> 2eph03 <
1
Fabrication and characterization of epitaxial
NbN/TaN/NbN Josephson junctions grown by
pulsed laser ablation
M. R. Nevala, I. J. Maasilta, K. Senapati and R. C. Budhani
Abstract—We report fabrication and characterization of
epitaxial NbN/TaN/NbN Josephson junctions grown by pulsed
laser ablation. These SNS junctions can be used as elements of
rapid-single-flux-quantum (RSFQ) logic, which is a promising
technology for high speed digital electronic devices. The
NbN/TaN/NbN trilayer films were prepared on a single crystal
MgO substrate by pulsed laser ablation, and patterned into
junctions using a novel process utilizing e-beam lithography,
chemical vapor deposition and e-beam evaporation. The quality
of junctions was tested by measuring the temperature
dependence of the junctions’ IcRn values, observed to be quite
close to theoretical values.
Index Terms—Josephson junctions, Pulsed laser deposition,
Superconducting device fabrication, Superconductor-normalsuperconductor devices.
form SNS Josephson junctions. TaN was chosen as barrier
material because of the tunability of its resistivity by varying
growth conditions (e.g pressure) and its observed high
resistivity [4], [5], [6] which results in higher resistance and
therefore possibly higher IcRn product. Another reason is that
its thermal, chemical and mechanical properties are quite
similar to NbN [5].
NbN was chosen because of its high critical temperature
(Tc) when fabricated by pulsed laser ablation (up to ~17 K),
and because of its good stability against strain, mechanical and
chemical attacks [5], [7]. Here, we have developed a simple
new fabrication process based on electron-beam lithography,
plasma-enhanced chemical vapor deposition (PECVD) and ebeam evaporation, which is shown to produce high-quality
junctions with a simple insulator alignment process, and which
could directly be extended to the submicron scale.
I. INTRODUCTION
II. LIKHAREV’S THEORY
single flux quantum (RSFQ) logic is a promising
technology for high speed digital electronic devices [1].
Devices with superconductor/normal conductor/superconductor (SNS) junctions have nonhysteretic [2] current I vs.
voltage V characteristics required for RSFQ logic. These kind
of SNS devices have several advantages. For example they
can be packed closely, their intrinsic switching speed is high,
and their fabrication is simpler than semiconductor transistors
with similar design [1].
In RSFQ circuitry the appropriate figure of merit of a SNS
junction is the product of the critical current Ic and the normal
resistance of the junction Rn, since it is inversely proportional
to SFQ pulse width [3], [4]. If we want to operate at 30-50
GHz frequency, the product IcRn should be greater than ~0.3
mV [3], [4].
We have used a tantalum nitride (TaN) barrier sandwiched
between superconducting niobium nitride (NbN) electrodes to
The density of Cooper pairs decreases with distance from
the superconductor in an SNS junction when electron pairs are
leaking from a superconductor to a normal conductor [2]. The
rate of decay is approximately exponential, and the distance
where the density has decreased by a factor 1/e is the
coherence length of the normal conductor [2] ξ n , expressed as
R
APID
Manuscript received 16 August 2008. This work was supported by the
Academy of Finland and the Technological foundation of Finland.
M. R. Nevala and I. J. Maasilta are with Nanoscience Center, Department
of Physics, University of Jyväskylä P.O. Box 35, FIN-40014 University of
Jyväskylä, Finland (phone: +358 14 260 4720; fax: +358 14 260 4756; e-mail:
minna.nevala@phys.jyu.fi).
K. Senapati, was with the Department of Physics, Indian Institute of
Technology Kanpur, Kanpur 208016, India. He is now with the Department of
Materials
Science
and
Metallurgy,
University
of
Pembroke Street, Cambridge, UK (e-mail: ks485@cam.ac.uk ).
Cambridge,
R. C. Budhani is with the Department of Physics, Indian Institute of
Technology Kanpur, Kanpur 208016, India (rcb@iitk.ac.in).
1/ 2
 hD 
(1)

 2π kT 
where D is the diffusion constant D = 1 /( e 2 g (ε F ) ρ ) , e is the
electron charge, g (ε F ) is the density of states at Fermi
ξn = 
energy ε F , ρ is the resistivity and h is the Planck’s constant
devided by 2π, k is the Boltzmann’s constant and T is
temperature.
Likharev developed a theory [8] for SNS junctions in the
temperature range 0.3Tc < T < Tc. Within that range and with
the condition [9] for N layer thickness L > 2ξ n (T c )
Likharev’s predictions follow quite closely the earlier de
Gennes’ theory [10] for SNS junctions.
Likharev’s theoretical prediction for the critical current Ic of
the junction when 0.3Tc < T < Tc can be approximated by
4 ∆ L
exp (− L / ξ n )
π eR n kT c ξ n
where ∆ is the energy gap parameter.
2
I c (T ; L ) ≅
(2)
> 2eph03 <
2
III. SAMPLE FABRICATION
NbN-TaN-NbN trilayers were fabricated on single crystal
(100) MgO substrates by pulsed laser ablation of high purity
Nb and Ta targets using a KrF excimer laser (λ=248 nm) in
high purity N2 environment. The turbo molecular pump based
deposition chamber was equipped with a multi-target carousel
for automated in-situ target exchange, to facilitate multilayer
depositions. Both NbN and TaN layers were deposited at a
substrate temperature of 600 °C and ~80 mTorr N2 pressure.
Pulse energy of the laser was kept fixed at ~145 mJ. The size
of laser spot on the target and the target-to-substrate distance
were optimized to achieve a deposition rate of 0.4 nm/sec for
NbN and ~0.5 nm/sec for TaN. Thickness of the top and base
NbN layers was kept constant at 200 nm while the thickness of
TaN was ~10 nm for all samples in this study.
Several NbN/TaN/NbN trilayer films were pattered by ebeam lithography as squares of different sizes from 3
micrometer × 3 micrometer to 10 micrometer × 10
micrometer, and reactive ion etched by a mixture of CHF3 and
O2 [11] to form the electrodes of SNS Josephson junctions.
The etching rate was ~10 nm/min at 55 mTorr and 150 W. A
thin layer of aluminum was used as an etch mask. The lateral
size and height of the etched pillars were checked with an
atomic force microscope.
E-beam lithography was again used for patterning the SiO2
insulator for the top Nb contact layer (Fig. 1). The growth of
SiO2 was done in a plasma enhanced chemical vapor
deposition (PECVD) reactor using a mixture of Silane/Argon
(5%SiH4/Ar) and nitrous oxide (N2O), after the surface was
cleaned gently with N2 plasma. The insulator deposition was
done in two steps: first at 150 °C for regions around the
devices, and then at 300 °C for areas below the bond pads,
both followed by a lift-off. Two steps were needed because
only the 300 °C SiO2 can withstand bonding, whereas accurate
small scale lift-off was only possible for the 150 °C SiO2.
Also, 150 °C is about the maximum temperature that could be
used to avoid burning the thinner e-beam resists around the
device active area. By using this unconventional technique (ebeam patterned PECVD lift-off), we achieved highly aligned
SiO2 patterns.
The top contact Nb layer was again pattered by e-beam
lithography and evaporated (2 Å/s) on top of the device with
an UHV ( ~ 9 × 10 − 9 mbar) e-beam evaporator. Before the Nb
deposition, we used a gentle O2 plasma cleaning and 1 kV Arion sputtering in situ to remove impurities and a possible thin
oxide layer from the NbN surface. The thickness of a possible
natural oxide on the NbN surface could be about 2 nm thick
[7]. In previous studies, Ar-ion cleaning has been shown to
reduce the NbN surface contact resistance [12], and was
observed to be necessary to make a good contact with the top
NbN layer also in this work. A direct bond far from the device
area served as the bottom NbN layer contact. A schematic
illustration of the final device structure is presented in Fig. 1.
Fig. 1. A schematic cross section of the device geometry. The NbN/TaN/NbN
trilayer was grown by pulsed laser ablation on single crystal (100) MgO.
Devices were pattered by e-beam lithography as squares and reactive ion
etched to form the electrodes of the SNS Josephson junctions. The growth of
highly aligned SiO2 insulator was performed by PECVD after e-beam
lithography patterning. The top contact Nb layer was e-beam evaporated in an
UHV chamber.
Fig. 2. Resistance versus temperature of a 8 micrometer × 8 micrometer
NbN/TaN/NbN Josephson junction. Inset shows a zoom-in of the Nb
transition. Critical temperature of NbN was ~15.7 K and for Nb ~ 8.8 K. The
high quality of the NbN film is reflected in its high transition temperature.
IV. EXPERIMENTAL RESULTS
Junction resistance (R) vs. temperature (T) was measured
for all samples, with a typical result shown in Fig. 2. Typically
critical temperatures of NbN (Tc ~15.7 K) and Nb (Tc ~ 8.8 K)
were observed. The high quality of the NbN film is reflected
in its high transition temperature.
The I-V measurements of the junctions were performed at
several different temperatures from 4.2 K upwards. The 3
micrometer × 3 micrometer junctions were not sputter cleaned
before the Nb deposition, which resulted in resistive behavior
even at zero bias. However, those 8 micrometer × 8
micrometer junctions that were cleaned showed a large
supercurrent branch, as exemplified in Figs. 3 and 4. In Fig. 3
we can notice a rounding of the I-V characteristics, possibly
due to thermal fluctuations [2]. In Fig. 4 less pronounced
rounding is seen; however, the critical currents are clearly
lower compared to Fig. 3. The difference between these two
devices (and others) is not well understood at the moment, as
fairly large variations were observed between devices
fabricated on the same substrate, with nominally identical
trilayer films.
The Rn values obtained from the I-V characteristics were
observed to be as high as 180 mΩ and relatively insensitive to
temperature below 7 K. The I-V data in Figs. 3 and 4
corresponds to values ~25 mΩ and ~ 42 mΩ, respectively. At
T ~11 K where Rn is dominated by the Nb contact Figs. 2, 3
> 2eph03 <
and 4 all give a consistent value ~100 mΩ. Using the I-V data,
we determined the IcRn values of four similar (nominally the
same trilayers) 8 micrometer × 8 micrometer devices at
several different temperatures from 4.2 K to 8.7 K. The results
are shown in Fig. 5. Measurements above 9 K were prohibited
by the finite resistance of the Nb lead.
3
the obtained D, 2 ξ n (T c ) ~ 2 nm < 10 nm = L, so that we are
well within the range of validity of Eq. (2). The dashed curve
shows the same analysis but with n = 2 × 10 21 cm-3, giving a
D ~ 7 . 5 × 10 − 6 m2s-1. We also checked that with these values
2 ξ n (T c ) ~ 1.5 nm < 10 nm = L. From Fig. 5 we can notice that
the theoretical curves follow fairly well the measured values.
However, one of the samples showed clearly lower IcRn
values, most likely due to a bad quality Nb-NbN interface.
Fig. 3. The I-V characteristic for a 8 micrometer × 8 micrometer
NbN/TaN/NbN Josephson junction at eight different temperatures from 4.2 K
to 11.0 K with a TaN barrier of 10 nm. The critical current at 4.2 K was ~3.2
mA.
Fig. 4. The I-V characteristic for a 8 micrometer × 8 micrometer
NbN/TaN/NbN Josephson junction at eight different temperatures from 4.2 K
to 11.8 K with a TaN barrier of 10 nm. The critical current at 4.2 K was ~1.2
mA.
Using the normal state resistance (~ 25 mΩ) of the junction
from Fig. 3 at 4.2 K we can estimate the resistivity of a 8
micrometer × 8 micrometer junction, giving ~16 mOhms cm.
To estimate a value for the diffusion constant D, we need in
addition a reliable value for the carrier density n. As it can
vary a lot as a function of the TaN composition [13], we have
to leave it as a fit parameter for our theoretical analysis. In
Fig. 5 we show two different theoretical curves based on Eq.
(2) with n as a parameter and L kept fixed at 10 nm. The solid
curve has n = 1× 10 21 cm-3, corresponding to a diffusion
constant D is ~ 9 . 5 × 10 − 6 m2s-1. In addition, we fixed a value
2 ∆ / kT c ≈ 4 .0 as observed for high quality NbN [14]. With
Fig. 5. The measured values of critical current Ic times normal layer
resistance Rn with different temperatures for four 8 micrometer × 8
micrometer junctions and two theoretical curves with different carrier density
n according to Likharev’s theory of SNS junctions. The product IcRn is the
appropriate figure of the merit for SNS junction and measured data follows
quite closely Likharev’s theory.
We also made I-V measurements at several different values
of magnetic flux density B from 0 T to 3.2 T at a temperature
4.2 K. The junctions were placed in the magnetic field so that
the field lines are along the N layer (90 degrees from the
direction of the current flow).
A typical measured curve of critical current Ic vs. magnetic
flux density B is presented in Fig. 6. Although the measured
curve in Fig. 6 does not show the Fraunhofer pattern of a clean
and small Josephson junction [2], it is clearly seen that
increasing magnetic field suppresses critical current Ic to zero
at ~3 T. The absence of the Fraunhofer pattern is the result of
a nonuniform critical current density at the junction area,
likely caused by the fact that the junction dimension (8 µm) is
slightly larger that the Josephson penetration depth, estimated
to be ~ 3 µm.
> 2eph03 <
4
Likharev’s theory of SNS junctions followed quite closely the
measurements, showing that this simple proximity effect
based model is likely a good enough design guide for NbN
based junctions.
REFERENCES
[1]
[2]
[3]
[4]
Fig. 6. The critical current (Ic) vs. magnetic flux density (B) for a 8
micrometer × 8 micrometer NbN/TaN/NbN Josephson junction with a TaN
barrier of 10 nm at the temperature 4.2 K.
[5]
[6]
V. CONCLUSIONS
We have grown high quality NbN/TaN/NbN trilayer films
with high critical temperature (~16 K) with the pulsed laser
ablation method, and developed an unconventional e-beam
lithography-based technique that uses lift-off of a high-quality
PECVD-grown SiO2 insulator layer to produce high alignment
accuracy. The method may allow a close packed structure on a
small area and a material friendly fabrication process for
submicron devices in the future. Ar-ion sputtering of the NbN
surface before the top wiring deposition improved the quality
of the junctions significantly by removing the native oxide and
surface contaminants from the NbN surface, proving to be
necessary for high quality junctions.
The I-V characteristics of the devices showed sharp features
at 4.2 K. However, the IcRn products were slightly smaller
although comparable to values measured before for sputtered
NbN films [4], and showed scatter between nominally
identical devices. Also, the highest values of IcRn at 4.2 K
were about three times lower than the desired ~0.3 mV. With a
further optimization of the TaN layer, improvements may be
possible, confirming the result that TaN is a promising barrier
material [4], [5]. In addition, theoretical curves based on
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
K. K. Likharev and V. K. Semenov, “RSFQ Logic/Memory Family: A
New Josephson-Junction Technology for Sud-Terahertz-ClockFrequency Digital systems,” IEEE Trans. Appl. Supercond., vol. 1, no.
1, pp. 3-28, Mar. 1991.
T. Van Duzer and C. W. Turner, Principles of Superconductive Devices
and Circuits. Prentice Hall, Upper Saddle River, New Jersey, USA,
1999.
V. K. Kaplunenko, “Fluxon interaction in an overdamped Josephson
transmission line,” Appl. Phys. Lett., vol. 66, pp. 3365-3367, 12 Jun.
1995.
A. B. Kaul, S. R. Whiteley, T. Van Duzer, L. Yu, N. Newman and J. M.
Rowell, “Internally shunted sputtered NbN Josephson junctions with a
TaNx barrier for nonlatching logic applications,” Appl. Phys. Lett., vol.
78, pp. 99-101, 1 Jan. 2001.
A. B. Kaul and T. Van Duzer, “NbN/TaNx/NbN SNS Josephson
Junctions by Pulsed Laser Deposition,” IEEE Trans. Appl. Supercond.,
vol. 11, pp. 88-91, 1 Mar. 2001.
R. Setzu, E. Baggetta and J. C. Villegier, ”Study of NbN Josephson
junctions with a tantalum nitride barrier tuned to the metal-insulator
transition”, J. Phys. Conf. Series, vol 97, 012077, 2008.
A. Darlinski and J. Halbritter, “Angle-resolved XPS Studies of Oxides at
NbN, NbC, and Nb Surfaces,” Surface and Interface Analysis, vol. 10,
pp. 223-237, 1987.
K. K. Likharev, “Superconducting weak links,” Rev. Mod. Phys., vol.
51, pp. 101-159, Jan. 1979.
K. A. Delin and A. W. Kleinsasser, “Stationary properties of highcritical-temperature proximity effect Josephson junctions,” Supercond.
Sci. Technol., vol. 9, pp. 227-269, 1996.
P. G. de Gennes, “Boundary effects in superconductors,” Rev. Mod.
Phys., vol. 36, pp. 225-237, Jan. 1964.
M. Radrapvar, M. J. Berry, R. E. Drake, S. M. Faris, S. R. Whiteley and
L. S. Yu, “Fabrication and Performance of all NbN Josephson Junction
Circuits,” IEEE Trans. Magn., vol. 23, no. 2, pp. 1480-1483, Mar. 1987.
M. Hajenius, J. J. A. Baselmans, J. R. Gao, T. M. Klapwijk, P. A. J. de
Korte, B. Voronov and G. Gol’tsman, “Low noise NbN superconducting
hot electron bolometer mixers at 1.9 and 2.5 THz,” Supercond. Sci.
Tech., vol. 17, pp. S224-S228, 2004.
L. Yu, C. Stampfl, D. Marshall, T. Eshrich, V. Narayanan, J. M. Rowell,
N. Newman and A. J. Freeman, “Mechanism and control of the metal-toinsulator transition in rocksalt tantalum nitride,” Phys. Rev. B., vol. 65,
245110, 2002.
A. Kawakami, Z. Wang and S. Miki, “Fabrication and characterization
of epitaxial NbN/MgO/NbN Josephson tunnel junctions”, J. Appl. Phys.,
vol. 90, pp. 4796-4799, 2001.