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Enhanced dielectric and piezoelectric properties of Mn doped

(Bi0.5Na0.5)TiO3(Bi0.5K0.5)TiO3SrTiO3 thin films

Wei Li a,b, Huarong Zeng c, Jigong Hao a, Jiwei Zhai a,

a Functional Materials Research Laboratory, Tongji University, Shanghai 200092, Chinab College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, Chinac Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

a r t i c l e i n f o

Article history:

Received 4 February 2013Received in revised form 13 May 2013Accepted 17 May 2013Available online 25 May 2013

Keywords:

Thin filmsSolgelBNTDielectricPiezoelectric

a b s t r a c t

Lead free 0.95(0.8Bi0.5Na0.5TiO30.2Bi0.5K0.5)TiO30.05SrTiO3 (BNTBKTST) + x mol% (x = 01.0) Mn thinfilms were deposited on Pt (111)/Ti/SiO2/Si substrates by a solgel processing technique. The effects ofMn content on the structural, ferroelectric, piezoelectric and dielectric properties of the BNTBKTST thinfilms were investigated in detail. The BNTBKTST thin films doped with different Mn content exhibitpseudo-cubic perovskite structure and doped with 0.5 mol% Mn exhibits the largest grain size and thebest dielectric properties. Reduced leakage current and improved piezoelectric constant (d33) wereobserved in the Mn doped BNTBKTST thin films. The maximum d33 value of BNTBKTST thin filmsdoped with 0.5 mol% Mn is approximately 93 pm/V, which is comparable to that of polycrystalline PZTthin films. The results indicated that Mn doped BNTBKTST thin films would be of great interest forlead-free piezoelectric devices.

2013 Elsevier B.V. All rights reserved.

1. Introduction

Lead-basedferroelectric materials suchas leadzirconate titanate(PZT) are the most popularly used piezoelectric ceramics due totheir excellent piezoelectric properties at the compositions closeto morphotropic phase boundaries (MPB). In consideration of thegrowing demand for green materials with minimized impacts onhealth and environment, researchers are making intensive effortsto replace lead-based ceramics with lead-free compositions[14].

Sodiumbismuth titanate Bi0.5Na0.5TiO3 (BNT) is considered to beone of the promising materials since its discovery by Smolenskiet al. in 1961[5]. Bulk ceramics of BNThaveshown strong ferroelec-tricity at room temperature (remnant polarization Pr = 38lC/cm

2

and coercive field Ec = 73 kV/cm). To decreasethe electrical conduc-tivity and also further improve the electromechanical responses,MPB solid solutions based on BNT were designed and fabricatedsuch as the Bi0.5 Na0.5 TiO3BaTiO3 (BNTBT), Bi0.5Na0.5TiO3Bi0.5-K0.5TiO3 (BNTBKT) which exhibited enhanced piezoelectric prop-erties [6,7]. Recently, innovative development of BNT-basedperovskites withcomplexcompositions shedlighton the potential-ity of lead-free piezoceramics for actuator applications. Zhang et al.[8] reported a giant strain response of 0.45% at 8 kV/mm drivingfield, equivalently 560 pm/V, in Bi1/2 Na1/2 TiO3BaTiO3K0.5Na0.5-NbO3 (BNTBTKNN) ceramics. Wang et al.[9] reported a large

unipolar strain of 0.36% (Smax/Emax = 600 pm/V) with a driving fieldof 6 kV/mm were obtained in BNTBKTST system. Through simul-taneously monitoring the longitudinal and transverse strain values,Jo et al. [6,10] excluded this assumption and attributed the highstrain response to the composition-induced non-polar phasearound room temperature.

In spite of the numerous studies on BNT-based single crystalsand ceramics, the BNT-based thin films research, still remains atpreliminary stage [11]. Piezoelectric thin films, offer a number ofadvantages in micro-electromechanical systems (MEMS), and thepreparation and characterization of piezoelectric thin films willbe hot topics in the present microelectronic industry [12,13]. Inthis regard, implementation of a lead-free thin film with strongpiezoelectric effect is required for many MEMS based devices.However, to the knowledge of the authors, the study of BNTBKTST thin films has not been reported in the literature. It maybe challenging to precisely control the complex composition ofBNTBKTST in a thin film configuration. Volatility of the A-siteelements (Na, K, and Bi) limited the evaluation of their electricalproperties due to relatively high leakage currents under high elec-tric fields [14]. In addition, the special characteristics of thin films(e.g., small grain sizes, formation of film-substrate interfaces, etc.)contribute to a marked reductionof their electrical properties com-pared with those of their counterpart bulk forms [15]. As a result,the potential piezoelectric performance of thin films based on BNTremains unclear. Recently, great efforts have been made in promot-ing the electrical properties of BNT-based thin films and BNTBKTthin films could be deposited on Pt/Ti/SiO2/Si substrates by solgel

0925-8388/$ - see front matter 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.05.127

Corresponding author. Tel./fax: +86 2165980544.

E-mail address: apzhai@tongji.edu.cn (J. Zhai).

Journal of Alloys and Compounds 580 (2013) 157161

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process together with rapid thermal annealing [16]. Improved rem-nant polarization and fatigue resistance were observed in La, V, Ce

and Mn doped thin film [11,17,18]. In these dopants, Mn is widelyused, as it can change chemical state from Mn2+ to Mn3+ or Mn4+,which could make the thin films with improved insulating charac-teristic against applied field [19,20]. In this work, the effects of Mncontent on the structural, ferroelectric, piezoelectric and dielectricproperties of the BNTBKTST thin films were investigated. Therelationship between the Mn content and the electrical propertiesfor the BNTBKTST thin films is expected to offer useful guidelinesto the design of the devices for the microelectronic industry.

2. Experimental processing

The 0.95(0.8Bi0.5Na0.5TiO30.2Bi0.5K0.5)TiO30.05SrTiO3 +x mol% (x = 0, 0.2, 0.5and 1.0) Mn thin films were prepared using a solgel processing. Stoichiometric

amountsof bismuth nitrate (Bi(NO3)35H2O) (98%, Alfa Aesar), sodium acetate (CH3-COONa) (99%, Alfa Aesar), potassium acetate (CH3COOK) (99%, Alfa Aesar), stron-tium acetate [Sr(CH3COO)20.5H2O] (98%, Alfa Aesar), manganese acetatetetrahydrate [Mn(CH3COO)24H2O] (99%, Alfa Aesar), and titanate isopropoxide[Ti(OC4H9)4] (97%, Alfa Aesar) were used as starting materials. To compensate forthe volatility of Bi, Na and K in the annealing process, all these three starting mate-rials were taken in a 10 mol% excessamount [12]. 2-Methoxyethanol and acetylace-tone were added to control the viscosity and cracking of films while ammoniasolutionwas chosenas ligand. The flow chart of the synthesis process of the precur-sor solutionis shown in Fig. 1. The concentration of the final solutions was adjustedto 0.3 M by adding acetic acid. After aging the precursor solution for 24 h, it wasdeposited on the Pt (1 11)/Ti/SiO2/Si substrates, by spin coating each layer at3000 rpm for 20 s. Each spin-coated layer was subsequently heat treated at400 C for 10 min in air. The coating and heat treatment procedures were repeateduntil the thin film reached a thickness of 600 nm. Finally, the samples wereannealed at 700 C for 30 min in air for crystallization.

The crystalline phase of the thin films was analyzed by an X-ray diffraction(XRD) (D/max2550 V, Rigaku, Japan) with Cu Ka radiation. The surface morphologyof the films was observed by a field emission scanning electron microscope (FESEM)(S-4700, HITACHI, Japan). Desired thickness was estimated by FESEM cross-sectionof the films. For electrical measurements, gold pads with0.50 mm in diameter werecoated on BNTBKTST films surface as the top electrodes by DC sputtering. Dielec-tric properties were measured using the precision LCR meter (4284A Agilent Inc.,USA). Currentvoltage (IV) curves were obtainedusing a Keithley 6517A electrom-eter. Hysteresis loops were acquired by a ferroelectric test system (Radiant Preci-sion Premier II). Displacement induced by the converse piezoelectric effect wasmeasured applying an AC voltage with amplitude of 10 V via the piezoresponseforce microscopy (PFM) (SPA 400, SPI3800N, Seiko, Japan) tip directly on the filmsurface without the top electrodes.

3. Results and discussion

The XRD patterns of the BNTBKTST thin films doped with dif-

ferent mol% Mn content are shown in Fig. 2. It is evident that all thefilms have well-developed pseudo-cubic perovskite structure, and

the (1 10) diffraction intensity is dominant as that of random-oriented BNTBKTST films. The ionic radii of Mn2+, Mn3+, andMn4+ are 0.96 , 0.65 , and 0.53 , respectively [11,21]. Whilethe ionic radii of K+, Na+, and Bi3+ are 1.78 , 1.39 , and 1.03 ,respectively. The diffraction peaks shift towards high angles withthe increase of Mn content, indicating the lattice contraction. It isin agreement with that most of Mn substitute on A-site of theperovskite structure [22].

The FESEM images of surface morphology of the BNTBKTSTthin films are shown in Fig. 3. The microstructure of BNTBKTSTfilm without Mn doping shows grain sizes of 40 nm. It is found that

the average grain size increases with the decrease of Mn concen-tration, reaching a maximum grain size of 80 nm for 0.5 mol%Mn doping and having a microstructure with a wide distributionindicating abnormal grain growth which is consistent with previ-ous reports on BCZT and BST [19,23]. The physical properties of fer-roelectric thin films can closely relate to the microstructure such asa grain size. Improved dielectric properties, remnant polarization,and leakage properties were observed with the increase of grainsize [17].

Fig. 4a shows the room temperature dielectric constant anddielectric loss versus the applied electric field for BNTBKTST thinfilms. All curves are butterfly-shaped, providing the evidence ofweak ferroelectricity of the BNTBKTST thin films at room tem-perature. The electrical properties of the BNTBKTST thin films

doped with different mol% Mn are listed in Table 1. The dielectricconstant under zero electric field are 410, 440, 460 and 420 for

Mixing

refluxing

100C, 1h

Mixing

refluxing

70C, 2h

Mixing

refluxing

70C, 1h

Sr(CH3COO)2

in acetic acid

Ti(OC4H9)4 in

2-methoxyethanol

and cetylacetone

CH3COONa

CH3COOK

and Bi(NO3)3

in acetic acid

Ti(OC4H9)4 in

2-methoxyethanol

and cetylacetone

ST

solution

BNT-BKT

solution

BNT-BKT-ST

solutionMn(CH3COO)2

ammonia solution

Fig. 1. Flow chart of preparation process of BNTBKTST solutions doped with different Mn content.

Pt (111) substrate

1.0%

0.5%

0.2%

21

0

O)

110

100

2

00

0%

Intensity(a.u.)

20 30 40 50 60 31 32 33 34

110

2 (

Fig. 2. XRD patterns of the BNTBKTST thin films doped with different Mn

content.

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the BNTBKTST thin films doped with 0, 0.2 0.5 and 1.0 mol% Mn,respectively. The highest dielectric constant of 0.5 mol%Mn-doped BNTBKTST thin film can be attributed to biggest grainsize of thin film among those with Mn doping [24]. In the case ofMn doping, significantly lower dielectric loss values were achievedcompared with the undoped BNTBKTST thin film. The BNTBKTST thin film doped with 0.5 mol% Mn has the smallest dielectricloss (0.035) not only because the film has a relative large grain sizeand uniform surface, but also because the space-charge effect,which will be discussed below. The dielectric constant and dielec-tric loss measured at room temperature as a function of frequencyranging from 1 kHz to 2000 kHz for Mn doped BNTBKTST thinfilms are shown in Fig. 4b.

The hysteresis loops of polarization versus electric field of theBNTBKTST thin films doped with different mol% Mn are shownin Fig. 5a. Mn doped BNTBKTST thin films, near-complete

polarization switching was achievable under 600 kV/cm. The rem-nant polarizations (2 Pr) increase to a maximum value 16lC/cm

2

at 0.5 mol% Mn doping and then decrease. The enhancement ofPrmight result from a decreased rhombohedral distortion and do-main size by Mn doping. The smaller of the domain size, the easier

200nm

(d)(c)

(b)(a)

200nm

200nm

200nm

Fig. 3. FESEM of the BNTBKTST thin films doped with (a): 0, (b): 0.2, (b): 0.5, and 1.0 mol% Mn.

-300 -200 -100 0 100 200 300

200

250

300

350

400

450

500

Dielectricloss

Dielectricconstant

E (kV/cm)

Mn-0%

Mn-0.2%Mn-0.5%Mn-1.0%

(a)

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

10 100 1000

320

340

360

380

400

420

440

460

480

Dielectricloss

Dielectricconstant

Frequency (kHz)

Mn-0%Mn-0.2%Mn-0.5%

Mn-1.0%

(b)

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Fig. 4. Dielectric constant and dielectric loss as function of (a) bias voltage and (b) frequency at room temperature for the BNTBKTST thin films doped with different Mncontent.

Table 1

The electrical properties of the BNTBKTST thin films doped with different mol% Mn.

Mn content(mol%)

Dielectricconstant

Leakage currentdensity (A/cm2)

Piezoelectriccoefficient (pm/V)

0 410 3.33 105 500.2 440 1.60 106 750.5 460 1.12 106 931.0 420 9.41 105 73

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it may be for domains to move during poling under an electric field,and thus a larger remnant polarization can be induced thatapproaches the saturation value [25].

Leakage current density was measured at room temperatureunder applied DC electric field. Leakage current properties of theBNTBKTST thin films doped with different mol% Mn are shownin Fig. 5b. The values of leakage current density were3.33 105, 1.60 106,1.12 106 and 9.41 105 A/cm2,respectively, at an electric field of 400 kV/cm for the BNTBKTST thin film capacitors doped with 0, 0.2, 0.5 and 1.0 mol% Mn.One of the most important characteristics of the ferroelectric thinfilms is the leakage current behavior since this provides informa-tion regarding the charge transport mechanisms and directly af-fects the applications of ferroelectric films [26]. The conductivityof donor-doped ABO3 oxide is heavily dependent on dopant typeand concentration, microstructure of the materials and appliedfield [27]. The plots of current density (J) versus electric field (E)are linear under 150, 400, 400 and 250 kV/cm for the BNTBKTST thin films doped with 0, 0.2, 0.5 and 1.0 mol% Mn, respectively.The conduction mechanism is thus ohmic at the low electric field.Abrupt increasing trend are observed at higher electric field for thefilms and the value of lnr change linearly with E1/2. It indicatesthat the leakage mechanism in these films may be dominated byPooleFrenkel emissions [28,29]. Due to the easy volatility of Na/K when annealed at high temperature in this system, chargecarriers sodium/potassium V0Na=K and oxygen V

O may exist in thefilm. Bismuth V000Bi; V

0

Na=K and oxygen V

O vacancies can lead to aspace-charge effect, enhancing the electrical conductivity [18].When Mn is incorporated into the A sites of BNT system, theconcentration of VO will be decreased by Mn

3+ or Mn4+ [20].Accordingly, space-charge conduction may be suppressed by theaddition of Mn, which should enhance the electrical resistivity.Interestingly, Mn might also substitute on the B-site of BNT

0-200

-30

-20

-10

0

10

20

30

P(C/cm

2)

E (kV/cm2)

Mn-0%Mn-0.2%Mn-0.5%Mn-1.0%

(a)

-600 -400 200 400 600

0 100 200 300 400

10-7

10-6

10-5

10-4

J

(A/cm

2)

E (kV/cm)

Mn-0%

Mn-0.2%

Mn-0.5%

Mn-1.0%

(b)

Fig. 5. Hysteresis loops of polarization versus (a) electric field and (b) leakagecurrent density of the BNTBKTST thin films doped with different Mn content.

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Applied Voltage (V)

Displacement(nm)

(a)

-100

-50

0

50

100

Effectived

33(pm/V)

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Applied Voltage (V)

Displacement(nm)

(b)

-100

-50

0

50

100

Effectived

33(pm/V)

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Applied Voltage (V)

Displacement(nm)

(c)

-100

-50

0

50

100

Effectived

33(pm/V)

-10 -5 0 5 10 -10 -5 0 5 10

-10 -5 0 5 10 -10 -5 0 5 10

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Effectived

33(pm/V)

Applied Voltage (V)

Displacement(nm)

(d)

-100

-50

0

50

100

Fig. 6. Electric field induced displacement of the BNTBKTST thin films doped with (a): 0, (b): 0.2, (c): 0.5, and (d): 1.0 mol% Mn.

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system, generating additional oxygen vacancies that coulddecrease the electrical resistivity when the Mn-doping concentra-tion is excessive [22].

The typical DV loops and the corresponding d33V loops of theBNTBKTST thin films are plotted in Fig. 6ad, respectively. Thedisplacements of the BNTBKTST thin films locate in the rangeof 00.8 nm calculated from the DV curves, and the d33 values ofthe corresponding Mn doping content (Mn = 0%, 0.2%, 0.5%, 1.0%)thin films are 50 pm/V, 75 pmV, 93 pm/V and 73 pm/V, respec-tively. The BNTBKT system is known to have good piezoelectricproperties at its morphotropic phase boundary, reported to be ataround 20 mol% BKT. Besides, the appropriate chemical modifica-tion (5 mol% ST) could shift the ferroelectric-to-relaxor phase tran-sition near room temperature, which makes maximum achievablestrain is possible at relatively low field [9]. More importantly, thesignificantlyimproved d33 coefficients, compared withthe undopedBNTBKTST films, were observed in Mn-doped samples. A reduc-tion in the coercive field is observed in Mn doped BNTBKTST thinfilms, whereas the maximum remanent polarization is essentiallyunchanged. It indicates that Mn doping result in combinatorialsoft piezoelectric characteristics in Mn doped BNTBKTST thinfilms due to the existence of mixed valence states of Mn3+ andMn2+ [30]. Moreover, the large grain size of the BNTBKTST thinfilm doped with Mn contributes to the improvement of the d33 va-lue. Forthe films with large grain size, a large piezoelectric responseis expected because it is easy for the domain to rotate. These resultsindicate that 0.5% mol Mn doped BNTBKTST thin films haspromise in lead free piezoelectric devices.

4. Conclusions

The BNTBKTST thin films doped with different Mn contentwere successfully obtained by solgel process. The effects of Mncontent on the structural, ferroelectric, piezoelectric and dielectricproperties of the BNTBKTST thin film were investigated. XRDpattern shows that the BNTBKTST thin films doped with differ-

ent Mn content have the pseudo-cubic perovskite phase. Themicrostructure and electric properties of BNTBKTST thin filmsare dependent on Mn content. The BNTBKTST thin film dopedwith 0.5% Mn exhibits the best dielectric property with dielectricconstant of 460 and dissipation factor of 0.035. Applied voltagedependence of leakage current density measurement indicates thatleakage current density of the BNTBKTST thin films doped withMn is lower than that of the film without doping Mn, which maydue to the large grain size and the reduction of oxygen vacancies.At the same time, piezoelectric constant of the BNTBKTST thinfilm doped with 0.5% Mn is 93 pm/V, which indicates that it is goodcandidate for the lead-free piezoelectric thin film.

Acknowledgments

This work was supported by the Specialized Research Fund forthe Doctoral Program of Higher Education of China (No.20120072130001), the National Natural Science Foundation of Chi-na (No. 50932007), and the Natural Science Foundation of Shan-dong Province of China (No. ZR2011EMQ015).

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