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ZnSb thin films for phase change memory application
Author’s Accepted Manuscript Improvement of phase change properties of stacked Ge2Sb2Te5/ZnSb thin films for phase change memory application Zifang He, Weihua Wu, Xinyi Liu, Jiwei Zhai, Tianshu Lai, Sannian Song, Zhitang Song www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)31481-1 http://dx.doi.org/10.1016/j.matlet.2016.09.021 MLBLUE21459
To appear in: Materials Letters Received date: 5 June 2016 Revised date: 13 August 2016 Accepted date: 10 September 2016 Cite this article as: Zifang He, Weihua Wu, Xinyi Liu, Jiwei Zhai, Tianshu Lai, Sannian Song and Zhitang Song, Improvement of phase change properties of stacked Ge2Sb2Te5/ZnSb thin films for phase change memory application, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.09.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improvement of phase change properties of stacked Ge2Sb2Te5/ZnSb thin films for phase change memory application Zifang Hea, Weihua Wua, Xinyi Liub, Jiwei Zhaia*, Tianshu Laib*, Sannian Songc, Zhitang Songc a
Functional Materials Research Laboratory, School of Materials Science & Engineering, Tongji
University, Shanghai 201804, China b
State Key Laboratory of Optoelectronic Materials and Technology, Department of Physics, Sun
Yat-Sen University, Guangzhou 510275, China c
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-system and
Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
Abstract Ge2Sb2Te5/ZnSb (GST/ZS) stacked thin films were proposed for high density phase change memories (PCM). Electrical and structural properties were studied by in-situ resistance measurements and X-ray diffraction (XRD), respectively. The films exhibited good thermal stability and two resistance steps during heating process. A picosecond laser pump-probe system was used to measure phase change speed. Phase change memory cells based on [GST(35 nm)/ZS(15 nm)]1 thin film were fabricated to test and verify multi-level switch between set and reset states. Keywords: Ge2Sb2Te5/ZnSb stacked thin films; High data storage density; High thermal stability; Phase change memory
1. Introduction PCM has been considered as one of the most potential candidates because of its high performances [1-3]. PCM is based on a reversible phase transition induced by electro-thermal stimulation [4]. Information storage can be realized through huge differences between amorphous state and crystalline state. In recent years, phase change material has attracted intensive exploration. GST is the most widely used material. However, several issues of GST limit it to provide high reliable devices for mobile memory applications. Firstly, GST has a quite low crystallization temperature (~150 °C) [2], leading to low thermal stability. So far, many studies have been developed to solve the problem by either doping 2
with the incorporated dopants including N [5], O [6], and Si [7] atoms, or superlattice-like (SLL) nanocompositing structure such as Ge8Sb92/Ga30Sb70 thin films [8]. Secondly, improving the storage capability is a permanent valuable issue. Amount of efforts recently have been made to increase data storage capability. PCM devices comprising two Ge2Sb2Te5 (GST) layers sandwiching a thermal insulating Ta2O5 barrier layer have been proved to exhibit multi-level phase change behavior [9]. Vanadium doped Sb2Te3 material with modified crystallization mechanism has observed two resistance steps [10]. In this study, using huge differences in phase change temperature between GST and ZnSb, by the way of stacking in nanoscale, realize a two-step transition to enhance storage density. In addition, the stacked GST/ZnSb thin films also show good thermal stability, fast phase change speed. All those make GST/ZnSb thin films a candidate for PCM applications.
2. Experimental details GST/ZnSb thin films ratios were deposited on SiO2/Si (100) wafers by using a radio-frequency magnetron sputtering system at room temperature. Thickness of each layer was controlled by deposition time. In-situ temperature-dependent resistances (R–T) were measured by using a custom-made setup at Ar atmosphere. Crystalline phases of films were analyzed by X-ray diffraction (XRD, Rigaku D/MAX 2550 V). A picosecond laser pump–probe system was used for real-time reflectivity measurements. PCM cells were fabricated by using the CMOS technology and cells properties were measured by using a TektronixAWG5012B arbitrary waveform generator and a Keithley 2602Aparameter analyzer.
3. Results and discussion R-T curves of stacked GST/ZnSb thin films are depicted in Fig. 1(a). A continuous decrease in resistance is observed until a sharp drop occurs at around 175 °C, which is attributed to phase transition from amorphous structure to metastable GST (Face-Centered Cubic: FCC) as discussed later. The second sudden drop appears at around 250 °C, during this process, crystallization of ZnSb dominates. GST and ZnSb crystallized at different temperatures, leading to two-step transition. Moreover, it is obvious that the crystallization temperature increases with ZnSb content increasing, indicating the 3
better thermal stability in stacked thin films. XRD pattern of [GST(35 nm)/ZS(15 nm)]1 is depicted in Fig. 1(b), just substrate peaks (SiO2) appear below 150 °C. With the increasing of temperature, SiO2 peak at around 33° disappeared. This is because the sample is placed in different angle during the testing process which may lead to the change of diffraction peak. At 200 °C, (2 0 0) and (2 2 0) diffraction peaks of metastable GST (FCC) have emerged, corresponding to the first transition in Fig. 1(a). After annealing at 250 °C, besides FCC phase of GST, (5 2 0) peak of metastable ZnSb (JCPDS no. 40-809) also appeared [11], corresponding to the second transition in Fig. 1(a). At around 300 °C, GST component in the film undergoes a transition from FCC to Hexagonal (HEX) phase, and both coexist. At 350 °C, (1 3 1), (2 2 3) peaks of stable ZnSb and (0 0 5), (1 0 3), (1 1 0), (2 0 3) peaks of HEX GST emerged. These results well demonstrate the unique crystallization process of the stacked thin films. Fig. 2(a) shows R-T curves of [GST(35 nm)/ZS(15 nm)]1 thin film at different heating rates. Crystallization activation energies (Ea), obtained from the slope of curves in Fig. 2(a), are 2.57 eV and 4.83 eV for the first and second transition, respectively. Higher activation energies indicate larger energy barrier the crystallization process need to overcome, and to show a better thermal stability. Fig. 2(c) and (d) show normalized resistance versus elapsed time at various isothermal annealing temperatures of two transitions. On the basis of Arrhenius law, it can be deduced that the extrapolated temperatures of 10-year retention lifetime for the amorphous state and intermediate state are 102 °C and 168 °C (Fig. 2(b)), respectively. In contrast, the value of GST thin film is only 90 °C [12]. It could be attributed to the effect of ZnSb which have an outstanding amorphous stability. In this regard, PCM cells based on GST(35 nm)/ZS(15 nm) possess an excellent reliability. Fig. 3(a) and (b) shows normalized reflectivity evolution of GST(35 nm)/ZS(15 nm). Phase change speed can be evaluated from the time difference of reflectivity’s sudden change induced by a picosecond laser pump-probe system. During the crystallization process (Fig. 3(a)), a 5.9 mJ/cm2 irradiation results in an abrupt increase in reflectivity within 9.1 ns. With a higher laser pulse fluence (7.8 mJ/cm2 ), amorphization transition is realized with a distinct decrease in reflectivity within 6.3 ns (Fig. 3(b)). In comparison, values of Ge2Sb2Te5 are 17.7 ns (crystallization) and 16.5 ns (amorphization) [13]. It is reported that GST thin film has a nucleation-dominated crystallization behavior [14], and nucleation process spends amount of time. However, Sb-rich thin films, such as a Ga2Te3Sb5 composition [15], and superlattice-like Ge2Sb2Te5/Sb thin films [16], have been elucidated to possess a 4
growth-dominated crystallization behavior. This allows even thin films to crystallize at higher speeds. In GST(35 nm)/ZS(15 nm) stacked thin film, ZnSb is a kind of Sb-rich material, and weak Zn-Te bonds (~155.2 kJ/mol) formed in the interface are easy to be broken to act as heterogeneous nucleating agents [17]. Both points are beneficial to speed up the phase change speed. T-shaped PCM cells based on GST(35 nm)/ZS(15 nm) thin films were fabricated to test and verify their electrical properties by using 0.18 μm CMOS technology. Fig. 3(c) shows current–voltage (I–V) curve. As the sweeping current increases, voltage snaps back to a smaller value twice, indicating twice negative-resistance behaviors. Resistance-voltage (R-V) curves with various pulse widths are plotted in Fig. 3(d). Two “RESET” processes can happen in succession when the applied pulse width is above 300ns. It should be pointed out that both “SET” and “RESET” processes have been realized with a 300ns pulse width. While it drops to 100ns, the second “RESET” process cannot be accomplished. This is due to a small pulse width cannot provide enough energy to accomplish the complete phase change process. The obvious two-step phase change transition in cells test makes GST(35 nm)/ZS(15 nm) a promising candidate for PCM applications with high data storage density.
4. Conclusions A novel stacked GST/ZnSb thin film showed improved phase change properties. As for [GST(35 nm)/ZS(15 nm)]1 thin film (optimized configuration), it has a good thermal stability and fast phase change speed. The multi-level data storage capability has been demonstrated by the PCM cell test. All these results indicated stacked GST/ZnSb thin films are promising candidates for PCM device applications.
Acknowledgments The authors would like to acknowledge financial support from the Natural Science Foundation of China under Grant No. 61474083.
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References [1] S. R. Ovshinsky, Reversible Electrical Switching Phenomena in Disordered Structures, Phys. Rev. Lett. 21 (1968) 1450. [2] I. Friedrich, V. Weidenhof, W. Njoroge, M. Wuttig, et al, Structural transformations of Ge2Sb2Te5 films studied by electrical resistance measurements, J. Appl. Phys. 87 (2000) 4130. [3] R. R. Liu, Z. F. He, J. W. Zhai, et al, Ultra-high speed and low-power superlattice-like Sn18Sb82– SnSe2 thin films for phase change memory applications, Mater. Lett. 163 (2016) 20-23. [4] X. L. Zhou, L. C. Wu, Z. T. Song, et al, Carbon-doped Ge2Sb2Te5 phase change material: A candidate for high-density phase change memory application, Appl. Phy. Lett. 101 (2012) 142104. [5] K. B. Borisenko, Y. X. Chen, S. A. Song, et al, Nanoscale Phase Separation and Building Blocks Ge2Sb2Te5N and Ge2Sb2Te5N2 Thin Films, Chem. Mater. 21 (2009) 5244. [6] K. B. Song, S. W. Sohn, J. Kim, et al, Chalcogenide thin-film transistors using oxygenated n-type and p-type phase change materials, Appl. Phy. Lett. 93 (2008) 043514. [7] E. Cho, S. Han, D. Kim, et al, Ab initio study on influence of dopants on crystalline and amorphous Ge2Sb2Te5, J. Appl. Phys. 109 (2011) 043705. [8] Z. F. He, R. R. Liu, J. W. Zhai, et al, High speed and high reliability in Ge8Sb92/Ga30Sb70 stacked thin films for phase change memory applications, J. Alloys. Compd. 653 (2015) 334-7. [9] A. Gyanathan, Y-C. Yeo, Multi-level phase change memory devices with Ge2Sb2Te5 layers separated by a thermal insulating Ta2O5 barrier layer, J. Appl. Phys. 110 (2011) 124517. [10] X. L. Ji, L. C. Wu, L. L. Cao, et al, Vanadium doped Sb2Te3 material with modified crystallization mechanism for phase-change memory application, Appl. Phy. Lett. 106 (2015) 243103. [11] Y. M. Chen, G. X. Wang, X. Shen, et al, Crystallization behaviors of ZnxSb100−x thin films for ultralong data retention phase change memory applications, CrystEngComm. 16 (2014) 757-62. [11] Q. Wang, B. Liu, Y. Y. Xia, et al, Cr-doped Ge2Sb2Te5 for ultra-long data retention phase change memory, Appl. Phy. Lett. 107 (2015) 222101. [13] Y. F. Hu, X. Y. Feng, J. W. Zhai, et al , Superlattice-likeSb50Se50/Ga30Sb70 thin films for high-speed and high density phase change memory application, Appl. Phy. Lett. 103 (2013) 152107. [14] H-Y. Cheng, S. Raoux, J. L. Jordan-Sweet, Crystallization properties of materials along the pseudo-binary line between GeTe and Sb, J. Appl. Phys. 115 (2014) 093101.
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[15] K-F. Kao, C-M. Lee, M-J. Chen, et al, Ga2Te3Sb5-A Candidate for Fast and Ultralong Retention Phase-Change Memory, Adv. Mater. 21 (2009) 1695-9.
[16] Y. F. Hu, H. Zou, J. H. Zhang, et al , Ge2Sb2Te5/Sb superlattice-like thin film for high speed phase change memory application, Appl. Phy. Lett. 107 (2015) 263105. [17] M. M. Tian, G. X. Wang, X. Shen, et al, Phase change properties of ZnSb-doped Ge2Sb2Te5 films, Acta. Phys. Sin. 64 (2015) 176802.
Fig. 1 (a) Resistance v.s. temperature for GST/ZnSb thin films, and monolayer thin films at a heating rate of 10 °C/min; (b) XRD patterns of [GST(35 nm)/ZnSb(15 nm)]1 thin films at as-deposited state and annealed at different temperatures for 10 min, respectively.
Fig. 2 (a) Resistance as a function of temperature for [GST(35 nm)/ZnSb(15 nm)]1 thin films at different heating rates; (b) Plots of failure time as a function of reciprocal temperature; The normalized resistance with elapsed time of the amorphous (c) and intermediate (d) states upon isothermal soaking at the shown temperatures.
Fig. 3 Reversible reflectivity evolution of [GST(35 nm)/ZnSb(15 nm)]1 thin film induced by consecutive picosecond laser pulses with different fluencies: (a) crystallization process, (b) amorphization process; (c) I–V and (d) R–V curves of the PCRAM cells based on the stacked [GST(35 nm)/ZnSb(15 nm)]1 thin film.
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Figure. 1
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Figure. 2
Figure. 3
Highlights multi-level phase change property. good thermal stability. fast phase change speed
unique multi-level crystallization mechanism
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PII: DOI: Reference:
S0167-577X(16)31481-1 http://dx.doi.org/10.1016/j.matlet.2016.09.021 MLBLUE21459
To appear in: Materials Letters Received date: 5 June 2016 Revised date: 13 August 2016 Accepted date: 10 September 2016 Cite this article as: Zifang He, Weihua Wu, Xinyi Liu, Jiwei Zhai, Tianshu Lai, Sannian Song and Zhitang Song, Improvement of phase change properties of stacked Ge2Sb2Te5/ZnSb thin films for phase change memory application, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.09.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improvement of phase change properties of stacked Ge2Sb2Te5/ZnSb thin films for phase change memory application Zifang Hea, Weihua Wua, Xinyi Liub, Jiwei Zhaia*, Tianshu Laib*, Sannian Songc, Zhitang Songc a
Functional Materials Research Laboratory, School of Materials Science & Engineering, Tongji
University, Shanghai 201804, China b
State Key Laboratory of Optoelectronic Materials and Technology, Department of Physics, Sun
Yat-Sen University, Guangzhou 510275, China c
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-system and
Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
Abstract Ge2Sb2Te5/ZnSb (GST/ZS) stacked thin films were proposed for high density phase change memories (PCM). Electrical and structural properties were studied by in-situ resistance measurements and X-ray diffraction (XRD), respectively. The films exhibited good thermal stability and two resistance steps during heating process. A picosecond laser pump-probe system was used to measure phase change speed. Phase change memory cells based on [GST(35 nm)/ZS(15 nm)]1 thin film were fabricated to test and verify multi-level switch between set and reset states. Keywords: Ge2Sb2Te5/ZnSb stacked thin films; High data storage density; High thermal stability; Phase change memory
1. Introduction PCM has been considered as one of the most potential candidates because of its high performances [1-3]. PCM is based on a reversible phase transition induced by electro-thermal stimulation [4]. Information storage can be realized through huge differences between amorphous state and crystalline state. In recent years, phase change material has attracted intensive exploration. GST is the most widely used material. However, several issues of GST limit it to provide high reliable devices for mobile memory applications. Firstly, GST has a quite low crystallization temperature (~150 °C) [2], leading to low thermal stability. So far, many studies have been developed to solve the problem by either doping 2
with the incorporated dopants including N [5], O [6], and Si [7] atoms, or superlattice-like (SLL) nanocompositing structure such as Ge8Sb92/Ga30Sb70 thin films [8]. Secondly, improving the storage capability is a permanent valuable issue. Amount of efforts recently have been made to increase data storage capability. PCM devices comprising two Ge2Sb2Te5 (GST) layers sandwiching a thermal insulating Ta2O5 barrier layer have been proved to exhibit multi-level phase change behavior [9]. Vanadium doped Sb2Te3 material with modified crystallization mechanism has observed two resistance steps [10]. In this study, using huge differences in phase change temperature between GST and ZnSb, by the way of stacking in nanoscale, realize a two-step transition to enhance storage density. In addition, the stacked GST/ZnSb thin films also show good thermal stability, fast phase change speed. All those make GST/ZnSb thin films a candidate for PCM applications.
2. Experimental details GST/ZnSb thin films ratios were deposited on SiO2/Si (100) wafers by using a radio-frequency magnetron sputtering system at room temperature. Thickness of each layer was controlled by deposition time. In-situ temperature-dependent resistances (R–T) were measured by using a custom-made setup at Ar atmosphere. Crystalline phases of films were analyzed by X-ray diffraction (XRD, Rigaku D/MAX 2550 V). A picosecond laser pump–probe system was used for real-time reflectivity measurements. PCM cells were fabricated by using the CMOS technology and cells properties were measured by using a TektronixAWG5012B arbitrary waveform generator and a Keithley 2602Aparameter analyzer.
3. Results and discussion R-T curves of stacked GST/ZnSb thin films are depicted in Fig. 1(a). A continuous decrease in resistance is observed until a sharp drop occurs at around 175 °C, which is attributed to phase transition from amorphous structure to metastable GST (Face-Centered Cubic: FCC) as discussed later. The second sudden drop appears at around 250 °C, during this process, crystallization of ZnSb dominates. GST and ZnSb crystallized at different temperatures, leading to two-step transition. Moreover, it is obvious that the crystallization temperature increases with ZnSb content increasing, indicating the 3
better thermal stability in stacked thin films. XRD pattern of [GST(35 nm)/ZS(15 nm)]1 is depicted in Fig. 1(b), just substrate peaks (SiO2) appear below 150 °C. With the increasing of temperature, SiO2 peak at around 33° disappeared. This is because the sample is placed in different angle during the testing process which may lead to the change of diffraction peak. At 200 °C, (2 0 0) and (2 2 0) diffraction peaks of metastable GST (FCC) have emerged, corresponding to the first transition in Fig. 1(a). After annealing at 250 °C, besides FCC phase of GST, (5 2 0) peak of metastable ZnSb (JCPDS no. 40-809) also appeared [11], corresponding to the second transition in Fig. 1(a). At around 300 °C, GST component in the film undergoes a transition from FCC to Hexagonal (HEX) phase, and both coexist. At 350 °C, (1 3 1), (2 2 3) peaks of stable ZnSb and (0 0 5), (1 0 3), (1 1 0), (2 0 3) peaks of HEX GST emerged. These results well demonstrate the unique crystallization process of the stacked thin films. Fig. 2(a) shows R-T curves of [GST(35 nm)/ZS(15 nm)]1 thin film at different heating rates. Crystallization activation energies (Ea), obtained from the slope of curves in Fig. 2(a), are 2.57 eV and 4.83 eV for the first and second transition, respectively. Higher activation energies indicate larger energy barrier the crystallization process need to overcome, and to show a better thermal stability. Fig. 2(c) and (d) show normalized resistance versus elapsed time at various isothermal annealing temperatures of two transitions. On the basis of Arrhenius law, it can be deduced that the extrapolated temperatures of 10-year retention lifetime for the amorphous state and intermediate state are 102 °C and 168 °C (Fig. 2(b)), respectively. In contrast, the value of GST thin film is only 90 °C [12]. It could be attributed to the effect of ZnSb which have an outstanding amorphous stability. In this regard, PCM cells based on GST(35 nm)/ZS(15 nm) possess an excellent reliability. Fig. 3(a) and (b) shows normalized reflectivity evolution of GST(35 nm)/ZS(15 nm). Phase change speed can be evaluated from the time difference of reflectivity’s sudden change induced by a picosecond laser pump-probe system. During the crystallization process (Fig. 3(a)), a 5.9 mJ/cm2 irradiation results in an abrupt increase in reflectivity within 9.1 ns. With a higher laser pulse fluence (7.8 mJ/cm2 ), amorphization transition is realized with a distinct decrease in reflectivity within 6.3 ns (Fig. 3(b)). In comparison, values of Ge2Sb2Te5 are 17.7 ns (crystallization) and 16.5 ns (amorphization) [13]. It is reported that GST thin film has a nucleation-dominated crystallization behavior [14], and nucleation process spends amount of time. However, Sb-rich thin films, such as a Ga2Te3Sb5 composition [15], and superlattice-like Ge2Sb2Te5/Sb thin films [16], have been elucidated to possess a 4
growth-dominated crystallization behavior. This allows even thin films to crystallize at higher speeds. In GST(35 nm)/ZS(15 nm) stacked thin film, ZnSb is a kind of Sb-rich material, and weak Zn-Te bonds (~155.2 kJ/mol) formed in the interface are easy to be broken to act as heterogeneous nucleating agents [17]. Both points are beneficial to speed up the phase change speed. T-shaped PCM cells based on GST(35 nm)/ZS(15 nm) thin films were fabricated to test and verify their electrical properties by using 0.18 μm CMOS technology. Fig. 3(c) shows current–voltage (I–V) curve. As the sweeping current increases, voltage snaps back to a smaller value twice, indicating twice negative-resistance behaviors. Resistance-voltage (R-V) curves with various pulse widths are plotted in Fig. 3(d). Two “RESET” processes can happen in succession when the applied pulse width is above 300ns. It should be pointed out that both “SET” and “RESET” processes have been realized with a 300ns pulse width. While it drops to 100ns, the second “RESET” process cannot be accomplished. This is due to a small pulse width cannot provide enough energy to accomplish the complete phase change process. The obvious two-step phase change transition in cells test makes GST(35 nm)/ZS(15 nm) a promising candidate for PCM applications with high data storage density.
4. Conclusions A novel stacked GST/ZnSb thin film showed improved phase change properties. As for [GST(35 nm)/ZS(15 nm)]1 thin film (optimized configuration), it has a good thermal stability and fast phase change speed. The multi-level data storage capability has been demonstrated by the PCM cell test. All these results indicated stacked GST/ZnSb thin films are promising candidates for PCM device applications.
Acknowledgments The authors would like to acknowledge financial support from the Natural Science Foundation of China under Grant No. 61474083.
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References [1] S. R. Ovshinsky, Reversible Electrical Switching Phenomena in Disordered Structures, Phys. Rev. Lett. 21 (1968) 1450. [2] I. Friedrich, V. Weidenhof, W. Njoroge, M. Wuttig, et al, Structural transformations of Ge2Sb2Te5 films studied by electrical resistance measurements, J. Appl. Phys. 87 (2000) 4130. [3] R. R. Liu, Z. F. He, J. W. Zhai, et al, Ultra-high speed and low-power superlattice-like Sn18Sb82– SnSe2 thin films for phase change memory applications, Mater. Lett. 163 (2016) 20-23. [4] X. L. Zhou, L. C. Wu, Z. T. Song, et al, Carbon-doped Ge2Sb2Te5 phase change material: A candidate for high-density phase change memory application, Appl. Phy. Lett. 101 (2012) 142104. [5] K. B. Borisenko, Y. X. Chen, S. A. Song, et al, Nanoscale Phase Separation and Building Blocks Ge2Sb2Te5N and Ge2Sb2Te5N2 Thin Films, Chem. Mater. 21 (2009) 5244. [6] K. B. Song, S. W. Sohn, J. Kim, et al, Chalcogenide thin-film transistors using oxygenated n-type and p-type phase change materials, Appl. Phy. Lett. 93 (2008) 043514. [7] E. Cho, S. Han, D. Kim, et al, Ab initio study on influence of dopants on crystalline and amorphous Ge2Sb2Te5, J. Appl. Phys. 109 (2011) 043705. [8] Z. F. He, R. R. Liu, J. W. Zhai, et al, High speed and high reliability in Ge8Sb92/Ga30Sb70 stacked thin films for phase change memory applications, J. Alloys. Compd. 653 (2015) 334-7. [9] A. Gyanathan, Y-C. Yeo, Multi-level phase change memory devices with Ge2Sb2Te5 layers separated by a thermal insulating Ta2O5 barrier layer, J. Appl. Phys. 110 (2011) 124517. [10] X. L. Ji, L. C. Wu, L. L. Cao, et al, Vanadium doped Sb2Te3 material with modified crystallization mechanism for phase-change memory application, Appl. Phy. Lett. 106 (2015) 243103. [11] Y. M. Chen, G. X. Wang, X. Shen, et al, Crystallization behaviors of ZnxSb100−x thin films for ultralong data retention phase change memory applications, CrystEngComm. 16 (2014) 757-62. [11] Q. Wang, B. Liu, Y. Y. Xia, et al, Cr-doped Ge2Sb2Te5 for ultra-long data retention phase change memory, Appl. Phy. Lett. 107 (2015) 222101. [13] Y. F. Hu, X. Y. Feng, J. W. Zhai, et al , Superlattice-likeSb50Se50/Ga30Sb70 thin films for high-speed and high density phase change memory application, Appl. Phy. Lett. 103 (2013) 152107. [14] H-Y. Cheng, S. Raoux, J. L. Jordan-Sweet, Crystallization properties of materials along the pseudo-binary line between GeTe and Sb, J. Appl. Phys. 115 (2014) 093101.
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[15] K-F. Kao, C-M. Lee, M-J. Chen, et al, Ga2Te3Sb5-A Candidate for Fast and Ultralong Retention Phase-Change Memory, Adv. Mater. 21 (2009) 1695-9.
[16] Y. F. Hu, H. Zou, J. H. Zhang, et al , Ge2Sb2Te5/Sb superlattice-like thin film for high speed phase change memory application, Appl. Phy. Lett. 107 (2015) 263105. [17] M. M. Tian, G. X. Wang, X. Shen, et al, Phase change properties of ZnSb-doped Ge2Sb2Te5 films, Acta. Phys. Sin. 64 (2015) 176802.
Fig. 1 (a) Resistance v.s. temperature for GST/ZnSb thin films, and monolayer thin films at a heating rate of 10 °C/min; (b) XRD patterns of [GST(35 nm)/ZnSb(15 nm)]1 thin films at as-deposited state and annealed at different temperatures for 10 min, respectively.
Fig. 2 (a) Resistance as a function of temperature for [GST(35 nm)/ZnSb(15 nm)]1 thin films at different heating rates; (b) Plots of failure time as a function of reciprocal temperature; The normalized resistance with elapsed time of the amorphous (c) and intermediate (d) states upon isothermal soaking at the shown temperatures.
Fig. 3 Reversible reflectivity evolution of [GST(35 nm)/ZnSb(15 nm)]1 thin film induced by consecutive picosecond laser pulses with different fluencies: (a) crystallization process, (b) amorphization process; (c) I–V and (d) R–V curves of the PCRAM cells based on the stacked [GST(35 nm)/ZnSb(15 nm)]1 thin film.
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Figure. 1
8
Figure. 2
Figure. 3
Highlights multi-level phase change property. good thermal stability. fast phase change speed
unique multi-level crystallization mechanism
9