Kinetics of the Glass Transition and Crystallization in Germanium-Antimony-Tellurium Ternary Amorphous Thin Films

We explore the Germanium-Antimony-Tellurium (GST) ternary phase diagram to determine the viability of as-deposited amorphous thin film growths at various growth temperatures. Using high resolution TEM imaging, we are able to demonstrate that some films are amorphous, while others are partially nanocrystalline. By comparing to composition and known coordination numbers for each element, we determine that the films most likely to yield stable amorphous phases have a typical coordination number of 2.45, consistent with theoretical predictions. Additionally, by determining heat capacity via fast scanning calorimetry, we show the kinetics of the glass transition and crystallization of various films. In doing so we exhibit the effects of growth temperature and composition on the thermodynamic and kinetic stability of the films. We demonstrate that higher substrate temperatures tend to yield more stable films, with higher glass transition temperatures, lower starting enthalpies, and decreased likelihood of crystallization. This ultrastability is also shown to correlate to increased densities and sound velocities. Finally, we correlate the high temperature thermodynamics to the low temperature heat capacity and mechanical loss effects, which are known to correlate with two-level systems.