PhD Thesis Arne Lüker final version V4 - Cranfield University

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150 PST as a buffer layer for PZT on SiO2 device structures such as lattice mismatch and thermal expansion coefficient mismatch between the buffer PST and the substrates, and strain in the film arising during deposition process [3, 12, 13]. For example, the lattice parameter (a-axis) of PST (60/40) composition is 0.3919 nm, giving a lattice mismatch of only ~2.9% between PZT (a = 0.4035 nm for 111) and PST whereas the lattice mismatch between Pt (a = 0.3923 nm) and PST is only ~0.11% [2, 8]. A systematic analysis of thermal stress between PZT and PST films with various Sr content and the nature of the resulting stress between those two, e.g. tensile or compressive caused by the thermal treatment including its relation with porosity and film density are still required to develop an in-depth knowledge of the films and subsequently the device properties which is believed to be still ongoing. Here and now only a qualitative analysis of the PST film as a barrier focusing on the possible integration in PZT transducer stacks could be initiated and compared with other commonly used barrier materials such as TiO2 films. The preliminary results on structural evaluation of the PZT/PST composites demonstrated that among other factors to grow high quality PZT thin films, the crystallisation temperature of the barrier PST layer plays a vital role to optimise the interfaces of substrate/barrier/electrode structures and ultimately the texture of the PZT films including its grain orientations. For example, PZT films without a seed layer often exhibit a texture consisting of rosette-like clusters that cause serious problems in the reliability of the FRAM devices due to inhomogeneous grain distribution of larger size (1- 3 µm in diameter) [14]. It is therefore necessary to control the microstructure and porosity of the barrier layer that is expected to develop into a smooth textured film with fine, dense and uniform grains under optimised thermal treatment. In particular, PST possesses perovskite-type structure similar to PZT films having lower lattice mismatch to PZT and Pt layers. PST films could provide energetically favourable growth conditions producing sufficient nucleation sites during its crystallisation on Si substrates that ensure a thermally stable PZT/PST interface to grow PZT thin film on its top subsequently. A controlled thermal treatment of underneath PST layer, which looked apparently in the range of 550- 600°C at onset of its crystallisation phase, could produce a crack-free smooth crystalline surface enhancing further the transformation of preferred perovskite phase of top PZT thin films without degrading the ferroelectric properties of PZT device structures.
Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications 8.3 Electrical Characterisation of PZT with a PST buffer layer The electrical characterisation of PZT/PST device structures was performed to assess the dielectric properties and polarisation hysteresis of the PZT composites with underneath PST film deposited at 550°C. It is to be noted that the capacitor design was comprised of PZT film sandwiched between top Cr/Au and bottom Pt electrodes, and its dielectric properties should not, in principle, be affected by the PST film properties, e.g. in terms of Sr content, tetragonal-cubic phase transformation and voltage-dependent dielectric properties typically as observed in PST device structures. The electrical performance was also analysed by comparing the dielectric properties of PZT composite having similar thickness of around 0.5-0.6 µm grown on a different template, in this case PZT/Pt/Ti/TiO2/SiNx/Si with TiO2 as a barrier layer. Fig. 8.8 shows the frequency dispersion of the dielectric constant (εr) and loss factor (tanδ) obtained from both types of PZT device structures. Permittivity 420 410 400 390 380 370 360 350 0 50 100 150 200 250 300 0 350 Frequency [kHz] Fig. 8.8: Permittivity and dielectric loss of PZT on (a), (c) PST and (b), (d) TiO2 as a buffer layer (c) (d) (a) (b) 0.07 0.06 0.05 0.04 0.03 0.02 0.01 Dielectric loss 151
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Sol-Gel derived Ferroelectric Thin
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Abstract Ferroelectric perovskite t
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Intensity [arb. Units] (g) (f) (e)
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PhD Thesis Arne Lüker final version V4 - Cranfield University

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