Tellurium self-diffusion in crystalline Ge$_2$Sb$_2$Te$_5$ phase change material

Ge$_2$Sb$_2$Te$_5$ is the most commonly used material for phase change random access memory. In this work, a chemically homogeneous 200 nm thick layer of amorphous Ge$_2$Sb$_2$Te$_5$ was grown on a single crystal Si wafer using DC magnetron sputtering and applying a stoichiometric target at room temperature. A metastable NaCl-type structure having a face-centered-cubic lattice was obtained by subsequent annealing at 473 K for 30 min. The crystal structure and microstructure were analyzed by X-ray diffraction and transmission electron microscopy. Te self-diffusion was measured by secondary ion mass spectroscopy applying a highly enriched natural $^{122}$Te isotope. The Te self-diffusion coefficients follow an Arrhenius law in the temperature range between room temperature and 353 K with an activation enthalpy of (125.0 ± 5) kJ/mol. The diffusion data are discussed in terms of either grain boundary diffusion contributions or, alternatively, in relation to volume diffusion enhanced by structural vacancies. In comparison to the amorphous counterpart, the Te self-diffusion rates in crystalline Ge$_2$Sb$_2$Te$_5$ are only marginally lower and exceed the volume diffusivities of Te in crystalline Te by more than four orders of magnitude, indicating that the structural vacancies seem to determine the measured diffusion rates.