Monolithic integration and optimization of waveguide silicon modulators and germanium photodetectors

Abstract

By tapping on the volume manufacturing capability of the Si CMOS platform, Si photonics can potentially offer costeffective yet high performance optical interface solutions, and will be especially important in short reach applications. Numerous challenges lie ahead for monolithically integrated Si photonics. There are process integration and thermal budget constraints when monolithically integrating individual Si photonic components such as modulators and photodetectors, and also CMOS on the same chip. In this work, the CMOS-compatible monolithic integration of Si modulators and Ge-on-Si photodetectors on the same wafer is demonstrated and the details of performance optimization are also discussed. Besides process compatibility, the modulators and photodetectors should possess high efficiency and the ability to operate at low power supply voltages. The methods to achieve this are also described. The carrier depletion type Si modulators achieved high modulation efficiency and speed (Vπ.Lπ = 2.6 V.cm, 10 Gbps). At 10 Gbps, an extinction ratio of 6 dB was measured in a modulator with 2-mm-long phase-shifters using single-ended drive (VRF = 5 V pp ). Low voltage operation at 3.125 Gbps was also demonstrated using differential drive, which allowed the drive voltage to be reduced to only 1 V (VRF = 1 Vpp). Ge-on-Si photodetectors were integrated by using a selective epitaxial Ge growth process. The performance of such photodetectors was evaluated in terms of speed, responsivity and dark current for different temperatures and operating voltages. It is shown that introducing a low thermal budget post-epitaxy anneal improves the performance of the Ge photodetectors, resulting in significantly improved dark current. The responsivity and speed in the low voltage regime are also enhanced, which enhances low voltage or even short-circuit ( V Bias = 0 V) operation.