Ag Coverage Research Articles

Synchrotron radiation photoemission study of the formation of the Ag/GaSb(110) interface

We have studied the chemistry and electronic properties of the Ag/GaSb(110) interface formed at room and low temperature with high resolution core level photoemission spectroscopy. The Ga 3d and Sb 4d core level spectra are deconvolved into bulk, surface, and reacted components. The relative changes in intensities of these components as a function of Ag coverage at room temperature (RT) and low temperature (LT) are studied in terms of interface morphology and chemistry. At LT, the Ga 3d and Sb 4d surface components disappear with about one monolayer of Ag coverage, confirming the two dimensional Ag growth. At RT, on the other hand, the surface components persist up to coverages of several monolayers, indicating a Volmer–Weber growth mode. At higher coverages, both the Ga 3d and the Sb 4d spectra are dominated by reacted components with lower binding energy. The intensity of the Ga 3d core level decays faster as a function of Ag coverage than that of Sb 4d, indicating out diffusion of Sb. Out diffusion is reduced at 80 K. Band bending at the GaSb surface is extracted from the shift of the bulk components. The final position of EF is found to be within 50 meV of the valence band maximum, in good agreement with previous ultraviolet photoemission results.

Si - noble metal (Au,Cu,Ag) interface formation studies by AES

Room temperature deposition of gold, copper and silver on the Si (111) surface has been studied by AES. The information on the changes of Si(3p) density of states in valence band with Au. Cu, Ag coverage on Si(111) surface have been obtained by means of the Auger Si(LVV) line structure analysis. Auger Si(LVV) line structure extraction have been produced by subtraction of Auger spectra for W-Au(Cu,Ag) system from those for Si-Au(Cu,Ag) at the same adsorbed metal surface concentrations. It has been shown that for Si-Au, Si-Cu systems the main density of Si(3p) states structure peculiarities can be explained by formation of (d-p) hybrid bonds between silicon and metal atoms at the interface, for Si-Ag system only weak interaction between the components has been observed which practically does not change the valence band structure. The differences in the properties of Si-Ag and Si-Au(Cu) interfaces can be explained by different metal d-electron binding energies. The Si-Au(Cu) interface chemical activity concentration dependence can be explained by the solid state electron energy structure formation processes.

Monte carlo simulation of temperature programmed desorption of Ag atoms from a Ru(001) surface

A Monte Carlo simulation method was applied to study the desorption process of submonolayer amounts of Ag atoms from Ru(001). The attractive lateral interactions between Ag atoms leading to island formation are also manifested in the desorption kinetics. First the equilibrium configuration of the Ag overlayer of hexagonal symmetry was simulated for different coverages. In the simplest case the desorption rate is evaluated as the sum of first order desorption rates of Ag atoms having 0, 1, 2, ...6 nearest Ag-Ag bonds, assuming the preexponential factor for desorption to be constant. The stimulated desorption curves exhibit quasi-zeroth order desorption kinetics for initial Ag coverages less than 0.35 ML. At higher coverages the desorption spectra are shifted towards higher temperatures and no unique slope can be identified. When the pre-exponential factor, however, is assumed to increase with Ag coverage according to a simple combinatory formula, the calculated desorption spectra exhibit the quasi-zeroth order behaviour also up to higher coverages in qualitative agreement with experimental results.

Initial stages of metal/semiconductor interface formation: Au and Ag on Si(111)

We have studied the atomic structures formed by monolayer coverages of Au and Ag on the Si(111) surface using primarily the technique of impact-collision ion scattering spectroscopy (ICISS). For the case of Au films annealed at 700°C, three different types of LEED patterns are formed depending on the fractional monolayer coverage: 5 × 1, √3 × √3, and 6×6. The ICISS data reveal that all the three surfaces are structurally similar: the Au atoms reside above the Si(111) plane, most likely in threefold-hollow sites, and the different surfaces appear to be characterized by rows (5×1) or a honeycomb network (√3×√3 and 6×6). In contrast, the Ag films deposited at elevated substrate temperature (480°C) display only a √3×√3 LEED pattern for coverages ranging from 0.25 to 35 monolayers. A trimer model appears to be more consistent with the low coverage Ag ICISS data rather than a honeycomb arrangement of the Ag atoms.

Growth of ultrathin Au and Ag films on a modified Ru surface

Using TDS, LEED and AES we have studied the adsorption of Ag and Au on an oxygen precovered Ru(001) surface in comparison with the growth mechanisms of these adsorbates on the non-modified substrate. Oxygen desorbs from Ru at higher temperatures than Ag and causes even the smallest detectable Ag coverage to sublimate like bulk silver (in contrast to the layer-by-layer growth on pure Ru). In turn, the TD-spectrum of Au is hardly affected because oxygen desorbs partially before gold. However, a quantitative analysis of AES intensities, taken at room temperature, as well as simple model calculations support a 3D island growth mode of Au on the oxygen precovered Ru surface as well.

Coverage dependence of the spin-resolved photoemission from epitaxially grown Ag layers on Pt(111)

Ag/Pt(111) has been studied by spin-, angle-, and energy-resolved photoemission with circularly polarized synchrotron radiation of BESSY for different Ag coverages. The prepared layers were characterized by AES and LEED and turned out to grow epitaxially. An independent layer thickness determination and some additional structure information were obtained from Rutherford backscattering studies in combination with channeling. The spin-resolved photoemission experiments were performed for normal incidence of circularly polarized light and normal emission of the photoelectrons. The spin information allowed a separation of spin-orbit split peaks of the Ag adsorbate layer and partly also a separation of Ag adsorbate peaks from the Pt substrate background intensity. This “method-induced improved resolution” was used to deduce information about the development of the Ag band structure in the Λ-direction from the coverage dependence of spin-resolved photoemission spectra. Dispersion of occupied bands was found to be already almost completely developed for a three-layer system. Information about the development of the band structure in its unoccupied part was obtained from the resonant behavior of peaks at about 6 eV below E F. The resonant behavior is already observed for three Ag layers and increases with layer thickness. We observe a peak in the photoemission spectra contributing to the resonances which is not predicted by existing theoretical models.

Surface segregation of Ba in polycrystalline YBa2Cu3O7 oxides: The effect of silver

We have investigated the effect of annealing polycrystalline YBa2Cu3O7−δ samples in a He ambient and in vacuum, on the composition of their internal surfaces. X-ray photoelectron spectroscopy (XPS) data indicate a decrease in the amount of BaCO3 but a higher proportion of Ba at the near-surface region upon low-temperature annealing in He or vacuum. The latter effect seems to derive from the presence of a surface barrier for oxygen outdiffusion, and can be prevented by diffusing Ag into the polycrystalline sample. Changes in the cutoff energies of the ultraviolet photoelectron spectra indicate that a submonolayer coverage of Ag is sufficient to produce a change in the work function of 0.3 eV, corresponding to the lowering of the surface barrier.

Probing the surface electrochemical properties of electrodeposited metal layers by chemisorption of redox-active species: iodine on silver-plated platinum

The surface electrochemical properties of Ag layers electrodeposited on smooth polycrystalline Pt electrodes has been investigated by observing the changes in the redox behavior of iodine chemisorbed on the bimetallic surface at critical stages of the electrodeposition process. It has been found that: (i) the electrochemical behavior of chemisorbed I at a given Ag coverage is the same whether deposition was performed before or after I adsorption; I is always the outermost layer. (ii) I −( aq) is oxidatively chemisorbed as iodine onto the electrodeposit regardless of Ag coverage. (iii) Chemisorbed iodine can be cathodically stripped from the Ag electrodeposit; the redox potential for the I( ads)/I −( aq) couple on Ag is shifted by about −1.5 V relative to that for the I 2( aq)/I −( aq) couple; the chemisorption strength of zerovalent iodine on Ag is 220 kJ mol −1 larger than that of I −. (iv) Deposition of 0.25 and 0.5 layer of Ag yielded results which could be rationalized in terms of I desorption from two chemisorbed states: one coordinated to Pt, the other to Ag. (v) Deposition of one layer of Ag exhibited unique I coverage vs potential plots which suggested that the bimetallic surface consisting of one Ag layer on Pt substrate possesses distinct interfacial properties. (vi) The redox behavior of I chemisorbed on two monolayers of Ag was very similar to those of bulk Ag, signifying that the surface chemistry of two monolayers of Ag is identical to that of the bulk material.

Annealing Ag on GaAs: Interplay between cluster formation and Fermi level unpinning

It has been shown that upon annealing a 2-monolayer coverage of Ag on UHV-cleaved GaAs at 500 °C, the Ag clusters into islands and the surface Fermi level moves back to within 0.2 eV of the bulk position. In this study, this Fermi level unpinning behavior has been investigated further by using various substrate doping levels and varying the anneal temperature. The Ag clustering process was observed using both ultraviolet photoelectron spectroscopy (UPS) and scanning electron microscopy. Through UPS, the surface Fermi level (Efs) movement was monitored simultaneously with the clustering process. No distinct temperature for the onset of clustering is observed. Rather, the clustering process occurs continuously over the range of temperatures studied (room temperature to 500 °C), with the Ag clusters growing and increasing in separation as anneals are performed at successively higher temperatures. A 10 min annealing time was sufficient to achieve a stable, equilibrium configuration at each annealing temperature. Movement of the Efs back to near the bulk position occurred between 375–450 °C for high doped n-GaAs (6×1018/cm3); whereas no movement of the Efs was observed for low doped n-GaAs (4×1016/cm3) up to 475 °C. Similar results were obtained when the experiment was repeated for high doped p-GaAs (1.4×1019/cm3) and low doped p-GaAs (5×1016/cm3). The absence of movement of Efs in the low doped GaAs is attributed to the longer substrate depletion length, implying that only the areas beneath and within the depletion length of the Ag clusters are pinned.

Effect of Ag on the surface barrier for depletion of oxygen from polycrystalline YBa2Cu3O7 oxides.

We have investigated the effect of evaporated Ag on the depletion of oxygen from polycrystalline $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ in a He ambient atmosphere by in situ resistivity experiments and surface spectroscopies. Our data indicate an increase in the rate of out-diffusion of oxygen due to the presence of the Ag, which lowers the surface barrier for the out-diffusion of oxygen by 0.2\ifmmode\pm\else\textpm\fi{}0.03 eV. We have performed x-ray photoelectron spectroscopy (XPS) analysis of the in situ fractured surfaces to indicate the presence of Ag at the internal surfaces in the oxide. XPS data from Ag-free in situ scraped samples show oxygen accumulation in the near-surface region upon vacuum annealing. This effect is prevented by evaporation of a few monolayers of Ag. Changes in the cutoff energies of the ultraviolet photoelectron spectra indicate that a submonolayer coverage of Ag is sufficient to produce a change in the work function of 0.3 eV, corresponding to the lowering of the surface barrier.

Determination of the bonding and growth of Ag on Si(100)-(2 x 1).

The bonding and growth of Ag on Si(100) were studied with high-resolution photoemission spectroscopy and high-energy electron diffraction. By monitoring the changes in the Si 2p core-level and Ag 4d valence-band line shapes, we have examined the chemical interaction between the Ag adatoms and Si surface dimer atoms. The Ag-to-Si bonding coordination number as a function of coverage was obtained. This number is 2 at low Ag coverages, which suggests that the bonding site for a Ag adatom is between two neighboring Si surface dimers along the direction of dimerization; the two dimer dangling bonds pointing at the Ag adatom join the two Ag sp hybrid orbitals to form two Ag-Si bonds. The growth mode was observed to be three dimensional at higher coverages. The Schottky-barrier height was measured.

Structural change of Si(100) and (111) surfaces after Ag deposition studied by MeV ion channeling

The structural change of Si(100) and (111) surfaces associated with the deposition of Ag thin films (0–5 ML thickness) has been studied by high energy ion channeling and low energy electron diffraction (LEED). The variation of the Si and Ag surface peak intensities has been measured using a 1 MeV He + ion beam as a function of Ag coverage ( gq) and heat treatment. With Ag deposition the reconstructed Si(100)2x1 structure relaxes to the bulk-like structure and the Si surface peak intensity decreases in the normal direction. In the case of Si(111), the Si surface peak intensity remains almost constant with Ag deposition over the range of film thicknesses examined.

Determination of an Adlayer Bonding Transition by Surface Extended X-Ray-Absorption Fine-Structure Spectroscopy: Cesium Adsorbed on Ag {111}

As the Cs coverage of Ag{111} is increased from approximately 0.15 to 0.3 monolayer the Cs-Ag bond length is found to increase from 3.20 \ifmmode\pm\else\textpm\fi{} 0.03 to 3.50 \ifmmode\pm\else\textpm\fi{} 0.03 \AA{}. This dramatic change is associated with a transition in the cesium bonding from predominantly ionic at low coverage towards a more covalent state at high coverage.

Resonant-photoemission study of the mechanism for room-temperature-alloyed interface formation of Au and Ag on Si(111)-(2 x 1).

Room-temperature deposition of gold and silver on the Si(111)-(2\ifmmode\times\else\texttimes\fi{}1) surface was studied by photoemission spectroscopy. We successfully observed, around the photon energy of the Si(2p) core excitation threshold (\ensuremath{\sim}100 eV), a resonancelike enhancement of the photoemission signal of Au(5d) at around monolayer coverage of Au, while no enhancement of the Ag(4d) signal was observed at around monolayer coverage of Ag. This result indicates the formation of a covalentlike bond between Au (deposited) and Si (substrate) atoms and not in Ag-Si, and the mechanism of room-temperature-alloyed interface formation at a metal-semiconductor interface can be described by the ``chemical-bonding model'' proposed before by the authors [M. Iwami, T. Terada, M. Kubota, H. Tochihara, and Y. Murata, Surf. Sci. 119, 115 (1988)].

Reversibility of Fermi level pinning

By depositing monolayer coverage of Ag on freshly cleaved, clean GaAs(110) surface, the surface Fermi level was found to pin at 0.45±0.05 eV above valence band maximum (VBM) for p-GaAs and 0.65±0.05 eV above VBM for n-GaAs, due to the formation of midgap electronic states. Upon annealing to 500 °C, the Ag on the surface clustered into islands and the surface Fermi level moved back to within 0.2 eV of the bulk position [conduction band minimum (CBM) and VBM for n and p types, respectively]. Further deposition of Ag on the annealed surface moved the surface Fermi level back to the previously pinned positions. The surface Fermi level movement was monitored using Ga 3d and As 3d photoemission signals. The Ag clustering was seen both by a dramatic decrease in the Ag 4d photoemission signal and by scanning Auger microscopy. Scanning electron microscopy also revealed that the Ag clustering formed hemispherical-like islands with a size distribution up to ∼500 Å in diameter. The average island separation is about a few thousand angstroms (∼2000 Å). As a comparison with a reactive metal, similar annealing experiments were performed for monolayer coverage of Al on GaAs. For the Al case, the metal remained unclustered after annealing and the surface Fermi level remained pinned.

Screening and delocalization effects in Schottky barrier formation

In order to assess the role of metallic screening at the metal–semiconductor interface, we have investigated the effects of Ag deposition on the band bending of GaAs(110) surfaces previously ‘‘pinned’’ with vanadium atoms. Their coverages ranged from 0.05 to 1.5 monolayers (ML), which positioned the surface Fermi level (EF)∼0.85–0.6 eV above the valence band maximum (VBM). For subsequent depositions of Ag additional changes in EF of up to 200 meV towards the VBM were observed on both n- and p-type GaAs. The onset of these changes, which occurs for mean Ag coverages of ∼0.1 ML, coincides with the appearance of metallic characteristics in the Ag clusters. These changes in EF can be explained in terms of a new model in which the interface impurity levels in the band gap of the semiconductor become delocalized through interaction with states of the metal overlayer. A functional expression is developed which relates the Fermi level to the metal and semiconductor parameters, and to an effective charge in each delocalized level. These levels, actually broadened resonances of the impurity-metal system, accommodate the charge densities required to equalize EF throughout the system. In analogy to a partially filled band, EF is determined by the average effective charge residing in each resonance. The resonances are nearly neutral and can accept or donate electronic charge as needed to compensate for the conductivity type of the semiconductor. Calculated changes of EF based on this model agree well with the experimental observations.

Initial growth modes of Ag on Ni(100) and Ni(111) determined by planimetry with adsorbed CO.

A novel photoemission technique is applied to examine the growth behavior of Ag on Ni(100) and Ni(111) substrates at room temperature. Adsorbed CO molecules are used to label the part of the Ni substrate surface not covered by Ag. From the measured photoemission intensity of the CO molecular orbitals, the area not covered by Ag is determined. The result shows that the growth is close to being layer by layer for the first 0.7 monolayer of Ag coverage, and deviates appreciably from the layer-by-layer mode at higher coverages.

Interaction of Cu, Ag and Au with van der Waals faces of WS, and SnS 2

The interactions of group IB metals with van der Waals faces of layered semiconductors have been investigated. Cu, Ag and Au were deposited on UHV cleaved (0001) faces of n-SnS 2 and n-WS 2. Copper reacts with SnS 2 to form a new bulk phase (CuSnS 2) whereas Ag and Au react only at the interface to form an interfacial phase containing reduced Sn. A bulk metal layer is formed for high Ag and Au coverages whereas copper continues to diffuse into the material even after deposition of hundreds of monolayers. An atomically abrupt semiconductor/metal interface is formed upon metal deposition onto WS 2, with no formation of an interfacial reaction phase, suggesting epitaxial growth of the metal overlayer. The contrasting interfacial reactivity is exploited to examine Schottky barrier formation on the two materials. The metal /semiconductor barrier height for WS 2 is close to the difference of metal and semiconductor work functions (Schottky limit). The barrier height for SnS 2 metal junctions deviates considerably from the work function difference (Fermi level pinning or Bardeen limit).

Photo-emission study of in situ intercalation of TiS2with Ag

The authors present angle-resolved ultraviolet photo-emission data on thin films of Ag evaporated onto the surface of TiS2. They find evidence for epitaxial growth of Ag on TiS2 and subsequent intercalation at room temperature and above. With submonolayer Ag coverage intercalation was indicated also at 220K. Changes in spectra were observed that can be understood as intercalation-induced changes in the TiS2 band structure. In particular, the appearance of a prominent new valence band structure was found to indicate formation of Ag-intercalated TiS2.

Summary Abstract: X-ray photoelectron spectroscopy and ion scattering spectroscopy of submonolayer coverage of Ag on SiO2

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