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Thickness dependence of switching for ferroelectric Langmuir films
ELSEVIER
Thin Solid Films 284-285 ( 1996) 474476
Thickness dependence of switching for ferroelectric Langmuir films L.M. Blinov *, V.M. Fridkin, S.P. Palto, A.V. Sorokin, S.G. Yudin Institute
ofCrystallography,
Russian Academy
ofSciences.
Leninsky prosp.
59, Moscow I 17333, Russia
Abstract Switchable ferroelectric Langmuir-Blodgett films prepared from the ferroelectric copolymer of vinylidene fluoride with trifluoroethylene P(VDF-TrFE) manifest the true b&ability for films thicker than 15 monolayers; thinner films are still switchable but monostable. The
crossover of the switching behaviour presents the first estimation of the ferroelectric correlation length, (- 75 8. Keywords:
Langmuir-Blodgett films; Ferroelectricity; Electric switching; Size effect
1. Introduction Since the discovery of ferroelectricity the investigation of very thin ferroelectric films has been attracting a great fundamental and applied interest. The fundamental aspect is related to the mechanism of the ferroelectric switching and phase transition. The switching nucleation-growth mechanism strongly depends on the film size [ 11. The screening conditions lead to the existence of “transition’ ’ or “correlation length”, below which the ferroelectric state in proper ferroelectrics becomes thermodynamically unstable [ 21. Thus, a fundamental question arises concerning the minimum size of the film or grain, which is consistent with the existence of proper ferroelectricity. To our knowledge, the thinnest switchable ferroelectric films made of conventional materials have thicknesses of about 600 A. Even those films have their coercive field considerably enhanced just to the limit of their breakdown. The potentialities of both the spin-coating technique (polymers) and vacuum evaporation (inorganic ferroelectrics) seem to be exhausted and novel approaches are needed to advance further to the molecular level. It should be noted that ferroelectric liquid crystals may be prepared in the form of free suspended films consisted of few monomolecular layers. Such films manifest ferroelectric properties [ 31. However, they are improper ferroelectrics and their spontaneous polarization is one or two orders of magnitude smaller than the values typical of conventional proper ferroelectrics. The question of the correlation length or the minimum film thickness compatible with proper ferroelectricity is still open. Recently we have shown [4,5] that Langmuir-Blodgett (LB) films may be prepared from the ferroelectric copoly* Corresponding author. 0040-6090/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSD10040-6090(95)08369-3
mers and those films manifest a well-defined ferroelectric transition and may be switched by an external electric field. The aim of the present work is an attempt to prepare LB ferroelectric films of various thickness (down to 2 monolayers) and to measure their pyroelectric properties and the switching behaviour. Pyroelectric measurements clearly show the existence of a critical film thickness (about 15 monolayers) below which the bistable ferroelectric switching is substituted by a monostable one. Thus, we believe, that, at last, the correlation length for proper ferroelectrics mentioned above has been estimated from experiment.
2. Experimental Films with thicknesses from 100 to 2 monolayers were prepared from the ferroelectric co-polymer P(VDF-TrFE) (70:30 mol.%) [6] by the horizontal lift technique. The procedure is described in detail in a previous paper [ 51. All measurements reported below were carried out on sandwich samples with two Al electrodes. The area of electrode overlapping was 1 X 1 mm’. During the pyroelectric measurements also described in detail in the previous paper [5] a d.c. voltage was applied for a short time in series with the load resistor to obtain the switching hysteresis loop in a coordinate electric field-pyroresponse. All measurements were performed at room temperature.
3. Results Due to the preparation method, freshly prepared films have a certain asymmetry and show pulse pyroelectric response
L. M. Biinov et al. /Thin Solid Films 284-285 (1996) 474476
-31 -15
1 -10
”
”
-5
0
”
“‘110
5
15
-?1W
Bias voltage, V Fig. 1. Pyroelectric hysteresis loops for films of thicknesses the reproducibility of the process.
Bias voltage. V
from 86 to 35 monolayers
even being never poled. On applying a d.c. voltage of various amplitude and sign hysteresis loops shown in Figs. 1 and 2 have been measured. For each particular point the d.c. voltage was applied only for a short (r = 4 s) time. The sign of the pyroresponse pulse is changed when a voltage corresponding to the coercive field is applied to a film.
monolayers
-101
’ -6
o
-4
’
-2
0
2
4
6
0.6
-0.4
-6
-4
-2
0
2
4
6
Bias voltage, V Fig. 2. Pyroelectric hysteresis loops for films of thicknesses 5 monolayers ( monostable switching).
475
of 15 and
(bistable switching).
Several switching cycles are shown to demonstrate
In Fig. 1 hysteresis loops are shown for LB films of thicknesses from 86 to 35 monolayers. All these films manifest bistability: after application of a certain bias a new state, no matter positive or negative, is rather stable, the pyroresponse does not change more than by 30% for 3 days. For thinner films, from 15 to 2 monolayers (Fig. 2) the bistability disappears. Now one state (say, p-state, corresponding, for example, to positive bias) is stable, the other, n-state, is achieved just on application of the negative bias and then relaxes back to the p-state for few minutes. The relaxation of the field induced states is shown in Fig. 3. The measurements were made as follows. First, a sample was poled for 4 s by a field twice as high as coercive one to achieve some initial state (shown by dotted lines). Then field of opposite sign of various strength was applied again for 4 s and the kinetics of the pyroresponse were measured. As examples, the data for a “thick” and “thin” films are displayed. For the thick film, a new state is memorized if the switching voltage is high enough (on relaxation, the pyroresponse pulse does not change its sign). For the thin film, even for a very strong switching field the pyroresponse pulse relaxed back to the initial polarity. However, after relaxation, the new state differs from the initial one (dotted lines), which might point to a certain role of a trapped charge in our films. The latter should be studied more carefully. It should be noted that a change from the bistable to monostable behaviour shown in Figs. 1 and 2 takes place at nearly the same thickness where the dielectric constant changes its magnitude [ 51. In our opinion, this thickness corresponds to the transition or correlation length mentioned in Section 1. The correlation length, according to our measurements, is
L. M. Blinov et af. /Thin
476
d
% g -1.0 k
0 o/>5
Solid Films 284-285
f I%%) 474-476
0 -5.7
__cLa
-7.1
1
x z -l.g
A
-2.91
“...‘L
35 monolayers
10
’
“‘.‘.I
,‘.“.I
100
0.2
-0
1000
I
2
1.0w108 30
-24.0
40
Number
P
60
70
of monolayers
60
90 100
1
N
Fig. 4. Coercive field strength as a function of film thickness expressed in numbers of monolayers.
20.1. ,/“-&3
$ i$ O.O
50
ferroelectric films are, indeed, a very interesting many applications.
b -0.1
system for
Z t -0.2 z
5 monolayers
-0.3 I
I . . . . . ..I
10
“..‘.S
100
4. Conclusion I
..,.“‘I
1000
Time, s Fig. 3. Relaxation of the pyroresponse from field-induced polarized states for LB films with thicknesses of 35 and 5 monolayers. Dotted lines show the pyroresponse of an initial state. The switching field is shown at thecurves in lo8 V m-t units.
estimated to be about 75 w ( 15 monolayers). To our knowledge, this is the first experimental estimation of the correlation length for ferroelectrics in general. Figs. 1 and 2 allow us to plot the thickness dependence of the coercive field,
In conclusion, ferroelectric properties of Langmuir-Blodgett multilayers composed from monolayers of ferroelectric copolymer P(VDF-TrFE) (70:30) manifest a remarkable thickness dependence. Films thicker than 15 monolayers show typical bistable switching. Thinner films are monostable. This critical thickness of about 75 A (observed also in the dielectric behaviour [ 51) may be considered as a characteristic correlation length found for the proper ferroelectrics for the first time.
Acknowledgements
(1) where A U is the half-width of the hysteresis loop and d is film thickness. The result is shown in Fig. 4. The dependence may be approximated by law E,aN-0.7. The thickness dependence of the coercive field may be compared with that observed for a conventional ferroelectric material where, for thicker films ( 1 000-5 000 A), the exponent found ( - 4/3) coincided with that anticipated from the Neel law in magnetic materials [ 71. The coercive field increases with decreasing film thickness and reaches the strength of (4-5) X lo8 V m-i. Fortunately, the breakdown field in LB films also increases for thinner films. The reason appears to be very simple: to produce a breakdown, free carriers should have high enough energy what requires for them a long enough distance to be accelerated; in any case they cannot acquire more energy (in eV> than the voltage applied to a film (few volts). Thus, ultrathin
We thank Dr. S. Iablonsky, Dr. K. Verkhovskaya and Dr. A. Bune for stimulating discussions. We also appreciate the financial support from the International Science Foundation (grant no. M35300) and the Russian Fund for Fundamental Researches (Grant no.95-02-054 1) .
References [I] J.F. Scott, H.M. Duiker, P.D. Beale, B. Pouligny, K. Dimm!er,M. Parris, D. Butlerand
S. Athens, Physica, BZ.50 ( 1988) 160. Phys. Rev. Len.. JO ( 1973)
[2] I.P. Batra, P. Wurfei and B.D. Silverman,
384.
[ 31 Ch. Bahr and D. Fliegner.
Ferroelecrrics. 147 ( 1993) 1. [4] S. Palto. L. Blinov, A. Bune, E. Dubovik, V. Fridkin, N. Petukhova, K. Verkhovskaya and S. Yudin. Ferroelecrrics Left, 19 (1995) 65. [5] L. Blinov, S. Palto. V. Fridkin, A. Sorokin and S. Yudin. Thin Solid Films, in press. [6] T. Furukawa, Phase Trans., 18 (1989) 143. [7] J.F. Scott, L.D. McMillan and CA. Araujo, Ferroelectrics, 93 (1989) 827.
Thin Solid Films 284-285 ( 1996) 474476
Thickness dependence of switching for ferroelectric Langmuir films L.M. Blinov *, V.M. Fridkin, S.P. Palto, A.V. Sorokin, S.G. Yudin Institute
ofCrystallography,
Russian Academy
ofSciences.
Leninsky prosp.
59, Moscow I 17333, Russia
Abstract Switchable ferroelectric Langmuir-Blodgett films prepared from the ferroelectric copolymer of vinylidene fluoride with trifluoroethylene P(VDF-TrFE) manifest the true b&ability for films thicker than 15 monolayers; thinner films are still switchable but monostable. The
crossover of the switching behaviour presents the first estimation of the ferroelectric correlation length, (- 75 8. Keywords:
Langmuir-Blodgett films; Ferroelectricity; Electric switching; Size effect
1. Introduction Since the discovery of ferroelectricity the investigation of very thin ferroelectric films has been attracting a great fundamental and applied interest. The fundamental aspect is related to the mechanism of the ferroelectric switching and phase transition. The switching nucleation-growth mechanism strongly depends on the film size [ 11. The screening conditions lead to the existence of “transition’ ’ or “correlation length”, below which the ferroelectric state in proper ferroelectrics becomes thermodynamically unstable [ 21. Thus, a fundamental question arises concerning the minimum size of the film or grain, which is consistent with the existence of proper ferroelectricity. To our knowledge, the thinnest switchable ferroelectric films made of conventional materials have thicknesses of about 600 A. Even those films have their coercive field considerably enhanced just to the limit of their breakdown. The potentialities of both the spin-coating technique (polymers) and vacuum evaporation (inorganic ferroelectrics) seem to be exhausted and novel approaches are needed to advance further to the molecular level. It should be noted that ferroelectric liquid crystals may be prepared in the form of free suspended films consisted of few monomolecular layers. Such films manifest ferroelectric properties [ 31. However, they are improper ferroelectrics and their spontaneous polarization is one or two orders of magnitude smaller than the values typical of conventional proper ferroelectrics. The question of the correlation length or the minimum film thickness compatible with proper ferroelectricity is still open. Recently we have shown [4,5] that Langmuir-Blodgett (LB) films may be prepared from the ferroelectric copoly* Corresponding author. 0040-6090/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSD10040-6090(95)08369-3
mers and those films manifest a well-defined ferroelectric transition and may be switched by an external electric field. The aim of the present work is an attempt to prepare LB ferroelectric films of various thickness (down to 2 monolayers) and to measure their pyroelectric properties and the switching behaviour. Pyroelectric measurements clearly show the existence of a critical film thickness (about 15 monolayers) below which the bistable ferroelectric switching is substituted by a monostable one. Thus, we believe, that, at last, the correlation length for proper ferroelectrics mentioned above has been estimated from experiment.
2. Experimental Films with thicknesses from 100 to 2 monolayers were prepared from the ferroelectric co-polymer P(VDF-TrFE) (70:30 mol.%) [6] by the horizontal lift technique. The procedure is described in detail in a previous paper [ 51. All measurements reported below were carried out on sandwich samples with two Al electrodes. The area of electrode overlapping was 1 X 1 mm’. During the pyroelectric measurements also described in detail in the previous paper [5] a d.c. voltage was applied for a short time in series with the load resistor to obtain the switching hysteresis loop in a coordinate electric field-pyroresponse. All measurements were performed at room temperature.
3. Results Due to the preparation method, freshly prepared films have a certain asymmetry and show pulse pyroelectric response
L. M. Biinov et al. /Thin Solid Films 284-285 (1996) 474476
-31 -15
1 -10
”
”
-5
0
”
“‘110
5
15
-?1W
Bias voltage, V Fig. 1. Pyroelectric hysteresis loops for films of thicknesses the reproducibility of the process.
Bias voltage. V
from 86 to 35 monolayers
even being never poled. On applying a d.c. voltage of various amplitude and sign hysteresis loops shown in Figs. 1 and 2 have been measured. For each particular point the d.c. voltage was applied only for a short (r = 4 s) time. The sign of the pyroresponse pulse is changed when a voltage corresponding to the coercive field is applied to a film.
monolayers
-101
’ -6
o
-4
’
-2
0
2
4
6
0.6
-0.4
-6
-4
-2
0
2
4
6
Bias voltage, V Fig. 2. Pyroelectric hysteresis loops for films of thicknesses 5 monolayers ( monostable switching).
475
of 15 and
(bistable switching).
Several switching cycles are shown to demonstrate
In Fig. 1 hysteresis loops are shown for LB films of thicknesses from 86 to 35 monolayers. All these films manifest bistability: after application of a certain bias a new state, no matter positive or negative, is rather stable, the pyroresponse does not change more than by 30% for 3 days. For thinner films, from 15 to 2 monolayers (Fig. 2) the bistability disappears. Now one state (say, p-state, corresponding, for example, to positive bias) is stable, the other, n-state, is achieved just on application of the negative bias and then relaxes back to the p-state for few minutes. The relaxation of the field induced states is shown in Fig. 3. The measurements were made as follows. First, a sample was poled for 4 s by a field twice as high as coercive one to achieve some initial state (shown by dotted lines). Then field of opposite sign of various strength was applied again for 4 s and the kinetics of the pyroresponse were measured. As examples, the data for a “thick” and “thin” films are displayed. For the thick film, a new state is memorized if the switching voltage is high enough (on relaxation, the pyroresponse pulse does not change its sign). For the thin film, even for a very strong switching field the pyroresponse pulse relaxed back to the initial polarity. However, after relaxation, the new state differs from the initial one (dotted lines), which might point to a certain role of a trapped charge in our films. The latter should be studied more carefully. It should be noted that a change from the bistable to monostable behaviour shown in Figs. 1 and 2 takes place at nearly the same thickness where the dielectric constant changes its magnitude [ 51. In our opinion, this thickness corresponds to the transition or correlation length mentioned in Section 1. The correlation length, according to our measurements, is
L. M. Blinov et af. /Thin
476
d
% g -1.0 k
0 o/>5
Solid Films 284-285
f I%%) 474-476
0 -5.7
__cLa
-7.1
1
x z -l.g
A
-2.91
“...‘L
35 monolayers
10
’
“‘.‘.I
,‘.“.I
100
0.2
-0
1000
I
2
1.0w108 30
-24.0
40
Number
P
60
70
of monolayers
60
90 100
1
N
Fig. 4. Coercive field strength as a function of film thickness expressed in numbers of monolayers.
20.1. ,/“-&3
$ i$ O.O
50
ferroelectric films are, indeed, a very interesting many applications.
b -0.1
system for
Z t -0.2 z
5 monolayers
-0.3 I
I . . . . . ..I
10
“..‘.S
100
4. Conclusion I
..,.“‘I
1000
Time, s Fig. 3. Relaxation of the pyroresponse from field-induced polarized states for LB films with thicknesses of 35 and 5 monolayers. Dotted lines show the pyroresponse of an initial state. The switching field is shown at thecurves in lo8 V m-t units.
estimated to be about 75 w ( 15 monolayers). To our knowledge, this is the first experimental estimation of the correlation length for ferroelectrics in general. Figs. 1 and 2 allow us to plot the thickness dependence of the coercive field,
In conclusion, ferroelectric properties of Langmuir-Blodgett multilayers composed from monolayers of ferroelectric copolymer P(VDF-TrFE) (70:30) manifest a remarkable thickness dependence. Films thicker than 15 monolayers show typical bistable switching. Thinner films are monostable. This critical thickness of about 75 A (observed also in the dielectric behaviour [ 51) may be considered as a characteristic correlation length found for the proper ferroelectrics for the first time.
Acknowledgements
(1) where A U is the half-width of the hysteresis loop and d is film thickness. The result is shown in Fig. 4. The dependence may be approximated by law E,aN-0.7. The thickness dependence of the coercive field may be compared with that observed for a conventional ferroelectric material where, for thicker films ( 1 000-5 000 A), the exponent found ( - 4/3) coincided with that anticipated from the Neel law in magnetic materials [ 71. The coercive field increases with decreasing film thickness and reaches the strength of (4-5) X lo8 V m-i. Fortunately, the breakdown field in LB films also increases for thinner films. The reason appears to be very simple: to produce a breakdown, free carriers should have high enough energy what requires for them a long enough distance to be accelerated; in any case they cannot acquire more energy (in eV> than the voltage applied to a film (few volts). Thus, ultrathin
We thank Dr. S. Iablonsky, Dr. K. Verkhovskaya and Dr. A. Bune for stimulating discussions. We also appreciate the financial support from the International Science Foundation (grant no. M35300) and the Russian Fund for Fundamental Researches (Grant no.95-02-054 1) .
References [I] J.F. Scott, H.M. Duiker, P.D. Beale, B. Pouligny, K. Dimm!er,M. Parris, D. Butlerand
S. Athens, Physica, BZ.50 ( 1988) 160. Phys. Rev. Len.. JO ( 1973)
[2] I.P. Batra, P. Wurfei and B.D. Silverman,
384.
[ 31 Ch. Bahr and D. Fliegner.
Ferroelecrrics. 147 ( 1993) 1. [4] S. Palto. L. Blinov, A. Bune, E. Dubovik, V. Fridkin, N. Petukhova, K. Verkhovskaya and S. Yudin. Ferroelecrrics Left, 19 (1995) 65. [5] L. Blinov, S. Palto. V. Fridkin, A. Sorokin and S. Yudin. Thin Solid Films, in press. [6] T. Furukawa, Phase Trans., 18 (1989) 143. [7] J.F. Scott, L.D. McMillan and CA. Araujo, Ferroelectrics, 93 (1989) 827.