AES study of room temperature oxygen interaction with near noble metal-silicon compound surfaces

Surface Soence 161 (1985) 1-11 North-Holland, Amsterdam

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AES STUDY OF ROOM TEMPERATURE OXYGEN INTERACTION WITH NEAR NOBLE METAL-SILICON COMPOUND SURFACES S. V A L E R I , U. D E L P E N N I N O , P. L O M E L L I N I a n d G. O T T A V I A N I Dlpartlmento dt Ftswa, Unwerslth dl Modena, Via Campz 213/.4, 1-41100 Modena, Italy

Received 20 February 1985, accepted for pubhcatlon 5 June 1985

We have camed out a comparatwe study of room temperature (RT) oxidation of near noble metal slhcldes slrmlar in StOlCtuometry (M2S1) and electromc structure. Core-valence-valence (CVV) Auger line measurements on N12S1, PdES1 and Pt2S1 surfaces before and after exposure to 104 L of oxygen were performed We compare the results with those for the oxadatlon of pure Sl In general oxygen interacts w~th SI atoms only In NI 2S1, however, features ascnbed to the onset of oxidation of N1 atoms appear m the NI(MVV) hne. In the N12S1 and Pd2SI case, the SI reaction rate is increased with respect to that of pure SI, the strongest oxidation enhancement being obtained in NIES1. However, S1 cannot be oxadlzed to SIO2 m these conditions, a SIOx (x < 2) phase is formed. The PtESl oxadatlon behavlour is close to that of pure SL The different catalytic effect of N1, Pd and Pt on Sx oxidation has been discussed We conclude that the mare effect of metal is to by-pass the klneUc bottleneck of pure Sl oxidation, Le the dIssocmtlon of the 02 molecule at the slhcide/gas interface The effect of different exposure procedures was also underhned.

1. Introduction The R T oxygen c h e m i s o r p t i o n a n d oxide f o r m a t i o n o n slhcides a n d S i - m e t a l i n t e r m i x e d phases has b e e n studied in a variety of experiments [1-10] a n d some i m p o r t a n t aspects, such as the Si preferred oxidation a n d the e n h a n c e m e n t of Si o x i d a t i o n with respect to pure Si, have been p o i n t e d out. Recently, the role of the Si-to-metal ratio a n d of the oxygen exposure m d e t e r m i n i n g the o x i d a t i o n kinetics a n d the c o m p o s i t i o n of the resulting surface oxide layer was investigated i n detail for the N i c o m p o u n d s [8,9]. I n these works, silicides a n d S i - m e t a l i n t e r m i x e d phases involving a variety of metals (e.g. A u [3,4,5], Ag [4], Cu [4], N i [8,9], Pd [1,2,4,6,7], Co [10]) have b e e n considered. I n spite of a generally similar behavlour, some differences can be identified, m a i n l y in the $1 oxidation i n t e n s i t y a n d in the c o m p o s i t i o n of the Si oxide layer. However, a realistic a n d q u a n t i t a t i v e c o m p a r i s o n b e t w e e n the results o n different S i - m e t a l systems c a n n o t be done, due to the difference in sample p r e p a r a t i o n , Si-to-metal ratio a n d o x i d a t i o n procedure 0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Dwis~on)

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Oxygen and near noble metal-sthcon compounds

A comparative study of the effect of different metals has been performed by oxidizing intermixed layers prepared by evaporating noble metals (Cu, Ag, Au) onto a $1 substrate and choosing the metal coverage so as to obtain a Si-to-metal ratio approximately equal to 1 [4]. In this case, however, due to the very thin layer investigated, the oxidation process is not controlled by the reaction at the S l - m e t a l / O 2 interface only, since even the reaction between the intermixed layer and the S1 substrate plays an important role. Indeed the stronger oxadatlon enhancement in Si-Cu than in other noble metal systems has been explained m terms of a greater energy available to disrupt the Si surface, thus making the $1 supply (and consequently the oxide growth) more efficient. It seems therefore timely to perform a comparative study of the RT oxldat~on of thick sihcides, in order to clarify the role of the different metals in the chemical reaction which take place at the interface between the sfliclde and the 0 2 atmosphere. To this end we have done a detailed experimental investigation of the oxidation m near-noble metal (Ni, Pd, Pt) sillc~des with the M2Si stoichiometry. We have chosen these materials for two reasons. First their electronic properties have been investigated throughly both from the experimental and from the theoretical viewpoint [11-13]. This fact can be of great help in understanding the modifications which occur during oxidation. Secondly these near noble metal sllicides are similar in their electronic structure. They have nearly filled d-bands and show a significant hybridization wxth Si p-states [12]. It ~s therefore expected that the microscopical mechanisms of the oxidation are the same in the three silicides and that any difference in their behavlour reflect a change in the interaction between oxygen and metal atoms belonging to different transition metal series. The U2S1 stoichiometry was chosen to put m evidence the Si oxidation enhancement (higher m metal-rich sllicldes [9]) thus making easier a comparative study. Our investigation was carried out using Auger Electron Spectroscopy (AES). This technique, being very sensitive to the surface, allows the early oxidation stages to be studied. Particularly fruitful is the analysis of the lines corresponding to transitions that involve valence electrons, like the Si(LVV) transition. As ~t is well known [14-16] these lines are very sensitive to the local chemical environment and can give useful information on the chemical species formed under oxygen exposure.

2. Experimental Surfaces of conventionally grown and characterized (Rutherford backscattering and X-ray diffraction) thick silicide films (3000 A) were cleaned in situ by Ar ÷ sputtering. It is well known that sputtering may induce modifica-

S Valert et al / Oxygen and near noble metal-slhcon compounds

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tion in the surface composition. The sputter cleaned surfaces can be charactenzed in stoichiometry by analysing the Si and metal Auger lines with the elemental standard method, which leads to a relation between the Auger signal intensity and the atomic concentration. The surface composition can be determined by this method with an accuracy of _+5%. In addition, information on the sputter induced modifications of the chemical bonding can be provided by core-valence-valence Auger lineshape analysis. It was found that a sputtering energy of about 0.7 keV preserves the correct stoichiometry of the NI2Si surface [17]. The PtSi surface sputtered at about 5 keV exhibits PtzSi-like stoichiometry and chemical bonding, over a depth of at least 30 ,~ [18]. The Pd2S1 surface is essentially unaltered by the sputtering process [15]. For comparison, a sputter cleaned Si film (2000 A) was also investigated. The use of polycrystalline instead of monocrystalline surfaces is not strictly ~mportant in chemisorption experiments, at least to understand the big features and not second order details [19]. The silicides and slhcon samples were simultaneously exposed at RT to an oxygen pressure of 5 × 10 - 6 Torr for 2000 s, which corresponds to an oxygen dose of 10 4 L (1 L = 106 Torr s). In order to have a minimum of excited oxygen the ion-gauge hot filament and the electron beam were turned off during oxygen exposure, and simultaneously the ion pump was valved off from the chamber. AES measurements were m a d e in an UHV chamber equipped for surface analysis (base pressure: 2 × 10-10 Torr). Spectra were recorded in the standard dN(E)/dE mode with a single pass CMA (energy resolution AE/E = 0.3%). The exciting source was an electron beam, Ep = 3 keV and Ip = 2/~A, coaxial with the CMA and at 60 ° with respect to the surface normal. We previously checked (monitoring the O(KLL) peak-to-peak height (Hpp) in all the investigated samples after an exposure of 5 L) that the maximum electron dose we used to record Auger spectra ( < 1019 electrons/cm 2) does not affect the chemisorbed layer. The systematics of the following Auger transitions was digitally recorded before and after oxygen exposure: Pt(64 eV), Ni(61 eV), Pd(43 eV), Pd(330 eV) and Si(92 eV) (2 gpp modulation) and 0(508 eV) (6 Vpp). All spectra were integrated and the background was subtracted with a spline method [20]. 3. Results

As already outlined [9] the O(KLL) Hpp is a good indication of the amount of chemlsorbed oxygen because the big features in the shape of this line remain essentially unmodified in a wide coverage range. The O(KLL) Auger line Hpp for the 10 4 L exposed silicides and Si are reported in table 1. In slicides there is an increase in the oxygen uptake with respect to Si: this enhancement is very slight in Pt2Si and more evident in Ni2Si and Pd2Si.

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S Valert et al / Oxygen and near noble metal-slhcon compounds

Table 1 Slhcides and pure Sa after 10 4 L oxygen exposure" O(KLL) mtensmes (Hop) normahzed to the value on NI2SI, percentages (Pox) of the SKLVV) sxgnal coming from oxadlzed $1 atoms, as evaluated by eq (1), and thackness X of the surface layer affected by the oxygen mteractaon

Nl2S1 Pd2S1 Pt2St S1

Hpp O(KLL) (au)

Pox (%)

X (A)

100 60 31 27

60 49 33 20

4 5 (_+0.5) 2 0 (±0.5) 1.2 (±0.5) 1 5 ( ± 0.5) a)

(+5) (+5) (±5) ( ± 5) a)

a) Tentative evaluation, see text

The Si(LVV) normalized spectra before and after oxygen exposure are shown in the N(E) form for NlzSi , PdzSi , Pt2Si and Si (fig. 1). In Ni2Si and Pd 2&, the Si(LVV) lineshape is strongly modified by oxygen exposure, with its I

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Fig. 2. Differencecurves between SI(LVV)Auger hnes in clean and oxidized slhclde spectra, the scahng factor has been chosen to have no contribution m the KE region around 88-89 eV. centre of gravity shifted toward lower kinetic energies (KE); this effect is less evident in Pt2Si and Si. The Si(LVV) line, in the N ( E ) form, has its maximum at about 89 eV in Si (87-88 eV in silicldes) and at about 74 eV in SiO2: the growth of structures in the intermediate region is indicative of the formation of Si suboxldes (Si20, SiO, Si203) [21,22]. The Si(LVV) difference spectra between the oxidized and clean silicides, reported in fig. 2 show several structures in the 70-85 eV range of KE, indicating that the formed oxide layer is a mixture of suboxides and SiO 2, conventionally named SiOx (x < 2). The main features appear at 76 eV for Ni2Si, 77 eV for Pd2Si, 80 eV for Pt2Si and, not reported here, at 81 eV for Si. The larger shift toward low K E of the Si(LVV) difference spectra suggests an higher coordination of Si atoms with oxygen ones (x.e. a Si oxidation state closere to SiO:) in N12Si and Pd2Si with respect to Pt 2Si and pure Si. It is noteworthy that the Si involvement in oxygen bonding in NieSi at 10 4 L, reflected in the Si(LVV) lineshape modification (fig. 3a) is higher than that reported previously [9], due to the different oxidation procedure ("abrupt" in the present work, and not "step by step"). This result is consistent with the findings of ref. [19] about pure Si RT oxidation behaviour. The effect of different exposure procedures is reflected even in the Ni(MVV) line, shown in fig. 3b in the derivative mode to put m evidence small changes in the lineshape. The structure at 48 eV and the dip at 40 eV appearing after

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Fig 3 Normalized SKLVV) (a) and NI(MW) (b) Auger hnes m N12S1, clean ( ) and oxadized with two different procedures abrupt exposure (-- -- --) (ttus work) and step by step exposure ( . . . . . . ) [9] adsorption are fingerprints of the initial state of N1 oxidation [8,9] and suggest an involvement of Ni atoms in the oxygen bonds. In spite of the slightly stronger line modification as a consequence of the abrupt exposure procedure, the Ni oxidation still results m a second order effect with respect to $1 oxidation in Ni2Si at RT. The metal Auger lineshapes in Pd2S1 and Pt2Si (not reported here) do not show any change and only attenuate after oxygen exposure. This is not sufficient to rule out an involvement of metal atoms in oxygen bonding, because in general the RT oxygen exposure does not affect appreciably the Auger lineshapes even in Pd and Pt metallic films due to the very weak oxygen-metal interaction [23-25] (we checked this on Pt and Pd polycrystalline samples). However, it is enough to suggest the chemisorbed oxygen to be mainly, or completely, Si-bonded [26]. The O(KLL) line shape can give important chemical information [27,28]. When oxygen atoms are close to a neon-like electronic configuration, O 2-, the spin-orbit coupling theory predicts five main features (apart from loss ones) which are related to different final states. If oxygen atoms are not close to such a configuration only three main peaks arise. We report in fig. 4 the d N ( E ) / d E form of O(KLL) for the investigated silicides. The five peaks expected for the neon-like configuration are labelled. We can see in fig. 4 taht the oxygen state has evolved toward 0 2- m N12Si and Pd2Si: four structures are well evident, the fifth (between 3p and 1D) is difficult to be observed even in bulk oxides, since the energy difference between the two 2s22p 4 states is small [27]. In Pt2S1, the structure 3p is inappreciable and the oxygen state is rather close to O.

S Valert et a l / Oxygen and near noble metal-sffwon compounds

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Auger spectra give also quantitative information. After oxygen exposure the main S I ( L V V ) feature of clean silicides (at about 88 eV KE) is reduced for two reasons: (i) part of the Si atoms are oxidized and their Auger contribution falls m a different spectral region; (ii) unoxidized $1 atom Auger emission is attenuated by the Si-O growing overlayer. An estimate of point (ii) is possible by the attenuation of metal Auger lines with low KE (i.e. with escape depth similar to the one of Si(LVV) Auger electrons). Therefore the percentage of Si(LVV) signal coming from oxidized Si atoms, Pox, can be given by [9]: Pox = 10011 - (I~JI~,)(ICet/I°et)] ,

(1)

where I~, is the Si(LVV) intensity in the clean sample, I~, the Si(LVV) intensity coming from unoxidized Si atoms (at about 88 eV) in the oxidized sample, I~et/I°et is the "attenuation factor" (I~et and I°~t are the intensities of the metal Auger line in clean and oxidized sample respectively). For pure Si, Pox can be tentatively evaluated using the attenuation factor of Pt2Si, in view of the similarity in the O(KLL) Hvp. The Pox values obtained via eq. (1) are reported in table 1. Using these values it is possible to calculate the thickness,

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Oxygen and near noble metal-slhcon compounds

X, of the surface layer affected by the oxygen interaction, as discussed in detail elsewhere [9]. We found 1.5 A in Si, 1.2 A in Pt2SI, 2.0 ,~ m PdzSi and 4.5 A in NizSI, with an estimated uncertainty of _+0.5 A. To summarize the experimental results: the oxygen uptake, the involvement of Si and O in chemical bonding and the oxide layer thickness are enhanced in near-noble metal silicides M2Si with respect to pure Si. The enhancement is very weak in Pt2Si and increases going from PdzSi to N12SI.

4. Discussion Thermodynamics gives a first qualitative explanation of the big features arising from experiments: heat of formation ( - AHf) of SIO 2 greater than that of near-noble metal oxides accounts for the preferential Si oxidation in silicides [6]. However, due to the importance of kinetic factors [29,30], it is not always possible to reach the final state predicted by thermodynamics. The role of different metals on the enhanced Si oxidation cannot be understood simply on the basis of thermodynamics. For Pd sllicides [6] and for C u - S i and P d - S i interrmxed phases [4] it was proposed that the driving force of the Si oxidation is proportional to the difference between - A H f (SiO2) and - A H r (silicides). Though there are controversial values of - A H f (sihcides) in the literature [31,32], in this approach we expect a SI oxidation enhancement in the sequence Ni2Si > Pt2Si > PdzSi. This trend is in disagreement with experimental results (N12S1 > Pd2S1 > Pt2Si) so that a dominant role of the - A H f balance should be ruled out. The breaking of the strong covalent SI sp 3 configuration (going from pure Si to silicides) was also invoked [3,4]. The metallic environment of Si atoms gives weaker, and easier to be broken, bonds, and thus an increased tendency to go toward the tetrahedral configuration of SiO 2 (Si atom coordinated with four oxygen atoms). In Ni sihcldes there is a larger covalent contribution to the chemical bonding with respect to Pd and Pt sihcldes [15] and, in this scheme, we expect Ni silicides to be less reactive, just the opposite of our findings. The formation of active adsorbate species at the gas/solid interface was found to be a rate limiting step for the R T S1 oxidation [21]. This suggests that a possible effect of the metal is the activation (i.e. the dissociation) of the 02 molecule. The tendency to dissociate dlatomic molecules increases, going above or to the left of Pt in the periodic table part of transition metals [18,33]. However the O 2 molecule is atomically chemisorbed at R T on Pt as on Pd and on Ni [24,25,29] so that the different catalytic activity of Ni, Pd and Pt should be related to the sticking probability, S, of oxygen on these metals. S ~s reported to be very different for Ni (S = 1), Pd (S = 0.3-0.4) and Pt (S < 0.1) [24,25,29]. The trend in these values is in agreement with our results and leads us to conclude that the main mechanism to catalyze the Si oxidation at R T is

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Table 2 Energy position of the "centre of gravity" of the d-band density of states m N12S1, Pd2S1 and Pt2Sl [37,38] with respect to the vacuum level (C), and its energy separation from the O2p level of O (E, =15 1 eV [361)

N12S1 Pd2S1 Pt2S1

C (eV)

[Ea-C [ (eV)

6.2 80 90

89 7.1 6.1

the dissociation of 02 at metal sites and the migration of atomic, more reactive, oxygen at Si atoms sites. Furthermore, the activation energy of oxygen atom migration from metal to sihcon sites was suggested to be very low [10]. In tlus scheme the main effect of the metal is to by-pass the kinetic bottleneck of pure SI oxidation, 1.e. the dissociation of O 2 at the surface/gas mterface. Of course the electronic structure of the metal in the Slllcides is not the same as in the pure metal and a further step is necessary in the discussion. A model was proposed for atoms and diatonuc molecules chemisorption on transition metals [33-35], in which the most important parameters were found to be the centre of gravity C of the d-band density of states and the energy E a (Em) of the first empty or partially filled level of the adatom (admolecule) (e.g. the 2p level of O and the 2~r antibonding level of O2). Increasing values of lEa - C I produce increasing heats of atomic chemisorption, due to a stronger charge transfer from d-band to the unfilled atomic orbitals. On the other hand heats of molecular chermsorption are rather insensitive to I E m - C I . In this model one expects the greater the value of l E a - C I the greater the tendency to dissociate. In the present case E a is the O 2p level of O, which hes about 15 eV below the vacuum level [36]. Experimental d-band density of states can be obtained by XPS, when the s - p photoionization cross-section is negligible with respect to the one of d-electrons. N12Si , PdzSi and Pt2Si d-band density of states is reported in refs. [37] and [38]. The C values normalized to the vacuum level are shown in table 2, together with the I Ea - C I values. We have a well defined trend because C approaches E a on going from NizSi to Pt2Si. This suggests again that the activity of the metal atoms (inside the s l h e M e ) to dissociate 02 follows the trend we found in the reactivity of the silicides. 5. Conclusions We have presented the results of a comparative investigation of oxygen interaction with several near noble silicides (M2Si) surfaces at RT. We have

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Oxygen and near noble metal-sthcon compounds

p r o v i d e d clear evidence that silicides of metals b e l o n g i n g to different t r a n s i t i o n m e t a l series exhibit different o x i d a t i o n behaviour, in spite of their similarity in s t o i c h i o m e t r y a n d electronic structure. Several o x i d a t i o n features (oxygen uptake, thickness of the f o r m e d o x i d e layer, o x i d a t i o n state of the affected Si a t o m s ) are e n h a n c e d in silicides with r e s p e c t to p u r e Si, in the sequence N i 2 S i > Pd2S1 >> P t z S i >/S1. This t r e n d c a n n o t b e u n d e r s t o o d s i m p l y in the t h e r m o d y n a m i c scheme, neither referred to the different " c o v a l e n t degree" of the $1 chemical b o n d i n g in sllicides of different metals. W e can ascribe the silicide o x i d a t i o n b e h a v i o u r to the c a p a b i l i t y of different m e t a l a t o m s to b r e a k the 0 2 molecule a n d to retain at the slllclde surface atomic, m o r e reactive oxygen. W e have also shown that in s i l i c o n - o x y g e n i n t e r a c t i o n e x p e r i m e n t s the e x p o s u r e p r o c e d u r e is a crucial p a r a m e t e r . It w o u l d b e interesting to investig a t e c o m p a r a t i v e l y the sllicide o x i d a t i o n in a wide t e m p e r a t u r e range. A s t u d y of this sort c o u l d p r o v i d e a d d i t i o n a l i n f o r m a t i o n to clarify the relevance a n d role of kinetic factors.

Acknowledgements T h e a u t h o r s w o u l d like to t h a n k C. C a l a n d r a a n d C. M a r i a n for critical r e a d i n g of the m a n u s c r i p t a n d for interesting discussions, a n d G. M a j n i a n d P. Sassaroll for the s a m p l e p r e p a r a t i o n . C o m p u t a t i o n a l s u p p o r t b y C e n t r o dl Calcolo, U n i v e r s i t h di M o d e n a , is gratefully a c k n o w l e d g e d . This w o r k was s u p p o r t e d b y G r u p p o N a z l o n a l e delle R i c e r c h e ( C N R ) , Italy.

References [1] L Ley and J D Raley,m. Proc 7th Intern Vacuum Congr. and 3rd Intern Conf. on Solid Surfaces, Vienna, 1977, p. 2031 [2] S D Bader, L Richter, M.B Brodsky, W E. Brower and G V Smith, Solid State Commun 37 (1981) 729. [3] A Cros, J Derrlen and F Salvan, Surface Scl. 110 (1981) 471 [4] I Abbati, G Ross1, L. Calharl, L Bralcovlch, I Llndau and W E Splcer, J. Vacuum Scl Technol 21 (1982) 1409 [5] J. Derrlen and F. Rmgelsen, Surface Scx 124 (1983) L35 [6] A Cros, R.A Pollak and K.N Tu, Thin Solid Films 104 (1983) 221 [7] R Butz and H. Wagner, J. Vacuum SCL Technol B1 (1983) 816 [8] S Valerl, U. del Pennmo and P Sassaroh, Surface ScL 134 (1983) L537. [9] S. Valerl, U del Penmno, P Lomelhm and P. Sassaroh, Surface SCL 145 (1984) 371 [10] G Castro, J.E. Hulse, J. Kuppers and A. Rodnguez Gonzalez-Ehpe, Surface SCL 117 (1982) 621 [11] G. Rubloff, Surface Scl 132 (1983) 268 [12] O Blsxand C Calandra, J Phys. C (Sohd State Phys.) 14 (1981) 5479. [13] C Calandra, O. Blsl and G Ottavmm, Surface Scl Rept. 4 (1984) 271

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