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The Ni∕AlGaN interfaces in AlGaN∕GaN Schottky diodes were investigated to explore the physical origin of postannealing effects in AlGaN∕GaN heterostructures using electron beam induced current (EBIC) and current-voltage characteristics. Black spot density in EBIC images of as-deposited Ni∕AlGaN∕GaN diodes is in the same order of 108cm−2 as the dislocation density of AlGaN∕GaN heterostructures characterized by atomic force microscopy, indicating that recombination sites near Ni∕AlGaN interface are related to dislocations. The EBIC images of the annealed diodes showed that the postannealing reduced the electrically active states at the Schottky metal/AlGaN interfaces. The thermal reaction near Ni∕AlGaN interface due to the postannealing induced passivation effect, leading to decrease in reverse leakage current density, ideality factor, and saturation current density of the diodes and increase in the Schottky barrier height. We suggest that the postannealing process is an effective way to passivate the AlGaN∕GaN heterojunction field effect transistors.