Avalanche gain in HgCdTe

Galileo Avionica S.p.A and SELEX Sensors & Airborne Systems Limited - Finmeccanica Companies

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

Galileo Avionica S.p.A and SELEX Sensors & Airborne Systems Limited - Finmeccanica Companies 2

Avalanche gain in HgCdTe

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

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.

Quite a useful component!


1

10

100

1000

0 2 4 6 8 10 12 14

Bias volts

Av

ala

nc

he

ga

in 2.5 μm

3 μm

3.5 μm

4 μm

4.5 μm

Avalanche gain v. bias volts and cutoff wavelength

Cut-off wavelength

[μm]

HgCdTe avalanche photodiodes at 77K


1

10

100

1000

0 2 4 6 8 10 12 14

Bias volts

Ava

lan

che

gai

n 2.5 μm

3 μm

3.5 μm

4 μm

4.5 μm

Avalanche gain v. bias volts and cutoff wavelength

Cut-off wavelength

[μm]

HgCdTe avalanche photodiodes at 77K

Used for Burst Illumination

LIDAR (BIL) imaging

Potential for low background flux

astronomy


LPE HgCdTe layer grown on CdZnTe substrate

HgCdTe monolith bonded to ROIC

n p

HgCdTe technology options for APDs

APD array using via-hole process

Multi-level APD designMOVPE HgCdTe layer grown on 75mm GaAs substrate

Bump bonded to ROIC

LPE material + via-hole hybrid technology- Currently gives best breakdown voltages

MOVPE material + mesa hybrid technology- Under development for APDs


Silicon multiplexer (ROIC) options

ME770 – Dual Mode

256x320 on 24µm pitch

Thermal imaging OR BIL imaging

ME780 - Swallow 3D

256x320 on 24µm pitch

3D intensity and range per pixel

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

Thermal image BIL image

BIL intensity image BIL range image


No avalanche gain

Gate - 3900ns

Avalanche gain - 4.6

Gate - 800ns

Avalanche gain - 13.8

Gate - 300ns

Avalanche gain - 38

Gate - 100ns

Pixel to pixel uniformity of avalanche gain

Short and long range uniformity of avalanche gain – no issue for data acquisition


Noise proportional to: Gain . sq rt (gate time . noise figure)

Detailed measurements give noise figure of 1.3 up to x97 gain

Noise after avalanche gain

Extra noise due to avalanche process negligible


Noise spatial distribution for typical BIL detector

Temp - 100K

Wavelength – 4.5 μm

Gate time - 160ns

Ava. gain - x25

Array operability performance – BIL compared with SW

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

Very few defects due to short gate time


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]


Noise-equivalent-photons (NEPh) - sensitivity figure of merit for APDs

5.02

..

.211

.2.

NFGT

FN

Q

NFNEPh

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10 12

Diode bias volts

NE

Ph

(p

ho

ton

s r

ms

)

NEPh

NEPh with CDS

Allows for photon noise


SW LPE HgCdTe layers

SELEX APD Pre-development Programme for ESO

3 variable jn hybrids5 full hybrids

2.64 μm

2.54 μm

2.50 μm

ME770 – Dual Mode

ME780 - Swallow 3D

2 variable jn hybrids4 full hybrids

2 FPAs to ESO in flatpacks

2 FPAs to ESO in flatpacks


Experimental hybrid with variable junction diameters

Variable junction diameter


0

2

4

6

8

10

12

0 2 4 6 8 10

Bias voltage

Sig

na

l (m

V)

D - 5.8

B - 6.2

F - 6.6

E - 7.0

C - 7.4

A - 7.8

0

2

4

6

8

10

12

14

0 2 4 6 8 10

Bias voltage

Gai

n

D - 5.8

B - 6.2

F - 6.6

E - 7.0

C - 7.4

A - 7.8

Result of variable junction diameter experiment

Better signal with smaller junction

No effect on avalanche gain

Conclusion: use small junction diameters on further arrays


ESO measurements on variable jn diameter array

ESO measurements show strong S/N benefit from using small junctions

Data:

Integration time – 3ms

Temperature – 60K

Cut-off – 2.64 μm


NEPh v. Bias Volts as function dark current- to set dark current specification

Target dark current specification is <1e-11 A/cm2 (360 e/s)

Data:


Temperature – 70K

Wavelength – 2.5 μm0

10

20

30

40

50

60

70

0 2 4 6 8 10 12

Diode bias volts

NE

Ph

(p

ho

ton

s r

ms

) 1.E-09

3.E-10

1.E-10

3.E-11

1.E-11

3.E-12

1.E-12

0

Dark current

(A/cm2)


1.E-12

1.E-11

1.E-10

1.E-09

0 20 40 60 80 100 120

Temperature (K)

Da

rk c

urr

en

t (A

/cm

^2)

SELEX

ESO

Comb

Comparison of SELEX and ESO measurements of dark current v. temperature

ESO measurements

Shows dark current specification is met for temperatures below 90K

Array data:

Cut-off wavelength – 2.64um

Target spec <1e-11 A/cm2

Trap-assisted tunnelling behaviour


ESO Electro-Optic Test Rig


Signal Noise

Typical output from ESO Test Rig

Shows that noise is limited by photon shot noise


ROIC – ME784

Bias – 7.1V

Temperature – 70K

TBB - 100ºC-50ºC

ESO measurement of uniformity under moderate gain


ESO measurement of Avalanche Gain – comparison with model

ROIC – ME770

Temperature – 70K

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)


ESO measurement of Quantum Efficiency – 70%

ROIC – ME770

Bias – 8.63V

Gain - 16x

Temperature – 70K


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


ESO measurement of noise at gain of 6.4

ROIC – ME784

Temperature – 60K

Aval. gain – 6.4



ESO measurement of noise at gain of 6.4

ROIC – ME784

Temperature – 60K

Aval. gain – x6.4


Theory for ME784Theory for custom ROIC


Dark current defect map under extreme conditions – effect of temperature

45K 60K 70K 80K

Reducing temperature reduces the number of high dark current pixels


Low photon flux imaging using avalanche gain

FPA at 60K

Average of 10 frames

6 electrons imaging

Readout with avalanche gain of x1.5

Readout with avalanche gain of x7


Modelled sensitivity based on measured data and with a custom ROIC

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10 12

Diode bias volts

NE

Ph

(p

ho

ton

s r

ms

)

NEPh

NEPh with CDS

Data:


Temperature – 77K

Cut-off – 2.5um

Avalanche gain offers an order improvement in NEPh


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

Date post:	01-Feb-2016
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Avalanche gain in HgCdTe

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