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5Physics of Complex Systems, 2023, vol. 4, no. 1 _www.physcomsys.ru
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Physics of Semiconductors. Physics of thin films
UDC 538.9
EDN KJGEWB
https://www.doi.org/10.33910/2687-153X-2022-4-l-30-35
Local environment of germanium atoms in Ge3Sb2Te6, Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7 amorphous and crystalline films
Yu. A. Petrushin™
1 Herzen State Pedagogical University of Russia, 48 Moika Emb., Saint Petersburg 191186, Russia
Author
Yuri A. Petrushin, ORCID: 0000-0002-9873-2685, e-mail: uraordie@mail.ru
For citation: Petrushin, Yu. A. (2023) Local environment of germanium atoms in Ge3Sb2Te6, Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7 amorphous and crystalline films. Physics of Complex Systems, 4 (1), 30-35. https://www.doi.org/10.33910/2687-153X-2022-4-1-30-35 EDN KJGEWB
Received 13 December 2022; reviewed 18 January 2023; accepted 18 January 2023. Funding: The study did not receive any external funding.
Copyright: © Yu. A. Petrushin (2023). Published by Herzen State Pedagogical University of Russia. Open access under CC BY-NC License 4.0.
Abstract. The valence state and local environment of germanium atoms in Ge3Sb2Te6, Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7 amorphous and crystalline films were determined by Môssbauer spectroscopy on the 119Sn isotope. In crystalline films, divalent germanium is located in octahedral positions in a rhombohedrally distorted NaCl-type crystal lattice. In amorphous films, tetravalent germanium atoms form a tetrahedral system of chemical bonds. In all the films, the nearest environment of germanium contains mainly tellurium atoms.
Keywords: local structure, Môssbauer spectroscopy, amorphous and crystalline films, valence state, local environment
Introduction
Due to a significant contrast in conductivity and reflectivity between the crystalline and amorphous phases, phase transition materials can be used to store and encode data for non-volatile memory (Lencer et al. 2011). It is believed that the compositions lying on the GeTe-Sb2Te3 pseudobinary line (Ge3Sb2Te6, Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7, let us denote them as GeSbTe) are the most promising materials for creating rewritable optical storage devices, since they have a short crystallization time, ideal reversibility between amorphous and crystalline states and high thermal stability (Qiao et al. 2019). To date, the crystal structures of GeSbTe alloys (let us denote them as c-GeSbTe) have been studied in detail (Lotnyk et al. 2016; Sun et al. 2007; Urban et al. 2013; Wang et al. 2017; Zhang et al. 2016; Zheng et al. 2019). Many studies have also been carried out in order to determine the short-range structure of GeSbTe amorphous alloys (let us denote them as ^-GeSbTe) (Baker et al. 2006; Jovari et al. 2008; Kolobov et al. 2004; Paesler et al. 2007); however, these structures are still being discussed (Qiao et al. 2019; Stellhorn et al. 2020).
Môssbauer spectroscopy (MS) can be an effective tool for changes detecting in the local environment of atoms and their electronic structure during GeSbTe compounds amorphization. However, apart from the works (Bordovskii et al. 2021; Marchenko et al. 2021) and the work (Ledda et al. 1988), which presents the 121Sb Môssbauer spectra of GeSb2Te4, Ge2Sb2Te5 and GeSb4Te7 crystalline compounds, there are no Môssbauer studies of GeSbTe ternary compounds.
This work is devoted to studying the nature of the local environment of germanium atoms in Ge3Sb2Te6, Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7 crystalline and amorphous films by absorption MS on 119Sn isotope.
Experimental technique
Undoped and tin-doped X-ray Ge3Sb2Te6, Ge2Sb2Te5, GeSb2Te4, GeSb4Te7, Ge2 95Sn005Sb2Te6, Ge195Sn0 05Sb2Te5, Ge0 95Sn005Sb2Te4 and Ge195Sn0 05GeSb4Te4 amorphous films (let us denote them as a-Ge(Sn)SbTe) were obtained by magnetron sputtering of polycrystalline targets. The films were annealed at 150 °C to obtain GeSbTe and Ge(Sn)SbTe crystalline films. The films composition was monitored by the X-ray fluorescence analysis.
The Mossbauer spectra were measured with a SM 4201 TerLab spectrometer at 80 K using a Ca119mmSnO source. Isomer shifts (S) of the 119Sn spectra are given relative to the CaSnO3 absorber.
Experimental results and their discussion
The Mossbauer spectra of 119Sn impurity atoms in Ge(Sn)SbTe amorphous and crystalline films are shown in Figs. 1 and 2. All spectra are single lines with a FWHM G ~ 1.30-1.36 mm/s. (The instrumental line width Gapp = 0.79(2) mm/s). The crystalline films spectra have isomer shifts S ~ 3.49-3.54 mm/s; for amorphous films S ~ 2.03-2.09 mm/s was obtained.
Fig. 1. Mossbauer spectra of 119Sn impurity atoms in Ge(Sn)Sb crystalline films and in the Ge0 98Sn0 02Te
crystalline compound
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Fig. 2. Môssbauer spectra of 119Sn in Ge(Sn)Sb amorphous films and in the crystalline germanium GeSn
The 119Sn impurity atoms spectra of GeSbTe crystalline films correspond to the ionic compounds of divalent tin. Fig. 1 shows the 119Sn impurity atoms spectrum in the Ge098Sn002Te compound for which 5 = 3.55(3) mm/s and G = 1.36(2) mm/s were obtained. These parameters correspond to divalent six-coordinated tin which isovalently replaces the divalent six-coordinated germanium atoms in the cationic node of the GeTe crystal lattice. GeTe compound crystallizes in a rhombohedrally distorted NaCl-type lattice, hence the broadening of the 119Sn impurity atoms spectrum in the Ge098Sn002Te solid solution. The broadening of ternary compounds spectra is also related with the presence of a high concentration of stoichiometric vacancies in the cationic sublattice of these compounds.
The data of Mossbauer spectroscopy on 119Sn impurity atoms for Ge(Sn)SbTe crystalline films comply with the results of the X-ray diffraction studies of metastable vacancy-disordered cubic GeSbTe crystalline compounds. Divalent tin Sn2+ (electronic configuration is 5s2px) replaces divalent germanium Ge2+ (electronic configuration is 4s2px) in positions 4 b of the rhombohedrally distorted NaCl-type crystal lattice, and there are only tellurium atoms in the nearest environment of six-coordinated germanium atoms. The latter circumstance explains the closeness of the isomer shifts of the 119Sn spectra in the GeSbTe crystalline compounds to the isomer shift of the 119Sn spectrum in the SnTe
compound (S = 3.54(1) mm/s and G = 0. 94(2) mm/s) and also to the isomer shift of the 119Sn impurity atom in the GeTe compound. The broadening of the spectra of c-GeSbTe ternary compounds compared to the spectral width of SnTe is due to the rhombohedral distortion of the lattices of c-GeSbTe compounds (six Ge-Te bonds at sites with octahedral symmetry are divided into three short and three long bonds, as in a GeTe crystal) as well as to a high concentration of randomly distributed stoichiometric vacancies in the cationic sublattice (Lotnyk et al. 2016; Sun et al. 2007; Urban et al. 2013; Wang et al. 2017; Zhang et al. 2016; Zheng et al. 2019).
The first problem that Môssbauer spectroscopy can solve is the determination of the valence state and local coordination number of germanium atoms in GeSbTe amorphous films. The isomer shifts of the 119Sn spectra of Ge(Sn)Sb amorphous films have values close to the values of the isomer shifts of the spectra of impurity tin atoms 119Sn in monocrystalline germanium (S = 1.79(1), see Fig. 2) and also close to the measured shift of the spectrum of gray tin a-Sn (S = 2.10(1)) that form the area of isomer shifts of tetravalent tin compounds with a tetrahedral sp3 system of chemical bonds. Thus, impurity tin atoms in the structure of Ge(Sn)Sb amorphous films isovalently replace tetravalent germanium atoms, which form a tetrahedral system of chemical bonds (the local coordination number of germanium atoms in amorphous films is four).
The second problem solved using Môssbauer spectroscopy data is the determination of the chemical nature of atoms in the nearest environment of germanium atoms in GeSbTe amorphous films. It should be noted that, if germanium atoms are also present in the nearest environment of germanium atoms (i. e. the formation of Ge-Ge chemical bonds), the isomer shift of the 119Sn spectra of amorphous films should be ~ 1.80 mm/s (as for the spectrum of impurity tin atoms in monocrystalline Ge). At the same time, the spectra of all amorphous films have isomer shifts within S ~ 2.06-2.09 mm/s. Note that the isomer shifts of the spectra of 119Sn impurity atoms in the Ge145Sn0 05Te8 5 glassy alloy are of the same range. Here the germanium (tin) atoms are tetravalent, forming a system of sp3 bonds (their coordination number is four) and having tellurium atoms in the local environment (Seregin et al. 1977; Seregina et al. 1977). This conclusion can be confirmed by the fact that the isomer shift of the 119Sn spectra of Ge(Sn) SbTe amorphous films monotonically increases from 2.03(1) mm/s for the Ge3Sb2Te6 compound (contains 27.3 at.% of Ge) to 2.07(1) mm/s for the GeSb4Te7 compound (contains 8.3 at.% of Ge). Based on all the above, it should be concluded that there are tellurium atoms in the local environment of germanium atoms in the Ge(Sn)SbTe and GeSbTe amorphous films.
Kolobov et al., based on the results of XAFS to describe the order-disorder transition in Ge2Sb2Te5 compound films, supposed that a crystalline film amorphization is accompanied by a germanium atom jump from octahedral positions in a crystalline film to tetrahedral positions with four Te atoms in the local environment (Kolobov et al. 2004). However, Baker et al. (Baker et al. 2006; Paesler et al. 2007), also using the EXAFS data, concluded that germanium atoms in Ge2Sb2Te5 amorphous films form the Te3Ge-GeTe3 structural units, where predominant formation of Ge-Ge bonds can be seen. The structure of Ge2Sb2Te5 and GeSb2Te4 amorphous films was also studied using the EXAFS method, in combination with high-energy X-ray diffraction and neutron diffraction, by the authors of (Jovari et al. 2008). Ge-Ge and Ge-Sb bonds were shown to be present while Te-Te and Sb-Sb bonds were not found. Germanium atoms have a fourfold coordination. Finally, the local structure of the Ge2Sb2Te5 amorphous phase was studied using anomalous X-ray scattering near the absorption K-edges of germanium, antimony, and tellurium atoms (Stellhorn et al. 2020). A half of the Ge atoms were found to have an octahedral environment similar to that in a crystalline phase. The remaining half of germanium atoms has tetrahedral symmetry and forms an energy barrier between the amorphous and crystalline phases, providing a long lifetime for the amorphous modification.
The Môssbauer spectroscopy data obtained by us comply with the ideas of the authors of (Kolobov et al. 2004) about the local structure of germanium atoms in Ge2Sb2Te5 amorphous films and allow us to extend these results to other GeSbTe amorphous compounds. Tetravalent germanium atoms form a tetrahedral system of chemical bonds in the structural network of the amorphous matrix and have only tellurium atoms in their nearest environment. The conclusion of the authors of (Jovari et al. 2008) that germanium atoms in Ge2Sb2Te5 and GeSb2Te4 amorphous compounds have fourfold coordination is also confirmed (with the only clarification that this is true for all the GeSbTe amorphous films).
Conclusions
The local environment of atoms in Ge3Sb2Te6, Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7 crystalline and amorphous films was determined by Môssbauer spectroscopy on the 119Sn isotope. The data of Môssbauer spectroscopy on 119Sn impurity atoms for crystalline films comply with the results of the X-ray diffraction studies: divalent tin replaces divalent germanium in a rhombohedrally distorted NaCl-type lattice.
Impurity tin atoms in the structure of GeSnSbTe amorphous films isovalently replace tetravalent germanium atoms, which form a tetrahedral system of chemical bonds (the local coordination number of germanium atoms in GeSbTe amorphous films is four), and the local environment of germanium contains mainly tellurium atoms.
Acknowledgements
I would like to thank my supervisor Prof. Seregin P. P. and Prof. Marchenko A. V. for their consistent support and guidance in carrying out this research.
Conflict of Interest
The author declares that there is no conflict of interest, either existing or potential.
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