Silicon-nitride surface passivation of submicrometer silicon waveguides for low-power optical switches

We report on a Si₃N₄-based surface-passivation technique for increasing free-carrier lifetimes in submicrometer silicon-on-insulator waveguides by up to 2 orders of magnitude. The carrier lifetime was measured by injecting free carriers into a critically coupled ring resonator through photoexcitation by short pump pulses and subsequently tracking the spectral detuning of the resonator during the carrier-recombination phase. Carrier lifetimes of up to 70 ns were obtained in waveguides passivated with a 45-nm-thick Si₃N₄coating. Such surface passivation is essential for low-power operation of optical switches based on carrier injection.