J. Phys. Chem. 1991,95,4783-4787

Nickel Silicide Formation and Dissociative Adsorption of Carbon Monoxide on NISI( 11 1)

1. Introduction

4783

Studied by UPS and XPS

Tetsuo Asakawa: Katsumi Tanaka,*** and Isamu Toyoshimas Catalysis Research Center, Hokkaido University, Sapporo, 060 Japan (Received: July 2, 1990)

The mechanism of nickel silicide formation was elucidated by adsorption of carbon monoxide (CO), using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Nickel was deposited on Si( 11 1) at room temperature and liquid N2 temperature, and the coverage of Ni(BNi) was determined by three different methods. On the surface at 8Ni I 2, adsorbed CO showed UPS spectra at 11.6 eV (4a) and 8.3 eV ( I T + 5a) below the Fermi level at 120 K. The latter peak shifted to 7.8 eV at 298 K, and additionally a new peak appeared at 6 eV. These results indicate that dissociative adsorption of CO occurs on the surface at 298 K. The cause of CO dissociation was considered on the surface. At BNi = 5, molecularly adsorbed CO desorbed at around 273 K. While on the surface at 8Ni L 10, CO adsorbed molecularly at 298 K. When the surface was heated once at 420 K, it lost the ability to adsorb CO at 298 K. These results indicate that nickel silicide is formed on Ni/Si(l11) surfaces with BNi = 5 at 273 K and with 6Ni 1 10 at 420 K. In conclusion, the mechanism of nickel silicide formation on Si( 11 1) can be summarized as follows. At BNi I 2, Ni deposits on clean Si( 11 1) without intermixing with Si, on which CO dissociates at 298 K. At BNi = 5 the Ni-Si intermixing takes place and nickel silicide is formed at 298 K, while at e N i 1 10 deposited Ni layers cover Si( 11 1) surface and they prevail over the nickel silicide formation at 298 K.

The mechanism of metal silicide formation is one of the sig- nificant subjects for understanding metalsemiconductor interfaces as well as for developing industrial applications of silicon device technology. The interface between nickel silicide and silicon has been intensively studied for understanding the relationship between electrical properties such as Schottky barrier height and the physical properties of the interface such as atomic structure or defects.l-' However, the initial stage of nickel silicide formation at 298 K still remains unsolved. A photoemission study has shown that the first monolayer of Ni on cleaved Si(l11) surfaces forms a compound markedly dissimilar to that formed by second stage of Ni depos i t i~n .~ On the other hand, a cluster growth model has been proposed on the basis of ion backscattering data?" These controversial interpretations are caused by the difficulty in picking up the chemical states of Ni or Si atoms in the first monolayer of the Ni/Si system. The presence of nickel silicide can be concluded by detecting an intermixing structure of Ni and Si with electron micro~copy '*~*~ or ion ~ c a t t e r i n g ~ . ~ and by measuring electronic properties of Si and Ni with AES,S,6 XPS? and UPS? However, the Ni amount is too small to detect in the intermixing layer at the initial stage of nickel silicide formation (ONi I 2). A LEED measurement suggested that Ni metals deposited on Si surfaces do not constitute a long-range ordered structure but an amorphous one.1° The appreciable binding energy (BE) shifts of the Ni 2p or 3d level were observed as a function of Ni coverage on Si.* However, these peak shifts are influenced not only by a compound formation but also by a metal cluster size. In general, a change of particle size and final relaxation process of pho- toemission with Ni coverage cause the peak shifts."-" In our previous experiment, the Ni 2p peak shifted (1.2 eV) on SO2/ Si( 1 11) with increase of Ni coverage, in which silicide formation did not 0 ~ c u r . I ~ The AES peak shape change of Si(LVV) is considerably too small to evaluate a nickel silicide formation in the initial stage of the deposition (6Ni I 2).5*6

In the present paper, we (i) studied the adsorption of CO in the initial stage of nickel deposition on clean Si( 1 11) in order to elucidate the mechanism of nickel silicide formation, because the CO adsorption is considered quite sensitive to a chemical state

To whom all correspondence should be addressed. 'Present address: Tosoh Co., Ltd., 1-8 Kasumi, Yokkaichi Mie, 510

Japan. $Resent address: Department of Electronics Engineering, The University

of Electro-Communications, 1-5-1 Chufugaouka Chofu, Tokyo, 182 Japan. 'Present address: Marubun Corporation, Dohkoh Building, 3-3-4 Mi-

namisuna, Kohtoh-ku Tokyo 136, Japan.

of substrate top layer, and (ii) found that C O dissociation a t 298 K on Ni/Si( 1 1 1) at ONi I 2 was probably caused by the interaction between CO molecule and the surface atom through oxygen, which is quite peculiar since CO molecules are known to be molecularly adsorbed on Ni metal a t 298 K.

2. Experimental Section In this study, a VG ESCA-3 Mark I1 photoelectron spectrom-

eter was used. Spectra of XPS and UPS were recorded by using a VG dual-anode X-ray source operating the Mg anode (hv = 1253.6 eV) and a He I1 (40.8 eV) UV source under a background pressure less than 1 X Torr. n- and p-type Si(l11) single crystals doped with P or B to the extent of 1 X 10l6 atoms/cm3 Si (purchased from Komatsu Denshi Kinzoku, Japan, and the specific resistivities of n- and p-type crystals are 4500 and 16 D cm, respectively) were treated in the preparation chamber. Carbon and oxygen impurities on a Si( 1 11) were removed by repeated Ar ion sputtering a t 298 K and successively the sample was annealed a t 1200 K. Ni was resistively heated to deposit on Si( 11 1) with a constant deposition rate a t 298 K or at liquid N2 temperature. Hereafter, a Si( 11 1) on which Ni was deposited is denoted as Ni/Si( 11 1). In our experimental condition, the layer-by-layer growth model in general evaluated by Auger in- tensities cannot be applied due to silicide formation. consequently, the amount of deposited Ni represented as Ni coverage 6Ni was apparent and was evaluated by the increase of Ni 2p intensity, decrease of Si 2p intensity, and a relative intensity of Ni 2p and Si 2p signals.

(1 ) Tung, R. T.; Gibson, J. M.; Poate, J. M. Phys. Rev. brr. 1983,50,429. (2) Tung, R. T. Phys. Rev. Leu. 1984, 54, 461. (3) Tung, R. T.; Gibson, J. M. J . VUC. Scf. Technol. 1985, A3, 987. (4) Liehr, M.; Schmid. P. E.; LeGoues, F. K.; Ho, P. S. Phys. Rev. Letr.

1985, 54, 2139. (5) Kobayashi, K. L. 1.; Sugaki, S.; Ishizuka, A.; Shiraki, Y.; Daimon, H.;

Murata, Y. Phys. Rev. B. 1982, 25, 1377. (6) van Locnen. E. J.; van der Veen, J. F.; Le Goua, K. L. Surf. Sci. 1985.

157, 1 . ( 7 ) Fischer, A. E. M. J.; Maree, P. M. J.; van der Veen, J. F. Appl. Sur/.

Sci. 1986, 27, 143. ( 8 ) Grunthaner, P. J.; Grunthaner, F. J.; Madhukar, A.; Mayer, J. M. J .

Vac. Sci. Technol. 1981, 19, 649. (9) Franciosi, A.; Weaver, J. W.; ONeill, D. G.; Chabal, Y.; Rowe, J. E.;

Poate, J. M.; Bisi, 0.; Calandra, C. J . VUC. Sei. Technol. 1982, 21, 624. (IO) Clabes, J. G. Surf. 1984, 145, 87. ( I I ) Baetzold, R. C.; Mason, M. G.; Hamilton, J. F. J . Chem. Phys. 1980,

72, 366, 6820. (12) Mason, M. G. Phys. Rev. B. 1983, 27, 748. (13) Egelhoff, F. W., Jr.; Tibetts, G. G. Phys. Rev. B. 1979, 19. 5029. (14) Asakawa, T.; Tanaka, K.; Toyoshima, I. Lungmuir 1988, I , 521.

0022-3654191 l2095-4783S02.50JO I , , 0 1991 American Chemical Society

4784 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 Asakawa et al.

CO 3000L -'A co iOOOL = c o 100-

1 l I I I I I I

282 284 286 288 Binding encrgy /eV Binding energy /eV

Figure 1. C Is and 0 Is peaks on Ni/Si(l11) at e,.,, = 2 exposed to 1 X IO2, 1 X lo3, and 3 X lo3 langmuirs of CO at 298 K.

From the increase of Ni 2p intensity, eNi is estimated as fol- lows:'5

dNi = d / d m = -0 .74x~ i hl {(Ib - I)/Ib)/2.2

Here I, Ib, XN~, and d, are the Ni 2p3(2 intensity of deposited Ni, that of bulk Ni, mean free path of Ni 2p, and mean thickness of Ni monolayer, respectively. We estimate XNi = 12.8 A and d , = 2.2 A.

With respect to decrease of Si 2p intensityi5 6Ni = d / d m = -0.74Xsi In (I/I,J/2.2

Here I,, and I are Si 2p intensities before and after Ni deposition, and we estimate Xsi = 24 A.

Considering the relative intensity of Ni 2p to Si 2p spectrum, the monolayer of Ni gives the value 1.35 by assuming the relative atomic sensitivity factor (ASF) of Ni 2pjlZ and Si 2p, ASFNizh,,/ASFsi2p, being 24.16 High Ni coverage was estimated from deposition time under the same condition.

Adsorption of CO was evaluated by monitoring the lr + 5a and 4u levels on UPS spectra or C 1s and 0 1s on XPS spectra. Binding energies (BE) of Ni 2p, 0 Is, and C 1s were all referred to the BE of Si 2p (99.5 eV).

3. Results 3.1. CO Adsorption on Ni/Si( 1 1 I ) . 3.1 . l a At Low Ni Cou-

erage, 8Ni I 2. Carbon Is and oxygen 1s spectra were recorded when a Ni/Si( 11 1) at BNi = 2 was exposed to 1 X lo2, 1 X lo3, and 3 X l@ langmuirs of CO at 298 K. Unless otherwise stated, Ni was deposited on Si( 1 1 1) at 298 K. The background-subtracted spectra are shown in Figure 1. Two kinds of C 1s peaks were observed at 284.5 and 285.8 eV. As the exposure of CO was increased from 1 X lo2 to 3 X lo3 langmuirs, the peak at 285.8 eV did not change but the peak intensity at 284.5 eV increased, whereas no appreciable change was observed in the 0 1s spectra. These two C 1s peaks can be assigned to carbon from dissociated CO (284.5 eV) and molecularly adsorbed CO (285.8 eV). The binding energy of molecularly adsorbed CO observed in this experiment is quite similar to that reported on Ni(100) (285.8 eV) and on Ni(l1 I ) (285.4 eV).I7 The result in Figure 1 dem- onstrates that co dissociation occurs on Ni/Si( 11 1) with ONi = 2 at 298 K, and the formed carbon increases with increasing exposure of CO. Oxygen 1 s spectra were observed at 53 1.3 and 533 eV, which are assigned to adoxygen formed from CO dis- sociation at low exposure and oxygen of molecularly adsorbed CO, respectively. It is found that the intensity of the 0 1s spectrum

( I S ) Hofmann, S. In Practical Surface Analysis by Auger and X-ray Photoelecrron Spectroscopy; Briggs. D., Seah, M. P., Eds.; John Wiley & Sons: New York, 1983; Chapter 4.

(16) Wagner, C. D.; Riggs, W. E.; Davis, L. E.; Moulder, J. F.; Mullen- berg. G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp., Physical Electronics: Eden Prairie, MN, 1979; p 80.

(17) Norton, P. R.; Tapping, R. T.; Goodale, J. W. Chem. Phys. Lert. 1976, 41, 247.

- W

z

0 2 4 6 8 10 12 14 16 Binding energy /eV

Figure 2. UPS spectra (He 11) of adsorbed CO molecules on Ni/Si(l11) at eNi = 1.5 following an exposure of 1 x lo2 langmuirs of CO at 120 K and successive heating to 298 K. RT represents 298 K.

at 531.3 eV increased slightly, whereas that of C 1s at 284.5 eV increased considerably as a function of CO exposure. UPS spectra of the adsorbed CO on a Ni/Si( 1 1 1) at dNi = 1.5

are shown in Figure 2. When Ni was deposited at 298 K and 1 X lo2 langmuirs of CO was introduced at 120 K on the Ni/ Si(l l1). two peaks were observed at 11.6 and 8.3 eV below the Fermi level, assigned to 4u and 1r + 5u of adsorbed CO mole- cules." Positions of the 5u and l r peaks were too close to separate; however, the 5u photoionization cross section was considerably smaller than that of the 1r under He I1 irradiation.'* The intensities of two peaks decreased with elevated temperatures up to 220 K, while at 273 K, the 1 r + 5u peak shifted to lower BE and a new peak appeared at 6 eV. The newly appeared peak can be assigned to 0 2p and/or C 2p.i9 The intensity of the peak at 6 eV increased at 298 K. These results imply that adsorbed CO begins to dissociate between 220 and 273 K on the Ni/Si( 11 1) at 6Ni = 1.5:

CO(a) - C(a) + O(a)

The peak separation between 4u and 1 r of adsorbed CO molecules, A(4u-lr), changed from 3.3 eV at 120 K to 3.8 eV at 298 K. The value at 298 K is considerably larger than that reported on Ni metal, 3.0-3.2 eV.17920

Although 1r and 4u levels are nonbonding, they reflect the carbon and oxygen bonding of CO molecule,2' and the A(4o-1r) value is, as a result, influenced by the carbon-oxygen bond length of adsorbed CO m o l e c ~ l e s . ~ ~ - ~ ~ The CO dissociation reaction occurs on the early transition metals, and they have larger A- (417-lr) values on dissociated CO species.23 These facts imply that the larger the value of h(40-1r), the greater reactivity for CO dissociation. The large value of A(4u-lr) (3.8 eV) at 273 K in Figure 2 suggests that molecularly adsorbed CO on Ni/ Si( 1 11) at eNi = 1.5 has a tendency to dissociate.

(18) Gustaffwn, T.; Plummer, E. W.; Eastman, D. E.; F r a t , J. L. Solid Srare Commun. 1 9 5 , 17, 391.

(19) Kuppen, J.; Ertl, G. Surf. Sei. 1978, 77, L647. Broden, G.; Gafner, G.; Bonzel, H. P. Surf. Sei. 1979, 84, 295; Appl. Phys. 1911, 13, 333.

(20) Eastman, D. A.; Cashion, J. K. Phys. Reo. Lett. 1971, 27, 1520. (21) Blyholder. B. J. Phys. Chem. 1964,68, 2772. Doyen, G.; Ertl, G.

Surf. Sei. 1974, 43, 197. (22) Broden, G.; Piru 0.; Bonzel, H. P. Chem. Phys. h r t . 1977,51,250.

Sci. 1976, 59, 593. (23) Broden, G.; R J in. T. N.; Brucker, C.; Benbow, R.; Hurych, 2. Sur/.

Nickel Silicide Formation The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4185

n C O / N i / S i ( I I I )

0 2 4 6 8 IO 12 14 16 Binding energy /eV

Figure 3. UPS spectra (He 11) of adsorbed CO molecules on Ni/Si( 11 1) at BNi = 5 following an exposure of 1 X IO2 langmuirs of CO at 120 K and heating to 298 K.

3.1.2. flNi = 5. UPS spectra of adsorbed CO molecules were recorded on a Ni/Si( 1 I 1) at eNi = 5, which had been prepared at 298 K. When the Ni/Si( 1 1 1) was exposed to 1 X 102 langmuirs of CO a t 120 K, two peaks were observed a t 8.3 eV ( 1 ~ + 5u) and 1 1.6 eV (4u) as shown in Figure 3. The intensity of these peaks decreased at 220 K and remained small at 273 K and then became zero at 298 K. During this experiment no new peaks appeared, and the A(4o-lr) value was constant at 3.3 eV. These results imply that nickel silicide covers the surface at 298 K, on which CO molecules adsorb weakly and desorb completely at 273 K.

3.1.3. eNi 1 IO. As the results of Figures 1-3 suggest, CO adsorption experiments provide the information of the chemical state of Ni deposited on Si(l11) at different temperatures. In this section, adsorption behavior of CO molecules on Ni deposited a t 298 K (spectra are not shown) and liquid nitrogen temperature (denoted as liq N2 temp) will be considered. CO adsorption was carried out a t 298 K on Ni/Si and eNi =

15, where Ni was deposited on Si( 1 1 1) 'at liquid N2 temperature and left at 298 K for 15 h. As shown in Figure 4a, CO molecules adsorbed molecularly and the value of A(44-177) was 3.2 eV. After heating the sample at 370 K for 1 h (b), adsorbed CO molecules completely desorbed. The sample was then cooled to 298 K and was dosed by a 1 X lo2 langmuirs of CO at the same temperature (c). It was found that CO adsorbed molecularly, but the intensity of UPS spectra decreased by factor of about 2 compared with (a). After heating the sample at 420 K for 1 h, CO molecules no longer adsorbed at 298 K (d). These results indicate that adsorbed CO molecules desorb from Ni below 370 K, and the intermixing reaction between nickel and silicon occurs slowly a t 370 K but predominantly at 420 K.

After a monolayer of Ni was further deposited at 298 K, 1 x IO2 langmuirs of CO was dosed on the surface at the same tem- perature. Molecularly adsorbed CO was observed as shown in spectrum e in Figure 4. The value of A(4u-1r) was 3.2 eV, and no peaks were observed at 6 eV due to 0 2p and/or C 2p. This result implies that Ni atoms (&i = 1) deposited on the nickel silicide/Si( 11 1) have the same chemical state as Ni metals re- garding CO adsorption behaviors and are unable to dissociate CO molecules, which is different from the surface of Ni/Si( 1 1 1) at

3.2. Ni Deposited on Si ( l I1) at Llquid Nitrogen Temperature. The effect of temperature on nickel silicide formation can be

6Ni = 1.5-2.

II \ - I i 370K

(a 1 I

COIN1 /SI( I I I )

0 2 4 6 8 IO 12 14 16 Binding Energy /eV

Figure 4. UPS spectra (He 11) of adsorbed CO molecules on Ni/Si( 1 1 1) at 8Ni = 15 obtained at 298 K: (a) 298 K, (b) after heating at 370 K for 1 h, (c) after exposing (b) to a 1 X IO2 langmuirs of CO, (d) after heating (c) at 420 K for 1 h and exposed to 1 X IO2 langmuirs of CO at 298 K, (e) after monolayer of Ni deposited on (d) and exposed to 1 X IO2 langmuirs of CO at 298 K.

w z -

NI d e p Ilq Nz temp 1, 0 2 ibi 4 6 8 IO 12 14 16

Binding energy /eV

Figure 5. UPS spectra (He 11) of adsorbed CO molecules on Ni/Si( l l l ) at eNi = 4 obtained for Ni deposition and exposure to 1 X IO2 langmuirs of CO at liquid N2 temperature and heating to 298 K.

studied at a nickel coverage between 2 and 10. Nickel was de- posited on Si(l l l ) with eNi = 4 at liquid N2 temperature, and CO was introduced at the same temperature. As shown in Figure 5b, CO adsorbed molecularly. When the sample was gradually heated to 298 K, about half of adsorbed CO still remained (d). Leaving the sample at 298 K for 15 h caused a considerable decrease of adsorbed CO molecules (e). These results mean that

Asakawa et al. 4186 The Journal of Physical Chemistry, Vol. 95, No. 12. 1991

TABLE I: Binding Energy (eV) of Ni 3d in Ni/Si(111) Obtained in UPS (He

Ni deposited (RT) 1.8 (D, M) 1.5 (N) 0.8 (M)

(6Ni 1-2) (6Ni = 4-5) (6Ni = 10-15) Ni deposited (77 K) 1.7 1.3-1.4 0.3

1.8 (D, M) 1.4 (N) 0.6-0.9 (M)

1.8 (N) 1.5 (N) 1.4 (N)

1.2 (M)

1

1

1

298 K

420 K

Ni deposited (298 K)

OCO adsorption behaviors are shown in parentheses: D = dissocia- tive adsorption; M = molecularly adsorption; N = no adsorption. b A monolayer of Ni was deposited at 298 K after annealing at 420 K.

nickel silicide formation does not occur at liquid N, temperature, whereas it does occur slowly at 298 K. This conclusion makes sense since CO molecules do not adsorb on nickel silicide at 298 K as shown in Figure 3.

3.3. Ni 3d Band Feature and Nickel Silicide Formation. The peak positions of Ni 3d below the Fermi level and types of CO adsorption at 298 K are summarized in Table I. The 3d band peak positions of Ni deposited on Si( 11 1) at 298 K changed as a function of Ni coverage. While 3d positions of Ni on Si( 1 1 1) at 77 K were constant up to 420 K at e N i = 1-2 (-1.8 eV) and BNi = 4-5 (- 1.4 eV), these values were almost the same as those of Ni deposited on Si( 11 1) at 298 K. However, at 8 N i = 10-15, an appreciable shift of Ni 3d position was observed. It is noted that the peak position at 420 K is coincident with that of Ni/ Si( 1 11) with eNi = 4-5 prepared at 298 K. This fact suggests that a nickel silicide forms from the Ni/Si(l11) interface (bulk) to the surface. It should also be noted that CO molecules do not adsorb at 298 K after annealing at 420 K (Figure 4). These results may imply that an intermixing of Ni and Si a t the surface can be detected by surveying the Ni 3d UPS shift at e N i = 10-15 or at the higher coverage. However, at a lower coverage between eNi = 1-2 and 4-5, the Ni 3d peak shift was observed as a function of NI coverage, but the nickel silicide formation cannot be detected by the shift.

4. Discussion 4.1. Dissociation of Carbon Monoxide Molecules. It is quite

interesting to elucidate the cause of CO dissociation on Ni/Si( 1 11) at 6Ni I 2. on the sputtered Ni( 11 I), decomposition of co occurs as well as desorption upon heating to -450 K, which is attributed to defects.24 Recently, CO dissociation was also observed on Ni( 11 1),25*26 Ni( 1 10),27*28 and Ni( 1 0 0 ) * ~ 3 single crystals at very high exposures. It is suggested that vibrational excitation of the CO molecule is required for CO dissociative ad~orption.~'

In our system, CO molecule adsorbed at 120 K changes to an activated state for dissociation at the higher temperatures as shown in Figure 2. Since 1 r and 4u molecular orbitals of CO are bonding orbitals,32 the shift of 1r to a lower binding energy directly demonstrates the weakening of the C-0 bond which favors the dissociation of the CO molecule. The center of the charge density in 4u orbital is located close to the oxygen atom, and the proportion of the peak shift in the 4u level is comparatively quite less than the Ir level.21*33 It is generally believed that the Sa orbital of

(6Ni = I ) b

(24) Eastman, D. E.; Demuth, J. E.; Baker, J. M. J . Vac. Sci. Techno!.

(25) Christman, K.; Schober, 0.; Ertl, G. J . Chem. fhys. 1973, 60, 4719. (26) Benziger, J. B.; Preston, R. E. Sur/. Sci. 1984, 141, 567. (27) Madden, H. M.; Ertl, G. Surf Scl. 1973. 35, 21 1. (28) Rosci, R.; Ciccacci, F.; Mcmeo, R.; Miriani, C.; Caputi, L. S.; Pa-

(29) Tracy, J. C. J. Chem. Phys. 1972, 56, 2736. (30) Goodman, D. W.; Kelly. R. D.; Madey, T. E.; White, J. M. J. Coral.

(3 1) Lee. M. B.; Btckerle, J. D.; Tang, S. L.; Ceyer, S. T. J . Chem. Phys.

(32) Hou, W. M. J . Chem. fhys. 1%5,43,624.

1974, I ! , 273.

pagno, L. J . Card. 1988,83, 19.

1980, 64, 479.

1987, 87, 723.

CO molecule interacts with unoccupied d orbitals of the metal (u-donation), and then the 2 r orbital of the CO molecule si- multaneously interacts with the highest occupied d orbital of the metal (Ir-back-donation) when the CO molecule is adsorbed with a carbon atom on the metal.*' Two reasons are proposed to interpret the CO dissociation. The first reason is based on strong r-back-donation. As the Fermi level approaches the vacuum level, the d orbitals become more diffuse and, as a consequence, a higher electron density can be stored in the CO 2 r Since the 27r orbital is antibonding of the CO molecule, the r-backdonation determines the C-0 bond weakening, which should cause a dominant I T orbital shift. This interpretation is based on the larger A(4u-1r) value on the early transition metals. The second interpretation is based on the direct interaction of the oxygen with the metal atom, in which CO is side-on bonded with its molecular axis either parallel or inclined toward the surface.34

If the CO dissociation reaction is caused by r-back-donation in the Ni/Si( 11 1) at 8 N i I 2, it is expected that the Fermi level in Ni rises and simultaneously the CO 2 r level has a higher electron density. This may be attributed to the fact that Ni atoms contact directly with the Si( 11 1) surface. However, this is not the case, because the nickel silicide in which the Ni atoms intermix with Si atoms neither adsorbs CO molecules nor dissociates at 298 K. According to recent theoretical calculations, the excess charges of Ni in nickel silicides are as low as -0.12 to 0.06 e per one Ni atom.35 These estimated values deny the r-back-donation for CO dissociation.

As shown in Figure 2, CO dissociation takes place following some portion of adsorbed CO molecules desorbing on Ni/Si( 1 1 1) at eNi I 2. This fact infers that vacant sites formed after CO desorption probably participate in CO dissociation. If this is the case, the vacant sites are not composed of Ni atoms since CO desorbs below 273 K on the sites. In addition, it is well-known that oxygen atoms strongly react with Si atoms.

In conclusion, we would like to suggest that the concerted interaction between the CO molecule and the surface atom through oxygen will be the cause of CO dissociation as follows.

/c-?, Ni Si

4.2. Nickel Silicide Formation. The CO adsorption is very sensitive to the chemical state of the top layer on Si( 1 11) and may be one of the best methods to elucidate the mechanism of nickel silicide formation. Based on this reason, the mechanism of nickel silicide formation can be studied in relation to results of CO adsorption and Ni 3d UPS peak shifts. It was concluded that at e N i I 2 Ni deposited on. clean Si(] 11) stayed on the surface without intermixing with Si atoms a t 298 K. This conclusion is basically attributed to the fact that nickel silicide, if formed, does not adsorb Co molecules at 298 K. At = 4-5, however, the intermixing takes place to form a nickel silicide. The intermixing at the surface is difficult to proceed completely at e N i 2 10 at 298 K, in which the surface is covered by sufficient layers of Ni atoms and the reasonable representation of the system is Ni/nickel silicide/Si( 11 1). The formation of nickel silicide, which takes place at the Ni/nickel silicide interface, transmits to the surface in the vertical direction through a diffusion of Si atoms at 298 K.

A cluster growth model is proposed for the silicide formation at 298 K, using high-resolution ion backscattering and Auger experiments.6.' According to the model, the N i s i islands, in which the top layer is covered with Si atoms, are formed in the initial stage. When silicon species cover nickel silicide in the initial stage, CO molecule should not adsorb. However, both molecularly adsorbed and dissociated CO molecules were observed at ONi I 2 in our system. Such discrepancies may be caused by a lower

(33) Sung, S. S.; Hoffman, R. J . Am. Chem. Soc. 1985. 107, 578. (34) dePaola, R. A.; Hrbek, J.; Hoffmann, F. M. J. Chem. Phys. 1985,

82, 2484. Eberhardt, W.; Hoffmann, F. M.; dePaola, R. A,; Heskett, D.; Strathy, I:; Plummer: E. W.; Moser. H. R. fhys. Reo. Lerr. 1985,51, 1856.

(35) Bmi, 0.; Chiao, L. W.; Tu, K. N. fhys. Rev. B. 1984, 30, 4664.

J. Phys. Chem. 1991,95,4787-4794 4787

limit on the sensitivity of ion backscattering and Auger mea- surements. The interface of nickel silicide has been studied with AES and UPS, and it is concluded that the Ni-Si intermixing reaction does not occur at low Ni merages and the first monolayer of Ni on Si( 1 1 1 ) is different from that in the second stage com- pound.s Although the peak change in the Si 4 , W A u g e r signal is observed in refs 5-7, the change is probably minor and is not necessarily reflected by the intermixing reaction of Ni on Si to form islands of nickel silicide.

The peak position of Ni at eNi = 4-5 deposited on s i ( 1 1 1 ) at 77 K does not shift following annealing at 420 K despite the fact that the nickel silicide formation does not occur at 77 K but proceeds at 298 K as shown in Figure 5. The electronic property of Ni which contacts directly to the Si( 1 1 1 ) surface without accompanying intermixing reaction might be similar to nickel silicide with a 3d peak shift. However, this is not the case because the peak position of Ni 3d at ONi = 1-2 has a higher binding energy than that a t eNi = 4-5 even when nickel silicide is not formed. In addition, CO adsorbs molecularly and dissociates at eNi = 1-2. Two reasons are deduced for causing Ni 3d peak shift as a function of Ni coverage. One is a size dependence of initial-state electronic property of Ni,11*'2 and the other is the photoelectron emission

Namely, the initial-state electronic property of Ni varies with Ni cluster size and the final-state relaxation process of

photoemission also varies with Ni cluster size as well as with electronic environment of Ni atom which is associated with surrounding Si atoms. As it is difficult to estimate the proportion of these two factors which contribute to the Ni 3d peak shift, it is concluded that nickel silicide formation cannot be evaluated from the Ni 3d peak shift at low Ni coverage. At a Ni coverage below 10, it is deduced that a Ni 3d peak shift due to treatment temperature is caused by a combination of electronic property of Ni and the final-state relaxation process associated with sur- rounding Si atoms in nickel silicide formation.

5. Conclusion The adsorption of CO molecule is one of the most powerful

methods to investigate the nickel silicide formation reaction, in which the exact chemical state of Ni atoms on the first monolayer of the surface in Ni/Si( 1 1 1 ) can be obtained. On the surface at BNi 5 2, deposited Ni situates on the Si(ll1) surface without intermixing with Si, on which CO molecules are able to dissociate at 298 K. Such a surface has a new function. On the surface at eNi = 4-5, a nickel silicide is formed and does not adsorb CO molecules at 298 K. On the surface at eNi 1 10, one Ni metal layer is present on the two-phase substrate, nickel silicide on Si( 1 1 l ) , and a CO molecule is adsorbed molecularly the same as on Ni metal.

Comparisons between Scanning Tunneling Microscopy and Outer-Sphere Electron-Transfer Rates at Pt( 11 1) Surfaces Coated with Ordered Iodine Adiayers

Si-Chung Chang, Shueh-Lin Yau, Bruce C. Schardt,* and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: July 6, 1990; In Final Form: November 26, 1990)

Rate parameters are reported for the electroreduction of eight CO*~'(NH,)~X complexes at ordered Pt(l11) surfaces coated with iodine adlayers whose structures are characterized by scanning tunneling microscopy (STM) in order to explore possible correlations between the outer-sphere electron-transfer kinetics and the spatially resolved adlattice properties as revealed by STM. The sixth ligands, X = NH3, F, OS03*-, OHz, acetate, and three cyclic organic carboxylates, were selected so to vary the reactant charge, and hence the magnitude of electrostatic double-layer effects, and to examine the effect of potential organic mediators. The ordered Pt( 1 11) surfaces were prepared by flame annealing, followed by cooling in a stream of nitrogen over iodine crystals (cf. ref 5 ) . Three types of iodine adlayer structures could be formed, one having a (d7Xd7)R19.I0 unit cell and two coexisting structures with (3 X 3) symmetry, as identified by STM. The real-space iodine adlattice structures extracted from these data are discussed (cf. ref 4) along with spatially dependent electron-tunneling parameters for each iodine adsorption site, also obtained from STM. For reactants containing only, inorganic ligands, the observed (apparent) rate constants k, are markedly (3-5-fold) larger on the ( 4 7 X 4 7 ) adlayer. This more facile electron mediation provided by the ( 4 7 X A) versus the (3 X 3) adlayers is rationalized in terms of the preponderance of threefold hollow iodine atoms in the former structure. Somewhat more facile electroreduction on the iodine adlayer surfaces is observed for complexes containing aromatic carboxylate substituents, although the kinetics in these cases are insensitive to the adlayer structure. This is attributed to the presence of specific interactions between the aromatic rings and the iodine adlayer. Comparisons are also made with corresponding rate parameters obtained at unmodified mercury and Pt( 1 1 1 ) electrodes.

An intriguing issue in surface electrochemistry concerns the relationships between the efficiency of electron transfer to and from solution redox couples a t metal surfaces and the electronic and molecular structure of the interfacial region. The significance of this question stems in part from the expectation that some electrochemical processes may proceed via nonadiabatic pathways Le., where the electron-tunneling probability within the transition state, tcCl. is less than unity, thereby impeding the reaction rate.' In addition, ratesurface environment variations can often arise from differences in the reaction energetics, associated especially with solvent reorganization and interfacial work terms

( 1 ) For a review, see: Weaver, M. J. In Comprehensiw Chemical Kinetics; Compton, R. G., Ed.; Elsevier: Amsterdam, 1987; Vol. 27, Chapter 1 .

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("double-layer" effects). Such electrochemical reactivity-inter- facial structural correlations are profitably pursued at ordered monccrystalline metal surfaces in view of their structural definition and uniformity. Such electrochemical kinetic measurements, however, are rare.

An interesting opportunity to examine such issues is provided by ordered Pt( 1 1 1) surfaces covered by iodine adlayers. The electrochemical properties of iodine-coated platinum are well- known,2 iodide (or iodine) yields densely packed adlayers held tenaciously via relatively strong covalent bonds to the metal substrate. Such adlayers oblige electrochemical reactions following

(2) (a) Lane, R. F.; Hubbard, A. T. J . Phys. Chem. 1975, 79.808. (b) Felter, T. E.; Hubbard, A. T. J . Electroanal. Chem. 1979, 100, 473. (c) Hubbard, A. T. Chem. Rev. 1988,88,633.

0 1991 American Chemical Society