Threshold Switching Devices Research Articles

Steep-Switching Fully Depleted Silicon-on-Insulator (FDSOI) Phase-Transition Field-Effect Transistor With Optimized HfO₂/Al₂O₃-Multilayer-Based Threshold Switching Device

The OFF-state resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> ) and threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) of a HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based threshold-switching (TS) device for phase-transition fully depleted siliconon-insulator (FDSOI) device on silicon substrate can be improved, e.g., ROFF is increased by implementing a multilayered HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (rather than HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer only). The HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer with the atomic layer deposition (ALD) cycle ratio of 3:1 (i.e., Hf:Al = 3:1) shows approximately 3.47 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> Ω of average ROFF and approximately 2.12 V of average VT. Next, VT can be modulated by adjusting the work function of the bottom electrode. We demonstrate that a high work function of the bottom electrode results in stronger built-in electric-field and tunneling, decreasing VT and ROFF. The Hf <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al1-based TS device with platinum electrode shows approximately 0.88 V of average VT and approximately 2.81 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> Ω of average ROFF. The phase-transition FDSOI device used in this work is fabricated by connecting the optimized TS device in series to the drain electrode of a baseline FDSOI device, whose channel length is 130 nm. The phase-transition FDSOI device shows the minimum subthreshold slope of 9 mV/decade owing to the abrupt resistive switching of the TS device. Additionally, it exhibits an increase in the ON/OFF-state current ratio by three times because of high R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> .

Enhancement of ovonic threshold switching characteristics using nanometer-scale virtual electrode formed within ultrathin hafnium dioxide interlayer

Herein, we investigate the effect of the active switching area on the electrical characteristics of an ovonic threshold switching (OTS) device. The active switching area was scaled-down below the conventional lithography limit by utilizing a nanometer-scale virtual electrode formed by the controlled breakdown of an ultrathin HfO2 interlayer. Improved switching characteristics were observed when the effective diameter of the virtual electrode was ∼6 nm. The bulk leakage current was blocked, which led to a reduction in the off-current by approximately two orders of magnitude. Furthermore, an improvement in the uniformity of the threshold voltage was observed. Our results showed that the optimal confinement of the active switching area is the key to achieve excellent performance, high reliability, and high tunability in an OTS device.

Improved Threshold Switching and Endurance Characteristics Using Controlled Atomic‐Scale Switching in a 0.5 nm Thick Stoichiometric HfO2Layer

AbstractElectrochemical metallization cell–based threshold switching (TS) devices are promising candidates for selectors in high‐density cross‐point memory arrays. However, TS characteristics in density‐ and stoichiometry‐engineered solid electrolyte systems have not been studied. By adopting TS‐based stoichiometric and substoichiometric solid electrolyte HfO2layers, the localized atomic scale movement of Ag ions can be effectively controlled in ultrathin bilayers. The stoichiometric HfO2thickness is crucial to this. This study proposes defect‐ and density‐engineered bilayer TS with a 0.5 nm critical thickness of the stoichiometric HfO2layer, which maximizes various switching characteristics. The unstable filament in the ultrathin stoichiometric HfO2layer prevents the formation of stable Ag clusters owing to limited Ag injection into the dense and stoichiometric HfO2layer. In addition, the substoichiometric HfO1.91(1.5 nm) buffer layer prevents direct injection of Ag ions from the top electrode into the dense HfO2layer. These factors enable the bilayer design to achieve a high turn‐off speed of 100 ns, excellent endurance above 107cycles, low off current of ≈pA, and tightVthdistributions even for sub‐2 nm devices. These exceptional results demonstrate the possibility of designing density‐graded bilayer TS devices with high stability, fast switching, and high endurance.

Improved Turn-Off Speed and Uniformity of Atomic Threshold Switch Device by AgSe Electrode and Bipolar Pulse Forming

To improve the turn-off speed and uniformity of atomic threshold switching (TS) devices, we propose the use of the AgSe electrode and controlled bipolar pulse forming method. Compared with TS devices with Ag and AgTe electrodes, TS device with the AgSe electrode shows an extremely fast turn-off speed, which can be explained by the limited injection of Ag into the switching layer. By applying positive bias (Icc = 500 nA, 1 ms) followed by negative bias (−0.1 to −0.2 V, $10~\mu \text{s}$ ), the TS device exhibits excellent switching uniformity. It can be explained by the formation of atomic-scale filament under the positive bias at low Icc and drift-back of excessive Ag by the negative bias. By designing the shape of the filament and concentration of the residual Ag, the TS device with the AgSe electrode shows promise for selector applications.

Abruptly-Switching MoS₂-Channel Atomic-Threshold-Switching Field-Effect Transistor With AgTi/HfO₂-Based Threshold Switching Device

A two-dimension (2D) atomic-threshold-switching field-effect transistor (ATS-FET) was implemented, by connecting an AgTi/HfO2-based threshold-switching (TS) device in series to the drain electrode of the 2D baseline-FET with molybdenum disulfide (MoS2) channel material. We optimized/developed the Ag/HfO2-based TS device because its characteristic is associated to the way how to achieve high performance of the 2D ATS-FET. By reducing the effective device area of the Ag/HfO2-based TS device down to $4~\mu \text{m}^{2}$ , low threshold voltage ( $\text{V}_{\mathrm {T}} \sim 0.42$ V), low threshold current (IT, drain current at the threshold voltage, $\sim 3.79 \times 10^{-11}$ A), and low VT variation (~0.09 V) were achieved. This is because the randomly formed filaments and electric field are better controlled with the scaled effective area. Next, the titanium (Ti)-injection barrier layer was inserted between the top electrode and the switching layer, while maintaining the optimized area in the TS device. The inserted Ti-injection barrier layer prevents the migration of Ag ions into the switching layer, enabling the stable TS operation even under the compliance current of $100~\mu \text{A}$ . Additionally, it locally restricts the region where the filaments are created inside the switching layer, resulting in a 17% lower VT variation and stable IT to approximately $\sim 1.5 \times 10^{-11}$ A in 100 cycles. Due to the low off-state leakage current and low variation characteristic of the optimized AgTi/HfO2-based TS device, the 2D ATS-FET (vs. 2D baseline-FET) shows the reduction of off-state leakage current (by $\sim 10^{2}$ in sub-threshold region) and the stable switching characteristic. The proposed 2D ATS-FET shows stably steep switching characteristics, e.g., sub-threshold swing under forward bias (~19 mV/decade) and reverse bias (~26 mV/decade), because of its abruptly switching characteristics of the TS device.

Different degradation mechanism by conduction region in AsTeGeSiN threshold switching device

AsTeGeSiN threshold switching devices, which have recently gained attention for their reliable operation, are investigated for the degradation mechanism under constant voltage stress (CVS). Threshold switching devices are essential for 3D stacking of high-density memories. The degradation mechanism can be divided into two conduction regions, i.e. before and after the threshold voltage ( V th ) by applying CVS. It is found that charge trapping, which can easily be recovered, is the main degradation mechanism in the low-conduction region. This is because Poole–Frankel conduction by deep traps occurred in the low-conduction region. However, trap generation may occur when going from deep traps to shallow traps, which is an important factor in the high-conduction region stress voltage. Switching stress for different duty cycles in two conduction regions is also performed. Therefore, when operating an AsTeGeSiN threshold-switching device as a selector, the read sequence is not a problem despite frequent usage. However, for the write sequence, reducing the number and duration of pulses helps extend the device's lifetime.

Ultrahigh drive current and large selectivity in GeS selector

Selector devices are indispensable components of large-scale nonvolatile memory and neuromorphic array systems. Besides the conventional silicon transistor, two-terminal ovonic threshold switching device with much higher scalability is currently the most industrially favored selector technology. However, current ovonic threshold switching devices rely heavily on intricate control of material stoichiometry and generally suffer from toxic and complex dopants. Here, we report on a selector with a large drive current density of 34 MA cm−2 and a ~106 high nonlinearity, realized in an environment-friendly and earth-abundant sulfide binary semiconductor, GeS. Both experiments and first-principles calculations reveal Ge pyramid-dominated network and high density of near-valence band trap states in amorphous GeS. The high-drive current capacity is associated with the strong Ge-S covalency and the high nonlinearity could arise from the synergy of the mid-gap traps assisted electronic transition and local Ge-Ge chain growth as well as locally enhanced bond alignment under high electric field.

Effect of Nitrogen Doping on Threshold Voltage in Amorphous Ga2Te3 for Application of Selector Devices

The implementation of ultrahigh‐density cross‐point array structures has received considerable interest as emerging storage devices, and threshold switching devices are regarded to be promising as to the suppression of leakage current in cross‐point array structures. Threshold switching devices need to modulate the threshold voltage (Vth) depending on the various memory elements to achieve proper selector device in cross‐point array structures. However, only limited methods are available for controlling Vth. Therefore, the nitrogen (N) doping effects on trap states, density of localized states, and Vth of the amorphous Ga2Te3 (a‐Ga2Te3) selector devices are investigated herein. Furthermore, the electrical conduction behavior is fitted using a trap‐controlled space charge limited conduction mechanism with two transition voltages, i.e., the space charge limited voltage (VSCL) and trap‐filled limited voltage (VTFL). The optical bandgap energy and optical Urbach energy are affected by the N doping in a‐Ga2Te3. In addition, N doping in a‐Ga2Te3 increases the density of localized states. These effects increase both the transition voltages (VSCL and VTFL). Thus, doping a‐Ga2Te3 with N reduces Vth when the trap states are changed. Furthermore, N‐doped a‐Ga2Te3 selector devices exhibit highly reliable switching up to 109 cycles.

Low-Power Artificial Neurons Based on Ag/TiN/HfAlOx/Pt Threshold Switching Memristor for Neuromorphic Computing

Threshold switching (TS) devices are promising candidates to build highly compact and energy efficient artificial neurons. Here, we present a Pt/Ag/TiN/HfAlOx/Pt (PATHP) device with excellent TS characteristics, including a large selectivity(1010), a wide range of operation current from 10 nA to 1 mA, an extremely steep slope (0.63 mV/dec) and fast turn-on speed (50 ns). The stable TS performance can be ascribed to the introduction of TiN buffer layer and the alternate atomic layer deposited HfAlOx layer. Further, we experimentally demonstrate the functions of leaky-integrate-and-fire neurons with low power feature based on a RC circuit and a single device, respectively, which are essential for constructing spiking neuromorphic systems.

CMOS Based Ovonic Threshold Switching Emulation Circuitry

Ovonic Threshold Switch (OTS) device is most popular switching device in PRAM. There are many OTS device research; however, it is hard to make reasonable OTS device which uses a circuit simulation and real device. In this work, we studied the OTS device emulation circuit, which can follow OTS characteristic, especially snapback current using 0.18 μm CMOS technology. This circuitry composes snapback current generator, cut off switch and output driver. Snapback current generator can make the current level up to 300 μA.

Understanding of the Abrupt Resistive Transition in Different Types of Threshold Switching Devices From Materials Perspective

In this article, we investigate dynamics of electrically driven abrupt and volatile resistive switching characteristics, which are called threshold switching (TS) characteristics, in insulator-to-metal transition NbO x switch, BTe x -based ovonic threshold switch, and Ag-based atomic switch by adopting field-induced nucleation theory. To verify field-induced nucleation phenomena in the initial electroforming and electrically driven TS processes of the TS devices, we apply constant square voltage pulses with various voltage magnitudes and measure the delay time required for the electrical switching process. During both the forming and switching processes, it is observed that all the TS devices exhibit switching delay time exponential in 1/voltage with different nucleation barrier energies ( ${W}_{{0}}$ ). In addition, we find that the ${W}_{{0}}$ for the first forming process appears to be larger than that for the subsequent switching process and the difference is observed to be greater if the formed device is crystallized and smaller if not. Moreover, we find that the effective barrier energy and the fastest speed that a device can have ( $\tau _{{0}}$ ) are closely related to switching speed of the TS devices. Also, we reveal that the hold voltage characteristics are affected by the ${W}_{{0}}$ and carrier concentration of the solid electrolyte.

Physically transient random number generators based on flexible carbon nanotube composite threshold switching

The resistive switching memories are in prosperity development on account of its simple structure and high performance. A primary problem of resistive switching memory reliability is discrete and random distribution of resistive switching parameters, which limits greatly the possible range of its digital memory application on account of uniformity of resistance parameters is poor and the distribution range is too large. But, from the other side, these random parameter distributions are eable be used to complete some computing tasks. Here, a random number generation is demonstrated with stochastic variation of Vset in a flexible threshold switching device by using the carbon nanotube and polyvinyl alcohol composite materials. Two reading voltages was applied to the two threshold switching devices, the electric potential of Vcin randomized either positive or negative in accordance with the cycle to cycle evolution of two threshold switching devices by the control of the voltage divider, which in return comes about a random bitstream after passing by the comparer. As well as, its transient characteristics have been illustrated for data security appilcation.

Investigation on Threshold Voltage Adjustment of Threshold Switching Devices with HfO2/Al2O3 Superlattice on Transparent ITO/Glass Substrate.

Threshold voltage adjustment in threshold switching (TS) devices with HfO2/Al2O3 superlattice (by means of changing the cycle ratio of HfO2 to Al2O3 in atomic layer deposition) is investigated to implement a transparent cross-point array. TS devices with different cycle ratios (i.e., 3:1, 3:2, and 3:3) were fabricated and studied. The threshold voltage of the devices was increased from 0.9 V to 3.2 V, as the relative contents of Al2O3 layer in the superlattice were increased. At the same time, it is demonstrated that the off-resistance values of the devices were enhanced from 2.6 × 109 to 6 × 1010 Ω as the atomic layer deposition (ALD) cycle ratio of HfO2 to Al2O3 layer was adjusted from 3:1 to 3:3. However, the hold voltage and the on-current values were almost identical for the three devices. These results can be understood using the larger barrier height of Al2O3 layer than that of HfO2 layer.

(Invited) Reliability and Scaling Perspectives of HfO2-Based OxRAM

Hafnium oxide OxRAM (Oxide Random Access Memory) is a non-volatile memory technology that has been widely investigated over the last 15 years. Its working mechanism relies on the formation and rupture of a tiny conductive filament through a HfO2 layer sandwiched between an active (Ti) and inert (TiN) electrodes. This causes the electrical resistance to very over three orders of magnitudes between a low resistance state (LRS) and a high resistance state (HRS).OxRAM is a strong candidate to replace embedded Flash technology at scaled nodes. This is justified by several considerations but mainly driven by the low cost of integration and very high compatibility with standard CMOS technology. Yet OxRAM faces tremendous challenges in terms of reliability, variability and scaling.In this paper we first show our latest findings in terms of reliability on a Ti/HfO2(5nm)/TiN cell. Controlling LRS and HRS resistance distributions can be achieved with a proper write/erase algorithm design (see Fig. 1) and it is a key point to enable large array integration. A proper write/erase sequence ensures long bit endurance up to 107 cycles. Data retention is then evaluated at four different temperatures (125, 165, 200 and 260 °C) to extract a 10 years retention temperature of 85°C.Next, we discuss OxRAM scaling perspectives. In this context, it is essential to take into account both the resistive cell and its selector (most typically a MOS transistor). As recently shown [2], so far the OxRAM scaling has been mainly limited by the programming current, which determines the MOS selector size. Even if the OxRAM cell is integrated in the BEOL above its selector, the bitcell size is mostly determined by the underlying transistor. In addition, the OxRAM programming voltage dictates the transistor EOT and challenges the transistor reliability for scaled nodes. We demonstrated our OxRAM cell integration with a thin gate oxide FDSOI transistor (GO1/SG) and we show how this paves the OxRAM integration in a 28nm node.Eventually we discuss the ultimate scaling perspective of an OxRAM array in a Cross-Point architecture. To enable such architecture the development of a BEOL selector is mandatory and we focused on Ovonic Threshold Switching device (OTS) for this scope. The OTS works switching between a low-conductive state and a metastable conductive one, which is voltage-triggered. We demonstrated (see Fig. 3) a 1S1R (one selector one resistor) OxRAM cell where the HfO2 memory is co-integrated with a series Ge-Sb-Se-N (GSSN) OTS. We eventually discuss the limits of this architecture showing that a trade-off exists between the half pitch of the Cross-Point array metal lines and the size of the memory bank.REFERENCE[1] G. Sassine et al., 2018 IEEE International Reliability Physics Symposium (IRPS). [2] J. Sandrini et al., 2019 IEEE Electronic Device Meeting (IEDM). [3] D. Alfaro Robayo et al., 2019 IEEE 11th International Memory Workshop (IMW). Figure 1

Cryogenic behavior of NbO2 based threshold switching devices as oscillation neurons

This Letter investigates the cryogenic behavior of NbO2 threshold switching devices. Pt/NbO2/Pt devices are demonstrated to be well functional as threshold switching devices at ultra-low temperature (4 K). When the temperature decreases, the OFF-state resistance of NbO2 increases and the switching voltage increases. With the extracted characteristics of NbO2 ranging from 4 K to 300 K, we continue to study the neuromorphic system using the crossbar array with resistive memories as resistive synapses and NbO2 as oscillation neurons at different temperatures through SPICE simulation. The simulation results show that the oscillation systems could still work properly at 4 K. The oscillation amplitude decreases as temperature increases. The oscillation frequency depends on both the temperature and the input voltage.

Modulating the filament rupture degree of threshold switching device for self-selective and low-current nonvolatile memory application

Resistive switching devices have tremendous potential for memory, logic, and neuromorphic computing applications. Cation-based resistive switching devices intrinsically show nonvolatile memory characteristics under high compliance current (ICC), while show volatile threshold switching (TS) selector characteristics under low ICC. However, separate researches about cation-based memory or selector are hard to evade the typical current-retention dilemma, which results in the hardship to obtain low-current memory and high-current selector. Here, we propose a novel strategy to realize nonvolatile storage characteristics in a volatile TS device by modulating the rupture degree of conductive filament (CF). Enlarging the rupture degree of the CF with a certain RESET process, as confirmed by transmission electron microscope and energy dispersive spectrometry results, the threshold voltage of the Ag/HfO2/Pt TS devices can be enlarged from 0.9 to 2.8 V. Generation of the voltage difference enables the volatile TS devices the ability of self-selective nonvolatile storage. Increasing the RESET magnitude and shrinking the device size have been proved effective ways to increase the read window of the TS memory (TSM) devices. Evading the limit of the current-retention dilemma, ultra-low energy dissipation can be obtained by decreasing ICC to nA level. With self-selective, low-energy, and potential high-density integration characteristics, the proposed TSM device can act as a potential supplement of novel storage class memories.

Threshold Voltage Drift in Te-Based Ovonic Threshold Switch Devices Under Various Operation Conditions

In this research, we investigate threshold voltage drift in a B-Te based OTS device under various operation conditions. To this aim, drift of threshold voltage after a switching process is examined. From the measurements, we find that the threshold voltage required to switch the device back after the first switching process decreases when the interval between the voltage pulsespplied for the switching is shorter than $10 \mu \text{s}$ , and becomes drift-free afterward. This result implies that a certain amount of heat is generated during the first switch which results in higher local temperature that lowers the threshold voltage, and is dissipated completely after $10 \mu \text{s}$ . A 3D finite element simulation is provided to demonstrate the thermal effect induced by the switching pulse. Our results demonstrate that the threshold voltage of the Te-based OTS device is affected by thermal effects during switching at short time scales, and thus indicate that the threshold voltage drift should be considered carefully in the fast device operation.

Field-induced nucleation switching in binary ovonic threshold switches

In this study, we investigate the initial electroforming and electrically driven threshold switching characteristics of a binary boron-tellurium-based ovonic threshold switch (OTS) device using field-induced nucleation theory and related material parameters. First, an analysis of the current-voltage characteristics of devices with various electrode areas shows that the OTS device exhibits filamentary switching, consistent with the field-induced nucleation framework. Further, we find that the OFF-state current depends on the device area, whereas the hold current remains constant regardless of the device area, indicating that the ON-state current flows through a local conduction path. For verifying field-induced nucleation phenomena in the forming and subsequent switching of the OTS device, we apply constant voltage pulses and measure the delay time required for electrical switching of the device. During both forming and switching processes, the switching time is exponential in inverse voltage with different nucleation energy (W0) values; W0 for forming is larger than that for switching. To verify this analysis, incremental voltage pulses with various pulse widths are also applied to confirm the relation between the threshold voltage and time. The W0 values for the two types of switching in this analysis are consistent, solidifying our understanding of the critical contribution of nucleation phenomena in OTS. The smaller W0 for switching can be explained by the generation of clusterlike nucleation sites during forming with lower nucleation energy than in the surrounding matrix.

A novel steep slope hybrid InGaZnO TFT with negative DIBL improvement based on the Ag/HfO2 threshold switching device

We present a steep slope hybrid thin film transistor (TFT) that integrates an Ag/HfO2 based threshold switching (TS) with a ferroelectric HfZrO gated InGaZnO TFT. The hybrid TFT shows a minimum subthreshold swing of 16 mV decade. The negative differential resistance effect (NDR) was observed during the switching process of the TS. And the InGaZnO TFT shows a negative drain-induced barrier lowering (DIBL) effect. Furthermore, a Verilog-A model is developed to simulate the hybrid TFT. All of these suggests the as-fabricated hybrid TFT is suitable for low power TFT based applications.

Improvement of NbOx-based threshold switching devices by implementing multilayer stacks

In this work the I–V characteristics of a niobium oxide-based threshold switching device were optimized to match the requirements for its application in neuromorphic circuits. Those neuromorphic circuits rely on coupled oscillators utilizing the volatile resistive switching effect of the memristor. A large voltage extension of the negative differential resistance region of the threshold switch enables enhanced signal amplification, and, furthermore, can lead to a better tolerance to device variability. A symmetric switching behavior as well as a high device stability for the operation in both voltage polarities is mandatory to allow the integration in circuits that utilize the connection of several threshold switching devices operated in different polarities. These properties are similarly important for the adoption of the threshold switches as selector devices in bipolar resistive memory arrays. Furthermore, a low forming voltage is desirable because it leads to a better control during the forming step. To meet all those requirements the application of multilayer stacks consisting of niobium and niobium oxide layers is proposed and their optimization is investigated in detail.