Sang Don Bu

Lanthanum-substituted bismuth titanate for use in non-volatile memories

Non-volatile memory devices are so named because they retain information when power is interrupted; thus they are important computer components. In this context, there has been considerable recent interest in developing non-volatile memories that use ferroelectric thin films—‘ferroelectric random access memories’, or FRAMs—in which information is stored in the polarization state of the ferroelectric material. To realize a practical FRAM, the thin films should satisfy the following criteria: compatibility with existing dynamic random access memory technologies, large remnant polarization (Pr) and reliable polarization-cycling characteristics. Early work focused on lead zirconate titanate (PZT) but, when films of this material were grown on metal electrodes, they generally suffered from a reduction of Pr (‘fatigue’) with polarity switching. Strontium bismuth tantalate (SBT) and related oxides have been proposed to overcome the fatigue problem, but such materials have other shortcomings, such as a high deposition temperature. Here we show that lanthanum-substituted bismuth titanate thin films provide a promising alternative for FRAM applications. The films are fatigue-free on metal electrodes, they can be deposited at temperatures of ∼650 °C and their values of Pr are larger than those of the SBT films.

Thin Film Magnesium Boride Superconductor with Very High Critical Current Density and Enhanced Irreversibility Field

Larbalestier †§ N. Rogado*, K.A. Regan*, M.A. Hayward*, T. He*, J.S. Slusky*, K. Inumaru*, M.K. Haas* and R.J. Cava* † Department of Materials Science and Engineering, Univer-sity of Wisconsin, 1509 University Avenue, Madison, WI 53706 USA § Applied Superconductivity Center, University of Wisconsin, 1500 Engineering Drive, Madison, WI 53706 USA * Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, NJ 08544 USA

Giant piezoelectricity on Si for hyperactive MEMS.

High-quality piezoelectric thin films are grown and exhibit superior properties for microelectromechanical systems. Microelectromechanical systems (MEMS) incorporating active piezoelectric layers offer integrated actuation, sensing, and transduction. The broad implementation of such active MEMS has long been constrained by the inability to integrate materials with giant piezoelectric response, such as Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT). We synthesized high-quality PMN-PT epitaxial thin films on vicinal (001) Si wafers with the use of an epitaxial (001) SrTiO3 template layer with superior piezoelectric coefficients (e31,f = –27 ± 3 coulombs per square meter) and figures of merit for piezoelectric energy-harvesting systems. We have incorporated these heterostructures into microcantilevers that are actuated with extremely low drive voltage due to thin-film piezoelectric properties that rival bulk PMN-PT single crystals. These epitaxial heterostructures exhibit very large electromechanical coupling for ultrasound medical imaging, microfluidic control, mechanical sensing, and energy harvesting.

DIFFERENCES IN NATURE OF DEFECTS BETWEEN SRBI2TA2O9 AND BI4TI3O12

X-ray photoemission spectroscopy measurements were executed to compare the nature of defects in SrBi2Ta2O9 (SBT) and Bi4Ti3O12 (BTO) films. In the SBT film, it was found that the oxygen ions at the metal–oxygen octahedra were much more stable than those at the Bi2O2 layers. On the other hand, for the BTO film, oxygen vacancies could be induced both at the titanium–oxygen octahedra and at the Bi2O2 layers. We suggested that the difference in stability of the metal–oxygen octahedra should be related to different fatigue behaviors of the SBT and the BTO films.

Very high upper critical fields in MgB2 produced by selective tuning of impurity scattering

We report a significant enhancement of the upper critical field Hc2 of different MgB2 samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities ρ ,w e sho wac lear trend of a ni ncrease in Hc2 as ρ increases. One particular high resistivity film had a zero-temperature Hc2(0) well above the Hc2 values of competing non-cuprate superconductors such as Nb3Sn and Nb–Ti. Our high-field transport measurements give record values H ⊥ c2 (0) ≈ 34 T and H || c2 (0) ≈ 49 T for high resistivity films and Hc2(0) ≈ 29 T for untextured bulk polycrystals. The highest Hc2 film also exhibits a significant upward curvature of Hc2(T ) and a temperature dependence of the anisotropy parameter γ( T ) = H || c2 /H ⊥ c2 opposite to that of single crystals: γ( T ) decreases as the temperature decreases, from γ( Tc) ≈ 2t o γ( 0) ≈ 1.5. This remarkable Hc2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB2 ,w hich offers special opportunities for further Hc2 increases by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of Hc2(T ) observed suggest that MgB2 can be made into a versatile, competitive high-field superconductor.

Formation of Co nanoclusters in epitaxial Ti0.96Co0.04O2 thin films and their ferromagnetism

Anatase Ti0.96Co0.04O2 films were grown epitaxially on SrTiO3 (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior was changed from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like one, which resulted in an increase in the saturation magnetization. These structural and magnetic changes were explained in terms of the formation of cobalt clusters whose existence was proved by transmission-electron-microscope studies. Our work clearly indicates that the cobalt clustering will cause room-temperature ferromagnetism in the Co-doped TiO2 films.

Critical thickness of ultrathin ferroelectric BaTiO3 films

To investigate the critical thickness of ferroelectric BaTiO3 (BTO) films, we fabricated fully strained SrRuO3∕BTO∕SrRuO3 heterostructures on SrTiO3 substrates by pulsed laser deposition with in situ reflection high-energy electron diffraction. We varied the BTO layer thickness from 3to30nm. By fabricating 10×10μm2 capacitors, we could observe polarization versus electric-field hysteresis loops, which demonstrate the existence of ferroelectricity in BTO layers thicker than 5nm. This observation provides an experimental upper bound of 5nm for the critical thickness. The BTO thickness-dependent scaling of the remanent polarization agrees with the predictions of recent first-principle simulations [J. Junquera and P. Ghosez, Nature 422, 506 (2003)].

Electronic anisotropy, magnetic field-temperature phase diagram and their dependence on resistivity in c-axis oriented MgB2 thin films

An important predicted, but so far uncharacterized, property of the new superconductor MgB2 is electronic anisotropy arising from its layered crystal structure. Here we report on three c-axis oriented thin films, showing that the upper critical field anisotropy ratio Hc2?/Hc2? is 1.8 to 2.0, the ratio increasing with higher resistivity. Measurements of the magnetic field-temperature phase diagram show that flux pinning disappears at H*?0.8Hc2?(T) in untextured samples. Hc2?(0) is strongly enhanced by alloying to 39 T for the highest resistivity film, more than twice that seen in bulk samples.

Ferroelectricity in highly ordered arrays of ultra-thin-walled Pb(Zr,Ti)O3 nanotubes composed of nanometer-sized perovskite crystallites.

We report the first unambiguous ferroelectric properties of ultra-thin-walled Pb(Zr,Ti)O 3 (PZT) nanotube arrays, each with 5 nm thick walls and outer diameters of 50 nm. Ferroelectric switching behavior with well-saturated hysteresis loops is observed in these ferroelectric PZT nanotubes with P r and E c values of about 1.5 microC cm (-2) and 86 kV cm (-1), respectively, for a maximum applied electric field of 400 kV cm (-1). These PZT nanotube arrays (10 (12) nanotubes cm (-2)) might provide a competitive approach toward the development of three-dimensional capacitors for the terabyte ferroelectric random access memory.

Different fatigue behaviors of SrBi2Ta2O9 and Bi3TiTaO9 films: Role of perovskite layers

To investigate the role of the perovskite layers on fatigue behaviors, SrBi2Ta2O9(SBT) and Bi3TiTaO9 (BTT) films were prepared by pulsed laser deposition using 15% Bi-excess bulk targets. The SBT and the BTT films grown at the similar deposition conditions showed similar growth behaviors, electrical properties, and retention characteristics. However, these films showed very different fatigue behaviors. The difference should come from the oxygen stability in the perovskite layer. Our work demonstrates that oxygen stability of the perovskite layers, as well as the self-regulating adjustment of the Bi2O2 layers, should be considered in the search for new candidate materials for nonvolatile ferroelectric memory devices.