Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping

Ge-rich GeSbTe alloys allowed overcoming temperature limitations of phase-change memory technology. In this paper, we present a thorough investigation of the structural evolution and the crystallization process of these alloys as a function of increasing temperature in annealing. We highlight the progressive rearrangement of the structure toward the demixing of Ge and GeSbTe phases. In particular, we show the stability of Sb–Te units and the development of Ge–Te bonds around these features. We observe the formation of a transient GeSbTe phase, which is driven by crystallization phenomena, leading to a gradual diffusion and expulsion of Ge. Therefore, the system moves toward the complete separation of Ge and Ge2Sb2Te5 stable phases. Furthermore, we investigate the effect of N-doping in Ge-rich GeSbTe, which induces the formation of Ge–N bonds. Such features are demonstrated to be responsible for a delayed structural reorganization to higher temperatures, thus affecting the entire process of crystallization and phase separation in the alloy.