We report on a ${\mathrm{Si}}_3{\mathrm{N}}_4$ -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 $70\mathrm{ns}$ were obtained in waveguides passivated with a 45-nm-thick ${\mathrm{Si}}_3{\mathrm{N}}_4$ coating. Such surface passivation is essential for low-power operation of optical switches based on carrier injection.