Ultrafast hot-carrier dynamics at chemically modified Ge interfaces probed by SHG.

Time-resolved second-harmonic generation (SHG) was used to study the hot-carrier dynamics and nonlinear optical properties of S-terminated and Cl-terminated Ge(111) interfaces on the femtosecond time scale. The hot-carrier second-order nonlinear optical susceptibilities were determined to be 720 +/- 50 times greater than the valence-band second-order nonlinear optical susceptibilities for the Ge(111)-S system and 880 +/- 100 times greater in the Ge(111)-Cl system. Furthermore, the ground- and excited-state second-order nonlinear optical susceptibilities are suggested to be out of phase for Ge(111)-S and Ge(111)-Cl systems, leading to a pump-induced decrease in the SHG signal as opposed to the increase in the SHG signal observed in the Ge(111)-GeO2 system. Although the SHG response reaches a steady state in 415 +/- 90 fs in the Ge(111)-GeO2 system, a faster response is observed in the Ge(111)-S system, 220 +/- 85 fs, and in the Ge(111)-Cl system, 172 +/- 50 fs. This suggests significantly faster carrier cooling at the Ge(111)-Cl and Ge(111)-S interfaces, with significant implications for hot-carrier mediated device degradation, and migration to high-K dielectrics.