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Min Hyeong KIM High-Speed Circuits and Systems Laboratory E.E. Engineering at YONSEI UNIVERITY 2011. 4. 13. 1
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[ Contents ] 1.Abstract 2.Introduction - Several laser structures 3.Device structure I.Lasing gain material II.Waveguides III.Bonding technology IV.Additional layer 4.Fabrication process 5.Experimental results 6.Conclusion & Summary 2
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1. Abstract A laser can be utilized on a silicon waveguide bonded to a multiple quantum wells(MQW). This structure allows the optical waveguide defined by CMOS technology to get an optical gain provided by Ⅲ - Ⅴ materials. It has a 1538nm laser, pulsed threshold of 30mW, and an output power of 1.4mW. How to implement this structure? How to operate?? Which principles??? 3
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2. Introduction It is challenge to build light-emitting devices on VLSI CMOS technology. Because Si has an indirect bandgap(E_g). How to overcome this challenges? 1.Raman laser 2.Using porous silicon or nanocrystalline-Si 3.SiGe quantum cascade structures 4.Er doped silica 5.Etc…. In this paper, we report the first demonstration of silicon evanescently** coupled laser structure. ** Evanescent wave An evanescent wave is a nearfield standing wave with an intensity that exhibits exponential decay with distance from the boundary at which the wave was formed. 4
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3. Device structure Silica Si substrate InP Cladding MQW gain material Si MQW laser + SL barrier + Bonding technology + SOI waveguide Light-emitting Process : Current or Laser Pumping >> MQW lasing >> wave evanescent to SOI waveguide >> output guiding 5
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3. Device structure Ⅰ. Lasing gain material – MQW(mutiple quantum wells) Ⅱ. Waveguides – SOI structure(last topic) A quantum well laser is a laser diode in which the active region of the device is so narrow that quantum confinement occurs. The wavelength of the light is determined by the width of the active region. Much shorter wavelengths can be obtained. Low threshold current. The greater efficiency. Ⅰ Ⅱ 6
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3. Device structure Ⅲ. Bonding technology – Plasma-Assisted Low Temperature Wafer Bonding Ⅲ Two samples are bonded together via oxygen plasma assisted wafer bonding Low temperature annealing(~250 ℃ ) preserves the optical gain of MQW. High temperature annealing makes (1) a surface non- uniformities and (2) gain reduction. Hydrophilic surface bonding : 125 ℃ Hydrophobic surface bonding : 400 ℃ Are better choices. 7
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3. Device structure Ⅳ Ⅳ. Additional layer – SL(Superlattice) barrier SL interposition Doped SL Non-intentionally doped SL Defect-blocking layer : It prevents the deep propagation of defects by fusing process. Luminescent properties are improved. 8
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3. Device structure _ detailed design InP cladding layer MQW absorber (500nm) MQW laser structure MQW absorber (50nm) InP cladding layer (110nm spacer) SL barrier (7.5nm) Si waveguide (W=1.3u, H=0.97u, L=0.78u) Silica layer (500nm) Si substrate For operating 1538nm wavelength 9
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4. Fabrication process 1.Form SiO2 layer on Si substrate _ thermal oxidation for 2 hours at 1050 ℃ 2.Form Si rib waveguides _ using inductively coupled plasma etching 3.Hetero-bond InP (already completed)/ Si _ Plasma-Assisted Low Temperature Wafer Bonding 4.Dice the device for mirroring 5.Polish and HR coat**(High-reflection coatings) for mirroring ** HR coating 10
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5. Experimental results [Experimental Conditions] 980nm laser diode pumping Through the top InP cladding layer Recorded on an IR camera through a polarizing beam splitter Laser diode Pumping [Results Pictures] Calculated TE mode profile TE near field image 11
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5. Experimental results A laser output almost occurs in the optical mode(Si waveguide) Slab mode(MQW) do not support lasing output The pumping threshold increase from 30mW to 50mW between 12 ℃ ~20 ℃. Quantum efficiency at 12 ℃ : about 3.2% Cavity length 600um Temperature 12 ℃ Pump power=1.4* threshold Group index=C/Vg=3.85 12
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6. Conclusion & Summary We can make the optically pumped Si evanescent laser consisting of MQW as active region bonded to Si waveguide as a passive device. (conclusion!) For operating at 1538nm, pump threshold is 30mW and slope efficiency is 3.2%. (conclusion!) On bonding process, use Plasma-Assisted Low Temperature Wafer Bonding to maintain the optical gain of gain material. By using SL(Superlattice) barrier, we can block the defects propagation from fusing(bonding) process. 13
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