Lattice Defects in Epitaxial Ba 2 Bi 4 Ti 5 O 18 Thin Films Grown by Pulsed Laser Deposition Onto LaNiO 3 Bottom Electro
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ABSTRACT Epitaxial, ferroelectric Ba 2 Bi 4Ti5 O1 8 films grown on LaNiO3 /CeO 2/ZrO 2 :Y20 3 epitaxial layers on Si(100) are investigated by cross-section high-resolution transmission electron microscopy (HRTEM). The films are perfectly oriented and consist of well-developed grains of rectangular shape. The grain boundaries are strained and contain many defects, especially a new type of defect, which can be described as a staircase formed by repeated lattice shifts of A = c/I 2 4.2 A in the [001] direction. This repeated shift results in seemingly bent ribbons of stacked Bi 2 O2 planes, involving, however, individual Bi 20 2 planes which remain strongly parallel to the (001) plane. These defects contain an excess of bismuth. Other defects found in the grain interior include mistakes in the stacking sequence originating from the presence of single, well-oriented, non-stoichionietric layers intergrown with the stoichiometric Ba 2Bi 4TisOj 8 film matrix. INTRODUCTION Ferroelectric thin films of bismuth-layer compounds like SrBi 2Ta 2O 9 gain more and more importance in view of their relevance to the development of non-volatile ferroelectric random access memories [I]. They are advantageous over usual perovskite materials in that they do not suffer from fatigue [2,3]. Though memories made of polycrystalline SBT films are already in use, the integration of Bi-layer type ferroelectric films into the silicon technology remains a challenge. The growth of epitaxial Bi-layer films on Si substrates is significant, because they allow fundamental and applied studies to be performed in order to probe the properties of these unconventional materials. As part of our studies of the structure-property relationships of various Bi-layer type ferroelectric films, like Bi4 Ti3 0 12 , SrBi 2Ta 2O9, or BaBi 4Ti4 Ol5 [4-6], we have prepared epitaxial Ba 2 Bi 4Ti5 Ol8 films on LaNiO 3 electrodes on CeO 2 /YSZ-buffered Si(100) and studied their ferroelectric and dielectric properties [7,8]. Here we report on cross-section HRTEM investigations of these films, with a strong emphasis on the lattice defects found. Ferroelectricity in the compound Ba 2 Bi 4Ti5 Ol 8 had been reported as early as in 1962 [9]. According to this work, the compound crystallizes in the tetragonal space group I4/mmm with the lattice parameters at = 3.88 A and ct = 50.3 A. Though the ferroelectric phase might indeed be orthorhombic, here we are keeping to that structure. To our knowledge, there are no reports on epitaxial Ba2Bi 4TisOl8 films in the literature, except preliminary information given in our two recent papers 17,8]. HRTEM investigations of lattice defects in bismuth-layer type perovskites have been performed rather rarely. Suzuki et al. [10] revealed twin domains and wavy c/6 translational boundaries in (001)- and (116)-oriented SrBi 2Ta 2O9 films. In our Ba 2Bi4 TiUO 1 8 films, similar types of defects involving shifts by A ct/1 2 along [001] are present, as described below. EXPERIMENTAL The YSZ and CeO 2 buffer layers, the LaNiO 3 (LNO) electrode layer, an
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