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SrRuO3 ultra-thin capacitors
Thin Solid Films 486 (2005) 149 – 152 www.elsevier.com/locate/tsf
Thickness-dependent ferroelectric properties in fully-strained SrRuO3/BaTiO3/SrRuO3 ultra-thin capacitors J.Y. Joa, Y.S. Kima, D.H. Kima, J.D. Kima, Y.J. Changa, J.H. Kongb, Y.D. Parkb, T.K. Songc, J.-G. Yoond, J.S. Junge, T.W. Noha,* a
ReCOE and School of Physics, Seoul National University, Seoul 151-747, Republic of Korea School of Physics and CSCMR, Seoul National University, Seoul 151-747, Republic of Korea c Department of Ceramic Science and Engineering, Changwon National University, Changwon 641-773, Republic of Korea d Department of Physics, University of Suwon, Suwon 445-743, Republic of Korea e Department of Physics, Soongsil University, Seoul 156-743, Republic of Korea b
Available online 29 January 2005
Abstract We have been able to gain insights into ferroelectric thin films from measurements of the thickness-dependence of various characteristics of these films, such as coercive field (E c), remnant polarization ( P r) and leakage current. Fully-strained SrRuO3 (SRO)/BaTiO3 (BTO)/SRO hetero-structures with ultra-thin BTO layers from 30 nm to 5 nm were deposited on SrTiO3 (001) substrates by pulsed laser deposition. Welldefined interfaces between the ferroelectric BTO layer and the electrode SRO layer were confirmed by a tunneling electron microscope image and by the thickness-independence of their coercive fields. The ferroelectric hysteresis loop was observed in the thinnest BTO layer (5 nm thickness). The decrease of ferroelectric polarization was observed as the BTO thickness decreased reduced, which agreed well with the theoretical prediction. Resistive switching behavior was not observed in the thinnest films, but was observed in thicker films after dielectric breakdown. D 2005 Elsevier B.V. All rights reserved. PACS: 77.80.-e; 77.22.Ej; 77.84.-s Keywords: BaTiO3; Remnant polarization; Coercive field; Resistive switching
1. Introduction Thickness-dependent physical properties of ferroelectric ultra-thin films such as coercive field (E c), remnant polarization ( P r), and leakage current have attracted tremendous scientific interest. They have also been a focus of technological interest, since they can have a major bearing on size reduction of ferroelectric devices [1]. Among the issues, the precise determination of the critical thickness for ferroelectricity has attracted the attention of many researchers. Recently, Junquera and Ghosez reported results of first-principle calculation for fully-strained SrRuO3 (SRO)/BaTiO3 (BTO)/SRO films on
4 Corresponding author. Tel.: +82 2 880 1385; fax: +82 2 875 1222. E-mail address: [email protected] (T.W. Noh). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.225
SrTiO3 (STO) substrates [2]. This work provided the first explicit case study to allow a direct and definite comparison between theoretical and experimental work. We estimated experimentally the critical thickness (t c) with fully-strained SRO/BTO/SRO films on STO substrates. Other groups have reported experimental critical thicknesses for ferroelectric material. However, due to leakage current, it is quite difficult to observe direct P–E hysteresis loops in ultra-thin films. Therefore, other researchers did not use electrical measurements, but instead used indirect methods, such as domain wall observation with X-ray diffraction [3], piezoresponse [4] and current transient [5], as well as other methods. Yanase et al. reported that P–E hysteresis loops ware observed in BTO layer thick nesses down to 12 nm [6]. In this paper, we will present the physical properties of fully-strained SRO/BTO/SRO in ultra-thin films.
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In addition, recently, Rodriguez et al. reported that resistivity hysteresis was found in current–voltage (I–V) curves for ferroelectric capacitors in ultra-thin film regions [7]. According to their report, in ultra-thin regions, ferroelectric capacitors did not show P–E ferroelectric hysteresis loops but they showed that phonon-assisted inelastic tunneling had started to occur, which was related to resistivity hysteresis in the I–V curve.
2. Experimental We deposited SRO/BTO/SRO films on STO (001) substrates with a high-quality interface. The step–terrace structure of the top SRO layer after post-annealing was clearly observed in the AFM image, as shown in Fig. 1(a). Fig. 1(b) is a line profile along the black line in Fig. 1(a), which indicates that the steps should be of one unit-cell height and the terraces should be atomically smooth. This strongly suggests that all the films were grown in a 2dimensional layer-by-layer mode, so atomically smooth surfaces were conserved during the growth of entire layers and the post-annealing processes. Smooth interfaces between SRO and BTO layer also could be confirmed with transmission electron microscope (TEM). Fig. 1(c) is a cross-sectional image of SRO (15 nm)/ BTO (5 nm)/SRO (30 nm) hetero-structure on STO substrate. As shown in Fig. 1(c), the interfaces between SRO/ BTO/SRO are clearly defined, which suggests that the ultra-thin BTO layer is grown without the interlayer diffusions. In order to test the predictions of Junquera and Ghosez precisely, it is important that all the layers of SRO/BTO/ SRO hetero-structures should be fully strained with the lattice of the STO substrate [2]. Fig. 2 shows an X-ray reciprocal space-mapping (X-RSM) around the asymmetric (1¯03) Bragg reflection for the hetero-structure with t BTO=30
Fig. 2. X-ray reciprocal space mapping around the asymmetric (1¯03) Bragg reflection of the hetero-structure with 30 nm thick BTO layer.
nm. In this figure, the horizontal and vertical axes represent the reciprocal lattice point along the [00l] and [h00] directions, which are the inverse of out-of-plane (c-axis) and in-plane (a-axis) lattice parameters, respectively. In the X-RSM results, the 458 streak through the STO substrate peak is believed to be from the resolution function of the diffractometer. The measurement was taken by Bruker D8 advance system that consisted of a 3 kW Cu Ka tube source and a Goebel mirror, which converted the diverging beam into a parallel beam. The system had a relatively weak signal compared to synchrotron sources and we had to take the map of our BTO ultra-thin films without monochromator crystals and with relatively large slit openings. The 458 streak would come from the size of the slit in front of the detector. Note that the peaks of the STO substrate, the SRO layers, and the BTO layer lie on a horizontal straight line, which demonstrates that the a-axis lattice parameters of all the deposited layers should be the same as that of the STO substrate, i.e. 0.3905 nm, which is within our experimental errors. This result indicates that all the films are fully strained in the ab-plane. Similar X-RSM results were
Fig. 1. (a) AFM image of the surface of the SRO/BTO/SRO/STO (001) hetero-structure with 6 nm thick BTO layer. (b) TEM image of the cross-section of the SRO/BTO/SRO/STO (001) hetero-structure with 5 nm thick BTO layer. (c) A line profile along the black line in the AFM image of (a).
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obtained for all the hetero-structures with various values of t BTO from 30 nm to 5 nm. The c-axis lattice constants of the BTO layers are larger than the bulk value, due to the in-plane biaxial compressive stress. For the BTO layers with t BTO z 9 nm, the c-axis lattice constant is about 0.427 nm, which is about 6% larger than the bulk BTO value, i.e. 0.40335 nm. For the BTO layer with t BTO=6 nm, the value was reduced to 0.42400 nm, which is still about 5% larger than the bulk value. These results demonstrate that all of the hetero-structures should be nearly fully-strained by the STO substrates with elongated c-axes. In order to measure the electrical properties of the SRO/ BTO/SRO hetero-structures, we fabricated square-shaped capacitors with areas of 1010 Am2 by conventional photolithography and ion-milling processes. Polarization vs. electric field ( P–E) loops were measured at 2 kHz with an aixACCT TF Analyzer 2000. The details of the heterostructure fabrication and film characterization were presented elsewhere.
3. Results and discussions Fig. 3(a) shows hysteresis loops for the BTO capacitors with t BTO of 6.5 and 15 nm. The P–E hysteresis loops provide direct evidence for the ferroelectricity of BTO thin films with t BTO down to 5 nm, which provides an experimental upper
Fig. 3. (a) Polarization vs. electric field ferroelectric hysteresis loops of the SRO/BTO/SRO capacitors with 15 nm (o), and 6.5 nm (n) BTO. (b) Normalized remnant polarization as a function of BTO thickness ranging from 30 to 5 nm is shown as filled squares. The solid line is the predicted function from the first-principle calculation by Junquera and Ghosez [2]. The dashed line is the converged value in the bulk region [1]. Coercive field as a function of BTO thickness ranging from 30 to 5 nm is shown as open circles.
Fig. 4. (a) I–V curve of SRO/BTO/SRO capacitor with BTO layer thickness 30 nm at room temperature. Inset shows polarization vs. electric field of SRO/BTO/SRO capacitor with BTO layer thickness 30 nm at room temperature. (b) I–V curve of SRO/BTO/SRO capacitor with BTO layer thickness 30 nm at 700 K. Inset shows polarization vs. electric field of SRO/BTO/SRO capacitor with BTO layer thickness 30 nm at 700 K.
bound for t c of 5 nm in the fully-strained SRO/BTO/SRO/ STO hetero-structure. Fig. 3(b) shows the t BTO-dependence of P r for our SRO/BTO/SRO hetero-structures. Note that the P r value was normalized to a converged value in the bulk region, i.e. to 31 AC/cm2. For the BTO film with t BTO=30 nm, the P r value is about 36 AC/cm2, which is larger than converged value in bulk region. The difference of about 15% between the P r values of our films and the theoretical value would have originated from the inaccuracy of the firstprinciple simulations in the thick region of t BTO or the possible existence of an extra elongation of the c-axis caused by oxygen vacancies in our films [8]. In Fig. 3(b), it is easy to see the thickness scaling effect on P r in the ultra-thin region of t BTO. Our experimental data, denoted by filled squares, clearly show strong t BTO-dependence in the region of t BTO b20 nm. We will compare the t BTO-dependence of P r for our BTO capacitors with a theoretical prediction. First-principle calculations, shown as the solid line in Fig. 3(b), can describe our t BTO-dependent scaling of P r in our BTO capacitors quite well. As shown in Fig. 3(b), the coercive field is almost independent of BTO layer thickness, ranging from 30 nm to 5 nm. According to Tagantsev’s model, interfacial dielectric layers between the ferroelectric layer and the electrodes result in the coercive field becoming thickness-dependent, so that the coercive field is inversely proportional to the thickness of the ferroelectric layer [9]. Our samples seem not to have these interfacial dielectric layers, so that the
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thickness-dependence of the coercive field does not appear. This may be evidence of perfect interfacial control in our samples. Resistivity hysteresis was reported in the I–V curve of ferroelectric capacitors in the ultra-thin film region [7]. According to the report, in ultra-thin film region, ferroelectric capacitors did not show P–E ferroelectric hysteresis loops but they showed that phonon-assisted inelastic tunneling had started to occur, which is related to resistivity hysteresis in the I–V curve. In particular, resistivity changes in a specific electric field, which is estimated to the coercive field. It demonstrated that the resistive switching behavior is a new phenomenon of ultra-thin capacitors. We carried out experiments to verify whether or not the polarization-related resistive switching occurs in an ultra-thin BTO capacitor, which is too thin to show a P–E hysteresis loop, due to leakage current, but was expected to have ferroelectric properties. In our ultra-thin capacitors that showed good hysteresis loops, we measured the current response after applying a triangular voltage wave to 1010 Am2 capacitors, with t BTO from 30 nm to 5 nm. As shown in Fig. 4, in the case of t BTO=30 nm capacitor, which showed a good P–E hysteresis loop, the magnitude of response current is smaller than Rodriguez’s result. Just after electric breakdown (5 mV, 10 kHz wave at 700 K), the magnitude of response current becomes larger than that of the original state and it shows similar behaviors to the polarization-related resistive switching, which is quite similar to the results of Rodriguez et al. [7]. In contrast to Rodriguez’s assertion, we observed the resistivity hysteresis behavior in capacitors with t BTO between 30 nm and 5 nm, after electric breakdown. This behavior is not limited to the ultra-thin film region. This suggested that the reported polarization-related resistive switching might not be due to intrinsic properties, such as phonon-assisted inelastic tunneling, but due to extrinsic effects, such as defects or charge traps related to electric breakdown or high leakage current.
4. Conclusions Fully-strained SrRuO3 (SRO)/BaTiO3 (BTO)/SRO hetero-structures with ultra-thin BTO layers from 30 nm to 5
nm were fabricated on SrTiO3 substrates by pulsed laser deposition. Epitaxial relation and strains induced by the substrates were observed in the X-ray reciprocal spacemapping. Well-defined interfaces between the ferroelectric BTO layer and the electrode SRO layer were confirmed by TEM images and the thickness-independence of the coercive fields. The ferroelectric hysteresis loop was observed in a 5 nm thick BTO layer capacitor of area 1010 Am2 fabricated by conventional photolithography and ion-milling processes. The decrease of ferroelectric polarization with decreasing BTO thickness was observed and agreed well with a theoretical prediction. Resistive switching behavior was not observed in the thinnest films. However, resistive switching behaviors were observed even in thick films, in which the BTO layer thickness is 30 nm, just after electric breakdown process. This resistive hysteretic behavior can be attributed to extrinsic defects.
Acknowledgements This work was financially supported by the Korean Ministry of Science and Technology through the Creative Research Initiative program and by KOSEF through CSCMR.
References [1] Zhongqing Wu, Ningdong Huang, Zhirong Liu, Jian Wu, Wenhui Duan, Bing-Lin Gu, Xiao-Wen Zhang, Phys. Rev., B 70 (2004) 104108. [2] J. Junquera, Ph. Ghosez, Nature 422 (2003) 506. [3] D.D. Fong, G.B. Stephenson, S.K. Streiffer, J.A. Eastman, O. Auciello, P.H. Fuoss, C. Thompson, Science 304 (2004) 1650. [4] T. Tybell, C.H. Ahn, J.-M. Triscone, Appl. Phys. Lett. 75 (1999) 856. [5] V. Nagarajan, S. Prasertchoung, T. Zhao, H. Zheng, J. Ouyang, R. Ramesh, W. Tian, X.Q. Pan, D.M. Kim, C.B. Eom, H. Kohlstedt, R. Waser, Appl. Phys. Lett. 84 (2004) 5225. [6] N. Yanase, K. Abe, N. Fukushima, T. Kawakubo, Jpn. J. Appl. Phys. 38 (1999) 5305. [7] J. Rodriguez Contreras, H. Kohlstedt, U. Poppe, R. Waser, C. Buchal, N.A. Pertsev, Appl. Phys. Lett. 83 (2003) 4595. [8] Y.S. Kim, D.H. Kim, J.D. Kim, Y.J. Chang, T.W. Noh, J.H. Hong, K. Char, Y.D. Park, S.D. Bu, J.-G. Yoon, J.S. Jung, Appl. Phys. Lett. (in press). [9] A.K. Tagantsev, I.A. Stolichnov, Appl. Phys. Lett. 73 (1999) 1326.
Thickness-dependent ferroelectric properties in fully-strained SrRuO3/BaTiO3/SrRuO3 ultra-thin capacitors J.Y. Joa, Y.S. Kima, D.H. Kima, J.D. Kima, Y.J. Changa, J.H. Kongb, Y.D. Parkb, T.K. Songc, J.-G. Yoond, J.S. Junge, T.W. Noha,* a
ReCOE and School of Physics, Seoul National University, Seoul 151-747, Republic of Korea School of Physics and CSCMR, Seoul National University, Seoul 151-747, Republic of Korea c Department of Ceramic Science and Engineering, Changwon National University, Changwon 641-773, Republic of Korea d Department of Physics, University of Suwon, Suwon 445-743, Republic of Korea e Department of Physics, Soongsil University, Seoul 156-743, Republic of Korea b
Available online 29 January 2005
Abstract We have been able to gain insights into ferroelectric thin films from measurements of the thickness-dependence of various characteristics of these films, such as coercive field (E c), remnant polarization ( P r) and leakage current. Fully-strained SrRuO3 (SRO)/BaTiO3 (BTO)/SRO hetero-structures with ultra-thin BTO layers from 30 nm to 5 nm were deposited on SrTiO3 (001) substrates by pulsed laser deposition. Welldefined interfaces between the ferroelectric BTO layer and the electrode SRO layer were confirmed by a tunneling electron microscope image and by the thickness-independence of their coercive fields. The ferroelectric hysteresis loop was observed in the thinnest BTO layer (5 nm thickness). The decrease of ferroelectric polarization was observed as the BTO thickness decreased reduced, which agreed well with the theoretical prediction. Resistive switching behavior was not observed in the thinnest films, but was observed in thicker films after dielectric breakdown. D 2005 Elsevier B.V. All rights reserved. PACS: 77.80.-e; 77.22.Ej; 77.84.-s Keywords: BaTiO3; Remnant polarization; Coercive field; Resistive switching
1. Introduction Thickness-dependent physical properties of ferroelectric ultra-thin films such as coercive field (E c), remnant polarization ( P r), and leakage current have attracted tremendous scientific interest. They have also been a focus of technological interest, since they can have a major bearing on size reduction of ferroelectric devices [1]. Among the issues, the precise determination of the critical thickness for ferroelectricity has attracted the attention of many researchers. Recently, Junquera and Ghosez reported results of first-principle calculation for fully-strained SrRuO3 (SRO)/BaTiO3 (BTO)/SRO films on
4 Corresponding author. Tel.: +82 2 880 1385; fax: +82 2 875 1222. E-mail address: [email protected] (T.W. Noh). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.225
SrTiO3 (STO) substrates [2]. This work provided the first explicit case study to allow a direct and definite comparison between theoretical and experimental work. We estimated experimentally the critical thickness (t c) with fully-strained SRO/BTO/SRO films on STO substrates. Other groups have reported experimental critical thicknesses for ferroelectric material. However, due to leakage current, it is quite difficult to observe direct P–E hysteresis loops in ultra-thin films. Therefore, other researchers did not use electrical measurements, but instead used indirect methods, such as domain wall observation with X-ray diffraction [3], piezoresponse [4] and current transient [5], as well as other methods. Yanase et al. reported that P–E hysteresis loops ware observed in BTO layer thick nesses down to 12 nm [6]. In this paper, we will present the physical properties of fully-strained SRO/BTO/SRO in ultra-thin films.
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J.Y. Jo et al. / Thin Solid Films 486 (2005) 149–152
In addition, recently, Rodriguez et al. reported that resistivity hysteresis was found in current–voltage (I–V) curves for ferroelectric capacitors in ultra-thin film regions [7]. According to their report, in ultra-thin regions, ferroelectric capacitors did not show P–E ferroelectric hysteresis loops but they showed that phonon-assisted inelastic tunneling had started to occur, which was related to resistivity hysteresis in the I–V curve.
2. Experimental We deposited SRO/BTO/SRO films on STO (001) substrates with a high-quality interface. The step–terrace structure of the top SRO layer after post-annealing was clearly observed in the AFM image, as shown in Fig. 1(a). Fig. 1(b) is a line profile along the black line in Fig. 1(a), which indicates that the steps should be of one unit-cell height and the terraces should be atomically smooth. This strongly suggests that all the films were grown in a 2dimensional layer-by-layer mode, so atomically smooth surfaces were conserved during the growth of entire layers and the post-annealing processes. Smooth interfaces between SRO and BTO layer also could be confirmed with transmission electron microscope (TEM). Fig. 1(c) is a cross-sectional image of SRO (15 nm)/ BTO (5 nm)/SRO (30 nm) hetero-structure on STO substrate. As shown in Fig. 1(c), the interfaces between SRO/ BTO/SRO are clearly defined, which suggests that the ultra-thin BTO layer is grown without the interlayer diffusions. In order to test the predictions of Junquera and Ghosez precisely, it is important that all the layers of SRO/BTO/ SRO hetero-structures should be fully strained with the lattice of the STO substrate [2]. Fig. 2 shows an X-ray reciprocal space-mapping (X-RSM) around the asymmetric (1¯03) Bragg reflection for the hetero-structure with t BTO=30
Fig. 2. X-ray reciprocal space mapping around the asymmetric (1¯03) Bragg reflection of the hetero-structure with 30 nm thick BTO layer.
nm. In this figure, the horizontal and vertical axes represent the reciprocal lattice point along the [00l] and [h00] directions, which are the inverse of out-of-plane (c-axis) and in-plane (a-axis) lattice parameters, respectively. In the X-RSM results, the 458 streak through the STO substrate peak is believed to be from the resolution function of the diffractometer. The measurement was taken by Bruker D8 advance system that consisted of a 3 kW Cu Ka tube source and a Goebel mirror, which converted the diverging beam into a parallel beam. The system had a relatively weak signal compared to synchrotron sources and we had to take the map of our BTO ultra-thin films without monochromator crystals and with relatively large slit openings. The 458 streak would come from the size of the slit in front of the detector. Note that the peaks of the STO substrate, the SRO layers, and the BTO layer lie on a horizontal straight line, which demonstrates that the a-axis lattice parameters of all the deposited layers should be the same as that of the STO substrate, i.e. 0.3905 nm, which is within our experimental errors. This result indicates that all the films are fully strained in the ab-plane. Similar X-RSM results were
Fig. 1. (a) AFM image of the surface of the SRO/BTO/SRO/STO (001) hetero-structure with 6 nm thick BTO layer. (b) TEM image of the cross-section of the SRO/BTO/SRO/STO (001) hetero-structure with 5 nm thick BTO layer. (c) A line profile along the black line in the AFM image of (a).
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obtained for all the hetero-structures with various values of t BTO from 30 nm to 5 nm. The c-axis lattice constants of the BTO layers are larger than the bulk value, due to the in-plane biaxial compressive stress. For the BTO layers with t BTO z 9 nm, the c-axis lattice constant is about 0.427 nm, which is about 6% larger than the bulk BTO value, i.e. 0.40335 nm. For the BTO layer with t BTO=6 nm, the value was reduced to 0.42400 nm, which is still about 5% larger than the bulk value. These results demonstrate that all of the hetero-structures should be nearly fully-strained by the STO substrates with elongated c-axes. In order to measure the electrical properties of the SRO/ BTO/SRO hetero-structures, we fabricated square-shaped capacitors with areas of 1010 Am2 by conventional photolithography and ion-milling processes. Polarization vs. electric field ( P–E) loops were measured at 2 kHz with an aixACCT TF Analyzer 2000. The details of the heterostructure fabrication and film characterization were presented elsewhere.
3. Results and discussions Fig. 3(a) shows hysteresis loops for the BTO capacitors with t BTO of 6.5 and 15 nm. The P–E hysteresis loops provide direct evidence for the ferroelectricity of BTO thin films with t BTO down to 5 nm, which provides an experimental upper
Fig. 3. (a) Polarization vs. electric field ferroelectric hysteresis loops of the SRO/BTO/SRO capacitors with 15 nm (o), and 6.5 nm (n) BTO. (b) Normalized remnant polarization as a function of BTO thickness ranging from 30 to 5 nm is shown as filled squares. The solid line is the predicted function from the first-principle calculation by Junquera and Ghosez [2]. The dashed line is the converged value in the bulk region [1]. Coercive field as a function of BTO thickness ranging from 30 to 5 nm is shown as open circles.
Fig. 4. (a) I–V curve of SRO/BTO/SRO capacitor with BTO layer thickness 30 nm at room temperature. Inset shows polarization vs. electric field of SRO/BTO/SRO capacitor with BTO layer thickness 30 nm at room temperature. (b) I–V curve of SRO/BTO/SRO capacitor with BTO layer thickness 30 nm at 700 K. Inset shows polarization vs. electric field of SRO/BTO/SRO capacitor with BTO layer thickness 30 nm at 700 K.
bound for t c of 5 nm in the fully-strained SRO/BTO/SRO/ STO hetero-structure. Fig. 3(b) shows the t BTO-dependence of P r for our SRO/BTO/SRO hetero-structures. Note that the P r value was normalized to a converged value in the bulk region, i.e. to 31 AC/cm2. For the BTO film with t BTO=30 nm, the P r value is about 36 AC/cm2, which is larger than converged value in bulk region. The difference of about 15% between the P r values of our films and the theoretical value would have originated from the inaccuracy of the firstprinciple simulations in the thick region of t BTO or the possible existence of an extra elongation of the c-axis caused by oxygen vacancies in our films [8]. In Fig. 3(b), it is easy to see the thickness scaling effect on P r in the ultra-thin region of t BTO. Our experimental data, denoted by filled squares, clearly show strong t BTO-dependence in the region of t BTO b20 nm. We will compare the t BTO-dependence of P r for our BTO capacitors with a theoretical prediction. First-principle calculations, shown as the solid line in Fig. 3(b), can describe our t BTO-dependent scaling of P r in our BTO capacitors quite well. As shown in Fig. 3(b), the coercive field is almost independent of BTO layer thickness, ranging from 30 nm to 5 nm. According to Tagantsev’s model, interfacial dielectric layers between the ferroelectric layer and the electrodes result in the coercive field becoming thickness-dependent, so that the coercive field is inversely proportional to the thickness of the ferroelectric layer [9]. Our samples seem not to have these interfacial dielectric layers, so that the
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thickness-dependence of the coercive field does not appear. This may be evidence of perfect interfacial control in our samples. Resistivity hysteresis was reported in the I–V curve of ferroelectric capacitors in the ultra-thin film region [7]. According to the report, in ultra-thin film region, ferroelectric capacitors did not show P–E ferroelectric hysteresis loops but they showed that phonon-assisted inelastic tunneling had started to occur, which is related to resistivity hysteresis in the I–V curve. In particular, resistivity changes in a specific electric field, which is estimated to the coercive field. It demonstrated that the resistive switching behavior is a new phenomenon of ultra-thin capacitors. We carried out experiments to verify whether or not the polarization-related resistive switching occurs in an ultra-thin BTO capacitor, which is too thin to show a P–E hysteresis loop, due to leakage current, but was expected to have ferroelectric properties. In our ultra-thin capacitors that showed good hysteresis loops, we measured the current response after applying a triangular voltage wave to 1010 Am2 capacitors, with t BTO from 30 nm to 5 nm. As shown in Fig. 4, in the case of t BTO=30 nm capacitor, which showed a good P–E hysteresis loop, the magnitude of response current is smaller than Rodriguez’s result. Just after electric breakdown (5 mV, 10 kHz wave at 700 K), the magnitude of response current becomes larger than that of the original state and it shows similar behaviors to the polarization-related resistive switching, which is quite similar to the results of Rodriguez et al. [7]. In contrast to Rodriguez’s assertion, we observed the resistivity hysteresis behavior in capacitors with t BTO between 30 nm and 5 nm, after electric breakdown. This behavior is not limited to the ultra-thin film region. This suggested that the reported polarization-related resistive switching might not be due to intrinsic properties, such as phonon-assisted inelastic tunneling, but due to extrinsic effects, such as defects or charge traps related to electric breakdown or high leakage current.
4. Conclusions Fully-strained SrRuO3 (SRO)/BaTiO3 (BTO)/SRO hetero-structures with ultra-thin BTO layers from 30 nm to 5
nm were fabricated on SrTiO3 substrates by pulsed laser deposition. Epitaxial relation and strains induced by the substrates were observed in the X-ray reciprocal spacemapping. Well-defined interfaces between the ferroelectric BTO layer and the electrode SRO layer were confirmed by TEM images and the thickness-independence of the coercive fields. The ferroelectric hysteresis loop was observed in a 5 nm thick BTO layer capacitor of area 1010 Am2 fabricated by conventional photolithography and ion-milling processes. The decrease of ferroelectric polarization with decreasing BTO thickness was observed and agreed well with a theoretical prediction. Resistive switching behavior was not observed in the thinnest films. However, resistive switching behaviors were observed even in thick films, in which the BTO layer thickness is 30 nm, just after electric breakdown process. This resistive hysteretic behavior can be attributed to extrinsic defects.
Acknowledgements This work was financially supported by the Korean Ministry of Science and Technology through the Creative Research Initiative program and by KOSEF through CSCMR.
References [1] Zhongqing Wu, Ningdong Huang, Zhirong Liu, Jian Wu, Wenhui Duan, Bing-Lin Gu, Xiao-Wen Zhang, Phys. Rev., B 70 (2004) 104108. [2] J. Junquera, Ph. Ghosez, Nature 422 (2003) 506. [3] D.D. Fong, G.B. Stephenson, S.K. Streiffer, J.A. Eastman, O. Auciello, P.H. Fuoss, C. Thompson, Science 304 (2004) 1650. [4] T. Tybell, C.H. Ahn, J.-M. Triscone, Appl. Phys. Lett. 75 (1999) 856. [5] V. Nagarajan, S. Prasertchoung, T. Zhao, H. Zheng, J. Ouyang, R. Ramesh, W. Tian, X.Q. Pan, D.M. Kim, C.B. Eom, H. Kohlstedt, R. Waser, Appl. Phys. Lett. 84 (2004) 5225. [6] N. Yanase, K. Abe, N. Fukushima, T. Kawakubo, Jpn. J. Appl. Phys. 38 (1999) 5305. [7] J. Rodriguez Contreras, H. Kohlstedt, U. Poppe, R. Waser, C. Buchal, N.A. Pertsev, Appl. Phys. Lett. 83 (2003) 4595. [8] Y.S. Kim, D.H. Kim, J.D. Kim, Y.J. Chang, T.W. Noh, J.H. Hong, K. Char, Y.D. Park, S.D. Bu, J.-G. Yoon, J.S. Jung, Appl. Phys. Lett. (in press). [9] A.K. Tagantsev, I.A. Stolichnov, Appl. Phys. Lett. 73 (1999) 1326.