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

Size: px

Start display at page:

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

Ezra Dawson
5 years ago
Views:

AMELIORATION DES PERFORMANCES DES LASERS A CASCADE QUANTIQUE - ETUDE DU CONFINEMENT OPTIQUE ET DES PROPRIETES THERMIQUES Jean-Yves Bengloan To cite this version:

atom-ph]. Université Paris Sud - Paris XI, 25. Français. <tel- 8418> HAL Id: tel-8418 https://tel.archives-ouvertes.

published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

1 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, 25. Français. <tel- 8418> HAL Id: tel Submitted on 5 Jul 26 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 Amélioration des performances des Lasers à Cascade Quantique : Étude du confinement optique et des propriétés thermiques J-Y Bengloan Thèse de doctorat de l Université de Paris XI Sud (Orsay) Thèse effectuée à Thales Research & Technology (TRT) à l Université de Paris VII

3 Performance optimisation of Quantum Cascade Lasers: Investigation of the optical confinement and thermal properties J-Y Bengloan Thèse de doctorat de l Université de Paris XI Sud (Orsay) Thèse effectuée à Thales Research & Technology (TRT) 2 à l Université de Paris VII

4 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 4. Conclusion Selective current injection by proton implantation Thick electro-plated gold 3

5 Quantum Cascade Lasers (QCLs) Main Properties Energy e - CB INTERSUBBAND transitions hn UNIPOLAR : only one type of carrier used (e - ) Distance (z) 4

6 Quantum Cascade Lasers (QCLs) Active Region (AR) grown by Molecular Beam Epitaxy (MBE) Transport zone Emission zone e - "MINIGAP" "MINIBAND" Transport zone Emission zone 3 "MINIGAP" 545 nm CASCADE scheme : Recycling of carriers N periods = N photons per carrier e e - "MINIBAND" 3 2 1

Spectral range of QCLs GaN/AlInGaN GaAs/GaInAs GaAs/AlGaInP

5 2 Classic Laser Diodes GaSb Near IR Quantum Cascade Lasers

5 µm < l < 24 µm and l> 65 µm Mid IR Far IR 5 1 15 2 25 1

sensitive Gas detection Environmental, Medical, Security

7 Spectral range of QCLs GaN/AlInGaN GaAs/GaInAs GaAs/AlGaInP InP/GaInAsP UV Classic Laser Diodes GaSb Near IR Quantum Cascade Lasers GaInAs/AlInAs/InP ; GaAs/AlGaAs ; InAs/AlSb PbSe (cryo) 3.5 µm < l < 24 µm and l> 65 µm Mid IR Far IR Wavelength (µm) Applications: Spectroscopy and high sensitive Gas detection Environmental, Medical, Security Free space optical communication Atmospheric transparency windows : 3-5 µm and 8-12 µm THz 6 Optical countermeasures

8 QCL History Bell Labs Thales Thales / CEM² QCL in GaInAs/AlInAs/InP QCL in GaAs/AlGaAs QCL in InAs/AlSb Room temperature CW operation for GaInAs/AlInAs/InP QCL 25 4mW room temperature CW operation for GaInAs/AlInAs/InP QCL 15K CW operation QCL in GaAs/AlGaAs QCL 7 CW: Continuous Wave

9 Gain optimisation First Laser operation Pulsed operation From intersubband emission to CW laser operation First CW operation CW operation 78K 78K 78K 3K Quantum engineering of Active Region Waveguide design optimisation Thermal management 8

10 GaAs/AlGaAs QCL Gain optimisation First laser operation Pulsed operation Thesis S.Barbieri - C.Becker From intersubband emission to CW laser operation First CW operation CW operation 78K 78K 78K 3K 1998 (Thales) Quantum engineering of Active Region 2 (3K) (TU Wien) Waveguide design optimisation Heat dissipation management InP-based QCL 1994 (Bell Labs) 1995 (8K) (Bell Labs) 22 (Neuchâtel) 9

11 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 4. Conclusion Selective current injection by proton implantation Thick electro-plated gold 1

12 AR J QCL Pulsed operation QCL devevopment: Reduction of the threshold current density J th J th = (a wg + a m ) / g G Optical power Quantum engineering of Active Region : g Waveguide design optimisation : a wg, G J th J 25 GaAs based QCL J th (ka/cm²) K 3K year

13 Waveguide principle n 3 Cladding n 2 n 2 > n 1 n 3 n 1 Guiding condition : n 1 > n 2 Core n 1 Cladding n 2 Waveguide optimisation by numerical simulations : 1D Simulations : Transfer Matrix Method (TMM) choice of appropriate layer compositions and thicknesses Increase figure of merit c= G / a w Decrease J th =(a w+ a m )/gg 12

14 Current GaAs QCL waveguides (1) GaAs Plasmon enhanced waveguide with highly doped cladding layers n eff =3.19 a = 17 cm -1 G = 28% l = 9.4µm Advantages c= 1.7 J th =15kA/cm 2 straight-forward MBE growth good electrical characteristics Drawbacks free carrier absorption (FCA) losses in highly doped layers Optical intensity (a.u.) n + AR n + GaAs Distance (µm) GaAs n - n Refractive index 13

15 Current GaAs QCL waveguides (2) Optical intensity (A.U.) 1 Normalised optical intensity (a.u.) (a.u) LOC=3,5 LOC=1; LOC=,5; LOC=2,6; a= ; a=177cm a=25 a=17cm 92, ; G=35%; G=28%; G=58%; G=63%; c=1,4 c=1,7 c=,6 c=,4 Plasm. LOC Active Region LOC Refractive index Plasm Vertical Distance dimension (µm) (µm) Refractive index Figure of merit, c χ = α w LOC, GaAs thickness (µm) Γ The χ optimisation is a trade-off between : Γ, decreasing with the GaAs thickness a, mainly due to FCA 14

16 Dielectric waveguide optimisation NECESSITY TO REDUCE OPTICAL LOSSES FROM CLADDINGS.3 h GaAs 3.5 Plasmon enhanced waveguide strengthened by dielectric layers : Normalised optical intensity (a.u).2.1 DIELECTRIC Distance (µm) h DIEL DIELECTRIC n + AR n Refractive index AlGaAs layers GaInP layers Maximum growth thickness ~1µm h GaAs +h Diel =3,5µm 15

17 Al x Ga 1-x As cladding waveguides With x Al : n GaAs/AlGaAs Γ α (mode overlap with highly doped layers reduced) c=g/a Normalised optical intensity (a.u.) AlGaAs Distance (µm) h GaAs h AlGaAs AlGaAs n + AR n Refractive index Figure of merit, c h AlGaAs (µm) 2 GaAs waveguide h GaAs (µm) 1 - x=2% - x=36% - x=7% - x=94% 16

18 Al x Ga 1-x As cladding waveguides With x Al : n GaAs/AlGaAs Γ α (mode overlap with highly doped layers reduced) c=g/a Normalised optical intensity (a.u)! Limitations : lattice mismatch constraint Al x Ga 1-x As thickness limit ~1/x Al AlGaAs h GaAs h AlGaAs n + AR n Distance (µm) AlGaAs Refractive index Figure of merit, c h AlGaAs (µm) 2 GaAs waveguide h GaAs (µm) 1 - x=2% - x=36% - x=7% - x=94% 17

19 Al.94 Ga.6 As cladding waveguides / QCL AL94 Al.94 Ga,6 As cladding waveguide 2 devices grown and processed identically: Normalised optical intensity (a.u.) a=12cm -1 ; G=35%; c=2,9 Distance (µm) Refractive index GaAs : GaAs waveguide AL94 : Al.94 Ga.6 As 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 2 µm c=2.9 J th ~2 times smaller than that of the GaAs plasmon enhanced waveguide. I H + implantation (insulating) 18

20 QCL AL94 Optical peak power (W) AL GaAs 2K 24K 3K 2K 24K 5kHz-1ns Current density (ka/cm²) Good optical performances for QCL AL94: Significant reduction of J th Agreement with simulations: J th (GaAs) / J th c(al94) / 2 Higher optical peak power 19

21 QCL AL94 Optical peak power (W) AL GaAs 2K 24K 3K 2K 24K 5kHz-1ns Bias (V) AL GaAs T=24K Current density (ka/cm²) Current (A) Good optical performances for QCL AL94: Significant reduction of J th Agreement with simulations: J th (GaAs) / J th c(al94) / 2 Higher optical peak power 3K operation Poor electrical characteristics for QCL AL94: Abnormally high knee: V c =14V (V c (GaAs)=5V) Bad ohmic contacts? Bad grading between GaAs and AlGaAs layers? Higher differential resistances 3xhigher for AL94 device compared to GaAs QCL High operating voltage 2

22 Al.36 Ga.64 As cladding waveguides Figure of merit, c h AlGaAs (µm) 2 GaAs waveguide h GaAs (µm) 1 - x=2% - x=36% - x=7% - x=94% Norm. optical intensity (a.u.) Al.36 Ga.64 As cladding waveguide a=15cm -1 ; G=37%; c=2,5 Distance (µm) c=2,5 Refractive index Electrical conductivity better than Al,94 Ga,6 As layers: better e - mobility lower effective mass 21

23 QCL AL36 Optical peak power (W) K - 3K AL36 AL Current density (ka/cm²) AL36 AL94 5kHz-1ns Bias (V) 4 3 AL AL36 GaAs 2 4 Current (A) 24K 26K 28K 3K Good optical performances for QCL AL36: J th significantly lower than J th (GaAs) J th slightly higher than J th (AL94) Higher optical peak power P max (AL36)=25 mw at 3K Electrical characteristics dependant on the temperature: Higher differential resistances dv/di= f (T) for AL36 QCL: E act =132meV High operating voltage for T<26K 22

24 Ga.51 In.49 P cladding waveguides Ga.51 In.49 P refractive Al.45 Ga.55 As refractive index Good n GaAs/GaInP for improved confinement: c=2,9 Good electrical conductivity Ga.51 In.49 P : lattice matched to GaAs no thickness limitation c=2,9 Norm. optical intensity (a.u) a=13cm -1 ; G=38%; c=2,9 MOVPE MBE MOVPE Refractive index Drawback : Ga.51 In.49 P re-growth by MOVPE at Thales growth in 3 steps 23 Distance (µm)

25 QCL GaInP Optical peak power (W) K - 3K AL36 AL36 GaInP GaInP 5kHz-1ns Good optical performances for QCL AL36: Current density (ka/cm²) J th significantly lower than J th (GaAs) J th (GaInP) higher than J th (AL36) High optical peak power at 78K P max (GaInP)=1,9W at 78K Optical peak lower than QCL AL36 at RT P max (GaInP)=15 mw at 3K Optical peak power (W) GaInP - - GaAs T=78K GaInP GaAs Current density (ka/cm²)

26 QCL GaInP Optical peak power (W) K - 3K AL36 AL36 GaInP GaInP 5kHz-1ns Bias (V) T=3K AL36 GaInP GaAs Current density (ka/cm²) Current (A) Good optical performances for QCL AL36: J th significantly lower than J th (GaAs) J th (GaInP) higher than J th (AL36) High optical peak power at 78K P max (GaInP)=1,9W at 78K Optical peak lower than QCL AL36 at RT P max (GaInP)=15 mw at 3K Electrical characteristics : Higher knee bias: V c =7V (V c (GaAs)=5V) Lower differential resistance than AL36 Higher operating voltage than GaAs 25

27 Summary of laser performances J th (ka/cm²) GaAs GaInP AL36 AL94 1kA/cm² T (K) Reduction of threshold current densities: Low J th Agreement with our predictions for AL94 and AL36: J th (GaAs) / J th c(diel.) / c(gaas) Wall-plug efficiency (%) GaInP AL36 AL94 - GaAs T=24K,1% Current (A) Better Wall-Plug efficiencies than LCQ GaAs WP(GaInP)=1%= 1xWP(GaAs) at 24K Optical performances improvements > 1% Electrical degradations 26 Best waveguide device : QCL AL36 Best QCLs : QCL AL36 for T> 25 K QCL GaInP for T< 25 K

28 Waveguide loss measurements Waveguide losses a w determined from J th =f(a m ) plot: 2 15 a w =21cm -1 QCL GaInP 3K J th =(a m +a w )/gg J th (ka/cm²) 1 5 a w =12cm -1 24K 18K 15K 78K AL36 GaInP GaAs w T<18Ka T 18K - 19 cm cm cm -1 2 cm -1 - a w increase at T 18K a m (cm -1 ) 27 Reduction of a w at low temperature compared to QCL GaAs

29 Gain coefficient Energie (mev) 28 Carrier leakage into the continuum? J th =f(a m ) plot gg =f(t) Observation of 2 operating regimes DE act =58meV Distance (nm) g.g (cm / ka ) E act =53.2meV Arrhenius diagram E act =57.7meV GaInP g t 3 exp(e AL36 act /kt) /T (K -1 ) Carrier leakage into the continuum Limitation of the conduction band discontinuity of GaAs/AlGaAs for room temperature operation

30 Waveguide study conclusion 25 GaAs based QCL Significant reduction of J th Best performances (J th, P max ) on GaAs-based QCLs Application of these waveguides on a bound-to-continuum AR QCL J th (ka/cm²) K 78K year Degradation of the electrical transport Limitation from the conduction band discontinuity of GaAs/AlGaAs underlined for room temperature operation High optical losses at Room Temperature 29

31 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 4. Conclusion Selective current injection by proton implantation Thick electro-plated gold 3

$Minigap Electron extraction Miniband Normalised optical intensity (a.u.) InP substrate AR InP n + Refractive index Distance (µm) 31 Beck et al.$

32 Application to InP-based QCLs Active Region: l~9µm 4 Quantum Wells Vertical Waveguide Structure: c~12 G~69% a~6 cm -1 Minigap Miniband Electron injection Minigap Electron extraction Miniband Normalised optical intensity (a.u.) InP substrate AR InP n + Refractive index Distance (µm) 31 Beck et al., Science 295, 31, (22)

33 Heat management in QCLs Standard ridge waveguide Buried heterostructure InP based QCLs InP InP InP No lateral heat dissipation Good lateral heat flow GaAs based QCLs Selective current injection H + implantation 32

34 Selective current injection D W H + implantation Intensity (a.u.) -7-3 Lateral mode profile D W 3 7 Distance (µm) For W D = Semi-insulating layers using proton implantation decrease electrically pumped area 6 14,5 Pumped area (A) : -5% Mode overlap (Γ) : -2% J th 1/Γ : +2% I th = J th x A : -4% 33 Decrease of injected electrical power

35 Selective current injection in InP QCLs: L-I-V pulsed characteristics Voltage (V) KHz - 1ns Current Density (ka/cm 2 ) T=78K Optical Power (W) Voltage (V) Current Density (ka/cm 2 ) 24K 26K 28K 3K 32K Optical Power (W) Current (A) Current I th =135 ma, J th =1,1 I th =5 ma, J th =4,3 ka/cm² Abnormally high increase of I th with temperature 34

36 T characteristics J th (ka.cm -2 ) InP without H + InP with H + Low T T =93K T =177K Implanted structure LEAKS LEAKS H T (K) Standard ridge waveguide Contact (Gold) SiO 2 (Insulator) T : Characteristic temperature from fit : J th = J th. exp(t/t ) n-inp 35

37 Current leakage implantation breakdown LCQ I leaks LCQ Active Region Normalised resistance (W.cm 2 ) T act =123K E act =86meV Temperature (K) Resistance (W) H + implantation creates shallow defects in n-doped InP material 36 GaAs E c E v H + implantation works well in GaAs but not in n-inp n-inp E c E v

38 Selective current injection for InP-based QCLs? Selective current injection by H+ implantation inefficient in InP-based QCLs Future : use of Fe-doped InP as insulating layer - Deep defects in InP bandgap - Fe cannot be deeply implanted growth of Fe-doped InP layer Fe doped InP 37

39 CW operation for H + implanted InP-based QCLs Tension (V) CW T = 78K Current (A) 4 35 mw Optical power (mw) Optical power (mw C CW 1 C 2 C 18 mw 2 C Current (A) High Temperature CW operation even with current leakage 38

Electroplated Gold (Au) devices Electroplated Au Device SiO 2 Gold 2 µm Very good heat dissipation device Best performances obtained on QCLs with this type

40 Electroplated Gold (Au) devices Electroplated Au Device SiO 2 Gold 2 µm Very good heat dissipation device Best performances obtained on QCLs with this type of device 24 µm Slivken et al, APL (24) 4 experimental arrangements: Without Au With electroplated Au epi up epi up epi down epi down + mirror L1 L2 L3 L4 39

41 Effect of Thick electroplated Au Average optical power (mw) T=1 C L1 epi up Duty cycle (%) L1 P max = 49mW for DC=12% 4

42 Effect of Thick electroplated Au Average optical power (mw) T=1 C L2 epi up L Duty cycle (%) L1 L2 P max = 49mW for DC=12% P max = 78mW for DC=25% 41

43 Effect of Thick electroplated Au Average optical power (mw) T=1 C L3 L2 epi down L Duty cycle (%) L1 L2 L3 P max = 49mW for DC=12% P max = 78mW for DC=25% P max = 12mW for DC=37% 42

44 Effect of Thick electroplated Au Average optical power (mw) T=1 C L4 L3 L2 epi down + mirror L1 L2 L3 L Duty cycle (%) 6 7 L4 P max = 49mW for DC=12% P max = 78mW for DC=25% P max = 12mW for DC=37% P max = 175mW for DC=4% 43

45 Thick electroplated Au - Summary R th (K.cm/ W) L1 epi up 6-5% L2 epi up 2,9 L3 epi down 1,8 ~ -4% L4 epi down 1,8 + mirror 44 Buried Heterostructure (Beck et al, Science295, 22) 1,45

46 Thick electroplated Au - Summary R th (K.cm/ W) Max. CW temperature L1 epi up 6 _ L2 epi up 2,9 13K L3 epi down 1,8 24K L4 epi down + mirror 1,8 278K 45 Buried Heterostructure (Beck et al, Science 22) 1,45 313K

47 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 4. Conclusion Selective current injection by proton implantation Thick electro-plated gold 46

48 Conclusion Significant performance improvements realised in GaAs-based QCLs owing to waveguide optimisation Use these waveguide on a bound to continuum structure Breakdown of selective current injection (H + implanted layers) in InP-based QCLs Application of Fe-doped InP layer Significant thermal improvements realised with thick electroplated Au R th close to that of buried heterostructure Thick electroplated Au on selective current injection devices 47

49 Mid IR QCL Material? GaAs has an intrinsic lower gain than GaInAs (m*>m*) Increase in J th I operation (Small dynamic current range) Higher losses Higher doping in the active region and the claddings Voltage(V) 1 5 InP REF_GaAs T=78K 1..5 Optical peak power (W) J (ka/cm 2 ) 48

50 Which material for which wavelength? Atmospheric windows III-V compounds phonon bands 5 4 Temperature (K) 3 2 InP based lasers Peltier GaAs based lasers 1 LN 2 Mid Infrared : AlInAs/GaInAs/InP QCLs InAs/AlSb QCLs Wavelength (µm) Far Infrared (THz) : GaAs/AlGaAs QCLs 49

51 Contributions to this work Thesis directed by Carlo Sirtori Simulation direction: Epitaxy realised by: A. De Rossi 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 5

MBE Growth of Terahertz Quantum Cascade Lasers Harvey Beere

MBE Growth of Terahertz Quantum Cascade Lasers Harvey Beere Cavendish Laboratory J J Thomson Avenue Madingley Road Cambridge, CB3 0HE United Kingdom People involved Harvey Beere, Chris Worrall, Josh Freeman,