Calorimetric measurements of phase transformations in thin films of amorphous Te alloys used for optical data storage

Abstract

Sputtered amorphous ${\mathrm{Ag}}_{\text{0.055}}$ ${\mathrm{In}}_{\text{0.065}}$ ${\mathrm{Sb}}_{\text{0.59}}$ ${\mathrm{Te}}_{\text{0.29}},$ ${\mathrm{Ge}}_4$ ${\mathrm{Sb}}_1$ ${\mathrm{Te}}_5,$ and ${\mathrm{Ge}}_2$ ${\mathrm{Sb}}_2$ ${\mathrm{Te}}_5$ thin films were studied by differential scanning calorimetry. The crystallization temperature and the heat of crystallization of the amorphous phases, the melting temperature and the heat of fusion of the crystalline phases, and the heat capacities of crystalline and liquid AgInSbTe were measured. The entropies of fusion are large $\text{(⩾2R),}$ which suggests a change of bonding type between liquid and crystal. In contrast to amorphous AgInSbTe and ${\mathrm{Ge}}_4$ ${\mathrm{Sb}}_1$ ${\mathrm{Te}}_5,$ which upon heating crystallize to a single phase within a small temperature interval, the crystallization of amorphous ${\mathrm{Ge}}_2$ ${\mathrm{Sb}}_2$ ${\mathrm{Te}}_5$ is complicated by a subsequent cubic-to-hexagonal transformation. No thermal evidence of a glass transition was found below the crystallization temperature. The ratio of the glass transition temperature (approximated as the crystallization temperature) to the liquidus temperature is 0.49–0.56, which identifies the materials as marginal glass formers. The heat capacity measurements on AgInSbTe were used to estimate the temperature dependence of the difference in enthalpy, entropy, and Gibbs free energy between the undercooled liquid and the crystal.