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Langmuir 2007, 23, 5386-5393
Epitaxial Growth of Single-Crystalline Al2O3 Films on Cr2O3(0001) Karifala Dumbuya,†,‡ Klaus Christmann,† and Sven L. M. Schroeder*,†,§ Institut fu¨r Chemie, Freie UniVersita¨t Berlin, Takustrasse 3, D-14195 Berlin, Germany, and Molecular Materials Centre, School of Chemistry, The UniVersity of Manchester, PO Box 88, SackVille Street, Manchester M60 1QD, U.K. ReceiVed May 22, 2006. In Final Form: January 24, 2007 Thin, crystallographically oriented single-crystalline Al2O3 films can be grown epitaxially on Cr2O3(0001) by codeposition of Al vapor and O2 at a substrate temperature of 825 K. The properties and growth of these films were monitored by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), low-energy ion scattering (LEIS), and X-ray photoelectron spectroscopy (XPS). Two routes of preparation were investigated: (i) stepwise growth by alternating deposition of Al at room temperature and subsequent exposure to O2 at elevated temperatures; (ii) codeposition of Al and O2 at T > 800 K. The first route was consistently found to result in the growth of a complex interfacial oxide followed by the growth of polycrystalline Al2O3. The second mode of preparation provided homogeneous and ordered, probably (0001)-oriented, films of Al2O3 that maintained a LEED pattern up to a thickness around 10 Å. The surface sensitive Cr MVV Auger transition at 34 eV was completely attenuated once the Al2O3 layer had reached a thickness of 6 Å, pointing to film homogeneity at an early stage. This was confirmed by the absence of a significant Cr signal in LEIS spectra.
Introduction ceramics,1
Aluminas are among the most important technical but they are also important industrial catalysts and widely used as supports for metal catalysts.2,3 To elucidate their surface chemistry, model studies with well-defined Al2O3 substrates are required.4,5 Oriented and polished R-Al2O3 bulk crystals are readily available and have been used as substrates for surface science investigations in the past.6,7 The surface structure of the (0001)-plane of R-Al2O3 crystals has been the object of a number of studies.8-11 However, the scope for characterization of these well-defined bulk oxide substrates is limited because the oxide crystals are electrical insulators, preventing the use of surface electron spectroscopies, which require electrically conducting samples. Consequently, oriented and single crystalline thin films of oxides on metal single-crystal substrates, which are electrically sufficiently conducting, are often preferred substrates for studying the surface chemistry of oxides.4 In the case of Al2O3, its growth by oxidation of Al single-crystal substrates has been investigated * To whom correspondence should be addressed at School of Chemical Engineering and Analytical Science, The University of Manchester, PO Box 88, Sackville Street, Manchester M60 1QD, U.K. E-mail: s.schroeder@ manchester.ac.uk. Fax: +44 (161) 306 4502. † Freie Universita ¨ t Berlin. ‡ Current address: Lehrstuhl fu ¨ r Physikalische Chemie II, Egerlandstr. 3, D-91058 Erlangen, Germany. § The University of Manchester. (1) Tasker, P. W. AdV. Ceram. 1984, 10, 176. (2) Catalyst Handbook, 2nd ed.; Twigg, M.V., Ed.; Manson Publishing: London, 1996. (3) Chen, P. J.; Goodman, D. W. Surf. Sci. 1994, 312, L767-L773. (4) Ba¨umer, M.; Freund, H. J. Prog. Surf. Sci. 1999, 61, 127-1998. (5) Boese, O.; Kemnitz, E.; Unger, W. E. S.; Schroeder, S. L. M. Phys. Chem. Chem. Phys. 2002, 4, 2824-2832. (6) Beitel, G.; Markert, K.; Wiechers, J.; Hrbek, J.; Behm, R. J. In Adsorption on Ordered Surfaces of Ionic Solids and Thin Films; Freund, H.-J., Umbach, E., Eds.; Springer-Verlag: Berlin, 1993; pp 71-82. (7) Bird, D. P. C.; de Castilho, C. M. C.; Lambert, R. M. Surf. Sci. 2000, 449, L221-L227. (8) Walters, C. F.; McCarty, K. F.; Soares, E. A.; van Hove, M. A. Surf. Sci. Lett. 2000, 464, L732-L738. (9) Soares, E. A.; van Hove, M. A.; Walters, C. F.; McCarty, K. F. Phys. ReV. B 2002, 65, 195405. (10) Gomes, J. R. B.; Moreira, I. D. R.; Reinhardt, P.; Wander, A.; Searle, B. G.; Harrison, N. M.; Illas, F. Chem. Phys. Lett. 2001, 341, 412-418. (11) Wander, A.; Searle, B.; Harrison, N. M. Surf. Sci. 2000, 458, 25-33.
extensively, but it results in not particularly well-defined overlayers, which are often polycrystalline or randomly oriented.12,13 Thin Al2O3 films with long-range surface order beyond the correlation length of electron diffraction have been obtained by selective oxidation of Al atoms present in NiAlx single-crystal alloys.4,14-18 Unfortunately, the crystallographic structure of these films is complex,17 which can make them somewhat cumbersome substrates for investigations of elementary surface reactions. Crystallographically simpler, ordered Al2O3(0001) films have more recently been obtained by growth on Nb(110)19 and on Ta(110) substrates.3 In this work, we take a different approach, viz., exploiting the fact that Al2O3 and Cr2O3 are isomorphous, both crystallizing in the corundum structure, and with a lattice parameter misfit of only a few percent. This mismatch is sufficiently low that singlecrystalline Cr2O3 surfaces can be suitable substrates for the heteroepitaxial growth of Al2O3. Cr is also one of only a few metallic elements where a low-energy surface and its oxide are commensurate,20,21 in this case due to the structure of the bcc(110) substrate surface, which exhibits an almost perfect uniaxial fit with the hexagonal lattice of the basal plane of Cr2O3. Electrically conducting single-crystalline films of Cr2O3(0001) can hence be grown readily by oxidation of Cr(110) surfaces under ultrahigh vacuum (UHV) conditions14,22,23 at T > 500 K.3 (12) Batra, I. P.; Kleinman, L. J. Electron Spectrosc. Relat. Phenom. 1984, 33, 175-241. (13) Gunter, P. L. J.; Niemantsverdriet, J. W.; Ribeiro, F. H.; Somorjai, G. A. Catal. ReV.sSci. Eng. 1997, 39, 77-168. (14) Jaeger, R. M.; Kuhlenbeck, H.; Freund, H. J.; Wuttig, M.; Hoffmann, W.; Franchy, R.; Ibach, H. Surf. Sci. 1991, 259, 235-252. (15) Gassmann, P.; Franchy, R.; Ibach, H. J. Electron Spectrosc. Relat. Phenom. 1993, 64-5, 315-320. (16) Bardi, U.; Atrei, A.; Rovida, G. Surf. Sci. 1992, 268, 87-97. (17) Stierle, A.; Renner, F.; Streitel, R.; Dosch, H.; Drube, W.; Cowie, B. C. Science 2004, 303, 1652-1656. (18) Stierle, A.; Renner, F.; Streitel, R.; Dosch, H. Phys. ReV. B 2001, 64, 165413. (19) Dietrich, C.; Koslowski, B.; Ziemann, P. J. Appl. Phys. 2005, 97. (20) Orent, T. W.; Bader, S. D. Surf. Sci. 1982, 115, 323-334. (21) Barbieri, A.; Weiss, W.; van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1994, 302, 259-279. (22) Xu, C.; Hassel, M.; Kuhlenbeck, H.; Freund, H. J. Surf. Sci. 1991, 258, 23-34. (23) Foord, J. S.; Lambert, R. M. Surf. Sci. 1986, 169, 327-336.
10.1021/la061434w CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007
Epitaxial Growth of Al2O3 Films on Cr2O3(0001)
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Such films have been used extensively as model systems for the Cr2O3(0001) surface.24-28 Experimental Section Surface Analysis. The experiments were performed in two separate ultrahigh vacuum (UHV) chambers, equipped with turbomolecular pumps that maintained a base pressure of 2 × 10-10 mbar (as measured by Bayard-Alpert ionization gauges). One apparatus was fitted with a four-grid electron optics for LEED, a cylindrical mirror analyzer (CMA, Varian) for AES, and a quadrupole mass spectrometer (Balzers QMG311). AES measurements were carried out using a normal incidence electron gun supplying electrons with a kinetic energy of 2.5 keV, with the sample positioned approximately 1 cm away from the electron gun. The second apparatus (Leybold) was equipped with facilities for XPS and LEIS and a separate chamber for preparation of samples. The XPS studies were performed using Mg KR radiation (1253.6 eV) from a nonmonochromatized X-ray source and a hemispherical electron analyzer (upgraded to a multichannel electron counter by Specs GmbH), operating at a pass energy of 20 eV. The Au 4f7/2 signal of Au metal at 83.9 eV was used to calibrate the binding energy (BE) scale. Quantitative analysis of the XPS data was performing using the XPSPeak package.29 All peaks were deconvoluted by fitting Gauss/ Lorentz curves after a Shirley background subtraction.30 LEIS measurements were undertaken with 450 eV He+ ions (He purity >99.999%), which were incident at an angle of 32° relative to the surface plane and detected at 90° scattering angle; the kinetic energy of the scattered ions was determined with the same hemispherical analyzer as in XPS but operating at a pass energy of 150 eV. LEIS experiments had to be conducted at a low ion energy, 450 eV, to minimize roughening of the surfaces due to the exposure to He+ ions. Surface roughening became very pronounced and rapid at higher ion energiesseven at ion energies of 1 k eV we observed significant structural degradation of the surfaces faster than the acquisition time of a single spectrum. Materials. The Cr(110) single crystal was mounted on the manipulators using 0.25 mm tantalum wires fed through four holes spark-eroded near the rim of the crystal. Resistive heating of the wires allowed heating to 1200 K, whereby a Ni/NiCr (K-type) thermocouple spot-welded to the edge of the crystal was used to monitor the temperature. All gases used (Messer Griesheim, O2, He, Ar) were of analytical grade. Aluminum wire of high purity (99.98%, Advent Research Materials, Ltd., Oxford, U.K.) was used for the evaporation of Al. The Al source was constructed by following a published design.31 Briefly, Al wire was positioned inside a small ceramic crucible (2 mm in diameter, 15 mm long) around which a tungsten wire for resistive heating was wound. The free ends of the wire were spot-welded to two stainless steel rods mounted on a CF35 flange with electrical feedthroughs rated for currents of 50 A. Prior to Al deposition the source was outgassed carefully until Al could be evaporated at pressures below 10-9 mbar. Preparation of Oxide Substrate. Prior to any deposition and/or oxidation experiments, the Cr(110) crystal was carefully cleaned at elevated temperature by Ar+ sputtering. The oxide film was subsequently grown by a procedure similar to those previously (24) Freund, H. J.; Kuhlenbeck, H.; Staemmler, V. Rep. Prog. Phys. 1996, 59, 283-347. (25) Cappus, D.; Xu, C.; Ehrlich, D.; Dillmann, B.; Ventrice, C. A., Jr.; Al, Shamery, K.; Kuhlenbeck, H.; Freund, H.-J. Chem. Phys. 1993, 177, 533-546. (26) Bender, M.; Ehrlich, D.; Yakovkin, I. N.; Rohr, F.; Ba¨umer, M.; Kuhlenbeck, H.; Freund, H. J.; Staemmler, V. J. Phys: Condens. Matter 1995, 7, 5289. (27) Rohr, F.; Ba¨umer, M.; Freund, H.-J.; Mejias, J. A.; Staemmler, V.; Mu¨ller, S.; Hammer, L.; Heinz, K. Surf. Sci. Lett. 1997, 372, L291-L297. (28) Rohr, F.; Ba¨umer, M.; Freund, H.-J.; Mejias, J. A.; Staemmler, V.; Mu¨ller, S.; Hammer, L.; Heinz, K. Surf. Sci. 1997, 389, 391. (29) Kwok, R. XPSPeak 4.1; Chinese University of Hongkong: Hong Kong, 1999. (30) Shirley, D. A. Phys. ReV. B 1972, 5, 4709. (31) Wytenburg, W. J.; Lambert, R. M. J. Vac. Sci. Technol., A 1992, 10, 3597-3598.
Figure 1. AES/LEED characterization of the Cr2O3(0001) and Cr(110) surfaces. Presented from left to right are (i) the Auger electron spectra of the highly surface-sensitive low-energy region from 20 to 70 eV, (ii) the less surface-sensitive region from 400 to 700 eV, and (iii) the corresponding LEED patterns measured at an electron energy of 69 eV. The upper LEED pattern is that of Cr2O3(0001), and the lower LEED pattern, that of Cr(110). described in the literature.22,23,26,32,33 Briefly, the clean Cr(110) surface was first annealed to 500 K and exposed to 5 × 10-6 mbar O2 for 3 min. Then the sample temperature was increased to 600 K and O2 dosed for 4 min at a partial pressure of 5 × 10-7 mbar. Subsequently, the sample was annealed further to 770 K for 1 min while keeping the O2 partial pressure constant. Finally, the crystal was annealed to 1000 K for 2 min in 5 × 10-7 mbar O2 and then cooled to 500 K while the O2 supply was switched off. Then the crystal was allowed to cool to room temperature while residual gases were pumped away. AE spectra and LEED images of the surface were taken immediately afterward to avoid significant residual gas adsorption. A well-ordered (1 × 1) LEED structure indicative of epitaxially grown Cr2O3(0001) was observed (Figure 1).
Results Cr(110) and Cr2O3(0001) Substrates. Figure 1 displays two Auger electron spectra (left) and the corresponding LEED patterns (right) of the clean Cr(110) surface (bottom) and the Cr2O3(0001) film. The latter spectrum exhibits, in addition to the characteristic Cr MVV emission at 37 eV and the LMM emission peaks (489, 527, and 571 eV), a peak at 508 eV that is due to the O KLL Auger emission. Worth noting is the attenuation of the Cr signals after oxidation and the well-known34 shift of the MVV emission to lower kinetic energies. The ordered surface structures are documented by the LEED patterns, both taken at an electron energy of 69 eV, which show the quasi-hexagonal symmetry of the Cr(110) surface and the hexagonal symmetry of the larger Cr2O3(0001) surface unit cell. The thickness of oxide films generated under conditions similar to those employed here has previously been estimated by quantitative XPS analysis35 and found to be in the range of several nanometers, i.e., larger than the escape depth of the O KLL, Cr LMM, and Cr MVV Auger electrons responsible for the spectra shown in Figures 1 and 2. Our data confirm this conclusion qualitatively, as the intensity of the Cr LMM Auger emission peaks from the oxide film is roughly 50% of the intensity observed for the clean Cr(110) metal substrate. As the oxide film has a Cr density approximately half of that in metallic Cr, we can conclude that the Auger spectra probe essentially the oxide only. (32) Foord, J. S. Ph.D. Thesis, University of Cambridge, 1980. (33) Kuhlenbeck, H.; Xu, C.; Dillmann, B.; Hassel, M.; Adam, B.; Ehrlich, D.; Wohlrab, S.; Freund, H. J.; Ditzinger, U. A.; Neddermeyer, H.; Neuber, M.; Neumann, M. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 15-27. (34) Ekelund, S.; Leygraf, C. Surf. Sci. 1973, 40, 179-199. (35) Maurice, V.; Cadot, S.; Marcus, P. Surf. Sci. 2000, 458, 195-215.
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Figure 2. Left: Low-energy (20-70 eV) AE spectra of Cr MVV and Al LVV transitions during deposition of the first Al2O3 layer on the Cr2O3(0001) surface. The spectra include, from bottom to top, the following: Cr2O3(0001); Cr2O3(0001) exposed to Al at 298 K (note the absence of the Al LVV transition from metallic Al at 67 eV); the same sample after exposure to 5 × 10-7 mbar O2 for 10 min; the same sample after subsequent annealing at 900 K for 10 min; the same sample after subsequent exposure to Al at 298 K (note the presence of the Al LVV transition of metallic Al at 67 eV). Right: Growth of a thick, polycrystalline Al2O3(0001) layer by repeated deposition of Al metal at room temperature and subsequent oxidation at 1023 K. The spectrum at the bottom is that of the final sample in Figure 2a after it had been additionally annealed for 10 min to 900 K in 5 × 10-6 mbar O2. Going upward, the spectra from the following samples are shown: same sample after annealing at 1025 K in 5 × 10-6 mbar O2; then several spectra obtained after successive 10 min deposition of Al at 298 K; the topmost spectrum corresponding to a final annealing step in 5 × 10-6 mbar O2 at 1023 K.
The growth of Cr2O3(0001) on Cr(110) has been extensively investigated.3,24,32 The oxide formed on Cr(110) at 825 K by exposure to O2 is (0001)-oriented single crystalline Cr2O3 with a hexagonal unit cell with parameters (0.47 ( 0.03 and 0.28 nm) that correspond to a well-ordered lattice of O2- ions with a Cr3+ termination layer.36,37 Ultrathin films of Cr2O3, e.g., those obtained by exposure to 20 L of O2 and annealing at temperatures of 673-773 K, may contain Cr species in lower oxidation states than Cr3+,38-40 but at the higher O2 pressures employed here the films contain essentially only Cr3+.41 Alternating Al Deposition and Oxidation at 300 K. Al was deposited in 10 min intervals at a rate of about 0.1 Å/min. Figure 2 presents several series of Auger electron spectra taken in the energy range between 20 and 70 eV, monitoring the Al/Al2O3 deposition process through the Cr MVV and Al LVV Auger transitions; details for all spectra are given in the diagrams and the figure caption. It can be seen that the Cr MVV transition gradually disappears (Figure 2, left), giving way to the Al LVV transitions, which finally dominate the spectra (Figure 2, right). The complete absence of a Cr MVV signal at the end of the deposition indicates that there is little intermixing of Cr and Al atoms in the topmost layer. The Cr MVV signal exhibits the well-known shift from 37 eV on the clean Cr(110) metal surface to 34 eV in the oxide (see (36) Rehbein, C.; Harrison, N. M.; Wander, A. Phys. ReV. B 1996, 54, 1406614070. (37) Brown, N. M. D.; You, H.-X. Surf. Sci. 1990, 233, 317-322. (38) Guo, D. H.; Guo, Q. L.; Altman, M. S.; Wang, E. G. J. Phys. Chem. B 2005, 109, 20968-20972. (39) Huggins, C. P.; Nix, R. M. Surf. Sci. 2005, 594, 163-173. (40) Priyantha, W. A. A.; Waddill, G. D. Surf. Sci. 2005, 578, 149-161. (41) Maetaki, A.; Kishi, K. Surf. Sci. 1998, 411, 35-45.
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Figure 1),34 which is accompanied by strong attenuation of the MVV emission because of the reduced valence electron count in Cr3+. After exposure to Al vapor at 300 K, an Al LVV AES signal at 56 eV appeared (Figure 2, left), which is indicative of Al3+.42,43 Parallel LEED investigations revealed that all surface order of the substrate had been lost, and exposing the sample to 5 × 10-7 mbar of O2 for 10 min at room-temperature did not lead to any visible change in the Al LVV peak position (Figure 2, left). After annealing of the sample to 900 K, surface order was restored (topmost spectrum in Figure 2, left). There are two possible explanations for the observed presence of oxidized Al after deposition of Al metal. Either Al metal reacts with oxygen ions from the Cr2O3(0001) substrate to form Al2O3 or it reacts with water or an oxygen-containing species from the residual gas. Reaction with the residual gas would be unlikely since depositions were carried out in ambient pressures below 10-9 mbar and because the evaporation of thicker Al layers led to the formation of metallic overlayers that were not oxidized (see below). It appears therefore that the observed formation of Al3+ is an interfacial manifestation of the first steps in the well-known aluminothermic reduction of Cr2O3.44-47 The most plausible oxidation states of Cr are Cr0 and Cr2+, so that either Al2O3 or CrO (possibly accompanied by some metallic Cr) is formed at the surface according to the reactions
2Al + 3Cr2O3 f Al2O3 + 6CrO
(1)
2Al + Cr2O3 f Al2O3 + 3Cr
(2)
and
or a nonstoichiometric mixed oxide is formed. A plausible example would be a reaction similar to
2Al + 3Cr2O3 f CrAl2O4·5CrO
(3)
but it appears unlikely that the reaction would actually follow a well-defined stoichiometry such as suggested by eq 3. Analogous redox reactions during deposition of reactive metals on oxides have previously been observed, e.g., during Cr deposition on ZnO.48,49 Interestingly, the aluminothermically formed initial oxide layer passivates the Cr2O3 substrate against further attack by Al metal under the conditions employed in our experiments. Further evaporation of Al led to the appearance of an AES signal at 67 eV (Figure 2, right), which is characteristic for metallic Al. The absence of any structure in the LEED pattern further indicated that there was no long-range order left. Long-range surface order could not be restored through subsequent oxidation and annealing cycles (Figure 2, right). These observations suggest that the uncontrolled aluminothermic surface reaction results in the formation of a defect-rich interfacial layer, with a distribution of nucleation centers that inhibit the subsequent growth of flat and ordered oxide films. Continuation of the process of alternating Al deposition and oxidation in O2 led to the disappearance of the Al AES emission (42) Wytenburg, W. J. Ph.D. Thesis, University of Cambridge, 1991. (43) Wytenburg, W. J.; Ormerod, R. M.; Lambert, R. M. Surf. Sci. 1993, 282, 205-215. (44) Sarangi, B.; Sarangi, A.; Ray, H. S. ISIJ Int. 1996, 36, 1135-1141. (45) Nelson, L. R. J. South Afr. Inst. Min. Metall. 1996, 96, 135-144. (46) Jayaraman, S.; Knio, O. M.; Mann, A. B.; Weihs, T. P. J. Appl. Phys. 1999, 86, 800-809. (47) Ray, H. S.; Sarangi, B.; Sarangi, A. Scand. J. Metall. 1996, 25, 256-264. (48) Spolveri, I.; Atrei, A.; Cortigiani, B.; Bardi, U.; Santucci, A.; Ghisletti, D. Surf. Sci. 1998, 413, 631-638. (49) Wagner, T.; Fu, Q.; Winde, C.; Tsukimoto, S.; Phillipp, F. Interface Sci. 2004, 12, 117-126.
Epitaxial Growth of Al2O3 Films on Cr2O3(0001)
at 67 eV, while the signals at 40 and 56 eV of Al3+ grew in intensity (Figure 2, right). The topmost Auger spectrum in Figure 2 corresponds to a fully oxidized, thick Al2O3 film. The continued absence of any LEED pattern indicated that it was polycrystalline with no appreciable long-range surface order. We did not identify any conditions under which a stepwise deposition/oxidation method led to single crystalline oxide films. In fact, the full oxidation of deposited Al metal layers was difficult, at least at the moderate oxygen partial pressures accessible by our experiments.13 The oxidation is presumably inhibited because the islands or clusters of any deposited Al metal are immediately passivated by a rapidly formed initial oxide overlayer. Codeposition of Al and O2. The codeposition of Al and O2 on a hot substrate was much more successful in generating ordered Al2O3 overlayers. Codeposition of Al and O2 was performed with the substrate preheated to 825 K. Al was then evaporated in the presence of O2 at a pressure of 5 × 10-7 mbar. After each evaporation run, the sample was let to cool while the O2 was simultaneously pumped off. We found that the surface temperature was quite critical: epitactic growth was only observed in a fairly narrow temperature window around 825 ( 100 K. Lower or higher substrate temperatures during deposition provided less ordered Al2O3 films or even films with no detectable order at all. Surface Order of Codeposited Films: LEED. After every Al2O3 deposition step the LEED pattern of the sample was recorded while the chemical composition of the surface region was monitored by AES (vide infra). We did not observe any evidence for significant disturbance of the film structure by electron bombardment under the conditions of LEED or AES probing. In Figure 3, the LEED patterns observed during a typical deposition series are assembled. They refer to the pure Cr2O3(0001) substrate and two Al2O3 films with AES-deduced thicknesses (vide supra) of approximately 5 Å (Figure 3b) and 10 Å (Figure 3c). Directly after Al deposition there was no AES evidence for metallic Al (see Figure 4 and discussion below) and LEED indicated that the hexagonal symmetry of the substrate was maintained. Noticeable, however, was a change in the quality of the patterns with increasing Al2O3 thickness. More precisely, the long-range order in the growing films decreased with increasing thickness. For example, the 5 Å thick film (Figure 3b) produced a still relatively sharp LEED pattern with bright spots and a weak background. The 10 Å film (Figure 3c), although still very satisfactorily oriented, exhibited already some imperfections, characterized by more diffuse LEED reflexes and a stronger background. Once the films had been prepared at 825 K, annealing at lower or higher temperatures did not improve their structural quality. Thickness of Codeposited Films: AES Analysis. On the right of Figure 4a we show a series of low-energy AE spectra obtained during one series of our codeposition experiments. In contrast to our observation during the stepwise deposition, the LVV Auger signal of oxidized Al (at 56 eV) was always maintained throughout the deposition process, even for the largest film thickness of approximately 10 Å. The spectra also exhibit a monotonic decrease of the Cr MVV and Cr LMM Auger signals with increasing Al2O3 coverage and a simultaneous rise in the Al LVV emission at 56 eV. The Cr Auger emission data allowed us to make an estimate for the Al2O3 film thickness. Particularly diagnostic was the Cr LMM transition at 571 eV (Figure 4b) because its peak does not overlap with that of any other Auger transition. For the quantitative analysis we assumed exponential attenuation of the Cr LMM Auger signal, ICrLMM, by a homogeneous, flat R-Al2O3-overlayer
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Figure 3. LEED patterns (from left to right) of the following: Cr2O3(0001), 108 eV; a 5 Å thick Al2O3 film, 113 eV; an approximately 10 Å thick Al2O3 film, 111 eV.
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Figure 4. Sequence of Auger electron spectra taken during the codeposition of Al and O2 method. The left figure (a) is the lowenergy region (20-70 eV) comprising Al LVV and Cr MVV transitions, while the right figure (b) contains the O KLL and Cr LMM transitions (400-700 eV). All spectra were obtained between 10 min Al2O3 deposition episodes with a primary electron energy of 2.5 keV. The bottom spectrum in each figure represents the substrate (Cr2O3), while the topmost spectra represent a 10 Å thick Al2O3 film as described in the text.
of thickness dAl2O3.50
(
)
dAl2O3 ICrLMM ) ICrLMM∞ exp λAl2O3(ECrLMM) cos θ
(4)
In this formula ICrLMM∞ is the intensity of the Cr LMM emission from the Cr2O3 substrate prior to the deposition of any Al2O3 overlayer, while θ is the electron emission angle relative to the surface normal,50 i.e., 42.3° for our cylindrical mirror analyzer.51 λAl2O3 represents the electron attenuation length (EAL) for the Cr LMM (571 eV) electrons in R-Al2O3, which is approximately 14 Å.52 A quantitative analysis of the Cr LMM Auger intensities, which were taken to be proportional to the peak-to-peak height of the differentiated signals,50 was then performed through a leastsquares fitting analysis, in which the thickness dAl2O3 was assumed to be linearly proportional to the deposition time. The result of this analysis is illustrated in Figure 5. It can be seen that the attenuation of the Cr LMM emission follows closely the predictions of eq 4. For example, the thickest films for which we observed LEED patterns corresponded to an attenuation of the Cr LMM emission by approximately 70%, which is compatible with an R-Al2O3 film thickness of approximately 10 Å. The most significant error in this analysis stems probably from the assumption that the deposited film has the electron attenuation properties of bulk R-Al2O3. This is quite likely to be inaccurate as the thickness of the deposited films never exceeded the size of one R-Al2O3 unit cell.53 For such thin oxide films it is plausible to expect a less dense phase than the bulk structure of the most (50) Seah, M. P. Quantification of AES and XPS. In Practical Surface Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; John Wiley and Sons Ltd.: New York, 1990; pp 201-255. (51) Rivie`re, J. C. Instrumentation. In Practical Surface Analysis, Vol. 1: Auger and X-ray Photoelectron Spectroscopy, 2nd ed.; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons: New York, 1990; pp 19-83. (52) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1991, 17, 911-926. (53) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, U.K., 1994.
Figure 5. Al2O3 film thickness calibration for the codeposition of Al and O2 using experimental data from Figure 4. The lower curve represents the experimental (empty circles) and TPP2-calculated (dotted line) signal attenuation of the Cr MVV Auger signal at 34 eV, while the upper curve is that of the experimental (filled circles) and TPP2-calculated (full line) attenuation of the Cr LMM Auger transition at 571 eV.
stable polymorph. For example, a defective Al2O3 phase similar to that observed on NiAl-alloy crystals may be present or a phase similar to γ-Al2O3. These phases are expected to be less dense than R-Al2O3, resulting in higher electron transmission and an underestimation of the true oxide film thickness by the analysis presented in Figure 5. Such phases will in all likelihood also have a smaller band gap and thus weaker electron attenuation power, compounding the likely underestimation of the thickness. On the other hand, TPP-2 calculations52,54 of the resulting inelastic mean free path variations reveal that the mentioned effects are very unlikely to introduce an error larger than approximately 30%. In fact, we found that our Cr LMM-derived thickness calibration is also very compatible with the attenuation of the Cr MVV emission at 34 eV, which is expected to be much more surface sensitive than the LMM emission. We found that the MVV emission had vanished at film thicknesses exceeding approximately 5-6 Å (Figure 5). An exponential best fit to the data (dotted line in Figure 5) predicted an electron attenuation length of 5 Å, which is in excellent agreement with predictions of the TPP-2 model.52 We mention here also that the rapid attenuation of the Cr MVV emission suggests that no Cr is present in the topmost region of the oxide films, i.e., that very little intermixing between the Cr2O3 substrate and the Al2O3 overlayer occurs. LEIS measurements were subsequently employed to ascertain that Cr is indeed absent from the topmost atomic layer (see below). Thickness of Codeposited Films: XPS Analysis. XPS measurements independently confirmed the information derived from AES. As expected, the Al 2p and Al 2s emission lines grow in intensity with increasing Al2O3 coverage (Figure 6) while the intensities of the Cr 3p (Figure 7) and Cr 2p lines (not shown) decrease concomitantly. The intensity of the O 2s emission (Figure 7) remains essentially unaltered, in agreement with expectations for the continued growth of an oxide overlayer with identical oxygen stoichiometry and mass density. For a more quantitative data analysis, the evolution of the Al 2s, Cr 2p, and Cr 3p signal intensities was compared to predictions derived from eq 4, using the IMFP values tabulated by Tanuma et al.55 In excellent agreement with the previous AES analysis, we found a monotonic development of all emission intensities (Figure 8) that is (54) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1993, 20, 77-89. (55) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1991, 17, 927-939.
Epitaxial Growth of Al2O3 Films on Cr2O3(0001)
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Figure 6. X-ray photoelectron spectra of the codeposition series showing the Al 2s and Al 2p regions as a function of deposition time. The Al 2p spectra are difference spectra obtained after subtraction of the Cr 3s photoelectron emission peak at a BE value of 76 eV and are, hence, of lower signal-to-noise quality.
Figure 7. X-ray photoelectron spectra of Cr 3p and O 2s regions from the codeposition measurements as a function of deposition time.
compatible with the growth of Al2O3 films up to approximately 10 Å in thickness. The binding energies of the Al 2p and the Al 2s emission lines (Figure 6) indicate the presence of Al3+ species only: Al 2p photoelectrons from zerovalent, metallic Al are known to appear at binding energies between 71 and 72 eV, while Al 2p emission from the Al3+ valence state is observed at binding energies around 75 eV.56,57 The Al 2p binding energy value of 75.8 eV observed for the thin Al2O3 film in this work thus agrees well with previous values reported in the literature. The energetic position of the Al 2s signal (120 eV) further confirms the Al3+ valence state. We did not find evidence for additional spectral components, e.g., through shoulders or line broadening, or for any chemical shift in either of the Cr and Al emission lines, suggesting that the codeposition method results in the growth of an oxide of welldefined stoichiometry close to that for Al2O3. (56) Practical Surface Analysis. Vol.1-Auger and X-ray Photoelectron Spectroscopy, 2nd ed.; John Wiley & Sons: Chichester, U.K., New York, Brisbane, Toronto, Singapore, 1990. (57) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-ray Photoelectron Spectroscopy; Perkin Elmer Corp.: Eden Prairie, MN, 1979.
Figure 8. Al2O3 film thickness calibration for the codeposition of Al and O2 using experimental XPS data from Figures 6 and 7. Shown are the following: experimental (empty circles) and TPP2-calculated (dotted line) signal attenuation of the Cr 3p emission; the experimental (gray filled circles) and TPP2-calculated (full line) attenuation of the Cr 2p emission; the experimentally observed (black squares) and TPP2-calculated (dashed line) increase of the Al 2s emission.
The analysis of the O 1s signals of the Al2O3 films and from the clean Cr2O3 substrate (Figure 9) indicates the presence of some adsorbed water or hydroxyl groups. O 1s BE values for Cr2O3 reported in the literature range from 530.0 to 530.8 eV,39,57-60 while those for Al2O3 appear around 531.6 eV.57 In line with these previous reports, our measurements resulted in O 1s peak positions of approximately 531 eV for the Cr2O3 films, while Al2O3 was characterized by an O 1s emission at a BE of 532 eV. The fitted O 1s peak for Cr2O3 shows a small structure at 533.5 eV that originates from hydroxide or adsorbed hydroxyl species. The O 1s signal from the thickest Al2O3 film, on the other hand, shows a comparatively more significant H2O contribution at 534.5 eV. This is not entirely unexpected since Al was deposited at a lower temperature (825 K) than the Cr2O3 (58) Allen, G. C.; Tucker, P. M.; Wild, R. K. J. Chem. Soc., Faraday Trans. 2 1978, 74, 1126-1140. (59) Gewinner, G.; Peruchetti, J. C.; Jae´gle´, A.; Kalt, A. Surf. Sci. 1978, 78, 439-458. (60) Grohmann, I.; Kemnitz, E.; Lippitz, A.; Unger, W. E. S. Surf. Interface Anal. 1995, 23, 887-891.
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Figure 11. Deconvolution of the low-energy ion scattering spectrum of the 6.3 Å Al2O3 film by fitted Gauss functions. Scattering from the Al atoms contains contributions from disordered (broad peak) and ordered domains (narrow peak). Note the absence of significant contributions from Cr scattering.
Figure 9. XP spectra of the O 1s region in Cr2O3 and an Al2O3 covered sample (∼12 Å thick) after Shirley background subtraction and fitting using the XPS peak fitting program (uppermost spectra). The lower two spectra are the Al 2s and 2p peaks of the same Al2O3 film.
Figure 10. Low-energy ion scattering spectra obtained for the codeposited Al2O3 films. The spectra were measured at a He partial pressure of 2 × 10-7 mbar.
substrate, which was grown by annealing in O2 to 1000 K. The deposition of Al2O3 was also slower than the Cr2O3 growth, resulting in longer exposure to H2O from the residual gas in the UHV chamber. Most importantly, however, the Al2O3 synthesis involved the evaporation and adsorption of metallic Al atoms, which are an efficient gettering material. Reaction with H2O from the residual gas thus competes efficiently with oxidation by O2 during the Al2O3 growth. Growth Mode and Morphology of Codeposited Films: LEIS. Ion scattering is extremely surface-sensitive, providing information on the chemical composition of the topmost atomic layer of the sample only.61 Shown in Figure 10 are the LEIS spectra of the codeposition series for which we already presented XP spectra in Figures 6 and 7. The results of our measurements (61) Armour, D. G. Ion Scattering Spectroscopy. In Methods of Surface Analysis; Walls, J. M., Ed.; Cambridge University Press: Cambridge, U.K., 1989; pp 283298.
indicate that deposition of an Al2O3 film with a thickness around 1 Å, i.e., far less than necessary to form a complete monolayer of basally oriented R-Al2O3 unit cells, attenuates the chromium LEIS signal at 398 eV almost completely. During the second and third deposition run this situation does not change significantly, in particular with respect to the attenuated (but still visible) Cr signal, even though the Al signal intensity steadily increases. One could perhaps invoke Cr cation exchange or diffusion to the surface to explain the observation of residual scattering at Cr. However, by the time the deposited oxide film reaches a thickness of approximately 6-7 Å, representing approximately a single layer of stoichiometric Al2O3, there appears to be no significant contribution of a Cr signal in the tail of the broadened Al scattering peak. This conclusion is illustrated in more detail in Figure 11, through a deconvolution of the topmost spectrum from Figure 10. It can be seen that the Al peak is characterized by a bimodal distribution of scattered ions, with a broad component that is probably due to the growth of some 3-dimensional crystallites and a more narrow contribution from the well-ordered epitaxially oriented planar domains visible in the LEED pattern. Note that both peak components are centered at exactly the same scattering angle, as one would expect of scattering from the same atomic species. From the absence of any additional feature or shoulder at the Cr position, we can conclude that the surface layer contains no more than a few % of Cr atoms. The absence of evidence for significant scattering from Cr in combination with the rapid attenuation of Cr-related electron emission in the AES and XPS data indicates that a thickness limit can be crossed beyond which Cr diffusion to the surface is no longer effective. At the chosen deposition rate, this corresponded to an Al deposition time of approximately 40-50 min. The broadening of the Al peak indicates that some 3D-growth disorder is intrinsic to the deposition process, in line with the conclusion drawn from the rather diffuse LEED patterns. However, surface roughening due to He+ ion bombardment during the LEIS measurements is likely to have caused additional disorder.
Discussion The simplest thermodynamically stable Al2O3 surface is the R-Al2O3(0001) plane, but its structure remains controversial.9 Compared to elemental materials such as metals, determining the surface structure of a compound involves several additional complicating factors. A compound may terminate along different planes, giving rise to inequivalent surface structures. In the case of Al2O3(0001), three different (0001)-plane terminations are possible: a single Al layer; an oxygen layer; a double Al layer.
Epitaxial Growth of Al2O3 Films on Cr2O3(0001)
First principle calculations predict an Al termination with the first layer atomic distances being greatly contracted (∼85%) relative to the bulk.62,63 X-ray diffraction and LEIS experiments also confirmed this assertion but with a much smaller contraction of the first interlayer.64 However, the models considered in these investigations were limited to the ideal (0001) surface. In another LEED study on R-Al2O3, it was concluded that a mixture of Aland O-terminated domains best explained the observed electron diffraction data.65 In contrast, a recent LEED-IV analysis of the R-Al2O3(0001) surface structure concluded that it was terminated by Al.9 A recent investigation3 of epitaxially grown ultrathin (∼8 Å) Al2O3 films on a Ta(110) substrate concluded that the films were probably terminated by oxygen ions because the surfaces were inert toward chemical interaction with a wide range of gases.3,66 The rationale behind this argument is that an oxygen-terminated surface has no unsaturated surface dangling bonds because the O2- ions have a closed shell of valence electrons. Oxygen ions are also much larger in diameter than the metal cations, efficiently shielding the more reactive metal centers from reaction with gas-phase species. We exposed our Al2O3 films to doses of 9 and 23 L of CO at room temperature and found no evidence for adsorption or dissociation of CO by XPS of the C 1s emission. It thus appears likely that our films have oxygen-terminated surfaces as well. The reverse system to the one investigated here, i.e., Cr deposition on R-Al2O3(0001), was recently studied in comparison with Cr growth on TiO2(110) and SrTiO3 surfaces.49 No oxidation of Cr by the Al2O3 substrate was detected at temperatures below 700 °C, in good agreement with the higher thermodynamic stability of Al2O3 relative to Cr2O3. In line with this result, we observed the thermodynamically expected interfacial reaction between Al and Cr2O3 already at room temperature. However, it quickly resulted in a passivating oxide layer that inhibited further reaction between Al and Cr2O3. In other words, the aluminothermic reaction between small amounts of Al metal in contact with a Cr2O3 surface in vacuum is self-terminating at room temperature. Furthermore, it does not appear to proceed at all in the presence of excess O2, which facilitates the formation of Al2O3 overlayers. Prima facie, these results, self-termination by an interfacial oxide and inhibition of the reaction by O2, contrast strongly with the reactivity of macroscopic Al/Cr2O3 mixtures at atmospheric pressure;44-47 once a mixture of Cr2O3 and Al has been ignited, the reduction of Cr proceeds to completion. Arguably, these (62) Batyrev, I.; Alavi, A.; Finnis, M. W. Faraday Discuss. Chem. Soc. 1999, 33-43. (63) Di Felice, R.; Northrup, J. E. Phys. ReV. B 1999, 60, R16287-R16290. (64) Ahn, J.; Rabalais, J. W. Surf. Sci. 1997, 388, 121-131. (65) Toofan, J.; Watson, P. R. Surf. Sci. 1998, 401, 162-172. (66) Bloss, F. D. Crystallography and Crystal Chemistry: An Introduction; Holt, Rinehart, and Winston: New York, 1971.
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differences reflect an example for a “pressure gap”67-69 between investigations of model systems in vacuum and practical systems under “real-world” conditions. This “gap” likely occurs because Al deposition under vacuum conditions permits only a limited flux of Al to the surface of the oxide substrate. A passivating layer can thus be formed that becomes quickly a kinetic barrier to the continuation of the aluminothermic process. Under practical conditions, there is no such limitation to the Al transport to the Cr2O3 because of the strong exothermicity of the aluminothermic process coupled to heat transfer limitations. This causes rapid heating of the reactants and melting of the Al component in the reaction mixture. Melting breaks up the native oxide film on metallic Al and facilitates its efficient transport to Cr2O3. The heat excess locally generated during the reaction is sufficient to also break up passivating interfacial oxide layers as those observed in this studysin line with the observation of Wagner et al. that even the reverse reaction, Al2O3 reduction by Cr oxidation, becomes feasible at high temperatures.49
Conclusions A combined study of the reaction between Al and a stoichiometric, single-crystalline Cr2O3 surface by XPS, LEED, AES, and LEIS revealed the growth characteristics and chemical composition of Al2O3 layers on this substrate. At room temperature, Al metal reacts spontaneously with Cr2O3 and forms a nonstoichiometric ultrathin oxide film that is disordered and passivates the underlying Cr2O3 against further reaction with Al. To facilitate epitaxial growth of ordered Al2O3(0001) the uncontrolled formation of this film must be prevented. This can be achieved by codeposition of Al and O2 at 825 K, which led to the epitaxial growth of thin, single-crystalline films of Al2O3, as shown by LEED, AES, XPS, and LEIS. The commensuracy of the Al2O3 films with the Cr2O3(0001) substrate suggests that they exhibit a (0001)-orientation. The results also revealed that there was no intermixing between the two oxides at Al2O3 coverages above a few Å. LEIS results allowed us to exclude the presence of Cr in the uppermost atomic layers at a coverage corresponding to a single layer of basally oriented stoichiometric Al2O3. Acknowledgment. We thank the DFG (Deutsche Forschungsgemeinschaft) for financial support under Contract No. SCHR677/1-1. We are grateful to Professor R. M. Lambert (University of Cambridge) for the loan of a Cr(110) single crystal. LA061434W (67) Over, H.; Muhler, M. Prog. Surf. Sci. 2003, 72, 3-17. (68) Thostrup, P.; Vestergaard, E. K.; An, T.; Laegsgaard, E.; Besenbacher, F. J. Chem. Phys. 2003, 118, 3724-3730. (69) Johansson, M.; Lundstrom, I.; Ekedahl, L. G. J. Appl. Phys. 1998, 84, 44-51.
