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Photoferroelectrics offer unique opportunities to explore light energy conversion based on their polarization-driven carrier separation and above-bandgap voltages. The problem associated with the wide bandgap of ferroelectric oxides, i.e., the vanishingly small photoresponse under visible light, has been overcome partly by bandgap tuning, but the narrowing of the bandgap is, in principle, accompanied by a substantial loss of ferroelectric polarization. In this article, we report an approach, ‘gap-state’ engineering, to produce photoferroelectrics, in which defect states within the bandgap act as a scaffold for photogeneration. Our first-principles calculations and single-domain thin-film experiments of BiFeO3 demonstrate that gap states half-filled with electrons can enhance not only photocurrents but also photovoltages over a broad photon-energy range that is different from intermediate bands in present semiconductor-based solar cells. Our approach opens a promising route to the material design of visible-light-active ferroelectrics without sacrificing spontaneous polarization., Overcoming the optical transparency of wide bandgap of ferroelectric oxides by narrowing its bandgap tends to result in a loss of polarization. By utilizing defect states within the bandgap, Matsuo et al. report visible-light-active ferroelectrics without sacrificing polarization.

Controlling and manipulating defects in materials provides an extra degree of freedom not only for enhancing physical properties but also for introducing additional functionalities. In ferroelectric oxides, an accumulation of point defects at specific boundaries often deteriorates a polarization-switching capability, but on the one hand, delivers interface-driven phenomena. At present, it remains challenging to control oxygen vacancies at will to achieve a desirable defect structure. Here, we report a practical route to designing oxygen-vacancy distributions by exploiting the interaction with transition-metal dopants. Our thin-film experiments combined with ab-initio theoretical calculations for BiFeO3 demonstrate that isovalent dopants such as Mn3+ with a partly or fully electron-occupied eg state can trap oxygen vacancies, leading to a robust polarization switching. Our approach to controlling oxygen vacancy distributions by harnessing the vacancy-trapping capability of isovalent transition-metal cations will realize the full potential of switchable polarization in ferroelectric perovskite oxides.

Superlattice-structured epitaxial thin films composed of Mn(5%)-doped BiFeO3 and BaTiO3 with a total thickness of 600 perovskite (ABO3) unit cells were grown on single-crystal SrTiO3 substrates by pulsed laser deposition, and their polarization and dielectric properties were investigated. When the layers of Mn-BiFeO3 and BaTiO3 have over 25 ABO3 unit cells (N), the superlattice can be regarded as a simple series connection of their individual capacitors. The superlattices with an N of 5 or less behave as a unified ferroelectric, where the BaTiO3 and Mn-BiFeO3 layers are structurally and electronically coupled. Density functional theory calculations can explain the behavior of spontaneous polarization for the superlattices in this thin regime. We propose that a superlattice formation comprising two types of perovskite layers with different crystal symmetries opens a path to novel ferroelectrics that cannot be obtained in a solid solution system.

Domain structures in polar materials provide an additional degree of freedom to tune ferroelectric photovoltaic (PV) effects. One of the approaches to control domain structures is to form a solid solution with analogs in different symmetries. In this study, we investigate the influence of domain structures on the PV properties of ferroelectric thin films in the xBaTiO3–(1−x)BiFeO3 (BT–BFO) solid-solution system (x = 0.1 and 0.2). We found that a substitution of BT for BFO substantially decreases the domain size down to several tens of nanometers, leading to an enhanced PV response owing to a marked contribution of the domain-wall PV effect.

We have investigated the local polarization switching behaviors of epitaxial (Bi1/2Na1/2)TiO3 (BNT) thin films obtained by a pulsed laser deposition (PLD). Using ozone gas as a deposition atmosphere of PLD, epitaxial growth of BNT films was achieved on SrRuO3/SrTiO3(100) electrode substrates. Piezoresponse force microscopy (PFM) revealed that BNT grains in the films have a single-domain structure without domain walls. These domain structures and the local polarization switching measured by PFM indicate that the oxidizing atmosphere of ozone gas is considered to suppress the generation of bismuth and oxygen vacancies in the deposited BNT layers. We propose that the PLD method using ozone is effective in obtaining high-quality single-phase BNT films with a less concentration of lattice vacancies.

We investigate the photovoltaic (PV) effects in ferroelectrics based on the polar structure in domains and the intrinsic symmetry breaking of ferroelastic domain walls (DWs). A comprehensive analysis for $\mathrm{BiFe}{\mathrm{O}}_{3}$ films with the single-domain and 71\ifmmode^\circ\else\textdegree\fi{} domain structures reveals a major contribution of the DWs superimposed on the bulk PV effect, which can explain the enhanced PV response in the multidomain film exhibiting photovoltages greater than the band gap. Our methodology can reveal the contributions of the bulk PV and the DW-PV effects quantitatively and uncover a design strategy for enhancing ferroelectric photovoltaics.

We investigate ferroelectric BaTiO3 capacitors with SrRuO3-based electrodes employing a (BaxSr1−x)TiO3 (BST) buffer layer on (100) SrTiO3 substrates prepared by pulsed laser deposition. Structural analysis by high-resolution X-ray diffraction reciprocal space mapping shows that the BST (x = 0.5 and 0.7) layers are relaxed and have their bulk in-plane lattices in the upper part owing to strain relief. The bottom electrodes on the buffer layers grow heteroepitaxially but cannot withstand a tensile stress and then show a diminution of the in-plane lattice. On the BST (x = 0.7) layer, compared with the capacitor with an SrRuO3 electrode, that with a Ba0.1Sr0.9RuO3 one has a reduced in-plane lattice relaxation of the BaTiO3 film and then exhibits a larger remanent polarization (Pr) of 34 μC/cm2 associated with a smaller shift of the hysteresis loop. The polarization hysteresis shift is attributed to a flexoelectric effect stemming from the coupling between out-of-plane polarization and a strain gradient in the BaTiO3 film. We conclude that a reduced misfit strain relaxation in the ferroelectric film achieved with the Ba0.1Sr0.9RuO3 electrode on the thicker BST (x = 0.7) buffer layer is the origin of an enhanced Pr with a smaller hysteresis shift.

Ferroelectric BaTiO3 (BT) thin-film capacitors with a buffer layer of (Ba1− x Sr x )TiO3 (BST) have been fabricated on (001) SrTiO3 (STO) single-crystal substrates by a pulsed laser deposition method, and the crystal structure and polarization hysteresis properties have been investigated. X-ray diffraction reciprocal space mapping shows that the BST buffer effectively reduces the misfit strain relaxation of the BT films on SrRuO3 (SRO) electrodes. The BT capacitor with the SRO electrodes on the BST (x = 0.3) buffer exhibits a well-saturated hysteresis loop with a remanent polarization of 29 µC/cm2. The hysteresis loop displays a shift toward a specific field direction, which is suggested to stem from the flexoelectric coupling between the out-of-plane polarization and the strain gradient adjacent to the bottom interface.

Photovoltaic (PV) properties of ferroelectric SrRuO3/BiFeO3/SrRuO3 (SRO/BFO/SRO) epitaxial thin-film capacitors are investigated. The experimental results of the markedly reduced PV response caused by the doping of Mn provide evidence that the PV properties originate not from the bulk-derived PV effect but from an interfacial band bending. We show that the capacitors having a defective layer composed of oxygen vacancies at the SRO/BFO interface exhibit a markedly large photocurrent. Our study demonstrates that a cooperative effect between the interface charges arising from ferroelectric polarization and the oxygen-vacancy-rich layer enhances the PV response in capacitor form in the BFO system.

We investigate the mechanism of a switchable diode behavior observed in ferroelectric SrRuO3/BiFeO3 (BFO)/SrRuO3 capacitors. We experimentally demonstrate that the switchable diode effect observed in the capacitors is induced by the polarization reversal in the BFO film. The conductivity in an Ohmic region in different oxidation states provides direct evidence that electron hole acts as the majority carrier, delivering p-type conduction. Density functional theory (DFT) calculations show that the p-type conduction arises from an unoccupied gap state of Fe4+ in an FeO5 pyramid which is derived from Bi vacancy. Our experimental and DFT study leads to the conclusion that the switchable diode effect originates from an asymmetric band bending in the top and bottom depletion layers modulated by ferroelectric polarization and oxygen vacancies.

Mn doping effect on the crystal structure and spontaneous polarization (Ps) in ferroelectric BiFeO3 (BFO) thin films are experimentally and theoretically investigated. Polarization hysteresis measurements for BiFe1−xMnxO3 (BFMO) films grown on (100) SrTiO3 substrates with SrRuO3 electrodes indicate that remanent polarization (Pr) remains unchanged in the range of 0 ≤ x ≤ 0.1. As a result of the density-functional theory (DFT) calculations for the rhombohedral, tetragonal, and monoclinic phases of BFO and BFMO (x = 0.5), we find that the monoclinic phase with compressive strain from the substrates yields the most reasonable Ps. The DFT calculations also indicate that heavy doping of up to 50% Mn does not lead to a considerable deterioration in Ps, which is consistent with the experiments. Our investigations demonstrate that the ferroelectric polarization in the BFMO system can be retained for a wide range of Mn content either in bulk or thin-film form.

The crystal structures and dielectric, polarization, and piezoelectric properties of high-density x(Bi0.5K0.5)TiO3–(1−x)BiFeO3 ceramics were investigated. The results obtained using x-ray and neutron powder diffractions and transmission electron microscopy showed that a morphotropic phase boundary between the rhombohedral (ferroelectric) and pseudocubic (ferroelectric) phases is present in 0.4

Crystal structures and microstructures associated with ferroelectric properties in a lead-free piezoelectric compound, x(Bi0.5K0.5)TiO3–(1-x)BiFeO3, have been investigated mainly by transmission electron microscopy (TEM). Complicated ferroelectric domain structures with a width of approximately ∼200 nm were found in the polar rhombohedral structure of the x = 0.30 compound. On the other hand, a tweed-like contrast, which stems from local lattice distortion, was observed in the pseudocubic structure of the x = 0.40, 0.46, and 0.50 compounds. The present results suggest that the pseudocubic phase is characterized by nanodomain structures consisting of non-polar cubic and polar rhombohedral structures.

Electrical conduction properties of SrRuO 3 (SRO)/BiFeO 3 (BFO)/SRO and SRO/10% Mn-doped BFO(BFMO)/SRO ferroelectric thin-film capacitors are investigated. The BFO capacitors exhibit a switchable diode effect accompanied by a conduction change from ohmic to space-charge-limited current with increasing external field. In contrast, the BFMO capacitors show only an ohmic conduction, arising from a considerable reduction in depletion layer width at the SRO/BFMO interfaces. These results suggest that the diode property can be tuned by Mn content in the BFO film. Our study opens the possibility of controlling the diode effect in BFO-based devices by a dilute Mn doping.