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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.

We present an angle resolved ultraviolet photoemission spectroscopy study of the adsorption of 2-butyne (CH3-C C-CH3) on Si(001)-2 x 1 at room temperature. We recorded valence band photoemission spectra for two azimuthal positions of a vicinal silicon surface, where all the rows formed by the surface silicon dimers are parallel. The photoemission symmetry selection rules allow the determination of the orientation of the molecular orbitals. The photoemission signal of the HOMO is enhanced when the electric field is parallel to the dimer rows. This showed that the pi orbital left intact after the cyclo-addition reaction of the molecule with one silicon dimer is parallel to the dimer rows. This indicates that each 2-butyne molecule adsorbs on one silicon dimer. In spite of the size of the system and the vicinity of the orbitals, the angle resolved study points out that no dispersion of the electronic bands occurs. Not all the surface dimers are reacted so some disorder still exists on the surface preventing the formation of Bloch states. (c) 2007 Elsevier B.V. All rights reserved.

Acetonitrile (CH3CN) adsorbs on Si(0 0 1)-2 × 1 at room temperature under two forms, a cycloaddition-like adduct (Si–C N–Si) and a pendent cyano (Si–CH2–C N) resulting from the decomposition of the molecule. Resonant Auger spectroscopy has been used to study the excited-state-dependent electron transfer from the N 1s core-excited molecular adsorbate to the silicon substrate, using the core-hole lifetime (∼6 fs) as an internal clock. It is shown that the πC N∗ NEXAFS state lies within the silicon bandgap because of a core-excitonic effect. Therefore no charge transfer of the excited electron to the substrate is observed. On the other hand the πC N∗ NEXAFS state is placed within the silicon conduction band. Excitation to this orbital leads to valence/Auger spectra in which both resonant and normal Auger contributions are observed. Therefore there is evidence for a charge transfer from the pendent C N to the silicon surface, on a timescale estimated to tens of femtoseconds.

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 room temperature oxidation of H-terminated Si(111) surfaces by activated oxygen has been investigated using high resolution (70 meV) Si 2p core-level spectroscopy with synchrotron radiation. The data are compared with previous studies using photoemission spectroscopy and high resolution electron energy loss spectroscopy. The metastability of the oxidized layer formed at room temperature is also examined.

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.

The structure of thin strained-layer (Ge n Si n ) p heterostructures on Si(001) is investigated by a multiple scattering approach to X-ray absorption spectroscopy (XAS) at Ge L 3 edge. Our results confirm the absence of sharp interfaces for those Ge–Si multilayers grown without Sb and, in the case of very thin Ge 2 Si 2 specimens, indicate the formation of a single phase, chemically ordered, Si–Ge alloy where the widely spaced (111) planes are occupied alternatively by the same atom type.

In comparison with oxidized bulk crystals, we show that the morphology of oxidized porous silicon can offer a unique opportunity of measuring suboxide distribution by conventional XPS and to characterize thoroughly interfacial defects by EPR.

We have studied by soft X-ray photoemission (hv = 145 eV) the modifications of the Si 2p core-level and of the valence band during the adsorption of water on Si(001)-2 × 1 and Si(111)-7 × 7 surfaces, at two substrate temperatures (300 and 90 K). In all cases, water reacts with silicon. We have followed the decay of Si 2p and valence-band surface states, as well as the growth of oxidation states, with increasing exposures to H2O. The breaking of H2O into -H and -OH fragments and their attachment to the triply coordinated surface atoms (silicon dimers on Si(001), adatoms and rest-atoms on Si(111)) is not the only reactional mechanism. Oxygen atoms are also inserted into SiSi bonds, so that oxidation offers an alternative channel to H2O dissociation and chemisorption. For both orientations, surface oxidation is facilitated at low temperature. The relative reactivity of the dangling-bonds of Si(111)-7 × 7 has been investigated: both valence band and Si 2p spectra indicate clearly that the rest-atoms are less reactive than the adatoms.

The strong resistance of the As-terminated Si(001) surface to oxidation in molecular oxygen at room temperature is the subject of this study. As 3d and Si 2p photoemission spectra, as well as valence band spectra, excited with synchrotron radiation (135 eV ≤ hv ≤ 165 eV), have been recorded as a function of increasing O2 exposures, over the range 2400–1.4 × 1013 L (langmuir). Spectral changes, simultaneously observed in the As 3d and Si 2p core-levels near ∼ 1012 L are indicative of the onset of oxidation. However a modification of the valence band, due to the contribution of O 2p states, is clearly evident from ∼ 1011 L, pointing to the presence of weakly bound oxygen species at that exposure. These results are confronted with a previous Auger study reporting an oxygen saturation coverage (0.5 monolayer) already in the 400–2000 L range, interpreted as the insertion of an oxygen atom between two dimerized arsenic atoms. On the contrary our data show that surface oxidation is a slow process, spatially inhomogeneous, as some dimerized As remain intact (∼ 24%) up to exposures as large as ∼ 1013 L. Oxidation is not limited to the outermost layer, as three arsenic oxidation states and the four silicon oxidation states (indicative of subsurface oxidation) are detected simultaneously. Moreover metallic arsenic shows up: this suggests a reduction of the arsenic oxide by silicon. Arsenic losses are also observed, probably via sublimation of As4O6 molecules.

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 chemical bonding and molecular orientation of copper phthalocyanine (CuN 8 C 32 H 16 ), deposited on clean single crystal silicon substrates of (111) and (001) orientation, were studied using X-ray photoemission spectroscopy (XPS) of the Cu 2p, C 1s, and N 1s core-levels and synchrotron radiation X-ray absorption spectroscopy (XAS) at the Cu L 2,3 edges. In the monolayer range, the strong interaction of the molecule with the Si (111) surface is mostly evidenced by changes in the Cu 2p XPS spectra that indicate a partial reduction (up to 30%) of the Cu(II) atoms to Cu 3d 10 . The linear dichroism of the XAS spectra shows that those molecules, whose copper is still Cu(II), lie down on the surface. On the other hand, the bonding of the molecule on Si(001) is weaker, as Cu 2p XPS spectra do not exhibit Cu(II) reduction, while XAS spectroscopy indicates a random orientation of the molecules with respect to the surface. N 1s XPS spectra exhibit, on both surfaces, very intense satellites which present, on Si(111), a sharp dependence on the electron take-off angle. With the growth of thicker films, the XPS line shapes of the molecular solid are recovered, and the average tilt angle of the molecules (around 70°) does not depend on the chosen Si surface. We conclude that the structure of the first deposited layer strongly depends on the specific substrate, but does not affect the successive growth of thick layers.

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.