Oxidation of molybdenum surfaces by reactive oxygen plasma and O2+ bombardment: an auger and XPS study

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 235È241 (1998)

Oxidation of Molybdenum Surfaces by ReactiveOxygen Plasma and Bombardment : an AugerO

2‘

and XPS Study

L. D. Lo� pez-Carren8 o,1 G. Ben•� tez,1 L. Viscido,1 J. M. Heras,1 F. Yubero,2 J. P. Espino� s2 andA. R. Gonza� lez-Elipe2,*1 Instituto de Investigaciones Fisicoqu•�micas (INIFTA), University of La Plata, C.C. 16, suc. 4, 1900-La Plata, Argentina2 Instituto de Ciencia de Materiales de Sevilla and Dpto. Q Inorga� nica (CSIC-Universidad de Sevilla), Avda. Ame� ricoVespucio s/n, 41092 Sevilla, Spain

The oxidation of molybdenum at room temperature with oxygen plasma or a beam of ions has been studiedO2‘

with Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS). Auger electron spectroscopyshows a progressive modiÐcation of the lineshape and the development of new features close to the intenseN

2, 3VV

Auger transitions of metallic Mo at 160, 186 and 221 eV. The e†ects are greater with the oxygen plasmaM4, 5

NNthan with the bombardment. X-ray photoelectron spectroscopy shows that the incorporation of oxygen intoO

2‘

the molybdenum gives rise to the formation of Mo6‘, Mo4‘ and a form of Mon‘ (n Æ 4). The(Mo4‘ + Mon‘)/Mo6‘ ratio was higher after ion bombardment than by treatment with the oxygen plasma,O

2‘

in which case the concentration of the Mo4‘ and Mo6‘ oxidation states was very similar. The analysis of the O 1sand O KLL Auger spectra under the di†erent conditions showed that whenever a high concentration of the Mo6‘

species is present, the O–Mo bond is more covalent in character. To evaluate the inÑuence of chemical and ballistice†ects in the two oxidation processes, additional experiments were carried out with This oxide was bom-MoO

3.

barded with Ar‘ and until a steady-state surface composition was reached. In both cases, the formation of aO2‘

considerable amount of Mo4‘ + Mon‘ (n Æ 4) and Mo0 species (i.e. reduction of Mo6‘ initially present in thesample) was detected. These results suggest that the high concentration of Mo4‘ + Mon‘ found upon bombard-ment of Mo0 with is rather produced by ballistic e†ects, which cause the reduction of the initially formedO

2‘

Mo6‘. 1998 John Wiley & Sons, Ltd.(

Surf. Interface Anal. Vol. 26, 235È241 (1998)

KEYWORDS: molybdenum oxides ; oxygen plasma; ballistic e†ects ; XPS

INTRODUCTION

Molybdenum oxides are of great technical interest incatalysis and electronics. In fact, and poly-MoO2molybdates are important compounds as catalysts or ascatalyst precursors1 because of their versatile catalyticproperties in both selective oxidation and selectivehydrogenation reactions. Accordingly, in the last twodecades surface science tools have been employed inseveral investigations of molybdenum oxides andmolybdates.2h10 Technically, these oxides are generallydeposited on high surface area substrates (i.e. Al2O3 ,

etc.)11,12 and they are used after slightTiO2 , SiO2 ,reduction. From the microelectronics point of view,pure molybdenum oxides and mixtures with tungstenoxide may be interesting as display materials becauseboth change their colour reversibly upon light irradia-tion.13 In all these Ðelds the study of the oxidation

* Correspondence to : A. R. Gonza� lez-Elipe, Instituto de Ciencia deMateriales de Sevilla and Dpto. Q. Inorga� nica (CSIC-Universidad deSevilla), Avda. Ame� rico Vespucio s/n, 41092 Sevilla, Spain. E-mail :agustin=cica.es.

states of molybdenum and the oxidation/reductionmechanism is of paramount importance.

The mechanism of chemical reduction of hasMoO3been studied by several authors.10,14h17 In these studiesMo6`, Mo0 and two forms of Mo4` appeared as themost usual oxidation states of molybdenum. Reductionof upon Ar` bombardment has also been report-MoO3ed.18 Electron beam-induced damage of the surface ofmolybdenum oxides has also been recognized.19 In fact,early investigations showed that the electron beam ofAuger equipment induces decomposition of the molyb-denum oxides, with a preferential loss of oxygen at thesurface.20

On the other hand, the interaction of oxygen withmetallic Mo surfaces has been the subject of severalinvestigations.9,15,21h27 These studies performed withXPS, AES, electron energy-loss spectroscopy (EELS)and Raman spectroscopy have focused on the oxidationstates of molybdenum and on the dependence of thethickness of the oxide on the temperature and oxygenpressure used during the oxidation processes. An alter-native oxidation procedure of molybdenum consists ofthe use of oxygen plasmas or oxygen ion beams. Withthese methods it is expected that oxidation can proceedto a large extent even at room temperature. However, in

CCC 0142È2421/98/040235È07 $17.50 Received 5 August 1997( 1998 John Wiley & Sons, Ltd. Accepted 30 October 1997

236 L. D. LOŠ PEZ-CARREN3 O ET AL .

connection with such processes it is worth mentioningthat while the papers dealing with the interaction ofnoble gas ions with oxide targets are very numer-ous,28h30 those reporting the interaction of reactivegases with metal or oxide targets are more scarce.31,32

In the present paper we study by AES and XPS thedi†erent oxidation processes occurring at room tem-perature on a polycrystalline Mo foil when it is sub-jected to the above-mentioned oxidation procedures.The results suggest that chemical and ballistic e†ects areinvolved in the processes. Moreover, the resultsobtained are of relevance for interpretation of thechemical e†ects produced during interaction of chargedions with oxides. Analysis of the chemical shiftsobserved by XPS for the di†erent oxidation states ofmolybdenum and the assumption of a certain distribu-tion scheme of these species after oxygen plasma or ionbeam treatment are two other aspects discussed in thispaper.

EXPERIMENTAL DETAILS

Two sets of experiments were performed in this work.In both cases a substrate of polycrystalline Mo foil(10 ] 10 ] 0.05 mm3 from Goodfellow Metals, 99.9%pure) was subjected to di†erent oxidation treatments asdescribed below. Also, powder samples were pre-MoO3pared ex situ according to the procedures described byHau†e33 and Fromhold34 (i.e. 1 atm. of oxygen at 700K). The quality of these powder samples was veriÐed byX-ray di†raction.

The Ðrst set of experiments consisted of AES mea-surements in a home-assembled ultrahigh vacuum(UHV) system in which a pressure of \5 ] 10~10 mbarcan be achieved routinely. Photoelectric work function(WF) studies were also available in situ. The electronenergy analyser (single-pass cylindrical mirror analyserof Physical Electronics) was operated at a resolution of0.6% E (1 eV modulation), which allows data acquisi-tion of the NNN Auger transitions of a clean Mosurface with enough resolution to distinguish clearlythose transitions at 15 and 25 eV (see Fig. 1). In thissystem, Auger spectra were obtained in derivative modeat high speed (D35 eV s~1) using a primary electronbeam of 2 keV with normal incidence on the unbiasedsample. No damage due to electron bombardment wasdetected in any of the samples studied. The cleansurface of the Mo sample foil for these studies wasobtained after successive cycles of Ar` bombardmentfollowed by Ñash heating in oxygen at up to 1300 K.Evolution of the chemical composition of the Mosurface during the cleaning procedure was followed byAES and WF measurements.

In a second set of experiments, XPS spectra wererecorded in an ESCALAB 210 spectrometer (base pres-sure 3 ] 10~10 mbar) working in the constant passenergy mode at 50 eV. Magnesium Ka radiation wasused as excitation source for the XPS measurements.The signal coming from the Mo 3d and O 1s core levelstogether with the x-ray-excited O KLL transitions weremonitored after the di†erent treatments. The cleaningprocedure of the Mo substrate consisted of Ar` sputter-ing with 3.5 keV ions. Ion bombardment with Ar` and

ions of 3.5 keV was performed in a preparationO2`chamber (base pressure 2] 10~7 mbar) connectedthrough a gate valve to the main chamber where theelectron analyser was located. The treatment of thesamples with oxygen plasma was also carried out inthe preparation chamber. Details about the productionof the oxygen plasma have been published elsewhere.35Here, it is enough to mention that the system for theproduction of the oxygen plasma consisted of a quartztube connected to the preparation chamber of thespectrometer. The plasma was generated by Ñowing O2(at a pressure of D10~2 Torr) through the quartz tubeplaced within a resonant microwave cavity suppliedwith a power of 100 W.

In order to calibrate the energy of the photoemissionspectra, the Mo peaks from metallic Mo (Mo0)3d5@2and (Mo6`) at 227.9 and 232.7 eV, respectively,MoO3were used as a reference. The spectra were backgroundcorrected (Shirley algorithm) prior to deconvolution.

The Mo 3d spectra recorded after ion or plasmatreatments show a complex shape. Di†erent chemicalstates of the Mo species present in the samples wereidentiÐed as follows. The Mo 3d photoemission spectrafrom Mo0 and Mo6` species were obtained from cleanMo and reference samples. For the rest of theMoO3samples, a linear combination of the Mo 3d levels ofMo0 and Mo6` chemical states was subtracted fromthe Mo 3d signal measured in each sample. Then, theremaining di†erence spectra were assigned to chemicalstates of Mo that di†er from Mo0 and Mo6`. This pro-cedure yields basically the same results as Ðttinganalysis. However, we prefer this way because it doesnot require any assumption about the actual number ofdi†erent chemical states present in the sample.

RESULTS

Auger electron spectroscopy

The adsorption of onto the Mo foil at room tem-O2perature or even at 100 K causes no detectable changein the Auger spectra up to exposures of D103 L.Walker et al.26 used 106 L at 873 K to form a thickoxide layer of D4 nm. Evidently, higher exposures ormore reactive methods are necessary to build up a thicksurface oxide.

Figure 1 shows the di†erences in the Mo Augerspectra of the clean Mo foil after being bombarded with

ions or exposed to a highly reactive oxygenO2`plasma. In both cases (spectra (b) and (c)) the Augertransitions at 15 eV and at 25 eVN1N2, 3V N2, 3VVturned into a broad peak because of a drastic change intheir relative intensities. In the region of Mo MNNtransitions (150È240 eV) three new peaks develop,located at energies 5È7 eV lower than those correspond-ing to the (160 eV), (186 eV) andM4, 5N1V M4, 5N2, 3V(221 eV) transitions in Mo0. These new tran-M4, 5VVsitions in the Auger spectra of oxidized Mo samples canbe associated with the formation of new chemical Mospecies at the topmost surface of the samples, as report-ed previously.20,24 Comparing Auger spectra (b) and (c)in Fig. 1, some facts are to be noted : the intensity of thenew features at 160 eV and 186 eV for the O2-plasma-

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 235È241 (1998) ( 1998 John Wiley & Sons, Ltd.

Mo OXIDATION BY OXYGEN PLASMA AND O2` 237

Figure 1. Auger spectra of a polycrystalline Mo foil : (a) cleansurface ; (b) after 11 min of exposure to an plasma; (c) after 90O

2min of bombardment. Details in text.O

2½

sample (spectrum (b)) is greater than that corre-treatedsponding to the sample (spectrum 1(c)) ;O2`-treatedand in spectrum (c) the Auger transition at 221M4, 5VVeV shifts slightly (2È3 eV) to higher kinetic energy.

X-ray photoelectron spectroscopy

Similarly to the AES experiments, exposing the sampleto 2.4 ] 106 L of oxygen at room temperature producesa scarcely detectable oxidation by XPS. However, oxi-dation occurs at room temperature by means of anoxygen plasma treatment or by bombardment with

ions.O2`Figure 2 shows the Mo 3d spectra (normal emission)after background subtraction in the case of a poly-crystalline Mo foil exposed to oxygen plasma during theindicated times. Evidently, this treatment produces aprogressive modiÐcation of the spectrum lineshape,which develops a signiÐcant contribution at the high-binding-energy side as the exposure increases. Theanalysis of the Mo 3d spectra according to the pro-cedure described in the experimental section reveals theformation of new chemical forms of molybdenum. TheMo binding energies of these forms, labelled A, B3d5@2and C, are : 232.7 eV (species A), 231.5 eV (species B)and 229.0 eV (species C) ; species C appears as a shoul-der to the right of the Mo peak of species B. The3d5@2spectra in Fig. 2 show that species A and B increasesimultaneously in intensity as the exposure to theoxygen plasma increases.

To gain some insight into the distribution in depth ofspecies A, B and C, spectra at grazing emission angles(60¡ relative to the surface normal) were taken. Figure 3shows a comparison of the Mo 3d spectra taken atnormal and grazing emission for the Mo foil exposed tooxygen plasma for 4 min. The spectra of Fig. 3 showthat the ratio between the intensities of species A and Bis practically constant (independent of grazing ornormal emissions). This fact indicates a homogeneousdistribution of the two species throughout the oxidizedoverlayer. However, this is not the case with species C(i.e. the shoulder at 229.0 eV), whose intensity at grazingemission angles does not increase accordingly. This

Figure 2. Molybdenum 3d photoelectron spectra taken at normalemission for a polycrystalline Mo foil exposed to a plasma ofoxygen at 298 K (——–) for the indicated periods of time: (· · · · ·)species A; (- - - - -) species B ½C; (– – – –) Mo0.

result would be consistent with the accumulation ofspecies C at the interface between the Mo metal and theoxidized overlayer.

Figure 4 shows the Mo 3d spectra corresponding tothe Mo foil subjected to bombardment until theO2`

Figure 3. Molybdenum 3d photoelectron spectra taken at normal(a) and 60¡ grazing emission (b) for a polycrystalline Mo foilexposed to a plasma of oxygen for 4 min (——–): (· · · · ·) speciesA; (- - - - -) species B ½C; (– – – –) Mo0.

( 1998 John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 26, 235È241 (1998)


Figure 4. Molybdenum 3d photoelectron spectra taken at normalemission for a polycrystalline Mo foil subjected to bombard-O

2½

ment at 298 K (——–) for the indicated period of time: (· · · · ·)species A; (- - - - -) species B ½C; (– – – –) Mo0.

steady state. As in the previous experiments withoxygen plasma, the formation of new chemical forms ofmolybdenum with higher binding energies than Mo0 isevident. Deconvolution of the experimental data indi-cates a small contribution of the component at the high-binding-energy side of the spectrum (species A). Therelative contribution of this component is practicallyconstant whatever the bombarding time. This is not thecase for the sum of species B and C, which increase inintensity as the bombardment proceeds. Remarkably, inthis type of experiment, species C is more intense thanspecies B. The presence of the latter is apparent by theshoulder corresponding to its component at 234.53d3@2eV. We would like to stress that, although Ðtting ofthese spectra with GaussianÈLorentzian curves is pos-sible, the result of the Ðtting was ambiguous because thewidth of elemental bands has to be changed as the O2`bombardment increases. This indicates a rather highcomplexity of the chemical environment around the Moions. Because of this, we have preferred the empiricalapproach described in the experimental section for theanalysis of the spectra. It is also interesting that theintensity ratio between species B and C is not modiÐedsubstantially in the spectra taken at grazing angles (notshown in Fig. 4). This fact indicates that both speciesare homogeneously distributed throughout the oxidizedlayer. A comparison of the Mo 3d spectra in Figs 2 and4 clearly shows that the type of oxidized species ofmolybdenum is di†erent in the two cases.

We expect that oxygen ions bonded to each of thesespecies should also be di†erent. This is shown in Fig. 5,where the x-ray-excited O KLL Auger spectra corre-sponding to the oxygen species formed on the Mo foilafter bombardment or exposure to the oxygenO2`plasma are depicted. From Fig. 5, the O KL2, 3L2, 3kinetic energies are 512.9 and 513.4 eV for the plasma-and the foil, respectively. The Ðrst value isO2`-treatedvery similar to that found in (upper curve). Simi-MoO3larly, binding energies of O 1s peaks (not shown) were530.2 and 529.7 eV for the plasma- and the O2`-treatedfoil, respectively. The former value (plasma-treated foil)is very similar to the value found for bulk MoO3 .Another interesting feature of the spectra in Fig. 5 is theshoulder at the high-kinetic-energy tail of the main

peak. This shoulder is more noticeable inKL2, 3L2, 3and in the plasma-treated foil than afterMoO3 O2`bombardment. The Ðrst-derivative spectra included inFig. 5 conÐrm this assessment clearly. In this Ðgure apeak at 521 eV is clearly visible in the spectra of MoO3and Mo0 foil treated with the plasma of oxygen. Verysimilar spectra could be obtained by using electrons asan excitation source in a conventional AES experiment.This shoulder is typical of compounds with an OÈMbond of considerable covalent character.36

The di†erent oxidation processes induced at roomtemperature by an oxygen plasma or a beam of oxygenions can be related to the ballistic e†ects induced by theaccelerated ions in the bombardment samples (i.e sput-tering, relocation, etc.) ; these processes are usuallyassociated with the interaction of high-energy ions withsolid surfaces.29 In trying to elucidate the incidence ofthese ballistic e†ects in our results, further bombard-ment experiments were carried out on the MoO3

Figure 5. (a) X-ray-induced O KLL Auger spectra, in integralform, of a polycrystalline Mo foil subjected to the indicated treat-ments. The spectrum of a sample is included for compari-MoO

3son. (b) First-derivative spectra that clearly show the featuresdiscussed. Vertical lines are included to guide the eyes.



Figure 6. Molybdenum 3d photoelectron spectra of an MoO3

sample subjected to Ar½ and bombardment up to the steadyO2½

state. The spectrum of is included for comparison: (· · · · ·)MoO3

species A; (- - - - -) species B ½C; (– – – –) Mo0.

sample. Figure 6 shows the Mo 3d spectra of an MoO3sample subjected to Ar` or bombardment until aO2`steady state was reached. Evidently, ion bombardmentleads to a partial reduction of the original MoO3sample in the two cases. According to Fig. 6 thisreduction involves the formation of Mo0 and species Band C, together with some contribution of speciesMo6`. The overall degree of reduction is higher withAr` than with However, it cannot be discardedO2`.that some amount of species B and C can be formed inthe latter case by re-oxidation of the original Mo0species due to exposure to the oxygen pressure requiredfor the ion gun.

DISCUSSION

As mentioned brieÑy in previous sections, the formationon an Mo foil of a thick oxide layer by means ofthermal oxidation requires high-temperature and high-pressure treatments. At room temperature and low pres-sures, it has been reported that the formation of up totwo monolayers (ML) of oxide requires very long timesof exposures.25 The present paper conÐrms the rela-tively inert character of Mo under similar conditions.Consequently, a Ðrst point to stress is that oxidation ofthe topmost layers of molybdenum is possible at roomtemperature and low pressure when using excitedspecies of oxygen (plasma or ions). The progressive oxi-dation of the Mo foil was monitored by AES. The

results in Fig. 1 show signiÐcant modiÐcations in thelineshape of the Mo MNN and NNN Auger transitions,while new peaks develop in the O KLL region (cf. Fig.5). According to the results of previous AESstudies,19,20 the spectra in these Ðgures can be taken ascorresponding to a partially oxidized Mo surface.Nevertheless, from the AES analysis only it is not easyto distinguish between the di†erent chemical forms ofMo present in each case.

X-ray photoelectron spectroscopy provides someinformation in this respect. Analysis of the spectra inFigs 2È4 is consistent with the assumption of three dif-ferent oxidized species of Mo (species A, B and C)besides Mo0. Comparing the spectra of Mo0, Mo6` in

(cf. Fig. 6) and previous data in the liter-MoO3ature,14h16,18,23,37,38 these species can be assigned asfollows : species A to Mo6`, species B to Mo4` andspecies C to Mon` (n \ 4). However, the di†erencebetween species B and C is not very clear. Two forms ofMo4` characterized by XPS14 have been reported inthe literature. The di†erence between their bindingenergy has been attributed to a di†erent coordinationstate of Mo4` and/or di†erent MoÈO distancesresulting from the rearrangement of an oxygen-defectivenetwork. In this respect, it is worth mentioning that inbulk molybdenum oxide, partial reduction/oxidationprocesses (i.e. Mo6`% Mo4`) occur easily throughshear plane mechanisms, so that di†erences in the dis-tribution of lattice de†ects in our case could contributeto the di†erences in binding energy and heterogeneity ofchemical species shown by the experimental results.However, the di†erence in binding energy found in ourcase between species B and C (i.e. D1.5 eV) is largerthan that reported for two types of Mo4` species inthese previous studies, so we think that between speciesB (with a binding energy typical of Mo4` in

and species C di†erences in atomicMoO2)14,15,18,42rearrangement of the MoÈO network and also in oxida-tion state might exist. Although not sufficiently clear atpresent, species C could be an Mo2` ion, as reportedpreviously for an experiment consisting of the thermaloxidation of Mo under low oxygen pressure38 or, ingeneral, Mon` species with n \ 4.

The experiment summarized in Fig. 3, correspondingto XPS spectra taken with normal and grazing angleson an Mo foil exposed to an oxygen plasma, providesadditional information about the in-depth distributionof species B and C. In Fig. 3, the relative contribution ofspecies C (and of course Mo0) is lower at grazing emis-sion angles. This indicates that species C is mainly con-centrated at the interface between the metal and theoxide overlayer. Scheme I of Fig. 7 crudely illustratesthe state of the Mo foil surface exposed to an oxygenplasma. The topmost layers may be formed by an inti-mate mixture of Mo6` and Mo4` species in a networkof oxygen ions. The thickness of this overlayer is D0.8j(j being the inelastic mean free path for electrons of1020 eV travelling in the oxide layer), as calculated fromthe intensity of the di†erent peaks in Fig. 3. In thesteady state [i.e. Fig. 2(c)] the thickness of the oxidelayer reaches a value of 1.2j and a very thin interlayerof D1È2 ML constituted by Mon` with n \ 4 (speciesC) seems to be formed between the outer oxide overlay-er and the metallic substrate. Thus, in the outer molyb-denum oxide layer the XPS results suggest the existence



Figure 7. Schematic representation of species distribution in anoxidized Mo foil after plasma treatment (scheme I) and bom-O

2½

bardment (Scheme II). More details in the text.

of an Mo4`ÈOÈMo6` distribution of ions, while at theinterlayer with the molybdenum metal the formation ofspecies of type Mon`ÈOÈ(Mo) (n \ 4) is expected. Thisspecies distribution model is conÐrmed by analysis ofthe results obtained upon bombardment (cf. Fig.O2`4). In this case the Mo4` and Mon` (n \ 4) ions are themain species in the steady state and the oxide overlayernetwork should be formed by structures of the typeMo4`ÈOÈMon` (n \ 4), plus Mo0 and, eventually,Mo6`. Accordingly, the relative concentration ofspecies C is higher than that of species B, as Fig. 4shows. Compared to the oxygen plasma experiment,bombardment with ions is expected to produceO2`considerable mixing of species in the modiÐed layer,therefore no stratiÐed structure of the type shown inscheme I of Fig. 7 is expected, but rather a structuresuch as that depicted in scheme II.

The e†ects of bombarding an Mo target with O2ìons of 3.5 keV energy are twofold : chemical and ballis-tic. The latter refer to the changes undergone by a solidtarget subjected to ion bombardment (atom displace-ment, sputtering, redeposition, etc.). Among them, pref-erential sputtering28,29 may lead to reduction of oxidetargets. Taking into account that ions are activat-O2èd oxygen species, the oxidation of Mo0 to a mixture ofMo4` and Mo6` should be expected, as in the case ofoxygen plasma. However, Mon` (n \ 4) constitutes themain species obtained upon ion bombardment.O2`From the point of view of mechanism, the experimentswith subjected to Ar` and bombardmentMoO3 O2`(cf. Fig. 6) provide a clue to this behaviour. In fact,Mo6` reduces to Mon` (n \ 4) and Mo0 upon bom-bardment either with Ar` or Reduction ofO2`. MoO3by Ar` bombardment has been reported in the liter-ature.30,39 The striking result in the present experimentis that a similar e†ect is found upon bombard-O2`ment. This fact can be rationalized by assuming the

existence of ballistic e†ects leading to the preferentialremoval of oxygen, which should be responsible of the

target reduction. In Mo0 the initial situation isMoO3slightly di†erent because no oxidized Mo is present.Consequently, in a Ðrst step, some oxidation of thetarget should occur. This oxidation of the metal targetmust be produced by the implantation oxygen atomsand the formation of some type of MoO3ÈMoO

xmixture. Such oxidation by implantation is likely tooccur through the formation and aggregation of defectclusters in the lattice (i.e. from which higherOÈMo

m),

oxides of molybdenum would likely grow. Simulta-neously, because of the above-mentioned ballistice†ects, molybdenum oxide is further reduced to MoO

xand, very likely, even to Mo0, although the latterprocess cannot be proved in this experiment because ofthe presence of Mo0 from the substrate.

The above-mentioned model, which attributes speciesB to Mo4` and C to Mon` (n \ 4), is consistent withthe di†erences in covalent character found for oxygenions of the overlayer when obtained by plasma treat-ment or bombardment. According to the AugerO2`spectra in Fig. 5 and the measured O 1s binding energyvalues, oxygen atoms in the oxide overlayer producedby plasma are more covalent than those in the overlayerproduced by bombardment. Assuming a similar Augerparameter in both cases, two facts support this conclu-sion : the higher binding energy of the O 1s peak for theplasma-treated sample, and the more intense shoulderat the high-kinetic-energy side of the peak.36KL2, 3L2, 3According to the Auger and O 1s photoelectron spectra,the covalent character of the oxygen atoms formedduring the plasma treated sample is very similar innature to that existing in (cf. Fig. 5). This is con-MoO3sistent with the high concentration of oxygen ionsbound to high oxidation states of Mo (i.e. Mo6` inspecies A and eventually Mo4` in species B, species Cbeing the minority under these conditions).

CONCLUSIONS

Oxidation of the surface of a polycrystalline Mo foil atroom temperature and low pressures can be performedusing reactive species such as oxygen plasma or low-energy ions. Nevertheless, the thickness of theO2òxide layers obtained is relatively small (1.2j) and thechemical composition involves Mon` (n O 4) and Mo6`species. Depending on the method employed, the actualsurface composition varies. With oxygen plasma, experi-mental data suggest that the surface oxide consists oftwo layers : an outer one formed by an intimate mixtureof Mo4` and Mo6` species immersed in a homoge-neous lattice of oxygen ions (ÈMo4`ÈO2~ÈMo6`È) ;and another, very thin, in direct contact with the metalsurface consisting mainly of Mon` (n \ 4). In the case of

ion bombardment, the oxide layer consists mainlyO2òf Mon` (n \ 4) species, probably produced by ballistic-related phenomena that cause the reduction of theMo6` species. In fact, the bombardment of withMoO3Ar` or ions induces similar reduction e†ects, theO2`degree of reduction being higher with Ar` than withO2`.



Acknowledgements

The authors gratefully acknowledge Ðnancial support from the Argen-tine and Spain Research Councils (CONICET and CSIC), as well asthe gift of equipment from the International Program for Physical

Sciences (IPPS, Sweden) and the Volkswagenwerk and von HumboldtFoundations (Germany). L.D.L-C is grateful to the InternationalCentre for Theoretical Physics (ICTP, Trieste, Italy) for Ðnancialassistance. Thanks are also given to the Spanish CICYT (project nosMAT94-1039-C02-01 and MAT97-0689) for Ðnancial support.

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Oxidation of molybdenum surfaces by reactive oxygen plasma and O2+ bombardment: an auger and XPS study