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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.

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.

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.

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 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.

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.

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.