Paper EM-ThP2
Direct Evidence for Post-Crystallization Germanium Precipitation in Thin Films of Phase-Change Material Ge₁₅Sb₈₅

Thursday, October 23, 2008, 6:00 pm, Room Hall D

We present evidence for the instability in the crystalline metallic phase of eutectic phase-change Ge₁₅Sb₈₅ thin films considered for integration into nonvolatile embedded memory cells. Te-free phase-change materials, owing to a combination of higher crystallization temperatures and the absence of easily diffusing chalcogen atoms, offer an advantage over the ternary chalcogenides, provided that the material is stable throughout the switching process. We find that while the amorphous (semiconducting) phase is robust until Ge₁₅Sb₈₅ film’s crystallization into a rhombohedral structure at 240^oC, at 350^oC, Ge rapidly precipitates out, aggregating at the film’s grain boundaries and interfaces. Ge precipitation, visualized directly with transmission electron microscopy and in-situ x-ray diffraction, is found to be exothermic by differential scanning calorimetry, and is found to affect films’ reflectivity, resistance, and stress. Such changes could impact switching reliability, with additional doping required to minimize the precipitation process.