Hetero-epitaxial BiFeO3/SrTiO3 nanolaminates with higher piezoresponse performanceover stoichiometric BiFeO3 filmsGeunhee Lee, Erika M. A. Fuentes-Fernandez, Guoda Lian, Ram S. Katiyar, and Orlando Auciello Citation: Applied Physics Letters 106, 022905 (2015); doi: 10.1063/1.4905871 View online: http://dx.doi.org/10.1063/1.4905871 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Orientation dependence on piezoelectric properties of Bi0.5Na0.5TiO3-BaTiO3-SrTiO3 epitaxial thin films Appl. Phys. Lett. 104, 172903 (2014); 10.1063/1.4874805 Strain-driven control of piezoelectricity in (Na,Bi)TiO3-BaTiO3 epitaxial thin films Appl. Phys. Lett. 102, 192901 (2013); 10.1063/1.4804135 Magnetic and structural properties of BiFeO3 thin films grown epitaxially on SrTiO3/Si substrates J. Appl. Phys. 113, 17D919 (2013); 10.1063/1.4796150 Piezoelectric response of nanoscale PbTiO 3 in composite PbTiO 3 − CoFe 2 O 4 epitaxial films Appl. Phys. Lett. 93, 074101 (2008); 10.1063/1.2969038 Epitaxial BiFeO 3 thin films on Si Appl. Phys. Lett. 85, 2574 (2004); 10.1063/1.1799234

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Hetero-epitaxial BiFeO3/SrTiO3 nanolaminates with higher piezoresponseperformance over stoichiometric BiFeO3 films

Geunhee Lee,1,a) Erika M. A. Fuentes-Fernandez,1 Guoda Lian,1 Ram S. Katiyar,2

and Orlando Auciello1,3,a)

1Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080,USA2Institute for Functional Nanomaterials, University of Puerto Rico, San Juan 00931-3343, Puerto Rico3Department of Bioengineering, University of Texas at Dallas, Richardson, Texas 75080, USA

(Received 24 November 2014; accepted 31 December 2014; published online 14 January 2015)

In this research, BiFeO3 (BFO) films are integrated into BFO/SrTiO3 (STO)/BFO nanolaminates

(BSB-NLs) featuring nanometer-scale thickness of BFO and STO layers. By introducing the STO

layer in between two BFO layers, the leakage current density is reduced by two orders of magnitude

with respect to relatively high leakage currents of current single BFO layers, i.e., from 10�5 A/cm2

to 10�7 A/cm2. The BSB-NL also shows very high piezoelectric response, which is �5 times higher

than that of the pure BFO with the same thickness. The highly strained state of the BFO layers con-

currently with the chemical/crystallographic state of the interfaces between the BFO and STO layers

contribute to the very high values of piezoresponse and very low leakage current observed in the

BSB-NLs. VC 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4905871]

Lead-free ferroelectric/piezoelectric BiFeO3 (BFO)

films have attracted much attention due to their properties

in both epitaxial and polycrystalline thin films for potential

applications to multiferroic film-based devices, e.g., non-

volatile ferroelectric memories,1 piezoelectric actuated

MEMS-based devices,1 and photovoltaic devices.2 The re-

manent polarization Pr and out-of-plane (d33) piezoelectric

coefficient of BFO are comparable to or greater than those of

tetragonal Ti-rich PZT, depending on the composition and/or

quality of the crystallinity and orientation of the film.3

However, stoichiometric BFO films generally exhibit large

coercive fields and a large leakage current,4 limiting the

applicability of BFO films in piezoactuated MEMS and

memory devices.5

Typically, the leaky nature of BFO films has been attrib-

uted to the presence of oxygen vacancies formed during film

growth.6 The oxygen vacancies cause some Fe3þ ions to

become Fe2þ, which are considered to be responsible for the

high leakage of BFO, increasing the amount of charged car-

riers, and resulting in degradation of the ferroelectric proper-

ties of BFO.6 In spite of extensive work, the leakage current

mechanism of BFO films is still unclear. For example, Pabst

et al.7 showed that the bulk limited Poole-Frenkel electron

emission is the dominant leakage mechanism for BFO films

integrated into Pt/SrRuO3 (SRO)/BFO/SRO/DyScO3 (DSO)

structures with symmetric BFO/SRO electrode interfaces.

However, the leakage mechanism is less clear for Pt/BFO/

SRO structures with asymmetric BFO/electrode interfaces.

On the other hand, Yang et al.8 proposed that the space

charge limitation (SCL) is the dominant leakage mechanism

for Pt/BFO/SRO asymmetric structure.

It is critical to reduce the leakage current and increase

the ferroelectric/piezoelectric properties of BFO films, to

integrate BFO into functional microelectronic, nanoelec-

tronic, MEMS, and photovoltaic devices. Attempts have

been made to reduce the leakage current and to improve

the ferroelectric/piezoelectric properties of BFO films via

doping by (1) substitution of Fe3þ with Ti4þ, Cr3þ, or simi-

lar ions, or Bi3þ by La3þ, Nd3þ, or similar ions,9 and (2)

solid solution formation with PbTiO3, BaTiO3, and other

perovskite oxides.10 On the other hand, putting an insulat-

ing layer at the top electrode/BFO/bottom electrodes inter-

faces to block charged carriers’ path was suggested as an

alternative approach to reduce leakage current and to

increase ferroelectricity and piezoelectricity of BFO

films.11,12

Following a complete scientific pathway, the research

discussed in this report reveals that the insertion of an insu-

lating crystalline SrTiO3 (STO) layer epitaxially grown in

the middle of two BFO layers with nanometer-scale thick-

ness, results in a BFO/STO/BFO nanolaminate (BSB-NLs)

structure that makes a major impact in the electrical and me-

chanical performance of BFO films-based devices in two

fundamental aspects: (1) order of magnitude reduction of

leakage current density, and (2) �5 times enhancement of

the piezoelectric response of the BSB-NLs, both properties

with respect to stoichiometric single BFO films. The

improvement on the two critical properties mentioned above

are attributed to negative and positive charge localization at

the top BFO/STO and the STO/bottom BFO interface,

respectively, which induces a high electric field opposing

carrier mobility through the STO layer, resulting in order of

magnitude reduction of the leakage current. In addition, the

BSB-NLs structure induces a highly strained epitaxial BFO

layer, which produces a substantial increase in the BFO

piezoresponse.

The synthesis of the BSB-NLs on a crystalline substrate

started by first growing electrically conductive bottom elec-

trode SRO layers epitaxially on single crystalline STO (100)

a)Authors to whom correspondence should be addressed. Electronic

addresses: geunhee.lee@utdallas.edu and orlando.auciello@utdallas.edu

0003-6951/2015/106(2)/022905/5/$30.00 VC 2015 AIP Publishing LLC106, 022905-1

APPLIED PHYSICS LETTERS 106, 022905 (2015)

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substrates, using a pulsed laser deposition (PLD) method,

which provides the synthesis of stoichiometric oxide films

with reproducible stoichiometry from oxide targets.13,14 All

stoichiometric oxide layers were grown at a substrate tem-

perature of 670 �C and oxygen pressure of 100 mTorr, in an

integrated cycle, without exposing any interface to atmos-

pheric pressure, by having all targets positioned in a rotata-

ble holder and exposing them sequentially to a KrF excimer

laser beam (248 nm wavelength and �2 J/cm2 energy den-

sity, 1 Hz pulse frequency). The SRO films, first grown epi-

taxially on STO substrates, provided a bottom electrode

template layer with lattice matching (3.93 A lattice) to the

BFO film (3.96 A lattice). Subsequently, the BFO films were

grown hetero-epitaxially on the SRO bottom electrode

layers, followed by the growth of the intermediate STO

layers and the top BFO layers to produce the BSB-NLs on

SRO bottom electrode layers. The whole BSB-NLs struc-

tures were subsequently cooled down slowly (10 �C/min)

from 670 �C to room temperature at 1 Torr of O2 for �1 h.

Patterned top Pt electrode (260 lm in diameter, 50 nm thick)

layers were deposited on the BFO top layers by electron

beam evaporation.

The structure of the individual layers and the BSB-NLs

was investigated by X-ray diffraction (XRD, Cu Ka radia-

tion, 1.542 A). The thickness of the films was measured via

cross-section scanning electron microscopy (SEM) and scan-

ning transmission electron microscopy (STEM). The surface

morphologies and piezoelectric domains of the films were

investigated using atomic force microscope (AFM) and pie-

zoresponse force microscopy (PFM), implemented in a MFP

3D scanning probe microscope (Asylum Research), respec-

tively. Leakage properties of the films were measured using

a Keithley 4200 system at room temperature. For all electri-

cal characterization, probing was done perpendicular to the

film plane with SRO as bottom electrode and Pt top electrode

with �260 lm in diameter (area of 1.4� 10�5 cm2) and

50 nm in thickness.

Figure 1(a) shows a schematic of the Pt (50 nm)/BFO

(40 nm)/STO (0, 12, 24, and 48 nm)/BFO (40 nm)/SRO

(55 nm)-based capacitors produced on STO (100) substrates

by controlling the number of laser beam pulses. Figure 1(b)

shows a cross-section SEM image of the BFO (40 nm)/SRO

(55 nm) initial layers. Figure 1(c) shows a low magnification

TEM image of BSB-NLs on the SRO bottom electrode layer,

with sharp interfaces, on an STO substrate. Figure 1(d)

shows a high-resolution STEM annular dark field (ADF)

image of the SRO bottom electrode layer on the STO sub-

strate; revealing an atomic scale, defect free, sharp crystal-

line interface (several areas were investigated to get

statistical data). However, subsequent layers and interfaces

(BFO/SRO Fig. 1(e), STO/BFO Fig. 1(f), and BFO/STO Fig.

1(g)) show some defects (dislocations), especially in the

STO layer. Also, the STO/BFO interface is not as atomically

sharp, as shown in Figs. 1(f) and 1(g).

Figures 2(a) and 2(b) show the XRD pattern for primary

peaks of BFO (001) and STO (100) layers of each epitaxially

grown BSB-NL and the pure BFO on SRO/STO substrate.

The X-ray diffraction (h–2h scan) of the BFO layer indicates

that the film is highly oriented along the c-axis epitaxially

related with the (100) STO substrate without any impurity

phases. The out-of-plane lattice parameter for the pure BFO

layer, calculated using Bragg’s law, was 4.06 A

(2h¼ 21.87�, 2.6%), which indicates an strained lattice,

because the equilibrium lattice parameter of the BFO bulk is

3.96 A in the pseudo-cubic structure without any strain.15

Table I shows a summary of the lattice parameter for the c-

axis of a pure BFO film and the set of BSB-NLs studied. The

crystalline quality of the BFO layer is within the range

observed in other reported work, as confirmed by the x scan

(Fig. 2(c)). Note that as the thickness of the STO layer

inserted between the BFO layers increases, the strain of the

BFO layer decreases, i.e., the diffraction peak of BFO from

the film is close to the bulk value (Fig. 2(a)). Since the STO

and SRO crystals have lattice parameters of 3.90 A and

FIG. 1. (a) Schematic of BSB-NLs

(Pt (50 nm)/BFO (40 nm)/STO (0, 12,

24, and 48 nm)/BFO (40 nm)/SRO

(55 nm)/STO (001 single crystalline

substrate)-based capacitors used for

electrical measurements); (b) SEM

image of BFO (40 nm)/SRO (55 nm)

initial layers on the (100) STO sub-

strate; (c) TEM image of BSB-NL

with 48 nm thick STO layer. (d)–(g)

HRTEM images at the interfaces of

SRO/STO, BFO/SRO, STO/BFO, and

BFO/STO, respectively.

022905-2 Lee et al. Appl. Phys. Lett. 106, 022905 (2015)

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3.93 A, respectively,16 the BFO film grown on the SRO/STO

substrate is compressively stressed horizontally and elon-

gated in the c-axis direction. So the pure BFO layer shows

the larger c-lattice spacing and strain compared with the

BSB-NLs. The diffraction analysis indicates that insertion of

the STO layer in between the BFO layers results in relaxa-

tion of stress in the layer, most probably due to a combina-

tion of lattice matching and crystallographic defects, such as

dislocations observed in the STO layer (Figs. 1(c) and 1(g)),

which contribute to release the strain in the BFO layer. This

hypothesis is supported by the observation that the relaxation

increases as the thickness of the STO layer increases.

The effect of the STO layer, inserted between the BFO

layers, on the electrical leakage of the whole BSB-NLs is

shown in Fig. 3. Figure 3(a) shows the J-V characteristics of

the pure BFO and the BSB-NLs, which reveals that the leak-

age current density is substantially reduced as the thickness

of the STO layer increases. When the thickness of the STO

layer is about 48 nm, the leakage current density is around

5� 10�7 A/cm2 at 1 V, which is two orders of magnitude

lower than the leakage of the pure BFO (�6� 10�5 A/cm2).

The explanation is that the highly insulating STO layer

blocks the current path for carriers in the pure leaky BFO

layer, inducing the substantial decrease in the leakage cur-

rent density of the BSB-NLs. The experimental raw J-Vpoints were re-plotted as the log10J vs log10E, based on the

SCL mechanism, as in Fig. 3(b), in which the current density

is expressed by17

JSCLC ¼9le0e

8

E2

d; (1)

where l is carrier mobility, E is the electric field applied in

the thin film, d is the thickness of thin film, e0 is the permit-

tivity of free space, and e is the optical dielectric permittivity

of the thin film. Therefore, if the SCL leakage mechanism is

dominant in the thin films, the log10J – log10E plot should

show linear relationships with a slope determined by the

exponent of E. Figure 3(b) shows a distinctive linear behav-

ior of the leakage current dependent on the applied voltage

range. The log10J – log10E data for the pure BFO film shows

clearly that SCL is the dominant leakage mechanism, in

agreement with prior reports6,7 as shown by the linear

increase with slope of �2 at low voltage range. However,

the leakage for the BSB-NLs exhibits a deviation from the

SCL mechanism to Ohmic conduction showing reduced

leakage current as the thickness of the STO layer increases,

which is confirmed by the slope of the line changes to �1.

Above �1.2 V (�150 kV/cm), the leakage current slop

changes abruptly as a function of the STO layer thickness.

The slopes of the J vs. E plots for the BSB-NLs indicate that

the insulating STO layer, most probably due to localized

space charges at the BFO/STO interface, largely controls

leakage current.

In relation to test the mechanical performance of the

BSB-NLs, the piezoresponse of pure BFO films and BSB-

NLs was measured using the PFM technique, which was ini-

tially systematically applied by Gruverman and Auciello for

measuring the piezoresponse of piezoelectric thin films18 and

more recently used by Nath et al. to measure the piezores-

ponse of BFO films without artifacts arising from the posi-

tioning of the laser beam on the AFM cantilever used to

measure the piezoresponse of films.19 Figure 4(a) shows a

comparison of the piezoresponse of BSB-NLs with three dif-

ferent STO layer thicknesses. The BFO/12 nm thick STO/

BFO NL exhibited the highest piezoresponse (�5� higher

than the pure BFO). However, as the thickness of the STO

layer increases the piezoresponse decrease, but still is always

higher than for the pure BFO film. The piezoresponse data

shown in Fig. 4(a), in view of the leakage current data of

Fig. 3, indicates that the leakage current affects the piezores-

ponse. In this respect, the analysis of the combined leakage

FIG. 2. (a) X-ray diffraction pattern of

BSB-NLs, showing the less out-of-

plane strained state for the BFO layer

grown on the thickest STO layer

(48 nm, green), and higher strained

BFO layer when grown on the thinner

STO layers; (b) wide range of XRD

pattern for BSB-NLs on the SRO/STO

(001) substrate, confirming hetero-

epitaxy of BSB-NLs on SRO/STO

(001) substrate; (c) rocking curve of the

primary BFO peak showing 0.2� full

width at half maximum (FWHM) of xangle, correlated with epitaxial quality.

TABLE I. Lattice parameters for the c-axis of pure BFO and BFO/STO/

BFO NLs.

Materials 2h (deg) d (A) e (%)

Pure BFO 21.87 4.064 2.6

BFO/12 nm STO/BFO 21.96 4.048 2.2

BFO/24 nm STO/BFO 22.08 4.026 1.7

BFO/48 nm STO/BFO 22.23 3.999 1.0

022905-3 Lee et al. Appl. Phys. Lett. 106, 022905 (2015)

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and piezoresponse data indicates that the insulating STO

layer inserted between the BFO layers contributes to the

substantial reduction of the leakage current, while simultane-

ously contributing to a substantial increase of piezoresponse

in the BSB-NLs. Understanding the fundamental physics

responsible for the performance of the BSB-NLs requires

a critical combined analysis of the leakage current and pie-

zoresponse data presented in Figs. 3 and 4, as described

below:

(1) Effect of the thickness of the STO layer: Increasing the

thickness of the STO layer results in reduction of the

leakage current, but with simultaneous reduction in the

piezoresponse. A potential explanation of this effect

relies on the effect of the strained state of the BFO layer

on the piezoresponse. It is well known that the highly

strained BFO film has higher polarization with higher

polarizability than the less strained film,20 with the

higher polarizability closely related to the higher

piezoresponse.21 As shown in Fig. 2(a), the BSB-NLs

with the 12 nm thick STO layer showed the highest

strained state compared with the other BSB-NLs (that is

with 24 and 48 nm thick STO layer), except for the pure

BFO film on the SRO electrode layer. The BSB-NLs

with the thinnest STO layer exhibit the highest piezores-

ponse among the BSB-NLs. Figures 4(b) and 4(c) show

the out-of plane piezoresponse domain structure of the

pure BFO film and the BFO/24 nm STO/BFO NL,

respectively. It also shows higher piezoresponse in the

BSB-NLs (1.34 nm) than the pure BFO (740 pm) as indi-

cated in the legends. Considering the sensitivity of the

cantilever used for the PFM measurement (�22.2 nm/V),

the slop of the plots in Fig. 4(a) can be changed to adapt

it to the piezoelectric coefficient as in Table II. Wang

et al.3 reported the piezoelectric coefficient of the pure

BFO (001) as �70 pm/V, while Fujino et al.22 showed a

value of around 110 pm/V for Sm-doped BFO.

Compared with the piezoelectric coefficient values of

FIG. 3. (a) Leakage current vs. applied

voltage for the BFO film and the BSB-

NLs, confirming the hypothesis that

the insulating STO layer contributes to

reduce the leakage current, semi-log

plot. (b) Re-plot of leakage current

curves based on the SCL (J / Ea,

log(J) vs. log(E) plot) conduction

mechanism, showing a linear behavior.

FIG. 4. (a) Piezoresponse vs drive voltage curves from the PFM measured piezoelectric response of single BFO film and BSB-NLs, showing the substantially

higher piezoelectric response of the nanolaminates. (b) and (c) Out-of-plane domain structure of the pure BFO and the BSB-NLs, respectively.

022905-4 Lee et al. Appl. Phys. Lett. 106, 022905 (2015)

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single BFO films reported by other authors,3,22 the single

BFO films produced here exhibit similar values of the

piezoelectric coefficient. By comparison, the BSB-NL

with �12 nm thick STO layer shows a much higher value

for the piezoelectric coefficient (�331 pm/V), which is

higher than any other reported value for single BFO

films,22 and comparable with the highest value for PZT

films (�500 pm/V) reported by Du et al.23 It is relevant

to point out that the piezoelectric coefficient reported by

Du et al.23 for PZT appears to be much higher than any

other values reported in the literature, so it may be

appropriate that the community look at this result crit-

ically and following the scientific process consider inves-

tigating if those results are reproducible or not in order

to clarify errors if they were made.

(2) Effect of domain size: The domain size in the BSB-NLs

is smaller than in the pure BFO films, in which the

domain size is normally proportional to the thickness of

the ferroelectric layer.24 The smaller domain size

observed in the BSB-NLs structure, can be correlated

with the fact that the BFO layer in the BSB-NLs has half

of the thickness (�40 nm), as shown in Fig. 1(a), com-

pared to the pure BFO (80 nm) film produced in the

research described in this paper.

In conclusion, the work reported here demonstrated that

BSB-NLs exhibit a simultaneous reduction in leakage cur-

rent by two orders of magnitude and 5� higher piezores-

ponse compared to pure BFO films. The results presented

here have great implication for potential applications of

BFO-based piezoactuated devices like MEMS or NEMS,

especially for biomedical applications (e.g., biosensors and

drug delivery systems) based on the lead-free biocompatibil-

ity of BFO piezoelectric components. In addition, applica-

tion of the BSB-NLs for non-volatile ferroelectric memories

and multiferroic BFO film-based devices, such as photovol-

taic cells, under investigation, may also warrant

investigation.

G.L. and R.S.K. acknowledge financial support in the

form of an NSF grant (NSF-RII-EPS-1002410). O.A.

acknowledges Endowed Chair support from UTD. All

primary data are archived in the Department of Materials

Science and Engineering of the University of Texas at

Dallas.

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TABLE II. Measured piezoelectric coefficient of pure BFO film and the

BFO/STO/BFO-NLs.

Materials

Approximated slope

from Figure 4(a) (mV/V)

Calculated piezoelectric

coefficient (pm/V)

Pure BFO �3.1 �68.9

BFO/12 nm STO/BFO �14.9 �330.8

BFO/24 nm STO/BFO �10.7 �237.5

BFO/48 nm STO/BFO �5.1 �113.2

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