Pulse Switching Measurements Research Articles

Hysteresis-Free Negative Capacitance Effect in Metal-Ferroelectric-Insulator-Metal Capacitors with Dielectric Leakage and Interfacial Trapped Charges

The negative capacitance (NC) stabilization of a ferroelectric (FE) material can potentially provide an alternative way to further reduce the power consumption in ultra-scaled devices and thus has been of great interest in technology and science in the past decade. In this article, we present a physical picture for a better understanding of the hysteresis-free charge boost effect observed experimentally in metal-ferroelectric-insulator-metal (MFIM) capacitors. By introducing the dielectric (DE) leakage and interfacial trapped charges, our simulations of the hysteresis loops are in a strong agreement with the experimental measurements, suggesting the existence of an interfacial oxide layer at the FE-metal interface in metal-ferroelectric-metal (MFM) capacitors. Based on the pulse switching measurements, we find that the charge enhancement and hysteresis are dominated by the FE domain viscosity and DE leakage, respectively. Our simulation results show that the underlying mechanisms for the observed hysteresis-free charge enhancement in MFIM may be physically different from the alleged NC stabilization and capacitance matching. Moreover, the link between Merz's law and the phenomenological kinetic coefficient is discussed, and the possible cause of the residual charges observed after pulse switching is explained by the trapped charge dynamics at the FE-DE interface. The physical interpretation presented in this work can provide important insights into the NC effect in MFIM capacitors and future studies of low-power logic devices.

Insertion of HfO2 Seed/Dielectric Layer to the Ferroelectric HZO Films for Heightened Remanent Polarization in MFM Capacitors

This article highlights the role of HfO2 seed/dielectric insertion layers on the ferroelectric properties of hafnium zirconium oxide (HZO)-based metal-ferroelectric-metal (MFM) capacitors. Maximum remanent polarization ( ${P}_{r}$ ) of $22.1~\mu \text{C}$ /cm2 was achieved when HfO2 seed layer of critical thickness (10 A) was inserted at the bottom of HZO films. However, as the bottom HfO2 seed layer thickness increases from 10 to 200 A, the ${P}_{r}$ was found to decrease. The same critical thickness of HfO2 seed layer when inserted at top of the HZO films was found to give a maximum ${P}_{r}$ of $19.6~\mu \text{C}$ /cm2. The obtained ${P}_{r}$ in both the cases (bottom and top HfO2 seed layer) was found to be better than the reference HZO device ( $17.7~\mu \text{C}$ /cm2). Moreover, increased coercive field $({E}_{c})$ was noticed due to the HfO2 seed layer insertion (1.11 MV/cm for bottom HfO2 and 1.09 MV/cm for top HfO2) when compared with the reference device (1.04 MV/cm). Short pulse switching measurements were carried out on the as-fabricated capacitors, and an enhanced ${E}_{c}$ and interfacial capacitance was observed in case of devices with HfO2 seed layer. The crystal structures, and interfacial layer effect was keenly observed and analyzed for the HZO films with and without the seed layer using grazing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS), respectively. The HfO2 seed layer was found to result in higher o-phase formation, and facilitates the diffusion/migration of oxygen atoms from seed layer to the substrate or vice versa in the HZO capacitors giving rise to enhanced ferroelectricity.

First-order reversal curve diagrams for characterizing ferroelectricity of Hf0.5Zr0.5O2 films grown at different rates

The ferroelectric properties of Hf0.5Zr0.5O2 (HZO) films deposited by atomic layer deposition using various growth rates were systematically investigated by analyzing first-order reversal curve (FORC) diagrams. The FORC diagram is a valuable methodology for characterization of ferroelectric switching by evaluation of subloop hysteresis. These diagrams are tremendously sensitive to changes in the hysteresis loops which can be used to describe switching characteristics. HZO films with a growth rate of 1.5 Å/cycle showed two oppositely biased regions in switching density plots. When decreasing the growth rate to 1.2 and 0.98 Å/cycle, these internal bias fields disappeared, showing one maximum switching current peak. Furthermore, the device fabricated with a growth rate of 1.2 Å/cycle showed the clearest switching current peak, indicating an increase in the magnitude of the switching current, as well as increased remanent polarization. In this paper, the FORC diagram and the result of pulse switching measurements suggest that the growth rate of the thin film is a crucial factor to determine the high quality of HZO ferroelectric films.

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.

Superior unipolar resistive switching in stacked ZrOx/ZrO2/ZrOx structure

This study investigates the performance of unipolar-switched ZrO2 RRAM, using an oxygen-deficient and amorphous ZrOx capping in a sandwich stack Al/ZrOx/ZrO2/ZrOx/Al structure. Superior high and low resistance switching and a resistance ratio (HRS/LRS) greater than 10 showed excellent dc endurance of 7378 switching cycles and 3.8 × 104 cycles in pulse switching measurements. Recovery behavior, observed in the I-V curve for the SET process (or HRS), led to HRS fluctuations and instability. A new resistance switching model for the stacked ZrO2 RRAM is proposed in this paper. In this model, oxygen-deficient and amorphous ZrOx film, capped on polycrystalline ZrO2 film, plays a key role and acts as an oxygen reservoir in making the oxygen ions redox easily for the SET process and in facilitating re-oxidation for the RESET process, resulting in excellent endurance. By improving the stability and recovery phenomena, engineering parameters of the current control may play a critical role during switching, and they can be correlated to the film’s thickness and the oxygen content of the amorphous ZrOx film.

Characterization and Modeling of the Conduction and Switching Mechanisms of HfOx Based RRAM

ABSTRACTThe conduction and switching mechanism of resistive random access memory (RRAM) is reviewed in this paper. The resistive switching in oxides is generally attributed to the conductive filament (made up of oxygen vacancies) formation and rupture in the oxide due to field assisted oxygen ion migration. As a model system for device physics study, HfOx based RRAM devices were fabricated and characterized. To identify the electron conduction mechanism, various electrical characterization techniques such as I-V measurements at various temperatures, low-frequency noise measurements, and AC conductance measurements were employed. It was suggested that the trap-assisted-tunneling is the dominant conduction mechanism. In order to explore the oxygen ion migration dynamics, pulse switching measurements were performed. An exponential voltage-time relationship was found between the switching time and the applied voltage. To obtain a first-order understanding of the variability of resistive switching, a Kinetic Monte Carlo (KMC) numerical simulator was developed. The generation/recombination/migration probabilities of oxygen vacancies and oxygen ions were calculated, and the conductive filament configuration was updated stochastically according to those probabilities. The KMC simulation can reproduce many experimental observations in the DC I-V sweep, pulse switching, endurance cycling, and retention baking, etc. The tail bits in the resistance distribution are attributed to the oxygen vacancy left over in the gap region due to a competition between the oxygen vacancy generation and recombination. To enable circuit and system development using RRAM, a compact device model was developed. The compact model, implemented in MATLAB, HSPICE, and Verilog-A, which can be employed in many commonly available circuit simulators using the SPICE engine.

Model Relating Thin Film Pzt Crystallographic Texture to Ferroelectric Switching Performance

AbstractThe fundamental physical property used for data storage in a ferroelectric non-volatile memory (FRAM) is the remanent polarization of a ferroelectric capacitor. In order to sense the data state of the capacitor, the charge pulse generated by either a switching or non-switching ferroelectric polarization is detected. Since the polarization of the ferroelectric is a vector property, the switchable polarization response of a ferroelectric capacitor is necessarily dependent on the texture of the ferroelectric material used to make the capacitor. A series of Ca, Sr, and La doped PbZr0.4Ti0.6O3 (PZT) capacitor test samples were produced with texture ranging from nearly complete {111} texture to random orientation. The volume fraction of the textured material was quantified by X-ray diffraction. An experimental correlation between texture and the switchable polarization was established using pulse switching measurements. A mathematical model was developed to explain the dependence of the switchable polarization on the crystallographic texture. It was found that the dependence of the saturated switchable polarization on texture required that there was no 90° domain reorientation. The impact of these findings will be related to the performance of FRAM.

Growth and characterization of epitaxial ferroelectric PbZrxTi1−xO3 thin film capacitors with SrRuO3 electrodes for non-volatile memory applications

Non-volatile FeRAMs based on thin film ferroelectric capacitors are emerging as an advantageous replacement for EEPROM and flash devices. In this work, ferroelectric Al/PbZrxTi1−xO3 (PZT)/SrRuO3 thin film capacitors were fabricated and its suitability for memory applications was tested. The PZT films, as well as the SrRuO3 bottom electrodes have been grown by pulsed laser deposition on LaAlO3(001) substrates. Epitaxial heterostructures with high crystalline quality were obtained. Top aluminum electrodes were thermally evaporated onto the PZT films. High dielectric constant of 700 and dielectric loss tangent of 0.03 were obtained. Good ferroelectric behavior with an activation field for switching γ=308 kV/cm and a low coercive voltage of 1.1 V at 40 Hz was observed. The reliability of the thin film capacitors has been tested by means of pulse switching measurements. The polarization shows very little fatigue with cumulative switching. Built-in imprint, arising from the use of asymmetric electrodes, along with induced imprint related to charge trapping at the interfaces were observed.

Characterization of metalorganic decomposition-derived SrBi2Ta2O9 thin films with different thicknesses

Ferroelectric capacitors having sputtered Pt bottom and top electrodes and metalorganic decomposition derived ferroelectric thin films of SrBi2Ta2O9 (SBT) were prepared. The thickness effects on the structure, surface morphology, and electrical properties of SBT thin films were studied and discussed. Structure and surface morphology were analyzed by x-ray diffraction and atomic force microscope. The electrical properties were characterized by the measurements of hysteresis, capacitance, and pulse switching. The crystalline structure, grain size, and electrical properties do not depend on film thickness when films are less than 440 nm thick. A larger grain size is observed for films thicker than 440 nm, which results in a larger remnant polarization. The thickness dependence of the coercive voltage and reciprocal capacitance can be explained by an interfacial layer model. The thickness-independent coercive field and dielectric constant, calculated from the interfacial layer model, are around 19 kV/cm and 343, respectively. Nonvolatile polarization (Pnv), obtained from pulse-switching measurements, increases with the applied electric field and finally saturates with a slight thickness effect. The relaxation of the remnant polarization that occurs at zero-voltage intervals between the pulses and the thickness-independent behavior of Pnv are explained based on depolarization fields within the ferroelectric due to nonswitching interfacial layers at the film/electrode interfaces.

Switching quality of thin-film PZT ferroelectric capacitors

The switching quality of PZT thin-film ferroelectric capacitors was studied, using hysteresis and pulse switching measurements. The ferroelectric properties of epitaxial Pt/PZT/LSCO/MgO capacitors are strongly affected by the LSCO material. A strong deviation between hysteresis and pulse measurements has been observed for one type of LSCO, corresponding to a strong relaxation of (oxygen-vacancy) space charge polarization. On the other hand, polycrystalline Pt/PZT/Pt show very rectangular hysteresis loops with high P, and pulse measurements reveal high speed switching and absence of rapid depolarization effects. These results prove the very high switching quality of Pt/PZT/Pt capacitors, required for their application as non-volatile memory elements.

Externally determined and intrinsic contributions to the polarization switching currents in SrBi2Ta2O9 thin films

It has been shown that ferroelectric materials are suitable for non volatile memories. To replace other technologies these materials have to fulfill several requirements, one is a sufficient switching velocity of the material. At present, memory access times amount to 10 to 50 ns. Hence, ferroelectric materials should exhibit access times similar to these values. As a result, the polarization switching must be completed within a few nanoseconds. This work presents fast pulse switching measurements with speeds equal to the speed in a real memory device. For all samples studied, a significant increase in the coercive voltage has been observed at rise times of the excitation signal of about one nanosecond. A qualitative model is suggested to explain the behavior of the material at highest speed compared to the hysteresis curve data recorded by low frequencies. It is distinguished between externally determined and intrinsic part of the polarization switching current.

Ferroelectric properties and fatigue of PbZr0.51Ti0.49O3 thin films of varying thickness: Blocking layer model

Ferroelectric capacitors having Pt bottom and top electrodes and a ferroelectric film of composition PbZr0.51Ti0.49O3 (PZT) were fabricated and investigated. The PZT films of thicknesses varying from 0.12 to 0.69 μm were prepared by organometallic chemical-vapor deposition. Annealed capacitors were investigated by capacitance, hysteresis, and pulse switching measurements. It is found that the thickness dependence of the reciprocal capacitance, the coercive voltage, and the polarization measured by pulse switching can all be explained by a blocking layer model, in which a dielectric layer of thickness dbl and relative permittivity εbl is situated between the PZT film and an electrode. It is shown that (i) the coercive field is independent of thickness having a value of 2.4 V/μm; (ii) the ratio εbl/dbl is in the range 20–28 nm−1; (iii) the voltage across the blocking layer is proportional to the polarization, Vbl=cP, where c=4.1±0.5 V m2/C; and (iv) the polarization depends on the electric field in the PZT layer, independent of thickness. Pulse switching endurance measurements showed that in the saturation range the fatigue for these ferroelectric capacitors is determined by the pulse voltage and is independent of the thickness.

Preparation and ferroelectric properties of PbZr0.53Ti0.47O3 thin films by spin coating and metalorganic decomposition

Films with a composition PbZr0.53Ti0.47O3 were prepared on different platinum bottom electrodes using a spin-coating process followed by metalorganic decomposition. The film morphology and structure were characterized by scanning electron microscopy and x-ray diffraction analysis. The morphology was strongly influenced by the heat cycle used to form the PbZr0.53Ti0.47O3 layer. Fast heating and high-fire temperatures produced smooth films, while slow heating and low-fire temperatures gave films having a rosette perovskite phase and an inter-rosette second phase. The films were characterized electrically by measuring hysteresis loops and capacitance and conductance versus bias voltage and by pulse-switching measurements. The dependence of the switched and nonswitched polarization on the number of switching cycles (i.e., the fatigue behavior) is found to be much better for fast-heated than for slowly heated films. Switching lifetimes exceeding 1011 cycles were measured. The type of platinum bottom electrode used was found to have a large influence on the ferroelectric properties of the lead zirconate titanate films.