Optical and electronic properties, nanoscale structural order, and transformation kinetics of phase change materials.

摘要

Phase change materials like Ge-Sb-Te and Ag-In-Sb-Te exhibit rapid and reversible amorphous-to-crystalline phase changes. The dramatic and controllable change in optical reflectivity and electrical conductivity is the basis of rewritable optical discs (CD-RW, DVD+/-RW, etc.) and nonvolatile phase change random access memories (PCRAMs). The fundamental understanding of these materials will serve as a much-needed roadmap for the efficient development of the technologies. This dissertation reveals various basic properties of these materials and the relationship between the transformation speed and nanoscale structural order. We determined that the optical bandgaps of the amorphous, FCC, and hexagonal phases of Ge2Sb2Te 5 are 0.7, 0.5, and 0.5 eV, respectively. The crystalline phases have a high concentration of holes down to ∼5 K, which implies that the Fermi level resides inside a band. The Hall coefficient of hexagonal phase at high temperatures is decided by both holes and electrons, even if it is p-type. Ge2Sb2Te5 also has a strong tendency to oxidize by a prolonged light illumination.;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.