Mg diffusion from p++ GaN cap layer degrades optically active acceptor density and radiative recombination in p-GaN

We report the adverse impact of Mg diffusion from a heavily doped p++ GaN cap layer on the optically active acceptor density and radiative recombination performance of the underlying p-GaN layer grown by metalorganic chemical vapor deposition. Secondary ion mass spectrometry depth profiling confirms significant longitudinal Mg diffusion from the p++ cap layer into p-GaN, leading to a total Mg concentration comparable to that of directly in situ doped samples. However, 30 K low-temperature photoluminescence measurements reveal that this diffusion-induced Mg increment hardly contributes to the density of optically active MgGa acceptors and instead introduces massive defect centers that suppress radiative recombination in p-GaN. The performance degradation is attributed to interstitial Mg and charged defect complexes introduced by diffusion, which induce strong ionized impurity scattering and reduce the radiative recombination probability of photogenerated carriers. This finding provides critical guidance for the p-type doping structure design of GaN-based optoelectronic and power electronic devices.