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Correlation between surface resistance of Yba2Cu3O7-δ thin films and oxygen pressure during deposition process
~
m
PHYSICA ©
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ELSEVIER
Physica C 282-287 (1997) 591-592
Correlation between surface resistance of Yba2Cu307_o thin films and oxygen pressure during deposition process * a
b
d
R.X. Wua, S.Z. Yang , Y.L. Zhou , B.B. Jin", M. Qiana, P.ll. Wua, J.H. Gu , and D.S. Dingd a Department
of Electronic Science and Engineering, Nanjing University, Nanjing 210093, P.R. CHINA Center for AdvancedStudies in Science and Technology of Microstructures, Nanjing University, Nanjing 210093, P.R. CHINA
b
Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P.R. CHINA
e Department of radio engineering, Nanjing University of Post and Telecommunication, Nanjing 210003, P.R. CHINA d
Laboratory of Molecular Bio-Molecular Electronics, Southeast University, Nanjing 210096, P.R. CHINA.
In this paper, we study the correlation between the depositing oxygen pressure and surface resistance Rs of YBa2Cu307_o (YBCO) films fabricated by pulsed-laser deposition method. It reveals that oxygen pressure has great influence on Rs. In terms of minimum Rs' there exists optimum value of oxygen pressure which is different from the optimum oxygen pressure for maximum Te. The crystalline orientation and surface morphologies of the films are helpful in understanding the variations of Rs with the oxygen pressure.
1. INTRODUCTION For the applications of high temperature superconducting (HTS) thin films, especially for passive microwave device applications, it is essential to be able to reproducibly deposit epitaxial thin films with low surface resistance Rs. For in-situ methods, the fabrication consists two processes, i.e. the deposition and the oxygenation, in both of which oxygen plays an important role. Previous studies on the oxygenation process have demonstrated that change in oxygen pressure has marginal effect on film's surface resistance Rs but has great effect on critical temperature Te [1-2]. For the deposition process we have found that oxygen pressure P(02) has great influences on Rs and T e' Reported here will be our main results and the comparisons between the influences on Rs and Te. 2. EXPERIMENT AND DISCUSSION To study the effects of oxygen pressure on the YBCO films during the deposition, it is important to
select a suitable deposition method. Pulsed-laser deposition (PLD) method has a major benefit that the thin film deposited has the same composition as that of the target except oxygen [3], making itself a good candidate for the experiments. We have made two sets of YBCO thin films with dimension of 10mm x 15mm. The substrates are Ystabilized Zironia (YSZ) for one set and LaAI03 (LAO) for another. For each set of the films, all fabrication conditions are the same except the oxygen pressure during the deposition. Each of the films ablated by PLD [4] is incorporated in turn into a TEo]o sapphire resonator as one of the end plate and HP8510 network analyzer is used to measure the unloaded quality factor of the resonator at 77K and 17.7 GHz so that the film's Rs can be found [7]. Then the critical temperature T e is measured by ac susceptibility at frequency I kHz. The results are summarized in Table I. From the Table, we can see that Rs values change with P(02) appreciably no mater what substrates the films are on. With the increase ofp(02) the Rs values
• This work is supported by the State Key Laboratory of Millimeter Waves, Southeast University, partly by the National Center for Research and Development on Superconductivity of China, and partly by the international collaborative research project of Telecommunications Advancement Organization (TAO) of Japan. 0921-4534/97/$17.00 © Elsevier Science B.Y. All rights reserved. PH S0921-4534(97)00399-7
RX Wu et a/.f Physica C 282-287 (1997) 591-592
592
Table I.
The results of the measurements YSZ
substrate
LAO
P(02) (Pa)
10
20
40
80
10
70
90
125
R. (mn)
2.6
2.4
2.0
275
64.5
2.0
1.4
14.6
Te (K)
84
87
86
85
83
90
87
87
orientation (axis)
c
c
c
c&a
c
c
c
c
1.03
0.63
0.63
0.65
FWHM (deg) RMS(nm)
32
10
6
decrease first and then increase, giving an optimum oxygen pressure somewhere in between which is substrate dependent. Similar to R., Te also changes with P(02). However, it is noticed that the optimum oxygen pressure for minimum R. is different from that for maximum Te. R. reaches its minimum after Tc reaches its maximum. Therefore, one should optimize the deposition process according to specific purpose. The decrease of Tc at high P(02) can be explained by oxygen over-doping [I]. To understand the dependence of the R. on P(02)' we check the film's structure by x-ray de fraction (XRD) and surface morphology by atomic force microscope (AFM). The results are also listed in Table 1. Evidently, the films on YSZ substrate are caxis oriented and the values of root-mean-square (RMS) roughness of the films decrease with the increase of P(02) until the optimum P(02) for minimum R. is reached. After that, the films have mixed c-axis and a-axis orientations and RMS of the films increase. Meanwhile, R. increases rapidly. On LAO substrates, the films are all c-axis oriented but the inplane alignment of the grains is different. The full width at half maximum (FWHM) of (005) peak decreases with the increase of P(02) until the R. reaches its minimum value and then FWHM increases again. It is evident that oxygen pressure has a great influence on crystalline quality and morphology of the YBCO films which, in tum, affects the film's surface resistance. At a proper oxygen pressure, the deposited film has a smooth surface, a preferred orientation and thus a low R..
15
3. SUMMARY We have studied the correlation between the surface resistance R. of the YBCO thin films and the oxygen pressure P(02) during the deposition. The experimental results show that R. is greatly affected by the P(02). The oxygen pressure during the deposition affects film's Tc as well. The optimum value of P(02) for minimum R. is different from that for maximum Te. Therefore, one should optimize the deposition conditions according to specific purpose. The change of R. with P(02) can be explained by the effects of P(02) on film's orientation and surface morphology.
ACKNOWLEDGMENT The authors thank J. Lu, X.Y. Ren and Q.S. Zhang (University of Electronic Science and Technology of China) for providing dielectric resonator.
REFERENCES 1. N.G. Chew, J.A. Edwards R.G. Humphreys, J.S. Satchell, S.W. Goodyear, B. Dew, and et aI., IEEE Trans. Appl. Supercond. 5 (1995) 1167 2. I.R. Gavaler. J. Talvacchio and R.W. Weinert, IEEE Trans. Appl. Supercond. 5 (1995) 1173 3. C.H. Stoessel, R.F. Bunshah, S. Prakash and H.R. Fetterman, J.Supercond. 6 (1993) 1 4. YJ. Tian, H.B. Lu, S.F. Xu, Z.H. Chen, D.F. Cui, Y.L, Zhou, Y.Z. Zhang, L. Li and G.Z. Yang, Physica C 220 (1994) 114. 5. J. Lu, X.Y. Ren and Q.S. Zhang, ICCS/'94 (1994) 959
m
PHYSICA ©
~
ELSEVIER
Physica C 282-287 (1997) 591-592
Correlation between surface resistance of Yba2Cu307_o thin films and oxygen pressure during deposition process * a
b
d
R.X. Wua, S.Z. Yang , Y.L. Zhou , B.B. Jin", M. Qiana, P.ll. Wua, J.H. Gu , and D.S. Dingd a Department
of Electronic Science and Engineering, Nanjing University, Nanjing 210093, P.R. CHINA Center for AdvancedStudies in Science and Technology of Microstructures, Nanjing University, Nanjing 210093, P.R. CHINA
b
Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P.R. CHINA
e Department of radio engineering, Nanjing University of Post and Telecommunication, Nanjing 210003, P.R. CHINA d
Laboratory of Molecular Bio-Molecular Electronics, Southeast University, Nanjing 210096, P.R. CHINA.
In this paper, we study the correlation between the depositing oxygen pressure and surface resistance Rs of YBa2Cu307_o (YBCO) films fabricated by pulsed-laser deposition method. It reveals that oxygen pressure has great influence on Rs. In terms of minimum Rs' there exists optimum value of oxygen pressure which is different from the optimum oxygen pressure for maximum Te. The crystalline orientation and surface morphologies of the films are helpful in understanding the variations of Rs with the oxygen pressure.
1. INTRODUCTION For the applications of high temperature superconducting (HTS) thin films, especially for passive microwave device applications, it is essential to be able to reproducibly deposit epitaxial thin films with low surface resistance Rs. For in-situ methods, the fabrication consists two processes, i.e. the deposition and the oxygenation, in both of which oxygen plays an important role. Previous studies on the oxygenation process have demonstrated that change in oxygen pressure has marginal effect on film's surface resistance Rs but has great effect on critical temperature Te [1-2]. For the deposition process we have found that oxygen pressure P(02) has great influences on Rs and T e' Reported here will be our main results and the comparisons between the influences on Rs and Te. 2. EXPERIMENT AND DISCUSSION To study the effects of oxygen pressure on the YBCO films during the deposition, it is important to
select a suitable deposition method. Pulsed-laser deposition (PLD) method has a major benefit that the thin film deposited has the same composition as that of the target except oxygen [3], making itself a good candidate for the experiments. We have made two sets of YBCO thin films with dimension of 10mm x 15mm. The substrates are Ystabilized Zironia (YSZ) for one set and LaAI03 (LAO) for another. For each set of the films, all fabrication conditions are the same except the oxygen pressure during the deposition. Each of the films ablated by PLD [4] is incorporated in turn into a TEo]o sapphire resonator as one of the end plate and HP8510 network analyzer is used to measure the unloaded quality factor of the resonator at 77K and 17.7 GHz so that the film's Rs can be found [7]. Then the critical temperature T e is measured by ac susceptibility at frequency I kHz. The results are summarized in Table I. From the Table, we can see that Rs values change with P(02) appreciably no mater what substrates the films are on. With the increase ofp(02) the Rs values
• This work is supported by the State Key Laboratory of Millimeter Waves, Southeast University, partly by the National Center for Research and Development on Superconductivity of China, and partly by the international collaborative research project of Telecommunications Advancement Organization (TAO) of Japan. 0921-4534/97/$17.00 © Elsevier Science B.Y. All rights reserved. PH S0921-4534(97)00399-7
RX Wu et a/.f Physica C 282-287 (1997) 591-592
592
Table I.
The results of the measurements YSZ
substrate
LAO
P(02) (Pa)
10
20
40
80
10
70
90
125
R. (mn)
2.6
2.4
2.0
275
64.5
2.0
1.4
14.6
Te (K)
84
87
86
85
83
90
87
87
orientation (axis)
c
c
c
c&a
c
c
c
c
1.03
0.63
0.63
0.65
FWHM (deg) RMS(nm)
32
10
6
decrease first and then increase, giving an optimum oxygen pressure somewhere in between which is substrate dependent. Similar to R., Te also changes with P(02). However, it is noticed that the optimum oxygen pressure for minimum R. is different from that for maximum Te. R. reaches its minimum after Tc reaches its maximum. Therefore, one should optimize the deposition process according to specific purpose. The decrease of Tc at high P(02) can be explained by oxygen over-doping [I]. To understand the dependence of the R. on P(02)' we check the film's structure by x-ray de fraction (XRD) and surface morphology by atomic force microscope (AFM). The results are also listed in Table 1. Evidently, the films on YSZ substrate are caxis oriented and the values of root-mean-square (RMS) roughness of the films decrease with the increase of P(02) until the optimum P(02) for minimum R. is reached. After that, the films have mixed c-axis and a-axis orientations and RMS of the films increase. Meanwhile, R. increases rapidly. On LAO substrates, the films are all c-axis oriented but the inplane alignment of the grains is different. The full width at half maximum (FWHM) of (005) peak decreases with the increase of P(02) until the R. reaches its minimum value and then FWHM increases again. It is evident that oxygen pressure has a great influence on crystalline quality and morphology of the YBCO films which, in tum, affects the film's surface resistance. At a proper oxygen pressure, the deposited film has a smooth surface, a preferred orientation and thus a low R..
15
3. SUMMARY We have studied the correlation between the surface resistance R. of the YBCO thin films and the oxygen pressure P(02) during the deposition. The experimental results show that R. is greatly affected by the P(02). The oxygen pressure during the deposition affects film's Tc as well. The optimum value of P(02) for minimum R. is different from that for maximum Te. Therefore, one should optimize the deposition conditions according to specific purpose. The change of R. with P(02) can be explained by the effects of P(02) on film's orientation and surface morphology.
ACKNOWLEDGMENT The authors thank J. Lu, X.Y. Ren and Q.S. Zhang (University of Electronic Science and Technology of China) for providing dielectric resonator.
REFERENCES 1. N.G. Chew, J.A. Edwards R.G. Humphreys, J.S. Satchell, S.W. Goodyear, B. Dew, and et aI., IEEE Trans. Appl. Supercond. 5 (1995) 1167 2. I.R. Gavaler. J. Talvacchio and R.W. Weinert, IEEE Trans. Appl. Supercond. 5 (1995) 1173 3. C.H. Stoessel, R.F. Bunshah, S. Prakash and H.R. Fetterman, J.Supercond. 6 (1993) 1 4. YJ. Tian, H.B. Lu, S.F. Xu, Z.H. Chen, D.F. Cui, Y.L, Zhou, Y.Z. Zhang, L. Li and G.Z. Yang, Physica C 220 (1994) 114. 5. J. Lu, X.Y. Ren and Q.S. Zhang, ICCS/'94 (1994) 959