Observation of compositional pulling phenomenon in Al_xGa_1-xN (0.4≪x≪1.0) films grown on (0001) sapphire substrates

High Al content Al_xGa_1-xN (0.4≪x≪1.0) films were grown at 1050 °C either on (0001) sapphire substrates or on AlN films by alternative supply of group III metalorganics (trimethylaluminum and trimethylgallium) and NH₃ in an inductively heated quartz reactor operated at atmospheric pressure. θ–2θ x-ray diffraction data show a compositional separation in the Al_xGa_1-xN films grown on low-temperature (LT) aluminum nitride (AlN) buffer layer-coated (0001) sapphire substrates. Absorption spectroscopic measurements also reveal double cutoff edges in the Al_xGa_1-xN sample grown directly on a LT AlN-coated (0001) sapphire substrate while only one absorption edge having higher energy was observed in the Al_xGa_1-xN film grown on a 0.1-μm-thick high-temperature (HT) AlN intermediate layer, which was deposited on top of the (0001) sapphire substrate. Cross-sectional transmission electron microscopic observations show the bilayer nature of the Al_xGa_1-xN films grown directly on the LT AlN-coated sapphire substrate. Such a compositional pulling phenomenon in the Al_xGa_1-xN film without a HT AlN intermediate layer is attributed to the extensive biaxial strain caused by the large mismatch between the Al_xGa_1-xN film and the (0001) sapphire subs- - trate. © 2003 American Institute of Physics.