The implantation of Si ions into undoped high-resistivity GaN films is of interest for the realization of high-performance digital and monolithic microwave integrated circuits. We report the effect of postimplant annealing conditions on the electrical, optical, and surface morphology of Si ion-implanted GaN films. We demonstrate high activation efficiencies for low-dose Si implants into unintentionally doped GaN/sapphire heteroepitaxial films. The Si ions were implanted through an epitaxial AlN cap layer at 100 keV and a dose∼5×1014 cm−2.Samples were subsequently annealed in an open-tube furnace for various times and temperatures. The postanneal electrical activation is correlated with the surface morphology of the film after annealing. The samples annealed at1150 °CinN2for 5 min. exhibited a smooth surface morphology and a sheet electron concentrationns∼6.8×1013 cm−2.©1998 American Institute of Physics.