Effect of thermal coupling on the photoresponse of YBCO infrared detector

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Effect of thermal coupling on the photoresponse
of YBCO infrared detector
Meng-Tsong HONG ( ? ? ? ) Department of
Electrical Engineering, Cheng-Shiu Institute of
Technology, Kaohsiung 833, Taiwan ( ? ? ? ? ? ? ?
? ? ) H. Chou, W. Y. Teng, C. Y. Ti, T. C. Wu, W.
C. Lu Department of Physics, National Sun Yat-Sen
University, Kaohsiung 804, Taiwan S. J.
Sun Department of Electronic Engineering,
Cheng-Shiu Institute of Technology, Kaohsiung
833, Taiwan
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Abstract
For an YBCO infrared detector, the irradiated
thermal energy is absorbed by the film and
finally drained to the heat sink. The intensity
of the photoresponse depends on the level of the
AC irradiation energy accumulated. It is
intuitively expected that weakening the thermal
coupling of the detector to the heat sink enhance
the photoresponse. However, in this study, we
found that the photoresponse was not enhanced but
was depressed by weakening the thermal coupling
of the setup. This could be attributed to the
nature of the low dissipation ability in the weak
thermal coupling configuration that accumulated a
higher level of DC Joule heat and gave rise to a
huge temperature gradient between the film and
the heat sink. As a result, the heat dissipation
was sped up and the photoresponse was depressed.
The National Science Council, Taiwan, R. O. C.
supports this work.
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1. Introduction The high-temperature
superconducting YBCO detector is one of the most
widely applicable infrared (IR) detectors. The
operating principle of the YBCO detector is based
upon the conversion of AC irradiated thermal
energy to a temperature rise ?T in the YBCO
detector. The magnitude of the AC thermal
signal ?T/WD indicates how efficiently the AC
irradiated power is converted to the temperature
rise in the detector, and is expected to be the
same for a specific thermal coupling setup.
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When a single straight bridge was prolonged to a
meander, the accumulation of DC Joule heat was
increased markedly due to the increase of
resistance at the midpoint transition. The AC
thermal signal, ?T/WD, could be understood as
reflecting the balance between the capability for
heat dissipation and the thermal energy generated
from AC irradiation and DC Joule heating. The
accumulation of DC Joule heat resulted in faster
heat dissipation and the suppression of the AC
thermal signal. How the AC thermal signal,
?T/WD, depends on the thermal coupling
configuration is technically and scientifically
important and worth studying.
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2. Experimental YBa2Cu3O7-? (YBCO) films were
deposited on SrTiO3(100) substrates by off-axis
RF sputtering. X-ray diffraction revealed the
grown films to be c-axis preoriented. The zero
resistance temperature Tco of the film was about
79 K. Conventional contact photolithography was
used to fabricate the desired geometrical
patterns, including a single straight bridge and
three meanders with 3, 5, and 11 strips. The
HTSC bolometer was mounted on the heat sink in a
closed cycle cryostat equipped with an optical
window.
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Two thermal coupling configurations of the
setup, strong and weak coupling configurations,
were constructed for measurement, as shown in
Fig. 1. In the strong thermal coupling
configuration, the substrate was directly
contacted with the cold finger, thus the heat was
conducted directly from the microstrips through
the substrate material to the cold finger. In
the weak thermal coupling configuration, a copper
ring was inserted between the substrate and the
cold finger, which resulted in the reduction of
the heat dissipation area and lengthened the heat
conduction paths.
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3. Results and Discussion The photoresponse ?V
of the YBCO detector can be expressed as ?V
?Ib(dR/dT)?T, where ? is the absorption
efficiency of the film, Ib is the bias current,
dR/dT is the slope of the resistance versus
temperature curve, and ?T is the temperature rise
due to the incident light. We found that the
(dR/dT)max values were all increased after the
copper ring was inserted between the substrate
and the cold finger. The relationships between
?T/WD and the number of strips in the microbridge
in the two different thermal coupling
configurations are shown in Fig. 2.
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The rapid dissipation of heat normally indicates
a small ?T/WD. The two dimensional (2D) heat
dissipation profiles of the bolometer with
different thermal coupling configurations are
shown in Fig. 1. With the substrate contacted
directly on the cold finger, as shown in Fig.
1(a), the thermal energy generated by the DC bias
currents and AC irradiation dissipated from the
film through the substrate to the cold finger.
The greater the number of strips contained in the
microbridge, the smaller the magnitude of the
?T/WD.
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As shown in Fig. 2, the 11-strip meander
exhibits the least value of ?T/WD, whereas the
three-strip meander has the highest ?T/WD. In
Fig. 1(b), for the case where the copper ring had
been inserted between the substrate and the cold
finger, the contact area of the substrate to the
cold finger was reduced and the behavior of heat
dissipation was altered. The perpendicular
dissipation of heat could almost be neglected.
Under the same measurement conditions, the DC
temperature rise reached a higher level than that
of the direct contact case.
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The magnitude of ?T/WD decreased monotonically
with the increasing of number of strips in the
weak thermal coupling configuration, as shown in
Fig. 2. In Fig. 3(a), and, Fig. 3(b), it is
found that the Tco was shifted to a lower
temperature on the same scale of 5 K when the
bias current was increased from 0.01 to 3 mA.
As the bias current increased from 0.01 to 3
mA, the normal state resistance, RN(85 K), was
increased by 5.2 and 9.2 and the Tc-onset was
lowered by 1 K and 5.8 K in strong and weak
thermal coupling configurations, respectively.
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It is expected that the AC thermal signal ?T/WD
is influenced by the change of the thermal
coupling configuration because of the variation
in heat dissipation. The bias current generated
DC power and ?T/WD values of the three-strip
meander with strong and weak thermal coupling
configurations are shown in Fig. 4. In the weak
thermal coupling configuration, the ?T/WD peak
shifts to a lower DC power. It is understood that
the dissipation capability of an assembly is
nearly fixed when the temperature gradient and
configuration are fixed.
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The increase of the temperature gradient in the
assembly indicates a rapid dissipation of heat
that lowers the accumulation of AC irradiation
energy in the microbridge. Therefore, the ?T/WD
drops sharply in the high DC power region. The
weak thermal coupling configuration exhibits a
peak ?T/WD at PDC 1 ?W, which is much lower
than that of the strong thermal coupling
configuration at PDC 300 ?W. This indicates
that the advantage of the weak thermal coupling
configuration is its low operation-power
consumption, whereas a higher ?T/WD value is
found for the strong thermal coupling
configuration.
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4. Conclusions The magnitude of ?T/WD was not
enhanced as expected but was depressed by
weakening the thermal coupling of the setup.
The enhancement of DC Joule heat accumulation
in the weak thermal coupling could be proved by
observing the increase of normal state resistance
and the sharpening of the transition width with
bias current. The AC thermal signal ?T/WD also
depended on the bias current and DC power
applied. In the weak thermal coupling
configuration, the peak of ?T/WD shifted to the
lower DC power region.
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Even though the AC thermal signal was lower in
the weak thermal coupling configuration than in
the strong one, the optimum operation condition,
where maximum ?T/WD is found, is located in the
lower bias current region. This creates an
advantage for the weak thermal coupling
configuration in that it can be operated with low
power consumption.  Acknowledgement This project
is supported by the National Science Council,
Taiwan, R. O. C.
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4