Shuai Wei

Abstract:

The phase-change materials (PCMs) such as Ge-Sb-Te alloys can be reversibly switched between amorphous and crystalline states on a timescale of nanoseconds. The phase switch provides the material basis for next-generation non-volatile phase-change memory devices. I show experimental studies of PCM kinetics spanning over 1000 K in temperature and 16 orders of magnitude in timescales. In the (supercooled) liquid states, this class of materials exhibits a variety of anomalous behaviors in thermodynamics and kinetics, such as heat capacity and density maxima, dynamic crossovers, and a breakdown of the Stokes-Einstein relations. These anomalies are related to a high- to low-density and metal-to-semiconductor liquid-liquid transition hidden below the melting temperature obscured by fast crystallization. I will bring the idea of tuning this transition through adjusting metallicities to attain desired switching kinetics for faster and more reliable phase-change memory applications.

1.Wei, S., Evenson, Z., Stolpe, M., Lucas, P. & Angell, C. A. Breakdown of the Stokes-Einstein relation above the melting temperature in a liquid phase-change material. Science Advances 4, eaat8632 (2018).
2.Wei, S., Coleman, G. J., Lucas, P. & Angell, C. A. Glass Transitions, Semiconductor-Metal Transitions, and Fragilities in Ge-V-Te (V=As, Sb) Liquid Alloys: The Difference One Element Can Make. Phys. Rev. Applied 7, 034035 (2017).
3.Wei, S., Lucas, P. & Angell, C. A. Phase change alloy viscosities down to Tg using Adam-Gibbs-equation fittings to excess entropy data: A fragile-to-strong transition. J. Appl. Phys. 118, 034903 (2015).