Isothermal Annealing Effects on $\beta$-Relaxations and Crystallization Behaviors in Amorphous GeTe

Abstract

A secondary $\beta$-relaxation process is often the dominant source of atomic dynamics below $T_\mathrm{g}$ in many glass forming systems. Recent studies reported the presence of $\beta$-relaxations in amorphous phase-change materials (PCMs) and showed that suppressing the $\beta$-relaxation via annealing in Ge$_{15}$Sb$_{85}$ can effectively slow down its crystallization kinetics. Yet, when Sb is replaced by Te, similar annealing protocol has little effect on the Te-rich alloy Ge$_{15}$Te$_{85}$. Here, we investigate amorphous GeTe that is a Sb-free PCM, but with faster crystallization kinetics than Ge$_{15}$Te$_{85}$. Using powder mechanical dynamic spectroscopy, we observe a clear reduction of the excess-wing in the loss modulus upon isothermal annealing, indicating a suppression of its $\beta$-relaxation. Ultrafast calorimetric analysis and time-resolved optical reflectivity measurements show that, whereas as-deposited GeTe exhibit stochastic crystallization behaviors, annealed samples crystallize more slowly with reduced stochasticity. Synchrotron X-ray scattering experiments reveal reinforced Peierls-like distortions in the amorphous structure after annealing, and demonstrate that, even if annealing introduces nucleation sites, it nonetheless slows down crystallization kinetics. These finding suggests that, in annealed GeTe, crystallization is limited by crystal growth rate, which is retarded through the suppression of $\beta$-relaxation.