Hybrid silicon evanescent photoreceiver

Research into silicon photonics has many motivations for employing mature CMOS processing technology to fabricate low-cost photonic and electronic integrated devices. High-volume manufacturing for data communication applications is just one example. Recently, we showed the utility of a hybrid silicon evanescent device in building lasers, amplifiers, and photodetectors. This structure combines a silicon waveguide and III-V semiconductor quantum wells to promote efficient light amplification and absorption through the III-V layers. The technique avoids the need for sophisticated alignment between the different substrates. Moreover, it enables production of multiple photonic active devices through a single bonding step, thus distinguishing itself from the individual III-V die attachment method with flip-chip bonding. Lasers,¹ amplifiers,² detectors,³ and racetrack lasers⁴ are some of the standalone devices that have been built using this approach.

Here, we combine photoreceivers with an amplifier and photodetector⁵ to illustrate the potential of photonic integration. Light detection is a major area of endeavor in silicon photonics. Germanium and silicon germanium, for instance, are good detector materials because of their absorption at the 1.3- and 1.5μm telecommunication wavelengths and their CMOS processing compatibility. Among the advances^6,7 made possible by these materials is a photoreceiver integrated with CMOS electronics.⁸ Silicon evanescent photodetectors are also interesting since their absorption edge can easily be extended beyond the 1600nm regime by engineering III−V quantum wells. In addition, they can be combined with lasers and amplifiers using the same materials and fabrication procedures.

Figure 1(a) shows an array of integrated amplifiers and detectors. For a 1.2mm−long structure, the maximum gain is 9.5dB at 300mA. The quantum efficiency of the 100μm detector is 50%. By putting the two together, the responsivity of the receiver increases to 5.7A/W with preamplification. The device shows 0.5dB saturation at a photocurrent of 25mA (see Figure 2). To reduce reflection in the amplifier, the ends of the III-V mesa are tapered to adiabatically (without mode mismatch loss) convert the hybrid mode to the silicon mode: see Figure 1(b). The reflectivity of these tapered junctions is estimated from the amplified spontaneous emission noise of the amplifier and is calculated to be <6× 10⁻⁴ with a coupling loss between 0.6 and 1.2dB for different devices across the die. The device bandwidth is measured as 3GHz by time domain impulse response, even though the resistance-capacitance–limited bandwidth is estimated to be 7.5GHz, indicating that the device speed is primarily constrained by the current III-V layer design. The quantum wells have a valence band offset of ∼105meV between the well and the barrier, which causes hole trapping and electrical field screening. A higher bandwidth can be achieved by using wells with a smaller valence band offset and a thinner separated confinement heterostructure layer to reduce the hole transit time.

Figure 1. (a) Top view of eight integrated devices with amplifiers (left) and detectors (right). (b) The III−V taper of the amplifier.

Figure 2. Saturation characteristics of the preamplified photodetector.

The hybrid silicon evanescent device platform provides a means of building active photonic devices on silicon by combining its mature, low-cost manufacturability with the optical functionality of III-V materials. The photoreceiver we have described shows an 8.5dB improvement in receiver sensitivity enabled by the integration of a hybrid silicon evanescent preamplifier and a waveguide photodetector. Potential benefits of this research include wavelength division multiplexing receivers with silicon passive wavelength demultiplexers and improved detector bandwidth.