Chemical and Structural Characterization of Programmed Ge-Rich GeSbTe Non-Volatile Memory Cells

Phase-Change Materials (PCMs), mainly represented by (GeTe)_x-(Sb₂Te₃)_y (GST) alloys, are used for high-density data storage in optical media and for solid-state non-volatile memories. Enrichment of GeSbTe alloys with germanium has been suggested as a suitable approach to increase the crystallization temperature and therefore to improve data retention in high-temperature applications of non-volatile phase change memories, such as embedded or automotive products. However, the tendency of Ge-rich GeSbTe alloys to decompose with the segregation of Ge still requires investigations on the processes involving element diffusion and compositional alterations both in thin films and devices.<br/>The failure mechanism of phase-change memory devices is strongly related with mass transport occurring during device cycling. Mass transport induces phase separation and segregations and indeed these issues has been reported both in heater-based architectures, like vertical mushroom cells, and in self-heating planar architectures, like line cells.<br/>The mass transport behavior observed in literature depends on the architecture and the programming conditions, but some general trends can be recognised. Therefore, one of the main efforts lies in finding compositional and operational condition such that the cell endurance is increased. To this aim, the structural and chemical analysis of devices after forming and cycling could provide an insight into the degradation mechanism.<br/>In this work we report the behavior of non-confined Ge-rich GST mushroom cells in reset and stuck reset states. In these Single Cell Vehicle (SCV) devices, TiN heaters have been fabricated with a diameter ranging from 50 to 100 nm.<br/>SCV devices has been characterized in terms of structural and chemical properties by High-Angle Annular Dark Field (HAADF) Scanning-Transmission Electron Microscopy (S-TEM), Energy X-ray Dispersive Spectroscopy (EDXS) and Electron Energy Loss Spectroscopy (EELS). The SCV devices were previously formed and programmed in the desired reset state and/or cycled up to 10⁶ times and electrical characterized. SCV TEM Lamellae have been obtained by Focused Ion Beam.<br/>The present study shows with high spatial resolution how the forming process modifies the initial elemental distribution.<br/>The RESET stucks, occurring after 10⁵-10⁶ cycles, seem to obey expected failure mechanisms. In this state, voids are formed on top of the heater, preventing a proper current path between heater and top electrode.