Optical and Electronic Properties, Nanoscale Structural Order, and Transformation Kinetics of Phase Change Materials

Abstract

One long standing puzzle was that, for a fixed composition, there may be different amorphous states that have greatly different crystallization kinetics. We provided clear evidence for the existence of nanoscale structural order in all amorphous states of Ge2Sb2Te5 and AgInSbTe, using Fluctuation electron microscopy (FEM) carried out in the transmission electron microscope. The nanoscale order is higher in the thermally annealed amorphous states than in the as-deposited (sputtered) state. Higher nanoscale order corresponds to more or larger ordered regions; then the structure has a greater probability to have supercritical nuclei that later grow into crystalline grains. The crystallization speeds of the annealed states are indeed higher than that of the as-deposited states. The greatly different effects of annealing on Ge2Sb2Te5 and AgInSbTe were explained by the difference in the nucleation rates. We also demonstrated that melt-quenched amorphous Ge2Sb2Te5 has clearly indexable crystalline particles of several nanometers, in addition to the amorphous matrix with high nanoscale order; the nanoparticles can act as supercritical nuclei that result in an order of magnitude faster crystallization.