Switching properties of epitaxial La 0.5 Sr 0.5 CoO 3 / Na 0.5 Bi 0.5 TiO 3 /La 0.5 Sr 0.5 CoO 3 ferroelectric capacitor

.


Introduction
Ferroelectric thin lm materials have been extensively investigated due to their good ferroelectric, dielectric, piezoelectric and optical properties for various kinds of applications, e.g. phase shiers, microelectromechanical systems, sensors, and ferroelectric random access memory (FeRAM). [1][2][3] Lead based Pb(Zr 1Àx Ti x )O 3 (PZT) lms are intensively studied worldwide including by our group due to their large remnant polarization and small coercive voltage. [4][5][6] However, PZT lms are not environmentally friendly materials, thus many efforts have been made to develop lead-free new type ferroelectric materials. 7,8 Sodium bismuth titanate, Na 0.5 Bi 0.5 TiO 3 (NBT), with A-site composite perovskite structure is considered as an excellent candidate to replace lead based ferroelectric materials. 9,10 Recently, we have successfully prepared a single-phase high quality NBT bulk material by solid state reaction method, and investigated the optical properties and ferroelectric properties. 11,12 The Na 0.5 Bi 0.5 TiO 3 ceramics have good ferroelectric properties at room temperature and relatively high Curie temperature (320 C), which makes ferroelectric NBT lm based device very promising. 13 Polycrystalline NBT lms with good ferroelectric remnant polarization ($11.9 mC cm À2 ) have been grown on Pt/Ti/SiO 2 /Si substrates using various methods such as radio-frequency magnetron sputtering and metal organic decomposition process. 14,15 Compared to the polycrystalline lms, it is believed that the highly oriented ferroelectric lms (especially the epitaxial lms) can possess higher polarization and less leakage current due to the lack of grain boundaries in the epitaxial lms. Highly (111) oriented NBT lm was prepared on Pt/Ti/SiO 2 /Si substrate by a sol-gel process, it is found that the remnant polarization and coercive eld are 20.6 mC cm À2 and 112 kV cm À1 , respectively. 16 (001) and (011) oriented epitaxial NBT lms have been successfully fabricated on platinum coated MgO and SrTiO 3 substrates by pulsed laser deposition, and very good dielectric and ferroelectric properties have been reported. 17,18 Compared to Pt electrode, the cheaper LSCO electrode can provide oxide/oxide (LSCO/NBT) interface, different from the metal/oxide (Pt/NBT) interface, which may further impact the NBT physical properties. From the viewpoint of our best knowledge, no results have been reported on the fabrication and characterization of epitaxial NBT capacitor sandwiched by oxide electrodes. In this paper, we report on the fabrication of the epitaxial LSCO/NBT/LSCO/SrTiO 3 (STO) heterostructure, and the structural and switching properties of the LSCO/NBT/LSCO ferroelectric capacitors have been systematically investigated.

Experimental
Briey, the Pt/LSCO/NBT/LSCO/STO heterostructure was prepared by a multi-step procedure.
Step 1: 60 nm-thick LSCO thin lm was deposited on (001) STO single crystal substrate by magnetron sputtering at room temperature under the following conditions: Ar : O 2 ¼ 3 : 1, power ¼ 30 W. Post-annealing was conducted at 550 C in a 1 atm oxygen-owing tube furnace.
Step 2: Na 0.5 Bi 0.5 TiO 3 target with 10% excessive bismuth and 10% sodium was used during deposition to compensate the loss of bismuth and sodium due to the strong volatility of bismuth and sodium. 250 nm-thick NBT lm was further deposited on the LSCO/STO heterostructure by pulsed laser deposition at 550 C and 7.5 Pa oxygen deposition pressure. The target and substrate distance was 4.5 cm; the laser energy density and repetition rate were 2 J cm À2 and 5 Hz, respectively. Step 3: 60 nm-thick LSCO lm and 70 nm thick Pt were sequentially deposited by magnetron sputtering on the surface of the NBT/LSCO/STO heterostructure through a shadow mask to get 7.85 Â 10 À5 cm 2 circular pads as the top electrodes of the capacitors. The Pt lm is used to improve the electrical contact between the measurement tip and LSCO. The Pt/LSCO/NBT/LSCO/STO heterostructure was further annealed at 550 C in an oxygen-owing tube furnace in order to make the as-grown top LSCO lm crystallized. The crystallinity and phase of the sample was characterized by X-ray diffraction (XRD) and the cross-sectional interfaces of NBT/LSCO/STO heterostructure were investigated using transmission electron microscopy (TEM). The ferroelectric properties and leakage current behavior of the LSCO/NBT/LSCO capacitor were studied using a precision LC unit from Radiant Technologies. The dielectric and piezoelectric properties were measured using an Agilent 4294A and piezoresponse force microscope (PFM-Asylum Research), respectively.

Results and discussion
diffraction peaks of NBT and LSCO are observed without any detectable secondary phase or diffraction peaks from other directions, indicating that both NBT and LSCO lms are highly (001) oriented. In order to further prove the epitaxial property of NBT lm on the LSCO/STO heterostructure, we performed the phi-scan of (110) plane of NBT lm. As shown in the inset of Fig. 1, the four evenly spaced peaks with spacing of 90 reveal that the NBT lm was epitaxially grown on NBT/LSCO/STO heterostructure. Fig. 1(b) presents the TEM image of the NBT/ LSCO and LSCO/STO interfaces, no obvious interdiffusion can be found at the interfaces. Selected area diffraction (SAED) of NBT and LSCO lms derived from fast Fourier transform (FFT) are respectively presented in the Fig. 1(b), indicating both NBT and LSCO are single crystalline lms. Fig. 1(c) shows a typical high-resolution TEM image of the LSCO/STO heterostructure and single crystalline LSCO lm is epitaxially grown on the surface of (001) STO substrate. The orientation relationship of these two layers is (001) Fig. 1(b), the lattice constants are 0.390 nm and 0.398 nm for the in-plane (a axis) and the out-plane (c axis), respectively. The c axis constant is consistent with the value, 0.397 nm, obtained from the XRD measurement. We should note that the out-plane lattice constant of NBT (0.397 nm) is larger than that of its bulk material (0.389 nm). A compressive force exists in the NBT lm when NBT is epitaxially grown on LSCO surface so that outplane lattice constant of NBT becomes larger. The polarization P s of a ferroelectric material is related to its tetragonality, (P s ) 2 f (c/a À 1) 2 , and the tetragonality (c/a ¼ 1.02) of the epitaxial NBT lm in the LSCO/NBT/LSCO heterostructure is larger than its bulk value (c/a ¼ 1.002), which is favorable for the NBT lm to have a larger remnant polarization.
A typical ferroelectric hysteresis loop of the LSCO/NBT/LSCO capacitor, measured at 250 kV cm À1 bias electric eld and 1 kHz frequency using capacitive method, is illustrated in Fig. 2(a). A saturated ferroelectric loop can be obtained with a remnant polarization, 15.6 mC cm À2 , and a small coercive eld, 47 kV cm À1 . The inset at the top le-hand side of Fig. 2(a) is the phase PFM images, which recorded for sample NBT aer square areas of (7 Â 7 mm 2 ) and (4 Â 4 mm 2 ) have been polarized (black region) and reverse polarized (white region) by applying bias electric eld À250 kV cm À1 and 250 kV cm À1 , respectively. We can see the polarization switching within these domains as square areas dened by black and white contrasts, characteristic of each orientation of the polarization can be observed. This means that (001) epitaxial NBT thin lm has good reversal memory properties. The switchable polarization, DP, is dened as the difference between the switched (P*) and nonswitched (P^) polarization. The inset at the bottom right-hand side of Fig. 2(a) demonstrates the relation of switchable polarization with applied electric eld. We can see that the switchable polarization increases rapidly with the increase of the applied electric eld, and it is saturated when the applied electric eld is greater than 100 kV cm À1 . Fig. 2(b) shows the pulse width dependence of the LSCO/NBT/LSCO capacitor measured at bias electric eld 250 kV cm À1 with a pulse delay time of 1 s. It can be seen that the switchable polarization slightly increases with the increase of the pulse width, which is favorable to the high speed ferroelectric memories. In addition, fatigue is also an important parameter to characterize ferroelectric lm for ferroelectric memories. In this experiment, the fatigue characteristic of the LSCO/NBT/LSCO capacitor was tested at bias electric eld 250 kV cm À1 and 1 MHz frequency with a pulse delay time of 1 s. As shown in Fig. 2(c), no obvious degradation can be found from the polarizations of the capacitor up to 10 10 switching cycles. The inset of Fig. 2(c) presents the hysteresis loops before and aer the fatigue test, in which no obvious difference can be found from the hysteresis loops, further indicating that the epitaxial LSCO/NBT/LSCO capacitor has very good fatigue resistance characteristic. Fig. 2(d) demonstrates the relation of dielectric constant and loss tangent with applied electric eld for the LSCO/NBT/LSCO capacitor at a frequency of 10 kHz. The tunability of 50.1% at the maximum applied eld of 250 kV cm À1 is obtained, in which the tunability is dened 19 In formula (1), 3 min is the minimum value of the dielectric constant at the maximum applied eld, 3 max is the maximum value of the dielectric constant at the applied eld of 0 kV cm À1 . The dielectric constant and dielectric loss of the (001) epitaxial NBT lm measured at 10 kHz are 559 and 0.19, respectively. To understand the local switching behavior of the NBT thin lms in ferroelectrics, the piezoresponse force microscope PFM (Asylum Research, Cypher, American) was used to characterize piezoelectric properties of the NBT lm. Since the effective piezoelectric coefficient d 33 can reect the most essential piezoelectric effect of piezoelectric lm material. 20 Measurement of d 33 was achieved by keeping the PFM tip xed above the local point of NBT lm and applying an electric eld from À500 to 500 kV cm À1 , while recording the piezoresponse signal amplitude (A) and applied eld (E). In the measurements, a bias voltage of 800 mV and a resonance frequency of 320 Hz were applied on the conductive test probe. According to the law of the converse piezoelectric effect, the relationship between amplitude (A) and applied eld (E) can be described as follows: 21 In formula (2), d is the initial thickness of the lms before deformation, A 0 and E 0 are the piezoelectric deformation and electric eld of the intersection, respectively. The piezoelectric hysteresis loop (d 33 -E) of NBT lm is calculated from the A-E curve based on eqn (1). Fig. 3(a) and (b) displays the corresponding piezoelectric phase-electric eld loop and typical piezoelectric response buttery loop. In the piezoelectric phase electric eld loop, the lm exhibits a signicant 180 domain ip. As can be seen, a typical well-shaped A-E "buttery" loop is obtained with a amplitude maximum of 1.82 nm appearing at 475 kV cm À1 . This result shows a strain as high as $0.7% (A/d) and indicates that the (00l) orientation of the epitaxial NBT lm has a very good piezoelectricity. In addition, at À205 kV cm À1 , the d 33 piezoelectric coefficients reached the 145 pm V À1 , which is relative higher than the reported NBT-based lms. [22][23][24] Leakage current is an important parameter of the electrical properties of ferroelectric capacitor related to thermal effect and energy loss so that minimum leakage current density is ideal for the real capacitors used in the ferroelectric memories. Four kinds of leakage mechanisms have been proposed for ferroelectric perovskite oxides: the ohmic conduction, the bulklimited space-charge-limited conduction (SCLC), the bulklimited Poole-Frenkel emission and the interface-limited Schottky emission. [25][26][27][28] Fig. 4 shows the relationship between the leakage current density J and the electric eld E for the LSCO/NBT/LSCO ferroelectric capacitor. The leakage current density at the applied electric eld of 250 kV cm À1 is 3.0 Â 10 À3 A cm À2 . The leakage current curve was re-plotted as shown in the inset at the le-hand side and the right-hand side of Fig. 4 in order to further characterize the conduction mechanisms of the LSCO/NBT/LSCO capacitor in different electric eld ranges. We can see two kinds of leakage behaviors account for the leakage current characteristic of the LSCO/NBT/LSCO capacitor, and 55 kV cm À1 is the transitional point for the two different leakage current mechanisms. The inset at the le-hand side of Fig. 4 demonstrates the linear relation of log(J) versus log(E) of the LSCO/NBT/LSCO capacitor for the low electric eld region, whose slope of 1.15 is close to 1.0, implying ohmic-like conduction for the low electric eld region. The relation between current density and electric eld for the SCLC is given by  In formula (3), J is the current density, E is electric eld, m is the magnetic permeability, 3 is the permittivity, d is lm thickness. Regarding high eld region, a straight line can be yielded using the relation of J versus E 2 , as shown in the inset at the right-hand side of Fig. 4, indicating SCLC conduction accounts for the conduction of the capacitor when the applied electric eld is higher than 55 kV cm À1 . Big differences can be found from the leakage current behavior between LSCO/NBT/ LSCO capacitor and Pt/NBT/Pt capacitors. 17 The current density of the LSCO/NBT/LSCO capacitor is 3.0 Â 10 À3 A cm À2 , while it is 1.4 Â 10 À2 A cm À2 for the Pt/NBT/Pt capacitor measured at the same electric eld of 250 kV cm À1 . The J-E curves of LSCO/NBT/LSCO capacitor for both positive and negative applied elds are very symmetric, however, very different leakage conduction behaviors can be found for the Pt/NBT/Pt capacitor. Moreover, the leakage current behavior of the Pt/NBT/Pt capacitor is mainly attributed to the interfacelimited Schottky emission for the high electric eld range, which is different from the leakage conduction behavior (SCLC) for the LSCO/NBT/LSCO capacitor. This difference can be ascribed to the fact that Pt and LSCO have different intrinsic property so that different interfaces are formed at Pt/NBT and LSCO/NBT interfaces, which further impact current conduction mechanism of the resulting capacitors. 29

Conclusions
Compared to the polycrystalline lms, the epitaxial LSCO/NBT/ LSCO capacitor possesses larger remnant polarization, high dielectric constant and effective piezoelectric coefficient as well as good fatigue resistance and small pulse width dependence. Moreover, the capacitor satises ohmic conduction behavior at electric elds lower than 55 kV cm À1 and SCLC behavior above 55 kV cm À1 . Our results pave a way for the research and development of lead-free sodium bismuth titanate ferroelectric memories.

Conflicts of interest
There are no conicts to declare.