In situ dynamic HR-TEM and EELS study on phase transitions of Ge2Sb2Te5 chalcogenides

The phase transition phenomena of Ge2Sb2Te5 chalcogenides were investigated by in situ dynamic high-resolution transmission electron microscopy (HR-TEM) and electron energy loss spectroscopy (EELS). A 300 kV field emission TEM and a 1250 kV high voltage TEM were employed for the in situ heating experiments from 20 to 500 °C for undoped and 3 wt% nitrogen-doped Ge2Sb2Te5 thin films deposited by DC sputtering. Crystallization of amorphous Ge2Sb2Te5 to its cubic structure phase started at 130 °C and then rapid crystal growth developed from cubic to hexagonal phase in the range of 130-350 °C; finally, the hexagonal crystals started to melt at 500 °C. For nitrogen-doped Ge2Sb2Te5, its crystallization from amorphous film occurred at higher temperature of ca. 200 °C, and the cubic and hexagonal phases were usually formed simultaneously without significant growth of crystals at further heating to 400 °C. EELS measurements showed that the electronic structures of Ge, Sb and Te stayed almost the same regardless of the amorphous, FCC and hexagonal phases. The nitrogen doped in Ge2Sb2Te5 was confirmed to exist as a nitride. Also, the doped nitrogen distributed homogeneously in both amorphous and crystalline phases. Localization of doped nitrogen was not found in the grain boundary of crystallized phases. The dynamic process of phase transition was enhanced by high-energy electron irradiation. Peeling of atomic layers in nitrogen-doped Ge2Sb2Te5 film was detected during heating assisted with electron beam irradiation.