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Formation and electronic structure of TiO2–Ag interface
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 91 (2007) 1051–1054 www.elsevier.com/locate/solmat
Formation and electronic structure of TiO22Ag interface Andriy Romanyuk, Peter Oelhafen Institute of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland Received 9 November 2006; accepted 19 February 2007 Available online 6 April 2007
Abstract In the present work the formation of the interface between polycrystalline silver and thin films of titanium oxide was studied with photoelectron spectroscopy (XPS, UPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Titanium oxide was deposited stepwise on 100 nm thick silver films by reactive magnetron sputtering allowing to study the evolution of the interface formation process. The process involves two steps: formation of thin layer of silver oxide and subsequent growth of the TiO2 film. For better understanding of the silver oxidation process, pure silver films were exposed to a low temperature Ar/O plasma for different time intervals providing a possibility to investigate early stages of the oxide film growth. r 2007 Elsevier B.V. All rights reserved. Keywords: Silver; Titanium oxide; Oxide–metal interface; Sputter deposition; Photoelectron spectroscopy
1. Introduction Transparent heat mirrors have been a subject of intense investigations due to their wide use for energy saving applications and solar energy collection [1–5]. The heat mirror usually consists of a thin noble metal film (Au, Ag, Cu) sandwiched between two antireflecting oxide layers among which titanium oxide is most commonly used [5,6]. As a metal component in such multilayer structure, thin silver films are the most promising candidates owing to silver’s high transmittance in the visible, and high reflection in the infrared range. TiO2 =Ag layer system is also a frequent choice in interference stacks as a part of optical band pass filters [7]. While a number of works are devoted to the growth of thin silver films on dielectrics, only few studies report on oxide preparation on silver [8–10]. In the case of oxide film deposition on metal substrate by reactive magnetron sputtering, the metal surface faces low temperature plasma that contains highly reactive oxygen atoms, ions and radicals. In these conditions the formation of interface phase, whose physical properties may differ from the bulk ones, is a primary reaction in the film growth process. Corresponding author. Tel.: +41 61 267 37 20; fax: +41 61 267 37 84.
E-mail address: [email protected] (A. Romanyuk). 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.02.016
Structure and composition of the interfacial region may play a critical role in applications of such optical thin film devices whose performance may be significantly impaired. In the present study we describe the formation of the interface between titanium oxide and polycrystalline silver films studied with in situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The in-depth composition analysis of interface components was performed with time-of-flight secondary ion mass spectrometry (TOF-SIMS). 2. Experimental details Thin films of silver (100 nm) were deposited on clean boron-doped (1 0 0)-oriented silicon wafer (resistivity 10 O cm) by direct current (DC) sputtering of an Ag target (purity 99.95%, +23 mm) in Ar atmosphere at pressure of 0.5 Pa. No carbon and oxygen contamination was detected in freshly sputtered Ag films. Titanium oxide was deposited stepwise on silver in the same experimental chamber by RF sputtering of titanium target (purity 99.95%, +23 mm) in Ar/O gas mixture containing 7% of oxygen. The gases were fed into the chamber via mass-flow controllers, while the gas total pressure in the chamber was controlled by a throttling valve and kept at 2.0 Pa. The RF power applied to the target was equal to 30 W. The deposition rate of
ARTICLE IN PRESS A. Romanyuk, P. Oelhafen / Solar Energy Materials & Solar Cells 91 (2007) 1051–1054
titanium oxide was fixed at 0:02 ML s1 and monitored by a quartz crystal monitor, calibrated through the thickness measurement with secondary electron microscopy in crosssection regime. According to our estimates, one monolayer (ML) corresponds to 0.24 nm of a titanium oxide. The sample was subsequently transferred into the spectrometer measurement chamber without breaking the vacuum under a background pressure below 1 107 Pa. The photoelectron spectroscopy analysis was performed on a VG ESCALAB 210 system equipped with a monochromatized Al Ka ð1486:6 eVÞ radiation source and a helium discharge lamp emitting in ultraviolet range (He I, 21:22 eV). The photoemission spectra were recorded at normal emission with an overall resolution of about 0.5 and 0.2 eV for XPS and for UPS, respectively. The energy positions of the spectra were calibrated with reference to the 4f 7=2 level of clean gold sample at 84.0 eV binding energy. In order to obtain the peak position of chemically shifted components, a fit procedure using Doniach–Sunjic functions [11] was applied after a Shirley background subtraction [12]. The stoichiometry of the deposited oxide films was evaluated from the O 1s and Ti 2p XPS peak ratio taking into account the ionization cross-sections of corresponding atoms. TOF-SIMS depth profiling was performed with an IONTOF IV system in dual beam mode with 1.0 keV Csþ sputtering and 10 keV Arþ primary ion beam for secondary ion generation.
π
HeI(hν=21.2eV) σ
Intensity (a.u.)
1052
5.0ML
2.0ML A B 1.0ML
0.5ML
AgFilm 10
8
6 4 2 Binding Energy (eV)
0
Fig. 1. Evolution of the valence band spectra (He I) as a function of the amount of titanium oxide deposited on polycrystalline silver film.
106
3. Results and discussion
107Ag
105
Intensity (a.u.)
Fig. 1 shows the valence band spectra (He I) recorded as a function of amount of deposited titanium oxide. The bottommost spectrum reveals the measurement on virgin polycrystalline Ag film and is dominated by the emission from the silver 4d band located above a binding energy of 4 eV. The deposition of small amounts of TiO2 (0.5 ML) results in substantial modification of Ag 4d spectrum and appearance of additional features located at about 3 and 2 eV denoted in Fig. 1 as ‘‘A’’ and ‘‘B’’, respectively. The shape and position of the spectrum is close to that observed for silver oxide suggesting the oxidation of silver surface [13]. With increasing amount of deposited TiO2 , the silver d band structure diminishes and the contribution from oxygen 2p electron states becomes more pronounced. After deposition of a total of 5 ML TiO2 the valence band consists of two broad overlapping peaks located at about 3.8 and 6 eV that correspond to non-bonding (p) and bonding (s) oxygen 2p orbitals with titanium d band [14]. Oxidation of silver and the formation of a thin oxide film is also confirmed by TOF-SIMS depth profile analysis of the structure with 20 nm of TiO2 on top of silver. As can be seen from Fig. 2 the interfacial region between silver and TiO2 is characterized by a significant increase in 123 ½AgO fragment ions intensity confirming the presence of the thin silver oxide layer.
64[TiO]
104
123[AgO]
103
102
101 10
20
30 Depth (nm)
40
50
Fig. 2. TOF-SIMS depth profile of 20 nm TiO2 deposited on Ag film.
In order to elucidate the dynamic of silver oxidation and oxide formation process, the freshly prepared silver film was exposed without titan sputtering to the argon plasma containing 7% of oxygen for different time intervals, allowing to investigate early stages of the oxide–metal
ARTICLE IN PRESS A. Romanyuk, P. Oelhafen / Solar Energy Materials & Solar Cells 91 (2007) 1051–1054
I1 s
where is the integrated signal intensity from Ag 3d line measured on unattenuated i.e. uncovered silver film, while I s represents the corresponding intensity from a sample covered with an overlayer of silver oxide of thickness d. y and l represent the electron take-off angle measured from the sample normal and electron inelastic mean free path, respectively. The value of l was derived from the predictive equation of Gries [17] and was found to be about 1.5 nm.
Ag3d5/2(hν=1486.6eV)
Ag+
Ag0
Intensity (a.u.)
90sec
45sec
AgFilm 370
369 368 367 Binding Energy (eV)
366
6
5
4
3
2
1
0 0
50 100 150 Plasma exposure time (sec)
200
Fig. 4. Oxide thickness as a function of plasma exposure time. The vertical error bar indicates the standard deviation of the corresponding intensities obtained by fit procedure. The uncertainty resulting from determination of electron inelastic mean free path is not included.
The results of this procedure is summarized in Fig. 4 which demonstrates the development of the oxide thickness upon exposure of the silver surface to the low temperature Ar/O plasma. As seen from the figure, in the early oxidation stages the oxide thickness increases rapidly, then the process saturates and the constant value of around 6 nm is reached after a total plasma exposure time of 180 s. The oxidation of silver may have serious impact on the optical performance of filters that use very thin ðo10 nmÞ silver films. It is well known that at this thickness range silver on dielectric substrates forms islands interconnected by thin ‘‘bridges’’ [18]. The oxidation of these originally conducting links results in the formation of isolated silver islands with markedly changed optical behavior. 4. Conclusions
18sec
371
7
Oxide thickness (nm)
interface formation process and the oxide film growth. In this experimental setup arrangement the magnetron with titanium target was replaced by an electrode that consisted of a graphite covered tantalum sheet. Fig. 3 shows the evolution of Ag 3d5=2 doublet component with plasma exposure time. The Ag 3d5=2 peak of pure silver film (bottommost spectrum) is nearly symmetric and positioned at a binding energy of 368.4 eV. The exposure of the Ag film to the reactive plasma results in peak broadening and shift in binding energy. The peak decomposition reveals the presence of a second component displaced in 0.4 eV towards lower binding energy which can be easily identified as Ag2 O [15]. With an increase in the plasma exposure time, the intensity of the oxide component increases gradually, reflecting continuous growth of the oxide film. The thickness of the oxide layer was evaluated with formula [16]: d Is ¼ I1 exp , s l cos y
1053
365
Fig. 3. The Ag 3d5=2 core-level spectra taken at normal emission for different plasma exposure times. Dashed lines represent the chemically shifted components after core-line decomposition.
The interface formation process between silver and titanium oxide has been studied with in situ photoelectron spectroscopy. It has been shown that sputtering of titanium oxide on silver, in the first reaction step, leads to silver oxidation and formation of thin silver oxide layer as confirmed by a TOF-SIMS depth profile analysis. The oxidation kinetics of silver surface has been studied stepby-step by exposing pure silver films to the low temperature plasma that contains Ar/O gas mixture. We have observed that the oxide thickness depends on oxidation time and in saturation regime reaches the value of around 6 nm. The oxidation process presented here has severe consequences for the design of stacks of optical films that make use of Ag
ARTICLE IN PRESS 1054
A. Romanyuk, P. Oelhafen / Solar Energy Materials & Solar Cells 91 (2007) 1051–1054
layers in the thickness range of a few nm. Therefore, in order to avoid possible discrepancy between estimated and observed optical performance of systems that consist of TiO2 =Ag layer combination, care in stack design should be taken. Acknowledgments The authors would like to thank Roland Steiner for technical support. Financial support of the Swiss Federal Office of Energy is gratefully acknowledged. References [1] J.C.C. Fan, F.J. Bachner, G.H. Foley, P.M. Zavracky, Appl. Phys. Lett. 25 (1974) 693. [2] J.C.C. Fan, F.J. Bachner, Appl. Opt. 15 (1976) 1012. [3] H. Ko¨stlin, G. Frank, Thin Solid Films 89 (1982) 287. [4] G.B. Smith, G.A. Niklasson, J.S.E.M. Svensson, C.G. Granqvist, J. Appl. Phys. 59 (1986) 571.
[5] T. Eisenhammer, M. Lazarov, M. Leutbecher, U. Scho¨ffel, R. Sizmann, Appl. Opt. 32 (1993) 6310. [6] C.G. Granqvist, Materials Science for Solar Energy Conversion Systems, Pergamon Press, Oxford, 1991. [7] H.A. Macleod, Thin Films Optical Filters, Macmillan Publishing Company, New York, 1986. [8] G. Lassaletta, A. Fernandez, A.R. Gonzalez-Elipe, J. Electron Spectrosc. Relat. Phenom. 87 (1997) 61. [9] S. Altieri, L.H. Tjeng, G.A. Sawatzky, Phys. Rev. B 61 (2000) 16948. [10] M. Caffio, B. Cortigiani, G. Rovida, A. Atrei, C. Giovanardi, A. di bona, S. Valeri, Surf. Sci. 531 (2003) 368. [11] S. Doniach, M. Sunjic, J. Phys. C 3 (1970) 285. [12] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [13] X. Bao, M. Muhler, Th. Schedel-Niedrig, R. Schlo¨gel, Phys. Rev. B 54 (1996) 2249. [14] Z. Zhang, S.-P. Jeng, V.E. Henrich, Phys. Rev. B 43 (1991) 12004. [15] L.H. Tjeng, M.B.J. Meinders, J. van Elp, J. Ghijsen, G.A. Sawatzky, Phys. Rev. B 41 (1990) 3190. [16] D. Briggs, M.P. Seah, Practical Surface Analysis by Auger and XRay Photoelectron Spectroscopy, Wiley, Chichester, 1983. [17] W.H. Gries, Surf. Interface Anal. 24 (1996) 38. [18] I. Dima, B. Popescu, F. Iova, G. Popescu, Thin Solid Films 200 (1991) 11.
Solar Energy Materials & Solar Cells 91 (2007) 1051–1054 www.elsevier.com/locate/solmat
Formation and electronic structure of TiO22Ag interface Andriy Romanyuk, Peter Oelhafen Institute of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland Received 9 November 2006; accepted 19 February 2007 Available online 6 April 2007
Abstract In the present work the formation of the interface between polycrystalline silver and thin films of titanium oxide was studied with photoelectron spectroscopy (XPS, UPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Titanium oxide was deposited stepwise on 100 nm thick silver films by reactive magnetron sputtering allowing to study the evolution of the interface formation process. The process involves two steps: formation of thin layer of silver oxide and subsequent growth of the TiO2 film. For better understanding of the silver oxidation process, pure silver films were exposed to a low temperature Ar/O plasma for different time intervals providing a possibility to investigate early stages of the oxide film growth. r 2007 Elsevier B.V. All rights reserved. Keywords: Silver; Titanium oxide; Oxide–metal interface; Sputter deposition; Photoelectron spectroscopy
1. Introduction Transparent heat mirrors have been a subject of intense investigations due to their wide use for energy saving applications and solar energy collection [1–5]. The heat mirror usually consists of a thin noble metal film (Au, Ag, Cu) sandwiched between two antireflecting oxide layers among which titanium oxide is most commonly used [5,6]. As a metal component in such multilayer structure, thin silver films are the most promising candidates owing to silver’s high transmittance in the visible, and high reflection in the infrared range. TiO2 =Ag layer system is also a frequent choice in interference stacks as a part of optical band pass filters [7]. While a number of works are devoted to the growth of thin silver films on dielectrics, only few studies report on oxide preparation on silver [8–10]. In the case of oxide film deposition on metal substrate by reactive magnetron sputtering, the metal surface faces low temperature plasma that contains highly reactive oxygen atoms, ions and radicals. In these conditions the formation of interface phase, whose physical properties may differ from the bulk ones, is a primary reaction in the film growth process. Corresponding author. Tel.: +41 61 267 37 20; fax: +41 61 267 37 84.
E-mail address: [email protected] (A. Romanyuk). 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.02.016
Structure and composition of the interfacial region may play a critical role in applications of such optical thin film devices whose performance may be significantly impaired. In the present study we describe the formation of the interface between titanium oxide and polycrystalline silver films studied with in situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The in-depth composition analysis of interface components was performed with time-of-flight secondary ion mass spectrometry (TOF-SIMS). 2. Experimental details Thin films of silver (100 nm) were deposited on clean boron-doped (1 0 0)-oriented silicon wafer (resistivity 10 O cm) by direct current (DC) sputtering of an Ag target (purity 99.95%, +23 mm) in Ar atmosphere at pressure of 0.5 Pa. No carbon and oxygen contamination was detected in freshly sputtered Ag films. Titanium oxide was deposited stepwise on silver in the same experimental chamber by RF sputtering of titanium target (purity 99.95%, +23 mm) in Ar/O gas mixture containing 7% of oxygen. The gases were fed into the chamber via mass-flow controllers, while the gas total pressure in the chamber was controlled by a throttling valve and kept at 2.0 Pa. The RF power applied to the target was equal to 30 W. The deposition rate of
ARTICLE IN PRESS A. Romanyuk, P. Oelhafen / Solar Energy Materials & Solar Cells 91 (2007) 1051–1054
titanium oxide was fixed at 0:02 ML s1 and monitored by a quartz crystal monitor, calibrated through the thickness measurement with secondary electron microscopy in crosssection regime. According to our estimates, one monolayer (ML) corresponds to 0.24 nm of a titanium oxide. The sample was subsequently transferred into the spectrometer measurement chamber without breaking the vacuum under a background pressure below 1 107 Pa. The photoelectron spectroscopy analysis was performed on a VG ESCALAB 210 system equipped with a monochromatized Al Ka ð1486:6 eVÞ radiation source and a helium discharge lamp emitting in ultraviolet range (He I, 21:22 eV). The photoemission spectra were recorded at normal emission with an overall resolution of about 0.5 and 0.2 eV for XPS and for UPS, respectively. The energy positions of the spectra were calibrated with reference to the 4f 7=2 level of clean gold sample at 84.0 eV binding energy. In order to obtain the peak position of chemically shifted components, a fit procedure using Doniach–Sunjic functions [11] was applied after a Shirley background subtraction [12]. The stoichiometry of the deposited oxide films was evaluated from the O 1s and Ti 2p XPS peak ratio taking into account the ionization cross-sections of corresponding atoms. TOF-SIMS depth profiling was performed with an IONTOF IV system in dual beam mode with 1.0 keV Csþ sputtering and 10 keV Arþ primary ion beam for secondary ion generation.
π
HeI(hν=21.2eV) σ
Intensity (a.u.)
1052
5.0ML
2.0ML A B 1.0ML
0.5ML
AgFilm 10
8
6 4 2 Binding Energy (eV)
0
Fig. 1. Evolution of the valence band spectra (He I) as a function of the amount of titanium oxide deposited on polycrystalline silver film.
106
3. Results and discussion
107Ag
105
Intensity (a.u.)
Fig. 1 shows the valence band spectra (He I) recorded as a function of amount of deposited titanium oxide. The bottommost spectrum reveals the measurement on virgin polycrystalline Ag film and is dominated by the emission from the silver 4d band located above a binding energy of 4 eV. The deposition of small amounts of TiO2 (0.5 ML) results in substantial modification of Ag 4d spectrum and appearance of additional features located at about 3 and 2 eV denoted in Fig. 1 as ‘‘A’’ and ‘‘B’’, respectively. The shape and position of the spectrum is close to that observed for silver oxide suggesting the oxidation of silver surface [13]. With increasing amount of deposited TiO2 , the silver d band structure diminishes and the contribution from oxygen 2p electron states becomes more pronounced. After deposition of a total of 5 ML TiO2 the valence band consists of two broad overlapping peaks located at about 3.8 and 6 eV that correspond to non-bonding (p) and bonding (s) oxygen 2p orbitals with titanium d band [14]. Oxidation of silver and the formation of a thin oxide film is also confirmed by TOF-SIMS depth profile analysis of the structure with 20 nm of TiO2 on top of silver. As can be seen from Fig. 2 the interfacial region between silver and TiO2 is characterized by a significant increase in 123 ½AgO fragment ions intensity confirming the presence of the thin silver oxide layer.
64[TiO]
104
123[AgO]
103
102
101 10
20
30 Depth (nm)
40
50
Fig. 2. TOF-SIMS depth profile of 20 nm TiO2 deposited on Ag film.
In order to elucidate the dynamic of silver oxidation and oxide formation process, the freshly prepared silver film was exposed without titan sputtering to the argon plasma containing 7% of oxygen for different time intervals, allowing to investigate early stages of the oxide–metal
ARTICLE IN PRESS A. Romanyuk, P. Oelhafen / Solar Energy Materials & Solar Cells 91 (2007) 1051–1054
I1 s
where is the integrated signal intensity from Ag 3d line measured on unattenuated i.e. uncovered silver film, while I s represents the corresponding intensity from a sample covered with an overlayer of silver oxide of thickness d. y and l represent the electron take-off angle measured from the sample normal and electron inelastic mean free path, respectively. The value of l was derived from the predictive equation of Gries [17] and was found to be about 1.5 nm.
Ag3d5/2(hν=1486.6eV)
Ag+
Ag0
Intensity (a.u.)
90sec
45sec
AgFilm 370
369 368 367 Binding Energy (eV)
366
6
5
4
3
2
1
0 0
50 100 150 Plasma exposure time (sec)
200
Fig. 4. Oxide thickness as a function of plasma exposure time. The vertical error bar indicates the standard deviation of the corresponding intensities obtained by fit procedure. The uncertainty resulting from determination of electron inelastic mean free path is not included.
The results of this procedure is summarized in Fig. 4 which demonstrates the development of the oxide thickness upon exposure of the silver surface to the low temperature Ar/O plasma. As seen from the figure, in the early oxidation stages the oxide thickness increases rapidly, then the process saturates and the constant value of around 6 nm is reached after a total plasma exposure time of 180 s. The oxidation of silver may have serious impact on the optical performance of filters that use very thin ðo10 nmÞ silver films. It is well known that at this thickness range silver on dielectric substrates forms islands interconnected by thin ‘‘bridges’’ [18]. The oxidation of these originally conducting links results in the formation of isolated silver islands with markedly changed optical behavior. 4. Conclusions
18sec
371
7
Oxide thickness (nm)
interface formation process and the oxide film growth. In this experimental setup arrangement the magnetron with titanium target was replaced by an electrode that consisted of a graphite covered tantalum sheet. Fig. 3 shows the evolution of Ag 3d5=2 doublet component with plasma exposure time. The Ag 3d5=2 peak of pure silver film (bottommost spectrum) is nearly symmetric and positioned at a binding energy of 368.4 eV. The exposure of the Ag film to the reactive plasma results in peak broadening and shift in binding energy. The peak decomposition reveals the presence of a second component displaced in 0.4 eV towards lower binding energy which can be easily identified as Ag2 O [15]. With an increase in the plasma exposure time, the intensity of the oxide component increases gradually, reflecting continuous growth of the oxide film. The thickness of the oxide layer was evaluated with formula [16]: d Is ¼ I1 exp , s l cos y
1053
365
Fig. 3. The Ag 3d5=2 core-level spectra taken at normal emission for different plasma exposure times. Dashed lines represent the chemically shifted components after core-line decomposition.
The interface formation process between silver and titanium oxide has been studied with in situ photoelectron spectroscopy. It has been shown that sputtering of titanium oxide on silver, in the first reaction step, leads to silver oxidation and formation of thin silver oxide layer as confirmed by a TOF-SIMS depth profile analysis. The oxidation kinetics of silver surface has been studied stepby-step by exposing pure silver films to the low temperature plasma that contains Ar/O gas mixture. We have observed that the oxide thickness depends on oxidation time and in saturation regime reaches the value of around 6 nm. The oxidation process presented here has severe consequences for the design of stacks of optical films that make use of Ag
ARTICLE IN PRESS 1054
A. Romanyuk, P. Oelhafen / Solar Energy Materials & Solar Cells 91 (2007) 1051–1054
layers in the thickness range of a few nm. Therefore, in order to avoid possible discrepancy between estimated and observed optical performance of systems that consist of TiO2 =Ag layer combination, care in stack design should be taken. Acknowledgments The authors would like to thank Roland Steiner for technical support. Financial support of the Swiss Federal Office of Energy is gratefully acknowledged. References [1] J.C.C. Fan, F.J. Bachner, G.H. Foley, P.M. Zavracky, Appl. Phys. Lett. 25 (1974) 693. [2] J.C.C. Fan, F.J. Bachner, Appl. Opt. 15 (1976) 1012. [3] H. Ko¨stlin, G. Frank, Thin Solid Films 89 (1982) 287. [4] G.B. Smith, G.A. Niklasson, J.S.E.M. Svensson, C.G. Granqvist, J. Appl. Phys. 59 (1986) 571.
[5] T. Eisenhammer, M. Lazarov, M. Leutbecher, U. Scho¨ffel, R. Sizmann, Appl. Opt. 32 (1993) 6310. [6] C.G. Granqvist, Materials Science for Solar Energy Conversion Systems, Pergamon Press, Oxford, 1991. [7] H.A. Macleod, Thin Films Optical Filters, Macmillan Publishing Company, New York, 1986. [8] G. Lassaletta, A. Fernandez, A.R. Gonzalez-Elipe, J. Electron Spectrosc. Relat. Phenom. 87 (1997) 61. [9] S. Altieri, L.H. Tjeng, G.A. Sawatzky, Phys. Rev. B 61 (2000) 16948. [10] M. Caffio, B. Cortigiani, G. Rovida, A. Atrei, C. Giovanardi, A. di bona, S. Valeri, Surf. Sci. 531 (2003) 368. [11] S. Doniach, M. Sunjic, J. Phys. C 3 (1970) 285. [12] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [13] X. Bao, M. Muhler, Th. Schedel-Niedrig, R. Schlo¨gel, Phys. Rev. B 54 (1996) 2249. [14] Z. Zhang, S.-P. Jeng, V.E. Henrich, Phys. Rev. B 43 (1991) 12004. [15] L.H. Tjeng, M.B.J. Meinders, J. van Elp, J. Ghijsen, G.A. Sawatzky, Phys. Rev. B 41 (1990) 3190. [16] D. Briggs, M.P. Seah, Practical Surface Analysis by Auger and XRay Photoelectron Spectroscopy, Wiley, Chichester, 1983. [17] W.H. Gries, Surf. Interface Anal. 24 (1996) 38. [18] I. Dima, B. Popescu, F. Iova, G. Popescu, Thin Solid Films 200 (1991) 11.