A series of Mg-doped thick InGaN layers with different Cp2Mg flows were grown on n-type GaN layers. The Mg doping effect on optical and electrical properties of InGaN:Mg was investigated through capacitance-voltage (C-V) measurements and temperature-resolved photoluminescence (PL). After annealing, p-type conductivity with acceptor concentrations about 3.5 x 10(18) cm(-3) and 9.5 x 10(17) cm(-3) were observed for the samples doped with little Cp2Mg. With the highest Cp2Mg flow, an inversion from p-type to n-type was observed by analysis of a Mott-Schottky (M-S) plot. The inversion of conductivity type was accompanied by a disappearance of InGaN band-to-band PL emission. It should be noted that annealing led to a substantial reduction of this band intensity. Thus, too high Mg doping is found to cause a strong compensation of p-type conductivity by nonradiative defects of n-type as it is seen from C-V and PL measurements. (c) 2012 Elsevier B.V. All rights reserved.