Evolution of subcritical nuclei in nitrogen-alloyed Ge₂Sb ₂Te₅

We analyze the evolution of subcritical nuclei as a function of nitrogen alloying and thermal annealing in the amorphous phase change material Ge ₂Sb₂Te₅. The existence of subcritical nuclei is inferred through measurement of the nucleation time in pulsed laser annealing, and is detected more directly using fluctuation transmission electron microscopy measurements that are sensitive to topological order on the nanoscale. In samples that are pre-annealed before crystallization experiments, the nanoscale order consistently increases and the nucleation times consistently decrease, in agreement with the interpretation that the nanoscale order corresponds to a population of subcritical nuclei that ripens upon annealing. However, this correlation is less obvious in as-deposited samples across a range of nitrogen contents: The quantity of nanoscale order diminishes only slightly with increased nitrogen alloying, whereas the nucleation times increase by nearly two orders of magnitude. We therefore interpret that nitrogen must affect either the critical size in order for a nucleus to be stable (the thermodynamics) or the rate at which nucleation and growth take place (the kinetics).