  A comprehensive first-principles study of the energetics, electronic and
magnetic properties of Co-doped GaN(0001) thin films are presented and the
effect of surface structure on the magnetic coupling between Co atoms is
demonstrated. It is found that Co atoms prefer to substitute the surface Ga
sites in different growth conditions. In particular, a CoN/GaN interface
structure with Co atoms replacing the first Ga layer is preferred under N-rich
and moderately Ga-rich conditions, while CoGax/GaN interface is found to be
energetically stable under extremely Ga-rich conditions. It's worth noted that
the antiferromagnetic coupling between Co atoms is favorable in clean GaN(0001)
surface, but the existence of FM would be expected to occur as Co concentration
increased in Ga-bilayer GaN(0001) surface. Our study provides the theoretical
understanding for experimental research on Co-doped GaN films and might promise
the Co:GaN system potential applications in spin injection devices.
