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Thermal properties of phase change material Ge2Sb2Te5 doped with Bi
NOC-16357; No of Pages 4 Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
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Thermal properties of phase change material Ge2Sb2Te5 doped with Bi A. Sherchenkov a, S. Kozyukhin b,⁎, A. Babich a, P. Lazarenko a a b
Department of Material Science, National Research University MIET, Moscow, Russia Kurnakov Institute of General and Inorganic Chemistry, RAS, Moscow, Russia
a r t i c l e
i n f o
Article history: Received 15 October 2012 Received in revised form 26 November 2012 Available online xxxx Keywords: Phase change memory materials; Chalcogenide; Thermal properties
a b s t r a c t The influence of Bi doping on the thermal properties and thermal stability of Ge2Sb2Te5 thin films was investigated. It was shown that Bi doping allows the change of thermal properties in wide range, and increase thermal stability of thin films. It was also shown that the doping of bismuth affects the electrical properties of the material. The existence of two Bi concentration ranges with two different doping mechanisms and influences on the film properties was established. In the range of low concentrations (0.5–1.0 wt.% of Bi) the anomalous deviations of properties from main tendencies were observed. This effect is explained with the use of percolation theory when at critical concentration (percolation threshold) formation of infinite cluster is accompanied by critical phenomena. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The phase change memory (PCM) technology including rewritable optical disks of various formats (DVD-RW, Blu-Ray) and the latest generation of electronic memories has provided prominent advances for the field of data storage, in particular high scalability, cyclability (up to 10 13 cycles) and high phase transformation speed [1–3]. These virtues were obtained with establishing of fast switching stable alloys of Ge\Sb\Te (GST) chalcogenide-based materials [4]. Though the switching power and RESET current consumption of GST based PCM cells are major factors in chip design of the data storage technology, the thermal stability of the PCM device can often be an obstacle in the path to the full commercialization. To overcome the problems PCM technology is needed to be improved. This requires effective methods for controlling the thermal, electrical, physical properties, and increasing stability. One of the main ways to control the properties of semiconductors is the introduction of the doping component in the host matrix of the material. However, most of chalcogenide glassy semiconductors are insensitive to doping because of Fermi level pinning [5]. In this case, the control of the physical properties of the PCM materials transforms into a complex problem. One of the possible effective ways is to use isomorphic element as an impurity. For Ge\Sb\Te system such an element is bismuth. In this case, it is a good reason to believe that bismuth can replace antimony positions, thus binding energy will decrease from 277.4 kJ/mol for Sb\Te bond to 232 kJ/mol for Bi\Te one [6]. However, it is difficult to predict a priori, how the bismuth doping will affect the PCM material properties, which are critical for operation of PCM device. Furthermore, ⁎ Corresponding author. Tel.: +7 95 952 2382; fax: +7 95 954 1279. E-mail address: [email protected] (S. Kozyukhin).
it should be noted that PCM devices require contradictory PCM material properties. For example, their amorphous phase has to be characterized by a low crystallization temperature to ensure a fast switching during writing, but on the contrary by a high crystallization temperature for high stability. In addition, there are only few experimental data about influence of bismuth on phase change materials [7–9], and for reasonable conclusions it is necessary to carry out further experimental studies. In this study we show that bismuth introduction to the amorphous Ge2Sb2Te5 (GST225) thin films may have an influence on thermal and electrical properties. 2. Experimental The initial Ge2Sb2Te5 alloys doped with different amounts of Bi (0.5, 1 and 3 wt.%) were prepared using synthesis method described in [10]. The materials (99.99% purity) were sealed in evacuated (5 · 10 −3 Pa) quartz ampoules then step by step heated to 850 °C in a rocking furnace to ensure homogeneity of the melt. Thin films were prepared using thermal deposition of these doped GST225s on c-Si (100) substrates in vacuum chamber. Residual pressure in the chamber was 10 −4 Pa. The maximum temperature during evaporation was kept below 630 °C. The films thicknesses were determined from the height of step using atomic force microscopy (NT-MDT SolverPro). The morphology of the layers was studied by scanning electron microscopy (Carl Zeiss NVision 40). The polycrystalline and amorphous structures of the synthesized alloys and as-deposited films, respectively, were checked by X-ray diffraction (XRD) (Rigaku D/MAX, Cu Kα λ = 0.15481 nm) [11]. Rutherford backscattering (RBS) (Ed = 1.0 and Eα = 2.7 MeV at 135° scattering angle) was used as an analytical method to determine the thin film compositions. We have used differential scanning calorimetry (DSC-50, Shimadzu) with different heating rates (q = 5–90°/min) to examine the thermal
0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.006
Please cite this article as: A. Sherchenkov, et al., Thermal properties of phase change material Ge2Sb2Te5 doped with Bi, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.006
2
A. Sherchenkov et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
properties and thermally induced transformations of GST materials. Temperature calibration was checked with In, Sn, Pb and Cd for all used heating rates.
3. Results First of all, we determined the film compositions, and the obtained values are listed in Table 1. As one can see, the films compositions are very close to those of bulk materials used for evaporation. DSC curves for initial synthesized crystalline undoped and doped GST225 indicate that there are no heat effects up to the temperature of 590 °C, above which endothermic peaks due to the melting of samples are observed with melting point decreasing with Bi doping (see Table 1). As it can be seen from Fig. 1 melting peaks for GST225 with high bismuth content (3 wt.%), and to some extent for 1 wt.% Bi are formed by overlapping of two peaks, which can be due to the phase separation at higher Bi concentrations. In the case of the as-deposited amorphous films measurements revealed a number of heat effects, which are shown in Fig. 2. The following common features can be pointed out for these samples: i) two exopeaks are connected with phase transitions; and ii) two endopeaks are being observed in the temperature ranges of 390– 415 °C and above 580 °C. Higher temperature endopeak is due to the melting. In the case of thin films melting temperatures are lower than those of synthesized crystalline samples, which can be attributed to the difference of the structures. The thermal characteristics determined from DSC measurements for GST thin films are summarized in Fig. 3. In general, the initial addition of Bi to GST225 leads to the decrease in the crystallization temperature, and value of the heat effect. Both of these effects can be attributed to the modification of binding energy and/or local atomic order. However, for high concentration of dopant (3 wt.%) these parameters begin to increase. The deviation from the tendency for high bismuth content samples can be connected with different mechanisms of impurity introduction in matrix. All the observed effects have thermally activated nature, and we used different isoconversional methods of Ozawa–Flynn–Wall, Kissinger–Akahira–Sunose, and Starink and Tang [12] to estimate the activation energy of the crystallization. These methods are based on the relations between heating rate and temperature for the fixed values of conversion. The following equation is used in the Ozawa–Flynn–Wall approach lnβ ¼ CW ðαi Þ−1:0518ðEðαi Þ=RTðαi ÞÞ
ð1Þ
where β is the heating rate, αi is fixed value of conversion (in our case the value of conversion was equal to the crystalline part), CW(αi) is the constant for fixed value of conversion αi, R is the gas constant, E(αi) is the effective activation energy of crystallization for fixed value of conversion αi, and T(αi) is temperature at which fixed value of conversion αi is reached for different heating rates.
Fig. 1. Endothermic peaks due to the melting of samples.
The relations for other approaches have close equations, for example, the Kissinger–Akahira–Sunose method h i 2 ln β=T ðαi Þ ¼ CK ðαi Þ–1:0518ðEðαi Þ=RTðαi ÞÞ;
ð2Þ
the Starink method h i 1:92 ln β=T ðαi Þ ¼ CS ðαi Þ–1:0008ðEðαi Þ=RTðαi ÞÞ;
ð3Þ
and the Tang method h i 1:894661 ln β=T ðαi Þ ¼ CT ðαi Þ–1:00145033ðEðαi Þ=RTðαi ÞÞ:
ð4Þ
We assumed that the total area of the peak corresponds to the heat effect of fully passed reaction (in our case crystallization). Then, the value of conversion can be calculated as the quotient of part of the peak area to its total area. It was found that the values of effective activation energies of crystallization estimated for GST225 + 0.5 wt.% Bi thin films with the use of these four methods differ less than 2%. So, hereafter we used only Ozawa–Flynn–Wall method for the remainder compositions. Fig. 4 shows the dependences of the effective activation energy on conversion for GST225 with different Bi contents. At low doping concentration effective activation energy increased sharply indicating on a retarding of crystallization, while at higher concentrations it decreased nearly to the values of undoped GST225. These results correlate with those of thermal stability measurements (see also Fig. 2c). Obtained high values of effective activation energies imply that crystallization is a diffusion controlled process. Earlier we have shown that multiple DSC measurements of GST225 lead to the appearance of endothermic peak in the temperature range of 390–415 °C, which can initiate reliability issue of PCM
Table 1 Compositions, thicknesses, and melting points of investigated thin film. Initial compounds (bulk)
GST225 GST225 + 0.5 wt.% Bi GST225 + 1 wt.% Bi GST225 + 3 wt.% Bi
Thin film compositions according to RBS spectroscopy (at. un.), accuracy ±5% Ge
Sb + Te
Bismuth
Oxygen
2 2 2 2
7 6.80 ± 0.20 6.86 ± 0.20 6.90 ± 0.20
0.024 ± 0.010 0.053 ± 0.010 0.14 ± 0.010
b(0.12 ± 0.04) b(0.20 ± 0.06) b(0.10 ± 0.04)
Bismuth content for thin film (wt.%, calculation)
Film thickness, nm
Melting point, °C
0.48 1.07 2.75
210 ± 10 170 ± 10 190 ± 10
624.5 617.0 615.8 615.9
Please cite this article as: A. Sherchenkov, et al., Thermal properties of phase change material Ge2Sb2Te5 doped with Bi, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.006
A. Sherchenkov et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
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Fig. 2. DSC scans of as-deposited amorphous GST thin films.
Fig. 4. Effective activation energy of crystallization for different thin films.
devices [13]. To investigate the influence of bismuth doping on the thermal stability of GST225, we carried out multiple DSC measurements of thin films. The endothermic peak in the temperature range of 390–415 °C appeared after the repeated DSC measurements for all doped materials also. The heat effect values increased with the number of DSC measurements. Fig. 3d shows the values of this effect for GST thin films after the five DSC measurements. However, it should be noted that endothermic peak kinetics for doped films is much slower than that of the undoped GST225. These results indicate the higher thermal stability of doped materials in comparison with undoped material. In addition, a material with low Bi concentration is more stable, than with high concentration. The nature of this endothermic peak for GST225 thin films is connected with the phase separation due to the diffusion of the
mobile Te atoms to the grain boundaries, and melting of Ge\Te eutectic phase, which is supported by the XRD measurements [14]. The temperatures of these endopeaks are quite close for undoped and Bi doped GST225, which indicates the similar nature of the processes leading to the appearance of these heat effects. In this case, doping can block the diffusion of the mobile Te atoms to the grain boundaries preventing the phase separation, and increasing the stability of the material. The d.c. resistivity measurements were carried out on planar structures, and showed that crystallization is accompanied by drastic decrease of resistivity for all compositions. Determined by resistivity measurements onset crystallization temperatures correlate with those measured by DSC method. Exponential temperature dependences of resistivity were established for all studied amorphous thin films. Analyses of these data, first of all of the value of pre-exponential factor σ0, showed that conductivity of GST amorphous films is determined by hopping mechanism of carriers in the tail states. The activation energy of conductivity for amorphous films decreases with bismuth doping up to 1 wt.% of Bi, and then remains nearly constant. The low bismuth content (up to 1 wt.%) leads to the increase in the resistivity of amorphous thin films, and the value of drop between the resistivities of amorphous and crystalline states, which is important for the reliable work of PCM cells. 4. Discussion
Fig. 3. Thermal characteristics of GST thin films: a — temperature of crystallization; b — heat effect of crystallization; c — effective activation energy of crystallization for α = 0.05; and d — value of endopeak in the temperature range of 390–415 °C after the five DSC measurements.
So, experimental concentration dependences of thermal, electrical properties, and thermal stability of Bi doped GST225 thin films indicate the existence in most cases of the turning points, which suggest that there are two mechanisms of introducing of bismuth atoms in the matrix: first when the impurity concentration is low, and second when impurity concentration is high. As the studied films are amorphous and accordingly the structure has no periodic lattice, the model of a continual percolation is applicable to them [15]. The main assumption of such approach consists of the following: the universality of critical indexes maintains moderate perturbations in the system, such as introduction of short order correlations or transformations of a lattice at an invariance of space dimension. In that case, assuming that the properties are isotropic and taking into consideration a short-range nature of impurity potential, for any type of the interaction between bismuth dopants one can designate the radius of impurity atom “action sphere” as R0 (see Fig. 5a) without taking into account the type of interactions as “impurity condensate” and the process of their formation as “condensation of impurity vapor”. The action radius of such spheres makes an order of about several coordination numbers for different solid solutions of tellurides [16].
Please cite this article as: A. Sherchenkov, et al., Thermal properties of phase change material Ge2Sb2Te5 doped with Bi, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.006
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Fig. 5. (a, b, c). Cluster formation diagram in GST material under introduction of bismuth atoms (explanation in the text).
In accordance with percolation theory (problem of spheres or model of random nodes which was used, for example, for description of phase transition in ferromagnetics [17]) there is an impurity (dopant) concentration at which will be formed a cluster of finite size (see Fig. 5b). The increase in concentration leads to the increase in the size of cluster accompanied by the creation at critical concentration (percolation threshold Pc) of the channels penetrating the whole system and appearance of the infinite cluster consisting of overlapping spheres of radius R0 (see Fig. 5c). The formation of infinite cluster is accompanied by critical phenomena, which must manifest themselves in the case of the solid solutions through anomalies in the concentration dependences. Reaching the critical concentration stimulates process of impurity atoms redistribution over the matrix so as to realize the configuration corresponding to minimum thermodynamical potentialordering of impurity atoms, complex formation and so on. In other words, the possible self-organization processes may include a longrange ordering of impurity atoms. The existence of such phenomena in the range of small impurity concentrations (less than 5 at.%) for some ternary solid solutions, for example, PbTe–CdTe, PbTe–GeTe, SnTe–InTe, PbTe–MnTe, etc. was proved in a number of studies [16,18]. 5. Conclusions The influence of Bi doping on the thermal and electrical properties and thermal stability of GST225 thin films was investigated. The existence of two Bi concentration ranges with two different doping mechanisms and influences on the film properties was established. In the range of low concentrations (0.5–1.0 wt.% of Bi) anomalous deviations of properties from main tendencies were observed. Doping of GST225 with Bi allows the increase in the range of material properties, which is important for the optimization of PCM technology. However, the choice of the impurity concentration is complicated by the contradictory requirements for the PCM material properties, and must be resolved taking into consideration characteristic features of future device.
Acknowledgments The study was supported by the Russian Foundation for Basic Research (project no. 11-03-00269) and Ministry of Education and Science of RF (projects no. 16.552.11.7086 and SC no. 8437).
References [1] E.R. Meinders, A.V. Mijiritskii, L. van Pieterson, M. Wuttig, Optical data storage phase-change media and recording, Philips Research Book Series, 4, Springer-Verlag, Berlin, 2006. [2] G.F. Burr, M.J. Breitwisch, M. Franceschini, et al., J. Vac. Sci. Technol. B 28 (2010) 223–262. [3] S. Raoux, W. Wojciech, D. Ielmini, Chem. Rev. 110 (2010) 240–267. [4] N. Yamada, E. Ohno, N. Akahira, K. Nishiuchi, K. Nagata, M. Takao, Jpn. J. Appl. Phys. 26 (Suppl. 26-4) (1987) 61–66. [5] N.F. Mott, E.A. Davis, Electron Processes in Non-Crystalline Materials, Clarendon Press, Oxford, 1979. [6] A. Kelly, G.W. Groves, P. Kidd, Crystallography and Crystal Defects (Rev. Ed.), Willey, Chichester, 2000. [7] T.-Y. Lee, Ki-B. Kim, B-Ki Cheong, et al., Appl. Phys. Lett. 80 (2002) 3313–3315. [8] K. Wang, D. Wamwangi, S. Ziegler, C. Steimer, M. Wuttig, J. Appl. Phys. 96 (2004) 5557–5562. [9] Ambika, P.B. Barman, J. Non-Oxide Glasses 3 (2012) 19–24. [10] N.Kh. Abrikosov, G.T. Danilova-Dobryakova, Izv. Akad. NaukUSSR, Ser. Neorg. Mater. 1 (1965) 204–206, (in Russ.). [11] S.A. Kozyukhin, A.A. Sherchenkov, E.V. Gorschkova, V.Kh. Kudoyarova, A.I. Vargunin, Phys. Status Solidi C 7 (2010) 848–851. [12] M. Abu El-Oyoun, J. Non-Cryst. Solids 357 (2011) 1729–1735. [13] A.A. Sherchenkov, S.A. Kozyukhin, E.V. Gorshkova, J. Optoelectron. Adv. Mater. 11 (2009) 26–33. [14] A.A. Sherchenkov, S.A. Kozyukhin, A.V. Babich, P.I. Lazarenko, Influence of doping on the properties of Ge–Sb–Te thin films for phase-change memory devices, Proc. II Int. Conf. on Modern Problems in Physics of Surface and Nanostructures, Yaroslavl, Russia, 2012. [15] D. Stauffer, Introduction to Percolation Theory, Taylor & Francis, London and Philadelphia, 1985. [16] E.I. Rogacheva, Jpn. J. Appl. Phys. 32 (Suppl. 32-3) (1993) 775–777. [17] A.A. Abrikosov, Adv. Phys. 29 (1980) 869–875. [18] E.I. Rogacheva, J. Phys. Chem. Solids 64 (2003) 1579–1583.
Please cite this article as: A. Sherchenkov, et al., Thermal properties of phase change material Ge2Sb2Te5 doped with Bi, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.006
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Thermal properties of phase change material Ge2Sb2Te5 doped with Bi A. Sherchenkov a, S. Kozyukhin b,⁎, A. Babich a, P. Lazarenko a a b
Department of Material Science, National Research University MIET, Moscow, Russia Kurnakov Institute of General and Inorganic Chemistry, RAS, Moscow, Russia
a r t i c l e
i n f o
Article history: Received 15 October 2012 Received in revised form 26 November 2012 Available online xxxx Keywords: Phase change memory materials; Chalcogenide; Thermal properties
a b s t r a c t The influence of Bi doping on the thermal properties and thermal stability of Ge2Sb2Te5 thin films was investigated. It was shown that Bi doping allows the change of thermal properties in wide range, and increase thermal stability of thin films. It was also shown that the doping of bismuth affects the electrical properties of the material. The existence of two Bi concentration ranges with two different doping mechanisms and influences on the film properties was established. In the range of low concentrations (0.5–1.0 wt.% of Bi) the anomalous deviations of properties from main tendencies were observed. This effect is explained with the use of percolation theory when at critical concentration (percolation threshold) formation of infinite cluster is accompanied by critical phenomena. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The phase change memory (PCM) technology including rewritable optical disks of various formats (DVD-RW, Blu-Ray) and the latest generation of electronic memories has provided prominent advances for the field of data storage, in particular high scalability, cyclability (up to 10 13 cycles) and high phase transformation speed [1–3]. These virtues were obtained with establishing of fast switching stable alloys of Ge\Sb\Te (GST) chalcogenide-based materials [4]. Though the switching power and RESET current consumption of GST based PCM cells are major factors in chip design of the data storage technology, the thermal stability of the PCM device can often be an obstacle in the path to the full commercialization. To overcome the problems PCM technology is needed to be improved. This requires effective methods for controlling the thermal, electrical, physical properties, and increasing stability. One of the main ways to control the properties of semiconductors is the introduction of the doping component in the host matrix of the material. However, most of chalcogenide glassy semiconductors are insensitive to doping because of Fermi level pinning [5]. In this case, the control of the physical properties of the PCM materials transforms into a complex problem. One of the possible effective ways is to use isomorphic element as an impurity. For Ge\Sb\Te system such an element is bismuth. In this case, it is a good reason to believe that bismuth can replace antimony positions, thus binding energy will decrease from 277.4 kJ/mol for Sb\Te bond to 232 kJ/mol for Bi\Te one [6]. However, it is difficult to predict a priori, how the bismuth doping will affect the PCM material properties, which are critical for operation of PCM device. Furthermore, ⁎ Corresponding author. Tel.: +7 95 952 2382; fax: +7 95 954 1279. E-mail address: [email protected] (S. Kozyukhin).
it should be noted that PCM devices require contradictory PCM material properties. For example, their amorphous phase has to be characterized by a low crystallization temperature to ensure a fast switching during writing, but on the contrary by a high crystallization temperature for high stability. In addition, there are only few experimental data about influence of bismuth on phase change materials [7–9], and for reasonable conclusions it is necessary to carry out further experimental studies. In this study we show that bismuth introduction to the amorphous Ge2Sb2Te5 (GST225) thin films may have an influence on thermal and electrical properties. 2. Experimental The initial Ge2Sb2Te5 alloys doped with different amounts of Bi (0.5, 1 and 3 wt.%) were prepared using synthesis method described in [10]. The materials (99.99% purity) were sealed in evacuated (5 · 10 −3 Pa) quartz ampoules then step by step heated to 850 °C in a rocking furnace to ensure homogeneity of the melt. Thin films were prepared using thermal deposition of these doped GST225s on c-Si (100) substrates in vacuum chamber. Residual pressure in the chamber was 10 −4 Pa. The maximum temperature during evaporation was kept below 630 °C. The films thicknesses were determined from the height of step using atomic force microscopy (NT-MDT SolverPro). The morphology of the layers was studied by scanning electron microscopy (Carl Zeiss NVision 40). The polycrystalline and amorphous structures of the synthesized alloys and as-deposited films, respectively, were checked by X-ray diffraction (XRD) (Rigaku D/MAX, Cu Kα λ = 0.15481 nm) [11]. Rutherford backscattering (RBS) (Ed = 1.0 and Eα = 2.7 MeV at 135° scattering angle) was used as an analytical method to determine the thin film compositions. We have used differential scanning calorimetry (DSC-50, Shimadzu) with different heating rates (q = 5–90°/min) to examine the thermal
0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.006
Please cite this article as: A. Sherchenkov, et al., Thermal properties of phase change material Ge2Sb2Te5 doped with Bi, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.006
2
A. Sherchenkov et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
properties and thermally induced transformations of GST materials. Temperature calibration was checked with In, Sn, Pb and Cd for all used heating rates.
3. Results First of all, we determined the film compositions, and the obtained values are listed in Table 1. As one can see, the films compositions are very close to those of bulk materials used for evaporation. DSC curves for initial synthesized crystalline undoped and doped GST225 indicate that there are no heat effects up to the temperature of 590 °C, above which endothermic peaks due to the melting of samples are observed with melting point decreasing with Bi doping (see Table 1). As it can be seen from Fig. 1 melting peaks for GST225 with high bismuth content (3 wt.%), and to some extent for 1 wt.% Bi are formed by overlapping of two peaks, which can be due to the phase separation at higher Bi concentrations. In the case of the as-deposited amorphous films measurements revealed a number of heat effects, which are shown in Fig. 2. The following common features can be pointed out for these samples: i) two exopeaks are connected with phase transitions; and ii) two endopeaks are being observed in the temperature ranges of 390– 415 °C and above 580 °C. Higher temperature endopeak is due to the melting. In the case of thin films melting temperatures are lower than those of synthesized crystalline samples, which can be attributed to the difference of the structures. The thermal characteristics determined from DSC measurements for GST thin films are summarized in Fig. 3. In general, the initial addition of Bi to GST225 leads to the decrease in the crystallization temperature, and value of the heat effect. Both of these effects can be attributed to the modification of binding energy and/or local atomic order. However, for high concentration of dopant (3 wt.%) these parameters begin to increase. The deviation from the tendency for high bismuth content samples can be connected with different mechanisms of impurity introduction in matrix. All the observed effects have thermally activated nature, and we used different isoconversional methods of Ozawa–Flynn–Wall, Kissinger–Akahira–Sunose, and Starink and Tang [12] to estimate the activation energy of the crystallization. These methods are based on the relations between heating rate and temperature for the fixed values of conversion. The following equation is used in the Ozawa–Flynn–Wall approach lnβ ¼ CW ðαi Þ−1:0518ðEðαi Þ=RTðαi ÞÞ
ð1Þ
where β is the heating rate, αi is fixed value of conversion (in our case the value of conversion was equal to the crystalline part), CW(αi) is the constant for fixed value of conversion αi, R is the gas constant, E(αi) is the effective activation energy of crystallization for fixed value of conversion αi, and T(αi) is temperature at which fixed value of conversion αi is reached for different heating rates.
Fig. 1. Endothermic peaks due to the melting of samples.
The relations for other approaches have close equations, for example, the Kissinger–Akahira–Sunose method h i 2 ln β=T ðαi Þ ¼ CK ðαi Þ–1:0518ðEðαi Þ=RTðαi ÞÞ;
ð2Þ
the Starink method h i 1:92 ln β=T ðαi Þ ¼ CS ðαi Þ–1:0008ðEðαi Þ=RTðαi ÞÞ;
ð3Þ
and the Tang method h i 1:894661 ln β=T ðαi Þ ¼ CT ðαi Þ–1:00145033ðEðαi Þ=RTðαi ÞÞ:
ð4Þ
We assumed that the total area of the peak corresponds to the heat effect of fully passed reaction (in our case crystallization). Then, the value of conversion can be calculated as the quotient of part of the peak area to its total area. It was found that the values of effective activation energies of crystallization estimated for GST225 + 0.5 wt.% Bi thin films with the use of these four methods differ less than 2%. So, hereafter we used only Ozawa–Flynn–Wall method for the remainder compositions. Fig. 4 shows the dependences of the effective activation energy on conversion for GST225 with different Bi contents. At low doping concentration effective activation energy increased sharply indicating on a retarding of crystallization, while at higher concentrations it decreased nearly to the values of undoped GST225. These results correlate with those of thermal stability measurements (see also Fig. 2c). Obtained high values of effective activation energies imply that crystallization is a diffusion controlled process. Earlier we have shown that multiple DSC measurements of GST225 lead to the appearance of endothermic peak in the temperature range of 390–415 °C, which can initiate reliability issue of PCM
Table 1 Compositions, thicknesses, and melting points of investigated thin film. Initial compounds (bulk)
GST225 GST225 + 0.5 wt.% Bi GST225 + 1 wt.% Bi GST225 + 3 wt.% Bi
Thin film compositions according to RBS spectroscopy (at. un.), accuracy ±5% Ge
Sb + Te
Bismuth
Oxygen
2 2 2 2
7 6.80 ± 0.20 6.86 ± 0.20 6.90 ± 0.20
0.024 ± 0.010 0.053 ± 0.010 0.14 ± 0.010
b(0.12 ± 0.04) b(0.20 ± 0.06) b(0.10 ± 0.04)
Bismuth content for thin film (wt.%, calculation)
Film thickness, nm
Melting point, °C
0.48 1.07 2.75
210 ± 10 170 ± 10 190 ± 10
624.5 617.0 615.8 615.9
Please cite this article as: A. Sherchenkov, et al., Thermal properties of phase change material Ge2Sb2Te5 doped with Bi, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.006
A. Sherchenkov et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
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Fig. 2. DSC scans of as-deposited amorphous GST thin films.
Fig. 4. Effective activation energy of crystallization for different thin films.
devices [13]. To investigate the influence of bismuth doping on the thermal stability of GST225, we carried out multiple DSC measurements of thin films. The endothermic peak in the temperature range of 390–415 °C appeared after the repeated DSC measurements for all doped materials also. The heat effect values increased with the number of DSC measurements. Fig. 3d shows the values of this effect for GST thin films after the five DSC measurements. However, it should be noted that endothermic peak kinetics for doped films is much slower than that of the undoped GST225. These results indicate the higher thermal stability of doped materials in comparison with undoped material. In addition, a material with low Bi concentration is more stable, than with high concentration. The nature of this endothermic peak for GST225 thin films is connected with the phase separation due to the diffusion of the
mobile Te atoms to the grain boundaries, and melting of Ge\Te eutectic phase, which is supported by the XRD measurements [14]. The temperatures of these endopeaks are quite close for undoped and Bi doped GST225, which indicates the similar nature of the processes leading to the appearance of these heat effects. In this case, doping can block the diffusion of the mobile Te atoms to the grain boundaries preventing the phase separation, and increasing the stability of the material. The d.c. resistivity measurements were carried out on planar structures, and showed that crystallization is accompanied by drastic decrease of resistivity for all compositions. Determined by resistivity measurements onset crystallization temperatures correlate with those measured by DSC method. Exponential temperature dependences of resistivity were established for all studied amorphous thin films. Analyses of these data, first of all of the value of pre-exponential factor σ0, showed that conductivity of GST amorphous films is determined by hopping mechanism of carriers in the tail states. The activation energy of conductivity for amorphous films decreases with bismuth doping up to 1 wt.% of Bi, and then remains nearly constant. The low bismuth content (up to 1 wt.%) leads to the increase in the resistivity of amorphous thin films, and the value of drop between the resistivities of amorphous and crystalline states, which is important for the reliable work of PCM cells. 4. Discussion
Fig. 3. Thermal characteristics of GST thin films: a — temperature of crystallization; b — heat effect of crystallization; c — effective activation energy of crystallization for α = 0.05; and d — value of endopeak in the temperature range of 390–415 °C after the five DSC measurements.
So, experimental concentration dependences of thermal, electrical properties, and thermal stability of Bi doped GST225 thin films indicate the existence in most cases of the turning points, which suggest that there are two mechanisms of introducing of bismuth atoms in the matrix: first when the impurity concentration is low, and second when impurity concentration is high. As the studied films are amorphous and accordingly the structure has no periodic lattice, the model of a continual percolation is applicable to them [15]. The main assumption of such approach consists of the following: the universality of critical indexes maintains moderate perturbations in the system, such as introduction of short order correlations or transformations of a lattice at an invariance of space dimension. In that case, assuming that the properties are isotropic and taking into consideration a short-range nature of impurity potential, for any type of the interaction between bismuth dopants one can designate the radius of impurity atom “action sphere” as R0 (see Fig. 5a) without taking into account the type of interactions as “impurity condensate” and the process of their formation as “condensation of impurity vapor”. The action radius of such spheres makes an order of about several coordination numbers for different solid solutions of tellurides [16].
Please cite this article as: A. Sherchenkov, et al., Thermal properties of phase change material Ge2Sb2Te5 doped with Bi, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.006
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Fig. 5. (a, b, c). Cluster formation diagram in GST material under introduction of bismuth atoms (explanation in the text).
In accordance with percolation theory (problem of spheres or model of random nodes which was used, for example, for description of phase transition in ferromagnetics [17]) there is an impurity (dopant) concentration at which will be formed a cluster of finite size (see Fig. 5b). The increase in concentration leads to the increase in the size of cluster accompanied by the creation at critical concentration (percolation threshold Pc) of the channels penetrating the whole system and appearance of the infinite cluster consisting of overlapping spheres of radius R0 (see Fig. 5c). The formation of infinite cluster is accompanied by critical phenomena, which must manifest themselves in the case of the solid solutions through anomalies in the concentration dependences. Reaching the critical concentration stimulates process of impurity atoms redistribution over the matrix so as to realize the configuration corresponding to minimum thermodynamical potentialordering of impurity atoms, complex formation and so on. In other words, the possible self-organization processes may include a longrange ordering of impurity atoms. The existence of such phenomena in the range of small impurity concentrations (less than 5 at.%) for some ternary solid solutions, for example, PbTe–CdTe, PbTe–GeTe, SnTe–InTe, PbTe–MnTe, etc. was proved in a number of studies [16,18]. 5. Conclusions The influence of Bi doping on the thermal and electrical properties and thermal stability of GST225 thin films was investigated. The existence of two Bi concentration ranges with two different doping mechanisms and influences on the film properties was established. In the range of low concentrations (0.5–1.0 wt.% of Bi) anomalous deviations of properties from main tendencies were observed. Doping of GST225 with Bi allows the increase in the range of material properties, which is important for the optimization of PCM technology. However, the choice of the impurity concentration is complicated by the contradictory requirements for the PCM material properties, and must be resolved taking into consideration characteristic features of future device.
Acknowledgments The study was supported by the Russian Foundation for Basic Research (project no. 11-03-00269) and Ministry of Education and Science of RF (projects no. 16.552.11.7086 and SC no. 8437).
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Please cite this article as: A. Sherchenkov, et al., Thermal properties of phase change material Ge2Sb2Te5 doped with Bi, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.006