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Piezoelectric and Dielectric Properties of Multilayered BaTiO/(Ba,Ca)TiO/CaTiO Thin Films 3

3

3

Xiaona Zhu, Ting Ting Gao, Xing Xu, Weizheng Liang, Yuan Lin, Chonglin Chen, and Xiang Ming Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05469 • Publication Date (Web): 12 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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ACS Applied Materials & Interfaces

Piezoelectric and Dielectric Properties of Multilayered BaTiO3/(Ba,Ca)TiO3/CaTiO3 Thin Films

Xiao Na Zhu,1,2 Ting Ting Gao,1 Xing Xu,2 Wei Zheng Liang3, Yuan Lin3, Chonglin Chen2,*, Xiang Ming Chen1, *

1

Laboratory of Dielectric Materials, Department of Materials Science and Engineering,

Zhejiang University, Hangzhou 310027, China 2

Department of Physics and Astronomy, University of Texas at San Antonio, Texas

78249, USA 3

State Key Laboratory of Electronic Thin films and Integrated Devices, University of

Electronic Science and Technology of China, Chengdu, Sichuan 610054, China

*

Corresponding authors: [email protected] & [email protected]. 1

ACS Paragon Plus Environment


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Abstract Highly oriented multilayered BaTiO3-(Ba,Ca)TiO3-CaTiO3 thin films were fabricated on Nb doped (001) SrTiO3 (Nb:STO) substrates by pulsed laser deposition. The configurations of multilayered BaTiO3-(Ba,Ca)TiO3-CaTiO3 thin films are designed with the thickness ratio of 1:1:1 and 2:1:1 and total thickness ~300 nm. Microstructural characterization by X-ray diffraction indicates that the as-deposited thin films are highly c-axis oriented and large in-plane strain is determined in BaTiO3 and CaTiO3 layer. Piezoresponse force microscopy (PFM) studies reveal an intense in-plane polarization component, whereas the out-of-plane shows inferior phase contrast.

The

optimized

combination

was

found

to

be

the

BaTiO3-(Ba0.85Ca0.15)TiO3-CaTiO3 structure with combination ratio 2:1:1, which displays the largest domain switching amplitude under DC electric field, the largest room temperature dielectric constant ~646, small dielectric loss of 0.03, and the largest dielectric tunability of ~50% at 400 kV/cm. These results suggest that the enhanced dielectric and tunability performance are greatly associated with the large in-plane polarization component and domain switching.

Keywords BaTiO3-CaTiO3, heretostrctures, ferroelectric thin films, PFM, dielectric properties

2


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INTRODUCTION Ferroelectric multilayers and compositional graded thin films which consist of periodic sequence of thin layers of different compounds have attracted intensive interests in recent years for their intriguing physical performance.1-4 Lots of theoretical calculations and experimental results revealed larger dielectric constant, 5,6 piezoresponse,7 polarization,8 tunability,9,10 in multilayered thin films compared to homogenous films. Through variation of the constituent materials or the stacking ratio of multilayers, the built-in interface and polarization gradient can be engineered for desired functional properties11. H. Tabata et al. revealed great dependence of dielectric constant as stacking periodicity.12 Liu et al. also show high dependence of microwave dielectric constant as stacking period numbers and layer thickness in multilayers.13 The average polarization could exceed the overall polarization of ferroelectric component, suggesting a new route for the enhancement of piezoelectricity or dielectric tunability. However, the piezo response investigation in multilayer films is still limited. One reason for the difficulty in investigation of piezoelectric response in nanostructures is that the piezoresponse is known to decrease with the feature size of ferroelectric nano structure. In addition, the piezo response is very likely to relax in a few seconds. Therefore, a precisely determination in thin films with nano features is difficult. BaTiO3 related supper lattices have been intensively investigated due to its excellent polarization behaviors even in nano scale. Multilayers consisting BaTiO3 and other paraelectric materials have attracted much attention for their outstanding polarization behavior compared to single BaTiO3 films.14-18 Lee et al. reported a 50% enhancement of the polarization in BaTiO3-SrTiO3-CaTiO3 supprlattices with respect to pure grown BaTiO3 film.2 Later, BaTiO3-CaTiO3 supperlattices have been proved superior in polarization behavior than BaTiO3-SrTiO3 supperlattices.19 It is also reported the increase density of interfaces could enhance the polarization in BaTiO3-CaTiO3 supperlattice.20, 21 Jo et al. reported a large strain (54 pm/V), using 3



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time-resolved synchrontron x-ray diffraction, in BaTiO3-CaTiO3 supperlattice.22 Thus, considering the large lattice mismatch between BaTiO3 and CaTiO3, the BaTiO3-CaTiO3 graded films could be a promise candidate in lead free piezo material for micro actuators and sensors applications. However, up to date, there are few investigations about the piezoelectric and dielectric properties with multilayer structure modulation in BaTiO3-CaTiO3 thin films. In order to explore the multilayer structure influence on piezo response and dielectric properties in BaTiO3-CaTiO3 multilayers, composition graded triple layer BaTiO3-(Ba,Ca)TiO3-CaTiO3 thin films were designed in this present work. In this configuration, top CaTiO3 layer, which suffers a tensile strain effect from the middle (Ba,Ca)TiO3 layer, could possibly be induced ferroelectric phase.19 The leakage and loss of the ferroelectric BaTiO3 layer could be controlled by the paraelectric CaTiO3 layer. Therefore, this structure would benefit both the ferroelectric and dielectric properties. In this present work, multilayered BaTiO3/(Ba,Ca)TiO3/CaTiO3 thin films with designed thickness were deposited on single crystal Nb doped (001) SrTiO3 substrate in turn, as seen in Figure 1. Systematical investigation of piezo performance, dielectric constant and its tunability have been performed. A large tunability ~50% and dielectric constant ~700, together with a low dielectric loss ~0.03 is achieved in the

optimized

BaTiO3-(Ba0.85Ca0.15)TiO3-CaTiO3

triple

layered

films

with

combination ratio 2:1:1. The large dielectric constant and tunability are close related to the large domain switching amplitude and in plane piezo response.

EXPERIMENTAL SECTION A KrF excimer pulsed laser deposition system with a wavelength of 248 nm with the energy density of about 2 J/cm2 and a repetition rate of 5 Hz was employed to fabricate all the ferroelectric multilayered thin films on single-crystal (001) Nb:STO substrates. Single-phase (Ba0.85Ca0.15)TiO3, (Ba0.75Ca0.25)TiO3, CaTiO3, and BaTiO3 targets were used for the deposition. The optimal growth conditions were found to be at the temperature of 840 °C with an oxygen pressure of 200 mTorr. The multilayered 4


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thin films were designed with distinct compositional layers: BaTiO3 (BTO), (Ba0.85Ca0.15)TiO3 (BCT15) and CaTiO3 (CTO) from bottom to top in turn with different combination ratios of 1:1:1(S1) and 2:1:1(S2). In comparison, the middle layer was also changed to (Ba0.75Ca0.25)TiO3 with combination ratios of 1:1:1(S3) and 2:1:1(S4).The total deposition times were set to be 30 min to ensure the same total film thicknesses of about 300nm. The surface morphologies, local ferroelectric properties were measured using an Asylum Research Cypher scanning probe microscope. The capacitance-voltage and dielectric studies were performed using Agilent Technologies impedance analyzer (4294A) from 100Hz to1MHz, using a fixed ac drive voltage of 50 mV.

RESULTS AND DISCUSSION Figure

2

presents

the

θ-2θ

X-ray

diffraction

patterns

of

the

BaTiO3-(Ba,Ca)TiO3-CaTiO3 multilayer thin films on Nb:STO(001) substrate. The peaks of Cu Kβ show around 41.7°, marked “ ”. All the films show (00l) peaks, indicating the films are highly c-axis oriented. Three peaks appear at 2θ~44.44o, 45.56o and 46.30o can be identified as satellite peaks for BaTiO3, (Ba,Ca)TiO3 layer, and CaTiO3 layer, respectively. The position of peak3 is close to the peak of substrate, and it is determined by Gauss fitting. According to Bragg equation, the d002 of each peak can be calculated, and also the in plane lattice constant of each layer. The calculation details are presented in supporting information (Table S1). Thus the in plane lattice constant misfit of each layer respect to its bottom layer can be calculated as well. The Table 1 shows the summary of the out of plane constant d001, the in plane constant, and the misfit of each layer. Take the BTO layer for example, the misfit is calculated using the equation below: (1.1)

where the

and

is the in plane constant of BTO layer and Nb:STO

substrate, respectively. As shown in Table 1, the BTO layer suffers 2% compress 5



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strain from bottom Nb:STO layer, whereas the CTO layer suffers 5% tensile strain from bottom BCT layer. The strain in middle layer of S1, S2 and S3, S4 is different, but small compared to the BTO and CTO layer. The large strain in CTO layer could be the origin of large strain response, which will be discussed later in the PFM result. The epitaxial quality and in-plane texture of the films with respect to substrate was confirmed by the XRD φ scan taken around the {101} planes. The peaks from the BTO/BCT/CTO multilayers display clear coincidence with those from Nb:STO substrate, as shown in the insets of Figure 2. Thus, the in-plane interface relationship between

the

triple

BTO/BCT/CTO

layers

and

the

substrate

is

(001)CTO//(001)BCT//(001)BTO//(001)Nb:STO, [100]CTO//[100]BCT//[100]BTO //[100]Nb:STO,, suggesting that the multilayered thin films have excellent epitaxial nature. The full width at half maximum (FWHM) of the rocking curve around (002) peak is 0.240, 0.187, 0.387, and 0.151 for sample S1, S2, S3 and S4, respectively. We used piezoelectric force microscopy (PFM) to observe ferroelectric domain structure. As shown in Figure 3, the surface morphology of the multilayers show nano featured grain size, and a grain shape corresponding nano domain structure in the out of plane direction. Compared to the OP images, the IP images show obviously higher amplitude and larger phase angle in all samples (see Figure 3&4). It indicates larger magnitude of in plane polarization. One reason of the large in-plane polarization here is the large tensile strain of the CaTiO3 layer suffered from the bottom layer (calculated in Table 1). Previous first principles study19 suggested that strong tensile strain applied to CaTiO3 would resulted in a large in-plane polarization, and it could reduce the out-of-plane polarization. The switching behavior is examined by a bias of 20 volt applied positively and negatively to the tip, with sample grounded. Figure 5 displays the OP amplitude (a) and phase (b) images when +20 V (upper part) and −20 V (bottom part) bias is applied to the sample surface, respectively. It is well known that detecting ferroelectric behavior at nanoscale is difficult, because nano grains would relax within a couple of seconds after poled. Typical PFM writes a DC voltage across the whole image and then reads out the polarization state with AC voltage. As for ferroelectric poling state 6


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at nano grains, it becomes relax as it read. Thus, here a new mode named SWAP was employed in this work, which read out the polarization state right following every DC line writing. As shown in Figure 5, intense out-of-plane amplitude signal is achieved, indicates the domains in all samples can be greatly switched with DC bias. We calculated the colour contrast of the phase images of the four samples (see Figure S3). The phase degree switched in S1, S2, S3 and S4 is 221o, 211o, 225o, and 216o, respectively. They are close in the four samples, with S1 and S3 slightly higher. The sample of S2 (CT(1)/BCT15(1)/BT(2)) shows the largest amplitude among all the prepared samples. This corroborates well to the following results that it has the largest dielectric constant and DC bias tunability. In order to precisely determine imprint, coercive bias, and saturation responses, and domain nucleation voltage on the nanoscale, a new technique named Switching Spectroscopy PFM (SS-PFM) was used for the selected individual grain of the multilayers.23 In SS-PFM, a sine wave is applied by a square wave which steps in magnitude with time. Between each ever-increasing voltage step, the offset is stepped back to zero with the AC bias still applied to determine the bias-induced change in polarization. Figure S1 show the loops used conventional method and SS-PFM determined loops. It can be noticed that the conventional loops have underestimated coercive field, overestimated amplitude, and reversely imprint direction in the amplitude loops. Therefore, the local phase and amplitude loops of all the samples were measured using SS-PFM, as shown in Figure 6. The hysteresis loop of the sample of S1 shifts to negative electric field, whereas that of the sample of S2 shifts to positive electric field. This indicates that the fixed polarizations of S1 and S2 are in opposite direction. This asymmetric behavior can be attributed to the presence of non-switchable domains pinned by the interfaces and built-in electric field. In contrast, the loops of the samples of S3 and S4 are symmetric. We summarized the coercive field and d33 from Figure 6 (see Table 2). It can be noticed that the average coercive filed of S1 and S3 is nearly twice of that of S2 and S4, respectively. This suggests that 2:1:1 configuration has a smaller coercive field compared to that of the 1:1:1 configuration. This also corroborates to the following 7



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results that S2 and S4 has relatively larger dielectric constant, because lower coercive field means that it needs lower electric field to active switchable polarization. The d33 values were estimated from the positive linear part of amplitude loops. In particular, the sample of S4 displays a large d33 value of 64.63±1.46 pm/V, and this value is relatively enhanced compared to homogeneous BTO films (up to 54 pm/V) and to Pb(Zr,Ti)O3 films(45 pm/V).24 This improvement in multilayered films are related to antiferrodistortive symmetry breaking, and the polarization gradient across the layered structure. It is noteworthy that the OP piezoresponse phase of sample S2&S4 can be reversibly switched 180o (as seen in Figure 6(b)&(d)), demonstrating a complete polarization switching. This may be associated with the increasing BaTiO3 layer in S2 and S4. In comparison, sample of S1 and S3 can be switched nearly 140o (as seen in Figure 6(a) &(c)). Room temperature dielectric constant and loss as a function of frequency are shown in Figure 7. The maximum values of dielectric constant were obtained in the sample of S2 (CT(1)/BCT15(1)/BT(2)) of ~646 with a low dielectric loss of ~0.03 from 10 kHz to1 MHz. The dielectric constant of S2 is bigger than S1, and that of S4 is also bigger than S3 above 100 kHz. This indicates that 2:1:1 configuration performs better in intrinsic dielectric constant. It can be understood

from strain coupling and

polarization coupling. The XRD results demonstrated (Ba,Ca)TiO3 show relatively small misfit to BTO layers, and larger strain is shown in BTO and CTO layer. Consider strain coupling in top CaTiO3 layer, with smaller in plane lattice parameter, it has much higher lattice mismatch (~5% respected to BCT15, ~4.8% respected to BCT25). Since the total thickness is fixed, the paraelectric CaTiO3 layer is thinner in 2:1:1 configuration than that of 1:1:1 configuration. Thus the strain gradient is larger in the 2:1:1 configuration, which is desired for paraelectric CaTiO3 layer to induce ferroelectricity. Considering the BTO layer, the strain gradient in BTO layer is inversely proportional to its thickness as well. Unlike the paraelectric layer, ferroelectric BaTiO3 layer favors less strain, because it would clamp the ferroelectric domain. Thus the 2:1:1 configuration show better in plane piezo response and also the dielectric behavior. The larger in plane polarization component in S2 and S4 is 8


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expected to associate with higher c-axis dielectric constant. This is similar to a single crystal BaTiO3, which has the largest dielectric constant always perpendicular to the direction of largest polarization. The Figure 8 summarized the dielectric constant and its tunability from 0 to 12V, the sample of S2 show best dielectric constant and tunability under 12V bias. It has a tunability of ~50% under 12V bias. This result corroborates to previous discussion that the sample of S2 has smallest coercive field, largest switching piezo response signal, and large in-plane piezo amplitude.

CONCLUSIONS Highly oriented multilayered BaTiO3-(Ba,Ca)TiO3-CaTiO3 thin films were fabricated on (001) (Nb:STO) substrates by pulsed laser deposition. Microstructural characterization by X-ray diffraction indicates that the as-grown hereostructures are highly c-axis oriented with large in-plane strain in BTO and CTO layer. Piezoresponse force microscopy (PFM) studies reveal an intense in-plane polarization component, whereas the out-of-plane shows inferior phase contrast. The optimized combination was found to be the BaTiO3-(Ba0.85Ca0.15)TiO3-CaTiO3 structure with combination ratio 2:1:1, which displays the best switching behavior, room temperature dielectric constant ~646, dielectric loss of 0.03, and large dielectric tunability of ~50% at 400 kV/cm. Our results suggest that the enhanced dielectric and tunability performance are greatly associated with the increasing of in-plane piezo response, stronger domain switching amplitude, and smaller coercive field.

Supporting Information Details of in-plane lattice constant calculations, comparison SS-PFM with conventional PFM, and distribution of color contrast of Figure 3, Figure 4, and Figure 5.

ACKNOWLEDGEMENTS The present work was supported by National Natural Science Foundation of China 9



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under grant numbers 51332006. We thank Doctor Jianjun Yao in Oxford Instruments for his kindly support in PFM measurements.

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Table 1 Peak position and the calculated out of plane lattice constant d001, in plane constant, and misfit of each layer respect to bottom layer. S1

S2

S3

S4

Peak1(2θ)

44.43

44.44

44.40

44.41

d001(BT)

4.073

4.072

4.075

4.075

In plane constant

3.980

3.980

3.978

3.978

d001 (Nb:STO)

3.899

3.899

3.899

3.899

Misfit respect to Nb:STO layer

0.021

0.021

0.020

0.020

Peak2(2θ)

45.57

45.38

45.36

45.53

d001(BCT)

3.977

3.992

3.994

3.980

In plane constant of BCT layer

3.993

3.986

3.963

3.970

Misfit respect to BTO layer

3.27*10-3

1.51*10-3

-3.77*10-3

-2.01*10-3

Peak3(2θ)

46.39

46.30

46.28

46.29

d001(CT)

3.910

3.917

3.919

3.918

In plane constant

3.785

3.781

3.780

3.781

Misfit respect to BCT layer

-0.05

-0.05

-0.046

-0.048

Table

2

Summarized

coercive

field

and

d33

value

of

triple

layered

BaTiO3-(Ba,Ca)TiO3-CaTiO3 films.

Coercive field (V)

S1

S2

S3

S4

V+

3.69

7.22

8.14

4.70

V-

-8.81

0.28

-8.73

-4.90

(V++V-)/2

6.25

3.75

8.435

4.8

d33 (pm/V)

39.17±1.60 22.31±1.88 9.81±0.64 64.63±1.46

Dielectric constant(@ 1MHz)

314.7

645.9

268.3

386.8

Dielectric loss(@ 1MHz)

0.022

0.033

0.058

0.032

50.5

30.9

21.9

Tunability(%，@12V，1MHz) 28.9

13



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Page 14 of 20

Figure 1 The sketch for the formula of the BaTiO3/(Ba,Ca)TiO3/CaTiO3 multilayers with different combination ratios.

Figure

2

X-ray

diffraction

pattern

for

(a)CT(1)/BCT15(1)/BT(1)

(b)CT(1)/BCT15(1)/BT(2)(c) CT(1)/BCT25(1)/BT(1)(d) CT(1)/BCT25(1)/BT(2)multilayers, inserted with phi scan taken around {101} plane for each sample.

14


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Figure 3 PFM images for (I)S1: CT(1)/BCT15(1)/BT(1) (II)S2: CT(1)/BCT15(1)/BT(2)(III) S3:

CT(1)/BCT25(1)/BT(1)(IV)S4:

CT(1)/BCT25(1)/BT(2)multilayers,

showing

(a)

topography images, (b) out-of-plane piezoresponse amplitude images, and (c) out-of-plane piezoresponse phase images, respectively.

15



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Page 16 of 20

Figure 4 PFM images for (I)S1: CT(1)/BCT15(1)/BT(1) (II)S2: CT(1)/BCT15(1)/BT(2)(III) S3:

CT(1)/BCT25(1)/BT(1)(IV)S4:

CT(1)/BCT25(1)/BT(2)multilayers,

showing

(a)

in-plane piezoresponse amplitude images, and (b) in-plane piezoresponse phase images, respectively.

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Figure

5

switching

behavior

of

(I)

S1:

CT(1)/BCT15(1)/BT(1)

(II)

S2:

CT(1)/BCT15(1)/BT(2) (III) S3: CT(1)/BCT25(1)/BT(1) (IV) S4: CT(1)/BCT25(1)/BT(2) multilayers showing (a)amplitude images, (b) phase images, when +20V DC bias is applied to the upper half, whereas -20V is applied to the lower half, respectively.

17



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Figure 6 Local Switching Spectroscopy PFM (SS-PFM) amplitude-voltage butterfly loops and phase-voltage hysteresis loops for (a) S1: CT(1)/BCT15(1)/BT(1) (b) S2: CT(1)/BCT15(1)/BT(2) (c) S3: CT(1)/BCT25(1)/BT(1) (d) S4: CT(1)/BCT25(1)/BT(2) multilayers.

Figure 7 Dielectric constant (a), and dielectric loss (b) of all the samples of different configurations.

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Figure 8 (a) Capacitance and (b) tunability under dc bias from 0-12V for all samples.

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Table of contents

20


Page 20 of 20

(Ba,Ca

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