Preparation of SrTiO3 thin films by the liquid phase ...

Materials Science and Engineering B99 (2003) 290 /293 www.elsevier.com/locate/mseb

Preparation of SrTiO3 thin films by the liquid phase deposition method Yanfeng Gao, Yoshitake Masuda, Tetsu Yonezawa, Kunihito Koumoto * Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Received 14 July 2002; received in revised form 22 September 2002

Abstract Successful preparation of strontium titanate (SrTiO3; STO) thin films has been realized by the liquid phase deposition method. The STO precursor solid thin films with close-packed grains smaller than 200 nm were fabricated from an aqueous solution of Sr(NO3)2/(NH4)2TiF6/H3BO3 /1/1/3 (molar ratio) at 50 8C on a Si substrate. X-ray photoelectron spectroscopy analysis showed that the as-deposited film contained fluorine as an impurity, which could be completely eliminated by annealing at 500 8C in air, accompanying the crystallization of the as-deposited amorphous thin film into perovskite-type STO. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Strontium titanate; Thin film; Liquid phase deposition

1. Introduction Strontium titanate (SrTiO3; STO) is a very attractive material for application into microelectronics because of its high charge storage capacity, good insulating properties and chemical stability [1]. It has been reported that STO is a promising material for dynamic random access memories in very large scale integrated devices [1], bypass capacitors in microwave monolithic integrated circuits [2] and high-k transistor gate dielectric for minimized metal-oxide semiconductor field-effect transistors [3]. In addition, it can also be applied as an insulating layer in thin film electroluminescent displays due to its excellent optical transparency in the visible region [4]. STO thin films are usually prepared by CVD, PVD, hydrothermal synthesis method, sputtering and MOCVD or by sol /gel method [3,5 /8]. Commercial implementation of CVD or PVD derived perovskite oxide thin films in integrated circuits is underway, and the first application in the market includes capacitors and ferroelectric non-volatile memories. However, both

* Corresponding author. Tel.: /81-52-789-3327; fax: /81-52-7893201. E-mail address: [email protected] (K. Koumoto).

CVD and PVD are multi-step processes installed in a vacuum system, which means high-energy consumption. For CVD deposition, precursors are usually transferred to reactors by liquid delivery technique, which makes it possible to use relatively involatile metallorganic precursors, but leads to difficulties in controlling the stoichiometry of the films. Sol/gel and hydrothermal methods can prepare thin films at relatively low temperatures, yet the former is a multi-step process including sol /gel preparation and coating thin films, while the latter needs special autoclaves which are usually operated under high pressures. The challenge, therefore, is to develop simple, energy-efficient, environment-friendly and cost-effective processes for preparing STO thin films. Liquid phase deposition (LPD) method is a low-cost method for film synthesis. LPD refers to the formation of oxide thin films from an aqueous solution of a metalfluoro complex which is slowly hydrolyzed by adding water, boric acid (H3BO3) or aluminum metal [9]. The addition of water forces precipitation of the oxide, and boric acid (BA) or aluminum acts as a fluorine scavenger destabilizing the fluoro complex and promoting precipitation of the oxide. A series of oxide thin films including simple oxides, multi-component oxides and compositionally graded oxides have been developed [10 /12]. Only two kinds of multi-component com-

0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00527-5

Y. Gao et al. / Materials Science and Engineering B99 (2003) 290 /293

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Fig. 1. XRD profiles of the as-deposited thin film and those annealed at different temperatures for 4.5 h in air (JCPDS card no.: 350734).

pounds were reported. One is a mixture composed of two simple oxides [10], the other is a complex compound deposited from its HF solution which was obtained by dissolving the compound in HF aqueous solution [11]. In this paper, we report a simple process to prepare STO thin films by the LPD method. This approach enables us to deposit nearly stoichiometric STO precursor solid thin films on a solid substrate with large area at room temperature.

2. Experimental procedures The substrates, one-side ground P-type Si (1 0 0) wafers (Shinetsu; resistivity 5 /10 V cm), were cleaned ultrasonically in acetone, ethanol and deionized water ( /17.6 MV cm) subsequently, followed by immersion into boiling water for 5 min. After dried at 50 8C, the substrates were irradiated by UV light (184.9 nm) for 2 h from a low pressure Hg lamp (5 W /4 Hg lamps, NL-

Fig. 2. XPS analysis of the as-deposited thin film; the inset one shows the XPS spectra of F1s range for the as-deposited thin film and those after annealing at different temperatures for 2 h in air.

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Fig. 3. SEM photographs of (a) the as-deposited thin film obtained for 4.5 h deposition; (b) after annealing at 500 8C for 2 h in air of (a); (c) crosssectional SEM photograph of (a); and (d) AFM image of (a).

UV253, Nippon Laser & Electronics Lab.) to improve the hydrophilic property of the substrate. The starting materials employed were ammonium hexafluorotitanate [(NH4)2TiF6 (AHFT), 98.5%, Mitsuwa Chemical Co., Japan], BA and strontium nitrate (SN) (purity: 98%, Kishida Chemical Co., Japan). They were separately dissolved in deionized water, and then mixed to form a homogenous solution containing 25 mM AHFT, 25 mM SN and 75 mM BA. The typical molar ratio was AHFT/SN/BA /1/1/3. The deposition of a solid phase was conducted by floating the substrate on the surface of the solution with the ground side upside down at 50 8C for 3/4.5 h. The substrate was floated to prevent particles formed in the solution from accumulating on the substrate surface. After deposition, the substrate was rinsed carefully in distilled water before air-drying. Scanning electron microscope (SEM; S-3000N, Hitachi) was used to observe the morphology. Scanning probe microscope (SPI3800N, Seiko Instruments Inc.)

was operated in AFM mode to observe the topography of the films; scans were taken at room temperature in air with a frequency of 2 kHz. The chemical composition of the deposited film was analyzed by X-ray photoelectron spectroscopy (XPS; Escalab210, VG Scientific Ltd) with Mg Ka as the X-ray source operated at pass energy of 20 eV. All spectra were referenced to the C1s signal at 284.6 eV. The structure and phase composition were characterized by X-ray diffraction (XRD; RAD-1C, Rigaku) with Cu Ka radiation (l /0.15418 nm) at a scanning speed of 18 min 1.

3. Results and discussion The molar ratio of AHFT/SN/BA was defined as 1/1/ 3 (pH :/3.4). In the solution composed of 0.025 M AHFT, 0.025 M SN and 0.075 M BA, obvious precipitation occurred after soaking at 50 8C for 30 min. XRD results (Fig. 1) for a film obtained after 4.5 h


deposition indicate that the as-deposited thin film was amorphous and became crystallized by annealing at 500 8C for 2 h in air. For a crystallized STO film, only diffraction peaks assigned to STO were observed, indicating no phase separation having occurred during annealing. Furthermore, even if the molar ratio of AHFT/SN/BA was changed from 1/1/3 to 1.3/1/3 or 1/ 1.3/3, the XRD pattern indicated only STO diffraction peaks, suggesting that the as-deposited film has a stoichiometric composition with respect to Sr/Ti ratio in spite of the different composition of a starting solution. The composition of the as-deposited film was further analyzed by XPS. As shown in Fig. 2, three peaks for Sr3d5/2, Ti2p3/2 and O1s were observed at about 133.5, 459.0 and 530.2 eV, respectively, which corresponded to binding energies for Sr2, Ti4 and O2 [13]. The atomic ratio of Sr/Ti /1.0/1.08 (by XPS analysis) for a specimen derived from a starting ratio of SN/AHFT / 1/1.3 further confirms that films with nearly stoichiometric composition tend to deposit. Si2p signals for the sample soaked for 4.5 h were not detected by XPS, suggesting that the film completely covered the substrate. A peak located at a binding energy of about 684.7 eV (inset of Fig. 2) was observed, which is the characteristic peak of F1s, indicating that the as-deposited film contained a quantity of fluorine. Heating process at 500 8C for 2 h in air resulted in the complete elimination of fluorine. Fig. 3(a) /(c) show morphology characteristics and the thickness of the thin film. The as-deposited thin film composed of closely packed grains (Fig. 3(a)). Some cracks appeared in the film annealed at 500 8C (10 8C min 1, Fig. 3(b)), which suggests optimization of the heating conditions is necessary to obtain highquality crystallized films. The thickness of the film was measured by SEM, indicating that the thickness of the film was about 200 nm (Fig. 3(c)). Topography of the as-deposited film was observed by AFM. The AFM image (Fig. 3(d)) reveals the formation of a uniform film of densely packed grains or agglomerates. The grain size is smaller than 200 nm (average grain size is 135 nm). The statistical roughness, root mean square for the whole measured area (/3 /3 mm2) is 7 /13 nm depending on the investigated area. Such

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topography and roughness are typical for the whole specimen and were easily reproduced.

4. Conclusions A novel process for preparing STO thin films was developed by the LPD method. The dense STO thin film composed of close-packed grains smaller than 200 nm in size were prepared on a Si substrate at 50 8C by using an aqueous solution of Sr(NO3)2 /(NH4)2TiF6 /H3BO3 (AHFT/SN/BA /1/1/3 and pH 3.4). The as-deposited thin film was amorphous and contained fluorine as major impurity. Annealing at 500 8C for 2 h in air resulted in complete elimination of fluorine and crystallization of the as-deposited thin film into perovskitetype STO.

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