60 H Hi ig gh h- -o ox xy yg ge en n- -p pr re es ss su ur re e C Cr ry ys st ta al l G Gr ro ow wt th h o of f F Fe er rr ro oe el le ec ct tr ri ic c B Bi i 4 4 T Ti i 3 3 O O 1 12 2 S Si in ng gl le e C Cr ry ys st ta al ls s Ferroelectric bismuth titanate (Bi 4 Ti 3 O 12 , BiT) has been regarded as a promising material for innovative semiconductor-based applications such as nonvolatile memories, electro-optic devices and uncooled infrared detectors because of its high Curie temperature, large spontaneous polarization ( P s ) and large electro-optic coefficient [1]. The control of polarization states is the underlying basis of these functional devices, and polarization switching is achieved through the nucleation of domains and the following domain-wall motion by applying an electric field ( E ). Leakage current arising from defects, however, interferes with the polarization switching of BiT-based materials [2]. In addition, oxygen vacancies are known to act as an obstacle to the polarization switching, and a resultant remanent polarization ( P r ) is suppressed by the clamping of the domain walls by oxygen vacancies [2]. The leakage current and domain clamping by oxygen vacancies make BiT unsuitable for the practical applications. Thus, a guiding principle of defect control is required to be established for obtaining high- quality BiT-based devices with a large P r as well as a low leakage current. Here, we show that high-oxygen- pressure crystal growth is an effective process for obtaining high-quality BiT crystals with a large P r and a low coercive field ( E c ) as well as low leakage current [3]. Synchrotron radiation powder diffraction experiments on the crushed powder of the crystals were performed using a large Debye-Scherrer camera installed at BL02B2 to investigate the precise crystal structure of BiT synthesized by the processing method. We used high-energy SR with a wavelength of λ = 0.035639(2) nm ( E ~ 35 keV) to reduce absorption by the samples. The BiT crystals under different P o 2 atmospheres had almost the same lattice parameters: a = 0.54505(5) nm, b = 0.54108(4) nm, c = 3.2834(3) nm. Rietveld analyses demonstrated that there was no significant difference in crystal structure in crystals grown under different pressures [3]. These experimental results indicate that high-oxygen- pressure crystal growth is suitable for obtaining defect-controlled ferroelectric crystals without any significant change in the main crystal structure. Domain observations by piezoresponse force microscopy (PFM) demonstrate that the clamping of 90° domains deteriorates P r for the crystals grown at 0.02 MPa oxygen, which is suggested to originate from the strong attractive interaction between 90° domain walls and oxygen vacancies. The vacancy formation of Bi and O during crystal growth at high temperatures is suppressed at a higher oxygen pressure, leading to a larger P r of 47 μ C/cm 2 and a lower E c of 26 kV/cm for the crystals grown at 1 MPa oxygen [3]. Figure 1 shows the polarization hysteresis loops measured along the a ( b )-axis (25°C, 1 Hz) [3]. The crystals ( P o 2 = 0.02 MPa) exhibited hysteresis with P r = 38 μ C/cm 2 and an E c = 38 kV/cm. The high- P o 2 - grown crystals had larger values of P r of 44 μ C/cm 2 ( P o 2 = 0.1 MPa) and 47 μ C/cm 2 ( P o 2 = 1 MPa). Note that the crystals grown at P o 2 = 1 MPa exhibited well- saturated polarization hysteresis with E c = 26 kV/cm. This E c value was much lower than those of the other crystals. Figure 2 shows P r and E c as a function of P o 2 during crystal growth [3]. With increasing P o 2 , P r monotonically increased, while the decrease in E c was marked over P o 2 = 0.1 MPa. Figure 3 shows the leakage current density ( J ) as a function of E along the a ( b )-axis (25°C) [3]. The 60 40 20 0 – 20 – 40 – 60 – 100 – 50 0 50 100 E (kV/cm) 25°C, 1 Hz E // a ( b ) axis P o 2 = 0.02 MPa P o 2 = 0. 1 MPa P o 2 = 1 MPa Cryst a l gr o wth a tm o sphere P ( C/cm 2 ) Fig. 1. Polarization hysteresis loops along the a ( b )- axis for the BiT crystals grown at a P o 2 of 0.02, 0.1, and 1 MPa. These crystals were annealed at 900°C for 10 h in air. The measurements were conduced at 25°C using an E at a frequency of 1 Hz. Materials science : Structure 10 -4 10 -5 10 -6 10 -7 10 -8 10 -9 10 -10 E (kV/cm) 25°C, E // a ( b ) axis P o 2 = 0.0 2 MPa P o 2 = 0. 1 MPa P o 2 = 1 MPa Crystal growth atmosphere Current Density ( A/cm 2 ) 0 20 40 60 80 Fig. 2. Remanent polarization ( P r ) and coercive field ( E c ) as a function of P o 2 during crystal growth (25°C, 1 Hz). The values of P r and E c are average in positive and negative regions in the polarization hysteresis loops (Fig. 1). 61 crystals ( P o 2 = 0.02 MPa) exhibite a high J of the order of 10 -7 to 10 -6 A/cm 2 . The increase in P o 2 to 0.1 MPa led to a drastic decrease in J to the order of 10 -9 A/cm 2 . The crystals grown at P o 2 = 1 MPa exhibited a relatively low J of the order of 10 -8 A/cm 2 . Here, we discuss the mechanism of domain clamping during the polarization switching along the a ( b )-axis. Figure 4 shows the PFM images observed on the a ( b )- c surface of the crystals grown at P o 2 = 0.02 MPa [3]. The in-plane PFM image of the as- annealed (nonpoled) crystals (Fig. 4(a)) exhibits 180° DWs parallel to the a - b plane. After an E of 100 kV/cm was applied along the a ( b )-axis at 25°C, the out-of-plane PFM image of the poled crystals was observed (Fig. 4(b)). A single domain state was not established for the poled crystals even though the applied E (100 kV/cm) is much higher than the E c value (38 kV/cm). Domains with P s(a) parallel to the poling direction were found. This is direct evidence that 90° domains are switched by applying an E of 100 kV/cm. Note that unswitched regions, i.e., 90° domains with P s(a) normal to the poling direction remained, and 90° DWs with an irregular structure appeared. The irregular-shaped 90° DWs have been reported to originate from the attractive interaction between V O ¨ and the electric field established near the 90° DWs due to the discontinuity of the P s component normal to the DWs. In the domains with P s(a) parallel to the poling direction, a small number of 180° domains with P s(a) antiparallel to the poling direction were observed. These 180° domains are a result of the domain backswitching due to the depolarization field. Our PFM observations lead to the conclusion that the clamping of 90° DWs plays a detrimental role in the P s(a) polarization switching in the BiT crystals. The vacancy formation at high temperatures is suppressed under a higher- P o 2 atmosphere, and then [V O ¨ ] becomes lower for the crystals grown at a higher P o 2 . The larger P r observed for the crystals ( P o 2 = 1 MPa) is found to originate from suppressed 90° domain clamping because of a lower [V O ¨ ]. In summary, the effects of P o 2 during the crystal growth of BiT on domain-switching behavior have been investigated through polarization measurements and domain observations by PFM. The crystal structure is investigated by high energy synchrotron radiation powder diffraction. The crystals grown at a high P o 2 of 1 MPa showed a large P r of 47 μ C/cm 2 and a low E c of 26 kV/cm. PFM observations demonstrate that the clamping of 90° DWs plays a detrimental role in polarization switching, leading to a low P r . High- P o 2 sintering is proposed as an effective process for suppressing the formation of vacancies of Bi and O without any change in the main crystal structure, leading to the realization of high-quality BiT- based devices with enhanced polarization-switching properties as well as low leakage current. Yuji Noguchi a , Masaru Miyayama a and Yoshihiro Kuroiwa b a Research Center for Advanced Science and Technology, The University of Tokyo b Department of Physical Science, Hiroshima University *E-mail: ynoguchi@crm.rcast.u-tokyo.ac.jp References [1] M. Soga et al .: Appl. Phys. Lett. 84 (2004 ) 100. [2] S.-J. Kim, C. Moriyoshi, S. Kimura, Y. Kuroiwa, K. Kato, M. Takata, Y. Noguchi and M. Miyayama: Appl. Phys. Lett. 91 (2007) 062913. [3] K. Yamamoto, Y. Kitanaka, M. Suzuki, M. Miyayama, Y. Noguchi, C. Moriyoshi and Y. Kuroiwa: Appl. Phys. Lett. 91 (2007) 162909. P r E c 50 45 40 35 30 50 40 30 20 10 0 0.01 0.1 1 Crystal-growth P o 2 (MPa) 25°C, 1 Hz E // a ( b ) axis E c ( kV/cm) P r ( C/cm 2 ) Fig. 3. Leakage current density ( J ) as a function of E along the a ( b )-axis for the BiT crystals (25°C, 1 Hz). (a) (b) Nonpoled crystal (in-plane) Poled crystal (out-of-plane) Bi 2 O 2 layer Perovskite layer poling direction 90° domain wall P s( a ) b ( a ) b ( a ) a ( b ) c c a ( b ) P s( a ) 1 m 1 m Fig. 4. PFM images of the a ( b )- c surface of the BiT crystals grown in air ( P o 2 =0.02 MPa); (a) in-plane image of the nonpoled (as-annealed) crystal, and (b) out-of-plane image of the poled crystal. The poling was conducted by applying an E of 100 kV/cm along the a ( b )-axis at 25°C.
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