Résumé
By performing nondegenerate pump- probe experiments, we study the relaxation dynamics of spin- polarized A and B excitons in wurtzite epitaxial GaN. By analyzing the differential reflectivity spectra (Delta R/R) of the two circularly polarized probe components, we are able to identify each spin relaxation channel ( electron, light- and heavy- hole spin flips ) separately and to extract characteristic times of the different spin relaxation processes. In addition, spectral oscillatory features are observed for negative delays. They show a rapid rise determined by the fast dephasing time T-2 of the excitonic transitions. We show that the high density of dislocations increases the spin relaxation of electrons and holes through the Elliot - Yafet mechanism and makes the exciton dephasing time very short. The measured heavy- hole relaxation time, which is not extremely short compared to the electron relaxation time, can be related to the band structure, in which the degeneracy between different spin valence bands is partially lifted.