Semiconductor Scintillators and Three-Dimensional …asdn.net/ngc2009/presentations/presentations...
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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 sourceangular 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
SiGe
CdTeInP
Atte
nuat
ion
Coe
ffici
ents
(cm
-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
ThalliumEC
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”
⎟⎟⎠
⎞⎜⎜⎝
⎛−+=
101
111cosEE
θ
iii
iii
EEEE
−=Δ−+=
−
−−−
1
1111cosθ
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
222
210 cos1
421
2 θE
θ3
θ1
Δ1
θ2
E0
1n̂
0n̂Δ2
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 systemof slightly smaller bandgap
• Readout ASICcustomized 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 detectorsnn
/1sin3.3
0 ==
θ
023.02sinsin41 02
0
0
≈⎟⎠⎞⎜
⎝⎛=∫ θθϕθ
π
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 resistp 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
AEFE
)cm10(for eV 0.23
(designed) eV 1.24300K) (Q;(expt) eV 1)300(
315F
G
A
−≈=
=≈
nE
EKE
as determined by CV
profiling both estimate jibe !
IV Characteristics of Diodes at 300 and 77 K
200
400
600
800
10001200
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
⎟⎟⎠
⎞⎜⎜⎝
⎛ +−∝
kTmeVTEI
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
( ) LKshMOSm
th2MOSDET
2 238ENC qIa
fCKa
gkTaCC ff τ
τ+⎥
⎦
⎤⎢⎣
⎡⋅
+⋅+=
shot noisethermal noiseENC –
currently: 3 ×
103
Next generation: < 103 ☺
low
ILK
≈
10 pAhigh CDET≈ 50 pF
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 shiftSuccess “mixed”
• Photon recyclingNearly ideal non-transparent scintillator
• Subband luminescence centersE.g., Yb3+ luminescent ion in InP ( 1μm emission)
• Impregnated “guest-host”
structuresNon-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 1.5Photon 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
TransmissionLuminescence
ReflectionLuminescence
Transm'nSpectrum
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 1.600.0
0.2
0.4
0.6
0.8
1.0
Trans'n Luminesc.
ReflectionLuminescence
Transmission spectrum
hv, eV
77 K
Aug 14, 2009 NGC/CSTC, Hamilton, ON 20
Transport of Holes/Photons
In a direct-gap n-type semiconductor, an event of interband absorption does not finish off the photon:
It emerges as a new hole and then again a new photon (spectrum)
Holes and photons inside are “interchangeable” entitiesAs a photon, its lifetime is limited by free-carrier absorption (rate ~ n). Key parameter — residual absorption coefficient μr
As a hole, its lifetime is limited by non-radiative processes, (rate ~ n2). Key parameter — radiative efficiency η
Photon “recycling”a.k.a. photon-assisted diffusion of holes
Aug 14, 2009 NGC/CSTC, Hamilton, ON 21
Photon recycling
( )[ ])(
)(1)( det
0det zP
zpPpzDn
n
ηξηηη+
=−×= ∑=z
d
D(z)
where
η
≈
0.9 is the radiative efficiency and ξ=1−η
h S hη η
ξ=1−η ξP(z)=pdet
+pfca
1−P 1−P
P(z)
Setc.
Aug 14, 2009 NGC/CSTC, Hamilton, ON 22
Problem with InP and possible remedy
The signal dependence on the position
D(z) presents a fundamental problem for the intended application:
How do we distinguish large faraway event from a smaller event closer to the detector surface?
Can we correct for the z dependence?
☺ Double-sided epi detector
zd
z
d − zd
if we know
z, we can correct for the attenuation J. H. Abeles & S. Luryi
US Pat. Prov. Appl. (May 2009)
Aug 14, 2009 NGC/CSTC, Hamilton, ON 23
Photon recycling with 2-sided detector
)()()(
)()()(
22
11
zPzpzD
zPzpzD
ηξηηξ
η
+=
+= z
d − zd
D1
(z)
D2
(z)
h S h Sη η
ξ=1−η ξP(z)
1−P 1−P
P(z)=p1
+p2
+pfca
etc.
Aug 14, 2009 NGC/CSTC, Hamilton, ON 24
Calculated attenuation ratio
0 20 40 60 80 100 120 140 160 1800.0
0.2
0.4
0.6
0.8
1.0
Atte
nuat
ion
Rat
io, ρ
Distance to nearest surface, μm
zd
D1
D2
)()()(
1
2
zDzDz ≡ρ
)()()(
)()()(
22
11
zPzpzD
zPzpzD
ηξηηξ
η
+=
+=
)()(
)()(
2
1
2
1
zpzp
zDzD
=
ratio of single-pass probabilities
Aug 14, 2009 NGC/CSTC, Hamilton, ON 25
Lightly doped InP: bright luminescence
Reflection geometry Transmission geometry
Aug 14, 2009 NGC/CSTC, Hamilton, ON 26
Lightly doped InP: low free-carrier abs’n
For ND
= 2×1017
cm-3, the measured αFCA
< 0.1 cm-1
0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.3002468
101214161820
cm-3 (Doping level)A
bsor
ptio
n co
effc
ient
, cm
-1
hν, eV
2E+17 2E+18 3.7E+18 4.3E+18 6.3E+18 8E+18
Aug 14, 2009 NGC/CSTC, Hamilton, ON 27
Ideal non-transparent scintillator
)()()(
)()()(
22
11
zPzpzD
zPzpzD
ηξηηξ
η
+=
+=
[ ]
[ ][ ]FCApzpzp
zpzpzP
zpzpzDzD
+++−+
=
++
=+
)()()1()()( PCE
)()()()()(
21
21
2121
ηηηηξ
η
zd
D1
D2
Based on photon recycling in low-doped (2×1017
cm-3) InP, where:
η
≈
98-99% and
FCA negligible
for η
≈
98%, PCE =91%
for η
≈
99%, PCE =95%
Unusual situation:
semiconductor is opaque, in the conventional sense, but photon collection efficiency approaches unity. No losses!
Aug 14, 2009 NGC/CSTC, Hamilton, ON 28
Pause for reflection
Two-sided detection pinpoints the z-coordinate precisely because of distance-dependent attenuation of single-pass photons
Would not work in a transparent
scintillator (except for the factor of 2)
Suppose we find semiconductor with much higher PCE?Is there advantage in 2-sided detection (apart from the factor of 2)?
Precision of z determination
is no longer ±
d, hence can make thick voxels!
Search for better PCEMust be accompanied by due regard to leg 2 of the triad (integrated PD)
Going beyond InP needs ideas for optically tight integration
Aug 14, 2009 NGC/CSTC, Hamilton, ON 29
Layered superlattice, e.g. InP/InGaAsPTransparency enhancement by the duty cycle factor, (d/a) ≤
100
Difficult to make
Need rapid growth (100 μm per hour)
HVPE ?
Hard to maintain lattice coherency in a thick free-standing structure: even a small mismatch...
Still worth trying !Any volunteers ?
holes migrate to the nearest well
diffusion time must much shorter than lifetime
d2/2D <<
τ
limits the duty cycle to about 100
Two-phase semiconductor structures
Emitted light
EG1 EG2
Host Guest
holes still migrate to the nearest inclusion
capture time must much shorter than lifetime
the duty cycle is practically unlimited
Random non-planar inclusions: “impregnations”
• Transparency gain scales as ratio of volumes
(d/a)3
• Lattice-matching requirement is removed
x'
d
x
a
Aug 14, 2009 NGC/CSTC, Hamilton, ON 30
Making a bulk impregnated structure ...
... any other ideas ?
• Direct growth with interruptions and self-organizatione.g. InGaAs quantum dots on GaAs substrate
• Phase separation via spinodal decomposition e.g. VPE grown InGaN shows subband emission (Shur)
• Transparent ceramics techniques (LLNL)Consolidation of nano-dot powders (e.g. ZnSe) by HIP
Two-phase ceramic, e.g. InP guest in CdTe matrix
Aug 14, 2009 NGC/CSTC, Hamilton, ON 31
Two-phase transparent ceramics
Mater. EG
(eV) Tm
(ºC)ZnS 3.68 1,850
ZnSe 2.822 1,100
ZnTe 2.394 1,240
CdS 2.50 1,750
CdSe 1.714 1,350
CdTe 1.474 1,041
InP 1.344 1,060
GaAs 1.424 1,240
Dots MatrixCdSe ZnTe
CdSe ZnSe
ZnTe ZnSe
CdS ZnSe
InP CdTe
impregnations must have smaller bandgap but higher melting point
Vacuum Sintering Hot Isostatic Pressing (HIP)
T ≈
0.9 Tm
Nano‐dots
matrix
Core/Shell
Nano‐mixture
InP impregnations
CdTe
Aug 14, 2009 NGC/CSTC, Hamilton, ON 32
Summary
Semiconductor scintillators – enabling technologyCompact and portable Compton telescope3D array of units, each endowed with integrated photoreceiverOptically tight integration, two-sided photodiode
Semiconductors “transparent” to own luminescenceDirect-gap semiconductors with photon recyclingSemiconductors activated with luminescent centersComposite “impregnated” systems
Aug 14, 2009 NGC/CSTC, Hamilton, ON 33
Sponsors
Energy and Spatial Correlation Effects in the Energy Resolution of Semiconductor Radiation Detectors (DTRA)
Semiconductor high-energy radiation detector with excellent isotope identification and directional capability (DHS)
Three-Dimensionally Pixellated Semiconductor Scintillator (DHS/NSF)
Aug 14, 2009 NGC/CSTC, Hamilton, ON 34
Going beyond InP... e.g., GaAs
GaAs pros• Higher radiative recombination coefficient, while nonradiative
similar
• Higher bandgap (good for low-noise photodetection at room temperature)
• May be cleaner, mature technology
GaAs cons• No proven epitaxial
detector
Wafer fusion technology may come to rescue
InPGaAs 7BB ≈ InPGaAs CC ≈
GaAs may be better than InP, but
Aug 14, 2009 NGC/CSTC, Hamilton, ON 35
no intermediate layer (wafer fusion)
Sacrificial (InP) substrate
Wafer bonding versus wafer fusion
Scintillator (GaAs) wafer
Photodiode (InP) heterostructure etchstop
intermediate bonding layer
Aug 14, 2009 NGC/CSTC, Hamilton, ON 36
What’s wrong with an intermediate layer ?
NOTHING
if it is really thin
ϕ
n3
n2d
BUT
n1GaAs
SiO2
InP
0 50 100 150 200 250 3000.0
0.2
0.4
0.6
0.8
1.0
Oxide Layer Thickness, nm
Mea
n Tr
ansm
issi
on C
oeffi
cien
t
0 2 4 6 8 10
0
2
4
6
8
10
“Frustrated”
Total Internal Reflection
λ
≈ 900 nm
naive comparison of the optical thickness with wavelength does not work for lower-index layers
even for 60 nm thick oxide layer, over 70% of GaAs scintillation will not go through
Aug 14, 2009 NGC/CSTC, Hamilton, ON 37
no intermediate layer (wafer fusion @ 600°C)
Sacrificial (InP) substrate Substrate removed (etched away using etchstop)
Photodiode integration by wafer fusion
Scintillator (GaAs) wafer
Photodiode (InP) heterostructure etchstop
Lithographic processing begins then the etchstop layer is removed too
Aug 14, 2009 NGC/CSTC, Hamilton, ON 38
InP
4. Fabrication of pin diode1. HIP on prepared InP substrate5. Flipping the picture upside down...
Optically tight photodiode integration
InGaAs etchstop layer
CdTe/InP transparent ceramic
InP sliver pin
diode
2. Etching away the InP substrate3. Etchstop layer goes away too
Aug 14, 2009 NGC/CSTC, Hamilton, ON 39
Optically tight photodiode integration
6. ... and looking at it again
p+
(Zn diffusion)n–
n+
i quaternary InGaAsP, 2 μm
Si3
N4
n+
InP slivern+
p metal contact
CdTe/InP transparent ceramic
n metal contact
Aug 14, 2009 NGC/CSTC, Hamilton, ON 40
Epitaxial pin diode fabrication
p+ n–
n+
i
n+
InP wafer
2 μmquaternary InGaAsP
Si3
N4
patterned resist
window etchingp metal contactZn diffusion
n metal contact backside patterning
epitaxial layers
bare scintillator slab
1 mm
Sarnoff – SBU collaboration
Aug 14, 2009 NGC/CSTC, Hamilton, ON 41
Compton Scattering
θ3
θ1
L3
L1
L2
θ2
E0
0.0
0.5
1.0
1.5
2.0
0
30
60
90
120
150
180
210
240
270
300
330
0.0
0.5
1.0
1.5
2.0
σ(θ)
θ γ ′
γ
e
γγ
θ′
−+=E
cmE
cm ee22
1cos
kinematics (Compton):
⎟⎟⎠
⎞⎜⎜⎝
⎛−+⎟
⎟⎠
⎞⎜⎜⎝
⎛=
′
′′ )(sin)( 2
2
0 θσθσγ
γ
γ
γ
γ
γ
EE
EE
EE
dynamics (Klein-Nishina):
γγ ′−= EEL)(Cs KeV 662
KeV 511137
2
=
=
γE
cme
Aug 14, 2009 NGC/CSTC, Hamilton, ON 42
Direction to source
( )n
n
N
j
j
nn N
npN
n δρρρr
vrr+−=≡ ∑
=
111
)(11
⎟⎟⎠
⎞⎜⎜⎝
⎛−+⎟⎟
⎠
⎞⎜⎜⎝
⎛= −
−−
)(sin)( 21
1
2
10 i
i
i
i
i
i
ii E
EEE
EE θσθσ
anisotropic scattering cross-section
dynamics (Klein-Nishina formula):
662 keV:
1ρr
Center of mass
cluster of n
interactions
0.0
0.5
1.0
1.5
2.0
0
30
60
90
120
150
180
210
240
270
300
330
0.0
0.5
1.0
1.5
2.0
σ(θ)