PhD Thesis Arne Lüker final version V4 - Cranfield University

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152 PST as a buffer layer for PZT on SiO2 The frequency range for these measurements was chosen between 1 KHz to 300 KHz applying a weak ac voltage excitation of 100 mV to preserve the polarisation state of the films. The dielectric constants in both PZT samples with buffer PST and TiO2 layers estimated at 1 kHz were 415 and 401 respectively and are quite similar to typically observed values in PZT films of 30/70 compositions [15]. These results revealed the fact that the buffer layer, in general, does not contribute to the dielectric properties of PZT thin film capacitors. As the frequency was increased, the dielectric constants in PZT samples with different types of templates exhibited a small variation of around 5% only, for example a higher value of εr = 390 at 100 KHz obtained from PZT stacks with buffer PST. This can be attributed to the interface effects produced by the different combinations of buffer layers and templates on PZT films including the lattice mismatch, thermal expansion mismatch, grain size and the possible contribution of domain structures [16]. In these particular structures under study, the PST thin film annealed at 550°C appeared to consist of mixed phases of nanocrystalline and amorphous structures, whereas TiO2 was fully crystallised into a stable rutile phase that is expected to influence the PZT film characteristics apparently. In fact, the dielectric loss measurements as illustrated in Fig. 8.8 gave rise to similar indication of interfacial effects showing a slightly higher loss factor, tanδ in PZT/PST structures compared to PZT/TiO2 stacks. However the dielectric loss was not significant in either type of structures and was very low only 0.034 and 0.025 at 100 KHz in PZT/PST and PZT/TiO2 stacks respectively showing good dielectric properties in agreement with published results obtained from typical PZT thin film devices [16, 17]. The ferroelectric behaviour of the PZT films with underneath PST was assessed by measuring the polarisation hysteresis loop as shown in Fig. 8.9. The saturation polarisation, remnant polarisation and coercive field were 39 µC/cm 2 , 28 µC/cm 2 and 104 kV/cm respectively at the maximum applied voltage of 19 V which is consistent with data obtained from PZT films on standard platinised substrates [3, 15]. As the piezoelectric coefficients (d31 and d33) are proportional to the remnant polarisation, the large value of the remnant polarisation obtained from the PZT/PST device structures is expected to produce stronger piezoelectric effect [17]. The initial characterisation of PZT device structures established that the PZT films with PST as a diffusion buffer had fully crystallized in the perovskite phase exhibiting good dielectric and ferroelectric behaviour. Although the piezoelectric
Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications coefficients of the PZT films were not measured directly in this study, it is envisaged from the experimental data of the dielectric constant and hysteresis loop that the PZT/PST composite has potential to provide good and comparable piezoelectric performances as typically observed in PZT device structures grown on commonly used barrier layers. Polarisation [µC/cm 2 ] 8.4 References PZT with PST buffer layer PZT on Pt/Ti -350 -300 -250 -200 -150 -100 -50 -10 50 100 150 200 250 300 350 1. P. Muralt, “Texture control and seeded nucleation of nanosize structures of ferroelectric thin films” J. Appl. Phys., Vol. 100, p.051605 (1-11), September, 2006. 2. K.H. Park, C.Y. Kim, Y.W. Jeong, H.K. Won, K.Y. Kim, J.S. Lee, S.T. Kim, “Microstructures and interdiffusion of Pt/Ti electrodes with respect to annealing in the oxygen ambient”, J. Material Res., Vol. 10 (7), p.1791-94, 1995 3. Z. Huang, Q. Zhang and R.W. Whatmore, “Low temperature crystallisation of lead zirconate titanate thin films by a sol-gel method”, J. Appl. Phys., Vol. 85 (10), p. 7355-61, May 1999 40 30 20 10 -20 -30 -40 Electric Field [kV/cm] Fig. 8.9: PE-loops of PZT 30/70 on a PST buffer layer and on a common Pt/Ti bottom electrode 153
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Sol-Gel derived Ferroelectric Thin
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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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