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Three-dimensional array of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+x single crystals
Physica C 372–376 (2002) 327–330 www.elsevier.com/locate/physc
Three-dimensional array of intrinsic Josephson junctions in Bi2Sr2CaCu2O8þx single crystals H.B. Wang a
a,b,*
, K. Maeda a, J. Chen
a,b
, P.H. Wu
a,c
, T. Yamashita
b,d
Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Japan c Research Institute of Superconductor Electronics, University of Nanjing, Nanjing 210093, China d New Industry Creation Hatchery Center, Tohoku University, Sendai 980-8579, Japan
Abstract Using a novel double-side fabrication method, a three-dimensional array of about 10,000 intrinsic Josephson junctions was fabricated and singled out from inside a bulk Bi2 Sr2 CaCu2 O8þx single crystal. The array was proved to be very promising for electronic applications in the sense that the junctions were homogeneous and the junctions were 3Darranged with high density. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Intrinsic Josephson junctions; 3D array; Integrated circuits
1. Introduction Many approaches have been adopted to fabricate intrinsic Josephson junctions (IJJs) in various high temperature superconductors for theoretical understanding and practical applications of intrinsic Josephson effects (IJEs) [1,2]. At the atomic layer level, IJJs of desirable number can be fabricated in single crystals, c-axis or non-c-axis films in the structures of mesa type, whisker type, microbridge type, lateral type, etc. [3–7]. However, studies on the junction number dependence of energy gap suggest that the suppression of gap, *
Corresponding author. Address: Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81-22-217-5474; fax: +81-22-217-5473. E-mail address: [email protected] (H.B. Wang).
which is mainly caused by quasiparticle injection and Joule heating, may be reduced by decreasing the junction number [8,9]. Therefore an array, in which many IJJs are grouped in separate stacks and the stacks are in series to each other via flexible connections, will be better than any other structures in which junctions of the same number are simply connected in series to each other. Recently we have developed a novel double-side fabrication method using which an IJJs stack can be singled out from inside a bulk Bi2 Sr2 CaCu2 O8þx (BSCCO) single crystal. The number of junctions involved is rather controllable, the junctions in the fabricated stack are very uniform, and they are quite sensitive to terahertz irradiations [10,11]. Two-dimentional (2D) or three-dimensional (3D) arrays of IJJs have been successfully fabricated adopting this approach. Reported in this paper is the fabrication of a 3D 256-stack array involving
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 6 6 1 - 5
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H.B. Wang et al. / Physica C 372–376 (2002) 327–330
more than 10,000 IJJs, as well as the c-axis transport properties of the array.
2. Sample preparation Although the fabrication of an array (Fig. 1) is quite similar to that of a single stack, we wish to describe it here briefly, stressing the differences. And for clarity, we show below the fabrication of two stacks in series as an example. To start our fabrication, we cleaved a piece of BSCCO single crystal and fixed it on a substrate, with c-axis perpendicular to the surface of the gold layer was sputtered onto the latter. A 500 A freshly cleaved BSCCO single crystal (side A) to prevent the BSCCO surface from possible deterioration in the following process. As shown in Fig. 1(a), after a photo resist was patterned on the sample surface, argon ion milling was used to etch the sample, resulting in a mesa 200 nm thick (Fig.
Fig. 1. A schematic description of the major steps in fabricating two stacks of IJJs in series with each other.
1(b)). The photo resist could be patterned into any desirable shape as discussed below, but for clarity we show here only a strip. After removing the photo resist, two separated pieces of photo resist were put on the mesa and ion milling was employed to form two vertical edges along the c-axis (Fig. 1(c)). The patterned mesa is then glued onto another substrate and cleaved from the big ped gold layer estal (Fig. 1(d) and (e)). Another 500 A was sputtered onto the sample (side B). This sample was then photolithographically etched to a depth as required by the desirable junction number, resulting in two IJJs stacks in series with each other (Fig. 1(f) and (g)). In the same procedure, four separate leads were made (Fig. 1(h)) for the current–voltage (I–V) measurements to follow. As the BSCCO slice from which the array was fabricated was about 200 nm thick, it was possible for us to observe samples under an optical microscope with either topside or backside illumination. Shown in Fig. 2 is the optical image of a sample of 256 IJJs stacks, taken with backside illumination after the whole process was completed. Two of the stacks are marked with open squares. One may see all stacks are with well-defined geometries and a–b plane sizes are about 4 lm by 4 lm. Total 256 stacks are in an area of 170 lm by 150 lm, obviously connected in series to each other via BSCCO strips. For future high
Fig. 2. An optical photo of a 256-stack 3D array of IJJs on MgO substrate (taken through an optical microscope with back-side illumination).
H.B. Wang et al. / Physica C 372–376 (2002) 327–330
frequency applications, a modified bow-tie antenna and choke filters were also superconductively connected to the array. 3. Experimental results Shown in Fig. 3 are the typical I–V curves of a 3D array. The I–V curves in Fig. 3(a) were measured by ramping a bias current up and down repeatedly. As usual, when we switched from a resistive branch to the next one, we had one more junction switching into voltage state. Thus the clear and gradual decrease of critical currents shown in Fig. 3(a) indicates that the more junctions were in voltage states, the smaller the critical current was; in other words, the critical current quite depended on how many junctions were in voltage states. It is consistent with the earlier observations, which attributed the phenomena to quasiparticle injection and Joule heating [8,9]. One may expect that the suppression of the critical currents and energy gap becomes more serious when the junction number is further increased.
329
However, in our experiments the critical currents kept decreasing only until about the 45th resistive branch. Then a new group of resistive branches appeared, the first of which corresponded to a critical current very close to what the first branch in the first group corresponded to. In this new group, the decrease tendency of the critical currents was similar to that in the first group. Due to the unavailability of large bias, we were not able to trace all the resistive branches of the array (about 250 V were required to trace all the branches). With 17 V available from the bias, we obtained the I–V curves shown in Fig. 3(b). The flatness of the I–V curve envelope indicates that the stacks in our array were quite homogeneous. However, there did exist a small variation of the critical currents, which enabled us to observe a few individual stacks switching to their voltage states. Based on this point, we estimated that there were about 45 junctions in one stack, which agreed very well with the thickness of the stack. And within an area of 170 lm by 150 lm, there were about 11,500 junctions in 256-stacks. The obtained results and the clear layout shown in Fig. 2 indicated we were able to achieve high density of junctions with uniform properties.
4. Conclusions Using a double-side fabrication method recently developed, a 3D array of IJJs in BSCCO was realized. Within an area of 170 lm by 150 lm, 256 stacks were connected in series with each other. I–V curves of the array indicated almost periodic decrease of the critical currents, and the periodicity revealed that in each stack there were about 45 junctions which agreed very well with the estimation based on the sample thickness. High density integration of IJJs can be achieved using the present technique, and such arrays are promising in many practical applications.
Acknowledgements Fig. 3. Typical I–V characteristics of a 256-stack array (a) the close-ups, and (b) the bias voltage up to 17 V.
This work was supported by CREST (Core Research for Evolutional Science and Technology)
330
H.B. Wang et al. / Physica C 372–376 (2002) 327–330
of Japan Science and Technology Corporation (JST) and Marubun Research Promotion Foundation, Japan, and partially carried out at the Laboratory for Electronic Intelligent Systems, Research Institute of Electrical Communication, Tohoku University, Japan.
References [1] R. Kleiner, F. Steinmeyer, G. Kunkel, P. M€ uller, Phys. Rev. Lett. 68 (1992) 2394. [2] G. Oya, N. Aoyama, A. Irie, S. Kishida, H. Tokutaka, Jpn. J. Appl. Phys. 31 (1992) L829. [3] Y.G. Xiao, R. Domel, C.L. Jia, C. Osthover, H. Kohlstedt, Supercond. Sci. Technol. 9 (1996) A22.
[4] Y.I. Latyshev, J.E. Nevelskaya, P. Monceau, Phys. Rev. Lett. 77 (1996) 932. [5] M. Veith, T. Eick, W. Brodkorb, M. Manzel, H. Bruchlos, T. Kohler, H.-G. Schmidt, E. Steinbeiss, H.-J. Fuchs, K. Schlenga, G. Hechtfischer, P. M€ uller, J. Appl. Phys. 80 (1996) 3396. [6] H.B. Wang, J. Chen, T. Tachiki, Y. Mizugaki, K. Nakajima, T. Yamashita, J. Appl. Phys. 85 (1999) 3740. [7] Y. Ishimaru, J. Wen, T. Utagawa, N. Koshizuka, Y. Enomoto, Physica C 293 (1997) 196. [8] M. Itoh, S. Karimoto, K. Namekawa, M. Suzuki, Phys. Rev. B 55 (1997) R12001. [9] A. Odagawa, M. Sakai, H. Adachi, K. Setsune, Jpn. J. Appl. Phys. 37 (1997) 486. [10] H.B. Wang, P.H. Wu, T. Yamashita, Appl. Phys. Lett. 78 (2001) 4010. [11] H.B. Wang, P.H. Wu, T. Yamashita, Phys. Rev. Lett. 87 (2001) 107002.
Three-dimensional array of intrinsic Josephson junctions in Bi2Sr2CaCu2O8þx single crystals H.B. Wang a
a,b,*
, K. Maeda a, J. Chen
a,b
, P.H. Wu
a,c
, T. Yamashita
b,d
Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Japan c Research Institute of Superconductor Electronics, University of Nanjing, Nanjing 210093, China d New Industry Creation Hatchery Center, Tohoku University, Sendai 980-8579, Japan
Abstract Using a novel double-side fabrication method, a three-dimensional array of about 10,000 intrinsic Josephson junctions was fabricated and singled out from inside a bulk Bi2 Sr2 CaCu2 O8þx single crystal. The array was proved to be very promising for electronic applications in the sense that the junctions were homogeneous and the junctions were 3Darranged with high density. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Intrinsic Josephson junctions; 3D array; Integrated circuits
1. Introduction Many approaches have been adopted to fabricate intrinsic Josephson junctions (IJJs) in various high temperature superconductors for theoretical understanding and practical applications of intrinsic Josephson effects (IJEs) [1,2]. At the atomic layer level, IJJs of desirable number can be fabricated in single crystals, c-axis or non-c-axis films in the structures of mesa type, whisker type, microbridge type, lateral type, etc. [3–7]. However, studies on the junction number dependence of energy gap suggest that the suppression of gap, *
Corresponding author. Address: Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81-22-217-5474; fax: +81-22-217-5473. E-mail address: [email protected] (H.B. Wang).
which is mainly caused by quasiparticle injection and Joule heating, may be reduced by decreasing the junction number [8,9]. Therefore an array, in which many IJJs are grouped in separate stacks and the stacks are in series to each other via flexible connections, will be better than any other structures in which junctions of the same number are simply connected in series to each other. Recently we have developed a novel double-side fabrication method using which an IJJs stack can be singled out from inside a bulk Bi2 Sr2 CaCu2 O8þx (BSCCO) single crystal. The number of junctions involved is rather controllable, the junctions in the fabricated stack are very uniform, and they are quite sensitive to terahertz irradiations [10,11]. Two-dimentional (2D) or three-dimensional (3D) arrays of IJJs have been successfully fabricated adopting this approach. Reported in this paper is the fabrication of a 3D 256-stack array involving
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 6 6 1 - 5
328
H.B. Wang et al. / Physica C 372–376 (2002) 327–330
more than 10,000 IJJs, as well as the c-axis transport properties of the array.
2. Sample preparation Although the fabrication of an array (Fig. 1) is quite similar to that of a single stack, we wish to describe it here briefly, stressing the differences. And for clarity, we show below the fabrication of two stacks in series as an example. To start our fabrication, we cleaved a piece of BSCCO single crystal and fixed it on a substrate, with c-axis perpendicular to the surface of the gold layer was sputtered onto the latter. A 500 A freshly cleaved BSCCO single crystal (side A) to prevent the BSCCO surface from possible deterioration in the following process. As shown in Fig. 1(a), after a photo resist was patterned on the sample surface, argon ion milling was used to etch the sample, resulting in a mesa 200 nm thick (Fig.
Fig. 1. A schematic description of the major steps in fabricating two stacks of IJJs in series with each other.
1(b)). The photo resist could be patterned into any desirable shape as discussed below, but for clarity we show here only a strip. After removing the photo resist, two separated pieces of photo resist were put on the mesa and ion milling was employed to form two vertical edges along the c-axis (Fig. 1(c)). The patterned mesa is then glued onto another substrate and cleaved from the big ped gold layer estal (Fig. 1(d) and (e)). Another 500 A was sputtered onto the sample (side B). This sample was then photolithographically etched to a depth as required by the desirable junction number, resulting in two IJJs stacks in series with each other (Fig. 1(f) and (g)). In the same procedure, four separate leads were made (Fig. 1(h)) for the current–voltage (I–V) measurements to follow. As the BSCCO slice from which the array was fabricated was about 200 nm thick, it was possible for us to observe samples under an optical microscope with either topside or backside illumination. Shown in Fig. 2 is the optical image of a sample of 256 IJJs stacks, taken with backside illumination after the whole process was completed. Two of the stacks are marked with open squares. One may see all stacks are with well-defined geometries and a–b plane sizes are about 4 lm by 4 lm. Total 256 stacks are in an area of 170 lm by 150 lm, obviously connected in series to each other via BSCCO strips. For future high
Fig. 2. An optical photo of a 256-stack 3D array of IJJs on MgO substrate (taken through an optical microscope with back-side illumination).
H.B. Wang et al. / Physica C 372–376 (2002) 327–330
frequency applications, a modified bow-tie antenna and choke filters were also superconductively connected to the array. 3. Experimental results Shown in Fig. 3 are the typical I–V curves of a 3D array. The I–V curves in Fig. 3(a) were measured by ramping a bias current up and down repeatedly. As usual, when we switched from a resistive branch to the next one, we had one more junction switching into voltage state. Thus the clear and gradual decrease of critical currents shown in Fig. 3(a) indicates that the more junctions were in voltage states, the smaller the critical current was; in other words, the critical current quite depended on how many junctions were in voltage states. It is consistent with the earlier observations, which attributed the phenomena to quasiparticle injection and Joule heating [8,9]. One may expect that the suppression of the critical currents and energy gap becomes more serious when the junction number is further increased.
329
However, in our experiments the critical currents kept decreasing only until about the 45th resistive branch. Then a new group of resistive branches appeared, the first of which corresponded to a critical current very close to what the first branch in the first group corresponded to. In this new group, the decrease tendency of the critical currents was similar to that in the first group. Due to the unavailability of large bias, we were not able to trace all the resistive branches of the array (about 250 V were required to trace all the branches). With 17 V available from the bias, we obtained the I–V curves shown in Fig. 3(b). The flatness of the I–V curve envelope indicates that the stacks in our array were quite homogeneous. However, there did exist a small variation of the critical currents, which enabled us to observe a few individual stacks switching to their voltage states. Based on this point, we estimated that there were about 45 junctions in one stack, which agreed very well with the thickness of the stack. And within an area of 170 lm by 150 lm, there were about 11,500 junctions in 256-stacks. The obtained results and the clear layout shown in Fig. 2 indicated we were able to achieve high density of junctions with uniform properties.
4. Conclusions Using a double-side fabrication method recently developed, a 3D array of IJJs in BSCCO was realized. Within an area of 170 lm by 150 lm, 256 stacks were connected in series with each other. I–V curves of the array indicated almost periodic decrease of the critical currents, and the periodicity revealed that in each stack there were about 45 junctions which agreed very well with the estimation based on the sample thickness. High density integration of IJJs can be achieved using the present technique, and such arrays are promising in many practical applications.
Acknowledgements Fig. 3. Typical I–V characteristics of a 256-stack array (a) the close-ups, and (b) the bias voltage up to 17 V.
This work was supported by CREST (Core Research for Evolutional Science and Technology)
330
H.B. Wang et al. / Physica C 372–376 (2002) 327–330
of Japan Science and Technology Corporation (JST) and Marubun Research Promotion Foundation, Japan, and partially carried out at the Laboratory for Electronic Intelligent Systems, Research Institute of Electrical Communication, Tohoku University, Japan.
References [1] R. Kleiner, F. Steinmeyer, G. Kunkel, P. M€ uller, Phys. Rev. Lett. 68 (1992) 2394. [2] G. Oya, N. Aoyama, A. Irie, S. Kishida, H. Tokutaka, Jpn. J. Appl. Phys. 31 (1992) L829. [3] Y.G. Xiao, R. Domel, C.L. Jia, C. Osthover, H. Kohlstedt, Supercond. Sci. Technol. 9 (1996) A22.
[4] Y.I. Latyshev, J.E. Nevelskaya, P. Monceau, Phys. Rev. Lett. 77 (1996) 932. [5] M. Veith, T. Eick, W. Brodkorb, M. Manzel, H. Bruchlos, T. Kohler, H.-G. Schmidt, E. Steinbeiss, H.-J. Fuchs, K. Schlenga, G. Hechtfischer, P. M€ uller, J. Appl. Phys. 80 (1996) 3396. [6] H.B. Wang, J. Chen, T. Tachiki, Y. Mizugaki, K. Nakajima, T. Yamashita, J. Appl. Phys. 85 (1999) 3740. [7] Y. Ishimaru, J. Wen, T. Utagawa, N. Koshizuka, Y. Enomoto, Physica C 293 (1997) 196. [8] M. Itoh, S. Karimoto, K. Namekawa, M. Suzuki, Phys. Rev. B 55 (1997) R12001. [9] A. Odagawa, M. Sakai, H. Adachi, K. Setsune, Jpn. J. Appl. Phys. 37 (1997) 486. [10] H.B. Wang, P.H. Wu, T. Yamashita, Appl. Phys. Lett. 78 (2001) 4010. [11] H.B. Wang, P.H. Wu, T. Yamashita, Phys. Rev. Lett. 87 (2001) 107002.