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Cu(Al) alloy interfaces—Influence of the crystallinity of alumina films
Applied Surface Science 256 (2010) 3051–3057
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The interface structure and band alignment at alumina/Cu(Al) alloy interfaces—Influence of the crystallinity of alumina films Michiko Yoshitake a,*, Weijie Song a, Jirˇı´ Libra b, Karel Masˇek b, Frantisˇek Sˇutara b, Vladimı´r Matolı´n b, Kevin C. Prince c a b c
National Institute for Materials Science, 3-12, Sakura, Tsukuba, 305-0003, Japan Charles University, Faculty of Mathematics and Physics, Dept. of Surface and Plasma Science, V Holesˇovicˇka´ch 2, 180 00 Prague 8, Czech Republic Sincrotrone Trieste, Strada Statale 14, km 163.5, 34012 Basovizza-Trieste, Italy
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 July 2009 Received in revised form 11 November 2009 Accepted 12 November 2009 Available online 2 December 2009
Both epitaxial and amorphous ultra-thin alumina films were grown on a Cu-9 at.%Al(1 1 1) substrate by selective oxidation of Al in the alloy in ultra high vacuum. The crystallinity of the alumina films was controlled by oxidation temperature. The photoelectron spectra of Al 2p, O 1s and valence band were measured in-situ during oxidation. The influence of the crystallinity on the interface structure between the alumina films and the substrate was discussed by analyzing the Al 2p spectra composed of multiple peaks. The energy difference between the Fermi level of the substrate and the valence band maximum of the alumina films (band offset) was derived from the valence band spectra. The energy band alignment at the interface between each of the two alumina films and the substrate was revealed by combining the binding energy values of the core levels with the band offset values. The influence of the alumina crystallinity on the band alignment was discussed. ß 2009 Elsevier B.V. All rights reserved.
PACS: 73.40.Ns Metal–nonmetal contacts Keywords: Epitaxial alumina film Cu–Al alloy Photoelectron spectroscopy In-situ oxidation Band alignment Interface termination
1. Introduction Oxide/metal interfaces are of great importance in a wide range of applications, such as thermal-barrier coatings, nanocomposites, electric and optical devices, various catalysis, and electrodes in fuel cells or batteries. Among various oxide/metal interfaces, the alumina/metal interface is one of the systems that have been extensively researched, because of the many desirable properties of alumina such as excellent electrical insulation, high thermal conductivity, hardness, and so forth. Epitaxial alumina/metal interfaces have been studied as a model material because alumina films can be grown epitaxially on several metals [1]. Among them, alumina/copper interfaces have been investigated in detail by both first-principles calculations and experiment. We have studied the epitaxial growth of alumina films on a Cu– Al alloy single crystal [2–6] as a model system of an oxide/metal gate structure in next-generation metal oxide semiconductors
* Corresponding author. Tel.: +81 29 863 5496; fax: +81 29 863 5571. E-mail address: [email protected] (M. Yoshitake). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.11.072
(MOS). It was demonstrated that the interface was terminated by aluminum atoms when a small amount of Al was alloyed with Cu [6], whereas oxygen atoms terminated the interface between pure Cu and alumina [7–9]. The band offset (the energy difference between the Fermi level and the valence band maximum, the Schottky barrier height for holes) at the interface between the epitaxial alumina and the Al-terminated Cu–Al alloy was measured and compared with the results of first-principles calculations. It was concluded that a large band offset was successfully achieved in our system [6] as the calculations predicted [10]. Besides alumina, most theoretical calculations on high-dielectric films for future gate oxides, so called high-k films, use single crystals as their models. Hence, experiments on epitaxial films are useful to make comparisons with these theoretical calculations. However, high-k films expected to be in use in the near future are amorphous [11] as the current SiO2 films are. Amorphous oxide films are mainly used to measure electric properties of high-k film/ metal gate structures, and these properties are determined by the band alignment at the interface. Therefore, it is important to study how the crystallinity of an insulating film influences the band alignment at an interface
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between an insulating film and a metal. In the present research, the interface structure and band offset at the interface between an amorphous film and a Cu-Al alloy single crystal, and an epitaxial alumina film and a Cu-Al alloy are investigated and compared. 2. Experimental The experiments were carried out at the Materials Science beam line, ELETTRA synchrotron light source, Trieste. The PHOIBOS 150 MCD-9 spectrometer (SPECS) of the beamline was used as an electron energy analyzer. The angle between the light and analyzer was 608. A photon energy of 106 eV, which corresponds to the Cooper minimum of the ionization cross-section for Cu 3p [12], was mainly used because the Cu 3p peak overlaps with Al 2p, which is our main interest in this study. The exact photon energy was checked using the Fermi edge of a Au reference sample. The XPS peak intensities were normalized to the incident photon flux. A Cu-9 at.%Al(1 1 1) single crystal was mechanically polished and introduced into the chamber. The temperature of the specimen was monitored by a thermocouple, which was attached to the back of the crystal. The specimen was first annealed at 770 K for 1 h, sputtered with 1 keV Ar ions for 5 min at room temperature and annealed at 770 K for 15 min. The cycle of 5 min Ar sputtering at room temperature and annealing at 770 K for 15 min was repeated several times until no trace of impurities was detected. With this treatment, the clean surface has a (H3 H3) R308 reconstruction, where one-third of the top surface is occupied by Al atoms [13]. The specimen was oxidized step-wise either at 670 K or at 910 K under a partial oxygen pressure of 6.6 106 Pa. It has already been confirmed from our previous studies with XPS [3] and AES [4] that only Al atoms in the alloy are oxidized and Cu atoms remain un-oxidized. When the substrate was oxidized at 670 K, the LEED pattern showed no spots but only a halo pattern, which we assign to amorphous alumina. Oxidation at 910 K resulted in an epitaxial alumina formation [4]. For spectral analysis, the free software ‘XPS peak’ [14] written by Dr. Raymond Kwok of The Chinese University of Hong Kong was used.
3. Results and discussion 3.1. Al 2p spectra: attribution and interface termination The Al 2p photoelectron spectra obtained with 106 eV light during oxidation at 670 K and 910 K are shown in Fig. 1. The binding energy for Cu 3p is about 75 eV but its intensity is negligible on the clean surface in Fig. 1, due to the small ionization cross section of the level at this photon energy. On the clean surface, a clear Al 2p3/2 and 2p1/2 doublet (peak a) is seen at binding energies of 72.40 eV and 72.80 eV, and there is a shoulder on both lines. As the oxidation progressed, the Al 2p3/2 and Al 2p1/2 doublet at 72.40 eV and 72.80 eV decreased while the shoulder became noticeable, and at last the shoulder turned into a weak doublet (peak b) at energies of 72.58 eV and 72.98 eV after 1024 L oxygen exposure. Upon oxidation, peaks at around 73.6 eV (peak c) and 75.0 eV (peak d) appeared. All the spectra were deconvoluted into four doublets (peaks a–d), due to Al 2p3/2 and 2p1/2, although the doublets are not observed separately in peaks c and d due to the large width of the lines. All the peaks in Fig. 1(a) have already been assigned [6]: peak a to a top surface component of the substrate, peak b to the substrate below the top surface, peak c originates at the interface between the substrate and outer layer (peak d), which was attributed to Al in Al2O3. The attribution of peaks a and b in Fig. 1(b) is the same as in Fig. 1(a) because both peaks originate from the substrate, which is the same for both experiments. To examine the attribution of peaks c and d, the spectra measured with 106 eV and 194 eV photons are compared for the epitaxial and amorphous alumina on the substrate in Fig. 2. The interface component (peak c) located deeper than the peak d component is more noticeable with 194 eV photons than with 106 eV ones in the epitaxial film (Fig. 2 (a)). Hence, a deeper layer is observed with 194 eV photons. For the amorphous film, peak c is more noticeable with 194 eV photons as seen in Fig. 2 (b). Therefore, it is also considered that peak c originates from the interface component in the amorphous film. In Table 1, the Al 2p binding energy of the outermost layer (peak d) is compared with reference values from various types of alumina
Fig. 1. XPS spectra of Al 2p region as a function of oxygen dosage during oxidation at (a) 910 K and (b) 670 K.
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Fig. 2. Al 2p spectra excited with 106 eV and 194 eV photons after 1024 L oxygen exposure for (a) epitaxial (910 K) and (b) amorphous (670 K) growth.
estimated as 1.62 + 7.11 = 8.73 J/m2, is larger than 4.32 J/m2 of Cu–O–Al (O-termination). Although the bonding of terminated atoms with Cu is stronger in Cu–O than that in Cu–Al, a very strong Al–O bonding makes a Cu–Al–O interface stable. Because of this, the interface termination is not influenced by the crystallinity of alumina.
[15–22], which vary from 73.7 eV to 75.5 eV. No difference in binding energy among bulk alpha (sapphire), gamma and amorphous alumina has been reported. The binding energies of peak d in our epitaxial and amorphous alumina films are within the reference values. The energy difference, O 1s–Al 2p, is also listed in Table 1, and varies from 456.6 eV to 457.2 eV. This variation is smaller than that of Al 2p, because the binding energy difference is less influenced by charging or an error in energy-scale calibration. Hence, the binding energy difference is a good quantity that measures the chemical state of compounds. The energy difference of both our films stays within the reference values. Our epitaxial alumina film has already been proved to be an oxide with Al in the 3+ valence state, so the valence of Al in our amorphous alumina is also considered to be 3+. We concluded that the attribution of all the peaks appearing during the amorphous film growth is the same as that during epitaxial growth. From the presence of the interface component, the interface formed by the amorphous film growth
0 Al2p Al2 O3
d2 ¼ l
sin u ln@
3.2. Oxide layer growth The thickness of the interface layer and the alumina film was estimated from the peak intensity obtained with 106 eV photons using the method described in reference [25]. With this method, the thickness of the interface layer (d1) is estimated from Eq. (1) and that of the alumina film (d2) from Eq. (2). Al2p
d1 ¼ lAlO sinu ln
!
Al2p Al2p ðIAlO =ICuAl Þ Al2p
Al2p
Al Al ðNAlO lAlO Þ=ðNCuAl lCuAl Þ
Al2p
Al2p Al2p Al Al Al ðIAlO =ICuAl Þ=ððNAlO lAlO Þ=ðNCuAl lCuAl ÞÞ þ 1 ððNAl
2 O3
was revealed to be terminated with Al, as for the epitaxial growth. This means that the interface termination is not influenced by the crystallinity of the alumina film. The relative thermodynamic energy for formation of the Al-terminated interface is estimated by comparing Cu–Al–O–Al bonding energy (Al-termination) with Cu– O–Al (O-termination). The theoretically calculated work of separation [23] of an alumina (0 0 0 1)/Al-terminated Cu interface has been reported to be 1.62 J/m2, while that of an O-terminated one to be 4.32 J/m2. The work of separation of bulk alumina estimated from the enthalpy of formation [24] is 7.11 J/m2. Therefore, the Cu–Al–O–Al bonding energy (Al-termination)
(1)
1
Al2 p Al2p IAl =ICuAl 2 O3 Al2p
þ1
Al2p
Al2p
Al lAl2 O3 Þ=ðNCuAl lCuAl ÞÞ
þ 1A
(2)
Al2p Al2p Al2p ; IAlO ; and ICuAl are Al 2p intensity from alumina where IAl 2 O3 (peak d), the interface (peak c) and Cu–Al alloy (peak b). The following values of the inelastic mean free path of the Al 2p photoelectron are used (kinetic energy of ca. 30 eV): alumina, Al2p lAl2p Al2 O3 1.24 nm; the interface layer, lAlO ; 1.24 nm; Cu–Al alloy, Al Al lAl2p ; 0.60 nm [26]. N ; N and NAl CuAl AlO Al2 O3 are atomic densities CuAl of Al in the Cu-Al alloy, in the interface layer and in the alumina. For the atomic density of Al in the interface layer, the average value between pure Al (6.02 1022/cm3) and alumina (4.69 1022/ Al Al cm3) was used. The values used for NAl CuAl ; NAlO and NAl2 O3 are 7.50 1021/cm3, 5.36 1022/cm3 and 4.69 1022/cm3. u is the take-off angle of photoelectrons and is 908 in this case. The
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Table 1 Binding energy of Al 2p and O 1s in various types of alumina (eV). Al 2p
O 1s
O 1s–Al 2p
Types of alumina
Refrence
74.7 74.4 74.2 73.7 74.0 74.3 75.5 74.1 75.0 75.2
531.6 531.0 531.0 530.9 530.9 531.3 532.1 531.3 531.7 532.0
456.9 456.6 456.8 457.2 456.9 457.0 456.6 457.2 456.7 456.8
Al2O3 Sapphire Sapphire Gamma Gamma Al2O3 Oxidized TiAl Al2O3 Our epitaxial Our amorphous
[15] [16] [17,18] [18] [19] [20] [21] [22] This study This study
thickness of the interface layer, the alumina film and the total film (interface + alumina) was calculated (Fig. 3). In the case of epitaxial alumina growth (Fig. 3(a)), the estimated thickness of the interface layer reached around 0.25 nm at 128 L and stayed almost constant for further oxygen exposure. This thickness, 0.25 nm, corresponds to the sum of one aluminum layer and one oxygen layer as discussed in the previous work [6]. The thickness of the alumina layer at 1024 L oxygen dose is approximately 1.3 nm, resulting in the total film thickness (peaks c and d) of 1.6 nm. In the case of the amorphous alumina growth, the thickness of the interface layer grew to 0.5 nm, which is almost twice as thick as the one in the epitaxial growth. The total film thickness at 1024 L is 1.6 nm, which is almost the same as in the epitaxial growth case. Concerning the binding energies of peak c and d, those in the amorphous alumina are more than 0.1 eV higher than in the
Fig. 4. Al 2p3/2 binding energy change of four peaks during oxidation for (a) epitaxial (910 K) and (b) amorphous (670 K) growth.
Fig. 3. The change of interface and oxide layer thickness during oxidation for (a) epitaxial (910 K) and (b) amorphous (670 K) growth.
epitaxial alumina as seen in Fig. 1. The results of the precise binding energy analysis of all the peaks are plotted in Fig. 4 as a function of oxygen dosage. As expected, the binding energy of peaks a and b was not influenced by the oxygen dosage and was the same for both types of alumina. The binding energy of peaks c and d slightly changed at the beginning of oxygen dosage approximately up to 128 L or 256 L in both types of alumina growth, at which dosage the total film thickness reached more than 0.5 nm. Although further oxygen dosage increased the total film thickness, no change in the binding energy was detected within the experimental error. It should be noted that the binding energy values of peaks c and d in the amorphous layer are larger than those in the epitaxial layer at high exposure, when the values are constant. Differences in the full width at half maximum (FWHM) of the Al 2p peaks were observed between the epitaxial and the amorphous alumina as plotted in Fig. 5(a) and (b). Both the FWHM value of peak c and its dependence on oxygen dosage are different between the epitaxial and the amorphous films. The FWHM was almost constant except for a larger value at the first oxidation step in the case of epitaxial growth. For the amorphous growth, the FWHM gradually decreased from the first value until approximately 500 L, before reaching a constant value. This difference is considered to be associated with the result that the thickness of the interface layer became constant in the early stages of the epitaxial growth, while it continued to increase until ca. 500 L for the amorphous case. The
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Fig. 5. The change of Al 2p3/2 full width at half maximum (FWHM) of four peaks during oxidation for (a) epitaxial (910 K) and (b) amorphous (670 K) growth.
FWHM became constant at high dosage for both growth modes, but its value for the amorphous film was larger than that of the epitaxial one. Concerning peaks a and b, which arise from the substrate, the FWHM is constant for all oxygen dosages and that of the amorphous film is almost the same as the epitaxial one. The FWHM of peak d does not differ much between the two types of alumina growth, but there may be a slight difference in the oxygen dosage dependence. The FWHM of the epitaxial film appears to increase slightly until 1024 L, while that of the amorphous one is almost constant for all the oxygen dosage except at the very beginning. 3.3. Valence band and O 1s spectra Spectra of the valence band acquired with 106 eV photons, and measured at normal emission before (0 L) and after 1024 L oxygen exposure at 670 K (amorphous) and 910 K (epitaxial), are shown in Fig. 6(a). In the figure, the spectrum at 0 L exposure is scaled by a
factor one-fifth, meaning that the peaks between 2 eV and 5 eV, mainly due to bands of Cu 3d character [27], are very intense. Peaks around 6 eV and 8 eV appeared after oxidation for both cases, which are attributed to O 2p. [28] On oxidation only an intensity decrease of Cu 3d and an increase of O 2p were observed, without any new feature appearing nor any apparent peak shifts. These
Table 2 Binding energy and work function change at 1024 L oxygen (eV).
Al 2p3/2 (int) (eV) Al 2p3/2(oxide) (eV) Work function change (eV) O 2p (eV) Valence band maximum (eV) O 1s (eV) Total film thickness (nm)
Epitaxial
Amorphous
73.63 74.9 0.52 6.45 4.4 531.7 1.6
73.75 75.07 0.51 6.65 4.6 532 1.6
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results suggest that there is no strong hybridization of Cu 3d and O 2p orbitals in both cases. The shape of the two spectra after oxidation is very similar but the position of O 2p (valence band of alumina) is approximately 0.2 eV higher in the amorphous case, while the position of Cu 3d is the same. The valence band maximum (VBM) of the epitaxial film is located 4.4 eV (0.2 eV) below the Fermi level, determined by subtracting the Cu 3d components from the spectrum [6]. Similarly, the VBM of the amorphous film is estimated to be located approximately 4.6 eV below the Fermi level. Although the absolute value has 0.2 eV uncertainty, it is quite certain that the VBM of the amorphous film is located deeper because of the fact that the valley between Cu 3d and O 2p in the spectra is deeper in the amorphous case. The O 1s spectra measured with 900 eV photons after 1024 L oxygen dosage at the two different temperatures are shown in Fig. 6(b). Due to the low ionization cross-section, the spectra are rather noisy, but the O 1s binding energy of the amorphous film is approximately 0.2 eV higher than that of the epitaxial one. 3.4. Band alignment
Fig. 6. Valence band (a) and O 1s (b) spectra of clean substrate (0 L), epitaxial and amorphous alumina films on the substrate.
The valence of Al in the alumina films is the same for the two types of alumina, but the Al 2p3/2 binding energies are different. The binding energy of O 2p and O 1s in the two films at the same total film thickness is also different as shown in Table 2. On the other hand, the binding energy difference between Al 2p (oxide) and O 2p for the amorphous film (68.42 eV) is almost the same as that for the epitaxial film (68.45 eV). The differences between O 2p and O 1s in the two films are also very similar. Therefore, it is concluded that the whole inner shell and the valence band levels are shifted parallel downwards by approximately 0.2 eV in the case of the amorphous alumina films, meaning that there is 0.2 eV difference in the band alignment. Furthermore, the Al 2p binding energy difference between interface and the oxide for the amorphous film, 1.32 eV, is rather similar to 1.27 eV for the epitaxial film. However, the Al 2p difference between the interface and the bulk alloy for the amorphous film, 1.17 eV, is significantly larger than for the epitaxial film, 1.05 eV. Therefore, the difference of the band alignment mainly occurs at the interface layer. This corresponds to
Fig. 7. The schematic energy band diagrams for (a) epitaxial and (b) amorphous alumina films on Cu-9Al substrate.
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the experimental results that the FWHM of peak c in the amorphous film was larger than that of the epitaxial film. The schematic band diagrams for the epitaxial and the amorphous alumina films on the substrate derived from the experiments are summarized in Fig. 7. The conduction band (CB) level is not shown in the figure, because band gap measurement by observing a loss feature of O 1s spectra [29] was not possible in our experiments. If we assume the band gap is the same for the two types of alumina films, the energy difference between the Fermi level and the CB minimum is smaller for the amorphous film by 0.2 eV. Because larger band gap values tend to be reported for crystalline films among various values for both crystalline and amorphous alumina [30–33], the energy difference for the amorphous film might be smaller by more than 0.2 eV than that for the epitaxial one. The band alignment difference of 0.2 eV is critical to the control of MOS operations. The difference of interface termination, which was calculated to result in the band alignment difference of 1.5 eV [10], is not the origin in the present case, because both films have Al-terminated interfaces as discussed in the previous section. In spite of the same interface termination, the difference in structure of the insulating film, whether it was crystalline or amorphous, made the band offset different. Therefore, in order to control the operation voltage of MOS devices, not only a gate insulating material, a gate metal and interface terminating atoms but also the crystallinity of the insulating film should be optimized. 4. Conclusion A Cu-9 at.%Al(1 1 1) alloy single crystal was oxidized in ultra high vacuum under carefully controlled conditions to grow selectively either an ultra-thin epitaxial alumina film at 910 K or an amorphous one at 670 K. Photoelectron spectra were measured in-situ during oxidation using synchrotron light. Precise analysis of Al 2p spectra was made possible by avoiding the overlap of the Cu 3p peak with the Al 2p peak using the Cooper minimum of Cu 3p. Four chemical states of Al were clearly separated: the top Al layer of the clean alloy, bulk Al in the alloy, the interface Al between the alloy and the alumina film, and Al in the alumina. It was concluded that the interface was Al terminated from the observation of interface Al. It was revealed that the Al 2p and O 1s peaks and the valence band spectra in the amorphous alumina film were all shifted to higher energy by approximately 0.2 eV compared to those in the epitaxial one. Comparison of the valence band spectra of the two alumina films showed that the valence band maximum of the amorphous alumina was located 4.6 eV below the Fermi level of the alloy. There were considerable differences in the interface layer between the two alumina films. The Al 2p binding energy difference between the interface component and the substrate is larger in the amorphous film. The interface layer in the amorphous film was twice as thick as that in the epitaxial one. The two major conclusions demonstrated here by electron spectroscopy are as follows: (1) the interface termination, which is governed by the thermodynamic stability of the interface chemical bonding, was not influenced by the crystallinity of the alumina films;
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(2) the band alignment, hence the Schottky barrier height, was affected by the oxide crystallinity even when the insulating material, the electrode metal and the interface termination were all the same. Acknowledgments One of the authors (M.Y.) greatly appreciates partial support by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (nos. 17560027, 20560027), from the Murata Science Foundation (A71162), and from The Mitsubishi Foundation. This work is a part of the research programs LC06058 that are financed by the Ministry of Education of the Czech Republic. The Materials Science Beamline is partially supported by the grant no. MSM 0021620834 that is financed by the Ministry of Education of the Czech Republic. References [1] R. Franchy, Surf. Sci. Rep. 38 (2000) 195–294. [2] M. Yoshitake, S. Bara, Y. Yamauchi, W. Song, J. Vac. Sci. Technol. A21 (2003) 1290– 1293. [3] W. Song, M. Yoshitake, S. Bera, Y. Yamauchi, Jpn. J. Appl. Phys. 42 (2003) 4716– 4720. [4] M. Yoshitake, S. Bera, Y. Yamauchi, W. Song, Surf. Interface Anal. 35 (2003) 824–828. [5] Y. Yamauchi, M. Yoshitake, W. Song, Appl. Surf. Sci. 237 (2004) 363–368. [6] M. Yoshitake, W. Song, J. Libra, K. Masˇek, F. Sˇutara, V. Matolı´n, K.C. Prince, J. Appl. Phys. 103 (2008), 033707-1-5. [7] T. Sasaki, K. Matsunaga, H. Ohta, J. Soc. Mater. Sci. Jpn. 52 (2003) 555–559. [8] T. Sasaki, K. Matsunaga, H. Ohta, Sci. Technol. Adv. Mater. 4 (2003) 575–584. [9] T. Sasaki, T. Mizoguchi, K. Matsunaga, Appl. Surf. Sci. 241 (2005) 87–90. [10] S. Shi, S. Tanaka, M. Kohyama, Mater. Trans. 47 (2006) 2696–2700. [11] H. Garcıa, S. Duenas, H. Castan, L. Bailon, K. Kukli, J. Aarik, M. Ritala, M. Leskela, J. Non-Cryst. Solids 354 (2008) 393–398. [12] X-ray DATA Booklet, Lawrence Berkeley National Laboratory, University of California, Berkerly, 2001, chapter 1.5. [13] J. Ferrante, Acta Metall. 19 (1971) 743–748. [14] http://www.phy.cuhk.edu.hk/surface/XPSPEAK/index.html. [15] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of XRay Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1979. [16] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, in: J. Chastain, R.C. King, Jr. (Eds.), Handbook of X-Ray Photoelectron Spectroscopy, Physical Electronics, Inc, 1995. [17] J.A. Taylor, J. Vac. Sci. Technol. 20 (1982) 751–755. [18] C.D. Wagner, D.E. Passoja, H.F. Hillery, T.G. Kinisky, H.A. Six, W.T. Jansen, J.A. Taylor, J. Vac. Sci. Technol. 21 (1982) 933–944. [19] T.L. Baar, Appl. Surf. Sci. 15 (1983) 1–35. [20] V.I. Nefedov, J. Electron Spectrosc. Relat. Phenom. 25 (1982) 29–47. [21] V. Maurice, G. Despert, S. Zanna, P. Josso, M.-P. Bacos, P. Marcus, Surf. Sci. 596 (2005) 61–73. [22] D. Leinen, A. Fernandez, J.P. Espinos, J.P. Holgado, A.R. Gonzales-Elipe, Appl. Surf. Sci. 68 (1993) 453–459. [23] R. Yang, S. Tanaka, M. Kohyama, Phil. Mag. Lett. 84 (2004) 425–434. [24] Formation enthalpy of Al2O3, 1675.7 kJ/mol, D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 74th ed., CRC Press, Inc, 1994. [25] T.J. Sarapatka, Thin Solid Films 226 (1993) 219–223. [26] The value of the inelastic mean free path for such low energy electron has not Al2p been well established. The values of lAl2p AlO and of lCuAl are approximated by Al2p lAl2p Al2 O3 and by lCu respectively, which were provided by private communication from Sigeo Tanuma. [27] H. Asonen, N. Lindroos, M. Pessa, R. Prasad, R.S. Rao, A. Bansil, Phys. Rev. B 25 (1982) 7075–7085. [28] H.P. Pinto, R.M. Nieminen, S.D. Elliott, Phys. Rev. B 70 (2004), 125402-1-11. [29] S. Miyazaki, J. Vac. Sci. Technol. B19 (2001) 2212–2216. [30] F.S. Ohuchi, R.E. French, J. Vac. Sci. Technol. A6 (1987) 1695–1696. [31] V. Rose, V. Podgursky, I. Costina, R. Franchy, Surf. Sci. 541 (2003) 128–136. [32] S. Andersson, P.A. Bruhwiler, A. Sandell, M. Frank, J. Libuda, A. Giertz, B. Brena, A.J. Mexwell, M. Baumer, H.-J. Freund, N. Martensson, Surf. Sci. 441 (1999) L964–L970. [33] M. Gautier, G. Renaud, L. Pham Van, B. Villette, M. Pollak, N. Thromat, F. Jollet, J.-P. Duraud, J. Am. Ceram. Soc. 77 (1994) 323–334.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The interface structure and band alignment at alumina/Cu(Al) alloy interfaces—Influence of the crystallinity of alumina films Michiko Yoshitake a,*, Weijie Song a, Jirˇı´ Libra b, Karel Masˇek b, Frantisˇek Sˇutara b, Vladimı´r Matolı´n b, Kevin C. Prince c a b c
National Institute for Materials Science, 3-12, Sakura, Tsukuba, 305-0003, Japan Charles University, Faculty of Mathematics and Physics, Dept. of Surface and Plasma Science, V Holesˇovicˇka´ch 2, 180 00 Prague 8, Czech Republic Sincrotrone Trieste, Strada Statale 14, km 163.5, 34012 Basovizza-Trieste, Italy
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 July 2009 Received in revised form 11 November 2009 Accepted 12 November 2009 Available online 2 December 2009
Both epitaxial and amorphous ultra-thin alumina films were grown on a Cu-9 at.%Al(1 1 1) substrate by selective oxidation of Al in the alloy in ultra high vacuum. The crystallinity of the alumina films was controlled by oxidation temperature. The photoelectron spectra of Al 2p, O 1s and valence band were measured in-situ during oxidation. The influence of the crystallinity on the interface structure between the alumina films and the substrate was discussed by analyzing the Al 2p spectra composed of multiple peaks. The energy difference between the Fermi level of the substrate and the valence band maximum of the alumina films (band offset) was derived from the valence band spectra. The energy band alignment at the interface between each of the two alumina films and the substrate was revealed by combining the binding energy values of the core levels with the band offset values. The influence of the alumina crystallinity on the band alignment was discussed. ß 2009 Elsevier B.V. All rights reserved.
PACS: 73.40.Ns Metal–nonmetal contacts Keywords: Epitaxial alumina film Cu–Al alloy Photoelectron spectroscopy In-situ oxidation Band alignment Interface termination
1. Introduction Oxide/metal interfaces are of great importance in a wide range of applications, such as thermal-barrier coatings, nanocomposites, electric and optical devices, various catalysis, and electrodes in fuel cells or batteries. Among various oxide/metal interfaces, the alumina/metal interface is one of the systems that have been extensively researched, because of the many desirable properties of alumina such as excellent electrical insulation, high thermal conductivity, hardness, and so forth. Epitaxial alumina/metal interfaces have been studied as a model material because alumina films can be grown epitaxially on several metals [1]. Among them, alumina/copper interfaces have been investigated in detail by both first-principles calculations and experiment. We have studied the epitaxial growth of alumina films on a Cu– Al alloy single crystal [2–6] as a model system of an oxide/metal gate structure in next-generation metal oxide semiconductors
* Corresponding author. Tel.: +81 29 863 5496; fax: +81 29 863 5571. E-mail address: [email protected] (M. Yoshitake). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.11.072
(MOS). It was demonstrated that the interface was terminated by aluminum atoms when a small amount of Al was alloyed with Cu [6], whereas oxygen atoms terminated the interface between pure Cu and alumina [7–9]. The band offset (the energy difference between the Fermi level and the valence band maximum, the Schottky barrier height for holes) at the interface between the epitaxial alumina and the Al-terminated Cu–Al alloy was measured and compared with the results of first-principles calculations. It was concluded that a large band offset was successfully achieved in our system [6] as the calculations predicted [10]. Besides alumina, most theoretical calculations on high-dielectric films for future gate oxides, so called high-k films, use single crystals as their models. Hence, experiments on epitaxial films are useful to make comparisons with these theoretical calculations. However, high-k films expected to be in use in the near future are amorphous [11] as the current SiO2 films are. Amorphous oxide films are mainly used to measure electric properties of high-k film/ metal gate structures, and these properties are determined by the band alignment at the interface. Therefore, it is important to study how the crystallinity of an insulating film influences the band alignment at an interface
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between an insulating film and a metal. In the present research, the interface structure and band offset at the interface between an amorphous film and a Cu-Al alloy single crystal, and an epitaxial alumina film and a Cu-Al alloy are investigated and compared. 2. Experimental The experiments were carried out at the Materials Science beam line, ELETTRA synchrotron light source, Trieste. The PHOIBOS 150 MCD-9 spectrometer (SPECS) of the beamline was used as an electron energy analyzer. The angle between the light and analyzer was 608. A photon energy of 106 eV, which corresponds to the Cooper minimum of the ionization cross-section for Cu 3p [12], was mainly used because the Cu 3p peak overlaps with Al 2p, which is our main interest in this study. The exact photon energy was checked using the Fermi edge of a Au reference sample. The XPS peak intensities were normalized to the incident photon flux. A Cu-9 at.%Al(1 1 1) single crystal was mechanically polished and introduced into the chamber. The temperature of the specimen was monitored by a thermocouple, which was attached to the back of the crystal. The specimen was first annealed at 770 K for 1 h, sputtered with 1 keV Ar ions for 5 min at room temperature and annealed at 770 K for 15 min. The cycle of 5 min Ar sputtering at room temperature and annealing at 770 K for 15 min was repeated several times until no trace of impurities was detected. With this treatment, the clean surface has a (H3 H3) R308 reconstruction, where one-third of the top surface is occupied by Al atoms [13]. The specimen was oxidized step-wise either at 670 K or at 910 K under a partial oxygen pressure of 6.6 106 Pa. It has already been confirmed from our previous studies with XPS [3] and AES [4] that only Al atoms in the alloy are oxidized and Cu atoms remain un-oxidized. When the substrate was oxidized at 670 K, the LEED pattern showed no spots but only a halo pattern, which we assign to amorphous alumina. Oxidation at 910 K resulted in an epitaxial alumina formation [4]. For spectral analysis, the free software ‘XPS peak’ [14] written by Dr. Raymond Kwok of The Chinese University of Hong Kong was used.
3. Results and discussion 3.1. Al 2p spectra: attribution and interface termination The Al 2p photoelectron spectra obtained with 106 eV light during oxidation at 670 K and 910 K are shown in Fig. 1. The binding energy for Cu 3p is about 75 eV but its intensity is negligible on the clean surface in Fig. 1, due to the small ionization cross section of the level at this photon energy. On the clean surface, a clear Al 2p3/2 and 2p1/2 doublet (peak a) is seen at binding energies of 72.40 eV and 72.80 eV, and there is a shoulder on both lines. As the oxidation progressed, the Al 2p3/2 and Al 2p1/2 doublet at 72.40 eV and 72.80 eV decreased while the shoulder became noticeable, and at last the shoulder turned into a weak doublet (peak b) at energies of 72.58 eV and 72.98 eV after 1024 L oxygen exposure. Upon oxidation, peaks at around 73.6 eV (peak c) and 75.0 eV (peak d) appeared. All the spectra were deconvoluted into four doublets (peaks a–d), due to Al 2p3/2 and 2p1/2, although the doublets are not observed separately in peaks c and d due to the large width of the lines. All the peaks in Fig. 1(a) have already been assigned [6]: peak a to a top surface component of the substrate, peak b to the substrate below the top surface, peak c originates at the interface between the substrate and outer layer (peak d), which was attributed to Al in Al2O3. The attribution of peaks a and b in Fig. 1(b) is the same as in Fig. 1(a) because both peaks originate from the substrate, which is the same for both experiments. To examine the attribution of peaks c and d, the spectra measured with 106 eV and 194 eV photons are compared for the epitaxial and amorphous alumina on the substrate in Fig. 2. The interface component (peak c) located deeper than the peak d component is more noticeable with 194 eV photons than with 106 eV ones in the epitaxial film (Fig. 2 (a)). Hence, a deeper layer is observed with 194 eV photons. For the amorphous film, peak c is more noticeable with 194 eV photons as seen in Fig. 2 (b). Therefore, it is also considered that peak c originates from the interface component in the amorphous film. In Table 1, the Al 2p binding energy of the outermost layer (peak d) is compared with reference values from various types of alumina
Fig. 1. XPS spectra of Al 2p region as a function of oxygen dosage during oxidation at (a) 910 K and (b) 670 K.
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Fig. 2. Al 2p spectra excited with 106 eV and 194 eV photons after 1024 L oxygen exposure for (a) epitaxial (910 K) and (b) amorphous (670 K) growth.
estimated as 1.62 + 7.11 = 8.73 J/m2, is larger than 4.32 J/m2 of Cu–O–Al (O-termination). Although the bonding of terminated atoms with Cu is stronger in Cu–O than that in Cu–Al, a very strong Al–O bonding makes a Cu–Al–O interface stable. Because of this, the interface termination is not influenced by the crystallinity of alumina.
[15–22], which vary from 73.7 eV to 75.5 eV. No difference in binding energy among bulk alpha (sapphire), gamma and amorphous alumina has been reported. The binding energies of peak d in our epitaxial and amorphous alumina films are within the reference values. The energy difference, O 1s–Al 2p, is also listed in Table 1, and varies from 456.6 eV to 457.2 eV. This variation is smaller than that of Al 2p, because the binding energy difference is less influenced by charging or an error in energy-scale calibration. Hence, the binding energy difference is a good quantity that measures the chemical state of compounds. The energy difference of both our films stays within the reference values. Our epitaxial alumina film has already been proved to be an oxide with Al in the 3+ valence state, so the valence of Al in our amorphous alumina is also considered to be 3+. We concluded that the attribution of all the peaks appearing during the amorphous film growth is the same as that during epitaxial growth. From the presence of the interface component, the interface formed by the amorphous film growth
0 Al2p Al2 O3
d2 ¼ l
sin u ln@
3.2. Oxide layer growth The thickness of the interface layer and the alumina film was estimated from the peak intensity obtained with 106 eV photons using the method described in reference [25]. With this method, the thickness of the interface layer (d1) is estimated from Eq. (1) and that of the alumina film (d2) from Eq. (2). Al2p
d1 ¼ lAlO sinu ln
!
Al2p Al2p ðIAlO =ICuAl Þ Al2p
Al2p
Al Al ðNAlO lAlO Þ=ðNCuAl lCuAl Þ
Al2p
Al2p Al2p Al Al Al ðIAlO =ICuAl Þ=ððNAlO lAlO Þ=ðNCuAl lCuAl ÞÞ þ 1 ððNAl
2 O3
was revealed to be terminated with Al, as for the epitaxial growth. This means that the interface termination is not influenced by the crystallinity of the alumina film. The relative thermodynamic energy for formation of the Al-terminated interface is estimated by comparing Cu–Al–O–Al bonding energy (Al-termination) with Cu– O–Al (O-termination). The theoretically calculated work of separation [23] of an alumina (0 0 0 1)/Al-terminated Cu interface has been reported to be 1.62 J/m2, while that of an O-terminated one to be 4.32 J/m2. The work of separation of bulk alumina estimated from the enthalpy of formation [24] is 7.11 J/m2. Therefore, the Cu–Al–O–Al bonding energy (Al-termination)
(1)
1
Al2 p Al2p IAl =ICuAl 2 O3 Al2p
þ1
Al2p
Al2p
Al lAl2 O3 Þ=ðNCuAl lCuAl ÞÞ
þ 1A
(2)
Al2p Al2p Al2p ; IAlO ; and ICuAl are Al 2p intensity from alumina where IAl 2 O3 (peak d), the interface (peak c) and Cu–Al alloy (peak b). The following values of the inelastic mean free path of the Al 2p photoelectron are used (kinetic energy of ca. 30 eV): alumina, Al2p lAl2p Al2 O3 1.24 nm; the interface layer, lAlO ; 1.24 nm; Cu–Al alloy, Al Al lAl2p ; 0.60 nm [26]. N ; N and NAl CuAl AlO Al2 O3 are atomic densities CuAl of Al in the Cu-Al alloy, in the interface layer and in the alumina. For the atomic density of Al in the interface layer, the average value between pure Al (6.02 1022/cm3) and alumina (4.69 1022/ Al Al cm3) was used. The values used for NAl CuAl ; NAlO and NAl2 O3 are 7.50 1021/cm3, 5.36 1022/cm3 and 4.69 1022/cm3. u is the take-off angle of photoelectrons and is 908 in this case. The
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Table 1 Binding energy of Al 2p and O 1s in various types of alumina (eV). Al 2p
O 1s
O 1s–Al 2p
Types of alumina
Refrence
74.7 74.4 74.2 73.7 74.0 74.3 75.5 74.1 75.0 75.2
531.6 531.0 531.0 530.9 530.9 531.3 532.1 531.3 531.7 532.0
456.9 456.6 456.8 457.2 456.9 457.0 456.6 457.2 456.7 456.8
Al2O3 Sapphire Sapphire Gamma Gamma Al2O3 Oxidized TiAl Al2O3 Our epitaxial Our amorphous
[15] [16] [17,18] [18] [19] [20] [21] [22] This study This study
thickness of the interface layer, the alumina film and the total film (interface + alumina) was calculated (Fig. 3). In the case of epitaxial alumina growth (Fig. 3(a)), the estimated thickness of the interface layer reached around 0.25 nm at 128 L and stayed almost constant for further oxygen exposure. This thickness, 0.25 nm, corresponds to the sum of one aluminum layer and one oxygen layer as discussed in the previous work [6]. The thickness of the alumina layer at 1024 L oxygen dose is approximately 1.3 nm, resulting in the total film thickness (peaks c and d) of 1.6 nm. In the case of the amorphous alumina growth, the thickness of the interface layer grew to 0.5 nm, which is almost twice as thick as the one in the epitaxial growth. The total film thickness at 1024 L is 1.6 nm, which is almost the same as in the epitaxial growth case. Concerning the binding energies of peak c and d, those in the amorphous alumina are more than 0.1 eV higher than in the
Fig. 4. Al 2p3/2 binding energy change of four peaks during oxidation for (a) epitaxial (910 K) and (b) amorphous (670 K) growth.
Fig. 3. The change of interface and oxide layer thickness during oxidation for (a) epitaxial (910 K) and (b) amorphous (670 K) growth.
epitaxial alumina as seen in Fig. 1. The results of the precise binding energy analysis of all the peaks are plotted in Fig. 4 as a function of oxygen dosage. As expected, the binding energy of peaks a and b was not influenced by the oxygen dosage and was the same for both types of alumina. The binding energy of peaks c and d slightly changed at the beginning of oxygen dosage approximately up to 128 L or 256 L in both types of alumina growth, at which dosage the total film thickness reached more than 0.5 nm. Although further oxygen dosage increased the total film thickness, no change in the binding energy was detected within the experimental error. It should be noted that the binding energy values of peaks c and d in the amorphous layer are larger than those in the epitaxial layer at high exposure, when the values are constant. Differences in the full width at half maximum (FWHM) of the Al 2p peaks were observed between the epitaxial and the amorphous alumina as plotted in Fig. 5(a) and (b). Both the FWHM value of peak c and its dependence on oxygen dosage are different between the epitaxial and the amorphous films. The FWHM was almost constant except for a larger value at the first oxidation step in the case of epitaxial growth. For the amorphous growth, the FWHM gradually decreased from the first value until approximately 500 L, before reaching a constant value. This difference is considered to be associated with the result that the thickness of the interface layer became constant in the early stages of the epitaxial growth, while it continued to increase until ca. 500 L for the amorphous case. The
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Fig. 5. The change of Al 2p3/2 full width at half maximum (FWHM) of four peaks during oxidation for (a) epitaxial (910 K) and (b) amorphous (670 K) growth.
FWHM became constant at high dosage for both growth modes, but its value for the amorphous film was larger than that of the epitaxial one. Concerning peaks a and b, which arise from the substrate, the FWHM is constant for all oxygen dosages and that of the amorphous film is almost the same as the epitaxial one. The FWHM of peak d does not differ much between the two types of alumina growth, but there may be a slight difference in the oxygen dosage dependence. The FWHM of the epitaxial film appears to increase slightly until 1024 L, while that of the amorphous one is almost constant for all the oxygen dosage except at the very beginning. 3.3. Valence band and O 1s spectra Spectra of the valence band acquired with 106 eV photons, and measured at normal emission before (0 L) and after 1024 L oxygen exposure at 670 K (amorphous) and 910 K (epitaxial), are shown in Fig. 6(a). In the figure, the spectrum at 0 L exposure is scaled by a
factor one-fifth, meaning that the peaks between 2 eV and 5 eV, mainly due to bands of Cu 3d character [27], are very intense. Peaks around 6 eV and 8 eV appeared after oxidation for both cases, which are attributed to O 2p. [28] On oxidation only an intensity decrease of Cu 3d and an increase of O 2p were observed, without any new feature appearing nor any apparent peak shifts. These
Table 2 Binding energy and work function change at 1024 L oxygen (eV).
Al 2p3/2 (int) (eV) Al 2p3/2(oxide) (eV) Work function change (eV) O 2p (eV) Valence band maximum (eV) O 1s (eV) Total film thickness (nm)
Epitaxial
Amorphous
73.63 74.9 0.52 6.45 4.4 531.7 1.6
73.75 75.07 0.51 6.65 4.6 532 1.6
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results suggest that there is no strong hybridization of Cu 3d and O 2p orbitals in both cases. The shape of the two spectra after oxidation is very similar but the position of O 2p (valence band of alumina) is approximately 0.2 eV higher in the amorphous case, while the position of Cu 3d is the same. The valence band maximum (VBM) of the epitaxial film is located 4.4 eV (0.2 eV) below the Fermi level, determined by subtracting the Cu 3d components from the spectrum [6]. Similarly, the VBM of the amorphous film is estimated to be located approximately 4.6 eV below the Fermi level. Although the absolute value has 0.2 eV uncertainty, it is quite certain that the VBM of the amorphous film is located deeper because of the fact that the valley between Cu 3d and O 2p in the spectra is deeper in the amorphous case. The O 1s spectra measured with 900 eV photons after 1024 L oxygen dosage at the two different temperatures are shown in Fig. 6(b). Due to the low ionization cross-section, the spectra are rather noisy, but the O 1s binding energy of the amorphous film is approximately 0.2 eV higher than that of the epitaxial one. 3.4. Band alignment
Fig. 6. Valence band (a) and O 1s (b) spectra of clean substrate (0 L), epitaxial and amorphous alumina films on the substrate.
The valence of Al in the alumina films is the same for the two types of alumina, but the Al 2p3/2 binding energies are different. The binding energy of O 2p and O 1s in the two films at the same total film thickness is also different as shown in Table 2. On the other hand, the binding energy difference between Al 2p (oxide) and O 2p for the amorphous film (68.42 eV) is almost the same as that for the epitaxial film (68.45 eV). The differences between O 2p and O 1s in the two films are also very similar. Therefore, it is concluded that the whole inner shell and the valence band levels are shifted parallel downwards by approximately 0.2 eV in the case of the amorphous alumina films, meaning that there is 0.2 eV difference in the band alignment. Furthermore, the Al 2p binding energy difference between interface and the oxide for the amorphous film, 1.32 eV, is rather similar to 1.27 eV for the epitaxial film. However, the Al 2p difference between the interface and the bulk alloy for the amorphous film, 1.17 eV, is significantly larger than for the epitaxial film, 1.05 eV. Therefore, the difference of the band alignment mainly occurs at the interface layer. This corresponds to
Fig. 7. The schematic energy band diagrams for (a) epitaxial and (b) amorphous alumina films on Cu-9Al substrate.
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the experimental results that the FWHM of peak c in the amorphous film was larger than that of the epitaxial film. The schematic band diagrams for the epitaxial and the amorphous alumina films on the substrate derived from the experiments are summarized in Fig. 7. The conduction band (CB) level is not shown in the figure, because band gap measurement by observing a loss feature of O 1s spectra [29] was not possible in our experiments. If we assume the band gap is the same for the two types of alumina films, the energy difference between the Fermi level and the CB minimum is smaller for the amorphous film by 0.2 eV. Because larger band gap values tend to be reported for crystalline films among various values for both crystalline and amorphous alumina [30–33], the energy difference for the amorphous film might be smaller by more than 0.2 eV than that for the epitaxial one. The band alignment difference of 0.2 eV is critical to the control of MOS operations. The difference of interface termination, which was calculated to result in the band alignment difference of 1.5 eV [10], is not the origin in the present case, because both films have Al-terminated interfaces as discussed in the previous section. In spite of the same interface termination, the difference in structure of the insulating film, whether it was crystalline or amorphous, made the band offset different. Therefore, in order to control the operation voltage of MOS devices, not only a gate insulating material, a gate metal and interface terminating atoms but also the crystallinity of the insulating film should be optimized. 4. Conclusion A Cu-9 at.%Al(1 1 1) alloy single crystal was oxidized in ultra high vacuum under carefully controlled conditions to grow selectively either an ultra-thin epitaxial alumina film at 910 K or an amorphous one at 670 K. Photoelectron spectra were measured in-situ during oxidation using synchrotron light. Precise analysis of Al 2p spectra was made possible by avoiding the overlap of the Cu 3p peak with the Al 2p peak using the Cooper minimum of Cu 3p. Four chemical states of Al were clearly separated: the top Al layer of the clean alloy, bulk Al in the alloy, the interface Al between the alloy and the alumina film, and Al in the alumina. It was concluded that the interface was Al terminated from the observation of interface Al. It was revealed that the Al 2p and O 1s peaks and the valence band spectra in the amorphous alumina film were all shifted to higher energy by approximately 0.2 eV compared to those in the epitaxial one. Comparison of the valence band spectra of the two alumina films showed that the valence band maximum of the amorphous alumina was located 4.6 eV below the Fermi level of the alloy. There were considerable differences in the interface layer between the two alumina films. The Al 2p binding energy difference between the interface component and the substrate is larger in the amorphous film. The interface layer in the amorphous film was twice as thick as that in the epitaxial one. The two major conclusions demonstrated here by electron spectroscopy are as follows: (1) the interface termination, which is governed by the thermodynamic stability of the interface chemical bonding, was not influenced by the crystallinity of the alumina films;
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(2) the band alignment, hence the Schottky barrier height, was affected by the oxide crystallinity even when the insulating material, the electrode metal and the interface termination were all the same. Acknowledgments One of the authors (M.Y.) greatly appreciates partial support by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (nos. 17560027, 20560027), from the Murata Science Foundation (A71162), and from The Mitsubishi Foundation. This work is a part of the research programs LC06058 that are financed by the Ministry of Education of the Czech Republic. The Materials Science Beamline is partially supported by the grant no. MSM 0021620834 that is financed by the Ministry of Education of the Czech Republic. References [1] R. Franchy, Surf. Sci. Rep. 38 (2000) 195–294. [2] M. Yoshitake, S. Bara, Y. Yamauchi, W. Song, J. Vac. Sci. Technol. A21 (2003) 1290– 1293. [3] W. Song, M. Yoshitake, S. Bera, Y. Yamauchi, Jpn. J. Appl. Phys. 42 (2003) 4716– 4720. [4] M. Yoshitake, S. Bera, Y. Yamauchi, W. Song, Surf. Interface Anal. 35 (2003) 824–828. [5] Y. Yamauchi, M. Yoshitake, W. Song, Appl. Surf. Sci. 237 (2004) 363–368. [6] M. Yoshitake, W. Song, J. Libra, K. Masˇek, F. Sˇutara, V. Matolı´n, K.C. Prince, J. Appl. Phys. 103 (2008), 033707-1-5. [7] T. Sasaki, K. Matsunaga, H. Ohta, J. Soc. Mater. Sci. Jpn. 52 (2003) 555–559. [8] T. Sasaki, K. Matsunaga, H. Ohta, Sci. Technol. Adv. Mater. 4 (2003) 575–584. [9] T. Sasaki, T. Mizoguchi, K. Matsunaga, Appl. Surf. Sci. 241 (2005) 87–90. [10] S. Shi, S. Tanaka, M. Kohyama, Mater. Trans. 47 (2006) 2696–2700. [11] H. Garcıa, S. Duenas, H. Castan, L. Bailon, K. Kukli, J. Aarik, M. Ritala, M. Leskela, J. Non-Cryst. Solids 354 (2008) 393–398. [12] X-ray DATA Booklet, Lawrence Berkeley National Laboratory, University of California, Berkerly, 2001, chapter 1.5. [13] J. Ferrante, Acta Metall. 19 (1971) 743–748. [14] http://www.phy.cuhk.edu.hk/surface/XPSPEAK/index.html. [15] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of XRay Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1979. [16] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, in: J. Chastain, R.C. King, Jr. (Eds.), Handbook of X-Ray Photoelectron Spectroscopy, Physical Electronics, Inc, 1995. [17] J.A. Taylor, J. Vac. Sci. Technol. 20 (1982) 751–755. [18] C.D. Wagner, D.E. Passoja, H.F. Hillery, T.G. Kinisky, H.A. Six, W.T. Jansen, J.A. Taylor, J. Vac. Sci. Technol. 21 (1982) 933–944. [19] T.L. Baar, Appl. Surf. Sci. 15 (1983) 1–35. [20] V.I. Nefedov, J. Electron Spectrosc. Relat. Phenom. 25 (1982) 29–47. [21] V. Maurice, G. Despert, S. Zanna, P. Josso, M.-P. Bacos, P. Marcus, Surf. Sci. 596 (2005) 61–73. [22] D. Leinen, A. Fernandez, J.P. Espinos, J.P. Holgado, A.R. Gonzales-Elipe, Appl. Surf. Sci. 68 (1993) 453–459. [23] R. Yang, S. Tanaka, M. Kohyama, Phil. Mag. Lett. 84 (2004) 425–434. [24] Formation enthalpy of Al2O3, 1675.7 kJ/mol, D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 74th ed., CRC Press, Inc, 1994. [25] T.J. Sarapatka, Thin Solid Films 226 (1993) 219–223. [26] The value of the inelastic mean free path for such low energy electron has not Al2p been well established. The values of lAl2p AlO and of lCuAl are approximated by Al2p lAl2p Al2 O3 and by lCu respectively, which were provided by private communication from Sigeo Tanuma. [27] H. Asonen, N. Lindroos, M. Pessa, R. Prasad, R.S. Rao, A. Bansil, Phys. Rev. B 25 (1982) 7075–7085. [28] H.P. Pinto, R.M. Nieminen, S.D. Elliott, Phys. Rev. B 70 (2004), 125402-1-11. [29] S. Miyazaki, J. Vac. Sci. Technol. B19 (2001) 2212–2216. [30] F.S. Ohuchi, R.E. French, J. Vac. Sci. Technol. A6 (1987) 1695–1696. [31] V. Rose, V. Podgursky, I. Costina, R. Franchy, Surf. Sci. 541 (2003) 128–136. [32] S. Andersson, P.A. Bruhwiler, A. Sandell, M. Frank, J. Libuda, A. Giertz, B. Brena, A.J. Mexwell, M. Baumer, H.-J. Freund, N. Martensson, Surf. Sci. 441 (1999) L964–L970. [33] M. Gautier, G. Renaud, L. Pham Van, B. Villette, M. Pollak, N. Thromat, F. Jollet, J.-P. Duraud, J. Am. Ceram. Soc. 77 (1994) 323–334.