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Quantum Well Infrared Detector
Jie Zhang, Win-Ching Hung
Department of Electrical and Computer Engineering
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Outline
IntroductionQuantum Well Infrared PhotodetectorsQWIP Focal Plane ArraysApplicationsSummary
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Atmospheric transmittance
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Space-Based Missions
Force Enhancement
Force Enhancement
Space ControlSpace
Control
Surveillance
Protection
of
Assets
Counter
Enemy
Capabilities
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Detecting Infrared RadiationHgCdTe semiconductorsSchottky barriers on SiSiGe heterojunctionsAlGaAs MQWsGaInSb strain layer superlatticesHigh T superconductorsSilicon Bolometers……..
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Classes of IR Detectors
Thermal Detectors
Photodetectors
Intrinsic Extrinsic Photoemissive Quantum Well
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Outline
IntroductionQuantum Well Infrared PhotodetectorsQWIP Focal Plane ArraysApplicationsSummary
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Semiconductors
INSULATOR
Conduction Band far from Valence Band.
Electrons not easily excited out of VB.
kT
CB
VB
kT
CB
VB
kT
CB
VB
METAL
Conduction Band close to Valence Band.
Electrons easily excited out of VB.
Electrons in CB free to move.
SEMICONDUCTOR
Conduction Band relatively close to Valence Band.
Electrons can be excited out of VB
under certain conditions.
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2-D Quantum ConfinementBulk Semiconductors
A B A
Epitaxial Layers
Conduction Bands
Valence Bands
Conduction Band
Valence Band
Discrete Energy Levels
A B A50 nm 50 nm5 nm
“Quantum Well”
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Multiple Quantum Wells
Bulk Semiconductor A
Bulk Semiconductor B
Semiconductor Heterostructure
Quantum Well Bandstructure
Grown atom-by-atom
in an MBE machine
(Molecular Beam Epitaxy)
A multi-quantum well layer structure used as a detectoris called a “QWIP” (Quantum Well Infrared Photodetector)
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PhysicsE
ner
gy
Hg1-xCdxTe
CB
VB
Quantum Well
BoundState
CB Quasi-BoundState
VB En
erg
y
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Design: Key Aspects
1-D arrays with the growth direction normal to the layers.
Vertical quantized quantum levels.
Horizontal planes exhibit a uniform energy state which allows electrons to move freely within the plane.
All electrons in a horizontal plane have the same transition energy
Only photons with energies corresponding to the selected energy gaps can be detected.
Well-depth can be altered by changing the properties of the layered materials.
Stacking wells allows for higher absorptions
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GaAs/ AlGaAs
AlGaAs
AlGaAs
AlGaAsGaAs
GaAs
AlGaAsGaAs
GaAs
0'
'2
* cos
sin
4)(
ro
wc
cnm
hfNdvva
96.0)(2 2
122
*
zEEm
f
Incidence angle
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Optical Coupling (1)Light waves that strike the layers perpend
icularly show no excitationOptions:45 degree wedgeBend the light inside the detectors with a roughe
d mirror on the back to scatter normal light.The mirror can be roughed randomly or periodic
ally
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Optical Coupling (2)
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Intersubband Absorption
Transitions between energy within same band
Intersubband transition energy
Transition energy inversely proportional to square of well-thickness.
Wide range wavelength Short-wave infrared (SWIR) λ~ 2μm Medium-wave infrared (MWIR) λ~ 4μm Long-wave infrared (LWIR) λ~10μm Very long-wave infrared (VWIR) λ>14μm
2*
22
122
3
wLmEE
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TransitionsBound to BoundBound to ContinuumBound to Quasi- Bound
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Bound-to-Continuum Excited bound state is situated in the contunuum
Photoexcited eletrons escape without tunneling Low bias voltage Low dark current
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Bound-to-Bound
Photo-excitation to another bound state within same energy band
Excited carriers escape out of well by tunneling
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QWIPs Vs. HgCdTe HgCdTe has higher absorption coefficient and l
ower thermal emission, especially at higher temperatures (>75K)
QWIPs show better capabilities as FPAs:High impedance, fast response time, long integration time, and low powe consumption
QWIPs have a greater potential in the VLWIR FPA operation with multi-color detection
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Outline
IntroductionQuantum Well Infrared PhotodetectorsQWIP Focal Plane ArraysApplicationsSummary
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Focal Plane Array
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Fabrication
1. Epitaxial growth of QWIP structure2. Processing of the QWIP array3. Fabrication of ROIC (readout integrated circuit)4. Processing of indium bumps5. Hybridization flip-chip bonding6. Mounting and wire bonding
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QWIP Camera•MWQs •Stacks of 50 n-doped GaAs well
with Al0.3Ga0.7As barriers•Uses bound to quasi-bound
transitions•Used low operating bias which
resulted in only a 1.4% QE•Used periodic mirror etching•Pixel size: 23x23 square micrometers•Cooled with closed-cycle Sterling Cooler•Consumes <45W•Operational temperature up to
70K
12-640x512 pixel arrays on a 3 inch GaAs wafer
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Cameras
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Outline
IntroductionQuantum Well Infrared PhotodetectorsQWIP Focal Plane ArraysApplicationsSummary
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Applications of IR Detector Arrays
Industrial
•Electronics
Medical
•Astronomy•Infrared target detection
Space Military
•Automotive Industry
•Weather Forecasting
(MWIR,LWIR)&VLWIR) (LWIR)(MWIR)&(LWIR)
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Application of VLWIR Detectors
Deep Space Astronomy
Early detection of long range missiles
Atmospheric pollution monitoring
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Conclusion
QWIPs vs. HgCdTe detectors - Better imaging applications - Easy fabrication and low cost
Physics of QWIPs - Quantum wells - Intersubband transition
Fabrication and characterizationApplicationsChanllenges
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Disadvantages
Requires low temperatures to operate.As with all photoconductors, noise is inevitable.