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Au doped Sb3Te phase-change material for C-RAM device
Applied Surface Science 254 (2008) 2281–2284 www.elsevier.com/locate/apsusc
Au doped Sb3Te phase-change material for C-RAM device Feng Wang a,b,*, Ting Zhang b, Chun-liang Liu a, Zhi-tang Song b, Liang-cai Wu b, Bo Liu b, Song-lin Feng b, Bomy Chen c b
a Key Laboratory of Physical Electronics and Devices of Ministry of Education, Xi’an Jiaotong University, Xi’an, 710049 ShaanXi, China Laboratory of Nanotechnology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 200050 Shanghai, China c Silicon Storage Technology, Inc., 1171 Sonora Court, Sunnyvale, CA 94086, USA
Received 5 September 2007; accepted 5 September 2007 Available online 11 September 2007
Abstract Au doped Sb3Te phase-change films have been investigated by means of in situ temperature-dependent resistance measurement. Crystallization temperature of 2 at.% Au doped Sb3Te has been enhanced to 161 8C, which leads to a better data retention. The physical stability of the film has been improved evidently after adding Au as well. Resistance contrast has been improved to 1.1 104, one order of magnitude higher than that of pure Sb3Te. X-ray diffraction patterns indicate the polycrystalline Au–SbTe series have hexagonal structure, similar with pure Sb3Te alloy, when Au doping dose is less than 9 at.%. # 2007 Elsevier B.V. All rights reserved. PACS : 64.70.Kb; 61.43.Dq; 73.61.Jc Keywords: Phase-change; Crystallization temperature; Data retention; Au doped Sb3Te
1. Introduction In recent years, chalcogenide random access memory (CRAM) has been developed to be one of the most promising candidates for the next generation nonvolatile memories due to its many advantages. The advantages of C-RAM include nonvolatility, high density, ability for scaling down and compatibility with complementary metal-oxide semiconductor (CMOS) process compared with other new memory technologies [1–4]. At present, high speed and good stability are desired qualities in C-RAM development. For stability, many researches focused on improving phase-change material [5– 7]. The essential is to optimize the material’s crystallization properties. There are two important aspects of the optimization, one is thermal stability and the other is physical stability. Preferable thermal stability and physical stability would make archival life longer and the phase-change film more reliable.
* Corresponding author. Tel.: +86 021 62511070x8408; fax: +86 021 62134404. E-mail address: [email protected] (F. Wang). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.013
However, improving stability may deteriorate other material properties such as resistance contrast or crystallization speed [5]. Suitable phase-change material should have good stability and other advantages at the same time. Eutectic Sb–Te material is widely used for C-RAM device and optical storage disk, and it has been proved to be suitable in high-speed applications. The most superior property of Sb–Te alloys compared with other materials is the fast crystallization speed due to its growth dominated crystallization mechanism which would lead to crystallization speed scales inversely with the size of contact area [8–10]. It has been found that the speed of crystallization increases with the Sb/Te ratio, but the thermal stability scales inversely with it [8,11]. In our former study, Sb3Te has preferable performance compared with other Sb–Te alloys. Hence, Sb3Te is discussed in this work. Although eutectic Sb–Te alloys have the above-mentioned superior properties, it has some obvious deficiencies, such as a relatively poor thermal stability. Thus, our investigation is mainly focused on the improvement of Sb–Te material performance combining high thermal stability with good physical stability and large resistance contrast by adding foreign element Au. Furthermore, the influence of Au content will be discussed to give the best composition.
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2. Experiments The Au doped Sb3Te films have been deposited by cosputtering single element Sb, Te and Au targets on SiO2/Si (1 0 0) substrates. The size of sample is about 3 cm 3 cm. Different power was applied on Au target in order to achieve different composition. In this experiment, the fixed Sb/Te ratio was obtained by applying the specific DC power applied on Sb and Te targets. The measured ratio is changeless with the value of 3:1 according to energy dispersive spectroscopy (EDS) measurement. The background pressure in the sputtering process was below 2 10 4 Pa, and the sputtering Ar pressure was 0.27 Pa. The thickness of the film is about 220 nm according to scanning electron microscope (SEM) crosssection observation. The films were annealed in N2 atmosphere at 250 8C for 1 min and 300 8C for 2 min in order to carry out X-ray diffraction (XRD) measurement and SEM surface observation, respectively. XRD was employed to characterize the structure of the film. The XRD patterns were taken in the 2u range of 20–708 using a Cu target with a scanning step of 0.028, and the data acquisition time in each step is 0.3 s. In situ temperature-dependent resistance measurement has been carried out in a vacuum chamber, inside which the temperature is regulated by a refrigerator [6]. In the refrigerator, high purity nitrogen is employed for refrigeration by Joule-Thomson effect, and a resistance wire is for Joule heating. The dependence of electrical resistance on temperature has been measured in discrete model with step of 2 K. 3. Results and discussion Fig. 1 shows the XRD patterns of pure and various Au doped Sb3Te after annealed at 250 8C for 1 min. When Au content is less than 4 at.%, it could be found that the crystalline structure of Au–SbTe series material is similar to pure Sb3Te. It indicates that Au doping does not change the type of lattice structure. All the materials are with hexagonal structure, and it accords with the reported research [12]. The peaks of Au–Sb compound would appear in the material with high Au content.
Fig. 2. Measured d(Log R)/dT as a function of temperature at a heating rate of 15 8C/min, where (A) represents pure Sb3Te, (B) 2 at.% Au doped Sb3Te, (C) 4 at.% Au doped Sb3Te and (D) 9 at.% Au doped Sb3Te.
An important effect of Au content is to increase thermal stability. Thermal stability is a basic property of phase-change material, and it will result in better data retention (long-term archival stability) and less thermal crosstalk in the C-RAM devices. In order to increase data retention of the phase-change material, a relatively high crystallization temperature (Tc) is desired [13]. In our research, Tc is obtained from in situ resistance measurement. The crystallization temperature is determined by the minimum in the derivative (d(Log R)/dT) [14]. Fig. 2 shows analyzed curves of d(Log R)/dT as a function of T, where R represents measured resistance and T temperature. Table 1 shows the crystallization temperature of Sb3Te and Au–SbTe series materials. For 2 and 4 at.% Au doped Sb3Te films, the crystallization temperature is 161 and 148 8C, respectively, when heating rate is fixed at 15 8C/min. Nevertheless Tc of pure Sb3Te is only 137 8C which is too low for practical applications. No obvious crystallization has been observed when Au concentration exceeds 9 at.%. The data retention of C-RAM is generally determined from the thermal stability of phase-change material. Therefore, Au– SbTe materials promise better data retention due to the higher Tc which offers better stability against spontaneous recrystallization at room temperature. Obtaining characteristic of data retention in this work is to evaluate the failure time (tf) at a certain annealing temperature [6,15,16]. Here tf is defined as the duration when the resistance of the material drops to 10% the value of amorphous state in annealing process. Resistance of films as a function of time at various temperatures is shown in Fig. 3. It is obvious that the Au–SbTe films have better data retention compared with Sb3Te since longer time needed to complete phase transition at the same temperature. Fig. 4 shows failure time of amorphous 2 at.% Au doped Sb3Te and pure Table 1 Crystallization temperature of various Au–SbTe films with different Au content
Fig. 1. X-ray diffraction patterns of Au–SbTe films on Si (1 0 0) substrate, annealed at 250 8C for 1 min, where (A) represents pure Sb3Te, (B) 2 at.% Au doped Sb3Te, (C) 4 at.% Au doped Sb3Te and (D) 9 at.% Au doped Sb3Te.
Composition
Crystallization temperature (8C)
Sb3Te 2 at.% Au doped Sb3Te 4 at.% Au doped Sb3Te
137 161 148
F. Wang et al. / Applied Surface Science 254 (2008) 2281–2284
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Fig. 4. Failure time as a function of temperature.
Fig. 3. Time-dependent electrical resistance at specific temperature: (a) Sb3Te at specific temperature from 135 to 150 8C. (b) 2 at.% Au doped Sb3Te at specific temperature from 135 to 155 8C.
Sb3Te. The failure time is extrapolated to 80 8C by fitting the data to an Arrhenius equation [8]. Although there would be some errors since it is far away from the measured range, the fitted curve indicates that doping Au can increase the failure time by several orders of magnitude at 80 8C. tf of Au–SbTe at 80 8C is above 2.7 1011 s, four orders of magnitude longer than that of pure Sb3Te of which is approximately 4.3 107 s. The reason of choosing 10% criteria is mainly based on the consideration that it will be very difficult to distinguish between low state and high state if the resistance drops to less than 10% of high state value. The result seems smaller than reported one because the different criteria of failure time. For example, tf was defined as the duration for complete crystallization in reported research [15]. Physical stability of film is a parameter which the device reliability is related to. Many factors such as volatilization or stress would result in crack of film. It would deteriorate the device reliability if the phase-change material film cracks. Fig. 5 shows the surfaces of annealed samples observed by SEM. Both of the films have been annealed at 300 8C for 2 min in N2 atmosphere. For Au–SbTe film, the film is with perfect surface, whereas many flaws turn up on the surface of Sb3Te film. This may be explained by release of stress inside the film during the annealing process, for there would be evident change in density of phase-change material during phase-change
Fig. 5. SEM image of film surface, annealed at 300 8C for 2 min. (a) Image of Sb3Te film. (b) Image of Au–SbTe film.
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identifying the states from high resistance state and low resistance state if the resistance of phase-change film is too low, considering the driver circuit of C-RAM chip is 1 kV. So, the limited content of Au dopant should be 4 at.% in practical applications. 4. Conclusions The properties of Au doped Sb–Te material have been investigated in detail. Good performance can be achieved from Au–SbTe series which combined good data retention with high physical stability and large resistance contrast. For 2 at.% Au doped Sb3Te, better performance has been achieved compared with pure Sb3Te, for Tc increases to 161 8C and the resistance contrast increases to 1.1 104. Failure time of 2 at.% Au doped Sb3Te is above 2.7 1011 s at 80 8C, several orders longer than that of pure one. Physical stability of film has been improved significantly as well. All these indicate that the Au–SbTe would be a suitable phase-change material for practical C-RAM applications. Acknowledgements
Fig. 6. Temperature-dependent electrical resistance of Sb3Te and Au–SbTe films: (a) pure Sb3Te and 2 at.% Au doped Sb3Te films. (b) 4 at.% Au doped Sb3Te and 9 at.% Au doped Sb3Te films.
process [17]. Since doping Au could increase the physical stability of Sb3Te film during phase-change process, Au doped Sb3Te would offer the potential for better reliability. The ratio of amorphous state resistance (Rhigh) and crystalline state resistance (Rlow) is referred to as resistance contrast. In practical applications, the resistance contrast must be large enough to distinguish the resistance of the material within memory chips correctly and easily. For 2 at.% Au doped Sb3Te, the ratio reaches to 1.1 104 with Rhigh 4.6 105 V and Rlow 41 V, an order of magnitude higher than pure Sb3Te with ratio of 6.2 103. Fig. 6 shows the electrical resistance of Au–SbTe film versus temperature measured in vacuum with heating rate of 15 8C/min. Before crystallization, the resistance decreases steadily when the temperature rises. It is obvious that the resistance decreases sharply near crystallization temperature. The resistance becomes not sensitive to temperature after crystallization. Although doping Au into Sb3Te would improve the performance, too high dose of dopant would make the performance just go to the opposite. Considering Tc shown in Fig. 2 and resistance contrast shown in Fig. 6, it is obvious that 2 at.% Au doped Sb3Te has best performance among Au– SbTe series, whereas the performance of 9 at.% Au doped Sb3Te becomes very poor. In addition, too much Au element would lead to very low resistance. It would go against
This work is supported by 973 Program (2007CB935400, 2006CB302700), National High Technology Development Program of China (2006AA03Z360), Science and Technology Council of Shanghai (06QA14060, 06XD14025, 0652nm003, 06DZ22017). References [1] M. Wuttig, C. Steimer, Appl. Phys. A 87 (2007) 411. [2] G. Muller, T. Happ, M. Kund, G.Y. Lee, N. Nagel, R. Sezi, IEEE P. Int. Electron. Devices Meet. (2004) 567. [3] A. Pirovano, A. Lacaita, A. Benvenuti, F. Pellizzer, S. Hudgens, R. Bez, IEEE P. Int. Electron. Devices Meet. (2003) 699. [4] S.G. Alberici, R. Zonca, B. Pashmakov, Appl. Surf. Sci. 231–232 (2004) 821. [5] K. Kao, H. Cheng, C. Jong, C. Lan, T. Chin, IEEE Trans. Mag. 43 (2007) 930. [6] T. Zhang, Z.T. Song, F. Wang, B. Liu, S.L. Feng, B. Chen, Jpn. J. Appl. Phys. 46 (2007) L602. [7] T. Zhang, Z.T. Song, F. Rao, G.M. Feng, B. Liu, S.L. Feng, B. Chen, Jpn. J. Appl. Phys. 46 (2007) L247. [8] M. Lankhorst, L. Pieterson, M. Schijndel, B. Jacobs, J. Rijpers, Jpn. J. Appl. Phys. 42 (2003) 863. [9] E.R. Meinders, M. Lankhorst, Jpn. J. Appl. Phys. 42 (2003) 809. [10] O. Noritake, A. Sumio, N. Naomasa, et al. Jpn. J. Appl. Phys. 41 (2002) 1695. [11] M. Lankhorst, B. Ketelaars, R. Wolters, Nat. Mater. 4 (2005) 347. [12] C. Sun, J. Lee, M. Youm, Y. Kim, Jpn. J. Appl. Phys. 45 (2006) 9157. [13] A. Pirovano, A. Redaelli, F. Pellizzer, F. Ottogalli, M. Tosi, D. Ielmini, A.L. Lacaita, R. Bez, IEEE Trans. Dev. Mater. Reliab. 4 (3) (2004) 422. [14] I. Friedrich, V. Weidenhof, W. Njoroge, P. Franz, M. Wuttig, J. Appl. Phys. 87 (2000) 4130. [15] L. Pieterson, M. Schijndel, J. Rijpers, Appl. Phys. Lett. 83 (2003) 1373. [16] T. Zhang, Z.T. Song, B. Liu, S.L. Feng, B. Chen, Solid-State Electron. 51 (2007) 950. [17] H. Lyeo, D. Cahill, B. Lee, J. Abelson, Appl. Phys. Lett. 89 (2006) 151904.
Au doped Sb3Te phase-change material for C-RAM device Feng Wang a,b,*, Ting Zhang b, Chun-liang Liu a, Zhi-tang Song b, Liang-cai Wu b, Bo Liu b, Song-lin Feng b, Bomy Chen c b
a Key Laboratory of Physical Electronics and Devices of Ministry of Education, Xi’an Jiaotong University, Xi’an, 710049 ShaanXi, China Laboratory of Nanotechnology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 200050 Shanghai, China c Silicon Storage Technology, Inc., 1171 Sonora Court, Sunnyvale, CA 94086, USA
Received 5 September 2007; accepted 5 September 2007 Available online 11 September 2007
Abstract Au doped Sb3Te phase-change films have been investigated by means of in situ temperature-dependent resistance measurement. Crystallization temperature of 2 at.% Au doped Sb3Te has been enhanced to 161 8C, which leads to a better data retention. The physical stability of the film has been improved evidently after adding Au as well. Resistance contrast has been improved to 1.1 104, one order of magnitude higher than that of pure Sb3Te. X-ray diffraction patterns indicate the polycrystalline Au–SbTe series have hexagonal structure, similar with pure Sb3Te alloy, when Au doping dose is less than 9 at.%. # 2007 Elsevier B.V. All rights reserved. PACS : 64.70.Kb; 61.43.Dq; 73.61.Jc Keywords: Phase-change; Crystallization temperature; Data retention; Au doped Sb3Te
1. Introduction In recent years, chalcogenide random access memory (CRAM) has been developed to be one of the most promising candidates for the next generation nonvolatile memories due to its many advantages. The advantages of C-RAM include nonvolatility, high density, ability for scaling down and compatibility with complementary metal-oxide semiconductor (CMOS) process compared with other new memory technologies [1–4]. At present, high speed and good stability are desired qualities in C-RAM development. For stability, many researches focused on improving phase-change material [5– 7]. The essential is to optimize the material’s crystallization properties. There are two important aspects of the optimization, one is thermal stability and the other is physical stability. Preferable thermal stability and physical stability would make archival life longer and the phase-change film more reliable.
* Corresponding author. Tel.: +86 021 62511070x8408; fax: +86 021 62134404. E-mail address: [email protected] (F. Wang). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.013
However, improving stability may deteriorate other material properties such as resistance contrast or crystallization speed [5]. Suitable phase-change material should have good stability and other advantages at the same time. Eutectic Sb–Te material is widely used for C-RAM device and optical storage disk, and it has been proved to be suitable in high-speed applications. The most superior property of Sb–Te alloys compared with other materials is the fast crystallization speed due to its growth dominated crystallization mechanism which would lead to crystallization speed scales inversely with the size of contact area [8–10]. It has been found that the speed of crystallization increases with the Sb/Te ratio, but the thermal stability scales inversely with it [8,11]. In our former study, Sb3Te has preferable performance compared with other Sb–Te alloys. Hence, Sb3Te is discussed in this work. Although eutectic Sb–Te alloys have the above-mentioned superior properties, it has some obvious deficiencies, such as a relatively poor thermal stability. Thus, our investigation is mainly focused on the improvement of Sb–Te material performance combining high thermal stability with good physical stability and large resistance contrast by adding foreign element Au. Furthermore, the influence of Au content will be discussed to give the best composition.
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2. Experiments The Au doped Sb3Te films have been deposited by cosputtering single element Sb, Te and Au targets on SiO2/Si (1 0 0) substrates. The size of sample is about 3 cm 3 cm. Different power was applied on Au target in order to achieve different composition. In this experiment, the fixed Sb/Te ratio was obtained by applying the specific DC power applied on Sb and Te targets. The measured ratio is changeless with the value of 3:1 according to energy dispersive spectroscopy (EDS) measurement. The background pressure in the sputtering process was below 2 10 4 Pa, and the sputtering Ar pressure was 0.27 Pa. The thickness of the film is about 220 nm according to scanning electron microscope (SEM) crosssection observation. The films were annealed in N2 atmosphere at 250 8C for 1 min and 300 8C for 2 min in order to carry out X-ray diffraction (XRD) measurement and SEM surface observation, respectively. XRD was employed to characterize the structure of the film. The XRD patterns were taken in the 2u range of 20–708 using a Cu target with a scanning step of 0.028, and the data acquisition time in each step is 0.3 s. In situ temperature-dependent resistance measurement has been carried out in a vacuum chamber, inside which the temperature is regulated by a refrigerator [6]. In the refrigerator, high purity nitrogen is employed for refrigeration by Joule-Thomson effect, and a resistance wire is for Joule heating. The dependence of electrical resistance on temperature has been measured in discrete model with step of 2 K. 3. Results and discussion Fig. 1 shows the XRD patterns of pure and various Au doped Sb3Te after annealed at 250 8C for 1 min. When Au content is less than 4 at.%, it could be found that the crystalline structure of Au–SbTe series material is similar to pure Sb3Te. It indicates that Au doping does not change the type of lattice structure. All the materials are with hexagonal structure, and it accords with the reported research [12]. The peaks of Au–Sb compound would appear in the material with high Au content.
Fig. 2. Measured d(Log R)/dT as a function of temperature at a heating rate of 15 8C/min, where (A) represents pure Sb3Te, (B) 2 at.% Au doped Sb3Te, (C) 4 at.% Au doped Sb3Te and (D) 9 at.% Au doped Sb3Te.
An important effect of Au content is to increase thermal stability. Thermal stability is a basic property of phase-change material, and it will result in better data retention (long-term archival stability) and less thermal crosstalk in the C-RAM devices. In order to increase data retention of the phase-change material, a relatively high crystallization temperature (Tc) is desired [13]. In our research, Tc is obtained from in situ resistance measurement. The crystallization temperature is determined by the minimum in the derivative (d(Log R)/dT) [14]. Fig. 2 shows analyzed curves of d(Log R)/dT as a function of T, where R represents measured resistance and T temperature. Table 1 shows the crystallization temperature of Sb3Te and Au–SbTe series materials. For 2 and 4 at.% Au doped Sb3Te films, the crystallization temperature is 161 and 148 8C, respectively, when heating rate is fixed at 15 8C/min. Nevertheless Tc of pure Sb3Te is only 137 8C which is too low for practical applications. No obvious crystallization has been observed when Au concentration exceeds 9 at.%. The data retention of C-RAM is generally determined from the thermal stability of phase-change material. Therefore, Au– SbTe materials promise better data retention due to the higher Tc which offers better stability against spontaneous recrystallization at room temperature. Obtaining characteristic of data retention in this work is to evaluate the failure time (tf) at a certain annealing temperature [6,15,16]. Here tf is defined as the duration when the resistance of the material drops to 10% the value of amorphous state in annealing process. Resistance of films as a function of time at various temperatures is shown in Fig. 3. It is obvious that the Au–SbTe films have better data retention compared with Sb3Te since longer time needed to complete phase transition at the same temperature. Fig. 4 shows failure time of amorphous 2 at.% Au doped Sb3Te and pure Table 1 Crystallization temperature of various Au–SbTe films with different Au content
Fig. 1. X-ray diffraction patterns of Au–SbTe films on Si (1 0 0) substrate, annealed at 250 8C for 1 min, where (A) represents pure Sb3Te, (B) 2 at.% Au doped Sb3Te, (C) 4 at.% Au doped Sb3Te and (D) 9 at.% Au doped Sb3Te.
Composition
Crystallization temperature (8C)
Sb3Te 2 at.% Au doped Sb3Te 4 at.% Au doped Sb3Te
137 161 148
F. Wang et al. / Applied Surface Science 254 (2008) 2281–2284
2283
Fig. 4. Failure time as a function of temperature.
Fig. 3. Time-dependent electrical resistance at specific temperature: (a) Sb3Te at specific temperature from 135 to 150 8C. (b) 2 at.% Au doped Sb3Te at specific temperature from 135 to 155 8C.
Sb3Te. The failure time is extrapolated to 80 8C by fitting the data to an Arrhenius equation [8]. Although there would be some errors since it is far away from the measured range, the fitted curve indicates that doping Au can increase the failure time by several orders of magnitude at 80 8C. tf of Au–SbTe at 80 8C is above 2.7 1011 s, four orders of magnitude longer than that of pure Sb3Te of which is approximately 4.3 107 s. The reason of choosing 10% criteria is mainly based on the consideration that it will be very difficult to distinguish between low state and high state if the resistance drops to less than 10% of high state value. The result seems smaller than reported one because the different criteria of failure time. For example, tf was defined as the duration for complete crystallization in reported research [15]. Physical stability of film is a parameter which the device reliability is related to. Many factors such as volatilization or stress would result in crack of film. It would deteriorate the device reliability if the phase-change material film cracks. Fig. 5 shows the surfaces of annealed samples observed by SEM. Both of the films have been annealed at 300 8C for 2 min in N2 atmosphere. For Au–SbTe film, the film is with perfect surface, whereas many flaws turn up on the surface of Sb3Te film. This may be explained by release of stress inside the film during the annealing process, for there would be evident change in density of phase-change material during phase-change
Fig. 5. SEM image of film surface, annealed at 300 8C for 2 min. (a) Image of Sb3Te film. (b) Image of Au–SbTe film.
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F. Wang et al. / Applied Surface Science 254 (2008) 2281–2284
identifying the states from high resistance state and low resistance state if the resistance of phase-change film is too low, considering the driver circuit of C-RAM chip is 1 kV. So, the limited content of Au dopant should be 4 at.% in practical applications. 4. Conclusions The properties of Au doped Sb–Te material have been investigated in detail. Good performance can be achieved from Au–SbTe series which combined good data retention with high physical stability and large resistance contrast. For 2 at.% Au doped Sb3Te, better performance has been achieved compared with pure Sb3Te, for Tc increases to 161 8C and the resistance contrast increases to 1.1 104. Failure time of 2 at.% Au doped Sb3Te is above 2.7 1011 s at 80 8C, several orders longer than that of pure one. Physical stability of film has been improved significantly as well. All these indicate that the Au–SbTe would be a suitable phase-change material for practical C-RAM applications. Acknowledgements
Fig. 6. Temperature-dependent electrical resistance of Sb3Te and Au–SbTe films: (a) pure Sb3Te and 2 at.% Au doped Sb3Te films. (b) 4 at.% Au doped Sb3Te and 9 at.% Au doped Sb3Te films.
process [17]. Since doping Au could increase the physical stability of Sb3Te film during phase-change process, Au doped Sb3Te would offer the potential for better reliability. The ratio of amorphous state resistance (Rhigh) and crystalline state resistance (Rlow) is referred to as resistance contrast. In practical applications, the resistance contrast must be large enough to distinguish the resistance of the material within memory chips correctly and easily. For 2 at.% Au doped Sb3Te, the ratio reaches to 1.1 104 with Rhigh 4.6 105 V and Rlow 41 V, an order of magnitude higher than pure Sb3Te with ratio of 6.2 103. Fig. 6 shows the electrical resistance of Au–SbTe film versus temperature measured in vacuum with heating rate of 15 8C/min. Before crystallization, the resistance decreases steadily when the temperature rises. It is obvious that the resistance decreases sharply near crystallization temperature. The resistance becomes not sensitive to temperature after crystallization. Although doping Au into Sb3Te would improve the performance, too high dose of dopant would make the performance just go to the opposite. Considering Tc shown in Fig. 2 and resistance contrast shown in Fig. 6, it is obvious that 2 at.% Au doped Sb3Te has best performance among Au– SbTe series, whereas the performance of 9 at.% Au doped Sb3Te becomes very poor. In addition, too much Au element would lead to very low resistance. It would go against
This work is supported by 973 Program (2007CB935400, 2006CB302700), National High Technology Development Program of China (2006AA03Z360), Science and Technology Council of Shanghai (06QA14060, 06XD14025, 0652nm003, 06DZ22017). References [1] M. Wuttig, C. Steimer, Appl. Phys. A 87 (2007) 411. [2] G. Muller, T. Happ, M. Kund, G.Y. Lee, N. Nagel, R. Sezi, IEEE P. Int. Electron. Devices Meet. (2004) 567. [3] A. Pirovano, A. Lacaita, A. Benvenuti, F. Pellizzer, S. Hudgens, R. Bez, IEEE P. Int. Electron. Devices Meet. (2003) 699. [4] S.G. Alberici, R. Zonca, B. Pashmakov, Appl. Surf. Sci. 231–232 (2004) 821. [5] K. Kao, H. Cheng, C. Jong, C. Lan, T. Chin, IEEE Trans. Mag. 43 (2007) 930. [6] T. Zhang, Z.T. Song, F. Wang, B. Liu, S.L. Feng, B. Chen, Jpn. J. Appl. Phys. 46 (2007) L602. [7] T. Zhang, Z.T. Song, F. Rao, G.M. Feng, B. Liu, S.L. Feng, B. Chen, Jpn. J. Appl. Phys. 46 (2007) L247. [8] M. Lankhorst, L. Pieterson, M. Schijndel, B. Jacobs, J. Rijpers, Jpn. J. Appl. Phys. 42 (2003) 863. [9] E.R. Meinders, M. Lankhorst, Jpn. J. Appl. Phys. 42 (2003) 809. [10] O. Noritake, A. Sumio, N. Naomasa, et al. Jpn. J. Appl. Phys. 41 (2002) 1695. [11] M. Lankhorst, B. Ketelaars, R. Wolters, Nat. Mater. 4 (2005) 347. [12] C. Sun, J. Lee, M. Youm, Y. Kim, Jpn. J. Appl. Phys. 45 (2006) 9157. [13] A. Pirovano, A. Redaelli, F. Pellizzer, F. Ottogalli, M. Tosi, D. Ielmini, A.L. Lacaita, R. Bez, IEEE Trans. Dev. Mater. Reliab. 4 (3) (2004) 422. [14] I. Friedrich, V. Weidenhof, W. Njoroge, P. Franz, M. Wuttig, J. Appl. Phys. 87 (2000) 4130. [15] L. Pieterson, M. Schijndel, J. Rijpers, Appl. Phys. Lett. 83 (2003) 1373. [16] T. Zhang, Z.T. Song, B. Liu, S.L. Feng, B. Chen, Solid-State Electron. 51 (2007) 950. [17] H. Lyeo, D. Cahill, B. Lee, J. Abelson, Appl. Phys. Lett. 89 (2006) 151904.