GaInNAs: A new Material in the Race for Long Wavelength VCSELs. Outline

Transcription

1 GaInNAs: A new Material in the Race for Long Wavelength VCSELs J. S. Harris S. Spruytte, C. Coldren, M. Larson, M. Wistey, V. Gambin, W. Ha Stanford University Agilent Seminar, Palo Alto, CA 14 February 21 JSH-1 Outline Introduction The Choices GaInNAs/GaAs Material Properties Work at Stanford MBE Growth Edge-emitting lasers Pulsed broad area VCSELs CW oxide-confined VCSELs GaInNAs as an Enabling Technology Summary JSH-2 Page 1 1

2 Exponential Growth in Bandwidth Demand JSH-3 Link Capacity Improvements JSH-4 Page 2 2

3 Skiing Optical Fiber Networks Bunny Beginner Intermediate Advanced Extreme OC Mbs OC Gbs OC Gbs OC Gbs OC Gbs After Prof. Alan Willner-USC JSH-5 Fiber Distance vs. Bit Rate Transmission Distance (km) Attenuation Limited Dispersion Limited Gigabit Ethernet 1 Gigabit Ethernet 155nm DFB-SMF 131nm DFB Or VCSEL-SMF 131nm FP-SMF VCSEL 131nm 5µm-MMF 85nm 5µm-MMF 85nm 62.5µm-MMF Bit Rate (Mb/s) JSH-6 Page 3 3

4 Need for Bandwidth Technological Trends Computer performance continues to follow Moore s Law Doubling of computing power every 18 months Device research indicates exponential growth until 21 (SIA roadmap) Network capacity is also increasing exponentially Gilder s Law Communication Capacity will triple every 12 months Growth at a rate greater than Moore s law requires new technologies Required network capacity Desktop bandwidth needs limited only by the human eye ~2 Gbps Internet traffic is non local Number of users and hosts is growing exponentially Need long wavelength, high speed, low cost VCSEL JSH-7 Long wavelength VCSELs Advantages of long wavelengths (>13nm) relative to short wavelengths (85nm): 1. Fiber system performance Installed multimode-fiber: optimized for 13nm Standard singlemode fiber: >126nm 2. Eye safety 3. CMOS voltage scaling compatibility 4. Si substrate transparency for integration Approach: Long wavelength active region on GaAs substrate Leverage 85nm VCSEL technology High performance GaAs/AlAs DBRs AlAs-oxide current confinement and optical mode control JSH-8 Page 4 4

5 Bandgap vs. Lattice Constant for III-V Materials 6. AlN (.21 µm) Bandgap energy (ev) (.31 µm) (.41 µm) α SiC (.62 µm) (1.24 µm) GaN InN Visible Spectrum ZnS SLE AlP GaP Si (Ge) GaAs BULK AlAs ZnSe Ge Lattice Constant (Å) InP CdS MgSe CdSe AlSb InAs ZnTe GaSb CdTe InSb JSH-9 Material Choices at 1.3µm Lattice Matched to GaAs 2.5 AlAs Bandgap (ev) Al x Ga 1-x As GaAs In x Al 1-x As InP In x Ga 1-x As In x Ga 1-x As y P 1-y 98nm 13nm 155nm.5 GaN y As 1-y Ga x In 1-x N y As 1-y on GaAs Lattice Parameter (Å) GaNAs bandgap bowing Rapid decrease in bandgap. N is small } control tensile/compressive strain on GaAs In is large InAs InN y As 1-y JSH-1 Page 5 5

6 GaInNAs/GaAs vs. InGaAsP/InP 1. Larger refractive index differences n(algaas)> n(ingaasp) Easier to make high reflectivity DBR mirrors n 1 n 2 n 1 n 1 - n 2 r n = n1 + n 2 R = r 1 + r 2 + r 3 + add in phase 2. Thermal conductivity AlAs/GaAs higher thermal conductivity than ternary or quaternary lattice matched to InP Heat removed through bottom DBR High termal conductivity DBR required N+ substrate 3. Larger conduction band offset Better thermal performance for lasers increasing N content 4. Better compositional control AlGaAs InGaNAs JSH-11 Overview MBE System Source of elemental Ga (.1-.8 monolayers/s) Source of elemental In (.2-.3 monolayers/s) Control nitrogen concentration Low concentration of impurities Nitrogen bypass Sharp QWs As cracker supplies As 2 (beam flux = 2 _ total beam flux of IIIelements) Radio frequency (rf) plasma supplies atomic N RHEED monitors growth Substrate temperature (phase segregation) JSH-12 Page 6 6

7 Characterization rf Plasma.2 Atomic Nitrogen Transitions.15.1 Molecular Nitrogen Transitions Ratio of intensity atomic nitrogen peak and intensity molecular nitrogen peak relative amounts of atomic and molecular nitrogen in plasma. Area under the peaks total amount of nitrogen in plasma. JSH-13 Growth Temperature and Phase Segregation no phase segregation MBE Window 1-2 phase segregation 1 2 Grown at 5 C Growth of GaN.5 As.95 thermodynamically stable at low temperature. high As 2 flux. 5% GaN Film grown at 7 C Grown at 7 C less N because 2N G N 2G. second phase observed. JSH-14 Page 7 7

8 Control of Nitrogen Composition by MBE HRXRD 2 SIMS HRXRD from superlattice Electron Microprobe Anal Nitrogen concentration inversely proportional to group III growth rate Unity N sticking coefficient!! JSH-15 Sharpness of GaInNAs Quantum Wells GaAs 65? Ga.8 In.2 N.15 As Å QW boundary : 2-3 ML thick QW thickness : 22 ML (=6.25 nm) Dislocation free GaAs JSH-16 Page 8 8

9 PL measurements for different N concentrations 13? 65? GaAs cap In.3 Ga.7 As QW GaAs Barrier.5.4 InGaAs wells InGaNAs wells.6 % Nitrogen 65? In.3 Ga.7 As QW 13? 65? GaAs barrier In.3 Ga.7 NAs QW GaAs Barrier at % Nitrogen 65? In.3 Ga.7 NAs QW.1 GaAs Substrate Luminescence efficiency of InGaNAs decreases with increasing N concentration JSH-17 1? 15? GaAs cap Al.33 Ga.66 As cap Effect of Anneal on PL properties of GaInNAs not annealed annealed 1? GaAs cap ? 7? 7? Ga.7 In.3 N.2 As.98 QW GaAs Barrier Ga.7 In.3 N.2 As.98 QW GaAs substrate Annealing (RTA 1 min at 76 C) of GaInNAs: Improves luminescence efficiency by factor 5. Shifts the peak to shorter wavelengths by 1 nm. Anneal during growth of subsequent layers. JSH-18 Page 9 9

10 Not annealed GaNAs QWs contain defects that trap carriers. ) NC-V (1 17cm not annealed 1. annealed Origin of low PL efficiency: C-V Measurements 2nm 5nm 1nm GaAs /cm 3 Be GaN.3 As /cm 3 Be GaAs /cm 3 Be GaAs p-substrate Depth (nm) 5nm 5nm 1nm NC-V (1 17cm -3) GaAs /cm 3 Si GaN.3 As /cm 3 Si GaAs /cm 3 Si GaAs n-substrate annealed not annealed Depth (nm) Annealing frees carriers (holes+electrons) in GaNAs QW. C-V measurements at 3 K and 1 MHz JSH-19 Origin of PL shift: SIMS Measurements 5 4 not annealed annealed 15 not annealed annealed Indium concentration profile Nitrogen concentration profile Indium diffusing less than Nitrogen. Shift of PL in GaInNAs caused predominantly by nitrogen diffusion. JSH-2 Page 1 1

11 Origin of low PL efficiency: Interstitial Nitrogen Channeling combined with Nuclear Reaction Analysis (NRA) to determine location of nitrogen. 14 N + 3 He 16 O + proton. 2.5 MeV 3 He analysis beam 1.5 MeV protons detected Counts Aligned <1> Unaligned Ratio Not annealed ±.3 Annealed ±.2 min =.6 for RBS channeling Nitrogen is not all substitutional. Amount of non-substitutional Nitrogen is decreased by annealing. JSH-21 In-plane Laser Results Broad area diode laser 7 Å Single QW laser J th = 45 A/cm µm X 84µm device 2 J th = 45A/cm =1.22µm 1.5 µm p-al.33 Ga.67 As.1 µm GaAs 7 nm.1 µm GaAs In.3 Ga.7 N.2 As.98 quantum well 1.5 µm n-al.33 Ga.67 As n + GaAs substrate Current (ma) JSH-22 Page 11 11

12 Wavelength (nm) Quantum Well Edge-Emitter Thermal Performance Wavelength vs. Temperature InGaNAs TQW Edge-Emitter L=8µm, lambda=1.24µm.4nm / º C Threshold Current (ma) Threshold vs.temperature InGaNAs TQW Edge-Emitter L=8µm, lambda=1.24µm T ~ 14K I th = I exp(t/t ) Heatsink Temperature ( C) Heatsink Temperature (K) Good thermal performance: high T JSH-23 High Power GaInNAs Edge Emitting Laser (Infineon/Ioffe) D.A. Livshits, A. Yu. Ergov and H. Riechert, Electonics Letters 36, 1382 (2) JSH-24 Page 12 12

13 Initial VCSEL Results Ti-Au 2 pair p-gaas/p-alas DBR GaInNAs/GaAs triple quantum well active region 22.5 pair n-gaas/n-alas DBR n+ GaAs substrate 5µm substrate emission Output Power (mw), Voltage (V) Intensity (a.u.) Wavelength (nm) Current (ma) Output power Voltage J th = 2.5 ka/cm 2 Efficiency.66W/A 1.2 m emission Pulsed operation JSH-25 GaInNAs Oxide-Confined VCSEL Device Structure Unoxidized aperture Ti/Au reflector/top contact 17 pairs p-gaas/alas protected from oxidation Process Highlights Mesas etch#1, SiO2 dep. SiO2 oxidation cap 3 pairs p-gaas/alas or GaAs/AlOx λ cavity Mesas etch#2 3 GaInNAs/GaAs 7Å/2Å active region 22 pair n-gaas/alas DBR Wet oxidation bottom n-contact Substrate emission JSH-26 Page 13 13

14 Oxide-confined GaInNAs VCSEL pulsed characteristics Typical L-I-V curves Output Power (mw) 1µm 15µm µm RT 2ns/1µs λ = 1.2µm Current (ma) µm square apertures 29µm.9-2 ma threshold current Current density down to 2 ka/cm 2 Slope Efficiency up to.9 W/A Voltage (V) Characteristics vs. size Current Density (A/cm2) Threshold Current (ma) Dimension (µm) Dimension (µm) JSH-27 Slope Eff. (W/A) Oxide-confined GaInNAs VCSEL Room Temp. CW Operation Output Power vs Current Output Power (mw) RT CW 3.6µm x 3.6µm 5µm x 5µm Ith = 1.32 ma.451 W/A Vth = 1.3 V Current (ma) Spectra (5µm x 5µm) Log Intensity (db, arb. ref.) 1.3 ma 3.5 ma 3 ma >4 db Wavelength (nm) 5µm aperture: 1.3 ma threshold current; 3.6µm: < 1 ma CW operation in spite of very high voltage drop (~9V) in top mirror Voltage (V) JSH-28 Page 14 14

15 Oxide-Confined VCSEL: CW Thermal Analysis Wavelength (nm) RT CW 5µm x 5µm.924 nm/mw 1.24 K/mW pulsed.2ns/1khz Dissipated Power (mw) ~6 K temperature rise at peak power Thermal Impedance Z T 1/diameter Z T (5µm) = 1.24 K/mW (Z T, 5µm etched pillar =.36 K/mW) Z T(5µm) / Z T(5µm) = 3.6 Temperature Rise ( C) I th(5µm) / I th(5µm) = 1/5 Reduce dissipated power in pdbr for higher output power JSH-29 Challenge for Long wavelength GaInNAs Post annealing process is required to improve material quality after growth Nitrogen out-diffuses from quantum wells Emission spectrum blue-shifts due to nitrogen loss JSH-3 Page 15 15

16 Advantages of GaNAs Barrier Decreased carrier confinement longer wavelengths. GaInNAs QW E1 Emission at GaInNAs QW E1 λ~1.36µm λ~1.44µm Decreased nitrogen out-diffusion during the anneal less shift of peak energy during anneal (65 mev versus 74 mev). Strain compensation possible (GaAsN tensile / GaInNAs compressive). HH1 GaAs Matrix HH1 GaNAs Matrix JSH-31 1? 15? 8? GaAs cap Al.33 Ga.66 As cap GaAs cap GaAsN Barrier PL Measurements 1.8 GaNAs barriers annealed GaAs barriers not annealed x GaAs barriers annealed 2? 65? 2? 65? 2? GaN.3 As.97 Barrier Ga.65 In.35 N.2 As.98 QW GaN.3 As.97 Barrier Ga.65 In.35 N.2 As.98 QW GaN.3 As.97 Barrier GaAs substrate Use of GaNAs barriers instead of GaAs barriers results in: PL peak at 1.44 m instead of 1.36 m for not annealed sample. PL peak at 1.34 m instead of 1.26 m for annealed sample. JSH-32 Page 16 16

17 GaAs Barrier GaInNAs Quantum Well 4 Ga.65 In.35 N.33 As.967 quantum wells and 2 InGaAs quantum wells were grown GaAs Barriers between Quantum wells As grown sample : PL peak at m Annealed at 78 o C(Highest PL intensity) : PL peak at m PL blue-shift : 64nm JSH-33 GaAs Barrier PL vs. Annealing Temperature 4GaInNAs & 2 InGaAs Quantum wells (GaAs barrier) PL intensity (a.u.) wavelength As grown 72C 1 min 74C 1min 76C 1min 78C 1min 8C 1min 82C 1min JSH-34 Page 17 17

18 GaNAs Barrier GaInNAs Quantum Well 4 Ga.65 In.35 N.33 As.967 quantum wells and 2 InGaAs quantum wells were grown GaNAs Barriers between Quantum wells As grown sample : PL peak at m Annealed at 78 o C(Highest PL intensity) : PL peak at m PL red-shift : 32nm JSH-35 GaNAs Barrier PL vs. Annealing Temperature n PL intensity (a.u.) As grown 72C 2 min 74C 1min 76C 1min 78C 1min 8C 1min 82C 1min Wavelength JSH-36 Page 18 18

19 In-Plane Lasers with GaNAs Barriers 12 Room Temperature Pulse Length: 2 ns, Period: 1 µ s 1 2 µ m x 8 µ m I th = 334mA, J th = 2.1 ka/cm 2 Slope efficiency =.186 W/A x Broad area in-plane laser. 3 Ga.65 In.35 N.2 As.98 QW s inserted between GaAsN barriers. Lasing wavelength: m. JSH-37 Characteristic Temperature GaAs Barriers GaNAs Barriers y = x R= y = x R= T (Kelvin) T (Kelvin) ln(i th ) ln(i th ) T = 15 K T = 146 K JSH-38 Page 19 19

20 GaInNAs: an Enabling Technology Many low cost photonic devices utilize vertical cavity configuration on GaAs substrates They don t operate at communications wavelengths Significant technology base for GaAs based modulators, detectors, tunable lasers, free space optical interconnects Require long wavelength QW active regions that are lattice matched to GaAs JSH-39 Tunable VCSELs for Switching and WDM Deformable Membrane Top Mirror p-contact Layer Spacer Layer Membrane Contact Pad p-contact Quantum Well Active Region & Cavity Spacer Air Gap n-distributed Bragg Reflector n-contact Layer Anti-Reflective Coating JSH-4 Page 2 2

21 In-Line Waveguide Coupled Semiconductor Active Devices DBR mirror GaAs Substrate Waveguide Core Side Polished Fiber Fiber Block Low insertion loss Coupling controlled by active region length Simple, low-cost bonding Applicable to detectors, lasers and switches JSH-41 Quantum Well Modulators Solder-Bonded to Si ICs GaAs p AlGaAs i MQW solder n + (1 Å) silicon epoxy silicon silicon epoxy anti-reflection coating K. W. Goossen et al., IEEE Photonics Tech. Lett. 7, (1995) JSH-42 Page 21 21

22 Summary GaInNAs/GaAs is a promising material for long wavelength VCSELs All epitaxial single-step growth High T active layer with high-contrast thermally-conductive mirrors Easily controlled composition by MBE Demonstrated low threshold, RT, CW GaInNAs VCSELs ~1 ma threshold current,.5 W/A slope efficiency at =1.2µm Peak output power limited by p-dbr resistance (~9V at I th ) Demonstrated edge emitting lasers at 1.3µm and PL beyond 1.3µm GaInNAs is the best candidate for low cost, 1.3 µm VCSELs Future work Rebuilding two chamber MBE system for high thruput, graded interface, C- doped mirrors and VCSEL growth Extend GaInNAs active QW material to tunable sources, modulators, semiconductor optical amplifiers, optical IC interconnects Apply GaInNAs active devices to Si optical interconnects JSH-43 Conclusion 2!!! The GOOD NEWS GaInNAs is easier to grow by MBE than by MO-CVD The BAD NEWS Grading & multiple Al composition GaAs/AlAs mirrors are difficult to grow by MBE Some would say this is (BAD NEWS) 2 JSH-44 Page 22 22