Transcript HgCdTe Avalanche Photodiode Arrays for Wavefront Sensing and Interferometry Applications Ian Baker* and Gert Finger** *SELEX Sensors and Airborne Systems Ltd, Southampton, UK **ESO,

HgCdTe Avalanche Photodiode Arrays for
Wavefront Sensing and Interferometry Applications
Ian Baker* and Gert Finger**
*SELEX Sensors and Airborne Systems Ltd, Southampton, UK
**ESO, Garching, Germany
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Avalanche gain in HgCdTe
HgCdTe – a unique material
• Electron/hole mass ratio very large – electron gets all the energy – single carrier cascade
process gives low added noise
• The conduction band of HgCdTe devoid of any low-lying secondary minima, which allows
for large electron energy excursions deep into the band, and hence the high probability of
impact ionization, with the generation of electron-hole pairs.
Avalanche photodiodes
• Voltage controlled gain at the point of absorption
• Almost no additional noise
• Near-zero power consumption
• Up to GHz bandwidth
• Requires no silicon real estate
Quite a useful component!
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Avalanche gain v. bias volts and cutoff wavelength
HgCdTe avalanche photodiodes at 77K
Cut-off
wavelength
1000
Avalanche gain
[μm]
2.5 μm
100
3 μm
3.5 μm
4 μm
10
4.5 μm
1
0
2
4
6
8
10
12
14
Bias volts
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Avalanche gain v. bias volts and cutoff wavelength
HgCdTe avalanche photodiodes at 77K
Cut-off
wavelength
Avalanche gain
1000
Used for Burst
Illumination
LIDAR (BIL)
imaging
100
[μm]
2.5 μm
3 μm
3.5 μm
4 μm
10
4.5 μm
Potential for low
background flux
astronomy
1
0
2
4
6
8
10
12
14
Bias volts
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HgCdTe technology options for APDs
n
LPE HgCdTe layer grown on
CdZnTe substrate
HgCdTe monolith bonded to ROIC
p
APD array using via-hole process
LPE material + via-hole hybrid technology
- Currently gives best breakdown voltages
Bump bonded to ROIC
MOVPE HgCdTe layer grown
on 75mm GaAs substrate
Multi-level APD design
MOVPE material + mesa hybrid technology
- Under development for APDs
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Silicon multiplexer (ROIC) options
ME770 – Dual Mode
256x320 on 24µm pitch
Thermal imaging OR BIL imaging
Thermal image
BIL image
ME780 - Swallow 3D
256x320 on 24µm pitch
3D intensity and range per pixel
BIL intensity image
BIL range image
Both ROICs can be configured to run in non-destructive readout.
Parasitic capacitance is higher than a custom ROIC but results
can allow for this.
Both used for ESO APD study
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Pixel to pixel uniformity of avalanche gain
No avalanche gain
Avalanche gain - 4.6
Avalanche gain - 13.8
Avalanche gain - 38
Gate - 3900ns
Gate - 800ns
Gate - 300ns
Gate - 100ns
Short and long range uniformity of avalanche gain – no issue for data acquisition
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Noise after avalanche gain
Noise proportional to:
Gain . sq rt (gate time . noise figure)
Detailed measurements give noise figure of 1.3 up to x97 gain
Extra noise due to avalanche process negligible
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Array operability performance – BIL compared with SW
Noise spatial distribution
for typical BIL detector
Temp - 100K
Wavelength – 4.5 μm
Very few defects due to short gate time
Gate time - 160ns
Ava. gain - x25
The low pixel defect count of BIL detectors is due to the short gate time. Wavefront
sensors need 3e5x longer integration time so dark current critical
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Avalanche gain for wavefront sensors
How does avalanche gain benefit wavefront sensors?
Typical requirement:
Integration time – 1.0 to 5.0 ms
Waveband – 1.0 to 2.5 µm
Multiple non-destructive readouts
Sensitivity in noise-equivalent-photons (NEPh) – 3 photons rms
[Note NEPh a better Figure of Merit for APDs]
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Noise-equivalent-photons (NEPh)
- sensitivity figure of merit for APDs
2 0.5 


NF 
 2.FN   
NEPh. 
1  1  

2.Q    T .G.NF   


80
Allows for photon noise
NEPh (photons rms)
70
60
50
NEPh
NEPh with CDS
40
30
20
10
0
0
2
4
6
8
10
12
Diode bias volts
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SELEX APD Pre-development Programme for ESO
ME770 – Dual Mode
2.50 μm
3 variable jn hybrids
5 full hybrids
2.54 μm
2 FPAs to ESO in flatpacks
ME780 - Swallow 3D
2.64 μm
SW LPE HgCdTe layers
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2 variable jn hybrids
4 full hybrids
2 FPAs to ESO in flatpacks
12
Experimental hybrid with variable junction diameters
Variable junction
diameter
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Result of variable junction diameter experiment
12
10
Better signal with smaller
junction
8
B - 6.2
F - 6.6
6
E - 7.0
C - 7.4
4
A - 7.8
No effect on avalanche gain
2
0
0
2
4
6
8
10
Bias voltage
14
12
10
Conclusion: use small
junction diameters on
further arrays
Gain
Signal (mV)
D - 5.8
D - 5.8
B - 6.2
8
F - 6.6
E - 7.0
6
C - 7.4
A - 7.8
4
2
0
0
2
4
6
8
10
Bias voltage
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ESO measurements on variable jn diameter array
Data:
Integration time – 3ms
Temperature – 60K
Cut-off – 2.64 μm
ESO measurements show strong S/N benefit
from using small junctions
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NEPh v. Bias Volts as function dark current
- to set dark current specification
Dark current
70
(A/cm2)
NEPh (photons rms)
60
1.E-09
50
3.E-10
1.E-10
40
3.E-11
1.E-11
30
3.E-12
1.E-12
0
20
10
Data:
Integration time – 5ms
Temperature – 70K
Wavelength – 2.5 μm
0
0
2
4
6
8
10
12
Diode bias volts
Target dark current specification is <1e-11 A/cm2 (360 e/s)
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Comparison of SELEX and ESO measurements
of dark current v. temperature
Dark current (A/cm^2)
1.E-09
Target spec <1e-11 A/cm2
1.E-10
SELEX
ESO
Comb
1.E-11
Array data:
Trap-assisted tunnelling behaviour
Cut-off wavelength – 2.64um
ESO measurements
1.E-12
0
20
40
60
80
100
120
Temperature (K)
Shows dark current specification is met for temperatures below 90K
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ESO Electro-Optic Test Rig
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Typical output from ESO Test Rig
Signal
Noise
Shows that noise is limited by photon shot noise
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ESO measurement of uniformity under moderate gain
ROIC – ME784
Bias – 7.1V
Temperature – 70K
TBB - 100ºC-50ºC
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ESO measurement of Avalanche Gain
– comparison with model
Measured data for 2.64 μm diode
Fitted: APD Gain = 0.0782*2(Vbias/1.126)+0.905
Model for 2.64 μm diode (green)
Model for 2.5 μm diode (red)
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ROIC – ME770
Temperature – 70K
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ESO measurement of Quantum Efficiency – 70%
ROIC – ME770
Bias – 8.63V
Gain - 16x
Temperature – 70K
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ESO measurement of electrons per ADU to calibrate
the detector test – 2.21 e/ADU
ROIC – ME784
Gain of 6.4
Temperature – 80K
Signal electrons – Q
Noise electrons – Q0.5
Signal V = Q.e.T/C
(Noise V)2 = Q.(e.T/C)2
Signal/(Noise)2 in ADUs =
electrons/ADU
T is pixel transfer function
C is integration cap
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ESO measurement of noise at gain of 6.4
ROIC – ME784
Temperature – 60K
Aval. gain – 6.4
Integration time – 5ms
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ESO measurement of noise at gain of 6.4
Theory for custom ROIC
Theory for ME784
ROIC – ME784
Temperature – 60K
Aval. gain – x6.4
Integration time – 5ms
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Dark current defect map under extreme conditions
– effect of temperature
45K
60K
70K
80K
Reducing temperature reduces the number of high dark current pixels
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Low photon flux imaging using avalanche gain
Readout with avalanche gain of
x1.5
Readout with avalanche gain of
x7
FPA at 60K
Average of 10 frames
6 electrons imaging
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Modelled sensitivity based on measured data and
with a custom ROIC
80
NEPh (photons rms)
70
60
50
NEPh
NEPh with CDS
40
30
Data:
20
Integration time – 5ms
10
Temperature – 77K
0
0
2
4
6
8
10
12
Cut-off – 2.5um
Diode bias volts
Avalanche gain offers an order improvement in NEPh
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Conclusions on avalanche gain for wavefront sensing
applications (A-O and interferometry)
Results so far
•
Avalanche gains up to x16 at 8.6V bias achieved in 2.64 μm material
•
6 electrons rms achieved with existing non-optimised ROIC and electronics
•
Optimised technology could provide 2-3 photons rms
•
All the aspirations of wavefront and interferometric applications can be met
by APD technology
Future work
•
Need to establish parameter space of APDs i.e. wavelength, temperature etc
•
Need to design custom ROIC
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