High-Throughput Calculations on the Decomposition Reactions of Off-Stoichiometry GeSbTe Alloys for Embedded Memories

Abstract

Chalcogenide GeSbTe (GST) alloys are exploited as phase change materials in a variety of applications ranging from electronic non-volatile memories to neuromorphic and photonic devices. In most applications, the prototypical Ge2Sb2Te5 compound along the GeTe-Sb2Te3 pseudobinary line is used. Ge-rich GST alloys, off the pseudobinary tie-line with a crystallization temperature higher than that of Ge2Sb2Te5, are currently explored for embedded phase-change memories of interest for automotive applications. During crystallization, Ge-rich GST alloys undergo a phase separation into pure Ge and less Ge-rich alloys. The detailed mechanisms underlying this transformation are, however, largely unknown. In this work, we performed high-throughput calculations based on Density Functional Theory (DFT) to uncover the most favorable decomposition pathways of Ge-rich GST alloys. The knowledge of the DFT formation energy of all GST alloys in the central part of the Ge-Sb-Te ternary phase diagram allowed us to identify the cubic crystalline phases that are more likely to form during the crystallization of a generic GST alloy. This scheme is exemplified by drawing a decomposition map for alloys on the Ge-Ge1Sb2Te4 tie-line. A map of decomposition propensity is also constructed, which suggests a possible strategy to minimize phase separation by still keeping a high crystallization temperature.

Crystallization and Electrical Properties of Ge-Rich GeSbTe Alloys

“…The free energy consisted of the total energy at zero temperature and the configurational free energy (at room temperature) due to disorder in the cubic sublattices. The calculated formation free energy is lower for alloys along the GeTe-Sb 2 Te 3 tie-line and on the Sb-GeTe isoelectronic line [15].…”

“…The experimental findings can be better understood with the support of DFT calculations. In a previous work [15], we have computed the DFT formation free energy, with respect to the elements in their standard states, of cubic GST alloys in the central region of the ternary phase diagram. The free energy consisted of the total energy at zero temperature and the configurational free energy (at room temperature) due to disorder in the cubic sublattices.…”

“…In Ref. [15], this scheme was exemplified by studying the decomposition pathways of Ge-rich alloys on the Ge-GeSb 2 Te 4 tie-line. Following the same methodology, we have computed the decomposition maps for the two alloys with high Ge amounts studied here.…”

“…Finally, we have computed the reaction free energy for the decomposition pathways of H-Ge-ST and H-Ge-GST alloys, by using the DFT data on the formation free energy of cubic alloys in the central part of the ternary Ge-Sb-Te phase diagram reported in our previous work, [15] which we refer to for all details.…”

“…Thus, such a phase separation process is more likely to occur upon crystallization. The phase separation of Ge-Ge 1 Sb 2 Te 4 alloys during crystallization has been very recently studied by means of high-throughput density functional theory (DFT) calculations [15]. It has been shown that the Ge-rich Ge 1 Sb 2 Te 4 alloys become more prone to decompose into crystalline Ge and a GST alloy less rich in Ge, as Ge amount is above 50 at %.…”