Effect of Nitrogen Doping on the Crystallization Kinetics of Ge2Sb2Te5

Abstract

Among the phase change materials, Ge2Sb2Te5 (GST-225) is the most studied and is already integrated into many devices. N doping is known to significantly improve some key characteristics such as the thermal stability of materials and the resistance drift of devices. However, the origin, at the atomic scale, of these alterations is rather elusive. The most important issue is to understand how N doping affects the crystallization characteristics, mechanisms and kinetics, of GST-225. Here, we report 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. Making use of AbStrain, a recently invented TEM-based technique, we evidence that the nanocrystals formed from the crystallization of N-doped amorphous GST-225 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 GST-225 is not 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.

Crystallization of Ge-Rich GeSbTe Alloys: The Riddle Is Solved

“…Then, they increase again for longer annealing time, probably as a result of the slight growth of the existing grains, again at the end of the phase separation process. It is important to note that these sizes are surprisingly large and well above any reported crystallite sizes observed during the nucleation-limited crystallization of pure GST-225. , …”

“…It is important to note that these sizes are surprisingly large and well above any reported crystallite sizes observed during the nucleation-limited crystallization of pure GST-225. 30,31 To set up and validate a scenario able to explain all the observed characteristics of the crystallization of GGST alloys, several questions must be answered. First of all, why is Ge crystallization observed as such a low temperature?…”

“…This scenario well explains why GST-225 crystals appear later and are larger than observed during the crystallization of pure GST-225. 30,31 In a GGST alloy, the crystallization of GST-225 is obtained only when very depleted Ge regions appear, that is,. when the phase separation is well advanced, which is finally obtained within larger domains.…”

An experimental study of Ge diffusion through Ge2Sb2Te5

“…The stack was protected against oxidation by depositing in situ, immediately after fabrication, a 10 nm thick SiN capping layer. Annealing experiments were performed either in situ in a TEM, following a procedure minimizing experimental artefacts, as presented in detail by Luong et al 32 , or ex situ, in a conventional annealing furnace, under nitrogen flux. XRD measurements were carried out using a Bruker D8 Discover diffractometer with a Cu -Ka1 source (λ = 1.5406 Å).…”

Revealing the Influence of Oxygen Doping on the Crystallization Kinetics of Sb-GeO2

“…In the past decade, many studies focus on the thermal stability of PCMs by introducing gas elements, such as oxygen − and nitrogen. − Especially, the oxygen-doped Sb-based alloys, such as O 2 -doped Ge-Sb-Te, − Ge-Sb, , Sn-Sb, , Zn-Sb, and Si-Sb, have exhibited an improvement in thermal stabilities. However, the excess O 2 doping would bring the negative effects in the phase-change performance, such as the low phase-transition speed.…”