Transient dielectric function and carrier related processes in doped cubic GaN determined by femtosecond pump–probe spectroscopic ellipsometry

Abstract

We employ time-resolved spectroscopic ellipsometry to determine the transient dielectric function of cubic GaN for delay-times between 200 fs and 4 ns after excitation. This pump and probe based technique uses a high energy laser pulse (266 nm, 35 fs fundamental pulse duration) to generate large electron–hole pair concentrations. These electrons relax down into the conduction band minimum with a characteristic relaxation time of 0.19 ps. The resulting phase-space band filling leads to a blue shift of the absorption edge, visible in the dielectric function for each delay-time. The greatest shift of the absorption edge is observed after ≈1 ps. Afterward, the free-electron concentration in the conduction band minimum decreases due to recombination and diffusion. After ≈100 ps, the absorption edge has nearly recovered into the steady-state case. In this study, we systematically investigate both doped cubic GaN and the effect of changing the pump intensity. This leads to a shift of the absorption edge between 200 and 500 meV for low and high pump intensities, respectively. Our analysis accounts for a free-carrier gradient within the cubic GaN (induced by the pump-pulse absorption), as well as the relaxation, recombination, and diffusion processes. More than 130 individual spectra are analyzed by the same modeling procedure to provide an objective comparison between different samples and pump power densities.