Low Noise Avalanche Photodiodes

Low Noise Avalanche Photodiodes John P. R. David

Electronic & Electrical Engineering University of Sheffield, U.K.

Talk TalkOutline Outline FF APD APD background background FF Low Low noise noise mechanisms mechanisms in in + + thin thin pp+-i-n -i-n+ss FF Temperature Temperature dependence dependence FF APD APD speed speed FF Conclusions Conclusions Low Noise Avalanche Photodiodes

National Centre for III-V Technologies, University of Sheffield, UK HH Established Establishedin inDepartment Departmentof ofElectronic Electronic&&

180 miles

Electrical ElectricalEngineering, Engineering,University Universityof of Sheffield Sheffieldin in1978 1978 HH Mission: Mission Mission:To Toprovide provideIII-V III-Vwafers wafers&& devices devicesto tothe theUK UKacademic academiccommunity community HH Current Capability CurrentCapability: Capability:22MBE, MBE,33MOVPE, MOVPE, Device DeviceFabrication, Fabrication,Characterisation Characterisation HH Staff: Staff Staff:10 10scientists, scientists,66technicians technicians HH Growth output Growthoutput: output:750 750wafers/year wafers/year HH Optical devices Opticalwafers wafers& &devices: devices:Lasers, Lasers,LEDs, LEDs, VCSELs, VCSELs,RC-LEDs, RC-LEDs,waveguides, waveguides, modulators, modulators,AFPMs, AFPMs,pins, pins,APDs, APDs,Q-Dot Q-Dot lasers, lasers,Q-Cascade Q-Cascadelasers lasers HH Electrical devices Electricalwafers wafers& &devices: devices:HBTs, HBTs, HEMTs, HEMTs,diodes diodes

Low Noise Avalanche Photodiodes

APD Research at Sheffield Physics • Impact ionization coefficient investigation • New materials and novel structures • ‘Dead-space’ characterization • Excess noise m easur ements • Temperature dependence • Analytical and numerical m odelling • Low -noise, high -speed avalanche photodiodes • Single photon avalanche photodiodes ( SPADs) Devices Low Noise Avalanche Photodiodes

Generic communication system Transmitter input data

Signal carrying medium

Receiver output

bits/s Repeater spacing X km

Communication capacity - Bit rate length product = repeater spacing*data rate {X [km]*Bandwidth [b/s]} Using an APD can increase repeater spacing Low Noise Avalanche Photodiodes

p-i-n vs. APD

Photocurrent

APD operating range

p-i-n operating range

Reverse Bias Voltage

p-i-n ] low voltage ] bias insensitive ] temperature insensitive ] simple bias circuit ] fast ] simple bias circuit ] cheap APD ] high sensitivity ] single photon detection (possibility) Low Noise Avalanche Photodiodes

Technology Comparison Photomultipliers + High gain (~106) + Low dark current + Low noise ? Reliability - Expensive - Poor efficiency - Complex filters - Bulky - Fragile - High voltage (~1kV)

Avalanche photodiodes + Inexpensive + Compact + Rugged + High detectivity + High reliability + Simpler, cheaper filters + Reasonable gain + High efficiency + Low voltage (50) 50) (< ) ( f3dB (Emmons) for all w and all M

Ü

Enhanced carrier speed dominates dead space Low Noise Avalanche Photodiodes

GBP comparison

gain-bandwidth product (GHz)

GBP (Monte Carlo) > GBP (Emmons) > GBP (vsat + dead space) 1000

v

Monte Carlo á more rapidly as w â than Emmons and (vsat + dead space)

v

GBP enhancement á as w â

v

Worst case is using (vsat + dead space)

Monte Carlo model Emmons’ model vsat + dead space

100

0.1

1 w (µm)


Published experimental f3dB ¤

¤

Lenox et al. (PTL 1999) measured f3dB of InAlAs RCE APDs ¤w

= 400 nm GBW = 130 GHz

¤w

= 200 nm GBW = 290 GHz

GBP200nm > 2 × GBP400nm Ü

Emmons’ model predicts GBP200nm = 2 × GBP400nm

Ü

But larger d/w in w = 200nm device slows frequency response

Ü

Suggests v200nm > v400nm Low Noise Avalanche Photodiodes

Conclusions H The excess noise decreases as the avalanche width decreases below 1µm, in disagreement with the theory of McIntyre H The low noise results from a more deterministic impact ionization process at high fields as dead space becomes more important H The carrier type initiating the multiplication becomes unimportant at high fields H Thin avalanching regions should be less temperature sensitive H Thin avalanching regions should be capable of high speed operation H 40 Gb/s APDs highly probable


Acknowledgements Graham , GrahamRees, Rees,Peter PeterRobson, Robson,Richard RichardTozer Tozer, Bob , Geoff BobGrey, Grey,Mark MarkHopkinson Hopkinson, GeoffHill, Hill,J.S. J.S.Roberts Roberts J.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, Groves,D.J. D.J.Massey, Massey, B. , C.N. B.Jacob, Jacob,P.J. P.J.Hambleton Hambleton, C.N.Harrison, Harrison,M. M.Yee, Yee, D.S. , C.K. , R. , K.F. , G.M. D.S.Ong Ong, C.K.Chia Chia, R.Ghin Ghin, K.F.Li, Li,S.A. S.A.Plimmer Plimmer, G.M.Dunn Dunn

IEEE IEEELEOS, LEOS,EPSRC, EPSRC,DERA, DERA,EU EU


UV Detection

UV-enhanced Si photodiodes ll Applications Applicationsrequiring requiring

UV UVdetection detection 2 2Atmospheric AtmosphericUV UVremote remote sensing sensing 2 2UV UVastronomy astronomy 2 2Combustion Combustioncontrol control 2 2Detection Detectionof offire, fire, corona coronadischarge dischargeon onHV HV lines lines 2 2Aircraft Aircraft& &missile missile detection detection


Motivation Why ? WhySiC SiCfor forUV UVAPDs APDs? llWide -SiC) Widebandgap bandgap(3.25eV (3.25eVfor for4H 4H-SiC)

⇒ ⇒excellent excellentfor forUV UVdetection detection ⇒ ⇒very verylow lowdark darkcurrent current ⇒ ⇒high hightemperature temperatureoperation operation

llLarge -SiC Largeβ/α β/αratio ratioin in4H 4H-SiC

⇒ ⇒desirable desirablefor forthick thickAPD APDstructures structures ⇒ ⇒performance performancein inthin thinstructures structuresunknown unknown


4H-SiC Device Structures

ll2° 2°+ve +vebevel beveledge edge&&multistep multistepjunction junctionextension extensiontermination termination

2 llSquare Squaremesas mesaswith withareas areas: :50 50××50 50~~210 210××210 210µm µm2 llPassivated Passivatedwith withSiO SiO2 2&&SiN SiNx x

llAl/ Al/Ti Titop topcontact contactwith withoptical opticalaccess access


Responsivity at Unity Gain, Beveled APDs 160

Responsivity (mA/W)

140 120

102

70%

101

60%

ll Similar Similarto totypical typical6H-SiC 6H-SiC

100

100

50% 250

80

300

350

400

40%

60

30%

40

20%

20

10%

photodiodes photodiodes ll Responsivity Responsivitycutoff cutoffat at ~380 ~380nm nm⇒ ⇒visible-blind visible-blind ll Peak Peakresponsivity responsivityof of144 144 mA/W mA/Wat at265 265nm nm⇒ ⇒ quantum quantumefficiency efficiencyof of~~ 67% 67%

0 230

250

270

290

310

330

350

370

Wavelength (nm) Low Noise Avalanche Photodiodes

Reverse IV Characteristics Beveled APDs, 160×160µm2 297 nm 365 nm 230 nm

10-4 10-5

Reach-Through APDs, 150×150mm2 10-4 297 nm -5 365 nm 10 230 nm 10-6

Current (A)

Current (A)

10-6 10-7 10-8 10-9

Dark

10-10

10-7 10-8 10-9

Dark

10-10

10-11

10-11

10-12 0

10

20

30

40

50

60

0

Reverse bias voltage (V)

20

40

60

80

100

120


ll Avalanche Avalanchebreakdown breakdownisissharp sharp&&well-defined well-definedat atVVbdbd==58.5V 58.5V&&124.0V 124.0V ll Carriers Carriersinjected injectedwith with230 230~~365 365nm nmlight lightto toinitiate initiatemultiplication multiplication ll II is 1 ~ 3 orders of magnitude > Idark ph ph is 1 ~ 3 orders of magnitude > I dark

ll AC ACmeasurements measurementscorroborate corroborateDC DCresults results


Multiplication Characteristics 20 18 16 14 12 10 8 6 4 2

Reach-Through APDs

Multiplication factor, M


Beveled APDs

365nm 297nm 265nm 250nm 240nm 230nm

25

30

35

40

45

50

55

60


20 18 16 14 12 10 8 6 4 2

365nm 297nm 265nm 250nm 240nm 230nm

50

60

70

80

90

100 110 120


ll M Mof of>>200 200measured measured ll M Mat atvarious variousλλmore moredisparate disparatefor forthicker thickerAPD APDstructure structure ll Smaller SmallerM Mfrom fromshorter shorterλλ

⇒ ⇒M Mhh>>M Mee⇒ ⇒ββ>>αα


Excess Avalanche Noise Characteristics Beveled APDs

Reach-Through APDs

25

60

¢ 230 nm ˜ 365 nm

Excess noise factor, F


30

k = 0.8

20 15 10 5

k = 0.1 10

20

30

40

50


¢ 230 nm ˜ 365 nm

k = 2.8

50 40 30 20

k = 0.15

10 5

10

15

20

25

30

35

40

45


ll Excess Excessnoise noisemeasured measuredfor forM M>>40 40

⇒ ⇒good goodquality qualityof ofAPDs, APDs,very verystable stableavalanche avalanchemultiplication multiplication ll Very Verylow lowexcess excessnoise noiseof ofkk==0.1 0.1&&0.15 0.15measured measuredwith with365 365nm nmlight light ll Excess Excessnoise noisefrom fromelectron electroninjection injection(230 (230nm) nm)gave gavekk==0.8 0.8&&2.8 2.8 Low Noise Avalanche Photodiodes

20 16 12

230 nm

8 4 25

30

35

40

45

50

55

60


20

k=0.4

10

365 nm

20


12

4H-SiC



Comparison with Si & Al0.8Ga0.2As k=0.3

k=0.2

Si and Al0.8Ga0.2As

8

k=0.1 6 4

k=0

2 10

20

30

40

50

60

70


16 12

Al0.8Ga0.2As

Si

8

Me

2

4

Al Al0.80.8Ga Ga0.20.2As Asrespectively respectively ll M Me &&M Mh closer closerfor forSi, Si,Al Al0.8 Ga Ga0.2 As As

Mh

4

0

ll V Vbdbdof of4H-SiC 4H-SiCisis10× 10×&&5× 5×of ofSi Si&&

6

e

8

10


12

14

0.8

h

0.2

ll 4H-SiC 4H-SiC⇒ ⇒lowest lowestexcess excessnoise noisein inaa

ww==0.1 0.1µm µmstructure structure


Conclusions

+ + 4H-SiC 4H-SiCAPD’s APD’sexhibit exhibitgood goodvisible-blind visible-blind performance performance + + Photomultiplication Photomultiplicationcharacteristics characteristics ⇒ ⇒Large LargeM Min inexcess excessofof 200 200measured measured ⇒ ⇒show showunambiguously unambiguouslythat thatββ >>αα ⇒ ⇒β/α β/α ratio ratioremains remainslarge largein inshort shortdevices devices

+ + Very Verylow lowexcess excessnoise noiseof ofkk==0.1 0.1achieved achievedwith with

mainly mainlyholes-initiated holes-initiatedmultiplication multiplication + + 4H-SiC 4H-SiCisisaasuitable suitablematerial materialfor forhigh-gain, high-gain,low low noise noiseUV UVavalanche avalanchephotodiodes photodiodes Low Noise Avalanche Photodiodes

Al0.8Ga0.2As : A Very Low Excess Noise Multiplication Medium for GaAs-based APDs

Motivation l AlxGa1-xAs material system is widely used in HBTs and IMPATTs l Use in telecom wavelength APDs limited by lack of lattice-matched material that absorbs at long wavelength l

GaInAsN has recently been demonstrated 1 absorbs long wavelength 1 lattice-matched to AlxGa1-xAs

l

GaAs-based APDs is possible and may require AlxGa1-xAs multiplication region for optimum performance


Al0.8Ga0.2As : A Very Low Excess Noise Multiplication Medium for GaAs-based APDs Device structures

Homojunction Homojunctionp-i-n/n-i-p p-i-n/n-i-p grown grownby byconventional conventionalMBE MBE with withw w==11µm µm ll 11heterojunction heterojunctionp-i-n p-i-nwith with w=0.8 w=0.8µm µmto toobtain obtainM Mee& &M Mhh from fromsame samediode diode ll Optical Opticalaccess accesswindow window fabricated fabricatedby bywet wetetching etching ll

Pure Purecarrier carrierinjection injectionobtained obtained with with442nm 442nm& &633nm 633nmlight light ll 542nm 542nmlight lightused usedto toproduce produce mixed mixedcarrier carrierinjection injection ll



Avalanche excess noise of homojunction p-i-n diodes 12

Me Mmixed

10

k=1

9


11


10

9 8 7 6 5 4 3 2

8 7 6 5 4 3

k=0

2

1

1

0

10

20

30

40


• Lower M for mixed carrier injection ⇒ α > β

50

2

4

6

8

10 12 14 16 18 20


• k~ 0.19 for electron multiplication • Larger F for mixed carrier injection Low Noise Avalanche Photodiodes


12

10

11

9

0.1µm

10

1µm

9 8 7 6 5 4 3 2



Avalanche excess noise of thin diodes k=1

8 7 6 5 4 3

k=0

2

1

1 2

3 4 5 6 7 8 10

20 30 40 60


• Vbd ↓ with decreasing w

2

4

6

8

10 12 14 16 18 20


• Comparable excess noise for bulk and thin diodes Low Noise Avalanche Photodiodes


Comparison with InP -based APDs ki=1

8

Commercial CommercialInP-based InP-based APD APDgive giveexcess excessnoise noiseof of kki=~0.7 with hole i=~0.7 with hole initiated initiatedmultiplication multiplication ll Much Muchlower lowerexcess excessnoise noise can canbe beobtained obtainedwith with Al Al0.80.8Ga Ga0.20.2As Asas asavalanche avalanche medium medium


ll

7 6

Commerical InP-based APD Fujitsu VN206

5 4 3 2

Al0.8Ga 0.2As

ki=0

1 1 2 3 4 5 6 7 8 9 10 11 12

Multiplication factor, M Low Noise Avalanche Photodiodes

Al0.8Ga0.2As : A Very Low Excess Noise Multiplication Medium for GaAs-based APDs Comparison with lower aluminium AlxGa1-xAs 1.00 0.80 0.70

Commercial InP-based APDs

0.60

ki

0.50 0.40 0.30

0.20

Al As (x ≤ 0.6) has large AlxxGa Ga1-x 1-xAs (x ≤ 0.6) has large avalanche avalancheexcess excessnoise noise ll Excess Excessnoise noiseof ofAl Al0.80.8Ga Ga0.20.2As As isismuch muchlower lower ll Al Al0.80.8Ga Ga0.20.2As Asalso alsohas haslower lower excess excessnoise noisethan thanaa commercial commercialInP-based InP-basedAPD APD ll At AtM=10, M=10,excess excessnoise noiseof of Al Al0.80.8Ga Ga0.20.2As Asisisat atleast least22times times lower lower ll

0.15 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Aluminium composition, x Low Noise Avalanche Photodiodes


-1

Impact ionization coefficients (cm )

Ionization coefficients Alx Ga1-xAs (x=0, 0.15, 0.3, 0.6)

105

llLarge Largeα/β α/β ratio ratioas as

compared As comparedto toAl AlxxGa Ga1-x 1-xAs of oflower lowerxx ⇒ ⇒Lower Lowerexcess excessnoise noise

104

103

llβ/α β/αratio ratioof ofInP InPis issmall small

⇒ ⇒Higher Higherexcess excessnoise noise

102

Al0.8Ga0.2As 101

1x10-6

2x10-6 3x10-6 4x10-6 5x10-6 Inverse electric field (cm/V) Low Noise Avalanche Photodiodes

Al0.8Ga0.2As : A Very Low Excess Noise Multiplication Medium for GaAs-based APDs Conclusions

+ Ga 0.2As +Bulk BulkAl Al0.8 Asdiodes diodesgive givelower lowerexcess excess 0.8Ga0.2 noise As (x ≤ 0.6) or InP noisethan thanAl AlxxGa Ga1-x As (x ≤ 0.6) or InP 1-x

+ +Consequence Consequenceof ofthe thelarger largerα/β α/βratio ratioin in Al Ga 0.2As Al0.8 As 0.8Ga0.2

+ +Low Lownoise noise APDs APDsmay maybe beachievable achievableon on GaAs GaAs substrates Ga 0.2As substratesusing usingAl Al0.8 Asas asthe thegain gain 0.8 Ga0.2 medium medium


APD noise measurement system

Excess Excessnoise noise

+ + 10 10MHz MHzcenter centerfrequency frequency + + ENBW ENBWof of44MHz MHz + + AC ACtechnique techniquewith with

modulated modulatedlight lightsource source + + FFfrom fromM M& &noise noisepower power + + Shot Shotnoise noiseof ofSi Si p-i-n p-i-nas as reference reference
