  We theoretically characterize the free-carrier plasma dispersion effect in
fully-etched silicon waveguides, with various dielectric material claddings,
due to fixed and interface charges at the silicon-dielectric interfaces. The
values used for these charges are obtained from the measured
capacitance-voltage (C-V) characteristics of SiO2, SiNx, and Al2O3 thin films
deposited on silicon substrates. The effect of the charges on the properties of
silicon waveguides is then calculated using the semiconductor physics tool
Silvaco in combination with the finite-difference time-domain (FDTD) method
solver Lumerical. Our results show that, in addition to being a critical factor
in the analysis of such active devices as capacitively-driven silicon
modulators, this effect should also be taken into account when considering the
propagation losses of passive silicon waveguides.
