Acceptor activation of Mg-doped GaN-Effects of N-2/O-2 vs N-2 as ambient gas during annealing

Here, we investigate the effects of O-2:N-2 (1:1) as ambient gas as compared with pure N-2 during activation annealing of Mg as p-type doping in GaN layers grown by MOCVD. The purpose is to understand the impact of O-2 on the resulting free hole concentration and hole mobility using SIMS, XRD, STEM, AFM, and Hall effect measurements. Even though the presence of O-2 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 N-2. 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 N-2 only as ambient gas, while annealing with O-2:N-2 (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.