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Analysis of persistent photoconductivity on oxygen deficient YBa2Cu3O7-δ thin films
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PHYSICA ®
ELSEVIER
Physica C 282-287 (1997) 671--672
Analysis of Persistent Photoconductivity on Oxygen Deficient YBa1Cu 307-x Thin Films D. Girata8, B. Arenasa, R. Hoyos8, 1. Osorio, M. E. Gomez, 1. Heirasb, and P. Prieto Departamento de Fisica, Universidad del Valle, A. A. 25360, Cali, Colombia Fully oxygenated epitaxial thin films ofYBa2Cu3O,_x with thicknesses between 100 and 300 DID were deposited by a dc high oxygen pressure sputtering method on (00 I) SrTi03 substrates. From these films we have obtained samples with different oxygen deficiency x between 0.2 to 0.8, using a thermal treatment. These films were illuminated at 10K, 95K, 273K and near room temperature with 632.8 DID laser light for more than three hours. The change of the conductivity during and after illumination was fitted according to the Kohlrausch expression L\cr(t) = L\cr(O)exp[-(tlt)~]. The relaxation time 't and the dispersion parameter ~ depend strongly upon oxygen content and temperature during illumination. This behavior indicate that persistent photoconductivity (pPC) in these films may be due to trapping of electrons in the Cu - 0 chains. This suggests the existence of a large energy barrier impeding the electrons to recombine and finally holes are transferred to the CU02 planes enhancing the conductivity. 1. INTRODUCTION
3. RESULTS AND DISCUSSION
Persistent Photoconductivity (PPC) and Photoinduced Superconductivity (PSC) [1,2] are the most important phenomena that appear when oxygen deficient YBa2Cu307_x thin films, 0 < x ~ I, are illuminated with visible light. These effects can be observed from resistance measurements during and after illumination. The behavior of the relative photoinduced conductivity and the photoinduced excess conductivity during the excitation and relaxation processes in these films can be explained by a model based on the trapping of electrons at oxygen vacancies at the Cu - 0 chains [3].
From four probe resistance measurements in a close-cycle He cryostat, the changes of the electrical conductivity of YBa2Cu307_x films were analyzed for different oxygen contents and temperatures, before and after illumination with 632.8 nm laser light, for several hours. The resistance of the films decreased with time and relaxed only if the temperature was above lOOK. For superconducting films with x between 0.2 to 0.55, we found an increase in Tc after illumination for eight hours. The relative photoinduced conductivity L\cr(t)/crdarlc, shows in Figure I (L\cr(t) = cr(t) - crdark). This figure
0.4 r--------;=====:=::;] 2. SAMPLE PREPARATION ~
High quality superconducting epitaxial YBa2Cu307_x thin films were grown in situ by a high pressure dc sputtering technique on SrTi03 substrates; experimental details of this method are described elsewhere [4]. The values of c-Iattice parameters calculated from x-ray spectra correspond to those expected for fully oxygenated thin films. Oxygen can be removed fum YBazCuJo,-x films by a treatment acca-ding the pressure-temperature phase diagram [5]. The procedure is similar to that described in [3]. The FWHM of the (005) peak fir the films was about 0.25 0 indicating that the film epitaxy had not been deteriorated by the deoxygenation process.
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6.6
6.8
B.v.
All rights reserved.
7.0
Oxygen content
Figure I. The relative photoinduced conductivity vs. oxygen content and temperature (inset).
"On leave lim1 tJnMmdad de Antioquia, Medellin, Colrmbia, b llM-UNAM, Mexioo. Suppooed by COLCIENCIAS, prqject 1115-{)5-409-94. 0921-4534/97/$17.00 © Elsevier Science PH S0921-4534(97)00489-9
•
'--~~-~~---'
672
D. Girata et al. / Physica C 282-287 (1997) 671-672
shows that the magnitude of the PPC decreases exponentially with the oxygen content and leads to enhance the conductivity for lower oxygen content and decreases with temperature faster than exponentially for near room temperature and slower for T < 273 K, as shown in the inset of Figure 1. The behavior of the photoinduced excess conductivity Acr~(t)/Acr~(O), where Acr~(t) is the change of the conductivity from an equilibrium value creq, can be fitted according to the Kohlrausch expression [1], Acr~(t) = Acreq(O) exp[-(tlt)P1 where t is the relaxation time and ~ is the dispersion parameter. While t decreases exponentially with the oxygen content, as shown in Figure 2, ~ increases as shown in the inset; notice that ~ varies slowly in the region Xc - 0.6, where the metal-insulator transition (MIT) takes place. 180r---------~============~
decay time t and lower energy barrier when the oxygen content decreases. 30~-------r========~ ~, )( = 0.75 1.0
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0.8
.. 20 ~
~
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0.6~/
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......
.. 10
275
300
325 T(K)
275
300
325
Temperature (K)
Figure 3. Relaxation times t and dispersion parameter ~ (inset) for several temperatures. Thus, the effect of illumination on conductivity is the same as that produced by oxygen doping: both increase the number of holes in the CU02 planes. The electron is trapped at the site of an oxygen vacancy in the Cu - 0 chains, producing a lattice distortion; this results in a large energy barrier preventing the reverse recombination with the hole. According to the model of induced vacancy capture [3] recombination of a trapped electron with a hole is possible at high enough temperatures since the electron may be thermally activated out of the trap.
Oxygen Content
Figure 2. Relaxation time t and dispersion parameter ~ (inset) for several oxygen contents. The relaxatioo time can be fitted with an expooential decay, as shown in Figure 3; this can be explained by a thermally activated relaxatioo process aaoos an energy bmier such that t(1) = to exp(MaT), where A = 0.673 eV. This value is lowt!f than 0.935 eV which was reported by Kudinov et al [1] for a sample with an oxygen content near the MIT. We found to = 3.5 X 10-8 s when the film with x = 0.75 relaxed at temperatures between 273K and 325K, after illumination at these temperatures. The dispersion parameter increased with temperature and can be fitted with a linear function, ~(T) = -1.15+T/157.5, as indicated with dashed line in the inset of Figure 3. These results, together with the analysis of till show an enhancement of PPC with smaller excitation time till, higher relative photoinduced conductivity, large
Acknowledgment: One of authors (D. Girata) wishes to thank the hospitality of the Schuller's group at the University of California, San Diego, where deoxygenation of the samples were carried out. REFERENCES
1. V. I. Kudinov, I. L. Chaplygin, A. I. Kirriyuk, N. M. Kreines, R. Laiho, E. Uhderanta, and C. Ayache, Phys. Rev. B 47,9017 (1993). 2. G. Nieva, E. Osquiguil, J. Guimpe~ M. Maenhoudt, B. Wuyts, Y. Brunyseraede, M. B. Maple, and I. K. Schuller, Appl. Pbys. Lett. 60, 2159 (1992). 3. J. Hasen, D. Lederman, I. K. Schuller, V. Kudinov, M. Maenhoudt and Y. Bruynseraede, Phys. Rev. B, 51 (2), 1342 (1995). 4. D. Girata, Y. Rojas, E. Bacca, M. Chac6n, W. lq1era, and P. Prieto, Sol. State Coot. 90, 539 (1994). 5. P. K. Gallagher, Adv. Ceramic Mat. 2, 632 (1987).
m ~
PHYSICA ®
ELSEVIER
Physica C 282-287 (1997) 671--672
Analysis of Persistent Photoconductivity on Oxygen Deficient YBa1Cu 307-x Thin Films D. Girata8, B. Arenasa, R. Hoyos8, 1. Osorio, M. E. Gomez, 1. Heirasb, and P. Prieto Departamento de Fisica, Universidad del Valle, A. A. 25360, Cali, Colombia Fully oxygenated epitaxial thin films ofYBa2Cu3O,_x with thicknesses between 100 and 300 DID were deposited by a dc high oxygen pressure sputtering method on (00 I) SrTi03 substrates. From these films we have obtained samples with different oxygen deficiency x between 0.2 to 0.8, using a thermal treatment. These films were illuminated at 10K, 95K, 273K and near room temperature with 632.8 DID laser light for more than three hours. The change of the conductivity during and after illumination was fitted according to the Kohlrausch expression L\cr(t) = L\cr(O)exp[-(tlt)~]. The relaxation time 't and the dispersion parameter ~ depend strongly upon oxygen content and temperature during illumination. This behavior indicate that persistent photoconductivity (pPC) in these films may be due to trapping of electrons in the Cu - 0 chains. This suggests the existence of a large energy barrier impeding the electrons to recombine and finally holes are transferred to the CU02 planes enhancing the conductivity. 1. INTRODUCTION
3. RESULTS AND DISCUSSION
Persistent Photoconductivity (PPC) and Photoinduced Superconductivity (PSC) [1,2] are the most important phenomena that appear when oxygen deficient YBa2Cu307_x thin films, 0 < x ~ I, are illuminated with visible light. These effects can be observed from resistance measurements during and after illumination. The behavior of the relative photoinduced conductivity and the photoinduced excess conductivity during the excitation and relaxation processes in these films can be explained by a model based on the trapping of electrons at oxygen vacancies at the Cu - 0 chains [3].
From four probe resistance measurements in a close-cycle He cryostat, the changes of the electrical conductivity of YBa2Cu307_x films were analyzed for different oxygen contents and temperatures, before and after illumination with 632.8 nm laser light, for several hours. The resistance of the films decreased with time and relaxed only if the temperature was above lOOK. For superconducting films with x between 0.2 to 0.55, we found an increase in Tc after illumination for eight hours. The relative photoinduced conductivity L\cr(t)/crdarlc, shows in Figure I (L\cr(t) = cr(t) - crdark). This figure
0.4 r--------;=====:=::;] 2. SAMPLE PREPARATION ~
High quality superconducting epitaxial YBa2Cu307_x thin films were grown in situ by a high pressure dc sputtering technique on SrTi03 substrates; experimental details of this method are described elsewhere [4]. The values of c-Iattice parameters calculated from x-ray spectra correspond to those expected for fully oxygenated thin films. Oxygen can be removed fum YBazCuJo,-x films by a treatment acca-ding the pressure-temperature phase diagram [5]. The procedure is similar to that described in [3]. The FWHM of the (005) peak fir the films was about 0.25 0 indicating that the film epitaxy had not been deteriorated by the deoxygenation process.
~ 0.3 ~ -.II:
~ 0.2
. '
~ 0.6 ~'.
~"O
x= 0.75
0.4·
..•
~
S
0.2
\
'r
-
0.0
Z'
S 0.1
o
150
300 T(K)
o.o~--~~~~---~-·---~~~~~-·~
6.2
6.4
6.6
6.8
B.v.
All rights reserved.
7.0
Oxygen content
Figure I. The relative photoinduced conductivity vs. oxygen content and temperature (inset).
"On leave lim1 tJnMmdad de Antioquia, Medellin, Colrmbia, b llM-UNAM, Mexioo. Suppooed by COLCIENCIAS, prqject 1115-{)5-409-94. 0921-4534/97/$17.00 © Elsevier Science PH S0921-4534(97)00489-9
•
'--~~-~~---'
672
D. Girata et al. / Physica C 282-287 (1997) 671-672
shows that the magnitude of the PPC decreases exponentially with the oxygen content and leads to enhance the conductivity for lower oxygen content and decreases with temperature faster than exponentially for near room temperature and slower for T < 273 K, as shown in the inset of Figure 1. The behavior of the photoinduced excess conductivity Acr~(t)/Acr~(O), where Acr~(t) is the change of the conductivity from an equilibrium value creq, can be fitted according to the Kohlrausch expression [1], Acr~(t) = Acreq(O) exp[-(tlt)P1 where t is the relaxation time and ~ is the dispersion parameter. While t decreases exponentially with the oxygen content, as shown in Figure 2, ~ increases as shown in the inset; notice that ~ varies slowly in the region Xc - 0.6, where the metal-insulator transition (MIT) takes place. 180r---------~============~
decay time t and lower energy barrier when the oxygen content decreases. 30~-------r========~ ~, )( = 0.75 1.0
•
~",
•'.
0.8
.. 20 ~
~
o
0.6~/
~
......
.. 10
275
300
325 T(K)
275
300
325
Temperature (K)
Figure 3. Relaxation times t and dispersion parameter ~ (inset) for several temperatures. Thus, the effect of illumination on conductivity is the same as that produced by oxygen doping: both increase the number of holes in the CU02 planes. The electron is trapped at the site of an oxygen vacancy in the Cu - 0 chains, producing a lattice distortion; this results in a large energy barrier preventing the reverse recombination with the hole. According to the model of induced vacancy capture [3] recombination of a trapped electron with a hole is possible at high enough temperatures since the electron may be thermally activated out of the trap.
Oxygen Content
Figure 2. Relaxation time t and dispersion parameter ~ (inset) for several oxygen contents. The relaxatioo time can be fitted with an expooential decay, as shown in Figure 3; this can be explained by a thermally activated relaxatioo process aaoos an energy bmier such that t(1) = to exp(MaT), where A = 0.673 eV. This value is lowt!f than 0.935 eV which was reported by Kudinov et al [1] for a sample with an oxygen content near the MIT. We found to = 3.5 X 10-8 s when the film with x = 0.75 relaxed at temperatures between 273K and 325K, after illumination at these temperatures. The dispersion parameter increased with temperature and can be fitted with a linear function, ~(T) = -1.15+T/157.5, as indicated with dashed line in the inset of Figure 3. These results, together with the analysis of till show an enhancement of PPC with smaller excitation time till, higher relative photoinduced conductivity, large
Acknowledgment: One of authors (D. Girata) wishes to thank the hospitality of the Schuller's group at the University of California, San Diego, where deoxygenation of the samples were carried out. REFERENCES
1. V. I. Kudinov, I. L. Chaplygin, A. I. Kirriyuk, N. M. Kreines, R. Laiho, E. Uhderanta, and C. Ayache, Phys. Rev. B 47,9017 (1993). 2. G. Nieva, E. Osquiguil, J. Guimpe~ M. Maenhoudt, B. Wuyts, Y. Brunyseraede, M. B. Maple, and I. K. Schuller, Appl. Pbys. Lett. 60, 2159 (1992). 3. J. Hasen, D. Lederman, I. K. Schuller, V. Kudinov, M. Maenhoudt and Y. Bruynseraede, Phys. Rev. B, 51 (2), 1342 (1995). 4. D. Girata, Y. Rojas, E. Bacca, M. Chac6n, W. lq1era, and P. Prieto, Sol. State Coot. 90, 539 (1994). 5. P. K. Gallagher, Adv. Ceramic Mat. 2, 632 (1987).