Ovonic Threshold Switching Device Research Articles

A novel approach to enhancing performance and endurance in GeS2 OTS devices using amorphous carbon doped W2N electrodes

Three–dimensional (3D) cross-point (X–point) memory has gathered interest for its fast data processing and high density, achieved by stacking memory with a selector device to prevent misinterpretation. Ovonic threshold switching (OTS) is a promising selector due to its reversible switching behavior. Although, OTS devices typically employ transition metal nitrides (TMN) such as TiNx, TaNx, and WNx for electrodes owing to their good stability, high melting points, and low resistivity, TMNs can diffuse and degrade device performance by recrystallizing with chalcogenide alloys. Amorphous carbon (a–C) can be a good alternative electrode material due to its low roughness, cost–effectiveness, high work function (WF), and excellent thermal stability. However, the high resistivity of a–C (~ 150 mΩ–cm) increases threshold voltage (Vth), causing high power consumption. Therefore, combining both a–C and TMN materials can effectively obtain their advantages. This study explores the effect of varying a–C content in W2N electrodes on GeS2–based OTS selectors. The (W2N)1-xCx (0 ≤ x ≤ 0.25) electrodes were deposited using DC magnetron co–sputtering. The phase of (W2N)1-xCx (0 ≤ x ≤ 0.25) films transformed from polycrystalline to amorphous with increasing x. Devices with (W2N)1-xCx/GeS2/W2N structure showed decreased Vth and off current, improving from 4.8 to 3.8V and 8.0 to 4.17nA, respectively. The subthreshold slope, distance between traps, and interface trap density (Nit) were extracted using the Pool–Frenkel model. The reduced Vth may be attributed to a higher WF and lower Nit with increasing x. The device’s lifetime improved up to 1.0 × 109 pulses for the highest a–C content in (W2N)1-xCx (0 ≤ x ≤ 0.25) electrodes.

GeSe ovonic threshold switch: the impact of functional layer thickness and device size

Three-dimensional phase change memory (3D PCM), possessing fast-speed, high-density and nonvolatility, has been successfully commercialized as storage class memory. A complete PCM device is composed of a memory cell and an associated ovonic threshold switch (OTS) device, which effectively resolves the leakage current issue in the crossbar array. The OTS materials are chalcogenide glasses consisting of chalcogens such as Te, Se and S as central elements, represented by GeTe6, GeSe and GeS. Among them, GeSe-based OTS materials are widely utilized in commercial 3D PCM, their scalability, however, has not been thoroughly investigated. Here, we explore the miniaturization of GeSe OTS selector, including functional layer thickness scalability and device size scalability. The threshold switching voltage of the GeSe OTS device almost lineally decreases with the thinning of the thickness, whereas it hardly changes with the device size. This indicates that the threshold switching behavior is triggered by the electric field, and the threshold switching field of the GeSe OTS selector is approximately 105 V/μm, regardless of the change in film thickness or device size. Systematically analyzing the threshold switching field of Ge–S and Ge–Te OTSs, we find that the threshold switching field of the OTS device is larger than 75 V/μm, significantly higher than PCM devices (8.1–56 V/μm), such as traditional Ge2Sb2Te5, Ag–In–Sb–Te, etc. Moreover, the required electric field is highly correlated with the optical bandgap. Our findings not only serve to optimize GeSe-based OTS device, but also may pave the approach for exploring OTS materials in chalcogenide alloys.

A self-selecting memory element based on a method of interconnected ovonic threshold switching device

A self-selecting memory element based on a method of interconnected ovonic threshold switching device

Ovonic threshold switching-based artificial afferent neurons for thermal in-sensor computing.

Artificial afferent neurons in the sensory nervous system inspired by biology have enormous potential for efficiently perceiving and processing environmental information. However, the previously reported artificial afferent neurons suffer from two prominent challenges: considerable power consumption and limited scalability efficiency. Herein, addressing these challenges, a bioinspired artificial thermal afferent neuron based on a N-doped SiTe ovonic threshold switching (OTS) device is presented for the first time. The engineered OTS device shows remarkable uniformity and robust endurance, ensuring the reliability and efficacy of the artificial afferent neurons. A substantially decreased leakage current of the SiTe OTS device by nitrogen doping results in ultra-low power consumption less than 0.3 nJ per spike for artificial afferent neurons. The inherent temperature response exhibited by N-doped SiTe OTS materials allows us to construct a highly compact artificial thermal afferent neuron over a wide temperature range. An edge detection task is performed to further verify its thermal perceptual computing function. Our work provides an insight into OTS-based artificial afferent neurons for electronic skin and sensory neurorobotics.

Excellent Reliability Characteristics of Ovonic Threshold Switch Device with Higher‐Temperature Forming Technique

Amorphous chalcogenide‐based ovonic threshold switch (OTS) has significant potential as a selector device in high‐density memory arrays. However, to realize the ideal selector device, OTS still needs to enhance its reliability characteristics such as device uniformity and threshold voltage (Vth) drift. Herein, the effect of elevated temperature forming on the selector characteristics of an OTS device is investigated. The findings indicate that the reliability performance of the device significantly improves when the forming process is conducted at higher temperature. In particular, it shows excellent Vth uniformity (σ = 32 mV) and a notable enhancement in the Vth drift characteristics. Based on trap analysis at cryogenic temperature (77 K), it is confirmed that improved reliability characteristics can be attributed to the stability of the activated trap formed by the HT forming process.

Improved Electrical Characteristics of Field Effect Transistors with GeSeTe-Based Ovonic Threshold Switching Devices.

Hyper-field effect transistors (hyper-FETs) are crucial in the development of low-power logic devices. With the increasing significance of power consumption and energy efficiency, conventional logic devices can no longer achieve the required performance and low-power operation. Next-generation logic devices are designed based on complementary metal-oxide-semiconductor circuits, and the subthreshold swing of existing metal-oxide semiconductor field effect transistors (MOSFETs) cannot be reduced below 60 mV/dec at room temperature owing to the thermionic carrier injection mechanism in the source region. Therefore, new devices must be developed to overcome these limitations. In this study, we present a novel threshold switch (TS) material, which can be applied to logic devices by employing ovonic threshold switch (OTS) materials, failure control of insulator-metal transition materials, and structural optimization. The proposed TS material is connected to a FET device to evaluate its performance. The results demonstrate that commercial transistors connected in series with GeSeTe-based OTS devices exhibit significantly lower subthreshold swing values, high on/off current ratios, and high durability of up to 108.

Improving the GeAsSe Ovonic Threshold Switching Characteristics by Carbon Buffer Layers for Ultralow Leakage Current (∼0.4 nA) and Low Drift Characteristics

Volatile Ovonic threshold switching (OTS) selectors have been regarded as the critical component of highly integrated three-dimensional (3D) cross-point array nonvolatile memory systems. However, relatively high leakage current hinders the further reduction of power consumption in the crossbar array. In addition, the threshold voltage drift phenomenon hinders the improvement of device reliability. Utilizing the buffer layer can effectively reduce the interaction between electrodes and the active layer in the cross-point architecture. Here, it manifests that leakage current can be reduced to ∼0.4 nA with a 5 nm thick amorphous carbon layer as a buffer layer in the GeAsSe-based OTS device, where the carbon layer stabilizes the composition of GeAsSe during the electrical switching cycles. It is also found that the carbon layer leads to a lower threshold voltage drift (35.6 mv/dec) and excellent endurance (>109 cycles with ∼0.4 nA ON-state current). The conduction mechanism analysis demonstrates that the inhibition of the carbon layer on drift originates from the high barrier height from delocalized states transformed into localized states. This work clearly demonstrates the role of the carbon layer and facilitates future 3D crossbar-storage technology applications.

Atomic-scale investigation on endurance mechanism of the GeTex-based OTS device by Si doping

The Ovonic Threshold Switching (OTS) devices have attracted great attention as a candidate for suppressing leakage current on 3D-PCM. However, the theories about improving the endurance of OTS devices are still lacking. Based on the theoretical study, we provide a strategic guide for developing high-endurance Si-doped GeTex devices efficiently. The contribution of Si doping on the bonding properties and atomic structure is systematically investigated by first-principal calculation. As we predicted, the strong Si–Te bonds, increasing tetrahedrons, and fewer 4-fold rings are found in the a-GTSi structure, where most of the atoms obey the “8-N” rules and transform toward 4-coordination to stiff the disordered structure. All these structural characteristics confirm that Si atoms improve amorphous stability. With the added Si atoms, the XPS identifies that Si–Te bonds and increasing numbers of the Ge–Te bonds are present in the Si: GeTex film. Experimentally, the device performs excellent endurance (∼107 cycles), which is well in agreement with the theory. This work provided an efficient approach to developing high-performance OTS devices.

A HydroDynamic Model for Trap-Assisted Tunneling Conduction in Ovonic Devices

Electrical conduction in ovonic threshold switching (OTS) devices is described by introducing a new physical model where the multiphonon trap-assisted tunneling (TAT) is coupled to a hydrodynamic theory. Static and transient electrical responses from GexSe <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}_{{1}-{x}}$ </tex-math></inline-formula> experimental devices are reproduced, outlining the role played by the material properties like mobility gap and defects in tuning the OTS performances. A clear physical interpretation of the mechanisms ruling the different OTS conduction regimes (off, threshold, on) is presented. A nanoscopic picture of the processes featuring the carrier transport is also given. The impact of geometry, temperature, and material modifications on device performance is discussed providing physical insight into the optimization of OTS devices.

Locally formed conductive filaments in an amorphous Ga2Te3 ovonic threshold switching device

Ovonic threshold switching (OTS) selector devices based on chalcogenide materials are promising candidates for addressing the sneak current in high-density cross-point array structures owing to their high selectivity, high endurance, and fast switching speed. However, the OTS mechanism remains controversial and needs to be clarified. In this study, the formation of local conductive filaments (CFs) during threshold switching in an amorphous Ga2Te3 OTS selector device was investigated by electrical measurements and conductive-atomic force microscopy (C-AFM). The amorphous Ga2Te3 OTS selector device requires a forming process before the threshold switching processes. In addition, the off-current density (JOFF) was dependent on the area of the bottom electrode. The difference between the threshold voltage (VTH) and the hold voltage (VH) increased as the applied higher electric field increased. The drift of VTH (VTH drift) depended on the relaxation time and measurement temperature. The requirements of the forming process, area dependence of the JOFF, the difference between the VTH−VH with the applied electric field, and VTH drift are expected to depend on locally formed CFs. In addition, the analysis of the C-AFM results strongly supports the formation of local CFs during threshold switching in an amorphous Ga2Te3 OTS selector device. The understanding of OTS behavior uncovered in this study may provide guidance for improving the characteristics of and designing materials for future applications of OTS selector devices.

Inside the ovonic threshold switching (OTS) device based on GeSbSeN: Structural analysis under electrical and thermal stress

In this article, we present the structural investigation by Raman spectroscopy of GeSbSeN ovonic threshold switching (OTS) material once integrated in selector devices featuring a top electrode based on a transparent and conductive indium tin oxide layer. The devices are characterized by standard electrical protocols, and the structural evolution of the material is investigated after several switching operations. The results are correlated with the spectra obtained from blanket samples annealed at increasing temperature and are supported by XRD and TEM analyses. We establish a link between the evolution of the material structure with the annealing process and the device behavior along cycling, bringing important advancement in the understanding of the switching mechanism and of the origin of the failure in OTS devices.

Controllable On-Resistance and its Thermal-Induced Channel Expansive Model in Ovonic Threshold Switch Selector

Ovonic threshold switch (OTS) selector is a key enabler for developing high-density nonvolatile memory with crossbar arrays. In this work, we investigated the resistance characteristics of the OTS device at the ON state, which is pivotal in the integration of the OTS and the memory element. The results indicate that the ON-resistance is not a constant value in the OTS device and is uncorrelated with the device areas but dependent on the real-time power applied on the device that is at ON state. Then, we proposed a thermal-induced compact model to describe the ON-resistance in the OTS device, referring to the simulation of the expansive conductive channel in OTS. The proposed model matches well with the measured ON-resistances with different conditions in the study. Our results contribute to further understanding of the threshold switching mechanism and selecting appropriate operation conditions for the OTS selector integrated in the high-density memory array.

Ge Content Optimization in Ge(SbSe)N OTS Materials for Selector Applications

In this article, we investigate the influence of germanium content in GeSbSeN-based ovonic threshold switching (OTS) selector devices. We performed physico-chemical analyses on five different Gex(SbSe)1–xN alloys to understand how the germanium content influences the material structure and its integrity once submitted to temperatures up to 400 °C. Thanks to the electrical characterization of Gex(SbSe)1–xN OTS devices, we analyze the evolution of the electrical parameters along cycling up to 108 cycles and before and after annealing at 400 °C. Cycle -to -cycle variability and drift phenomenon are also investigated. Finally, we demonstrate how Ge content should be properly tuned to improve the thermal stability of the alloy without affecting the leakage current and the electrical parameters’ variability.

Monolithic TCAD simulation of phase-change memory (PCM/PRAM) + Ovonic Threshold Switch (OTS) selector device

Owing to the increasing interest in the commercialization of phase-change memory (PCM) devices, a number of TCAD models have been developed for their simulation. These models formulate the melting, amorphization and crystallization of phase-change materials as well as their extreme conductivity dependence on both electric field and temperature into a set of self-consistently-solved thermoelectric and phase-field partial-differential equations. However, demonstrations of the ability of such models to match actual experimental results are rare. In addition, such PCM devices also require a so-called selector device – such as an Ovonic Threshold Switching (OTS) device – in series for proper memory operation. However, monolithic simulation of both the PCM and OTS selector device in a single simulation is largely absent from the literature, despite its potential value for material- and design-space explorations. It is the goal of this work to first characterize a PCM device in isolation against experimental data, then to demonstrate the qualitative behavior of a simulated OTS device in isolation and finally to perform a single monolithic simulation of the PCM + OTS device within the confines of a commercially available TCAD solver: GTS Framework.

Screening Switching Materials with Low Leakage Current and High Thermal Stability for Neuromorphic Computing

AbstractNeuromorphic computing implemented with spiking neural networks is an energy efficient computing paradigm to break through the Von Neumann bottleneck in the future. Ovonic threshold switching (OTS) selector is considered to be a promising spiking neuron candidate. As ≈1011 artificial neurons are needed for brain‐inspired computing, leakage current of OTS devices would waste enormous power. OTS devices with ultralow leakage current are deeply desired. Since the leakage current is closely related to the bandgap of OTS materials and only the amorphous one shows the volatile switching property, here binary gallium sulfide (GaS), characterized by 2.53 eV large band‐gap and ≈550 °C high crystallization temperature is singled out, as the most appealing OTS material. High‐density trap states, an essential prerequisite of the OTS material, are detected in the amorphous GaS. Indeed, the OTS behaviors of the GaS‐based device are directly observed. The simple OTS device could provide 21.23 MA cm−2 large ON current density, and ≈10−8 A low OFF current and ≤10 ns switching speed. Furthermore, it is also used to build GaS‐based leakage integrate and fire neurons for future neuromorphic computing. This study provides a new idea for material design before device preparation and takes a neuron as an example.

Ovonic threshold switching device and its application to logic gate function and steep slope in field effect transistors

We report the poly(p-vinylphenol) (PVP)-based ovonic threshold switching devices. By optimizing the effective device area of the PVP-based device, low threshold voltage and low threshold voltage variation were achieved. By changing the compliance current to affect the strength of the conductive filaments, the threshold switching characteristics of the devices were tested at four different compliance currents of 1 × 10−4 A, 5 × 10−5 A, 1 × 10−5 A, 5 × 10−6, the results show that the lower the limiting current, the lower the on state current. The PVP-based device is used in the design of logic gates to realize the function of “AND” and “OR”. Furthermore, a steep slope FET was realized by connecting the PVP based device in series to the gate electrode of the graphene oxide-field effect transistor. The proposed device achieves an extremely low subthreshold swing of 23.7 mV/decade and a high on-off ratio of∼105 due to the steep switching of the threshold switching device at the gate region.

Enhanced Switching Characteristics of an Ovonic Threshold Switching Device With an Ultra-Thin MgO Interfacial Layer

In this study, the effect of a MgO interfacial layer on the electrical characteristics of an ovonic threshold switching (OTS) device was investigated. Compared to the control OTS device, the device with the MgO interfacial layer showed excellent switching uniformity and ultralow off-state leakage current (<inline-formula> <tex-math notation="LaTeX">$\sim 440$ </tex-math></inline-formula>pA) without noticeable changes in the electrical forming process. By extracting the activation energy before and after the forming process, we confirmed that the enhanced switching characteristics were obtained by reducing the interaction between the OTS film and electrodes. Furthermore, the 1S-1R operation with TaO_x-based resistive random access memory (ReRAM) showed successful suppression of the leakage current, which is suitable for a cross-point memory array application.

Epitaxial growth and optical band gap variation of ultrathin ZnTe films

Zinc telluride (ZnTe) has attracted interests for its semiconducting, optoelectronic, and electrical switching properties. However, the growth mechanism of ultrathin epitaxial films is not well established. Here we present a systematic study of the growth ultrathin ZnTe films on GaAs (001) by molecular-beam epitaxy. In situ reflection high-energy electron diffraction and synchrotron based high-resolution X-ray diffraction showed that both surface atomic ordering and single crystalline phase aligned to the substrate orientation with small variation of c-axis lattice in the ultrathin films. While the deviation of chemical compositions depended on the growth conditions, information on the variation of the band gap and in-gap states was obtained through spectroscopic ellipsometry analysis. Our study showed that single crystal ZnTe films can serve as a model system in the development of Ovonic threshold switching devices for cross-point device applications.

A Discharge-Path-Based Sensing Circuit With OTS Snapback Current Protection for Phase Change Memories

A discharge-path-based sensing circuit is proposed to reduce the damage caused by an ovonic threshold switch (OTS) snapback current to a phase-change memory (PCM). OTS devices are used as access devices (selectors) in most PCM systems to increase the sensitivity and resolve the leakage current problem that occurs during sensing of the PCM cell. Snapback current, which occurs during an OTS phase change by using the OTS device, causes damage to the PCM device and deteriorates the read performance; thus, this study proposes a discharge path circuit as a new sensing method to reduce the damage inflicted on the PCM. In addition, a gate-coupled PMOS (GCPMOS) and a current mirror using a feed-forward technique are designed to reduce the peak value of the energy applied to the PCM. The discharge path circuit, GCPMOS, and the current mirror are designed in a 180-nm CMOS process and occupy an area of 0.19-mm2. The measurement results after fabrication show that, compared to the conventional sense amplifier based scheme, the discharge path circuit reduces the amount of energy applied to the PCM by 21.8%, and the discharge path circuit with the GCPMOS reduces the total energy consumption by 27.7%. Furthermore, the discharge path circuit with a feed-forward current mirror reduces the initial peak level of the PCM current by 10.1% and the total energy consumption by 34.6%. The proposed sensing circuit is the first snapback protection circuit reported to the public domain.

Effect of Stochastic Resonance on Classification Accuracy of Neural Networks Utilizing Inherent Stochasticity in Threshold Voltage of Ovonic Threshold Switching Device

In this study, stochastic resonance (SR) exploits the inherent stochastic characteristics of the OTS threshold voltage to enhance the inference performance of neural networks. First, the threshold switching of the OTS device is characterized, and a signal detection using an OTS device is proposed. Next, we investigate the impact of stochasticity in the threshold voltage on detecting weak signals in the SR system. Finally, by evaluating the inference performance of the artificial neural network, we confirm that the inherent stochasticity can effectively restore the degraded MNIST image in poor visibility conditions in the OTS device. As a result, the recognition accuracy was improved from 10.28% to 95.78% when the stochasticity characteristic was reflected. These results show that stochasticity in the device can improve system performance.