Non-quadratic intensity dependence of the second harmonic signal from the p+-Si/SiO2 interface due to ultrafast photo-induced charge carrier screening

The Si/SiO₂ interface is the most widely used system in modern electronic devices. Electrically active defects at this interface play an important role in device performance and reliability. In a previous study we have shown that in the case of highly boron doped Si, these interfacial defect states are ionized and result in a built-in interfacial electric field, which gives rise to an instantaneous electric field induced second harmonic (EFISH) signal, I^(2ω)(E₀), upon irradiation with femtosecond laser pulses (73±5 fs, 35 GW/cm² ≤ I^(ω)_peak ≤ 115 GW/cm²) [1]. We now show that the observed power law dependence of the instantaneous EFISH signal on the incident intensity, I^(2ω)(E₀) ∝ (I^(ω)_peak)ⁿ, reveals 1.2 ≤ ≤ 2.1 for six fs laser wavelengths 741.2 nm ≤ λ ≤ 801.0 nm. The lowest value is observed at λ = 752.4 nm (2-hv = 3.3 eV). Its deviation from = 2 is attributed to shielding of by electron-hole pairs generated via two-photon absorption (TPA).