Effect of thermal annealing on the optical and structural properties of γ-Al2O3 films prepared on MgO substrates by MOCVD

Metalorganic chemical vapor deposition (MOCVD) of γ-Al2O3 films is performed on MgO (110) and (111) substrates by using trimethylaluminum and O2 as the precursors. The effects of post-deposition annealing on the microstructure and epitaxial relationship of the films are investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Schematic diagrams are proposed to illuminate the epitaxial relationships between the γ-Al2O3 films and MgO substrates. The γ-Al2O3 films annealed at 1000 °C exhibit the best crystalline quality, for which clear epitaxial relationships of γ-Al2O3 (110)∥MgO (110) with γ-Al2O3[1¯10]∥MgO [1¯10] and γ-Al2O3 (111)∥MgO (111) with γ-Al2O3[11¯0]∥MgO [11¯0] have been ascertained. The average transmittance of the obtained samples in the visible range is over 85%. The optical band gaps of the γ-Al2O3 films annealed at 1000 °C on MgO (110) and (111) substrates are about 5.81 and 5.80 eV, respectively, which are a bit smaller than those of the as-deposited films.