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SbSe nanocomposite multilayer films
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Scripta Materialia 68 (2013) 522–525 www.elsevier.com/locate/scriptamat
Phase-change behaviors of Sb80Te20/SbSe nanocomposite multilayer films Mingcheng Sun,a,b Sannian Song,a,⇑ Zhitang Song,a Jiwei Zhai,b,⇑ Guangfei Liangc and Yiqun Wuc a
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-System and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China b Functional Materials Research Laboratory, Tongji University, Shanghai 200092, China c Key Laboratory of High Power Laser Materials, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Received 20 October 2012; revised 1 December 2012; accepted 2 December 2012 Available online 8 December 2012
The crystallization temperatures and 10 year data retention temperatures of the Sb80Te20/SbSe nanocomposite multilayer films can be modulated by varying the thickness ratio between Sb80Te20 and SbSe layers. The crystallization dynamics of the films induced by nanosecond laser pulses are studied using in situ reflectivity measurement. The set and reset operation for phase change memory device based on [Sb80Te20 (4 nm)/SbSe (10 nm)]7 multilayer film can be achieved by an electric pulse as short as 50 ns. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Multilayer film; Crystallization; Thermal stability; Optical reflectivity; Electric resistivity
Phase-change memory (PCM) has been considered as one of the most promising candidates for nextgeneration nonvolatile memory due to such features as fast speed, low power consumption, high scalability and good compatibility with the complementary metaloxide semiconductor (CMOS) process [1–3]. The phase-change materials in PCM can be transformed reversibly between amorphous (high resistance) and crystalline (low resistance) states by Joule heating with nanosecond electrical pulses. This high and low resistivity can be employed to store the data “1” and “0”. A high and short electrical pulse is applied to achieve the amorphous state (the RESET process) and a lower but slightly longer pulse is used to convert to the polycrystalline state (the SET process) [4]. Generally, good data retention and a high switching speed for PCM are hard to achieve at the same time because of the trade-off between the thermal stability and crystallization speed of the phase-change materials [5]. In recent years, nanocomposite multilayer films have attracted considerable attention due to certain characteristics that are not observed in monolayer materials [6,7].
⇑ Corresponding
authors; e-mail addresses: [email protected]. ac.cn; [email protected]
They have a lower thermal conductivity in both the horizontal and vertical directions because of the phonon scattering at their interfaces [8,9]. The thinness of the layers, and thus the high specific surface area, leads to high heterogeneous crystallization rates in nanostructured phase-change layers [10]. Owing to these advantages, the use of nanocomposite multilayer films could be an effective way to solve the issue by developing a phasechange layer with high crystallization speed and thermal stability. Sb–Te alloys are well known for their rapid crystallization speed due to the growth-dominated crystallization mechanism [11]. Of the Sb–Te alloys, Sb80Te20 has the fastest crystallization speed, which originates from its high atomic ratio of Sb to Te [12,13]. However, it shows poor amorphous phase stability due to its low crystallization temperature. According to the results of Kang et al. [14], SbSe would be a good candidate for PCM as it has good thermal stability. In the present study, Sb80Te20/SbSe nanocomposite multilayer films were fabricated and investigated for high speed and good data retention PCM. Sb80Te20/SbSe nanocomposite multilayer films with different thicknesses of the Sb80Te20 layers, monolayer Sb80Te20 film and monolayer SbSe film were deposited on 0.5 mm thick Si(1 0 0) wafers (covered by 300 nm thick SiO2) using a radio-frequency (RF) magnetron
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.12.001
M. Sun et al. / Scripta Materialia 68 (2013) 522–525
sputtering system at room temperature with Sb80Te20 and SbSe targets, respectively. The total thickness of all films was about 100 nm. The thickness of each individual layer was controlled by the deposition time. Prior to the growth of the multilayer films, Sb80Te20 and SbSe deposition rates were obtained by depositing a single layer on the corresponding target and measured its thickness using an Alpha-Step 500 profiler (Tencor Instrument). All deposition processes were carried out in an Ar atmosphere with a pressure of 0.2 Pa, a flow of 30 s ccm and an RF power of 20 W for both targets. The substrate was rotated at a speed of 20 rpm to guarantee a uniform deposition. In situ temperature-dependent resistance (R–T) was measured in an Ar atmosphere by a two-point-probe set-up to obtain the crystallization temperature. The films were kept at different temperatures for isothermal R–T measurements to estimate the data retention time and the activation energy for crystallization (Ea) by the Arrhenius equation. A nanosecond laser pumpprobe system was used for real-time reflectivity measurements. The light source used for irradiating the sample was a mode-locked Nd:YAG laser with a pulse duration of approximately 8 ns and a wavelength of 532 nm. The probing laser was a 632.8 nm wavelength continuous light beam from an He–Ne laser. The reflected probe beam was collected using a high-speed silicon avalanche photodiode and a fast digital phosphor oscilloscope with a total time resolution of approximately 2 ns. The PCM devices based on a [Sb80Te20 (4 nm)/SbSe (10 nm)]7 nanocomposite multilayer film and a SbSe monolayer film, with a tungsten heating electrode of 260 nm diameter, were fabricated by 0.18 lm CMOS technology. Between the storage medium and the top electrode, an 20 nm thick TiN film was deposited by direct current magnetron sputtering. Resistance–voltage (R–V) measurements were conducted using a Keithley 2400 semiconductor parameter analyzer and an Agilent 81104 A programmable pulse generator. The R–T measurements were carried out with a heating rate of 10°C min1 from room temperature to 250 °C for all films, as shown in Figure 1. At the beginning of the measurements, the resistance decreased slowly as the temperature increased. This can be attributed to thermally assisted trap-limited conduction [15]. A rapid drop then appeared when the temperature
Figure 1. Resistance as a function of temperature for Sb80Te20/SbSe nanocomposite multilayer films with different thicknesses of the Sb80Te20 layers at a heating rate of 10°C min1.
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approached the crystallization temperature (Tc), which can be defined as the temperature at which the derivative of resistance with respect to temperature (dR/dT) reaches a minimum value [16]. The Tc of the Sb80Te20/ SbSe multilayer films was between those of the Sb80Te20 and SbSe monolayer films, and changed with the different thickness ratios of the Sb80Te20 to the SbSe layers as a result of the thermal stabilities of the SbSe and Sb80Te20 films. Raoux et al. [17] reported that the crystallization temperature increases with decreasing of layer thickness due to the interfacial effect. Hence, the Tc of Sb80Te20/SbSe multilayer films depends on the interfacial effect and the thickness ratio between the Sb80Te20 and SbSe layers. As can be seen in Figure 1, the resistance of the as-deposited films decreased with increasing Sb80Te20 layer thickness. However, the ratio of amorphous resistance to crystalline resistance exceeds three orders of magnitude, which is large enough for the ON/OFF ratio in PCM applications. A data retention capability, which could be evaluated by the time-dependent resistance measurements under isothermal conditions, is vital for PCM devices. It is generally determined by the thermal stability of amorphous films. The failure time is defined as the time when the resistance decreases to half of its initial value at a specific isothermal temperature [18]. Figure 2(a) shows the normalized resistance as a function of annealing time at various isothermal annealing temperatures for a [Sb80Te20 (4 nm)/SbSe (10 nm)]7 multilayer film. The other Sb80Te20/SbSe multilayer films and the SbSe monolayer film were measured in the same way. The failure time is related to the activation energy by the Arrhenius equation [19]:
Figure 2. (a) Normalized resistance as a function of annealing time at various temperatures for [Sb80Te20 (4 nm)/SbSe (10 nm)]7 nanocomposite multilayer film. (b) Plots of failure time as a function of reciprocal temperature for SbSe and Sb80Te20 monolayer films, and Sb80Te20/SbSe nanocomposite multilayer films.
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M. Sun et al. / Scripta Materialia 68 (2013) 522–525
t ¼ A expðEa =kT Þ where t is the failure time, A is a proportional time constant, Ea is the activation energy for crystallization and k is Boltzmann’s constant. Lines fitted for plots of the failure time vs. the reciprocal temperature [20] according to the equation are shown in Figure 2(b) to estimate the data retention capability of the SbSe, Sb80Te20 and Sb80Te20/SbSe films. From Figure 2(b), we can see that Sb80Te20 film has poor data retention capability (about 20 °C), which is not enough for use in PCM applications. Compared with the SbSe film, the 10 year data retention temperature of the Sb80Te20/SbSe multilayer films declined gradually with increasing thickness of the Sb80Te20 layer. This can be ascribed to the extremely low thermal stability of Sb80Te20. However, when the thickness of the Sb80Te20 layer does not exceed 4 nm, the 10 year data retention temperature could reach 94 °C, which is higher than that of traditional Ge2Sb2Te5 film. By means of the extrapolated fitting lines, the calculated activation energies are 2.92, 1.36,3.66, 3.08, 2.86 and 1.85 eV for the SbSe and Sb80Te20 monolayer films, and the [Sb80Te20 (2 nm)/SbSe (10 nm)]9, [Sb80Te20 (4 nm)/SbSe (10 nm)]7, [Sb80Te20 (6 nm)/SbSe (10 nm)]6 and [Sb80Te20 (8 nm)/SbSe (10 nm)]5 multilayer films, respectively. The Ea of the Sb80Te20/SbSe films decreased with increasing thickness of the Sb80Te20 layer due to the small crystalline barrier of Sb80Te20. Also, the interfacial energy that exists in multilayer films [21] means that the [Sb80Te20 (2 nm)/SbSe (10 nm)]9 and [Sb80Te20 (4 nm)/ SbSe (10 nm)]7 films both have a larger Ea than the SbSe film. That is to say, the activation energies of Sb80Te20/ SbSe films depend on the trade-off between the interfacial effect and the thickness ratio of Sb80Te20 to SbSe. Figure 3 shows the crystallization dynamics of the SbSe, [Sb80Te20 (4 nm)/SbSe (10 nm)]7 and Sb80Te20 films induced by a nanosecond laser pulse. For the purpose of comparison, the real reflectivities (y-axis) of the SbSe and [Sb80Te20 (4 nm)/SbSe (10 nm)]7 films were increased by 0.25 and 0.1, respectively, as shown in Figure 3. For all the measured films, the initial reflectivity was about 0.2. With a fluence of 34.4 mJ cm2 irradiation, the reflectivity increased to a higher stable level after a longer incubation time. Since the phase change process is closely linked to the optical properties, real-time reflectivity measurements can provide an indirect observation of the
Figure 3. The transient reflectivity during crystallization of SbSe, [Sb80Te20 (4 nm)/SbSe (10 nm)]7 and Sb80Te20 films induced by nanosecond laser pulses with a pulse fluence of 34.4 mJ cm2.
structural evolution process in phase-change materials. The reflectivity change corresponds to the amorphousto-crystal phase transformation [22]. Since they were irradiated by the same laser pulse, the incubation time before the reflectivity change could reveal the crystallization speed of the films [23]. The incubation times of the SbSe, [Sb80Te20 (4 nm)/SbSe (10 nm)]7 and Sb80Te20 films were 65, 40 and 28 ns, respectively (inset in Fig. 3). This means that the [Sb80Te20 (4 nm)/SbSe (10 nm)]7 multilayer film possesses an extremely rapid crystallization speed – presumably because of the incorporation of Sb80Te20 – which has a favorable effect on the switching speed for PCM applications. In order to evaluate the properties in a PCM application, T-shaped PCM devices based on [Sb80Te20 (4 nm)/ SbSe (10 nm)]7 and SbSe films were fabricated using 0.18 lm CMOS technology. R–V curves of the cells and a schematic diagram of the cross-sectional cell structure are shown in Figure 4. It can be observed that the reversible phase-change process can be completed for the PCM cells by different electronic pulses. The resistance ratio between the RESET and SET states exceeds two orders of magnitude, which offers a substantial margin for identifying the high and low resistance states. When the test pulse was 100 ns, the PCM cell based on the [Sb80Te20 (4 nm)/SbSe (10 nm)]7 multilayer film had smaller SET and RESET threshold voltages (1.2 and 3.2 V) than the cell based on the SbSe film (1.7 and 3.8 V). Thus, the PCM devices based on [Sb80Te20 (4 nm)/SbSe (10 nm)]7 could consume less power than those based on the SbSe film. An important reason for this is that the large amount of phonon scattering that occurs at the interfaces in the multilayer structure leads to a decrease in thermal conductivity [24], which could improve the energy usage. Compared with the PCM cell based on the SbSe film, which requires a 100 ns pulse to complete the RESET/SET operations, the cell based on the [Sb80Te20 (4 nm)/SbSe (10 nm)]7 multilayer film requires only a 50 ns pulse. That is to say, the PCM devices based on the Sb80Te20/SbSe multilayer films possessed a faster switching speed. This result is consistent with the real-time optical reflectivity measurements. In summary, Sb80Te20/SbSe nanocomposite multilayer films with different thickness of Sb80Te20 were investigated for PCM application. The crystallization temperature could be changed by adjusting the thickness ratio of the Sb80Te20 to SbSe layers. The data retention capabilities for Sb80Te20/SbSe multilayer films were as
Figure 4. R–V curve of PCM cells based on [Sb80Te20 (4 nm)/SbSe (10 nm)]7 nanocomposite multilayer film and SbSe monolayer film.
M. Sun et al. / Scripta Materialia 68 (2013) 522–525
certained for common environments. Crystallization dynamics measurements revealed the reflectivity change during the phase transition of the films. The crystallization speed was improved for Sb80Te20/SbSe multilayer films because of the incorporation of Sb80Te20 layers. Reversible phase-change can be achieved for PCM cells based on [Sb80Te20 (4 nm)/SbSe (10 nm)]7 film using different electronic pulses. Further, the PCM cells exhibited a faster switching speed and less power consumption than those based on monolayer films. The authors acknowledge the financial support of the Foundation Project by Science and Technology Council of Shanghai (Grant No. 1052nm07200). [1] M. Wuttig, N. Yamada, Nat. Mater. 6 (2007) 824. [2] M. Lankhorst, B. Ketelaars, R. Wolters, Nat. Mater. 4 (2005) 347. [3] S.H. Lee, Y. Jung, R. Agarwal, Nat. Nanotechnol. 2 (2007) 626. [4] J.D. Maimon, K.K. Hunt, L. Burcin, J. Rodgers, IEEE Trans. Nucl. Sci. 50 (2003) 1878. [5] L. Desmond, L. Shi, W. Wang, R. Zhao, H. Yang, L.-T. Ng, K.-G. Lim, T.-C. Chong, Y.-C. Yeo, Nanotechnology 22 (2011) 254019. [6] J. Feng, Y. Lai, B. Qiao, B. Cai, Y. Yin, T. Tang, B. Chen, Jan. J. Appl. Phys. 46 (2007) 5724. [7] T.C. Chong, L.P. Shi, R. Zhao, P.K. Tan, J.M. Li, H.K. Lee, X.S. Miao, A.Y. Du, C.H. Tung, Appl. Phys. Lett. 88 (2006) 122114. [8] Y. Chen, D. Li, J.R. Lukes, Z. Ni, M. Chen, Phys. Rev. B 72 (2005) 174302.
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[9] M. Mazumder, T.B. Tasciuc, S.C. Teehan, E. Stinzianni, H. Efstathiadis, S. Solovyov, Appl. Phys. Lett. 96 (2010) 093103. [10] S.-H. Lee, Y. Jung, R. Agarwal, Nano Lett. 8 (2008) 3303. [11] Y.-S. Hsu, Y.-C. Her, Scripta Mater. 61 (2009) 1129. [12] H.Y. Cheng, K.F. Kao, C.M. Lee, T.S. Chin, Thin Solid Films 516 (2008) 5513. [13] H.Y. Cheng, K.F. Kao, C.M. Lee, T.S. Chin, IEEE Trans. Magn. 43 (2007) 927. [14] M.J. Kang, T.J. Park, D. Wamwangi, K. Wang, C. Stetmer, S.Y. Choi, M. Wutting, Microsyst. Technol. 13 (2007) 153. [15] D. Ielmini, Y. Zhang, Appl. Phys. Lett. 90 (2007) 192102. [16] V. Friedrich, W. Weidenhof, P. Njoroge, M. Franz, J. Wuttig, Appl. Phys. 87 (2000) 4130. [17] S. Raoux, H.-Y. Cheng, J.L. Jordan-Sweet, B. Munoz, M. Hitzbleck, Appl. Phys. Lett. 94 (2009) 183114. [18] K.F. Kao, C.M. Lee, M.J. Chen, M.J. Tsai, T.S. Chin, Adv. Mater. 21 (2009) 1695. [19] K.F. Kao, C.C. Chang, F.T. Chen, M.J. Tsai, T.S. Chin, Scripta Mater. 63 (2010) 855. [20] F. Rao, Z. Song, K. Ren, X. Li, L. Wu, W. Xi, B. Liu, Appl. Phys. Lett. 95 (2009) 032105. [21] N. Ohshima, J. Appl. Phys. 83 (1998) 5244. [22] J. Siegel, A. Schropp, J. Solis, C.N. Afonso, Appl. Phys. Lett. 84 (2004) 2250. [23] C. Wang, S. Li, J. Zhai, B. Shen, M. Sun, T. Lai, Scripta Mater. 64 (2011) 645. [24] D. Loke, L. Shi, W. Wang, R. Zhao, L.-T. Ng, K.-G. Lim, H. Yang, T.-C. Chong, Y.-C. Yeo, Appl. Phys. Lett. 97 (2010) 243508.
Scripta Materialia 68 (2013) 522–525 www.elsevier.com/locate/scriptamat
Phase-change behaviors of Sb80Te20/SbSe nanocomposite multilayer films Mingcheng Sun,a,b Sannian Song,a,⇑ Zhitang Song,a Jiwei Zhai,b,⇑ Guangfei Liangc and Yiqun Wuc a
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-System and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China b Functional Materials Research Laboratory, Tongji University, Shanghai 200092, China c Key Laboratory of High Power Laser Materials, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Received 20 October 2012; revised 1 December 2012; accepted 2 December 2012 Available online 8 December 2012
The crystallization temperatures and 10 year data retention temperatures of the Sb80Te20/SbSe nanocomposite multilayer films can be modulated by varying the thickness ratio between Sb80Te20 and SbSe layers. The crystallization dynamics of the films induced by nanosecond laser pulses are studied using in situ reflectivity measurement. The set and reset operation for phase change memory device based on [Sb80Te20 (4 nm)/SbSe (10 nm)]7 multilayer film can be achieved by an electric pulse as short as 50 ns. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Multilayer film; Crystallization; Thermal stability; Optical reflectivity; Electric resistivity
Phase-change memory (PCM) has been considered as one of the most promising candidates for nextgeneration nonvolatile memory due to such features as fast speed, low power consumption, high scalability and good compatibility with the complementary metaloxide semiconductor (CMOS) process [1–3]. The phase-change materials in PCM can be transformed reversibly between amorphous (high resistance) and crystalline (low resistance) states by Joule heating with nanosecond electrical pulses. This high and low resistivity can be employed to store the data “1” and “0”. A high and short electrical pulse is applied to achieve the amorphous state (the RESET process) and a lower but slightly longer pulse is used to convert to the polycrystalline state (the SET process) [4]. Generally, good data retention and a high switching speed for PCM are hard to achieve at the same time because of the trade-off between the thermal stability and crystallization speed of the phase-change materials [5]. In recent years, nanocomposite multilayer films have attracted considerable attention due to certain characteristics that are not observed in monolayer materials [6,7].
⇑ Corresponding
authors; e-mail addresses: [email protected]. ac.cn; [email protected]
They have a lower thermal conductivity in both the horizontal and vertical directions because of the phonon scattering at their interfaces [8,9]. The thinness of the layers, and thus the high specific surface area, leads to high heterogeneous crystallization rates in nanostructured phase-change layers [10]. Owing to these advantages, the use of nanocomposite multilayer films could be an effective way to solve the issue by developing a phasechange layer with high crystallization speed and thermal stability. Sb–Te alloys are well known for their rapid crystallization speed due to the growth-dominated crystallization mechanism [11]. Of the Sb–Te alloys, Sb80Te20 has the fastest crystallization speed, which originates from its high atomic ratio of Sb to Te [12,13]. However, it shows poor amorphous phase stability due to its low crystallization temperature. According to the results of Kang et al. [14], SbSe would be a good candidate for PCM as it has good thermal stability. In the present study, Sb80Te20/SbSe nanocomposite multilayer films were fabricated and investigated for high speed and good data retention PCM. Sb80Te20/SbSe nanocomposite multilayer films with different thicknesses of the Sb80Te20 layers, monolayer Sb80Te20 film and monolayer SbSe film were deposited on 0.5 mm thick Si(1 0 0) wafers (covered by 300 nm thick SiO2) using a radio-frequency (RF) magnetron
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.12.001
M. Sun et al. / Scripta Materialia 68 (2013) 522–525
sputtering system at room temperature with Sb80Te20 and SbSe targets, respectively. The total thickness of all films was about 100 nm. The thickness of each individual layer was controlled by the deposition time. Prior to the growth of the multilayer films, Sb80Te20 and SbSe deposition rates were obtained by depositing a single layer on the corresponding target and measured its thickness using an Alpha-Step 500 profiler (Tencor Instrument). All deposition processes were carried out in an Ar atmosphere with a pressure of 0.2 Pa, a flow of 30 s ccm and an RF power of 20 W for both targets. The substrate was rotated at a speed of 20 rpm to guarantee a uniform deposition. In situ temperature-dependent resistance (R–T) was measured in an Ar atmosphere by a two-point-probe set-up to obtain the crystallization temperature. The films were kept at different temperatures for isothermal R–T measurements to estimate the data retention time and the activation energy for crystallization (Ea) by the Arrhenius equation. A nanosecond laser pumpprobe system was used for real-time reflectivity measurements. The light source used for irradiating the sample was a mode-locked Nd:YAG laser with a pulse duration of approximately 8 ns and a wavelength of 532 nm. The probing laser was a 632.8 nm wavelength continuous light beam from an He–Ne laser. The reflected probe beam was collected using a high-speed silicon avalanche photodiode and a fast digital phosphor oscilloscope with a total time resolution of approximately 2 ns. The PCM devices based on a [Sb80Te20 (4 nm)/SbSe (10 nm)]7 nanocomposite multilayer film and a SbSe monolayer film, with a tungsten heating electrode of 260 nm diameter, were fabricated by 0.18 lm CMOS technology. Between the storage medium and the top electrode, an 20 nm thick TiN film was deposited by direct current magnetron sputtering. Resistance–voltage (R–V) measurements were conducted using a Keithley 2400 semiconductor parameter analyzer and an Agilent 81104 A programmable pulse generator. The R–T measurements were carried out with a heating rate of 10°C min1 from room temperature to 250 °C for all films, as shown in Figure 1. At the beginning of the measurements, the resistance decreased slowly as the temperature increased. This can be attributed to thermally assisted trap-limited conduction [15]. A rapid drop then appeared when the temperature
Figure 1. Resistance as a function of temperature for Sb80Te20/SbSe nanocomposite multilayer films with different thicknesses of the Sb80Te20 layers at a heating rate of 10°C min1.
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approached the crystallization temperature (Tc), which can be defined as the temperature at which the derivative of resistance with respect to temperature (dR/dT) reaches a minimum value [16]. The Tc of the Sb80Te20/ SbSe multilayer films was between those of the Sb80Te20 and SbSe monolayer films, and changed with the different thickness ratios of the Sb80Te20 to the SbSe layers as a result of the thermal stabilities of the SbSe and Sb80Te20 films. Raoux et al. [17] reported that the crystallization temperature increases with decreasing of layer thickness due to the interfacial effect. Hence, the Tc of Sb80Te20/SbSe multilayer films depends on the interfacial effect and the thickness ratio between the Sb80Te20 and SbSe layers. As can be seen in Figure 1, the resistance of the as-deposited films decreased with increasing Sb80Te20 layer thickness. However, the ratio of amorphous resistance to crystalline resistance exceeds three orders of magnitude, which is large enough for the ON/OFF ratio in PCM applications. A data retention capability, which could be evaluated by the time-dependent resistance measurements under isothermal conditions, is vital for PCM devices. It is generally determined by the thermal stability of amorphous films. The failure time is defined as the time when the resistance decreases to half of its initial value at a specific isothermal temperature [18]. Figure 2(a) shows the normalized resistance as a function of annealing time at various isothermal annealing temperatures for a [Sb80Te20 (4 nm)/SbSe (10 nm)]7 multilayer film. The other Sb80Te20/SbSe multilayer films and the SbSe monolayer film were measured in the same way. The failure time is related to the activation energy by the Arrhenius equation [19]:
Figure 2. (a) Normalized resistance as a function of annealing time at various temperatures for [Sb80Te20 (4 nm)/SbSe (10 nm)]7 nanocomposite multilayer film. (b) Plots of failure time as a function of reciprocal temperature for SbSe and Sb80Te20 monolayer films, and Sb80Te20/SbSe nanocomposite multilayer films.
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M. Sun et al. / Scripta Materialia 68 (2013) 522–525
t ¼ A expðEa =kT Þ where t is the failure time, A is a proportional time constant, Ea is the activation energy for crystallization and k is Boltzmann’s constant. Lines fitted for plots of the failure time vs. the reciprocal temperature [20] according to the equation are shown in Figure 2(b) to estimate the data retention capability of the SbSe, Sb80Te20 and Sb80Te20/SbSe films. From Figure 2(b), we can see that Sb80Te20 film has poor data retention capability (about 20 °C), which is not enough for use in PCM applications. Compared with the SbSe film, the 10 year data retention temperature of the Sb80Te20/SbSe multilayer films declined gradually with increasing thickness of the Sb80Te20 layer. This can be ascribed to the extremely low thermal stability of Sb80Te20. However, when the thickness of the Sb80Te20 layer does not exceed 4 nm, the 10 year data retention temperature could reach 94 °C, which is higher than that of traditional Ge2Sb2Te5 film. By means of the extrapolated fitting lines, the calculated activation energies are 2.92, 1.36,3.66, 3.08, 2.86 and 1.85 eV for the SbSe and Sb80Te20 monolayer films, and the [Sb80Te20 (2 nm)/SbSe (10 nm)]9, [Sb80Te20 (4 nm)/SbSe (10 nm)]7, [Sb80Te20 (6 nm)/SbSe (10 nm)]6 and [Sb80Te20 (8 nm)/SbSe (10 nm)]5 multilayer films, respectively. The Ea of the Sb80Te20/SbSe films decreased with increasing thickness of the Sb80Te20 layer due to the small crystalline barrier of Sb80Te20. Also, the interfacial energy that exists in multilayer films [21] means that the [Sb80Te20 (2 nm)/SbSe (10 nm)]9 and [Sb80Te20 (4 nm)/ SbSe (10 nm)]7 films both have a larger Ea than the SbSe film. That is to say, the activation energies of Sb80Te20/ SbSe films depend on the trade-off between the interfacial effect and the thickness ratio of Sb80Te20 to SbSe. Figure 3 shows the crystallization dynamics of the SbSe, [Sb80Te20 (4 nm)/SbSe (10 nm)]7 and Sb80Te20 films induced by a nanosecond laser pulse. For the purpose of comparison, the real reflectivities (y-axis) of the SbSe and [Sb80Te20 (4 nm)/SbSe (10 nm)]7 films were increased by 0.25 and 0.1, respectively, as shown in Figure 3. For all the measured films, the initial reflectivity was about 0.2. With a fluence of 34.4 mJ cm2 irradiation, the reflectivity increased to a higher stable level after a longer incubation time. Since the phase change process is closely linked to the optical properties, real-time reflectivity measurements can provide an indirect observation of the
Figure 3. The transient reflectivity during crystallization of SbSe, [Sb80Te20 (4 nm)/SbSe (10 nm)]7 and Sb80Te20 films induced by nanosecond laser pulses with a pulse fluence of 34.4 mJ cm2.
structural evolution process in phase-change materials. The reflectivity change corresponds to the amorphousto-crystal phase transformation [22]. Since they were irradiated by the same laser pulse, the incubation time before the reflectivity change could reveal the crystallization speed of the films [23]. The incubation times of the SbSe, [Sb80Te20 (4 nm)/SbSe (10 nm)]7 and Sb80Te20 films were 65, 40 and 28 ns, respectively (inset in Fig. 3). This means that the [Sb80Te20 (4 nm)/SbSe (10 nm)]7 multilayer film possesses an extremely rapid crystallization speed – presumably because of the incorporation of Sb80Te20 – which has a favorable effect on the switching speed for PCM applications. In order to evaluate the properties in a PCM application, T-shaped PCM devices based on [Sb80Te20 (4 nm)/ SbSe (10 nm)]7 and SbSe films were fabricated using 0.18 lm CMOS technology. R–V curves of the cells and a schematic diagram of the cross-sectional cell structure are shown in Figure 4. It can be observed that the reversible phase-change process can be completed for the PCM cells by different electronic pulses. The resistance ratio between the RESET and SET states exceeds two orders of magnitude, which offers a substantial margin for identifying the high and low resistance states. When the test pulse was 100 ns, the PCM cell based on the [Sb80Te20 (4 nm)/SbSe (10 nm)]7 multilayer film had smaller SET and RESET threshold voltages (1.2 and 3.2 V) than the cell based on the SbSe film (1.7 and 3.8 V). Thus, the PCM devices based on [Sb80Te20 (4 nm)/SbSe (10 nm)]7 could consume less power than those based on the SbSe film. An important reason for this is that the large amount of phonon scattering that occurs at the interfaces in the multilayer structure leads to a decrease in thermal conductivity [24], which could improve the energy usage. Compared with the PCM cell based on the SbSe film, which requires a 100 ns pulse to complete the RESET/SET operations, the cell based on the [Sb80Te20 (4 nm)/SbSe (10 nm)]7 multilayer film requires only a 50 ns pulse. That is to say, the PCM devices based on the Sb80Te20/SbSe multilayer films possessed a faster switching speed. This result is consistent with the real-time optical reflectivity measurements. In summary, Sb80Te20/SbSe nanocomposite multilayer films with different thickness of Sb80Te20 were investigated for PCM application. The crystallization temperature could be changed by adjusting the thickness ratio of the Sb80Te20 to SbSe layers. The data retention capabilities for Sb80Te20/SbSe multilayer films were as
Figure 4. R–V curve of PCM cells based on [Sb80Te20 (4 nm)/SbSe (10 nm)]7 nanocomposite multilayer film and SbSe monolayer film.
M. Sun et al. / Scripta Materialia 68 (2013) 522–525
certained for common environments. Crystallization dynamics measurements revealed the reflectivity change during the phase transition of the films. The crystallization speed was improved for Sb80Te20/SbSe multilayer films because of the incorporation of Sb80Te20 layers. Reversible phase-change can be achieved for PCM cells based on [Sb80Te20 (4 nm)/SbSe (10 nm)]7 film using different electronic pulses. Further, the PCM cells exhibited a faster switching speed and less power consumption than those based on monolayer films. The authors acknowledge the financial support of the Foundation Project by Science and Technology Council of Shanghai (Grant No. 1052nm07200). [1] M. Wuttig, N. Yamada, Nat. Mater. 6 (2007) 824. [2] M. Lankhorst, B. Ketelaars, R. Wolters, Nat. Mater. 4 (2005) 347. [3] S.H. Lee, Y. Jung, R. Agarwal, Nat. Nanotechnol. 2 (2007) 626. [4] J.D. Maimon, K.K. Hunt, L. Burcin, J. Rodgers, IEEE Trans. Nucl. Sci. 50 (2003) 1878. [5] L. Desmond, L. Shi, W. Wang, R. Zhao, H. Yang, L.-T. Ng, K.-G. Lim, T.-C. Chong, Y.-C. Yeo, Nanotechnology 22 (2011) 254019. [6] J. Feng, Y. Lai, B. Qiao, B. Cai, Y. Yin, T. Tang, B. Chen, Jan. J. Appl. Phys. 46 (2007) 5724. [7] T.C. Chong, L.P. Shi, R. Zhao, P.K. Tan, J.M. Li, H.K. Lee, X.S. Miao, A.Y. Du, C.H. Tung, Appl. Phys. Lett. 88 (2006) 122114. [8] Y. Chen, D. Li, J.R. Lukes, Z. Ni, M. Chen, Phys. Rev. B 72 (2005) 174302.
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