near-infrared detector arrays m. robberto (with several slides grabbed from j. beletic, k. hodapp et...
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Near-Infrared Detector Arrays
M. Robberto
(with several slides grabbed from J. Beletic, K. Hodapp et al.)
Intrinsic materials
material λc(m) Eg(eV)
AgCl 0.39 3.20
CdS 0.52 2.40
GaP 0.55 2.24
CdSe 0.69 1.80
CdTe 0.71 1.45
GaAs 1.35 0.92
Si 1.11 1.12
Ge 1.85 0.67
PbS 2.95 0.42
InAs 3.18 0.39
PbTe 5.0 0.25
PbSe 5.40 0.23
InSb 5.40 0.23
Pb1-xSnxTe <12.4 >0.10
Hg1-xCdxTe <12.4 >0.10
The bandgap depends on the temperature
0 ( <0)g gE E T
e.g. for InSb: Eg = 0.24 eV and β= -2x10-4eV
NASA CDR 05-08-01
Extrinsic materials
P-type ◄ ► N-type
material λc(m) Eg(eV)
Ge:Au 8.27 0.15
Ge:Hg 13.8 0.09
Ge:Cd 20.7 0.06
Ge:Cu 30.2 0.041
Ge:Zn 37.6 0.033
Ge:Be 40 0.03
Ge:B 119.2 0.0104
Ge:Ga 120 0.01
Ge:Li 140 0.009
Si:In 8.00 0.165
Si:Mg 14.3 0.087
Si:Ga 17.1 0.0723
Si:Bi 17.6 0.0706
Si:Al 18.1 0.0685
Si:As 23.1 0.0537
Homework 1
• Small fractional changes in x lead to large fractional changes in the gap energy. How well we need to control x at room Temperature to have a 2% uncertainty in response at cutoff for
– HgCdTe 1.72micron cutoff at 145K [WFC3]
– HgCdTe 2.5micron cutoff at 77K [ground based]
– HgCdTe 5micron cutoff at 35K [JWST]
– HgCdTe 10micron cutoff at 35K [NEOCAM]
• Among these, that is the most demanding material to grow?
Cross-section of HgCdTe detectorp-on-n
P-on-N design
PN junctionSemiconductor
EF = Fermi Level => ½ occupancy at high T
PN junction
N-typeP-type
Doped semiconductors
Impurities (doping) move the EF closer to the valence (P-type) or conduction (N=type) bands.
PN junction
N-typeP-type
P-N Junction
When the two materials are brought into electrical contact, the electrons and hole diffuse. Recombination occurs until the Fermi levels are in equilibrium.
Depletion or Space Charge region
E
Depletion Region
• Not Neutral: there is an electric field from the N-type (+ charged) to the P-type (- charged)
• Free (depleted) of mobile carriers: extremely low conductivity, or high resistivity.
• An insulator between two charge distributions is a capacitance.
• The development of the electric field eventually stops the diffusion: “built-in voltage” or “contact potential”
• The electric field facilitates the flow of charges in one direction and prevents in the other: diode
E
PN junctionReverse biased P-N Junction
Reverse bias: apply voltage with the same polarity of the contact potential
+ Voltage to the N-type
- Voltage to the P-type
makes depletion region wider and increases the resistance of the junction.
(but do not exagerate! => breakdown)
Forward bias: smaller depletion region, eventually no E: high conductivity
N-typeP-type
E+Eb
PN junction illuminatedReverse biased P-N Junction
Assume a photon is absorbed BY THE BULK MATERIAL on the P-type side, creating a hole-electron pair. They will eventually recombine. However, if the electron (minority
carrier in the P-type material), reaches the junction before recombination, it will be swept on the other side. There it becomes a majority carrier. It will be sensed out if the bias is
kept constant, or recombines with a hole and discharges the junction
If the bias is “floating”, the other original hole, a majority carrier in the sea of holes, will drift until recombination, calling an electron from ground. A current is generated in the
reverse direction with respect to the original one that set the junction.
(Same is true for photogenerated holes in N-type material).
N-typeP-type
PN junction illuminatedReverse biased P-N Junction
Assume a photon is absorbed BY THE BULK MATERIAL on the P-type side, creating a hole-electron pair. They will eventually recombine. However, if the electron (minority
carrier in the P-type material), reaches the junction before recombination, it will be swept on the other side. There it becomes a majority carrier. It will be sensed out if the bias is
kept constant, or recombines with a hole and discharges the junction
If the bias is “floating”, the other original hole, a majority carrier in the sea of holes, will drift until recombination, calling an electron from ground. A current is generated in the
reverse direction with respect to the original one that set the junction.
(Same is true for photogenerated holes in N-type material).
N-typeP-type
PN junction illuminatedReverse biased P-N Junction
Assume a photon is absorbed BY THE BULK MATERIAL on the P-type side, creating a hole-electron pair. They will eventually recombine. However, if the electron (minority
carrier in the P-type material), reaches the junction before recombination, it will be swept on the other side. There it becomes a majority carrier. It will be sensed out if the bias is
kept constant, or recombines with a hole and discharges the junction
If the bias is “floating”, the other original hole, a majority carrier in the sea of holes, will drift until recombination, calling an electron from ground. A current is generated in the
reverse direction with respect to the original one that set the junction.
(Same is true for photogenerated holes in N-type material).
N-typeP-type
Back to zero bias and beyond
N-typeP-type
P-N Junction
Eventually the junction is discharged but photons are still absorbed. The diffusion current pushes back to maintain the built-in bias. Dark and photocurrent therefore work in
different directions. An equilibrium is reached: saturation.
Reset
Photon detection
Do you see the cross-talk/MTF problem?
End of integration
Reading out the generated charges• “Hybrid CMOS sensors”
• Indium bumps are aligned, squeezed and distorted to establish electric contact between detector layer and multiplexer: COLD-WELDING
• The addressing and readout electronics is built on Silicon. More standard technology (still >107 transistors).
HAWAII-2: Photolithographically Abut 4 CMOS Reticles to Produce Each 20482 ROIC
Twelve 20482 ROICs per 8” Wafer
20482 Readout Provides Low Read Noise for Visible and MWIR
Canon 16mm x 14 mm
GCA 20mm x 20 mm
ASML 22mm x 27.4 mm
Reticle-Stitching: 2048x2048 ROIC
Submicron Stepper Options
RSC Approach
H A W A I I - H A W A I I - 2 R G2 R G
H A W A I I - H A W A I I - 2 R G2 R G
• HgCdTe detector – substrate removed to achieve 0.6 µm sensitivity
HgCdTe Astronomy Wide Area Infrared Imager with 2k2 Resolution, Reference pixels and Guide Mode
• Specifically designed multiplexer– highly flexible reset and readout options – optimized for low power and low glow operation– three-side close buttable
• Two-chip imaging system: MUX + ASIC– convenient operation with small number of clocks/signals– lower power, less noise
HAWAII-2RG
Block Diagram
5 MHz column buffersfast normal shift register + logic
glow
and
cro
ssta
lk s
hiel
d
g low and crosstalk shield
Additional row of reference pixels for diagnostic purposes
2048 x 2048 pixel array(2040 x 2040 sensitive pixels)
4 rows and columns containing reference pixels
4 rows and columns containing reference pixels
serial interface
clock buffers
fast guide shift register + logic
Slow
gui
de s
hift
regi
ster
+ lo
gic
Slow
nor
mal
shi
ft re
gist
er +
logi
c
decoders for horizontal start and stop address
I/O Pads & output buffers
deco
ders
for v
ertic
al s
tart
and
sto
p ad
dres
s
• All pads located on one side (top)
• Approx. 110 doubled I/O pads (probing and bonding)
• Three-side close buttable
• 18 µm pixels
• Total dimensions: 39 x 40.5 mm²
Output Options
Slo
w s
can
dire
ctio
n se
lect
able
Single output for all2048 x 2048 pixels
(guide mode always uses single output)
Fast scan direction selectable
Single Output ModeSingle Output Mode
default scan directions
Fast scan direction individuallyselectable for each subblock
Separate output for each subblock of 512 x 2048 pixels
S
low
sca
n di
rect
ion
sele
ctab
le
4 Output Mode4 Output Mode
default scan directions
Output Options (2)Sl
ow s
can
dire
ctio
n se
lect
able
32 Output Mode32 Output Mode
Separate output for each subblock of 64 x 2048 pixels
Four different patterns for fast scan direction selectable
default scan directions
Interleaved readout of full field and guide window
Guide windowGuide window
Full fieldFull field
FPAFPA• Switching between full field and guide window is possible at any time
any desired interleaved readout pattern can be realized• Three examples for interleaved readout:
1. Read guide window after reading part of the full field row
2. Read guide window after reading one full field row
3. Read guide window after reading two or more full field rows
Pixel by pixel reset Line by line reset Global reset
Full field
Guidewindow
Pixel by pixel reset Line by line reset Global reset
Full field
Guidewindow
Reset Schemes
MIRI Detectors: Si:As IBC
• Extrinsic (vs. HgCdTe, intrinsic)
• Blocked Impurity Band (BIB) extrinsic (vs. “Bulk”)
READOUT INTEGRATED CIRCUIT (ROIC)
• 1024 × 1024 / 25 μm pixels
• 7 K Operation
• Source-Follower-per-Detector (SFD) PMOS input circuit
• Low Noise: 10 – 12 e- rms with Fowler-8
• Low Read Glow
• Low Power: < 0.5 mW
• 4 outputs with interleaved columns
• Reference pixels on all outputs mimic "dark" detectors
• Reference output averages noise from 8 "dark" reference pixels
• 2.75 second read time at 10 μsec per sample (100 kHz pixel data rate)
Time
Dio
de B
ias
Vol
tage
0.5 V
0 V
Res
et
Readout
Re
se
t
kTC Noise
Reset-Read Sampling
Reset Noise in Capacitors
Energy stored in a capacitor:
Noise floor energy:
E_n = ½kT
Noise Charge: E=En
Problem:
Calculate the Reset noise for JWST detectors, assuming: C= 50 fF, T=37 K
Time
Dio
de B
ias
Vol
tage
0.5 V
0 V
Re
se
t
Open Shutter Close Shutter
Readout
Re
se
t
Readout
kTC noise
CD
S S
ign
al
Double Correlated Sampling
Time
Dio
de B
ias
Vol
tage
0.5 V
0 V
Res
et
Readout
Re
se
t
Readout
kTC noise
MC
S S
ign
al
Fowler (multi) Sampling
Time
Dio
de B
ias
Vol
tage
0.5 V
0 V
Res
et
Re
se
t
Up-the-ramp Readout
kTC noise
MC
S S
ign
al
Up-the-Ramp Sampling
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