We investigated the effects of thermal annealing of graphene on its electrical properties with the aim of improving the electrical contacts between graphene and the p-GaN layer. Compared with pristine graphene which shows ambipolar characteristics, graphene thermally annealed under NH3 atmosphere exhibited obvious p-type characteristics and increased conductance, representing the hole doping effect in the graphene. This result is contrary to the expectation for substitutional doping of nitrogen in graphene, and can be attributed to the physisorption of gas molecules on the graphene surface. Due to the lowering of the Fermi level and the enhancement in the conductivity of annealed graphene, electrical contact to the p-type GaN has been improved, with the specific contact resistance decrease from 1.02 Omega.cm(2) to 0.49 Omega.cm(2). (C) 2013 Elsevier B.V. All rights reserved.