Semiconductor Lasers on Silicon John E. Bowers

Dept. of ECE, University of California, Santa Barbara, 93106 bowers@ece.ucsb.edu

Abstract

Rapid advances in silicon photonics are being driven by a combination of a need for more complex, higher functionality and lower cost photonic integrated circuits. The advances are also driven by the needs for silicon electronics to limit the pin count and power limits of electronic chips, as summarized in the International Technology Roadmap for Semiconductors (ITRS). The need to make a useful laser in or on silicon is significant. In direct bandgap materials, such as GaAs and InGaAsP, radiative recombination occurs rapidly and efficiently. However, silicon, like germanium, is an indirect bandgap material, and has a light emission efficiency on the order of 10-6. Hybrid Silicon Lasers: We have developed a hybrid approach to lasers on silicon that combines InGaAlAs layers on silicon

waveguides[1]. We wafer bond a compound semiconductor wafer to a SOI wafer using the same process used to make SOI substrates themselves. This combines the superior gain characteristics of compound semiconductors with the superior passive waveguide characteristics of silicon waveguides and allows photonics to gain the advantages of CMOS processing on silicon wafers. We have demonstrated wafer bonding for substrates up to 150 mm in diameter, and the photoluminescence characteristics of the bonded compound semiconductor films actually improve after bonding. The hybrid silicon process has been used to demonstrate Fabry Perot lasers that operate at temperatures up to 105 °C, and with powers up to 30 mW. Distributed feedback (DFB), distributed Bragg reflector (DBR) and segmented grating DBR (SGDBR) lasers have also been demonstrated on silicon substrates. For optical interconnects, small size and low power consumption are critical and this requires a short cavity design. Compact microring lasers with 50 µm diameter have cw thresholds as low as 4 mA and reasonable output powers (3.5 mW) (Fig. 1). Further scaling down of the laser diameter and reducing scattering loss is necessary to reduce the threshold to the 100 µA level Hybrid Silicon Photonic Integrated Circuits: One major advantage of silicon CMOS is integration, and in addition to lasers, many other optical elements have been demonstrated for next generation optical PICs. 50 Gbit/s hybrid silicon electroabsorption modulators have been demonstrated. Hybrid silicon Mach Zehnder modulators operating at 40 Gbit/s were demonstrated with a Vπ of 4 V. A variety of switches on silicon have been demonstrated for use in on-chip interconnects. Hybrid silicon optical amplifiers with 18 dB chip gain and 12 dBm saturation power have been demonstrated. SiGe PIN and APD photodetectors as well as hybrid silicon photodetectors, have been demonstrated. Modest levels of integration have been demonstrated, including arrays of eight DFB and eight EA modulators and photodetectors as well as eight hybrid silicon optical amplifiers integrated with eight photodetectors.

1. Liang, Bowers, Nature Photonics (2010).

Fig. 1: (a) Hybrid silicon ring laser threshold currents. Inset: SEM image of a 15 µm diameter laser (b) Part of a 150 mm diameter wafer containing ring cavity lasers and a 1 cm2 chip containing microring lasers.