Si(111)7 × 7 interface

Surface

258

Science 251/252

(1991) 258-261 North-Holland

Empty states of the NifSi( 111) 7 x 7 interface J.-Y.

Veuillen, T.A. Nguyen Tan

LEPES/CNRS,

Received

BP 166. 38042 Grenoble Cedex, Frrrnce

1 October

1990; accepted

for publication

23 November

1990

Up to now, the evotution of the empty states of the metal/silicon interface has received only little attention. It is however necessary to investigate both occupied and unoccupied states in order to get a complete picture of the interface electronic structure. We present a combined X-ray photoemission (~~~/bre~tr~~ung isochromat spectroscopy (BIS) study of the Ni/Siflflf7 X7 interface formation at room temperature. Valence and conduction band densities of states, as well as metal and Si core fevefs, were recorded for coverages (8) ranging from 1 to 1oQ ML. Above 1 ML, the data reveal an intermixed phase. Its composition is close to Ni,Si at low coverage (0 = 3-6 ML), as testified by both XPS and BIS spectra. (For higher coverages, we found evidence for composition changes induced by the e-beam.) In the I ML range the situation seems to be more complex, with possible specific empty interface states.

We present a study of the electronic structure of the Ni/Si(llI)7 X 7 interface at room temperature (RT) in which we have investigated both the occupied and the unoccupied density of states (DOS). The experimental DOS’s were obtained using direct and inverse photoe~ssio~ in the Xray regime: X-ray photoemission spectroscopy (XI’S) and bremsstralhung isochromat spectroscopy (BIS) respectively. These techniques have been shown to be quite sensitive to the early stages of interface formation [1,2]. Ni/Si(lll) is a convenient system for such a study because the reference XPS and BIS spectra for some of the Ni silicides exist in the literature [3-S]. The Ni/Si interface has been found to be very reactive, even at RT, and various models have been proposed to describe the early stages of its formation [3]. Our data will be compared to existing experimental [3,7--101 and theoretical results.

The experiments were carried out in an UHV system manufactured by VSW (base pressure 2 X I()-IO mbar). For BIS the measurements were made by an X-ray monochromator tuned on the Al Ktw line. The BIS e-gun was operated at a low target current (about 60 @A) since higher sample currents may induce changes in the electronic structure of the interface [l]. Under these conditions the count rate was of the order of a few cps at threshold, and a rather long acquisition time (12 h) was necessary to get a statisfactory signaito-noise ratio. The vafence band (VB) and corelevel (CL) XPS spectra were recorded before and after each BIS run to make sure that no changes occurred in the electronic structure of the interface during the BIS experiment. The Fermi level was determined by measuring the edge of the BIS spectrum of a Pd sample for each coverage.

J.-Y. Vet&en et al. / Empty states of the Ni/Si(ill)7X

7 interface

259

The clean Si(111)7 X 7 surface was prepared by ion bombardment and annealing. Ni layers were then deposited by electron bombardment of a Ni wire wrapped on a W tip. The evaporation rate was of the order of 0.5 A/mm. In this paper, metal thicknesses are given in monolayers {ML) units (1 ML = 7.8 X 1014 atoms cme2). Nisi, was formed by annealing at 650 o C for 20 min a 60 A thick Ni layer deposited at RT on Si(111)7 X 7 [7]. This annealed layer showed a symmetrical 1 X 1 LEED diagram, characteristic of an epitaxial disilicide film of mixed A + B character (71.

3. Results

-Nisi

0

5

IO

Ni thickness

The valence band (XPS) and conduction band (BIS) spectra of bulk compounds (Si, Ni and Nisi,, curves a, g and e respectively) and of the interface are reported in fig. 1. XPS and BIS spectra of Si and Ni are similar to those found in the literature [l-4]. The XPS spectrum of the disilicide is dominated by a peak at -3.1 eV, attributed to Ni 3d non-bonding states. The shoulder at -5 eV is related to bonding (Ni3dSi3p) states and the structure near the Fermi level (EF) to the occupied antibonding (Ni3d-Si3p) states [5,6,12]. The main cont~bution to the weak structure near 10 eV comes from Si3s states. The BIS spectrum of Nisi, is much less structured

Energy

f E-E,)

(eV)

Fig. 1. Valence band XPS and BIS spectra of bulk materials (Si: curve a, Ni: curve g, Nisi 2: curve e), and of the interface for various Ni coverages (curve b: 1 ML, c: 3 ML, d: 6 ML, f: 10 ML).

2

(ML)

Fig. 2. Binding energy (BE) of Si2p and Ni2p,,* core levels and kinetic energy (KE) of the NiL,W Auger tine for various Ni thickness (0). The corresponding values for bulk Ni and NiSi z are indicated by (-).

than the XPS one. From band structure calculations [12], the empty DOS consists mainly of antibonding states with strong Si3s and 3p character. For very thin Ni layers deposited on Si(ll1) (l-6 ML, curves b, c and d in fig. l), the VB XPS spectra are dominated by a structure at - 1.8 eV (labelled a in figs. 1 and 3). The energy location of this peak does not seem to depend much on the metal thickness 8 in the range l-6 ML. From cross section considerations [5], it should be related to Ni3d derived states. In this coverage range, we found that both the binding energy (BE) and the linewidth of the Ni2p,,, core-level (CL) peak are essentially constant, and different from both bulk Ni and Nisi, ones (see fig. 2). For higher coverages (B = 10 ML, curve f in fig. l), the maximum of the d band shifts towards E,, and the BE of the Ni2p,,, line becomes closer to that of bulk Ni. These results reflect the reactive nature of the Ni/Si(lll) interface at low coverages. We now examine the BIS spectra of fig. 1. In the early stages of the interface formation (1-6 ML), we note an increase of the emission in the vicinity of E, (O-2 eV) relative to clean Si. However, the spectra remain rather structureless even for 8 = 10 ML, and do not show any sharp peak

260

1. Y. Yeuilien et ai. / Empty

statesof the Ni /

at the Fermi level as for bulk Ni. This is another indication of the reactive nature of the interface. (However, for 6 = 10 ML, the BIS e-beam may have induced some overreaction of the RT interface.) At this stage, we can try to determine the nature of the intermixed phase from our results. If we consider the VB spectrum of the 6 ML deposit (curve d of fig. 1, which should be less sensitive than the 3 ML one to thickness effects or to substrate contribution), we see that it is dominated by a structure “a”, located at 1.8 eV BE and asy~et~c on the high BE side. This excludes as disilicide-like compound, since it is very different from curve e of fig. 1. Reference XPS spectra of Ni,Si and Nisi exist in the literature [5,6]. The main peak of the Nisi spectrum is located at more than 2 eV BE and is rather symmetric, whereas that of the Ni,Si spectrum is found at 1.6-1.7 eV BE and is strongly asymmetric. Our VB XPS results thus favour the formation of a Ni,Si-like phase for 6 = 6 ML. Moreover, the BE of the Ni2p,,, peak for the 6 ML deposit is 0.7 eV larger than that of bulk Ni. This BE shift is closer to the value reported for Ni,Si (0.6 eV relative to Ni), than to the one published for Nisi (1.1 eV relative to Ni) (ref. [S]). The XPS results thus indicate that a Ni,Si-like phase is formed at RT for B = 6 ML. We believe that this should also be true for 6 = 3 ML, for the following reasons: (i) the structure “a” is located at the same BE for both coverages and is clearly asymmetric for 3 ML; (ii) the BE (see fig. 2) and the lineshape of the Ni2p,,, peak are identical for B = 3 and 6 ML: (iii) the ICE (see fig. 2) and the lineshape of the Ni L,W Auger line are quite similar for both coverages. This is in part a consequence of points (i) and (ii), and this furthermore shows that correlation and relaxation effects are of the same order in these interfacial compounds. We have thus established that a Ni,Si-like interfacial compound is formed on top of Si for f? = 3 and 6 ML. This is in agreement with the findings of ref. [9]. where the growth of Ni,Si island in this coverage range has been inferred from an ion scattering experiment. Our conclusion does not contradict the results of a previously

Sifl 1 I) 7 x 7 interface

- NiiSi 6ML 3ML -

1

-10

I

-5

.

/L 1ML.

0

Energy (E-E,)

0

5

..J

10

(eVi

Fig. 3. Valence band XPS and BIS spectra after subtraction of the substrate contribution for small coverages (I. 3 and 6 ML: full lines). The dashed curve in the upper part of the figure (left) is the valence band XPS spectrum of Ni,Si (from ref. [6]).

published UPS study [lo]. For increasing coverages (10 ML) the surface of the film becomes more metal rich. In order to have further insight into the evolution of both the occupied and empty states at the Ni/Si interface the substrate contribution to XPS and BIS spectra has been subtracted from the experimental data. The Si contribution has been attenuated by assuming the growth of a uniform Ni,Si layer and an electron mean free path of 15 A. (The result of the subtraction is in fact not very sensitive to this attenuation factor.) The subtracted spectra are shown in fig. 3. As expected to VB XPS spectra for 3 and 6 ML look similar to that of Ni,Si. The corresponding BIS spectra are step-like. This is the anticipated result for Ni,Si since neither the theoretical DOS nor the calculated one-particule spectrum show any strong peak near the Fermi level. (Although inverse photoemission in the UV range of ref. [13] shows a small structure near E,.) The 1 ML BIS spectrum of fig. 3 is more intriguing. It shows a prominent peak “b” in the vicinity of the Fermi level (O-2.5 eV). This structure may also be present for @ = 3 ML is an attenuated form since the 3 ML spectrum shows an enhanced intensity within 3 eV of E, relative to the 6 ML one. This suggests that peak “b” might be related to an interface state. The electronic structure of the Ni/Si( 111) in-

J. - Y. Veuilien et al. / Empty states of the Ni / Si(l1 I)7 X 7 interface

terface has been calculated in the 1 ML range for various geometrical models [ll]. From a comparison of their theoretical densities of states with UPS spectra, Bisi et al. came to the conclusion that only two models give a satisfactory description of the experimental results. (Namely one ML of Nisi, on Si(ll1) with B orientation and the Nisi, adamantane structure.) Our VB XPS spectrum for 1 ML seems to support this conclusion. However, the agreement between our BIS spectrum (8 = 1 ML) and the computed empty DOS is rather poor for both models (peak b is not reproduced by ~lculation). This suggests that the actual interface geometry for B = 1 ML may be different from those considered in ref. [ll]. For instance if Ni,Si islands are formed in the very early stages of interface formation [9].

4. Conclusion Our experimental techniques give evidence of the growth of a Ni,Si-like phase at the Ni/ Si(111)7 x 7 interface at RT in the coverage range 3-6 ML. In the 1 ML range, our BIS results indicate the existence of an empty interface state located at about 1.5 eV above the Fermi level.

261

References [l] J.-Y. Veuillen, T.A. Nguyen Tan, R. Cinti, M. De Crescenzi, J. Derrien, Phys. Rev. B 39 (1989) 8015. [2] J.-Y. Veuillen, T.A. Nguyen Tan, R. Cinti, S. D’Addato, S. Turchini, S. Nannarone, A. Santaniello and G. Rossi, Vacuum 41 (1990) 702. [3] C. Calandra, 0. Bisi and G. Ottaviani, Surf. Sci. Rep. 4 (1984) 271. [4] W. Speier, J.C. Fuggle, R. Zeller, B. Ackermann, K. Szot, F.U. Hillbrecht and M. Campagna, Phys. Rev. B 30 (19840 6921. [5] W. Speier, E. van Leuken, J.C. Fuggle, D.D. Sarma, L. Kumar, B. Dauth and K.H.J. Bushow, Phys. Rev. B 39 (1989) 6008; D.D. Sarma, W. Speier. R. Zeller, E. van Leuken, R.A. de Groot and J.C. Fuggle, J. Phys. Condensed Matter 1 (1989) 913i. [6] 0. Bisi, C. Calandra, U. de1 Pennino, P. Sassaroli and S. Valeri, Phys. Rev. B 30 (1984) 5696. [7] R.T. Tung, J. Vat. Sci. Technol. A 5 (1987) 1840. [8] N.W. Cheung, P.J. Grunthaner, F.I. Grunthaner, J.W. Mayer and B.M. Ulrich, J. Vat. Sci. Technol. 18 (1981) 917. [9] E.J. van Loenen, J.W.M. Frenken and J.F. van der Veen, Appl. Phys. Lett. 45 (1984) 41. IlO] K.L.I. Kobayashi, S. Sugaki, A. Ishizaka, Y. Shiraki, H. Daimon and Y. Murata, Phys. Rev. B 25 (1982) 1377. [ll] 0. Bisi, L.W. Chiao and K.N. Tu, Phys. Rev. 30 (1984) 4664. [12] J. Tersoff and D.R. Hamann, Phys. Rev. B 28 (1983) 1168. f13] M. A&an, R. Baptist, G. Chauvet and T.A. Nguyen Tan, Solid State Commun. 57 (1986) 1.