Coaxial recess integration of InGaAs edge emitting laser diodes with waveguides on silicon substrates : a complete solution to laser integration on ICs

Abstract

In this thesis, the first demonstration of the full integration of 1.55[mu]m InGaAs/InP edge emitting platelet laser diodes with SiON/SiO2 dielectric waveguides on a silicon substrate is presented. Small footprint laser platelets (300[mu]m long by 150[mu mwide and 6.3[mu]m high), are integrated and bonded in recesses etched in SiO2 deposited on a Si substrate, and are coaxially coupled to the dielectric waveguides fabricated on the same wafer. Lasers assembled in 6.5[mu]m deep recesses are securely solder-bonded in place with a thin film Al/In bonding layer, which also brings the laser platelet back side n-contact to the wafer front side for measurements. The Al/In bonding layer composition and thickness are carefully optimized to provide highly reproducible vertical alignment to maximize the coupling of the laser output beam to the dielectric waveguide. Lasers are bonded into the recesses with this solder-bonding layer during a pressure assisted temperature cycle at 220°C. The low temperature nature of the bonding phase makes this integration technique CMOS compatible. The integrated lasers show lasing operation with threshold currents of Ith=17mA and Ith=19mA for pulsed and continuous wave drives respectively, at T=15°C. The output spectrum shows single mode lasing near 1550[mu]m, and a side mode suppression ratio of 25dB which is significantly higher than typical Fabry Perot cavity laser diodes. Furthermore, the integrated lasers have a characteristic temperature, T0, of 76K which is improved from 60K for non-integrated lasers. Also the integrated lasers consistently show lower threshold currents compared to their non-integrated counterparts. The coupling loss between the laser and dielectric waveguide is extracted to be as low as 1dB, a value that can be further reduced by improved horizontal alignment and better matching the widths of laser stripe and dielectric waveguide. Overall, this recess integration approach is CMOS compatible, is highly modular, compact and flexible, permits testing and selection of devices prior to integration, and allows integration of lasers emitting at different wavelengths on the same chip. It eliminates the need for wafer bonding III/V substrates to the host Si IC along with added complexity and cost it involves, and can be implemented using easily accessible technologies.