Ab initio study on influence of dopants on crystalline and amorphous Ge2Sb2Te5

The pronounced effects of dopants such as Si, N, and O atoms, on material properties of Ge2Sb2Te5 are investigated at the atomic level using ab initio calculations. In the crystalline phase, stable doping sites are determined by characteristic chemical bonds such as Ge–N and Ge–O. The comparison of lattice parameters between theory and experiment supports the existence of dopants at vacant or interstitial positions. The electronic density of states indicate that the localization at the valence top increases with N- or O-doping, explaining the increase of resistivity in experiments. The amorphous structures of doped Ge2Sb2Te5 are obtained by melt-quench simulations and they are well understood by selective bonds between dopants and host atoms. The chemical bonds around dopants are more favorable in the amorphous phase than in the crystalline state, accounting for increased amorphous stability of doped Ge2Sb2Te5. The atomic and electronic structures of amorphous Ge2Sb2Te5 do not support a viewpoint that the bonding nature is changed toward the covalent character upon doping. The recrystallization of doped Ge2Sb2Te5 is also simulated and it is directly confirmed that the crystallization process of doped Ge2Sb2Te5 is slowed down by dopants.