Effect of Cu deposition on structural and electronic properties of cleaved Si(111) surfaces

Surface Science 152/153 (1985) 1228-1238 North-Holland, Amsterdam

EFFECT OF Cu DEPOSITION ON STRUCTURAL PROPERTIES OF CLEAVED Si( 111) SURFACES A. TALEB-IBRAHIMI, and P. CHEN **

V. MERCIER,

L.uhoratoire de Physique da Solides, Associk F-75230 Paris CPdex 05 France

Received

2 April 1984; accepted

for publication

AND ELECTRONIC

C.A. Sl?BENNE,

au CNRS

D. BOLMONT

154, UnroersitC Pierre et Marie

*

Curre,

30 May 1984

Photoemission yield spectroscopy (PYS), LEED and AES measurements are performed on a set of UHV cleaved Si(lll)-2 X 1 reconstructed surfaces from n- and p-type crystals as a function of Cu coverage 0. The Cu layers are obtained by evaporation at calibrated rates on the substrate kept at room temperature in UHV. 0 varies from about IO-* monolayer (ML) to several tens of ML. In LEED, one observes successively the Si(lll)-2X 1 at 8 = 0, a Si(lll)-4X 1 from 0 = 0.5 to 5 ML, a Cu(lll)-fi Xfi from 5 to 10 ML and a Cu(lll)-1 Xl beyond 10 ML. In AES, the Si peak at 91 eV is split at low Cu coverages and remains observable up to 50 ML. Both the work function and the ionization energy decrease by about 0.1 eV in the 1 ML coverage range, while the effective density of surface states is markedly modified by Cu adsorption. These results are understood within a two-step Cu deposition process: a two-dimensional alloy between Cu and Si forms first; then an epitaxial growth of Cu is observed.

1. Introduction A significant effort has been made to understand the microscopic nature of the d metal-Si interfaces. The growing amount of experimental data points out the complexity of the interaction at the boundary between the metal and the substrate. A large number of information on the Ag-Si(ll1) and Au-Si(ll1) systems is already available while less studies have been devoted to the Cu-Si(ll1) interfaces. Sticking to the case of the room temperature deposition of the metal onto the 2 x 1 reconstructed Si(ll1) surface, it has been shown that Ag forms an essentially abrupt interface at the atomic scale, that is exempt from any alloy formation [l]. On the contrary, the Au-Si(ll1) system is characterised by the

* Present address: ** Present address:

ISEA, Universite de Hte Alsace, F-68093 Mulhouse Cedex, France. Dpt. of Physics, University of Fudan, Shanghai, People’s Republic of China.

0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

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presence of an alloyed phase the composition and bonding nature of which are not fully ascertained yet. The presence of an alloy and the segregation of Si to the surface are now accepted but there are still some discrepancies concerning the Au growth process [2-71. The first work on the Cu-Si(lll)-2 x 1 system [8] gives evidence, from photoelectron spectroscopy, of a strong intermixing for interfaces prepared at RT and even for those obtained at liquid nitrogen temperature; ordered structures have been observed by low energy electron diffraction (LEED) [8]. Other investigations using Auger electron spectroscopy and photoemission point out that a silicide-like compound is formed at RT and remains stable after annealing up to = 300°C [9,10]. From these results the Cu-Si(lll)-2 X 1 system appears to be more ordered and more stable than Au-Si. Our purpose in this paper is to report on the Cu deposition at RT on cleaved 2 X 1-Si(lll) surfaces as studied by LEED, Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS). We present results concerning the structural properties of the system and give information on the electronic density of states at the forming interface, especially in the gap,and its evolution in the 1 ML range (1 ML = 7.84 x 1014 atoms cm-* with respect to the substrate). The variation of the work function and ionization energy are also determined and the corresponding surface barrier height is compared to the Schottky barrier measured through electrical characteristics of the same system by Thandilakis [ll].

2. Experimental details

The multiple-technique ultra-high vacuum (UHV) apparatus used for the experiments has been described elsewhere [l]. In the present work, a set of oriented monocrystalline silicon bars has been studied, the free carrier densities of which are, respectively, n = 2.4 x lOI cmp3 and p+ = 1 x 1019 cmp3. The 2 x 1 reconstructed surface was obtained by cleavage along the [?ll] direction in the (111) plane. Copper was evaporated from a Knudsen-type cell. The evaporation rate was carefully calibrated using a quartz balance. The reproducibility and the stability of the Cu flux have been checked and were regularly tested with the quartz balance which could replace the sample at its exact position. A deposition rate of lo-* ML s-’ was used. For PYS, photon energies between 4 and 6.6 eV were used. The measurements were performed after cleavage, then after each Cu deposit. The contamination by residual gases was carefully checked and during measurements, as well as during evaporations, the total pressure in the vessel remained always below 4 X lo-” Torr.

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3. Results

3.1.LEED Fig. 1 shows characteristic steps in the evolution of the LEED diagram upon increasing copper coverage 0. After cleavage, low energy electron diffraction shows the well known 2 x 1 diagram of the clean cleaved silicon surface. For 8 -( 0.5 ML, additional spots appear in the [211] direction between the integerorder and half-order spots, leading to the formation of a 4 X 1 structure. While the 2 x 1 spots remain quite sharp, the new fractional-order spots appear blurred and elongated along the fractional-order line, i.e., the [2ll] direction (fig. 1a). Beyond - 1 ML, faint half-order streaks. parallel to [211], are observed in the pattern indicating a 4 X 2 reconstruction (fig. 1b). This pattern

Fig. 1. Selected LEED diagrams of a Si(ll I)-2 X 1 surface covered with Cu: (a) B = 1 ML, E = 37 eV; (b) 8 = 2 ML, E = 37 eV; (c) 0 = 10 ML, E = 65 eV; (d) I3 = 20 ML, E = 69 eV.

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remains observable up to 8 = 5 ML where the Cu(lll)- fi X fiR30’ diagram starts to be seen (fig. lc), with very large fractional-order spots on a high background, and stays up to 0 = 10 ML. Beyond that coverage, the high background disappears, and an hexagonal pattern corresponding to the unreconstructed Cu(ll1) plane is observed rotated by 30” with respect to Si(lll)-1 x 1 (fig. Id). 3.2. AES As in Au/Si(lll)-2 X 1 system [7], a small coverage of copper induces a strong change in the shape of the Si(LW 91 eV) peak: at 0 slightly below 1 ML it is already considerably broadened, and at 0 - 3 ML, a splitting into two peaks, at 90 and 95 eV respectively, starts to show up. From Hiraki et al.‘s study [12] this splitting is typical of the presence of a Cu-Si alloy. Because of the splitting, the evolution in intensity of the low energy Si Auger peak has been determined by measuring the intensity of the single peak observed at low coverages, and, at higher coverages when a double structure is resolved, by averaging over the intensities of the two observed peaks [7]. Under these conditions, quantitative information from this evolution have to be considered cautiously. On the contrary, the high energy Si Auger peak at 1619 eV undergoes only an intensity change upon Cu deposition and appears easier to use to get quantitative information on the growth process. Fig. 2 shows the evolution of the relative intensity of both peaks, Si (“91 eV”) and Si (1619 eV) as a function of Cu coverage 8, each peak being referred to its after cleavage value.

Fig. 2. Auger intensities versus copper coverage of Si (“91 eV”) and Si (1619 eV) relative to their after cleavage values; Cu (55 eV) relative to its value at 50 ML coverage. the dashed curve represents the plot of 1 - exp( - d/X) for a copper metal layer, and with X = 4 A.

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The Si (1619 eV) peak shows an exponential decrease, up to the highest coverages, from which an electron escape depth of (15 + 2) A through Cu metal is found. (One should recall that the atomic density of Cu is larger than that of Si by a factor of about 2.5, and 1 ML referred to Cu metal is equivalent to about 2.5 ML referred to the Si substrate as far as the number of atoms per cm2 is concerned. In this paper, we refer the “monolayer” to the substrate.) On the other hand, the Si (“91 eV”) peak decreases initially quite fast, then it remains nearly constant between about 5 and 10 ML before decreasing again up to the highest coverages (- 50 ML). This is different from what was observed with Au [7] and in the present case there is no indication of a segregation process of Si on the outer surface of the system. However, the “91 eV” Si peak is still observed, split, at the highest coverages: as the copper coverage increases, the doublet keeps the shape it had reached at about 10 ML and its attenuation rate is not compatible with the growth of a uniform layer of pure Cu. This observation suggests either that some Si may be dispersed in the Cu layer at the beginning of its formation over a Cu-Si alloy, the Si-rich Cu region being progressively burried in the Cu layer; or that Cu forms islands which enlarge and spread more and more over a Cu-Si layer. In fig. 2, the intensity of the Cu Auger peak at 55 eV, referred to its value at 19= 50 ML, has been also plotted versus 8. This curve has been fitted with a law of the form 1 - exp( -d/h), where d is the overlayer thickness and X the electron escape length. The best fit corresponds to h = 4 A for a Cu metal overlayer. It should be noted that the escape depth values of 15 and 4 A, which have been determined from AES, are quite satisfactory for electrons having an energy of about 1620 and 55 eV respectively. 3.3. PYS Looking now at the photoemission yield results, fig. 3 shows a typical set of the effective density of filled states N*(E) at increasing Cu coverages. These data are obtained by taking the first derivative with respect to photon energy of the corresponding photoemission yield spectra. The principle of the method has been discussed at length earlier [13]. In this procedure, the photon energy is used as an energy scale referenced to the vacuum level which allows a direct determination of the work function [13]. This is one of the advantages of our method in comparison with the usual energy distribution curve measurements, where the work function value + is much less accurately obtained. From the comparison of similar sets of curves as in fig. 3 obtained on differently doped substrates, it is also possible [1,13] to obtain the ionization energy @ at increasing Cu coverages 8, as far as the coverage can be considered as statistically uniform, that is, in the present case, up to about 1 or 2 ML. Both the variations of the work function 4” or $, and ionization energy @ are shown in fig. 4 in the case of the n or p doped samples. In fig. 4, the

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PHOTON

ENERGY

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6 (EV)

Fig. 3. Effective density of filled states N*(E) versus photon energy for a lightly doped cleaved silicon sample (n = 2.4 X lOI cmm3) at different copper coverages. Both semilog and linear plots are presented: curves 1, 0 = 0 ML; curves 2, B = 0.5 ML; curves 3, 0 = 1 ML; curve 4, 0 = 5 ML; curve 5,B = 10 ML. The spectra are shown displaced with respect to one another along the N* axis for each type of plot.

surface barrier height +a, as deduced from the values of &, and @, is also shown, in the case of our low doped n-type substrates. Beyond 0 = 2 ML, the determination of the ionization energy becomes more difficult and not very significant since it would depend on the actual growth process. @ is independent of the Si substrate type and doping while the work function +, which is already only slightly type and doping dependent on the clean surface, appears even less so as the Cu coverage increases. Such a relative pinning of the Fermi level, at a given coverage 8, means that the density of surface states at Fermi level remains always above the:,10i3 states cm-* eV_’ range. In such cases, the surface density of charge and band bending are determined from the data given in fig. 4 once the silicon type and doping are

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YAC”“l.4 LEVEL CulSi

(111) 2x1

z

Fig. 4. Variations of the work functions @I” and $, for n- and p-type samples respectively (n = 2.4~70~~ cmm3, p =1019 cm-‘). the ionization energy Q, (left scale) and surface barrier height +a (right scale) as a function of copper coverage on a Si(ll1) cleaved surface.

l

‘2

a 0.3

0.1 :

/

0 - 0.5

Ai

IB

/ 0

0.5

ENERGY

(EV)

1

Fig. 5. Variations of the effective density of filled surface states AN*(E) in the silicon valence band region upon increasing copper coverage. E,, is the surface position of the Si valence band

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given (in the present case of an n-type sample with 2 X 1014 carriers per cm3, the band bending varies slightly from 0.34 eV at 8 = 0 to 0.38 eV at 8 = 1 ML, the positive surface charge densities being respectively 4.1 X 10” and 4.3 X 10” electron charges per cm*). More information on the variation of the density of filled states upon Cu coverage can be extracted from the sets of curves as in fig. 3, in particular in the energy range corresponding to the upper part of the Si valence band. Actually, in fig. 3, only the changes of the density of filled surface states in the gap, which corresponds to the low photon energy part of the curves, are clearly seen. Now, if the contribution of the Si valence band states is removed, changes in N*(E) in the upper energy range investigated will then be visualized. Such differences, AN*(E) at Cu coverage B, obtained by subtracting a fitted contribution of the form k(hv - 0)5”2, are represented in fig. 5, using the Si valence band edge at the surface, E,,, as the new energy scale origin. This procedure has been checked and discussed before in ref. [l] and the curves in fig. 5 give a good picture of the density of filled surface states at different Cu coverages. The curve at zero coverage is the well known dangling bond surface state band of clean cleaved Si(ll1) with its double structure at about 0.2 and 0.6 eV below the valence band edge E,,, labeled A and B in fig. 5.

4. Discussion LEED and AES results can be interpreted in the frame of a two-step growth process. The first step would correspond to a strong Cu-Si interaction leading to the formation of a Cu-Si alloy. The second step would be the growth of metallic copper over the alloyed region at the interface. During the first step of the growth process, an ordered surface layer is formed: it is characterized by a 4 x 1 then 4 X 2 unit mesh, with disorder in the [211] direction, which is already clearly observable below half monolayer coverage. the formation of an alloy is evidenced by the splitting of the low energy Si Auger line which is resolved at B = 2 ML but induces a broadening of the observed Auger structure at coverages as low as 0.7 ML. In fact, the strong Si-Cu interaction starts from the very beginning of the Cu deposition. This is revealed by the initial exponential decrease of the 91 eV Si peak (fig. 2), at coverages below - 1 ML when the Auger signal arises mostly from pure Si. If this decrease originated from the attenuation through a uniform cu metal layer, it would give an escape depth X of 1.8 A which is far too small for electrons in the 90 eV energy range. However, a satisfactory value of - 4 A for h is obtained provided the electrons go through a layer having the atomic density of Si. Putting it another way, the unusually fast initial decrease of the 91 eV peak can be understood if the Auger signal coming from the pure Si substrate is attenuated through a layer thicker than the actual Cu deposited

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layer. The thicker layer is the two-dimensional compound formed by the strong interaction between Cu and Si. The Si atoms involved in this two-dimensional compound contribute no longer to the low energy LVV Auger signal because of their new bonding state or states from which the Auger double structure will arise. The Si KLL Auger transition is far less sensitive to the change in the chemical environment and hence the high energy peak at 1619 eV decreases normally with the Cu layer thickness. The observation that the evolution of the Cu (55 eV) Auger signal at low coverages can be described by taking into account only the Cu layer thickness indicates that the deposited Cu atoms do not penetrate into the Si substrate. The second step would correspond to the formation of a rough layer of crystalline copper. Keeping in mind that about 2.5 ML of Cu, referred to the substrate, are needed to form one single layer of Cu metal, it is not surprising to find a somewhat distorted and disordered copper layer below 10 ML as demonstrated by the LEED diagram, which reorders nicely into a 1 x 1 pattern beyond 10 ML. At the same time, the electron escape depths which are deduced from the evolution of Si (1619 eV) and Cu (55 eV) Auger signal are compatible with the formation of a uniform metallic Cu layer. If a two-step model of the Cu/Si(lll)-2 x 1 interface formation can be proposed, the composition and atomic arrangement within the two-dimensional compound remain unknown. In LEED, the Cu( 11 l)- fi X fi pattern is observed first at a coverage of about 5 ML together with faint spots of the two-dimensional compound 4 X 1 pattern. This means that at 5 ML, that part of the Cu atoms which has formed the metallic copper layer is close to a monolayer of Cu metal, i.e., slightly below 2.5 ML. Then, we suggest that about 3 ML of Cu are needed to complete the first step. If the alloyed region corresponds to the eutectic composition Cu,Si which has been observed after high temperature annealings [10,12], it means that a monolayer of Si atoms is involved in the two-dimensional compound. In the light of such a growth process, let us now discuss the results deduced from photoemission yield spectroscopy, limiting ourselves to Cu coverages up to 2 ML since the determination of Si bulk states contribution to the effective density of states becomes problematic beyond that coverage. Considering first the ionization energy @ displayed in fig. 4, a slight decrease, reaching 0.15 eV, is observed upon 0.4 ML deposition of Cu. then it stays constant at 5.20 eV up to 2 ML. As in the Au/Si(lll)-2 X 1 system it means that the surface alloy formation does not induce a large change of the surface dipole; in particular it is smaller than the change of 0.4 eV observed under the same conditions in the Ag/Si(lll)-2 X 1 system [l] where no alloying effect occurs. The work functions +,, for the lightly doped n-type sample, and +,,, for the highly doped p-type (fig. 4) remain unchanged upon Cu deposition up to 0.1 ML. Both +” and +r increase slightly (0.11 eV for +, and 0.16 eV for &,)

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reaching the same value of 4.76 eV at 8 = 0.4 ML, then the work function keeps the same value for both n- and p-type samples. Such a pinning of the Fermi level is justified by the non-zero density of surface states in the gap which is clearly observed at the low energy edge of the effective density of states in fig. 3. The density of surface states at Fermi level is even seen to increase with Cu coverage. We should also-point out, that the distance of the Fermi level from the valence band edge. (E,,), equal to @ - $, decreases from 0.48 eV for the clean surface to 0.38 eV (for the n-type) and 0.33 eV (for the p-type) and then an increase of the distance between the Fermi level and the valence band edge is observed, reaching 0.44 eV at 0 = 0.4 ML. This behaviour is different from that observed for Ag and Au under similar experimental conditions and disagrees with calculated predictions made in the case of an abrupt interface [14]. The corresponding barrier height $B defined in a Schottky structure as the distance between the conduction band edge and the Fermi level at the interface, that is here $Ia = E, - (@ - $), appears to be constant in the 1 ML range, as shown in fig. 4. Its value there is found to be r#~~= 1.12 - 0.44 = 0.68 + 0.05 eV in agreement with the (mu values obtained from electrical measurements [ll] on fully grown Schottky structures prepared under similar preparation conditions, these values being 0.62 eV from conductance measurements and 0.69 eV from capacitance measurements. Now, looking at the changes of the effective density of surface states, in fig. 5 one can see that from 0.10 ML coverage the two well-known peaks, at 0.2 and 0.6 eV below the valence band edge E,,, labeled A and B respectively, and which characterise the clean cleaved (111) surface of silicon, are progressively replaced by a single smaller peak, labeled C, at 0.4 eV below E,,, very similar to what was obtained in the case of Ag [6] and Au [12] under the same conditions. The initial step of the interface formation at room temperature on the cleaved Si(l11) surface differs markedly for Ag, where a monoatomic, ordered, non-metallic Ag layer forms between the Si(ll1) substrate and the epitaxial metal, for Au, where a silicon-gold alloyed layer segregates from the substrate upon formation of an intermediate Au metal layer and, here, for Cu which forms an ordered two-dimensional compound. In all cases, however, the characteristics of the future Schottky barrier appear to be settled in the one monolayer coverage range. It also seems that the initial structure of the clean silicon surface is more important for the interface crystallography [15] than for its electronic properties, at least upon room temperature preparation conditions. Thermal effect may induce true interdiffusion which would change significantly the electronic properties. Acknowledgment

The help of Dr. F. Proix through many fruitful discussions reading of the manuscript is gratefully acknowledged.

and the critical

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References [l] [2] [3] [4] [S] [6] [7] [8] [9] [IO] [II] [12] [13] [14] [15]

D. Bolmont, P. Chen, C.A. Sebenne and F. Proix, Phys. Rev. B24 (1981) 4552. K. Okuno, T. Ito, M. Iwami and A. Hiraki, Solid State Commun. 34 (1980) 493. A. Cros, J. Derrien and F. Salvan, Surface Sci. 110 (1981) 471. P. Perfetti, S. Nannarone, F. Patella, C. Quaresima, A. Savoia and G. Ottaviani, Phys. Rev. B26 (1982) 1125. F. Houzay, G.M. Guichar, A. Cros, F. Salvan, R. Pinchaux and J. Derrien, J. Phys. C (Solid State Phys.) 15 (1982) 7065. 0. Bisi, C. Calandra, L. Braicovich, G. Rossi, I. Abbati, 1. Lindau and W.E. Spicer, J. Phys. C (Solid State Phys.) 15 (1982) 4707. A. Taleb-Ibrahimi, C.A. Sebenne, D. Bolmont and P. Chen, Surface Sci. 146 (1984) 229. I. Abbati and M. Grioni, J. Vacuum Sci. Technol. 19 (1981) 631. G. Rossi, T. Kendelewicz, I. Lindau and W.E. Spicer, J. Vacuum Sci. Technol. Al (1983) 987. G. Rossi and I. Lindau, Phys. Rev. B28 (1983) 3597. A. ThanaIIakis, J. Phys. C (Solid State Phys.) 8 (1975) 655. A. Hiraki, A. Shimizu. M. Iwami. T. Narusawa and S. Komiya, Appl. Phys. Letters 26 (1975) 57. CA. Sebenne, Nuovo Cimento 39B (1977) 768. F. Guinea, J. Sanchez-Dehesa and F. Flares, J. Phys. C (Solid State Phys.) 16 (1983) 6499. F. Ringeisen, J. Derrien, E. Daugy, J.M. Layet, P. Mathiez and F. Salvan, J. Vacuum Sci. Technol. Bl (1983) 546.