Download Single photon detectors based on quantum dot field effect

Single photon detectors based on semiconductor
devices with InAs quantum dots
Beata E Kardynał
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Quantum Information Group, Toshiba Research Europe Ltd
Patrick See
Neil Beattie
Ian Farrer
Andrew J Shields
James Blakesley
Dave Ritchie
Cambridge Research Laboratory
Toshiba Research Europe Ltd
Cambridge, UK
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Semiconductor Physics
Cavendish Laboratory
University of Cambridge
Cambridge, UK
Quantum Information Group, Toshiba Research Europe Ltd
Motivation
‰ Development of single photon detector for quantum cryptography
and more generally for quantum information systems.
‰ Requirements:
high detection efficiency
very low dark count rate
fast >10 MHz and faster better
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Quantum Information Group, Toshiba Research Europe Ltd
Overview
‰ Devices for single photon detection:
ƒ resonant tunnelling diodes containing InAs quantum dots (QDRTD)
ƒ field effect transistors containing InAs quantum dots (QDFET)
‰ Principle of Operation: sensing electrostatic potential due to
captured photo-hole.
‰ Single photon counting.
‰ Conclusions.
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Quantum Information Group, Toshiba Research Europe Ltd
Quantum Dots Field Effect Transistor
photon (semi)-transparent
gate
20nm GaAs
electron channel
h
‰ Photon absorption takes place in
quantum well (20nm) and the
e
optional absorber under the well
quantum
dot layer
n+ GaAs back
gate
(up to 200nm).
photon
absorber
AlGaAs
barrier Si doping
‰ Photo-holes are captured on
negatively charged dots.
V(x,y)
‰ At 4K the effect is persistent.
(x,y)
I
EF
EC
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Quantum Information Group, Toshiba Research Europe Ltd
Effect of Light on QDFET
100
dark
101
100
-0.5
0.0
105
10-1
10-2
10-3
0.5
103
101
10-1
illuminated
-0.5
0.0
0.5
Conductance (µS)
Conductance (µS)
102
0 .0 V
- 0 .0 2 V
- 0 .0 4 V
- 0 .0 8 V
10
- 0 .1 2 V
- 0 .1 4 V
- 0 .1 8 V
- 0 .2 V
0
Gate Voltage (V)
200
400
600
800
Time (s)
‰ Negative charge on the dots shifts the I-V characteristics of the FET. Reduced by
illumination charge on the dot results in increased conductance of the FET.
‰ Electrical detection of single photon possible if the FET is small (about 1 – 2 µm
mesa, 1 – 2 µm gate).
‰ Steps in current are up to 10% of the total current.
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Quantum Information Group, Toshiba Research Europe Ltd
0.2
‰ Detection efficiency of up to 1.0% at 650
nm.
‰ Limited by:
0.1
‰ NiCr gate (25% transmission).
0.0
0
5
10
Incident Photon Flux
15
(s-1)
‰ Both GaAs and AlGaAs absorption detected.
‰ No losses due to the tunnelling of holes
from GaAs.
‰ 20-25 nm of AlGaAs around the dots
“active” assuming uniform collection
efficiency.
Abs20nm
Abs25nm
1
1
0.1
400
600
Wavelength (nm)
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Quantum Information Group, Toshiba Research Europe Ltd
800
Absorption near Dots (%)
‰ Low absorption of 20nm thick GaAs
well at 650 nm (6%).
Quantum Efficiency (%)
Detection rate (s-1)
Spectral Response of the QDFET
Quantum Dots Resonant Tunnelling Diode
50nm n+ GaAs
collector
quantum
dot layer
‰ Photon absorption in the absorber on the
collector side of the device (100 - 600 nm)
photon
absorber
AlGaAs /GaAs
RTD
n+ GaAs emitter
‰ Photo-holes are captured on negatively
charged dots.
‰ Photo-holes recombine on the dots
producing photons of lower energy that
GaAs bandgap – no re-absorption.
‰ Tunnelling current repopulates depleted
dot with τ ∝ Itunnel.
‰ Independent contacts are obtained using
cross-wire geometry of the device.
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Quantum Information Group, Toshiba Research Europe Ltd
Effect of Light on QDRTD
‰ Charged dots detune the device (in
I
e
emitter
emitter- collector voltage bias) from the
resonance compared with RTDs with no
collector
dots.
h
‰ Reduction of the charge on a dot with a
0.02
0.5
0.00
Current Density
-2
(Acm )
0.04
photo-hole brings the RTD locally closer to
0.0
the resonance: tunnelling current rises
sharply.
0.5
1.0
Bias (V)
-0.2
‰ Electrical detection of single photon
possible if the RTD is small (1 – 2 µm
diameter).
-0.4
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Quantum Information Group, Toshiba Research Europe Ltd
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0.04
‰ RTD has 300 nm absorber region under
50nm n+ collector.
λ=550nm
3
‰ The best fit between the measurement and
theory is obtained when it is assumed that
only bottom 150 nm of the absorber is
“active”.
0.03
2
0.02
0.01
20
0
0.00
0.0
0.1
0.2
0.3
0.4
Incident Photon Flux (s-1)
‰ Maximum efficiency of 15 % is measured at
620 nm wavelength.
‰ Efficiency is limited by:
ƒ photon reflection from the surface.
ƒ losses in the collector contact layer.
Quantum Efficiency (%)
Detection rate (s-1)
Spectral response of RTD
Absorption in 150nm
GaAs above InAs dots
15
10
5
0
400
600
Wavelength (nm)
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Quantum Information Group, Toshiba Research Europe Ltd
800
Single photon counting – amplifier
Source-Drain Bias
Gate Voltage
+ Reset Pulse
Vout
Vac
SPD ISPD
Band-Pass Amp
VDC
LHe cryostat
Low-Pass Amp
Gain (mV/nA)
Optic Fibre
Cascode Bias
100
10
1k
10k
100k
1M
10M
Frequency (Hz)
‰ Transimpedance amplifier is used with the detector for single photon counting.
‰ Amplifier is configured to operate with high resistance of the device.
‰ Amplifier input referred noise is about 0.14 pA/Hz0.5 and is dominated by the input
transistor.
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Quantum Information Group, Toshiba Research Europe Ltd
100
0.0
50
0.0
0.5
‰ Typical traces of the output signals from
the low-pass and ac amplifiers captured
on the oscilloscope.
AC output
10.0
1.0
Time (s)
Current (nA)
Low-pass amp output (nA)
20.0
Band-pass amp output (mV)
Single photon counting – amplifier II
185
180
175
0
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Quantum Information Group, Toshiba Research Europe Ltd
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4
6
Time (ms)
8
10
Counting Setup
Pulsed Laser Power
Supplier
Laser Sync Signal
‰ Minimum time gate 5 ns.
Time Delay
‰ Laser wavelength 684 nm.
Laser Diode
Discriminator Level
‰ Pulse filling factors: 10-4 - 103
photons.
SPD
‰ T = 4.2K.
Amp
Period of the Measurement
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Quantum Information Group, Toshiba Research Europe Ltd
Photon Counting with QDRTD
a)
‰ Device with 3 nm Al0.33Ga0.67As barriers =>
Counts (a.u.)
1.0
Light induced step size about 15nA.
0.5
‰ Jitter (FWHM) measured to be 150 ns =>
solely the circuit property, not the detection
process speed.
0.0
0
1
2
Time (µs)
‰ Dark counts: amplifier noise and RTS noise
1
Count rate (pulse)
10
of the device .
0
10
-1
10
-2
10
light
dark
-3
10
-4
10
20
40
60
Discriminator level (mV)
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Quantum Information Group, Toshiba Research Europe Ltd
Photon Counting with QDRTD
a)
dark counts (per gate)
-2
10
-3
10
‰ Maximum counting efficiency at 684 nm
-4
10
-5
laser wavelength 9% and 1% at a very low
-6
dark count of 10-6 per gate (10-8 ns-1).
10
10
-7
10
-8
10
0
2
4
6
8
10
approximate quantum efficiency (%)
gain : (step height) / (reset rate)/e =2 ·105
6
10
5
10
4
10
3
10
2
10
1
10
0
10
-1
10
-2
10
-3
10
‰ From the current step and reset rate we get
-2
peak current (Acm )
-2
modelled current (Acm )
step height (nA)
-1
reset rate (s )
for 3 nm barriers device.
‰ We measure current: more important is the
step height itself.
3
4
5
6
7
8
Barrier thickness (nm)
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9
10
‰ Exponential increase of photon-induced
signal with RTD barrier thickness.
Quantum Information Group, Toshiba Research Europe Ltd
Jitter at strong signal
Current (nA)
Count rate (arb. units)
illumination
400
300
200
100
0
0
200
400
600
data
Gaussian fit
1.0
0.5
0.0
100
Time (s)
120
140
160
180
200
Time after laser pulse (ns)
‰ The jitter was measured (for FET) to be 6.5 ns (FWHM) and it is still limited by the
capacitance of the circuit.
‰ Frequency of operation limited by width and jitter of the signal: 100% of count in 25 ns
gate.
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Quantum Information Group, Toshiba Research Europe Ltd
Counting with QDFET
10
10
10
0
light
dark
-1
-2
-3
10
-4
10
-5
20
40
60
Dark count rate (per gate)
Count rate (per gate)
10
-2
10
-3
10
-4
10
-5
10
-6
10
-7
10
0.0
Discrimintor level (mV)
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Quantum Information Group, Toshiba Research Europe Ltd
0.5
Quantum efficiency (%)
1.0
Summary
‰ Maximum quantum efficiency is obtained in the QDRTD and is about 15% at
620nm. It should increase to 65% at 475 nm in optimised devices.
‰ Maximum counting efficiency at 684nm laser was measured to be about 9%.
‰ Dark count rate is very low for both devices: at 1% counting efficiency dark
count rate is 10-8 ns.
‰ Jitter of the counting is limited by the signal to noise ratio and the minimum
jitter was measured to be 150 ns FWHM when the current step was about 15
nA. We expect reduction of the jitter to 100 ps – 1 ns in optimised circuit.
‰ Wavelength of detection can be tuned with material system used.
InGaAs/InAlAs on InP can be used for telecommunication wavelengths.
‰ Since the absorption region is different from the detection region it may be
possible to increase the absorption area.
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Quantum Information Group, Toshiba Research Europe Ltd