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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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 µm
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Avalanche gain v. bias volts and cutoff wavelength
HgCdTe avalanche photodiodes at 77K
Cut-off wavelength µm
Used for Burst Illumination LIDAR (BIL) imaging
Potential for low background flux astronomy
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HgCdTe technology options for APDs
n
p
LPE HgCdTe layer grown on CdZnTe substrate
HgCdTe monolith bonded to ROIC
APD array using via-hole process
LPE material via-hole hybrid technology -
Currently gives best breakdown voltages
Bump bonded to ROIC
Multi-level APD design
MOVPE HgCdTe layer grown on 75mm GaAs substrate
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 Gate - 3900ns
Avalanche gain - 4.6 Gate - 800ns
Avalanche gain - 13.8 Gate - 300ns
Avalanche gain - 38 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 Gate
time - 160ns Ava. gain - x25
Very few defects due to short gate time
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
Allows for photon noise
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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
2 variable jn hybrids 4 full hybrids
SW LPE HgCdTe layers
2 FPAs to ESO in flatpacks
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Experimental hybrid with variable junction
diameters
Variable junction diameter
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Result of variable junction diameter experiment
Better signal with smaller junction No
effect on avalanche gain
Conclusion use small junction diameters on
further arrays
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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 (A/cm2)
Data Integration time 5ms Temperature
70K Wavelength 2.5 µm
Target dark current specification is lt1e-11 A/cm2
(360 e/s)
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Comparison of SELEX and ESO measurements of dark
current v. temperature
Target spec lt1e-11 A/cm2
Array data Cut-off wavelength 2.64um
Trap-assisted tunnelling behaviour
ESO measurements
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
0.07822(Vbias/1.126)0.905 Model for 2.64 µm
diode (green) Model for 2.5 µm diode (red)
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
Data Integration time 5ms Temperature
77K Cut-off 2.5um
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