Ovonic Threshold Switching Selector Research Articles

Disclosing the basis of the ratio of the firing voltage to the threshold voltage in Ovonic Threshold Switching selectors: Urbach energy

Disclosing the basis of the ratio of the firing voltage to the threshold voltage in Ovonic Threshold Switching selectors: Urbach energy

The role of arsenic in the operation of sulfur-based electrical threshold switches

Arsenic is an essential dopant in conventional silicon-based semiconductors and emerging phase-change memory (PCM), yet the detailed functional mechanism is still lacking in the latter. Here, we fabricate chalcogenide-based ovonic threshold switching (OTS) selectors, which are key units for suppressing sneak currents in 3D PCM arrays, with various As concentrations. We discovered that incorporation of As into GeS brings >100 °C increase in crystallization temperature, remarkably improving the switching repeatability and prolonging the device lifetime. These benefits arise from strengthened As-S bonds and sluggish atomic migration after As incorporation, which reduces the leakage current by more than an order of magnitude and significantly suppresses the operational voltage drift, ultimately enabling a back-end-of-line-compatible OTS selector with >12 MA/cm2 on-current, ~10 ns speed, and a lifetime approaching 1010 cycles after 450 °C annealing. These findings allow the precise performance control of GeSAs-based OTS materials for high-density 3D PCM applications.

Unveiling the structure and electronic characteristics of amorphous GeS for high performance threshold switching

Ovonic threshold switching selectors are widely studied owing to the essential application in high density phase-change memory. Amorphous GeS is proposed as a potential candidate for the excellent performance. However, the knowledge of amorphous GeS is still insufficient up to date. Here, we have studied the structure and electronic characteristics of GeS in the amorphization process, by using ab initio molecular dynamics simulations. The results indicate that the amorphous GeS is mainly made up of Ge–S bonds. The Ge- and S-centered clusters are dominantly in the form of octahedral structures in liquid GeS. During the amorphization process, most of Ge-centered clusters become highly coordinated octahedrons while a small number of Ge-centered clusters change to tetrahedrons, and the S-centered clusters deviate from the octahedral structure gradually. In addition, the large bandgap and the relatively small mid-gap states in amorphous GeS lead to a high switching voltage.

Thermally Stable and High-Speed Ge-Te Based Ovonic Threshold Switching Selector With a Ge Intercalated Structure

Thermal stability and switching speed of ovonic threshold switching (OTS) selector are key elements for its high-speed memory application. However, current methods tend to introduce over complicated elements and even toxic elements which will improve fabrication process complexity and be environment unfriendly. In this work, we studied a Ge-Te based OTS selector with Ge intercalated layers inserted in OTS layer. Through Poole-Frenkel model fitting, we found that the Ge intercalated layers introduced energy barrier, reducing device off-current while improving its thermal stability. The optimized device showed a thermal stability improvement of more than 180°C. In the meantime, the optimized device had good switching performance with fast switching speed of 8.2 ns, low off-current of 10 nA, and endurance of 1.3×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> . After a 350°C annealed process, the device performance showed good uniformity and almost no degradation. This work provides experiment guidance of the high thermal stability requirement for OTS selectors.

A refresh operation method for solving thermal stability issues and improving endurance of ovonic threshold switching selectors

A refresh operation method for an OTS selector to ensure a long memory chip life.

Failure mechanism investigation and endurance improvement in Te-rich Ge–Te based ovonic threshold switching selectors

The endurance of ovonic threshold switching (OTS) selectors is a key element for memory application. However, multi-element system for OTS in recent studies will induce element or phase segregation and lead to device failure. Since pure Te based device characterizes relatively high off current, in this work, we studied a Te-rich Ge–Te based OTS selector. We first conducted a failure analysis on Ge–Te based OTS selector. Through first-principles calculations, we found that a relatively larger Ge concentration in the Ge–Te system may lead to a worse device endurance after continuous operation due to the migration of Ge atoms. Experiments further proved that device endurance can be improved more than two orders of magnitude through decreasing Ge concentration and the element segregation is greatly weakened by the composition close to elemental. Finally, a significantly improved endurance of 2 × 1010 was realized in Ge10Te90 based OTS selectors. In the meantime, the Ge10Te90 based OTS selectors show good switching performance and potential for use in memory applications.

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.

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.

Performance Improvement of GeTex-Based Ovonic Threshold Switching Selector by C Doping

In this letter, we fabricated a C-doped GeTe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ovonic threshold switching (OTS) selector in a via-hole structure. The material composition is simple without toxic element doping. Performance of the GeTe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> -based selector is improved to be comparable to commercial GeSe-based selectors through C doping. A selector with advantages of low off-current (2 nA), low threshold voltage (1.26 V) accompanying with a big voltage window (0.55 V), high selectivity ( > 4.2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> ), satisfactory endurance (> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cycles with 2 mA on-current), low cycle-to-cycle V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> variation and fast operating speed (8.5 ns) was fabricated. Besides, selector with self-limited on-current character was found. Next, reasons behind off-current reduction and consistency improvement by using C doping were studied by First-principles calculations.

The enhanced performance of a Si–As–Se ovonic threshold switching selector

Si–As–Se based OTS materials are studied, the increasing content of silicon leads to a larger threshold voltage and smaller off-state current.

Constructing reliable PCM and OTS devices with an interfacial carbon layer

Ovonic memory switching (OMS) and ovonic threshold switching (OTS) are the most attractive characteristics of the chalcogenides, where the former implements nonvolatile data storage in phase change materials (PCM) and the latter enables the high-density 3D integration of PCM. Wide-range temperature variation during device fabrication and electrical operation brings risks of stress and diffusion induced device failure, that urges the improvement of reliability in OTS and OMS devices. In this work, carbon layers with different thickness have been inserted between Ge2Sb2Te5 chalcogenide and TiN electrode, to improve the interface reliability. Carbon layer with optimized thickness can reduce the elements diffusion and lower the peeling risk at the interface. While, excessively thick carbon layers will cause Ge precipitation from the chalcogenide layer, leading to a fragile interface. Moreover, a low power consumption has been achieved in PCM cells with carbon layers, attributed to the blocked heat dissipation at the amorphous carbon layer and the consequential high-power efficiency. The application of carbon layer not only ensures the good reliability in OTS selectors and PCM devices, but also improves the power efficiency.

Ovonic threshold switching selectors for three-dimensional stackable phase-change memory

Abstract

Conformal deposition of GeTe films with tunable Te composition by atomic layer deposition

Atomic layer deposition (ALD) of chalcogenide amorphous films has attracted attention, thanks to its ability to deposit highly conformal and uniform thin films on three-dimensional structures; these film properties are critical for the development of ultrahigh-density integrated electronic circuits such as nanoscale 3D cross-point memory stacks. Control over the composition of chalcogenide compounds is indispensable for fabricating functional electronic devices. In this work, the authors used trichlorogermane (HGeCl3) and bis(trimethylsilyl)telluride [(Me3Si)2Te] as ALD precursors to develop conformal and uniform germanium telluride (GeTe) films at low temperature (60 °C). The growth of GeTe resulted in moderately Ge rich films independently of the ALD precursor injection time and the use of single or discrete (Me3Si)2Te doses. To achieve control over the GeTe film composition, the authors developed a unique ALD of elemental tellurium (Te) using tellurium ethoxide [Te(OEt)4] and (Me3Si)2Te as precursors. Combining GeTe and elemental Te in an ALD supercycle fashion, they obtained GeTex films with a controllable Te content showing a uniform and conformal morphology on high aspect ratio trench structures. A detailed analysis of the variations in Ge and Te contents upon exposure of GeTex films to Te(OEt)4 revealed that an interfacial replacement of Ge atoms to form volatile Ge(OEt)2 takes place and produces a decrease in Ge and an increase in Te content. The ALD studies of GeTe and Te films described in this work provide a facile route for deposition of GeTe with controllable composition. This material system is important for the development of ovonic threshold switching selectors and phase change memories.

From octahedral structure motif to sub-nanosecond phase transitions in phase change materials for data storage

Phase change random access memory (PCRAM) has been successfully applied in the computer storage architecture, as storage class memory, to bridge the performance gap between DRAM and Flash-based solid-state drive due to its good scalability, 3D-integration ability, fast operation speed and compatible with CMOS technology. Focusing on phase change materials and PCRAM for decades, we have successfully developed 128 Mb embedded PCRAM chips, which can meet the requirements of most embedded systems. 3D Xpoint (3D PCRAM), invented by Intel and Micron, has been regarded as a new breakthrough in the last 25 years since the application of NAND in 1989, which represents state-of-the-art memory technology. This technology has some remarkable features, such as the confined device structure with 20 nm size, the metal crossbar electrodes to reduce the resistance variations in PCRAM arrays, and the ovonic threshold switching selector that can provide a high drive current and a low leakage current. A good understanding of phase change mechanism is of great help to design new phase change materials with fast operation speed, low power consumption and long-lifetime. In this paper, we firstly review the development of PCRAM and different understandings on phase change mechanisms in recent years, and then propose a new view on the mechanism, which is based on the octahedral structure motifs and vacancies. Octahedral structure motifs are generally found in both amorphous and crystalline phase change materials. They are considered to be the basic units during phase transition, which are severely defective in the amorphous phase. These configurations turn into more ordered ones after minor local rearrangements, the growth of which results in the crystallization of rocksalt (RS) phase with a large amount of vacancies in the cation sites. Further driven by thermodynamic driving force, these vacancies move and layer along certain directions; consequently, the metastable RS structure transforms into the stable hexagonal (HEX) structure. Based on our results, we find that reversible phase transition between amorphous phase and RS phase, without further changing into HEX phase, would greatly decrease the required power consumption. Robust octahedra and plenty of vacancies in both amorphous and RS phase, respectively avoiding large atomic rearrangement and providing necessary space, are crucial to achieve the nanosecond or even sub-nanosecond operation of PCRAM.

Nanosecond threshold switching of GeTe6 cells and their potential as selector devices

Time-resolved threshold switching characteristics including transient parameters such as delay time and holding voltage are reported for a nanoscale GeTe6 Ovonic threshold switching (OTS) device. The voltage dependence of the threshold switching process has been studied, revealing switching in less than 5 ns in the fastest case. A constant holding voltage is observed for the different voltage pulses applied, which is an indicative for a stable on state in the amorphous phase. In addition, the potential of GeTe6 devices as OTS selectors is validated.