PhD Thesis Arne Lüker final version V4 - Cranfield University

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10 Introduction gel synthesis and characterization of BST. Reasons for a reorientation towards (Pb,Sr)TiO3, which is a relative new material for tunable applications and to a lesser extent characterized, are given and explained. PST offers a higher tunability and a lower loss than BST and has some remarkable properties, which are unique, therefore the main focus in this thesis will be on PST. This thesis is presented in nine chapters. Chapter 1 deals with the history of ferroelectrics and sol-gel techniques and reviews the state-of-the-art of materials for tunable applications with the main focus on varactors. The purpose of this chapter is to give an easy to read introduction in these fields of science. The subject of chapter 2 are physical and chemical issues which did not fit in chapter 1 e.g. the theory of the dielectric response according to the Landau-Theory, a theoretical treatment of losses in ferroelectric crystals, the crystallography of perovskite materials and the theory of sol-gel synthesis. Chapter 3, entitled Characterisation of Ferroelectric Thin Films, describes the experimental details within this work. BaxSr1-xTiO3 is the subject of chapter 4. Because this material is of scientific interest for decades now an individual literature review of sol-gel prepared BST films and applications is given here. Despite numerous attempts this material could not be deposited successfully. Serious crack formation were a major obstacle and led at the end to a reorientation towards (Pb,Sr)TiO3, the subject of chapter 5. Apart from the crystallisation pathway and dielectric properties of PST 50/50 thin films a simple application is described in this chapter: a varactor using copper electrodes to boost the overall quality factor. Chapter 6 deals with the effect of B site doping of PST 40/60. Three different theories from the literature are presented, one was adopted to explain the properties of Mn doped PST. The growth of PST directly on SiO2 is the subject of chapter 7. Actually the ability of PST to grow crack free on SiO2 was discovered by accident while working with a patterned Pt bottom electrode. BST and PZT crack and blister directly on SiO2 and the question “Why does PST grow without an additional interlayer like Pt?” led to an in- depth analysis of the interfacial regions between PST/SiO2 and PZT/SiO2 by Auger Emission Spectroscopy.
Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications The multifunctionality of PST is the topic of chapter 8, where PST serves as a buffer layer for piezoelectrical PZT. PST has a similar perovskite-type structure and a lattice mismatch of only ~3% to PZT. The specialty of PST to grow crack free on SiO2 would help to incorporate PZT with patterned bottom electrodes, a matter which is still fairly complex and difficult. The thesis ends with a conclusion chapter that also looks towards the future and assesses the ramifications of this research. 11
Page 1 and 2: Sol-Gel derived Ferroelectric Thin
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Page 5 and 6: Acknowledgements I want to take thi
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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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Tunability [%] 100 90 80 70 60 50 4
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(g) (f) (e) (d) (c) (b) (a) Sol-Gel
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Internships: Sol-Gel derived Ferroe
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PhD Thesis Arne Lüker final version V4 - Cranfield University

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