Boron and nitrogen co-doped diamond (BNDD) has attracted extensive research attention owing to its promising potential for n-type conductivity. However

boron–nitrogen (B–N)-related complexes in BNDD lead to highly complex optoelectronic properties

which hinders its practical applications. In this study

we present a systematic investigation of the electrical and optical properties of BNDDs with graded nitrogen doping

both in the as-grown state and after high-pressure high-temperature (HPHT) annealing. Nitrogen incorporation is found to sharply increase the resistivity of boron-doped diamond

dominated by enhanced hole scattering from nitrogen-associated defects and impurities. Following HPHT annealing

all BNDDs maintain stable p-type conduction with an elevated hole concentration

which benefits from the combined effects of neutral boron acceptor activation and intrinsic B–N complex dissociation. Strikingly

donor–acceptor pair (DAP) recombination luminescence appears concurrently with a drop in hole concentration in high-nitrogen BNDDs annealed above 2000 °C

directly confirming neutral nitrogen donor activation under HPHT conditions. This work provides a practical strategy for the performance modulation and optimization of BNDD

and reveals the activation effect of HPHT annealing on nitrogen impurities

providing a feasible path toward the realization of n-type diamond.