oxide interface formation: a vibrational and electronic investigation by electron spectroscopies

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265 (1992) 31-38

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Copper/ oxide interface formation: a vibrational and electronic investigation by electron spectroscopies T.

Conard, J. Ghijsen, J.M. Vohs

‘, P.A.

Thiry,

R. Caudano

Lahoratoire Interdisciplinaire de Spectroscopic Electronique, Institute for Studies in Interface Science, Facult& Unir>ersitairesNotre Dame de la Pair, B-5000 Namur, Belgium

and R.L.

Johnson

lJniL:ersitiitHamburg, II. Institut fiir Experimentalphysik, D-2000 Hamburg 50, Germany Received

29 April

1991; accepted

for publication

29 November

1991

In this study, we deposited copper on a MgO(100) surface at room temperature (using a Knudsen cell) and studied the interface formation using electron spectroscopy. The evolution of the AES peak intensities showed that copper grows on MgO(100) in the Stranski-Krastanov mode. In HREELS experiments, the intensity and the position of the energy loss corresponding to the MgO surface optical phonon at 80.7 meV, both decrease with increasing Cu coverage. These results agree with theoretical spectra simulated from the dielectric theory by considering a Cu,O overlayer on a semi-infinite MgO crystal substrate at the beginning of the growth. From the HREELS data, both the formation of a homogeneous Cu metallic overlayer or a CuO overlayer on MgO can be ruled out. The synchrotron-radiation (SRI photoemission measurements were performed in the vicinity of the Cu3p-3d resonance. The positions of the Cu resonance peaks as a function of Cu coverage on MgO show that at low coverage the difference in energy between the main Cu3d peak and the resonance peak is close to that found in Cu,O and at higher coverage close to metallic copper indicating the formation of an interacting phase at the beginning followed by the growth of metallic copper.

1. Introduction Interfaces between metal and oxide have been studied over the last few years due to their importance in many applications such as catalytic reactions, ultrathin films, microelectronics and metallurgy. Since the discovery by Bednorz and Miiller in 1986 111 of a family of copper oxide ceramics with a high superconducting critical temperature much work has been devoted to the copper/ oxide interface formation because copper oxide planes are found in nearly all these ’ Permanent address: Department of Chemical University of Pennsylvania, Towne Building, street, Philadelphia, PA 19104.6393, USA.

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ceramics and because the best electrical properties are obtained in thin films. Hence copper/ oxide interface formation provides a good model of the conditions of epitaxial growth of these superconducting ceramics on oxides substrates. Up to now, the studies in the copper/oxide interface field were oriented towards determining the growth mode of copper on several oxides by Auger electron spectroscopy (AES) or in determining the electronic structure of the copper/ oxide interface by X-ray photoelectron spectroscopy (XPS), or ultraviolet photoelectron spectroscopy (UPS), or electron energy loss spectroscopy (EELS). M@ller et al. studied the Cu/MgO(lOO) interface formation by Auger spectroscopy and the charging effects [2] induced

B.V. All rights

reserved

by the electron irradiation of the Cu/MgO system. They found that the growth mode of coppet on magnesium oxide is of the Stranski-Krastanov type [31.The Cu/MgO( 100) system was also studied by XPS [4] to follow the evolution of the valence state of copper and showed that at very low coverage the Auger signature of copper in this system is similar to the Auger parameter of copper in Cu,O (1840.5 eV1. An EELS study of the Cu/ZnO( 1010) system was also performed by He and Moller [5] and they found an electronic resonance of Cuf. Some photoemission studies were performed on the deposition of copper on oxidized metals like aluminium [6] and chromium [7] where no interactions were detected, and on manganese [8] oxide where an interacting phase was observed at low coverage. More recently Moller and Wu studied the formation of the interface between Cu and TiO, by AES, EELS, and UPS [9] and by LEED [lO]. They found a Stranski-Krastanov growth mode with very limited interaction between the Cu3d level and the substrate. In this study, WC evaporated copper on a MgO(100) sample using a Knudsen-type effusion ceil. We first performed an AES study of the growth mode of copper on a MgO(lO0) single crystal followed by high-resolution electron cnergy loss spectroscopy (HREELS) and resonant photoemission spectroscopy (RPES) studies to compare the valence state of Cu found by these two techniques with the valence state deduced by other techniques.

2. Experimental

set-up

The LEED and Auger experiments wet-c performed in an Ultrahigh-vacuum [l l] set-up consisting of a preparation chamber with a base pressure of 2 x lO_” mbar where the metallization of the sample was performed and an analyser chamber containing an Auger spectrometer and a four-grid LEED apparatus. The pressure in the latter chamber was 3 x lO_ “’ mbar. A cylindrical mirror Auger analyzer (Varian CMA) was used

with a 3 keV beam energy and an emission current of 0.1 PA. All the spectra were recorded in the integral mode.

The HREELS UHV system consists of a preparation chamber (base pressure in the low lo-” mbar range) and an analyser chamber (base pressure in the low lOme” mbar range). The HREEL spectrometer used in this study is of the double hemispherical design (SEDRA-ISA Riber). This study was performed in the specular geometry with a beam of 5 eV electrons and an incidence angle of 45”. Spectra with an energy resolution (FWHM) of 8 meV were obtained. In electron spectroscopies, when dealing with insulators, charging effects have to be compensated. This was done by irradiating the sample with a high-energy (I -2 keV) electron beam [ 121. Typical energies and currents used for the neutralization gun in this work were 1.7 kcV and 0.1 PA.

The resonant photoemission spectroscopy work was performed at the Flipper II bcamlinc on the DORIS storage ring at HASYLAB (Hamburg, Germany). The details of the spectrometer arc described clsewherc [13]. In order to limit the charging effects, WC had to close the exit slit 01 the monochromator (100 pm) and insert an Al foil (I000 A) in the beam to reduce its intensity. In addition. an electron tlood gun was used with an energy of 6 eV. The electron analyser is a double-pass cylindrical mirror analyser (CMA). A four-grid LEED was also available. The overall energy resolution is better than 0.25 eV but this figure is somewhat worsened by the charging effects (see below).

The samples used in this study are MgO 4N single crystals ingots cleaved in air along the ( 100) plane. Thcrcafter, samples were introduced into the UHV chamber and heated by electron bomhardmcnt (AES and HREELS) or radiativcly

T. Conrad et al. / Copper /oxide

33

interface formation

ENERGY (cm-‘)

(RPES) for one hour to a temperature of about 800°C. After this treatment clean and wellordered surfaces were obtained as indicated by the Auger spectrum and LEED pattern. Little carbon contamination was observed by Auger spectroscopy. The deposition of copper was performed using a Knudsen cell built with a pBN crucible radiatively heated by a tantalum wire. The thickness of the deposited layer was monitored using a quartz crystal oscillator.

3. Auger and HREELS results During the Auger electron spectroscopy (AES) study, the 0 KLL, Cu LMM and Mg KLL lines were recorded [14] as a function of the copper coverage. The evolution of the intensities (areas) of the 0 KLL and Cu LMM Auger lines is characteristic of the growth of one monolayer of copper followed by islands formation, known as the Stranski-Krastanov growth mode, confirming earlier results [2]. Fig. 1 shows HREELS data for increasing sopper coverage for selected coverages up to 20 A as determined by the quartz crystal oscillator. With increasing coverage, we observed a shift in the position of the phonon peak to the lower energy side (this can best be seen on the double-loss peak, that moves from 1306 to 1137 cm- ‘> with a decrease of the intensity ratio of the loss peak to the elastic peak. All the spectra were normalized to the height of the elastic peak to allow comparison of the intensities. All spectra in fig. 1 were recorded with the same measurement time. This shows that as the coverage of copper increases the counting rate decreases. This could be related to the charging effect observed in Auger spectroscopy. The fact that, for metal-covered samples, we cannot neutralize the charging of the surface with the same accuracy as for the clean sample indicates that charging is not homogeneous at the surface due to the distribution of trapping sites induced by the copper atoms. These results were interpreted from the dielectric theory developed by Lambin et al. [15]. Following this theory, we are able to simulate a

-200

-100

0

100

200

300

400

ENERGY (meV) Fig. 1. HREEL spectra of the formation of the Cu/Mg0(100) interface. Data recorded in the specular geometry (0 = 45”) with an incident electron energy of 5 eV.

HREEL spectrum for a multilayered system, provided that the dielectric functions of the components are known. This theory can only give a qualitative comparison for this study because it can only consider a homogeneous layer and not the growth of islands on a surface as for the second step of the growth of copper on MgO. For the dielectric function of MgO, we used published infrared optical constants determined by HREELS [16]. Magnesium oxide has only one surface optical phonon at an energy of 651 cm- ‘. No cuprous oxide HREELS data are available so we used infrared reflectivity data. These constants are shown in table 1 [17] together with the magnesium oxide optical constants. The optical damping constant of cuprous oxide was slightly adjusted. Indeed, it is generally observed that the

34

T. Conrad et al. / Copper /oxide intrrfuce formation

damping coefficients measured in HREEL spectra are higher than the infrared values [18]. This is the reason why for the simulation of the spectra we increased the infrared damping ratios by a factor of two. Two vibrational modes were found at 149 and 636 cm-’ in the HREELS spectrum calculated for a semi-infinite Cu,O crystal. The exhibited intensities of these two peaks were considerably weaker intensities than the corresponding magnesium oxide modes. Recent measurements were performed on CuO powder and single crystal by HREELS showing mainly one mode at 592 cm-’ [19]. However, due to the poor quality of the recorded spectra, the parameters of the dielectric function of cupric oxide could not be determined so that no HREELS theoretical simulations were available for the interface between MgO and CuO. As discussed before, the energy and the intensity ratio of the Ioss peak to the elastic peak decrease with increasing copper coverage. In fig. 2 we present theoretical and experimental data for a layer of cuprous oxide on magnesium oxide. The first-order theoretical spectrum was calculated for a 5 A Cu,O thickness. It is dominated by the interface phonon of the MgO slightly shifted to lower frequency with respect to the MgO surface mode. Due to the very small strengths of the C&O oscillators no observable contribution from interface modes can be seen and only two surface modes are visible at 149 and 636 cm-‘. However when convoluting the data in order to produce a theoretical spectrum that can be compared to the experimental one, the small contribution of the 149 cm-’ surface peak is smeared out by the convolution process and the

Table 1 Dielectric function parameters of MgO determined by HREELS and of CuaO determined by infrared spectroscopy (in this table w. AC, and Y are the resonant frequency, oscillator strength, and damping coefficient of the phonons: and E, is the high frequency dielectric constant) u (cm-‘)

dE

Y

MgO

393

6.85

0.089

CUzO

146.3 609

0.4 0.7

0.02 0.06

% -__l_ 2.95 6.5

Energy (cm-‘) -?SO

-250

0

250

750

9

.I’30

-50

0

50

10013

_-

105

Energy (meV)

Fig. 2. Theoretical spectra simulated for a 5 A Cu,O overlayer on a semi-infinite substrate of MgO. Spectrum a is the first-order Born approximation and spectrum b is spectrum a convoluted to be compared with experimental spectra. The inset in the figure shows the changes in the position of the double toss peak in experimental HREELS spectra during the growth of Cu on MgOflOO) and in simulated spectra of the growth of Cu,O on MgO.

636 cm-’ one merges in the MgO interface peak and slightly red-shifts its frequency. The inset of fig. 2 shows the energy shift of a measured loss peak as a function of coverage together with theoretical values derived from simulated spectra. The values shown in the inset were taken from the double loss peak (at 1306 cm-’ (161 meV) for clean MgO) because it has a larger energy shift and is therefore more precisely determined.

4. RPES results The photoemission data were collected for photon energies in the vicinity of the Cu3p ionization threshold. This takes advantage of the Fano resonance observed in transition metals, rare earths and actinides [201. In the case of copper, the first step is the promotion of a 3p electron to 3d level. This excited state decays through an Auger process to a final state that could also be reached through direct photoemission from the 3d shell. The observed resonance results from the interference between these two

T. Conrad et al. / Copper /oxide interface formation

-71

:

-I

1

30

20

10

Binding energy

0 (eV)

Fig. 3. Photoemission spectra of 0.25 A Cu on MgO(100) at 70 and 75.5 eV photon energy showing the resonance peak at about 12.5 eV binding energy.

channels leading to the same final state. The main interest of this process is that depending on the chemical binding the energies of the resonant features are sensitive to the copper chemical state. It is then possible to use this property to distinguish the different valence states of copper [21,22]. In addition, due to the large difference in the photoemission cross sections of Cu3d and 02p levels [23], (at hv = 80 eV, (~(02~) = 2.064 Mb and a(Cu3d) = 8.72 Mb) informatio? can be obtained for thicknesses as low as 0.25 A copper as shown in fig. 3. When we studied the formation of the interface between copper and magnesium oxide, we had to solve the charging problem due to the high intensity of the synchrotron radiation. We observed shifts of the photoemission peaks of about 70 eV at an incident photon energy of 75.5 eV, so we had to use a flood gun to neutralize the surface charge. Unfortunately, even using the flood gun and by reducing the intensity of the synchrotron radiation we were not able to neutralize totally the surface potential of the sample and shifts up to 10 eV were still observed during the growth of the interface. In addition, a second problem was caused by the fact that the intensity of the synchrotron radiation decays with time. The consequence is that the surface potential is

35

not constant with time and corresponding shifts of the photoemission peaks were observed. To compensate this effect and to obtain sufficient counting statistics, we were forced to take many individual scans that were afterwards computeradded together. This procedure resulted in some loss of resolution due to the addition of the spectra. Absolute values of the binding energy of the different levels cannot be determined because of the variable charging effect so the spectra were aligned (as for the other photoemission results presented in this paper) on the position of the Cu3d level determined by a Lorentzian-Gaussian peak curve fitting of the spectra. Following XPS valence band results, the binding energy of the Cu3d level was fixed at 2 eV. The spectra

iI

-I

!

I

I

30

1

20

10

Binding energy

0 (eV)

Fig. 4. RPES spectra at 75.5 eV photon energyofor coverage of 0.25, 0.50, 0.75, 1.0 and 1.50 A.

copper

very low coverage (below 1 A), the energy difference between the two peaks is close to that found in cuprous oxide and then decrease to reach the value found in metallic copper.

5. discussion

id-

.~

30

__..-

20

.!

10

Binding energy Fig. 5. RPES

spectra

at 755

eV

0 W/t

photon

ene!gy

for copper

coverage of 0. I, 2. .i. 4, 8. and 12 A.

obtained for two different sets of mcasur~ments are shown in figs. 4 and 5. The changes in the valence band spectra with increasing copper coverage are shown in fig. 6. At

CUaO

0

IS,

l

2nd

. . ;

2

s

cue

10 -

5

z zii r5

I=

8, 0

5

10

15

coverage

Fig. 6. Changes in the valence

band spectra as a function

the Cu thickness.

of

The coverage dependence of the HREELS peaks positions compared to theoretical values found by simulating the growth of Cu,O on MgO shows that copper interacts with magnesiunl oxide at low coverage and probably forms an oxide layer. Fig. 2 shows that copper in this oxidized form is in a valence state similar to copper in cuprous oxide (CuzO). From the inspection of these data the energy shift was attributed mainly to the overlapping of the Cu,O surface phonon modes and MgO interface phonon modes as seen on the simulated spectra of the first-order Born approximation. From the HREELS data the valence state Cu” as in cupric oxide (CuO) can be excluded because due to positions of the main mode seen in HREELS [19] the shape of the phonon mode should be much more modified and there should appear an obvious shoulder on the low-energy side of the phonon peak. The formation of a hom~}geneous metallic layer on the surface of MgO can also be totally excluded because such a layer would efficiently screen the electric field associated with the optical phonons of the substrate. The formation of a mixed oxide of the type Cu,Mg, _,O could also occur at the interface. According to Chang and Mitra [24] this would result in a single phonon mode behaviour as expected for ionic materials with a frequency shifting linearly from 651 cm -. ’ (MgO) to 5 10 on the copper concentracm -’ (CuO) depending tion. This linear shift is not observed in our experiments. The RPES data (fig. 6) show that the energy of the resonance peak is not at all that of cupric oxide [22] or other Cu’+ compounds like the c’u dihalides where energy differences of 5.5, X.5, and 9.0 eV were found for CuF2. CuCl, and

T. Conrad et al. / Copper/oxide

respectively [251. The energy difference CuBr,, between the Cu3d main peak and satellite we found is also much higher than for cuprate superconductors [26] (X.4 eV for YBa,Cu,O, and La,,,Sr,,,,CuO,). This excludes both the formation of a mixed oxide and of a cupric oxide layer. It is more difficult to decide from the photoemission measurements if Cu+ or metallic Cu is present at the surface due to the similarities in the electronic structure of Cu,O and Cu [21,221 and to the experimental limitations due to the charging effects as mentioned above. At low coverage, the energy difference of the resonance peak and the Cu3d peak is close to the one found in CU,~O or in CuCl [27], while at higher coverages (> 2 A) it comes closer to the value associated with metallic copper. This is consistent with the picture coming from the AES study which indicates the formation of islands at the surface. Therefore, the formation of the interface between copper and magnesium oxide begins with the formation of a reacted phase similar to Cu,O and is then followed by the growth of a metallic phase. Our interpretation agrees with the XPS results of Alstrup and Moller [4] who also conclude from examining the Auger parameter of copper that $opper is in a Cu’ state at coverage below a few A and changes progressively from cuprous oxide to metallic copper as the coverage increases. The same experiment performed by EELS [5] showed at low coverage a loss peak which could also be attributed to a Cut state. A reacted phase was also observed for low coverages when copper was evaporated on MnO [7]. Finally, we shall conclude by raising an unsolved problem related to the model presented here. It is clear that extra oxygen is needed to account for the copper oxidation. Unfortunately, because of charging effects, we were not able to study by RPES the reduction of the magnesium. Furthermore AES and XPS results do not show significant reduction of magnesium. At present, no definite explanation can be given, as the residual oxygen and water concentrations in the vacuum chamber are not sufficient to explain the oxygen needed for the reaction. Further dosing experiments arc clearly necessary to solve this problem.

inte
37

6. Conclusion We have examined both the vibrational (HREELS) and electronic structure (AES, RPES) aspects of copper/ magnesium oxide interface formation. The results obtained with AES indicate that the growth of copper on magnesium oxide follows the Stranski-Krastanov mode. The combined HREELS and RPES results show that copper grows on magnesium oxide at low coverages in a highly interacting phase which has to be an oxidized species similar to Cu,O. The formation of an initial metallic layer or of CuO is excluded by the experimental observations. F$lowing formation of the oxidized layer (- 2 A) metallic copper is found for larger coverages.

Acknowledgements One of us (J.G.) is a research associate of the NFSR (Belgium). This work has been supported by the Belgian Prime Minister’s Services-Science Policy Programming within the framework of the “Concerted Research Action on Interfacial Mathe “Interuniversity Attraction Pole in terials”, Interface Science”, and the “Impulse Program on High T, Superconductors”, and by the German Federal Minister for Science and Technology (BMFT) under grant no. 05 490 CAB

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and K.A. Miiller. Z. Phys. B 64 (1986) 189. [2] P.J. Moller and J.-W. He, Nucl. Instrum. Methods B I7 (1986) 137. [3] J.-W. He and P.J. MQller, Surf. Sci. 17X (1986) 934. [4] 1. Al&up and P.J. Mgller. Appl. Surf. Sci. 33/34 (19%) 143. [5] J.-W. He and P.J. Meller. Surf. Sci. 1X0 (19X7) 411. [h] V. Di Castro, G. Polzonetti and R. Zanoni, Surf. Sci. I62 (1985) 34X. [7] V. Di Castro, C. Furlani, G. Polzonetti and c‘. Cozza. J. Electron Spectrosc. Relat. Phenom. 46 (198X) 2Y7. [8] V. Di Castro and G. Polzonetti. Chem. Phys. Lett. I39 (1987) 21s. [9] M.-C. Wu and P.J. Mc?ller. Surf. Sci. 224 (lYX9) 250. [IO] P.J. Molter and M.-C. Wu, Surf. Sci. 224 (IYXY) 265. [II] R. Sporken, PhD thesis, Namur, 198X. unpublished.

3x

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[12] M. Liehr, P.A. Thiry. J.J. Pireaux and R. Caudano, Phys. Rev. B. 33 (1986) 56X2. [13] R.L. Johnson and J. Reichardt, Nucl. Instrum. Methods 208 (1983) 791. [14] T. Conard, J.M. Vohs, P.A. Thiry and R. Caudano. Surf. Interface Anal. 16 (1990) 446. [15] P. Lambin, J.P. Vigneron and A.A. Lucas, Phys. Rev. B 32 (1985) 8203. [16] P.A. Thiry, PhD thesis, Namur, 1984, unpublished. [17] P. Dawnson, M.M. Hargreave and G.R. Wilkinson, J. Phys. Chem. Solids 34 (1973) 2201. [18] M. Liehr, P.A. Thiry, J.J. Pireaux and R. Caudano, J. Vat. Sci. Technol. A 2 (1984) 1079. [19] Yu Li-Ming, A. Degiovanni, T. Conard. J.L. Dewandel, P.A. Thiry. J. Ghijsen and R. Caudano, Surf. Sci., in press. [20] L.C. Davis, J. Appl. Phys. 59 (1986) R25; U. Fano, Phys. Rev. 124 (1961) 1866.

[21] M.R. Thiiler, R.L. Benbow and Z. Hurych. Phya. Rev. B 26 (1982) 669. [22] J. Ghijsen, L.H. Tjeng, H. Eskes, G.A. Sawatzky and R.L. Johnson, Phys. Rev. B 42 (1990) 2268. [23] J.J. Yeh and I. Lindau, At. Data Nucl. Data Tables 32 (1985) 1. [24] I.F. Chang and S.S. Mitra, Adv. Phys. 20 (1071) 359. [25] G. van der Laan. PhD thesis, Groningen, 1982, unpublished; Solid State Commun. 42 (1982) 165. [26] Z.-X. Shen, J.W. Allen, J.J. Yeh, J.-S. Rang, W. Ellis, W. Spicer, 1. Lindau. M.B. Maple, Y.D. Dalichaouch, MS. Torikachvili, J.Z. Sun and T.H. Geballe, Phys. Rev. B 36 (1987) 8414. [27] A. Goldmann, J. Tejeda, N.J. Shevchik and M. Cardona. Phys. Rev. B 10 (1974) 4388.