Epitaxial Ferroelectric Films
Abstract
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Intrinsic and extrinsic ferroelectric properties of defect-free and defect-containing epitaxial Pb(Zr,Ti)O3 films:
I. Vrejoiu, G. Le Rhun, L. Pintilie, D. Hesse, and M. Alexe
(Top) Cross section TEM image of an epitaxial PZT film on a SrRuO3 electrode on a single-crystal SrTiO3 substrate, grown from an oxygen-deficient target. The film contains ferroelectric 90° a-c boundaries (white arrows), threading dislocations (black arrows) and dislocation half loops (white double arrow).
(Bottom) Cross section TEM of an epitaxial PZT film layer-by-layer grown from an oxygen-stoichiometric target onto an SrRuO3 electrode on a vicinal SrTiO3 substrate. The film is almost free from extended defects.
Macroscopic ferroelectric hysteresis loops of the defect-containing epitaxial PZT film (a), and the defect-free epitaxial PZT film (b). From the difference of the loops, the intrinsic and extrinsic contributions to the ferroelectric properties can be derived.
For details, see, e. g. Phil.Mag. 86 (2006) 4477
and Advanced Materials 18 (2006) 1657.
Epitaxial ferroelectric thin films:
H.N. Lee, K. Satyalakshmi, A. Pignolet, A.R. James, M. Alexe, and D. Hesse
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.
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.
For details, see, e. g. Applied Physics Letters 80 (2002) 1040-1042.
Epitaxial growth of non-c-axis-oriented bismuth-layered perovskite films:
H.N. Lee, A. Pignolet, D.H. Bao, N.D. Zakharov, and D. Hesse
Ferroelectric bismuth-layered perovskite films like SrBi2Ta2O9 and La-substituted Bi4Ti3O12 [click here to see their crystal structure] 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.
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.
Recently, (100) - oriented La-substituted Bi4Ti3O12 films on SrRuO3 - electroded, YSZ- buffered Si (100) have been achieved, with a remanent polarization of 32 µC/cm2
For details, see, e. g. Science 296 (2002) 2006-2009.
Microstructure of epitaxial complex oxide thin films:
N.D. Zakharov, K. Satyalakshmi, A. Pignolet, S. Senz, A.R. James, X.H. Zhu, and D. Hesse
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).
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.
For details, see, e. g. Cryst.Res.Tech. 35 (2000) 641-651.
(Large area) pulsed laser deposition:
A. Pignolet, H.N. Lee, S. Senz, M. Alexe, and D. Hesse
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.
3D-plot of the thickness uniformity of a SrBi2Ta2O9 (SBT) film deposited by large area PLD on a 3-inch wafer.
For details, see, e. g. Ferroelectrics 225 (1999) 201-220.