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Phase change material W0.04(Sb4Te)0.96 for application in high-speed phase change memory
Journal of Alloys and Compounds 594 (2014) 82–86
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Phase change material W0.04(Sb4Te)0.96 for application in high-speed phase change memory Kun Ren a,b,⇑, Feng Rao a,⇑, Zhitang Song a, Shilong Lu a, Cheng Peng a,b, Min Zhu a,b, Liangcai Wu a, Bo Liu a, Songlin Feng a a State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Micro-System and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China b Graduate University of the Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 12 November 2013 Received in revised form 6 January 2014 Accepted 9 January 2014 Available online 21 January 2014 Keywords: High speed phase change memory W doped Sb4Te
a b s t r a c t W-doped Sb4Te is proposed for high-speed phase change memory (PCM). The crystallization speed of W0.04(Sb4Te)0.96 is characterized in both the film and device, confirmed to be 30 ns and 6 ns. The crystallization temperature and data retention have been increased to 192 and 112 °C. The melting point is 550 °C, 75 °C lower than that of GST. The grain size is controlled to be 30–50 nm, lowering the stress and improving the cycling ability. The small grains, good thermal stability, high crystallization speed and low melting temperature have made the W0.04(Sb4Te)0.96 a potential candidate for high-speed PCM application. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In the past decades, phase change materials have been extensively studied for application of nonvolatile memories for its outstanding optical and electrical application [1,2]. It is utilized extensively as rewritable optical storage media because of the remarkable distinction of optical characteristics between amorphous and crystalline phases and the capacity of reversible switching rapidly between the two phases. Meanwhile, PCM is considered as one of the most promising candidates for the next generation nonvolatile solid-state memory. The key feature of PCM is the reversible phase transition, induced by an electric pulse, between the amorphous (high resistivity, reset) and the crystalline (low resistivity, set) states. A large resistance ratio can be utilized to identify the stored data ‘‘1’’ and ‘‘0’’. Although PCM will be possibly commercialized in the near future, there are still some problems with regard to phase change materials that need to be solved. On one hand, attractive market opportunities would arise, however, if nonvolatile memory can be developed that reach dynamic random access memory ⇑ Corresponding authors. Address: State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Micro-System and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China (K. Ren). Tel.: +86 21 62511070x8406; fax: +86 21 62134404. E-mail addresses: [email protected] (K. Ren), [email protected] (F. Rao). http://dx.doi.org/10.1016/j.jallcom.2014.01.044 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
(DRAM)-like switching speed of around 10 ns [3]. On the other hand, to compete with NOR-flash memory, the data retention at 10 year should be higher, for example, around 80 °C for embedded storage application for computer or even higher (120 °C) for some potential application in automobile electronics [4]. Ge2Sb2Te5 is the most widely studied and used phase change material due to its overall qualified performance. However, the phase change speed of around 50 ns [5] and the data retention of around 85 °C [6] make it difficult for its application in high speed PCM and high thermal stable PCM. Most of the Sb-rich phase change materials have a growth-dominant crystallization behavior, whose crystallization is consisted of a long nucleus incubation duration and the following fast grain growth process [7,8]. However, in PCM, the residual nucleus and the electrode interfaces will lower the activation energy for nucleus formation, reducing the incubation time, leading to a fast crystallization speed [9]. Therefore, the Sb-rich phase change material is a good candidate for the high speed PCM application [10,11]. Although the operation speed of PCM is improved by its crystallization behavior, the great grain size will induce large stress in the material and the interfaces, leading to the cavities formation and the poor interface adhesion. These two problems are the main factors that responsible for the failure of PCM cycling [12]. And there are several works of improving the thermal stability and material uniformity of Sb4Te by doping, such as Ti and O doping, with the crystallization speed remaining high [10,11]. In this work, W is doped into the Sb4Te phase change material, expecting to get a better thermal stability
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and a smaller grain size, without sacrificing its high phase change speed. 2. Experiments The Sb4Te and W-Sb4Te films researched in this work were prepared by sputtering method at room temperature. The films for X-ray diffraction (XRD), real-time reflectivity measurement and resistance–temperature (R–T) tests are about 300 nm thick, deposited on the Si/SiO2 (1 1 0) substrates. The crystallization temperature (Tc) and melting temperature (Tm) of amorphous W0.04(Sb4Te)0.96 is studied by differential scanning calorimetry (DSC) with a heating rate of 5 °C/min. The light source used for irradiating the sample is Nd:YAG laser with a duration of 8 ns and a wave length of 532 nm. The probing laser is a continuous He–Ne laser beam with a wavelength of 632.8 nm. The transmission electron microscopy (TEM) samples of Sb4Te and W-Sb4Te films are fabricated by depositing 30 nm-thick films on carbon supporting membranes. T-shaped PCM cell fabricated by 0.13 lm CMOS technology was utilized to verify the electrically induced phase change ability of the materials, as shown in inset of Fig. 2b. The resistance–voltage (R–V) of the PCM cell was monitored with Keithley 2400m and Tektronix AWG 5002B.
3. Results and discussion The R–T tests are carried out to characterize the sheet resistance change during heating and cooling processes, as shown in Fig. 1a. The Tc of Sb4Te is 151 °C, which rises with the increase of the doped W (192 °C for W0.04(Sb4Te)0.96 and 228 °C for W0.09(Sb4Te)0.91). The sheet resistance of amorphous Sb4Te film is 4 105 X/h, about 3 magnitudes higher than its crystalline sheet resistance (2 102 X/h). The W doping will cause the decrease of the sheet resistance of the amorphous film and the increase of the sheet resistance of the crystalline film, leading to a smaller resistance margin. As the doped W reaches 14 at%, the resistances of the amorphous and crystalline film almost stay at the same
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level, showing an upper limit of 14 at% W in the W-Sb4Te phase change materials. Data retention, another important parameter for PCM application, is characterized by extrapolating the isothermal Arrhenius plots, which are obtained by the logarithm of failure time vs. the reciprocal of isothermal temperature. In the isothermal process, the failure time are defined as the duration that the sheet resistance of the film reaches half of its initial value. By extrapolation of the failure time to 10 years, 10 years lifetime temperatures are calculated to be 41, 112 and 180 °C for Sb4Te, W0.04(Sb4Te)0.96 and W0.09(Sb4Te)0.91 films, respectively. The W0.04(Sb4Te)0.96 has a better data retention than that of Ge2Sb2Te5, indicating that it is thermally stable enough for embedded storage application. And its resistance margin of two magnitude is large enough for distinguishing high and low resistance. Although W0.09(Sb4Te)0.91 film has a good data retention that even qualified the application in automobile electronics, the one magnitude resistance margin seems to be too small for PCM application. Thus, more detailed characterizations should be carried out on the W0.04(Sb4Te)0.96 material. The Tc and Tm of amorphous W0.04(Sb4Te)0.96 is studied by DSC with a heating rate of 5 °C/min. The Tc and Tm of W0.04 (Sb4Te)0.96 are determined to be 185 °C and 550 °C, respectively. The Tm of W0.04(Sb4Te)0.96 is 75 °C lower than that of GST (625 °C), which will require a lower power consumption for the W0.04(Sb4Te)0.96 based PCM. The dynamic crystallization behavior of W0.04(Sb4Te)0.96 film is characterized by the real-time reflectivity measurement, as shown in Fig. 2a. The as-deposit amorphous film is in a low reflective state. After the laser pulse irradiated the film, the heat generated by the pulse triggered the crystallization of the film, making the film more reflective. Between the two stable states, there is the crystallization process, which is consisted of the nucleus
Fig. 1. (a) Sheet resistance of Sb4Te, W0.04(Sb4Te)0.96 and W0.09(Sb4Te)0.91 films as functions of temperature, with a heating rate of 10 °C/min. (b) The Arrhenius extrapolation at 10 years of data retention for Sb4Te, W0.04(Sb4Te)0.96 and W0.09(Sb4Te)0.91 films. (c) The DSC results of the W0.04(Sb4Te)0.96 material, with the heating rate of 5 °C/min.
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Fig. 2. (a) The transient reflectivity during crystallization of W0.04(Sb4Te)0.96 film induced by a nanosecond laser pulse with a width of 8 ns. The inset shows the transient part of the reflectivity curve. (b) R–V performance of the PCM cell based on W0.04(Sb4Te)0.96 materials. The inset shows the cross section of a PCM cell, with the scale bar presenting 200 nm. (c) Resistance–pulse width performance of the PCM cell based on W0.04(Sb4Te)0.96 material with different pulse height.
incubation and the grain growth. As shown in inset of Fig. 2a, the duration of the transition is 30 ns, which is shorter than the time for Ge2Sb2Te5 crystallization (50 ns) [5]. The nucleus incubation takes a large proportion of the entire crystallization time for phase change materials with a growth-dominated crystallization behavior [7]. When it is in the PCM cell, the residual nucleus and the interface induced easy crystallization will shorten the incubation time, making the crystallization much faster. T-shape PCM cell based on W0.04(Sb4Te)0.96 is fabricated to study the electrical triggered phase change ability of the material, as shown in inset of Fig. 2b and c. The R–V tests, as shown in Fig. 2b, are carried out to characterize the electric pulses triggered set and reset operations. When the pulse duration is 100 and 200 ns, the voltages for set and reset operations are 1.0–1.2 V and 2.7 V, respectively. The two magnitude of resistance margin is agree with the R–T results. The set state resistance of the W0.04(Sb4Te)0.96 based cell is higher than that of Ge2Sb2Te5 based cell using the same fabrication technology [13], which will benefit to lower the operation current, leading to a lower power consumption. Due to the lower crystallization speed of the phase change material than the melting speed, the operation speed of the PCM is usually determined by the speed of set operation. Thus, the set speed of W0.04(Sb4Te)0.96 based cell is characterized to define the operation speed, as shown in Fig. 2c. When the voltage is fixed in 1.0 V, the set operation can be completed by a 100 ns pulse. As the voltage increases from 1.0 V to 1.7 V, the pulse width required to complete the set operation decreases from 100 ns to 6 ns. Further increase the pulse height will possibly shorten the set duration, which cannot be detected due to the limit of the test equipments. The set operation can be finished in 6 ns when P1.7 V pulse is applied, indicating the fast speed of the W0.04(Sb4Te)0.96 based PCM cell. The 6-ns speed has satisfied the 10-ns speed requirement for
DRAM, making the W0.04(Sb4Te)0.96 a potential candidate for high speed PCM application. XRD method is carried out to characterize the lattice information of annealed Sb4Te and W0.04(Sb4Te)0.96 films with a thickness of 300 nm, as shown in Fig. 3. The Si diffraction peak in the curve is caused by the SiO2/Si substrate. In the curves corresponding to the 200 °C and 350 °C annealed Sb4Te and W0.04(Sb4Te)0.96 films, the diffraction peaks are indexed to be rhombohedral Sb, with no peak corresponding to W metal or W-containing compound being observed. The grain sizes in the two samples are estimated by analyzing the full width at half maximum (FWHM) of the diffraction peaks based on Scherrer’s formula [12]. The average sizes of grains in Sb4Te and W0.04(Sb4Te)0.96 films are calculated to be 59 and
Fig. 3. (a) XRD curves of 200 and 350 °C annealed Sb4Te and W0.04(Sb4Te)0.96 films on Si/SiO2 (1 0 0) substrates. The inset shows the calculated grain size.
K. Ren et al. / Journal of Alloys and Compounds 594 (2014) 82–86
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Fig. 4. (a) TEM picture of 250 °C annealed Sb4Te sample. The inset shows the SAED result of a selected range about 200 nm. (b) HRTEM picture of 250 °C annealed Sb4Te sample. (c) TEM picture of 350 °C annealed W0.04(Sb4Te)0.96 sample. The inset shows the SAED result of a selected range about 200 nm. (d) HRTEM picture of 350 °C annealed W0.04(Sb4Te)0.96 sample, with the amorphous regions enclosed by the dash lines.
34 nm, respectively. There will be more grain boundary when the grain size decreases, which helps to enhance the carriers scattering, thus leading to a higher resistance. Therefore, the decrease of the grain size is one of the main reasons for the higher cell resistance in set state, helping to decrease the power consumption of the reset operation. The smaller grains will cause less stress, suppressing the cavities formation during phase change, providing more adhesive interfaces. Thus, the smaller grains will help to lower the power consumption and improve the reliability of the PCM. TEM method is applied to have a direct observation of the distribution and size of the grains in the 250 °C annealed Sb4Te sample and 350 °C annealed W0.04(Sb4Te)0.96 sample, as shown in Fig. 4. By indexing the selected area electron diffraction (SAED) patterns in inset of Fig. 4a, the crystals can be assigned to hexagonal Sb4Te. The SAED pattern of Sb4Te sample with a selected range of about 200 nm suggests that the grain size is at least larger than 200 nm. The dark stripes in Fig. 4a have been reported in previous works, whose appearance usually accompanies the formation of large grains [7]. The high resolution TEM (HRTEM) result of the Sb4Te sample shows a long-range ordered atomic arrangement, with very few defects being observed, showing a large continuous Sb4Te grain, as shown in Fig. 4b. By W doping, the grain size is greatly decreased, as confirmed by the diffraction rings in inset of Fig. 4c. Meanwhile, the dark stripes cannot be observed in Fig. 4c. Although the grains are still assigned to rhombohedral Sb, there are a lot of amorphous regions embedded in the grains, as shown in Fig. 4d, ruining the integrity of the grain, thus lowering the grain size. The smaller grains provide more interfaces that can scatter the carriers when current flows, increasing the resistivity of the material and enhancing the heat efficiency. Higher heat efficiency will lower the current required for reset operation, resulting in a
low power consumption. According to the existence of amorphous regions, it is possible that W remain amorphous after the crystallization of the material. However, the atom radius of W (139 pm) is similar to Sb (145 pm) and Te (140 pm), it is also possible that W atoms are in the crystal as substitutional impurities. The accurate location of W atoms in the material still need further study. 4. Conclusions In conclusion, W doped Sb4Te materials are studied for phase change memory. By doping, the thermal stability of W0.04 (Sb4Te)0.96 has been improved, showing a Tc of 192 °C and a data retention of 112 °C. The crystallization speed is 30 ns by laser irradiation, which can be less than 6 ns in the PCM cell. The grains in the crystalline W0.04(Sb4Te)0.96 film are controlled to be 30–50 nm, with some amorphous region embedded in. The small grains will reduce the stress in the material and increase the crystalline resistivity, benefitting to a better cycling ability and a lower operation current. The low Tm will benefit to lower the power consumption of W0.04(Sb4Te)0.96 base PCM. Therefore, W0.04(Sb4Te)0.96 is considered to be a potential candidate for high speed PCM application. Acknowledgements Supported by National Key Basic Research Program of China(2013CBA01902, 2010CB934300, 2011CBA00602, 2011CB 932800), National Integrate Circuit Research Program of China (2009ZX02023-003), National Natural Science Foundation of China (60906004, 60906003, 61006087, 61076121), Science and Technology Council of Shanghai (1052nm07000, 12QA1403900).
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References [1] N. Yamada, E. Ohno, K. Nishiuchi, N. Akahira, M. Takao, J. Appl. Phys. 69 (1991) 2849. [2] M. Wuttig, N. Yamada, Nat. Mater. 6 (2007) 824. [3] H. Wong, S. Raoux, S. Kim, J. Liang, J. Reifenberg, B. Rajendran, M. Asheghi, K. Goodson, Proc. IEEE 98 (2010) 2201. [4] K. Kao, C. Lee, M. Chen, M. Tsai, T. Chin, Adv. Mater. 21 (2009) 1695. [5] R. Kojima, N. Yamada, Jpn. J. Appl. Phys. 40 (2001) 5930. [6] S. Raoux, W. Welnic, D. Ielmini, Chem. Rev. 110 (2010) 240. [7] B. Kooi, J. Hossom, J. Appl. Phys. 95 (2004) 4714.
[8] Y. Cheng, Z. Song, Y. Gu, S. Song, F. Rao, L. Wu, B. Liu, S. Feng, Appl. Phys. Lett. 99 (2011) 261914. [9] W. Wang, D. Loke, L. Shi, R. Zhao, H. Yang, L. Law, L. Ng, K. Lim, Y. Yeo, T. Chong, A. Lacaita, Sci. Rep. 2 (2012) 360. [10] M. Zhu, L. Wu, F. Rao, Z. Song, X. Ji, D. Yao, Y. Cheng, S. Song, B. Liu, L. Xu, J. Appl. Phys. 114 (2013) 124302. [11] Y. Hu, M. Sun, S. Song, Z. Song, J. Zhai, J. Alloys Comp. 551 (2013) 551. [12] A. Patterson, Phys. Rev. 56 (1939) 978. [13] C. Peng, L. Wu, F. Rao, Z. Song, P. Yang, H. Song, K. Ren, X. Zhou, M. Zhu, B. Liu, J. Chu, Appl. Phys. Lett. 101 (2012) 122108.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Phase change material W0.04(Sb4Te)0.96 for application in high-speed phase change memory Kun Ren a,b,⇑, Feng Rao a,⇑, Zhitang Song a, Shilong Lu a, Cheng Peng a,b, Min Zhu a,b, Liangcai Wu a, Bo Liu a, Songlin Feng a a State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Micro-System and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China b Graduate University of the Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 12 November 2013 Received in revised form 6 January 2014 Accepted 9 January 2014 Available online 21 January 2014 Keywords: High speed phase change memory W doped Sb4Te
a b s t r a c t W-doped Sb4Te is proposed for high-speed phase change memory (PCM). The crystallization speed of W0.04(Sb4Te)0.96 is characterized in both the film and device, confirmed to be 30 ns and 6 ns. The crystallization temperature and data retention have been increased to 192 and 112 °C. The melting point is 550 °C, 75 °C lower than that of GST. The grain size is controlled to be 30–50 nm, lowering the stress and improving the cycling ability. The small grains, good thermal stability, high crystallization speed and low melting temperature have made the W0.04(Sb4Te)0.96 a potential candidate for high-speed PCM application. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In the past decades, phase change materials have been extensively studied for application of nonvolatile memories for its outstanding optical and electrical application [1,2]. It is utilized extensively as rewritable optical storage media because of the remarkable distinction of optical characteristics between amorphous and crystalline phases and the capacity of reversible switching rapidly between the two phases. Meanwhile, PCM is considered as one of the most promising candidates for the next generation nonvolatile solid-state memory. The key feature of PCM is the reversible phase transition, induced by an electric pulse, between the amorphous (high resistivity, reset) and the crystalline (low resistivity, set) states. A large resistance ratio can be utilized to identify the stored data ‘‘1’’ and ‘‘0’’. Although PCM will be possibly commercialized in the near future, there are still some problems with regard to phase change materials that need to be solved. On one hand, attractive market opportunities would arise, however, if nonvolatile memory can be developed that reach dynamic random access memory ⇑ Corresponding authors. Address: State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Micro-System and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China (K. Ren). Tel.: +86 21 62511070x8406; fax: +86 21 62134404. E-mail addresses: [email protected] (K. Ren), [email protected] (F. Rao). http://dx.doi.org/10.1016/j.jallcom.2014.01.044 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
(DRAM)-like switching speed of around 10 ns [3]. On the other hand, to compete with NOR-flash memory, the data retention at 10 year should be higher, for example, around 80 °C for embedded storage application for computer or even higher (120 °C) for some potential application in automobile electronics [4]. Ge2Sb2Te5 is the most widely studied and used phase change material due to its overall qualified performance. However, the phase change speed of around 50 ns [5] and the data retention of around 85 °C [6] make it difficult for its application in high speed PCM and high thermal stable PCM. Most of the Sb-rich phase change materials have a growth-dominant crystallization behavior, whose crystallization is consisted of a long nucleus incubation duration and the following fast grain growth process [7,8]. However, in PCM, the residual nucleus and the electrode interfaces will lower the activation energy for nucleus formation, reducing the incubation time, leading to a fast crystallization speed [9]. Therefore, the Sb-rich phase change material is a good candidate for the high speed PCM application [10,11]. Although the operation speed of PCM is improved by its crystallization behavior, the great grain size will induce large stress in the material and the interfaces, leading to the cavities formation and the poor interface adhesion. These two problems are the main factors that responsible for the failure of PCM cycling [12]. And there are several works of improving the thermal stability and material uniformity of Sb4Te by doping, such as Ti and O doping, with the crystallization speed remaining high [10,11]. In this work, W is doped into the Sb4Te phase change material, expecting to get a better thermal stability
K. Ren et al. / Journal of Alloys and Compounds 594 (2014) 82–86
and a smaller grain size, without sacrificing its high phase change speed. 2. Experiments The Sb4Te and W-Sb4Te films researched in this work were prepared by sputtering method at room temperature. The films for X-ray diffraction (XRD), real-time reflectivity measurement and resistance–temperature (R–T) tests are about 300 nm thick, deposited on the Si/SiO2 (1 1 0) substrates. The crystallization temperature (Tc) and melting temperature (Tm) of amorphous W0.04(Sb4Te)0.96 is studied by differential scanning calorimetry (DSC) with a heating rate of 5 °C/min. The light source used for irradiating the sample is Nd:YAG laser with a duration of 8 ns and a wave length of 532 nm. The probing laser is a continuous He–Ne laser beam with a wavelength of 632.8 nm. The transmission electron microscopy (TEM) samples of Sb4Te and W-Sb4Te films are fabricated by depositing 30 nm-thick films on carbon supporting membranes. T-shaped PCM cell fabricated by 0.13 lm CMOS technology was utilized to verify the electrically induced phase change ability of the materials, as shown in inset of Fig. 2b. The resistance–voltage (R–V) of the PCM cell was monitored with Keithley 2400m and Tektronix AWG 5002B.
3. Results and discussion The R–T tests are carried out to characterize the sheet resistance change during heating and cooling processes, as shown in Fig. 1a. The Tc of Sb4Te is 151 °C, which rises with the increase of the doped W (192 °C for W0.04(Sb4Te)0.96 and 228 °C for W0.09(Sb4Te)0.91). The sheet resistance of amorphous Sb4Te film is 4 105 X/h, about 3 magnitudes higher than its crystalline sheet resistance (2 102 X/h). The W doping will cause the decrease of the sheet resistance of the amorphous film and the increase of the sheet resistance of the crystalline film, leading to a smaller resistance margin. As the doped W reaches 14 at%, the resistances of the amorphous and crystalline film almost stay at the same
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level, showing an upper limit of 14 at% W in the W-Sb4Te phase change materials. Data retention, another important parameter for PCM application, is characterized by extrapolating the isothermal Arrhenius plots, which are obtained by the logarithm of failure time vs. the reciprocal of isothermal temperature. In the isothermal process, the failure time are defined as the duration that the sheet resistance of the film reaches half of its initial value. By extrapolation of the failure time to 10 years, 10 years lifetime temperatures are calculated to be 41, 112 and 180 °C for Sb4Te, W0.04(Sb4Te)0.96 and W0.09(Sb4Te)0.91 films, respectively. The W0.04(Sb4Te)0.96 has a better data retention than that of Ge2Sb2Te5, indicating that it is thermally stable enough for embedded storage application. And its resistance margin of two magnitude is large enough for distinguishing high and low resistance. Although W0.09(Sb4Te)0.91 film has a good data retention that even qualified the application in automobile electronics, the one magnitude resistance margin seems to be too small for PCM application. Thus, more detailed characterizations should be carried out on the W0.04(Sb4Te)0.96 material. The Tc and Tm of amorphous W0.04(Sb4Te)0.96 is studied by DSC with a heating rate of 5 °C/min. The Tc and Tm of W0.04 (Sb4Te)0.96 are determined to be 185 °C and 550 °C, respectively. The Tm of W0.04(Sb4Te)0.96 is 75 °C lower than that of GST (625 °C), which will require a lower power consumption for the W0.04(Sb4Te)0.96 based PCM. The dynamic crystallization behavior of W0.04(Sb4Te)0.96 film is characterized by the real-time reflectivity measurement, as shown in Fig. 2a. The as-deposit amorphous film is in a low reflective state. After the laser pulse irradiated the film, the heat generated by the pulse triggered the crystallization of the film, making the film more reflective. Between the two stable states, there is the crystallization process, which is consisted of the nucleus
Fig. 1. (a) Sheet resistance of Sb4Te, W0.04(Sb4Te)0.96 and W0.09(Sb4Te)0.91 films as functions of temperature, with a heating rate of 10 °C/min. (b) The Arrhenius extrapolation at 10 years of data retention for Sb4Te, W0.04(Sb4Te)0.96 and W0.09(Sb4Te)0.91 films. (c) The DSC results of the W0.04(Sb4Te)0.96 material, with the heating rate of 5 °C/min.
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Fig. 2. (a) The transient reflectivity during crystallization of W0.04(Sb4Te)0.96 film induced by a nanosecond laser pulse with a width of 8 ns. The inset shows the transient part of the reflectivity curve. (b) R–V performance of the PCM cell based on W0.04(Sb4Te)0.96 materials. The inset shows the cross section of a PCM cell, with the scale bar presenting 200 nm. (c) Resistance–pulse width performance of the PCM cell based on W0.04(Sb4Te)0.96 material with different pulse height.
incubation and the grain growth. As shown in inset of Fig. 2a, the duration of the transition is 30 ns, which is shorter than the time for Ge2Sb2Te5 crystallization (50 ns) [5]. The nucleus incubation takes a large proportion of the entire crystallization time for phase change materials with a growth-dominated crystallization behavior [7]. When it is in the PCM cell, the residual nucleus and the interface induced easy crystallization will shorten the incubation time, making the crystallization much faster. T-shape PCM cell based on W0.04(Sb4Te)0.96 is fabricated to study the electrical triggered phase change ability of the material, as shown in inset of Fig. 2b and c. The R–V tests, as shown in Fig. 2b, are carried out to characterize the electric pulses triggered set and reset operations. When the pulse duration is 100 and 200 ns, the voltages for set and reset operations are 1.0–1.2 V and 2.7 V, respectively. The two magnitude of resistance margin is agree with the R–T results. The set state resistance of the W0.04(Sb4Te)0.96 based cell is higher than that of Ge2Sb2Te5 based cell using the same fabrication technology [13], which will benefit to lower the operation current, leading to a lower power consumption. Due to the lower crystallization speed of the phase change material than the melting speed, the operation speed of the PCM is usually determined by the speed of set operation. Thus, the set speed of W0.04(Sb4Te)0.96 based cell is characterized to define the operation speed, as shown in Fig. 2c. When the voltage is fixed in 1.0 V, the set operation can be completed by a 100 ns pulse. As the voltage increases from 1.0 V to 1.7 V, the pulse width required to complete the set operation decreases from 100 ns to 6 ns. Further increase the pulse height will possibly shorten the set duration, which cannot be detected due to the limit of the test equipments. The set operation can be finished in 6 ns when P1.7 V pulse is applied, indicating the fast speed of the W0.04(Sb4Te)0.96 based PCM cell. The 6-ns speed has satisfied the 10-ns speed requirement for
DRAM, making the W0.04(Sb4Te)0.96 a potential candidate for high speed PCM application. XRD method is carried out to characterize the lattice information of annealed Sb4Te and W0.04(Sb4Te)0.96 films with a thickness of 300 nm, as shown in Fig. 3. The Si diffraction peak in the curve is caused by the SiO2/Si substrate. In the curves corresponding to the 200 °C and 350 °C annealed Sb4Te and W0.04(Sb4Te)0.96 films, the diffraction peaks are indexed to be rhombohedral Sb, with no peak corresponding to W metal or W-containing compound being observed. The grain sizes in the two samples are estimated by analyzing the full width at half maximum (FWHM) of the diffraction peaks based on Scherrer’s formula [12]. The average sizes of grains in Sb4Te and W0.04(Sb4Te)0.96 films are calculated to be 59 and
Fig. 3. (a) XRD curves of 200 and 350 °C annealed Sb4Te and W0.04(Sb4Te)0.96 films on Si/SiO2 (1 0 0) substrates. The inset shows the calculated grain size.
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Fig. 4. (a) TEM picture of 250 °C annealed Sb4Te sample. The inset shows the SAED result of a selected range about 200 nm. (b) HRTEM picture of 250 °C annealed Sb4Te sample. (c) TEM picture of 350 °C annealed W0.04(Sb4Te)0.96 sample. The inset shows the SAED result of a selected range about 200 nm. (d) HRTEM picture of 350 °C annealed W0.04(Sb4Te)0.96 sample, with the amorphous regions enclosed by the dash lines.
34 nm, respectively. There will be more grain boundary when the grain size decreases, which helps to enhance the carriers scattering, thus leading to a higher resistance. Therefore, the decrease of the grain size is one of the main reasons for the higher cell resistance in set state, helping to decrease the power consumption of the reset operation. The smaller grains will cause less stress, suppressing the cavities formation during phase change, providing more adhesive interfaces. Thus, the smaller grains will help to lower the power consumption and improve the reliability of the PCM. TEM method is applied to have a direct observation of the distribution and size of the grains in the 250 °C annealed Sb4Te sample and 350 °C annealed W0.04(Sb4Te)0.96 sample, as shown in Fig. 4. By indexing the selected area electron diffraction (SAED) patterns in inset of Fig. 4a, the crystals can be assigned to hexagonal Sb4Te. The SAED pattern of Sb4Te sample with a selected range of about 200 nm suggests that the grain size is at least larger than 200 nm. The dark stripes in Fig. 4a have been reported in previous works, whose appearance usually accompanies the formation of large grains [7]. The high resolution TEM (HRTEM) result of the Sb4Te sample shows a long-range ordered atomic arrangement, with very few defects being observed, showing a large continuous Sb4Te grain, as shown in Fig. 4b. By W doping, the grain size is greatly decreased, as confirmed by the diffraction rings in inset of Fig. 4c. Meanwhile, the dark stripes cannot be observed in Fig. 4c. Although the grains are still assigned to rhombohedral Sb, there are a lot of amorphous regions embedded in the grains, as shown in Fig. 4d, ruining the integrity of the grain, thus lowering the grain size. The smaller grains provide more interfaces that can scatter the carriers when current flows, increasing the resistivity of the material and enhancing the heat efficiency. Higher heat efficiency will lower the current required for reset operation, resulting in a
low power consumption. According to the existence of amorphous regions, it is possible that W remain amorphous after the crystallization of the material. However, the atom radius of W (139 pm) is similar to Sb (145 pm) and Te (140 pm), it is also possible that W atoms are in the crystal as substitutional impurities. The accurate location of W atoms in the material still need further study. 4. Conclusions In conclusion, W doped Sb4Te materials are studied for phase change memory. By doping, the thermal stability of W0.04 (Sb4Te)0.96 has been improved, showing a Tc of 192 °C and a data retention of 112 °C. The crystallization speed is 30 ns by laser irradiation, which can be less than 6 ns in the PCM cell. The grains in the crystalline W0.04(Sb4Te)0.96 film are controlled to be 30–50 nm, with some amorphous region embedded in. The small grains will reduce the stress in the material and increase the crystalline resistivity, benefitting to a better cycling ability and a lower operation current. The low Tm will benefit to lower the power consumption of W0.04(Sb4Te)0.96 base PCM. Therefore, W0.04(Sb4Te)0.96 is considered to be a potential candidate for high speed PCM application. Acknowledgements Supported by National Key Basic Research Program of China(2013CBA01902, 2010CB934300, 2011CBA00602, 2011CB 932800), National Integrate Circuit Research Program of China (2009ZX02023-003), National Natural Science Foundation of China (60906004, 60906003, 61006087, 61076121), Science and Technology Council of Shanghai (1052nm07000, 12QA1403900).
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