Paraelectric Layer Research Articles

A negative capacitance field effect transistor with a modified gate stack and drain-side cavity for label-free biosensing

Dielectrically modulated (DM) negative capacitance field effect transistor (NCFET)-based label-free biosensors have emerged as promising devices for accurate detection of various biomolecules, where the sensitivity of DM architectures strongly depends on the sensing mechanism as well as on the size of the nanocavity. Therefore, to achieve higher sensitivity along with reduced fabrication complexity, we propose to utilize a pre-existing drain-side spacer region as a nanocavity, in a fully depleted silicon-on-insulator-based NCFET architecture. The ferroelectric (FE) layer in the metal-ferroelectric-insulator-semiconductor configuration meaningfully alters the impact of the drain’s electric field on the source-side electrostatics, which results in higher sensitivity. Having quantified the sensitivity of an FE-dielectric (FE-DE) gate-stack-based NCFET biosensor, we now propose to include a paraelectric (PE) layer between the FE and DE materials, thus modifying the gate stack from FE-DE to FE-PE-DE with an equivalent negative capacitance seen from both stacks; here, a remarkable improvement is seen in the FE-PE-DE gate-stack-based NCFET, with nearly identical linearity performance, as seen from the high Pearson’s coefficient value ( r2⩾ 0.9). Therefore, to illustrate the efficacy of the proposed sensing mechanism and the modified gate stack (FE-PE-DE), DE constant ( kBio ) values in the range of kBio = 4.5 to kBio = 75.99 are considered. Finally, the effect of scaling the channel length ( Lg ) on the sensitivity of the FE-PE-DE NCFET device is shown, and a high value, particularly at lower permittivity, demonstrates the versatility and wide applicability of the proposed NCFET biosensor.

Tuning Topology Phases by Controlling Effective Screening and Depolarization in Oxide Superlattices.

Polar topological phases in oxide superlattices attracted significant attention due to their unique properties. Previous work revealed that a polar vortex and polar skyrmions exist in (PTO)/(STO) superlattices under different elastic constraints, i.e., on different substrates. Here, our phase-field simulation demonstrates that manipulating the PTO and STO layers' thickness can control the effective screening provided by STO and the depolarization degree in PTO, thus switching the system among the polar skyrmions, vortex labyrinth, or paraelectric phase without changing elastic constraints. Additionally, reducing the STO thickness creates interlayer coupling among PTO layers, generating the long-range order of topological phases within superlattices. Furthermore, we construct a PTO-STO thickness topological phase diagram. These findings offer insights into the polar topological phases' formation in oxide superlattices, elucidating the roles of ferroelectric and paraelectric layers in their formation.

Energy storage properties of multilayer thin films based on relaxor-ferroelectric (Ba0.9Sr0.1)(Zr0.4Ti0.6)O3 and paraelectric Sr(Zr0.1Ti0.9)O3 layers

Lead-free dielectric film capacitors with high recoverable energy-storage density (Ur) and large energy efficiency (η) are highly desired for the development of next-generation energy-storage devices for environmental sustainability. In this work, the impact of inserting SrZr0.1Ti0.9O3 layers in the structure and properties of [(Ba0.9Sr0.1)Zr0.4Ti0.6O3/SrZr0.1Ti0.9O3]N = 4/(Ba0.9Sr0.1)Zr0.4Ti0.6O3 ([BSZT/SZT]N = 4/BSZT) multilayer films was systematically investigated. The results indicated that both maximum polarization and breakdown strength are enhanced in the multilayer structure. This is attributed to the strain gradient, dielectric constant gradient, and interface blocking effect in the interfaces between relaxor BSZT and paraelectric SZT layers. Consequently, a high Ur of about 107.3 J/cm3 and an excellent η of about 84.2% were achieved in the multilayer films (Ur and η values were about 73.7 J/cm3 and 73.1%, and 40.9 J/cm3 and 89.6%, respectively, for the single-layered BSZT and SZT films).

Analysis and Mitigation of Negative Differential Resistance effects in Double-Gate Silicon-on-Insulator Negative Capacitance Field Effect Transistor with improved analog performance

Negative Capacitance Field Effect Transistors (NCFETs) offer the possibility of enhancing channel length scalability, with a key challenge being to ensure hysteresis and Negative Differential Resistance (NDR) free behavior. In this context, through TCAD-based simulations, for a symmetric Double-gate Silicon-on-Insulator (DG-SOI) baseline architecture with a Metal–Ferroelectric–Insulator–Semiconductor (MFIS) configuration, we first examine the root cause of NDR as being the FE layer with a small number of strongly coupled domains, which allows the drain to lower the surface potential (ΨSs) and internal gate voltage (Vint) on the source side. With this understanding of NDR, the inclusion of a paraelectric (PE) layer on the drain side of the gate stack, by splitting the stack into two halves laterally, while keeping the thickness of the stack unchanged, minimizes the drain’s influence on the source-sided electrostatics. This results in the mitigation of NDR effects along with superior analog performance parameters, thus making these FE–PE gate stack-based NCFET devices suitable for analog circuit applications.

The Impact of a Paraelectric layer in the FE/DE Stack on Performance of NCFET

In this paper, through TCAD simulations, we show that the introduction of a thin paraelectric (PE) layer between the ferroelectric (FE) and dielectric (DE) layers in an MFIS structure, expands the design space for the FE layer enabling hysteresis-free and steep subthreshold behavior, even with a thicker FE layer. This can be explained by analyzing the FE-PE stack from a capacitance perspective where the thickness of the PE layer in the FE-PE stack has the effect of reducing the FE layer thickness, while also reducing the remnant polarization. Finally, for the same FE-PE-DE stack, analog performance parameters such as \(\frac {G_{m}}{G_{ds}}\) and \(\frac {G_{m}}{I_{d}}\) are analyzed, showing good characteristics over a wide range of gate lengths, at low drain voltages, thus demonstrating applicability for low power applications.

All-polymeric multilayer para/ferroelectric dielectric films utilizing a gradient structure toward concurrent high discharge efficiency and energy density

Polymeric dielectrics exhibit huge prospects for power electronic devices due to their ultrafast charge-discharge speed. However, a persisting challenge is that enhanced energy density always brings about deteriorated efficiency. Herein, a novel design of all-organic dielectric films, consisting of gradient transition layers sandwiched between a paraelectric layer and a ferroelectric layer, are prepared. It is interesting to find that the breakdown behaviors of the multilayer films are strongly dependent on the configuration of gradient transition layers. Consequently, via adjusting the configuration of gradient transition layers, a high breakdown strength of 623 kV/mm, an excellent efficiency of 89.5%, along with a high-energy density of 7.34 J/cm3 are achieved. This work offers a unique route to develop polymeric dielectrics with concurrent high breakdown strength, high-energy density, and high efficiency.

Largely Improved Breakdown Strength and Discharge Efficiency of Layer‐Structured Nanocomposites by Filling with a Small Loading Fraction of 2D Zirconium Phosphate Nanosheets

AbstractDielectric film capacitors have aroused considerable attention on account of the fast development of pulsed power systems. However, enhanced energy density is always acquired at the cost of deteriorated charge/discharge efficiency. Herein, well balanced energy density and efficiency are achieved in a series of reasonably designed bilayer composites consisting of a ferroelectric layer and a paraelectric layer at the meantime. It is interesting to find that, when merely 1.6 wt% Zr(HPO4)2 nanosheets are introduced into the ferroelectric layer, a substantially improved energy density of 11.22 J cm−3, which is about 165% that of the bilayer composite without Zr(HPO4)2 nanosheets, is achieved at 650 kV mm−1. Meanwhile, a high charge/discharge efficiency of 89.8% and a low loss tangent of 0.024@10 kHz which is much lower than the pristine ferroelectric polymer layer (0.058@10 kHz) is maintained. Furthermore, finite element simulation reveals that the electric breakdown paths will develop along the macroscopical‐interfaces between adjoining layers and the microcosmic‐interfaces between the Zr(HPO4)2 nanosheets and polymer matrix, which can effectively increase the length of breakdown paths and contribute to improved breakdown strength. This work demonstrates that the Zr(HPO4)2 nanosheets can be promising fillers for other high‐performance dielectric composites.

Fatigue and leakage current characteristics of lead free bilayer thin film structures

Multilayer thin films of (Ba0.50Sr0.50)TiO3 (BST) and Ba(Zr0.15Ti0.85)O3 (BZT) were deposited on Pt(111)/SiO2/Si(111) substrates using Pulsed LASER Deposition (PLD) technique. This manuscript reports on the role of a paraelectric layer in the stack of multilayers on certain key aspects related to crystallographic orientation, interface morphology, tunability, and energy storage properties. The incorporation of paraelectric layer is found to strongly influence the electrical properties of a ferroelectric layer whose composition is at the morphotropic phase boundary. A possible methodology to obtain enhanced electrical properties in multilayer structures based on electric field distribution across different layers in the stack is demonstrated. Incorporation of a paraelectric layer results in restricting the migration of oxygen vacancies due to the presence of the internal bias field. Retention of remanent polarization when driven by 109 bipolar switching cycles and other fatigue properties are reported. Leakage current characteristics were evaluated and the fitting of the experimental data with various conduction mechanisms indicate that different mechanisms are responsible for leakage current conduction as a function of bias voltage. A relatively high power density of >12 MW/cm3 with an energy storage density efficiency of >70%, tunability values comparable with multilayer thin films grown on single crystal substrates are some of the attributes of the multilayer structure.

Static Negative Capacitance Response in Ferroelectric/Paraelectric Superlattices

Recently Zubko et al.(1) reported a negative capacitance effect in PbTiO3/SrTiO3 superlattices. The negative capacitance of ferroelectrics is proposed as a solution to overcome the “Boltzmann tyranny” that holds back the process of miniaturization. Nowadays, the transistors are becoming too small, which increased the energy dissipation hence the self-heating of the electronic chips. The phenomenon of negative capacitance is gaining a great interest since it has been proposed as a way to reduce the power consumption of field-effect transistors(2), by replacing the conventional SiO2 gate insulator by an insulator that provides a negative capacitance effect.Actually, in ferroelectrics, “the negative capacitance state” takes place at the frontier between the two stable polarization states, during the transient processes of switching, and therefore is intrinsically thermodynamically unstableHence, negative capacitance cannot be evidenced for single layer ferroelectrics but should be stabilized in a series capacitance system. The ferroelectric capacitance can be stabilized in the negative regime, when a paraelectric capacitor and a ferroelectric capacitor are connected in series. Hence, the paraelectric/ferroelectric superlattices provide a model system to investigate the negative capacitance effect.The negative capacitance was shown to be strongly related to the existence of a 180° stripe domain structure. In fact, the depolarization field inside such structure is very strong and induces 180° stripe nanodomains formation in order to screen this field and to maintain the ferroelectric state. This nanodomains, in particularly, the domain wall motion greatly influences the dielectric properties of the heterostructure.Here we report an experimental study on a series of Pb(Zr0.2Ti0.8)O3 /SrTiO3 superlattices in which we evidenced 180° stripe nanodomains. The investigation of such system is motivated by the fact that leakage currents are reduced in PZT compared to the ferroelectric PbTiO3, investigated by Zubko et al.,and also by the possibility to vary the misfit strain related to PbTiO3 /SrTiO3 superlattices . For this study the thickness of the ferroelectric layers was fixed to 4 unit cells and those of the paraelectric layer was varied from 3 to 10 unit cells. The domain’s electrostatic effect in the superlattice is confirmed by a noticeable reduction of the Curie temperature. We will present an in-depth analysis of the dielectric response of these superlattices by impedance spectroscopy. The temperature dependent dielectric measurements will be presented and will emphasize a negative capacitance effect in these superlattices. References - Zubko et al., Nature, 2016, 534, 524–528.- S. Salahuddin and S. Datta, Nano Lett.8, 405 (2008)

A Comparative Analysis of Asymmetric (BaTiO3) (1 − x)Λ/(BaZrO3) xΛ Superlattices via X‐Ray Diffraction and Raman Spectroscopy

This work is aimed at studying asymmetric ((BT) (1 − x)Λ/(BZ) xΛ) superlattices, grown by pulsed laser deposition onto (001) MgO substrates. The thicknesses of BT (ferroelectric) and BZ (paraelectric) layers were varied so that х ranged from 0 to 1 at a modulation period Λ of about 80 Å. The films were 400 nm thick. The out‐of‐plane lattice parameters of constituents were assessed using X‐ray diffraction. The lattice dynamic peculiarities of superlattices were probed via Raman spectroscopy; special attention is paid to the analysis of E(1TO) and A1(2TO) ferroelectric soft modes. A comparative analysis of data acquired via both experimental techniques reveals the enhancement of stress between BT and BZ layers with a decrease in symmetry from the tetragonal to a monoclinic phase due to strains induced by the lattice parameter mismatch between the constituents.

Spectral and polarization properties of a planar multiferroic structure

The effect of magnetic and electric fields control on the reflection, transmission, and absorption spectra of a planar symmetric ferrite–paraelectric–ferrite structure in the magnetic resonance region of ferrite layers has been investigated. A special feature of the structure has been the high permittivity (εp≈103) of the paraelectric layer in the considered range of temperatures and electric fields. The spectral characteristics substantially depend on the type of circular polarization (right or left) of propagating eigenwaves of a longitudinally magnetized structure, which makes it possible to use it to create a controlled splitter of both transmitted and reflected waves. It has been established that the angles of rotation of the polarization plane and ellipticity can be controlled by changing not only the external fields, but also the temperature.

Pinning of a ferroelectric Bloch wall at a paraelectric layer.

The phase-field simulations of ferroelectric Bloch domain walls in BaTiO3–SrTiO3 crystalline superlattices performed in this study suggest that a paraelectric layer with a thickness comparable to the thickness of the domain wall itself can act as an efficient pinning layer. At the same time, such a layer facilitates the possibility to switch domain wall helicity by an external electric field or even to completely change the characteristic structure of a ferroelectric Bloch wall passing through it. Thus, ferroelectric Bloch domain walls are shown to be ideal nanoscale objects with switchable properties. The reported results hint towards the possibility to exploit ferroelectric domain wall interaction with simple nanoscale devices.

Enhanced tunneling electroresistance effects in HfZrO-based ferroelectric tunnel junctions by high-pressure nitrogen annealing

Greatly improved ferroelectricity with an excellent remanent polarization of 20 μC/cm2 and enhanced tunneling electroresistance (TER) were achieved with TiN/HfZrO(HZO)/p-type Ge ferroelectric tunnel junctions (FTJs) annealed at a high pressure of 200 atmosphere (atm.). We found that the enhanced ferroelectric characteristics can be ascribed to the effective formation of an orthorhombic phase at high pressures. This was verified by the combined study of grazing angle incidence X-ray diffraction, transmission electron microscopy, and hysteresis polarization curve analyses. In addition, using pulse switching measurements, we quantitatively evaluated the interfacial paraelectric capacitance (Ci) of HZO FTJs according to the annealing temperature. HZO films annealed at 550 °C and 200 atm. exhibited an excellent TER effect ratio of 20 due to the extra paraelectric layer between the ferroelectric layer and the bottom electrode and a relatively high remanent polarization.

Bending influence of the electrocaloric effect in a ferroelectric/paraelectric bilayer system

The influence of bending behavior on the electricaloric (EC) effect is investigated by adopting a thermodynamic model of ferroelectric (FE)/paraelectric (PE) bilayer. The polarization transitions driven by the bending effect occur at a certain temperature range, glass-like transitions, which are found to dramatically impact the EC response. A large range of EC response can be obtained from the FE/PE bilayer which is attributed to the varying misfit strain across the film thickness arising from the bending effect. The results suggest that careful choice of thickness ratio between FE and PE layers allows one to harness the intrinsic and extrinsic contributions to obtain the optimized EC response, which presents great potential for solid refrigeration application.

Strong influence of non-ideality of electrodes on stability of single domain state in ferroelectric-paraelectric superlattices

We study the phase stabilities with respect to small perturbations in ferroelectric-paraelectric superlattices and show that nature of the electrodes characterized by a deviation from the ideal behavior strongly influences the possibility to obtain single-domain state in ferroelectric-paraelectric superlattices. To demonstrate this, we analyze the limit of stability of the paraelectric and the single domain state in ferroelectric-paraelectric superlattices in contact with top and bottom electrodes with finite screening lengths. The combined analytical and numerical analyses of one bilayer and two bilayer systems are carried out using the Landau-Ginzburg-Devonshire formalism and equations of electrostatics. The BaTiO3/SrTiO3 system was considered as an example. Unlike the case of ideal electrodes where the stability limits are independent of the system size, the stability analysis in a multilayer with real electrodes should take into account explicitly the number of the repeating units that makes the algebra very cumbersome, forcing us to consider systems with one and two bilayer stacks only. Extrapolating the difference between the two systems to the cases of many repeating units gives us the possibility to make qualitative but feasible predictions related to those with many repeating units. We observe that in systems with nearly equal thicknesses of the ferroelectric and paraelectric layers, the electrodes with realistic screening lengths lead to dramatic widening of the parametric region where the single-domain state is absolutely unstable expelling the single-domain state to unphysical layer thicknesses and temperatures. This region grows when one goes from a single bilayer to two bilayer system, implying that obtaining a single domain state becomes even less feasible in systems with many bilayers. When electrode properties approach that of ideal in addition to increasing the volume fraction of the ferroelectric component, the effect of growth of the region of absolute instability of the single domain state may remain very strong for relatively thin repeating units (a few nanometers). This tendency will continue with increasing the number of the repeating units.

Electrostatic tuning of oxygen octahedral rotations in ferroelectric perovskites

Using first-principles methods, we investigate the joint effects of epitaxial strain and electrostatic coupling on the appearance of oxygen octahedral rotations, O6, in ferroelectric perovskites. We find that by varying the relative content of ferroelectric and paraelectric layers in epitaxially strained superlattices the size of O6 can be efficiently tuned.

Pyro-paraelectricity

The electrical responses of materials and devices subjected to thermal inputs, such as the Seebeck effect and pyroelectricity, are of great interest in thermal-electric energy conversion devices. Of particular interest are phenomena which exploit heterogeneities in the mechanics of heterostructured materials and systems for novel and unexplored thermal-electric responses. Here we introduce a new mechanism for converting thermal stimuli into electricity via structural heterogeneities, which we term “pyro-paraelectricity”. Specifically, when a paraelectric material is grown on a substrate with a different lattice constant, the paraelectric layer experiences an inhomogeneous strain due to the lattice mismatch, establishing a strain gradient along the axis of the layer thickness. This strain gradient, induced via the lattice mismatch, can be multiple orders of magnitude higher than strain gradients in bulk materials imparted by mechanical bending (0.1 m-1). Consequently, charge separation is induced in the paraelectric layer via flexoelectricity, leading to a polarization in proportion to the dielectric constant. The dielectric constant, and thus the polarization, in turn changes with temperature. Therefore, when a strained metal–insulator–metal (MIM) heterostructure is subjected to a thermal input, changes in the permittivity generate an electrical response. We demonstrate this concept of “pyro-paraelectricity” by employing a MIM heterostructure with a high permittivity sputtered barium strontium titanate (BST) film as the insulating layer in a platinum sandwich. The resulting strain gradient of more than 104 m-1 due to the structural heterogeneity was verified by an X-ray diffraction scan. To demonstrate “pyro-paraelectricity”, the MIM heterostructure was subjected to a thermal input, thereby generating current which was highly correlated to the thermal input. A theoretical model was found to be consistent with the experimental data. These results prove the existence of “pyro-paraelectricity”.

Electrocaloric properties of ferroelectric-paraelectric superlattices controlled by the thickness of paraelectric layer in a wide temperature range

As functions of the paraelectric layer thickness, misfit strain and temperature, the electrocaloric properties of ferroelectric-paraelectric superlattices are investigated using a time-dependent Ginzburg-Landau thermodynamic model. Ferroelectric phase transition driven by the relative thickness of the superlattice is found to dramatically impact the electrocaloric response. Near the phase transition temperature, the magnitude of the electrocaloric effect is maximized and shifted to lower temperatures by increasing the relative thickness of paraelectric layer. Theoretical calculations also imply that the electrocaloric effect of the superlattices depends not only on the relative thickness of paraelectric layer but also on misfit strain. Furthermore, control of the relative thickness of paraelectric layer and the misfit strain can change availably both the magnitude and the temperature sensitivity of the electrocaloric effect, which suggests that ferroelectric-paraelectric superlattices may be promising candidates for use in cooling devices in a wide temperature range.

Interface-Induced Modifications of Polarization in Nanoscale Ferroelectric Superlattices

A thermodynamic model to study the effect of interface intermixing on polarization of ferroelectric superlattices composed of a ferroelectric and paraelectric layers is developed. Interface intermixing forms an intermixed layer with property different from its individual layers, leading to inhomogeneous ferroelectric properties in the superlattices. Polarization induces near the interface of paraelectric layers, which extends into the layer over a distance governed by its characteristic length. Dependence of polarization on periodic thickness indicates an interface-induced modification of ferroelectricity in superlattices. Enhancement in polarization of superlattices is shown to be possible, if certain interface property and periodic thickness met.

Voltage-readable nonvolatile memory cell with programmable ferroelectric multistates in organic inverter configuration

We demonstrate a voltage-readable nonvolatile memory cell with programmable ferroelectric multistates in an organic inverter configuration. The intermediate memory states of a ferroelectric gate insulator, varying with the magnitude of the programming voltage, allow the multilevels of the drain current at zero gate-source voltage in a ferroelectric organic field-effect transistor (OFET). The current output from the ferroelectric memory is directly converted into the voltage-readable output in a zero-gate load inverter configuration where both a driving paraelectric OFET having a paraelectric buffer layer and a load ferroelectric OFET are monolithically integrated in a single substrate. The multilevel voltage-readable output characteristics are obtained from the ferroelectric multistates as a function of the programming voltage.