- Email: [email protected]
SrTiO3 superlattices
Vacuum 66 (2002) 397–401
Dependence of dielectric and ferroelectric behaviors on growth orientation in epitaxial BaTiO3/SrTiO3 superlattices O. Nakagawaraa,*, T. Shimutaa, T. Makinoa, S. Araia, H. Tabatab, T. Kawaib a
Murata Manufacturing Company Limited, R & D Division, 2-26-10, Tenjin, Nagaokakyoshi, Kyoto 617-8555, Japan b ISIR Sanken, Osaka University, 8-1 Mihogaoka Ibaraki, Osaka 567-0047, Japan
Abstract Epitaxial BaTiO3/SrTiO3 dielectric superlattices have been prepared with molecular layer order accuracy by ArF pulsed laser deposition. Superlattices with various periods of stacking layer are constructed both on (0 0 1) and (1 1 1) Nb-doped SrTiO3 conductive substrates to elucidate orientation dependence of their dielectric behaviors. Results of reflection high-energy electron diffraction patterns combined with X-ray diffraction profiles show that the superlattices are completely epitaxial with the substrate. Superlattices with (0 0 1) orientation have large remanent polarization of 2Pr ¼ 46 mC/cm2 in BaTiO3 (6.0 nm)/SrTiO3 (1.2 nm) imbalance thickness ratio. As for (1 1 1)-oriented superlattices, the relative dielectric constant goes up to 594 upon decreasing the period of the layer, which is twice as large as that of (1 1 1)-oriented (Ba0.5,Sr0.5)TiO3 solid-solution film. Typical remanent polarization 2Pr is 2.7 mC/cm2. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Artificial dielectric superlattices are recently within the bounds of possibility to create materials whose properties are superior to those of a solid– solution film. A number of studies concerning BaTiO3/SrTiO3 and related materials [1–3], Bibased layered oxides [4] and KTaO3/KNbO3 [5] superlattices have been performed to control crystal growth and to improve their dielectric properties. In particular, (0 0 1)-oriented BaTiO3/ SrTiO3 superlattices are of great interest to many workers [6–9] because lattice strain along the polarized direction may enhance their ferroelectric behaviors. It was reported that remanent polarization in (0 0 1)-oriented BaTiO3/SrTiO3 superlattice increased with decreasing the modulation period *Corresponding author. E-mail address: [email protected] (O. Nakagawara).
[10]. These dielectric behaviors were explained to be due to large lattice strains in the heteroepitaxial interface region. In order to set up film characteristics in the manner electronic devices would expect, our attention is focused on the variation of orientation. A large remanent polarization is expected to appear on (0 0 1) superlattice and a high dielectric constant in (1 1 1). This study is conducted to fabricate (0 0 1)- and (1 1 1)-oriented BaTiO3/ SrTiO3 superlattices with various modulation periods and to elucidate their fundamental dielectric behaviors under a low-frequency range.
2. Experimental The BaTiO3/SrTiO3 multilayered thin films were deposited both on (0 0 1) and (1 1 1) Nb-doped
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 1 6 1 - 6
O. Nakagawara et al. / Vacuum 66 (2002) 397–401
3. Results and discussion 3.1. BaTiO3(0 0 1)/SrTiO3(0 0 1) superlattice The RHEED patterns of all films showing fourfold symmetry indicate that the films are in
complete epitaxy with SrTiO3(0 0 1) substrate. The in-plane epitaxial relationship between the film and substrate is identified to be /1 0 0S// /1 0 0S. A main peak and satellite peaks are observed in the ‘‘balanced’’ structure with 1.2/1.2, 2.0/2.0 and 4.0 nm/4.0 nm periodicity. Calculated periods of ðdBT þ dST Þ from the angle interval between satellite peaks almost agree with the designed periods. Broad peaks are observed instead of satellite peaks with the 24 nm/24 nm periodicity because the stacking thickness is not enough for an interfacial X-ray diffraction intensity. Fig. 1 shows the XRD patterns of the ‘‘imbalanced’’ superlattices with 15/3, 10/3, 3/10 and 3 ML/15 ML stacking periodicity. A main peak and satellite peaks can be observed in XRD profiles of all the superlattices. Two theta position
substrate (200)
3/15
-1st
3/10
+1st
main
-2nd -1st
-3rd -2nd
main
BaTiO3/SrTiO3(001)
{
main
3/3
40
42
+1st
10/3
15/3
+1st
main
-1st
-1st
main
+1st
-1st
SrTiO3 conductive substrates that could be also available for bottom electrodes. Multilayered films were formed by an ArF pulsed-laser deposition technique. A laser with an energy density of 2 J/ cm2 is focused on the stoichiometric BaTiO3 and SrTiO3 ceramic targets alternately with pulse frequency of 1 Hz and base pressure in the vacuum chamber, 1 105 Pa. Total thickness of the multilayered films was fixed at 100 nm. The films were prepared at 6001C in an oxygen/ozone (6%) ambient at a pressure of 1 Pa. Patterned platinum thin films were deposited as a top electrode to complete the setup of a metal–insulator–metal structure. A solid–solution film was also fabricated for comparison with superlattices as for dielectric behaviors of (0 0 1) and (1 1 1) orientation. As for (0 0 1)-oriented superlattices, thickness ratio is changed to improve the remanent polarization. We propose superlattice structure with BaTiO3 layer thickness dBT unequal to dST ; which is called ‘‘imbalanced’’ structure. Meanwhile, superlattice with dBT equal to dST is called ‘‘balanced’’ structure. ‘‘Balanced’’ superlattices vary in the stacking periodicity from 1.2/1.2 to 27 nm/27 nm and ‘‘imbalanced’’ with 6.0/1.2, 4.0/1.2, 1.2/4.0 and 1.2 nm/6.0 nm corresponding to 15/3, 10/3, 3/ 10 and 3 ML/15 ML (monolayer), respectively. Dependence of ferroelectric properties on the crystal structure and thickness ratio has been investigated with ‘‘imbalanced’’ strained superlattice. For (1 1 1), stacking periodicity varies from 0.45/0.45 to 10 nm/10 nm. Periodicity dependence of spacing of the (1 1 1) plane and the dielectric properties were investigated. Analyses of reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) were carried out to identify crystallinity of the films. Electrical properties were measured by an impedance analyzer HP4194A at room temperature. Polarization–electric field hysteresis was measured by Radiant RT6000.
Intensity [a.u.]
398
44
46
2 [deg.]
48
50
Fig. 1. XRD patterns of the ‘‘imbalanced’’ BaTiO3/ SrTiO3(0 0 1) superlattices. The numbers shown in the Figs. 1–3 mean the thickness ratio of BaTiO3/SrTiO3 described by ML (number of monolayer).
O. Nakagawara et al. / Vacuum 66 (2002) 397–401
500
50
15/3
BaTiO3/SrTiO3(001)
r 2Pr
400
0.412
0.408
r
10/3
3/3
15/3
300
30
200
0.404
40
20
10/3
2Pr [ C/cm2]
0.416
Lattice parameter c [nm]
399
3/3 3/15
0.400
100
3/10
10
3/15 3/10 BaTiO3/SrTiO3(001)
0 0.396
0
SrTiO3
20
40
60
80
100
BaTiO3
Proportion of BaTiO 3 in superlattices [mol%] Fig. 2. Mean lattice parameter c of BaTiO3/SrTiO3(0 0 1) superlattices against the proportion of BaTiO3 in superlattices. Open squares denote the lattice parameter c of the BaTiO3 and SrTiO3 single-phase films prepared under the same deposition conditions as superlattices. Broken line shows the weighted mean of the lattice parameters c of BaTiO3 and SrTiO3 films.
of the main peak decreases from 45.2 to 43.71 as the thickness ratio dBT =dST increases. The periods of the ‘‘imbalanced’’ superlattices also agree with the designed periods. Fig. 2 shows the mean lattice parameter c calculated from the main peak positions as a function of the proportion of BaTiO3 in superlattices. The solid line is determined by least square method. Open squares denote the lattice parameters c of the BaTiO3 and SrTiO3 single phase films prepared under the same deposition conditions as superlattices. The mean lattice parameter c of superlattices is larger than the weighted mean of the lattice parameters c of BaTiO3 and SrTiO3 films (broken line in Fig. 2), which shows a tendency to be more remarkable as the proportion of BaTiO3 increases. Relative dielectric constant er at 1 MHz of ‘‘balanced’’ BaTiO3/SrTiO3 superlattices increases as the layer thickness decreases. A maximum value of relative dielectric constant is, however, only 240, which is as large as those of the BaTiO3 (er ¼ 260) and SrTiO3 (er ¼ 230) single
0 0.1
1
10
dBT/dST Fig. 3. Relative dielectric constant er and remanent polarization 2Pr of ‘‘imbalanced’’ BaTiO3/SrTiO3(0 0 1) superlattices as a function of lattice parameter ratio of BaTiO3 to SrTiO3, dBT =dST :
phase films prepared under the same deposition conditions as superlattices. Remanent polarization, as well as the relative dielectric constant, increases as the layer thickness decreases. The remanent polarization of the superlattice with the stacking periodicity of 1.2 nm/ 1.2 nm (2Pr ¼ 18 mC/cm2 exceeds that of the BaTiO3 single phase film (2Pr ¼ 14 mC/cm2) though the compositional proportion of BaTiO3 in the superlattice, which strongly contributes to its ferroelectricity, is only 50%. Fig. 3 shows the relative dielectric constant er at 1 MHz and the remanent polarization 2Pr of the ‘‘imbalanced’’ superlattices as a function of dBT =dST thickness ratio. The relative dielectric constant is 220–270 and there is no correlation between the relative dielectric constant and dBT =dST : However, the remanent polarization increases as dBT =dST increases. The remanent polarization 2Pr of the superlattice with the stacking periodicity of BaTiO3 15 ML/SrTiO3 3 ML is 46 mC/cm2, which is about three times larger than that of the BaTiO3 single phase film (2Pr ¼ 14 mC/cm2 ) and is comparable to that of bulk BaTiO3 (2Pr ¼ 50 mC/cm2). Both the increase in lattice parameter c by the introduction of
O. Nakagawara et al. / Vacuum 66 (2002) 397–401
interface strain and in the compositional proportion of ferroelectric BaTiO3 lead to the large remanent polarization of BaTiO3-rich ‘‘imbalanced’’ superlattice.
0.2298
BaTiO3 /SrTiO3 (111)
0.2294
d 111 [ nm ]
400
0.2290 0.2286 0.2282
3.2. BaTiO3(1 1 1)/SrTiO3(1 1 1) superlattice
0.2278
Fig. 4 shows XRD of 2y scan profiles from ‘‘balanced’’ BaTiO3(1 1 1)/SrTiO3(1 1 1) superlattice. Different patterns of RHEED observed from ½0 1 1% and ½1 1 2% incident direction indicate that an in-plane-oriented epitaxial film has been fabricated. Results of RHEED pattern combined with XRD profiles verifying (1 1 1) orientation show that multilayered films are completely epitaxial on the substrate. Satellite peaks based on a superlattice structure are observed at the side of the main (1 1 1) peak of the samples with 2.0/2.0, 4.0/ 4.0 and 10 nm/10 nm period. Periods of superlattice evaluated from diffraction angles of satellite peaks were 21.3, 8.1 and 3.7 nm, corresponding to the designed values of 20, 8 and 4 nm, respectively. No satellite peak was observed in the profile of 0.45 nm/0.45 nm, a minimum period. We have not come to a conclusion on whether some kind of interfacial irregularities or the essential crystal
main sub
Intensity [a.u.]
satellite 4/4nm sub
main
BaTiO3/SrTiO3(111)
25
35
0.45/0.45nm
45
55
2 [degree] Fig. 4. X-ray diffraction profiles of BaTiO3/SrTiO3(1 1 1) superlattices film with each modulation period of 4/4 and 0.45 nm/0.45 nm, respectively.
0.1
1
10
100
modulation period [ nm ] Fig. 5. The change of spacing of BaTiO3/SrTiO3(1 1 1) plane ( d111 ) as a function of the modulation period.
structure results in the disappearance of satellite peaks. The variation of the spacing of (1 1 1) (d111 value) of ‘‘balanced’’ superlattice as a function of the modulation period is shown in Fig. 5. The d111 value determined from the diffraction angle of the main peak increases from 0.2281 up to 0.2294 nm with decreasing of the modulation period. All d111 values of the multilayered films are larger than that of (Ba0.5,Sr0.5)TiO3 solid–solution in the bulk (0.2260 nm). The increase of d111 is attributed to an in-plane pressure effect due to a lattice mismatch between BaTiO3 and SrTiO3. The largest d111 value is observed in the film with the shortest period of 0.45 nm/0.45 nm, which refers to the existence of lattice strain in the heteroepitaxial interface region of the film. Fig. 6 shows the changes in the relative dielectric constant of the ‘‘balanced’’ superlattices at 1 MHz as a function of the modulation period. The relative dielectric constant increases as the period decreases, from a minimum of 334 with 10 nm/ 10 nm period to a maximum of 594 with the shortest 0.45 nm/0.45 nm period. What should be emphasized is that dielectric constants of the films in the range between 4.0/4.0 and 0.45 nm/0.45 nm stacking periods are larger than that of (Ba0.5,Sr0.5)TiO3 solid–solution prepared under the same deposition conditions as those of superlattices. Relative dielectric constant of (1 1 1)oriented (Ba0.5,Sr0.5)TiO3 solid–solution thin film was 341 with 100 nm thickness. Considering these
O. Nakagawara et al. / Vacuum 66 (2002) 397–401
401
4. Conclusions
700 BaTiO3/SrTiO3(111)
600
r
500 400 300 200 0.1
Solid-solution film
1 10 modulation period [ nm ]
100
Fig. 6. Relative dielectric constant er of BaTiO3/SrTiO3(1 1 1)oriented superlattices with various periods.
results, the stress at the interface region between BaTiO3 and SrTiO3 layers plays an important role in the enhancement of relative dielectric constant of (1 1 1)-oriented superlattice structure. Lattice strain caused by the induced stress may enhance the ionic shift in the perovskite structure against an electric field, attributing to the increase of dielectric constant. It has been already reported that the pressure induced by the interface in (0 0 1)oriented BaTiO3/SrTiO3 superlattices is estimated to be about 400–500 MPa from elastic calculations [6]. As for the (0 0 1) BaTiO3/SrTiO3 superlattices in the Ref. [6], the dielectric constant has a maximum at 0.8 nm modulation layer. The reason is that the original crystal structure of BaTiO3 is maintained in the superlattice with 0.8 nm period, but not with 0.4 nm (monolayer) period. For (1 1 1) superlattices, original crystal structures of BaTiO3 and SrTiO3 are maintained with 0.45 nm/0.45 nm period judging from a layer structure of /1 1 1S direction. So the lattice strain is effective for the dielectric constant of (1 1 1) superlattices even with 0.45 nm period. Remanent polarization values of the other films are o2.7 mC/cm2. It is worthy of attention that (1 1 1)-oriented BaTiO3/SrTiO3 superlattice has a much smaller remanent polarization than those of (0 0 1)-oriented superlattices, which is mainly due to anisotropy in remanent polarization of the ferroelectric BaTiO3/SrTiO3 superlattice.
Dependence of dielectric and ferroelectric behaviors on growth orientation in epitaxial BaTiO3/ SrTiO3 superlattices has been elucidated. What improves ferroelectricity of (0 0 1)-oriented strained superlattice is a structure with an ‘‘imbalanced’’ thickness ratio. A maximum remanent polarization 2Pr is 46 mC/cm2 which is comparable to that of bulk BaTiO3. We have also prepared (1 1 1)-oriented superlattices which have a high relative dielectric constant up to 594 and a low remanent polarization of 2.7 mC/cm2. The superlattice with (0 0 1) orientation seems to be promising for ferroelectric devices owing to its large remanent polarization, while (1 1 1) for high frequency applications if a dielectric behavior can be found for them to give low dielectric relaxation in the microwave range. Lattice strain has mainly influenced ferroelectricity of superlattices oriented along the polarization direction and the relative dielectric constant of those with (1 1 1) orientation. We should control the orientation to obtain properties desired for each device application.
References [1] Iijima K, Terashima T, Bando Y, Kamigaki K, Terauchi H. J Appl Phys 1992;72:2840. [2] Zhu A, Li C, Zou K, Zeng Y, Lu H, Wang H, Cui D, Chen Z, Mai Z. J Mater Sci Lett 1997;16:1898. [3] Tabata H, Kawai T. Appl Phys Lett 1997;70:321. [4] Tabata H, Tanaka H, Kawai T. Jpn J Appl Phys 1995;34:5146. [5] Specht ED, Christen H-M, Norton DP, Boatner LA. Phys Rev Lett 1998;80:4317. [6] Tabata H, Tanaka H, Kawai T. Appl Phys Lett 1994;65:1970. [7] Tabata H, Tanaka H, Kawai T, Okuyama M. Jpn J Appl Phys 1995;34:544. [8] Tsurumi T, Mitarai S, Nam S-M, Ishibashi Y, Fukunaga O. IEEE 10th International Symposium on Applied Ferroelectric, vol. 1. 1996. p. 499. [9] Yang G-Z, Lu H-B, Zhou Y-L, Lei Z-L, Chen Z-H. Acta Phys Sin (Overseas Ed.) 1998;7:623. [10] Wang Z, Yasuda T, Hatatani S, Oda S. Proc Electrochem Soc 1999;99–2:1054.
Dependence of dielectric and ferroelectric behaviors on growth orientation in epitaxial BaTiO3/SrTiO3 superlattices O. Nakagawaraa,*, T. Shimutaa, T. Makinoa, S. Araia, H. Tabatab, T. Kawaib a
Murata Manufacturing Company Limited, R & D Division, 2-26-10, Tenjin, Nagaokakyoshi, Kyoto 617-8555, Japan b ISIR Sanken, Osaka University, 8-1 Mihogaoka Ibaraki, Osaka 567-0047, Japan
Abstract Epitaxial BaTiO3/SrTiO3 dielectric superlattices have been prepared with molecular layer order accuracy by ArF pulsed laser deposition. Superlattices with various periods of stacking layer are constructed both on (0 0 1) and (1 1 1) Nb-doped SrTiO3 conductive substrates to elucidate orientation dependence of their dielectric behaviors. Results of reflection high-energy electron diffraction patterns combined with X-ray diffraction profiles show that the superlattices are completely epitaxial with the substrate. Superlattices with (0 0 1) orientation have large remanent polarization of 2Pr ¼ 46 mC/cm2 in BaTiO3 (6.0 nm)/SrTiO3 (1.2 nm) imbalance thickness ratio. As for (1 1 1)-oriented superlattices, the relative dielectric constant goes up to 594 upon decreasing the period of the layer, which is twice as large as that of (1 1 1)-oriented (Ba0.5,Sr0.5)TiO3 solid-solution film. Typical remanent polarization 2Pr is 2.7 mC/cm2. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Artificial dielectric superlattices are recently within the bounds of possibility to create materials whose properties are superior to those of a solid– solution film. A number of studies concerning BaTiO3/SrTiO3 and related materials [1–3], Bibased layered oxides [4] and KTaO3/KNbO3 [5] superlattices have been performed to control crystal growth and to improve their dielectric properties. In particular, (0 0 1)-oriented BaTiO3/ SrTiO3 superlattices are of great interest to many workers [6–9] because lattice strain along the polarized direction may enhance their ferroelectric behaviors. It was reported that remanent polarization in (0 0 1)-oriented BaTiO3/SrTiO3 superlattice increased with decreasing the modulation period *Corresponding author. E-mail address: [email protected] (O. Nakagawara).
[10]. These dielectric behaviors were explained to be due to large lattice strains in the heteroepitaxial interface region. In order to set up film characteristics in the manner electronic devices would expect, our attention is focused on the variation of orientation. A large remanent polarization is expected to appear on (0 0 1) superlattice and a high dielectric constant in (1 1 1). This study is conducted to fabricate (0 0 1)- and (1 1 1)-oriented BaTiO3/ SrTiO3 superlattices with various modulation periods and to elucidate their fundamental dielectric behaviors under a low-frequency range.
2. Experimental The BaTiO3/SrTiO3 multilayered thin films were deposited both on (0 0 1) and (1 1 1) Nb-doped
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 1 6 1 - 6
O. Nakagawara et al. / Vacuum 66 (2002) 397–401
3. Results and discussion 3.1. BaTiO3(0 0 1)/SrTiO3(0 0 1) superlattice The RHEED patterns of all films showing fourfold symmetry indicate that the films are in
complete epitaxy with SrTiO3(0 0 1) substrate. The in-plane epitaxial relationship between the film and substrate is identified to be /1 0 0S// /1 0 0S. A main peak and satellite peaks are observed in the ‘‘balanced’’ structure with 1.2/1.2, 2.0/2.0 and 4.0 nm/4.0 nm periodicity. Calculated periods of ðdBT þ dST Þ from the angle interval between satellite peaks almost agree with the designed periods. Broad peaks are observed instead of satellite peaks with the 24 nm/24 nm periodicity because the stacking thickness is not enough for an interfacial X-ray diffraction intensity. Fig. 1 shows the XRD patterns of the ‘‘imbalanced’’ superlattices with 15/3, 10/3, 3/10 and 3 ML/15 ML stacking periodicity. A main peak and satellite peaks can be observed in XRD profiles of all the superlattices. Two theta position
substrate (200)
3/15
-1st
3/10
+1st
main
-2nd -1st
-3rd -2nd
main
BaTiO3/SrTiO3(001)
{
main
3/3
40
42
+1st
10/3
15/3
+1st
main
-1st
-1st
main
+1st
-1st
SrTiO3 conductive substrates that could be also available for bottom electrodes. Multilayered films were formed by an ArF pulsed-laser deposition technique. A laser with an energy density of 2 J/ cm2 is focused on the stoichiometric BaTiO3 and SrTiO3 ceramic targets alternately with pulse frequency of 1 Hz and base pressure in the vacuum chamber, 1 105 Pa. Total thickness of the multilayered films was fixed at 100 nm. The films were prepared at 6001C in an oxygen/ozone (6%) ambient at a pressure of 1 Pa. Patterned platinum thin films were deposited as a top electrode to complete the setup of a metal–insulator–metal structure. A solid–solution film was also fabricated for comparison with superlattices as for dielectric behaviors of (0 0 1) and (1 1 1) orientation. As for (0 0 1)-oriented superlattices, thickness ratio is changed to improve the remanent polarization. We propose superlattice structure with BaTiO3 layer thickness dBT unequal to dST ; which is called ‘‘imbalanced’’ structure. Meanwhile, superlattice with dBT equal to dST is called ‘‘balanced’’ structure. ‘‘Balanced’’ superlattices vary in the stacking periodicity from 1.2/1.2 to 27 nm/27 nm and ‘‘imbalanced’’ with 6.0/1.2, 4.0/1.2, 1.2/4.0 and 1.2 nm/6.0 nm corresponding to 15/3, 10/3, 3/ 10 and 3 ML/15 ML (monolayer), respectively. Dependence of ferroelectric properties on the crystal structure and thickness ratio has been investigated with ‘‘imbalanced’’ strained superlattice. For (1 1 1), stacking periodicity varies from 0.45/0.45 to 10 nm/10 nm. Periodicity dependence of spacing of the (1 1 1) plane and the dielectric properties were investigated. Analyses of reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) were carried out to identify crystallinity of the films. Electrical properties were measured by an impedance analyzer HP4194A at room temperature. Polarization–electric field hysteresis was measured by Radiant RT6000.
Intensity [a.u.]
398
44
46
2 [deg.]
48
50
Fig. 1. XRD patterns of the ‘‘imbalanced’’ BaTiO3/ SrTiO3(0 0 1) superlattices. The numbers shown in the Figs. 1–3 mean the thickness ratio of BaTiO3/SrTiO3 described by ML (number of monolayer).
O. Nakagawara et al. / Vacuum 66 (2002) 397–401
500
50
15/3
BaTiO3/SrTiO3(001)
r 2Pr
400
0.412
0.408
r
10/3
3/3
15/3
300
30
200
0.404
40
20
10/3
2Pr [ C/cm2]
0.416
Lattice parameter c [nm]
399
3/3 3/15
0.400
100
3/10
10
3/15 3/10 BaTiO3/SrTiO3(001)
0 0.396
0
SrTiO3
20
40
60
80
100
BaTiO3
Proportion of BaTiO 3 in superlattices [mol%] Fig. 2. Mean lattice parameter c of BaTiO3/SrTiO3(0 0 1) superlattices against the proportion of BaTiO3 in superlattices. Open squares denote the lattice parameter c of the BaTiO3 and SrTiO3 single-phase films prepared under the same deposition conditions as superlattices. Broken line shows the weighted mean of the lattice parameters c of BaTiO3 and SrTiO3 films.
of the main peak decreases from 45.2 to 43.71 as the thickness ratio dBT =dST increases. The periods of the ‘‘imbalanced’’ superlattices also agree with the designed periods. Fig. 2 shows the mean lattice parameter c calculated from the main peak positions as a function of the proportion of BaTiO3 in superlattices. The solid line is determined by least square method. Open squares denote the lattice parameters c of the BaTiO3 and SrTiO3 single phase films prepared under the same deposition conditions as superlattices. The mean lattice parameter c of superlattices is larger than the weighted mean of the lattice parameters c of BaTiO3 and SrTiO3 films (broken line in Fig. 2), which shows a tendency to be more remarkable as the proportion of BaTiO3 increases. Relative dielectric constant er at 1 MHz of ‘‘balanced’’ BaTiO3/SrTiO3 superlattices increases as the layer thickness decreases. A maximum value of relative dielectric constant is, however, only 240, which is as large as those of the BaTiO3 (er ¼ 260) and SrTiO3 (er ¼ 230) single
0 0.1
1
10
dBT/dST Fig. 3. Relative dielectric constant er and remanent polarization 2Pr of ‘‘imbalanced’’ BaTiO3/SrTiO3(0 0 1) superlattices as a function of lattice parameter ratio of BaTiO3 to SrTiO3, dBT =dST :
phase films prepared under the same deposition conditions as superlattices. Remanent polarization, as well as the relative dielectric constant, increases as the layer thickness decreases. The remanent polarization of the superlattice with the stacking periodicity of 1.2 nm/ 1.2 nm (2Pr ¼ 18 mC/cm2 exceeds that of the BaTiO3 single phase film (2Pr ¼ 14 mC/cm2) though the compositional proportion of BaTiO3 in the superlattice, which strongly contributes to its ferroelectricity, is only 50%. Fig. 3 shows the relative dielectric constant er at 1 MHz and the remanent polarization 2Pr of the ‘‘imbalanced’’ superlattices as a function of dBT =dST thickness ratio. The relative dielectric constant is 220–270 and there is no correlation between the relative dielectric constant and dBT =dST : However, the remanent polarization increases as dBT =dST increases. The remanent polarization 2Pr of the superlattice with the stacking periodicity of BaTiO3 15 ML/SrTiO3 3 ML is 46 mC/cm2, which is about three times larger than that of the BaTiO3 single phase film (2Pr ¼ 14 mC/cm2 ) and is comparable to that of bulk BaTiO3 (2Pr ¼ 50 mC/cm2). Both the increase in lattice parameter c by the introduction of
O. Nakagawara et al. / Vacuum 66 (2002) 397–401
interface strain and in the compositional proportion of ferroelectric BaTiO3 lead to the large remanent polarization of BaTiO3-rich ‘‘imbalanced’’ superlattice.
0.2298
BaTiO3 /SrTiO3 (111)
0.2294
d 111 [ nm ]
400
0.2290 0.2286 0.2282
3.2. BaTiO3(1 1 1)/SrTiO3(1 1 1) superlattice
0.2278
Fig. 4 shows XRD of 2y scan profiles from ‘‘balanced’’ BaTiO3(1 1 1)/SrTiO3(1 1 1) superlattice. Different patterns of RHEED observed from ½0 1 1% and ½1 1 2% incident direction indicate that an in-plane-oriented epitaxial film has been fabricated. Results of RHEED pattern combined with XRD profiles verifying (1 1 1) orientation show that multilayered films are completely epitaxial on the substrate. Satellite peaks based on a superlattice structure are observed at the side of the main (1 1 1) peak of the samples with 2.0/2.0, 4.0/ 4.0 and 10 nm/10 nm period. Periods of superlattice evaluated from diffraction angles of satellite peaks were 21.3, 8.1 and 3.7 nm, corresponding to the designed values of 20, 8 and 4 nm, respectively. No satellite peak was observed in the profile of 0.45 nm/0.45 nm, a minimum period. We have not come to a conclusion on whether some kind of interfacial irregularities or the essential crystal
main sub
Intensity [a.u.]
satellite 4/4nm sub
main
BaTiO3/SrTiO3(111)
25
35
0.45/0.45nm
45
55
2 [degree] Fig. 4. X-ray diffraction profiles of BaTiO3/SrTiO3(1 1 1) superlattices film with each modulation period of 4/4 and 0.45 nm/0.45 nm, respectively.
0.1
1
10
100
modulation period [ nm ] Fig. 5. The change of spacing of BaTiO3/SrTiO3(1 1 1) plane ( d111 ) as a function of the modulation period.
structure results in the disappearance of satellite peaks. The variation of the spacing of (1 1 1) (d111 value) of ‘‘balanced’’ superlattice as a function of the modulation period is shown in Fig. 5. The d111 value determined from the diffraction angle of the main peak increases from 0.2281 up to 0.2294 nm with decreasing of the modulation period. All d111 values of the multilayered films are larger than that of (Ba0.5,Sr0.5)TiO3 solid–solution in the bulk (0.2260 nm). The increase of d111 is attributed to an in-plane pressure effect due to a lattice mismatch between BaTiO3 and SrTiO3. The largest d111 value is observed in the film with the shortest period of 0.45 nm/0.45 nm, which refers to the existence of lattice strain in the heteroepitaxial interface region of the film. Fig. 6 shows the changes in the relative dielectric constant of the ‘‘balanced’’ superlattices at 1 MHz as a function of the modulation period. The relative dielectric constant increases as the period decreases, from a minimum of 334 with 10 nm/ 10 nm period to a maximum of 594 with the shortest 0.45 nm/0.45 nm period. What should be emphasized is that dielectric constants of the films in the range between 4.0/4.0 and 0.45 nm/0.45 nm stacking periods are larger than that of (Ba0.5,Sr0.5)TiO3 solid–solution prepared under the same deposition conditions as those of superlattices. Relative dielectric constant of (1 1 1)oriented (Ba0.5,Sr0.5)TiO3 solid–solution thin film was 341 with 100 nm thickness. Considering these
O. Nakagawara et al. / Vacuum 66 (2002) 397–401
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4. Conclusions
700 BaTiO3/SrTiO3(111)
600
r
500 400 300 200 0.1
Solid-solution film
1 10 modulation period [ nm ]
100
Fig. 6. Relative dielectric constant er of BaTiO3/SrTiO3(1 1 1)oriented superlattices with various periods.
results, the stress at the interface region between BaTiO3 and SrTiO3 layers plays an important role in the enhancement of relative dielectric constant of (1 1 1)-oriented superlattice structure. Lattice strain caused by the induced stress may enhance the ionic shift in the perovskite structure against an electric field, attributing to the increase of dielectric constant. It has been already reported that the pressure induced by the interface in (0 0 1)oriented BaTiO3/SrTiO3 superlattices is estimated to be about 400–500 MPa from elastic calculations [6]. As for the (0 0 1) BaTiO3/SrTiO3 superlattices in the Ref. [6], the dielectric constant has a maximum at 0.8 nm modulation layer. The reason is that the original crystal structure of BaTiO3 is maintained in the superlattice with 0.8 nm period, but not with 0.4 nm (monolayer) period. For (1 1 1) superlattices, original crystal structures of BaTiO3 and SrTiO3 are maintained with 0.45 nm/0.45 nm period judging from a layer structure of /1 1 1S direction. So the lattice strain is effective for the dielectric constant of (1 1 1) superlattices even with 0.45 nm period. Remanent polarization values of the other films are o2.7 mC/cm2. It is worthy of attention that (1 1 1)-oriented BaTiO3/SrTiO3 superlattice has a much smaller remanent polarization than those of (0 0 1)-oriented superlattices, which is mainly due to anisotropy in remanent polarization of the ferroelectric BaTiO3/SrTiO3 superlattice.
Dependence of dielectric and ferroelectric behaviors on growth orientation in epitaxial BaTiO3/ SrTiO3 superlattices has been elucidated. What improves ferroelectricity of (0 0 1)-oriented strained superlattice is a structure with an ‘‘imbalanced’’ thickness ratio. A maximum remanent polarization 2Pr is 46 mC/cm2 which is comparable to that of bulk BaTiO3. We have also prepared (1 1 1)-oriented superlattices which have a high relative dielectric constant up to 594 and a low remanent polarization of 2.7 mC/cm2. The superlattice with (0 0 1) orientation seems to be promising for ferroelectric devices owing to its large remanent polarization, while (1 1 1) for high frequency applications if a dielectric behavior can be found for them to give low dielectric relaxation in the microwave range. Lattice strain has mainly influenced ferroelectricity of superlattices oriented along the polarization direction and the relative dielectric constant of those with (1 1 1) orientation. We should control the orientation to obtain properties desired for each device application.
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