Applied Surface Science 255 (2008) 2129–2132

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Applied Surface Science

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Ba0.5Sr0.5TiO3/Bi1.5Zn1.0Nb1.5TiO7 multilayer thin films prepared bysol–gel method

Xin Yan *, Wei Ren, Peng Shi, Xiaoqing Wu, Xiaofeng Chen, Xi Yao

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

Article history:

Received 25 October 2007

Received in revised form 14 April 2008

Accepted 2 July 2008

Available online 10 July 2008

PACS:

77.22.Ch

77.22.Gm

77.55.+F

81.20.Fw

Keywords:

Sol–gel processes

Polycrystalline deposition

Perovskite

Dielectric materials

Ferroelectric materials

A B S T R A C T

Ba0.5Sr0.5TiO3/Bi1.5Zn1.0Nb1.5O7 (BST/BZN) multilayer thin films were prepared on Pt/Al2O3 substrates by

sol–gel method. The structure, morphology, and tunable dielectric properties of BST/BZN thin films were

investigated. X-ray diffraction results showed that the structure of BST/BZN multilayer thin films was

composed of a cubic BZN pyrochlore phase and a cubic BST perovskite phase. The diffraction pattern

confirmed that there was no measurable reaction occurred between the BST and BZN layers. The field-

emission scanning electron microscope (FESEM) showed that the surface of BST/BZN multilayer thin

films was crack-free and compact. The dielectric constant and loss tangent of the BST/BZN multilayer thin

films were 106 and 0.011 at 10 kHz, respectively. The dielectric tunability was 10% under dc bias field of

355 kV/cm at 10 kHz. The medium dielectric constant, low loss tangent and tunability of the dielectric

constant suggest that BST/BZN multilayer thin films have potential application for tunable microwave

device applications.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Recently, strong interest has focused on the development oftunable dielectric materials for microwave device applications[1,2]. Ba0.5Sr0.5TiO3 (BST) thin films, paraelectric at roomtemperature, are attractive candidates for microwave devices, suchas phase shifters, filters, varactors, delay lines, etc. [3–6]. However,significant reductions in loss at high frequencies together with theimproved dielectric tunability are needed for their realization indevices. Recent attempts to improve the dielectric properties,i.e. increase tunability and decrease the dielectric loss, of BST thinfilms have focused on films doped [7,8], improvement of thedielectric–electrode interface [9,10], alteration/control of film stress[11,12], and the modification of the films composition [13,14].Cubic pyrochlore phase Bi1.5Zn1.0Nb1.5O7 (BZN) has attracted muchattention as a dielectric material for microwave tunable applicationsmainly due to its very low dielectric loss (�5� 10�4 in bulk) andmoderate tunability [15–17]. It is expected that introducing BZN inBST films to form BST/BZN multilayer thin films will reduce

* Corresponding author. Tel.: +86 29 82668679; fax: +86 29 82668794.

E-mail address: yanxin@mail.xjtu.edu.cn (X. Yan).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.07.045

dielectric loss and dielectric constant of BST films and meanwhilekeep high tunability.

BST thin films have been prepared by a variety of techniquesinvolving pulsed laser deposition (PLD) [18], RF-magnetronsputtering [19], and sol–gel process [20,21]. Among thesetechniques, the sol–gel technique offers significant advantagesover others in such as purity, better homogeneity, and precisionstoichiometry control, large area deposition, and ease of doping.

In the present work, BST/BZN multilayer thin films wereprepared on Pt/Al2O3 substrate by sol–gel method. The phasecomposition, microstructure, dielectric properties and tunability ofBST/BZN multilayer thin films have been studied.

2. Experimental

BZN and BST thin films were prepared by sol–gel method,respectively. Barium acetate, strontium acetate and titanium tetra-n-butoxide were used as starting materials for BST precursors.Equal moles of barium acetate and strontium acetate weredissolved in heated glacial acetic acid to form Ba–Sr solutions.The mixtures of titanium tetra-n-butoxide and acetyl acetone(used as a chelating agent) were dropped into Ba–Sr solutionsunder constant stirring at 80 8C. 2-Methoxyethanol was added to

Fig. 1. XRD pattern of BST, BZN and BST/BZN multilayer thin films on Pt/Al2O3

substrates annealed at 750 8C.

X. Yan et al. / Applied Surface Science 255 (2008) 2129–21322130

adjust the viscosity of the solution. The final concentration of BSTsolution was 0.5 mol/L. The starting materials for BZN werebismuth acetate, zinc acetate dehydrate, and niobium ethoxide.The mixtures of 2-methoxyethonal, pyridine, and glacial aceticacid were used as solvent. The detailed synthesis procedures of theBZN precursor solution can be found in Ref. [15]. The concentrationof BZN solution was 0.3 mol/L.

BZN films and BST films were deposited on Pt/Al2O3 substratesby a spin-coating technique with a spin rate of 3000 rpm for 30 s.After the spin-coating, each layer was pre-baked at 350 8C for2 min and annealed at 750 8C for 2 min in a rapid thermal furnacebefore the next layer was deposited. The spin-coating processeswere repeated to reach the multilayer structure of BZN/BZN/BZN/BST/BST/BST. By comparison, pure BZN and pure BST thin filmswere also prepared by the same process. The thickness ofmultilayer BST/BZN thin films, pure BZN thin films and pure BSTthin films was 675 nm, 500 nm and 400 nm, respectively.

The phase composition of the thin films was characterized by aRigaku D/Max-2400 X-ray diffractometer (XRD) with Cu Karadiation. A field-emission scanning electron microscope (FESEM,JEOL JSM-6700F) was used to investigate the surface and cross-section morphology of the thin films. The thickness of the thinfilms was measured by a step profiler (Ambios Inc., XP-2). For thedielectric measurements, top Au electrodes with a diameter of1 mm were DC-sputtered on the films via a shadow mask to form ametal–insulator–metal (MIM) structure. The dielectric propertiesand capacitance–voltage curves were measured with an Agilent4294A impedance analyzer.

3. Results and discussion

The XRD patterns of Al2O3 substrate, and BST, BZN, BST/BZNfilms annealed at 750 8C are shown in Fig. 1. The results showedthat pure BST films were cubic perovskite polycrystalline structure,and pure BZN films were cubic pyrochlore polycrystallinestructure, respectively. There was no preferential orientation forboth two films. The phase composition of BST/BZN multilayer thinfilms was composed of a perovskite BST phase and a cubicpyrochlore BZN phase. The diffraction pattern confirms that therewas no measurable reaction occurred between BST and BZN layerannealing at 750 8C. It can be seen that the peaks corresponding toBZN phase were suppressed in BST/BZN multilayer thin films. Thecrystallization degree of BZN was affected on different substrate.The BZN layer within the multilayer thin films was deposited onthe BST layer, and the pure BZN thin films were deposited on Ptsubstrate.

Fig. 2. FESEM images of pure BST and BZN thin films on Al2O3 substra

Fig. 2 shows the surface morphology images of pure BST, pureBZN thin films on Al2O3 substrates annealed at 750 8C. It can beseen that the surface of BST films is smooth and crack-free, and thegrain size is about 40 nm. The small and fine grains may contributeto the short-time annealing and crystallization with a rapidthermal processing. It is very similar to the results reported in Refs.[22,23]. The surface morphology of BZN thin films shows the grainsize is around 100 nm, which is larger than that of BST thin films.The key reason should be the lower crystallization temperature forBZN films comparing with the relative higher of BST films, resultingin better crystallization of BZN thin films at the same annealingtemperature. Fig. 3 shows the surface morphology and cross-section FESEM images of BST/BZN multilayer thin films. Morphol-ogy of the BST/BZN multilayer thin films indicates that the films arecompact, crack-free. The morphology of BST/BZN multilayer thinfilms is different from that of pure BZN thin films, since they weredeposited on different substrates. The pure BZN thin films weredeposited on (1 1 1) preferential orientation Pt substrate, and theBZN layer in the multilayer thin films were deposited onpolycrystalline BST layer. The difference can cause the BZN graindifferent growth mechanism. Moreover, the interfaces in themultilayer thin films can affect the BZN grain growth. We can alsosee the effect from above the XRD results. Further work needs toclarify the mechanism. The distinct interfaces can be seen in thecross-section image. No obvious diffusions between the BZN andBST layers have been observed.

Fig. 4 shows the dielectric properties of pure BST, BZN thin filmsand BST/BZN multilayer thin films. The dielectric constants of BST,

tes annealed at 750 8C: (a) BST thin films and (b) BZN thin films.

Fig. 3. FESEM images of BST/BZN multilayer thin films on Al2O3 substrates annealed at 750 8C: (a) surface image of BST/BZN films and (b) cross-section image of BST/BZN films.

X. Yan et al. / Applied Surface Science 255 (2008) 2129–2132 2131

BZN and BST/BZN thin films are 138, 111 and 106 at 10 kHz,respectively. The smaller dielectric constants of the BZN/BST thinfilms can be a result of the presence of BZN phase in BST, which issimilar to the cases of BST doped with oxides [7,8]. The BST/BZNmultilayer thin films can be considered as the BST and BZNcapacitors connected in series. Therefore the dielectric constant ofthe multilayer thin film can be estimated using Eq. (1) as follows:

1

e ¼nBST

eBSTþ nBZN

eBZN(1)

where eBST and eBZN are dielectric constants of pure BST and pureBZN thin films and nBST and nBZN are the volume fractions of eachmaterial in the multilayer films. Since the multilayer thin filmshave the same surface area, the volume fractions of each materialin the multilayer films can be substituted by the thickness fractions(thicknesses of three-layer BZN and three-layer BST layers are375 nm and 300 nm, respectively). Taking dielectric constant ofthe pure BST film as 138 and BZN as 111, the resultant value of themultilayer films is 121. The measured value of the multilayer thinfilm is 106, which is close to calculated one. The result shows thatthe BST/BZN multilayer thin films can keep moderate dielectricconstant.

The loss tangent of BST, BZN and BST/BZN multilayer thin filmsis 0.053, 0.008 and 0.011 at 10 kHz, respectively. The loss tangentof the multilayer thin films is between that of the BST films and the

Fig. 4. Dielectric constant and loss tangent as a function of frequency for BST, BZN

and BST/BZN multilayer thin films.

BZN thin films. It indicates that the low loss tangent of themultilayer films benefits from the BZN layer in BST/BZN thin films.

The dielectric properties of BST, BZN and BST/BZN films havebeen investigated as a function of a DC bias field. Fig. 5 gives thebias field dependence of the normalized dielectric constant for thethin films annealed at 750 8C. The measuring frequency is 10 kHz.The dielectric constant decreases with the bias field, while the losstangent varies little with the bias field. The dielectric tunability ofBZN, BST and BST/BZN thin films is 1.6%, 24% and 10% at same biasfield of 355 kV/cm, respectively. The results show that thetunability of BST/BZN multilayer thin films is lower than that ofBST thin films. At the same time, the dielectric loss tangent isconsiderably reduced compared with that of BST thin films. Thefigure of merit (FOM) of the BST/BZN thin films, BST thin films andBZN thin films is 9, 4.5 and 2, respectively. Since the dielectric losstangent is considerably reduced, the FOM of the BST/BZN thin filmsannealed at 750 8C has been optimized, which is 350% of higherthan that of BZN films and about 2 times of BST films. The work onBST/BZN multilayer thin films fabricated by PLD was reported byWang et al. [24] which showed dielectric loss below 0.01 andtunability (11%) under a bias of 770 kV/cm. The tunability value of10% in our experiments was close to Wang results, but the bias fieldapplied in our experiment was only 355 kV/cm and there also wasno evidence to show saturation in tunability up to the maximummeasurement bias voltage. Therefore, high tunability could beexpectably realized by further increasing the bias voltage.

Fig. 5. Normalized dielectric constant vs. DC bias field plots for BST, BZN and BST/

BZN multilayer thin films.

X. Yan et al. / Applied Surface Science 255 (2008) 2129–21322132

4. Conclusions

BST/BZN multilayer thin films have been prepared on Pt/Al2O3

substrates by sol–gel method. The BST/BZN multilayer thin filmsexhibited a medium dielectric constant around 106 and a low losstangent of 0.011 at 10 kHz. The result shows introducing BZNin BST films to form BST/BZN multilayer thin films can reducedielectric loss of BST films. The multilayer thin films annealed at750 8C had a 10% tunability of measured with dc bias filed of355 kV/cm at 10 kHz. The medium dielectric constant, low losstangent and tunability of the dielectric constant suggest that BST/BZN multilayer thin films have potential application for tunablemicrowave device applications.

Acknowledgements

This work was supported by National Natural Science Founda-tion of China (Grant Nos. 50572086, 50602034 and U0634006).

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