Ferroelectric Junctions Research Articles

Heterogeneous integration photoferroelectrics for self-powered photoelectric detectors

The anomalous photovoltaic effect inherent in ferroelectric materials brings promising opportunities for self-powered photoelectric detectors. However, the photoelectric detection performances in photoferroelectrics are limited by the low photocurrent output due to the recombination and low separation ability of photogenerated carriers. This work proposes a heterogeneous integration technology for self-powered photoelectric detectors by designing a P-type/N-type/P-type (PNP) ferroelectric junction with sandwich structure, in which not only the photogenerated carrier separation is strengthened due to the enhanced polarization derived from the interface compressive stress, but also the carrier recombination is suppressed by the built-in electric field in the depletion layer. Thus, the photocurrent output in the PNP heterojunction is increased by more than seven times at 0 V bias compared with N-type ferroelectric film and exhibits excellent responsivity and detectivity. The temperature stability for repeatable time-resolved self-powered photocurrent demonstrates applicability in high temperature environments. This work provides a feasible strategy for high-performance self-powered photoelectric detectors by heterogeneous integration in photoferroelectrics, promoting ferroelectric photonic applications.

Advances in Organic Multiferroic Junctions

Typically, organic multiferroic junctions (OMFJs) are formed of an organic ferroelectric layer sandwiched between two ferromagnetic electrodes. The main scientific interest in OMFJs focuses on the magnetoresistive properties of the magnetic spin valve combined with the electroresistive properties associated with the ferroelectric junction. In consequence, memristive properties that couple magnetoelectric functionalities, which are one of the most active fields of research in material sciences, are opening a large spectrum of technological applications from nonvolatile memory to elements in logic circuits, sensing devices, energy harvesting and biological synapsis models in the emerging area of neuromorphic computing. The realization of these multifunctional electronic elements using organic materials is presenting various advantages related to their low-cost, versatile synthesis and low power consumption functioning for sustainable electronics; green disintegration for transient electronics; and flexibility, light weight and/or biocompatibility for flexible electronics. The purpose of this review is to address the advancement of all OMFJs including not only the achievements in the charge and spin transport through OMFJs together with the effects of electroresistance and magnetoresistance but also the challenges and ways to overcome them for the most used materials for OMFJs.

Ultrathin Nitride Ferroic Memory with Large ON/OFF Ratios for Analog In-memory Computing.

Computing in the analog regime using nonlinear ferroelectric resistive memory arrays can potentially alleviate the energy constraints and complexity/footprint challenges imposed by digital von Neumann systems. Yet the current ferroelectric resistive memories suffer from either low ON/OFF ratios/imprint or limited compatibility with mainstream semiconductors. Here, for the first time, ferroelectric and analog resistive switching in an epitaxial nitride heterojunction comprising ultrathin (≈5nm) nitride ferroelectrics, i.e., ScAlN, with potentiality to bridge the gap between performance and compatibility is demonstrated. High ON/OFF ratios (up to 105 ), high uniformity, good retention, (<20% variation after >105 s) and cycling endurance (>104 ) are simultaneously demonstrated in a metal/oxide/nitride ferroelectric junction. It is further demonstrated that the memristor can provide programmability to enable multistate operation and linear analogue computing as well as image processing with high accuracy. Neural network simulations based on the weight update characteristics of the nitride memory yielded an image recognition accuracy of 92.9% (baseline 96.2%) on the images from Modified National Institute of Standards and Technology. The non-volatile multi-level programmability and analog computing capability provide first-hand and landmark evidence for constructing advanced memory/computing architectures based on emerging nitride ferroelectrics, and promote homo and hybrid integrated functional edge devices beyond silicon.

High valence states of Pd supported on ferroelectric BaTiO3 driven by electric polarization

At a metal–ferroelectric junction, it has been considered that the electric polarization of the ferroelectric material can affect the electronic structure of the neighboring metal. Here, we demonstrate that the valence state of Pd can be shifted to the unstable high value of 4+ by the electric field of electric polarization in ferroelectric BaTiO3. Study of the absorption fine structure of both hard and soft x rays revealed the existence of Pd4+ states on the surface of Pd oxide nanoparticles. The positions of Pd and oxygen atoms are shifted in opposite directions by the electric field due to the electric polarization of ferroelectric BaTiO3. The atomic displacement of Pd and O forms a zigzag structure, in which the coordination number of Pd atoms is changed from four to six, producing a quadrivalent state. This report presents experimental evidence that ferroelectric polarization can control the electronic states of neighboring metal atoms, and we suggest that using the ferroelectric support effect may produce a new type of catalyst.

Perspectives on Atomic-Scale Switches for High-Frequency Applications Based on Nanomaterials.

Nanomaterials science is becoming the foundation stone of high-frequency applications. The downscaling of electronic devices and components allows shrinking chip’s dimensions at a more-than-Moore rate. Many theoretical limits and manufacturing constraints are yet to be taken into account. A promising path towards nanoelectronics is represented by atomic-scale materials. In this manuscript, we offer a perspective on a specific class of devices, namely switches designed and fabricated using two-dimensional or nanoscale materials, like graphene, molybdenum disulphide, hexagonal boron nitride and ultra-thin oxides for high-frequency applications. An overview is provided about three main types of microwave and millimeter-wave switch: filament memristors, nano-ionic memristors and ferroelectric junctions. The physical principles that govern each switch are presented, together with advantages and disadvantages. In the last part we focus on zirconium-doped hafnium oxide ferroelectrics (HfZrO) tunneling junctions (FTJ), which are likely to boost the research in the domain of atomic-scale materials applied in engineering sciences. Thanks to their Complementary Metal-Oxide Semiconductor (CMOS) compatibility and low-voltage tunability (among other unique physical properties), HfZrO compounds have the potential for large-scale applicability. As a practical case of study, we present a 10 GHz transceiver in which the switches are FTJs, which guarantee excellent isolation and ultra-fast switching time.

Blocking of Conducting Channels Widens Window for Ferroelectric Resistive Switching in Interface‐Engineered Hf0.5Zr0.5O2 Tunnel Devices

AbstractFilms of Hf0.5Z0.5O2 (HZO) contain a network of grain boundaries. In (111) HZO epitaxial films on (001) SrTiO3, for instance, twinned orthorhombic (o‐HZO) ferroelectric crystallites coexist with grain boundaries between o‐HZO and a residual paraelectric monoclinic (m‐HZO) phase. These grain boundaries contribute to the resistive switching response in addition to the genuine ferroelectric polarization switching and have detrimental effects on device performance. Here, it is shown that, by using suitable nanometric capping layer deposited on HZO film, a radical improvement of the operation window of the tunnel device can be achieved. Crystalline SrTiO3 and amorphous AlOx are explored as capping layers. It is observed that these layers conformally coat the HZO surface and allow to increase the yield and homogeneity of ferroelectric junctions while strengthening endurance. Data show that the capping layers block ionic‐like transport channels across grain boundaries. It is suggested that they act as oxygen suppliers to the oxygen‐getters grain boundaries in HZO. In this scenario it could be envisaged that these and other oxides could also be explored and tested for fully compatible CMOS technologies.

Molecular-Beam Epitaxy of Two-Dimensional In2Se3 and Its Giant Electroresistance Switching in Ferroresistive Memory Junction.

Ferroelectric thin film has attracted great interest for nonvolatile memory applications and can be used in either ferroelectric Schottky diodes or ferroelectric tunneling junctions due to its promise of fast switching speed, high on-to-off ratio, and nondestructive readout. Two-dimensional α-phase indium selenide (In2Se3), which has a modest band gap and robust ferroelectric properties stabilized by dipole locking, is an excellent candidate for multidirectional piezoelectric and switchable photodiode applications. However, the large-scale synthesis of this material is still elusive, and its performance as a ferroresistive memory junction is rarely reported. Here, we report the low-temperature molecular-beam epitaxy (MBE) of large-area monolayer α-In2Se3 on graphene and demonstrate the use of α-In2Se3 on graphene in ferroelectric Schottky diode junctions by employing high-work-function gold as the top electrode. The polarization-modulated Schottky barrier formed at the interface exhibits a giant electroresistance ratio of 3.9 × 106 with a readout current density of >12 A/cm2, which is more than 200% higher than the state-of-the-art technology. Our MBE growth method allows a high-quality ultrathin film of In2Se3 to be heteroepitaxially grown on graphene, thereby simplifying the fabrication of high-performance 2D ferroelectric junctions for ferroresistive memory applications.

Electronic switching by metastable polarization states in BiFeO3 thin films

We present an approach to control resistive switching in metal-ferroelectric contacts using a radially symmetric electric field. In ferroelectrics with significant polarization along the corresponding field lines, the field above a critical threshold will induce polarization discontinuity, a corresponding nanoscale volume of space charge, and a conducting junction. We demonstrate this principle using nanoscale polarization switching of a conventional (001)-oriented thin film of $\mathrm{BiFe}{\mathrm{O}}_{3}$. Without any optimization, the conducting state created in this regime of resistive switching exhibits local currents of $\ensuremath{\sim}1\text{--}10\phantom{\rule{0.16em}{0ex}}\mathrm{nA}$, approaching the $\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{nA}$ threshold required for device implementation [J. Jiang et al., Nat. Mater. 17, 49 (2018)]. The corresponding electronic function is that of a volatile resistive switch, which is directly compatible with neuristor functionality that encodes the functioning basis of an axon [M. D. Pickett et al., Nat. Mater. 12, 114 (2013)]. Phase-field modeling further reveals that in the strongly charged local configuration, $\mathrm{BiFe}{\mathrm{O}}_{3}$ locally undergoes a rhombohedral-tetragonal (R-T) phase transition, in part due to substantial piezoelectric expansion of the lattice. The estimated local charge density can be as high as $\ensuremath{\sim}{10}^{21}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$, which would be extremely difficult to achieve by conventional doping approaches without altering other material properties. Therefore, this method for creating stable and reproducible strongly charged ferroelectric junctions enables more systematic studies of their physical properties, such as the possibility of structural and electronic phase transitions, and it can lead to new ferroelectric devices for advanced information functions.

Atomistic Simulations of Methylammonium Lead Halide Layers on PbTiO3 (001) Surfaces

The substantial increase in the power conversion efficiency of hybrid perovskite solar cells, to date reaching more than 20% in the laboratory, has strongly motivated research on this class of organic–inorganic materials and related devices, particularly based on CH3NH3PbI3–xXx/TiO2 heterostructures (X = Cl,Br). Taking under consideration that a ferroelectric substrate may act as an efficient electron transporter, positively influencing charge collection across the interface and allowing the tuning of the halide perovskite (HP) - ferroelectric junction, we performed extensive density functional theory calculations on CH3NH3PbI3–xClx layers deposited on tetragonal PbTiO3 (PTO) (001) surfaces, to study their structural and electronic properties. The main findings of this study are as follows. (i) A ferroelectric polarization pointing from the PTO/HP interface to the PTO is favorable for the photogenerated electrons transfer across the interface and their transport to the collecting electrode. (ii) The PTO i...

Evaluation of the spectroscopic ellipsometry and dielectric properties of Cr2O3 nanoparticles doped PVDF thin films for future application of organic ferroelectric junctions

Here, we report the synthesis of Cr2O3 nanoparticles via ethylene glycol assisted microwave approach. Thin films of Cr2O3 nanoparticles doped Poly(vinylidene fluoride) (PVDF), at a variety concentrations (0.01–0.09wt%), were prepared by spin-coating technique on quartz substrate with thickness of 200nm. The influence of the concentrations of Cr2O3 nanoparticles on the structural, morphological, optical and dielectric properties of the PVDF is examined. The optical properties of the Cr2O3/PVDF thin films were examined by the ellipsometry and fitted by Forouhi–Bloomer model. The doping of PVDF by 0.05wt% of Cr2O3 nanoparticles showed an enhancement of the index of refraction compared to the neat PVDF. A remarkable enhancement of the dielectric constant was achieved when the PVDF film doped with 0.05wt% of Cr2O3 nanoparticles with 35 orders of magnitude compared to the neat PVDF film, owing to the Maxwell–Wagner–Sillars polarization effect. This results in an enhancement of the ferroelectric properties of the PVDF by increasing its remnant polarization from 8.7μC/cm2 to 12.1μC/cm2. These superior properties may enable the developed Cr2O3/PVDF films to be a potential candidate in the field of organic ferroelectric random access memory junctions.

Tunnel electroresistance in BiFeO3 junctions: size does matter

In ferroelectric tunnel junctions, the tunnel resistance depends on the polarization orientation of the ferroelectric tunnel barrier, giving rise to tunnel electroresistance. These devices are promising to be used as memristors in neuromorphic architectures and as non-volatile memory elements. For both applications, device scalability is essential, which requires a clear understanding of the relationship between polarization reversal and resistance change as the junction size shrinks. Here we show a robust tunnel electroresistance in BiFeO3-based junctions with diameters ranging from 1200 to 180 nm. We demonstrate that the tunnel electroresistance and the corresponding fraction of reversed ferroelectric domains change drastically with the junction diameter: while the micron-size junctions display a reversal in less than 10% of the area, the smallest junctions show an almost complete polarization reversal. Modeling the electric-field distribution, we highlight the critical role of the bottom electrode resistance which significantly diminishes the actual electric field applied to the ferroelectric barrier in the mixed polarization state. A polarization-dependent critical electric field below which further reversal is prohibited is found to explain the large differences between the ferroelectric switchability of nano- and micron-size junctions. Our results indicate that ferroelectric junctions are downscalable and suggest that specific junction shapes facilitate complete polarization reversal.

Ferroelectricity in Covalently functionalized Two-dimensional Materials: Integration of High-mobility Semiconductors and Nonvolatile Memory.

Realization of ferroelectric semiconductors by conjoining ferroelectricity with semiconductors remains a challenging task because most present-day ferroelectric materials are unsuitable for such a combination due to their wide bandgaps. Herein, we show first-principles evidence toward the realization of a new class of two-dimensional (2D) ferroelectric semiconductors through covalent functionalization of many prevailing 2D materials. Members in this new class of 2D ferroelectric semiconductors include covalently functionalized germanene, and stanene (Nat. Commun. 2014, 5, 3389), as well as MoS2 monolayer (Nat. Chem. 2015, 7, 45), covalent functionalization of the surface of bulk semiconductors such as silicon (111) (J. Phys. Chem. B 2006, 110 , 23898), and the substrates of oxides such as silica with self-assembly monolayers (Nano Lett. 2014, 14, 1354). The newly predicted 2D ferroelectric semiconductors possess high mobility, modest bandgaps, and distinct ferroelectricity that can be exploited for developing various heterostructural devices with desired functionalities. For example, we propose applications of the 2D materials as 2D ferroelectric field-effect transistors with ultrahigh on/off ratio, topological transistors with Dirac Fermions switchable between holes and electrons, ferroelectric junctions with ultrahigh electro-resistance, and multiferroic junctions for controlling spin by electric fields. All these heterostructural devices take advantage of the combination of high-mobility semiconductors with fast writing and nondestructive reading capability of nonvolatile memory, thereby holding great potential for the development of future multifunctional devices.

First-principles study of interface doping in ferroelectric junctions.

Effect of atomic monolayer insertion on the performance of ferroelectric tunneling junction is investigated in SrRuO3/BaTiO3/SrRuO3 heterostrucutures. Based on first-principles calculations, the atomic displacement, orbital occupancy, and ferroelectric polarization are studied. It is found that the ferroelectricity is enhanced when a (AlO2)− monolayer is inserted between the electrode SRO and the barrier BTO, where the relatively high mobility of doped holes effectively screen ferroelectric polarization. On the other hand, for the case of (LaO)+ inserted layer, the doped electrons resides at the both sides of middle ferroelectric barrier, making the ferroelectricity unfavorable. Our findings provide an alternative avenue to improve the performance of ferroelectric tunneling junctions.

Colossal resistance switching in Pt/BiFeO3/Nb:SrTiO3 memristor

We report reversible resistance switching behaviors in Pt/BiFeO3/Nb:SrTiO3 memristor. The resistance of the junctions can be tuned up to about five orders of magnitude by applying voltage pulses at room temperature, which exhibits excellent retention and anti-fatigue characteristics. The high performances are promising for employing ferroelectric junctions in nonvolatile memory and logic devices. The nonvolatile resistance switching behaviors could be attributed to the formation and annihilation of trap centers in the BFO films, resulting in Poole–Frenkel emission for low resistance state and the thermionic emission for high resistance state, respectively.

Dynamic Control of Tunneling Conductance in Ferroelectric Tunnel Junctions

We investigate the dynamic characteristics of electric polarization P(t) in a ferroelectric junction under ac applied voltage and stress, and calculate the frequency response and the cut-off frequency f0, which provides a reference for the upper limit of the working frequency. Our study might be significant for sensor and memory applications of nanodevices based on ferroelectric junctions.

Interface effects at a ferromagnetic and ferroelectric junction

Magnetoelectric effect at Fe3Si/BaTiO3 (001) interfaces is investigated by using the first-principles calculations. Interfacial separation works reveal that the FeSi/TiO2 and modified FeFe/TiO2 and SiSi/TiO2 interfaces are more stable than the Fe/TiO2 interface. While for the modified SiSi/TiO2 interface, the paraelectric phase is recovered in BaTiO3 slab due to its symmetrical structure in the central TiO2 layer. Compared with the original Fe/TiO2 and FeSi/TiO2 interfaces, more net change magnetizations are achieved at modified FeFe/TiO2 interface. The predicted magnetoelectric effect opens a direction to control magnetic properties of thin-film layered structure by electric fields.

Effects of ferroelectric dead layer on the electron transport in ferroelectric tunneling junctions

Ferroelectric dead layer is intrinsic and inevitable at the metal-ferroelectrics interface. In general, it is detrimental to the application of nanoscale devices; however, in ferroelectric tunneling junctions with ferroelectric dead layer, an enhanced tunneling electroresistance (TER) can be achieved when the nonswitchable interface polarization at both metal-ferroelectrics interfaces points to the ferroelectric center. The larger the interface polarization, the stronger the TER. In addition, low dielectric constant of dead layer will favor such enhancement. Our results provide an alternative route for achieving larger TER in ferroelectric junctions.

Impact of applied strain on the electron transport through ferroelectric tunnel junctions

Combining nonequilibrium Green’s functions with density-functional theory, we have investigated the effect of external strain field on the tunneling electroresistance (TER) of ferroelectric material sandwiched between Pt electrodes. The results show that the strain induced para/ferroelectric phase transitions play an important role in the electronic transport properties of the junction. Sizable enhancements in the resistance are found for the strained ferroelectric junctions with a TER ratio of 9000%. Detail analyses show that the Ti–O displacements along the transport direction in ferroelectric barrier change the effective potential profile, resulting in a giant piezoelectric resistance in the ferroelectric tunnel junctions.