INVESTIGATIONS ON PHASE-TRANSITIONS WITHIN THIN AL2O3 LAYERS ON NIAL(001) - HREELS ON ALUMINUM-OXIDE FILMS

We have investigated the phase transitions and the structure of thin a luminum oxide layers on NiAl(001) using high-resolution electron energ y loss spectroscopy (HREELS), low-energy electron diffraction (LEED) a nd Auger electron spectroscopy (AES). The oxide films were prepared ei ther by oxygen adsorption at 300 K and annealing to different temperat ures or by oxidation at different temperatures up to 1500 K. We have d emonstrated that in connection with LEED and AES data the HREEL spectr a allow the determination of the structure of the alumina layers. Diff erent phases of the Al2O3 layers have been observed depending on annea ling or oxidation temperature. The observed energy losses are high-fre quency branches omega(+) of Fuchs-Kliewer (FK) surface phonons. Calcul ated spectra based on dielectric theory reproduce the measured spectra very well. The thickness of the well-ordered theta-Al2O3 layer is est imated to be about 10 Angstrom.