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Research Activities
Microstructure-property relations of nanostructured ferroelectrics: |
Cross section transmission electron microscope image of an epitaxial lead zirconate titanate island ("PZT") grown by chemical solution deposition onto a strontium titanate substrate ("STO"), figure (a). The white "T" symbols designate misfit dislocations visualized in cross section. The high-resolution electron microscopy image figure (b) shows a section of the undistorted crystal lattice far from the misfit dislocations. Figure (c) shows a computer-processed representation of the deformations eyy which are located in a tube of 8 by 4 nm cross section around a misfit dislocation. The tube is seen in cross section. The peaks in yellow and red mark the highly deformed parts of the crystal lattice, whereas the green and blue sections represent the undistorted lattice. Ferroelectricity vanishes in the highly deformed parts, resulting in a modified size effect. For details, see
Nature materials 3 (2004) 87-90.
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Ferroelectric Thin Films: |
The material class of the bismuth-layered perovskites (also called Aurivillius phases) has proven to show very promising properties, especially regarding their application to integrated non-volatile ferrolectric memories. In contrast to most of the simple ferroelectric perovskite materials (BaTiO3, PbTiO3, PZT), thin films of bismuth-layered perovskites exhibit a very high endurance to fatigue (i.e. to spontaneous polarization loss after a certain number of polarization reversals) even on metallic electrodes. High quality films of the bismuth-layered perovskites SrBi2Ta2O9 (SBT), Bi4Ti3O12 (BiT), Bi3.25La0.75Ti3O12 (BLT), BaBi4Ti4O15 (BBiT) and Ba2Bi4Ti5O18 (B2BiT)
are grown by PLD and structurally as well as electrically characterized.
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Ferroelectric hysteresis loops of epitaxial Bi3.25La0.75Ti3O12 thin films deposited by PLD on SrRuO3 bottom thin-film electrodes in dependence on the crystallographic orientation of the ferroelectric film, viz. (001)-, (118)-, (104)- and (100)-orientation. Red curve for a 10 nm thin SrRuO3 bottom electrode on a YSZ(100) buffer layer on a Si(100) wafer; the other curves for a 50 nm thick SrRuO3 bottom electrode on a single-crystal SrTiO3 substrate. |
Ferroelectric Ultrathin Films and Multilayers: |
| Since actual efforts are concentrated to reduce the size of electronic devices, another challenge is to find new materials or to improve the dielectric properties of existing materials used in fabrication of ceramic capacitors and dynamic random access memories (DRAMs) which require high dielectric constant at small dimensions. For this purpose, multilayers of BaTiO3/SrTiO3 are deposited by PLD on vicinal SrTiO3 substrates. The microstructure of the grown films plays an important role in the dielectric behaviour. It is also important to take into account the influence of the interface of different multilayers on the dielectric properties.
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High-resolution TEM picture of BaTiO3/SrTiO3 multilayers deposited on a SrTiO3 substrate. The thickness of the BaTiO3 film is increased from 0.5 nm to 15 nm. |
Nanostructuring of Ferroelectric Thin Films: |
The ever-higher integration of microelectronic devices also addresses both questions of technological interest and of fundamental importance. Structuring devices at a submicron scale is an ambitious task intensively researched all over the world. On the other hand the questions to know whether ferroelectric structures that have submicron size (lateral and in height) are still ferroelectric, are still switching, and whether they still possess an equilibrium domain pattern are fundamental questions to be addressed before scaling down ferroelectric devices. Nanostructures as small as 50 nm in size have been fabricated by the Direct Electron Beam Writing method. First results show that these nanostructures are still ferroelectric and are still fully switching.
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SEM picture of several arrays of PZT nanostructures with various sizes obtained by direct electron beam writing . |
Recently, arrays of fully switchable ferroelectric mesoscopic structures of 300 nm in lateral size have been prepared by a modfied nanoimprint lithography method.
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Topography image of an array of ferroelectric PZT cells prepared by imprint lithography (left) and piezoresponse image of the same array (at the same magnification) after switching serveral cells using d.c. puls applied to the tip (right) . |
Piezoreponse Scanning Force Microscopy: |
Voltage-modulated scanning force microscopy (SFM), also called Piezoreponse SFM, is a powerful method for imaging ferroelectric domains. It allows the characterization of ferroelectric thin films at a nanoscopic scale, as well as the investigation of their ferroelectric behavior in dependence on their micro- or nano-structure. The materials under investigation are various bismuth layered perovskite thin films with different crystallographic orientations, as well as ferroelectric nanostructures made of PZT and other materials.
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Figure 1:
The well-known lamellar domain structure in simple perovskites. Within micron-sized grains, domains as small as 50 nm in lateral size are present. 90° as well as 180° domain walls are also visible.
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Figure 2:
Ferroelectric domains in twinned-epitaxial bismuth titanate films. A rectangular (110)-oriented crystallite embedded in a flat c-oriented matrix is shown in the 3-D surface plot. The color scale corresponds to the piezoresponse signal reflecting the out-of-plane polarization component. Half of the polarization in the structure was oriented upward, by applying a +30V pulse to the bottom electrode.
A weaker response of the matrix shows that there is a non-zero component of polarization along the [001] direction.
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Figure 3:
http://www.mpi-halle.mpg.de/~charna/results/bt.htm
Piezoresponse microscopy is a powerfull technique that allows to image the in-plane component of the polarization and therefore to obtain information about the three-dimensional distribution of polarization. |
Epitaxy of Aurivillius phases: |
Microelectronic devices are becoming smaller and smaller. This fosters an ever higher integration as well as more stringent requirements on the materials used. Therefore, the need of epitaxial and single crystalline films of functional materials is quickly growing. Bismuth-layered perovskites (alternatively called Aurivillius phases) are extremely anisotropic materials [click here to see their crystal structure (102 KB)]. The study of the dependence of their properties on their crystallographic orientation is thus of utmost importance. We are growing films of various bismuth-layered perovskite materials with several crystallographic orientations and are investigating the relationships between their ferroelectric properties and their crystallographic orientation.
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TEM plan view analysis of an epitaxial
Bi4Ti3O12 / LaNiO3 / SrTiO3 (001) structure. |
Epitaxial Growth of non-c-axis-oriented bismuth-layered perovskite films: |
Ferroelectric bismuth-layered perovskite films like SrBi2Ta2O9 and La-substituted Bi4Ti3O12 are presently being studied for use in digital memory systems. However, due to their highly anisotropic structure epitaxial thin films of these materials easily grow with the [001] axis perpendicular to the film plane, i.e. in the so called c-axis orientation. However, c-axis-oriented films do have no (or a negligibly small) polarization component along the film normal, because the vector of the (major) spontaneous polarization in these layered perovskite materials is along the a axis. If bismuth-layered perovskite films are to be used in ferroelectric thin film capacitors with plane electrodes on the top and bottom as in the geometry used for dynamic random access memories (DRAMs), a polarization component oriented normally to the electrode plane is, however, essential. Therefore, we concentrate on the growth of epitaxial films in one of the so-called non-c-axis orientations.
We succeeded, using PLD, the deposition of (116)- and (103)-oriented SrBi2Ta2O9 epitaxial films as well as the deposition of (118)- and (104)-oriented Bi4Ti3O12 and La-substituted Bi4Ti3O12 epitaxial thin films both on SrTiO3 substrates and on silicon-buffered substrates
Recently, (100) - oriented La-substituted Bi4Ti3O12 films on SrRuO3 - electroded, YSZ- buffered Si (100) have been achieved, with a remanent polarization of 32µC/cm2
Science 296 (2002) 2006-2009
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Schematic drawings (top) and corresponding x-ray diffraction pole figures (bottom) of non-c-axis-oriented La-substituted Bi4Ti3O12 (BLT) epitaxial thin films. The BLT films having the (118) (left) and (104) (right) orientations were grown on SrTiO3(011) and (111) substrates, respectively. The pole figures were recorded using the BLT 117 reflection. |
Microstructure of epitaxial complex oxide thin films: |
High resolution plan-view image of an epitaxial SrRuO3 thin film grown on a (001)SrTiO3 single crystal substrate. The film was grown by laser deposition at a substrate temperature of 850 °C. "BC" is an antiphase boundary (APB) extending along the pseudotetragonal [100] direction; viewing direction is [001]. The arrows indicate the lattice shift along the APB. The inset shows a structure model (left) and a computer-simulated image (right) performed on the base of this structure model. As a result, the antiphase boundary contains an extra SrO layer and thus contributes to the resistivity of the film.(Cooperation with G.Koren, Technion, Haifa, Israel).
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XTEM picture of a SRO film deposited on a STO substrate by PLD.
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HRTEM cross section image (left) of a tile boundary in a (001)-oriented epitaxial Ba2Bi4Ti5O18 thin film grown by PLD on an epitaxial LaNiO3 electrode on a CeO2/YSZ-buffered Si(100) wafer. Near the boundary (running approximately vertically) bright ribbons - containing dark lines corresponding to the Bi2O2 (001) layers - are seemingly bending upwards. However the Bi2O2 layers (sharp dark lines) remain strongly parallel to the (001) plane, even in the bending regions of the ribbons. This is due to a specific stairlike stacking order, which is shown in the scheme (right). The scheme also demonstrates that the tile boundaries are bismuth-rich compared with regular regions of the Ba2Bi4Ti5O18 lattice
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HRTEM cross section image and stucture model of a Ba2Bi4Ti5O18 film near a tile boundary |
(Large Area) Pulsed Laser Deposition: |
Thin films of various complex oxides, in particular of various ferroelectric bismuth-layered perovskites (also called Aurivillius phases), are deposited by large area PLD on substrates as large as 3-inch in diameter. Several techniques have been used and developed, among them rocking target PLD and axis-offset PLD. Large area PLD combines the high-quality of the films usually obtained by PLD and the large area deposition usually not achievable with this technique. The films show a good thickness uniformity (Figure) as well as a good compositional uniformity across the entire wafer.
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3D-plot of the thickness uniformity of a SrBi2Ta2O9 (SBT) film deposited by large area PLD on a 3-inch wafer. |
Direct Wafer Bonding of Ferroelectrics: |
Ferroelectric thin films have to be deposited and/or processed at relatively high temperatures, causing various damages (interdiffusion, strain, presence of structural defects) at the interface with the substrate or underlying layer. If the ferroeletric films are deposited directly on top of silicon, the high temperature processing ruins the quality of the semiconductor directly in contact with the ferroelectric material. Deposition of ferroelectric films directly on top of a semiconductor material is, however, a requirement for ferroeletric field effect - based devices. On the other hand, it is known that Direct Wafer Bonding (DBW) allows to intimately join dissimilar materials at relatively low temperature without any "glue" of any kind, provided that their surface is smooth and flat enough. Direct wafer bonding was therefore applied for the fabrication of ferroelectric layers in intimate contact with silicon, and the interface trap densitiy, for instance, has indeed been improved by a factor 500.
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XTEM picture of the interface between a PZT film grown by CSD and a Si (100) 3-inch wafer bonded by direct wafer bonding. |
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