H. Stafast

Ionization and shielding of interface states in native p+-Si/SiO2 probed by electric field induced second harmonic generation

Electric field induced second harmonic measurements applying femtosecond laser pulses (1.59 eV, 80±5 fs, 80 MHz) to substantially boron doped p+-Si/SiO2 interfaces reveal a temporal evolution of the second harmonic (SH) signal, which differs drastically from that of weakly doped samples. A significant initial SH signal is observed in native p+-Si/SiO2 interfaces for boron doping concentrations >7.5×1017 cm−3. This SH signal is attributed to a built-in interfacial electric field E0 generated by the doping induced accumulation of charges at the Si/SiO2 interface following the ionization of interface defect states. A sign reversal is observed in the azimuthal SH anisotropy pattern of the initial SH signal relative to that of the saturated SH signal in p+-Si/SiO2 indicating that the doping related and electron induced interfacial field components oppose each other. Furthermore, the intensity dependence of the initial SH signal in p+-Si/SiO2 is found to be nonquadratic and, in particular, shows a nonmonotonic ...

Optical second-harmonic imaging of PbxCd1−xTe ternary alloys

We employ femtosecond laser pulses (80 fs, 1.59 eV, and 80 MHz) to study the optical second-harmonic (SH) response of PbxCd1−xTe ternary alloys (x about 0.2) grown by the vertical Bridgman method. The alloy segregates into a Pb-rich and a Cd-rich phase, the latter dominating the SH response of the ternary alloy by at least two orders of magnitude. Several sample regions show a regular layer-by-layer accommodation of the Pb-rich and Cd-rich phases as seen by a periodic alternation of the alloy’s SH response on a ∼10‐μm length scale. Furthermore, we employ polarization-resolved SH imaging as well as SH imaging at different azimuthal angles to obtain spatially resolved mappings of the sample, which are sensitive to the composition as well as the growth orientation of the PbxCd1−xTe material system. We observe an azimuthal phase shift of approximately 30° between coherent macroscopic regions (several mm2) in the Cd-rich phase of the ternary alloy. We interpret these regions as large area crystalline grains of...

Ultraviolet pulse laser induced modifications of native silicon/silica interfaces analyzed by optical second harmonic generation

Native silicon/silica (Si∕SiO2) interfaces are investigated by electric field induced second harmonic (EFISH) generation employing near infrared femtosecond laser pulses (782.8nm, 80fs, 10nJ, 80MHz repetition rate). Here, the temporal EFISH evolution induced by the femtosecond laser irradiation is recorded with a time resolution of 0.2s. Comparative EFISH studies are performed with Si∕SiO2 interfaces, preexposed to ultraviolet (UV) laser pulses (308nm, 16ns, 0.45–2.8J∕cm2) as well as virgin Si∕SiO2. After UV irradiation the femtosecond laser induced electron injection and trapping in the ultrathin oxide is found to be drastically accelerated in contrast to the essentially unaffected interfacial hole dynamics. This result is explained by an enhanced interfacial electron trap density caused in most cases by the UV laser induced melting and recrystallization of the near-interface silicon. Furthermore, three-dimensional second harmonic imaging reveals a saturation effect of the UV induced sample modification ...

Non-quadratic intensity dependence of the second harmonic signal from the p + -Si/SiO 2 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).

Femtosecond laser diagnostics of the built-in electric field across the p+-Si/SiO2 interface and its ultrafast shielding

Ultrafast shielding of the built-in electric field E0 across the p+-Si/SiO2 interface of boron doped Si upon near infrared femtosecond (fs) laser pulse irradiation (73 ± 5 fs, 35 GW/cm2≤ Ipeak(ω) ≤ 115 GW/cm2) is shown to be dominated by electron-hole (e-h) pairs generated via two-photon absorption (TPA), whereas contributions from one-photon absorption (OPA) appear negligible. E0 shows up in the instantaneous signal I(2ω)(t≈0)≈I(2ω)(E0) of the Electric Field Induced Second Harmonic (EFISH). Its power law is derived from the linear log I(2ω)(E0) vs. log (Ipeak(ω))n plots of six fs laser wavelengths 741.2 nm ≤ λ ≤ 801.0 nm for the first time. These reveal 1.2 ≤ n(λ) ≤ 2.1 with the minimum at λ = 752.4 nm (2hν = 3.3 eV) related to resonantly enhanced TPA. Shielding of E0 by e-h pairs from OPA cannot be detected by EFISH in the same fs laser pulse as their generation requires relatively slow electron-phonon coupling.