Materials Design of OTS Selectors and Phase-Change Materials for 3D Cross-point Memory Technology

Abstract

With the increasing demand for high-capacity and high- performance memory, storage class memory (SCM) technology has aimed to fill the gap between storage and memory in the computer storage hierarchy of a modern system. The capability to fast read and write data in SCM is very important for big data analysis and AI applications. However, to boost the memory density, SCM technology requires NVM densely packed in “crosspoint” arrays to achieve $4F^2$ footprint with selecting devices capable of delivering high current and power. In addition, it is possible to integrating a repeating sequence of phase change memory layer and selector layer, achieving a 3D stackable structure increasing dramatically the density of SCM technology. Ovonic Threshold Switching (OTS) material/device show potential to meet all the selector requirements both from electrical and 3D integration point of view. In the first part of this paper, we will discuss the role that the OTS selector plays in minimizing the leakage current through the many non-selected cells, while also delivering the right voltages and currents to the selected cell. We will focus on how to design OTS materials to meet the selector criteria enabling cross-point memory operation. OTS material based on Te-As-Ge-Si system have been demonstrated for a selector device since 1968 [1] and have been the primarily studied materials. However, insufficient cycling endurance and low thermal stability remains a key hurdle that inhibits these materials to be used in a large cross-point array. Different Ge, Si and As concentrations in Te-As-Ge-Si system [2] are tuned and optimal composition is proposed to reach endurance performance to $10^{11}$ cycles. However, the high leakage current $(I_{OFF})$ and high threshold voltage $(V_{th})$ drift are still the main concerns for TeAsGeSi-based materials. Se- based chalcogenides have demonstrated to be good selectors [3]. The doping effect of B, C, S, Si as well as In dopants into AsSeGe chalcogenide will be systematically discussed [4] where it will be providing the guideline for material engineering of OTS for 3D crosspoint memory. In the second part of this paper, we will discuss the phase-change materials (PCM) suitable for storage-class memory with fast switching speed and high endurance. We will also focus on the need for researching new PCM material with uniform and stable $V_{tS}$ (system threshold voltage when PCM is in SET state) and $V_{tR}$ (system threshold voltage when PCM is in RESET state) distribution as well as lack of degradation of $V_{tS}$ and $V_{tR}$ memory window during the write cycling operation. The materials design for PCM based on the crosspoint memory specification will be reviewed and discussed, together with the impact that OTS have on PCM materials during programming operation and how it may affect $V_{tS}$ and $V_{tR}$ drift performance [5]. Therefore, it is desirable to find new PCM which can be matched with candidate OTS selector to optimize the 3D crosspoint memory performance.