III-nitride materials, such as ternary alloys of gallium nitride (GaN) and aluminum nitride (AlN), are the prominent semiconductor systems in research and industry due to their importance for optoelectronic applications using ultraviolet (UV) spectral range. Although significant efforts have been made over the last two decades, the main drawback of epitaxial structures hindering their full potential in devices is still associated with obtaining reasonably good p-doping control. Here, an effect of acceptor activation by post-growth treatments, that is conventional and rapid thermal annealing, was studied, revealing that while selecting inappropriate conditions p-type AlGaN structures with microstructural degradation, surface precipitation, Mg out-diffusion and poor electrical properties are achieved. The observed planar segregation in a form of pyramidal domains (Mg-rich features), associated with Mg overdose and its limited solubility in AlGaN (similar to 5 x 10(19) cm(-3)) results in a decrease of the hole concentration. However, rapid thermal annealing in oxidizing and then reducing atmospheres leads to controlled oxygen co-doping of a p-type layer, and at the same time acceptor activation is enhanced and the carrier concentration is increased, >10(18) cm(-3). Therefore, rapid thermal annealing of Mg-doped AlGaN structures, in particular using oxygen atmosphere, is advantageous to obtain relatively high carrier concentration and p-type conduction.