Measuring the role of surface chemistry in silicon microphotonics
  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. 
Matthew Borselli, Thomas J. Johnson, and Oskar Painter