High Operation Temperature (HOT) High Operation Temperature (HOT) Split-off Band IR Detectors Split-off Band IR Detectors

Viraj Jayaweera

1. Introduction IR Range, Applications, Types of IR detectors

2. Interfacial Workfunction Internal Photoemission (IWIP) Detectors

Detector Structure, HIWIP, HEWIP Mechanism

3. Detector Measurements and Characterization

4. Split-off Band Detectors

5. Possible Material Systems to Extend Spectral Range.

6. Conclusion and Future Studies

Outline Outline

Sir Frederick William Herschel (1738-1822)

musician and an astronomer

famous for his discovery of the planet Uranus in 1781

Discover “calorific rays” in 1800 later renamed as “Infrared rays”

Discovery of InfraredDiscovery of Infrared

What is Infrared (IR) ?What is Infrared (IR) ?(the prefix infra means `below‘)

The electromagnetic spectrum includes gamma rays, X-rays, ultraviolet, visible, infrared, microwaves, and radio waves. The only difference between these different types of radiation is their wavelength or frequency.

Visible Micro wave near-IRnear-IR mid-IRmid-IR Far-IRFar-IR

= 0

.75

m

Can’t see (human eye)

Infrared is usually divided Infrared is usually divided into 3 spectral regionsinto 3 spectral regions

0.8 – 5 m 5 - 40 m 40 - 250 m

Some animals can "see" in the infrared. For example, snakes in the pit viper family (e.g. rattlesnakes) have sensory "pits," which are used to detect infrared light. This allows the snake to find warm-blooded animals.

This is the radiation produced by the motion of atoms and molecules in an object.

Any object which has a temperature above absolute zero (0 K) radiates infrared.

Landing space shuttle

person

holding burning match

Cat Infrared image of Orion

Application: biophysics, communication, remote sensing, medical imaging, security and astrophysics.

Human & vehicle at total darkness thermal image in white=hot mode

same image in Black=hot mode

Human Suspect climbing over fence at 2:49 AM in total darkness

Suspect attempting to burglarize vehicle at 1:47 AM in total darkness.

IR Detectors

Types of IR DetectorTypes of IR Detector

Pyroelectric Detectors

Photon Detectors

Photo Conductiv

e

Photovoltaic

BolometerPhoto Conducti

ve

Interfacial Workfunction Internal Photoemission (IWIP) Detectors

Homojunction IWIP = HIWIP

Heterojunction IWIP = HEIWIP

Thermal Detectors

Thermopile

Real DetectorReal Detector

Substrate

Bottom Contact p++ GaAs

p+ GaAs (emitter)

AlGaAs (barrier)

Top Contact p++ GaAs

N Period

s

400 μm 400 μm

Structure of the Interfacial Workfunction Internal Structure of the Interfacial Workfunction Internal Photoemission Detector.Photoemission Detector.

Heterojunction

GaAs (barrier)p+ GaAs (emitter)

Homojunction

(photo conductive (photo conductive type)type)

Au contact layers

<2.5μm

~1.5mm

Barrier formed by Homojunction (n-type)

(Δ comes from doping)

n+ doped GaAs

GaAs

Δ

zero bias

e-

in+

ECn

EF

hν

Δ

biased

JAP 77, 915 (1995)

HIWIPHIWIP(Homojunction Interfacial Workfunction Internal Photoemission

Detector)

Absorption is due to free carriersInterface is sharp (no space charge)

HEIWIPHEIWIP(HEterojunction Interfacial Workfunction Internal Photoemission

Detector)

Barrier formed by Heterojunction (p-type)

(Δ comes from Al fraction and doping)

APL 78, 2241 (2001)

APL 82, 139 (2003)

AlGaAs

p+ GaAs

Δh+

ip+

hν Δ

biased zero bias

(not quantized)

Measurements and Measurements and CharacterizationCharacterization(IVT) Current Voltage Temperature

measurements

Radiation shield

Cool finger

Vacuum

Sample

Switching System

Source Meter

Temperature Controller

He close cycle refrigerator head

PC

V

Log (

I)

Using IVT measurements

• Uniformity of sample (dark current density vs. voltage plot).

• Dark Current Variation with bias Voltage and Temperature.

• Background Limited Performance (BLIP) Temperature.

• Experimental Δ (slope of ln(I/T1.5) vs. 1/T plot)

Measurements and Measurements and CharacterizationCharacterizationSpectral

Response

Moving mirror

Fixed mirror

Source

Beam splitter

Sample

RL

Output

time (mirror position)

outp

ut

energ

y

Wave numberoutp

ut

energ

y

Furrier transformation

FTIR Spectrometer

o

Threshold wavelength

2.5 5.0 7.5 10.0 12.5 15.00.00

0.01

0.02Split-off

Response

Free Carrier

Response

Qua

ntum

Effi

cien

cy

Wavelength (µm)

Sample 1332

T = 50K

Split-off ResponseSplit-off Response

0

0.2

0.4

0.6

1 2 3 4 5Wavelength (um)

Re

sp

on

se

(A

/W)

80K

90K

105K

120K

100K

130K

Split-off Response of the Detector HE0204 Split-off Response of the Detector HE0204 Under Different TemperaturesUnder Different Temperatures

E

k

Heavy Hole Band

Light Hole Band

Split-off Band

Ef

ESO

Detector mechanism consisting of three processes

1. Photoabsorption. (produces excited carriers)

2. Carrier escape.

3. Sweep out and collection of the escaped carriers.

Ek

Heavy Hole Band Light

Hole Band

Split-off Band

Ef

ΔL/H escapeFree Carrier Absorption

Light/Heavy Hole Band

Split-off Band

ΔSO

Response Mechanism I Response Mechanism I

The photoexcitation process consists of the standard free carrier absorption.

Ek

Heavy Hole Band

Split-off Band

Ef

ΔL/H

escape

Split-off Absorption

Light/Heavy Hole Band

Split-off Band

scattering

ΔSO

Light Hole Band

direct photoabsorption to the split-off band, followed by a scattering to the light/heavy hole band.

Response Mechanism II Response Mechanism II

Ek

Heavy Hole Band

Split-off Band

Ef

ΔL/H

escape

Split-off Absorption

Light/Heavy Hole Band

Split-off Band

ΔSO

Light Hole Band

Response Mechanism III Response Mechanism III

Single indirect photoabsorption into the split-off band.

Ek

Heavy Hole Band

Split-off Band

Ef

ΔL/H

escape

Split-off Absorption

Light/Heavy Hole Band

Split-off Band

scattering

ΔSO

Light Hole Band

Response Mechanism IV Response Mechanism IV

indirect photoabsorption, followed by a scattering event to the light or heavy hole band.

MaterialΔSO

(meV)λSO

(μm)Elh

(meV)Eso

(meV)

InN 3 410 -790 -793

GaN 20 62 -1840 -1860

AlN 19 65 -2640 -2660

InP 108 11 -140 -248

GaP 80 16 -470 -550

AlP 70 18 -940 -1010

InAs 390 3.2 +210 -180

GaAs 340 3.6 +0 -340

AlAs 280 4.4 -530 -810

The Split-off Band Offset Energy for The Split-off Band Offset Energy for Different MaterialsDifferent Materials

The energies of the light/heavy hole band (Elh) and the split-off hole band (ESO) relative to the valance band maximum of GaAs.

Conclusion and Future StudiesConclusion and Future Studies

1. High Operating Temperature

The devices tested with a threshold of ~20 µm showed a maximum operating temperature of 130 K. By reducing the threshold to ~5 µm, the operating temperature should be increased to 300 K with D* of ~5×109 Jones.

2. Increase Quantum efficiency

Absorption efficiency can be increase by

• Increasing the no of emitter layers

• Increasing the doping to the maximum possible value

3. Device Design for a 15 μm Detector Operating at 200K

Device will based on p-doped GaP emitters and undoped AlGaP barriers.

Thank Thank YouYou

-12

-10

-8

-6

-4

-6 -4 -2 0 2 4 6Bias Voltage (V)

Lo

g (

Dar

k C

urr

ent)

20 K 25 K

30 K 35 K

40 K 45 K

50 K 55 K

60 K 65 K

70 K 75 K

80 K 85 K