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Crystallization Kinetics of Nitrogen Doped Ge2Sb2Te5

Reinforcing the nucleation-dominated crystallization of Ge2Sb2Te5 alloys by N doping

par Guy Molénat - publié le

N doping significantly improves some key characteristics of PCM devices, but the origin, at the atomic scale, of these alterations was elusive. Using a combination of ex-situ and in-situ TEM techniques, we show that the observed higher crystallization temperature and finer grain morphology result from the bonding of N with Ge which renders amorphous Ge2Sb2Te5 more viscous and impedes Ge diffusion in both the amorphous and crystalline phases during annealing.

Researchers at CEMES-CNRS, supported by Léti/CEA engineers, have reported the results of a combination of in situ and ex situ transmission electron microscopy (TEM) investigations carried out on specifically designed samples to evidence the influence of N concentration on the crystallization kinetics and resulting morphology of the alloy. Beyond the known shift of the crystallization temperature and the observation of smaller grains, we show that N renders the crystallization process more “nucleation dominated” and ascribe this characteristic to the increased viscosity of the amorphous state. This increased viscosity is linked to the mechanical rigidity and the reduced diffusivity resulting from the formation of Ge–N bonds in the amorphous phase. During thermal annealing, N hampers the coalescence of the crystalline grains and the cubic to hexagonal transition.

Left, Dark Field TEM image of the sample after annealing at 180°C for 30 min showing the difference in crystal sizes in the N implanted (top) and in the unimplanted (bottom) regions. Middle : differences in nucleation and growth probabilities in pure and N implanted GST-225. Right : Strain experienced by the Ge2Sb2Te5 nanocrystals in the N implanted and unimplanted regions.


Making use of AbStrain, a TEM-based technique recently invented at CEMES, we evidence that the nanocrystals formed from the crystallization of N-doped amorphous Ge2Sb2Te5 are under tension, which suggests that N is inserted in the lattice and explains why it is not found at grain boundaries. Globally, all these results demonstrate that the origin of the effect of N on the crystallization of Ge2Sb2Te5 is not to be attributed to the formation of a secondary phase such as a nitride, but to the ability of N to bind to Ge in the amorphous and crystalline phases and to unbind and rebind with Ge along the diffusion path of this atomic species during annealing.


Contact : Alain Claverie, claverie [at] cemes.fr

Publication : Effect of Nitrogen Doping on the Crystallization Kinetics of Ge2Sb2Te5, Minh Anh Luong, Nikolay Cherkashin, Béatrice Pecassou, Chiara Sabbione, Frédéric Mazen and Alain Claverie, Nanomaterials 2021, 11, 1729.

Link : https://doi.org/10.3390/nano11071729