Thin Solid Films 520 (2011) 789–792

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Thin Solid Films

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Enhanced tunable dielectric properties of Ba0.5Sr0.5TiO3/Bi1.5Zn1.0Nb1.5O7 multilayerthin films by a sol–gel process

Xin Yan a,b,c,⁎, Wei Ren c, Peng Shi c, Xiaoqing Wu c, Xi Yao c

a School of Materials Science and Engineering, Chang'an University, Xi'an 710064, Chinab Shenzhen Key Laboratory of Special Functional Materials, Shenzhen University, Shenzhen 518060, Chinac Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education, Xi'an Jiaotong University, Xi'an 710049, China

⁎ Corresponding author at: School of Materials ScienUniversity, Xi'an 710064, China. Tel./fax: 86 29 8233734

E-mail address: xinyan@chd.edu.cn (X. Yan).

0040-6090/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tsf.2011.04.118

a b s t r a c t

a r t i c l e i n f o

Available online 28 April 2011

Keywords:Dielectric materialsMultilayer thin filmsRapid thermal annealingSol–gel processTunable dielectric properties

Ba0.5Sr0.5TiO3(BST)/Bi1.5Zn1.0Nb1.5O7(BZN) multilayer thin films were prepared on Pt/Ti/SiO2/Si substrates bya sol–gel method. The structures and morphologies of BST/BZN multilayer thin films were analyzed by X-raydiffraction (XRD) and field-emission scanning electron microscope. The XRD results showed that theperovskite BST and the cubic pyrochlore BZN phases can be observed in the multilayer thin films annealed at700 °C and 750 °C. The surface of the multilayer thin films annealed at 750 °C was smooth and crack-free. TheBST/BZNmultilayer thin films annealed at 750 °C exhibited a medium dielectric constant of around 147, a lowloss tangent of 0.0034, and a relative tunability of 12% measured with dc bias field of 580 kV/cm at 10 kHz.

ce and Engineering, Chang'an0.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Recently, Ba1−xSrxTiO3(BST) thin films have been intensivelyinvestigated for applications in tunable microwave components, suchas frequency-agile filters, voltage-controlled oscillators, phase shiftersand antennas, because of their large electric field-dependent tunabilityand the adjustable dielectric properties from different doping ratio of SrandBa [1–6]. However, the relatively large dielectric loss and the limitedfigure of merit (FOM) (FOM is defined as the ratio of tunability and losstangent at room temperature) restrict the practical applications of BSTthin films in tunable microwave elements. Researchers have found thatBST films with high tunability, low loss at zero bias, and high dielectricbreakdown fields can be grown using pulse laser deposition (PLD) withjudiciously chosen process parameters [7–9]. It has been found thatdoping of low loss oxides into ferroelectric materials is an effective wayto reduce the loss tangent. Various oxides, such as MgO, ZrO2, TiO2, andAl2O3, have been used as additives to reduce the loss tangent of BST thinfilms [10–16]. But in many cases, the reduction in the dielectric loss bydoping is limited and the tunability of BST decreases substantially at thesame time.

Recently, cubic pyrochlore phase Bi1.5Zn1.0Nb1.5O7 (BZN) hasattracted much attention as a dielectric material for microwave tunableapplicationsmainly due to its very lowdielectric loss [17–20]. SandwichBZN/BST/BZN films deposited by radio frequencymagnetron sputtering

have been reported [21]. The BZN/Mn-BST heterolayer films depositedon Nb:SrTiO3 substrates by PLD has been reported [22]. These resultssuggest that BZN–BST composite thin films may have advantages intunablematerials design by using the compatibility and flexibility of thecomposites.

In our study, BST/BZN alternating multilayer thin films have beenprepared on Pt/Ti/SiO2/Si substrates by a sol–gel method. The phasecomposition, microstructure, dielectric properties and tunability ofthe resulting thin films have been investigated.

2. Experimental details

BSTandBZN thinfilmswerepreparedusinga sol–gelprocessing. Thecomposition of BST used was Ba0.5Sr0.5TiO3. The precursor materialsused to prepare BST were barium acetate, strontium acetate andtitanium tetra-n-butoxide. Glacial acetic acid and 2-methoxy ethanolwere used as the solvents. Equi-molar mounts of barium and strontiumacetate were dissolved in heated glacial acetic acid. Titanium tetra-n-butoxide was mixed with acetyl acetone (chelating agent) and thenmixed with the barium and strontium acetate solutions under constantstirring. 2-Methoxyethanol was added to the sol to adjust its viscosity.The prepared solwas stirred for 30 min to allow complex formation. Thefinal concentration of the BST solwas 0.5 mol/L. The composition of BZNused was Bi1.5Zn1.0Nb1.5O7. The starting materials for the BZN sol werebismuth acetate, zinc acetate dehydrate, and niobium ethoxide.2-Methoxyethonal, pyridine, and glacial acetic acid were used assolvents. The detailed synthesis procedures of used to produce theBZN sol are described in ref. [19]. The concentration of the BZN sol was0.2 mol/L. BZNfilms andBSTfilmsweredeposited alternatively onPt/Ti/SiO2/Si substrates by a spin-coating technique with a spin rate of3000 rpm for 30 s. Each layer of wet films was pre-baked at 350 °C for

790 X. Yan et al. / Thin Solid Films 520 (2011) 789–792

3 min and then fired at 650–750 °C for 3 min in a rapid thermal furnacebefore the next layer was deposited. Thicknesses of multilayer BST/BZNthin films are 675 nm. Pure BZN and BST thin films were also preparedby the same process for comparison. Thicknesses of Pure BZN and BSTlayers are 400 nm and 500 nm respectively.

The structures of the thin films were characterized using a RigakuD/Max-2400 X-ray diffractometer (XRD) with CuKα radiation at40 kV and 100 mA. The surface and cross-section morphologies of thethin films were examined using a field-emission scanning electronmicroscope (FESEM, JEOL JSM-6700F) at an operating voltage of5.0 kV. The thickness of the thin films was measured using a stepprofiler (Ambios Inc, XP-2). For the dielectric measurements, top Auelectrodes with a diameter of 1 mm were dc-sputtered on the filmsvia a shadow mask to form a metal-insulator-metal structure. Thedielectric properties and capacitance–voltage curves were measuredwith an Agilent 4294A impedance analyzer.

3. Results and discussion

3.1. Crystallization and phases

Fig. 1 shows the XRD patterns of multilayer BST/BZN thin filmsannealed at different temperatures. The peaks present in the patterns ofthemultilayer thinfilms are believed to arise fromboth the BST andBZNlayers. The cubic pyrochlore BZN phase can be observed in themultilayer thin films annealed at 650 °C using a rapid thermal process,but there were no peaks of perovskite BST phase in the same annealingtemperature. The BZN thin films were crystalline at lower annealingtemperatures than the BST layer,which is consistentwith others reports[19]. The BST thin films were amorphous after rapid thermal annealingat 650 °C, so a higher temperature was required to produce crystallineBST. As the annealing temperature was increased, the perovskite BSTphase emerged, and can be observed in the multilayer thin filmsannealed at 700 °C and 750 °C. At the same time, the intensities of theBZN diffraction peaks increased, but the layer of BZN maintained theircubic pyrochlore structure. The diffraction patterns confirm that nomeasurable reaction occurred between the BST and BZN componentsafter annealing at temperatures up to 750 °C for 3 min.

3.2. Morphology

Fig. 2 shows the surface morphology of BST films, BZN filmsannealed at 750 °C. The surface morphology of BST films shows thatthe grain size is small and about 20 nm. The surface of BST films is

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Fig. 1. XRD pattern of multilayer BST/BZN thin films annealed at different temperature.

compact and crack-free. The surface morphology of BZN films showsthe grain size is around 100 nm. It is to be noted that pores appearinside the large grains in BZN films annealed at 750 °C.

The surfacemorphology of BST/BZNmultilayer thin films annealedat 650–750 °C and a cross-section image of the BST/BZN multilayerthin films annealed at 750 °C are presented in Fig. 3. The terminatinglayer of the BST/BZNmultilayer thin films is a BZN layer, so the surfacemorphology of the BST/BZNmultilayer thin films is similar to that of apure BZN thin film. The grain size is small in the film annealed at650 °C, and it becomes larger as the annealing temperature increases.Pores appear in the BST/BZNmultilayer thin films when the annealingtemperature is above 650 °C. It is assumed that these pores formedduring the thermal processing of the BZN thin films. Further workneeds to clarify the mechanism of pore formation. The FESEM imagesof the BST/BZNmultilayer thin films indicate that the films are smoothand crack-free. A cross-section FESEM image of the BST/BZNmultilayer thin films (Fig. 3(d)) shows that each layer in the filmshas distinct interfaces. No obvious diffusion between the BZN and BSTlayers is observed.

3.3. Dielectric properties

The dielectric properties of pure BST and BZN and multilayer thinfilms are presented in Table 1. The BST/BZN multilayer thin filmsannealed at 650 °C shows the lowest dielectric constant because thelayers of BST are amorphous and the layers of BZN show lowcrystalline degree. The dielectric constants of BST/BZNmultilayer thinfilms increased with the annealing temperature, but are smaller thanthat of a pure BST thin film. The smaller dielectric constants of theBZN/BST thin films are a result of the presence of BZN phase in BST,which is similar to those observed for BST dopedwith oxides [23]. TheBST/BZN multilayer thin films can be considered as the BST and BZN

Fig. 2. SEM images of pure (a) BST, (b) BZN thin films on Pt/Ti/SiO2/Si substrates.

Fig. 3. SEM images of BST/BZN multilayer thin films annealed at (a)650 °C, (b)700 °C, (c)750 °C, (d) cross-section SEM image of BST/BZN multilayer thin films annealed at 750 °C.

791X. Yan et al. / Thin Solid Films 520 (2011) 789–792

capacitors connected in series. Therefore the dielectric constant of themultilayer thin film can be estimated using Eq. (1) as follows:

ε =εBSTεBZN

vBSTεBZN + vBZNεBSTð1Þ

where BST and BZN are dielectric constants of pure BST and pure BZNthin films. Since the multilayer thin films have the same surface area,the volume fractions of each material in the multilayer films can besubstituted by thickness fractions (Thicknesses of BZN and BST layersare 300 nm and 375 nm respectively). Taking dielectric constant ofthe pure BST film as 230 and BZN as 109, the resultant values of themultilayer films should be 150. The measured value of the multilayerthin film is 147, which is close to the calculated value.

The loss tangent of the BST/BZN multilayer thin films decreased asthe annealing temperature increased, which is caused by theincreased crystalline degree of both BST and BZN at higher annealingtemperature. According to above capacitors connected in seriesmodel, it is easy to understand the BZN layer contributes to low losstangents of the multilayer films.

The tunability and FOM of BST, BZN, BST/BZN multilayer thin filmsannealed at 650 °C, 700 °C, and 750 °C are presented in Table 1.Although the tunability of the BZN thin films was rather low (4%)compared with other reported results of as high as 55% with a bias

Table 1Dielectric properties of pure BST, BZN and BST/BZN multilayer thin films at 10 kHz.

Films Dielectric constant Loss tangent Tunability (%) FOM

BST 230 0.026 38(600 kV/cm) 14.6BZN 109 0.0047 4(550 kV/cm) 8.5BST/BZN (650 °C) 56 0.0079 1(580 kV/cm) 1.3BST/BZN (700 °C) 111 0.0039 7(580 kV/cm) 17.9BST/BZN (750 °C) 147 0.0034 12(580 kV/cm) 35.3

field of 2.4 MV/cm [17], the bias field applied in our experiments wasonly 550 kV/cm. It is expected that higher tunability could be realizedby increasing the bias field. The tunability of the BST/BZN multilayerthin film annealed at 650 °C is very low (1%). The reason is that BSTthin films are amorphous phase in multilayer thin films and the BZNthin films shows low tunability in low bias field. As the annealingtemperature is increased, the crystalline degree of the BST and BZNlayers increases, so the tunability of BST/BZNmultilayer thin films alsoincreases. The tunability of the BST/BZN multilayer thin filmsannealed at 750 °C is 12% (580 kV/cm). The tunability of the BST/BZN multilayer films is lower than that of the pure BST thin films.However, the dielectric loss tangent is considerably smaller than thatin the BST thin film. The FOM of the BST/BZN thin films annealed at750 °C has been optimized, which is 140% higher than that of a BSTfilm and about 4 times that of a BZN film. The tunability of the BZN/BST multilayer thin films in the present study is higher than reportedvalues (12%, 770 kV/cm) for BZN/BST/BZN sandwich films under asimilar applied bias field [21].

4. Conclusions

BST/BZN multilayer thin films were prepared on Pt/Ti/SiO2/Sisubstrates using a sol–gel method and rapid thermal processing. Thecubic pyrochlore BZN and perovskite BST phases can be observed inthe multilayer thin films annealed at 750 °C. The BST/BZN multilayerthin films annealed at 750 °C exhibited a medium dielectric constantof around 147, a low loss tangent of 0.0034, and a relative tunability of12%, which was measured under a dc bias field of 580 kV/cm at10 kHz. The highest FOM was achieved for the BST/BZN thin filmannealed at 750 °C, and is 140% higher than that of a BST film andabout 4 times that of a BZN film. The relative large dielectric constant,low loss tangent, and tunability suggest that BST/BZN multilayer thinfilms have potential application for tunable microwave deviceapplications.

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Acknowledgment

This work was supported by National Natural Science Foundation ofChina (Grant Nos. 50572086 and U0634006) and the FundamentalResearch Funds for the Central Universities (Grant No. CHD2009JC010).The project T201009 was supported by Shenzhen Key Laboratory ofSpecial Functional Materials, Shenzhen University, Shenzhen.

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