Ge-Rich GeSbTe Alloys—Material Properties and Devices

Enrichment of GeSbTe alloys with germanium has been proposed as a valid approach to increase the crystallization temperature and therefore to address high temperature applications<b> </b>of non-volatile phase change memories, such as embedded or automotive. Despite the achievement of higher crystallization temperature, Ge-rich GeSbTe alloys commonly suffer of segregation of pure Ge, with the formation of less Ge-rich compositions that may adversely affect the device cyclability and endurance. With the aim to find some possible routes to limit the Ge segregation, starting from the compositions along the GeTe-Sb₂Te₃ pseudo-binary line, we investigate several GeSbTe alloys enriched with low or high amount of Ge. The temperature dependence of the electrical properties of the amorphous alloys and the formation of the crystalline phases have been investigated in thin films by in situ electrical measurements and ex-situ structural analysis, determining the formed phases. The segregation and decomposition processes have been also discussed on the basis of density functional theory calculations, identifying the compositions which are expected to be less prone to decompose with Ge segregation. Single cell memory devices with the most promising material have been manufactured and tested. The characterization of the devices confirm the expected material performance. After a forming process, the device operates between two distinct logic states (SET and RESET) with one order of magnitude of resistance contrast, and can be reversible switched for up to 10⁶ cycles. Upon annealing for 1 h, the one order of magnitude resistance contrast is preserved up to 240°C, and the complete crystallization of the RESET state is not observed even for annealing at temperature up to 280°C.