  The silicon/silicon dioxide (Si/SiO2) interface plays a crucial role in the
performance, cost, and reliability of most modern microelectronic devices, from
the basic transistor to flash memory, digital cameras, and solar cells. Today
the gate oxide thickness of modern transistors is roughly 5 atomic layers, with
8 metal wire layers required to transport all the signals within a
microprocessor. In addition to the increasing latency of such reduced-dimension
metal wires, further "Moore's Law" scaling of transistor cost and density is
predicted to saturate in the next decade. As a result, silicon-based
microphotonics is being explored for the routing and generation of
high-bandwidth signals. In comparison to the extensive knowledge of the
electronic properties of the Si/SiO2 interface, little is known about the
optical properties of Si surfaces used in microphotonics. In this Letter, we
explore the optical properties of the Si surface in the
telecommunication-relevant wavelength band of 1400-1600 nm. Utilizing a high
quality factor (Q ~ 1.5x10^6) optical microresonator to provide sensitivity
down to a fractional surface optical loss of 10^-7, we show that optical loss
within Si microphotonic components can be dramatically altered by Si surface
preparation, with fraction loss of 2 x 10^-5 measured for chemical oxide
surfaces as compared to <2 x 10^-6 for hydrogen-terminated Si surfaces. These
results indicate that the optical properties of Si surfaces can be
significantly and reversibly altered by standard microelectronics treatments,
and that stable, high optical quality surface passivation layers will be
critical in future Si micro- and nano-photonic systems.
