A Combined Photoelectron Spectroscopy and ab Initio Study of the

J. Phys. Chem. C 2008, 112, 19995–20006

19995

A Combined Photoelectron Spectroscopy and ab Initio Study of the Adsorbate System O2/PbTe(001) and the Oxide Layer Growth Kinetics Lada V. Yashina,*,† Tatiana S. Zyubina,‡ Ralph Pu¨ttner,§ Alexander S. Zyubin,‡ Vladimir I. Shtanov,†,⊥ and Egor V. Tikhonov† Department of Chemistry, Moscow State UniVersity, Leninskie gory, 119991 Moscow, Russia, Institute of Problems of Chemical Physics RAS, 142432 ChernogoloVka, Moscow Region, Russia, and Institut fu¨r Experimentalphysik, Freie UniVersita¨t Berlin, Arnimallee 14, D-14195 Berlin-Dahlem, Germany ReceiVed: May 11, 2008; ReVised Manuscript ReceiVed: September 13, 2008

The adsorption of molecular oxygen on PbTe(001) surfaces has been investigated using synchrotron radiation induced photoelectron spectroscopy (BESSY II, Berlin). The behavior of adsorbate system O2/PbTe(001) was also modeled in complementary DFT-B3LYP studies using a large-cluster approach. For several possible adsorption structures, the adsorption enthalpies, the changes of the effective charges, and the core-level shifts induced by adsorption were calculated. For the energetically most favorable adsorption structures, the calculated chemical shifts are in good agreement with the experimental observations. Oxidation was observed at exposures above ∼105 L of O2. In total, three steps of oxidation were observed. The first step of the reaction is associated with the formation of Te-O-Pb bonds resulting in Te0 states. The subsequent second step is characterized by a Te0 fTe4+ transformation with three O atoms attaching to each surface Te atom. As a result, -TeO32species are formed. The third step is associated with the growth of the planar layer of PbTeO3. In the range of exposures from 1011 to 1015 L, the kinetics of this layer growth can be described with the time-logarithmic law. The oxide growth rate does not depend on the presence of water vapor. 1. Introduction Modern studies of surface reactivity and chemisorption mechanisms at the atomic scale dictate to exploit a combined approach of experimental observations and ab initio modeling of the adsorbate system. Therefore, quantum chemical approaches became one of the most powerful tools to interpret experimental spectra in surface science.1,2 They can substantially support both the comprehensive interpretation of experimental results and the quantitative assessment of different groups of experimental data like spectroscopic results, which are sensitive to chemical bond formation, and like microscopic or diffraction data, which provide direct information on the atomic-scale geometry. Because of the high photon-energy resolution provided by third-generation synchrotron radiation facilities, photoelectron spectroscopy became a widely used tool to identify both the reaction products and the chemisorption mechanisms.2 One possibility to deduce the adsorption structures from the observed chemical shifts is to model the adsorption behavior with an ab initio approach and to calculate the chemical shifts that correspond to different possibilities for the bond geometries. A comparison of the calculated and experimental chemical shifts has the potential to provide the comprehensive understanding of the chemisorption mechanism.3 PbTe is a semiconductor compound which is of interest for both fundamental science4-6 and applications, for example, in the fields of IR optoelectronics and thermoelectricity.6 The geometry and the energy levels for the clean PbTe(001) surface were studied in detail both experimentally and theoretically.7-10 * To whom correspondence should be addressed. Phone: +7 495 939 2086; fax: +7 495 939 0998; e-mail: [email protected]. † Moscow State University. ‡ Institute of Problems of Chemical Physics RAS. § Freie Universita ¨ t Berlin. ⊥ Deceased.

Investigations on the adsorption of oxygen on semiconductors like PbTe are also of great importance since the oxygen has a direct impact on the performance of devices fabricated from these materials. In this context, numerous electrophysical studies of thin films, such as the Hall coefficient or conductivity measurements, showed that the surface oxidation of semiconductors results in dramatic changes of different device properties, see for example refs 11 and 12. Though these studies clearly demonstrate that these properties are sensitive to the interaction of oxygen with the materials, they lack detailed surface information which is essential to understand the underlying interaction mechanism. On the basis of previous adsorption studies, it is known that the oxygen adsorption on PbTe(001) surface proceeds in several steps.13-15 Classical adsorption studies performed by Green and Lee13 associated the first step with the formation of a peroxidelike surface complex that covers up to 70% of the PbTe(001) surface (at exposures of ∼105-106 L) and the second step with an oxide formation. Ultraviolet spectroscopy (UPS) studies of epitaxial (111) and polycrystalline films of PbTe have shown that the oxygen coverage increases at exposures around 105 L drastically from 40 to 70%. At exposures between 103 and 105 L of oxygen, PbTe valence band bends up at the surface because of electron transfer from the PbTe bulk to the adsorbed oxygen, which acquires a negative charge.14 At higher exposures, pronounced changes in the core-level spectra are observed. An oxygen coverage of 100% was observed at 106 L. The O-O bonds are broken already at an exposure of 3 · 107 L according to UPS and X-ray photoelectron spectra (XPS) reported by Hagstro¨m and Fahlman.15 Measurements performed in the range of 105-1012 L showed that the slope of the oxygen concentration curve decreases at exposures of around 109 L. In summary, these data suggest that at least three different steps can be distinguished, which correspond approximately to the ranges of

10.1021/jp804153g CCC: $40.75  2008 American Chemical Society Published on Web 11/21/2008

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103-105, 105-109, and 109-1012 L of oxygen exposure. At high exposures, a layer of PbTeO3 is formed.16 However, the chemical changes at the PbTe surface owing to oxygen exposure are not yet understood in detail especially in the range of 105-109 L. In two recent publications,16,17 the adsorption behavior of O2 on PbTe was modeled using small clusters and only one oxygen molecule. In these studies, it was found that (1) peroxide-like structures are stable; (2) oxygen is bonded not only with tellurium but also with lead; (3) the structures with both oxygen atoms bonded with different tellurium atoms are lower in energy than those where two oxygen atoms are bonded to the same tellurium; and (4) the chemical shift for the Te 4d level in the most stable oxidation structures amounts to ∼1 eV relative to pure PbTe. However, this simple model is not sufficient to describe the reactions occurring at the PbTe(001) surface in detail. First, structures with more than two oxygen atoms have to be considered to model reaction products like PbTeO3. Second, larger clusters are expected to provide sufficiently accurate estimations for the core-level shifts induced by the chemisorption, since in this case surface atoms have the correct coordination, and possible influences of the cluster border are minimized. From the experimental point of view, the chemical changes within one monolayer, that is, at low exposures, should be investigated since this situation matches the quantum chemical modeling. XPS studies using synchrotron radiation are characterized by a unique energy resolution required for such type of studies as well as by a high and tuneable surface sensitivity which allows one to observe the chemical modification of the uppermost surface layer in detail. The present report is devoted to combined photoemission and quantum-chemical modeling studies of the adsorption of oxygen on the surfaces of lead telluride. Besides, the oxide layer growth kinetics has been studied in a wide range of exposures using a laboratory Al KR source. By combining all data, a comprehensive description for the oxidation mechanism of PbTe(001) surface with O2 is reached. 2. Experimental Details The X-ray photoelectron spectra with high surface sensitivity were recorded at the synchrotron radiation facility BESSY II in Berlin, Germany, using the Russian-German beamline (RGBL). This beamline is equipped with a Petersen-type planegrating monochromator and provides high energy resolution.18 The spectra were taken with an experimental setup that consists of a preparation chamber and an analysis chamber. The latter one is equipped with a CLAM 4 (Thermo VG Scientific) electron energy analyzer. PbTe single crystals were obtained by the vapor-liquid-solid (VLS) method which is described in detail in ref 19. The crystals were p- and n-type samples with a typical dislocation density of 2 · 105 cm-2; low-angle grain boundaries and inclusions were practically absent. Clean surfaces were obtained by cleaving the crystals in the preparation chamber along the (001) plane. For the cleavage procedure and during the measurements, the sample holder was cooled with liquid nitrogen since this leads to better cleavage conditions and a decrease of the phonon broadening in the photoemission spectra. The cooling is also beneficial to prevent the possible evaporation of the oxidation products at UHV conditions. The quality of the cleaved surfaces was confirmed by low-energy electron diffraction (LEED) studies. The Pb 5d and Te 4d spectra were recorded using a photon energy of hν ) 125 eV and an analyzer pass energy (PE) of

Figure 1. The Te 4d spectra of (a) a clean PbTe(001) surface as well as (b) a PbTe surface after O2 exposure e1, (c) a PbTe surface after exposure e2, and (d) a PbTe surface after exposure e3. The solid lines through the data points represent the fit results and the subspectra of the contributions of the different spectral features. Vertical dotted lines labeled sx show for the different structures x ) 1-20 presented in Figure 4 the calculated positions of Te 4d5/2 lines.

2.5 eV; in this way, an overall (monochromator and analyzer) energy resolution of about 40 meV full width at half-maximum (fwhm) was obtained. The absolute energy scale is limited by the mechanics of the monochromator and is ≈0.2 eV. The detection angle was ≈80°, that is, close to normal emission. The surface area of the crystal was =5 × 5 mm2 while the diameter of the X-ray spot was estimated to be about 100 µm; its exact position at the sample surface was not precisely controlled. The exposure of the samples to O2 was performed in the preparation chamber at room temperature using high

Study of the Adsorbate System O2/PbTe(001)

J. Phys. Chem. C, Vol. 112, No. 50, 2008 19997

U(E) ) a + sS(E) + tT(E)

(1)

where S(E) is a Shirley background and T(E) is a Tougaard background with the parameter C ) 1643 eV2. For the spectra measured with the achromatic Al KR source, the satellites were subtracted prior to the fit analysis. Additional details on the fitting procedure are given below. 3. Computational Details

Figure 2. The Pb 5d spectra of (a) a clean PbTe(001) surface as well as (b) a PbTe surface after O2 exposure e1, (c) a PbTe surface after exposure e2, and (d) a PbTe surface after exposure e3. The solid lines through the data points represent the fit results and the subspectra of the contributions of the different spectral features.

purity gas (99.998 vol %). The surfaces of cleaved PbTe were studied directly after cleavage as well as after several consecutive oxygen exposures, namely, exposure e1 which corresponds to 2.5 · 105 L of O2, exposure e2 which corresponds to 109 L, and exposure e3 which corresponds to 1011 L. The oxidation of four additional p-type samples of PbTe was studied at essentially higher exposures (1011-1015 L) using an ESCALAB MKII spectrometer equipped with a hemispherical electron energy analyzer. For this, the Pb 5d, Pb 4f, Te 3d, Te 4d, and O 1s spectra were recorded in a constant analyzer pass energy mode (20 eV) using achromatic Al KR emission. The samples were cleaved in the preparation chamber at a base pressure of ∼10-8 mbar and were exposed for defined periods of time to dry and water-vapor-saturated oxygen at a pressure of 1 bar. For the data analysis, the spectra were fitted by the Gaussian-Lorentzian convolution functions with simultaneous optimization of the background parameters. The background was described by the equation

The interaction of a PbTe(001) surface with O2 molecules was also studied theoretically using density functional theory (DFT). In these calculations, the geometry optimization and vibrational frequency calculations were performed using the hybrid density functional B3LYP method, that is, Beck’s threeparameter nonlocal exchange functional with the nonlocal correlation functional of Lee, Yang, and Parr, with a LanL2DZ basis augmented by d-polarization functions (with the exponents equal to 0.237 (d) for Te, 0.164 (d) for Pb17); the pseudopotential LanL2 for Te and Pb atoms; and a 6-31G* basis for the O atoms (B3LYP/LanL2DZ*). We used GAUSSIAN-0320 programs to perform the calculations. We considered effective atomic charges calculated by Mulliken. The applied level of calculations allows to obtain a calculated length for gaseous PbTe molecules which agrees with the experimental values within an accuracy of 0.02 Å.21 The chemical shifts were estimated for each atom of interest in the initial state approximation as the difference in the electric potential (EP) at the site of the nucleus before and after the reaction with oxygen, which is calculated at the B3LYP level. These results are compared with the values extracted from the experimental spectra. With this approach, the atomic geometry of the absorption species was deduced and the mechanism for the surface reactions could be described on an atomic scale. The initial state approximation was chosen for calculations since the core-level shifts obtained within the static final-state approximation for these materials are usually considerably larger than the experimental values. The reason for this is that correlation effects are hardly taken into account within the latter approximation, and the correlation energy has an opposite sign with the relaxation energy. In this situation, even application of the simplest Koopmans’ theorem (and hence more complex approaches like calculations employing electric potentials) yields more realistic results than calculations performed within the pure final-state approximation. On the other hand, the experimental values of relaxation energy, for example, for the Te 3d level, can be evaluated from the Auger parameters of different compounds.22 As it has been shown for PbTe, the difference in Te 3d relaxation energies for different compounds, for example, PbTe, Te, and TeO2, deduced in such a way does not exceed 0.2-0.4 eV,23 which is comparable with the errors induced during experimental determination of chemical shift. In contrast to this, in the case of Pb lines (e.g., Pb 4f), the corresponding difference of Auger parameters between different compounds is essentially higher; it exceeds 1 eV. Thus, we can expect that the evaluated values of chemical shifts for tellurium levels should be more reasonable and more comparable with experiment than those calculated for lead levels. 4. Results and Discussion 4.1. Adsorption Mechanism. 4.1.1. The High-Resolution Photoelectron Spectra. The Te 4d and Pb 5d XP spectra of clean PbTe(001) surfaces and after the consecutive oxygen exposures e1, e2, and e3 are measured using photon energy of 125 eV and are shown in Figures 1 and 2. In detail, the Te 4d spectrum of a clean PbTe(001) surface is shown in Figure 1a


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TABLE 1: Summary of the Results for the Te 4d and Pb 5d Features Observed in PbTe Spectra after O2 Exposurea relative intensities feature

assignment/structure (see Figure 4 and Table 2)

Te I Te II Te III Te IV Te V Te VI

Te2-–Pb surface Te2-–Pb bulk Te0/4 Te0/18 Te4+/16 Te4+/18

Pb I Pb II Pb III

Pb-Te Pb-O-Te0/4 Pb-O-Te4+/18

ChStheo, meV

ChSexp, meV

exposure e2b

clean surface

exposure e1

1070 1580 4430 4540

Te 4d 0 308 1068 1476 3949 4260

0.59 0.41 0 0 0 0

0.38 0.36 0.24 0 0.02 0

0 0.23 0.24 0.32 0.12 0.09

410 570

Pb 5d 0 901 1046

1 0 0

0.84 0.16 1

0.32 0.48 0.20

0 0.18 0.22 0.28 0.20 0.12

exposure e3 0 0.1 0 0 0 0.9 0.10 0 0.90

a Given are the assignments, chemical shifts, ChS, and relative intensities of the different features in Te 4d and Pb 5d spectra obtained after different exposures to O2. The error bars for the chemical shifts are estimated to be less than 70 meV. b Left column refers to a p-type sample, right column to an n-type one.

and consists of two spin-orbit split peaks. In a thorough data analysis, these peaks can be decomposed into a surface (Te I) and a bulk (Te II) component with a surface core-level shift of 0.3 eV. The spectrum subsequent to exposure e1 is displayed in Figure 1b and consists of an additional spectral feature Te III, which is shifted to higher energies by 1.07 ( 0.05 eV relative to the feature Te I. The obtained peak parameters are summarized in Table 1. The higher binding energy for the feature Te III indicates a surrounding with a lower electron density, that is, the corresponding Te atoms are oxidized. After exposure e1, this is the case for about 30% of Te surface atoms. Exposure e2 results in the total modification of the surface layer, that is, the spectral feature Te I completely disappeared; see Figure 1c. After exposure e3, all tellurium surface atoms reacted forming the final oxidation product PbTeO3, which corresponds to spectral feature Te VI with a chemical shift of 3.96 eV relative to the feature Te I. Additionally, the features Te IV and Te V with the intermediate values of 1.48 ( 0.07 and 3.64 ( 0.05 eV, respectively, for the chemical shifts have to be employed for a satisfying curve fitting of spectrum subsequent to exposure e2, see Figure 1c. An assignment of the spectral features is given below. The Pb 5d spectra are presented in Figure 2. The spectrum of a clean surface consists of the spin-orbit split feature Pb I. The exposure to oxygen induces the spectral features Pb II and Pb III with chemical shifts 0.90 ( 0.07 eV and 1.046 ( 0.07 eV, respectively. The latter feature can be correlated with the final oxidation product while the feature Pb II is required to describe the spectrum after exposure e2. In addition, the Pb 5d level superimposes with the O 2s level, which increases in intensity with the amount of oxygen exposure. The obtained peak parameters are summarized in Table 1. We found no surface enrichment in lead after oxygen exposures e1, e2, and e3. As it follows from Table 1, the spectra obtained for n- or p-PbTe samples after exposure e2 show no essential difference within accuracy limits. Therefore, at least for this exposure, the oxidation rate and mechanism does not depend significantly on bulk carrier type, n or p. In summary, these experimental data suggest two different steps for the adsorption of oxygen on the PbTe(001) surface. The first step corresponds to the spectral feature Te III with chemical shift of 1.1 eV, and the second step corresponds to the features Te IV and Te V with chemical shifts of 3.6-4.0 eV.

4.1.2. The DFT Results. To predict the most stable adsorption structures, the behavior of an adsorbate system can be described in the first approximation in terms of the adsorption enthalpy. In this work, the adsorption enthalpy was estimated to be

∆Hads ) Estructure - (Ecluster + nEO2 ⁄ 2)

(2)

with Estructure being total cluster energy after adsorption, Ecluster being total cluster energy before adsorption, EO2 being total energy of O2 molecule, and n being the number of adsorbed oxygen atoms. The PbTe(001) surface was modeled as a cluster cut from the bulk structure. For the calculations, the two (PbTe)34 25/ 25/9/9 clusters displayed in Figure 3 (structures 1 and 2) were used. These structures differ by the atom in the center of the surface layer, Te for structure 1 and Pb for structure 2. These two possibilities allow to model accurately the reaction centers associated with either Te or Pb atoms. Both clusters have five central surface and subsurface atom positions each (5/5) being fully optimized. Neighboring atom positions were optimized only in the vertical direction, whereas the boundary atoms were fixed in their bulk positions. For separated adsorption geometries, the results were also tested using the larger cluster (PbTe)52 36/36/16/16 with a similar shape (structure 3); in this case, eight central atom positions have been fully optimized. It was found that an increase of the cluster size does not lead to significant changes in the geometry, the adsorption energy, the core-level shifts (changes of less than 0.2 eV), and the effective charges (changes of less than 0.03 e). Besides, it was found that chemisorption induces chemical shifts of less than 0.05 eV for all atoms, which are not directly involved in the bonds. Therefore, the oxygen adsorption can be treated easily as a number of local adsorption events on the surface. Peroxidelike structures are not considered in the present report, since the present spectra do not show any direct evidence for them; see below. In the calculations, structures modeling the adsorption of up to six oxygen atoms were taken into account. In Table 2, the central fragments of the structure and the adsorption energies of the different isomers corresponding to the different number of adsorbed oxygen atoms are displayed. Structures 4, 7, 10-12, 14, 16-18, and 20 show the adsorption on cluster 1, and structures 5, 6, 8, 9, 13, 15, 19, and 21 show the adsorption on cluster 2. The calculated chemical shifts and the corresponding total energies are also given in Table 2.

Study of the Adsorbate System O2/PbTe(001) TABLE 2: Summary of the DFT Results


20000 J. Phys. Chem. C, Vol. 112, No. 50, 2008 TABLE 2: Continued

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TABLE 2: Continued

a The structures containing ‘a’ in their titles refer to (PbTe)52 cluster. Those without ‘a’ in the structure title refer to (PbTe)34 cluster. For different clusters modeling the O2 adsorption, the energies, ∆Hads, are given relative to the value for the (PbTe)m cluster with nO2 molecules at infinite distance. The changes of the electric potential, ∆EP, are presented relative to the value for the (PbTe)m cluster for Te and Pb and relative to the O-Te bond in structure 4 for O. Given also are the Mulliken atomic charges Z*. b Subsurface Pb. c Te is bonded with subsurface O. d O is bonded with subsurface Pb. e Te is bonded with surface O. f O is bonded with surface Pb.


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Figure 3. Optimized geometry for three clusters modeling the PbTe(001) surface. Clusters 1 and 2 differ by the species of the central atom.

In the following, we shall discuss the results for the different structures. In this discussion, the chemical shifts for the Pb 5d and the Te 4d levels are given relative to the energy position of the surface atoms in structures 1-3. Structures 4-6 in Table 2 describe the different geometries which can be caused by one oxygen atom. The formation of structure 4 results in the minimal value of ∆Hads and it may, therefore, presumably be formed at low oxygen coverage. In this structure, the O atom is bonded with a surface Te atom and a subsurface Pb atom. The resulting values of chemical shifts are 1.07 eV for the Te 4d level and 0.41 eV for the Pb 5d level. Structure 5, where the O atom is positioned at the surface, is energetically less favorable and structure 6, which supposes on-top oxygen bonded to lead, is thermodynamically unstable. Structures 7-15 are the different isomers obtained after the adsorption of two oxygen atoms. The formation of structures 7-11 with both oxygen atoms located between the surface and the subsurface atomic layers of PbTe gives a substantial decrease of the total energy; among them, structure 7 is the most stable one. For this structure, the chemical shifts for the Te 4d and the Pb 5d levels are equal to 2.99 eV and 0.52 eV, respectively. In the isomeric structures 12-15, both oxygen atoms are on the surface, but their formation is energetically less favorable. In general, the chemical shift for tellurium atoms that are bonded with two oxygen atoms amounts to ∼3 eV, whereas the value for Te atoms bonded with only one O atom is in the range of 1.1-1.7 eV.

Figure 4. (a) The calculated chemical shift and (b) effective charge of a Te atom as a function of O atoms bonded to Te (lower x-axis) and the formal oxidation state of Te atom (upper x-axis). Experimental values for the chemical shifts taken from the NIST database24 are indicated with blue diamonds.

The adsorbate geometries 16 and 17 are composed of three oxygen atoms bonded with the same tellurium atom with the isomer 16 being more stable. In this structure, two O atoms are bonded in addition to the Te atom with two surface Pb atoms, and the third O atom is bonded with one subsurface Pb atom. For these two structures, the chemical shifts are 4.3 and 4.7 eV for the Te 4d level as well as about 0.5 eV for the Pb 5d level. Structures 18-20 describe the cluster bonded to four oxygen atoms. Structure 20 is characterized by four oxygen atoms that are bonded with the same tellurium atom [4O-Te]. It is energetically less favorable than structure 19, where all oxygen atoms are bonded to different Te atoms [4(Te-O)], and structure 18, where three oxygen atoms are bonded to one Te atom and the fourth oxygen atom is bonded to a different Te atom [3O-Te + O-Te]. For structure 20, the chemical shift for the Te 4d level is maximal, namely, 6.26 eV. The adsorption of six O atoms (see, e.g., structure 21) is combinations of the structures considered above, that is, the O2/PbTe(001) adsorbate system behaves additively and can be described as a combination of the local elemental events given above. 4.1.3. Comparison with Experiment. The theoretical results have shown that the chemical shift of the 5d level in a Pb atom

Study of the Adsorbate System O2/PbTe(001) bonds to oxygen dependents predominantly on the atomic position within the cluster. For a Pb atom at the surface, the chemical shift is relatively small, namely, only 0.1-0.4 eV. For subsurface Pb atoms, the chemical shift is higher at 0.7 eV. The experimentally observed values are 0.9-1.05 eV higher than the calculated ones. However, the Pb 5d spectra are in general not sensitive to the oxygen adsorption since lead remains in the same formal oxidation state. In contrast to this, the Te 4d spectra provide much more information on the interaction with oxygen since the formal oxidation state of tellurium changes. This can be seen by the vertical dotted lines in Figure 1, which indicate for all structures in Table 2 the theoretical positions of the Te 4d5/2 lines. Obviously, the theoretically estimated chemical shifts can be separated into four groups, namely, 1.1-1.7, 2.2-2.9, 4.4-4.7, and 6.3-6.4 eV. Each of these groups is defined by tellurium atoms which have the same number of oxygen atoms in their environment. This can also be seen in Figure 4, where the calculated chemical shifts and Mulliken charges are given as a function of the number of oxygen atoms attached to the Te atom. Both these plots show a linear behavior with only minor scattering, that is, the correlations between the effective charge of the Te atom and the chemical shift can clearly be established. So, for example, the chemical shift for a Te atom bonded to one oxygen atom is in the range of 1.1-1.7 eV; the exact value depends on adsorbate system geometry. This is in line with the spectral features Te III and Te IV of the experimental spectra presented in Figure 1. In this case, the tellurium atoms are formally transformed to the neutral charge state since its effective charge is close to zero. Figure 4a also shows experimental chemical shifts (blue diamonds) for several Te compounds taken from the NIST database.24 These data also show a linear correlation between the chemical shift for the formal charge state. In particular, they exhibit a chemical shift of approximately 1 eV between PbTe and elemental tellurium, which confirms the given assignments. In summary, this plot illustrates a useful correlation between the calculated chemical shifts and the formal oxidation states of Te. If two oxygen atoms are attached to one Te atom, its Mulliken charge acquires a value of 0.6 e. This corresponds within our findings to the formal oxidation state Te2+ and results in a chemical shift of 2.2-2.9 eV. This oxidation state cannot be found in the Te 4d spectra of Figure 1. This is in agreement with our calculations since for such structures the calculated adsorption enthalpy per oxygen atom is generally higher than for the structures with O-Te as well as 3O-Te bond formations. Finally, stable bulk compounds of Te2+ are not known so that experimental chemical shifts are not available. The configuration with one Te atom bonded to three oxygen atoms turned out to be the most stable one in our calculations. The corresponding chemical shift and the effective charge are 4.5 eV and 1.4 e, respectively. This situation fits to the formal charge state Te4+, and the chemical shifts agree reasonably well with those of the spectral features Te V and Te VI in Figure 1. Moreover, the latter spectral feature describes the final oxidation product. Four O atoms bonded to one Te atom lead to the formal charge state Te6+. The resulting effective charge is equal to 1.9 e and the chemical shift is equal to 6.3 eV; however, no experimental chemical shift of such value is observed. This is in line with the theoretical adsorption enthalpy values discussed above and the fact that Te6+ is only found after high-temperature oxidation of PbTe powder.25

J. Phys. Chem. C, Vol. 112, No. 50, 2008 20003 The formation of possible peroxide-like structures would lead to chemical shifts that are lower than those for the structures considered above since the electron density transfer from Pb and Te atoms to O atoms should be less in this case. In the present studies, we did not find spectral features which could be related to peroxide-like structures, however, a final answer on the existence of such structures subsequent to the adsorption of oxygen on PbTe(001) surfaces requires additional studies both experimentally and theoretically. In particular, in situ experiments are required since the present experiments do not allow observing peroxide-like structures if the O-O bonds break on a time scale of minutes. Thus, on the basis of the experimental data and DFT results, the following oxygen adsorption mechanism can be proposed. The O-O bond breaks on a time scale which is too short to be observed with the given experimental setup. At low coverage, each O atom forms a chemical bond with one Te atom leading to a Te2-fTe0 transformation. The calculated adsorption enthalpies for structures with one and two oxygen atoms attached to the cluster show that it is energetically more beneficial to form two O-Te bond structures instead of one 2O-Te bond structure. Therefore, we expect that the surface is uniformly covered with O-Te bond structures. Once all tellurium atoms at the surface are bonded to one oxygen atom, additional exposure will lead to the attachment of two more oxygen atoms to one Te atom resulting in Te4+. The presence of the spectral features Te IV and Te V with chemical shifts slightly different from those for Te III and Te VI allows us to assume that this transformation is performed via different geometries including surface and subsurface oxygen positions. 4.2. Oxidation Kinetics. The oxidation kinetics was studied using higher oxygen exposures (3 · 1011 to 1 · 1015 L) and laboratory equipment that provides Al KR radiation. In the spectra of freshly cleaved surfaces (not shown here), no individual states are resolved because of the low-energy resolution and surface sensitivity. Therefore, we describe the structure in the Te 3d5/2 spectrum (Te i) with one singlet each (the spin-orbit splitting of the Te 3d level is 10.34 eV) and the spin-orbit split structures in the Pb 4f (Pb i), Pb 5d, and Te 4d spectra with one doublet each; all these features were assigned to the respective bulk state. Figure 5 shows a sequence of spectra taken at different angles for a PbTe(001) surface which has undergone an exposure of 1015 L. The Te 3d5/2 spectrum consists of the features Te i and Te ii which correspond to Te2- and Te4+ states, respectively, with the separation of 3.9 eV. The Pb 4f spectra also consist of two features (Pb i and Pb ii) which are split by 1.1 eV. Obviously, the spectral features Te ii and Pb ii can be related to the oxidation products. The PbTe(001) oxidation kinetics is illustrated in Figure 6. Figure 6a displays in a semilogarithmic presentation the relative intensities of the spectral features related to oxidation as a function of the exposure obtained for the different samples. Two samples (marked as samples 1 and 2 in Figure 6a) were exposed to water-saturated oxygen. Nevertheless, they do not show any systematic deviation in the oxidation kinetics from the samples exposed to dry oxygen. Therefore, one can conclude that the presence of water vapor in oxygen does not influence the oxidation rate within our accuracy limits. The oxide layer composition was estimated to be PbTeO3. This composition was obtained from the intensities of the O 1s peak (not shown here) and the relative intensities of the spectral features Te ii and Pb ii as well as the atomic sensitivity factor ratios Pb/Te, O/Pb, and O/Te. These values were obtained from


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Figure 5. Te 3d5/2 and Pb 4f spectra of PbTe(001) after oxygen exposure of 1015 L recorded at different detection angles (0° corresponds to normal emission). Solid lines show the fit results.

An analysis of angular dependence of the relative intensities of the oxygen-related features of the spectra in Figure 6 is presented in Figure 7. The angular dependence is in agreement with the Hill equation

(

h ) λ cos θ ln 1 +

Figure 6. PbTe(001) oxidation kinetics: (a) oxidized species fraction in different spectra for all experiments and (b) oxide layer thickness as calculated from the different spectral lines. Part of the data points origin are from refs 14 and 15.

many reference samples, namely, PbTe, PbTeO3, TeO2, PbSO4, PbSeO3, and PbSeO4. Besides, thorough analysis of the data obtained for all sets of experiments showed that the intensity ratios Pb 4f/Te 3d5/2 and Pb 5d/Te 4d do not change as already observed for the oxidation of the first monolayer. Therefore, one can exclude the evaporation of oxidation products or any kind of surface segregation.

Isurface Ibulk

)

(3)

where h corresponds to the layer thickness and λ ) 28.8 Å (Pb 4f), 30.8 Å (Pb 5d), 21.5 Å (Te 3d), and 30.4 Å (Te 4d) to the inelastic mean free path estimated using the Tanuma-PowellPenn (TPP 2m) formula.26 In addition, θ is the detection angle, Isurface is the intensity of the signal originating from the oxidized layer, and Ibulk is the intensity from the unoxidized atoms. The agreement of the experimental data with the Hill equation suggests that the layer has a uniform thickness, that is, it does not have to be described with islands. This finding is supported by complementary scanning tunneling microscopy (STM) studies (not shown here), which show monatomic steps at the surface after an exposure of 1012 L, that is, rougher surface structures have not been formed. These results allow the calculation of the oxide layer thickness on the basis of the Hill equation. The calculated oxide layer thicknesses are presented in Figure 6b together with data obtained from the literature.14,15 The layer thicknesses obtained from different spectral lines (Pb 4f, Pb 5d, Te 3d, and Te 4d) as well as for different crystal samples are in a good agreement with each other with a maximal scattering of (2 Å. The data obtained using a photon energy of 125 eV are also shown. As it can be seen from the solid line in Figure 6b, the oxidation kinetics can be described in the range of exposures from 1010 to 1016 L with the direct logarithmic law

1 1 h(t) ) ln(ab) + ln(t + t0) b b

(4)

which is the solution of the Elovich equation.27 Here, a ) 2 · 10-8 and b ) 0.85 are constants, and t0 ) 2 · 1010 L approximately corresponds to the oxygen adsorption process occurring within one monolayer.


J. Phys. Chem. C, Vol. 112, No. 50, 2008 20005 as well as with data available from literature, we were able to derive a coherent picture of the PbTe(001) surface oxidation. From this, we can describe the oxidation process in four steps from which we have studied steps 2-4. The first step (range A in Figure 6) is evidently related to the formation of peroxidelike structures17 which occurs at exposures below ∼105 L;13 it is accompanied by gradual bending up of the valence band because of adsorbate-related surface states.14 The present study gives no additional information on this matter. Under oxygen exposures of 2 · 105 to 2 · 1010 L (ranges B and C in Figure 7), chemical reactions within the uppermost monolayer are observed. Thus, the second step is associated with a formation of relatively stable oxide structures with one oxygen atom attached to each tellurium atom. This gives rise to a Te0 state (range B). The third step includes the formation of -TeO3 particles in the surface layer (range C). As soon as the uppermost surface layer is completely modified, the final oxidation product PbTeO3 starts to grow as a layer of uniform thickness. In the range above 2 · 1010 L, the oxide growth kinetics obeys the time-logarithm law (range D). In summary, we arrived at a consistent picture for the surface oxidation of PbTe(001) including the oxide layer growth kinetics. In this context, DFT calculations using large-cluster approach turned out to be a useful tool to predict the structures of the oxygen-related surface species as well as to understand the spectral features observed in X-ray photoelectron studies.

Figure 7. The angular dependence of the oxidized species fraction (a) Te 3d5/2 and (b) Pb 4f.

Although the direct logarithmic law describes rather accurately the oxidation of several metals, metal alloys, and semiconductors at low temperature, the theoretical interpretation of this law has been a challenge to surface scientists. This can be seen by the fact that several models have been proposed to explain it.27,28 Three examples given in these publications as explanation are (1) an oxidation while the diffuse oxide-substrate interface is formed,27 (2) a progressive roughening of the surface, and (3) an oxidation limited by the tunneling of the electrons from interfacial substrate atoms to the O2 molecule.28 Therefore, the exact physical meaning of the constants a and b in eq 4 depends on the detailed oxidation mechanism and requires additional knowledge as well as an accurate solution of all kinetic equations. Nevertheless, the constant b expresses from the practical point of view the oxidation rate in the exponential oxidation regime and allows to estimate or control oxidation in semiconductor processing. 5. Summary and Conclusions In the present studies, X-ray photoelectron spectra and a theoretical description of the adsorbate system O2/PbTe(001) were obtained, and it turned out that the theoretical results describe the spectral features quite well. In particular, the theoretical chemical shifts for the tellurium agrees well with the experimental values. In addition, experimentally observed tellurium oxidations states (Te0 and Te4+) correspond to the most stable structures predicted by the calculations. By comparing the quantum chemical calculations with the experimental results

Acknowledgment. The SXPS experiments were performed as a part of the bilateral Program Russian-German Laboratory at BESSY II. The calculations were performed at the calculation center of IPCP RAS and at the ZEDAT of the Freie Universitat Berlin. The authors would like to thank the Russian Foundation for Basic Research for partial financial support. We also thank M. V. Poygin and V. S. Neudachina for participation in the experiments and Dr. S. Yu. Vassiliew for the STM work. References and Notes (1) Bagus, P. S.; Illas, F.; Pacchioni, G.; Parmigiani, F. J. Electron Spectrosc. Relat. Phenom. 1999, 100, 215. (2) Surface Analysis by Auger and X-ray Spectroscopy; Briggs, D.; Grand, J. T., Eds.; IMPublication: Chichester, U.K., 2003. (3) Becker, U.; Hochella, M. F., Jr. Geochim. Cosmochim. Acta 1996, 60, 2413. (4) Ahmad, S.; Mahanti, S. D.; Hoang, K.; Kanatzidis, M. G. Phys. ReV. B 2006, 74, 155205. (5) Hummer, K.; Grueneis, A.; Kresse, G. Phys. ReV. B 2007, 75, 195211. (6) Lead Chalcogenides: Physics and Applications; Khokhlov, D., Ed.; Gordon&Breach: New York, 2002. (7) Ma, J.; Jia, Yu.; Song, Y.; Liang, E.; Wu, L.; Wang, F.; Wang, X.; Hu, X. Surf. Sci. 2004, 551, 91. (8) Satta, A.; de Gironcoli, S. Phys. ReV. B 2000, 63, 033302. (9) Lazarides, A. A.; Duke, C. B.; Paton, A.; Kahn, A. Phys. ReV. B 1995, 52, 14895. (10) Yashina, L V.; Neudachina, V. S.; Tikhonov, E. V.; Shtanov, V. I.; Molodtsov, S. L.; Poyguine, M. V. BESSY Annu. Rep. 2003, 431. (www.bessy.de). (11) Rogacheva, E. I.; Tavrina, T. V.; Nashchekina, O. N.; Volobuev, V. V.; Fedorov, A. G.; Sipatov, A. Yu.; Dresselhaus, M. S. Thin Solid Films 2003, 423, 257. (12) Kreizman, R.; Traistman, N.; Shaked, M.; Dashevsky, Z.; Dariel, M. P. Key Eng. Mater. 2007, 336-338, 875. (13) Green, M.; Lee, M. J. J. Phys. Chem. Solids 1966, 27, 797. (14) Sun, T. S.; Byer, N. E.; Chen, J. M. J. Vac. Sci. Technol. 1978, 15, 585. (15) Hagstro¨m, A. L.; Fahlman, A. Appl. Surf. Sci. 1978, 1, 455. (16) Yashina, L. V.; Tikhonov, E. V.; Neudachina, V. S.; Zyubina, T. S.; Chaika, A. N.; Shtanov, V. I.; Kobeleva, S. P.; Dobrovolsky, Yu. A. Surf. Interface Anal. 2004, 36, 993. (17) Zyubina, T. S.; Neudachina, V. S.; Yashina, L. V.; Shtanov, V. I. Surf. Sci. 2005, 574, 52.

20006 J. Phys. Chem. C, Vol. 112, No. 50, 2008 (18) Gorovikov, S. A.; Molodtsov, S. L.; Follath, R. Nucl. Instrum. Methods A 1998, 411, 506. (19) Yashina, L. V.; Shtanov, V. I.; Yanenko, Z. G. J. Cryst. Growth 2003, 252, 68. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;

Yashina et al. Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (21) Benran, K. Bond lengths and angles in gas-phase molecules, 3rd ed.; Maruzen Company, LTD, 1984. (22) Moretti, G. J. Electron Spectrosc. Relat. Phenom. 1998, 95, 95. (23) Waddington, S. D.; Weightman, P.; Matthew, J. A. D.; Grassie, A. D. C. Phys. ReV. B 1989, 39, 10239. (24) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble, J. R. NIST XPS database, version 3.4. http:// srdata.nist.gov/xps (Accessed April 2008). (25) Tananaeva, O. I.; Sapozhnikov, R. A.; Novoselova, A. V. Inorg. Mater. (Russ.) 1969, 5, 737. (26) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1993, 21, 165. (27) Cerofolini, G. F.; La Bruna, G.; Meda, L. Appl. Surf. Sci. 1995, 89, 361. (28) Cerofolini, G. F.; Mascolo, D.; Vlad, M. O. J. Appl. Phys. 2006, 100, 054308.

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