F. Rochet

Copper phthalocyanine on Si(111)-7×7 and Si(001)-2×1 surfaces : an X-ray photoemission spectroscopy and synchrotron X-ray absorption spectroscopy study

Abstract 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.

Contrasted behavior of Si(001) and Si(111) surfaces with respect to NH3 adsorption and thermal nitridation: a N 1s and Si 2p core level study with synchrotron radiation

Abstract 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.

Copper phthalocyanine on Si(111)-7 × 7 and Si(001)-2 × 1: an XPS/AES and STM study

Abstract 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.

Suboxides at the Si/SiO2 interface: a Si2p core level study with synchrotron radiation

Abstract Synchrotron radiation X-ray photoemission spectroscopy (SRXPS), used with an optimized surface and interface sensitivity, is a unique tool to determine the chemical composition and spatial extension of the suboxide layer present at the Si/SiO2 interface. The bonding at ‘thermal interfaces’ appear to be essentially dependent on the Si crystal orientation. For Si(001)/SiO2 a detailed photon energy-dependent and angle-dependent. SRXPS study (in the 120 ÷ 175 eV photon energy range) has been performed. Previous results on higher-oxidation states cross-section resonances around hv = 130 eV are re-examined. The various oxide states do not present photoelectron diffraction peaks, when polar scans are performed in the (110) azimuthal plane. On the other hand, there is evidence that substrate silicon atoms close to the interface retain orientational order. Limits in the vertical distribution of the suboxides are given. The compatibility of our experimental findings with models of the current literature — in particular the so-called ‘dimerized interface’ model — is examined.

Adsorption of water on Si(001)-2 × 1 and Si(111)-7 × 7 surfaces at 90 and 300 K: A Si 2p core-level and valence band study with synchrotron radiation

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.

Metal phthalocyanines (MPc, MNi, Cu) on Cu(001) and Si(001) surfaces studied by XPS, XAS and STM

Abstract The growth mode of metal phthalocyanines (MPc, MNi, Cu) on clean Cu(001) and Si(001) (i.e. bonding and adsorption geometry) and their electronic structure are studied by a combination of spectroscopic techniques: x-ray photoemission, x-ray absorption at Metal L 2,3 edges and scanning tunnelling microscopy.

Acetylene gas as a carbon source: An x-ray photoemission spectroscopy and near-edge x-ray absorption fine structure spectroscopy study of its stability on Si(111)-7×7

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.

Theory assisted interpretation of copper phthalocyanine core levels XPS spectra

Abstract X-ray photoemission spectroscopy (XPS) and ab initio quantum mechanical calculations (SCF, CI, MCSCF) are used in combination to study the electronic structure of copper phthalocyanine (CuPc). The XPS core level spectra as well as the molecular orbital calculations suggest that in this molecule, the CuN bonds are ionic to a large extent. In the Cu2p spectrum, the final state corresponding to the main line, at the lower binding energy side, appears as a large mixing of two configurations; in one of them the core hole is fully screened by a charge transfer from the ligand 2p orbitals to the Cu3d x 2 − y 2 orbital. For the Cu3s and Cu3p core holes the screening by the ligand to metal (L → M) charge transfer is less important. For the Cu2p and Cu3p core levels, the higher binding energy states (satellites) correspond to shake-up processes exhibiting L → M charge transfer as well as M → M transitions, whereas in the case of Cu3s these are purely L → M.

The As-terminated Si(001) surface and its oxidation in molecular oxygen: an Si 2p and As 3d core-level study with synchrotron radiation

Abstract 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 O 2 exposures, over the range 2400–1.4 × 10 13 L (langmuir). Spectral changes, simultaneously observed in the As 3d and Si 2p core-levels near ∼ 10 12 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 ∼ 10 11 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 ∼ 10 13 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 As 4 O 6 molecules.

Heteroepitaxial growth of InAs on GaAs(100) mediated by Te at the interface

Abstract The heteroepitaxy of InAs on a Te covered GaAs surface is investigated by photoelectron spectroscopy. Core-level spectra probed with synchrotron radiation show that Te remains at the interface between InAs and GaAs in a concentration much higher than the solubility limit of this element in III–V compounds, suggesting that a new compound is being formed. We propose that this interlayer is responsible for the observed changing in the growth mode of the InAs overlayer from islands to layer by layer.