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In the era of digital information, the increasing demand for the high-efficiency memory storage hints a huge hardware revolution arising with the explosively boosting data volume and the swiftly advanced technologies such as neuromorphic computing and artificial intelligence. The chalcogenide-based phase-change materials (PCMs), as one of the novel memory materials, are widely applied to large capacity rewritable optical and electronic data storages. Based on the reversible transformation between the amorphous and crystalline phases upon laser heating or electric current, the profound resistive differences in two states are utilized to record the digital data. The high scalability, fast operation speed and proper unit costs make the PCM-based random access memories (PRAMs) the most promising and outstanding candidates for the next generation of universal non-volatile memories, which surpasses the flash-based slid-state drivers (SSD) and magnetic-based hard disk drivers (HDD) as well as competes with the fast but volatile static and random access memories (SRAMs and DRAMs), ultimately eliminating the distinction between fast volatile memories and conventional non-volatile storages in the memory hierarchy.
The representative PCM is Ge-Sb-Te (GST), located on the pseudo-binary line between GeTe and Sb2Te3 compounds, which has a rather fast switch speed up to tens of nanoseconds. However, to further increase the operation speed of the PCM device, the ultrafast crystallization during the SET (writing) process remains as a bottleneck limited by the fundamental properties of the PCMs, such as nucleation rate and growth speed. Massive efforts have been paid by the researchers in the field to furthermore accelerate the SET speed. D. Loke et al. proposed a pre-programming scheme for fast switch between amorphous and crystalline states of rock-salt GST, which hints the huge significance of the incubation for the stabilized crystalline precursors. Following the ideas that transition metals into antimony telluride can result in the superior crystallization speed by contrast with the typical PCMs, Rao et al. performed a screening for doped transition metal and fabricated a newly designed PCM alloy Sc0. 2Sb2Te3 (SST), which has been prove to have the sub-nanoseconds crystallization speed. In SST, the introduction of the doped scandium into the rock-salt Sb2Te3, stabilizes the nuclei with stronger local chemical bonds as well as better thermodynamic stability. Here the parents phase antimony telluride Sb2Te3 is a metastable rock-salt state with 1/3 of the lattice vacancies in the crystalline phase and with defective octahedrons as its most probable local structural motifs. With the significant similarity in both crystalline and amorphous phases between Sb2Te3 and scandium telluride Sc2Te3, the doped Scandium into the Sb2Te3 ensures the extreme geometrical matches in two states.
The ultrafast crystallization speed differing from the original Sb2Te3 system results from the decreased nucleation stochasticity triggered by doped Scandium based on the robust scandium telluride bonds and more stable crystal precursors. Unlike its parent state Sb2Te3 which has extensively been studied both from experiments and simulations, the role of scandium representing in the alloy as well as the other compounds during the crystallization process remains a huge mystery with only few researches to be published. It is of paramount significance to understand the mechanism that, even though the Sb2Te3 and Sc2Te3 have almost the same crystalline and amorphous structural features, the Sc could be a crucial driving force for the formation of the stable nucleation nuclei leading to an extremely fast crystallization. As a matter of fact, instead of taking a tiny glance at the local structure around doped Scandium in SST alloy, it is of pressing necessity to have a comprehensive view on the amorphous properties of scandium telluride as compared with the similar structure of antimony telluride.
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