Piezoelectric response of epitaxial Pb(Zr<sub>0.2</sub>Ti<sub>0.8</sub>)O<sub>3</sub> films measured by scanning tunneling microscopy

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Piezoelectric response of epitaxial Pb(Zr

0.2

Ti

0.8

)O

3

films measured by scanning tunneling microscopy

KUFFER, Olivier, et al.

Abstract

We report on scanning tunneling microscopymeasurements of the piezoelectric response in ferroelectricheterostructuresgrown by off-axis rf magnetronsputtering. The samples are composed of a single-crystalline ferroelectricfilm of Pb(Zr0.2Ti0.8)O3deposited on a conducting substrate and covered with an ultrathin metallic film of gold. The high quality of the c-axis oriented ferroelectric layer is evidenced by sharp polarization hysteresis loops. By applying a voltage to the bilayer and recording the inverse piezoelectric effect with the scanning tunneling microscope, we demonstrate the ability to measure the phase response as well as the ferroelectric switching. We obtained strain-field plots with a butterfly loop shape, and a quantitative measurement of the longitudinal piezoelectric coefficient (d33).

KUFFER, Olivier, et al . Piezoelectric response of epitaxial Pb(Zr

0.2

Ti

0.8

)O

3

films measured by scanning tunneling microscopy. Applied Physics Letters , 2000, vol. 77, no. 11, p. 1701

DOI : 10.1063/1.1309017

Available at:

http://archive-ouverte.unige.ch/unige:35911

Disclaimer: layout of this document may differ from the published version.

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Piezoelectric response of epitaxial Pb(Zr 0.2 Ti 0.8 )O 3 films measured by scanning tunneling microscopy

O. Kuffer, I. Maggio-Aprile, J.-M. Triscone, O/. Fischer, and Ch. Renner

Citation: Applied Physics Letters 77, 1701 (2000); doi: 10.1063/1.1309017 View online: http://dx.doi.org/10.1063/1.1309017

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/77/11?ver=pdfcov Published by the AIP Publishing

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Piezoelectric response of epitaxial PbZr

0.2

Ti

0.8

O

3

films measured by scanning tunneling microscopy

O. Kuffer,a)I. Maggio-Aprile, J.-M. Triscone, and Ø. Fischer

DPMC, Universite´ de Gene`ve, Quai Ernest-Ansermet 24, 1211 Gene`ve 4, Switzerland Ch. Renner

NEC Research Institute, 4 Independence Way, Princeton, New Jersey 08540

Received 12 May 2000; accepted for publication 13 July 2000

We report on scanning tunneling microscopy measurements of the piezoelectric response in ferroelectric heterostructures grown by off-axis rf magnetron sputtering. The samples are composed of a single-crystalline ferroelectric film of Pb

Zr0.2Ti0.8

O3deposited on a conducting substrate and covered with an ultrathin metallic film of gold. The high quality of the c-axis oriented ferroelectric layer is evidenced by sharp polarization hysteresis loops. By applying a voltage to the bilayer and recording the inverse piezoelectric effect with the scanning tunneling microscope, we demonstrate the ability to measure the phase response as well as the ferroelectric switching. We obtained strain-field plots with a butterfly loop shape, and a quantitative measurement of the longitudinal piezoelectric coefficient (d33). © 2000 American Institute of Physics.

S0003-6951

00

02037-4

In the past several years, ferroelectric thin films have been broadly investigated for several applications such as nonvolatile memory elements,1pyroelectric detectors,2elec- tromechanical actuators,3 and field-effect devices.4 Local probe techniques, such as atomic-force microscopy

AFM

, have been widely and successfully used to investigate and help understand their physical properties at the nanometer scale.5–9 In particular, AFM is a powerful tool to study the local inverse piezoelectric response and can be employed to image and study ferroelectric domains structures.5–9 These studies are directly related to the longitudinal piezoelectric coefficient (d33), whose small amplitude requires very high- resolution measurements. Most AFM piezoelectric measure- ments are based on the phase detection of the response rather than its magnitude,5–9and a quantitative measurement of the piezoelectric response coefficient is limited by the difficulty in estimating accurately the electric field between the tip and the sample.10A possible way to overcome these problems is to use a top flat electrode in order to have a well-defined field.11,12

Nonlocal studies, such as double-beam laser interferom- etry, pneumatic pressure rig, or the wafer flexure technique, have been performed by other groups,13–15with the advan- tage of taking into account the strain due to the substrate, which complicates the determination of the piezoelectric co- efficients, since it can add a component due to the simulta- neous bending of the substrate.

Using a scanning tunneling microscope

STM

for local and quantitative piezoelectric response measurements ap- pears to be an interesting alternative approach, because of the extremely high vertical resolution of the STM. Previous studies have reported on such work, but all of them were done on nonperovskite materials.16–18 Applying this tech- nique to high-quality bilayer heterostructures, composed of a perovskite ferroelectric film covered with an ultrathin elec-

trode, has the advantage of giving clear and reproducible piezoelectric response measurements.

In this letter, we present piezoelectric response measure- ments made by STM on lead zirconate titanate thin films.

Most of the quantitative local probe measurements of the piezoelectric coefficients of these compounds, using AFM or a tunneling acoustic microscope,10,19 were performed near the morphotropic phase boundary, because of their high pi- ezoelectric coefficient values.20

A key parameter to reduce strain effects is to choose a composition of PZT having a low lattice mismatch with the substrate. Epitaxial films of Pb

Zr0.2Ti0.8

O3

PZT

were de- posited by off-axis radio-frequency magnetron sputtering on conducting

001

Nb-doped SrTiO3

NSTO

single-crystal substrates. The PZT growth is achieved in an argon and oxy- gen discharge at a pressure of 200 mTorr

75% Ar/25% O2

, with a substrate temperature of

500 °C.21 X-ray measure- ments performed on 300-Å-thick samples displayed finite- size effects on the PZT 001 and 002 reflections, which dem- onstrate the high-crystalline quality8 and allow us to precisely calibrate the deposition rate

250 Å/h

.

For the measurements discussed here, we typically grew thicknesses of PZT of the order of 3000–4000 Å in order to reduce leakage current effects. A 60-Å-thick layer of gold is deposited in situ at room temperature. Figure 1

a

shows the x-ray diffraction analysis of one of the bilayers with a␪–2␪ scan revealing epitaxial,22single-crystalline growth of c-axis oriented PZT with a c-axis lattice parameter of 4.16 Å. The in-plane lattice parameter value is 3.95 Å, giving rise to a mismatch with NSTO of about 1%. The full width at half maximum of the rocking curve taken around the PZT 001 reflection has a value of 0.087°, indicating the high degree of crystallinity. Low-angle measurements allow us to pre- cisely measure the thickness of the gold electrode

60 Å

and demonstrate the high quality of the interface between the ultrathin metallic film and the PZT surface. We then per- formed STM topography images on the patterned gold elec-

aElectronic mail: olivier.kuffer@physics.unige.ch

APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 11 11 SEPTEMBER 2000

1701

0003-6951/2000/77(11)/1701/3/$17.00 © 2000 American Institute of Physics

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trode

typical size 0.04– 0.09 mm2

revealing smooth sur- faces with a roughness lower than 6 Å rms

共⬍

15 Å peak to valley

over 1␮m⫻1␮m areas, as seen in Fig. 1

b

. To further characterize the bilayer, polarization hysteresis loops were taken through standard Sawyer–Tower measure- ments,23as shown in Fig. 1

c

. We obtained a remanent po- larization of 35␮C/cm2and a coercive voltage of about 4 V corresponding to a coercive field of 100 kV/cm for a 4000- Å-thick PZT film.

The experimental setup for measuring the local piezo- electric activity using a STM is shown in Fig. 2. We use a

homemade STM head in ambient conditions24 with electro- chemically etched Ir tips. The tip is brought in constant cur- rent mode (It0.5 nA) and with a bias voltage (Vt⫽0.5 V) on top of the ultrathin gold electrode. The film is prepoled before any piezoelectric response measurement. A low- frequency sawtooth voltage

10 Hz

is applied to the sub- strate

NSTO

with the top electrode

Au

as the ground reference. The frequency is chosen slow enough to ensure constant current operation mode of the STM, since in this configuration, the tip will follow accurately the motion of the PZT layer. Recording this vertical movement directly mea- sures the induced strain

z response

, that is the inverse pi- ezoelectric response of the bilayer. A topographic image is taken to measure the slope of the sample, and thus, to verify that the vertical movement of the tip is perpendicular to the surface. The misalignment is always found to be less than 1°.

To demonstrate the technique, we show in Fig. 3 a record of the piezoelectric response of the bilayer, as a func- tion of time, when the STM tip is regulating at a fixed loca- tion. If the driving voltage is smaller than the coercive volt- age of the PZT film, the strain response is linear, and in or out of phase with respect to the excitation, depending on the polarization direction. In this case, part I of Fig. 3 shows the sawtooth in-phase response as a function of time. At time t1, the applied voltage is increased above the coercive voltage of the PZT film, and the ferroelectric switching is observed by a doubling of the frequency response

part II

. Finally, at time t2

part III

, the applied voltage is again reduced below the coercive voltage, and the response is out of phase, as seen by the 180° jump of the response (

⌬⌽

) with respect to the signal obtained in part I.

When the PZT film is poled, we found the piezoelectric response to be homogeneous at various positions of the tip on the sample. We emphasize that the Au/PZT interface and the thickness of the gold electrode play an important role in the reproducibility. The ultrathin gold electrode allows one to minimize any artifact such as intergrain or elastic defor- mation effects in the measurement. Furthermore, the surface of the electrode (0.04– 0.09 mm2) is large enough to neglect clamping effects present at the edges.25

We next investigated the longitudinal piezoelectric coef-

FIG. 1.aCu K x-ray–2diffractogram of 60 Å Au/ 4000 Å PZT/

NSTO showing the first two sets of reflections. The size effect at low angle is due to the 60-Å-thick gold electrode.bSTM topography.cSawyer–

Tower measurement of the polarization hysteresis loop of the bilayer. Pr

35C/cm2and Ec100 kV/cm.

FIG. 2. Setup for the piezoelectric measurement. A sawtooth voltage is applied to the ferroelectric layer, while the STM tip, in constant current mode on the gold contactgrounded, can directly sense the inverse piezo- electric effect at a fixed position.

FIG. 3. Three-dimensional STM image of the piezoelectric responseverti- cal scaleof the bilayer, measured at one point on the sample, as a function of timehorizontal scales. The image was taken by stacking consecutive segments of constant duration共time axis txnext to one another共time axis ty. Parts I and III of the image show the in- and out-of-phase piezoelectric response for a driving voltage (3 V) lower than the coercive voltage4 V. Part II shows the switching response with a doubling of the frequency for a driving voltage of10 V.

1702 Appl. Phys. Lett., Vol. 77, No. 11, 11 September 2000 Kufferet al.

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ficient (d33) of the bilayer. When the driving voltage is larger than the coercive voltage, we obtain clear strain-field plots in Fig. 4

a

, by representing the switching response

see part II in Fig. 3

as a function of the applied voltage. This butterfly loop shows both the inverse piezoelectric effect and the polarization switching of the ferroelectric PZT films. Its numerical derivative

Fig. 4

b

兲兴

gives the d33hysteresis loop.

This kind of measurement yields a d33 value of 50

⫾2 pm/V.26 This value is in good agreement with a recent local probe measurement,27 and can also be compared with double-beam interferometry measurements.28 A striking re- sult is that the coercive voltage values and the shape of the local d33 hysteresis loop

Fig. 4

b

兲兴

, including the slight asymmetry, perfectly match the macroscopic polarization hysteresis loop measurement obtained in Fig. 1

c

, although the nature of the two measurements is different.

In conclusion, we have shown that a STM can be used to probe locally the piezoelectric response of a ferroelectric film covered with an ultrathin electrode. The sample quality is a key point in this technique, since the interfaces and the sur- face corrugations have to be optimized in order to perform tunneling in good and reproducible conditions. For epitaxial single-crystalline thin films, the high resolution of the instru- ment allows one to obtain clear strain-field plots and gives a direct and quick access to the measurement of the piezoelec- tric d33 coefficient. This technique opens up promising po- tentials for local investigations of ferroelectric memory de- vices.

The authors would like to thank C. H. Ahn and T. Tybell for fruitful discussions, and J.-G. Bosch and A. Stettler for

valuable technical help. This work was supported in part by the Swiss National Science Foundation.

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Appl. Phys. Lett., Vol. 77, No. 11, 11 September 2000 Kufferet al.

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