Optical response characteristics arising from delocalized electrons in phase change materials

Phase change materials have great potential for data storage applications owing to the large optical and electrical contrasts during rapid switching between their amorphous and crystalline phases. Hence, significant efforts have been made to identify and understand their unique characteristics. Here, we report a distinct optical characteristic of phase change materials and explain its presence via electronic structure considerations. The optical response of phase change materials and non-phase change materials are investigated via experiments. Annealing them from room temperature to 400 °C, we observed that Sb₂Te₃ exhibits phase change properties but not Bi₂Te₃, despite their similar crystal structures. A red shift in the absorption spectra is observed for crystalline Sb₂Te₃ with respect to its amorphous phase. From first-principles calculations, we explain that the delocalized electrons in crystalline Sb₂Te₃ films are responsible for this red shift. In contrast, the electrons in Bi₂Te₃ films are localized and no absorption red shift is observed. The absorption red shift is also observed in other phase change materials from the GeTe–Sb₂Te₃ pseudo-binary system; hence the detection of such a red shift may be used for identifying potential phase change materials.