Relationship between Ta Oxidation State and Its Local Atomic

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C: Physical Processes in Nanomaterials and Nanostructures

Relationship between Ta Oxidation State and Its Local Atomic Coordination Symmetry in a Wide Range of Oxygen Non-Stoichiometry Extent of TaO x

Sergey Kasatikov, Elena O. Filatova, Sergei Sakhonenkov, Aleksei S. Konashuk, and Anna Makarova J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12053 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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The Journal of Physical Chemistry

Relationship between Ta Oxidation state and Its Local Atomic Coordination Symmetry in a Wide Range of Oxygen Non-stoichiometry Extent of TaOx S. Kasatikov1, E. Filatova1,*, S. Sakhonenkov1, A. Konashuk1, A. Makarova2 1) Institute of Physics, St-Petersburg State University, Ulyanovskaya Str. 3, Peterhof 198504, St. Petersburg, Russia; 2) Institute for Solid State and Materials Physics, Technische Universität Dresden, 01062 Dresden, Germany. *Corresponding author email: [email protected] Abstract Evolution of electronic and atomic structure of amorphous Ta2O5 during sputtering by Ar+ ions was investigated by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. XPS Ta 4f spectra were analyzed through spectral decomposition with paying special attention to inelastic scattered electron background subtraction from the Ta 4f spectra. The decomposition revealed formation of Ta4+, Ta3+, Ta2+, Ta1+ and Ta0 chemical states during the sputtering. The dynamics of the Ta chemical states evolution was analyzed and referred to the modification of the local atomic structure revealed by NEXAFS O K – edge absorption spectra analysis. The transformation of the O K – edge spectra during the sputtering suggests preservation of a significant part of octahedrons (the structural units of stoichiometric amorphous Ta2O5), which serve as a matrix for low-symmetry structural units transformed from the octahedra during the sputtering. Possible mechanisms of the octahedra transformation were discussed basing on the O K – edge spectra and orbital correlation diagrams. It is noteworthy that the experiment showed a possibility of complete Ta metallization by Ar+ ions bombardment of amorphous Ta2O5 on Si substrate. 1 ACS Paragon Plus Environment

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1. Introduction Knowledge of electronic and local atomic structure is a fundamental aspect for studies of materials applied in electronic devices. Electronic properties of these materials are directly related to the chemical state of the constituent atoms and their local arrangement. Stoichiometric tantalum oxide Ta2O5 and non-stoichiometric TaOx have been largely studied during the last decades due to the wide range of its possible applications in electronic devices. Ta2O5 is a well-known high-k dielectric material that is used in commercial high-density capacitors for dynamic random access memory (DRAM) applications.1,2 Another attractive property of tantalum pentoxide is a capability to preserve oxygen deficiency (Ta2O5-x), which is provided by oxygen vacancies in the material. This feature is efficiently exploited for purposes of resistive random access memory applications (ReRAM).3–7 One of the most perspective type of ReRAM is based on the resistive switching mechanism induced by oxygen vacancies migration in a memorizing layer. The switching consists of growth/disruption of conductive filament formed by oxygen vacancies depending on the applied voltage. Ta2O5/TaOx structure appeared to be a perspective system for the recording medium application.5,8–10 The non-stoichiometric TaOx layer with relatively high conductivity, where x is less than 2.5, is supposed to store O vacancies, while the memory cell is in the highest resistance state, and to emit the vacancies to the Ta2O5 layer with further formation of the conductive filaments. Moreover, Ta2O5/TaOx bi-layered structure is also considered as a material for bidirectional selectors in ReRAM devices, which are necessary for the memory cells operation in high-density ReRAM arrays.5 To date, the electronic and local atomic structure of Ta2O5 and nonstoichiometric TaOx has been investigated in numerous works by XPS and XAS methods.9,11–24 The major part of these studies is focused on the effect of oxygen 2 ACS Paragon Plus Environment

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deficiency on the electronic properties of tantalum oxide. These works can be divided into two diametrically opposed groups according to the process of oxygen deficiency creation. The first group took advantage of reactive sputtering deposition of Ta films on a substrate varying the rate of O2 gas flow in the deposition chamber to produce TaOx with different extent of oxygen deficiency.12,14,19 The second one exploited the effect of oxygen preferential sputtering by Ar+ ions in Ta2O5 to reduce the concentration of O.9,15,18,21–24 Despite the difference in the approach to the nonstoichiometric TaOx preparing, Ta 4f photoelectron spectra of TaOx reported in previous studies show a common tendency: intermediate Ta oxidation states appear with variation of oxygen concentration. However, there is a lot of controversy about the reported results on the Ta chemical states dynamic and the diversity of the Ta chemical states formed during the variation of oxygen concentration.22 This discrepancy indicates a strong dependency of the Ta oxidation states on experimental conditions of the non-stoichiometric oxide film preparing. Nevertheless, the valence band (VB) photoelectron spectra show quite similar behavior with variation of the oxygen

concentration

regardless

the

differences

in

the

TaOx production

techniques.14,19,25 The non-stoichiometry of tantalum oxide is accompanied by the formation of defect electron states about 2 eV above the VB maximum, which are ascribed to the presence of oxygen vacancies basing on theoretical calculations.25,26 Tsuchiya et al. thoroughly investigated the local atomic structure of amorphous TaOx with different x (2.95 – 1.86) using near edge absorption fine structure (NEXAFS), extended absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS) and Raman scattering spectroscopies (RSS).12 The work is focused on the evolution of the polyhedral structure of the reactive sputtered amorphous TaOx film with variation of the non-stoichiometry extent. The study revealed that the non-stoichiometry of TaOx with 2.5 > 𝑥 > 1.86 is achieved by both the formation of Ta – O polyhedra with lower coordination number (an octahedral 3 ACS Paragon Plus Environment


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TaO6 is assumed to transform to a square pyramid TaO5p) and variation of the polyhedra conjunction from corner- to edge- and plane-sharing. At the same time, a small admixture of intermediate chemical states of Ta atoms appears as one can see from the XPS Ta 4f photoelectron spectra presented in the paper. Denny et al. also studied the local atomic structure of amorphous TaOx film deposited by radio frequency sputtering method at different O2 gas flow rate which provides different x in TaOx (2.44 - 2.50).19 In this paper, Ta – O average bond length depending on the different x was analyzed by EXAFS spectroscopy. The authors report about a decrease of coordination number of the first Ta – O shell and a considerable growth of the average Ta – O bond distance with oxygen concentration decreasing. However, so far, no any thoroughly study of the relation between Ta oxidation state and its local atom coordination symmetry in a wide range of the oxygen nonstoichiometry extent has been done. In this work we performed a simultaneous analysis of the Ta chemical state evolution along with transformation of the local Ta atom coordination symmetry during Ar+ ions sputtering utilizing XPS and NEXAFS techniques. An amorphous Ta2O5 film was successively sputtered by Ar+ ions in order to provide a gradual decrease of the x value in TaOx. NEXAFS and XPS methods were used after each step of the sputtering procedure to obtain information on the electronic structure of TaOx and symmetry of the local surroundings of the Ta atoms. Moreover, special attention was paid to the XPS spectra deconvolution procedure to trace the relation between the oxidation state of Ta atom in TaOx with different x (𝑥 ≤ 2.5) and its local surroundings formed by O atoms. 2. Experimental

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The studied amorphous Ta2O5 layer of 50 nm thickness was fabricated on Si (100) substrate using reactive sputtering deposition technique. To trace the dynamic of the appearance of intermediate Ta oxidation states, Ar+ ion sputtering was applied using in situ ion beam with the sputtered area size being 15 mm × 15 mm that exceeded the size of the sample (10 mm × 10 mm). The energy of the incident Ar+ ion beam was varied between 2.0 keV at early stages and 2.9 keV at later steps of the sputtering to maintain the process of O preferential sputtering. The grazing incident angle of the beam was 200. The spatial profile of the beam was Gaussian and the density of the ions differed less than by 10% at the edges of the sample compared to the middle point of the sample. In order to provide a gradual decrease of the x in TaOx, the sample was sputtered gradually by many steps. During the experiment, direct measuring of the ion beam current density was not available due to the experimental station configuration. However, this parameter was being controlled indirectly by measuring the current from the sample. The value of the sample current was being maintained at 3.5 mkA at each sputtering step. Prior to the measurements, the surface of the sample was cleaned by Ar+ ions sputtering at energy of 0.5 keV during 10 minutes and at grazing incident angle of 200. Structural and chemical characterization was done using X-ray photoelectron spectroscopy (XPS) and the near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The XPS and NEXAFS measurements were performed at RGL-station on Russian-German beamline (RGBL) of BESSY II synchrotron light source of Helmholtz-Zentrum Berlin. The XPS spectra of Ta 4f line and VB were taken at the excitation photon energy of 200 eV using a hemispherical electron energy analyzer (Specs Phoibos 1500). All the photoelectron spectra were collected with the combined analyzer and monochromator energy resolution better than 430 meV. Also, a survey spectrum of 5 ACS Paragon Plus Environment


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the sample was acquired at the excitation photon energy of 700 eV at each step of the sputtering. The binding energy scale of the spectra was calibrated by Si 2p photoelectron line (Eb = 99.5 eV) recorded at the final steps of the sputtering when the signal from the Si substrate was significant due to the thickness reduction of the tantalum oxide layer. The energy scale of the spectra at earlier sputtering stages was calibrated by the binding energy position of Ta5+ 4f7/2 peak. The position of Ta5+ 4f7/2 (26.3 eV) was derived from the survey spectrum of stoichiometric Ta2O5 obtained in a wide binding energy scale (0 – 650 eV) calibrated by C 1s line position (Eb = 284.8 eV) of adventitious carbon contamination. The NEXAFS spectra were obtained by monitoring the total electron yield (TEY) in a drain current mode and partial electron yield (PEY) with 400 eV retarding grid potential. These two different methods of spectrum recording provide information both from the surface region (PEY) and from the bulk (TEY). The probing depth of the methods is mainly defined by inelastic mean free path of the electrons collected and was about 10 nm and 3 nm in TEY and PEY (with 400 eV retarding potential), respectively. Calibration of the photon energy scale was performed by measurement of Au 4f7/2 photoelectron peak (Eb = 83.95 eV) in 1st and 2nd orders of diffraction. The actual photon energy was equated to a difference between the 1st order and 2nd order Au 4f7/2 kinetic energies. The XPS and NEXAFS measurements were performed after each step of the sputtering. The photon energy during the XPS measurements and the voltage of the retarding grid potential of PEY mode were chosen in such a manner to achieve an equal probing depth of the XPS spectra and NEXAFS spectra in the vicinity of O K – edge absorption and to obtain the spectra exactly from the near-surface region that is modified by Ar+ ions sputtering. According to Benito and Palacio,22 the width of the modified layer is expected to be about 3 nm. 6 ACS Paragon Plus Environment

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3. Results and Discussion 3.1 XPS Ta 4f spectra Figure 1 shows Ta 4f photoelectron spectra measured at the excitation photon energy of 200 eV from the pristine Ta2O5 sample and those observed after different sputtering steps. The spectra were normalized to the total integral intensity value of Ta 4f line of the Ta2O5, which corresponds to Ta5+ state. Such normalization makes the Ta 4f spectra comparable and allows us to trace the redistribution of Ta 4f peaks intensity between Ta5+ and other tantalum states appeared during the sputtering. The analysis of the Ta 4f spectra of stoichiometric Ta2O5 before the sputtering reveals a double peak structure presence with spin–orbit splitting of 1.91 eV and the integral intensity ratio of 4 3 between the components which correspond to the spinorbit doublet of Ta5+. During the sputtering, new features appeared in the spectrum at lower binding energy compared to the Ta5+ peaks position. These changes reflect new intermediate Tan+ chemical states appearance in TaOx, where n is less than 5. As a result of charge transfer between O and Ta atoms, the electrostatic core potential of the Ta atom is differently screened at different oxidation states, which leads to different binding energy of the Ta 4f doublet peaks position limited by the highest Eb assigned to the highest possible oxidation state of Ta in tantalum oxide (Ta5+) and the lowest Eb associated with metallic Ta, which is formally Ta0. As follows from Figure 1, at the early stages of the sputtering (0 – 30 min.) the Ta 4f photoelectron spectrum of the TaOx shows smooth transformation to a mixture of signal from Ta5+ and intermediate oxidation states. However, after 30 minutes of total sputtering time the spectrum changes weakly and stagnation in the spectrum evolution is observed (30 – 80 min.), which points to some stable mixture of the stoichiometric oxide and sub-oxides. Nevertheless, further increase in total sputtering time finally leads to a strong alternation in concentration of Ta5+ and intermediate Ta 7 ACS Paragon Plus Environment


oxidation states with lower n in Tan+ appears in the spectrum. Due to the complexity of the spectra, their deconvolution was carried out to trace the evolution of the components

corresponding

to

different

Ta

chemical

states.

before sputtering 20 min. 30 min. 50 min. 80 min. 120 min. 230 min. 480 min.

Normalized intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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32

30

28

26

24

22

20

18

Binding energy, eV

Figure 1. Ta 4f photoelectron spectra from the Ta2O5 sample before and after different Ar+ ion sputtering steps (after 20, 30, 50, 80, 120, 230 and 480 min. of total sputtering time).

3.2 Fitting results of Ta 4f spectra 3.2.1 Inelastic scattered photoelectrons background.


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Up to now, plenty works have been published where TaOx with different x is analyzed by means of XPS and deconvolution of XPS Ta 4f spectra is performed.9,13,15,20,21,23–25 However, many of them neglect the role of inelastic scattered photoelectrons background in the Ta 4f spectrum deconvolution. Mostly, classical Shirley’s subtraction of the background is widely used as a standard procedure,27,28 which is based on the artificial relation between the intensity of the photoelectron peaks and the background intensity and does not consider the material properties accounting for the inelastic scattering of the photoelectrons. Taking into account the complexity of the Ta 4f spectra at the latest stages of the sputtering due to the presence of intermediate Ta chemical states and, thus, large number of principle components, one has to apply a more physically justified model of the background description to minimize the effect of the background subtraction uncertainty on further fitting results. In principle, an inelastic scattered electrons background of an XPS spectrum can be calculated according to the formula:

𝑇(𝐸) =

∫

∞

𝐹(𝐸′ ― 𝐸)𝑆(𝐸′)𝑑𝐸′ ,

𝐸

(1) where F(x) is an energy loss function (ELF) representing the probability that a photoelectron will undergo inelastic scattering and lose energy (𝐸 ― 𝐸′); 𝑆(𝐸′) is the measured spectrum.29 According to Tougaard,29 the ELF can be determined experimentally by reflection electron energy loss spectroscopy (REELS) instead of calculations based on the dialectical formalism. However, the ELF obtained by this method tends to overestimate the effect of surface excitations on electron energy losses in comparison 9 ACS Paragon Plus Environment


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to the electrons in XPS since the backscattered electrons pass the surface region twice in REELS. Tougaard also supposed that, in order to get higher accuracy of description of the photoelectron energy losses in XPS spectrum, the REELS spectrum has to be taken at higher primary energy of incident electrons than the XPS peak energy. This is so because the ELF determined by REELS at higher primary energy has less contribution from the surface region and, hence, the energy-loss processes have a higher resemblance to those taking place in XPS. Moreover, the ELF shows a general tendency to vary slowly with primary energy of REELS electrons.29 Another important contribution of Tougaard to the XPS spectra analysis is the introduction of a Universal cross-section concept. The idea consists in dividing solids into universality classes based on the width of the dominating shape of their inelastic electron cross-section. The Universal cross-section is expressed mathematically by a two or three parameter function depending on the class of materials. This approach allows avoiding of time-consuming calculations and measurements for XPS background determination and introduces flexibility to the model of the background description, which is needed when a composition of a studied sample is not known precisely or changes during an experiment. According to the classification suggested by Tougaard,29 Ta2O5 can be attributed to a class of materials with FWHM of the dominating shape of the ELF lying in the 15 - 20 eV range. Considering the isolating properties of amorphous Ta2O5 and its band gap being about 4.3 eV, 25,30 a three-parameter function with an additional band gap parameter T0 offered by Tougaard is suitable for the approximation of the experimental ELF of Ta2O5:

𝜆𝐾 = 𝜃(𝑇 ― 𝑇0) ∙

𝐵𝑇 (𝐶 ― 𝑇2)2 + 𝐷𝑇2

, (2)


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where B, C, D are parameters, 𝜃(𝑇 ― 𝑇0) = 1 for 𝑇 > 𝑇0 and 0 for 𝑇 < 𝑇0; 𝑇0 is the band gap width.29 So far, Ta2O5 and TaOx have been studied by REELS and EELS in several works at different energy of the incident electron beam (primary electron energy) (200 eV – 40 keV).31–34 The photoelectrons contributing to the XPS Ta 4f line taken at excitation photon energy of 200 eV have primary kinetic energy of about 170 eV. However, there is a lack of experimental REELS or EELS spectra of Ta2O5 acquired at low primary energies in literature. Shvets et al. performed REELS measurements of Ta2O5 for 200 eV incident electron beam, however, the spectrum is expressed in arbitrary units which makes the task of the ELF calculation from the spectrum rather complicated.32 Moreover, according to Tougaard, such low primary energy of the electron beam in REELS can cause a strong deviation of the obtained ELF from that of Ta 4f photoelectrons due to a significant contribution of surface excitations, which tends to be overestimated by REELS. Fadanelli et al. performed calculation of the normalized ELF of Ta2O5 from REELS spectrum for electrons with primary energy of 5 keV.31 Other studies of Ta2O5 by REELS/EELS available in literature contain spectra taken at energy of the incident electron beam not lower than 5 keV without the ELF calculation.33,34 One should notice that Vos et al. and Sanz et al. studied TaOx with different oxygen concentration by REELS and EELS.33,34 The studies revealed considerable modification of the REELS and EELS spectra with the x variation in TaOx. This fact also suggests that considerable changes of the ELF have to be expected during the sputtering. Thus, one has to expect significant evolution of the background shape in a Ta 4f XPS spectrum of TaOx during the sputtering. In this work, the ELF obtained by Fadanelli et al. for 5 keV was approximated by the three-parameter function offered by Tougaard (eq 2). The obtained parameters 11 ACS Paragon Plus Environment


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were used for the background simulation of Ta 4f spectrum of the Ta2O5 using CasaXPS software. During the following fitting procedure the parameters of the ELF approximation were slightly changed at each step of the sputtering in order to provide a reasonable fit of the simulated background and experimental spectrum. The B and T0 of the cross-section function (eq 2) were considered as the major parameters of the background refinement due to the following reasons. Firstly, owning to the Ta 4f spectra normalization approach performed in the present work, the intensity scale is expressed in arbitrary units. The B parameter represents a simple multiplication of a signal from the inelastically scattered photoelectrons by a constant value. Since an XPS signal from a particular element is directly proportional to a number of this element atoms in a sample, a variation of the B parameter allows us to fit the simulated background to the experimental one as the background signal would be from a larger or lower amount of Ta atoms of TaOx compared to the background obtained directly from the ELF approximation. The second parameter T0 is associated with a band gap width of a sample, which may be modified during the sputtering with approaching low concentrations of O in the TaOx. The T0 value affects only the energy position of the simulated background onset. However, the approximation of the ELF by the model function (eq 2) reduces the accuracy of the simulated background. As a result, the fine structure i.e. plasmon excitations of the ELF cannot be described precisely. Due to this fact, the plasmon excitations in the Ta 4f spectra were described by additional peaks introduced to the fitting procedure. 3.2.3 Fitting strategy At the first stage of the fitting procedure, the spectrum of the pristine Ta2O5 sample was decomposed. The Ta 4f doublet accounting for Ta5+ state was fitted by 12 ACS Paragon Plus Environment

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two peaks with fixed intensity ratio between the components Ta 4f7/2 Ta 4f5/2 = 4 3 and the same pseudo-Voigt line-shape i.e. by a sum of Gaussian and Lorentzian lineshape with different weighting. The best-fit line-shape GL(60) was taken as a function used for Ta 4f peaks approximation of the sputtered TaOx spectra. The full width at half maximum (FWHM) parameter of the peaks was set to be identical for the Ta 4f doublet but was not restricted to any value. The energy positions of the peaks also were not strictly constrained during the refinement. O 2s line was described by a GL(60) pseudo-Voigt function without any restrictions. Then, the spectrum obtained after the last sputtering step, where only metallic Ta0 remained, was decomposed using asymmetric line-shape (LA(0.85,1.7,25)) for the Ta0 4f doublet approximation. The splitting energy of the doublet components appeared to be the same as one obtained previously for the pristine Ta2O5 spectrum (1.91 eV). The first stage of the fitting procedure provided starting parameters of the Ta 4f and O 2s peaks for the deconvolution of the Ta 4f spectra obtained during the sputtering. The spin–orbit splitting (1.91 eV) of the Ta 4f doublet and the integral intensity ratio of the doublet components (Ta 4f7/2 Ta 4f5/2 = 4 3) were set to be constant for all the Tan+ chemical states used for the deconvolution of the following sputtering steps spectra. The FWHM parameter of the Ta 4f peaks was limited by a minimum value being equal to the FWHM of the Ta 4f peaks derived from the pristine spectrum fit. The FWHM of the Ta 4f peaks representing Tan+ (𝑛 < 5) is expected to increase with Ta reduction due to life time broadening of the line-shape, thus, the parameter was not constrained to a constant value. Also, the FWHM of the Ta5+ doublet peaks was not limited to a constant value to consider the effect of possible slight changes of the Ta5+ doublet binding energy owning to supposed



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evolution of Ta atom local surroundings and, hence, averaging of the initial position of Ta5+ 4f due to crystal field potential modification. In order to minimize the basis set of pseudo-Voigt functions used for the spectra deconvolution, special attention was paid to physical justification of the Tan+ components introduced to the fitting model. According to literature, Ta2O5 is the only stable tantalum oxide whereas numerous reported metastable phases such as TaO2, TaO and other sub-oxides TaxO, where 𝑥 ≤ 2, cannot be stabilized in a pure state without undergoing special conditions or impurities.35,36 Garg et al. supposed that metastable oxides may be formed by “kinetically influenced transformations”.36 Zhai et al. performed joint experimental and computational study of tritantalum oxide clusters (Ta3On-) and established that Ta3O7- cluster contains Ta3+ center site. The authors assumed that this fact might serve as a model for reduced defect sites in bulk tantalum oxide.37 Nevertheless, according to the previous results of the Ta 4f spectra deconvolution of TaOx with oxygen deficiency, Ta4+ and Ta2+ are the chemical states that are always present as principle fitting components in contrast to Ta3+ and Ta1+ states.9,22–25 Basing on this fact, Ta5+, Ta4+ and Ta2+ were considered as initial components forming a minimal basis for the non-stoichiometric oxide spectra deconvolution. It is noteworthy that a Ta0 component was regarded as a necessary component for the basis extension due to the direct observation of a metallic Ta line after the last stage of the sputtering. 3.2.4 Discussion of the Ta 4f photoelectron spectra decomposition Figure 2 presents results of the Ta 4f photoelectron spectra decomposition of the Ta2O5 recorded at different steps of the sputtering. Energy positions and other parameters of the components are presented in Table 1. The deconvolution of the spectra reveals the following dynamic of the Ta chemical state changing during the 14 ACS Paragon Plus Environment

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sputtering: i) after the first step of the sputtering (20 min.), Ta4+/3+ and Ta2+ components emerge as a result of the oxygen concentration reduction; ii) the following stages of the Ta 4f spectra transformation (30 – 80 min.), where the stagnation of the spectra evolution is observed, are accompanied by the presence of a Ta1+ component; iii) further sputtering results in

a strong redistribution of the

components intensity between Ta5+ and other Ta 4f components emerged at the earlier sputtering stages (Ta4+/3+, Ta2+, Ta1+) leading to the prevalence of the Ta1+, Ta5+, Ta2+ components (120 min.); iv) after 230 min. of total sputtering time, the metallic component Ta0 becomes dominating and the Ta4+/3+ component intensity has significantly increased whereas the Ta5+, Ta2+ components have attenuated comparing to the previous spectrum (120 min.). One should note that the Ta4+/3+ component was designated so due to the itinerant character of the component: its energy position during the sputtering can be classified into two groups of values. The first group (24.4 eV and 24.6 eV for 20 and 50 min., respectively) is characteristic for Ta3+ chemical state while the second one (25.1 eV and 25.4 eV for 120 and 230 min., respectively) can be assigned to Ta4+. This idea is illustrated in Fig. 3 where the relative energy shift value Δ of the Ta 4f components (Δ = Eb(Ta5 + ) – Eb(Tan + ), where 𝑛 ≤ 5 ) is presented as a function of a Ta chemical state (5+, 4+, 3+, 2+, 1+, 0). While the charge of a Ta ion reduces proportionally to electron charge along with a reduction of the its oxidation state, one can expect a linear dependency of a Tan+ component energy shift on the n value in a first approximation.38 Figure 3 demonstrates a consistent linear behavior of the components energy shift for all the fits if the Ta4+/3+ component is defined as Ta3+ in for the first two spectra (20 and 50 min.) and Ta4+ for the spectra obtained after 120 and 230 min. The discrepancy between the energy shifts of the components assigned to a particular chemical state can be a consequence of the influence of the surroundings symmetry of the Tan+ ion on the binding energy of the Tan+ 4f XPS line 15 ACS Paragon Plus Environment


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and possible imperfectness of the fitting model.38 In addition, one has to mention that any attempts of simultaneous introduction of Ta4+ and Ta3+ components into the fitting model has not provided any reasonable result if the described above fitting strategy was followed (Sec. 3.2.3).


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Ta5+

t = 0 min.

Ta4+/3+ Ta5+ Ta2+

t = 120 min.

Intensity

Intensity

Ta1+

bulk plasmon surface plasmon

45

40

35

30

25

20

45

40

35

Binding energy,eV

30

25

20

Binding energy,eV Ta5+

t = 20 min.

O 2s

bulk plasmon surface plasmons

O 2s

t = 230 min.

Ta4+/3+2+ Ta Ta1+ Ta 5+

O 2s

Intensity

Intensity

Ta0

Ta4+/3+ Ta2+ bulk plasmon surface plasmon

45

40

35

bulk plasmon

O 2s

30

25

20

45

surface plasmons

40

Binding energy, eV

35

30

25

20

Binding energy, eV Ta5+

Ta0

t = 480 min.

Intensity

t = 50 min.

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ta4+/3+ Ta2+ Ta1+ O 2s

bulk plasmon

surface plasmon

bulk plasmon surface plasmon 45

40

35

30

25

20

45

Binding energy, eV

40

35

30

Binding energy, eV


25

20


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Figure 2. Experimental and fitted spectra of the Ta2O5 measured before and after different Ar+ ion sputtering steps (after 20, 30, 50, 80, 120, 230 and 480 min. of total sputtering time). The black dots represent an experimental spectrum while the red solid line is an envelope of the fitting components sum. Total sputtering time, min. 0

20

50

120

230

480

Fitting component

Position, eV

FWHM, eV

Line shape

Ta5+ 4f7/2 O 2s Ta5+ 4f7/2 Ta4+/3+ 4f7/2 Ta2+ 4f7/2 O 2s Ta5+ 4f7/2 Ta4+/3+ 4f7/2 Ta2+ 4f7/2 Ta1+ 4f7/2 O 2s Ta5+ 4f7/2 Ta4+/3+ 4f7/2 Ta2+ 4f7/2 Ta1+ 4f7/2 O 2s Ta5+ 4f7/2 Ta4+/3+ 4f7/2 Ta2+ 4f7/2 Ta1+ 4f7/2 Ta0 4f7/2 O 2s

26,3 22,7 26,3 24,6 24,0 22,6 26,3 24,4 23,7 22,7 22,0 26,3 25,1 23,7 22,6 21,0 26,3 25,4 23,9 22,7 21,4 21,2

1,08 4,21 1,34 1,63 1,70 4,35 1,60 1,64 1,61 1,73 4,38 1,80 1,80 1,66 1,40 3,98 1,90 1,56 1,58 1,53 1,37 4,50

GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60) GL(60)

Ta0 4f7/2

21,6

0,62

LA(0.85,1.7,25)

Table 1. Peak Parameters of the Principle Components Derived through the Ta 4f Spectra Deconvolution of Ta2O5 before the Sputtering and after 20, 50, 120, 230, 480 Minutes of Total Sputtering Time.


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20 min. 50 min. 120 min. 230 min.

5

4

, eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

2

1

0 5

4

3

2

1

0

Formal oxidation state of Ta, n+

Figure 3. An energy shift (Δ) of the Tan+ 4f deconvolution components employed in the peak fitting procedure of the Ta 4f spectra of the Ta2O5 sputtered for 20, 50, 120 and 230 min. compared to the corresponding Ta5+ 4f component position. The black line represents a linear approximation of the data points obtained for the Ta 4f spectrum after 230 min. of the sputtering.

3.3Valence band spectra The photoelectron spectra of the valence band collected at the excitation photon energy of 200 eV from the pristine Ta2O5 sample and after different sputtering steps are plotted in Figure 4. It is well known that the valence band of Ta2O5 consists of two sub-bands.25,26 In the present work, we address the position of the valence band 19 ACS Paragon Plus Environment


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maximum (VBM), so we will discuss only the upper sub-band. This sub-band is formed by O 2p states mixed with Ta 5d and 6s states. As one can see from Figure 4, the shape and energy position of the VB are changing during the sputtering. One can allocate two main features A and B in the upper VB. According to literature, the band A originates predominantly from Ta 5d states. The band B is mainly formed by O 2p states of mixed with 5d and 6s states of Ta.25,26 Interestingly, the most considerable VB spectrum transformation occurs at the early sputtering stages (20, 30, 50 min.). While the 20 min. spectrum is almost identical to the pristine one, there is a strong change in the VB spectrum shape after 30 and 50 min. of the sputtering steps, which correspond to the stagnation period in the Ta 4f spectrum evolution. This implies that the reason of the spectrum changes originates only from structural changes and not from the Ta chemical state transformation. During the sputtering process a peak (C) appears at about 2 eV above the VB maximum and its intensity increases with increasing the sputtering time. According to literature, this peak is mainly attributed to Ta 5d electron states. Ivanov et al. performed calculations of the electronic structure of δ-Ta2O5 with oxygen vacancies Ovac demonstrating that the presence of Ovac leads to the formation of the defect states by Ta 5d electrons with an admixture of Ta 6s electrons about 2 eV above the VB top.25 In addition, the authors showed the applicability of the δ-Ta2O5 calculations to an analysis of amorphous Ta2O5 electronic structure due to similarity of their electronic structure. It is noteworthy that the position of the defect states corresponds to the d-band of metallic Ta. Moreover, previously it has been established that this defect peak appears in a result of the metallic d-band transformation during oxidation of a metallic Ta film.14


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C

Intensity, a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


230 min. 120 min. 100 min. 50 min. 30 min.

A

B

20 min. before sputtering

14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 -1 -2 -3 -4 -5

Binding energy, eV

Figure 4. Evolution of XPS valence band spectra of Ta2O5 with increase of total sputtering time. All the spectra are shifted along the ordinate axis for clarity. A and B indicate the features of the pristine spectrum mainly attributed to Ta 5d and O 2p, respectively. C points to a defect state peak induced by the sputtering.

3.4

NEXAFS spectra

Figure 5 shows the O K – edge absorption spectra of the pristine Ta2O5 sample and after different sputtering steps measured in PEY mode. All the NEXAFS spectra were normalized to an absorption jump regarding the background caused by 21 ACS Paragon Plus Environment


absorption below the O K – edge. From Figure 6 one can see that the spectra of the pristine Ta2O5 sample obtained in TEY and PEY modes in the present work are in a good agreement with each other. This implies the homogeneity of the sample before the sputtering. The spectra are also consistent with the NEXAFS O K – edge spectra of amorphous Ta2O5 presented in literature. 11,17

Normalized intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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420 min. 300 min. 230 min. 120 min. 100 min. 80 min. 50 min. 30 min. 20 min. before sputtering

A'

A

B'

B

0.7 eV 520

525

530

535

540

545

550

555

560

565

570

Photon energy, eV

Figure 5. O K – edge absorption spectra of Ta2O5 measured at different steps of Ar+ ions sputtering by monitoring the partial electron yield. All the spectra are shifted along the ordinate axis for clarity. The dashed lines indicate the positions of the A, A' features and the B' - B band of the spectrum obtained before Ar+ ions sputtering. The dotted lines represent the positions of the A feature and B' - B band at the late stages of the sputtering.


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before sputtering, TEY before sputtering, PEY Normalized intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


520

525

530

535

540

545

550

555

560

565

570

Photon energy, eV

Figure 6. O K – edge absorption spectra of pristine Ta2O5 recorded by monitoring the partial (PEY) and total (TEY) electron yield.

Amorphous Ta2O5 has an octahedral symmetry of local atomic structure where O atoms are placed in the vertices of the octahedron and Ta atom is in the geometrical center of the equatorial plane.39 This oxide has d0 configuration that results to the absence of d-electron interaction and, therefore, simple structure of O K – edge spectrum. It is known that in octahedral complexes the metal eg orbitals are directed toward the corners of the octahedra and have a stronger overlap with orbitals of neighboring atoms. As a result, O K near edge structure is very sensitive to changes in 23 ACS Paragon Plus Environment


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the local bonding environment. The O K – edge absorption spectrum of amorphous Ta2O5 originates from electron transitions to the unoccupied MOs formed by 2p states of O mixed with Ta 5d states split by the crystal field into triple degenerated t2g and double degenerated eg states (main broad band A′ − A) and with Ta 6sp states (broad features B′ − B). The energy separation of the first two peaks directly reflects the ligand-field splitting and their intensity ratio is mainly defined by the number of unoccupied states of t2g and eg orbitals, which are 3 and 2 for Ta2O5 octahedral ligandfield, respectively.39 Another important factor that influences the intensity ratio of the spectrum details is O 2p – Ta 5d hybridization strength, which strongly depends on spatial orientation of the atomic orbitals. Deviation of the intensity ratio of the first two peaks from the statistical 3/2 in Ta2O5 may indicate distortions of the octahedral polyhedron, which were previously reported in literature.39 The O K – edge absorption spectra of TaOx at different total sputtering time are plotted in Figure 5, Figure 7a and Figure 7b. A joint analysis of these spectra reveals some interesting tendencies. On the whole, the intensity ratio of the features assigned to t2g and eg states and the energy distance between them changes with total sputtering time. The evolution of the spectra can be divided into three stages. The first step is represented by the spectrum obtained after 20 minutes of the sputtering. It can be characterized by an unchanged main structure of the spectrum with a slightly decreased energy distance between the t2g and eg details compared to the spectrum of the pristine Ta2O5. The second stage of the spectra transformation (after 30 and 50 minutes) involves: i) significant gap smoothing between the first two peaks (A′ − A); ii) broadening of the peaks associated with t2g and eg states; iii) decrease of the first two peaks energy separation; iv) transformation of the broad band (B′ − B) to two peaks. Interestingly that the corresponding Ta 4f XPS spectrum, after 30, 50 and 80 minutes of total sputtering time, shows stagnation in its transformation and, thus, in 24 ACS Paragon Plus Environment

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the evolution of the Ta chemical states. The third stage of the evolution of the O K – edge absorption spectra (80 – 420 min.) can be described by preservation of the main three-peak structure accompanied by reduction of the energy distance between the A′ − A peaks by 0.7 eV and a gradual decrease of the A′/A peak intensity ratio. In addition, the width of the A′ and A peaks is increased and the asymmetry of the peaks is strongly enhanced pointing to the presence of an unresolved fine structure in the A′ − A region. One should notice that the near-edge absorption spectrum represents an integral signal coming from all the structural units presenting in the sample. Due to this fact, the following analysis of the spectra is based on deducing possible structural units emerging during the sputtering and ruling out ones that do not fit the experimental observed O K – edge spectra. 3.4 Discussion and analysis of the NEXAFS spectra. Evolution of O K – edge NEXAFS spectra of transition metal oxides can be analyzed qualitatively by means of molecular orbital model approach. According to the model, an energy position of features of a NEXAFS O K – edge spectrum of transition metal oxide can be understood by interaction degree of the metal d electrons and oxygen 2p electrons, which is mainly defined by overlap extent of the metal and oxygen orbitals.39 The metal orbital eg (dz2,dx2 ― y2 ) overlaps stronger with O orbitals (mainly 2pz) than the t2g because in octahedral environment metal eg orbitals point directly to the O atoms (σ bonds with O 2pz) whereas metal t2g (dxy, dxz, dyz) are not directed towards the ligand atoms (π bonds with O 2px, 2py). Thus, the energy position of the eg peak is higher than that of the t2g, since the orbitals are antibonding in nature. The observed decrease in the energy position of the eg peak (the decrease of the t2g and eg energy 25 ACS Paragon Plus Environment


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splitting) reflects weakening of the overlap between the Ta 5dz2, 5dx2 ― y2 and O 2pz orbitals. Also, the mentioned above broadening of the first two peaks, gap smoothing between the first two peaks and the appearance of the peaks asymmetry point to an emergence of new spectral features in the region of the main band A/ – A during the sputtering. The early stages of the spectra evolution can be rationalized by means of consideration of octahedral distortions with formation of new structural units with lower symmetry and coordination number. The XPS analysis revealed formation of Ta sub-oxides, which must lead to deviation from d0 configuration of the octahedral complex. In terms of ligand field theory approach, this implies filling of the lowest energy orbital (t2g) with electrons. In turn, the modification of the dn configuration can result in fact that Oh symmetry is no longer energy beneficial for Ta surroundings. There are several typical distortions of octahedral complexes. One of them is tetragonal elongation/compression originating from Jahn-Teller (J – T) effect, which can be summarized in the following statement: any non-linear molecule is unstable in a degenerated state and will distort to remove the degeneracy.40,41 This theorem is also particularly relevant for transition-metals complexes. The effect is most pronounced for d4, d7, d9 configurations of octahedral complexes since in this case the degeneracy will be removed from eg states, which are directed towards the ligands. Thus, removing of the eg degeneracy will provide higher difference in net energy of the d electrons and, hence, stronger distortions. Nevertheless, the effect also appears in d1 and d2 configurations but to less extent due to the direction of the t2g orbitals between the ligands in octahedral complexes. In principle, the reduction of Ta5+ state to Ta4+ (d1) or Ta3+(d2) oxidation states can lead to removing of the t2g degeneracy leading either to z – axis compression (d1) or z – axis elongation (d2) (Figure 8). However, according to the orbital correlation diagram presented in Figure 8, such a type of distortion also leads to energy splitting 26 ACS Paragon Plus Environment

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of the eg states. This implies an appearance of a new spectral feature with higher energy position originating from one of the eg orbitals member. Due to this fact, the J – T distortion scheme is not consistent with the experimental spectra changes after the early sputtering stages (20 – 30 min.) (Figure 7a). Nevertheless, this distortion type may occur at the later sputtering steps (80 – 480 min.) (Figure 7b) where the A' and A peaks become significantly broadened without changing their energy position.

a

before sputtering 20 min. 30 min. 50 min.

b

Normalized intensity, a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c

d A'

530

532

A

534

536

0.5 eV

538

540

f

B' 542

B 544

546

548

550

Photon energy, eV

Figure 7a. NEXAFS O K – edge spectra of TaOx at the early steps of sputtering by Ar+ ions. The arrows point to the most prominent features appeared after 50 min. of the sputtering. The dashed lines indicate the positions of the Aꞌ and A peaks of the pristine spectrum. The dotted line shows the position of the A peak after 20 min. of the sputtering.


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before sputtering 50 min. 80 min. 100 min. 120 min. 230 min. 300 min. 420 min.

(A', A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Normalized intensity, a. u.


d

A

A' 530

532

534

536

538

B' 540

542

B 544

546

548

550

Photon energy, eV Figure 7b. NEXAFS O K – edge spectra of TaOx at the later steps of sputtering by Ar+ ions. The dashed lines show the approximate positions of the A′ and A peaks. The arrow points to the feature (d) appeared after 80 min. of the sputtering. In addition, the redistribution of the A′ and A peaks intensity is illustrated in the figure by comparison of the highest (before sputtering) and lowest (300 min.) intensity difference ΔI(A′, A) between the A′ and A features.

Albright et al. considered and discussed plenty of structural types typical for TM complexes starting from highly symmetric octahedral complexes with further conversion to less symmetric geometries within a wide range of coordination numbers (6 - 2).42 According to this work, there are other common Oh distortions typical for d2 and d4 complexes. One is a decrease of one trans L – M – L γ angle from 1800 as shown in Figure 9. The second one is variation of cis L – M – L α angle from 900. A 28 ACS Paragon Plus Environment

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combination of both types of deformation results in conversion of an octahedron to a bicapped tetrahedron. Deviation of the γ and α angles from the ideal values (1800 and 900, respectively) will stabilize a one member of the t1u (energy of the antibonding orbital increases) and destabilize a one component of the eg states (energy of the antibonding orbital decreases). Also these cis and trans L – M – L angle distortions split the degeneracy of the t2g. The behavior of the t2g, eg and t1u antibonding orbitals within the frameworks of the trans and cis L – M – L angles distortion is in good agreement with the experimental transformation of the O K – edge spectrum after 20, 30 and 50 min. of the sputtering. The degeneracy removing from the t2g states correlates with the increase of the first peak FWHM and appearance of the shoulder a (Figure 7a). The features c and f (Figure 7a) can be assigned to a destabilized member of the eg orbital and one stabilized component of the t1u, respectively, as predicted by the L – M – L angles distortion scheme described above. The relative decrease of the intensity of the d feature in the B' – B band correlates with the t1u states degeneracy reduction that occurs due to this distortion type.

Figure 8. Orbital correlation diagram of a Jahn-Teller distorted (tetragonal distortion) TM octahedral complex. Only the d orbitals region is presented.



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Figure 9. Orbital correlation diagram for bending one trans L – M – L angle in an octahedral transition metal complex. Only the antibonding and non-bonding orbitals region is presented. The diagram is based on the calculations from literature.42

Another reasoning explaining the evolution of the O K – edge spectra is based upon a partial conversion of octahedral structural units to square pyramids with some extent of trans L – M angle distortions analogous to ones of Oh described above except for one missing ligand. Figure 10 shows an orbital correlation diagram of Oh to C4v ML5 (square pyramid) conversion (left). When one trans ligand is removed from an octahedron the major perturbation occurs with the z2 component of the eg set: the energy position of the component shifts to lower energy. Also the degeneracy of the t2g states is removed and dxz and dyz components rise slightly in energy. Although the diagram does not include the metal p-orbitals region, removal of the t1u states degeneracy can be expected while the conversion of an octahedron to a square 30 ACS Paragon Plus Environment

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pyramid. In addition, a correlation diagram for a square pyramid bending, consisting in decreasing of an angle between basal and apical ligand sites, is presented in Figure 10 (right). Previous studies showed that d0 – d2 square pyramid TM complexes are expected to have 𝛾 > 900.42 Increasing of the γ angle leads to an even stronger shift of the z2 component to lower energy than in the case of the octahedron conversion to a square pyramid. Moreover, the dx2 ― y2 state, which is another member of the eg set in Oh, displaces to lower energy position along with energy position raise of the dxz and dyz states. Taking account of the presence of the Ta3+ chemical state at the early sputtering stages (20, 30, 50 min.), such a distortion can occur during the formation of a square pyramid based on Ta3+ ion (d2 configuration). The feature b in Figure 7a, appearing after 50 min. of the sputtering, points to a presence of a significant contribution from states located in between the octahedral t2g and eg states. Moreover, the c feature, which appeared after 30 min. of the sputtering, can be explained by the decrease of the dx2 ― y2 component energy position due to the bending of the square pyramid whereas the position of the main A feature is the same at the 20, 30, 50 min. sputtering stages. It is worth to connect together all the observation derived from the Ta 4f and VB XPS and O K – edge absorption spectra analysis at the early sputtering stages (0 – 80 min.). The stagnation of the Ta 4f spectra after 30, 50 and 80 min. points to unchanged Ta chemical states during the sputtering stages. Whereas the analysis of the VB and O K – edge absorption spectra obtained after 30 and 50 min. of the sputtering reveals the significant structural modification. Basing on the consideration of possible structural transformations at these stages, following conclusions can be made. The changes of the Ta chemical state at the earliest sputtering stage (20 min.) are firstly appeared in the absence of strong modification of Ta local surroundings symmetry, for instance, through rearrangement of the octahedra (polymerization) and creation of oxygen vacancies. Then, conversion of the octahedra to more beneficial 31 ACS Paragon Plus Environment


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types of polyhedrons occurs without any variation of the Ta chemical states (30, 50 min.). As was shown above, formation of square pyramids and bended octahedra can be expected during this structural change.

Figure 10. Orbital correlation diagram for an octahedron to square pyramid conversion (left) with further bending of the pyramid (γ is an angle between apical and basal ligand sites). Only the d orbital part is shown. The diagram of the pyramid distortion is based on calculations presented in literature.42

After 80 min. of the sputtering (Figure 7b), the spectra have a considerably lower signal contribution from the states that presumably may originate from a square pyramid coordination (Figure 7a, feature b). One should also notice that the Bꞌ – B band restores its intensity and shape compared to the 30 and 50 min. spectra (Figure 7b).

These changes indicate a decrease in the concentration of the previously

emerged type of structural units (before 80 min.) whose presence leads to the appearance of the b and f features. The decreased Aꞌ/A intensity ratio at the late 32 ACS Paragon Plus Environment

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sputtering stages compared to the early stages indicates filling of the lowest unoccupied molecular orbitals, which is consistent with the strong Ta oxidation state reduction observed by means of XPS at the late sputtering stages (120, 230, 480 min.). The decrease of the Aꞌ and A features energy separation compared to the pristine spectrum by about 0.7 eV can be explained through an O – Ta distances increase due to the weaker electrostatic attraction of the Tan+ and O2- ions induced by Tan+ ion charge reduction. The broadening of the Aꞌ and A along with smoothing of the gap between these features suggest a presence of an unresolved fine structure in the Aꞌ – A region. The discussed above changes of the O K – edge spectra (after 80 min. of sputtering and later) are also correlated with Ta chemical states evolution revealed by XPS Ta 4f spectra fitting (Figure 2): Ta1+ emerges and Ta3+ component is replaced by Ta4+ state. Moreover, Ta1+ and Ta2+ become dominating components after 120 min of sputtering. Taking into account the preservation of the main three-peak structure of the O K – edge spectra (Figure 5), the presence of Ta1+ and Ta2+ chemical states can be realized by formation of polyhedrons with low coordination numbers (e.g. ML4, ML3, ML2) serving as bridges between the distorted octahedrons. Thus, a considerable part of the octahedrons persists during the sputtering providing a matrix to lowsymmetry polyhedrons serving as bridges between the octahedra. This assumption is supported by a relatively weak alteration of the A′/A intensity ratio (A′/A > 1) during the whole sputtering process). However, if one assumes octahedral surroundings of Ta2+ and Ta1+ (d3 and d4 configurations, respectively), the statistical ratio of the t2g and eg peaks will change to ¾ for d3 and to ½ for low-spin d4 and 1 for high-spin d4 configuration. This suggests formation of low-symmetry structural units based on Ta2+ and Ta1+ ions. According to Albright et al., there are many possible conversions of Oh and square pyramid (C4v ML5) to less-symmetric structural units.42 Orbital correlation 33 ACS Paragon Plus Environment


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diagrams of these transformations can be characterized by multi component structure, which appears due to lowering of the symmetry and, therefore, degeneracy removing. Thus, due to integral nature of the absorption spectra, complicated structure of molecular orbital diagrams and diversity of possible low symmetry Ta surroundings, any reasonable analysis of the O K – edge spectra at the intermediate and late sputtering steps (80 – 480 min.) is not possible in the frameworks of the molecular orbital approach. Nevertheless, several assumptions on the structural units with low coordination number emerging during the sputtering can be made by means of consideration of theoretical calculations performed on the relation between configuration dn and stereochemistry of TM compounds. As has been discussed earlier, the deconvolution of the Ta 4f XPS spectra reveals a reduction of Ta oxidation state to Ta0, that is, a variation of the Ta complex configuration from d0 to d5. The d3 and d4 configurations correspond to Ta2+ and Ta1+ chemical states, which prevail at the intermediate and late sputtering stages (Ta1+ in particular). Cicera et al. performed a computational study of stereochemistry of four-coordinated homoleptic TM complexes associated with each possible spin state.43 It was established that for d4 coordination the most stable coordination is square planar, whereas Td or C2v symmetry is more stable for d3 configuration. Thus, one can assume formation of square planar Ta1+ surroundings at the late sputtering stages along with possible Td or C2v Ta2+ surroundings formation. However, it is noteworthy that a rearrangement of the polyhedrons net structure can also be expected along with further decrease in Ta coordination number. Moreover, the formation of low-symmetry polyhedrons can results in significant reduction of O 2p and Ta 5d states hybridization strength leading to a decrease of OK spectrum sensitivity to these structural changes due to dipole selection rules. 4. Conclusion 34 ACS Paragon Plus Environment

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Evolution of electronic and atomic structure of amorphous Ta2O5 during Ar+ ion sputtering was studied by means of XPS and NEXAFS spectroscopy. The XPS spectra decomposition reveals appearance of Ta intermediate oxidation states (Ta4+, Ta3+, Ta2+, Ta1+ ) with increasing of total sputtering time. Moreover, significant amount of Ta0 appears at the last steps of sputtering. Basing on the results of the decomposition, a scheme of Ta oxidation states evolution was offered. The scheme is divided into four steps. At the first step, Ta atoms partially change their chemical state from Ta5+ to a mixture of Ta3+ and Ta2+ (20 min. of total sputtering time). After that, a stagnation of the Ta 4f spectrum transformation is observed (30 – 80 min.). It is notable that a small contribution of Ta1+ oxidation state to the Ta 4f spectrum appears after 30 min of the sputtering. At the third step, the concentration of the Ta2+, Ta1+ oxidation states increases and Ta4+ replaces Ta3+ chemical state (80 – 120 min.). Further sputtering leads to formation of considerable amount of formal metallic Ta0 state (230 min.). It was established that complete metallization of Ta can be achieved by Ar+ ion sputtering of amorphous Ta2O5 film deposited on a Si substrate. The analysis of the XPS valence band spectra revealed a redistribution of the VB states with formation of a defect peak about 2 eV above the VB maximum. These states are ascribed to Ta 5d electrons and can be attributed to the formation of oxygen vacancies. NEXAFS spectra of O K – edge were analyzed to trace the local atomic environment modification of Ta in the tantalum oxide during the sputtering. A possible model of structural changes was offered. Basing on molecular orbital correlation diagrams of octahedron distortions and conversions to polyhedrons of lower symmetry, trans and cis L – M – L angle distortion of the octahedra along with formation of bended square pyramids was proposed as a possible scenario that occurs at the early stages of the sputtering (20, 30, 50 min. of total sputtering time). 35 ACS Paragon Plus Environment


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Interestingly, the O K – edge spectra corresponding to the early stages are correlated with the retention of Ta chemical states revealed by XPS and the presence of Ta3+ oxidation state. The analysis of the spectra at the following stages reveals preservation of significant part of the octahedra during the sputtering, which suggests formation of low-symmetric structural units incorporated to a matrix of distorted octahedra. Possible geometries were also overviewed basing on orbital correlation diagrams and relationship between the d3 and d4 configurations of a TM complex and its stereochemistry. It is notable that the predominance of the Ta0 component at the late sputtering stages (after 230 min. of sputtering) exhibits no any essential influence on the corresponding O K – edge spectra. Although the presented results do not allow us to conclude about the atomic surroundings of Ta0 atoms, one can assume that some kind of clusterization of tantalum atoms in formal metallic chemical state (Ta0) occurs in the matrix of distorted octahedra. Acknowledgements We gratefully acknowledge the assistance from the bilateral Program “RussianGerman Laboratory” at HZB BESSY II. We gratefully acknowledge the financial support by Helmholtz Zentrum Berlin (HZB) and also thank HZB for the allocation of synchrotron radiation beamtime. References (1)

Atanassova, E.; Paskaleva, A. Challenges of Ta2O5 as High-k Dielectric for Nanoscale DRAMs. In 2006 25th International Conference on Microelectronics; IEEE, 2006; pp 45–52. https://doi.org/10.1109/ICMEL.2006.1650894. 36 ACS Paragon Plus Environment

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Relationship between Ta Oxidation State and Its Local Atomic

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