Doping of AlxGa1−xN

$\mathrm{N-}$ type ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ exhibits a dramatic decrease in the free-carrier concentration for $\text{x⩾0.40.}$ Based on first-principles calculations, we propose that two effects are responsible for this behavior: (i) in the case of doping with oxygen (the most common unintentional donor), a $\mathrm{DX}$ transition occurs, which converts the shallow donor into a deep level; and (ii) compensation by the cation vacancy ${\mathrm{(V}}_{\mathrm{Ga}}$ or $V_{\mathrm{Al}}\mathrm{),}$ a triple acceptor, increases with alloy composition $\mathrm{x.}$ For $\mathrm{p-}$ type doping, the calculations indicate that the doping efficiency decreases due to compensation by the nitrogen vacancy. In addition, an increase in the acceptor ionization energy is found with increasing $\mathrm{x.}$