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Formation and stability of the Cu(110)+c(2×2)-Si surface alloy studied by high resolution XPS
Surface Science 454–456 (2000) 778–782 www.elsevier.nl/locate/susc
Formation and stability of the Cu(110)+c(2×2)-Si surface alloy studied by high resolution XPS C. Rojas b, F.J. Palomares a, M.F. Lo´pez c, A. Goldoni d, G. Paolucci d, J.A. Martı´n-Gago a, * a Instituto Ciencia de Materiales de Madrid-CSIC, Cantoblanco, 28049-Madrid, Spain b Institut de Physique Applique´e, EPFL, CH-1015 Lausanne, Switzerland c Centro Nacional de Investigaciones Metalurgicas-CSIC, Avda. Gregorio del Amo 8, 28040-Madrid, Spain d Sincrotrone Trieste, S.c.p.A., S.S. 14 Km 163., in Area Science Park, 34012 Basovizza-Trieste, Italy
Abstract High-resolution synchrotron radiation photoemission has been used to investigate the formation of the Cu(110)+c(2×2)-Si surface alloy. The complex spectra of the Si 2p core-level are analyzed as multiple component spectra for different Si coverages and annealing temperatures of the surface alloy. The results show that c(2×2) islands are formed from the very beginning of the growth and that Si has a high diffusion length on Cu. The thermal stability of the surface alloy has been studied by measuring real-time photoemission spectra at different temperatures. The surface alloy is stable up to 180°C. Above this temperature disruption of the surface alloy and clustering of the Si atoms can be observed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Copper; Growth; Metal–semiconductor interfaces; Silicon; Surface structure, morphology, roughness, and topography; Synchrotron radiation photoelectron spectroscopy
1. Introduction Synchrotron radiation photoemission spectroscopy is one of the most important tools for establishing relationships between structural and electronic properties at surfaces. Thus, the surface core-level shifts (CLS) are related to charge transfer, electronic screening, geometrical structure and other basic properties of the electronic structure [1]. The ascription of the XPS components to atomic features is of great importance because it can be used to follow processes on surfaces such as surface dynamics and chemisorption [1–3]. * Corresponding author. Fax: +34-1-3720623. E-mail address: [email protected] (J.A. Martı´n-Gago)
However, a clear interpretation of the origin of the CLS is not always straightforward. Actually, many adsorbed or alloyed systems present small shifted components with binding energies close to each other that bring about additional difficulties to resolve. The high flux and resolving power of the new third generation synchrotron radiation sources allow for a clear identification very close shifted components even for very low coverage [4]. Therefore, it is possible to use the CLS components to gather information about the first stages of the growth and the evolution of different systems. In this paper we exploit CLS to obtain structural information about the growth mode and thermal stability of the Cu(110)+c(2×2)-Si surface alloy.
0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 00 ) 0 01 8 3 -7
C. Rojas et al. / Surface Science 454–456 (2000) 778–782
The Cu(110)+c(2×2)-Si surface alloy is formed by room temperature deposition of about 0.5 Si ML on a Cu(110) surface. This system is of fundamental importance because it incorporates two kinds of different atoms in an unusual configuration, i.e. a semiconductor deposited on a metal surface. Its geometrical structure has been determined recently by X-ray photoelectron diffraction ( XPD) [5,6 ] and low energy electron diffraction (LEED) [7] experiments. It consists of an alternative replacement of the Cu surface atoms of the [110] rows by the deposited Si atoms, forming thus a c(2×2) superstructure. The Si atoms are found to be inward relaxed with respect to the surface Cu ones [5–7]. In a previous study, our group have concluded that up to four different components can be unequivocally separated from the Si 2p core-level peak [8]. By a combination of scanning tunneling microscopy (STM ), XPD and X-ray photoemission ( XPS) techniques these components were assigned to the different atomic environments observed by STM. In the present work we take advantage of the previous Si 2p assignment [8] to study the surface alloy formation from the very beginning of the growth. Thus, by measuring the evolution of the Si 2p core-level with increasing Si coverage we give a picture of the alloying process. We will show that strong diffusion takes place to form surface alloy type of bonds from the first arriving atoms. Furthermore, we will show some real-time Si 2p core-level spectra as a function of the annealing temperature recorded to gather information about the thermal stability of all the previously described atomic environments. We have found that the surface alloy is stable up to 180°C while above this temperature the surface alloy layer disrupts leading to Si clustering and to the formation of high coverage phases.
2. Experimental details Experiments were carried out at the SuperESCA beamline at the ELETTRA synchrotron radiation facility [9]. The photon energy was varied between 178 and 800 eV. The end station is an ultrahigh vacuum chamber equipped with an hemispherical
779
electron analyzer, LEED apparatus and a computer-assisted sample manipulator. The overall energy resolution (beam line+analyzer) at 178 eV was around 70 meV. Cu(110) samples were prepared by repeated cycles of ion bombardment and annealing at 550° C. After the cycles the surface exhibited the characteristic 1×1 sharp LEED pattern. XPS confirmed the absence of O, C and S impurities at the surface prior and after deposition. Si was evaporated in situ using an electron bombardment Si cell previously calibrated by a quartz crystal. The Si coverage was additionally estimated by measuring the Cu 3p to Si 2p corelevels intensity ratio.
3. Experimental results and discussion Fig. 1 shows some selected Si 2p core-level spectra for various Si coverages recorded at room temperature. The coverage ranges from 0.04 ML (bottom spectrum) to around six Si atomic layers (top spectrum). The spectrum labeled as 0.5 ML corresponds to the completion of the surface alloy, i.e. when the half-order LEED spots are narrowest and sharpest. A mathematical fit of every spectrum using a convolution of Doniach–Sunjic∞ functions with gaussians is indicated in the figure by a thin line. The relevant parameters for the decomposition of the Si 2p core-level peak are well known from scientific literature [1–3] and our previous work[8]. The overall fit is represented by the solid line overlapping the data points. In the figure, we can distinguish clearly two different kinds of spectra, those which were recorded for coverage lower and higher than 0.5 ML (i.e. before and after the completion of the surface alloy). The low coverage spectra are characterized by very narrow peaks and they are very similar to each other. The small gaussian broadening indicates a high degree of order from the first moments of the growth. Particularly, the Si 2p peak has an experimental full width at half maximum ( FWHM ) at room temperature ranging from 200 to 150 meV for lower (<0.4 Si ML) and higher coverage (0.4– 0.5 ML), respectively. All of them have been fitted using the same line-shape parameters and components as those for the surface alloy of Ref. [8],
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C. Rojas et al. / Surface Science 454–456 (2000) 778–782
Fig. 1. Si 2p core-level photoemission spectra obtained for various Si coverages on Cu(110). Points represent experimental data and lines the result of the fit. Dotted curves at the bottom of every spectrum represent the four different components used for the decomposition of the peaks. The Cu(110)+c(2×2)-Si surface alloy is obtained for a coverage of 0.5 ML. The upper spectrum is for an approximate coverage of 6 ML.
that is, four spin–orbit doublets, which are shown at the bottom of every spectrum (dotted lines). Just the relative intensities have been allowed to vary. In a previous work, these components were related with different sites at the surface. Using STM we have concluded that the final morphology of the surface alloy presents large c(2×2) terraces, and in addition islands and Si clusters. The islands grow on top of the surface alloy terraces with the same atomic structure. Thus, they present surface
alloy structure both in the surface and underneath. All these features are reflected in the Si 2p photoemission spectrum as small shifted components from the main peak [8]. In that work, we have shown that the main component in the Si 2p corelevel peak corresponds to the c(2×2) terraces (S1), the second and the third to Si atoms on top (S2) and under (S3) the islands. The buried Si atoms of the islands have a different atomic environment which will be reflected in the XPS spectra as a shifted component with respect to S2. The fourth (S4), shifted around −0.4 eV with respect to the main peak, corresponds to small Si clusters that appear spread out on the surface. This issue is thoroughly discussed in Ref. [8]. We have compared the integrated area of the main component (S1) with the area of the other small peaks (S2, S3, S4). The average ratio between them for all coverages is approximately constant: S4/S1=0.2±0.1; S3/S1=0.3±0.15 and S2/S1= 0.2±0.1. These values have been found by averaging different experiments. We believe that the error bars are related to uncontrolled changes in the experimental conditions, such surface roughness, evaporation rate, surface temperature, etc. The fact that from the very beginning all the four components are in a constant ratio in all spectra suggests that both islands and clusters grow from the very beginning. Then, the alloy formation cannot be considered as a pure layer-by-layer growth but the system prefers rather to minimize its energy by growing two layers at the same time. This growth mode operates from the beginning until the completion of the surface alloy layer. A similar behavior has been found in Co–Cu(111) growth [10]. It is interesting to remark that we present a high resolution Si 2p core-level recorded for 0.04 Si ML. For this low coverage it is possible to measure a well-structured core-level spectrum with the same four components at the same binding energies. The corresponding LEED pattern showed the unmodified (1×1) reconstruction without any traces from the c(2×2) indicating the absence of long-range order at this low coverage. Wide and weak half-order spots are appreciated for coverages higher than 0.2 ML. Thus, for this low coverage one could expect to have just a few
C. Rojas et al. / Surface Science 454–456 (2000) 778–782
atoms spread on the surface, although the binding energy of the Si 2p core-level spectrum is the same as that of the complete surface alloy (within the experimental resolution). The fact that all spectra are composed of peaks having the same binding energy indicates that the atomic structure does not change during growth. Thus, surface diffusion has clustered together all Si atoms forming small alloy islands. This suggest that the diffusion length of the arriving Si atoms on the Cu surface should be high. These small islands have been already observed by STM [11], although it was not possible to conclude using this technique whether or not they present order inside. For coverages higher than 0.5 ML (i.e. an Si film on the surface alloy), the Si 2p peak becomes broader. This fact indicates the formation of disordered phases. The same set of components cannot be used any more for fitting the spectra. The component corresponding to the surface alloy (S1) is observed in the upper spectra as a small contribution. The maximum of the peak appears at a relative binding energy around −0.4 eV. This value corresponds to SiMSi bonding as can be inferred for the upper spectrum where a six-layer thick Si film has been examined. The intermediate component in the 1.7 ML peak has no a straightforward assignment and it might be related to the reported (2×2) phase which appears for coverages between 0.6 and 0.9 ML [11]. The shift at −0.4 eV could be interpreted as a charge transfer process in the 2D alloy from the Cu to the Si atoms. However, small electronegativity differences, as is the case of Si and Cu, may not correctly predict the direction of the charge transfer [2,12]. In addition, extra atomic relaxation effects should be very important in this case, where the Si atoms are embedded in a metallic matrix. Particularly, core-level shifts of about 5 eV induced by screening have been theoretically predicted for a Si adatom on a high density metal surface ( jellium) [13] Additionally, a 0.4 eV shift in the Si 2p peak from a metallic ErSi1.7 exclusively induced by extra-atomic effects has been reported [14]. Therefore, the core-level shifts are not adequate on their own for extracting insights about charge transfer upon alloying. Fig. 2 shows the behavior of the Si 2p core-level
781
Fig. 2. Intensity evolution of the four photoemission components of the Si 2p core-level peak from the Cu(110)+c(2×2)-Si surface alloy for different annealing temperatures.
components for an Si surface coverage of 0.6 ML, as result of a real-time annealing experiment (further details about the experimental method can be found in Ref. [4]). The sample was continuously heated from room temperature to 300°C. The measuring time per spectrum was around 10 s. This permits the recording of spectra in a very small temperature interval. In Fig. 2 we show the area of the four referred peaks as a function of the substrate temperature in the range from 30 to 300°C. This method is particularly interesting to evaluate the stability of the surface alloy and other Si environments with the substrate temperature. However, this is not an easy task because of the close binding energy of the components. To produce Fig. 2 we have fitted the spectra using the same components with the same parameters, except for the intensity and the gaussian width because the phonon broadening factor increase continuously with temperature. At first sight three different regions can be distinguished. In the first one up to 180°C all the components were approximately stable. However, a continuous decrease of the component S2 and an increase of the component S3 is observed. These two components were assigned to the formation of islands, and so both of them should behave on
782
C. Rojas et al. / Surface Science 454–456 (2000) 778–782
the same way. Although we do not have a definitive explanation for this difference, we believe that other extra peaks with similar binding energies and related to other structures could appear. These new peaks could mask the straight interpretation of those small components. For instance, formation of Si chains induced by temperature for coverages higher than 0.5 ML has been observed by STM [11]. This could be related with the intermediate component observed in the spectrum corresponding to 1.7 Si ML in Fig. 1. In a second stage the Si–Cu surface alloy decreases in intensity and at the same time component S4 increases. This is a clear indication of a disruption of the surface alloy, both in terraces and islands. The Si atoms removed from the alloy diffuse at this temperature to form clusters. In region 3 all components decrease indicating Si diffusion towards the Cu bulk. However, the component S2 increases importantly. We have verified by STM that this increase could be related to the formation of a (3×4) surface reconstruction consisting of Si chains rather than to an increase in the area of the islands. This reconstruction could be responsible for a component in the Si 2p peak at this particular binding energy. Summarizing, we have investigated the formation of the Cu(110)+c(2×2)-Si surface alloy by high resolution synchrotron radiation photoemission. From the very beginning of the growth c(2×2) islands are formed indicating a very high diffusion length of Si in Cu. The c(2×2) is stable up to 180°C. Above this temperature there is a disruption of the surface alloy and Si clustering.
Acknowledgements This work is partially supported by the Spanish DGCYT project PB98/1224 and by the European Union under contract No. ERBCHGECT920013 (Access to Large Scale Installations). FJP acknowledges the support from the grant Comunidad Auto´noma de Madrid 07N/0062/98 (Spain).
References [1] F.J. Himpsel, Surf. Sci. 299–300 (1994) 525. [2] W.F. Egeholf, Surf. Sci. Rep. 6 (1986) 253. [3] J.A. Carlisle, T. Miller, T.C. Chiang, Phys. Rev. B 45 (1992) 3811. [4] A. Baraldi, S. Lizzit, D. Cocco, G. Paolucci, Phys. Rev. B 57 (1988) 3811. [5] J.A. Martı´n-Gago, R. Fasel, J. Hayoz, R.G. Agostino, D. Naumovic∞, P. Aebi, L. Schlarapach, Phys. Rev. B 55 (1997) 12 896. [6 ] C. Rojas, C. Polop, E. Roma´n, J.A. Martı´n-Gago, R. Gunnella, B. Brena, D. Cocco, G. Paolucci, Phys. Rev. B 57 (1998) 4493. [7] C. Polop, C. Rojas, E. Roma´n, J.A. Martı´n-Gago, B. Brena, D. Cocco, G. Paolucci, Surf. Sci. 497 (1998) 268. [8] J.A. Martı´n-Gago, C. Rojas, C. Polop, J.L. Sacedo´n, E. Roma´n, A. Goldoni, G. Paolucci, Phys. Rev. B 59 (1999) 3070. [9] A. Abrami et al., Rev. Sci. Instrum. 66 (1995) 1618. [10] J. de la Figuera, J.E. Prieto, C. Ocal, R. Miranda, Phys. Rev. B 47 (1993) 13043. [11] C. Polop, J.L Sacedo´n, J.A. Martı´n-Gago, Surf. Sci. 402 (1998) 245. [12] G. Le Lay, M. Gothelid, T.M. Grehk, M. Bjorkquist, U.O. Karlsson, W. Tuaristov, Phys. Rev. B 50 (1994) 14 277. [13] A.R. Willians, N.D. Lang, Surf. Sci. 68 (1977) 138. [14] J.A. Martı´n-Gago, J.M. Go´mez-Rodrı´guez, J.Y. Veuillen, Phys. Rev. B 55 (1997) 5136.
Formation and stability of the Cu(110)+c(2×2)-Si surface alloy studied by high resolution XPS C. Rojas b, F.J. Palomares a, M.F. Lo´pez c, A. Goldoni d, G. Paolucci d, J.A. Martı´n-Gago a, * a Instituto Ciencia de Materiales de Madrid-CSIC, Cantoblanco, 28049-Madrid, Spain b Institut de Physique Applique´e, EPFL, CH-1015 Lausanne, Switzerland c Centro Nacional de Investigaciones Metalurgicas-CSIC, Avda. Gregorio del Amo 8, 28040-Madrid, Spain d Sincrotrone Trieste, S.c.p.A., S.S. 14 Km 163., in Area Science Park, 34012 Basovizza-Trieste, Italy
Abstract High-resolution synchrotron radiation photoemission has been used to investigate the formation of the Cu(110)+c(2×2)-Si surface alloy. The complex spectra of the Si 2p core-level are analyzed as multiple component spectra for different Si coverages and annealing temperatures of the surface alloy. The results show that c(2×2) islands are formed from the very beginning of the growth and that Si has a high diffusion length on Cu. The thermal stability of the surface alloy has been studied by measuring real-time photoemission spectra at different temperatures. The surface alloy is stable up to 180°C. Above this temperature disruption of the surface alloy and clustering of the Si atoms can be observed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Copper; Growth; Metal–semiconductor interfaces; Silicon; Surface structure, morphology, roughness, and topography; Synchrotron radiation photoelectron spectroscopy
1. Introduction Synchrotron radiation photoemission spectroscopy is one of the most important tools for establishing relationships between structural and electronic properties at surfaces. Thus, the surface core-level shifts (CLS) are related to charge transfer, electronic screening, geometrical structure and other basic properties of the electronic structure [1]. The ascription of the XPS components to atomic features is of great importance because it can be used to follow processes on surfaces such as surface dynamics and chemisorption [1–3]. * Corresponding author. Fax: +34-1-3720623. E-mail address: [email protected] (J.A. Martı´n-Gago)
However, a clear interpretation of the origin of the CLS is not always straightforward. Actually, many adsorbed or alloyed systems present small shifted components with binding energies close to each other that bring about additional difficulties to resolve. The high flux and resolving power of the new third generation synchrotron radiation sources allow for a clear identification very close shifted components even for very low coverage [4]. Therefore, it is possible to use the CLS components to gather information about the first stages of the growth and the evolution of different systems. In this paper we exploit CLS to obtain structural information about the growth mode and thermal stability of the Cu(110)+c(2×2)-Si surface alloy.
0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 00 ) 0 01 8 3 -7
C. Rojas et al. / Surface Science 454–456 (2000) 778–782
The Cu(110)+c(2×2)-Si surface alloy is formed by room temperature deposition of about 0.5 Si ML on a Cu(110) surface. This system is of fundamental importance because it incorporates two kinds of different atoms in an unusual configuration, i.e. a semiconductor deposited on a metal surface. Its geometrical structure has been determined recently by X-ray photoelectron diffraction ( XPD) [5,6 ] and low energy electron diffraction (LEED) [7] experiments. It consists of an alternative replacement of the Cu surface atoms of the [110] rows by the deposited Si atoms, forming thus a c(2×2) superstructure. The Si atoms are found to be inward relaxed with respect to the surface Cu ones [5–7]. In a previous study, our group have concluded that up to four different components can be unequivocally separated from the Si 2p core-level peak [8]. By a combination of scanning tunneling microscopy (STM ), XPD and X-ray photoemission ( XPS) techniques these components were assigned to the different atomic environments observed by STM. In the present work we take advantage of the previous Si 2p assignment [8] to study the surface alloy formation from the very beginning of the growth. Thus, by measuring the evolution of the Si 2p core-level with increasing Si coverage we give a picture of the alloying process. We will show that strong diffusion takes place to form surface alloy type of bonds from the first arriving atoms. Furthermore, we will show some real-time Si 2p core-level spectra as a function of the annealing temperature recorded to gather information about the thermal stability of all the previously described atomic environments. We have found that the surface alloy is stable up to 180°C while above this temperature the surface alloy layer disrupts leading to Si clustering and to the formation of high coverage phases.
2. Experimental details Experiments were carried out at the SuperESCA beamline at the ELETTRA synchrotron radiation facility [9]. The photon energy was varied between 178 and 800 eV. The end station is an ultrahigh vacuum chamber equipped with an hemispherical
779
electron analyzer, LEED apparatus and a computer-assisted sample manipulator. The overall energy resolution (beam line+analyzer) at 178 eV was around 70 meV. Cu(110) samples were prepared by repeated cycles of ion bombardment and annealing at 550° C. After the cycles the surface exhibited the characteristic 1×1 sharp LEED pattern. XPS confirmed the absence of O, C and S impurities at the surface prior and after deposition. Si was evaporated in situ using an electron bombardment Si cell previously calibrated by a quartz crystal. The Si coverage was additionally estimated by measuring the Cu 3p to Si 2p corelevels intensity ratio.
3. Experimental results and discussion Fig. 1 shows some selected Si 2p core-level spectra for various Si coverages recorded at room temperature. The coverage ranges from 0.04 ML (bottom spectrum) to around six Si atomic layers (top spectrum). The spectrum labeled as 0.5 ML corresponds to the completion of the surface alloy, i.e. when the half-order LEED spots are narrowest and sharpest. A mathematical fit of every spectrum using a convolution of Doniach–Sunjic∞ functions with gaussians is indicated in the figure by a thin line. The relevant parameters for the decomposition of the Si 2p core-level peak are well known from scientific literature [1–3] and our previous work[8]. The overall fit is represented by the solid line overlapping the data points. In the figure, we can distinguish clearly two different kinds of spectra, those which were recorded for coverage lower and higher than 0.5 ML (i.e. before and after the completion of the surface alloy). The low coverage spectra are characterized by very narrow peaks and they are very similar to each other. The small gaussian broadening indicates a high degree of order from the first moments of the growth. Particularly, the Si 2p peak has an experimental full width at half maximum ( FWHM ) at room temperature ranging from 200 to 150 meV for lower (<0.4 Si ML) and higher coverage (0.4– 0.5 ML), respectively. All of them have been fitted using the same line-shape parameters and components as those for the surface alloy of Ref. [8],
780
C. Rojas et al. / Surface Science 454–456 (2000) 778–782
Fig. 1. Si 2p core-level photoemission spectra obtained for various Si coverages on Cu(110). Points represent experimental data and lines the result of the fit. Dotted curves at the bottom of every spectrum represent the four different components used for the decomposition of the peaks. The Cu(110)+c(2×2)-Si surface alloy is obtained for a coverage of 0.5 ML. The upper spectrum is for an approximate coverage of 6 ML.
that is, four spin–orbit doublets, which are shown at the bottom of every spectrum (dotted lines). Just the relative intensities have been allowed to vary. In a previous work, these components were related with different sites at the surface. Using STM we have concluded that the final morphology of the surface alloy presents large c(2×2) terraces, and in addition islands and Si clusters. The islands grow on top of the surface alloy terraces with the same atomic structure. Thus, they present surface
alloy structure both in the surface and underneath. All these features are reflected in the Si 2p photoemission spectrum as small shifted components from the main peak [8]. In that work, we have shown that the main component in the Si 2p corelevel peak corresponds to the c(2×2) terraces (S1), the second and the third to Si atoms on top (S2) and under (S3) the islands. The buried Si atoms of the islands have a different atomic environment which will be reflected in the XPS spectra as a shifted component with respect to S2. The fourth (S4), shifted around −0.4 eV with respect to the main peak, corresponds to small Si clusters that appear spread out on the surface. This issue is thoroughly discussed in Ref. [8]. We have compared the integrated area of the main component (S1) with the area of the other small peaks (S2, S3, S4). The average ratio between them for all coverages is approximately constant: S4/S1=0.2±0.1; S3/S1=0.3±0.15 and S2/S1= 0.2±0.1. These values have been found by averaging different experiments. We believe that the error bars are related to uncontrolled changes in the experimental conditions, such surface roughness, evaporation rate, surface temperature, etc. The fact that from the very beginning all the four components are in a constant ratio in all spectra suggests that both islands and clusters grow from the very beginning. Then, the alloy formation cannot be considered as a pure layer-by-layer growth but the system prefers rather to minimize its energy by growing two layers at the same time. This growth mode operates from the beginning until the completion of the surface alloy layer. A similar behavior has been found in Co–Cu(111) growth [10]. It is interesting to remark that we present a high resolution Si 2p core-level recorded for 0.04 Si ML. For this low coverage it is possible to measure a well-structured core-level spectrum with the same four components at the same binding energies. The corresponding LEED pattern showed the unmodified (1×1) reconstruction without any traces from the c(2×2) indicating the absence of long-range order at this low coverage. Wide and weak half-order spots are appreciated for coverages higher than 0.2 ML. Thus, for this low coverage one could expect to have just a few
C. Rojas et al. / Surface Science 454–456 (2000) 778–782
atoms spread on the surface, although the binding energy of the Si 2p core-level spectrum is the same as that of the complete surface alloy (within the experimental resolution). The fact that all spectra are composed of peaks having the same binding energy indicates that the atomic structure does not change during growth. Thus, surface diffusion has clustered together all Si atoms forming small alloy islands. This suggest that the diffusion length of the arriving Si atoms on the Cu surface should be high. These small islands have been already observed by STM [11], although it was not possible to conclude using this technique whether or not they present order inside. For coverages higher than 0.5 ML (i.e. an Si film on the surface alloy), the Si 2p peak becomes broader. This fact indicates the formation of disordered phases. The same set of components cannot be used any more for fitting the spectra. The component corresponding to the surface alloy (S1) is observed in the upper spectra as a small contribution. The maximum of the peak appears at a relative binding energy around −0.4 eV. This value corresponds to SiMSi bonding as can be inferred for the upper spectrum where a six-layer thick Si film has been examined. The intermediate component in the 1.7 ML peak has no a straightforward assignment and it might be related to the reported (2×2) phase which appears for coverages between 0.6 and 0.9 ML [11]. The shift at −0.4 eV could be interpreted as a charge transfer process in the 2D alloy from the Cu to the Si atoms. However, small electronegativity differences, as is the case of Si and Cu, may not correctly predict the direction of the charge transfer [2,12]. In addition, extra atomic relaxation effects should be very important in this case, where the Si atoms are embedded in a metallic matrix. Particularly, core-level shifts of about 5 eV induced by screening have been theoretically predicted for a Si adatom on a high density metal surface ( jellium) [13] Additionally, a 0.4 eV shift in the Si 2p peak from a metallic ErSi1.7 exclusively induced by extra-atomic effects has been reported [14]. Therefore, the core-level shifts are not adequate on their own for extracting insights about charge transfer upon alloying. Fig. 2 shows the behavior of the Si 2p core-level
781
Fig. 2. Intensity evolution of the four photoemission components of the Si 2p core-level peak from the Cu(110)+c(2×2)-Si surface alloy for different annealing temperatures.
components for an Si surface coverage of 0.6 ML, as result of a real-time annealing experiment (further details about the experimental method can be found in Ref. [4]). The sample was continuously heated from room temperature to 300°C. The measuring time per spectrum was around 10 s. This permits the recording of spectra in a very small temperature interval. In Fig. 2 we show the area of the four referred peaks as a function of the substrate temperature in the range from 30 to 300°C. This method is particularly interesting to evaluate the stability of the surface alloy and other Si environments with the substrate temperature. However, this is not an easy task because of the close binding energy of the components. To produce Fig. 2 we have fitted the spectra using the same components with the same parameters, except for the intensity and the gaussian width because the phonon broadening factor increase continuously with temperature. At first sight three different regions can be distinguished. In the first one up to 180°C all the components were approximately stable. However, a continuous decrease of the component S2 and an increase of the component S3 is observed. These two components were assigned to the formation of islands, and so both of them should behave on
782
C. Rojas et al. / Surface Science 454–456 (2000) 778–782
the same way. Although we do not have a definitive explanation for this difference, we believe that other extra peaks with similar binding energies and related to other structures could appear. These new peaks could mask the straight interpretation of those small components. For instance, formation of Si chains induced by temperature for coverages higher than 0.5 ML has been observed by STM [11]. This could be related with the intermediate component observed in the spectrum corresponding to 1.7 Si ML in Fig. 1. In a second stage the Si–Cu surface alloy decreases in intensity and at the same time component S4 increases. This is a clear indication of a disruption of the surface alloy, both in terraces and islands. The Si atoms removed from the alloy diffuse at this temperature to form clusters. In region 3 all components decrease indicating Si diffusion towards the Cu bulk. However, the component S2 increases importantly. We have verified by STM that this increase could be related to the formation of a (3×4) surface reconstruction consisting of Si chains rather than to an increase in the area of the islands. This reconstruction could be responsible for a component in the Si 2p peak at this particular binding energy. Summarizing, we have investigated the formation of the Cu(110)+c(2×2)-Si surface alloy by high resolution synchrotron radiation photoemission. From the very beginning of the growth c(2×2) islands are formed indicating a very high diffusion length of Si in Cu. The c(2×2) is stable up to 180°C. Above this temperature there is a disruption of the surface alloy and Si clustering.
Acknowledgements This work is partially supported by the Spanish DGCYT project PB98/1224 and by the European Union under contract No. ERBCHGECT920013 (Access to Large Scale Installations). FJP acknowledges the support from the grant Comunidad Auto´noma de Madrid 07N/0062/98 (Spain).
References [1] F.J. Himpsel, Surf. Sci. 299–300 (1994) 525. [2] W.F. Egeholf, Surf. Sci. Rep. 6 (1986) 253. [3] J.A. Carlisle, T. Miller, T.C. Chiang, Phys. Rev. B 45 (1992) 3811. [4] A. Baraldi, S. Lizzit, D. Cocco, G. Paolucci, Phys. Rev. B 57 (1988) 3811. [5] J.A. Martı´n-Gago, R. Fasel, J. Hayoz, R.G. Agostino, D. Naumovic∞, P. Aebi, L. Schlarapach, Phys. Rev. B 55 (1997) 12 896. [6 ] C. Rojas, C. Polop, E. Roma´n, J.A. Martı´n-Gago, R. Gunnella, B. Brena, D. Cocco, G. Paolucci, Phys. Rev. B 57 (1998) 4493. [7] C. Polop, C. Rojas, E. Roma´n, J.A. Martı´n-Gago, B. Brena, D. Cocco, G. Paolucci, Surf. Sci. 497 (1998) 268. [8] J.A. Martı´n-Gago, C. Rojas, C. Polop, J.L. Sacedo´n, E. Roma´n, A. Goldoni, G. Paolucci, Phys. Rev. B 59 (1999) 3070. [9] A. Abrami et al., Rev. Sci. Instrum. 66 (1995) 1618. [10] J. de la Figuera, J.E. Prieto, C. Ocal, R. Miranda, Phys. Rev. B 47 (1993) 13043. [11] C. Polop, J.L Sacedo´n, J.A. Martı´n-Gago, Surf. Sci. 402 (1998) 245. [12] G. Le Lay, M. Gothelid, T.M. Grehk, M. Bjorkquist, U.O. Karlsson, W. Tuaristov, Phys. Rev. B 50 (1994) 14 277. [13] A.R. Willians, N.D. Lang, Surf. Sci. 68 (1977) 138. [14] J.A. Martı´n-Gago, J.M. Go´mez-Rodrı´guez, J.Y. Veuillen, Phys. Rev. B 55 (1997) 5136.