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Temperature dependent measurements of the electrical resistance have been employed to study structural changes in sputtered Ge2Sb2Te5 films. The pronounced changes of film resistance due to structural changes enable a precise determination of transition temperatures and activation energies. Furthermore the technique is sensitive enough to measure the influence of ultrathin capping layers on the transformation kinetics. With increasing temperature the Ge2Sb2Te5 films undergo a structural change from an amorphous to rock salt structure (Fm3m) around 140 °C and finally a hexagonal structure (p3̄m) around 310 °C. Both structural changes are accompanied by a major drop of resistance. Applying the Kissinger method Anal. Chem. 29, 1702 (1957) the activation energy for crystallization to the rock salt structure is determined to be 2.24±0.11 eV, and for the phase transformation to the hexagonal phase to be 3.64±0.19 eV, respectively. A thin capping layer of ZnS–SiO2 leads to an increase of the first transition temperature as well as of the corresponding activation energy (2.7±0.2 eV).
Friedrich et al. (Mon,) studied this question.