Surface Science 458 (2000) 229–238www.elsevier.nl/locate/susc

Spectroscopic characterisation and chemical reactivity ofsilicon monoxide layers deposited on Cu(100)

F. Yubero a,*, A. Barranco a, J.A. Mejıas b, J.P. Espinos a, A.R. Gonzalez-Elipe aa Instituto de Ciencia de Materiales de Sevilla (CSIC–Univ. Sevilla) and Dpto. Quımica Inorganica C/Americo Vespucio s/n. E-41092,

Sevilla, Spainb Facultad de Ciencias Experimentales, Universidad Pablo de Olavide, Carretera de Utrera Km 1, E-41013, Sevilla, Spain

Received 22 December 1999; accepted for publication 17 March 2000

Abstract

The chemical reactivity of SiOx

species on Cu(100) and oxygen-passivated Cu(100) surfaces has been studied bymeans of synchrotron photoemission (PES), X-ray photoemission ( XPS), X-ray absorption ( XAS) spectroscopiesand ab initio density functional calculations. When silicon monoxide is deposited on a clean Cu(100) surface, belowthe equivalent coverage of ~2 A, partial dismutation of Si2+ species into Si0 plus Si4+ species takes place. Thepresence of Si0 as patches of Si–Cu(100) surface alloy and SiO monomers and polymeric forms of Si2+ species hasbeen explained theoretically. The observation of Si4+ species is explained by a two-step process: first the reductionof Si2+ to Si0 plus O adsorbed on the Cu(100) surface; second the adsorbed O reacts with the incorporated SiO tooxidised to Si4+. The mechanism would be similar to that proposed for the reaction CO�CO2 on O-Cu(100) surface.On the other hand, when very low coverages (equivalent thickness<0.5 A) of SiO are deposited on an oxygen-passivated Cu(100) surface, only Si2+ species are stabilised, probably in an (SiO)

npolymeric form. The theoretical

calculations have confirmed very weak interaction between the SiO structures and the passivated surface. The Si-Kabsorption edge of these Si2+ species is characterised by only two peaks at 1840 and 1849 eV. They have been assignedto the p and s orbitals of the SiO bond, by similarity with the CO molecule. Preferential orientation of the SiO unitsparallel to the surface is found by XAS and confirmed by the theoretical calculations. © 2000 Elsevier Science B.V.All rights reserved.

Keywords: Chemisorption; Density functional calculations; Near edge extended X-ray absorption fine structure (NEXAFS);Photoelectron spectroscopy; Silicon oxides

1. Introduction SiOx

compounds can be used to study the evolutionof the electronic structure from the semiconductoramorphous silicon to the insulator amorphousNon-stoichiometric silicon oxides (SiO

xwith

SiO2.x<2) have been widely studied because of theirFormation of ultrathin SiO

xlayers (<5 A) hasvarious applications as optical coatings [1], passiv-

been claimed at the interface between metallication layers [2] or interlayers in electronics [3]. Insilicon and SiO2 [4–6 ]. All the possible oxidationaddition to the technologically motivated interest,states of Si (i.e., Sin+, with n=0 to 4) have beendetected in these layers. The relative intensity of* Corresponding author. Fax: +34 95 4460665.

E-mail address: yubero@cica.es (F. Yubero) the Sin+ species at this interface depends on the

0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S0039-6028 ( 00 ) 00443-X

230 F. Yubero et al. / Surface Science 458 (2000) 229–238

thickness of the oxide layer, the temperature and chamber did not exceed 1×10−9 mbar duringthe crystallographic surface of Si. evaporation.

Growth of thick layers of SiOx

layers with The substrate for the silicon oxide deposits wascontrolled stoichiometry has also been reported a Cu(100) single crystal. Prior to each evaporation,[7–14]. It is considered that tetrahedral units the surface of the Cu(100) single crystal wasSiM[Si4−mO

m] (m=0 to 4) form these films, where cleaned and reconstructed by several cycles of

an Si atom is bonded to four other atoms of either Ar+ sputtering and annealing at 600°C. The typi-Si or O, while the O atoms are bonded to two Si cal sharp squared pattern of the Cu(100) surfaceatoms of different tetrahedra [8,14]. The number was detected by low energy electron diffractionof oxygen and silicon neighbours in the coordina- (LEED) examination after the cleaning procedure.tion tetrahedron of each silicon atom defines its A first series of experiments consisted of theformal oxidation state. Thus, different oxidation evaporation of silicon monoxide under UHV con-states of silicon from Si0 to Si4+ are detected in ditions on a freshly reconstructed Cu(100) surface.these compounds [7–14]. In a second series, the reconstructed Cu(100)

The preparation and thermal stability of ultra- surface was exposed to oxygen (1000 L) at roomthin films of pure SiO2 on Mo has been reported temperature prior to the silicon monoxide depos-by Goodman and co-workers [15,16 ]. On the other ition. According to the literature, such an adsorp-hand, Martin-Gago and co-workers [17–19] have tion of oxygen produces the formation of astudied the interface between the Cu(110) surface 2E2×E2R45°-O reconstruction of the Cu(100)and metallic silicon. However, not much is known surface [22,23].about the chemistry and properties of oxidised Photoemission spectra (PES) of the Si 2p andspecies of silicon different from Si4+ and Si0. A Cu 3p core lines were recorded using 150 eV pho-systematic study of the intermediate silicon oxides tons at the TGM3 beamline of BESSY (Berlin).in pure form is still missing. In particular, silicon Under these experimental conditions, the kineticmonoxide is a very interesting compound because

energy of the Si 2p and Cu 3p photoelectrons arein molecular form it is isoelectronic with CO,

~50 and ~75 eV, respectively. Thus, their corre-probably the most widely studied probe moleculesponding IMFP for electrons travelling in SiO arein surface science [20,21].4.4 A and 5.1 A, respectively [24]. Thus, thisThis paper reports the preparation and charac-experiment is extremely surface sensitive. Both theterisation of ultrathin silicon oxide deposits withincidence angle of the photons and the collectioncontrolled stoichiometry on clean and oxygen-angle of the photoelectrons, were 45° with respectpassivated Cu(100) surfaces. The chemical sta-to the surface normal. This was done to achievebility and the electronic structure of these depositsthe strongest surface sensitivity to better-character-have been investigated by means of photoemissionised submonolayer coverages of the silicon oxideand X-ray absorption spectroscopies and ab initiodeposits. The energy resolution of the PES meas-density functional calculations.urements was such that the full width at half-maximum (FWHM) of the Si0 species of the Si 2ppeaks was of 1.1 eV (combined energy resolution2. Experimentalof monochromator plus electron analyser~0.8 eV ).The experiments presented in this paper were

X-ray photoelectron spectra ( XPS) were alsoperformed in several types of ultrahigh vacuumrecorded on an ESCALAB210 electron spectrome-(UHV ) equipment. The base pressure in all ofter using unmonochromated Mg Ka (1253.6 eV )them was less than 3×10−10 mbar. Silicon monox-radiation as excitation source and collectionide deposits were evaporated in situ from a home-normal to the surface. The FWHM of pure Si0made Knudsen cell filled with a nominal siliconspecies was 1.4 eV for this characterisation. Simonoxide powder from Goodfellows. The cell was

carefully outgassed so that the pressure in the Auger parameters were calculated by measuring

231F. Yubero et al. / Surface Science 458 (2000) 229–238

the Si 2p binding energies and the Si KLL kineticenergies (the latter excited with the bremstrahlungof the X-ray source). Stoichiometry of the siliconoxide deposits (i.e., O/Si ratio) was determined bymeasuring the areas of the Si 2p and O 1s peaksand referring the obtained values to that of anSiO2 reference sample. This consisted of an SiO2layer (~350 A thick) thermally grown on a Si(111)wafer, which was cleaned by mild bombardmentwith 500 eV O+

2. The amount of the silicon oxide

deposited on the copper substrate was estimatedby measuring the Si 2p and Cu 2p peak areas.

Throughout the paper, the amount of siliconoxide deposited on the surface has been expressedin terms of an equivalent thickness x0. A layer-by-layer growth of the deposit was assumed and theusual exponential laws for the intensities of theCu 3p and Si 2p photoelectron peaks for the PESmeasurements, and the Cu 2p and Si 2p for theXPS measurements, normalised by the correspond-ing sensitivity factors [25], were considered.

X-ray absorption spectra at the Si-K edge(1835–1850 eV ) were recorded by total electronyield detection (i.e., drain current) at the SA32beamline of SuperACO (LURE, Orsay). Thephoton energy of the radiation was selected by anInSb(111) double crystal monochromator (reso-

Fig. 1. (a) Raw Si 2p photoelectron spectra normalised to thelution of 0.75 eV at 2000 eV ). photon flux for increasing amounts of silicon monoxide depos-

ited on Cu(100). (b) Result of the fitting analysis of the spectrain (a) after background subtraction and normalised to the samepeak area.3. Results

Fig. 1a shows a series of raw Si 2p photoelectron Si4+species. This result agrees with previous exper-imental findings [4,5]. Fig. 1b shows the fittingspectra, normalised to the photon flux, excited

with 150 eV photons for successive evaporations analysis of the previous spectra. The best fit of thewhole series was achieved locating the Si 2p3/2of silicon monoxide on a clean Cu(100) surface.

Each spectrum in Fig. 1a depicts a series of peaks peaks at 99.2, 100.2, 101.3, 102.1 and 102.9 eVbinding energies for the Si0, Si1+, Si2+, Si3+ andthat extend from 98 to 105 eV binding energy.

They correspond to the Si 2p contributions of Si4+ species, respectively. Note that the positionof the maximum of an Si 2p species is at ~0.2 eVdifferent Sin+species (n=0, 1, 2, 3, 4). In order to

quantify the relative amount of the Sin+ compo- higher binding energy than its correspondingSi 2p3/2 component. The energy separationnents, each Sin+ species has been simulated by two

gaussians with 2:1 intensity ratio (2p3/2 and 2p1/2 between successive Sin+ species follows theaccepted energy separation of ~1 eV per chemicalcomponents) separated by the spin–orbit splitting

of 0.6 eV [5]. It is found that the FWHM of the state [8,11,13], although it does not fully coincidewith that reported by Himpsel et al. [5,6 ] forSi 2p peak corresponding to each Sin+ species

increases as higher oxidation states of Si are con- Sin+ species at the SiO2/Si interface. This mightbe due to an effect of second neighbours on thesidered from 1.1 eV for Si0 to 1.5 eV for

232 F. Yubero et al. / Surface Science 458 (2000) 229–238

Fig. 2. Relative amount of each Sin+ component in Fig. 1b asa function of the amount of silicon oxide deposited.

binding energy of the Sin+ species. In fact,Banaszak Holl and co-workers [26,27] havereported that second neighbour effects can shift aformal Si0 species by ~1.0 eV. Such second neigh-bour contribution cannot be discarded here andmight also explain the observed energy shifts.

The relative concentration of each Sin+ speciesagainst the amount of deposited silicon oxide isrepresented in Fig. 2. For x0≤2.5 A, Si0, Si2+ andSi4+ are the majority species in the deposits. Therelative amount of Si2+ and Si4+ remain almost Fig. 3. (a) Raw Si 2p photoelectron spectra normalised to the

photon flux for increasing amounts of silicon monoxide depos-constant (~31% and ~37%, respectively) up toited on a oxygen-passivated Cu(100) surface. (b) Result of thex0#2.5 A, and that of Si0 decreases slightly fromfitting analysis for the spectra in (a) after background subtrac-23% to 15% of the total amount of Sin+ species.tion and normalised to the same peak area.

This behaviour suggests that Si0 species are formedat the earliest stages of the deposition. Meanwhile,Si+ and Si3+ species remain at levels of the 5% shifts to higher binding energies from 101.5 tofor x0≤2.5 A. Beyond this point (i.e., x0> 103.1 eV for increasing values of x0. The FWHM2.5 A), Si3+ species increases drastically at the of these features increase from 1.4 eV forexpense of mainly the Si2+ oxidation states. Note x0=0.1 A to 1.9 eV for x0=2.6 A so differentthat the thickness x0#2.5 A defines a kind of Sin+ species contribute to these spectra. We havebreakdown point in our experiment. applied the same deconvolution procedure as that

In a second experiment, silicon monoxide was used in the first series of spectra, using the sameevaporated at a partial pressure of 10−8 mbar of peak position and width for the parameters

describing each Sin+ component. The result of theO2 on a 2E2×E2R45°-O reconstruction of theCu(100) surface [22,23] (i.e., on a Cu(100) surface fitting procedure is shown in Fig. 3b, while the

variation of the relative contribution of each Sin+with oxygen preadsorbed on its surface). The extraO2 was supplied to avoid any reduction of the species against x0 is depicted in Fig. 4. For x0 ≤

0.1 A, only Si2+ species stabilise on the surface.evaporated Sin+ species. The Si 2p photoelectronspectra obtained are depicted in Fig. 3a. In this For equivalent thicknesses x0>0.3 A, Si3+ species

start to be formed while the relative contributioncase, the spectra show a single maximum that

233F. Yubero et al. / Surface Science 458 (2000) 229–238

Fig. 4. Relative amount of each Sin+ component in Fig. 3b asa function of the amount of silicon monoxide deposited.

of Si2+ species decreases slowly as the amount ofFig. 5. Evolution of (a) the O/Si ratio and (b) aSi∞ for silicondeposited silicon monoxide increases. Finally, formonoxide deposited on an oxygen-reconstructed Cu(100) sur-x0>1.5 A, the contribution of Si4+ species to theface against the amount of deposit.spectra (~10%) becomes significant. It is interes-

ting that by recording PES spectra at normal andgrazing emission for a sample with x0#0.7 A content in the silicon oxide layer increases up to

an O/Si ratio of ~1.6 when a thick layer (several(spectra not shown), the Si3+ species appear prefer-entially in the outermost region of the surface. tens of A) of silicon oxide is deposited. On the

other hand, it is apparent in Fig. 5b that aSi∞ variesThe same experiments were also performed inour laboratory with conventional XPS, as men- from 1714.6 to 1713.4 eV for x0≤3 A. Hence, a

shift in this parameter of ~1.2 eV occurs throughtioned in Section 2. In this case, the quantificationwas done through the Cu 2p, O 1s and Si 2p photo- all these initial stages of the deposition experiment.

The initial value, i.e., 1714.6 eV can be assignedelectron peaks. Additional information about thestoichiometry of the silicon oxide deposit (i.e., to pure Si2+ species on a passivated Cu(100)

surface. The energy shift can be explained by theO/Si ratio) and the modified Auger parameter ofsilicon aSi∞ , (aSi∞ =binding energy Si 2p+kinetic combined effect of the decrease of the extra-atomic

relaxation energy at the Si sites and the appearanceenergy Si KVV ) was obtained. This information isparticularly relevant when silicon monoxide is of higher oxidation states of Si (i.e., Si3+ and

Si4+) for increasing amount of deposited siliconevaporated on the oxygen-reconstructed Cu(100)surface. The O/Si ratio has been estimated from oxide. Note that, for coverages x0#3 A, one

obtains again by XPS a kind of breakdown pointthe O 1s and Si 2p peak areas as mentioned inSection 2. The evolution of these parameters with as it was found previously by PES.

Si-K edge absorption spectra were also recordedthe amount of deposited silicon monoxide is shownin Fig. 5. for the silicon oxide deposited on the

For x0<1 A, the stoichiometry (i.e., O/Si ratio 2E2×E2R45°-O reconstruction of the Cu(100)in Fig. 5a) of the silicon oxide deposited on the surface. Spectra corresponding to successive depos-

itions are shown in Fig. 6 ( left). The spectra (a)2E2×E2R45°-O reconstruction of the Cu(100)surface is ~1 within the error bars, which agrees and (b) correspond to samples with equivalent

thicknesses x0#0.5 A and #2.0 A, respectively.with the majority presence of Si2+ species, as wasdeduced previously from the fitting analysis in The spectrum (c) corresponds to a thick layer of

SiO1.3 where Si+ and Si3+ are majority species, asFig. 3b. It is also worth mentioning that the oxygen

234 F. Yubero et al. / Surface Science 458 (2000) 229–238

in Fig. 6 (right), suggesting preferential orientationof the SiMO bond parallel to the surface.

4. Discussion

The previous experimental results have revealedthat the deposition of silicon monoxide yields twocompletely different behaviours depending on theoxidation state of the copper surface. Thus, Si2+species coexists at the Cu(100) surface with Si0and Si4+ species for coverages x0 below ~2 A,probably due to an strong interaction of the depos-ited SiO units on the Cu(100) surface. In contrast,non-dissociative SiO adsorption takes place whenthe deposition is done on the oxygen-reconstructed

Fig. 6. Left: Si-K edge XAS spectra for increasing amounts of Cu(100) surface, with the SiMO bond preferen-silicon monoxide deposited on an oxygen-reconstructed

tially oriented parallel to the surface.Cu(100) surface (a) 0.5 A, (b) 2.0 A and (c) thick SiO1.3 To account for these experimental findings, wedeposit. Right: Si-K edge of 0.3 A of SiO deposited on anoxygen-reconstructed Cu(100) surface taken at normal and have studied the structure and energetics of SiOCugrazing (60°) incidence of the synchrotron light with respect to surface interaction by means of ab initio densitythe surface. functional calculations within the generalised gra-

dient approximation [29]. These calculations wereperformed using ultrasoft pseudopotentials and awe have previously reported [14]. It is apparent

from the inspection of the shape of these spectra plane waves basis set with a kinetic energy cutoffof 340 eV. All the structures were fully relaxedthat a progressive evolution occurs when passing

from the low to the high coverage situations. All with the symmetry constrains imposed by thesurface unit cell. Within the calculations, the cellspectra are characterised by two features at

~1840 eV and ~1847 eV. These features become parameter was kept fixed at the value obtainedfrom a bulk Cu relaxation, i.e., 3.6 A.broader as the amount of deposited silicon oxide

increases. In addition, a new feature at ~1844 eV One of the main issues is whether the adsorptionof SiO on Cu(100) is dissociative or molecular.develops and increases in intensity. A preliminary

assessment of the evolution of the shape of these Therefore, we started our calculations from con-figurations in which Si and O where adsorbed ontospectra can be made by comparison with the

photoemission results in Fig. 3. Thus, spectrum (a) the different surface sites and the SiMO bondswere dissociated. However, in all cases a geometryin Fig. 6 ( left) corresponds to Si2+ species, while

the appearance of the shoulder at ~1844 eV indi- relaxation led to bonded SiO molecules, suggestingthat the presence of adsorbed neighbouring Si0cates the progressive formation of Si3+ species.

The features at 1840 and 1847 eV can be interpre- and O atoms is unlikely. This adsorption is ratherexothermic (362 kJ mol−1). In the most stableted as the p and s orbitals of the SiO bond, by

similarity with the CO molecule [28]. The preferen- geometry of absorption, the oxygen atom of theSiO unit would be bonded to the fourfold hollowtial orientation of the SiO units was tested in a

sample with x0#0.3 A of SiO deposited on the site of the unit cell of the Cu(100) surface. The Oatom would be lifted ~0.9 A above the highestoxygen-reconstructed copper surface by measuring

the Si-K edge with normal and grazing incidence Cu atomic plane. The tilt angle of the SiO bondwith respect to the surface normal is ~70°, withof the synchrotron light to the surface. In fact,

resonance of the s contribution to the Si-K edge the Si atom approximately on top of a Cu atom(SiMCu distance of ~2.3 A). Under these condi-was observed at normal incidence, as it is shown

235F. Yubero et al. / Surface Science 458 (2000) 229–238

Fig. 7. Schematic representation of the most stable geometry of adsorption of (a) SiO monomers on Cu(100), (b) (SiO)n

polymericchains on Cu(100), (c) SiO monomers on units on 2E2×E2R45°-O Cu(100), and (d) (SiO)

npolymeric chains on

2E2×E2R45°-O Cu(100).

tions, the SiMO distance is ~1.7 A, similar to the These two results for isolated SiO units and(SiO)

npolymeric chains deposited on Cu(100) aregas phase bond length (i.e., ~1.6 A). An schematic

representation of this situation is drawn in Fig. 7a. consistent with the presence of some of the Siatoms as Si2+species at low coverages (x0<3 A),Another possibility we have considered is

whether the interaction between SiO units results as was shown in Figs. 1 and 2.The existence of dissociated Si and O atoms inin a change of the chemical state of the surface

silicon. This may happen by dismutation neighbouring sites on Cu(100) is not a stablesituation. However, the dissociation of the (SiO)

n(2SiO�Si0+SiO2) of the SiO molecules at thesurface. This process is kinetically hindered chains adsorbed on Cu(100) into a CuSi surface(although exothermic) in bulk-like SiO

xfilms, but alloy [17–19] plus the passivated 2E2×

here it could be favoured by interaction with the E2R45°-O Cu(100) areas is an exothermic processsurface. Geometry relaxations started from dissoci- by 5 J m−2. This supports the presence of Si0 atated configurations led to the formation of the earliest stages of deposition (see Figs. 1 and 2).(SiO)

npolymeric chains, in which the oxidation Our current calculations do not support the

state of silicon is two (i.e., Si2+). The calculations presence of Si4+ species at the early stages ofindicate a strong SiO–SiO interaction on the deposition of SiO on Cu(100) (see Figs. 1 and 2).Cu(100) surface. The energy of formation of these The experimental results on the reconstructed sur-polymeric chains from the adsorbed monomers face (Figs. 3 and 4) as well as the calculationsdescribed before is 277 kJ mol−1. Their most stable below are strong evidence against the catalyticadsorption geometry consists of Si atoms on top oxidation of Si2+ to Si4+ on the 2E2×of the fourfold hollow site with the Si atoms E2R45°-O Cu(100) surface. However, other~1.6 A above the highest Cu plane and oxygens oxygen-covered phases on the Cu(100) surfacebridging each pair of Si atoms. The SiMO distance could be active for silicon oxidation. This is knownin the (SiO)

npolymeric chains is ~1.6 A with the to be the case for the oxidation of CO, for which

reactive phases of O on Cu(100) other thanSiMO bonds ~50 off normal (see Fig. 7b).

236 F. Yubero et al. / Surface Science 458 (2000) 229–238

Table 1c(2×2)-O or 2E2×E2R45°-O are responsible forHeat of adsorption (kJ mol−1) of SiO monomers on thethe catalytic process [30]. Similar reactive phases,Cu(100) and 2E2×E2R45°-O Cu(100) surfaces and the heat

which are currently under investigation, may be of formation of polymeric (SiO)n

chains from the adsorbedformed in this case due to SiO dissociation pro- monomers on the same surfaces (1 eV=96.5 kJ mol−1)vided the process SiOMCu(100)�CuSi(100)+

SiO monomer (SiO)n

polymerOMCu(100) is exothermic as mentioned below.Then, in a second step, the oxygen-covered surface Cu(100) 362 277would oxidise the SiO units: OMCu(100)+SiO� 2E2×E2R45°-O Cu(100) 56 256

Cu(100)+SiO2. The net reaction would be a dis-proportionation of SiO into Si and SiO2. Such a

isolated Si2+ species detected experimentally whenmechanism would explain the presence of Si4+ asSiO is deposited on oxygen-passivated Cu(100)SiO2 from oxidation of SiO at the reactive oxygen(see Figs. 3 and 5) correspond to these weaklycovered areas (see Figs. 1 and 2).adsorbed SiO monomers and (SiO)

npolymericTherefore, the mechanisms described above

chains. The weak interaction between SiO and thewould explain the presence of the different oxida-oxygen-passivated Cu(100) surface seems to be ation states observed: Si2+ as monomers or poly-critical condition that prevents a further reactionmeric chains interacting strongly with Cu(100), Siof SiO (as monomer or polymer) with the surface.as CuSi surface alloy patches and SiO2 fromThe energetics of the different transformationsoxidation of SiO at the reactive oxygen covereddescribed in the paper are summarised in Table 1.areas.

Hence, the previous experimental and theoreti-The situation is different when the SiO mon-cal results indicate that Si2+species are preferen-omers are deposited on the 2E2×E2R45°-Otially formed when evaporating silicon monoxideCu(100) reconstruction. The heat of absorptionon an oxygen-reconstructed Cu(100) surface for

of SiO monomers on this 2E2×E2R45°-Oequivalent thicknesses up to ~0.5 A, before SiO

Cu(100) reconstructed surface is 56 kJ mol−1, con- monomers and (SiO)n

chains start to interact withsiderably weaker than on Cu(100). In the most them. According to the theoretical calculationsstable configuration for one adsorbed SiO unit on and XAS measurements, the structure formed bythe 2E2×E2R45°-O Cu(100) cell, the SiO mon- the SiO units lies with the SiMO bond parallel toomer sits on top of the missing row of Cu atoms the surface. Preferential enhancement of the swith a tilt angle of 86°off normal, i.e., nearly with respect to p orbitals at the Si-K edge wasparallel to the surface Both Si and O atoms point observed by recording at grazing and normaltowards the step edges, with the CuMSi and angles of incidence of the radiation (Fig. 6 (right)).CuMO distances of 3.55 and 3.50 A, respectively We would like to emphasise that, to our knowl-(see Fig. 7c). edge, this is the first time that the formation of an

The formation of linear (SiO)n

chains from almost pure silicon monoxide layer has beenthese adsorbed SiO units, 3.2 A above the reported in literature.OMCu(100) surface, with SiMO bond lengths of The increasing amount of Si3+ and Si4+ species1.8 A within the polymer, is again rather exother- on the oxygen-passivated Cu(100) surface is cer-mic (256 kJ mol−1). In contrast, the interaction tainly due to the oxygen available during the SiOenergy of this polymer with the surface is only evaporation and the thermodynamic and kinetic6 kJ mol−1. Since this 2E2×E2R45°-O Cu(100) constrains of the process. These effects have beenreconstructed surface is already passivated, it is studied by our group and they will be publishedunlikely that it would dissociate (SiO)

ninto Si- elsewhere.

and O-covered domains. This accounts for the To conclude, it is worth remarking that theabsence of Si0 species when SiO is deposited on energetics of the density functional calculationsthe oxygen-passivated Cu(100) surface (see Figs. 3 performed in this paper is consistent with spectro-

scopic characterisation performed in these systems.and 4). In addition, these results suggest that the

237F. Yubero et al. / Surface Science 458 (2000) 229–238

However, further investigation should be done in orientation of the SiO units is preferentially paral-lel to the surface, as determined by XAS andorder to confirm the different geometric arrange-

ments of the SiO deposited on the Cu(100) and confirmed by the theoretical calculations.2E2×E2R45°-O Cu(100) surfaces addressed inthis paper by the calculations. A first confirmationthat the SiO bond is preferentially oriented parallelto the oxygen-reconstructed Cu(100) surface has Acknowledgementsalready been observed by crude XAS measure-ments, as is shown in Fig. 6 (right). Nevertheless, We thank the CICYT (pr. No. MAT97-689)characterisation by in situ AFM or STM micro- for their financial support. We also thank to thescopies or photoelectron diffraction on these sys- TMR program for their financial support for thetems would be desirable. measurements at BESSY and LURE. Thanks are

also given to the staff of these synchrotrons fortheir technical support. J.A.M. is grateful for sup-port from Plan Andaluz de Investigacion (gr.FQ205) and the DGES (PB98-0326).

5. Conclusions

The chemical reactivity of SiOx

species onCu(100) and 2E2×E2R45°-O Cu(100) surfaces Referenceshas been studied by means of spectroscopic charac-terisation by PES, XPS and XAS and ab initio [1] G. Hassand, C.D. Salzberg, J. Opt. Soc. 44 (1954) 181.

[2] M.J. O’Leary, J.H. Thomas III, J. Vac. Sci. Technol. A 5density functional calculations. It is found that(1987) 106.when SiO is deposited on a clean Cu(100) surface,

[3] C.R. Helms, B.E. Deal (Eds.), The Physics and Chemistrybelow the equivalent coverage of ~2A, an appar-of SiO2 and the Si–SiO2 Interface, Plenum, New York,

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Lagarde, J.P. Connerade, Z. Phys. D 21 (1991) 357.reaction CO�CO2 on OMCu(100) surface [30]. [10] V. Belot, R.J.P. Corriu, D. Leclerq, P. Lefevre, P.H.

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