TGSW2017
2017/9/26

Status of Photo-Detector Developments Based on STJsfor COBAND Project

K.Takemasa, S.H.Kim, Y.Takeuchi, T.Iida , K.Nagata, C.Asano, R.Wakasa
A.Kasajima, H.Kanno (Univ. of Tsukuba)
For COBAND collaboration

▪ We are developing STJ detectors and cryogenic amplifiers for COBAND (COsmic BAckgroundNeutrino Decay search) experiment.

▪ Brief introduction of COBAND experiment▪ measure the neutrino mass by observing the neutrino decay of cosmic background neutrino.
▪ COBAND experiment consists of two type of measurement: rocket experiment and satellite experiment▪ Requirements for the detector▪ Continuous spectrum of photon energy around E=25 meV(=50m)▪ Energy measurement for a single photon with a better than 2%

resolution for E=25meV to identify the sharp edge in the spectrum

STJ can achieves requirements.
Nb/Al-STJ for the rocket experiment.
Hf-STJ for the satellite experiment.
2▪ Structure▪ Superconductor/Insulator/Superconductor

Overhead view of STJ detector▪ Size: dozens～hundreds m square and 500 nm height▪ Working principle▪ Incident photon is absorbed in the superconductor and excites

cooper pairs.
▪ Excited cooper pairs become quasi-particles.
▪ Quasi-particles go through insulator by tunnel effect.
▪ Number of quasi-particles is determined by energy of incident
particle.
▪ Thus, we can measure the energy of incident particle bymeasuring the tunnel current.
Quasi-particles

Josephson current.

w/ light
w/o light

Incident photon
Cooper pairs

: superconducting
gap energy

Superconductor/Insulator/Superconductor

Leakage current
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IV characteristic of STJ▪ Statistical fluctuation in number of quasi-particles determines STJ energy resolution.
▪ Smaller superconducting gap energy  yields better energy resolution.

𝜎𝐸 =Δ: Superconducting gap energy
F: fano factor
ε: Photon energy

1.7Δ 𝐹εMaterial

Tc(K)(meV)

Si-1200

Niobium

9.20

1.550

Aluminum

1.14

0.172

Hafnium

0.16

0.021

Tc : Superconducting critical temperature
Need ~1/10Tc for practical operation

NbHf▪ Well established as Nb/Al-STJ▪ Hf-STJ as a photon detector is not established▪ Nq.p. = 25meV/1.7 =9.5

▪ Nq.p. = 25meV/1.7 =735

▪ poor resolution but counting is possible▪ 2% energy resolution is achievable if fano factor <0.34

Performance of our Nb/Al-STJ sample

Our Nb/Al-STJ is fabricated with CRAVITY at AIST
Structure: Nb/Al/AlOx/Al/Nb = 100nm/70nm/1nm/70nm/200nm▪ Leakage current▪ Requirement: Ileak < 100pA to detect a single far-infrared photon(=40-80m)▪ Measured:

STJ size# of samples

Ileak@0.3mV2020m2

739  13pA

1010m2

2014  7pA

Nb/Al-STJ meets the requirements of the rocket experiment !
5▪ Earlier version of our Hf-STJ▪ Structure: Hf/HfOx/Hf = 250nm/1.5nm/300nm▪ Δ ~ 20μeV▪ Leakage current at 20μV = 20μA@50mK (100μm sq. sample)
Leakage current is too large.
Required leakage current = 10pA@50mK
Necessary to perform improvements very much.
100μm×100μm
T= 50～60mKTEM
Hf

HfOx

HfOx

HfSi wafer

Hf~1.5nm

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▪ We add thin (a few nm) Al layer between the insulator and the upper Hf layer.
▪ Josephson current is observed and it’s suppressed by magnetic field.
▪ Δ = 20~30μeV.
▪ Ileak = 5μA@128mK (200μm × 200μm sample)▪ Ileak becomes 16 times smaller than old sample.

▪ Response to visible laser pulses is observed.
200μm×200μm
Hf-STJ
2μA/div

2μA/div

10μV/div

10μV/div

T = 128mK
B = 0 Gauss

T = 128mK
B = 5 Gauss

10μA/div
20μV/div

Apply magneticfield(5 Gauss)

10μA/div

Response to laser pulses

20μV/div▪ =465nm▪ 5MHz oscillation 5μs per 100ms.

7▪ To reduce leakage current, We modified the sputtering condition to make smooth Hf layers.
▪ Rough surfaces of Hf layer cause defects of insulator.

▪ We made a Hf-STJ consisting of Hf layers with a small roughness of 2.5 nm RMS deposited a newspattering condition.

Result (200m sq. sample):▪ Leakage current: 6A , 16 times smaller than the old Hf-STJ.
▪ Response to laser continuous illumination is observed.
Hf-STJ(RMS=2.5nm)
200m 200m
~25eV
T~140mK7A increase

This Hf-STJ works as a STJ photon detector.
However, leakage current is still large, need more improvement.

Response to visible(=465nm) DC-like laser light
T = 140mK, 9Gauss B field is applied.

8▪ Our Nb/Al-STJ achieves the requirement.
▪ But, we haven't succeeded in detecting a far infrared single photon due to readout noise.
▪ To improve the signal-to-noise ratio, we are developing cold amplifier.
▪ Requirements for cold amplifier▪ Operation at cryogenic temperaturenoise fromrefrigerator.
Measurement
device @roomtemp▪ To reduce STJ’s thermal noise(∝√𝑻 𝒆(−𝜟/𝒌𝒃𝑻) ), we need to make cooler 300mK.
▪ Cold amplifier should be able to operate at 300mK.

▪ Low power consumption

60K▪ Typical cooling power of our refrigerator is 100μW@300mK.
▪ Power consumption of the amplifier should be less than cooling power of our refrigerator

3K▪ Response speed▪ The integration time of charge of Nb/Al-STJ is a few μs.

▪ Amplification gain should be large enough up to 1MHz.

STJ & amp.
@300mK Cold amplifier using SOI(Silicon-On-Insulator) technology can achieve these requirements
9▪ SOI device consists of devices on silicon thin film that exists on insulating film.
▪ FD(Fully depleted) SOI: thin SOI layer(normally<50nm). All body areas under the channel are depleted.

▪ FD-SOI device was proved to operate at 4.2K by a JAXA group.
▪ Characteristics:▪ Low power consumption▪ High speed▪ Easy large scale integration▪ Suppression of charge-up by high mobility carrier due to thin depletion layer(~50nm)

FD-SOI-CMOS

Id-Vg curve of W/L=10m/0.4m at |Vds|=1.8V

Both p-MOS and n-MOS show excellent performance at 3K and below.

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Circuit design
Common source
AmplifierBufferInput and Amplified Output
Input (1mV/div)

Time scale 2ms/div

OutputInput

Amplified Output (50mV/div)
Gain vs. Frequency
at 300mK

Gain of 80 was achieved for a signal frequency
up to 400kHz signals at 300mK.

400kHz11▪ We connect 20m sq. Nb/Al-STJ and SOI amplifier on the cold stage through a capacitance
T~350mK
PW~230μW

Inverting amplification
Gain: 66
512xaveraged

We observe 20m sq. Nb/Al-STJ responses to laser pulses of
=465nm amplified by SOI amplifier situated at T=350mK12▪ R&D of STJ detectors and SOI-cryogenic amplifier are underway.
▪ Nb/Al-STJ satisfied our requirement for leakage current less than 0.1nA▪ Leakage current of Hf-STJ becomes 1/16 by using smooth Hf layer▪ Cryogenic amplifier with the SOI technology worked at 300mK▪ We have succeeded in amplifying the STJ signal with the SOI cryogenicamplifier.

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