# Single-photon detection in the mid-infrared up to 10 micron wavelength using
tungsten silicide superconducting nanowire detectors

V. B. Verma<EMAIL_ADDRESS>National Institute of Standards and Technology,
Boulder, CO, USA.  B. Korzh<EMAIL_ADDRESS>Jet Propulsion Laboratory,
California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA, USA  A.
B. Walter  Jet Propulsion Laboratory, California Institute of Technology, 4800
Oak Grove Dr., Pasadena, CA, USA  A. E. Lita  National Institute of Standards
and Technology, Boulder, CO, USA.  R. M. Briggs  Jet Propulsion Laboratory,
California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA, USA  M.
Colangelo  Department of Electrical Engineering and Computer Science,
Massachusetts Institute of Technology, Cambridge, MA, USA.  Y. Zhai  National
Institute of Standards and Technology, Boulder, CO, USA.  E. E. Wollman  Jet
Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr.,
Pasadena, CA, USA  A. D. Beyer  Jet Propulsion Laboratory, California
Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA, USA  J. P. Allmaras
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove
Dr., Pasadena, CA, USA  B. Bumble  Jet Propulsion Laboratory, California
Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA, USA  H. Vora
National Institute of Standards and Technology, Boulder, CO, USA.  D. Zhu
Department of Electrical Engineering and Computer Science, Massachusetts
Institute of Technology, Cambridge, MA, USA.  E. Schmidt  Jet Propulsion
Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena,
CA, USA  K. K. Berggren  Department of Electrical Engineering and Computer
Science, Massachusetts Institute of Technology, Cambridge, MA, USA.  R. P.
Mirin  National Institute of Standards and Technology, Boulder, CO, USA.  S.
W. Nam  National Institute of Standards and Technology, Boulder, CO, USA.  M.
D. Shaw  Jet Propulsion Laboratory, California Institute of Technology, 4800
Oak Grove Dr., Pasadena, CA, USA

###### Abstract

We developed superconducting nanowire single-photon detectors (SNSPDs) based
on tungsten silicide (WSi) that show saturated internal detection efficiency
up to a wavelength of 10 $\upmu\mathrm{m}$. These detectors are promising for
applications in the mid-infrared requiring ultra-high gain stability, low dark
counts, and high efficiency such as chemical sensing, LIDAR, dark matter
searches and exoplanet spectroscopy.

††preprint: AAPM/123-QED

Efficient single-photon counting, with a detection efficiency greater than 50%
has, to date, been achieved only at wavelengths shorter than 2
$\upmu\mathrm{m}$ [1]. Extension of such performance to the mid-infrared has
potential for new applications in astronomy [2] as well as LIDAR [3], dark
matter searches [4] and the fundamental study of fast molecular dynamics and
chemistry [5, 6, 7, 8]. Interest in the mid-infrared stems primarily from the
presence of numerous absorption signatures for molecules such as water vapor,
carbon dioxide, oxygen and ozone, methane, and nitrous oxide. These molecules
are not only important for understanding of the evolution of our own planet,
but may also be signatures of life or indicate the potential habitability of
planets outside of our solar system. The spectroscopy of exoplanet atmospheres
is a prime example of an application requiring single-photon-counting
detectors with ultra-stable gain and high efficiency due to the photon-starved
nature of the measurements [9, 10].

Here we report on superconducting nanowire single photon detectors (SNSPDs)
based on tungsten silicide (WSi) with saturated internal detection efficiency
up to a wavelength of 10 $\upmu\mathrm{m}$. Internal detection efficiency is
defined as the probability that a photon absorbed in the nanowire generates an
electrical output pulse from the detector. Prior work in the near-infrared has
shown that achieving saturated internal detection efficiency is a critical
step in making high efficiency devices [11, 12]. Our results indicate that it
may be possible to achieve high system detection efficiency in the mid-
infrared with WSi SNSPDs by optimizing the absorption in the nanowires and the
coupling to the active area of the detectors, as has been done at near-
infrared wavelengths.

Previous approaches to achieving sensitivity in the mid-infrared were based on
SNSPDs fabricated from niobium nitride (NbN) thin films. NbN has historically
been the material of choice for SNSPDs due to its relatively high
superconducting transition temperature (8
$\mathrm{K}\mathrm{e}\mathrm{l}\mathrm{v}\mathrm{i}\mathrm{n}$ to 12
$\mathrm{K}\mathrm{e}\mathrm{l}\mathrm{v}\mathrm{i}\mathrm{n}$) allowing
operation at 4 $\mathrm{K}\mathrm{e}\mathrm{l}\mathrm{v}\mathrm{i}\mathrm{n}$,
and its fast thermal recovery time allowing operation at high count rates
[13]. In order to improve the energy sensitivity of the nanowire detectors and
extend the photo-response to longer wavelengths, there are several possible
approaches. One is to decrease the cross-sectional area of the nanowire,
making the film thinner or the nanowire narrower. This results in a higher
probability of a hotspot (non-superconducting domain) being generated since
the energy per unit area is larger, and thermal conductivity along the length
of the nanowire is smaller. Using this approach, saturated internal detection
efficiency has been demonstrated with NbN SNSPDs up to a wavelength of
approximately 3 $\upmu\mathrm{m}$ using ultra-narrow 30 nm-wide nanowires
[14]. In another report, single-photon sensitivity was achieved at a
wavelength of 10.6 $\upmu\mathrm{m}$ using 40 nm-wide NbN nanowires, although
the internal detection efficiency was not saturated[15].

The second approach to improving energy sensitivity is to engineer the
material to reduce the free carrier density. Reducing the free carrier density
(increasing resistivity) of the material results in the deposited energy being
divided amongst fewer quasiparticles, thus increasing the effective
temperature of each quasiparticle. Simultaneously, the thermal impedance along
the length of the nanowire is increased, which more effectively localizes the
deposited energy along its length. This approach is preferable to reducing the
width and thickness of the nanowires from a fabrication and yield perspective,
as detector yield begins to degrade significantly at widths below about 80 nm
[16]. Below this width, edge roughness and constrictions in the width of the
nanowire due to fabrication imperfections begin to dominate and significantly
degrade the device performance. The constrictions result in a suppressed
switching current, the current at which the critical current density in the
wire is exceeded and the superconductor switches to the normal (non-
superconducting) state.

Yet another approach to improving energy sensitivity is reducing the
superconducting gap energy. This results in a larger number of broken Cooper
pairs (quasiparticles) for a given amount of deposited energy in the
superconductor [17]. However, reducing the superconducting gap energy implies
a lower superconducting transition temperature ($T_{\mathrm{c}}$) and thus a
lower operating temperature for the detectors. This is generally undesirable
as it results in an increase in the complexity and cost of the cryogenics.

As outlined above, the most promising approach to lowering the energy
threshold is to increase the resistivity of the superconducting film, which is
typically achieved through variation of the stoichiometry. For amorphous WSi,
the $T_{\mathrm{c}}$ is not strongly dependent on stoichiometry as shown in
Fig. 1, which allows a significant variation in resistivity while maintaining
a fixed operating temperature of the system. In addition, the films remain
amorphous over a wide range of sputtering parameters and stoichiometries [18],
as verified by x-ray diffraction measurements.

Figure 1: Superconducting transition temperature ($T_{\mathrm{c}}$) as a
function of sputtering power on the silicon target during co-sputtering of the
WSi film. The W sputtering power is fixed at 100 W. Measurements were
performed on bulk films with thickness greater than or equal to 50 nm.

We fabricated detectors from high-resistivity WSi films using a Si-rich
stoichiometry. The films are co-sputtered from separate W and Si targets,
allowing the composition of the films to be tuned by adjusting the relative
sputtering powers. Typical sputtering powers for the W and Si targets are 100
and 180 W, respectively, which in our deposition system results in a silicon
content of 15%. We increased the Si sputtering power from 180 to 260 W, while
maintaining the W sputtering power at 100 W. This resulted in a silicon
composition of 35±7% as estimated by secondary ion mass spectroscopy (SIMS).
The film thickness was 3.2 nm, and the superconducting transition temperature
was 3.1 K. Note that the $T_{\mathrm{c}}$ of the thin film is significantly
lower than that shown in Fig. 1 due to the decrease of $T_{c}$ with film
thickness. Each detector is a single 10 $\upmu\mathrm{m}$-long nanowire
instead of a large-area meander, in order to reduce the probability of
fabrication defects or constrictions along the length of the wire for the
relatively narrow widths investigated ranging from 50 to 80 nm. A 2
$\upmu\mathrm{H}$ inductance consisting of a 10 $\upmu\mathrm{m}$-wide
meandering wire was patterned into the WSi film in series with the nanowire to
prevent latching to the non-superconducting state [19]. Scanning electron
micrographs of the chip layout are shown in Fig. 2. The small active area of
the device prevents it from achieving a high coupling efficiency in mid-
infrared applications, however, it serves as a well controlled experiment to
study the optimum nanowire material and geometry to achieve the maximum
intrinsic detection efficiency.

Figure 2: Scanning electron micrographs of the chip layout.

The SNSPDs were measured at a temperature of 0.9 K, and flood illuminated
using various quantum cascade lasers (QCLs) also mounted inside the cryostat,
at the 4 Kelvin stage. Figure 3 shows the measurement setup. The three QCLs
(4.8, 7.4, and 9.9 $\upmu\mathrm{m}$) are mounted side by side, such that it
is possible to study the response to different wavelengths during the same
cooldown of the cryostat. The QCLs are pulsed with 260 $\upmu\mathrm{s}$
pulses and a 4% duty cycle. To collect the photon response curves, a 200
$\upmu\mathrm{s}$ gate is applied on a counter, synchronized with the middle
of the laser pulses. The background rate (BCR) is collected by synchronizing
the gate with the period when the laser is turned off. The signal from the
SNSPD is amplified with a cryogenic amplifier operating at the 4 kelvin stage,
with a gain of 45 dB, bandwidth of 1.5 GHz and noise temperature of less than
6 K. An additional room temperature amplifier with 25 dB of gain and 500 MHz
bandwidth was used, followed by a 120 MHz low-pass filter. This readout scheme
enabled pulses to be readout while biased with currents as low as 200 nA.
Figure 4 shows normalized photon count rate (PCR) vs. bias current curves for
SNSPDs fabricated with a film deposited with a sputtering power on the silicon
target of 260 W. PCR curves are shown for two different nanowire widths (50
$\mathrm{n}\mathrm{m}$ and 70 nm), and measurements were obtained at three
wavelengths (4.8, 7.4, and 9.9 $\upmu\mathrm{m}$). As shown in the figure, the
70 nm-wide nanowire showed the onset of a saturation plateau even at a
wavelength as long as 9.9 $\upmu\mathrm{m}$. The absence of this feature for
narrower wires is likely due to the presence of a fabrication defect such as a
constriction in the width of the nanowire.

Figure 3: Quantum cascade laser (QCL) setup inside the cryostat, flood
illuminating the sample. Typically the 10.6 $\upmu\mathrm{m}$ bandpass filter
was omitted, in order to enable the use of all three lasers on the same
cooldown. But it was used to verify the response at 9.9 $\upmu\mathrm{m}$ with
and without it.  Figure 4: Normalized photoresponse count rate vs. bias
current curves for SNSPDs fabricated from a WSi film with a higher silicon
content (sputtering power on silicon target increased from 180 to 260 W). PCR
curves are shown for two different nanowire widths (50 and 70 nm) and
measurements were obtained at three wavelengths (4.8, 7.4, and 9.9
$\upmu\mathrm{m}$). Black squares correspond to measurement of the background
count rates.

In order to further explore the effect of film stoichiometry on energy
sensitivity, we fabricated a second set of devices with an even higher
sputtering power on the silicon target of 320 W. Analysis by secondary ion
mass spectroscopy indicated a silicon composition of 48 ± 10%. The film
thickness was determined to be 2.61 nm, slightly thinner than the previous set
of devices with the lower silicon sputtering power, with a marginally lower Tc
of 2.8 K compared to the previous film with a Tc of 3.1 K. Fig. 5 shows the
photon count rate vs. bias current curves and dark count measurements for two
nanowires having widths of 70 nm and 50 nm and a length of 10
$\upmu\mathrm{m}$. Both geometries show a significant improvement in
saturation of the internal detection efficiency at all three wavelengths. Note
that the shape of the 4.8 $\upmu\mathrm{m}$ PCR curve for the 50 nm wire is
distorted at the lowest bias currents because the readout threshold was about
200 nA, hence any pulses created at lower currents were missed by the counter.

Figure 5: Normalized photoresponse count rate vs. bias current curves for
SNSPDs fabricated from a WSi film with a further increase of the silicon
content (sputtering power on silicon target increased from 260 W to 320 W)
compared to the devices in Fig. 4. Two different nanowire widths (50 and 70
nm) are presented and measurements were obtained at three wavelengths (4.8,
7.4, and 9.9 $\upmu\mathrm{m}$). Black squares correspond to measurement of
the background count rates. The switching currents are lower, however, the
relative saturation of the internal efficiency is better for all wavelengths
and nanowire geometries.

In conclusion, we have fabricated WSi SNSPDs with a saturated internal
detection efficiency at mid-infrared wavelengths up to 10 $\upmu\mathrm{m}$.
Increasing film resistivity by tuning the film stoichiometry appears to be a
promising approach to improving energy sensitivity. Demonstrating saturated
internal efficiency is the first important step to obtaining high system
detection efficiency by efficient design of an optical stack around the
detector to enhance absorption [12], and optimized self-aligned fiber coupling
[20] or free-space coupling [21]. However, with both fiber-coupling and free-
space coupling, filtering blackbody radiation from the environment presents a
significant engineering challenge in the mid-infrared, requiring the use of
cold filters or gratings. While the initial data presented here appears
promising, further work is required to investigate the scalability and yield
of narrow wires on the order of 50 – 80 nm over larger areas, which will be
required for kilopixel and larger arrays required for applications such as
exoplanet spectroscopy.

##  I Acknowledgements

Support for this work was provided in part by the Defense Advanced Research
Projects Agency, Defense Sciences Office, through the Detect program. A. B.
Walter’s research was supported by an appointment to the NASA Postdoctoral
Program at the Jet Propulsion Laboratory, administered by Universities Space
Research Association under contract with NASA. Support for this work was
provided in part by the NASA ROSES-APRA program. Part of this research was
performed at the Jet Propulsion Laboratory, California Institute of
Technology, under contract with the National Aeronautics and Space
Administration (80NM0018D0004). The authors would like to thank Pierre
Echternach for technical discussions.

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