High Operating Temperature Split-off Band IR Detectors Viraj Jayaweera Department of Physics and Astronomy Georgia State University

Outline Types of IR Detectors Difference Between Mechanisms of Other types of detectors and Split-off Detector Detector Structure and Experimental Results Advantages for the Split-off detectors Conclusion

Types of Infrared Detectors IR Detectors Photon Detectors Thermal Detectors Pyroelectric Detectors Bolometer Thermopile Photo Conductive Photovoltaic QWIP (Quantum Well Infrared Photodetectors) DWELL (Quantum Dots-in-a-Well Infrared Photodetectors) T-QDIP (Tunneling Quantum Dot Infrared Photodetectors) HIWIP (Homojunction Interfacial Workfunction Internal Photoemission) HEIWIP (Heterojunction Interfacial Workfunction Internal Photoemission)

Detector Mechanisms Intrinsic (InSb, HgCdTe) Quantum Well Conduction Band E Conduction Band E k k ESO Heavy Hole Band Light Hole Band Heavy Hole Band Light Hole Band Split-off Band Split-off Band Intrinsic (InSb, HgCdTe) Quantum Well

Split-off Mechanism IR Photon excites holes from the light/heavy hole bands to the split-off band (Solid Arrow) Excited holes may escape in split-off band or, May scatter into the light/heavy hole bands and then escape (Dashed Arrow) Conduction Band E Transition is entirely in hole bands Carrier energies are continuous not quantized Split-off response is inherently broadband k Ef ΔL/H Heavy Hole Band Light Hole Band ΔSO Split-off Band

Response Mechanism I E escape Free Carrier Absorption Light/Heavy Hole Band Split-off Band k EF ΔL/H Heavy Hole Band Light Hole Band ΔSO 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 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.

Detector Structure (HE0204) i p+ hν Δ After processing Substrate ~1000 A Metal p GaAs AlGaAs ++ + n Periods Top Contact Barrier Emitter

Detector Structure (HE0204) 400 μm Au contact layers Top Contact p++ GaAs <2.5μm ~1.5mm N Periods GaAs (barrier) p+ GaAs (emitter) Homojunction Heterojunction AlGaAs (barrier) p+ GaAs (emitter) Bottom Contact p++ GaAs Substrate

Absorption (HE0204) Absorption Wavelength (mm)

Split Off Response (HE0204) Mechanism III Mechanism II/IV Responsivity (A/W) Wavelength (mm)

Quantum Efficiency of Detector “1332” 50K Reducing threshold to 10 µm should increase operating temperature to ~200 K Increasing emitter doping should further increase T

Advantages h+ h+ h+ Δ Δ ESO ESO ESO Δ Δ Δ Increased operating Temperature Use of the split-off band provides increased absorption at short wavelengths Increased escape due to high carrier energies Increased gain due to impact ionization from high energy carriers h+ i p+ h+ i p+ h+ i p+ Δ Δ Dark Current ~e-Δ/kT ESO ESO ESO Δ Δ Δ

Advantages Different material will cover different split-off ranges Antimonides – 1-2 µm Arsinides – 3-5 µm Phosphides – 8-15 µm Nitrides – 40-60 µm

Conclusion SO detector shows improved response Optimized devices should operate near room temperature for 3-5 µm detector Use of other materials will allow tailoring of the response range

End

Advantages over other 3-5 µm detectors Arsenides will be used for this range Have advantage of allowing integrated electronics Present Detector Advantage Proposed Split-off Detector InSb 77 K Operating Temperature 300 K HgCdTe 77-240 K ~4% Bad Pixels (256x256) Uniformity ~0.1% Bad Pixels (600x512) PbSe Threshold depends on Temperature Better Stability Threshold fixed by split-off energy

Advantages over other 8-14 µm detectors Phosphides will be used for this range Present Detector Advantage Proposed Split-off Detector Quantum Wells 77 K Narrow Response Operating Temperature Response width 200 K Broadband Response HgCdTe

Advantages over other 30-60 µm detectors Nitrides will be used for this range Nitrides are radiation hard, allowing high background operation Present Detector Advantage Proposed Split-off Detector BIB Detectors 4.2 K Operating Temperature 77 K Ge:Ga Extrinsic Split-off detectors will be much faster than thermal detectors

Differences from other approaches Intrinsic Detectors (InSb, HgCdTe) Transition is entirely in hole bands Quantum Wells Carrier energies are continuous not quantized SO Response is inherently broadband