AMELIORATION DES PERFORMANCES DES LASERS A CASCADE QUANTIQUE - ETUDE DU CONFINEMENT OPTIQUE ET DES PROPRIETES THERMIQUES

(1)

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AMELIORATION DES PERFORMANCES DES LASERS A CASCADE QUANTIQUE - ETUDE DU CONFINEMENT OPTIQUE ET DES PROPRIETES

THERMIQUES

Jean-Yves Bengloan

To cite this version:

Jean-Yves Bengloan. AMELIORATION DES PERFORMANCES DES LASERS A CASCADE QUANTIQUE - ETUDE DU CONFINEMENT OPTIQUE ET DES PROPRIETES THERMIQUES. Physique Atomique [physics.atom-ph]. Université Paris Sud - Paris XI, 2005. Français. �tel-00084018�

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Amélioration des performances des Lasers à Cascade Quantique :

Étude du confinement optique et des propriétés thermiques

Thèse effectuée à Thales Research & Technology (TRT) à l’Université de Paris VII

J-Y Bengloan

(3)

Performance optimisation of Quantum Cascade Lasers:

Investigation of the optical confinement and thermal properties

J-Y Bengloan

Thèse effectuée à Thales Research & Technology (TRT) Thèse de doctorat de l’Université de Paris XI – Sud (Orsay)

(4)

3

PLAN

1. Introduction

2. Waveguide Optimisation in GaAs/AlGaAs QCLs

GaAs based guides (plasmon enhanced) / Limitations AlGaAs and GaInP Guides

3. Enhancement of thermal dissipation properties of GaInAs/AlInAs/InP

QCLs

Selective current injection by proton implantation Thick electro-plated gold

(5)

Quantum Cascade Lasers (QCLs)

• INTERSUBBAND transitions

• UNIPOLAR : only one type of carrier used (e-₎

Main Properties

hν e -CB Distance (z) Energy

(6)

5

45 nm

Quantum Cascade Lasers (QCLs)

Active Region (AR) grown by Molecular Beam Epitaxy (MBE)

"MINIGAP"

Transport zone Emission zone

3 2 1

e

-"MINIBAND" "MINIGAP"

Transport zone Emission zone

3 2 1

e

-e

-"MINIBAND" • CASCADE scheme : Recycling of carriers

(7)

Spectral range of QCLs

InP/GaInAsP Wavelength (µm) 0 5 10 15 20 25 100 0.5 1 1.5 2 Classic Laser Diodes

GaN/AlInGaN GaAs/GaInAs_GaAs/AlGaInP

GaSb PbSe (cryo)

UV Near IR

THz

Mid IR Far IR

Quantum Cascade Lasers

GaInAs/AlInAs/InP ; GaAs/AlGaAs ; InAs/AlSb

3.5 µm < λ < 24 µm and λ> 65 µm

Applications:

• Spectroscopy and high sensitive Gas detection Environmental, Medical, Security

• Free space optical communication

(8)

7

QCL History

1994 QCL in GaInAs/AlInAs/InP QCL in GaAs/AlGaAs 2003 1998 2002 QCL in InAs/AlSb Room temperature CW operation for GaInAs/AlInAs/InP QCL 2005 150K CW operation QCL in GaAs/AlGaAs QCL 400mW room temperature CW operation for GaInAs/AlInAs/InP QCL CW: Continuous Wave

(9)

From intersubband emission

to CW laser operation

First Laser

operation _operationFirst CW

Pulsed operation CW operation Gain optimisation

78K

78K 78K 300K

Waveguide design optimisation

Thermal management Quantum engineering of Active Region

(10)

9

Thesis S.Barbieri - C.Becker

From intersubband emission

to CW laser operation

First laser

operation _operationFirst CW

Pulsed operation CW operation Gain optimisation 78K 78K 78K 300K InP-based QCL 1994 (Bell Labs) 2002 (Neuchâtel) 1995 (80K) (Bell Labs)

Heat dissipation management Quantum engineering of Active Region

1998

(Thales)

GaAs/AlGaAs

QCL 2000 (30K)_{(TU Wien)}

(11)

PLAN

1. Introduction

2. Waveguide Optimisation in GaAs/AlGaAs QCLs

3. Enhancement of thermal dissipation properties of GaInAs/AlInAs/InP

QCLs

(12)

11

Pulsed operation QCL devevopment:

Reduction of the threshold current density J

_th

J_th = (α_wg+ α_m) / g Γ QCL J AR J Optical power J_th

Waveguide design optimisation : α_wg, Γ Quantum engineering of Active Region : g

0 5 10 15 20 25 1998 2000 2002 year J th (kA/cm²) 300K 78K GaAs based QCL

(13)

Waveguide principle

n₂> n₁≥n₃ n₁ n₃ Cladding n₂ Core n1 Cladding n2

Guiding condition : n

₁

> n

₂

Increase figure of merit

χ= Γ / α

w

Waveguide optimisation by numerical simulations :

• 1D Simulations : Transfer Matrix Method (TMM)

choice of

appropriate layer

compositions and

thicknesses

Decrease

J

_th

=(

α

_w+

α

_m

)/g

Γ

(14)

13

Current GaAs QCL waveguides (1)

GaAs Plasmon enhanced waveguide with highly doped cladding layers

n_eff=3.19 α = 17 cm-1 Γ = 28% λ = 9.4µm

χ= 1.7

J

_th

=15kA/cm

2 Distance (µm) -12 -10 -8 -6 -4 -2 ₀ 0 0.2 0.4 0.6 0.8 1

Optical intensity (a.u.)

0 2 4 6 8 10 Refractive index AR n+ n+ GaAs GaAs n- n -èAdvantages

l straight-forward MBE growth l good electrical characteristics

èDrawbacks

l free carrier absorption (FCA) losses

(15)

Vertical dimension (µm) -12 -10 -8 -6 -4 -2 ₀ 0 0.2 0.4 0.6 0.8 1

Optical intensity (A.U.)

0 2 4 6 8 10 Refractive index Active _Region LOC LOC Plasm. Plasm.

Normalised optical intensity

(a.u)

Distance (µm)

Refractive

index

LOC=0,5; α=177cm-1_;Γ=63%; χ=0,4

(a.u)

Distance (µm)

Refractive

index

LOC=1; α= 92,5 cm-1_;Γ=58%; χ=0,6

(a.u)

Distance (µm)

Refractive

index

LOC=2,6; α=25 cm-1_;Γ=35%; χ=1,4

(a.u.) Distance (µm) Refractive index LOC=3,5 ; α=17cm-1_;Γ=28%; χ=1,7 0 0.5 1.0 1.5 2.0 0 2 4 60 0.5 1.0 1.5 2.0

LOC, GaAs thickness (µm)

w α χ = Γ

Figure of merit,

χ

Current GaAs QCL waveguides (2)

The χ optimisation is a trade-off between : • Γ, decreasing with the GaAs thickness

(16)

15

Dielectric waveguide optimisation

NECESSITY TO REDUCE OPTICAL LOSSES FROM CLADDINGS

è Plasmon enhanced waveguide

strengthened by dielectric layers :

q

AlGaAs layers

q

GaInP layers -12 -10 -8 -6 -4 -2 0 0 0.1 0.2 0.3 -12 -10 -8 -6 -4 -2 0 2 2.5 3 3.5 hGaAs

hDIEL Refractive index

Distance (µm)

Normalised optical intensity (

a.u

)

AR

n+ n+

DIELECTRIC

DIELECTRIC Maximum growth thickness ~10µm

(17)

1 2 3 4 0 1 2 3 0 1 2 3 Figure of merit, χ h_GaAs(µm)

−

x=20%

−

x=36%

−

x=70%

−

x=94% hAlGaAs(µm) GaAs waveguide -12 -10 -8 -6 -4 -2 0 0 0.1 0.2 0.3 -12 -10 -8 -6 -4 -2 02 2.5 3 3.5 hGaAs

hAlGaAs Refractive index

Distance (µm)

a.u. ) AR n+ n+ AlGaAs AlGaAs

Al

_x

Ga

_1-x

As cladding waveguides

With x_Al Þ:

⇒

∆n_GaAs/AlGaAsÞ

⇒

Γ Þ

(18)

17

Al

_x

Ga

_1-x

As cladding waveguides

-12 -10 -8 -6 -4 -2 0 0 0.1 0.2 0.3 -12 -10 -8 -6 -4 -2 02 2.5 3 3.5 hGaAs

hAlGaAs Refractive index

Distance (µm)

a.u ) AR n+ n+ AlGaAs AlGaAs

Limitations : lattice mismatch constraint â Al

_x

Ga

_1-x

As thickness limit ~1/x

_Al

!

1 2 3 4 0 1 2 3 0 1 2 3 Figure of merit, χ h_GaAs(µm)

−

x=20%

−

x=36%

−

x=70%

−

x=94% hAlGaAs(µm) GaAs waveguide With x_Al Þ:

⇒

∆n_GaAs/AlGaAsÞ

⇒

Γ Þ

(19)

Al

_0.94

Ga

_0.06

As cladding waveguides / QCL AL94

χ=2.9

è J_th ~2 times smaller than that of the GaAs plasmon enhanced waveguide.

Al_0.94Ga_0,06As cladding waveguide

(a.u.) Distance (µm) Refractive index α=12cm-1_;Γ=35%; χ=2,9 20 µm I H+_implantation (insulating)

2 devices grown and processed identically: èGaAs : GaAs waveguide

èAL94 : Al_0.94Ga_0.06As waveguide

• Identical 3 quantum-well AR ( same growth set ) • Double trench ridge devices

• H+ implanted for selective current channelling • Low duty cycle to avoid device heating

(20)

19 0 0.2 0.4 0.6 0 5 10 15 20

Optical peak power (W)

Current density (kA/cm²)

200K 240K 200K 240K 300K

−

AL94 − −GaAs

QCL AL94

5kHz-100ns

Good optical performances for QCL AL94:

è Significant reduction of J_th

è Agreement with simulations:

J_th(GaAs) / J_th(AL94) ≅ χ(AL94) / χ(GaAs)≅ 2 è Higher optical peak power

(21)

0 0.2 0.4 0.6

0 5 10 15 20

200K 240K 200K 240K 300K

−

AL94 − −GaAs

QCL AL94

5kHz-100ns

è Significant reduction of J_th

è Agreement with simulations:

J_th(GaAs) / J_th(AL94) ≅ χ(AL94) / χ(GaAs)≅ 2 è Higher optical peak power

è 300K operation 0 5 10 15 20 25 30 0 1 2 3 4 5 T=240K Bias (V) Current (A)

−

AL94 − − GaAs

Poor electrical characteristics for QCL AL94:

è Abnormally high knee: V_c=14V (V_c(GaAs)=5V)

Bad ohmic contacts?

Bad grading between GaAs and AlGaAs layers?

è Higher differential resistances

3x higher for AL94 device compared to GaAs QCL

(22)

21

Al

_0.36

Ga

_0.64

As cladding waveguides

Electrical conductivity better than Al_0,94Ga_0,06As layers:

better e- mobility lower effective mass

Al_0.36Ga_0.64As cladding waveguide

χ=2,5

Norm. optical intensity (

a.u. ) Refractive index Distance (µm) α=15cm-1; Γ=37%; χ=2,5 1 2 3 4 0 1 2 3 0 1 2 3 Figure of merit, χ hGaAs(µm)

−

x=20%

−

x=36%

−

x=70%

−

x=94% hAlGaAs(µm) GaAs waveguide

(23)

QCL AL36

5kHz-100ns

è J_th significantly lower than J_th(GaAs) è J_th slightly higher than J_th(AL94)

è Higher optical peak power P_max(AL36)=250 mW at 300K

Electrical characteristics dependant on the temperature:

è Higher differential resistances

èdV/dI= f (T) for AL36 QCL: E_act=132meV è High operating voltage for T<260K

0 10 20 30 40 0 2 4 240K Bias (V) Current (A) AL94 AL36 GaAs 260K 280K 300K 0 0.2 0.4 0.6 0.8 0 5 10 15

−

240K

−

300K AL94 AL94 AL36 AL36

(24)

23

Ø Ga_0.51In_0.49P refractive index ≅ Al_0.45Ga_0.55As refractive index

è Good ∆n_GaAs/GaInP for improved confinement: χ=2,9

Ø Good electrical conductivity

Ø Ga_0.51In_0.49P : lattice matched to GaAs

è no thickness limitation

Ga

_0.51

In

_0.49

P cladding waveguides

Drawback : Ga_0.51In_0.49P re-growth by MOVPE at Thales χ=2,9 è growth in 3 steps

Norm. optical intensity (

a.u ) Refractive index Distance (µm) MBE MOVPE MOVPE α=13cm-1; Γ=38%; χ=2,9

(25)

QCL GaInP

5kHz-100ns

è J_th significantly lower than J_th(GaAs) è J_th(GaInP) higher than J_th(AL36)

è High optical peak power at 78K P_max(GaInP)=1,9W at 78K

è Optical peak lower than QCL AL36 at RT P_max(GaInP)=150 mW at 300K 0 0.2 0.4 0.6 0.8 0 5 10 15 20

−

240K

−

300K GaInP AL36 AL36 GaInP 0 0.5 1.0 1.5 2.0 0 5 10 15 20

GaAs GaInP

−

GaInP

− − GaAs

(26)

25

QCL GaInP

5kHz-100ns

Electrical characteristics :

è Higher knee bias: V_c=7V (V_c(GaAs)=5V)

è Lower differential resistance than AL36 è Higher operating voltage than GaAs

0 5 10 15 0 1 2 3 4 Bias (V) Current (A) GaInP AL36 GaAs T=300K 0 0.2 0.4 0.6 0.8 0 5 10 15 20

−

240K

−

300K GaInP AL36 AL36 GaInP

è J_th significantly lower than J_th(GaAs) è J_th(GaInP) higher than J_th(AL36)

è High optical peak power at 78K P_max(GaInP)=1,9W at 78K

è Optical peak lower than QCL AL36 at RT P_max(GaInP)=150 mW at 300K

(27)

Reduction of threshold current densities:

è Low J_th

è Agreement with our predictions for AL94 and AL36:

J_th(GaAs) / J_th(Diel.)≅ χ(Diel.) / χ(GaAs)

Summary of laser performances

Better Wall-Plug efficiencies than LCQ GaAs

èWP(GaInP)=1%= 10xWP(GaAs) at 240K 0 0.2 0.4 0.6 0.8 1.0 0 1 2 3 4 5 Current (A) Wall -plug efficiency (%)

−

GaInP_AL36

−

AL94

−

GaAs T=240K 0,1%

Best waveguide device :

Best QCLs : QCL AL36 for T> 250 K Electrical degradations

>

Optical performances improvements 0 2 4 6 8 10 12 14 100 200 300 Jth (kA/cm²) T (K) GaAs GaInP AL36 AL94

•

10kA/cm² 1%

(28)

27

Waveguide loss measurements

0 5 10 15 20 -20 -10 0 10 20 αm (cm-1₎ Jth (kA/cm²) 300K 240K 180K 150K 78K

Waveguide losses α_w determined from

J_th=f(α_m) plot: J_th=(α_m+α_w)/gΓ α_w=21cm-1 α_w=12cm-1 QCL GaInP 21 cm-1 12 cm-1 GaInP 19 cm-1 -AL36 -20 cm-1 GaAs

α

w T≥180K T<180K Reduction of α_w at low temperature compared to QCL GaAs α_w increase at T ≥ 180K

(29)

0 200 400 600 0 20 40 60 Energie (meV ) 3 2 1

Gain coefficient

Carrier leakage into the continuum ? Observation of 2 operating regimes

g ∝ τ₃∝ exp(E_act/kT)

∆Eact=58meV

Carrier leakage into the continuum

Limitation of the conduction band discontinuity of GaAs/AlGaAs for room

temperature operation J_th=f(α_m) plot _ gΓ =f(T) 2 4 6 8 0.002 0.004 0.006 0.008 0.010 0.012 g. Γ (cm / kA ) 1 /T (K-1₎ E_act=57.7meV E_act=53.2meV GaInP AL36

•

Arrhenius diagram

(30)

29

Waveguide study conclusion

Best performances (J_th, P_max) on GaAs-based QCLs

Limitation from the conduction band discontinuity of

GaAs/AlGaAs underlined for room temperature operation

Application of these waveguides on a bound-to-continuum AR QCL Significant reduction of J_th 0 5 10 15 20 25 1998 2000 2002 2004 2006 year J th (kA/cm²) 300K 78K GaAs based QCL

Degradation of the electrical transport

(31)

PLAN

1. Introduction

2. Waveguide Optimisation in GaAs/AlGaAs QCLs

3. Enhancement of thermal dissipation properties of GaInAs/AlInAs/InP

QCLs

(32)

31

Application to InP-based QCLs

Minigap Miniband 1 2 3 4 Electron injection Electron extraction Miniband Minigap

Beck et al., Science 295, 301, (2002)

Active Region: λ~9µm

4 Quantum Wells

Vertical Waveguide Structure: χ~12 Γ~69% α~6 cm-1

(a.u. ) Refractive index Distance (µm) AR n+ InP substrate InP

(33)

No lateral heat dissipation

Standard ridge waveguide

Good lateral heat flow

Buried heterostructure

InP InP based QCLs

Heat management in QCLs

InP InP H+ _implantation

Selective current injection

(34)

33

Selective current injection

Lateral mode profile

W D Distance (µm)0 3 7 -7 -3 Intensity (a.u.)

0,5

14

6 D

W

≈

=

For

H+ _implantation W D

Semi-insulating layers using proton implantation

decrease electrically pumped area

⇒

Pumped area (A)

à

: -50%

⇒

Mode overlap (

Γ)

à

: -20%

⇒

J

_th

∝ 1/

Γ

Þ

: +20%

⇒

I

_th

= J

_th

x A

à

: -40%

(35)

Selective current injection in InP QCLs:

L-I-V pulsed characteristics

2 4 6 8 1 0 0 0.1 0 .2 0.3 0 .4 0.5 0 .6 0.7 0 .8 0.9 1 .0 0 1 2 3 4 5 6 7 8 0 0 .1 0 .2 0 .3 0 .4 0 .5 Voltage (V) Optical Power (W ) Current (A) T=78K

Current Density (kA/cm2₎

5KHz - 100ns 0 2 4 6 8 10 0 0.5 1.0 1.5 0 1 2 3 4 5 6 7 8 9 10 11 12 0 0.1 0.2 0.3 0.4 0.5 Voltage (V) Optical Power (W ) Current (A) 320K 240K 260K 280K 300K

Current Density (kA/cm2₎

@78K: I_th=135 mA, J_th=1,1 kA/cm² @300K: I_th=500 mA, J_th=4,3 kA/cm²

(36)

35 0 1 2 3 4 5 6 7 100 200 300 J th (kA.cm -2) T (K) T₀=93K

T

₀

characteristics

T₀: Characteristic temperature from fit : J_th= J_th0. exp(T/T₀)

LEAKS LEAKS 0 1 2 3 4 5 6 7 100 200 300 J th (kA.cm -2) T (K) T₀=177K T₀=93K InP without H+ InP with H+

•

SiO₂(Insulator) n-InP Contact (Gold)

Standard ridge waveguide

Low T₀

H+

(37)

Current leakage – implantation breakdown

I_leaks LCQ LCQ Active Region 0.1 0.2 0.3 100 2 00 30 0 10 20 50 Normalised resistance ( Ω .cm 2 ) Temperature (K) Resistance ( Ω ) T_act=1023K E_act=86meV

H

+

_{implantation creates shallow defects in n-doped InP material}

H

+

implantation

works well in

GaAs but not

in n-InP

E

_c

E

_v

GaAs

E

_c

E

_v

n-InP

(38)

37

Selective current injection for InP-based QCLs ?

Selective current injection by H+ implantation inefficient in InP-based QCLs

Fe doped InP

Future : use of Fe-doped InP as insulating layer - Deep defects in InP bandgap

(39)

CW operation for H

+

_{implanted InP-based QCLs}

4 6 8 10 0 0.2 0.4 0.6 0.8 1.0 1.2 0 100 200 300 400 Tension (V) Optical power ( mW ) Current (A) CW T = 78K 0 20 40 60 0 0.2 0.4 0.6 0.8 1.0 1.2 Optical power ( mW Current (A) CW 0°C 10°C 20°C 350 mW 18 mW 20°C

High Temperature CW operation

even with current leakage

(40)

39

Electroplated Gold (Au) devices

24 µm Gold 20 µm SiO₂

epi down

epi up

epi down + mirror

4 experimental arrangements:

Without Au With electroplated Au

L1 L2 L3 L4

Electroplated Au Device

Very good heat dissipation device

Best performances obtained on QCLs with this type of device Slivken et al, APL (2004)

(41)

Effect of Thick electroplated Au

0 50 100 150 200 0 10 20 30 40 50 60 70 Average optical power (mW) Duty cycle (%) T=10°C

epi up

L1

P

_max

= 49mW for DC=12%

L1

(42)

41

Effect of Thick electroplated Au

epi up

L1 L2

P

_max

= 49mW for DC=12%

L1

P

_max

= 78mW for DC=25%

L2

(43)

Effect of Thick electroplated Au

epi down

L1 L2 L3

L1

P

max

= 49mW for DC=12%

P

_max

= 102mW for DC=37%

L3

P

_max

= 78mW for DC=25%

L2

(44)

43

Effect of Thick electroplated Au

epi down

+ mirror

L1 L2 L3 L4

P

_max

= 49mW for DC=12%

L1

P

_max

= 175mW for DC=40%

L4

P

_max

= 102mW for DC=37%

L3

P

_max

= 78mW for DC=25%

L2

(45)

Thick electroplated Au - Summary

epi down

+ mirror

epi down

epi up

L4

L3

L2

L1

R

_th

(K.cm/ W)

6 2,9

1,8

-50%

~ -40%

Buried Heterostructure

(Beck et al, Science295, 2002)

(46)

45

Thick electroplated Au - Summary

epi down

+ mirror

epi down

epi up

L4

L3

L2

L1

R

_th

(K.cm/ W)

6 2,9

1,8

Max. CW

temperature

_

130K

240K

278K

Buried Heterostructure

(Beck et al, Science 2002)

(47)

PLAN

1. Introduction

2. Waveguide Optimisation in GaAs/AlGaAs QCLs

3. Enhancement of thermal dissipation properties of GaInAs/AlInAs/InP

QCLs

(48)

47

Conclusion

Use these waveguide on a bound to continuum structure

Thick electroplated Au on selective current injection devices

Breakdown of selective current injection (H+ _{implanted layers) in InP-based QCLs} Significant performance improvements realised in GaAs-based QCLs owing

to waveguide optimisation

Application of Fe-doped InP layer

Significant thermal improvements realised with thick electroplated Au R_th close to that of buried heterostructure

(49)

Mid IR QCL Material ?

0 5 10 0 5 10 15 200 0.5 1.0 Voltage(V)

Optical peak power (W

)

REF_GaAs

J (kA/cm2₎

InP

T=78K GaAs has an intrinsic lower gain than GaInAs (m*>m*)

Increase in J_th I_operation(Small dynamic current range)

Higher doping in the active region and the claddings Higher losses

(50)

49

Which material for which wavelength ?

0 100 200 300 400 2 5 10 20 Temperature (K) III-V compounds phonon bands LN₂ Peltier Atmospheric windows 500 Wavelength (µm) 50 100

GaAs based lasers InP based lasers

200 Far Infrared (THz) : GaAs/AlGaAs QCLs Mid Infrared : AlInAs/GaInAs/InP QCLs InAs/AlSb QCLs

(51)

Contributions to this work…

Thesis directed by Carlo Sirtori

Epitaxy realised by: X. Marcadet (MBE)

M. Lecomte, O. Parillaud (MOVPE)

Devices processing: M. Calligaro, M. Carbonnelle

Y. Robert, C. Darnazian

Characterisations realised with the help of :

C. Faugeras, L. Sapienza, S. Forget, E. Boër-Duchemin