Oxidation Process in Ge-Rich GeSbTe Alloy for Phase-Change Memory: Mechanism, Kinetic and N-Doping Influence
Authors: Goffart Ludovic , Vallée Christophe , Navarro Gabriele , Reynard Jean-Philippe , Pelissier Bernard , Lefèvre Gauthier 
Published: December 2020

Phase-change materials (PCMs) based on GeSbTe alloys are well known and widely used in optical storage devices. The recent progress in scaling and material engineering made PCMs also promising candidates for high-speed non-volatile memory applications [1]. Indeed, Phase-Change Memory (or PCM) devices feature high writing and reading speed and compatibility with high working and storage temperatures.
Ge-rich GeSbTe is a promising material for embedded applications requiring high data stability at high temperature, thanks to its higher crystallization temperature wrt standard PCMs such as Ge2Sb2Te5
[2-3]. Moreover, N doping in Ge-rich GeSbTe was also introduced in order to stabilize even more the material at high temperature, and improve its programming performances [4-5]. However, fabrication steps like etching or stripping that imply an exposure of the PCMs to air, could affect the material properties and the final device performance [6]. It is known that Ge is particularly sensitive to oxidation, and this makes even more important to well understand and study such phenomena in Ge-rich GeSbTe. Recent observations already reported the appearing of segregation mechanisms due to surface oxidation in Ge2Sb2Te5 and Ge-rich GeSbTe [7]. Such material degradation could significantly affect the final device performance and the electrical parameters variability. Therefore, the understanding of the physics of these mechanisms become fundamental for improving the fabrication steps for future further improvements of the device (e.g. new barriers, new etching strategies, etc.).
This study focuses on the oxidation mechanism and on its kinetic in Ge-rich GeSbTe, and how it is influenced by N doping. We analyzed the oxidation in amorphous and crystalline samples, at room temperature and after annealing of increasing duration at high temperature in order to accelerate oxidation process. Using surface characterization by pARXPS (parallel angular resolved XPS), we monitor with precision the evolution in time of the oxide thickness (figure 1) and of the composition of different N-doped Ge-rich GeSbTe samples (figure 2). We report the evolution of our samples along more than one year, showing a double steps oxidation mechanism, with a kinetic strongly dependent on N content. Oxidation speed and thickness are then correlated with N content, leading to an analytical model that is in agreement with all the data acquired (figure 3), and that allows to describe the ongoing oxidation and segregation phenomena. TEM images completed by EDX analyses (figure 4) are here shown to support our findings.
In this work, we present the study of the oxidation mechanism and kinetic in N-doped Ge-rich GeSbTe materials by pARXPS, TEM and EDX analyses. We propose an analytical model for the oxidation kinetic based on our data, highlighting the strong impact of N doping concentration on the parameters of our model.

[1] G. Navarro et al., “Phase-Change Memory: Performance, Roles and Challenges,” in 2018 IEEE International Memory Workshop (IMW), Kyoto, 2018, pp. 1–4, doi: 10.1109/IMW.2018.8388845.

[2] P. Zuliani et al., "Overcoming Temperature Limitations in Phase Change Memories With Optimized GexSbyTez" in IEEE Transactions on Electron Devices, vol. 60, no. 12, pp. 4020-4026, Dec. 2013.

[3] J. Kluge et al., “High Operating Temperature Reliability of Optimized Ge-Rich GST Wall PCM Devices,” 2016, pp. 1–4, doi: 10.1109/IMW.2016.7495273

[4] G. Navarro et al., “Trade-off between SET and data retention performance thanks to innovative materials for phase-change memory,” 2013, pp. 21.5.1-21.5.4, doi: 10.1109/IEDM.2013.6724678.

[5] A. Kiouseloglou et al., “A Novel Programming Technique to Boost Low-Resistance State Performance in Ge-Rich GST Phase Change Memory,” IEEE Transactions on Electron Devices, vol. 61, no. 5, pp. 1246–1254, May 2014, doi: 10.1109/TED.2014.2310497.

[6] P. Noé, C. Sabbione, N. Bernier, N. Castellani, F. Fillot, and F. Hippert, “Impact of interfaces on scenario of crystallization of phase change materials,” Acta Materialia, vol. 110, pp. 142–148, May 2016, doi: 10.1016/j.actamat.2016.03.022.

[7] M. Agati, C. Gay, D. Benoit, and A. Claverie, “Effects of surface oxidation on the crystallization characteristics of Ge-rich Ge-Sb-Te alloys thin films,” Applied Surface Science, vol. 518, p. 146227, Jul. 2020, doi: 10.1016/j.apsusc.2020.146227.

Figure 1