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Fast crystallization of Mg-doped Sb4Te for phase change memory
Vacuum 112 (2015) 33e37
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Fast crystallization of Mg-doped Sb4Te for phase change memory Xiang Shen a, b, *, Junjian Li b, Guoxiang Wang b, Zhanshan Wang a, Yegang Lu b, Shixun Dai b a b
Institute of Precision Optical Engineering, Department of Physics, Tongji University, Shanghai 200092, China Laboratory of Infrared Material and Devices, The Advanced Technology Research Institute, Ningbo University, Ningbo 315211, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 August 2014 Received in revised form 29 October 2014 Accepted 10 November 2014 Available online 18 November 2014
We prepared Mg-doped Sb4Te films and investigated their structural, electrical and optical properties. It was found that Mg could increase the crystallization temperature and improve the activation energy of crystallization as well as amorphous state stability of the Sb4Te film. Compared with Ge2Sb2Te5, the optimal composition of Mg19.8(Sb4Te)80.2 exhibits a higher crystallization temperature (~187 C), and better data retention ability (keeping the amorphous state at 113.6 C for ten years). Moreover, fast full crystallization (~20 ns at a laser power of ~60 mW) in the Mg19.8(Sb4Te)80.2 film is confirmed, which is essential to achieve rapid data recording in phase change memory. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Thin films Crystallization Optical reflectivity Fast crystallization
1. Introduction Flash memory is encountering scaling limits currently although it has been successfully used as nonvolatile semiconductor memory. New memory technologies based on amorphous chalcogenide semiconductor films have been investigated to overcome the limit of the flash memory. In fact, chalcogenide films have been widely used in compact disk, digital versatile disk rewritable, digital versatile disk-random access memory and blue-ray disk due to a large change in refractive index and reflectivity [1,2]. On the other hand, a larger difference in resistivity between the amorphous and crystalline state during phase change process makes the chalcogenide films be the best candidate to fabricate electrical nonvolatile memory device known as phase change memory (PCM) [3]. Being one of the key materials to realize PCM, chalcogenide films with different compositions have been widely investigated. Especially, Ge2Sb2Te5 has been used as a phase-change layer applied in PCM device based on its outstanding properties [4,5]. Nevertheless, several issues, including low crystallization temperature and low crystalline resistance that have led to poor data retention ability
and large RESET current in the PCM device, need to be addressed, and further improvement of the material properties is necessary. It is well known that the eutectic SbeTe material exhibits fast crystallization speed but poor thermal stability. However, the SbeTe material can be improved by doping other elements, such as Si [6], Ga [7], Al [8], Zn [9], In [10], Ti [11], Se [12]. In addition, our previous investigation has confirmed that Mg is effective in forming covalent bonds and reducing the atomic diffusivity, and thus improving amorphous stability of the film [13]. In this paper, Mg-doped Sb4Te films were prepared and studied systematically in order to explore the best composition for PCM applications. The effects of Mg doping on crystallization temperature, electrical resistance, and thermal stability of the films were analyzed by various experimental methods including the measurements of sheet resistance versus temperature (ReT), X-ray diffraction (XRD) and Transmission electron microscopy (TEM) patterns. The crystallization mechanism of Mg-doped Sb4Te films could be obtained with the help of a laser-induced static tester.
2. Experiments
* Corresponding author. Institute of Precision Optical Engineering, Department of Physics, Tongji University, Shanghai 200092, China. Tel.: þ86 574 87609873; fax: þ86 574 87600946. E-mail address: [email protected] (X. Shen). http://dx.doi.org/10.1016/j.vacuum.2014.11.012 0042-207X/© 2014 Elsevier Ltd. All rights reserved.
The Mg-doped Sb4Te films were deposited by radio-frequency magnetron sputtering system onto SiO2/Si (100) wafers by using Mg and Sb4Te alloy targets. In each run of experiment, the chamber was evacuated to 2 104 Pa, and then Ar gas was introduced to 0.35 Pa for the deposition. The DC power on the Mg target was
34
X. Shen et al. / Vacuum 112 (2015) 33e37
tuned in a range from 3 to 15 W while the RF power on the Sb4Te target was fixed to be 80 W in order to obtain the different Mg doping content. The thickness of films was determined to be about 200 nm by the surface profiler. The compositions of the Mg-doped Sb4Te films were measured by energy dispersive spectroscopy (EDS). In situ temperature-dependent resistance at a heating rate of 40 K/min and time-dependent resistance at isothermal annealing measurement had been carried out in a vacuum chamber. XRD and TEM were employed to characterize the structure of the asdeposited and annealed films. The diffraction patterns were taken in the 2q range of 20e60 using Cu Ka radiation with a wavelength of 0.154 nm and performed under Bragg conditions for samples. The crystallization behavior on a nanosecond scale was measured using a static tester with a wavelength of 658 nm (PST-1, NANOSstorage Co. Ltd., KOREA). And the penetration depth is about 25 nm obtaining by using the equation: d ¼ 1/a ¼ l/4pk [14], a is the absorption coefficient, l is the wavelength (658 nm) of the laser, k is the extinction coefficient. 3. Results and discussion Fig. 1(a) presents the curves of sheet resistance versus the annealing temperature (ReT) in Mg-doped Sb4Te films at a heating rate of 40 K/min. The sheet resistance of the Mg-doped Sb4Te films increases with increasing Mg content at room temperature. With
Fig. 1. (a) Sheet resistance as function of temperature for MgeSb4Te films. (b) The extrapolated data retention time of MgeSb4Te films.
increase of temperature, the sheet resistance of all films decreases gradually with a quick drop at the crystallization temperature (Tc), indicating a phase transition from the amorphous phase to the crystalline phase. The Tc values of the Mg4.7(Sb4Te)95.3, Mg13.9(Sb4Te)86.1, Mg19.8(Sb4Te)80.2 and Mg24.6(Sb4Te)75.4 films is ~147, ~172, ~187 and ~176 C, respectively, all of which are higher than Tc of Sb4Te at ~135 C. The Tc is enhanced significantly by the addition of Mg content, which helps to improve the thermal stability of the amorphous SbeTe material. However, when Mg content increases to 24.6 at% in the Mg-doped Sb4Te films, the Tc and crystalline resistance begins to decrease, as shown in Fig. 1(a). Thus, considering the high crystallization temperature and crystalline resistance, the composition of Mg19.8(Sb4Te)80.2 is suitable as a phase-change layer for PCM application. The data retention of Mg4.7(Sb4Te)95.3, Mg13.9(Sb4Te)86.1, Mg19.8(Sb4Te)80.2 and Mg24.6(Sb4Te)75.4 films is characterized by isothermal change in resistance on the basis of the Arrhenius equation [15]. By the linear fitting of the logarithm of failure time versus reciprocal temperature (1/kT) for Mg-doped Sb4Te films, we can determine the crystallization activation energy and the temperature for 10-year archival life, as shown in Fig. 1(b). It can be seen that the temperature for the optimal composition Mg19.8(Sb4Te)80.2 is 113.6 C with the largest activation energy of 3.81 eV. Obviously, the film has better thermal stability than GST
Fig. 2. XRD patterns of (a) as-deposited and (b) 200 C-annealed Mg-doped Sb4Te films.
X. Shen et al. / Vacuum 112 (2015) 33e37
film (88 C, 2.96 eV) [13]. Thus, it can be expected that PCM device based on Mg19.8(Sb4Te)80.2 film should store information longer and safer. In order to analyze the effect of Mg on the structure of Sb4Te films, the XRD patterns of the MgeSb4Te films was shown in Fig. 2. No crystallization peaks can be observed in the as-deposited Mg4.7(Sb4Te)95.3, Mg13.9(Sb4Te)86.1 and Mg19.8(Sb4Te)80.2 films, as shown in Fig. 2(a), confirming an amorphous nature of the films. On the other hand, when Mg4.7(Sb4Te)95.3, Mg13.9(Sb4Te)86.1 and Mg19.8(Sb4Te)80.2 films are annealed at 200 C, they crystallized into a crystal Sb phase as shown in Fig. 2(b). Each crystalline peak can be indexed according to the PDF card (JCPDS NO. 71-1173). With increasing Mg content to 19.8 at%, there is no change in the peak positions, indicating that when Mg atoms are doped into Sb4Te films, the lattice structure remains unchanged. Moreover, the intensity of the peak is suppressed and the width becomes broader, implying that crystal grains become smaller. There will be more grain boundary when the grain size decreases, which helps to enhance the carrier scattering, thus leading to a higher resistance. A bright field TEM image and a selected area electron diffraction (SAED) pattern of the Mg19.8(Sb4Te)80.2 film are shown in Fig. 3(a) and (b), respectively. The TEM sample was fabricated on a microgrid and annealed at 200 C in Ar atmosphere for 3 min. It is found that the film displays many crystal grains with a size of 20e40 nm as shown in Fig. 3(a), which is in good agreement with that estimated from the line-width of XRD patterns. Fig. 3(b) shows that a mono-crystal ring could be identified as the phase of Sb, which is consistent with the XRD results. High-resolution transmission electron microscopy (HRTEM) images as shown in Fig. 3(c) and (d) reveals that the measured interplanar distances match with those in crystalline Sb phase very well.
35
To investigate the crystallization behavior in nano-second scale, a static tester was employed. The static tester uses pulsed laser irradiation with laser power from 5 to 70 mW and pulse width from 5 to 250 ns for phase transformation [16]. Fig. 4(a)e(c) shows Power-Time-Effect (PTE) diagram for Mg4.7(Sb4Te)95.3, Mg19.8(Sb4Te)80.2 and Mg24.6(Sb4Te)75.4 films. The relative reflectivity changes DC is defined as DC ¼ (RafterRbefore)/Rbefore [17], where Rafter and Rbefore are the reflectivity after and before irradiation, respectively. At a lower laser power and/or a short pulse width, the relative reflectivity change does not change in the initial region I in Fig. 4(a)e(c). It indicates that the applied laser is not sufficient to initiate crystallization. With increasing the laser power and/or pulse width, DC increases up to the maximum value which means that the film becomes to a full crystallization in the region II in Fig. 4(a)e(c). In order to better characterize the speed of the phase transition, we measured the relative crystallization (defined as D ¼ DC/DCmax) [18] at various laser power and pulse width of Mg-doped Sb4Te films. Fig. 5(a)e(f) shows the minimum pulse duration to initiate crystallization (tsc) and the minimum pulse duration to complete crystallization (tc) of the as-deposited films. The relative crystallization changes increases when the film evolves from the amorphous phase to the crystalline phase in Mg4.7(Sb4Te)95.3, Mg19.8(Sb4Te)80.2 and Mg24.6(Sb4Te)75.4 films. In Fig. 5(a), the relative crystallization starts to increase at the power of 10 mW. The tsc decreases from 239 to 104 ns gradually with the increase of Mg content. With laser power increasing to 20 mW in Fig. 5(b), the change in tendency of tsc is similar to the result with the laser power of 10 mW. Differently, D reaches the maximum value which means that the films have fully crystallized. Among them, the tc of the Mg19.8(Sb4Te)80.2 film has the shortest time to reach the complete
Fig. 3. (a) The bright field TEM image. (b) The SAED pattern of the Mg19.8(Sb4Te)80.2 film annealed at 200 C for 3 min (c) and (d) The HRTEM images of the same film.
X. Shen et al. / Vacuum 112 (2015) 33e37
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crystallization, which is determined to be ~70 ns at a laser power of ~20 mW, as shown in Fig. 4(b). When the power increases to 30 mW, both tsc (30 ns) and tc (45 ns) of Mg19.8(Sb4Te)80.2 film become shortest as shown in Fig. 5(c). Subsequently, the tsc of the Mg19.8(Sb4Te)80.2 film decreases from ~20 to ~10 ns and the tc of the Mg19.8(Sb4Te)80.2 film from ~30 to ~20 ns as the laser power increases further from 40 to 60 mW, as shown in Fig. 5(d)e(f). According to Ref. [19], tsc and tc of GST is ~40 ns and ~260 ns at a laser power of 70 mW, respectively. As for Mg21.5(Sb4Te)78.5 film, the lower laser power of 60 mW is large enough to induce the film crystallized. tsc is determined to be only ~10 ns and tc is only ~20 ns.
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It indicates that the Mg19.8(Sb4Te)80.2 film exhibit faster crystallization and lower power compared with GST film [19]. To verify the structural stability during reversible phase change between crystalline phases and amorphous phases of the materials, the optical switching behavior was tested. The set power of 35 mW and pulse width of 250 ns was chosen on the basis of the PTE diagram. In order to achieve the reset process, more joule energy should be required to apply in the materials,
X. Shen et al. / Vacuum 112 (2015) 33e37
thus we adopted the power of 70 mW and pulse width of 150 ns in this case. Fig. 5(g) shows the optical contrast DR (DR ¼ RafterRbefore) values for Mg19.8(Sb4Te)80.2 film under repeating laser pulses (up to 20) for crystalline and amorphous phases. We can see the differences in DR between crystallization and amorphization are sustained by relatively constant value during switching cycles. Here, we should note that although we demonstrated the ability of reversible phase change in this material, it seems incompatible with the results as indicated in PTE diagram, where if the laser power of 70 mW and pulse width of 150 ns is applied on the film, the film is still on the crystalline state. The reason may be ascribed to the different thermal conductivity between amorphous and crystalline state of film. When only 25 nm depth in the surface is melted, amorphization can be happened in the reversible switching since the initial state of film is crystalline, the crystalline material usually exhibits higher thermal conductivity, and then higher quenching rate will be possible. In comparison, the initial state of film is amorphous as presented in the PTE diagram, and its low quenching rate may lead to crystallize in the materials.
4. Conclusions In summary, we prepared Mg-doped Sb4Te films and investigated their structural, electrical and optical properties systematically over a wide range of Mg doping from 0 to 24.6 at%. We found that proper Mg doping could increase crystalline resistance, crystallization temperature and activation energy. The best performance can be achieved in Mg19.8(Sb4Te)80.2 film with high Tc of 187 C as well as good data retention (keeping for 10 yr at ~113.6 C). Furthermore, the complete crystallization time and required power of the as-deposited Mg19.8(Sb4Te)80.2 film (~20 ns at a laser power of ~60 mW) was found to be much faster and lower than that of the GST film. All these properties indicate that the Mg19.8(Sb4Te)80.2 film would be promising for applications in the fast-speed and low-power PCM.
37
Acknowledgments This work was financially supported by the National Program on Key Basic Research Project (973 Program) (Grant No. 2012CB722703), the National Natural Science Foundation of China (Grant Nos. 61377061, 61306147), the Young Leaders of academic climbing project of the Education Department of Zhejiang Province (Grant No. pd2013092), the Program for Innovative Research Team of Ningbo City (Grant No. 2009B217), and sponsored by K. C. Wong Magna Fund in Ningbo University. References [1] Ohta T, Nishiuchi K, Narumi K. Jpn J Appl Phys 2000;39:770e4. [2] Maeda T, Terao M, Shimano T. Jpn J Appl Phys 2003;42:1044e51. [3] Wełnic W, Pamungkas A, Detemple R, Steimer C, Blugel S, Wuttig M. Nat Mater 2006;5:56e62. [4] Kyrsta S, Cremer R, Neuschutz D, Laurenzis M, Haring BP, Kurz H. Appl Surf Sci 2001;179:55e60. [5] Wang GX, Nie QH, Shen X, Wang RP, Wu LC, Fu J, et al. Appl Phys Lett 2012;101:051906. [6] Gu YF, Cheng Y, Song SN, Zhang T, Song ZT, Liu XY, et al. Scr Mater 2011;65:622e5. [7] Kao KF, Chu YC, Tsai MJ, Chin TS. J Appl Phys 2012;111:102808. [8] Peng C, Wu LC, Song ZT, Rao F, Zhu M, Li XL, et al. Appl Surf Sci 2011;257: 10667e70. [9] Shen X, Wang GX, Wang RP, Dai SX, Wu LC, Chen YM, et al. Appl Phys Lett 2013;102:131902. [10] Jang MH, Park SJ, Lim DH, Park SJ, Cho MH, et al. Appl Phys Lett 2010;96: 05112. [11] Zhu M, Wu LC, Rao F, Song ZT, Xia MJ, Ji XL, et al. Appl Phys Lett 2014;104:063105. [12] Nakayama K, Kojima K, Imai Y, Kakimoto Y, Suzuki M. Jpn J Appl Phys 2003;42:404e8. [13] Fu J, Shen X, Nie QH, Wang GX, Wu LC, Dai SX, et al. Appl Surf Sci 2013;264:269e72. [14] Kittel C. Introduction to solid state physics. 8th ed. Beijing: Chemical Industry Press; 2005. [15] Rao F, Song ZT, Ren K, Zhou XL, Cheng Y, Wu LC, et al. Nanotechnology 2011;22:145702. [16] Coombs JH, Jongenelis AP, Es-Spiekman WV, Jacobs BA. J Appl Phys 1995;78: 4906. [17] Kang MJ, Choi SY, Wamwangi D, Wang K, Steimer C, Wuttig M. J Appl Phys 2005;98:014904. [18] Ziegler S, Wuttig M. J Appl Phys 2006;99:064907. [19] Wang GX, Shen X, Nie QH, Wang RP, Wu LC, Lu YG, et al. Appl Phys Lett 2013;103:031914.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Fast crystallization of Mg-doped Sb4Te for phase change memory Xiang Shen a, b, *, Junjian Li b, Guoxiang Wang b, Zhanshan Wang a, Yegang Lu b, Shixun Dai b a b
Institute of Precision Optical Engineering, Department of Physics, Tongji University, Shanghai 200092, China Laboratory of Infrared Material and Devices, The Advanced Technology Research Institute, Ningbo University, Ningbo 315211, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 August 2014 Received in revised form 29 October 2014 Accepted 10 November 2014 Available online 18 November 2014
We prepared Mg-doped Sb4Te films and investigated their structural, electrical and optical properties. It was found that Mg could increase the crystallization temperature and improve the activation energy of crystallization as well as amorphous state stability of the Sb4Te film. Compared with Ge2Sb2Te5, the optimal composition of Mg19.8(Sb4Te)80.2 exhibits a higher crystallization temperature (~187 C), and better data retention ability (keeping the amorphous state at 113.6 C for ten years). Moreover, fast full crystallization (~20 ns at a laser power of ~60 mW) in the Mg19.8(Sb4Te)80.2 film is confirmed, which is essential to achieve rapid data recording in phase change memory. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Thin films Crystallization Optical reflectivity Fast crystallization
1. Introduction Flash memory is encountering scaling limits currently although it has been successfully used as nonvolatile semiconductor memory. New memory technologies based on amorphous chalcogenide semiconductor films have been investigated to overcome the limit of the flash memory. In fact, chalcogenide films have been widely used in compact disk, digital versatile disk rewritable, digital versatile disk-random access memory and blue-ray disk due to a large change in refractive index and reflectivity [1,2]. On the other hand, a larger difference in resistivity between the amorphous and crystalline state during phase change process makes the chalcogenide films be the best candidate to fabricate electrical nonvolatile memory device known as phase change memory (PCM) [3]. Being one of the key materials to realize PCM, chalcogenide films with different compositions have been widely investigated. Especially, Ge2Sb2Te5 has been used as a phase-change layer applied in PCM device based on its outstanding properties [4,5]. Nevertheless, several issues, including low crystallization temperature and low crystalline resistance that have led to poor data retention ability
and large RESET current in the PCM device, need to be addressed, and further improvement of the material properties is necessary. It is well known that the eutectic SbeTe material exhibits fast crystallization speed but poor thermal stability. However, the SbeTe material can be improved by doping other elements, such as Si [6], Ga [7], Al [8], Zn [9], In [10], Ti [11], Se [12]. In addition, our previous investigation has confirmed that Mg is effective in forming covalent bonds and reducing the atomic diffusivity, and thus improving amorphous stability of the film [13]. In this paper, Mg-doped Sb4Te films were prepared and studied systematically in order to explore the best composition for PCM applications. The effects of Mg doping on crystallization temperature, electrical resistance, and thermal stability of the films were analyzed by various experimental methods including the measurements of sheet resistance versus temperature (ReT), X-ray diffraction (XRD) and Transmission electron microscopy (TEM) patterns. The crystallization mechanism of Mg-doped Sb4Te films could be obtained with the help of a laser-induced static tester.
2. Experiments
* Corresponding author. Institute of Precision Optical Engineering, Department of Physics, Tongji University, Shanghai 200092, China. Tel.: þ86 574 87609873; fax: þ86 574 87600946. E-mail address: [email protected] (X. Shen). http://dx.doi.org/10.1016/j.vacuum.2014.11.012 0042-207X/© 2014 Elsevier Ltd. All rights reserved.
The Mg-doped Sb4Te films were deposited by radio-frequency magnetron sputtering system onto SiO2/Si (100) wafers by using Mg and Sb4Te alloy targets. In each run of experiment, the chamber was evacuated to 2 104 Pa, and then Ar gas was introduced to 0.35 Pa for the deposition. The DC power on the Mg target was
34
X. Shen et al. / Vacuum 112 (2015) 33e37
tuned in a range from 3 to 15 W while the RF power on the Sb4Te target was fixed to be 80 W in order to obtain the different Mg doping content. The thickness of films was determined to be about 200 nm by the surface profiler. The compositions of the Mg-doped Sb4Te films were measured by energy dispersive spectroscopy (EDS). In situ temperature-dependent resistance at a heating rate of 40 K/min and time-dependent resistance at isothermal annealing measurement had been carried out in a vacuum chamber. XRD and TEM were employed to characterize the structure of the asdeposited and annealed films. The diffraction patterns were taken in the 2q range of 20e60 using Cu Ka radiation with a wavelength of 0.154 nm and performed under Bragg conditions for samples. The crystallization behavior on a nanosecond scale was measured using a static tester with a wavelength of 658 nm (PST-1, NANOSstorage Co. Ltd., KOREA). And the penetration depth is about 25 nm obtaining by using the equation: d ¼ 1/a ¼ l/4pk [14], a is the absorption coefficient, l is the wavelength (658 nm) of the laser, k is the extinction coefficient. 3. Results and discussion Fig. 1(a) presents the curves of sheet resistance versus the annealing temperature (ReT) in Mg-doped Sb4Te films at a heating rate of 40 K/min. The sheet resistance of the Mg-doped Sb4Te films increases with increasing Mg content at room temperature. With
Fig. 1. (a) Sheet resistance as function of temperature for MgeSb4Te films. (b) The extrapolated data retention time of MgeSb4Te films.
increase of temperature, the sheet resistance of all films decreases gradually with a quick drop at the crystallization temperature (Tc), indicating a phase transition from the amorphous phase to the crystalline phase. The Tc values of the Mg4.7(Sb4Te)95.3, Mg13.9(Sb4Te)86.1, Mg19.8(Sb4Te)80.2 and Mg24.6(Sb4Te)75.4 films is ~147, ~172, ~187 and ~176 C, respectively, all of which are higher than Tc of Sb4Te at ~135 C. The Tc is enhanced significantly by the addition of Mg content, which helps to improve the thermal stability of the amorphous SbeTe material. However, when Mg content increases to 24.6 at% in the Mg-doped Sb4Te films, the Tc and crystalline resistance begins to decrease, as shown in Fig. 1(a). Thus, considering the high crystallization temperature and crystalline resistance, the composition of Mg19.8(Sb4Te)80.2 is suitable as a phase-change layer for PCM application. The data retention of Mg4.7(Sb4Te)95.3, Mg13.9(Sb4Te)86.1, Mg19.8(Sb4Te)80.2 and Mg24.6(Sb4Te)75.4 films is characterized by isothermal change in resistance on the basis of the Arrhenius equation [15]. By the linear fitting of the logarithm of failure time versus reciprocal temperature (1/kT) for Mg-doped Sb4Te films, we can determine the crystallization activation energy and the temperature for 10-year archival life, as shown in Fig. 1(b). It can be seen that the temperature for the optimal composition Mg19.8(Sb4Te)80.2 is 113.6 C with the largest activation energy of 3.81 eV. Obviously, the film has better thermal stability than GST
Fig. 2. XRD patterns of (a) as-deposited and (b) 200 C-annealed Mg-doped Sb4Te films.
X. Shen et al. / Vacuum 112 (2015) 33e37
film (88 C, 2.96 eV) [13]. Thus, it can be expected that PCM device based on Mg19.8(Sb4Te)80.2 film should store information longer and safer. In order to analyze the effect of Mg on the structure of Sb4Te films, the XRD patterns of the MgeSb4Te films was shown in Fig. 2. No crystallization peaks can be observed in the as-deposited Mg4.7(Sb4Te)95.3, Mg13.9(Sb4Te)86.1 and Mg19.8(Sb4Te)80.2 films, as shown in Fig. 2(a), confirming an amorphous nature of the films. On the other hand, when Mg4.7(Sb4Te)95.3, Mg13.9(Sb4Te)86.1 and Mg19.8(Sb4Te)80.2 films are annealed at 200 C, they crystallized into a crystal Sb phase as shown in Fig. 2(b). Each crystalline peak can be indexed according to the PDF card (JCPDS NO. 71-1173). With increasing Mg content to 19.8 at%, there is no change in the peak positions, indicating that when Mg atoms are doped into Sb4Te films, the lattice structure remains unchanged. Moreover, the intensity of the peak is suppressed and the width becomes broader, implying that crystal grains become smaller. There will be more grain boundary when the grain size decreases, which helps to enhance the carrier scattering, thus leading to a higher resistance. A bright field TEM image and a selected area electron diffraction (SAED) pattern of the Mg19.8(Sb4Te)80.2 film are shown in Fig. 3(a) and (b), respectively. The TEM sample was fabricated on a microgrid and annealed at 200 C in Ar atmosphere for 3 min. It is found that the film displays many crystal grains with a size of 20e40 nm as shown in Fig. 3(a), which is in good agreement with that estimated from the line-width of XRD patterns. Fig. 3(b) shows that a mono-crystal ring could be identified as the phase of Sb, which is consistent with the XRD results. High-resolution transmission electron microscopy (HRTEM) images as shown in Fig. 3(c) and (d) reveals that the measured interplanar distances match with those in crystalline Sb phase very well.
35
To investigate the crystallization behavior in nano-second scale, a static tester was employed. The static tester uses pulsed laser irradiation with laser power from 5 to 70 mW and pulse width from 5 to 250 ns for phase transformation [16]. Fig. 4(a)e(c) shows Power-Time-Effect (PTE) diagram for Mg4.7(Sb4Te)95.3, Mg19.8(Sb4Te)80.2 and Mg24.6(Sb4Te)75.4 films. The relative reflectivity changes DC is defined as DC ¼ (RafterRbefore)/Rbefore [17], where Rafter and Rbefore are the reflectivity after and before irradiation, respectively. At a lower laser power and/or a short pulse width, the relative reflectivity change does not change in the initial region I in Fig. 4(a)e(c). It indicates that the applied laser is not sufficient to initiate crystallization. With increasing the laser power and/or pulse width, DC increases up to the maximum value which means that the film becomes to a full crystallization in the region II in Fig. 4(a)e(c). In order to better characterize the speed of the phase transition, we measured the relative crystallization (defined as D ¼ DC/DCmax) [18] at various laser power and pulse width of Mg-doped Sb4Te films. Fig. 5(a)e(f) shows the minimum pulse duration to initiate crystallization (tsc) and the minimum pulse duration to complete crystallization (tc) of the as-deposited films. The relative crystallization changes increases when the film evolves from the amorphous phase to the crystalline phase in Mg4.7(Sb4Te)95.3, Mg19.8(Sb4Te)80.2 and Mg24.6(Sb4Te)75.4 films. In Fig. 5(a), the relative crystallization starts to increase at the power of 10 mW. The tsc decreases from 239 to 104 ns gradually with the increase of Mg content. With laser power increasing to 20 mW in Fig. 5(b), the change in tendency of tsc is similar to the result with the laser power of 10 mW. Differently, D reaches the maximum value which means that the films have fully crystallized. Among them, the tc of the Mg19.8(Sb4Te)80.2 film has the shortest time to reach the complete
Fig. 3. (a) The bright field TEM image. (b) The SAED pattern of the Mg19.8(Sb4Te)80.2 film annealed at 200 C for 3 min (c) and (d) The HRTEM images of the same film.
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crystallization, which is determined to be ~70 ns at a laser power of ~20 mW, as shown in Fig. 4(b). When the power increases to 30 mW, both tsc (30 ns) and tc (45 ns) of Mg19.8(Sb4Te)80.2 film become shortest as shown in Fig. 5(c). Subsequently, the tsc of the Mg19.8(Sb4Te)80.2 film decreases from ~20 to ~10 ns and the tc of the Mg19.8(Sb4Te)80.2 film from ~30 to ~20 ns as the laser power increases further from 40 to 60 mW, as shown in Fig. 5(d)e(f). According to Ref. [19], tsc and tc of GST is ~40 ns and ~260 ns at a laser power of 70 mW, respectively. As for Mg21.5(Sb4Te)78.5 film, the lower laser power of 60 mW is large enough to induce the film crystallized. tsc is determined to be only ~10 ns and tc is only ~20 ns.
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It indicates that the Mg19.8(Sb4Te)80.2 film exhibit faster crystallization and lower power compared with GST film [19]. To verify the structural stability during reversible phase change between crystalline phases and amorphous phases of the materials, the optical switching behavior was tested. The set power of 35 mW and pulse width of 250 ns was chosen on the basis of the PTE diagram. In order to achieve the reset process, more joule energy should be required to apply in the materials,
X. Shen et al. / Vacuum 112 (2015) 33e37
thus we adopted the power of 70 mW and pulse width of 150 ns in this case. Fig. 5(g) shows the optical contrast DR (DR ¼ RafterRbefore) values for Mg19.8(Sb4Te)80.2 film under repeating laser pulses (up to 20) for crystalline and amorphous phases. We can see the differences in DR between crystallization and amorphization are sustained by relatively constant value during switching cycles. Here, we should note that although we demonstrated the ability of reversible phase change in this material, it seems incompatible with the results as indicated in PTE diagram, where if the laser power of 70 mW and pulse width of 150 ns is applied on the film, the film is still on the crystalline state. The reason may be ascribed to the different thermal conductivity between amorphous and crystalline state of film. When only 25 nm depth in the surface is melted, amorphization can be happened in the reversible switching since the initial state of film is crystalline, the crystalline material usually exhibits higher thermal conductivity, and then higher quenching rate will be possible. In comparison, the initial state of film is amorphous as presented in the PTE diagram, and its low quenching rate may lead to crystallize in the materials.
4. Conclusions In summary, we prepared Mg-doped Sb4Te films and investigated their structural, electrical and optical properties systematically over a wide range of Mg doping from 0 to 24.6 at%. We found that proper Mg doping could increase crystalline resistance, crystallization temperature and activation energy. The best performance can be achieved in Mg19.8(Sb4Te)80.2 film with high Tc of 187 C as well as good data retention (keeping for 10 yr at ~113.6 C). Furthermore, the complete crystallization time and required power of the as-deposited Mg19.8(Sb4Te)80.2 film (~20 ns at a laser power of ~60 mW) was found to be much faster and lower than that of the GST film. All these properties indicate that the Mg19.8(Sb4Te)80.2 film would be promising for applications in the fast-speed and low-power PCM.
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Acknowledgments This work was financially supported by the National Program on Key Basic Research Project (973 Program) (Grant No. 2012CB722703), the National Natural Science Foundation of China (Grant Nos. 61377061, 61306147), the Young Leaders of academic climbing project of the Education Department of Zhejiang Province (Grant No. pd2013092), the Program for Innovative Research Team of Ningbo City (Grant No. 2009B217), and sponsored by K. C. Wong Magna Fund in Ningbo University. References [1] Ohta T, Nishiuchi K, Narumi K. Jpn J Appl Phys 2000;39:770e4. [2] Maeda T, Terao M, Shimano T. Jpn J Appl Phys 2003;42:1044e51. [3] Wełnic W, Pamungkas A, Detemple R, Steimer C, Blugel S, Wuttig M. Nat Mater 2006;5:56e62. [4] Kyrsta S, Cremer R, Neuschutz D, Laurenzis M, Haring BP, Kurz H. Appl Surf Sci 2001;179:55e60. [5] Wang GX, Nie QH, Shen X, Wang RP, Wu LC, Fu J, et al. Appl Phys Lett 2012;101:051906. [6] Gu YF, Cheng Y, Song SN, Zhang T, Song ZT, Liu XY, et al. Scr Mater 2011;65:622e5. [7] Kao KF, Chu YC, Tsai MJ, Chin TS. J Appl Phys 2012;111:102808. [8] Peng C, Wu LC, Song ZT, Rao F, Zhu M, Li XL, et al. Appl Surf Sci 2011;257: 10667e70. [9] Shen X, Wang GX, Wang RP, Dai SX, Wu LC, Chen YM, et al. Appl Phys Lett 2013;102:131902. [10] Jang MH, Park SJ, Lim DH, Park SJ, Cho MH, et al. Appl Phys Lett 2010;96: 05112. [11] Zhu M, Wu LC, Rao F, Song ZT, Xia MJ, Ji XL, et al. Appl Phys Lett 2014;104:063105. [12] Nakayama K, Kojima K, Imai Y, Kakimoto Y, Suzuki M. Jpn J Appl Phys 2003;42:404e8. [13] Fu J, Shen X, Nie QH, Wang GX, Wu LC, Dai SX, et al. Appl Surf Sci 2013;264:269e72. [14] Kittel C. Introduction to solid state physics. 8th ed. Beijing: Chemical Industry Press; 2005. [15] Rao F, Song ZT, Ren K, Zhou XL, Cheng Y, Wu LC, et al. Nanotechnology 2011;22:145702. [16] Coombs JH, Jongenelis AP, Es-Spiekman WV, Jacobs BA. J Appl Phys 1995;78: 4906. [17] Kang MJ, Choi SY, Wamwangi D, Wang K, Steimer C, Wuttig M. J Appl Phys 2005;98:014904. [18] Ziegler S, Wuttig M. J Appl Phys 2006;99:064907. [19] Wang GX, Shen X, Nie QH, Wang RP, Wu LC, Lu YG, et al. Appl Phys Lett 2013;103:031914.