Semiconductor Quantum Dots: A Multifunctional Gain Material for Advanced Optoelectronics

Transcription

1 Semiconductor Quantum Dots: A Multifunctional Gain Material for Advanced Optoelectronics Johann Peter Reithmaier Technische Physik, University of Würzburg, Germany Quantum Dots: A New Class of Gain Material for Different Application Areas Quantum Dot Formation and Basic Properties Application Examples for Different Wavelength Ranges Summary, Conclusions and Prospects J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 1

2 Influence of Dimensionality on Gain E c 3D (bulk) E c 2D (QW) E c 0D (Qdot) D(E c ) D(E c ) D(E c ) Photon D(E v ) D(E v ) D(E v ) E v E v E v D(E) ~ E D(E) ~ const D(E) ~ δ(e) Much higher carrier density at transition energy possible J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 2

3 Carrier Density at Transition Energy E QW E QDs E F E F excited state of same dot ensemble N N low transparency carrier density reduced temperature dependence (T 0 ) homogeneously broadend gain of one dot ensemble J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 3

4 Size Distribution of Quantum Dots Single Dot Spectroscopy 100 nm 100 nm norm. intensity 200 nm 300 nm reference 1,25 1,30 1,35 1,40 Broad emission spectrum of dot ensemble due to size fluctuations energy [ev] Single PL line due to dot selection J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 4

5 Distributed Spectral Gain at RT inhomogeneously broadend gain function due to size fluctuations multi-wavelength amplification due to weak overlap between gain functions of different dot ensembles homogeneous linewidth 5-10 mev inhomogeneous linwidth mev homogeneously broadend gain of one dot ensemble J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 5

6 QDot Density of State Function dot wire wel dot wire bulk wel bulk j Enhanced gain at transition wavelength Asada et et al., al., JQE JQE 22, 22, (1986) Higher material gain reduction of laser threshold QW: 60 A/cm 2 QDot: < 40 A/cm 2 J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 6

7 Predictions of Special Dot Properties h + e - QW e - h + QDots Reduced diffusion: No diffusion to surfaces Reduced active volume: Low absorption and inversion density Refractive index decoupled from carrier density: No Chirp α = 4π λ dn dg dn dn J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 7

8 Consequences for Applications Small active volume and high density of states low threshold current density (lasers, SOAs) Discrete energy states low temperature sensitivity of threshold and emission wavelength (DFB lasers with n(t) / g(t) const) Inhomogeneous broadening broad gain bandwidth (ECLs, SOAs, DFB lasers) Symmetric gain function + small active volume small chirp factor (high speed direct modulation, low filamentation in high power lasers) Localized carrier storage by higher order QD states high speed amplification (SOAs, lasers) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 8

9 QD Formation and Basic Properties 980 nm QDot Material on GaAs 1.3 µm QDot Material on GaAs µm QDot Material on InP J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 9

10 980 nm QDot Material MBE growth of Ga0.4In0.6As dots on GaAs dot density: cm-2 About 3 times broader gain spectrum due to dot size distribution much larger tuning range for DFB lasers J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 10

11 980 nm QDot Lasers Substitution of QW in active region by single QD layer Operation temperature > 210 C J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 11 APL APL 74, 74, (1999)

12 Gain of 980 nm QD and QW-Laser Inversion condition already achieved at lower carrier densities j tr = 36 A/cm 2 (α i = 2.2 cm -1 ) j th = 54 A/cm 2 (2mm,HR/HR) Integrated gain for single dot layer limited Threshold gain : g th = α i + 1 L ln 1 R R J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil APL APL 77, 77, (2000)

13 Control of temperature sensitivity of λ active layer waveguide Γ (%) quantum well 430 nm dot layers 800 nm dot layer 430 nm dot layer 800 nm 0.18 new design 800 nm 0.16 QW: λ/ T = nm/k QD (d.l.): λ/ T = QD (s.c.): λ/ T = QD (w.c.): λ/ T = QD (n.d.): λ/ T = Γ QW/QD waveguide Lichtwelle QDLs show high temperature stability of emission wavelength (QWL: 0.33 nm/k, QDL: 0.14 nm/k) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 13

14 Gain of QW vs. QD Layers g mat (E) D(E) f(e,µ) QW µ = 1.24 ev to 1.40 ev QD Reduced blue shift due to high total gain J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 14 After saturation of first transition large blue shift Photonics West,

15 Growth of 1.3 µm InAs/GaInAs QDs "Dots in a Well"-concept (Liu et al., EL 35, 1163 (1999) / Ustinov et al., APL 74, 2815 (1999)) InAs embedded in GaInAs buffer layers Room temperature emission at 1.3 µm High quantum dot density GaAs 5 nm Ga 1-x In x As 2-3 ML InAs 5 nm Ga 1-x In x As E C Growth rate: r GaAs = 1 µm/h r InAs = 140 to 260 nm/h Growth temperature: T = 510 C J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 15

16 Growth of 1.3 µm Quantum Dots InAs-Dots on GaAs cm -2 intensity (a.u.) wavelength (nm) T = 20 C In content of QW (%) InAs-Dots on Ga 0.85 In 0.15 As cm -2 energy (ev) High dot densities for InAs on GaInAs mev line width 60 mev level distance Longer wavelength at higher In content J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 16

17 1.3 µm Quantum Dot Laser GRINSCH by multiperiod SSL 6 InAs/GaInAs Q-Dot layers with 50 nm GaAs spacers 650 nm cavity width GRINSCH with SSL structure 1,6 µm Al 0.4 Ga 0.6 As cladding layers J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil nm spacing (40 nm GaAs, 10 GaInAs) lens shape InAs QDots XTEM XTEM performed by by M. M. Schowalter, D. D. Gerthsen, University of of Karlsruhe

18 1.3 µm Quantum Dot Laser High gain at transition energy already at low current density Fundamental transition saturates and higher order transitions contribute to the gain maximum gain of cm -1 (6 dot layers stack) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 18

19 Uncoated Ridge Waveguide Laser voltage (V) d > 45 % output power per facet (mw) pulsed current (ma) 30 ma threshold current for 800 µm long uncoated laser (6 Q-Dot layers) Laser operation up to 156 C (record value) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 19

20 Coated Ridge Waveguide Lasers Ridge width: 4 µm output power at front facet (mw) intensity (a.u.) wavelength (µm) Resonator length: 400 µm! Reflectivities: 83 % and 95 % Laser operation on ground state I th = 4.4 ma, η d = 0.21 W/A (record value) Short cavities with high η d possible by HR coatings and very low internal absorption losses (< 1-2 cm -1 ) current (ma) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 20 JJAP JJAP 41, 41, (2002)

21 Lasers with improved QD structures Symmetric DWELLs QD growth direction Asymmetric DWELLs QD J th,inf < 120 A/cm 2 (6 QDLs) (< 20 A/cm 2 per QDL) threshold current density (A/cm 2 ) J th, = A/cm 2 J th, = A/cm reciprocal cavity length (1/cm) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 21

22 Broad Area Laser (L = 1.3 mm) Symmetric DWELLs QD growth direction Asymmetric DWELLs QD intensity per facet (mw) normalized intensity wavelength (nm) sym. DWELL asym. DWELL Half of threshold current About 30% higher slope efficiency current (ma) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 22

23 Improved T 0 -values at RT Improved T 0 -value due to new QD layer design (from 60 K 130 K) Lower threshold current densities over the whole operation range Emission from fundamental transition Emission wavelengths: µm J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 23

24 Introduction of p-doping D. Deppe et al. Univ. of Texas Doping enhances population inversion and increases gain at low pumping levels undoped n - doped N d = 10 per QD p - doped N a = 10 per QD Assume: Quasi-equilibrium Ε e Charge neutrality m = ev Ε h J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 24

25 p-doped QD Layers (RWG-Laser) D. Deppe et al. Univ. of Texas Further improvement of T 0 -value by p-doping to > 200 K up to 70 C Thershold Current (ma) Threshold Current versus Temperature For a P-doped 5-QD stack Laser QD Edge Emitter W = 5 µm L c = 970 µm CW, T o = 213 K pulsed, T o = 232 K L c = 650 µm CW, T o = 200 K Temperature (K) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 25

26 Epitaxy of InP Based QDs (λ > 1.4 µm) nm top view SEM image of InAs quantum dashes (100) InP substrates 200 nm InGaAlAs buffer 5 MLs InAs Irregular shaped quantum dashes Dashes preferentially aligned along the (0-11) direction Dash formation: MLs J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 26

27 Wide Emission Wavelength Range Control of emission wavelength by dash layer thickness Nearly symmetric low temperature PL spectra Wide wavelength range: µm Already realized: Laser emission at RT between 1.5 and 1.79 µm J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 27

28 Threshold and Efficiency of 1.5 µm Lasers Threshold Current Density BA-laser (L = 100 µm) T = 20 C, pulsed Differential Efficiency BA-laser (L = 100 µm) T = 20 C, pulsed Transparency threshold current density: 330 A/cm 2 (< 100 A/cm 2 per dash layer) Internal quantum efficiency: 62% Absorption: 8.5 cm -1 J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 28

29 Temperature Sensitivity Strong reduction of temperature shift of emission wavelength Reason based on gain saturation: slight dependence on cavity length Temperature shift as low as refractive index change: QD: 0.12 nm/k, QW: 0.53 nm/k J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 29

30 Examples of Device Applications 980 nm QD pump laser Temperature stable QD-DFB laser 1.3 µm QD-DFB lasers 1.3 µm QD-VCSEL Quantum Dot SOAs J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 30

31 980 nm High Power QD Laser 2 mm 100 µm broad area laser Record value of 4 W cw output power (HR/AR coating, > 8 MW/cm 2 ) Wall plug efficiency > 50 % at 1 W J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 31 Emission by fundamental mode High temperature stability Reduced wavelength shift EL EL 37, 37, (2001)

32 High Temperature Laser Performance T 0 for QWLs is higher due to SSL-barriers but constant for QD lasers No difference in temperature dependence for constant output power (T 1 ) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 32

33 Single Mode Emitting QDot Lasers Complex coupled DFB-Laser fabricated by e-beam Wavelength selection by grating period (SMSR = 52 db) I th < 20 ma for all periods ( λ = 33 nm) EL EL 35, 35, (1999) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 33

34 Temperature Stable DFB-Laser stable single mode emission no mode hopping single mode operation over 194 K temperature range reason: λ well (T) λ dot (T) quantum film quantum dots 3 times larger bandwidth 2 times lower temperature shift of wavelength J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 34

35 1.3 µm QD-DFB-Lasers Ridge waveguide lasers with lateral metal gratings defined by e-beam lithography Complex coupled distributed feedback Device dimensions: L = 800 µm, R = 83% / 95 % EL EL 37, 37, (2001) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 35

36 Characteristics of 1.3 µm QD-DFB-Lasers Single mode operation with SMSRs well above 40 db 20 ma threshold current Stable sidemode suppression J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 36

37 Wavelength Tuning by Grating Period Wavelength controlled by grating period ( λ = 36 nm) Linear wavelength dependence No SMSR degradation over whole tuning range Constant device performance J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 37 JJAP JJAP 41, 41, (2002)

38 High Frequency Properties of 1.3 µm Lasers intensity (db) f 3dB = 5.9 GHz I=40, 130 ma odulation frequency (GHz) EL EL 37, 37, (2001) f r (GHz) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 38 f 3dB =7.53GHz 0.50 GHz/ (ma) I ma Large modulation bandwidth for 800 µm long HR/HR coated device 3dB bandwidth thermally limited

39 Improved HF-Properties by p-doping D. Deppe et al. Univ. of Texas 6 3 (a) 10 x I th T = 300 K p-doped 5-stacks h ω = 30 mev L = 400 µm Simulated Small Signal Modulation Response P-doped 5-QD stack Laser E = 30 mev Calculated Modulation Response (db) (b) 14.6 GHz 10 x I th 50 x I th 50 x I th 33.0 GHz T = 373 K p-doped 5-stacks h ω = 30 mev L = 400 µm GHz 24.7 GHz Frequency (GHz) 40 J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 39

40 1.3 µm Quantum Dot VCSEL Laser light InGaAs Quantum Dots Metal contacts 1.75 λ (p)gaas Quantum Dot µ-cavity 1.75 λ (n)gaas DBR Al(Ga)Ox GaAs AlGaAs GaAs Substrate GaAs/Al(Ga)Ox Bragg mirrors with buffer layers for improved stability (rapid thermal annealing) Tapered apertures Cavity design optimized for low optical losses N. Ledentsov (Ioffe) J. Lott (AFIT, USA) D. Bimberg (TUB) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 40 CW output power up to 1 mw 14 % wall plug efficiency Max. diff. eff. > 90%

41 1.3-µm InAs Quantum-Dot SOAs Cross-sectional view Electrode p-al0.7ga0.3as n-al 0.7Ga 0.3As n-gaas Substrate Sugawara et al. GaAs SCH Electrode Photograph of chip No.1 Input Output Electrode 10-layer InAs quantum dots Surface Cross section 100 nm 100 nm Output [dbm] dB/25mm ASE 100 nm Signal Wavelength [nm] J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 41

42 Pattern-Effect-Free Operation of QD-SOA Sugawara et al. Amplifier gain [db] Power 10Gb/s input 20ps/div. Time 1 ka/cm 2 Linear gain Bulk InGaAsP SOA: -2.0dB 20ps/div Output power [dbm] Quantum-Dot SOA -0.9dB -1.4 db Max db -4.7 db 20ps/div. 20ps/div. 20ps/div. J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 42

43 Prospects for QD-SOAs and Switches Sugawara et al. Experiments Pattern effect free up to Gb/s (λ = 1.3 µm) Wavelength conversion by cross gain 10 Gb/s From simulation results expected (λ = 1.55 µm) About 15-dB improvement in output power compared to bulk SOAs High-speed optical switching up to 160 Gb/s J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 43

44 InP based RWG QDash Laser T = 20 C, cw CW operating ridge waveguide lasers 0.11 W/A per facet (15 mw per facet) 67 ma treshold current 2 mm x 3 µm J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 44

45 Summary Main advantages of QDot-Lasers: lower inversion carrier density, low temperature sensitivity broad gain spectrum, low chirp, multi-wavelength amplification, high speed Major Application Areas: 980 nm high power lasers (4 W cw from 100 µm facet, 1 W up to 110 C) Ultra-temperature-stable DFB lasers (T op. > 210 C ) High performance 1.3 µm RWG lasers (I th = 4.4 ma, T op. > 150 C) 1.3 µm DFB lasers with wide tuning range (I th = 20 ma, SMSR > 50 db) 1.3 µm QD-VCSEL (P cw 1 mw) Pattern free 10 GBit/s amplification at 1.3 µm by QD-SOAs µm QD-lasers based on InP with broad gain spectrum and very low temperature sensitivity of emission wavelength (dλ/dt = 0.12 nm/k) J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 45

46 Conclusions and Prospects 980 nm high brightness pump sources: Main advantages: low filamentation and temperature sensitivity Already near to commercialization Open points: demonstration of improved output powers, reliability 1.3 µm edge emitters and VCSELs: Main advantages: GaAs substrate, low threshold, low temperature sensitivity, broad gain spectrum In many device properties superior to existing technologies Edge emitters at commercialization step (e.g. Zhia, Albuquerque) Open points: high modulation speeds (solutions on the way) 1.5 µm InP based edge emitters and SOAs: Main advantages: broad gain spectrum, multi-wavelength amplification, low chirp, low temperature sensitivity of emission wavelength, high speed Device related research just started but fast progress Open points: improvement of material quality, confirmation of properties J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 46

47 Acknowledgements Research Group at Technische Physik, University of Würzburg: F. Klopf, R. Krebs, St. Deubert (GaAs based QD lasers) R. Schwertberger, D. Gold (InP based QD lasers) S. Parusel, A. Wolf (technicians) M. Kamp, M. Emmerling (lithography group) Financial Support: European Community: IST projects BigBand Ultrabright German Federal Ministry of R&E (BmBF "KomLaser") State Goverment of Bavaria ("Bavarian Research Grant") D. Deppe et al. Univ. of Texas N. Ledentsov (Ioffe) J. Lott (AFIT, USA) D. Bimberg (TUB) Sugawara et al. J.P. Reithmaier, Universität Würzburg, jpr\powerpoint\2002\2002_optimist\optimist, Foil 47