Local structures of free-standing Al_xGa_1-xN thin films studied by extended x-ray absorption fine structure

Local structural information for the first two atomic shells surrounding Ga atoms in free standing Al_xGa_1-xN alloy films has been obtained by extended x-ray absorption fine structure spectroscopy. For an AlN mole fraction ranging from 0 to 0.6, we found that the first shell Ga-N bond length had only a weak composition dependence, roughly one quarter of that predicted by Vegard's Law. In the second shell, the Ga-Ga bond length was significantly longer than that of Ga-Al (Δ∼0.04-0.065 Å). A bond-type specific composition dependence was observed for the second shell cation-cation distances. While the composition dependence of the Ga-Ga bond length is ∼70% of that predicted by Vegard's Law, the Ga-Al bond length was essentially composition independent. These results suggested that local strain in Al_xGa_1-xN was also accommodated by lattice distortion in the Al cation sublattice.