Modal and Thermal Characteristics of 670nm VCSELs Klein Johnson Mary Hibbs-Brenner Matt Dummer Vixar Photonics West 09 Paper: Opto: 7229-09 January 28, 2009
Overview Applications of red VCSELs Device performance / limitations Thermal management Improved oxide VCSEL mode control Results / Performance Data Summary
Who Is Vixar? Vixar founded in late 2004 (Maple Grove, Minnesota) - Focused on 660nm-800nm VCSELs - Biomedical, industrial, commercial and military sensors - Founding team has an extensive history in VCSEL R&D and productization - Outsource Model: Fabless Opto
Applications/Value Propositions Laser Printing 2D arrays for speed enhancement Preferred wavelength for photosensitive materials Oximetry/BioSensors Low power for wireless sensors High yields to wavelength specs; Low dλ/dt Industrial Sensors Superior beam characteristics compared to LEDs Reduced cost of optics and increased mechanical robustness Arrays can eliminate mechanical scanning Residential/Consumer POF High speed (>2Gb/sec) modulation Low NA for efficient fiber coupling Spectroscopy Narrow linewidth, polarization stable Medical Diagnostics and Imaging Large scale linear arrays MEMS Integration (beam scanners) Vertical emission simplifies packaging
Typical Device Structure Al 0.5 Ga 0.5 As / Al x Ga (1-x) As DBRs: x>0.92 1-3λ cavity GaInP QW s (compressive strain) Tensile strained barriers and spacer transition 50% 70% graded AlGaInP SCH Zn doping in p-spacer Misoriented substrates
Well Known Technical Challenges Small conduction band offset AlGaInP Thermal carrier overflow at high J,T Need for ~50% AlGaAs in DBRs Poor thermal conductivity (high thermal impedance) Low index contrast DBR (resistance, thermal impedance) Reduced mobility (increased resistance) Zn diffusion in active region Burn-in effects Reliability concerns Oxygen Incorporation Reduction in radiative efficiency
Demonstrated CW Performance (Proton) Single-mode 2.8mW @ 671nm: 4.5mA, 20C Divergence = 5.5 to 7.5 deg FWHM SMSR > 45dB -20 Multi-mode -30 11.5mW @ 673nm, 20C -40-50 Peak WPE of 19.9% at 18mA. -60-70 Temperature -80 670nm Lasing to 79C 90 >1mW @ 60C 80 70 Efficiency 60 50 Peak WPE 22.9% 40 30 Polarization stability 20 10 PER 20-25dB typical Tmax [deg C] Relative Optical Power (dbm) 666 667 668 669 670 671 672 673 674 675 676 Wavelength (nm) 0 4 6 8 10 12 14 16 18 20 22 Aperture Diameter [microns]
Proton VCSEL Limitations For many applications, proton performance is fine Good single-mode power Adequate temperature performance But Proton VCSELs don t modulate very well Severe DCD for low duty cycle, high ER applications Thermal lensing Example: Laser Printing Desire >20db (i.e. infinite) extinction ratio Minimal turn-on delay 5-10ns pulse widths Low duty cycles (<<1%)
The Oxide Alternative Red oxide VCSELs modulate well But Low single-mode power Exhibit poorer temp performance Goal: Improve red oxide VCSEL performance through 1) Improved thermal management 2) Improved mode control
High Temp Pulsed Operation Performance under pulsed conditions 50nS Pulse; 1% Duty Peak Power [mw] 10 9 8 7 6 5 4 3 2 1 0 0 5 10 15 20 25 30 Peak Current [ma] 20 40 60 80 100 120 DC @ 20C
Thermal Model Planar and etched mesa device structures Extraction of thermal impedance Anisotropic DBR thermal conductivity K r = (K AlAs +K AlGaAs )/2 = (90 + 10)/2 K z = 2 /(1/K AlAs + 1/K AlGaAs ) = 2/(1/90+1/10) = 50 W/mK = 18 W/mK
Substrate Removal Common practice in HB LEDs Thermal Impedance (C/W) 2.4 2.3 2.2 2.1 2 1.9 1.8 1.7 1.6 0 50 100 150 200 Substrate Thickness (um) Rth vs. Substrate Thickness (modeled) 0.0035 0.003 0.0025 0.002 0.0015 0.001 0.0005 0 Average of Lmax 3. 2. 1. 20 35 50 Temp Lmax vs. Temp (experimental) Comment Au Plated-no mount UnModified Au Plated-InDiamond Not terribly effective for red VCSELs ~10% reduction in Rth reasonable Not attractive for processing 1. Substrate removed 2. Au plated 3. Diamond fused
Aperture/Metal Overlap Strong effect on Rth Thermal Impedance (C/W) 2.9 2.7 2.5 2.3 2.1 140 135 130 125 120 115 110 105 Junction Temp (C) Thermal Impedance (C/W) 2.4 2.3 2.2 2.1 2 1.9 1.8 1.7 1.6 1.9 100-4 -3-2 -1 0 1 2 Metal/Aperture Overlap (um) (modeled) 1.5 0 0.5 1 1.5 2 2.5 Metal/Aperture Overlap (um) (experimental) Penalty in output power due to vignetting Alignment-related uniformity variation
Lateral Mesa Heatsinking Mesa etch with metal plating Thermal Impedance (C/W) 2.5 2.4 2.3 2.2 2.1 2 1.9 1.8 1.7 1.6 1.5 0 1 2 3 4 5 6 7 Mesa Depth Zm (um) 120 118 116 114 112 110 108 106 104 102 100 Junction Temp (C) Potentially effective (~15% reduction in Rth) Increased process complexity Demonstration in process @CSU (Lear)
Improved Mirror Design Depart from quarter wave stack ~30% reduction in Rth Minimal observable increase in Ith 99.4 99.2 99 98.8 Reflectivity 98.6 98.4 98.2 98 97.8 97.6 Stop Band Width (nm) 97.4 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 DBR Layer Thickness Ratio (Zh/Zl) 20 18 16 14 12 10 8 6 4 2 0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 DBR Layer Thickness Ratio (Zh/Zl) Thermal Impedance (C/mW) 2.7 2.5 2.3 2.1 1.9 1.7 1.5 Design 1 Design 2 Design 3 Measured Data Predicted
Junction Heating Color map of heat flux
Junction Heating Time Constant 1λ AlAs Cavity: τ=3.13us 2λ AlAs Cavity: τ=2.99us 850nm: τ=3.47us
Oxide VCSEL Mode Control Displace oxide layer in P-DBR Reduced index confinement Increased scattering loss High order mode suppression Delta neff 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0 n n eff eff λ = * λ 0 0 5 10 15 20 25 30 Oxide Pos Above Active Region Threshold Gain [cm-1] 1150 1100 1050 1000 950 900 850 Fundamental 1st Order *Hadley, JQE, v32, n4, p607, 1996. 800 0 2 4 6 8 10 12 14 16 Oxide Position (DBR Periods Above Active Region)
Mode Control Results 8 th period oxide 7um aperture
Mode Control Results 12 th period oxide 7um aperture Excessive current leakage
DCD Results Duty Cycle Disortion DCD is a dependence of peak pulse power on duty cycle Proton VCSELs have issues with DCD due to thermal effects 12 th period oxide ER=20dB 10nS pulsewidth 0.10% duty cycle 10nS pulsewidth 50% duty cycle
Uniformity 670nm oxide VCSEL 8x8 array uniformity 12 th period oxide
High Speed Modulation 2.125GB/s PRBS 2 7 ~10dB ER 670nm Unfiltered Eye @ 2.125G
Nitride Hydration Surprise 80801X01: 4 devices before hydration S6BS-4 80801X01: 4 devices after hydration S6BS-4 0.0025 0.0025 0.002 0.002 Power [W] 0.0015 0.001 Power [W] 0.0015 0.001 0.0005 0.0005 0 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 0 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 Current [A] Current [A]
Summary Investigated multiple thermal management techniques Surface and lateral mesa heatsinking promising Substrate removal cost/benefit not compelling Improved DBR design is highly effective Direct heatsining of active region inconclusive Raised oxide highly effective for mode control 2mW, 7um aperture, full operating range Minimal DCD Need to reduce leakage