Nanoscale Characterization of Ge-Rich GeSbTe in Thin Films, Memory Cells and Heterostructures

Phase-Change Materials (PCMs), mainly represented by (GeTe) -(Sb₂Te₃) (GST) alloys, are used for high-density data storage in optical media and for solid-state non-volatile memories. Among all, Ge-rich GeSbTe alloys has been proposed to increase the crystallization temperature and therefore to improve data retention in high-temperature applications of non-volatile PCM memories. However, Ge-rich GeSbTe alloys tend to decompose, producing Ge segregation and compositional changes, as a consequence of elemental diffusion.<br/>In PCM devices, the failure mechanism is strongly related with mass transport occurring during programming. Due to this elemental transport both phase separation and segregations can occur 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/>While the details of such mass transport strictly depend on the architecture and the programming conditions, some general trends can be recognized. To this aim, the local structural and chemical analysis of memory cells after forming and cycling could provide an insight into the degradation mechanism.<br/>In this work, Ge-rich PCMs have been investigated at the nanoscale, by means of conventional Transmission Electron Microscopy (TEM), High Angular Annular Dark Field (HAADF) Scanning/TEM (S/TEM) and correlated Energy X-ray Dispersive Spectroscopy (EDXS) and Electron Energy Loss Spectroscopy (EELS). The behavior of several Ge-rich GST systems is discussed, starting from Ge-rich thin films to memory cells and heterostructures.<br/>Ge-rich GST thin films with high Ge amount (&gt;40%) are characterized by a high crystallization temperature (270 °C). By EELS mapping and electron diffraction, we studied the phase separation as function of the as grown alloy composition.<br/>Single Cell Vehicle (SCV) mushroom Ge-rich GST cells, with a TiN heater in the range of 50-100nm, have been prepared in fresh reset, cycled reset and stuck reset states. The present study shows with high spatial resolution how the forming process modifies the initial elemental distribution and how it evolves upon cycling.<br/>Finally, with a focus on S/TEM and EELS analyses, we present our last results on PCM heterostructures upon annealing. Chalcogenide heterostructures, fabricated by a combination of PCM and Confinement Materials (CMs), offer indeed the opportunity of tuning different physical properties, possibly combining features ascribed to the constituent single thin films. For multilayers containing Ge-rich GST we have observed a minimized Ge segregation, while increasing the crystallization temperature with respect to GST225.