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Doped HfO2-based ferroelectric-aided charge-trapping effect in MFIS gate stack of FeFET
Journal of Applied Physics
Abstract
The ferroelectric field effect transistor (FeFET) is a very promising candidate for low-power and non-volatile memory. However, the co-existing effect of ferroelectric polarization and interface charge trapping in the FeFETs is demonstrated and many efforts have been made to eliminate this charge-trapping effect, which is usually treated as a deleterious effect. In contrast, we have found that the charge-trapping effect can play a dominant role in ferroelectric gates. In this work, we have verified that the charge-trapping effect of the ferroelectric/insulator interface could induce a memory window as the main physical mechanism in the TiN/Hf0.5Zr0.5O2/SiO2/p-Si (MFIS) structure, in which the ferroelectric characteristics of HZO thin films was verified through a reverse-grown MFIS structure. We also demonstrated that 2.5 nm SiO2 is optimal for the charge tunneling effect and the device has the largest memory window. Moreover, in order to enlarge the memory window of MFIS capacitors, we utilized the stress-enhanced ferroelectric polarization characteristics of Hf0.5Zr0.5O2 to improve the charge-trapping effect. Such a finding demonstrates that the ferroelectric-aided charge-trapping devices are potential to be used in non-volatile memories.
... This resulted in a shift in the threshold voltage, a reduction in the memory window, and a decrease in the drain current. 8,9 Furthermore, ferroelectric properties can be negatively impacted by alterations in the interfacial characteristics between the electrode material and the ferroelectric material, such as the TiO x N y interlayer formed at the interface between the zirconium-hafnium-based ferroelectric film and the TiN metal electrode. Liao et al. were able to tune the P r for ferroelectric devices within the range of 6.4-25.9 ...
Hf0.5Zr0.5O2 based capacitors with a metal–ferroelectric–metal structure have been fabricated in this study, with or without ammonia plasma nitridation at the metal/oxide interfaces. The purpose of this work is to study the role nitrogen plays in the ferroelectric properties of these materials. It is observed that ferroelectricity is enhanced after the interface treatment with NH3 plasma nitridation. The material analysis results indicate that this ferroelectricity improvement results from the diffusion of nitrogen atoms into the Hf0.5Zr0.5O2 film at the metal/oxide interface. Further, the nitrogen binds to the oxygen vacancies at the interface and forms a nitrogen-doped Hf0.5Zr0.5O2 film, enabling oxygen vacancies movement. After 90 s of NH3 plasma interfacial treatment, the metal–ferroelectric–metal capacitor device achieves a maximum memory window of 3.7 V. The results of the electrical test and physical phase analysis demonstrate that NH3 plasma interfacial treatment of ferroelectric thin films can effectively improve the interfacial effect between ferroelectric materials and the top electrode, which in turn enhances the storage performance of the device.
In this study, we investigated the impact of unstable and stable interface trap charges (
on
switching in metal–ferroelectric–insulator–Si (MFIS) ferroelectric field-effect transistors (FeFETs), which vary with the thickness of the insulator. We also examine how these variations ultimately affect the various performance metrics of MFIS FeFETs. To achieve this, we varied the thickness of the insulator (
in MFIS FeFETs to 1.5, 2.0, and 2.5 nm, thereby controlling the amount of
injected from the channel into the ferroelectric (FE)/insulator interface. As
decreases, the amount of
increases, which amplifies the electric field across the FE layer. As a result,
switching enhances, and consequently, the MW characteristics of MFIS FeFETs improve. Furthermore, to analyze this in detail, we employed
–
measurements on MFIS FeFETs to simultaneously extract unstable and stable
as well as
and MW. The results show that as
increases to 1.5, 2.0, and 2.5 nm,
during program/erase (PGM/ERS) operations decreases to 100%, 61%, and 54%, respectively. This leads to a corresponding decrease in
to 100%, 59%, and 52%. Additionally, after sufficient delay following the PGM/ERS operations, we observe that the proportion stable
compared to
is 91%, regardless to
and the remaining 9% of
contributes to the MW property. Consequently, as
increases to 1.5, 2.0, and 2.5 nm, the net charge decreases to 100%, 61%, and 54%, resulting in MW values of 1.85, 1.05, and 0.85 V, respectively. Finally, we analyzed the impact of
generation as a function of
on the variability and endurance characteristics of MFIS FeFETs.
The magneto-electric coupling (MEC) effect has been considered an effective method for the voltages controlled magnetic anisotropy in traditional ferroelectric/ferromagnetic structures. Unlike traditional perovskite ferroelectrics, the ferroelectric hafnium-based oxides hold great potential for use in the complementary metal oxide semiconductors (CMOS) circuit with the advantages of CMOS compatibility and easy scaled-down and lower leakage current. In this article, the MEC effects in the PtCoRu/Hf0.5Zr0.5O2 (HZO) heterostructure have been investigated using the polar magneto-optical Kerr effect microscopy and anomalous Hall effect. The major modification of perpendicular magnetic anisotropy of the PtCoRu thin film was controlled obviously within the ±4 V polarized voltages of the Hf0.5Zr0.5O2 (HZO) film, accompanying with the coercivity field and remnant magnetization significantly decreased. The Hall voltages of PtCoRu in Hall bar devices were also controlled effectively under ±3 V polarized voltages. Such a finding proposes a more optimized method for the magnetic logic gates and memories based on voltage-controlled magnetic anisotropy in future.
In this paper, we propose a method to improve the performance of TiN/Hf0.5Zr0.5O2 (HZO)/TiN Nano-capacitors used in memory devices. Instead of direct fabrication of the TiN/HZO/TiN device, our method involves an intermediate step in which W metal is used as a capping material to induce a large in-plane tensile strain during rapid thermal annealing, resulting in a total suppression of the monoclinic phase and the appearance of the ferroelectric phase. Consequently, after removing the W capping electrode through an etching process and the post-deposition of a TiN top electrode at room temperature, a high remnant polarization of approximately 40 μC cm-2 and a 65% increase of coercive field were obtained. Moreover, the leakage current was reduced by an order of magnitude compared to the normal TiN/HZO/TiN capacitor; this result is attributed to the presence (absence) of the W/HZO (TiN/HZO) top interface during thermal annealing. The formation of a TiO x interfacial layer at elevated temperatures, which pulls oxygen from the HZO layer, resulting in the formation of oxygen vacancies, is the main cause of the high leakage current through the TiN/HZO/TiN stacks. It was confirmed that the re-capped TiN/HZO/TiN capacitor has a comparable endurance to a normal capacitor. Our results offer the re-capping process as a promising approach to fabricating HfO2-based ferroelectric memory devices with various electrode materials.
Ultrathin ferroelectric materials could potentially enable low-power perovskite ferroelectric tetragonality logic and nonvolatile memories1,2. As ferroelectric materials are made thinner, however, the ferroelectricity is usually suppressed. Size effects in ferroelectrics have been thoroughly investigated in perovskite oxides—the archetypal ferroelectric system3. Perovskites, however, have so far proved unsuitable for thickness scaling and integration with modern semiconductor processes4. Here we report ferroelectricity in ultrathin doped hafnium oxide (HfO2), a fluorite-structure oxide grown by atomic layer deposition on silicon. We demonstrate the persistence of inversion symmetry breaking and spontaneous, switchable polarization down to a thickness of one nanometre. Our results indicate not only the absence of a ferroelectric critical thickness but also enhanced polar distortions as film thickness is reduced, unlike in perovskite ferroelectrics. This approach to enhancing ferroelectricity in ultrathin layers could provide a route towards polarization-driven memories and ferroelectric-based advanced transistors. This work shifts the search for the fundamental limits of ferroelectricity to simpler transition-metal oxide systems—that is, from perovskite-derived complex oxides to fluorite-structure binary oxides—in which ‘reverse’ size effects counterintuitively stabilize polar symmetry in the ultrathin regime. Enhanced switchable ferroelectric polarization is achieved in doped hafnium oxide films grown directly onto silicon using low-temperature atomic layer deposition, even at thicknesses of just one nanometre.
Memory devices with high speed and high density are highly desired to address the ‘memory wall’ issue. Here we demonstrated a highly scalable, three-dimensional stackable ferroelectric diode, with its rectifying polarity modulated by the polarization reversal of Hf0.5Zr0.5O2 films. By visualizing the hafnium/zirconium lattice order and oxygen lattice order with atomic-resolution spherical aberration-corrected STEM, we revealed the correlation between the spontaneous polarization of Hf0.5Zr0.5O2 film and the displacement of oxygen atom, thus unambiguously identified the non-centrosymmetric Pca21 orthorhombic phase in Hf0.5Zr0.5O2 film. We further implemented this ferroelectric diode in an 8 layers 3D array. Operation speed as high as 20 ns and robust endurance of more than 10⁹ were demonstrated. The built-in nonlinearity of more than 100 guarantees its self-selective property that eliminates the need for external selectors to suppress the leakage current in large array. This work opens up new opportunities for future memory hierarchy evolution.
Ferroelectric field effect transistors (FeFETs) based on lead zirconate titanate (PZT) ferroelectric material and amorphous-indium-gallium-zinc oxide (a-IGZO) were developed and characterized. The PZT material was processed by a sol-gel method and then used as ferroelectric gate. The a-IGZO thin films, having the role of channel semiconductor, were deposited by radio-frequency magnetron sputtering, at a temperature of ~50°C. Characteristics of a typical field effect transistor with SiO
2
gate insulator, grown on highly doped silicon, and of the PZT-based FeFET were compared. It was proven that the FeFETs had promising performances in terms of I
on
/I
off
ratio (i.e., 10
6
) and IDS retention behavior.
We demonstrated polarization induced switching in lead zirconium titanate (PZT) back gated multilayer MoS2 ferroelectric field effect transistors (Fe-FETs) for low power non-volatile memory devices. In this work, we investigate the influence of the interface between ferroelectric and 2D (MoS2) materials on the transistor characteristics for non-volatile memory applications. We fabricated Fe-FET devices with multilayer MoS2 as a channel material and polycrystalline and single crystal PZT thin films as the ferroelectric gate materials. The Fe-FET with polycrystalline PZT as a gate shows good memory characteristics; however, the hysteresis in transfer characteristics of the Fe-FET is in the clockwise direction (anti-hysteresis), which indicates that the transistor characteristics are not purely controlled by the ferroelectric charge. On the other hand, the Fe-FET devices with a single crystal PZT as the gate shows stable memory characteristics with a clear anti-clockwise hysteresis, which shows that the transistor characteristics are purely controlled by the ferroelectric charge. To understand the source of the anti-hysteresis and the interface between the ferroelectric and 2D materials, we have performed polarization switching at nanoscale using piezo force microscopy (PFM). The local piezoelectric hysteresis loop measured by PFM reveal that the polarization reversal in polycrystalline MoS2/PZT heterostructures is due to inhomogeneous polarization distribution, oxygen vacancies and high surface roughness of the polycrystalline PZT thin films. Our MoS2 Fe-FET devices with epitaxial PZT as a gate exhibit low switching voltages ≤2 V (much lower than the reported 2D-FET devices (8–20 V)), high ON-OFF ratios ≥10⁴ (comparable to state of the art Fe-FET devices) and reproducible hysteresis behaviour with good sustainability over 200 cycles of switching operations. This study helps with the understanding of the fundamental phenomenon for anti-hysteresis, and the realization of low power and reliable non-volatile memory devices.
We report the integration of a ferroelectric (FE) silicon-doped hafnium oxide material in ferroelectric field-effect transistor (FeFET) devices fabricated with an optimized interfacial layer in a gate-first scheme. The effect of increasing the permittivity (k) value of the interface layer on the performance of the metal-ferroelectric-insulator-semiconductor (MFIS)-FE-HfO₂ FeFET is studied in terms of its switching characteristics, endurance, and retention. In contrast to the previous work, the FE Si:HfO₂-integrated FeFET devices show a low-power operation capability as well as an improved endurance characteristics without jeopardizing high-temperature retention. The utilization of an optimized SiON interface layer for MFIS-HfO₂ FeFET stack is discussed, and the improvements are outlined with reference to a standard low-k SiO₂ interface.
In this letter, effects of top electrodes (TEs) on ferroelectric properties of Hf
0.5
Zr
0.5
O
2
(HZO) thin films are examined systematically. The remnant polarization (Pr) of HZO thin films increases by altering TEs with lower thermal expansions coefficient (α). The largest 2P
r
value of 38.72 μC/cm
2
is observed for W TE with α = 4.5 × 10
-6
/K, while the 2P
r
value is only 22.83 μC/cm
2
for Au TE with α = 14.2 × 10
-6
/K. Meanwhile, coercive field (E
c
) shifts along the electric field axis and the offset is found to be dependent on the difference of workfunctions (WFs) between TE and TiN bottom electrode (BE). E
c
shifts toward negative/positive direction, when the WF of TE is larger/smaller (Pt, Pd, Au/W, Al, Ta) than TiN BE. This letter provides an effective way to modulate HfO
2
-based device performance for different requirements in actual application.
In this letter, ferroelectric memory performance of TiN/Y-doped-HfO2 (HYO)/TiN capacitors is investigated under proton radiation with 3 MeV energy and different fluence (5e13, 1e14, 5e14 and 1e15 ions/cm2). X-ray diffraction (XRD) patterns confirm that the orthorhombic phase Pbc21 of HYO film has no obvious change after proton radiation. Electrical characterization results demonstrate slight variations of the permittivity and ferroelectric hysteresis loop after proton radiation. The remanent polarization (2Pr) of the capacitor decreases with increasing proton fluence. But the decreasing trend of 2Pr is suppressed under high electric fields. Furthermore, the 2Pr degradation with cycling is abated by proton radiation. These results show that the HYO-based ferroelectric memory is highly resistive to proton radiation, which is potentially useful for space applications.
We fabricate, characterize, and establish the critical design criteria of Hf0.5Zr0.5O2 (HZO)-based ferroelectric field effect transistor (FeFET) for nonvolatile memory application. We quantify VTH shift from electron (hole) trapping in the vicinity of ferroelectric (FE)/interlayer (IL) interface, induced by erase (program) pulse, and VTH shift from polarization switching to determine true memory window (MW). The devices exhibit extrapolated retention up to 10 years at 85 °C and endurance up to 5 x 10⁶ cycles initiated by the IL breakdown. Endurance up to 10¹² cycles of partial polarization switching is shown in metal-FE-metal capacitor, in the absence of IL. A comprehensive metal-FE-insulator-semiconductor FeFET model is developed to quantify the electric field distribution in the gate-stack, and an IL design guideline is established to markedly enhance MW, retention characteristics, and cycling endurance.
All-electrical and programmable manipulations of ferromagnetic bits are highly pursued for the aim of high integration and low energy consumption in modern information technology. Methods based on the spin-orbit torque switching in heavy metal/ferromagnet structures have been proposed with magnetic field, and recently are heading toward deterministic switching without external magnetic field. Here we demonstrate that an in-plane effective magnetic field can be induced by an electric field without breaking the symmetry of the structure of the thin film, and realize the deterministic magnetization switching in a hybrid ferromagnetic/ferroelectric structure with Pt/Co/Ni/Co/Pt layers on PMN-PT substrate. The effective magnetic field can be reversed by changing the direction of the applied electric field on the PMN-PT substrate, which fully replaces the controllability function of the external magnetic field. The electric field is found to generate an additional spin-orbit torque on the CoNiCo magnets, which is confirmed by macrospin calculations.
Ferroelectric Si-doped HfO2 thin films are integrated into three different device stacks with a p+ Ge substrate, a p+ Si substrate, and a TaN bottom metal gate. The ferroelectric behavior of the Si-doped HfO2 thin films are strongly dependent on the bottom interfaces. Si-doped HfO2 thin films have favorably improved ferroelectric properties on the p+ Ge substrate due to the lack of a dielectric interfacial layer between HfO2 and Ge. The low voltage operation and cycling stability of Si-doped HfO2 ferroelectric thin films on Ge can lead to the realization of high performance, robust Ge ferroelectric field effect transistors for nonvolatile memory applications.
A ferroelectric field-effect transistor (FeFET), capable of logic and memory functionalities in a single device, is a promising three-terminal memtransistor that enables high-performance in-memory computing for non Von Neumann architectures. Among all HfO2-based ferroelectric materials, HfZrO2(HZO) has attracted the most attention due to the low process temperature of ≤500 °C; however, it has relatively weak polarization. Many prior works claimed that the way to improve HZO-based FeFET characteristics is to enhance HZO ferroelectric properties, while they did not account for the fundamental compromise on dielectric breakdown strength (BDS), transistor ON/OFF current (ION/IOFF) ratio, and memory window (MW) due to the enhanced polarization. In this work, we propose an approach for controlling the ferroelectric orthorhombic phase (O phase) and the corresponding polarization in optimal value by engineering both the surface morphology and stress of HZO layer by a thermal expansion mismatch with a TiN/W stacked capping layer, to improve the BDS, ION/IOFFratio, and MW. Through electrode surface optimization and stress memorization we achieved an 18% HZO ferroelectricity increase with a high BDS value of ≥4.8 MV/cm. Our optimized FeFET shows good electrical characteristics and supports operation in an identical pulse programming (IPP) mode, showing good potentiation and depression nonlinearity (-0.84 and -2.04) with an asymmetry factor of 1.2. A simulation based on the proposed FeFET array demonstrates the high potential of application in an artificial neural network (ANN).
In recent years, several experimental approaches have been adopted to study and understand the mechanism and improve the ferroelectricity of fluorite-type hafnia-based ferroelectric materials. In this regard, significant efforts have been made to elucidate the role of top electrode and bottom electrode (TE and BE) materials in defining the ferroelectricity in such systems, especially in terms of induced mechanical tensile stress by these materials during the process of annealing. However, the effect of the electrode material was not investigated both at TE and BE, and despite numerous efforts, there is still a lack of accurate and systematic understanding. In this report, we have carried out a systematic investigation on the effect of TE and BE materials having different coefficient of thermal expansion (CTE), by changing the electrode material one at a time, both at the top and bottom. The influence of the TE was confirmed using [TE/Hf
0.5
Zr
0.5
O
2
(HZO)/TiN] structure in which the BE was fixed as TiN, and the influence of the BE was confirmed using [TiN/HZO/BE] structure by fixing TiN as the TE. As revealed by polarization versus electric field and residual stress analysis, smaller CTE of the electrode was found to result in higher tensile stress in the HZO films during the annealing process, facilitating the formation of higher ferroelectric o-phase and thereby resulting in greater ferroelectricity. Although the influence of TE and BE on the ferroelectric property of HZO films was found to show similar trends according to the CTE value of the electrodes, the influence of TE on the ferroelectric property of the HZO capacitors is found to be mainly due to the variation in the induced mechanical tensile stress; pulse switching measurement and X-ray photoelectron spectrometer (XPS) analysis suggest that in case of BE, both the induced mechanical tensile stress and the interfacial dead layer were found to play a significant part. As a result, BE was found to have a greater influence on ferroelectricity of the HZO capacitors when compared with that of TE. The highest remnant polarization of 48.2 and
/cm
2
was obtained for W with the lowest of CTE of
in both the configurations. The results obtained in this article are expected to provide a new way out to optimize the interface quality and ferroelectricity in HZO-based capacitors.
This article theoretically investigates the impact of charges at the ferroelectric/interlayer interface on the depolarization field of ferroelectric FET (FeFET) with metal/ferroelectric/interlayer/Si gate structure. The interfacial charges include fixed charges and trapped/detrapped charges. We find that the positive or negative interfacial charges (~10
13
cm
-2
) can align the directions of the depolarization field and corresponding polarization in the ferroelectric. This article may provide insight into the device design of FeFET.
The ferroelectric polarization switching in ferroelectric hafnium zirconium oxide (Hf0.5Zr0.5O2, HZO) in the HZO/Al2O3 ferroelectric/dielectric stack is investigated systematically by capacitance-voltage and polarization-voltage measurements. The thickness of dielectric layer is found to have surprisingly unexpected but determinant impact on the ferroelectric polarization switching of ferroelectric HZO. A suppression of ferroelectricity is observed with thick dielectric layer. The difference between thick and thin dielectric layer is attributed to the leakage-current-assist switching through the dielectric layer and a theoretical explanation is provided. The interfacial polarization coupling effect between the ferroelectric layer and dielectric layer is observed for the first time in HZO material system, showing a capacitance enhancement compared to ferroelectric capacitor and dielectric capacitor in series. This work provides some insights on understanding of some of reported negative capacitance field-effect transistors (NC-FETs) experiments where a ultrathin dielectric is applied as a capacitance matching layer and can play a similar role as in HZO/Al2O3 stack. Finally, we show that ferroelectric field-effect transistors, which are fundamentally different from NCFETs with physical ferroelectric switching, can also provide DC enhancement in both on-state and off-state, suggesting the potential of using ferroelectric-gated field-effect transistors for low-power logic applications.
Hf0.5Zr0.5O2 (HZO) based ferroelectric-field-effect transistor (FeFET) with a ZrO2seed layer was demonstrated. It was found that the ZrO2seed layer could effectively improve ferroelectric properties of the HZO thin film. The remanent polarization and the coercive voltage of the metal-ferroelectric-insulator-semiconductor (MFIS) structure with the ZrO2-seed layer were larger than those without the seed layer. Moreover, the FeFET with the ZrO2seed layer showed wider counter-clockwise hysteresis loops in the transfer characteristics than that without the seed layer, achieving a large memory window of about 2.8 V. These results validate the advantages of the ZrO2 seed layer in promotion of FeFET performance and thus warrant further study.
Epitaxial ferroelectric Hf0.5Zr0.5O2 films have been successfully integrated in a capacitor heterostructure on Si(001). The orthorhombic Hf0.5Zr0.5O2 phase, [111] out-of-plane oriented, is stabilized in the films. The films present high remnant polarization Pr close to 20 μC/cm2, rivaling with equivalent epitaxial films on single crystalline oxide substrates. Retention time is longer than 10 years for writing field of around 5 MV/cm, and the capacitors show endurance up to 10⁹ cycles for writing voltage of around 4 MV/cm. It is found that the formation of the orthorhombic ferroelectric phase depends critically on the bottom electrode, being achieved on La0.67Sr0.33MnO3 but not on LaNiO3. The demonstration of excellent ferroelectric properties in epitaxial films of Hf0.5Zr0.5O2 on Si(001) is relevant towards fabrication of devices that require homogeneity in the nanometer scale, as well as for better understanding of the intrinsic properties of this promising ferroelectric oxide.
High-performance negative capacitance p-type FinFETs (p-FinFETs) with a 3-nm-thick ferroelectric (FE) hafnium zirconium oxides (Hf
0.5
Zr
0.5
O
2
) layer are fabricated based on a conventional high-κ metal gate FinFETs fabrication flow. The devices show improved subthreshold swing values [34.5 mV/dec for 500-nm gate length (L
G
) and 53 mV/dec for 20-nm-L
G
devices] and slight hysteresis voltages (~9 mV for L
G
= 500 nm and ~40 mV for L
G
= 20-nm transistors). With the integrated FE film, a strong driving current enhancement (up to 260%) is also obtained compared with that of conventional FinFETs. The inherent reasons for the improved characteristics contribute to the low-interface state density (D
it
) and the perfect channel electrostatic integrity.
In this letter, the write disturb of Hf
0.5
Zr
0.5
O
2
-based 1T-FeFET nonvolatile AND memory array is experimentally investigated for
/2 and
/3 inhibition bias schemes to determine the worst-case memory sensing condition. Read margin analysis reveals that the increased leakage current in the low-
erased state and the increased read current of the high-
programmed state are the key factors that limit the maximum array size.
Recent demonstration of aggressively scaled HfO
2
-based ferroelectric field effect transistors (FE-HfO
2
-FETs) has illustrated a pathway to fabricate FeFETs that enjoy COMS-compatibility, low power, fast switching speed, scalability, and long retention. One potential issue of this promising technology is its limited endurance, which has been attributed to the degradation of gate stack before the fatigue of polarization in the ferroelectric HfO
2
layer. Some associated work has identified charge trapping and trap generation as key villains, but a clear understanding of two aforementioned underlining mechanisms is still missing. In this letter, we initiated this letter to investigate the roles of charge trapping and trap generation in causing endurance failure of FE-HfO
2
FETs.
Graphene-based field effect transistor (FET) is fabricated by employing ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) as a gate insulator. The co-existed effects of ferroelectric gating and interface charge trapping on the transport properties of graphene are investigated based on the FET structure. The sheet resistance (Rs) of graphene shows a slight decay under a small bias voltage which is much less than the coercive voltage of the ferroelectric PMN-PT, suggesting the non-negligible charge trapping effects. On the other hand, when the bias voltage is increased up to the value larger than the coercive voltage, Rs exhibits three states, including an initial rapid change followed by a slow nearly exponential evolution, and then a saturated state when the bias voltage is retained or after it is released. More importantly, a high-resistance state is finally reached due to the ferroelectric gating, implying that ferroelectric effects are dominated in this process. The underlying physical mechanism is fully investigated to well address the observed evolution of time-dependent Rs. Such a finding provides one an opportunity to understand co-existed effects of ferroelectric gating and charge trapping and tune the transport properties of graphene through the interface effects.
In this work, we report on the electron transport properties of ultrathin (~ 2.5 nm) ferroelectric polycrystalline Hf0.5Zr0.5O2 (HZO) films grown by the Atomic Layer Deposition (ALD) technique directly on the highly doped Si substrate. On the devices with TiN top electrode we demonstrate the reversible electroresistance change R↑/R↓ up to 3 across HZO layer following the polarization reversal. We further correlate the electroresistance switching with the polycrystalline structure of the the HZO layer, the polarization measurements, the current lateral distribution over the HZO layer as well as DFT calculations of the electronic band structure. Such analysis reveals that the polarization switching in ferroelectric HZO grains contribute to the current transport in TiN/HZO/n⁺-(p⁺-)Si devices.
Ir/Ca0.2Sr0.8Bi2Ta2O9 (CSBT)/HfO2/Si ferroelectric-gate field-effect transistors (FeFETs), which were appropriate for low-voltage 3.3 V operations, were developed. The key to the success was the use of N2-dominant gas mixed with a small amount of O2 in a gas flow during the annealing of the FeFETs at 780 °C for CSBT polycrystallization. The Ir gate was newly developed for overcoming the problem of Pt peeling off from the CSBT surface during the novel annealing process. For maximizing the memory windows of the FeFETs, the optimum flow rate of O2 mixed with 1000-sccm-fixed N2 was found to be as low as 0.5 sccm. The novel annealing process suppressed the SiO2 interfacial layer growth to 2.6 nm thickness. The annealing also improved CSBT ferroelectricity. A 10⁹ cycle endurance and a 10⁵ s retention were demonstrated by 3.3 V writing of the FeFETs.
Recent discovery of ferroelectricity in thin hafnium oxide films has led to a resurgence of interest in ferroelectric memory devices. Although both experimental and theoretical studies on this new ferroelectric system have been undertaken, much remains to be unveiled regarding its domain landscape and switching kinetics. Here we demonstrate that the switching of single domains can be directly observed in ultra-scaled ferroelectric field effect transistors. Using models of ferroelectric domain nucleation we explain the time, field and temperature dependence of polarization reversal. A simple stochastic model is proposed as well, relating nucleation processes to the observed statistical switching behavior. Our results suggest novel opportunities for hafnium oxide based ferroelectrics in non volatile memory devices.
Ferroelectric field effect transistors (FeFETs) based on ferroelectric hafnium oxide (HfO2) thin films show high potential for future embedded nonvolatile memory applications. However, HfO2 films besides their recently discovered ferroelectric behavior are also prone to undesired charge trapping effects. Therefore, the scope of this paper is to verify the possibility of the charge trapping during standard operation of the HfO2 -based FeFET memories. The kinetics of the charge trapping and its interplay with the ferroelectric polarization switching are analyzed in detail using the single-pulse ID–VG technique. Furthermore, the impact of the charge trapping on the important memory characteristics such as retention and endurance is investigated.
Due to their immense scalability and manufacturability potential, the HfO2-based ferroelectric films attract significant attention as strong candidates for application in ferroelectric memories and related electronic devices. Here, we report the ferroelectric behavior of ultrathin Hf0.5Zr0.5O2 (HZO) films, with the thickness of just 2.5 nm, which makes them suitable for use in ferroelectric tunnel junctions, thereby further expanding the area of their practical application. Transmission electron microscopy (TEM) and electron diffraction analysis of the films grown on highly doped Si substrates confirms formation of the fully crystalline non-centrosymmetric orthorhombic phase responsible for ferroelectricity in Hf0.5Zr0.5O2. Piezoresponse force microscopy and pulsed switching testing performed on the deposited top TiN electrodes provide further evidence of the ferroelectric behavior of the Hf0.5Zr0.5O2 films. The electronic band line-up at the top TiN/Hf0.5Zr0.5O2 interface and band bending at the adjacent n+-Si bottom layer attributed to the polarization charges in Hf0.5Zr0.5O2 have been determined using in situ XPS analysis. The obtained results represent a significant step toward the experimental implementation of Si-based ferroelectric tunnel junctions.
Reliability characteristics of ferroelectric thin films (10 nm) based on Si-doped HfO2 have been investigated with focus on potential memory applications. Extensive retention, imprint, and endurance data for this new type of ferroelectric material are presented for the first time. The variability of reliability characteristics in terms of capacitor annealing temperatures as well as excitation amplitude effects is analyzed. Stable ferroelectric switching behavior can be observed in a wide temperature range from 80 K up to 470 K. Bake tests at 125 degrees C show almost no retention loss for saturated polarization states up to cumulative testing times of 1000 h. In addition to the same-state retention, opposite-state retention was observed to be equally stable. Traditional imprint behavior of the programmed state occurs after a few hours of baking time, and stable behavior could be verified until the end of the 1000-h retention test. Endurance characteristics of the ferroelectric thin films are shown to depend significantly on the annealing temperature of the capacitors and on the cycling voltage during testing. In thin films which had been annealed at 1000 degrees C, breakdown at 2 MV/cm limits endurance after 10(8) cycles. A lower annealing temperature of 650 degrees C could improve the breakdown-limited endurance to 10(10) cycles.
Metal-ferroelectric-insulator-semiconductor (MFIS) capacitors with a Pb(Zr0.53,Ti0.47)O3 ferroelectric layer and a hafnium oxide insulator layer have been fabricated and characterized. The size of the capacitance-voltage memory windows was investigated. The memory window first increases to a saturated value of 0.7 V with the sweep voltage and then decreases due to charge injection. The oxide trapped charges in the ferroelectric∕insulator layers are studied by a voltage stress method. The flatband voltage (VFB) is measured before and after the voltage stress. The ΔVFB is 0.59 V at a negative stress voltage pulse of −5 V for 30 s. The ΔVFB under positive voltage stress was much less and was 0.06 V at a stress voltage of +5 V for 5 min. The energy-band diagram of the MFIS structure at inversion and accumulation modes are plotted and the VFB shift can be explained by the trapping or detrapping of charges. The current-density versus stress time (J‐t) characteristics were also measured. The result is consistent with the charge trapping model.
The charge trapping is studied in metal-ferroelectric-insulator-semiconductor (MFIS) capacitors with SrBi2Ta2O9 (SBT)∕Al2O3∕SiO2 gate stack by high-frequency and pulsed capacitance-voltage (CV) measurements. The ferroelectric polarization is observed by high-frequency CV. Under fast gate voltage sweep in pulsed CV, the delay of electron trapping detrapping in the buffer layer induces an opposite CV hysteresis direction than that of the ferroelectric polarization. For memory programming, the hole trapping in the gate stack limits the electric field in SBT. Furthermore, the electron trapping during stress induces serious threshold voltage instability as well as erratic memory read out. All these charge trapping problems are important for the practical application and reliability of the memory with MFIS structure.
Ferroelectric properties of Si-doped HfO2 thin films (10 nm) have been investigated. The focus of this letter is to evaluate the potential applicability of these thin films for future 3-D ferroelectric random access memory capacitors. Polarization switching was tested at elevated temperatures up to 185 degrees C and showed no severe degradation. Domain switching dynamics were electrically characterized with pulse-switching tests and were not in accordance with Kolmogorov-Avrami-type switching. Nucleation-limited switching is proposed to be applicable for these new types of ferroelectric thin films. Furthermore, same-state and opposite-state retention tests were performed at 125 degrees C up to 20 h. It was found that samples that had previously been annealed at 800 degrees C showed improved retention of the written state as well as of the opposite state. In addition, fatigue measurements were carried out, and no degradation occurred for 10(6) programming and erase cycles at 3 V.
Incipient ferroelectricity is known to occur in perovskites such as SrTiO3, KTaO3, and CaTiO3. For the first time it is shown that the intensively researched HfO2 thin films (16 nm) also possess ferroelectric properties when aluminium is incorporated into the host lattice. Polarization measurements on Al:HfO2 based metal–insulator–metal capacitors show an antiferroelectric-to-ferroelectric phase transition depending on annealing conditions and aluminium content. Structural investigation of the electrically characterized capacitors by grazing incidence X-ray diffraction is presented in order to gain further insight on the potential origin of ferroelectricity. The non-centrosymmetry of the elementary cell, which is essential for ferroelectricity, is assumed to originate from an orthorhombic phase of space group Pbc21 stabilized for low Al doping in HfO2. The ferroelectric properties of the modified HfO2 thin films yield high potential for various ferroelectric, piezoelectric, and pyroelectric applications. Furthermore, due to the extensive knowledge accumulated by various research groups regarding the HfO2 dielectric, an immediate relevance of ferroelectric hafnium oxide thin films is anticipated by the authors.
We investigated phase transitions in ferroelectric silicon doped hafnium oxide (FE-Si:HfO2) by temperature dependent polarization and x-ray diffraction measurements. If heated under mechanical confinement, the orthorhombic ferroelectric phase reversibly transforms into a phase with antiferroelectric behavior. Without confinement, a transformation into a monoclinic/tetragonal phase mixture is observed during cooling. These results suggest the existence of a common higher symmetry parent phase to the orthorhombic and monoclinic phases, while transformation between these phases appears to be inhibited by an energy barrier.
The transition metal oxides ZrO(2) and HfO(2) as well as their solid solution are widely researched and, like most binary oxides, are expected to exhibit centrosymmetric crystal structure and therewith linear dielectric characteristics. For this reason, those oxides, even though successfully introduced into microelectronics, were never considered to be more than simple dielectrics possessing limited functionality. Here we report the discovery of a field-driven ferroelectric phase transition in pure, sub 10 nm ZrO(2) thin films and a composition- and temperature-dependent transition to a stable ferroelectric phase in the HfO(2)-ZrO(2) mixed oxide. These unusual findings are attributed to a size-driven tetragonal to orthorhombic phase transition that in thin films, similar to the anticipated tetragonal to monoclinic transition, is lowered to room temperature. A structural investigation revealed the orthorhombic phase to be of space group Pbc2(1), whose noncentrosymmetric nature is deemed responsible for the spontaneous polarization in this novel, nanoscale ferroelectrics.
The switching behavior of ferroelectric capacitors experiencing arbitrary time‐dependent electric fields and arbitrary initial conditions is investigated both theoretically and experimentally. A general approach for modeling incomplete dipole switching in ferroelectric capacitors is used to derive equations describing the electrical behavior of a simple characterization circuit with arbitrary initial conditions and arbitrary time‐dependent applied voltages. The equations include four experimentally determined parameters: the remanent and spontaneous polarizations, the coercive field, and the ferroelectric dielectric constant. Once these model parameters are determined from a single high‐frequency sinusoidal hysteresis loop, model predictions are made with no adjustable parameters. The circuit behavior for both sinusoidal and trapezoidal input signals is computed, including asymmetric and nonperiodic signals as well as several different initial conditions. The accuracy of the model predictions is quantitatively verified with experimental data. The approach is also utilized to model the switching behavior of a ferroelectric capacitor containing a sheet of space charge. It is found that hysteresis loop distortions resulting from ionizing radiation resemble those caused by a sheet of space charge. This quantitative modeling capability facilitates the optimization of the design of ferroelectric memory circuits by minimizing the amount of required electrical testing and characterization. It can also be used to facilitate the identification and understanding of degradation mechanisms occurring in ferroelectric thin films.
Operation is demonstrated of a field‐effect transistor made of transparant oxidic thin films, showing an intrinsic memory function due to the usage of a ferroelectric insulator. The device consists of a high mobility Sb‐doped n‐type SnO 2 semiconductor layer, PbZr 0.2 Ti 0.8 O 3 as a ferroelectric insulator, and SrRuO 3 as a gate electrode, each layer prepared by pulsed laser deposition. The hysteresis behavior of the channel conductance is studied. Using gate voltage pulses of 100 μs duration and a pulse height of ±3 V, a change of a factor of two in the remnant conductance is achieved. The dependence of the conductance on the polarity of the gate pulse proves that the memory effect is driven by the ferroelectric polarization. The influence of charge trapping is also observed and discussed. © 1996 American Institute of Physics.
The non-volatile polarization of a ferroelectric is a promising candidate for digital memory applications. Ferroelectric capacitors have been successfully integrated with silicon electronics, where the polarization state is read out by a device based on a field effect transistor configuration. Coupling the ferroelectric polarization directly to the channel of a field effect transistor is a long-standing research topic that has been difficult to realize due to the properties of the ferroelectric and the nature of the interface between the ferroelectric and the conducting channel. Here, we report on the fabrication and characterization of two promising capacitor-less memory architectures.
Ferroelectric field effect devices offer the possibility of nonvolatile active memory elements. Doped rare-earth manganates, which are usually associated with colossal magnetoresistive properties, have been used as the semiconductor channel material of a prototypical epitaxial field effect device. The carrier concentration of the semiconductor channel can be "tuned" by varying the manganate stochiometry. A device with La0.7Ca0.3MnO3 as the semiconductor and PbZr0.2Ti0.8O3 as the ferroelectric gate exhibited a modulation in channel conductance of at least a factor of 3 and a retention loss of 3 percent after 45 minutes without power.