Local Structure of Amorphous and Crystalline Ge 2 Sb 2 Te 5 Films
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Structure of Amorphous and Crystalline Ge2Sb2Te5 Films A. V. Marchenkoa, E. I. Terukovb,c, F. S. Nasredinovd, and P. P. Seregina* a
b
Herzen State Pedagogical University, St. Petersburg, 191186 Russia Ioffe Physical Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia c St. Petersburg Electrotechnical University “LETI,” St. Petersburg, 197376 Russia d Peter the Great St. Petersburg Polytechnic University, St. Petersburg, 195251 Russia *e-mail: [email protected] Received March 17, 2020; revised March 17, 2020; accepted June 30, 2020
Abstract—It is shown using Mössbauer spectroscopy on 119Sn, 121Sb, and 125Te isotopes that the local structure of germanium atoms changes during crystallization of amorphous Ge2Sb2Te5 films (tetrahedral symmetry changes to the octahedral one), whereas the environment of antimony and tellurium atoms remains the same (the environment of antimony atoms it close to that in the Sb2Te3 compound). Keywords: phase memory, Ge2Sb2Te5, Mössbauer spectroscopy. DOI: 10.1134/S1063785020100090
Modern phase-memory technology is mainly based on the use of films of Ge2Sb2Te5-type compounds, the crystalline and amorphous states of which are characterized by high optical contrast. Therefore, these materials can be used for erasable optical disks. It is believed that the aforementioned properties of Ge2Sb2Te5 films are due to structural features of amorphous and crystalline phases. In particular, a possible model explaining the mechanism of fast reversible transition from the crystalline to amorphous state for Ge2Sb2Te5 films was proposed in [1]. This model is based on the assumption that the transition under consideration is accompanied by a change in the coordination state of germanium atoms, while the local environments of tellurium and antimony atoms remain the same. Obviously, this statement requires independent experimental confirmation. Mössbauer spectroscopy is an efficient method for studying local structural transformations in solids during their amorphization, which makes it possible to identify unambiguously changes in the valence and coordination states of a Mössbauer probe [2, 3]. A fundamental requirement to the Mössbauer probe used in these studies is its a priori localization in a specific site of the crystal lattice or a structural network of an amorphous material. The purpose of this Letter was to study the local structure of crystalline and amorphous Ge2Sb2Te5 films on 125Te, 121Sb, and 119Sn isotopes (obviously, 125Te and 121Sb probes occupy tellurium and antimony sites, and the isovalent substitution of impurity tin atoms for germanium atoms in the structure of glassy and crystalline germanium tellurides was reliably
established [2, 3]). Crystalline and amorphous Ge1.5Te8.5 films have also been investigated for comparison. X-ray amorphous Ge2Sb2Te5, Ge1.5Te8.5, Ge1.95Sn0.05Sb2Te5, and Ge1.45Sn0.05Te8.5 films were obtained by dc magnetron sputtering of a polycrystalline target of the corresponding composition in nitrogen. The Ge1.95Sn0.05Sb2Te5 and Ge1.45Sn0
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