Silicon Optical Modulators in Silicon-on-Insulator Substrate Based on the p–i–n Waveguide Structure

We report the fabrication and characterization of the P–P-–N optical waveguide modulators fabricated on silicon-on-insulator (SOI) substrate. The modulation scheme was achieved via the carrier injection, or plasma dispersion effect. The corresponding P and N regions were defined in both types of SOI substrates (P-type SOI and highly resistive N-type SOI substrates with respective resistivities of ρ~7–10 Ω cm and ρ~7000–10000 Ω cm) using the spin-on-dopant (SOD) technique conducted at 900–1000 °C in a nitrogen ambient. The resultant dopant concentrations and diffusion depths were found to be critically dependent on the diffusion time/temperature and the resistivity of SOI substrate used. For the modulators fabricated with various waveguide widths and electrode lengths, the corresponding modulation index was enhanced in response to an increase in the electrode (or modulation) length and/or a decrease in waveguide width. The highest modulation index of ~4.15% was successfully achieved for a silicon P–P-–N waveguide modulator with 5-µm-wide waveguide and 7-mm-long modulation electrode.