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

• Published in: Scientific reports Vol. 7; no. 1; pp. 8234 - 12
• Main Authors: Mantovan, R., Fallica, R., Mokhles Gerami, A., Mølholt, T. E., Wiemer, C., Longo, M., Gunnlaugsson, H. P., Johnston, K., Masenda, H., Naidoo, D., Ncube, M., Bharuth-Ram, K., Fanciulli, M., Gislason, H. P., Langouche, G., Ólafsson, S., Weyer, G.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 15-08-2017; Nature Publishing Group; Nature Portfolio
• Subjects: 639/301/1005/1008; 639/301/119/2795; Crystallization; Humanities and Social Sciences; multidisciplinary; Phase transitions; Science; Science (multidisciplinary); Spectroscopy; Thin films
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/s41598-017-08275-5

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.