Multi-level phase-change memory with ultralow power consumption and resistance drift

In situ characterization of vacancy ordering in Ge-Sb-Te phase-change memory alloys

Tailoring the degree of structural disorder in Ge-Sb-Te alloys is important for the development of non-volatile phase-change memory and neuro-inspired computing. Upon crystallization from the amorphous phase, these alloys form a cubic rocksalt-like structure with a high content of intrinsic vacancies. Further thermal annealing results in a gradual structural transition towards a layered structure and an insulator-to-metal transition. In this work, we elucidate the atomic-level details of the structural transition in crystalline GeSb2Te4 by in situ high-resolution transmission electron microscopy experiments and ab initio density functional theory calculations, providing a comprehensive real-time and real-space view of the vacancy ordering process. We also discuss the impact of vacancy ordering on altering the electronic and optical properties of GeSb2Te4, which is relevant to multilevel storage applications. The phase evolution paths in Ge-Sb-Te alloys and Sb2Te3 are illustrated using a summary diagram, which serves as a guide for designing phase-change memory devices.

A Complicated Route from Disorder to Order in Antimony-Tellurium Binary Phase Change Materials

The disorder-to-order (crystallization) process in phase-change materials determines the speed and storage polymorphism of phase-change memory devices. Only by clarifying the fine-structure variation can the devices be insightfully designed, and encode and store information. As essential phase-change parent materials, the crystallized Sb-Te binary system is generally considered to have the cationic/anionic site occupied by Sb/Te atoms. Here, direct atomic identification and simulation demonstrate that the ultrafast crystallization speed of Sb-Te materials is due to the random nature of lattice site occupation by different classes of atoms with the resulting octahedral motifs having high similarity to the amorphous state. It is further proved that after atomic ordering with disordered chemical occupation, chemical ordering takes place, which results in different storage states with different resistance values. These new insights into the complicated route from disorder to order will play an essential role in designing neuromorphic devices with varying polymorphisms.

Electrochemical Ionic Synapses: Progress and Perspectives

Artificial neural networks based on crossbar arrays of analog programmable resistors can address the high energy challenge of conventional hardware in artificial intelligence applications. However, state-of-the-art two-terminal resistive switching devices based on conductive filament formation suffer from high variability and poor controllability. Electrochemical ionic synapses are three-terminal devices that operate by electrochemical and dynamic insertion/extraction of ions that control the electronic conductivity of a channel in a single solid-solution phase. They are promising candidates for programmable resistors in crossbar arrays because they have shown uniform and deterministic control of electronic conductivity based on ion doping, with very low energy consumption. Here, the desirable specifications of these programmable resistors are presented. Then, an overview of the current progress of devices based on Li+ , O2- , and H+ ions and material systems is provided. Achieving nanosecond speed, low operation voltage (≈1 V), low energy consumption, with complementary metal-oxide-semiconductor compatibility all simultaneously remains a challenge. Toward this goal, a physical model of the device is constructed to provide guidelines for the desired material properties to overcome the remaining challenges. Finally, an outlook is provided, including strategies to advance materials toward the desirable properties and the future opportunities for electrochemical ionic synapses.

Microstructure characterization, phase transition, and device application of phase-change memory materials

Phase-change memory (PCM), recently developed as the storage-class memory in a computer system, is a new non-volatile memory technology. In addition, the applications of PCM in a non-von Neumann computing, such as neuromorphic computing and in-memory computing, are being investigated. Although PCM-based devices have been extensively studied, several concerns regarding the electrical, thermal, and structural dynamics of phase-change devices remain. In this article, aiming at PCM devices, a comprehensive review of PCM materials is provided, including the primary PCM device mechanics that underpin read and write operations, physics-based modeling initiatives and experimental characterization of the many features examined in nanoscale PCM devices. Finally, this review will propose a prognosis on a few unsolved challenges and highlight research areas of further investigation.

Ultra-Stable, Endurable, and Flexible Sb2TexSe3-x Phase Change Devices for Memory Application and Wearable Electronics

Flexible memory and wearable electronics represent an emerging technology, thanks to their reliability, compatibility, and superior performance. Here, an Sb₂Te_xSe_3-x (STSe) phase change material was grown on flexible mica, which not only exhibited superior nature in thermal stability for phase change memory application but also revealed novel function performance in wearable electronics, thanks to its excellent mechanical reliability and endurance. The thermal stability of Sb₂Te₃ was improved obviously with the crystallization temperature elevated 60 K after Se doping, for the enhanced charge localization and stronger bonding energy, which was validated by the Vienna ab initio simulation package calculations. Based on the ultra-stability of STSe, the STSe-based phase change memory shows 65 000 reversible phase change ability. Moreover, the assembled flexible device can show real-time monitoring and recoverability response in sensing human activities in different parts of the body, which proves its effective reusability and potential as wearable electronics. Most importantly, the STSe device presents remarkable working reliability, reflected by excellent endurance over 100 s and long retention over 100 h. These results paved a novel way to utilize STSe phase change materials for flexible memory and wearable electronics with extreme thermal and mechanical stability and brilliant performance.

Charge Configuration Memory Devices: Energy Efficiency and Switching Speed

Current trends in data processing have given impetus for an intense search of new concepts of memory devices with emphasis on efficiency, speed, and scalability. A promising new approach to memory storage is based on resistance switching between charge-ordered domain states in the layered dichalcogenide 1T-TaS2. Here we investigate the energy efficiency scaling of such charge configuration memory (CCM) devices as a function of device size and data write time τW as well as other parameters that have bearing on efficient device operation. We find that switching energy efficiency scales approximately linearly with both quantities over multiple decades, departing from linearity only when τW approaches the ∼0.5 ps intrinsic switching limit. Compared to current state of the art memory devices, CCM devices are found to be much faster and significantly more energy efficient, demonstrated here with two-terminal switching using 2.2 fJ, 16 ps electrical pulses.

Designing Conductive-Bridge Phase-Change Memory to Enable Ultralow Programming Power

Phase-change material (PCM) devices are one of the most mature nonvolatile memories. However, their high power consumption remains a bottleneck problem limiting the data storage density. One may drastically reduce the programming power by patterning the PCM volume down to nanometer scale, but that route incurs a stiff penalty from the tremendous cost associated with the complex nanofabrication protocols required. Instead, here a materials solution to resolve this dilemma is offered. The authors work with memory cells of conventional dimensions, but design/exploit a PCM alloy that decomposes into a heterogeneous network of nanoscale crystalline domains intermixed with amorphous ones. The idea is to confine the subsequent phase-change switching in the interface region of the crystalline nanodomain with its amorphous surrounding, forming/breaking "nano-bridges" that link up the crystalline domains into a conductive path. This conductive-bridge switching mechanism thus only involves nanometer-scale volume in programming, despite of the large areas in contact with the electrodes. The pore-like devices based on spontaneously phase-separated Ge13 Sb71 O16 alloy enable a record-low programming energy, down to a few tens of femtojoule. The new PCM/fabrication is fully compatible with the current 3D integration technology, adding no expenses or difficulty in processing.

Tailoring the Structural and Optical Properties of Germanium Telluride Phase-Change Materials by Indium Incorporation

Chalcogenide phase-change materials (PCMs) based random access memory (PCRAM) enter the global memory market as storage-class memory (SCM), holding great promise for future neuro-inspired computing and non-volatile photonic applications. The thermal stability of the amorphous phase of PCMs is a demanding property requiring further improvement. In this work, we focus on indium, an alloying ingredient extensively exploited in PCMs. Starting from the prototype GeTe alloy, we incorporated indium to form three typical compositions along the InTe-GeTe tie line: InGe3Te4, InGeTe2 and In3GeTe4. The evolution of structural details, and the optical properties of the three In-Ge-Te alloys in amorphous and crystalline form, was thoroughly analyzed via ab initio calculations. This study proposes a chemical composition possessing both improved thermal stability and sizable optical contrast for PCM-based non-volatile photonic applications.