PhD Thesis Arne Lüker final version V4 - Cranfield University

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156 Final Discussion, Conclusions and Future Work Final Discussions, Conclusions and Future Work Fig. 9.1 represents the journey we took within this work in terms of the figure of merit (FOM) and different ferroelectric perovskite thin films. FOM Fig. 9.1: The figure of merit of different ferroelectric perovskite thin films presented in this thesis. We started with BST 40/60 thin films with a very low FOM of 1.4 which is a result of severe film cracking during the high temperature annealing bake. With the change to PST 50/50, a system which is very friendly in terms of film cracking, the FOM increased rapidly up to a value of 16.6. A small improvement of 1.65 could be made with the change of the electrode from Pt to Cu. This was explained with the lower resistivity of Cu which increased the quality factor of the overall material system. But is was observed that the response of PST 50/50 to an applied electric field showed a hysteresis (see fig. 5.9) indicating that the thin film is in the ferroelectric tetragonal state with a permanent electric dipole moment which in turn increases the loss. By decreasing the Pb content down to 40% the Curie temperature was ensured to be below room temperature viz. the film was in the favoured paraelectric state without a permanent dipole moment. However, the FOM decreased slightly. Doping of the B site with Mn altered the charge balance inside the perovskite film.
Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications Fig. 9.2: Tunability and figure of merit at 10V and dielectric constant and Curie temperature at zero bias Fig. 9.2 summarises the effect of Mn doping in more detail. The dielectric constant reaches a maximum of 1100 with 3 mol% Mn; the maximum value of the tunability with 10 V is 76.72% with 1 mol% Mn and the FOM reaches 23.96 with 3 mol% Mn. This compares well with results from Du et al. [1], who reported a tunability of 80% and a FOM of 14.17 in pure PST 40/60 and 70% and 7 in La doped PST, or Sun et al. [2], who achieved a tunability of 69.4% and a FOM of 28.9 in (Pb0.25Ba0.05Sr0.7)TiO3. All these values drop significantly when the Mn doping level exceeds 3 mol% and we identified two possible reasons for this behaviour. First, Mn 2+ doping consumes oxygen vacancies to get incorporated as Mn 3+ and/or Mn 4+ at the Ti 4+ site of the (Pb,Sr)Ti03 perovskite crystal structure. The negative charged Mn ions balance the positive induced charge of the oxygen vacancies leading to a more “perfect” (cubic) and electronically saturated perovskite. At the same time, more polarisation path may be provided when the lattice structure becomes more perfect. This results in an increase of the dielectric constant, tunability and FOM. of PST 40/60 with different Mn content At a doping level of 2 mol% Mn the crystal is totally saturated. With further doping a hopping conduction due to the hopping of the charge carriers between Mn sites begins to occur and becomes distinct in 5 mol% doped films. This type of hopping process is therefore associated with a certain amount of Mn dopant and more Mn amount can 157
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Sol-Gel derived Ferroelectric Thin
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Abstract Ferroelectric perovskite t
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Table of Content Introduction and M
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Intensity [arb. Units] (g) (f) (e)
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Relative diel. diel. constant Const
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PhD Thesis Arne Lüker final version V4 - Cranfield University

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