Measurement of crystal growth velocity in a melt-quenched phase-change material

Abstract

Phase-change materials are the basis for next-generation memory devices and reconfigurable electronics, but fundamental understanding of the unconventional kinetics of their phase transitions has been hindered by challenges in the experimental quantification. Here we obtain deeper understanding based on the temperature dependence of the crystal growth velocity of the phase-change material AgInSbTe, as derived from laser-based time-resolved reflectivity measurements. We observe a strict Arrhenius behaviour for the growth velocity over eight orders of magnitude (from ~10 nm s−1 to ~1 m s−1). This can be attributed to the formation of a glass at elevated temperatures because of rapid quenching of the melt. Further, the temperature dependence of the viscosity is derived, which reveals that the supercooled liquid phase must have an extremely high fragility (>100). Finally, the new experimental evidence leads to an interpretation, which comprehensively explains existing data from various different experiments reported in literature.

Crystal growth within a phase change memory cell

“…In general, DG(T) will be larger than 0 for ToT melt (that is, the crystalline phase is energetically more favourable than the liquid phase), equal to 0 at T ¼ T melt , and smaller than 0 for T4T melt (that is, the liquid phase is more favourable than the crystalline phase). The expression for DG commonly used for phase change materials is the ThompsonSpaepen approximation 16 , which was also employed by Orava et al 14 and Salinga et al 15 :…”

“…These measurements were conducted on as-deposited phase change materials. A more recent effort used laser-based time-resolved reflectivity measurements 15 . Despite pushing up the temperature range over which the crystallization rate was measured, both techniques were limited to temperatures below the temperature of the maximum growth velocity and far below the melting temperature.…”

“…A widely accepted model describing the temperature dependence of crystal growth is 15 v g ðTÞ ¼ 4r atom Á k B T 3pl 2 R hyd Á 1 ZðTÞ Á 1 À exp À DGðTÞ…”

Growth of spherulites: foundation of the DSC analysis of solidification

“…The existence of diffusion-controlled crystal growth is evident from the proportionality of crystal growth rate and diffusivity for crystallization solution and from the inverse proportionality between crystallization rate and viscosity for crystallization from melt, observed at large enough undercooling [9]. In conclusion, temperature changes are able to affect solidification from melt of homogeneous system when they cause an appreciable viscosity change of the liquid phase [10]. In other words, crystallization rate is also controlled by the mobility that atoms or molecules have in the liquid state.…”

“…The function (10), determined by the analysis of DSC crystallization peaks explained in paragraph 3, is indeed almost universally found to have a typical ''sigmoidal'' shape (the adjective sigmoidal is here used to indicate a monotonically crescent function with an inflection point).…”