1. Quantum Well Infrared Detector
Jie Zhang, Win-Ching Hung
Department of Electrical and Computer Engineering

2. Outline Introduction Quantum Well Infrared Photodetectors
QWIP Focal Plane Arrays
Applications
Summary

3. Atmospheric transmittance

4. Space-Based Missions Surveillance Protection of Assets Counter Enemy
Force Enhancement
Space Control
Protection of
Assets
Counter
Enemy
Capabilities

5. Detecting Infrared Radiation
HgCdTe semiconductors
Schottky barriers on Si
SiGe heterojunctions
AlGaAs MQWs
GaInSb strain layer superlattices
High T superconductors
Silicon Bolometers
……..

6. Classes of IR Detectors
Thermal Detectors
Photodetectors
Intrinsic
Extrinsic
Photoemissive
Quantum Well

7. Outline Introduction Quantum Well Infrared Photodetectors
QWIP Focal Plane Arrays
Applications
Summary

8. Semiconductors METAL INSULATOR Conduction Band close to Valence Band.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.
INSULATOR
Conduction Band far from Valence Band.
Electrons not easily excited out of VB. kT CB VB
SEMICONDUCTOR
Conduction Band relatively close to Valence Band.
Electrons can be excited out of VB
under certain conditions.

9. 2-D Quantum Confinement
Bulk Semiconductors
Epitaxial Layers A B A B A
50 nm
5 nm
50 nm
Conduction Bands
Conduction Band
Discrete Energy Levels
Valence Band
Valence Bands
“Quantum Well”

10. Multiple Quantum Wells
Bulk Semiconductor A
Bulk 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

11. Physics Energy Energy Q u a n t m W e l H g C d T e C B C B V B V B
Quasi-Bound
State H g 1 - x C d T e C B B o u n d S t a e
Energy
Energy V B V B

12. 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

13. GaAs/ AlGaAs Incidence angle AlGaAs GaAs AlGaAs GaAs AlGaAs GaAs

14. 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

15. Optical Coupling (2)

16. 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

17. Transitions
Bound to Bound
Bound to Continuum
Bound to Quasi- Bound

18. Bound-to-Continuum Excited bound state is situated in the contunuum
Photoexcited eletrons escape without tunneling
Low bias voltage
Low dark current

19. Bound-to-Bound
Photo-excitation to another bound state within same energy band
Excited carriers escape out of well by tunneling

20. 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

21. Outline Introduction Quantum Well Infrared Photodetectors
QWIP Focal Plane Arrays
Applications
Summary

22. Focal Plane Array

23. 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

25. 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

26. Cameras

27. Outline Introduction Quantum Well Infrared Photodetectors
QWIP Focal Plane Arrays
Applications
Summary

28. Applications of IR Detector Arrays
Industrial
Electronics
Medical
Astronomy
Infrared target detection
Space
Military
Automotive Industry
Weather Forecasting
(MWIR,LWIR)&VLWIR)
(LWIR)
(MWIR)&(LWIR)

29. Application of VLWIR Detectors
Deep Space
Astronomy
Atmospheric
pollution monitoring
Early detection of
long range missiles

30. Conclusion QWIPs vs. HgCdTe detectors Physics of QWIPs
- Better imaging applications
- Easy fabrication and low cost
Physics of QWIPs
- Quantum wells
- Intersubband transition
Fabrication and characterization
Applications
Chanllenges

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