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Dependence of magnetic tunnel junction properties on tunnel barrier roughness
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 304 (2006) e300–e302 www.elsevier.com/locate/jmmm
Dependence of magnetic tunnel junction properties on tunnel barrier roughness Jang Roh Rhee Department of Physics, Sookmyung Women’s University, Seoul 140-742, South Korea Available online 28 February 2006
Abstract Magnetic tunnel junctions (MTJs) consisting of Si/SiO2/Ta/Ru/IrMn/CoFe/Ru/CoFe/Al-O/CoFe/NiFe/Ru with different surface roughness of bottom electrode were prepared, and the dependence of tunneling magnetoresistance (TMR) ratio and resistance area product (RA) on surface roughness of tunnel barrier and the tunneling characteristics of these junction devices were investigated. The MTJ with rough tunnel barrier of 12 A˚ root mean square roughness (Rrms) showed TMR ratio of 4% and RA of 2.2 kO mm2. In contrast, the MTJ with uniform tunnel barrier of Rrms ¼ 3 A˚ showed TMR ratio of 40% and RA of 14 kO mm2. As MTJs had more uniform tunnel barrier, TMR ratio and resistance were higher, while the interlayer coupling field decreased. It was confirmed that the smooth surface of bottom electrode was a basic requirement for MTJs. r 2006 Elsevier B.V. All rights reserved. PACS: 73.40.Gk; 85.75.Dd; 75.75.+a Keywords: Magnetic tunnel junction; Tunneling magnetoresistance; Tunnel barrier; Barrier width; Barrier height
Magnetic tunnel junction (MTJ) is attractive for use as bit cells for nonvolatile magnetic random access memory (MRAM) due to their large tunneling magnetoresistance (TMR) ratio at room temperature [1,2]. The MTJ consists of two ferromagnetic (FM) electrodes separated by a tunnel barrier that exhibits TMR due to spin polarized tunneling [3]. The bottom FM electrode is an important factor in drawing out the desirable performance of MTJ cells, such as high TMR ratio, good resistance uniformity, high breakdown voltage, and weak bias dependence of TMR. Of several microstructural features related with the bottom electrode, the surface roughness is considered one of the most critical ones. In this study, MTJs of Si/SiO2/Ta/Ru/IrMn/CoFe/ Ru/ CoFe/Al-O/CoFe/NiFe/Ru using antiferromagnetic Ir22Mn78 as the pinning layer and synthetic antiferromagnetic CoFe/Ru/CoFe as the pinned layer with different surface roughness of the bottom electrode are prepared. We present the dependence of MTJs characteristics on Corresponding author. Tel.: +82 2 710 9404; fax: +82 2 718-2337.
E-mail address: [email protected]. 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.028
different surface roughness and tunneling characteristics of these junctions. MTJs varying surface roughness of the bottom electrode by surface treatment using ion beam are deposited on oxidized Si (1 0 0) substrates by a 7-target DC and RF magnetron sputtering system under base pressure below 2.0 108 Torr and Ar pressure of 2 mTorr. A magnetic field of 100 Oe is applied to induce the uniaxial magnetic anisotropy in FM layer. Junctions are fabricated by photolithographic patterning procedure and ion beam etching. A series of annealing is done at 200 1C under a static magnetic field of 1 kOe in vacuum furnace with a base pressure 8.0 107 Torr. The insulating layer is formed by depositing an 8 A˚ thick Al layer followed by RF plasma oxidation [4,5] in a load lock chamber under a base pressure of 1.2 107 Torr. TMR ratio and resistance are measured using four-point probe station. The crosssectional views of MTJs with various surface roughnesses are analyzed using transmission electron micrograph (TEM). Fig. 1 shows the influence of surface roughness (root mean square roughness; Rrms ¼ 3–12 A˚) of tunnel barrier
ARTICLE IN PRESS J.R. Rhee / Journal of Magnetism and Magnetic Materials 304 (2006) e300–e302
e301
0.006
Ip Iap
0.002
120 80 40 0 -0.6
0.000
40
-0.4
(a)
36 32
-0.002
28 24
-0.004
20 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Bias Voltage (V)
-0.2 0.0 0.2 Bias Voltage (V)
0.4
-0.006 0.6
0.04
32 28
Ip Iap
Rp Rap
0.02
on TMR ratio and resistance. TMR ratio and resistance increase as the surface roughness of tunnel barrier decreases. As MTJs have more uniform tunnel barrier, TMR ratio and resistance are higher, while interlayer coupling field (Hint) between the free layer and pinned layer decreases. For the MTJ with rough surface (with more than Rrms ¼ 10 A˚), resistance area product (RA) greatly reduces. Fig. 2(a) and (b) show bias voltage dependence of resistance and current for the 10 10 mm2-sized MTJ with tunnel barrier roughness (Rrms) of 3 A˚ and 12 A˚, respectively. Here, Rap, Iap and Rp, Ip are resistances and currents of the MTJ when two FM layers have their magnetizations antiparallel and parallel, respectively. RA and TMR ratio at bias voltage of 100 mV for MTJs with tunnel barrier of Rrms ¼ 3 A˚ and Rrms ¼ 12 A˚ are 14 kO mm2, 40% and 2.2 k O mm2, 4%, respectively. To further prove that a MTJ is a perfect junction, one should look at the bias voltage dependence of its TMR ratio, since only junctions with identical electrodes will show a symmetric pattern [6]. Many factors such as the quality of the barrier and the materials of FM electrodes used in the junction can affect the bias dependence of TMR as stated in a previous report [7]. For a practical junction, a high TMR value is a strong indication that a high quality of the tunnel barrier and its two interfaces are obtained. When this high TMR value is coupled with a high symmetry of the bias voltage dependence, it provides a necessary condition of having an optimized barrier and two interfaces. If on the other hand large asymmetric conditions were observed, it would suggest that at least one of the two interfaces could be further optimized [8]. In the MTJ with tunnel barrier of Rrms ¼ 3 A˚, the current-voltage (I–V) curve and bias voltage dependence of TMR ratio are
20 16 12 8 4 0 -0.6
(b)
0.00
4.8 TMR ratio (%)
Resistance (Ω )
24
Fig. 1. Typical minor TMR curves of MTJs with tunnel barrier roughness (Rrms) of 3, 4, 8 and 12 A˚, respectively.
Current (A)
0.004
160
TMR ratio (%)
Resistance (Ω)
200
Rp Rap
4.4 4.0 3.6
Current (A)
240
-0.02
3.2 -0.6-0.4-0.2 0.0 0.2 0.4 0.6 Bias Voltage (V)
-0.4
-0.2 0.0 0.2 Bias Voltage (V)
0.4
-0.04 0.6
Fig. 2. Bias voltage dependence of resistance and current for MTJs with (a) tunnel barrier roughness (Rrms) of 3 A˚ and (b) tunnel barrier roughness (Rrms) of 12 A˚. Bias voltage dependence of TMR ratio is shown in the insets.
non linear and symmetric as shown in Fig. 2(a) and the inset of Fig. 2(a), respectively. In contrast, in the case of the MTJ with tunnel barrier of Rrms ¼ 12 A˚, the I–V curve is nearly linear and the bias voltage dependence of TMR is asymmetric, suggesting that the MTJ has rough tunnel barrier, and it is difficult to achieve the optimum TMR ratio and RA. The bias voltage, where the TMR ratio is reduced to half of its value, is about 600 mV for the MTJ with tunnel barrier of Rrms ¼ 3 A˚. The barrier thicknesses of the MTJs with tunnel barrier of Rrms ¼ 3 A˚ and Rrms ¼ 12 A˚ show 12.3 A˚ and 11.8 A˚, the barrier heights show 3.07 eV and 4.65 eV, respectively, which are obtained by fitting I–V curves to the Simmon’s model [9]. Fig. 3 shows the cross-sectional TEM image of MTJs with tunnel barrier of Rrms ¼ 3 A˚ (a) and tunnel barrier of Rrms ¼ 12 A˚ (b). Fig. 3(a) and (b) correspond to MTJs of Fig. 2(a) and (b), respectively. The rough surface of the bottom electrode possibly serves the preferential conduction channels activated through the barrier, shunting the conduction to spinindependent path. The high TMR ratio and its low variation with bias are crucial for stable sensing of signal
ARTICLE IN PRESS e302
J.R. Rhee / Journal of Magnetism and Magnetic Materials 304 (2006) e300–e302
Top electrode Al2O3
(a)
Bottom electrode
References
20nm (b)
and the tunneling characteristics of these junctions. As MTJs had more smooth surface of bottom electrode, TMR ratio and resistance were higher, while Hint decreased. The I–V curve and TMR ratio versus bias voltage curve of the MTJ with tunnel barrier of Rrms ¼ 12 A˚ were linear and asymmetric, respectively, but in the case of the MTJ with tunnel barrier of Rrms ¼ 3 A˚, these curves were non-linear and symmetric, respectively. It was confirmed that the smooth surface of bottom electrode was a basic requirement for MTJs.
Top electrode
Al2O3 Bottom electrode
20 nm Fig. 3. TEM cross-sectional views of MTJs with (a) tunnel barrier of Rrms ¼ 3 A˚ and (b) tunnel barrier of Rrms ¼ 12 A˚.
from a MRAM, and the smooth surface of bottom electrode is a basic requirement for them. In conclusion, we investigated the dependence of MTJs characteristics on surface roughness of bottom electrode
[1] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 74 (1995) 3273. [2] W.J. Gallapher, S.S.P. Parkin, Yu. Lu, X.P. Bian, A. Marley, K.P. Roche, R.A. Altman, S.A. Rishton, C. Jahrnes, T.M. Shaw, Gang Xiao, J. Appl. Phys. 81 (1977) 3741. [3] M. Julliere, Phys. Lett. A 54 (1975) 225. [4] M. Sato, H. Kikuchi, K. Kobayashi, J. Appl. Phys. 83 (1998) 6691. [5] J.S. Moodera, L.R. Kinder, J. Appl. Phys. 79 (1996) 4724. [6] X.H. Xiang, T. Zhu, J. Du, G. Landry, J.Q. Xiao, Phys. Rev. B 66 (2002) 174407. [7] K. Li, Y. Wu, J. Qiu, G. Han, Z. Guo, T. Chong, J. Magn. Magn. Mater. 241 (2002) 89. [8] D. Wang, C. Nordman, J.M. Daughton, Z. Qian, J. Fink, IEEE Trans. Magn. 40 (2004) 2269. [9] J.G. Simmons, J. Appl. Phys. 34 (1963) 1793.
Journal of Magnetism and Magnetic Materials 304 (2006) e300–e302 www.elsevier.com/locate/jmmm
Dependence of magnetic tunnel junction properties on tunnel barrier roughness Jang Roh Rhee Department of Physics, Sookmyung Women’s University, Seoul 140-742, South Korea Available online 28 February 2006
Abstract Magnetic tunnel junctions (MTJs) consisting of Si/SiO2/Ta/Ru/IrMn/CoFe/Ru/CoFe/Al-O/CoFe/NiFe/Ru with different surface roughness of bottom electrode were prepared, and the dependence of tunneling magnetoresistance (TMR) ratio and resistance area product (RA) on surface roughness of tunnel barrier and the tunneling characteristics of these junction devices were investigated. The MTJ with rough tunnel barrier of 12 A˚ root mean square roughness (Rrms) showed TMR ratio of 4% and RA of 2.2 kO mm2. In contrast, the MTJ with uniform tunnel barrier of Rrms ¼ 3 A˚ showed TMR ratio of 40% and RA of 14 kO mm2. As MTJs had more uniform tunnel barrier, TMR ratio and resistance were higher, while the interlayer coupling field decreased. It was confirmed that the smooth surface of bottom electrode was a basic requirement for MTJs. r 2006 Elsevier B.V. All rights reserved. PACS: 73.40.Gk; 85.75.Dd; 75.75.+a Keywords: Magnetic tunnel junction; Tunneling magnetoresistance; Tunnel barrier; Barrier width; Barrier height
Magnetic tunnel junction (MTJ) is attractive for use as bit cells for nonvolatile magnetic random access memory (MRAM) due to their large tunneling magnetoresistance (TMR) ratio at room temperature [1,2]. The MTJ consists of two ferromagnetic (FM) electrodes separated by a tunnel barrier that exhibits TMR due to spin polarized tunneling [3]. The bottom FM electrode is an important factor in drawing out the desirable performance of MTJ cells, such as high TMR ratio, good resistance uniformity, high breakdown voltage, and weak bias dependence of TMR. Of several microstructural features related with the bottom electrode, the surface roughness is considered one of the most critical ones. In this study, MTJs of Si/SiO2/Ta/Ru/IrMn/CoFe/ Ru/ CoFe/Al-O/CoFe/NiFe/Ru using antiferromagnetic Ir22Mn78 as the pinning layer and synthetic antiferromagnetic CoFe/Ru/CoFe as the pinned layer with different surface roughness of the bottom electrode are prepared. We present the dependence of MTJs characteristics on Corresponding author. Tel.: +82 2 710 9404; fax: +82 2 718-2337.
E-mail address: [email protected]. 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.028
different surface roughness and tunneling characteristics of these junctions. MTJs varying surface roughness of the bottom electrode by surface treatment using ion beam are deposited on oxidized Si (1 0 0) substrates by a 7-target DC and RF magnetron sputtering system under base pressure below 2.0 108 Torr and Ar pressure of 2 mTorr. A magnetic field of 100 Oe is applied to induce the uniaxial magnetic anisotropy in FM layer. Junctions are fabricated by photolithographic patterning procedure and ion beam etching. A series of annealing is done at 200 1C under a static magnetic field of 1 kOe in vacuum furnace with a base pressure 8.0 107 Torr. The insulating layer is formed by depositing an 8 A˚ thick Al layer followed by RF plasma oxidation [4,5] in a load lock chamber under a base pressure of 1.2 107 Torr. TMR ratio and resistance are measured using four-point probe station. The crosssectional views of MTJs with various surface roughnesses are analyzed using transmission electron micrograph (TEM). Fig. 1 shows the influence of surface roughness (root mean square roughness; Rrms ¼ 3–12 A˚) of tunnel barrier
ARTICLE IN PRESS J.R. Rhee / Journal of Magnetism and Magnetic Materials 304 (2006) e300–e302
e301
0.006
Ip Iap
0.002
120 80 40 0 -0.6
0.000
40
-0.4
(a)
36 32
-0.002
28 24
-0.004
20 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Bias Voltage (V)
-0.2 0.0 0.2 Bias Voltage (V)
0.4
-0.006 0.6
0.04
32 28
Ip Iap
Rp Rap
0.02
on TMR ratio and resistance. TMR ratio and resistance increase as the surface roughness of tunnel barrier decreases. As MTJs have more uniform tunnel barrier, TMR ratio and resistance are higher, while interlayer coupling field (Hint) between the free layer and pinned layer decreases. For the MTJ with rough surface (with more than Rrms ¼ 10 A˚), resistance area product (RA) greatly reduces. Fig. 2(a) and (b) show bias voltage dependence of resistance and current for the 10 10 mm2-sized MTJ with tunnel barrier roughness (Rrms) of 3 A˚ and 12 A˚, respectively. Here, Rap, Iap and Rp, Ip are resistances and currents of the MTJ when two FM layers have their magnetizations antiparallel and parallel, respectively. RA and TMR ratio at bias voltage of 100 mV for MTJs with tunnel barrier of Rrms ¼ 3 A˚ and Rrms ¼ 12 A˚ are 14 kO mm2, 40% and 2.2 k O mm2, 4%, respectively. To further prove that a MTJ is a perfect junction, one should look at the bias voltage dependence of its TMR ratio, since only junctions with identical electrodes will show a symmetric pattern [6]. Many factors such as the quality of the barrier and the materials of FM electrodes used in the junction can affect the bias dependence of TMR as stated in a previous report [7]. For a practical junction, a high TMR value is a strong indication that a high quality of the tunnel barrier and its two interfaces are obtained. When this high TMR value is coupled with a high symmetry of the bias voltage dependence, it provides a necessary condition of having an optimized barrier and two interfaces. If on the other hand large asymmetric conditions were observed, it would suggest that at least one of the two interfaces could be further optimized [8]. In the MTJ with tunnel barrier of Rrms ¼ 3 A˚, the current-voltage (I–V) curve and bias voltage dependence of TMR ratio are
20 16 12 8 4 0 -0.6
(b)
0.00
4.8 TMR ratio (%)
Resistance (Ω )
24
Fig. 1. Typical minor TMR curves of MTJs with tunnel barrier roughness (Rrms) of 3, 4, 8 and 12 A˚, respectively.
Current (A)
0.004
160
TMR ratio (%)
Resistance (Ω)
200
Rp Rap
4.4 4.0 3.6
Current (A)
240
-0.02
3.2 -0.6-0.4-0.2 0.0 0.2 0.4 0.6 Bias Voltage (V)
-0.4
-0.2 0.0 0.2 Bias Voltage (V)
0.4
-0.04 0.6
Fig. 2. Bias voltage dependence of resistance and current for MTJs with (a) tunnel barrier roughness (Rrms) of 3 A˚ and (b) tunnel barrier roughness (Rrms) of 12 A˚. Bias voltage dependence of TMR ratio is shown in the insets.
non linear and symmetric as shown in Fig. 2(a) and the inset of Fig. 2(a), respectively. In contrast, in the case of the MTJ with tunnel barrier of Rrms ¼ 12 A˚, the I–V curve is nearly linear and the bias voltage dependence of TMR is asymmetric, suggesting that the MTJ has rough tunnel barrier, and it is difficult to achieve the optimum TMR ratio and RA. The bias voltage, where the TMR ratio is reduced to half of its value, is about 600 mV for the MTJ with tunnel barrier of Rrms ¼ 3 A˚. The barrier thicknesses of the MTJs with tunnel barrier of Rrms ¼ 3 A˚ and Rrms ¼ 12 A˚ show 12.3 A˚ and 11.8 A˚, the barrier heights show 3.07 eV and 4.65 eV, respectively, which are obtained by fitting I–V curves to the Simmon’s model [9]. Fig. 3 shows the cross-sectional TEM image of MTJs with tunnel barrier of Rrms ¼ 3 A˚ (a) and tunnel barrier of Rrms ¼ 12 A˚ (b). Fig. 3(a) and (b) correspond to MTJs of Fig. 2(a) and (b), respectively. The rough surface of the bottom electrode possibly serves the preferential conduction channels activated through the barrier, shunting the conduction to spinindependent path. The high TMR ratio and its low variation with bias are crucial for stable sensing of signal
ARTICLE IN PRESS e302
J.R. Rhee / Journal of Magnetism and Magnetic Materials 304 (2006) e300–e302
Top electrode Al2O3
(a)
Bottom electrode
References
20nm (b)
and the tunneling characteristics of these junctions. As MTJs had more smooth surface of bottom electrode, TMR ratio and resistance were higher, while Hint decreased. The I–V curve and TMR ratio versus bias voltage curve of the MTJ with tunnel barrier of Rrms ¼ 12 A˚ were linear and asymmetric, respectively, but in the case of the MTJ with tunnel barrier of Rrms ¼ 3 A˚, these curves were non-linear and symmetric, respectively. It was confirmed that the smooth surface of bottom electrode was a basic requirement for MTJs.
Top electrode
Al2O3 Bottom electrode
20 nm Fig. 3. TEM cross-sectional views of MTJs with (a) tunnel barrier of Rrms ¼ 3 A˚ and (b) tunnel barrier of Rrms ¼ 12 A˚.
from a MRAM, and the smooth surface of bottom electrode is a basic requirement for them. In conclusion, we investigated the dependence of MTJs characteristics on surface roughness of bottom electrode
[1] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 74 (1995) 3273. [2] W.J. Gallapher, S.S.P. Parkin, Yu. Lu, X.P. Bian, A. Marley, K.P. Roche, R.A. Altman, S.A. Rishton, C. Jahrnes, T.M. Shaw, Gang Xiao, J. Appl. Phys. 81 (1977) 3741. [3] M. Julliere, Phys. Lett. A 54 (1975) 225. [4] M. Sato, H. Kikuchi, K. Kobayashi, J. Appl. Phys. 83 (1998) 6691. [5] J.S. Moodera, L.R. Kinder, J. Appl. Phys. 79 (1996) 4724. [6] X.H. Xiang, T. Zhu, J. Du, G. Landry, J.Q. Xiao, Phys. Rev. B 66 (2002) 174407. [7] K. Li, Y. Wu, J. Qiu, G. Han, Z. Guo, T. Chong, J. Magn. Magn. Mater. 241 (2002) 89. [8] D. Wang, C. Nordman, J.M. Daughton, Z. Qian, J. Fink, IEEE Trans. Magn. 40 (2004) 2269. [9] J.G. Simmons, J. Appl. Phys. 34 (1963) 1793.