Sn Overlayer Research Articles

Structural investigation of flat overlayer and surface alloy of Sn on Mo(110)

Atomic structures of Sn on Mo(110) are investigated using low-energy electron diffraction (LEED) and density functional theory (DFT) calculation. Sn atoms occupy substitutional sites at elevated temperatures in (3 × 1) structure, corresponding to a coverage of 0.33 ML. The substitutional model agrees with previous LEED, workfunction, and STM results. For (1 × 3) structure at 0.67 ML, Sn atoms adsorb in the nearest three-fold hollow sites. With increasing Sn coverage, the (1 × 4) structure at 0.75 ML, Sn atoms start to fill four-fold hollow sites. On both overlayer phases, Sn structures are flat without significant buckling. Compared with a two-dimensional tin, stanene, which is a stacking of α-Sn(111), Sn overlayer on Mo(110) is not related to α-Sn(111) but to β-Sn(100), which is normal metallic

In situ X-ray absorption spectroscopy of Sn species adsorbed on platinized platinum electrode in perchloric acid solution containing stannous ions

In situ X-ray absorption spectroscopy (XAS) was applied to investigate the Sn adlayer on platinized (pl-) Pt electrode in deaerated 0.2 M HClO4 solution containing 10−3 M Sn2+ in relation to the effect of Sn addition on electrocatalysis of Pt. A periodical emersion method under potentiostatic polarization, using pl-Pt plate (roughness factor Sr = 770) as a working electrode was employed to detect sensitively the sub-monolayer coverage of Sn on Pt. The Sn K-edge absorption spectra in a scanning XAS mode were measured by monitoring the Sn Kα1 fluorescence line. The Sn K-edge absorption near-edge structure (XANES) has indicated that the Sn species adsorbed on the pl-Pt electrode is partly oxygenated in the Sn-underpotential (UPD) region between − 0.05 and 0.25 V (RHE) which is overlapped with the UPD region of hydrogen. The extended X-ray absorption fine structure (EXAFS) analysis has supported a Sn overlayer model in which Sn atom occupies the hollow site of the nearest neighbor Pt atoms and is further bound with oxygen atoms in the Sn-UPD region. The coordination number of the Sn–Pt bond or Sn–Sn bond decreases with increasing potential, while the coordination number of the Sn–O bond increases reversely. In the potential region between 0.45 and 0.85 V (RHE), the EXAFS analysis has suggested that two-dimensional surface Sn oxide forms on the pl-Pt electrode, which is supported from the potential-pH equilibrium diagram of the Sn/H2O system.

Antiferromagnetic superexchange mediated by a resonant surface state in Sn/Si(111)

The Sn overlayer on the Si(111) surface has been considered as a prototypical system for exploring two-dimensional (2D) correlated physics on the triangular lattice. Most of the previous theoretical studies were based on the presumption that the surface state dominantly originates from Sn dangling-bond (DB) electrons, leading to a strongly correlated 2D electronic system. By contrast, our density-functional theory calculations show that the Sn DB state significantly hybridizes with Si substrate states to form a resonant state. The strong resonance between the $\mathrm{Sn}\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}5{p}_{z}$ and $\mathrm{Si}\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}3{p}_{z}$ orbitals facilitates the recently observed antiferromagnetic order through superexchange interactions, giving rise to a band-gap opening. It is thus demonstrated that the insulating ground state of Sn/Si(111) can be characterized as a Slater-type insulator via band magnetism.

Structural analysis of the Cu(100)–p(3[formula omitted])R45°–Sn surface using low and medium energy ion scattering spectroscopies

The atomic structure of the Cu(100)–p(32×2)R45°–Sn surface has been studied using medium energy ion scattering (MEIS), co-axial impact collision ion scattering spectroscopy (CAICISS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The p(32×2)R45° reconstruction was observed with LEED following the deposition of 0.42ML of Sn from a Knudsen cell on to the clean Cu(100) surface. The existence of sub-surface Sn atoms was immediately ruled out by the lack of blocking dips in the MEIS Sn scattered yield. Previously reported Sn overlayer and Cu–Sn surface layer alloy models were tested against the medium and low energy ion scattering data, with the best fit realised following structural optimization of the surface alloy model proposed by Pussi [K. Pussi et al., Surf. Sci. 549 (2004) 24]. Different out-of-plane shifts were observed for Sn atoms depending on their proximity to the missing Cu row.

Low pressure oxidation of ordered Sn/Pd(110) surface alloys

The reaction of oxygen at low pressure with the Sn/Pd(110) system has beenexamined by photoelectron spectroscopy using synchrotron radiation. Thec(2 × 2) and(3 × 1) reconstructions of the Sn/Pd(110) surface at 0.5 and 0.7 monolayers(ML) Sn coverage and a 1.75 ML Sn overlayer on the Pd(110) surface afterflashing to 470 K were studied. The Sn 4d core level is strongly affected byO2 adsorption while the Pd 3d core level shows very little change other than a decreasein intensity. Starting with a 10 L dose of oxygen, prominent changes in thespectra were observed for all Sn/Pd(110) surface alloys. Analysis of the Sn 4dcore levels indicates that oxidation proceeds with the formation of well-definedstates of Sn, which were identified as a Pd–Sn–O interface layer, SnO andSnO2 oxides. The valence band spectra confirm this assignment. TheSn2+ andSn4+ component signals originate from the topmost surface layer, i.e. tin atoms in more highlyoxidized states constitute the topmost surface layer on top of the Pd–Sn–O interface.The presence of a sub-surface PdSn intermetallic alloy facilitates the tin oxideformation; the Sn–O phase formation is accompanied by Pd–Sn bond dissociation.

Metastable surface structures of the bimetallic Sn/Pt(100) system

An ordered Sn overlayer on Pt(1 0 0) and several metastable Sn/Pt(1 0 0) surface alloys were studied using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). These surfaces were studied previously using alkali-ion scattering spectroscopy (ALISS), LEED, and CO chemisorption, in order to determine conditions for alloy formation [Surf. Sci. 330 (1995) 193]. In those earlier studies, structural transitions with increasing annealing temperatures from a c(2 × 2)-Sn overlayer to a c(2 × 2)-Sn/Pt(1 0 0) surface alloy and finally to a (3√2×√2)R45° alloy structure were observed. Here STM is used to determine surface morphologies and atomic scale structures of these alloys. Sn (0.67 ML) deposited on the reconstructed Pt(1 0 0)-hex surface and annealed to 550 K formed a c(2 × 2)-Sn overlayer with compact, monolayer-high islands with step edges oriented along the [1 0 0] and [0 1 0] azimuths. Annealing the sample to 750 K initiated alloying and a c(2 × 2) surface alloy formed locally. However, the c(2 × 2) alloy structure does not appear to be stable even at this early stage of alloying and narrow channels along the 〈1 0 0〉 crystallographic directions form. For higher annealing temperatures, these channels evolve to form a two-domain structure with almost equidistant channels along the [1 0 0] and [0 1 0] directions, respectively. A dominant channel separation of 3√2 times the surface-lattice constant of Pt(1 0 0) is observed. This channel separation gives rise to a streaky (3√2×√2)R45° LEED pattern. The channels are characterized by three missing atomic rows. Between these channels are narrow terraces that are predominantly three-atomic-rows wide. The step edges of these terraces are Pt-terminated, while Sn is alloyed in the center of these terraces. A local p(2 × 2) ordering of alloyed Sn is often observed for wider terraces. Annealing to 1000 K results in “fragmentation” of the narrow terraces to form small, often square, islands with side-lengths as small as √2 times the surface-lattice constant of Pt(1 0 0). The structural characterization of these Sn/Pt(1 0 0) surfaces is used to interpret previous chemical adsorption studies.

Metastable surface structures of the bimetallic Sn/Pt(1 0 0) system

An ordered Sn overlayer on Pt(1 0 0) and several metastable Sn/Pt(1 0 0) surface alloys were studied using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). These surfaces were studied previously using alkali-ion scattering spectroscopy (ALISS), LEED, and CO chemisorption, in order to determine conditions for alloy formation [Surf. Sci. 330 (1995) 193]. In those earlier studies, structural transitions with increasing annealing temperatures from a c(2 × 2)-Sn overlayer to a c(2 × 2)-Sn/Pt(1 0 0) surface alloy and finally to a (3√2×√2)R45° alloy structure were observed. Here STM is used to determine surface morphologies and atomic scale structures of these alloys. Sn (0.67 ML) deposited on the reconstructed Pt(1 0 0)-hex surface and annealed to 550 K formed a c(2 × 2)-Sn overlayer with compact, monolayer-high islands with step edges oriented along the [1 0 0] and [0 1 0] azimuths. Annealing the sample to 750 K initiated alloying and a c(2 × 2) surface alloy formed locally. However, the c(2 × 2) alloy structure does not appear to be stable even at this early stage of alloying and narrow channels along the 〈1 0 0〉 crystallographic directions form. For higher annealing temperatures, these channels evolve to form a two-domain structure with almost equidistant channels along the [1 0 0] and [0 1 0] directions, respectively. A dominant channel separation of 3√2 times the surface-lattice constant of Pt(1 0 0) is observed. This channel separation gives rise to a streaky (3√2×√2)R45° LEED pattern. The channels are characterized by three missing atomic rows. Between these channels are narrow terraces that are predominantly three-atomic-rows wide. The step edges of these terraces are Pt-terminated, while Sn is alloyed in the center of these terraces. A local p(2 × 2) ordering of alloyed Sn is often observed for wider terraces. Annealing to 1000 K results in “fragmentation” of the narrow terraces to form small, often square, islands with side-lengths as small as √2 times the surface-lattice constant of Pt(1 0 0). The structural characterization of these Sn/Pt(1 0 0) surfaces is used to interpret previous chemical adsorption studies.

A robust method of calculating surface atomic geometry

We present a robust method for self-consistent solution of the Kohn-Sham equations for both the electronic and ionic degrees of freedom. The solution is based on a supercell technique with local density functional pseudopotential theory. From a given initial set of atomic positions, the (self-consistent) charge density is calculated using Broyden's scheme. A conjugate gradient technique is then used to minimise the total energy by moving along the Born-Oppenheimer subspace and hence the equilibrium geometry is achieved. The algorithm is applied to a number of III–V semiconductor substrate systems with and without various metal overlayer coverages. Our results for clean GaAs(110) and InP(110) surfaces compare well with recent Car-Parrinello type calculations, as well as those obtained via LEED analysis. In addition we present the results of our calculations for both GaAs(110) and InP(110) substrates, with one and two ordered monolayer coverage of Bi and Sn. We find that in general the Bi overlayers practically remove the surface tilt angle, leaving a very small vertical shear on the adsorbate atoms. In contrast the Sn overlayer is characterised by a large vertical shear.

Analysis of monolayer films during molecular beam epitaxy by reflection electron energy loss spectroscopy

Reflection electron energy loss spectroscopy at high electron energies is an analytic technique well-suited to compositional analysis at the monolayer level during molecular beam epitaxial growth. For submonolayer-thickness Ge films grown on (2 × 1)-reconstructed Si (001) substrates, the variations in core loss intensities with film thickness and scattering angle are well described by a simple geometrical model for scattering, in the absence of surface resonance effects. Surface resonant scattering leads to an enhancement in the core loss intensity detected for thin epitaxial films, as the azumithal incidence direction is varied around the [110] azimuth, at a fixed incidence angle relative to the substrate plane. Increased surface roughness leads to a decreased inelastic scattering contribution relative to smooth surfaces, with low loss spectra that are more characteristic of transmission scattering. Detection limits for C have been established for reflection electron energy loss spectroscopy, enabling in situ analysis of surface cleaning prior to silicon molecular beam epitaxy. Short range structural information has been obtained using extended edge energy loss fine structure (EXELFS) for a newly-observed Sn reconstruction on a Si(001) surface, a (3 × 1) Sn overlayer on clean, hydrogen-terminated (1 × 1) Si (001).

Tin deposition on ruthenium (001): a multitechnique surface science study

Tin deposition on the Ru(001) surface was studied by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and low-energy ion scattering spectroscopy (LEISS). Tin deposition (AES signal intensity) versus time plots obtained over the substrate temperature (T[sub s]) range 330-670 K indicate that the Sn adatoms exhibit several different deposition growth modes. At T[sub s] = 330 K the Sn deposition follows a Stranski-Kranstanov growth mode (uniform monolayer followed by three-dimensional growth). Two LEED patterns are seen for T[sub s] = 330 K Sn depositions that are interpreted in terms of ordered Sn overlayers in the submonolayer coverage regime. For Sn depositions at T[sub s] = 500 K the Sn overlayer growth is also approximated by a Stranski-Kranstanov growth mode with the second and subsequent layers exhibiting less three-dimensional nucleation than at 330 K. For specific Sn depositions at T[sub s] > 600 K and upon annealing to 1,000 K, ordered surfaces with ([radical]3[times][radical]3)R30[degrees] and p(2[times]2) LEED patterns are obtained depending upon initial Sn precoverages and annealing temperatures and are suggested to arise from surface alloys. LEISS data indicate that the ([radical]3[times][radical]3)R30[degrees] surface alloy has a surface SnRu atom ratio of 2:1. 30 refs., 8 figs.

Electronic structure of Sn-doped C60 film

The Sn-doped C60 film was obtained by annealing a 100A˚thick C60 film with 6A˚thick Sn overlayer at 200°C. Photoelectron spectra show that the ration of Sn atoms to C60 molecules in the film is about 3:1. The valence band and C 1s core level spectra of the Sn-doped C60 film are similar to that of C60 film. In contrast with the expectation of the reported superconductive phase of Sn- doped C60, no metallic conduction band with a distinct Fermi level cutoff has been observed in the valence bands portion.

Scanning tunneling microscopy and spectroscopy of thin metal films on the GaAs(110) surface

The scanning tunneling microscope is used to study the properties of metal films with thicknesses in the range 1–10 monolayers (ML). Results are presented for two materials—antimony and tin, on the GaAs(110) surface. The acquisition and analysis methods used in the spectroscopic measurements are described in detail. For Sb films, a layered structure is observed. Very low conductivity is found at the Fermi-level, indicating semimetallic behavior. For Sn films, a layered structure with semimetallic nature is observed below 2 ML. It is found that the existence of an adatom layer is important for layer by layer growth of the Sn overlayer. At Sn coverages above 3 ML, a structural phase transition is observed, accompanied by an electronic transition to a metallic state.

Orientation Dependence of the Stability of Strained Sn Eplayers Grown On Cd0.8Zn0.2Te Substrates

ABSTRACTThe strain energy density in epitaxial overlayers depends strongly on the substrate's surface orientation. The affect of a growth orientation dependent strain energy contribution to the phase stability was studied by growing overlayers of metastable, semiconducting a-Sn films on the (100) and (111) surfaces of Cd0.8Zn0.2Te substrates. On the (100) surface the α-phase of Sn remains stable for coverages up to and beyond 125 ML. On the (111) surface though, the Sn overlayer shows metallic behavior from a partial α→β phase separation for coverages lower than 100 ML. Static calculations using the stress-strain relationship bear out these experimental results by showing that the strain energy density for the (111) growth direction is 38% larger than for the (100) direction.

Scanning tunneling microscopy and first-principles theory of the Sn/GaAs(110) surface.

Scanning tunneling microscopy of Sn overlayers on the GaAs(110) surface shows patterns with local order of nominal (3\ifmmode\times\else\texttimes\fi{}3) periodicity and an absence of long-range order. Using first-principles calculations, we show that these observations can be explained in terms of a double-layer structure which consists of a complete (1\ifmmode\times\else\texttimes\fi{}1) Sn layer covered by Sn adatoms. This structure results from a subtle balance between electronic energy and lattice strain. Spectroscopic studies indicate that this Sn overlayer exhibits a narrow gap (0.2 eV) centered in the gap region of the GaAs substrate.

Oxidation of tin on silicon substrate by rapid isothermal processing

We report an oxidation study of an Sn overlayer on Si(100) carried out at 400 °C by rapid isothermal processing (RIP) and furnace processing. Single oxide phase SnO2 could be obtained only by rapid isothermal processing. Based on x-ray diffraction, x-ray photoelectron spectroscopy, high-frequency capacitance-voltage characteristics, and breakdown measurements, improved quality of dielectric films was obtained by RIP. A possible explanation based on the difference in the radiation spectrum of the two sources of energy is also given.

Fermi-level pinning at the interfaces of Sb, Sn, and Ge on GaAs(110) surfaces

The interfaces of Sb, Sn, and Ge on GaAs(110) prepared at room temperature (RT) and 80 K low temperature (LT) have been studied using photoelectron spectroscopy. It is found that all the overlayers grow in a Stranski–Krastanov mode at RT, and the same growth mode holds at LT too. In contrast to many metals on GaAs(110) studied before, lack of temperature dependence of the Fermi-level pinning behavior has been observed. Two distinct pinning positions are found (0.75 and 0.5 eV above the valence-band maximum for n- and p-type GaAs, respectively) at RT and LT. Defects are responsible for those pinnings. Additional change in the Fermi-level position at the Sn/GaAs interface is related to the metallization of Sn overlayer.

Interface formation and thermal stability of Sn overlayers grown on cubic SiC(100)

We present an angle resolved synchrotron radiation photoemission study in the photon energy range hν=40–140 eV of the Sn/SiC(100) interface formation. Valence-band and core level photoemission spectra suggest that the first monolayer of Sn grows as a two-dimensional layer in the tetrahedrally bonded α phase. Subsequent layers grow as metallic β-Sn in a three-dimensional cluster mode. Heating a 60 Å Sn/SiC(100) interface grown at room temperature to 400 °C leads to a Sn-Si reaction with a breakdown of the α phase and qualitative changes in the sizes and/or shapes of the Sn overlayer clusters. Heating to 1000 °C accelerates the Sn-Si reaction but does not seem to completely remove the β-Sn clusters.

Adsorption and growth of Sn on Si(100) from synchrotron photoemission studies.

Photoemission spectroscopy was used to study the initial growth and interaction of Sn on Si(100). The as-deposited surfaces were annealed at about 550 /sup 0/C, generating the following coverage-dependent reconstructions: c(4 x 4), (6 x 2), c(8 x 4), and (5 x 1). Changes in the Si 2p core-level line shape were identified as the selective modification of Si surface dimer atoms by the presence of Sn adatoms. A relative measure of the Sn-adatom--Si-substrate interaction is presented by quantification of the average number of Si surface sites modified in the presence of a Sn adatom for various coverages. The Sn 4d core-level spectra revealed two components for Sn coverages below 1 monolayer (ML), indicating the existence of two inequivalent Sn sites, whose relative occupancy is seen to vary with coverage. The Sn 4d core level for the majority sites roughly follows the movement of the Fermi level within the band gap of Si for increasing Sn coverages, while that for the minority sites remains at a constant binding energy relative to the Si valence-band maximum. Implications concerning the structure of the Sn overlayer are discussed. The Sn-induced band bending and the Schottky barrier height are obtained. Photoemission from the valencemore » bands near the Fermi level shows that the Sn-covered Si(100) surface has little density of states at the Fermi level for coverages below 1 ML.« less

Growth of Group IV-IV Heterostructures: Initial Stages of Interface Formation

ABSTRACTThe two major issues in the growth of a heterostructure are (1) the degree of perfection of the overlayer and (2) the sharpness of the interface. The initial stages of interface formation play a crucial role in this respect. Relevant questions are addressed under atomically clean conditions in the Si/Ge Si/Si and Ge/Sn systems, using ion scattering surface analysis, low energy electron diffraction and Auger electron spectroscopy. Of particular interest with respect to (1) is the general role of reconstruction in epitaxial growth: A necessary condition for perfect growth is the reordering of the substrate surface reconstruction. We show that the deposition temperature necessary to achieve this reordering depends strongly on the topography of the substrate reconstruction. For example, Ge deposition at room-temperature reorders the Si(100)2×1 reconstruction but not the Si(111)7×7, implying different epitaxial temperatures for these two substrates. To illustrate (2) we discuss the complex growth and anomalous diffusion found in the Ge/Sn system. Below a certain critical coverage Θc (1.15·1015 cm−2) no indiffusion of the Sn overlayer takes place, even at 700 K, although above Θc severe indiffusion does occur at this temperature. This result is discussed in terms of theories of surface segregation.

Adsorption of Sn on cleaved Si(111) surfaces

The initial steps of ultrahigh vacuum deposition of Sn on clean cleaved Si(111) surfaces kept at room temperature were studied using low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS). AES shows the formation of a seemingly uniform Sn overlayer up to 2 ML followed by the growth of metallic islands. During the adsorption stage, up to 2 ML, the initial 2 × 1 reconstructed surface is first replaced by a √3 × √3 R30° pattern in the 1 3 ML range and then by a 1 × 1 diagram beyond 0.5 ML. Size and valency effects are observed in the very first interaction process up to 1 3 ML . The electronic properties deduced from PYS reveal a monotonous decrease of the ionization energy up to 1 ML, a slight displacement of the Fermi level position towards the top of the valence band at that coverage, and the replacement of the double-structured dangling bond peak of Si(111) 2 × 1 by an apparently unique peak of Sn-induced dangling bond states. A remarkable situation occurs at 1 ML coverage with a very well ordered overlayer and the possible opening of a gap in the induced surface states.