Investigation of changes in band gap and density of localized states on phase transition for Ge₂Sb₂Te₅ and Si _3.5Sb₂Te₃ materials

Upon phase transition, the resistivity changes of Ge₂Sb ₂Te₅ (GST) and Si_3.5Sb₂Te ₃ (SST) are proved to be closely related to the variations of band gap and density of localized states. Amorphous SST has a slightly more localized state than amorphous GST; however, the larger band gap of SST material causes relatively difficult phase transition processes. Therefore, the phase change memory (PCM) cell based on the SST film shows larger threshold voltages for both set and reset operations than that of the GST-based PCM cell. The formation of amorphous Si-rich segregated areas in the SST film during phase transitions increases the randomness of the whole film microstructure, which leads to a different Urbach tail absorption result to that of the GST film.