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Interpretation of the Changing the Band Gap of Al2O3 Depending on Its Crystalline Form: Connection with Different Local Symmetries Elena O. Filatova* and Aleksei S. Konashuk

Downloaded by GEORGETOWN UNIV on August 25, 2015 | http://pubs.acs.org Publication Date (Web): August 21, 2015 | doi: 10.1021/acs.jpcc.5b06843

St. Petersburg State University, Ul’yanovskaya Str. 1, Peterhof, 198504 St. Petersburg, Russia ABSTRACT: The valence and conduction bands of am- and γ-Al2O3 films grown by the atomic layer deposition technique were studied simultaneously in identical experimental conditions using high-resolution near -edge X-ray absorption fine structure and soft X-ray photoelectron spectroscopy. The valence band maximum was found to be centered at 3.64 ± 0.04 eV for amAl2O3 and 3.47 ± 0.04 eV for γ-Al2O3. The band gap of Al2O3 was determined to be 7.0 ± 0.1 and 7.6 ± 0.1 eV for measured am- and γ-Al2O3, respectively. The main role in changing the band gap belongs to a shift of the bottom of conduction band depending on Al2O3 crystalline form. The position of the bottom of the conduction band is governed by the charge transfer from Al atom to the oxygen that depends strongly on the Al atom coordination symmetries. A strong p−d hybridization allowed for Td symmetry but forbidden for Oh symmetry plays the decisive role in the formation of the bottom of the conduction band.

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INTRODUCTION Aluminum oxide (Al2O3) is an important dielectric, ceramic, and catalyst. Because of its wide band gap, low leakage current, and modest value of dielectric constant, Al2O3 is considered to be a suitable insulator for various electronic applications ranging from gate dielectric in metal-oxide-semiconductor (MOS) transistors to trapping or blocking insulator in charge trapping nonvolatile memory cells. Notice that developing nanoscale memory-bit cells for nonvolatile random access memory is one of a key technological step now. It is well known that various metastable polymorphs of Al2O3 exist (transition aluminas like γ, η, δ, θ, and χ phases) in addition to the thermodynamically stable α-Al2O3 form (corundum).1 From the technological application point of view, amorphous (am), gamma (γ), and alpha (α) Al2O3 forms are of greatest interest. It is important that each structural form is characterized by its own value of the band gap. The reported experimental band gap value for α-Al2O3 is 8.8 eV,2 for γ-Al2O3 is 7.0−8.7 eV,3,4 and for am-Al2O3 is 5.1−7.1 eV.4 It is noteworthy that the band gap value depends on the method of synthesis. For instance, the method of atomic layer deposition (ALD) allows us to synthesize the amorphous films with band gap of 6.2 eV,5 while the am-Al2O3 film, grown by spray pyrolysis has a band gap of 5.6 eV.6 Currently the controversial points on the main factor determining the value of band gap in different Al2O3 phases exist in the literature4,7,8 that stimulates further spectroscopic experiments on this material. According to refs 4 and 7, the band gap varies mainly due to shift of the bottom of the conduction band. Following the calculations made in ref 8, the energy position of the conduction band is changed only to the value of 0.5 eV, while 80% of the variation observed is due to the valence band shifts. By the present moment, numerous experimental9−20 and theoretical7,23−29 studies of am-, γ-, and α-Al2O3 in the vacuum © XXXX American Chemical Society

ultraviolet and soft X-ray regions have been accumulated. X-ray absorption,10−18 reflection,19 and emission13,20 specroscopies, X-ray stimulated luminescence,21 electron energy loss,22 have been employed to better understand the electronic properties of this compound. Nevertheless, most of the photoabsorption data are obtained from the inner 2p level of aluminum, and only a small number of works are aimed at studying the photoabsorption from the 1s level of oxygen. Moreover, the joint investigation of the valence and conduction bands under identical experimental conditions using soft X-ray emission and total electron yield spectroscopies can be found only in the ref 13 for α- and γ-Al2O3 and ref 30 for am-Al2O3. These studies were performed with help of laboratory spectrometers. Also, the soft X-ray emission and reflection study was performed for α-Al2O3 in ref 31 using synchrotron radiation. In the present work, we report the results on the valence and conduction band states of the am- and γ-Al2O3 obtained by means of two independent X-ray spectroscopic techniques, in particular, high-resolution X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopies (XPS), which have become powerful tools for probing empty and occupied electronic states, respectively. All of the measurements were performed jointly under identical experimental conditions. Remember that NEXAFS spectroscopy provides the information about local (associated with a hole localization in the core−shell) and partial (allowed for certain angular momentum symmetry) electronic density of states of the conduction band. In photoelectron spectroscopy, the electrons from the valence band and core levels are excited in a continuous spectrum at fixed excitation photon energy. In this Received: July 15, 2015 Revised: August 12, 2015

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DOI: 10.1021/acs.jpcc.5b06843 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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RESULTS AND DISCUSSION Figure 1 shows the measured Al L2,3- and O K-absorption spectra of γ-Al2O3 and am-Al2O3. The Al L2,3 spectra were


case, a hard relationship between the photoionization crosssection, wave function symmetry, and excitation photon energy exists and should be considered.32 The goal of the current paper is a careful analysis of the energy positions of the top of the valence band (EV) and the bottom of the conduction band (EC) of the am- and γ-Al2O3 to reveal the factor providing different values of their band gap. To gain further insight into the electronic structure of different Al2O3 crystal forms the Al L2,3- and O K-absorption spectra were aligned in energy using the energy separation between the O 1s and Al 2p3/2 core levels in am- and γ-Al2O3 derived from XPS measurement. The precise determination of the band gap of Al2O3 is essential to determine the position of EC that can be calculated based on the EV taken from photoemission spectrum. The band gap of the am- and γ-Al2O3 was established from measured energy-loss spectra. The principles of this method can be found in refs 33−35.

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EXPERIMENTAL METHODS

Am-Al2O3 and γ-Al2O3 films with thickness 12 nm were grown by atomic layer deposition (ALD) technique from trimethylaluminum Al(CH3)3 and water precursors at 300 °C. The samples were prepared on the identically treated [IMECcleaned] n-type Si(100) surfaces, as described in details elsewhere.36,37 Annealing at 1000 °C for 60 s in a nitrogen atmosphere resulted in the crystallization of the amorphous film in the γ phase. Near-edge X-ray absorption fine structure (NEXAFS) and Xray photoelectron spectroscopy (XPS) measurements were performed at the RGL-station on the Russian−German beamline at the BESSY II synchrotron light source of the Helmholtz-Zentrum Berlin.38 The XPS spectra were taken at the excitation photon energy of 700 eV using a hemispherical electron energy analyzer (Specs Phoibos 1500). All photoemission spectra were collected with the analyzer and monochromator energy resolution better than 430 meV. The binding energy scale was referenced to the value of Au 4f7/2 photoelectron peak position (83.95 eV).39,40 The charge neutralization system was used to neutralize charging during photon excitation. Ar+ ion sputtering was applied in situ using a 1 keV ion beam directed to the surface at a small grazing incident angle to clean the surface of the studied samples. NEXAFS spectra were measured at the incident angle of 45° in the vicinity of Al L2,3- and O K-absorption edges with energy resolution better than E/ΔE = 3500. The spectra were obtained by monitoring the total electron yield from the samples in current mode. Calibration of the photon energy scale was performed by measurement of Au 4f7/2 photoelectron peak with photon excitation in the first and second orders of diffraction. The actual photon energies were equated to the difference between the first-order and second-order Au 4f7/2 kinetic energies. The films were also characterized by X-ray diffraction (XRD). XRD pattern was obtained in the asymmetric geometry of a grazing incidence (grazing incidence diffraction, GID). Xray tube with a copper anode was used as a source of X-rays. The measurements were carried out on Cu Kα1 (0.154056 nm). The angular resolution of the diffractometer was 0.03°. The studies confirmed the amorphous and gamma phases of the studied films, correspondingly.

Figure 1. Al L2,3 absorption spectra of γ- and am-Al2O3. α-Al2O3 spectrum measured with high-energy resolution borrowed from the work41 (a). Al L2,3 and O K absorption spectra of γ-Al2O3 (b) and amAl2O3 (c) aligned in energy using the energy separation (456.6 eV and 456.8, respectively) between the O 1s and Al 2p3/2 core levels in γ- and am-Al2O3 derived from XPS measurement. The Al L2,3 and O K spectra for α-Al2O3 measured simultaneously in the identical experimental conditions borrowed from the work13 are also shown (d). Al L2,3 and O K spectra are shown in red and blue colors, respectively, in all parts of the Figure.

aligned in energy to O K spectra using the energy separation of 456.6 and 456.8 eV between the O 1s and Al 2p3/2 core levels of γ- and am-Al2O3 films, respectively, derived from XPS measurement performed in the current work. Figure 2 shows the Al 2p and O 1s photoelectron spectra for the γ- and am-Al2O3 films. Panels show the fitted data, indicating an assumed spin−orbit splitting of 0.47 eV41 and a branching ratio of 2:1. Because in the Al 2p core-level line these peaks are unresolvable, the spin−orbit splitting and branching ratio were fixed during fitting. Excellent fits were obtained with a Gauss and Lorentz functions in ratio 50/50%. The binding energy of the Al 2p3/2 for the γ- and am-Al2O3 is 74.7 and 74.50 eV, respectively, which agrees well with the reported value of 74.4 eV for bulk Al2O3.42 Notice that in α-Al2O3 the Al atom can occupy only octahedrally coordinated sites in contrast with γ- and am-Al2O3, where the Al atom can occupy both tetrahedrally ([AlO4]−5) and octahedrally ([AlO6]−9) coordinated sites in different proportion. That is why for completeness of the discussion we also present the Al L2,3 and O K absorption spectra for α-Al2O3 borrowed from the work13 measured simultaneously by total B


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The Journal of Physical Chemistry C

of the 3d states in the AlO6 octahedron (10Dq) is given by 10Dq = 5eq3d/3R5, where q is the charge on each ligand atom (oxygen), 3d is the expectation value of the operator r4 for the Al 3d wave function, and R is the interatomic distance R(Al−O).46−48 It is well known49−51 that corundum is constructed of distorted Oh symmetry fragments, and as a consequence the band d, connected to doubly degenerate eg state, is always split into two bands d−d′. The presence of small details in the band c is connected to the existence of long-range order in the structure52 and splitting of triply degenerated state t2g. Turning to the joint analysis of the Al L2,3-absorption spectra of α-, γ-, and am-Al2O3 crystal forms, recall that contrary to αAl2O3 the Al atom in other modifications can occupy both tetrahedrally and octahedrally coordinated sites. As follows from refs 43, 45, 53, in going from an octahedral to a tetrahedral geometry, the symmetry species a1g, t1u, t2g, and eg map directly into the species a1, t2 (p-like), t2 (d-like), and e; however, there are two important differences: (i) in tetrahedral symmetry (without central symmetry), the p → p-like transitions are allowed; (ii) in tetrahedral field, the orientation of e symmetry state is energetically favored over t2 symmetry state that means the features c and d show the inverted structures with respect to octahedral field (features c and d are assigned to the transitions to e and t2 (d-like), respectively). A joint analysis of the Al L2,3-absorption spectra shows the increasing intensity of the peak b relative to the peak a with increasing the content of AlO4 tetrahedra in the structure. In an orderly sequence of α-Al2O3, γ-Al2O3, and am-Al2O3 (the content of the AlO4 tetrahedra increases), the ratio of the intensities of the peaks a and b (Ia/Ib) decreases from 1.65 (αAl2O3) to 0.73 (γ-Al2O3) and further to 0.68 (am-Al2O3). (The uncertainty was about 0.01, and it was specified by the accuracy of determining the values of the intensities of peaks a and b.) One can conclude that the variation in the relative intensity of features a and b is due to the selection rules for the different symmetries (octahedral and tetrahedral symmetries), but also the coordination site distortion significantly contributes to enhance the intensity of the feature b assigned to forbidden p → p transition in the centrally symmetric systems. This result correlates well with the studies carried in refs 41 and 54. In the α- and γ-Al2O3, the feature a has two clearly resolved components, but the ratio of relative intensities of the components is opposite in the studied crystal phases. In the α-Al2O3 the a and a″ components reflect the spin−orbit splitting of Al 2p state and correspond to the L3 and L2 edges, respectively. The energy splitting is 0.47 eV.41 In γ-Al2O3, the Al atom can occupy both tetrahedrally and octahedrally coordinated sites. As follows from ref 41, the site mixing effects in the materials in which AlO6 coordination coexists with AlO4 coordination leads to three peaks in the feature a. a′ at the lowest energy is assigned to the L3-edge of AlO4 coordination, a″ at the highest energy is assigned the L2-edge of AlO6, and the intermediate feature a is assigned to a superposition of the L2-edge of AlO4 and the L3-edge of AlO6 coordinations, respectively. Also, it was established in ref 41 that the shift to lower energy of the edge offset and the spin−orbit splitting depends strongly on the nominal charge on the oxygen atoms. A joint analysis of the spectra shown in the Figure 1 points to the correlation of the energy position of features a in the spectra of the α- and γ-Al2O3. One can assume that the broadening in the low-energy region of the feature b in the spectrum of γ-Al2O3 corresponds to the feature a″ in the

Figure 2. Experimental and fitted Al 2p and O 1s photoelectron spectra of am-Al2O3 (a,c) and γ-Al2O3 (b,d) films measured at an excitation energy of 700 eV and normal emission angle after cleaning the surface. (a,b) Al 2p spectra and (c,d) O 1s spectra. The band gap of Al2O3 was determined to be 7.0 ± 0.1 and 7.6 ± 0.1 eV for am- and γ-Al2O3, respectively.

electron yield. These spectra were aligned using original XPS data from the ref 13. As follows from the Figure 1, the Al L2,3-absorption spectra of different Al2O3 crystal forms exhibit a well-developed fine structure (the main features are labeled a−d) that results from the Al 2p electron transitions to unoccupied molecular orbitals (MOs). Because of the local nature of the X-ray absorption process the main role in the absorption spectra fine structure formation is played by the characteristics (atomic and electronic) of the nearest surroundings of the atoms that participate in absorption. For this reason the cluster or quasimolecular approach to absorption spectra calculation is often used.43−45 In this approach the problem of finding the wave functions, energy, and transition probability matrix element is solved for the quasi-molecular or cluster fragment that satisfies the translation symmetry principle. Within the quasi-molecular approach to the main fine structure features in α-Al2O3, where the Al atom can occupy only octahedrally coordinated sites, the features a−d are associated with transitions from Al 2p states to the [AlO6]−9 cluster excited states of a1g (Al 3s), t1u (Al 3p), t2g (Al d), and eg (Al d) symmetry.43−45 The states of the aluminum ion giving the main contributions to the wave functions of the molecular states are shown in the brackets. In an ideal octahedron, p → p-like transitions are forbidden by dipolar selection rules, and one can expect a low intensity peak b representing the transitions to unoccupied MOs of t1u symmetry (formed from the Al 3p orbitals with an admixture of O 2p orbitals) in the Al L2,3-absorption spectrum of α-Al2O3, as is observed in the Figure 1. Note that the features c and d are connected to transitions to the t2g and eg continuum states and result from the influence of the surroundings (the molecular fields of the oxygen atoms) on the aluminum atom photoabsorption process. These MOs are formed by covalent mixing of the Al 3d with the O 2p states, and as a consequence, the corresponding peaks can be observed in the absorption spectra of both aluminum and oxygen atoms. The ligand-field splitting C


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The Journal of Physical Chemistry C spectrum of α-Al2O3. It is plausible to assume that the feature a reflects the L3-edge of AlO6 coordination in γ-Al2O3. The broad unresolved feature a′ can be assigned to a superposition of the L2,3-edge of AlO4 coordinations. Also, one can see that in amAl2O3 the feature a is a broad peak without separation on a′ and a″ components. Besides the intensity changes the energy changes in the considered sequence are traced also: (i) a strong dependence on the crystal form of the energy distance between features a and b (0.9, 1.6, and 2.8 eV for α-, γ-, and am-Al2O3, respectively) and (ii) the energy position of absorption edge onset shifts to lower energy as a content of tetrahedral coordination in the structure increases. As follows from experimental and theoretical analyses carried out in the refs 13, 41, and 49, the differences in the Al L2,3-absorption spectra of the phases containing tetrahedral (AlO4) and octahedral (AlO6) coordinations result from the strong p−d hybridization allowed for Td symmetry but forbidden for Oh symmetry49 due to reasons of symmetry. Nevertheless, when the octahedron is distorted as in the case of corundum, some p−d hybridization is allowed. Another very important reason for the established difference connects with the presence of a core hole in the final state, which affects the absorption spectra by creating an attractive potential (which is determined by both the core hole−photoelectron attraction and the amount of screening of the core hole by valence electrons) that shifts the edge toward lower energy. The position of the edge onset is related to the effective charge on the Al atoms in the different coordination symmetries. According to refs 41 and 49, the core hole is screened in Oh symmetry by a charge transfer from oxygen to Al that is forbidden in Td symmetry. All of the discussed regularities can be traced in the shape of the d feature in the discussed spectra: a gradual disappearance of the d−d′ splitting with increasing the proportion of tetrahedrally coordinated sites in the structure and the absence of the d−d′ splitting in the spectrum of amorphous alumina that is an additional proof of the excess of tetrahedra in this structure. Let us turn to the consideration of the O K-absorption spectra of the discussed structures that are also shown in the Figure 1. It is known that in octahedral (tetrahedral) complexes the metal eg(t2) orbitals are directed toward the corners of the octahedra (tetrahedra) and have a stronger overlap with orbitals of neighboring atoms (2p orbitals of oxygen atoms). As a result, O K-near edge structure is very sensitive to changes in the local bonding environment. According to ref 30, the O K-absorption spectrum of corundum originates from transitions to the unoccupied MOs formed by 2p states of O mixed with Al 3d states split by the crystal field into Al t2g and Al eg states (main broad band A′−A) and with Al 4s and Al 4p states (broad features B′−B, respectively).13,30 In the γ- and am-Al2O3, both coordination sites (octahedral and tetrahedral) exist in different ratio. The orientation of a d-like state of e symmetry in a tetrahedral field is energetically favored over that of the d-like state with t2 symmetry, and thus in γ- and am-Al2O3 the broad band A′−A is assigned to transition into the unoccupied states formed by 2p states of O mixed with t2g(e) and eg(t2) electronic states following the proportion of the tetrahedral and octahedral coordination sites in the structure. Notice that a low-energy shift of the O K-absorption edge in an orderly sequence of α-Al2O3, γ-Al2O3, and am-Al2O3 is observed. The same regularity was established in Al L2,3-absorption spectra.

A joint analysis of the studied Al L2,3- and O K-absorption spectra (Figure 1) reveals some interesting tendencies. First of all, the feature a in the Al L2,3-spectrum, that is connected to the Al 3s states, is located in the lower energy region with respect to the O K-absorption edge position. Such experimental evidence supports other results,2,30,55 according to which the bottom of the conduction band in Al2O3 is formed solely by Al 3s states. Second, for each studied crystal form the energy shift between Al L2,3- and O K-absorption edges is observed: the more tetrahedra content, the more energy shift. In the spectrum of corundum, the energy position of the feature b (assigned to the forbidden p → p transition and occurs due to distortion of octahedra AlO6) is located at energy lower than that of the inflection point of O K-spectrum. With the increase in the tetrahedra content in the structure, the overlapping between the band A−A′ of the O K-spectrum and feature b of the Al L2,3-spectrum becomes greater. It is plausible to conclude that in the tetrahedral coordination site the overlapping between Al 3p orbitals and O 2p orbitals, which, in turn, are mixed with Al 3d orbitals, increases as compared with an octahedral coordination site. While Al p−d hybridization is allowed only in tetrahedral coordination,49 the established tendency is not surprising and correlates with refs 41 and 49. Finally, we can conclude that the relative position of both the initial core level and the unoccupied final state is affected by the charge transfer from Al atom to the oxygen, which depends strongly on the Al atoms coordination symmetries. Also, the presence of a core hole during the absorption process differently affects the ground-state conduction-band onset depending on the coordination symmetry. The photoelectron spectra of the valence band of studied γand am-Al2O3 films are plotted in the Figure 3. It is well known

Figure 3. Experimental photoelectron spectra of the valence band (upper subband) of γ- and am-Al2O3 films measured at an excitation energy of 700 eV and normal emission angle after cleaning the surface. The uncertainty in VBM position is ±0.04 eV.

that the valence band of Al2O3 consists of two subbands.50,56,57 In the present work, we address the position of the valence band maximum (VBM), so we will discuss only upper subband. This subband is formed by O 2p states mixed with Al 3s, 3p, and 3d states. As shown in the Figure 3, the shape and energy position of the VB depends on the Al2O3 crystal form. One can allocate two main features A and B in the upper VB. According to calculations presented in ref 58, the band A originates predominantly from antibonding 2pπ states of the oxygen. The band B is assigned to the bonding 2pσ states of the oxygen mixed with 3s, 3p, and 3d states of aluminum in octahedral coordinated sites. The bonding O 2pσ states mixed with 3s, 3p, and 3d states of aluminum in tetrahedral coordinated sites are allocated between features A and B. Obviously, the observed D


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The Journal of Physical Chemistry C changes in the VB agree well with the proportion of the tetrahedral and octahedral coordination sites in the structure. Because the value of the VBM is sensitive to the choice of points on the used leading edge to obtain the regression line,59,60 we have selected several different sets of points over the linear region of the leading edge to perform regressions. The uncertainty of derived values was found to be in the range of 0.04 eV. The VBM was found to be centered at 3.64 ± 0.04 eV for am-Al2O3 film and 3.47 ± 0.04 eV for γ-Al2O3 film. Notice that according to ref 61 the VBM of corundum is centered at 3.3 eV. That means in the considered sequence of Al2O3 crystal phases (α-, γ-, and am-Al2O3) a lower energy shift of the VBM is observed. Now let us turn to the Figure 2. As follows from Figure 2 the band gap of measured am- and γ-Al2O3 films is equal to 7.0 ± 0.1 and 7.6 ± 0.1 eV, respectively. Count these values from the VBM to estimate the position of the bottom of the conduction band and then compare these positions with the position of Al L2,3-absorption edges, respectively. Figure 4 shows the Al L2,3-

Two important facts follow from the joint consideration of the spectra plotted in the Figure 4. First, a high-energy shift of the bottom of the conduction band in the sequence am-, γ-, and α-Al2O3 is established. Between am- and γ-Al2O3 the shift is equal to 0.77 eV; between γ- and α-Al2O3 the shift is equal to 1.37 eV. Second, the feature a (assigned to transitions from Al 2p states to the Al 3s states) in the Al L2,3-absorption spectra of all of the studied Al2O3 crystal forms is located under the bottom of conduction band and can be interpreted as being due to the creation of an exciton localized on the aluminum atom. Such conclusion agrees well with the predictions in ref 16. One can conclude that the decisive role in the formation of the band gap plays the conduction band. The position of the bottom of the conduction band is governed by the charge transfer from Al atom to the oxygen, which depends strongly on the Al atoms coordination symmetries. A strong p−d hybridization allowed for Td symmetry but forbidden for Oh symmetry due to reasons of symmetry plays the decisive role in the formation of the bottom of the conduction band.

Figure 4. Al L2,3 and O K absorption spectra of α-, γ-, and am-Al2O3 films on a common energy scale where the Fermi level is chosen as zero point of the energy scale. The vertical lines indicate the position of the bottom of the conduction band.

CONCLUSIONS The valence and conduction bands of different Al2O3 crystalline forms were studied. The VBM was found to be centered at 3.64 ± 0.04 eV for am-Al2O3, 3.47 ± 0.04 eV for γ-Al2O3, and 3.3 eV for α-Al2O3.61 High energy shift of the bottom of the conduction band in the sequence am-, γ-, and α-Al2O3 is established. (Between am- and γ-Al2O3 the shift is equal to 0.77 eV; between γ- and α-Al2O3 the shift is equal to 1.37 eV.) The feature a (assigned to transitions from Al 2p states to the Al 3s states) in the Al L2,3-absorption spectra of all of the studied Al2O3 crystal forms is located under the bottom of conduction band and can be interpreted as being due to the creation of an exciton localized on the aluminum. The main role in changing the band gap belongs to the shift of the bottom of the conduction band depending on the Al2O3 crystalline form. The position of the bottom of the conduction band is governed by the charge transfer from the Al atom to the oxygen, which depends strongly on the Al atoms coordination symmetries.

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and O K-absorption spectra on a common energy scale, where the Fermi level is chosen as zero point of the energy scale. The vertical lines indicate the position of the bottom of the conduction band. The correct combination was achieved with help of XPS data. In the XPS method, it is assumed that there is a thermodynamic equilibrium in the system “sample-spectrometer” so that the Fermi levels of a sample and spectrometer are equalized. Then, if the binding energies, EB, of a given state are measured from common Fermi level the energy conservation reads hν = E B + φ + E kin

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +7 (812) 428 43 52. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by SPbSU Grant 11.37.656.2013. We gratefully acknowledge assistance from the bilateral Program “Russian-German Laboratory at HZB-BESSY II.” We also thank the staff of HZB for valuable technical assistance.

(1)

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where hν is the energy of X-ray photon, φ is the photoelectric work function, and Ekin is the kinetic energy of the electrons ejected from the EB level. Equation 1 allows calculating the binding energies EB of all of the electrons participating in the photoeffect based on measured Ekin. The binding energies, obtained from the XPS spectra (Figure 2) measured for γ-Al2O3 and am-Al2O3, were used to compare the X-ray absorption spectra of different atoms constituting the film on the common energy scale. The XPS data for α-Al2O3 were taken from the work.13 The Fermi level was chosen as zero-point of the energy scale.

REFERENCES

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