Quantum Well
Infrared Detector
Jie Zhang, Win-Ching Hung
Department of Electrical and Computer Engineering
Outline
Introduction
Quantum Well Infrared Photodetectors
QWIP Focal Plane Arrays
Applications
Summary
Atmospheric transmittance
Space-Based Missions
Surveillance
Force Enhancement
Protection
of
Assets
Space Control
Counter
Enemy
Capabilities
Detecting Infrared Radiation
HgCdTe semiconductors
Schottky barriers on Si
SiGe heterojunctions
AlGaAs MQWs
GaInSb strain layer superlattices
High T superconductors
Silicon Bolometers
……..
Classes of IR Detectors
Thermal Detectors
 Photodetectors




Intrinsic
Extrinsic
Photoemissive
Quantum Well
Outline
Introduction
Quantum Well Infrared Photodetectors
QWIP Focal Plane Arrays
Applications
Summary
Semiconductors
CB
CB
kT
kT
VB
VB
INSULATOR
Conduction Band far from Valence Band.
Electrons not easily excited out of VB.
METAL
Conduction Band close to Valence Band.
Electrons easily excited out of VB.
Electrons in CB free to move.
CB
kT
VB
SEMICONDUCTOR
Conduction Band relatively close to Valence Band.
Electrons can be excited out of VB
under certain conditions.
2-D Quantum Confinement
Bulk Semiconductors
A
B
A
Epitaxial Layers
A
B
A
50 nm 5 nm 50 nm
Conduction Bands
Conduction Band
Discrete
Energy
Levels
Valence Band
Valence Bands
“Quantum Well”
Multiple Quantum Wells
Bulk
Bulk
Semiconductor A Semiconductor B
Grown atom-by-atom
in an MBE machine
(Molecular Beam Epitaxy)
Semiconductor Heterostructure
A multi-quantum well layer
structure used as a detector
is called a “QWIP” (Quantum
Well Infrared Photodetector)
Quantum Well Bandstructure
Physics
Quantum Well
CB
CB
Bound
State
Energy
VB
Quasi-Bound
State
VB
Energy
Hg1-xCdxTe
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
GaAs/ AlGaAs
AlGaAs
GaAs
AlGaAs
GaAs
GaAs
AlGaAs
GaAs
AlGaAs
  c N w hf
 a(v)dv   4 m*cn
0
r
 o

 sin 2  '

 cos '

2m*
f  2 ( E2  E1 )  z  2  0.96

Incidence angle



Optical Coupling (1)
Light waves that strike the layers
perpendicularly show no excitation
Options:
 45 degree wedge
 Bend the light inside the detectors with a
roughed mirror on the back to scatter normal
light.
 The mirror can be roughed randomly or
periodically
Optical Coupling (2)
Intersubband Absorption
 Transitions between energy within same band
 Intersubband transition energy
3 2 2
E2  E1 
2m* L2w
 Transition energy inversely proportional to square of wellthickness.
 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
Transitions
Bound to Bound
Bound to Continuum
Bound to Quasi- Bound
Bound-to-Continuum
 Excited bound state is situated in the contunuum
 Photoexcited eletrons escape without tunneling
 Low bias voltage
 Low dark current
Bound-to-Bound
Photo-excitation to another bound state within
same energy band
Excited carriers escape out of well by
tunneling
QWIPs Vs. HgCdTe
 HgCdTe has higher absorption coefficient and
lower 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
Outline
Introduction
Quantum Well Infrared Photodetectors
QWIP Focal Plane Arrays
Applications
Summary
Focal Plane Array
Fabrication
1. Epitaxial growth of QWIP
structure
2. Processing of the QWIP
array
3. Fabrication of ROIC
(readout integrated circuit)
4. Processing of indium
bumps
5. Hybridization flip-chip
bonding
6. Mounting and wire
bonding
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
Cameras
Outline
Introduction
Quantum Well Infrared Photodetectors
QWIP Focal Plane Arrays
Applications
Summary
Applications of IR Detector Arrays
•Automotive
Industry
•Electronics
Industrial
(MWIR)&(LWIR)
•Weather
Forecasting
•Infrared target
detection
•Astronomy
Space
(MWIR,LWIR)&VLWIR)
Medical
(LWIR)
Military
Application of VLWIR Detectors
Deep Space
Astronomy
Early detection of
long range missiles
Atmospheric
pollution monitoring
Conclusion
QWIPs vs. HgCdTe detectors
- Better imaging applications
- Easy fabrication and low cost
Physics of QWIPs
- Quantum wells
- Intersubband transition
Fabrication and characterization
Applications
Chanllenges
Disadvantages
Requires low temperatures to
operate.
As with all photoconductors, noise
is inevitable.