Ion Scattering Spectroscopy Research Articles

Interface formation in W/Si multilayers studied by Low Energy Ion Scattering

We studied the formation of interfaces in W/Si multilayer structures deposited by magnetron sputtering. In vacuo Low Energy Ion Scattering Spectroscopy was used to determine the materials growth profile with a close to atomic resolution. For that purpose, bi-layer structures, W-on-Si and Si-on-W were grown, with Si being deposited using Ar as the sputter gas and W being deposited in either Ar or Kr. The measurements revealed strongly asymmetric interfaces, with W-on-Si being significantly broader than Si-on-W. However, no difference was found in the W-on-Si interface width depending on the sputter gas, Ar or Kr, used for W deposition, suggesting that it is not a ballistic effect of the backscattered gas ions that is responsible for the wider W-on-Si interface. However, strong surface segregation of Si atoms upon the deposition of W in Kr on Si was observed. Mechanisms responsible for the observed effects are discussed.

Surface Reduction State Determines Stabilization and Incorporation of Rh on α‐Fe2O3(11¯02)

AbstractIron oxides (FeOx) are among the most common support materials utilized in single atom catalysis. The support is nominally Fe2O3, but strongly reductive treatments are usually applied to activate the as‐synthesized catalyst prior to use. Here, Rh adsorption and incorporation on the () surface of hematite (α‐Fe2O3) are studied, which switches from a stoichiometric (1 × 1) termination to a reduced (2 × 1) reconstruction in reducing conditions. Rh atoms form clusters at room temperature on both surface terminations, but Rh atoms incorporate into the support lattice as isolated atoms upon annealing above 400 °C. Under mildly oxidizing conditions, the incorporation process is so strongly favored that even large Rh clusters containing hundreds of atoms dissolve into the surface. Based on a combination of low‐energy ion scattering (LEIS), X‐ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) data, as well as density functional theory (DFT), it is concluded that the Rh atoms are stabilized in the immediate subsurface, rather than the surface layer.

Thickness measurement of nm HfO2 films

Main textA pilot study for the thickness measurement of HfO2 films was performed by the Surface Analysis Working Group (SAWG) of the Consultative Committee for Amount of Substance (CCQM). The aim of this pilot study was to ensure the equivalency in the measurement capability of national metrology institutes for the thickness measurement of HfO2 films. In this pilot study, the thicknesses of six HfO2 films with nominal thickness from 1 nm to 4 nm were measured by X-ray Photoelectron Spectroscopy (XPS), X-ray Reflectometry(XRR), X-ray Fluorescence Analysis (XRF), Transmission Electron Spectroscopy (TEM), Spectroscopic Ellipsometry (SE) and Rutherford Backscattering Spectrometry (RBS). The reference thicknesses were determined by mutual calibration of a zero-offset method (Medium Energy Ion Scattering Spectroscopy (MEIS) of KRISS) and a method traceable to the length unit (the average thicknesses of three XRR data except the thinnest film). These reference thicknesses are traceable to the length unit because they are based on the traceability of XRR. For the thickness measurement by XPS, the effective attenuation length of Hf 4f electrons was determined. In the cases of XRR and TEM, the offset values were determined from a linear fitting between the reference thicknesses and the individual data by XRR and TEM. The amount of substance of HfO2, expressed as thickness of HfO2 films (in both linear and areal density units), was found to be a good subject for a CCQM key comparison.To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by the CCQM.

(Invited) Area-Selective Spatial ALD of SiO2 Interleaved with Etch-Back Supercycles: Quantification of Area Selectivity By Low Energy Ion Scattering

Area-selective ALD interleaved with etch-back steps in a supercycle fashion has recently been reported as very effective in achieving high selectivity. Such supercycles can result in lower defectivity on the non-growth area and thicker layers on the growth area, as compared to solely area-selective ALD.1, 2 Here, we use low energy ion scattering (LEIS) to probe the selectivity of the first supercycle, consisting of plasma-enhanced selective spatial-ALD of SiO2 and conventional CF4-based reactive ion etching (RIE). Given its extreme sensitivity to the top monolayer(s) of a thin film, LEIS can reliably quantify the selectivity/defectivity on the non-growth areas in terms of surface coverage and derived thickness.A three-step approach was used for the area-selective spatial ALD of SiO2,3 which consisted of inhibitor, silicon precursor (BDEAS), and O2 plasma exposures. The process selectivity as a function of the number of cycles was probed using LEIS to monitor the SiO2 surface coverage on the non-growth area. After 20 spatial ALD cycles, no silicon was detected on the non-growth area (detection limit 2 % SiO2 surface coverage), implying excellent process selectivity. Increasing the number of ALD cycles resulted in a gradual loss of selectivity. Interestingly after 110 ALD cycles, the SiO2 coverage was 86%, indicating a not yet closed layer. At the same time, a SiO2 thickness of 11 nm was measured on the growth area, using spectroscopic ellipsometry. As the last step of the super-cycle, a 3 seconds RIE was sufficient to bring the SiO2 coverage back to zero on the non-growth area, while an 8 nm thick SiO2 layer was left on the growth area.The data presented in this work demonstrate the effectiveness of combining area-selective spatial ALD + etch-back corrections to achieve extreme SiO2 selectivity while retaining high deposition rates. Finally, we have extended the plasma-enhanced area-selective spatial ALD of SiO2 to other oxides and metals non-growth areas. REFERENCES 1. R. Vallat et al., JVSTA, 35, 01B104 (2017).2. S. K. Song et al., Chem. Mater. 31 4793-4804 (2019).3. A. Mameli et al., ACS Nano, 11, 9303-9311 (2017). Figure 1

Calcium and fluorine signals in HS-LEIS for CaF2(111) and powder—Quantification of atomic surface concentrations using LiF(001), Ca, and Cu references

The powder of CaF2 has been proposed as a practical reference for the quantitation of Ca and F in low energy ion scattering (LEIS) analysis. It is chemically inert, easy to clean, and inexpensive. LEIS is extremely surface sensitive. Thus, in contrast to surface analytic techniques such as x-ray photoelectron spectroscopy and secondary ion mass spectroscopy, the surface termination of a sample is clearly reflected in the LEIS results. It is, thus, unlikely that in LEIS, the F/Ca ratio for CaF2 is 2.0. This paper supports the reliability of the CaF2 powder reference by evaluating the calcium and fluorine atomic surface concentrations, roughness factor and shows that the surface termination of the powder is the same as that of CaF2(111). The CaF2 samples are treated by annealing at 725 K and measured at 625 K. The presented spectra are practically free of contamination. Ion scattering by LiF (001), an evaporated Ca layer, and a Cu foil are used as basic references for Ca and F. The atomic sensitivity factors and the relative sensitivity factors have been determined for F, Ca, and Cu (3 keV He+, 145°). The F/Ca atomic ratio is found to be the same (2.3 ± 0.1) for CaF2(111) and its powder. For the powder, the Ca and F signals are reduced by a factor of 0.77 ± 0.03 in comparison with those for the single crystal.

Thermal stability of oxidized ultrathin Li films on TZM for plasma facing components

For improved understanding of the behavior of Li plasma facing components (PFCs) in the presence of oxygen impurities, we report on the thermal stability of ultrathin (up to 1.0 nm) LiOx films on a titanium-zirconium-molybdenum (TZM) alloy substrate over the temperature range of 310–1400 K. LiOx films were prepared by either post-oxidation of Li films or Li films deposited on an oxygen-precovered TZM surface. Thermal stability of these films was studied using a combination of temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and low-energy ion scattering (LEIS). For post-oxidized Li films, TPD showed that no Li desorption occurred until temperatures above 620 K, and then Li desorbed from the surface via at least three desorption peaks. The O 1s XPS spectra on the post-oxidized films at 310 K indicate the formation of lithium oxide (Li2O) and peroxide/hydroxide (Li2O2/LiOH). The peroxide/hydroxide converts to oxide after heating to 680 K with no desorption of Li or O2, and then this film decomposes to liberate Li into the gas phase while leaving oxygen at the TZM surface. Heating the LiOx films to 1070 K led to a solid-state reaction with surface Mo atoms of the substrate to form a condensed binary lithium molybdenum oxide (LixMoOy) phase, which upon further heating decomposed to liberate gas phase binary lithium molybdenum oxide species. Li deposition on an oxygen-precovered TZM surface formed a complex, highly stable, oxygen rich, Li-O-Mo interfacial oxide. Li desorption from multilayer Li films on oxygen-precovered TZM surfaces occurred in a metallic Li multilayer peak and three other oxide-derived peaks. These results support the improved thermal stability of oxidized lithium as compared to metallic Li and have implications for Mo impurity transport into the plasma at high temperatures.

Quantitative analysis of calcium and fluorine by high‐sensitivity low‐energy ion scattering: Calcium fluoride

Low‐energy ion scattering (LEIS) probes the atomic composition of the outer surface. Well‐defined reference samples are used for the quantitation. For elements like fluorine and calcium, it is not easy to find suitable, clean, and homogeneous references, since fluorine is a gas and calcium is a very reactive metal. In contrast to surface analytic techniques such as XPS, the extreme surface sensitivity of LEIS makes it difficult to use stable compounds like CaF2 as reference, since these compounds are not homogeneous at the atomic scale. With LEIS, CaF2 is not expected to show an atomic ratio F/Ca = 2.0. Thus, before CaF2 can be used as reference, its atomic surface concentrations have to be determined. Here, 3‐keV He+ scattering by a LiF(001) single crystal, an evaporated layer of Ca, and a Cu foil are used as basic references. High‐purity CaF2 is available in two forms: a single crystal and a powder. For a practical reference, powders are preferred, since if bulk impurities segregate to the surface, they will be dispersed over a large surface area. It is found that both CaF2 (111) and powder have similar F/Ca atomic ratios. This confirms the F termination for both samples. For the powder, the F and Ca signals are reduced by 0.77 ± 0.03 in comparison with those for the single crystal. The atomic sensitivity factors and relative sensitivity factors have been determined for F, Ca, and Cu.

Improved accuracy in multicomponent surface complexation models using surface-sensitive analytical techniques: Adsorption of arsenic onto a TiO2/Fe2O3 multifunctional sorbent

Novel composite materials are increasingly developed for water treatment applications with the aim of achieving multifunctional behaviour, e.g. combining adsorption with light-driven remediation. The application of surface complexation models (SCM) is important to understand how adsorption changes as a function of pH, ionic strength and the presence of competitor ions. Component additive (CA) models describe composite sorbents using a combination of single-phase reference materials. However, predictive adsorption modelling using the CA-SCM approach remains unreliable, due to challenges in the quantitative determination of surface composition. In this study, we test the hypothesis that characterisation of the outermost surface using low energy ion scattering (LEIS) improves CA-SCM accuracy. We consider the TiO2/Fe2O3 photocatalyst-sorbents that are increasingly investigated for arsenic remediation. Due to an iron oxide surface coating that was not captured by bulk analysis, LEIS significantly improves the accuracy of our component additive predictions for monolayer surface processes: adsorption of arsenic(V) and surface acidity. We also demonstrate non-component additivity in multilayer arsenic(III) adsorption, due to changes in surface morphology/porosity. Our results demonstrate how surface-sensitive analytical techniques will improve adsorption models for the next generation of composite sorbents.

The influence of oxygen on the neutralization of slow helium ions scattered from transition metals and aluminum surfaces

Low energy ion scattering (LEIS) was employed for the analysis of thin films of Mo, Ru, Hf, Al and their oxides. Measurements with different He+ energies showed that the characteristic velocities for neutralization of the transition metal atoms change when the metal binds with oxygen. However, such behavior was not observed for aluminum. We suggest that the increased neutralization in oxidized Ru, Hf and Mo originates from the presence of the O 2s band. This band is in resonance with the He 1s level, which allows for a quasi-resonant neutralization mechanism (qRN). On the other hand, a decrease of the strong Auger neutralization for metallic Al upon oxidation may compensate for the increase in neutralization by qRN, leading to similar neutralization behavior of Al in both states. We also demonstrate the dependence of characteristic velocity on oxygen content and discuss how this effect can be used to select proper reference samples for quantitative surface analysis by LEIS.

Temporally resolved LEIS measurements of Cr segregation after preferential sputtering of WCrY alloy

The dynamic behaviour of thermally driven segregation of Cr to the surface of WCrY smart alloy is studied with low energy ion scattering (LEIS). Sputtering the WCrY sample with 500 eV D2+ ions at room temperature results in preferential removal of the lighter alloy constituents and causes an almost pure W surface layer. At elevated temperatures above 700 K the segregation of Cr atoms towards the surface sets in and prevents the formation of a pure W layer. The simultaneous heating and sputtering of the sample leads to a surface state which reflects the balance between sputter removal and segregation flux, and deviates from the equilibrium due to thermally driven segregation. Stopping the sputter ion beam allows the system to relax and develop toward the segregation equilibrium. The time constants for the temporal changes of W and Cr surface coverage are obtained from a series of LEIS measurements. The segregation enthalpy is determined from the time constants obtained for various sample temperatures.

Energetics and Structure of Nickel Atoms and Nanoparticles on MgO(100)

The growth morphology and interfacial energetics of vapor-deposited Ni on the MgO(100) surface at 300 and 100 K have been studied using single crystal adsorption calorimetry (SCAC), He+ low-energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At 300 K, the Ni atoms grow as three-dimensional nanoparticles with a saturation number density of 5 × 1016 particles/m2. The differential heat of adsorption at 300 K increases rapidly with coverage, from 276 (initially) to 335 kJ/mol by 0.4 ML. Thereafter, it slowly increases asymptotically to the sublimation enthalpy of bulk Ni (430 kJ/ml) by 9 ML. The Ni 2p3/2 XPS peak binding energy at 300 K is initially (i.e., at 0.16 ML) 1.4 eV higher than that for bulk Ni(solid), but it decreases to that value at high coverage. The Ni atoms form a metastable hcp phase at 300 K when in nanoparticles with diameter <2.5 nm, and the adhesion energy of such Ni nanoparticles to MgO(100) was found to be 3.05 J/m2. At 100 K, the Ni atoms form single adatoms and then 0.17 nm thick 2D islands at low coverage with fewer Ni–Ni bonds compared to the Ni nanoparticles formed at 300 K. Thus, the initial heat (i.e., for the first ∼0.03 ML) is 148 kJ/mol at 100 K, 128 kJ/mol lower than that at 300 K, and remains lower for the 2D islands. With increasing coverage at 100 K, the tiny 2D Ni islands grow in size to cover nearly the entire surface before thickening. The XPS Ni 2p3/2 peak binding energy for 0.21 ML Ni on MgO(100) at 100 K is 2.2 eV higher than that for bulk Ni(solid), suggesting charge transfer from Ni to MgO(100) and formation of Ni2+ at very low coverage.

Sodium tungstate-promoted CaMnO3 as an effective, phase-transition redox catalyst for redox oxidative cracking of cyclohexane

Oxidative cracking, which combines catalytic oxidation and cracking reactions, represents a promising approach to reduce the energy and carbon intensities for light olefin production from naphtha. The need to co-feed gaseous oxygen with hydrocarbons, however, leads to significant COx formation and safety concerns. The cost and energy consumption associated with air separation also affects its economic attractiveness. In this study, we investigated a redox oxidative cracking (ROC) scheme and evaluated perovskites (La0.8Sr0.2FeO3 and CaMnO3) and Na2WO4-promoted perovskite (La0.8Sr0.2FeO3@Na2WO4 and CaMnO3@Na2WO4) as the redox catalysts for ROC. CaMnO3@Na2WO4 redox catalyst shows high activity, selectivity, and stability for light olefin production from cyclohexane. Operated under a redox oxidative cracking (ROC) scheme, CaMnO3@Na2WO4 enhances the catalytic cracking of cyclohexane, while showing high selectivity towards hydrogen combustion with its built-in, active lattice oxygen. Over three-fold increase in olefin yield compared to thermal cracking and 35% yield increase compared to conventional O2-cofeed oxidative cracking were achieved. Low energy ion scattering (LEIS), X-ray photoelectric spectroscopy (XPS), and differential scanning calorimetry (DSC) indicated a core-shell structure, where a molten Na2WO4 layer covers the CaMnO3 core. Na2WO4 modifies the oxygen donation behavior of CaMnO3 and provides a catalytically active surface for cyclohexane activation. In-situ XRD revealed that CaMnO3@Na2WO4 exhibited excellent structural stability and regenerability. The transformation of Mn4+ ↔ Mn3+ ↔ Mn2+ in CaMnO3, facilitated by reversible phase transition to (Ca/Mn)O solid solution, is responsible for the lattice oxygen donation and uptake during redox cycles. Electrochemical impedance spectroscopy (EIS) measurements further confirmed that the oxygen species were transported through the molten Na2WO4 layer to participate in ROC. These findings offer mechanistic insights to design effective redox catalysts for hydrocarbon valorization using the chemical looping strategy.

Ni Nanoparticles on CeO2(111): Energetics, Electron Transfer and Structure by Ni Adsorption Calorimetry, Spectroscopies and DFT

The morphology, interfacial bonding energetics and charge transfer of Ni clusters and nanoparticles on slightly-reduced CeO2-x(111) surfaces at 100 to 300 K have been studied using single crystal adsorption calorimetry (SCAC), low-energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and density functional theory (DFT). The initial heat of adsorption of Ni vapor decreased with the extent of pre-reduction (x) of the CeO2-x(111), showing that stoichiometric ceria adsorbs Ni more strongly than oxygen vacancies. On CeO1.95(111) at 300 K, the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Ni clusters on stoichiometric steps, followed by the Ni particles spreading onto less favorable terrace sites. At 100 K, the clusters nucleate on terraces due to slower diffusion. Adsorbed Ni monomers are in the +2 oxidation state, and they bind by ~45 kJ/mol more strongly to step sites than terraces. The measured heat of adsorption versus average particle size on terraces is favorably compared to DFT calculations. The Ce 3d XPS lineshape showed an increase in Ce3+/Ce4+ ratio with Ni coverage, providing the number of electrons donated to the ceria per Ni atom. The charge transferred per Ni is initially large but strongly decreases with increasing cluster size for both experiments and DFT, and shows large differences between clusters at steps versus terraces. This charge is localized on the interfacial Ni and Ce atoms in their atomic layers closest to the interface. This knowledge is crucial to understanding the nature of the active sites on the surface of Ni-CeO2 catalysts for which metal-oxide interactions play a very important role in the activation of O-H and C-H bonds. The changes in these interactions with Ni particle size (metal loading) and the extent of reduction of the ceria help to explain how previously reported catalytic activity and selectivity change with these same structural details.

The interplay between surface facet and reconstruction on isopropanol conversion over SrTiO3 nanocrystals

Strontium titanate (SrTiO3) is an extensively investigated perovskite for various applications due to its optical, electrical and chemical properties. To gain an in-depth understanding of the active sites involved in heterogeneous catalysis over the broadly used SrTiO3 (STO), we studied a model reaction, isopropanol conversion, on three differently shape-controlled nanocrystals: cube, truncated cube and dodecahedra. SEM, XRD and XPS confirmed the morphology, phase and composition of STO shapes. Low energy ion scattering (LEIS) revealed the occurrence of surface reconstruction over STO shapes during O2 pretreatment at different temperatures. Based on the catalytic activities, scanning transmission electron microscopy images and density functional theory calculations, the step sites on STO derived from surface reconstruction were proposed to be the active sites for isopropanol conversion. This was further confirmed by steady state isotopic kinetic analysis (SSITKA) which demonstrated similar intrinsic turnover frequencies (TOFs) for the differently reconstructed STO shapes. It is concluded that the crystal facets impose an indirect effect on the catalysis of STO via controlling the degrees of surface reconstruction: the less stable STO (1 1 0) facet (dodecahedra) leads to more step sites after reconstruction and hence higher overall reaction rate than the more stable (1 0 0) facet (cube). This work highlights the important interplay between the crystal facet and surface reconstruction in controlling the nature and density of active sties and thus catalysis over complex oxides.

Single-layer graphene on epitaxial FeRh thin films

Graphene is a 2D material that displays excellent electronic transport properties with prospective applications in many fields. Inducing and controlling magnetism in the graphene layer, for instance by proximity of magnetic materials, may enable its utilization in spintronic devices. This paper presents fabrication and detailed characterization of single-layer graphene formed on the surface of epitaxial FeRh thin films. The magnetic state of the FeRh surface can be controlled by temperature, magnetic field or strain due to interconnected order parameters. Characterization of graphene layers by X-ray Photoemission and X-ray Absorption Spectroscopy, Low-Energy Ion Scattering, Scanning Tunneling Microscopy, and Low-Energy Electron Microscopy shows that graphene is single-layer, polycrystalline and covers more than 97% of the substrate. Graphene displays several preferential orientations on the FeRh(0 0 1) surface with unit vectors of graphene rotated by 30°, 15°, 11°, and 19° with respect to FeRh substrate unit vectors. In addition, the graphene layer is capable to protect the films from oxidation when exposed to air for several months. Therefore, it can be also used as a protective layer during fabrication of magnetic elements or as an atomically thin spacer, which enables incorporation of switchable magnetic layers within stacks of 2D materials in advanced devices.

Experimental characterization of hydrogen adsorption sites for H/W(111) using low energy ion scattering

Low energy ion scattering (LEIS) and direct recoil spectroscopy (DRS) are among the few experimental techniques that allow for the direct detection of hydrogen on a surface. The interpretation of LEIS and DRS measurements, however, is often made difficult by complexities that can arise from complicated scattering processes. Previously, these complexities were successfully navigated to identify the exact binding configurations of hydrogen on a few surfaces using a simple channeling model for the projectile ion along the surface. For the W(111) surface structure, this simple channeling model breaks down due to the large lateral atomic spacing on the surface and small interlayer spacing. Instead, our observed hydrogen recoil signal can only be explained by considering not just channeling along the surface but also scattering from subsurface atoms. Using this more complete model, together with molecular dynamics (MD) simulations, we determine that hydrogen adsorbs to the bond-centered site for the W(111)$+\text{H}$(ads) system. Additional MD simulations were performed to further constrain the adsorption site to a height $h=1.0\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{3.33333pt}{0ex}}\AA{}$ and a position ${d}_{\mathrm{BC}}=1.6\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{3.33333pt}{0ex}}\AA{}$ along the bond between neighbors in first and second layers. Our determination of the hydrogen adsorption site is consistent with density functional theory simulation results in the literature.

Calcium fluoride and gold reference by high sensitivity-low energy ion scattering

Information about the outermost atomic layer of a material is critically important for many processes and materials, including in catalysis, tribology, wetting, corrosion, and thin film growth. Low energy ion scattering (LEIS) is an extremely surface sensitive technique that can quantitatively analyze the outermost atomic layer of a material. Recent developments in LEIS have resulted in a particularly high sensitivity version of the technique known as high sensitivity LEIS (HS-LEIS). In LEIS and HS-LEIS, reference and/or standard materials allow straightforward quantitation of the spectra. In this submission, the authors show the HS-LEIS spectrum of a high purity calcium fluoride powder. CaF2 is a useful reference material for calcium in LEIS because it is relatively inexpensive, not hygroscopic, and available in high purity. Due to the applications/uses of calcium in different areas of science, including in catalysis, polymer light emitting diodes, organic photovoltaics, solid oxide fuel cells, and solar cells, these data may enable future work in these areas. A reference spectrum for gold is included with this submission.

Oxidation of metal thin films by atomic oxygen: A low energy ion scattering study

In this study, we combine low-energy ion scattering (LEIS) static and sputter depth profiles for characterization of the oxidation kinetics on Zr, Mo, Ru, and Ta films of various thicknesses, followed by exposure to atomic oxygen at room temperature (∼20 °C). A method for nondestructive determination of the oxide growth rate via LEIS static depth profiling (static DP) is presented in detail. This method shows high sensitivity to the oxide thickness formed, and the results are in agreement with those obtained by X-ray reflectometry and sputter depth profiling (sputter DP). Sequential exposures of oxygen isotopes in combination with LEIS sputter DP are applied to elucidate the growth mechanism of the oxide films. The results indicate that the oxidation kinetics at the applied experimental conditions is directly influenced by the metal work function, characterizing a Cabrera-Mott growth type. The maximum thickness of the formed oxide and oxide growth rate are in the order Zr ≈ Ta &amp;gt; Mo &amp;gt; Ru. The combining of analysis by LEIS static DP and isotope tracing sputter DP is decisive in the characterization of oxidation kinetics in the room temperature regime.

In-situ studies of silicide formation during growth of molybdenum-silicon interfaces

The growth development of nanometer thick Mo and Si layers was studied using in situ laser deflection and Low Energy Ion Scattering (LEIS). The growth stress obtained from changes in wafer curvature during growth is correlated to changes in the surface stochiometry monitored by LEIS. For Si on Mo, the compressive-tensile-compressive stress development could be explained by the formation of interfacial silicide compounds and the transition between these and the bulk growth of Si. For Mo on Si, a strong initial tensile stress due to silicide formation saturates upon reduced availability of free Si at the growing Mo surface, followed by a near instantaneous tensile increase in stress related to the amorphous-to-crystalline phase transition, which coincides with the end of the compound formation, as determined with LEIS.

Selective growth of Al2O3 on size-selected platinum clusters by atomic layer deposition

In heterogeneous catalysis, atomic layer deposition (ALD) has been developed as a tool to stabilize and reduce carbon deposition on supported nanoparticles. Here, we discuss use of high vacuum ALD to deposit alumina films on size-selected, sub-nanometer Pt/SiO2 model catalysts. Mass-selected Pt24 clusters were deposited on oxidized Si(100), to form model Pt24/SiO2 catalysts with particles shown to be just under 1 nm, with multilayer three dimensional structure. Alternating exposures to trimethyl-aluminum and water vapor in an ultra-high vacuum chamber were used to grow alumina on the samples without exposing them to air. The samples were probed in situ using X-ray photoelectron spectroscopy (XPS), low-energy ion scattering spectroscopy (ISS), and CO temperature-programmed desorption (TPD). Additional samples were prepared for ex situ experiments using grazing incidence small angle x-ray scattering spectroscopy (GISAXS). Alumina growth is found to initiate at least 60 times more efficiently at the Pt24 cluster sites, compared to bare SiO2/Si, with a single ALD cycle depositing a full alumina layer on top of the clusters, with substantial additional alumina growth initiating on SiO2 sites surrounding the clusters. As a result, the clusters were completely passivated, with no exposed Pt binding sites.