We have systematically investigated the n-type doping characteristics of O-implanted GaN and AlGaN from the viewpoint of annealing temperature. The n-type regions were produced in undoped materials by O+ implantation and subsequent annealing with an SiO2 encapsulation layer at temperatures between 1000 and 1300degreesC. From room-temperature Hall-effect measurements, both materials showed an increase in sheet carrier concentration with the rising of annealing temperature. The effective activation efficiency tended to decrease from 1.3 to 0.5% with increasing Al content in AlxGa1-xN up to 10%, even after a 1300degreesC anneal. Furthermore, O-implanted AlGaN displayed a large increase of electron mobility with increasing annealing temperature, which was clearly different from the situation of O-implanted GaN. These results indicate that the high-temperature annealing for O-implanted AlGaN probably enables an improvement of crystallinity in addition to an increase of O-doping characteristics. (C) 2004 The Electrochemical Society.