  The hot-wall metal-organic chemical vapor deposition (MOCVD), previously
shown to enable superior III-nitride material quality and high performance
devices, has been explored for Mg doping of GaN. We have investigated the Mg
incorporation in a wide doping range ($2.45\times{10}^{18}~cm^{-3}$ up to
$1.10\times{10}^{20}~cm^{-3}$) and demonstrate GaN:Mg with low background
impurity concentrations under optimized growth conditions. Dopant and impurity
levels are discussed in view of Ga supersaturation which provides a unified
concept to explain the complexity of growth conditions impact on Mg acceptor
incorporation and compensation. The results are analysed in relation to the
extended defects, revealed by scanning transmission electron microscopy (STEM),
X-ray diffraction (XRD), and surface morphology, and in correlation with the
electrical properties obtained by Hall effect and capacitance-voltage (C-V)
measurements. This allows to establish a comprehensive picture of GaN:Mg growth
by hot-wall MOCVD providing guidance for growth parameters optimization
depending on the targeted application. We show that substantially lower H
concentration as compared to Mg acceptors can be achieved in GaN:Mg without any
in-situ or post-growth annealing resulting in p-type conductivity in as-grown
material. State-of-the-art $p$-GaN layers with a low-resistivity and a high
free-hole density (0.77 $\Omega$.cm and $8.4\times{10}^{17}~cm^{-3}$,
respectively) are obtained after post-growth annealing demonstrating the
viability of hot-wall MOCVD for growth of power electronic device structures.
