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The growth of ultrathin films of copper on polycrystalline ZrO2
Surface Science 452 (2000) 58–66 www.elsevier.nl/locate/susc
The growth of ultrathin films of copper on polycrystalline ZrO 2 D. Sotiropoulou *, S. Ladas Department of Chemical Engineering, University of Patras and ICE/HT-FORTH, P.O. Box 1414, Gr-26500 Rion, Patras, Greece Received 3 July 1999; accepted for publication 18 January 2000
Abstract X-ray photoelectron spectroscopy ( XPS/XAES ) and work function measurements ( WF ) have been used to study ˚ at room temperature (RT ) and up to 8 A ˚ at the Cu/ZrO interface. Copper was deposited up to a thickness of 26 A 2 673 K. At RT deposition it was found that, after the formation of just about 1 ML, copper starts to form threedimensional clusters, exhibiting a Stranski–Krastanov growth process. At 673 K, 3D copper clusters begin to form from the very early stages of deposition, which reveals a stronger 3D character. The XPS results did not show any strong interaction between ZrO and Cu. Copper clusters exhibited core-level shifts due to both final and initial state 2 effects. Work function measurements carried out during deposition at 673 K yielded additional information about the copper growth mode and the copper–zirconia interaction, which is consistent with the XPS results. Furthermore, the work function of clean ZrO was measured as 3.1±0.1 eV from the width of the ultraviolet photoelectron spectrum 2 ( UPS). © 2000 Elsevier Science B.V. All rights reserved. Keywords: Copper; Growth; Interface states; Work function measurements; X-ray photoelectron spectroscopy
1. Introduction The physics, chemistry and structure of the interfacial region between metals and ceramics play an important role in many applications, including thin film technology, semiconductor devices, ceramic–metal joining technology and heterogenous catalysis. The materials at the interface may show no structural changes if the interaction is only of a physical nature, further referred to as physical interaction, but may show dramatic changes if chemical reactions occur [1–6 ]. Concerning heterogenous catalysis, support interactions can play an important role in determining * Corresponding author. Fax: +3061-993255. E-mail address: [email protected] (D. Sotiropoulou)
the catalytic properties of highly dispersed metal particles on oxides. Such interactions may cause an electron transfer between the metal and the support and influence the metal adsorption properties [7]. In recent years, an increasing interest in Me/ZrO systems has been noticed, because these 2 systems are used in many catalytic reactions [8– 10]. For zirconia this is an especially interesting possibility, since zirconia is also a solid electrolyte [11,12]. The study of the Cu/ZrO interface is of 2 interest since zirconia-supported copper catalysts are active and selective for methanol synthesis [13– 16 ] or for NO reduction [17]. Despite the technological interest in Cu/ZrO systems, the study of 2 copper deposition on ZrO under ultrahigh 2 vacuum ( UHV ) conditions is quite limited [18]. In this work, X-ray photoelectron spectroscopy
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( XPS/XAES) and work function measurements ( WF ) were used, in order to study the Cu/ZrO 2 interface. Copper was vapour deposited on ZrO 2 in UHV at RT and at 673 K. The growth mode of copper layers on ZrO at the early stages of 2 deposition was studied at both temperatures using XPS. Energy shifts in XPS and XAES have been measured in order to investigate the metal–support interaction. The work function measurements, taken during deposition at 673 K, provided additional useful information on copper growth mode on ZrO and on deposit–substrate interaction. 2 2. Experimental The XPS measurements were performed in a turbopumped UHV chamber (base pressure 4×10−10 mbar) equipped with a Leybold LHS-12 hemispherical electron energy analyser and a twinanode X-ray gun for XPS. Photoelectrons were excited using the unmonochromated Mg Ka line at 1253.6 eV and the analyser was used at constant DE mode with 100 eV pass energy and a sampling area of 5×3 mm2. The ultraviolet photoelectron ( UP) spectra were taken in the same UHV chamber with a specs UVS 10/35 source using the HeI line at 21.2 eV. A Kelvin probe with a gold-plated vibrating reference electrode, which was piezoelectrically driven, was used to monitor work function changes. The precision in the measurements was estimated to be ±10 meV. Drift in the signal due to adsorption of the residual gases was of the order of 0.5 meV min−1, therefore it can be neglected. The substrate was high-purity, polycrystalline ZrO discs, stabilized with 8 mol% Y O and fabri2 2 3 cated by tape casting and subsequent dense sintering at 1870 K. The sample was precleaned before entering the vacuum system in a dilute HCl solution and then in a methanol solution using ultrasonic equipment. The sample was mounted on a Mo plate, which could be heated to 1100 K, and was UHV cleaned by cycles of Ar+ sputtering and annealing. Copper evaporation was performed with a home-built evaporation source, which consisted of
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a tungsten filament wrapped with a Cu wire (purity 99.99%). The Cu was melted initially to improve the thermal contact with tungsten. The sample was placed at a distance of 6 cm away from the Cu source. The equivalent thickness of deposited Cu was estimated indirectly from the attenuation of the Zr3d signal during the early stages of deposition ˚ ). This method is exact when using l(ECu=12.4 A Zr the growth mode is of a layer-by-layer type (FM ), but underestimates the coverage for 3D particle growth. 1 ML of copper corresponds to a thickness ˚. of 2.28 A The uncertainty in the determination of the binding energy values was estimated to be about 0.2 eV.
3. Results and discussion 3.1. Growth mode The geometric and electronic structure of metal films change with coverage as the metal film islands or clusters change from a single atomic layer to a multilayer thickness. The two limiting growth modes are the layer-by-layer [Frank van der Merwe ( FM )] [19] and the Volmer Weber ( VW ) in which 3D particles are formed [20]. The other types of growth mode are intermediate cases, like the Stranski–Krastanov growth where 3D particles begin to form after an initial layer-type growth. In order to determine the growth mode of Cu on the ZrO substrate, the Cu2p/Zr3d XPS experi2 mental intensity ratio was plotted as a function of Cu equivalent thickness (Fig. 1). Deposition data were obtained both at RT and at 673 K. In the same figure, the solid line represents the theoretical curve for the FM growth mode. This curve has ˚ ) and been obtained using the values l(ECu =5 A Cu2p ˚ ) for the inelastic mean l(ECu =12.4 A free paths Zr3d [21], as well as an experimental I /I ratio Cu,2 Zr,2 for bulk Cu and clean ZrO equal to 3.15. 2 In Fig. 1, it is clear that the RT data fall on the FM theoretical curve and then deviate towards a rather 3D particle growth. This deviation is actually more pronounced than that shown in Fig. 1, to the extent that the Cu thickness is underesti-
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Fig. 1. The Cu2p/Zr3d XPS intensity ratio as a function of Cu equivalent thickness at RT and at 673 K. The solid line represents the theoretical curve for a layer-by-layer growth mode.
mated. Because of the experimental error and the limited set of data, it is difficult to decide on the exact point where the change from a predominantly layer-type to a 3D particle growth mode takes place (inset in Fig. 1), but it is approximately near the completion of 1 ML (Stranski–Krastanov growth). At 673 K, it is obvious that copper clusters are formed from the very early stages of deposition and the deviation from the FM curve is more pronounced than at RT, indicating a stronger 3D character. Comparison of the Cu/ZrO with the Ni/ZrO 2 2 growth mode, investigated by the same experimental procedure [22], leads to the conclusion that the two transition metals show similar behaviour. Both tend to form 3D particles after the formation of
0.5 to 1 ML at RT and at high temperature (HT ). This is in accordance with various bulk thermodynamic analyses, according to which the oxide ZrO is not wetted by the liquid Cu and Ni metals. 2 Furthermore, it is also in accordance with our experimental values of contact angles of Cu and Ni on ZrO , which were determined between 117 2 and 123° for both systems, using the sessile drop technique [23]. The poor wetting (h>90) and therefore the low adhesion between the oxide and the metals suggest the absence of chemical reaction and no strong interaction between them. This means that the metal does not form a continuous film on the oxide, but particles characterized by some measurable contact angle between the oxide and the metal. However, an initial stable mono-
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layer formation cannot be excluded, as contact angle measurements usually do not distinguish among Stranski–Krastanov (SK ) and Volmer– Weber growth mode [6 ]. 3.2. XPS and XAES energy shifts Because of the insulating character of ZrO at 2 room temperature and the concomitant sample charging, we faced the problem of referencing the kinetic energy scale. It is known that insulating samples are charged and the charging changes during metal deposition. So it is difficult to distinguish between a chemical shift and a shift caused by varying charging. The choice of C1s as intrinsic reference was impossible since the surface was clean and the C1s XPS signal too small to be detected. To cope with the problem, we decided to choose the Zr3d core-level binding energy at 5/2 182.6 eV as internal reference, assuming negligible chemical interaction between the Zr cation and the Cu. This assumption is supported by the invariance of the Zr peak shape during deposition, as well as by previous work [22,24,25]. Concerning the deposition at 673 K, we did not face the charging problem because ZrO was conducting. 2 Fig. 2 shows the XP spectra of the O1s and Cu2p core levels for various Cu amounts at RT. In Fig. 2a it is obvious that there is no bordering of the O1s peak within the experimental error of 0.1 eV, and there is also no changing of the binding energy of O1s during deposition. This is an indication that there is no strong interaction between substrate and deposit at the early stages of deposition. At the Cu2p spectra (Fig. 2b) no shake-up satellite features characteristic of CuO are observed, even at the lowest coverages, indicating that no Cu(II ) is formed. The Cu2p binding 3/2 energy decreases from 933.535 eV to 932.6 eV ˚. when the coverage increases from 0.02 to 25.8 A The value 932.6 eV corresponds to metallic Cu as measured on a Cu foil. The shifts of the Auger peaks of the deposit also give important information for the growth mode and the interaction
Fig. 2. a) (top left) and b) ( lower left) XPS core-level spectra of the Cu/ZrO system for different Cu deposition times. 2
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Fig. 3. Variation of the Cu2p binding energy and Auger 3/2 Cu(LMV ) kinetic energy as a function of Cu equivalent thickness at room temperature.
Fig. 4. Variation of the Cu2p binding energy and Auger 3/2 Cu(LMV ) kinetic energy as a function of Cu equivalent thickness at 673 K.
between the metal and the oxide. In the case of the Cu/ZrO interface, unfortunately the main 2 Auger peak of the Cu is overlapped by the Zr3p peak, making detection of the Cu(LVV ) peak impossible. For this reason, we have recorded the Cu(LMV ) Auger peak, which has a small signal, so we do not have measurements at the very early stages of deposition. Figs. 3 and 4 show the Cu2p binding energy (BE) and Cu(LMV ) 3/2 kinetic energy as a function of Cu thickness at RT and 673 K. Fig. 3 shows that the BE decreases towards the bulk value for the metallic Cu measured on a Cu foil (932.6 eV ), whereas the Auger kinetic energy ( KE ) increases. This is general behaviour for metal deposits on inert substrates. The shifts of BE and Auger KE reflect initial and final state effects. The initial state effect is due to electron exchange with the substrate and/or intrinsic size effect. The final state effect reflects the core hole screening, which is more effective when the average size of the clusters increases, resulting in the decrease of the BE close to the bulk value
[26,27]. On the other hand, Fig. 4 shows that from the early stages of deposition the BE and KE values are near the bulk value of metallic Cu. This result indicates that at 673 K big clusters are formed from the very early stages of deposition. In Fig. 4, it is also shown that the BE and Auger KE approach values slightly different from the bulk copper values. Although this deviation is within the experimental error, it may lead to the conclusion that Cu O (BE=932.4 eV ) has 2 been formed. In order to clarify this point we determined another sensitive parameter, the Auger parameter a, which is defined as a=BE(Cu2p )+KE[Cu(LMV )]. 3/2 The use of the Auger parameter is important as it is very sensitive to chemical and physical changes and insensitive to surface charging on insulating samples. The derived value for the final step of deposition is 1771.2 eV for RT deposition and 1771.3 eV for HT deposition. Taking into account that the experimental value for the Auger Cu(LMV ) parameter, measured on a copper foil,
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imations the following equations hold: DBE(Cu2p )=De−DR 3/2 DKE[Cu(LMV )]=−De+3DR
(1) (2)
where De is the chemical shift due to the initial charge distribution (initial state effect) and DR the relaxation shift (final state effect). Taking into account the definition of the Auger parameter, Eq. (3) is derived: DR=Da/2.
Fig. 5. Auger parameter variation of Cu versus the Cu equivalent thickness.
is 1771.2±0.1 eV, we conclude that metallic copper is also formed on the ZrO surface at 673 K. The 2 small deviation (0.2 eV ) of BE and Auger KE from the metallic value of Cu may be attributed to the fact that at 673 K the mobile oxygen ions are concentrated near the Cu/ZrO interface, 2 giving rising to an additional potential. The a values of Cu clusters are plotted in Fig. 5 as a function of the thickness of deposited copper. It is obvious that the a increases owing to the fact that the increase in Auger KE is larger than the corresponding decrease in Cu2p BE. Small oscil3/2 lations observed in Fig. 5 can be attributed to instability of the deposit, e.g. 2D to 3D cluster reconstruction, which causes a rearrangement of the electronic density of Cu clusters. The Auger parameter analysis was used to estimate roughly the chemical shift (or the electron exchange) between Cu and ZrO during deposition. 2 This method, developed by Kao et al. [28,29], correlates the BE and Auger KE shifts of the substrate to the initial and final effects at a metal/semiconductor interface. With some approx-
(3)
Therefore, by measuring the binding energy shift and the Auger kinetic energy shift with respect to the bulk, one can calculate the chemical shift and the relaxation energy shift. Positive values of De indicate that the deposited clusters at very low coverage are negatively charged with respect to the bulk and vice versa. When the coverage of Cu ˚ at RT deposition, increases from 1.35 to 25.8 A we obtain DBE=−0.15 eV and DKE=1.15 eV, hence De=0.35 eV and DR=0.50 eV. (At cover˚ we do not have measureages lower than 1.35 A ments of the Auger KE.) From the deposition data at 673 K with similar initial Cu coverage, we obtained DBE#0, DKE=0.24, hence De=0.12 eV and DR=0.12 eV. The final state contribution at RT is larger in magnitude than that at 673 K, consistent with smaller Cu particles at room temperature. The derived values of De are comparable with initial state shifts for small Cu clusters dispersed on Al O (0.4 eV ) [30] and on carbon (up to 2 3 0.5 eV ) [31] at RT. The positive values indicate that the copper atoms carry initially a negative charge, relative to the bulk. It is known that initial state effects include charge-transfer phenomena between substrate and deposit as well as intrinsic changes which depend on cluster size. The sign of the derived values of De is the same as that of the surface core-level shift (SCLS) of metallic Cu, which is mainly an intrinsic initial state effect [32]. The SCLS is equal to 0.24 eV [30], which indicates the presence of a size effect in our De values [33]. On the other hand, De has been associated with a charge transfer between substrate and deposit [28,29], the direction of which depends on the sign of De. In our case, the difference between the De
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value at RT and the SCLS of Cu clusters could be attributed to a charge transfer from ZrO to 2 Cu. Taking into account that De would be even larger if we had measurements for smaller Cu ˚ , then the clusters at coverages lower than 1.35 A contribution of charge transfer is strengthened. The smaller value of De at 673 K reflects a size effect. At this point it should be noted that in both cases, RT and 673 K, the magnitude of is De small, precluding the formation of a new compound at the interface, and indicating a very weak interaction between Cu and ZrO . 2 This result is in very good agreement with our proposed model for metal/ceramic interfaces [24,25], which predicts for Cu/ZrO system a weak 2 interaction. The De values are comparable, within experimental error, with the respective value 0.15 eV provided by a previous work for the Ni/ZrO 2 system [22]. It is not possible to make an exact comparison, because of the lack of measurements in the same coverage region. We compared the De ˚ thickness range of deposvalue for the 1.3 to 3 A ited metal for the two systems at room temperature and noticed that De is 0.1 eV higher in the case of the Ni/ZrO system. Taking into account that 2 0.1 eV is within the experimental error, we can say with some caution that the previous result is consistent with the higher value of work of adhesion (stronger interaction between substrate and metal ) in the case of the Ni/ZrO system [23]. 2
ponds to a work function change D( WF ) of the sample surface. It was observed that the WF of clean ZrO was 2 initially not stable in UHV and for this reason the clean ZrO was allowed to stabilize by keeping it 2 at 673 K in UHV for many hours. The CPD gradually decreased and after 48 h reached saturation. Then the deposition experiment was carried out. The results of the WF variation are shown in Fig. 6 where it is shown that the WF increases monotonically. This behaviour is typical for a metal deposited onto a substrate with lower WF than that of the metal and with no substantial intermixing or any other strong interaction between the two compounds. The absolute value of the WF for polycrystalline copper is given in the literature between 4.6 and 4.8 eV [35,36 ]. Concerning the WF of pure polycrystalline ZrO , to our knowledge, there exists 2 only one measurement at 3.12 eV, corresponding to the high-field apparent work function of a patchy surface of ZrO [37]. In another more 2 recent work, it has been assumed to lie near the
3.3. Work function measurements The adsorption of atoms or molecules on a solid surface, which takes place at the first step of the formation of a solid/solid interface, is generally associated with a change of work function [34]. In the case of the metal vapour deposition, the investigation of the WF variation of the substrate surface gives useful information about the growth process. The WF variation of the ZrO surface during 2 deposition was measured at 673 K where ZrO is 2 conductive by the Kelvin probe method, which gives the contact potential difference (CPD) between a conductive sample surface and a reference electrode. The change of this potential corres-
Fig. 6. Work function variation with Cu equivalent thickness during deposition at 673 K.
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value of the metal Zr (3.9 eV ) [38]. We measured the WF of ZrO following stabilization at 673 K 2 from the low energy cut-off of the UP spectra with the sample negatively polarized, in order to obtain a sharp secondary electron edge. The derived value, 3.1±0.1 eV, is lower than that for polycrystalline Cu. Thus the WF behaviour in Fig. 6 is justified. It must be added that the value of WF for bulk Cu is not yet reached within the figure. Using the simple relationship for the average work function of the Cu/ZrO surface eQ=heQ +(1−h)eQ , 2 Cu ZrO2 where h is the fraction of surface covered by Cu clusters and taking into account that at this point of deposition copper and ZrO have the bulk 2 values for WF, we find that h#0.45. Using the respective XPS data and the relationship I /I =(1−h)+h exp−[(d /h)/l(ECu )], where Zr Zr,2 eq Zr d is the equivalent copper thickness, we also eq found the value 0.45 eV for h. This indicates that the WF results are consistent with the XPS results. The initial variations of the WF show a positive slope which means that the Cu adatoms are negatively charged. This result agree with the results of the Auger parameter analysis, where the De sign also denotes that the copper is slightly negatively charged. The WF changes of clean ZrO up to saturation 2 could not be explained in this study. The XP spectra of Zr3d and O1s peaks obtained simultaneously with the work function measurements did not show any change of the peaks. Probably the work function of the ZrO is very sensitive to 2 oxygen concentration in the residual gas of the atmosphere or to the mobility of the O2− ions. In a recent work, WF changes of the ZrO surface 2 were discussed in terms of defect chemistry of the near surface layers [39]. This defect chemistry depends on the Y O content, on the partial pres2 3 sure of oxygen, on the doping or not of ZrO , and 2 it is reported that the CPD behaviour is generally complicated.
4. Conclusions The mechanism of growth of Cu on ZrO at 2 room temperature seems to be a Stranski– Krastanov process. At 673 K 3D particles of
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copper start to form from the very early stages of deposition, and they are larger in size. The XP spectra did not indicate any strong interaction between deposit and substrate. The copper clusters showed small core-level shifts due to both initial and final state effects, and they were negatively charged because of the initial state effect. The absolute value of the work function of ZrO was 2 found by UPS to be 3.1 eV at 673 K. The study of the work function variation with copper thickness during deposition at 673 K showed that WF increases towards the bulk value of copper. After deposition of almost 4 ML of copper, saturation of the WF change did not occur, which verifies the strong 3D character of the copper deposit. The initial slope of the WF curve also suggests that the copper clusters are negatively charged.
References [1] J.A. Pask, A.P. Tomsia, in: J.A. Pask, A. Evans ( Eds.), Surfaces and Interfaces in Ceramic and Ceramic–Metal Systems, Plenum Press, New York, 1981, p. 411. [2] M.W. Finnis, Acta Metall. Mater. 40 (1992) S25. [3] J.-G. Li, J. Am. Ceram. Soc. 75 (1992) 3118. [4] M. Nicholas, in: S.D. Peteves ( Ed.), Designing Interfaces for Technological Applications, Elsevier Applied Science, London, 1989, p. 49. [5] K. Foger, in: J.R. Anderson, M. Boudart ( Eds.), Catalysis. Science and Technology Vol. 6, Springer, Berlin, 1984, pp. 227–305. [6 ] C.T. Campell, Surf. Sci. Rep. 27 (1997) 1. [7] S. Roberts, R.J.J. Corte, Chem. Phys. 93 (1990) 5337. [8] H.C. Yao, H.K. Stepien, H.S. Gandhi, J. Catal. 61 (1980) 1. [9] M. Ichikawa, Bull. Chem. Soc. Jpn. 51 (1978) 2268. [10] D. Sotiropoulou, C. Yiokari, C.G. Vagenas, S. Ladas, Appl. Catal. A: General 183 (1999) 15. [11] S. Bebelis, C.G. Vagenas, J. Catal. 118 (1989) 125. [12] J. Michaels, C.G. Vagenas, J. Catal. 85 (1984) 477. [13] C. Frolich, R.A. Koppel, A. Baiker, M. Kilo, A. Wokaun, Appl. Catal. A: General 106 (2) (1993) 275. [14] P.D.L. Mercera, J.G. Van Ommen, E.B.M. Doesburg, A.J. Burggraaf, J.R.H. Ross, Appl. Catal. 57 (1990) 127. [15] R.A. Koeppel, A. Baiker, A. Wokaun, Appl. Catal. A: General 84 (1992) 77. [16 ] C. Schild, A. Wokaun, A. Baiker, J. Mol. Catal. 63 (1990) 243. [17] F. Figueras, B. Coq, E. Ensuque, D. Tachon, G. Delahay, in: Proc. 214th National Meeting of the American Ceramic Society (1997) 805. [18] Q. Ge, J. Moller, Thin Solid Films 254 (1995) 10.
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[19] J. Werckmann, A. Mosser, J.P. Deville, Analysis 9 (1981) 213. [20] C. Argille, G.E. Rhead, Surf. Sci. Rep. 10 (1989) 277. [21] D. Penn, J. Electron Spectrosc. Relat. Phenom. 9 (1976) 29. [22] D. Sotiropoulou, S. Ladas, Surf. Sci. 408 (1998) 182. [23] P. Nikolopoulos, D. Sotiropoulou, J. Mater. Sci. Lett. 6 (1987) 1429. [24] D. Sotiropoulou, P. Nikolopoulos, J. Mater. Sci. 28 (1993) 356. [25] D. Sotiropoulou, S. Agathopoulos, P. Nikolopoulos, J. Adhes. Sci. Technol. 10 (1996) 989. [26 ] Ch. Kuhrt, M. Harsdorff, Surf. Sci. 245 (1991) 173. [27] C.R. Henry, Surf. Sci. Rep. 31 (1998) 231. [28] C.G. Kao, S.C. Tsai, M.K. Bahl, Y.W. Chyng, Surf. Sci. 95 (1980) 268. [29] M.K. Bahl, S.C. Tsai, Y.W. Chyng, Phys. Rev. B 21 (1980) 1344.
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The growth of ultrathin films of copper on polycrystalline ZrO 2 D. Sotiropoulou *, S. Ladas Department of Chemical Engineering, University of Patras and ICE/HT-FORTH, P.O. Box 1414, Gr-26500 Rion, Patras, Greece Received 3 July 1999; accepted for publication 18 January 2000
Abstract X-ray photoelectron spectroscopy ( XPS/XAES ) and work function measurements ( WF ) have been used to study ˚ at room temperature (RT ) and up to 8 A ˚ at the Cu/ZrO interface. Copper was deposited up to a thickness of 26 A 2 673 K. At RT deposition it was found that, after the formation of just about 1 ML, copper starts to form threedimensional clusters, exhibiting a Stranski–Krastanov growth process. At 673 K, 3D copper clusters begin to form from the very early stages of deposition, which reveals a stronger 3D character. The XPS results did not show any strong interaction between ZrO and Cu. Copper clusters exhibited core-level shifts due to both final and initial state 2 effects. Work function measurements carried out during deposition at 673 K yielded additional information about the copper growth mode and the copper–zirconia interaction, which is consistent with the XPS results. Furthermore, the work function of clean ZrO was measured as 3.1±0.1 eV from the width of the ultraviolet photoelectron spectrum 2 ( UPS). © 2000 Elsevier Science B.V. All rights reserved. Keywords: Copper; Growth; Interface states; Work function measurements; X-ray photoelectron spectroscopy
1. Introduction The physics, chemistry and structure of the interfacial region between metals and ceramics play an important role in many applications, including thin film technology, semiconductor devices, ceramic–metal joining technology and heterogenous catalysis. The materials at the interface may show no structural changes if the interaction is only of a physical nature, further referred to as physical interaction, but may show dramatic changes if chemical reactions occur [1–6 ]. Concerning heterogenous catalysis, support interactions can play an important role in determining * Corresponding author. Fax: +3061-993255. E-mail address: [email protected] (D. Sotiropoulou)
the catalytic properties of highly dispersed metal particles on oxides. Such interactions may cause an electron transfer between the metal and the support and influence the metal adsorption properties [7]. In recent years, an increasing interest in Me/ZrO systems has been noticed, because these 2 systems are used in many catalytic reactions [8– 10]. For zirconia this is an especially interesting possibility, since zirconia is also a solid electrolyte [11,12]. The study of the Cu/ZrO interface is of 2 interest since zirconia-supported copper catalysts are active and selective for methanol synthesis [13– 16 ] or for NO reduction [17]. Despite the technological interest in Cu/ZrO systems, the study of 2 copper deposition on ZrO under ultrahigh 2 vacuum ( UHV ) conditions is quite limited [18]. In this work, X-ray photoelectron spectroscopy
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( XPS/XAES) and work function measurements ( WF ) were used, in order to study the Cu/ZrO 2 interface. Copper was vapour deposited on ZrO 2 in UHV at RT and at 673 K. The growth mode of copper layers on ZrO at the early stages of 2 deposition was studied at both temperatures using XPS. Energy shifts in XPS and XAES have been measured in order to investigate the metal–support interaction. The work function measurements, taken during deposition at 673 K, provided additional useful information on copper growth mode on ZrO and on deposit–substrate interaction. 2 2. Experimental The XPS measurements were performed in a turbopumped UHV chamber (base pressure 4×10−10 mbar) equipped with a Leybold LHS-12 hemispherical electron energy analyser and a twinanode X-ray gun for XPS. Photoelectrons were excited using the unmonochromated Mg Ka line at 1253.6 eV and the analyser was used at constant DE mode with 100 eV pass energy and a sampling area of 5×3 mm2. The ultraviolet photoelectron ( UP) spectra were taken in the same UHV chamber with a specs UVS 10/35 source using the HeI line at 21.2 eV. A Kelvin probe with a gold-plated vibrating reference electrode, which was piezoelectrically driven, was used to monitor work function changes. The precision in the measurements was estimated to be ±10 meV. Drift in the signal due to adsorption of the residual gases was of the order of 0.5 meV min−1, therefore it can be neglected. The substrate was high-purity, polycrystalline ZrO discs, stabilized with 8 mol% Y O and fabri2 2 3 cated by tape casting and subsequent dense sintering at 1870 K. The sample was precleaned before entering the vacuum system in a dilute HCl solution and then in a methanol solution using ultrasonic equipment. The sample was mounted on a Mo plate, which could be heated to 1100 K, and was UHV cleaned by cycles of Ar+ sputtering and annealing. Copper evaporation was performed with a home-built evaporation source, which consisted of
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a tungsten filament wrapped with a Cu wire (purity 99.99%). The Cu was melted initially to improve the thermal contact with tungsten. The sample was placed at a distance of 6 cm away from the Cu source. The equivalent thickness of deposited Cu was estimated indirectly from the attenuation of the Zr3d signal during the early stages of deposition ˚ ). This method is exact when using l(ECu=12.4 A Zr the growth mode is of a layer-by-layer type (FM ), but underestimates the coverage for 3D particle growth. 1 ML of copper corresponds to a thickness ˚. of 2.28 A The uncertainty in the determination of the binding energy values was estimated to be about 0.2 eV.
3. Results and discussion 3.1. Growth mode The geometric and electronic structure of metal films change with coverage as the metal film islands or clusters change from a single atomic layer to a multilayer thickness. The two limiting growth modes are the layer-by-layer [Frank van der Merwe ( FM )] [19] and the Volmer Weber ( VW ) in which 3D particles are formed [20]. The other types of growth mode are intermediate cases, like the Stranski–Krastanov growth where 3D particles begin to form after an initial layer-type growth. In order to determine the growth mode of Cu on the ZrO substrate, the Cu2p/Zr3d XPS experi2 mental intensity ratio was plotted as a function of Cu equivalent thickness (Fig. 1). Deposition data were obtained both at RT and at 673 K. In the same figure, the solid line represents the theoretical curve for the FM growth mode. This curve has ˚ ) and been obtained using the values l(ECu =5 A Cu2p ˚ ) for the inelastic mean l(ECu =12.4 A free paths Zr3d [21], as well as an experimental I /I ratio Cu,2 Zr,2 for bulk Cu and clean ZrO equal to 3.15. 2 In Fig. 1, it is clear that the RT data fall on the FM theoretical curve and then deviate towards a rather 3D particle growth. This deviation is actually more pronounced than that shown in Fig. 1, to the extent that the Cu thickness is underesti-
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Fig. 1. The Cu2p/Zr3d XPS intensity ratio as a function of Cu equivalent thickness at RT and at 673 K. The solid line represents the theoretical curve for a layer-by-layer growth mode.
mated. Because of the experimental error and the limited set of data, it is difficult to decide on the exact point where the change from a predominantly layer-type to a 3D particle growth mode takes place (inset in Fig. 1), but it is approximately near the completion of 1 ML (Stranski–Krastanov growth). At 673 K, it is obvious that copper clusters are formed from the very early stages of deposition and the deviation from the FM curve is more pronounced than at RT, indicating a stronger 3D character. Comparison of the Cu/ZrO with the Ni/ZrO 2 2 growth mode, investigated by the same experimental procedure [22], leads to the conclusion that the two transition metals show similar behaviour. Both tend to form 3D particles after the formation of
0.5 to 1 ML at RT and at high temperature (HT ). This is in accordance with various bulk thermodynamic analyses, according to which the oxide ZrO is not wetted by the liquid Cu and Ni metals. 2 Furthermore, it is also in accordance with our experimental values of contact angles of Cu and Ni on ZrO , which were determined between 117 2 and 123° for both systems, using the sessile drop technique [23]. The poor wetting (h>90) and therefore the low adhesion between the oxide and the metals suggest the absence of chemical reaction and no strong interaction between them. This means that the metal does not form a continuous film on the oxide, but particles characterized by some measurable contact angle between the oxide and the metal. However, an initial stable mono-
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layer formation cannot be excluded, as contact angle measurements usually do not distinguish among Stranski–Krastanov (SK ) and Volmer– Weber growth mode [6 ]. 3.2. XPS and XAES energy shifts Because of the insulating character of ZrO at 2 room temperature and the concomitant sample charging, we faced the problem of referencing the kinetic energy scale. It is known that insulating samples are charged and the charging changes during metal deposition. So it is difficult to distinguish between a chemical shift and a shift caused by varying charging. The choice of C1s as intrinsic reference was impossible since the surface was clean and the C1s XPS signal too small to be detected. To cope with the problem, we decided to choose the Zr3d core-level binding energy at 5/2 182.6 eV as internal reference, assuming negligible chemical interaction between the Zr cation and the Cu. This assumption is supported by the invariance of the Zr peak shape during deposition, as well as by previous work [22,24,25]. Concerning the deposition at 673 K, we did not face the charging problem because ZrO was conducting. 2 Fig. 2 shows the XP spectra of the O1s and Cu2p core levels for various Cu amounts at RT. In Fig. 2a it is obvious that there is no bordering of the O1s peak within the experimental error of 0.1 eV, and there is also no changing of the binding energy of O1s during deposition. This is an indication that there is no strong interaction between substrate and deposit at the early stages of deposition. At the Cu2p spectra (Fig. 2b) no shake-up satellite features characteristic of CuO are observed, even at the lowest coverages, indicating that no Cu(II ) is formed. The Cu2p binding 3/2 energy decreases from 933.535 eV to 932.6 eV ˚. when the coverage increases from 0.02 to 25.8 A The value 932.6 eV corresponds to metallic Cu as measured on a Cu foil. The shifts of the Auger peaks of the deposit also give important information for the growth mode and the interaction
Fig. 2. a) (top left) and b) ( lower left) XPS core-level spectra of the Cu/ZrO system for different Cu deposition times. 2
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Fig. 3. Variation of the Cu2p binding energy and Auger 3/2 Cu(LMV ) kinetic energy as a function of Cu equivalent thickness at room temperature.
Fig. 4. Variation of the Cu2p binding energy and Auger 3/2 Cu(LMV ) kinetic energy as a function of Cu equivalent thickness at 673 K.
between the metal and the oxide. In the case of the Cu/ZrO interface, unfortunately the main 2 Auger peak of the Cu is overlapped by the Zr3p peak, making detection of the Cu(LVV ) peak impossible. For this reason, we have recorded the Cu(LMV ) Auger peak, which has a small signal, so we do not have measurements at the very early stages of deposition. Figs. 3 and 4 show the Cu2p binding energy (BE) and Cu(LMV ) 3/2 kinetic energy as a function of Cu thickness at RT and 673 K. Fig. 3 shows that the BE decreases towards the bulk value for the metallic Cu measured on a Cu foil (932.6 eV ), whereas the Auger kinetic energy ( KE ) increases. This is general behaviour for metal deposits on inert substrates. The shifts of BE and Auger KE reflect initial and final state effects. The initial state effect is due to electron exchange with the substrate and/or intrinsic size effect. The final state effect reflects the core hole screening, which is more effective when the average size of the clusters increases, resulting in the decrease of the BE close to the bulk value
[26,27]. On the other hand, Fig. 4 shows that from the early stages of deposition the BE and KE values are near the bulk value of metallic Cu. This result indicates that at 673 K big clusters are formed from the very early stages of deposition. In Fig. 4, it is also shown that the BE and Auger KE approach values slightly different from the bulk copper values. Although this deviation is within the experimental error, it may lead to the conclusion that Cu O (BE=932.4 eV ) has 2 been formed. In order to clarify this point we determined another sensitive parameter, the Auger parameter a, which is defined as a=BE(Cu2p )+KE[Cu(LMV )]. 3/2 The use of the Auger parameter is important as it is very sensitive to chemical and physical changes and insensitive to surface charging on insulating samples. The derived value for the final step of deposition is 1771.2 eV for RT deposition and 1771.3 eV for HT deposition. Taking into account that the experimental value for the Auger Cu(LMV ) parameter, measured on a copper foil,
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imations the following equations hold: DBE(Cu2p )=De−DR 3/2 DKE[Cu(LMV )]=−De+3DR
(1) (2)
where De is the chemical shift due to the initial charge distribution (initial state effect) and DR the relaxation shift (final state effect). Taking into account the definition of the Auger parameter, Eq. (3) is derived: DR=Da/2.
Fig. 5. Auger parameter variation of Cu versus the Cu equivalent thickness.
is 1771.2±0.1 eV, we conclude that metallic copper is also formed on the ZrO surface at 673 K. The 2 small deviation (0.2 eV ) of BE and Auger KE from the metallic value of Cu may be attributed to the fact that at 673 K the mobile oxygen ions are concentrated near the Cu/ZrO interface, 2 giving rising to an additional potential. The a values of Cu clusters are plotted in Fig. 5 as a function of the thickness of deposited copper. It is obvious that the a increases owing to the fact that the increase in Auger KE is larger than the corresponding decrease in Cu2p BE. Small oscil3/2 lations observed in Fig. 5 can be attributed to instability of the deposit, e.g. 2D to 3D cluster reconstruction, which causes a rearrangement of the electronic density of Cu clusters. The Auger parameter analysis was used to estimate roughly the chemical shift (or the electron exchange) between Cu and ZrO during deposition. 2 This method, developed by Kao et al. [28,29], correlates the BE and Auger KE shifts of the substrate to the initial and final effects at a metal/semiconductor interface. With some approx-
(3)
Therefore, by measuring the binding energy shift and the Auger kinetic energy shift with respect to the bulk, one can calculate the chemical shift and the relaxation energy shift. Positive values of De indicate that the deposited clusters at very low coverage are negatively charged with respect to the bulk and vice versa. When the coverage of Cu ˚ at RT deposition, increases from 1.35 to 25.8 A we obtain DBE=−0.15 eV and DKE=1.15 eV, hence De=0.35 eV and DR=0.50 eV. (At cover˚ we do not have measureages lower than 1.35 A ments of the Auger KE.) From the deposition data at 673 K with similar initial Cu coverage, we obtained DBE#0, DKE=0.24, hence De=0.12 eV and DR=0.12 eV. The final state contribution at RT is larger in magnitude than that at 673 K, consistent with smaller Cu particles at room temperature. The derived values of De are comparable with initial state shifts for small Cu clusters dispersed on Al O (0.4 eV ) [30] and on carbon (up to 2 3 0.5 eV ) [31] at RT. The positive values indicate that the copper atoms carry initially a negative charge, relative to the bulk. It is known that initial state effects include charge-transfer phenomena between substrate and deposit as well as intrinsic changes which depend on cluster size. The sign of the derived values of De is the same as that of the surface core-level shift (SCLS) of metallic Cu, which is mainly an intrinsic initial state effect [32]. The SCLS is equal to 0.24 eV [30], which indicates the presence of a size effect in our De values [33]. On the other hand, De has been associated with a charge transfer between substrate and deposit [28,29], the direction of which depends on the sign of De. In our case, the difference between the De
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value at RT and the SCLS of Cu clusters could be attributed to a charge transfer from ZrO to 2 Cu. Taking into account that De would be even larger if we had measurements for smaller Cu ˚ , then the clusters at coverages lower than 1.35 A contribution of charge transfer is strengthened. The smaller value of De at 673 K reflects a size effect. At this point it should be noted that in both cases, RT and 673 K, the magnitude of is De small, precluding the formation of a new compound at the interface, and indicating a very weak interaction between Cu and ZrO . 2 This result is in very good agreement with our proposed model for metal/ceramic interfaces [24,25], which predicts for Cu/ZrO system a weak 2 interaction. The De values are comparable, within experimental error, with the respective value 0.15 eV provided by a previous work for the Ni/ZrO 2 system [22]. It is not possible to make an exact comparison, because of the lack of measurements in the same coverage region. We compared the De ˚ thickness range of deposvalue for the 1.3 to 3 A ited metal for the two systems at room temperature and noticed that De is 0.1 eV higher in the case of the Ni/ZrO system. Taking into account that 2 0.1 eV is within the experimental error, we can say with some caution that the previous result is consistent with the higher value of work of adhesion (stronger interaction between substrate and metal ) in the case of the Ni/ZrO system [23]. 2
ponds to a work function change D( WF ) of the sample surface. It was observed that the WF of clean ZrO was 2 initially not stable in UHV and for this reason the clean ZrO was allowed to stabilize by keeping it 2 at 673 K in UHV for many hours. The CPD gradually decreased and after 48 h reached saturation. Then the deposition experiment was carried out. The results of the WF variation are shown in Fig. 6 where it is shown that the WF increases monotonically. This behaviour is typical for a metal deposited onto a substrate with lower WF than that of the metal and with no substantial intermixing or any other strong interaction between the two compounds. The absolute value of the WF for polycrystalline copper is given in the literature between 4.6 and 4.8 eV [35,36 ]. Concerning the WF of pure polycrystalline ZrO , to our knowledge, there exists 2 only one measurement at 3.12 eV, corresponding to the high-field apparent work function of a patchy surface of ZrO [37]. In another more 2 recent work, it has been assumed to lie near the
3.3. Work function measurements The adsorption of atoms or molecules on a solid surface, which takes place at the first step of the formation of a solid/solid interface, is generally associated with a change of work function [34]. In the case of the metal vapour deposition, the investigation of the WF variation of the substrate surface gives useful information about the growth process. The WF variation of the ZrO surface during 2 deposition was measured at 673 K where ZrO is 2 conductive by the Kelvin probe method, which gives the contact potential difference (CPD) between a conductive sample surface and a reference electrode. The change of this potential corres-
Fig. 6. Work function variation with Cu equivalent thickness during deposition at 673 K.
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value of the metal Zr (3.9 eV ) [38]. We measured the WF of ZrO following stabilization at 673 K 2 from the low energy cut-off of the UP spectra with the sample negatively polarized, in order to obtain a sharp secondary electron edge. The derived value, 3.1±0.1 eV, is lower than that for polycrystalline Cu. Thus the WF behaviour in Fig. 6 is justified. It must be added that the value of WF for bulk Cu is not yet reached within the figure. Using the simple relationship for the average work function of the Cu/ZrO surface eQ=heQ +(1−h)eQ , 2 Cu ZrO2 where h is the fraction of surface covered by Cu clusters and taking into account that at this point of deposition copper and ZrO have the bulk 2 values for WF, we find that h#0.45. Using the respective XPS data and the relationship I /I =(1−h)+h exp−[(d /h)/l(ECu )], where Zr Zr,2 eq Zr d is the equivalent copper thickness, we also eq found the value 0.45 eV for h. This indicates that the WF results are consistent with the XPS results. The initial variations of the WF show a positive slope which means that the Cu adatoms are negatively charged. This result agree with the results of the Auger parameter analysis, where the De sign also denotes that the copper is slightly negatively charged. The WF changes of clean ZrO up to saturation 2 could not be explained in this study. The XP spectra of Zr3d and O1s peaks obtained simultaneously with the work function measurements did not show any change of the peaks. Probably the work function of the ZrO is very sensitive to 2 oxygen concentration in the residual gas of the atmosphere or to the mobility of the O2− ions. In a recent work, WF changes of the ZrO surface 2 were discussed in terms of defect chemistry of the near surface layers [39]. This defect chemistry depends on the Y O content, on the partial pres2 3 sure of oxygen, on the doping or not of ZrO , and 2 it is reported that the CPD behaviour is generally complicated.
4. Conclusions The mechanism of growth of Cu on ZrO at 2 room temperature seems to be a Stranski– Krastanov process. At 673 K 3D particles of
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copper start to form from the very early stages of deposition, and they are larger in size. The XP spectra did not indicate any strong interaction between deposit and substrate. The copper clusters showed small core-level shifts due to both initial and final state effects, and they were negatively charged because of the initial state effect. The absolute value of the work function of ZrO was 2 found by UPS to be 3.1 eV at 673 K. The study of the work function variation with copper thickness during deposition at 673 K showed that WF increases towards the bulk value of copper. After deposition of almost 4 ML of copper, saturation of the WF change did not occur, which verifies the strong 3D character of the copper deposit. The initial slope of the WF curve also suggests that the copper clusters are negatively charged.
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