The recombination leakage current induced by planar isolation ofn‐p+AlGaAs graded heterojunction area with oxygen ion implantation followed by annealing at 500–650 °C has been investigated. The recombination leakage current is smaller than that induced by conventional isolation with hydrogen ion implantation; specifically, it is about one order of magnitude smaller after the higher temperature (600–650 °C) annealing. The main origin of the remaining recombination leakage current is probably not oxygen deep levels but rather recombination centers related to the ion implantation damage that remains after annealing. A point‐defect complex level, which may be related to the recombination center, is detected at an activation energy of around 0.8 eV by deep level transient spectroscopy.