Aug 14, 2009 NGC/CSTC, Hamilton, ON 1

Semiconductor Scintillators and Three-Dimensional Integration

• Isotope identification spectroscopic energy resolution

• Direction to source angular resolution

critical needs:

Aug 14, 2009 NGC/CSTC, Hamilton, ON 2

X-ray (γ-ray) attenuation

1 mm

1 cm

1 dm

absorption length

Aug 14, 2009 NGC/CSTC, Hamilton, ON 3

1E-3 0.01 0.1 1 10

0.1

1

10

100

1000

10000

100000

1 mm

Si Ge

CdTe InP

A tte

nu at

io n

C oe

ffi ci

en ts

(c m

-1 )

hν (MeV)

Element Z

Si 14×2

Ga/As 31/33

Ge 32×2

In/P 49/15

Cd/Te 48/52

X-ray (γ-ray) attenuation by materials

Aug 14, 2009 NGC/CSTC, Hamilton, ON 4

Semiconductors and scintillators

Thallium EC

EV

hν

= 3 eVEG

= 7 eV

NaI scintillator

> 200 nS

38,000 ph/MeV

Si or Ge pin

diode

cm

> 100 nS

77K

> 10 kV

up to 300,000 e-h/MeV

p

n

Aug 14, 2009 NGC/CSTC, Hamilton, ON 5

Gamma spectroscopy

One measures the number N of electrons and holes produced by incident gamma particle

N ~ Eγ

That number fluctuates.

If the statistics of N

were Poisson,

var

(N) = N

but due to correlations (in semiconductors)

var

(N) =

F N

the Fano factor, F ≈

0.1

Aug 14, 2009 NGC/CSTC, Hamilton, ON 6

Non-proportionality in scintillators

Luminescence in dielectric scintillators is controlled by reactions nonlinear in N (exciton formation, Auger, etc.)

This is one of the reasons γ

spectroscopy with scintillators is not as accurate as it is with semiconductor (diodes).

In semiconductor scintillators, every reaction on the way to luminescence is linear

in the N

of minority carriers

Expect

no non-proportionality effects!

S. A. Payne et al (LLNL group), preprint, 2009

Need: N ~ Eγ

but ...

Aug 14, 2009 NGC/CSTC, Hamilton, ON 7

3D scintillator array

Semiconductor scintillators, each endowed with its own photoreceiver

10 × 10 × 10 array contemplated

Enables both

isotope discrimination and determination of the direction to source

A different way of determining Eγ

(unlike γ

spectroscopy)

Aug 14, 2009 NGC/CSTC, Hamilton, ON 8

3D pixellation of response to a single γ

photon

Upon analog-to-digital conversion each unit reports not a 1 ns pulse but an information-carrying signal:

• where ionization occurred

• time of the event

• amplitude of the event

Photosensitive Layer

γ

- Sensing

Semiconductor

Scintillator

Stack of Scintillator

Slabs

2D pixel

γ

Aug 14, 2009 NGC/CSTC, Hamilton, ON 9

Compton “telescope”

⎟⎟ ⎠

⎞ ⎜⎜ ⎝

⎛ −+=

10 1

111cos EE

θ

iii

iii

EE EE

−=Δ −+=

−

−− −

1

11 11cosθ

keV) 5112 =cme(in units of

The energy E0

of the incident γ-photon

Compton kinematics: two equations at each interaction site

i

The

incident

direction

⎟⎟ ⎠

⎞ ⎜⎜ ⎝

⎛ − Δ

+Δ+ Δ

+Δ= 2

22 2

2 10 cos1

4 2 1

2 θ E

θ3

θ1

Δ1

θ2

E0

1n̂

0n̂ Δ2

γ

Δ3

Aug 14, 2009 NGC/CSTC, Hamilton, ON 10

What is needed

• Semiconductor scintillator “transparent”

to its own luminescence

• Integrated (optically tight) surface photoreceiver system of slightly smaller bandgap

• Readout ASIC customized to the photoreceiver system

The triad:

I will focus on the first two legs of the triad

Aug 14, 2009 NGC/CSTC, Hamilton, ON 11

Semiconductor scintillator: transparency

• Moss-Burstein shift

• Photon re-absorption suppressed

• Radiative decay time ≈

10−9

s

• Need material transparent to its own fundamental light emission

• Photons must be delivered to the surface

EC

EV

EF

hνInP

“Conventional”

transparency

Aug 14, 2009 NGC/CSTC, Hamilton, ON 12

Need for optically-tight photoreceiver

θ0InP

Air

Total Internal Reflection

escape cone

θ0 =17º

2% for free-space detectorsn n

/1sin 3.3

0 = =

θ

023.02sinsin4 1 02

0

0

≈⎟ ⎠ ⎞⎜

⎝ ⎛=∫ θθϕθπ

θ

dd

escaping fraction of photons

Optically-tight integrated detectors collect the entire scintillating radiation

Epitaxial InGaAsP diodes on InP

Aug 14, 2009 NGC/CSTC, Hamilton, ON 13

Epitaxially integrated pin diode

p+ n –

n+ i

n+

InP wafer

2 μmquaternary InGaAsP

Si3

N4

patterned resist p metal contact

n metal contact backside patterning

epitaxial layers

bare scintillator slab

Sarnoff – SBU collaboration

Aug 14, 2009 NGC/CSTC, Hamilton, ON 14

Characteristics of quaternary epi diodes

(Q) GE

(InP) GE

AE FE

)cm10(for eV 0.23

(designed) eV 1.24300K) (Q; (expt) eV 1)300(

315 F

G

A

−≈=

= ≈

nE

E KE

as determined by CV

profiling both estimate jibe !

IV Characteristics of Diodes at 300 and 77 K

200

400

600

800

1000 1200

1400

1600

1800

2000

‐2 ‐1.5 ‐1 ‐0.5 0 0.5 1 1.5

Voltage (V)

Current (pA) 295 K

77 K

0.56 V

⎟⎟ ⎠

⎞ ⎜⎜ ⎝

⎛ +− ∝

kTm eVTEI

2 )(exp A

< 10 pA

at 300 K (1 pA

in best diodes)

Light collection efficiency ≈

85%

Aug 14, 2009 NGC/CSTC, Hamilton, ON 15

Read-Out Circuits

Output digital readout data

ASIC

IPD

CDET

Peak detector

Preamplifier

Pulse-shapingPhotodetector

ADC

Vss

( ) LKsh MOSm

th2 MOSDET

2 2 3 8ENC qIa

fC Ka

g kTaCC ff τ

τ +⎥

⎦

⎤ ⎢ ⎣

⎡ ⋅

+⋅+=

shot noisethermal noiseENC –

currently: 3 ×

103

Next generation: < 103 ☺

low

ILK

≈

10 pAhigh CDET≈ 50 pF

Presenter Presentation Notes Equivalent Noise Charge (ENC) quantifies sensitivity of the electrical readout circuitry in terms of the charge at the output of the detector that would produce a signal output equal to the total noise contribution.

Aug 14, 2009 NGC/CSTC, Hamilton, ON 16

Single-quanta response

Train of scintillator pulses recorded as voltage waveform in the read-out circuit

α-particles from 241Am

α-particle from 241Am

γ-photon from 137Cs

Aug 14, 2009 NGC/CSTC, Hamilton, ON 17

Making semiconductor transparent

• Moss-Burstein shift Success “mixed”

• Photon recycling Nearly ideal non-transparent scintillator

• Subband luminescence centers E.g., Yb3+ luminescent ion in InP ( 1μm emission)

• Impregnated “guest-host”

structures Non-layered two-phase random systems

... to its own fundamental luminescence

... new ideas are welcome

Aug 14, 2009 NGC/CSTC, Hamilton, ON 18

Transparency: theory vs. reality

EF

EG

Conduction band

Valence band

hν

EC

EV

Momentum non-conservation in heavily-doped InP

“Average”

(over the emission spectrum) photon mean free path is about 0.1 mm

1.3 1.4 1.5 Photon energy (eV)

Emission spectrum

Log Transparency

EF

no free lunch

Aug 14, 2009 NGC/CSTC, Hamilton, ON 19

Luminescence Experiments

Excitation beam

Reflection Luminescence

Transmission Luminescence

Monochromator InP wafer

Monochromator 1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.600.0

0.2

0.4

0.6

0.8

1.0

Transmission Luminescence

Reflection Luminescence

Transm'n Spectrum

hv, eV

300 K

InP, ND

=6.3×1018

cm-3

(S)

Heavily doped n-type InP wafers from Nikko materials (ACROTEC)

1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60 0.0

0.2

0.4

0.6

0.8

1.0

Trans'n Luminesc.

Reflection Luminescence