, but for thicker films a new oxide phase is detected (TiO x , x 7 ML, only TiO 2 is detected.

Radiolysis of water ice in the outer solar system: Sputtering and trapping of radiation products

We performed quantitative laboratory radiolysis experiments on cubic water ice between 40 and 120 K, with 200 keV protons. We measured sputtering of atoms and molecules and the trapping of radiolytic molecular species. The experiments were done at fluences corresponding to exposure of the surface of the Jovian icy satellites to their radiation environment up to thousands of years. During irradiation, O 2 molecules are ejected from the ice at a rate that grows roughly exponentially with temperature; this behavior is the main reason for the temperature dependence of the total sputtering yield. O 2 trapped in the ice is thermally released from the ice upon warming; the desorbed flux starts at the irradiation temperature and increases strongly above 120 K. Several peaks in the desorption spectrum, which depend on irradiation temperature, point to a complex distribution of trapping sites in the ice matrix. The yield of O 2 produced by the 200 keV protons and trapped in the ice is more than 2 orders of magnitude smaller than used in recent models of Ganymede. We also found small amounts of trapped H 2 O 2 that desorb readily above 160 K.

AES and factor analysis study of silicide growth at the Pd/c-Si interface

We have measured the evolution of a palladium/silicon interface under consecutive annealing periods, performed at 200°C in UHV conditions. The interface was analyzed by means of Auger electron spectroscopy combined with factor analysis applied in a sequential way. We found that silicide appears only after annealing and evolves until all the palladium is consumed. A silicon compound different from silicide, identified as PdxSi with x<2 is found at the interface Pd/Si and Pd2Si/Si, before and after annealing respectively.

Relevant effects of localized atomic interactions and surface density of states on charge transfer in ion-surface collisions

Through a time-dependent quantum-mechanical calculation of the H + scattering by a highly oriented pyrolitic graphite (HOPG) surface, we are able to satisfactorily reproduce the interesting features we observed in ion scattering experiments in H + /HOPG system. We found that the combined effects of the semimetal character of HOPG together with the localized nature of the carbon atom states primarily determine the angular dependence and the magnitude of the ion fractions for large outgoing angles. The spin fluctuation effects (not considered in the present calculation) are discussed as one of the the main causes of the disagreement between the spinless theory results and the experiments for small exit angles.

Chemical reactivity of alkali-semiconductor interfaces: the case of KGaAs(110) and KSi(100) interfaces

Abstract The growth at room temperature of K GaAs (110) and K Si (100) interfaces have been studied using Auger electron spectroscopy and principal component analysis. The PCA of the K Auger line of K GaAs (110) reveals the existence of three pure independent components. The splitting, without lineshape modifications, of the first two of them suggests the formation of complexes of different size at the surface. The third component has remarkable lineshape modifications with respect to the others and it is observed from the beginning of the evaporation process indicating the reactivity of the K GaAs (110) interface. This is not the case for the K Si (100) interface, where PCA only reveals the splitting of different sized complexes.

Ion mixing and backscattering effect in AES depth profiling

Abstract By combining Auger electron spectroscopy and 2 keV Ar + ion bombardment we have obtained the depth profile of a Si-Ta interface. We have also simulated this depth profile through a Monte Carlo calculation. Through comparison between experimental results and the calculation we studied the Auger matrix and interface broadening effects on AES depth profiles. We found that the backscattering effect strongly depends on the impinging electron angle and energy.

Diffusion of implanted nitrogen in the Cu(0 0 1) surface

Auger electron spectroscopy (AES) was used to study the thermally activated diffusion process of low energy N+ implanted on the Cu(0 0 1) surface, a proposed model system for self-assembled nanostructures. The nitrogen diffusion was characterized as a function of the substrate temperature and the N+ implantation energy. An experiment that combines a temperature evolution followed by an AES depth profiling was done to clarify the type of diffusion. A comparison of the experimental results with kinetic Monte Carlo simulations suggests a preferential diffusion of the nitrogen to the surface followed by nitride decomposition.

Adsorption of potassium on GaAs(110)

The adsorption of potassium on the GaAs(110) surface has been studied by ion scattering and recoiling spectroscopy (SARS) and Auger electron spectroscopy (AES). The SARS measurements indicate that a major part of the potassium atoms adsorb along the [001] gallium rows, in a region close to the sites of a new arsenic layer, and that only a minor part of the potassium atoms adsorb along the [001] arsenic rows. In agreement with this suggestion, the energy shifts in the substrate Auger peaks indicate that, at the beginning of the adsorption, the potassium atoms react preferentially with gallium atoms.

An STM and Monte Carlo study of the AlF3 thin film growth on Cu(1 1 1)

We report measurements of AlF3 thin film growth on Cu(1 1 1) at room temperature by means of scanning tunneling microscopy. The growth proceeds by the formation of fractal islands characterized by a very corrugated surface. Through uncovered zones and island density we determined a diffusion length of ~25 nm for the adsorbed molecules. Even with this large diffusion length the step-edges do not appear fully decorated. These experimental results are contrasted with simulations based on a limited diffusion aggregation model and Metropolis Monte Carlo.Additionally, the results of the AlF3 sub-monolayer growth on Cu(1 1 1) are compared with our previous results on Cu(1 0 0), finding that both systems show more differences than similarities. Thus, while the growth on Cu(1 0 0) shows fully decorated step-edges, on Cu(1 1 1), they present non-covered zones even at coverages as high as 0.7 monolayers. Supported on MC simulations we suggest that the qualitative difference between both faces is due to different step-edge behaviour.

A detailed Auger electron spectroscopy study of the first stages of the growth of C60 thin films

Fil: Vidal, Ricardo Alberto. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Santa Fe. Instituto de Fisica del Litoral. Universidad Nacional del Litoral. Instituto de Fisica del Litoral; Argentina