Ferroelectric Thin Film Capacitors Research Articles

Nonlinear domain wall velocity in ferroelectric Si-doped HfO2 thin film capacitors

We investigate the nonlinear response of the domain wall velocity (v) to an external electric field (Eext) in ferroelectric Si-doped HfO2 thin film capacitors using piezoresponse force microscopy (PFM) and switching current measurements. We verified the reliability of the PFM images of ferroelectric domain switching by comparing the switched volume fraction in the PFM images with the time-dependent normalized switched polarization from the switching current data. Using consecutive time-dependent PFM images, we measured the velocity of the pure lateral domain wall motion at various Eext. The Eext-dependent v values closely follow the nonlinear dynamic response of elastic objects in a disordered medium. The thermally activated creep and flow regimes were observed based on the magnitude of Eext. With a dynamic exponent of μ = 1, our thin film was found to have random-field defects, which is consistent with the Lorentzian distribution of characteristic switching time that was indicated in the switching current data.

Transfer of Ferroelectric Thin Film Capacitor Using Internal Stress of Plated Film

The purpose of this article is for the establish of the releasing technologies of ferroelectric thin film capacitors using the internal stress of plated film. The stress analysis in simulation and the confirmation of experiment was carried out. As a result, it was demonstrated that the thin film capacitor can be actually released off. It was also confirmed that the performance as a capacitor after releasing was also good for the expected application.

Enhanced energy storage properties of amorphous BiFeO3/Al2O3 multilayers

The energy storage density of ferroelectric thin film capacitors is mainly limited by the breakdown strength. Here we demonstrate that the high breakdown strength and high energy storage density can be achieved by constructing BiFeO3/Al2O3 ferroelectricity-insulators heterojunction. The breakdown strength, leakage current density and energy storage performance are investigated systematically of the multilayer thin films. The relatively high breakdown strength is achieved in the Al2O3/BiFeO3/Al2O3/BiFeO3 thin film, which is benefited from the amorphous structure, built-in electric field and boundary conditions of electromagnetic field. At the same time, the excellent energy storage density of the thin film is obtained mainly due to the effect of high breakdown strength. The proposed structure of this work provides an useful method to improve the energy storage density of thin film capacitors.

Effect of electrode on dielectric susceptibility and pyroelectric properties of a ferroelectric thin film capacitor using landau-khalatnikov theory

The influence of electrodes on the dielectric susceptibility and pyroelectric properties of a ferroelectric thin film with surface transition layers has been investigated within the framework of Landau-Khalatnikov dynamic theory. The contribution of the electrodes is reflected by the depolarization field in the free-energy function. The large electrode effect implies the strong depolarization field in ferroelectric thin films. The results show that the electrode materials can greatly impact the dynamic dielectric and pyroelectric properties of a ferroelectric thin film.

Leakage current characteristics of SrTiO3/LaNiO3/Ba0.67Sr0.33TiO3/SrTiO3 heterostructure thin films

There is an urgent demand to explore new approaches of improving the electric breakdown strength of the ferroelectric thin-film capacitor without the degradation of the high dielectric constant. In this work, LaNiO3/Ba0.67Sr0.33TiO3 (LNO/BST) thin film and SrTiO3(STO)/LNO/BST/SrTiO3 (STO) were prepared by using radio frequency (RF) magnetron sputtering technique and ultrathin STO insulator layers were inserted between the LNO/BST and metal electrodes (Au or Pt) to improve the electric breakdown strength and the leakage current of the LNO/BST thin film. X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations revealed that these multilayer thin films show compact, smooth and uniform morphologies. The leakage current density of STO/LNO/BST/STO thin film was decreased by one order of magnitude and breakdown strength of STO/LNO/BST/STO thin film was enhanced from 0.72 to 1.26 MV·cm−1 compared with that of LNO/BST thin film. Moreover, the dielectric constant of the STO/LNO/BST/STO thin film keeps at the same level as that of LNO/BST thin film, and dielectric loss of the STO/LNO/BST/STO thin film was decreased slightly compared with that of LNO/BST thin film.

Strain engineering of dischargeable energy density of ferroelectric thin-film capacitors

Abstract Ferroelectric oxide thin-film capacitors find applications in microelectronic systems, mobile platforms, and miniaturized power devices. They can withstand higher electric fields and display significantly larger energy densities than their bulk counterparts and exhibit higher maximum operating temperatures and better thermal stabilities than polymer-based dielectric capacitors. However, ferroelectric oxide thin films typically possess large remanent polarization and exhibit significant dielectric loss, thereby limiting their dischargeable energy densities. Here we demonstrate, using phase-field simulations, that strain can be utilized to modify the polarization response to electric field and thus optimize the energy-storage performance of ferroelectric thin-film capacitors. For example, an in-plane tensile strain can significantly narrow hysteresis loops by reducing the remanent polarization without significantly decreasing the out-of-plane saturated polarization. As a result, both the dischargeable energy density and charge-discharge efficiency can be significantly enhanced. We analysed the domain structures and energy surfaces to understand the underlying mechanisms for the enhancements. We also propose a bending strategy to further improve the dischargeable energy density, which can be achieved, e.g., by growing ferroelectric thin films on a flexible substrate (e.g., mica). This work provides a general strategy to optimize the energy-storage performance of ferroelectric thin-film capacitors for high-energy/power-density storage applications.

Total ionizing dose effects of 60Co γ-rays radiation on HfxZr1−xO2 ferroelectric thin film capacitors

The behavior of ferroelectric thin film capacitors under radiation environments is significant for the development of rad-hard ferroelectric random access memory (FeRAM). Here, we fabricated the ferroelectric thin film capacitors with 12-nm-thick HfxZr1−xO2 (HZO) and investigated the total ionizing dose (TID) effects of 60Co γ-rays radiation on them. It is found that after 60Co γ rays radiation with a total dose as high as 10 Mrad (Si), the hysteresis loops of the HZO ferroelectric thin film capacitors slightly shift to the positive bias direction, and the remanent polarization decreases by 5%. Besides, negligible changes in surface morphology, leakage current density, relative dielectric constant, dielectric loss, endurance and retention characteristics are observed. These results suggest that the HZO ferroelectric thin film capacitors have great potential in the application for highly rad-hard FeRAM.

High-performance La-doped BCZT thin film capacitors on LaNiO3/Pt composite bottom electrodes with ultra-high efficiency and high thermal stability

Abstract Dielectric capacitors possessing large energy storage density, high efficiency and high thermal stability simultaneously are very attractive in modern electronic devices to be operated in harsh environment. Here, it is demonstrated that large energy storage density (W ∼ 15.5 J/cm3), ultra-high efficiency (η ∼93.7%) and high thermal stability (the variation of both W from 20 °C to 260 °C and η from 20 °C to 140 °C is less than 5%) have been simultaneously achieved in the La-doped (Ba0.904Ca0.096)0.9775+xLa0.015(Zr0.136Ti0.864)O3 (x = 0.0075) lead-free relaxor ferroelectric thin film capacitors deposited on LaNiO3/Pt composite bottom electrodes by using a sol-gel method. The good energy storage property of the thin film capacitors at x = 0.0075 is mainly ascribed to the diversity of the structure of the nano-clusters around the three-phases coexisting component point (Ba0.904Ca0.096)(Zr0.136Ti0.864)O3 where cubic, tetragonal and rhombohedral phases coexisted, as well as the ultra-high quality of thin film due to the utilization of the LaNiO3/Pt composite bottom electrode, making it a promising candidate for dielectric capacitors working in harsh environments.

Radiation-induced changes of vacancy-type defects in ferroelectric capacitors as revealed by Doppler broadening positron annihilation spectroscopy

Thin film ferroelectric capacitors of composition Pb(Zr0.52Ti0.48)O3 were exposed to Fe3+ radiation (1011 to 1013 ions/cm2), and the change in the defect structure was investigated by Doppler broadening positron annihilation spectroscopy and other characterization techniques. As the radiation fluence increases, a systematic drop of the S parameter of the positron annihilation photopeak is observed and attributed to an increase in the Zr- and Ti-site related vacancies relative to the Pb-sites. The results demonstrate that the radiation has a more significant influence on the Zr- and Ti-sites relative to the Pb-sites. It is also observed that the S parameter of the Mn-doped samples is higher than the undoped counterparts. Coupled with ferroelectricity measurements and X-ray diffraction, the results suggest that the Mn dopant modifies the initial structure of the material and leads to a different functional response.

Fabrication and characterization of (Ba, Sr)RuO3 ceramic targets and thin films for ferroelectric BaTiO3 thin-film capacitors

We report the preparation of (Ba, Sr)RuO3 ceramic targets in a conventional process in air at ambient pressure and their epitaxial thin-film growth on Ba0.7Sr0.3TiO3 buffered SrTiO3 substrates. We found that a simultaneous addition of CuO and SiO2 is effective in obtaining the relatively dense ceramics. The microstructural and chemical investigations show that the synergetic effect of a liquid-phase (Ba-Sr-O) assisted grain growth accelerated by Cu-derived Ru charge inhomogeneity along with a rearrangement of grains promoted by a liquid-phase silicate is the mechanism of the (Ba, Sr)RuO3 densification. We demonstrate that all epitaxial BaTiO3 thin-film capacitors with (Ba1−xSrx)RuO3 electrodes can be obtained up to x of 0.6.

Dispersion in Ferroelectric Switching Performance of Polycrystalline Hf0.5Zr0.5O2 Thin Films.

Interests in nanoscale integrated ferroelectric devices using doped HfO2-based thin films are actively reviving in academia and industry. The main driving force for the formation of the metastable non-centrosymmetric ferroelectric phase is considered to be the interface/grain boundary energy effect of the small grains in polycrystalline configuration. These small grains, however, can invoke unfavorable material properties, such as nonuniform switching performance. This study provides an in-depth understanding of such aspects of this material through careful measurement and modeling of the ferroelectric switching kinetics. Various previous switching models developed for conventional ferroelectric thin-film capacitors cannot fully account for the observed time- and voltage-dependent switching current evolution. The accurate fitting of the experimental results required careful consideration of the inhomogeneous field distribution across the electrode area, which could be acquired by an appropriate mathematical formulation of polarization as a function of electric field and time. Compared with the conventional polycrystalline Pb(Zr,Ti)O3 film, the statistical distribution of the local field was found to be three times wider. The activation field and characteristic time for domain switching were larger by more than 1 order of magnitude. It indicates that doped HfO2 is inhomogeneous and "hard" ferroelectric material compared with conventional perovskite-based ferroelectrics.

Enhanced electrical response in ferroelectric thin film capacitors with inkjet-printed LaNiO3 electrodes

We describe the inkjet printing of lanthanum nickelate (LaNiO3) top electrodes onto ferroelectric Pb(Zr,Ti)O3 films on platinized silicon substrates. The developed ink formulation enables the deposition of well-defined, smooth, and flat layers with minimal inter-diffusion at the LaNiO3–Pb(Zr,Ti)O3 interface. The capacitors exhibit better polarization switching characteristics, improved fatigue properties, and about 40% larger dielectric constant than those with sputtered Au top electrodes. The Rayleigh analysis of dielectric response reveals enhanced domain wall motion in capacitors with printed LaNiO3 electrodes, most probably due to (i) effective compensation of oxygen vacancies by the oxide electrode, (ii) thinner depletion regions at the electrode–ferroelectric interface, and (iii) lower concentration of charged defects introduced by processing.

Control of misfit strain in ferroelectric BaTiO3 thin-film capacitors with SrRuO3-based electrodes on (Ba, Sr)TiO3-buffered SrTiO3 substrates

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.

Ultrahigh energy storage in lead-free BiFeO3/Bi3.25La0.75Ti3O12 thin film capacitors by solution processing

Thin film ferroelectric capacitors (TFFCs) with excellent energy storage have attracted increasing attention due to the electronic devices toward miniaturization and integration. BiFeO3 (BF)/Bi3.25La0.75Ti3O12 (BL) based thin films are prepared by chemical solution deposition for energy storage. Ultrahigh energy storage with a recoverable energy density Ure of 54.9 J/cm3 and an efficiency η of 74.4% is observed in the bilayered BF/BL thin films. Further improvement of energy storage is realized in trilayered BL/BF/BL thin films with a Ure of 65.5 J/cm3 and an efficiency η of 74.2% at an electric field of 2753 kV/cm as well as excellent fatigue endurance up to 109 cycles. The results suggest that BF/BL based thin films can be used as lead-free TFFCs in energy storage applications.

BiFeO3-doped (K0.5,Na0.5)(Mn0.005,Nb0.995)O3 ferroelectric thin film capacitors for high energy density storage applications

Environmentally benign lead-free ferroelectric (K0.5,Na0.5)(Mn0.005,Nb0.995)O3 (KNMN) thin film capacitors with a small concentration of a BiFeO3 (BF) dopant were prepared by a cost effective chemical solution deposition method for high energy density storage device applications. 6 mol. % BF-doped KNMN thin films showed very slim hysteresis loops with high maximum and near-zero remanent polarization values due to a phase transition from the orthorhombic structure to the pseudo-cubic structure. Increasing the electric field up to 2 MV/cm, the total energy storage density (Jtotal), the effective recoverable energy density (Jeff), and the energy conversion efficiency (η) of lead-free KNMN-BF thin film capacitors were 31.0 J/cm3, 28.0 J/cm3, and 90.3%, respectively. In addition, these thin film capacitors exhibited a fast discharge time of a few μs and a high temperature stability up to 200 °C, proving their strong potential for high energy density storage and conversion applications.

Assessment of TID Effect of FRAM Memory Cell Under Electron, X-Ray, and Co- $60~\gamma $ Ray Radiation Sources

This paper investigates the total ionizing dose (TID) effect of the memory cell of the ferroelectric random access memory (FRAM) under electron, X-ray, and Co- $60~\gamma $ ray radiation sources. An electron accelerator and an X-ray microbeam from synchrotron, which are used to simulate the given radiation environments, offer local irradiation on the FRAM memory cell. In addition, the Co- $60~\gamma $ ray source provides global irradiation on the full chip of FRAM. The FRAM memory cell is proved to have a lower failure threshold for TID effect than the ferroelectric thin-film capacitor due to the performance degradation of nMOS transistor in memory cell. The failure phenomenon is studied according to the experimental results of different radiation sources, and the failure mechanism is discussed based on the technology and the characteristics of FRAM memory cell in circuit-level. The difference of device performance is also analyzed for electron irradiation and X-ray irradiation.

Four-State Anti-Ferroelectric Random Access Memory

Ferroelectric random access memory (FRAM) is a two-state non-volatile memory, in which information is digitally encoded using switchable remanent polarization states within a ferroelectric thin film capacitor. Here, we propose a novel non-volatile memory based on anti-ferroelectric polycrystalline ceramics, termed anti-FRAM (AFRAM). The AFRAM memory cell architecture is similar to FRAM, but it is an operation protocol. Our initial experimental demonstration of the memory effect in anti-ferroelectric ceramic shows, remarkably, that the AFRAM technology encodes data in both ferroelectric sublattices of the anti-ferroelectric medium. This results in a four-state non-volatile memory capable of storing two digital bits simultaneously, unlike the FRAM technology that has two-memory states and it is capable to store one digital bit per cell.

Nanodomain Engineering in Ferroelectric Capacitors with Graphene Electrodes.

Polarization switching in ferroelectric capacitors is typically realized by application of an electrical bias to the capacitor electrodes and occurs via a complex process of domain structure reorganization. As the domain evolution in real devices is governed by the distribution of the nucleation centers, obtaining a domain structure of a desired configuration by electrical pulsing is challenging, if not impossible. Recent discovery of polarization reversal via the flexoelectric effect has opened a possibility for deterministic control of polarization in ferroelectric capacitors. In this paper, we demonstrate mechanical writing of arbitrary-shaped nanoscale domains in thin-film ferroelectric capacitors with graphene electrodes facilitated by a strain gradient induced by a tip of an atomic force microscope (AFM). A phase-field modeling prediction of a strong effect of graphene thickness on the threshold load required to initiate mechanical switching has been confirmed experimentally. Deliberate voltage-free domain writing represents a viable approach for development of functional devices based on domain topology and electronic properties of the domains and domain walls.

Crystal structure and polarization hysteresis properties of ferroelectric BaTiO3 thin-film capacitors on (Ba,Sr)TiO3-buffered substrates

Ferroelectric BaTiO3 (BT) thin-film capacitors with a buffer layer of (Ba1−xSrx)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.

Ferroelectric/Semiconductor/Tunnel-Junction Stacks for Nondestructive and Low-Power Read-Out Memory

We demonstrate that a tunnel junction connected in series to the ferroelectric (FE) via a semiconductor (SC) layer can be used to 1 probe the polarization state in an FE thin-film capacitor. A thermodynamic analysis is carried out to demonstrate the carrier depletion and accumulation at the SC/tunnel-junction interface as a function of the polarization direction in the FE layer. Our results indicate that the tunnel junction’s polarization and electroresistance depend on those of the main FE bit for a carefully chosen doping of the SC. This would allow measurement of the tunnel current to probe the polarization state of the FE bit nondestructively with much lower power consumption than the destructive technique where the displacement current at the switching of polarization is read. The proposed approach considers an FE/SC/tunnel-junction stack with top and bottom electrodes and is analogous to the Giant Magnetoresistance (GMR) stacks where the resistance change is indicative of magnetization orientation of the pinned and soft layers, implying the binary bit in the storage medium.