The Technology of Selective Area Growth and How its Simulation Makes Photonic Integrated Circuit Possible

1. The Technology of Selective Area Growth and How its Simulation Makes Photonic Integrated Circuit Possible Muhammad A. Alam and Roosevelt E. People Silicon Lab, Agere Systems Outline Introduction: why is PIC difficult ? SAG and its Simulation How Agere designs PIC Circuits PIC for short-haul systems Conclusions

2. Collaborators • S. Sputz, Eric Issacs, Y. Vandenberg, K. Evans-Lutterodt • J. Johnson, L. Ketelsen, and J. Eng • M. Hybertsen, Kent Smith, M. Pinto • L. Gruezke and L. Reynolds

3. Long distance vs. local area network long-distance metro LAN Wide Area Network : high performance circuits (part 1) Local Area Network: lower performance but also lower cost (part II)

4. Introduction: A Simple Optical Communication System Receiver Transmitter Laser Modulator Coupler Amplifier Detector Amplifier data in data out

5. Introduction: silicon vs. III-V Integration GaAs H-MESFET Chips (1996) ~ 1 million Transistors ~ 4 level of metals Silicon DSP Chip: ~ 10 million Transistors ~ 5/6 level of metals When devices are the same, high integration is possible when devices are dissimilar, integration becomes difficult

6. PIC issue 1: Integration of Dissimilar Devices (e.g. EML) Top View x z Laser Modulator Side View y z How to make devices of different thicknesses on the same wafer ?

7. PIC issue 2: Optical Interconnect (e.g. EML) Coupling (D) Mode Shift ( dD) Radiation Loss ( L/D ) FCA ( L ) How to make a low loss optical interconnect ?

8. glass WG glass WG glass WG InP laser PIC issue 3: Coupling to Fiber poor Simple Coupling InP laser Good but costly and lossy Lens Coupling InP laser 5 mm Integrated and excellent Expanded-Beam Coupling 30mm How to ensure good coupling between PIC and the fiber ? (L. Ketelsen)

9. isolators Laser array Amplifer/modulator fiber Ball lens in V-grove p-i-n- Monitor diode Planar lightwave circuit How to Solve these Three Problems (~ 1992/1993), (Tom Koch, U. Koren ...) (~1999/2000), (L. Ketelsen, J. Johnson ...)

10. Traditional Device Integration (with Etch and Regrowth Photo-Resist QW

11. Traditional Interconnect Integration (Issue 2) Multiple Step Etch Shadow Deposition Photo-Resist

12. SAG Basics: A Schematic Diagram of a MOCVD Reactor H2 Carrier Gas + AsH3 + PH3 TMIn TMGa SiH4 Stagnant Layer Heater Wafer

13. Model: Description of SAG MOCVD System

14. The Empirical Approach to SAG A team of about ten people trying to see - a. did we get the correct thickness variation b. did we get the right composition variation (index) ?

15. Model: Simulation of SAG MOCVD Dj Dnj(r)=0, kjnj(0)=J(0) Dj ~ T(3/2) [(mjmH)/(mj+mH) ]s P P = pressure, D = diffusion coefficient mH=masses

16. SAG Model: Simulation of Gas Dynamics in Boundary Layer x z Ga Flow Pattern D/k=180 um In Flow Pattern D/k = 25 um

17. Properties of the Composite Films Enhancement Factor: E In (x,y) = n In (x,y,0) / n In (x0,y0,0) Thickness Enhancement: E Q (x,y) = E In (x,y) (1-s0) + E Ga (x,y)s0 Composition 1 - s(x,y) = ( E In (x,y)/E Q (x,y) ) (1-s0) Q0 = In(1-s0)Ga(s0)As(1-t0)P(t0) Q=In(1-s)Ga(s)As(1-t)P(t)

18. Basics: Experimental Techniques WYCO Micro-Xray Micro-PL Thickness Bandgap Strain E. Issacs, J. Vandenberg, K. Lutterodt R. Scotti, S. Sputz S. Sputz

19. Experimental Verification Thickness In, Bandgap Ga ; Thickness, bandgap, and strain has same profile

20. A Puzzle ( 30 mm ) Laser x z Alley 300 mm T. Siegrist, J. Vandenberg, T. Pernell, and S. Sputz Intuition: Bandgap follows thickness in the laser section Puzzle: It does the opposite in the alley Process vs. characterization

21. Puzzle Resolved Thickness (nm) Line Cut Expt Wavelength (nm) Sim. • Explanation: • In decay 25 um; Ga decay 180 um • Expander is Ga rich • Intrinsic property, not a material • problem Strain(%)

22. Prediction: Spatial Oscillation in Growth

23. Rectangular Laser • Hammerhead Relative Thickness • Final Design Distance (micron) Design Process: An Example for WSL Laser array fiber

24. Design Verified: Mask design for the WSL Project z x

25. Isolator modulator amplifier fiber p-i-n- Monitor diode Design: Mask design for the XBAM Lens no longer necessary self-aligned, reduced cost

26. Design: Mask design for the XBAM prev best Ultra-short oxide masks, impossible without a simulator

27. Design: Mask design for the Tunable Laser Project Isolator modulator amplifier fiber p-i-n- Monitor diode Laser array

28. Design: Mask design for the Tunable Laser Project Side view Top view mask layer - first use of two level SAG - 26 parameter optimization - OFC best new product award

29. This is what we have now

30. where do we stand now on long-haul device integration ? 1. Simulation tools available on the web. Automatically evaluates designs and tells one if how closely the objectives can be made. 2. All the designers know how to use it, and since 1997 all the PIC designs are based on it. 3. Instead of months, design of PIC integration now takes only days. 4. All optical processing for > 40G circuits will also be completely integrated from the very beginning - this in contrast to our competitors “…. SAG modeling has become so remarkably accurate and predictive that we will no longer use characterization as a means for verifying SAG processing step ... “

31. Long distance vs. local area network fiber transceiver long-distance metro LAN ethernet and fiber-Channel - direct modulation - very low cost (detector/transmitter) - short distances (amplifier-less)

32. Small footprint lasers, Packing Density and Radiation Loss Minimum inter-device spacing is 300 micron (NTT, 1996)!

33. Small footprint lasers: Radiation Loss and Redesign 1. Main contribution to radiation is from the transition region. 2. Rectangular to special mask profile

34. Lateral taper Small footprint lasers: after fabrication Quantum well active Underlying thick waveguide Blocking structure Laser Facet 20:1 lateral active taper Underlying thick waveguide Expander Facet No active, thinned waveguide

35. expanded beam laser standard laser Small footprint lasers: coupled power 3x times greater XBL Far Field ~16x9 degrees • Coupled power is ~3x greater • Narrow far field provides increased coupling • power and increased alignment tolerance FP Far Field: ~30x30 degrees

36. Small footprint lasers: increased packing density Prev. “theory” limit - 50 percent more devices with 3x higher performance - a win/win situation - Part of Agere’s new Transceiver chip-set, by far the best in the industry - J. Eng did laser design

37. fiber transceiver PIC of a Self-aligned Transponder for LAN I/O Port Laser Detector Transceiver Oxide Mask In Distribution

38. PIC for coarse-WDM Transceiver for LAN fiber transceiver Direct modulation must be below 20 GHz EML or external modulation too expensive Multiple wavelength is a solution - 8-10 colors over a multi-mode/ single- mode fiber

39. LAN-CWDM Solution: Multiple Wavelength VCSEL Array Mirror QW Mirror l1 l1 l2 l3 Simultaneous matching of Cavity Mode and Gain Peak is difficult !

40. LAN-CWDM VCSEL ARRAY : Design of Quantum Wells Bottom SCH Mirror

41. LAN-CWDM VCSEL: Design of Cavity and Matching Top SCH Bottom SCH Mirror Z-axis (um) SAG allows perfect matching of cavity and emission modes for all wavelengths !

42. Conclusions • Photonic Integrated Circuits are becoming integral part of modern communication systems, and SAG-MOCVD has been an enabling process technique to fabricate these PIC. • The 3D SAG model has been remarkably successful in designing these PIC, and has reduced design cycle time from months to days. • As part of a tool suite (laser simulation for QW and Bragg-grating design, beam propagation for waveguide design, etc. ), makes PIC design less of a trial-an-error process.