  Metalorganic chemical vapor deposition (MOCVD) of GaN layers doped with Mg
atoms to the recognized optimum level of [Mg] $\sim2 \times 10^{19}$ cm$^{-3}$
has been performed. In a sequence of MOCVD runs, operational conditions,
including temperature and flow rate of precursors, have been maintained except
for intentionally larger flows of hydrogen carrier gas fed into the reactor. By
employing the largest hydrogen flow of 25 slm in this study, the performance of
the as-grown Mg-doped GaN layers has been certified by a room-temperature hole
concentration of $p\sim2 \times 10^{17}$ cm$^{-3}$ in the absence of any
thermal activation treatment. Experimental evidence is delivered that the large
amounts of hydrogen during the MOCVD growth can regulate the incorporation of
the Mg atoms into GaN in a significant way so that MgH complex can co-exist
with a dominant and evidently electrically active isolated MgGa acceptor.
