UC Berkeley

Ferroelectrics are an important group of functional materials which support various applications including memory, dielectrics, and piezoelectrics. The definitive feature for classical ferroelectrics is the spontaneous polarization that can be switched to different directions under electric fields, and the characteristic properties include the polarization-electric field (P-E) hysteresis loops and the phase transition at the Curie temperature. Microscopically, classical ferroelectrics exhibit a long-range polar order with identical local polarization in each unit cell. The disordered cousins of classical ferroelectrics, relaxor ferroelectrics, however, form a nanoscale polar order which produces distinctive properties, such as slim, tilted P-E loops, diffuse phase transitions, and multiple critical temperatures. Epitaxial thin films of relaxor ferroelectrics pave a pathway to manipulating the nanoscale polar order and properties in such materials, providing multiple tuning parameters including strain, thickness, orientation, and multilayer/superlattice geometry. In this dissertation, I first present a systematic study of crystal-orientation effects in relaxor thin films. While PbMg1/3Nb2/3O3 thin films appear relatively insensitive to crystal orientation, BaZr0.5Ti0.5O3 thin films exhibit strong orientation dependence, with (111)-oriented films showing enhanced dielectric responses and elevated critical temperatures compared to (001)-oriented films. Next, I discuss the research efforts on the relaxor-based superlattices, [BaTiO3]m/[BaZrO3]n (m, n = 4-12), in which the chemical order is deterministically, precisely controlled, and consequently the polar order is manipulated. With decreasing BaTiO3 layer thickness, a transition from ferroelectric- to relaxor-like behaviors was observed driven by the enhanced random-field strength. Finally, I demonstrate the potential of relaxor-based multilayers as high-performance thin-film piezoelectrics, focusing on PbZr0.2Ti0.8O3/0.68PbMg1/3Nb2/3O3-0.32PbTiO3/PbZr0.2Ti0.8O3 trilayers. The strain state in the ferroelectric top/bottom layers is tuned such that they exhibit a mixed phase between c/a and a1/a2 domain structures, and the facilitated field-induced transition from a1/a2 to c/a domain structures underpins the piezoelectric response. Introducing the relaxor middle layers further improves the dielectric strength, thus enhancing the maximum field-induced strain up to 2.10%. Ultimately, in epitaxial relaxor thin films and relaxor-based multilayers, the combination of strain, thickness, orientation, and multilayer/superlattice engineering provides a powerful toolset to manipulate the nanoscale polar order and properties, and opens up a new playground for the design of future thin-film dielectric and piezoelectric materials.