PhD Thesis Arne Lüker final version V4 - Cranfield University

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26 Introduction based on ferroelectrics [44, 45]. In fact, ferroelectrics are gaining ground for tunable microwave applications, including lumped element (device sizes much smaller than the microwave wavelength) and distributed element devices, such as varactors, phase shifters, delay lines, tunable filters, antennas, etc... Nevertheless, problems such as hysteresis, temperature dependence of parameters (such as permittivity of the ferroelectric material), etc., still need adequate treatment. One of the fundamental problems concerning the practical use of tunable ferroelectric devices has to do with the Fig. 1.17: (a) BZN structure with random offcentering of ions (yellow: O, green: Bi). (b) Applied bias field dependence of the permittivity and dielectric loss of a BZN film on Al2O3/Pt at 1 MHz [46] strong temperature dependence of the parameters of tunable ferroelectric devices associated with the inherent temperature dependence of the ferroelectric’s permittivity. Recently the group around Stammer et al. introduced a new non-ferroelectric material: bismuth zinc niobate (BZN), which has a cubic pyrochlore structure [46]. To understand the origin of the tunability of BZN, which would aid in the discovery of new tunable materials, models were developed that describe the temperature dependence of the tunability of non-ferroelectrics. BZN films thus have a practical advantage over tunable ferroelectric materials, as they would allow temperature-stable tunable devices. It was also shown that, in a first approximation, at temperatures above the dielectric relaxation, the temperature dependence of the BZN film tunability can be described by a simple model of hopping dipoles under the influence of random fields. This is consistent with the chemical and displacive randomness of the BZN structure that controls the dielectric behavior [47, 48]. However, the origin of the large electric field tunability of the permittivity of BZN is not yet understood.
Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications Nevertheless, ferroelectrics are the materials of main interest for voltage tunable devices today and BST is, despite the aforementioned problems, the most likely candidate to make the way towards commercially available applications since it is more studied both in terms of electrical performance, manufacturability and reliability. Against the trend towards lead-free materials a new perovskite ferroelectric material, namely Lead Strontium Titanate, (Pb, Sr)TiO3, has attracted the interest of researchers nowadays. Although there are health and environmental concerns with lead-based materials, this material has remarkable advantages compared to BST or related ferroelectrics. It is easy to deposit crack-free via the sol-gel route, has relatively low loss and a high tunability (~ 80%), and can even be deposited directly on SiO2 or SiNx, facts that make it very interesting for direct applications or the integration of other frequently used perovskites like PZT which can not be deposited directly on SiO2. 1.4 References 1. Maurice Soenen, La Pharmacie à La Rochelle. Les Seignettes et le sel polychreste. Thèse de doctorat de l’Université de Bordeaux, 1910 2. Simon Boulduc, Compt. rendus Acad. Roy. des Sciences 1731 3. David Brewster, Edinbg. J. of Science, Vol. 1, No. II. 208-218 (1824). Observation on the Pyro-Electricity in Minerals 4. Jacques et Pierre Curie, Compt. rend. 91, 294-295, 383-387 (1880). Développement par pression de l’électricité polaire dans des crystaux hémiédriques à faces inclinées. 5. A.M. Nicholson, Piezophony. U.S. Patent 1, 495, 429. Filed April 10, 1918. Patentd May 27, 1924. A.M. Nicholson, Generating and Transmitting electric Currents , U.S. Patent 2, 212, 845. Filed April 10, 1918. Patented August 27, 1940 6. Paul Langevin, Procédé et appareils d’émission et de réception des ondes élastiques sous-marines à l’aide des propriétés piézoélectriques du quartz. Brevet d’Invention No. 505,703. Demandé le 17 septembre 1918. Délivré le 14 mai 1920 7. P. Debye, Physik. Zeitschr. XIII, 97-100 (1912). Einige Resultate einer kinetischen Theorie der Isolatoren (Vorläufige Mitteilung). 27
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Intensity [arb. Units] (g) (f) (e)
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Relative diel. diel. constant Const
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Intensity [arb. Units] Intensity [a
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PhD Thesis Arne Lüker final version V4 - Cranfield University

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