Realizing strain enhanced dielectric properties in BaTiO3 films by liquid phase assistedgrowthDavid T. Harris, Matthew J. Burch, Jon F. Ihlefeld, Peter G. Lam, Jing Li, Elizabeth C. Dickey, and Jon-PaulMaria Citation: Applied Physics Letters 103, 012904 (2013); doi: 10.1063/1.4813270 View online: http://dx.doi.org/10.1063/1.4813270 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structural and dielectric properties of laser ablated BaTiO3 films deposited over electrophoretically dispersedCoFe2O4 grains J. Appl. Phys. 116, 164112 (2014); 10.1063/1.4900516 Strong growth orientation dependence of strain relaxation in epitaxial (Ba,Sr)TiO3 films and the resultingdielectric properties J. Appl. Phys. 109, 091605 (2011); 10.1063/1.3581203 Combined effect of thickness and stress on ferroelectric behavior of thin BaTiO 3 films J. Appl. Phys. 93, 2855 (2003); 10.1063/1.1540225 Epitaxial growth and dielectric properties of functionally graded ( Ba 1−x Sr x ) TiO 3 thin films with stoichimetricvariation J. Vac. Sci. Technol. A 20, 1796 (2002); 10.1116/1.1503787 Dielectric properties of Mg-doped Ba0.96Ca0.04Ti0.84Zr0.16O3 thin films fabricated by metalorganicdecomposition method Appl. Phys. Lett. 78, 3517 (2001); 10.1063/1.1375002

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Realizing strain enhanced dielectric properties in BaTiO3 films by liquidphase assisted growth

David T. Harris,1,a) Matthew J. Burch,1 Jon F. Ihlefeld,2 Peter G. Lam,1 Jing Li,1

Elizabeth C. Dickey,1 and Jon-Paul Maria1

1Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina27606, USA2Materials Science and Engineering Center, Sandia National Laboratories, Albuquerque, New Mexico 87185,USA

(Received 14 March 2013; accepted 18 June 2013; published online 3 July 2013)

The addition of a liquid-forming flux to barium titanate thin films promotes densification and grain

growth, improves nonlinear dielectric properties, and allows residual strain to be sustained in

polycrystalline films without cracking at thicknesses relevant to device fabrication. Relative tuning,

an excellent indicator of crystalline quality and an important material property for tunable

microwave devices, increases from 20% to 70%. Films exhibit 0.15% residual differential thermal

expansion mismatch strain, resulting in a shift to the paraelectric-ferroelectric phase transition of

50 �C. This result is in excellent agreement with theory, demonstrating the ability to tune ferroic

transitions without epitaxial approaches. VC 2013 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4813270]

The electronic properties of crystalline materials can be

predictively and reproducibly tuned by the presence of me-

chanical strain. With strain, it is possible to achieve or

enhance desirable properties in semiconducting,1 ferromag-

netic,2 and superconducting3 thin films. While the impact of

strain on a ferroelectric transition has been known since the

1950s,4 only recently thin-film strain engineering has

become an area of substantial interest; in large part following

the predictions of phase stability as a function of epitaxial

strain.5 Experimental work shows that the stability window

of functional ferroic phases can be expanded by tens or hun-

dreds of degrees, and that it is possible to realize large

increases in the polarization of ferroelectric films through

the application of large compressive strains.6–11

While the ability to tune properties via mechanical

boundary conditions presents exciting possibilities for prop-

erty enhancement, these possibilities can only be harnessed

when the crystalline quality is high and when strains can be

stabilized in layers of technologically relevant thickness and

substrates that are consistent with the practical concerns of

device fabrication and commercialization. Epitaxial embodi-

ments feature excellent crystallinity; however, stabilizing

strain in thicknesses greater than several 10 s of nm remains

a challenge. Furthermore, substrates that produce large epi-

taxial strains to most perovskites are limited with respect to

availability, size, and cost. This may result in limited utility

of the epitaxial approach to little more than scientific

curiosity.

As an alternative, one may consider thermal expansion

mismatch as a comparable origin of in-plane strain that is

generic to a wide palette of film/substrate combinations,

including polycrystalline films on mainstream semiconduc-

tor/insulate substrates. As such, preparing crack-free layers

of thermally strained material of excellent crystallinity

would provide a parallel avenue for property engineering via

strain. However, in many such systems, there are limitations

to achieving sufficiently high crystal quality and a suffi-

ciently refined microstructure so that transitions are sharp

and cracking is avoided. These limitations are particularly

evident in polycrystalline BaTiO3 thin films, where crystal

structure and microstructure are difficult to control when

synthesis temperatures are limited to <900 �C, and where

dielectric properties are particularly sensitive to perturba-

tions from the ideal crystalline state.

In this report, we demonstrate that the crystallinity and

microstructure of BaTiO3 can be enhanced dramatically by

the addition of a high-temperature liquid phase that promotes

crystalline perfection and homogeneous densification. This

concept builds upon the successes of both epitaxial12 and

polycrystalline systems13 and enables one to overcome

directly the limitations in realizing the full non-linear dielec-

tric response of BaTiO3, and the ability to tune the ferroelec-

tric phase transition with residual strain. For BaTiO3 on

sapphire, a common commercial substrate for semiconductor

device fabrication, we demonstrate a three-fold increase in

permittivity in a film microstructure that can sustain �0.15%

biaxial strain that increases the Currie temperature (Tc) by

�50 �C in a 500 nm thick layer. Furthermore, we show that a

small volume fraction of BaO-B2O3 (BBO) flux provides the

mass transport to enhance crystal perfection and to homoge-

nize the grain structure such that large strains are sustained

without failure of film mechanical integrity. This is achieved

in film thicknesses conventional for device applications and

in excess of that possible via epitaxial growth.

Thin films were prepared from BaTiO3 targets contain-

ing 0, 1, 3, and 5% BBO 869 �C eutectic composition flux on

sapphire substrates using pulsed laser deposition. A post-

anneal in air at 900 �C was used to crystallize the films.

Grain growth and permittivity were maximized at a deposi-

tion temperature of 400 �C; this provided a dense pre-

crystalline layer. The 869 �C eutectic composition of

a)Author to whom correspondence should be addressed. Electronic mail:

dtharri2@ncsu.edu

0003-6951/2013/103(1)/012904/5/$30.00 VC 2013 AIP Publishing LLC103, 012904-1

APPLIED PHYSICS LETTERS 103, 012904 (2013)

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barium-borate was chosen for its low melting point.14 The

flux was prepared by a modified form of chemical

solution precipitation.15 Barium hydroxide octahydrate

(Ba(OH2)8H2O; 22 g) was boiled in approximately 200 ml of

distilled water then the insolubles were filtered using light

vacuum suction. A solution of boric acid (B(OH)3;26 g) and

distilled water (225 ml) was mixed, heated, and then poured

into the hot, stirring barium solution. The final solution was

removed from heat and allowed to precipitate overnight then

dried at 120 �C. Using a differential scanning calorimeter

(Netzsch STA 449 F3), the onset of the barium-borate melt-

ing was found to be 863 �C, in good agreement with the

expected value. Further details on target preparation, film

growth, and characterization are available in the supplemen-

tal information.16

Figure 1 provides a microstructure summary of the BT-

BBO flux series. Porosity is evident in films prepared from

the 0% BBO target, while those with BBO appear fully

dense. Grain size increases for all films grown from fluxed

targets, but is greatest for those grown from the 3% BBO tar-

get. Furthermore, the final microstructure shows the charac-

teristic features of BaTiO3 assisted by a high-temperature

liquid phase; these include the bimodal distribution of grain

sizes and faceting observed by SEM.17 Measurements using

the linear intercept method show that average grain size

increases from 50 nm to 320 nm for films grown from the 3%

target. The formation of secondary phases, often at grain

boundaries and triple points, is commonly seen when large

amounts of a glass flux are added.13,18–21 Extensive HRTEM

analysis of the 3% sample revealed clean grain boundaries

throughout the material with no evidence of residual glassy

or crystalline phases. HRTEM images of a representative

grain boundary and triple junction are shown in Figure 2.

B2O3 is well known to be volatile at elevated temperatures,22

and this volatility has previous been used to explain the lack

of post-synthesis secondary phases at sintering temperatures

above 1150 �C.18 To further explore the possibility of boron

loss and its spatial distribution, boron depth profiles of

BaTiO3 with 3% BBO were measured by SIMS for films

annealed at 900 �C for 1 h and 20 h. SIMS data reveal an

overall reduction in total B content with annealing time, and

a general shift of boron from the substrate interface to the

film surface. Though quantitative analysis using a known

reference was not performed, the SIMS data suggest a total

boron concentration below 1 mol%.

Although second phases were not observed at grain

boundaries or triple junctions, epitaxial BaAl2O4 was

detected using XRD. TEM cross-sections reveal an interface

layer of BaAl2O4 in all films. Despite the strong x-ray reflec-

tions observed, the interface layer was found to range from a

uniform layer of approximately 3 nm in the 0% samples, to

patchy interface grains that extend as much as 100 nm from

the original BT/sapphire interface for films with 3% BBO.

Selected area diffraction and electron energy loss spectros-

copy (EELS) confirmed the structure, epitaxial nature of the

second phase, and the absence of titanium. The high Tc

superconducting community has explored the role of inter-

face reactions with sapphire for YBa2Cu3O7�d films proc-

essed in a variety of conditions.23–25 In all studies, aluminum

diffusion into a Ba-containing oxide produces BaAl2O4 at

the interface. In the case of the high Tc phases, the supercon-

ducting properties are severely diminished. In the present

case, however, the presence of BaAl2O4 does not seem to in-

hibit improved grain growth or dielectric properties. The sec-

ond phase is confined primarily to the substrate-film

interface and makes up a small volume fraction of the total

film—we estimate less than 10%. Due to our interdigitated

capacitor structure, the second phase’s location at the inter-

face, and its low permittivity of about 15,26 we can view

BaAl2O4 as an extension of the substrate that results in

slightly decreased effective film thickness. The overall

impact on dielectric properties, especially the non-linear

response, should be minimal. The inclusion of the BBO flux

seems to facilitate BaAl2O4 formation, but the kinetics of

grain growth and the atomistic mechanisms that regulate the

physical location and temperature window of the liquid

remain under investigation and will be the subject of future

reports.

The role of BBO in microstructure enhancement is evi-

dent by TEM and XRD measurements, however at this stage,

its ability to enhance crystallinity is unknown. A particularly

effective means to estimate such effects is to compare the

magnitude of the non-linear dielectric response and broaden-

ing of the ferroelectric phase transition. A strong non-linear

response and a sharp phase transition are indicators of

well-crystallized ferroelectric materials. The former is

FIG. 1. TEM cross-sections of BaTiO3 films containing (a) 0%, (b) 1%,

(c) 3%, and (d) 5% barium borate flux. Top platinum electrodes and the sap-

phire substrate are visible at the top and bottom of each image. Grain size

increases from �50 nm in the pure samples to �320 nm in the 3% BBO

sample.

012904-2 Harris et al. Appl. Phys. Lett. 103, 012904 (2013)

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accomplished routinely by capacitance-voltage measure-

ments, which reveal directly the extrinsic contribution of do-

main walls to the total permittivity. Relative tuning

measured at applied voltages up to 35 V (117 kV cm�1) is

shown in Figures 3(a)–3(d) for interdigitated capacitors with

a constant lateral spacing of 3 lm. Relative tuning increased

from 20% for pure BaTiO3 films to 70% for those grown

from the 3% fluxed target. The large increase in tunability is

a direct indicator of domain walls and their motion. Both the

number of domains and their mobility increase with grain di-

ameter and crystal perfection, respectively, this is consistent

with the larger-grained microstructures enabled by a liquid

flux.

In order to compare phase transitions, dielectric meas-

urements were taken as a function of temperature from

�183 �C to 227 �C, the data are shown in Figure 3(e). For

the low-flux and non-flux films, the temperature dependence

of permittivity is extremely broad, consistent with scaling

effects observed in many reports, and we are unable to

resolve distinct phase transitions beyond a broad maxima

�110 �C.27,28 For BT samples with 3% and 5% flux, the tem-

perature dependence is pronounced and the cubic-tetragonal

and the tetragonal-orthorhombic transitions are clearly visi-

ble. Furthermore, the primary phase transformation tempera-

ture is shifted to between 50 �C and 70 �C higher than

expected for bulk material. To explain these observations,

and to relate them to a specific chemical or microstructural

origin in a self-consistent manner, we must consider simulta-

neously the possible impacts of Ba/Ti nonstoichiometry,

boron-substitution, and residual strain, as each are present to

finite extent in all samples containing BBO.

Ba/Ti ratio is considered first. In the 5% BBO targets,

enough excess barium oxide is added to change the Ba/Ti ra-

tio to 1.02. This level of stoichiometry change is similar to

those studied previously in bulk29 and films.30 In these

reports, increasing Ba/Ti by adding excess BaO enhanced

grain growth and permittivity by increased solid transport.30

Lee et al. and Ihlefeld et al. achieved three-fold increase in

permittivity, but found that the para-ferroelectric transition

was broadened, in contrast to the sharpening of the transition

observed in the present work. It is suggested that 5% excess

BaO is soluble in BaTiO3 and results in a Tc depression of up

to 20 �C31 or not at all.29

While we cannot rule out the impact of increased Ba/Ti

ratio, we believe it is a negligible contributor for two

reasons: (1) the direction of Tc shift is inconsistent with Ba

excess and (2) the present films are processed at 900 �C. In

the aforementioned study of Ihlefeld et al. permittivity and

grain growth enhancement by Ba excess were only observed

FIG. 2. HRTEM images of a character-

istic (a) grain boundary and (b) triple

junction in a BaTiO3 film prepared

form a 3% BBO target. Residual glassy

material is not present in either

structure.

FIG. 3. (a)–(d) Capacitance-voltage curves show that tunability is increased

to 70% for samples grown from the 3% target, a strong indication of

increased crystalline quality. (e) Capacitance measurements as a function of

temperature reveal a three-fold increase in the permittivity. Additionally, the

para-ferroelectric transition temperature (Tc) has been shifted >50 �C higher

due to the residual tensile strain of the films.

012904-3 Harris et al. Appl. Phys. Lett. 103, 012904 (2013)

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for films heat treated at 1100 �C and higher suggesting that

higher temperatures than utilized in the present study are

necessary to activate the increased solid-state transport.30

Another possible explanation for an elevated Tc is boron

substitution on the BaTiO3 lattice. Previous studies on ce-

ramic BaTiO3 doped with B2O3 vapor revealed Tc elevated

by a maximum of 2 �C32 or a negligible impact on the transi-

tion temperature.18 As such, the >50 �C shifts observed pres-

ently cannot be attributed to boron incorporation.

The final candidate mechanism is mechanical strain,

since cooling a BaTiO3/sapphire interface from 900 �C to

room temperature introduces approximately 0.16% biaxial

tensile strain from thermal expansion mismatch.33 XRD

sin2w analysis was undertaken in order to quantify the strain

in these films. Since the expected levels of strain are >0.1%,

it is not appropriate to assume that the w¼ 0� reference d-

spacing measurement is unstrained.34 To obtain an accurate

unstrained lattice parameter (d0), films were prepared using

identical processing conditions but on copper foils as

described previously.35 The extremely low shear modulus of

Cu, especially after a high temperature anneal, enables plas-

tic deformation in the substrate to minimize residual stress.

Using this reference, sin2w measurements of fluxed films

reveal a residual strain of (0.15 6 0.02)%, which agrees well

with expected strain from thermal expansion mismatch. The

d-spacing data and curve fit are shown in Figure 4.

Residual strain can be related to the film/substrate ther-

momechanical properties, which in turn provides a quantita-

tive link to Tc. Pertsev et al. calculated the strain dependence

of the structural phase transition temperatures for epitaxial

(001)-oriented BaTiO3 films.5 Although our film is neither

epitaxial nor single-domain as assumed by Pertsev et al., the

calculations apply to all mechanisms of biaxial strain.

Pertsev predicts that approximately 0.14% biaxial tensile

strain is needed to increase Tc by 50 �C, a predication within

the error of the present strain measurements. Consequently,

from the sin2w measurements, the Tc shift, and calculated

thermal expansion mismatch, we conclude that with the as-

sistance of a liquid phase flux, polycrystalline BaTiO3 films

exhibit a sufficiently homogeneous and dense microstructure

to sustain strains >0.1% with no evidence of cracking under

optical or scanning electron microscopy. Furthermore, the

flux improves crystal quality to provide a concomitant sharp-

ening of the ferroelectric phase transition allowing one to

resolve clearly a> 50 �C increase in Tc.

Strain engineering of ferroelectric thin films provides an

attractive avenue to modify and enhance material properties.

However, the conventional epitaxial approach is often lim-

ited by a reliance on exotic single crystalline substrates (e.g.,

DyScO3). In these situations, dislocation relaxation mecha-

nisms during high temperature growth establish a critical

thickness above which strains relax.36 As a comparative

example, for SrRuO3 on SrTiO3, the epitaxial strains are

�0.5% and the critical thickness was measured �10 nm.37

For the present case, strains of similar order appear to be sta-

ble at film thicknesses >500 nm.

In this report, we demonstrate that adding a high temper-

ature flux to a BaTiO3 thin films introduces liquid phase

transport and enhances densification, grain growth, and the

non-linear dielectric response; a nearly 4X increase in dielec-

tric tunability is observed for a fully dense and large-grained

flux-assisted material. Since the liquid forms at temperatures

approaching 900 �C, this becomes the temperature range for

mechanical equilibration and strain relaxation. X-ray and

SEM measurements reveal that upon cooling, in-plane ten-

sile strains of 0.15% evolve without crack formation. These

strains are identical to that predicted by differential thermal

expansion. These strains increase the stability window of the

ferroelectric phase by >50 �C, similar to that observed for

epitaxial systems and agree with phenomenological predica-

tions. Ultimately, this fluxing approach offers a chemical

pathway to improved crystal quality and microstructure, and

a comparable means of studying strain effects in a much

broader range of materials since epitaxial registry is not

required.

We would like to thank Ali Moballegh of North

Carolina State University for assistance with the TEM sam-

ple preparation; Professor Brady Gibbons of Oregon State

University for polarization measurements; Lauren Garten

and Professor Susan Trolier-McKinstry of Penn State

University for capacitance-temperature measurements.

Sandia National Laboratories is a multiprogram laboratory

managed and operated by Sandia Corporation, a wholly

owned subsidiary of Lockheed Martin Corporation, for the

U.S. Department of Energy’s National Nuclear Security

Administration under Contract No. DE-AC04-94AL85000.

Funding was provided by Defense MicroElectronics Activity

(H94003-10-0-1002).

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