Phase Separation in Ge-Rich GeSbTe at Different Length Scales: Melt-Quenched Bulk versus Annealed Thin Films

Abstract

Integration of the prototypical GeSbTe (GST) ternary alloys, especially on the GeTe-Sb2Te3 tie-line, into non-volatile memory and nanophotonic devices is a relatively mature field of study. Nevertheless, the search for the next best active material with outstanding properties is still ongoing. This search is relatively crucial for embedded memory applications where the crystallization temperature of the active material has to be higher to surpass the soldering threshold. Increasing the Ge content in the GST alloys seems promising due to the associated higher crystallization temperatures. However, homogeneous Ge-rich GST in the as-deposited condition is thermodynamically unstable, and phase separation upon annealing is unavoidable. This phase separation reduces endurance and is detrimental in fully integrating the alloys into active memory devices. This work investigated the phase separation of Ge-rich GST alloys, specifically Ge5Sb2Te3 or GST523, into multiple (meta)stable phases at different length scales in melt-quenched bulk and annealed thin film. Electron microscopy-based techniques were used in our work for chemical mapping and elemental composition analysis to show the formation of multiple phases. Our results show the formation of alloys such as GST213 and GST324 in all length scales. Furthermore, the alloy compositions and the observed phase separation pathways agree to a large extent with theoretical results from density functional theory calculations.

Synthesis, Properties and Applications of Germanium Chalcogenides

“…The alloy compositions and the observed phase separation pathways reported in [ 4 , 6 ] agree to a large extent with the theoretical results from the density functional theory calculations, as presented in [ 8 ], where O. Abou El Kheir and M. Bernasconi perform high-throughput calculations to uncover the most favorable decomposition pathways of Ge-rich GST alloys. They also construct a map of decomposition propensity, suggesting a possible strategy to minimize phase separation while still maintaining a high crystallization temperature.…”

“…Five articles of this Special Issue focus on Ge-rich GST alloys, exploring their electronic and electrical properties [ 4 , 5 , 6 , 7 ] as well as decomposition pathways, including from a theoretical point of view [ 8 ].…”

“…The contribution of D. Tadesse Yimam et al [ 6 ] investigates the phase separation of GST523 into multiple phases in melt quenched bulk and annealed thin films, identifying the formation of GST123 and GST324 alloys in all length scales.…”

Characterizing optical phase-change materials with spectroscopic ellipsometry and polarimetry

“…Several authors have already used dynamic ellipsometry to establish the crystallization temperature of several PCMs such as Sb [42], Sb 2 Te 3 or Ge 2 Sb 2 Te 5 [43]. To illustrate this procedure, and as an example, Fig.…”

“…5 shows a scheme of this technique and recent results of its application to the observation of transient effects in crystalline PCM thin film. A wide range of events happens during light-induced phase transitions of PCM between time scales from 0.1 -100 ps [42][43][44][45][46][47][48][49][50][51][52][53][54] and time-resolved ellipsometry in complement with ultrafast structural methods are ideal tools for probing these dynamics.…”