Résumé
We present experimental results obtained on wurtzite epitaxial GaN layers grown on sapphire and SiC substrates. Thanks to a set of samples with different values of the residual strain, we demonstrate that the high dislocation density enhances the spin relaxation rate through the Elliott-Yafet mechanism. This fact is validated by the T-1 temperature dependence of the spin-relaxation times. The influence of the valence-band structure on the hole-spin relaxation is also highlighted. In particular, a decrease in the hole-spin relaxation rate, accompanied by a strong polarization rate (similar to 50%) of the differential reflectivity signal (Delta R/R), is observed when the splitting Delta E-AB between the heavy-hole and the light-hole bands is larger than the broadening Gamma(A) of the A excitonic transition. On the contrary, the overlap of the A and B resonances for Gamma(A)>Delta E-AB is responsible for a decrease in the Delta R/R polarization rate (similar to 10%) and an enhancement of the spin relaxation rate.