SUBPICOSECOND KINETICS OF BAND-EDGE ABSORPTION IN AL0.25GA0.75AS

We report on transient photoinduced transmission in the vicinity of the fundamental absorption edge of a 200-nm-thick Al0.25Ga0.75As layer after excitation with a 150-fs optical pulse. The peak of the differential transmission spectrum appears at 1.75 eV independent of the excitation energy which was varied between 1.75 and 1.81 eV. The theoretical modeling of the photoinduced transmission requires consideration of both the exciton and unbound-electron-hole-pair contributions. Calculations of the transient spectra taking into account band filling, band-gap renormalization, and screening of the Coulomb interaction reveal the dominant influence of Coulomb screening on the band-edge-absorption kinetics. Coulomb screening results in a reduction of the exciton binding energy and a decrease in the Coulomb enhancement factor describing the reduced absorption of the continuum state. The plasma dynamics in the nonthermalized regime are described with use of a full dynamic screening of all scattering processes. The dynamic screening critically reduces the plasma thermalization time with respect to the results obtained for the static screening. The agreement with experimental data is significantly improved.