Abstract: Phase change materials (PCMs) are semiconducting alloys that rapidly and reversibly switch between an amorphous, or glassy state and a crystalline, or ordered state via electrical and optical pulses . The two phases have optical and electrical properties that vary by orders of magnitude, which means that they can be used in fast, non-volatile memory devices . However, a fundamental understanding of the complex nucleation and growth processes that underlie the rate-limiting step of switching from the amorphous to crystalline state remains to be developed .
In the first part of the talk, I will demonstrate via in situ electron diffraction that femtosecond optical excitation above a threshold fluence of the amorphous, as-deposited, PCM Ge2Sb2Te5creates grains 2 orders of magnitude larger than previously reported . This was confirmed with transmission electron microscopy coupled with custom MATLAB analysis. I will present a theoretical model which shows that this arises from a crossover from a nucleation-dominated crystallization régime to a growth-dominated crystallization régime, and I will show that the measured grain size is consistent with Johnson–Mehl–Avrami–Kolmogorov (JMAK) crystallization theory for temperatures near the melting temperature. In the second part of the talk I will show, using the MeV Ultrafast Electron Diffraction (MeV-UED) instrument at the SLAC National Accelerator Laboratory, preliminary time-resolved crystallization data of pure Sb, which has shown promise as a PCM. The single-shot nature of the experiment necessitates the aligning and the centering of noisy, amorphous diffraction patterns, and I will show how that was done, thereby allowing us to analyze our data. This work will give us a better understanding of the rate-limiting step in these materials and what gives PCMs their unique properties.
 H. S. P. Wong, S. Raoux, et al., “Phase Change Memory,” Proc. IEEE, 98 (2010).
 A. K. U. Michel, P. Zalden, et al., “Reversible Optical Switching of Infrared Antenna Resonances with Ultrathin Phase-Change Layers Using Femtosecond Laser Pulses,” ACS Photonics, 1 (2014).
 M. Zajac, A. Sood et al., “Synthesis of Macroscopic Single Crystals of Ge2Sb2Te5 via Single-Shot Femtosecond Optical Excitation,” ACS Crystal Growth and Design, 20 (2020).