DX-center formation in wurtzite and zinc-blendeAlxGa1−xN

The transition from shallow to deep centers as a function of pressure or alloying is investigated for oxygen and silicon donors in GaN and AlN, based on first-principles total-energy calculations. The stability of the localized deep state ($\mathrm{DX}$ center) is found to depend on interactions between the impurity and third-nearest-neighbor atoms, which occur in different positions in the zinc-blende and the wurtzite phase. $\mathrm{DX}$-center formation is suppressed in the zinc-blende phase, as well as for silicon donors. The results strengthen the identification of oxygen as the unintentional dopant in $n$-type GaN, and shed new light on the driving force for $\mathrm{DX}$ formation.