Low Noise Avalanche Photodiodes

Low Noise AvalanchePhotodiodesJohn P. R. DavidElectronic & Electrical EngineeringUniversity of Sheffield, U.K.Talk Outline✦ APD background✦ Low noise mechanisms in inthin p + + -i-n + + s✦ Temperature dependence✦ APD speed✦ ConclusionsLow Noise Avalanche Photodiodes

National Centre for III-V Technologies,University of Sheffield, UK180 miles★ Established in in Department of of Electronic &Electrical Engineering, University of ofSheffield in in 1978 1978★ Mission: To To provide III-V wafers &devices to to the the UK UK academic community★ Current Capability: 2 MBE, 3 MOVPE,Device Fabrication, Characterisation★ Staff: 10 10 scientists, 6 technicians★ Growth output: 750 750 wafers/year★ Optical wafers & devices: Lasers, LEDs,VCSELs, RC-LEDs, waveguides,modulators, AFPMs, pins, APDs, Q-Dotlasers, Q-Cascade lasers★ Electrical wafers & devices: HBTs,HEMTs, diodesLow Noise Avalanche Photodiodes

Generic communication systeminputdataTransmitterSignalcarryingmediumReceiveroutputbits/sRepeaterspacing X kmCommunication capacity - Bit rate length product= repeater spacing*data rate {X [km]*Bandwidth [b/s]}Using an APD can increaserepeater spacingLow Noise Avalanche Photodiodes

p-i-n vs. APDPhotocurrentp-i-noperatingrangeAPDoperatingrangeReverse Bias Voltagep-i-nlow voltagebias insensitivetemperatureinsensitivesimple bias circuitfastsimple bias circuitcheap✽✽✽✽✽✽✽APD✽ high sensitivity✽ single photon detection(possibility)Low Noise Avalanche Photodiodes

Technology ComparisonPhotomultipliersAvalanchephotodiodes+ High gain (~10 6 )+ Low dark current+ Low noise? Reliability+ Inexpensive+ Compact+ Rugged+ High detectivity+ High reliability+ Simpler, cheaper filters+ Reasonable gain+ High efficiency+ Low voltage (

Comparison of three detector typesPIN-FET Ge APD InGaAs APDSensitivity X dBm (X+4) dBm (X+8) dBmCost Moderate Moderate HighWavelength 1.3mm & 1.5mm 1.3mm 1.3mm & 1.5mmReliability 10 11 hrs 10 6 hrs 10 6 hrsu An InGaAs APD requires 8dB less opticalpower to produce the same signal as a PIN-FET.u PIN-FET is cheaper, faster and more reliable.Low Noise Avalanche Photodiodes

p-i-n vs. APDComparison of sensitivitySensitivity ( (dBm dBm)ð APDs can have anextra -8dBmsensitivity c.f. p-i-nð APD advantagereduces as as speedincreasesBit Rate (GB/s)Low Noise Avalanche Photodiodes

InGaAs/InP SAM-APDLow Noise Avalanche Photodiodes

APD VendorsVendors Sensitivity V BD (V) D V BD (V/ C) 3DB bandwidth Misc:(dBm)(GHz)JDSU ERM577 -32 40-70 1.8 10nA@ V BD -1.58.5uA/uW@ V BD -1.5Mitsubishi-33 35-75 0.12 1.9FU319SPAAgere-34 45-70 0.07-0.14 2P173AFujitsu-34 50 0.12 2.5FRM5W232BSNova Crystals - 29-32 0.03 Fused InGaAs/SiNVR251NEC 8501 40-80 0.1 3 7nA@0.9 V BDAlcatel 1914 30-75 3.5 70nA@0.9 V BDSensors Unlimited -24 25-42 6 5nA@ V BD -1SU-10ATRMitsubishi-25 20-40 0.05 7FU321SPAJDSU ERM578 -22 20-40 7NEC 4270 -25 16-32 0.02 8 Superlattice APD(k=0.4)Low Noise Avalanche Photodiodes

Ionization - Threshold energyFor ionization:1) energy and momentum conservation2) minimisation of energyImpact ionization schematicdiagramvalence electronholefinal electronEghot electronE thm em hhm s-om lhE g = Band gapE th = Threshold energy‘minimum energy forionization’For parabolic bandsand equal masses,E th = 1.5 E gLow Noise Avalanche Photodiodes

Multiplication processtimeMultiplication buildup timerequired to achieve M1injectedelectronHigh field region7collectedelectronsM e = 7✪ Multiplication=current out/current in in✪ Avalanche takes timeto to build-upelectronholeLow Noise Avalanche Photodiodes

Excess avalanche noiseMean Multiplication, 35302520151050Multiplication buildup timerequired to achieve MMean Multiplication Noise on Multiplication0 10 20 30 40 50 60‣ An APD can give us usgain.‣ Unfortunately theavalanche noise candegrade the S/N ratio.‣ An optimum value for exists.Reverse bias in VoltsLow Noise Avalanche Photodiodes

An ideal avalanche photodiodeIphotonoise current=2eIBphotoAPDbiasedto Gain = MMIphotoMean SquareNoise current=2eIBMphoto2✏Non ideal APDS = 2eI photo photoBM 2 Fwhere F = excess noise factor✏For an ideal APD F = 1Low Noise Avalanche Photodiodes

BackgroundLog (Power)Mean multiplicationIdealRealReverse bias voltageAPD signalAPD noiseOptimumSNRPreamplifiernoise floorM=1 M optMultiplication factor➲ APD’s rely rely on on internalgain to to improve S/N S/N ratio➲ Impact ionization processstochasticavalanchenoise➲ Excess avalanche noiselimits APD’s maximumuseful gain, M➲ In In bulk structure, largeb/a b/a (or (or a/b) ratio requiredfor for low low excess noiseLow Noise Avalanche Photodiodes

Photodetectors - S/Np-i-n photodiode and amplifierI ph__i 2 AMPA[W] V o( Iph)S=Nequiv. amp. noisesource( i )amp22Avalanche photodiode and amplifierI ph MA[W] V o__i 2 AVAL (M)__i 2 AMPSN=i( MI )2amp+2ph2iAVALLow Noise Avalanche Photodiodes

McIntyre’s avalanche noise theory (1966)F ( M ) = M ( 1 +( 1 − k ) ⎛ M − 1⎞2⎜ ⎟ )k ⎝ M ⎠wherek = β αê a = electron probability of ionization per unit length [m -1 ]ê b = hole probability of ionization per unit length [m -1 ]Assumes:➊ Multiplication process does not dependon carrier history.➋ k = b /a is a constantLow Noise Avalanche Photodiodes

McIntyre’s model for electron injectionF v M eExcess Noise Factor, F5.04.54.03.53.02.52.01.5k = 10.60.40.2k = 0k = b / a k = b / aThe excess noise dependsonly on the ionizationcoefficient ratio (k) and themultiplication value.Larger ionizing carrier typeshould initiate avalanche.1.01 2 3 4 5 6 7 8 910Multiplication, MLow Noise Avalanche Photodiodes

GaAs Ionization CoefficientsField dependence of GaAsionization coefficientsIonization coefficients (cm -1 )10 510 410 3k = b/a = 1 0.9 0.810 6 ba»b0.1mm0.5mm1mmav Most III-V semiconductorshave 0.4 £ k £ 2.5v High excess noise expected,especially at higher electricfields when kfi110 20e+0 2e-6 4e-6 6e-61/E (cm/V)Low Noise Avalanche Photodiodes

Ionization CoefficientsPhotomultiplication (arb. units)M e and M h for 1mm Si and GaAs pins65M e(Si)M h(Si)M e(GaAs)4M h(GaAs)321Simple relationship between a, b andmultiplication characteristics in pins.α =β =1 ⎡ M e − 1⎢W ⎣ Me− M1 ⎡ Mh− 1W⎢⎣ Mh− Mh⎤ ⎛⎥ ln⎜⎦ ⎝⎤ ⎛⎥ ln⎜⎦ ⎝MMMMGaAs & Si have similar V bd butvery different a/b ratios.eehhe⎞⎟⎠⎞⎟⎠0 5 10 15 20 25 30 35Reverse Voltage (V)Low Noise Avalanche Photodiodes

Excess noise in Si and GaAs, M e5.0k=1.00.80.6Excess Noise Factor, F4.54.03.53.02.52.01.5McIntyreGaAsSi0.40.2k=0Ü In thick structures,the excess noise F isdetermined by k, theb/a ratio.Ü Silicon has a small kcompared to GaAs,hence low noise.1.01 2 3 4 5 6 7 8 910Multiplication, MLow Noise Avalanche Photodiodes

Enhancement of Ionization by MQW’sChin et al. (1980)postulated that a largeêEc would enhance aCapasso et al. (1982) reported alarge enhancement in a ?inAlGaAs/GaAs MQW’sp+electronsGaAsDEvDEcAlGaAsEcn+EvElectron ionisation coefficients (/cm)10 510 4Bulk GaAs10 310 2MQWCapasso's data10 10.2 0.3 0.4 0.5 0.6Inverse electric field (x10 -5 cm/V)Low Noise Avalanche Photodiodes

p-i-n diode schematichvVR1µm p+intrinsic1µm n+hvM hM eIntrinsic thickness, w variesfrom 1mm to 0.05mm.Pure M e & M h obtained byilluminating thick p+ & n+layers with shortwavelength illumination.n + -i-p + s also grown toobtain M h more easily.Low Noise Avalanche Photodiodes

Multiplication from GaAs p + -i-n + s4Multiplication factors‣ M e and M h were measuredin different thickness p + -in+ s.Multiplication, M3210.025mm1mm0.5mm0.05mm0.1mmblack: Me, red: Mh‣ Lock-in techniques allow M eand M h to be determined inthe presence of large darkcurrents.‣ M e » M h as ‘w’ decreases,suggesting that a » b00 5 10 15 20 25 30 35Reverse bias (V)Low Noise Avalanche Photodiodes

Excess noise in GaAs p + -i-n + sExcess Noise Factor, FExpected convergencewith decreasing w54321w = 1µmw = 0.5µmw = 0.3µmw = 0.2µmw = 0.1µmw = 0.049µmk = 1 0.6 0.4k = 01 2 3 4 5 6 7 8 910Multiplication, M e0.2Electron initiated noisemeasurements showedunexpected andsignificant noisereduction as w becamesmallerThe excess noisedecreases as wdecreases, instead ofincreasing as kfi1Low Noise Avalanche Photodiodes

Excess noise in GaAs n + -i-p + sExpected convergencewith decreasing wExcess Noise Factor, F54321w = 0.2µmw = 0.1µmw = 0.05µmk = 1 0.6 0.4k = 01 2 3 4 5 6 7 8 910Multiplication, M h0.2The excess noise decreasesas w decreases, instead ofincreasing according to k.Hole initiated noisemeasurements alsoshowed unexpected andsignificant noisereduction as w becamesmallerBehavior cannot beexplained by McIntyretheoryLow Noise Avalanche Photodiodes

Multiplication characteristics in InPMultiplication factors108FujitsuSAM-APDMultiplication, M642w = 0.24mmw = 0.33mmw = 0.48mmw = 0.90mmw = 2.40mmMeasured M e(symbols)e Calculated M e(solide lines)using bulk ionization coefficients00 20 40 60 80 100Reverse bias (V)Low Noise Avalanche Photodiodes

Excess noise factor in InPExcess noise factor, FF(M)15k=2.4Fujitsu SAM APD10k=1decreasing w5k=0.4k=01 10Multiplication, MðððSame symbols as as beforeNoise measured usingwrong (electron) carriertype typeFujitsu SAM-APD givesb/a b/a = 1.4 1.4 \ k eff =eff 0.7 0.7ð Structure with with w = 0.24 0.24gives k eff =eff 0.4 0.4 --muchbetter than than SAM-APDwith with hole hole multiplicationðLow Low noise possible even evenwith with electron injectionwith with thin thin wLow Noise Avalanche Photodiodes

Multiplication characteristics in SiliconMultiplication factorsMultiplication, M108642w = 0.12mmw = 0.18mmw = 0.32mmw = 0.84mm(n + -i-p + )Measurement of M h andM mix on 0.84mm n + -i-p +Measurement of M e andM mix on 0.32, 0.18,0.12mm p + -i-n + s05 10 15 20 25 30Reverse bias (V)Low Noise Avalanche Photodiodes

Local noise model prediction vs. experiment insubmicron Si p + -i-n + sExcess Noise Factor, F108642F e(M e)blue: M eblack: McIntyre modelk = 0.2k = 02 4 6 8 10Multiplication, M4 Local field noisemodel givesincreasing excessnoise from k = 0.4-0.7 as w decreasesfrom 0.32-0.12mm.4 Experiment showsthat F(M e ) howeveris virtually constantat k » 0.2.Low Noise Avalanche Photodiodes

Modeling of thinAPD behaviorLow Noise Avalanche Photodiodes

McIntyre Noise ModelProbabilityof ionizationProbabilityof ionizationProbability density functionof ionization1b1aElectronsHolesx★ McIntyre’s noise modelassumes that a carrier’sionization probability isindependent of distanceprobability density function(PDF) is exponential★ This assumption leads to theMcIntyre expression forexcess noise factor★ Avalanche noise depends onthe b /a ratioxLow Noise Avalanche Photodiodes

Dead Space ModelsProbabilityof ionizationProbabilityof ionizationProbability density functionof ionizationdeadspacedeadspace1a1bxx˜More realistic picture ofionization probabilityshows significant deadspace at high electric field˜Presence of dead spacereduces CoV makesmultiplication moredeterministic less noisy˜A significant dead spacereduces the importance ofthe b/a ratio & the carriertype initiatingmultiplicationLow Noise Avalanche Photodiodes

Monte Carlo Estimation of F• Multiplication viaimpact ionizationw1N eN h=( M + M + ... + M +1 2N − 1MN−2)NM trial = 1 + N e + N h2=+ ... + MN − 1+ MN 2)N2Excess Noise Factor,F=Low Noise Avalanche Photodiodes

Probability distribution of electronionization path lengths ( = 5)Normalised Probability, µm -1Probability density functionof ionization32100.0 0.5 1.0 1.5 2.060High Field: E = 960 kV/cm4020ddLow Field: E = 380 kV/cm = 0.39mmCoV = 0.86 = 0.032mmCoV = 0.31(mm)Ü At low fields ð relativelysmall dead space & lowionization probabilityÜ At high fields ð relativelylarge dead space & higherionization probability ðnarrow ionizationprobability distribution.CoV = stand. dev. in in ll e e//

Distribution of Multiplication for = 51Probability function ofmultipliplicationProbability function of Multiplication, P e (M)0.10.010.0010.00010.10.010.0010.0001w = 0.5µmE = 380 kV/cmF = 2.933w = 0.05µmE = 960 kV/cmF = 2.0080 10 20 30 40 50 60 70✒ There are more highorder multiplicationevents at lowerelectric fields, givingrise to more noiseMultiplication, M eLow Noise Avalanche Photodiodes

Typical path lengths as a function of electric fieldPath length, mm10.10.01Monte Carlo modelresults400 600 800 1000Electric Field kV/cm10 410 310 210 1Scatters/ionization event✒ Scattering becomesless important as theelectric field increases✒ Ionization tendstowards ballisticideal, i.e. like PMTScatters per ionization eventBallistic dead spaced = 2.1eV/qEMean ionization path length Low Noise Avalanche Photodiodes

Excess noise in p + -n diodeshvM e➸Reducing w in p + -i-n + sreduces excess noise.V R1µm p+p / n type1µm n+➸How does increasingdoping i.e. electricfield gradient affectnoise?P or N doping varies from2 x 10 16 cm -3 to 3 x 10 17 cm -3 .Low Noise Avalanche Photodiodes

Simulated excess noise in P + N junctions(... primary electrons are injected into the HIGH field)Simulated excess noise resultsExcess Noise Factor, F15105N d=2x10 16 cm -3N d=5x10 16 cm -3N d=1x10 17 cm -3N d=2x10 17 cm -3The excess noise is reducedby increasing the dopingFor the same total depletionthickness, F(P + N) < F(P + -I-N + )0P-I-Ns: w=1µm & 0.1µm0 5 10 15 20Electron Multiplication, M eLow Noise Avalanche Photodiodes

Simulated excess noise in PN + junctions(... primary electrons are injected into the LOW field)Excess Noise Factor, F15105N A=2x10 16 cm -3N A=5x10 16 cm -3N A=1x10 17 cm -3N A=2x10 17 cm -3The excess noise is againreduced by increased dopingFor the same dopingmagnitude, F(PN + ) isslightly greater than F(P + N)00 5 10 15 20Electron Multiplication, M eLow Noise Avalanche Photodiodes

Experimental results with electron injectionExcess Noise Factor, F432P + N:N D=6x10 16 cm -3PN + :N A=1x10 17 cm -3PN + :N A=3x10 17 cm -3Increasing the doping in thePN + devices reduces noiseExperiment corroborates theory10 5 10Electron multiplication, M eLow Noise Avalanche Photodiodes

Effect of temperaturevariation on APDperformanceLow Noise Avalanche Photodiodes

Temperature dependence of avalanche multiplicationV APD (V)Bias required for M = 4 & M =12 at different temperatures6560555045M=12M=4M = 12M = 4● APD multiplication is verytemperature sensitiveNot a problem when inputsignal is large - BER increaseswhen at the limit ofsensitivity● Breakdown variation is ~0.06-0.2V/°C4035-10 0 10 20 30 40 50 60 70Temperature ( 0 C)Active circuit required tovary bias to ensureconstant multiplicationLow Noise Avalanche Photodiodes

Temperature dependent I-V for 1mm GaAsMeasured current (A)10 -3 Increasingtemperature10 -410 -510 -610 -710 -810 -910 -1010 -111mm GaAs p-i-nPhotocurrentDark current0 5 10 15 20 25 30 35 40 45Reverse bias (V)● Photocurrent, dark currentand breakdown measuredon different thickness GaAsp-i-n diodes, from 20K-500K● Sharp V bd observed at alltemperatures.● Dark currents increase withtemperature● Avalanche multiplicationreduces with increasingtemperatureLow Noise Avalanche Photodiodes

Temperature dependent I-V for 0.5mm & 0.1mm GaAs0.5mm GaAs p-i-n0.1mm GaAs p-i-nMeasured current (A)10 -3 Increasing10 -4temperature10 -510 -610 -710 -810 -910 -1010 -11PhotocurrentDark current0 5 10 15 20Reverse bias (V)Measured current (A)10 -3 Increasing10 -4 temperature10 -510 -610 -710 -810 -910 -1010 -11PhotocurrentDark current0 1 2 3 4 5 6 7 8 9Reverse bias (V)• Similar behavior seen in thinner avalanche width structures• Thinner devices are less affected by changes in temperatureLow Noise Avalanche Photodiodes

Change in V bd with TemperatureBreakdown Voltage (V)w = 1.0 mm, 0.5mm & 0.1 mm45403530252015105w=1µmw=0.5µmw=0.1µm0 100 200 300 400 500Temperature (K)V bd (T) / V bd (300K) (%)120110100908070Percentage change in V bdDecreasing AvalancheWidth0 100 200 300 400 500Temperature (K)The breakdown change ismore significant in thickerstructuresTemperature coefficientdecreases from 0.032V/ o C to0.004V/ o CLow Noise Avalanche Photodiodes

Temperature dependent ionization coefficientsIonization Coefficients (cm -1 )10 510 4 IncreasingTemperature10 3GaAs ionization coefficients1 2 3 4 51/E (x10 -6 cm/V)500KabGaAs300K100K●●●●●Ionization coefficientsderived from multiplicationdataIonization coefficientsdecrease with increasingtemperatureThe change is much larger atlower electric fieldsThinner avalanche widthsoperate at higher electricfieldsPhonon scattering relativelyless important at higherelectric fieldsLow Noise Avalanche Photodiodes

Temperature dependent I-V in thin InPMeasured current (A)10 -410 -510 -610 -710 -810 -910 -1010 -1110 -12InP I-V characteristics10 -210 -3IncreasingphotocurrentI darktemperature0 2 4 6 8 10 12 14 16 18Reverse bias (V)● Similar measurements onw = 0.3mm InP p-i-n from20K-300K● Low dark current and● Sharp breakdownobserved● V bd decreases astemperature decreases● Similar results observedin structure with w =0.5mm.Low Noise Avalanche Photodiodes

InP temperature coefficient of V bdV bd (T)/V bd(300K) (%)InP percentage change in V bd100908070605040w = 0.30mmw = 0.50mmZappa et al.Fujitsu SAM APD0 50 100 150 200 250 300Temperature (K)➩ Lower temperaturecoefficient of breakdownvoltage, h o as w decreases.➩ Zappa et al (IPRM’96): h o ~0.225V/ o C➩ Fujitsu SAM-APD: h o ~ 0.09- 0.15V/ o C➩ w = 0.3mm: h o ~ 0.012V/ o C➩ w = 0.5mm: h o ~ 0.02V/ o CLow Noise Avalanche Photodiodes

Effect of thinavalanching widthson APD speedLow Noise Avalanche Photodiodes

Gain-bandwidth CharacteristicsBandwidth (GHz)10Carrier trappingGain -Bandwidth~120GHzv Bandwidth decreases atlow multiplication -carrier trappingv Bandwidth decreases athigh multiplication -multiple transits11 10Multiplication Gain (M)Low Noise Avalanche Photodiodes

APD speed limitations-multiplication build-up time❖ APD is slow c.f. p-i-n diodes due to multipletransits required for high gains❖ Difficult to achieve 10 Gb/s operation withthick avalanching structureselectronpositionw 10w 1injected 0 M = 3M = 5timetimeLow Noise Avalanche Photodiodes

Thin avalanche region multiplication build-up timeelectroninjectedposition0 w 1M = 3timeelectroninjectedtimew 2

APD limitations - frequency responseFrequency response of APDsfor fixed reverse biasNormalised Gain (dB)0-5-10M = 5B = 8GHzM = 10B = 4GHzM = 1B = 40GHz0.1 1 10 100Frequency (GHz)◆ APD frequency responseapproximates a 1st ordersystem◆ Figure of merit - Gainbandwidth product (GBP)◆ Motivation of thinavalanche regions < 1mm toincrease GBPLow Noise Avalanche Photodiodes

Multiplication-limited bandwidth¯ Factors affecting APD speed: Carrier transit time,RC time constant, carrier diffusion andmultiplication buildup time (f 3dB)normalized bandwidth (2p f 3dBt)10.1Constant GBP [Emmons]a/ b = 1oincreasing a/ b10 100electron multiplication factor, M✪ Conventional (Emmons’) model[Emmons, 1967]✪ Negligible dead space ð d = 0✪ Constant carrier speed ð v = v sat(saturated drift velocity)✪ Constant gain-bandwidthproduct,GBP✪ GBP scales with t , carriertransit timeLow Noise Avalanche Photodiodes

Effect of dead space on speed¬ Comparison of d = 0 cf. nonlocalmodel with d „ 0Impulse current (A)10 -6 without dead spacewith dead space10 -710 -810 -90 10 20 30 40 50time (ps)¯ v = v sat¯ Avalanching regionwidth of w = 0.1mm¯ M = 12.5¯ d ñ, avalanche currentimpulse response decaysmore slowly ð lower f 3dB[Hayat and Saleh, 1992]Low Noise Avalanche Photodiodes

Effects of dead space● f m of APDs with avalanche width of w, a = b and = 20● Compare d/w = 0, 0.2 and 0.3fm (GHz)1050Emmons' analysisd/w = 0d/w = 0.2d/w = 0.30.0 0.5 1.0 1.5 2.0w (mm)❖ Obtain f m by Fouriertransforming the currentimpulse response❖ f m obtained agrees withEmmons’ prediction ford/w = 0❖ As d/w increases, f m fallsbelow the predicted values❖ d/w is larger in thin APDsð absolute decrease in f m islarger in thin APDsLow Noise Avalanche Photodiodes

Carrier speed assumptionsv Monte Carlo model cf. constant v = v sat modelv Same dead space, dImpulse current (A)10 -6 Monte Carlo model10 -710 -810 -9constant v = v sat0 10 20 30 40 50time (ps)¯ w = 0.1mm, M = 12.5¯ Enhanced speed inMC model leads tofaster decay ofcurrent impulseresponse ð higher f 3dB¯ Dead space and enhancedspeed effects compete![Hambleton et al, 2002]Low Noise Avalanche Photodiodes

Simulation result comparisons10w = 1.00mm w = 0.20mm w = 0.05mm100f3dB(GHz)f3dB(GHz)10f3dB(GHz)100Monte CarloEmmons1 10M1 10M101 10MÜÜÜConstant GBPf 3dB(Monte Carlo) > f 3dB(Emmons) for all w and all MEnhanced carrier speed dominates dead spaceLow Noise Avalanche Photodiodes

GBP comparisongain-bandwidth product (GHz)GBP (Monte Carlo) > GBP(Emmons) > GBP (v sat+ deadspace)1000100Monte Carlo modelEmmons’ modelv sat+ dead spacevvvMonte Carlo á morerapidly as w â thanEmmons and (v sat +dead space)GBP enhancement áas w âWorst case is using (v sat+ dead space)0.1 1w (mm)Low Noise Avalanche Photodiodes

Published experimental f 3dB¤ Lenox et al. (PTL 1999) measuredf 3dB of InAlAs RCE APDs¤w = 400 nm GBW = 130 GHz¤w = 200 nm GBW = 290 GHz¤ GBP 200nm > 2 · GBP 400nmÜÜÜEmmons’ model predictsGBP 200nm = 2 · GBP 400nmBut larger d/w in w = 200nmdevice slows frequency responseSuggests v 200nm > v 400nmLow Noise Avalanche Photodiodes

Conclusions★ The excess noise decreases as the avalanche width decreasesbelow 1mm, in disagreement with the theory of McIntyre★ The low noise results from a more deterministic impactionization process at high fields as dead space becomesmore important★ The carrier type initiating the multiplication becomesunimportant at high fields★ Thin avalanching regions should be less temperaturesensitive★ Thin avalanching regions should be capable of high speedoperation★ 40 Gb/s APDs highly probableLow Noise Avalanche Photodiodes

AcknowledgementsGraham Rees, Peter Robson, Richard Tozer,Bob Grey, Mark Hopkinson, , Geoff Hill, J.S. RobertsJ.S. J.S. Ng, Ng, B.K. B.K. Ng, Ng, C.H. C.H. Tan, Tan, K.S. K.S. Lau, Lau, C. C. Groves, D.J. D.J. Massey,B. B. Jacob, P.J. P.J. Hambleton, , C.N. C.N. Harrison, M. M. Yee,Yee,D.S. D.S. Ong, , C.K. C.K. Chia, , R. R. Ghin, , K.F. K.F. Li, Li, S.A. S.A. Plimmer, , G.M. DunnIEEE LEOS, EPSRC, DERA, EUEULow Noise Avalanche Photodiodes

UV DetectionUV-enhancedSi photodiodes● Applications requiringUV detection✒ Atmospheric UV remotesensing✒ UV astronomy✒ Combustion control✒ Detection of of fire,corona discharge on on HVlines✒ Aircraft & missiledetectionLow Noise Avalanche Photodiodes

MotivationWhy SiC for UV APDs?● Wide bandgap (3.25eV for 4H-SiC)⇒ excellent for for UV detection⇒ very low dark current⇒ high temperature operation● Large b/a ratio in in 4H-SiC⇒ desirable for for thick APD structures⇒ performance in in thin structures unknownLow Noise Avalanche Photodiodes

4H-SiC Device Structures●●2 2 +ve +vebevel edge edge & multistep junction extension termination●●Square mesas with with areas :: 50 50 · 50 50 ~ 210 210 · 210 210 mm mm 22●●Passivated with with SiO SiO 2 & 2 SiN SiN xx●●Al/ Ti Ti top top contact with with optical accessLow Noise Avalanche Photodiodes

Responsivity at Unity Gain, Beveled APDs16014070%10 2Responsivity (mA/W)1201008060402060%50%40%30%20%10%10 110 0250 300 350 400● Similar to to typical 6H-SiCphotodiodes● Responsivity cutoff at at~380 nm nm visible-blind● Peak responsivity of of 144 144mA/W at at 265 265 nm nmquantum efficiency of of ~67%0230 250 270 290 310 330 350 370Wavelength (nm)Low Noise Avalanche Photodiodes

Reverse IV CharacteristicsCurrent (A)Beveled APDs, 160·160mm 2 Reach-Through APDs, 150·150mm 210 -4 297 nm10 -4365297 nm10 nm-5230 nm10 -5365 nm230 nm10 -610 -610 -710 -710 -810 -810 -9Dark10 -9Dark10 -1010 -1110 -120 10 20 30 40 50 60Reverse bias voltage (V)Current (A)●●Avalanche breakdown is is sharp sharp & well-defined at at V bd=bd 58.5V 58.5V & 124.0V 124.0V●●Carriers injected with with 230 230 ~ 365 365 nm nm light light to to initiate initiate multiplication10 -1010 -11●●II phisphis 1 ~ 3 orders orders of of magnitude > II dark dark●●AC AC measurements corroborate DC DC results results0 20 40 60 80 100 120Reverse bias voltage (V)Low Noise Avalanche Photodiodes

Multiplication CharacteristicsMultiplication factor, M2018161412108642365nm297nm265nm250nm240nm230nmBeveled APDsMultiplication factor, M2018161412108642Reach-Through APDs365nm297nm265nm250nm240nm230nm25 30 35 40 45 50 55 60Reverse bias voltage (V)50 60 70 80 90 100 110 120Reverse bias voltage (V)● M of of > 200 200 measured● M at at various l more disparate for for thicker APD structure● Smaller M from shorter lM h>h M ebe> aLow Noise Avalanche Photodiodes

Excess Avalanche Noise CharacteristicsExcess noise factor, F30252015105Beveled APDs¢ 230 nmk = 0.8˜ 365 nmk = 0.1Excess noise factor, F605040302010Reach-Through APDsk = 2.8 ¢ 230 nm˜ 365 nmk = 0.1510 20 30 40 50Multiplication factor, M5 10 15 20 25 30 35 40 45Multiplication factor, M●●Excess noise measured for for M > 40 40good good quality of of APDs, very very stable avalanche multiplication●●Very low low excess noise of of k = 0.1 0.1 & 0.15 0.15 measured with with 365 365 nm nm light light●●Excess noise from from electron injection (230 (230 nm) nm) gave gave k = 0.8 0.8 & 2.8 2.8Low Noise Avalanche Photodiodes

Comparison with Si & Al 0.8 Ga 0.2 AsMultiplication factor, MMultiplication factor, M201612844H-SiC20 25 30 35 40 45 50 55 60Reverse bias voltage (V)20161284SiM e365 nm230 nmM hAl 0.8Ga 0.2As0 2 4 6 8 10 12 14Reverse bias voltage (V)Excess noise factor, F12108642k=0.4 k=0.3Si andAl AsAl 0.8Ga 0.2 Ask=0.2k=0.1k=010 20 30 40 50 60 70Multiplication factor, M●●V bdofbdof 4H-SiC is is 10· 10· & 5· 5· of ofSi Si &Al Al 0.8Ga0.8Ga 0.2As0.2As respectively●●M e&e M hcloserh for for Si, Si, Al Al 0.8Ga0.8Ga 0.2As0.2As●●4H-SiClowest excess noise in in aw = 0.1 0.1 mm mm structureLow Noise Avalanche Photodiodes

Conclusions☞ 4H-SiC APD’s APD’sexhibit good good visible-blindperformance☞ Photomultiplication characteristicsLarge Large M in in excess excess of of 200 200 measuredshow show unambiguously that that β > αβ/α β/αratio remains remains large large in in short short devices devices☞ Very Very low low excess excess noise noise of of k = 0.1 0.1 achieved with withmainly mainly holes-initiated multiplication☞ 4H-SiC 4H-SiC is is a suitable material for for high-gain, low lownoise noise UV UV avalanche photodiodesLow Noise Avalanche Photodiodes

Al 0.8 Ga 0.2 As : A Very Low Excess Noise MultiplicationMedium for GaAs-based APDs● Al x Ga 1-x As material system is widely used in HBTs andIMPATTs● Use in telecom wavelength APDs limited by lack oflattice-matched material that absorbs at long wavelength●MotivationGaInAsN has recently been demonstrated✑ absorbs long wavelength✑ lattice-matched to Al x Ga 1-x As● GaAs-based APDs is possible and may require Al x Ga 1-x Asmultiplication region for optimum performanceLow Noise Avalanche Photodiodes

Al 0.8 Ga 0.2 As : A Very Low Excess Noise MultiplicationMedium for GaAs-based APDsDevice structures● Homojunction p-i-n/n-i-pgrown by by conventional MBEwith w = 1 mm mm● 1 heterojunction p-i-n withw=0.8 mm mm to to obtain M e & e M hhfrom same diode● Optical access windowfabricated by by wet etching● Pure carrier injection obtainedwith 442nm & 633nm light● 542nm light used to to producemixed carrier injectionLow Noise Avalanche Photodiodes

Al 0.8 Ga 0.2 As : A Very Low Excess Noise MultiplicationMedium for GaAs-based APDsAvalanche excess noise of homojunction p-i-n p n diodesMultiplication factor, M121110987654321M eM mixed0 10 20 30 40 50Reverse bias voltage (V)Excess noise factor, F10987654321k=1k=02 4 6 8 10 12 14 16 18 20Multiplication factor, M• Lower M for mixed carrierinjection a > b• k~ 0.19 for electron multiplication• Larger F for mixed carrier injectionLow Noise Avalanche Photodiodes

Al 0.8 Ga 0.2 As : A Very Low Excess Noise MultiplicationMedium for GaAs-based APDsAvalanche excess noise of thin diodesMultiplication factor, M1211109876543210.1mm1mm2 3 4 5 6 78 10 20 30 40 60Reverse bias voltage (V)Excess noise factor, F10987654321k=1k=02 4 6 8 10 12 14 16 18 20Multiplication factor, M• V bdfl with decreasing w• Comparable excess noise for bulkand thin diodesLow Noise Avalanche Photodiodes

Al 0.8 Ga 0.2 As : A Very Low Excess Noise MultiplicationMedium for GaAs-based APDsExcess noise factor, FComparison with InP-based APDs8765432CommericalInP-based APDFujitsu VN206k i=1Al 0.8Ga 0.2Ask i=0●●Commercial InP-basedAPD give give excess noise of ofk i=~0.7i with hole holeinitiated multiplication●●Much lower excess noisecan can be be obtained withAl Al 0.8Ga0.8Ga 0.2As0.2As as as avalanchemedium11 2 3 4 5 6 7 8 9 10 11 12Multiplication factor, MLow Noise Avalanche Photodiodes

Al 0.8 Ga 0.2 As : A Very Low Excess Noise MultiplicationMedium for GaAs-based APDsComparison with lower aluminium Al x Ga 1-x As1.00k i0.800.700.600.500.400.300.20CommercialInP-based APDs0.150.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Aluminium composition, x●●Al xGaxGa 1-xAs1-xAs (x (x £ 0.6) 0.6) has has largeavalanche excess noise●●Excess noise of of Al Al 0.8Ga0.8Ga 0.2As0.2Asis is much lower●●Al 0.8Ga0.8Ga 0.2As0.2As also also has has lowerexcess noise than acommercial InP-based APD●●At M=10, excess noise of ofAl Al 0.8Ga0.8Ga 0.2As0.2As is is at at least 2 timeslowerLow Noise Avalanche Photodiodes

Al 0.8 Ga 0.2 As : A Very Low Excess Noise MultiplicationMedium for GaAs-based APDsIonization coefficientsImpact ionization coefficients (cm -1 )Al xGa 1-xAs (x=0, 0.15, 0.3, 0.6)10 5 Al 0.8Ga 0.2As10 410 310 210 1● Large a/b a/bratio as ascompared to to Al Al x Ga x Ga 1-x As 1-x Asof of lower xLower excess noise● b/a b/aratio of of InP is is smallHigher excess noise1x10 -6 2x10 -6 3x10 -6 4x10 -6 5x10 -6Inverse electric field (cm/V)Low Noise Avalanche Photodiodes

Al 0.8 Ga 0.2 As : A Very Low Excess Noise MultiplicationMedium for GaAs-based APDsConclusions☞ Bulk Al 0.8 0.8Ga 0.2 0.2As diodes give lower excessnoise than Al xx Ga 1-x 1-xAs (x (x £ 0.6) or or InP☞ Consequence of of the larger a/b ratio in inAl 0.8 0.8Ga 0.2 0.2As☞ Low noise APDs may be be achievable on GaAssubstrates using Al 0.8 0.8Ga 0.2 0.2As as as the gainmediumLow Noise Avalanche Photodiodes

APD noise measurement systemExcess noise☞ 10 10 MHz center frequency☞ ENBW of of 4 MHz☞ AC AC technique withmodulated light source☞ F from M & noise power☞ Shot noise of of Si Si p-i-n as asreferenceLow Noise Avalanche Photodiodes