Structural ferroelectric and piezoelectric properties of epitaxial PMN-PT thin films deposited by pulsed laser deposition

Abstract

Ferroelectric oxides, most notably PZT, are typically used for the fabrication of high performing piezoelectric MEMS devices for the purpose of sensing and actuation. An alternative to PZT is the relaxor ferroelectric such as PMN-PT. Bulk PMN-PT shows a 5-10 times larger piezoresponse compared to bulk PZT ceramics and has a large electromechanical coupling coefficient of k33≈0.9. Therefore, this material is an auspicious candidate for hyperactive MEMS devices for sensor and actuator applications. This thesis deals with the study of epitaxial PMN-PT films for the purpose of sensing and actuation. Perovskite phase and (001) orientation pure epitaxial PMN-PT films were fabricated by carefully controlling the fabrication conditions. Within a relative narrow process window to achieve phase and orientation pure films, the effect of the growth conditions like substrate temperature, fluence, target-substrate distance and ambient gas pressure was investigated. This study shows that polarization is more sensitive to substrate temperature then to target distance and much less to fluence and least to gas pressure. It is known that better lattice matching can broaden the processing window. Superior lattice matching was achieved using a PZT buffer layer between the PMN-PT and SRO bottom electrode resulting in a significant improvement in crystallinity and consequently in properties of the PMN-PT films. Furthermore the effect of lattice strain was investigated by varying the PZT interfacial layer composition in a wide range. The PMN-PT films showed giant self-bias field which can be tuned by the composition the of PZT interfacial layer. This is of significance for devices like energy scavengers and low energy consumption sensors that preferably operate at zero bias voltage. The highest longitudinal piezoelectric coefficient (d33) was achieved for PMN-PT films deposited on PZT (50/50) interfacial layer with lowest self-bias field. For some applications, it is desirable to have a higher d33 as well as a high self-bias. It is known that doped PZTs can produce selfbias field. Therefore, PMN-PT films were deposited on Nb and Fe doped PZT interfacial layers. PMNPT films on doped PZT interfacial layer show a higher self-bias along with a higher d33. Furthermore the effect of Nb-doped PZT interfacial layer thickness on the properties of PMN-PT was investigated, which showed a variation in properties associated with epitaxial strain. Besides epitaxial strain, the residual strain (substrate induced strain) due to the difference of thermal expansion coefficients between substrate and deposited film, show an influence on the ferro and piezoelectric properties of epitaxial PMN-PT films. To study the effect of substrate induced strain on the ferro and piezoelectric properties, PMN-PT films were prepared on a wide range of substrates. This study shows that substrate induced strain has a significant effect on the structural properties of PMN-PT films and consequently on the ferro and piezoelectric properties of these films. The knowledge of strain effect on properties is useful to engineer a PMN-PT film with certain desired properties. Silicon is the gold standard substrate for the fabrication of MEMS sensors. To study the effect of orientation on properties, epitaxial PMN-PT films with different out of plane orientation [(001) and (110)] were prepared using a CeO2/YSZ bilayer buffer and symmetric SrRuO3 electroded silicon substrates by carefully controlling the growth conditions. The ferroelectric and piezoelectric properties of these ferroelectric capacitors exhibited orientation dependence. The (001) oriented films show a relatively large self-bias voltage compared to the (110) oriented films. This is ascribed to a strain gradient layer in the relaxor at the interface with the bottom electrode. The aging behaviour of (001) films was superior compared to that of (110) films due to less structural defects. Nevertheless a superior effective, longitudinal piezoelectric coefficient (d33,eff) for (110) oriented films was observed compared to (001) oriented films, due to a less dense columnar structure. Furthermore, a PMN-PT thin film based sensing device (cantilever) was also fabricated and characterized with respect to ferroelectric and piezoelectric response which showed excellent properties.