PhD Thesis Arne Lüker final version V4 - Cranfield University

Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications
Barium Strontium Titanate (BST)
It should be well known by now that BaTiO3 is a ferroelectric perovskite and has been
well studied in bulk ceramic form where the measured permittivities are well in the
thousand (see Fig. 2.4). By substituting the the A-site atom (Ba-atom) with Sr the Curie
temperature, TC, can be shifted down from 120°C to around room temperature (Fig. 1.12).
The linear drop of TC is ca. 3.4°C per mol%. Therefore, 30 mol% Sr (x=0.3) would bring
the TC of Ba1-xSrxTiO3 down to room temperature. Due to this fact BST was and still is an
attractive candidate for voltage tunable devices like varactors, phase shifters, DRAMs
etc., where the ferroelectric crystal or thin film should be in the paraelectric state viz. the
operating temperature should be above TC.
BST films are, like PST, polycrystalline. Their properties heavily depend on
composition, stoichiometry, microstructure (grain size and size distribution), film
thickness and homogeneity of the film. The BST or/and PST thin film growth method
significantly affects the composition, stoichiometry, crystallinity, and grain size of the
film and, consequently, its dielectric properties. A variety of techniques have been used
to deposit these films (see introduction section of chapter 2.4).
BST 60/40 was chosen as a starting material in this study because it is widely
investigated and used for voltage tunable thin films. A wide range of publications makes
it relatively easy to gain access in this topic.
In this chapter a short review of BST deposition is given, followed by the description
of the synthesis and characterisation of BST thin films developed in the beginning of this
study. It concludes with a comparison between two different approaches of BST sol-gel
synthesis, pointing out the dominant role of the interfacial layer and the capacitance-
density.
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