Surface modification of yttria-stabilized-zirconia thin films under various oxygen partial pressures

Thin Solid Films 520 (2012) 5826–5831

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Surface modification of yttria-stabilized-zirconia thin films under various oxygen partial pressures J.S. Bae a, S.-S. Park b, B.S. Mun c, S.H. Park b, E.S. Hwang b, J. Kim d, J. Huh e, H.J. Park f, J.S. Kim f, H.J. Yun g, H.G. Kim a, S.Y. Jeong h, J. Hwang b, i, S. Park b,⁎ a

Busan Center, Korea Basic Science Institute, Busan 609-735, Republic of Korea Department of Physics, Pusan National University, Busan 609-735, Republic of Korea Department of Applied Physics, Hanyang University, ERICA, Ansan 426-791, Republic of Korea d National Core Research Center for Extreme Light Applications, Gwangju Institute Science & Technology, Gwangju 500-712, Republic of Korea e Chonnam National University, Yeosu 550-749, Republic of Korea f Research Center for Dielectric & Advanced Matter, Pusan National University, Busan 609-735, Republic of Korea g Jeunju Center, Korea Basic Science Institute, Jeonju 561-756, Republic of Korea h Department of Cogno-mechatronics Engineering, Pusan National University, Miryang 627-706, Republic of Korea i Department of Physics, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, Republic of Korea b c

a r t i c l e

i n f o

Article history: Received 9 January 2011 Received in revised form 3 February 2012 Accepted 8 February 2012 Available online 21 February 2012 Keywords: Oxide thin films X-ray diffraction Spectroscopy Surface structure Optical band-gap

a b s t r a c t This report discusses the structural and spectroscopic analysis of yttria-stabilized-zirconia (YSZ) thin films grown on Al2O3(0001) substrates. It is found that the changes of oxygen partial pressure during the growth are closely related to the surface chemical compositions and the surface crystal orientations of the thin films. The presence of oxygen partial pressure produces a polycrystalline structure on the thin film while a preferred orientation of crystal structures is formed under no oxygen partial pressure. Difficulty arises in identifying the structure of the thin films due to the broad characteristics of the x-ray diffraction (XRD) peaks; however, the XRD rocking scan suggests the existence of two lateral domain sizes. The chemical analysis of the thin films from x-ray photoelectron spectroscopy measurements indicates the enrichment of surface yttrium-oxide as the oxygen partial pressure increases. The detailed analysis of valence band spectra also suggests that the thin films undergo a surface structural phase transition, i.e., transforming from a single tetragonal structure to a mixed (cubic + monoclinic) structure. Furthermore, the optical data display the small increments of the band gap as the oxygen partial pressure increases, which reflects the presence of the structural phase transition of the thin films. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Among many oxide systems, yttria-stabilized-zirconia (YSZ) thin films showed many promising characteristics as a replacement material for SiO2 gate dielectrics of Si-based devices [1] and as an electrolyte for solid oxide fuel cell applications [2–4]. Recently, it was shown that the epitaxial YSZ thin films exhibited unusual transport properties at the interface of oxide fuel cell, caused by the large ionic (oxygen) conductivities. The discovery of these new physical properties garnered high interest in these oxide materials [5]. In fact, there have been many efforts to utilize the anisotropy of oxide single crystals for the growth of nanocrystalline thin films with different crystal orientations, which exhibited various physical properties due to the different surface energies [6]. Especially, in the

⁎ Corresponding author. Tel.: + 82 51 510 2595; fax: + 82 51 513 7664. E-mail address: [email protected] (S. Park). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.02.049

growth of thin film, it is well known that the growth environments are closely related to the physical properties of the thin film. For example, depending on the growth conditions, two thin films with identical compositions can display very different physical properties. Consequently, the investigation on the growth parameters of oxide thin films, such as the partial pressure of oxygen, growth temperature, and growth methods [7–15,31,16], has become an important research field for the study of oxide thin films. Previously, we demonstrated that the YSZ thin films grown on Si substrate using a pulsed laser deposition (PLD) process maintained excellent chemical stoichiometry [16,17]. Among many other growth methods of oxide thin films, a PLD method is known to be one the best techniques to replicate the composition of the target materials [18]. Due to the different vapor pressures of the materials the maintaining consistent chemical stoichiometry between target materials and thin films is a challenging issue. In addition, the knowledge of those physical parameters facilitates the realization of new functionalities in future oxide electronics. Furthermore, the fabrication of the

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functional oxide materials at a nanometer scale becomes an important subject for utilizing new multi-functional physical properties in the fields of electronics, non-linear optics, and magnetism [19]. Therefore, it is important to investigate the physical properties of thin films as a function of growth parameters so that optimum conditions for the fabrication of ideal thin films can be obtained. Among many growth parameters, the variation of oxygen partial pressure is one of the key parameters to fabricate high-quality thin films due to its critical roles of oxygen in the functionality of thin films [20–22]. In this paper, we investigate on the structural and spectroscopic characteristics of YSZ thin films grown by PLD system with different oxygen partial pressures to examine the oxygen partial pressure dependent on the physical properties of YSZ thin films. With various state-of-the-art experimental tools, the important chemical and physical properties of YSZ thin films, such as the modifications of surface structure, surface compositions, and optical characteristics, are investigated as a function of oxygen partial pressure.

(a) (b)

(c) (d)

2. Experiments Various YSZ thin films were deposited onto Al2O3(0001) substrates using PLD system (λKrF = 248 nm, 5 Hz, 200 mJ; Nano-PLD-1000, PVD) at 700 °C to examine the growth environments (in particular oxygen partial pressure) dependence of structural and spectroscopic characteristics of the YSZ thin films. The oxygen partial pressure was varied from 0.0 mTorr to 200 mTorr and the deposition time was kept as 25 min. For the target material, a YSZ (8% yttrium doped) (100) single crystal (SC) substrate with a 2 inch diameter was used [23]. The base pressure of the PLD system was maintained under 2.8 × 10− 6 Torr and the distance between target and substrate was kept as 4.5 cm. The crystal structure of the YSZ thin films was examined using an xray diffractometer (PANalytical, X'pert PRO MRD) and Pohang light source (5C2 beamline) [24]. To examine both the chemical structures and the electronic structures of the sample surface, the x-ray photoelectron spectroscopy (XPS) was utilized, one of the most effective tools for those purposes [25,26]. Both the core-level and the valence band spectra of thin films were obtained as function of oxygen partial pressure. The monochromatized Al K (1486.6 eV) radiation (AXIS-NOVA, Kratos, Inc) was used as the x-ray source. The effective irradiation area of x-ray focusing on sample was a 400 μm diameter. The base pressure inside the XPS analysis chamber was 4.2 × 10− 9 Torr. The 40 eV pass energy and the dwell time of 100 ms were chosen to obtain an optimum count-rate of XPS with the resolution of 0.46 eV. The flood gun was applied as a charge neutralizer when necessary. The data were acquired before and after 30 s Ar+ etching at 2 kV. The binding energy was calibrated using the C (1s) line (284.5 eV) for the XPS spectra of the films, measured before the etching. Since the reference C (1s) peak was removed after etching process, it was hard to determine the exact binding energy of each element for the etched thin films unless there exists a particular element which can be used as a reference binding energy. Therefore, the aforementioned spectra (etched ones) were used not for locating the absolute position, but for performing comparisons of FWHM and shape of the spectra. Finally, UV–VIS transmission spectra of the YSZ thin films were measured to examine the characteristics of optical properties and band-gaps of thin films as a function of oxygen partial pressure.

Fig. 1. X-ray diffraction (Bragg–Brentano geometry) patterns for the samples grown at 700 °C with (a) 0 mTorr, (b) 30 mTorr, (c) 100 mTorr, and (d) 200 mTorr oxygen partial pressures. The vertical dotted lines represent two major peak positions and their higher order peaks.

another peak (peak B ≈34.7 ± 0.1° and its higher order peak B′), suggesting that the sample transformed into a polycrystalline as the oxygen pressure increased. An accurate measurement of the peak position is important to identify the crystal structures of thin films. However, the measured x-ray diffraction peaks were too broad to identify whether the thin films were tetragonal in structure or cubic in structure. This ambiguity is resultant of the close proximity of the peaks. For example, the known cubic YSZ (111) (YSZ(200)) 2θ peak is 30.00 (34.780) degree and the known tetragonal YSZ (101) (YSZ (002)) 2θ peak is 30.170 (34.692) degree [27]. Therefore, the measured x-ray diffraction pattern did not allow us to determine the exact structure of YSZ thin films. Furthermore, the grazing incident x-ray diffraction measurements and ϕ-scan (not shown) also did not provide enough information to distinguish whether the YSZ thin films have a cubic or tetragonal structure. However, the existence of a slight increase in the peak position (peak A in Fig. 1) suggests that the increase of oxygen partial pressure possibly affects the variation of the structure by either a compressive strain (since 2θ increases) or structural change. It has been reported that the diffraction peak position also shifts to a higher angle as the oxygen (yttrium) concentration increases (decreases) in the cubic and tetragonal YSZ powders [27]. Therefore, it is possible to have the formation of different stoichiometries of the YSZ thin films as oxygen partial pressure increases. In addition, the intensity of peak B, relative to that of peak A, increased up to the sample grown at 100 mTorr, indicating the increase of the population of different crystal orientations. The full width at half maximum (FWHM) of the peaks, obtained from Gaussian fitting, also increased as the oxygen partial pressure increased, suggesting the variation of growth morphologies (e.g., grain sizes). Table 1 shows the Table 1 The 2θ values and FWHM of the two major peaks (A and B) obtained from Gaussian fitting and their grain size (D) obtained from Scherrer formula.

3. Results and discussion 3.1. Structural analysis Fig. 1 shows the x-ray diffraction pattern of the YSZ thin films grown on Al2O3(0001) substrates under various oxygen partial pressures. Without additional oxygen, the thin film exhibited a diffraction peak (peak A ≈29.7 ± 0.3°) and its higher order peak (peak A′). As the partial pressure of oxygen increased during the growth, the YSZ films exhibited

0 mTorr 30 mTorr 100 mTorr 200 mTorr

Peak Peak Peak Peak Peak Peak Peak Peak

A B A B A B A B

2θ (°)

FWHM (°)

D (Å)

29.5

0.48

171.

29.3 34.6 29.9 34.8 30.0 34.8

0.27 0.24 0.66 0.32 0.98 0.42

304. 347. 125. 260. 83.9 198.

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FWHM of the two main peaks (peak A and peak B) of the YSZ thin films and the corresponding grain sizes (D) obtained from Scherrer formula, D ¼ β Kλ , where λ is incident x-ray wavelength (Cu–K line), β0 is a 0 cosθ width (radian unit) of the peak (FWHM), and K = 0.9 [28–30]. As the partial pressure of oxygen increased, both peaks became broader, suggesting that the system possessed finer grains. Interestingly, the FWHM (grain size) of the YSZ became the smallest (largest) for the sample growth at 30 mTorr oxygen partial pressure. The oxygen partial pressure dependent on the measured x-ray rocking curves (circles) and fitted results (solid curves) of the YSZ thin films is shown in Fig. 2. The rocking curves for both peaks (the rocking curves of peak B are not shown here) exhibit two distinct features as the amount of oxygen partial pressure increases. The samples, deposited above 100 mTorr oxygen show one sharp (narrow) peak and one broad peak, indicating the system contained two different domain distributions. The sharp (broad) feature is associated with the larger (smaller) and highly (less) oriented structure. Table 2 shows the estimated FWHM for individual peaks after Lorentzian fitting. 3.2. Spectroscopic analysis The core-level XPS spectra of the Zr 3d, Y 3d, and O 1s from the YSZ thin films grown on Al2O3(0001) under the various oxygen partial pressures are shown in Fig. 3. The peak positions (i.e., binding energies) obtained from the curve fitting, after removing background, are listed in Table 3 [32]. The spin-orbit-splitting (SOS) energy was set to 2.43 eV and 2.05 eV for Zr 3d5/2 and Y 3d5/2 [33]. The values of FWHM of each spectra are ~1.5 eV, ~1.5 eV, and ~1.4 eV for Zr 3d, Y 3d, and O 1s elements, respectively. The reference O 1s and Zr 3d5/2 binding energies (BEs) (the vertical lines in Fig. 4) in the ZrO2 matrix were known as 530.0 eV and 182 eV, respectively [34–36]. The solid curves represent the best fitted results [32]. The measured binding energy of Zr 3d5/2 peak of the thin films was ~0.2 eV lower than the binding energy of the reference sample. However, it is ~0.5 eV higher than the binding energy of Zr 3d5/2 peak of the YSZ SC (not shown). The lower binding energy in comparison to

Fig. 2. Oxygen partial pressure dependent rocking curves of peak A of the YSZ thin films. The solid curves are representing the fitted results. For the samples grown above 100 mTorr, we used two peaks to fit the data.

Table 2 The FWHM of the x-ray rocking curve of the peaks A and B shown in Fig. 2. Partial pressure

Peak A FWHM (°)

Peak B FWHM (°)

0 mTorr 30 mTorr 100 mTorr

4.98 1.28 0.20 3.44 0.17 5.67

6.31 0.11 4.56 0.16 6.00

200 mTorr

the reference binding energy of Zr is possibly due to the presence of yttrium in surface layer. That is, the increase of surface yttrium modifies the chemical bonding between zirconium–oxygen and generates the shifts of the Zr 3d5/2 binding energy to the lower one. One thing to note is that a pair of single component of 3d peak is enough to complete the deconvolution of Zr 3d. The discussion of this corelevel shift of Zr will be also presented below with the analysis of Y 3d spectra. Previously, Majumdar and Chatterjee reported that the physical origin of the core-level shift of binding energy in the bulk YSZ samples (powders and pellets) was attributed to the increase of surface yttrium concentrations i.e., surface segregation [37]. In the case of Y 3d spectra, the binding energy of Y 3d5/2 in the YSZ thin films slightly increased as the oxygen partial pressure increased. Below 100 mTorr oxygen partial pressure, the Y 3d5/2 binding energy was smaller than the Y 3d5/2 binding energy in the Y2O3 (156.70 eV [35,36]) matrix (reference line in Fig. 3(b)). For the samples grown above 100 mTorr, the binding energy shifted slightly higher than the reference values, indicating that there exists a small variation in the chemical environments of yttrium neighbor due to the presence of large amount of oxygen. In fact, as shown in Fig. 3, the Y 3d spectra show the additional chemical component on higher binding energy side when the oxygen partial pressure reached 100 mTorr, which is different from the Zr 3d spectra. The amount of this chemical shift of Y 3d is almost ~1.0 eV and ~1.3 eV under 100 mTorr and 200 mTorr, respectively. Under the higher oxygen partial pressure, there are more changes on the chemical environment of Y element than Zr element. This shows the possible enhancement of yttrium concentration at the surface and/or the significant changes of yttrium–oxygen chemical bonding near surface, which reveals the presence of surface segregation between Y and Zr elements under high oxygen pressure. The physical origin of the chemical shifts and the surface segregation between those two elements can be attributed to the change of the Madelung potential at the sample surface. Previously, the formation of the yttrium hydroxide at the surface was reported under similar conditions to our experiment [39,40]. The details of yttrium–oxygen bonding will be given below with the discussion of O 1s core-level spectra. Unlike other core-level spectra, most of the oxygen XPS spectra (shown in Fig. 3(c)) deconvoluted into multiple peaks. A lowest binding energy (~529.5± 0.1 eV) of the O 1s, i.e., main peak, is attributed to the oxygen in the thin films. The position of the main O 1s peak is located between those of the ZrO2 (530 eV) and the O 1s binding energy of the Y2O3 (529.10 eV) matrix [35,36]. Furthermore, as the oxygen partial pressure increased, the main peak slightly decreased and was close to the oxygen binding energy of Y2O3, supporting the increases of yttrium-oxygen bonding at the surface. In the first two O 1s spectra in Fig. 3(c), an extra peak, i.e., a secondary peak, at 531.47 eV (531.74 eV) for 0 mTorr (30 mTorr), can be found at higher binding energy side. This secondary peak can be assigned as a surface hydroxide since the thin films are exposed to the air before the XPS measurement. However, it is also possible that this secondary peak contains the information of Zr–O–Y bondings as in the case of O 1s spectra of higher oxygen partial pressure [41]. In this report, we assume that those secondary peaks from the

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(a)

(b)

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(c)

Fig. 3. (a) Zr 3d5/2, (b) Y 3d5/2, and (c) O 1s XPS spectra of YSZ thin films for various oxygen partial pressures. The solid curves are the best fitted results after Shirley background subtraction. Dashed lines denote the reference binding energy value obtained from the literatures [35,36].

low oxygen partial pressure are coming from the surface hydroxide since the amounts of the peak intensity are relatively low and the peaks disappeared after surface etching. As the oxygen partial pressure increases, the additional oxygen peaks formed in the higher binding energy side, 531.35 eV and 532.46 eV (531.15 eV and 532.30 eV) for the sample grown at 100 mTorr (200 mTorr) and remained after surface etching. Furthermore, it is clear that the peak areas of the higher binding energy side are increased as the oxygen partial pressure increased. Therefore, this observation suggests that the extra amount of oxygen affects the chemical environments of the thin films, i.e., the changes on Zr–O–Y bonding [33,34,41]. The precise assignment of these two peaks at higher binding energy is a non-trivial problem. However, it can be safely estimated that these peaks are related to the formation of Y(Zr)2 − xO3 + x on the surface due to the high oxygen partial pressure condition

Table 3 The fitted binding energies (BEs) of the Zr 3d5/2, Y 3d5/2, and O 1s XPS profile of the YSZ thin films for various oxygen partial pressures. The FWHM used for fitting was ~ 1.5 eV. Partial pressure

Element

BE1 (eV)

0 mTorr

Zr 3d5/2 Y 3d5/2 O 1s Zr 3d5/2 Y 3d5/2 O 1s Zr 3d5/2 Y 3d5/2 O 1s Zr 3d5/2 Y 3d5/2 O 1s

181.76 156.63 529.52 181.88 156.67 529.57 181.86 156.68 529.54 181.72 156.68 529.42

30 mTorr

100 mTorr

200 mTorr

BE2 (eV)

BE3 (eV)

531.47

531.74 157.62 531.35

532.46

157.99 531.15

532.30

[33,41]. However, an important note to consider is that the amount of chemical shifts of Y 3d spectra shows significant changes as oxygen partial pressure increased while the one of Zr 3d spectra showed little changes. This finding clearly supports that the interaction of oxygen vacancies is mostly done via Y element. It is well known that the substituted Y3+ cations create vacancies in the oxygen sublattice since Y3+ cations have a lower valence than Zr4+ [3]. Therefore, the extra amount of oxygen caused a decrease in the number of oxygen vacancies created by the substituted Y3 + cations and plays an important role to stabilize the thin films [9,10]. In fact, this can be explained with the additional chemical component of Y 3d peaks at higher oxygen partial pressure in Fig. 3(b) and the presence of two extra oxygen peaks in Fig. 3(c). Now, those two extra oxygen peaks can be understood as the intermediate chemical states due to the decrease of oxygen vacancies under oxygen partial pressure. The valence bands of various samples are shown in Fig. 4(a). For the thin film grown at 0 mTorr, there exist two peaks (~4.5 eV and ~6.5 eV), which possess different peak heights. Previously, it was reported that the higher peak (around 6.5 eV) height relative to the lower peak (around 4.5 eV) height in YSZ pellets is a fingerprint of the tetragonal (or cubic) structure of the sample [37]. Therefore, it is thought that the thin film grown at 0 mTorr may have a tetragonal (or cubic) structure at the surface. As the oxygen partial pressure increased, the higher peak height (around 6.5 eV) decreased, suggesting the existence of the structural change in the film surface. Although it is difficult to explain the detail features of the valence bands without band calculations, the observed spectra are sufficient indication for the existence of surface structural changes. In comparison to the hypothetical valence bands [38] and the observed YSZ SC spectrum ( ), the YSZ SC surface may consist of the standard monoclinic ZrO2 and standard cubic Y2O3 since the lower peak height is greater than the higher peak height. Therefore, the observed changes

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(a)

(b)

Fig. 4. (a) The oxygen partial pressure dependent XPS valence spectra of the as-grown YSZ thin films, YSZ SC target, and Al2O3 substrate. The inset shows valence band spectra of the etched YSZ thin films. (b) The oxygen partial pressure dependent band offset energy of the samples obtained from linear extrapolations near the band offset. The error bar for each band offset energy value is smaller than the size of the symbol.

in the valance band spectra may be attributed to the change of the surface crystal structure from tetragonal (or cubic) to mixed (monoclinic ZrO2 + cubic Y2O3) as oxygen partial pressure increased. This surface structure change is also associated with the large yttrium– oxygen concentration at the surface. For the etched YSZ thin films, the valence band spectra are shown in the inset of Fig. 4(a) for comparison. It is worth noting that the scale of x-axis is not shown here due to the difficulty of the binding energy calibration of the etched films. However, the figure clearly shows that the spectra remain the tetragonal (or cubic) surface structure with extra amounts of oxygen, indicating that the observed structure change for the as-grown thin films is limited to the surface. The valence band offset energy obtained from the linear extrapolations near the band offset is shown in Fig. 4(b). As oxygen partial pressure increased, the valence band offset energy remained unchanged and rested between the cubic YSZ SC and Al2O3(0001) substrate, suggesting that the change of the surface structure does not significantly impact on the offset of the valence band offset energy. Fig. 5 shows the measured UV–VIS transmission spectra of the YSZ thin films. The direct band gap of bulk -Al2O3 is 8.7 eV [42], which is much larger than the measured band gap of YSZ thin films. All the samples show near 80% transmission in the visible region. The clear oscillation features in two spectra of the samples grown at 0.0 mTorr and 30 mTorr oxygen partial pressure are originated from the multiple internal reflection in the YSZ film. There are similar oscillations for the other two samples but they are much weaker and have larger periods. From the period of the oscillation and the

Fig. 5. The oxygen partial pressure dependent transmission of the YSZ thin films. The inset shows optical band-gap of the sample obtained from Tauc plot.

known dielectric constant (ϵ = 3.92 at 3.12 eV) [10,43], the thickness of the films can be estimated; ~260 nm for 0 mTorr and ~240 nm for 30 mTorr sample. Since the other two films show larger periods, they are thinner than the thin films grown at the lower oxygen partial pressure. We note that for the samples grown at the higher oxygen partial pressure, the films became thinner due to the increase of scattering between target vapors and gas molecules (oxygen). This is also confirmed from x-ray reflectivity measurements (not shown). The optical band gaps were extracted from the transmission spectra by using a Tauc plot [44]. The extracted oxygen dependent energy gap is displayed in the inset of Fig. 5. The higher oxygen pressure causes, the larger band gaps of the thin film. The measured band-gap energy of the YSZ SC is4.86 eV, which is consistent with the reported values [45–47]. However, the band-gap energy of the YSZ thin films possessed larger values than YSZ SC and varied from 5.63 to 5.77 eV, depending on the oxygen partial pressure. It has been reported that the optical band gap of ZrO2 and Y2O3 is 5–7 eV [48–50] and 5.6– 5.75 eV [51], respectively. Furthermore, French et al. showed that the variation of the direct optical band-gap energy of ZrO2 is related to the crystal structure of the samples [50]. They mentioned that the cubic ZrO2 has greater band-gap energy than the tetragonal ZrO2. Based on the valence band analysis, we can speculate that the increases of optical band-gap with oxygen partial pressure may be associated with the structural changes of the YSZ thin films from tetragonal to mixed phases. 4. Summary The YSZ thin films, grown by PLD, exhibit different crystal structures as the oxygen partial pressure varied during the growth. As oxygen partial pressure increased, the polycrystalline nature and the shift of the peak position with two different grain sizes were observed from x-ray diffraction. The XPS measurements showed the core-level shifts of Y 3d and O 1s spectra, revealing the presence of interaction between oxygen vacancies and yttrium element. It is estimated that the presence of oxygen during the growth of the films will directly affect the growth kinetics through the collision between oxygen molecules and the source clusters, which result in different atomistic configurations. In turn, this variation of surface chemical stoichiometry may result in the surface structural changes from tetragonal (or cubic) to mixed structure (monoclinic ZrO2 + cubic Y2O3) as the oxygen partial pressure increased. Finally, the increase of the optical band gap of the YSZ thin films with oxygen partial pressure may also open the additional potential application utilizing optical tunability.

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Acknowledgments The authors are grateful for the support of the National Research Foundation of Korea Grants (2010-0018374, 2011-0002273, R012008-000-21092-0, and KRF-2006-005-J02803) funded by the Korean Government. In particular, authors, J. S. Kim and H. J. Park were granted from the post-doctoral program of MOEHRD (KRF-2006-005-J02803). S.-S. Park is currently at Seoul Semiconductor Corp., Ansan, Kyunggi-do 425-851, Korea. References [1] S.J. Wang, C.K. Ong, S.Y. Xu, P. Chen, W.C. Tjiu, J.W. Chai, A.C.H. Huan, W.J. Yoo, J.S. Lim, W. Feng, W.K. Choi, Appl. Phys. Lett. 78 (2001) 1604. [2] L.R. Pederson, P. Singh, X.-D. Zhou, Vacuum 80 (2006) 1066 and references therein. [3] F.W. Fergus, R. Hui, Z. Li, D.P. Wilkinson, J. Zhang (Eds.), Solid Oxide Fuel Cells; Materials Properties and Performance, CRC Press, 2009. [4] A.S. Nesaraj, J. Sci. Ind. Res. 69 (2010) 169 and references therein. [5] J. Garcia-Barriocanal, A. Rivera-Calzada, M. Varela, Z. Sefrioui, E. Iborra, C. Leon, S.J. Pennycook, J. Samtamaria, Science 321 (2008) 676. [6] K.H.L. Zhang, A. Walsh, C.R.A. Catlow, V.K. Lazarov, R.G. Egdell, Nano Lett. 10 (2010) 3740. [7] S.B. Ogale (Ed.), Thin Films and Heterostructures for Oxide Electronics, Springer, 2005. [8] M. Fanciulli, G. Scarel (Eds.), Rare Earth Oxide Thin Films: Growth, Characterization, and Applications, Topics in Applied Physics, Springer, 2006. [9] C.S. Kim, W.J. Kim, S.J. Jo, S.W. Lee, S.J. Lee, H.K. Baik, Electrochem. Solid-State Lett. 9 (2006) G96. [10] D.K. Fork, D.B. Fenner, G.A.N. Connell, J.M. Phillips, T.H. Geballe, Appl. Phys. Lett. 57 (1990) 1137. [11] R. Bachelet, A. Boulle, B. Soulestin, F. Rossignol, R. Guinebretière, A. Dauger, Thin Solids Films 515 (2007) 7080. [12] F. Lallet, N. Olivi-Tran, L.J. Lewis, Phys. Rev. B: Condens. Matter 79 (2009) 035413. [13] O. Pons-Y, J. Moll, E. Perriere, R.M. Millon, D. Defourneau, B. Defourneau, A. Vincent, A. Boudrioua Essahlaoui, W. Seiler, J. Appl. Phys. 92 (2002) 4885. [14] A. Huignard, A. Aron, P. Aschehoug, B. Viana, J. Thery, A. Laurent, J. Perriere, J. Mater. Chem. 10 (2000) 549. [15] S. Kaneko, K. Akiyama, T. Oguni, T. Ito, Y. Hirabayashi, S. Ohya, K. Seo, Y. Sawada, H. Funakubo, M. Yoshimoto, Jpn. J. Appl. Phys. 45 (2006) L1328 and references therein. [16] S.-S. Park, J.S. Bea, S. Park, J. Phys. Condens. Matter 22 (2010) 015002. [17] S.S. Park, M.S. Thesis, Pusan National University (2009). [18] D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition, John Wiley and Sons, New York, 1994. [19] E.W. Plummer, Ismail, R. Matzdorf, A.V. Melechko, J.P. Pierece, J. Zhang, Surf. Sci. 500 (2002) 1.

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[20] T. Dittrich, J. Weldmann, F. Koch, I. Uhlendorf, I. Lauemann, Appl. Phys. Lett. 75 (1999) 3980. [21] S.Y. Chow, S. Wang, Ceram. Int. 30 (2004) 1257. [22] Y.-P. Xiong, H. Kishimoto, K. Yamaji, M. Yoshinaga, T. Horita, M.E. Brito, H. Yokokawa, Electrochem. Solid-State Lett. 13 (2010) B21. [23] The high temperature structural stability can be achieved by adding optimal yttrium (~8%). [24] http://pal.postech.ac.kr. [25] D. Briggs, M.P. Seah, Practical Surface Analysis, vol. 1, Wiley, 1990. [26] J.F. Watts, J. Wolstenolme, An Introduction to Surface Analysis by XPS and AES, Wiley, 2003. [27] JCPDS — International Center for Diffraction Data, , 2002. [28] B.D. Cullity, Elements of X-ray Diffraction, 2nd ed. Addison-Wesley Pub. Com. Inc., 1978 [29] J. Als-Nielsen, D. McMorrow, Elements of Modern X-ray Physics, Wiley, 2001. [30] M. Birkholz, Thin Film Analysis by X-ray Scattering, Wiley-VCH, 2006. [31] J.H. Kim, J.H. Lee, Y.W. Heo, J.J. Kim, Ju-O Park, J. Electroceram. 23 (2009) 169. [32] XPSPEAK4.1 (written by Raymond Kwok) was used to analyze the measured XPS data (http://www.uksaf.org/software.html). [33] J.F. Moulder, W.F. Stickle, P.E. Soble, K.D. Boomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics Inc., 1992 [34] F. Parmigiani, L.E. Depero, L. Sangaletti, G. Samoggia, J. Electron. Spectrosc. Relat. Phenom. 63 (1993) 1 and references therein. [35] B.F. Dzhurinskii, D. Gati, N.P. Sergushin, V.I. Nefedov, Y.A.V. Salyn, Russ. J. Inorg. Chem. 20 (1975) 2307. [36] http://lasurface.com. [37] D. Majumdar, D. Chatterjee, J. Appl. Phys. 70 (1991) 988; Thin Solid Films 206 (1991) 349. [38] D. Majumdar, D. Chatterjee, Thin Solid Films 236 (1993) 164. [39] W. Uwamino, T. Ishizuka, H. Yamatera, J. Electron Spectrosc. Relat. Phenom. 34 (1984) 67. [40] R. Reichl, K.H. Gaukler, Appl. Surf. Sci. 26 (1986) 196. [41] I. Valov, R.A. De Souza, C.Z. Wang, A. Börger, C. Korte, M. Martin, K.-D. Becker, J. Janek, J. Mater. Sci. 42 (2007) 1931. [42] F.S. Ohuchi, R.H. French, J. Vac. Sci. Technol., A 6 (1987) 195. [43] A.K. Chin, A. Satyanarayan, J.H. Zarrabi, W. Vetterling, J. Appl. Phys. 64 (1988) 994. [44] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1966) 627. [45] P.D.C. King, T.D. Veal, S.A. Hatfield, P.H. Jefferson, C.F. McConville, C.E. Kendrick, C.H. Swartz, S.M. Durbin, Appl. Phys. Lett. 91 (2007) 112103. [46] S.E. Paje, J. Llopis, Appl. Phys. A 57 (1993) 225. [47] V.R. PaiVerneker, A.N. Petelin, F.J. Crowne, D.C. Nagle, Phys. Rev. B 40 (1989) 8555. [48] S.E. Kul'kova, O.N. Murzhnikova, Inorg. Mater. 36 (2000) 38. [49] L.Q. Zhu, Q. Fang, G. He, M. Liu, L.D. Zhang, J. Phys. D: Appl. Phys. 39 (2006) 5285. [50] R.H. French, S.J. Glass, F.S. Ohuchi, Y.-N. Xu, W.Y. Ching, Phys. Rev. B: Condens. Matter 49 (1994) 5133. [51] W.C. Wang, M. Jadylevich, V.V. Afanas'ev, A. Stesmans, C. Adelmann, S. Van Elshocht, J.A. Kittl, M. Lukosius, Ch. Walczyk, Ch. Wenger, Appl. Phys. Lett. 95 (2009) 132903.