Download Polarization reversal anti-parallel to the applied electric field

APPLIED PHYSICS LETTERS
VOLUME 84, NUMBER 2
12 JANUARY 2004
Polarization reversal anti-parallel to the applied electric field observed
using a scanning nonlinear dielectric microscopy
Takeshi Moritaa) and Yasuo Cho
Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai,
Miyagi 980-8577, Japan
共Received 19 May 2003; accepted 8 November 2003兲
Ultrahigh-density storage devices consisting of poling reversed nanodots are examined using a
scanning nonlinear dielectric microscope 共SNDM兲. By using the SNDM, real-time observation of
the poling direction was attempted and an unexpected phenomenon was discovered. For lithium
tantalite films thicker than 350 nm, poling directions were aligned anti-parallel to the poling electric
field. The critical thickness is thought to be dependent on the material properties, the probe radius,
the applied voltage and the pulse duration. This anti-parallel poling phenomenon disagrees with
previous poling reversal mechanisms from the conventional plate capacitor model. At present, the
reason for and details of anti-parallel poling reversal are unclear, but may be related to the
concentrated electric field near the cantilever. © 2004 American Institute of Physics.
关DOI: 10.1063/1.1637938兴
As is well known, the spontaneous polarization of ferroelectric material is an intrinsic property that is used in nonvolatile memory devices. The poling direction can be reversed in a nanometer size area using the conductive
cantilever of a scanning probe microscope. As a method for
detecting nanodot patterns, the scanning nonlinear dielectric
microscope 共SNDM兲1,2 is superior to the piezoresponse
microscope3 in terms of resolution.4
By using SNDM, Cho et al. have demonstrated data
storage in inverted domain dots in ferroelectric material at a
data density of 1.5 Tbit/in2 . 5,6 The advantage of ferroelectric
data storage over ferromagnetic storage is the thin domain
wall that has been found to be only a few lattice constants
wide by SNDM.7
To realize a much higher speed, a higher reliability nanodot writing and reading system, fundamental investigations
into nanodot formation mechanisms are indispensable. Indeed, a number of unexplained phenomena have been observed in polarization reversal, such as a back-switching8 and
ring-shaped poling reversal dots that are sometimes observed
during nanodot patterning. Hence, a conductive cantilever
was fixed on the ferroelectric material and poling reversal
phenomenon under the cantilever was observed.
Figure 1 shows a SNDM system for real-time poling
reversal observation that is an atomic force microscope connected to a self-oscillator 共around 1.2 GHz兲, a FM demodulator and a lock-in amplifier. Ferroelectric materials exhibit
strong nonlinear permittivity that is a function of applied
electric field. For a sinusoidal external electric field 共in this
study, 8 kHz兲 from the bottom electrode to the cantilever tip,
the ferroelectric capacitor under the cantilever is slightly
changed due to the nonlinear permittivity being proportional
to the electric field. By monitoring the excited frequency
shift of the self-oscillator, the direction of the poling direction can be measured because the sign of the corresponding
nonlinear permittivity depends on the poling direction. A
a兲
Electronic mail: [email protected]
large electrical voltage pulse 共a duration of 50 ms兲 was applied to the ferroelectric thin film to affect the poling direction while the poling direction was detected using a minute
electric field 共8 kHz 3 Vop ) by SNDM measurement.
Single crystal lithium tantalate film was used as a substrate with an applied voltage of ⫹10 V. Positive voltage
represents an electric field from the bottom electrode to the
cantilever. The film thickness was 63 nm and initial polarizations were aligned from the top to the bottom surface. This
poling situation is observed as a ⫹90 degree signal from the
lock-in amplifier, and the other is ⫺90 degrees. As shown in
Fig. 2共a兲, the phase changed from ⫹90 degrees to ⫺90 degrees at the rising edge of the applied voltage. This indicates
that the poling direction reversed parallel to the applied electric field, as Cho et al. demonstrated.5 For the negative voltage 共⫺10 V兲, the phase difference did not change indicating
that the polarization direction was not affected by negative
voltage. These results match the plate capacitor model, and
agree with the conventional P-E hysteresis curve.
Similar experiments were carried out using a thicker film
共1.3 ␮m兲 whose initial state was the same as the previous
one. When a ⫹100 V pulse was applied to the bottom electrode, polarization reversed in the rising edge of the voltage
pulse and then backswitched in the deceasing edge as shown
FIG. 1. Real-time measurement set up for observing poling reversal using a
scanning nonlinear dielectric microscope.
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© 2004 American Institute of Physics
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258
Appl. Phys. Lett., Vol. 84, No. 2, 12 January 2004
FIG. 2. Poling reversal observation in 63 nm thick lithium tantalate, showing an electric field 共a兲 anti-parallel and 共b兲 parallel to the initial
poling direction.
in Fig. 3共a兲. The signal from SNDM only describes the poling direction of a shallow surface region. Thus, the external
electric field is thought to have not surpassed the coersive
electric field to reverse the polarization and the reversed poling did not penetrate through the entire film.
These results mentioned above are explained with a normal poling reversal mechanism, however, ⫺100 V voltage
pulses resulted in unexpected polarization reversal, as shown
in Fig. 3共b兲. This shows that for an electric field from the
cantilever to the bottom electrode, the poling direction
aligned to become anti-parallel to the electric field. Different
to the normal polarization reversal mechanism observed in a
63 nm thick film, this abnormal reversal in 1.3 ␮m thick film
occurred during the falling edge of the applied voltage pulse.
Large undershoots and unexpected voltage were not confirmed to be generated while the pulse voltage was applied.
T. Morita and Y. Cho
Even if the anti-parallel poling reversal phenomenon observed in Fig. 3共b兲 was caused by undershoot or other unexpected pulse voltage, the pulse voltage would have to exceed
⫹100 V, because even ⫹100 V was not sufficient to reverse
the poling direction, as shown in Fig. 3共a兲.
The critical thickness of anti-parallel poling reversal was
examined using lithium tantalate films of various thickness,
and for films thicker than around 350 nm, anti-parallel poling
reversal was confirmed. This value is expected to be dependent on material parameters, including permittivity of the
ferroelectric material, cantilever curvature radius 共in this
study the curvature radius is around 25 nm兲, the applied voltage and pulse duration.
To examine whether anti-parallel poling reversal is a
general phenomenon among ferroelectric materials, lead zirconium titanate thin film 共PZT兲 was used as a substrate. This
was deposited by the sol-gel method and had a thickness of
130 nm. Anti-parallel poling reversal was found and poling
direction reversed when the input voltage pulse returned to
zero, which is a characteristic of the anti-parallel poling direction reversal shown in Fig. 3共b兲. The final poling directions were anti-parallel to the applied electric field. Hence
the anti-parallel poling reversal is not peculiar to lithium
tantalate but may be generally applicable to ferroelectric materials. For a 130 nm PZT thin film, only anti-parallel poling
reversal was observed, and so critical thickness of the PZT
medium is smaller than lithium tantalate.
Anti-parallel poling reversal is an unexpected phenomenon, although a similar phenomenon has been already reported by Abplanalp et al.9 using a 4 ␮m thick barium titanate thin film. They detected the anti-parallel poling reversal
phenomenon using a piezoresponse scanning force microscope, and reported that this polarization reversal was related
to the large pressing force of the cantilever probe because a
1.9 ␮N pressing force was required to detect the reversal.
Using a 40 nN pressing force, only normal poling reversal
was observed. In our case, pressing force was only a few nN,
and the SNDM was used to observe poling directions. As
mentioned earlier, compared to the piezoresponse scanning
force microscope, SNDM is superior in terms of the resolution, and the latter is based entirely on electrical information
without mechanical displacement induced by an inverse piezoelectric effect. Hence, in the previous study, anti-parallel
poling reversal may, in fact, have occurred with a smaller
pressing force, but was overlooked due to an unsuitable
method for detecting poling directions. Of course, the study
of Abplanalp et al. contributed to anti-parallel poling reversal, with the most important suggestion being that this unexpected phenomenon is related to the mechanical force. Mechanical strain is expected to be concentrated near the
cantilever due to the inverse piezoelectric effect. This cannot
be explained qualitatively at present, although numerical calculations including an inverse piezoelectric effect could reveal the important clues. The thickness dependency of poling
reversal found in this study might also suggest that antiparallel poling reversal is related to the concentrated strain
near the cantilever because a thinner film is clamped by the
bottom electrode and less piezoelectric strain is induced than
a thicker film.
By utilizing the antiparallel poling reversal, a nanometer
FIG. 3. Poling reversal observation in 1.3 ␮m thick lithium tantalate, showing an electric field 共a兲 anti-parallel and 共b兲 parallel to the initial
poling direction.
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Appl. Phys. Lett., Vol. 84, No. 2, 12 January 2004
FIG. 4. Patterned nanodots utilizing antiparallel poling reversal. Each dot
was written with different input voltage. The initial poling statement is plus
surface 共indicated as black兲 and reversed poling nanodot is indicated as a
bright color.
dot pattern was written and read with a conventional scanning process to examine the dot radius in regard to the input
voltage and pulse width. The ferroelectric substrate was a
lithium tantalate thin film whose thickness was 2– 4 ␮m and
the initial poling direction was the plus surface. The input
voltage had a modified sinusoidal shape and the electric field
direction was the same as the initial poling direction. Before
the scanning experiments, the cantilever was fixed on the
thin film, and then by monitoring the phase signal from the
lock-in amplifier it was confirmed that the antiparallel poling
reversal occurred. With the opposite-direction pulse that
causes a normal poling reversal, the poling direction was not
affected using this substrate. The formed dot pattern shown
T. Morita and Y. Cho
259
in Fig. 4 indicates that the large peak value and wide pulse
result in a large reversed dot and this tendency agrees with
the normal poling reversal method.
In conclusion, anti-parallel poling reversal phenomenon
was observed using a nonlinear dielectric microscope. This
poling reversal was unexpected because the poling direction
was aligned anti-parallel to the applied electric field. This
was only detected for thicker films with the critical thickness
in the case of lithium tantalate film being about 350 nm. In
thinner films, only normal poling reversal was observed.
Normal polarization reversal was observed as the applied
voltage increased and anti-parallel polarization reversal was
observed during the falling edge. Although the anti-parallel
poling reversal mechanism is unknown, mechanical stress or
strain are thought to play a role. Usage of the cantilever
probe results in a concentrated electric field and this situation
is much different from the conventional plate type ferroelectric capacitor model.
Y. Cho, Integr. Ferroelectr. 50, 89 共2002兲.
Y. Cho, S. Kazuta, and H. Ito, Appl. Phys. Lett. 79, 2955 共2001兲.
3
A. Gruverman, O. Auciello, R. Ramesh, and T. Tokumoto, Nanotechnology 8, A38 共1997兲.
4
K. Matsuura, Y. Cho, and H. Odagawa, Jpn. J. Appl. Phys. 40, 3534
共2001兲.
5
Y. Hiranaga, K. Fujimoto, Y. Wagatsuma, Y. Cho, A. Onoe, K. Terabe, and
K. Kitamura, Mater. Res. Soc. Symp. Proc. 748, U5.5.1 共2003兲.
6
Y. Cho, K. Fujimoto, Y. Hiranaga, Y. Wagatsuma, A. Onoe, K. Terabe, and
K. Kitamura, Appl. Phys. Lett. 81, 4401 共2002兲.
7
Y. Cho, K. Matsuura, N. Valanoor, and R. Ramesh, Proceedings of the 7th
International Symposium on Ferroic Domains and Mesoscopic Structures
(ISFD7), 2002, B50P03.
8
F. Jona and G. Shirane, Ferroelectric Crystals 共Macmillan company, New
York, 1962兲, p. 46.
9
M. Abplanalp, J. Fousek, and P. Gunter, Phys. Rev. Lett. 86, 5799 共2001兲.
1
2
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