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Infrared response of granular YBCO superconducting films
Solid State Communications, Vol. 89, No. 6, pp. 535 537, 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038 1098/94 $6.00 + .00
Pergamon
0038-1098(93)E0047-2
I N F R A R E D RESPONSE O F G R A N U L A R YBCO S U P E R C O N D U C T I N G FILMS Fangqiao Zhou, Jianhua Hao, Handong Sun, Xingrong Zhao, Zhihong Mai and Xinjian Yi Department of Optical Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, P.R. China
(Received 31 August 1993 by A.H. MacDonald) Granular superconducting films of YBa2Cu307_ x are investigated as photodetectors for infrared radiation. These films may be thought of as a random network of grain boundary Josephson junctions. Two optical responses with different response time have been observed. Responsivity of the typical detector is 8 x 103 V W -l at the range of 8 - 1 4 #m wavelength, and the detector is capable of responding up to 120 KHz.
1. I N T R O D U C T I O N OPTICAL detectors based on two different responsitive mechanisms have previously demonstrated in granular films of both low- and high-temperature superconductors such as BaPbBiO and YBa2Cu307_ x [1-3]. In the bolometric mode of operation, the abrupt change of resistance of the superconductor at its resistance transition edge is used to sense radiation. However, these films may be thought of as a random network of grain boundary Josephson junctions [4]. When the film is fully superconducting via Josephson coupling of the grains, the absorption of infrared photons destroy the phase coherence of Cooper pair wave functions between the grains, causing a phase slip to represent a resistive loss in the superconducting films which is measured as a voltage change at constant current. When the film patterned as a microbridge is biased at current lower than its critical one, a nonequilibrium optically responsive component across the random network of boundary Josephson junctions upon irradiation has been observed. In this paper we report optical response measurements of predominantly c-axis oriented crystalline YBa2Cu3OT_x granular films prepared by electron beam multilayer evaporation to infrared radiation of wavelength ranges of 3 - 5 #m and 8 - 1 4 # m . 2. E X P E R I M E N T S
2.1. Sample preparation The films were prepared on the substrates of Y-stabilized ZrO2 single crystal by electron beam
multilayer evaporation. An electron gun was used to evaporate BaF2, Y203 and Cu sequentially for three periods on the substrates maintained at temperature of 250°C. The deposition rates and thickness of each layer were monitored by a quartz crystal oscillator. The deposition rates were 0.25 nm s -l, 0.6 nm s -1 and 0 . 2 n m s -I for Y203, BaF2 and Cu respectively, and the total thickness was about l # m . The composition of the films was adjusted by varying the relative thickness of each layer to accomplish the stoichiometrical component of YBa2Cu307_ x. The films were calcined in a tube furnace at temperature of 700°C in flowing oxygen mixed with water vapour for 30 min, and then annealed at 900°C. All runs were controlled by a computer. The films were patterned into 100 x 4 0 # m 2 or 100 x 10/zm 2 microbridge patterns by optical lithography and chemical etching in a diluted aqueous solution of H3PO 4. The electric contacts were formed by indium solder connections. 2.2. Measurement procedure As-prepared microbridge film samples were placed in a LN-cooled metal dewar with access via an optical filter. Using a blackbody or InGaAsP laser as radiation sources, the responsivity, signal to noise ratio and response dependence on chopping frequency of the samples which operated at real temperature o f 80 K in vacuum were measured. The radiation from the blackbody was mechanically chopped at frequency range from 10Hz to 2 K H z , and InGaAsP laser was electrically modulated up to
535
536
I N F R A R E D RESPONSE OF G R A N U L A R YBCO FILMS 3K
Vol. 89, No. 6
1.0 0.8
oO o
~°0.6
Q
ooo m
1.5K
•
•
•
O
o
•
•
Q
•
0.4
Z~
g ~
6'
f~
0.2
i.."
0 0 21)
n
a
•
30
40
50
n
80
I
120
160
20( °)
Fig. 1. X-ray diffraction pattern of the YBCO film including that of ZrO2 substrate. 120KHz. The output from the microbridge sample was observed on a lock-in amplifier. 3. RESULTS A N D DISCUSSION 3.1. Film characters
As-prepared films were characterized by scanning electron microscopy (SEM), X-ray diffraction and conventional four probe resistance measurement. A typical X-ray diffraction pattern of the film is shown in Fig. 1. The major (0 0 h) peaks of the YBCO crystal structure are clearly observed except for the peak of ZrO2 and a few other peaks. The film is highly oriented with the C-axis perpendicular to the substrate surface. Figure 2 is a SEM photograph of as-prepared YBCO films. It can be clearly seen that the morphology consists of oriented arrays of interconnecting bar grains. This morphology of the films is similar to Wang's results in oriented YBa2CuaOT_x films [5]. The temperature dependence of resistance was measured by the conventional d.c. four probe method with a current density of 13 A cm -2, Fig. 3 shows the
200
240
300
T (K)
61)
Fig. 3. Temperature dependence of the d.c.-resistance for the YBCO film. experimental curve in which the zero resistance transition temperature is 90 K for a typical sample. 3.2. Optical response The dependences of infrared response on the bias current and chopping frequency were measured for various detectors. Figure 4 shows the bias current dependences of infrared responses for the microbridge samples which operated in both bolometric and nonbolometric modes. As shown in Fig. 4, curve (a) indicated that major component of the response in this case is a change in the film resistance which is due to equilibrium heating of the sample. By contrast, a response peak at a bias volume of 3.5 mA near the critical current of measured sample exists in curve (b) as shown in Fig. 4, and then decrease with the further increase of the bias current. In this case, the simple picture of response corresponds to a phase slip to take place while the sample remained in d.c.-zero resistance state. This is similar to Enomoto et al.'s observation in the granular BaPbBiO films [2]. Figure 5 shows chopping frequency dependences of response for a typical sample. Because of the limit
25-
- 100
20
80
15 -
---- (b) /- -
60 ~" ::t.
j / / e / o /
4o >~
(a)
20
..v~o~t 2
t
I
3
4
0 5
Ib (mA)
Fig. 2. SEM photograph of the YBCO film used to fabricate infrared detector.
Fig. 4. Bias current dependences of 8 - 1 4 #m infrared response at chopping frequency of 4 8 0 H Z for samples which operated in (a) bolometric mode and (b) nonbolometric mode.
Volt 89, No. 6
INFRARED RESPONSE OF GRANULAR YBCO FILMS
25 2O --, 15 =1. v 10 O~ Q~'S.-... 5
0
O
, ~ ~ O
I 400
•
I 800
•
•
•
I 1200
I 1600
I 2000
f (Hz)
Fig. 5. Frequency dependences of 8 - 1 4 # m infrared response for samples of microbridge at 3mA respectively.
537
400 Hz. The response volume of a typical sample of sensitive area 100 x 10/zm -2 is4.8 x 103VW -l, from infrared radiation at wavelength of 8 - 1 4 # m for chopping frequency of 400 Hz. The detectivity of these microbridge granular films which were used as infrared detectors has been determined by measuring the signal to noise ratio. Typical volumes of D* about 6.8 x 108 cm Hz 1/2 W -I for the sample of 100 x 40#m 2 and 1.5 x 109cmHz -1 for the sample of 100 × 10 #m 2 were availed respectively. A detector's responsivities in the wavelength ranges of 3-5#m and 8-14#m have been measured respectively. The responsivities in two ranges were about the same. 4. SUMMARY
of maximum chopping frequency, the upper limit of measured frequency was just only 2000 Hz. As shown in Fig. 5, when the chopping frequency is low, the response drops sharply with increasing frequency; but when the frequency is higher than 400Hz, the response drops very slowly and tends to be fiat. The infrared response appears to be almost unchanged in the frequency range of 400-2000 Hz in Fig. 5. In the case of electrical chopping laser radiation at wavelength of 1.3 #m, obvious optical response for even higher chopping frequency up to 120kHz was observed. It is showing that the sample has two optical response mechanisms with different response time. A thermal equilibrium effect is considered to change the superconducting energy gap with increasing temperature resulting from thermal radiation, and another nonequilibrium optical response is considered to phase slip process. The thermal equilibrium response is dominated at chopping frequency lower than 400 Hz, and then become not rather too important at frequency higher than
In summary, we have investigated characteristics of the YBCO films prepared by electron beam multilayer evaporation and given some experimental results on the infrared detectors from high Tc YBCO superconducting film. Our work suggests that it is possible to improve the responsivity by optimizing the design and fabricating technology in the structure of the detector. Such approaches are now in progress. REFERENCES I. 2. 3. 4. 5.
M.G. Forrester, M. Gottlieb, J.R. Gravaler & A.I. Braginski, Appl. Phys. Lett. 53, 1332 (1988). K. Tanabe, Y. Enomoto, M. Suzuki, T. Iwata & A. Yamah, Jpn. J. Appl. Phys. 29, 1446 (1990). U. Strom, IEEE Trans. Magn. 25, 1315 (1989). Y. Enomoto & T. Murakami, J. Appl. Phys. 11, 3807 (1986). X.K. Wang, K.C. Sheng, S.J. Lee, Y.H. Shen, S.N. Song, D.X. Li, R.P.H. Chang & J.B. Ketterson, Appl. Phys. Lett. 54, 1573 (1989).
Pergamon
0038-1098(93)E0047-2
I N F R A R E D RESPONSE O F G R A N U L A R YBCO S U P E R C O N D U C T I N G FILMS Fangqiao Zhou, Jianhua Hao, Handong Sun, Xingrong Zhao, Zhihong Mai and Xinjian Yi Department of Optical Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, P.R. China
(Received 31 August 1993 by A.H. MacDonald) Granular superconducting films of YBa2Cu307_ x are investigated as photodetectors for infrared radiation. These films may be thought of as a random network of grain boundary Josephson junctions. Two optical responses with different response time have been observed. Responsivity of the typical detector is 8 x 103 V W -l at the range of 8 - 1 4 #m wavelength, and the detector is capable of responding up to 120 KHz.
1. I N T R O D U C T I O N OPTICAL detectors based on two different responsitive mechanisms have previously demonstrated in granular films of both low- and high-temperature superconductors such as BaPbBiO and YBa2Cu307_ x [1-3]. In the bolometric mode of operation, the abrupt change of resistance of the superconductor at its resistance transition edge is used to sense radiation. However, these films may be thought of as a random network of grain boundary Josephson junctions [4]. When the film is fully superconducting via Josephson coupling of the grains, the absorption of infrared photons destroy the phase coherence of Cooper pair wave functions between the grains, causing a phase slip to represent a resistive loss in the superconducting films which is measured as a voltage change at constant current. When the film patterned as a microbridge is biased at current lower than its critical one, a nonequilibrium optically responsive component across the random network of boundary Josephson junctions upon irradiation has been observed. In this paper we report optical response measurements of predominantly c-axis oriented crystalline YBa2Cu3OT_x granular films prepared by electron beam multilayer evaporation to infrared radiation of wavelength ranges of 3 - 5 #m and 8 - 1 4 # m . 2. E X P E R I M E N T S
2.1. Sample preparation The films were prepared on the substrates of Y-stabilized ZrO2 single crystal by electron beam
multilayer evaporation. An electron gun was used to evaporate BaF2, Y203 and Cu sequentially for three periods on the substrates maintained at temperature of 250°C. The deposition rates and thickness of each layer were monitored by a quartz crystal oscillator. The deposition rates were 0.25 nm s -l, 0.6 nm s -1 and 0 . 2 n m s -I for Y203, BaF2 and Cu respectively, and the total thickness was about l # m . The composition of the films was adjusted by varying the relative thickness of each layer to accomplish the stoichiometrical component of YBa2Cu307_ x. The films were calcined in a tube furnace at temperature of 700°C in flowing oxygen mixed with water vapour for 30 min, and then annealed at 900°C. All runs were controlled by a computer. The films were patterned into 100 x 4 0 # m 2 or 100 x 10/zm 2 microbridge patterns by optical lithography and chemical etching in a diluted aqueous solution of H3PO 4. The electric contacts were formed by indium solder connections. 2.2. Measurement procedure As-prepared microbridge film samples were placed in a LN-cooled metal dewar with access via an optical filter. Using a blackbody or InGaAsP laser as radiation sources, the responsivity, signal to noise ratio and response dependence on chopping frequency of the samples which operated at real temperature o f 80 K in vacuum were measured. The radiation from the blackbody was mechanically chopped at frequency range from 10Hz to 2 K H z , and InGaAsP laser was electrically modulated up to
535
536
I N F R A R E D RESPONSE OF G R A N U L A R YBCO FILMS 3K
Vol. 89, No. 6
1.0 0.8
oO o
~°0.6
Q
ooo m
1.5K
•
•
•
O
o
•
•
Q
•
0.4
Z~
g ~
6'
f~
0.2
i.."
0 0 21)
n
a
•
30
40
50
n
80
I
120
160
20( °)
Fig. 1. X-ray diffraction pattern of the YBCO film including that of ZrO2 substrate. 120KHz. The output from the microbridge sample was observed on a lock-in amplifier. 3. RESULTS A N D DISCUSSION 3.1. Film characters
As-prepared films were characterized by scanning electron microscopy (SEM), X-ray diffraction and conventional four probe resistance measurement. A typical X-ray diffraction pattern of the film is shown in Fig. 1. The major (0 0 h) peaks of the YBCO crystal structure are clearly observed except for the peak of ZrO2 and a few other peaks. The film is highly oriented with the C-axis perpendicular to the substrate surface. Figure 2 is a SEM photograph of as-prepared YBCO films. It can be clearly seen that the morphology consists of oriented arrays of interconnecting bar grains. This morphology of the films is similar to Wang's results in oriented YBa2CuaOT_x films [5]. The temperature dependence of resistance was measured by the conventional d.c. four probe method with a current density of 13 A cm -2, Fig. 3 shows the
200
240
300
T (K)
61)
Fig. 3. Temperature dependence of the d.c.-resistance for the YBCO film. experimental curve in which the zero resistance transition temperature is 90 K for a typical sample. 3.2. Optical response The dependences of infrared response on the bias current and chopping frequency were measured for various detectors. Figure 4 shows the bias current dependences of infrared responses for the microbridge samples which operated in both bolometric and nonbolometric modes. As shown in Fig. 4, curve (a) indicated that major component of the response in this case is a change in the film resistance which is due to equilibrium heating of the sample. By contrast, a response peak at a bias volume of 3.5 mA near the critical current of measured sample exists in curve (b) as shown in Fig. 4, and then decrease with the further increase of the bias current. In this case, the simple picture of response corresponds to a phase slip to take place while the sample remained in d.c.-zero resistance state. This is similar to Enomoto et al.'s observation in the granular BaPbBiO films [2]. Figure 5 shows chopping frequency dependences of response for a typical sample. Because of the limit
25-
- 100
20
80
15 -
---- (b) /- -
60 ~" ::t.
j / / e / o /
4o >~
(a)
20
..v~o~t 2
t
I
3
4
0 5
Ib (mA)
Fig. 2. SEM photograph of the YBCO film used to fabricate infrared detector.
Fig. 4. Bias current dependences of 8 - 1 4 #m infrared response at chopping frequency of 4 8 0 H Z for samples which operated in (a) bolometric mode and (b) nonbolometric mode.
Volt 89, No. 6
INFRARED RESPONSE OF GRANULAR YBCO FILMS
25 2O --, 15 =1. v 10 O~ Q~'S.-... 5
0
O
, ~ ~ O
I 400
•
I 800
•
•
•
I 1200
I 1600
I 2000
f (Hz)
Fig. 5. Frequency dependences of 8 - 1 4 # m infrared response for samples of microbridge at 3mA respectively.
537
400 Hz. The response volume of a typical sample of sensitive area 100 x 10/zm -2 is4.8 x 103VW -l, from infrared radiation at wavelength of 8 - 1 4 # m for chopping frequency of 400 Hz. The detectivity of these microbridge granular films which were used as infrared detectors has been determined by measuring the signal to noise ratio. Typical volumes of D* about 6.8 x 108 cm Hz 1/2 W -I for the sample of 100 x 40#m 2 and 1.5 x 109cmHz -1 for the sample of 100 × 10 #m 2 were availed respectively. A detector's responsivities in the wavelength ranges of 3-5#m and 8-14#m have been measured respectively. The responsivities in two ranges were about the same. 4. SUMMARY
of maximum chopping frequency, the upper limit of measured frequency was just only 2000 Hz. As shown in Fig. 5, when the chopping frequency is low, the response drops sharply with increasing frequency; but when the frequency is higher than 400Hz, the response drops very slowly and tends to be fiat. The infrared response appears to be almost unchanged in the frequency range of 400-2000 Hz in Fig. 5. In the case of electrical chopping laser radiation at wavelength of 1.3 #m, obvious optical response for even higher chopping frequency up to 120kHz was observed. It is showing that the sample has two optical response mechanisms with different response time. A thermal equilibrium effect is considered to change the superconducting energy gap with increasing temperature resulting from thermal radiation, and another nonequilibrium optical response is considered to phase slip process. The thermal equilibrium response is dominated at chopping frequency lower than 400 Hz, and then become not rather too important at frequency higher than
In summary, we have investigated characteristics of the YBCO films prepared by electron beam multilayer evaporation and given some experimental results on the infrared detectors from high Tc YBCO superconducting film. Our work suggests that it is possible to improve the responsivity by optimizing the design and fabricating technology in the structure of the detector. Such approaches are now in progress. REFERENCES I. 2. 3. 4. 5.
M.G. Forrester, M. Gottlieb, J.R. Gravaler & A.I. Braginski, Appl. Phys. Lett. 53, 1332 (1988). K. Tanabe, Y. Enomoto, M. Suzuki, T. Iwata & A. Yamah, Jpn. J. Appl. Phys. 29, 1446 (1990). U. Strom, IEEE Trans. Magn. 25, 1315 (1989). Y. Enomoto & T. Murakami, J. Appl. Phys. 11, 3807 (1986). X.K. Wang, K.C. Sheng, S.J. Lee, Y.H. Shen, S.N. Song, D.X. Li, R.P.H. Chang & J.B. Ketterson, Appl. Phys. Lett. 54, 1573 (1989).