GaN layers grown on sapphire substrates were characterized using high resolution optical reflectivity and absorption spectroscopy in the region of ground and excited exciton states. The main exciton parameters are deduced from calculations of reflectivity contours for A and B exciton S-states. The parameters of the Γ5 state of the A-exciton as well as those of the Γ5 and Γ1 states of the B-exciton are determined from a comparative analysis of reflectivity and absorption spectra in thin layers. The influence of strains inherent to MOCVD-grown GaN layers on the exciton parameters including effective masses and longitudinal-transverse splitting is discussed. Electron transitions from the three (Γ9, Γ7, Γ7) upper valence bands to the second Ec2 conduction band of Γ3 symmetry were evidenced.
