An interface specific investigation, by time-resolved second-harmonic generation, shows that photoexcited carrier dynamics at Si(111) interfaces depend strongly on surface termination. Oxide- and H-terminated surfaces show distinct transient behavior, with a surface recombination velocity <103cm/s. Incompletely H-terminated Si(111) shows faster dynamics, correlating with less interface passivation. A simple model reveals that the second-order nonlinear optical susceptibility of photoexcited carriers is two orders of magnitude greater than that of the valence band electrons.

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