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NEXAFS data [S. Rangan et al., Phys. Rev. B 71 (2005) 165319] and FTIR data [M.P. Schwartz, R.J. Hamers, Surf. Sci. 601 (2007) 945] apparently do not converge in the identification of the reaction products of acetonitrile (CH 3 CN) with Si(0 0 1)-2 × 1 at room temperature. Using DFT calculations of core-excited/core-ionized spectra and of IR vibrational frequencies and intensities, we show the consistency of the body of experimental data. Three species are present on the surface in equivalent amounts, a C N moiety, a pendent C N and a C C N ketenimine submitted to a strong twist imposed by the Si bond directionality. More generally, the paper shows the usefulness of spectroscopic data simulations in the elucidation of complex surface chemistry problems.

This paper focuses on the use and value of XPS and NEXAFS spectroscopies to unveil the nature of the chemical bond of various bifunctional nitrile molecules adsorbed on Si (001) 2×1 at 300 K. The adsorption modes are also discussed in the light of recent theoretical publications devoted to optimized geometries and reaction paths of these molecules on Si (001).

The present work is a comparative study of the adsorption modes of acetonitrile and of the conjugated molecule acrylonitrile on Si(0 0 1)-2×1 at room temperature, using synchrotron radiation photoemission spectroscopy (PES) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Both molecules chemisorb on the surface. Besides dissociated species revealed by PES, NEXAFS spectroscopy shows that a large fraction of acetonitrile molecules are di-σ bonded to the surface, e.g. after a [2+2]-like cycloaddition of the CN group with a silicon dimer. In the case of acrylonitrile, PES indicates that most of the molecules chemisorb non-dissociatively. Nevertheless multiple bonding geometries must be envisaged to account for the NEXAFS spectra: the major reaction products has a flat lying geometry, the two outer atoms of the molecule skeleton being bonded to two silicon atoms. Other geometries, which correspond to the reaction of the CC bond with a silicon dimer or to the formation of a dative bond through the nitrogen lone pair with one silicon atom have also to be envisaged.

Pseudomorphic c-SiC alloys on Si(1 1 1) were grown by exposure to acetylene (C 2 H 2 ) in ultra-high-vacuum conditions. The behavior of C incorporation in the lattice for different substrate growth temperatures was studied by UHV electronic techniques. We found that C atoms occupy tetrahedral substitutional sites in the Si lattice when the substrate temperature is kept below 650°C. With increasing temperature, up to 950°C, we have observed a substantial increase of the C atoms mainly bounded to silicon in nonsubstitutional sites in a nearly planar sp 2 configuration. This is evidenced by the analysis of the line shape of the near-edge energy loss and by extended energy loss fine structure measurements which are particularly sensitive to the bond length around each carbon atom.

The Si 2p soft X-ray photoemission study of various oxidized surfaces (thermal oxidation and room temperature (RT) oxidation) shows the benefit derived from the use of high-energy resolution (70 meV). Interesting structural information can be retrieved from an analysis of the line widths of the oxides, suboxides, and elemental Si peaks. In particular the binding energy (BE) of elemental silicon layers adjacent to the oxidized layer can be distinguished from that of deeper silicon layers (the two prominent `interfacial' elemental Si lines are shifted by about +0.2 and −0.2 eV). These new data show a possible effect of oxygen second neighbors on elemental Si 2p binding energies. Moreover, changes in the oxide/suboxide line widths – as seen in a comparative study of the Si(1 1 1)-7×7 surface thermally oxidized in O2 and exposed at RT to O2 or H2O – may be attributed to static disorder, i.e., variations in Si–O bond lengths.

We report a study using X-ray Photoemission Spectroscopy (XPS) of a nanocomposite which is a mixture of silicon nitride (Si3N4) micrometric grains (mean diameter of 0.2 μm) and silicon carbide (SiC) nanoparticles (from 0.01 μm to 0.06 μm). A differential charge effect was detected in this compound for the C1s and Si2p core levels. We concluded that the existence of such an effect was because the first raw acquired spectra display selective partial reductions of the charge for 6–7 h. Contributions to the XPS core levels from silicon carbide (Si–C4 and C–Si4) are not affected by charge shifts, in contrast to other contributions. This differential charge effect was not observed for SiCN nanopowders which display a rigid charge shift of all the contributions in the same core levels. This is a good fingerprint for the structural organization in the compound. We concluded the existence of SiC nanoparticles dispersed in a vitreous YSiAlON[C] phase located at the grain boundaries in the material which made up a conducting cement/skeleton surrounding large silicon nitride grains (0.2 μm).

The different carbonization processes of a Si(111) 7 x 7 reconstructed surface exposed to acetylene (C 2 H 2 ) were studied by ultrahigh-vacuum electronic and luminescence techniques. We found that the molecule breaks at about 450°C and for temperature up to 600°C a Si 1-x C x alloy is formed in the near-surface region of the sample. For higher temperatures, up to 850°C, crystalline SiC crystallite precipitation epitaxially grown on the Si surface is observed. The luminescence spectra obtained at each stage of carbonization revealed the formation of well defined Si 1-x C x phases (with x equal to a few per cent) and it clearly demonstrates that the exposure to C 2 H 2 is highly efficient procedure to change in a continuous way the near-infrared optical properties of Si surfaces in the spectral range from 0.9 to 1.1 eV. The observation at 650°C exposure of an intense and broadened photoluminescence emission in the range 1.2-1.6eV is particularly interesting. This indicates the possibility that in the near-surface region a local concentration of the first-nearest-neighbour Si-C bond of between 15-20% is present, indicating the formation of a high C-rich layer.

The carbonization process of a Si(111) 7×7 reconstructed surface exposed to acetylene (C 2 H 2 ) has been investigated by combining several structural, electronic, and luminescence techniques. The aim was to give a comprehensive scenario of the different carbonaceous phases formed on the clean silicon surface starting from the C 2 H 2 molecule chemisorbed at room temperature up to the formation of a crystalline c-SiC compound. We found that the molecule breaks at about 450°C and for temperatures up to 600°C a Si 1− x C x alloy is formed inside the sample. For higher temperatures, up to 850°C, c-SiC crystallite precipitation epitaxially grown on the Si surface is observed. The near-edge-energy loss spectra at the carbon K edge ensures that for C 2 H 2 deposition temperatures higher than 600°C the carbon bonds are sp 3 -like, thus demonstrating the absence of non-substitutional carbon concentration. The luminescence spectra performed at each stage of carbonization revealed the formation of well-defined Si 1− x C x phases (with x of a few percent) and clearly demonstrate that the exposure to C 2 H 2 is a highly efficient method to change in a continuous way the near-infrared optical properties of silicon surfaces in the spectral range from 0.9 to 1.1 eV.

We report here on the determination of depth profiles of oxygen and nitrogen with nanometer resolution in silicon oxynitride films with thicknesses below 3 nm. We have studied oxynitride films grown by rapid thermal processing of Si (0 0 1) wafers in various Nitric Oxide (NO) atmospheres. Angle Resolved X-Ray Photoelectron Spectroscopy (ARXPS) analyses were carried out to determine the chemical composition of the films as well as the spatial distribution of the chemical bonds. The nuclear depth profiling method uses narrow, isolated resonances found in the cross section of the nuclear reactions 18 O(p,α) 15 N at 152 keV and 15 N(p,αγ) 12 C at 429 keV. The excitation curves of the nuclear reactions in the neighbourhood of the resonance energies can be used to determine the concentration versus depth profiles by means of SPACES, a personal computer implementation of the stochastic theory of energy loss. We discuss aspects of the simulation of excitation curves and the effect of NO pressure on the structure of the oxynitride film.

Silicon surfaces with different reconstruction, Si(111)-7 × 7 and Si(001)-2 × 1, are exposed to a molecular beam of copper phthalocyanine. On Si(111) the adsorbate-substrate interaction is strong, as suggested by a partial reduction of copper shown in the XPS spectra and by the evidence provided by STM of a chemical reaction involving some of the Si adatoms. As a consequence, the molecules are not imaged at atomic scale by STM. At variance with this behaviour, on the single-domain Si(001)-2 × 1 face, the adsorbed molecules are clearly imaged by STM, suggesting an adsorption state more weakly bonded. This is the first observation of Cu phthalocyanine molecules imaged on a silicon surface, and it further illustrates the potential of STM for surface chemistry.

The role of surface structure in controlling NH3 surface chemistry has been investigated by N 1s and Si 2p core level photoemission on Si(001)-2 × 1 and Si(111)-7 × 7, taking advantage of the surface sensitivity provided by synchrotron radiation tunability. Sequential treatments, i.e. 90 K adsorption/300 K annealing/1193 K nitridation, have been carried out. A common feature of low-temperature NH3 adsorption on Si(001) and Si(111) is the growth of a solid NH3 layer over a decomposed ammonia interlayer where, among other species, atomic nitrogen is present. Increasing the temperature causes this solid NH3 layer to desorb. A 300 K annealing of the Si(001) surface eliminates also the adsorbed atomic nitrogen species to reach a situation in which only H atoms and NH2 fragments decorate the silicon dimer broken bonds. In contrast to the Si(001) case, a 300 K annealing of the rougher Si(111) surface does not lead to a unique adsorption site/NH3 species: in particular atomic N remains. The formation of higher subnitride states (already at 90 K) is also evidenced on Si(111) with respect to Si(001). The situation of greater complexity (due to the 7 × 7 reconstruction) of the Si(111) surface with respect to Si(001), when one considers low-and room-temperature adsorption processes, is strikingly reversed when one deals with thermal nitridation. The Si(111) subnitride distribution is compatible with an ideal abrupt interface, when the Si 3 N 4 Si(001) interface appears as rougher. Using the clean and 300 K annealed surfaces as templates, the amounts (per unit area) of subnitrides at the Si 3 N 4 Si interface are estimated.

X-ray photoelectron spectroscopy (XPS) is used to characterize copper sulphides inserted into Langmuir-Blodgett (LB) films of behenic acid. Comparisons are made between samples exhibiting very different electrical conductivity but synthesized by the same method. Sulphur atoms with two different environments are found: divalent sulphur S 2- and dimer sulphur (S 2 ) 2- , higher concentrations of dimer sulphur (S 2 ) 2- being present in insulating samples. Chemical bonding of copper in LB films is compared with CuS and Cu 2 S powders. The copper to sulphur ratio and the divalent to dimer sulphur ratio are obtained from various samples and lead to the formula: Cu 2 S, x S 2 with x ranging from 0.5 to 0.8.

In this work, we examine the possibility of using core-level spectroscopies, X-ray photoabsorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) to obtain a (semi)quantitative measurement of the distribution of copper oxidation states in copper oxide layers of overall stoichiometry CuOy, otherwise precisely determined by ion beam analysis (IBA), a technique independent of the chemistry. It turns out that the agreement between the analysis of XAS spectra and IBA measurements is excellent as long as y>0.5, that is for mixtures of CuO and Cu2O. The limits of the applicability of XPS are also discussed. In the field of material sciences, the application of XAS (and also of XPS, to some extent) to determine the Cu(II)/Cu(I) ratios is thus promising in those cases which do not allow precise stoichiometry measurements by ion beam techniques.

Rapid thermal nitridation (RTN) of silicon dioxide films and silicon covered with its native oxide was performed at atmospheric pressure in a lamp heated furnace in NH3 at a temperature of 1000 ° C. Physicochemical characterizations of grown films were carried out by nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS). For the films formed by the nitridation of silicon with its native oxide, the XPS measurements, using normal and grazing emergence angles, indicate that a stoichiometric silicon nitride appears between two nitroxide (SiOxNy) films at the silicon/dielectric interface and at the surface of the dielectric. The depth profile of nitrogen in films formed by the rapid thermal nitridation of a 36 A silicon dioxide was determined using chemical etching associated with NRA and estimated through the XPS technique. A homogeneous nitroxide was evidenced for the most part of the dielectric film, and a monolayer (∼ 0.7 × 1015 atoms cm−2) of stoichiometric silicon nitride was observed at the interface between the silicon substrate and the nitroxide film.

The aim of this work is to determine if, after oxidation in pure O2, a vicinal surface Si(001) miscut 5° towards the [1 1 0] direction, exhibits a preferential SiO2 formation on the [110] ledges. In a first stage, combining XPS (X-ray photoelectron spectroscopy) and LEED (low-energy electron diffraction) available in an auxiliary chamber, we examine the cleaning conditions leading to a non-faceted stepped Si surface with dominant (2 × 1) single domains. This surface preparation is reproduced in the synchroton facility chamber where ARPR-single-resolved photoemission spectroscopy) is performed: after cleaning the Si surface and studying the starting situation (san- states, initial contamination), an ultra-thin oxide film (∼ 1.5 × 1015 oxidised Si atoms/cm2) is grown at 785–800°C, under Torr of oxygen. The absence of anisotropy in the ARPES emission of the Si 2p core levels taken at the minimum electron escape depth (hv=130eV), points to no extra-growth of the oxide on the ledges. The in-depth distribution of the oxide states seems to be homogeneous, within the sensitivity of the probe. On the other hand, after partial thermal desorption of the ultra-thin oxide film in vacuum, a strong anisotropy in the emission of electrons from SiO2 is seen in areas where oxide patches coexist with bare silicon. It is shown that this ensues from a significant step bunching and (111) faceting of the surface. A correlation is made with SEM (scanning electron microscopy) images of the surface. The problem of step bunching, inasmuch as it can alter the interpretation of shadowing effects, is emphasized.

X-ray photoelectron spectroscopy (XPS) is applied to the physico-chemistry study of inserted HgS into multilayers of behenic acid. Comparison is made with precursors used during the in-situ synthesis: behenic acid Langmuir-Blodgett films, mercury behenate Langmuir-Blodgett films and the two allotropic forms α-HgS and δ-HgS. The results obtained show that the mercury sulfide is stoichiometric, close to the δ-HgS form and a correlation has been established between sample conductivity and XPS spectra.

We have studied the adsorption geometries of benzonitrile (C{sub 6}H{sub 5}-C{identical_to}N) deposited on Si(001)-2x1 at 300 K, using synchrotron radiation x-ray photoemission spectroscopy and x-ray absorption spectroscopy. The experimental data are interpreted through density functional theory calculations of ground and excited states. It is shown that the molecule chemisorbs under multiple forms. While the literature indicated that adsorption is highly selective at 110 K (bonding via the cyano), at 300 K, cyano bonding is observed concomitantly with other geometries for which the molecule interacts with the surface by the phenyl ring. Among the phenyl-bonded species, the cyano group can remain unreacted or can make a dativelike bond with the substrate via its nitrogen end (the latter species is a minority species at surface saturation). Real-time kinetic studies show that all species are present on the surface with the same distribution from the very beginning of the adsorption process.

This paper presents the results of X-ray photoelectron spectroscopy and secondary ion mass spectrometry experiments performed to obtain information about the sensing mechanism involved in the detection of phosphine by conducting Langmuir-Blodgett (LB) films based on TCNQ and EDT-TTF derivatives. In contrast to the EDT-TTF based LB films, it is shown that with the TCNQ derivative the action of phosphine (PH3) results in the insertion of phosphine subproducts in the LB film. This information is correlated with the general behaviour of each film towards phosphine detection.

The x-ray photoelectron spectra (XPS) of copper tetraazaporphyrin (CuTAP), used as a model of copper phthalocyanine(CuPc) are studied by theoretical simulations. The nature of the main and satellit ...

Si 2pcore-level spectroscopy is a unique tool to determine the chemical composition and spatial extension of the suboxide layer present at the Si/SiO2interface. In the case of ultra-thin oxide films (thickness n+(withn=1,…,4). After a brief review of two decades of XPS studies on the Si/SiO2interface, we give an account of the recent debate concerning the possible contribution of the second oxygen neighbor shell to the chemical shift, which, if effective, would modify the picture of the interface. Then, we examine the benefit derived from the use of very high energy resolution (70 meV athv=130 eV), and we try to determine, for this system, what are the limits of this spectroscopy. To illustrate the latter point, among various case studies (thermal oxides, room temperature adsorption etc.), we treat in more detail the case of the H-terminated Si(1 11) surface oxidized by atomic oxygen, and discuss our data in the light of previous XPS and vibrational spectroscopy studies.

Rapid thermal oxynitridation in nitric oxide (NO) of a thick (14 nm) Si02 film grown on Si(001) is studied as a first stage towards understanding of atomic transport mechanisms occuring during NO annealing of thin SiO2 films. The SiO2 films were grown in an ultra high vacuum rapid thermal processing (RTP) furnace in static pressure of natural O2 (16O2). These films were then annealed in N and 180-enriched NO (15N18O) for 20 and 80 s. Total amounts of nitrogen and oxygen (areal densities in at.cm-2) and heavy isotopes depth distribution were measured using non resonant and resonant nuclear reactions analysis. The results are discussed in terms of atomic depth profiles and growth mechanisms. These first results are more likely explained by two mechanisms occuring in parallel. In the first one, NO diffuses through the silica network without reacting with it and both N and O are fixed in the near interface region. In the second one, 18O is fixed near the oxide surface due to a mechanism related with a step-by-step motion of network oxygen atoms, by a simple diffusion process, induced by the presence of network defects, involving O only. This latter mechanism leads mostly to an exchange of oxygen atoms between the oxide network and the gas phase.

The Cu 2p and N is X-ray photoelectron spectra of nitrogen square planar copper(II) complex, derived from copper phthalocyanine and especially substituted to produce Langmuir-Blodgett (LB) films, are reported and compared with those of commercial copper phthalocyanine (CuPc) and selected porphyrin compounds. Although the copper atom is found primarily in the Cu(II) state, the authors observe the presence of the reduced Cu(I) form, with a great variety of relative intensities, because of a concomitant ability to reduction. In contrast, in the LB films, the copper atom remains in the Cu(II) state. The authors discuss this different behavior in terms of the reduction degree of the molecule, the localization of the additional electrons on the metal or the macrocycle, and a different nitrogen geometry around copper. 25 refs., 4 figs., 1 tab.

Reflection high-energy electron diffraction (RHEED) and high-resolution transmission electron microscopy (HRTEM) show that, when vicinal Si(001) surfaces, cut with a 5° tilt about [110], are thermally oxidized in very dry O2, an epitaxial crystalline phase of oxide grows along [110] ledges. X-ray photoemission confirms that the oxide is stoichiometric SiO2, This crystalline SiO2 is not a standard phase of silica. We show that it can have a cubic Bravais cell (ao=8·8 A). We propose simple models derived from β-cristobalite, coherent with the projected Si positions given by HRTEM. Structure factors are calculated and compared with the RHEED diagrams of oxide grown on both Si(001) vicinal surfaces and ‘rough’ Si surfaces of various orientations. Finally we discuss the process involved in the growth of very thin oxide films.

The thermal oxidation of silicon is generally modelled by Deal and Grove's theory based on the assumption that the oxygen molecules dissolve in silicon in interstitial positions and migrate to the Si-SiO2 interface where they react with the silicon substrate. Experimental results for oxidation in dry oxygen agree with this theory only for thick oxide films. The growth of very thin oxide films exhibits particular features which are discussed in this paper. For these films, the growth mechanism is different from that of thick films; this difference is possibly associated with the transport of oxygen atoms through the silica network. The effect of hydrogenated impurities is also discussed.

Thermal silicon nitridation mechanisms in ammonia gas at 950°C and under 2 × 10−5 Torr were investigated by using 15N as a tracer. Nitride layers first grown in 14NH3 were further grown in 15NH3. 15N profiling was obtained by combining nuclear microanalysis with step-by-step chemical dissolution of the nitride layer. Thus one can show that the nitride film consists of a 14N layer located near the Si/nitride interface and a 15N layer located near the external surface of the film. It was also shown that there is no isotope exchange between the nitrogen of the nitride network and of the NH3 gas. These results indicate that the growth does not occur by interstitial transport through the nitride film of the NH3 molecules nor by any other nitrogenated species resulting from a decomposition of the ammonia gas. A possible incorporation of hydrogenated species was also examined using ND3 nitridation. As the relative concentration of deuterium in such a film was less than 10−4, ammonia most probably dissociates at the outer surface and hydrogenated species do not react appreciably with the network. From these experiments, it can be concluded that the growth of the silicon nitride layer occurs by either: - transport of silicon atoms through the layer which then react with the nitridant gas; - or a step-by-step migration of a nitrogen network defect (vacancy or interstitialcy) in only one direction. In such a case, the defect should be electrically charged and the hoping direction should be imposed by a strong electrical field arising from the charging of nitrogenated species adsorbed at the external nitride surface by electrons coming from the silicon substrate because of the very small thickness of the nitride film (< 40 A) (tunnel or thermionic effects) .

We have studied the structural characteristics of very thin silicon oxide films thermally grown (at 930 °C, 10 Torr) in 18O enriched dry oxygen for times ranging from 0.5 to 22.5 h (corresponding to equivalent thicknesses ranging from 3.2 to 20.8 nm). Chemical etching combined with nuclear microanalysis, x‐ray photoelectron spectroscopy (XPS), and reflection high‐energy electron diffraction (RHEED) has been applied to the samples. A layer that is very slow to dissolve (of equivalent thickness ∼1.1 nm) was observed for oxide films grown for oxidation times of less than 1 h (equivalent thicknesses under 4.6 nm). As indicated by XPS this layer seems to be related to a structure different from what is observed for thicker films; its existence is also correlated to a structural order as seen by RHEED.

Catalytic behaviour of nickel particles smaller than 1.3 nm, encaged in X and Y zeolites, in the presence of Ce3 +, is related to their electronic properties. In general the rates of reaction are lower than those of a standard catalyst made of 2.5 nm nickel particles on SiO2; this is attributed to the strong localization of the Ni 3d states due to the size effect. However a high rate, more than twice the standard, is found for Ni in X zeolite, in relation with a Ni 3d 10 configuration. On the other hand the fragmentation of butane is deeper with Ni in Y support than in X support and this corresponds to a high density of Ni d states at the Fermi level.

The thermal growth of 3CSiC films at 820°C in acetylene (chosen among other hydrocarbons because of its high reactivity with Si) was performed and the resulting films were analyzed by nuclear reaction analysis (NRA), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Films were grown on clean single domain 2 × 1 reconstructed Si (0 0 1) vicinal surfaces either in natural (C2H2) or in 99% deuterium enriched (C2D2) acetylene under 2 × 10−6Torr. In order to characterize the electronic structure and the short range order of the films, XPS and its by-product X-ray photodiffraction (XPD) were performed in situ in an analysis chamber connected to the preparation chamber. The growth kinetics was followed by measuring, ex situ, the amount of incorporated carbon using the 12C(d,p)13C reaction at 970 keV, while for determining the hydrogen incorporation in the films the D(3He,p)4He reaction at 700 keV was used. Also ex situ, the film morphology was followed by SEM as the thickness of the layer increased. By using these complementary techniques, phenomena like H incorporation, C in-diffusion and 3CSiC nucleation have been evidenced at the very beginning of the growth (amount of 12C incorporated in the films smaller than 1016 atoms/cm2). Besides, it was observed that the imperfect coalescence of 3CSiC nuclei determine the morphology of films grown during longer times.

The electronic structure and bonding geometry of acetylene adsorbed at room temperature on Si(111)-7×7 is studied by a combination of synchrotron radiation x-ray photoemission spectroscopy and of near-edge x-ray absorption fine structure spectroscopy. Then the stability of the molecule, submitted to thermal annealings and to synchrotron white beam irradiation is examined. The possibility of using acetylene gas as a carbon source for the fabrication of silicon-carbon compounds (or for the formation of abrupt carbon/silicon interfaces) is discussed.

The thermal oxidation of silicon is generally modelled by Deal and Grove's theory based on the assumption that the oxygen molecules dissolve in silicon in interstitial positions and migrate to the Si-SiO2 interface where they react with the silicon substrate. Experimental results for oxidation in dry oxygen agree with this theory only for thick oxide films. The growth of very thin oxide films exhibits particular features which are discussed in this paper. For these films, the growth mechanism is different from that of thick films; this difference is possibly associated with the transport of oxygen atoms through the silica network. The effect of hydrogenated impurities is also discussed.