Atomic Migration Induced Crystal Structure Transformation and Core-Centered Phase Transition in Single Crystal Ge2Sb2Te5 Nanowires

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

By controlling the amorphous-to-crystalline relative volume, chalcogenide phase-change memory materials can provide multi-level data storage (MLS), which offers great potential for high-density storage-class memory and neuro-inspired computing. However, this type of MLS system suffers from high power consumption and a severe time-dependent resistance increase ("drift") in the amorphous phase, which limits the number of attainable storage levels. Here, we report a new type of MLS system in yttrium-doped antimony telluride, utilizing reversible multi-level phase transitions between three states, i.e., amorphous, metastable cubic and stable hexagonal crystalline phases, with ultralow power consumption (0.6-4.3 pJ) and ultralow resistance drift for the lower two states (power-law exponent < 0.007). The metastable cubic phase is stabilized by yttrium, while the evident reversible cubic-to-hexagonal transition is attributed to the sequential and directional migration of Sb atoms. Finally, the decreased heat dissipation of the material and the increase in crystallinity contribute to the overall high performance. This study opens a new way to achieve advanced multi-level phase-change memory without the need for complicated manufacturing procedures or iterative programming operations.

Ultrafast interfacial transformation from 2D- to 3D-bonded structures in layered Ge-Sb-Te thin films and heterostructures

Two-dimensional van-der-Waals-bonded chalcogenide heterostructures have recently received a lot of attention due to promising applications in the fields of photonics, plasmonics and data storage. Of particular interest is the interfacial switching process inherent in these structures, which is assumed to occur locally at the van-der-Waals interfaces and thus represents an intracrystalline transition. However, detailed experimental studies on the underlying mechanism are still lacking. In this work, epitaxially grown thin films consisting of van-der-Waals-bonded Ge-Sb-Te and GeTe/Sb2Te3 based heterostructures are employed as a model system to investigate structural changes induced by a single ns-laser pulse. A combined approach using X-ray diffraction and advanced transmission electron microscopy is applied to study phase transitions within the Ge-Sb-Te-based thin films in detail. The results reveal ultrafast transitions from 2D-bonded layered structures to 3D-bonded structures via a transient molten phase. Moreover, the interface between the 2D- and 3D-bonded structures is well defined by a single van-der-Waals gap, suggesting that the transition can be controlled very precisely in its spatial extent by an appropriate choice of the laser fluence. Overall, the results of this work offer a new perspective on the switching mechanism in Ge-Sb-Te-based materials and demonstrate the potential of van-der-Waals-bonded Ge-Sb-Te compounds to be applied for novel phase-change memory concepts.

Characterization of interfacial barrier charging as a resistive switching mechanism in Ag/Sb2Te3/Ag heterojunctions

In this study, bipolar memristive behaviors were systematically characterized in Ag/Sb2Te3/Ag hetero-junctions. By using in situ Raman and photoluminescence spectroscopy, a direct observation of the bonding environment and band structure confirmed that resistive switches are strongly related to the electronic valence changes in Sb2Te3 and the formation of Schottky barriers at Ag/Sb2Te3 interfaces. Band movement of Sb2Te3 acquired by first-principles calculations also supports the electrostatic barrier charging as a memristive mechanism of Ag/Sb2Te3/Ag heterocells. Independent resistance-switching behaviors that can be utilized in both amorphous and crystalline Sb2Te3 lead to multiple resistance values with a large memory window (104-105) and low read voltage (∼0.2 V), giving rise to a unique multi-level memory concept. This study based on Ag/Sb2Te3/Ag hetero-junctions offers a significant understanding to promote the use of Sb2Te3 and other chalcogenide memristors as promising candidates for compatible high-density memory applications.