Effects of Underlayer Engineering on Crystallization Kinetics of Ge-Rich GeSbTe Phase-Change Memory Alloys

Phase change memory (PCM) is recognized as one of the most advanced and promising non-volatile memory technologies due to its high scalability, endurance, and retention time [1]. Among phase-change materials, Ge-rich GeSbTe (GGST) chalcogenide alloys are particularly noteworthy for their superior thermal stability compared to congruent compositions, a key attribute to address the strict reliability specifications of embedded automotive applications [2].<br/>There is a growing interest in understanding the effects on crystallization kinetics of GGST in presence of various interfacial systems. Encapsulation layers can function as oxidation barriers or promote heterogeneous nucleation of Ge through interfacial chemical reactions [3]. Underlayers (UL) composed of Ge-deficient alloys serve as pathways for Ge atomic diffusion from GGST via grain boundaries, providing a crucial engineering tool to modulate the crystallization kinetics of GGST layers [4]. However, the impact of ULs with varying starting crystallization states and stoichiometries on GGST has not yet been comprehensively studied.<br/>In this work, we investigate the effects of GST-based and Ge-based ULs, prepared under different thermal conditions, on the crystallization process of GGST bulk phase-change layers. The evolution of the UL/PCM interface under different temperature profiles reveals the atomic Ge absorption kinetics of the bottom layers, resulting in a delay in the stack crystallization temperature. This phenomenon is distinctly observed through film resistivity measurements using four-point probes coupled with a hot plate. The Ge concentration mismatch after diffusion influences the crystallographic arrangement of Sb-Te and Ge grains within the bulk material, as evidenced by a combination of physicochemical characterization techniques, including Raman spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM/EDX) analyses.<br/>In conclusion, our analysis of GGST systems interfaced with various ULs highlights the important effect of UL structure and stoichiometry on the phases’ segregation and crystallization kinetics in Ge-rich GeSbTe alloys. We believe that these results can provide important insights into possible engineering pathways for delaying and controlling segregation phenomena in Ge-rich GeSbTe systems.<br/><br/>[1] H.-S. P. Wong et al., « Phase Change Memory », Proc. IEEE, vol. 98, no 12, p. 2201-2227, dec. 2010, doi: 10.1109/JPROC.2010.2070050.<br/>[2] P. Zuliani et al., « Overcoming Temperature Limitations in Phase Change Memories With Optimized GexSbyTez », IEEE Trans. Electron Devices, vol. 60, no 12, p. 4020-4026, dec. 2013, doi: 10.1109/TED.2013.2285403.<br/>[3] O. Daoudi et al., « Encapsulation Effects on Ge-Rich GeSbTe Phase-Change Materials at High Temperature », Physica Rapid Research Ltrs, p. 2300448, févr. 2024, doi: 10.1002/pssr.202300448.<br/>[4] M. Luong, E. Rahier, S. Ran, et A. Claverie, « Crystallization of Amorphous N-Doped Ge-Rich GST Layers Deposited on a Polycrystalline GST Template », Physica Rapid Research Ltrs, p. 2300421, fev. 2024, doi: 10.1002/pssr.202300421.