We have fabricated and tested submillimeter-wave superconductor–insulator–superconductor (SIS) mixers using very high-current-density Nb/AlN/Nb tunnel junctions (Jc[approximate]30 kA cm–2). The junctions have low-resistance-area products (RNA[approximate]5.6 Omega µm2), good subgap-to-normal resistance ratios Rsg/RN[approximate]10, and good run-to-run reproducibility. From Fourier transform spectrometer measurements, we infer that omegaRNC = 1 at 270 GHz. This is a factor of 2.5 improvement over what is generally available with Nb/AlOx/Nb junctions suitable for low-noise mixers. The AlN-barrier junctions are indeed capable of low-noise operation: we measure an uncorrected double-sideband receiver noise temperature of TRX = 110 K at 533 GHz for an unoptimized device. In addition to providing wider bandwidth operation at lower frequencies, the AlN-barrier junctions will considerably improve the performance of THz SIS mixers by reducing rf loss in the tuning circuits

Bumble, Bruce

Chen, Jian

Kawamura, Jonathan

LeDuc, Henry G.

Miller, David

Stern, Jeff A.

Zmuidzinas, Jonas

English

Caltech Authors - Main

APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 15 10 APRIL 2000Very high-current-density NbÕAlNÕNb tunnel junctions for low-noise
submillimeter mixers
Jonathan Kawamura,a) David Miller, Jian Chen,b) and Jonas Zmuidzinas
California Institute of Technology, 320-47, Pasadena, California 91125
Bruce Bumble, Henry G. LeDuc, and Jeff A. Stern
Center for Space Microelectronics Technology, Jet Propulsion Laboratory, Pasadena, California 91108
~Received 27 October 1999; accepted for publication 14 February 2000!
We have fabricated and tested submillimeter-wave superconductor–insulator–superconductor ~SIS!
mixers using very high-current-density Nb/AlN/Nb tunnel junctions (Jc’30 kA cm22). The
junctions have low-resistance-area products (RNA’5.6 V mm2), good subgap-to-normal resistance
ratios Rsg /RN’10, and good run-to-run reproducibility. From Fourier transform spectrometer
measurements, we infer that vRNC51 at 270 GHz. This is a factor of 2.5 improvement over what
is generally available with Nb/AlOx/Nb junctions suitable for low-noise mixers. The AlN-barrier
junctions are indeed capable of low-noise operation: we measure an uncorrected double-sideband
receiver noise temperature of TRX5110 K at 533 GHz for an unoptimized device. In addition to
providing wider bandwidth operation at lower frequencies, the AlN-barrier junctions will
considerably improve the performance of THz SIS mixers by reducing rf loss in the tuning circuits.
© 2000 American Institute of Physics. @S0003-6951~00!04915-9#Niobium-based superconductor–insulator–superconduc-
tor ~SIS! tunnel junction mixers have now achieved near-
quantum-limited noise performance in the near-millimeter
bands,1 as was predicted to be possible theoretically.2,3 This
success relies heavily on advances in the technology for fab-
ricating high-quality small-area niobium junctions with alu-
minum oxide barriers ~Nb/AlOx/Nb!, along with integrated
Nb thin-film superconducting tuning circuits. The tuning cir-
cuits are necessary to overcome the frequency response limi-
tation imposed by the finite resistance–capacitance (RNC)
product of SIS junctions: the 3 dB roll off in the frequency
response (vRNC51) of an untuned Nb/AlOx/Nb junction
occurs at approximately 100 GHz. Here, RN is the junction
normal resistance and C is its capacitance.
An inductive tuning circuit with inductance L allows the
center frequency n05v0/2p of the response to be chosen
arbitrarily, according to the usual condition v0
2LC51, with a
response bandwidth that is limited by the RNC product. Ei-
ther a series or parallel inductance can be used, and is most
often provided by a section of superconducting microstrip
transmission line. In practice, rf losses in the tuning circuit
seriously limit the performance at higher frequencies. Tuning
circuits made from Nb become ineffective above 700 GHz,
due to the onset of large Ohmic losses ~surface resistance!
above the superconducting energy-gap frequency ngap
52D/h . At present, one of the main goals of SIS mixer
development, driven by the needs of airborne and space-
based astronomy projects, is to extend their low-noise per-
formance to frequencies above 1 THz.
For operation above 700 GHz, a high conductivity nor-
mal metal ~e.g., Al! can be used in place of niobium for the
tuning circuit.4 Another possibility is to use a low-loss su-
a!Electronic mail: jhk@caltech.edu
b!On leave from the Research Institute of Electrical Communication, Tohuku
University, Sendai 980-8577, Japan.2110003-6951/2000/76(15)/2119/3/$17.00
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject tperconductor with a larger energy gap, such as NbTiN.5 In
either approach, the resulting resonant circuit requires a large
quality factor (Q;10 at 1 THz! for ideal performance and is,
therefore, quite sensitive to Ohmic loss. For instance, in mix-
ers which use normal metal tuning circuits, much of the rf
power is still dissipated in the tuning circuit, and only a
small fraction ~;20%! of the signal is absorbed ~detected! by
the tunnel junction. High-frequency Ohmic loss is an impor-
tant issue even for superconducting films ~e.g., NbTiN!,
since the loss is often a sensitive function of film quality,
composition, and microstructure, and can be difficult to con-
trol. In many SIS mixer designs, superconducting microstrip
lines are also used for the impedance transformers which
match the circuit to the rf feed, which can either be a planar
antenna structure or a waveguide-to-microstrip transition.
Although there may be losses in these transformers as well,
the mixer’s overall performance depends most critically on
the inductive circuit which tunes out the junction capaci-
tance, due to its high-Q requirement.
Thus, an important parallel strategy is to reduce the junc-
tion RNC product, so that the Q required for the tuning cir-
cuit is reduced and the mixer performance is less sensitive to
losses. The RNC product is independent of junction area, but
depends critically on the thickness of the tunnel barrier. Be-
cause the tunneling resistance decreases exponentially as the
barrier thickness t is reduced, while the capacitance should
increase only as 1/t , thinner tunnel barriers are needed to
reduce the RNC product. Two parameters are commonly
used to characterize the tunnel barrier: the Josephson critical
current density Jc and the resistance-area product RNA .
However, there is a serious practical difficulty associated
with increasing the current density ~reducing RNA): at some
point, the junction quality degrades rapidly.
An important figure of merit for SIS junctions used in
low-noise mixer applications is the subgap-to-normal resis-
tance ratio Rsg /RN , which is a measure of the junction leak-9 © 2000 American Institute of Physics
o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
2120 Appl. Phys. Lett., Vol. 76, No. 15, 10 April 2000 Kawamura et al.age current. This quantity controls the conversion efficiency
and noise of the mixer.3 For Nb/AlOx/Nb junctions, the
subgap-to-normal resistance ratio degrades rapidly for cur-
rent densities much higher than 10 kA cm22.6 This corre-
sponds to a resistance-area product of RNA’20 V mm2.
There is good evidence7 that the quality degradation of Nb/
AlOx/Nb tunnel junctions is caused by pinhole defects in the
barrier. For RNA’20 V mm2, the junction specific capaci-
tance is Cs’85 fF mm22; combining these quantities, one
obtains the roll-off frequency (2pRNC)21’100 GHz
quoted above.
It is possible to fabricate high-quality junctions with
higher current densities by using AlN as the barrier
material.8 We have recently fabricated Nb/AlN/Nb junctions
with current densities as high as Jc’50 kA cm22. The high-
current-density AlN junctions have also been incorporated
into submillimeter mixer circuits, using an existing design.
This design, which was optimized for Nb/AlOx/Nb junctions
with Jc’10 kA cm21, has a well-understood behavior and
has demonstrated excellent performance.9 We assumed that
this approach would be adequate, since the specific capaci-
tance of AlN junctions was previously estimated to be
roughly similar to that of AlOx junctions,8 and since the
capacitance should only be a slow function of current den-
sity.
The fabrication of the mixers largely followed the steps
used to make the previous devices, with one exception. In-
stead of following the Al deposition by oxidation to form an
oxide barrier, an AlN barrier was formed by plasma
nitridation.10 In this process, 7 nm of Al is sputtered on a Nb
ground plane, and the nitride barrier is formed by exposing it
to a pure nitrogen plasma. The ground plane and wiring layer
Nb film are 170 and 300 nm thick, respectively. The junc-
tions are square and have a nominal area in the range A
51.2– 2.25 mm2. The target current density is Jc
’30 kA cm22, which yields junctions with RNA
55.6 V mm2. We obtain good subgap-to-normal resistance
ratios, Rsg /RN’10. The current–voltage characteristics of
one of the mixers are shown in Fig. 1. A batch of mixers
with lower current density was also fabricated, with RNA
520 V mm2. These mixers have very similar Rsg /RN ratios
as the high-current-density devices, which allow the mixer
characteristics to be directly compared.
The mixer configuration we use is a quasioptical planar
twin-slot antenna coupled to a two-junction tuning circuit.11
The completed mixer chip is attached to an antireflection-
coated hyperhemispherical Si lens. The lens and substrate are
fitted into a copper mixer block assembly, which is mounted
on a liquid-helium-cooled cold plate. The input beam passes
through a high-density polyethylene lens at 4.2 K and several
layers of infrared filtering provided by porous Teflon on the
77 K radiation shield. The vacuum window is a 25-mm-thick
mylar film. The local oscillator is coupled to the input beam
with a 12-mm-thick mylar beam splitter at room temperature.
The intermediate frequency is centered at 1.5 GHz and the
first-stage amplification is provided by a high-electron-
mobility transistor amplifier with an input noise of T if
’5 K.
The spectral response of 650 GHz mixers with both high
and normal current densities was measured with Fourier
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject ttransform spectrometer ~FTS!, and the results are shown in
Fig. 2. The difference between the two mixers is quite dra-
matic, as the response of the mixer with higher current den-
sity spans nearly an octave. FTS measurements were made
on a number of other devices, and are summarized in Fig. 3.
The response curves were fitted with a mixer model, which
takes into account the frequency-dependent impedance of the
slot antenna and the behavior of the tuning circuit. The sur-
face impedance of the niobium films used in the microstrip
transmission lines was calculated using Mattis–Bardeen
theory in the local limit.12 The model response was multi-
plied by the antenna main-beam efficiency to simulate the
mixer’s coupling to the FTS. In general, the main-beam ef-
ficiency varies only slowly near the center frequency but
decreases rapidly at high frequencies due to the onset of
large sidelobes in the antenna pattern.
The junction capacitance and normal-state resistivity of
the niobium films are the main physical parameters that are
derived by adjusting the mixer model to fit the measure-
FIG. 1. Current–voltage (I – V) characteristics of a high-current-density 550
GHz mixer. The bath temperature is 4.2 K. Shown are the I – V character-
istics traced with ~dashed heavy! and without ~solid heavy! local-oscillator
power applied. The LO frequency is 533 GHz, and the photon step begins at
V’0.6 mV. The if power in response to 295 K ~solid light! and 77 K
~dashed light! loads is shown as a function of voltage bias. The bumps in the
if power response near V’1.2 mV reflect insufficient supression of Joseph-
son currents. The mixer is normally biased near 2.0 mV for best perfor-
mance. For this measurement, a 12.5 mm Mylar beam splitter was used to
couple the optimum amount of LO power to the mixer, and TRX5110 K.
FIG. 2. Direct detection FTS response of Nb/AIN/Nb mixers with Jc
’30 kA cm22 ~heavy! and Jc’10 kA cm22 ~light! current densities. The
measured receiver responses are indicated by histograms, and the model fits
to these are the dashed curves. For the high-current-density mixer, we infer
vRC51 at 270 GHz, whereas for the low current density mixer, vRC
51 at 110 GHz.o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
2121Appl. Phys. Lett., Vol. 76, No. 15, 10 April 2000 Kawamura et al.ments. In our simulations, the normal-state resistivity rn is
only used as a scaling parameter for the Mattis–Bardeen
complex conductivity, so adjusting rn is equivalent to chang-
ing the penetration depth. The FTS curves are quite well
described by the model, and provide tight constraints on the
physical parameters. The junction specific capacitance Cs
can be calculated by dividing the junction capacitance used
in the mixer model by the junction area. The junction areas
are inferred from the junction resistance RN , by assuming
that the resistance-area product RNA is constant across the
wafer and is given by the value deduced from a set of test
junctions with a wide range of junction areas.
From the mixer models, we find r54 mV cm for the Nb
film normal-state resistivity and Cs5105 fF mm22 for the
specific capacitance of the high-current-density junctions.
The specific capacitance for the low-Jc AlN junctions is
found to be Cs585 fF mm22, which is very similar to that of
AlOx barrier junctions with the same current density. The
value for the normal-state resistivity is consistent with that
actually measured for similar films. By combining the junc-
tion capacitance determined by modeling with the junction
resistance ~trivially determined from dc measurements!, we
find that vRNC51 at 270 GHz for the high-current-density
mixers. For comparison, for the low-current-density mixers,
we find vRNC51 at 110 GHz, which is close to that at-
tained with Nb/AlOx/Nb junctions. Thus, the RNC product
of the high-current-density junction mixers represents an im-
provement by a factor ;2.5 over what is presently attainable
from Nb/AlOx/Nb junctions. Note that our determination of
the RNC product does not depend on accurate knowledge of
the junction area; the area is needed only to calculate the
specific capacitance.
Finally, the sensitivity of a high-current-density mixer
was measured using the standard Y-factor technique. For a
FIG. 3. Direct detection FTS response plotted with mixer model calcula-
tions for a number of different mixers with very high-current-density junc-
tions: ~a! a 550 GHz mixer with junction area A50.8 mm2; ~b! 550 GHz
mixer, A51.3 mm2; ~c! 650 GHz mixer, A51.7 mm2; and ~d! 750 GHz
mixer, A51.0 mm2.Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject tmixer designed for operation near 550 GHz, we measured a
double-sideband receiver noise temperature of TRX5110 K
at 530 GHz, with no corrections applied. The spectral re-
sponse of this mixer is shown in Fig. 3~b!, and its current–
voltage characteristics and intermediate frequency ~if! output
power in response to hot ~295 K! and cold ~77 K! loads are
shown in Fig. 1. We stress that the mixer circuit design was
optimized for a junction with RNA’20 V mm2, so the cou-
pling is not optimal at either the if or the rf. Regardless, the
performance at 530 GHz is still excellent and is comparable
to our best results with this mixer design and receiver con-
figuration when employing lower-Jc Nb/AlOx/Nb mixers.
This measurement confirms that the high-current-density
junctions used in our study are capable of low-noise mixing.
The use of high-current-density SIS mixers should
greatly advance the development of low-noise terahertz mix-
ers. Generally, it should greatly relax the requirements on the
losses in tuning circuits. In particular, it should be possible to
improve the noise performance of a mixer that uses a
normal-metal tuning circuit by a factor of at least 2 near 1
THz. Finally, the reduction in the RNC product naturally
allows the construction of mixers with very large input band-
widths. This is important especially at higher frequencies
where the wider bandwidth allows a given frequency range
to be covered with fewer mixers, resulting in simplified in-
strument designs.
This work was supported in part by NASA/JPL and its
Center for Space Microelectronics Technology, by NASA
Grant Nos. NAG5-4890, NAGW-107, and NAG2-1068, by
the NASA/USRA SOFIA instrument development program,
and by the Caltech Submillimeter Observatory ~NSF Grant
No. AST-9615025!. One of the authors ~J.C.! acknowledges
support from the Japanese Ministry of Education, Science,
Sports, and Culture.
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Very high-current-density Nb/AlN/Nb tunnel junctions for low-noise submillimeter mixers

http://authors.library.caltech.edu/8933/1/KAWapl00.pdf

Very high-current-density Nb/AlN/Nb tunnel junctions for low-noise submillimeter mixers

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