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Study on WSb3Te material for phase-change memory applications
Applied Surface Science 355 (2015) 667–671
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Study on WSb3 Te material for phase-change memory applications Yun Meng a,b , Xilin Zhou b , Peigao Han a,∗ , Zhitang Song b , Liangcai Wu b,∗ , Chengqiu Zhu b , Wenjing Guo a , Ling Xu c , Zhongyuan Ma c , Lianke Song a a Shandong Province Key Laboratory of Laser Polarization and Information Technology, School of Physics and Engineering, Qufu Normal University, 273165 Qufu, People’s Republic of China b State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, 200050 Shanghai, People’s Republic of China c National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 24 January 2015 Received in revised form 14 June 2015 Accepted 11 July 2015 Available online 15 July 2015 Keywords: W doping Refined grain size High adhesive strength High SET/RESET speed
a b s t r a c t The phase-change performance of Wx Sb3 Te material were systemically investigated by in situ resistancetemperature measurement, X-ray diffraction (XRD), Raman scattering, adhesive strength test and transmission electron microscope (TEM) in this paper. Experimental results show that the thermal stability of Sb3 Te was increased significantly with W doping. XRD and TEM results prove that the incorporation of W plays a role in suppressing the crystallization of Sb3 Te films, causing smaller grain size. Furthermore, the adhesive strength between W electrode and phase-change material was increased obviously by W addition and a relatively rapid SET/RESET operation of 10 ns is realized with large sensing margin. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Phase-change random access memory (PCM) has attracted much attention for its merits of high switching speed, high scalability and low power consumption, becoming the most feasible candidates for the next generation universal memory [1–4]. Data storage in PCM is achieved by utilize the resistivity contrast between high resistance RESET state (amorphous) and low resistance SET state (crystalline) phase of the chalcogenide material. Ternary alloy Ge2 Sb2 Te5 (GST) is the most popular compound for PCM applications, but there are some issues limiting its development. For example, its 10-year data retention temperature (∼89 ◦ C) cannot quite meet the demand for the automobile electronics [5,6]. The crystallization speed of GST (50 ns) is not enough for the high speed PCM application [7–9]. In order to overcome those obstacles, screening a proper new phase-change material is an effective way for current PCM technology [10]. The Sb–Te binary alloy has shown relatively fast phasechange ability with growth-dominated crystallization mechanism. However, its thermal stability is poor due to the low crystallization temperature (<100 ◦ C) [11]. It was reported that the
∗ Corresponding authors. E-mail addresses: [email protected] (P. Han), [email protected] (L. Wu). http://dx.doi.org/10.1016/j.apsusc.2015.07.069 0169-4332/© 2015 Elsevier B.V. All rights reserved.
thermal stability of Sb–Te materials can be improved obviously by doping, such as Ti [12], Cu [13], N [14] and W [15,16]. Among them, W doped Sb–Te material has a better thermal stability with good phase-change performance. In this paper, the phase-change performances of Wx Sb3 Te material were systemically investigated by in situ resistance-temperature measurement, XRD, Raman scattering, adhesive strength test and TEM. XRD and TEM results prove that the incorporation of W plays a role in suppressing the crystallization of Sb3 Te films, causing smaller grain size. The smaller grain size is benefit to thermal stability and device performance. The Raman peaks shifting show the substitution of Sb or Te atoms by W atoms in crystal lattice, which inhibits further crystallization. Furthermore, the adhesive strength between Sb3 Te film and W electrodes could be significantly improved by W doping. The higher adhesive strength indicates higher device reliability in PCM application. It is worth to point out that, the reversible phase change can be trigged by 10 ns electrical pulse, which is faster than that of GST. 2. Experimental The Wx Sb3 Te (x = 0, 0.17, 0.31, 0.38, 0.46 at.%) films were deposited on Si/SiO2 by radio frequency co-sputtering of W and Sb3 Te targets at room temperature. For adhesive strength test, W film with a thickness of about 50 nm was deposited on Si substrate by sputtering, which was followed by the deposition of Sb3 Te or
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Y. Meng et al. / Applied Surface Science 355 (2015) 667–671
Fig. 1. (a) R–T curves of Wx Sb3 Te films at heating rate of 20 ◦ C/min; (b) the linear fitted curves based on logarithmic resistivity versus reciprocal temperature in amorphous state; (c) Kissinger’s plot of Ea evaluation; (d) plot of the failure time for 10 years data retention.
W0.31 Sb3 Te films of 400 nm. We marked those films as Sb3 Te/W and W0.31 Sb3 Te/W respectively. The background and sputtering pressures were 2.2 × 10−4 Pa and 0.2 Pa, respectively. The concentration of W was varying with different sputtering power applied to the targets. The sputtering power of Sb3 Te was fixed to 20 W while the radio frequency power of W target was rated at 0 W, 5 W, 8 W, 10 W, and 12 W, respectively. The compositions and thickness of the deposited films were determined using energy dispersive X-ray spectroscopy (EDS) and scanning electron microscope (SEM), respectively. The sheet resistance as a function of the temperature (R–T) was measured in situ in a vacuum chamber with a fixed heating/cooling rate of 20 ◦ C/min. Films annealed for 2 min at 140 ◦ C, 180 ◦ C, and 220 ◦ C respectively were employed to characterize the structure by XRD. To further research the mechanism action of W atoms, Raman scattering spectra were recorded in back scattering geometry at room temperature. The Ar ion laser was used for excitation at a wavelength of 785 nm. Films used in Raman scattering spectra were annealed at 180 ◦ C and 220 ◦ C for 10 min, respectively. To investigate the microstructures of Wx Sb3 Te films, the films of Sb3 Te, W0.31 Sb3 Te and W0.46 Sb3 Te annealed at 220 ◦ C for 3 min were used in TEM, high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). The adhesive strength between W and phase-change material was tested by Nano Indenter® . Ttype PCM devices were fabricated with electrode diameters of 190 nm. The resistance–voltage (R–V) test was carried out by Keithley 2400 m and Tektronix AWG5002B parameter analyzer.
3. Results and discussions Fig. 1(a) shows the sheet resistances of Wx Sb3 Te films as a function of annealing temperature at a fixed heating rate of 20 ◦ C/min.
The crystallization temperature (Tc ) is determined by derivative of sheet resistance with respect to temperature. As presented in Fig. 1(a), Tc increases from 152.7 ◦ C to 223.7 ◦ C (see Table 1), which indicates a more stable amorphous phase thus better data retention capacity with increased W content. The temperature dependence for the resistivity in a semiconductor can be expressed by = 0 exp(−E /kB T), where 0 is a pre-exponential factor and E is the activation energy for electrical conduction. As shown in Fig. 1(b), the as-deposited E are 0.29 eV, 0.22 eV, 0.15 eV, 0.12 eV and 0.10 eV for pure Sb3 Te, W0.17 Sb3 Te, W0.31 Sb3 Te, W0.38 Sb3 Te, and W0.46 Sb3 Te films, respectively. The decreasing of E corresponds to the change of resistance after W doping. Activation energy upon crystallization (Ea ) of the Wx Sb3 Te films was obtained from a Kissinger’s method: ln[(dT/dt)/T2 ] = C + Ea /(kb Tc ), where dT/dt and C are the heating rate and constant, respectively. In Fig. 1(c), Ea increases with W content adding, and then better thermal stability of the amorphous Wx Sb3 Te could be achieved for the PCM application. In isothermal process, data retention of the film is determined by the relationship of isothermal change and failure time in sheet resistance. It could be calculated from extrapolation of the isothermal Arrhenius plots: t = exp(Ea /kB T). The failure time is defined as Table 1 Composition of Wx Sb3 Te films and the corresponding Tc , Tr , Ea , and E . Material
Sb3 Te W0.17 Sb3 Te W0.31 Sb3 Te W0.38 Sb3 Te W0.46 Sb3 Te
Non-isothermal
Isothermal
Tc (◦ C)
Ea (eV)
E (eV)
Tr (◦ C)
Ea (eV)
152.7 154.8 185.8 206.8 223.7
3.04 3.19 3.52 3.83 4.23
0.29 0.22 0.15 0.12 0.10
78.7 81.3 114.6 134.2 153.9
3.14 3.30 3.73 4.04 4.42
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Fig. 2. (a) XRD curves of 140 ◦ C, 180 ◦ C, and 220 ◦ C-annealed W0.31 Sb3 Te films and 180 ◦ C-annealed Sb3 Te film on Si/SiO2 substrates, (b) 220 ◦ C-annealed Sb3 Te, W0.31 Sb3 Te and W0.46 Sb3 Te films.
the time during that the sheet resistance reaches the half of its initial magnitude. By extrapolation of the failure time to 10 years, the 10-year data retention temperature (Tr ) could be calculated. As presented in Fig. 1(d), the Tr of Wx Sb3 Te film are about 78.7 ◦ C, 81.3 ◦ C, 114.6 ◦ C, 134.2 ◦ C, and 153.9 ◦ C for Sb3 Te, W0.17 Sb3 Te, W0.31 Sb3 Te, W0.38 Sb3 Te, and W0.46 Sb3 Te films, respectively. Comparing with pure Sb3 Te, the thermal stability of W-doped Sb3 Te film is improved obviously. The W0.31 Sb3 Te has been considered satisfying the demand of automotive systems and portable electric products. The high Tc , Ea , and Tr have proved the high thermal stability of W-doped Sb3 Te film. Furthermore, it is noted that Ea as summarized in Table 1 is closed to that calculated by the Kissinger’s method, suggesting that the fitted failure curves of Tr are reliable. Composition of Wx Sb3 Te films and the corresponding Tc , Tr , Ea , and E were summarized in Table 1. XRD was used to investigate the crystal structure of W0.31 Sb3 Te films. With the temperature increases, the film crystallized into crystallites, as shown in Fig. 2(a). Fig. 2(b) shows, all diffraction peaks belong to Sb3 Te, while no other diffraction peak corresponding to W or Te is found. It indicates the amorphous nature of most W atoms in the crystalline, while keeping the crystal structure unchanged. In the figure, with the addition of W, intensities of (0 1 6), (1 1 0) and (1 1 4) peaks decrease sharply, even the (0 0 4) and (0 0 5) peaks disappeared. As evaluated from Scherrer equation, the grain size of pure Sb3 Te film, W0.31 Sb3 Te film and W0.46 Sb3 Te film are about 81 nm, 21 nm and 10 nm. The suppressed peaks and the smaller grain size could be indicated the incorporated W atom plays a role in suppressing crystallization of the Sb3 Te films, which correspond to the increased Tc and Ea of Wx Sb3 Te films. In practical applications, the smaller grain could reduce the power consumption of the cell device [17]. To investigate the effect of W doping on the structure of Sb3 Te, the films annealed at 180 ◦ C and 220 ◦ C were used in Raman scattering spectroscopy. As presented in Fig. 3, the 180 ◦ C annealed W0.31 Sb3 Te film shows a single broad peak located at around 140 cm−1 , indicating the amorphous state of the film. On the contrast, the 180 ◦ C annealed Sb3 Te film is in crystalline state with two Raman modes located at around 120.2 cm−1 and 149.2 cm−1 , which are corresponding to Eg modes and A1g modes, respectively, according to the references [18–20]. The Eg symmetry modes represent in-plane (shear) vibrations, while the Ag symmetry modes vibrate out-of-plane (breathing) along the c axis of the crystal. Different degree of crystallization of the two films at the same temperature shows crystallization process of Sb3 Te film was inhibited by W doping. When annealed at 220 ◦ C, the W0.31 Sb3 Te film is in crystalline state with two Raman peaks at 115.8 cm−1 and 146.7 cm−1 ,
as shown in Fig. 3. Due to the fact that the W atom is heavier than Sb or Te atom, the shifting of the two peaks to a relatively lower frequency with W doping could be caused by the substitution of Sb or Te atoms by W atoms in the structure units, inhibiting further crystallization [21]. The morphology of crystalline Sb3 Te and W doped Sb3 Te films were characterized by TEM, HRTEM, and SAED, as shown in Fig. 4. In Fig. 4(a), it can be seen crystalline Sb3 Te film consists of irregular shape grains. The grain size reaches several hundred nanometers. Compared to pure Sb3 Te, the grain size of Wx Sb3 Te in Fig. 4(d) and (g) is getting smaller and smaller with increasing of W content. The smaller grains will cause less stress, suppressing the cavities formation during phase change, thus it will help to reduce the power consumption and improve the reliability of the PCM. As presented in Fig. 4(b), (e) and (h), the spacing of lattice fringes are calculated ˚ 2.16 A˚ and 1.76 A, ˚ corresponding to (2 1¯ 1¯ 0), (1 1 0) and to be 2.13 A, (0 2 3) planes of HEX phase Sb3 Te, which indicates that the film is crystallized with homogeneous polygonal grains. The SAED pattern of pure Sb3 Te is single crystal like diffraction spots, as shown in Fig. 4(c). With W doped, the SAED pattern exhibits polycrystalline diffraction rings, as presented in Fig. 4(f) and (i). The polycrystalline rings of the SAED pattern suggest that the grain size is significantly refined by W doping. The adhesive strength between phase change film and electrodes is important for the application of PCM. Vestiges nick micrographs of the Sb3 Te/W and Wx Sb3 Te/W samples annealed
Fig. 3. Raman scattering spectra of 180 ◦ C and 220 ◦ C-annealed films.
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Fig. 4. TEM BF images of (a) Sb3 Te, (d) W0.31 Sb3 Te and (g) W0.46 Sb3 Te, HRTEM images of (b) Sb3 Te, (e) W0.31 Sb3 Te and (h) W0.46 Sb3 Te, SAED patterns of (c) Sb3 Te, (f) W0.31 Sb3 Te and (i) W0.46 Sb3 Te.
at 200 ◦ C are shown in Fig. 5. In these figures, square curve and black curve indicate the penetration depth and the vertical load, respectively, as a function of scratch distance from 100 to 700 m. In the Nano Indente® test equipment, the point where the film on the substrate begins to peel off and crack is defined as C point. The load corresponding to the C point is the critical load (Lc ). The Lc of Sb3 Te/W sample is 8.5 mN as shown in Fig. 5(a), while 11.6 mN is obtained for W0.31 Sb3 Te/W sample as presented in Fig. 5(b). It is obvious that, the adhesive strength between phase-change film and W film has been increased 36.5% after W doping, which indicates that the adhesive strength between Sb3 Te film and W electrodes
could be significantly improved by W doping. In applications of PCM, higher adhesive strength indicates higher device reliability. At the end, the electrical phase-change ability of W doped Sb3 Te material was evaluated. Fig. 6 shows the SET and RESET operations of the PCM cells based on W0.31 Sb3 Te materials. The operation is realized with a pulse width ranging from 300 to 10 ns. It can be observed that the cell resistance value in set and reset states are ∼5×103 and ∼3×105 , respectively. The voltage margin of SET and RESET operation is as large as 2.8 V and the resistance ratio is about 2 orders of magnitude, which offers adequately large sensing margin for PCM. Furthermore, the reversible phase change could
Fig. 5. Scratch test curves for (a) Sb3 Te/W sample; (b) W0.31 Sb3 Te/W sample.
Y. Meng et al. / Applied Surface Science 355 (2015) 667–671
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Fig. 6. (a) R–V characteristics of the device cells based on W0.31 Sb3 Te material; (b) endurance characteristic based on W0.31 Sb3 Te material.
be achieved by electrical pulse of 10 ns and a cycling ability up to 3.9 × 104 was demonstrated. 4. Conclusion The W-doped Sb3 Te phase-change material has been prepared and systematically investigated. The thermal stability of amorphous Sb3 Te material is significantly improved by W doping. XRD and TEM results prove that the incorporation of W plays a role in suppressing the crystallization of Sb3 Te films, causing smaller grain size, which is benefit for thermal stability and device performance. Furthermore, the adhesive strength between Wx Sb3 Te film and W electrodes can be significantly improved by W doping. The high SET/RESET operation speed of Wx Sb3 Te based PCM cells makes the W-doped Sb3 Te a potential material for high speed applications. Acknowledgements This work is supported by National Key Basic Research Program of China (2011CBA00607) and National Natural Science Foundation of China (11104160, 11104161). References [1] M. Wuttig, Phase-change materials – towards a universal memory, Nat. Mater. 4 (2005) 265. [2] J. Tomforde, et al., Thin films of Ge–Sb–Te-based phase change materials:microstructure and in situ transformation, Chem. Mater. 23 (2011) 3871. [3] E.R. Meinders, M.H.R. Lankhorst, Determination of the crystallization kinetics of fast-growth phase-change materials for mark-formation prediction, J. Appl. Phys. 42 (February (2B)) (2003) 809. [4] I. Friedrich, et al., Structural transformations of Ge(2)Sb(2)Te(5) films studied by electrical resistance measurements, J. Appl. Phys. 87 (2000) 4130. [5] L. Perniola, V. Sousa, A. Fantini, E. Arbaoui, A. Bastard, M. Armand, A. Fargeix, C. Jahan, J.F. Nodin, A. Persico, D. Blachier, A. Toffoli, S. Loubriat, E. Gourvest, G.B. Beneventi, H. Feldis, S. Maitrejean, S. Lhostis, A. Roule, O. Cueto, G. Reimbold, L. Poupinet, T. Billon, B. De Salvo, D. Bensahel, P. Mazoyer, R. Annunziata, P. Zuliani, F. Boulanger, IEEE Electron Device Lett. 31 (2010) 488. [6] D.M. Trichês, S.M. Souza, C.M. Poffo, J.C. de Lima, T.A. Grandi, R.S. de Biasi, Structural instability and photoacoustic study of AlSb prepared by mechanical alloying, J. Alloys Compd. 505 (2) (2010) 762.
[7] V. Weidenhof, I. Friedrich, S. Ziegler, M. Wuttig, Laser induced crystallization of amorphous Ge2 Sb2 Te5 films, J. Appl. Phys. 89 (2001) 3168. [8] K.B. Borisenko, Y.X. Chen, S.A. Song, D.J.H. Cockayne, Atomistic origin of the enhanced crystallization speed and n-type conductivity in bi-doped Ge–Sb–Te phase-change materials, Chem. Mater. 21 (2009) 5244. [9] E. Cho, S. Han, D. Kim, H. Horii, H.S. Nam, Ab initio study on influence of dopants on crystalline and amorphous Ge2 Sb2 Te5 , J. Appl. Phys. 109 (2011) 043705. [10] M.H.R. Lankhorst, B.W.S.M.M. Ketelaars, R.A.M. Wolters, Low-cost and nanoscale non-volatile memory concept for future silicon chips, Nat. Mater. 4 (April (4)) (2005) 347. [11] N. Oomachi, S. Ashida, N. Nakamura, K. Yusu, K. Ichihara, Recording characteristics of Ge doped eutectic SbTe phase change discs with various compositions and its potential for high density recording, Jpn. J. Appl. Phys. 41 (March (3B)) (2002) 1695–1697. [12] M. Zhu, L. Wu, F. Rao, Z. Song, X. Ji, D. Yao, Y. Cheng, S. Lv, S. Song, B. Liu, L. Xu, The effect of tiantium doping on the structure and phase change characteristics of Sb4 Te, J. Appl. Phys. 114 (2013) 124302. [13] S. Raoux, M. Salinga, J.L.J. Sweet, A. Kellock, Effect of Al and Cu doping on the crystallization properties of the phase change materials SbTe and GeSb, J. Appl. Phys. 101 (2007) 044909. [14] Y. Yin, H. Sone, S. Hosaka, Characterization of nitrogen-doped Sb2 Te 3 films and their application to phase change memory, J. Appl. Phys. 102 (2007) 064503. [15] C. Peng, L. Wu, F. Rao, Z. Song, P. Yang, H. Song, K. Ren, X. Zhou, M. Zhu, B. Liu, J. Chu, W–Sb–Te phase-change material: a candidate for the trade-off between programming speed and data retention, Appl. Phys. Lett. 101 (2012) 122108. [16] K. Ren, F. Rao, Z. Song, S. Lu, C. Peng, M. Zhu, L. Wu, B. Liu, S. Feng, Phase change material W0.04 (Sb4 Te)0.96 for application in high-speed phase change memory, J. Alloys Compd. 594 (2014) 82. [17] W.J. Wang, D. Loke, L.T. Law, L.P. Shi, R. Zhao, M.H. Li, L.L. Chen, H.X. Yang, Y.C. Yeo, A.O. Adeyeye, T.C. Chong, A.L. Lacaita, Engineering grains of Ge2 Sb2 Te5 for realizing fast-speed, low-power, and low-drift phase-change memories with further multilevel capabilities, in: 2012 IEEE International Electron Devices Meeting (IEDM), 2012. [18] U. Rossow, U. Frotscher, N. Esser, U. Resch, T. Muller, W. Richter, D.A. Woolf, R.H. Williams, Growth mode of ultrathin Sb layers on Si studied by spectroscopic ellipsometry and Raman-scattering, Appl. Surf. Sci. 63 (January) (1993) 35–39. [19] G.X. Wang, X. Shen, Q.H. Nie, T.F. Xu, S.X. Dai, Y.G. Lu, Y.M. Chen, J.J. Li, Characterization of physical properties for Zn-doped Sb3 Te films, Appl. Phys. Express 6 (2013) 095801. [20] X. Zhou, L. Wu, Z. Song, F. Rao, K. Ren, C. Peng, S. Song, B. Liu, L. Xu, S. Feng, Phase transition characteristics of Al–Sb phase change materials for the phase change memory application, Appl. Phys. Lett. 103 (2013) 072114. [21] S. Guo, Z. Hu, X. Ji, T. Huang, X. Zhang, L. Wu, Z. Song, J. Chu, Temperature and concentration dependent crystallization behavior of Ge2 Sb2 Te5 phase change films: tungsten doping effects, RSC Adv. 4 (2014) 57218.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Study on WSb3 Te material for phase-change memory applications Yun Meng a,b , Xilin Zhou b , Peigao Han a,∗ , Zhitang Song b , Liangcai Wu b,∗ , Chengqiu Zhu b , Wenjing Guo a , Ling Xu c , Zhongyuan Ma c , Lianke Song a a Shandong Province Key Laboratory of Laser Polarization and Information Technology, School of Physics and Engineering, Qufu Normal University, 273165 Qufu, People’s Republic of China b State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, 200050 Shanghai, People’s Republic of China c National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 24 January 2015 Received in revised form 14 June 2015 Accepted 11 July 2015 Available online 15 July 2015 Keywords: W doping Refined grain size High adhesive strength High SET/RESET speed
a b s t r a c t The phase-change performance of Wx Sb3 Te material were systemically investigated by in situ resistancetemperature measurement, X-ray diffraction (XRD), Raman scattering, adhesive strength test and transmission electron microscope (TEM) in this paper. Experimental results show that the thermal stability of Sb3 Te was increased significantly with W doping. XRD and TEM results prove that the incorporation of W plays a role in suppressing the crystallization of Sb3 Te films, causing smaller grain size. Furthermore, the adhesive strength between W electrode and phase-change material was increased obviously by W addition and a relatively rapid SET/RESET operation of 10 ns is realized with large sensing margin. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Phase-change random access memory (PCM) has attracted much attention for its merits of high switching speed, high scalability and low power consumption, becoming the most feasible candidates for the next generation universal memory [1–4]. Data storage in PCM is achieved by utilize the resistivity contrast between high resistance RESET state (amorphous) and low resistance SET state (crystalline) phase of the chalcogenide material. Ternary alloy Ge2 Sb2 Te5 (GST) is the most popular compound for PCM applications, but there are some issues limiting its development. For example, its 10-year data retention temperature (∼89 ◦ C) cannot quite meet the demand for the automobile electronics [5,6]. The crystallization speed of GST (50 ns) is not enough for the high speed PCM application [7–9]. In order to overcome those obstacles, screening a proper new phase-change material is an effective way for current PCM technology [10]. The Sb–Te binary alloy has shown relatively fast phasechange ability with growth-dominated crystallization mechanism. However, its thermal stability is poor due to the low crystallization temperature (<100 ◦ C) [11]. It was reported that the
∗ Corresponding authors. E-mail addresses: [email protected] (P. Han), [email protected] (L. Wu). http://dx.doi.org/10.1016/j.apsusc.2015.07.069 0169-4332/© 2015 Elsevier B.V. All rights reserved.
thermal stability of Sb–Te materials can be improved obviously by doping, such as Ti [12], Cu [13], N [14] and W [15,16]. Among them, W doped Sb–Te material has a better thermal stability with good phase-change performance. In this paper, the phase-change performances of Wx Sb3 Te material were systemically investigated by in situ resistance-temperature measurement, XRD, Raman scattering, adhesive strength test and TEM. XRD and TEM results prove that the incorporation of W plays a role in suppressing the crystallization of Sb3 Te films, causing smaller grain size. The smaller grain size is benefit to thermal stability and device performance. The Raman peaks shifting show the substitution of Sb or Te atoms by W atoms in crystal lattice, which inhibits further crystallization. Furthermore, the adhesive strength between Sb3 Te film and W electrodes could be significantly improved by W doping. The higher adhesive strength indicates higher device reliability in PCM application. It is worth to point out that, the reversible phase change can be trigged by 10 ns electrical pulse, which is faster than that of GST. 2. Experimental The Wx Sb3 Te (x = 0, 0.17, 0.31, 0.38, 0.46 at.%) films were deposited on Si/SiO2 by radio frequency co-sputtering of W and Sb3 Te targets at room temperature. For adhesive strength test, W film with a thickness of about 50 nm was deposited on Si substrate by sputtering, which was followed by the deposition of Sb3 Te or
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Y. Meng et al. / Applied Surface Science 355 (2015) 667–671
Fig. 1. (a) R–T curves of Wx Sb3 Te films at heating rate of 20 ◦ C/min; (b) the linear fitted curves based on logarithmic resistivity versus reciprocal temperature in amorphous state; (c) Kissinger’s plot of Ea evaluation; (d) plot of the failure time for 10 years data retention.
W0.31 Sb3 Te films of 400 nm. We marked those films as Sb3 Te/W and W0.31 Sb3 Te/W respectively. The background and sputtering pressures were 2.2 × 10−4 Pa and 0.2 Pa, respectively. The concentration of W was varying with different sputtering power applied to the targets. The sputtering power of Sb3 Te was fixed to 20 W while the radio frequency power of W target was rated at 0 W, 5 W, 8 W, 10 W, and 12 W, respectively. The compositions and thickness of the deposited films were determined using energy dispersive X-ray spectroscopy (EDS) and scanning electron microscope (SEM), respectively. The sheet resistance as a function of the temperature (R–T) was measured in situ in a vacuum chamber with a fixed heating/cooling rate of 20 ◦ C/min. Films annealed for 2 min at 140 ◦ C, 180 ◦ C, and 220 ◦ C respectively were employed to characterize the structure by XRD. To further research the mechanism action of W atoms, Raman scattering spectra were recorded in back scattering geometry at room temperature. The Ar ion laser was used for excitation at a wavelength of 785 nm. Films used in Raman scattering spectra were annealed at 180 ◦ C and 220 ◦ C for 10 min, respectively. To investigate the microstructures of Wx Sb3 Te films, the films of Sb3 Te, W0.31 Sb3 Te and W0.46 Sb3 Te annealed at 220 ◦ C for 3 min were used in TEM, high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). The adhesive strength between W and phase-change material was tested by Nano Indenter® . Ttype PCM devices were fabricated with electrode diameters of 190 nm. The resistance–voltage (R–V) test was carried out by Keithley 2400 m and Tektronix AWG5002B parameter analyzer.
3. Results and discussions Fig. 1(a) shows the sheet resistances of Wx Sb3 Te films as a function of annealing temperature at a fixed heating rate of 20 ◦ C/min.
The crystallization temperature (Tc ) is determined by derivative of sheet resistance with respect to temperature. As presented in Fig. 1(a), Tc increases from 152.7 ◦ C to 223.7 ◦ C (see Table 1), which indicates a more stable amorphous phase thus better data retention capacity with increased W content. The temperature dependence for the resistivity in a semiconductor can be expressed by = 0 exp(−E /kB T), where 0 is a pre-exponential factor and E is the activation energy for electrical conduction. As shown in Fig. 1(b), the as-deposited E are 0.29 eV, 0.22 eV, 0.15 eV, 0.12 eV and 0.10 eV for pure Sb3 Te, W0.17 Sb3 Te, W0.31 Sb3 Te, W0.38 Sb3 Te, and W0.46 Sb3 Te films, respectively. The decreasing of E corresponds to the change of resistance after W doping. Activation energy upon crystallization (Ea ) of the Wx Sb3 Te films was obtained from a Kissinger’s method: ln[(dT/dt)/T2 ] = C + Ea /(kb Tc ), where dT/dt and C are the heating rate and constant, respectively. In Fig. 1(c), Ea increases with W content adding, and then better thermal stability of the amorphous Wx Sb3 Te could be achieved for the PCM application. In isothermal process, data retention of the film is determined by the relationship of isothermal change and failure time in sheet resistance. It could be calculated from extrapolation of the isothermal Arrhenius plots: t = exp(Ea /kB T). The failure time is defined as Table 1 Composition of Wx Sb3 Te films and the corresponding Tc , Tr , Ea , and E . Material
Sb3 Te W0.17 Sb3 Te W0.31 Sb3 Te W0.38 Sb3 Te W0.46 Sb3 Te
Non-isothermal
Isothermal
Tc (◦ C)
Ea (eV)
E (eV)
Tr (◦ C)
Ea (eV)
152.7 154.8 185.8 206.8 223.7
3.04 3.19 3.52 3.83 4.23
0.29 0.22 0.15 0.12 0.10
78.7 81.3 114.6 134.2 153.9
3.14 3.30 3.73 4.04 4.42
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Fig. 2. (a) XRD curves of 140 ◦ C, 180 ◦ C, and 220 ◦ C-annealed W0.31 Sb3 Te films and 180 ◦ C-annealed Sb3 Te film on Si/SiO2 substrates, (b) 220 ◦ C-annealed Sb3 Te, W0.31 Sb3 Te and W0.46 Sb3 Te films.
the time during that the sheet resistance reaches the half of its initial magnitude. By extrapolation of the failure time to 10 years, the 10-year data retention temperature (Tr ) could be calculated. As presented in Fig. 1(d), the Tr of Wx Sb3 Te film are about 78.7 ◦ C, 81.3 ◦ C, 114.6 ◦ C, 134.2 ◦ C, and 153.9 ◦ C for Sb3 Te, W0.17 Sb3 Te, W0.31 Sb3 Te, W0.38 Sb3 Te, and W0.46 Sb3 Te films, respectively. Comparing with pure Sb3 Te, the thermal stability of W-doped Sb3 Te film is improved obviously. The W0.31 Sb3 Te has been considered satisfying the demand of automotive systems and portable electric products. The high Tc , Ea , and Tr have proved the high thermal stability of W-doped Sb3 Te film. Furthermore, it is noted that Ea as summarized in Table 1 is closed to that calculated by the Kissinger’s method, suggesting that the fitted failure curves of Tr are reliable. Composition of Wx Sb3 Te films and the corresponding Tc , Tr , Ea , and E were summarized in Table 1. XRD was used to investigate the crystal structure of W0.31 Sb3 Te films. With the temperature increases, the film crystallized into crystallites, as shown in Fig. 2(a). Fig. 2(b) shows, all diffraction peaks belong to Sb3 Te, while no other diffraction peak corresponding to W or Te is found. It indicates the amorphous nature of most W atoms in the crystalline, while keeping the crystal structure unchanged. In the figure, with the addition of W, intensities of (0 1 6), (1 1 0) and (1 1 4) peaks decrease sharply, even the (0 0 4) and (0 0 5) peaks disappeared. As evaluated from Scherrer equation, the grain size of pure Sb3 Te film, W0.31 Sb3 Te film and W0.46 Sb3 Te film are about 81 nm, 21 nm and 10 nm. The suppressed peaks and the smaller grain size could be indicated the incorporated W atom plays a role in suppressing crystallization of the Sb3 Te films, which correspond to the increased Tc and Ea of Wx Sb3 Te films. In practical applications, the smaller grain could reduce the power consumption of the cell device [17]. To investigate the effect of W doping on the structure of Sb3 Te, the films annealed at 180 ◦ C and 220 ◦ C were used in Raman scattering spectroscopy. As presented in Fig. 3, the 180 ◦ C annealed W0.31 Sb3 Te film shows a single broad peak located at around 140 cm−1 , indicating the amorphous state of the film. On the contrast, the 180 ◦ C annealed Sb3 Te film is in crystalline state with two Raman modes located at around 120.2 cm−1 and 149.2 cm−1 , which are corresponding to Eg modes and A1g modes, respectively, according to the references [18–20]. The Eg symmetry modes represent in-plane (shear) vibrations, while the Ag symmetry modes vibrate out-of-plane (breathing) along the c axis of the crystal. Different degree of crystallization of the two films at the same temperature shows crystallization process of Sb3 Te film was inhibited by W doping. When annealed at 220 ◦ C, the W0.31 Sb3 Te film is in crystalline state with two Raman peaks at 115.8 cm−1 and 146.7 cm−1 ,
as shown in Fig. 3. Due to the fact that the W atom is heavier than Sb or Te atom, the shifting of the two peaks to a relatively lower frequency with W doping could be caused by the substitution of Sb or Te atoms by W atoms in the structure units, inhibiting further crystallization [21]. The morphology of crystalline Sb3 Te and W doped Sb3 Te films were characterized by TEM, HRTEM, and SAED, as shown in Fig. 4. In Fig. 4(a), it can be seen crystalline Sb3 Te film consists of irregular shape grains. The grain size reaches several hundred nanometers. Compared to pure Sb3 Te, the grain size of Wx Sb3 Te in Fig. 4(d) and (g) is getting smaller and smaller with increasing of W content. The smaller grains will cause less stress, suppressing the cavities formation during phase change, thus it will help to reduce the power consumption and improve the reliability of the PCM. As presented in Fig. 4(b), (e) and (h), the spacing of lattice fringes are calculated ˚ 2.16 A˚ and 1.76 A, ˚ corresponding to (2 1¯ 1¯ 0), (1 1 0) and to be 2.13 A, (0 2 3) planes of HEX phase Sb3 Te, which indicates that the film is crystallized with homogeneous polygonal grains. The SAED pattern of pure Sb3 Te is single crystal like diffraction spots, as shown in Fig. 4(c). With W doped, the SAED pattern exhibits polycrystalline diffraction rings, as presented in Fig. 4(f) and (i). The polycrystalline rings of the SAED pattern suggest that the grain size is significantly refined by W doping. The adhesive strength between phase change film and electrodes is important for the application of PCM. Vestiges nick micrographs of the Sb3 Te/W and Wx Sb3 Te/W samples annealed
Fig. 3. Raman scattering spectra of 180 ◦ C and 220 ◦ C-annealed films.
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Fig. 4. TEM BF images of (a) Sb3 Te, (d) W0.31 Sb3 Te and (g) W0.46 Sb3 Te, HRTEM images of (b) Sb3 Te, (e) W0.31 Sb3 Te and (h) W0.46 Sb3 Te, SAED patterns of (c) Sb3 Te, (f) W0.31 Sb3 Te and (i) W0.46 Sb3 Te.
at 200 ◦ C are shown in Fig. 5. In these figures, square curve and black curve indicate the penetration depth and the vertical load, respectively, as a function of scratch distance from 100 to 700 m. In the Nano Indente® test equipment, the point where the film on the substrate begins to peel off and crack is defined as C point. The load corresponding to the C point is the critical load (Lc ). The Lc of Sb3 Te/W sample is 8.5 mN as shown in Fig. 5(a), while 11.6 mN is obtained for W0.31 Sb3 Te/W sample as presented in Fig. 5(b). It is obvious that, the adhesive strength between phase-change film and W film has been increased 36.5% after W doping, which indicates that the adhesive strength between Sb3 Te film and W electrodes
could be significantly improved by W doping. In applications of PCM, higher adhesive strength indicates higher device reliability. At the end, the electrical phase-change ability of W doped Sb3 Te material was evaluated. Fig. 6 shows the SET and RESET operations of the PCM cells based on W0.31 Sb3 Te materials. The operation is realized with a pulse width ranging from 300 to 10 ns. It can be observed that the cell resistance value in set and reset states are ∼5×103 and ∼3×105 , respectively. The voltage margin of SET and RESET operation is as large as 2.8 V and the resistance ratio is about 2 orders of magnitude, which offers adequately large sensing margin for PCM. Furthermore, the reversible phase change could
Fig. 5. Scratch test curves for (a) Sb3 Te/W sample; (b) W0.31 Sb3 Te/W sample.
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Fig. 6. (a) R–V characteristics of the device cells based on W0.31 Sb3 Te material; (b) endurance characteristic based on W0.31 Sb3 Te material.
be achieved by electrical pulse of 10 ns and a cycling ability up to 3.9 × 104 was demonstrated. 4. Conclusion The W-doped Sb3 Te phase-change material has been prepared and systematically investigated. The thermal stability of amorphous Sb3 Te material is significantly improved by W doping. XRD and TEM results prove that the incorporation of W plays a role in suppressing the crystallization of Sb3 Te films, causing smaller grain size, which is benefit for thermal stability and device performance. Furthermore, the adhesive strength between Wx Sb3 Te film and W electrodes can be significantly improved by W doping. The high SET/RESET operation speed of Wx Sb3 Te based PCM cells makes the W-doped Sb3 Te a potential material for high speed applications. Acknowledgements This work is supported by National Key Basic Research Program of China (2011CBA00607) and National Natural Science Foundation of China (11104160, 11104161). References [1] M. Wuttig, Phase-change materials – towards a universal memory, Nat. Mater. 4 (2005) 265. [2] J. Tomforde, et al., Thin films of Ge–Sb–Te-based phase change materials:microstructure and in situ transformation, Chem. Mater. 23 (2011) 3871. [3] E.R. Meinders, M.H.R. Lankhorst, Determination of the crystallization kinetics of fast-growth phase-change materials for mark-formation prediction, J. Appl. Phys. 42 (February (2B)) (2003) 809. [4] I. Friedrich, et al., Structural transformations of Ge(2)Sb(2)Te(5) films studied by electrical resistance measurements, J. Appl. Phys. 87 (2000) 4130. [5] L. Perniola, V. Sousa, A. Fantini, E. Arbaoui, A. Bastard, M. Armand, A. Fargeix, C. Jahan, J.F. Nodin, A. Persico, D. Blachier, A. Toffoli, S. Loubriat, E. Gourvest, G.B. Beneventi, H. Feldis, S. Maitrejean, S. Lhostis, A. Roule, O. Cueto, G. Reimbold, L. Poupinet, T. Billon, B. De Salvo, D. Bensahel, P. Mazoyer, R. Annunziata, P. Zuliani, F. Boulanger, IEEE Electron Device Lett. 31 (2010) 488. [6] D.M. Trichês, S.M. Souza, C.M. Poffo, J.C. de Lima, T.A. Grandi, R.S. de Biasi, Structural instability and photoacoustic study of AlSb prepared by mechanical alloying, J. Alloys Compd. 505 (2) (2010) 762.
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