R&D of Hf-STJ as FIR single-photon spectrometer for
COBANDTGSW2022 session 5-9
Universe Evolution and Matter Origin
onlineSep. 27, 2022

Y. Takeuchi (TCHoU, Univ. of Tsukuba)
for the COBAND collaboration

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COBAND(Cosmic Background Neutrino Decay)
Search for Neutrino decay in Cosmic background neutrino➔To be observed as FIR photons around ~50m𝜸

𝝂𝟐

𝝂𝟑

Neutrino Decay

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Neutrino Decay signal and backgrounds
Surface Brightness I [MJy/sr]

CIB summary from Matsuura et al.(2011)
ZE

AKARI

CMB

COBE

Expected Neutrino
Decay signal

ZL

𝜏 = 1 × 1014 yr𝑠
𝒎𝟑 = 𝟓𝟎 𝐦𝐞𝐕

ISD
DGL

ZodiacalEmission

SL

Wavelength [m]Current experimental
lower limit is 1012 yrs

E [meV]

We can identify  decay signal by highly precise measurement of photon energy
spectrum around ~50m
3➔ Require for the detector to detect and measure single photon energy at ~50m

Superconducting Tunnel Junction (STJ)
Superconductor / Insulator /Superconductor (SIS)
Josephson junction device
SuperconductorE2

Insulator
Ns(E)Superconductor
Superconductor
Insulator

Δ: Superconducting gap energy

A constant bias voltage (|V|<2Δ/e) is applied across the junction.

A photon absorbed in the superconductor breaks Cooper pairs and creates tunneling current ofquasi-particles proportional to the deposited photon energy.
• Much lower gap energy (Δ) than FIR photon ➔ Can detect FIR photon and measure its energy.
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STJ current-voltage characteristics
Current

E

W/ light illumination
No light

2Δ-2/eleak current

2/e

DC Josephson current

Suppressed by BfieldVoltage
B field
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STJ candidates

SiNbAlNb/Al-STJTc[K]
9.23
1.20
• Well-established and commonly used.
• Δ~0.6meV by the proximity effect from Al Δ[meV] 1100 1.550 0.172• Operation temp. <400mK• Back-tunnelling gain G ~10• Nq.p.=25meV/1.7Δ10~ 250
E/E~10% for E=25meV (~50m)➔25meV single-photon detection is feasible ideally.
➔Candidate for the rocket experiments with diffraction grating.

Hf0.165
0.020

Hf-STJ• Not established as a practical photo-detector yet by any group• Nq.p.=25meV/1.7Δ~ 735• 2% energy resolution for a 25meV single-photon is achievable➔Spectrum measurement without a diffraction grating.
➔Developing for a future satellite experiment6

Superconducting device process equipment at KEK clean room
Hf-STJ samples are fabricated at KEK clean room byour group.
• Successful in etching Hf layer in 2008.
• Confirm SIS junction by Hf-HfOx-Hf in 2010.
• Confirm Hf-STJ response to visible light pulse in 2013.

ICP-RIE

Aligner in yellow hat

Metal film sputtering

7X-ray illumination test at IBS CUP in Jun. 2019

Adiabatic Demagnetization Refrigeration(ADR) at temperature down to 30mK
◼ Hf-STJ I-V◼ Hf-STJ response to X-ray photon (55Fe)◼

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X-ray illumination test at IBS CUP in Jun. 2019
On ADR cold stage◼ STJ samples◼ Cu collimator◼ X-ray source (55Fe) sealedin polyester tape
◼ Solenoid coil with persistentcurrent switch

X-ray source

STJCu Collimator

cold stage
200m×200m(250nm)(~2nm)

25mm(400nm)chip carrier

9200m sq. Hf-STJ sample under test
Anode oxidation

Al wireAl wireHf Upper layer (250nm)

300nm

HfOx (~2nm)
Hf Lower layer (400nm)
Si wafer (100)10Setup for I-V curve and X-ray response measurements at the IBS CUPCurrentSource

Differential
Amplifier+-Hf−STJI
constant current

ADR

V
STJ I-V curve w/ light w/o light◼

◼
◼
◼

Sinusoidal current on STJ for I-V measurement
Constant current on STJ for X-ray response measurement
Voltage of STJ is read with differential amp. placed at room
temp.
X-ray signal is shown up as a negative pulse in STJ voltage.
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I-V curve about 34mK w/ B field
200m square Hf-STJ @ T～34mK• Applied magnetic field ～10G on STJ• Leak current of ～3A(@20V)

This leakage is actually much larger than our expectation. (Our goal is ～pA)

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I-V curve about 30mK w/o and w/ B field
200m square Hf-STJ @ T～30mK• DC Josephson current is shown up without magnetic field, and it issuppressed with magnetic field (～10G).
I-V curve at 30mK in no magnetic field

I-V curve at 31mK in magnetic field

Magnet Off

Magnet Onzoom

This confirms the SIS junction on the STJ.

zoom

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μA

Temperature dependence of I-V curve
214.3mK

197.5mK

μV
150.2mK64.7mK

14Temperature dependence of Gap energy

4VV (μeV)

Define V from I-V curve

Dot: DataCurve: 𝜅 (BCS theory)
Δ = Δ(𝑇; Δ0 ) from gap eqn.

Temp. (mK)• Temperature dependence of V matched the BCS theory very well.
• Found to be 𝑇𝑐 = 250.31 ± 0.03𝑚𝐾

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Temperature dependence of Leakage
𝐼𝑡ℎ = 𝑃0Δ 𝑇
Δ 𝑇𝑐
exp −+ 𝑃2
𝑘𝑇𝑐 𝑇𝑐𝑘𝑇𝑐 𝑇

Leak current (μA)

Assuming 𝑇𝑐 = 250.31𝑚𝐾 ，Δ0~1.7639𝑘𝑇𝑐

BCS theory (P.W.Epperlein 1978)

Δ = Δ(𝑇; Δ0 ) from gap eqn.

𝑉 = 10, 15, 20, 25, 30𝜇𝑉

Extracted Δ0 from fit

Dot：DataCurve：BCS theory

Temp. (mK)10V: 27.59 eV
15V: 28.38 eV
20V: 29.52 eV
25V: 30.63 eV
30V: 31.62 eV• Temperature dependence of leakage matched the BCS theory.
• Found to be Δ0 ~30𝜇𝑉

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Imaginary component in gap energy?
• Because of quasi-particle lifetime, gap energy Δ has imaginary part.
(Mitrovic 2007).

DoS (a.u.)• In this case, density of state distribution changes2

2Re()Im()=0.1Re()17Imaginary component in gap energy?
• Assume the following

normal region resistance: 3 = 30eV + i ・ 3 eV• Also assume existence of parallel normal current path of resistance: 6 
40μA

-40μA

-100μV

100μV

Not perfectly, but closer I-V curve is reproducible on these assumptions. 18

Response to X-ray photon

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X-ray Signal waveform template
Apply I=1A (corresponding to V～9.2V) on Hf-STJ @ T～32mK• Set trigger at -1.1V for falling pulse in AC coupled readout• Get 1390 events in ～2 hours• Obtain the signal template from these events.

Normalized so that the peak is at -1VTrigger time since the first event(sec)20

Fit observed signal to the template
Use the region from -50s to 200s in the template (1250points)• Amplitude and baseline are fitting parameters.
• Scan t from -50s) to +50 s) for RSS minimum
𝑅𝑆𝑆 ≡ ෍ 𝐴𝑣 𝑡𝑖 + 𝑉0 − 𝑉 𝑡𝑖 + Δ𝑡2𝑡𝑖 ∈ [−50𝜇𝑠, 200𝜇𝑠]

𝑖𝑣 𝑡𝑖 : template waveform at 𝑡𝑖𝑉 𝑡𝑖 : observed waveform at 𝑡𝑖
𝐴: Amplitude
𝑉0 : Baseline
From each event, we extract
Amplitude, t, and baseline.

Fit region
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Example of Signal waveform fits0000010.bin

22Distributions of t, baseline, and amplitude
t (s)Amplitude

baseline (V)

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Residual Sum of Squares (RSS) distribution
Least squares fitting of 1250pts

Selection criteria|baseline| <0.2V
RSS <90V224

Energy distribution
After selection on fit parameters for candidates, 1138 events left.
• K1+K2 X-rays with energy of 5.9 keV and a probability about 24.4%,• K X-rays with nominal energy of 6.5 keV and a probability about 2.85%
Assuming K:K ratio and peak energies
and same  for K and K,We fit the distribution and scaled.

～6.7%(39622eV)

We found the energy resolution is
6.7% (39622eV)

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Pedestal distribution
We estimate pedestal distribution by forging pedestal waveforms from triggered events themselves.
• Once fit nominal waveform to signal template, and get fit parameters ( amplitude, baseline, t)• Use amplitude and t (not baseline), subtract signal component from waveform.
• After subtraction, fit again to the signal template.

Subtracted signal component
and fit again

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Mean and r.m.s. in each time bin after subtraction
for the subtraction methodology check

For subtracted waveforms 𝑉𝑗 (𝑡𝑖 ), calculate mean and rms after candidate selection
Mean: 𝑉ത 𝑡𝑖

1≡ σ𝑗 𝑉𝑗 (𝑡𝑖 )𝑁r.m.s.: rms 𝑡𝑖 ≡1
σ 𝑉 𝑡
𝑁 𝑗 𝑗 𝑖− 𝑉𝑗

21𝑛𝑉𝑗 ≡ σ𝑖 𝑉𝑗 (𝑡𝑖 )Meanr.m.s.

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Signal component subtraction seems to work fine to mimic pedestal events.

Amplitude distribution extracted from fits oversignal subtracted events
～6.1%(3597eV)• Sigma of measured energy distribution for pedestal samples forged fromsubtraction events is found to be 6.1% (359eV), which is almost comparable tosigma of signal events.
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Summary• Hf-STJ is under development for application to COBAND project, aiming at farinfrared single photon detector and spectrometer.
• We successfully fabricated Hf-STJs with confirmed SIS junction and tested themwith X-ray source.
• We measured and compare temperature dependence of I-V curve of Hf-STJ withBCS theory. Unexpectedly large leakage is likely due to large imaginary
component of gap energy as well as normal current path other than SIS junction.
• We confirmed the clear signals for X-ray single photon from 55Fe with a Hf-STJ
sample and found that energy resolution is about 6.7% for 5.9keV.
These are the world first results for Hf-STJ.
• Currently the energy resolution is dominated by pedestal noise. This could be
due to readout noise, where we have a plenty room to improvement on this.
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