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Night Vision

Transmission ModePhotocathodes

Covering the Spectral Range6/19/2002

New Developments in Photodetection3rd Beaune Conference June 17-21, 2002

Arlynn Smith, Keith Passmore, Roger Sillmon, Rudy BenzITT Industries - Night Vision

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Introduction• Goal: Produce negative electron affinity transmission mode cathodes covering

a broad spectral range and possessing high quantum efficiency• Low dark count and low cost are secondary goals

• Results are transmission mode photocathodes, semiconductor bonded to glass

• Photocathode material characterization tools• Material composition, doping density (SIMS, Transmission)

• Photogenerated carrier decay rate (TRPL)

• Photocathode material systems investigated and the spectral ranges• GaAs

• Standard spectral range (450 nm - 880 nm)

• UV GaAs (250 nm - 880 nm)

• InGaAs• 960 nm peak (500 nm - 990 nm)

• 1060 nm (600 nm - 1100 nm)

• InGaP (high QE above GaAs)• 400 nm - 680 nm

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Review of Photocathode Physics• Photons are absorbed in semiconductor regions creating electron

hole pairs

• The electrons must diffuse to the emission surface to be emitted tovacuum

• Electrons recombine in the bulk with a characteristic rate

• Electron recombination at a surface is described by surfacerecombination velocity (front surface recombination velocity {FSRV})

• Emission probability at the vacuum surface is the final process toeffect the photoresponse of a negative electron affinity photocathode

• In all cases, except the recombination velocity, the parameters shouldbe maximized to maximize photoresponse

• Presenting results of applying a design methodology for attempting tomaximize photoresponse

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Entrance Windows Studied• 7056 - Standard glass faceplates for Night Vision systems

• Fiber optics of similar glass are also in common use

• Thermal coefficient of expansion 51.5x10-7 cm/cm/ºC

• 9741 - UV glass for higher blue response• Thermal coefficient of expansion 38.0x10-7 cm/cm/ºC

• Sapphire - in an exploratory bonding and epitaxial growth phase

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250 300 350 400

Wavelength (nm)

Nor

mal

ized

Tra

nsm

issi

on

7056 Glass9741 Glass

Glass fluorescence

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Characterization of Photocathode Material• Monitor normal elemental constituents

• Check for known lifetime limitingconstituents

• Checks for diffusion of dopant species

Wavelength (nm)

800 850 900 950 1000 1050 1100

% T

rans

mis

sion

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1.425 eV, GaAs 1.286 Ev, 9.7% In 1.214 Ev, 14.7% In

• Verify cutoff wavelength of material

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Photogenerated Carrier Decay Rate• Photo-excite double layer heterostructure with fast laser pulse and observe

light emitted from the material

• Rate of decay is an indication of minority carrier lifetime.• Time resolved photoluminescence (TRPL), inverse of decay rate can be thought of as lifetime

• Lifetime and mobility are factors in diffusion length

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0.0E+00 1.0E-08 2.0E-08 3.0E-08 4.0E-08 5.0E-08Time (sec)

Phot

on C

ount

Laser 1 High PowerLaser 1 Low PowerLaser 2 High PowerLaser 2 Low PowerLaser 3 High PowerLaser 3 Low Powerfit 11.3 nsecfit 8.1 nsec

• Using power can saturatetrap levels

• Using laser wavelengthscan probe different depths

• Want all curves to producesame decay rate andpossess a long timeconstant

• Can be performed onbonded and unbondedstructures

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TRPL Lifetime for GaAs Photocathodes• Minority carrier lifetime depends upon intrinsic material properties

• Band to band and Auger coefficients

• Lifetime also has a dependence upon the grown structure• Doping dependence

• Material quality

• Interface quality

• Typical values for GaAs• Low doped material 11 nsec

• Typical photocathode 1.5 to 3 nsec

• Poor quality photocathode < 300 psec to 1 nsec

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GaAs Photocathode with QE > 50%

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400 450 500 550 600 650 700 750 800 850 900 950Wavelength (nm)

Qua

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Effi

cien

cy (%

) • Material improvement forlow FSRV

• Lessons learned from TRPLexperiments

• UHV process improvements

• Non-standard UHVproduction process

• Now can the blue portion ofthe spectrum be enhancedby using a differentphotocathode structure

Field strength> 40 kV/cm

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FSRV and Decay Rate Effect QE Shape

• High FSRV effects blueportion of spectrum fortransmission mode

• integrated photoresponsewas 3X lower

• TRPL lifetime was 6Xlower due to contaminatedgrowth source

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QE

Hi FSRV, Low TRPL GaAsBest GaAs

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Extended Blue GaAs Photocathode

• Thinned the AlGaAs window layer forimproved blue transmission

• Thickness can not be reduced to zeroas FSRV increases

• Applied most of the lessons learnedfrom best GaAs photocathodes

• Used production process• Lower response but repeatable

• Lower red response is due to using athinner photocathode

• Still not getting to cutoff of 7056 glass• Fall off in blue is 50 nm to soon

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Wavelength (nm )

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Effi

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cy (%

)

Extended blue 2

Typical GaAs

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Transmission Mode UV GaAs Design• First attempt was a windowless GaAs structure (similar to UV Silicon systems)

• Failed for low photoresponse

• Can GaAs process be improved to the glass cutoff for UV response

• Refine structure from “Spectral Characteristics of GaAs Solar Cells Grown by LPE” in JEMVol. 28 #1, pp. 35-38, 1999.

• Higher doping concentrations possible using MOVPE compared to LPE

7.0E-01

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0.0E+00 5.0E-09 1.0E-08 1.5E-08 2.0E-08 2.5E-08

Distance from Faceplate (m)

Con

duct

ion

Ban

dedg

e (e

V)

100 A {4e18}, 100 A {4e18}100 A {1e19}, 100 A {4e18}50 A {1e19}, 100 A {4e18}50 A {1e19}, 100 A {1e19}50 A {1e19}, 50 A {1e19}50 A {3e19}, 50 A {1e19}

GaAs thickness {doping}, AlAs thickness {doping}

• Trade off absorptioncharacteristics for FSRVreduction

• Settled on structurebetween red and black

• Based on current dopingcapability

• Going to be very sensitiveto FSRV

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Reached Cutoff Wavelength of Glass

• TRPL lifetime of 2.42 to 2.48nsec before bonding to glass(roughly standard values)

• Response in the 275 nm to 375nm higher than extended blue

• 375 nm to 475 nm responselower

• Postulated due to FSRV pullingresponse down

• If true then higher responsepossible with higher doping orlower FSRV

• Now attempt to push cutoffwavelength by using differentglass

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275 375 475 575 675 775 875

Wavelength (nm)

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QE

(to 6

00 n

m)

UV GaAs Structure

Extended Blue GaAs

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UV GaAs Structure on 9741 Results• Result was low integrated photoresponse, typically 3X lower than the

same structure on 7056 glass

• Spectral response shows very low blue response• Shape is indicative of high FSRV

• TCE mismatch of faceplate/semiconductor puts GaAs in tension• Widens the bandgap and decreases doping effectiveness

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(to p

eak

QE)

7056 Glass Faceplate9741 Glass Faceplate

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High QE GaAs Photocathode Summary• Have demonstrated QE in excess of 50% for GaAs

photocathodes over a broad spectral range

• Broadened the spectral range to 250 nm - 900 nm throughthe use of photocathode design

• First attempts to reach 200 nm were unsuccessful due tophysical properties of glass• Looking for glass with UV transmission, suitable physical

characteristics and TCE equal to or greater than GaAs

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Extended NIR Photocathodes

6% In 16% In

• Move to the longer wavelength side of the GaAs spectrum

• Substitution of InGaAs for GaAs material in cathode (no other changes)• In concentration controls cutoff wavelength (960, 1060 nm)

• High In concentration leads to higher lattice mismatch in epitaxial layer• Chose material system due to cost and experience with the GaAs epitaxial growth

• Lattice mismatch creates defects which reduce minority carrier properties• Substrate orientation can also play a role in lattice mismatch

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TRPL Lifetimes for InGaAs SamplesTRPL RT broad band PL

Cathode avg. τ τ τ τ (ns) Avg. Intensity (rel.)Std. GaAs 2.00 200

1.5% In 2.00 ---6% In Structure 1 0.32 10**6% In Structure 2 0.42 19**8% In Structure 3 1.07 100**8% In Structure 3 0.75 75**16% In Structure 4 <0.12 Near noise level16% In Structure 5 <0.12 Near noise level16% In Structure 6 0.28 1 ---16% In Structure 6 0.125 2 ---

1 substrate orientation 1 2 substrate orientation 2** uncorrected for detector response relative to GaAs

• TRPL lifetimes sensitive to:• Indium concentration

• Structure• Buffer layer

• Doping concentration

• Growth conditions

• Substrate

• Optimization results shown inTable, best structuresprocessed into tube

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Results of Extended NIR Photocathodes

• Peak QE of 18% on 960 nmphotocathode structure (13% at 960nm)

• Peak QE of ~5% on 1060 nmphotocathode structure (1% at 1060nm)

• Further InGaAs material optimizationpossible

• UHV processing optimizationpossible

• Lower band gap and low lifetimeaffecting performance

• Switching to lattice matched materialcould improve minority carrierproperties

• Lattice matched material would alsoremove visible cross hatch pattern0.01

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960 nm Response Tube 1960 nm Response Tube 2960 nm Response Tube 31060 nm Response TubeS1 PhotocathodeHigh QE GaAs

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Preliminary InGaP Material Results• Why use InGaP:

• Wider band gap should give higher QE over smaller spectral range

• Lattice matched to GaAs

• First hurdle was growth conditions for lattice match to GaAs

• Second step is structure with acceptable TRPL and PL

Structure TRPL PL IntensityFree surface/InGaP <0.2 1 Window Structure 1 <0.2 10 Window Structure 2 <0.2 100 Window Structure 3 <0.2 1000

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Initial InGaP Photocathode Results

• Spectral response is again indicative of a high FRSV or lowminority carrier lifetime

• Does not eliminate the possibility of using material in reflectionmode photocathodes

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Effi

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)Surface Etch 1Surface Etch 2

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Conclusions• High QE transmission mode GaAs photocathodes are possible

• Can be extended into the UV range• Further work must be performed to find suitable faceplate

• Lattice mis-matched InGaAs can provide extended red transmissionmode photocathodes

• Lattice matched material could improve performance

• Initial InGaP transmission mode photocathodes processed• Quantum efficiency indicative of low diffusion length or high FSRV• Experiments planned for reflection mode tests

• Confirm structure is FSRV limited

• Design experiments to investigate methods of reducing FSRV fortransmission mode cathodes