DX-CENTER FORMATION IN WURTZITE AND ZINCBLENDE 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 A lN, based on first-principles total-energy calculations. The stability of the localized deep state (DX center) is found to depend on interac tions between the impurity and third-nearest neighbor atoms, which occ ur in different positions in the zinc-blende and the wurtzite phase. D X-center formation is suppressed in the zinc-blende phase, as well as for silicon donors. The results strengthen the identification of oxyge n as the unintentional dopant in n-type GaN, and shed new light on the driving force for DX formation.