Applications In Nonvolatile Memory Devices Research Articles

Switchable Photovoltaic Effect and Robust Nonlinear Optical Response in a High-Temperature Molecular Ferroelectric [C8N2H22][PbI4

Hybrid organic-inorganic molecular ferroelectrics (HOIMFs) have garnered significant attention for their potential applications in nonvolatile memory and spintronic devices. However, few efforts have been devoted to the photoelectric properties of lead halide molecular ferroelectrics, despite the fact that robust ferroelectricity and flexibility are desirable for thin-film photoelectric devices. Herein, we present a novel lead halide molecular ferroelectric [C8N2H22][PbI4] (1) synthesized hydrothermally. A polar monoclinic structure of 1 was solved by single-crystal X-ray diffraction and second-harmonic generation (SHG) tests. A direct band gap of 2.36 eV was confirmed by UV-vis spectrum and theoretical calculation. Hysteresis measurements demonstrated inherent room-temperature (RT) ferroelectricity in 1 with a spontaneous polarization (Ps) of 3.2 μC/cm2. The 1-based photoelectric device shows a notable photovoltaic (PV) effect with Voc ∼ 0.27 V, Jsc ∼ 38 nA/cm2 under AM 1.5 G illumination, and a rapid response time of ∼1.5 ms. A considerable enhancement in PV performance has been achieved by adjusting the ferroelectric polarization, resulting in a maximum Voc ∼ 0.75 V, Jsc ∼ 2.28 μA/cm2. Notably, 1 exhibits a rather large SHG signal, which is approximately 2.61-fold higher than that of KH2PO4 (KDP) upon a 1064 nm laser radiation. This study offers a bright avenue for lead halide molecular ferroelectrics as promising optoelectronic devices and SHG materials.

(Invited) Role of Oxidant Gas for Atomic Layer Deposition of HfxZr1−XO2 Thin Films on Ferroelectricity of Metal-Ferroelectric-Metal Capacitors

Since the first report of ferroelectricity in HfxZr1−xO2 (HZO) in 2011 [1], a lot of attention has been devoted for future non-volatile memory device applications due to its scalability (< 10 nm), low thermal budget (< 400°C) and matured atomic layer deposition (ALD) technique [2]. The thermal ALD process using H2O or O3 is generally employed for the fabrication of ferroelectric HZO thin films. On the other hand, the effect of the plasma-enhanced ALD process using O2 plasma gas on the crystallinity and ferroelectricity has not been systematically clarified. In this paper, we report the importance of ALD oxidant gases for HZO films using H2O or O2 plasma on the ferroelectricity of metal–ferroelectric–metal (MFM) capacitors.MFM capacitors with the H2O- and O2 plasma-based HZO films were fabricated as follows. A 10-nm-thick HZO films were deposited on a TiN bottom-electrode (BE-TiN) by ALD at 300°C using H2O or O2 plasma as oxidant gases and (Hf/Zr)[N(C2H5)CH3]4 (Hf:Zr = 1:1) cocktail precursor. The Hf/Zr ratios in both types of HZO films were estimated to be 0.4/0.6 using X-ray photoelectron spectroscopy (XPS). Next, a post-deposition annealing (PDA) process was carried out at 400 °C for 1 min under a N2 atmosphere. Finally, a TiN top-electrode was deposited by DC sputtering.First, the crystallinity and remanent polarization (2P r = P r + − P r −) of the H2O- and O2 plasma-based HZO films were investigated. Nanocrystals with ferroelectric orthorhombic (O), tetragonal (T), cubic (C) phases partially existed in the O2 plasma-based film after the ALD process, while an amorphous structure dominantly existed in the H2O-based film, analyzed by the grazing-incidence X-ray diffraction analysis and cross-sectional transmission electron microscopy images [3]. This could be because of a strong oxidation power and high energy ion/electron bombardment of O2 plasma. After the PDA process, the O phase was formed more in the O2 plasma-based HZO film than the H2O-based film. Therefore, the nanocrystal grains in the as-grown O2 plasma-based HZO film could play an important role as nuclei for the crystallization and formation of O phase [3,4]. From the polarization–electric field hysteresis curves, the PDA-treated O2 plasma-based capacitor exhibited 2P r of 20 µC/cm2, which was 1.5 times higher than that (13 µC/cm2) of the H2O-based capacitor. Thus, these results indicate that the crystallization and O phase formation of HZO films were promoted by using O2 plasma gas as an ALD oxidant, resulted in a higher 2P r.Next, endurance properties of these MFM capacitors were evaluated. In the wake-up state, both capacitors showed almost the same increase rate (~8%) of switching polarization (P sw). In the fatigued state, on the other hand, the O2 plasma-based capacitor exhibited less P sw degradation (~33%) after 106 cycles, whereas the P sw of H2O-based capacitor decreased by ~47%. To clarify these different fatigue properties, the TiOxNy interface reaction layer (IRL) formation at the HZO/BE-TiN interface was analyzed using synchrotron hard X-ray photoelectron spectroscopy (HAXPES). For the HAXPES Ti 2p spectra, the Ti-O peak intensity for the O2 plasma-based capacitor was larger than that of the H2O-based capacitor [5]. Therefore, an oxygen-rich TiON-IRL could be formed at the HZO/BE-TiN interface during ALD process due to the stronger oxidizing power of O2 plasma. Additional oxygen vacancies (VO) were reported to formed in HfO2-based films during field cycling due to the interface reaction between the HfO2-based film and TiN electrode. Moreover, one of the origins of the P sw degradation was thought to be domain pinning caused by VO [6]. Therefore, an oxygen-rich TiON-IRL for the MFM capacitor with the O2 plasma-based HZO film could prevent the interface reaction and formation of additional VO in HZO films, led to superior fatigue properties.In summary, the MFM capacitor with the O2 plasma-based HZO film showed superior 2Pr and fatigue resistance compared to those of the H2O-based one. Based on these experimental results, it is important to consider the selection of an ALD oxidant for fabrication of HZO-based MFM capacitors.This work was partly supported by JSPS KAKENHI (Nos. JP21J01667, JP20H02189, and JP18J22998) and MEXT Leading Initiative for Excellent Young Researchers (No. JPMXS0320220213). The HAXPES measurements were performed under approval of the NIMS Synchrotron X-ray Station (2020A4602 and 2020A4651).[1] J. Müller et al., Appl. Phys. Lett. 99, 112901 (2011).[2] J.-H. Kim et al., ACS Appl. Electron. Mater. 5, 4726 (2023).[3] T. Onaya et al., APL Mater. 10, 051110 (2022).[4] T. Onaya et al., Microelectron. Eng. 215, 111013 (2019).[5] T. Onaya et al., Solid State Electron. 210, 108801 (2023).[6] M. Pešić et al., Adv. Funct. Mater. 26, 4601 (2016).

Ferroelectric properties and current mechanisms of BaTiO3/Nb:SrTiO3 thin films

[Formula: see text] thin film oxides have been deposited on [Formula: see text] substrates using a pulsed laser deposition system. Different annealing methods were thereafter used to assess the ferroelectricity of the grown films. The residual polarization value, 2Pr, of 4.205 [Formula: see text]C/cm2 and saturated residual polarization value, 2Pmax, of 15.484 [Formula: see text]C/cm2 were obtained using in-situ annealing. After performing rapid thermal annealing (RTA) annealing, the residual polarization value rose to 4.676 [Formula: see text]C/cm2 and the saturated residual polarization value rose to 18.723 [Formula: see text]C/cm2. Furthermore, a comparison of results showed that the saturation test frequency of the in-situ annealing was 10 times higher than the saturation test frequency of the RTA. This could be explained by the fact that the secondary high-temperature annealing led to a decrease in the conductivity of the doped oxide and, thereby, a decrease in the effective electric field that was applied to both ends of the BTO ([Formula: see text]) film. Thus, a lower frequency was required to ensure a flip of all ferroelectric domains within the BTO film. By testing the fatigue frequency at 100 kHz, [Formula: see text] stable cycles in both annealing methods and the results confirmed the good stability of the device performance. A linear fit analysis of the [Formula: see text] – [Formula: see text] curves showed under in-situ annealing the presence of both a bulk-limited current mechanism that was dominated by the space charge limited current (SCLC) and Pool–Frenkel (PF) mechanisms, and an interface-limited current that was dominated by the Fowler–Nordheim (FN) mechanism in the structure. After RTA annealing, the conducting mechanism is ohmic contact at low electric field and the SCLC mechanism at high field strength. The results show that after RTA annealing, the quality and ferroelectric properties of the films are significantly improved compared to those under in-situ annealing. These results showed that the BTO/NSTO/STO ([Formula: see text]) structure had a standard test frequency as well as reliable stability. The BTO/NSTO/STO structure showed, therefore, promising applications in future nonvolatile information memory devices.

Inherent D-A Architecture in Indoloquinoxalines with an Array of Substituents for Non-Volatile Memory Device Applications.

Donor-acceptor (D-A)-based architecture has been the key to increase storage capability efficiency through the enhanced charge transportation in the fabricated device. We have designed and synthesized a series of functionalized indoloquinoxalines (IQ) for non-volatile organic memory devices. The investigation on UV-visible spectra reveals the absorption maxima of the compounds around 420 nm, attributed to the intramolecular charge transfer between indole and quinoxaline moiety. The irreversible anodic peak in the 1.0 to 1.5 V range indicates the indole moiety's oxidation ability. Besides, the cathodic peak in the range of -0.5 to -1.0 V, contributed to the stability of the reduced quinoxaline unit. All the compounds exhibited uniformly covered thin film in SEM analysis, potentially facilitating the seamless charge carrier migration between adjacent molecules. The methoxyphenyl substituted compound exhibited the binary write-once read-many (WORM) memory behavior with the lowest threshold voltage of -0.81 V. The molecular simulations displayed the efficient intramolecular charge transfer, providing the fabricated device's distinctive conductive states. Except for the tert-butylphenyl compound, which showed volatile dynamic random-access memory (DRAM) behavior, all the other compounds exhibited non-volatile WORM memory behavior, suggesting IQs potential as an intrinsic D-A molecule in organic memory devices on further structural refinement.

Frequency dependence of wake-up and fatigue characteristics in ferroelectric Al0.93B0.07N thin films

B-doped AlN ferroelectric thin films are promising candidates for non-volatile memory device applications, due to their relatively low deposition temperatures, large remanent polarizations (up to 120 μC/cm2), and outstanding polarization retention properties. In this research, the polarization switching fatigue characteristics in epitaxial B-doped AlN thin films were investigated. Films were deposited via reactive RF magnetron sputtering onto W-coated c-plane sapphire substrates. Polarization switching fatigue measurements were conducted with triangular waveforms for frequencies between 50 and 1000 Hz from room temperature to 200°C. On repeated cycling, the films undergo wake-up; the wake-up process was characterized by an activation energy of 0.15 ± 0.05 eV. After a period of normal switching, the film leakage current increased with additional cycling, and finally the films underwent dielectric breakdown. Unpatterned Al0.93B0.07N films with 100 nm thick Pt top electrodes survived ∼104 bipolar cycles, whereas films with 1000 nm W top electrodes survived ∼105 cycles before dielectric breakdown. Modeling was used to design a field plate, which improved the performance to ∼106 fatigue cycles. It was found that dielectric failure during fatigue was not due to surface flashover but was associated with hard breakdown events in the dielectric. Failure for 100 Hz cycling was not associated with device self-heating but was exacerbated by field concentrations; at higher frequencies, the instantaneous temperature rise during the switching becomes prominent. Based on the fatigue process under 100 Hz under various ambient temperatures, the activation energy of the polarization switching fatigue of B-doped AlN thin films was found to be Ea = 0.10 ± 0.03 eV.

Photoinduced growth of the crystalline phase of tellurium on a 1T′-MoTe2 matrix

Due to the growing demand for miniaturization and energy efficiency in modern electronic devices, there is a renewed interest for optoelectronic memories and sensors based on 2D materials. In particular, the molybdenum ditelluride (MoTe2) is one of the most promising materials for applications in nonvolatile phase-change memory devices, as its properties can be controlled by visible-light illumination. Among the several ways to synthesize MoTe2, the molybdenum oxide tellurization through isothermal close space sublimation (CSS) annealing in gas atmosphere is a simple and low-cost effective method for large-scale production of devices based on this layered material. Therefore, the understanding of the physical properties of MoTe2 thin films produced by this technique is crucial for future applications. Surprisingly, our results indicate that there is a photoinduced growth of the crystalline phase of tellurium on the 1T′-MoTe2 matrix even when the power density of the laser is low. From Raman spectroscopy investigations, we were able to show that nanometer-sized tellurium crystallites work as seed sites for the photocrystallization of tellurium. By assuming that the overall crystallization process is described by a kinetic approach that is based on the Kolmogorov–Johnson–Mehl–Avrami theory, our results indicate that the process is governed by an anisotropic organization of the tellurium atoms in helical structures during the crystal growth.

High tunneling electroresistance in ferroelectric tunnel junctions based on two-dimensional α-In2Se3/MoTe2 van der Waals heterostructures.

Ferroelectric polarization-controlled band alignment can be realized in van der Waals heterostructures (vdWHs), which can be used to create new types of ferroelectric tunnel junctions (FTJs). In this work, we design six probable configurations of two-dimensional vdWHs based on a two-dimensional α-In2Se3 ferroelectric material which has two opposite polarization states P↑ and P↓, and the semiconductor MoTe2. First-principles calculations show robust ferroelectric polarization-controlled switching behavior between the high conductance state in configuration AA-P↓ and the low conductance state in configuration AA-P↑ in the most stable AA stacked vdWHs. Based on this vdWH, a two-dimensional transverse FTJ with AA-P↓ or AA-P↑ as the tunneling barrier and (In0.5Sn0.5)2Se3 monolayers (n-type doped) as electrodes is designed. The tunneling electroresistance ratio of the FTJs at the Fermi level reaches 1.22 × 104% when the tunneling barrier contains two repeating units N = 2 and can be greatly increased by increasing the thickness of the ferroelectric layer. Analysis of the work function, charge redistribution, and local density of states is performed to interpret the above phenomena. The findings suggest the great potential of the AA stacked α-In2Se3/MoTe2 vdWH in the design of high-performance FTJs and application in high-density non-volatile memory devices.

Physical mechanism of Ge doping enhanced Ruddlesden-Popper structure quasi-2D Sr&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt; ceramic hybrid improper ferroelectricity

Hybrid improper ferroelectricity with quasi-two-dimensional (quasi-2D) structure has attracted much attention recently due to its great potential in realizing strong magnetoelectric coupling and room-temperature multiferroicity in a single phase. However, recent studies show that there appears high coercive field and low remnant polarization in ceramics, which severely hinders the applications of this material. In this work, high-quality Sr&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt; and Sr&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;1.99&lt;/sub&gt;Ge&lt;sub&gt;0.01&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt; ceramics with a Ruddlesden-Popper (R-P) structure are successfully prepared, and their crystal structures and electrical properties are investigated in detail. It is found that the Sr&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt; ceramic exhibits a lower coercive field that is close to that of Sr&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt; single crystal. Moreover, via a small amount of Ge doping, the polarization reaches 0.34 μC/cm&lt;sup&gt;2&lt;/sup&gt; for Sr&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt; and 0.61 μC/cm&lt;sup&gt;2&lt;/sup&gt; for Sr&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;1.99&lt;/sub&gt;Ge&lt;sub&gt;0.01&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt;. Combining crystal lattice dynamic studies, we analyze the Raman and infrared responses of the samples, showing the information about the tilting and rotation of the oxygen octahedra in the samples. The improved ferroelectricity after doping may be attributed to the increased amplitude of the tilt mode and the reduced amplitude of rotation mode. Besides, the enhanced ferroelectric properties through Ge doping and its mechanism are further investigated by the Berry phase approach and the Born effective charge method. Furthermore, via the UV-visible spectra, the optical bandgap is determined to be 3.91 eV for Sr&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt; ceramic and 3.95 eV for Sr&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;1.99&lt;/sub&gt;Ge&lt;sub&gt;0.01&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt; ceramic. Using the Becke-Johnson potential combined with the local density approximation correlation, the bandgap is calculated and is found to be in close agreement with the experimental result. And the electronic excitations can be assigned to the charge transfer excitation from O 2p to Sn 5s (Ge 4s). The effects of Ge doping on the ability of Sr&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt; to gain and lose electrons and the bonding strength of Sn-O bond are analyzed via two-dimensional charge density difference. In conclusion, this study provides insights into the synthesis method and modulation of ferroelectric properties of hybrid improper ferroelectrics Sr&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt;, potentially facilitating their widespread applications in various capacitors and non-volatile memory devices.

Exploring the potential of malononitrile functionalized donor-acceptor systems for non-volatile memory device applications.

A novel series of D-bridge-A type organic small molecules has been designed, synthesized, and evaluated for non-volatile resistive switching write-once read-many (WORM) memory application. This study explores structure-property relationships by coupling electron-deficient malononitrile units with donors such as dibenzofuran, dibenzothiophene, and triphenylamine. Photophysical investigations revealed significant intramolecular charge transfer interaction, while electrochemical analyses demonstrated optimal band gaps ranging from 2.20 to 3.10 eV. All synthesized compounds exhibited robust, non-volatile, resistive switching memory capabilities, with ON/OFF ratios spanning 102 to 103. The lowest recorded threshold voltage was -1.25 V, and devices demonstrated substantial stability with retention times of 103 s. Notably, triphenylamine-based compounds displayed superior memory performance compared to their counterparts. The solubility of the compounds in common organic solvents suggests that they are viable for cheap fabrication techniques. Density functional theory calculations were used to visualize the key molecular orbitals and support the proposed mechanisms for resistive switching. The strategic implementation of equipotential donors and acceptors is highly desirable. This well-rounded approach guarantees optimal performance and fosters broader applicability of these devices.

(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).

The g-C3N4-TiO2 nanocomposite for non-volatile memory and artificial synaptic device applications

Recently, 2D layered materials like graphitic carbon nitride (g-C3N4) are gaining significant attention due to their excellent structural and electronic properties. Metal oxide and g-C3N4 composite can enhance chemical and electronic properties which can ultimately enhance device performance. Given this, we report the synthesis of g-C3N4–TiO2 nanocomposite by the ex-situ (solid state reaction) method for resistive switching (RS) applications. Initially, the TiO2 nanoparticles (NPs) were optimized by using annealing them at different temperatures and composited with the g-C3N4. The devices fabricated using g-C3N4–TiO2 nanocomposite shows good RS properties at very low SET and RESET voltages (< 0.5 V), similar to the biological voltage scale. The distribution of switching voltages of all devices was studied using the cumulative probability and Weibull distribution techniques. Moreover, switching voltages were modeled and predicted using the statistical time series analysis method. All fabricated devices demonstrated the double-valued charge flux characteristics, suggesting the presence of a non-ideal memristor effect. The optimized g-C3N4-TiO2 (TiO2 NPs annealed at 450 °C) device shows good endurance and retention memory properties. Moreover, the optimized g-C3N4-TiO2 device can mimic the various bio-synaptic properties such as potentiation-depression, excitatory postsynaptic current, and paired-pulse facilitation. The charge transport results reveal that Ohmic and Child's square law dominated the conduction of the device. Based on electrical and charge transport results, a plausible filamentary RS effect is presented. This study suggested that the g-C3N4-TiO2 is beneficial for both memory and neuromorphic computing applications.

Preparation of Remote Plasma Atomic Layer-Deposited HfO2 Thin Films with High Charge Trapping Densities and Their Application in Nonvolatile Memory Devices.

Optimization of equipment structure and process conditions is essential to obtain thin films with the required properties, such as film thickness, trapped charge density, leakage current, and memory characteristics, that ensure reliability of the corresponding device. In this study, we fabricated metal-insulator-semiconductor (MIS) structure capacitors using HfO2 thin films separately deposited by remote plasma (RP) atomic layer deposition (ALD) and direct-plasma (DP) ALD and determined the optimal process temperature by measuring the leakage current and breakdown strength as functions of process temperature. Additionally, we analyzed the effects of the plasma application method on the charge trapping properties of HfO2 thin films and properties of the interface between Si and HfO2. Subsequently, we synthesized charge-trapping memory (CTM) devices utilizing the deposited thin films as charge-trapping layers (CTLs) and evaluated their memory properties. The results indicated excellent memory window characteristics of the RP-HfO2 MIS capacitors compared to those of the DP-HfO2 MIS capacitors. Moreover, the memory characteristics of the RP-HfO2 CTM devices were outstanding as compared to those of the DP-HfO2 CTM devices. In conclusion, the methodology proposed herein can be useful for future implementations of multiple levels of charge-storage nonvolatile memories or synaptic devices that require many states.

Giant Nonlinear Optical Response via Coherent Stacking ofIn-Plane Ferroelectric Layers.

Thin ferroelectric materials hold great promise for compact nonvolatile memory and nonlinear optical and optoelectronic devices. Herein, an ultrathin in-plane ferroelectric material that exhibits a giant nonlinear optical effect, group-IV monochalcogenide SnSe, is reported. Nanometer-scale ferroelectric domains with ≈90°/270° twin boundaries or ≈180° domain walls are revealed in physical-vapor-deposited SnSe by lateral piezoresponse force microscopy. Atomic structure characterization reveals both parallel and antiparallel stacking of neighboring van der Waals ferroelectric layers, leading to ferroelectric or antiferroelectric ordering. Ferroelectric domains exhibit giant nonlinear optical activity due to coherent enhancement of second-harmonic fields and the as-resulted second-harmonic generationwasobserved to be 100 times more intense than monolayer WS2 . This work demonstratesin-plane ferroelectric ordering and giant nonlinear optical activity in SnSe, which paves the way for applications in on-chip nonlinear optical components and nonvolatile memory devices.

A Facile Hydrothermal Synthesis and Resistive Switching Behavior of α-Fe2O3 Nanowire Arrays.

A facile hydrothermal process has been developed to synthesize the α-Fe2O3 nanowire arrays with a preferential growth orientation along the [110] direction. The W/α-Fe2O3/FTO memory device with the nonvolatile resistive switching behavior has been achieved. The resistance ratio (RHRS/RLRS) of the W/α-Fe2O3/FTO memory device exceeds two orders of magnitude, which can be preserved for more than 103s without obvious decline. Furthermore, the carrier transport properties of the W/α-Fe2O3/FTO memory device are dominated by the Ohmic conduction mechanism in the low resistance state and trap-controlled space-charge-limited current conduction mechanism in the high resistance state, respectively. The partial formation and rupture of conducting nanofilaments modified by the intrinsic oxygen vacancies have been suggested to be responsible for the nonvolatile resistive switching behavior of the W/α-Fe2O3/FTO memory device. This work suggests that the as-prepared α-Fe2O3 nanowire-based W/α-Fe2O3/FTO memory device may be a potential candidate for applications in the next-generation nonvolatile memory devices.

Magnetic-field-controlled spin valve and spin memory based on single-molecule magnets

A single-molecule magnet is a long-sought-after nanoscale component because it can enable us to miniaturize nonvolatile memory storage devices. The signature of a single-molecule magnet is switching between two bistable magnetic ground states under an external magnetic field. Based on this feature, we theoretically investigate a magnetic-field-controlled reversible resistance change active at low temperatures in a molecular magnetic tunnel junction, which consists of a single-molecule magnet sandwiched between a ferromagnetic electrode and a normal metal electrode. Our numerical results demonstrate that the molecular magnetism orientation can be manipulated by magnetic fields to be parallel/antiparallel to the ferromagnetic electrode magnetization. Moreover, different magnetic configurations can be “read out” based on different resistance states or different spin polarization parameters in the current spectrum, even in the absence of a magnetic field. Such an external magnetic field-controlled resistance state switching effect is similar to that in traditional spin valve devices. The difference between the two systems is that one of the ferromagnetic layers in the original device has been replaced by a magnetic molecule. This proposed scheme provides the possibility of better control of the spin freedom of electrons in molecular electrical devices, with potential applications in future high-density nonvolatile memory devices.

Optimization of the Cycle Numbers of TiO2 Resistive Random-Access Memory Devices by Annealing

Resistance switching (RS) devices that are based on titanium dioxide (TiO2) nanomaterials have recently attracted substantial research interest because of their simple preparation technology and potential applications in the applications of memory devices. In this paper, the RS characteristics of TiO2-based memristors were regulated by changing the crystal structure of the TiO2 film. Resistive switching memory, consisting of Ag/amorphous TiO2/Ti fabricated by anodic oxidation, exhibited poor cycling stability. Annealing was applied to the anodized sample over the temperature range of 230–680 °C, and the structure of TiO2 was investigated throughout the process. Amorphous TiO2 transformed into the anatase phase as the annealing temperature was increased to 280 °C and the stable cycle number of the device peaked at 51. By annealing at >450 °C, TiO2 was gradually transformed into the rutile phase, and the stable cycle number of the device decreased to 7. X-ray photoelectron spectroscopy confirmed that the concentration of oxygen vacancies in rutile TiO2 decreased with increasing temperature compared with that in anatase TiO2, resulting in the declining performance of the device. This work presents a low-cost annealing process to prepare stable anatase TiO2-based memristors, which will promote the industrialization of the devices for nonvolatile memory applications in the future.

Emerging Fluorite-Structured Antiferroelectrics and Their Semiconductor Applications

The ferroelectric properties of fluorite-structured oxides have attracted significant attention from researchers because of their potential applications in nonvolatile memory devices, which are enabled by their compatibility with the complementary metal oxide semiconductor technology and physical scalability to a thickness below 10 nm. Another important emerging property of these materials is their antiferroelectricity, which originates from the field-induced transition between the polar and nonpolar phases. Various applications of fluorite-structured antiferroelectrics, such as those in volatile logic devices and cell capacitors for dynamic random-access memory, engineered nonvolatile memory, and energy storage/conversion devices, have been recently proposed, although they have not been investigated in detail compared to fluorite-structured ferroelectrics. The volatile nature of fluorite-structured antiferroelectrics with a characteristic field-induced phase transition, which clearly distinguishes them from ferroelectrics, have endowed these materials with unique properties that cannot be achieved by ferroelectrics. Therefore, the emerging antiferroelectricity of fluorite-structured oxides is comprehensively reviewed in this paper from its fundamentals to various semiconductor applications based on the existing literature.

Dielectric and bipolar resistive switching properties of Ag doped As–S–Se chalcogenide for non-volatile memory applications

In this study, dielectric and bipolar resistive switching properties of the chalcogenide from the Ag–As40S30Se30 system were investigated for potential application in non-volatile memory devices. Preparation of the glasses was done with the use of melt-quenching technique. Current–voltage (I–V) characteristics were determined at different temperatures, and stability of the Ag/chalcogenide sample/Ag structure through constant value of memory window was noticed. Namely, it was observed that a bipolar resistive switching effect is present in the prepared samples with noticeable transitions between high-resistance and low-resistance states when the voltage polarity was changed. Temperature dependent I–V measurements indicated that resistive switching voltage decreases as temperature increases suggesting thermally activated motion of charge carriers. The conduction model fitting results suggested that the Ohmic and space-charge limited current conduction are responsible for resistive switch. Based on the analysis of the obtained results, resistive switching mechanism was explained by using filamentary conduction. In addition, study of dielectrical properties indicated presence of space charge polarization. The presented results show that the synthesized material has properties that suggest them for potential application in non-volatile memory devices.

Fast switching of spontaneous polarization in a microporous molecular rotor ferroelectric

A microporous structural molecular rotor ferroelectric with fast polarization switching (25 kHz) originates from the unique mechanism of fast static–rotating–static motional process showing potential application in non-volatile memory devices.

Tri-level resistive switching characteristics and conductive mechanism of HfO&lt;sub&gt;2&lt;/sub&gt;/NiO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;/HfO&lt;sub&gt;2&lt;/sub&gt; stacks

&lt;sec&gt;With the extensive integration of portable computers and smartphones with “Internet of Things” technology, further miniaturization, high reading/writing speed and big storage capacity are required for the new-generation non-volatile memory devices. Compared with traditional charge memory and magnetoresistive memory, resistive random access memory (RRAM) based on transition metal oxides is one of the promising candidates due to its low power consumption, small footprint, high stack ability, fast switching speed and multi-level storage capacity.&lt;/sec&gt;&lt;sec&gt;Inspired by the excellent resistive switching characteristics of NiO and HfO&lt;sub&gt;2&lt;/sub&gt;, NiO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; films are deposited by magnetron sputtering on the Pt&lt;inline-formula&gt;&lt;tex-math id="Z-20230629144836"&gt;\begin{document}$\langle111\rangle $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144836.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144836.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; layer and the polycrystalline HfO&lt;sub&gt;2&lt;/sub&gt; film, respectively. Their microstructures, resistive switching characteristics and conductive mechanisms are studied. X-ray diffractometer data show the &lt;inline-formula&gt;&lt;tex-math id="Z-20230629144852"&gt;\begin{document}$\langle111\rangle $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144852.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144852.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; preferred orientation for the NiO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; film deposited on the Pt&lt;inline-formula&gt;&lt;tex-math id="Z-20230629144904"&gt;\begin{document}$\langle111\rangle $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144904.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144904.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; layer but the &lt;inline-formula&gt;&lt;tex-math id="Z-20230629144913"&gt;\begin{document}$\langle100\rangle $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144913.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230629144913.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; preferred one for the film deposited on the polycrystalline HfO&lt;sub&gt;2&lt;/sub&gt; layer. X-ray photoelectron depth profile of Ni 2p core level reveals that the NiO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; film is the mixture of oxygen-deficient NiO and Ni&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;. NiO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;(111) films show bipolar resistive switching (RS) characteristics with a clockwise current-voltage (&lt;i&gt;I-V&lt;/i&gt;) loop, but its ratio of the high resistance to the low resistance (&lt;i&gt;R&lt;/i&gt;&lt;sub&gt;H&lt;/sub&gt;/&lt;i&gt;R&lt;/i&gt;&lt;sub&gt;L&lt;/sub&gt;) is only ~10, and its endurance is also poor. The NiO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;(200)/HfO&lt;sub&gt;2&lt;/sub&gt; stack exhibits bipolar RS characteristics with a counterclockwise &lt;i&gt;I-V&lt;/i&gt; loop. The &lt;i&gt;R&lt;/i&gt;&lt;sub&gt;H&lt;/sub&gt;/&lt;i&gt;R&lt;/i&gt;&lt;sub&gt;L&lt;/sub&gt; is greater than 10&lt;sup&gt;4&lt;/sup&gt;, the endurance is about 10&lt;sup&gt;4&lt;/sup&gt; cycles, and the retention time exceeds 10&lt;sup&gt;4&lt;/sup&gt; s. In the initial stage, the HfO&lt;sub&gt;2&lt;/sub&gt;/NiO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;(200)/HfO&lt;sub&gt;2&lt;/sub&gt; stack shows similar bi-level RS characteristics to the NiO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;(200)/HfO&lt;sub&gt;2&lt;/sub&gt; stack. However, in the middle and the last stages, its &lt;i&gt;I-V&lt;/i&gt; curves gradually evolve into tri-level RS characteristics with a “two-step Setting process” in the positive voltage region, showing potential applications in multilevel nonvolatile memory devices and brain-like neural synapses. Its &lt;i&gt;I-V&lt;/i&gt; curves in the high and the low resistance state follow the relationship of ohmic conduction (&lt;inline-formula&gt;&lt;tex-math id="Z-20230714031758-1"&gt;\begin{document}$ I \propto V $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230714031758-1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230714031758-1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;), while the &lt;i&gt;I-V&lt;/i&gt; curves in the intermediate resistance state are dominated by the space-charge-limited-current mechanism (&lt;inline-formula&gt;&lt;tex-math id="Z-20230714031758-2"&gt;\begin{document}$ I \propto V^2 $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230714031758-2.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230331_Z-20230714031758-2.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;). The tri-level RS phenomena are attributed to the coexistence of the oxygen-vacancy conductive filaments in the NiO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;(200) film and the space charge limited current in the upper HfO&lt;sub&gt;2&lt;/sub&gt; film.&lt;/sec&gt;