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