Migration Phenomena in Ge-Rich GeSbTe Alloys for Embedded Memory Applications

Phase change memory is based on the peculiar properties of the phase change materials adopted as active media, that strongly depend on the material composition. Among the others, Ge₂Sb₂Te₅ has been often considered the alloy of choice because of its fast switching properties, high resistance contrast and stability. However, the low crystallization temperature (around 150°C) prevents the use of Ge₂Sb₂Te₅ for high temperature applications, such as embedded or automotive. Recently, Ge enrichment of GeSbTe alloys 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. In normal operation, the PCM cell is subject to high current density to raise the local temperature above the melting point. This produces repetition of electrical and thermal stress during the device lifetime, causing the motion of the different atoms and leading to compositional variations over time and/or across the device, which may have impact on performance and reliability. Such a process may be enhanced in Ge rich GeSbTe since these 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 we investigated several Ge enriched GeSbTe alloys. The temperature dependence of the electrical properties of the amorphous alloys and the formation of the crystalline phases have been studied in thin films by in situ electrical measurements and ex-situ structural analysis. 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. The most promising composition has been adopted to manufacture single cell memory devices, with a device structure enhancing the thermal and electrical stress and therefore suitable to study the atomic migration. The characterization of the devices confirms the expected material performance. After a forming process, the devices operate 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. Despite the Ge enrichment, the drift coefficient in the SET state resembles that of Ge₂Sb₂Te₅. After cycling, the atomic distribution in the device has been investigated by scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS), indicating that a compositional reconfiguration of the material in the active region occurs.