Effect of (Ba+Sr)/Ti ratio on dielectric and tunable properties of Ba_0.6Sr_0.4TiO₃ thin film prepared by sol-gel method

School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China

Received 10 April 2006; accepted 25 April 2006

Abstract: Ba_0.6Sr_0.4TiO₃ (BST) thin films were fabricated on Pt coated Si (100) substrates by sol-gel techniques with molar ratio of (Ba+Sr) to Ti changing from 0.76 to 1.33. The effect of (Ba+Sr)/Ti ratio deviating from the stoichiometry on microstructure, grain growth, dielectric and tunable properties of BST thin films were investigated. TiO₂ and (Ba,Sr)₂TiO₄ were found as a second phase at the ratios of 0.76 and 1.33, respectively. The variation of the ratio reveals more significant effect on the grain size in B-site rich samples than that in A-site rich samples. The dissipation factor decreases rapidly from 0.1 to 0.01 at 1 MHz with decreasing (Ba+Sr)/Ti ratio. The tunability increases with decreasing ratio from 1.33 to 1.05, and then decreases with decreasing ratio from 1.05 to 0.76. The film with (Ba+Sr)/Ti ratio of 1.05 has a maximum tunability of 32% and a dissipation factor of 0.03 at 1 MHz.

Key words: Ba_0.6Sr_0.4TiO₃ thin film; (Ba+Sr)/Ti ratio; sol-gel method; dielectric properties

1 Introduction

Barium strontium titanate (Ba_1-xSr_xTiO₃, BST) thin films are being extensively investigated as potential candidates of applications for tunable microwave devices. The large dielectric nonlinearity (permittivity vs electrified dependence) and low dielectric loss of this film make it possible to be integrated in electronically tunable filters, delay lines, and phase shifters[1-3]. For these applications, it must satisfy several critical requirements, such as high dielectric tunability, low dielectric loss in microwave frequency range, high temperature stability of dielectric properties and so on. Many investigators have found the dielectric properties of BST thin films are strongly affected by composition, substrates, stress and grain size [4-8]. Recently, it has been found that the dielectric properties are notably dependent on the (Ba+Sr)/Ti ratio deviating from the stoichiometric value at a given Ba/Sr ratio [9-11]. IM et al[9] reported that (Ba_xSr_1-x)Ti_1+yO_3+z thin films with a (Ba+Sr)/Ti ratio of 0.73 prepared by rf magnetron sputtering exhibited substantially reduced dielectric loss(tan δ=0.004 7) and the largest figure of merit(FOM=101). JIA et al[10] found that the dielectric loss of epitaxial Ba_0.6Sr_0.4TiO₃ films could be reduced by controlling the amount of TiO₂ doped. On the other hand, Ba_xSr_1-xTiO₃ thin film with slightly Ti-poor composition prepared by ion beam sputtering showed a higher dielectric constant than the stoichiometric film, but the dielectric loss had no obvious dependence on the (Ba+Sr)/Ti ratio[11]. However, these reports on the study of (Ba+Sr)/Ti ratio effect show ambiguous results and the ratio effect on the films fabricated by sol-gel methods is lack of discussion. Moreover, there are few further researches about the (Ba+Sr)/Ti ratio effect on the grain growth, defect mechanisms and dielectric properties.

In this paper, the effect of (Ba+Sr)/Ti ratio on the crystallographic structure, microstructures and dielectric behaviors of BST thin films prepared by sol-gel methods were presented and discussed. The relation between defect mechanisms of nonstoichiometry and dielectric properties was preliminarily analyzed.

2 Experimental

A series of Ba_0.6Sr_0.4Ti_1±yO₃ thin films with different (Ba+Sr)/Ti ratio (changing from 0.76 to 1.33) were fabricated on Pt(111)/Ti/SiO₂/Si substrates by sol-gel techniques, respectively. The starting materials were barium acetate (Ba(AC)₂), strontium acetate (Sr(AC)₂) and titanium-tetrabutoxide (Ti(OC₄H₉)₄). Glacial acetic acid and 2-methoxyethanol were used as the solvents. The Ba(AC)₂ and Sr(AC)₂ solutions were mixed with the Ti(OC₄H₉)₄ solutions, to form BST precursor. The precursor solution was spun on substrates at 4 000 r/min for 30 s. After spin coating, the films were heated at 650 ℃ for 30 min to be crystallized into perovskite phase. The spinning-heating cycles were repeated until the film thickness was up to required value. The deposited films were annealed in air at 750 ℃ for 1 h.

The crystal structure of films was examined by X-ray diffraction technique (XRD, Rigaku-D/Max, Japan).The microstructure and morphology were observed by field emission scanning electron microscope (FE-SEM, XL30 FEG). The dielectric properties and dielectric tunability were measured by using a precision impedence analyzer (Agilent 4294A, Japan).

3 Results and discussion

Fig.1 shows the XRD patterns of Ba_0.6Sr_0.4Ti_1±yO₃ films with different (Ba+Sr)/Ti ratios. The films with the ratios ranging from 0.83 to 1.18 show a single perovskite phase. The film with the ratio of 1.33 shows a peak (■) near 28.8° besides the perovskite peaks. This second phase is considered to be (Ba,Sr)₂TiO₄ phase. The film with the ratio of 0.76 shows a peak (●) near 26.1? which is considered to be the TiO₂ phase. The lattice constant does not change with the ratio in the range from 0.76 to 1.0, which is approximately 0.396 nm. The diffraction peaks shift toward small angles as the ratio increases from 1.0 to 1.33, which implying the increase of the lattice constant. For the A-site rich films, the excess A-site ions could substitute for B-site ions and the lattice constant increases, owing to the difference of ionic radii between the Ba²⁺, Sr²⁺ and the Ti⁴⁺. When Ba²⁺, Sr²⁺ content increases, this substitution would induce Ruddlesden-Poper(RP) faults in the films which

Fig.1 XRD patterns of Ba_0.6Sr_0.4Ti_1±yO₃ films with different (Ba+Sr)/Ti ratios

have been confirmed by theoretical calculation and experiments[12,13]. Moreover, (Ba,Sr)₂TiO₄ second phase forms even at higher (Ba+Sr)/Ti ratio. On the other hand, for the B-site rich films, Ti⁴⁺ could not substitute for the A-site ions and form amorphous or ill-crystallized. When excess Ti in films are enormous, Ti ions segregate into grain boundaries and the second phase (TiO₂) forms.

Fig.2 shows the FE-SEM photographs for both surface and cross-section of the Ba_0.6Sr_0.4Ti_1±yO₃ thin films with different (Ba+Sr)/Ti ratios. It is found that surface morphologies of all films are uniform, smooth and crack free. The thickness of the films is about 500 nm. As shown in Figs.2(a)-(d), the average grain size of the thin films is 50, 100, 110 and 110 nm, respectively. The variation of the ratio has dramatic effect on the grain size. In B-site rich samples, the excess Ti segregate into grain boundaries and form ill-crystallized, so grain boundaries become curvy and grain growth will be suppressed, which is agreement with that reported by CHO et al[14]. However, in A-site rich samples, the excess Ba²⁺, Sr²⁺  are accommodated into the BST lattice or generate RP faults, which have less influence on the grain growth. The corresponding cross-section FE-SEM images of all films are shown in Figs.2 (a′)-(d′). As the ratio increases, the texture of the films changes from layered to columnar.

The dielectric properties of Ba_0.6Sr_0.4Ti_1±yO₃ films as a function of the DC bias electric field at frequency of 1 MHz are shown in Fig.3. It can be seen that the dielectric tunability and dissipation factor depend on the (Ba+Sr)/Ti ratio. The dielectric tunability increases with the decrease of the ratio from 1.33 to 1.05, and then decreases with the continuous decrease of the ratio from 1.05 to 0.76. The film with the ratio of 1.05 reveals a maximum tunability of 32%. Generally, the dielectric tunability is related to the composition and grain size of the specimen, especially in paraelectric regions. In A-site excess samples, the grain size changes insignificantly as shown in Fig.2. The reason causing the maximum dielectric tunability is in grain interior, such as the anharmonic effect and electric dipoles. Ba²⁺, Sr²⁺  substitute for Ti⁴⁺ and form electric dipole  complex. At the same time, the difference of ionic radii results in an increase in the residual stress, which causes the cavity of oxygen octahedron increasing in adjacent lattices. These phenomenon enhance the anharmonic effect and amplify the effective dipole moment. Consequently, the dielectric tunability increases. Similar situation has been reported by WU et al[15] and CHIU et al[16]. The maximum dielectric tunability is observed in the films with the ratio of 1.05. However, with the increase of A-site excess amount, the excess Ba, Sr in the BST lattice form the RP

Fig.2 FE-SEM surface and cross-section photographs of Ba_0.6Sr_0.4Ti_1±yO₃ thin films: (a), (a′) Ratio=0.76; (b), (b′) Ratio=1.0; (c), (c′) Ratio=1.05; (d), (d′) Ratio=1.33

structure and induce interlayers in BST lattice. The presence of such interlayers, which are expected to be less polarisable than the pervoskite lattice, contributed to the decrease of dielectric tunability[17]. As excess Ba, Sr in films are enormous, the formation of (Ba,Sr)₂TiO₄ second phase with low permittivity is another factor reducing dielectic tunability[18]. In B-site rich samples, Excess Ti in films segregate into grain boundaries and suppress grain growth. Therefore, dielectric constant and tunability decrease with increasing amount of excess Ti. Further more, the second phase (TiO₂) formes, which has lower dielectric constant compared with BST is another factor reducing dielectric tunability. From Fig.3(b), it can be noted that the dissipation factor decreases rapidly

Fig.3 Dielectric constant (a) and dissipation factor (b) of Ba_0.6Sr_0.4Ti_1±yO₃ thin films with different (Ba+Sr)/Ti ratio as a function of bias voltage at frequency of 1 MHz

from 0.10 to 0.01 with decreasing ratio of (Ba+Sr)/Ti. In A-site rich samples, electric dipoles, owing to substitution and the anharmonic effect, high defect concentration and (Ba,Sr)₂TiO₄ second phase which has large dielectric loss[19] are factors increasing the dissipation factor. In B-site rich sample, excess Ti segregate into grain boundaries and reduce the dielectric loss of the films, as mentioned by IM et al[9] and STREIFFER et al [20]. The dissipation factor of BST film with the ratio of 0.76 is 0.011, which is the lowest dielectric loss in these samples.

Fig.4 shows the dielectric tunability and dissipation factor (tanδ) of BST thin films as function of (Ba+Sr)/Ti ratio. The film with the ratio of 1.05 has a maximum tunability of 32% and a dissipation factor of 0.03 at 1 MHz, which have been improved in a certain extent compared with the stoichiometric sample. The film with the ratio of 0.76 reveals the lowest dissipation factor of 0.011. Although the dissipation factor is larger than that reported by IM et al[9], it is reasonable for the films prepared on Pt coated Si(100) substrates by sol-gel method.

Fig.4 Dielectric tunability and dissipation factor of BST thin films as function of (Ba+Sr)/Ti ratio at 1 MHz

4 Conclusions

Ba_0.6Sr_0.4Ti_1±yO₃ thin films with different (Ba+Sr)/Ti ratios were fabricated on Pt(111)/Ti/SiO₂/Si substrates by sol-gel techniques. The (Ba+Sr)/Ti ratio plays an important role in the crystallographic structure, microstructures and dielectric behaviors of BST thin films. Excess A-site ions have little influence on the grain growth, but excess Ti suppress grain growth. The dissipation factor decreases rapidly with the decrease of the (Ba+Sr)/Ti ratio. The film with the ratio of 1.05 has a maximum tunability of 32% and a dissipation factor of 0.03 at 1 MHz, which have been improved in a certain extent compared with the stoichiometric sample.

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(Edited by YANG Hua)

Foundation item: Project (50332030) supported by the National Natural Science Foundation of China

Corresponding author: CHENG Jin-rong; Tel: +86-21-56332704; Fax: +86-21-56332694; E-mail: jrcheng@staff.shu.edu.cn