Ferroelectric-electrode Interface Research Articles

Physical chemistry of the TiN/Hf0.5Zr0.5O2 interface

Ferroelectric hafnia-based thin films are promising candidates for emerging high-density embedded nonvolatile memory technologies, thanks to their compatibility with silicon technology and the possibility of 3D integration. The electrode–ferroelectric interface and the crystallization annealing temperature may play an important role in such memory cells. The top interface in a TiN/Hf0.5Zr0.5O2/TiN metal–ferroelectric–metal stack annealed at different temperatures was investigated with X-ray photoelectron spectroscopy. The uniformity and continuity of the 2 nm TiN top electrode was verified by photoemission electron microscopy and conductive atomic force microscopy. Partial oxidation of the electrode at the interface is identified. Hf is reduced near the top interface due to oxygen scavenging by the top electrode. The oxygen vacancy (VO) profile showed a maximum at the top interface (0.71%) and a sharp decrease into the film, giving rise to an internal field. Annealing at higher temperatures did not affect the VO concentration at the top interface but causes the generation of additional VO in the film, leading to a decrease of the Schottky Barrier Height for electrons. The interface chemistry and n-type film doping are believed to be at the origin of several phenomena, including wake-up, imprint, and fatigue. Our results give insights into the physical chemistry of the top interface with the accumulation of defective charges acting as electronic traps, causing a local imprint effect. This may explain the wake-up behavior as well and also can be a possible reason of the weaker endurance observed in these systems when increasing the annealing temperature.

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.

Surface modification and effects of organic ferroelectrics with blending hyperbranched polymer

The surface modification of ferroelectric films is expected to improve the properties of fatigue, which is important for application in memory devices. In this study, we fabricated thin insulators at an electrode–ferroelectric interface by the phase separation of a ferroelectric polymer and an insulator. The surface and bulk characterization indicated that the insulators consisting of a hyperbranched polymer spontaneously phase-separated from the organic ferroelectric polymer by thermal annealing. It was revealed that the separated layers were composed of three layers and had a lower surface energy than the ferroelectric films. The annealing time evolution of the surface contact angle and dielectric spectra indicated the phase separation dynamics and structural behavior. The fatigue properties of the surface-modified ferroelectric films improved, but the remanent polarization and coercive electric field value resulted in a trade-off.

Effect of top electrode material on radiation-induced degradation of ferroelectric thin film structures

The effects of gamma irradiation on the dielectric and piezoelectric responses of Pb[Zr0.52Ti0.48]O3 (PZT) thin film stacks were investigated for structures with conductive oxide (IrO2) and metallic (Pt) top electrodes. The samples showed, generally, degradation of various key dielectric, ferroelectric, and electromechanical responses when exposed to 2.5 Mrad (Si) 60Co gamma radiation. However, the low-field, relative dielectric permittivity, εr, remained largely unaffected by irradiation in samples with both types of electrodes. Samples with Pt top electrodes showed substantial degradation of the remanent polarization and overall piezoelectric response, as well as pinching of the polarization hysteresis curves and creation of multiple peaks in the permittivity-electric field curves post irradiation. The samples with oxide electrodes, however, were largely impervious to the same radiation dose, with less than 5% change in any of the functional characteristics. The results suggest a radiation-induced change in the defect population or defect energy in PZT with metallic top electrodes, which substantially affects motion of internal interfaces such as domain walls. Additionally, the differences observed for stacks with different electrode materials implicate the ferroelectric–electrode interface as either the predominant source of radiation-induced effects (Pt electrodes) or the site of healing for radiation-induced defects (IrO2 electrodes).

Multistability in Bistable Ferroelectric Materials toward Adaptive Applications

Traditionally thermodynamically bistable ferroic materials are used for nonvolatile operations based on logic gates (e.g., in the form of field effect transistors). But, this inherent bistability in these class of materials limits their applicability for adaptive operations. Emulating biological synapses in real materials necessitates gradual tuning of resistance in a nonvolatile manner. Even though in recent years few observations have been made of adaptive devices using a ferroelectric, the principal question as to how to make a ferroelectric adaptive has remained elusive in the literature. Here, it is shown that by locally controlling the nucleation energy distribution at the ferroelectric–electrode interface multiple‐addressable states in a ferroelectric can be created, which is necessary for adaptive/synaptic applications. This is realized by depositing a layer of nonswitchable ZnO on top of thin film ferroelectric PbZr x Ti(1– x )O3. This methodology of interface‐engineered ferroelectric should enable realising brain‐like adaptive/synaptic memory in complementary metal‐oxide‐semiconductor (CMOS) devices. Furthermore, the temporally stable multistability in ferroelectrics should enable the designing of multistate memory and logic devices.

TaN interface properties and electric field cycling effects on ferroelectric Si-doped HfO2 thin films

Ferroelectric HfO2-based thin films, which can exhibit ferroelectric properties down to sub-10 nm thicknesses, are a promising candidate for emerging high density memory technologies. As the ferroelectric thickness continues to shrink, the electrode-ferroelectric interface properties play an increasingly important role. We investigate the TaN interface properties on 10 nm thick Si-doped HfO2 thin films fabricated in a TaN metal-ferroelectric-metal stack which exhibit highly asymmetric ferroelectric characteristics. To understand the asymmetric behavior of the ferroelectric characteristics of the Si-doped HfO2 thin films, the chemical interface properties of sputtered TaN bottom and top electrodes are probed with x-ray photoelectron spectroscopy. Ta-O bonds at the bottom electrode interface and a significant presence of Hf-N bonds at both electrode interfaces are identified. It is shown that the chemical heterogeneity of the bottom and top electrode interfaces gives rise to an internal electric field, which causes the as-grown ferroelectric domains to preferentially polarize to screen positively charged oxygen vacancies aggregated at the oxidized bottom electrode interface. Electric field cycling is shown to reduce the internal electric field with a concomitant increase in remanent polarization and decrease in relative permittivity. Through an analysis of pulsed transient switching currents, back-switching is observed in Si-doped HfO2 thin films with pinched hysteresis loops and is shown to be influenced by the internal electric field.

Phase transitions and domain structures in thin ferroelectric films

Recent advances in technology made available high quality thin ferroelectric films and ferroelectric–paraelectric multilayers. But understanding of the properties of these systems is far from being complete. In particular, it is not clear why various anomalies observed at phase transitions here are different from those in high quality bulk systems. The aim of the paper is to discuss some specific features of ferroelectric phase transitions in thin films and multilayers which are taken into account only partially by the theory. The discussion is limited to revealing the character of ferroelectric state forming just after the transition, i.e. if it is single-domain or multi-domain, if it is single-phase or two-phase. First, thin films with electrodes and on a substrate are discussed. The role of non-ideality of electrodes in realization of one of the first pairs of possibilities is emphasized. A more recent idea is that even for ideal electrodes a domain formation is possible when some conditions on the electrode–ferroelectric interface and the material constants of the material are met. This seems to be quite possible for the considered systems. Clamping by the substrate may lead to formation of a two-phase state which is practically unexplored for very thin films on substrates. Second, ferroelectric–paraelectric multilayers are considered which are more challenging for theoretical study than the thin films. Indeed, despite periodicity in the composition the domain structures are almost never periodic along the multilayer. The non-ideality of electrodes seems to lead to practical impossibility of single-domain ferroelectric phase transition in the multilayers of the considered type.

Disappearance of ferroelectric critical thickness in epitaxial ultrathinBaZrO3films

The intrinsic critical ferroelectric thickness of epitaxial ultrathin capacitors of incipient ferroelectric $\mathrm{BaZr}{\mathrm{O}}_{3}$ (BZO) films with realistic $\mathrm{SrRu}{\mathrm{O}}_{3}$ (SRO) electrodes is investigated by first-principles calculations based on density functional theory. We reveal that polarization can stably exist even in one-unit-cell thick BZO films, i.e., absence of critical thickness, whereas the widely investigated proper ferroelectrics like $\mathrm{BaTi}{\mathrm{O}}_{3}$ and $\mathrm{SrTi}{\mathrm{O}}_{3}$ films have no polarization. The influences of realistic ferroelectric-electrode interface and misfit strain on the ionic and electronic structures of the BZO-SRO thin film system have been examined under the short-circuited boundary condition. It is found that the ionic polarization of conductive SRO electrodes can effectively strengthen the screening of bound charges at the interface, which greatly reduces the depolarization field in the BZO films. Furthermore, the epitaxial misfit strain remarkably enhances the polarization through the enhancement of hybridization of $\mathrm{Zr}$ and $\mathrm{O}$ electron orbitals, resulting in the disappearance of ferroelectric critical thickness. Our findings are beyond the critical thickness of proper ferroelectrics and are thus promising for future nanometer-scale ferroelectric device such as high-density ferroelectric memory.

Electrophysical properties of lead zirconate titanate films doped with lanthanum

The influence of lanthanum doping of lead zirconate titanate films (PZTL) on their polarization properties and leakage currents is studied. The films of Pb1 − x La x (Zr0.48Ti0.52)1 − x/4O3 with x = 0, 0.02, 0.05, 0.08, and 0.1 and a thickness of 240 nm were produced using the sol-gel method at an annealing temperature of 650°C on the substrates with a Si-SiO2-TiO2-Pt structure. It is found that the residual polarization and the coercive field are weakened after doping with La. The films with x = 0.02 exhibit satisfactory ferroelectric characteristics (7 μC/cm2, 25 kV/cm) that make them suitable for use in ferroelectric memory. The current-voltage curves of PZTL films have two characteristic regions. In weak fields (up to 80–90 kV/cm), the leakage current is attributable primarily to the high resistance of the depletion region of the reverse-biased Schottky barrier at the electrode-ferroelectric interface, and the leakage current is thus practically independent of the La content. When the threshold voltage is exceeded, the Schottky barrier breakdown occurs, and the leakage current in strong fields is defined by the density of free carriers in the bulk of the film. This density depends on the La doping level. The leakage current at x = 0.02 is reduced by about two orders of magnitude compared to undoped PZT film. Further increases in the La content are accompanied by an increase in the leakage current in strong fields. This effect is attributed to a change in the conduction type and an increase in the density of n-type carriers.

Electrical properties of lead-free KNN films on SRO/STO by RF magnetron sputtering

Abstract Highly (001) oriented (K,Na)NbO 3 lead-free piezoelectric films were fabricated on SrRuO 3 buffered SrTiO 3 (001) single crystal substrates by RF magnetron sputtering. The films showed a stable frequency dependence of dielectric permittivity and loss tangent in the range of 1 kHz–1 MHz. A fairly high tunability of 69.7% (@400 kV/cm) was obtained in the films. Well saturated P – E hysteresis loops were obtained at E =250 kV/cm, the remnant polarization P r was 8 μC/cm 2 , and coercive field E c was only 40 kV/cm. These films showed a fatigue-free behavior up to 10 10 switching cycles. The decreased coercive field, improved dielectric and fatigue properties may be related to SRO bottom electrodes, which can efficiently suppress the “dead layer” near the ferroelectric-electrode interface. The leakage current density of the films was only 3.48×10 −6 A/cm 2 at the electric field of 250 kV/cm, which is even lower than that of Mn-doped KNN films. In addition, the piezoelectric constant d 33 was estimated to be 36 pm/V.

Factors for the polarization lifetime in metal–ferroelectric–insulator–semiconductor capacitors

Depolarization field in metal–ferroelectric–insulator–semiconductor (MFIS) capacitors with a ferroelectric–electrode interface layer was derived theoretically in this work. The polarization relaxation characteristics were investigated in details based on Lou’s polarization retention model. It is found that the retention time of ferroelectric field-effect transistors (FETs) can be affected significantly by the dielectric constant and the thickness of ferroelectric thin film, and by the interface layer thickness. The results may provide some insights into the design and the retention property improvement of MFIS–FET as nonvolatile memory.

Structural defects and local chemistry across ferroelectric–electrode interfaces in epitaxial heterostructures

We present a detailed investigation of the chemistry at the growth interface between the bottom electrode and ferroelectric film in (001)-oriented epitaxial ferroelectric thin-film heterostructures. Three different ferroelectric systems, namely PbZr0.2Ti0.8O3, PbZr0.52Ti0.48O3, and BaTiO3 deposited on SrRuO3/SrTiO3, were investigated to compare and contrast the role of lattice mismatch versus the volatility of the deposited cation species. A combination of transmission electron microscopy-based imaging and spectroscopy reveals distinct correlations among the ferroelectric thin-film composition, the deposition process, and chemical gradients observed across the ferroelectric–electrode interface. Sr diffusion from the electrode into the ferroelectric film was found to be dominant in PbZr0.2Ti0.8O3/SrRuO3/SrTiO3 thin films. Conversely, Pb diffusion was found to be prevalent in PbZr0.52Ti0.48O3/SrRuO3/SrTiO3 thin films. The BaTiO3/SrRuO3/SrTiO3 heterostructure was found to have atomically sharp interfaces with no signature of any interdiffusion. We show that controlling the volatility of the cation species is as crucial as lattice mismatch in the fabrication of defect-free ferroelectric thin-film devices.

Ferromagneticlike Closure Domains in Ferroelectric Ultrathin Films: First-Principles Simulations

We simulate from first principles the energetic, structural, and electronic properties of ferroelectric domains in ultrathin SrRuO_{3}/BaTiO_{3}/SrRuO_{3} ferroelectric capacitors in short circuit. The domains are stabilized down to two unit cells at zero temperature, adopting the form of a domain of closure, common in ferromagnetic thin films. The domains are closed by the in-plane relaxation of the atoms in the first SrO layer of the electrode, which behaves more like SrO in highly polarizable SrTiO3 than in metallic SrRuO3. Even if small, these lateral displacements are very important to stabilize the domains and might provide some hints to explain why some systems break into domains while others remain in a monodomain configuration. An analysis of the electrostatic potential reveals preferential points of pinning for charged defects at the ferroelectric-electrode interface, possibly playing a major role in film fatigue.

Interface-induced phenomena in polarization response of ferroelectric thin films

This article reviews the existing theoretical models describing the interface-induced phenomena which affect the switching characteristics and dielectric properties of ferroelectric thin films. Three groups of interface-induced effects are addressed—namely, “passive-layer-type” effects, ferroelectric-electrode contact potential effects, and the poling effect of the ferroelectric-electrode interface. The existing experimental data on dielectric and switching characteristics of ferroelectric thin film capacitors are discussed in the context of the reviewed theories. Special attention is paid to the case of internal bias field effects.

Surface-Stimulated Nucleation of Reverse Domains in Ferroelectrics

We present a model for reverse domain nucleation in ferroelectrics, which takes into account ferroelectric-electrode coupling in both homogeneous and random cases. The model provides a solution to the coercivity paradox--i.e., the large discrepancy between the observed and predicted coercive fields, common to many systems. We demonstrate the possibility of not thermally activated nucleation of reverse domains. We find that small inhomogeneities in the ferroelectric-electrode interface may lead to an exponentially wide spectrum of waiting times for switching. The model predicts that switching is facilitated near morphotropic phase boundaries in perovskite-type ferroelectrics.

Size effects in ultrathin epitaxial ferroelectric heterostructures

In this letter we report on the effect of thickness scaling in model PbZr0.2Ti0.8O3(PZT)∕SrRuO3 heterostructures. Although theoretical models for thickness scaling have been widely reported, direct quantitative experimental data for ultrathin perovskite (<10nm) films in the presence of real electrodes have still not been reported. In this letter we show a systematic quantitative experimental study of the thickness dependence of switched polarization in (001) epitaxial PZT films, 4to80nm thick. A preliminary model based on a modified Landau Ginzburg approach suggests that the nature of the electrostatics at the ferroelectric–electrode interface plays a significant role in the scaling of ferroelectric thin films.

Improvement in ferroelectric properties of Pt/PZT/Pt capacitors etched as a function of Ar/O 2 gas mixing ratio into Cl 2/CF 4 plasma

In this work, to investigate improvement of the damage using oxygen containing plasma, we etched PZT films as a function of Ar ( x%)/O 2 ( y%) gas mixing ratio in Cl 2 (56%)/CF 4 (14%) plasma (where the sum of x and y is 30). The maximum etch rate of the PZT thin films was 146 nm/min for Ar (30%)/O 2 (0%) added into the Cl 2/CF 4 plasma. After the etching, the plasma-induced damages were characterized in terms of hysteresis curves, leakage current, switching polarization and retention capacity as a function of the gas mixing ratio. When the ferroelectric properties of PZT films were etched as a function of O 2 and Ar and the gas mixing ratios were compared, the value of remnant polarization in O 2 (30%) added Cl 2/CF 4 plasma is higher than that in Ar (30%). The results showed that after the etching the charges accumulated by oxygen vacancies prevented further domain switching at the top electrode–ferroelectric interface and created leakage current because of modification of the interfacial Schottky barrier during the etching process. The physical damage to the near surface and the crystal structure of the etched PZT thin films was evaluated by using X-ray diffraction (XRD). The remnant polarization, leakage current, retention and fatigue properties are improved with increasing O 2 content. From XRD results, the improvement in the ferroelectric properties of PZT capacitors etched in O 2 containing plasma was consistent with the increased intensities of the (100) and (200) peaks.

Critical thickness for ferroelectricity in perovskite ultrathin films.

The integration of ferroelectric oxide films into microelectronic devices, combined with the size reduction constraints imposed by the semiconductor industry, have revived interest in the old question concerning the possible existence of a critical thickness for ferroelectricity. Current experimental techniques have allowed the detection of ferroelectricity in perovskite films down to a thickness of 40 A (ten unit cells), ref. 3. Recent atomistic simulations have confirmed the possibility of retaining the ferroelectric ground state at ultralow thicknesses, and suggest the absence of a critical size. Here we report first-principles calculations on a realistic ferroelectric-electrode interface. We show that, contrary to current thought, BaTiO3 thin films between two metallic SrRuO3 electrodes in short circuit lose their ferroelectric properties below a critical thickness of about six unit cells (approximately 24 A). A depolarizing electrostatic field, caused by dipoles at the ferroelectric-metal interfaces, is the reason for the disappearance of the ferroelectric instability. Our results suggest the existence of a lower limit for the thickness of useful ferroelectric layers in electronic devices.

Direct observation of inversely polarized frozen nanodomains in fatigued ferroelectric memory capacitors

The direct observation of blocked polarization domains at the electrode–ferroelectric interface of electrically fatigued ferroelectric films is reported. Blocked nanodomains are believed to be the origin of polarization fatigue in ferroelectric nonvolatile memories but have not been directly observed so far due to the required upper metal electrode which impedes the direct access to the surface of the ferroelectric film. This problem has been solved by using low temperature melting metal as removable top electrode. After fatigue and subsequent top electrode removal it was possible to observe the polarization state of the fatigued capacitor and its depth profile by means of detection of the local piezoelectric activity with a conductive atomic force microscope tip. Blocked polarization domains with opposite polarization compared to the film body could be directly observed at the upper ferroelectrics surface.

Polarization imprint and size effects in mesoscopic ferroelectric structures

Piezoresponse scanning force microscopy measurements performed on lead zirconate titanate mesoscopic structures revealed a negative shift of the initial piezoelectric hysteresis loop. The shift is dependent on the size of the structure and is most probably due to the pinning of ferroelectric domains at the free lateral surface and ferroelectric–electrode interface. Considering a simple model, the thickness of the pinned domain layers is found to be about 15 and 70 nm at the ferroelectric–electrode interface and lateral free surface, respectively.