Sol-gel synthesis of textured lanthanum titanate thin films
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Control over crystallographic orientation in thin films is important, particularly with highly anisotropic structures. Because of its ferroelectric nature, the layered perovskite La2Ti2O7 has interesting piezoelectric and electrooptic properties that may be exploited when films are highly textured. Sol-gel films with an orientation factor of greater than 95% were fabricated without relying on epitaxial (lattice-matching) growth from the substrate. Film orientation and crystallization were confirmed by x-ray diffraction, scanning electron microscopy, atomic force microscopy, and optical measurements. The particle sizes in all precursor solutions were measured by dynamic light scattering experiments. Experimental results indicate that film orientation is a function of precursor solution concentration, size of the molecular clusters in the solution, and film thickness.
I. INTRODUCTION
Lanthanum titanate, La2Ti2O7 (LaT), is a member of the perovskite layer structure (PLS) family of ferroelectrics sometimes referred to as the strontium pyroniobate family. LaT possesses an extremely high Curie temperature (Tc ⳱ 1500 °C) and exhibits piezoelectric and electrooptic effects.1,2 The temperature stability, coupled with good piezoelectric properties, makes LaT suitable for possible use as a high-temperature transducer material, while the low-dielectric loss at microwave frequencies makes it a candidate material for high-frequency applications.3 Like various polymorphs of all A2B2O7 PLS compounds, LaT originates from a paraelectric prototype structure whose space group is Cmcm.4 It is characterized by perovskite slabs stacked along the a axis, which are made up of corner-sharing BO6 octahedra and 12 coordinated A cations. Each slab is four octahedra thick and is linked to a neighboring slab by A cations lying near the boundary.4,5 As LaT cools through Tc, it undergoes a ferroelectric phase change to orthorhombic, Cmc21, in which the spontaneous polarization appears along the b axis due to an asymmetric deformation of the TiO6 octahedra. In the temperature range of 780–720 °C, there is another ferroelectric–ferroelectric phase change from orthorhombic to monoclinic, P21.5,6 This transition is characterized by a rotation that occurs through the Ti4+ ions without deforming the shapes of the octahedra, resulting in a small change in magnitude of the dipole moment in each respective TiO6 octahedron.5 Figure 1 is an illustration of the room-temperature modification of LaT. J. Mater. Res., Vol. 18, No. 2, Feb 2003
The first LaT films reported were sol-gel films deposited on fused silica and Si(100) substrates by a spincoating process resulting in films that showed a substantial amount of (100) grain orientation on the Si(100) substrate.7 Films of lesser orientation have been grown by metalorganic decomposition on platinized silicon substrates.8 X-ray diffraction (XRD) patterns revealed the presence of a second phase, La4Ti9O24, along with the major phase, La2Ti2O7. The amount of the second phase present and the grain orientation wa
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