Justin McLaughlin
Solid State Dr. Perera
Autumn 2011
INFRARED DETECTION UTILIZING SPLIT-OFF BAND PHOTODETECTORS
SUMMARY
• Need for IR Detectors • QWIPs/HEIWIP • Ideas of operation • Architecture • Device characteristics • High temperature operation
ELECTROMAGNETIC SPECTRUM
USES FOR INFRARED DETECTORS • Astronomy • Fiber Optic Communication • Military use • Thermal Imaging • IR Spectroscopy
PROBLEMS WITH DETECTION
• Air
• Heat
PROBLEMS WITH DETECTION
DETECTORS: QWIPS (QUANTUM WELL INFRARED PHOTODETECTORS
• Potential Well creates quantized energy levels • Design based energy
band separation • Normal incidence
not allowed
• Absorption by free carriers • Threshold wavelength is determined by doping of emitter and aluminum
percentage of barrier • Allows for customizable detection threshold
p+-GaAs AlxGa1-xAs
ΔE
VB EF
Emitter Barrier
HETEROJUNCTION INTERFACIAL WORKFUNCTION INTERNAL PHOTOEMISSION
(HEIWIP)
Split-off
HEIWIP
2 3 4 5 0.00
0.02
0.04
Abs
orpt
ion
Wavelength ( µ m)
Sample 1332 λ=17 µm
Increased absorption not due to free carriers Occurring at shorter wavelength
VALENCE BAND
• Split off band offers additional energy level for holes
• 0.341 eV for GaAs • 3.64 µm
SPLIT OFF BAND MECHANISM
SO
Ef
EBL/H LH
CB
HH
EBSO
k
Indirect absorption followed by scattering and escape through barrier Threshold Energy EESO - Ef
Direct absorption followed by scattering and escape through barrier Threshold Energy EESOf - Ef
Indirect absorption high enough for escape into barrier SO band Threshold Energy EBSO - Ef
EESO
E
p+-GaAs AlGaAs
SO
L /H
SPLIT OFF MECHANISM
USEFULNESS
• Any possible benefits? • Increased Aluminum content raises barrier,
making SO transition the more dominant mode of absorption
• Higher barrier makes thermal excitation over the barrier more difficult
• 3 Devices with varying Aluminum Fractions
• 30 periods of GaAs/AlGaAs heterojunctions
DEVICE ARCHITECTURE
Device Al
Fraction x
Δ (meV)
λt (μm)
Responsivity (mA / W)
Highest Operating
Temperature
SP1 0.28 155 8 2.3 ± 0.1 140
SP2 0.37 207 6 2.7 ± 0.1 190
SP3 0.57 310 4 0.29 ± 0.01 300
155 meV
SO Band
L /H Band
340 meV
SP1 SP2
207 meV 340
meV
SP3
310 meV 340
meV
DEVICE CHARACTERISTICS
CONCLUSION • Use of SO band provides an additional mechanism for absorption in p-doped
semiconductors • By raising barrier height, dark current is reduced at higher temperatures,
allowing for higher temperature operation • Future work includes testing responsivity improvements for devices (graded
barriers, increased doping, etc)
THANK YOU
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