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Contact resistivity of amorphous and crystalline GeCu2Te3 to W electrode for phase change random access memory
Materials Science in Semiconductor Processing 47 (2016) 1–6
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Contact resistivity of amorphous and crystalline GeCu2Te3 to W electrode for phase change random access memory S. Shindo a, Y. Sutou a,n, J. Koike a, Y. Saito b, Y.-H. Song c a
Department of Materials Science, Tohoku University, 6-6-11 Aoba-yama, Aoba-ku, Sendai 980-8579, Japan Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan c Department of Electronic Engineering, Hanyang University, Seoul 133-791, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 November 2015 Received in revised form 10 February 2016 Accepted 11 February 2016 Available online 18 February 2016
We have investigated the contact resistivity of GeCu2Te3 (GCT) phase change material to a W electrode using the circular transfer length method (CTLM). The contact resistivity ρc of as-deposited amorphous GCT to W was 3.9 10 2 Ω cm2. The value of ρc drastically decreased upon crystallization and crystalline GCT that annealed at 300 °C showed a ρc of 4.8 10 6 Ω cm2. The ρc contrast between amorphous (as-deposited) and crystalline (annealed at 300 °C) states was larger in GCT than in conventional Ge2Sb2Te5 (GST). Consequently, it was suggested from a calculation based on a simple vertical structure memory cell model that a GCT memory cell shows a four times larger resistance contrast than a GST memory cell. & 2016 Elsevier Ltd. All rights reserved.
1. Introduction A new class of non-volatile memory (NVM) has been widely studied in an attempt to overcome the scaling limit of flash memory technology [1]. Phase change random access memory (PCRAM) has attracted much attention as a new class of NVM because of its high scalability, rapid access speed and large resistance contrast [1,2]. In PCRAM, data are recorded as differences in electrical resistance between the high resistance amorphous and low resistance crystalline phases of phase change material (PCM). Phase transition between amorphous and crystalline states can be achieved by way of Joule heating induced by an electrical pulse through electrodes. Ge2Sb2Te5 (GST) has been widely studied for application in PCRAM technology because of its high crystallization speed and the good repeatability of its phase transition [3–6]. However, GST shows a low crystallization temperature of about 160 °C [3], which limits high temperature data retention capability. Recently, Sutou et al. have proposed a GeCu2Te3 (GCT) compound as PCM. GCT has not only a higher crystallization temperature (230 °C) but also a lower melting point (500 °C) than GST (630 °C) [3,7]. Therefore, GCT shows better retention capability and lower power consumption for amorphization than GST [7,8]. In addition, GCT shows a fast phase change speed, comparable to GST [9]. Hence, GCT is a promising PCM to n
Corresponding author. E-mail addresses: [email protected] (S. Shindo), [email protected] (Y. Sutou), [email protected] (Y.-H. Song). http://dx.doi.org/10.1016/j.mssp.2016.02.006 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
realize PCRAM with high performance and reliability. With the scaling down of PCRAM cells, contact resistance between PCM and an electrode becomes a dominant factor in determining the memory cell resistance [10,11]. Kang et al. have investigated the contact resistance of TiN/GeSb2Te4 using the transfer length method (TLM) and found that reduction of the contact resistance leads to improvement of the cycling endurance and the fluctuations in memory cell performance such as threshold voltage [12]. Roy et al. measured the contact resistivity of GST and doped Sb2Te to a Ti30W70 (TiW) electrode using a circular transfer length method (CTLM) and a cross-bridge Kelvin resistor (CBKR) configuration [13–16]. They demonstrated that the contact resistivity depended on the annealing temperature, applied voltage and the thickness of the GST and doped Sb2Te. Moreover, Chua et al. investigated the contact resistivity of GeTe to various electrodes (W, Ni, TiW) using CTLM and found that the contact resistivity was dependent on the work function of electrode materials and that higher work function produced lower contact resistivity [17]. More recently, Hwang et al. demonstrated in GST and GeTe nanowire phase change memory that large contact resistance resulted in a reduction of the reset current [18]. Thus, in future PCRAM technology, the contact resistance will become an essential issue. However, the contact resistivity of GCT has not yet been studied. In this work, we report the contact resistivity of GCT to a W electrode in amorphous and crystalline states using CTLM. Based on the obtained contact resistivity, we discuss the effect of the contact resistivity on the resistance contrast of a simple GCT memory cell.
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2. Experimental 2.1. Film preparation A GCT film with a thickness of 200 nm was deposited on a SiO2 (100 nm)/Si substrate by RF co-sputtering of GeTe and CuTe targets [19]. A GST film was also prepared by sputtering a Ge2Sb2Te5 alloy target for comparison. In this study, prior to fabrication of CTLM patterned samples, the as-deposited amorphous GCT and GST films were heated up to a predetermined temperature in an Ar atmosphere at a heating rate of 10 °C min 1 in order to investigate the contact resistivity in amorphous and crystalline states. In the paper, this process is called “annealing”. In this study, the following four conditions were selected as annealing conditions: (1) temperatures before crystallization, (2) temperatures just after crystallization, (3) higher temperatures than the crystallization temperature and (4) 300 °C. The annealing temperatures for the GCT and GST films were individually selected based on the temperature dependence of the resistance obtained by two-point probe measurement [8]. Fig. 1 shows the temperature dependence of the resistance in the GCT and GST films. It can be seen that the GCT film has a crystallization temperature (Tc) of about 235 °C, while the GST film has a Tc of about 185 °C. Based on these results, we selected the annealing temperatures (1) 230 °C, (2) 250 °C, (3) 270 °C and (4) 300 °C for the GCT film and (1) 150 °C, (2) 188 °C, (3) 250 °C and (4) 300 °C for the GST film. 2.2. CTLM sample preparation In this study, CTLM was used to measure the contact resistivity of GCT to a W electrode. CTLM has the following advantages: (1) the current crowding effect can be eliminated [20], (2) the CTLM pattern can be easily fabricated by lithography [14], and (3) this method can be applied for both Ohmic and Schottky contacts [21]. Fig. 2(a) shows a schematic diagram of the CTLM pattern, where EA is the inner W electrode and EB is the outer W electrode. The fabrication methods for CTLM patterned sample are as follows: Photolithography was employed to fabricate the CTLM pattern on the as-deposited and the annealed GCT and GST films. Since the maximum process temperature (110 °C) was much lower than the crystallization temperature of the GCT and GST films [8], crystallization of the as-deposited films could be prevented during
Fig. 2. (a) Schematic diagram of CTLM pattern. EA is the inner W electrode, EB is the outer W electrode. (b) CTLM patterns with L ¼ 200 mm and various gap spacing d of 10–45 mm. L is the radius of EA and d is the gap spacing between EA and EB.
the lithography process. The native surface oxide layer of the GCT and GST films was removed by reverse sputtering to a depth of 30 nm and then a W electrode was deposited with a thickness of 200 nm by sputtering in the same chamber. Finally, a CTLM pattern with a W electrode, as shown in Fig. 2(a), was obtained by removing the photoresist layer. Additionally, the entire surface of each CTLM sample was cleaned by reverse sputtering and then coated with a carbon layer (a few nm) to prevent the surface oxidation of films. 2.3. CTLM measurement Fig. 2(b) shows the top view of the actual fabricated CTLM pattern observed with an optical microscope. The radius L of the inner circle electrode was fixed to 200 mm and the gap spacing d was varied from 10 to 45 mm, where the actual sizes of L and d were measured with the optical microscope. To evaluate the contact resistivity, current-voltage (I-V) measurements were carried out using the four-point probe method with a semiconductor parameter analyzer (Agilent, 4155 C) at room temperature. Meanwhile, for the case of amorphous GST, I-V measurements were done using a two-point probe method because the resistance of the amorphous GST was too high (more than 106 Ω) to be measured by the four-point probe method with the semiconductor parameter analyzer. From I-V results, the measured resistance RM between EA and EB for the CTLM pattern was calculated. The RM can be described as [22]:
Fig. 1. Temperature dependence of the resistance in the GCT and GST films obtained by two-point probe measurement in Ar atmosphere.
RM =
Rsh (d + 2L t )C , 2π L
(1)
S. Shindo et al. / Materials Science in Semiconductor Processing 47 (2016) 1–6
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Fig. 3. Measured resistance RM of (a) as-deposited GCT and (b) as-deposited GST CTLM patterns as a function of gap spacing d, before correction (filled circles) and after correction (open triangles).
where Rsh is the sheet resistance of the PCM layer, Lt is the transfer length and C is the correction factor described as [22]:
C=
L ⎛ d⎞ ln⎜1 + ⎟. d ⎝ L⎠
(2)
RM as a function of the gap spacing d shows a non-linear dependence due to the circular shape of the electrode. The obtained curve can be corrected into a linear curve by dividing RM by C to evaluate the contact resistivity. Fig. 3 shows the measured resistance RM of (a) as-deposited GCT and (b) as-deposited GST as a function of gap spacing d, before correction (filled circles) and after correction (open triangles). The transfer length Lt and the sheet resistance Rsh were obtained from the x-intercept and the slope of the fitting line, respectively. The contact resistivity, ρc, of the PCM to the electrode and the resistivity, ρ, of the PCM were calculated by the following equations:
ρc = Rsh⋅L t2,
(3)
ρ = Rsh⋅t,
(4)
where t is the film thickness of the PCM layer. In this study, the film thickness t was measured using an atomic force microscope (AFM).
3. Results Fig. 4 shows the annealing temperature dependence of the resistivity ρ of the GCT and GST films obtained by CTLM at room temperature. The ρ of the as-deposited amorphous GST film is 2.2 103 Ω cm and shows a sharp drop upon crystallization to a metastable fcc structure, which is similar to that obtained by in situ resistance measurements as shown in Fig. 1 [8,23]. Even after crystallization, ρ decreases continuously with increasing annealing temperature due to the strong annealing temperature dependence of fcc-GST [24]. The ρ contrast between the as-deposited amorphous GST and the crystalline GST annealed at 300 °C is 4.6 104. The ρ of the as-deposited amorphous GCT film is 2.0 Ω cm, which is about three orders of magnitude lower than that of the as-deposited GST film. Similar to the GST film, the ρ of the GCT film drops due to crystallization at around 235 °C. After
Fig. 4. Annealing temperature dependence of the resistivity of the GCT and GST films obtained by CTLM measurement at room temperature.
crystallization, the ρ of the crystalline GCT film varies little with increasing annealing temperature. This tendency is similar to that obtained by in situ resistance measurement (Fig. 1) [8]. The ρ contrast between the amorphous GCT and the crystalline GCT annealed at 300 °C is 2.7 103 which is about one order of magnitude smaller than that of the GST film. Fig. 5 shows the annealing temperature dependence on ρc for the GCT and the GST films to the W electrode. The ρc of the asdeposited amorphous GST is 3.2 10 2 Ω cm2 and shows a sharp decrease due to crystallization at around 185 °C, and then a gradual decrease with increasing annealing temperature. This tendency is similar to the reported results of ρc for GST to a TiW electrode obtained by CTLM and CBKR measurements [14,15]. The ρc contrast upon crystallization is 2.3 103, 20 times smaller than the ρ contrast. Meanwhile, the ρc of as-deposited amorphous GCT is 3.9 10 2 Ω cm2 followed by a sharp drop due to crystallization at around 235 °C. After crystallization, the ρc of crystalline GCT varies little with increasing annealing temperature. The ρc
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Fig. 6. Simple cell model for the calculation of the total resistance in a PCRAM cell.
Fig. 5. Annealing temperature dependence of contact resistivity of GCT and GST films to the W electrode obtained by CTLM measurement at room temperature. Table 1 Resistivity and contact resistivity of PCMs to W measured by CTLM. PCM GeCu2Te3
Ge2Sb2Te5
Annealing temperature [°C] ρ [Ω cm]
ρc [Ω cm2]
0
2
As-deposited 230 250 270 300 As-deposited 150 188 250 300
2.0 10 4.9 10 1 1.3 10 3 1.0 10 3 7.5 10 4 2.2 103 2.9 103 2.9 10° 2.0 10 1 4.8 10 2
3.9 10 1.2 10 2 3.8 10 6 2.8 10 6 4.8 10 6 3.2 10 2 5.2 10 2 6.1 10 4 2.7 10 5 1.4 10 5
γ ¼ρc/ρ [cm] 2.0 10 2 2.4 10 2 2.9 10 3 2.8 10 3 6.4 10 3 1.5 10 5 1.8 10 5 2.1 10 4 1.4 10 4 2.9 10 4
contrast upon crystallization is 8.1 103, 3 times larger than the ρ contrast. The experimental data is summarized in Table 1. Here, the ρc /ρ ratio is defined as γ, a dominance factor of contact resistivity. As shown in Table 1, the γ of GCT is larger than that of GST in both amorphous and crystalline states. This means that the contact resistance is more dominant in the total resistance of the GCT memory cell than in that of the GST memory cell. Moreover, the γ of amorphous GCT is higher than that of crystalline GCT, while the γ of amorphous GST is lower than that of crystalline GST. That is, it is suggested that the contribution of the contact resistance to the total resistance of the GCT memory cell in the amorphous state is larger than that of the GCT memory cell in the crystalline state.
4. Discussion Based on the obtained results of ρ and ρc for GCT and GST films with the W electrode, the contribution of the contact resistance to the total resistance of PCRAM cell can be evaluated. Generally, it is known that in a real PCRAM cell, only a partial region of PCM layer at around the bottom electrode contact exhibits phase transition and shows a dome-like shape. And, the size of the dome-like shaped active region depends on the PCM layer thickness and the cell structure [25–28]. In this case, we need to consider the contact resistance only between PCM and bottom electrode to evaluate the contribution of the contact resistance to the total resistance of
PCRAM cell, although we have to additionally consider the contact resistance between dome-like shaped amorphous phase and the crystalline matrix of PCM. Meanwhile, in an advanced PCRAM cell structure, e.g., the 20 nm confined cell structure, the entire PCM layer transforms from the crystalline to amorphous state and vice versa, which significantly improves the number of endurance cycles [29]. In this case, we have to consider the contact resistance between both the top electrode/PCM and bottom electrode/PCM interfaces to evaluate the contribution of the contact resistance to the total resistance of PCRAM cell. In this paper, we assumed that a simple memory cell model composed of PCM and electrodes as shown in Fig. 6 and that the whole area of the PCM layer reversibly transforms between amorphous and crystalline states. The total resistance of the PCRAM cell, RT, can be calculated by the following equation:
RT = RPCM + 2R C = ρ⋅
2ρ t + c, A A
(5)
where RPCM and ρ are the resistance and the resistivity of the PCM; RC and ρc are the contact resistance and the contact resistivity between PCM and the W electrode; and t is the PCM thickness; A is the contact area of the PCM and the W electrode. The resistance of the W electrode layers can be ignored because of its very low value. Eq. (5) can be rewritten to calculate the proportion of the contact resistance, 2RC, to the total resistance, RT,
2R C = RT
2ρc A ρ⋅t A
+
2ρc A
=
1 t ρ ⋅ 2 ρc
+1
=
1 t 2γ
+1
. (6)
The 2RC /RT ratio is described only with t and γ. Fig. 7(a) shows the dependence of 2RC /RT on t for the amorphous and crystalline states of GCT and GST. In this calculation, the ρ and ρc values of the as-deposited and annealed films at 300 °C are used for amorphous and crystalline states, respectively, for the following reason. It is noted that GST used in this study showed crystallization to a metastable fcc structure at around 185 °C followed by crystal structural change to a stable hcp at around 350 °C [8]. Here, it is known that crystalline GST in the phase change area of actual PCRAM cells shows a metastable fcc structure, not a stable hcp structure [25]. Therefore, the values at 300 °C in Table 1 are taken for the crystalline state of the fcc structure. In the GST cell, RT is dominated by RC in the region where to1 mm. This trend for the GST cell has been already pointed out by Huang et al. [10,11]. Meanwhile, in the GCT cell, it is found that the contribution of RC to RT is much larger than in the GST cell and consequently, RT is fully dominated by RC in the region of
S. Shindo et al. / Materials Science in Semiconductor Processing 47 (2016) 1–6
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Fig. 7. (a) Proportion of the contact resistance RC to the total resistance RT of GCT and GST cells as a function of PCM thickness t. (b) The proportion of the RC to the RT as a function of γ a t ¼ 100 nm.
to 0.1 mm. This is due to γ being larger in the GCT film than in the GST film, as shown in Table 1. It is found that the contribution of the RC in the RT of amorphous GCT is larger than that of crystalline GCT at any given thickness because of the higher γ of the amorphous GCT than the crystalline GCT. Fig. 7(b) shows the ratio of 2RC to RT as a function of γ at the PCM thickness t ¼100 nm that is the PCM layer thickness in conventional PCRAM devices [25–27]. It is seen that in the GCT cell, RT is dominated by RC in both amorphous and crystalline states. On the other hand, in the GST cell, RT is fully dominated by RC in the crystalline state, while the proportion of the RC to the RT is no more than 75% in the amorphous state because of its lower γ. The figure indicates that RT is fully dominated by the contact resistance when γ is larger than 10 4 cm, while RT is fully dominated by the resistance of PCM itself when γ is lower than 10 7 cm. Next, we evaluate the resistance contrast of PCRAM cells. The resistance contrast is calculated by the following equation:
ρamo ⋅t + 2ρcamo RTamo = . RTcry ρcry ⋅t + 2ρccry
(7)
The superscripts in the equation represent the amorphous and crystalline states. The equation indicates that the resistance contrast is dominated by ρamo /ρcry for a thick film and by ρcamo /ρccry for a thin film. Fig. 8 shows the scaling effect of the resistance contrast of GCT and GST cells. The resistance contrast of the GST cell is 17 times larger than that of the GCT cell at t¼ 10 2 m because of the dominance by the ρ of PCM for a thick film. It is seen that the resistance contrast of the GST cell decreases with decreasing t, whereas that of the GCT cell increases with decreasing t. This is because of the dominance by the ρc of W/PCM for a thin film. Consequently, the resistance contrast of the GCT cell becomes 4 times larger than that of the GST cell at to10 nm for a highly scaled PCRAM cell size. This result indicates that the GCT cell is expected to show better accuracy in data reading than the GST cell in highly scaled PCRAM cells.
5. Conclusion We investigated the contact resistivity, ρc, of GCT and GST to W electrode using CTLM. The ρc of W/amorphous GCT (as-deposited) was 3.9 10 2 Ω cm2, whereas that of W/crystalline GCT
Fig. 8. Scaling effect of the resistance contrast of GCT and GST cells.
(annealed at 300 °C) was 4.8 10 6 Ω cm2. The ρc of W/amorphous GST (as-deposited) was 3.2 10 2 Ω cm2, whereas that of W/crystalline GST (annealed at 300 °C) was 1.4 10 5 Ω cm2. From a simple calculation based on the obtained results, the resistance contrast of the GCT cell is expected to be about four orders of magnitude, which is four times larger than that of the GST cell. Such a large resistance contrast in the GCT cell is desirable for better accuracy in data reading operation.
Acknowledgement The authors thank Dr. P. Fons of AIST, Japan, for help in the preparation of the manuscript. This work was supported by KAKENHI (Grant no. 15H04113) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, CT and Future Planning (NRF2015R1A2A2A01007289).
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Contact resistivity of amorphous and crystalline GeCu2Te3 to W electrode for phase change random access memory S. Shindo a, Y. Sutou a,n, J. Koike a, Y. Saito b, Y.-H. Song c a
Department of Materials Science, Tohoku University, 6-6-11 Aoba-yama, Aoba-ku, Sendai 980-8579, Japan Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan c Department of Electronic Engineering, Hanyang University, Seoul 133-791, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 November 2015 Received in revised form 10 February 2016 Accepted 11 February 2016 Available online 18 February 2016
We have investigated the contact resistivity of GeCu2Te3 (GCT) phase change material to a W electrode using the circular transfer length method (CTLM). The contact resistivity ρc of as-deposited amorphous GCT to W was 3.9 10 2 Ω cm2. The value of ρc drastically decreased upon crystallization and crystalline GCT that annealed at 300 °C showed a ρc of 4.8 10 6 Ω cm2. The ρc contrast between amorphous (as-deposited) and crystalline (annealed at 300 °C) states was larger in GCT than in conventional Ge2Sb2Te5 (GST). Consequently, it was suggested from a calculation based on a simple vertical structure memory cell model that a GCT memory cell shows a four times larger resistance contrast than a GST memory cell. & 2016 Elsevier Ltd. All rights reserved.
1. Introduction A new class of non-volatile memory (NVM) has been widely studied in an attempt to overcome the scaling limit of flash memory technology [1]. Phase change random access memory (PCRAM) has attracted much attention as a new class of NVM because of its high scalability, rapid access speed and large resistance contrast [1,2]. In PCRAM, data are recorded as differences in electrical resistance between the high resistance amorphous and low resistance crystalline phases of phase change material (PCM). Phase transition between amorphous and crystalline states can be achieved by way of Joule heating induced by an electrical pulse through electrodes. Ge2Sb2Te5 (GST) has been widely studied for application in PCRAM technology because of its high crystallization speed and the good repeatability of its phase transition [3–6]. However, GST shows a low crystallization temperature of about 160 °C [3], which limits high temperature data retention capability. Recently, Sutou et al. have proposed a GeCu2Te3 (GCT) compound as PCM. GCT has not only a higher crystallization temperature (230 °C) but also a lower melting point (500 °C) than GST (630 °C) [3,7]. Therefore, GCT shows better retention capability and lower power consumption for amorphization than GST [7,8]. In addition, GCT shows a fast phase change speed, comparable to GST [9]. Hence, GCT is a promising PCM to n
Corresponding author. E-mail addresses: [email protected] (S. Shindo), [email protected] (Y. Sutou), [email protected] (Y.-H. Song). http://dx.doi.org/10.1016/j.mssp.2016.02.006 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
realize PCRAM with high performance and reliability. With the scaling down of PCRAM cells, contact resistance between PCM and an electrode becomes a dominant factor in determining the memory cell resistance [10,11]. Kang et al. have investigated the contact resistance of TiN/GeSb2Te4 using the transfer length method (TLM) and found that reduction of the contact resistance leads to improvement of the cycling endurance and the fluctuations in memory cell performance such as threshold voltage [12]. Roy et al. measured the contact resistivity of GST and doped Sb2Te to a Ti30W70 (TiW) electrode using a circular transfer length method (CTLM) and a cross-bridge Kelvin resistor (CBKR) configuration [13–16]. They demonstrated that the contact resistivity depended on the annealing temperature, applied voltage and the thickness of the GST and doped Sb2Te. Moreover, Chua et al. investigated the contact resistivity of GeTe to various electrodes (W, Ni, TiW) using CTLM and found that the contact resistivity was dependent on the work function of electrode materials and that higher work function produced lower contact resistivity [17]. More recently, Hwang et al. demonstrated in GST and GeTe nanowire phase change memory that large contact resistance resulted in a reduction of the reset current [18]. Thus, in future PCRAM technology, the contact resistance will become an essential issue. However, the contact resistivity of GCT has not yet been studied. In this work, we report the contact resistivity of GCT to a W electrode in amorphous and crystalline states using CTLM. Based on the obtained contact resistivity, we discuss the effect of the contact resistivity on the resistance contrast of a simple GCT memory cell.
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2. Experimental 2.1. Film preparation A GCT film with a thickness of 200 nm was deposited on a SiO2 (100 nm)/Si substrate by RF co-sputtering of GeTe and CuTe targets [19]. A GST film was also prepared by sputtering a Ge2Sb2Te5 alloy target for comparison. In this study, prior to fabrication of CTLM patterned samples, the as-deposited amorphous GCT and GST films were heated up to a predetermined temperature in an Ar atmosphere at a heating rate of 10 °C min 1 in order to investigate the contact resistivity in amorphous and crystalline states. In the paper, this process is called “annealing”. In this study, the following four conditions were selected as annealing conditions: (1) temperatures before crystallization, (2) temperatures just after crystallization, (3) higher temperatures than the crystallization temperature and (4) 300 °C. The annealing temperatures for the GCT and GST films were individually selected based on the temperature dependence of the resistance obtained by two-point probe measurement [8]. Fig. 1 shows the temperature dependence of the resistance in the GCT and GST films. It can be seen that the GCT film has a crystallization temperature (Tc) of about 235 °C, while the GST film has a Tc of about 185 °C. Based on these results, we selected the annealing temperatures (1) 230 °C, (2) 250 °C, (3) 270 °C and (4) 300 °C for the GCT film and (1) 150 °C, (2) 188 °C, (3) 250 °C and (4) 300 °C for the GST film. 2.2. CTLM sample preparation In this study, CTLM was used to measure the contact resistivity of GCT to a W electrode. CTLM has the following advantages: (1) the current crowding effect can be eliminated [20], (2) the CTLM pattern can be easily fabricated by lithography [14], and (3) this method can be applied for both Ohmic and Schottky contacts [21]. Fig. 2(a) shows a schematic diagram of the CTLM pattern, where EA is the inner W electrode and EB is the outer W electrode. The fabrication methods for CTLM patterned sample are as follows: Photolithography was employed to fabricate the CTLM pattern on the as-deposited and the annealed GCT and GST films. Since the maximum process temperature (110 °C) was much lower than the crystallization temperature of the GCT and GST films [8], crystallization of the as-deposited films could be prevented during
Fig. 2. (a) Schematic diagram of CTLM pattern. EA is the inner W electrode, EB is the outer W electrode. (b) CTLM patterns with L ¼ 200 mm and various gap spacing d of 10–45 mm. L is the radius of EA and d is the gap spacing between EA and EB.
the lithography process. The native surface oxide layer of the GCT and GST films was removed by reverse sputtering to a depth of 30 nm and then a W electrode was deposited with a thickness of 200 nm by sputtering in the same chamber. Finally, a CTLM pattern with a W electrode, as shown in Fig. 2(a), was obtained by removing the photoresist layer. Additionally, the entire surface of each CTLM sample was cleaned by reverse sputtering and then coated with a carbon layer (a few nm) to prevent the surface oxidation of films. 2.3. CTLM measurement Fig. 2(b) shows the top view of the actual fabricated CTLM pattern observed with an optical microscope. The radius L of the inner circle electrode was fixed to 200 mm and the gap spacing d was varied from 10 to 45 mm, where the actual sizes of L and d were measured with the optical microscope. To evaluate the contact resistivity, current-voltage (I-V) measurements were carried out using the four-point probe method with a semiconductor parameter analyzer (Agilent, 4155 C) at room temperature. Meanwhile, for the case of amorphous GST, I-V measurements were done using a two-point probe method because the resistance of the amorphous GST was too high (more than 106 Ω) to be measured by the four-point probe method with the semiconductor parameter analyzer. From I-V results, the measured resistance RM between EA and EB for the CTLM pattern was calculated. The RM can be described as [22]:
Fig. 1. Temperature dependence of the resistance in the GCT and GST films obtained by two-point probe measurement in Ar atmosphere.
RM =
Rsh (d + 2L t )C , 2π L
(1)
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Fig. 3. Measured resistance RM of (a) as-deposited GCT and (b) as-deposited GST CTLM patterns as a function of gap spacing d, before correction (filled circles) and after correction (open triangles).
where Rsh is the sheet resistance of the PCM layer, Lt is the transfer length and C is the correction factor described as [22]:
C=
L ⎛ d⎞ ln⎜1 + ⎟. d ⎝ L⎠
(2)
RM as a function of the gap spacing d shows a non-linear dependence due to the circular shape of the electrode. The obtained curve can be corrected into a linear curve by dividing RM by C to evaluate the contact resistivity. Fig. 3 shows the measured resistance RM of (a) as-deposited GCT and (b) as-deposited GST as a function of gap spacing d, before correction (filled circles) and after correction (open triangles). The transfer length Lt and the sheet resistance Rsh were obtained from the x-intercept and the slope of the fitting line, respectively. The contact resistivity, ρc, of the PCM to the electrode and the resistivity, ρ, of the PCM were calculated by the following equations:
ρc = Rsh⋅L t2,
(3)
ρ = Rsh⋅t,
(4)
where t is the film thickness of the PCM layer. In this study, the film thickness t was measured using an atomic force microscope (AFM).
3. Results Fig. 4 shows the annealing temperature dependence of the resistivity ρ of the GCT and GST films obtained by CTLM at room temperature. The ρ of the as-deposited amorphous GST film is 2.2 103 Ω cm and shows a sharp drop upon crystallization to a metastable fcc structure, which is similar to that obtained by in situ resistance measurements as shown in Fig. 1 [8,23]. Even after crystallization, ρ decreases continuously with increasing annealing temperature due to the strong annealing temperature dependence of fcc-GST [24]. The ρ contrast between the as-deposited amorphous GST and the crystalline GST annealed at 300 °C is 4.6 104. The ρ of the as-deposited amorphous GCT film is 2.0 Ω cm, which is about three orders of magnitude lower than that of the as-deposited GST film. Similar to the GST film, the ρ of the GCT film drops due to crystallization at around 235 °C. After
Fig. 4. Annealing temperature dependence of the resistivity of the GCT and GST films obtained by CTLM measurement at room temperature.
crystallization, the ρ of the crystalline GCT film varies little with increasing annealing temperature. This tendency is similar to that obtained by in situ resistance measurement (Fig. 1) [8]. The ρ contrast between the amorphous GCT and the crystalline GCT annealed at 300 °C is 2.7 103 which is about one order of magnitude smaller than that of the GST film. Fig. 5 shows the annealing temperature dependence on ρc for the GCT and the GST films to the W electrode. The ρc of the asdeposited amorphous GST is 3.2 10 2 Ω cm2 and shows a sharp decrease due to crystallization at around 185 °C, and then a gradual decrease with increasing annealing temperature. This tendency is similar to the reported results of ρc for GST to a TiW electrode obtained by CTLM and CBKR measurements [14,15]. The ρc contrast upon crystallization is 2.3 103, 20 times smaller than the ρ contrast. Meanwhile, the ρc of as-deposited amorphous GCT is 3.9 10 2 Ω cm2 followed by a sharp drop due to crystallization at around 235 °C. After crystallization, the ρc of crystalline GCT varies little with increasing annealing temperature. The ρc
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Fig. 6. Simple cell model for the calculation of the total resistance in a PCRAM cell.
Fig. 5. Annealing temperature dependence of contact resistivity of GCT and GST films to the W electrode obtained by CTLM measurement at room temperature. Table 1 Resistivity and contact resistivity of PCMs to W measured by CTLM. PCM GeCu2Te3
Ge2Sb2Te5
Annealing temperature [°C] ρ [Ω cm]
ρc [Ω cm2]
0
2
As-deposited 230 250 270 300 As-deposited 150 188 250 300
2.0 10 4.9 10 1 1.3 10 3 1.0 10 3 7.5 10 4 2.2 103 2.9 103 2.9 10° 2.0 10 1 4.8 10 2
3.9 10 1.2 10 2 3.8 10 6 2.8 10 6 4.8 10 6 3.2 10 2 5.2 10 2 6.1 10 4 2.7 10 5 1.4 10 5
γ ¼ρc/ρ [cm] 2.0 10 2 2.4 10 2 2.9 10 3 2.8 10 3 6.4 10 3 1.5 10 5 1.8 10 5 2.1 10 4 1.4 10 4 2.9 10 4
contrast upon crystallization is 8.1 103, 3 times larger than the ρ contrast. The experimental data is summarized in Table 1. Here, the ρc /ρ ratio is defined as γ, a dominance factor of contact resistivity. As shown in Table 1, the γ of GCT is larger than that of GST in both amorphous and crystalline states. This means that the contact resistance is more dominant in the total resistance of the GCT memory cell than in that of the GST memory cell. Moreover, the γ of amorphous GCT is higher than that of crystalline GCT, while the γ of amorphous GST is lower than that of crystalline GST. That is, it is suggested that the contribution of the contact resistance to the total resistance of the GCT memory cell in the amorphous state is larger than that of the GCT memory cell in the crystalline state.
4. Discussion Based on the obtained results of ρ and ρc for GCT and GST films with the W electrode, the contribution of the contact resistance to the total resistance of PCRAM cell can be evaluated. Generally, it is known that in a real PCRAM cell, only a partial region of PCM layer at around the bottom electrode contact exhibits phase transition and shows a dome-like shape. And, the size of the dome-like shaped active region depends on the PCM layer thickness and the cell structure [25–28]. In this case, we need to consider the contact resistance only between PCM and bottom electrode to evaluate the contribution of the contact resistance to the total resistance of
PCRAM cell, although we have to additionally consider the contact resistance between dome-like shaped amorphous phase and the crystalline matrix of PCM. Meanwhile, in an advanced PCRAM cell structure, e.g., the 20 nm confined cell structure, the entire PCM layer transforms from the crystalline to amorphous state and vice versa, which significantly improves the number of endurance cycles [29]. In this case, we have to consider the contact resistance between both the top electrode/PCM and bottom electrode/PCM interfaces to evaluate the contribution of the contact resistance to the total resistance of PCRAM cell. In this paper, we assumed that a simple memory cell model composed of PCM and electrodes as shown in Fig. 6 and that the whole area of the PCM layer reversibly transforms between amorphous and crystalline states. The total resistance of the PCRAM cell, RT, can be calculated by the following equation:
RT = RPCM + 2R C = ρ⋅
2ρ t + c, A A
(5)
where RPCM and ρ are the resistance and the resistivity of the PCM; RC and ρc are the contact resistance and the contact resistivity between PCM and the W electrode; and t is the PCM thickness; A is the contact area of the PCM and the W electrode. The resistance of the W electrode layers can be ignored because of its very low value. Eq. (5) can be rewritten to calculate the proportion of the contact resistance, 2RC, to the total resistance, RT,
2R C = RT
2ρc A ρ⋅t A
+
2ρc A
=
1 t ρ ⋅ 2 ρc
+1
=
1 t 2γ
+1
. (6)
The 2RC /RT ratio is described only with t and γ. Fig. 7(a) shows the dependence of 2RC /RT on t for the amorphous and crystalline states of GCT and GST. In this calculation, the ρ and ρc values of the as-deposited and annealed films at 300 °C are used for amorphous and crystalline states, respectively, for the following reason. It is noted that GST used in this study showed crystallization to a metastable fcc structure at around 185 °C followed by crystal structural change to a stable hcp at around 350 °C [8]. Here, it is known that crystalline GST in the phase change area of actual PCRAM cells shows a metastable fcc structure, not a stable hcp structure [25]. Therefore, the values at 300 °C in Table 1 are taken for the crystalline state of the fcc structure. In the GST cell, RT is dominated by RC in the region where to1 mm. This trend for the GST cell has been already pointed out by Huang et al. [10,11]. Meanwhile, in the GCT cell, it is found that the contribution of RC to RT is much larger than in the GST cell and consequently, RT is fully dominated by RC in the region of
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Fig. 7. (a) Proportion of the contact resistance RC to the total resistance RT of GCT and GST cells as a function of PCM thickness t. (b) The proportion of the RC to the RT as a function of γ a t ¼ 100 nm.
to 0.1 mm. This is due to γ being larger in the GCT film than in the GST film, as shown in Table 1. It is found that the contribution of the RC in the RT of amorphous GCT is larger than that of crystalline GCT at any given thickness because of the higher γ of the amorphous GCT than the crystalline GCT. Fig. 7(b) shows the ratio of 2RC to RT as a function of γ at the PCM thickness t ¼100 nm that is the PCM layer thickness in conventional PCRAM devices [25–27]. It is seen that in the GCT cell, RT is dominated by RC in both amorphous and crystalline states. On the other hand, in the GST cell, RT is fully dominated by RC in the crystalline state, while the proportion of the RC to the RT is no more than 75% in the amorphous state because of its lower γ. The figure indicates that RT is fully dominated by the contact resistance when γ is larger than 10 4 cm, while RT is fully dominated by the resistance of PCM itself when γ is lower than 10 7 cm. Next, we evaluate the resistance contrast of PCRAM cells. The resistance contrast is calculated by the following equation:
ρamo ⋅t + 2ρcamo RTamo = . RTcry ρcry ⋅t + 2ρccry
(7)
The superscripts in the equation represent the amorphous and crystalline states. The equation indicates that the resistance contrast is dominated by ρamo /ρcry for a thick film and by ρcamo /ρccry for a thin film. Fig. 8 shows the scaling effect of the resistance contrast of GCT and GST cells. The resistance contrast of the GST cell is 17 times larger than that of the GCT cell at t¼ 10 2 m because of the dominance by the ρ of PCM for a thick film. It is seen that the resistance contrast of the GST cell decreases with decreasing t, whereas that of the GCT cell increases with decreasing t. This is because of the dominance by the ρc of W/PCM for a thin film. Consequently, the resistance contrast of the GCT cell becomes 4 times larger than that of the GST cell at to10 nm for a highly scaled PCRAM cell size. This result indicates that the GCT cell is expected to show better accuracy in data reading than the GST cell in highly scaled PCRAM cells.
5. Conclusion We investigated the contact resistivity, ρc, of GCT and GST to W electrode using CTLM. The ρc of W/amorphous GCT (as-deposited) was 3.9 10 2 Ω cm2, whereas that of W/crystalline GCT
Fig. 8. Scaling effect of the resistance contrast of GCT and GST cells.
(annealed at 300 °C) was 4.8 10 6 Ω cm2. The ρc of W/amorphous GST (as-deposited) was 3.2 10 2 Ω cm2, whereas that of W/crystalline GST (annealed at 300 °C) was 1.4 10 5 Ω cm2. From a simple calculation based on the obtained results, the resistance contrast of the GCT cell is expected to be about four orders of magnitude, which is four times larger than that of the GST cell. Such a large resistance contrast in the GCT cell is desirable for better accuracy in data reading operation.
Acknowledgement The authors thank Dr. P. Fons of AIST, Japan, for help in the preparation of the manuscript. This work was supported by KAKENHI (Grant no. 15H04113) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, CT and Future Planning (NRF2015R1A2A2A01007289).
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