n n

n n 1

Page 1

STANFORD

JSH-1

GaInNAs:A new Material in the Race for

Long Wavelength VCSELsJ. S. Harris

S. Spruytte, C. Coldren, M. Larson,M. Wistey, V. Gambin, W. Ha

Stanford University

Agilent Seminar, Palo Alto, CA14 February 2001

STANFORD

JSH-2

Outline

l Introductionl The Choicesl GaInNAs/GaAs Material Propertiesl Work at Stanford

l MBE Growthl Edge-emitting lasersl Pulsed broad area VCSELsl CW oxide-confined VCSELs

l GaInNAs as an Enabling Technologyl Summary

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JSH-3

Exponential Growth inBandwidth Demand

STANFORD

JSH-4

Link Capacity Improvements

n n

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Page 3

STANFORD

JSH-5

Skiing Optical Fiber Networks

After Prof. Alan Willner-USC

Beginner Intermediate AdvancedBunny Extreme

OC-482.5 Gbs

OC-19210 Gbs

OC-76840 Gbs

OC-3072160 Gbs

OC-12622 Mbs

STANFORD

JSH-6

Fiber Distancevs. Bit Rate

0.01

0.1

1

10

100

1000

0.1 1 10 100 1000 104

Attenuation Limited

Dispersion Limited

Gigabit Ethernet

1550nm DFB-SMF

1310nm DFBOr VCSEL-SMF

1310nm FP-SMF

Bit Rate (Mb/s)

Tra

nsm

issi

on D

ista

nce

(km

)

850nm 50µm-MMF

VCSEL1310nm 50µm-MMF

850nm 62.5µm-MMF

10 Gigabit Ethernet

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STANFORD

JSH-7

Need for BandwidthTechnological Trends

l Computer performance continues to follow Moore’s Lawl Doubling of computing power every 18 monthsl Device research indicates exponential growth until 2010 (SIA roadmap)

l Network capacity is also increasing exponentiallyl Gilder’s Law – “Communication Capacity will triple every 12 months”l Growth at a rate greater than Moore’s law requires new technologies

l Required network capacityl Desktop bandwidth needs limited only by the human eye ~2 Gbpsl Internet traffic is non locall Number of users and hosts is growing exponentially

l Need long wavelength, high speed, low cost VCSEL

STANFORD

JSH-8

● Advantages of long wavelengths (>1300nm) relative to shortwavelengths (850nm):1. Fiber system performance

Installed multimode-fiber: optimized for 1300nmStandard singlemode fiber: >1260nm

2. Eye safety3. CMOS voltage scaling compatibility4. Si substrate transparency for integration

● Approach: Long wavelength active region on GaAs substrateLeverage 850nm VCSEL technology

High performance GaAs/AlAs DBRsAlAs-oxide current confinement and optical mode control

Long wavelength VCSELs

n n

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STANFORD

JSH-9

Bandgap vs. Lattice Constantfor III-V Materials

InAs

AlSb

GaSbGeBULKSLE

(Ge)

α–SiC

GaSbInSb

AlSb CdTe

InAs

InN

GaN

AlN

ZnS MgSe

CdSAlP

CdSe

ZnTe

ZnSe

GaAs

GaPAlAs

InPSi

Lattice Constant (Å)

(.41 µm)

(.62 µm)

6.0

4.0

3.0

2.0

1.0

3.0 3.2 3.4 5.4 5.6 5.8 6.0 6.2 6.4

Ban

dgap

ene

rgy

(eV

)

(1.24 µm)

VisibleSpectrum

(.31 µm)

(.21 µm)

STANFORD

JSH-10

● GaNAs bandgap bowing Rapid decrease in bandgap.● N is small● In is large

Material Choices at 1.3µmLattice Matched to GaAs

control tensile/compressive strain on GaAs}

InxGa1-xAsyP1-y

InxGa1-xAs

InP

GaNyAs1-y

InNyAs1-y

980nm

GaxIn1-xNyAs1-y

on GaAs

AlAs

InAs

InxAl1-xAsAlxGa1-xAs

GaAs

0

0.5

1

1.5

2

2.5

5.4 5.5 5.6 5.7 5.8 5.9 6 6.1 6.2

Ban

dgap

(eV

)

1300nm1550nm

Lattice Parameter (Å)

n n

n n 6

Page 6

STANFORD

JSH-11

GaInNAs/GaAs vs.InGaAsP/InP

2. Thermal conductivity●AlAs/GaAs higher thermal conductivity thanternary or quaternary lattice matched to InP●Heat removed through bottom DBR●High termal conductivity DBR required

increasing Ncontent

AlGaAs InGaNAs

1. Larger refractive index differences● n(AlGaAs)> n(InGaAsP)●Easier to make high reflectivity DBR mirrors

3. Larger conduction band offset●Better thermal performance for lasers

4. Better compositional control

N+ substrate

n1

n2

n1

R = r1 + r2 + r3 + …add in phase

n1- n2rn= n1+ n2

STANFORD

JSH-12

Overview MBE System

n Source of elemental Ga(0.1-0.8 monolayers/s)

n Source of elemental In(0.2-0.3 monolayers/s)

n As cracker supplies As2(beam flux = 20 _ totalbeam flux of III-elements)

n Radio frequency (rf)plasma supplies atomic N

n RHEED monitors growthn Substrate temperature

(phase segregation)

n Control nitrogen concentrationn Low concentration of impuritiesn Nitrogen bypassn Sharp QWs

n n

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Page 7

STANFORD

JSH-13

Characterization rf Plasma

n Ratio of intensity atomic nitrogen peak and intensitymolecular nitrogen peak relative amounts of atomic andmolecular nitrogen in plasma.

n Area under the peaks total amount of nitrogen in plasma.

6000 7000 8000 90000

0.05

0.1

0.15

0.2Atomic Nitrogen Transitions

Molecular NitrogenTransitions

STANFORD

JSH-14

Growth of GaN0.05As0.95thermodynamically stable atl low temperature.l high As2 flux.

300 400 500 600 700 800 900 100010

-25

10-20

10-15

10-10

10-5

5% GaN

no phase segregation

phase segregation

MBE Window

Growth Temperature andPhase Segregation

Film grown at 700 °Cl less N because 2NG N2

G.l second phase observed.

65 70 75 80 8510

0

102

104

106

Grown at 500 °C

Grown at 700 °C

n n

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Page 8

STANFORD

JSH-15

Control of NitrogenComposition by MBE

Nitrogen concentration inversely proportional to group IIIgrowth rate Unity N sticking coefficient!!

0 0.5 1 1.5 2 2.5 30

2

4

6

8

10

HRXRD SIMS HRXRD from superlatticeElectron Microprobe Anal

STANFORD

JSH-16

Sharpness of GaInNAsQuantum Wells

l QW boundary : 2-3 ML thickl QW thickness : 22 ML (=6.25 nm)l Dislocation free

GaAs

Ga0.8In0.2N0.015As0.985

65?

GaAs10 Å

n n

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Page 9

STANFORD

JSH-17

PL measurements fordifferent N concentrations

l Luminescence efficiency of InGaNAs decreaseswith increasing N concentration

In0.3Ga0.7NAs QW

In0.3Ga0.7NAs QW

GaAs Barrier

GaAs Substrate

GaAs barrier

13

0?

65

?6

5?

In0.3

Ga0.7

As QW

GaAs Barrier

In0.3

Ga0.7

As QW

GaAs cap

13

0?

65

?6

5?

1 1.1 1.2 1.3 1.40

0.1

0.2

0.3

0.4

0.5InGaAs wells InGaNAs wells

0.6 % Nitrogen

1 at % Nitrogen

STANFORD

JSH-18

Effect of Anneal onPL properties of GaInNAs

1.15 1.2 1.25 1.3 1.35 1.40

0.5

1

1.5

2

2.5

3

3.5

4not annealedannealed

Annealing (RTA 1 min at 760 °C) of GaInNAs:l Improves luminescence efficiency by factor 50.l Shifts the peak to shorter wavelengths by 100 nm.

Anneal during growth of subsequent layers.

GaAs cap

GaAs cap

GaAs Barrier

GaAs substrate

Al0.33

Ga0.66

As cap

Ga0.7In0.3N0.02As0.98 QW

Ga0.7In0.3N0.02As0.98 QW

10

0?

15

0?

10

00

?7

0?

20

0?

70

?

n n

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Page 10

STANFORD

JSH-19

Origin of low PL efficiency:C-V Measurements

l Not annealedGaNAs QWscontain defectsthat trapcarriers.

GaAs 3.1016/cm3 Be

GaAs 3.1016/cm3 Be

GaN0.03As0.97 3.1016/cm3 Be

GaAs p-substrate

200nm

50nm

1000nm

C-V measurements at300 K and 1 MHz

GaAs 3.1016/cm3 Si

GaAs 3.1016/cm3 Si

GaN0.03As0.97 3.1016/cm3 Si

GaAs n-substrate

500nm

50nm

1000nm

100 200 300 4000.0

0.2

0.4

0.6

0.8

1.0annealednot annealed

N C-V

(1017

cm

-3)

Depth (nm)

200 400 600 800 1000 12000.0

0.2

0.4

0.6

0.8

1.0

1.2 not annealedannealed

N C-V

(1017 c

m-3 )

Depth (nm)

l Annealing freescarriers(holes+electrons)in GaNAs QW.

STANFORD

JSH-20

Origin of PL shift:SIMS Measurements

Indium diffusing less than Nitrogen.ñShift of PL in GaInNAs caused predominantly by

nitrogen diffusion.

120 130 140 150 160 170 1800

100

200

300

400

500not annealedannealed

120 130 140 150 160 170 1800

5000

10000

15000not annealedannealed

Indium concentration profile Nitrogen concentration profile

n n

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Page 11

STANFORD

JSH-21

l Nitrogen is not all substitutional.l Amount of non-substitutional Nitrogen is

decreased by annealing.

Counts Aligned <100> Unaligned Ratio

Not annealed 102 390 0.262 ± 0.03

Annealed 59 369 0.160 ± 0.02

Origin of low PL efficiency:Interstitial Nitrogen

l Channeling combined with Nuclear ReactionAnalysis (NRA) to determine location of nitrogen.

l 14N + 3He 16O + proton.l 2.5 MeV 3He analysis beaml 1.5 MeV protons detected

min = 0.06for RBS channeling

STANFORD

JSH-22

In-plane Laser Results

l Broad area diodelaser

l 70 Å Single QW laserl Jth = 450 A/cm2

1.5 µmp-Al0.33Ga0.67As

1.5 µmn-Al0.33Ga0.67As

n+ GaAssubstrate

0.1 µmGaAs

0.1 µmGaAs

7 nmIn0.3Ga0.7N0.02As0.98

quantum well

0

10

20

30

40

50

60

0 1 0 0 2 0 0 3 0 0 4 0 0

Current (mA)

20µm X 840µm deviceJth = 450A/cm 2

=1.22µm

n n

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Page 12

STANFORD

JSH-23

Quantum Well Edge-Emitter Thermal Performance

Wav

elen

gth

(n

m)

Wavelength vs. Temperature

1235

1240

1245

1250

1255

1260

30 40 50 60 70 80 90Heatsink Temperature (˚C)

InGaNAs TQW Edge-EmitterL=800µm, lambda=1.24µm

0.4nm / º C

Th

resh

old

Cu

rren

t (m

A)

Threshold vs.Temperature

120

140

160

180

200

220

240

290 300 310 320 330 340 350 360

Heatsink Temperature (K)

InGaNAs TQW Edge-Emitter L=800µm, lambda=1.24µm

T0 ~ 140K

Ith = I0exp(T/T0)

Ü Good thermal performance: high T0

STANFORD

JSH-24

High Power GaInNAs EdgeEmitting Laser (Infineon/Ioffe)

D.A. Livshits, A. Yu. Ergov and H. Riechert, Electonics Letters 36, 1382 (2000)

n n

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Page 13

STANFORD

JSH-25

Initial VCSEL Results

n+ GaAs substrate

22.5 pair n-GaAs/n-AlAs

DBR

20 pair p-GaAs/p-AlAs

DBR

Ti-Au

GaInNAs/GaAs triple quantum well

active region

substrate emission

50µm

0

4

8

12

16

20

0 100 200 300 400 500

Output power

Voltage

Ou

tpu

t P

ow

er (

mW

), V

olt

age

(V)

Current (mA)

1190 1195 1200 1205 1210

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

l Jth = 2.5 kA/cm2

l Efficiency 0.066W/Al 1.2 m emissionl Pulsed operation

STANFORD

JSH-26

GaInNAs Oxide-ConfinedVCSEL

l Device Structure

Ti/Au reflector/top contact

17 pairs p-GaAs/AlAsprotected from oxidation

22 pair n-GaAs/AlAs DBR

3 GaInNAs/GaAs70Å/200Å active region

λ cavity

3 pairs p-GaAs/AlAsor GaAs/AlOx

SiO2 oxidation cap

bottom n-contact

Substrate emission

Mesas etch#1, SiO2 dep.

Mesas etch#2

Wet oxidation

l Process Highlights

Unoxidized aperture

n n

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Page 14

STANFORD

JSH-27

Oxide-confined GaInNAsVCSEL pulsed characteristics

l 3.6 - 29 µm square aperturesl 0.9 - 20 mA threshold currentl Current density down to 2 kA/cm2

l Slope Efficiency up to 0.09 W/A

0.1

1

10

100

Thr

esho

ld C

urre

nt (

mA

)

Dimension (µm)

0

0.5

1

1.5

2

2.5

0

5

10

15

20

0 10 20 30 40 50

Out

put P

ower

(m

W)

Vol

tage

(V

)

Current (mA)

RT 200ns/100µsλ = 1.2µm

4.3µm

10µm

15µm

29µm

0

1000

2000

3000

4000

5000

6000

7000

0.05

0.06

0.07

0.08

0.09

0.1

0 5 10 15 20 25 30

Cur

rent

Den

sity

(A

/cm

2)

Slo

pe E

ff. (

W/A

)

Dimension (µm)

l Characteristics vs. sizel Typical L-I-V curves

STANFORD

JSH-28

Oxide-confined GaInNAs VCSELRoom Temp. CW Operation

Output Power vs Current

-90

-80

-70

-60

-50

-40

1195 1200 1205 1210 1215

Log

Inte

nsity

(dB

, arb

. ref

.)

Wavelength (nm)

1.3 mA

3 mA

3.5 mA

>40 dB

Spectra (5µm x 5µm)

l 5µm aperture: 1.3 mA threshold current; 3.6µm: < 1 mAl CW operation in spite of very high voltage drop (~9V) in top mirror

0

0.02

0.04

0.06

0.08

0.1

0

2

4

6

8

10

12

14

16

0 1 2 3 4 5

Out

put P

ower

(m

W)

Vol

tage

(V

)

Current (mA)

RT CW

5µm x 5µmIth = 1.32 mA0.0451 W/AVth = 10.3 V

3.6µm x 3.6µm

n n

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Page 15

STANFORD

JSH-29

Oxide-Confined VCSEL:CW Thermal Analysis

l ~60°K temperature rise atpeak power

l Thermal ImpedanceZT 1/diameter

l ZT (5µm) = 1.24 K/mW(ZT, 50µm etched pillar = 0.36 K/mW)

l ZT(5µm) / ZT(50µm) = 3.6l Ith(5µm) / Ith(50µm) = 1/50l Reduce dissipated power in

pDBR for higher outputpower1200

1201

1202

1203

1204

1205

0 10 20 30 40 50

0

10

20

30

40

50

60

Wav

elen

gth

(nm

)

Dissipated Power (mW)

RT CW5µm x 5µm0.0924 nm/mW1.24 K/mW

Tem

pera

ture

Ris

e (°

C)

pulsed 0.2ns/10kHz

STANFORD

JSH-30

Challenge for Longwavelength GaInNAs

l Post annealing process is required to improvematerial quality after growth

l Nitrogen out-diffuses from quantum wellsl Emission spectrum blue-shifts due to nitrogen

loss

n n

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Page 16

STANFORD

JSH-31

Advantages ofGaNAs Barrier

l Decreased carrier confinement Emission atlonger wavelengths.

l Decreased nitrogen out-diffusion during theanneal less shift of peak energy duringanneal (65 meV versus 74 meV).

l Strain compensation possible (GaAsN tensile /GaInNAs compressive).

E1

HH1

λ~1.36µm

GaInNAsQW

GaAs Matrix

E1

HH1

λ~1.44µm

GaInNAsQW

GaNAs Matrix

STANFORD

JSH-32

1.2 1.25 1.3 1.35 1.4 1.450

0.2

0.4

0.6

0.8

1GaNAs barriers annealed GaAs barriers not annealed xGaAs barriers annealed

GaAsN BarrierPL Measurements

Use of GaNAs barriers instead of GaAs barriers results in:l PL peak at 1.44 m instead of 1.36 m for not annealed sample.l PL peak at 1.34 m instead of 1.26 m for annealed sample.

GaAs cap

GaAs cap

GaN0.03As0.97 Barrier

GaAs substrate

Al0.33Ga0.66As cap

Ga0.65In0.35N0.02As0.98 QW

Ga0.65In0.35N0.02As0.98 QW

100?

150?

800?

65?

200?

65?

GaN0.03As0.97 Barrier200?

GaN0.03As0.97 Barrier200?

n n

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Page 17

STANFORD

JSH-33

GaAs Barrier GaInNAsQuantum Well

l 4 Ga0.65In0.35N0.033As0.967 quantum wells and 2InGaAs quantum wells were grown

l GaAs Barriers between Quantum wellsl As grown sample : PL peak at 1.286 ml Annealed at 780oC(Highest PL intensity) : PL

peak at 1.222 ml PL blue-shift : 64nm

STANFORD

JSH-34

GaAs Barrier PL vs.Annealing Temperature

4GaInNAs & 2 InGaAs Quantum wells (GaAs barrier)

00.00050.001

0.00150.002

0.00250.003

0.00350.004

0.0045

8000 9000 10000 11000 12000 13000 14000

wavelength

PL in

tens

ity

(a.u

.) As grown720C 1 min740C 1min760C 1min780C 1min800C 1min820C 1min

n n

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Page 18

STANFORD

JSH-35

GaNAs Barrier GaInNAsQuantum Well

l 4 Ga0.65In0.35N0.033As0.967 quantum wells and 2InGaAs quantum wells were grown

l GaNAs Barriers between Quantum wellsl As grown sample : PL peak at 1.336 ml Annealed at 780oC(Highest PL intensity) : PL

peak at 1.368 ml PL red-shift : 32nm

STANFORD

JSH-36

GaNAs Barrier PL vs.Annealing Temperature

n149

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

8000 9000 10000 11000 12000 13000 14000

Wavelength

PL in

tensi

ty (

a.u.) As grown

720C 2 min740C 1min760C 1min780C 1min800C 1min820C 1min

n n

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Page 19

STANFORD

JSH-37

In-Plane Lasers with GaNAsBarriers

l Broad area in-plane laser.l 3 Ga0.65In0.35N0.02As0.98 QW’s inserted between GaAsN

barriers.l Lasing wavelength: 1.296 m.

0 100 200 300 4000

2

4

6

8

10

12Room TemperaturePulse Length: 200 ns, Period: 10 s20 m x 800 m

Ith

= 334mA, Jth

= 2.1 kA/cm2

Slope efficiency = 0.186 W/A

µ µ µ

1.29 1.295 1.3 1.3050

1

2

3

4

5

6

7x 10

-11

STANFORD

JSH-38

Characteristic Temperature

T0 = 105°K T0 = 146°K

GaAs Barriers GaNAs Barriers

280

290

300

310

320

330

340

350

360

5.2 5.3 5.4 5.5 5.6 5.7 5.8

y = -494.1 + 145.98x R= 0.94335

T (

Kel

vin)

ln(Ith)

280

290

300

310

320

330

340

350

360

5.6 5.7 5.8 5.9 6 6.1 6.2

y = -312.3 + 104.74x R= 0.83115

T (

Kel

vin)

ln(Ith)

n n

n n 20

Page 20

STANFORD

JSH-39

GaInNAs: anEnabling Technology

l Many low cost photonic devices utilize vertical cavityconfiguration on GaAs substrates

l They don’t operate at communications wavelengths

l Significant technology base for GaAs based modulators,detectors, tunable lasers, free space optical interconnects

l Require long wavelength QW active regions that are latticematched to GaAs

STANFORD

JSH-40

Tunable VCSELs forSwitching and WDM

Space r Laye r

n-Dis t r ibu ted Bragg Re f lec to r

Quan tum We l l Ac t i ve Reg ion

& Cav i t y Space r

M e m b r a n e Contac t Pad

Defo rmab leM e m b r a n e Top Mi r ro r

Ai r Gap

p-con tac t

p -Con tac t Laye r

n -Con tac t Laye r Ant i -Ref lec t ive Coat ing

n n

n n 21

Page 21

STANFORD

JSH-41

l Low insertion lossl Coupling controlled by active region lengthl Simple, low-cost bondingl Applicable to detectors, lasers and switches

GaAs Substrate

DBR mirror

Waveguide Core

Fiber BlockSide Polished

Fiber

In-Line Waveguide CoupledSemiconductor Active Devices

STANFORD

JSH-42

solder (1000 Å)n +

silicon

GaAsp AlGaAs

epoxy

i MQW

silicon

epoxy

silicon

anti-reflectioncoating

Quantum Well ModulatorsSolder-Bonded to Si ICs

K. W. Goossen et al.,IEEE Photonics Tech.Lett. 7, 360 - 362(1995)

n n

n n 22

Page 22

STANFORD

JSH-43

Summary

l GaInNAs/GaAs is a promising material for long wavelengthVCSELs

l All epitaxial single-step growthl High T0 active layer with high-contrast thermally-conductive mirrorsl Easily controlled composition by MBE

l Demonstrated low threshold, RT, CW GaInNAs VCSELsl ~1 mA threshold current, 0.05 W/A slope efficiency at =1.2µml Peak output power limited by p-DBR resistance (~9V at Ith)l Demonstrated edge emitting lasers at 1.3µm and PL beyond 1.3µml GaInNAs is the best candidate for low cost, 1.3 µm VCSELs

l Future workl Rebuilding two chamber MBE system for high thruput, graded interface, C-

doped mirrors and VCSEL growthl Extend GaInNAs active QW material to tunable sources, modulators,

semiconductor optical amplifiers, optical IC interconnectsl Apply GaInNAs active devices to Si optical interconnects

STANFORD

JSH-44

GaInNAs is easier to grow by MBEthan by MO-CVD

Some would say this is(BAD NEWS)2

The GOOD NEWS

Conclusion 2!!!

The BAD NEWSGrading & multiple Al composition GaAs/AlAs

mirrors are difficult to grow by MBE