Virtually every device that makes use of the new ceramic superconductors will need normal conductor to superconductor contacts. The current-voltage and electrical noise characteristics of these contacts could become important design considerations. I-V and low frequency electrical noise measurements are presented on contacts between a normal conductor and superconducting polycrystalline YBa2Cu3O7. The contacts were formed by first sputtering gold palladium pads onto the surface of the bulk superconductor and then using silver epoxy to attach a wire(s) to each pad. Voltage across the contacts was found for small current densities. The voltage spectral density, S sub v(f), a quantity often used to characterize electrical noise, very closely followed an empirical relationship given by S sub v(f) = C(VR)sq/f, where V is the DC voltage across the contact, R is the contact resistance, F is frequency, and C is a contant found to be 2 x 10(exp -10)/Omega sq at 78 K. This relationship was found to be independent of contact area, contact geometry, sample fabrication technique, and sample density

Chen, T. M.

Hall, J.

English

NASA Technical Reports Server

N92-21624
LOW FREQUENCY ELECTRICAL NOISE ACROSS CONTACTS BETWEEN A
NORMAL CONDUCTOR AND SUPERCONDUCTING BULK YBa2CuaOr
J. Hall and T.M. Chen
Noise and Reliability Research Lab, Electrical Engineering Department
University of South Florida, Tampa, Florida 33620
Abstract- Virtually every practical device that makes use of the new ceramic superconductors will need normal
conductor to superconductor contacts. The current-voltage and electrical noise characteristics of these contacts could
become important design considerations. This paper presents I-V and low frequency electrical noise measurements on
contacts between a normal conductor and superconducting polycrystalhne YBa2Cu307. For small current densities,
current through the contacts was found to be proportional to V 17. The voltage spectral density, Sv(f), very closely
followed an empirical relationship given by, Sv(f) = C(V_Rc)2/f, where Vc is the DC voltage across the contact, Re
is the contact resistance, f is frequency, and C is a constant found to be 2 x 10-1°/f_ _ at 78° K. This relationship
was found to be independent of contact area, contact geometry, sample fabrication technique, and sample density.
INTRODUCTION
In the past few years the discovery of new high-
transition temperature (high-T_) superconductors has
led to an abundance of research on the fundamental
properties of these materials. For practical devices to be
constructed from these materials the electrical contacts
to the superconductor need to be investigated. Several
researchers have investigated the theoretical[I,2] and ex-
perimental[3,4,5] characteristics of normal metal to low-
temperature superconductors. The normal metal to bulk
superconducting YBa2Cu307 contact is of particular im-
portance because the material is not a traditional metal.
One method of investigation is the current-voltage (I-
V) characteristics of the contact. This information is
needed for device work if the contact resistance is not a
constant. Another important tool in investigating ma-
terials and junctions is noise measurement. Noise mea-
surements can be used to define the minimum sensitiv-
ity of a device as well as illuminate physical processes
occurring in a material or junction. The Noise and Re-
liability Research Laboratory at the University of South
Florida has been investigating the noise characteristics in
YBa2Cu307 superconductors since these materials were
discovered.
In this paper, we investigate the current-voltage and
noise characteristics of the normal--superconductor (N-
S) contacts at liquid nitrogen (LN2) temperatures for
several contacts of different geometries and contact re-
sistances. The noise experiments lead to an important
empirical formula for predicting the noise voltage in an
N-S contact.
NORMAL METAL
CONTACT
SUPERCONDUCTOR I
Figure 1: Typical sample contact configuration for nor-
mal metal (Ag) to superconductor (bulk YBa_Cu307)
junction.
EXPERIMENTAL METHODS
For this experiment, the YBa2Cu307 bulk samples
used were of three different densities (4.44, 4.46, and
5.02 g/cm 3) and two different fabrication techniques: ox-
Mate precipitation (oe) and solid-state reaction (SSR).
All the samples had transition temperatures around 90K
and were verified to be deep within the superconducting
region at the experimental temperatures (78K).
Each sample consisted of at least four contacts with
a typical contact configuration show in figure 1. The
contacts made to these samples were made in the follow-
ing manner. The bulk YBa2CuaO7 surface was sanded
with very fine grain sandpaper to remove any of the ex-
posed surface which may have oxidized. It was then
ultrasonically cleaned for two minutes in a 100% ethyl
145
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Table 1: Parameters for the 5 different contacts used in
this investigation.
Con- Con- Max
Samp Samp Dens Are_ tact tact Res
no. name g/cm 3 mm2 shape no. (t_)
1 Trak 4.46 15 Rect 1 1.1
2 Trak 4.46 15 Rect 2 0.65
3 Trak 4.46 15 Rect 4 2.5
4 Fsu25 5.07 12 Semicr 1 0.91
5 Fsu26 4.44 7 Circ 2 1.6
alcohol bath. The contact mask was then fastened to the
sample and it was then sputtered with a 300 nm layer
of gold-palladium (AuPd). The AuPd pads were then
covered with silver epoxy and silver coated wires were
pressed into the epoxy. The epoxy was cured at 1800C
for 1 hour. These contacts usually yielded a contact re-
sistance between 0.5-2.5_ for a 0.25 cm 2 area contact.
Table 1 shows the various parameters for the sample con-
tacts used.
The samples were then wired and connected to the
measurement equipment. Each sample was suspended
about 1 inch above the LN2 level in a 3-liter MVE dewar.
The temperature at this level was 78K and was found to
remain constant for at least 12 hours. The sample was
first connected to verify superconductivity in the region
to be measured. Refering to figure 1, the current was
passed from 1 through 4 and the voltage was checked
acro_ contacts 2 and 3 for zero resistance. After super-
conductivity was verified, the current is passed from 2 to
4 and the voltage is measured across contacts 2 and 3.
Since the bulk material in that region was already found
to have no resistance or noise, any resistance or noise
generated must come from the junction between the nor-
mal metal at contact 2 (now current carrying) and the
superconducting YBa2Cu3Or. The sample contact was
then subjected to I-V measurements. The contacts were
connected to a Keithley Constant Current Source with a
Keithley Nanovoltmeter for reading the contact voltage
(Vc). The measurement equipment was controlled by a
PC-compatible computer with software written for this
experiment. The noise measurement system was con-
structed as shown in figure 2. The system consisted of
a DC biasing circuit consisting of 2-12V gel cells con-
nected in parallel and various wirewound resistors for
selecting different bias currents through the sample con-
tact. The current flowing in the system was measured by
reading the voltage across V, and dividing by /_ontrol-
RoonLr_ Rb4u I$ CB I0:I000
PAR 1900 PAR 113
Transformer Low -noise
Amp
tSuRlrcopductoriV,
at 78K {i, bore LN| level)
HP 3561
Spectrum Analyzer
Figure 2: Experimental apparatus for contact noise mea-
surements.
The RbiM resistor was used to adjust the biasing current
in the system. The current was passed through contact
2 to contact 4. The blocking capacitor, CB, was used
to block DC current from magnetizing PAR-1900 trans-
former. The PAR-1900 transformer was connected from
the 10:1000 turns ratio giving a gain of 100 through the
transformer. It was connected to a PAR-113 Low-noise
Amplifier with the gain set at 100. This combination
of the transformer and amplifier gave an overall gain of
10 000. The HP 3561 Spectrum Analyzer was used to
collect the noise traces. The traces were collected in the
low-frequency region of 1-51 Hz with 25 averages per
trace to yield more than sufficient resolution.
Excess noise checks were made at various stages of
the experiment to assure that the noise measured was
coming from the N-S contact and not an element of the
measurement system. Also the silver epoxy was tested
and showed no excess noise.
RESULTS AND DISCUSSION
I. Current-voltage experiments
The results obtained for the I-V relationship of these
contacts were a little surprising. These contacts showed
a decreasing resistance as the voltage across the contact
increased. This same effect was observed by Van Schevi-
coven and De Waele [6] for bulk point contacts between
a normal metal and high-To superconductor. Moreland,
et al[7], also found similar results in break junctions in
a bulk YBa_Cu3Or sample. Figure 3 shows a typical I-
V curve for the bulk contacts used in this investigation.
The figure shows the parabolic nature of the curve as
well as the symmetry in both the positive and negative
voltage regimes. The parabolic shape of the curve was
common for all the contacts investigated and led to an
146
Table 2: Experimental results of current-voltage mea-
surements.
Sample II a I a [zero-biasII
numberII I I Res. II
II 1 9.57 1.68 1.1
11 2 13.81.760.653 3.47 1.66 2.5
II 5 3.34 1.57 1.6
0.S
0.4-
0,3-
< 0,2
.._ 0.1
I_ 0.0
% -0.1
_ -o.,
-0.3
-0.4
-0.8
(a)
--0.111 --0.12 --O.OI 0.00 O,Oe
Voltage (V)
1.2.
0.12 0.16
_'_ 1.0.
t O.I.
0.8.
0.7-
,_ 0.6.
_ 0.4-
0.3 :
-D.1B
-0.12 -'0._6 0,00 O,Og 0.12 0.111
Voltage (V)
Typical electrical characteristics of a nor-Figure 3:
mal metal-superconducting YBa2Cu307 contact; (a)
Current-Voltage graph; (b) Resistance-Voltage graph.
empirical formula
I = AV_ (1)
where I is the current flowing through the contact, Ve
is the voltage across the contact, A is a constant related
to the resistance of the sample, and A is a power factor
which is around 1.7 for the samples tested. Table 2 gives
the A and A values for the various contacts used in this
investigation. Van Schevicoven and De Waele attribute
this parabolic behavior to normal electron tunneling be-
tween the normal metal contact to high-Te superconduc-
tor.
II. Noise experiments
The noise measurements were taken over a 0.1 to 30
mV bias voltage range for the five contact samples. The
Table 3: Contact noise measurement results for the equa-
tion Sv (f) = CV_ R_/I.
I Sample H Zero-bias Cnu ber II _ ( ) (l/a _)
1 1.1 2.1 x 10 -1°
2 0.65 3.3 x 10 -l°
3 2.5 2.1 x 10 -1°
4 0.91 1.6 x 10 -l° I
5 1.6 7.8 x 10 -11
Empirical
formula
1,90
2.06
1.98
1.90
1.98
2.0 x 10 -1° 2.00
noise traces were dumped from the HP 3561 into a lab
computer where they were analyzed using a least-squares
line fitting method to determine the slope as a func-
tion of frequency. All the noise traces were "l/f noise"
where "l/f noise" is defined as noise proportional to
1If a where a ranges between 0.8 and 1.2. After this
data was analyzed, the value of each trace at 10 Hz was
recorded and compared to the bias voltage, current, re-
sistance, and other parameters. All the experimental
noise current spectral densities were found to follow an
empirical formula. Figure 4 shows the plot of the noise
current spectral density (S#(f) or Sv(I)/R_) vs. the
contact voltage (Ve) for the five sample contacts used.
The log St(f) vs log Ve forms a straight line for which
all the data collected lies in close proximity. The empir-
ical formulas for the noise voltage spectral density and
the noise current spectral density, respectively, are
C(Vene) _
Sv (f) -- f (2)
St(f)- CVe_ (3)
y
where C is an empirical constant found to be approx-
imately 2 x 10-1°/_ _, Ve is the contact voltage, Re is
the contact resistance, and f is the frequency. Table
3 shows the experimental values of C for the various
contacts. The noise which was measured in the con-
tacts was very different from the results found in bulk
YBa2Cu30_ which has been reported by several experi-
menters[8,9,10]. The implications of this empirical equa-
tion are that the noise voltage spectral density can be
predicted by measuring the contact resistance which may
be of great advantage when practical devices are to be
fabricated from high-Te materials.
CONCLUSIONS
147
IE-13-
II_-14,
N
"N 1_-15-
.<
_ 11-16
m
1E-17
11-18
51_-19
O_
s Vlz 4.46g/cm3(SSR)
0 4.4a cn (SSR)
a Vz3 4.46 g/cn_(SSR) .d._
v Vlz 5.07 g/cn_(OP} ,, +._%_ n
m Vzs 4.44 g/cm_(OP) ,:+_e,-o
--- Empirical formula , _,_ o
, p,'-o
, A-+_n
::o,::! I
o I i i I p I
0.6 1.0 2.0 6.0 10.0 20.0
Voltage(mY)
Figure 4: Graph of noise current spectral density
(St(f)or Sv(f)/R _)fornormal metal-superconducting
YBa_Cu30¢ contacts.The dotted lineshows the empir-
icalformula obtained from the data.
Five normal metal-superconducting YBa2CuaOz
sample contacts were thoroughly studied for their
current-voltage and noise characteristics. The I-V mea-
surements led to an empirical formula, I = AV _, to de-
scribe the parabolic relationship between I and V. The
noise measurements yielded an empirical equation of the
form, Sv(,f) = C(VcRc)2/], which describes the noise
voltage spectral density for a contact with resistance,
Re, and voltage, V_.
The N-S contact characteristics are very important
and need to be thoroughly understood for any practical
device made from superconducting YBa2Cu3Oz. The
noise limits the sensitivity of any device and since the
superconductor itself yields no measurable noise in the
superconducting region, the normal metal contact noise
would most likely be the source of excess noise in such a
device.
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microjunctions between normal metals and super-
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(1968).
3. G.E. Blonder, M. Tinkham, T.M. Klapwijk, "Tran-
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charge imbalance, and supercurrent conversion,"
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5. T.Y. Itsiang, J. Clarke, "Boundary resistance of
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ing measurements of the high-T¢ superconductor
YBa2Cu3Oz," Phys Rev B, 35, 8856, (1987).
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YBa2CusOz," Proceedings of the 1989 SOUTH-
EASTCON, 3, 1057, (1989).
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This work was partially supported by DARPA Su-
perconductivity Grant No. MDA 972-88-J-I006. The
authors wish to thank Dr. H. Hickman and Dr. A.J.
Dekker for their help in preparing and performing the
experiments presented in this manuscript.
148


Low frequency electrical noise across contacts between a normal conductor and superconducting bulk YBa2Cu3O7

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