Metal-insulator transition and doping-induced phase change in Ge 2 Sb 2 Se 5 x Te 5 - 5 x

Ge2Sb2Te5 (GST-225), a phase change material (PCM) with vast differences in the electrical and optical characteristics between its amorphous and crystalline phases, is revisited to explore its properties with Se doping. GST crystallizes in a layered hexagonal ground state, while the precursor to the amorphous state is a distorted rock salt like structure with vacancies at the Ge/Sb sites. Upon doping, liquid nitrogen quenched Ge 2 Sb 2 Se 5 x Te 5 - 5 x (GSST-225) exhibits a direct hexagonal-to-amorphous phase change above x > 0.8, whereas the rock salt like structure appears as a second phase with a volume fraction that does not change as a function of the doping. The phase change is accompanied by a metal-to-insulator transition (MIT), with a several orders of magnitude increase in the resistivity on approaching the amorphous state. A similar MIT is observed even without the phase change in hexagonal crystals with doping levels above x > 0.8. On warming amorphous GSST (x = 0.9) above room temperature, a reversal to the hexagonal phase occurs with a re-crystallization onset temperature (Tc) above 300 °C, much higher than the Tc (∼ 170 °C) of amorphous GST and an activation energy of 1.47 eV, which is comparable to good glass formers.