Negative Capacitance Effect Research Articles

Thickness‐Dependent Asymmetric Potential Landscape and Polarization Relaxation in Ferroelectric HfxZr1−xO2 Thin Films through Interfacial Bound Charges

AbstractThe negative capacitance (NC) effect in ferroelectric materials provides a possible solution to break the Boltzmann tyranny and realize steep‐slope field‐effect transistors (FETs) with sub‐60 mV dec−1 subthreshold slope (SS). HfO2‐based ferroelectrics (FE) have attracted great attention as dielectric candidates for NCFETs due to their excellent scalability and compatibility with complementary metal–oxide–semiconductor technology. However, understanding of the ferroelectric properties of HfO2‐based FEs, especially at reduced thickness, is still at an early stage. The quasistatic polarization relaxation behavior of ferroelectric HfxZr1−xO2 (HZO) thin films below ≈50 nm is reported. Universal asymmetries in coercive voltage and remanent polarization in HZO films deposited on various substrates are observed. The asymmetry is strongly thickness dependent and is related to interface bound charges. These findings suggest that the symmetric double‐well free energy landscape in ideal FEs is not a priori applicable to thin films, which has important implications for HZO‐based devices. It is found, through numerical simulations, that the asymmetric potential landscape can reduce SS in NCFETs without suffering from hysteresis. These results provide new insights toward understanding the mechanisms of FE devices and highlight the need for further efforts to investigate interface‐related phenomena in HZO.

Modeling of Negative Capacitance in Ferroelectric Thin Films.

The negative capacitance (NC) effect in ferroelectric thin films has attracted a great deal of attention from the material and semiconductor device communities because it could be a possible solution to the impending problems related to field-effect transistor power consumption and dynamic random-access memory charge loss. A short discussion on the fundamental premise of the NC effect is presented. A phase-field model based on the time-dependent Ginzburg-Landau (TDGL) formalism in conjunction with the Chensky-Tarasenko (C-T) formalism for multidomain configuration is then developed to reveal the subtle correlation between the domain wall motion and NC effect for different thicknesses of ferroelectric and dielectric films. When a ferroelectric film becomes thin enough, a stripe domain structure can be achieved through competition between the electrostatic energy and domain wall energy. This stripe domain structure is quite resilient to transition to a homogeneous polarization state, making it very useful for (quasi-)static NC operation. Finally, the physical implications of the numerical results are explored with analytical modeling. It is identified that the domain wall motion in the stripe domain structure remains dominated by the external field, even when the entire film is in the (quasi-)static NC state.

Compact Model for Negative Capacitance Enhanced Spintronics Devices

Although spin transfer torque (STT)-based magnetic tunneling junction (MTJ) owns advantages of nonvolatility, nonlimited endurance, and fast write/read, its demand for high current density significantly casts a shadow over its future prospects. Recently, a novel three-terminal MTJ cell that combines voltage-controlled magnetic anisotropy (VCMA) effect and negative capacitance (NC) effect is proposed. Drawing support from the NC amplified VCMA effect and the three-step operation scenario, this novel MTJ cell can dramatically lower the energy consumption to fJ as well as keep high operation speed within nanoseconds. The feasibility of the proposed NC enhanced VCMA spintronics device for memory and logic application is proved by extensive physical simulation in our previous work. However, a SPICE compatible compact model of the proposed NC enhanced VCMA spintronics device is still demanded for circuit and system level evaluation. In this paper, we provide an accurate and fast compact model of NC enhanced VCMA device for both memory and logic applications.

Formation, negative capacitance and negative conductance effects in Selenium stacked layers sandwiched with Ag nanosheets

In this work, we report the nature of formation in the presence and absence of Ag nanosheets being inserted between of two stacked layers of Se thin films which are grown onto Au substrates. The Se/Se and Se/Ag/Se films which are prepared by the thermal evaporation technique under vacuum pressure of 10−5 mbar are studied by means of x-ray diffraction, conductance and capacitance spectroscopy techniques in the frequency domain of 0.01–1.80 GHz. Metal inducted crystallization processes from amorphous to hexagonal phases are achieved by using the Au substrate. The presence of Ag nanosheets of thicknesses of 50 nm between two 500 nm thick stacked layers of Se strongly affects the structural parameters through increasing the lattice constants, the microstrain and the defect density and decreasing the crystallite size. While the two stacked layers of Se sandwiched between Au and In metals displayed negative conductance effect associated with resonance in the capacitance and maximum microwave cutoff frequency () of 0.68 GHz near 1.31 GHz, the insertion of Ag nanosheets forced the two stacked layers to exhibit higher positive conductance values and increased the values to 17.4 GHz. Ag nanosheets also caused negative capacitance (NC) effect in all the studied frequency domain. NC effect is associated with resonance-anti-resonance phenomena in the region of 1.33–1.37 GHz. The features of the selenium stacked layers make them attractive for use in microwave circuits as cavities, noise reducers, parasitic capacitance cancellers and bandpass filters.

Negative Capacitance Transistor with Two‐Dimensional Channel Material (Molybdenum disulfide, MoS2)

Negative capacitance field effect transistor with two‐dimensional channel material (2D‐NCFET) has received lots of attention as a steep switching device for low power application. In this work, a P(VDF0.75‐TrFE0.25) ferroelectric capacitor is connected in series to the gate electrode of baseline FET with two‐dimensional channel material, i.e., molybdenum disulfide (MoS2). With the help of negative capacitance effect in ferroelectric material (herein, notice that the NC effect is physically originated from the polarization switching in ferroelectric material), it is experimentally demonstrated that the 2D‐NCFET enables to achieve the sub‐60 mV/decade subthreshold swing (SS = 32 mV/decade at 300 K). Note that the SS of baseline FET used for the 2D‐NCFET is 110 mV/decade at 300 K.

Ferroelectric Characteristics of Hf0.5Zr0.5O2-Based Mfis and Mfmis Capacitors for Steep-Slope Ferroelectric Field Effect Transistor Applications

Research of hafnium oxide-based ferroelectric materials has attracted much attention in the community with superior advantages of their compatibility with silicon. Also, negative capacitance (NC) effect from ferroelectric films attracts a great deal of attention because it could be one of the solutions for the high power-consumption issue of field effect transistors (FET). However, since the non-linear Q-V response of semiconductor in the FET structure in conjunction with the domain formation in the ferroelectric layer resulted in complicated device behaviors, it is necessary to understand the physics of the ferroelectric characteristics related to the interactions among the layers such as the depolarization field and charge trapping/injection for sub-60mV/dec subthreshold swing (SS) operation. In this study, the electrical properties of a Metal-Ferroelectric-Insulator-Semiconductor (MFIS) capacitor with a 7-10nm Hf0.5Zr0.5O2 (HZO) ferroelectric layer and an ultrathin interfacial oxide layer (SiO2) were examined in order to understand the underlying physics of the ferroelectric characteristics (Figure 1 (a)). These properties were compared with those of a Metal-Ferroelectric-Metal-Insulator-Semiconductor (MFMIS) capacitor to analyze the interactions among each layer in the two different structures by pulse measurement. In addition, the ferroelectric characteristics of HZO on semiconductor related to the charge injection through the interfacial oxide layer with different thicknesses and the non-linear Q-V response of semiconductor have been investigated (Figure 1 (b)). Lastly, the effects of different PMA temperatures and HZO thicknesses were also examined and the experimental data were verified by the charge injection-controlled simulation model which was used to simulate the transfer characteristics based on different experimental conditions. (Figure 2). Figure 1

Structural and optoelectronic properties of CdS/Y/CdS thin films

In the current study, the structural, optical, photoelectrical and electrical properties of CdS/Y/CdS thin films are investigated. The current design include the evaporation of a layer of 70 nm thick yttrium between two layers of CdS. Each CdS layer is of thickness of 500 nm. It is observed that the yttrium slab increased the microstrain, defect density, stacking faults and decreased the grain size and redshifts the indirect allowed transitions energy band gap of CdS. In addition an enhancement by ~5 times in the light absorbability is detected at 1.74 eV. The enhanced absorbance results in increasing the photocurrent by ~ 21 times and changed the recombination mechanism from a trap assisted recombination to supralinear recombination mechanisms. Moreover, the ac signal analysis in the frequency domain of 10–1800 MHz has shown that the yttrium forces the CdS to exhibit negative capacitance effect and make it behave as band stop filter with notch frequency of 1520 MHz. The quality of the CdS/Y/CdS films as microwave cavities are screened by the evaluation of the return loss which revealed good features of the nanostructured films as microwave receivers.

High-temperature dielectric relaxations in (Ce, Y) codoped BaZrO3 ceramics

(Ce, Y) codoped BaZrO3 ceramics, BaZr0.1Ce0.7Y0.2O3-δ and BaZr0.4Ce0.4Y0.2O3-δ ceramics were synthesized via solution combustion based on the glycine nitrate process. The dielectric properties of the samples have been investigated in the temperature range from 300 to 1000 K and frequency range from 102 Hz to 106 Hz. It is observed that the samples exhibit colossal dielectric behavior in the temperature below ∼600 K. The colossal dielectric behavior is composed of two sets of relaxations with the low- and high-temperature relaxations being caused by the dipole pairs of -Y and -Y, respectively. In the temperature higher than ∼600 K, the samples show two dielectric anomalies with extremely large dielectric permittivity over 106. It is argued that the low-temperature dielectric anomaly is the typical pseudo-relaxor behavior caused by ion-hopping-motion. While the high-temperature dielectric anomaly results from the negative capacitance effect.

Analysis of Negative-Capacitance Germanium FinFET With the Presence of Fixed Trap Charges

In this paper, the effect of negative-capacitance (NC) on germanium FinFET (GeFinFET) with the presence of fixed trap charges ( ${N}_{\text {ftc}}$ ) has been studied. The analysis of various parameters of NCGeFinFET has shown that the positive trap charges improve the device characteristics whereas the negative trap charges (NTC) degrade the performance of devices. Therefore, we have optimized properties of ferroelectric (FE) to suppress the degradation caused by NTC. Furthermore, the impact of ${N}_{\text {ftc}}$ has been studied on the output characteristics of NCGeFinFET inverter, and it has been shown that the degradation due to NTC reduces by optimizing the FE properties. It has also been demonstrated that NCGeFinFET exhibits excellently improved characteristics as compared to GeFinFET.

Negative Capacitance Effect in the Low-Frequency Impedance of Barium Strontium Titanate Semiconductor Ceramics

Polycrystalline (Ba0.85Sr0.15)0.999Ce0.001TiO3 samples prepared by a standard ceramic processing technique for positive temperature coefficient of resistance materials have been characterized by impedance spectroscopy. Major attention has been paid to the inductive character of the impedance in the low-frequency range of the measuring signal. In the literature, such an impedance is interpreted in terms of a negative capacitance effect. Based on analysis of the existing notions of the structure of grain boundaries in ceramics, we have proposed an equivalent circuit that models frequency dependences of the immitance in good agreement with experimental data.

Ferroelectric negative capacitance

The capacitor is a key element of electronic devices and is characterized by positive capacitance. However, a negative capacitance (NC) behaviour may occur in certain cases and implies a local voltage drop opposed to the overall applied bias. Therefore, a local NC response results in voltage enhancement across the rest of the circuit. Within a suitably designed heterostructure, ferroelectrics display such an NC effect, and various ferroelectric-based microelectronic and nanoelectronic devices have been developed, showing improved performance attributed to NC. However, the exact physical nature of the NC response and direct experimental evidence remain elusive or controversial thus far. In this Review, we discuss the physical mechanisms responsible for ferroelectric NC, tackling static and transient NC responses. We examine ferroelectric responses to voltage and charge, as well as ferroelectric switching, and discuss proof-of-concept experiments and possibilities for device implementation. Finally, we highlight different approaches for the optimization of the intrinsic NC response to maximize voltage amplification. Ferroelectrics-based materials can display a negative capacitance (NC) effect, providing an opportunity to implement NC in electronic circuits to improve their performance. In this Review, the authors discuss static and transient NC responses in ferroelectrics and highlight proof-of-concept experiments and possibilities for device implementation.

Structural, Optical, Dielectric and Electrical Properties of Al-Doped ZnSe Thin Films

In this work, the heavy aluminum doping effects on the compositional, structural, optical, dielectric and electrical properties of ZnSe thin films are investigated. It is observed that the Zn/Se compositional ratio increases with increasing Al content. The major cubic phase of ZnSe becomes more pronounced compared to the hexagonal phase. In addition, the presence of Al in the structure of ZnSe causes lattice constant contraction, decreased the grain size and increased both of the strain and defect density. Optically, the Al doping increased the light absorbability and widens both of the energy band gap and energy interbands which are present in the band gap of ZnSe films. Moreover, the Al doping into ZnSe lowers the high frequency dielectric constant and enhances the optical conductivity. On the other hand, the capacitance spectra which are studied in the frequency domain of 0.01–1.80 GHz displayed negative capacitance effect associated with resonance–antiresonance phenomena upon doping of ZnSe with Al. Such enhancements in the physical properties of ZnSe that are achieved via Al doping make the zinc selenide thin films more appropriate for electronic and optoelectronic technological applications.

Transient Negative Capacitance Effect in Atomic‐Layer‐Deposited Al2O3/Hf0.3Zr0.7O2 Bilayer Thin Film

AbstractThe negative capacitance (NC) effect is now attracting a great deal of attention in work towards low‐power operation of field effect transistors and extremely large capacitance density in dynamic random access memory. However, to date, observation of the NC effect in dielectric/ferroelectric bilayer capacitors has been limited to the use of epitaxial ferroelectric thin films based on perovskite crystal structures, such as Pb(Zr,Ti)O3 and BaTiO3, which is not compatible with current complementary metal oxide semiconductor technology. This work, therefore, reports on the transient NC effect in amorphous‐Al2O3/polycrystalline‐Hf0.3Zr0.7O2 bilayer systems prepared using atomic layer deposition. The thin film processing conditions are carefully tuned to achieve the appropriate ferroelectric performances that are a prerequisite for the examination of the transient NC effect. Capacitance enhancement is observed in a wide voltage range in 5–10 nm thick Al2O3/Hf0.3Zr0.7O2 bilayer thin films. It is found that the capacitance of the dielectric layer plays a critical role in the determination of additional charge density induced by the NC effect. In addition, inhibition of the leakage current is important for stabilization of nonhysteretic charge–discharge behavior of the bilayers. The mean‐field approximation combined with classical Landau formalism precisely reproduces the experimental results.

Enhancement of the performance of the Cu2Se band filters via Yb nanosandwiching

AbstractIn this article, we report the experimental and theoretical modeling on the band pass filters that are made of two thin film layers of Cu2Se coated onto aluminum substrates and nanosandwiched with 50 nm ytterbium layers. The nanosandwiching of Yb between two layers of Cu2Se is found to decrease the lattice constant, the defect density, and the strain and increase the grain size in the Cu2Se. Electrically, it is observed that, Al/Cu2Se/Al structure exhibits wave trap characteristics with notch frequency of 1.31 GHz. The Yb‐layers improved the performance of the band pass filters by increasing the amplitude of the reflection coefficients, increasing the return loss values and decreasing the voltage standing wave ratios. The calculated conduction and wave trapping parameters nominate the Yb‐nanosandwiched Cu2Se films for use in communication technology as they exhibit negative capacitance effect and narrow band pass range.

A TCAD device simulator for exotic materials and its application to a negative-capacitance FET

A new device simulator named Impulse TCAD was developed, being built on top of a nonlinear finite volume method solver, which is further based on the Python script language and its associated scientific libraries. The user can fully customize the device properties and/or equations using scripts, which allows ready handling of exotic materials with nonstandard physical models. As a demonstration, a transient analysis of a negative-capacitance field-effect transistor (NC FET) is presented. The ferroelectric material in the NC FET is handled by using the time-dependent Ginzburg–Landau–Devonshire (GLD) equation, while standard device equations are applied for the rest of the device. The simulations show that, starting from the spontaneous polarization state, the ferroelectric regions evolve into the negative-capacitance regime, showing the expected characteristics of a NC FET. They also indicate that the Ginzburg term in the GLD equation, which is related to the correlation radius $$r_{\mathrm{c}}$$ , plays an important role in the appearance of the negative-capacitance effect. When $$r_{\mathrm{c}}$$ is too short compared with the channel length, the ferroelectric region becomes segregated into multiple polarization domains, resulting in loss of the NC FET characteristic.

Voltage and frequency dependence of negative capacitance behavior in a Graphene-TiO2 nanocomposite photoanode based on quantum dot sensitized solar cells

In this letter, we investigated negative capacitance (NC) effect in nanocomposite based quantum dot sensitized solar cells. TiO2 and graphene-TiO2 nanocomposite photoanode based on quantum dot sensitized solar cells (QDSSCs) were fabricated and the capacitance-voltage (C–V), capacitance-frequency (C-f) and conductance-voltage (G/ω-V) characteristics were studied by attention the series resistance (Rs) effect. Admittance spectroscopy of QDSSCs were measured in a variable voltage and frequency ranges of (−2 V) - (+2 V) and 10 kHz-10 MHz, respectively. Capacitance values are exhibited a behavior transition from the positive to negative value after 50 kHz. As the capacitance values decreases, conductance increases in the NC region. Negative capacitance effect of meaning that QDSSC indicates an inductive behavior. Inductive behavior, which means that electrons injects from fluorine-doped tin oxide (FTO) conductive glass electrode to photoanode (graphene-TiO2). Also, NC phenomenon has not form significant effect on Rs.

Investigation of Gate-Stress Engineering in Negative Capacitance FETs Using Ferroelectric Hafnium Aluminum Oxides

In this paper, we reported a ferroelectric HfAlO x negative capacitor transistor with gate-stress (GS) engineering. Compared to control device, the HfAlO x transistor with GS engineering percentage of $\text{I}_{ \mathrm{\scriptscriptstyle ON}}$ enhancement and 27% $\text{V}_{T}$ reduction under the assistance of the negative capacitance (NC) effect. The orthorhombic phase transition played a crucial role in realizing the NC effect. From the results of the material analysis, the theoretical Landau simulation and electrical measurement, we demonstrated that the GS is favorable for inducing orthorhombic phase crystallization of HfAlOx and stabilizing the NC effect, which shows the potential for the application of advanced technology node.

Analysis on Performance of Ferroelectric NC-FETs Based on Real-Space Gibbs-Free Energy With Atomic Channel Structure

Ferroelectric (FE) negative capacitance (NC) MOSFETs is investigated in depth with a full real-space atomic simulator. For the first time, Gibbs-free energy of the whole atomic system is employed to determine the appropriate operation region in polarization-electric-field ( ${P}$ – ${E}$ ) path of the FE layer for FE MOSFETs. With different FE thicknesses, the operation path of the FE layer in MOS structures can be hysteresis or hysteresis-free. For MOSFET structures, the operation path of FE layer will vary with different FE materials. With proper device parameters, the NC effect in FE layer can be obtained with minimum Gibbs-free energy of the MOS and MOSFET structures. The body factor, ${dV}_{G}/{d} \Psi _{S}$ , is obtained with the real-space structure to reflect the influence of the FE layer and optimize the device performance. With the FE layer of the same thickness, the NC-FET with longer channel has a larger body factor which causes the tradeoff between low tunneling current and small body factor. This tradeoff makes it difficult to improve the subthreshold swing by changing the gate length. In addition, the effect of floating metal layer between FE and SiO2 layers in NC-FETs is studied in detail with atomic level real-space analysis for different gate lengths. The metal layer has a stronger influence on the electric characteristics of NC-FETs with longer channel.

Time-resolved simulation of the negative capacitance stage emerging at the ferroelectric/semiconductor hetero-junction

Recently, a number of papers have demonstrated sub-60 mV/decade switching by using the negative capacitance (NC) effect in ferroelectric-gate FETs. However, the physical picture is not yet understood. In this paper, an alternative physical picture for emerging NC is proposed and the development of the NC stage at the ferroelectric/semiconductor hetero-junction is described. Proposed physical picture is based on two factors, 1. “decrease in an additional voltage originated from the depolarization field by surface potential of semiconductor” and 2. “Change in the distribution ratio of gate voltage (VG) to voltage applied to the ferroelectric layer (VF) and surface potential of the semiconductor (ψS) due to the capacitance change of semiconductor.” With considering these two essential phenomena, time-resolved simulations of the NC stage emerging at the ferroelectric/semiconductor hetero-junction were carried out. This NC phenomena expressed by the negative differential of the DF for the VF, i.e. (∂DF/∂VF<0), emerging in the MFS (metal/ferroelectric/semiconductor) capacitor without inserting dielectric layer, are dynamically simulated to discuss the proposed NC process. The simulation results clearly reveal that the NC stage is originated from the existence of additional voltage caused by the depolarization field by surface potential of semiconductor originated from the existence of remanent polarization of ferroelectric layer, and change in the capacitance of the semiconductor during polarization switching. The different physical picture from steady-state NC and transient NC can be clearly shown.

Switching dynamics and modeling of multi-domain Zr-Doped HfO2 ferroelectric thin films

HfO2-based ferroelectrics have attracted attention as promising materials for advanced memory applications owing to their negative capacitance effect, high scalability, and full-CMOS compatibility. Accordingly, the switching dynamics of HfO2 thin films have been actively discussed and simulated using the Landau-Khalatnikov equation (LK model). Although the simulated results agree with experimental results in many studies, there is a slight dissimilarity near the coercive field in the polarization-electric field curve. For accurate and general modeling, a new model that combines the conventional LK model and Euler's equation was proposed in this work. The model was examined under single-domain and multi-domain conditions. The simulated curves using the Landau-Euler method better fit with measured curves than those using the LK model.