Mӧssbauer Studies of the Local Surrounding of Atoms in Amorphous and Crystalline Ge2Sb2Te5 Films

Abstract

The local environment of atoms in crystalline and amorphous films Ge₂Sb₂Te₅ was determined by method of MÓ§ssbauer spectroscopy with the ¹¹⁹Sn, ¹²¹Sb and ¹²⁵Te isotopes. The current stage of phase memory (FP) research based on chalcogenide semiconductors is associated with the use of incongruently melting compounds in the Ge-Sb-Te system lying on the GeTe-Sb₂Te₃ quasi-binary section line, and the composition of Ge₂Sb₂Te₅ is of the greatest interest.

Keywords

Ge₂Sb₂Te₅ ; X-ray absorption fine-structure spectroscopy; Magnetron; Quartz

Introduction

An important property of the Ge₂Sb₂Te₅ film compound is its ability to rapid and reversible transitions between crystalline and amorphous states under the action of low-energy influences, and one of the tasks to improve the technology of FP is related to the study of the crystallization process of amorphous Ge₂Sb₂Te₅ films. It is obvious that it is fundamentally important to obtain information about the local structure of the amorphous film.

Crystal structures of the Ge₂Sb₂Te₅ alloys were studied in detail [1-4]. As for their amorphous modifications, many studies have been conducted to determine the structure of the short-range order in them [5-8], however, a comparison of all experimental data shows that these structures are still the subject of discussion [1,2,8]. In particular, there are obvious contradictions in the interpretation of experimental results obtained in the study of amorphous Ge₂Sb₂Te₅ films by X-ray Absorption Fine-structure Spectroscopy (XAFS) [5-7]. This indicates the need to use additional experimental methods that are sensitive to minor changes in the local structure and in the population of the electron shells of atoms during the transition from the amorphous to the crystalline state.

MÓ§ssbauer spectroscopy is an effective method for studying structural rear-arrangements in solids. An important requirement for the MÓ§sbauer probe used for such studies is its a’priori localization in a specific node of the crystal lattice or structural grid of an amorphous material. This requirement can be fulfilled by studying the local structure of crystalline and amorphous Ge₂Sb₂Te₅ films by absorption MÓ§ssbauer spectroscopy with the ¹²⁵Te, ¹²¹Sb and ¹¹⁹Sn isotopes.

In our work [9], the absorption and emission techniques described above were used to study structural rearrangements in Ge₂Sb₂Te₅ films. This allowed us to obtain information about structural rearrangements in the local environment of Ge, Sb and Te atoms during the crystallization of amorphous material.

Literature Review

Neodymium Ge₂Sb₂Te₅ and Ge₁.95Sn_0.05Sb₂Te₅ compounds were synthesized from elemental substances in quartz ampoules evacuated to 10^-3 mm Hg at 1050°C. X-ray amorphous films were obtained by magnetron sputtering of polycrystalline targets of similar composition at constant current in a nitrogen atmosphere on a silicon substrate. To obtain the Ge₁.95Sn_0.05Sb₂Te₅ films, the ¹¹⁹Sn isotope with an enrichment of 92% was used. The thickness of the films ranged from 40 to 120 microns. Crystallization of amorphous films was carried out at a temperature of 310°C (with the formation of the hcp phase).

The ^119mSn MÓ§ssbauer source based on the Ge₂Sb₂Te₅ crystalline films were prepared by diffusion of the ¹¹⁹Sb or ^119mTe isotopes into an amorphous film at a temperature of 310°C for 10 hours. The MÓ§ssbauer spectra were measured at 80 K.

The Ca^119mmSnO₃, Ca121SnO₃ and Zn¹²⁵mTe sources were used for measuring the absorption spectra of ¹¹⁹Sn, ¹²¹Sb and ¹²⁵Te, respectively. Isomeric Shifts (IS) of the ¹¹⁹Sn, ¹²¹Sb and ¹²⁵Te spectra are given relative to the spectra of the CaSnO₃, InSb and ZnTe absorbers, respectively. The composition of amorphous and crystalline films was controlled by X-ray fluorescence analysis.

Absorption Mössbauer spectroscopy data

The spectra of the ¹¹⁹Sn impurity atoms of amorphous and polycrystalline plates are single widened lines (line width at half-height G~1.15-1.35 mm/s).

The spectra of the ¹¹⁹Sn amorphous films have an isomer shift of IS=2.06(3) mm/s. These values are typical for isomeric shifts in the spectra of impurity atoms of tin in a Ge_1.5Te_8.5 glassy alloy and compounds of tetravalent tin with a tetrahedral system of chemical bonds [10]. Based on the values of the isomeric shifts of the ¹¹⁹Sn spectra, it is concluded that tin atoms and germanium atoms replaced by them in the structural grid of Ge₂Sb₂Te₅ films form a tetrahedral system of sp3 chemical bonds. Since germanium atoms can have only tellurium atoms in their local surroundings in the structural grid of the amorphous Ge_1.5Te_8.5, the proximity of isomeric shifts of all the studied amorphous films indicates that germanium atoms are connected only with tellurium atoms in the structural grid of the amorphous Ge₂Sb₂Te₅ film.

A characteristic feature of the spectra of ¹¹⁹Sn impurity atoms in the amorphous Ge₂Sb₂Te₅ films is their broadening. Two reasons for broadening are considered. Firstly, possible distortions of the angles between the bonds of tin atoms with atoms in its immediate environment should lead to a broadening of the spectrum due to unresolved quadrupole splitting. The second reason for the broadening of the spectra is the fluctuations in the lengths of the Ge-Te bonds while maintaining tetrahedral bond angles. This will lead to a broadening of the spectrum due to an inhomogeneous isomeric shift.

The spectra of the ¹¹⁹Sn polycrystalline films have an isomer shift of IS=3.53(3) mm/s. This value is close to the isomeric shift of the spectrum of compounds of divalent hexoordinated tin with tellurium (IS=3.55(3) mm/s). Consequently, the crystallization of films does not lead to a change in the chemical nature of atoms in the local environment of germanium (tin), but the valence state of germanium (tin) changes. The width of the spectra of the polycrystalline films is significantly larger than the approximate width of the spectral line of ¹¹⁹Sn. This indicates that in the polycrystalline phases, tin does not form an SnTe compound (a crystal lattice of the NaCl type), but is part of the fcc and hcp phases, for which the MÓ§ssbauer spectra are broadened due to unresolved quadrupole splitting.

The ¹²¹Sb spectra of the crystalline and amorphous Ge₂Sb₂Te₅ films, as well as the spectrum of the Sb₂Te₃ polycrystalline compound, are single somewhat widened lines (the maximum broadening is observed for an amorphous film G~5.1 mm/s), whose isomeric shifts (IS~5.1–5.4 mm/s) are typical for the spectra of the ¹²¹Sb trivalent compounds of antimony. Since the crystallization of the amorphous film does not lead to a significant change in the parameters of the spectra of ¹²¹Sb, and also considering that these parameters are close to the parameters of the spectrum of the Sb₂Te₃ compound, it should be concluded that the local structure of the antimony atoms in all the studied materials is approximate.

The spectrum of the ¹²⁵Te amorphous films is a quadrupole doublet (quadrupole splitting QS=4.42(8) mm/s), whose isomeric shifts (IS~0.35(6) mm/s) are typical for the Mossbauer spectra of divalent tellurium compounds. The spectrum of the Ge₂Sb₂Te₅ crystalline film can be described by a single poorly resolved quadrupole doublet with IS=0.30(6) mm/s, QS=4.30(8) mm/s and G=6.52(8) mm/s parameters corresponding to the divalent tellurium. At the same time, the immediate environment of tellurium atoms in amorphous and crystalline films remains unchanged.

Emission Mössbauer spectroscopy data

In the process of the diffusion doping of the amorphous Ge₂Sb₂Te₅ films with the ¹¹⁹Sb and ^119mTe impurity atoms at a temperature of~300°C, the films crystallize to form the hcp phase.

In the case of parent ¹¹⁹Sb atoms, the spectrum is a single broadened line (G=1.32(2) mm/s), whose isomeric shift (IS=3.47(2) mm/s) corresponds to divalent tin Sn²⁺. The spectrum of the ^119mSn impurity atoms formed after the radioactive decay of the parent ¹¹⁹Sb atoms in the antimony nodes of the Sb₂Te₃ crystal lattice has similar parameters, and it can be concluded that in both cases tellurium atoms are located in the local environment of the ^119mSn²⁺ atoms. This is consistent with the hcp data of the structure of Ge₂Sb₂Te₅ crystal films, which is based on a 9-layer trigonal packing of atoms-Te-Sb-Te-Ge-Te-Ge-Te-Sb, so that tellurium atoms are located in the local environment of antimony atoms.

Discussion and Conclusion

In the case of parent ^119mTe atoms, the spectrum is an overlap of two broadened lines (G=1.41-1.46 mm/s). A more intense line with an isomeric shift of IS=2.42(2) mm/s, lying in the region of isomeric shifts of the spectra of intermetallic tin compounds, corresponds to the ^119mSn⁰ centers formed after the decay of the parent ^119mTe atoms in the tellurium nodes. The less intense line (IS=3.51(2) mm/s) corresponds to the ^119mSn²⁺ centers formed after the decay of the parent ^119mTe atoms, which shifted from tellurium nodes to antimony, germanium or tellurium nodes due to the recoil energy accompanying the radioactive decay of the tellurium isotope. The large width of these lines is explained either by a set of possible atoms (antimony, germanium, tellurium) in the local environment of tellurium nodes, or by a similar set of nodes into which the daughter atom ¹¹⁹Sb is displaced. The spectra of the ^119mSn impurity atoms formed after the radioactive decay of the parent ^119mTe atoms in the tellurium nodes of the Sb₂Te₃ crystal lattice have a similar structure, and it can be concluded that in many cases only tellurium atoms are in the local environment of the ^119mSn²⁺ atoms.

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