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Josephson effects in a-axis oriented YBa2Cu3O7−y SIS tunnel junctions with double layer barrier
ELSEVIER
Physica C 341-348 (2000) 2741-2742
www.elsevier.nl/Iocate/physc
J o s e p h s o n E f f e c t s in A - a x i s O r i e n t e d YBa2Cu307_y S I S T u n n e l J u n c t i o n s w i t h D o u b l e L a y e r Barrier You-song Jiang, Tadayuki Kobayashi and Toshinari Goto The University of Electro-Communications, Chofu-Shi, Tokyo, 182-8585, Japan
In this study, we have fabricated vertical SIS tunnel junctions with a-axis oriented YBa2Cu307.y(YBCO) electrodes. In order to prevent the pinhole in the barrier layer, double layer consists of PrGaO3(PGO) and CeO2 was used. The standard lithography technology and ion milling were employed to define the junctions with area of 31am× 151am. The junctions with barriers 5nm thick PGO and 2.5nm thick CeO2 showed supercurrents and RSJ like voltage-current(I-V) characteristics. Fraunhofer like magnetic field dependence of Ic were observed, and the results proved the existence of Josephson effects in these junctions. 1.Introduction High Tc trilayer SIS junctions have been studied, and it seemed to be difficult to fabricate the Josephson devices due to the short coherence length and the existence of pinhole like short circuits. In this experiment, we fabricated SIS junctions with a-axis oriented YBCO thin films, because the coherence length in the a-axis direction is ten times as long as in the c-axis direction. SrTiO3 substrate was chosen to use for obtaining a-axis oriented YBCO thin films with high critical current density (Jc). Double layer barrier was employed to prevent pinhole short circuit, which consisted of PrGaO3 (PGO) and CeO2. Both PGO and CeO2 have well lattice match with YBCO thin film to support epitaxial growth of later [1]-[3].
respectively. Then 150nm thick YBCO thin film as counter electrode was deposited at 720degree. Finally, a protection layer of 20nm thick Au was sputtered on top of the four-layer structure. Standard photolithography was used to'fabricate the junctions. They were delineated and patterned down to the base electrode by ion milling. The deposition and lift-off of amorphous MgO as the insulator layer were prepared before Au was put down for contacts to the counter electrodes of junctions. Lastly, the lead wires of Au were separated for four-terminal measurements. The X-ray diffraction measurement showed that base and counter electrodes of YBCO films were a-axis oriented and the multilayered structure was epitaxially grown.
2.Experiment We deposited a-axis oriented YBCO thin films on SrTiO3 substrates using DC hollow cathode sputtering method by the temperature gradient and deposition rate variation method that had been previously described in detail [4]. The thickness of the base YBCO was about 300nm. The barriers of PGO and CeO2 were deposited using ion beam sputtering method at substrate temperature of 680degree; their thickness was 5nm and 2.5nm,
3.Results and discussion Figure 1 shows the typical I-V characteristics of the junction measured at 45K. The area of the junction was 3Xl5~tm 2. The I-V curve is in agreement with RSJ-model. The critical current density Jc and the normal resistance normalized by °the junction area (RnS) are 0.9x 103A/cm2 and 0.077~tQcm 2 respectively. The IcRn product at 45K is 0.07mV. The magnetic field dependence of the critical
0921-4534/00/$ - see front matter © 2000 Elsevier Science B . ~ PI1 S0921-4534(00)01478-7
All rights reserved.
2742
Y-S. Jiang et al./Physica C 341-348 (2000) 2741-2742
current for our junction measured at 4.2K is shown in figure 2. The solid line is the Fraunhofer pattern fit to the curve with an offset of 0.83mA, indicating that modulation rate is 65%. The Ic for the junction as a function of temperature is shown in figure 3. As can be observed, Josephson current disappears at the temperature higher than 62K, which suggested that the transition temperature of the interface or the counter electrode for the junction is lower than the Tc of base electrode of YBCO film. Figure I. A typical I-V characteristic of the junction with double layer barrier at 45K. 3
25
2
3 15
1
o5
0 3
i
,
i
t
,
-2
-I
0
1
2
~/~,
4.Conclusion We had investigated the fabrication and properties of all a-axis YBCO tunnel junctions with double layer barrier of PGO and CeO2. The I-V characteristics of the junctions were in agreement with the RSJ model. The values of Jc, RnA and IcRn product for the junction were 0.9Xl03A/cm 2, 0.077p Q cm 2 and 0.07mV at 45K, respectively. The observed suppercurrent was modulated by an applied magnetic field, which was close to the Fraunhofer pattern. It is found that the double layer barrier structure is useful to suppress the effect of pinhole.
Acknowledgement
Figure 2. Magnetic-field dependence of critical current(Ic) for the a-axis junction with double layer barrier at 4.2K. J u n c d o n size is 3 x 15vrn2. 25
T h e a u t h o r s w o u l d like to t h a n k Mr. T. Aoki for t e c h n i c a l s u p p o r t . This work was supported by Grant-in-Aid for Scientific Research on Priority Area "Vortex Electronics".
Reference
2
1
o5
A
o i0
20
30 ~em~mtlax~
40
50
60
~)
Figure 3. Temperature dependence of critical current (Ic).
[1] P.A. Nilsson, G.Brorsson, E. Olsson, Z.G. Ivanov and T. Claeson, IEEE Trans. Appl. Supercond., 3 (1993) 2958. [2] G.Brorsson, E.Olsson, EA. Nilsson and T.Claeson, J. Appl. Phys. 75, (1994) 827. [3] T. Goto, T. Kuji, Y.S. Jiang, M. Moriya, K. Usami and T. Kobayashi, IEEE Trans. Appl. Supercond., 9, (1999) 1653. [4] Y.S. Jiang, T. Kobayashi and T. Goto, IEEE Trans. Appl. Supercond., 9, (1999) 1657.
Physica C 341-348 (2000) 2741-2742
www.elsevier.nl/Iocate/physc
J o s e p h s o n E f f e c t s in A - a x i s O r i e n t e d YBa2Cu307_y S I S T u n n e l J u n c t i o n s w i t h D o u b l e L a y e r Barrier You-song Jiang, Tadayuki Kobayashi and Toshinari Goto The University of Electro-Communications, Chofu-Shi, Tokyo, 182-8585, Japan
In this study, we have fabricated vertical SIS tunnel junctions with a-axis oriented YBa2Cu307.y(YBCO) electrodes. In order to prevent the pinhole in the barrier layer, double layer consists of PrGaO3(PGO) and CeO2 was used. The standard lithography technology and ion milling were employed to define the junctions with area of 31am× 151am. The junctions with barriers 5nm thick PGO and 2.5nm thick CeO2 showed supercurrents and RSJ like voltage-current(I-V) characteristics. Fraunhofer like magnetic field dependence of Ic were observed, and the results proved the existence of Josephson effects in these junctions. 1.Introduction High Tc trilayer SIS junctions have been studied, and it seemed to be difficult to fabricate the Josephson devices due to the short coherence length and the existence of pinhole like short circuits. In this experiment, we fabricated SIS junctions with a-axis oriented YBCO thin films, because the coherence length in the a-axis direction is ten times as long as in the c-axis direction. SrTiO3 substrate was chosen to use for obtaining a-axis oriented YBCO thin films with high critical current density (Jc). Double layer barrier was employed to prevent pinhole short circuit, which consisted of PrGaO3 (PGO) and CeO2. Both PGO and CeO2 have well lattice match with YBCO thin film to support epitaxial growth of later [1]-[3].
respectively. Then 150nm thick YBCO thin film as counter electrode was deposited at 720degree. Finally, a protection layer of 20nm thick Au was sputtered on top of the four-layer structure. Standard photolithography was used to'fabricate the junctions. They were delineated and patterned down to the base electrode by ion milling. The deposition and lift-off of amorphous MgO as the insulator layer were prepared before Au was put down for contacts to the counter electrodes of junctions. Lastly, the lead wires of Au were separated for four-terminal measurements. The X-ray diffraction measurement showed that base and counter electrodes of YBCO films were a-axis oriented and the multilayered structure was epitaxially grown.
2.Experiment We deposited a-axis oriented YBCO thin films on SrTiO3 substrates using DC hollow cathode sputtering method by the temperature gradient and deposition rate variation method that had been previously described in detail [4]. The thickness of the base YBCO was about 300nm. The barriers of PGO and CeO2 were deposited using ion beam sputtering method at substrate temperature of 680degree; their thickness was 5nm and 2.5nm,
3.Results and discussion Figure 1 shows the typical I-V characteristics of the junction measured at 45K. The area of the junction was 3Xl5~tm 2. The I-V curve is in agreement with RSJ-model. The critical current density Jc and the normal resistance normalized by °the junction area (RnS) are 0.9x 103A/cm2 and 0.077~tQcm 2 respectively. The IcRn product at 45K is 0.07mV. The magnetic field dependence of the critical
0921-4534/00/$ - see front matter © 2000 Elsevier Science B . ~ PI1 S0921-4534(00)01478-7
All rights reserved.
2742
Y-S. Jiang et al./Physica C 341-348 (2000) 2741-2742
current for our junction measured at 4.2K is shown in figure 2. The solid line is the Fraunhofer pattern fit to the curve with an offset of 0.83mA, indicating that modulation rate is 65%. The Ic for the junction as a function of temperature is shown in figure 3. As can be observed, Josephson current disappears at the temperature higher than 62K, which suggested that the transition temperature of the interface or the counter electrode for the junction is lower than the Tc of base electrode of YBCO film. Figure I. A typical I-V characteristic of the junction with double layer barrier at 45K. 3
25
2
3 15
1
o5
0 3
i
,
i
t
,
-2
-I
0
1
2
~/~,
4.Conclusion We had investigated the fabrication and properties of all a-axis YBCO tunnel junctions with double layer barrier of PGO and CeO2. The I-V characteristics of the junctions were in agreement with the RSJ model. The values of Jc, RnA and IcRn product for the junction were 0.9Xl03A/cm 2, 0.077p Q cm 2 and 0.07mV at 45K, respectively. The observed suppercurrent was modulated by an applied magnetic field, which was close to the Fraunhofer pattern. It is found that the double layer barrier structure is useful to suppress the effect of pinhole.
Acknowledgement
Figure 2. Magnetic-field dependence of critical current(Ic) for the a-axis junction with double layer barrier at 4.2K. J u n c d o n size is 3 x 15vrn2. 25
T h e a u t h o r s w o u l d like to t h a n k Mr. T. Aoki for t e c h n i c a l s u p p o r t . This work was supported by Grant-in-Aid for Scientific Research on Priority Area "Vortex Electronics".
Reference
2
1
o5
A
o i0
20
30 ~em~mtlax~
40
50
60
~)
Figure 3. Temperature dependence of critical current (Ic).
[1] P.A. Nilsson, G.Brorsson, E. Olsson, Z.G. Ivanov and T. Claeson, IEEE Trans. Appl. Supercond., 3 (1993) 2958. [2] G.Brorsson, E.Olsson, EA. Nilsson and T.Claeson, J. Appl. Phys. 75, (1994) 827. [3] T. Goto, T. Kuji, Y.S. Jiang, M. Moriya, K. Usami and T. Kobayashi, IEEE Trans. Appl. Supercond., 9, (1999) 1653. [4] Y.S. Jiang, T. Kobayashi and T. Goto, IEEE Trans. Appl. Supercond., 9, (1999) 1657.