Preparation and characterization of Ba2TiSi2O8 ferroelectric films produced by sol–gel method

Preparation and characterization of Ba2TiSi2O8 ferroelectric films produced by sol–gel method

Materials Letters 58 (2004) 2927 – 2931 www.elsevier.com/locate/matlet Preparation and characterization of Ba2TiSi2O8 ferroelectric films produced by...

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Materials Letters 58 (2004) 2927 – 2931 www.elsevier.com/locate/matlet

Preparation and characterization of Ba2TiSi2O8 ferroelectric films produced by sol–gel method Wukun Dai a, Mankang Zhu a,*, Y.D. Hou a, Hao Wang a, Hui Yan a, Mingming Shao b, Xiaoyang Chen b, J.B. Xu c a

Key Lab of Advanced Functional Materials, Institute of Materials Science and Engineering, Beijing University of Technology, 100 Pingleyuan, Choyang District, Beijing 100022, China b Beijing Changfeng Surface Acoustic Wave Device Corporation, Beijing 100891, China c Department of Electronic Engineering, CUHK, Shatin, Hong Kong, China Received 13 February 2004; received in revised form 17 May 2004; accepted 20 May 2004 Available online 23 June 2004

Abstract Fresnoite (Ba2TiSi2O8, BTS) thin films were grown on polished Si(100) substrates by sol – gel method. The films were characterized using Fourier transform infrared spectroscopy (FTIR), Raman scattering spectroscopy, X-ray diffraction (XRD) and atom force microscopy (AFM). The results reveal that the crystallinity of fresnoite thin films increases and their structures become more compact as post-annealing temperature increases. Combined with XRD data, the strong FTIR peaks and Raman bands assigned to Ti – O and Si – O vibration indicate the formation of fresnoite phase in the films at a temperature of 750 jC. Besides, the AFM observation showed the films have a smooth surface, fine grains and dense structure. D 2004 Elsevier B.V. All rights reserved. Keywords: Fresnoite; Ba2TiSi2O8; Thin film; Sol – gel; Crystallization

1. Introduction Fresnoite, Ba2TiSi2O8 (BTS), is a pyroelectric mineral 2 belonging to the space group P4bm-C4V and having lattice constants a = b = 0.852 nm and c = 0.521 nm, and a tetragonality c/a of 0.61 [1,2]. The positive and negative charges in the BST lattice are asymmetrically distributed, providing a spontaneous dipole of 1.14  10 26 C m and good piezoelectric properties. Researchers studied the synthesis and properties of fresnoite, such as its optical second harmonic generation (SHG) [3,4], UV luminescence with long-decay time [5], and, recently, the fresnoite ceramic powders showed ferroelectric hysteresis [6]. Particularly, Yamauchi et al. [7] reported that fresnoite thin films with c-axis orientation possess zero temperature coefficient of delay (TCD) and excellent surface acoustic wave (SAW) properties. Many efforts had been made to * Corresponding author. Tel.: +86-10-67392733; fax: +86-1067392412. E-mail address: [email protected] (M. Zhu). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.05.021

prepare fresnoite thin film. Yamauchi et al. [8] and Kawa [9] studied the deposition of fresnoite thin films from a ceramic target. Li et al. [10] produced fresnoite thin film with orientation by rf sputtering. They deposited fresnoite thin-film with c-axis orientation on Si (100) and Si (111) substrate by elevating the substrate temperature to 845 jC, and found the remarkable anisotropy in grain growth of fresnoite thin films. Ding et al. [11] crystallized Sr2TiSi2O8 and fresnoite thin-film from a glass bulk by the use of ultrasonic surface treatment, and observed strong optical SHG from the STS thin films. However, sol –gel-derived fresnoite thin film has not yet been reported because of the difficulties in obtaining even, completely crystallized films. In this paper, we fabricated fresnoite film by a sol – gel process with starting materials of Ba(CH 3 COO) 2 , Si(OC2H5)4 and Ti(OC3H7)4. The effect of heat-treating temperature on the crystallinity of the fresnoite films was investigated. The fresnoite thin films were characterized by FT-IR, Raman scattering, XRD and AFM. It was found that crystalline fresnoite film was obtained at 750 jC and

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Fig. 1. FT-IR spectra of BTS thin films after post-thermal treatment at different annealing temperature.

its crystallization was enhanced with increasing heat-treating temperature.

Fig. 2. The FMHW of absorption at 900 and 858 cm temperature.

1

versus annealing

ited films were observed in ambient using AFM; the images were digitized into 512  512 pixels with a scanning frequency of 0.75 Hz.

2. Experimental 3. Results and discussion High-purity barium acetate Ba(CH3COO)2, tetra-butyl titanate Ti(OC3H7)4 and tetra-methyl silicate Si(OC2H5)4 were used as starting materials, and C2H5OH was selected as solvent. Tetra-butyl titanate and tetra-methyl silicate were separately dissolved in ethanol and deionized water and mixed in a two-neck, round-bottom flask under stirring at 40 jC for 2 h. Then, barium acetate solution of 0.5 M concentration was dropped into the above mixed solution gradually. After refluxing for 4 h, the final colorless transparent solution was obtained and the concentration was adjusted to 0.1 M. The precursor solution is stable and no visible precipitation occurs even in several months. After aging the partially hydrolyzed solution for 1 week, BTS solution was spin-coated directly onto a Si (100) substrate. The rotation speed and the spin time were fixed at 3000 rpm and 45 s, respectively. The wet films were pre-annealed at 120 jC in air for 20 min. Finally, the annealing of the films was carried out in a tube furnace with oxygen atmosphere, keeping the temperature in the range of 700 – 900 jC for 15 min. The vibration characteristics of fresnoite films were determined by FT-IR and Raman scattering. The crystallization and crystal structure of the thin films were determined at room temperature by XRD in a h –2h configuration using Cu Ka radiation. The morphologies of the as-deposTable 1 IR absorption band (cm

3.1. FT-IR analysis IR spectroscopy is a powerful method for determining the bonding structures in matters. Fresnoite is believed to be a silicate mineral with specific [TiO5] pyramid and layered [Si2O7] structure. Fig. 1 shows the FT-IR spectra of fresnoite thin films annealed at different temperatures. In the wave number region of 400 –1000 cm 1, five peaks at 964, 904 (the strongest), 858, 580 and 478 cm 1, respectively, were observed for all specimens. As the annealing temperature increased from 750 jC to 890 jC, all the peaks were enhanced gradually, which means that the post-annealing plays an important role on the structure formation of fresnoite thin films. There are many structures in the Si– O groups, such as isolated, chained, layered, ring and skeleton, such that the vibration of Si– O bonds is much diversified. For example, in ring [SinO4(n 1)] structures, the strongest peak emerges at about 920– 980 cm 1, like in garnet; and in isolated [SiO4] structure, its strongest absorption emerges at 890– 900 cm 1 [12]. According to the above analysis, the strong absorption at about 900 cm 1 of the fresnoite films, as seen in Fig. 1, is assigned to the Si– O structure similar to the isolated [SiO4] structure. Table 1 [13] lists the IR vibration

1

) for different vibration modes in Ba2TiSi2O8 crystal

o(Ba – O)

y(Ti – O)

y(Si – O)

y(Ti – O)

y(Si – O – Si)

o(Ti – O)

o(Ti – O)

o(Si – O)

100 – 300

300 – 400

400 – 550

550 – 650

583

749

858

860 – 1024

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Fig. 3. Raman spectra of BTS thin films after post-thermal treatment at 830 jC.

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Fig. 5. The FWHM of XRD peaks at different annealing temperatures.

3.2. Raman study absorptions of different modes in crystal Ba2TiSi2O8. It is indicated that the vibration of symmetric Si –O – Si stretching of [Si2O7] leads to IR peak at 583 cm 1 and that of Ti– O bonds emerged at 749 and 858 cm 1. Hence, the IR absorption at 582 and 860 cm 1 in Fig. 1 is in accordance with the vibration modes of [Si2O7] and [Ti– O], which means the fresnoite films annealed above 750 jC have similar [Si – O] and [Ti – O] structure to that of crystal fresnoite. To further study the temperature effect, the full width of half maximum (FMHW) of the main IR absorptions was measured. Fig. 2 gives the FMHWs of different IR absorption o(Ti –O) at 858 cm 1 and o(Si – O) at 900 cm 1. It is obvious that the FWHMs of the o(Ti –O) and o(Si– O) absorption become small as the annealing temperatures increase. It indicates that the crystallinity of the films is enhanced as the temperature increases.

Fig. 4. XRD pattern of BTS thin-film annealed at different temperatures.

Markgraf and Sharma [14] had studied the Raman spectra of the polarized single-crystal fresnoite. It is reported that the oS(Si – O – Si) mode of [Si2O7] group is found at 666 cm 1 and is relatively weak. The strongest bands in the fresnoite single crystal spectra were found at 858 and 873 cm 1. Both of them were attributed to the o(SiO3) modes and the stretching of a short Ti –O bond, with mixing between them being evident. The vibrational characteristics of fresnoite are dominated by the molecular groups. Fig. 3 is the Raman spectrum of fresnoite thin films annealed at 830 jC. In the spectrum, there are three typical vibration bands at 869, 662 and 593 cm 1, respectively. The strongest band at 869 cm 1 in the film’s spectra is assigned to the vibration of o(SiO3) and stretching of a short Ti –O bond. Furthermore, two other relatively weak modes are corresponding to oS(Si –O – Si) mode. These show the good consistency in Raman spectra between the films and fres-

Fig. 6. Lattice parameter of fresnoite phase in the thin films annealed at different temperatures.

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Fig. 7. Interatomic distances in (a) the (001) plane of fresnoite and (b) (100) plane of Si.

noite crystal. In a word, the Raman spectra, together with FTIR spectra, show the formation of fresnoite phase in the film prepared by sol – gel method when it was annealed at temperature over 750 jC. 3.3. XRD analysis X-ray diffraction (XRD) technique is an effective method for identifying the crystallography of a matter. Fig. 4 is the XRD patterns of fresnoite thin films annealed at 700, 750, 790, 830, 870 and 890 jC, respectively. As seen from Fig. 3, the XRD pattern of the sample annealed at 700 jC shows the amorphous feature, therefore, high-temperature annealing is necessary to transform the thin film from the amorphous to the crystalline. When the temperatures increase, three strong peaks at 29.5, 27.5, and 34.9 jC, respectively, were observed in the XRD patterns as seen from Fig. 4. Compared to the JCPDS file (38-222), these three peaks corresponded to the diffraction of the (201), (211), (002) planes of the BTS phase, and other weaker peaks can also be

assigned to the planes of the fresnoite phase, showing the formation of the phase at 750 jC, which is much lower than those in literatures [10,15]. Fig. 5 gives the variation of FWHMs for the (201), (211) and (002) diffractions of fresnoite thin films. It is obvious that the FWHMs of all three diffractions decrease as the temperature increases, indicating the grains grow up. Compared to the FMHWs in the FT-IR spectra in Fig. 3, it is obvious that the FWHMs in the XRD spectra have the same trends. This consistency in XRD and FT-IR affirms the crystallinity enhancement due to the crystal growth at elevated temperatures. Besides, as the annealing temperature increases, the lattice parameters of the thin films change in different ways as shown in Fig. 6. It can be seen from Fig. 6 that there is no obvious variation observed for the a-axis; the lattice parameter reduces only from 0.5150 to 0.5145 nm. However, the cell parameter for the c-axis reduces sharply from 0.8394 to 0.8365 nm. These changes mean the shrinkage of the c-axis lattice of the BST phase as the temperature rises. Compared to the parameters a = 0.5210 nm and c = 0.852 nm of the crystal fresnoite, the c-axis of the cell is significantly lowered, which causes the reduction of the tetragonality c/a of the fresnoite. It is believed that the lattice shrinkage is related to the misfit between fresnoite film and Si substrate. For example, for the BST grains grown on the Si (100) substrate, there is a misfit between the BST lattice and the Si lattice. Fig. 7 shows the interatomic distances in the (100) plane of Si and in the (001) plane of BST. As shown in the figure, the parameters of the BST lattice are shorter than that of the Si substrate; the misfit of the lattice parameters of fresnoite and Si reaches 36%. Therefore, the lattice misfit will affect the lattice of the BST films severely and causes the lattice shrinkage of the BST phase. Generally, the large piezoelectric effect of fresnoite is believed to be due to its large tetragonality. Hence, this reduction in the tetragonality of

Fig. 8. AFM micrographs recorded from fresnoite thin films annealed at 830 jC; (a) top view and (b) surface plot.

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the fresnoite phase may influence the dielectric and piezoelectric properties of fresnoite thin films.

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Scientific and Technological Development Project of Beijing Education Committee and the Natural Science Foundation of China (Grant No. 10104004).

3.4. AFM analysis Fig. 8 shows AFM micrographs taken from fresnoite films deposited on a Si (100) substrate annealing at 830 jC. The film thickness was determined to be about 0.2 Am by UV reflectance measurement. It can be seen in Fig. 8 that the crystal grains had separated out obviously although grain boundaries clearly did not . The grain size was in the range 0.30 –0.50 Am. It was also observed from Fig. 8 that the film had a relatively smooth surface and compact grain, which is important for its application. In addition, there was no obvious crack on the film surface, which was one of the usual problems during the preparation of the fresnoite films by sol –gel method.

4. Conclusion In this paper, fresnoite (Ba2TiSi2O8) thin films were successfully grown on Si (100) substrates using a sol – gel technique. The FTIR and Raman spectra show the fresnoite phase is formed in the thin films. X-ray analysis indicates the amorphous thin films transform into crystalline state when they were annealed at over 750 jC. The crystallinity of the samples is enhanced, and the tetragonality of the fresnoite phase is reduced as the annealing temperature increases. Moreover, AFM observation found that the thin films have a relatively smooth surface, and the grain size was in the range 0.3 –0.5 Am.

Acknowledgements We would like to thank Dr. Tao Liu for his assistance in the XRD identification. The paper was sponsored by the

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