N Content and Stoichiometry Impact on Crystallization Kinetics of Ge-Rich GeSbTe Alloys

Phase–Change Memory (PCM) is a mature technology based on chalcogenide alloys, which has proven its suitability for next generation of non–volatile memory, in particular targeting embedded applications [1]. This result was achieved thanks to the intrinsic features of PCM in terms of scalability, fast programming and high endurance. Enrichment of GeSbTe (GST) alloys by Ge (GGST) and the introduction of light elements demonstrated to be a successful solution to enhance the data retention in PCM. This opens the possibility for this technology to target automotive applications, featuring strict specifications in terms of stability in high temperature environment [2-4]. The optimized combination of N introduction and increased Ge content delayed phase separation and crystallization processes of Ge and GST phases. This delay can be attributed to the reduced mobility of Ge atoms caused by the formation of Ge-N bonds [4, 5].<br/>In this work, we performed <i>in situ</i> X-ray diffraction to assess how N and stoichiometry influence GGST materials during thermal annealing. Thus, the need for a beam with high energy and rapid measurements were necessary for following the amorphous-to-crystalline transition effectively.<br/>These experiments took place at BM2-D2AM Beamline of the ESRF in November 2022. A monochromatic beam with energy of 10.9 keV and a 2D detector were used to record the scattered intensity. The result patterns were subsequently corrected and integrated to derive 1D diffraction patterns for analyses and interpretation. The diffraction peaks were fitted with Lorentzian function to extract different parameters (area, height and full width at half maximum) giving information about the crystallization evolution. In the present work, we performed annealing with different heating ramp ups for recovering activation energy by using Kissinger law [6], and then quantifying the impact of N in the different compositions.<br/>These measurements allowed us to observe the following phenomena:<br/>N increases the energy barrier for the crystallization of Ge cubic phase in all the GGST systems studied, while a limited impact is observed on the crystallization kinetic of the GST phase;<br/>- The growth rate of the Ge phase is confirmed to be really high but likely limited at the interface of GeSbTe grains (i.e. where the heterogeneous nucleation of Ge takes place), i.e. the Ge grains size reaches very fast a plateau, and starts to slowly increase again only at temperatures higher than 450 °C;<br/>- The GST phase shows a linear evolution (with respect to Ge phase), with dependent growth rate on the initial stoichiometry of the deposited alloy.<br/>- The evolution of the parameters extracted from our measurements (i.e. crystallization temperatures, crystals sizes etc.) will be presented comparing different GGST compositions. These results will be supported with <i>ex situ</i> Raman and IR spectroscopies as well as TEM-EDX analyses.<br/>These results bear new insights for a better understanding of the crystallization process in Ge-rich GST and the impact of N introduction in such alloys.<br/>Haut du formulaire<br/><br/>[1] P. Cappelletti, et al., JAP 53, 193002 (2020).<br/>[2] P. Zuliani, et al. IEEE TED 60, 4020–4026 (2013).<br/>[3] I. Yang et al., H. J. Electrochem. Soc. 157, H483 (2010).<br/>[4] L. Prazakova et al., JAP 128, 215102 (2020).<br/>[5] G. Navarro et al., (IMW), (2016).<br/>[6] R. L. Blaine et al., (Thermochimica Acta), (2012).