Correlating ultrafast calorimetry, viscosity, and structural measurements in liquid GeTe and Ge15Te85

Abstract

Two distinct trends in the temperature dependence of viscosity, measured directly and inferred from calorimetry by analyzing crystallization kinetics, can be correlated with the temperature evolution of the height of the first peak of the x-ray total structure factor for liquid GeTe and Ge 15 Te 85 . The phase-change chalcogenide GeTe is a high-fragility liquid with the kinetic fragility value of 76, at the glass-transition temperature, being between those for liquid (Ag,In)-doped Sb 2 Te and Ge 2 Sb 2 Te 5 . The viscosity of the high-temperature liquid shows Arrhenius kinetics on cooling to the melting point, and the structure factor conforms to the fragile liquid. For liquid Ge 15 Te 85 , the temperature evolution of the structure factor suggests a transition in the temperature range of about 100 K above the melting. The crystallization shows a wide range of Arrhenius kinetics in the supercooled liquid region. This finding combined with the dynamic viscosity measurements is interpreted by invoking a weak fragile-to-strong crossover on cooling the liquid Ge 15 Te 85 . The differences in structures and dynamics of liquid GeTe and Ge 15 Te 85 appear closely correlated to their distinctly different crystallization mechanisms.

Recipe for ultrafast and persistent phase-change memory materials

“…The enhanced kinetic contrast due to the FTS crossover enables high atomic mobility at elevated temperatures for rapid crystal growth in a less-viscous matrix while suppressing atomic diffusion near room temperature for good data retention in a more-viscous state. If the FTS crossover occurs at either below T g (e.g., in Ge 2 Sb 2 Te 5 ) 17 or above T m (e.g., in Ge 15 Te 85 ) 37 , it would cause relatively less-or more-viscous flow with inferior data retention or slower crystal growth, respectively. Regarding this, a desirable FTS crossover for an ultrafast and persistent PCM should occur immediately below the typical programming temperatures for crystallization (<~600-700 K) and considerably above its T g (>~400 K), as exemplified by the cases of Sc x Sb 2 Te 3 (x =~0.1-0.3).…”

Switching between Crystallization from the Glassy and the Undercooled Liquid Phase in Phase Change Material Ge2Sb2Te5

“…PCMs like Ge 2 Sb 2 Te 5 , GeTe, and (Ag,In)‐doped Sb 2 Te (AIST) generally show high fragilities and bad glass‐forming abilities, while related chalcogenides like GeSe are non‐PCMs that exhibit better glass‐forming abilities with lower fragilities and the presence of a calorimetric glass transition . The question arises what is the origin of such distinctive differences between PCMs and non‐PCMs, since GeTe and GeSe are isolelectronic and should hence have similar bonding mechanisms.…”

Kinetics Features Conducive to Cache-Type Nonvolatile Phase-Change Memory

“…The FTS crossover was also observed in many other chalcogenide films, such as Ge 15 Te 85 , and AIST, ,, as well as GST and GeTe nanoparticles, which may originate from the increase of Peierls distortion in short range, and the formation of an energetically favorable network at or beyond medium range, during quenching of the supercooled liquid. , As Peierls distortion increases, electrons become more localized between atoms, exerting constraints on atomic migration and consequently increasing the activation barrier of viscous flow. An enhanced kinetic contrast due to FTS crossover thus enables high atomic mobility at elevated temperatures for rapid crystal growth and suffocated atomic diffusion near room temperature for good data retention.…”