Crystallization of Ge-rich GeSbTe Alloys

The riddle is solved

August 31, 2022

Ge-rich GeSbTe (GST) alloys are of considerable interest for embedded digital memories and neuromorphic devices. Up to now, the mechanisms by which such alloys crystallize remained unclear and very controversial. Using in situ synchrotron X-ray diffraction during isothermal annealing and advanced TEM techniques, we unveiled the mechanisms leading to the overall crystallization of such alloys.

Our previous ex situ XRD studies have shown that the incubation time, i.e. the time after which the onset of crystallization is observed, is of about several tens of minutes at 330°C. We decided to investigate the 310-330°C temperature range in detail, in an effort to slow down and decompose the different mechanisms which intervene in the crystallization process. The great angular and temporal resolution offered in diffraction by the synchrotron allow us to characterize these different steps.

Left: Synchrotron X-ray diffraction map obtained during in-situ isothermal annealing at 310°C on a Ge-rich GST sample as a function of annealing time.
Right: Graphical representation of the crystallisation process of Ge-rich GST alloys proposed on the basis of experimental research and calculations.

During in-situ annealing at 310°C, the initially homogeneous and amorphous material undergoes a progressive phase separation leading to the formation of Ge-rich regions of different compositions. During this decomposition, first formed GeTe embryos crystallize and trigger the heterogeneous crystallization of the Ge cubic phase. As the phase separation proceeds, these embryos dissolve and the Ge phase gradually builds up through the nucleation of small grains. Only when this Ge cubic phase is largely formed, the remaining amorphous matrix may locally reach the Ge2Sb2Te5 composition at which it can crystallize as large grains. Our density functional theory calculations confirm that the quite exotic Pnma GeTe structure we have experimentally identified is more stable than the regular R3m structure at nanometric sizes.

These results unveil the intriguing characteristics of the three-step crystallization of Ge-rich alloys and close a long-lasting controversy concerning the order and the scenario of crystallization in these alloys.

Crystallization of Ge-Rich GeSbTe Alloys: The Riddle Is Solved
Eloïse Rahier, Sijia Ran, Nicolas Ratel Ramond, Shuangying Ma, Lionel Calmels, Sabyasachi Saha, Cristian Mocuta, Daniel Benoit, Yannick Le Friec, Minh Anh Luong, and Alain Claverie
ACS Appl. Electron. Mater. 4, 6, 2682 (2022)

Alain Claverie – alain.claverie[at]
Eloïse Rahier – eloise.rahier[at]

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