AVS2001 Session EL-MoM: Ferroelectric

Monday, October 29, 2001 9:40 AM in Room 130

Monday Morning

Start	Invited?	Item
9:40 AM		EL-MoM-1 Quantitative Analysis of Piezoresponse Force Microscopy: Electrostatic vs. Electromechanic Interactions S.V. Kalinin, D.A. Bonnell (University of Pennsylvania) One of the critical aspects of the behavior of ferroelectric materials, both in the context of nano domains and for thin films, is the inter relation between atomic polarization and compensation charge. Piezoresponse force microscopy (PFM) has provided insight on domain structure and polarization reversal processes on the nanometer level. However, the imaging mechanism in PFM is complex in that both electrostatic and electromechanic interactions can contribute to image contrast. Here we analyze the electrostatic and electromechanical contrast in PFM using analytical solutions for the electrostatic sphere-dielectric plane problem coupled to Hertzian contact for the piezoelectric indentation problem. With this analysis local piezo electric properties can be quantified from traditional PFM and we suggest a novel variant that reduces the effects of system resonances to the contrast. The latter allows the measurement excitation and the domain switching bias to be decoupled. We use this approach on BaTiO₃ (100) to examine compensation charge and switching behavior.
10:20 AM		EL-MoM-3 Template Sol-Gel Synthesis and Characterization of BaTiO₃ Nanostructures B.A. Hernandez, K.C. Chang, E.R. Fisher, P.K. Dorhout (Colorado State University) Barium titanate in both its cubic and tetragonal form has shown relevance in many applications. A high dielectric constant, ferroelectric behavior, and electro-optical properties make this material a candidate for many applications in the microelectronics industry (e.g. capacitors and DRAMs). Recently, thin films and submicron size particles of BaTiO₃ have been studied and have shown that dielectric and ferroelectric properties change as a function of grain size and density of the particles. To explain these effects, Landau-Devonshire theory has been applied to both thin film and particle geometry. As far as we know, no one has studied size effects in high aspect ratio BaTiO₃ nanostructure materials. Thus we have developed a synthetic strategy to produce high aspect ratio nanostructures via a sol-gel template method. A chelate sol-gel solution was made using Ba(OAc)₂ and Ti(Oprⁱ)₄. Whattman Anodisc Membranes with a 200 nm pore size were dipped into the solution, allowed to air dry, and then calcined to the appropriate temperature for the tetragonal or cubic phase. SEM analysis shows that the structures can be thought of as fibers with a 200 nm diameter and 50µm length. Powder X-ray diffraction, Raman Spectroscopy, and Differential Scanning Calorimetry of the structures confirmed the production of cubic and tetragonal phases when fired at 700°C and 900°C respectively. Dielectric behavior and polarization data also will be presented.
10:40 AM		EL-MoM-4 Low-temperature Process of (Y_0.95,Bi_0.05)MnO₃ Ferroelectric Thin Film and its Structural and Electrical Properties T.J. Choi, Y.S. Kim, J. Lee (Sung Kyun Kwan University, Korea) (Y_0.95,Bi_0.05)MnO₃ (YBM) on Pt and Y₂O₃ buffered Si (100) have been prepared by pulsed laser deposition. Addition of Bi in YMnO₃ enhanced crystallization of YMnO₃ thin films. The c-axis oriented YBM films have been obtained on Pt and Y₂O₃/Si at 700 °C, which is lower deposition temperature than that of typical YMnO₃ films. The processing conditions of depositing oxygen partial pressure and cooling atmosphere have affected crystallization behavior and electrical properties of YBM films. At low oxygen partial pressures, (111), (112) and c-axis oriented polycrystalline nature were observed. As the oxygen pressure increased, YBM films intended to grow with c-axis preferred orientation. YBM deposited in oxygen pressure of 100 mTorr was strongly oriented along the c-axis at the substrate temperature of 700 °C. YBM grown on Y₂O₃/Si had a hysteresis curve with a clockwise direction, which indicates that the C-V hysteresis curve is caused by ferroelectric polarization switching. The memory window was about 2 V at a sweep voltage of 8 V.
11:00 AM		EL-MoM-5 Effects of Bi/Sr Stoichiometric Ratio on Electrical Properties of Pt/SrBi₂Nb₂O₉/Si Ferroelectric Gate Structure Y.T. Kim, S.I. Kim (Korea Institute of Science and Technology); I.H. Choi, H.S. Choi (Korea University); C.W. Lee (Kookmin University, Korea) In comparison with high dielectric constant (k) isotropic ferroelectric materials such as Pb(Zr,Ti)O₃ (PZT), and PLZT for the storage capacitor, Bi-layered perovskite materials have relatively low k and some advantages such as excellent fatigue resistance, low leakage current. Especially, for MFSFETs, SrBi₂Nb₂O₉ (SBN) seems to be a promising candidate among the Bi-layered perovskite family because it has relatively lower k than SBT.¹ It is well known that the lower k, the higher electric field is applied to the ferroelectric thin film.² The high electric field causes greater memory window, which becomes now an issue for the application of MFSFETs as cell devices in the non destructive readout ferroelectric random access memory (NDRO-FRAM) with low voltage operation. In this work, in order to control the Bi/Sr stoichiometric ratio, we used SrNb₂O₇ and Bi₂O₃ targets with different powers of rf magnetrons. As a result, we have found that electrical properties are strongly sensitive to the Bi content. The capacitance-voltage (C-V) characteristics and memory windows of Pt/SBN/Si gate were investigated with various Bi/Sr content ratios. The memory window of the Pt/SBN/Si gate with the Bi/Sr ratio of 3.1 becomes 1.8 V at applied voltage of 3 V, which is the greatest memory window so far. However, the memory window gradually increases with increasing the Bi/Sr ratio in the SBN thin films, but when the Bi/Sr ratio becomes over than 3.1 the electrical properties such as memory window and breakdown voltage becomes to be degraded. The SBN thin film with Bi/Sr ratio of 3.1 has the (008) preferred orientation after annealing at 600°C for 1hr in O₂ ambient, which is relatively lower recrystallization temperature than that of other Bi-layered perovskite family. Particularly, in order to improve the memory window, it has been used to insert buffer insulators such as CeO₂, Y₂O₃, and YMnO₃,2). However, in this work, we can obtain the greatest memory window without a buffer insulator. ¹ Y. T. Kim and D. S. Shin, Appl. Phys. Lett. 71, 3507 (1997) ² H. N. Lee, Y. T. Kim and Y. K. Park, Appl. Phys. Lett. 74, 3887 (1999)
11:20 AM	Invited	EL-MoM-6 Recent Progress of Ferroelectric Memory Materials and FET-type FeRAMs H. Ishiwara (Tokyo Institute of Technology, Japan) A new class of ferroelectric materials was synthesized by adding sol-gel solution of a dielectric material to that of conventional ferroelectric materials such as PbZr_1-XTi_XO₃, SrBi₂Ta₂O₉, and Bi₄Ti₃O₁₂. It was found that the crystallization temperature of the new materials was decreased by 150°C to 200°C compared to the original materials and that the surface of the crystallized film was extremely flat. It was also found that the ferroelectric properties were almost the same as or even better than the original ones. Thus, a 13-nm-thick Bi₄Ti₃O₁₂-based film with a saturation polarization voltage of 0.5 V was obtained. Concerning the FET-type FeRAMs (ferroelectric random access memories), improvement of the data retention characteristics is most important. To improve the retention time, the buffer layer material, which was necessary to insert between the ferroelectric gate film and a Si substrate for preventing interdiffusion of the constituent elements, was optimized, as well as the structure of the ferroelectric-gate FET was carefully designed. As a result, MFIS (metal-ferroelectric-insulator-semiconductor) diodes with excellent characteristics were fabricated by combination of a Si₃N₄ buffer layer and a c-axis-oriented Bi₄Ti₃O₁₂ film. In order to further improve the retention characteristics, a prototype of the 1T2C cell was also fabricated, in which two ferroelectric capacitors with the same area were connected to the gate terminal of an MOSFET so that the depolarization field was not generated. In this cell, nondestructive readout operation up to 10⁴ times and the excellent data retention up to 17 hours were realized.