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Marta Agati1 Daniel Benoit2 Alain Claverie1

1, Center for Materials Elaboration and Structural Studies - Centre National de la Recherche Scientifique (CEMES-CNRS), Toulouse, , France
2, STMicroelectronics, Crolles, , France

Phase Change Materials (PCM) have been widely recognized as effective building-block for alternative memory devices, where the information bit is stored in the form of two distinct resistive states, namely the high-resistance amorphous phase and the low-resistance crystalline phase. Switching between the different phases is achieved through the heating of the PCM, by means of the application of voltage pulses via the Joule effect. Phase Change Memories have exhibited sub-10 ns switching speed, scalability to sub-10 nm dimensions, extremely good cyclability and endurance, data retention ability as well as integration in large-arrays, thus representing potential candidates for future substitute devices to flash memories. Among the PCM, the investigation on pseudo-binary chalcogenide-based alloys, in particular Ge2Sb2Te5 (GST), has been encouraged because of the combination of their adequate resistivity contrast and rapid switching between the two phases. Nevertheless, some issues still subsist, concerning the reliability of GST-based memories as well as their thermal stability for PCM applications which require quite high working-temperatures, hence motivating further studies. In this context, it has been found that GST can be enhanced by means of Ge enrichment or by doping it with nitrogen or carbon, in order to increase the crystallization temperature and provide a better thermic stability of the amorphous phase. After such modifications, variations of the phase-change mechanism and of the resistance trend are expected. In this work, focus has been put on Ge-enriched N-doped GST materials, especially on their structural properties at the nanoscale and the dynamics of their phase change. To this aim, in-situ Transmission Electron Microscopy (TEM) analyses at different temperatures have been systematically pursued in order to delineate the phase change process during the thermal transient and describe the physics beyond the mechanisms of nucleation and grain growth. The impact of surrounding media on the physical mechanism has been considered in terms of interfacial energy balance and comparisons have been made with the more known Ge2Sb2Te5. Furthermore, ex-situ measurements on samples previously annealed in a hot tube have been realized, in particular X-Ray Diffraction spectroscopy and ex-situ structural TEM characterizations, i.e. diffraction, energy filtered TEM and High Resolution imaging. Hence, TEM-Energy Dispersive X-Ray mapping has been performed to characterize the chemical implications of the phase change mechanism. This study represents a step toward a major understanding of the physical phenomena beyond the phase transformation of PCM at nanoscale resolution, which reveals to be at the basis of the behavior of the final device. The attentive examination of the different stages of the crystallization process may also support the application of GST as multilevel data storage employing more than two states of electrical resistance.

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