Composition/structure/property relations of multi-ion-beam reactive sputtered lead lanthanum titanate thin films: Part I
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K. L. More High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6064 (Received 9 December 1992; accepted 5 May 1993)
This paper, the second of three parts, presents a detailed analysis of the crystallographic texture observed in lead lanthanum titanate (PLT) thin films deposited by multi-ion-beam reactive sputtering. Since films were deposited at a substrate temperature of 100 °C, they exhibit an amorphous structure that can be described by the structure zone model. The as-deposited microstructure is transformed via crystallization of the perovskite phase and PbO evaporation during postdeposition annealing. Transmission electron microscopy was used to determine that phase pure PLT films develop textured clusters (as large as 700 nm in diameter) consisting of (100) aligned 10 nm nanocrystals. As excess PbO is added to the PLT film, the textured cluster size decreases until only isolated PLT nanocrystals exist. Below a critical quantity of excess PbO in the as-deposited film (approximately 15 mole %), the textured cluster structure produces a (100) texture through the film thickness, which generates x-ray diffraction patterns typical of textured microstructures. At high PbO contents, the excess PbO causes the formation of clusters that are smaller than the film thickness, resulting in a polycrystalline-type x-ray diffraction pattern. A qualitative model describing microstructure development is presented.
I. INTRODUCTION Because of the anisotropic nature of ferroelectric crystals, crystallographic texture in polycrystalline materials can strongly influence the material properties. In ferroelectric thin films, crystallographic texture must be carefully controlled to obtain reproducible electrical properties necessary for ferroelectric device applications. This paper, the second of a three part series of papers, presents a detailed study of the relationships among composition, crystallographic texture, and microstructure development in lead lanthanum titanate (PLT) thin films deposited by multi-ion-beam reactive sputtering (MIBERS). The perovskite PLT solid solution is described by the formula: P b 30-ao L a
3*
3+x(l.S-a)
3+x(1.5-a)
•
xj2a-l.S)
X
•_
o.
(1)
a
'Present address: Ecole Polytechnique Federate de Lausanne, Laboratoire de Ceramique, MX-D Ecublens, CH-1015, Lausanne, Switzerland. b 'Also with Department of Engineering Science and Mechanics. J. Mater. Res., Vol. 8, No. 9, Sep 1993
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with 0.9 < a < 1.5 and x < 0.30. The variable a is called the lead elimination factor, x is the molar La content, and • indicates a site vacancy. The La content of PLT changes the unit cell measured at room temperature from tetragonal (for x = 0, a0 = 0.390, c 0 = 0.415, and the space group is PAmm) to cubic (for x = 0.30, a 0 = 0.392 nm, and the space group is Pm3m) and shifts the ferroelectric to paraelectric transition temperature of PLT from 490 °C (x = 0) to - 9 0 °C (x = 0.30). The changes in crystal structure accompa
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