| The dynamics of carrier relaxation in Group-III Nitrides on a femtosecond time scale were investigated.; A novel technique for the spectro-temporal characterization of a UV white-light continuum, which was used as a probe pulse in pump-probe spectroscopy, was developed, namely the time-resolved transient grating technique. The characterization of a 700 nm broad white-light continuum with a 25-fs time resolution was demonstrated. The limits of the time resolution of this technique were thoroughly investigated. Optical quenching of UV-photo conductivity by a near-IR beam in a GaN thin film MSM detector was observed. A 4-level rate-equation model was developed to explain its dependence on the IR-intensity. A midgap metastable defect level with an estimated relaxation time of approx. 10 μs is found responsible for the photoconductivity quenching.; A rate equation model with 6 different levels was developed to analyze the carrier relaxation and nonlinear excitation mechanisms in a InGaN/GaN multiple quantum well. Parameters like carrier relaxation times and nonlinear absorption coefficients were determined quantitatively.; Intraband relaxation of electrons and carrier trapping into bandtail states occurs within 700–900 fs. The observed radiative decay time of 5 ps for the saturable yellow photoluminescence is faster than usually reported in literature.; The near-IR nonlinear photoresponse showing a power-law dependence of order p = 1,2–3,3 and close to 4 is explained. 3-photon interband absorption (p = 3) is the dominant carrier excitation process. Saturable carrier trapping results in an effective 4 th-order response.; The strong localization of the relaxation behavior and the photoresponse on a 1–2 μm length scale is explained as clustering of the VGa-SiN defect complex around extended defects.; The 3-photon and the non-degenerate 2-photon absorption coefficient in GaN are determined and compared to a 2-parabolic-band model for semiconductors. The measured values show reasonable agreement with the model.; Fast carrier trapping of conduction band carriers in bandtail states was found to occur within 2 ps, carrier relaxation between different bandtail states with a time constant of 500 fs.; The free-carrier induced refractive index change Δn can be accounted for with a band-filling model, including contributions from free-carrier absorption and interband dipole-transitions. |
