Studying phase change memory devices by coupling scanning precession electron diffraction and energy dispersive X-ray analysis

Abstract

A new simultaneous acquisition software for Precession Electron Diffraction (PED) and Energy Dispersive X-ray Spectroscopy (EDX) mapping has been developed. This approach has been used to study the RESET operation in Ge-rich Ge-Sb-Te (Ge-rich GST) non-volatile Phase-Change Memory (PCM). Such an operation consists in the local amorphization of the device's active volume, which defines the memory state. Using PED patterns indexing, we were able to segment the amorphous area, and access its composition through EDX quantification. Moreover, both the spatial distribution and the chemical composition of the two crystalline phases overlapped in the chalcogenide layer were retrieved by using the spectral unmixing approach and PED patterns indexing. It evidences that Ge and Ge2Sb1Te2 (GST-212) crystals are separated in the studied devices. Despite this, we show that Ge-enrichment is preserved in the active volume of a functional device, which is crucial to guarantee the RESET state stability at high temperature. We also link the origin of the failure of the RESET operation observed in a dysfunctional device to the larger Ge-depleted area.