Capacitively-Induced Free-Carrier Effects in Nanoscale Silicon Waveguides for Electro-Optic Modulation

We fabricate silicon waveguides in silicon-on-insulator (SOI) wafers clad with either silicon dioxide, silicon nitride, or aluminum oxide, and by measuring the electro-optic behavior of ring resonators, we characterize the cladding-dependent and capacitively-induced free-carrier effects in each of these waveguides. By comparing our measured data with simulation results, we confirm that the observed voltage dependencies of the transmission spectra are due to changes in the concentrations of holes and electrons within the semiconductor waveguide, and we show for the first time how strongly these effects depend on the cladding material which comes into contact with the silicon waveguide. Additionally, the waveguide loss is found to have a particularly high sensitivity to the applied voltage, and may therefore find use in a wide range of applications which require low- or high-loss propagation. Collectively, these phenomena may be incorporated into more complex waveguide designs in the future to create high-efficiency electro-optic modulators.