Characterization of supercooled liquid Ge2Sb2Te5 and its crystallization by ultrafast-heating calorimetry

Abstract

Differential scanning calorimetry (DSC) is widely used to study the stability of amorphous solids, characterizing the kinetics of crystallization close to the glass-transition temperature T(g). We apply ultrafast DSC to the phase-change material Ge(2)Sb(2)Te(5) (GST) and show that if the range of heating rates is extended to more than 10(4) K s(-1), the analysis can cover a wider temperature range, up to the point where the crystal growth rate approaches its maximum. The growth rates that can be characterized are some four orders of magnitude higher than in conventional DSC, reaching values relevant for the application of GST as a data-storage medium. The kinetic coefficient for crystal growth has a strongly non-Arrhenius temperature dependence, revealing that supercooled liquid GST has a high fragility. Near T(g) there is evidence for decoupling of the crystal-growth kinetics from viscous flow, matching the behaviour for a fragile liquid suggested by studies on oxide and organic systems.

Crystal growth within a phase change memory cell

“…Despite the tremendous increase of the temperature range over which recent studies have investigated the crystal growth velocity, still only the left side of T cryst had been probed 14,15 . Going beyond those studies, we also probed the dynamic behaviour of the meltquenched material around and above the maximum of the growth velocity all the way up to the melting temperature.…”

“…A fixed value of n E was assumed for all temperatures and can be estimated from experiments conducted at low ambient temperatures at which the crystal growth rate is negligible. The threshold switching voltage data V th (t) (shown in Supplementary Note 1) was then fitted to equation (14) in conjunction with equations (15) and (5) with v g 0 ðTÞ, E 0 th and Q as fitting parameters. The resulting u a (t) and v g 0 ðTÞ are shown in Fig.…”

“…Most experimental investigations on the crystal growth velocity have been limited to low temperatures, where the crystallization rate is small. A recent effort to measure the growth rate is that by Orava et al 14 , who used ultrafast differential scanning calorimetry. These measurements were conducted on as-deposited phase change materials.…”

“…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 :…”

Glass Transitions, Semiconductor-Metal Transitions, and Fragilities in Ge−V−Te ( V=As , Sb) Liquid Alloys: The Difference One Element Can Make

“…The knowledge of glass transition behavior and kinetic properties of the supercooled liquid state are crucial for applications and the understanding of this class of materials, as they are related to structural relaxations, aging, stability, processability, crystallization kinetics [4], etc. In particular, the temperature dependence of viscosity plays an important role in nucleation and growth rates of crystals and is receiving increasing attention in the field of phase-change materials [5].…”