Faculté des sciences et techniques de l'ingénieur STI, Programme doctoral Sciences et Génie des matériaux, Institut des matériaux IMX (Laboratoire de céramique LC)

Polarisation reversal in ferroelectric PVDF and PZT films

Gysel, Roman ; Setter, Nava (Dir.) ; Tagantsev, Alexandre (Dir.)

Thèse Ecole polytechnique fédérale de Lausanne EPFL : 2008 ; no 4097.

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    Ferroelectrics are characterised by a spontaneous polarisation that can be reversed by an external electric field. The stability of the polarisation states and the possibility for controlled switching between the states render ferroelectric materials very attractive for memory applications, in which polarisation states are associated to a binary information "0" or "1". Fast, dense, and reliable data storage has gained enormous importance in our information technology based society. In this context, the fundamental physics of ferroelectric materials attracted great interest with the ultimate goal of reaching memory elements with shortest access times and highest densities. Two model systems of ferroelectric materials were studied in this thesis on the polarisation reversal: an inorganic, classical perovskite and an organic polymer. Despite the fundamental structural differences between lead zirconate titanate [Pb(Zr,Ti)O3] and the investigated polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE)] copolymer, the switching kinetics in these materials can be described by a similar formalism. Our approach of a combination of macroscopic and microscopic techniques enables an insight into the mechanisms of the polarisation reversal and allows for a better understanding of macroscopic effects governed by mechanisms occurring on the nanoscale. The experimental techniques developed for the ferroelectric capacitor structures have been successfully applied for studying a first hybrid PVDF/silicon ferroelectric transistor for non-volatile information storage. The major accomplishments of this thesis are the following: We demonstrated for the first time a piezoelectric scanning probe study on the cross section of ferroelectric thin films. In combination with switching pulses, this allowed us to directly image the polarisation reversal in ferroelectric films quasi-dynamically. The utility of cross-sectional domain imaging as a powerful tool for the study of the polarisation reversal in ferroelectrics has been corroborated. The apparent contradiction between observations interpreted as a fast sideways growth and the classical scenario of a fast forward growth could be explained by an oblique polarisation direction with respect to the film plane. A surface-stimulated nucleation of reverse domains, with preference on the electrode with a negative potential was observed. It was interpreted in terms of a built-in field due to a depletion layer at the ferroelectric-electrode interface. A combination of microscopic and macroscopic methods used for the first time in organic ferroelectric thin films enabled a better insight into polarisation reversal of the copolymer of vinylidene fluoride and trifluoroethylene P(VDF-TrFE). The polarisation reversal was found to be impeded by a restricted domain growth. This limits the applicability of the conventional Kolmogorov-Avrami model as well as the nucleation-limited switching model with a broad distribution of nucleation times. We demonstrated that an interface-adjacent passive layer may impact on the switching properties and gives rise to a retardation of the polarisation reversal. Furthermore, an extraordinary dielectric constant increase was observed in the films with a passive layer due to an additional domain wall contribution. We fabricated for the first time a hybrid silicon/ferroelectric polymer ferroelectric single transistor memory element with non-volatile operation. The observed difference of the polarisation reversal kinetics of the ferroelectric in the simple capacitor and in the transistor structure was explained by the gate oxide buffer layer in the transistor. Our previously developed tools and approaches enabled a better understanding of retention loss mechanism. A semi-quantitative retention model was suggested that explains the observed exponential retention loss. The investigation of relevant fundamental aspects of ferroelectric materials and the development of novel characterisation techniques in this work can be of interest for further development of non-volatile memory devices.