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

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

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”

(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.

Infrared is usually divided into 3 spectral regions Micro wave Visible near-IR mid-IR Far-IR 0.8 – 5 m 5 - 40 m 40 - 250 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.

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.

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.

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

Real Detector

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)

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)

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)

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)

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

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

# 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

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

Split-off Response of the Detector HE0204 Under Different Temperatures

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.

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.

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.

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.

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.

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.

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.

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