1. High Operating Temperature (HOT) Split-off Band IR Detectors
Viraj Jayaweera

2. Outline Introduction IR Range, Applications, Types of IR detectors
Interfacial Workfunction Internal Photoemission (IWIP) Detectors
Detector Structure, HIWIP & HEWIP Mechanisms
Detector Measurements and Characterization
Split-off Band Detectors
Possible Material Systems to Extend Spectral Range
Conclusion and Future Studies

3. Discovery of Infrared Sir Frederick William Herschel (1738-1822)
musician and an astronomer
famous for his discovery of the planet Uranus in 1781
Discovered “calorific rays” in 1800 later renamed as “Infrared rays”

4. (the prefix infra means `below‘)
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.

5. Infrared is usually divided into 3 spectral regions
Micro wave
Visible
near-IR
mid-IR
Far-IR
0.8 – 5 m m m
Can’t see (human eye)
= 0.75 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.

6. 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.
person
holding burning match
Cat
Infrared image of Orion
Landing space shuttle
Application: biophysics, communication, remote sensing, medical imaging, security and astrophysics.

8. 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.

9. Pyroelectric Detectors
Types of IR Detector
IR Detectors
Photon Detectors
Pyroelectric Detectors
Thermal Detectors
Photo Conductive
Photo Conductive
Photovoltaic
Bolometer
Thermopile
Interfacial Workfunction Internal Photoemission (IWIP) Detectors
Homojunction IWIP = HIWIP
Heterojunction IWIP = HEIWIP

10. Real Detector

11. Structure of the Interfacial Workfunction Internal Photoemission Detector.
~0.5mm
Au contact layers
Top Contact p++ GaAs
N Periods
p+ GaAs (emitter)
GaAs (barrier)
Homojunction
Heterojunction
p+ GaAs (emitter)
AlGaAs (barrier)
Bottom Contact p++ GaAs
Substrate
(photo conductive type)

12. HIWIP Barrier formed by Homojunction (n-type)
(Homojunction Interfacial Workfunction Internal Photoemission Detector) n+ i
n+ doped GaAs
GaAs e- hν Δ Δ EF
ECn
zero bias
biased
Barrier formed by Homojunction (n-type)
(Interfacial Workfunction Δ comes from doping)
JAP 77, 915 (1995)

13. Barrier formed by Heterojunction (n-type)
HEIWIP
(HEterojunction Interfacial Workfunction Internal Photoemission Detector) n+ i
n+ GaAs
AlGaAs e- hν Δ Δ
zero bias
biased
Barrier formed by Heterojunction (n-type)
Interfacial Workfunction Δ comes from Al fraction and doping (Δ = Δd + Δx)
Absorption is due to free carriers
Interface is sharp (no space charge)
APL 78, 2241 (2001)
APL 82, 139 (2003)

14. Barrier formed by Heterojunction (p-type)
HEIWIP
(HEterojunction Interfacial Workfunction Internal Photoemission Detector)
p+ GaAs
AlGaAs h+ i p+ hν Δ
biased Δ
zero bias
Barrier formed by Heterojunction (p-type)
Interfacial Workfunction Δ comes from Al fraction and doping (Δ = Δd + Δx)
Absorption is due to free carriers
Interface is sharp (no space charge)
APL 78, 2241 (2001)
APL 82, 139 (2003)

15. Measurements and Characterization
(IVT) Current Voltage Temperature measurements
Using IVT measurements …
Uniformity of sample (dark current density vs. voltage plot)
Dark Current Variation with bias Voltage and Temperature
Background Limited Infrared Photon detector (BLIP) Temperature
Interfacial Workfunction (Δ) (slope of ln(I/T1.5) vs. 1/T plot)
Switching System
Source Meter
He closed-cycle refrigerator head
Vacuum
Sample
Temperature Controller PC V
Log (I)
Cold finger
Radiation shield

16. time (mirror position)
Measurements and Characterization
time (mirror position)
output energy
Spectral Response
Source
Moving mirror
Sample RL
Output
Wave number
output energy
Fourier transformation
Beam splitter o
Threshold wavelength
Fixed mirror
FTIR Spectrometer

17. # of electrons produced Quantum Efficiency (η) = # of photons insident
Detector Output
Responsivity (R) =
Radiation Input
# of electrons produced
Quantum Efficiency (η) =
# of photons insident
Radiant flux necessary to give an output signal equal to the r.m.s. noise output of the detector
Noise Equivalent Power (NEP) = 1
Detectivity (D*) =
NEP

18. Split-off Response Quantum Efficiency Wavelength (µm) Sample 1332
2.5
5.0
7.5
10.0
12.5
15.0
0.00
0.01
0.02
Split-off
Response
Free Carrier
Quantum Efficiency
Wavelength (µm)
Sample 1332
T = 50K

19. Split-off Response of the Detector HE0204 Under Different Temperatures

20. Detector mechanism consisting of three processes k
Heavy Hole Band
Light Hole Band
Split-off Band Ef
ESO
Detector mechanism consisting of three processes
Photoabsorption. (produces excited carriers)
Carrier escape.
Sweep out and collection of the escaped carriers.

21. Free Carrier Absorption
Response Mechanism I
Light/Heavy Hole Band E k Ef
ΔL/H
Free Carrier Absorption
escape
Heavy Hole Band
Light Hole Band
ΔSO
Split-off Band
Split-off Band
The photoexcitation process consists of the standard free carrier absorption.

22. Response Mechanism II Light/Heavy Hole Band E k Ef ΔL/H
Split-off Absorption
Heavy Hole Band
escape
Light Hole Band
scattering
ΔSO
Split-off Band
Split-off Band
direct photoabsorption to the split-off band, followed by a scattering to the light/heavy hole band.

23. Response Mechanism III
Light/Heavy Hole Band E k Ef
ΔL/H
Split-off Absorption
Heavy Hole Band
Light Hole Band
ΔSO
Split-off Band
Split-off Band
escape
Single indirect photoabsorption into the split-off band.

24. Response Mechanism IV Light/Heavy Hole Band E k Ef ΔL/H
Split-off Absorption
Heavy Hole Band
escape
Light Hole Band
scattering
ΔSO
Split-off Band
Split-off Band
indirect photoabsorption, followed by a scattering event to the light or heavy hole band.

25. The Split-off Band Offset Energy for Different Materials
ΔSO (meV)
λSO
(μm)
Elh
(meV)
Eso
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 energies of the light/heavy hole band (Elh) and the split-off hole band (ESO) relative to the valance band maximum of GaAs.

26. Conclusion and Future Studies
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.
Increase Quantum efficiency
Absorption efficiency can be increase by
Increasing the no of emitter layers
Increasing the doping to the maximum possible value
Device Design for a 15 μm Detector Operating at 200K
Device will based on p-doped GaP emitters and undoped AlGaP barriers.

27. Thank You