Fragile‐to‐Strong Transition in Phase‐Change Material Ge₃Sb₆Te₅

Abstract

Chalcogenide phase‐change materials combine a remarkable set of properties that makes them promising candidates for future non‐volatile memory applications. Binary data storage exploits the high contrast in electrical and optical properties between the covalent amorphous and metavalent crystalline phase. Here the authors perform an analysis of the liquid phase kinetics of the phase‐change material Ge3Sb6Te5, which is the key to ultrafast switching speeds. By employing four experimental techniques, the viscosity is measured over sixteen orders of magnitude despite its propensity for fast crystallization. These measurements reveal that the liquid undergoes a transition in viscosity–temperature dependence associated with a liquid–liquid phase transition. The system exhibits a shallow viscosity change with temperature near the glass transition which stabilizes the memory cells in the amorphous state and which limits the severity of relaxation processes. Meanwhile, when heated during the writing process, the fragility increases to more than double, causing the viscosity to drop rapidly enabling a nanosecond crystallization speed. This change in viscosity–temperature dependence is highly unusual among glass forming liquids and is reminiscent of the behavior of water. This viscosity transition is also key to the technological success of phase‐change materials for computer memory applications.

Resistance Drift Convergence and Inversion in Amorphous Phase Change Materials

“…We selected this material because it has recently shown promise as a PCM due to the very high crystal growth speeds mandatory for memory applications. [ 52 ] Furthermore, it features a fragile‐to‐strong transition (FST), [ 52 ] which is beneficial for fast crystallization at high temperatures and amorphous phase stability at low temperatures. [ 53 ] The change in excess specific heat capacity C p exc ( T ) of the Ge 3 Sb 6 Te 5 glassy phase is shown in Figure a during upscans at 40 °C min −1 similar to Ge 15 Te 85 shown in Figure 1 a.…”

“…The annealing temperature was then increased by 10 °C to 60 °C and the sample was annealed for another hour and cooled back to RT and so forth. The resistivity relaxation was monitored during each isothermal annealing step (see Supporting Information [ 52 ] for the complete data set). Since the relaxation time is large at temperatures far below T g , the glass is not expected to stabilize within one hour but the fictive temperature should continuously decrease.…”

“…step (see Supporting Information [52] for the complete data set). Since the relaxation time is large at temperatures far below T g , the glass is not expected to stabilize within one hour but the fictive temperature should continuously decrease.…”

Glassy and liquid Sb2S3: insight into the structure and dynamics of a promising functional material

“…Benchmark telluride PCMs are characterized by low viscosity and high fragility of the melt [112][113][114][115][116] ensuring fast atomic mobility at elevated temperature, which enables rapid crystallization within the nanosecond range, and a good retention of the amorphous phase in the vicinity of T g and at lower T. The simulated dynamics in Sb 2 S 3 reveals fast diffusion at high temperatures and a sudden decrease of diffusivity approaching the glass transition.…”

“…The m(As 2 S 3 ) = 26 is typical for canonical chalcogenide glasses with a network structure. [113][114][115][116]125,126 Antimony sesquisulfide exhibits a higher fragility, m (Sb 2 S 3 ) = 59, approaching that of telluride PCMs. [113][114][115][116] We should also note good agreement between the experimental Z(T) and calculated Z FPMD (T) viscosity.…”