We have investigated the growth of magnesium-doped GaP (GaP:Mg) layers on GaN by metalorganic chemical vapor deposition. The hole carrier concentration increased linearly from 0.8 x 1018 to 4.2 x 10(18) cm(-3) as the Bis(cyclopentadienyl) magnesium (Cp2Mg) mole flow rate increased from 1.2 x 10(-7) to 3.6 x 10(-7) mol/min. However, the hole carrier concentration decreased when the CP2Mg mole flow rate was further increased. The double crystal X-ray diffraction (DCXRD) rocking curves showed that the GaP:Mg layers were single crystalline at low CP2Mg molar flow. However, the GaP:Mg layers became polycrystalline if the CP2Mg molar flow was too high. The decrease in hole carrier concentration at high CP2Mg molar flow was due to crystal quality deterioration of the GaP layer, which also resulted in the low hole mobility of the GaP:Mg layer. (C) 2010 Elsevier B.V. All rights reserved.