Atomic structures of supersaturated ZnO- Al2 O3 solid solutions

Supersaturated ZnO- Al2 O3 (>20 at. % Al) thin films are grown by pulsed laser deposition technique on silica glass substrates at 600 °C. They are characterized by combining x-ray diffraction, Al-K edge x-ray absorption near edge structures (XANESs), high resolution transmission electron microscope (TEM) imaging, TEM analysis, and a series of first principles calculations. The films are composed of textured wurtzite grains with c planes parallel to the substrate. The distance between c planes expands significantly when the Al concentration is greater than 10 at. %. The expansion disappears after annealing the films at above 800 °C. High density of dislocationlike defects is found in the as deposited film. Any segregation of Al cannot be detected either at the grain boundaries or inside the grains. The lattice expansion toward c axis and the experimental XANES can be satisfactorily explained by taking a hypothetical homologous model with the composition of (ZnO)3 (Al2 O3) as the local environment of Al in the supersaturated solid solution. Simplified substitutional models with Al at the Zn site in wurtzite ZnO cannot explain these experimental results. First principles calculations show that the homologous phase is energetically more favorable than the simplified substitutional models, although decomposition into ZnO and Zn Al2 O4 is more favorable than the homologous phase. The local atomic structures of the supersaturated solid solution are therefore concluded to be analogous to the metastable homologous phase.