Growth conditions are used to control the residual carbon impurity incorporation in p(++) -GaN layers. Specific contact resistance (rho c) with various residual carbon concentrations has been investigated through the circular transmission line model (CTLM) method and secondary ion mass spectroscopy (SIMS) analysis. A correlation between residual carbon and rho c indicates that incorporation of proper carbon impurity can be an advantage for Ohmic contact, although carbon can also act as a compensating donor to worsen the Ohmic contact at a very high concentration. Finally, rho c is improved to 6.80 x 10 (5) Omega x cm(2) with a carbon concentration of 8.3 x 10(17) cm (3) in p(++) -GaN layer, when the growth temperature, pressure and flow rate of CP2Mg and TMGa are 940 degrees C, 100 Torr, 3 mu mol/min and 28 mu mol/ min, respectively. (C) 2017 Elsevier B.V. All rights reserved.