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1 MANUSCRIPT COVER PAGE Abstract ID: yclee11 or Program ID: EP406 Title of Paper: Microstructure and Phase Transformation of Zinc Titanate Thin Films Keywords: ZnTiO 3 ; thin film; amorphous; magnetron sputtering; post-annealing Corresponding Author: Ying-Chieh Lee Full Mailing Address: Department of Materials Engineering, National Pingtung University of Science & Technology, 1 Shuefu Road, Neipu, Pingtung,91201 Taiwan, R.O.C. YCLee@mail.npust.edu.tw Telephone: ext.7556 Fax: Estimation of the length of the manuscript (The pages of the manuscript should be numbered) Number of words Length of the text 1) (using "electronic word count") 2550 Number of Tables 2) Number of Figures 3) TOTAL NUMBER OF WORDS 4) ) Including abstract and list of references. 2) A full-page table counts as 300 words, half-page table as 150 words, etc. 3) Each figure including Fig. a, Fig. b, Fig. c, etc. must be counted separately. 4) Total number of words: Contributed papers are limited to 4000 words. Manuscripts exceeding these word limits will not be considered for publication. The articles should not exceed four published pages.

2 Suggested reviewers: 1. Name: S. Marinel (S. Marinel). Address: Laboratoire CRISMAT, ENSICaen, 6 Bd Mare chal Juin, Caen Cedex, France 2. Name: Sam Zhang msyzhang@ntu.edu.sg (S. Zhang). Address: School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore , Singapore. 3. Name: Jin Hyeok Kim jinhyeok@chonnam.ac.kr (J.H. Kim) Address: Photonic and Electronic Thin Films Laboratory, School of Materials Science and Engineering, Chonnam National University, 300 Yongbong-Dong, Puk-Gu, Gwangju, , South Korea. 4. Name: Dongfeng Xue dfxue@chem.dlut.edu.cn (D. Xue). Address: State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, Dalian University of Technology, Dalian , China

3 Microstructure and Phase Transformation of Zinc Titanate Thin Films Ying-Chieh Lee 1 *, Yen-Lin Huang 2, Ching-Li Hu 3 1 Department of Materials Engineering, National Pingtung University of Science & Technology, 1 Shuefu Road, Neipu, Pingtung,91201 Taiwan, R.O.C. 2 Engineering, National Chung Hsing University, 250 Kuo Kuang Road., Taichung 402, Taiwan R.O.C. 3 Electronic Ceramics Research Center, Yageo, Kaohsiung 811, Taiwan Department of Materials * Contacting Author: YCLee@mail.npust.edu.tw Tel: ext.7556 Fax: Abstract In this study, the microstructures and phase transformations of zinc titanate thin films were investigated. Zinc titanate thin films were synthesized on SiO 2 /Si(100) substrates using RF magnetron sputtering and a sintered ceramics target of ZnTiO 3 with various Ar to O 2 ratio (10:0, 9:1, 8:2). The substrate temperatures ranging from 30 to 400 o C and the as-deposited films were annealed at temperatures ranging from 600 to 1000 o C. The samples were analyzed using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). A single phase of ZnTiO 3 (hexagonal) films was obtained, and was found to exist at substrate temperature of 400 o C and annealing temperatures between 700 and 800 o C. However, the hexagonal ZnTiO 3 decomposed into cubic Zn 2 TiO 4 and rutile TiO 2 at an annealing temperature of 900 o C. This result differed from bulk ZnTiO 3 ceramics, in which the hexagonal ZnTiO 3 phase remained stable at temperatures below 945 o C, and then decomposes to cubic Zn 2 TiO 4 and TiO 2 phases at 945 o C, as indicated in the phase diagram of ZnO-TiO 2. On the other hand, with the increasing oxygen partial pressure (the Ar-to-O 2 ratio was decreased from 10:0 to 8:2) at a substrate temperature of 400 o C, where the phase transformations versus temperature were also investigated. Two phases were present at 800 o C when doped with O 2 : Zn 2 Ti 3 O 8 and TiO 2 phase. This differs from the phase diagram of ZnO-TiO 2, since TiO 2 and Zn 2 Ti 3 O 8 would be present at 945 o C and below 700 o C, respectively.

4 Keywords:ZnTiO 3 ; thin film; amorphous; magnetron sputtering; post-annealing 1. Introduction The ZnO-TiO 2 system exists in three forms: zinc meta-titanate (ZnTiO 3 ) with a hexagonal illuminate structure; zinc ortho-titanate (Zn 2 TiO 4 ) with a cubic spinel crystal structure; and zinc polytitanate (Zn 2 Ti 3 O 8 ) with a cubic defect spinel structure [1]. Zn 2 Ti 3 O 8 can only be formed from the Zn 2 TiO 4 phase, and has been observed to be a low-temperature form of ZnTiO 3 that exists at temperatures, T < 820 o C [2, 3]. It is known that h-zntio 3 decomposes into Zn 2 TiO 4 and rutile TiO 2 at T > 945 o C [4]. Single phase ZnTiO 3 has been prepared using zinc oxide and retile hydrate at T = 850~900 o C [1]. However, according to Yamaguchi et al., the cubic to hexagonal transformation of ZnTiO 3 occurs at temperatures above 820 o C, and the compounds are characterized by octahedral TiO 6 groups [5]. Pure h-zntio 3 has superior dielectric properties in the microwave range. It has a perovskite-type oxide structure, which may be useful as a microwave resonator material [6]. Furthermore, ZnTiO 3 can be sintered at 1100 o C without the use of sintering aids [4,6]. Moreover, when a sintering aid is added, it can be fired at temperatures below 900 o C [7,8]. ZnTiO 3 has potential applications in gas sensors that detect ethanol or carbon monoxide. It is also a promising candidate for the use in nonlinear optics, as a luminescent material and in various photocatalytic roles [9-10]. In this study, substrate temperature from 30 to 400 o C was used. In addition, the effect of post-annealing on microstructures and phase transformations of ZnTiO 3 thin films were studied, where the as-deposited films were annealed at temperatures ranging from 600 to 1000 o C. The impact of the atmosphere during sputtering (Ar/O 2 ratio) was also investigated.

5 Characterizations of the films were performed using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). 2. Experimental procedure Zinc titanate thin films were fabricated onto the SiO 2 /Si substrates using a MHz, 600 W RF magnetron sputtering system. A bulk zinc titanate target was synthesized by conventional solid-state methods from 1:1 molar mixtures high-purity oxide powders ZnO and TiO 2 (> 99.9 %). The sintered powder was then pressed into disks with a diameter of 3 inch and thickness of 3 mm, These were subsequently used as ZnTiO 3 targets. The sputtering chamber was evacuated by an oil diffusion pump to a base pressure of torr. The gas (Ar + O 2 ) flow was fixed at 50 sccm. The O 2 gas flow was changed from 0 to 10 sccm. The working pressure and the RF power were torr and 200 W, respectively. The films were deposited at substrate temperatures ranging from 30 to 400 o C for 120 minutes. The as-deposited films were annealed at 600 to 1000 o C for 2 hours with a heating rate of 5 o C/min in air. Crystallinity of the films were analyzed by X-ray diffraction (XRD, Bruker D8A Germany), with Cu Kα radiation for 2θ from 20 o to 70 o at a scan speed of 3 o min -1, and a grazing angle of 0.5 o under 40 kv and 40 ma. X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe) was applied to determine the chemical composition of the films, and cleaned the surface of films by pre-sputtering with 15 second. Microstructure of the films and the ZnTiO 3 /SiO 2 interfaces were investigated by filed-emission transmission electron microscopy (FE-TEM, FEI E.O. Tecnai F20) at an acceleration voltage of 200 kv, equipped with energy-dispersive spectroscopy (EDS). Atomic force microscopy (AFM, Veeco CP-II) was used to study the surface topography, with a sacn scope of 5 5 μm.

6 3. Results and discussion 3-1 The effect of substrate temperatures on the phase transformation of ZnTiO 3 thin films The XRD patterns of the as-deposited zinc titanate films with and without annealing are shown in Fig. 1. All the specimens were prepared with a substrate temperature of 400 C. As shown in Fig. 1(a), no diffraction peaks were observed, indicating that the film structure is amorphous. When the annealing treatment was at 600 C, the (311) peak of the Zn 2 Ti 3 O 8 phase appeared in the diffraction pattern, indicating the presence of the crystalline phase in Fig. 1(b). As the annealing temperature increased to 700 C, the (104), (116), (214) peaks of hexagonal ZnTiO 3 phase were observed, as shown in Fig. 1(c). The (104) peak was the greatest, indicating that there is highly (104)-oriented ZnTiO 3 on the SiO 2 /Si (100) substrates. Chen et. al. [11] had shown that (100)-oriented MgTiO 3 films were obtained on the Si substrate. Atoms arranged according to the preferred orientation tend to reduce the free energy of the system. When the annealing temperature was further increased to 900 C, the hexagonal ZnTiO 3 decomposed into cubic Zn 2 TiO 4 and TiO 2 (rutile) as shown in Fig. 1 (e). Unlike bulk ZnTiO 3 ceramics, this result shows that the hexagonal ZnTiO 3 phase remained stable at temperatures below 945 C, followed by decomposition to cubic Zn 2 TiO 4 and TiO 2 phases at 945 C as indicated by the phase diagram of ZnO-TiO 2 [4]. The reason for this is that the nano-scale particles are composed of thin films, where particles of a nanometer scale with a larger surface area lead to chemical reactions. However, the (104) peak shifted to the left when the annealing temperature was increased. In addition, both Zn 2 SiO 4 and rutile phases were observed at 1000 o C as shown in Fig. 1(g). This is attributed to cubic Zn 2 TiO 4 decomposing into ZnO and TiO 2, and ZnO preferentially react with SiO 2 /Si substrates to form Zn 2 SiO 4 with high temperature annealing. The result indicates the presence of more TiO 2 as the intensity of the TiO 2 peaks increased. The chemical reactions occurred includes:

7 Zn 2 TiO 4 2ZnO + TiO (1) 2ZnO + SiO 2 Zn 2 SiO (2) The thin films with the pure ZnTiO 3 phase were prepared for samples with a substrate temperature of 400 C and annealing temperature of 800 C. To solidify this result, high resolution TEM was used to analyze these ZnTiO 3 thin films. Fig. 2 shows the cross-section of the ZnTiO 3 thin films on a SiO 2 /Si substrate. The thickness of the film illustrated was found to be about 130 nm. The HRTEM of region 1 shows the d spacing of the h-zntio 3 phase, which is d 003 = nm; thus the SiO 2 films is amorphous. The XRD analysis has also revealed a similar result. Comprehensively, the results confirmed that the ZnTiO 3 thin films were successfully prepared at annealing temperatures ranging from 700 to 800 C. TEM bright-field images of the zinc titanate thin films are shown in Fig. 3. The samples were deposited on substrates at 400 C and annealed at 900 C. The grain (1) in Fig. 3 (a) is the ZnTiO 3 phase, and grain (2) was considered to be the Zn 2 TiO 4 according to the electron diffraction patterns in Fig. 3 (b) and Fig. 3 (c). This result indicates that the ZnTiO 3 and Zn 2 TiO 4 phases co-existed in the samples when annealed at 900 C, and coincided with the XRD results (Fig 1). However, it is noted that no Zn 2 Ti 3 O 8 was detected. It was reported by Bartram et. al. [1] and Kim et. al. [3] that the compound formation in the ZnO-TiO 2 system differed slightly to the results in our study. It is interesting that different phase transformation temperatures were observed for zinc titanates synthesized with different initial particles sizes. The comparison of phase transformation between nanometer particles and micrometer particles of zinc titanates were performed, and the results are summarized in Table 1. At the nanometer scale, the results revealed that the pure ZnTiO 3 (hexagonal phase) were obtained, and existed at temperatures between 700 and 800 C. However, after a further increase in temperature to 900 C, hexagonal ZnTiO 3 decomposed into cubic Zn 2 TiO 4 and rutile TiO 2. This result is unlike

8 ZnTiO 3 at the micrometer scale, in which the hexagonal ZnTiO 3 phase remained stable at temperatures below 945 C, followed by decomposition to the cubic Zn 2 TiO 4 and TiO 2 phases at 945 C, as indicated in the phase diagram of ZnO-TiO 2. Interesting phase transformations of the thin films deposited on different substrate temperatures annealed at 800 o C were observed, and the results are shown in Fig. 4. Two types of phase formation was found there; one is the Zn 2 Ti 3 O 8 phase at substrate temperatures ranging from 30 to 300 C, the other one is ZnTiO 3 phase at substrate temperatures of 400 C. The result indicates that the ZnTiO 3 phase can be formed at 400 C substrate temperature and annealing at 800 C. In addition, the main Zn 2 Ti 3 O 8 phase with a minor ZnTiO 3 phase are found at a substrate temperature 300 C, however, the intensity of the ZnTiO 3 peak is very small. Further study of the microstructure using transmission electron microscopy may clarify the issue. 3-2 The effect of oxygen concentration on the phase transformation of ZnTiO 3 thin film XRD measurements were performed in order to examine the variation of structural properties with varying oxygen partial pressures (PO 2 ). Fig. 5 shows the XRD spectra of zinc titanate thin films grown under different Ar and O 2 ratios. The films were deposited at a substrate temperature 400 o C and were annealed at 800 C. For an Ar-to-O 2 ratio of 9:1 (10 % O 2 ), the ZnTiO 3 and Zn 2 Ti 3 O 8 phases co-existed as shown in Fig. 5 (b). This result indicates that 10 % O 2 dopant leads to the remaining Zn 2 Ti 3 O 8 phase at 800 o C, because the Zn 2 Ti 3 O 8 phase is stable below 800 o C without oxygen doping, as seen in Fig. 1. However, when the O 2 partial pressure was increased to 20 % (Ar-to-O 2 ratio of 8:2), the major and minor phases were still ZnTiO 3 and TiO 2, respectively. This result indicates that 20 % O 2 dopant leads to the decomposition of the Zn 2 Ti 3 O 8 phase into ZnTiO 3 and TiO 2 at 800 o C as shown in Fig. 5 (c), the

9 chemical reaction is listed below: Zn 2 Ti 3 O 8 phase 2ZnTiO 3 + TiO (3) It has to be noted that, there are two phases present at 800 C when O 2 is used as a dopant: Zn 2 Ti 3 O 8 and TiO 2. This differs from the phase diagram of ZnO-TiO 2, since TiO 2 appears at 945 C and Zn 2 Ti 3 O 8 exists below 700 C. Table 2 lists the element analysis of thin film using ESCA equipment. It is found that the Zn/Ti ratio decreased gradually with an increase in substrate temperatures for temperature ranging from 200 to 400 C. It can be explained that the zinc atoms have facile evaporation at high substrate temperatures due to the low melting point of zinc (419.6 C). On the other hand, the Zn/Ti ratio decreased significantly when O 2 was doped into the chamber. This may be attributed to a lower deposition rate and poorer crystallinity (Fig. 5) of films when O 2 is present. The morphologies of the thin film with varying O 2 partial pressures are shown in Fig. 6. The grain size decreased with increasing O 2 partial pressure. According to XRD analysis, the phases of the thin film also differ with O 2 partial pressure. The surface morphologies of zinc titanite films has been observed with AFM, and the results are shown in Fig. 7, corresponding to the samples prepared at oxygen partial pressures of 0 % and 10 %. The left figures display the surface morphologies and the pictures on the right depicts typical three-dimensional representations (5000 nm by 5000 nm surface plots). All the films present a rough surface texture, consisted of particles fused together, building up high mountains and deep valleys. All the films can be described as a contiguous network of particles and aggregates with significant roughness. It is also shown that the TiO 2 particles decrease in size with an increase in oxygen partial pressure, which may be ascribed to the deposition rate. 4. Conclusion

10 ZnTiO 3 thin films were successfully prepared by RF magnetron sputtering on a SiO 2 /Si substrate at temperatures lower than the bulk sintering temperature. It is shown that crystallization of the ZnTiO 3 phase occurred at a substrate temperature of 400 C and annealing temperature of 700 C over 2 h. It is found that the as-deposited films were amorphous, as confirmed by XRD results. The onset of crystallization also took place at an annealing temperature of 600 C. At low temperature (600 C), cubic phase Zn 2 Ti 3 O 8 was observed. Raising the temperature to 700 C, a Zn 2 Ti 3 O 8 phase to hexagonal ZnTiO 3 phase transformation occurred. However, as the annealing temperature exceeded 900 C, the ilumenite ZnTiO 3 decomposed into cubic Zn 2 TiO 4 and rutile TiO 2. On the other hand, with increasing oxygen partial pressure (Ar-to-O 2 ratio decreased from 10:0 to 8:2) at 400 C of substrate temperature, the phase transformations versus temperatures changed. At an Ar-to-O 2 ratio of 9:1, ZnTiO 3 and Zn 2 Ti 3 O 8 phases co-existed at 800 C. By increasing the PO 2 partial pressure to 8:2, the ZnTiO 3 phase remained as the main phase, accompanied by a TiO 2 minor phase at 800 C. Reference 1. S.F. Bartram and R.A. Slepetys, J. Am. Ceram. Soc. 44, 493 (1961) 2. U. Steinike and B. Wallis, Cryst. Res. Technol. 32, 187 (1997) 3. H.T. Kim, Y. Kim, M. Valant and D. Suvorov, J. Am. Ceram. Soc. 84, 1081 (2001) 4. F.H. Dulin and D.E. Rase, J. Am. Ceram. Soc. 43, 125 (1960) 5. O. Yamaguchi, M. Morimi, H. Kawabata and K. Shimizu, J. Am. Ceramic Soc. 70, C97 (1987) 6. H. T. Kim, J. D. Byun and Y. H. Kim, Mater. Res. Bull. 33, 963 (1998) 7. Y.C. Lee and W.H. Lee, Jpn. J. Appl. Phys. 44, 1838 (2005) 8. H.T. Kim, S.H. Kim, S. Nahm, J.D. Byun and Y.H. Kim, J. Am. Ceram. Soc. 82, 3043

11 (1999) 9. Lew, K. Jothimurugesan and M. F. Stephanopoulos, Ind. Eng. Chem. Res. 28, 535 (1989) 10. H. K. Jum, T. J. Lee, S. O. Ryu and J. C. Kim, Ind. Eng. Chem. Res. 40, 3547 (2001) 11. Y. B. Chen and C. L. Huang, J. Crystal Growth 282, 482 (2005) Figure and Table Captions: Table 1. The comparison of phase transformation between nanometer particles and micrometer particles of zinc titanates using different procedures. Table 2. Lists the element analysis of zinc titanate thin films at different parameters using ESCA equipment Fig. 1. X-ray diffraction patterns of the zinc titanate thin films (a) as-deposited at RF power of 200 W, sputtering time of 2 h, substrate temperature of 400 o C, and different annealing temperatures (b) 600 o C, (c) 700 o C, (d) 800 o C, (e) 900 o C, (f) 950 o C and (g) 1000 o C. Fig. 2. (a) Cross-section TEM micrographs and (b) HRTEM image of the zinc titanate thin films deposited at a substrate temperature of 400 o C and annealing temperature of 800 o C. Fig. 3. (a) Plane-view TEM micrographs, (b) ZnTiO 3 nano-beam diffraction and (c) Zn 2 TiO 4

12 nano-beam diffraction of the zinc titanate thin films deposited at substrate temperature of 400 o C and annealing temperature of 900 o C. Fig. 4. X-ray diffraction patterns of the zinc titanate thin films at different substrate temperatures 30 to 400 o C, and annealed 800 o C. Fig. 5. X-ray diffraction patterns of the zinc titanates thin films deposited at RF power: 200 W, sputtering time: 2 h, substrate temperature: 400 o C, annealing temperature: 800 o C with Ar to O 2 ratio (a) 10:0, (b) 9:1 and (c) 8:2. Fig. 6. The film thickness of the zinc titanate deposited at 400 o C of substrate temperature and 800 o C annealing with Ar to O 2 ratio (a) 10:0, (b) 9:1 and (c) 8:2. Fig. 7. Plane-view SEM micrographs of zinc titanate thin film deposited at 400 o C of substrate temperature and 800 o C annealing with Ar to O 2 ratio (a) 10:0, (b) 9:1 and (c) 8:2. Fig. 8. AFM surface morphologies and the right pictures depict typical three-dimensional representations (5000 nm 5000 nm surface plots) of the zinc titanate thin films deposited at 400 o C of substrate temperature and 800 o C annealing: (a) and (b) is Ar to O 2 ratio 10:0, and (c) and (d) is Ar to O 2 9:1.

13 Table 1 Substrate temp. ( C) Particles of nanometer scale Annealing Oxygen concentration (%) temp. ( C) Particles of micrometer scale [1, 3] Calcining temp. Phase ( C) Zn 2 Ti 3 O 8 ~ ZnTiO 3 + Zn 2 Ti 3 O 8 ZnTiO 3 + Zn 2 Ti 3 O ZnTiO 3 ZnTiO 3 + Zn 2 Ti 3 O 8 ZnTiO 3 + TiO 2 ~850 ZnTiO ZnTiO 3 + Zn 2 TiO 4 + TiO 2 ~945 ZnTiO 3 + Zn 2 TiO 4 + TiO 2

14 Table 2 Substrate temp.( C) Oxygen partial pressure (Ar to Surface element composition, X% O 2 ratio) Zn2p Ti2p O1s Zn/Ti 30 50/ / / / / /

15 Fig. 1 g) f) (110) (220) ZnTiO 3 Zn 2 TiO 4 Zn 2 Ti 3 O 8 TiO 2 Zn 2 SiO 4 (113) (223) (410) (220) (342) (211) (220) (311) (104) Intensity (a.u.) e) d) (110) (220) (311) (024) (422) (116) (511) (214) c) b) a) (220) (311) θ (degrees)

16 Fig. 2

17 Fig. 3

18 Fig. 4 (104) ZnTiO 3 Zn 2 Ti 3 O 8 Intensity (a. u.) Intensity 400 (211) (200) (311) (024) (116) (422) (214) (511) (214) (440) (208) RT(30 ) θ 2-Theta (degrees) - Scale

19 Fig. 5 Ar: O 2 (sccm) (a)50: 0 (b)45: 5 (c)40: 10 (104) ZnTiO 3 Zn 2 Ti 3 O 8 TiO 2 (Rutile) Intensity Intensity (a. u.) (a) 0 (a)0% % (b) (b)10% % (200) (311) (024) (116) (422) (214) (511) (214) (440) (214) (208) (c) (c)20% % (110) (301) θ 2-Theta (degrees) - Scale

20 Fig. 6

21 Fig. 7