DX-center formation in wurtzite and zinc-blende AlxGa1-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 (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. 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 DX formation.