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Photoemission study on formation of YCu intermetallic compound on Cu(1 0 0) surface
Physica B 226 (1996) 399-405
ELSEVIER
Photoemission study on formation of Y-Cu intermetallic compound on Cu(1 0 O) surface Liu Xianming*, Wu Jianxin, Zhu Jingsheng Structure Research Laboratory, University of Science and Technology of China, Anhui, Hefei 230026, China
Received 26 March 1996
Abstract Deposition of Y on the Cu(1 0 0) surface at room temperature has been studied by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and work function measurement. The formation of a relatively thick mixed interface, > 100 A, is observed. The Y/Cu atomic ratio of the mixed interface is found to be about 1 : 3 at the Y coverage of 2.0 ML. Based on the Y-Cu phase diagram we suggest that an intermetallic compound Y C u 3 is formed in the top layer of the interface in the range of 0.8-2.0 ML. For the coverage more than 2.0 ML, the Y atoms are stopped to diffuse into the intermetallic compound and form a yttrium layer on it.
1. Introduction In the past decades, the diffusion of metal atoms on the metal surface has been researched to understand the thermodynamic and kinetic behavior of surface a t o m transport [1-5]. Recent studies have shown that, when a rare-earth metal is deposited on other metal substrate, an intermixed interface or alloying overlayer can be formed at r o o m temperature or just above and the overlayer has amorphous structure [6-13]. For example, the L a - A u system was found to form a m o r p h o u s alloy over a wide composition range [6]. F o r the rare-earth metal-Cu systems, such as C e - C u [7], S m - C u [11], Nd-Cu(1 00) [12] and Nd-Cu(1 1 1) [13] systems, an intermetallic compound or alloy was observed after the deposition of the rare-earth metal on Cu substrate. It is difficult to predict the nature of an interface when one element is deposited onto * Corresponding author.
another. This is due to the variety of the chemical and physical properties that must be considered, such as surface energy, interface energy, heat of formation of compound or alloy and diffusion properties of one element in another. If the surface energy of the deposited metal is considerably smaller than that of the substrate, a homogeneous overlayer can be formed. On the other hand, if the surface energy of the deposited metal is larger than the substrate surface energy, the most stable state may be one where the deposited atoms nucleate on the surface. There is a possibility that the energetically most favorable state is one where an interface alloy or intermetallic compound has formed. The metal atoms of Cu, Ag, Au, Re, Ir, Pt, Co and Fe have been shown to possess very high diffusion coefficients in the rare-earth metals. When a rare-earth metal is deposited on one of these surfaces, it reacts with the substrate and forms an interdiffusion layer. The onset of interdiffusion can be very different, for example, in the Ce/Si(1 1 0)
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system [14] there is a critical coverage of 0.6 monolayer (ML) for the interdiffusion to happen, while there is no critical coverage observed in the Ce/Si(100) system [15]. In this paper, we used X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS) and work function measurement to study the formation of Y/Cu(10 0) mixed interface. An intermetallic compound was formed on Cu(1 00) surface.
2. Experimental details The experiments were performed at room temperature in a VG ESCALAB MKII electron spectrometer with a base pressure of 2 x 10 lo mbar. In addition to XPS and UPS, it is equipped a spherical sector analyzer whose energy resolution is 0.8 eV in XPS and 0.2 eV in UPS, respectively. A magnesium Kc~ X-ray (1253.6eV) and a HeI ultraviolet (21.2 eV) source were used. The photoemission spectra were recorded using 20 eV pass energy for XPS. UPS binding energies are referenced to the Fermi level, and XPS binding energies are calibrated with respect to the Cu 2p3/2 emission at 932.70eV and Au4fT/2 emission at 84.00eV. Changes in surface work function were also obtained in this system by monitoring the shifts of the onset of secondary electron emission during UPS experiments, with the sample biased at - 3.0 V. Cu(10 0) substrate, treated by mechanical polishing and ultrasonic washing, was first bombarded with argon ions of 2 keV x 30 ~tA for 1 h, and then annealed at 800°C for 30 min in the UHV chamber. This procedure was repeated several times till no contaminates can be detected by XPS and UPS. Yttrium was evaporated in a tungsten basket and the whole evaporation apparatus was carefully degassed before experiments. During the deposition, the evaporating current was kept constant at 10.5A, and the pressure was never exceeded 1 × 1 0 - 9 mbar. The evaporation rate of yttrium was estimated to be about 1.4 M L per minute. In order to avoid the accumulated contamination, we bombarded yttrium off the substrate after spectra for each coverage had taken, rather than evaporating yttrium successively.
Yttrium coverage was estimated from the XPS core levels intensities using of a nonreactive layerby-layer model. In this model, it is assumed that the deposited Y atoms reside on the Cu(1 0 0) surface in a regular stack of monolayers successively. For Y coverage 0 in the submonolayer range, the expressions for the intensities of Y 3d and Cu 2p3/2 core-levels are:
Iv = 0I°[1 - exp( - a/2v cos ~)],
(1)
Icu = OI°u exp( - a/2cu cos e) + (1 - 0) Ic°u.
(2)
For coverage 0 above one monolayer, we used the following equations to determine the thickness x: Iy
=
I°v[1 - exp( - x/2v cos ~)],
lcu = lc°uexp( - X/2cu cos c0,
(3) (4)
where oa is the thickness of one Y monolayer taken as 2.9 A (this is roughly half the c lattice parameter in the hcp structure of yttrium) [16], Iv and Icu are the measured intensities, I ° and I°u are the respective intensities for the pure yttrium and copper obtained under the same photoemission condition, they correspond to modified sensitivity factors, I°v for the Y 3d5/2 level is 1.76 [17] and the value of I°u for the Cu 2p3/2 level is 4.2 [17], 2v and 2Cu are the mean free paths of the core-level photoelectron and were taken as 24.3 [18] and 8 A [7], respectively. The takeoff angle e was 15 ° in our experiments. In submonolayer cases, Eq. (1) can be divided by Eq. (2) to solve for 0, while in overlayer cases, x can be calculated by dividing Eq. (3) by Eq. (4). The calculated data of Y coverages was listed in the table. Such model is incorrect for the case of interdiffusion or island formation. For the Y/Cu(1 00) system, the number of Y layers obtained from this model should be smaller than the actual value.
3. Results and discussion The interaction of yttrium with the Cu(100) surface was studied for yttrium coverages ranging from 0 = 0 to 0 = 3.4 ML. The XPS spectra of Y3d and Cu 2p3/2 core levels recorded for each Y coverage are shown in Fig. 1. For the clean Cu(100) surface, a sharp peak of Cu2p3/2 core level was
401
L. Xianming et al. / Physica B 226 (1996) 399-405
I
I ~x
Cu LMM hv=1253.6eV
/
/ I 330.0 J 928
I 933
(a)
I
I 938
I
I 943
I
I 948
I
I 953
{b)
I
I
336.0
I
338.0
I
340.0
Binding Energy (eV)
Y3d
5
150.0
334.0
Fig. 2. Spectra ofCu LMM transition at different Y coverages of (1) clean surface, (2) 0.4 ML, (3) 0.8 ML, (4) 1.1 ML, (5) 2.0 ML, (6) 3.4 ML.
Binding Energy (eV) B
332.0
153.0
156.0
159.0
162.0
165.0
Binding Energy (eV)
Fig. 1. (a) Spectra of the Cu 2p3/2 core level at different Y coverages of (1) clean surface, (2) 0.4 ML, (3) 0.8 ML, (4) 1.1 ML, (5) 2.0 ML, (6) 3.4 ML. (b) Spectra of the Y 3d core level at different Y coverages of (1) 0.4 ML, (2) 0.8 ML, (3) 1.1 ML, (4) 2.0 ML, (5) 3.4 ML.
observed at 932.7 eV corresponding to the zerovalent copper. As yttrium is e v a p o r a t e d on the surface (0 = 0.4 ML), this peak shifts in position towards higher binding energy to 932.85 eV. The
binding energy of Y3ds/2 core level is 156.2eV. This represents a 2.1 eV shift from the metallic state of yttrium found at 154.0 eV [18]. F u r t h e r increase in Y coverage (up to 0 = 3.4 M L ) causes successively increase in the C u 2 p 3 / : binding energy. The m a x i m u m value of the shift for Cu 2p3/2 core level is 0.5 eV, with respect to the clean Cu(1 00) surface. At the same time, the Auger L3VV transition of Cu induced by X-ray (shown in Fig. 2) had the same tendency shifting to higher binding energy (or to lower kinetic energy). However, the Y 3ds/a core level with the binding energy of 156.2 eV observed at low Y coverage (0 = 0.4 ML) almost remains constant as the Y coverage increases. Fig. 3 displays the shifts of Cu 2p3/2 and CuL3VV transition related to the clean C u ( 1 0 0 ) surface against Y coverage. The continuous increase in the binding energy of the Cu2p3/2 core level even for large coverages m a y be indicative of a concentration gradient across the interface. Core level shifts to higher binding energy usually indicate that the charge transfers away from the corresponding atoms. In metallic systems, however, final state screening plays an i m p o r t a n t role, and shifts of core levels m a y be complicated. Because
L. Xianming et al. / Physica B 226 (1996) 399-405
402
Cu2p3a(.)
(BE,eV)
1.2
CuL3VV(o)
933.2
335.1
933.1
335.0
933.0
334.9
932.9
334.8
932.8
334.7
Y/Cu Atomic Ratio 1.0 0.8 0.6
932.7
0
1
2
3
4
5
6
334.6
Evaporation time (min) Fig. 3. Changes in binding energy of Cu2p3/2 (o) and CuL3VV (0) core levels with evaporation times.
0.4 0.2 0.0
0
1
2
3
4
5
6
Evaporation time (min) Fig. 4. Variation of Y/Cu atomic ratio in the surface with evaporation times.
copper is more electronegative than yttrium, negative charge would transfer from yttrium to copper atoms leading to a strong Y-Cu bonding. Similar core level shifts, as we observed in Y/Cu(100) system, of the Cu 2p photoemission to higher binding energy have previously been observed for Ce-Cu system [7, 19]. This is also consistent with the work function consideration. While the net charge flows onto Cu atoms, the decrease of d character, which accompanies the increase of non-d count, produces Cu core level shift to higher binding energy usually associated with a loss of charge [20]. The continuous shift of Cu 2p3/2 photoemission and large shift of Y 3d core levels even at low coverage indicate that there is an interaction between overlayer and substrate in the interface. Rare-earth metals can form alloys or intermetallic compounds at room temperature deposited onto Cu substrate, such as Ce-Cu system [7,19], Nd-Cu(100) [12] and N d - C u ( l l l ) [13]. The yttrium can react with copper to form bulk YCux intermetallic compounds (x = 1, 2, 3, 5) on certain condition based on the Y-Cu phase diagram. In the present work, the interdiffusion between Y deposited and Cu substrate was observed and an intermetallic compound was formed on the mixed interface even at low coverages. For the intermetallic compounds of CeNi~ and CeAu~, the Ce3d core
levels shift to higher binding energy with the concentration x increase [21]. Considering the spectra of Y 3d core levels and the work function changes (discussed below), we assume that only a YCux intermetallic compound of single phase exists in the top layer of the mixed interface during Y deposition. The Y/Cu atomic ratio versus the evaporation times was plotted in Fig. 4. In the composition calculation we used the sensitivity factors of pure copper and yttrium as those of the metals in the YCux intermetallic compound. It is obviously not accurate but can give the variation tendency and the approximate value of the atomic ratio in the surface. The curve can be divided into three regions as shown in the figure. The atomic ratio increases linearly in the region I and there is a plateau of the Y/Cu atomic ratio curve in the continuous region for Y coverages up to 2.0 ML. At the initial deposition, the interdiffusion happened immediately in the Y/Cu(100) interface. But the interdiffusion layer is so thin that the deposited Y can diffuse into the substrate rapidly. This results in the atomic ratio gradually increasing with evaporation time. In region II, although yttrium was successively deposited onto the Cu substrate, the Y/Cu atomic ratio nearly kept constant and was equal to
L. Xianming et al. / Physica B 226 (1996) 399-405
0.47:1 at the Y coverage of 2.0 ML. Because the mean free path of the Y 3d photoelectrons is much longer than that of Cu 2p photoelectrons, the real value of the Y/Cu atomic ratio is certainly less than the measured value and may be equal to 1 : 3. We estimated the thickness of the mixed interface to be o more than 100 A. At this stage the rates of yttrium evaporation and interdiffusion seem to be equal. It makes the concentration gradient across the interface get equilibrium. As the evaporation time is over than 5 min, the thick mixed interface prevented the yttrium from diffusing into the Cu substrate successively. We consider that a Y overlayer covered on the Y-Cu intermetallic compound because of the rapid increase of Y/Cu atomic ratio. In the same time, the electronic charge transfer from yttrium to copper still took place in terms of the continuous shifts of Cu2p core levels to higher binding energy and the unchanged position and lineshape of Y 3d photoemission. We believe that a YCu3 intermetallic compound formed in the top layer of the mixed interface [-12]. We also studied the valence band structure of the Y/Cu(100) interface by UPS (He I). The data is presented in Fig. 5. For clean Cu(1 00) surface, the character of the Cu 3d-band and 4s-band can be seen clearly. At submonolayer coverages the Cu d-character is quickly attenuated. When the Cu(100) surface was covered with 0.4ML Y, a dominant peak located at about 3.0 eV binding energy below Fermi level was found. Furthermore, this peak shifted to higher binding energy with increasing Y coverage and finally stopped at 3.9 eV as the Y coverage was 3.4 ML. We argue that the shift of this peak is caused by the charge transfer and the formation of an intermetallic compound. A new photoemission peak at 6.1 eV binding energy was observed in the valence band spectrum of 0.8 ML coverage and became more and more intense but did not shift with the Y coverage. This peak was observed in previous works [22, 23]. For the clean surface of single crystal or polycrystal yttrium specimens, there was no the peak of ,-~ 6.0 eV binding energy in UPS. After a light Ar + bombardment, this peak was observed. This result indicates that the peak at --~6.0 eV binding energy is not induced by oxygen contamination. The peak at ~ 6.0 eV binding energy was also observed in the
403
UPS
hv=21.2eV
I
~I
t i
-2.0
~
~
1
0.0
I
2.0
I
4.0
I
I
6.0
I
I
8.0
I
I
10.0
I
I
12.0
Binding Energy (eV) Fig. 5. Spectra of valence band of the Y/Cu(1 00) interface at different Y coverages of(l) clean surface, (2) 0.4 ML, (3) 0.8 ML, (4) 1.1 ML, (5) 2.0 ML, (6) 3.4 ML.
He I UP spectrum of the Y overlayer. The intensity of this peak attenuated rapidly under annealing at 800°C. In other words, crystallization of the amorphous Y overlayer attenuates the intensity of this peak. By all the results above, we argue that this peak is closely relative to the surface order. The amorphous structure of the Y overlayer may enhance the photoionization cross-section at hv = 21.2 eV for the Y4d state. Under our experimental conditions, the oxygen concentration was below the limit of detectability of XPS just after the Y evaporation. No trace of oxygen-induced peaks is present in Fig. I(B). We also observed the O 2p emission in the range of 4.9-5.5 eV binding energy (to be published elsewhere) in the work on oxidation of the Y-Cu intermetallic compound. Therefore, we believe that the peak at 6.1 eV binding energy was attributed to Y4d electrons and the effect of oxygen contamination is negligible. At large Y coverage, 0 = 3.4 ML, the surface of the sample was completely covered with yttrium. Because only the photoelectrons emitted from two
404
L. Xianming et al. /Physica B 226 (1996) 399-405
or three monolayers can be detected in UPS experiment, these two peaks must be attributed to the direct photoexcitation of Y4d electrons. From Fig. 5 we can see that the peak just below the Fermi level grows stronger and does not move as the Y coverage increase, which corresponds to the photoemission of Y5s electrons, and may contain some character of Cu 4s-band. The corresponding work function data are listed in Table 1 measured by UPS during Y deposition at room temperature. The work function decreased rapidly before attaining a constant value at the point of 0.8 ML Y coverage. In the coverage range from 0.8 to 2.0 ML, the work function was about 3.1 eV (taking the work functions of clean Cu(100) surface and pure yttrium to be 4.59 eV [16] and 3.1 eV [16], respectively). Although this value is equal to that of bulk yttrium, it is the work function of the intermetallic compound formed on the Cu(100) surface, rather than that of deposited yttrium. We know that the surface work function of a material is very sensitive to the physical and chemical properties of the surface, such as the composition, crystal and electronic structure, charge transfer, chemical environment, etc. No change of the work function in the wide range indicates that only an intermetallic compound of single phase exists in the detecting depth of UPS. After the Y coverage was more than 2.0 ML, the work function decreased slightly which was attributed to the electronic charge transfer from yttrium to the compound. This successive decrease demonstrates the formation of a Y layer on the intermetallic compound. Therefore, the result of work function change improves our suggestion again that during the Y deposition there is a single phase intermetallic
Table 1 The list of Y coverages and work function changes during Y deposition Coverage (1)
Evaporation time (min.)
Work function (2)
0
0.5
1.0
1.5
3
(1) 0 (ML) (2) - A4~(eV)
0 0
0.4 1.0
0.8 1.5
1.1 1.5
1.7 2.0 3.4 1.5 1.7 1.6
5
6
compound growing in the top layer of the mixed interface and a yttrium layer is formed on the compound for the Y coverage more than 2.0 ML.
4. Conclusion
Yttrium was deposited on clean copper and the interaction of the Y overlayer with the Cu(100) surface was studied for coverages ranging from 0 to 3.4 ML. The present data of core level photoemission indicates that the interaction takes place through electronic charge transfer from Y overlayer to the Cu substrate, causing a strong Y-Cu bonding. For the coverage less than 2.0 ML, the deposited Y interdiffuses into the substrate to form a mixed interface and an intermetallic compound YCu 3 exists in the top layer of the mixed interface. After this point, the Y atoms are stopped to diffuse into the intermetallic compound and form a yttrium layer on it.
References [1] A.G. Naumovets and Yu.S. Vedula, Surf. Sci. Rep. 4 (1985) 365. [2] R. Gomer, Rep. Prog. Phys. 53 (1990) 917. [3] Y.L. He, J.K. Zuo, G.C. Wang and J.J. Low, Surf. Sci. 255 (1991) 269. [4] A. Brodde, K. Dreps, J. Binder, Ch. Lunau and H. Neddermeyer, Phys. Rev. B 47 (1993) 6609. [5] Yi-Ren Tzeng, Hao-tse Wu, Keh-Dong Shiang and Tien T. Tsong, Phys. Rev. B 47 (1993) 5549. [6] R.B. Schwarz and W.L. Johnson, Phys. Rev. Lett. 51 (1983) 415. [7] N.A. Braaten, J.K. Grepstad and S. Raaen, Phys. Rev. B 40 (1989) 7969. [8] S. Raaen, Solid State Commun. 73 (1990) 389. [9] G.M. Roe, C.M.C. de Castilho and R.M. Lambert, Surf. Sci. 301 (1994) 39. [10] J. Tang, J.M. Lawrence and J.C. Hemminger, Phys. Rev. B 47 (1993) 16477. [11] A. Rosengren and B. Johansson, Phys. Rev. B 26 (1982) 3068. [12] Roger M. Nix and Richard M. Lambert, Surf. Sci. 186 (1987) 163. [13] R.M. Nix, R.t W. Judd and R.M. Lambert, Surf. Sci. 203 (1988) 307. [14] A. Fujimori, M. Grioni, J.J. Joyce and J.H. Weaver, Phys. Rev. B 36 (1987) 1075. [15] F.U. Hillebrecht, Appl. Phys. Lett. 55 (1989) 227.
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[16] R.C. Weast, Ed., CRC Handbook of Chemistry and Physics, 58th ed. (CRC Press, Boca Raton, 1977). [17] D. Briggs and M.P. Seah, ed., Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy (Wiley, New York, 1983). [18] A. Mesarwi and A. Igatiev, Surf. Sci. 244 (1991) 15. [19] S. Raaen, C. Berg and N.A. Braaten, Surf. Sci. 269/270 (1992) 953.
405
[20] R.E. Watson and M.L. Pedman, Struct. Bonding 24 (1975) 83. [21] J.C. Fuggle, F.U. Hillebrecht, Z, Zoluierek and R. Laser, Phys. Rev. B 27 (1983) 7330. [22] S.D. Barrett and R.G. Jordan, Z. Phys. B-Condensed Matter 66 (1987) 375. 1-23] R. Baptist, A. Pellissier and G. Chauvet, Z. Phys. BConden. Matter 73 (1988) 107.
ELSEVIER
Photoemission study on formation of Y-Cu intermetallic compound on Cu(1 0 O) surface Liu Xianming*, Wu Jianxin, Zhu Jingsheng Structure Research Laboratory, University of Science and Technology of China, Anhui, Hefei 230026, China
Received 26 March 1996
Abstract Deposition of Y on the Cu(1 0 0) surface at room temperature has been studied by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and work function measurement. The formation of a relatively thick mixed interface, > 100 A, is observed. The Y/Cu atomic ratio of the mixed interface is found to be about 1 : 3 at the Y coverage of 2.0 ML. Based on the Y-Cu phase diagram we suggest that an intermetallic compound Y C u 3 is formed in the top layer of the interface in the range of 0.8-2.0 ML. For the coverage more than 2.0 ML, the Y atoms are stopped to diffuse into the intermetallic compound and form a yttrium layer on it.
1. Introduction In the past decades, the diffusion of metal atoms on the metal surface has been researched to understand the thermodynamic and kinetic behavior of surface a t o m transport [1-5]. Recent studies have shown that, when a rare-earth metal is deposited on other metal substrate, an intermixed interface or alloying overlayer can be formed at r o o m temperature or just above and the overlayer has amorphous structure [6-13]. For example, the L a - A u system was found to form a m o r p h o u s alloy over a wide composition range [6]. F o r the rare-earth metal-Cu systems, such as C e - C u [7], S m - C u [11], Nd-Cu(1 00) [12] and Nd-Cu(1 1 1) [13] systems, an intermetallic compound or alloy was observed after the deposition of the rare-earth metal on Cu substrate. It is difficult to predict the nature of an interface when one element is deposited onto * Corresponding author.
another. This is due to the variety of the chemical and physical properties that must be considered, such as surface energy, interface energy, heat of formation of compound or alloy and diffusion properties of one element in another. If the surface energy of the deposited metal is considerably smaller than that of the substrate, a homogeneous overlayer can be formed. On the other hand, if the surface energy of the deposited metal is larger than the substrate surface energy, the most stable state may be one where the deposited atoms nucleate on the surface. There is a possibility that the energetically most favorable state is one where an interface alloy or intermetallic compound has formed. The metal atoms of Cu, Ag, Au, Re, Ir, Pt, Co and Fe have been shown to possess very high diffusion coefficients in the rare-earth metals. When a rare-earth metal is deposited on one of these surfaces, it reacts with the substrate and forms an interdiffusion layer. The onset of interdiffusion can be very different, for example, in the Ce/Si(1 1 0)
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system [14] there is a critical coverage of 0.6 monolayer (ML) for the interdiffusion to happen, while there is no critical coverage observed in the Ce/Si(100) system [15]. In this paper, we used X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS) and work function measurement to study the formation of Y/Cu(10 0) mixed interface. An intermetallic compound was formed on Cu(1 00) surface.
2. Experimental details The experiments were performed at room temperature in a VG ESCALAB MKII electron spectrometer with a base pressure of 2 x 10 lo mbar. In addition to XPS and UPS, it is equipped a spherical sector analyzer whose energy resolution is 0.8 eV in XPS and 0.2 eV in UPS, respectively. A magnesium Kc~ X-ray (1253.6eV) and a HeI ultraviolet (21.2 eV) source were used. The photoemission spectra were recorded using 20 eV pass energy for XPS. UPS binding energies are referenced to the Fermi level, and XPS binding energies are calibrated with respect to the Cu 2p3/2 emission at 932.70eV and Au4fT/2 emission at 84.00eV. Changes in surface work function were also obtained in this system by monitoring the shifts of the onset of secondary electron emission during UPS experiments, with the sample biased at - 3.0 V. Cu(10 0) substrate, treated by mechanical polishing and ultrasonic washing, was first bombarded with argon ions of 2 keV x 30 ~tA for 1 h, and then annealed at 800°C for 30 min in the UHV chamber. This procedure was repeated several times till no contaminates can be detected by XPS and UPS. Yttrium was evaporated in a tungsten basket and the whole evaporation apparatus was carefully degassed before experiments. During the deposition, the evaporating current was kept constant at 10.5A, and the pressure was never exceeded 1 × 1 0 - 9 mbar. The evaporation rate of yttrium was estimated to be about 1.4 M L per minute. In order to avoid the accumulated contamination, we bombarded yttrium off the substrate after spectra for each coverage had taken, rather than evaporating yttrium successively.
Yttrium coverage was estimated from the XPS core levels intensities using of a nonreactive layerby-layer model. In this model, it is assumed that the deposited Y atoms reside on the Cu(1 0 0) surface in a regular stack of monolayers successively. For Y coverage 0 in the submonolayer range, the expressions for the intensities of Y 3d and Cu 2p3/2 core-levels are:
Iv = 0I°[1 - exp( - a/2v cos ~)],
(1)
Icu = OI°u exp( - a/2cu cos e) + (1 - 0) Ic°u.
(2)
For coverage 0 above one monolayer, we used the following equations to determine the thickness x: Iy
=
I°v[1 - exp( - x/2v cos ~)],
lcu = lc°uexp( - X/2cu cos c0,
(3) (4)
where oa is the thickness of one Y monolayer taken as 2.9 A (this is roughly half the c lattice parameter in the hcp structure of yttrium) [16], Iv and Icu are the measured intensities, I ° and I°u are the respective intensities for the pure yttrium and copper obtained under the same photoemission condition, they correspond to modified sensitivity factors, I°v for the Y 3d5/2 level is 1.76 [17] and the value of I°u for the Cu 2p3/2 level is 4.2 [17], 2v and 2Cu are the mean free paths of the core-level photoelectron and were taken as 24.3 [18] and 8 A [7], respectively. The takeoff angle e was 15 ° in our experiments. In submonolayer cases, Eq. (1) can be divided by Eq. (2) to solve for 0, while in overlayer cases, x can be calculated by dividing Eq. (3) by Eq. (4). The calculated data of Y coverages was listed in the table. Such model is incorrect for the case of interdiffusion or island formation. For the Y/Cu(1 00) system, the number of Y layers obtained from this model should be smaller than the actual value.
3. Results and discussion The interaction of yttrium with the Cu(100) surface was studied for yttrium coverages ranging from 0 = 0 to 0 = 3.4 ML. The XPS spectra of Y3d and Cu 2p3/2 core levels recorded for each Y coverage are shown in Fig. 1. For the clean Cu(100) surface, a sharp peak of Cu2p3/2 core level was
401
L. Xianming et al. / Physica B 226 (1996) 399-405
I
I ~x
Cu LMM hv=1253.6eV
/
/ I 330.0 J 928
I 933
(a)
I
I 938
I
I 943
I
I 948
I
I 953
{b)
I
I
336.0
I
338.0
I
340.0
Binding Energy (eV)
Y3d
5
150.0
334.0
Fig. 2. Spectra ofCu LMM transition at different Y coverages of (1) clean surface, (2) 0.4 ML, (3) 0.8 ML, (4) 1.1 ML, (5) 2.0 ML, (6) 3.4 ML.
Binding Energy (eV) B
332.0
153.0
156.0
159.0
162.0
165.0
Binding Energy (eV)
Fig. 1. (a) Spectra of the Cu 2p3/2 core level at different Y coverages of (1) clean surface, (2) 0.4 ML, (3) 0.8 ML, (4) 1.1 ML, (5) 2.0 ML, (6) 3.4 ML. (b) Spectra of the Y 3d core level at different Y coverages of (1) 0.4 ML, (2) 0.8 ML, (3) 1.1 ML, (4) 2.0 ML, (5) 3.4 ML.
observed at 932.7 eV corresponding to the zerovalent copper. As yttrium is e v a p o r a t e d on the surface (0 = 0.4 ML), this peak shifts in position towards higher binding energy to 932.85 eV. The
binding energy of Y3ds/2 core level is 156.2eV. This represents a 2.1 eV shift from the metallic state of yttrium found at 154.0 eV [18]. F u r t h e r increase in Y coverage (up to 0 = 3.4 M L ) causes successively increase in the C u 2 p 3 / : binding energy. The m a x i m u m value of the shift for Cu 2p3/2 core level is 0.5 eV, with respect to the clean Cu(1 00) surface. At the same time, the Auger L3VV transition of Cu induced by X-ray (shown in Fig. 2) had the same tendency shifting to higher binding energy (or to lower kinetic energy). However, the Y 3ds/a core level with the binding energy of 156.2 eV observed at low Y coverage (0 = 0.4 ML) almost remains constant as the Y coverage increases. Fig. 3 displays the shifts of Cu 2p3/2 and CuL3VV transition related to the clean C u ( 1 0 0 ) surface against Y coverage. The continuous increase in the binding energy of the Cu2p3/2 core level even for large coverages m a y be indicative of a concentration gradient across the interface. Core level shifts to higher binding energy usually indicate that the charge transfers away from the corresponding atoms. In metallic systems, however, final state screening plays an i m p o r t a n t role, and shifts of core levels m a y be complicated. Because
L. Xianming et al. / Physica B 226 (1996) 399-405
402
Cu2p3a(.)
(BE,eV)
1.2
CuL3VV(o)
933.2
335.1
933.1
335.0
933.0
334.9
932.9
334.8
932.8
334.7
Y/Cu Atomic Ratio 1.0 0.8 0.6
932.7
0
1
2
3
4
5
6
334.6
Evaporation time (min) Fig. 3. Changes in binding energy of Cu2p3/2 (o) and CuL3VV (0) core levels with evaporation times.
0.4 0.2 0.0
0
1
2
3
4
5
6
Evaporation time (min) Fig. 4. Variation of Y/Cu atomic ratio in the surface with evaporation times.
copper is more electronegative than yttrium, negative charge would transfer from yttrium to copper atoms leading to a strong Y-Cu bonding. Similar core level shifts, as we observed in Y/Cu(100) system, of the Cu 2p photoemission to higher binding energy have previously been observed for Ce-Cu system [7, 19]. This is also consistent with the work function consideration. While the net charge flows onto Cu atoms, the decrease of d character, which accompanies the increase of non-d count, produces Cu core level shift to higher binding energy usually associated with a loss of charge [20]. The continuous shift of Cu 2p3/2 photoemission and large shift of Y 3d core levels even at low coverage indicate that there is an interaction between overlayer and substrate in the interface. Rare-earth metals can form alloys or intermetallic compounds at room temperature deposited onto Cu substrate, such as Ce-Cu system [7,19], Nd-Cu(100) [12] and N d - C u ( l l l ) [13]. The yttrium can react with copper to form bulk YCux intermetallic compounds (x = 1, 2, 3, 5) on certain condition based on the Y-Cu phase diagram. In the present work, the interdiffusion between Y deposited and Cu substrate was observed and an intermetallic compound was formed on the mixed interface even at low coverages. For the intermetallic compounds of CeNi~ and CeAu~, the Ce3d core
levels shift to higher binding energy with the concentration x increase [21]. Considering the spectra of Y 3d core levels and the work function changes (discussed below), we assume that only a YCux intermetallic compound of single phase exists in the top layer of the mixed interface during Y deposition. The Y/Cu atomic ratio versus the evaporation times was plotted in Fig. 4. In the composition calculation we used the sensitivity factors of pure copper and yttrium as those of the metals in the YCux intermetallic compound. It is obviously not accurate but can give the variation tendency and the approximate value of the atomic ratio in the surface. The curve can be divided into three regions as shown in the figure. The atomic ratio increases linearly in the region I and there is a plateau of the Y/Cu atomic ratio curve in the continuous region for Y coverages up to 2.0 ML. At the initial deposition, the interdiffusion happened immediately in the Y/Cu(100) interface. But the interdiffusion layer is so thin that the deposited Y can diffuse into the substrate rapidly. This results in the atomic ratio gradually increasing with evaporation time. In region II, although yttrium was successively deposited onto the Cu substrate, the Y/Cu atomic ratio nearly kept constant and was equal to
L. Xianming et al. / Physica B 226 (1996) 399-405
0.47:1 at the Y coverage of 2.0 ML. Because the mean free path of the Y 3d photoelectrons is much longer than that of Cu 2p photoelectrons, the real value of the Y/Cu atomic ratio is certainly less than the measured value and may be equal to 1 : 3. We estimated the thickness of the mixed interface to be o more than 100 A. At this stage the rates of yttrium evaporation and interdiffusion seem to be equal. It makes the concentration gradient across the interface get equilibrium. As the evaporation time is over than 5 min, the thick mixed interface prevented the yttrium from diffusing into the Cu substrate successively. We consider that a Y overlayer covered on the Y-Cu intermetallic compound because of the rapid increase of Y/Cu atomic ratio. In the same time, the electronic charge transfer from yttrium to copper still took place in terms of the continuous shifts of Cu2p core levels to higher binding energy and the unchanged position and lineshape of Y 3d photoemission. We believe that a YCu3 intermetallic compound formed in the top layer of the mixed interface [-12]. We also studied the valence band structure of the Y/Cu(100) interface by UPS (He I). The data is presented in Fig. 5. For clean Cu(1 00) surface, the character of the Cu 3d-band and 4s-band can be seen clearly. At submonolayer coverages the Cu d-character is quickly attenuated. When the Cu(100) surface was covered with 0.4ML Y, a dominant peak located at about 3.0 eV binding energy below Fermi level was found. Furthermore, this peak shifted to higher binding energy with increasing Y coverage and finally stopped at 3.9 eV as the Y coverage was 3.4 ML. We argue that the shift of this peak is caused by the charge transfer and the formation of an intermetallic compound. A new photoemission peak at 6.1 eV binding energy was observed in the valence band spectrum of 0.8 ML coverage and became more and more intense but did not shift with the Y coverage. This peak was observed in previous works [22, 23]. For the clean surface of single crystal or polycrystal yttrium specimens, there was no the peak of ,-~ 6.0 eV binding energy in UPS. After a light Ar + bombardment, this peak was observed. This result indicates that the peak at --~6.0 eV binding energy is not induced by oxygen contamination. The peak at ~ 6.0 eV binding energy was also observed in the
403
UPS
hv=21.2eV
I
~I
t i
-2.0
~
~
1
0.0
I
2.0
I
4.0
I
I
6.0
I
I
8.0
I
I
10.0
I
I
12.0
Binding Energy (eV) Fig. 5. Spectra of valence band of the Y/Cu(1 00) interface at different Y coverages of(l) clean surface, (2) 0.4 ML, (3) 0.8 ML, (4) 1.1 ML, (5) 2.0 ML, (6) 3.4 ML.
He I UP spectrum of the Y overlayer. The intensity of this peak attenuated rapidly under annealing at 800°C. In other words, crystallization of the amorphous Y overlayer attenuates the intensity of this peak. By all the results above, we argue that this peak is closely relative to the surface order. The amorphous structure of the Y overlayer may enhance the photoionization cross-section at hv = 21.2 eV for the Y4d state. Under our experimental conditions, the oxygen concentration was below the limit of detectability of XPS just after the Y evaporation. No trace of oxygen-induced peaks is present in Fig. I(B). We also observed the O 2p emission in the range of 4.9-5.5 eV binding energy (to be published elsewhere) in the work on oxidation of the Y-Cu intermetallic compound. Therefore, we believe that the peak at 6.1 eV binding energy was attributed to Y4d electrons and the effect of oxygen contamination is negligible. At large Y coverage, 0 = 3.4 ML, the surface of the sample was completely covered with yttrium. Because only the photoelectrons emitted from two
404
L. Xianming et al. /Physica B 226 (1996) 399-405
or three monolayers can be detected in UPS experiment, these two peaks must be attributed to the direct photoexcitation of Y4d electrons. From Fig. 5 we can see that the peak just below the Fermi level grows stronger and does not move as the Y coverage increase, which corresponds to the photoemission of Y5s electrons, and may contain some character of Cu 4s-band. The corresponding work function data are listed in Table 1 measured by UPS during Y deposition at room temperature. The work function decreased rapidly before attaining a constant value at the point of 0.8 ML Y coverage. In the coverage range from 0.8 to 2.0 ML, the work function was about 3.1 eV (taking the work functions of clean Cu(100) surface and pure yttrium to be 4.59 eV [16] and 3.1 eV [16], respectively). Although this value is equal to that of bulk yttrium, it is the work function of the intermetallic compound formed on the Cu(100) surface, rather than that of deposited yttrium. We know that the surface work function of a material is very sensitive to the physical and chemical properties of the surface, such as the composition, crystal and electronic structure, charge transfer, chemical environment, etc. No change of the work function in the wide range indicates that only an intermetallic compound of single phase exists in the detecting depth of UPS. After the Y coverage was more than 2.0 ML, the work function decreased slightly which was attributed to the electronic charge transfer from yttrium to the compound. This successive decrease demonstrates the formation of a Y layer on the intermetallic compound. Therefore, the result of work function change improves our suggestion again that during the Y deposition there is a single phase intermetallic
Table 1 The list of Y coverages and work function changes during Y deposition Coverage (1)
Evaporation time (min.)
Work function (2)
0
0.5
1.0
1.5
3
(1) 0 (ML) (2) - A4~(eV)
0 0
0.4 1.0
0.8 1.5
1.1 1.5
1.7 2.0 3.4 1.5 1.7 1.6
5
6
compound growing in the top layer of the mixed interface and a yttrium layer is formed on the compound for the Y coverage more than 2.0 ML.
4. Conclusion
Yttrium was deposited on clean copper and the interaction of the Y overlayer with the Cu(100) surface was studied for coverages ranging from 0 to 3.4 ML. The present data of core level photoemission indicates that the interaction takes place through electronic charge transfer from Y overlayer to the Cu substrate, causing a strong Y-Cu bonding. For the coverage less than 2.0 ML, the deposited Y interdiffuses into the substrate to form a mixed interface and an intermetallic compound YCu 3 exists in the top layer of the mixed interface. After this point, the Y atoms are stopped to diffuse into the intermetallic compound and form a yttrium layer on it.
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[16] R.C. Weast, Ed., CRC Handbook of Chemistry and Physics, 58th ed. (CRC Press, Boca Raton, 1977). [17] D. Briggs and M.P. Seah, ed., Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy (Wiley, New York, 1983). [18] A. Mesarwi and A. Igatiev, Surf. Sci. 244 (1991) 15. [19] S. Raaen, C. Berg and N.A. Braaten, Surf. Sci. 269/270 (1992) 953.
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