AlGaN semiconductor with p-i-n structure was grown on sapphire subst rate using metal-organic chemical vapor deposition (MOCVD) system.Mesa-structure ultravio let detectors were fabricated by conventional technology.Ni/Au/Ni/Au (20nm/20nm/20nm/20nm) met al schemes were deposited by electron beam evaporation as p-type contact.After first annealing in air ambient at 550℃ for 3min and secondary annealing in N_2ambient at 750℃ for 30s,an ohmic contact formed.Analysis on the microstructure evolution of the annealed p-electrode is carried out by high -resolution transmission electron microscopy (HRTEM) and energy dispersive spectrometer (EDS ).The results reveal that after first annealing,elements diffusion occurrs in p-electrode ,but metal schemes maintain the initial layers of the as-deposited sample.And a 4nm-thick amo rphous layer appears on the interface of metal and semiconductor.While after secondary annealing,the m etal schemes change sharply and the amorphous layer disappears.Microstructure a nalysis on the metal-semiconductor interface shows a semi-coherent relationship and enhanced orderliness.Ni outdi ffused to the surface of p-electrode,Au indiffused to the surface of GaN,and Ga diffused to metal elec trode,which results in the enrichment of Au and Ga near the interface.These phe nomena contribute to the formation of p-AlGaN ohmic contact.