The structures and defects are studied in arsenic-ion-implanted GaAs(As+–GaAs) films annealed at temperatures higher than 600 °C by using transmission electron microscopy, deep level transient spectroscopy, temperature-dependent conductance, and photoluminescence. The estimated concentration of arsenic precipitates in films decreases from ∼4×1016 cm-3 to ∼6×1015 cm-3 and the corresponding size increases from ∼3 to ∼10 nm as the annealing temperature increases from 600 to 800 °C. A defect with an energy level at about 0.3 eV from the band edge is found and its concentration increases with the increasing annealing temperatures. The electrical transport of free carriers is replaced by hopping conduction, through the defect band at about 0.26 eV below conduction band, when the film is annealed at temperature 800 °C. It indicates that during high-temperature annealing the defect of the arsenic and gallium vacancies due to the diffusion of As and Ga atoms is the dominant factor to change its electrical and structural properties. © 1997 American Institute of Physics.