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Abstract
This dissertation presents systemic studies on the ferroelectric properties of ultra-thin films under different interfacial conditions. In the first part, we focused on the engineering of the polarization rotation in ultra-thin bilayered (tetragonal PbZr0.3Ti0.7O3/rhombohedral PbZr0.55Ti0.45O3) epitaxial (001) PZT film. Robust and reversible in-plane and out-of-plane polarization can be observed at nano scale. The top PZT-T layer is decoupled from the clamping effect of substrate due to an inserted PZT-R layer. Moreover, the PZT-R layer reduces the symmetry of PZT-T layer by inducing an in-plane tensile strain. These facilitate large-scale reversible polarization rotation and enhanced dielectric and electromechanical responses. Next, the effect of an intentional a large depolarization field on PZT (PbZr0.2Ti0.8O3) is investigated. An intentional depolarization field is achieved by introducing SrTiO3 (STO) in two 3 nm thick PZT ultra-thin film on (001)-oriented STO substrate. By varying the STO spacer thickness (3 to 10 unit cells), the d-spacing and as-grown domain state of PZT layers are significantly affected. A 6 nm thick single PZT film was also deposited as reference. This ‘reference’ sample shows elongated PZT c lattice parameter (0.416 nm) with mostly monodomain polarization with downward orientation. It also shows significant imprint in the switching loops under external bias. By contrast, STO spacer changes the domain state from monodomain to stripe-like 180° polydomain in virgin state which reduces the imprint by 80%. In addition, the time duration of external electric field to cause domain switching is decreased dramatically compared to the ‘reference’ sample. In the third part, the 2 nm bottom electrode is replaced by 20 nm metal-like LSMO in PZT films. The PFM results indicate that the 180° polydomain configuration changes from stripe-like to bubble-like. The nano bubble domains are induced by the net effect of polarization field and external screening conditions in the ultra-thin ferroelectric film. Further, the bubble-like domains turn into strip-like domains by simply repeated scanning. In summary, this thesis shows that it is possible to engineer novel domains in ultra-thin ferroelectric films and hence functional performances via interfacial engineering of mechanical and electrical boundary conditions.