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Domain switching effects in epitaxial films of ferroelectric bismuth titanate
Thin Solid Films, 36 (1916) 509-512 0 Elsevier Sequoia S.A., Lausanne-Piinted
DOMAIN SWITCHING EFFECTS IN EPITAXIAL BISMUTH TITANATE*
S. Y. WU, M. H. FRANCOMBE Westinghouse (Received
1.
FILMS OF FERROELECTRIC
AND W. J. TAKE1
Research Laboratories,
August
509
in Switzerland
Pittsburgh,
Pa. 15235 (U.S.A.)
25, 1975)
INTRODUCTION
In previous publications we described the method of growth] and the structural, optical and electrical properties 2-4 of epitaxial films of ferroelectric bismuth titanate Bi4Ti30i2. Stoichiometric epitaxial layers of this compound of thickness up to 30 pm are grown by r.f. sputtering from a bismuth oxide rich target onto single-crystal substrates such as MgO and MgA1204. In particular, growth at temperatures above the Curie point (67.5 “C) on the (110) face of MgO crystals yields optically transparent films possessing a stripe twin structure with, in adjacent stripes, the a (or b) and c axes of the pseudo-orthorhombic structure lying parallel to the substrate plane. Growth on the (110) face of MgAla O4 yields untwinned layers with the single u-c epitaxial orientation. The ferroelectric polar axis and the major axis of the index ellipsoid lie in the u-c plane, making angles of 4” with the a axis and 25” with the c axis, respectively (Fig. 1). Reversal of the c-axis component of polarization P, causes the index ellipsoid to rotate through approximately 50”, and when viewing along the b axis using crossed polarizers a near-optimum change in optical contrast is achieved4. In the present studies, domain switching in a-c epitaxially oriented films was studied with two electric field configurations. In the first, the field was applied perpendicular
(a)
Ibl
Fig. 1. (a) The orientation of the polarization P, with respect to the crystallographic axes of BbTi30I2; (b) the orientation of the index ellipsoid axes in the a-c plane. The broken lines indicate the change in ellipsoid orientation due to the reversal of the Pc polarization component. * Paper presented at the Third International Conference on Thin Films, “Basic Problems, tions and Trends”, Budapest, Hungary, August 25-29, 1975; Paper 12-20.
Applica-
510
S. Y. WLI, hl. 11. FKAN(‘OfvlB~~., W. J. TAK1-I
to the filtn plane so as to cause switching of P, in and out of the plane. ln the second, the field was applied parallel to the film plane and at 45” to the a and c axes3. _. ’
k
Two epitaxial film samples were used. Both were grown by r.f. [reactive sputtering on single-crystal substrates at 700 “c’, using a bismuth oxide rich ceramic target as described previously’. The first sample BTR 15X was gt-own to a thickness of IS pm on ( 1 10) MgO, and the second BTR 224 to a thickness of 10 ,um on ( 1 IO) MgA1,04 Sections of the films that were large enough for domain studies wet-c removed by the combined use of thermal cycling and a razot- blade. The stripe twins in detached pieces of sample BTR IS8 were removed by thermal cycling through the Curie point3. Two electrode configurations were employed. The lirst, for sample BTR 158, involved the use of semitransparent evaporated gold electrodes (200 ,& thick) and allowed fields to be applied nortnal to the filtn plane. In the second arrangement, used with BTR 224, pairs of electrodes were deposited on opposite f&e areas of the f&n leaving a small gap 25 pm wide, across which a field parallel to the film plane was applied at 45” to the LIand c axes. The changes in domain structure WCI-cexamined in white light with the films between crossed polarirers. 3.
I‘IS ANL) I)ISCUSSION
For fields normal to the film plane no effects were observed until the field was raised to about 70 kV cm-‘. Slow switching frotn “h-up” to “a-up” then occttt-red in small randomly distributed regions of the Film, associated with a change in color. The
Fig. 2. The successive stages in domain switching for a field perpendicular to the film which is initially in the sin&z u-c orientation condition. showin? (a) nucleation and (h), (c) propapation 9lY switched regions. I:ig. 3. The measured lateral 90” domain wall velocity perpendicular to the film surface in Bi4Ti30t2 films.
1’s. the reciprocal
of the electric
field
of
511
EPITAXIAL FILMS OF Bi4Ti3012
E \ -C (a) T
La Fig. 4. The appearance of 1 80° domain depoled condition; (b) the c component
boundaries for u-c plane switching: (a) the electrically poled; (c) both components poled.
“u-up” regions could be switched back with a smaller reversed field in much shorter times. With fields closer to the coercive field (90 kV cm-‘) for about 20 s, the whole film was converted to “a-up” (Fig. 2(c)). However, shortly after this, fine cracks were observed. Once cracking had occurred, it was not possible to switch back completely to the “b-up” orientation. The cracks, which ran parallel to the c axis, started from the lower surface (Fig. 2(c)) and propagated into the film, terminating before reaching the upper surface. The development of these cracks can be explained in terms of the crystal structure of Bi4Ti30r2. As the lattice parameters along the a and b axes are 5.448 and 5.410 A, respectively, the generation of “o-up” domains of the shape shown in Fig. 2(b) will cause a relative contraction of the lower surface of the film. (The film can be seen to curve up under the microscope, as illustrated in Fig. 2(b).) Part of the tensile stress at the lower surface is relieved by the development of the tine cracks. Once these cracks are formed they appear to act as nucleating centers for the generation of “a-up” domains. After complete switching to “u-up” has occurred, field reversal causes nucleation of “b-up” regions on the upper surface at the previous points ot coalescence of the “u-up” domains (Fig. 2(c)). The lateral velocity of the 90” twin walls was measured for different applied fields up to about 100 kV cm-‘. The results are shown in Fig. 3. At low fields, i.e. E= 50 kV cm -l, the twin wall velocity appears to be nucleation controlled and can be expressed by v = vo exp (6/E) with an activation field F of about 42 kV cm-‘. At higher fields there is a linear dependence on the electric field, expressed as z: = @’ 6,) where E. = 46 kV cm-‘, and the wall mobility /1 has a low value of about 1.67 x 10e9 cm2 V-’ s-l. More recent results based on tine pictures for more perfect films grown on MgAL204 indicate much faster switching, with velocities 200-300 times higher for the same range of fields. With fields applied parallel to the film surface (Fig. 4) two distinct domain structures were observed with boundaries parallel to the c and a axes, respectively. Initial electrical depoling with high a.c. fields leads to the crossed-wall structure shown in Fig. 4(a). With the subsequent application of a gradually increased d.c. field, poling of the low coercive field c component of P, occurs, and walls parallel to the c axis
S.\I'. WII.Xl. II. I~KAN(‘OMBl~.. W. J.TAKb:l
512
disappear (Fig. 4(b)). Further increase in the field to the point where the component along the (I axis exceeds the Pa (Fig. 1) coercive value Causes walls parallel to this axis to disappear (Fig. 4(c)).
1 2 3 4
W. J. W. J. S. Y. S. Y.
Takei, N. Takei, N. Wu, W. J. Wu. W. J.
P. bormigoni and M. H. Francombe, J. lat. Sci. T?chrzol., ’ (1970) 442. I’. Pormig;ni and M. M. Francombe, Appl. 1’1l?;s. I.eri., 1.5 (1969) 256. Takei, M. Il. I:rancombe and S. F. Cummins, E‘wroclectrics, 3 (I 972) 217. Takei and M. H. I:rancombe, .4ppI. Phys. I.c,tf., 3_1 (19731 26.
DOMAIN SWITCHING EFFECTS IN EPITAXIAL BISMUTH TITANATE*
S. Y. WU, M. H. FRANCOMBE Westinghouse (Received
1.
FILMS OF FERROELECTRIC
AND W. J. TAKE1
Research Laboratories,
August
509
in Switzerland
Pittsburgh,
Pa. 15235 (U.S.A.)
25, 1975)
INTRODUCTION
In previous publications we described the method of growth] and the structural, optical and electrical properties 2-4 of epitaxial films of ferroelectric bismuth titanate Bi4Ti30i2. Stoichiometric epitaxial layers of this compound of thickness up to 30 pm are grown by r.f. sputtering from a bismuth oxide rich target onto single-crystal substrates such as MgO and MgA1204. In particular, growth at temperatures above the Curie point (67.5 “C) on the (110) face of MgO crystals yields optically transparent films possessing a stripe twin structure with, in adjacent stripes, the a (or b) and c axes of the pseudo-orthorhombic structure lying parallel to the substrate plane. Growth on the (110) face of MgAla O4 yields untwinned layers with the single u-c epitaxial orientation. The ferroelectric polar axis and the major axis of the index ellipsoid lie in the u-c plane, making angles of 4” with the a axis and 25” with the c axis, respectively (Fig. 1). Reversal of the c-axis component of polarization P, causes the index ellipsoid to rotate through approximately 50”, and when viewing along the b axis using crossed polarizers a near-optimum change in optical contrast is achieved4. In the present studies, domain switching in a-c epitaxially oriented films was studied with two electric field configurations. In the first, the field was applied perpendicular
(a)
Ibl
Fig. 1. (a) The orientation of the polarization P, with respect to the crystallographic axes of BbTi30I2; (b) the orientation of the index ellipsoid axes in the a-c plane. The broken lines indicate the change in ellipsoid orientation due to the reversal of the Pc polarization component. * Paper presented at the Third International Conference on Thin Films, “Basic Problems, tions and Trends”, Budapest, Hungary, August 25-29, 1975; Paper 12-20.
Applica-
510
S. Y. WLI, hl. 11. FKAN(‘OfvlB~~., W. J. TAK1-I
to the filtn plane so as to cause switching of P, in and out of the plane. ln the second, the field was applied parallel to the film plane and at 45” to the a and c axes3. _. ’
k
Two epitaxial film samples were used. Both were grown by r.f. [reactive sputtering on single-crystal substrates at 700 “c’, using a bismuth oxide rich ceramic target as described previously’. The first sample BTR 15X was gt-own to a thickness of IS pm on ( 1 10) MgO, and the second BTR 224 to a thickness of 10 ,um on ( 1 IO) MgA1,04 Sections of the films that were large enough for domain studies wet-c removed by the combined use of thermal cycling and a razot- blade. The stripe twins in detached pieces of sample BTR IS8 were removed by thermal cycling through the Curie point3. Two electrode configurations were employed. The lirst, for sample BTR 158, involved the use of semitransparent evaporated gold electrodes (200 ,& thick) and allowed fields to be applied nortnal to the filtn plane. In the second arrangement, used with BTR 224, pairs of electrodes were deposited on opposite f&e areas of the f&n leaving a small gap 25 pm wide, across which a field parallel to the film plane was applied at 45” to the LIand c axes. The changes in domain structure WCI-cexamined in white light with the films between crossed polarirers. 3.
I
For fields normal to the film plane no effects were observed until the field was raised to about 70 kV cm-‘. Slow switching frotn “h-up” to “a-up” then occttt-red in small randomly distributed regions of the Film, associated with a change in color. The
Fig. 2. The successive stages in domain switching for a field perpendicular to the film which is initially in the sin&z u-c orientation condition. showin? (a) nucleation and (h), (c) propapation 9lY switched regions. I:ig. 3. The measured lateral 90” domain wall velocity perpendicular to the film surface in Bi4Ti30t2 films.
1’s. the reciprocal
of the electric
field
of
511
EPITAXIAL FILMS OF Bi4Ti3012
E \ -C (a) T
La Fig. 4. The appearance of 1 80° domain depoled condition; (b) the c component
boundaries for u-c plane switching: (a) the electrically poled; (c) both components poled.
“u-up” regions could be switched back with a smaller reversed field in much shorter times. With fields closer to the coercive field (90 kV cm-‘) for about 20 s, the whole film was converted to “a-up” (Fig. 2(c)). However, shortly after this, fine cracks were observed. Once cracking had occurred, it was not possible to switch back completely to the “b-up” orientation. The cracks, which ran parallel to the c axis, started from the lower surface (Fig. 2(c)) and propagated into the film, terminating before reaching the upper surface. The development of these cracks can be explained in terms of the crystal structure of Bi4Ti30r2. As the lattice parameters along the a and b axes are 5.448 and 5.410 A, respectively, the generation of “o-up” domains of the shape shown in Fig. 2(b) will cause a relative contraction of the lower surface of the film. (The film can be seen to curve up under the microscope, as illustrated in Fig. 2(b).) Part of the tensile stress at the lower surface is relieved by the development of the tine cracks. Once these cracks are formed they appear to act as nucleating centers for the generation of “a-up” domains. After complete switching to “u-up” has occurred, field reversal causes nucleation of “b-up” regions on the upper surface at the previous points ot coalescence of the “u-up” domains (Fig. 2(c)). The lateral velocity of the 90” twin walls was measured for different applied fields up to about 100 kV cm-‘. The results are shown in Fig. 3. At low fields, i.e. E= 50 kV cm -l, the twin wall velocity appears to be nucleation controlled and can be expressed by v = vo exp (6/E) with an activation field F of about 42 kV cm-‘. At higher fields there is a linear dependence on the electric field, expressed as z: = @’ 6,) where E. = 46 kV cm-‘, and the wall mobility /1 has a low value of about 1.67 x 10e9 cm2 V-’ s-l. More recent results based on tine pictures for more perfect films grown on MgAL204 indicate much faster switching, with velocities 200-300 times higher for the same range of fields. With fields applied parallel to the film surface (Fig. 4) two distinct domain structures were observed with boundaries parallel to the c and a axes, respectively. Initial electrical depoling with high a.c. fields leads to the crossed-wall structure shown in Fig. 4(a). With the subsequent application of a gradually increased d.c. field, poling of the low coercive field c component of P, occurs, and walls parallel to the c axis
S.\I'. WII.Xl. II. I~KAN(‘OMBl~.. W. J.TAKb:l
512
disappear (Fig. 4(b)). Further increase in the field to the point where the component along the (I axis exceeds the Pa (Fig. 1) coercive value Causes walls parallel to this axis to disappear (Fig. 4(c)).
1 2 3 4
W. J. W. J. S. Y. S. Y.
Takei, N. Takei, N. Wu, W. J. Wu. W. J.
P. bormigoni and M. H. Francombe, J. lat. Sci. T?chrzol., ’ (1970) 442. I’. Pormig;ni and M. M. Francombe, Appl. 1’1l?;s. I.eri., 1.5 (1969) 256. Takei, M. Il. I:rancombe and S. F. Cummins, E‘wroclectrics, 3 (I 972) 217. Takei and M. H. I:rancombe, .4ppI. Phys. I.c,tf., 3_1 (19731 26.