Download Ultrafast polarization switching in thin-film ferroelectrics

APPLIED PHYSICS LETTERS
VOLUME 84, NUMBER 7
16 FEBRUARY 2004
Ultrafast polarization switching in thin-film ferroelectrics
J. Li, B. Nagaraj, H. Liang, W. Cao, Chi. H. Lee, and R. Ramesha)
Materials Research Science and Engineering Center, University of Maryland, College Park, Maryland 20742
共Received 3 July 2003; accepted 3 December 2003兲
We present an experimental approach to study the ultrafast polarization switching dynamics in
thin-film ferroelectrics. A semiconductor photoconductive switch with femtosecond laser
illumination is used as a ‘‘pulse generator’’ to produce jitter-free, sub-100 ps rise time
step-function-like electrical pulses. Quantitative measurements yield a polarization switching time,
t s , of ⬃220 ps when measured with a 5 V, 68 ps rise time input electrical pulse. Modeling of the
switching transients using the Merz–Ishibashi model and Merz–Shur model of switching kinetics
yields a quantitative estimate of the characteristic switching time constant, t 0 , of ⬃70–90 ps.
© 2004 American Institute of Physics. 关DOI: 10.1063/1.1644917兴
The application of ferroelectric capacitors in nonvolatile
ferroelectric random access memories 共FeRAMs兲 is due to
their switchable remanent polarization, which can be reversed by application of a short voltage pulse.1,2 In such an
application, the issue of how fast the polarization can be
switched is of great interest. With this method, Larsen and
co-workers have reported a polarization switching time of
⬃390 ps for PZT films.3,4 Nucleation and growth kinetics
based theoretical models predict the switching time to be of
the order of ⬃70 ps.5–7 Further, our earlier studies8 showed
that the polarization switching time changed dramatically
with the rise time of the input electrical pulse, suggesting
that it is convoluted with the circuit parasitics. The resulting
switching times were of the order of the input pulse rise
time, showing that much faster input pulses were required.
We report in this letter results of switching experiments in
thin PNZT films using a femtosecond laser triggered experimental setup that has enabled us to probe the intrinsic
switching dynamics.
A semiconductor photoconductive switch is used as a
fast ‘‘pulse generator’’ to produce step-like pulse with fast
rise times. The resistance of the photoconductive switch in
its dark state 共or high resistance state兲 without laser illumination is ⬃100 k⍀ to M⍀. With laser illumination, it is in its
conductive state, with an ‘‘on-state’’ resistance of a few ⍀.
The photoconductive switch produces fast electrical pulses
with rise time of ⬃50–100 ps that depends on the laser
power, bias level, etc. A Si photoconductive switch is chosen
to generate the step-function-like pulse because the carrier
lifetime in Si is of the order of microseconds. This enables us
to systematically create pulses of up to several hundred
nanoseconds. Si switches with interdigitated fingers were
patterned into a coplanar transmission line structure with 50
⍀ characteristic impedance in order to transmit wide band
signals with little dispersion.
Fully integrated Pt/共La0.5Sr0.5)CoO3 /Pb共Nb0.04Zr0.28
Ti0.68)O3 /(La0.5Sr0.5)CoO3 /Pt ferroelectric capacitors with
thickness of 200 nm were used in this study. Details of the
growth and long-term properties are reported elsewhere.8
Large bond pads were wire bonded and the device was inserted into a 50 ⍀ microwave transmission line setup. A
a兲
Electronic mail: [email protected]
HP54750A digitizing oscilloscope with time resolution of
⬃20 ps was used to monitor the polarization switching response. The internal 50 ⍀ impedance of the oscilloscope is
matched to the transmission line in order to ensure that no
signals reflects back to the ferroelectric device.
A schematic layout of the experimental setup is shown in
Fig. 1. A femtosecond Ti:sapphire laser 共frequency⫽10 kHz兲
illuminates both the photodiode 共PD兲 trigger and the Si photoconductive switch 共PCS兲. The PD generates a trigger signal
to synchronize the oscilloscope and two pulse generators.
Conventional pulse generators 1 and 2 produce pulses with
rise time of ⬃7 ns. Generator 1 creates a write pulse train
which sets the ferroelectric device to the stable positive or
negative polarization state by applying a positive or negative
square pulse. In our experiment, pulses with 8 V amplitude
and 1 ms duration were used as the write pulses. Pulse generator 2 generates a biasing pulse train for the Si switch. The
delay of this pulse train is tuned so that it arrives at the Si
switch coincident with the laser pulse. The Si switch generates a read pulse train, the timing of which is controlled by
the relative delay between generators 1 and 2. The delay is
tuned to ensure that the writing pulses occur in between the
reading pulses. The resulting output pulse train to the ferroelectric device is a combination of the write and read pulse
trains. The new pulse train can be a sequence of positive–
positive pulses or negative–positive pulses by varying the
FIG. 1. Experimental setup with a Si photoconductive switch as the fast
pulse generator.
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© 2004 American Institute of Physics
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Appl. Phys. Lett., Vol. 84, No. 7, 16 February 2004
FIG. 2. Input electrical pulse generated by a Si photoconductive switch and
corresponding remanent (⌬ P) responses of a 4.5 ␮m⫻5.4 ␮m LSCO/
PNZT/LSCO capacitor. The amplitude of the electric field applied is 250
kV/cm 共5 V兲.
polarity of the writing pulse generator. The oscilloscope is
used to monitor the displacement current flowing through the
ferroelectric capacitor. Due to the small dispersion from
transmission lines and the high bandwidth of the monitoring
oscilloscope, the time resolution of this setup depends on the
rise time of the pulse generated by the Si switch, which can
be less than 100 ps. A negative–positive pulse sequence applied to the ferroelectric device produces a switched response
P * whereas a positive–positive sequence gives the nonswitched response P∧ . ⌬ P⫽ P * ⫺ P ∧ is the remanent polarization, which is directly connected to domain switching dynamics. For each capacitor size, a minimum of five
capacitors was measured; each capacitor was tested a minimum of five times to obtain data at each voltage.
Figure 2 shows the early stages of the 5 V input electrical pulse with a rise time 共10%–90%兲 of 68 ps and the corresponding output voltage response of the P * ⫺ P ∧ for a 4.5
␮m⫻5.4 ␮m capacitor; the inset shows the full input pulse.
The switching time, t s , which is defined as the time from the
onset of switching to a point 90% from the maximum, is
⬃220 ps. We find that this switching transient is quite sensitive to the capacitor size, the rise time of the input pulse, and
the pulse shape. An important fundamental question is the
limit of the switching speed in thin film ferroelectrics. If we
assume that the domain walls propagate with field depen-
Li et al.
1175
dent speed v ⫽ ␮ E and mobility ␮ of ⬃2.4⫻10⫺4 m2 /V s, 9
then the switching time in a 200 nm PNZT film is ⬃80 ps,
corresponding to a 5 V pulse. The switching process can be
described in terms of a classical solid-state phase transformation that involves nucleation and growth of reverse domains that causes polarization reversal.5,10,11 The volume
fraction of the domains switched is given by f (switched)
⫽1⫺exp关⫺⌽(E)⌿(t) 兴 , where ⌽(E) represents the field
dependence and ⌿(t) represents the time dependence of
the switching process. Combining the Merz model with that
of Ishibashi’s, yields f (switched)⫽1⫺exp兵⫺关exp(⫺␣/E)
⫻(t/t0)n兴其, where t 0 is the characteristic switching time and
n is the dimensionality of the nucleating domain. Shur et al.
refined this model to account for finite-size corrections of
the KA theory through the insertion of an impingement
time, given by the following equation:
f 共 switched兲 ⫽1⫺exp兵 ⫺ 关 exp共 ⫺ ␣ /E 兲共 t/t 0 兲 n 共 1⫺t/t m 兲兴 其 ,
共1兲
where both t 0 and n have the same definition as in the Ishibashi model and t m accounts for impingement of the growing domains on the boundary of the media.
We have used these two related models to fit the experimental current transients for the 4.5 ␮m⫻5.4 ␮m capacitor
as a function of the field applied. The f (switched) vs t data
are obtained by integrating the current transients and normalizing with respect to the maximum polarization. This normalized switched domain fraction is plotted as a function of
the transient time. A 500 kV/cm activation field8 was used to
fit the experimental data. Figures 3共a兲 and 3共b兲 show plots of
normalized, switched polarization versus time, and the corresponding fit using both Ishibashi and Shur et al. models.
The general approach for fitting the switching current transients using the Ishibashi model is by varying both t 0 and n.
As a result, a nonintegral value of dimensionality n is typically obtained. In our preliminary fitting of the experimental
data, we varied both t 0 and n, which yielded a n value of
3.1. However we recognized that interpretation of the
switching data is more meaningful when n is an integer,
thereby enabling us to identify the actual nucleation and
growth mechanism. Therefore in our fit, we fixed n to an
integer value of 3.0 and extracted the t 0 value from experimental current transient data. The excellent fit of the experi-
FIG. 3. 共a兲, 共b兲 Plots of measured 共dotted line兲 and
fitted 共solid line兲 f (t) curves of a 4.5 ␮m⫻5.4 ␮m
LSCO/PNZT/LSCO capacitor. The fit was carried out
with both 共a兲 Ishibashi and 共b兲 Shur et al. models. 共c兲
Characteristic switching time t 0 as a function of the
voltage applied.
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Li et al.
Appl. Phys. Lett., Vol. 84, No. 7, 16 February 2004
FIG. 4. 共a兲 Remanent (⌬ P) response of a 4.5 ␮m⫻5.4
␮m LSCO/PNZT/LSCO capacitor with different rise
times of input electric pulse. 共b兲 Characteristic switching time t 0 as a function of the rise time of the input
electric pulse. The amplitude of the electric field applied is 250 kV/cm.
mental data with the model, Fig. 3共a兲, is noteworthy. Furthermore, we did not observe any significant changes in the
regression coefficient of the fit as a consequence of changing
the n value from 3.1 to 3.0, thus supporting that validity of
fixing n⫽3. t 0 of 68 ps is obtained for this capacitor with
n⫽3 and the field dependence is shown in Fig. 3共c兲. We have
also attempted to fit the switching transients to the Shur et al.
model, Fig. 3共b兲, in which the polarization switching process
is separated into two or three stages with different dimensionalities.12 All stages of the kinetic process can be described by the same equation, Eq. 共1兲, except that the dimensionality changes by integral values for each stage. However,
our data could not be fitted well with such a multistage
switching process.
What is the meaning of the observed value of n⫽3? Two
limiting situations of the reversal process are usually considered in both Ishibashi11 and Shur et al.12 models, called ␣
and ␤ models. In the ␣ model, reverse nuclei arise throughout the whole process and in the ␤ model reversal is caused
by nuclei that pre-exist at the beginning of the process. The
fitting equations for ␣ and ␤ models are the same except that
dimensionality n is different. Therefore, n⫽3 corresponds to
two-dimensional 共2D兲 growth of reverse domains in the ␣
model and three-dimensional 共3D兲 growth in the ␤ model.
Differentiating between these two possibilities is complicated by the polycrystalline nature of the films used in this
study; however, we suspect that 3D reversal is unlikely 共as
surmised by Ishibashi and Takagi11兲. We note that the value
of n determined from fitting the switching transients is extremely sensitive to the size of the capacitor; although not
presented in this letter, increasing the capacitor size systematically lowers the n value, presumably due to significant
convolution from capacitive RC effects. We have observed
that the capacitor size and therefore the RC-time constant
directly influence the shape of the ⌬ P response 共i.e., it produces a long ‘‘tail’’兲, and consequently the fitting parameters.
Therefore, it is clearly important to reduce the RC-time constant of the test structure to a value smaller than the expected
t 0 value 共i.e., smaller than ⬃50 ps兲. For the 4.5 ␮m⫻5.4 ␮m
size capacitor, we estimate an RC-time constant value of
about 45 ps.
Finally, we show that the rise time of the input pulse
significantly impacts the switching dynamics. Figure 4共a兲
shows the ⌬ P response with different rise times of the input
electrical pulse. Figure 4共b兲 shows the rise time dependence
of characteristic switching time t 0 for this 4.5 ␮m⫻5.4 ␮m
capacitor and another 12.5 ␮m⫻14 ␮m capacitor, obtained
by fitting the transients to the Ishibashi model. We have not
been able to observe saturation in the t 0 value as a function
of the rise time, since the smallest rise time we have been
able to obtain is of the order of 50 ps.
In conclusion, we demonstrated direct measurement of
the ferroelectric polarization switching time, t s of ⬃220 ps
using a femtosecond optically generated electrical pulse with
a rise time of 68 ps. We estimated the characteristic switching time t 0 to be ⬃70–90 ps with fixed dimensionality n⫽3.
The switching dynamics are impacted most by the rise time
of the input electric pulses to the FE device and the size of
the capacitor, i.e., the RC-time constant of the capacitor. In
order to obtain the real intrinsic polarization switching time,
the rise time of the input electric pulses has to be smaller
than or at least of the same order as the intrinsic switching
time, while the capacitor size has to be small enough so that
the RC-time constant is equal to or less than the intrinsic
switching time. The polarization switching time measurement is limited by the larger of the two parameters, namely,
the RC-time constant and the rise time of the input electric
pulse. In other words, the rise time becomes important only
when the capacitor size is small enough. For larger capacitors, a fast rise time will still result in a large switching time
since it is now limited by the RC-time constant of the capacitor. For smaller capacitors, a longer rise time will still
result in a larger switching time. Only when the above two
parameters work together can one obtain a switching time
that is close to the intrinsic switching time.
This work was supported by the NSF-MRSEC under
Contract No. DMR-00-80008.
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