GeTe-Sb 2 Te 3 pseudobinary system, especially Ge 2 Sb 2 Te 5 alloy, is the most desirable material to be commercialized in phase change random access memory. Directly resolving the local atomic arrangement of Ge 2 Sb 2 Te 5 during intermediate steps is an effective method to understand its transition mechanism from face-centered-cubic to hexagonal phases. In this study, we provide insights into the atomic arrangement variation during face-centered-cubic to hexagonal transition process in Ge 2 Sb 2 Te 5 alloy by using advanced atomic resolution energy dispersive X-ray spectroscopy. Induced by thermal annealing, randomly distributed germanium and antimony atoms would migrate to the specific (111) layer in different behaviors, and antimony atoms migrate earlier than germanium atoms during the phase transition process, gradually forming intermediate structures similar to hexagonal lattice. With the migration completed, the obtained stable hexagonal structure has a partially ordered stacking sequence described as below:-Te-Sb x /Ge y-Te-Ge x /Sb y-Te-Ge x /Sb y-Te-Sb x /Ge y-Te-(x > y), which is directly related to the migration process. The current visual fragments suggest a gradual transition mechanism, and guide the performance optimization of Ge 2 Sb 2 Te 5 alloy.
Many experiments have shown that three-dimensional-confined grain refinement (GR) textures in phase-change memory reduce power consumption and improve endurance performance. However, a lack of knowledge on the GR mechanisms and their influence on device performances challenges designs that concurrently enhance the comprehensive device performances using the same impurity-doped strategy. Here, we experimentally observe dramatic GR in carbon-doped Ge2Sb2Te5 (GST), which also presents reduced power consumption and enhanced endurance performances. We provide low power consumption evidence that thermal conductivity controls the thermal transport heat loss and is proportional to the size of nanoscale grains because the boundary severely scatters phonons. Our simulations indicate that the short carbon chains in the boundary interlace with each other and trend to form trialkyl carbon atoms that constitute the basic local environment of graphene. The stable sheet consists of aggregated carbon, which is even stable above the melting temperature of GST and acts as a second-phase drag to refine the grain size. The enhanced endurance is explained by the restricted migration from the stable carbon sheet, which is verified by the greatly reduced diffusion coefficient of the host atoms in the boundary because of the less shielding effect from the core electrons in carbon and strong bonds formed between host and carbon atoms. Our findings show that the reduced power consumption and enhanced endurance from GR engineering are feasible in phase-change memory, which has been largely overlooked.
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