Mg-doped p-GaN films with various Mg flow rates were grown on a sapphire substrate by metal-organic chemical-vapor deposition (MOCVD) at a very low reactor pressure of 50 mbar. The surface roughness of R-rms decreased from 23.4 to 0.72 nm with a decrease in the Mg flow rate from 118 nmol min(-1) to 30 nmol min(-1). In addition, a high Mg activation efficiency of similar to 5% was achieved by an optimized Mg flow rate. The photoluminescence (PL) and (0 0 2) X-ray diffraction (XRD) measurements indicated that the density of Mg-Ga-V-N complexes and screw-type dislocations in the p-GaN films were reduced under a low Mg flow rate. Fitting the variable-temperature Hall data indicated that the acceptor activation energy and the compensation ratio were similar to 151 meV and similar to 10%, respectively. It is therefore believed that the decrease in the compensation effect, by decreasing the density of the compensating center such as Mg-Ga-V-N complex and/or screw dislocation, plays an important role in improving the Mg acceptor activation efficiency in the low pressure growth of p-GaN films. (C) 2012 Elsevier B.V. All rights reserved.