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 aluminum oxide layers on NiAl(001) using high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The oxide films were prepared either by oxygen adsorption at 300 K and annealing to different temperatures or by oxidation at different temperatures up to 1500 K. We have demonstrated that in connection with LEED and AES data the HREEL spectra allow the determination of the structure of the alumina layers. Different phases of the Al2O3 layers have been observed depending on annealing or oxidation temperature. The observed energy losses are high-frequency branches omega(+) of Fuchs-Kliewer (FK) surface phonons. Calculated spectra based on dielectric theory reproduce the measured spectra very well. The thickness of the well-ordered theta-Al2O3 layer is estimated to be about 10 Angstrom.