Strong Bulk Photovoltaic Effect in Planar Barium Titanate Thin Films

The bulk photovoltaic effect (BPE) leads to the generation of a photocurrent from an asymmetric material. Despite drawing much attention due to its ability to generate photovoltages above the band gap ($E_g$), it is considered a weak effect due to the low generated photocurrents. Here, we show that a remarkably high photoresponse can be achieved by exploiting the BPE in simple planar BaTiO$_3$ (BTO) films, solely by tuning their fundamental ferroelectric properties via strain and growth orientation induced by epitaxial growth on different substrates. We find a non-monotonic dependence of the responsivity ($R_{\rm SC}$) on the ferroelectric polarization ($P$) and obtain a remarkably high BPE coefficient ($\beta$) of $\approx$10$^{-2}$ 1/V, which to the best of our knowledge is the highest reported to date for standard planar BTO thin films. We show that the standard first-principles-based descriptions of BPE in bulk materials cannot account for the photocurrent trends observed for our films and therefore propose a novel mechanism that elucidates the fundamental relationship between $P$ and responsivity in ferroelectric thin films. Our results suggest that practical applications of ferroelectric photovoltaics in standard planar film geometries can be achieved through careful joint optimization of the bulk structure, light absorption, and electrode-absorber interface properties.Comment: 12 pages, 8 figure

Emergent Ferroic Order at Oxide Heterointerfaces

This work focuses on complex-oxide superlattices as a pathway to numerous forms of emergent phenomena due to strong interfacial interactions present in unit-cell precise structures. Superlattices based on ferroelectric and dielectric materials can elicit new forms of ferroelectricity, exotic dipolar textures, and distinctive domain structures. Here, I place two different ferroelectric phases in the PbZr1-xTixO3 system and use superlattice design as a proxy for local composition – asking what happens when the overall chemistry is that of the morphotropic phase boundary, but the individual layers are far away from that boundary? The intimate interfacing of these dissimilar materials results in a unique combination of effects: simultaneous large polarization magnitude and large permittivity. The material effectively acts like a combination of the robust parent ferroelectrics and an interfacial region that akin to the phase-boundary. Next, I analyze relaxor-like behavior, which is typically associated with chemical inhomogeneity and complexity in solid solutions, in atomically precise (BaTiO3)n/(SrTiO3)n symmetric superlattices. Dielectric studies reveal frequency dispersion of dielectric response which increases in magnitude as the periodicity decreases. Techniques such as Vogel-Fulcher analysis and bond valence molecular dynamics simulations reveal that relaxor-like behavior in short-period superlattices arises from temperature-driven size variations of antipolar stripe domains in contrast to the more thermally stable dipolar configurations in long-period superlattices. Moreover, the size and shape of antipolar domains are tuned by superlattice periodicity following Kittel’s Law thus providing an artificial route to relaxor-like behavior which may expand the ability to control desired properties in these complex systems. Ultimately, I show that unit-cell-precise deposition provides a pathway to design novel heterostructures and thus access interfacial-driven phenomena, posing the question: can control at a single unit-cell be achieved with designer chemical ordering and structure? Such questions will motivate the community’s further pursuit of these matters. The current work paves the way for future research in the realization of novel approaches to ferroelectricity stemming from interfacial interactions, which will continue to be more relevant as devices get smaller