Phase-Change Memory (PCM) is a mature non-volatile technology that has demonstrated its suitability for scaling, fast programming speed, low power consumption, and high endurance. The ternary Ge₂Sb₂Te₅ (GST-225) chalcogenide system has been widely used in PCM because of its fast switching capabilities and significant resistivity contrast between the amorphous and crystalline states. However, its low crystallization temperature does not meet the requirements of embedded applications. Material engineering, particularly Ge-enrichment and N-doping, has led to GeSbTe-based (GST) PCM devices with improved thermal stability and data retention that meet the stringent specifications required for automotive applications. However, the use of non-stoichiometric alloys can cause elemental or phase segregation during the fabrication steps, which can be a source of device-to-device variability in highly scaled devices in the next technology nodes. In addition, Ge-enrichment results in an increased structural relaxation at the grain boundaries of the crystalline phase (i.e. device programmed in the SET state), leading to a gradual increase in the resistivity of the PCM cell and consequent retention failure (i.e. SET drift). Understanding the factors that influence the segregation and subsequent crystallization kinetics is important for developing strategies to mitigate or control these phenomena.

The aim of this work is to investigate innovative chalcogenide materials based on Ge, Sb and Te through in situ resistivity measurements and physico-chemical analyses (Raman and infrared spectroscopy, in situ and ex situ XRD and TEM-EDX analyses), with the goal of optimizing these materials for integration into next generation of PCM technology. First, we focus on the effects of the encapsulation on the crystallization kinetics of Ge-rich GeSbTe materials annealed at temperatures compatible with the Back-End-Of-Line (BEOL) of CMOS fabrication. We show how the nature and thickness of the encapsulation layer affects the crystallization kinetics and microstructure of the chalcogenide film. We then investigate the key role of stoichiometry by varying the Sb/Te ratio in defining the crystalline structure of the segregated GST phases. We demonstrate that the high thermal stability of the amorphous phase can be combined with rapid crystalline growth and uniformity in Ge-rich GST with a high Sb/Te ratio. These results are significant for improving crystalline morphology (i.e., fewer grain boundaries) to reduce the SET drift phenomenon compared to the polycrystalline GST-225 phase. Finally, we explore the possibility of targeting Ge-rich GST alloys by multilayer deposition combining GeSbTe layers  with high and low Sb/Te ratios with Ge layers. Thanks to the knobs offered by this approach, we demonstrate the possibility to better control the evolution of the morphology at high temperature.

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