Search

In order to analyse the interplay between ferroelectric switching and parasitic charge trapping in ferroelectric doped hafnium oxide, a metal ferroelectric metal capacitor and grain boundary model is implemented in TCAD. In our study, we present how bulk traps inside the ferroelectric material, traps at the interface with the electrodes are impacting the ferroelectric switching behaviour. Switching peak splitting was observed experimentally, and reproduced by TCAD simulation. Furthermore, the influence of DC voltage stress on the switching behaviour is investigated experimentally and compared to the already known AC-behaviour. DC stress of the FeCAP redistributes the charge within the device influencing the kinetics of the ferroelectric switching and causing the peak split. [ABSTRACT FROM AUTHOR]

Harnessing ferroelectric negative capacitance in Hf0.5Zr0.5O2‐based thin films is promising for applications in nanoscale electronic devices with ultralow power dissipation, due to their ultimate scalability and semiconductor process compatibility. However, so far, it has been unclear if negative capacitance is an intrinsic material property of ferroelectric Hf0.5Zr0.5O2, or if it is an extrinsic effect caused by specific domain configurations and lateral domain wall motion as seen in perovskite ferroelectrics. Here, symmetric and asymmetric Hf0.5Zr0.5O2/Al2O3‐based ferroelectric/dielectric heterostructures are investigated to understand the relationship among depolarization, interfacial charge, domain formation, and negative capacitance. To achieve this, detailed electrical characterization is combined with structural data, analytical modeling, and numerical simulations. The findings suggest that negative capacitance in these ferroelectric/dielectric heterostructures is an intrinsic property of the Hf0.5Zr0.5O2 layer, which has important implications for potential applications. Furthermore, it is experimentally observed that the energy barrier for polarization switching in Hf0.5Zr0.5O2 is largely independent of the domain configuration and layer thickness, which confirms recent predictions by first principles calculations. [ABSTRACT FROM AUTHOR]

• Reliability measurements on hafnium based ferroelectric devices. • Characterization of differential negative capacitance/ hysteresis free charge amplification of negative capacitance capacitors. • Degradation of hysteresis free charge amplification in unipolar operation of MFIM devices. A measurement procedure is presented to characterize the degradation of the hysteresis free charge amplification in unipolar operation of negative capacitance capacitors. Two different degradation processes are identified from these measurements and are related to remanent switching of domains and a change in the electrical internal bias field. [ABSTRACT FROM AUTHOR]

• Negative capacitance operation leads to new perspectives for CMOS nanoscale devices. • Lack of TCAD numerical modeling of ferroelectric materials. • Numerical models and methods proposed for ferroelectric materials. • Use of ferroelectric devices in High Energy Physics experiments detection systems. • Modeling of radiation damage effects at the dielectric/ferroelectric interface. In this work advanced TCAD (Technology Computer Aided Design) modeling will be used aiming at investigating the potentiality of Negative Capacitance (NC) devices in non-conventional application domains (e.g., radiation detection). A device-level approach to simulate the electrical characteristics of ferroelectric Hf 0.5 Zr 0.5 O 2 (HZO) has been developed. The validation of the models and of the adopted numerical methods relies on the comparison between simulations and measurements of Metal-Ferroelectric-Metal and Metal-Ferroelectric-Insulator-Metal capacitors. By introducing the experimentally observed dielectric/ferroelectric interfacial trapped charges, our simulations are in a strong agreement with experimental measurements. These charges compensate the bound ferroelectric polarization charge when no external electric field is applied. The ferroelectric/dielectric interface could be therefore studied before and after X-ray irradiation. The goal will be to investigate the suitability of innovative NC devices to be used in High Energy Physics experiments detection systems, featuring self-amplified segmented, high granularity detectors. [ABSTRACT FROM AUTHOR]

Hafnium oxide is widely used for resistive switching devices, and recently it has been discovered that ferroelectricity can be established in (un-)doped hafnium oxide as well. Previous studies showed that both switching mechanisms are influenced by oxygen vacancies. For resistive switching, typically amorphous oxide layers with an asymmetric electrode configuration are used to create a gradient of oxygen vacancies. On the other hand, ferroelectric switching is performed by having symmetric electrodes and requires crystalline structures. The coexistence of both effects has recently been demonstrated. In this work, a detailed analysis of the reversible interplay of both switching mechanisms within a single capacitor cell is investigated. First, ferroelectric switching cycles were applied in order to drive the sample into the fatigued stage characterized by increased concentration of oxygen vacancies in the oxide layer. Afterwards, a forming step that is typical for the resistive switching devices was utilized to achieve a soft breakdown. In the next step, twofold alternation between the high and low resistance state is applied to demonstrate the resistive switching behavior of the device. Having the sample in the high resistance state with a ruptured filament, ferroelectric switching behavior is again shown within the same stack. Interestingly, the same endurance as before was observed without a hard breakdown of the device. Therefore, an effective sequence of ferroelectric—resistive—ferroelectric switching is realized. Additionally, the dependence of the forming, set, and reset voltage on the ferroelectric cycling stage (pristine, woken-up and fatigued) is analyzed giving insight into the physical device operation. [ABSTRACT FROM AUTHOR]

Transient negative capacitance effects in epitaxial ferroelectric Pb(Zr0.2Ti0.8)O3 capacitors are investigated with a focus on the dynamical switching behavior governed by domain nucleation and growth. Voltage pulses are applied to a series connection of the ferroelectric capacitor and a resistor to directly measure the ferroelectric negative capacitance during switching. A time-dependent Ginzburg-Landau approach is used to investigate the underlying domain dynamics. The transient negative capacitance is shown to originate from reverse domain nucleation and unrestricted domain growth. However, with the onset of domain coalescence, the capacitance becomes positive again. The persistence of the negative capacitance state is therefore limited by the speed of domain wall motion. By changing the applied electric field, capacitor area or external resistance, this domain wall velocity can be varied predictably over several orders of magnitude. Additionally, detailed insights into the intrinsic material properties of the ferroelectric are obtainable through these measurements. A new method for reliable extraction of the average negative capacitance of the ferroelectric is presented. Furthermore, a simple analytical model is developed, which accurately describes the negative capacitance transient time as a function of the material properties and the experimental boundary conditions. [ABSTRACT FROM AUTHOR]

Doping of HfO2 thin films is a powerful means for stabilizing ferroelectricity in this material. Yet, the amount of doping is critical to achieving the best ferroelectric response. This article investigates the impact of the Si doping content on the key performance metrics of ferroelectric field-effect transistors (FeFETs). We report on its considerable effect on the memory window, switching uniformity and steepness, number of intermediate conductance states, and data retention. Based on the experimental results, we introduce simple figures of merit for a rapid assessment of the FeFET performance. Finally, we identify an important tradeoff between the abovementioned properties as a function of the dopant content and discuss possibilities for specialized device design for nonvolatile memory applications and potential neuromorphic devices based on FeFETs. [ABSTRACT FROM AUTHOR]

Ferroelectric hafnium oxide (HfO2) has been extensively studied for over a decade, especially as a CMOS-compatible material in emerging memory applications. Most recently, it has gained a lot of attention for being applied in the field of beyond von-Neumann computing. The specific nature of different nonvolatile storage elements, particularly ferroelectric capacitors (FeCaps), tunnel junctions (FTJs), and field-effect transistors (FeFETs), dictates the boundary conditions of their adoption in "beyond von-Neumann" circuits and applications. While, for example, the scaling limits restrict the minimum feature size of FeCaps and FTJs, FeFETs have been proven to be highly scalable. On the other hand, HfO2-based FeFETs exhibit a lower cycling endurance than FeCaps, which has strong implications on their integration into circuits establishing beyond von-Neumann computing. This perspective points out some of the challenges that HfO2-based FeFETs encounter in reality and how they might be overcome by a suitable tuning of the device. Likewise, it is demonstrated how a suitable choice of application can partially or completely mitigate the imposed challenges. [ABSTRACT FROM AUTHOR]

Accumulative switching (AS) in ferroelectric field-effect transistors (FeFETs), which takes place under a train of subcritical voltage pulses, represents not only a concern in terms of disturbs for classical nonvolatile memory but also a source of novel opportunities for neuromorphic applications. Here, we report a detailed experimental investigation of AS for large-area and scaled devices and compare it with the conventional one-shot switching. We show that both of them obey the same exponential dependence between the switching time and voltage, irrespective of the pulse polarity, the investigated interval duration between the pulses, and the device size. This unified physical picture sheds new light on the FeFET operation and holds promise to considerably simplify the modeling of both memory and neuromorphic functionalities in these devices. [ABSTRACT FROM AUTHOR]

Long data retention is a critical requirement for many of the potential applications of HfO2-based ferroelectric field-effect transistors (FeFETs). However, methods for its rapid assessment are still missing. In this article, we report a detailed investigation of the retention and switching properties of FeFETs fabricated in the 28-nm high-k metal gate technology. We identify a clear correlation between the two properties and propose a method for fast prediction of the device retention behavior, which can be easily adopted to judge different fabrication processes. Finally, we extend the validity of the method to a direct assessment of disturb-free operating conditions, which may be particularly valuable for FeFET array operation. [ABSTRACT FROM AUTHOR]

Hafnium oxide (HfO2)-based ferroelectric field-effect transistor (FeFET) is an attractive device for nonvolatile memory. However, when compared to the well-established flash devices, the memory window (MW) of FeFETs reported so far is rather limited, which might be an obstacle to practical applications. In this article, we report on FeFETs fabricated in the 28-nm high- ${k}$ metal gate (HKMG) bulk technology with 90 and 80 nm for the channel length and width, respectively, which show a large MW of nearly 3 V. This is achieved by adopting 20-nm-thick HfO2 films in the gate stack instead of the usually employed 10-nm-thick films. We show that such a thickness increase leads to only a moderate increase of the switching voltages, and to a significantly improved resilience of the memory characteristics upon the parasitic charge trapping. The devices display a good retention at high temperatures and endure more than $10^{{5}}$ bipolar cycles, thus supporting this technology for a future generation of FeFET memories. [ABSTRACT FROM AUTHOR]

Abstract The discovery of ferroelectricity in hafnium oxide has revived the interest in ferroelectric memories as a viable option for low power non-volatile memories. However, due to the high coercive field of ferroelectric hafnium oxide, instabilities in the field cycling process are commonly observed and explained by the defect movement, defect generation and field induced phase transitions. In this work, the optical and transport experiments are combined with ab-initio simulations and transport modeling to validate that the defects which act as charge traps in ferroelectric active layers are oxygen vacancies. A new oxygen vacancy generation leads to a fast growth of leakage currents and a consequent degradation of the ferroelectric response in Hf 0.5 Zr 0.5 O 2 films. Two possible pathways of the Hf 0.5 Zr 0.5 O 2 ferroelectric property degradation are discussed. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Ferroelectric Tunnel Junctions (FTJs) are promising electron devices which can be operated as memristors able to realize artificial synapses for neuromorphic computing. In this work, after a thorough validation of the in-house-developed experimental setup, novel methodologies are devised and employed to investigate the large- and small-signal responses of FTJs, whose discrepancies have proven difficult to interpret in previous literature. Our findings convey a significant insight into the contribution of the irreversible polarization switching to the bias-dependent differential capacitance of the ferroelectric–dielectric stack. • Versatile experimental setup is validated for both large and small signal analyses. • Triangular AC small signal helps distinguish the irreversible polarization switching. • AC sinusoid over a triangular waveform can probe both large and small signal behaviour. • Large and small signal responses can be split by proper filtering the overall current. • Triangular frequency can link large and small signal switching capacitance peaks. [ABSTRACT FROM AUTHOR]

To further reduce the power dissipation in nanoscale transistors, the fundamental limit posed by the Boltzmann distribution of electrons has to be overcome. Stabilization of negative capacitance in a ferroelectric gate insulator can be used to achieve this by boosting the transistor gate voltage. Up to now, negative capacitance is only directly observed in polymer and perovskite ferroelectrics, which are incompatible with semiconductor manufacturing. Recently discovered HfO2-based ferroelectrics, on the other hand, are ideally suited for this application because of their high scalability and semiconductor process compatibility. Here, for the first time, a direct measurement of negative capacitance in polycrystalline HfO2-based thin films is reported. Decreasing voltage with increasing charge transients are observed in 18 and 27 nm thin Gd:HfO2 capacitors in series with an external resistor. Furthermore, a multigrain Landau-Khalatnikov model is developed to successfully simulate this transient behavior in polycrystalline ferroelectrics with nucleation limited switching dynamics. Structural requirements for negative capacitance in such materials are discussed. These results demonstrate that negative capacitance effects are not limited to epitaxial ferroelectrics, thus significantly extending the range of potential applications. [ABSTRACT FROM AUTHOR]

Novel hafnium oxide (HfO2)-based ferroelectrics reveal full scalability and complementary metal oxide semiconductor integratability compared to perovskite-based ferroelectrics that are currently used in nonvolatile ferroelectric random access memories (FeRAMs). Within the lifetime of the device, two main regimes of wake-up and fatigue can be identified. Up to now, the mechanisms behind these two device stages have not been revealed. Thus, the main scope of this study is an identification of the root cause for the increase of the remnant polarization during the wake-up phase and subsequent polarization degradation with further cycling. Combining the comprehensive ferroelectric switching current experiments, Preisach density analysis, and transmission electron microscopy (TEM) study with compact and Technology Computer Aided Design (TCAD) modeling, it has been found out that during the wake-up of the device no new defects are generated but the existing defects redistribute within the device. Furthermore, vacancy diffusion has been identified as the main cause for the phase transformation and consequent increase of the remnant polarization. Utilizing trap density spectroscopy for examining defect evolution with cycling of the device together with modeling of the degradation results in an understanding of the main mechanisms behind the evolution of the ferroelectric response. [ABSTRACT FROM AUTHOR]

An important source of degradation in thin dielectric material layers is the generation and migration of oxygen vacancies. We investigated the formation of Frenkel pairs (FPs) in HfO2 as the first structural step for the creation of new defects as well as the migration of preexisting and newly built oxygen vacancies by nudged elastic band (NEB) calculations and stress induced leakage current (SILC) experiments. The analysis indicates, that for neutral systems no stable intimate FPs are built, whereas for the charge states q = ± 2 FPs are formed at threefold and at fourfold coordinated oxygen lattice sites. Their generation and annihilation rate are in equilibrium according to the Boltzmann statistics. Distant FPs (stable defects) are unlikely to build due to high formation energies and therefore cannot be accounted for the measured gate leakage current increase of nMOSFETs under constant voltage stress. The negatively charged oxygen vacancies were found to be very immobile in contrast to positively charged V0's with a low migration barrier that coincides well with the experimentally obtained activation energy. We show that rather the activation of preexisting defects and migration towards the interface than the defect generation are the cause for the gate oxide degradation. [ABSTRACT FROM AUTHOR]

The recently discovered ferroelectric behavior of HfO2-based dielectrics yields the potential to overcome the main challenges of the ferroelectric field-effect transistors (FeFETs)—CMOS compatibility as well as scalability to the state-of-the-art technology nodes of logic transistors. In this paper, we study the impact of scaling on the memory performance of FeFET devices employing Si:HfO2 ferroelectric films. The operation capability was proven down to a gate length of 28 nm. Program/erase characteristics, endurance behavior, and retention properties were analyzed for FeFETs with gate lengths scaled down to 32 nm. The detected difference in the performance between the long and short channel devices could be for the most part attributed to transistor short channel effects. In addition, the effect of temperature on the device properties of Si:HfO2-based FeFETs was investigated in detail. The program/erase speed was ascertained to be independent of temperature. On the other hand, increase in temperature resulted in reduced initial memory window accompanied by its slightly accelerated decay with time. [ABSTRACT FROM AUTHOR]

Theoretical and experimental methods are applied to investigate the degradation of SiO2/O2 gate-stacks in state-of-the-art MOSFETs. A combination of density functional theory and nonequilibrium Green's function formalism has been applied to the atomic scale calculation of the leakage current through SiO2/O2 dielectrics. Samples with different dielectric stacks have been taken into account to study the thickness dependence of SiO2 and HfO2 on the leakage current. The calculated results show a good agreement with the leakage current and constant voltage stress measurements. The current influenced by oxygen vacancies, particularly in the High- $k$ dielectric close to the SiO2/O2 interface has been analyzed. Comparison between the measurement and simulation results show that oxygen vacancy defects in the HfO2 are a likely cause for progressive stress-induced leakage current in MOSFETs with ultrathin High- $k$ gate-stack. [ABSTRACT FROM PUBLISHER]

Highlights: [•] Doped HfO2 exhibits ferroelectricity. [•] Ferrolectric HfO2 was integrated in 500 nm and 100 nm FETs. [•] Ferroelectricity induces a VT shift enabling non-volatile data storage. [•] Endurance is 20k cycles and retention can be extrapolated to 10 years. [ABSTRACT FROM AUTHOR]

Time-dependent dielectric breakdown and stress-induced leakage current (SILC) behavior of a 28-nm hafnium-based high-k/metal gate n-type MOS transistors are investigated under dc and ac conditions. Compared with the constant voltage stress results under dc conditions, a decrease in device lifetime is observed for low-frequency ac stress. Degradation enhancement and SILC behavior are attributed to defect generation in the SiO2 interface layer. For high-frequency ac stress, a lifetime improvement is observed, because of reduced trap generation. [ABSTRACT FROM PUBLISHER]

BiFeO3 thin films were grown on Pt/c-sapphire substrates by pulsed laser deposition with different growth rates. With increasing growth rate the leakage current is decreased and the conduction mechanism changes from bulk-limited Poole-Frenkel emission to interface-limited Schottky emission. In the present letter, we show that only the growth rate of the BiFeO3 films close to the metal contacts has to be increased in order to reduce the leakage current and to observe saturated polarization-electric field hysteresis loops. [ABSTRACT FROM AUTHOR]

The increasing demand for efficient storage of electrical energy is one of the main challenges in the transformation toward a carbon neutral society. While electrostatic capacitors can achieve much higher power densities compared to other storage technologies like batteries, their energy densities are comparatively low. Here, it is proposed and demonstrated that negative capacitance, which is present in ferroelectric materials, can be used to improve the energy storage of capacitors beyond fundamental limits. While negative capacitance was previously mainly considered for low power electronics, it is shown that ferroelectric/dielectric capacitors using negative capacitance are promising for energy storage applications. Compared to earlier results using (anti)ferroelectric materials for electrostatic energy storage, much higher efficiencies of more than 95% even for ultrahigh energy densities beyond 100 J cm−3 are demonstrated using nonepitaxial thin films suitable for integration on 3D substrates. Stable operation up to 150 °C and 108 charging/discharging cycles is further demonstrated. [ABSTRACT FROM AUTHOR]

BiFeO3 thin films have been deposited on Pt/sapphire and Pt/Ti/SiO2/Si substrates with pulsed laser deposition using the same growth conditions. Au was sputtered as the top electrode. The microscopic structure of the thin film varies by changing the underlying substrate. Thin films on Pt/sapphire are not resistively switchable due to the formation of Schottky contacts at both the top and the bottom interfaces. However, thin films on Pt/Ti/SiO2/Si exhibit an obvious resistive switching behavior under forward bias. The conduction mechanisms in BiFeO3 thin films on Pt/sapphire and Pt/Ti/SiO2/Si substrates are discussed to understand the different resistive switching behaviors. [ABSTRACT FROM AUTHOR]