High-density Ferroelectric Memories Research Articles

Nonvolatile Memory Device Based on the Ferroelectric Metal/Ferroelectric Semiconductor Junction.

The ferroelectric tunnel junction (FTJ) is a competitive candidate for post-Moore nonvolatile memories due to its low power consumption and nonvolatility, with its performance being strongly dependent on the conditions for contact between the ferroelectric material and the metal electrode. The development of two-dimensional materials in recent years has offered new opportunities such as functional metal layers, which is challenging for traditional FTJ systems. Here, we introduce the newly discovered ferroelectric metal WTe2 as the electrode to construct WTe2/α-In2Se3/Au ferroelectric semiconductor junctions. The interplay between the ferroelectricity in the van der Waals electrode and tunnel junction leads to the emergence of novel device characteristics, including concomitant multiresistance levels, a low switching voltage (<2 V), and a high on/off ratio (>105). Using ferroelectric metal as the electrode for the ferroelectric tunnel/semiconductor junction offers an alternative approach to low-power and high-density ferroelectric memory devices.

Epitaxial Orientation-Controlled High Crystallinity and Ferroelectric Properties in Hf0.5Zr0.5O2 Films.

Hafnium-based binary oxides are essential for fabricating nanoscale high-density ferroelectric memory devices. However, effective strategies to control and improve their thin-film single crystallinity and metastable ferroelectricity remain elusive, hindering potential applications. Here, using NdGaO3 (NGO) substrates with four crystalline orientations, we report a systematic study of the structural characterizations and ferroelectric properties of epitaxial Hf0.5Zr0.5O2 (HZO) films, demonstrating orientation-controlled high crystallinity and enhanced ferroelectric properties. HZO films grown on NGO(001) and NGO(110) substrates exhibit relatively low crystallinity and a significant presence of the monoclinic phase. In contrast, HZO films grown on NGO(100) and NGO(010) possess high single crystallinity and a dominant ferroelectric phase. These differences are attributed to the surface symmetry of the NGO substrate, which favors the formation of 4- or 2-fold domain configurations. Moreover, the optimized HZO films exhibit a large polarization (2Pr) of ∼50 μC/cm2, enhanced fatigue behavior up to 1011 cycles, improved retention of 2Pr ∼ 40 μC/cm2 after 10 years, and characteristic polarization switching speeds in the submicrosecond range. Our results highlight the importance of modulating the single crystallinity and ferroelectric phase fraction of HfO2-based films to enhance ferroelectric properties, further revealing the potential of epitaxial symmetry engineering.

Giant tunneling resistance and robust switching behavior in ferroelectric tunnel junctions of WS2/Ga2O3 heterostructures: The influence of metal–semiconductor contacts

The ferroelectric tunneling junctions (FTJs) are widely recognized as one of the non-volatile memories with significant potential. Ferroelectricity usually fades away as materials are thinned down below a critical value, and this problem is particularly acute in the case of shrinking device sizes, thus attracting attention to two-dimensional ferroelectric materials (2DFEMs). In this work, we designed 2D ferroelectric Ga2O3-based FTJs with out-of-plane polarization, and the influence of metal–semiconductor contact in the electrode region on the system is considered. Here, using density functional theory combined with the non-equilibrium Green's function approach to quantum transport calculations, we demonstrate robust ferroelectric polarization-controlled switching behavior between metallic and semiconducting states in Ga2O3/WS2 ferroelectric heterostructures. The potential barrier of the metal–semiconductor contact in the electrode region is lower than that of the intrinsic material, thereby resulting in an increased probability of electron tunneling. Our results reveal the crucial role of 2DFEMs in the construction of FTJs and highlight the significant impact of electrode contact types on performance. This provides a promising approach for developing high-density ferroelectric memories based on 2D ferroelectric semiconductor heterostructures.

Polarization fatigue mechanism of laminated hafnium zirconium oxide ferroelectric thin films

Laminated HfO2-based ferroelectric thin films (FE-HfO2) have offered unexpected opportunities to implement the high-density ferroelectric memory and on chip piezoelectric or pyroelectric devices. But the polarization fatigue performance is poor and the underlying mechanism is unknown. In this work, we comprehensively investigate the polarization fatigue behaviors of Hf0.5Zr0.5O2(HZO)/Al2O3/HZO laminated thin films. Several possible mechanisms for the fatigue performances are discussed in detail. On basis of the analysis of micro-structures and domain switching dynamics before and after fatigue, we confirm that domain wall pinning phenomenon exists in the fatigued laminated HZO, without field-induced phase transition as well as thickness and chemical properties changes of interfacial layers. Moreover, we discover by a pulse measurement method that domain wall pinning induced by the increase of Al2O3/HZO interface trap charge is responsible for the fatigue performances of the laminated films. This work will provide useful guidance for designing high-reliability laminated FE-HfO2, and contribute to understand the physical mechanisms of FE-HfO2.

Adjustable onset voltages of embedded LiNbO3 domain-wall selectors for large-scale memory integration

A ferroelectric domain-wall memory has dual functionalities, where the volatile interfacial domain nearby the electrode can function as an embedded selector in contrast to the nonvolatile domain within an inner cell for information storage. However, most of crossbar memories require independent adjustments of the onset voltage of the selector and the coercive voltage of the inner domains at the same node fabrication technology. Here, we fabricated a LiNbO3 mesa-like domain-wall device to touch two top-to-top triangular-like Cu side electrodes. The readout wall current is more than 40 μA in write time as short as 20 ns. With the reduction of the apex angle of the Cu electrode from 180° to 30°, the onset voltage increases continuously from 0.68 to 3.05 V irrespective of the coercive voltage along with the improvement of polarization retention. The underlying physics is discussed on thickening of an effective interfacial layer projected along the applied electric-field direction. This finding enables the crossbar connection of high-density ferroelectric domain-wall memory.

(Invited) Fabrication Technique of Ferroelectric HfxZr1−XO2 Thin Films Using ALD-ZrO2 Nucleation Layers

Ferroelectric HfxZr1−xO2 (HZO) thin films have attracted considerable attention towards high-density and low-power ferroelectric memory devices of ferroelectric random access memory (FeRAM) and ferroelectric field-effect transistors (FeFETs) [1]. To obtain ferroelectric orthorhombic (O) phase of HZO films, several techniques have been investigated, such as the TiN stressor layer technique [2] and controlled annealing conditions [3]. On the other hand, we pay attention to the ZrO2 nucleation layer technique because ZrO2 films deposited by atomic layer deposition (ALD) method formed a nanocrystalline structure and consisted dominantly of O, tetragonal (T), and cubic (C) phases, whose lattice constants are considerably close to that of ferroelectric O phase for HfO2 [4]. In this paper, we report the crystallinity and ferroelectricity of HZO films fabricated using the ZrO2 nucleation layer technique for metal-ferroelectric-metal (MFM) and metal-ferroelectric-semiconductor (MFS) capacitors.First, four types of MFM capacitors of TiN/HZO/TiN (MFM w/o), TiN/HZO/bottom-ZrO2/TiN (MFM B-ZrO2), TiN/top-ZrO2/HZO/TiN (MFM T-ZrO2), and TiN/top-ZrO2/HZO/bottom-ZrO2/TiN (MFM D-ZrO2) were systematically investigated for the FeRAM applications [5]. The thicknesses of the ALD-grown HZO and ZrO2 films were 10 and 2 nm, respectively. The post-deposition annealing was performed at 600°C. The remanent polarization (2P r = P r + − P r −) of the MFM capacitors increased in the following order: MFM w/o (12 µC/cm2) < MFM B-ZrO2 (15 µC/cm2) < MFM T-ZrO2 (23 µC/cm2) < MFM D-ZrO2 (29 µC/cm2), while all capacitors showed almost the same coercive electric field (E c) value of around 1.2 MV/cm. Therefore, maximum 2P rvalue of the MFM capacitor was obtained using the top- and bottom-ZrO2 nucleation layers. For the MFM capacitors with ZrO2 nucleation layers, epitaxial-like grain growth of the HZO film occurred on the nanocrystalline top- and bottom-ZrO2 layers. Moreover, the relative ratio of O/T/C phases in the ferroelectric film, which was estimated using the peak area ratio of O(111)/T(101)/C(111)/{M(−111)+ O(111)/T(101)/C(111)+M(111)}, increased in the following order: MFM w/o < MFM B-ZrO2 < MFM T-ZrO2 < MFM D-ZrO2. Therefore, the top- and bottom-ZrO2 layers play an important role as nucleation layers for the crystallization of the HZO film, resulting in a larger relative ratio of O/T/C phases in the ferroelectric film compared to w/o. Moreover, these results indicate that the top-ZrO2 layer was more effective to form the ferroelectric phase compared to the bottom-ZrO2 layer.Next, two types of MFS capacitors of TiN/HZO/Si (MFS w/o) and TiN/top-ZrO2/HZO/Si (MFS T-ZrO2) were fabricated by low-temperature processes at 300°C of ALD and post-metallization annealing (PMA) for the FeFET applications [6]. The thicknesses of both HZO and ZrO2 films were 10 nm. Both MFS w/o and MFS T-ZrO2 were found to form an extremely thin SiO2 interfacial layer with a thickness of one or two monolayers between the HZO film and Si substrate due to a low temperature fabrication process of 300°C. The HZO film of the MFS T-ZrO2 formed O/T/C phases even with a low thermal budget of 300°C, while that of the MFS w/o had an amorphous structure. This result is attributed to the beneficial role of the nanocrystalline top-ZrO2 nucleation layer for the crystallization of an amorphous HZO film during the PMA process. Therefore, a higher 2P r value of 15 µC/cm2 was obtained for the MFS T-ZrO2 than that (2.2 µC/cm2) for the MFS w/o. Moreover, the endurance properties of the MFS T-ZrO2 were free from the wake-up effect and exhibited less degradation of switching polarization (P sw). This could be attributed to the insertion of a ZrO2 film at the TiN/HZO interface, which promoted the preferential formation of the ferroelectric orthorhombic phase and prevent the formation of oxygen vacancies at the ZrO2/HZO interface. Thus, we concluded that superior ferroelectricity with wake-up free properties and higher fatigue resistance of HZO-based MFS capacitors were achieved by using the ZrO2 nucleation layer technique.These results suggest that the fabrication technique of ferroelectric HZO films using nanocrystalline ALD-ZrO2 nucleation layers is one of the promising processes for the next-generation non-volatile memory device applications.This work was supported by JSPS KAKENHI (Nos. JP21J01667, JP20H02189, and JP18J22998) and MEXT Leading Initiative for Excellent Young Researchers (No. JPMXS0320220213).[1] T. Mikolajick et al., J. Appl. Phys. 129, 100901 (2021).[2] S. J. Kim, et al., Appl. Phys. Lett. 111, 242901 (2017).[3] S. Migita et al., Jpn. J. Appl. Phys. 58, SBBA07 (2019).[4] T. Onaya et. al., Appl. Phys. Express. 10, 081501 (2017).[5] T. Onaya et al., APL Mater. 7, 061107 (2019).[6] T. Onaya et al., APL Mater. 10, 051110 (2022).

Ultra-High-Density Ferroelectric Array Formed by Sliding Ferroelectric Moiré Superlattices.

2D van der Waals (vdW) materials offer infinite possibilities for constructing unique ferroelectrics through simple layer stacking and rotation. In this work, we stack nonferroelectric GeS2 and ferroelectric CuInP2S6 to form heterostructures by combining sliding ferroelectric polarization with displacement ferroelectric polarization to achieve multiple polarization states. First-principles calculations reveal that the polarization reversal of the CuInP2S6 component in the GeS2/CuInP2S6/GeS2 heterostructure can simultaneously drive the switching of sliding ferroelectric polarization, displaying a robust coupling of the two polarizations and leading to the overall polarization switching. Based on this, ferroelectric arrays with a density of 6.55 × 1012 cm-2 (equivalent to a storage density of 0.7 TB cm-2) were constructed in a moiré superlattice, and the polarization strength of array elements was 11.77 pC/m, higher than that of all reported 2D vdW out-of-plane ferroelectrics. High density, large polarization, and electrically switchable array elements in ferroelectric arrays provide unprecedented opportunities to design 2D high-density nonvolatile ferroelectric memories.

Wake-Up Free Ultrathin Ferroelectric Hf0.5Zr0.5O2 Films.

The development of the new generation of non-volatile high-density ferroelectric memory requires the utilization of ultrathin ferroelectric films. The most promising candidates are polycrystalline-doped HfO2 films because of their perfect compatibility with silicon technology and excellent ferroelectric properties. However, the remanent polarization of HfO2 films is known to degrade when their thickness is reduced to a few nanometers. One of the reasons for this phenomenon is the wake-up effect, which is more pronounced in the thinner the film. For the ultrathin HfO2 films, it can be so long-lasting that degradation occurs even before the wake-up procedure is completed. In this work, an approach to suppress the wake-up in ultrathin Hf0.5Zr0.5O2 films is elucidated. By engineering internal built-in fields in an as-prepared structure, a stable ferroelectricity without a wake-up effect is induced in 4.5 nm thick Hf0.5Zr0.5O2 film. By analysis of the functional characteristics of ferroelectric structures with a different pattern of internal built-in fields and their comparison with the results of in situ piezoresponse force microscopy and synchrotron X-ray micro-diffraction, the important role of built-in fields in ferroelectricity of ultrathin Hf0.5Zr0.5O2 films as well as the origin of stable ferroelectric properties is revealed.

Enhancing polarization retention in BiFeO3 domain wall nanodevices through controlled space charge redistribution

Ferroelectric thin films with binary switchable polarization offer promising applications in domain wall (DW) memory. However, achieving reliable polarization retention in size-reduced devices remains challenging. In this study, we present a planar ferroelectric BiFeO3 DW nanodevice demonstrating stable polarization retention through electrical cycling. By precisely controlling the device size, including the width and gap of planar Pt electrodes, we observed a back-switching process in switched polarization within 20 min, as characterized by ex-situ piezoresponse force microscopy (PFM). To overcome this issue, long-time alternating voltage pulse cycling effectively suppressed the back-switching behavior, leading to symmetrized polarization coercive voltages of the device. PFM characterization further confirmed the persistence of the artificially created DW between the electrode pair. This improved polarization retention was attributed to the migration of mobile charged defects, such as oxygen vacancies, at the domain boundary. The presence of these defects generated a local built-in field, which screened the net polarization induced by the charged DW. Upon erasing the DW, the local built-in field gradually dissipated during opposite voltage poling, resulting in restored poor polarization retention and asymmetric coercive voltage in the nanodevices. This work presents a promising approach to utilize defect redistribution for stabilizing the switched polarization in the development of high-density ferroelectric DW memory.

Size-dependent polarization retention in ferroelectric BiFeO3 domain wall memories

Reliable polarization retention in a single crystal-like ferroelectric thin film is crucial for the effective implementation of ferroelectric domain wall (DW) memories. This ensures that the saved information can be non-destructively read out by identifying the resistive state of the memory with a voltage signal below the coercive voltage (Vc) of the ferroelectric film. However, stabilizing a small volume of switched polarization for ultra-high-density information storage applications is still challenging due to its back-switching behavior at zero electric field. In this study, we investigated the polarization retention of ferroelectric BiFeO3 (BFO) thin film devices with size-variable Pt electrodes. With the scaling down of the device size, the switched polarizations became more susceptible to depolarization field and ferroelastic stress arising at the artificially created domain boundary. We found that BFO devices featuring a switched polarization area larger than 7 × 106 nm2 demonstrated both long-term polarization retention and a current rectification ratio of 4.5:1 induced by DW manipulation. As the device size decreases, the retention time of the switched polarization reduces gradually, which results in the disappearance of the device current due to the DW being eliminated by the back-switching of the polarization. A plot was created to illustrate the relationship between the current retention time and the switched polarization area on the surface of the BFO thin films. Furthermore, the unsustainable polarization retention facilitates a higher current rectification ratio of 48:1, which is suitable for non-destructive information readout with a voltage pulse larger than Vc in high-density ferroelectric DW memories.

Ferroelectric Transistors for Memory and Neuromorphic Device Applications.

Ferroelectric materials have been intensively investigated for high-performance nonvolatile memory devices in the past decades, owing to their nonvolatile polarization characteristics. Ferroelectric memory devices are expected to exhibit lower power consumption and higher speed than conventional memory devices. However, non-complementary metal-oxide-semiconductor (CMOS) compatibility and degradation due to fatigue of traditional perovskite-based ferroelectric materials have hindered the development of high-density and high-performance ferroelectric memories in the past. The recently developed hafnia-based ferroelectric materials have attracted immense attention in the development of advanced semiconductor devices. Because hafnia is typically used in CMOS processes, it can be directly incorporated into current semiconductor technologies. Additionally, hafnia-based ferroelectrics show high scalability and large coercive fields that are advantageous for high-density memory devices. This review summarizes the recent developments in ferroelectric devices, especially ferroelectric transistors, for next-generation memory and neuromorphic applications. First, the types of ferroelectric memories and their operation mechanisms are reviewed. Then, issues limiting the realization of high-performance ferroelectric transistors and possible solutions are discussed. The experimental demonstration of ferroelectric transistor arrays, including 3D ferroelectric NAND and its operation characteristics, are also reviewed. Finally, challenges and strategies toward the development of next-generation memory and neuromorphic applications based on ferroelectric transistors are outlined.

In-plane ferroelectric tunnel junctions based on 2D α-In2Se3/semiconductor heterostructures

Ferroelectric tunnel junctions (FTJs) have great potential for application in high-density non-volatile memories. Recently, α-In2Se3 was found to exhibit robust in-plane and out-of-plane ferroelectric polarizations at a monolayer thickness, which is ideal to serve as a ferroelectric component in miniaturized electronic devices. In this work, we design two-dimensional van der Waals heterostructures composed of an α-In2Se3 ferroelectric and a hexagonal IV–VI semiconductor and propose an in-plane FTJ based on these heterostructures. Our first-principles calculations show that the electronic band structure of the designed heterostructures can be switched between insulating and metallic states by ferroelectric polarization. We demonstrate that the in-plane FTJ exhibits two distinct transport regimes, tunneling and metallic, for OFF and ON states, respectively, leading to a giant tunneling electroresistance effect with the OFF/ON resistance ratio exceeding 1 × 104. Our results provide a promising approach for the high-density ferroelectric memory based on the 2D ferroelectric/semiconductor heterostructures.

The tunneling electroresistance effect in a van der Waals ferroelectric tunnel junction based on a graphene/In2Se3/MoS2/graphene heterostructure.

In recent years, α-In2Se3 has attracted great attention in miniaturizing nonvolatile random memory devices because of its room temperature ferroelectricity and atomic thickness. In this work, we construct two-dimensional (2D) van der Waals (vdW) heterostructures α-In2Se3/MoS2 with different ferroelectric polarization and design a 2D graphene (Gr)/In2Se3/MoS2/Gr ferroelectric tunnel junction (FTJ) with the symmetric electrodes. Our calculations show that the band alignment of the heterostructures can be changed from type-I to type-II accompanied by the reversal of the ferroelectric polarization of In2Se3. Furthermore, the ferroelectricity persists in Gr/In2Se3/MoS2/Gr vdW FTJs, and the presence of dielectric layer MoS2 in the FTJs enables the effective modulation of the tunneling barrier by altering the ferroelectric polarization of α-In2Se3, which results in two distinct conducting states denoted as "ON" and "OFF" with a large tunneling electroresistance (TER) ratio exceeding 105%. These findings suggest the importance of ferroelectric vdW heterostructures in the design of FTJs and propose a promising route for applying the 2D ferroelectric/semiconductor heterostructures with out-of-plane polarization in high-density ferroelectric memory devices.

First-principles investigation of ferroelectric and piezoelectric properties in one-dimensional transition metal oxytetrahalides

We perform first-principles density functional theory calculations to systematically investigate electronic structures of a quasi-one-dimensional $M\mathrm{O}{X}_{4}$ ($M=\mathrm{Cr},\mathrm{Mo},\mathrm{W}$, and $X=\mathrm{F},\mathrm{Cl},\mathrm{Br}$) materials consisting of weakly coupled one-dimensional chains. The compositional dependence on the ferroelectric and piezoelectric properties such as switching polarization, energy barriers, Born effective charges, and piezoelectric coefficients are investigated. We find that the majority of the $M\mathrm{O}{X}_{4}$ systems exhibit sizable polarization and large piezoelectric coefficients comparable to conventional ferroelectrics. More importantly, we find that the energy cost of forming a ${180}^{\ensuremath{\circ}}$ domain walls is negligibly small due to the weak interchain coupling, having an atomically thin domain wall with almost no reduction in the local polarization. The robust ferroelectric switching, atomically thin ferroelectric domain wall, and large piezoelectric coefficients make the $M\mathrm{O}{X}_{4}$ family of materials potential candidates for applications for efficient electronic devices such as high-density ferroelectric memories and high-resolution pressure sensors.

Structural, magnetic and ferroelectric properties of VOBr&lt;sub&gt;2 &lt;/sub&gt;monolayer: A first-principles study

On the basis of first-principles calculations, the structure, magnetism and ferroelectricity of VOBr&lt;sub&gt;2&lt;/sub&gt; monolayer are studied systematically in the present work. The calculation results indicate that a spontaneous ferroelectric distortion takes place at low temperature, causing the structure of VOBr&lt;sub&gt;2&lt;/sub&gt; to transform from a centrosymmetric paraelectric phase to a ferroelectric one. In contrast with its sister compound VOI&lt;sub&gt;2&lt;/sub&gt;, the dimerization of V is unstable in VOBr&lt;sub&gt;2&lt;/sub&gt; and may quench the local magnetic moment on V ions. Additionally, the easy magnetization axis of VOBr&lt;sub&gt;2&lt;/sub&gt; monolayer is in-plane along the &lt;i&gt;a&lt;/i&gt;-axis, and the magnetic coupling between adjacent local moments is antiferromagnetic both along the &lt;i&gt;a&lt;/i&gt;-axis and along the &lt;i&gt;b&lt;/i&gt;-axis. Moreover, the ferroelectric displacement of V ions occurs in the &lt;i&gt;a&lt;/i&gt;-axis, along the V—O—V chains direction, resulting in a polarization of about 40 μC/cm&lt;sup&gt;2&lt;/sup&gt;. Comparing with the ferro-to-paraelectric reversal pathway, the energy barrier can be effectively reduced for ferroelectric switching on partial or individual V—O—V chains. It is reasonable to believe that the dipole moment flipping on specific chain can be achieved through a moderate external field, thereby providing new direction for designing the low-energy-consumption and high-density ferroelectric memory device.

Excellent ferroelectric Hf0.5Zr0.5O2 thin films with ultra-thin Al2O3 serving as capping layer

Hf0.5Zr0.5O2 (HZO) has become one of the most popular HfO2 based ferroelectric thin films due to its huge potential to integrate low-cost high-density nonvolatile ferroelectric memory. Most researchers sandwiched the HZO between metals, such as TiN, and then adopted post-deposition high temperature anneal to improve the ferroelectricity and reliability of the film. In this work, the effect of a thin dielectric Al2O3 layer with different thicknesses to replace a metallic capping layer on the ferroelectric properties of a HZO (10 nm) thin film is evaluated, and we also compared the effects of TiN and W bottom electrodes on the properties of a capacitor. The results showed that the TiN/Al2O3 (1 nm)/HZO/W capacitor performed the best with a maximum 2Pr as high as 31.4 μC/cm2 at an electric field of ±3 MV/cm and very low leakage currents. In addition, the fatigue studies demonstrated the capacitor's excellent endurance properties with continuous cycling up to 1010 cycles. The use of an ultra-thin Al2O3 layer with excellent capping effects would significantly simplify the integration process of HfO2-based ferroelectric memory.

Controlled manipulation of conductive ferroelectric domain walls and nanoscale domains in BiFeO3 thin films

Recently, there is a surge of research interest in configurable ferroelectric conductive domain walls which have been considered as possible fundamental building blocks for future electronic devices. In this work, by using piezoresponse force microscopy and conductive atomic force microscopy, we demonstrated the controlled manipulation of various conductive domain walls in epitaxial BiFeO3 thin films, e.g. neutral domain walls (NDW) and charged domain walls (CDWs). More interestingly, a specific type of nanoscale domains was also identified, which are surrounded by highly conductive circular CWDs. Similar nanoscale domains can also be controlled created and erasured by applying local field via conductive probe, which allow nondestructive current readout of different domain states with a large on/off resistance ratio up to 102. The results indicate the potential to design and develop high-density non-volatile ferroelectric memories by utilizing these programable conductive nanoscale domain walls.

Study of SiO2 Interfacial Layer Growth during Fabrication Process of Ferroelectric HfxZr1−XO2-Based Metal-Ferroelectric Semiconductor

Recently, ferroelectric HfxZr1−xO2 (HZO) thin films have attracted much attention towards high-density and low-power ferroelectric memories such as ferroelectric field-effect transistors (FeFETs) for artificial intelligence (AI). [1] An annealing process at ≥ 300 °C is typically required to form the ferroelectric orthorhombic (O) phase of HZO films. [2, 3] It remains a big issue of an abnormal SiO2 interfacial layer (SiO2-IL) growth at the HZO/Si interface during metal-ferroelectric-semiconductor (MFS) fabrication process. It has been reported that a SiO2-IL growth led to degradation of remanent polarization (2P r) and reliability of MFS capacitors. [4] However, it is unclear how the process conditions of MFS structures, such as an atomic layer deposition (ALD) of HZO films and annealing process, affect the SiO2-IL formation in detail. In this work, we investigated a SiO2-IL growth as a function of the process temperature of ALD and annealing processes and a SiO2-IL growth of TiN/HZO/Si structures.P+-Si substrate was first cleaned using a buffered hydrofluoric acid to remove native oxide, and then an HZO film was deposited by ALD using H2O gas and (Hf/Zr)[N(C2H5)CH3]4 (Hf:Zr = 1:1) cocktail precursor. The ALD growth temperature was varied from 200 to 300 °C. A 100-nm-thick TiN film was then fabricated as gate electrode by DC sputtering. Finally, the post-metallization annealing (PMA) was carried out at 300–600 °C in an N2 atmosphere. A 3-nm-thick HZO film was used to evaluate the SiO2-IL growth using XPS analysis. On the other hand, TiN-gated MFS capacitor with a 10-nm-thick HZO film was prepared to examine electrical properties.First, the formation of the SiO2-IL was found to be negligible small at HZO (3 nm)/Si interface regardless of the ALD growth temperatures because no significant peak originating from Si-O bonds was observed in XPS spectra of Si 2p. Therefore, we employed 300 °C as an ALD temperature of HZO films for the fabrication of MFS capacitors. Next, the SiO2-IL growth as a function of PMA temperature was evaluated. The SiO2-IL growth increased as PMA temperature increased from 400 to 600 °C. Therefore, the annealing process at a lower temperature suppressed a SiO2-IL growth for the fabrication of MFS structures.Finally, polarization–electric field (P– E) measurements were conducted to evaluate the ferroelectricity of TiN/HZO (10 nm)/p+-Si MFS capacitors after the PMA process at 300–600 °C. The MFS capacitors after the PMA process at 500 and 600 °C showed breakdown during the P– E measurement due to a large leakage current. The MFS capacitor after the PMA process at 400 °C showed significantly higher 2P r value of 26 µC/cm2 compared to that (7.6 µC/cm2) after the PMA process at 300 °C. This is because the HZO films of the MFS capacitor after the PMA process at 400 °C were crystallized with the ferroelectric O phase while the HZO films had an amorphous-like structure after the PMA process at 300 °C, evaluated by X-ray diffraction (XRD). These experimental results indicated that the PMA temperature significantly affects the growth of the SiO2-IL at the HZO/Si interface while almost no difference was observed between the ALD temperature conditions.This work was partially supported by JSPS KAKENHI (JP18J22998 and JP20H02189).[1] J. Y. Kim et al., APL Mater. 9, 021102 (2021).[2] T. Onaya et al., Microelectron. Eng. 215, 111013 (2019).[3] S. J. Kim, et al., S. J. Kim et al., Appl. Phys. Lett. 111, 242901 (2017).[4] J. Mohan et al., Appl. Phys. Lett. 118, 102903 (2021).

Nonvolatile operation of vertical ferroelectric gate-all-around nanowire transistors

Ferroelectric gate-all-around (GAA) transistors with a nanowire (NW) channel standing vertically on the substrate would be a potential breakthrough to overcome limitations in the high-integration of ferroelectric memories. In the present study, we fabricated vertical ferroelectric GAA NW transistors (VFGAANWTs) with ZnO NWs (average diameters: 53–193 nm) as the channel, (Hf,Zr)O2 film (average thicknesses: 9.3–58 nm) as the gate ferroelectric, and Ti as the gate electrode. The channel length was 100–300 nm. The VFGAANWTs showed n-channel operation with on/off ratios of ∼10, and their on/off states were retained for at least 2 min by the ferroelectric polarization of (Hf,Zr)O2 after the gate bias was removed, demonstrating the nonvolatile electric field effect in the VFGAANWTs. The results confirm the strong potential of the VFGAANWTs in high-density ferroelectric memory applications.

Nanoscale bubble domains with polar topologies in bulk ferroelectrics

Multitudinous topological configurations spawn oases of many physical properties and phenomena in condensed-matter physics. Nano-sized ferroelectric bubble domains with various polar topologies (e.g., vortices, skyrmions) achieved in ferroelectric films present great potential for valuable physical properties. However, experimentally manipulating bubble domains has remained elusive especially in the bulk form. Here, in any bulk material, we achieve self-confined bubble domains with multiple polar topologies in bulk Bi0.5Na0.5TiO3 ferroelectrics, especially skyrmions, as validated by direct Z-contrast imaging. This phenomenon is driven by the interplay of bulk, elastic and electrostatic energies of coexisting modulated phases with strong and weak spontaneous polarizations. We demonstrate reversable and tip-voltage magnitude/time-dependent donut-like domain morphology evolution towards continuously and reversibly modulated high-density nonvolatile ferroelectric memories.