© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim p s s current topics in solid state physics c st a tu s so li d i www.pss-c.comp h y si caPhys. Status Solidi C 9, No. 2, 290–293 (2012) / DOI 10.1002/pssc.201100315 Arrays of 850 nm photodiodes and vertical cavity surface emitting lasers for 25 to 40 Gbit/s optical interconnects J. A. Lott*,1, A. S. Payusov2, 3, S. A. Blokhin2,4, P. Moser3, N. N. Ledentsov1, and D. Bimberg3 1 Vertically Integrated Systems GmbH, Hardenbergstrasse 7, 10623 Berlin, Germany 2 St. Petersburg Academic Univ. Nanotech. Research & Edu. Centre RAS, Khlopin St. 8/3, 195220 St. Petersburg, Russian Federation 3 Institut für Festkörperphysik und Zentrum für Nanophotonik, Technische Universität Berlin, 10623 Berlin, Germany 4 Connector Optics LLC, Domostroitelnaya Street 16 lit. B, 194292 St. Petersburg, Russian Federation Received 5 July 2011, revised 12 August 2011, accepted 5 October 2011 Published online 18 November 2011 Keywords vertical cavity surface emitting lasers, photodiodes, data communications, optical interconnects * Corresponding author: e-mail
[email protected], Phone: +49 (30) 308 31 42 40, Fax: +49 (30) 308 31 43 59, Web: www.v-i-systems.com We report the design and characterization of arrays of compact, energy efficient, multi- and single-mode GaAs- based 850 nm vertical cavity surface emitting lasers (VCSELs) and complementary p-i-n photodiodes (PDs) for 25 to 40 Gb/s optical interconnection applications in computing and data communications. Using foundry ser- vice epitaxial growth and processing we obtain over 50k operational devices from each 76.2 mm-diameter wafer. Using randomly selected on-wafer-probed, individually- probed, and packaged VCSEL and PD dice for single- channel link tests we consistently achieve record error- free operation (defined as a bit error ratio (BER) of less than 1x10-12) at 25 Gbit/s over up to 500 m of OM3 mul- timode optical fiber. We use our high speed VCSELs to characterize our high speed PDs and vice versa, and achieve open optical eye diagrams at up to 40 Gbit/s. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Commercial state-of-the-art short- reach (up to about 300 m) optical interconnects based on 850 nm-range photodiodes (PDs) and multimode vertical cavity surface emitting lasers (VCSELs) operate reliably and “error-free” (defined as a bit error ratio (BER) of less than 1x10-12) at 10 to 14 Gb/s per channel. Experimental high-speed multimode (MM) 850 nm VCSEL-based links [1-9] using the standard nonreturn-to-zero (NRZ) voltage modulation scheme include the demonstration of back-to- back (~2 m) MM VCSEL-based links at 25 Gb/s [7] and at 40 Gb/s [8, 9]. Other published experimental results in- clude a MM link at 15.6 Gb/s over 1 km [10] and single- mode links at 10 Gb/s over 2.8 km [11-12] and 5 km [13]. As the bandwidth capacity of commercial optical inter- connects scales toward faster and faster data rates - from for example the current 14 GBaud Fibre Channel (FC), 26 Gbit/s (26G) Infiniband (IB) eight data rate (EDR), and 40 and 100G Ethernet (GbE) standards toward respectively FC 32G, 68G Infiniband (obtained by extrapolating the IB data rate to the year 2014), and 400GbE to 1TbE standards, it becomes increasingly essential for manufacturers to pro- duce a reliable, energy efficient, and cost competitive VCSEL-based array transmitter technology wherein the per channel data rate is 25 Gbit/s and eventually 40 to 50 Gbit/s. Also of keen commercial interest is the ability to produce Nx25G arrays (with N=1, 4, 12, etc.) as a ubiqui- tous commodity for high performance computing (“super- computer”) applications. Equally important is the devel- opment of a complimentary high-speed PD and array tech- nology with high receiver sensitivity to minimize the sys- tem cost and operating power consumption. Possible future high-speed photodetector solutions include reduced area p- i-n photodiodes (PDs), highly sensitive but bandwidth lim- ited resonant cavity p-i-n PDs, or advanced MSM detectors, avalanche PDs, or waveguide-integrated PDs. Herein we report 25-40 Gbit/s 850 nm VCSEL and PD components for efficient optical interconnects that may be mass-produced as individual dice or as NxM arrays. We demonstrate 25 Gb/s links at up to 500 m with record low energy-to-data ratios approaching 100 fJ/bit. Phys. Status Solidi C 9, No. 2 (2012) 291 www.pss-c.com © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Contributed Article 2 Device design We produce our VCSELs and PDs using foundry partners for epitaxial growth and device processing according to our epitaxial designs [1-5], growth procedures, fabrication mask sets, and fabrication recipes and methods. Our VCSELs and PDs are arranged in a two- dimensional square array pattern such that the devices may be diced into standard 1x1 chips or 1xN arrays where the device-to-device pitch in both the x- and y-directions is 250 µm. The VCSELs and PDs are suitable for wire- bonding or flip mounting. Also, both device types may be tested directly on-wafer before thinning and dicing to a thickness of ~150 µm using a standard 50 ohm GS (ground-source) or GSG microprobe head with a standard tip spacing of 75 or 100 µm. We show in Fig. 1 and in Fig. 2 a schematic cross- section and two microscope images, respectively, of our 850 nm VCSEL. The VCSEL has a ground (G) cathode contact and a source or signal (S) anode contact. Both of these metal contacts reside on a relatively thick (~8-9 µm) dielectric to reduce pad capacitance, except for the portion of the cathode metal that fills an etched trench to make an ohmic contact to the VCSEL’s n-doped buffer/contact layer just above the substrate. In Fig. 3 we show a sche- matic diagram of our p-i-n PD (without the complete source contact). We produce PDs with active region di- ameters from 15 up to 45 µm (and up to 50 µm for test de- vices) suitable for detection at 850 nm from 20 to greater than 40 Gbit/s. As with our VCSELs we employ an un- doped substrate, double mesa structure, and a quasi- coplanar transmission line compact (250 µm x 250 µm per die before dicing) GS pad configuration suitable for high frequency packaging or on-wafer testing. Figure 1 Processed 850 nm VCSEL schematic cross-section. Figure 2 Images of processed VCSELs of identical design: left) a white light microscope image; and right) a confocal la- ser scanning microscope 3D surface profile image. Figure 3 Schematic cross-section diagram of a high-speed PD. Inset: initial small-signal RLC circuit model for the PD including the quasi-coplanar contact pads. 3 Device characterization We show in Fig. 4 the static light power-current-voltage (L-I-V) characteristics of our 850 nm VCSELs with ~6 µm-diameter oxide apertures. We achieve lasing at temperatures from 20 up to 170 °C. Near 20 °C the threshold current is below 0.5 mA. In Fig. 5 we show the emission spectra for this same VCSEL at cur- rents of 1 to 9 mA in 1 mA steps, and note that the RMS spectral width increases moderately from ~0.36 nm at 1 mA to ~0.54 nm at 9 mA. In Fig. 6 we show the details of the emission spectrum at 7 mA. Figure 4 L-I-V characteristics vs. temperature from 20 to 170 °C for an 850 nm VCSEL with ~6 µm-diameter oxide apertures. Figure 5 Emission spectra at 1 to 9 mA in 1 mA steps for the 850 nm VCSEL in Fig. 4. 852 854 856 858 860 862 864 9 mA 8 mA 7 mA 6 mA 5 mA 4 mA 3 mA 2 mA 1 mA In te ns ity (5 0 dB /d iv ) Wavelength (nm) 25°C 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 70°C 170°C 150°C 130°C 100°C 40°C O ut pu t P ow er (m W ) Current (mA) 20°C30°C 20°C 0 1 2 3 4 V ol ta ge (V ) VI Systems GmbH S G VI Systems GmbH image courtesy of G. Almuneau, LAAS 292 J. A. Lott et al.: Arrays of 850 nm photodiodes and vertical cavity surface emitting lasers © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com p h ys ic ap s sstatus solid i c Figure 6 Emission spectrum at 7 mA for the VCSEL in Fig. 5. The root-mean-square spectral width (ΔλRMS) is ~0.45 nm. In Fig. 7 and Fig. 8 we again show –I–V and spectral emission data for VCSELs made from the same epitaxial material as in Figs. 4-6 but with oxide aperture diameters of ~3.0 µm. These VCSELs exhibit single-mode or quasi- single mode emission with values of ΔλRMS less than ~0.2 nm. In Fig. 8 the side-mode-suppression-ratio (SMSR) is about 25 dB from 25 to 85 °C. Figure 7 L-I-V characteristics from 25 to 85 °C for an 850 nm VCSEL with ~3.0 µm-diameter oxide apertures. Figure 8 Emission spectra above threshold for the quasi-single- mode VCSEL described in Fig. 7 showing a SMSR of 25 dB. In Fig. 9 we show the relaxation resonance frequency (fR) and the -3 dB frequency (f3dB) for a single-mode (SM) 850 nm VCSEL with a SMSR > 36 dB. We note that the fR saturates at ~30 GHz at ~3 mA, whereas for our ~6 and ~9 µm diameter oxide VCSELs the fR saturates at ~22 and ~16 GHz, respectively – see also the data in Fig. 10. For the SM VCSEL we show the extracted D-factor (small cur- rent linear region: D = 16.3 GHz/mA1/2) and the modula- tion conversion efficiency factor MCEF = 20 GHz/mA1/2. The peak f3dB value indicates that damping due to heating limits the speed of our SM VCSELs despite the ultra-high fR. We perform large signal modulation experiments using a (27-1) pseudorandom binary sequence (PRBS) in a NRZ format and achieve open optical eye diagrams up to 35-40 Gbit/s (a record bit rate for SM 850 nm-range VCSELs). Figure 9 Extracted frequencies (fR and f3dB) against the square- root of current above threshold for a SM VCSEL with an oxide aperture of ~2 to 3 µm. Inset: measured optical eye diagrams. Figure 10 Comparison of ~peak f3dB and inverse D-factor for VCSELs with oxide aperture diameters of ~3, 6, and 10 µm. Next in Fig. 11 we show the results of bit error ratio (BER) tests at 25 Gbit/s over Draka MaxCap-OM3 multi- mode fiber using our 850 nm SM VCSEL dice on a probe station as the light transmitters. The VCSEL emission is directed into a standard multimode fiber (MMF). As before we use a (27-1) PRBS and an NRZ format, but now with a forward current bias of I = 1.375 mA (and a forward volt- age of V = 2.081 Volts) and a peak-to-peak modulation voltage of 0.766 Volts. We achieve error-free transmission over 500 m of OM3 MMF with an energy-to-data ratio IV/BR (where BR is the bit rate) of only 114.5 fJ/bit. Finally we characterize our high-speed PDs. In Fig. 12 we show the measured dark I-V curves against temperature for a 25 µm-dia. active area PD. The reverse dark current 0.0 0.5 1.0 1.5 2.0 2.5 0 5 10 15 20 25 30 Fr eq ue nc y (G H z) MCEF=20 GHz/mA1/2 (I-Ith) 1/2 (mA)1/2 D=16.3 GHz/mA1/2 852 854 856 858 860 862 -70 -60 -50 -40 -30 -20 -10 0 857.24 nm 857.61 nm 856.47 nm 858.41 nm 858.58 nm 859.47 nm In te ns ity (d B ) Wavelength (nm) 25°C 7 mA 860.23 nm 854 855 856 857 858 859 860 861 862 863 -70 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 C ou pl ed P ow er (m W ) Wavelength (nm) 85 oC 80 oC 70 oC60 oC50 oC40 oC 30 oC 25 oC ( )1/2 R th fD I I = − ( ) 3 1/2 dB th fMCEF I I = − 40 Gb/s 35 Gb/s 30 Gb/s 28 Gb/s 25 Gb/s all 10 ps/div ~2-3 µm oxide dia. ~6 µm ~9 µm 0 1 2 3 4 5 6 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 20 oC~3.0 µm oxide aperture 25 oC 30 oC 40 oC 50 oC 60 oC 70 oC 80 oC 85 oC P ow er (m W ) Current (mA) 85 oC 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 V ol ta ge (V ) fR f3dB 2 3 4 5 6 7 8 9 10 11 15 16 17 18 19 20 21 22 Oxide Aperture Diameter (μm) In ve rs e D -fa ct or (m A 1/ 2 /G H z) Pe ak -3 d B F re qu en cy (G H z) 0.05 0.10 0.15 0.20 Phys. Status Solidi C 9, No. 2 (2012) 293 www.pss-c.com © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Contributed Article is below 1 nA below ~50 °C. In Fig. 13 we show measured optical eye diagrams at 28 Gbit/s for the entire set of GSG PD test devices. We achieve open eyes even for the largest 50 µm-diameter PD. In Fig. 14 we show a small-signal equivalent circuit model for our PDs, and in Table 1 the extracted model parameters. In Fig. 15 we plot the model PD junction capacitance C3 versus PD junction area. Figure 11 Bit error ratios at 25 Gbit/s over Draka MaxCap-OM3 MMF using an 850 nm SM VCSEL biased at 1.375 mA. We achieve error-free transmission over a record 500 m. Figure 12 Measured dark I-V characteristics for a 25 µm- diameter p-i-n PD from 20 to 90 °C. Figure 13 Optical eye diagrams for a test set of GSG PDs with active region diameters of 20 to 50 µm. Each PD is excited by a VIS MM VCSEL via a 50 µm-core MMF with a lens-tipped end. Figure 14 Small-signal RLC circuit model for the photodiodes. Table 1 Extracted PD small-signal circuit model parameters. dia. (µm) C1 (fF) C3 (fF) L1 (pH) R1 (Ω) R2 (Ω) R3 (Ω) 15 22.6 64.1 6.48 354 0 247k 20 22.1 76.4 12.8 393 0 240k 25 25.7 94.6 13.2 297 0 220k 30 24.9 118.8 2.2 321 1 125k 35 21.2 137.9 1.0 416 3 83.8k 45 29.9 174.6 1.0 165 5 215k Figure 15 Small-signal RLC model capacitance C3 for the PDs as a function of active junction area, as extracted from fitting the s-parameter measurements to the RLC model. References [1] J. A. Lott et al., in: Proc. CLEO/QELS, 2010, CME2. [2] S. A. Blokhin et al., Electron. Lett. 45(10), 501 (2009). [3] A. Mutig et al., Appl. Phys. Lett. 95, 131101 (2009). [4] G. Fiol et al., Electron. Lett. 47(14) (2011). [5] P. Moser et al., Appl. Phys. Lett. 98, 231106 (2011). [6] C. Ji et al., IEEE Photon. Tech. Lett. 22(10), 670 (2010). [7] N. Y. Li et al., in: Proc. OFC/NFOEC 2010, OTuP2. [8] P. Westbergh et al., Electron. Lett. 45, 366 (2009). [9] P. Westbergh et al., IEEE JSTQE, 2114642 (2011). [10] P. Pepeljugoski et al., IEEE PTL 14(5), 717 (2002). [11] G. Giaretta et al., in: Proc. CLEO/QELS, 2000, CPD13. [12] R. Michalzik et al., in: Proc. ECOC, Munich, 2000. [13] H. Hasegawa et al., IEICE Electr. 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