Ge-Rich GST Phase-Change Alloys: Thorough Investigation of Structural Evolution in Temperature
Authors: Prazakova Lucie , Nolot Emmanuel , Martinez Eugénie , Fillot Frederic , Rouchon Denis , Bernier Nicolas , Rochat Névine , Sabbione Chiara , Morel Damien , Bernard Mathieu , Cyrille Marie-Claire , Navarro Gabriele 
Published: December 2020

Phase-Change Memory (PCM) is considered the most mature among the emerging Non-Volatile Memory technologies. Indeed, PCM features fast programming speed, excellent scalability down to nm scale and high endurance. It relies on the reversible and fast transition between a highly resistive amorphous phase and a low resistive crystalline phase of a chalcogenide phase-change alloy. PCM’s high maturity is demonstrated by its commercialization in Storage Class Memory (SCM) market [1] and by the recent demonstration of its manufacturability and reliability in 28 nm technology node for Automotive applications [2]. Moreover, PCM is considered the best-suited device for emerging neuromorphic systems and in-memory computing applications [3].
For several years, PCM has been considered not suitable to target stability at high temperature: indeed, devices based on standard material Ge2Sb2Te5 (GST225) trigger crystallization at 150°C, limiting the target applications portfolio. With the development of Ge-rich GeSbTe (Ge-rich GST) materials [4], improved PCM stability was demonstrated, fulfilling the strict requirements in terms of data retention of embedded applications, rising the questions on the origin of such improvements.
Recent works on Ge-rich GST showed the effect of Ge enrichment and N doping in such alloys, in particular highlighting the increased crystallization temperature and the phase segregation at high temperature [5, 6]. However, the fine engineering of Ge-rich alloys requires a deep investigation of their structure and the understanding of its evolution in temperature.
In this work, we present a thorough investigation of GeSbTe system, comparing the alloys with and without Ge enrichment, and with and without N doping. Structural analysis is performed by a wide set of complementary techniques: Raman spectroscopy, X-Ray Diffraction (XRD) and FTIR spectroscopy, TEM/EDX analyses and electrical resistivity measurements. All the analyses were performed on as-deposited amorphous samples and on samples annealed at several temperatures ranging up to more than 400°C.
Comparing the Raman spectra of Ge-rich alloys with the spectra of GST225, we highlight the importance of Sb-Te structural units and their stability at all the steps of the evolution of the layer structure in temperature. The rearrangement of Ge-Te bonds, with the following crystallization of GeSbTe phase, is accompanied by the evolution of amorphous Ge phase in the layer towards the crystalline one (e.g. Fig. 1), demonstrated also in XRD spectra and TEM images. Ge single-element layer analyses allow us to evidence the important impact of the presence of Sb-Te bonds on Ge crystallization.
Correlation of the Ge-N modes in N-doped Ge and Ge-rich GST samples by FTIR spectroscopy shows the effect of N on the formation of a GeN phase in GeSbTe alloy and its evolution in temperature (e.g. Fig. 2). N doping retards the rearrangement of Ge-Te bonds and the consecutive crystallization of Ge, which is delayed at higher temperature with regard to undoped sample. The sequence of phases that appear in the system with the increasing temperature is demonstrated by XRD spectra, performed on both doped and undoped layers.
In summary, in this work we present a thorough investigation of the structural evolution of Ge-rich GST system in temperature thanks to the correlation of spectra and data coming from a wide set of techniques. These results help in understanding the mechanisms behind the crystallization and layer segregation, and in designing next-generation of PCM for automotive applications.
[1] Huai-Yu Cheng et al., 2019 J. Phys. D: Appl. Phys. 52 473002.
[2] Paolo Cappelletti et al., 2020 J. Phys. D: Appl. Phys. 53 193002.
[3] T. Kim et al., IEEE TED, vol. 67, no. 4, pp. 1394-1406.
[4] P. Zuliani et al., 2019 IEEE 11th International Memory Workshop, pp. 1-4.
[5] V. Sousa et al., 2015 VLSI Technology, pp. T98-T99.
[6] M. Agati et al., J. Mater. Chem. C, 2019, 7, 8720.

Figure 1