Hole conductivity and compensation in epitaxial GaN:Mg layers

The concentration p and the mobility mu of holes in metal-organic chemical vapor deposition (MOCVD) GaN:Mg layers were studied by room temperature Hall-effect measurements as a function of the Mg concentration N-A in the range 3 x 10(18) cm(-3) less than or equal to N-A less than or equal to 1 x 10(20) cm(-3). The hole density first increases with increasing N-A, reaches a maximum value p(max) approximate to 6 x 10(17) cm(-3) at N-A approximate to 2 x 10(19) cm(-3), decreases for larger N-A values, and drops to very small values at N-A approximate to 1 x 10(20) cm(-3). The hole mobility decreases monotonically with increasing N-A. The p(N-A) data provide strong evidence for self-compensation, i.e., for a doping driven compensation of the Mg acceptor by intrinsic donor defects. This effect becomes significant when N-A exceeds a value of 2 x 10(19) cm(-3). A semiquantitative self-compensation model involving nitrogen vacancies is developed. It accounts satisfactorily for the measured p(N-A) dependence and suggests that self-compensation limits the hole conductivity in bulklike MOCVD GaN:Mg layers grown near 1300 K to about 1.2 (Ohm cm)(-1).