Te-based chalcogenide materials for selector applications

One-Selector-One-Resistor Integrated Memory Cells Based on Two-Dimensional Heterojunction Memory Selectors

Selectors are critical components for reducing the sneak path leakage currents in emerging resistive random-access memory (RRAM) arrays. Two-dimensional (2D) materials provide a rich choice of materials with van der Waals stacking to form the heterostructure selectors with controllable energy barriers. Here, we experimentally demonstrate 2D-material-based heterostructure selectors with exponential current-voltage (I-V) relationships and integrate them with hafnium oxide (HfOx)-based RRAMs, forming one-selector-one-resistor (1S1R) cells. The multilayer graphene (MG)/tungsten disulfide (WS2)/platinum (Pt) selector contains two asymmetric heterojunctions with different Schottky barriers, which lead to highly nonlinear and asymmetric I-V characteristics. The 2D selectors in 1S1R cells can successfully drive RRAMs, reduce sneak path leakage current by more than 100 times, and provide the set compliance current. The 1S1R cells are further modeled and integrated into both planar and 3D memory arrays, with circuit-level simulations demonstrating that the presence of 2D selectors in large memory arrays can reduce the power consumption by up to 86%, improve the read/write margin by up to 31%, and avoid write failure. Such a platform holds high potential for constructing 3D high-density memories and performing in-memory computing.

A comprehensive review of advanced trends: from artificial synapses to neuromorphic systems with consideration of non-ideal effects

A neuromorphic system is composed of hardware-based artificial neurons and synaptic devices, designed to improve the efficiency of neural computations inspired by energy-efficient and parallel operations of the biological nervous system. A synaptic device-based array can compute vector-matrix multiplication (VMM) with given input voltage signals, as a non-volatile memory device stores the weight information of the neural network in the form of conductance or capacitance. However, unlike software-based neural networks, the neuromorphic system unavoidably exhibits non-ideal characteristics that can have an adverse impact on overall system performance. In this study, the characteristics required for synaptic devices and their importance are discussed, depending on the targeted application. We categorize synaptic devices into two types: conductance-based and capacitance-based, and thoroughly explore the operations and characteristics of each device. The array structure according to the device structure and the VMM operation mechanism of each structure are analyzed, including recent advances in array-level implementation of synaptic devices. Furthermore, we reviewed studies to minimize the effect of hardware non-idealities, which degrades the performance of hardware neural networks. These studies introduce techniques in hardware and signal engineering, as well as software-hardware co-optimization, to address these non-idealities through compensation approaches.

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.

Chalcogenide Ovonic Threshold Switching Selector

Today's explosion of data urgently requires memory technologies capable of storing large volumes of data in shorter time frames, a feat unattainable with Flash or DRAM. Intel Optane, commonly referred to as three-dimensional phase change memory, stands out as one of the most promising candidates. The Optane with cross-point architecture is constructed through layering a storage element and a selector known as the ovonic threshold switch (OTS). The OTS device, which employs chalcogenide film, has thereby gathered increased attention in recent years. In this paper, we begin by providing a brief introduction to the discovery process of the OTS phenomenon. Subsequently, we summarize the key electrical parameters of OTS devices and delve into recent explorations of OTS materials, which are categorized as Se-based, Te-based, and S-based material systems. Furthermore, we discuss various models for the OTS switching mechanism, including field-induced nucleation model, as well as several carrier injection models. Additionally, we review the progress and innovations in OTS mechanism research. Finally, we highlight the successful application of OTS devices in three-dimensional high-density memory and offer insights into their promising performance and extensive prospects in emerging applications, such as self-selecting memory and neuromorphic computing.

Mechanism and Impact of Bipolar Current Voltage Asymmetry in Computational Phase-Change Memory

Nanoscale resistive memory devices are being explored for neuromorphic and in-memory computing. However, non-ideal device characteristics of read noise and resistance drift pose significant challenges to the achievable computational precision. Here, it is shown that there is an additional non-ideality that can impact computational precision, namely the bias-polarity-dependent current flow. Using phase-change memory (PCM) as a model system, it is shown that this "current-voltage" non-ideality arises both from the material and geometrical properties of the devices. Further, we discuss the detrimental effects of such bipolar asymmetry on in-memory matrix-vector multiply (MVM) operations and provide a scheme to compensate for it.

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.

Thermally stable threshold selector based on CuAg alloy for energy-efficient memory and neuromorphic computing applications

As a promising candidate for high-density data storage and neuromorphic computing, cross-point memory arrays provide a platform to overcome the von Neumann bottleneck and accelerate neural network computation. In order to suppress the sneak-path current problem that limits their scalability and read accuracy, a two-terminal selector can be integrated at each cross-point to form the one-selector-one-memristor (1S1R) stack. In this work, we demonstrate a CuAg alloy-based, thermally stable and electroforming-free selector device with tunable threshold voltage and over 7 orders of magnitude ON/OFF ratio. A vertically stacked 64 × 64 1S1R cross-point array is further implemented by integrating the selector with SiO₂-based memristors. The 1S1R devices exhibit extremely low leakage currents and proper switching characteristics, which are suitable for both storage class memory and synaptic weight storage. Finally, a selector-based leaky integrate-and-fire neuron is designed and experimentally implemented, which expands the application prospect of CuAg alloy selectors from synapses to neurons.

Great Potential of Si-Te Ovonic Threshold Selector in Electrical Performance and Scalability

The selector is an indispensable section of the phase change memory (PCM) chip, where it not only suppresses the crosstalk, but also provides high on-current to melt the incorporated phase change material. In fact, the ovonic threshold switching (OTS) selector is utilized in 3D stacking PCM chips by virtue of its high scalability and driving capability. In this paper, the influence of Si concentration on the electrical properties of Si-Te OTS materials is studied; the threshold voltage and leakage current remain basically unchanged with the decrease in electrode diameter. Meanwhile, the on-current density (J_on) increases significantly as the device is scaling down, and 25 MA/cm² on-current density is achieved in the 60-nm SiTe device. In addition, we also determine the state of the Si-Te OTS layer and preliminarily obtain the approximate band structure, from which we infer that the conduction mechanism conforms to the Poole-Frenkel (PF) model.

Highly Reliable Ovonic Threshold Switch with TiN/GeTe/TiN Structure

A new architecture has become necessary owing to the power consumption and latency problems of the von Neumann architecture. A neuromorphic memory system is a promising candidate for the new system as it has the potential to process large amounts of digital information. A crossbar array (CA), which consists of a selector and a resistor, is the basic building block for the new system. Despite the excellent prospects of crossbar arrays, the biggest obstacle for them is sneak current, which can cause a misreading between the adjacent memory cells, thus resulting in a misoperation in the arrays. The chalcogenide-based ovonic threshold switch (OTS) is a powerful selector with highly nonlinear I-V characteristics that can be used to address the sneak current problem. In this study, we evaluated the electrical characteristics of an OTS with a TiN/GeTe/TiN structure. This device shows nonlinear DC I-V characteristics, an excellent endurance of up to 10⁹ in the burst read measurement, and a stable threshold voltage below 15 mV/dec. In addition, at temperatures below 300 °C, the device exhibits good thermal stability and retains an amorphous structure, which is a strong indication of the aforementioned electrical characteristics.

Highly Tunable Lateral Homojunction Formed in Two-Dimensional Layered CuInP2S6 via In-Plane Ionic Migration

As basic building blocks for next-generation information technologies devices, high-quality p-n junctions based on van der Waals (vdW) materials have attracted widespread interest. Compared to traditional two-dimensional (2D) heterojunction diodes, the emerging homojunctions are more attractive owing to their intrinsic advantages, such as continuous band alignments and smaller carrier trapping. Here, utilizing the long-range migration of Cu+ ions under an in-plane electric field, a lateral p-n homojunction was constructed in the 2D layered copper indium thiophosphate (CIPS). The symmetric Au/CIPS/Au devices demonstrate an electric-field-driven resistance switching (RS) accompanied by a rectification behavior without any gate control. Moreover, such rectification behavior can be continuously modulated by poling voltage. We deduce that the reversable rectifying RS behavior is governed by the effective lateral build-in potential and the change of the interfacial barrier during the poling process. Furthermore, the CIPS p-n homojuction is evidenced by the photovoltaic effect, with the spectral response extending up to the visible region due to the better photogenerated carrier separation efficiency. Therefore, this work provides a facile route to fabricate homojunctions through electric-field-driven ionic migration.

Low-voltage ultrafast nonvolatile memory via direct charge injection through a threshold resistive-switching layer

The explosion in demand for massive data processing and storage requires revolutionary memory technologies featuring ultrahigh speed, ultralong retention, ultrahigh capacity and ultralow energy consumption. Although a breakthrough in ultrafast floating-gate memory has been achieved very recently, it still suffers a high operation voltage (tens of volts) due to the Fowler-Nordheim tunnelling mechanism. It is still a great challenge to realize ultrafast nonvolatile storage with low operation voltage. Here we propose a floating-gate memory with a structure of MoS2/hBN/MoS2/graphdiyne oxide/WSe2, in which a threshold switching layer, graphdiyne oxide, instead of a dielectric blocking layer in conventional floating-gate memories, is used to connect the floating gate and control gate. The volatile threshold switching characteristic of graphdiyne oxide allows the direct charge injection from control gate to floating gate by applying a nanosecond voltage pulse (20 ns) with low magnitude (2 V), and restricts the injected charges in floating gate for a long-term retention (10 years) after the pulse. The high operation speed and low voltage endow the device with an ultralow energy consumption of 10 fJ. These results demonstrate a new strategy to develop next-generation high-speed low-energy nonvolatile memory.

Elemental electrical switch enabling phase segregation-free operation

[Figure: see text].

Electronic, optical and thermoelectric properties of a novel two-dimensional SbXY (X = Se, Te; Y = Br, I) family: ab initio perspective

Recent developments in the synthesis of highly crystalline ultrathin BiTeX (X = Br, Cl) structures [Debarati Hajra et al., ACS Nano 14, 15626 (2020)] have led to the exploration of the atomic structure, dynamical stability, and electronic, optical, and thermoelectric properties of SbXY (X = Se, Te; Y = Br, I) monolayers via density functional calculations. The calculated phonon spectrum, elastic stability conditions, and cohesive energy verified the stability of the studied SbXY monolayers. The mechanical properties reveal that all studied monolayers are stable and brittle. Based on PBE (PBE + SOC) functional calculations, the SbXY monolayers are semiconductors with indirect bandgaps. The calculated bandgaps using HSE (HSE + SOC) for SbSeBr, SbSeI, SbTeBr, and SbTeI monolayers are between 1.45 and 1.91 eV, which are appealing for applications in nanoelectronic devices. The signature of the Rashba effect appears in the SbXY monolayer. The SbXY monolayers are visible-light active. Hole doping can be an efficient way to increase the electricity production of SbXY monolayers from waste heat energy. This study suggests that SbXY (X = Se, Te; Y = Br, I) monolayers represent promising new electronic, optical, and energy conversion systems.

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 and optical properties of amorphous Si-Ge-Te 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.

Ultrathin Three-Monolayer Tunneling Memory Selectors

High-density memory arrays require selector devices, which enable selection of a specific memory cell within a memory array by suppressing leakage current through unselected cells. Such selector devices must have highly nonlinear current-voltage characteristics and excellent endurance; thus selectors based on a tunneling mechanism present advantages over those based on the physical motion of atoms or ions. Here, we use two-dimensional (2D) materials to build an ultrathin (three-monolayer-thick) tunneling-based memory selector. Using a sandwich of h-BN, MoS₂, and h-BN monolayers leads to an "H-shaped" energy barrier in the middle of the heterojunction, which nonlinearly modulates the tunneling current when the external voltage is varied. We experimentally demonstrate that tuning the MoS₂ Fermi level can improve the device nonlinearity from 10 to 25. These results provide a fundamental understanding of the tunneling process through atomically thin 2D heterojunctions and lay the foundation for developing high endurance selectors with 2D heterojunctions, potentially enabling high-density non-volatile memory systems.

Ab initio prediction of semiconductivity in a novel two-dimensional Sb2X3 (X= S, Se, Te) monolayers with orthorhombic structure

[Formula: see text] and [Formula: see text] are well-known layered bulk structures with weak van der Waals interactions. In this work we explore the atomic lattice, dynamical stability, electronic and optical properties of [Formula: see text], [Formula: see text] and [Formula: see text] monolayers using the density functional theory simulations. Molecular dynamics and phonon dispersion results show the desirable thermal and dynamical stability of studied nanosheets. On the basis of HSE06 and PBE/GGA functionals, we show that all the considered novel monolayers are semiconductors. Using the HSE06 functional the electronic bandgap of [Formula: see text], [Formula: see text] and [Formula: see text] monolayers are predicted to be 2.15, 1.35 and 1.37 eV, respectively. Optical simulations show that the first absorption coefficient peak for [Formula: see text], [Formula: see text] and [Formula: see text] monolayers along in-plane polarization is suitable for the absorption of the visible and IR range of light. Interestingly, optically anisotropic character along planar directions can be desirable for polarization-sensitive photodetectors. Furthermore, we systematically investigate the electrical transport properties with combined first-principles and Boltzmann transport theory calculations. At optimal doping concentration, we found the considerable larger power factor values of 2.69, 4.91, and 5.45 for hole-doped [Formula: see text], [Formula: see text], and [Formula: see text], respectively. This study highlights the bright prospect for the application of [Formula: see text], [Formula: see text] and [Formula: see text] nanosheets in novel electronic, optical and energy conversion systems.

Semiconducting Chalcogenide Alloys Based on the (Ge, Sn, Pb) (S, Se, Te) Formula with Outstanding Properties: A First-Principles Calculation Study

Very recently, a new class of the multicationic and -anionic entropy-stabilized chalcogenide alloys based on the (Ge, Sn, Pb) (S, Se, Te) formula has been successfully fabricated and characterized experimentally [Zihao Deng et al., Chem. Mater. 32, 6070 (2020)]. Motivated by the recent experiment, herein, we perform density functional theory-based first-principles calculations in order to investigate the structural, mechanical, electronic, optical, and thermoelectric properties. The calculations of the cohesive energy and elasticity parameters indicate that the alloy is stable. Also, the mechanical study shows that the alloy has a brittle nature. The GeSnPbSSeTe alloy is a semiconductor with a direct band gap of 0.4 eV (0.3 eV using spin-orbit coupling effect). The optical analysis illustrates that the first peak of Im(ε) for the GeSnPbSSeTe alloy along all polarization directions is located in the visible range of the spectrum which renders it a promising material for applications in optical and electronic devices. Interestingly, we find an optically anisotropic character of this system which is highly desirable for the design of polarization-sensitive photodetectors. We have accurately predicted the thermoelectric coefficients and have calculated a large power factor value of 3.7 × 10¹¹ W m^-1 K^-2 s^-1 for p-type. The high p-type power factor is originated from the multiple valleys near the valence band maxima. The anisotropic results of the optical and transport properties are related to the specific tetragonal alloy unit cell.

A Compact Model of Ovonic Threshold Switch Combining Thermal Dissipation Effect

Ovonic threshold switch (OTS) has received great attention in neuromorphic computing due to its support for high-density synapse array as a selector and leaky-integration-firing functions Hodgkin-Huxley neurons. However, there is no simple and complete model for device simulation and integrated circuit design, which hindered application until now. In this work, we developed a compact physical model of OTS based on the Poole-Frenkel effect accompanied by the thermal dissipation effect for the first time. The thermal dissipation effect describes the energy flow between the device and the environment so that the model is more practical. Compared with previous experiments, the numerical results fairly fitted the electrical characteristics, demonstrating the model validity. In addition, the relation of the device performance with material and structure was deduced, which can facilitate optimizing the OTS device. The model will be useful for device design and implemented with high speed for simplicity.

Resistive switching in diamondoid thin films

The electrical transport properties of a thin film of the diamondoid adamantane, deposited on an Au/W substrate, were investigated experimentally. The current I, in applied potential V, from the admantane-thiol/metal heterstructure to a wire lead on its surface exhibited non-symmetric (diode-like) characteristics and a signature of resistive switching (RS), an effect that is valuable to non-volatile memory applications. I(V) follows a hysteresis curve that passes twice through [Formula: see text] linearly, indicating RS between two states with significantly different conductances, possibly due to an exotic mechanism.

Bidirectional-nonlinear threshold switching behaviors and thermally robust stability of ZnTe selectors by nitrogen annealing

Three-dimensional stackable memory frames involving the integration of two-terminal scalable crossbar arrays are expected to meet the demand for high-density memory storage, fast switching speed, and ultra-low power operation. However, two-terminal crossbar arrays introduce an unintended sneak path, which inevitably requires bidirectional nonlinear selectors. In this study, the advanced threshold switching (TS) features of ZnTe chalcogenide material-based selectors provide bidirectional threshold switching behavior, nonlinearity of 104, switching speed of less than 100 ns, and switching endurance of more than 107. In addition, thermally robust ZnTe selectors (up to 400 ℃) can be obtained through the use of nitrogen-annealing treatment. This process can prevent possible phase separation phenomena observed in generic chalcogenide materials during thermal annealing which occurs even at a low temperature of 250 ℃. The possible characteristics of the electrically and thermally advanced TS nature are described by diverse structural and electrical analyses through the Poole-Frankel conduction model.

Reactive ion etching of an ovonic threshold switch (OTS) material using hydrogen-based plasmas for non-volatile phase change memories

Etch characteristics of ovonic threshold switch (OTS) materials composed of Ge-As-Te for a phase-change random access memory (PCRAM) has been investigated using reactive ion etching via hydrogen-based gases such as H2, CH4, NH3, CH4 + H2, and CH4 + NH3. Among the investigated hydrogen-based gases, NH3 showed the highest etching rate of about 0.52 nm s-1, but the formation of nitride compounds and the increased roughness were observed on the OTS surface by nitrogen. The use of other hydrogen-based gases such as CH4 and CH4 + H2 showed the deposition and low OTS etch rate, respectively, due to the presence of carbon in CH4. Even though H2 showed the better etch characteristics due to the no surface residues or compounds on the OTS surface related to carbon or nitrogen in the etch gases, the best OTS etch characteristics such as the second highest etch rate of 0.45 nm s-1, the lowest surface roughness of 0.21 nm, and no surface residues or compounds were observed with CH4 + NH3 due to the removal of carbon and nitrogen on the surface by the formation of volatile CN compounds while maintaining a high hydrogen atomic concentration in the plasma.

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.

Composition-Controlled Atomic Layer Deposition of Phase-Change Memories and Ovonic Threshold Switches with High Performance

Chalcogenide compounds are the main characters in a revolution in electronic memories. These materials are used to produce ultrafast ovonic threshold switches (OTSs) with good selectivity and moderate leakage current and phase-change memories (PCMs) with excellent endurance and short read/write times when compared with state-of-the-art flash-NANDs. The combination of these two electrical elements is used to fabricate nonvolatile memory arrays with a write/access time orders of magnitude shorter than that of state-of-the-art flash-NANDs. These devices have a pivotal role for the advancement of fields such as artificial intelligence, machine learning, and big-data. Chalcogenide films, at the moment, are deposited by using physical vapor deposition (PVD) techniques that allow for fine control over the stoichiometry of solid solutions but fail in providing the conformality required for developing large-memory-capacity integrated 3D structures. Here we present conformal ALD chalcogenide films with control over the composition of germanium, antimony, and tellurium (GST). By developing a technique to grow elemental Te we demonstrate the ability to deposit conformal, smooth, composition-controlled GST films. We present a thorough physical and chemical characterization of the solids and an in-depth electrical test. We demonstrate the ability to produce both OTS and PCM materials. GeTe₄ OTSs exhibit fast switching times of ∼13 ns. Ge₂Sb₂Te₅ ALD PCMs exhibit a wide memory window exceeding two orders of magnitude, short write times (∼100 ns), and a reset current density as low as ∼10⁷ A/cm²-performance matching or improving upon state-of-the-art PVD PCM devices.

Nanoscale 3D Stackable Ag-Doped HfOx-Based Selector Devices Fabricated through Low-Temperature Hydrogen Annealing

Electrochemical metallization-based threshold switching devices with active metal electrodes have been studied as a selector for high-density resistive random access memory (RRAM) technology in crossbar array architectures. However, these devices are not suitable for integration with three-dimensional (3D) crossbar RRAM arrays due to the difficulty in vertical stacking and/or scaling into the nanometer regime as well as the asymmetric threshold switching behavior and large variation in the operating voltage. Here, we demonstrate bidirectional symmetric threshold switching behaviors from a simple Pt/Ag-doped HfO_x/Pt structure. While fabricating the Pt/Ag-doped HfO_x/Pt film using a 250 nm hole structure, filaments composed of Ag nanoclusters were constructed through a low-temperature (∼200 °C) hydrogen annealing process where the shape of the film in a nanoscale via a hole structure was maintained for integration with 3D stackable crossbar RRAM arrays. Finite Ag filament paths in the HfO_x layer led to uniform device-to-device performances. Moreover, we observed that the hydrogen annealing process reduced the delay time through the reduction of the oxygen vacancies in the HfO_x layer. Consequently, the proposed Pt/Ag-doped HfO_x/Pt-based nanoscale selector devices exhibited excellent performance of high selectivity (∼10⁵), ultralow OFF current (∼10 pA), steep turn-on slope (∼2 mV/decade), and short delay time (3 μs).

Thermally Stable Amorphous Oxide-based Schottky Diodes through Oxygen Vacancy Control at Metal/Oxide Interfaces

Amorphous oxide semiconductor (AOS)-based Schottky diodes have been utilized for selectors in crossbar array memories to improve cell-to-cell uniformity with a low-temperature process. However, thermal instability at interfaces between the AOSs and metal electrodes can be a critical issue for the implementation of reliable Schottky diodes. Under post-fabrication annealing, an excessive redox reaction at the ohmic interface can affect the bulk region of the AOSs, inducing an electrical breakdown of the device. Additionally, structural relaxation (SR) of the AOSs can increase the doping concentration at the Schottky interface, which results in a degradation of the rectifying performance. Here, we improved the thermal stability at AOS/metal interfaces by regulating the oxygen vacancy (V_O) concentration at both sides of the contact. For a stable quasi-ohmic contact, a Cu-Mn alloy was introduced instead of a single component reactive metal. As Mn only takes up O in amorphous In-Ga-Zn-O (a-IGZO), excessive V_O generation in bulk region of a-IGZO can be prevented. At the Schottky interfaces, the barrier characteristics were not degraded by thermal annealing as the Ga concentration in a-IGZO increased. Ga not only reduces the inherent V_O concentration but also retards SR, thereby suppressing tunneling conduction and enhancing the thermal stability of devices.

Towards a 3D GeSbTe phase change memory with integrated selector by non-aqueous electrodeposition

We have recently reported a new method for the electrodeposition of thin film and nanostructured phase change memory (PCM) devices from a single, highly tuneable, non-aqueous electrolyte. The quality of the material was confirmed by phase cycling via electrical pulsed switching of both 100 nm nano-cells and thin film devices. This method potentially allows deposition into extremely small confined cells down to less than 5 nm, 3D lay-outs that require non-line-of-sight techniques, and seamless integration of selector devices. As electrodeposition requires a conducting substrate, the key condition for electronic applications based on this method is the use of patterned metal lines as the working electrode during the electrodeposition process. In this paper, we show the design and fabrication of a 2D passive memory matrix in which the word lines act as the working electrode and nucleation site for the growth of confined cells of Ge-Sb-Te. We will discuss the precursor requirement for deposition from non-aqueous, weakly coordinating solvents, show the transmission electron microscopy analysis of the electrodeposition growth process and elemental distribution in the deposits, and show the fabrication and characterisation of the Ge-Sb-Te memory matrix.

Materials Selection and Mechanism of Non-linear Conduction in Chalcogenide Selector Devices

The electronic structure and conduction mechanism of chalcogenide-based Ovonic threshold switches (OTS) used as selectors in cross-point memory arrays is derived from density functional calculations and quasi-Fermi level models. The switching mechanism in OTS is primarily electronic. This uses a specific electronic structure, with a wide tail of localized states below the conduction band edge. In amorphous GeSe_2−x the conduction band consists of Ge-Se σ*states with a low effective mass, and with a broad tail of localized Ge-Ge σ* states below this band edge. This leads to the OTS behavior. At high fields the electron quasi-E_F moves up through these tail states, lowering the conductivity activation energy, and giving the non-linear switching process. The 4:2 coordinated GeSe_2−x based alloys are the most favorable OTS material because they have the correct network connectivity to give a high electron mobility and lack of crystallization, a favorable band structure to produce the non-linear conduction, an optimum band gap, and with nitrogen or carbon alloying, a sufficiently low off-current.

Chemical Vapor-Deposited Vanadium Pentoxide Nanosheets with Highly Stable and Low Switching Voltages for Effective Selector Devices

Recently, attempts to overcome the physical limits of memory devices have led to the development of promising materials and architectures for next-generation memory technology. The selector device is one of the essential ingredients of high-density stacked memory systems. However, complicated constituent deposition conditions and thermal degradation are problematic, even with effective selector device materials. Herein, we demonstrate the highly stable and low-threshold voltages of vanadium pentoxide (V₂O₅) nanosheets synthesized by facile chemical vapor deposition, which have not been previously reported on the threshold switching (TS) properties. The electrons occupying trap sites in poly-crystalline V₂O₅ nanosheet contribute to the perfectly symmetric TS feature at the bias polarity and low-threshold voltages in V₂O₅, confirmed by high-resolution transmission electron microscopy measurements. Furthermore, we find an additional PdO interlayer in V₂O₅ nanodevices connected with a Pd/Au electrode after thermal annealing treatment. The PdO interlayer decreases the threshold voltages, and the I_on/ I_off ratio increases because of the increased trap density of V₂O₅. These studies provide insights into V₂O₅ switching characteristics, which can support low power consumption in nonvolatile memory devices.

Ag:SiO xN y-Based Bilayer ReRAM Structure with Self-Limiting Bidirectional Threshold Switching Characteristics for Cross-Point Array Application

We fabricate a Pt/Ag:SiO _xN _y/Ti programmable metallization cell exhibiting bidirectional threshold switching (TS) or nonvolatile resistive switching (RS) characteristics through a simple thermal annealing process. The cell composed of pristine Ag:SiO _xN _y layers showed self-limiting TS characteristics with high selectivity and extremely low OFF currents, whereas the same cell showed typical RS characteristics after thermal annealing at 250 °C. The operating mechanism was investigated using scanning transmission electron microscopy and X-ray photoelectron spectroscopy. Next, a Ag:SiO _xN _y-based one selector-one resistor device was fabricated by employing both TS and RS characteristics in a single cell, which exhibited excellent self-rectifying memory performance.

A C-Te-based binary OTS device exhibiting excellent performance and high thermal stability for selector application

In this letter, we demonstrate a new binary ovonic threshold switching (OTS) selector device scalable down to ø30 nm based on C-Te. Our proposed selector device exhibits outstanding performance such as a high switching ratio (I_on/I_off > 10⁵), an extremely low off-current (∼1 nA), an extremely fast operating speed of <10 ns (transition time of <2 ns and delay time of <8 ns), high endurance (10⁹), and high thermal stability (>450 °C). The observed high thermal stability is caused by the relatively small atomic size of C, compared to Te, which can effectively suppress the segregation and crystallization of Te in the OTS film. Furthermore, to confirm the functionality of the selector in a crossbar array, we evaluated a 1S-1R device by integrating our OTS device with a ReRAM (resistive random access memory) device. The 1S-1R integrated device exhibits a successful suppression of leakage current at the half-selected cell and shows an excellent read-out margin (>2¹² word lines) in a fast read operation.

High speed chalcogenide glass electrochemical metallization cells with various active metals

We fabricated electrochemical metallization cells using a GaLaSO solid electrolyte, an InSnO inactive electrode and active electrodes consisting of various metals (Cu, Ag, Fe, Cu, Mo, Al). Devices with Ag and Cu active metals showed consistent and repeatable resistive switching behaviour, and had a retention of 3 and >43 days, respectively; both had switching speeds of <5 ns. Devices with Cr and Fe active metals displayed incomplete or intermittent resistive switching, and devices with Mo and Al active electrodes displayed no resistive switching ability. Deeper penetration of the active metal into the GaLaSO layer resulted in greater resistive switching ability of the cell. The off-state resistivity was greater for more reactive active metals which may be due to a thicker intermediate layer.