Atomic-scale study of the amorphous-to-crystalline phase transition mechanism in GeTe thin films

The underlying mechanism driving the structural amorphous-to-crystalline transition in Group VI chalcogenides is still a matter of debate even in the simplest GeTe system. We exploit the extreme sensitivity of (57)Fe emission Mössbauer spectroscopy, following dilute implantation of (57)Mn (T½ = 1.5 min) at ISOLDE/CERN, to study the electronic charge distribution in the immediate vicinity of the (57)Fe probe substituting Ge (Fe(Ge)), and to interrogate the local environment of Fe(Ge) over the amorphous-crystalline phase transition in GeTe thin films. Our results show that the local structure of as-sputtered amorphous GeTe is a combination of tetrahedral and defect-octahedral sites. The main effect of the crystallization is the conversion from tetrahedral to defect-free octahedral sites. We discover that only the tetrahedral fraction in amorphous GeTe participates to the change of the Fe(Ge)-Te chemical bonds, with a net electronic charge density transfer of ~ 1.6 e/a(0) between Fe(Ge) and neighboring Te atoms. This charge transfer accounts for a lowering of the covalent character during crystallization. The results are corroborated by theoretical calculations within the framework of density functional theory. The observed atomic-scale chemical-structural changes are directly connected to the macroscopic phase transition and resistivity switch of GeTe thin films.