  We show that undoped silicon waveguides may suffer of up to 1.8 dB/cm
free-carrier absorption caused by improper surface passivation. To verify the
effects of free-carriers we apply a gate field to the waveguides. Smallest
losses correspond to higher electrical sheet resistances and are generally
obtained with non-zero gate fields. The presence of free carriers for zero gate
field is attributed to surface transfer doping. These results open new
perspectives for minimizing propagation losses in silicon waveguides and for
obtaining low-loss and highly conductive silicon films without applying a gate
voltage.
