Ovonic Threshold Switching Research Articles

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.

Three-Terminal Ovonic Threshold Switch (3T-OTS) with Tunable Threshold Voltage for Versatile Artificial Sensory Neurons.

Inspired by information processing in biological systems, sensor-combined edge-computing systems attract attention requesting artificial sensory neurons as essential ingredients. Here, we introduce a simple and versatile structure of artificial sensory neurons based on a novel three-terminal Ovonic threshold switch (3T-OTS), which features an electrically controllable threshold voltage (Vth). Combined with a sensor driving an output voltage, this 3T-OTS generates spikes with a frequency depending on an external stimulus. As a proof of concept, we have built an artificial retinal ganglion cell (RGC) by combining a 3T-OTS and a photodiode. Furthermore, this artificial RGC is combined with the reservoir-computing technique to perform a classification of chest X-ray images for normal, viral pneumonia, and COVID-19 infections, releasing the recognition accuracy of about 86.5%. These results indicate that the 3T-OTS is highly promising for applications in neuromorphic sensory systems, providing a building block for energy-efficient in-sensor computing devices.

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.

Rational engineering of a switching material for an Ovonic threshold switching (OTS) device with mitigated electroforming

Toward the development of an energy-efficient artificial neuron device, a study of the mechanism of electroforming in OTS and mitigation of the electroforming by doping Sn in GeSe was conducted.

Amorphous Hf–O–Te as a selector via a modified conduction mechanism by Te content control

The resistive switching of an Ovonic threshold switch (OTS) material is characterized by Poole-Frenkel (PF) conduction and atomic rearrangement originating from chalcogen defects; thus, most OTS materials contain Se and/or Te. In addition to these chalcogen elements, As is included to form rigid amorphous networks. However, since As and Se are toxic, the development of As- and Se-free OTS materials is strongly desired. To realize As-free OTS materials that exhibit a comparable or even superior performance, a new strategy for material development should be established. In this study, we aimed to add Te to a HfO2 insulator to convert the electric conduction mechanism to PF conduction for realizing a selector function. The optical and electrical characteristics of Hf–O–Te ternary amorphous films with various Te contents were investigated. By changing the Te content, the optical bandgap was found to be tuned in the range of 0.46–5 eV, concomitantly with the modification of electrical properties. The electrical characteristics of the films demonstrated a strong compositional dependence, and Hf0.24O0.55Te0.21 was found to exhibit PF conduction, resulting in a selector function with a selectivity of approximately two orders of magnitude. These results indicate that the inclusion of a heavy chalcogen, such as Te, is effective in altering the conduction mechanism of transition-metal oxides and realizing a selector function.

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.

Relaxation Oscillation Effect of the Ovonic Threshold Switch on the SET Characteristics of Phase-Change Memory in Cross-Point Structure

In this letter, the relaxation oscillation (RO) effect of the Ovonic Threshold Switch (OTS) on the RESET (amorphous)-to-Set (crystalline) characteristics of Phase-change memory (PCM) is discussed. RO, which happens when the current flowing through the OTS is below hold current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{h}{)}$ </tex-math></inline-formula> , may increase the minimum SET current and the fail bit rate (FBR) of RESET-to-SET transition in the Cross-point structure PCM (XP-PCM). The RO from the single XP-PCM cell was successfully measured using a low capacitance (low-z) probe and a high frequency oscilloscope, and the corresponding current oscillation is calculated from the voltage, discharge time, and write current. The voltage RO was proportionally increased as the applied current was decreased below the hold current of the OTS ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{h}{)}$ </tex-math></inline-formula> . The increasing RO behaviors were well matched with the increasing FBR of RESET-to-SET transition in the 64k bit array. The results indicate that the current oscillation not only makes the current supply for the crystallization unstable to generate SET fail but may even induce the partial amorphization (RESET) due to the series of high current – short pulses during the oscillation.

Capacitance Behavior With Voltage Bias in Phase-Change Memory for Fast Operation

In this work, the time-delay effect on device operation caused by capacitance is studied in terms of phase-change memory (PCM) integrated with an ovonic threshold switch (OTS) selector. The capacitance studied in this work is the intrinsic capacitance associated with capacitive reactance of the PCM itself. The capacitance of the PCM in the amorphous state was measured, and it presents an exponential dependence on voltage bias. Through the simulation model of an OTS-PCM integrated device that considering the measured capacitance behavior, the time-delay characteristics of the integrated device for the SET process with various pulses were investigated. Results indicate that the onset of the threshold switching (TS) is delayed in the PCM but occurs early in the OTS with capacitance in the integrated device. In addition, it is found that the variable capacitance behavior can not only minimize the delay effect, but also accelerate the SET operation under certain conditions in the integrated device. Our results can provide practical guidance for the design of fast operating devices.

Tuning of the thermal stability and ovonic threshold switching properties of GeSe with metallic and non-metallic alloying elements

In order to make 3D crossbar memory architectures viable, selector elements with highly non-linear current–voltage characteristics are required. Ovonic Threshold Switching (OTS) is a highly non-linear phenomenon observed in amorphous chalcogenides, such as GeSe, that shows promise for application in selectors. In this paper, the impact of alloying with metallic (Zr), metalloid (B, Sb), and non-metallic (C, N) elements as a function of their concentration on the thermal stability and switching properties of alloyed GeSe layers is studied. In the case of the thermal stability analysis, the key parameter that is tracked is the crystallization temperature (Tc) of the as-deposited amorphous films since OTS only occurs in amorphous materials. Using a simple metal–insulator–metal type test structure where the bottom electrode is scaled to 6 μm, the OTS properties of the alloyed layers are also compared. The pristine leakage current (Ipris), the first fire voltage (VFF), and the threshold voltage (Vth) were determined using DC and pulsed (AC) measurements. Results indicate that C alloying in combination with sufficiently high nitrogen incorporation can extend the thermal stability above 600 °C with only low dependence on the C content. Among the metallic and metalloid elements, crystallization temperature is strongly dependent on alloying concentration. In general, larger concentrations are needed to obtain a Tc above 400 °C as compared to CN alloying. Electrical characterization indicates strong dependence of the first fire voltage and the leakage current on the metallicity of the alloying element with only small to moderate concentrations required to influence electrical properties.

Arsenic-free ovonic threshold switch for next-generation selectors in 3-dimensional cross-point memories

Ovonic threshold switch (OTS) has been considered for use as a selector in 3-dimensional (3D) cross-point memories. Arsenic-containing materials such as the Ge-As-Si-Te-N system have been proposed for the switching of OTS devices. However, as is toxic to both humans and the environment, such materials are generally avoided. Here, high-performance OTS devices are demonstrated using As-free material, i.e., a Ga-N-Sb-Te layer, which exhibits promising threshold switching characteristics. The Ga-N-Sb-Te thin film was deposited by co-sputtering process at 298 K. The OTS device based on the Ga-N-Sb-Te material exhibited a large ON current density (>10 MA/cm2) with long cycling endurance (>107), thus can be considered a promising candidate for use as a selector for 3D cross-point memories.

Cycling Induced Metastable Degradation in GeSe Ovonic Threshold Switching Selector

Ovonic threshold switching (OTS) selector is a promising candidate to suppress the sneak current paths in emerging memory arrays, but there is still a gap between its performance and the rigorous requirement from memory devices, especially its endurance improvement is hindered by insufficient understanding of the mechanism. In this work, cycling induced degradation of GeSe-based OTS selectors is studied with electrical characterization techniques. The existence of metastable state between the on- and off-state during cycling is observed and statistically analyzed alongside with the gradual off-state leakage current increase. Such metastable degradation may be attributed to the generation of unstable Ge-Ge bonds that might be induced by element segregation, which is also responsible for the higher off-state leakage current in GeSe selectors after large cycling or with higher Ge component. This work provides experimental guidance for optimizing OTS selectors.

Sub-Picosecond Non-Equilibrium States in the Amorphous Phase of GeTe Phase-Change Material Thin Films.

The sub-picosecond response of amorphous germanium telluride thin film to a femtosecond laser excitation is investigated using frequency-domain interferometry and ab initio molecular dynamics. The time-resolved measurement of the surface dynamics reveals a shrinkage of the film with a dielectric properties' response faster than 300 fs. The systematic ab initio molecular dynamics simulations in non-equilibrium conditions allow the atomic configurations to be retrieved for ionic temperature from 300 to 1100 K and width of the electron distribution from 0.001 to 1.0 eV. Local order of the structures is characterized by in-depth analysis of the angle distribution, phonon modes, and pair distribution function, which evidence a transition toward a new amorphous electronic excited state close in bonding/structure to the liquid state. The results shed a new light on the optically highly excited states in chalcogenide materials involved in both important processes: phase-change materials in memory devices and ovonic threshold switching phenomenon induced by a staticfield.

Evaluating Ovonic Threshold Switching Materials with Topological Constraint Theory.

The physical properties of ovonic threshold switching (OTS) materials are of great interest due to the use of OTS materials as selectors in cross-point array nonvolatile memory systems. Here, we show that the topological constraint theory (TCT) of chalcogenide glasses provides a robust framework to describe the physical properties of sputtered thin film OTS materials and electronic devices. Using the mean coordination number (MCN) of an OTS alloy as a comparative metric, we show that changes in data trends from several measurements are signatures of the transition from a floppy to a rigid glass network as described by TCT. This approach provides a means to optimize OTS selector materials for device applications using film-level measurements.

Structural features of chalcogenide glass SiTe: An ovonic threshold switching material

The state-of-the-art phase-change memory is usually composed of ovonic threshold switching (OTS) material and ovonic memory switching (OMS) material for selective and data storage, respectively. OMS materials have been intensely studied, while the knowledge of the OTS mechanism is still inadequate. In this article, we have explored the local structure and electronic property of a simple OTS material, the amorphous (a-) SiTe, by first-principles calculations. The results reveal that most of the atoms in a-SiTe obey the “8-N” rule in contrast to a-GeTe, a well-studied OMS material. 76.5% of Si-centered configurations are in the form of randomly distributed tetrahedral clusters, while Te-centered configurations are relatively disordered without notable conformation. Furthermore, a large number of fivefold rings are found in a-SiTe. All of these structural characteristics lead to the high stability of a-SiTe, prohibiting its crystallization. In addition, the p state of Te also contributes much to the mid-gap states, which may be relevant for OTS behavior. Our findings provide an in-depth understanding of the structural signature and electronic properties of a-SiTe, having important implications for the design and applications of OTS materials.

Extended endurance performance and reduced threshold voltage by doping Si in GeSe-based ovonic threshold switching selectors

Selectors are essential part of the high-density storage architectures to operate the storage device in the crossbar arrays by providing a desired net voltage drop and suppressing undesired current flow. Germanium selenide -based Ovonic Threshold Switching (OTS) selector shows a promising candidate for simple binary composition and high selectivity, however, the high threshold voltage and thermal budget lower than required for Complementary Metal Oxide Semiconductor process are needed to overcome. In this paper, we demonstrate the effect of silicon doping Ge42Se58 thin films on their electrical characteristics of OTS devices and the electronic structure of materials. Si doping allows low threshold voltages ranging from 1.5 to 3 V and improved thermal stability, Si-Ge-Se with Si 13% can cycle steadily more than 5 × 106 cycles. In addition, the optical energy gap of films decreases with increasing Si content. These results, being interpreted as Si atoms modify the bonding type of Ge42Se58 thin film as well as electronic structure includes the energy gap and trap states, suggest an effective method to modulate threshold voltage of the GeSe-based selector by using Si as dopant for various OTS device applications. According to the unified analytical model (Poole-Poole-Frenkel) based on trap-limited conduction, the trap density of Si-Ge-Se thin films in the switching process decreases from 1.29×1017cm−3 to 2.88×1016cm−3with increasing Si content.

Structural and optical properties of amorphous Si\u2013Ge\u2013Te thin films prepared by combinatorial sputtering

The lack of order in amorphous chalcogenides offers them novel properties but also adds increased challenges in the discovery and design of advanced functional materials. The amorphous compositions in the Si–Ge–Te system are of interest for many applications such as optical data storage, optical sensors and Ovonic threshold switches. But an extended exploration of this system is still missing. In this study, magnetron co-sputtering is used for the combinatorial synthesis of thin film libraries, outside the glass formation domain. Compositional, structural and optical properties are investigated and discussed in the framework of topological constraint theory. The materials in the library are classified as stressed-rigid amorphous networks. The bandgap is heavily influenced by the Te content while the near-IR refractive index dependence on Ge concentration shows a minimum, which could be exploited in applications. A transition from a disordered to a more ordered amorphous network at 60 at% Te, is observed. The thermal stability study shows that the formed crystalline phases are dictated by the concentration of Ge and Te. New amorphous compositions in the Si–Ge–Te system were found and their properties explored, thus enabling an informed and rapid material selection and design for applications.

Ovonic threshold switching induced local atomic displacements in amorphous Ge60Se40 film probed via in situ EXAFS under DC electric field

In an effort to verify whether the Ovonic threshold switching in amorphous chalcogenide film is concomitant with any noticeable atomic rearrangements, we analyze EXAFS spectra of amorphous chalcogenide Ge60Se40 (at%) film measured in situ under DC electric field. The in situ EXAFS measurements are performed with specifically prepared specimens, in which 100-nm-thick amorphous Ge60Se40 layer is sandwiched between two metallic electrode layers, all the while a constant DC electric bias being applied. Our EXAFS analysis indicates that only Debye-Waller factor undergoes a noticeable change upon the Ovonic threshold switching, keeping both coordination number and interatomic distance unaltered. The changes in Debye-Waller factors of Ge-Se and Ge-Ge pairs appear to be reversible during the ON/OFF cycle; however, heteropolar Ge-Se pair experiences a conspicuously greater change in its Debye-Waller factor than homopolar Ge-Ge counterpart. Increase of thermal disorder due to Joule heating is insufficient to support this observation, which instead is caused by a preferential increase of static disorder associated with heteropolar Ge-Se bonds in the ON state. The heteropolar Ge-Se pair is supposed to feel the electrical wind force (momentum transfer by conducting electrons) as well as the Coulombic force more strongly than the homopolar Ge-Ge pair.

Study on Hydrogen-Based Reactive Ion Etching of Ovonic Threshold Switch (OTS) Materials for Phase Change Memory Devices

Phase change random access memory (PCRAM) is one of the most industrially applicable memory devices because of its advantages such as non-volatility, high speed, scalability, and low power consumption. Ovonic threshold switch (OTS) materials are used in selector devices in PCRAM circuits and are generally composed of chalcogenide compounds. In this study, etch characteristics of OTS material composed of Ge-As-Te has been investigated using reactive ion etching (RIE) by hydrogen-based gases such as H2, CH4, NH3, etc. Among the investigated hydrogen-based plasmas, NH3 showed the highest etch rate due to the high vapor pressures of the hydrides of the OTS material, but the formation of nitride compound and the increased roughness were observed on the OTS surface. In the case of CH4 plasma, due to the carbon on the OTS surface, a polymer layer was deposited. Even though H2 showed the better surface smoothness due to the no surface residues or compounds on OTS surface related to carbon or nitrogen, the etch rates were lower than NH3. In this presentation, the effects of various hydrogen-based gases on the etch characteristics of OTS material will be discussed in details.

Understanding of forming and switching mechanism using trap distribution model for ovonic threshold switch device

In this study, we investigate the electrical characteristics of a W/Si–Te–As–Ge/W ovonic threshold switch (OTS) under various conditions to reveal the origin of its forming and its effect on the subsequent threshold switching. Our results indicate that the forming of the OTS is a process of generating a metastable filamentary conductive path with a statistical nature, which leaves behind the lowest energy level of the activated trap, thereby significantly affecting the subsequent conduction as well as the dynamics of the threshold switching in the OTS. In addition, we apply a hot forming technique to control the distribution and energy level of the residual traps. By forming at elevated temperatures, the overshoot damage due to abrupt switching can be reduced. As a result, the trap distance and activation energy can be increased, the off current decreases from 54 to 7 nA (1/2 Vth read), and the threshold voltage increases from 1.5 to 1.7 V.