Scientific Reports (Aug 2017)

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

  • R. Mantovan,
  • R. Fallica,
  • A. Mokhles Gerami,
  • T. E. Mølholt,
  • C. Wiemer,
  • M. Longo,
  • H. P. Gunnlaugsson,
  • K. Johnston,
  • H. Masenda,
  • D. Naidoo,
  • M. Ncube,
  • K. Bharuth-Ram,
  • M. Fanciulli,
  • H. P. Gislason,
  • G. Langouche,
  • S. Ólafsson,
  • G. Weyer

DOI
https://doi.org/10.1038/s41598-017-08275-5
Journal volume & issue
Vol. 7, no. 1
pp. 1 – 12

Abstract

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Abstract 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 57Fe emission Mössbauer spectroscopy, following dilute implantation of 57Mn (T½ = 1.5 min) at ISOLDE/CERN, to study the electronic charge distribution in the immediate vicinity of the 57Fe probe substituting Ge (FeGe), and to interrogate the local environment of FeGe 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 FeGe-Te chemical bonds, with a net electronic charge density transfer of ~ 1.6 e/a0 between FeGe 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.