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Seminar | X-Ray Science Division

Characterization of the Crystallization Dynamics of Phase-Change Materials (PCMs)via Single-Shot Ultrafast Electron Diffraction (UED)

XSD/TRR Seminar

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 [1]. 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 [2]. 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 [1].

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 [3]. 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. 

References

[1] H. S. P. Wong, S. Raoux, et al., Phase Change Memory,” Proc. IEEE, 98 (2010). 

[2] 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).

[3] 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).