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Near-infrared (NIR) Single Photon Counting Detectors (SPADs)
Chong Hu, Minggou Liu, Joe C. Campbell & Archie Holmes
ECE DepartmentUniversity of Virginia
Outline
Introduction to Single Photon Counting Detectors (SPADs)
Current States of near-infrared SPADs
Summary and Future Goals
Mature Novel
PMT
• High gain• Low dark current• Low noise• Low quantum efficiency• Large, bulky• Expensive• High voltage • Fragile• Ambient light catastrophic
2000 V
APD
t
• Electron & Hole avalanche multiplication
• Good efficiency• Acceptable dark counts• Afterpulsing
Superconductors
• Hotspot Generation and Resistive Barrier
• High efficiency• Low dark counts• No afterpulsing• T < 1K!!
Geiger mode - APD functions as a switchAnalog DigitalResponsivity Single photon detection
efficiencyDark current Probability of dark count
Concept of excess noise does not apply!
Current
Voltage
Current
Linearmode
Geigermode
Vbr
on
off
Current
Voltage
Current
Linearmode
Geigermode
Vbr
on
off
timetime
timetime
time
Single photon input
APD output
Discriminatorlevel
Digital comparator output
Successfulsingle photondetection
Photon absorbed but insufficient gain – missed count
Dark count – from dark current
Geiger-mode operation
Current
Voltage
Current
Linear
mode
Geiger
mode
Vbr
on
off
Current
Voltage
Current
Linear
mode
Geiger
mode
Vbr
on
avalanche
off
quench
armVdc + V
Performance Parameters Photon detection efficiency
(PDE)The probability that a single
incident photon initiates a current pulse that registers in a digital counter
Dark count Rate (DCR)/Probability (DCP) The probability that a count
is triggered by dark current instead of incident photons
timetime
timetime
time
Single photon input
APD output
Discriminatorlevel
Digital comparator output
Successfulsingle photondetection
Photon absorbed but insufficient gain – missed count
Dark count – from dark current
Photon Detection EfficiencyPDE = external x collection x Pavalanche
V/Vbr
0.0 0.1 0.2 0.3
Bre
ak
do
wn
Pro
ba
bili
ty
0.0
0.2
0.4
0.6
0.8
1.0
InP: 1110 nm
InGaAsP 1.06, 1.3 m
InGaAs 1.55 m
Dark current• 0.18nA at 95% of Vbr at 297K• 0.15pA at 95% of Vbr at 200KGood enough?
40m-diameter In0.53Ga0.47As/InP
Reverse Bias (V)-60-55-50-45-40-35-30
Dar
k cu
rren
t (A
)
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5220K 230K 240K 250K 260K 270K 280K 290K 300K
300K Photocurrent
Dark current
1.06 μm SPADs: DCR vs. PDE InGaAsP absorber lower generation-recombination dark currentDCR approaching Si SPAD DCR with greatly increased PDE
Si SPADs have PDE < 2% at 1.06 μm
101
102
103
104
0% 10% 20% 30% 40% 50%Photon Detection Efficiency
Dar
k Co
unt R
ate
(Hz)
237 K
259 K
250 K
80 μm dia. InGaAsP SPAD 1-ns gated operation500 kHz repetition rate0.1 photon/pulseAP probability < 10-4 per gate
105
273K – 295K
**300K
0 5 10 15 20 25 30 35 40 45 5010 -7
10 -6
10 -5
10 -4
10 -3
10 -2
Dar
k Co
unt P
roba
bilit
y
Single Photon Detection Efficiency (%)
InGaAs/InP, 300K1550 nm (2007)
InGaAs/InP (2007)
Dark Count Probability versus Photon Dark Count Probability versus Photon Detection EfficiencyDetection Efficiency
200K
240K
Reverse Bias (V)-60-55-50-45-40-35-30
Dar
k cu
rren
t (A
)
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5220K 230K 240K 250K 260K 270K 280K 290K 300K 300K_photo
215K
268K
Current
Voltage
Current
Linear
mode
Geiger
mode
Vbr
on
off
Current
Voltage
Current
Linear
mode
Geiger
mode
Vbr
on
avalanche
off
quench
armVdc + V
Quenching Techniques
Quenching circuit– Quench avalanche current– Reset the device
• Passive Quenching– Quenched by discharging
capacitance– Slow recharge
• Active Quenching– Raise the anode voltage– Quick recharge
• Gated Quenching
hh
RRLL=300k=300k
RRss=50=50
VVbb ++VVExEx
Comparator
Cs
hh
RRLL=300k=300k
RRss=50=50
VVbb ++VVExEx
RRLL=300k=300k
RRss=50=50
VVbb ++VVExEx
Comparator
Cs
hh
RRss
VVbb ++VVExEx
Comparator
VVqq
Quenchdriverhh
RRss
VVbb ++VVExEx
Comparator
VVqq
Quenchdriver
Vq>VEX
Afterpulsing
Initial avalanche Released carriers from traps
Number of trapped carriers
1E+3
1E+4
1E+5
1E+6
1E+7
1E+8
1 10 100 1000Hold-off Time (μs)
220K200K
175K150K
6 V overbias
1E+3
1E+4
1E+5
1E+6
1E+7
1E+8
1 10 100 1000
Dar
k C
ou
nt
Rat
e (H
z)
Biasing scheme
Reduction of Afterpulsing: Decreasing Charge Flow
hh
RRLL=100k=100k
RRss=50=50
VVbb ++VVExEx
Comparator
Cs
hh
RRLL=100k=100k
RRss=50=50
VVbb ++VVExEx
RRLL=100k=100k
RRss=50=50
VVbb ++VVExEx
Comparator
Cs
The total charges flowing through device:Q=(Cs+Cd)Vex
Cd: device capacitanceCs: stray capacitance
Passive quenching
Time (ns)0 100 200 300 400 500 600 700
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.0
0.2
0.4
0.6
0.8
1.0
Passive quench with 100k
PQAR
Released carriers from traps
Passive quench with 100k
PQAR
Released carriers from traps
Time (ns)
0 100 200 300 400 500 600 700
Exc
ess
vo
lta
ge
(V
)0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Ca
rrie
r em
iss
ion
ra
te (
a.u
.)
0.0
0.2
0.4
0.6
0.8
1.0
Passive quench 100k
Released carriers from traps
Passive quench 10M
Passive Quenching with Active Reset (PQAR)
Time (ns)
0 100 200 300 400 500 600 7000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Car
rier
em
issi
on
rat
e (a
.u.)
0.0
0.2
0.4
0.6
0.8
1.0
0 100 200 300 400 500 600 700
Exc
ess
volt
age
(V)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.0
0.2
0.4
0.6
0.8
1.0
Conventional passive quench
100k
PQARactive reset
Released carriers from traps
PQARpassive quench
10M
10MΩ
Vb+Vex
10nFRs =50 Ω Amplifier
Trigger in
Transistor
PulseGenerator
Counter
A
Output
10MΩ
Vb+Vex
10nFRs =50 Ω Amplifier
Trigger in
Transistor
PulseGenerator
Counter
A
OutputOutputOutput
16
0
10
20
30
40
50
60
70
1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04CW laser power (fW)
Me
as
ure
d c
ou
nts
x 1
00
0 (
/s)
Vex=2.6V
Vex=2.0V
Vex=1.6V
PQAR at 230K
bdoffholdd
d PRN
NDCR
1
( )1
td b
t hold off
NTotalCR R QE P
N
PCR TotalCR DCR PDE
Hold-off = 15s
1.E+00
1.E+01
1.E+02
10 15 20 25 30 35 40 45PDE (%)
230K240K gated mode
10-4
10-5
10-6
Dar
k co
unt
prob
abili
ty (
/ns)
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08
Photon flux (#/s)
Ph
oto
n c
ou
nt
rate
(#
/s)
Vex=2.6V
Vex=2.0V
Vex=1.6V
34s 15sVb
Voltage on device Compare with gated mode results
NEP ~ 10-16 W/Hz1/2
Gated Quenching of a SPAD
hh
RRloadload
RRss=50=50
VVDCDC
Comparator
AC pulse
hh
RRloadload
RRss=50=50
VVDCDC
RRloadload
RRss=50=50
VVDCDC
Comparator
AC pulse
Dark Count RatePhoton Detection Efficiency
Laser pulse
Total bias on APD
Avalanchepulse
Avalanche pulse due to dark carriers
(false positive)
Avalanche pulse due to incident photon
Missed photon
Excess bias(V ex)
AC pulse width
Time
V dc
Vbr
Time
Time
(false negative)
Gated-PQAR
10MΩ
Cag
Cac
Vbias
Rs=50ΩAmplifier
A Counter
Vgate
Compared to PQAR
• Suppressed dark counts by gated bias
• Reduced complexity
• Array operation: recharge together, quench separately
The transistor can be HBT monolithically integrated on the SPAD, and the its gate/base input can be shared over the whole array.
0 5 10 15 20 25 30 351
2
4
6
8
10
Dar
k C
oun
t R
ate
(kH
z)
Photon Detection Efficiency (%)
220K 200K 180K 220K 200K 180K gated-quenching
gated-PQAR
Gated Quenching and Gated PQARGated Quenching and Gated PQAR
Gated Quenching and Gated PQARGated Quenching and Gated PQAR
0.01 0.1 1 1010
-5
10-4
10-3
Dar
k C
oun
t P
rob
abil
ity
Repetition Rate (MHz)
220K, Vex = 5.6%
200K, Vex = 6.0%
180K, Vex = 6.2%
220K, Vex = 5.6%
200K, Vex = 6.0%
180K, Vex = 6.2%
gated-quenchinggated-PQAR
Conclusions
Performance of Geiger-mode APDs is improving rapidlyAcceptable detection efficiencies and dark count
probability levelsGetting a better control over the afterpulsing
problem
Future Goals
Move closer to quantum limited detectionDark Current 0Quantum Efficiency 100%Read Noise 0
Move to longer wavelengths
Do photon number resolving
High QE Structure
Type-II Quantum Wells (QWs)Formed between materials with staggered band line-ups Electrons and holes are confined in adjoining layers Spatially indirect absorption and emission Smaller effective bandgap for long-wavelength operation
CB
VB
GaAs0.5Sb0.5
0.79eV
Ga0.47In0.53As0.77eV
ΔEc=0.236eV
ΔEv=0.247eV
0.49eV ≈ 2.5 μm
E
x
Where we are now (pin devices)
Rogalski, A., Progress in Quant. Elec., 27(2-3), pp 59 (2003)
-2 V bias(RT)
-2 V bias(200K)
26
AC
output
photon photon photon
Gated Quench
output
photon photon photon
Gated-PQAR
Vgate
Vbias
Transistor off: high resistance for fast passive quench
Transistor on: low resistance for fast reset
Vdiode
Compared to gated quench
• Comparable circuit complexity
• Wider AC pulses: easier to generate and synchronize
• Uniform output pulse shape, good for photon-number-resolution with multiplexing
Gated-PQAR for Synchronized DetectionGated-PQAR for Synchronized Detection
Questions??
Simulated Breakdown ProbabilitiesSimulated Breakdown Probabilities
V/Vbr
0.0 0.1 0.2 0.3
Bre
akd
ow
n P
rob
abili
ty
0.0
0.2
0.4
0.6
0.8
1.0
InAlAs 190nm PbrE InP 190nm PbrHInAlAs 1110nm PbrE InP 1110nm PbrH
J. P. R. David, University of SheffieldJ. P. R. David, University of Sheffield
InP
AlInAs
Decreasing thickness: 0.2 m, 0.5 m, 1.0 m
Excess bias (%)
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Dar
k C
ou
nt
Rat
e (k
Hz)
0
20
40
60
80
100
120
140
160
Reverse Bias (V)-52-50-48-46
Dar
k C
urr
ent
(A)
10-13
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
• 4 x 4 Subarray – uniform single-photon response
• 20 x 20 Array – 98% yield
Afterpulsing Probability vs. Total Charge
30
AC pulse
Laser pulse
Delay1s
Period
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
Total charge (pC)
Aft
erp
uls
ing
pro
ba
bili
ty Duration
Magnitude
31
Total Charge Flow
0 1 2 3 4 5 6 7 8 9
1
10
100
1000
no
rma
lize
da
fte
rpu
lsin
g p
rob
ab
ilit
yexcess bias (V)
gated-quench with 2ns gates PQAR with 0.23 pF of total capacitance
0 1 2 3 4 5 6 7 8 90.1
1
10
100
tota
l c
ha
rge
(p
C)
excess bias (V)
gated-quench with 2 ns gates PQAR with 0.23 pF of total capacitance
36x reduction
0 1 2 3 4 5 6 7 8 90.1
1
10
100
tota
l c
ha
rge
(p
C)
excess bias (V)
gated-quench with 2 ns gates PQAR with 0.23 pF of total capacitance
36x reduction
32
“Effective Excess Noise Factor”
2
2
2
(peak signal)Effective excess noise factor 1
peak signal
11.6 1 1.0004
572
mV
mV
Pixel Level Monolithic Integration of Active Switching Elements
• Reduced parasitics
• Faster quenching
• Reduced afterpulsing
• Increased transmission and
sampling rates
• Packaging advantages
Vbias
Rs =50
Counter
Vbias
Rs =50
Counter
Active switching element