Core-level Photoelectron Spectroscopy Research Articles

Studies of amorphous carbon using X-ray photoelectron spectroscopy, near-edge X-ray-absorption fine structure and Raman spectroscopy

We report core level and valence band X-ray photoelectron spectroscopy (XPS), carbon K and oxygen K near-edge X-ray-absorption fine structure spectroscopy (NEXAFS), and Raman spectroscopy results of plasma-deposited amorphous carbon generated from fullerene C 60. In comparison with evaporated C 60, the C 1 s peak is broader and asymmetric for the amorphous carbon film and its shake-up satellites disappear. The valence band shows three fairly broad peaks. Only one prominent π* resonance occurs in the NEXAFS spectrum. Recognizable structures appear in the σ* region indicating the formation of new bonds. In the Raman spectrum the typical D and G bands were observed. The amorphization of C 60 and post-plasma functionalization of the surface after exposure to atmosphere cause changes in the electronic structure.

Hydrogen-modified metal–silicon interfaces

Wet chemical preparation of well-ordered hydrogen-terminated silicon surfaces recently led to many new studies of the surface, its oxidation and the formation of metal–semiconductor interfaces based on hydrogen-terminated surfaces. When dealing with hydrogen-modified metal–silicon interfaces the key issue is to evaluate the influence of hydrogen termination on interface formation and to clarify the role of the interfacial hydrogen as a passivating layer or as a surfactant which may be eliminated during interface growth. In this article three systematic studies using core-level photoelectron spectroscopy are presented and related to earlier studies for a more general appreciation of hydrogen effect at hydrogen-modified metal–silicon interfaces.

Surface reconstructions of 3C-SiC(001) studied by high-resolution core-level photoemission

The different surface reconstructions of the 3C-SiC(001) surface have been systematically studied by high-resolution core-level photoelectron spectroscopy. For the Si-rich 3×2 surface, three different Si 2p surface components are clearly identified supporting the structure model with 2 3 ML of Si ad-dimers. It is confirmed that the c(2×2) and 2×1 surfaces are respectively C- and Si-terminated by observing a single dominant surface component in the C 1s or Si 2p spectra with binding energy shifts of 1.04 eV and −0.50 eV respectively relative to the bulk component. The atomic origins of these surface core levels are assigned and the core-level shifts are compared with previous measurements and recent theoretical calculations.

Magnetic circular dichroism in photoelectron angular distributions from Gd(0001)

Abstract We report on the influence of photoelectron diffraction effects on magnetic circular dichroism in angular distribution as measured using core-level photoelectron spectroscopy. As an example system we study the effect by measuring azimuthal angle scans of the 4d core-level on a thick Gd(0001) film grown on W(110). In particular, for photoelectron emission in off-normal directions we observe deviations from the pure atomic picture which can be interpreted in terms of a multiple scattering theory. We also evaluate different measuring methods to see their influence on the angular behavior. Using high resolution data we can even study the state-specific contributions to the magnetic dichroism signal. The knowledge of these effects should help in interpreting absolute dichroism values on the basis of sum rules.

Sb desorption from Sb/GaAs(001) and GaSb(001) analyzed by core-level photoelectron spectroscopy

Abstract The process of Sb desorption from GaSb(001) and Sb-terminated GaAs(001) was investigated by using a real-time analysis system that uses time-resolved core-level photoelectron spectroscopy. From the photoelectron intensity analysis, two competing processes with fast and slow time constants were found on the Sb desorption from both substrates and the activation energies were determined to be 0.71 eV and 0.90 eV for GaSb(001) and 1.1 eV and 2.5 eV for Sb-terminated GaAs(001). We investigated the relationship between the activation energies and surface bonding structure using spectrum differences based on the assignment of conventional core-level photoelectron spectroscopy of the surfaces whose Sb coverage is well controlled. This indicated that the activation energy of the desorption related to the transition from 2×8 to 2×4 of an Sb-terminated GaAs(001) is larger than the other desorption processes and it can be explained that the large potential barrier for bonding structure rearrangement between 2×8 superstructure and 2×4 superstructure exist in addition to Sb atoms desorption.

Arsenic interlayers at the Sn/InP(001) interface

The reconstructed InP(001)-(2×4) surface and the arsenic-modified InP(001)-(1×1)-As surface were included in a comparative study in order to investigate the influence of arsenic interlayers on the interface formation. Interface reactions and the morphology of tin films on InP(001) have been investigated using core level photoelectron spectroscopy (SXPS) and LEED. A strong reaction between InP and the deposited tin is observed on the Sn/InP(001)-(2×4) interface. An arsenic interlayer passivates the substrate against this reaction. Arsenic gives rise to layer-by-layer growth mode up to Sn layer thicknesses of 1 nm of Sn on InP(001), while without this interlayer only one flat monolayer of Sn was formed. Beyond these limits three-dimensional metallic tin islands grow on both surfaces. Annealing of the tin films to 650 K after the last coverage initiates coagulation of the tin and lifts the passivating effect of the arsenic interlayer.

Alkyl-terminated Si(111) surfaces: A high-resolution, core level photoelectron spectroscopy study

The bonding of alkyl monolayers to Si(111) surfaces has been studied with high-resolution core level photoelectron spectroscopy (PES). Two very different wet-chemical methods have been used to prepare the alkyl monolayers: (i) Olefin insertion into the H–Si bond of the H–Si(111) surface, and (ii) replacement of Cl on the Cl–Si(111) surface by an alkyl group from an alkyllithium reagent. In both cases, PES has revealed a C 1s component shifted to lower binding energy and a Si 2p component shifted to higher binding energy. Both components are attributed to the presence of a C–Si bond at the interface. Along with photoelectron diffraction data [Appl. Phys. Lett. 71, 1056, (1997)], these data are used to show that these two synthetic methods can be used to functionalize the Si(111) surface.

High-temperature arsenic adsorption on InP(110)—surface bonding and surface structure

Abstract A study including core level photoelectron spectroscopy and photoelectron diffraction was performed in order to obtain information on the high-temperature arsenic adsorption on the InP(110) surface. Surface bonding, surface structure and interface reactions were investigated in comparison to InP(110). An arsenic–phosphorus exchange reaction replacing the first layer phosphorus atoms by arsenic and forming a monolayer of InAs was observed using photoelectron spectroscopy. A polar angle scanned photoelectron diffraction study was performed in order to obtain structural information on this arsenic-exchanged surface. Comparison of simulations with experimental angle distribution curves applying double scattering calculations supports a bond angle rotation of this surface with a tilt angle of ω=29°.

The surface core-level shift of the Nb(110) surface

The surface core-level shift (SCLS) of the Nb(110) surface has been determined by means of high-resolution core-level photoelectron spectroscopy of the Nb 3d5/2 core level. A SCLS of 180 +- 15 meV towards higher binding energy with respect to the bulk component was estimated by curve fitting the Nb 3d5/2 core-level spectra taken using several photon energies. The sign of the SCLS agrees with recent ab initio calculations including both initial- and finalstate effects, but the shift is significantly smaller than predicted in these calculations.

Short Range Order and the Nature of Defects and Traps in Amorphous Silicon Oxynitride Governed by the Mott Rule

Using valence band and Si $2p$ core level photoelectron spectroscopy, it is shown that the short range order in amorphous silicon oxynitride $(a\ensuremath{-}{\mathrm{SiO}}_{x}{\mathrm{N}}_{y})$ is governed by the Mott rule. According to this rule, each Si atom is coordinated by four O and/or N atoms, each O atom (as in ${\mathrm{SiO}}_{2}$) is coordinated by two Si atoms, and each N atom (as in ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$) is coordinated by three Si atoms. The nature of the removal of Si-Si bonds (hole traps) at the interface of ${\mathrm{SiO}}_{2}/\mathrm{Si}$ by nitridation and the origin of Si-Si bond creation near the top surface of gate oxynitride in metal-oxide-semiconductor devices are understood for the first time by the Mott rule.

Gallium phosphate thin solid films: structural and chemical determination of the oxygen surroundings by XANES and XPS

Amorphous dielectric gallium phosphate thin films of various chemical compositions were deposited on silicon substrates by the `pyrosol' process. This paper reports the results of a structural study of these deposits before and after thermal annealing. Oxygen K-edge X-ray Absorption Near Edge Structures (XANES) measurements and oxygen 1s (O 1s) core-level X-ray Photoelectron Spectroscopy (XPS) data collection were carried out in order to probe the oxygen surroundings and chemical states. In P-rich thin films, the O atoms are dicoordinated, with the coexistence of Ga–O–P and P–O–P bridges. In Ga-enriched deposits, the oxygen atoms are mainly dicoordinated to gallium atoms. The annealed P-rich films are composed of dicoordinated oxygen atoms forming only Ga–O–P linkages, while in the annealed Ga-rich deposits, the O atoms present a mixed surroundings constituted of di, tri and tetracoordinated oxygen atoms.

Huge charge transfer from potassium on H:Si(111)-(1×1)

Potassium adsorption on the hydrogen-terminated H:Si(111)-(1×1) surface is studied by high-resolution core-level and valence-band photoelectron spectroscopy. New light is shed on the interpretation of core-level spectra at the K/Si(111) interface. Low potassium coverages induce a large downward band bending. The Fermi level moves ∼0.26 eV above the conduction band minimum, which is explained by a huge charge transfer from the adsorbate to the semiconductor bulk. For room-temperature saturation coverage, the Fermi level drops back into the semiconductor band gap. This leads to a vanishing n-type Schottky barrier height Φ (n)=0.02±0.03 eV in agreement with theoretical results.

Structural and hydriding properties of amorphous MgNi with interstitially dissolved carbon

MgNiC x with an amorphous structure were synthesized up to x=1.31 by mechanical alloying of amorphous (a-)MgNi and graphite as starting materials. The core-level X-ray photoelectron spectroscopy revealed that the original bonding states with metallic character of a-MgNi remained even in the sample with high carbon content although the graphite dissolved as interstitial carbon and it mainly bounded with Mg in a-MgNi. The crystallization temperature of MgNiC x increased with increasing carbon content up to x=0.85, which was accompanied by a drastic change in the crystallization processes. Upon hydrogenation of MgNiC x , the atomic ratio of hydrogen plus carbon to metal (H+C)/M remained a constant value of ≃0.9 through the whole system, indicating that carbon atoms dissolved into the sites that otherwise would have been occupied by hydrogen atoms.

Real-time analysis for MBE by time-resolved core-level photoelectron spectroscopy.

A system has been developed for the real-time analysis of surface reactions during molecular beam epitaxial growth which uses photoelectron spectroscopy with VUV light taken from synchrotron radiation. This system consists of a synchrotron radiation beamline and growth/analysis apparatus in which photoelectron spectroscopy is performed with sub-second time resolution. In this system, photoelectron spectra are measured in sequence by a 'non-scanning' measurement method that enables the acquisition of snapshot photoelectron spectra using a multi-channel detector. This non-scanning measurement method was enabled by equipping an electric field correction grid. This system was used to monitor the photoelectron spectra of a GaSb(001) surface.

Au/H:Si(111)−(1×1)interface versusAu/Si(111)−(7×7)

The room-temperature growth of gold on Si(111) surfaces has been directly compared for nonreconstructed hydrogen-terminated surfaces and the bare $7\ifmmode\times\else\texttimes\fi{}7$-reconstructed surface by means of high-resolution core-level and valence-band photoelectron spectroscopy and Auger spectroscopy. The growth modes show strong differences with gold island formation on the passivated $\mathrm{H}:\mathrm{S}\mathrm{i}(111)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)$ surface. Silicon segregated on top of the deposited gold film is seen for both interfaces. The Schottky barrier is only weakly affected by the hydrogen termination.

Time-resolved core-level photoelectron spectroscopy on Sb-terminated GaAs(001) under Sb supply control at growth temperature

The Sb desorption process from Sb-terminated GaAs(001) substrates has been analysed by time-resolved core-level photoelectron spectroscopy to investigate the influence of the substrate on the desorption. From the time dependence of the Ga 3d intensity, and the spectrum changes of Ga 3d and Sb 4d under Sb flux and during Sb desorption, three series of time constants for Sb desorption were found and activation energies of 2.5 eV and 1.1 eV were obtained for two of the desorption series. Spectrum analysis clarified that Sb double layers lie on the surface under Sb flux exposure and that Sb atoms corresponding to activation energies of 2.5 and 1.1 eV desorb from the topmost site of the Sb double layers. This is the same in the case of GaSb (001) substrate without the activation energy and shows that the bonding strength between the Sb atoms of the topmost layer site and the second layer site depends on the substrate materials.

Adsorption of SO 2 on Cu(100) and Cu(100)- c(2 × 2)- O surfaces studied with photoelectron spectroscopy

The adsorption of SO 2 on clean Cu(100) and Cu(100)- c(2 × 2)- O surfaces at room temperature has been studied using high resolution core-level photoelectron spectroscopy. On both surfaces, two types of atomic S and two types of SO 3 are found as shown by their well separated S 2p binding energies. On the Cu(100)- c(2 × 2) O surface, pre-adsorbed oxygen inhibits the SO 2 decomposition and accelerates the SO 2 reaction with the substrate which benefits SO 3 formation.

NH3onSi(111)7×7:Dissociation and surface reactions

Core-level and valence-band photoelectron spectroscopy on the dissociative adsorption of ammonia on Si(111) 7 X 7 is presented. Adsorption at room temperature produces three nitrogen 1s components, ...

Formation of the interface between c-Si(111) and diamond-like carbon studied with photoelectron spectroscopy

The hydrogen-terminated Si(111) surface was used as a substrate to grow amorphous hydrogenated carbon (a-C:H) films by means of layer-by-layer plasma deposition. Electronic and structural properties of the forming interface were determined by in situ core level and valence band photoelectron spectroscopy. From the core level spectra the gradual formation of an initially carbon-rich and eventually stoichiometric silicon carbide interface layer is inferred with a final thickness of 18Å. The silicon core level data indicate that the substrate is hydrogen-passivated owing to the plasma exposure. On the final heterostructure, the valence band edge of the diamond-like carbon is found to be 0.4 eV below that of the interfacial a-SiC:H and also below that of the silicon, with the offset amounting to 0.7 ± 0.1 eV.

C60-terminated Si surfaces: Charge transfer, bonding, and chemical passivation

The interaction of ${\mathrm{C}}_{60}$ with the Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) and Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surfaces has been investigated using synchrotron radiation core-level and valence-band photoelectron spectroscopy. ${\mathrm{C}}_{60}$ induces distinct spectral changes in the $\mathrm{S}\mathrm{i}\ensuremath{-}2p$ core-level emission from both surfaces, indicative of charge transfer to the adsorbed fullerene molecules. Our results suggest that ${\mathrm{C}}_{60}$ adsorption on Si(111) induces a redistribution of charge within the (7\ifmmode\times\else\texttimes\fi{}7) unit cell involving electron transfer from rest atom to adatom dangling bonds. For a one monolayer coverage [on both Si(111) and Si(100)], broad ${\mathrm{C}}_{60}$-induced chemically shifted components are present in the core-level spectra. Valence-band spectra, however, show no evidence for a high degree of electron occupation of the ${\mathrm{C}}_{60}$ lowest unoccupied molecular orbital. We present core-level data which illustrate that adsorption of a ${\mathrm{C}}_{60}$ monolayer inhibits ambient oxidation of the Si surface.