Quantum Well Infrared Detector

Jie Zhang, Win-Ching Hung

Department of Electrical and Computer Engineering

Outline

IntroductionQuantum Well Infrared PhotodetectorsQWIP Focal Plane ArraysApplicationsSummary

Atmospheric transmittance

Space-Based Missions

Force Enhancement

Force Enhancement

Space ControlSpace

Control

Surveillance

Protection

of

Assets

Counter

Enemy

Capabilities

Detecting Infrared RadiationHgCdTe semiconductorsSchottky barriers on SiSiGe heterojunctionsAlGaAs MQWsGaInSb strain layer superlatticesHigh T superconductorsSilicon Bolometers……..

Classes of IR Detectors

Thermal Detectors

Photodetectors

Intrinsic Extrinsic Photoemissive Quantum Well

Outline

IntroductionQuantum Well Infrared PhotodetectorsQWIP Focal Plane ArraysApplicationsSummary

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.

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”

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)

PhysicsE

ner

gy

Hg1-xCdxTe

CB

VB

Quantum Well

BoundState

CB Quasi-BoundState

VB En

erg

y

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

AlGaAs

AlGaAsGaAs

GaAs

AlGaAsGaAs

GaAs

0'

'2

* cos

sin

4)(

ro

wc

cnm

hfNdvva

96.0)(2 2

122

*

zEEm

f

Incidence angle

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

Optical Coupling (2)

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

TransitionsBound to BoundBound to ContinuumBound 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 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

Outline

IntroductionQuantum Well Infrared PhotodetectorsQWIP Focal Plane ArraysApplicationsSummary

Focal Plane Array

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

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

IntroductionQuantum Well Infrared PhotodetectorsQWIP Focal Plane ArraysApplicationsSummary

Applications of IR Detector Arrays

Industrial

•Electronics

Medical

•Astronomy•Infrared target detection

Space Military

•Automotive Industry

•Weather Forecasting

(MWIR,LWIR)&VLWIR) (LWIR)(MWIR)&(LWIR)

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 characterizationApplicationsChanllenges

Disadvantages

Requires low temperatures to operate.As with all photoconductors, noise is inevitable.