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88Physics of Complex Systems, 2021, vol. 2, no. 2 _www.physcomsys.ru
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Condensed Matter Physics. Semiconductor Physics
UDC 538.9
https://www.doi.org/10.33910/2687-153X-2021-2-2-61-67
Local structure of amorphous and crystalline Ge2Sb2Te5 films
Yu. A. Petrushin1, A. V. Marchenko1, P. P. Seregin™
1 Herzen State Pedagogical University of Russia, 48 Moika Emb., Saint Petersburg 191186, Russia
Authors
Yuri A. Petrushin, e-mail: [email protected]
Alla V. Marchenko, ORCID: 0000-0002-9292-2541, e-mail: [email protected] Pavel P. Seregin, ORCID: 0000-0001-5004-2047, e-mail: [email protected]
For citation: Petrushin, Yu. A., Marchenko, A. V., Seregin, P. P. (2021) Local structure of amorphous and crystalline Ge2Sb2Te5 films. Physics of Complex Systems, 2 (2), 61-67. https://www.doi.org/10.33910/2687-153X-2021-2-2-61-67 Received 10 February 2021; reviewed 10 March 2021; accepted 10 March 2021.
Copyright: © The Authors (2021). Published by Herzen State Pedagogical University of Russia. Open access under CC BY-NC License 4.0
Abstract. An effective way to examine structural rearrangement in solids is Mossbauer spectroscopy. A key requirement to Mossbauer probes used for these purposes is the possibility of their localization in a certain site of the crystal lattice or in the structural network of the amorphous material. When absorption spectroscopy is used to examine the local structure of crystalline and amorphous Ge2Sb2Te5 films, this requirement is satisfied for 119Sn isotope. Tin atoms 119Sn isovalently substitute germanium atoms in the structure of both vitreous and crystalline germanium tellurides. The absorption Mossbauer spectroscopy on 119Sn impurity centers shows that germanium atoms in the structure of amorphous and polycrystalline Ge2Sb2Te5 films have different local symmetries (tetrahedral in the amorphous phase and octahedral in the crystalline).
Keywords: Mossbauer spectroscopy, phase-memory, Ge2Sb2Te5, local structure, X-ray fluorescence analysis.
Introduction
Phase-memory (PM) devices based on chalcogenide semiconductors are mostly used at present for reversible transitions from the amorphous to the crystalline state of thin Ge—Sb—Te films, with the composition Ge2Sb2Te5 attracting most interest. The compound Ge2Sb2Te5 cannot be obtained as bulk glass, but the magnetron sputtering of a target can produce amorphous films. The improvement of PM devices and the technology of their production should be based on the results of a study of the crystallization of amorphous Ge2Sb2Te5 films. Obtaining information about the local structure of an amorphous film and comparing it with the crystal structure is paramount to such studies. It is impossible to describe the PM mechanism without knowing the structural transformations in reversible phase transitions between the amorphous and crystalline states. For example, a model of a fast reversible transition from the crystalline to the amorphous state was suggested in the early studies of Ge2Sb2Te5 films by the XANES method (X-ray absorption near-edge structure) (Kolobov et al. 2004).
The reversible transition from the amorphous state to the cubic crystalline phase is most frequently used in Ge2Sb2Te5-based PM devices. However, the operating temperatures of these devices are limited to 120 °C because of the low thermal stability of the amorphous phase. It was suggested in a recent study (Hu et al. 2020) to replace the amorphous-cubic phase transition with a transition from the metastable cubic to stable hexagonal phase in the same films. This replacement provides a combination of high optical contrast, thermal stability, and a small change in density. It also raises the maximum working temperature of optics to 240 °C. The authors of (Hu et al. 2020) attribute the high optical contrast to an increase in the difference in structural disorder on passing from the cubic phase to the hexagonal. This necessitates the analysis of the structure and structural disorder of both crystalline phases of Ge2Sb2Te5.
An effective way to examine structural rearrangement in solids is Mossbauer spectroscopy (Bobokhu-zhaev et al. 2020). A key requirement to Mossbauer probes used for these purposes is the possibility of their localization in a certain site of the crystal lattice or in the structural network of the amorphous material. When absorption spectroscopy is used to examine the local structure of crystalline and amorphous Ge2Sb2Te5 films, this requirement is satisfied for 125Te, 121Sb as well as 119Sn isotopes. 125Te and 121Sb can serve as probes for tellurium and antimony sites, whereas tin atoms 119Sn isovalently substitute germanium atoms in the structure of both vitreous and crystalline germanium tellurides, as was previously shown (Seregina et al. 1977; Micoulaut et al. 2014; Marchenko et al. 2019). Additionally, it is possible to use emission Mossbauer spectroscopy for the 119mSn isotope with 119Sb and 119mTe parent nuclei. The parent nuclei are introduced into antimony and tellurium sites, respectively. The daughter Mossbauer probe 119mSn formed according to the decay scheme of 119Sn and 119Te in Fig. 1 can inherit either antimony sites (if the 119Sb isotope is used) or tellurium sites (if the 119mTe isotope is used). This process allows to obtain the models of antisite defects with tin (as an analog of germanium) at antimony or tellurium sites.
Fig. 1. Scheme of the decay of 119mmSn, 119Sb, and 119mTe parent isotopes
In this study, we examine the structural rearrangements in Ge2Sb2Te5 films by the above-described absorption and emission procedures. The goal of the study is to obtain information about the following:
• structural rearrangements in the local environment of germanium, antimony, and tellurium atoms during the crystallization of amorphous films;
• the nature of tin defects in the structure of crystalline films.
To interpret the obtained results, we also carry out similar studies of the crystalline compounds Sb2Te3, GeTe, and vitreous alloy GeL5Te85.
Experiment
The compounds under study, Ge2Sb2Te5, Ge195Sn0 05Sb2Te5, Sb2Te3, and GeTe as well as Ge145Sn0 05Te8 5, and Ge15Te8 5 alloys were synthesized from elementary substances at 1050 °C in quartz cells evacuated to 10-3 mm Hg.
X-ray-amorphous films of Ge2Sb2Te5, Ge195Sn0 05Sb2Te5, Ge145Sn0 05Te8 5, and Ge15Te8 5 were produced by dc magnetron sputtering of the target of the corresponding compositions in an atmosphere of nitrogen. The Ge195Sn005Sb2Te5 and Ge145Sn005Te85 were deposited by using a 119Sn preparation enriched to 92%. Amorphous Ge2Sb2Te5 and Ge195Sn0 05Sb2Te5 films were crystallized at 150 °C (to give a cubicfcc (face-centered cubic) phase) or at 310 °C (to give a hexagonal hcp (hexagonal cubic) phase) (Kato, Tanaka 2005; Shelby, Raoux 2009; Siegrist et al. 2011; Sousa 2011). The amorphous Ge15Te85 and Ge145Sn005Te85 films were crystallized at 250 °C.
Mossbauer 119raSn sources based on Ge2Sb2Te5 crystalline films (hcp phase) were prepared via the diffusion of carrierless 119Sb or 119raTe isotopes into thin amorphous films at a temperature of 310 °C for 10 h. 119mSn Mossbauer sources based on Sb2Te3 and GeTe were prepared via fusion of the corresponding compound with carrierless 119Sb or 119Te in sealed cells.
The 119Sb and 119Te isotopes were produced, respectively, by the reactions 119Sn(p, n)U9Sb and 117Sn(a, 2n)119raTe, with the subsequent chromatographic isolation of carrierless preparations 119Sb and 119Te.
The emission spectra were measured with CaSnO3 as an absorber (surface density in terms of tin 5 mg/cm2). The spectrum of this absorber with a source of the same composition was a single line with a full width at the half-height G = 0.79(1) mm/s, which was taken to be the instrumental width of the spectral line. For the sources prepared with 119raTe, the spectra were measured after dynamic radioactive equilibrium between the 119Sb and 119raTe isotopes was attained. The isomer shift of the Mossbauer spectra of 119mSn and 119Sn is presented relative to the CaSnO3 absorber. All the Mossbauer spectra were measured with a CM 4201 TerLab spectrometer at 80 K.
The compositions of the amorphous and crystalline films as well as of the target were monitored by the X-ray fluorescence analysis.
Experimental results and their discussion
Data of the absorption Mossbauer spectroscopy on 119Sn
The typical spectra of 119Sn impurity atoms in amorphous (vitreous) and polycrystalline materials, shown in Figs. 2 and 3, are single broadened lines (G ~ 1.15-1.35 mm/s).
-. sn-iv jc^^.
Ge2Sb2Te5:119Sn \ /
amorhous film
v 'V'. Sn° '
polycrystalline film fee structure \ t
•• • * •»' ; ."'•. . .'. * V.
polycrystalline film hep structure \ J
■7 -3.5 0 3.5 7
Velocity, mm/s
Fig. 2. Absorption Mossbauer spectra of Sn impurity atoms in amorphous and polycrystalline Ge2Sb2Te5 films. The positions of spectral lines associated with Sn-IV and Sn0 centers
■7 -3.5 0 3.5 7
Velocity, mm/s
Fig. 3. Absorption Mossbauer spectra of 119Sn impurity atoms in the vitreous and polycrystalline Ge1 5Te8 5 alloy.
The positions of spectral lines associated with Sn-IV and Sn centers
The spectra of 119Sn for amorphous Ge2Sb2Te5 and vitreous Gex 5Te85 show isomer shifts IS ~ 2.06-2.09 mm/s. These isomer shifts are typical of the spectra of 119Sn compounds of tetravalent tin with a tetrahedral system of Sn-IV chemical bonds (Bobokhuzhaev et al. 2020; Seregina et al. 1977). The 119Sn spectra of the polycrystalline Ge2Sb2Te5 samples in both phases, fcc and hcp, and of Ge15Te85 show isomer shifts IS ~ 3.49-3.52 mm/s, which are close to the isomer shift of the 119Sn compound of divalent tin with tellurium, IS = 3.55(2) mm/s, which has the octahedral system of chemical bonds.
The values of the isomer shift of the 119Sn spectra suggest that tin atoms and germanium atoms replaced by tin atoms in the structural network of amorphous Ge2Sb2Te5 and vitreous Ge15Te85 form a tetrahedral sp3 system of chemical bonds. Because germanium (tin) atoms can have only tellurium atoms in their local environment in the structural network of the vitreous alloy, the close values of the isomer shift for all the amorphous materials under study indicate that germanium (tin) atoms are bonded only to tellurium atoms in the structural network of amorphous Ge2Sb2Te5. The broadening of the 119Sn spectra of all the amorphous materials under study is due to the lack of long-range order in the position of atoms in these materials—a characteristic property of the Mossbauer spectra of disordered structures.
The fact that the isomer shifts in the 119Sn spectra of polycrystalline Ge2Sb2Te5 and Ge1#5Te8#5 are close to those for the SnTe compound indicates that only tellurium atoms remain upon crystallization in the local environment of germanium (tin) atoms. The widths of the spectra of the polycrystalline samples substantially exceed the instrumental width. This indicates that tin does not form the SnTe compound (crystal lattice of NaCl type) in their composition, but enters into the composition of Gex - xSnxTe solid solutions (in the Ge15Te85 alloy) or into thefcc or hcp phases (in Ge2Sb2Te5 films). According to the X-ray diffraction data, the Gex - xSnxTe solid solutions and the fee Ge2Sb2Te5 phase have rhombohedrally distorted lattices of the NaCl type, and the hep Ge2Sb2Te5 phase has a lattice with the 9-layered trigonal packing of atoms —Te—Sb—Te—Ge—Te—Te—Ge—Te—Sb— (Kato, Tanaka 2005; Shelby, Raoux 2009; Siegrist et al. 2011; Sousa 2011). Noncubic distortion of the lattices must lead to a quadrupole splitting of the Mossbauer spectra of 119Sn by an amount that is smaller in the given case than the spectral line width.
Data of the emission Mossbauer spectroscopy on 119Sn
In the process of the diffusion doping of Ge2Sb2Te5 amorphous films with 119Sb and 119mTe impurity atoms at a temperature of ~300 °C, the films crystallize to give the hcp phase (Kato, Tanaka 2005; Shelby, Raoux 2009; Siegrist et al. 2011; Sousa 2011). The typical spectra of 119mSn impurity atoms formed
after the radioactive decay of 119Sb atoms at antimony sites and 119mTe at tellurium sites of the crystal lattice are presented in Fig. 4.
Sn2
Sb2T e3:119Sb
*
» *
Sb2Te3: 119mT e \ Sn° V*v
- , -*v
* " * " * V/".
GeTe: n9mTe \ Sn°
■7 -3.5 0 3.5 7
Velocity, mm/s
Fig. 4. Emission Mossbauer spectra of 119mSn impurity atoms formed after the radioactive decay of 119Sb at antimony sites and 119mTe at tellurium sites of a crystalline (hcp -phase) Ge2Sb2Te5 film. The positions of spectral
lines associated with Sn2+ and Sn0 centers
In the case of 119Sb parent atoms, the spectrum has the form of a single broadened line (G = 1.32(2) mm/s). The isomer shift of this spectrum (IS = 3.47(2) mm/s) corresponds to divalent tin Sn2+. The spectrum of 119mSn impurity atoms formed upon the radioactive decay of 119Sb parent atoms at antimony sites of the crystal lattice of Sb2Te3 has similar parameters. Hence follows the conclusion that, in both cases, tellurium atoms are in the local environment of 119mSn2+ atoms. This agrees with the data for the hcp structure of Ge2Sb2Te5 crystalline films (Kato, Tanaka 2005), according to which tellurium atoms are in the local environment of antimony atoms. A conclusion can also be made that there are only tellurium atoms in the local environment of 119mSn2+ in both cases.
In the case of 119raTe atoms, the spectrum is a superposition of two broadened lines (G = 1.41-1.46 mm/s). The higher intensity line with the isomer shift IS = 2.42(2) mm/s falling within the range of isomer shifts of the spectra of intermetallic compounds of tin corresponds to 119mSn° centers formed after the decay of 119mTe mother atoms at tellurium sites. The layered lattice of the hep phase of Ge2Sb2Te5 has three types of tellurium layers (Micoulaut et al. 2014), which gives rise to an inhomogeneous isomer shift in addition to a quadrupole splitting and significant broadening of the spectral line.
The weak intensity line with IS = 3.51(2) mm/s is associated with 119mSn2+ centers formed after the decay of 119mTe mother atoms shifted from tellurium sites to Sb or Ge sites due to the recoil energy accompanying the radioactive decay of the 119mTe isotope. The set of sites to which the daughter atom of 119Sb is shifted also leads to an inhomogeneous isomer shift and substantial broadening of the spectral line.
A similar structure is observed for the spectra of 119mSn impurity atoms formed after the radioactive decay of 119mTe atoms at tellurium sites of the crystal lattices of Sb2Te3 and GeTe (see Fig. 5), and a conclusion can be made that only tellurium atoms are in all cases in the local environment of 119mSn2+ atoms.
Sn2 J^'
Sb2T e3:119Sb
«
Sb2Te3: 119mT e \ Sn°
, -*v
GeTe: n9mTe \ Sn°
■7 -3.5 0 3.5 7
Velocity, mm/s
Fig. 5. Emission Mossbauer spectra of 119mSn impurity atoms formed after the radioactive decay of 119Sb at antimony sites and 119mTe at tellurium sites of the Sb2Te3 and GeTe compounds. The positions of spectral lines
associated with Sn2+ and Sn0 centers
The 119mSn atoms that are fixed, due to electron capture from 119Sb or to a chain of electron-capture events from 119mTe, at Sb or Te sites of the hep lattice of Ge2Sb2Te5 can be regarded as models of antisite defects because an electronic analog of an atom from one sublattice (germanium) is found at the site of the other sublattice.
Conclusions
It was shown that tin atoms and germanium atoms substituting the former in the structure of amorphous and polycrystalline Ge2Sb2Te5 and Ge1 5Te8 5 have different local environment symmetries (tetrahedral in the amorphous phase and octahedral in the crystalline phase). The method of emission Mossbauer spectroscopy on 119mSn impurity centers formed after the radioactive decay of 119Sb and 119mTe mother atoms was used to identify antisite tin defects at antimony and tellurium sites of Ge2Sb2Te5 crystalline films. The broadening of the spectra of the antisite defects is accounted for either by a set of possible atoms (antimony, germanium, tellurium) in the local environment of tellurium sites, or by a similar set of sites to which the daughter 119Sb atom is shifted.
References
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