Surface encapsulation for low-loss silicon photonics

Encapsulation layers are explored for passivating the surfaces of silicon to reduce optical absorption in the $1500\mathrm{nm}$ wavelength band. Surface-sensitive test structures consisting of microdisk resonators are fabricated for this purpose. Based on previous work in silicon photovoltaics, coatings of $\mathrm{Si}{\mathrm{N}}_x$ and $\mathrm{Si}{\mathrm{O}}_2$ are applied under varying deposition and annealing conditions. A short dry thermal oxidation followed by a long high-temperature ${\mathrm{N}}_2$ anneal is found to be most effective at long-term encapsulation and reduction of interface absorption. Minimization of the optical loss is attributed to simultaneous reduction in sub-band-gap silicon surface states and hydrogen in the capping material.