Multi-GHz Operation of Tilted- Charge LED for Optical Interconnect

Multi-GHz Operation of Tilted- Charge LED for Optical Interconnect Chao-Hsin Wu 吳肇欣 Department of Electrical Engineering & Graduate Institute of Photonics and Optoelectronics & Graduate Institute of Electronics Engineering National Taiwan University June 28 th, 2012 Integrated Optoelectronic Device Lab, NTU 1

Outline Introduction and Issues From Transistor to Transistor Laser Charge Control Model: Carrier Dynamics Multi-GHz Operation Tilted-Charge Light- Emitting Diode Conclusion Integrated Optoelectronic Device Lab, NTU 2

How do we connect all these devices and data? HDTV Kiosks Medical Imaging Cloud Server Network Appliances Laptop Digital Signage In-Vehicle Infotainment Security Surveillance Test & Measurement Integrated Optoelectronic Device Lab, NTU 3

What are the issues? Processor bandwidth doubles each 2 years due to silicon scaling. Electrical interconnect delay is the main obstacle for increasing the system s clock speed (fast source, wiring delay). Media-rich content drives the market for higher-speed short-reach interconnects. Wikipedia K. Emsley et al, Proc. SPIE, 5246, 8-11 (2003) Integrated Optoelectronic Device Lab, NTU 4

Moving From Copper to Optical Interconnect Problem: copper reaches its limits because of signal degradation and power consumption. Solution: optical interconnect since the speed of light is limited by the dielectric property. USB connector with optical interconnect. Intel Lightpeak introduced Jan 2010. Integrated Optoelectronic Device Lab, NTU 5

Maximum Commercial I/O Data Rate per channel (Gb/s) I/O Interface: Optical Interconnect The fiber optic market is being reborn, driven by demand for high bandwidth applications at very short distances which are served by multiple billions of connectors. 100 Major Standards Infiniband Fiber Channel 40G I/O 25G I/O Optical technology 10 Ethernet USB/Lightpeak 1 2010 2011 2012 2013 2014 2015 Year Copper technology Serial I/O transmission speed historically increases ~ 4-fold each 4-5 years. More efficient and high bandwidth light sources are required. Integrated Optoelectronic Device Lab, NTU 6

Why Optics? Disadvantages of Electrical Interconnect: Physical Problems (at high frequencies/high noise environments) Electromagnetic Interference (EMI) Cross-talk Signal Distortion Reflections High Power Consumption Heat Generation RC Delay Limited Bandwidth Advantages of Optical Interconnect: Capable to provide high bandwidths Free from electrical short-circuits Immune to EMI Essentially no crosstalk between adjacent signals No impedance matching required Low-loss transmission at high frequencies Long-haul fiber communication system Toward short-distance, high-volume range. Integrated Optoelectronic Device Lab, NTU 7

How to Use Lightwave to Transmit Information Simplified phasor representation of EM wave E t cos (ωt + θ) Amplitude Frequency Phase We can modulate/detect the three variables listed above to achieve data transmission. The easiest way is to do amplitude modulation. Basic Optical Interconnect Transmitter: LED or Laser Transmission Medium: Fiber (MM/SM), Waveguide (Polymer, semiconductor), orfree space Receiver: Photo diode or Transistor Integrated Optoelectronic Device Lab, NTU 8

Light Source Selection Laser (Light Amplification by Stimulated Emission of Radiation) Coherence Photons have fixed phase relationship Relative narrow spectra Low divergence after collimation Higher cost Large modulation bandwidth (> GHz) LED (Light-Emitting Diodes) Spontaneous emission Incoherence Photons with random phase Relative broad spectra Low cost Size can be small Small modulation bandwidth (~ MHz) Integrated Optoelectronic Device Lab, NTU 9

Semiconductor Laser Edge-emitting laser Contact layers Light output (elliptical) VCSEL (Vertical Cavity Surface Emitting Laser) Light output (circular) Grown mirror stacks Characteristics Facet coating Gain layers Contact layers Integrated Optoelectronic Device Lab, NTU 10

Limitation of Diode Laser: slow τ rec resonance peak Diode Laser Transmitter Module Example Waveforms: large resonance 3.125 Gb/s 10 Gb/s Input Waveform Resonance Diode Laser Closed Eye Bias Current Control Input Stage (AC coupling) Voltage-to -Current Converter Modulation Current Control Matching Network Monitor PD to Compensation Circuit Power consumption > 1W 1 Resonance frequency results in: - closed eye diagram -bad BER (bit error rate) 1 Picolight data sheet Integrated Optoelectronic Device Lab, NTU 11

VCSELs Large Resonant Peak (12 db) One of the reported Fastest VCSELs today 12 db resonance 1. Extreme bias! I/I TH = 31x 2. Utilize high resonance peak, filtered with 30-GHz low-pass filter. 3. Eye opens but suffer slow rise-fall time. Output signal degrades severely. Output signal (red) 35 Gb/s eye Y.C. Chang, C.S. Wang, and L.A. Coldren, Elec. Lett. 43, 19 (2007). Slow rise-fall time Input signal (blue) Integrated Optoelectronic Device Lab, NTU 12

Pros: Can LED be a Light Source? No threshold current Low power consumption Simple process, robust, low cost High yield, lifetime, reliability Simple circuit design Less directivity No resonance Cons: Slow modulation speed (commercial LED < 1 GHz) Relatively weak light output (resonance cavity, array) For the last two decades, spontaneous recombination life time was presumably in the nano second range. How long does it take to emit a photon spontaneously? We observe a whole system effect of carrier recombination process. nano second. Integrated Optoelectronic Device Lab, NTU 13

Recombination Lifetime, τ rec R r B n p B: recombination rate of an isolated e-h pair n, p: active layer concentration of electron and holes 1. B: Purcell effect: time-resolved photoluminescence measurements => NO electrical data. (First presented in this work!) 2. n, p: increase dopant concentrations => nonradiative recombination that decreases internal efficiency. (Absent in this work!) Previous best LED Modulation Bandwidth (in1999) Sub-100 ps recombination lifetime is hard to achieve N a : 7x10 19 cm 3 h= 10 % N a = 2x10 19 cm 3 h= 25 % t =100 psec C.H. Chen, et al,appl. Phys. Lett., 74, 3140, 1999 D. Fattal et al., Appl. Phys. Lett. 93, 24350, 2008 Integrated Optoelectronic Device Lab, NTU 14

Outline Introduction and Issues From Transistor to Transistor Laser Charge Control Model: Carrier Dynamics Multi-GHz Operation Tilted-Charge Light- Emitting Diode Conclusion Integrated Optoelectronic Device Lab, NTU 15

Transistor Invention by Bardeen and Brattain (1947) - The birth of Microelectronics, Integrated Circuits and Computer Industry What is known in 1947? Identification of minority carrier injection, recombination in the base and collection underlying bipolar transistor operation What is not known in 1947? Indirect-Gap (Ge) transistor base recombination process produce phonons (heat) Direct-Gap (GaAs & InP) transistor base recombination produced photons (light) Integrated Optoelectronic Device Lab, NTU 16

III-V Alloy Semiconductor LED & Laser (1962) -The Birth of Optoelectronics Comm. Display & Solid State Lighting Semiconductor Laser (1962) GaAs, Hall (GE), Nathan (IBM), Rediker (MIT-Lincoln) GaAsP : Holonyak, (GE) First Visible Red Light Laser and LED (1962) (Holonyak s GaAsP Alloy Semiconductor) Heterojunction Laser (1969) Alferov (Ioffe), Kromer, Panish and Hayashi (Bell) Quantum Well Laser (1977) Holonyak and Rezek (Illinois) Integrated Optoelectronic Device Lab, NTU 17

Combination of Electrical and Optical Capability Electrical Optical Integrated Optoelectronics High Volume Low Cost Highly- Integrated Scalable Large Bandwidth Low Loss Without EM Interference Great Potential! Integrated Optoelectronic Device Lab, NTU 18

Multi-GHz Light-Emitting Transistor and Resonancefree (< 5dB) Transistor Laser with t rec < 30 ps HBLET (2004) Light output from base ~MHz operation QW-HBLET (2004) Incorporated QWs to enhance optical output QW-HBLET Laser (2005) Edge-emitting Transistor Laser Resonance-free M. Feng et al, Appl. Phys. Lett. 84, 151 (2004) M. Feng et al, Appl. Phys. Lett. 84, 1953 (2004) M. Feng et al, Appl. Phys. Lett. 89, 113504 (2006) Unique three-terminal characteristics for electrical-optical integration Integrated Optoelectronic Device Lab, NTU 19

BJT Bipolar Junction Transistor vs. Heterojunction Bipolar Transistor qv N Emitter I E qv P Homojunction: ΔE g =0 qv EB qv N : electron barrier hight qv P : hole barrier hight I n I p I s Base I r I B I C qv CB Collector I n : Injected electron current I P : Injected hole current I s : Saturation current at EB junction I r : Recombination current at Base region Current gain: Ic In Ir In max I I I I I B p BJT: Depends on Doping Input: Base (I B ), Output: Collector (I C ) Requires high doping of emitter to achieve high current gain max I I n p N N d a s v v nb pe r p Integrated Optoelectronic Device Lab, NTU 20

Bipolar Junction Transistor vs. Heterojunction Bipolar Transistor HBT I n : Injected electron current qv N Emitter ΔE g I E qv P qv EB I n I s Base I r I C qv CB I P : Injected hole current I s : Saturation current at EB junction I r : Recombination current at Base region Current gain: Ic In Ir In max I I I I I B p s r p Heterojunction: ΔE g >0 qv N : electron barrier hight qv P : hole barrier hight I p I B Collector HBT: Also depends on ΔE g max I I n p N N d a v v nb pe exp[( E g ) / kt] Emitter Base BJT Highly Doped Lowly Doped HBT Lightly Doped low junction capacitance Highly doped can be made thin Reduce base transit time, improve rf performance Integrated Optoelectronic Device Lab, NTU 21

Laterally Scaled HBT Fabrication Process Emitter size determines intrinsic device size (active area) Current devices: 0.25-0.5 μm width Extrinsic contact areas determine parasitics, present contact resistance vs. junction capacitance tradeoff m-bridge Base Contact B Remove extrinsic material to eliminate capacitance E C 10 Breakdown Voltage [V] 1 InP/InGaAs Type-I DHBT InP phemt SiGe HBT UCSB Type-I InP/GaAsSb Type-II DHBT Illinois InP SHBT ETHZ Illinois InP Type-II DHBT Illinois InP PHBT Fujitsu phemt f T = 690 GHz BV CEO = 3.2 V f T = 765 GHz BV CEO = 1.65 V Minimal Extrinsic Area 100 1000 f T [GHz] Integrated Optoelectronic Device Lab, NTU 22

From Heterojunction Bipolar Transistor (HBT) to Light- Emitting Transistor (LET) n-p-n Heterojunction Bipolar Transistor (HBT) n-p-n Light-Emitting Transistor (LET) E FN I E E C E FN I C E V E FP N + - In 0.49 Ga 0.51 P p - GaAs n - GaAs I B Base QW enhances recombination and plays the role of an optical collector, competing with the electrical collector (I C ) within the base transit time t t of few ps. The existence of the third terminal, collector, which functions as a carrier collector for slow recombination carriers. Transistor current gain I I C B t B t t t B : carrier recombination lifetime in Base region t t : carrier transit time from Emitter to Collector Integrated Optoelectronic Device Lab, NTU 23

Light-Emitting Transistor Base Active Region Design Current gain, I C /I B, reduces significantly in samples with single or multiple QW(s) of varying sizes, W QW inserted in base, and increased base doping, N A Change by varying base QW design and doping H.W. Then et al, Appl. Phys. Lett. 91, 033505 (2007). Integrated Optoelectronic Device Lab, NTU 24

Reduced recombination lifetime (<100 ps) in LET Recombination rate can be tailored t B < 100 ps is possible Based on bimolecular recombination lifetime : 1 t v N QW as a collector : t B 1 GW QW N A th t v th, thermal velocity σ, effective carrier capture cross section N r, density of possible recombination sites G, constant = 2.3x10-3 cm 2 /s W QW, quantum well width N A, base doping t B 1 GW QW N A B 1 N rad A, B rad 1.910 Auger Recombination Process :C ~ 710 10 cm -30 3 cm / s for Bulk GaAs 6 / s, t B,Auger ~ 90 ps Integrated Optoelectronic Device Lab, NTU 25

Enhanced Light Output by Reducing Current Gain Changing by varying base QW design and doping For spontaneous radiative The light output P P W QW N τ QW B 1 τ B n QW 1 W QW τ B A QW recombinat ion I E = I B + I C = (1 + ) I B at same I E, I B Light t rec < 100 ps unheard of in LED community, but not uncommon in transistor community Multi-GHz spontaneous light emitter possible Integrated Optoelectronic Device Lab, NTU 26

Outline Introduction and Issues From Transistor to Transistor Laser Charge Control Model: Carrier Dynamics Multi-GHz Operation Tilted-Charge Light- Emitting Diode Conclusion Integrated Optoelectronic Device Lab, NTU 27

Charge Control Model of LET and TL BC jn == electrical collector QW == optical collector Q1 affects the radiative output Q2 affects transistor gain. I B Q t 1 t,1 I C Q t 2 t, 2 I B 1 t B Q1 Q2 Q1 Q t B t bulk 1 Q1 1 t Q Q t bulk 1 2 2 t,1 Q t 1 t, 1 Boundary conditions: Eg. t t,2 = W EC2 /2D = 1.56 ps (W EC = W B 900 Å) t t,1 = W EQW2 /2D = 1.05ps (W EQW 740 Å) n/t = 0, n ~ 0 at BC junction, I C = qad(n/x) BC at BC junction, I E = qad(n/x) EB at EB junction, I E = I B + I C M. Feng et al, Appl. Phys. Lett. 91, 053501 (2007). Integrated Optoelectronic Device Lab, NTU 28

Carrier Dynamics in (p-i-n) Light-Emitting Diode and Diode Laser Carriers must wait to recombine (no unidirectional flow ) Saturated operation. Charges governed by recombination only. Holes injected. Recombination i p n Integrated Optoelectronic Device Lab, NTU 29

Carrier Dynamics in (n-p-n) Light-Emitting Transistor and Transistor Laser: Carriers that are in dynamic flow tilted charge population. Fast carriers recombine, slow carriers exit via collector. Base charging governed by both collector and QW. Holes built-in by p-doping. 1 Charge control - Uniquely different from LED/DL 2 Photon-carrier Interaction / laser - Uniquely different from HBT Integrated Optoelectronic Device Lab, NTU 30

Room Temperature CW Operation Transistor Laser (Sept. 2005) Integrated Optoelectronic Device Lab, NTU 31

Outline Introduction and Issues From Transistor to Transistor Laser Charge Control Model: Carrier Dynamics Multi-GHz Operation Tilted-Charge Light- Emitting Diode Conclusion Integrated Optoelectronic Device Lab, NTU 32

Tilted-Charge Light-Emitting Diode Base population in the n-p-n HBLET Emitter (n) Base (p) Collector (n) Tilted population in the base region due to electrically reverse-biased Base- Collector boundary condition. Tilted population is established by the built-in reverse field between Base- Drain boundary due to common contact metallization. Carriers that are too slow to recombine within transit time t t =W B2 /2D, will be drained by the drain layer, resulting a drain current I D, I E =I B +I D. Two-port configuration can be readily implemented with the current systems. Integrated Optoelectronic Device Lab, NTU 33

TC-LED Array: increase optical output while maintain high-speed characteristics The emitter mesa diameter of the single LED is 10 mm, while the four-led array consists four separate (electrically-isolated) single LEDs biased by a common electrical input, I E. The distance between two adjacent LEDs is 30 mm. The maximum optical output of the four-led array and its corresponding bias current is about 3.3 times higher than that of the single LED. The current distribution is not fully uniform (3.3x signal vs. 4x) because of heating, especially at higher bias currents. Wu et al, IEEE Photon Tech. Lett.., 21, 1834 (2009) Integrated Optoelectronic Device Lab, NTU 34

TC-LED Array: increase optical output while maintain high-speed characteristics The input impedance of four-led array is 3.3 times smaller than that of the single LED, which is poorer impedance-match (to 50 Ω) for rf modulation. The data show an excellent fit to the form, H(f) = Ao/(1+jf/f 3 db ). Both the single LED and four-led array, though more poorer less impedance-matched, show a -3 db bandwidth, f 3dB, of 4.3 GHz at similar I E current density. The 4.3 GHz modulation bandwidth of the four-led array indicates that the array design indeed provides higher optical output with no loss in the modulation bandwidth. Integrated Optoelectronic Device Lab, NTU 35

7 GHz Modulation Bandwidth of Tilted-Charge LED Emitter size is 10x10 mm 2 No temperature control. We obtain a -3 db bandwidth, f 3dB, of 7 GHz, corresponding to an effective t B = 23 ps. World Record! G. Walter et al, Appl. Phys. Lett., 94, 231125 (2009) Integrated Optoelectronic Device Lab, NTU 36

Outline Introduction and Issues From Transistor to Transistor Laser Charge Control Model: Carrier Dynamics Multi-GHz Operation Tilted-Charge Light- Emitting Diode Conclusion Integrated Optoelectronic Device Lab, NTU 37

Conclusion Sub-100 ps carrier recombination lifetime can be easily achieved in the light-emitting transistor and transistor laser. Multi-GHz spontaneous modulation bandwidth has been demonstrated in the tilted-charge LED format. Light output can be improved in an 2x2 LED array while maintaining the high-speed characteristics. To further incorporate with Si-photonic process, the high-speed and low-cost TC-LED has the potential application for future optical interconnect. Integrated Optoelectronic Device Lab, NTU 38