Simultaneous Transmission Electron Microscopy and Nanocalorimetry Capturing the Thermodynamics and Kinetics of the Amorphous-to-Crystalline Transition of Phase Change Materials

Phase change materials (PCMs) are marginal glass formers that may be reversibly switched from an amorphous to crystalline state. Due to the large difference in resistivity between the amorphous and crystalline phases and the fast switching, they may be used as thin films in memristor and neuromorphic memory. Until recently, it had not been possible to image and measure the crystal growth rate, above the glass transition temperature due to the high rate of crystallization. Methods that measure the crystallized fraction, such as differential scanning calorimetry and reflectivity measurements have been used to extract growth rate, but these methods require assumptions about the relative contribution of nucleation and growth. Some of the experimental results and analysis of the calorimetry have led to the questioning of classical glass behavior models. This current work utilized nanocalorimeters designed and fabricated at the National Institute of Standards and Technology to fit in a transmission electron microscope (TEM) in combination with high-speed electron detectors at the National Center for Electron Microscopy. Thin films of Ag₃In₄Sb₇₆Te₁₇, GeTe and Ge₇Sb₉₃ were deposited in the nanocalorimeters, and high-frame-rate imaging (up to 30,000 frame/s) and the fast heating (from 1,0000 to 9,000 K/s) enabled the collection of calorimetric data along with simultaneous imaging of grain growth for temperatures above the reported glass transitions for these PCMs. The growth rates were resolved in millisecond time scales along with the heat capacity and enthalpy of crystallization. These measurements help complete the picture of the crystallization kinetics of the PCMs and suggest that assumptions made about nucleation in the analysis of ultrafast calorimetry experiments have led to the overestimation of the fragility of these PCMs.