Growth of Ni-Al alloys on Ni(1 1 1): (I) Formation of epitaxial Ni3Al from ultra-thin Al deposits

In this paper we describe the alloying process of ultra-thin Al layers (below 8 × 10¹⁵ Al/cm²) deposited on Ni(1 1 1). For this purpose Auger electron spectroscopy, low energy electron diffraction, and ion beam analysis-channelling measurements have been performed in situ in an ultra-high vacuum chamber. Al deposits formed at low temperature (about 130 K) are strained defective crystalline layers retaining the substrate orientation. Alloying takes place, with very progressive Ni enrichment, in a very broad temperature range between 250 K and 570 K. This feature shows that diffusion of the alloy species is more and more difficult when the Ni concentration increases. At 570 K a crystallographically and chemically ordered Ni₃Al phase is formed, and its order continuously improves upon annealing, up to 750 K. We have shown by ion beam methods that this alloy is three-dimensional, extending up to 16 (1 1 1) planes for the thickest deposits. The Ni₃Al phase can also be obtained directly by Al deposition at 750 K, but its crystalline quality is lower and the layer is probably formed of grains elongated along 〈1 1 −2〉 directions. The Al content of the thin Ni₃Al layers formed mostly dissolves in the bulk above 800 K. However a small amount of Al remains segregated at the Ni crystal surface.     

Growth of Ni-Al alloys on Ni(1 1 1), from Al deposits of various thicknesses: (II) Formation of NiAl over a Ni3Al interfacial layer

This paper describes the second part of a study devoted to the growth of thin Ni-Al alloys after deposition of Al on Ni(1 1 1). In the previous paper [S. Le Pévédic, D. Schmaus, C. Cohen, Surf. Sci. 600 (2006) 565] we have described the results obtained for ultra-thin Al deposits, leading, after annealing at 750 K, to an epitaxial layer of Ni₃Al(1 1 1). In the present paper we show that this regime is only observed for Al deposits smaller than 8 × 10¹⁵ Al/cm² and we describe the results obtained for Al deposits exceeding this critical thickness, up to 200 × 10¹⁵ Al/cm². Al deposition was performed at low temperature (around 130 K) and the alloying process was followed in situ during subsequent annealing, by Auger electron spectroscopy, low energy electron diffraction and ion beam analysis-channeling measurements, in an ultra-high vacuum chamber connected to a Van de Graaff accelerator. We evidence the formation, after annealing at 750 K, of a crystallographically and chemically well-ordered NiAl(1 1 0) layer (whose thickness depends on the deposited Al amount), over a Ni₃Al “interfacial” layer (whose thickness—about 18 (1 1 1) planes—is independent of the deposited Al amount). The NiAl overlayer is composed of three variants, at 120° from each other in the surface plane, in relation with the respective symmetries of NiAl(1 1 0) and Ni₃Al(1 1 1). The NiAl layer is relaxed (the lattice parameters of cc-B2 NiAl and fcc-L1₂ Ni₃Al differ markedly), and we have determined its epitaxial relationship. In the case of the thickest alloyed layer formed the results concerning the structure of the NiAl layer have been confirmed and refined by ex situ X-ray diffraction and information on its grain size has been obtained by ex situ Atomic Force Microscopy. The kinetics of the alloying process is complex. It corresponds to an heterogeneous growth leading, above the thin Ni₃Al interfacial layer, to a mixture of Al and NiAl over the whole Al film, up to the surface. The atomic diffusion is very limited in the NiAl phase that forms, and thus the progressive enrichment in Ni of the Al film, i.e. of the mean Ni concentration, becomes slower and slower. As a consequence, alloying is observed to take place in a very broad temperature range between 300 K and 700 K. For annealing temperatures above 800 K, the alloyed layer is decomposed, Al atoms diffusing in the bulk of the substrate.     

Growth and structure of vanadium oxide on anatase (1 0 1) terraces

The interaction of vanadium oxide with epitaxial anatase films exposing (1 0 1) terraces was characterized. The TiO₂ films were grown on vicinal LaAlO₃ (1 1 0) substrates by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE); reflection high energy and low energy electron diffraction (RHEED and LEED) indicated that the films exposed (1 0 1) terraces of the anatase TiO₂ polymorph. When a vanadium oxide monolayer was deposited onto the anatase surface by OPA-MBE at 725 K, only (1 × 1) RHEED and LEED patterns were observed. The V X-ray photoelectron spectroscopy (XPS) peak intensities indicated that the monolayer wetted the anatase surface and so the diffraction patterns were attributed to an epitaxial vanadia layer. Analysis of the vanadium oxide monolayer by X-ray and ultraviolet photoelectron spectroscopies revealed that the V was predominantly 5+. When the vanadia coverage was increased at 725 K, Auger electron spectra showed only very slow attenuation of the anatase Ti peaks while spots began to develop in RHEED patterns recorded along the LaAlO₃ direction; both indicative of 3-D cluster formation. In the orthogonal direction, the RHEED patterns showed unusual diagonal streaks. Meanwhile, the (1 × 1) LEED pattern persisted even after 30 nm of vanadia was deposited. This was attributed to gaps between the 3-D clusters exposing the epitaxial monolayer. Core level XPS spectra of the 3-D clusters revealed a broad V 2p_3/2 peak that was centered at the position expected for V⁴⁺ but could be deconvoluted into three peaks corresponding to V³⁺, V⁴⁺, and V⁵⁺. It is shown that crystallographic shear that accommodates such variations in the oxygen content of V oxides can lead to the diagonal streaks in RHEED patterns recorded along the LaAlO₃ [0 0 1] direction even as the pattern in the orthogonal direction shows sharp transmission spots. The results show that vanadia growth on anatase (1 0 1) proceeds through the Stranski-Krastanov mode with a strong vanadia-titania interaction stabilizing a dispersed vanadia monolayer. The results are compared with previous data for vanadia growth on anatase (0 0 1) where smooth, epitaxial VO₂ films grow ad infinitum.     

Initial growth of platinum on oxygen-covered Ni(1 1 0) surfaces

The initial growth of Pt on the Ni(1 1 0)-(3 × 1)-O and NiO(1 1 0) surfaces has been studied by coaxial impact collision ion scattering spectroscopy (CAICISS), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). Prior to Pt deposition, the atomic structure of the near-surface regions of the Ni(1 1 0)-(3 × 1)-O and NiO(1 1 0) structures were studied using CAICISS, finding changes to the interlayer spacings due to the adsorption of oxygen. Deposition of Pt on the Ni(1 1 0)-(3 × 1)-O surface led to a random substitutional alloy in the near-surface region at Pt coverages both below and in excess of 1 ML. In contrast, when the surface was treated with 1800 L of atomic oxygen in order to form a NiO(1 1 0) surface, a thin Pt layer was formed upon room temperature Pt deposition. XPS and LEED data are presented throughout to support the CAICISS observations.     

Growth of Co nanoparticles on a nanostructured θ-Al2O3 film on CoAl(1 0 0)

We have investigated the growth of Co nanoparticles on θ-Al₂O₃/CoAl(1 0 0) by means of Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (EELS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Due to Volmer-Weber growth, Co forms particles with a mean diameter of approximately 2.5 nm and height of 0.8 nm. Even on the entirely covered oxide, there is no Ostwald ripening and Co particles stay structurally isolated. The nanoparticles exhibit a small size distribution and tend to form chains, as predetermined by the streak structure of the oxide template. For sufficient high coverages Co-core-CoO-shell nanoparticles may be evidenced, which is explained as a result of surfactant oxygen. The nanostructured particles may open the door to numerous applications, such as in catalysis and magnetoelectronic applications, where large areas of ordered nanodots are desired.     

Co on thin Al2O3 films grown on Ni3Al(1 0 0)

The growth of Co on thin Al₂O₃ layers on Ni₃Al(1 0 0) was investigated by Auger electron spectroscopy, high resolution electron energy loss spectroscopy (EELS), and scanning tunneling microscopy. At 300 K, Co grows in three-dimensional clusters on top of the Al₂O₃ layer. A defect structure of the alumina layer plays a crucial role during the early stage of Co growth. After deposition of 10 Å of Co, a complete screening of the dipoles of the Al₂O₃ layer due to the Co film is found in the EELS measurements. Annealing the Co film reveals a process of coalescence of Co clusters and, above 700 K, diffusion of the Co atoms through the oxide film into the substrate takes place.     

Substitutional adsorption geometry for Pb(1 1 1)-(√3 × √3)R30°-K

Intermixed structures for alkalis (larger than Li) on close-packed substrates have previously been observed only on Al(1 1 1). This study shows that K forms an ordered intermixed structure on Pb(1 1 1). The structures of clean Pb(1 1 1) and Pb(1 1 1)-(√3 × √3)R30°-K were studied using dynamical low-energy electron diffraction (LEED). The clean Pb(1 1 1) surface at 47 K was found to be a relaxed version of the bulk structure, in agreement with an earlier study of the same surface [Y.S. Li, F. Jona, P.M. Marcus, Phys. Rev. B 43 (1991) 6337]. At room temperature, adsorption of K on this surface results in a (√3 × √3)R30° structure, which was shown using dynamical LEED to consist of K atoms substituted in surface vacancies. The K-Pb bond length was found to be 3.62 ± 0.3 Å, with no significant change to the Pb interlayer spacings.     

Holographic diffuse LEED image reconstruction for simultaneous occupation of different oxygen adsorption sites on Ni(1 1 1)

We determined the local adsorption structure of disordered oxygen on the Ni(1 1 1) surface by means of diffuse holographic LEED. The measurements have been performed above the critical temperature (T_c=450 K) for the oxygen order-disorder phase transition at 500 K and at a coverage of Θ=0.25 ML. At this temperature we found, in agreement with a previous LEED-IV-analysis [Surf. Sci. 349 (1996) 185], that besides the fcc threefold sites also hcp sites are occupied. In addition, a small amount seems to be located also at top and bridge sites. Reconstructing the holographic wavefield information, the different oxygen adsorption geometries are superimposed in the real space image. Nevertheless, a spatial resolution of 0.5-1 Å was sufficient to clearly distinguish between them. The influence of algorithmic parameters on the image quality was tested.     

Ultra-thin film growth of titanium dioxide on W(1 0 0)

We have employed low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy to follow the epitaxial growth of thin films of TiO₂ on W(1 0 0). The films were grown both by metal vapour deposition of titanium onto the substrate in UHV with subsequent annealing in a low partial pressure of oxygen, and by metal vapour deposition in a low partial pressure of oxygen. LEED patterns showed the characteristic patterns of (1 1 0) oriented rutile. A systematic spot splitting was observed and attributed to a stepped surface. The calculated step height was found to be in good agreement with that expected for rutile TiO₂(1 1 0), 3.3 Å. Titanium core level shifts were used to identify oxidation states as a function of film thickness allowing the interpretation in terms of a slightly sub-stoichiometric interface layer in contact with the substrate. In combination with the LEED patterns, the film structure is therefore determined to be (1 1 0) oriented rutile with a comparable level of stoichiometry to UHV prepared bulk crystals. The ordered step structure indicates considerable structural complexity of the surface.     

Epitaxial C60 thin films on Bi(0 0 0 1)

We have used the Bi(0 0 0 1)/Si(1 1 1) template to grow highly ordered C₆₀ epitaxial thin films and analyzed them using scanning tunneling microscopy and low-energy electron microscopy. The in situ low-energy electron microscope investigations show that the initial nucleation of the C₆₀ islands on the surface takes place at surface defects, such as domain boundaries and multiple steps. The in-plane lattice parameters of this C₆₀ film turns out to be the same as that of the bulk fcc(1 1 1) C₆₀. The line-on-line epitaxial structure is realized in spite of a weak interaction between the C₆₀ molecules and Bi(0 0 0 1) surface, while scanning tunneling spectroscopy indicates that there is a negligible charge transfer between the molecules and the surface.     

Ordered growth of nanodots on a pre-structured metallic template: Au/N/Cu(0 0 1)

We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) the growth of gold particles on the N/Cu(0 0 1) self-organized surface. This template consists of nitrogen islands separated by bare Cu lines and forming a regular 2D array of period 5 nm. When Au is evaporated onto this surface, it mainly grows at the intersections between the Cu lines. The islands organization reproduces then the substrate 2D ordering.However, if the substrate temperature is too low, islands form everywhere. On the contrary, if the substrate temperature is too high, some nucleation sites are empty. By following the intensity of the diffraction satellites during the growth, we have observed that the ordering of the Au particles is optimum when the substrate temperature is between 210 and 290 K. Using both an analytical treatment based on the rate equations and kinetic Monte-Carlo simulations, we have determined the activation energy for the diffusion process and the energy of the traps.     

Surface alloy formation after deposition of Ce on Rh(1 1 0)

Vapour deposition of Ce onto a Rh(1 1 0) single crystal at room temperature is studied by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED). The thicknesses of the deposited Ce layers are estimated to be between 2 and 9 Å. To study the changes in the Ce-Rh surface layer, the samples are annealed at temperatures between 500 and 1000 °C after Ce deposition.After heating, a c(2 × 2) LEED pattern appears for the sample with the thinnest deposited Ce layer (2.4 Å). For samples with thicker Ce-films, the LEED pattern co-exists of a c(2 × 2) structure and a more diffuse 6% contracted (2 × 1) structure. This appears at the same temperature as the Ce 3d and Rh 3d core levels exhibit sharp intensity changes and binding energy shifts.The intensity of the f⁰, f¹ and f² multiplets in the Ce 3d core level spectra change when the annealing temperature is increased. The relative intensity of the Ce 3d f⁰ and f² features compared to the Ce 3d f¹ features is largest after annealing to 500 °C. This is below the temperature at which the ordered surface alloy is formed. When the sample is heated above the formation temperature of the surface alloy, the relative intensity of the Ce 3d f⁰ and f² features decrease.     

The effect of Pt on Ni3Al surface oxidation at low-pressures

The fully-oxidized surface that forms on (1 1 1) oriented Ni₃Al single crystals, with and without Pt addition, at 300-900 K under oxygen pressures of ca. 10⁻⁷ Torr was studied using XPS, AES, and LEIS. Two main types of surfaces form, depending upon oxidation temperature. At low-temperature, the predominant oxide is NiO, capped by a thin layer of aluminum oxide, which we refer to generically as Al_xO_y. At high-temperature (i.e., 700-800 K), NiO is replaced by a thick layer of Al_xO_y. By comparing samples that contain 0, 10 and 20 at.% Pt in the bulk, we find that the effect of Pt is to: (1) reduce the maximum amount of both NiO and Al_xO_y; and (2) shift the establishment of the thick Al_xO_y layer to lower temperatures. Platinum also decreases the adsorption probability of oxygen on the clean surface.     

Structure and composition of the NiPd(1 1 0) surface

The NiPd(1 1 0) alloy surface was studied using low energy electron diffraction to measure the structure and composition of the first three atomic layers. The surface layer is highly enriched in Pd and has a significantly buckled structure. The second layer is also buckled, with displacements even larger than the surface layer. The second layer also exhibits intralayer segregation (chemical ordering), with alternate close-packed rows of atoms being Ni enriched and Pd enriched. The third layer has a structure and composition close to that of the bulk alloy. These results are compared with results for the other low index faces of NiPd, the extensive literature on NiPt alloy surfaces, and the growing body of theoretical literature for NiPd alloy surfaces.     

Formation of oxygen-induced Si(1 1 3)-3 × 2 facets on the Si(5 5 12) surface

We report the formation of Si(1 1 3)-3 × 2 facets upon exposing oxygens on the Si(5 5 12) surface at an elevated temperature. These facets are found to form only for a limited range of oxygen exposure and exhibit a well-defined 3 × 2 LEED pattern. We also find the surface electronic state unique only to the facets in the valence band. The spectral feature of these electronic states and the behavior of a (1/3 1/2) LEED spot upon oxygen contents in the facets indicate that the formation is a heterogeneous mixture of the clean Si(1 1 3) facets free of oxygens and other facets containing oxygen atoms.     

Elemental steps in the growth of thin β-Ga2O3 films on CoGa(1 0 0)

The oxidation of CoGa(1 0 0) at 700 K was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy, low energy electron diffraction and Auger electron spectroscopy (AES). At 700 K, thin well-ordered β-Ga₂O₃ films grow on CoGa(1 0 0). The EEL spectrum of the Ga-oxide films exhibit Fuchs–Kliewer phonons at 305, 455, 645, and 785 cm⁻¹. For low oxygen exposure (<0.2 L), the growth of oxide-islands starts at step edges and on defects. The oxide films have the shape of long, rectangular islands and are oriented in the [1 0 0] and [0 1 0] directions of the substrate. For higher oxygen exposure, islands of β-Ga₂O₃ are found also on the terraces. After an exposure of 200 L O₂ at 700 K, the CoGa(1 0 0) surface is homogeneously covered with a thin film of β-Ga₂O₃.     

Adsorption of Pb on NiAl(1 1 0): energetics and structure

The adsorption of Pb onto a NiAl(1 1 0) single crystal surface at 300 K has been studied by Auger electron spectroscopy (AES), Low energy electron diffraction (LEED), molecular beam/surface scattering and single crystal adsorption calorimetry (SCAC). AES indicates a Stranski-Krastanov growth mode, i.e., Pb initially grows on NiAl(1 1 0) two-dimensionally until the first layer completes at 0.89 ML, where a superstructure is observed by LEED, followed by 3D islanding. Measurements of the Pb gas that does not stick indicate that Pb sticks on NiAl(1 1 0) with an initial probability of 0.99. The initial heat of adsorption of Pb on NiAl(1 1 0) is 249 ± 10 kJ/mol. Due to the repulsive interactions between Pb adatoms, the heat of adsorption decreases within the first layer to a value identical to the heat of sublimation of bulk Pb (195 kJ/mol), where it remains at higher coverages. This first application of adsorption calorimetry on such a thick sample (75 μm versus 0.2-8 μm previously) demonstrates that adsorption calorimetry can be extended to a wider range of surfaces, since this thickness can be achieved with nearly any single crystal material by simple mechanical thinning.     

Growth and alloy formation of ultrathin Ni films on Co/Pt(1 1 1)

Low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) were used to study the growth and the structural evolution of Ni/Co/Pt(1 1 1) following high-temperature annealing. From the oscillation of the specular beam of the LEED and Auger uptake curve, we concluded that the growth mode of thin Ni films on 1 ML Co/Pt(1 1 1) is at least 2 ML layer-by-layer growth before three-dimensional island growth begins. The alloy formation of Ni/1 ML Co/Pt(1 1 1) was analyzed by AES. The temperature for the intermixing of Ni and Co layers in the upper interface without diffusing into the bulk of Pt is independent of the thickness of Ni when a Co buffer is one atomic monolayer. After the temperature was increased, formations of Ni-Co-Pt alloy, Ni-Pt alloy and Co-Pt alloy were observed. The temperature required for the Ni-Co intermixing layer to diffuse into Pt bulk increases with the thickness of Ni. The interlayer distance as a function of annealing temperature for 1 ML Ni/1 ML Co/Pt(1 1 1) was calculated from the I-V LEED. The evolution of LEED patterns was also observed at different annealing temperatures.     

An ordered surface ternary alloy of a c(6 × 4) structure formed on Cu(0 0 1) by substitutional coadsorption of Mg and Bi

A c(6 × 4) structure formed on Cu(0 0 1) by the coadsorption of Mg and Bi atoms at room temperature has been determined by a tensor low energy electron diffraction analysis. It is an ordered surface ternary alloy with a thickness of single layer, in which Mg, Bi and Cu atoms are mixed in the top layer. In the primitive unit cell, there are one Mg, four Bi, six Cu atoms and one vacancy in the top layer, and substituted Mg and Bi atoms form MgBi₄ plane clusters being arranged in the c(6 × 4) order. Structural parameters show that Mg-Bi bond distances in the MgBi₄ cluster are 3.01 and 3.07 Å, which are shorter than the summation of metallic radii of Mg and Bi. It is concluded that a direct, attractive interaction between Mg and Bi atoms plays critical role in the formation of the c(6 × 4) structure.     

RT growth of acetonitrile and acrylonitrile on Si(0 0 1)-2 × 1 studied by XPS and LEED

In this work we show the adsorption of acetonitrile (CH₃CN) and acrylonitrile (CH₂CHCN) on Si(0 0 1)-2 × 1 at room temperature by increasing the molecular doses. Especially, by means of XPS and LEED data, we stress the action of these molecules on the silicon surface locating the dangling-bonds quasi-saturation within 10 L. The shortage of nitrogen XPS signal and some anomalies in carbon spectra point to an invading action from a traditional X-ray source (Al-K_α line) against chemisorbed molecules. In particular, we think that a long exposure to this radiation could break carbon-silicon bonds changing some adsorption geometries and making desorb molecular fragments.