Bi1.5Zn1.0Nb1.5O7 multilayer thin films by a sol–gel process

Thin Solid Films 520 (2011) 789–792

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Enhanced tunable dielectric properties of Ba0.5Sr0.5TiO3/Bi1.5Zn1.0Nb1.5O7 multilayer thin films by a sol–gel process Xin Yan a,b,c,⁎, Wei Ren c, Peng Shi c, Xiaoqing Wu c, Xi Yao c a b c

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

a r t i c l e

i n f o

Available online 28 April 2011 Keywords: Dielectric materials Multilayer thin films Rapid thermal annealing Sol–gel process Tunable dielectric properties

a b s t r a c t Ba0.5Sr0.5TiO3(BST)/Bi1.5Zn1.0Nb1.5O7(BZN) multilayer thin films were prepared on Pt/Ti/SiO2/Si substrates by a sol–gel method. The structures and morphologies of BST/BZN multilayer thin films were analyzed by X-ray diffraction (XRD) and field-emission scanning electron microscope. The XRD results showed that the perovskite BST and the cubic pyrochlore BZN phases can be observed in the multilayer thin films annealed at 700 °C and 750 °C. The surface of the multilayer thin films annealed at 750 °C was smooth and crack-free. The BST/BZN multilayer thin films annealed at 750 °C exhibited a medium dielectric constant of around 147, a low loss tangent of 0.0034, and a relative tunability of 12% measured with dc bias field of 580 kV/cm at 10 kHz. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Recently, Ba1 − xSrxTiO3(BST) thin films have been intensively investigated for applications in tunable microwave components, such as frequency-agile filters, voltage-controlled oscillators, phase shifters and antennas, because of their large electric field-dependent tunability and the adjustable dielectric properties from different doping ratio of Sr and Ba [1–6]. However, the relatively large dielectric loss and the limited figure of merit (FOM) (FOM is defined as the ratio of tunability and loss tangent at room temperature) restrict the practical applications of BST thin films in tunable microwave elements. Researchers have found that BST films with high tunability, low loss at zero bias, and high dielectric breakdown fields can be grown using pulse laser deposition (PLD) with judiciously chosen process parameters [7–9]. It has been found that doping of low loss oxides into ferroelectric materials is an effective way to reduce the loss tangent. Various oxides, such as MgO, ZrO2, TiO2, and Al2O3, have been used as additives to reduce the loss tangent of BST thin films [10–16]. But in many cases, the reduction in the dielectric loss by doping is limited and the tunability of BST decreases substantially at the same time. Recently, cubic pyrochlore phase Bi1.5Zn1.0Nb1.5O7 (BZN) has attracted much attention as a dielectric material for microwave tunable applications mainly due to its very low dielectric loss [17–20]. Sandwich BZN/BST/BZN films deposited by radio frequency magnetron sputtering

⁎ Corresponding author at: School of Materials Science and Engineering, Chang'an University, Xi'an 710064, China. Tel./fax: 86 29 82337340. E-mail address: [email protected] (X. Yan). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.118

have been reported [21]. The BZN/Mn-BST heterolayer films deposited on Nb:SrTiO3 substrates by PLD has been reported [22]. These results suggest that BZN–BST composite thin films may have advantages in tunable materials design by using the compatibility and flexibility of the composites. In our study, BST/BZN alternating multilayer thin films have been prepared on Pt/Ti/SiO2/Si substrates by a sol–gel method. The phase composition, microstructure, dielectric properties and tunability of the resulting thin films have been investigated. 2. Experimental details BST and BZN thin films were prepared using a sol–gel processing. The composition of BST used was Ba0.5Sr0.5TiO3. The precursor materials used to prepare BST were barium acetate, strontium acetate and titanium tetra-n-butoxide. Glacial acetic acid and 2-methoxy ethanol were used as the solvents. Equi-molar mounts of barium and strontium acetate were dissolved in heated glacial acetic acid. Titanium tetra-nbutoxide was mixed with acetyl acetone (chelating agent) and then mixed with the barium and strontium acetate solutions under constant stirring. 2-Methoxyethanol was added to the sol to adjust its viscosity. The prepared sol was stirred for 30 min to allow complex formation. The final concentration of the BST sol was 0.5 mol/L. The composition of BZN used was Bi1.5Zn1.0Nb1.5O7. The starting materials for the BZN sol were bismuth acetate, zinc acetate dehydrate, and niobium ethoxide. 2-Methoxyethonal, pyridine, and glacial acetic acid were used as solvents. The detailed synthesis procedures of used to produce the BZN sol are described in ref. [19]. The concentration of the BZN sol was 0.2 mol/L. BZN films and BST films were deposited alternatively on Pt/Ti/ SiO2/Si substrates by a spin-coating technique with a spin rate of 3000 rpm for 30 s. Each layer of wet films was pre-baked at 350 °C for

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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 furnace before the next layer was deposited. Thicknesses of multilayer BST/BZN thin films are 675 nm. Pure BZN and BST thin films were also prepared by the same process for comparison. Thicknesses of Pure BZN and BST layers are 400 nm and 500 nm respectively. The structures of the thin films were characterized using a Rigaku D/Max-2400 X-ray diffractometer (XRD) with CuKα radiation at 40 kV and 100 mA. The surface and cross-section morphologies of the thin films were examined using a field-emission scanning electron microscope (FESEM, JEOL JSM-6700F) at an operating voltage of 5.0 kV. The thickness of the thin films was measured using a step profiler (Ambios Inc, XP-2). For the dielectric measurements, top Au electrodes with a diameter of 1 mm were dc-sputtered on the films via a shadow mask to form a metal-insulator-metal structure. The dielectric properties and capacitance–voltage curves were measured with an Agilent 4294A impedance analyzer. 3. Results and discussion

compact and crack-free. The surface morphology of BZN films shows the grain size is around 100 nm. It is to be noted that pores appear inside the large grains in BZN films annealed at 750 °C. The surface morphology of BST/BZN multilayer thin films annealed at 650–750 °C and a cross-section image of the BST/BZN multilayer thin films annealed at 750 °C are presented in Fig. 3. The terminating layer of the BST/BZN multilayer thin films is a BZN layer, so the surface morphology of the BST/BZN multilayer thin films is similar to that of a pure BZN thin film. The grain size is small in the film annealed at 650 °C, and it becomes larger as the annealing temperature increases. Pores appear in the BST/BZN multilayer thin films when the annealing temperature is above 650 °C. It is assumed that these pores formed during the thermal processing of the BZN thin films. Further work needs to clarify the mechanism of pore formation. The FESEM images of the BST/BZN multilayer thin films indicate that the films are smooth and crack-free. A cross-section FESEM image of the BST/BZN multilayer thin films (Fig. 3(d)) shows that each layer in the films has distinct interfaces. No obvious diffusion between the BZN and BST layers is observed.

3.1. Crystallization and phases 3.3. Dielectric properties Fig. 1 shows the XRD patterns of multilayer BST/BZN thin films annealed at different temperatures. The peaks present in the patterns of the multilayer thin films are believed to arise from both the BST and BZN layers. The cubic pyrochlore BZN phase can be observed in the multilayer thin films annealed at 650 °C using a rapid thermal process, but there were no peaks of perovskite BST phase in the same annealing temperature. The BZN thin films were crystalline at lower annealing temperatures than the BST layer, which is consistent with others reports [19]. The BST thin films were amorphous after rapid thermal annealing at 650 °C, so a higher temperature was required to produce crystalline BST. As the annealing temperature was increased, the perovskite BST phase emerged, and can be observed in the multilayer thin films annealed at 700 °C and 750 °C. At the same time, the intensities of the BZN diffraction peaks increased, but the layer of BZN maintained their cubic pyrochlore structure. The diffraction patterns confirm that no measurable reaction occurred between the BST and BZN components after annealing at temperatures up to 750 °C for 3 min.

The dielectric properties of pure BST and BZN and multilayer thin films are presented in Table 1. The BST/BZN multilayer thin films annealed at 650 °C shows the lowest dielectric constant because the layers of BST are amorphous and the layers of BZN show low crystalline degree. The dielectric constants of BST/BZN multilayer thin films increased with the annealing temperature, but are smaller than that of a pure BST thin film. The smaller dielectric constants of the BZN/BST thin films are a result of the presence of BZN phase in BST, which is similar to those observed for BST doped with oxides [23]. The BST/BZN multilayer thin films can be considered as the BST and BZN

3.2. Morphology

10

20

30

40

50

BZN(622)

BZN(444)

BZN(622)

650oC

BZN(622)

o

BZN(444)

BZN(440)

o

BZN(444)

Pt(200) BST(200) BST(200)

700 C

BZN(440)

750 C

BZN(440)

Pt(111) Pt(111)

Pt(111)

Au Au Au

BZN(400)

BST(110) BST(110)

BZN(400)

BZN(400)

BZN(222) BZN(222)

BST(100)

BST(100)

BZN(111)

Intensity (a.u.)

1000

BZN(222)

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

60

2 Theta (deg.) Fig. 1. XRD pattern of multilayer BST/BZN thin films annealed at different temperature.

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

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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.

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

εBST εBZN vBST εBZN + vBZN εBST

ð1Þ

where BST and BZN are dielectric constants of pure BST and pure BZN thin films. Since the multilayer thin films have the same surface area, the volume fractions of each material in the multilayer films can be substituted by thickness fractions (Thicknesses of BZN and BST layers are 300 nm and 375 nm respectively). Taking dielectric constant of the pure BST film as 230 and BZN as 109, the resultant values of the multilayer films should be 150. The measured value of the multilayer thin film is 147, which is close to the calculated value. The loss tangent of the BST/BZN multilayer thin films decreased as the annealing temperature increased, which is caused by the increased crystalline degree of both BST and BZN at higher annealing temperature. According to above capacitors connected in series model, it is easy to understand the BZN layer contributes to low loss tangents of the multilayer films. The tunability and FOM of BST, BZN, BST/BZN multilayer thin films annealed 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 1 Dielectric properties of pure BST, BZN and BST/BZN multilayer thin films at 10 kHz. Films

Dielectric constant

Loss tangent

Tunability (%)

FOM

BST BZN BST/BZN (650 °C) BST/BZN (700 °C) BST/BZN (750 °C)

230 109 56 111 147

0.026 0.0047 0.0079 0.0039 0.0034

38(600 kV/cm) 4(550 kV/cm) 1(580 kV/cm) 7(580 kV/cm) 12(580 kV/cm)

14.6 8.5 1.3 17.9 35.3

field of 2.4 MV/cm [17], the bias field applied in our experiments was only 550 kV/cm. It is expected that higher tunability could be realized by increasing the bias field. The tunability of the BST/BZN multilayer thin film annealed at 650 °C is very low (1%). The reason is that BST thin films are amorphous phase in multilayer thin films and the BZN thin films shows low tunability in low bias field. As the annealing temperature is increased, the crystalline degree of the BST and BZN layers increases, so the tunability of BST/BZN multilayer thin films also increases. The tunability of the BST/BZN multilayer thin films annealed 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 that in the BST thin film. The FOM of the BST/BZN thin films annealed at 750 °C has been optimized, which is 140% higher than that of a BST film 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 reported values (12%, 770 kV/cm) for BZN/BST/BZN sandwich films under a similar applied bias field [21]. 4. Conclusions BST/BZN multilayer thin films were prepared on Pt/Ti/SiO2/Si substrates using a sol–gel method and rapid thermal processing. The cubic pyrochlore BZN and perovskite BST phases can be observed in the multilayer thin films annealed at 750 °C. The BST/BZN multilayer thin films annealed at 750 °C exhibited a medium dielectric constant of around 147, a low loss tangent of 0.0034, and a relative tunability of 12%, which was measured under a dc bias field of 580 kV/cm at 10 kHz. The highest FOM was achieved for the BST/BZN thin film annealed at 750 °C, and is 140% higher than that of a BST film and about 4 times that of a BZN film. The relative large dielectric constant, low loss tangent, and tunability suggest that BST/BZN multilayer thin films have potential application for tunable microwave device applications.

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Acknowledgment This work was supported by National Natural Science Foundation of China (Grant Nos. 50572086 and U0634006) and the Fundamental Research Funds for the Central Universities (Grant No. CHD2009JC010). The project T201009 was supported by Shenzhen Key Laboratory of Special Functional Materials, Shenzhen University, Shenzhen. References [1] J. Oh, S. Delprat, M. Ismail, M. Chaker, E.E. Djoumessi, K. Wu, Integr. Ferroelectr. 112 (2009) 24. [2] M.B. Okatan, M.W. Cole, S.P. Alpay, J. Appl. Phys. 104 (2008) 10417. [3] U.C. Chung, C. Elissalde, M. Maglione, C. Estournes, M. Pate, J.P. Ganne, Appl. Phys. Lett. 92 (2008) 042902. [4] C.L. Chen, H.H. Feng, Z. Zhang, A. Brazdeikis, Z.J. Huang, W.K. Chu, C.W. Chu, F.A. Miranda, F.W. Van Keuls, R.R. Romanofsky, Y. Liou, Appl. Phys. Lett. 75 (1999) 412. [5] C.L. Chen, J. Shen, S.Y. Chen, G.P. Luo, C.W. Chu, F.A. Miranda, F.W. Van Keuls, J.C. Jiang, E.I. Meletis, H.Y. Chang, Appl. Phys. Lett. 78 (2001) 652. [6] M.W. Cole, P.C. Joshi, M. Ervin, M. Wood, R.L. Pfeffer, J. Appl. Phys. 92 (2002) 3967. [7] S.K. Dey, P. Majhi, J.S. Horwitz, S.W. Kirchoefer, W.J. Kim, Int. J. Appl. Ceram. Tec. 2 (2005) 59. [8] N. Navi, J.S. Horwitz, R.C.Y. Auyeung, S.B. Qadri, H.D. Wu, Thin Solid Films 510 (2006) 115.

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