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Time-dependent femtosecond pump-probe spectroscopic ellipsometry studies onzincblende gallium-nitride (zb-GaN) are performed and analyzed between2.9-3.7eV. An ultra-fast change of the absorption onset (3.23eV for zb-GaN) isobserved by investigating the imaginary part of the dielectric function. The266nm (4.66eV) pump pulses induce a large free-carrier concentration up to$4\times 10^{20}$cm$^{-3}$, influencing the transition energy betweenconduction and valence bands due to many-body effects, like band filling andband gap renormalization, up to $\approx$500meV. Additionally, the absorptionof the pump-beam creates a free-carrier profile within the 605nm zb-GaN layer.This leads to varying optical properties from sample surface to substrate,which are taken into account by grading analysis for an accurate description ofthe experimental data. A temporal resolution of 100fs allows in-depthinvestigations of occurring ultra-fast relaxation and recombination processes.We provide a quantitative description of the free-carrier concentration andabsorption onset at the sample surface as a function of relaxation,recombination, and diffusion yielding a characteristic relaxation time of0.19ps and a recombination time of 26.1ps.

Femtosecond pump-probe absorption edge spectroscopy of cubic GaN