Materials Science Research Lecture
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. One of the central goals is to increase the crystallization speed at an elevated temperature for a higher writing speed. By contrast, amorphous metals, also called metallic glasses discovered at Caltech back to 1960s, have been long battling crystallization, and engineered to slow down crystallization. Here I present a thermodynamic and structure approach to understand the diverse kinetics, and show how the kinetics can be tuned for technological innovations.
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 attributed to a high- to low-density and metal-to-semiconductor liquid-liquid transition hidden below the melting temperature Tm obscured by fast crystallization. Unlike bulk metallic glassformers, where the supercooled regime may be accessible to the technique of electrostatic levitations, PCMs are such poor glass formers that require probing techniques with a cooling rate of 109 K/s to avoid crystallization. Using ultrafast pump-probe X-ray, the structural evolution of the transition can be directly detected in nanoseconds timescale before crystallization interferes. I show that the liquid-liquid transitions in PCMs much resemble the phenomenology of well-known anomalous supercooled water. This implies the presence of a special temperature window being crucial for ultrafast crystallizations (switch speed) at a high temperature, and meanwhile, ensuring amorphous phase stabilities (data retention) at low temperatures. I will bring the idea how this temperature window can be tuned through adjusting component metallicities to attain desired switching kinetics. Finally, below the glass transition Tg, where structural relaxations take hours or years, the presence of β-relaxations is discovered in PCMs, representing a fast process of atomic motions even in frozen-in glasses. Such a finding in "covalent" glasses is striking, and can be rationalized in term of a special bonding mechanism related to the concept of resonance bonding by Linus Pauling. While β-relaxations in amorphous metals have been extensively discussed, for PCMs I will point out their relevance to the switching behavior in memory applications.
More about the Speaker:
Dr. Shuai Wei obtained his Ph.D. degree in Materials Science and Engineering at Saarland University in Germany in 09/2014. After completing his Ph.D., he moved to the School of Molecular Sciences at Arizona State University as a Feodor-Lynen Research Fellow awarded by the A.v. Humboldt-Foundation for a two-year research stay in the US. After the Humboldt fellowship, he returned to Germany in 2016 and joined the Institute of Physics for New Materials at RWTH Aachen University as a research fellow. His research has a strong focus on kinetic and thermodynamic properties and their links to atomic-scale structures in Phase-Change Materials and amorphous metals. He combines multiple approaches to understand these novel materials in a broad context of glass sciences and metallurgy, which holds the promise to solve major scientific problems in amorphous materials and drive technological innovations. His awards and honors include RWTH Start-Up fund (2017), Feodor-Lynen Fellowship of Humboldt Foundation (2014-2017), Dr.-Eduard-Martin Award (2015), MRS Graduate Student Silver Award (2013), and the Kühborth Foundation Award (2010).