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Depth-Dependent Electrical Impedance Distribution in Al2O3 Films on Al(111)sDetection of an Inner Barrier Layer I. Popova, V. Zhukov, and J. T. Yates, Jr.* Department of Chemistry, Surface Science Center, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received July 17, 2000 A method to probe the depth dependence of the electrical impedance of an Al2O3 film on Al metal has been developed. The deposition of high electron affinity species (O3, NO2, Cl2) at 90 K on the outer surface of an oxide film (d ) 20-25 Å) on Al(111) produces negatively charged adsorbate molecules as a result of the trap-mediated electron tunneling from the metal. The capacitor produced in this way exhibits an electrostatic field gradient across the Al2O3 film which has been depth profiled by measuring the shifts in the Al3+(2p) and O2-(1s) X-ray photoelectron spectroscopy (XPS) features originating from the film. Analysis of the XPS peak shifts and shape variation shows that most of the potential gradient exists in the inner thin layer of Al2O3 (d ) 7 Å) adjacent to the metal surface. We postulate that the inner oxide layer detected here is the crucial region for corrosion protection of Al.
I. Introduction Al and its alloys are important materials, and their corrosion protection is a significant problem. In addition to applied research concerning improving Al corrosion resistance, the fundamentals of Al oxidation have been studied extensively.1-8 Under ambient conditions, Al metal is covered with an oxide film.1 The electrical conductivity of this film is a critical physical property governing the kinetics of continued oxide layer growth as well as corrosion processes, where the oxide film is the corrosion product as well as the corrosion inhibitor. Electrical fields across the depth of the thin oxide films may be induced by placing adsorbates on the outer surface of the oxide layer. If the film is sufficiently thin, and if the adsorbate presents an empty acceptor density of states which extends below the Fermi edge of the metal substrate, then charge transfer from the metal into the adsorbate orbitals will occur. The production of a charged capacitor, with the negative charge on the surface of the oxide, will result in an electrostatic potential gradient between the grounded metal and the outer surface of the oxide layer. In the presence of the potential gradient across the oxide film (within the escape depth of photoelectrons), changes in X-ray photoelectron spectroscopy (XPS) peak shape and position are observed for the elements comprising the film. The field distribution within the film can be deduced from the XPS spectrum as is done for the semiconductor space charge region.9-11 The variation of the potential within * To whom correspondence should be addressed. (1) Wefers, K.; Misra, C. Oxides and Hydroxides of Aluminum; Alcoa Technical Paper No.19; Alcoa Laboratories, 1987. (2) Batra, I. P.; Kleinman, L. J. Electron Spectrosc. Relat. Phenom. 1984, 33, 175. (3) O′Connor, D. J.; Wouters, E. R.; Denier van der Gon, A. W.; Vrijomoeth, J.; Zagwijn, P. M.; Slijkerman, W. F. J.; Frenken, J. M. N.; van der Veen, J. F. Surf. Sci. 1993, 287/288, 438. (4) Trost, J.; Brune, H.; Wintterlin, J.; Behm, R. J.; Ertl, G. J. Chem. Phys. 1998, 108, 1740. (5) Ocal, C.; Basurco, B.; Ferrer, S. Surf. Sci. 1985, 157, 233. (6) Barr, T. L. J. Phys. Chem. 1978, 82, 1801. (7) Cabrera, N.; Mott, N. F. Rep. Prog. Phys. 1948, 12, 163. (8) Fehlner, F. P.; Mott, N. F. Oxid. Met. 1970, 2, 59.
the film thickness should therefore reflect the electrical and structural properties of the oxide film.9 The formation of an electrostatic potential across an Al2O3 film upon deposition of a high electron affinity species was previously observed for the Au/Al2O3/Al system where Au atoms (electron affinity (EA) ) 2.8 eV12) were postulated to be negatively charged by the electrons tunneling from the underlying metal.13 The potential gradient produced within the oxide film by the polarized Au/Al2O3/Al capacitor was observed to cause a 0.4 eV decrease in the Al3+(2p) core level photoelectron binding energy. It was proposed that a linear electric field gradient existed across the Al2O3 layer. However, these studies were conducted on relatively thin (4-5 monolayer (ML)) Al2O3 films,13 having structure significantly different from the Al2O3 layers studied here (d ≈ 20 Å or 9 ML). In the present study, we investigated Al2O3 films with a thickness close to that produced under atmospheric conditions. The investigation of the Al2O3 film is particularly suited for the XPS technique, because the limiting thickness of the grown layer does not exceed the photoelectron escape depth.14 We have extended the work in ref13 by using three high electron affinity adsorbates Cl2, NO2, and O3 (with EA ) 2.1-2.4 eV12). The adsorption of these molecules was observed to produce large equivalent shifts in both oxidic O2-(1s) and Al3+(2p) XPS features. An analysis of the peak shifts and shape changes permitted the measurement of the electrostatic field distribution in the film, as was previously reported for various semiconductor and insulator samples.9,10 Our results show a nonlinear distribution of the electrostatic potential across the oxide film, with the inner region of the film exhibiting a high electrical potential gradient compared to the outer region. These results are consistent with the double layer structure of Al2O3. Such a structure was postulated and experimentally (9) Barr, T. L. J. Vac. Sci. Technol. A 1989, 7, 1677. (10) Lau, W. M. J. Appl. Phys. 1989, 65, 2047. (11) Lau, W. M. J. Appl. Phys. 1990, 67, 1504. (12) CRC Handbook of Chemistry and Physics, 71st ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1990. (13) Ocal, C.; Ferrer, S.; Garcia, N. Surf. Sci. 1985, 163, 335. (14) Penn, D. R. J. Electron Spectrosc. Relat. Phenom. 1976, 9, 29.
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found for the thick Al oxide films grown in the presence of water (ref 1 and references therein). This work is the first experimental detection of the barrier-type layer on the thin, dry O2-grown Al2O3 films. II. Experimental Section The experiments were carried out in a two-level ultrahigh vacuum (UHV) system described in detail previously.15 The upper preparation chamber, separated by a gate valve from the lower level analytical chamber, was used for oxidation procedures and gas exposures. The base pressure of the chamber was in the range of (2-3) × 10-10 Torr. The system was equipped with low-energy electron diffraction (LEED) as well as XPS, Auger electron spectroscopy (AES), and high-resolution electron energy loss spectroscopy (HREELS) instruments, and a shielded quadrupole mass spectrometer.15 The Al KR line X-ray radiation (1486.7 eV) was used for XPS studies. A hemispherical electron energy analyzer with the normal to the crystal oriented at 35° to the electron-optical axis was used for XPS experiments. The preparation of the Al(111) crystal for these measurements and its mounting and cleaning procedures were described in ref 15. In these experiments a thick Al2O3 layer (d ≈ 20 Å) was grown on Al(111) by exposures to controlled fluxes of ozone gas through an all-glass doser (flux ) 4 × 1013 molec/s‚cm2). Ozone, rather than O2 gas, was used for Al(111) oxidation at 300 K, because it produces thicker oxide layers.16 The thickness of the film grown in this way (and thus the absolute oxygen coverage) was measured with the oxidic Al3+(2p) and O2-(1s) XPS signals using atomic sensitivity factors (ASF)17 and experimental photoelectron attenuation lengths.14 The calculated oxide film thickness (∼20 Å) was confirmed by analyzing the angular dependence of the XPS intensities. The accuracy of the binding energy scale was ensured by calibration against the known binding energy for the Au(4f7/2) level (Eb ) 83.8 eV) of a clean Au foil and the Al0(2p) level of a clean Al(111) sample (Eb ) 72.6 eV).17 We estimate the error to be (0.1 eV. The grown oxide layer was annealed to 723 K in a vacuum prior to all the experiments to ensure reproducible film morphology. The film morphology is known to depend on the thermal treatment of the film.5 After the sample was annealed, it was cooled to 90 K and exposed to one of the high electron affinity molecules (NO2, O3, Cl2). At this temperature the molecules are condensed onto the oxide surface. Upon exposure, the coverage of the adsorbed species was monitored by XPS (O(1s), N(1s), Cl(1s)) features for O3, NO2, and Cl2, correspondingly). The observed binding energy of the O2-(1s) and Al3+(2p) features (hereafter referred to as oxidic O(1s) and Al(2p) features) were reversibly altered in the adsorption and desorption processes.
III. Results A. Electrostatic Effects Due to Adsorption. The deposition of the high electron affinity molecules on top of the oxide film was monitored by recording the corresponding XPS peaks (Cl(1s), N(1s), O(1s), and Al(2p)). The XPS features for the Al2O3 layer prior to adsorption are shown in Figure 1 (spectrum (a)). After 48 L of Cl2 exposure (1 L ) 1 × 10-6 Torr‚s, uncorrected for ion gauge sensitivity) both of the oxidic Al(2p) and O(1s) spectral features shifted by an identical amount to lower binding energy (spectrum (b)). The value of the shift is shown as a function of exposure to Cl2 at 90 K in Figure 2. The magnitude of the shift is found to saturate at 1.3 eV, after a 40-50 L Cl2 exposure. Similarly to the chlorine adsorption, both NO2 and O3 adsorption caused the oxidic Al(2p) and O(1s) features to shift to lower binding energy by 1.4 (15) Zhukov, V.; Popova, I.; Fomenko, V.; Yates, J. T., Jr. Surf. Sci. 1999, 441, 240. (16) Popova, I.; Zhukov, V.; Yates, J. T., Jr. Manuscript in preparation. (17) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-ray Photoelectron Spectroscopy; Muilenberg, G. E., Ed.; Perkin-Elmer Corp., Physical Electronics Division: Eden Prairie, MN, 1979.
Figure 1. XPS O2-(1s) and Al(2p) (Al0 and Al3+) peaks for (a) clean Al2O3 layer; (b) after 48 L Cl2 exposure at 90 K. The O(1s) and Al(2p) oxide peaks shift together to lower binding energy. The Al0(2p) feature corresponding to metallic Al underneath the oxide layer is not perturbed and remains at constant binding energy.
Figure 2. Parallel shift of both oxidic O2-(1s) and Al3+(2p) peaks upon exposure to Cl2. The maximum shift (1.3 eV) is achieved after 40-50 L exposure, at 90 K.
and 1.3 eV, respectively. The sign of the peak shift (all in the same direction) corresponds to negative charging of the outer part of the oxide film. In contrast, the Al0(2p) XPS feature, originating from the metallic Al substrate, was unshifted by adsorption, remaining at constant binding energy (72.6 eV). Electrical charging of the insulator films may be induced by photoelectron ejection during the XPS process.9,10,18 As a result, a sufficiently low conductivity film would be expected to charge positively, causing the XPS features to shift to higher binding energy, which is the opposite to our observations. To investigate the possibility of contributions to surface charging from the photoelectron ejection, we varied the X-ray flux by a factor of 6. To within 0.1 eV, no shift or peak shape changes were observed for the oxidic XPS features. Thus, significant charging of the oxide film due to photoelectron ejection does not occur in these experiments. The observed changes in the XPS spectrum due to the adsorption of the high electron affinity gases were found to be reversible, and both oxidic features shifted by the same amount gradually back to their original energies upon heating in a vacuum, as shown in Figure 3. Here the positions of both oxidic O(1s) and Al(2p) peaks are shown as a function of the annealing temperature. (18) Smentkowski, V. S.; Ja¨nsch, H.; Henderson, M. A.; Yates, J. T., Jr. Surf. Sci. 1995, 330, 207.
Electrical Impedance Distribution in Al2O3 Films
Figure 3. Gradual removal of the charging effect upon annealing, as both oxidic Al3+(2p) and O2-(1s) features rigidly shift together by 1.3 eV to higher binding energies back to their original electron binding energies.
Figure 4. Decrease of the Cl(1s) XPS peak area upon annealing. The amount of Cl2 on the oxide surface above 400 K is close to the detection limit of our XPS spectrometer and is approximated to be 0 ML. The line is drawn to guide the eye.
The Cl(1s) spectra indicated the complete thermal desorption of the chlorine from the oxide surface, proving that the oxidic XPS peak shifts are directly connected to the presence of adsorbed species. Figure 4 shows the changes in the Cl(1s) XPS feature area as a function of the annealing temperature in vacuum. On the basis of the extrapolation (Figure 4), we estimate that the coverage of chlorine approaches zero at 500-600 K and after annealing to 673 K no signal in the Cl(1s) region is observed. Similar results were observed for NO2 and O3 adsorption. For O3 adsorption, the integrated O(1s) intensity (oxygen coverage) was found to increase by approximately 1015% along with the peak shift (1.4 eV). The observed oxygen coverage change is small compared to the value found for the saturated oxide layer (grown at 300 K) and is associated with the condensation of molecular ozone onto the oxide surface at these low temperatures. In our experiments the charging of the oxide surface upon deposition of the three high electron affinity molecules is always observed. This causes an electric field to develop across the film, leading to equivalent shifts of oxidic Al(2p) and O(1s) XPS features. Thus, adsorbate (high electron affinity molecules) deposition is a tool for the creation of an electrostatic potential drop across the oxide layer. As will be shown in the Discussion, this study of the depth distribution of the electrostatic potential is a key to understanding the structure of the oxide film.
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Figure 5. XPS oxidic O2-(1s) feature shift to lower binding energy upon deposition of NO2 at 90 K as a function of oxide layer thickness. Each point corresponds to a separate experiment, where 300 L of NO2 was deposited on the surface.
B. Effect of the Film Thickness on the Magnitude of the Electrostatic Potential. The oxide layer growth was found to stop at the limiting thickness of ∼20 Å, when grown by exposures to ozone.16 Therefore, Al2O3 layers of higher thickness were grown by exposing the crystal to high pressures of O2 gas (up to 760 Torr). The maximum oxide film thickness produced in these experiments did not exceed 40 Å. The oxide films were subsequently annealed at 723 K in vacuum prior to exposure to the high electron affinity molecules. The XPS shift (O(1s) oxidic feature) observed upon exposure to NO2 is shown in Figure 5 as a function of the film thickness. As expected, the XPS peak shift, related to the tunneling probability (transmission coefficient), decays with increasing oxide film thickness (see Discussion). IV. Discussion Before discussing the mechanisms for the change in electrostatic potential within the oxide layer depth due to the adsorption of high electron affinity molecules, we need to address the question of the type of interaction between the adsorbed molecules and the Al2O3 film. In separate sets of experiments Cl2, NO2, and O3 molecules were completely removed from the oxide surface upon annealing to relatively low temperature (example, Figure 4) and no signs of additional oxidation (growth of oxidic Al(2p) feature) were observed upon deposition of the investigated molecules. Thus, the three high electron affinity species are weakly adsorbed on the outer surface of the oxide. This interaction is responsible for the observed XPS peak shifting (Figures 1-3). Below we discuss the mechanism of the potential buildup and derive the potential distribution within the Al2O3 film. A. Mechanism of Negative Charging of the Oxide Layer. The mechanism of the potential increase across the oxide film thickness is similar to the mechanism of formation of the so-called Cabrera-Mott capacitor in the theory of low-temperature metal oxidation (LTO).7,8,19 In our experiments, the molecule adsorbing on top of the oxide layer interacts with the Al metal underneath. The strength of molecule-metal interaction depends on the relative positions of electronic levels in the Al and in the adsorbed moleculessthe degree of the overlap of the metal conduction band and the molecule’s electron affinity (19) Fromhold, A. T., Jr. Theory of Metal Oxidation, Part 1s Fundamentals; Defects in Crystalline Solids Series, v. 9; North-Holland Publishing Company: Amsterdam, 1976.
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Figure 6. Schematic potential energy diagram of electron tunneling from the Al metal to the broadened and lowered affinity level of the X molecule. The tunneling causes charging of the adsorbed molecules (Xδ-).
orbitals. The orbital overlap strongly depends on the initial position of the molecule’s affinity level, the strength of molecule-surface interaction, the available energy levels for the tunneling electrons, and the molecule-metal distance. A schematic of the molecule-surface interaction is given in Figure 6. The narrow electron affinity level of the molecule (X) (when infinitely separated from the metal) is broadened and lowered upon interaction with the surface (due to lifetime and image effects). The degree of broadening and shifting reflects the strength of moleculesurface interaction. This broadened affinity level is thus partially located below the metal’s Fermi level and has a nonzero overlap with valence electron orbitals of the metal, allowing electron tunneling to occur. Tunneling electrons produce ionization (by electron attachment) of the adsorbed molecules, resulting in a more negative electrostatic potential in the film at larger distances from the metal. The equilibrium negative electrical potential (so-called Mott potential8) is established. The amount of the XPS peak shift (∆Eb ) -1.3 to - 1.5 eV) was found to be comparable in all three high electron affinity molecules (EA ) 2.1-2.4 eV12). In contrast, other adsorbate molecules of various types with lower electron affinity (O2, SF6, and Xe with EA values of 0.45, 1.05, and 0 eV, respectively12) did not cause any shift of the oxidic O(1s) and Al(2p) features, and therefore, did not cause any measurable electrostatic potential gradient across the Al2O3 film when adsorbed at 90 K. Thus, we propose that the oxide layer charging is caused by the electron tunneling to the broadened and sufficiently lowered molecular affinity levels. The molecules themselves act as the electron-accepting centers, rather than their dissociated counterparts, as proposed in the Cabrera-Mott theory7,8 for the metal oxidation process itself. From the XPS oxidic O(1s) and Al(2p) peak positions and half-width changes observed here, we can determine the electrostatic potential distribution through the film thickness, allowing us to differentiate regions of different structure within the oxide layer. B. The Potential Gradient across the Oxide Film Thickness. The kinetic energy of a photoelectron will shift in response to the local electrostatic potential experienced by an atom (ion) in a solid. Within the escape depth of the photoelectrons sampled, both peak shifting and shape changes will occur when an electrostatic field gradient is applied across the sampled region. This effect has been used to study band bending in semiconductors10,11 as well as morphological details of multicomponent disperse systems.9 In our studies, binding energy shifts are introduced by the presence of the field across the film. The value of the shift is connected to the magnitude of the field and the peak broadening depends on the homogeniety of the potential distribution, and thus variation of the electro-
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static potential throughout the sampled region. For a linear potential variation within the film, a significant peak broadening, due to the contributions from photoelectrons emitted from the different depths, should be present. In contrast, in our experiments, a 1.3 eV oxidic O(1s) peak shift along with a relatively small broadening (∼0.2 eV) are observed. This indicates that there is no potential gradient in the major fraction of the film and that most of the potential drop occurs across a small fraction of oxide layer thickness. The XPS spectrum samples electrons from all of these regions and is a composite of multiple features. A deconvolution procedure was used to separate different contributions and to derive the potential distribution function from the depth and angular dependence of the shifted XPS features. The method is fully justified by the value of the shift (1.3-1.5 eV) which is above the resolution of our XPS spectrometer, allowing us to confidently separate spectral features representing photoelectrons from the different depths. The analysis is not limited by the film thickness, which is comparable to the electron escape depth (λ ≈ 17 Å).14 For the Al2O3 films studied, the Al0(2p) feature from the underlying metal substrate is always observed in the spectrum, indicating that the whole depth of the film is being analyzed. For the purpose of the potential deconvolution,20 the film (d ) 23 Å) is divided into 2-ML-thick slabs (ds ) 4.6 Å), with the assumption that the electrostatic potential within each slab is uniform. The spectral feature from each of the slabs is taken as a Gaussian of the full width at half-maximum (fwhm) characteristic for these measurements (1.79 eV, the same as the peak fwhm for Al2O3 before charging induced by adsorbing molecules). The depth-dependent attenuation of the XPS signal is then included in the attenuation factors. Then, using a curvefitting procedure, for each group of possible slab potentials, components are fitted into the total peak area, and a staircase-type function, describing the potential distribution within the film, is derived. Oxygen XPS O(1s) peaks are used to obtain the potential distribution in the film, because the use of Al3+(2p) is complicated by its proximity to the metallic Al0(2p) peak. The following procedures were used to estimate depth and angular dependent attenuation factors,17 describing the reduction of the electron emission, I, as a function of emission depth:
I(z, θ) ) I0 exp(-z/λ cos θ)
(1)
where z is the emission depth, λ is inelastic mean free path of a photoelectron (λ ≈ 17 Å), and θ is the photoemission detection angle. The emission depth for each slab is calculated as
z ) (n - 0.5)ds
(2)
where ds is the thickness of the correspondent slab, and n is its number (n ) 1 for the surface layer). A least-squares analysis of the total resultant peak shape after the fitting (containing five components, one from each individual slab) was carried out. It has been shown that two groups of slabs, with a combined thickness of approximately 7 and 16 Å, are present within the escape depth of the photoelectrons. The derived potential dis(20) Microcal Origin Multiple Gaussian Fitting option was used in these experiments. The O-coverage-dependent fwhm of the fitted peaks were kept the same as those of the original O(1s) peak of Al2O3, with the assumption of small changes in O(1s) intensity upon adsorption of high EA molecules.
Electrical Impedance Distribution in Al2O3 Films
Figure 7. Schematic diagram showing the electrostatic potential distribution within the oxide film and the resultant XPS spectrum. One of the possible potential distribution functions (dotted line) is shown along with the derived staircasetype function (solid line). Two distinct layers (7 ML (∼16 Å) and 3 ML (∼7 Å)) are derived from the XPS O2-(1s) peak deconvolution (shown below the diagram). Most of the potential drop (1.5 V) occurs across the thin layer next to the Al-Al2O3 interface.
tribution within the film along with the XPS O(1s) feature used to derive it are shown in Figure 7. The obtained staircase distribution function involves the assumption of a constant potential within every slab. All of the observed potential drop is seen to occur across the thin (d ) 7 Å) inner part of the oxide layer. There is no significant potential change across the outer part of the film (d ) 16 Å). The XPS signal from the inner region is shifted 1.5 eV to lower binding energy. The value of the field corresponding to the derived potential (4.3 × 106 V/cm) is close to the breakdown voltage of bulk alumina (3-13 × 106 V/cm).21 This suggests that the inner layer possesses very good dielectric properties. In contrast, the outer part may be a poorer insulator or may be porous and more easily permeable by the charged species (see section D). The derived staircase potential distribution function is an approximation and may not reflect the actual potential in the inner layer close to Al metal. If the electrostatic potential of the middle point is reflected by the XPS spectrum, the actual value of the potential at the interface between the oxide film and Al metal may be slightly higher than the value found (shown as a dashed line in the diagram, Figure 7). C. Electrostatic Effects for Different Film Thicknesses. Variations in the oxide thickness change the distance between the adsorbed species and the Al metal, thus altering the tunneling distance (Figure 5). As expected, the electrostatic potential, proportional to the tunneling probability (transmission coefficient), decays with increasing the oxide film thickness (d ) 10-40 Å). Electron tunneling through the thin Al2O3 films (d e 50 Å) was investigated in detail for Al-Al2O3-Al junctions.22 The observed tunneling currents were found to be (21) Klein, N.; Albert, M. J. Appl. Phys. 1982, 53, 5840. (22) Chopra, K. L. Thin Film Phenomena; McGraw-Hill Book Co.: New York, 1969.
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higher than those predicted theoretically for a given oxide thickness. It was proposed that in the presence of ionic defects and trapping centers (known to exist in the thin alumina films1) the tunneling conditions fluctuate and an effective (rather than actual) film thickness is involved. The importance of the trap-mediated tunneling was emphasized because an average mean free path for the tunneling electrons was found to be only 5 Å, much lower than the thickness of the Al2O3 film used.22 Thus, the mechanism of the observed electron tunneling is defined by the presence of the defects in the oxide layer structure. D. Structural Effects in the Oxide Film Influencing the Electrostatic Potential Distribution. Our results suggest the existence of the two layers within the Al oxide film, showing different behavior in the presence of the electrostatic charge. A high gradient of the electric field is observed across an inner layer, suggesting its high electrical impedance. In contrast, the outer layer is shown not to exhibit any electrostatic potential gradient. Two possible explanations for the observed potential distribution involving electrical (impedance) differences, or structural (porosity) differences for the two layers can be proposed. 1. Electrical Impedance. Most of the potential drop across the oxide layer occurs in the inner region of the film (7 Å) adjacent to the metal surface. This inner layer is proposed to have an inherently higher electrical impedance for ionic and electronic transport, compared to the outer layer. Charge transport was shown to slow through the wellordered oxide structures. In contrast, grain boundaries and vacancies were shown to increase the rate of the electron transfer for the growing oxide film.5 Thus, higher impedance of the inner layer suggests its higher degree of ordering and less defective structure, causing its superior corrosion resistance. 2. Porosity of the Layer. A schematic of the capacitor produced by the electrically charged adsorbate molecules is shown in Figure 7, with all of the adsorbate concentrated on the outer surface of the oxide. No electrical field gradient is observed across the outer part (16 Å) of Al2O3 in agreement with the lower impedance that was postulated. However, permeation of this outer part of the film by the negatively charged adsorbate molecules is possible due to the porosity of the layer. Entry of the Xδ-adsorbate species into a porous structure would have a tendency to level the electrostatic potential gradient across this porous outer region of the film. The less porous inner layer would then experience a high electrostatic field across its depth. Thus, both impedance and porosity of the inner and outer layers of Al oxide may vary, contributing to their different electrical characteristics. The structure of the oxide layer may determine its electrical properties, making the less dense and porous layer a poorer insulator. Thus, we propose the existence of an inner dense oxide layer with high impedance, covered by a porous outer oxide layer with poorer insulating properties. E. Connection to Al Corrosion Passivation by Al2O3 Films. In our studies, two insulating regions within an Al2O3 film on Al(111) were distinguished on the basis of the electrostatic potential distribution produced by electron attachment to weakly adsorbed test molecules added to the film. Most of the potential drop across the film occurs in the region adjacent to the metal, as shown in Figure 7. The difference between the two layers is explained on the basis of their porosity and/or electrical impedance (section D) and may be related to different growth mechanisms for the two regions.23 The relation of (23) Atkinson, A. Rev. Mod. Phys. 1985, 57, 437.
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the film structure to its electrochemical corrosion behavior has been previously proposed24 and transmission electron studies are underway.25 It is likely that corrosion passivation of an oxidized Al surface may be attributed to special structural properties of the Al2O3 inner film, giving it higher electrical impedance and an associated lower porosity. This way, under ionic corrosion conditions, the layer can better impede ions in solution from penetrating to the clean Al metal, thus preventing its corrosion (dissolution). The presence of an inner layer of Al2O3 possessing superior insulating and corrosion resistance properties has been postulated previously for the thick anodized Al oxide films.1 This inner layer has been called the barrier layer in studies of Al corrosion passivation by alumina films [ref 1 and references therein]. The experimental evidence reported in this study suggests that the existence of the barrier-type layer is not limited to the partially hydrolyzed Al oxide films (grown in the presence of water or water vapor).1 In contrast, our results show that this layer is an inherent part of the Al2O3 film structure and is present independent of the film thickness or method of preparation. This study is the first experimental observation of the barrier-type layer in the thin alumina films (d (24) Kuznetsova, A.; Burleigh, T. D.; Zhukov, V.; Blachere, J.; Yates, J. T., Jr. Langmuir 1998, 14, 2502. (25) Kuznetsova, A.; Yates, J. T., Jr.; et al. Submitted to Langmuir.
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e 20-25 Å). It links together extensive literature on the structure of the thick Al2O3 films1,23 and surface-science studies of the mechanisms of initial stages of Al metal oxidation.2-5,15 V. Conclusions An electrostatic field across the depth of an Al2O3 film on Al(111) is produced by trap-mediated electron tunneling from the Al metal to the affinity level of adsorbed high electron affinity molecules. The potential gradient across the film manifests itself in the nonuniform shifting and distortion of the XPS peaks associated with the Al2O3 film. The electrostatic potential distribution through the film depth indicates the existence of two layers, with significantly different structural properties. A thin (7 Å) layer at the oxide-Al interface bears most of the potential drop (1.5 V) and is covered by an outer layer (16 Å) which experiences little or no potential drop across its thickness. We propose that this inner part of the oxide film is the so-called “barrier“ layer, postulated to provide the corrosion resistance of the thick Al2O3 films (ref 1 and references therein). This paper reports the first experimental detection of the inner barrier layer under nonaqueous conditions. Acknowledgment. We thank the Air Force Office of Scientific Research for support of this work. We also thank Dr. Leon Sanche for helpful discussion. LA001009H
