- Email: [email protected]
LaNiO3 superlattices
Scripta Materialia 179 (2020) 102–106
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Significantly enhanced ferroelectric and dielectric properties in BaTiO3 /LaNiO3 superlattices Jun Liang Lin a,b, Zhan Jie Wang a,b,c,∗, Xiang Zhao a, Zhi Dong Zhang b a
School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China c School of Material Science and Engineering, Shenyang University of Technology, Shenyang 110870, China b
a r t i c l e
i n f o
Article history: Received 27 July 2019 Revised 7 December 2019 Accepted 8 January 2020 Available online 20 January 2020 Keywords: BaTiO3 Superlattice Laser deposition Microstructure Electrical properties
a b s t r a c t BaTiO3 /LaNiO3 (BTO/LNO) ferroelectric superlattices with different stacking periods have been prepared by pulsed laser deposition (PLD). Compared to the pure BTO films, the BTO/LNO superlattices have significantly enhanced ferroelectric and dielectric properties. This is because, in addition to the strain effect, the accumulated oxygen vacancies in each ultra-thin LNO layer can change the depolarization field and reduce the leakage current in the superlattices. The results indicate that the use of ultra-thin metallic oxide layers as the constituent material of the ferroelectric superlattices is a feasible and effective way to improve the properties of superlattices.
Ferroelectric thin films have been widely applied in engineering applications such as nonvolatile ferroelectric random access memories (NV-FRAM), ferroelectric field effect transistors (FeFET) and micro-electromechanical systems (MEMS) [1–3]. In recent years, with the rapid development of thin film preparation technology, great progress has been made in the study of the structure and properties of ferroelectric superlattices. Many special phenomena and new functional characteristics have been found in ferroelectric superlattices, such as the switchable polar spirals [4], the negative capacitance [5], the improper ferroelectricity [6], and the enhanced electrical properties [7,8]. In ferroelectric superlattices, the interface effects are crucial to the electrical performances because of the large number of artificial heterointerfaces with the specific chemical composition and microstructure. Up to now, several kinds of ferroelectric superlattices, such as ferroelectric/paraelectric, ferroelectric/ferroelectric and ferroelectric/metallic superlattices, have been studied. However, most studies have focused on the first two superlattice systems. The results show that the enhanced electrical properties of ferroelectric superlattices can be achieved by strain effect, which is caused by the different crystal structures or large lattice mismatch of the constituent materials. There is a common problem in the ferroelectric superlattices, that is, the existence of depolarization fields induced by the discontinuity of polariza∗ Corresponding author at: Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China. E-mail addresses: [email protected], [email protected] (Z.J. Wang).
https://doi.org/10.1016/j.scriptamat.2020.01.010 1359-6462/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
tion between constituent materials [9,10]. The depolarization fields have a negative effect on the electrical properties of ferroelectric superlattices. For the ferroelectric/metallic superlattices, metallic oxide layers are usually used as electrodes of ferroelectric films, so metallic oxide layers as constituent materials are expected to play a charge compensation role to reduce the depolarization field. However, it should be noted that metallic oxide layers with high conductivity may increase the leakage current of ferroelectric superlattices. It has been reported that BaTiO3 /LaNiO3 superlattices have an obvious enhancement in dielectric properties by controlling the stacking periodicity [11,12], but the ferroelectric properties may not be obtained in measurement due to the large leakage current. Therefore, the research on ferroelectric/metallic superlattice system is not deep enough. In recent years, it has been demonstrated that the metal-insulator transition (MIT) can occur in the ultrathin LNO film, and the electrical conductivity of ultrathin LNO film can be significantly reduced when the thickness decreases below 5 unit cells [13,14]. Therefore, when the metallic oxide layers are ultrathin, the leakage current in the ferroelectric/metallic superlattice can be expected to be reduced to obtain measurable ferroelectric properties. In this study, we attempt to prepare BaTiO3 /LaNiO3 (BTO/LNO) ferroelectric/metallic superlattices with different LNO thicknesses ranging from 1 to 5 unit cells by PLD, and study the effect of LNO thickness on the ferroelectric properties. The experimental results show that significantly enhanced ferroelectric polarization and dielectric constant are obtained in the BTO/LNO superlattices.
J.L. Lin, Z.J. Wang and X. Zhao et al. / Scripta Materialia 179 (2020) 102–106
103
Fig. 1. (a) A cross-sectional TEM image and (b) a high-resolution cross-sectional TEM image of the BTO30 /LNO2 superlattice. The inset in b shows the enlarged interface area of the box. (c) The Fast Fourier Transform (FFT) patterns transformed from the HRTEM image.
Fig. 2. (a) XRD patterns of the BTO/LNO superlattices with the LNO thickness of 2 and 3 unit cells and the pure BTO film, (b) enlarged XRD patterns around the (002) diffraction peak. (c) The average out-of-plane lattice parameters (cav ) and the calculated out-of-plane lattice parameters of the BTO layers (cal-cBTO ) in the BTO/LNO superlattices.
Asymmetric BTO30 /LNOx (where x = 1 to 5 unit cells) superlattices were grown on (001)-oriented Nb-doped SrTiO3 single-crystal substrates (Nb:SrTiO3 with 0.7 wt% of Nb, abbreviated as NSTO) by PLD. The BTO/LNO superlattices were deposited using a KrF excimer laser (λ = 248 nm) with a repetition rate of 5 Hz and a laser energy density of about 1.4 J/cm2 . The BTO and LNO layers were alternately deposited using two separated targets at a substrate temperature of 750 °C under an oxygen pressure of 15 Pa. The total superlattice thickness was fixed at about 300 nm. In addition, a BTO (15 nm)/LNO (30 nm) bilayer film was grown on the NSTO (001) substrate under the same growth conditions for the Xray photoelectron spectroscopy (XPS) measurement. Crystalline structure of the BTO/LNO superlattices was analyzed by X-ray diffraction (XRD; D/max-20 0 0, Rigaku). The crosssectional microstructures were characterized by transmission electron microscopy (TEM; Tecnai G2 F20, FEI). The valence state of the elements across the BTO/LNO interface was determined by Xray photoelectron spectroscopy (XPS; ESCALAB 250, Therma, Al Kα source, 1486.60 eV, energy step: 0.1 eV, resolution: 400 meV). Pt films (diameter: 0.5 mm) were prepared by sputtering as top electrodes of the superlattices for electrical measurements. Electrical properties were measured by using a standard ferroelectric testing system (TF20 0 0E; Aixacct) and an impedance analyzer LCR Hitester (HIOKI 3532-50). The microstructures of the BTO/LNO superlattices have been characterized by TEM. For instance, Fig. 1(a) shows a cross-
sectional TEM image of the BTO30 /LNO2 superlattice with an overall thickness of about 300 nm. It is clearly exhibited that a layered structure with periodically alternating BTO and LNO layers is well formed in the superlattice. The high-resolution TEM (HRTEM) image (Fig. 1(b)) shows that the BTO and LNO layers are epitaxially grown on each other with flat and sharp interfaces. Besides, by enlarging the interface area of the box in Fig. 1(b), it can be seen that the thickness of the LNO layer is two unit cells, as shown in the inset of Fig. 1(b). The Fast Fourier Transform (FFT) patterns transformed from the HRTEM image can also confirm the epitaxial relationship between BTO and LNO layers in the superlattices, as shown in Fig. 1(c). The elongated diffraction spots of the BTO/LNO superlattices along the out-of-plane direction are shown in the inset in Fig. 1(c), which also exhibits the typical structural characteristics of the superlattices. Fig. 2(a) shows XRD patterns of the BTO/LNO superlattices with the LNO thickness of 2 and 3 unit cells and the pure BTO film. Only strong (00l) diffraction peaks can be observed in these samples, implying that the pure BTO films and the BTO/LNO superlattices are epitaxially grown on the NSTO(001) substrates. Fig. 2(b) shows the enlarged XRD patterns around the (002) diffraction peak, and the structural characteristic of the superlattices is also represented by the appearance of symmetrically distributed satellite peaks (denoted as ±1, ±2, …) around the (002) main peak (labeled as “0”). In addition, it can be seen that the position of the main peaks shifts toward lower angles slightly with increasing the
104
J.L. Lin, Z.J. Wang and X. Zhao et al. / Scripta Materialia 179 (2020) 102–106
Fig. 3. (a) P–E hysteresis loops of the pure BTO film and the BTO/LNO superlattices with the LNO thickness of 2 and 3 unit cells. (b) Dielectric constant as a function of the frequency for the pure BTO film and the BTO/LNO superlattices with the LNO thickness of 2 and 3 unit cells. (c) Leakage current density (J) as a function of the electric field (E) of the pure BTO film and the BTO/LNO superlattices with different LNO thicknesses. (d) The ln J vs ln E curves for the pure BTO film and the BTO30 /LNO3 superlattice at the positive bias (the slope values of the fitted lines are marked), the inset is the P–F fit for the pure BTO film.
LNO thickness, and the average out-of-plane lattice parameter (cav ) of each sample can be calculated according to the corresponding main peak position. The cav of the pure BTO film, the BTO30 /LNO2 superlattice and the BTO30 /LNO3 superlattice is 4.0 01, 4.0 03 and ˚ respectively. The lattice mismatching between bulk BTO 4.006 A, ˚ and LNO (apc = 3.86 A) ˚ is about 3.3%, so the BTO lay(a = 3.99 A) ers are in the in-plane compressive strain state, and the LNO layers are in the in-plane tensile strain state in the superlattices. In other words, the out-of-plane lattice parameter of the BTO layers and the LNO layers should be increased and reduced, respectively. To simplify the analysis, assuming that the out-of-plane lattice parameter of the LNO layers remains unchanged, the out-of-plane lattice parameter of the BTO layers (cal-cBTO ) in the superlattices can also be approximately calculated by the formula: cav = (1 - x) cBTO + x cLNO , where x = n/(30 + n). As shown in Fig. 2(c), the cal-cBTO for ˚ the BTO30 /LNO2 and BTO30 /LNO3 superlattices is 4.013 and 4.021 A, respectively, which is increased compared with the cav of the pure BTO film, indicating a larger tetragonality (c/a) of the BTO layers. The above results of TEM and XRD analysis show that we have obtained BTO/LNO superlattices that meet our design requirements. Fig. 3(a) shows the P–E hysteresis loops of the BTO/LNO superlattices with the LNO thickness of 2 and 3 unit cells and the pure BTO film measured at room temperature. It can be seen that the polarization properties of the BTO/LNO superlattices increase as the thickness of the LNO layers increases from 2 to 3 unit cells. And the remanent polarization (Pr ) of the BTO30 /LNO3 superlattice can reach 13.04 μC/cm2 , which shows a significant enhancement of the ferroelectric polarization compared to the pure BTO film (Pr = 5.41 μC/cm2 ). However, when the thickness of the LNO
layers is further increased to 4 unit cells, the increased conductivity of the LNO layers will lead to an abnormal shape of the hysteresis loop. Fig. 3(b) shows that there is also a large enhancement of the dielectric properties in the BTO/LNO superlattices. The dielectric constant measured at 1 kHz for the pure BTO film and the BTO30 /LNO3 superlattice is 483 and 917, respectively. Obviously, the LNO layers in the superlattices have a great influence on the electrical properties. Considering of the different physical characteristics between the LNO and the BTO layers, the effect of the mismatch strain may not be the only factor to improve the electrical properties. To further investigate the enhancement mechanism of the electrical properties, we have measured the leakage current density of the BTO/LNO superlattices with different thicknesses of LNO and the pure BTO film. Fig. 3(c) shows the leakage current density (J)−electric field (E) curves of the pure BTO film and the BTO/LNO superlattices. With increasing the thickness of the LNO layers from 1 unit cell to 5 unit cells, the leakage current density of the BTO/LNO superlattices decreases at first and then increases compared to the pure BTO films. For the BTO30 /LNO3 superlattice, the leakage current density can be reduced by about two orders of magnitude. Apparently, the magnitude of leakage current is related to the thickness of the LNO layers. It has been reported that the electrical conductivity of LNO film decreases with the decrease of the thickness, especially when the thickness is about 3 unit cells, the metal-insulating transition will occur [14]. Therefore, when the thickness of the LNO layers in the BTO/LNO superlattices is more than 3 unit cells, the relatively large conductivity will cause the large leakage current. Fig. 3(d) shows the lnJ vs lnE curves for the BTO30 /LNO3 superlattice and the pure BTO film at the positive bias.
J.L. Lin, Z.J. Wang and X. Zhao et al. / Scripta Materialia 179 (2020) 102–106
105
Fig. 4. (a) O 1s and (b) Ni 2p XPS spectra across the BTO/LNO interface of BTO(15 nm)/LNO(30 nm) bilayer thin film.
For the BTO30 /LNO3 superlattice, the slope of the curve is about 1 at low electric fields, indicating that the leakage current in the BTO30 /LNO3 superlattice is in agreement with Ohm’s law. However, with the increase of electric field intensity, the slope increase to about 2.6, which is in accordance with the trap-controlled space charge limited current (SCLC) mechanism [15]. The slope of a lnJ– lnE plot is a fitting parameter related with the characteristic energy of trap distribution, and it has been shown that the slope is usually between 2 and 3 when the leakage behavior is dominated by the interfaces accompanied with accumulated defects in composite films [15–17]. For the pure BTO film, the slope of the curve is about 1 at low electric fields and is about 3.3 with the increase of electric field intensity. The slope value of 3.3 in the pure BTO film indicates that the leakage behavior is different from that of the BTO/LNO superlattices. The leakage current of the pure BTO film can be further fitted by the Poole–Frenkel (P–F) emission mechanism (see the inset of Fig. 3(d)), which is a common bulk-limited leakage mechanism of dielectric materials [18]. Based on the above results, it can be suggested that the distribution of charged defects in the BTO/LNO superlattices is different from that in the pure BTO film. In the BTO/LNO superlattices, the charged defects may mainly concentrate at the BTO/LNO interfaces, which can act as potential wells to trap charge carriers passing through the BTO/LNO interfaces, thus reducing leakage current during conduction process. To investigate the possible charged defects in the BTO/LNO superlattices, we have prepared BTO(15 nm)/LNO(30 nm) bilayer films, and determined the valence states of elements across the BTO/LNO interface by XPS during the etching process. In the process of measurement, the analysis region gradually approached the interface, and the etching rate was about 0.1 nm/s. Fig. 4(a) shows the O 1 s XPS spectra with different etching times. The O 1s spectra can be deconvoluted into two signals: the low binding energy peak is assigned to lattice oxygen at around 529.9 eV and the high binding energy peak is assigned to oxygen vacancies (Vo ) at around
532 eV [19]. When the analysis position is close to the BTO/LNO interface region (150 s), the peak intensity of Vo begins to increase significantly and is stronger in the LNO layer. In addition, Fig. 4(b) shows that not only the Ni3+ 2p peaks but also the Ni2+ 2p peaks can be observed in the LNO layer. The appearance of Ni2+ state is accompanied by the formation of oxygen vacancies, which may be due to the thermodynamic instability of Ni3+ state in the crystallization process [20]. The oxygen-deficient LNO layers may possess the slightly modified lattice parameters but the same pseudocubic perovskite structure, as compared with the bulk LNO [21]. Although the XPS results are obtained in the BTO/LNO bilayer film, the distribution of vacancy defects in the BTO/LNO superlattices should be similar to these results. Therefore, in the BTO/LNO superlattices, the oxygen vacancies accumulated in each ultra-thin LNO layer can change the depolarization field and reduce the leakage current. In summary, BTO/LNO ferroelectric/metallic superlattices with different LNO thicknesses have been prepared by pulsed laser deposition. The experimental results showed that significantly enhanced ferroelectric polarization and dielectric constant are observed in the BTO/LNO superlattices compared to the pure BTO films. This is due to that in addition to the strain effect, the accumulated oxygen vacancies in each ultra-thin LNO layers could modify the depolarization field and reduce the leakage current in the superlattices, resulting the enhanced electrical properties. Therefore, the use of ultra-thin metal oxide layers as the constituent material of ferroelectric superlattices is a new and effective method to improve the properties of superlattices. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
106
J.L. Lin, Z.J. Wang and X. Zhao et al. / Scripta Materialia 179 (2020) 102–106
Acknowledgments This work was supported by the Key Research and Development Plan of Liaoning Province (No. 2017104002), the Basic Scientific Research Projects of Colleges and Universities of Liaoning Province of China (No. LZGD2017005), the major project of Industrial Technology Research Institute of Liaoning Colleges and Universities (No. 201824010) and the National Basic Research Program of China (No. 2017YFA0206302). References [1] J.F. Scott, Science 315 (2007) 954–959. [2] H. Mulaosmanovic, T. Mikolajick, S. Slesazeck, ACS Appl. Mater. Interfaces 10 (2018) 23997–24002. [3] I. Kanno, Jpn. J. Appl. Phys. 57 (2018) 040101. [4] Z.J. Hong, L.Q. Chen, Acta Mater. 164 (2019) 493–498. [5] W. Gao, A. Khan, X. Marti, C. Nelson, C. Serrao, J. Ravichandran, R. Ramesh, S. Salahuddin, Nano Lett. 14 (2014) 5814–5819. [6] E. Bousquet, M. Dawber, N. Stucki, C. Lichtensteiger, P. Hermet, S. Gariglio, J.M. Triscone, P. Ghosez, Nature 452 (2008) 732–736.
[7] M. Benyoussef, J. Belhadi, A. Lahmar, M.E. Marssi, Mater. Lett. 234 (2019) 279–282. [8] Y. Bastani, N. Bassiri-Gharb, Acta Mater. 60 (2012) 1346–1352. [9] M. Dawber, E. Bousquet, MRS Bull. 38 (2013) 1048–1055. [10] K.G. Lim, K.H. Chew, D.Y. Wang, L.H. Ong, M. Iwata, Europhys. Lett. 108 (2014) 67011. [11] J.Y. Tseng, T.B. Wu, Mater. Chem. Phys. 88 (2004) 433–437. [12] Y.C. Liang, T.B. Wu, H.Y. Lee, Y.W. Hsieh, J. App. Phys. 96 (2004) 584–589. [13] R. Scherwitzl, S. Gariglio, M. Gabay, P. Zubko, M. Gibert, J.M. Triscone, Phys. Rev. Lett. 106 (2011) 246403. [14] E. Sakai, M. Tamamitsu, K. Yoshimatsu, S. Okamoto, K. Horiba, M. Oshima, H. Kumigashira, Phys. Rev. B 87 (2013) 075132. [15] W.W. Li, R. Zhao, R.J. Tang, A.P. Chen, W.R. Zhang, X. Lu, H.Y. Wang, H. Yang, ACS Appl. Mater. Interfaces 6 (2014) 5356–5361. [16] B. He, Z.J. Wang, ACS Appl. Mater. Interfaces 8 (2016) 6736–6742. [17] J.L. Lin, Z.J. Wang, X. Zhao, W. Liu, Z.D. Zhang, Ceram. Int. 44 (2018) 20664–20670. [18] F.C. Chiu, Adv. Mater. Sci. Eng. 2014 (2014) 578168. [19] S.A. Ansari, M.M. Khan, M.O. Ansari, S. Kalathil, J. Lee, M.H. Cho, RSC Adv. 4 (2014) 16782–16791. [20] K. Tsubouchi, I. Ohkubo, H. Kumigashira, Y. Matsumoto, T. Ohnishi, M. Lippmaa, H. Koinuma, M. Oshima, Appl. Phys. Lett. 92 (2008) 262109. [21] L. Qiao, X. Bi, Thin Solid Films 519 (2010) 943–946.
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Significantly enhanced ferroelectric and dielectric properties in BaTiO3 /LaNiO3 superlattices Jun Liang Lin a,b, Zhan Jie Wang a,b,c,∗, Xiang Zhao a, Zhi Dong Zhang b a
School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China c School of Material Science and Engineering, Shenyang University of Technology, Shenyang 110870, China b
a r t i c l e
i n f o
Article history: Received 27 July 2019 Revised 7 December 2019 Accepted 8 January 2020 Available online 20 January 2020 Keywords: BaTiO3 Superlattice Laser deposition Microstructure Electrical properties
a b s t r a c t BaTiO3 /LaNiO3 (BTO/LNO) ferroelectric superlattices with different stacking periods have been prepared by pulsed laser deposition (PLD). Compared to the pure BTO films, the BTO/LNO superlattices have significantly enhanced ferroelectric and dielectric properties. This is because, in addition to the strain effect, the accumulated oxygen vacancies in each ultra-thin LNO layer can change the depolarization field and reduce the leakage current in the superlattices. The results indicate that the use of ultra-thin metallic oxide layers as the constituent material of the ferroelectric superlattices is a feasible and effective way to improve the properties of superlattices.
Ferroelectric thin films have been widely applied in engineering applications such as nonvolatile ferroelectric random access memories (NV-FRAM), ferroelectric field effect transistors (FeFET) and micro-electromechanical systems (MEMS) [1–3]. In recent years, with the rapid development of thin film preparation technology, great progress has been made in the study of the structure and properties of ferroelectric superlattices. Many special phenomena and new functional characteristics have been found in ferroelectric superlattices, such as the switchable polar spirals [4], the negative capacitance [5], the improper ferroelectricity [6], and the enhanced electrical properties [7,8]. In ferroelectric superlattices, the interface effects are crucial to the electrical performances because of the large number of artificial heterointerfaces with the specific chemical composition and microstructure. Up to now, several kinds of ferroelectric superlattices, such as ferroelectric/paraelectric, ferroelectric/ferroelectric and ferroelectric/metallic superlattices, have been studied. However, most studies have focused on the first two superlattice systems. The results show that the enhanced electrical properties of ferroelectric superlattices can be achieved by strain effect, which is caused by the different crystal structures or large lattice mismatch of the constituent materials. There is a common problem in the ferroelectric superlattices, that is, the existence of depolarization fields induced by the discontinuity of polariza∗ Corresponding author at: Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China. E-mail addresses: [email protected], [email protected] (Z.J. Wang).
https://doi.org/10.1016/j.scriptamat.2020.01.010 1359-6462/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
tion between constituent materials [9,10]. The depolarization fields have a negative effect on the electrical properties of ferroelectric superlattices. For the ferroelectric/metallic superlattices, metallic oxide layers are usually used as electrodes of ferroelectric films, so metallic oxide layers as constituent materials are expected to play a charge compensation role to reduce the depolarization field. However, it should be noted that metallic oxide layers with high conductivity may increase the leakage current of ferroelectric superlattices. It has been reported that BaTiO3 /LaNiO3 superlattices have an obvious enhancement in dielectric properties by controlling the stacking periodicity [11,12], but the ferroelectric properties may not be obtained in measurement due to the large leakage current. Therefore, the research on ferroelectric/metallic superlattice system is not deep enough. In recent years, it has been demonstrated that the metal-insulator transition (MIT) can occur in the ultrathin LNO film, and the electrical conductivity of ultrathin LNO film can be significantly reduced when the thickness decreases below 5 unit cells [13,14]. Therefore, when the metallic oxide layers are ultrathin, the leakage current in the ferroelectric/metallic superlattice can be expected to be reduced to obtain measurable ferroelectric properties. In this study, we attempt to prepare BaTiO3 /LaNiO3 (BTO/LNO) ferroelectric/metallic superlattices with different LNO thicknesses ranging from 1 to 5 unit cells by PLD, and study the effect of LNO thickness on the ferroelectric properties. The experimental results show that significantly enhanced ferroelectric polarization and dielectric constant are obtained in the BTO/LNO superlattices.
J.L. Lin, Z.J. Wang and X. Zhao et al. / Scripta Materialia 179 (2020) 102–106
103
Fig. 1. (a) A cross-sectional TEM image and (b) a high-resolution cross-sectional TEM image of the BTO30 /LNO2 superlattice. The inset in b shows the enlarged interface area of the box. (c) The Fast Fourier Transform (FFT) patterns transformed from the HRTEM image.
Fig. 2. (a) XRD patterns of the BTO/LNO superlattices with the LNO thickness of 2 and 3 unit cells and the pure BTO film, (b) enlarged XRD patterns around the (002) diffraction peak. (c) The average out-of-plane lattice parameters (cav ) and the calculated out-of-plane lattice parameters of the BTO layers (cal-cBTO ) in the BTO/LNO superlattices.
Asymmetric BTO30 /LNOx (where x = 1 to 5 unit cells) superlattices were grown on (001)-oriented Nb-doped SrTiO3 single-crystal substrates (Nb:SrTiO3 with 0.7 wt% of Nb, abbreviated as NSTO) by PLD. The BTO/LNO superlattices were deposited using a KrF excimer laser (λ = 248 nm) with a repetition rate of 5 Hz and a laser energy density of about 1.4 J/cm2 . The BTO and LNO layers were alternately deposited using two separated targets at a substrate temperature of 750 °C under an oxygen pressure of 15 Pa. The total superlattice thickness was fixed at about 300 nm. In addition, a BTO (15 nm)/LNO (30 nm) bilayer film was grown on the NSTO (001) substrate under the same growth conditions for the Xray photoelectron spectroscopy (XPS) measurement. Crystalline structure of the BTO/LNO superlattices was analyzed by X-ray diffraction (XRD; D/max-20 0 0, Rigaku). The crosssectional microstructures were characterized by transmission electron microscopy (TEM; Tecnai G2 F20, FEI). The valence state of the elements across the BTO/LNO interface was determined by Xray photoelectron spectroscopy (XPS; ESCALAB 250, Therma, Al Kα source, 1486.60 eV, energy step: 0.1 eV, resolution: 400 meV). Pt films (diameter: 0.5 mm) were prepared by sputtering as top electrodes of the superlattices for electrical measurements. Electrical properties were measured by using a standard ferroelectric testing system (TF20 0 0E; Aixacct) and an impedance analyzer LCR Hitester (HIOKI 3532-50). The microstructures of the BTO/LNO superlattices have been characterized by TEM. For instance, Fig. 1(a) shows a cross-
sectional TEM image of the BTO30 /LNO2 superlattice with an overall thickness of about 300 nm. It is clearly exhibited that a layered structure with periodically alternating BTO and LNO layers is well formed in the superlattice. The high-resolution TEM (HRTEM) image (Fig. 1(b)) shows that the BTO and LNO layers are epitaxially grown on each other with flat and sharp interfaces. Besides, by enlarging the interface area of the box in Fig. 1(b), it can be seen that the thickness of the LNO layer is two unit cells, as shown in the inset of Fig. 1(b). The Fast Fourier Transform (FFT) patterns transformed from the HRTEM image can also confirm the epitaxial relationship between BTO and LNO layers in the superlattices, as shown in Fig. 1(c). The elongated diffraction spots of the BTO/LNO superlattices along the out-of-plane direction are shown in the inset in Fig. 1(c), which also exhibits the typical structural characteristics of the superlattices. Fig. 2(a) shows XRD patterns of the BTO/LNO superlattices with the LNO thickness of 2 and 3 unit cells and the pure BTO film. Only strong (00l) diffraction peaks can be observed in these samples, implying that the pure BTO films and the BTO/LNO superlattices are epitaxially grown on the NSTO(001) substrates. Fig. 2(b) shows the enlarged XRD patterns around the (002) diffraction peak, and the structural characteristic of the superlattices is also represented by the appearance of symmetrically distributed satellite peaks (denoted as ±1, ±2, …) around the (002) main peak (labeled as “0”). In addition, it can be seen that the position of the main peaks shifts toward lower angles slightly with increasing the
104
J.L. Lin, Z.J. Wang and X. Zhao et al. / Scripta Materialia 179 (2020) 102–106
Fig. 3. (a) P–E hysteresis loops of the pure BTO film and the BTO/LNO superlattices with the LNO thickness of 2 and 3 unit cells. (b) Dielectric constant as a function of the frequency for the pure BTO film and the BTO/LNO superlattices with the LNO thickness of 2 and 3 unit cells. (c) Leakage current density (J) as a function of the electric field (E) of the pure BTO film and the BTO/LNO superlattices with different LNO thicknesses. (d) The ln J vs ln E curves for the pure BTO film and the BTO30 /LNO3 superlattice at the positive bias (the slope values of the fitted lines are marked), the inset is the P–F fit for the pure BTO film.
LNO thickness, and the average out-of-plane lattice parameter (cav ) of each sample can be calculated according to the corresponding main peak position. The cav of the pure BTO film, the BTO30 /LNO2 superlattice and the BTO30 /LNO3 superlattice is 4.0 01, 4.0 03 and ˚ respectively. The lattice mismatching between bulk BTO 4.006 A, ˚ and LNO (apc = 3.86 A) ˚ is about 3.3%, so the BTO lay(a = 3.99 A) ers are in the in-plane compressive strain state, and the LNO layers are in the in-plane tensile strain state in the superlattices. In other words, the out-of-plane lattice parameter of the BTO layers and the LNO layers should be increased and reduced, respectively. To simplify the analysis, assuming that the out-of-plane lattice parameter of the LNO layers remains unchanged, the out-of-plane lattice parameter of the BTO layers (cal-cBTO ) in the superlattices can also be approximately calculated by the formula: cav = (1 - x) cBTO + x cLNO , where x = n/(30 + n). As shown in Fig. 2(c), the cal-cBTO for ˚ the BTO30 /LNO2 and BTO30 /LNO3 superlattices is 4.013 and 4.021 A, respectively, which is increased compared with the cav of the pure BTO film, indicating a larger tetragonality (c/a) of the BTO layers. The above results of TEM and XRD analysis show that we have obtained BTO/LNO superlattices that meet our design requirements. Fig. 3(a) shows the P–E hysteresis loops of the BTO/LNO superlattices with the LNO thickness of 2 and 3 unit cells and the pure BTO film measured at room temperature. It can be seen that the polarization properties of the BTO/LNO superlattices increase as the thickness of the LNO layers increases from 2 to 3 unit cells. And the remanent polarization (Pr ) of the BTO30 /LNO3 superlattice can reach 13.04 μC/cm2 , which shows a significant enhancement of the ferroelectric polarization compared to the pure BTO film (Pr = 5.41 μC/cm2 ). However, when the thickness of the LNO
layers is further increased to 4 unit cells, the increased conductivity of the LNO layers will lead to an abnormal shape of the hysteresis loop. Fig. 3(b) shows that there is also a large enhancement of the dielectric properties in the BTO/LNO superlattices. The dielectric constant measured at 1 kHz for the pure BTO film and the BTO30 /LNO3 superlattice is 483 and 917, respectively. Obviously, the LNO layers in the superlattices have a great influence on the electrical properties. Considering of the different physical characteristics between the LNO and the BTO layers, the effect of the mismatch strain may not be the only factor to improve the electrical properties. To further investigate the enhancement mechanism of the electrical properties, we have measured the leakage current density of the BTO/LNO superlattices with different thicknesses of LNO and the pure BTO film. Fig. 3(c) shows the leakage current density (J)−electric field (E) curves of the pure BTO film and the BTO/LNO superlattices. With increasing the thickness of the LNO layers from 1 unit cell to 5 unit cells, the leakage current density of the BTO/LNO superlattices decreases at first and then increases compared to the pure BTO films. For the BTO30 /LNO3 superlattice, the leakage current density can be reduced by about two orders of magnitude. Apparently, the magnitude of leakage current is related to the thickness of the LNO layers. It has been reported that the electrical conductivity of LNO film decreases with the decrease of the thickness, especially when the thickness is about 3 unit cells, the metal-insulating transition will occur [14]. Therefore, when the thickness of the LNO layers in the BTO/LNO superlattices is more than 3 unit cells, the relatively large conductivity will cause the large leakage current. Fig. 3(d) shows the lnJ vs lnE curves for the BTO30 /LNO3 superlattice and the pure BTO film at the positive bias.
J.L. Lin, Z.J. Wang and X. Zhao et al. / Scripta Materialia 179 (2020) 102–106
105
Fig. 4. (a) O 1s and (b) Ni 2p XPS spectra across the BTO/LNO interface of BTO(15 nm)/LNO(30 nm) bilayer thin film.
For the BTO30 /LNO3 superlattice, the slope of the curve is about 1 at low electric fields, indicating that the leakage current in the BTO30 /LNO3 superlattice is in agreement with Ohm’s law. However, with the increase of electric field intensity, the slope increase to about 2.6, which is in accordance with the trap-controlled space charge limited current (SCLC) mechanism [15]. The slope of a lnJ– lnE plot is a fitting parameter related with the characteristic energy of trap distribution, and it has been shown that the slope is usually between 2 and 3 when the leakage behavior is dominated by the interfaces accompanied with accumulated defects in composite films [15–17]. For the pure BTO film, the slope of the curve is about 1 at low electric fields and is about 3.3 with the increase of electric field intensity. The slope value of 3.3 in the pure BTO film indicates that the leakage behavior is different from that of the BTO/LNO superlattices. The leakage current of the pure BTO film can be further fitted by the Poole–Frenkel (P–F) emission mechanism (see the inset of Fig. 3(d)), which is a common bulk-limited leakage mechanism of dielectric materials [18]. Based on the above results, it can be suggested that the distribution of charged defects in the BTO/LNO superlattices is different from that in the pure BTO film. In the BTO/LNO superlattices, the charged defects may mainly concentrate at the BTO/LNO interfaces, which can act as potential wells to trap charge carriers passing through the BTO/LNO interfaces, thus reducing leakage current during conduction process. To investigate the possible charged defects in the BTO/LNO superlattices, we have prepared BTO(15 nm)/LNO(30 nm) bilayer films, and determined the valence states of elements across the BTO/LNO interface by XPS during the etching process. In the process of measurement, the analysis region gradually approached the interface, and the etching rate was about 0.1 nm/s. Fig. 4(a) shows the O 1 s XPS spectra with different etching times. The O 1s spectra can be deconvoluted into two signals: the low binding energy peak is assigned to lattice oxygen at around 529.9 eV and the high binding energy peak is assigned to oxygen vacancies (Vo ) at around
532 eV [19]. When the analysis position is close to the BTO/LNO interface region (150 s), the peak intensity of Vo begins to increase significantly and is stronger in the LNO layer. In addition, Fig. 4(b) shows that not only the Ni3+ 2p peaks but also the Ni2+ 2p peaks can be observed in the LNO layer. The appearance of Ni2+ state is accompanied by the formation of oxygen vacancies, which may be due to the thermodynamic instability of Ni3+ state in the crystallization process [20]. The oxygen-deficient LNO layers may possess the slightly modified lattice parameters but the same pseudocubic perovskite structure, as compared with the bulk LNO [21]. Although the XPS results are obtained in the BTO/LNO bilayer film, the distribution of vacancy defects in the BTO/LNO superlattices should be similar to these results. Therefore, in the BTO/LNO superlattices, the oxygen vacancies accumulated in each ultra-thin LNO layer can change the depolarization field and reduce the leakage current. In summary, BTO/LNO ferroelectric/metallic superlattices with different LNO thicknesses have been prepared by pulsed laser deposition. The experimental results showed that significantly enhanced ferroelectric polarization and dielectric constant are observed in the BTO/LNO superlattices compared to the pure BTO films. This is due to that in addition to the strain effect, the accumulated oxygen vacancies in each ultra-thin LNO layers could modify the depolarization field and reduce the leakage current in the superlattices, resulting the enhanced electrical properties. Therefore, the use of ultra-thin metal oxide layers as the constituent material of ferroelectric superlattices is a new and effective method to improve the properties of superlattices. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
106
J.L. Lin, Z.J. Wang and X. Zhao et al. / Scripta Materialia 179 (2020) 102–106
Acknowledgments This work was supported by the Key Research and Development Plan of Liaoning Province (No. 2017104002), the Basic Scientific Research Projects of Colleges and Universities of Liaoning Province of China (No. LZGD2017005), the major project of Industrial Technology Research Institute of Liaoning Colleges and Universities (No. 201824010) and the National Basic Research Program of China (No. 2017YFA0206302). References [1] J.F. Scott, Science 315 (2007) 954–959. [2] H. Mulaosmanovic, T. Mikolajick, S. Slesazeck, ACS Appl. Mater. Interfaces 10 (2018) 23997–24002. [3] I. Kanno, Jpn. J. Appl. Phys. 57 (2018) 040101. [4] Z.J. Hong, L.Q. Chen, Acta Mater. 164 (2019) 493–498. [5] W. Gao, A. Khan, X. Marti, C. Nelson, C. Serrao, J. Ravichandran, R. Ramesh, S. Salahuddin, Nano Lett. 14 (2014) 5814–5819. [6] E. Bousquet, M. Dawber, N. Stucki, C. Lichtensteiger, P. Hermet, S. Gariglio, J.M. Triscone, P. Ghosez, Nature 452 (2008) 732–736.
[7] M. Benyoussef, J. Belhadi, A. Lahmar, M.E. Marssi, Mater. Lett. 234 (2019) 279–282. [8] Y. Bastani, N. Bassiri-Gharb, Acta Mater. 60 (2012) 1346–1352. [9] M. Dawber, E. Bousquet, MRS Bull. 38 (2013) 1048–1055. [10] K.G. Lim, K.H. Chew, D.Y. Wang, L.H. Ong, M. Iwata, Europhys. Lett. 108 (2014) 67011. [11] J.Y. Tseng, T.B. Wu, Mater. Chem. Phys. 88 (2004) 433–437. [12] Y.C. Liang, T.B. Wu, H.Y. Lee, Y.W. Hsieh, J. App. Phys. 96 (2004) 584–589. [13] R. Scherwitzl, S. Gariglio, M. Gabay, P. Zubko, M. Gibert, J.M. Triscone, Phys. Rev. Lett. 106 (2011) 246403. [14] E. Sakai, M. Tamamitsu, K. Yoshimatsu, S. Okamoto, K. Horiba, M. Oshima, H. Kumigashira, Phys. Rev. B 87 (2013) 075132. [15] W.W. Li, R. Zhao, R.J. Tang, A.P. Chen, W.R. Zhang, X. Lu, H.Y. Wang, H. Yang, ACS Appl. Mater. Interfaces 6 (2014) 5356–5361. [16] B. He, Z.J. Wang, ACS Appl. Mater. Interfaces 8 (2016) 6736–6742. [17] J.L. Lin, Z.J. Wang, X. Zhao, W. Liu, Z.D. Zhang, Ceram. Int. 44 (2018) 20664–20670. [18] F.C. Chiu, Adv. Mater. Sci. Eng. 2014 (2014) 578168. [19] S.A. Ansari, M.M. Khan, M.O. Ansari, S. Kalathil, J. Lee, M.H. Cho, RSC Adv. 4 (2014) 16782–16791. [20] K. Tsubouchi, I. Ohkubo, H. Kumigashira, Y. Matsumoto, T. Ohnishi, M. Lippmaa, H. Koinuma, M. Oshima, Appl. Phys. Lett. 92 (2008) 262109. [21] L. Qiao, X. Bi, Thin Solid Films 519 (2010) 943–946.