Snowmass 2001 1

Avalanche Photodiodes

from the Start.R. Rusack

The University of Minnesota

Snowmass 2001 2

Early History

•Avalanche Photodiodes were invented by R. McIntyre at RCA in Canada in the late sixties.

•First considered in HEP at an Isabel meeting.

•Considered at the SDC for the readout of the EM shower max detector.•RCA ( GE EG&G) •API•RMD

Snowmass 2001 3

How they work

+

-

h+

eSi

Ar coating

Contact layer

Collection Region

Avalanche region

Drift

Substrate

Contact layer

Electric Field

p

n

n++

Electrons generated by the incident light are multiplied in the high field region at the junction.

Snowmass 2001 4

Why CMS selected APD’s

Snowmass 2001 6

APD’s with crystals

High side tail suppressed by APD.

Snowmass 2001 7

APD’s in the CMS detector

PbWO4 crystal

Snowmass 2001 8

Light Output from PbWO4

0

20

40

60

80

100

350 400 450 500 550 600 650

Wavelength (nm)

Rel

ativ

e ap

litu

de(%

)

Output is peaked at 420 nm.

~50 photons/MeV from the

Light out from Bogorodisk PbWO4 crystal.

Snowmass 2001 9

APD’s for CMS

Manufacturer:•Hamamatsu Photonics, Japan.

Quantity:•Two APD’s per crystal– 124,000 APD’s

with spares.

Accessibility during operation:•None.

Radiation levels:•Maximum expected dose 200 kGy and

2 1013 neutrons/cm2.

Crystal Light Outout:•~ 50 photons/MeV on a 4.5 cm2 area.

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Basic APD Structure:

APD is grown epitaxially on an n++ wafer.

Junction

Si2N4 AR coating

Groove to minimize

surface leakage current.

5 5 mm2

active area

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APD properties

Active area 5x5 mm2

Operating voltage (Vr) ~380 V

Capacitance 70 pF

Serial resistance 3

Dark Current <10 nA

Quantum efficiency 72% @ 420 nm

1/M*dM/dV (M=50) 3.3 %

1/M*dM/dT (M=50) -2.3%

Excess Noise Factor (M=50) 2 Distance to break-down (Vb-Vr) 30-40V Effective Thickness (M=50) 5m Gain range up to 1000

Snowmass 2001 12

APD parameters I

APD#303

0.1

1

10

100

1000

10000

0 100 200 300 400 500

Bias [V]

Gai

n

0

10

20

30

40

50

60

0 500 1000 1500 2000

Gain

1/M

*dM

/dV

[%

]

-30

-25

-20

-15

-10

-5

0

0 500 1000 1500 2000Gain

1/M

*dM

/dT

[%

]

Snowmass 2001 13

1

3

5

7

9

11

13

15

0 500 1000 1500 2000

Gain

Exc

ess

Noi

se F

acto

r

APD parameters II

10

100

1000

10000

0 50 100 150 200 250 300 350 400

Voltage [V]

Ca

pa

cit

an

ce

[p

F]

0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000Wavelength [nm]

Qu

antu

m E

ffic

ien

cy [

%]

9.25

11.25

13.25

15.25

X distance in mm

9.25

11.25

13.25

15.25

Y distance in mm

0

1´10-7

2´10-7

3´10-7

A

9.25

11.25

13.25

15.25

X distance in mm

Response uniformity at 420 nm.

Quantum Efficiency

Capacitance v Bias

Excess Noise Factor v Gain

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Problems Solved

Radiation Damage:•Neutron • Ionizing radiation.•API effect.

Quantum efficiency drift.•Change to epoxy

Lifetime:•Failure due to poor surface

connectivity.

Electrical Characteristics.•High Capacitance.

Snowmass 2001 15

Irradiation Tests.

Irradiation with protons:•All irradiation so far has been with an 70

MeV protons beam at PSI – Switzerland.•2 1013 neutrons/cm2 in ~ 1 hour.

Irradiation with gammas.•All irradiation with 60C0 source.

Irradiation with neutrons.•Setting up a Californium source (252Cf) for

irradiation at the University of Minnesota.•2 1013 neutrons/cm2 in ~ 2 days.

Snowmass 2001 16

Device failure

0

5

10

15

20

25

30

35

0 1000 2000 3000 4000 5000 6000 7000 8000Time (s)

Da

rk C

urr

en

t (

A)

Irradiation in a 70 MeV proton beam.

Snowmass 2001 17

Diagnosis

Breakdown at this point when irradiated. High current at the SiO2-

Al interface.

Solution: increase spacing of Al deposit.

n++

nn++

Al SiO2/SiN

p

p++

Snowmass 2001 18

Lessons

To bring a new technology to reality requires:• Time 1987 to 1996.• Early resources. TNLRC/SSC to show viability.

To go from a possibility to an established technology takes:• Time 1996 to 2001.• Resources ~ $500k.• A manufacturer who sees this technology as a future money earner and does not expect to

recover all costs of development from the experiment.

Expect the unexpected.