Instrumentation & Measurement Magazine 24-5 - 69

Demonstration of Single
Photon Detection in Amorphous
Molybdenum Silicide / Aluminium
Superconducting Nanostrip
Daniela Salvoni, Loredana Parlato, Mikkel Ejrnaes, Francesco Mattioli, Alessandro Gaggero,
Francesco Martini, Giovanni Ausanio, Davide Massarotti, Domenico Montemurro,
Halima Giovanna Ahmad, Luigi Di Palma, Francesco Tafuri, Roberto Cristiano, and Giovanni Piero Pepe
S
ingle photon detectors (SPD) are largely used when
the signal to be measured is particularly low or if
high accuracy, timing resolution and low noise are
required. About twenty years ago, a new type of SPD based
on superconducting materials appeared, and today these devices
present the best performances in terms of efficiency,
noise, counting rate, detectable wavelength, dead time, and
timing jitter. In this work, we first describe the working principle
of Superconducting Nanostrips Single Photon Detectors
(SNSPDs) and provide a brief overview of the main properties
and applications. Then, we present the results of the
fabrication and characterization of Molybdenum Silicide
SNSPDs with a cover layer of a thin Aluminium film (MoSi/
Al). MoSi has already been adopted to fabricate SNSPDs, and
in this work, we prove that the additional Al layer enhances
the operating conditions without compromising the detection
likelihood and leaving unaltered the properties of the device,
even over a temporal window of two years and after several
thermal cycles.
Superconducting Nanostrip Single
Photon Detectors
Superconductors were proposed for single photon detection as
they exhibit an energy gap which is of the order of a few meV,
about three orders of magnitude lower than in semiconductors,
that are the materials on which the most common SPD are
based. This means that, in principle, it is possible to enlarge by
a factor of 1000 the maximum detectable photon wavelength
band with the use of superconducting materials.
In 2001, Gregory Gol'tsman, Roman Sobolewski and colleagues
first demonstrated single photon counting in visible
and infrared wavelength domain with a Superconducting
Nanostrip Single Photon Detector (SNSPD) [1]. The device consisted
of a 5 nm thick and 200 nm wide NbN nanostrip, cooled
at 4.2 K to lead the material in the superconducting state.
Since that measurement, many important results have been
achieved, and now it is possible to realize SNSPDs which are
extremely precise and are being adopted worldwide for their
highly attractive performances.
August 2021
Today, SNSPDs can offer up to 98% system detection efficiency
at λ = 1550 nm [2], less than 1 cps dark counts rate, about
10 ns dead time (maximum counting rate of hundreds MHz),
3 ps temporal resolution [3].
From the properties just listed, one can notice that these devices
can guarantee an optimum signal to noise ratio. Similar
values can be reached with conventional single photon detectors
just in the visible/UV domain. Conversely, SNSPDs'
energy gap of few meV opens the way to the detection of infrared
photons, and measurements up to λ = 10 μm have been
carried out. The largest detectable wavelength of the photon
depends on the energy gap Δ and on other parameters as
follows:
λ 

1

Ic
2N D wd 1

I


where N0 represents the density of states, D the diffusion constant,
τ the thermalization time, Ic the nanostrip critical current,
and d and w the thickness and the width of the nanostrip, respectively
[4]. As one can see in (1), the detectable wavelength
is strongly affected by the energy gap value.
The only disadvantage in the use of SNSPD was often
identified in the cryogenic temperature required to reach the
operative settings; however, today, good performances can
also be reached with simple closed cycle cryostats that are not
as expensive as in the past.
Operating Principle
A superconductor is a material that, when cooled below its
critical temperature Tc
(1)
, enters in a state of zero resistance.
Superconducting state can be destroyed by increasing the temperature
or, similarly, by increasing either the bias current or
the magnetic field applied to the device over the critical values.
SNSPDs consist of a thin superconducting nanostrip, whose
thickness and width are around 10 nm and 100 nm, respectively.
The scales are set by superconductor coherence length
ξ and London penetration depth λL
. The length of the strip is
typically of the order of hundreds of μm, and the nanostrip are
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