Growth, Structure, and Properties of Uniformly a- Axis Oriented Ferroelectric Bi 3.25 La 0.75 Ti 3 O 12 Thin Films on Si
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Growth, Structure, and Properties of Uniformly a-Axis Oriented Ferroelectric Bi3.25La0.75Ti3O12 Thin Films on Si(100) Substrates D. Hesse, H. N. Lee 1, N. D. Zakharov, and U. Gösele Max-Planck-Institut für Mikrostrukturphysik, D-06120 Halle (Saale), Germany 1 Now with Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA ABSTRACT Uniformly a-axis-oriented, epitaxially twinned Bi3.25 La 0.75Ti3O12 (BLT) thin films having the major spontaneous polarization entirely along the film normal were grown by pulsed laser deposition on yttria-stabilized zirconia-buffered Si(100) substrates covered with very thin SrRuO 3 bottom electrodes. Using SrRuO 3 bottom electrodes of a specific low thickness in combination with a relatively high growth rate and a high oxygen pressure, the volume fraction of the BLT (100) orientation, which is competing with the BLT (118) orientation, was increased up to 99%. In this way the growth of fully a-axis-oriented BLT epitaxial films was achieved, attaining a remanent polarization of 32 µC/cm2. Initial fatigue experiments indicated hardly any fatigue after 109 switching cycles.
INTRODUCTION Ferroelectric films of bismuth-layered perovskite compounds like SrBi 2Ta2O9 (SBT) or (Bi,La)4Ti 3O12 (BLT) are important for future high-density non-volatile ferroelectric random access memories (NV-FRAMs) [1-3]. Though commercial low-density memories made of polycrystalline SBT films are already in use, the integration of Bi-layered type ferroelectric films into the silicon technology and the increase of memory density (up to the Gbit-size range) remain a challenge. In particular, the growth of uniformly oriented films on Si(100) substrates is significant, because such films will be required, if the lateral size of ferroelectric memory cells will drop down to below 100 nm for Gbit memories [2,4]. This cell size being of the order of the grain size of polycrystalline films, the arising non-uniformity problems with randomly oriented films [5] can be solved by using uniformly oriented films. If the film is to be used in a ferroelectric thin-film capacitor geometry, the presence of a nonzero polarization component perpendicular to the film plane is essential. This presence is, however, not trivial, because the bismuth-layered perovskite compounds strongly prefer to grow in the c-axis orientation but the vector of the spontaneous polarization is (almost or entirely) perpendicular to the c-axis in these materials. As a consequence non-c-axis oriented films should be grown, particularly on Si(100). The growth of non-c-axis oriented SBT, SrBi 2Nb 2O9 (SBN) or Bi4Ti 3O12-based bismuth-layered perovskite films on complex-oxide single crystal substrates has been reported by several groups [6-14]. The growth of non-c-axis oriented Bi-layered perovskite films on electroded Si(100) substrates was achieved only very recently [15-21]. Mostly thin SrRuO3 bottom electrodes were used on Si(100) [15-20]. Sometimes, however, thin platinum electrodes on Si(100) were applied [21]. Since non
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