In this paper, recent progress of phase change memory (PCM) is reviewed. The electrical and thermal properties of phase change materials are surveyed with a focus on the scalability of the materials and their impact on device design. Innovations in the device structure, memory cell selector, and strategies for achieving multibit operation and 3-D, multilayer high-density memory arrays are described. The scaling properties of PCM are illustrated with recent experimental results using special device test structures and novel material synthesis. Factors affecting the reliability of PCM are discussed.

Phase Change Memory

The global demand for data storage and processing has increased exponentially in recent decades. To respond to this demand, research efforts have been devoted to the development of non-volatile memory and neuro-inspired computing technologies. Chalcogenide phase-change materials (PCMs) are leading candidates for such applications, and they have become technologically mature with recently released competitive products. In this Review, we focus on the mechanisms of the crystallization dynamics of PCMs by discussing structural and kinetic experiments, as well as ab initio atomistic modelling and materials design. Based on the knowledge at the atomistic level, we depict routes to improve the parameters of phase-change devices for universal memory. Moreover, we discuss the role of crystallization in enabling neuro-inspired computing using PCMs. Finally, we present an outlook for future opportunities of PCMs, including all-photonic memories and processors, flexible displays with nanopixel resolution and nanoscale switches and controllers. Chalcogenide phase-change materials (PCMs) are leading candidates for non-volatile memory and neuro-inspired computing devices. This Review focuses on the crystallization mechanisms of PCMs as well as the design principles to achieve PCMs with high switching speeds and good data retention, which will aid the development of PCM-based universal memory and neuro-inspired devices.

Designing crystallization in phase-change materials for universal memory and neuro-inspired computing

Operation speed is a key challenge in phase-change random-access memory (PCRAM) technology, especially for achieving subnanosecond high-speed cache memory. Commercialized PCRAM products are limited by the tens of nanoseconds writing speed, originating from the stochastic crystal nucleation during the crystallization of amorphous germanium antimony telluride (Ge2Sb2Te5). Here, we demonstrate an alloying strategy to speed up the crystallization kinetics. The scandium antimony telluride (Sc0.2Sb2Te3) compound that we designed allows a writing speed of only 700 picoseconds without preprogramming in a large conventional PCRAM device. This ultrafast crystallization stems from the reduced stochasticity of nucleation through geometrically matched and robust scandium telluride (ScTe) chemical bonds that stabilize crystal precursors in the amorphous state. Controlling nucleation through alloy design paves the way for the development of cache-type PCRAM technology to boost the working efficiency of computing systems.

https://science.sciencemag.org/content/sci/early/2017/11/08/science.aao3212.full.pdf

Reducing the stochasticity of crystal nucleation to enable subnanosecond memory writing

The role of local structure in dynamical arrest

There are at least three fundamental states of matter, depending upon temperature and pressure: gas, liquid, and solid (crystal). These states are separated by first-order phase transitions between them. In both gas and liquid phases a complete translational and rotational symmetry exist, whereas in a solid phase both symmetries are broken. In intermediate phases between liquid and solid, which include liquid crystal and plastic crystal phases, only one of the two symmetries is preserved. Among the fundamental states of matter, the liquid state is the most poorly understood. We argue that it is crucial for a better understanding of liquids to recognize that a liquid generally has the tendency to have a local structural order and its presence is intrinsic and universal to any liquid. Such structural ordering is a consequence of many-body correlations, more specifically, bond angle correlations, which we believe are crucial for the description of the liquid state. We show that this physical picture may naturally explain difficult unsolved problems associated with the liquid state, such as anomalies of water-type liquids (water, Si, Ge, ...), liquid-liquid transition, liquid-glass transition, crystallization and quasicrystal formation, in a unified manner. In other words, we need a new order parameter representing a low local free-energy configuration, which is a bond orientational order parameter in many cases, in addition to a density order parameter for the physical description of these phenomena. Here we review our two-order-parameter model of liquid and consider how transient local structural ordering is linked to all of the above-mentioned phenomena. The relationship between these phenomena is also discussed.

Bond orientational order in liquids: Towards a unified description of water-like anomalies, liquid-liquid transition, glass transition, and crystallization: Bond orientational order in liquids.

Phase-change materials are key components in rewritable optical disks and are promising for non-volatile electronic memories. The very different structure and ultrafast recrystallization dynamics of another class of phase-change materials, Sb–Te-based alloys, now suggests their use in future memory applications.

From local structure to nanosecond recrystallization dynamics in AgInSbTe phase-change materials

The most common materials used for phase-change recording today are GeTe-Sb2Te 3 (GST) pseudobinary compounds [1] and Sb-Te binary compounds with small amounts of In, Ag and/or Ge, for example Ag 3.5In 3.8Sb75. 0Te 17.7 (AIST) [2]. These materials possess sufficiently fast recrystallization speed; however, it is well known that recrystallization in the two groups is strikingly different. In the former (GST) group it proceeds mainly by way of nucleation inside the amorphous marks (bits), in the latter (AIST) group by way of crystal growth from the rim. Our present structural analysis for the amorphous AIST revealed that this contrast in crystallization stems from difference in both amorphous structures. In the GST amorphous materials, crystal-like atomic arrangements, in other words, crystalline fragments are scattered, hidden in the extensive atomic network; these act as crystalline nuclei for the amorphous mark to be instantaneously crystallized. On the other hand, that AIST amorphous atomic configuration consists of 3 + 3 coordination polyhedra as well as in its crystalline phase [5]. Once sufficient heat energy is given to this amorphous material, bond interchanges are caused in the polyhedra, which changes them into crystalline ones. This rearrangement swiftly propagates from neighbor to neighbor forming the crystalline atomic configuration, which finally shows needle-like crystallization texture

/pdf/structural-investigation-for-a-aist-clarify-the-difference-tovb23chma.pdf

Tetsuo Honma

Papers

From local structure to nanosecond recrystallization dynamics in AgInSbTe phase-change materials

Structural investigation for a-AIST clarify the difference between the two types of contrastive phase-change mechanisms