We demonstrate a high-speed silicon Mach-Zehnder modulator (MZM) with low insertion loss, based on the carrier depletion effect in a lateral PN junction. A 1.9 dB on-chip insertion loss and a VπLπ < 2 V·cm were achieved in an MZM with a 750 μm-long phase shifter by properly choosing the doping concentration and precisely locating the junction. High-speed modulations up to 45-60 Gbit/s have been demonstrated with an additional 1.6 dB optical loss, indicating a total insertion loss of 3.5 dB. A high extinction ratio of 7.5 dB was also realized at the bit rate of 50 Gbit/s with an acceptable insertion loss of 6.5 dB.

A detailed analysis of the photoluminescence ͑PL͒ from Si nanocrystals ͑NCs͒ embedded in a silicon-rich SiO 2 matrix is reported. The PL spectra consist of three Gaussian bands ͑peaks A , B, and C͒, originated from the quantum confinement effect of Si NCs, the interface state effect between a Si NC and a SiO 2 matrix, and the localized state transitions of amorphous Si clusters, respectively. The size and the surface chemistry of Si NCs are two major factors affecting the transition of the dominant PL origin from the quantum confinement effect to the interface state recombination. The larger the size of Si NCs and the higher the interface state density ͑in particular, Siv O bonds͒, the more beneficial for the interface state recombination process to surpass the quantum confinement process, in good agreement with Qin's prediction in Qin and Li ͓Phys. Rev. B 68, 85309 ͑2003͔͒. The realistic model of Si NCs embedded in a SiO 2 matrix provides a firm theoretical support to explain the transition trend.

A high-speed depletion-mode silicon-based microring modulator with interleaved PN junctions optimized for high modulation efficiency and large alignment tolerance is demonstrated. It is fabricated using standard 0.18 μm complementary metal-oxide-semiconductor processes and provides low V(π)L(π)s of 0.68 V·cm to 1.64 V·cm with a moderate doping concentration of 2 × 10(17) cm(-3). The measured modulation efficiency decreases by only 12.4% under ± 150 nm alignment errors. 25 Gbit/s non-return-zero modulation with a 4.5 dB extinction ratio is experimentally realized at a peak-to-peak driving voltage of 2 V, demonstrating the excellent performance of the novel doping profile.