crystallization of ion amorphised Ge2Sb2Te5 in nano-structured thin films

The crystallization kinetics of nano-structured amorphous Ge2Sb2Te5 (GST) regions (20-100 nm in diameter) obtained by Electron Beam Lithography (EBL) and 40 KeV Ge+ 1014/cm2 ion irradiation of crystalline 20 nm thick films was investigated. The amorphous regions, surrounded by crystalline cubic (fcc) or hexagonal (hcp) phase, were recrystallized by isothermal annealing in the temperature range 80°C - 120°C and by focused electron beam irradiation. The process was followed in situ by transmission electron microscope (TEM). The recrystallization is governed by the growth of the surrounding f.c.c. crystalline interface with a velocity of 2×10-2 nm/s at 110°C. The interface velocity is higher in the h.c.p. substrate less than a factor ten. Local focused electron beam irradiation induces instead crystalline nucleation inside the nano amorphous regions. Similar experiments have been performed on planar ion amorphized thin films lying on both GST crystalline phases. In both cases the recrystallization is mainly associated to the movement of the amorphous-crystalline interface. These results indicate that the stability of the amorphous region, generated by ion irradiation, is severely affected by the adjacent crystalline structure and by the size of the amorphous area, critically involved in the scaling of the PCM-based devices.