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 Fe-57 emission Mssbauer spectroscopy, following dilute implantation of Mn-57 (T1/2 = 1.5 min) at ISOLDE/CERN, to study the electronic charge distribution in the immediate vicinity of the Fe-57 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 similar to 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.