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  • 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

    = 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

    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