In this work the effect of carrier gas and post-growth activation conditions on the electrical properties of Mg-doped p-GaN single layers grown in a vertical flow close-coupled showerhead MOCVD system is investigated. The results of Hall effect measurements show that although the optimal Mg precursor flow rate depends on the growth atmosphere and is smaller when N-2 is used as a carrier gas, similar electrical properties could be realized with both H-2 and N-2 carrier gases. The results of SIMS analysis demonstrate that while Mg incorporation is higher for N-2 carrier gas, the efficiency of Mg activation is higher when H-2 is used as carrier gas. We have also observed that the structural quality of N-2 grown layers is substantially lower compared to the H-2 case. (c) 2006 Elsevier B.V. All rights reserved.