Observation of Titania and Titanate Phase Changes in Oxidation

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Observation of Titania and Titanate Phase Changes in Oxidation-Controlled ZnO/TiN and HfO/TiN Thin Films: an X-ray Absorption Spectroscopy Study 2

Doyeong Kim, Minji Lee, Seung-Yub Song, Dae Hyun Kim, Tae Joo Park, and Deok-Yong Cho J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06565 • Publication Date (Web): 03 Aug 2016 Downloaded from http://pubs.acs.org on August 8, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Observation of Titania and Titanate Phase Changes in Oxidation-controlled ZnO/TiN and HfO2/TiN Thin Films: an X-ray Absorption Spectroscopy Study Doyeong Kim1, Minji Lee1, Seung-Yub Song1, Dae Hyun Kim2, Tae Joo Park2,3, Deok-Yong Cho1,* 1

IPIT & Department of Physics, Chonbuk National University, Jeonju 54896, Republic of Korea

2

Department of Advanced Materials Engineering, Hanyang University, Ansan 15588, Republic

of Korea 3

Department of Materials Science and Chemical Engineering, Hanyang University, Ansan

15588, Republic of Korea Abstract

We investigated the local electronic structures of oxidation-controlled TiN thin films for preparation of photocatalytic titania using soft X-ray absorption spectroscopy. It is shown that the TiN layers on top of oxides such as HfO2 or ZnO, are easily oxidized by heat during postdeposition annealing (PDA) so as to form nitrogen-incorporated titania (N-TiO2). The local

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structures of the oxidized films evolved significantly depending on the bottom oxide and the PDA conditions; When TiN was deposited on HfO2, which is less reactive than ZnO, PDA at 700°C stabilized a mixture of rutile and anatase phases under almost any gas (N2 or O2) environments. On the other hand, when TiN was deposited on ZnO, which is reactive enough to oxidize the TiN layers substantially, the PDA resulted in a rich phase diagram according to the gas environment - under N2 environment, an anatase local structure is dominant, while under O2 environment, a rutile or yet another local structure with a high symmetry as in perovskite ZnTiO3 or ilmenite, becomes dominant. As for the origin of the phase changes, the abundance of the oxide phases are found to be strongly correlated with the averaged grain sizes in the films. The changes in the local structures resulted in the blue- or red-shifts of conduction band, implying that we can engineer the electronic properties in the N-TiO2 films by properly choosing the bottom oxide.


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1. Introduction Titania (TiO2) has drawn tremendous attention for its versatility in many types of applications including pigment1, photocatalysis2, optoelectronics3, or electronic devices.4,5 In efforts to enhance the photo-response on visible light for use in photocatalytic devices or solar cells, it has been attempted to fabricate TiO2 doped with light atoms such as B, C, N or F.6-9 Among them, nitrogen-doped TiO2 (N-TiO2) has gained particular interests after the realization of visible light absorption by Sato10. So far, various physical or chemical methods, including ion implantation11, hydrolysis12, or oxidation of TiN13, have been adopted to implement N-TiO2. In particular, the oxidation method14-17 has a strong advantage in regard of securing the structural stability, since the moderate kinetics of oxygen migration during the thermal oxidation would hardly deliver a structural damage as strong as in the dopant implantation. Furthermore, the activity of the oxidation process can be tuned easily by adjusting the gas environments (N2 or O2) for the growth of titania.17-20 In a very recent report, even an epitaxial TiO2 film was realized from oxidizing a TiN film.21 Therefore, it is highly probable to control the optoelectronic properties of N-TiO2 within the oxidation method. When TiN is conjugated with some oxide layers below, its oxidation can be facilitated even more. The oxide layers beneath the TiN layers can provide oxygen ions for TiN oxidation, if the interface between the bottom oxide and TiN is thermodynamically unstable. The stability of the interface can be estimated by comparing the value of the standard molar heat of formation (∆fH0) of the bottom oxide to the difference between the values for TiO2 (~-944.0 kJ/mol)22 and TiN (337.65 kJ/mol)23, namely, ~-606 kJ/mol. For instance, a HfO2/TiN interface, which is often implemented for high-k dielectric gate stack24, should be stable because ∆fH0 of HfO2 is -1144.7


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kJ/mol (Ref. 22), much lower than -606 kJ/mol. Thus, the TiN oxidation due to oxygen donation from HfO2, should be minimal. On the other hand, a ZnO/TiN interface should be less stable so that a few TiN layers near the interface can be oxidized almost completely, because ∆fH0 of ZnO is -350.5 kJ/mol (Ref. 22) being comparable to a half of -606 kJ/mol, that is, the energy cost to fully oxidize TiN into TiO2 per O atom in formula unit. Thus, TiN deposition on some less stable oxides (here, ZnO) can be an alternative way to oxidize TiN efficiently. Therefore, this work focused on investigating the influence of the bottom oxides (representatively, HfO2 and ZnO) as well as the post-deposition annealing (PDA) on the chemical states and structural properties of the oxidized TiN films. For the interface reaction can reach typically within a few nm, the thickness of the TiN layers examined in this work, was restricted to ~5 nm to minimize the contribution of less-affected bulk region in thicker TiN. The structural orders in such thin TiN layers should not be strong enough to be measurable by diffraction techniques. Hence, we employed soft X-ray absorption spectroscopy (XAS) to examine the local structures in the top layers, because it is a powerful technique to identify the titania microstructures17,25-27. The results of the XAS analyses clearly showed a systematic structural evolution of the topmost layers into various oxide phases including rutile28,29, anatase28,29, and a novel titanate phase, by PDA under various N2 or O2 environments. This suggests that it is feasible to control the optoelectronic properties of the oxidized TiN films by adjusting the abundances of those titania and titanate phases.30-32

2. Experimental details


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Sample Preparation. HfO2 and ZnO were deposited on a dilute HF-cleaned boron-doped ptype Si(100) wafers using atomic layer deposition (ALD) techniques. For HfO2, ALD was processed up to a thickness of ~9 nm at 280oC using Tetrakis(ethylmethylamino)hafnium (TEMAHf) as the source and ozone with the concentration of 180 g/m3 as the oxygen source. Meanwhile, for ZnO, ALD was done for the same thickness at 150oC using diethylzinc (DEZ) source and the H2O source. The thicknesses of the oxide films were estimated by spectroscopic ellipsometry (Nanoview Co., MG-1000). Then 5 nm-thick TiN layers were deposited on top of the oxides by DC reactive magnetron sputtering with a Ti target under an N2+Ar ambiance. The resistivity of TiN prepared by the same method but with 100 nm thickness, measured by 4 point probe was ~461 µΩ cm, which is good enough for use as an electrode. X-ray diffraction (XRD) measurement for the 100 nm-thick TiN film clearly showed the peaks for a crystalline TiN, suggesting that the deposited TiN is almost stoichiometric. PDA was processed at 700oC for 1 min under various O2/N2 gas pressures in a rapid thermal processing facility (SNTEK Co., SRS5000). The base pressure during the PDA was ~1 x 10-3 Torr. Results of the XRD measurement showed no signatures of crystallization on the thin TiN layers due to “thin”-ness of the samples. Measurements. XAS at Ti L2,3-, O K-, and N K-edges was performed at the 2A beamline in the Pohang Light Source. The absorption coefficients near the edges were measured in total electron yield mode at room temperature with normal-incidence geometry. Field emission scanning electron microscope (FESEM; Carl Zeiss, SUPRA 40VP) was used to identify the surface structure. For the FESEM measurement, a few nm-thick osmium layers were covered on top of the samples by chemical vapor deposition to prevent the sample charging effects during the electron beam irradiation. A low kinetic energy (2 keV) electron beam was used to enhance the signals from the topmost TiN layers.


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3. Results A. Ti L2,3-edge XAS Fig. 1 shows the Ti L2,3-edge XAS spectra of 5 nm-thick TiN thin films on top of the HfO2 (a) and ZnO (b) layers. Ti L3(L2)-edge XAS reflects the electron excitation from a Ti 2p3/2(2p1/2) core level to the Ti 3d unoccupied states, the features of which are mostly determined by the site symmetry of the Ti-O coordination33,34. It is shown that the lineshapes of the XAS spectra evolve depending on both the bottom oxide and the details in the PDA conditions. This suggests that the valence and local structures of the Ti atoms can change sensitively upon the oxygen supply from both the bottom oxide and the ambiance. The results of the XAS analyses will be demonstrated further in detail in the following figures. Fig. 2 shows the Ti L2,3-edge XAS spectra of the HfO2/TiN films after the PDA under N2 (a) or O2 (b) environment. The HfO2/TiN interface should be thermodynamically more stable than the ZnO/TiN interface, so that the oxidation of TiN due to the interface reaction should be minimal. The main peaks located at hν = 455-462 eV and 463-469 eV should be attributed to the L3- and L2-edge absorption, respectively, while the small satellite peaks at hν = 470-480 eV should be to the complex higher energy multiplets due to charge transfer excitations26. Figs. 2(d) and (e) show the magnified views of the satellite peaks in Figs. 2(a) and (b), respectively. For those figures, the higher energy tails of the main peaks are subtracted arbitrarily from the original data assuming Lorentzian lineshapes.


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The overall lineshapes and peak intensities of Ti L2,3-edge XAS reflect the local structures of the Ti ions, being the fingerprints for various titanium compounds25. For comparison, the spectra of a rutile and an anatase (TiO2) taken from Ref. 26, and the spectra of a clean TiN film (~15 nm) are appended in Figs. 2(c) and (f). The representative features from the respective references are highlighted by filled triangles, open triangles, and open squares. The spectra of the films annealed without external gas supply (annealed in vacuum) and with a partial pressure of P(N2)=0.1 Torr in Fig. 2(a), appear to be similar to the spectrum of the reference TiN film [Fig. 2(c)]. This suggests that TiN structure is roughly maintained for those samples. However, at the same time, the peaks highlighted by the open squares are broadened and the first main peaks near hν = 458 eV are enhanced, compared to the spectrum of TiN. The signatures of oxidation (triangles for the rutile and anatase local structures) are shown more evidently in Fig. 2(d). Therefore, the two HfO2/TiN samples are oxidized only in part. Meanwhile, all the other spectra appear fundamentally different from the spectrum of the reference TiN. The lineshapes of the spectra from the samples for P(N2)’s higher than 0.1 Torr (Figs. 2(a) and (d)) and for all P(O2)’s (Figs. 2(b) and (e)), are similar to each other except minor differences in peak intensities. The prominent peaks highlighted by the triangles in Figs. 2(d) and (e), suggest that those films underwent significant oxidations. Seeing that the peak intensities of the filled and open triangles are overall comparable, we can judge that the samples are composed of mixtures of rutile and anatase. The oxidation by the PDA under O2 environment (Fig. 2(b) and (e)) can be easily understood as the consequence of oxygen migration from ambient O2 gas. In contrast to the case of the vacuum annealing, the O2 PDA should effectively oxidize the TiN films. However, the oxidation by the PDA under N2 environment (Fig. 2(a) and (d)) appears not plausible, because an increase


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of P(N2) should have stabilized TiN preventing oxidation.20 Rather, the oxidation for the P(N2)=1,2, and 5 Torr samples, but not for P(N2)=0.1 Torr or 0, might be from some not intrinsic but adventitious origins we cannot engineer. Interestingly, it was reported that TiN can be oxidized even under N2 gas environment.35 It was argued that a very small amount of O2 gas existed as impurities even in a “pure” N2 gas cylinder as to result in TiN oxidation.35 The remaining O2 gas in the thermal processing chamber under the base pressure (~10-3 Torr) can contribute to the oxidation as well. Therefore, the oxidation of the films can be promoted (but not significantly) as the P(N2) increases. Likewise, Fig. 3 shows the Ti L2,3-edge XAS spectra of the ZnO/TiN films after the PDA under N2/O2 environment. The spectra with various P(N2)’s and P(O2)’s are shown in Figs. 3(a) and (b), respectively. Their magnified views for the high-energy satellites are presented in Figs. 3(c) and (d) after subtracting Lorentzian slopes as was done in Figs. 2(d) and (e). The spectrum of the vacuum-annealed ZnO/TiN sample shows a similar lineshape with that of anatase, suggesting that the TiN layers in contact with ZnO are readily oxidized to form an anatase-like local structure. The stronger tendency of oxidation in ZnO/TiN than in HfO2/TiN is reasonable in that ZnO is thermodynamically less stable compared to HfO2. It is also observed in Figs. 3(a) and (c) that for all P(N2)’s, the lineshapes are very similar to that of anatase. This indicates the predominance of anatase-like local structure in the N2 PDA ZnO/TiN samples. Certainly, donation of oxygen from ZnO should be the dominant mechanism of the TiN oxidation. Meanwhile, for the case of O2 PDA ZnO/TiN samples, oxidation was further promoted by the external O2 supply. Figs. 3(b) and (d) show that the O2 PDA samples show a variety of oxide phases with increasing P(O2)’s. For P(O2)=0.1 Torr, a mixture of rutile and anatase phases prevails, as is highlighted by both the filled and open triangles (for rutile and anatase,


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respectively). For P(O2)=1 Torr, the features of the open triangles become weakened suggesting that the rutile phase prevails. As P(O2) increases further, the doublet near hν = 460 eV starts to merge to constitute a sharper peak (highlighted by stars). Generally, merging of features is a signature of transformation to a higher site symmetry.26,36 In the anatase and rutile structures, TiO6 octahedra are distorted sharing the four and two edges, respectively, resulting in lower site symmetries of D4h’s.34 On the other hand, in the case of perovskite titanate as in SrTiO3 or ilmenite (FeTiO3), TiO6 octahedra share their corners only being almost free from structural distortions (Oh symmetry)25. For the comparison, the spectrum of SrTiO3 (Ref. 26) is appended. The spectrum of P(O2)=2 or 5 Torr ZnO/TiN sample is very similar to that of SrTiO3. This implies that under a high oxygen abundance, TiN can transform into an oxide which has a perovskite-like local structure. It will be demonstrated shortly in the following paragraphs that the titanate is most plausibly ZnTiO3.

B. O K-edge XAS Fig. 4 shows the O K-edge XAS spectra of the HfO2/TiN (a,b) and ZnO/TiN (d,e) films after the PDA under N2 (a,d) or O2 (b,e) environments. Similar to Ti L2,3-edge XAS, O K-edge XAS reflects the unoccupied electronic structure of the Ti ions that are bound with O 2p orbitals through the O 2p – Ti 3d (or 4sp) orbital hybridizations.37 The first two main peaks at hν = 530535 eV for each of the spectra can be attributed to the unoccupied Ti 3d-t2g and 3d-eg manifolds, whose intensities might change depending on the details of the local structures (see e.g. Fig. 4(c)). The next main peaks at hν = 537-548 eV represent the unoccupied Ti 4sp states, whose lineshapes are known to vary significantly depending the details of the local structures as


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well.38,39 The overall intensities of the three spectra from the HfO2/TiN samples prepared with PDA under vacuum and P(N2)=0.1 Torr, and the ZnO/TiN sample prepared with PDA under vacuum, are relatively weak due to the incomplete oxidation. The O K-edge XAS spectra of SrTiO3, rutile and anatase taken from Ref. 26 are also appended in Fig. 4(c) and (f). As in Figs. 2 and 3, the signatures of the three reference materials are highlighted by stars, filled triangles, and open triangles, respectively. The results of the fingerprint analyses for the O K-edge XAS data are identical to the conclusions drawn from the Ti L2,3-edge XAS analyses; for HfO2/TiN, a mixture of rutile and anatase phases prevails, while for ZnO/TiN, an anatase phase is dominant for N2-PDA samples but a systematic evolution in local structure (rutile+anatase, rutile, and yet a high symmetry phase as in a perovskite titanate) with increasing O2 pressure for the O2-PDA samples. Particularly notable is the similarity in the spectra between the P(O2)=2 or 5 Torr ZnO/TiN sample and SrTiO3 shown in Fig. 4(e). It is known, however, that the doublet highlighted by stars near hν = 536 eV originated from the Sr 4d orbitals, which cannot exist in the ZnO/TiN system.40 Instead, the doublet can be attributed to an involvement of Zn in the titania film, namely, formation of a zinc titanate (ZnTiO3) local structure. The Zn 4sp orbitals can be localized enough to contribute to the sharp features near hν = 536 eV.41 Although it is unclear how the external O2 gas plays a role in stabilizing the ZnTiO3-like local structure, it can be told that the high reactivity of ZnO would facilitate the Zn incorporation to form N-ZnTiO3 phase. Fig. 5 shows the schematic phase diagram for the local structures in the films with increasing P(O2). For HfO2/TiN, only the mixtures of rutile and anatase were observed for all P(O2)’s except for the sample annealed in vacuum. On the other hand, for ZnO/TiN, a variety of phases


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was observed; an anatase local structure observed at P(O2)=0, and the local structure gradually transforms into a rutile local structure [P(O2)= 1 Torr] and does further into a titanate local structure (ZnTiO3) [P(O2)=2 and 5 Torr]. It is shown for ZnO/TiN, the mixed rutile + anatase phase exists only in a narrow P(O2) range (near 0.1 Torr), while the similar mixed phases prevail in most P(O2) ranges in the HfO2/TiN system. This suggests that ZnO/TiN system, compared to HfO2/TiN, undergoes the phase changes much faster with increasing P(O2). The faster phase changes in ZnO/TiN signify that the chemical instability of the interface can somehow facilitate the phase evolutions toward the anatase, rutile, and titanate phases. At this moment, it is unclear how such the phase changes are correlated with the interface stability. Extensive theoretical and experimental studies on the structural and charge configurations at the interface might be needed to clarify this correlation. In the next section, we discuss the consequence of the local structural change on the electrical properties (Sec.4.A) and the possible origin of such peculiar evolution in the titania phases upon the PDA (Sec.4.B). Also, we demonstrate the nature of the nitrogen incorporation in the semiconducting titania films, addressing the chemical states of nitrogen ions remaining after the thermal treatments (Sec. 4.C).

C. DISCUSSION A. Electronic Properties The electronic structure above the chemical potential can be scrutinized by examining the low energy parts in the O K-edge XAS spectra. One of the advantages of using O K-edge XAS is that


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we can deduce the electronic structure directly due to negligible final state effects.38,42 Fig.6 shows the magnified views of the O K-edge XAS spectra of the ZnO/TiN samples near the conduction band (CB) edges. The edge energies were estimated by extrapolations at the highest slopes to the abscissa. The edge energies of all the N2 PDA ZnO/TiN samples (Fig. 6(a)) remained at ~529.7 eV, while those of the O2 PDA samples (Fig. 6(b)) exhibit substantial shifts; they were ~529.6 eV for P(O2)= 0.1 and 2 Torr samples, ~529.5 eV for P(O2)= 1 Torr sample, and ~529.7 eV for P(O2)= 5 Torr sample. For low P(O2) samples, the edge energy tends to decrease with increasing P(O2)’s. Compared to the case of the vacuum-annealed sample, the edge energies of P(O2)= 0.1 Torr and 1 Torr samples changed by -0.1 eV and -0.2 eV, respectively. This is very consistent with the anatase to rutile phase change, in that the edge energy for rutile is ~0.2 eV lower than for anatase.26 Thus, the lower energy shift for low P(O2) samples most plausibly originate from the change in the phase abundance within the regime of titania (see Fig. 5). In contrast, for higher P(O2) samples, the edge energy starts to increase with increasing P(O2)’s. Compared to the edge energy of P(O2)=1 Torr sample, those of P(O2)= 2 and 5 Torr samples increased by +0.1 eV and +0.2 eV, respectively. This turnover should be related to the emergence of the new ZnTiO3-like perovskite phase (titanate). Ti site symmetry in the perovskite-like local structure should be higher than that in rutile or anatase. Suppression of the multiplet splitting in the electronic structure due to high symmetry would result in an increased lower bound of the CB energies. Thus the higher energy shift for higher P(O2) samples might also originate from a phase evolution; but this time it is related to entering into the regime of the titanate (displacement in a vertical way in the right panel of Fig. 5). Therefore, it can be


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concluded that the edge shifts observed in Fig. 6(b) are in accordance with the phase changes from anatase to rutile, and further to the titanate local structures. It should be also noted that practically, for sketching the density of states (DOS), the energies need to be readjusted with respect to the chemical potentials, because the values of hν’s for given electron states in the O K-edge XAS data are the differences in energy from the O 1s level, not referenced with the chemical potential.43 For instance, according to the results of the recent calculations,30 the CB edge of rutile should be higher than that of anatase by 0.22 eV when the energy is referenced by the chemical potential. This appears seemingly contradictory to our observation in Fig. 5(b), that is, a lower energy shift for rutile-like local structure. However, the controversy can be resolved by considering the shift of the chemical potential since the samples we prepared are, in fact, contains nitrogen, which can give rise to a doping effect.16,44 (See Sec. 4.C for more details on the nitrogen detection.) This implies that the effects of band alignments for the energy readjustment could be crucial for deducing the CB energies correctly.

B. Morphology In order to investigate the origin of the local structural evolution, we examined the microstructures at the surfaces of the ZnO/TiN films using FESEM. Fig. 7 shows the surface morphology of the titania films after PDA under N2 (a-c) or O2 (d-f) environment. As the P(N2) increases (from Fig. 7(a) to (c)), the average sizes of the grains tend to increase slightly but they remain below 30 nm. In contrast, as the P(O2) increases (from Fig. 7(d) to (f)), large-sized grains with average radius of ~50 nm emerge for the sample of P(O2)= 1 Torr, and the grains become far bigger (over 100 nm) for the P(O2)= 2 Torr sample. By comparing with the images obtained


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by alternative detectors installed in a grazing-exit geometry (not shown), we confirmed that the huge grains are formed indeed on the top of the surface. It is well known that the stability of the titania phases changes with increasing crystal size; when the crystal size is smaller than 11 nm, an anatase structure is most stable due to enhanced contribution of surface free energy along with unsatisfied surface charge distributions, while when the crystal size exceeds 35 nm, a rutile structure becomes stabilized.45,46 The sizes of the grains estimated in Fig. 7 would represent roughly the sizes of the crystallinity of the titania. As the grains become smaller, the contribution of surface (of the grains) with lack of atomic periodicity becomes significant compared to that of the bulk. This can lead to a substantial degree of distortion in the Ti-O coordination being accompanied by edge sharing between two adjacent octahedra. In the anatase structure, TiO6 octahedra share four edges with a substantial distortion, while in the rutile structure, TiO6 share only two edges with less distortion.38 We speculate for even larger grain sizes, the octahedra can have a far more relaxed structure in which the octahedra do not share the edges but the corners only. This is consistent with the perovskite ZnTiO3-like local structures, which is what we have suggested from the XAS results. Therefore, it can be concluded that the evolution of the grain sizes with the gas environment during the PDA, is very consistent with the observation in the XAS spectra (Figs. 3 and 4).

C. Nitrogen Incorporation Oxidation of TiN will lead to the ionic exchange between N3- ions in TiN and O2- ions either from the oxides in contact or from the ambiance. The chemistry and the local structural environments of the N atoms after the PDA are examined by N K-edge XAS. Fig. 8 shows the N


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K-edge XAS spectra of the HfO2/TiN (a) and ZnO/TiN (b) films for representative P(N2)/P(O2) values during the PDA. For the case of the vacuum-annealed and P(N2)= 0.1 Torr samples in Figs. 8(a) and (b), the spectra reflect mostly the electronic structures of TiN.17,37 In TiN (N3-), N 2p orbitals are almost filled but partially unfilled due to N 2p – Ti 3d orbital hybridization. Therefore, the N K-edge XAS spectra reflect the unoccupied electronic structure of Ti 3d or 4sp states in TiN. It is known that TiN has a rocksalt structure, so that the energies of the Ti orbitals states are determined by the TiN6 octahedral coordination. This is very similar to the case of O K-edge XAS for titania, where TiO6 octahedral coordination prevails. As a result, the N K-edge spectrum of TiN appears to be the same as the O K-edge spectrum of TiO2. Therefore, the features at hν = 401-410 eV should be attributed to the Ti 3d-t2g and eg manifolds for TiN, while those at hν = 410-420 eV should be to the Ti 4sp, same as in Fig. 4. Besides, also observed is the exceptionally sharp features near hν = 407 eV. This can hardly originate from peculiar electronic structure of anionic nitrogen, because in TiN, N 2p shells are almost filled (2p6) so that no such well-defined quantum states (and so with the sharp atomic-like features) can exist. Rather, the sharp features have been frequently observed in the spectra of molecules that contain nitrogen atoms.37 According to literatures, however, peaks for π- or σbonding in common N-containing molecules (N2 or NO) cannot have such an energy; that is, the resonance positions for π(N*≡N) or π(N*≡O) should be below 401 eV,47,48 whereas the resonance positions for σ(N*≡N) or σ(N*≡O) should be above 414 eV.47,48 The most appropriate assignment for the peaks could be to certain TiNx remaining in molecular forms among TiO2 or TiN. Since the Ti-N bonds in the molecular forms would be more covalent than in TiN crystal, the


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electronegativity of the N ions in the molecular forms would be higher than those in the lattice so as to have a higher energy than the t2g manifolds (near hν = 404 eV). Interestingly, the lineshapes of the two P(O2)= 0.1 Torr samples in both figures are very different from those of the others: P(N2)=0.1 Torr or vacuum-annealed samples. We notice that the features from the structurally ordered TiN are all disappeared but only the features from the molecular residues remain. The small bumps near hν = 410 eV in the spectra of the P(O2)= 0.1 Torr samples, can be assigned as the ionization potentials of the molecular residues.49 We also found that the spectra of all the other samples, i.e. the HfO2/TiN and ZnO/TiN samples of P(O2)’s or P(N2)’s higher than 0.1 Torr (not shown in Fig. 8) appear to be very similar to those of the P(O2)= 0.1 Torr samples. This suggests the oxidation due to the O2 PDA accomplished the N↔O exchange and then drove out the N atoms very efficiently. Therefore, it is confirmed that the TiN films transformed after the PDA into titanium oxides incorporated by nitrogen (N-TiO2 or N-ZnTiO3). It is noteworthy that the lineshape of the N K-edge spectrum is not correlated with the abundances of the oxide phases. For example, the spectra of HfO2/TiN and ZnO/TiN with P(N2)= 0.1 Torr appear alike, although the former and the latter has been revealed to be almost TiN (lightly oxidized) and an anatase (TiO2), respectively. This can happen because XAS at N K-edge captures the signals from the N-containing lattices or molecules only. Furthermore, no apparent spectral evolution was observed in the ZnO/TiN samples with increasing P(O2)’s from 0.1 Torr to 5 Torr (not presented in Fig. 8) in spite of the titania phase changes evidenced in Figs. 3 and 4. This implies that N concentration in the formed N-TiO2’s can be controlled hardly within the TiN-oxidation method. Thus, the controllability of “N doping” for tailoring the electronic properties might not be effective.


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This study showed that the electronic structure of the titania and titanate films evolved in accordance with the local structural evolution rather than the nitrogen concentrations. The oxidation of TiN films deposited on ZnO, could not be prevented even with annealing under N2 environment. This suggests the reaction at the interfaces between ZnO and TiN is more important than the external gas environment in tailoring the local structures in the oxide thin films. Therefore, adopting an appropriate bottom (or interface) oxide in contact with TiN, can be a pathway to implement the phase controls on the N-TiO2 films.

CONCLUSION XAS and FESEM studies on the TiN thin films deposited on HfO2 and ZnO after thermal treatments showed that the films become N-incorporated titanium oxides. The oxide phases evolved with the gas environment during the PDA. In the case of HfO2/TiN system, a mixture of rutile and anatase-like local structures prevail after PDA with abundant N2/O2 gas. In the case of ZnO/TiN system, various phases emerge depending on the gas environment; an anatase-like local structure for the N2-PDA samples, while rutile + anatase, rutile, and a perovskite ZnTiO3like local structure for the O2-PDA samples, with the increasing order of the O2 partial pressures. These local structural evolutions are correlated with the evolution in the grain sizes and the conduction band structures, suggesting that the electronic properties of the N-TiO2 or N-ZnTiO3 films can be tuned efficiently by the choice of bottom oxides as well as the thermal treatment.


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FIGURES

Figure 1: (Color online) Ti L2,3-edge XAS spectra of the HfO2/TiN (a) and ZnO/TiN (b) films annealed at 700oC under N2 or O2 environment or without additional gas supply. A clear evolution of lineshapes upon the oxide and the PDA condition suggests that the oxidation strongly influences the chemistry and local structures of the TiN films.


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Figure 2: (Color online) Ti L2,3-edge XAS spectra of the HfO2/TiN films annealed under N2 (a) or O2 (b) environment, and the magnified views of the higher energy satellites in (d) and (e), respectively. For comparison, the spectra of a clean TiN film, and a rutile and an anatase taken from Ref. 26, are displayed in (c) and (f), and their signatures are highlighted by □’s, ▼’s, and ▽’s, respectively. It is clearly shown that all the HfO2/TiN films are readily oxidized and remained as mixtures of rutile + anatase TiO2, except for the vacuum annealed sample, in which TiN is oxidized only in part.


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Figure 3: (Color online) Ti L2,3-edge XAS spectra of the ZnO/TiN films annealed under N2 (a) or O2 (b) environment, and the magnified views of the higher energy satellites in (c) and (d), respectively. The spectra of a SrTiO3 taken from Ref. 26 are appended for comparison. The signatures of SrTiO3, rutile, and anatase, are marked by stars, ▼’s, and ▽’s, respectively. The ZnO/TiN films showed a variety of oxide phases ranging from an anatase (N2 PDA in (a)) to a rutile and a perovskite-like high symmetry phase (O2 PDA in (b)).


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Figure 4: (Color online) O K-edge XAS spectra of the HfO2/TiN (a,b) and ZnO/TiN (d,e) films annealed at 700oC under N2 (a,d) or O2 (b,e) environment. For comparison, the spectra of a SrTiO3, rutile and anatase TiO2’s taken from Ref. 26 are appended in (c) and (f). The signatures of SrTiO3, rutile, and anatase TiO2 are marked by stars, ▼’s, and ▽’s, respectively.


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Figure 5: (Color online) Schematic phase diagram for HfO2/TiN and ZnO/TiN with increasing P(O2) during the PDA at 700oC for 1 min.


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Figure 6: (Color online) Magnified views of the O K-edge XAS spectra near the conduction band edges. The spectra of the samples annealed under N2 environment (a) showed no evolution in the edge energies, while those of the samples annealed under O2 environment (b) showed a significant evolution in accordance with the changes in the local structures from anatase-like structure toward rutile and ZnTiO3-like local structures with the order of increasing P(O2).


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Figure 7: The morphologies of the film surfaces after the PDA treatments, examined by field emission SEM. The sizes of the grains tend to increase moderately (limited by less than 30 nm) with increasing P(N2)’s (a-c), whereas they increase abruptly (over 100 nm) with increasing P(O2)’s (d-f).


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Figure 8: (Color online) N K-edge XAS spectra of the HfO2/TiN (a) and ZnO/TiN (b) films. The sharp peaks near hν = 407 eV can be attributed to N-ions in certain molecular forms, while the broad features in the spectra of the P(N2)=0.1 Torr and the vacuum-annealed samples can be to TiN. The broad features almost disappeared for the P(O2)=0.1 Torr samples and for all the other samples not presented here, reflecting that the TiN films became mostly oxidized with incorporation of nitrogen in the molecular forms.


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AUTHOR INFORMATION Corresponding Author *The author to whom correspondence should be addressed. [email protected], Tel: +82 63 270 3444.

Author Contributions D.-Y.C. conceived the experiments and supervised the research. D. K, M. L, S.-Y. S. and D.Y.C. conducted the XAS and analyzed the data. D. H. and T. J. P. fabricated the samples. D.K. and D.-Y.C. performed the FESEM measurement and wrote the manuscript. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015R1C1A1A02037514 and 2015R1A5A1037548). D.-Y.C. acknowledges the support from Research Base Construction Fund Support Program funded by Chonbuk National University in 2016.


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REFERENCES (1) Winklet, J. Titanium dioxide, production, properties and effective usage; Vincentz Networks: Hanover, 2013. (2) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37−38. (3) O’Regan, B.; Gratzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353, 737-740. (4) Xin, X.; Zhou, X.; Wu, J.; Yao, X.; Liu, Z. Scalable synthesis of TiO2/graphene nanostructured composite with high-rate performance for lithium ion batteries. ACS Nano 2012, 6, 11035−11043. (5) Kim, S. K.; Kim, K. M.; Jeong, D. S.; Jeon, W.; Yoon, K. J.; Hwang, C. S. Titanium dioxide thin films for next-generation memory devices. J. Mater. Res., 2013, 28, 313-325. (6) Zhao, W.; Ma, W.; Chen, C.; Zhao, J.; Shuai, Z. Efficient degradation of toxic organic pollutants with Ni2O3/TiO2-xBx under visible irradiation. J. Am. Chem. Soc. 2004, 126, 47824783. (7) Sakthivel, S.; Kisch, H. Daylight photocatalysis by carbon-modified titanium dioxide. Angew. Chem., Int. Ed. 2003, 42, 4908-4911. (8) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001, 293, 269-271. (9) Yu, J. C.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Effects of F-doping on the photocatalytic activity and microstructures of nanocrystalline TiO2 powders, Chem. Mater. 2002, 14, 38083816. (10) Sato, S. Photocatalytic activity of NOx-doped TiO2 in the visible light region. Chem. Phys. Lett. 1986, 123, 126-128. (11) Ghicov, A.; Macak, J. M.; Tsuchiya, H.; Kunze, J.; Haeublein, V.; Frey, L.; Schmuki, P. Ion implantation and annealing for an efficient N-doping of TiO2 nanotubes. Nano Lett., 2006, 6, 1080-1082. (12) Yang, G.; Jiang, Z.; Shi, H.; Xiao, T.; Yan, Z. Preparation of highly visible-light active Ndoped TiO2 photocatalyst. J. Mater. Chem. 2010, 20, 5301-5309. (13) Saha, N.C.; Tompkins, H.G. Titanium nitride oxidation chemistry: an x-ray photoelectron spectroscopy study. J. Appl. Phys. 1992, 72, 3072-3079. (14) Di Valentin, C.; Finazzi, E.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Paganini, M. C.; Giamello, E. N-doped TiO2: theory and experiment. Chem. Phys. 2007, 339, 44-56.


27


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Page 28 of 31

(15) Wittmer, M.; Noser, J.; Melchior, H. Oxidation kinetics of TiN thin films. J. Appl. Phys. 1981, 52, 6659. (16) Zhou, X.; Peng, F.; Wang, H.; Yu, H.; Yang, J. Preparation of nitrogen doped TiO2 photocatalyst by oxidation of titanium nitride with H2O2. Mater. Res. Bull. 2011, 46, 840-844. (17) Chen, S. C.; Sung, K. Y.; Tzeng, W. Y.; Wu, K. H.; Juang, J. Y.; Uen, T. M.; Luo, C. W.; Lin, J-Y.; Kobayashi, T.; Kuo, H. C. Microstructure and magnetic properties of oxidized titanium nitride thin films in situ grown by pulsed laser deposition. J. Phys. D: Appl. Phys. 2013, 46, 075002. (18) Vaz, F.; Cerqueira, P.; Rebouta, L.; Nascimento, S.M.C.; Alves, E.; Goudeau, Ph.; Rivière, J.P. Preparation of magnetron sputtered TiNxOy thin films, Surface and Coatings Technology, 2003, 174–175, 197-203. (19) Mohamed, S. H.; Kappertz, O.; Niemeier, T.; Drese, R.; Wakkad, M. M.; Wuttig, M. Effect of heat treatment on structural, optical and mechanical properties of sputtered TiOxNy films. Thin Solid Films, 2004, 468, 48-56. (20) Ponon, N. K.; Appleby, D. J. R.; Arac, E.; King, P. J.; Ganti, S.; Kwa, K. S. K.; O'Neill, A. Effect of deposition conditions and post deposition anneal on reactively sputtered titanium nitride thin films. Thin solid Films. 2015, 578, 31-37. (21) Moatti, A.; Bayati, R.; Narayan, J. Epitaxial growth of rutile TiO2 thin films by oxidation of TiN/Si{100} heterostructure. Acta Materialia 2016, 103, 502-511. (22) CRC Handbook of chemistry and physics, 81st ed., 2000. (23) Chase, M.W., Jr., NIST-JANAF thermochemical tables, fourth edition, J. Phys. Chem. Ref. Data, Monograph 9, 1998, 1-1951. (24) Wenger, Ch.; Lukosius, M.; Weidner, G.; Müssig, H. -J.; Pasko, S.; Lohe, Ch. The role of the HfO2-TiN interface in capacitance-voltage nonlinearity of metal-insulator-metal capacitors. Thin Solid Films. 2009, 517, 6334-6336. (25) De Groot, F. M. F.; Figueiredo, M. O.; Basto, M. J.; Abbate, M.; Petersen, H.; Fuggle, J. C. 2p X-ray absorption of titanium in minerals. Phys. Chem. Miner.1992, 19, 140-147. (26) Van der Laan, G. Polaronic satellites in x-ray-absorption spectra. Phys. Rev. B. 1990, 41, 12366. (27) Kucheyev, S. O.; van Buuren, T.; Baumann, T. F.; Satcher, Jr. J. H.; Willey, T. M.; Meulenberg, R. W.; Felter, T. E.; Poco, J. F.; Gammon, S. A.; Terminello, L, J. Electronic structure of titania aerogels from soft x-ray absorption spectroscopy. Phys. Rev. B. 2004, 69, 245102. (28) Diebold, U. The surface science of titanium dioxide, Surf. Sci. Rep. 2003, 48, 53-229.


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Page 29 of 31

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


(29) Wyckoff, R.W.G. Crystal Structures Volume 1; John Wiley & Sons: New York, 1964. (30) Scanlon, D. O.; Dunnill, C. W.; Buckeridge, J.; Shevlin, S. A.; Logsdail, A. J.; Woodley, S. M.; Catlow, C. R. A.; Powell, M. J.; Palgrave, R. G.; Parkin, I. P. et al. A. Band alignment of rutile and anatase TiO2. Nat. Mater. 2013, 12, 798-801. (31) Di Paola, A.; Bellardita, M.; Palmisano, L. Brookite, the least known TiO2 photocatalyst. Catalysts, 2013, 3, 36-73. (32) Reyes-Coronado, D.; Rodríguez-Gattorno, G.; Espinosa-Pesqueira, M. E.; Cab, C.; de Coss, R.; Oskam, G. Phase-pure TiO2 nanoparticles: anatase, brookite and rutile. Nanotechnology, 2008, 19, 145605. (33) Crocombette, J. P.; Jollet, F. Ti 2p x-ray absorption in titanium dioxides (TiO2): the influence of the cation site environment. J. Phys.: Condens. Matter. 1994, 6, 10811-10821. (34) Thakur, H.; Kumar, R.; Thakur, P.; Brookes, N. B.; Sharma, K. K.; Singh, A. P.; Kumar, Y.; Gautam, S.; Chae, K. H. Modifications in structural and electronic properties of TiO2 thin films using swift heavy ion irradiation. J. Appl. Phys. 2011, 110, 083718. (35) Lu, F.; Lo, J. The influences of oxygen impurity contained in nitrogen gas on the annealing of titanium nitride. J. Eur. Ceram. Soc. 2002, 22, 1367-1374. (36) Waychunas, G.A. Synchrotron radiation XANES spectroscopy of Ti in minerals: Effects of Ti bonding distances, Ti valence, and site geometry on absorption edge structure. Am. Mineral. 1987, 72, 89-101. (37) Soriano, L.; Abbate, M.; Fuggle, J. C.; Prieto, P.; Jiménez, C.; Sanz, J. M.; Galán, L.; Hofmann, S. Thermal oxidation of TiN studied by means of soft x-ray absorption spectroscopy. J. Vac. Sci. Technol., A. 1993, 11, 47. (38) Mizoguchi, T.; Tanaka, I.; Yoshioka, S.; Kunisu, M.; Yamamoto, T.; Ching, W. Y. Firstprinciples calculations of ELNES and XANES of selected wide-gap materials: Dependence on crystal structure and orientation. Phys. Rev. B 2004, 70, 045103. (39) Kronawitter, C. X.; Kapilashrami, M.; Bakke, J. R.; Bent, S. F.; Chuang, C.-H.; Pong, W.-F.; Guo, J.; Vayssieres, L.; Mao, S. S. TiO2-SnO2:F interfacial electronic structure investigated by soft x-ray absorption spectroscopy. Phys. Rev. B 2012, 85, 125109. (40) Tang, Y.-H.; Tsai, M.-H.; Jan, J. C.; Pong, W. F. First-principles and x-ray absorption studies of the electronic structures of Ba1-xSrxTiO3 alloys. Chin. J. Phys. 2003, 41, 167-176. (41) Cho, D.-Y.; Kim, J. H.; Na, K. D.; Song, J.; Hwang, C. S.; Park, B.-G.; Kim, J.-Y.; Min, C.H.; Oh, S.-J. Spectroscopic evidence for limited carrier hopping interaction in amorphous ZnO thin film. Appl. Phys. Lett. 2009, 95, 261903. (42) Kang, Y.; Lee, S.; Sim, H.; Sohn, C. H.; Park, W. G.; Song, S. J.; Kim, U. K.;Hwang, C. S.; Han, S.; Cho, D.-Y. The impact of orbital hybridization on the electronic structure of crystalline


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InGaZnO: a new perspective on the compositional dependence. J. Mater. Chem. C 2014, 2, 9196-9204. (43) Cho, D.-Y.; Jung, H.-S.; Hwang, C. S. Structural properties and electronic structure of HfO2-ZrO2 composite films. Phys. Rev. B 2010, 82, 094104. (44) Cong, Y.; Zhang, J.; Chen, F.; Anpo, M. Synthesis and characterization of nitrogen-doped TiO2 nanophotocatalyst with high visible light activity. J. Phys. Chem. C. 2007, 111, 6976-6982. (45) Kandiel, T. A.; Robben, L.; Alkaim, A.; Bahnemann, D. Brookite versus anatase TiO2 photocatalysts: phase transformations and photocatalytic activities. Photochem. Photobiol. Sci. 2013, 12, 602-609. (46) Zhang, H. Z.; Banfield, J. F. Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: insights from TiO2, J. Phys. Chem. B, 2000, 104, 3481-3487. (47) Hitchcock, A. P.; Brion, C. E. K-shell excitation spectra of CO, N2, and O2. J. Electron Spectrosc. Relat. Phenom. 1980, 18, 1-21. (48) Wight, G. R.; Brion, C. E. K-shell energy loss spectra of 2.5 keV electrons in CO2 and N2O. J. Electron Spectrosc. Relat. Phenom. 1974, 3, 191-205. (49) Stöhr, J. NEXAFS spectroscopy; Springer-Verlag: Berlin, 1992.


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Observation of Titania and Titanate Phase Changes in Oxidation

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