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Effect of hydrogen doping on electrical properties of YBaCuO thin films
Physica C 235 240 (1994) 587-588 North-Holland
PHYSICA
(~
Effect of Hydrogen Doping on Electrical Properties of Y-Ba-Cu-O Thin Films + w . Kula* and Roman Sobolewski Department of Electrical Engineering and Laboratory for Laser Energetics, University of Rochester, Rochester, NY 14627, USA We report our studies on the influence of hydrogen doping on the electrical properties of epitaxial YBa2Cu30 x (YBCO) thin films of various oxygen content. We have found that hydrogenation increased the YBCO's normal-state resistivity, as well as broadened its superconducting transition and decreased the T c. The above effects were most pronounced in YBCO with low oxygen contents. However, they could be reversed by a subsequent annealing of the samples in oxygen. We associate the observed behavior with the hydrogen-diffusioninduced modification of the free-hole concentration in YBCO CuO 2 planes. Electrical properties of YBa2Cu30 x (YBCO) are very sensitive to the c o m p o u n d ' s free-carrier concentration, which can be significantly affected by a proper chemical doping. One of the most interesting dopants of YBCO is hydrogen, which, as it was recently demonstrated for superconducting YBCO ceramics [1] and thin films [2], can be easily incorporated into Y B C O ' s crystalline structure. Hydrogen doping can be done at relatively low temperatures, and it does not cause any severe chemical degradation of YBCO. In this work we report our study on the influence of hydrogen doping on the electrical properties of epitaxial YBCO thin films of various oxygen content. Motivation of this study is to better understand the charge-transfer process in YBCO. It is also related to potential applications of hydrogenation for controlled modifications of YBCO electrical properties. Our test structures consisted of 150-nm-thick YBCO-on-LaA103 films with fully oxygenated (T c -90 K), partially oxygen-depleted (T c = 20-60 K), and oxygen-poor (semiconducting) YBCO regions, patterned in each sample by a laser-writing technique [3]. The structures were hydrogenated by furnace annealing at 120°C-200°C for 0.5-48 h in a flow of a 4%-H2/96%-N 2 gas mixture. Some of the samples were subsequently re-oxygenated either by furnace
annealing at 280°C, or laser writing. In latter cases, we were interested in reversibility of our hydrogenation procedure. Figure l(a) shows resistance versus temperature R(T) dependencies for a sample consisting of a series connection of the oxygen-rich and partially oxygen depleted (with T c --- 50 K) regions. The curves were registered before and after the sample hydrogenation at 180°C for 5 h. As we see, the hydrogenation resulted in a widening of the superconducting transition of the fully oxygenated YBCO phase and a T c decrease in the o x y g e n - d e p l e t e d region. Figure l(a) also shows that the observed changes were to some extent reversible and a few days after the hydrogenation, this sample, as well as any others, exhibited a "healing" process that resulted in a partial reversal of the hydrogenation-induced changes. It is interesting to note that during that healing period, sample's conductivity turned out to be very sensitive to the external electric field [4]. The effect of hydrogen doping was most pronounced in YBCO with lower oxygen content (i.e., in semiconducting YBCO), whose resistivity increased after hydrogenation by an order of magnitude [Fig. 1(b)]. The hydrogenation process did not, however, affect the mechanism of electrical transport in semiconducting YBCO. Both before and
+ Work supported by the Air Force Office of Scientific Research under Grant No. F49620-94-1-0094. Also at the Institute of Physics of the Polish Academy of Sciences, PL-02 668 Warszawa, Poland. 0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)00855-8
588
lg Ku/a. R. Sobolewski/t'hvsica (' 235 240 (/994) 587 5,'¢~
after hydrogenation, oxygen-poor samples exhibited a variable-length hopping conductivity, as it is indicated in the inset in Fig. l(b). We have also found that a prolonged, e.g., 48-h hydrogenation had a destructive impact on our samples (Fig. 2), since it resulted in a large increase of the normal-state resistivity and disappearance of superconductivity even in the oxygen-rich YBCO phase. This
destruction, however, could be to a large extem reversed, as it is shown in Fig. 2, by subsequent annealing of the sample in oxygen. Finally, we note that properties of control samples, annealed tbr 5 h at 180°C in pure argon atmosphere, remained unchanged, demonstrating that all the discussed above effects were due to the hydrogen doping of our samples.
600
80
~ 400
(a)
(D
,
,
,
~
108
60
106
40
104
--
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"~ 200
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l
f
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20
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0 30 z1772
50 70 Temperature (K)
() ZI777
109
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K~ , ~
1
0.2 / 4
0.4 )
0,(
Z1773
100 200 Temperature (K)
100 150 200 250 300 Temperature (K)
-
103 0
50
Fig. 2. Resistivity vs temperature for a fully oxygenated YBCO film before hydrogenation (solid line), after hydrogenation at 200°C for 48 h (dotted line), and after subsequent annealing in oxygen at 280°C for 24 h (dashed line).
5" 1°7 e-, ea
~
I 00
0
90
10 2 I
300
Fig. 1. Resistivity vs temperature of a laser-written YBCO test structure registered before (dashed lines), immediately after (dotted lines), and seven days after (solid lines) the sample hydrogenation at 180°C for 5 h. (a) superconducting YBCO, (b) semiconducting YBCO.
In conclusion, our results support the concepl that hydrogen atoms incorporated into YBCO easily contribute additional electrons and decrease the freehole concentration in the YBCO CuO 2 planes, thus decreasing the normal-state conductivity and T:. I. 2.
3. 4.
E. K Shalkova, Yu. M. Baikov, and T A Ushakova, Superconductivity, 5 (1992) 22 G. Dortmann, J. Erxmeyer, S. Blfisser, J. Steiger, T. Paatsch, A. Weidinger, H. Karl, and B. Stritzker, Phys. Rev. B, 49 (1994) 600. R. Sobolewski, W. Xiong, W. Kula, and J. R Gavaler, Appl. Phys. Lett., 64 (1994) 643 . W. Kula and R. Sobolewski, Phys. Rew B, 49 (1994) 6428.
PHYSICA
(~
Effect of Hydrogen Doping on Electrical Properties of Y-Ba-Cu-O Thin Films + w . Kula* and Roman Sobolewski Department of Electrical Engineering and Laboratory for Laser Energetics, University of Rochester, Rochester, NY 14627, USA We report our studies on the influence of hydrogen doping on the electrical properties of epitaxial YBa2Cu30 x (YBCO) thin films of various oxygen content. We have found that hydrogenation increased the YBCO's normal-state resistivity, as well as broadened its superconducting transition and decreased the T c. The above effects were most pronounced in YBCO with low oxygen contents. However, they could be reversed by a subsequent annealing of the samples in oxygen. We associate the observed behavior with the hydrogen-diffusioninduced modification of the free-hole concentration in YBCO CuO 2 planes. Electrical properties of YBa2Cu30 x (YBCO) are very sensitive to the c o m p o u n d ' s free-carrier concentration, which can be significantly affected by a proper chemical doping. One of the most interesting dopants of YBCO is hydrogen, which, as it was recently demonstrated for superconducting YBCO ceramics [1] and thin films [2], can be easily incorporated into Y B C O ' s crystalline structure. Hydrogen doping can be done at relatively low temperatures, and it does not cause any severe chemical degradation of YBCO. In this work we report our study on the influence of hydrogen doping on the electrical properties of epitaxial YBCO thin films of various oxygen content. Motivation of this study is to better understand the charge-transfer process in YBCO. It is also related to potential applications of hydrogenation for controlled modifications of YBCO electrical properties. Our test structures consisted of 150-nm-thick YBCO-on-LaA103 films with fully oxygenated (T c -90 K), partially oxygen-depleted (T c = 20-60 K), and oxygen-poor (semiconducting) YBCO regions, patterned in each sample by a laser-writing technique [3]. The structures were hydrogenated by furnace annealing at 120°C-200°C for 0.5-48 h in a flow of a 4%-H2/96%-N 2 gas mixture. Some of the samples were subsequently re-oxygenated either by furnace
annealing at 280°C, or laser writing. In latter cases, we were interested in reversibility of our hydrogenation procedure. Figure l(a) shows resistance versus temperature R(T) dependencies for a sample consisting of a series connection of the oxygen-rich and partially oxygen depleted (with T c --- 50 K) regions. The curves were registered before and after the sample hydrogenation at 180°C for 5 h. As we see, the hydrogenation resulted in a widening of the superconducting transition of the fully oxygenated YBCO phase and a T c decrease in the o x y g e n - d e p l e t e d region. Figure l(a) also shows that the observed changes were to some extent reversible and a few days after the hydrogenation, this sample, as well as any others, exhibited a "healing" process that resulted in a partial reversal of the hydrogenation-induced changes. It is interesting to note that during that healing period, sample's conductivity turned out to be very sensitive to the external electric field [4]. The effect of hydrogen doping was most pronounced in YBCO with lower oxygen content (i.e., in semiconducting YBCO), whose resistivity increased after hydrogenation by an order of magnitude [Fig. 1(b)]. The hydrogenation process did not, however, affect the mechanism of electrical transport in semiconducting YBCO. Both before and
+ Work supported by the Air Force Office of Scientific Research under Grant No. F49620-94-1-0094. Also at the Institute of Physics of the Polish Academy of Sciences, PL-02 668 Warszawa, Poland. 0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)00855-8
588
lg Ku/a. R. Sobolewski/t'hvsica (' 235 240 (/994) 587 5,'¢~
after hydrogenation, oxygen-poor samples exhibited a variable-length hopping conductivity, as it is indicated in the inset in Fig. l(b). We have also found that a prolonged, e.g., 48-h hydrogenation had a destructive impact on our samples (Fig. 2), since it resulted in a large increase of the normal-state resistivity and disappearance of superconductivity even in the oxygen-rich YBCO phase. This
destruction, however, could be to a large extem reversed, as it is shown in Fig. 2, by subsequent annealing of the sample in oxygen. Finally, we note that properties of control samples, annealed tbr 5 h at 180°C in pure argon atmosphere, remained unchanged, demonstrating that all the discussed above effects were due to the hydrogen doping of our samples.
600
80
~ 400
(a)
(D
,
,
,
~
108
60
106
40
104
--
F e,.)
"~ 200
"
J
l
f
I I
20
S,'-7
0 30 z1772
50 70 Temperature (K)
() ZI777
109
! 105 ! ._~
K~ , ~
1
0.2 / 4
0.4 )
0,(
Z1773
100 200 Temperature (K)
100 150 200 250 300 Temperature (K)
-
103 0
50
Fig. 2. Resistivity vs temperature for a fully oxygenated YBCO film before hydrogenation (solid line), after hydrogenation at 200°C for 48 h (dotted line), and after subsequent annealing in oxygen at 280°C for 24 h (dashed line).
5" 1°7 e-, ea
~
I 00
0
90
10 2 I
300
Fig. 1. Resistivity vs temperature of a laser-written YBCO test structure registered before (dashed lines), immediately after (dotted lines), and seven days after (solid lines) the sample hydrogenation at 180°C for 5 h. (a) superconducting YBCO, (b) semiconducting YBCO.
In conclusion, our results support the concepl that hydrogen atoms incorporated into YBCO easily contribute additional electrons and decrease the freehole concentration in the YBCO CuO 2 planes, thus decreasing the normal-state conductivity and T:. I. 2.
3. 4.
E. K Shalkova, Yu. M. Baikov, and T A Ushakova, Superconductivity, 5 (1992) 22 G. Dortmann, J. Erxmeyer, S. Blfisser, J. Steiger, T. Paatsch, A. Weidinger, H. Karl, and B. Stritzker, Phys. Rev. B, 49 (1994) 600. R. Sobolewski, W. Xiong, W. Kula, and J. R Gavaler, Appl. Phys. Lett., 64 (1994) 643 . W. Kula and R. Sobolewski, Phys. Rew B, 49 (1994) 6428.