Synthesis and Real Structure Group

Phase Change Materials

Torben Dankwort

Thin Ge-Sb-Te films are promising materials for application in non-volatile data storage systems due to their unique properties combination, e.g. fast crystallization speed, high optical/resistivity contrast etc. The strong correlation between real structure and materials properties fosters basic research interest in this field.

We are investigating structural changes of Ge-Sb-Te facilitating in situ transmission electron microscopy (TEM) and in situ X-ray diffraction (XRD) experiments.

For Ge6Sn2Sb2Te11 we observed that after the initial crystallization the stabilization of a rock salt type phase with strong disorder in cationic sites was found in the temperature regime from ~120 °C ‑ ~150 °C. At 150 °C structural vacancy starts to order and thus a transition to a primitive rhombohedral phase is observed. At 260 °C a transition to the thermodynamically stable phase is observed. Diffuse scattering and HRTEM micrograph indicate that the obtained sample is still to some extend disordered highlighted by the formation of so called bi layer defects. These type of defects are of large interest as it is proposed that these defects facilitate a resistivity change in solid state for GeTe/Sb2Te3 superlattices.   At 280 °C we were able to observe for the first time in situ the movement of these defects. We could verify the suspection that the movement of these defects is associated with an increase in ordering for these materials.

We extended our work on bi-layer defects in cooperation with Andriy Lotnyk (IOM Leipzig) and could verify our initial observation of the bi layer defect movement and add more details to the transition process for Ge2Sb2Te5 and superlattice materials. Thus we observed that vacancy layers form in GeTe/Sb2Te3 superlattices at elevated temperature highlighting a possible new mechanism for the change in resistivity.