Observation of compositional pulling phenomenon in AlxGa1-xN (0.4<x<1.0) films grown on (0001) sapphire substrates

High Al content AlxGa1-xN (0.4<1.0) films were grown at 1050 degreesC 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. theta-2theta x-ray diffraction data show a compositional separation in the AlxGa1-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 AlxGa1-xN sample grown directly on a LT AlN-coated (0001) sapphire substrate while only one absorption edge having higher energy was observed in the AlxGa1-xN film grown on a 0.1-mum-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 AlxGa1-xN films grown directly on the LT AlN-coated sapphire substrate. Such a compositional pulling phenomenon in the AlxGa1-xN film without a HT AlN intermediate layer is attributed to the extensive biaxial strain caused by the large mismatch between the AlxGa1-xN film and the (0001) sapphire substrate. (C) 2003 American Institute of Physics.