Local structure of (Ga,Fe)N and (Ga,Fe)N:Si investigated by x-ray absorption fine structure spectroscopy

X-ray absorption fine-structure (XAFS) measurements supported by ab initio computations within the density functional theory (DFT) are employed to systematically characterize Fe-doped as well as Fe- and Si-codoped films grown by metalorganic vapor-phase epitaxy. The analysis of extended-XAFS data shows that depending on the growth conditions, Fe atoms either occupy Ga substitutional sites in GaN or precipitate in the form of epsilon-Fe3N nanocrystals, which are ferromagnetic and metallic according to the DFT results. Precipitation can be hampered by reducing the Fe content, by increasing the growth rate, or by codoping with Si. The near-edge region of the XAFS spectra provides information on the Fe charge state and shows its partial reduction from Fe+3 to Fe+2 upon Si codoping, in agreement with the Fe electronic configurations expected within various implementations of DFT.