R. Anton

The role of neutral oxygen radicals in the oxidation of Ag films

Silver films with thickness between 2 and 500 nm were exposed to an excited gas mixture of argon and oxygen from a microwave ECR ion source. Oxidation was observed, even with the extraction optics grounded, with a positive or negative bias voltage on the film, and without direct sight between the film and the source. Thus, the reaction must be caused by neutral oxygen species, probably atoms. The oxygen uptake was monitored with a quartz microbalance, and yielded, after a delay of ambiguous origin, a linear increase with time until all Ag had reacted to nominally Ag2O. The rate strongly depended on the gas pressure. Simultaneous measurements of the optical transmittance, reflectance, and of the electrical resistivity suggested a layer growth mode of the oxide. Comparison of the experimental dependence of the transmittance on film thickness with computer simulations yielded a bes fit with n=2.5 ± 0.1 and k=0,11 ± 0.02 for the oxide. Further oxygen absorption was slower, and approached an equilibrium of the production and decay of AgO.

On the origin of a lattice expansion in palladium and PdAu vapour deposits on various substrates

Abstract In order to gain more insight into the possible lattice expansion mechanism in vapour deposits of palladium, vapour deposits of palladium and of PdAu alloy particles and films were produced under well-controlled conditions, either in ultrahigh vacuum or in high vacuum, with and without the admission of H2O, on amorphous films of carbon and SiO2 as well as on single-crystal surfaces of NaCl and KBr. In all cases, lattice expansion values of up to 2.6% were measured, depending on the condition of degassing of the substrate and on the residual gas composition during deposition, especially with respect to water vapour. It was found that the lattice expansion was correlated with the presence of hydrogen on or in the substrate materials, as detected by secondary-ion mass spectroscopy (SIMS). X-ray photoelectron spectroscopy (XPS) analyses of a satellite of the main palladium peaks indicated a correlation with the concentration of hydrogen on the samples. Other impurities such as carbon or oxygen were excluded as being responsible for the lattice expansion by SIMS and XPS data. Furthermore, as no systematic difference in the lattice expansions was found for epitaxially grown deposits on alkali substrates and for non-oriented deposits on other substrate materials, the effect of pseudomorphism was shown not to be significant, too. Therefore the only remaining mechanism for lattice expansion is believed to be the formation of hydride, although the stabilization of this phase still needs to be clarified.

Auger electron spectroscopy investigations of segregation in AuPd and AgPd alloy thin films

The surface composition of vapour-deposited thin films of Ag-Pd and of Au-Pd was determined by Auger electron spectroscopy. Some supporting measurements were also made with secondary ion mass spectroscopy. Alloy films were deposited on carbon substrates in situ by simultaneous evaporation of the pure components. At temperatures up to about 323 K (50°C), the Auger compositions nearly matched the bulk compositions, which were determined by X-ray fluorescence analysis. Significant surface enrichment of Ag or Au respectively was found during annealing up to 873 K (600 °C), and this was most pronounced in the range of low to medium bulk concentrations of the segregating metal. The Auger intensity ratios were converted to surface compositions by using a monolayer model of segregation, and taking into account the electron attenuation in subsurface layers. The results were compared with data from the literature. In particular, for Ag-Pd alloys, the converted Auger compositions were found to correspond fairly well to quantitative data on the catalytic activity, which was determined in our laboratory during ongoing studies of the oxidation of C support films by Pd-containing alloy particles at elevated temperature under O. Thus, the results demonstrate that investigations of segregation are valuable for the understanding of catalytic properties of alloys.

Nucleation and growth of binary alloys on substrates

Abstract The nucleation and growth of silver-gold alloys on rock-salt cleavage surfaces was investigated systematically by evaporating the two metals from different sources so that the alloy was formed on the substrate surface. Measurements were carried out on the time dependences of the crystallite populations and size distributions and of the composition of the condensed material. It was found that the composition varies strongly with crystallite diameter and with the deposition time. The results were interpreted in terms of an extended kinetic nucleation model from the difference between the single atom adsorption energy and the activation energy for surface diffusion of the components. The adsorption energy for gold on rock-salt was found to be about 0.08 eV higher than for silver, whereas the activation energy for surface diffusion was about 0.02 eV lower for gold than for silver. These results were compared with data from other workers and show some discrepancies which may be due to different surface conditions.

Extension of the kinetic nucleation model to binary alloys

Abstract The kinetic nucleation model in its simplest form, neglecting coalescence effects and the mobility of clusters, was extended to systems with two components deposited simultaneously from different evaporation sources. By this extension of the model, the composition at the very beginning of the condensation process can be predicted as a function of the evaporation rates of the components, the substrate temperature and the differences in their adsorption and activation energies for surface diffusion. Calculations were made for the AuAg system on alkali halides using energy values given by several authors. The results demonstrated that it is possible to determine the differences in the energy parameters with some accuracy by the experiments described.

Interaction of gold, palladium and Au-Pd alloy deposits with oxidized Si(100) substrates

The growth of thin gold, palladium and Au-Pd alloy deposits from the vapour phase on Si(100) wafer material covered with thermally grown or “native” oxide layers was studied by means of Auger electron spectroscopy and reflection high energy electron diffraction as well as by scanning electron microscopy and Auger sputter profiling. The mean thickness of the metal and alloy deposits ranged from 3 to 15 nm. The temperature dependence of the growth modes and crystallographic texture was investigated in the range from room temperature to 620°C. Post- deposition annealing experiments were carried out at temperatures of up to 620°C for up to 60 h. Palladium was found to penetrate through “native” oxide interfaces, probably at inhomogeneities, and palladium silicide formation was observed starting at temperatures as low as 220°C. Almost no diffusion of gold through the “native” oxide was detected. In the case of alloy deposits, segregation led to an enrichment of gold at the surface while considerable amounts of palladium diffused into the silicon substrate to form silicide. Almost no diffusion of gold and palladium through thermally grown oxide interfaces of thickness 7 nm could be detected under our experimental conditions. However, indications for a diffusive Pd-SiO2 interaction were found by an enrichment of palladium and oxygen near the alloy- oxide interface in annealed deposits even on this type of oxide.

Scattering experiments with silver on NaCl, KBr, amorphous carbon and polycrystalline tungsten

Abstract Experimental results for the nucleation and growth of thin films from the vapour phase are commonly interpreted on the basis of a kinetic nucleation model. It is assumed that the adatoms form a quasi two-dimensional gas on the substrate surface and that nucleation takes place by collisions of single atoms. During the initial stages, comdensation is incomplete and a large fraction of adatoms may re-evaporate before being trapped by other adatoms or clusters. The mean residence time is determined by the adatom adsorption energy Ea and the surface vibration frequency τ0−1 via the Frenkel equation. We investigated the residence times of silver atoms on various substrates by time-of-flight spectroscopy. In addition, the angular and velocity distributions of the scattered atoms were examined. For silver on NaCl and on KBr the residence time was below the detection limit of 40 μs for temperatures above 25 °C, leading to estimated adsorption energies below 0.6 eV and 0.46 eV respectively. Within the error limits, these data are consistent with results from nucleation experiments. For silver on amorphous carbon, two different binding states were indicated: a dominant state with Ea=1.3 eV and a less populated state with Ea=1.8 eV. For silver on polycrystalline tungsten, at a somewhat lowered detection limit, residence times in the range from 30 μs to 0.3 s were measured for temperatures between 1450 and 900 °C, resulting in Ea=2.9 eV and τ0=7.9×10−14 s, in close agreement with data from other researchers. In all cases cosine distributions of the scattered atoms were found. This indicates, together with measured velocity distributions, almost complete accommodation of the adatoms.

Nucleation and growth of Au-Cu binary alloys from the vapour phase on NaCl single crystals

Abstract The nucleation and growth of copper-gold alloys on rock salt surfaces was investigated. The metals were evaporated simultaneously from two different sources in ultrahigh vacuum. During the early stages of condensation the alloy deposits were enriched in gold compared with the ratio of the vapour beam fluxes. The experimental results were interpreted using an extended nucleation model for alloys. This kinetic model allows the determination of the difference between the adsorption energies Ea and between the activation energies Ed for surface diffusion of the two components. Using values for Ea and Ed for gold from previous investigations the corresponding values for copper were determined and found to be Ea = 0.86 eV and Ed = 0.62 eV. Discrepancies between theoretical and experimental results regarding the dependence of the composition of the alloy deposits on deposition time is discussed.

The nucleation and growth of PdAu alloy particles on NaCl(100)

Abstract The early stages of thin film formation of PdAu alloys by simultaneous deposition of the components onto NaCl(100) cleavage surfaces had been investigated both experimentally and by a theoretical model in terms of rate equations, combined with an adatom diffusion model. Whereas the condensation coefficient of gold was initially very low, that of palladium was near unity even at an early stage. As a straightforward analytical solution of the rate equation approach is not feasible for such a complex case, we simulated the initial stages of condensation numerically by calculating the composition of the particles from atomic capture rates, taking into account the development in time of the adatom and particle number densities, and of the growth rates. Comparison of the results was also made with Monte Carlo calculations, which will presented in more detail elsewhere. A close fit of experimentally measured compositions and particle growth rates was obtained by an appropriately chosen difference of the adatom energies for adsorption and for surface diffusion of the components, which is (Ea-Ed)Pd−(Ea−Ed)Au = 0.12±0.02 eV. However, the experimentally measured particle number densities varied much less with the deposition fluxes than was theoretically expected. We suspect that this is caused by dimer and cluster mobilities and/or defect decoration, which are not included in our simplified model. It is argued that this may affect the absolute values of the energy parameters, but is only of minor importance for the difference given above.

Temperature dependence of AuPd thin film alloy contact formation on oxidized silicon substrates

Abstract The formation of metallic contacts of very thin vapour-deposited AuPd alloy films on Si(111) substrates covered with thin oxide layers was studied by means of surface analysis techniques (Auger electron spectroscopy, reflection high energy electron diffraction and transmission electron microscopy) as well as by electrical measurements (current density versus voltage characteristics). On deposition of the metals at temperatures below 100°C onto samples covered with native oxide, metal/ insulator/semiconductor-like behaviour was found. An increasing contribution of Schottky-like contacts was observed for deposition temperatures above 100°C, whereas for deposition temperatures above 200°C an increase in the ohmic contribution and a decrease in the photoresponse were observed. These effects were explained by the selective reaction of palladium with the oxide interface and the formation of palladium silicide. Segregation of alloy films produced an intimate contact between the metal overlayer and the silicide. The minimum amount of palladium necessary to produce nearly ideal ohmic contacts was estimated to be a few nanometres. Hardly any ohmic contribution was obtained when the depositions were carried out onto thermally oxidized samples with an oxide thickness of 7 nm, even for substrate temperatures of 620°C.