Search

Neuromorphic computing has garnered significant attention because it can overcome the limitations of the current von-Neumann computing system. Analog synaptic devices are essential for realizing hardware-based artificial neuromorphic devices; however, only a few systematic studies in terms of both synaptic materials and device structures have been conducted so far, and thus, further research is required in this direction. In this study, we demonstrate the synaptic characteristics of a ferroelectric material-based thin-film transistor (FeTFT) that uses partial switching of ferroelectric polarization to implement analog conductance modulation. For a ferroelectric material, an aluminum-doped hafnium oxide (Al-doped HfO2) thin film was prepared by atomic layer deposition. As an analog synaptic device, our FeTFT successfully emulated short-term plasticity and long-term plasticity characteristics, such as paired-pulse facilitation and spike timing-dependent plasticity. In addition, we obtained potentiation/depression weight updates with high linearity, an on/off ratio, and low cycle-to-cycle variation by adjusting the amplitude and number of input pulses. In the simulation trained with optimized potentiation/depression conditions, we achieved a pattern recognition accuracy of approximately 90% for the Modified National Institute of Standard and Technology (MNIST) handwritten data set. Our results indicated that ferroelectric transistors can be used as an alternative artificial synapse.

We investigated chemical vapor-deposited (CVD) two-dimensional (2D) niobium diselenide (NbSe2) material for the resistive switching and synaptic characteristics. Three different atomic switch devices with Ag/HfO2/Pt, Ag/Ti/HfO2/Pt, and Ag/NbSe2/HfO2/Pt were studied as both memory and neuromorphic devices. Both the inserted Ti and NbSe2 buffer layers effectively control the stochastic Ag-ion diffusion, leading to suppressed variation of switching characteristics, which is a critical issue in an atomic switch device. Especially, the device with the 2D NbSe2 buffer layer strikingly enhanced the device reliability in both endurance and retention. In conjunction with scanning transmission electron microscopy (STEM) and energy-dispersive spectrometry (EDS) analysis of the control of the Ag-ion migration, it was understood that filament connection is interrelated with the SET and RESET processes. Besides resistive behaviors in the memory device, various synapse functions such as spike-rate-dependent plasticity (SRDP), forgetting curve, potentiation, and depression were demonstrated with an atomic switch with the 2D NbSe2 buffer layer. Furthermore, the emulated long-term synaptic property was simulated using the MNIST 28 × 28 pixel database. Using adopting a CVD 2D NbSe2 blocking layer, the stochastic Ag-ion diffusion behavior is well-controlled and therefore stable switching and synapse functions are attained.

The development of bioinspired electronic devices that can mimic the biological synapses is an essential step towards the development of efficient neuromorphic systems to simulate the functions of the human brain. Among various materials that can be utilized to attain electronic synapses, the existing semiconductor industry-compatible conventional materials are more favorable due to their low cost, easy fabrication and reliable switching properties. In this work, atomic layer deposited HfO2-based memristor synaptic arrays are fabricated. The coexistence of threshold switching (TS) and memory switching (MS) behaviors is obtained by modulating the device current. The TS characteristics are exploited to emulate essential synaptic functions. The Ag diffusive dynamics of our electronic synapses, analogous to the Ca2+ dynamics in biological synapses, is utilized to emulate synaptic functions. Electronic synapses successfully emulate paired-pulse facilitation (PPF), post-tetanic potentiation (PTP), spike-timing-dependent plasticity (STDP), short-term potentiation (STP), long-term potentiation (LTP) and transition from STP to LTP with rehearsals. The psychological memorization model of short-term memory (STM) to long-term memory (LTM) transition is mimicked by image memorization in crossbar array devices. Reliable and repeatable bipolar MS behaviors with a low operating voltage are obtained by a higher compliance current for energy-efficient nonvolatile memory applications.

The resistive switching random access memory (RRAM) device has received a great interest for the next-generation non-volatile memory application, and resistive switching (RS) characteristics are mainly affected by conductive oxygen vacancies ([V··]) within switching material. Various effective approaches with materials, doping, and thermal treatments have been attempted to achieve the stable RS behaviors. Particularly, thermal annealing is considered as an efficient knob to control [V··] compared to other approaches. However, research on thermal treatment still lacks results and further research efforts are still needed to improve the RS characteristics of the devices. In this work we investigated the RS characteristics of Ti/ZrOx/Pt-structured RRAM device in comparison without and with postrapid thermal annealing (RTA) temperature range under oxygen ambient. The as-fabricated device with atomic layer-deposited ZrOx switching layer exhibited conducting characteristics, which is related to a relatively high amount of [V··] within switching medium. It indicates that moderate amount of [V··] apparently determines the appropriate RS behaviors. With this regard, we modulated the [V··] in ZrOx thin films by employing RTA in the ranges of 500 °C to 800 °C at the oxygen ambient for 60 s. Unlike device without RTA, we observed the stable RS characteristics from device with RTA and device annealed at 700 °C exhibits the excellent bipolar RS characteristics such as higher Ron/Roff, smaller cycle-to-cycle switching variation, better endurance, and longer retention among RTA conditions, indicating moderate amount of [V··] formed within ZrOx thin film layer. Moreover, increasing ALD ZrOx thickness shows the further improvement in the RS characteristics and RTA on the thicker ZrOx device still improves the RS behaviors. This research indicates that modulating [V··] by fast thermal annealing on the ALD zirconia material can provide the proper RS characteristics of the non-volatile memory applications.

Neuromorphic computing has garnered significant attention because it can overcome the limitations of the current von-Neumann computing system. Analog synaptic devices are essential for realizing hardware-based artificial neuromorphic devices; however, only a few systematic studies in terms of both synaptic materials and device structures have been conducted so far, and thus, further research is required in this direction. In this study, we demonstrate the synaptic characteristics of a ferroelectric material-based thin-film transistor (FeTFT) that uses partial switching of ferroelectric polarization to implement analog conductance modulation. For a ferroelectric material, an aluminum-doped hafnium oxide (Al-doped HfO

In this report we perform the irradiation of argon (Ar) plasma on the surface of amorphous atomic layer deposited HfO2 thin film intentionally to modulate interface between Ti and HfO2 having influence on resistive switching as well as synaptic characteristics within the Ti/HfO2/Pt memristor structure. Due to structural change in sub-TiOx interfacial layer as well as oxygen vacancy modulation at the HfO2/Ti interface, compared to the pristine HfO2 thin film, the Ar plasma-irradiated HfO2 thin film exhibits an electro-forming free repeatable switching with digital SET and multilevel voltage-induced RESET characteristics under smaller positive and negative sweeps, respectively. The multilevel resistance states are induced by applying different stopping voltages during the process of RESET. To investigate synaptic characteristics of the Ar plasma-treated device, we carried out the transient electrical measurement by designing the input stimulation with different pulse parameters in order to achieve the symmetric and near-linear conductance change. Finally, we emulated one of the important learning rules of biological synapse called spike-time-dependent-plasticity (STDP) by applying specially designed spikes at the top and bottom electrode by adjusting the time intervals between pre- and post-spikes.

We report the dependence of the thickness of amorphous boron nitride (a-BN) on the characteristics of conductive bridge random access memory (CBRAM) structured with the Ag/a-BN/Pt stacking sequence. The a-BN thin film layers of three different thicknesses of 5.5, 11, and 21.5 nm were prepared by the sputtering deposition. Depending on the thickness of the a-BN layer, the devices are found to be in either low-resistance state (LRS) or high-resistance state (HRS) prior to any consecutive switching cycle. All devices with 5.5 nm thick a-BN switching layer are in LRS as the pristine state, while devices with 21.5 nm thick a-BN layer are found to be in HRS as the pristine state. To attain reliable switching cycles, initial RESET and electroforming process are necessarily required for the devices with 5.5 and 21.5 nm thick a-BN layer, respectively. However, the devices with the a-BN layer of thickness between 5.5 and 21.5 nm in pristine states are in either HRS or LRS. This dependence of the a-BN thickness on different resistance states in the pristine state can be explained by in situ Ag diffusion during its sputter deposition to form a top electrode on the a-BN layer. Our finding shows a detailed investigation and a deep understanding of the switching mechanism of Ag/a-BN/Pt CBRAM devices with respect to different a-BN thicknesses for the future computing system.

We demonstrate inherent biorealistic synaptic plasticity functions in the Pt/n-ZnO/SiO2–x/Pt heterostructures, where the n-ZnO semiconductor is geometrically cone-shaped in the size of a few nanometers. The synaptic functions were achieved within a two-terminal, electroforming-free, and low-power rectifying diode-like resistive switching device. The important rate-dependent synaptic functions, such as the nonlinear transient conduction behavior, short- and long-term plasticity, paired-pulse facilitation, spike-rate-dependent plasticity and sliding threshold effect, were investigated in a single device. These characteristics closely mimic the memory and learning functions of those in biosynapses, where frequency-dependent identical spiking operations are mostly taking place, and we emulate these characteristics in the “Learning-Forgetting-Relearning” synaptic behavior. The switching dynamics in the cone-shaped n-ZnO semiconductor are correlated with the transport mechanism along the grain boundaries of the charged ion species, namely, oxygen vacancies and charged oxygen. The diffusion and generation/recombination of these defects have specific time scales of self-decay by virtue of the asymmetric profile of the n-ZnO cone defects. Finally, the essential biorealistic synaptic plasticity functions were discovered for the perspectives of dynamic/adaptive electronic synapse implementations in hardware-based neuromorphic computing. An electrical component that remembers its past and has potential use in an artificial brain has been developed by researchers in South Korea. Unlike a conventional resistor, the electrical resistance of a so-called memristor depends on the current passing through it, and this property is stored when the current is switched off. This behavior mimics that of synapses in the brain, and so this technology represents an opportunity to replicate the information-processing ability of the brain in an artificially engineered system. Changhwan Choi from Hanyang University, Seoul, and colleagues created their biorealistic memristors by depositing a thin film of zinc oxide on a layer of silicon oxide covered with nanometer-scale conical protrusions. The team measured the electrical response of their device and identified short-term and long-term memory retention characteristics. In this research, we utilize unique cone-shaped structure of crystalline n-ZnO semiconductor, fabricated by controlled wet etching conditions to realize near to bio-synapse size (~20–40 nm) electronic synapse device. The comprehensive “synaptic plasticity behavior” was realized in the device via homogeneous oxygen vacancies accumulation and relaxation states such as paired-pulse facilitation (PPF), short-term plasticity (STP) to long-term plasticity (LTP) memory transition, and important “learning-experience” synaptic functioning.

The effects of in-situ NH 3 plasma passivation on the interface between atomic layer deposited HfO 2 and Ga-face n-GaN substrate in metal-oxidesemiconductor (MOS) devices were investigated by varying plasma power and exposure time and compared with GaN MOS device without plasma passivation. ALD HfO 2 -GaN device with in-situ NH 3 plasma treatment shows improved electrical characteristics including negligible frequency dispersion at the near flat-band voltage region, lower hysteresis (∼10 mV), suppressed oxide capacitance dispersion in the accumulation (2.2%), lower leakage current density (5.21 × 10 −2 A/cm 2 at 1 V), and low interface state density (D it ) of ∼6.77 × 10 11 eV −1 cm −2 at E c -E t = 0.3 eV using an optimized plasma passivation exposure time of 10 min and power of 50 W. These results are attributed that NH 3 plasma treatment could eliminate carbon species and detrimental sub-GaO x as well as passivate the surface and bulk defects on GaN caused by Ga-N dissociation.

We investigated chemical vapor-deposited (CVD) two-dimensional (2D) niobium diselenide (NbSe

The development of bioinspired electronic devices that can mimic the biological synapses is an essential step towards the development of efficient neuromorphic systems to simulate the functions of the human brain. Among various materials that can be utilized to attain electronic synapses, the existing semiconductor industry-compatible conventional materials are more favorable due to their low cost, easy fabrication and reliable switching properties. In this work, atomic layer deposited HfO2-based memristor synaptic arrays are fabricated. The coexistence of threshold switching (TS) and memory switching (MS) behaviors is obtained by modulating the device current. The TS characteristics are exploited to emulate essential synaptic functions. The Ag diffusive dynamics of our electronic synapses, analogous to the Ca2+ dynamics in biological synapses, is utilized to emulate synaptic functions. Electronic synapses successfully emulate paired-pulse facilitation (PPF), post-tetanic potentiation (PTP), spike-timing-dependent plasticity (STDP), short-term potentiation (STP), long-term potentiation (LTP) and transition from STP to LTP with rehearsals. The psychological memorization model of short-term memory (STM) to long-term memory (LTM) transition is mimicked by image memorization in crossbar array devices. Reliable and repeatable bipolar MS behaviors with a low operating voltage are obtained by a higher compliance current for energy-efficient nonvolatile memory applications.

Although they have attracted enormous attention in recent years, software-based and two-dimensional hardware-based artificial neural networks (ANNs) may consume a great deal of power. Because there will be numerous data transmissions through a long interconnection for learning, power consumption in the interconnect will be an inevitable problem for low-power computing. Therefore, we suggest and report 3D stackable synaptic transistors for 3D ANNs, which would be the strongest candidate in future computing systems by minimizing power consumption in the interconnection. To overcome the problems of enormous power consumption, it might be necessary to introduce a 3D stackable ANN platform. With this structure, short vertical interconnection can be realized between the top and bottom devices, and the integration density can be significantly increased for integrating numerous neuromorphic devices. In this paper, we suggest and show the feasibility of monolithic 3D integration of synaptic devices using the channel layer transfer method through a wafer bonding technique. Using a low-temperature processible III-V and composite oxide (Al2O3/HfO2/Al2O3)-based weight storage layer, we successfully demonstrated synaptic transistors showing good linearity (αp/αd = 1.8/0.5), a high transconductance ratio (6300), and very good stability. High learning accuracy of 97% was obtained in the training of 1 million MNIST images based on the device characteristics.

We have demonstrated the co-existence of reliable analog and digital switching characteristics with tantalum oxide based memristor by appropriate rapid thermal annealing (RTA). The device without RTA exhibits a digital SET and multilevel RESET for positive and negative sweeps, respectively. On the other hands, the device shows only analog switching characteristics such that current level increases and decreases gradually for successive positive and negative voltage sweeps, respectively, before any electroforming process with the RTA in the nitrogen ambient at the crystalline temperature of tantalum oxide which is 700 °C for 60 s. Once electroforming process is done, the device exhibits a reliable digital switching with SET at positive sweep and RESET at negative sweep voltages. In the analog state of the device we successfully emulate the synaptic characteristic of the device like spike-rate dependent plasticity (SRDP), pulse-paired facilitation (PPF) and post-tetanic potentiation (PTP). Finally, the Hermann Ebbinghaus forgetting curve is obtained from these devices. The conversion of the device from the digital SET and multilevel RESET to analogue switching is attributed to structural transition of amorphous tantalum oxide to polycrystalline tantalum oxide, different defect density and interface variation in the device.

We report a modulation of oxygen vacancies profile in atomic layer deposition (ALD) HfO2-x thin films by reducing oxidant pulse time (0.7 s–0.1 s) and study its effect on resistive switching behavior with a Ti/HfO2-x/Pt structure. Hf 4f spectra of x-ray photoelectron microscopy (XPS) and depth profile confirm varied oxygen vacancies profiles by shifts of binding energies of Hf 4f5/2 and Hf 4f7/2 main peaks and its according HfO2-x sub-oxides for each device. The ultraviolet photoelectron spectroscopy (UPS) confirms different electron affinity (χ) of HfO2 and HfO2-x thin films, implying that barrier height at Ti/oxide interface is reduced. Current transport mechanism is dictated by Ohmic conduction in fully oxidized HfO2 thin films - Device A (0.7 s) and by Trap Filled Space Charge Limited Conduction (TF-SCLC) in less oxidized HfO2-x thin films - Device B (0.3 s) and Device C (0.1 s). A switching mechanism related to the oxygen vacancies modulation in Ti/HfO2-x/Pt based resistive random access memory (RRAM) devices is used to explain carefully notified current transport mechanism variations from device-to-device. A proper endurance and long-time retention characteristics of the devices are also obtained.

A two terminal semiconducting device like a memristor is indispensable to emulate the function of synapse in the working memory. The analog switching characteristics of memristor play a vital role in the emulation of biological synapses. The application of consecutive voltage sweeps or pulses (action potentials) changes the conductivity of the memristor which is considered as the fundamental cause of the synaptic plasticity. In this study, a neuromorphic device using an in-situ growth of sub-tantalum oxide switching layer is fabricated, which exhibits the digital SET and analog RESET switching with an electroforming process without any compliance current (compliance free). The process of electroforming and SET is observed at the positive sweeps of +2.4 V and +0.86 V, respectively, while multilevel RESET is observed with the consecutive negative sweeps in the range of 0 V to −1.2 V. The movement of oxygen vacancies and gradual change in the anatomy of the filament is attributed to digital SET and analog RESET switching characteristics. For the Ti/Ta2O3−x/Pt neuromorphic device, the Ti top and Pt bottom electrodes are considered as counterparts of the pre-synaptic input terminal and a post-synaptic output terminal, respectively.

As the demand for data increases rapidly, neuromorphic computing, which can process parallel data and consume low power, is attracting to mimic the human nervous system. Resistive switching memory is considered as a viable element to realize neuromorphic computing due to high power and area efficiency. Although many previous studies have emphasized the linearity of analog switching, which is essential for realizing an ideal artificial synapse, it is still limited to control the diffusion of oxygen vacancies and suppress abrupt switching. Switching mechanism depends on the growth and dissolution of oxygen-deficient filaments, which could cause random variation. In this study, to further improve linear conductance change characteristics, we investigate the role of inserting a diffusion limiting layer in the synapse device operated by the migration of metal cations. Inserting thin SiO2 was prepared by plasma enhanced atomic layer deposition at the Cu/ZrO2 interface. This effectively mitigates the random diffusion of Cu ions and can perform multi-level RESET switching characteristics, which significantly improve the linearity of conductance with the number of pulses. In addition, various synaptic characteristics of potentiation and depression, short-term memory to long-term memory transition, pulse-paired facilitation, and post-tetanic potentiation, spike timing dependent plasticity were successfully emulated.

The comparison of resistive switching (RS) storage in the same device architecture is explored for atomic layer deposition (ALD) Al2O3, HfO2 and HfAlOx-based resistive random access memory (ReRAM) devices. Among them, the deeper high- and low- resistance states, more uniform VSET-VRES, persistent ROFF/RON (>102) ratio and endurance up to 105 cycles during both DC and AC measurements were observed for HfAlOx-based device. This improved behavior is attributed to the intermixing of amorphous Al2O3/HfO2 oxide layers to form amorphous thermally stable HfAlOx thin films by consecutive-cycled ALD. In addition, the higher oxygen content at Ti/HfAlOx thin films interface was found within the energy dispersive spectroscopy analysis (EDS). We believe this higher oxygen content at the interface could lead to its sufficient storage and supply, leading to the stable filament reduction-oxidation operation. Further given insight to the RS mechanism, SET/RESET power necessities and scavenging effect shed a light to the enhancement of HfAlOx-based ReRAM device as well.

The charge trapping behaviors of ammonium poly-sulfide, (NH4)2Sx, passivated GaN MOS device with atomic layer deposited HfAlOx gate dielectric and DC-sputtered TiN gate electrode are investigated and compared with those of GaN MOS device with hydrochloric acid, HCl, passivation. Compared to non-S passivated device, higher peak capacitance (>20% @1MHz), reduced frequency dispersion (~30% ), lower hysteresis (~34% ), higher breakdown (1.3), lower stress induced flat-band voltage (VFB) shift (~30% ), and lower interface state density (Dit), stronger immunity Dit generation (Dit) against gate bias voltage are attained with S-passivated device. Further improved characteristics are observed with post deposition annealing at 400C for 10min. These behaviors are mainly attributed to higher bond strength energy of SO and SN than those of ClO and ClN bonds. Display Omitted Sulfur passivation induces higher Cox, reduced frequency dispersion, lower hysteresis, higher VBD, and lower Dit.Stress induced VFB and Dit are significantly suppressed with sulfur-passivation.Further improvement is still observed after post annealing at 400 oC.

We have investigated the effects of metal gate process temperature on the effective work function (Weff) of MOS devices with all ALD HfO2/TiN gate stack and its correlation with grain size of PEALD TiN metal gate is presented with other electrical characteristics. Ti precursor and reactant were used with tetrakis-dimethyl-amino titanium (TDMAT) and H2/N2 mixture for TiN while tetrakis-ethylmethyl-amino hafnium (TEMA-Hf) and H2O were used for HfO2. With increasing TiN deposition temperature, the Weff of TiN electrode is positively shifted up to ~200meV while EOT is kept as 1.2nm. These findings could be attributed to the combining effects from crystal structure change (i.e., increased grain size) and different chemical composition. Unlike PVD TiN system, in ALD TiN system, higher VFB is observed with increasing Ti ratio in the film, ascribed to the small amount of carbon residue (69at%) within the TDMAT precursor. Display Omitted HfO2-MOS device with ALD TiN using TDMAT precursor and H2/N2 mixture shows higher VFB by 200mV with 1.2nm EOT.TDMAT ALD TiN shows higher VFB with increasing [Ti]/[Ti+N2] ratio due to the combining effects of carbon residue from the precursor and increased grain size.

For the potential switching bio-memory device application using DNA composite thin film, we fabricated and characterized the transparent and biocompatible resistive switching random access memory (RRAM) device within the structure stacking of Pt/Cu

We have investigated the switching mechanism of conductive bridge random access memory (CBRAM) with Ag/SnO2/Pt, Ag/InGaZnO(IGZO)/Pt and their hybrid oxide devices with different stacking sequence (Ag/SnO2/IGZO/Pt and Ag/IGZO/SnO2/Pt). Typical bipolar resistive switching is observed in single layered devices and an Ag/SnO2/IGZO/Pt hybrid device. Interestingly, a stable and reproducible unipolar resistive switching is observed for a hybrid device with a stacking sequence of Ag/IGZO/SnO2/Pt. This result suggests that the staking sequence of dielectrics in the IGZO and SnO2 electrolyte determines unipolar or bipolar switching. The different switching types in the hybrid electrolyte are based on different migration or diffusion rates of Ag ions in the solid electrolyte and redox reaction rates at the electrodes. And as compared to single layered devices, the hybrid structured devices exhibit low operation voltages, higher ION/IOFF ratio, uniform switching cycles and better endurance and retention characteristics. The results and switching mechanisms demonstrated here in hybrid devices can be extended to other hybrid devices based on CBRAM device.

The enhanced resistive switching (RS) characteristics of the flexible TaOx/InGaZnO (IGZO)-based bilayer memristor deposited on the polyethylene terephthalate (PET) substrate are demonstrated in the Ti/TaOx/IGZO/Pt heterostructure. The Ar ion (Ar+) plasma treatment was intentionally induced on the surface of TaOx/IGZO thin films to improve RS filament consistency and results displayed the improved uniform RS behavior of the plasma-treated device. Further, this device is investigated as an artificial electronic synapse and important synaptic functions such as long term potentiation (LTP), long term depression (LTD), paired-pulse facilitation (PPF), post-tetanic potential (PTP) and spike-timing dependent plasticity (STDP) characteristics are well emulated and analogues to the biological synapse behavior. The bending test to assess influence of stress/strain induction on the RS properties is also demonstrated. Further, bulk and angle-resolved x-ray photoelectron spectroscopy (XPS) is utilized to shed a light on the influence of Ar+ plasma on the surface of TaOx/IGZO thin films, as well as AFM analysis, and this insight is argued in the elaborated RS mechanism discussion.

Lithospermum erythrorhizon has long been used as a traditional oriental medicine. In this study, the acute and 28-day subacute oral dose toxicity studies of hexane extracts of the roots of L. erythrorhizon (LEH) were performed in Sprague-Dawley rats. In the acute toxicity study, LEH was administered once orally to 5 male and 5 female rats at dose levels of 500, 1,000, and 2,000 mg/kg. Mortality, clinical signs, and body weight changes were monitored for 14 days. Salivation, soft stool, soiled perineal region, compound-colored stool, chromaturia and a decrease in body weight were observed in the extract-treated groups, and no deaths occurred during the study. Therefore, the approximate lethal dose (ALD) of LEH in male and female rats was higher than 2,000 mg/kg. In the subacute toxicity study, LEH was administered orally to male and female rats for 28 days at dose levels of 25, 100, and 400 mg/kg/day. There was no LEH-related toxic effect in the body weight, food consumption, ophthalmology, hematology, clinical chemistry and organ weights. Compound-colored (black) stool, chromaturia and increased protein, ketone bodies, bilirubin and occult blood in urine were observed in the male and female rats treated with the test substance. In addition, the necropsy revealed dark red discoloration of the kidneys, and the histopathological examination showed presence of red brown pigment or increased hyaline droplets in the renal tubules of the renal cortex. However, there were no test substance-related toxic effects in the hematology and clinical chemistry, and no morphological changes were observed in the histopathological examination of the kidneys. Therefore, it was determined that there was no significant toxicity because the changes observed were caused by the intrinsic color of the test substance. These results suggest that the no-observed-adverse-effect Level (NOAEL) of LEH is greater than 400 mg/kg/day in both sexes.

For the potential switching bio-memory device application using DNA composite thin film, we fabricated and characterized the transparent and biocompatible resistive switching random access memory (RRAM) device within the structure stacking of Pt/Cu2+ doped salmon DNA/FTO, where Cu2+ doping into salmon DNA was solution-processed. The device shows good bipolar switching characteristics with SET and RESET processes at negative and positive sweeps, respectively, with switching memory window greater than 103 ratios. The device was observed to be in low resistance state as its pristine state and an initial RESET state was necessary to achieve programmable SET and RESET cycles. Based on the electrical characteristics of the Cu2+-doped salmon DNA-based RRAM device we propose a switching mechanism with the formation and rupture of conductive filaments due to the migration of Cu2+ during the electrical stress. Our understanding could contribute to the engineering of biomaterial memory switching medium for the environmentally benign, biocompatible and biodegradable memory storage devices.

We performed various pulse measurements on an atomic layer deposited (ALD) HfO2-based resistive switching random access memory (RRAM) device and investigated its electronic synaptic characteristics. Unlike requirements for RRAM device application, to achieve the multi-state conductance changes required for the synaptic device, we employed additional sputtered TaOx thin film formation on the ALD HfO2 switching medium, which leads to engineering the concentration of oxygen vacancies and modulating the conductive filaments. With this TaOx/HfO2 bi-layered device, we attained gradual resistive switching, linear and symmetric conductance change, improved endurance and reproducibility characteristics compared to a single HfO2 device. Finally, we emulated spike-timing-dependent plasticity based learning rule with pulses inspired by neural action potential, indicating its potential as an electronic synaptic device in a hardware neuromorphic system.