A comparison of Ge, Sb and Te thermal diffusion through Ge2Sb2Te5

Chalcogenide Ge-Sb-Te alloys are used in a new generation of nonvolatile memories named Phase Change Memories which can be thermally switched from low resistivity to high resistivity states by feeding the memory cells with appropriate electrical pulses. During their lifetime, the heterogeneous redistribution of the elements within the active region of the cells is at the origin of the drift of their electrical characteristics and, ultimately, of their failure. Up to now, little is known about the relative diffusivities of the atomic elements that compose the alloys, what hampers the modeling and optimization of the materials and devices. Here, we report an experimental study of the thermal diffusion of Ge, Sb and Te through Ge2Sb2Te5. For this purpose, specific structures consisting of a layer of Ge, Sb or Te sandwiched between two Ge2Sb2Te5 layers have been fabricated and annealed at various temperatures and times. Using transmission electron microscopy based techniques, we evidence that Te diffusion can be activated from about 160 °C while Ge and Sb needs much higher temperatures, 220 °C and 350 °C, respectively. These results are in disagreement with previous theoretical simulations predicting that Ge is the most diffusive element while Te is the least.