Metal-Insulator Transition and Doping-Induced Phase Change in Ge₂Sb₂Se_5xTe_5-5x

A phase change material (PCM), Ge2Sb2Te5 (GST-225), 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 has 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 Se doping, liquid nitrogen quenched Ge2Sb2Se5xTe5-5x (GSST-225) exhibit a direct hexagonal-to-amorphous phase change above x > 0.8. The rock-salt like structure appears as a second phase with its 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 several orders of magnitude increase in the resistivity on approaching the amorphous state. On warming amorphous GSST (x = 0.9) above room temperature, a reversal to the crytsalline hexagonal phase occurs with a re-crystallization onset temperature (Tc) above 300 degree, much higher than the Tc (around 170 degree) of amorphous GST and an activation energy of 1.47 eV, which is comparable to good glass formers.