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(1)Materials Transactions, Vol. 43, No. 11 (2002) pp. 2880 to 2884 c 2002 The Japan Institute of Metals. Microstructure and Dielectric Properties of Barium Titanate Film Prepared by MOCVD Tetsuro Tohma ∗ , Hiroshi Masumoto and Takashi Goto Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Barium titanate (BaTiO3 ) films were prepared on (100)Pt/(100)MgO substrates by metal-organic chemical vapor deposition (MOCVD). The effects of deposition temperature (Tdep ) and O2 partial pressure (PO2 ) on the microstructure and dielectric properties of the films were investigated. The BaTiO3 films prepared at Tdep = 873 K showed a randomly orientated granular structure, while those prepared at Tdep = 973 K showed a significant (001) orientation with a columnar structure. The grain size was strongly affected by PO2 and showed the maximum at PO2 = 66 to 93 Pa. The dielectric constant increased from 93 to 640 with increasing grain size from 20 to 130 nm, showing a broad peak at 350 to 380 K. (Received August 22, 2002; Accepted September 25, 2002) Keywords: barium titanate, metal-organic chemical vapor deposition, film, microstructure, dielectric constant, grain size. 1. Introduction Barium titanate (BaTiO3 ) films have attracted much attention for the application of dynamic random access memories and electro-optic devices due to excellent dielectric and optical properties.1, 2) Several methods such as rf-magnetron sputtering,3, 4) reactive evaporation,5) laser deposition,6) chemical solution deposition,7) sol-gel.8–10) and metal-organic chemical vapor deposition (MOCVD)11–15) have been applied to prepare BaTiO3 films. BaTiO3 films generally consisted of fine grains ranging from several tens to hundreds of nanometers in size, whose dielectric constant might change from several tens to thousands decreasing with decreasing the grain size.3, 7, 10, 14) However, the relationship between dielectric properties and microstructure of BaTiO3 films has not been well understood. MOCVD is advantageous to control the microstructure of films by changing deposition conditions. Many papers have been published on the preparation of BaTiO3 films by MOCVD, and reported that the dielectric properties were significantly affected by deposition conditions and microstructure.11–15) In the MOCVD process, many deposition conditions such as partial pressure of source gases, deposition temperature and total pressure should be controlled to obtain high-quality BaTiO3 films. In published papers, however, the effect of oxygen partial pressure on the dielectric properties has not been reported. We have prepared BaTiO3 films by MOCVD. This paper reports the effect of oxygen partial pressure (PO2 ) on the microstructure and dielectric properties of the BaTiO3 films. 2. Experimental Procedure A horizontal hot-wall type CVD furnace was used to prepare films. The detailed experimental setup was reported elesewhere.16) Bis-dipivaloylmethanato-barium (Ba(DPM)2 ) and bis-isopropoxy-bis-dipivaloylmethanato-titanium (Ti(O∗ Graduate. Student, Tohoku University.. i-C3 H7 )2 (DPM)2 ) as Ba and Ti sources were vaporized at 503 to 523 K and 333 to 373 K, respectively. The source vapors were separately transported to the reaction chamber with Ar gas whose flow rate was 0.75 × 10−6 m3 ·s−1 . O2 gas was introduced as an oxidant and mixed with source vapors near the substrate. The O2 gas flow rate (FRO2 ) was changed from 0 to 1.0 × 10−6 m3 ·s−1 to control PO2 from 0 to 160 Pa. The total gas flow rate was kept at 2.5 × 10−6 m3 ·s−1 , and the total pressure in the reaction chamber was fixed at 400 Pa. The deposition temperatures (Tdep ) were 873 and 973 K. The deposition time was 3.6 ks at most. In order to measure the dielectric properties, Pt films were prepared on the (100)MgO substrates as a bottom electrode by dc sputtering, where the (100)Pt  (100)MgO relation was observed. Pt films were prepared again on the BaTiO3 films as a top electrode (0.5 mm in diameter). The phase and preferred orientation were examined by Xray diffraction (XRD: Rigaku RAD-C). The thickness was measured by a thickness tester (Tayler Hobson, Talystep). The surface and cross-sectional morphologies were observed by scanning electron microscopy (SEM: JOEL JSM-5400F). Dielectric properties were measured by impedance spectroscopy (Solartron 1260 and 1294) in the frequency ( f ) range from 10−1 to 106 Hz. 3. Results and Discussion Figure 1 shows the effect of PO2 on the XRD patterns of BaTiO3 films prepared at Tdep = 873 and 973 K. When O2 gas was not introduced into the chamber, the films were amorphous and black-colored due to a carbon residue. The films prepared at PO2 ≥ 13 Pa were transparent and all XRD peaks were identified to BaTiO3 . To understand the effect of PO2 on the carbon residue in the deposits, the equilibrium compositions of solid phases in the Ba–Ti–C–H–O–Ar system were calculated as shown in Fig. 2. Detailed procedure of the calculation was reported elsewhere.16) 17 solid species (BaTiO3 , Ba2 TiO4 , BaCO3 , etc.) and 20 gas species (O2 , CO2 , H2 O, etc.) were considered in the calculation. The equilibrium state is calculated from minimizing the total Gibbs free energy un-.

(2) Microstructure and Dielectric Properties of Barium Titanate Film Prepared by MOCVD. 2881. Fig. 1 XRD patterns of BaTiO3 films prepared at (a) Tdep = 873 K and (b) Tdep = 973 K.. Deposition Temperature, Tdep / K. 1000. 900. BaTiO3. 800. 700. Carbon 600 0.01. BaTiO3 0.1. 1. 10. O2 Partial Pressure, PO2 / Pa Fig. 2 Calculated solid phases as functions of Tdep and PO2 . (Ptot = 400 Pa, (Ba(DPM)2 + Ti(O-i-C3 H7 )2 (DPM)2 )/(Ar + O2 ) = 1/2.5 × 104 ).. der the mass conservation conditions. Ba and Ti source contents were assumed as the same as the experimental condition (i.e., (Ba(DPM)2 +Ti(O-i-C3 H7 )2 (DPM)2 )/(Ar+O2 ) = 1/2.5 × 104 ). The carbon phase might be calculated to remain in the BaTiO3 phase at PO2 = 0.17–0.2 Pa. Since PO2 in the experiments was more than 13 Pa, the O2 content could be enough to oxidize the carbonates in metal-organic sources, and no residue carbon might be deposited. The BaTiO3 films prepared at Tdep = 873 K showed no preferred orientation, or XRD peaks were too small to be identified. The lattice parameter of films prepared at Tdep = 873 K and PO2 = 13 to 120 Pa was 0.401 nm in a cubic expression. The BaTiO3 films prepared at Tdep = 973 K showed a significant (001) orientation. The lattice parameter calculated from the (00l) diffractions was 0.402 nm which was close to that of BaTiO3 sintered body (c = 0.404 nm).5) Several paper reported that (001) oriented films were prepared on. Fig. 3 Surface morphology of BaTiO3 films prepared at Tdep = 973 K and (a) PO2 = 13 Pa, (b) PO2 = 93 Pa and (c) PO2 = 160 Pa.. (100)Pt/(100)MgO substrates by sputtering,4) reactive evporation5) and laser deposition.6) They explained that the (001) orientation might be caused of the close lattice parameters between BaTiO3 (a = 0.399 nm, c = 0.404 nm) and Pt (0.392 nm) and MgO (0.421 nm).5) This might be caused the (001) orientation at Tdep = 973 K in this study. Figure 3 shows the effect of PO2 on the surface morphology of the films prepared at Tdep = 973 K. The grain size of the films prepared at Tdep = 973 K and PO2 = 13 Pa was about 40 nm, and increased with increasing PO2 . Figure 4 shows the relationship between PO2 and the grain size of the films. The grain size of the films prepared at Tdep = 973 K showed the maximum of 130 nm at PO2 = 66 Pa, and was almost constant about 100 nm at PO2 > 90 Pa. At Tdep = 873 K, the grain size showed the maximum of 70 nm at PO2 = 93 Pa, and it decreased with increasing PO2 . Figure 5 shows the crosssections of the films prepared at PO2 = 93 Pa. The BaTiO3 film prepared at Tdep = 873 K (Fig. 5(a)) had a granular structure, and that prepared at Tdep = 973 K a columnar struc-.

(3) 2882. T. Tohma, H. Masumoto and T. Goto 150. 50. 0 0. 50. 100. 150. Tdep : 873 K : 973 K 0.2. 0.1. 0.0 0. 200. 50. 100. 150. 200. O2 Partial Pressure, PO2 / Pa. O2 Partial Pressure, PO2 / Pa. Fig. 6 Effect of PO2 on deposition rate of BaTiO3 films.. Relative Dielectric Constant, ’. Fig. 4 Relationship between PO2 and grain size of BaTiO3 films.. 0.5. 500 : 298 K : 523 K. 400. 0.4. 300. 0.3. 200. 0.2. 100. 0.1. 0 10-1. 100. 101. 102. 103. 104. 105. Loss Tangent, tan. Grain Size, d / nm. 100. Deposition Rate, R / nm s-1. 0.3. T dep : 873 K : 973 K. 0.0 106. Frequency, f / Hz Fig. 7 Frequency dependence of ε and tan δ of BaTiO3 films prepared at Tdep = 873 K and PO2 = 13 Pa.. Fig. 5 Cross-sections of BaTiO3 films prepared at PO2 = 93 Pa and (a) Tdep = 873 K and (b) Tdep = 973 K.. ture. In the CVD process, the morphology of deposits often changes from a granular to a columnar microstructure with increasing Tdep due to significant grain growth at high temperatures.17) The morphology shown in Fig. 5 was in agreement with the general trend of CVD process. Figure 6 shows the effect of PO2 on the deposition rates of BaTiO3 films. The deposition rates at Tdep = 873 K were 0.16 to 0.21 nm·s−1 almost independent of PO2 , while those at Tdep = 973 K showed the maximum of 0.22 nm·s−1 at PO2 = 70 Pa. The deposition rates of BaTiO3 films by MOCVD on literatures11–15) have ranged from 0.02 to 0.4 nm which were close to those of this study. Figure 7 shows the frequency dependence of relative dielectric constant (ε ) and loss tangent (tan δ) of the BaTiO3 film prepared at Tdep = 873 K and PO2 = 13 Pa. The ε and tan δ at 298 K were almost constant in the frequency range from 10−1 to 106 Hz, but those at 523 K increased significantly with decreasing frequency below 104 Hz. This appar-. ent increase of ε at the low frequency region might be caused of the effect of d.c. conduction. The ε and tan δ slightly increased at more than 105 Hz. This may be caused of the effect of inductance component of the measurement cell which becomes significant at high frequencies.18, 19) The ε and tan δ at 104 to 105 Hz should be the intrinsic value of BaTiO3 films. Figure 8 shows the temperature dependence of ε of BaTiO3 films at 105 Hz. The ε showed broad peaks at 350 to 380 K close to the Curie temperature of BaTiO3 (TC = 393 K). The ε values were smaller than those of bulk sintered bodies,20) and the temperature dependence of ε became less significant with decreasing ε . The ε values of the films prepared at PO2 = 40 Pa were greater than those prepared at PO2 = 13 and 160 Pa. It is known that the tetragonal ferroelectric phase of BaTiO3 would change to a cubic paraelectric phase with decreasing grain size.20–23) The critical grain size disappearing the ferroelectricity of BaTiO3 was reported to be 25 to 120 nm.20–23) The small ε values and the weak temperature dependence of ε shown in Fig. 8 might be caused of the small grain size of BaTiO3 films. Figure 9 shows the effect of PO2 on ε and tan δ of BaTiO3 films at 105 Hz and 298 K. The ε of films prepared at Tdep = 873 and 973 K showed the maximum of 535 and 640.

(4) Tdep = 873 K 800. : P O2 = 13 Pa : P O2 = 40 Pa : P O2 = 160 Pa. 600. 400. 200. 0. 350. 300. 400. 450. 600. 400. 200. 0. 500. Tdep : 873 K : 973 K. 0. 50. Temperature, T / K. 0.08 0.06 0.04 0.02. 0 0. 50. 100. 150. Relationship between grain size and ε of BaTiO3 films.. tween Tdep = 873 K and 973 K could be caused of the effect of preferred orientation. Table 1 summarized the reported deposition conditions and properties of BaTiO3 films. The dielectric properties, particularly ε , of BaTiO3 films were strongly affected by preparation methods and deposition conditions. Figure 11 shows the relationship between grain size and ε of BaTiO3 prepared by several methods. The ε of BaTiO3 bulk sintered bodies showed the maximum at the grain size of 800 to 1000 nm.20) There is the general trend that the ε decreases with decreasing grain size in the region less than 700 nm. The relationship between ε and grain size in this study was in agreement with the general trend. Figure 12 shows the relationship between leakage current density (J ) and electric field (E) at 298 K. The BaTiO3 films showed significant low leakage current density below 1 MV·m−1 . Above the critical electric field (E crit ), J increased exponentially with increasing E indicating nonohmic conduction. The E crit of films prepared at Tdep = 873 K showed greater E crit than those prepared at Tdep = 973 K. Tsai et al. reported that the E crit of (Ba, Sr)TiO3 films increased with decreasing grain size.25) The smaller grain size means more numbers of grain boundary in the films. They assumed that the grain boundary could be more insulative. 0 200. O2 Partial Pressure, PO2 / Pa Fig. 9. 150. 0.10. 400. 200. Fig. 10. Loss Tangent, tan. Relative Dielectric Constant, ’. Tdep : 873 K : 973 K. 600. 100. Grain Size, d /nm. Fig. 8 Temperature dependence of ε of BaTiO3 films at 105 Hz. 800. 2883. 800. Tdep = 973 K. : P O2 = 13 Pa : P O2 = 40 Pa : P O2 = 160 Pa. Relative Dielectric Constant, ’. Relative Dielectric Constant, ’. Microstructure and Dielectric Properties of Barium Titanate Film Prepared by MOCVD. Effect of PO2 on ε and tan δ of BaTiO3 films at 105 Hz and 298 K.. at PO2 = 93 and 66 Pa, respectively. The tan δ of BaTiO3 films were 0.01 to 0.02 close to those of BaTiO3 films prepared by a sol-gel method.9) Figure 10 shows the relationship between grain size and ε at 298 K. The ε clearly increased with increasing grain size. The trend at Tdep = 973 K was slightly different from that at Tdep = 873 K. It is reported that the ε to the 001 direction of BaTiO3 is smaller than those to other direction.24) Since the films prepared at Tdep = 973 K had the significant (001) orientation, the different trend be-. Table 1 Deposition conditions and properties of BaTiO3 films prepared by several methods. Method. Substrate. Tdep /K. Orientation. Thickness /nm. Grain size /nm. ε. Ref.. rf-magnetron sputtering reactive evaporation laser deposition CSD sol-gel MOCVD MOCVD MOCVD MOCVD PE-MOCVD. Pt (100)Pt/(100)MgO (100)Pt/(100)MgO Pt/ZrO2 /SiO2 /Si stainless steel ITO glass (100)Pt/(100)MgO (100)Si p-type Si Pt/Ti/SiON/Si. 973 973 1073 973 973–1123 673 1073 973 823 873. (110) (001) (001) random random amorphous random (100) random random. 210–2000 20–800 950 200 1600 80–100 1000 1000 350 600. 50–180. 500 60 100 200. 750–2500 100–700 350 250–850 205–401 12.9 1040 107 250 300. 3) 5) 6) 7) 10) 11) 12) 13) 14) 15). MOCVD. (100)Pt/(100)MgO. 873 973. random (001). 520–750 240–800. 20–66 40–130. 96–555 249–658. This study. CSD: chemical solution deposition, PE-MOCVD: plasma enhanced MOCVD. 100 30–170 15–25.

(5) 2884. T. Tohma, H. Masumoto and T. Goto. slightly smaller ε than those prepared at Tdep = 873 K. The smaller ε might be associated with the significant (001) orientation of the films prepared at Tdep = 973 K. The ε of films showed broad peaks at 350 to 380 K. The films prepared at Tdep = 873 K had smaller leakage current density than those prepared at Tdep = 973 K.. Relative Dielectric Constant, ’. 6000 : MOCVD (This study) : MOCVD (12)-(15) : sputtering (3) : CSD (7) : sol-gel (10) : bulk (20). 4000. Acknowledgements 2000. 0 10. 100. 1000. Grain Size, d /nm Fig. 11 Relationship between grain size and ε of BaTiO3 at room temperature.. This research was partly supported by a Grant-in-Aid for Scientific Research (No. 12555174 and 13650729) from the Ministry of Education, Culture, Sports, Science and Technology and a Foundation for Promotion of Material Science and Technology of Japan. The authors thanks to Mr. E. Aoyagi and Mr. Y. Hayasaka of Institute for Materials Research, Tohoku University for the SEM measurements. REFERENCES. 102. Current Density, J / A m -2. PO2 : 40 Pa : 120 Pa. 100. Tdep = 973 K 10-2. 10-4. Tdep = 873 K. 10-6 0.1. 1. 10. Electric Field, E / MV m-1 Fig. 12. Relationship between J and E at 298 K.. than the grain. This must have caused the better insulative properties.25) As shown in Fig. 5, the grain size of the film prepared at Tdep = 873 K was smaller than that prepared at Tdep = 973 K. This might cause the difference of E crit shown in Fig. 12. 4. Summary BaTiO3 films were prepared on (100)Pt/(100)MgO substrates by MOCVD. The BaTiO3 films prepared at Tdep = 873 K showed a randomly oriented granular structure. The grain size of films prepared at Tdep = 873 K increased with increasing PO2 and showed the maximum of 65 nm at PO2 = 93 Pa. The BaTiO3 films prepared at Tdep = 973 K showed a significant (001) orientation with a columnar structure. The grain size of films prepared at Tdep = 973 K showed the maximum of 130 nm at PO2 = 66 Pa. The ε increased from 93 to 640 with increasing grain size from 20 to 130 nm. The films prepared at Tdep = 973 K had larger grain sizes and. 1) G. H. Haertling: J. Vac. Sci. Technol. A 9 (1991) 414–420. 2) H. A. Lu, L. A. Wills, B. W. Wessels, W. P. Lin, T. G. Zhang, G. K. Wong, D. A. Neumayer and T. J. Marks: Appl. Phys. Lett. 62 (1993) 1314–1316. 3) J. W. Jang, S. J. Chung, W. J. Cho, T. S. Hahn and S. S. Choi: J. Appl. Phys. 81 (1997) 6322–6327. 4) K. Abe, S. Komatsu, N. Yanase, K. Sano and T. Kawakubo: Jpn. J. Appl. Phys. 36 (1997) 5846–5856. 5) Y. Yano, K. Iijima, Y. Daitoh, Y. Bando, Y. Watanabe, H. Kasatani and H. Terauchi: J. Appl. Phys. 76 (1994) 7833–7838. 6) K. Kaemmer, H. Huelz, B. Holzapfel, W. Haessler and L. Schultz: J. Phys. D 30 (1997) 522–526. 7) S. Hoffmann and R. Waser: J. Europ. Ceram. Soc. 19 (1999) 1339–1343. 8) T. Hayashi, N. Oji and H. Maiwa: Jpn. J. Appl. Phys. 33 (1994) 5277– 5280. 9) H. B. Sharma and H. N. K. Sarma: Thin Solid Films 330 (1998) 178– 182. 10) M. C. Cheung, H. L. W. Chan, Q. F. Zhou and C. L. Choy: Nano Struct. Mater. 11 (1999) 837–844. 11) Y. S. Yoon, D. H. Lee, T. S. Kim, M. H. Oh and S. S. Yom: J. Vac. Sci. Technol. A 12 (1994) 751–753. 12) H. Nakazawa, H. Yamane and T. Hirai: Jpn. J. Appl. Phys. 30 (1991) 2200–2203. 13) J. Zeng, H. Wang, M. Wang, S. Shang, Z. Wang and C. Lin: Thin Solid Films 322 (1998) 104–107. 14) I. T. Kim, C. H. Lee and S. J. Park: Jpn. J. Appl. Phys. 33 (1994) 5125– 5128. 15) P. C. v. Buskirk, R. Gardiner, P. S. Kirlin and S. Krupanidhi: J. Vac. Sci. Technol. A 10 (1992) 1578–1583. 16) T. Tohma, H. Masumoto, T. Hirai and T. Goto: Mater. Trans. 42 (2001) 702–706. 17) C. E. Morosanu: Thin Films by Chemical Vapor Deposition, (ELSEVIER, 1990) pp. 163–176. 18) P. C. Joshi and S. B. Desu: Thin Solid Films 300 (1997) 289–294. 19) F. M. Ponte, E. Longo, E. R. Leite and J. A. Varela: Thin Solid Films 386 (2001) 91–98. 20) G. Arlt, D. Hennings and G. d. With: J. Appl. Phys. 58 (1985) 1619– 1625. 21) K. Uchino, E. Sadanaga and T. Hirose: J. Am. Ceram. Soc. 72 (1989) 1555–1558. 22) M. H. Frey and D. A. Payne: Appl. Phys. Lett. 63 (1993) 2753–2755. 23) S. Tsunekawa, S. Ito, T. Mori, K. Ishikawa, Z.-Q. Li and Y. Kawazoe: Phys. Rev. B 62 (2000) 3065–3070. 24) W. J. Mertz: Phys. Rev. 76 (1949) 1221–1225. 25) M. S. Tsai, S. C. Sun and T. Y. Tseng: J. Appl. Phys. 82 (1997) 3482– 3487..

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Figure

Fig. 1XRD patterns of BaTiO3 films prepared at (a) Tdep = 873 K and (b)Tdep = 973 K.
Fig. 4Relationship between PO2 and grain size of BaTiO3 films.
Table 1Deposition conditions and properties of BaTiO3 films prepared by several methods.
Fig. 11Relationship between grain size and ε′ of BaTiO3 at room temper-ature.

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