ALUMINUM SUPERCONDUCTING HOT-ELECTRON BOLOMETER MIXERS FOR THz APPLICATIONS
University, 15 Prospect Street, New Haven, Connecticut 06520-8284, A.
Skalare, B.S. Karasik, W.R. McGrath, P. Echternach, and H.G. LeDuc, Center for
Space Microelectronics Technology, Jet Propulsion Laboratory, California
Institute of Technology, Pasadena, California 91109
We report on microwave measurements of superconducting
aluminum hot-electron bolometers (Al HEBs). Diffusion-cooled Al HEB mixers are
ideal candidates for space-borne and terrestrial remote-sensing applications in
the Terahertz frequency range since they are predicted to have small local
oscillator (LO) power requirements, intermediate frequency (IF) bandwidths
10 GHz, and a noise temperature lower than that of Nb and NbN HEBs.1
Mixer measurements were made at an LO frequency ~30 GHz LO, with an IF in the
range 0.1-7.3 GHz. For T < 0.8 K, a magnetic field H=0.1-0.3T was applied to
suppress the superconductivity in the contact pads, and partly in the bridge.
For a 0.6 μm long Al HEB, we measure an IF bandwidth of 4 GHz, a conversion
efficiency η = -8dB, and a mixer noise temperature Tm
4K, DSB (Tmixer=Toutput noise/2η). These results are
shown to be in quantitative agreement with simple theoretical predictions.
Recent studies on Nb and NbN hot-electron bolometer (HEB)
mixers have demonstrated that they are excellent candidates for Terahertz
spectroscopy applications.2-4 For Nb HEB mixers, the largest
intermediate frequency (IF) bandwidths are obtained for devices much shorter
than the inelastic electron-phonon length. These rely on the out-diffusion of
hot electrons from the microbridge into cold reservoirs as the dominant mode of
energy relaxation.5 Diffusion-cooled Nb mixers have demonstrated IF
bandwidths up to 10 GHz, with the local oscillator (LO) power needed for optimal
operation typically ~ tens of nW at Terahertz frequencies. The noise performance
of diffusion-cooled Nb devices is excellent, with an achieved receiver noise
temperature TR=1800K, DSB at 2.5THz. 3
Recently, HEBs employing superconductors with a lower
transition temperature than Nb (Tc ~ 6K) have been proposed.1
The devices studied here are diffusion-cooled HEBs based on Al, with Tc
~ 1.5 to 2.4K. Improvements in mixer performance are predicted since clean Al
films have a lower transition temperature and a higher diffusivity D than Nb
We present measurements for Al HEB mixers at microwave
frequencies. The frequency of the LO source used is ~ 30 GHz. The primary
motivation for studying mixing at microwave frequencies is that much of the
device physics relevant to THz mixing can be explored with the simpler microwave
measurements. Previous microwave studies of Nb HEBs has been useful in this
We present here predictions for mixer performance of Al HEB1,2
devices. The IF bandwidth of the HEB mixer can be estimated from the thermal
time constant th of the device. The thermal relaxation rate
has a term due to inelastic electron-phonon scattering, and one due to the
"out" diffusion rate -- th-1
In our devices, electron-phonon scattering is negligible, and the thermal time constant
is given by the diffusion time2
and the –3dB intermediate frequency rolloff is thus:
f -3dB =1/(2eff
) = 1/(2th).
L is the length of the bolometer. Eq. (1) applies when
electro-thermal feedback is small, so that eff = th.
The higher the diffusivity, the larger the IF bandwidth.
Calculations for devices several coherence lengths long indicate that an IF
bandwidth ¤ 10 GHz should be attainable.
Al HEBs are also promising since the LO power required for operation is
predicted to be lower than that of Nb and NbN mixers. The LO power for a
diffusion-cooled device is given by2,6
where £ = 2.45x10-8 Watt-Ohm/K2 is the Lorenz
constant and R the device resistance. At 2.5 THz, for Nb HEBs, PLO ~
20 nW3 and PLO ~ 100 nW7 for NbN phonon-cooled
HEBs. The LO power dissipated in the mixer in Al should be ~ 0.2 nW based on
scaling of the data obtained for Nb at 20 GHz2, and ~2nW for THz
Though HEB mixer theories for noise are currently under
discussion, we discuss here two main thermal noise sources: thermal fluctuation
noise and Johnson noise. The contribution of thermal fluctuation noise to the
total device noise is proportional to the critical temperature10, and
should thus be smaller in Al devices than in Nb ones. Lowering the Tc
of the HEB will similarly result in a decrease of the Johnson noise. Quantum
noise, however, must also be considered. A lower bound on the contribution to
the mixer noise is TMQ » h/k
11.At the microwave frequencies we used, the quantum noise is almost negligible,
~1K. At Terahertz frequencies, the quantum noise limit is not negligible. TM
Q= 120K at 2.5 THz. Since the measured mixer noise of Nb HEBs is much greater
than TMQ, we believe that reducing the two thermal
contributions, by use of Al HEBs, will reduce TM. This should hold
true even for more advanced noise theories. The mixer noise temperature at 30
GHz due to thermal sources is predicted to be ~8 K by scaling the best results
obtained with Nb at 20 GHz by Tc.
II. Devices and Measurement Setup
The devices consist of a thin, narrow Al microbridge with
dimensions d=13-17nm, W=0.1μm, and L=0.2-1μm, where d,W, and L are the
thickness, width, and length, respectively. Thick contacts consist of a
tri-layer of Al, Ti, and Au with thickness ~ 68nm, 28nm, 28nm respectively on
top of the thin Al film. The fabrication details can be found in Ref. 12. The
device parameters are summarized in Table I.
Table I: Device parameters. Diffusion constant value of devices A and D
are measured, while those for B and C are inferred from the resistivity.
The device width is 0.1m. For mixer tests, Tc=1.0K for device A in
a magnetic field to 2.4K for device B in zero field.
The superconducting transition temperature of the Al
microbridges in zero field ranged from ~ 1.5-2.4 K depending on length and
resistivity. The contact pads are a combination of normal and superconducting
metals, and have a transition temperature which is lower than that of the
microbridge, with Tc,contact pads »
0.6-1.0K. For tests below Tc,contact pads a perpendicular magnetic
field is applied to suppress the superconductivity in the contact pads.
The devices are mounted on the cold stage of a variable
temperature 3He cryostat. The bath temperature was varied from
0.25-1.6K for the mixing experiments, and up to 40K for Johnson noise
calibrations and other measurements. A schematic of the measurement setup is
shown in Fig. 1.
A. IF Bandwidth
The IF bandwidth depends on the bias point used. Measurements
reported here are for bias points in the resistive state where conventional HEB
mixing models can be applied. The measured IF bandwidth ranged from 1.2-6 GHz.
In Fig. 2,
a comparison is made between the measured IF bandwidth and the value
predicted from a calculation of the diffusion time
The bias points considered in determining the IF bandwidth
were the ones which gave the maximum conversion efficiency in the resistive
state. We can see good agreement with the prediction for a diffusion-cooled
B. Optimum LO Power
The LO power used in the mixing experiments is in the range
of õ 1.0 nW delivered to the mixer block.
Values of the conversion efficiency and mixer noise are presented as a function
of LO power in Fig. 3.
The mixer noise temperature is calculated from the output
noise of the device and the conversion efficiency: Tm
The LO power needed for optimum conversion efficiency is approximately the same
value that gives the best noise performance. Experimentally this is the case
since the output noise is slowly varying with bias voltage and thus the dominant
factor in determining the voltage dependence of the mixer noise is the
conversion efficiency. Measurements of the temperature dependence of the optimum
LO power were also made, and are in agreement with the relation presented in Eq.
C. Mixer Noise
In Fig. 4,
the dependence of mixer noise and of conversion
efficiency on bias voltage is shown for device A, using Tm=Tout/2.
The minimum of the mixer noise temperature is ~4 K for device A. At the LO
frequency used, this is ~ 3 hv/k. The mixer noise temperature with a 20 GHz LO
in Nb HEBs was ~ 120 h/k in the case with a finite critical current, but
33 h/k when the critical current was fully suppressed by PLO.6
This mixer noise temperature is somewhat lower than predicted
by simply scaling Nb data at 20 GHz according to Tc. However, in the
Nb measurements, there was excess noise, the origin of which was not explained.
For the Al HEBs, the total output noise is consistent with thermal fluctuation
and Johnson noise contributions with Johnson noise of the magnitude expected for
Results for mixing with Al HEBs at microwave frequencies are
very good. The IF signal bandwidth scales with device length and diffusivity as
predicted in the diffusion cooling model, Eq. (1). The LO power needed for
mixing scales approximately linearly with Tc. The measured mixer
noise is somewhat lower than that predicted by scaling Nb HEB results. The
measured IF bandwidth and optimum LO power are in good agreement with lumped
Currently, a major design issue for space-borne application
of HEB mixer receivers is the availability of an appropriate LO source.
Molecular lasers are heavy and need high-power sources. Other possibilities at
present are photomixer sources and multipliers. A successful traveling-wave THz
photomixer has been shown to have an output power of at least ~ 10nW above 1
THz.13 This is not enough for mixing with Nb HEBs. But our results
for the optimum LO power for Al HEB mixers indicate that there is real
possibility for integrating a THz Al HEB mixer with such a photomixer.
In actual receivers, saturation effects have to be
considered. Since the bias voltage range over which good performance is observed
is tens of microvolts, output saturation due to background noise or a large
input signal might be an issue. Choosing a smaller mixer bandwidth for
situations when high input power is present is a potential solution. Further
work is needed to quantify at which power levels saturation effects are
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