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Zn-doped Sb70Se30 thin films with multiple phase transition for high storage density and low power consumption phase change memory applications
Scripta Materialia 178 (2020) 324–328
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Zn-doped Sb70 Se30 thin films with multiple phase transition for high storage density and low power consumption phase change memory applications Ruirui Liu a,b, Anya Hu a,b, Zihan Zhao c, Haitao Zhou a,b,∗, Jiwei Zhai c,∗∗, Xiao Zhou d,∗∗, Sannian Song e, Zhitang Song e a
School of Materials Science and Engineering, Central South University, Changsha 410083, PR China Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Central South University, Changsha 410083, PR China c Shanghai Key Laboratory for R&D and Application of Metallic Functional Materials, School of Materials Science & Engineering, Tongji University, Shanghai 201804, PR China d Institute of Mechanical Engineering, École polytechnique fédérale de Lausanne, 1015 Lausanne, Switzerland e State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, PR China b
a r t i c l e
i n f o
Article history: Received 15 October 2019 Revised 24 November 2019 Accepted 24 November 2019
Keywords: Zn44 Sb39 Se17 thin film Phase change memory Multiple phase change Power consumption
a b s t r a c t The single layer Zn44 Sb39 Se17 thin film, being fabricated by Sb70 Se30 and Zn using co-sputtering method, exhibits double phase change transition. The double phase change processes are mainly attributed to the crystallization of the Sb and ZnSb phases. The potential operating temperature for ten years of two phase change processes are 64 °C and 179 °C, of which the good stability of the second phase change can be sufficient for the autoelectronic applications. Meanwhile we also discovered the Zn44 Sb39 Se17 thin film presents apparent low power consumption in comparison with Ge2 Sb2 Te5 and other reported Sb-Se based phase change materials.
The heightened need for data storage and information processing drives the urgent search for new computing devices. Specifically, memory unit is of great importance for data shuffling and data storage in computing devices. Among emerging memories, phase change random access memory (PCRAM) is considered as one of the most promising candidates for the next generation nonvolatile memory since PCRAMs exhibit the faster data operation and higher data storage capacity with relative low power consumption in comparison with volatile dynamic random memory (DRAM) and other memories. In particular, phase change materials (PCMs), the fundamental component in PCRAM, directly determine the performance and service life of PCRAMs. Usually, PCMs can rapidly and reversibly switch between amorphous (disorder) and (meta)-stable crystalline (order) states under the different pulsed electrical field and present different electrical resistance and optical reflectance [1–5]. Upon subjecting to a small-magnitude and
∗ Corresponding author at: School of Materials Science and Engineering, Central South University, Changsha 410083, PR China. ∗∗ Corresponding authors. E-mail addresses: [email protected] (H. Zhou), [email protected] (J. Zhai), x.zhou@epfl.ch (X. Zhou).
https://doi.org/10.1016/j.scriptamat.2019.11.054 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
long-duration pulse, the amorphous state is locally crystallized (i.e., SET operation). While a large-magnitude and small-duration pulse would render the crystalline state being heated above the melting temperature and quickly cooled to an amorphous state (i.e., RESET operation). Therefore, the composition optimization of PCM that enables the transformation between SET and RESET operation stable and fast becomes one important goal in the development of PCRAMs. Among different categories of PCMs, Ge2 Sb2 Te5 (GST) compound became the best choice for commercial product because of the good stability and the mature fabrication in industry. However, due to the increasing demand on higher stability, faster speed and lower power consumption in design of PCRAM, it is still necessary to further study the properties of PCMs. In recent years, some progress has been achieved on improvement of the thermal stability and phase change speed [6–11]. However, there is no apparent breakthrough regarding increase of storage density and reduction of power consumption. Recent studies suggested multiple phase transition in PCM is one effective way to promote the storage density, which can be achieved by fabricating multilayer thin films with superlattice-liked structure [12–15]. However, the unavoidable lattice mismatch within the interfaces between layers in the film would give rise to strong
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Fig. 1. (a) Temperature-dependent sheet resistance curves of Sb70 Se30 , ZnSb and Zn44 Sb39 Se17 thin films at heating rate of 10 °C/min. (b) Failure time vs reciprocal temperature for Zn44 Sb39 Se17 thin film.
degradation in device stability. In this work, we discovered Sb-rich Sb70 Se30 single layer thin films with Zn dopant and without artificial interfaces could evidently increase data density by triggering multiple phase transition (similar to multilayer thin film). Most importantly, lattice mismatch is effectively eliminated due to the lack of interfaces in single layer Zn doped Sb70 Se30 thin film. To our knowledge, it is the first report that Sb-Se PCM system with element dopant can cause multiple phase transition and increase data density without the introduction of detrimental lattice mismatch in the film, which is one different strategy to increase data density in comparison with multilayer thin film. Furthermore, we also found Zn-doped Sb70 Se30 single layer film shows very low power consumption among reported Sb-Se PCM systems. Both of these findings suggested Zn-doped Sb70 Se30 single layer film is one good candidate in PCRAM application. Zn-doped Sb-Se thin film was fabricated on a SiO2 /Si (100) substrate by means of co-sputtering with Zn and Sb70 Se30 targets (a.t. 99.99%). After component identification by energy dispersion spectrum (EDS), the thin film is composed as Zn44 Sb39 Se17 . The layer thickness was controlled as about 50 nm. The deposition process was carried out in an Ar atmosphere at a pressure of 0.2 Pa, with a flow of 30 sccm and a sputtering power of 20 W. The substrate was rotated at an autorotation speed of 20 rpm to ensure deposition uniformity. The custom-made two-point-probe setup was used to measure the in-situ temperature-dependent resistances (R-T) of the thin films in the Ar atmosphere, and the annealing samples were analyzed by this technique as well. Raman spectroscopy was performed to reveal the amorphous structure of the thin film. The crystalline phase of the Zn44 Sb39 Se17 thin film before and after annealing was observed by X-ray diffraction (XRD) with Cu Kα radiation, where 2θ is ranged from 20 to 60° with a 2° scanning rate. X-ray photoelectron spectroscopy (XPS) was employed to determine the binding states of the components. A PCM cell based on a Zn44 Sb39 Se17 thin film was fabricated by complementary metal oxide semiconductor (CMOS) technology, and the resistancevoltage property was measured by a Tektronix AWG5012B arbitrary waveform generator and a Keithley 2602A parameter analyzer. Fig. 1(a) shows the temperature-dependent sheet resistance curves (R-T) of Sb70 Se30 , ZnSb and Zn44 Sb39 Se17 thin films at the rate of 10 °C/min. The gradual decrease of the resistance with the increase of annealing temperature initially in R-T curves originates from the intrinsic semiconductor property of amorphous thin films [16]. Besides, the resistance experiences an abrupt drop with the further increase of temperature, which corresponds to Tc , indicating a transient transition from the disorder state to the order state. According to the R-T curve of Sb70 Se30 , it can be seen that only one phase change occurs in Sb70 Se30 in which the Tc is about 181 °C. In contrast, there are two phase change processes observed in Zn44 Sb39 Se17 . In particular, the first phase change takes place around 125 °C, which corresponds to the crystallization of
Sb phase. This phase transition would be further analyzed with the assistant of other experimental techniques in the following section. It is worthy to note there is another phase change appearing around 270 °C. We found this temperature is coincident with the phase change temperature of ZnSb, indicative of the formation of ZnSb in our examined Zn44 Sb39 Se17 thin film [17,18]. In order to explore the amorphous stability of Zn44 Sb39 Se17 thin film, ten-years data retention temperature (Tten ) was calculated by Arrhenius equation based on the failure time t, which is expressed as,
t = τ0 exp[Ea /kb T ]
(1)
where the failure time t corresponds to the time when half of the initial resistance remained. τ 0 is the pre-exponential factor depending on the material’s properties, Ea defines as the crystallization activation energy, kb is Boltzmann’s constant and T is the annealing temperature to process the Zn44 Sb39 Se17 thin film, which is slightly lower than Tc . The Arrhenius plot of Zn44 Sb39 Se17 thin film with two phase change processes is shown in Fig. 1(b). It is worth noting that the Tten of the first phase change of Zn44 Sb39 Se17 thin film is about 64 °C, slightly lower than that of GST. However, the Tten of the second phase change is 179 °C, much higher than that of GST, being sufficient for the autoelectronic applications (120 °C) [19]. As for the clarification of electrical properties of Zn44 Sb39 Se17 thin film, Raman spectra was used to detect the atomic information during phase transition in Zn44 Sb39 Se17 thin film, which is shown in Fig. 2(a). The structure of Zn44 Sb39 Se17 thin film in different treatment conditions was analyzed firstly. As shown in Fig. 2(a), there is only one broad Raman peak for as-deposited (amorphous) Zn44 Sb39 Se17 thin film, which appears at around 145 cm−1 , corresponding to the A7 phase Raman mode of fluctuation in the short-range order of Sb component [18,20,21]. While the Raman peak has a slight red shift in Zn44 Sb39 Se17 thin film after annealing under 50 °C (143 cm−1 ) and 150 °C (142 cm−1 ). The red shift indicates the decrease of the bandgap of Zn44 Sb39 Se17 thin film, mainly coming from the formation of more Sb crystalline phase with the increase of the annealing temperature [18,20,21]. When the annealing temperature of Zn44 Sb39 Se17 thin film rises to 300 °C, the single Raman peak is split into two peaks at 114 cm−1 and 149 cm−1 respectively. The Raman peak of 114 cm−1 corresponds to the vibrational peak of Sb phase and the Raman peak of 149 cm−1 is associated with vibrational peak of ZnSb phase. In addition, we also investigated the crystalline phase of Zn44 Sb39 Se17 thin film by XRD (see Fig. 2(b)) after annealing at various temperatures up to 400 °C (completion of second phase change). It can be seen that there is only Sb phase in the thin film after annealing at 50 °C and 150 °C. In combination with the Raman data, we can comfim that the first phase change origins from the formation of Sb phase. When the crystallization
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Fig. 2. (a) Raman spectra of the Zn44 Sb39 Se17 thin film in amorphous state (Am) and in the state after annealing at 50 °C, 150 °C and 300 °C for 5 min. (b) XRD of Zn44 Sb39 Se17 thin film in amorphous state and in the state after anealed in different temperatures for 15 min.
Fig. 3. XPS spectra of Sb 3d for Zn44 Sb39 Se17 thin film in (a) its amorphous state, (b) annealled at 150 °C for 1 min and (c) annealled at 300 °C for 5 min.
of Zn44 Sb39 Se17 thin film was completely finished, Sb, ZnSb and Sb2 Se3 phases are identified where in ZnSb and Sb are major crystalline phases and little amount of Sb2 Se3 exists in the thin film. The stable ZnSb and Sb2 Se3 phases play the critical role in determining the stability of Zn44 Sb39 Se17 thin film in second phase transition. However, it should be emphasized that the occurrence of second phase transition mainly dominated by the formation of ZnSb rather than Sb2 Se3 phase due to the limited formation of Sb2 Se3 during the crystallization. Apart from Raman and XRD data, the bond arrangement of Zn44 Sb39 Se17 thin film was analyzed by XPS spectra, wherein the
Sb 3d5/2 for Zn44 Sb39 Se17 thin film in three different states (amorphous state, after the first phase and after the second phase change) are shown in Fig. 3. It can be seen that there are two peaks in the amorphous state of the Sb 3d5/2 , being located at 528.2 eV and 530.6 eV, which corresponds to Sb-Sb bond and SbSe bond respectively [22–25]. After annealing at 150 °C for 1 min (completely finish the first phase change without starting the second phase change), the XPS peaks are still associated with SbSb bond and Sb-Se bond. Meanwhile, the fraction of Sb-Sb bond clearly increase in this state, further demonstrating the first phase change refers to the formation of Sb phase. As the annealing at
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Fig. 4. (a) Current-voltage curve for Zn44 Sb39 Se17 thin film, (b) resistance-voltage characteristics of the cells based on Zn44 Sb39 Se17 thin film.
Table 1 The device properties of GST and other Sb-Se based PCMs. t is RESET time, VRESET is RESET voltage, RRESET is RESET resistance and ERESET is the RESET energy. Systems
t (ns)
VRESET (V)
RRESET (Ω)
Zn44 Sb39 Se17 (This work) GST [26] Sb65 Se35 [27] Al19 Sb54 Se27 [28] Ga1 Sb6 Se3 [29] Sb65 Se6 Te29 [30] Ga13 Sb32 Se55 [31] Si0.20 (Sb2 Se)0.80 [32]
100 100 100 200 150 50 500 20
1.5 3.1 1.28 2.68 1.8 1.8 2.5 2.9
1.5 3.2 1.7 1.2 2.2 2.1 4.3 7.1
× × × × × × × ×
109 106 106 107 106 107 107 106
ERESET (J) 1.5 3.0 9.6 1.2 2.2 7.7 7.3 2.4
× × × × × × × ×
10−16 10−13 10−14 10−13 10−13 10−15 10−14 10−14
300 °C for 5 min, the XPS peaks change to 527.4 eV and 529.9 eV. The peak in 527.4 eV is in correspondence with Zn-Sb bond and the peak in 529.9 eV is also in consistence with the Sb-Se bond [22–25]. The red shift of the XPS peak (529.9 eV) mainly originates from the influence of Zn-Sb bond [22–24]. In conclusion, the XPS data further confirms that the two phase change processes of Zn44 Sb39 Se17 thin film derives from the crystallization of Sb and ZnSb phases. PCRAM cells based on Zn44 Sb39 Se17 thin film was fabricated by using 0.18 μm CMOS technology. The obtained current-voltage (I-V) and resistance-voltage (R-V) characteristics of the film are shown in Fig. 4. As revealed in I-V curve, a threshold switching phenomenon is detected from high-resistance to a low-resistance (negative resistance behavior) when the voltage goes beyond threshold voltage (Vth ). The Vth s of the two phase changes for Zn44 Sb39 Se17 thin film are 0.65 V and 0.19 V. Furthermore, R-V curve exposes that the high-resistance switches twice to the lowresistance state (SET operation, VSET ) and then changes back to the high-resistance (RESET operation, VRESET ) with the electric pulse width of 100 ns. The device data confirms that the PCRAM based on Zn44 Sb39 Se17 thin film can realize the multiple phase change processes, effective increasing of the storage density. More importantly, it is worhty to note the RESET threshold voltage is appreantly lower than commerical GST and other reported Sb-Se based PCMs (see Table 1). This intresting result would give rise to much lower power consumption in RESET process. In order to quantitatively evaluate the power consuption, we calculated the 2 RESET energy (ERESET ) by VRE SE T /RRE SE T × tRE SE T . The RESET energy of Zn44 Sb39 Se17 in this work and of other PCMs are shown in Table 1. It can be easily seen that the RESET energy of our Zn44 Sb39 Se17 thin film is about 1.5 × 10−16 J, being 2~3 orders of magnitude lower than that of GST and other reported Sb-Se based PCMs. This result further indicates the low power consumption in our studied Zn44 Sb39 Se17 thin film. In summary, we systematically investigates the phase change properites of the Zn44 Sb39 Se17 thin film. The results demonstrate that the Zn44 Sb39 Se17 thin film can take place twice phase change processes. The first phase change refers to the crystallization of Sb
phase and the second phase change corresponds to the crystallization of the ZnSb phase. These two phase change processes have relative good thermal stability, especially for the second phase change process, which can satisfy the need of the autoelectronic applications. The multiple phase change property indicates the high storage density of the single layer Zn44 Sb39 Se17 thin film. Furthermore, the low power consumption is also achieved. Therefore, Our Zn doped Sb-Se thin film can be considered as a good candidate in PCRAM applications. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Ruirui Liu is a postdoc researcher in Central South University. This work is supported by the Fundamental Research Funds of Central South University for postdoc researcher (214835) and China Postdoctoral Science Foundation grant (2019M652796). The authors also would like to acknowledge the support by Shanghai Municipal Science and Technology Commission under Grant No. 19JC1416803. References [1] M. Wuttig, N. Yamada, Nat. Mater. 6 (11) (2007) 824–832. [2] M. Wuttig, Nat. Mater. 4 (4) (2005) 265–266. [3] T. Siegrist, P. Jost, H. Volker, M. Woda, P. Merkelbach, C. Schlockermann, M. Wuttig, Nat. Mater. 10 (3) (2011) 202–208. [4] H.-S.P. Wong, S. Raoux, S. Kim, J. Liang, J.P. Reifenberg, B. Rajendran, M. Asheghi, K.E. Goodson, Proc. IEEE 98 (12) (2010) 2201–2227. [5] M.K. Qureshi, V. Srinivasan, J.A. Rivers, Scalable high performance main memory system using phase-change memory technology, ACM SIGARCH Comput. Archit. News 37 (2009) 24–33. [6] K.-F. Kao, C.-C. Chang, F.T. Chen, M.-J. Tsai, T.-S. Chin, Scr. Mater. 63 (8) (2010) 855–858. [7] M. Zhu, L. Wu, Z. Song, F. Rao, D. Cai, C. Peng, X. Zhou, K. Ren, S. Song, S. Feng, Applied Physics Letters 100 (12) (2012) 122101. [8] F. Rao, K. Ding, Y. Zhou, Y. Zheng, M. Xia, S. Lv, Z. Song, S. Feng, I. Ronneberger, R. Mazzarello, W. Zhang, E. Ma, 358(6369) (2017) 1423–1427. [9] Z. He, R. Liu, P. Wu, J. Zhai, T. Lai, S. Song, Z. Song, J. Alloys Compd. 653 (2015) 334–337. [10] M. Zhu, M. Xia, F. Rao, X. Li, L. Wu, X. Ji, S. Lv, Z. Song, S. Feng, H. Sun, S. Zhang, Nat. Commun. 5 (1) (2014) 4086. [11] R. Liu, X. Zhou, J. Zhai, J. Song, P. Wu, T. Lai, S. Song, Z. Song, ACS Appl. Mater. Interfaces 9 (32) (2017) 27004–27013. [12] 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 (12) (2006) 122114. [13] Z. He, P. Wu, R. Liu, J. Zhai, T. Lai, S. Song, Z. Song, CrystEngComm 18 (7) (2016) 1230–1234. [14] R. Liu, Z. He, J. Zhai, S. Song, Z. Song, X. Zhou, Mater. Lett. 163 (2016) 20–23. [15] R. Liu, P. Wu, Z. He, J. Zhai, X. Liu, T. Lai, Thin Solid Films 625 (2017) 11–16. [16] D. Ielmini, Y. Zhang, J. Appl. Phys. 102 (5) (2007) 054517. [17] Z. He, W. Wu, X. Liu, J. Zhai, T. Lai, S. Song, Z. Song, Mater. Lett. 185 (2016) 399–402. [18] Y. Chen, G. Wang, X. Shen, T. Xu, R. Wang, L. Wu, Y. Lu, J. Li, S. Dai, Q. Nie, CrystEngComm 16 (5) (2014) 757–762.
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Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Zn-doped Sb70 Se30 thin films with multiple phase transition for high storage density and low power consumption phase change memory applications Ruirui Liu a,b, Anya Hu a,b, Zihan Zhao c, Haitao Zhou a,b,∗, Jiwei Zhai c,∗∗, Xiao Zhou d,∗∗, Sannian Song e, Zhitang Song e a
School of Materials Science and Engineering, Central South University, Changsha 410083, PR China Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Central South University, Changsha 410083, PR China c Shanghai Key Laboratory for R&D and Application of Metallic Functional Materials, School of Materials Science & Engineering, Tongji University, Shanghai 201804, PR China d Institute of Mechanical Engineering, École polytechnique fédérale de Lausanne, 1015 Lausanne, Switzerland e State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, PR China b
a r t i c l e
i n f o
Article history: Received 15 October 2019 Revised 24 November 2019 Accepted 24 November 2019
Keywords: Zn44 Sb39 Se17 thin film Phase change memory Multiple phase change Power consumption
a b s t r a c t The single layer Zn44 Sb39 Se17 thin film, being fabricated by Sb70 Se30 and Zn using co-sputtering method, exhibits double phase change transition. The double phase change processes are mainly attributed to the crystallization of the Sb and ZnSb phases. The potential operating temperature for ten years of two phase change processes are 64 °C and 179 °C, of which the good stability of the second phase change can be sufficient for the autoelectronic applications. Meanwhile we also discovered the Zn44 Sb39 Se17 thin film presents apparent low power consumption in comparison with Ge2 Sb2 Te5 and other reported Sb-Se based phase change materials.
The heightened need for data storage and information processing drives the urgent search for new computing devices. Specifically, memory unit is of great importance for data shuffling and data storage in computing devices. Among emerging memories, phase change random access memory (PCRAM) is considered as one of the most promising candidates for the next generation nonvolatile memory since PCRAMs exhibit the faster data operation and higher data storage capacity with relative low power consumption in comparison with volatile dynamic random memory (DRAM) and other memories. In particular, phase change materials (PCMs), the fundamental component in PCRAM, directly determine the performance and service life of PCRAMs. Usually, PCMs can rapidly and reversibly switch between amorphous (disorder) and (meta)-stable crystalline (order) states under the different pulsed electrical field and present different electrical resistance and optical reflectance [1–5]. Upon subjecting to a small-magnitude and
∗ Corresponding author at: School of Materials Science and Engineering, Central South University, Changsha 410083, PR China. ∗∗ Corresponding authors. E-mail addresses: [email protected] (H. Zhou), [email protected] (J. Zhai), x.zhou@epfl.ch (X. Zhou).
https://doi.org/10.1016/j.scriptamat.2019.11.054 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
long-duration pulse, the amorphous state is locally crystallized (i.e., SET operation). While a large-magnitude and small-duration pulse would render the crystalline state being heated above the melting temperature and quickly cooled to an amorphous state (i.e., RESET operation). Therefore, the composition optimization of PCM that enables the transformation between SET and RESET operation stable and fast becomes one important goal in the development of PCRAMs. Among different categories of PCMs, Ge2 Sb2 Te5 (GST) compound became the best choice for commercial product because of the good stability and the mature fabrication in industry. However, due to the increasing demand on higher stability, faster speed and lower power consumption in design of PCRAM, it is still necessary to further study the properties of PCMs. In recent years, some progress has been achieved on improvement of the thermal stability and phase change speed [6–11]. However, there is no apparent breakthrough regarding increase of storage density and reduction of power consumption. Recent studies suggested multiple phase transition in PCM is one effective way to promote the storage density, which can be achieved by fabricating multilayer thin films with superlattice-liked structure [12–15]. However, the unavoidable lattice mismatch within the interfaces between layers in the film would give rise to strong
R. Liu, A. Hu and Z. Zhao et al. / Scripta Materialia 178 (2020) 324–328
325
Fig. 1. (a) Temperature-dependent sheet resistance curves of Sb70 Se30 , ZnSb and Zn44 Sb39 Se17 thin films at heating rate of 10 °C/min. (b) Failure time vs reciprocal temperature for Zn44 Sb39 Se17 thin film.
degradation in device stability. In this work, we discovered Sb-rich Sb70 Se30 single layer thin films with Zn dopant and without artificial interfaces could evidently increase data density by triggering multiple phase transition (similar to multilayer thin film). Most importantly, lattice mismatch is effectively eliminated due to the lack of interfaces in single layer Zn doped Sb70 Se30 thin film. To our knowledge, it is the first report that Sb-Se PCM system with element dopant can cause multiple phase transition and increase data density without the introduction of detrimental lattice mismatch in the film, which is one different strategy to increase data density in comparison with multilayer thin film. Furthermore, we also found Zn-doped Sb70 Se30 single layer film shows very low power consumption among reported Sb-Se PCM systems. Both of these findings suggested Zn-doped Sb70 Se30 single layer film is one good candidate in PCRAM application. Zn-doped Sb-Se thin film was fabricated on a SiO2 /Si (100) substrate by means of co-sputtering with Zn and Sb70 Se30 targets (a.t. 99.99%). After component identification by energy dispersion spectrum (EDS), the thin film is composed as Zn44 Sb39 Se17 . The layer thickness was controlled as about 50 nm. The deposition process was carried out in an Ar atmosphere at a pressure of 0.2 Pa, with a flow of 30 sccm and a sputtering power of 20 W. The substrate was rotated at an autorotation speed of 20 rpm to ensure deposition uniformity. The custom-made two-point-probe setup was used to measure the in-situ temperature-dependent resistances (R-T) of the thin films in the Ar atmosphere, and the annealing samples were analyzed by this technique as well. Raman spectroscopy was performed to reveal the amorphous structure of the thin film. The crystalline phase of the Zn44 Sb39 Se17 thin film before and after annealing was observed by X-ray diffraction (XRD) with Cu Kα radiation, where 2θ is ranged from 20 to 60° with a 2° scanning rate. X-ray photoelectron spectroscopy (XPS) was employed to determine the binding states of the components. A PCM cell based on a Zn44 Sb39 Se17 thin film was fabricated by complementary metal oxide semiconductor (CMOS) technology, and the resistancevoltage property was measured by a Tektronix AWG5012B arbitrary waveform generator and a Keithley 2602A parameter analyzer. Fig. 1(a) shows the temperature-dependent sheet resistance curves (R-T) of Sb70 Se30 , ZnSb and Zn44 Sb39 Se17 thin films at the rate of 10 °C/min. The gradual decrease of the resistance with the increase of annealing temperature initially in R-T curves originates from the intrinsic semiconductor property of amorphous thin films [16]. Besides, the resistance experiences an abrupt drop with the further increase of temperature, which corresponds to Tc , indicating a transient transition from the disorder state to the order state. According to the R-T curve of Sb70 Se30 , it can be seen that only one phase change occurs in Sb70 Se30 in which the Tc is about 181 °C. In contrast, there are two phase change processes observed in Zn44 Sb39 Se17 . In particular, the first phase change takes place around 125 °C, which corresponds to the crystallization of
Sb phase. This phase transition would be further analyzed with the assistant of other experimental techniques in the following section. It is worthy to note there is another phase change appearing around 270 °C. We found this temperature is coincident with the phase change temperature of ZnSb, indicative of the formation of ZnSb in our examined Zn44 Sb39 Se17 thin film [17,18]. In order to explore the amorphous stability of Zn44 Sb39 Se17 thin film, ten-years data retention temperature (Tten ) was calculated by Arrhenius equation based on the failure time t, which is expressed as,
t = τ0 exp[Ea /kb T ]
(1)
where the failure time t corresponds to the time when half of the initial resistance remained. τ 0 is the pre-exponential factor depending on the material’s properties, Ea defines as the crystallization activation energy, kb is Boltzmann’s constant and T is the annealing temperature to process the Zn44 Sb39 Se17 thin film, which is slightly lower than Tc . The Arrhenius plot of Zn44 Sb39 Se17 thin film with two phase change processes is shown in Fig. 1(b). It is worth noting that the Tten of the first phase change of Zn44 Sb39 Se17 thin film is about 64 °C, slightly lower than that of GST. However, the Tten of the second phase change is 179 °C, much higher than that of GST, being sufficient for the autoelectronic applications (120 °C) [19]. As for the clarification of electrical properties of Zn44 Sb39 Se17 thin film, Raman spectra was used to detect the atomic information during phase transition in Zn44 Sb39 Se17 thin film, which is shown in Fig. 2(a). The structure of Zn44 Sb39 Se17 thin film in different treatment conditions was analyzed firstly. As shown in Fig. 2(a), there is only one broad Raman peak for as-deposited (amorphous) Zn44 Sb39 Se17 thin film, which appears at around 145 cm−1 , corresponding to the A7 phase Raman mode of fluctuation in the short-range order of Sb component [18,20,21]. While the Raman peak has a slight red shift in Zn44 Sb39 Se17 thin film after annealing under 50 °C (143 cm−1 ) and 150 °C (142 cm−1 ). The red shift indicates the decrease of the bandgap of Zn44 Sb39 Se17 thin film, mainly coming from the formation of more Sb crystalline phase with the increase of the annealing temperature [18,20,21]. When the annealing temperature of Zn44 Sb39 Se17 thin film rises to 300 °C, the single Raman peak is split into two peaks at 114 cm−1 and 149 cm−1 respectively. The Raman peak of 114 cm−1 corresponds to the vibrational peak of Sb phase and the Raman peak of 149 cm−1 is associated with vibrational peak of ZnSb phase. In addition, we also investigated the crystalline phase of Zn44 Sb39 Se17 thin film by XRD (see Fig. 2(b)) after annealing at various temperatures up to 400 °C (completion of second phase change). It can be seen that there is only Sb phase in the thin film after annealing at 50 °C and 150 °C. In combination with the Raman data, we can comfim that the first phase change origins from the formation of Sb phase. When the crystallization
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Fig. 2. (a) Raman spectra of the Zn44 Sb39 Se17 thin film in amorphous state (Am) and in the state after annealing at 50 °C, 150 °C and 300 °C for 5 min. (b) XRD of Zn44 Sb39 Se17 thin film in amorphous state and in the state after anealed in different temperatures for 15 min.
Fig. 3. XPS spectra of Sb 3d for Zn44 Sb39 Se17 thin film in (a) its amorphous state, (b) annealled at 150 °C for 1 min and (c) annealled at 300 °C for 5 min.
of Zn44 Sb39 Se17 thin film was completely finished, Sb, ZnSb and Sb2 Se3 phases are identified where in ZnSb and Sb are major crystalline phases and little amount of Sb2 Se3 exists in the thin film. The stable ZnSb and Sb2 Se3 phases play the critical role in determining the stability of Zn44 Sb39 Se17 thin film in second phase transition. However, it should be emphasized that the occurrence of second phase transition mainly dominated by the formation of ZnSb rather than Sb2 Se3 phase due to the limited formation of Sb2 Se3 during the crystallization. Apart from Raman and XRD data, the bond arrangement of Zn44 Sb39 Se17 thin film was analyzed by XPS spectra, wherein the
Sb 3d5/2 for Zn44 Sb39 Se17 thin film in three different states (amorphous state, after the first phase and after the second phase change) are shown in Fig. 3. It can be seen that there are two peaks in the amorphous state of the Sb 3d5/2 , being located at 528.2 eV and 530.6 eV, which corresponds to Sb-Sb bond and SbSe bond respectively [22–25]. After annealing at 150 °C for 1 min (completely finish the first phase change without starting the second phase change), the XPS peaks are still associated with SbSb bond and Sb-Se bond. Meanwhile, the fraction of Sb-Sb bond clearly increase in this state, further demonstrating the first phase change refers to the formation of Sb phase. As the annealing at
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Fig. 4. (a) Current-voltage curve for Zn44 Sb39 Se17 thin film, (b) resistance-voltage characteristics of the cells based on Zn44 Sb39 Se17 thin film.
Table 1 The device properties of GST and other Sb-Se based PCMs. t is RESET time, VRESET is RESET voltage, RRESET is RESET resistance and ERESET is the RESET energy. Systems
t (ns)
VRESET (V)
RRESET (Ω)
Zn44 Sb39 Se17 (This work) GST [26] Sb65 Se35 [27] Al19 Sb54 Se27 [28] Ga1 Sb6 Se3 [29] Sb65 Se6 Te29 [30] Ga13 Sb32 Se55 [31] Si0.20 (Sb2 Se)0.80 [32]
100 100 100 200 150 50 500 20
1.5 3.1 1.28 2.68 1.8 1.8 2.5 2.9
1.5 3.2 1.7 1.2 2.2 2.1 4.3 7.1
× × × × × × × ×
109 106 106 107 106 107 107 106
ERESET (J) 1.5 3.0 9.6 1.2 2.2 7.7 7.3 2.4
× × × × × × × ×
10−16 10−13 10−14 10−13 10−13 10−15 10−14 10−14
300 °C for 5 min, the XPS peaks change to 527.4 eV and 529.9 eV. The peak in 527.4 eV is in correspondence with Zn-Sb bond and the peak in 529.9 eV is also in consistence with the Sb-Se bond [22–25]. The red shift of the XPS peak (529.9 eV) mainly originates from the influence of Zn-Sb bond [22–24]. In conclusion, the XPS data further confirms that the two phase change processes of Zn44 Sb39 Se17 thin film derives from the crystallization of Sb and ZnSb phases. PCRAM cells based on Zn44 Sb39 Se17 thin film was fabricated by using 0.18 μm CMOS technology. The obtained current-voltage (I-V) and resistance-voltage (R-V) characteristics of the film are shown in Fig. 4. As revealed in I-V curve, a threshold switching phenomenon is detected from high-resistance to a low-resistance (negative resistance behavior) when the voltage goes beyond threshold voltage (Vth ). The Vth s of the two phase changes for Zn44 Sb39 Se17 thin film are 0.65 V and 0.19 V. Furthermore, R-V curve exposes that the high-resistance switches twice to the lowresistance state (SET operation, VSET ) and then changes back to the high-resistance (RESET operation, VRESET ) with the electric pulse width of 100 ns. The device data confirms that the PCRAM based on Zn44 Sb39 Se17 thin film can realize the multiple phase change processes, effective increasing of the storage density. More importantly, it is worhty to note the RESET threshold voltage is appreantly lower than commerical GST and other reported Sb-Se based PCMs (see Table 1). This intresting result would give rise to much lower power consumption in RESET process. In order to quantitatively evaluate the power consuption, we calculated the 2 RESET energy (ERESET ) by VRE SE T /RRE SE T × tRE SE T . The RESET energy of Zn44 Sb39 Se17 in this work and of other PCMs are shown in Table 1. It can be easily seen that the RESET energy of our Zn44 Sb39 Se17 thin film is about 1.5 × 10−16 J, being 2~3 orders of magnitude lower than that of GST and other reported Sb-Se based PCMs. This result further indicates the low power consumption in our studied Zn44 Sb39 Se17 thin film. In summary, we systematically investigates the phase change properites of the Zn44 Sb39 Se17 thin film. The results demonstrate that the Zn44 Sb39 Se17 thin film can take place twice phase change processes. The first phase change refers to the crystallization of Sb
phase and the second phase change corresponds to the crystallization of the ZnSb phase. These two phase change processes have relative good thermal stability, especially for the second phase change process, which can satisfy the need of the autoelectronic applications. The multiple phase change property indicates the high storage density of the single layer Zn44 Sb39 Se17 thin film. Furthermore, the low power consumption is also achieved. Therefore, Our Zn doped Sb-Se thin film can be considered as a good candidate in PCRAM applications. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Ruirui Liu is a postdoc researcher in Central South University. This work is supported by the Fundamental Research Funds of Central South University for postdoc researcher (214835) and China Postdoctoral Science Foundation grant (2019M652796). The authors also would like to acknowledge the support by Shanghai Municipal Science and Technology Commission under Grant No. 19JC1416803. References [1] M. Wuttig, N. Yamada, Nat. Mater. 6 (11) (2007) 824–832. [2] M. Wuttig, Nat. Mater. 4 (4) (2005) 265–266. [3] T. Siegrist, P. Jost, H. Volker, M. Woda, P. Merkelbach, C. Schlockermann, M. Wuttig, Nat. Mater. 10 (3) (2011) 202–208. [4] H.-S.P. Wong, S. Raoux, S. Kim, J. Liang, J.P. Reifenberg, B. Rajendran, M. Asheghi, K.E. Goodson, Proc. IEEE 98 (12) (2010) 2201–2227. [5] M.K. Qureshi, V. Srinivasan, J.A. Rivers, Scalable high performance main memory system using phase-change memory technology, ACM SIGARCH Comput. Archit. News 37 (2009) 24–33. [6] K.-F. Kao, C.-C. Chang, F.T. Chen, M.-J. Tsai, T.-S. Chin, Scr. Mater. 63 (8) (2010) 855–858. [7] M. Zhu, L. Wu, Z. Song, F. Rao, D. Cai, C. Peng, X. Zhou, K. Ren, S. Song, S. Feng, Applied Physics Letters 100 (12) (2012) 122101. [8] F. Rao, K. Ding, Y. Zhou, Y. Zheng, M. Xia, S. Lv, Z. Song, S. Feng, I. Ronneberger, R. Mazzarello, W. Zhang, E. Ma, 358(6369) (2017) 1423–1427. [9] Z. He, R. Liu, P. Wu, J. Zhai, T. Lai, S. Song, Z. Song, J. Alloys Compd. 653 (2015) 334–337. [10] M. Zhu, M. Xia, F. Rao, X. Li, L. Wu, X. Ji, S. Lv, Z. Song, S. Feng, H. Sun, S. Zhang, Nat. Commun. 5 (1) (2014) 4086. [11] R. Liu, X. Zhou, J. Zhai, J. Song, P. Wu, T. Lai, S. Song, Z. Song, ACS Appl. Mater. Interfaces 9 (32) (2017) 27004–27013. [12] 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 (12) (2006) 122114. [13] Z. He, P. Wu, R. Liu, J. Zhai, T. Lai, S. Song, Z. Song, CrystEngComm 18 (7) (2016) 1230–1234. [14] R. Liu, Z. He, J. Zhai, S. Song, Z. Song, X. Zhou, Mater. Lett. 163 (2016) 20–23. [15] R. Liu, P. Wu, Z. He, J. Zhai, X. Liu, T. Lai, Thin Solid Films 625 (2017) 11–16. [16] D. Ielmini, Y. Zhang, J. Appl. Phys. 102 (5) (2007) 054517. [17] Z. He, W. Wu, X. Liu, J. Zhai, T. Lai, S. Song, Z. Song, Mater. Lett. 185 (2016) 399–402. [18] Y. Chen, G. Wang, X. Shen, T. Xu, R. Wang, L. Wu, Y. Lu, J. Li, S. Dai, Q. Nie, CrystEngComm 16 (5) (2014) 757–762.
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