Acceptor activation of Mg-doped GaN—Effects of N₂/O₂ vs N₂ as ambient gas during annealing

Here, we investigate the effects of O₂:N₂ (1:1) as ambient gas as compared with pure N₂ during activation annealing of Mg as p-type doping in GaN layers grown by MOCVD. The purpose is to understand the impact of O₂ on the resulting free hole concentration and hole mobility using SIMS, XRD, STEM, AFM, and Hall effect measurements. Even though the presence of O₂ in the ambient gas during annealing is very effective in reducing the H level of the Mg-doped GaN layers, the maximum achievable hole concentration and mobility is still higher with pure N₂. The differences are explained by an in-diffusion of O to the GaN layer acting as n-dopant and, thus, giving rise to a compensation effect. The Mg–H complexes at substitutional (Mg_Ga), i.e., the electrically active acceptor sites that provide free holes, are preferentially activated by annealing with N₂ only as ambient gas, while annealing with O₂:N₂ (1:1) also dissociates electrically inactive Mg–H complexes resulting in much less residual H. At the lower growth pressure of 150 mbar compared to higher growth pressure of 300 mbar, an increasing carbon incorporation leads to a compensation effect drastically reducing the free hole concentration while the mobility is unaffected.