Mann-Ho Cho | Searcharxiv

arXiv2022

Advanced interfacial phase change material: structurally confined and interfacially extended superlattice

Interfacial Phase Change Memory (iPCM) retrench unnecessary power consumption due to wasted heat generated during phase change by reducing unnecessary entropic loss. In this study, an advanced iPCM (GeTe/Ti-Sb2Te3 Superlattice) is synthesized by doping Ti into Sb2Te3. Structural analysis and density functional theory (DFT) calculations confirm that bonding distortion and structurally well-confined layers contribute to improve phase change properties in iPCM. Ti-Sb2Te3 acts as an effective thermal barrier to localize the generated heat inside active region, which leads to reduction of switching energy. Since Ge-Te bonds adjacent to short and strong Ti-Te bonds are more elongated than the bonds near Sb-Te, it is easier for Ge atoms to break the bond with Te due to strengthened Peierls distortions (Rlong/Rshort) during phase change process. Properties of advanced iPCM (cycling endurance, write speed/energy) exceed previous records. Moreover, well-confined multi-level states are obtained with advanced iPCM, showing potential as a neuromorphic memory. Our work paves the way for designing superlattice based PCM by controlling confinement layers.

cond-mat.mtrl-sci↗

arXiv2022

Conversion between metavalent and covalent bond in metastable superlattices composed of 2D and 3D sublayers

Reversible conversion over multi-million-times in bond types between metavalent and covalent bonds becomes one of the most promising bases for universal memory. As the conversions have been found in metastable states, extended category of crystal structures from stable states via redistribution of vacancies, researches on kinetic behavior of the vacancies are highly on demand. However, they remain lacking due to difficulties with experimental analysis. Herein, the direct observation of the evolution of chemical states of vacancies clarifies the behavior by combining analysis on charge density distribution, electrical conductivity, and crystal structures. Site-switching of vacancies gradually occurs with diverged energy barriers owing to a unique activation code-the accumulation of vacancies triggers spontaneous gliding along atomic planes to relieve electrostatic repulsion. Study on the behavior can be further applied to multi-phase superlattices composed of Sb2Te3 (2D) and GeTe (3D) sublayers, which represent the superior memory performances but their operating mechanisms were still under debates due to their complexity. The site-switching is favorable (suppressed) when Te-Te bonds are formed as physisorption (chemisorption) over the interface between Sb2Te3 (2D) and GeTe (3D) sublayers driven by configurational entropic gain (electrostatic enthalpic loss). Depending on the type of interfaces between sublayers, phases of the superlattices are classified into metastable and stable states, where the conversion could be only achieved in the metastable state. From this comprehensive understanding on the operating mechanism via kinetic behaviors of vacancies and the metastability, further studies towards vacancy engineering are expected in versatile materials.

cond-mat.mtrl-sci↗