Investigation of Crystallization Behavior of Sputter-Deposited Nitrogen-Doped Amorphous Ge₂Sb₂Te₅ Thin Films

The crystallization behavior of nitrogen-doped amorphous Ge₂Sb₂Te₅-(N) phase-change thin films was studied by utilizing differential scanning calorimetry, in situ ellipsometry and in situ transmission electron microscopy. The combined analysis of in situ ellipsometry isotherms of amorphous Ge₂Sb₂Te₅-(N) films and the Johnson-Mehl-Avrami equation revealed that the crystallization process of amorphous Ge₂Sb₂Te₅-(N) films changes depending on the nitrogen content. The crystallization behavior of Ge₂Sb₂Te₅ film revealed a two-step process that includes spherical-nucleation and disc-shaped grain growth. In contrast, nitrogen-doping into Ge₂Sb₂Te₅ thin films suppresses the second step and the crystallization of Ge₂Sb₂Te₅-(N) becomes a one-step process that is the primary nucleation process. The number of nucleation sites during the crystallization of amorphous Ge₂Sb₂Te₅-(N) films, increased markedly with the annealing temperature in the spherically shaped nuclei and eventually saturated. The effective crystallinity of Ge₂Sb₂Te₅-(N) alloy films decreased with the increase in nitrogen content, mainly due to the grain-size refinement.