Overcoming the Fundamental Barrier Thickness Limits of Ferroelectric Tunnel Junctions through BaTiO3/SrTiO3 Composite Barriers.

Ferroelectric tunnel junctions (FTJs) have attracted increasing research interest as a promising candidate for nonvolatile memories. Recently, significant enhancements of tunneling electroresistance (TER) have been realized through modifications of electrode materials. However, direct control of the FTJ performance through modifying the tunneling barrier has not been adequately explored. Here, adding a new direction to FTJ research, we fabricated FTJs with BaTiO3 single barriers (SB-FTJs) and BaTiO3/SrTiO3 composite barriers (CB-FTJs) and reported a systematic study of FTJ performances by varying the barrier thicknesses and compositions. For the SB-FTJs, the TER is limited by pronounced leakage current for ultrathin barriers and extremely small tunneling current for thick barriers. For the CB-FTJs, the extra SrTiO3 barrier provides an additional degree of freedom to modulate the barrier potential and tunneling behavior. The resultant high tunability can be utilized to overcome the barrier thickness limits and enhance the overall CB-FTJ performances beyond those of SB-FTJ. Our results reveal a new paradigm to manipulate the FTJs through designing multilayer tunneling barriers with hybrid functionalities.

Flexoelectric rectification of charge transport in strain-graded dielectrics.

Flexoelectricity is emerging as a fascinating means for exploring the physical properties of nanoscale materials. Here, we demonstrated the unusual coupling between electronic transport and the mechanical strain gradient in a dielectric epitaxial thin film. Utilizing the nanoscale strain gradient, we showed the unique functionality of flexoelectricity to generate a rectifying diode effect. Furthermore, using conductive atomic force microscopy, we found that the flexoelectric effect can govern the local transport characteristics, including spatial conduction inhomogeneities, in thin-film epitaxy systems. Consideration of the flexoelectric effect will improve understanding of the charge conduction mechanism at the nanoscale and may facilitate the advancement of novel nanoelectronic device design.

Nanoscale mapping of temperature-dependent conduction in an epitaxial VO2 film grown on an Al2O3 substrate.

Vanadium dioxide (VO2) is one of the extensively studied strongly correlated oxides due to its intriguing insulator-metal transition near room temperature. In this work, we investigated temperature-dependent nanoscale conduction in an epitaxial VO2 film grown on an Al2O3 substrate using conductive-atomic force microscopy (C-AFM). We observed that only the regions near the grain boundaries are conductive, producing intriguing donut patterns in C-AFM images. Such donut patterns were observed in the entire measured temperature range (300-355 K). The current values near the grain boundaries increased by approximately two orders of magnitude with an increase in the temperature, which is consistent with the macroscopic transport data. The spatially-varied conduction behavior is ascribed to the coexistence of different monoclinic phases, i.e., M1 and M2 phases, based on the results of temperature-dependent Raman spectroscopy. Furthermore, we investigated the conduction mechanism in the relatively conductive M1 phase regions at room temperature using current-voltage (I-V) spectroscopy and deep data analysis. Bayesian linear unmixing and k-means clustering showed three distinct types of conduction behavior, which classical C-AFM cannot resolve. We found that the conduction in the M1 phase regions can be explained by the Poole-Frenkel mechanism. This work provides deep insight into IMT behavior in the epitaxial VO2 thin film at the nanoscale, especially the coexistence and evolution of the M1 and M2 phases. This work also highlights that I-V spectroscopy combined with deep data analysis is very powerful in investigating local transport in complex oxides and various material systems.

A New Approach to the Fabrication of Memristive Neuromorphic Devices: Compositionally Graded Films.

Energy-efficient computing paradigms beyond conventional von-Neumann architecture, such as neuromorphic computing, require novel devices that enable information storage at nanoscale in an analogue way and in-memory computing. Memristive devices with long-/short-term synaptic plasticity are expected to provide a more capable neuromorphic system compared to traditional Si-based complementary metal-oxide-semiconductor circuits. Here, compositionally graded oxide films of Al-doped MgxZn1-xO (g-Al:MgZnO) are studied to fabricate a memristive device, in which the composition of the film changes continuously through the film thickness. Compositional grading in the films should give rise to asymmetry of Schottky barrier heights at the film-electrode interfaces. The g-Al:MgZnO films are grown by using aerosol-assisted chemical vapor deposition. The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the films show self-rectifying memristive behaviors which are dependent on maximum applied voltage and repeated application of electrical pulses. Endurance and retention performance tests of the device show stable bipolar resistance switching (BRS) with a short-term memory effect. The short-term memory effects are ascribed to the thermally activated release of the trapped electrons near/at the g-Al:MgZnO film-electrode interface of the device. The volatile resistive switching can be used as a potential selector device in a crossbar memory array and a short-term synapse in neuromorphic computing.

Selective control of multiple ferroelectric switching pathways using a trailing flexoelectric field.

Flexoelectricity is an electromechanical coupling between electrical polarization and a strain gradient 1 that enables mechanical manipulation of polarization without applying an electrical bias2,3. Recently, flexoelectricity was directly demonstrated by mechanically switching the out-of-plane polarization of a uniaxial system with a scanning probe microscope tip3,4. However, the successful application of flexoelectricity in low-symmetry multiaxial ferroelectrics and therefore active manipulation of multiple domains via flexoelectricity have not yet been achieved. Here, we demonstrate that the symmetry-breaking flexoelectricity offers a powerful route for the selective control of multiple domain switching pathways in multiaxial ferroelectric materials. Specifically, we use a trailing flexoelectric field that is created by the motion of a mechanically loaded scanning probe microscope tip. By controlling the SPM scan direction, we can deterministically select either stable 71° ferroelastic switching or 180° ferroelectric switching in a multiferroic magnetoelectric BiFeO3 thin film. Phase-field simulations reveal that the amplified in-plane trailing flexoelectric field is essential for this domain engineering. Moreover, we show that mechanically switched domains have a good retention property. This work opens a new avenue for the deterministic selection of nanoscale ferroelectric domains in low-symmetry materials for non-volatile magnetoelectric devices and multilevel data storage.

Oxygen Partial Pressure during Pulsed Laser Deposition: Deterministic Role on Thermodynamic Stability of Atomic Termination Sequence at SrRuO3/BaTiO3 Interface.

With recent trends on miniaturizing oxide-based devices, the need for atomic-scale control of surface/interface structures by pulsed laser deposition (PLD) has increased. In particular, realizing uniform atomic termination at the surface/interface is highly desirable. However, a lack of understanding on the surface formation mechanism in PLD has limited a deliberate control of surface/interface atomic stacking sequences. Here, taking the prototypical SrRuO3/BaTiO3/SrRuO3 (SRO/BTO/SRO) heterostructure as a model system, we investigated the formation of different interfacial termination sequences (BaO-RuO2 or TiO2-SrO) with oxygen partial pressure (PO2) during PLD. We found that a uniform SrO-TiO2 termination sequence at the SRO/BTO interface can be achieved by lowering the PO2 to 5 mTorr, regardless of the total background gas pressure (Ptotal), growth mode, or growth rate. Our results indicate that the thermodynamic stability of the BTO surface at the low-energy kinetics stage of PLD can play an important role in surface/interface termination formation. This work paves the way for realizing termination engineering in functional oxide heterostructures.

Interface Control of Ferroelectricity in an SrRuO3 /BaTiO3 /SrRuO3 Capacitor and its Critical Thickness.

The atomic-scale synthesis of artificial oxide heterostructures offers new opportunities to create novel states that do not occur in nature. The main challenge related to synthesizing these structures is obtaining atomically sharp interfaces with designed termination sequences. In this study, it is demonstrated that the oxygen pressure (PO2) during growth plays an important role in controlling the interfacial terminations of SrRuO3 /BaTiO3 /SrRuO3 (SRO/BTO/SRO) ferroelectric (FE) capacitors. The SRO/BTO/SRO heterostructures are grown by a pulsed laser deposition method. The top SRO/BTO interface, grown at high PO2 (around 150 mTorr), usually exhibits a mixture of RuO2 -BaO and SrO-TiO2 terminations. By reducing PO2, the authors obtain atomically sharp SRO/BTO top interfaces with uniform SrO-TiO2 termination. Using capacitor devices with symmetric and uniform interfacial termination, it is demonstrated for the first time that the FE critical thickness can reach the theoretical limit of 3.5 unit cells.

Energy landscape scheme for an intuitive understanding of complex domain dynamics in ferroelectric thin films.

Fundamental understanding of domain dynamics in ferroic materials has been a longstanding issue because of its relevance to many systems and to the design of nanoscale domain-wall devices. Despite many theoretical and experimental studies, a full understanding of domain dynamics still remains incomplete, partly due to complex interactions between domain-walls and disorder. We report domain-shape-preserving deterministic domain-wall motion, which directly confirms microscopic return point memory, by observing domain-wall breathing motion in ferroelectric BiFeO3 thin film using stroboscopic piezoresponse force microscopy. Spatial energy landscape that provides new insights into domain dynamics is also mapped based on the breathing motion of domain walls. The evolution of complex domain structure can be understood by the process of occupying the lowest available energy states of polarization in the energy landscape which is determined by defect-induced internal fields. Our result highlights a pathway for the novel design of ferroelectric domain-wall devices through the engineering of energy landscape using defect-induced internal fields such as flexoelectric fields.

Flexoelectric control of defect formation in ferroelectric epitaxial thin films.

Flexoelectric control of defect formation and associated electronic function is demonstrated in ferroelectric BiFeO3 thin films. An intriguing, so far never demonstrated, effect of internal electric field (Eint ) on defect formation is explored by a means of flexoelectricity. Our study provides novel insight into defect engineering, as well as allows a pathway to design defect configuration and associated electronic function.

Flexoelectric effect in the reversal of self-polarization and associated changes in the electronic functional properties of BiFeO(3) thin films.

Flexoelectricity can play an important role in the reversal of the self-polarization direction in epitaxial BiFeO3 thin films. The flexoelectric and interfacial effects compete with each other to determine the self-polarization state. In Region I, the self-polarization is downward because the interfacial effect is more dominant than the flexoelectric effect. In Region II, the self-polarization is upward, because the flexoelectric effect becomes more dominant than the interfacial effect.

Multilevel data storage memory using deterministic polarization control.

Multilevel non-volatile memory for high-density date storage is achieved by using the deterministic control of ferroelectric polarization. In a real ferroelectric thin-film system, eight stable and reproducible polarization states are realized (i.e., 3-bit data storage) by adjusting the displacement current. This approach can be used to triple or quadruple the memory density, even at existing feature scales.

Active control of ferroelectric switching using defect-dipole engineering.

Active control of defect structures and associated polarization switching in a ferroelectric material is achieved without compromising its ferroelectric properties. Based on dipolar interaction between defect dipole and polarization, the unique functionality of the defect dipole to control ferroelectric switching is visualized. This approach can provide a foundation for novel ferroelectric applications, such as high-density multilevel data storage.

Polarization relaxation induced by a depolarization field in ultrathin ferroelectric capacitors.

Time-dependent polarization relaxation behavior induced by a depolarization field E(d) was investigated on high-quality ultrathin SrRuO3/BaTiO3/SrRuO3 capacitors. The E(d) values were determined experimentally from an applied external field to stop the net polarization relaxation. These values agree with those from the electrostatic calculations, demonstrating that a large E(d) inside the ultrathin ferroelectric layer could cause severe polarization relaxation. For numerous ferroelectric devices of capacitor configuration, this effect will set a stricter size limit than the critical thickness issue.