Alloy formation in the Au{1 1 1}/Ni system - An investigation with scanning tunnelling microscopy and medium energy ion scattering

Using the techniques of scanning tunnelling microscopy (STM) and medium energy ion scattering (MEIS), we examine the growth and annealing behaviour of ultrathin Ni films on Au{1 1 1} at 300 K. As has been shown previously, submonolayer growth of Ni on Au{1 1 1} is strongly influenced by the presence of the herringbone reconstruction with two-dimensional clusters nucleating at herringbone elbows. Second layer growth commences prior to the completion of the monolayer. After multiple layers have been deposited, the surface morphology retains a similar cluster-like appearance. Annealing produces surfaces exhibiting long range Moiré structures and, at higher temperature, triangular misfit dislocations. We use MEIS to examine the composition and structure of these surface alloy phases and conclude that in each case, they consist of an essentially pure Au surface layer on a bimetallic second layer.     

Growth of Ce on Rh(1 1 1)

We have studied the growth of cerium films on Rh(1 1 1) using STM (scanning tunneling microscopy), LEED (low energy electron diffraction), XPS (X-ray photoelectron spectroscopy) and AES (Auger electron spectroscopy). Measurements of the Ce films after room temperature deposition showed that Ce is initially forming nanoclusters in the low coverage regime. These clusters consist of 12 Ce atoms and have the shape of pinwheels. At a coverage of 0.25 ML (monolayer, ML) an adatom layer with a (2 × 2) superstructure is observed. Above 0.4 ML, Rh is diffusing through pinholes into the film, forming an unstructured mixed layer. Annealing at 250 °C leads to the formation of ordered Ce-Rh compounds based on the bulk compound CeRh₃. At a coverage of 0.1 ML, small ordered (2 × 2) surface alloy domains are observed. The exchanged Rh atoms form additional alloy islands situated on the pure Rh(1 1 1) surface, showing the same (2 × 2) superstructure as the surface alloy. At a coverage of 0.25 ML, the surface is completely covered by the surface alloy and alloy islands. The (2 × 2) structure is equivalent to a (1 1 1)-plane of CeRh₃, contracted by 6%. Annealing a 1 ML thick Ce layer leads to a flat surface consisting of different rotational domains of CeRh₃(1 0 0). The Rh needed for alloy formation comes from 50 Å deep pits in the substrate. Finally we show that LEIS (low energy ion scattering) is not suitable for the characterization of Ce and CeRh films due to strong effects of neutralization.     

Morphology of Ni ultrathin films on Mo(1 1 0) and W(1 0 0) studied by LEED and STM

The morphology of ultrathin Ni films on Mo(1 1 0) and W(1 0 0) has been studied by low-energy electron diffraction and scanning tunneling microscopy. Ni films grow pseudomorphically on Mo(1 1 0) at 300 K for a coverage of 0.15 ML. A (8 × 1) structure is found at 0.4 ML, which develops into a (7 × 1) structure by 0.8 ML. The film undergoes a structural change to fcc Ni(1 1 1) at 6 ML. The growth mode switches from layer-by-layer to Stranski-Krastanov between 4 ML and 6 ML. Annealing at around 850 K results in alloying of submonolayer films with the substrate, while for higher coverages the Ni agglomerates into nanowedge islands. Ni films grow pseudomorphically on W(1 0 0) up to a coverage of around 2 ML at 300 K, above which there is a structural change from bcc to hcp Ni with the epitaxial relationship . This is accompanied by the formation of orthogonal domains of uniaxial strain-relieving dislocations from the third layer of the film. For coverages up to 1 ML the growth proceeds by formation of two-dimensional islands, but shifts to three-dimensional growth by 2 ML with rectangular islands aligned along the 〈0 1 1〉 substrate directions. Annealing at around 550 K results in agglomeration of Ni into larger islands and increasing film roughness.     

STM observation of surface phases of Sn/Cu(0 0 1)

Geometrical structures of the Sn-adsorbed Cu(0 0 1) surfaces are studied with scanning tunneling microscopy. There are four phases in the Sn coverage range between 0.2 and 0.5 mono-atomic layer (ML). On the basis of the observed atomic images in this range, we propose structural models for the phases with 0.33 and 0.375 ML of Sn. All the phases consist of embedded Sn atoms in the Cu surface, forming two-dimensional surface alloy structures. On the surface with ∼0.4 ML of Sn, a novel one-dimensional structure is observed.     

The epitaxial growth of Pd electrodeposition on Au(1 0 0) studied by LEED and RHEED

The epitaxial growth of Pd adlayers electrochemically deposited onto Au(1 0 0) has been studied by LEED, RHEED and AES. For the first 6 ML, the Pd deposits grow pseudomorphically on Au(1 0 0) with a lateral expansion of 4.5% with respect to bulk Pd. The strain in the expanded commensurate (1 × 1) Pd layers on Au(1 0 0) begins to be relieved at the Pd coverage between 6 and 9 ML range by formation of a compressed Pd film with respect to Au(1 0 0) surface and the compression increases continuously with thickness. At ca. 20 ML Pd the lattice constant of the film approaches to the bulk Pd and three-dimensional Pd islands develop since around 30 ML coverage. No superstructure due to the Pd-Au surface alloy can be found for coverages from monolayer up to 30 ML Pd on Au(1 0 0). A c(2 × 2) phase has been observed on the Pd-deposited Au(1 0 0) electrodes, which is ascribed to an ordered Cl adlayers adsorbed on Pd adlayers rather than a Pd-Au surface alloy.     

Preferential heights in the growth of Ag islands on Si(1 1 1)-(7 × 7) surfaces

Growth behavior of thin Ag films on Si substrates at room temperature has been investigated by scanning tunneling microscopy and reflection high energy electron diffraction. In the layer-plus-island growth Ag islands show strongly preferred atomic scale heights and flat top. At low coverage (1 ML), islands containing two atomic layers of Ag are overwhelmingly formed. At higher coverages island height distribution shows strong peaks at relative heights corresponding to an even number (2, 4, 6, …) of Ag atomic layers. Beyond some coverage the height preference vanishes due to the appearance of screw dislocations and spiral growth.     

Growth and thermal stability of ultra-thin Ag and Au layers on Mo(1 1 1) surface

The influence of temperature on the growth process of ultra-thin Ag and Au layers on the Mo(1 1 1) surface was investigated. At 300 K growth of the Stranski-Krastanov type was found for Ag; for Au growth of the monolayer plus simultaneous multilayers type was found, where a base layer is one physical layer. The first three geometrical adsorbed layers for Ag are thermally stable. For annealed Au layers triangle features with base side length from 15 to 35 Å were formed for θ < 6 monolayer (ML), and for θ > 6 ML part of the Au formed a flat adlayer with Au atoms grouped in equilateral triangles with side length 7 Å. The presence of Au layers does not cause faceting, layers are not smooth which could be caused by the fact that Au does not wets the substrate. For Ag thick layers reversible wetting/non-wetting transition was observed at 600 K. Ag layers on Mo(1 1 1) surface did not lead to faceting.     

Study of c(2 × 2)-MnAu(0 0 1) layers on Mn(0 0 1) by means of scanning tunneling microscopy/spectroscopy

Intermixing, growth, geometric and electronic structures of gold films grown on antiferromagnetic stacking body-centered-tetragonal manganese (0 0 1) films were studied by means of scanning tunneling microscopy/spectroscopy at room temperature in ultra-high vacuum. We found stable ordered c(2 × 2)-MnAu(0 0 1) alloy layers after depositing Au on pure Mn layers. Since at the fourth layer (5 × 23)-like Au reconstruction appears instead of the c(2 × 2) structure and local density of states peaks obtained on the c(2 × 2)-MnAu surface disappear, pure Au layers likely grow from the fourth layer.     

Electron and lattice structure of ultra thin Ag films on Si(1 1 1) and Si(0 0 1)

We studied the low temperature (T ? 130 K) growth of Ag on Si(0 0 1) and Si(1 1 1) flat surfaces prepared by Si homo epitaxy with the aim to achieve thin metallic films. The band structure and morphology of the Ag overlayers have been investigated by means of XPS, UPS, LEED, STM and STS. Surprisingly a (√3 × √3)R30° LEED structure for Ag films has been observed after deposition of 2-6 ML Ag onto a Si(1 1 1)(√3 × √3)R30°Ag surface at low temperatures. XPS investigations showed that these films are solid, and UPS measurements indicate that they are metallic. However, after closer STM studies we found that these films consists of sharp Ag islands and (√3 × √3)R30°Ag flat terraces in between. On Si(0 0 1) the low-temperature deposition yields an epitaxial growth of Ag on clean Si(0 0 1)-2 × 1 with a twinned Ag(1 1 1) structure at coverage’s as low as 10 ML. Furthermore the conductivity of few monolayer Ag films on Si(1 0 0) surfaces has been studied as a function of temperature (40-300 K).     

Growth and morphology of Cobalt thin films on Pd(1 1 1)

We report on the growth of thin Co films on Pd(1 1 1) at three different temperatures 180 K, 300 K, and 550 K. The structure and morphology was determined by scanning tunneling microscopy and low energy electron diffraction. The growth mode was found to vary with temperature. For 180 K and 300 K, we observed a tendency to double layer growth for the initial layers while at elevated temperatures, the initial film grows in single layer. For most conditions, non-ideal three-dimensional growth was observed. Two-dimensional growth was only found for growth temperature of 550 K and coverages above 5 ML. Depending on temperature, the Co islands at low coverages exhibit three principally different shapes: dendritic at 180 K, hexagonal at 300 K and triangular at 550 K. For growth at 550 K and coverages above 5 ML, the islands changed to an irregular shape. This transition is most likely responsible for the transition to 2D growth. Further, the large strain is relaxed by the creation of a dislocation network with mixed fcc and hcp stacking. Depending on the temperature and coverage, a hexagonal or a triangular network was observed. Finally, we have investigated the effect of annealing Co films prepared at 180 K and 300 K. Heating to 490 K leads to coarsening and intermixing.     

Epitaxial growth of Co(0 0 0 1)hcp/Fe(1 1 0)bcc magnetic bi-layer films on SrTiO3(1 1 1) substrates

Co(0 0 0 1)_hcp/Fe(1 1 0)_bcc epitaxial magnetic bi-layer films were successfully prepared on SrTiO₃(1 1 1) substrates. The crystallographic properties of Co/Fe epitaxial magnetic bi-layer films were investigated. Fe(1 1 0)_bcc soft magnetic layer grew epitaxially on SrTiO₃(1 1 1) substrate with two type variants, Nishiyama–Wasserman and Kurdjumov–Sachs relationships. An hcp-Co single-crystal layer is obtained on Ru(0 0 0 1)_hcp interlayer, while hcp-Co layer formed on Au(1 1 1)_fcc or Ag(1 1 1)_fcc interlayer is strained and may involve fcc-Co phase. It has been shown possible to prepare Co/Fe epitaxial magnetic bi-layer films which can be usable for patterned media application.     

(2 × 1)-Na surface reconstruction induced by NaCl dissociation on Ag(1 1 0) during LEED analysis

This study first reports the initial growth stages of sodium chloride (NaCl) on Ag(1 1 0) at room temperature. NaCl grows in bi-layer mode along its [1 0 0] axis and gives rise to (4 × 1) and (1 × 2) reconstructed domains for coverages lower than two monolayers (ML), a minimal thickness inducing a bi-dimensional closed film. In addition, a 10 ML NaCl film has been examined by low energy electron diffraction (LEED). LEED analysis leads to the dissociation of the NaCl deposit in a few minutes. The NaCl dissociation implies Cl desorption from the surface and Na remaining on it. The residual Na is arranged in the form of a (2 × 1) surface reconstruction and is found to be strongly bounded to the Ag substrate. These findings have been established by using the X-ray photoelectron spectroscopy technique.     

Sulfur-induced mobilization of Au surface atoms on Au(1 1 1) studied by real-time STM

The interaction of sulfur with gold surfaces has attracted considerable interest due to numerous technological applications such as the formation of self-assembled monolayers and as a chemical sensor. Here, we report on the interaction of sulfur with Au(1 1 1) at two different temperatures (300 K and 420 K) studied by real-time scanning tunnelling microscopy, low energy electron diffraction and Auger electron spectroscopy. In the low coverage regime (<0.1 ML), S adsorption lifts the herringbone reconstruction of the clean Au(1 1 1) surface indicating a lateral expansion of the surface layer. An ordered (√3 × √3)R30° sulfur adlayer develops as the coverage reaches ∼0.3 ML. At higher S coverages (>0.3 ML) gold surface atoms are removed from regular terrace sites and incorporated into a growing gold sulfide phase. At 300 K this process leads to the formation of a rough pit and mound surface morphology. This gold sulfide exhibits short-range order and an incommensurate, long-range ordered AuS phase develops upon annealing at 450-525 K. In contrast, formation of an ordered AuS phase via rapid step-retraction rather than etch pit formation is observed during S-interaction with Au(1 1 1) surfaces at 420 K. Our results shed new light on the S-Au(1 1 1) interaction.     

Structure and stability of Sb/Au(1 1 0)-c(2 × 2) surface phase

Adsorption of 0.5 monolayers (ML) of Sb on the Au(1 1 0) surface resulted in the formation of a c(2 × 2) surface reconstruction. Analysis of surface X-ray diffraction data by a direct method revealed the existence of an ordered substitutional surface alloy, with every other hollow site occupied by Au and Sb atoms. Quantitative conventional χ² refinement showed a contraction of 0.12 ± 0.03 Å in the spacing of the first Au layer to the second, an expansion of 0.13 ± 0.03 Å in the second-to-third layer distance, and an inward Sb displacement (rumpling) of 0.21 ± 0.04 Å. This surface phase proved to be extremely robust, with the long-range order of this arrangement remaining up to substrate temperatures of 900 K.     

Initial stages of iron silicide formation on the Si(1 0 0)2 × 1 surface

The initial stages of iron silicide growth on the Si(1 0 0)2 × 1 surface during solid-phase synthesis were investigated by photoelectron spectroscopy using synchrotron radiation. The experiments were made on iron films of 1-50 monolayer (ML) thickness in the temperature range from room temperature to 750 °С. Our results support the existence of three stages in the Fe deposition on Si(1 0 0) at room temperature, which include formation of the Fe-Si solid solution, Fe₃Si silicide and an iron film. The critical Fe dose necessary for the solid solution to be transformed to the silicide is found to be 5 ML. The solid-phase reaction was found to depend on the deposited metal dose. At 5 ML, the reaction begins at 60 °С, and the solid-phase synthesis leads to the formation of only metastable silicides (FeSi with the CsCl-type structure, γ-FeSi₂ and α-FeSi₂). A specific feature of this process is Si segregation on the silicide films. At a thickness of 15 ML and more, we observed only stable phases, namely, Fe₃Si, ε-FeSi and β-FeSi₂.     

Thickness dependent stripe structure stability of Ag films on Si(1 1 1)-(4 × 1)-In substrate

The thickness dependent stripe structure stabilization of Ag films on Si(1 1 1)-(4 × 1)-In substrate is thermodynamically considered. It is found that for the stability of the structure, there is a competition between the sum of elastic energy and stacking fault energy in the film and the film-substrate interface energy. The presence of equilibrium of them leads to a critical film thickness. Beyond it, the stripe structure will transform into a flat one. Our prediction for n_c of Ag films shows reasonable agreement with experimental data. In addition, according to the established model, it is predicted that Au could also form the above stripe structure on this substrate with a similar n_c value of Ag.     

Electrical conductance at initial stage in epitaxial growth of Pb, Ag, Au and In on modified Si(1 1 1) surface

The electronic properties of thin metallic films of Pb, Ag, Au and In atoms deposited at 105 K on well defined metallic surface, i.e. Si(1 1 1)-(6 × 6)Au surface with 10 ML of annealed Pb, were investigated using four-point probe method in UHV condition. The structure of the substrate and deposited metals were monitored by the RHEED system. The electrical conductance, measured during the deposition of In and Pb atoms, shows the local minimum for the coverage equals about 0.3 ML whereas for Au and Ag atoms the conductance decreases during the first monolayer growth. For Au atoms the local maximum in the conductance was observed for the coverage about 0.55 ML, which can be connected with localized states. To describe theoretically the conductance behavior the tight-binding Hamiltonian and equation of motion for the Green’s function were used and good qualitative agreement was obtained.     

Growth morphology of ultra-thin Ni films on Pd(1 0 0)

A series of thin Ni films, with thicknesses between 0.2 ML to 13 ML, were deposited on a Pd(1 0 0) substrate (a = 3.89 Å) at room temperature (RT). The growth morphology was investigated using scanning tunneling microscopy (STM). STM images indicate the existence of three different growth modes as a function of increasing coverage. Up to 6.5 ML, the films grow pseudomorphically, consistent with a face-centered tetragonal (fct) structure. From 6.5 ML to 10.5 ML a new apparent interlayer distance of 1.0 ± 0.1 Å is established. The new structure is accompanied by the appearance of an arrangement of filaments on the top layer surface. These filaments are presumably related to a strain relief mechanism of the fct films. Finally above 10.5 ML the Ni films recover the face-centered cubic (fcc) lattice constants. The filaments evolve, as a function of coverage, to form a net-like structure over the whole 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.     

Array of double Au-Ag chains on the Si(5 5 7) surface

Using scanning tunneling microscopy we study the topographic properties of Ag structure on the Au induced, highly ordered Si(5 5 7) surface. Topography measurements show that a small amount of Ag (0.25 ML) deposited on that surface leads to considerable modifications of the one-dimensional structure induced by Au atoms. In particular, we observe two different chains on each terrace, which are identified as Si adatoms and Ag chain structures. The STM topography of those chains strongly depends on the bias voltage, indicating an important role of electronic effects in this system.