Type: 
SSO Seminar
Date-Time: 
Wednesday, January 8, 2020 - 16:00 to 17:00
Location: 
Weniger 304
Event Speaker: 
Prof. Melissa Santala
Local Contact: 
Heidi Schellman
Abstract: 

Phase change materials (PCMs) are semi-conducting alloys with distinct optical and electrical properties in the amorphous and crystalline phases that make them useful for memory applications. In memory devices, amorphous bits are crystallized in nanoseconds by either laser or Joule heating, but the amorphous phase must also be stable against crystallization for long-term data retention. Crystal growth rates relevant to memory devices span orders of magnitude and fundamental questions regarding PCM crystallization mechanisms remain open, partly due to the difficulty in measuring crystallization kinetics in certain temperature regimes. Only a small set of materials satisfy the requirements of adequate contrast in properties, amorphous stability at low temperatures, and rapid crystallization. This talk will cover the application of multiple imaging techniques used by my group to directly quantify crystal growth rates in PCMs over a broad range of temperatures. The measureable growth rates from the different techniques span from ~10-9 to >10 m/s and the in situ imaging techniques applied include optical microscopy, conventional transmission electron microscopy, and dynamic TEM (DTEM), a photo-emission TEM technique with nanosecond-scale time resolution. The use of complementary experimental techniques allow the crystal growth rates to be mapped over a large temperature range and can give insights into the crystallization kinetics of PCMs and other marginal glass formers. Challenges associated with integrating results from different techniques will be discussed.

The crystal growth rate, u, is plotted as a function of temperature for a representative PCM. The kink at Tg solid is due the change from a solid-solid to a liquid-solid growth mechanism. The region just above Tg is important due to its defining relationship with the liquid fragility. The limiting behavior at umax is difficult to observe directly because it exceeds 1 m/s for most PCMs.­ Multiple imaging methods may be used to map isothermal growth rates, u, over a temperature range capturing key limiting behaviors in PCM alloys. In situ nanocalorimetry will be combined with the TEM techniques