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The atomic and electronic characteristics of homoepitaxial C(100) thin films and their reactions with very thin erbium deposits have been studied by low-energy electron diffraction (LEED) and photoelectron spectroscopy (X-ray photoelectron spectroscopy [XPS] and ultraviolet photoelectron spectroscopy [UPS]). These films, of 3 μm thick, are grown by microwave chemical vapor deposition (CVD) and p-doped (1017 B/cm3). Measurements are made on two types of surfaces: plasma-hydrogenated and chemically oxidized. The hydrogenated surfaces exhibit 2(2×1) LEED pattern and negative electron affinity (NEA). Under annealing at high temperature (500–650°C) in oxygen (1 to 5×10−5 mbar), the hydrogenated surface is transformed slowly into an oxidized one which has the same atomic and electronic structures as the chemically oxidized surface, namely a (1×1) LEED diagram and a positive electron affinity (PEA). Under annealing at high temperature, erbium deposits react with the hydrogenated surface and not with the oxidized one. The reaction is not complete and produces a very thin interface erbium carbide layer. Internal photoemission measurements performed on erbium carbide/diamond contacts, protected against oxidation by a layer of erbium silicide, show potential barrier heights close to 1.9 eV.

The Fermi surface of ErSi1.7(0001) thin films grown epitaxially on Si(111) has been measured by angular photoemission intensity distribution maps at different photon energies using synchrotron radiation. Valence-band spectra at different collection angles along the main symmetry directions of the surface Brillouin zone (SBZ) have been measured, and they have allowed us to find that the Fermi surface is characterized by hole and electron pockets at particular high-symmetry points of the SBZ. The shape of the experimental Fermi surface for different photon energies is similar, indicating a bidimensional electronic character, in contrary to the expectation based upon band-structure calculations and the crystallographic structure. Moreover, Si 2p core-level photoemission spectra taken at surface-sensitive conditions reveal only one chemical state for the Si atoms at the surface layer.

Silicon oxide (SiOx) and erbium oxide (ErOx) layers in the form of SiOx/ErOx/SiOx/Si structures were sequentially deposited onto silicon substrates by reactive RF-sputtering without breaking the vacuum. The structures were subsequently heat treated at 800°C under an argon pressure of 10 −3 mbar. XPS measurements revealed that the layers thus obtained are homogeneous. The relative intensities of the Si 2p, Er 4d and O 1s core level peaks suggest a Er:Si:O composition ratio equal to 2:2:7. Furthermore, the chemical shifts observed for the Si 2p and Er 4d peaks showed the formation of a compound in which silicon (Si) and erbium (Er) are, respectively, in tetrahedral and octahedral oxygen environments. XRD measurements showed the formation of erbium pyrosilicate (Er 2 Si 2 O 7 ) which is consistent with the XPS results.

The optical and transport properties of rhenium silicide thin films epitaxially grown on Si(111) have been studied. On the one hand, the analysis of the optical spectrum above the band gap energy confirms the semiconducting character of the samples. Moreover, the absorption coefficient prefactor measured for films of various thicknesses in the range of 50–475 A depend on the thickness itself, possibly reflecting a two-dimensional quantization of states related to the existence of a quantum well due to the band offsets at the heterojunction. On the other hand, electrical measurements on high resistivity silicon substrates are explained by the existence of an impurity band. The concentration of electrically active impurities decreases by one order of magnitude when the sample is annealed at 850°C under a hydrogen partial pressure.

The structure and electronic properties of the interfaces formed by Re evaporation on clean Si(111)-7 × 7 and subsequent annealing were investigated by low energy electron diffraction (LEED), Photoelectron spectroscopy (XPS/XAES, UPS) and work function (WF) measurements. Rhenium was deposited from submonolayer up to 50 ML coverages and gradually annealed up to 1100 K. Deposition causes rapid disappearance of the 7 × 7 reconstruction and the substrate UPS features, a gradual increase in the WF from 4.5 to 5.1 eV and negligible chemical shift in the XPS/XAES peaks. The interface remained abrupt with no intermixing, as metallic Re grew in a simultaneous multilayers fashion above 0.8 ML coverage. Annealing of 50 ML of Re causes a Si enrichment of the overlayer above 600 K and the formation at around 950 K of a continuous layer of silicide which remains stable up to 1100 K, whereas annealing of Re layers close to 1 ML creates ReSi 2 islands on the substrate surface.

The atomic structure of tantalum (Ta)-amorphous silicon (a-Si) interfaces has been investigated using a-Si/Ta/a-Si multilayers, sputtered on unheated crystalline silicon (c-Si) substrates without breaking the vacuum. The back and the top Si layers were respectively 1000 and 500 A thick, while the Ta thickness was 250 A. The concentration profiles and the crystallographic structure were obtained using three complementary techniques: X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction and specular X-ray reflectometry. In particular, we demonstrate that Ta/a-Si and a-Si/Ta interfaces are diffuse with an interpenetration-depth of the order of 20 A. The Ta layer is not well crystallised and its density is 12% lower than in the bulk material.

Oxygenated silicon-carbon alloy (Si x C 1 − x :O) films have been prepared by reactive r.f. sputtering. These films were deposited, in a gas mixture of argon and oxygen, from a composite target consisting of a silicon disc on which graphite chips had been placed. The structural and optical properties have been investigated using infrared (IR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and visible-near-infrared optical absorption. XPS as well as IR measurements show that the films are essentially composed of silicon, carbon and oxygen as expected. The atomic fractions of these elements, deduced from quantitative XPS analysis, are of the order of 50%, 28% and 22% for Si, C and O atoms respectively. A study of the chemical shifts of C 1 s and Si 2p core level spectra reveals that the films contain not only Si atoms bound to C and O but also free silicon and free carbon clusters. The Raman spectroscopy results confirm the presence of free silicon and indicate that the films are amorphous. The optical gap of these films is around 1.3 eV and the refractive index is found to follow the Sellmeier law with a value of about 2.05 at λ = 2 μm.

ErSi2−x silicides, bulk and epitaxially grown on Si(111), are investigated by X-ray absorption spectroscopy at Er L edges in order to determine the effects of the relaxation due to the silicon vacancies on the local environment of erbium in those materials. The value of the shortest Er-Si bond length derived from the L1 edge data is found to be 2.95 ± 0.03 A, in agreement with previous neutron diffraction data giving an orthorhombic-hexagonal deformation of the AlB2 structure where the Si vacancies are ordered. The same bond length is deduced from L3 edge data recorded from epitaxial film. This result is consistent with a simple model for the relaxation due to the vacancies in these epitaxial films, where the hexagonal symmetry is preserved. The effect of the strain due to the mismatch to the Si substrate is also evidenced from EXAFS. Preliminary results on a two-dimensional erbium silicide layer are given, showing the strong anisotropy of Er environment for one monolayer Er coverage.

The formation of erbium silicide ErSi1.7 interface and thin films on plane and stepped Si(100) surfaces have been investigated by low-energy electron diffraction (LEED), photoelectron spectroscopy (XPS and UPS) and Auger electron spectroscopy (AES). For the interfaces, approximately 0.3–2 ML of Er was evaporated on the substrate and subsequently annealed at 400–600°C. For the thin films the co-evaporation method was used. On stepped Si(100) the surface state disappears less rapidly than on a plane one, suggesting a preferential clustering of the silicide along the steps. Upon annealing of the deposits (~ 50 to 100 A) a diffuse (2 × 2) LEED pattern is observed at 600–800°C. This pattern corresponds to a (2 × 1)ErSi 1.7 (10 1 2) . The valence bands show two main peaks at 1.3 and 2.8 eV and no dispersion in the surface plane, confirming the poor crystallinity of the film. The results are discussed with those obtained for ErSi 1.7 Si (111) .

The electronic structure of erbium silicide ultra-thin films epitaxially grown on Si(111) has been studied by means of angle-resolved ultra-violet photoemission spectroscopy (ARUPS) in the coverage range 0.2–4 monolayers (ML). Some peaks probably related to the silicide surface atomic structure are observed at any coverage. Features that appear at normal emission in connection with an R3 superstructure are ascribed to vacancy-induced states in the silicon surface plane. No true interface states could be identified in this study.

The reaction of Si atoms with the ErSi1.7(0001) surface and the reepitaxy of Si on the ErSi1.7(0001)/Si(111) structure have been studied by LEED, XPS and angle-resolved UPS. For the interfaces, variable amounts of Si atoms, from ∼ 0.3 monolayer (ML) to ∼3 ML have been deposited on samples maintained at room temperature and the measurements have been carried out before and after annealing at 400–500°C. LEED observations show a fairly good stability of the surface structure with respect to deposited Si atoms. The characteristic √3 × √3 R30° pattern of ErSi1.7(0001) remains visible until a surface coverage θ ≈ 1.5 ML and a (1×1) diagram is superimposed when deposits ≥ 0.75 ML are annealed. The surface states of the silicide are attenuated upon Si deposition but emerge again on the UPS spectra after annealing. XPS measurements indicate that small quantities of Si atoms are diffused into the silicide. Epitaxial growth of Si on the ErSi1.7(111) structure is obtained when ∼ 20 to 30 A of Si are annealed at 600°C on the latter. The epitaxial layer is characterized by its (7×7) LEED pattern and its surface states in the UPS valence band. A second ErSi1.7(0001) thin layer has been also grown on the epitaxial Si, demonstrating the possibility of fabricating multilayers.

This work deals with the formation of a sputtered Er/amorphous Si (a-Si) interface and its behaviour when the a-Si/Er/a-Si/c-Si structures are annealed at high temperature. Er and a-Si layers were sequentially sputtered without breaking the vacuum and the interfaces were investigated by depth-profile XPS, grazing incidence X-ray diffraction (GIXD) and scanning electron microscopy (SEM). The evolution of the Er4d, Si2p and Si KLL spectra with abrasion time indicates a diffuse and reactive interface. GIXD reveals that the interfacial compound is Er5Si3. The formation of this compound is enhanced upon annealing at 400°C. Minute amount of ErSi2 is also detected at this temperature. At 600°C all the Er layer is consumed and GIXD detects aside Er5Si3 weak reflections of hexagonal ErSi2. At 800°C the only formed phase is ErSi2. SEM observations indicate that for small thicknesses of Er and a-Si the silicide layers are continuous, otherwise the films are rather granular. The results are compared to the ones obtained with Er films evaporated on Si.

We have investigated the growth and the electronic structure of ultra-thin (0.2 to 6 ML) Er films deposited at room temperature on a Si(111)7 × 7 surface and subsequently annealed to 600°C using LEED, ARUPS, AES and XPS. Our aim was to study the early stages of the (epitaxial) erbium silicide/Si(111) interface formation. We have found two different regimes: 1. - In the submonolayer range, our data indicate the growth of two-dimensional islands having a (1 X 1) symmetry. The band structure of this phase has been mapped. - Above 1 ML, the film surface shows an asymmetric 3 × 3 R30° superstructure. The ARUPS spectra rapidly evolve towards those of thick silicide layers, although some differences remain clearly observable close to the Γ point. These results - complemented with AES and XPS data - will be discussed in relation with our present knowledge of the erbium silicide structure.

Here, we investigate the influence of the barrier height morphology upon the electrical properties of ErSi 1.7 /Si-p junctions. Solid-phase epitaxy of the silicide is achieved in ultra-high vacuum by annealing a layer whose composition is stoichiometric within the accuracy of the X-ray photoemission spectrometer. In principle, this procedure should avoid any silicon consumption from the substrate. Two annealing temperatures are used: 800 and 600°C. Current-voltage and capacitance-voltage methods give results which are analysed by comparison with two models. One of them takes into account the barrier height inhomogeneities and the other the various kinds of interface states, some of them spreading just far enough from the interface to be in equilibrium with the semiconductor. Comparison of these data with admittance measurements under forward bias permits one to distinguish between these two effects. Anomalies in internal photoemission yield are also evidenced. All these electrical characterizations show that, although these silicides display an epitaxial relationship with the substrate, damages result from the preparation procedure. Thus, these ex-situ methods are useful complements of the structural and spectroscopic in-situ surface characterizations.

The very low lattice mismatch between the ErSi1.7(0001) and Si(111) allows a convenient epitaxial growth of the former on silicon substrates. However, the morphology of the films formed is found to depend greatly on the epitaxy techniques used. We have thus undertaken a study of the different possible techniques (single metal deposition, alternate evaporation and co-evaporation) so as to determine the procedure which gives silicide thin films of good quality. Best results are obtained by co-evaporation: the films are free of pinholes and present a flat surface. The formation of the silicide films was followed in-situ by LEED and XPS. The ErSi1.7 silicide is characterized by a sharp ( 3 × 3 )R30° LEED pattern which always appears after annealing at temperatures ⩾ 650°C whatever deposition technique is used. The Si KLL spectra, on the other hand, are found very appropriate to detect the presence of holes. The texture of the films was studied ex-situ by SEM. Glancing incidence X-ray diffraction and SIMS have also been used to get insights into the crystalline parameters and the in-depth composition of the silicide layers. The lattice of the epitaxial film is laterally extended to adapt to the Si(111) lattice and compressed axially. No evidence of ordered Si vacancies in the silicide volume is detected. The Er : Si ratio is the same throughout the film. The formation of ErSi1.7 is discussed in relation to the nucleation controlled reaction and the atomic structure of the compound.

Er metal has been evaporated on clean Si(111)(7 × 7) substrates at room temperature and the ErSi interaction at the interface studied by low energy electron diffraction (LEED). X-ray photoemission spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES). The use of the low and high energy X-ray lines given by the Zr anode (Zr Mξ and Lα) have allowed us to detect respectively the surface sensitive Si2p core-level and the chemical state sensitive Si KLL Auger transition, and hence, to evidence the inter-diffusion and to determine the nature of the reacted phase(s). A three-step mechanism has been found: formation of a chemisorbed layer until a critical coverage θ c ≈ 0.8 ML (monolayer) is attained, inter-mixing reaction in the ∼ 0.8–2.5 ML range giving a silicide-like layer, and growth of a nearly pure Er upper layer.

The present work deals with the formation of molybdenum silicides in all-sputtered a-Si/Mo/a-Si/Mo/a-Si/c-Si(100) multilayers. The structure were prepared without breaking the vacuum and the substrates were not intentionally heated during the sputtering. After deposition, the multilayers were annealed at 500 or 700°C. Grazing incidence X-ray diffraction and XPS techniques were used to characterize the Mo/a-Si and a-Si/Mo structures first and then the a-Si/Mo/a-Si/Mo/a-Si/c-Si(100) multilayers. The essential results can be summarized as follows: 1. (1) The nature of the interface formed in the unannealed structures depends strongly on the order of the deposits: if Si is sputtered onto sputtered Mo films, the a-Si/Mo interface is found to be abrupt while in the reverse order the deposit leads to a diffuse Mo/a-Si interface. 2. (2) The thermal treatment has a significant effect only if the temperature is of the order of 700°C. In this case, the result depends on the a-Si and Mo thickness: for thickness lower than ≈200 A only MoSi2 is formed, with predominance of the hexagonal phase, while for thicknesses greater than ≈200°C, we simultaneously observe the tetragonal phases MoSi2 and Mo5Si3.

The physico-chemical properties of interfaces between sputtered aluminum and cadmium sulfide films are investigated. Structural and chemical analysis are made using grazing-incidence X-ray diffraction (GIXD) and quantitative X-ray photoelectron spectroscopy (XPS), respectively. The effect of the order of deposits is also investigated (CdS on Al or the reverse). It is found that when Al is deposited first, the native oxide layer prevents a chemical reaction between sulfur and aluminium but a thin microcrystalline CdS layer is formed at the interface. When CdS is deposited first, a chemical reaction takes place between sulfur and the aluminum overlayer.

The oxidation of ErSi 1.7 thin films, epitaxially grown on Si(111)(7×7) surfaces by solid phase epitaxy, has been investigated by X-ray and UV photoelectron spectroscopies. Oxidation has been carried out at room temperature under low pressure (≤ 2×10 −5 mbar) and 1 atm of oxygen, and at 700°C under 2 × 10 −5 mbar of oxygen. In all cases, both Si and Er react with oxygen. At room temperature the reaction depends on the pressure. Under low pressure the silicide surface is rather inert: a chemisorption phase of oxygen on Si and Er is detected only after exposures 3 langmuir. High oxygen pressure produces a thin layer of mixed SiO 2 , Si su☐ides and Er 2 O 3 . At 700°C, SiO 2 and Er 2 O 3 are simultaneously formed, thus implying the decomposition of the silicide. The oxide layer has a SiO 2 termination at the surface. Only about half of the decomposed Si atoms react with oxygen. The preferential oxidation of Er is attributed to the high value of the heat of formation of Er 2 O 3 . An oxidation mechanism is proposed.

The interaction of Tb with Si(111)7 × 7 at room temperature for various Tb coverages has been studied by photoelectron spectroscopy (UPS, XPS), AES and WF measurements. The experimental results indicate that Tb forms a two-dimensional layer up to 2Aas it is shown by the shift of Tb 4f and 4d states. For greater coverages a reaction of Tb and Si starts evidenced by the reacted Si 2p component, and the change of the Si Auger KLL line shape. The interacted interface extends until 12Aof Tb thickness where metallic Tb begins to build up.

The electronic properties of erbium silicide thin films epitaxially grown on Si(111) have been investigated by X-ray and UV photoemission. The crystalline quality has been checked by low-energy electron diffraction. XPS indicates very weak charge transfer and metallic bonding in the silicide phase. The Si 2p core-level and the Auger transition Si KLL present double structures revealing two types of Si sites, the first one attributed to Si atoms in normal sites in the silicide and the second one to Si atoms in the vicinity of the vacancies and (or) the Si substrate portions seen through the holes of the film. The UPS valence band of about 4 eV width and formed of Er(6s5d)-Si(3s3p) hybridized states disperses weakly in the direction perpendicular to the surface and strongly in the surface plane. This valence band is compared to the ones already measured on YSi −1.7 and GdSi −1.7 and to the calculations made for YSi 2

We report UPS, AES and WF measurements on the interaction of K and O 2 on thin semiconducting epitaxial layers of β-FeSi 2 on Si(111). Oxygen was adsorbed on clean and K-covered β-FeSi 2 at 300 K and the system was heated up to 900 K with the suicide remaining stable. A correlation of the experimental results indicates that at RT on clean β-FeSi 2 oxygen chemisorbs dissociatively on silicon sites and does not interact with iron. In the presence of K oxygen chemisorbs on iron and on silicon sites with an increased sticking coefficient. No potassium oxide was formed. Upon heating up to 800 K SiO 2 is formed on the suicide film under conditions in which the suicide remains stable.

The unoccupied electronic states of the interfaces formed by the heavy refractory metals (W, Mo, Ta and Nb) with a silicon substrate have been investigated by inverse photoemission in the UV range. Emission from metals dominates the IPE spectra. In the submonolayer range the four metals give alike unoccupied densities of states, consisting of a broad structure centered at about 2–3 eV above the Fermi level, which is attributed to the antibonding states of the chemisorbed metal atoms. At higher surface coverage θ, two types of spectrum evolution are observed. For Mo and W on the one hand, which form abrupt interfaces with Si, the IPE spectra tend rapidly towards the metal ones, already attained at θ ⩾ 3 ML. For Nb and Ta on the other hand, which form “surface disilicide” thin layers and diffuse interfaces, the metal spectra develop at much higher θ. The conduction bands of the “surface disilicides” are different from bulk silicide ones, indicating a lack of long range order.

Potassium adsorption on thin β - FeSi 2 epitaxially grown on Si(111) was studied with ultra-violet photoemission spectroscopy (UPS), Auger electron spectroscopy (AES) and work function (WF) measurements. A value of (4.65 ± 0.03) eV was measured for the absolute WF of the silicide surface. A combination of the experimental results upon K deposition indicates a plausible mode of K adsorption on β - FeSi 2 . Initially K adsorbs on Si triangular sites with a Fe atom underneath up to a coverage of ∼ 2 × 10 14 atoms cm −2 and then on Si triangular sites with no Fe underneath up to a saturation coverage of ∼ 2 × 10 14 atoms cm −2 .

Electron energy loss spectroscopy (EELS) has been applied to investigate the Co/Si(111) interface formation at room temperature. The results allow to conclude that at very low coverage θ ⩽ 2 − 3 monolayers a CoSi 2 -like phase is already formed. With higher coverages the topmost layer displays an average composition richer in Co and finally for θ ⩾ 10 monolayers a film of almost pure Co is growing on top of the interface. During the interface development, an interface plasmon coupling effect has also been evidenced by the EELS technique.

XPS, UPS and LEED have been used to study the interface formation and epitaxy of very thin CaF 2 films (⩽63 A ) on thin layers of CoSi 2 (∼ 70 A ) , formed by solid phase epitaxy on Si(111). XPS core levels (Si 2s, Co 2p, Ca 2p, F 1s) and Si(KLL) Auger transition indicate that both Ca and F are bonded to Si atoms only, whether the CoSi 2 surface is Si or Co-rich. At room temperature CaF 2 grows layer-by-layer on CoSi 2 (111), and no ordered surface structure is observed by LEED. XPS valence bands reveal that the electronic structure of CoSi 2 is not affected by the formation of the CaF 2 upper layer. Upon annealing thin layers of CaF 2 in the 550–650°C range, LEED shows good (1 × 1) patterns of CaF 2 (111). The thin epitaxial films are stable up to ∼ 650° C . Above this temperature the CaF 2 valence band undergoesrapid changes indicating a degradation of the insulator layer.

Recent developments in the epitaxial growth of metallic and semiconducting silicides on silicon are reviewed. The structural, electronic and electrical properties of these silicide-silicon interfaces are examined with the aid of results obtained with a large variety of in-situ and ex-situ surface techniques. The paper will focus on two topics: (i) recent progress on the epitaxial growth of thin metallic CoSi 2 films on Si(111), (ii) epitaxial growth of thin semiconducting β-FeSi 2 films on Si(111), (100) and (100) vicinal faces.

XPS and UPS have been used for studying the oxidation of TaSi2 films formed by annealing, under ultrahigh-vacuum, of Ta deposits (≈70A) on polycrystalline Si. Oxidation has been carried out at room-temperature and at 700°C with an oxygen pressure of 2×10-5 mbar. Both Ta and Si react with oxygen. At room-temperature, O chemisorbs on the surface giving a mixed phase of Si suboxides and Ta oxides (TaO, TaO2 and Ta2O5). At 700°C, a very thin Ta2O5 layer (

XPS, work-function and LEED have been used to study the interface formation of the heavy refractory metals (Nb, Mo, Ta and W) with Si substrates. Metal evaporation has been carried out in uhv, on clean Si(111) (7 × 7) surfaces maintained at room temperature. Deposit thickness θ has ranged from a fraction of a monolayer (ML) to ≈ 100 ML. Intensity variations of the substrate and deposit XPS signals, work-function changes and core level binding energy shifts as a function of θ agree so as to indicate the different natures of the interfaces, and different growth modes of the thin films. Mo and W form abrupt interfaces with Si(111) and grow layer-by-layer. On the contrary, Nb and Ta give very reactive interfaces; interdiffusion takes place, after the completion of one 2D disordered chemisorbed metal layer, leading to mixed (1 metal: 2 Si) phases until ≈ 6 ML of Nb or ≈ 4 ML of Ta are deposited. Then, pure Nb condenses on top of the mixed layer, while the Ta overlayer presents a gradually decreasing concentration of Si. These different interfacial reactions can be explained by a qualitative model which takes into account both the Hiraki electrostatic screening mechanism and the Brillson metal-semiconductor bond strength concept.

Fe thin films have been deposited on stepped Si(100) surfaces under ultra high vacuum (uhv) conditions and investigated by X-ray-induced Auger electron spectroscopy (XAES) and photoelectron spectroscopy (XPS). The results suggest that even at room temperature a slight Si interdiffusion occurs into the Fe layers, giving rise to an intermixed FeSi phase. With increasing coverage (Θ ⩾ 15 A ) the surface composition becomes Fe richer, and an almost pure Fe film grows on top of the interfacial compound.

The interfaces formed by Si evaporated onto a Ni(100) surface have been studied by ultraviolet photoemission and inverse photoemission spectroscopy in the VUV range. It is found that at room temperature the first stage of compound formation corresponds to a “Ni 2 Si” type silicide; as more and more silicon is deposited on the clean Ni surface, a NiSi-like phase is obtained and then finally amorphous silicon is observed for high coverage.

Direct photoemission (XPS and UPS), inverse photoemission (IPE) and SiL 2,3 soft X-ray emission have been used to investigate the electronic structure and the chemical bonding in NbSi 2 , TaSi 2 , MoSi 2 and WSi 2 thin films formed on Si(111) clean surfaces. Valence bands and conduction bands reveal strong similarities between NbSi 2 and TaSi 2 on the one hand, and MoSi 2 and WSi 2 on the other hand. For the two former silicides, the density of the occupied states fits qualitatively to the calculations of Bisi and Chiao for isoelectronic and isostructural VSi 2 , while discrepancies are observed for the unoccupied states. Experimental spectra of MoSi 2 and WSi 2 agree well with the theoretical band structure calculated by Bhattacharyya et al. on both sides of the Fermi level. Moreover, the presence of the Si3d states hybridized with the metal d states in the valence band as predicted by these calculations is confirmed by SiL 2,3 emission experiments. Compared to pure metals and silicon the binding energy shifts of the metal and Si core levels are weak and are not simply related to charge transfer. Using the Si “Auger parameter” variation, we find a small charge loss of ∼ 0.1–0.2 electron by each Si atom. This means that the bonding between silicon and the metal is predominantly covalent.

XPS, XAES, UPS and LEED have been used to study the interface formation of W evaporated onto Si(111) (7×7) surfaces, with samples maintained at room temperature and after subsequent annealing of the deposits. The intensity variations of the core levels (Si 2p and W 4f) and of the Auger signals (Si KLL, Si LVV and W NOO) as a function of surface coverage θ correspond to an abrupt junction and a layer-by-layer growth mode. This hypothesis is confirmed by work function measurements. The W 4 f 7 2 undergoes an energy shift of − 0.4 eV (towards lower binding energy) at low coverage, θ ⪷ 1 monolayer (ML), and reaches its bulk metal value at θ ≈ 10 ML; this shows in addition a rather short transition region. Meanwhile, the only variations observed with Si core levels are energy shifts of −0.2 eV for Si 2p and +0.2 eV for Si KLL, which take place at the very first deposition of a submonolayer amount of W. These shifts are attributed to a surface band bending variation. When the metal deposit attains a surface coverage θ ≈ 0.1 ML the Si characteristic surface states disappear from the UPS spectra, while the (7×7) LEED pattern is replaced by a (1×1) diagram which afterwards vanishes in the background for θ⪸1 ML . UPS indicates a rapid predominance of d-metal emission, and a W valence band is fully developed at θ ⪸ 4 ML . Upon annealing of 5–100 ML deposits at temperatures above 750°C, XPS and UPS testify the completion of WSi2 formation and LEED patterns indicate an epitaxial overlayer.

The interfaces formed by Si evaporated onto Ni(100) and Mo polycrystalline surfaces are studied by XPS, XAES and UPS, with the samples at room temperature (RT) and after annealing of deposits of ∼ 20 monolayers. The RT interfacial reactions are different for the two metals; with Ni strong interaction occurs leading to an intermixed and gradual interface, while the junction is rather abrupt with Mo. On both metals, the Si 2p energy shift is negligible in the monolayer range and attains ∼ −0.3 eV in “thick” deposit. In comparison with the inverse systems, i.e. metals deposited on Si, it is found that the Si-metal and metal-Si interfaces formed at RT are similar, but their subsequent annealings produce fairly different composite layers.

Iron deposition up to ~ 25 A on planar and stepped Si(001) surfaces was studied by LEED, AES, UPS, and ELS measurements. On both surfaces iron nucleation is of 3D type with notable interdiffusion at room temperature. Annealing of the overlayers up to 800 K enhances the interdiffusion and above 800 K semiconducting FeSi 2 is formed on both surfaces, unstructured on the planar and epitaxially on the stepped one.

We report UPS, work-function and LEED results on the interfacial reaction between evaporated Ta and Si(111)(7×7) surface under ultrahigh vacuum conditions. At room temperature, a dosordered chemisorbed phase is formed at low coverage, θ≲1 monolayer (ML), and is characterized by an UPS Ta 5d peak shifted at about −1.2 eV as compared to bulk Ta. For 1≲θ≲4 ML, although AES and XPS indicate a 1 Ta:2 Si composition, the valence band spectra are still different from the TaSi2 one. At higher coverage, the electronic structure of polycrystalline Ta is progressively recovered. Annealing of the deposits leads to silicide formation by interdiffusion and surface atomic ordering at temperatures from 650°C to 850°C, depending on the thickness. All the deposits in the 0.8–100 ML range, including the smallest ones, are stable until ∼500°C and the silicide formation reaction begins only at higher temperature, reflecting an activation energy barrier. For θ≲5 ML, clustering of the TaSi2 phase occurs. For thicker deposits, continuous TaSi2 overlayers are obtained. The electronic structure of this silicide is discussed in relation with existing models.